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Porous Anodic Aluminum Oxide: Anodization and TemplatedSynthesis of Functional NanostructuresWoo Lee*,†,‡ and Sang-Joon Park†

†Korea Research Institute of Standards and Science (KRISS), Yuseong, 305-340 Daejeon, Korea‡Department of Nano Science, University of Science and Technology (UST), Yuseong, 305-333 Daejeon, Korea

CONTENTS

1. Introduction 74872. Types of Anodic Aluminum Oxide (AAO) 74883. Ionic Conduction in Anodic Oxide Films 7490

3.1. High-Field Conduction Theory 74903.2. Elementary Interfacial Reactions 74903.3. Transport Numbers 74913.4. Stress-Driven Ionic Transport 7492

4. Electrolytic Breakdown 74934.1. Factors Influencing Breakdown 7493

4.1.1. The Nature of Anodized Metal 74944.1.2. Electrolyte Conditions 74944.1.3. Current Density (j) 74944.1.4. Other Factors Influencing Breakdown 7494

4.2. Models for Breakdown 74954.2.1. Electron Avalanche Multiplication 74954.2.2. Stress-Driven Breakdown 7496

5. Structure of Porous Anodic Aluminum Oxide(AAO) 74975.1. General Structure 7497

5.1.1. Pore Diameter (Dp) 74975.1.2. Interpore Distance (Dint) 74995.1.3. Barrier Layer Thickness (tb) 7499

5.2. Structure of Pore Wall (Anion Incorporation) 74995.3. Effect of Heat Treatments 7502

6. Growth of Porous Anodic Aluminum Oxide (AAO) 75026.1. Stress Generation in Anodic Oxide Films 7502

6.1.1. Volume Expansion 75026.1.2. Stress Measurements 75036.1.3. Effects of External Stresses on Pore

Growth 75056.2. Initial-Stage Pore Formation 7506

6.2.1. Qualitative Description on Pore Forma-tion 7506

6.2.2. Kinetics of Porosity Initiation 7506

6.2.3. Morphological Instability 75076.3. Steady-State Pore Formation 7509

6.3.1. Joule’s Heat-Induced Chemical Dissolu-tion 7509

6.3.2. Field-Assisted Oxide Dissolution 75096.3.3. Average Field Model for Steady-State

Pore Structure 75106.3.4. Direct Cation Ejection Mechanism 75116.3.5. Flow Model for Steady-State Pore

Formation 75127. Self-Ordered Porous Anodic Aluminum Oxide

(AAO) 75137.1. Mild Anodization (MA) 75147.2. Hard Anodization (HA) 75167.3. Pulse Anodization (PA) 75187.4. Cyclic Anodization (CA) 75217.5. Anodization of Thin Aluminum Films De-

posited on Substrates 75218. Long-Range Ordered Porous AAO 75249. AAO Template-Based Synthesis of Functional

Nanostructures 75289.1. Electrochemical Deposition (ECD) 75289.2. Electroless Deposition (ELD) 75319.3. Sol−Gel Deposition 75319.4. Surface Modification 75339.5. Template Wetting 75389.6. Mask Techniques 75399.7. Chemical Vapor Deposition (CVD) 75409.8. Atomic Layer Deposition (ALD) 7541

10. Closing Remarks and Outlook 7543Author Information 7544

Corresponding Author 7544Notes 7544Biographies 7545

Acknowledgments 7545Abbreviations 7545References 7547

1. INTRODUCTION

In ambient atmospheres, aluminum becomes rapidly coatedwith a compact 2−3 nm thick oxide layer. This native oxidelayer prevents the metal surface from further oxidation. Becauseof the surface native oxide, aluminum generally has goodcorrosion resistance. However, local corrosion of metal canoccur in rather aggressive outdoor environments, containing

Received: January 2, 2014Published: June 13, 2014

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corrosive chemicals (e.g., chlorides or sulfates). In 1857, Bufffirst found that aluminum can be electrochemically oxidized inan aqueous solution to form an oxide layer that is thicker thanthe native one.1 This phenomenon has been called “anodiza-tion” because the aluminum part to be processed constitutesthe anode in an electrolytic cell. In the early 1920s, thephenomenon observed by Buff was exploited for industrial scaleapplications, for example, protection of seaplane parts fromcorrosive seawater.2 In general, the anodic aluminum oxide(AAO) films form with two different morphologies (i.e.,nonporous barrier-type oxide films and porous-type oxidefilms) depending mainly on the nature of the anodizingelectrolyte.3 Because the process was first implemented forprotection purposes, the anodization of aluminum and itsalloys, particularly porous-type anodization, has receivedconsiderable attention in the industry because of its extensivepractical applications. Many desirable engineering propertiessuch as excellent hardness, corrosion, and abrasion resistancecan be obtained by anodizing aluminum metals in acidelectrolytes.4 In addition, due to its high porosity, the porousoxide films formed on the metals serve as a good adhesion basefor electroplating, painting, and semi-permanent decorativecoloration. The anodized products can be easily found inelectronic gadgets, electrolytic capacitors, cookware, outdoorproducts, plasma equipment, vehicles, architectural materials,machine parts, etc. Recently, this nearly century-old industrialprocess has been drawing increasing attention from scientists inthe field of nanotechnology. This trend originated with theseminal works of Masuda and co-workers, who reported onself-ordered porous AAO in 19955 and the subsequentdevelopment of the two-step anodization process in 1996.6

Porous AAO film grown on aluminum is composed of a thinbarrier oxide layer in conformal contact with aluminum, and anoverlying, relatively thick, porous oxide film containingmutually parallel nanopores extending from the barrier oxidelayer to the film surface.7 Each cylindrical nanopore and itssurrounding oxide region constitute a hexagonal cell alignednormal to the metal surface. Under specific electrochemicalconditions, the oxide cells self-organize into hexagonal close-packed arrangement, forming a honeycomb-like structure.5−7

Pore diameter and density of self-ordered porous AAOs aretunable in wide ranges by properly choosing anodizationconditions: pore diameter = 10−400 nm and pore density =108−1010 pores cm−2. The novel and tunable structural featuresof porous AAOs have been intensively exploited for synthesiz-ing a diverse range of nanostructured materials in the forms ofnanodots, nanowires, and nanotubes, and also for developingfunctional nanodevices.The objective of this Review is to provide a solid information

source for researchers entering this field and to establish abroad and deep knowledge base. This Review introduces thefundamental electrochemical processes associated with anodicoxidation of aluminum, and discusses the recent progress onanodization of aluminum for the development of orderedporous AAOs, and nanotechnology applications of porousAAOs. We organize this Review as follows: after discussing thegrowth characteristics of two different types of AAOs (section2), we will describe the theory of ionic conductions andelementary interfacial reactions (section 3), followed byelectrolytic breakdown (section 4) to understand thefundamental electrochemistry associated with anodic oxidationof aluminum. Next, the electrochemical factors defining thegeometric and chemical structures of porous AAOs will be

discussed (section 5). Anodization of aluminum is a volumeexpansion process, and thus is accompanied by mechanicalstresses. Recent studies have indicated that the stresses haveprofound implications not only on the ionic transport, but alsoon the self-ordering behavior of oxide nanopores. We willdiscuss in detail the effect of stress on pore growth (section6.1), the kinetics of pore initiation, and morphologicalinstability associated with the early stage of anodization(section 6.2), and recent models describing steady-state poreformation (section 6.3). After that, recent progress onanodization of aluminum used in fabricating self-orderedporous AAO and also for engineering internal pore structureswill be discussed (section 7). In addition, various approaches tolong-range order porous AAO will be reviewed (section 8). Inthe last part of this Review (section 9), various chemicalapproaches for the syntheses of low-dimensional functionalnanostructures and the fabrications of advanced nanodeviceswill be discussed. These approaches include electrochemicaldeposition (ECD), electroless deposition (ELD), sol−geldeposition, surface modification, template wetting, shadowmask techniques, chemical vapor deposition (CVD), andatomic layer deposition (ALD). Chemistry issues encounteredin the template-based synthesis of functional nanostructureswill be discussed in detail. Finally, we will present thechallenges and future prospects of the field (section 10).

2. TYPES OF ANODIC ALUMINUM OXIDE (AAO)Anodization of aluminum in aqueous electrolytes forms anodicoxide films with two different morphologies, that is, thenonporous barrier-type oxide films and the porous-type oxidefilms. The chemical nature of the electrolytes mainlydetermines the morphology of AAOs.3,7,8 A compact non-porous barrier-type AAO films can be formed in neutralelectrolytes (pH 5−7), such as borate, oxalate, citrate,phosphate, adipate, tungstate solution, etc., in which the anodicoxide is practically insoluble.9,10 Meanwhile, porous-type AAOsare formed in acidic electrolytes, such as selenic,11 sulfuric,12

oxalic,12 phosphoric,7,12,13 chromic,12,14 malonic,12,15−17 tarta-ric,12,18 citric,12,17−20 malic acid,12,18 etc., in which anodic oxideis slightly soluble. Early models describing anodic oxide growthwere developed on the basis of the barrier-type oxide.21−24

Moreover, in the early stage of porous-type oxide growth, theformation of the initial barrier oxide is followed by theemergence of incipient pores. Therefore, in this Review, we willmention the barrier-type oxide growth to the extent needed forunderstanding porous-type oxide formation. Some excellentreview articles covering the barrier-type anodic oxide films aregiven in refs 3 and 25.The two types of anodic oxides (i.e., barrier- vs porous-type

AAO) differ in their oxide growth kinetics. In the case ofbarrier-type oxide formation under potentiostatic conditions(i.e., U = constant), current density (j) decreases exponentiallywith time (t). Correspondingly, the film growth rate decreasesalmost exponentially with time (t), which places a limit on themaximum film thickness obtainable for barrier-type AAO films(Figure 1). It has been experimentally verified that thethickness of barrier-type film is directly proportional to theapplied potential (U). On the other hand, current density (j) inporous-type anodization under potentiostatic conditionsremains almost constant within a certain range of values duringthe anodization process, due to the constant thickness of thebarrier layer at the pore bottom. The thickness of the resultingporous oxide film is linearly proportional to the total amount of

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charge (i.e., anodization time, t) involved in the electrochemicalreaction.Radiotracer studies, employing an immobile marker (125Xe),

have indicated that, in the case of barrier-type oxide formation,anodic alumina grows simultaneously at the oxide/electrolyteinterface and at the metal/oxide interface, through Al3+ egressand O2−/OH− ingress, respectively, under a high electric field(E).26−28 In the case of porous-type anodic alumina formation,on the other hand, oxide grows at the metal/oxide interface viathe inward migration of O2−/OH− ions. 18O tracer studies haveshown that outwardly migrating Al3+ cations do not contributeto the oxide growth at the oxide/electrolyte interface, but are allshed into the anodizing electrolyte via direct ejectionmechanism (see section 6.3.4).10,29−31 Otherwise, egressingAl3+ ions would form anodic alumina at the oxide/electrolyteinterface to heal any developing or embryonic pores there.Schematic diagrams illustrating the dimensional changes of

aluminum during the barrier-type and porous-type anodic oxideformation are shown in Figure 2.32 An immobile marker layer isimplanted into the starting aluminum with a native oxide layer(Figure 2a). When a barrier-type film is formed at 100% currentefficiency (ηj) by anodization of the marker-implanted

aluminum, new oxide forms above and below the markerlayer (Figure 2b). The marker layer is located at a depth of 40%of the film thickness in a plane corresponding to that of theoriginal metal surface. On the other hand, when a barrier-typefilm is formed at 60% current efficiency (ηj), the plane of themarker layer is immobile and 40% of the Al3+ cations are shedinto the electrolyte via direct cation ejection mechanismwithout contributing to the oxide formation (Figure 2c). In thiscase, anodic oxide grows at the metal/oxide interface via theinward migration of O2−/OH− ions. When porous-type anodicoxide forms at 60% current efficiency, the marker plane islocated above that of the original metal surface (Figure 2d). Inthis case, the metal/oxide interface is also the oxide growthfront, and 40% of Al3+ ions are ejected into the solution.Because cations are being shed into the electrolyte, the

current efficiency (ηj) of porous-type oxide growth is typicallymuch lower than that of the barrier-type. Accordingly, thePilling−Bedworth ratio (PBR = the ratio of molar volume ofthe grown oxide to molar volume of the consumed metal; seesection 6.1.1) for the initial barrier oxide formation in porous-type oxide growth at the early stage of anodization is lower thanthat for barrier-type oxide growth: PBR = 0.90 for porous-typeoxide growth at ηj = 53.5% in phosphoric acid solution andPBR = 1.7 for barrier-type oxide growth at ηj = 100% in neutraladipate solution.9,33 Shimizu et al.33 suggested that the initialbarrier oxide grows under increasing tensile stress (PBR < 1),which causes local oxide cracking most probably at therandomly present metal protrusions. The generated surfacecracks were considered to be local paths for electrolytepenetration, causing non-uniform local thickening of the initialbarrier oxide. Non-uniform thickening of the initial oxide causesconcentration and redistribution of the current lines into therelatively thin oxide regions between the protrusions (i.e., alocal increase in electric field, E). Consequently, localizedscalloping of the metal/oxide interface takes place. Shimizu etal.33 pointed out that the non-uniform thickening of anodicoxide (i.e., morphological instability) in the initial barrier oxideis “one of the most distinctly different growth features betweenporous- and barrier-type AAO films”. Unlike porous-type oxidegrowth in acid electrolytes, anodic oxides in neutral electrolytesgrow highly uniformly on surface finished aluminum,maintaining flat metal/oxide and oxide/electrolyte interfaces.Even the smoothing of initially rough aluminum surfaces duringthe growth of barrier oxide films has been experimentallyobserved.34

Figure 1. Two different types of anodic aluminum oxide (AAO)formed by (a) barrier-type and (b) porous-type anodizations, alongwith the respective current (j)−time (t) transients under potentiostaticconditions.

Figure 2. Schematic diagrams illustrating dimensional changes of an aluminum specimen following anodizing. (a) Initial aluminum with a thin air-formed oxide film. The red dashed line represents an immobile marker layer implanted into the initial aluminum with a thin air-formed oxide film.(b) Anodized at 100% efficiency with formation of a barrier-type anodic film. (c) Anodized at just above 60% efficiency with formation of a barrier-type anodic film. (d) Anodized at 60% efficiency with formation of a porous anodic film. Reproduced with permission from ref 32. Copyright 2006The Electrochemical Society.





3. IONIC CONDUCTION IN ANODIC OXIDE FILMS

3.1. High-Field Conduction Theory

When a valve-metal is anodized under either potentiostatic orgalvanostatic condition, anodic oxide film forms on the metal.For anodizing aluminum (Al) and tantalum (Ta), an empiricalexponential dependence of the ionic current density (j) on theelectric field (E) is established. Ionic current density (j) underhigh-field conditions, which is the case for anodic oxide growth,can be associated with the movement of charged ions in thebarrier oxide, and can be related to the potential drop (ΔU)across the barrier oxide through the exponential law ofGuntherschulze and Betz, as follows:21,35

β β= = Δj j E j U texp( ) exp( / )0 0 b (1)

where j0 and β are material-dependent constants at a giventemperature, and ΔU/tb is the effective electric field (E,typically 106−107 V cm−1) impressed on the barrier layer withthickness tb. For anodic alumina, a large range of j0 and β valueshas been reported: j0 = 3 × 104 to 1 × 10−18 A cm−2 and β = 0.1× 10−6 to 5.1 × 10−6 cm V−1.25

For anodic oxidation of metal in an electrolyte, three theoriesbased on the following possible rate-determining steps for oxideformation have been developed:3 ion transfers (i) across themetal/oxide interface (Mott−Cabrera theory),23,24 (ii) throughthe oxide bulk (Verwey theory),22 and (iii) across the oxide/electrolyte interface (Dewald theory).36,37 In the point defectmodel of Macdonald et al.,38 the oxide film is assumed tocontain a high concentration of non-interacting positive andnegative point defects, and the rate-determining step for theoxide growth is assumed to be the transport of metal and oxidevacancies across the oxide film. All of these theories can explainthe empirical exponential relationship proposed by Gunter-shultz and Betz. On the other hand, transient experimentsfavorably indicate that the rate-determining step is themovement of charged ions within the oxide.25

On the basis of the rate-determining movement of ionswithin the oxide, the high-field model relates the parameters j0and β in eq 1 to the nature of oxide materials. The high-fieldconduction model is based on a hopping mechanism, in whichthe activation energy for hopping ions is dependent on electricfield E (Figure 3).25 Ions at regular sites or interstitial positionsjump to vacancies or other interstitial positions in theirneighborhood. The model assumes that the oxide is defect-freeand of homogeneous composition. When the electric field (E)

in the oxide is high enough (e.g., 106−107 V cm−1), the ioniccurrent density (j) can be expressed as25

ρ α= −⎜ ⎟⎜ ⎟⎛⎝

⎞⎠⎛⎝

⎞⎠j v a

WRT

azFERT

exp(2)

where v is the hopping attempt frequency of the ion, ρ is thedensity of concentration of mobile charge in C cm−3, a is thehopping inter-distance, W is the hopping activation energy atzero field, α is a parameter describing the asymmetry of theactivation barrier at non-zero field, z is the valence of themobile ions, and F is Faraday’s constant. From eqs 1 and 2, thefollowing relations can be obtained:

υρ= −⎜ ⎟⎛⎝

⎞⎠j a

WRT

exp0 (3)

β α= azFRT (4)

Because the parameter a can be related to the inter-atomicdistance in the oxide, one can expect that the electric fieldstrength (E) increases when the oxygen ion density increases(i.e., a decrease in parameter a) provided that the otherparameters are constant. Equation 1 can be modified to obtaina Tafel equation:

β= +j j Eln ln 0 (5)

For a constant oxide thickness tb, a constant Tafel slope β isobtained.The electric field (E) in the oxide can be related to the

applied (or measured, in the potentiostatic condition) electrodepotential (U). The measurable potential drop between themetal and the electrolyte is equal to

= Δ + Φ + ΦU U m/o o/e (6)

where ΔU is the potential drop in the oxide, and Φm/o and Φo/eare the potential drops at the metal/oxide and oxide/electrolyteinterfaces, respectively.39 In a typical anodization, the potentialdrops at the metal/oxide and oxide/electrolyte interfaces arequite small, as compared to the several tens of volts of potentialdrop in the oxide (i.e., ΔU ≫ Φm/o + Φo/e). Therefore, thefollowing approximation for the electric field (E) is possible forthe high-field ionic transport:

= Δ ≈E U t U t/ /b b (7)

where tb is the thickness of oxide.3.2. Elementary Interfacial Reactions

As was already discussed in section 2, it is now widely acceptedthat for the anodic growth of alumina both Al3+ cations andoxygen-containing anions (e.g., O2− or OH−) are mobile withinthe anodic oxide under high electric field (E).10,26−28,40 Al3+

ions migrate outwardly toward the oxide/electrolyte interface,while O2− or OH− anions move inwardly toward the metal/oxide interface. Therefore, one can consider both (i) the metal/oxide and (ii) the oxide/electrolyte interfaces as the growthfront of anodic oxide during anodization of a valve-metal. Foranodizing aluminum, the following elementary reactions areconsidered to be possibly occurring at the interfaces (Figure 4).(i) At the metal/oxide interface:

→ ++ −Al Al 3e(ox)3

(8)

+ →+ −2Al 3O Al O(ox)3

(ox)2

2 3 (9)

Figure 3. Influence of the electric field strength (E) on the activationenergy of hopping ions. Reproduced with permission from ref 25.Copyright 1993 Elsevier.



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(ii) At the oxide/electrolyte interface:

+ →+ −2Al 3O Al O(ox)3

(ox)2

2 3 (10)

+ → ++ +Al O 6H 2Al 3H O2 3 (aq) (aq)3

2 (1) (11)

→+ +Al Al(ox)3

(aq)3

(12)

+ +− −2O O 4e(ox)2

2(g) (13)

+ + +− − +2H O O OH 3H2 (1) (ox)2

(ox) (aq) (14)

Reactions 9 and 10 correspond to the formation of anodicoxide at the metal/oxide and oxide/electrolyte interfaces,respectively. Reaction 11 describes dissolution of anodicalumina by Joule’s heat-induced oxide dissolution and/orfield-induced oxide dissolution, which will be discussed insection 6.3.1 and section 6.3.2, respectively. On the other hand,reaction 12 occurs through field-assisted direct ejection of Al3+

ions from the metal/oxide interface through oxide into theelectrolyte, which will be discussed in detail in section 6.3.4.Reactions 11−13 decrease the net current efficiency (ηj)associated with the anodic oxide formation. Reaction 14describes the heterolytic dissociation of water molecules at theoxide/electrolyte interface, which supplies oxygen anions to themetal/oxide interface to form anodic oxide. By assuming thatall oxide anions from the dissolution of Al2O3 at the oxide/electrolyte interface migrate to the metal/oxide interface toreform Al2O3, and that all oxide anions from the dissociation ofwater contribute to the oxide formation, Su et al. proposed thefollowing overall reaction at the oxide/electrolyte interface:41

+ → + − −

+ + −

+ −

− +

n n x

x n x

Al O H O 2Al (3 )O

OH (2 )3H

2 3 2 (1) (aq)3

(ox)2

(ox) (aq) (15)

where n denotes the amount of water dissociated per mole ofAl2O3 that is dissolved at the same time. Su et al. claimed thefield-dependent nature of the heterolytic dissociation of waterin reaction 14 and related the dissociation rate of water to theporosity (P) of AAO, which will be touched upon in section7.1.3.3. Transport Numbers

As mentioned in previous sections, anodic oxide formation canoccur at both the metal/oxide and the oxide/metal interfaces.The relative amount of mobile ions transported to the oxideforming interfaces is called the “transport number”: t+ for cation

and t− = 1 − t+ for anion. Transport numbers can bedetermined by employing a “marker layer”, whose position inthe anodic oxide film indicates the extent of oxide that wasformed at each interface. If the metal ions are the only mobilespecies, new oxide should be formed at the oxide/electrolyteinterface on top of the marker layer. On the other hand, ifoxygen anions are the only mobile species, the new oxideshould be formed at the metal/oxide interface below themarker layer. Davies et al.26 stated that the ideal marker atomsfor determination of transport numbers should fulfill thefollowing requirements: “The markers should be (i) uncharged,so that they do not migrate in the oxide under the influence ofthe applied field; (ii) large in size, so that they do not diffusesignificantly within the oxide lattice; (iii) present in traceamount, so that the macroscopic properties of the tagged oxideremain unaltered; and (iv) detectable, in order to assess theirdepth in the oxide.”These conditions can be satisfied by implanting radioisotopes

125Xe inert gas atoms or 222Rn, which are heavier than typicalvalve-metals and oxygen, in a preformed thin oxide film andsubsequently anodized.26,27,42 Radioactive tracers allow theposition of the buried marker to be assessed by monitoring theenergy of emitted α- or β-particles.26,27,42,43 Other techniquesto measure the buried marker position in oxide includeRutherford backscattering spectrometry (RBS)40 or directobservation of voids formed by implanted Xe by employingcross-sectional transmission electron microscopy (TEM).28,44 Arepresentative cross-sectional TEM image showing an immobileXe marker layer is given in Figure 5. The sample in the figurewas formed in near-neutral potassium phosphate electrolyte at ahigh current efficiency (ηj ≈ 100%).44 The approximately 10-nm-thick straight Xe marker layer is located at about themidpoint of the film. The anodic oxide above the marker layerformed by the field-driven outward migration of Al3+ ions andthat beneath the marker layer by the field-driven inwardmigration of oxygen carrying anions, O2−/OH−. Assuming thatall egressing Al3+ ions contribute to the oxide formation, thecation transport number was directly estimated to be t+ =0.49.44 For anodizing conditions under which oxide grows withappreciable metal dissolution, however, TEM-based directmeasurement may underestimate the cation transport number.In such cases, Al3+ ions dissolved in anodizing electrolyteshould be quantified to estimate the equivalent oxide thickness.Davies et al.26 pointed out that the location of an immobilemarker in anodic oxide markedly depends on the anodizationconditions, such as current density (j) and the nature of

Figure 4. Schematic diagrams showing elementary interfacial reactions for (a) barrier-type and (b) porous-type anodic oxide.




electrolyte. The authors performed transport number experi-ments using radioisotope 125Xe marker atoms and alsoquantitative analyses on dissolving Al3+ ions during anodizationin two different near-neutral electrolytes (i.e., sodiumtetraborate and ammonium citrate). According to theirexperiments, both metal and oxygen ions are mobile duringoxide growth. In the borate solution, anodic alumina grew withhigh current efficiencies (i.e., negligible cation loss), and thusthe immobile 125Xe markers were completely buried in theoxide. The mean cation transport number (t+) was estimated tobe t+ = 0.58 for anodic alumina formed at the current densityrange of 0.1−10 mA cm−2. On the other hand, in citratesolution, the amount of aluminum passed into the solution wasas high as 40% of the total oxidized metal at low currentdensity, but decreased as the current density increased. The125Xe markers in anodic alumina remained very close to theouter surface. The cation transport number varied with currentdensity, from about t+ = 0.37 at 0.1 mA cm−2 to t+ = 0.72 at 10mA cm−2.In the case of porous AAO, the transport numbers of mobile

ions can be estimated by the so-called “pore-filling method”,which was originally used to determine the porosity (P) ofporous AAO by Dekker and Diddelhoek.45,46 In this method,aluminum is first anodized in an acid electrolyte to formporous-type anodic oxide and subsequently re-anodized in aneutral electrolyte electrolyte to form barrier-type oxide under agalvanostatic condition. During anodizations, potential (U)−time (t) transients are monitored. During the barrier-typeanodizing (i.e., re-anodizing process), new oxide graduallyforms simultaneously within the pores and underneath thebarrier layer of the pre-formed porous anodic oxide, becauseboth Al3+ and O2− ions contribute to the oxide formation at themetal/oxide and oxide/electrolyte interfaces, respectively. As aresult, the cell potential gradually increases with time during there-anodizing process. Figure 6 schematically shows (a) themovement of Al3+ and O2− and (b) the cell potential (U)−time(t) profile during the re-anodizing process.46 The non-zerovalue of U at t = 0 is due to the original barrier layer of pre-formed porous AAO. The complete filling of pores isaccompanied by the change of the slope in the U−t curve attime tp due to the sudden increase of the oxide/electrolyteinterfacial area. For the time t < tp, the following relation can beobtained:46

ρ + =+ −⎡

⎣⎢⎤⎦⎥P

ht

ht

jMnFk

dd

dd (16)

where ρ (=2.95 g cm−3) is the density of oxide, P is the porosityof porous AAO, dh+/dt and dh‑/dt are, respectively, the rates ofthe increase of the barrier oxide thickness at the oxide/electrolyte and metal/oxide interfaces, j is the current density,M is the atomic weight of Al, n (=3) is the number of electronsinvolved in oxidation reaction, F is Faraday’s constant, and k(=0.505) is the weight fraction of aluminum in the oxide. Thecation transport number is given by the ratio of the weight ofnew oxide formed within the pores per unit time to the totalweight of new oxide formed per unit time:46

= +++ + −⎛

⎝⎜⎞⎠⎟

⎛⎝⎜

⎞⎠⎟t P

ht

Pht

ht

dd

/dd

dd (17)

The slopes m1 and m2 of the U−t transient in Figure 6b aregiven by46

= ++ −⎛

⎝⎜⎞⎠⎟m

ARht

ht

1 dd

dd1

(18)

= ++ −⎛

⎝⎜⎞⎠⎟m

ARP

ht

ht

1 dd

dd2

(19)

where AR is the anodizing ratio (=the ratio of the barrier layerthickness to the cell potential, in nm V−1), and assumed to be aconstant. From eqs 16−19, the porosity (P) of porous AAO isgiven by

=− −

+

+Pt m m

t m m( / )

1 (1 )( / )2 1

2 1 (20)

For porous AAO formed in 1.125 M oxalic acid at 30 V,Takahashi and Nagayama reported that the transport numbersof mobile Al3+ and O2− ions are t+ = 0.4 and t− = 0.6,respectively.46

3.4. Stress-Driven Ionic Transport

The high-field conduction model describes the relationbetween ionic current density (j) and the electric field (E)well. However, stress gradients in the oxide may possiblycontribute to the ionic transport. Hebert and Houser47,48 have

Figure 5. Cross-sectional transmission electron microscopy (TEM)image showing immobile 125Xe marker layer. The sample (i.e., barrier-type anodic oxide film) was formed at a constant current density of 1mA cm−2 to 100 V in near-neutral potassium phosphate electrolyte.Adapted from ref 44 with permission. Copyright 1987 Taylor &Francis (www.tandfonline.com).

Figure 6. Schematics showing (a) the movement of Al3+ and O2− ionsduring the re-anodizing process and (b) the corresponding cellpotential (U)−time (t) curve. Reproduced with permission from ref46. Copyright 1978 Elsevier.



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developed a model for ionic transport in growing amorphousanodic alumina films, in which ion migration in the oxide isdriven by gradients of mechanical stress as well as electricpotential. It also considers the viscoelastic creep of the oxide. Inother words, both stress gradient-driven ionic migration andstress gradient-driven creep are considered in the model. It isassumed that stress originates at the metal/oxide interface dueto the volume change upon oxidation. For stress gradient-driven ionic migration, the empirical high-field conductionrelation is generalized by considering the dependence of theionic current density on the gradient of the ionic chemicalpotential ∇μi:

47−49

μμ

μ= −∇|∇ |

|∇ |⎜ ⎟⎛⎝

⎞⎠J C u

aRT

2 sinhii

ii i

0i

(21)

where Ji, Ci, and ui0 are, respectively, the flux, the concentration,

and the pre-exponential velocity of ion “i” (i = M and O formetal and oxygen, respectively), and a is the migration jumpdistance in the oxide. The chemical potential μ is related to themean stress (σ) and electrical potential (ϕ) as follows:48

μ ϕ σ= + − u z F Vi i0

i i (22)

where ui0, zi, and Vi are the standard chemical potential, the

charge number, and the molar volume of ion i, respectively. Forbarrier-type anodic alumina film, the mean normal stress isdefined according to σ = 1/3(σxx + σyy) = 2/3σxx, where x- andy-directions are parallel to the interface.47 For the stressgradient-driven oxide creep, the model enforces the con-servation of electrical charge and volume and the momentumbalance in a Newtonian fluid. For galvanostatic anodization ofaluminum at the applied current density j, the constraint ofcharge conservation can be written as follows:

= − +j FJ FJ2 3O M (23)

On the other hand, the volume balance is

Ω= − −

jF

V J v3

MO O (24)

where ΩM is the molar volume of the Al atom in the metal andv is the creep velocity in the oxide. By employing the Maxwellviscoelastic model and also by assuming a large elastic modulus,the momentum balance in a Newtonian fluid is expressed as47

ησ= ∇ + ∇ + ∇ ∇·v v0

1 13

( )2

(25)

where η is the viscosity.For porous AAO film growing under steady-state,47 the

model predicted that a large compressive interfacial stresscauses the lateral flow of oxide materials from the center ofpore base toward the cell boundaries and the upward flow inthe pore wall oxide, as in the flow pattern experimentallyobserved from W tracer studies (see section 6.3.5). Simulationresults indicated that the stress field driving the flow resultsfrom the following three origins: “the volume expansionoccurring at the metal/oxide interface, nonlinearity of theequations governing conduction of mobile ions (i.e., Al3+ andO2−/OH−), and incorporation of electrolyte-derived anionicspecies within the anodic oxide near the oxide/electrolyteinterface”.47

For barrier-type anodic alumina film,48 the model predictedthe average stress in the oxide to be compressive when thecurrent density is smaller than 0.5 mA cm−2 (i.e., compressive

to tensile transition at 0.5−1.0 mA cm−2), while average tensilestress of the order of 50 MPa was predicted above 1 mA cm−2,which is in good agreement with the experimental stress data ofBradhurst and Leach.50 In addition, by taking into consid-eration the viscous flow of oxide material, the model predictedthe increases of the cation transport number (t+) as a functionof current density (j). On the basis of experimental evidencethat cation transport number (t+) is largely dependent on theelectrolyte condition, Hebert and Houser pointed out that theoxide viscosity and conduction parameters may depend on thesolution composition as a result of electrolyte anionincorporation into the anodic oxide film. They suggested thatbulky electrolyte anions disrupt the local packing of oxygenions and influence transport properties by the introduction ofadditional free volume into the amorphous oxide.48

4. ELECTROLYTIC BREAKDOWN

When valve-metals (e.g., Al, Ta, Nb, Zr, etc.) are anodizedunder galvanostatic conditions, the thickness of the oxide filmsincreases linearly with time. Correspondingly, the appliedpotential (U) increases linearly with time to keep the electricfield (E) constant during the process. Under this condition, theanodizing potential (U) finally reaches a value at which visiblesparking on the anode starts appearing, and local thickening,cracking, blistering, or even burning of oxide film commences.This local event is called “electrolytic breakdown”, which notonly prevents the uniform growth of anodic films over themacroscopic metal surface, but also permanently degrades thedielectric properties of the oxide. The anodizing potential at theonset of this local event is called breakdown potential (UB).Because the oxide thickness increases linearly with theanodizing potential (U) in galvanostatic conditions, thebreakdown is dependent on the oxide thickness and occurs ata critical oxide thickness. Breakdown during anodization can beassociated with a number of phenomena. These include theappearance of visible sparking/luminescence,51−59 the localcrystallization of oxide,60−66 oxygen evolution at theanode,63,67,68 retardation of potential rise,69,70 occurrence ofaudible cracking,71 and rapid voltage fluctuations.69,70,72 Inporous AAO growth, breakdown can occur under high currentdensity anodizing conditions.4 If the reaction heat cannot beadequately dissipated from the anode, electrolyte heating maycause local increase in conductivity and a current “run away”process. This results in local thickening or burning of anodicoxide, terminating uniform growth of porous AAO. The anodicoxide in the burnt area exhibits typically a different color fromthe burnt-free areas. For a given anodizing electrolyte, on theother hand, porous AAO formed at a potential just belowbreakdown value (i.e., U < UB) exhibits the best self-ordering ofpores (section 7.1).18 Improving the breakdown characteristicsof anodic oxide films through proper control of the electrolytecomposition, surface state of the starting aluminum, andreaction heat can allow one not only to explore new anodizingconditions for self-ordered pore growth, but also to engineerinternal pore structures (see sections 7.2−7.4). In this section,we discuss some of the electrochemical factors influencingbreakdown, and models that explain the breakdown phenom-ena.

4.1. Factors Influencing Breakdown

In general, the breakdown potential (UB) is dependent on thenature of the metal being anodized, the current density (j), andthe composition (or resistivity) of the electrolyte. Meanwhile,



the electrolyte temperature, stirring rate, and history of anodicoxide have no influence on the breakdown potential.4.1.1. The Nature of Anodized Metal. Wood and

Pearson investigated metals whose anodization in 3%ammonium tartrate ended in sparking, and associated thebreakdown potential (UB) with the ionic bonding character-istics of the anodic oxides by employing the criteria of Paulingand Wells. They established a descending order of UB accordingto the melting point of the corresponding oxide: Zr (300 V) >Al (245 V) > Ta (200 V) > Nb (190 V).72 However, Alwitt andVijh reported a different descending order of UB foranodizations of the same metals in the same conditions: Al(350 V) > Zr (315 V) > Ta (275) > Nb (190 V).73 Theycorrelated the increase in UB with the increasing heat offormation per equivalent (−ΔHf/equiv) of oxide, which isapproximately equal to one-half the value of the forbidden bandgap of the oxide. Further, they noted that the dependence of UBon the band gap would reflect the electronic nature of thebreakdown phenomena. As such, rather conflicting reports havebeen published for the dependence of UB on the intrinsic solid-state properties of anodic oxides. Iknopisov et al.69 pointed outthat the dependence of UB on the nature of the metal isconsiderably smaller than the dependence on the electrolyteresistivity (ρe).4.1.2. Electrolyte Conditions. Early studies have reported

that the breakdown potential (UB) increases linearly with thelogarithm of the electrolyte resistivity (ρe) with the followingequation:

ρ= +U A B logB e (26)

where A and B are the constants depending on the electrolytecomposition and the anodized metal.69,71,74,75 Figure 7 showsthe dependence of UB on log ρe during anodization of Nb, Ta,Al, and Zr.69 It appears from the figure that the differentinfluences of ρe on UB defeat attempts to set the metals in serieswith respect to the breakdown characteristics of their anodic

film. For anodic oxide of aluminum, some authors havereported that anion concentration (CA

−) influences UB.76,77

Kato et al. showed that at a fixed solution resistivity UBdecreases linearly with an increase in the logarithm of theanion concentration, or more specifically the anion charge withthe following relation:77

= − −U A B ClogB A (27)

On the basis of anodization experiments with tantalum insulfuric, phosphoric, and hydrochloric acids, Arifuku et al.78

reported that UB is dependent upon the detailed distributionprofiles of incorporated anions in the anodic oxide. Later, therole of incorporated electrolyte species in the electricalbreakdown was emphasized by Albella et al., who have putforward a theory of avalanche breakdown during anodicoxidation.79−81

4.1.3. Current Density (j). For tantalum anodization inammonium sulfate, Yahalom and co-workers76,82 reported thatthe breakdown potential (UB) is almost independent of thecurrent density (j). For anodic films on aluminum, Ikonopisovet al.69 also reported that a 500-fold increase of current density(j) only lowers the breakdown potential (UB) by 15%. On theother hand, Di Quarto et al.74,75,83 pointed out the occurrenceof two different kinds of breakdown, that is, “mechanical” and“electrical” breakdown. For anodic oxides of tungsten,74

zirconium,75 and titanium84,85 under limited conditions, theynoted that anodic oxides grew with an increasing number ofdefects at a retarded rate (i.e., reduced slope in U−t curve)during galvanostatic anodizations, until electrical breakdown(EB) eventually occurs with visible sparks. They termed thischaracteristic growth as mechanical breakdown (MB). Forelectrical breakdown (EB), they reported that current density(j) has little effect on the breakdown potential (UEB), which isin line with the reports of Yahalom et al. and Ikonopisov etal.69,76,82 In the case of mechanical breakdown (MB), however,they observed that current density (j) has a significant effect onthe breakdown potential (UMB) according to the followingequation:

= +U A B jlogMB MB MB (28)

where AMB and BMB are constants, which depend mainly on thekind of anion in the electrolyte and slightly upon pH andconcentration of electrolyte: BMB > 0 for anodic oxides ofzirconium and titanium75,85 and BMB < 0 for anodic oxide oftungsten.74

4.1.4. Other Factors Influencing Breakdown. Thesurface state of the starting metal (i.e., the surface defects(flaws), purity, processing history, etc.) also strongly influencesthe breakdown potential (UB).

61,86 In general, the surfacedefects unavoidably cause a decrease of the breakdownpotential (UB) with the commencement of sparks.87 On theother hand, post-breakdown anodization experiments haveshown that breakdown characteristics are independent of thehistory of the anodic oxide film.72,88,89 When a valve-metal wasanodized in electrolyte A until breakdown occurred at UB,A, andthen the resulting sample was re-anodized in electrolyte B witha higher breakdown potential (UB,B), the film formation duringthe post-breakdown anodization continued at normal kineticsuntil breakdown occurred at UB,B.

88,89 Temperature (T) is oneof the easily controllable parameters of the electrolyte.Ikonopisov formulated the temperature dependence of thebreakdown potential UB (section 4.2.1).

90 However, a change inthe electrolyte temperature can alter both the electrolyte

Figure 7. Dependence of the breakdown potential (UB) on thelogarithm of electrolyte resistivity (ρe) for anodizations of Ta, Nb, Al,and Zr in solutions of ammonium salicyalte in dimethylformamide.Reproduced with permission from ref 69. Copyright 1979 Elsevier.




resistivity (ρe) and the property of the growing anodic oxide.When the dependence of electrolyte resistivity (ρe) isconsidered, no clearly pronounced dependence of the break-down potential (UB) on the temperature (T) was ob-tained.71,77,91

4.2. Models for Breakdown

4.2.1. Electron Avalanche Multiplication. The firstattempt to develop a quantitative model of breakdown wasmade by Iknopisov.90 He considered experimentally observedbreakdown characteristics, and noting that the breakdownpotential (UB) mainly depends on the nature of the anodizedmetal and the electrolyte resistivity (ρe), he inferred thatbreakdown is dependent upon the solid-state properties of theanodic oxide and is controlled by electrochemical reactions atthe oxide/electrolyte interface. In his model, the initialelectrons are injected from the electrolyte into the oxideconduction band (CB) by either a Fowler−Nordheim or aSchottky mechanism (Figure 8). The injected electrons

accelerate and multiply in avalanche during their travel in theoxide of thickness (tox) to the anode, until the avalanche currentreaches a critical value for breakdown. In this multiplicationprocess, the electronic current (je) depends on the traveldistance (x) with x = 0 being the oxide/electrolyte interface,which can be expressed by

α ψ= == = =j j E t j rqEtexp[ ( ) ] exp[ / ]x t x xe, e, 0 ox e, 0 ox iox (29)

where α(E) is the impact ionization coefficient at the electricfield E, r is a recombination constant (r < 1), q is the electroncharge, and ψi is the threshold energy for impact ionization.Breakdown occurs if the electronic current (je) exceeds a criticalvalue je,B at a critical oxide thickness (tox,B). Because UB = Etox,B,the breakdown potential (UB) is given by

ψ= − =U rq j j( / )(ln ln )xB i e,B e, 0 (30)

The dependences of je,x=0 on the electric field (E),69,92

temperature (T), and electrolyte resistivity (ρe)93 were

empirically determined to be, respectively:

α α==j Eexp[ ]xe, 0 1 21/2

(31)

β β= +=j Tln /xe, 0 1 2 (32)

γρ= γ=

−j xe, 0 1 e2

(33)

From eqs 30 and 31, the dependence of breakdown potential(UB) on the electric field (E) is given by

ψ α ψα= − −U rq j rq E( / )(ln ln ) ( / )B i e,B 1 i 2 (34)

This equation explains a slight decrease of UB with increasingcurrent density (j). Regarding the relation between thebreakdown potential (UB) and temperature (T), the followingexpression is obtained by combining eqs 30 and 32:

ψ β β= − −U rq j T( / )(ln / )B i e,B 1 2 (35)

Equation 35 predicts that UB is dependent on the temperature(T), which conflicts with experimental observations.71,77,91 Forthis discrepancy, Ikonopisov pointed out the interplay betweentemperature (T) and the solution resistivity (ρe). For thedependence of breakdown potential (UB) on the electrolyteresistivity (ρe), from eqs 30 and 33, one may obtain

ψ γ γ ρ

ψ γ ψγ ρ

= − +

= − +

U rq j

rq j rq

( / )(ln ln ln )

( / )(ln ln ) (2.3 / ) log

B i e,B 1 2 e

i e,B 1 i 2 e (36)

Equation 36 has exactly the same form as eq 26, whichdescribes an empirical relation between UB and ρe.Although Ikonopisov’s model explains some of the

experimental results, it has been criticized by many authors.Shimizu pointed out the unrealistic value of the mean free pathλ(E) of ionized electrons, which from eqs 26, 29, and 36 isgiven by

λ α ψ γ= = =E E rqE E B( ) 1/ ( ) / (1/2.3 )( / )i 2 (37)

By using the experimental values from Ikonopisov et al. for E(=8.7 × 106 V cm−1) and B/γ2 (=1000 V), Shimizu obtained λ= 500 nm, which roughly corresponds to the thickness of oxidefilms formed up to the potential 400 V and indicates theabsence of the electron avalanche capable of causing thebreakdown.94 Albella et al. questioned the origin of electrons inIkonopisov’s model.87 They pointed out Ikonopisov’s modellacked a reasonable explanation of the role of the electrolyteand the absence of specific electrochemical reactions requiredfor the injection of the initial electrons.Albella et al. explicitly considered the effect of the anodizing

electrolyte by posulating that the initial electrons for theavalanche come from the electrolyte species incorporated intoanodic oxide.79−81 The incorporated electrolyte species act asimpurity centers close to the oxide conduction band (CB),releasing electrons to the conduction band via the field-assistedPoole−Frenkel mechanism (Figure 9).80,87,95,96 In the model ofAlbella et al., the total current density (jt) consists of threecomponents:

= + +j j j jt 1 2 e (38)

where j1 is the oxidation current density, j2 is the currentdensity consumed by the incorporated electrolyte species and isassumed to be a constant faction γ of j1 (i.e., j2 = γj1), and je isthe electronic current density. The electronic current density(je) at the anode can be expressed as

Figure 8. Schematic representation of the band structure and theavalanche breakdown in Ikonopisov’s model. Reproduced withpermission from ref 79. Copyright 1984 The Electrochemical Society.




α αβ= == =j j t j Uexp[ ] exp[ ]x xe e, 0 ox e, 0 (39)

where α is the impact ionization coefficient, and β is the ratio ofthe oxide thickness to the anodization potential (U) and isequal to the inverse of the electric field E (i.e., β = 1/E = AR,the anodizing ratio). Because the initial electronic currentoriginates from incorporated species, je,x=0 should be a constantfraction η of oxyanion current (j2) and is je,x=0 = ηj2 = ηγj1.Under the assumption that the critical current density is afraction z of the oxidation current j1, the breakdown potential(UB) should satisfy the following relation:

αβ ==j U zjexp[ ]xe, 0 B 1 (40)

Accordingly, the breakdown potential (UB) is given by

αβ α= = ηγU zj j E z(1/ ) ln( / ) ( / ) ln( / )B 1 0 (41)

The time derivative of the potential is given by

ρ= +⎜ ⎟

⎛⎝⎜⎜

⎞⎠⎟⎟⎛⎝

⎞⎠⎛⎝⎜⎜

⎞⎠⎟⎟U

tEF

Mx y

jMx y

jdd

1

ox

1

1 11

2

2 22

(42)

where ρox is the oxide density, F is the Faraday constant, andM1 and M2 are the molecular weights of the oxide and theincorporated species, respectively, whose corresponding anionand cation valences are x1,y1 and x2,y2, respectively. Combiningeqs 38 and 42 yields the following differential equation:

φγ γ αβ= + + + γη −U t Kj Ud /d (1 )[1 exp( )]t1

(43)

where K is the unitary rate of anodization for oxide withoutelectrolyte incorporation and given by M1E/x1y1ρoxF, and φ isthe ratio of the equivalent weight of the incorporated species tothat of oxide, that is, φ = (M2/x2y2)/(M1/x1y1). Assuming aconstant field in the oxide, the integration of eq 43 yields therelation between the anodizing potential (U) and time (t):

= − ΔU t V t V t( ) ( ) ( ) (44)

with V(t) and ΔV(t) given by

φγγ

= ++

V t Kj t( )11 t (45)

γηγ α

αΔ =+

−V tE

U E( )1

[exp( / ) 1](46)

The factor (1 + φγ)/(1 + γ) in eq 45 describes the correctionof the anodizing rate due to the incorporation of electrolytespecies. On the other hand, eq 46 enforces deviation ofpotential from the linearity due to the avalanche effect.Accordingly, eq 44 predicts a gradual decrease of slope (dU/dt) of the potential−time curve during galvanostatic anodiza-tion. Albella et al. confirmed the validity of eq 44 by fitting it onthe experimental results of tantalum anodization (Figure 10).80

Further, by fitting eq 44 on the experimental potentialevolutions in different electrolyte concentrations (C), theyobtained a relation between γ and C:95

γ ≈ aCb(47)

with a and b being electrolyte-dependent constants. From eqs41 and 47, the concentration dependence of the breakdownpotential (UB) is given by

α η≈ −U E z a b C( / )[ln( / ) ln ]B (48)

which is in good agreement with the experiments.4.2.2. Stress-Driven Breakdown. Sato97 distinguished five

different possible contributions to the mechanical stresses inanodic oxide: (a) electrostriction pressure, (b) interfacialtension of the film, (c) internal stress caused by the volumeexpansion, (d) internal stress due to partial hydration/dehydration of the anodic oxide, and (e) local stress causedby impurities. By considering the first two contributions as themost general factors for breakdown, he mathematically deriveda thermodynamic model of stress-driven breakdown, and

Figure 9. Band diagram showing the avalanche multiplication ofelectrons in the model of Albella et al. The impurity level inconduction band (CB) is denoted by “A”. Reproduced withpermission from ref 81. Copyright 1987 Elsevier.

Figure 10. Experimental result for the evolution of the potential (U)as a function of time (t) during tantalum anodization in 1.2 M H3PO4at 1.78 mA cm−2. The theoretical curve (solid line) has been fittedaccording to eqs 44−46. Reproduced with permission from ref 80.Copyright 1985 Elsevier.





explained the breakdown potential (UB) and the effect ofadsorbed anionic species on it. According to the model,97 thestress (ΔP) accumulated in the oxide film is equal to

ε επ

γΔ = − −Pt

( 1)8 ox (49)

where ε is the oxide permittivity and γ is the surface tension. Ineq 49, the first term represents the electrostriction effect andthe second the interfacial tension effect. According to Sato, foran oxide dielectric with ε > 10, an electric field (E) of 5 × 106 Vcm−1 produces a compressive electrostriction pressure exceed-ing 1000 kg cm−2, which is higher than the critical mechanicalstrength of oxides and thus may be a cause of their mechanicalfailure. Because the compressive stress within the anodic oxideincreases with thickness, there is a limiting oxide thickness(tox,B) above which breakdown occurs. The incorporatedanionic species causes a decrease in the surface tension (γ),and thus increases the stress by lowering the value of thesecond term in eq 49. The dependence of the breakdownpotential (UB) on the anion concentration (CA

−) in theelectrolyte was also established, as follows:

πε ε

= −+

Γ−

−U

CkTd

d ln8

( 1)B

AA

(50)

where ΓA− is the anion density at the oxide surface at the

breakdown potential (UB). The model predicts a lowerbreakdown potential for electrolytes having higher anionconcentration. Kato et al. have also used electrostriction toexplain the enhancement of breakdown by incorporated anionicimpurities (see eq 27).77 They suggested that the incorporatedanions lead to additional electrostrictive input into themechanical stress in oxide films.Sato noted three different forms of mechanical breakdown

depending on the mechanical property of the films: brittle crackfor rigid anhydrous anodic oxides, and plastic deformation orflow for visco-plastic hydrous anodic oxides (Figure 11).97 Hesuggested that the formation of porous AAO films onaluminum is associated with continuous mechanical breakdown,accompanied by a continuous plastic flow of oxide under high

film pressure. The alignment of pores in a close-packed patternwas considered to be a process of relieving the film stress. Asthe breakdown in the anodic oxidation of aluminum proceeds,therefore, a porous oxide layer progressively forms and thickenson the compact barrier oxide layer, of which thickness remainsconstant with continuous plastic deformation. This electro-striction-stimulated breakdown model is somewhat in line withthe recent flow model accounting for the steady-state formationof porous AAO film by Skeldon et al. (see section 6.3.5).98

5. STRUCTURE OF POROUS ANODIC ALUMINUMOXIDE (AAO)

5.1. General Structure

Figure 12 shows schematically an idealized structure of porousAAO, together with scanning electron microscopy (SEM)images of each part of the porous AAO. Porous AAO has ahoneycomb-like structure. Porous oxide layer formed onaluminum substrate contains a large number of mutuallyparallel pores. Each cylindrical nanopore and its surroundingoxide constitute a hexagonal cell aligned normal to the metalsurface. Each nanopore at the metal/oxide interface is closed bya thin barrier oxide layer with an approximately hemisphericalmorphology. Under proper anodization conditions, the oxidecells are self-organized to form a hexagonally close-packedstructure.7 On the other hand, the surface of the aluminumafter complete removal of the porous oxide layer is texturedwith arrays of concave features. The thickness of the porousAAO layer on aluminum is proportional to the total charge(Qc) involved in the electrochemical oxidation. Therefore, thedepth of oxide nanopores is easily tunable from a few tens ofnanometers up to hundreds of micrometers by controllinganodization time (t). In general, the structure of self-orderedporous AAO is often defined by several structural parameters,such as interpore distance (Dint), pore diameter (Dp), barrierlayer thickness (tb), pore wall thickness (tw), pore density (ρp),and porosity (P). For ideally ordered porous AAO, thefollowing relationships can be drawn by simple geometricconsideration:

= +D D t2 (in nm)int p w (51)

ρ = × −⎛⎝⎜

⎞⎠⎟D

23

10 cmPint2

14 2

(52)

π= ×⎛⎝⎜

⎞⎠⎟⎛⎝⎜

⎞⎠⎟P

D

D(%)

2 3100p

int (53)

These structural parameters of porous AAO are known to bedependent on the anodizing conditions: the type of electrolyte,anodizing potential (U), current density (j), temperature (T),etc. Among those, anodizing potential (U) and current density(j) are the most important electrochemical parameters. Areview on this matter has recently been published by Sulka.99

Here, we briefly discuss the major structural parameters ofporous AAO and the electrochemical factors influencing them.

5.1.1. Pore Diameter (Dp). O’Sullivan and Wood usedelectron microscopy to quantitatively study the morphology ofporous AAO potentiostatically formed in phosphoric acid(H3PO4) electrolyte.100 The pore diameter (Dp), interporedistance (Dint), and barrier layer thickness (tb) were observed tobe directly proportional to the anodizing potential (U). Theirmicroscopic analysis revealed that the pore diameter increases

Figure 11. Three modes of mechanical breakdown of surface films.Reproduced with permission from ref 97. Copyright 1971 Elsevier.




at the rate (ζp) of 1.29 nm V−1 with respect to the anodizingpotential (U):

ζ= · = ·D U U1.29P p (54)

They pointed out that the electrolyte concentration does notsignificantly influence the pore diameter (Dp), while thetemperature of the electrolyte is positively correlated withpore diameter.100 On the other hand, theoretical modeling ofporous AAO growth performed by Parkutik and Shershulskypredicted a decrease in pore diameter with decreasingelectrolyte pH (i.e., increasing electrolyte concentration) dueto the enhanced dissolution velocity of anodic oxide at the porebase.101 Moreover, a recent study by Sulka and Parkołaindicated that the pore diameter decreases with decreasingtemperature.102 In general, pore diameter close to the surface ofa porous AAO film is larger than that close to the pore bottoms(i.e., truncated pore channels), especially when anodization isconducted at an elevated temperature and/or for an extendedperiod of time. This can be attributed to the chemicaldissolution of the pore wall oxide by acid electrolyte.Accordingly, pore diameter measured from the pore bottom(not from the surface of AAO film) is more relevant forinvestigating the intrinsic effect of electrochemical parameterson the structure of porous AAO.Recently, Lee et al.103 reported that pore diameter (Dp)

increases with current density (j) under potentiostaticanodization conditions (Figure 13). Under specific experimen-tal conditions, they observed spontaneously oscillating current

during potentiostatic hard anodization (HA) at the potentialrange of 140−200 V (see section 7.2). They suggested that at agiven potentiostatic condition (i.e., U = constant) the

Figure 12. Schematic structure of (a) porous anodic aluminum oxide (AAO) on Al foil and (b) cross-sectional view. (c−e) SEM images of porousAAO, showing top surface, barrier layer, and bottom surface, respectively. Scale bars are 1 μm. Panels c−e were reprinted with permission from ref111. Copyright 2006 Macmillan Publishers Ltd.: Nature Materials.

Figure 13. Cross-section SEM micrographs of AAOs prepared fromtwo separate anodization experiments, whose reactions wereterminated near j = 86 mA cm−2 and j = 881 mA cm−2 in sinusoidallyoscillating currents under potentiostatic condition (U = 200 V). (c) Aschematic cross-section of AAO on Al. (d) The parameters definingthe geometry of the pore bottom. Scale bars = 250 nm. Reproducedwith permission from ref 103. Copyright 2010 Wiley-VCH VerlagGmbH & Co. KGaA, Weinheim.





distribution of the current lines and electric field (E) may besensitively varied by the geometric details of the barrier oxidelayer. That would influence the movement rates of theelectrolyte/oxide and oxide/metal interfaces, hence the porediameter (Dp).

103

5.1.2. Interpore Distance (Dint). It has long beenestablished that the interpore distance (Dint) is also linearlyproportional to the anodizing potential (U).7,100,104−109 Adetailed study on this matter was performed for sulfuric andoxalic acid by Ebihara et al.104,105 Their empirical expressionson the relationship between the interpore distance (Dint) andanodizing potential (U) are as follows:

= + · = −D U Ufor sulfuric acid: 12.1 1.99 ( 3 18 V)int(55)

= + · ≤D U Ufor oxalic acid: 14.5 2.00 ( 20 V)int(56)

=− + · ≥U U1.70 2.81 ( 20 V) (57)

For oxalic acid-based anodizations in the potential range of 20−60 V, Hwang et al. reported that interpore distance onlydepends on anodizing potential (U), not on the temperature ofthe electrolyte:107

= − + · = −D U Ufor oxalic acid: 5.2 2.75 ( 20 60 V)int(58)

This temperature independence of the interpore distance is inline with the results of Keller et al.,7 but conflicts with theexperimental results of Sulka and Parkoła,102 who observed thatinterpore distance is positively correlated with temperature forself-ordered porous AAOs formed by sulfuric acid-basedanodization; interpore distance at an elevated temperature(10 °C) is about 10% larger than that at a low temperature (i.e.,−8 to 1 °C). O’Sullivan and Wood100 reported for phosphoricacid-based anodization that increasing the temperature or theelectrolyte concentration decreases the interpore distance. Forself-ordered porous AAOs formed by mild anodization (MA)conditions using sulfuric, oxalic, and phosphoric acid, it hasgenerally been accepted that the interpore distance (Dint) islinearly proportional to the anodizing potential (U) with aproportionality constant ζMA of 2.5 nm V−1 (see section7.1):109

ζ= · = ·D U U2.5int MA (59)

However, this empirical formula is not valid for hardanodization (HA), under conditions in which a high electricfield (E) is exerted across the barrier layer due to high currentdensity (j) during anodization.16,110−113 This will be discussedin detail in section 7.2.5.1.3. Barrier Layer Thickness (tb). The thickness of the

barrier layer (tb) is one of the most important structuralparameters of porous AAO for understanding the kinetics ofthe electrochemical oxidation of aluminum. Like otherstructural parameters, barrier layer thickness (tb) is alsodependent on the anodizing potential (U). The potentialdependence of the barrier layer thickness has also been knownas “anodizing ratio (AR = tb/U)”, the inverse of whichcorresponds to the electric field (E) across the barrier layer, andit determines the ionic current density (j) (see eq 1).Accordingly, at a given anodizing potential (U), current density(j) increases exponentially as a function of the inverse of theanodizing ratio AR (i.e., the electric field strength E). Earlierstudies have indicated that the anodizing ratio equals 1.2 nm

V−1.7,100,114 For self-ordered porous AAOs formed by mildanodization (MA, see section 7.1) in oxalic and phosphoric acidat various anodizing potentials (U), Vrublevsky et al. havereported empirical equations based on results from their re-anodizing experiments.115−117 They used the equations forestimating the barrier layer thickness by assuming the anodizingratio, ARMA = 1.14 nm V−1. On the other hand, reducedanodizing ratios for sulfuric and oxalic acid have recently beenreported for hard anodization (HA); ARHA = 0.6−1.0 nm V−1

(see Figure 13d and section 7.2).18,111,113,118,119 Chu et al.18

determined the anodizing ratio for less-popular anodizingelectrolytes (e.g., tartaric, citric, glycolic, and malic acids) byperforming what they termed “critical-potential anodization”.The anodizing ratio for various electrolytes was determined tobe AR ≈ 1 nm V−1 (Figure 14). It should be noted that thisvalue of anodizing ratio is the averaged proportionality constantdetermined from mild and hard anodization experiments ofaluminum.

5.2. Structure of Pore Wall (Anion Incorporation)

The incorporation of electrolyte-derived anions into anodicalumina is considered a general phenomenon for both barrier-and porous-type anodization, occurring least for the former andgreatest for the latter.3 For three major pore-forming acidelectrolytes (e.g., H2SO4, H2C2O4, and H3PO4), incorporationof acid anions occurs via inward migrations under an electricfield (E) during the anodization of aluminum. The incorpo-rated acid anions influence the chemical, optical, andmechanical properties of the resulting porous AAO. Forexample, incorporated oxalate (C2O4

2−) anions together withsingly ionized oxygen vacancies (F+ center) have been knownto contribute to the blue photoluminescence (PL) of porousAAO formed in oxalic acid solution.120−122 The mechanicalproperties (e.g., hardness, wear resistance, and elasticity) ofanodic alumina are also known to be affected by theincorporated chemical species (e.g., water and acidanions).123−125 The amount of incorporated acid anions andtheir distribution in anodic alumina depend on the anodizationpotential (U), current density (j), and temperature (T), as well

Figure 14. Effect of anodizing potential (U) on the barrier layerthickness (tb) for porous AAO formed in different acid electrolytes.(Solid symbols, measured values; open symbols, calculated values fromthe half-thickness of the pore walls). Reproduced with permissionfrom ref 18. Copyright 2006 The Electrochemical Society.




as the type and concentration of electrolytes.44,117,126−129

Accordingly, the chemical structure of the pore wall of AAOsvaries with the anodization conditions. Han et al.129 haverecently reported that, even at a steady-state growth condition(i.e., fixed U, j, and T), the content of anionic impurities andtheir incorporation depth decrease as a function of anodizationtime due to the progressive reduction of the electrolyteconcentration, which markedly affects pore widening as well asthe opening of the barrier oxide layer by wet-chemical etching.On the basis of TEM investigations of disordered porous

AAOs formed in sulfuric, oxalic, phosphoric acid solutions,Thompson and co-workers suggested that pore wall oxide has aduplex structure in terms of chemical composition: an acid-anion contaminated outer oxide layer next to the pores and arelatively pure inner oxide layer, as schematically shown inFigure 15a.130 TEM micrographs of porous AAO films formedin phosphoric and oxalic acids (i.e., H3PO4-AAO and H2C2O4-AAO) showed a cell structure with cell-boundary bands,although the cell-boundary band in H2C2O4-AAO did notappear as well-defined as that observed in H3PO4-AAO. Thecell-boundary bands became markedly more apparent uponcontinued exposure to the electron beam due to the preferentialcrystallization of the cell boundary regions. Meanwhile, thepresence of a cell-boundary band was not apparent for porousAAO films formed in sulfuric acid (i.e., H2SO4-AAO). Theduplex structure of the pore walls was confirmed by Ono andMasuko,131 who reported that the depth and the amount ofanion incorporation in H3PO4-AAO increased linearly withanodizing potential (U), but the presence of a duplex structurewas not confirmed by TEM for samples formed at U < 10 V.They also found that the crystallization rate of pore wall oxideunder a strong electron-beam irradiation decreases with theincreasing content of incorporated electrolyte species.132

Scanning transmission electron microscopy (STEM) andenergy dispersive X-ray (EDX) point analysis on H3PO4-AAOby Thornton and Furneaux revealed that the cell-boundary

bands are composed of relatively pure alumina, whereas thematerial adjacent to the pores contains incorporated phosphatespecies from the electrolyte.133,134 Recent microscopic chemicalanalyses of the pore wall material of highly ordered H3PO4-AAO by Le Coz et al.135 clearly indicated the presence ofphosphorus-free cell-boundary bands (Figure 15b−e). Thedifferent parts of the unit cell were found to have aheterogeneous chemical composition of Al2O3·0.197AlPO4·0.034H2O, which supports the results of the previous works byThompson et al.10,130 The work also highlighted, as a newfinding, that there is an interstitial rod material with acomposition of Al2O3·0.018AlPO4·xH2O at the triple junctionconnecting three cells (Figure 15f).135

Thompson and Wood related the steady-state anodizingbehavior of porous AAO films formed in the major anodizingacids to the distribution of the acid anions within the barrierlayers and the true field strengths across the relatively purealumina regions.14 Starting with the knowledge that thethickness ratio of the inner to outer pore wall oxide layerincreases in the order sulfuric acid < oxalic acid < phosphoricacid < chromic acid, they depicted the same order of thicknessratio for the barrier oxide at the bottom of pores and correlatedit to the rates of oxide formation at the same anodizingpotential (U). The averaged effective electric field (E = ΔU/tb)across the barrier layer is approximately constant for the barrierlayers of AAOs formed in different acid electrolytes, becausethe measured anodizing ratios are similar (ARMA = tb/ΔU ≈ 1.2nm V−1). On the other hand, the potential drop (ΔU) isgreater and linear across the relatively pure alumina region andsmaller across the outer acid anion-contaminated region, wherethe potential decreases progressively toward the oxide/electrolyte interface (Figure 16). Therefore, the true electricfield across the relatively pure alumina region of the barrierlayer is in the order:

Figure 15. (a) Schematics illustrating the duplex structure of pore walls of porous AAO: vertical (left) and transverse (right) cross-sections. TEMplane view (b) of H3PO4-AAO and the corresponding X-ray maps of the elements: (c) phosphorus, (d) oxygen, and (e) aluminum. (f) TEM planeview of H3PO4-AAO, showing the different parts of the pore wall (i.e., the outer pore wall, cell-boundary band, and interstitial rod). Reproduced withpermission from ref 135. Copyright 2009 Elsevier.




− > − > −

> −

H SO AAO H C O AAO H PO AAO

H CrO AAO2 4 2 2 4 3 4

2 4

Accordingly, the rate of formation of the anodic oxide filmsin the different acids varies with the same order given above ifsolid-state ionic migration across the inner layer is the rate-determining step.14 The current density (j) is related to thepotential drop (ΔU) across the barrier oxide by the high-fieldconduction theory (eq 1). Among the acid electrolytes above,sulfuric acid will give the highest current density (j), because ofthe highest potential drop across the relatively pure aluminaregion adjacent to the metal interface.The duplex nature of the pore wall can be experimentally

evidenced by investigating the rate of pore widening. For agiven set of etching conditions (i.e., temperature andconcentration of an etchant solution, typically H3PO4), therate of etching is dependent on the chemical composition of thepore wall oxide of AAO.129 Figure 17 shows (a) the evolutionof pore diameter (Dp) as a function of pore wall etching time(tetch) for porous AAO formed in 0.3 M oxalic acid (H2C2O4),together with (b) representative SEM micrographs. Aspresented in Figure 17a, Dp versus tetch plot is characterizedby an inflection point, at which the slope of the curve changes.Pore wall oxide in the early stage is etched at a higher rate (1.04nm min−1) than that (0.36 nm min−1) in the later stage. Theretarded rate of etching in the later stage can be attributed tothe relatively pure nature of the inner pore wall oxide, ascompared to the less dense outer pore wall oxide due to theincorporation of anionic species. As shown in Figure 17b, the

ability to precisely control the pore diameter by the porewidening process is one of the most attractive features ofporous AAO for template-based nanofabrication. This featureallows one to systematically investigate the size dependence ofchemical or physical properties of ordered arrays of nanodots,nanowires, or nanotube materials prepared from porous AAOtemplates.As-prepared porous AAOs are amorphous and contain

varying amounts of water depending on anodizing condition.3

The local coordination environments of aluminum inamorphous AAOs have been extensively studied using X-rayradial distribution analysis,136 electron-yield extended X-rayabsorption fine structure (EXAFS) spectroscopy,137 and magic-angle spinning nuclear magnetic resonance (27Al MASNMR).138−141 Amorphous AAOs have been considered tohave a close structural relation to spinel (fcc) γ-Al2O3 withtetra- and hexa-coordinated aluminum cations in the mixingratio of 1:2. Farnan et al.138 have reported that the coordinationnumbers of aluminum cations are dependent on the anodizingelectrolyte: hexa-coordination for porous AAO formed inchromic acid (i.e., H2CrO4-AAOs), tetra-, penta-, and hexa-

Figure 16. Distribution of the potential drop and electric field, E(slope of the voltage−distance plot), across barrier layers of porousAAOs formed in (a) sulfuric, (b) oxalic, (c) phosphoric, and (d)chromic acid. Reproduced with permission from ref 14. Copyright1981 Macmillan Publishers Ltd.: Nature.

Figure 17. (a) The evolution of pore diameter (Dp) as a function oftime (tetch) upon wet-chemical etching of porous AAOs formed in 0.3M oxalic acid at 40 V. Wet-chemical etchings of pore wall oxide wereperformed in 5 wt % H3PO4 at 29 °C. The numbers in the plot are theslopes (in nm min−1) of the corresponding linear fits. (b) SEM imagesshowing systematic increase in pore diameter (Dp) as a function oftime (tetch) upon wet-chemical etching in 5 wt % H3PO4 (29 °C).Reproduced with permission from ref 129. Copyright 2013 TheAmerican Chemical Society.





coordination for H2SO4- and H2C2O4-AAOs, and tetra- andpenta-coordination for H3PO4-AAOs. It is now generallyaccepted that the aluminum exists in four-, five-, and six-foldcoordination with oxygen, although the ratio of thecoordination numbers varies in different samples formed indifferent anodizing conditions. Farnan et al.138 attributed suchvariations of aluminum coordination to the presence ofhydroxyl groups (−OH) within alumina, with an increase ofanodizing temperature favoring the hexa-coordination. Incorpo-rated electrolyte species (e.g., acid anions, proton, and water)may be more responsible for such variations of aluminumcoordination. Yet, a clear explanation for such variation ofcoordination number has not been given yet.

5.3. Effect of Heat Treatments

The surfaces of pore walls are hydrophilic due to the surface-bound hydroxyl (−OH) groups, and this feature allows easymodifications of the surface property via self-assembly ofvarious functional molecules. Extensive recent reviews on thismatter are given in refs 142 and 143. On the other hand, as-prepared porous AAOs are highly labile to both acid and baseattack. Proper high temperature heat treatments of as-preparedporous AAOs markedly improve their thermal stability andresistance against acid, base, and other corrosive chemicals,allowing the resulting AAOs to be useful as starting materialsfor developing various devices, which will operate in hightemperatures or harsh environments. TEM and X-raydiffraction (XRD) studies have indicated that the porousAAOs undergo a series of polymorph transformations uponheat treatment up to 1500 °C in air with the following route(see Figure 18 for XRD): amorphous AAO → γ-Al2O3 → δ-Al2O3 → θ-Al2O3 → α-Al2O3.

121,139,140,144−147 AmorphousAAOs crystallize into almost pure γ-Al2O3 at a temperaturerange of 820−900 °C, and then undergo successive trans-formations through metastable ccp δ- and θ-Al2O3 until

thermodynamically stable hcp α-Al2O3 (corundum) is obtainedabove 1150 °C. Crystalline phase transformations of porousAAOs formed in different electrolytes have also beeninvestigated by thermogravimetric (TG) and differentialscanning calorimetry (DSC) analyses.139,140,144,147−151 Theresults have indicated that the type of anodizing electrolytes(i.e., the nature of incorporated acid anions) strongly influencestransformation temperatures during heat treatment. ForH2C2O4-AAOs, Mardilovich et al.144 reported that dehydrationoccurs up to 400 °C, dehydroxylation occurs at 400−700 °C,and incorporated oxalates pyrolyze at higher temperaturesbefore the final transformation into thermodynamically stableα-Al2O3. A comparative study of H2SO4-, H2C2O4-, andH3PO4-AAOs by Mata-Zamora et al.150 has shown thatincorporated phosphates are stable up to 1400 °C, whilesulfates and oxalates decompose into gaseous SO2 and CO2 at900 and 870 °C, respectively. Later, Kirchner et al.140

confirmed decomposition of sulfates in H2SO4-AAOs at around900 °C by mass spectrometry (MS).In general, the initial crystallization and subsequent phase

transformation of porous AAO are accompanied by changes inthe morphology of pores and also in mechanical proper-ties.139,144,151 MacQuaig et al. reported ∼6.7% loss of porecircularity, ∼15% increase in pore diameter (Dp), ∼13%decrease in pore density (ρ), and about a 2-fold increase inmicrohardness (from 2.5 to 4.7 GPa) upon heat-treatment ofas-prepared H2C2O4-AAO up to 1200 °C.151 They attributedthe changes of the overall pore structure to the densification ofpore wall materials, which is associated with dehydration,dehydroxylation, and the loss of incorporated acid anions in thecourse of the phase transformations from amorphous tocrystalline α-Al2O3. Mardilovich et al. observed sharp decreasesin the flexibility of H2C2O4-AAO films at the onset ofcrystallization at 820−840 °C and on phase transformationfrom metastable ccp-alumina to hcp-alumina (i.e., α-Al2O3) at1100−1150 °C.144 They pointed out that the fragility of porousα-Al2O3 membranes is so high that they cannot be consideredfor routine practical use as membranes. High temperaturetreatment of initially planar porous AAO membranes oftenleads to serious mechanical deformation (e.g., curling/rolling)or even cracking.139,144,148 Therefore, careful heat-cyclingprocessing is required. Recently, Chang and co-workersreported that severe deformation of porous AAO membranescan be prevented by removing the acid-anion contaminatedouter pore wall oxide before annealing at high temperatures.152

A proper hydrothermal treatment of H3PO4-AAOs improvedcrystallinity of the relatively pure inner oxide layer (i.e., the cell-boundary band in Figure 15), which enhanced the etchingcontrast between the inner and outer oxide layers, after whichthe outer pore wall oxide was selectively removed from the unitcells of the AAO membranes by wet-chemical etching.Deformation-free porous α-Al2O3 membranes could beobtained by annealing of the resulting samples at 1300 °C(see Figure 19).

6. GROWTH OF POROUS ANODIC ALUMINUM OXIDE(AAO)

6.1. Stress Generation in Anodic Oxide Films

6.1.1. Volume Expansion. Oxidation of aluminum is avolume expansion process. The volume expansion duringanodization can be quantitatively expressed by the Pilling−Bedworth ratio (PBR). The PBR is rigorously defined by the

Figure 18. XRD spectra of heated H3PO4-AAO (Co Kα radiation).Key: AlP = AlPO4, A = α-Al2O3, T = θ-Al2O3. Reproduced withpermission from ref 139. Copyright 2005 Elsevier.




molar volume ratio of grown oxide (Vox) to the consumedmetal (Vm) as follows:

ρρ

= =VV

MnM

PBR ox

m

ox m

m ox (60)

where Mox is the molecular weight of oxide, Mm is the atomicweight of metal, n is the number of atoms of metal per oneformula of the oxide, and ρm and ρox are the densities of metaland oxide, respectively. In science on the corrosion of metals,PBR has been the basis for judging the protectiveness of apassivating oxide: if PBR < 1, the passivating oxide is undertensile stress and easily cracked; if 1 < PBR < 2, the oxidecovers the metal uniformly and is protective; if PBR > 2, thepassivating oxide is under too much compressive stress andeasily crumbles (e.g., iron oxide on iron).153,154 For anodicalumina growth, PBR can be experimentally determined fromthe current efficiency (ηj) of oxide formation and the density(ρAAO) of the resulting AAO, provided that the composition ofanodic oxide is well-defined. The densities of barrier- andporous-type AAOs have been reported to be in the range ofρAAO = 2.7−3.5 g cm−3.104,111,155−157 Assuming compositionstoichiometry of Al2O3 and ρAAO = 3.0 g cm−3, PBR for AAOgrowth is 1.70 at 100% current efficiency (ηj). For porous-typeAAO growth, on the other hand, PBR can vary between 1.02and 1.58 due to the lower current efficiency (ηj = 60−93%).158−161 The exact determination of PBR is rathercomplicated by the composition of anodic oxide, its density,and the current efficiency (ηj) of oxide formation. Con-sequently, volume expansion in anodic oxide growth has beenconsidered instead, by employing a volume expansion factor(kv), and a simple thicknesses ratio:116,162−164

=k h h/v AAO Al (61)

where hAAO and hAl are the vertical heights of the AAO and theconsumed aluminum, respectively (Figure 20).

For three major pore-forming acid electrolytes (i.e., H2SO4,H2C2O4, and H3PO4), a wide range of volume expansionfactors (kv) between 0.86 and 1.90 have been reported (Table1).106,116,162,163,165−168 For anodizations of aluminum inH2SO4, Jessensky et al.165 investigated the relation betweenthe volume expansion factor (kv) and the degree of self-ordering of pores. They reported that optimal conditions forthe best ordered arrays of pores are accompanied by a moderatevolume expansion (i.e., kv = 1.22), while contraction or verystrong volume expansion can result in the disordering of pores.Further, they proposed that the mechanical stress at the metal/oxide interface, which is associated with volume expansion andincreases with the anodizing potential (U), is the driving forcefor the formation of ordered hexagonal pore arrays, by causingrepulsive forces between the neighboring pores. Li et al.reported a similar line of experimental results, noting that thevolume expansion factor (kv) for the optimum anodizationconditions required for ordered pore arrays should be close to1.4, irrespective of the anodizing electrolytes (i.e., H2SO4,H2C2O4, or H3PO4).

106 Later, on the other hand, Nielsch et al.proposed that the best self-ordering of pores requires a porosity(P) of 10%, the condition that corresponds to a volumeexpansion of porous AAO of about 1.23, independent of theelectrolyte.109

More systematic investigations on the effect of anodizationconditions on the volume expansion factor (kv) have beenperformed by Vrublevsky and co-workers. For porous AAOgrowths in H2SO4 and H2C2O4 electrolytes, they determinedthat the volume expansion factor (kv) has a linear dependenceon the anodizing potential (U) and also noted a linear relationbetween the logarithm of the current density (i.e., ln j) and theinverse volume expansion factor (i.e., 1/kv).

116,163,167 Foranodizations under galvanostatic conditions, an increase oftemperature led to a decrease of the volume expansion factor(kv) due to the corresponding decrease of the formationpotential (U), in which the slopes of ln j versus 1/kv curves areinvariant with respect to the anodizing temperature.163,167

However, the slopes of curves were found to be different fordifferent electrolytes, which was explained by the influence ofacid-anion incorporation on the volume expansion: the larger isthe amount of incorporated acid anions, the larger is thevolume expansion factor (kv).

167 On the basis of theexperimental results of Vrublevsky et al., one may obtain thefollowing relation:

6.1.2. Stress Measurements. Anodic alumina is adielectric material. During anodization, a very large electricfield (typically, 106−107 V cm−1) is impressed on the oxidefilm. The electric field drives the inward movement of O2− ionsand the outward migration of Al3+ ions within the anodic oxide.The resistance to these counter-migrations and the attraction of

Figure 19. A photograph of the porous α-Al2O3 membranes obtainedby annealing the porous AAOs with (a) and without (b) acid-anioncontaminated outer pore walls. (c−e) SEM images of porous Al2O3membranes obtained by annealing the porous AAOs after removal ofouter pore walls at (c) 1115 °C, (d) 1250 °C, and (e) 1300 °C; thescale bars are 1 μm. Reproduced with permission from ref 152.Copyright 2012 The Royal Society of Chemistry.

Figure 20. A schematic illustration of volume expansion duringanodization of aluminum.





the charged species result in a compressive electrostatic stressalong the direction of the electric field. The compressive stressnormal to the oxide surface is proportional to the dielectricconstant (ε) and the square of the electric field (E):169

σ επ

=⊥ E8

2(62)

This compressive stress occurs only under the electric field.Thus, the stress will relax when the anodization is stopped.Proost et al. have determined electrostatic stress in anodic oxidefilms of thickness tb as follows:

67,170

σ εε α α

= −−

− +vv

Et1

( )2

ES0

1 22

b (63)

where v is the Poisson coefficient of the oxide film, ε is itsrelative dielectric constant, εo is the vacuum permittivity, and α1and α2 are both characteristic electrostriction constants.Porous AAO has been known to incorporate water as well as

varying amounts of electrolyte-derived anionic species (seesection 5.2). Compositional effects arising from anionincorporation and oxide inhomogeneities may be sources ofstress in anodic oxides. The stress during anodization can bemeasured by monitoring the changes of the substratedeflection. Stoney related the measured deflection (i.e., theradius of curvature) to the stress (σ) as follows:171

σ =ΔYt

kt6

2

b (64)

where Y is Young’s modulus of the substrate metal, t is thethickness of the metal foil, tb is the thickness of the oxide on themetal, and Δk is the radius of curvature. Equation 64 has beenmodified over the decades to account for lateral strain,differences in the elastic moduli of the oxide and its metalsubstrate, and non-uniformity of the stress distribution in theoxide. Vermilya172 applied the Stoney method for anodizingdifferent metals. Upon applying potential, the substratedeflected, indicating tensile stress. The stress observed withthe forming voltage applied was always more compressive thanat zero voltage except for tungsten. Higher stress was observedwhen the rate of oxide formation was high. Vermilya attributedthe observed stress to a dynamic hydration process. As theoxide film is buried by newly generated oxide, it is dehydratedby proton migration, producing tension in the film. Bradhurst

and Leach50 conducted deflection measurements on aluminumand reported the effects of current density and oxide thicknesson the stress. During anodization of aluminum in ammoniumborate and ammonium citrate solutions, they observed that thestress can be either tensile or compressive depending on thecurrent density. Compressive stress was observed below 1 mAcm−2, while tensile stress was observed at higher currentdensities. A similar line of results was obtained by Nelson andOriani,169 who performed deflection measurements duringanodization of aluminum and titanium in 0.1 M H2SO4. Thedeflection was related to the stress as follows:169

σ =Δ

−E t k

v L t3(1 )M

2

M2 2

b (65)

where EM and vM are the elastic modulus and Poisson’s ratio ofmetal strip of length, L, and thickness, t; tb is the thickness ofthe oxide layer where the stress is generated; Δk is the radius ofcurvature of the metal strip. The compressive stress due toelectrostriction was found to increase linearly with anodizingpotential during oxide growth. Slowly grown oxides havegreater electrostrictive deflections than more rapidly grownoxides. For aluminum anodized in 0.1 M sulfuric acid, thedeflections are compressive at low current densities andbecome tensile above 0.6 mA cm−2. The development oftensile stress was attributed to the volume difference betweenthe metal being oxidized and the oxide formed at the metal/oxide interface.169 Moon and Pyun investigated the effect ofelectrolyte concentration and current density on the deflectionbehavior of aluminum in sulfuric acid.173,174 Their experimentsfound that the deflections become more tensile as the currentdensity increases. Compressive stresses were always observed atthe relatively low current density (j = 2 mA cm−2). On theother hand, at larger concentrations of sulfuric acid, the rates ofdeflection with respect to the current density became nonlinear.The observed compressive stress at low current densities andtensile stress at higher current densities was explained in termsof the annihilation of cation vacancies and the formation ofoxygen vacancies at the metal/oxide interface, respectively. Theauthors suggested that the stresses that developed are notdistributed over the entire oxide film, but are limited to anarrow region of the metal/oxide interface below 1 nm.174 Anopposite evolution of internal stress has recently been reported.Proost et al.175 measured the internal stress of porous AAO

Table 1. Volume Expansion Factors (kv) Reported for Porous Anodic Aluminum Oxide (AAO) Growth in Three Major Pore-Forming Acid Electrolytes

electrolyte concentration temp (°C) Ua or jb kvc refs

H2SO4 20 wt % 1 18−25 V 0.86−1.62 16520 wt % 1 19 V 1.41 1061.7 wt % 1 25 V 1.36 1061.7 wt % 10 25 V 1.40 10610 wt % mixture solutiond 6−20 mA cm−2 1.4−1.6 167

18−22 50 V 1.72 1620 100 V 1.76 162

300 V 1.90 162H2C2O4 2.7 wt % 1 40 V 1.42 106

0.34 M 15 40 V 1.18 1680.22−0.92 M 16−18 1.62−4.8 mA cm−2 1.25−1.42 163,167

H3PO4 0.1−2.9 M 18 2−13 mA cm−2 1.2−1.62 166aU = anodizing potential (in V). bj = current density (in mA cm−2). ckv = volume expansion factor. dA mixture solution of 0.41 M H2SO4 and 0.16M H3BO3.



during the very initial stage of pore formation in 0.4 Mphosphoric acid by employing a high-resolution in situcurvature measurement technique based on multiple-beamdeflectometry. The internal stress was found to remain constantduring the initial barrier growth. However, the internal stressvalue increases in the compressive direction with increasingcurrent density. When the current density is lower than 4 mAcm−2, the internal stress in the barrier oxide is tensile, while athigher current densities, the internal stress becomes compres-sive. The different stress evolutions in different electrolytesystems have not yet been fully understood. More systematic insitu measurements of stress as a function of the anodizingconditions at the very initial stage of pore formation arerequired to resolve conflicting stress evolutions reported in theliterature. A meaningful advance in this direction has recentlybeen made by Hebert and co-workers,176 who demonstratedthe effectiveness of phase-shifting curvature interferometry as anew technique for high-resolution in situ monitoring of stressevolution during anodization. The newly developed metrologyallows extremely stable and reliable measurement of curvaturechanges at a curvature resolution of 10−3 km−1, which iscomparable to or higher than that of the high-resolutionmultiple-beam deflectometry technique.175,177−179 From stressmeasurements during galvanostatic anodizing of aluminum in0.4 M phosphoric acid, Hebert and co-workers176 found thatthe apparent stress in the barrier oxide is tensile (+100 MPa) atlow current density but became increasingly compressive athigher current densities. In addition, they observed thattransition from tensile to compressive stress occurs at currentdensity of 4 mA cm−2, in good agreement with the report ofProost and co-workers.175

6.1.3. Effects of External Stresses on Pore Growth. Theeffect of an applied external tensile stress on the self-ordering ofpores was first studied by Sulka and co-workers.180 It was foundthat the magnitude of applied tensile stress influences theordering degree of pores. At a relatively low external tensilestress, regular hexagonal arrangement of pores was observed.However, a high tensile stress completely destroyed the porearrangement of porous AAO. Large holes and pits, rather thannanopores, appeared on a highly stressed surface. Thisexperimental result revealed that the mutually repulsivemechanical force between neighboring pores, which isassociated with volume expansion due to oxidation of themetal, may be the driving force for the self-organized formationof hexagonally close-packed arrangement of nanopores, assuggested by Jessensky et al.165

As to the effects of compressive stress on pore growth, Parket al.181 have performed the anodization of aluminum confinedin micrometer-sized vertical trench patterns. The authorsobserved that during the anodization of the confined aluminum,the anodization rate is significantly retarded at the verticalsidewall of the trench. Because of the retarded anodization rate,most of the aluminum at the edge part of the structure remainsand its thickness decreases gradually with increasing distancefrom the vertical sidewall toward the central part of the trenchstructure (Figure 21). The authors attributed this phenomenonto the accumulation of compressive stress at the verticalsidewall of the trench structure, where linear vertical volumeexpansion is severely prohibited by additional stress. Theauthors noted that because compressive stress is an additionalkinetic barrier to the electrochemical oxidation of aluminum,the anodization kinetics of aluminum should be severelyretarded.

Retardation of anodization rate has also been reported forlateral anodization processes. In that process, an aluminum thinfilm deposited on a substrate is sandwiched by a rigid insulatingtop layer, and then the side edges of the aluminum are anodizedto produce horizontal arrays of pores on the substrate.182−186

Oh and Thompson186 reported abnormal behavior in theanodic oxidation of aluminum in mechanically confinedstructures used for the formation of horizontal nanoporousAAO. The authors observed that dendrites, periodic internalpore structures (see Figure 22a), formed with a 5% retardedgrowth rate, as compared to its value during bulk anodizationunder the same conditions. They attributed the observedanomalies to the suppressed volume expansion and a plasticflow of anodic oxide confined by an insulating top layer;because volume expansion by plastic flow in the pore growth

Figure 21. Cross-sectional SEM images of the AAO confined in amicrometer-sized vertical trench pattern: (a) low and (b) highmagnification image. Scale bar = 1 μm. Reproduced with permissionfrom ref 181. Copyright 2006 The Electrochemical Society.

Figure 22. (a) Cross-sectional SEM image of AAO nanopore formedby horizontal anodization. Scale bar = 100 nm. (b) Schematicillustration of the formation mechanism of horizontal AAO withdendritic internal pore structure. Reproduced with permission from ref186. Copyright 2011 The Electrochemical Society.





direction is prohibited by traction at the insulating top layer,the extra volume of newly formed anodic alumina is extrudedinside the primary pores, resulting in a dendritic structure, asschematically illustrated in Figure 22b.

6.2. Initial-Stage Pore Formation

6.2.1. Qualitative Description on Pore Formation.Porous AAO can be easily fabricated by anodization ofaluminum in acid electrolytes either under a constant potential(i.e., potentiostatic) or a constant current (i.e., galvanostatic)condition. In general, potentiostatic anodization is widelyemployed for the fabrication of self-ordered porous AAO,because of the linear relation between the applied potential (U)and the structural parameters of the resulting AAO (i.e., porediameter Dp, interpore distance Dint, and barrier layer thicknesstb, section 5). Figure 23 shows (a) a typical current (j)−time (t)curve for potentiostatic anodization, (b) potential (U)−time (t)curve for galvanostatic anodization, together with (c) schematicillustrations of the stages of porous structure development.101

When a constant anodic potential (U) is applied, a thincompact barrier oxide starts to grow over the entire aluminumsurface (stage I). Thickening of the initial barrier oxide overtime (t) results in an increase of the series resistance (R) of theanodization circuit. Current (j) is initially maintained at thelimiting current (jlimit) of the power supply, and correspond-ingly potential (U = jR) increases linearly with time (t) (see theinset of Figure 23a). When the thickness (or the resistance, R)of the compact barrier oxide layer reaches a certain value,current (j) drops rapidly to hit the minimum value (stage II).For this stage, O’Sullivan and Wood100 suggested that current(i.e., electric field) concentrates on local imperfections (e.g.,defects, impurity, pits) existing on the initial barrier oxide,resulting in non-uniform oxide thickening and pore initiation atthe thinner oxide areas. Thompson and co-workers10,33,44 haveproposed that local cracking of the initial barrier oxide due toaccumulated tensile stress (PBR < 1) may develop the paths forelectrolyte penetration. Local increase in field strength at thepenetration paths effectively polarizes the Al−O bonds,facilitating field-assisted oxide dissolution there (section6.3.2),100 and eventually leads to development of individualpenetration paths into embryo pores.10,44 Accordingly, furtheranodization leads to a gradual increase in current (j) to a local

maximum due to the ready diffusion of electrolyte (stage III).After that, current (j) reaches a steady value after passingthrough an overshoot (stage IV). The appearance of currentovershoot has been related to the decrease of the initial poredensity with the steady-state growth of major pores:168 poresincrease in size by persistent merging with adjacent pores. For agiven set of anodization conditions, the rate of potentialincrease at the very beginning of anodization, the value of theminimum current, the time needed for anodizing current toreach a steady value, and the appearance of the currentovershoot have been known to be directly dependent on theanodizing potential (U), electrolyte pH and temperature, andthe initial surface state of the aluminum.168,187,188

For the case of galvanostatic anodization, a similarprogression can be observed for stages I−IV, while thepotential (U) changes as a function of time (Figure 23b).Under constant-current conditions, the oxide growth rateshould be proportional to the applied current density (j) andconstant according to the Faraday’s law. In addition, a constantelectric field (E = U/tb) is required to sustain the appliedconstant current (j).76 Accordingly, the potential (U) increaseswith the thickness of the growing barrier oxide (tb), as shown inthe inset of Figure 23b. However, in practice, the evolution ofpotential (U) deviates from a simple linear increase with time,as shown in Figure 23b. For convenience, various mechanismsgoverning such a deviation have been referred to as growthinstabilities, which include, for example, mechanical breakdownduring zirconium anodization, and surface undulation/poreinitiation during aluminum anodization.178 Figure 23b shows agradual retardation of potential (U) increase at stage II. Such apotential evolution can be attributed to a morphologicalinstability, that is, transition from the stage of barrier oxidegrowth to the stage of porous oxide growth.178 In the followingsections, we will discuss in detail the kinetics and morphologicalinstability involved in the early stage of anodization, which havebeen systematically investigated by Proost and co-work-ers.160,175,178,189,190

6.2.2. Kinetics of Porosity Initiation. Recent publicationshave shown that the growth of a porous AAO and its self-organization are most likely driven by the internal stressesdeveloping in the anodic oxide.47,190−192 Proost and co-workershave investigated the initiation of porosity during galvanostatic

Figure 23. Schematic diagram of the kinetics of porous AAO growth in (a) potentiostatic and (b) galvanostatic conditions: (a) Current (j)−time (t)curves for potentiostatic anodization (i.e., U = constant) and (b) potential (U)−time (t) curve for galvanostatic anodization (i.e., j = constant).




anodization of aluminum thin film on a silicon substrate in 1 Msulfuric acid (22 °C) at j = 5.0 mA cm−2 by in situ monitoringof the internal stress-induced curvature of the substrate.190

They observed that the magnitude of the substrate curvature(ΔK, in km−1) increases at a constant rate in the compressivedirection up to a transition time (ca. 7.9 s) at which the rate ofcurvature change suddenly increases (Figure 24a). Theobserved curvature transition was attributed to the initiationof porosity accompanied by an increase of the oxide growthrate. This was confirmed by a quantitative analysis of the radialpower spectral density (PSD) distributions of the anodizedsurfaces as presented in Figure 24b, in which the first (λ1) andsecond (λ2) correlation breaks are associated with thealuminum grain morphology, and the appearance of porosity,respectively. Further, the authors pointed out that the increasein growth rate after the curvature transition would not beexpected if the porous layer grew by dissolution of the oxide atthe pore base.190

Hoar and Yahalom76,187 have proposed that the poreinitiation occurs at a sufficiently low electric field due toproton entry into the initial barrier oxide at preferred sites,where concentrated electric field (i.e., current) accelerates oxidedissolution to develop pores. The authors187,193 suggested thatthe Al3+ ions found in the solution originated from the oxide atthe pore base as a result of field-assisted (rather than “thermal”)oxide dissolution (see section 6.3.2). Yet later studies29,30,161

have revealed that Al3+ ions in the electrolyte are the result ofdirect ejection of Al3+ ions from the metal/oxide interfacethrough the oxide into the solution (see section 6.3.4), ratherthan coming from the field-assisted oxide dissolution process.Recently, Proost et al. have for the first time correlated thekinetics of Al3+ loss to the morphological changes occurringduring the very early stage of galvanostatic anodization ofaluminum in 1 M sulfuric acid160 and 0.4 M phosphoric acid189

by employing in situ inductively coupled plasma opticalemission spectrometry (ICP-OES). For both anodizationcases, the authors observed three distinct regimes of Al3+ loss.However, the evolution of the Al3+ loss rate turned out to bemarkedly different, as follows. For the same current density, therate of Al3+ loss is higher in phosphoric acid than in sulfuric acidduring the barrier layer growth stage (regime A, stage I inFigure 23b). On the other hand, the Al3+ loss rate is lowerduring the pore initiation stage (regime B, stage II in Figure23b), as compared to the barrier oxide layer formation stage

(i.e., regime A). This implies that the oxide formation efficiency(ηj) during the initiation of porosity is higher than that duringthe initial barrier oxide formation, which is in line with theincrease in the oxide growth rate upon pore initiation, as shownby in situ monitoring of the internal stress-induced curvature ofthe substrate (vide supra).190 On the other hand, the authorsattributed the lower Al3+ loss rate during the pore initiationstage to the non-uniformity of current distribution uponcommencement of pore initiation.189 In sulfuric acid, the Al3+

loss rate during stages II and III was constant, and increasedslightly at the beginning of stage IV. In phosphoric acid,however, the cation loss rate decreased again during stage IIand then remained more or less constant during stages III andIV (regime C). The authors associated the difference in theobserved evolution of Al3+ loss rate with the morphologicaldifferences of the growing anodic oxide (i.e., pore size andspacing).189 In sulfuric acid, the rate of Al3+ loss during thesteady-state pore growth stage was similar to the level of thebarrier layer growth stage, whereas in phosphoric acid bothrates were markedly different; there was a higher rate during thebarrier layer growth stage. For both cases of anodization, therates of Al3+ loss were observed to linearly increase with thecurrent density (j). Proost et al.189 suggested that the directproportionality between the rate of Al3+ loss and the currentdensity (j) can be attributed to the direct ejection of Al3+ ionsinto the electrolyte, because the field-assisted contribution maybe relatively independent of the current density according tothe high field conduction theory.25 For both cases ofanodizations, the oxide formation efficiency (ηj) during theporous oxide growth stage was found to increase with currentdensity (j). For phosphoric acid anodizing, however, theefficiency (ηj) during the barrier oxide growth stage did notincrease with current density (j), and remained constant in thecurrent range j = 2.0−10 mA cm−2 (Figure 25), which isdifferent from the case of sulfuric acid anodizing.160 Theabnormal efficiency values at 1.0 mA cm−2 in Figure 25 weresuggested to occur when field-assisted dissolution (see section6.3.2) rather than direct cation ejection (see section 6.3.4)becomes the predominant mechanism of Al3+ loss for currentdensities lower than 2.0 mA cm−2.164,189

6.2.3. Morphological Instability. As was briefly men-tioned at the end of section 6.2.1, at the very early stage ofanodization, pores initiate as a result of the morphologicalinstability of growing anodic oxide. This becomes evident at an

Figure 24. (a) Curvature change and cell voltage evolution as a function of anodization time at 5 mA cm−2 in 1 M H2SO4 (22 °C); (b) radial powerspectral density (PSD) of samples anodized for different times. Reprinted with permission from ref 190. Copyright 2009 AIP Publishing LLC.




oxide thickness of a few nanometers as shown in Figure26.67,161,168,178,190,194 Previous research has suggested that

internal stress may play a role in the growth instability ofanodic oxides.64,65,75,192,195 Van Overmeer and Proost178 haveinvestigated the relation between the internal stress, themorphological instability, and the pore initiation during thegrowth of porous AAO. They employed a high-resolution insitu curvature measurement technique to monitor the internalstress during anodization of a thin aluminum film on a siliconsubstrate in 1.0 M H2SO4 at 5 mA cm−2. The anodization curve

(i.e., potential vs time) was characterized by three well-definedregimes, as discussed in the previous section. By converting thecurvature versus time curve (see Figure 24a) into a stress−thickness versus potential curve, the authors obtained internalstress data corresponding to the barrier layer growth regimefrom the slope of the stress−thickness versus potential curve(Figure 27). On the basis of the anodizing ratio (AR) of 1.1 nm

V−1 for aluminum anodizing in 1.0 M H2SO4, they then couldplot the thickness at pore initiation (the first vertical dashedline in Figure 27a) as a function of stress for a set of anodizingexperiments performed at different current densities, j = 1.5−25mA cm−2 (Figure 27b). It turned out that oxide thickness at thepoint of pore initiation increases with the compressive stress.The authors claimed that such a dependency revealed that“pore initiation itself does not correspond to a stress-affectedinstability, although further development of the instability intoan ordered steady-state pore morphology can be considered tobe stress-affected (i.e., porosity-induced stress relaxation).”178

Instead, as an alternative instability criterion, they suggestedthat electrostatic energy acts as a driving force not only forporosity initiation, but also for selection of the interporedistance (Dint) of anodic oxides.

175

More recently, Hebert and co-workers194 performed a linearstability analysis of an instability mechanism controlled by oxidedissolution and ionic migration at the initial stage of poreformation. The authors claimed that previous mod-els101,192,196−198 based on nonlinear interface kinetics may beunrealistic, because the oxide formation efficiency (ηj) is weaklydependent on the current density (j).199,200 Their modelpredicted that the range of oxide formation efficiencies (ηj)producing pattern selection depends on the cation charge (z)and PBR; patterns with a minimum pore spacing occur within anarrow range of the oxide formation efficiency (ηj = 65−70%for porous anodic alumina and 50−58% for anodic titania),which occurs if z > 2. According to the model, the wavelengthfor the maximum disturbance growth rate is proportional to thethickness of anodic oxide, which quantitatively explains theproportionality of interpore distance (Dint) to anodizingpotential (U). This holds for both disordered and self-orderedporous AAOs, and also for diverse anodizing electrolytes.

Figure 25. Dependence of the oxide formation efficiency (ηj) oncurrent density (j) for Al anodizing in 0.4 M H3PO4 during barriergrowth (regime A, i.e., stage I in Figure 23b, ■) and steady-stateporous growth (regime C, i.e., stages III−IV in Figure 23b, □).Reproduced with permission from ref 189. Copyright 2011 TheElectrochemical Society.

Figure 26. Cross-sectional TEM micrographs of anodic oxide film onAl anodized in 0.4 M H3PO4 at 4.5 mA cm−2 for (a) 17 s, (b) 34 s, and(c) 55 s. Adapted with permission from ref 161. Copyright 2010 TheElectrochemical Society.

Figure 27. (a) Stress−thickness versus cell potential curves duringgalvanostatic anodization of aluminum for 20 s in 1.0 M H2SO4 (22°C) at 5 mA cm−2. The vertical dashed line indicates the moment ofpore initiation. (b) Oxide thickness at pore initiation versusinstantaneous internal stress in the barrier. The numbers near datapoints indicate corresponding current density in mA cm−2. Reprintedwith permission from ref 178. Copyright 2011 Elsevier.






6.3. Steady-State Pore Formation

6.3.1. Joule’s Heat-Induced Chemical Dissolution. Forsteady-state pore growth, the movement rates of the metal/oxide interface and the oxide/electrolyte interface should bebalanced, keeping the barrier layer thickness (tb) constant. Ithas long been believed that this balance is achieved by adynamic equilibrium between the oxide formation at the metal/oxide interface and the removal of oxide at the oxide/electrolyte interface.100,101,168,193 Joule’s heat-induced chemicaldissolution of the barrier oxide by acid electrolyte has beensuggested.7,127,168,187,201 However, this thermal mechanismdoes not reasonably explain the dynamic balance of themovement rates of the interfaces (i.e., the metal/oxide andoxide/electrolyte interfaces), because the rate of chemicaldissolution is typically much lower than that of oxide formationeven at an elevated temperature.29 Generated heat has beenconsidered to play a role in enlarging pores by assistingdissolution of the pore wall oxide, resulting in truncated porechannels.202 From the experimentally determined oxideformation rate (372.5 nm/min) in 1.5 M H2SO4 (21 °C) at20 mA cm−2 and the chemical dissolution rate (0.084 nm/min)in the same electrolyte, Hunter and Fowle201 inferred that theelectrolyte condition at the pore base must locally change to theequivalent of boiling of 5.10 M H2SO4 at 124 °C to satisfy therequired rate of chemical dissolution (i.e., 372.5 nm/min). Onthe other hand, on the basis of their calculations of steady-statetemperature distribution, Nagayama and Tamura203 reportedthat local temperature rise due to Joule heating under acomparable anodization condition (i.e., 1.0 M H2SO4, 27 °C,and j = 9.4 mA cm−2) is not more than ΔT = 0.07 °C. Theyclaimed that the high rate of local oxide dissolution should beinterpreted as a consequence of high electric field (E)impressed on the barrier layer, as had been suggested byHoar and Mott.193 Recent in situ measurements of anodetemperature during anodizations in 0.3 M H2C2O4 at 40 V haveshown that the maximum temperature change is ΔT ≈ 1 °C,which contradicts Joule’s heat-induced chemical dissolution ofanodic oxide at the pore base.204

6.3.2. Field-Assisted Oxide Dissolution. Hoar and Mottproposed193 that the thickness of the barrier oxide layer ismaintained by the dynamic rate balance between the followingtwo processes occurring at the oxide electrolyte interface: (1)the oxide formation by the reaction between O2− ions and Al3+

ions migrated from the metal/oxide interface, as in theformation of barrier-type oxide, and (2) the oxide dissolution.The authors assumed that oxide formation takes place both atthe metal/oxide interface and at the oxide/electrolyte interface,as in the barrier-type oxide formation. In their model, oxide atthe oxide/electrolyte interface is decomposed to Al3+ and O2−

ions. The resulting Al3+ ions go into the electrolyte. Meanwhile,O2− ions in contact with acid electrolyte become OH− andmove through the oxide to form new oxide at the metal/oxideinterface. The proton (H+) released by the oxide formationreaction would then diffuse back to the electrolyte by protontransfer between the lattice O2− ions. Since the oxide ions fromthe oxide/electrolyte interface are spread over a larger area atthe metal/oxide interface, the oxide dissolution occurs at agreater rate compared to oxide formation at the metal/oxideinterface. In other words, the net result of process (2) is theprogressive thinning of the barrier oxide layer due to therequirement of oxygen volume conservation in the oxide, whichis compensated by the oxide formation through process (1) tokeep the thickness of the barrier layer constant during

anodization. Hoar et al.187,193 suggested that the local oxidedissolution is assisted by the increased electric field (E) due tothe geometry of the pore base. The proposed model suffersdisadvantages, however, in that it does not specify the detailsassociated with field-assisted oxide dissolution and it adopts theidea that the outwardly migrating Al3+ ions contribute to theoxide formation at the oxide/electrolyte interface, whichactually does not occur in the case of porous-type oxidegrowth (see section 6.3.4).29,30

O’Sullivan and Wood100 proposed a detailed physicalmechanism for field-assisted chemical dissolution of anodicoxide, and qualitatively explained the dependence of the porousmorphology on anodizing conditions. They explained the field-assisted oxide dissolution in terms of the effective polarizationof Al−O bonds in the lattice at the oxide electrolyte interfaceunder the field (Figure 28).100 In this model, the electric field

(E) across the barrier oxide can effectively polarize (i.e.,stretch) the Al−O bond along the applied field direction,lowering the effective activation energy for bond dissociation.Solvation of Al3+ ions by water molecules via activated complex(i.e., Al(H2O)6

3+) and the removal of O2− ions by H3O+ ions as

H2O are facilitated. Because the electric field (E) isconcentrated on the pore base, the oxide dissolution rate isthe greatest there, and a dynamic equilibrium between theoxide formation and the oxide dissolution can be established.This model has popularly been cited in the literature to explainthe growth and morphology of porous AAO.Several models based on field-assisted oxide dissolution have

been developed. In their theoretical modeling, Parkhutik andShershulsky101 considered the three-dimensional (3D) distri-bution of electric field and current in the barrier oxide layer andincluded the field-assisted dissolution at the oxide/electrolyteand oxide formation at the metal/oxide interface as boundaryconditions to predict the steady-state pore morphology. Theirmodel predicted that the movement rate of the oxide/electrolyte interface by field-assisted dissolution is pH-depend-ent and has an exponential dependence on the electric field (E).

Figure 28. Schematics of field-assisted dissolution mechanism byO’Sullivan and Wood: (a) before polarization, (b) after polarization,(c) removal of Al3+ and O2− ions, and (d) remaining oxide. Adaptedwith permission from ref 100. Copyright 1970 Royal SocietyPublishing.




The model also predicted a linear dependence of interporedistance (Dint) on anodizing potential (U).Thamida and Chang198 were also able to predict the linear

dependence of the interpore distance (Dint) on the anodizingpotential (U). They performed a perturbation analysis of theequation describing the movements of the metal/oxideinterface by oxide formation, and oxide/electrolyte interfaceby oxide dissolution, the rate of which was considered to begoverned by the local electric field. The electric field at bothinterfaces was considered to be dependent on the shape ortopography of the interfaces. Their model predicted that theratio of pore diameter (Dp) to interpore distance (Dint) is afactor independent of anodizing potential (U), but varies withthe electrolyte pH.198 Singh et al.192 have further put forwardprevious models. The main element of their model is theButler−Volmer relation, which describes the exponentialdependence of the current on the overpotential and thedependence of the activation energy of oxide dissolutionreaction on the Laplace pressure and the elastic stress withinanodic oxide.192,205 Their model predicted ordered hexagonalpore arrays for a range of volume expansion factors close to theone observed for ordered porous films.Recently, Friedman et al.206 performed a systematic

experimental investigation to study the stability phase diagramas a function of pH and anodizing potential (U) in an attemptto validate the above-mentioned theoretical models. Byconsidering the discrepancies between the previous theoreticalmodels and their experimental results, they concluded that theprevious models must include an appropriate weighting factorto account for the oxide formation and dissolution mechanismduring the pore formation. The discrepancies may originatefrom the fact that these models were based on pore formationby field-assisted oxide dissolution at the pore base. In fact, thefield-assisted dissolution mechanism for steady-state poreformation has recently been rejected by several authors (seesections 6.3.4 and 6.3.5).For the initial stage of anodization, on the other hand, Oh

and Thompson207 have recently reported direct experimentalevidence of the impact of the electric field (E) on the oxidedissolution rate and the existence of a threshold electric field(E*) for pore initiation. To assess the effect of an electric field(E) on the Al2O3 dissolution rate, the authors re-anodized aplanar pre-formed Al2O3 layer (thickness h0 = 160 nm) onaluminum and investigated the evolution of thickness of thepreformed oxide layer as a function of time (Figure 29). Thethickness of the planar barrier oxide was found to havedecreased from 160 to 131 nm after re-anodization in 5 wt %H3PO4 at 86 V for 49 min, without changing the thickness ofthe aluminum (i.e., no anodic oxidation of aluminum). Inaddition, the dissolution rate of the barrier oxide of a fixedinitial thickness was found to increase with electric field (E).The results indicated that field-assisted oxide dissolution isoperative at the initial stage of anodization. On the basis of theinvariance of the thickness of aluminum and the morphology ofthe metal/oxide interface, Oh and Thompson suggested thatthe formation of the incipient pores is associated with field-induced instability at the oxide/electrolyte interface atsufficiently high electric field. In a separate work, Skeldonand co-workers have reported confirming experimental resultsof field-assisted oxide dissolution at the initial stage of poreformation during potentiostatic anodization of aluminum inphosphoric acid.208 In their study of pore initiation, the authorsemployed immobile arsenic species as tracers. 18O-labeled

barrier-type oxide films were first formed in sodium arsenate(Na2HAsO4·7H2O) solution, and subsequently the resultingsamples were re-anodized in phosphoric acid under electricfields (i) below the threshold electric field (E*) reported by Ohand Thompson for the formation of incipient pores, (ii) closeto the threshold field to induce significant anodic oxidation ofaluminum, and (iii) well above the threshold field. Frommicroscopic analyses of the arsenic and 18O contents, and thepore morphologies and arsenic distribution in the resultinganodic oxide films, it was found that the field-assisted oxidedissolution is mainly responsible for the formation of incipientpores at oxide formation efficiency (ηj) = ∼20−30%, whilefield-assisted flow of oxide materials is operative for the growthof major pores at oxide formation efficiency (ηj) = ∼57−66%.208 The authors suggested that the preferential growth ofincipient pores locally increases the current density at the porebases, which may influence the transport numbers of mobileions, insertion of electrolyte-derived anionic species into theanodic oxide, and thus oxide viscosity.208

6.3.3. Average Field Model for Steady-State PoreStructure. As discussed in section 6.2.1, after the formation ofincipient pores, some large incipient pores develop into majorpores by increasing their size through persistent merging withneighboring smaller incipient pores. As the anodizationproceeds, the growing major pores readjust their sizes andspatial arrangement to establish equilibrium morphology (i.e.,hexagonally close-packed pore distribution). During this period,merging, dying, or even branching of the pores may occur. Fora given set of anodization conditions, the barrier layer thickness(tb), pore size (Dp), and interpore distance (Dint, or cell size) inthe equilibrium pore structure are mainly determined by theanodizing potential (U). O’Sullivan and Wood suggested a so-

Figure 29. (a,b) SEM images showing field-assisted dissolution ofanodic oxide under an electric field (E): (a) before and (b) after re-anodization of a planar preformed Al2O3 layer in a 5 wt % H3PO4solution at 86 V for 49 min. Scale bar = 200 nm. (c) Changes in thethickness of a planar preformed Al2O3 layer due to electric field- andtime-dependent dissolution behavior in a 5 wt % H3PO4 solution.Reprinted with permission from ref 207. Copyright 2011 Elsevier.




called “average field model” to explain the establishment of theequilibrium structure of growing pores.100 The essence of themodel is that field-assisted dissolution at the base of growingpores should occur to some different extents until the electricfield (E) across the barrier layers eventually becomes the samefor every pore, the condition under which pores of anequilibrium size grow at a constant rate, being determined bythe average field. The model can be qualitatively explained asfollows. Figure 30 schematically shows three adjacent pores

growing with different pore sizes (Dp1, Dp2, and Dp3) andbarrier layer thicknesses (tb1, tb2, and tb3). We assume that pore(b) has the equilibrium dimension of a given set of anodizationconditions, in which tb2 and R2 are the equilibrium barrier layerthickness and radius of curvature of the metal/oxide interface,respectively. Although O’Sullivan and Wood assumed that thebarrier layer thickness (tb) and the angle (w = 45.7°) in apotentiostatic anodization are constant irrespective of the poresizes (Dp), we suppose here that only w remains constant whiletb changes with Dp, which represents the real cases better. Forthe three cases given in Figure 30, the electric field (E) acrossthe barrier layers is different, because at a given anodizingpotential (U) the field (E) is inversely proportional to thebarrier layer thickness (i.e., E = U/tb). Accordingly, field-assisted oxide dissolution at the pore base would occur to agreater extent for pore (a), as compared to pores (b) and (c),while anodic oxidation of metal would take place to the sameextent at the metal/oxide interface due to higher currentdensity (j) according to eq 1, until Dp1, R1, and tb1 have reachedtheir equilibrium values (i.e., Dp2, R2, and tb2). For pore (c), onthe other hand, field-assisted oxide dissolution and anodicoxidation reactions would be progressively retarded because oflower electric field (E), until pore (c) has the equilibriumdimension. Unoxidized metal near the hemispherical concaveridges (i.e., the points marked with “A” in Figure 30) is actedupon by two lateral field components of different magnitudesfrom two neighboring pores. The lateral field component ofgreater magnitude drives the field-assisted dissolution andanodic oxidation to a greater extent. The corresponding pore

would be enlarged in the lateral direction. Accordingly, for allthree cases in Figure 30, rearrangement of pores in conjunctionwith the self-adjustment of pore size (Dp) and barrier layerthickness (tb) would take place until the electric field (E) acrossthe barrier layers of every pore approaches the average field.

6.3.4. Direct Cation Ejection Mechanism. The averagefield model discussed above is based on the field-assisted oxidedissolution.100 However, this dissolution mechanism alone doesnot adequately explain some of the experimental results, inparticular, that the quantity of Al3+ ions in the electrolyte afteranodization exceeds that of Al3+ ions originating from theformation of the pores. On the basis of pore-filling experiments,Takahashi and Nagayama reported that transport numbers forAl3+ and O2− ions moving across the barrier layer are t+ = 0.4and t− = 0.6, respectively.46 The result implies that both Al3+

and O2− ions migrate in anodic oxide, with 40% of the ioniccurrent carried by the Al3+ ions and the remainder by the O2−

ions, and further that about 40% of the Al3+ ions are lost intothe electrolyte without contributing to the porous oxideformation. Considering the porosity (P ≈ 10%) of porousAAO formed in typical anodization conditions,109 30% of Al3+

ions should be lost through a mechanism different from field-assisted oxide dissolution, even if the latter is still operative.Siejka and Ortega studied the pore formation mechanism by

employing 18O tracing techniques.30 They first formed acompact base oxide film on aluminum in electrolyte enriched inH2

18O and subsequently anodized the sample in H216O-

enriched H2SO4 electrolyte for pore formation. From thenuclear microanalyses for 18O-isotope content and depthdistribution, the 18O tracer was found to be located at thefilm surface conserving its initial isotopic concentration (Figure31b), which was attributed to oxide decomposition inside the

oxide and reincorporation of the released oxygen to form newoxide at the metal/oxide interface. 18O-tracer studies along withthe analyses of the components of ionic current (jtot) havefurther revealed that the number of oxygen in porous AAOaccounts for about 60% of the total ionic current (i.e., jox =60%·jtot), which is numerically equal to the current efficiency(ηj) of porous AAO formation.29,30 The remainder is associatedwith the loss of Al3+ ions into the electrolyte (i.e., jloss = 40%·jtot). Siejka and Ortega claimed that pore formation in theabsence of oxygen losses should be associated with the directejection (jAl = 30%·jtot) of Al3+ ions from the metal/oxideinterface into the electrolyte across the barrier oxide and theoutward movement (jdec = 10%·jtot) of Al3+ ions produced by

Figure 30. Schematic representation of growing three adjacent pores(a, b, and c) with different pore sizes (Dp1, Dp2, and Dp3) and barrierlayer thicknesses (tb1, tb2, and tb3). R1, R2, and R3 are the radii ofcurvature of the metal/oxide interfaces of the respective pores. ω(∼45.7°) is the angle subtended from the center of curvature to thepore bases. The red lines extending from the metal/oxide interface tothe oxide/electrolyte interface represent current lines. The regionsmarked with “A” correspond to the area of unoxidized metal near theconcave ridges.

Figure 31. Schematic diagrams illustrating findings of oxygen tracerexperiments for the growth of (a) barrier and (b) porous anodic filmson aluminum. In each case, the anodizing is carried out first inelectrolyte enriched in H2

18O and second in electrolyte enriched inH2

16O. Adapted with permission from ref 32. Copyright 2006 TheElectrochemical Society.





oxide decomposition inside the barrier layer toward theelectrolyte: jloss = jAl + jdec = 40%·jtot.

30 Consistent resultshave recently been reported by Skeldon and co-workers,161 whoobserved that the amounts of 18O tracer species in films formedby sequential anodizing in phosphoric acid do not changesignificantly during the formation of anodic oxide between thebarrier and porous stages. The direct cation ejectionmechanism is also in line with the recent report by Wu et al.,who have experimentally shown that the formation of poresdoes not occur through oxide dissolution process at the oxide/electrolyte interface, but through direct ejection of Al3+ ionsinto the electrolyte.31 However, the process involved in oxidedecomposition inside the barrier layer and its role in thedynamic balance between the movement rates of interfaces (i.e.,the metal/oxide and the oxide/electrolyte interfaces) need tobe specified.6.3.5. Flow Model for Steady-State Pore Formation.

Skeldon et al.32,98,209,210 have investigated the development ofpores in porous AAO by performing tracer studies. Theyanodized Al/W-tracer/Al substrates (W-tracer layer; 3−5 nmthick, Al−30 at. % W, Figure 32a) in H3PO4 solution andinvestigated the movement of the tracer band (WO3) by TEMand RBS.32,98 During anodic oxidation, tungsten speciesincorporated into the anodic oxide migrated toward theoxide/electrolyte interfaces at about 0.38 of the rate of Al3+

ions.211 It was observed that the incorporated tracer band

immediately beneath the pores initially lies slightly below theadjacent tracer near cell wall regions (Figure 32b). Upwarddisplacement of the tracer band at the cell wall regions becamepronounced upon further anodizations, as compared to thetracer below the pores, where the band is getting fainterbecause of a reduced tungsten concentration (Figure 32c,d). Itwas suggested that the fine-tungsten lines along the cellboundaries beneath the tracer layer are associated with thetungsten enrichment in the aluminum adjacent to thealuminum/film interface, because formation of WO3 requireshigher Gibbs free energy as compared to Al2O3.

98,212,213 Theauthors pointed out that the observed behavior of the tracerband is contrary to what one would expect from theconventional dissolution-based model of porous AAO growthas follows.32,98 According to the conventional model of poreformation, tungsten tracer should be incorporated into theanodic oxide first at the regions immediately below the poresand then near the cell boundary regions because of thescalloped geometry of the barrier oxide layer. Thus, the tracerband at the pore regions should lie ahead of the tracer at thepore wall regions because of the outward migration of tungstenspecies, contrary to the experimental observations. If field-assisted oxide dissolution occurred at the oxide/electrolyteinterface, to maintain a constant barrier layer thickness (tb), atungsten-rich layer with a sharp image contrast (instead ofdiminished contrast, as in Figure 32c) would have been

Figure 32. Cross-section transmission electron micrographs (TEM) of (a) the sputter-deposited aluminum containing a tungsten tracer layer, andfollowing anodization for (b) 180 s, (c) 240 s, and (d) 350 s at 5 mA cm−2 in 0.4 M H3PO4 at 293 K. Reprinted with permission from ref 98.Copyright 2006 Elsevier. The distributions of tungsten tracer in anodic oxide are schematically illustrated above the respective TEM images. (e)Simulated current lines and potential distribution for porous AAO growth in oxalic acid at 36 V (color scale is potential in volts). (f) Simulated flowvelocity vectors and mean stress (color scale is dimensionless stress). Panels (e) and (f) reproduced by permission from ref 200. Copyright2009Macmillan Publishers Ltd.: Nature Materials.




observed by TEM at the pore bases due to preferentialdissolution of Al3+ ions of a higher mobility in anodic oxide.On the basis of the results of tracer experiments, Skeldon and

co-workers32,98 proposed the flow mechanism of poregeneration, as an alternative process to conventional field-assisted oxide dissolution. According to the new model, theconstant thickness of the barrier layer (tb) during porous AAOformation is maintained by viscous flow of oxide materials fromthe pore base to the cell boundary. It was suggested by theauthors that the displacement of oxide materials is driven bylarge compressive stresses (∼100 MPa) from electrostrictiondue to a high electric field (E ≈ 106 V cm−1)97 and also byvolume expansion due to oxidation of aluminum,165 and thatcan be facilitated by the involvement of most of the oxideconstituents in ionic transport (i.e., the plastic flow of oxidematerials).97,214,215

In most anodizations for porous AAO formation, acid anionsfrom electrolytes are incorporated into the barrier layer throughinward migration. They migrate slowly in the barrier oxide, ascompared to O2−/OH− ions, due to their relatively large size.As a consequence, the barrier layer exhibits a duplex structurein terms of chemical composition, with an acid anioncontaminated outer oxide layer and a relatively pure inneroxide layer adjacent to the metal/oxide interface (see section5.2).14 The relative thickness of the outer oxide layer to theinner one depends mainly on the nature of the anodizingelectrolyte in a given set of anodizations and is invariantthroughout anodization. The flow model accounts forincorporation of electrolyte anions into anodic oxide and alsotheir migration behavior in the barrier layer. Incorporatedanionic species are transported toward the cell walls in additionto their inward migration in the oxide without dissolution-related loss; otherwise, the barrier oxide would eventuallycontain no incorporated acid anions because of their slowmigration. The flow model considers Al3+ ions as the only ionicspecies being lost into the electrolyte by field-assisted directejection.30 Garcia-Vergara et al. have pointed out that bulkyacid anions play a key role in the flow of oxide materials,influencing not only the pore and cell dimensions but also theself-organization of pores through the redistribution ofstress.32,98,216 The isotopic order and the absence of tracerloss in the earlier 18O tracer studies of Siejka and Ortega (seesection 6.3.4)30 have also been recently reinterpreted in termsof the field-induced plastic flow of oxide materials from thepore base toward the cell boundary, during which most of theinitially incorporated 18O tracers are displaced to the cellwalls.161 The formation of dendritic (or fish-bone-like) pores ina mechanically constrained environment has also beenexplained in the framework of the flow mechanism.186,217

The flow mechanism of steady-state pore generation hasbeen supported by the theoretical modeling of Houser andHebert,47,199,200 which highlighted not only the ionic migrationunder the gradients of mechanical stress and electric potential,but also its implication on the Newtonian viscoelastic flow ofoxide materials from the pore base toward the pore bottomsand further into the cell walls (see Figure 32e,f). Yet, the modeldoes not take into account the volume expansion stress at themetal/oxide interface for the viscous flow of the oxide materials.The authors concluded that the compressive stress at the porebase drives the flow of oxide materials in association with thecompetition of strong anion adsorption with deposition ofoxygen ions.200

Skeldon and co-workers213,218 have also investigated theeffect of the types of tracer elements on the distributions oftracer layers in anodic alumina. Figure 33 schematically

compares the distributions of hafnium-, neodymium-, andtungsten-containing tracer layers in porous films formed inphosphoric acid. Contrary to what was observed from the tracerstudies using tungsten, the hafnium and neodymium exhibitedthe types of movement behaviors that one might expect fromfield-assisted oxide dissolution. The authors attributed theobserved behaviors of tracer bands to the fast movement ratesof the tracer cations (i.e., Hf4+ and Nd3+) in anodic alumina ascompared to W6+ ions and also to the loss of fast moving tracercations into the electrolyte through direct ejection mechanism;Hf4+ ions migrated outward in anodic alumina at about thesame rate as Al3+ ions,219 while Nd3+ ions migrated about twiceas fast as Al3+ ions (i.e., 6 times faster than W6+ ions).220

Although the authors’ explanation of the unexpected distortionseems to be plausible within the present knowledge ofanodization, more systematic experimental investigationsusing other electrolyte systems and theoretical study arerequired to fully verify the oxide flow model.

7. SELF-ORDERED POROUS ANODIC ALUMINUMOXIDE (AAO)

Studies on porous-type anodization have been mainly led bythe surface finishing industry, and hence focused on thedevelopment of cost-effective anodizing processes and theimprovement of engineering properties of anodized products.Although various anodizing processes have been intensivelyexplored by industry, the size uniformity and spatial ordering ofpores have not been considered a major concern. Typicalporous AAOs formed by industrial processes, represented byhard anodization (HA), exhibit disordered pore structures withnumerous micrometer-sized cracks.221−224 Therefore, classicalHA processes have not been implemented in the currentnanotechnology research, until the recent development of newHA processes. On the other hand, the mild anodization (MA)

Figure 33. Schematic diagram comparing (a) hafnium-, (b) neo-dymium-, and (c) tungsten-containing tracer layers in porous anodicfilms formed on aluminum in phosphoric acid. Reprinted withpermission from ref 213. Copyright 2009 Elsevier.




of aluminum produces self-ordered porous AAOs with uniformpore size (Dp) and interpore distance (Dint), which can be easilytuned by an appropriate selection of anodization conditions.Thus, that process has been intensively utilized in academicresearch for a wide variety of nanotechnology applications. Yetthe MA process is slow (film growth rate = 2−10 μm h−1), andcan be conducted within a narrow range of anodizingconditions. In the following sections, we will discuss theconventional MA and the newly developed HA processes,factors governing the structure of porous AAO, and somecharacteristics of porous AAO that are relevant to nano-technology applications. In addition, recent attempts toengineer the internal pore structure of AAO by pulseanodization (PA) and cyclic anodization (CA) will bediscussed.

7.1. Mild Anodization (MA)

Masuda and Fukuda found that the bottom part of porousAAOs films produced by anodizing of aluminum in 0.3 M oxalicacid at 40 V exhibits a self-ordered pore structure as a result ofthe gradual rearrangement of the initially disordered pores.5 Onthe basis of this experimental finding, Masuda and Satohdeveloped the so-called “two-step anodization” process, bywhich porous AAOs could be obtained with a highly orderedarrangement of uniform nanopores (Figure 34).6 In a typicaltwo-step anodization, the first anodization process is conductedfor more than 24 h. The resulting porous AAO with disorderedpores in its top part (Figure 34a) is selectively removed by theso-called “PC-etching” process using an aqueous mixture of 0.5M H3PO4 and 0.2 M CrO3 at 80 °C.225 The surface of theresulting aluminum is textured with arrays of almost hemi-spherical concaves (Figure 34b). The second anodization iscarried out with the textured aluminum under the samecondition employed for the first anodizing. Pores nucleate atthe centers of each concave feature and grow normal to the

aluminum substrate maintaining their directional coherency(Figure 34c). Position-defined pore generation during thesecond anodization has been attributed to the relatively thinnative oxide at the bottom of each concave, where the electricfield (E) is the highest and the resistance is the lowest.168 Ingeneral, porous AAOs formed by two-step MA process exhibit apolydomain structure (Figure 34d).226 The lateral size of thedefect-free domain increases with the anodizing time, but islimited to several micrometers.Since the development of the two-step process from oxalic

acid-based anodization, a lot of studies have been conductednot only to produce porous AAOs with different pore sizes anddensities with an improved arrangement of pores, but also tounderstand the mechanism responsible for the growth and self-organizing behavior of pores during anodization. Particularefforts have been devoted to exploring the optimum conditionsfor pore ordering, mainly with sulfuric, oxalic, or phosphoricacid. Studies have indicated that the self-organized growth ofordered pores occurs within a relatively narrow window(known as the “self-ordering regime”) of anodizing conditions(see Figure 36). For a given electrolyte system, in the case ofMA, there is an optimum anodizing potential (U) for the bestordering of pores: (i) sulfuric acid (0.3 M H2SO4) at U = 25 Vfor an interpore distance (Dint

MA) = 65 nm;106,227 (ii) oxalic acid(0.3 M H2C2O4) at 40 V for Dint

MA = 103 nm;6,106,168,227,228 (iii)selenic acid (0.3 M H2SeO4) at 48 V for Dint

MA = 112 nm;11 and(iv) phosphoric acid (0.3 M H3PO4) at 195 V for Dint

MA = 500nm.109,229 Considerable efforts have been made to explore newself-ordering regimes in a wider range of Dint

MA. With the idea ofcontrolling self-organization by adjusting the mechanical stressbetween the metal/oxide interface, Shingubara et al. employeda mixture solution of 0.3 M sulfuric acid and 0.3 M oxalic acid(v/v = 1:1) to change the density of the resulting porous AAOand obtained self-ordered porous AAO with Dint

MA = 73 nm at U

Figure 34. (Left) A schematic showing a conventional two-step mild anodization (MA) process for self-ordered porous anodic aluminum oxide(AAO): (i) the first long-term anodization, (ii) removal of disordered porous AAO, and (iii) the second anodization at the identical condition to thefirst one. Representative surface SEM micrographs of the respective samples are shown in panels (a), (b), and (c), respectively. Reprinted withpermission from ref 228. Copyright 1998 Springer Science and Business Media. (d) A color-coded SEM image of AAO formed by two-step MAusing 0.3 M oxalic acid at 40 V, showing a poly-domain structure. An area with the same color consists of a domain. The pores are color-coded onthe basis of the average angle to the six nearest neighbors. Pores that have no apparent hexagonal coordination (i.e., defect pores) are marked withwhite. Reprinted with permission from ref 226. Copyright 2008 The American Chemical Society.




= 37 V.230 More recently, Sun and co-workers231 reported anH3PO4-based MA process using aluminum oxalate ((Al-C2O4)2C2O4) as an additive to suppress breakdown of porousAAO during anodization at high potential and temperature.They showed that the interpore distance (Dint

MA) of porousAAOs can be continuously tuned from 410 to 530 nm byperforming anodization with the mixture solution at ananodization potential (U) ranging from 180 to 230 V. Onoet al.17,232 demonstrated the fabrication of porous AAOs withan interpore distance range of 300−600 nm by MA in organicacid electrolytes: Dint

MA = 300 nm for 5 M malonic acid at 120 V,Dint

MA = 500 nm for 3 M tartaric acid at 195 V, and DintMA = 600

nm for 2 M citric acid at 240 V. Yet the spatial ordering and sizeuniformity of pores of the resulting porous AAOs were by farinferior to those formed by conventional MA processes usingsulfuric, oxalic, or phosphoric acid, which would limit thepractical applications of such porous AAOs to nanotechnologyresearch.For self-ordered porous AAO formed by MA processes,

recent morphological studies indicated that anodizing potential(U) determines the interpore distance Dint

MA, barrier layerthickness tb

MA, and pore diameter DpMA.106,109,116 The interpore

distance (DintMA) and barrier layer thickness (tb

MA) increaselinearly with anodizing potential (U) with proportionalityconstants ζMA = 2.5 nm V−1 for Dint

MA and ARMA = 1.2 nm V−1

for tbMA.106,109,114 The observed potential dependence of Dint

MA

and tbMA is consistent with the earlier reports for disordered

porous AAO.7,100,114 Linear dependence of the pore diameter(Dp

MA) on anodizing potential (U) has also been reported bothfor disordered and for self-ordered porous AAOs. O’Sullivanand Wood100 reported that pore diameter of disordered AAOsincreases with anodizing potential at a rate of ζp = 1.29 nm V−1.For self-ordered porous AAO, on the other hand, Nielsch etal.109 proposed that self-ordering of pores requires a porosity(PMA) of about 10% regardless of the specific anodizingconditions by assuming Dint

MA = 2.5 nm V−1. Because porosity(PMA) of AAO is given by eq 53, 10% porosity requirement ofNielsch et al. dictates a linear increase of the pore diameter withthe anodizing potential at a late of ζp = 0.83 nm V−1,irrespective of anodizing conditions. However, there are anumber of recent reports, indicating that the self-ordering ofpores can occur at other porosity (PMA) values ranging from0.8% to 30% depending on the MA conditions, and thus thatthe anodizing potential (U) is not the only parameterdetermining the pore diameter (Dp).

11,102,232,233 For example,Nishinaga et al. have recently reported fabrication of porousAAO with Dp

MA = 10.4 nm and DintMA = 112 nm (porosity, PMA =

0.8% according to eq 53) by H2SeO4-based anodization at 48 Vand 0 °C.11 The authors noted that the solubility of anodizingacid electrolyte has important effects on pore diameter andinterpore distance, and suggested that the weak solubility ofselenic acid under the anodization condition employed causedthe formation of 10-nm-scale pores.11 In another example,Chen et al. reported a continuous decrease of the porediameter, which did not affect either the interpore distance orthe barrier layer thickness, when increasing the concentration ofpolyethylenglycol (PEG) additive in phosphoric acid electro-lyte.233 Because the porosity (P) of AAO is determined by theratio of the pore diameter to the interpore distance (i.e., Dp/Dint in eq 53), increasing the PEG concentration in theanodizing electrolyte corresponds to decreasing the porosity ofAAO. The use of an organic additive to reduce the porediameter has recently been extended by Martin et al., who

demonstrated fabrication of porous AAOs with pore diameterless than 15 nm by using an aqueous mixture of sulfuric acidand ethylene glycol (EG).234 Chen et al. noted two possibleorigins for the decreasing porosity with the addition of organicadditive:233 (i) the increase of the effective electric field (E) dueto the reduction of the dielectric constant (ε) of the anodizingelectrolyte upon addition of an organic modulator, and (ii) theweak chemical dissolution of the pore wall oxide under theprotection of organic molecules. Among them, the former is inline with the earlier report by Ono et al. that the Dp/Dint ratiodecreases with increasing electric field strength (E) duringanodization.232 Su et al. have further explored the effect ofelectric field (E) on porosity (P). In a series of papers,235−237

they proposed a mechanistic model, proposing that the porosity(P) of AAO is directly governed by the relative rate of waterdissociation at the oxide/electrolyte interface, according to thefollowing relation:236

= +P n3/( 3) (66)

where n is the amount of water that dissociates per mole ofAl2O3 in the oxide dissolution reaction given in reaction 15.The model regards both the dissociation of water and thedissolution of oxide at the oxide/electrolyte interface asimportant processes, although the latter has recently beenlargely disputed, as discussed in sections 6.3.4 and 6.3.5. Byperforming quantum-chemical model computations, theauthors showed that the electric field (E) can significantlyfacilitate heterolytic dissociation of properly oriented watermolecules at the oxide surface, that is, field-enhanced waterdissociation.41 Under a stronger electric field (E), thedissociation rate of water will be increased (i.e., an increaseof the n value in eq 66), and thus the porosity will be reduced.As such, the model appears to adequately explain thedependence of porosity (P) on electric field (E). The aboverelation was derived from a simple geometric consideration ofgrowing AAO and the mass balance of total oxide-formingoxygen anions produced by the dissociation of water anddissolution of Al2O3. In other words, the original model by Suet al. postulates that all of the oxygen anions produced by waterdissociation and oxide dissolution should contribute to theoxide formation at the metal/oxide interface.236 However, byitself this assumption does not fulfill the necessary conditionrequired for the formation of the steady-state pore morphology,that is, the constant barrier layer thickness (tb) maintained bythe dynamic balance of the movement rates of metal/oxide andoxide/electrolyte interface, as follows. A close inspection ofreaction 15 used in the authors’ model reveals there is adifferent field dependence for the water dissociation than forthe oxide dissolution process. To increase the n value under astronger electric field (E), water dissociation should occur at agreater rate than oxide dissolution. Under such circumstances,one may expect a progressive increase of the barrier oxide layerthickness (tb) as the anodiziation proceeds. Therefore, otherionic processes may be required to explain the constantthickness of the barrier layer during anodization. It wasassumed that Al3+ ions are directly ejected from the metal/oxide interface without contributing to the oxide formation.235

When MA is conducted outside the self-ordering regime, thedegree of spatial ordering of nanopores decreases drastically.For a given acid electrolyte, there is a breakdown potential(UB), above which anodization is accompanied by localthickening, burning, and cracking of the anodic oxide film,caused by a catastrophic flow of current (j) and consequent



evolution of a large amount of heat:4,18,79,89,94,97,238,239 UB = 27,50, and 197 V for sulfuric, oxalic, and phosphoric acid,respectively. It has been known for the MA process thatanodizations just below the breakdown potentials (UB) yieldporous AAOs with the best self-ordered pore structures.18 Onoand co-workers232,240 investigated the self-ordering of nano-pores at the local areas of burnt protrusions with a largenumber of cracks. They found that the locally thickened areasformed by breakdown exhibit domains of highly ordered cellarrangement. In addition, the cell size (Dint) and the barrierlayer thickness (tb) of porous AAO at the burnt area wereobserved to be remarkably smaller than those at the burnt-freearea, which is in line with the earlier report by Tu et al.239

These experimental observations imply that current density (j)primarily determines the degree of spatial ordering of pores andthe structural parameters (i.e., cell size Dint and the barrier layerthickness tb) of porous AAOs.

7.2. Hard Anodization (HA)

As mentioned in section 4.1, stable anodization in a givenelectrolyte is difficult to maintain over breakdown potential(UB) due to the occurrence of burning or breakdown of thegrowing anodic oxide film. Classical hard anodization (HA)processes adopted in industry have typically been conducted atpotentials (U) higher than breakdown value (UB) at theexpense of randomly occurring local breakdown of anodic oxidefilm. The processes take advantage of high speed growth(typically 50−100 μm h−1) of oxide films due to the highcurrent density (j) at an increased anodizing potential (U).However, the resulting porous anodic films are characterized byseverely buckled (or uneven) surfaces with numerous cracksand non-uniform (or distorted) pores. Control of the pore size(Dp), interpore distance (Dint), and the aspect ratio of thenanopores is also very difficult. For these reasons, the classicalHA processes have not been employed for nanotechnologyapplications.Various attempts have been made to overcome the problems

associated with local breakdown events during HA. Theresearch activity to date includes extending anodizing potential(U) over breakdown potentials (UB) by appropriately tuningthe three major pore-forming acid electrolytes (i.e., H2SO4,H2C2O4, and H3PO4), by searching for new anodizingelectrolytes, and by efficiently removing the reaction heat.The current density (j) in HA process is typically 1 or 2 ordersof magnitude higher than that of conventional MA processes.Thus, Joule’s heating (Q) during HA is 2 or 4 orders ofmagnitude greater than the ordinary MA processes. Theexcessive heat during the HA process can not only promoteacidic dissolution of the resulting porous AAO, but also triggerlocal breakdown events.4,239

Chu and co-workers18,110 reported that the breakdownpotential (UB = 27 V) in a sulfuric acid-based anodizationsystem can be increased up to 70 V by experimentally aging theelectrolyte after a long-term anodizing (i.e., pre-electrolyzing)at 10−20 A hours per liter. Because of the high anodizingpotential (U), the current density (j) for stable anodizationcorrespondingly rose significantly, and thus led to a high-speedfilm growth. The authors were able to fabricate self-orderedporous AAOs with Dint = 90−130 nm at 40−70 V and 160−200 mA cm−2 at 0.1−10 °C. That condition is far from those ofMA (H2SO4: 25 V and 2−15 mA cm−2) but similar to those ofclassical HA,221,223,241−243 although the authors named thisprocess “high-field anodization”. Porous AAOs formed by this

process exhibited poor mechanical stability, which greatly limitstheir practical application as templates for various nano-fabrications. The produced porous AAOs exhibited weak celljunction strength (Figure 35a), and thus the individual aluminananotubes could be easily separated upon applying a weakexternal stress (Figure 35b), unlike the case of porous AAOsformed by MA processes (Figure 35c). Chu et al. pointed outthat the concentration of Al3+ ions dissolved in an aged solutionplays a key role in the stable growth of porous AAOs at highpotentials (U > Ub) and current densities (j), avoiding theburning or breakdown of porous AAO films.18,110 However, theelectrochemical action of the aged solution has not yet beenclearly understood. In an attempt to determine the effect ofaged solutions on breakdown potential (UB), Schwirn etal.113,119 performed a series of HA experiments at the potentialrange of U = 27−80 V by using sulfuric acid containingdifferent concentrations of Al3+ ions. Their results have shownthat stable anodization at high potentials (U > UB) is actuallydetermined by the initial limiting current density (jlimit), not bythe solution state.For an oxalic acid-based anodization system, Lee et al.111

have shown that the self-ordering regimes can be extended byperforming HA of aluminum. By introducing a thin (ca. 400nm) porous oxide layer onto an aluminum substrate, graduallyincreasing the anodizing potential (U) to a target value (100−160 V) at the rate of 0.5−0.9 V s−1, and effectively removingthe reaction heat through direct thermal contact of thealuminum substrate with an underlying cooling plate, theauthors could suppress the breakdown of oxide films and growmechanically stable highly ordered porous AAOs at anodizingpotentials of U > 100 V. Their HA process established a newself-ordering regime with widely tunable interpore distances:Dint

HA = 220−300 nm at U = 110−150 V and j = 30−250 mAcm−2. They suggested that current density j (i.e., the electricfield E) is a key parameter governing the self-ordering of pores

Figure 35. (a,b) SEM images showing weak cell junction strength ofporous AAOs formed by H2SO4-hard anodization (HA). (c) Aschematic illustration showing two different fracture modes of porousAAOs formed by H2SO4-hard (HA) and mild anodization (MA): theA−A′ cleavage plane for HA-AAO and the B−B′ cleavage plane forMA-AAO. Panel (a) reprinted with permission from ref 18. Copyright2006 The Electrochemical Society. Panel (b) reprinted withpermission from ref 113. Copyright 2008 The American ChemicalSociety.




at a given anodizing potential (U), which is in line with thesuggestion by Ono et al.232,240 The authors also found that HAprocess produces porous AAOs at 25−35 times faster growthrates, as compared to conventional MA processes. Interestingly,the porosity (PHA) of HA-AAOs was found to be 3.3−3.4%,which is about one-third of the porosity value (PMA ≈ 10%)that was proposed as a requirement for self-ordered MA-AAOsby Nielsch et al.109 The experimental method has also beenapplied to sulfuric and malonic acid-based HAs.16,113

As mentioned above, HA of aluminum is accompanied by alarge amount of reaction heat. Accordingly, the reaction heatshould be properly removed for stable anodization at highpotential (U > UB) and current density (j). To this end, Li etal.118,122 added ethanol (C2H5OH) to the aqueous anodizingelectrolytes (e.g., H2SO4, H2C2O4, and H3PO4) and conductedstable HA at high potentials (U) and current densities (j). Intheir HA processes, the added ethanol served not only as an

agent for lowering the freezing point of the electrolyte down to−10 °C, but also as a coolant for removing a large amount ofreaction heat through its vaporization from the metal/oxideinterface. The process allowed the fabrication of self-orderedporous AAOs with various interpore distances: Dint

HA = 70−140nm for H2SO4-HA at 30−80 V, 225−450 nm for H2C2O4-HAat 100−180 V, and 320−380 nm for H3PO4-HA at 195 V.118,122

Microscopic investigations of HA-AAOs have shown that thebarrier layer thickness (tb) increases at a rate of ARHA = 0.6−1.0nm V−1 with respect to anodizing potential (U) depending onthe current density (j),18,111,113,118,119 which is smaller thanARMA ≈ 1.2 nm V−1 for MA-AAOs.7,100,114 Lee et al.111

attributed the reduced ARHA (i.e., anodizing ratio) to the highcurrent density (j) involved in the HA process in accordancewith the high field conductivity theory, which predicts aninversely proportional dependence of tb to the logarithm ofcurrent density (j) at a given anodization potential (U) (see eq1). Figure 36 summarizes the self-ordering potentials (U) andcorresponding interpore distances (Dint) of porous AAOsformed by MA and HA in three major pore-formingelectrolytes (i.e., H2SO4, H2C2O4, and H3PO4). It was foundthat HA-AAOs exhibit a reduced potential (U) dependence ofthe interpore distance (Dint) with a proportionality constantζHA = 1.8−2.1 nm V−1,16,110,111,113,118,122,244,245 as compared toself-ordered MA-AAOs (i.e., ζMA = 2.5 nm V−1).109 On theother hand, HA experiments performed at potentiostaticconditions (i.e., U = constant) have also shown that theinterpore distance (Dint

HA) of HA-AAOs decreases with thecurrent density (j), indicating that anodizing potential (U) isnot the only parameter determining the cell size of porousAAOs.16,111,113,118 Lee et al. assumed that high mechanicalstress at the metal/oxide interface due to high current density j(i.e., high electric field strength, E) may be responsible for thereduced ζHA.

111 In other words, energetically unfavorable field-induced mechanical stress (i.e., electrostriction pressure) wassuggested to be accommodated by reducing the cell size toincrease the surface area of the metal/oxide interface. This isreminiscent of the decrease of critical wavelength (λc) of surfaceundulations upon increase in the internal stress (σ) for aninitially flat surface (i.e., λc → ∞ as σ → 0) according to themodel of Srolovitz:246,247

λ π γ σ= M /c2

(67)

where γ is the surface energy and M is the biaxial modulus of asolid. On the other hand, on the basis of the experimentallyobserved tensile to compressive stress transition duringgalvanostatic anodization in 0.4 M H3PO4, Proost et al. havereported that the factor controlling the interpore distance (Dint)is not likely to be an internal stress-induced surfaceperturbation, but rather an electrostatic energy-induced surfaceinstability with the critical perturbation wavelength (λc) givenby the following equation:175

λ π γ ε ε= t E(4 / )c b 0 r2 1/2

(68)

where tb is the barrier layer thickness, ε0 and εr are the vacuumand the relative permittivities of the oxide, respectively, and E isthe electric field.248 The instability equation predicts that theinterpore distance (Dint) is a function of (tb/E

2)1/2 with theslope of 2π/(γ/ε0εr)

1/2. Interpore distances measured duringthe growth of porous anodic alumina and titania agree closelywith those predicted by their electrostatic energy-basedperturbation criterion.175

Figure 36. (a−f) Representative SEM images of MA- and HA-AAOs.Panel (a) reproduced with permission from ref 226. Copyright 2008The American Chemical Society. Panel (b) reproduced withpermission from ref 106. Copyright 1998 AIP Publishing LLC.Panel (c) reproduced with permission from ref 229. Copyright 1998The Japan Society of Applied Physics. Panel (d) reprinted withpermission from ref 110. Copyright 2005 Wiley-VCH Verlag & Co.KGaA, Weinheim. Panel (e) reproduced with permission from ref 111.Copyright 2006 Nature Publishing Group. Panel (f) reproduced withpermission from ref 118. Copyright 2006 IOP Publishing. Reprintedwith permission from IOP Publishing. (g) Self-ordering regimes in MA(filled symbols) and HA (open symbols) by using H2SO4 (blacksymbols), H2C2O4 (red symbols), H2SeO4 (green symbol), andH3PO4 (blue symbols). The black solid lines represent the linearregressions of the data with correlation parameters of ζMA = 2.5 nmV−1 and ζHA = 1.8−2.0 nm V−1. Data for H3PO4-HA (△) showcurrent density (j) dependence of the interpore distance (Dint) at afixed anodizing potential (U = 195 V). Data derived from refs 106,227for H2SO4-MA, refs 5,106,168 for H2C2O4-MA, ref 11 for H2SeO4, refs109,229 for H3PO4-MA, ref 113 for H2SO4-HA, ref 111 for H2C2O4-HA, and ref 118 for H3PO4-HA.




Recently, Lee et al.103 accidently found that porous AAOsthat experienced spontaneous current oscillations (amplitude∼800 mA cm−2) during a potentiostatic HA under unstirredelectrolyte conditions exhibit modulated pore structures. Fromanodizing experiments performed at potential (U) = 140−200V, the authors observed that peak profiles of the oscillatingcurrents are symmetrically sinusoidal with relatively small butincreasing amplitudes at the early stage, while asymmetric withlarger and uniform amplitudes at the lager stage of anodization(Figure 37a). The oscillation period was observed to increasewith time. Microscopic investigation of the resulting porousAAOs revealed that the pattern of pore modulations matchesexactly with the detailed profile of oscillating current peaks(Figure 37b); the pore diameter and the segment length ofmodulated pores increase with the amplitude and the period ofoscillating currents, respectively. The authors attributed thespontaneous current oscillations to the periodic concentrationchange of anodizing electrolyte at the pore bottom underunstirred electrolyte (i.e., a diffusion-controlled anodicoxidation of aluminum). On the basis of the observeddependence of the pore diameter on the anodizing currentdensity, they suggested that the internal pore structures ofporous AAOs can be tailored by judiciously controlling the

current density (j) during anodization of aluminum underpotentiostatic conditions.

7.3. Pulse Anodization (PA)

Ordered porous AAOs with tailor-made internal pore structuresmay provide a new degree of freedom in templated syntheses ofnovel nanomaterials,249−251 and also serve as ideal platforms forinvestigating the adsorption and separation behaviors of diverseparticles, ions, or biologically important molecules.252−254 Onthe basis of the experimental finding that HA of aluminumproduces porous AAOs with one-third lower porosity (P) thanMA (i.e., PHA ≈ 3% for HA and PMA ≈ 10% for MA), Lee andco-workers111 have demonstrated fabrication of AAOs havingperiodically modulated nanopore diameters along the pore axesby combining MA and HA processes (Figure 38). In theirmethod, each step for modulation of pore diameter (Dp)required a tedious manual exchange of the anodizingelectrolytes to satisfy both MA and HA conditions. In anattempt to avoid this problem, they have recently developed anapproach for continuous modulations of internal pore structureof porous AAOs by pulse anodization (PA) of aluminum undera potentiostatic condition using sulfuric or oxalic acidsolution.112 Historically, pulse anodization (PA) of aluminumwas developed in the early 1960s.255,256 The process has

Figure 37. (a) Current (j)−time (t) transient showing spontaneous current (j) oscillation during hard anodization of aluminum at 200 V with anunstirred 0.3 M oxalic acid. An instant solution agitation was introduced at the time indicated by the arrow in (a). (b) An enlarged j−t curve after theelectrolyte agitation, together with the corresponding pore structure of AAO. Reprinted with permission from ref 103. Copyright 2010 Wiley-VCHVerlag GmbH & Co. KGaA, Weinheim.




popularly been employed in the aluminum industry to produceanodic films of high technical quality (i.e., improved corrosionand abrasion resistance) at an efficient rate of produc-tion.257−259 However, the process has been out of view inacademic research in the past four decades, and has not beenapplied to the development of nanostructured materials due tothe non-uniform and disordered pore structures of the resultinganodic alumina.In a newly developed PA process, a low potential (UMA) and

a high potential (UHA) are alternately pulsed to achieve MA andHA conditions, respectively. A typical pulse profile isschematically shown in Figure 39. A current recovery behavioris observed for MA pulses, similarly to the current evolution atthe early stage of ordinary MA processes (Figure 23a). Currentis high at the initial stage, drops to a minimum value, and thengradually increases to reach a steady value after passing anovershoot.112,260,261 On the other hand, upon applying an HApulse, the current density (j) increases steeply (typically, jHA ≈10−102 × jMA) for a short period of time and then decreasesexponentially, which is the typical anodization kinetics of HA ofaluminum under a continuous potentiostatic condition.111 Inthe anodization of aluminum, Joule’s heat (Q) for a givenperiod of time (t) is proportional to the square of the currentdensity (j):

= =Q Ujt R j tb2

(69)

where Rb is the resistance of the barrier layer.262 HA ofaluminum produces a large amount of reaction heat due to thehigh anodic current density (j). The generated heat wouldincrease the proton activity of the acid electrolyte, and thus maycause undesired acidic dissolution of pore wall oxide or evenlocal burning of the growing porous AAO via catastrophic localflow of current (i.e., electrolytic breakdown). Therefore, theheat should be properly removed during anodization at highcurrent density (j). PA provides an effective way of dissipation

of the Joule’s heat. The heat generated by an HA-pulsing can beeffectively dispersed during the subsequent MA-pulsing.224,263

In a typical PA process, the current density (j) changesperiodically to the values determined by pulsed potentials (i.e.,jMA for UMA and jHA for UHA; jMA < jHA). As a consequence, theresulting porous AAO exhibits a lamellar structure withalternate stacking of MA-AAO slab with a smaller porediameter and HA-AAO slab with a larger pore diameter(Figure 40a−c), where the thickness of each oxide layer isdetermined by the pulse durations (i.e., τMA for MA-pulse andτHA for HA-pulse).112,224 In addition to such a structuralmodulation, porous AAOs formed by PA also exhibit a periodic

Figure 38. SEM image of porous AAO with modulated pore diameters prepared by cycling mild anodization (MA) and hard anodization (HA). Amagnified image of the area marked with a white rectangle is shown as an inset. Reproduced with permission from ref 111. Copyright 2006 NaturePublishing Group.

Figure 39. Schematics showing potential (U)−current (j) relationduring potentiostatic pulse anodization (PA).





compositional modulation along the pore axes. TEM-EDXpoint analysis of porous AAO formed by sulfuric acid-based PArevealed that the amount of anionic impurities (mostly SO4

2−)in HA-AAO slabs is about 88% higher than that in MA-AAOslabs, which was attributed to the high current density (jHA)during HA-pulsing.112 As discussed in section 5.2, anodic oxidecontaminated with anionic impurities exhibits a poor chemicalstability against an oxide etchant (e.g., 5 wt % H3PO4). Bytaking advantage of higher level of anionic impurities in HA-AAO slabs, Lee et al.112 could completely separate the MA-AAO slabs from a single as-prepared porous AAO byperforming selective chemical etching of HA-AAO slabs (Figure40d).Structural modulation of porous AAOs can also be achived

by galvanostatic PA, where current pulses satisfying MA andHA conditions are periodically applied. As discussed in section7.2, H2SO4-AAO formed under HA conditions shows fairlyweak junction strength between cells.110,112,113 On the basis ofthis experimental finding, Lee and co-workers263 explored aconvenient route for the mass preparation of uniform aluminananotubes with prescribed lengths. The authors appliedperiodically galvanic MA and HA pulses to achieve continuousmodulations of pore diameter and also to weaken the junctionstrength between cells. Alumina nanotubes, the length of whichis determined by the HA-pulse duration (τHA), could beobtained by immersing the resulting porous AAO into anaqueous mixture of 0.2 M CuCl2 and 6.1 M HCl, followed by

ultrasonic treatment to separated individual nanotubes from thesample.As was discussed in section 7.2, MA and HA exhibit different

dependences of the barrier layer thickness (tb) on anodizingpotential (U); ARMA ≈ 1.2 nm V−1 for MA and ARHA = 0.6−1.0nm V−1 for HA. According to eq 1, the anodizing currentdensity (j) is exponentially proportional to the inverse of thebarrier layer thickness (tb). In PA of aluminum, therefore, whenthe anodizing potential is changed from a higher UHA to a lowerUMA, the current density drops abruptly from jHA to a minimumvalue and then increases gradually to a value (jMA) determinedby UMA (i.e., current recovery). It was reported that the timerequired for a complete recovery of anodizing current dependson the chemical nature of the barrier oxide (i.e., the content ofanionic impurities), the electrolyte temperature, and thepotential difference between UHA and UMA.

112,264 For PA inthree major pore-forming acid electrolytes, Lee and Kim notedthat the time required for current recovery increases in theorder: H2SO4 < H2C2O4 < H3PO4.

264 Further, they pointed outthat PA of aluminum in H2C2O4 or H3PO4 electrolyte isdifficult to achieve continuously within a reasonable period oftime because of the retarded current recovery, especially at alow temperature and a large potential difference.264 To resolvethe problem associated with the slow current recovery, theyincreased gradually anodizing potential prior to pulsing a highanodizing potential.264 Porous AAOs with tailor-made internalpore geometries could be conveniently prepared by employingpotential pulses with deliberately designed periods and

Figure 40. (a) Scheme for the preparation of porous AAOs with modulated pore diameters by pulse anodization (PA). (b) False-colored SEM imageof AAO formed by H2SO4 PA. AAO slabs formed by MA and HA pulses are indicated by MA-AAO and HA-AAO, respectively. (c) SEM imageshowing modulated pore diameter. (d) SEM image of 3D stacks of MA-AAO slabs, obtained by selective removal of HA-AAO slabs by chemicaletching. Reproduced with permission from ref 112. Copyright 2008 Nature Publishing Group.




amplitudes (Figure 41). This capability for engineering internalpore geometry may provide a unique opportunity in templatedsynthesis of nanowires and nanotubes with modulateddiameters, and also in utilization of porous AAOs with 3Dperiodic pore structures in photonic applications.

7.4. Cyclic Anodization (CA)

The concept of structural engineering of porous AAO bycombining MA and HA has been put forward by Losic etal.,265,266 who developed a new anodizing process, called cyclicanodization (CA). They employed periodically oscillatingcurrent signals with different cyclic parameters (i.e., period,amplitude, and profile) during anodization of aluminum toachieve structural modulations in porous AAOs (Figure 42).265

Porous AAOs with modulated pores of different shapes(circular- or ratchet-type), diameters, and lengths wereprepared by applying current signals of deliberately chosencyclic parameters. Microscopic investigation of AAOs formedby CA processes indicated that the structural details of poresfollow exactly the applied current profiles. The authors pointedout that the transitional anodization (TA) mode, the transitionfrom MA to HA condition (Figure 42d), is important forstructural engineering of pores in CA process. It was proposedthat the TA mode creates a pore, whose internal geometry isdirected by the characteristics (i.e., profile, period, andamplitude) of the applied cyclic signal. They further showednanostructuring of porous AAOs with distinctive, hierarchicalinternal pore structures by employing multiprofiled currentsignals: for example, three different successive CA steps,

beginning with first cycle having a gradually increasingamplitude of current, then a double-profiled cycle, and last aseries of triangular galvanic cycles.265 AAOs with periodicallyperforated pores (i.e., pores with nanoholes along horizontaldirections) were also fabricated by chemical etching of cyclicanodized porous AAOs.266

7.5. Anodization of Thin Aluminum Films Deposited onSubstrates

In addition to overcoming their brittle characteristics, AAOsgrown with thin Al films deposited on substrates of choicewould potentially offer much broader application than those onbulk aluminum foils. The substrate could be insulators (e.g.,glass),267−270 semiconductors (Si and TiN),271−277 non-valve-metal (e.g., Cu, Ag, Au, Pt, etc.)270,278−283 or valve-metal (e.g.,Ti, W, Nb, Zr, Ta, etc.)-coated Si substrates,284−290 andtransparent indium thin oxide (ITO).291−298 AAOs formed byanodizing thin Al films on substrates have been utilized notonly as patterning masks, but also as templates for fabricatingvarious functional nanostructures, including arrays of Sinanoholes,271 carbon nanotubes (CNT),287 magnetic nano-wires,289,293 thermoelectric nanowires,270 photocatalytic nano-wires or nanotubes,293,296 and valve-metal oxide nanodots ornanorods.277,284,286,288

Previous works have indicated that the anodizing behavior ofthe interfacial area of the Al and substrate material depends onthe underlying material in terms of anodization kinetics andstructure of the barrier layer (Figure 43). When anodizing Alfilms on insulating substrates (e.g., SiO2) in three major pore-

Figure 41. Schematics showing (a) the experimental process for the fabrication of AAO with tailor-made pore structures by pulse anodization (PA)and (b) a generalized form of a potential pulse employed in pulse anodizations. Uj and τij define the repeating unit of potential waves, where Uj = thepotential at the time tj with U1 = U5 (j = 1−4), τij = tj+1 − tj, i = the pulse number (i = 1, 2, 3...). Representative SEM image of porous AAOs withmodulated pores prepared by pulse anodization; (c) τ11 = 36 s, (d) τ11 = 144 s, (e) τ11 = τ21 = 36 s, τ31 = τ41 = 144 s, and (f) τ11 = τ21 = 144 s, τ31 =36 s, τ41 = 144 s, τ51 = τ61 = 36 s. Other parameters were fixed at U1 = 80 V, U2 = 140 V, U3 = U4 = 160 V, τi2 = τi4 = 0 s, τi3 = 0.2 s. The repeatingunits of pulses are shown as insets in the respective images. Reprinted with permission from ref 264. Copyright 2010 IOP Publishing.




forming electrolytes (i.e., H2SO4, H2C2O4, and H3PO4, Figure43a,d), the completion of the anodization is marked by a sharpdecrease in current density (j) and color change.267−270 Thebarrier layer has a U-shaped morphology like that of AAOformed from bulk Al foils. Al just underneath the cell boundaryremains unoxidized in the form of discrete nanometer-sizedparticles, which are trapped between the alumina barrier layerand the insulating substrate. Unlike bulk Al foils, however, thebarrier layers of AAOs formed by anodization of Al films onconductor-coated substrates (e.g., Si, Pt, Au, ITO, etc.) arecharacterized by inverted morphology with interfacial voids(Figure 43b,e).270−274,276−282,291−297 On the other hand, poresof AAOs formed on valve-metal-coated substrates (e.g., Ti, W,

Nb, Zr, Ta, etc.) are filled with corresponding oxide nanodotsor nanorods, protruding from the barrier layer (Figure43c,f).284−290

Among the above-mentioned substrate materials, theanodization of Al film on a Si substrate has been the mostextensively investigated. When anodizing an Al/Si substrate,after the complete anodization of the Al film, oxidation of the Sisurface forms SiO2 nanodots under an inverted barrier oxidelayer. The thickness of the inverted barrier layer is significantlyreduced as compared to that formed on bulk Alfoils.271,272,299,300 Seo et al. suggested that formation ofinterfacial voids and the inversion of the barrier layer has amechanical origin, involving multiple process stages, as follows

Figure 42. (a) Schematics of the galvanostatic cyclic anodization (CA) for structural modulation of porous AAO. (b,c) SEM images of AAOs withmodulated pores fabricated by asymmetrical current signals (i.e., exponential saw-tooth) with two different amplitudes. (d) Influence of currentamplitude on the pore shape and the length of modulated pore segments. Anodization modes (i.e., MA, TA, HA) associated with correspondinganodization currents are marked on the pore structures and graphs. Reprinted with permission from ref 265. Copyright 2009 Wiley-VCH VerlagGmbH & Co. KGaA, Weinheim.

Figure 43. Schematic current (j)−time (t) curves during anodization of thin Al films on (a) insulating substrates, (b) non-valve-metal-coated (orITO) substrates, and (c) valve-metal-coated substrates. (d−f) Schematic cross-sections of the respective porous AAOs. The solid curve in (b)corresponds to j−t curve during anodization of silicon substrate without conducting surface layer.





(Figure 44).276 When the barrier layer touches the Si surface,the residual Al underneath the cell boundary region is laterallyconfined and thus cannot accommodate the volume expansionstress due to the rigid Si substrate without interfacialrestructuring to create the necessary additional space (i.e.,void). The driving force for void nucleation is the stresspushing the Si substrate downward. On the other hand, upwardstresses are exerted on the bottom of each pore, resulting in theinversion of the barrier layer curvature (i.e., detachment of thebarrier layer). The electric field for anodizing the remaining Alis locally concentrated on the pore edges. As a result, adendritic branching occurs at the edge of each pore bottom.The upward stress increases due to void growth duringanodizing of the residual Al. The inversion behavior allows thebarrier layer to be farther away from the Si substrate, whichrelieves the stress burden while the barrier layer becomesthinner upon further anodizing.276 From TEM nanoprobeenergy dispersive X-ray spectroscopy (EDS), Seo et al.276 foundthat the inverted barrier layer surrounding a void has acompositional bilayer structure with a thin Al-rich region nearthe void and relatively thick Al-poor region near the porebottom, the origin of which is likely to be associated with thedetachment process of the barrier layer during void formation.Anodizing Al film/Si substrate for an extended period of time

results in a local oxidation of Si by electrolyte infiltratingthrough the channels between the pore bottoms and voids,forming SiO2 nanodots just underneath the voids,276,300

accompanied by a violent evolution of oxygen gas bubbles.As a result of the gas evolution, the porous AAO typicallydelaminates from the Si substrate. Similar phenomena have alsobeen observed for anodization of Al films on ITO/glasssubstrates, although Chu et al.291,292,295,298 have successfullyanodized thin Al films on ITO/glass substrate. This may beascribed to improved physical bonding of the AAO as a resultof the sputter deposition of highly energetic Al atoms on theITO/glass substrate. In the case of Al films on non-valve-metal-coated substrates (e.g., Cu, Pd−Au, Ag, Pt), prolongedanodization results in detrimental breakdown or dissolutionof metal during the anodization.270 Accordingly, anodization fora prescribed time is required for each substrate.For electronic applications, it is desirable to grow a barrier-

layer-free AAO directly on conductor-coated substrates. Sanderand Tan demonstrated the fabrication of a barrier-layer-freeAAO membrane by anodizing Al film deposited on an Au-coated substrate, followed by chemical etching of the barrierlayer in 5 wt % H3PO4.

278 Yet their chemical etching processresulted in enlargement of the pore diameter, due to the

isotropic nature of anodic alumina etching. Moreover, severalminutes of anodization of the sample resulted in detachment ofthe AAO from the substrate surface. Yang et al. anodized an Al/Au/Si substrate in an effort to circumvent the need to open thebarrier layer.281 Yet, the Al/Au bilayer system formed Au−Alintermetallic phases, which catalyzed a deleterious oxygenevolution reaction, causing detachment of the AAO membranefrom the substrate.281,283 For anodization of Al/Pt/Sisubstrates, a barrier layer thinning process based on stepwisevoltage reduction279 or the use of a reverse bias in KOH270

have been employed to minimize pore widening. However, thestepwise voltage reduction process resulted in the bifurcation ofpores near the barrier layer. Under the reverse polarizationconditions, the Pt underlayer catalyzed the electrolysis of water,violently evolving H2 gas and thus delaminating porous AAOfrom the substrate. To improve physical bonding between theporous AAO and a conductor-coated substrate, a thin interlayer(typically, ca. 5-nm-thick Ti) between the Al and the conductorlayer has been introduced.282,283,301 Yasui et al. introduced athin layer of Ti (1.5 nm) between the Al and Pt layer to serveboth as an adhesion promoter and as a barrier eliminating thinTiO2 in 5 wt % H3PO4.

282 Oh and Thompson have successfullydemonstrated a selective barrier perforation process byanodizing Al films on a W(60 nm)/Ti(15 nm)-coated siliconsubstrate.289,290 Their process is based on selective dissolutionof the anodized valve-metal oxide (i.e., WO3) in a pH 7phosphate buffer solution. During removal of the WO3,isotropic pore wall etching did not take place due to theneutral nature of the etching solution. After the selectiveremoval of WO3 from the pore base, Oh and Thompson coulddirectly grow Ni or Pt nanowires by electrodeposition into thepores of AAO, in which the exposed base metal layers served ascathode.289,290

Pringle215 first theoretically analyzed the anodizing behaviorsof superimposed valve-metal layers (i.e., bilayered metal films).He predicted that the metal order would be conserved if theanodic oxide of the superimposed metal was less resistive, whilethe metal order would be partially inverted through fingerpenetration of oxide by the underlying anodic oxide if thesuperimposed anodic oxide was more resistive. He also pointedout that these phenomena may be changed by the effects oftransport number, relative rate of cation migrations, oxidestructure, and PBR.215 Later, this model was experimentallyconfirmed by Shimizu and co-workers,214,302−304 who observedthat the random penetrations of lanceolate oxide fingers fromthe less resistive underlying oxide layer into the more resistiveupper oxide layer take place. Recent works by Mozalev and co-

Figure 44. Cross-sectional SEM images (top) and a schematic (bottom) showing the process of void formation: (a) touching of the barrier layerwith Si surface, (b) flattening of pore bottoms due to stress accumulation, (c) void nucleation to minimize the stress, and (d) formation of invertedbarrier layer. SEM images were adapted with permission from ref 276. Copyright 2007 Elsevier.




workers have indicated that in certain electrolytes, anodizing ofthin Al deposited on a layer of valve-metals (e.g., W, Ti, Ta,Nb) results in the filling of AAO pores with nanodots ornanorods of the corresponding metal oxides (Figure45).20,286,305−307 More recently, Chu et al.288 obtained similarexperimental results from anodization of Al film on a Zr-coatedglass substrate. Mozalev et al.307 pointed out that the filling ofAAO pores with other valve-metal oxides is possible due to thehigher PBR and cation transport number (t+), as compared tothose of Al; for example, the PBR for Ta/Ta2O5 = 2.5 and t+Ta= 0.24−0.5.

8. LONG-RANGE ORDERED POROUS AAO

Porous AAOs formed under the self-ordering conditions exhibita poly-domain structure, where each domain containshexagonally ordered nanopores of an identical orientationand is separated by the boundaries. The domain boundaries arecharacterized by defect pores (white dots in Figure 34d). Formost nanotechnology applications, porous AAOs with uniformpore size and long-range ordering of pores are required.Masuda et al.308 first reported fabrication of ideally orderedporous AAOs with a single-domain configuration over a fewmm2 area. The process involved pre-patterning of thealuminum surface by transferring the pattern of a hard SiC

stamp (mold) onto the aluminum by mechanical pressure (i.e.,nanoimprinting), followed by anodization. The SiC imprintstamps were fabricated with an array of convex features ofdesired arrangements in a limited dimension by electron beamlithography (EBL) technique.308−311 Each shallow indentationformed on the aluminum surface defined the position of poregrowth by initiating pore nucleation at the initial stage ofanodization, and thus led to a perfect arrangement of poreswithin the patterned area (Figure 46a). Masuda et al. furtherextended the method to fabricate pore array architectures withsquare- or triangle-shaped pore openings in square or triangulararrangements (Figure 46c,d).309 In a pre-pattern on aluminum,the missing sites of the pattern can be compensatedautomatically during anodization, if the distance between themissing site and the nearest patterned sites satisfies thepotential (U)−interpore distance (Dint) relation required forthe self-ordering of pores (i.e., ζMA = Dint/U = 2.5 nm V−1,section 7.1).311 The size of the pores formed at those missingsites is smaller than that of pores formed at the patterned sites(Figure 46e,f). By utilizing this self-compensation ability inAAO growth, Smith and co-workers have demonstrated thefabrication of porous AAOs with hybrid circular-diamond andcircular-triangular-diamond pore cross-sections.312 By anodiz-ing aluminum with surface pre-patterns in nonequilibrium

Figure 45. SEM images of anodic tantala nanorods formed by multi-step anodization of Al/Ta/Si substrate: (a) before and (b) after removal ofAAO. (c−e) Schematic cross-sectional views of (c) the Al/Ta bilayer anodized at 53 V in 0.2 M H2C2O4, (d) after potentiostatic re-anodizing from53 to 310 V in 0.5 M H3BO3, and (e) after pore widening followed by re-anodizing from 53 to 310 V in 0.5 M H3BO3. Reprinted with permissionfrom ref 307. Copyright 2004 The Electrochemical Society.




tessellation arrangement (Figure 46f,g), the authors found thatthe cell geometries in the resulting porous AAOs aredetermined by the arrangements of the unpatterned andpatterned pore sites and also direct the cross-sectional shape ofpores: circular, diamond, and triangular pores, respectively, inregular, elongated, and partially compressed hexagonal cells(Figure 46i,j). The authors attributed the evolution of diamondand triangular pore cross-sections to a coupled effect betweenthe thick pore wall oxide and the longer segment length of cell-boundary bands (section 5.2), suppressing or eliminating theinfluence of the smaller cell segments.312 Porous AAOs withsharp-featured non-circular pore cross-sections can be used astemplates in synthesizing functional nanostructures forenhanced sensing in localized surface plasmon resonance(LSPR) and surface-enhanced Raman spectroscopy (SERS).313

Stimulated by the works of Masuda and co-workers, severalgroups have developed various surface pre-patterning methodsto fabricate single-domain AAOs with tunable interporedistances, Dint (Table 2). Earlier research in this direction wasmostly devoted to the development of an economic way ofproducing hard imprint stamps with large lateral dimensions.Surface pre-patterning by mechanical nanoindentations usingoptical diffraction grating,314 Si3N4 mold fabricated by deep-UVlithography,315 wafer-scale Ni mold fabricated by laserinterference lithography (LIL),317 and self-assembled mono-or multi-layer of nanospheres316,327 is effective in terms ofprocess cost and pre-patterning area. However, theseapproaches have an intrinsic limitation, in that they rely onthe transfer of stamp patterns onto the aluminum surface underhigh mechanical pressure (typically, 5−100 kN cm−2) andtherefore are not suited for pre-patterning of thin aluminumlayers deposited on technologically more relevant substrates fordevice integration, such as fragile silicon or glass. In general, asthe pattern density on an imprint stamp increases, the higherthe required applied pressure must be in this pattern transferprotocol. In addition to severely deforming the aluminum and

cracking the underlying brittle substrate, high mechanicalpressure also causes structural damage of the imprint stampafter several times of pattern transfer.In attempts to resolve the above-mentioned problems, other

pre-patterning techniques have been proposed: evaporation ofthin aluminum film on ordered arrays of self-assembled Fe2O3nanoparticles (NPs) on silicon followed by mechanicalstriping,318 resist-assisted or direct patterning by focused-ion-beam (FIB) lithography,319,328−330 and direct nanoindentationusing a tip of a scanning probe microscope (SPM).320,331 Yetthese methods are not practical for pre-patterning over anextended area due to the limited ordered area (ca. 2 μm2) ofnanoparticles for the first technique, and due to the serialnature of the patterning process for the other two techniques.For large-scale fabrications of long-range ordered porous AAOson fragile substrates, pre-patterning of thin aluminum films byholographic lithography,321 block-copolymer lithography,322

and nanosphere lithography (NSL)323 has been developed.Fabrication of single domain porous AAOs over 2-in. siliconwafer has been also realized by directly anodizing thinaluminum films deposited on a lithographically generatedSiO2 mask, in which ordered SiO2 holes underneath thealuminum film define the position of pore growth by effectivelyguiding the electric field during anodization.332 Most of thesetechniques have demonstrated their effectiveness in fabricationsof ideally ordered porous AAOs with tunable interpore distance(Dint) on brittle substrates over an extended dimension.However, they are inherently incapable of generating multiplecopies of surface pre-patterns on aluminum.Soft lithography utilizing elastomeric poly-dimethylsiloxane

(PDMS) stamp or nanoimprint lithography (NIL) onpolymeric resist has been demonstrated as a versatile methodfor multiple transfers of a master pattern onto varioussubstrates.325,333,334 Pattern transfer in these lithographictechniques can be achieved at pressures 103−105 times lessthan those used in hard stamping directly onto aluminum. Lai

Figure 46. (a) Schematics showing process for ideally ordered porous AAO. (b−d) SEM micrographs of porous AAOs with different hole arrayarchitectures: (b) circular, (c) square, and (d) triangular pore openings. (e−j) SEM micrographs showing self-compensated pore formation; (e−g)surface prepatterns with missing sites of pattern and (h−j) the respective porous AAOs formed by anodization. The black arrows in (h) indicatepores formed at the missing sites of pattern in (e). SEM micrographs shown in (f,g,i,j) highlight formation of porous AAOs with (f,i) hybrid circular-diamond and (g,j) hybrid cicular-diamond-triangle pore cross-sections; the scale bars are 500 nm. Panel (b) was reprinted with permission from ref308. Copyright 1997 AIP Publishing LLC. Panels (c,d) were reprinted with permission from ref 309. Copyright 2001 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim. Panels (e,h) were reprinted with permission from ref 311. Copyright 2001 AIP Publishing LLC. Panels (f,g,i,j) were reprintedwith permission from ref 312. Copyright 2008 AIP Publishing LLC.




Table

2.Metho

dsof

SurfacePre-patterningof

Aluminum

forIdeally

Ordered

Porou

sAno

dicAluminum

Oxide

(AAO)

pre-patterning

methods

Alfilm

onsubstratea

patternedarea

reported

min

Dint

(nm)

remarks

refs

SiCstam

pX

3mm

×3mm

63SiCimprintstam

pwas

fabricated

usingelectron

beam

lithography

(EBL);stam

ping

pressure:28

kNcm

−2

308,310

opticaldiffractiongrating

X5mm

×5mm

481

Alsurfacewas

pre-patternedby

atwo-step

press-in

procedureof

opticaldiffractiongrating

314

Si3N

4stam

pX

4-in.w

afer

500

stam

pwith

arrays

ofSi

3N4pyramidswas

replicated

from

asilicon

master;stam

ping

pressure:5kN

cm−2

315

nanosphere

crystals

X>cm

2area

150

hexagonalarraypatternof

self-assembled

nanosphere

crystalwas

transferredonto

Alsurface;

stam

ping

pressure:98

kNcm

−2

316

Nistam

pX

wafer

scale

200

Niimprintstam

pswerereplicated

from

masterpatterns,fabricated

bylaserinterference

lithography

(LIL);stam

ping

pressure:5−

25kN

cm−2

317

Fe2O

3nanoparticles

(NPs)

X2μm

×2μm

136-μm

-thick

Alwas

depositedon

self-assembled

2Darrays

ofFe

2O3NPs

onSi,and

separatedby

usingan

adhesive

tape

foranodization

318

focused-ion-beam

(FIB)

lithography

O100

pre-patterning

ofAlsurfacewas

achieved

byfocusedGa+-ionbeam

319

probe-tip

-based

direct

patterning

O1μm

×1μm

100

thesurfaceof

Alwas

directlypatternedby

nanoindentationusingthetip

ofascanning

probemicroscope(SPM

);indentationload:40

μN320

holographiclithography

O1cm

×1cm

∼300

thin

Alfilmswith

perio

dicsurfaceundulatio

nsweregeneratedby

depositin

gAlonto

thephotoresist-grating-patterns,d

eveloped

byholographic

lithography

321

block-copolymer

lithography

O45

highlyorderedholearraypatternof

thin

blockcopolymer

film

was

transferredto

theAlsurfaceby

reactiveionetching(RIE)

322

nanosphere

lithography

(NSL

)O

>1mm

2area

84pre-patterning

ofAlfilm

onasubstratewas

achieved

bychem

icaletchingof

Althroughthemaskform

edby

sputter-depositio

nof

tungsten

onthe

orderedarrays

ofpolystyrenesphere

323

mold-assisted

chem

ical

etching

O2cm

×2cm

277

pre-patterning

ofAlwas

achieved

bycontactin

gwith

Aletchant-absorbed

PDMSmold

324

nanoimprintlithography

(NIL)

O∼10

cm2area

100

Nim

oldwas

replicated

from

amasterpatternfabricated

byEB

L,andits

patternwas

transferredonto

athermoplasticresist;subsequently,the

resulting

resistpatternwas

transferredonto

anAlsurfaceby

Ar-ionbeam

milling;

imprintpressure:0.5kN

cm−2

325

step

andflashimprint

lithography

(SFIL)

Owafer

scale

100

orderedarrays

ofnanoindentsweregeneratedby

wet-etching

ofAlthroughpre-patternedpolymer

masks

that

werecreatedby

SFIL

onAlsurface;

imprintpressure:<1

0−3kN

cm−2

326

aPre-patterning

capabilityof

thin

Alfilm

depositedon

fragile

substrates.



et al.324 reported a generic method of pre-patterning of analuminum surface by mold-assisted chemical etching. Theirtechnique is based on the reaction-diffusion wet-stamping (RD-WETS) process, which creates ordered arrays of shallow etchpits on aluminum by the absorption/liberation of aluminumetchant adsorbed in a PDMS stamp. Single-domain porousAAOs with a square lattice configuration over 2 × 2 cm2 areawere fabricated by anodization of the patterned aluminum.Recently, Kustandi and co-workers have demonstrated

fabrication of single-domain porous AAOs with arbitraryinterpore distances (Dint) and different pore lattice config-uration on 4-in. silicon wafer (Figure 47).326 Their approach isbased on the step and flash imprint lithography (SFIL) using apatterned quartz template. The SFIL was employed to pattern apolymer mask layer on aluminum film. Pre-patterning of thealuminum surface was achieved by transferring the pattern ofthe polymer mask onto the underlying aluminum films by wet-chemical etching. The authors claimed that the demonstrated

Figure 47. (a) Schematic diagrams showing fabrication procedures of ideally ordered porous AAO. (b−e) SEM images of porous AAOs fabricatedby the procedures shown in (a): (b) an overview image, (c) an image showing clear distinction between the pre-patterned (left) and non-patterned(right) pore arrangement, (d) an SEM image of porous AAO with square arrangement of pores, and (e) a cross-sectional image of porous AAOgrown on silicon wafer. (f) A photograph of AAO template on a 4-in. silicon wafer. Panels (a−f) were reproduced with permission from ref 326.Copyright 2010 The American Chemical Society.

Figure 48. (a) Cross-sectional SEM image of porous AAO formed by anodization of pre-patterned aluminum at an outside self-ordering condition:pattern interval = 200 nm, anodization at U = 80 V using 0.04 M H2C2O4 (17 °C). (b) Dependence of the depth of ordered pores on the anodizingpotential (U): pattern intervals = 70, 100, 150, 200, and 250 nm for U = 28, 40, 60, 80, and 100 V, respectively. Anodizing electrolyte = 0.3 MH2C2O4 for U = 28, 40, 60 V and 0.04 M H2C2O4 for U = 80 and 100 V. Reproduced with permission from ref 335. Copyright 2001 TheElectrochemical Society.





process provides significant merits over existing surface pre-patterning approaches in terms of patterning area, processsimplicity, robustness, and throughput, allowing up to 10 000times of pattern transfers to the surface of aluminum filmsdeposited on fragile substrates.326

As discussed above, anodization of pre-patterned aluminumis indeed an effective way for obtaining ideally ordered porousAAO. It should be noted, however, that there is a majorlimitation of the process; to maintain the initially ideallyordered pore configuration, the obtainable maximum aspectratio of uniform nanopores depends critically on theanodization conditions. For anodization of aluminum of agiven pattern interval, one should properly select anodizingparameters (e.g., potential and electrolyte) to fulfill the self-ordering requirement of pores. As discussed in section 7, forMA and HA using three major pore-forming acid electrolytes,there are specific ranges of anodizing potential (U), which giveself-ordered pore structure. Under other applied potentials, thedepth of nanopores is limited in terms of ideal ordering (seeFigure 48).335

9. AAO TEMPLATE-BASED SYNTHESIS OFFUNCTIONAL NANOSTRUCTURES

While nanostructured materials are considered to have hugeapplication potential, cost-effective, high-throughput, andreproducible synthetic strategies are an essential prerequisite.The synthesis of both simple and complex low-dimensionalnanostructures (e.g., nanodots, wires, tubes, core/shell NPs,organic−inorganic nanohybrids, etc.) relies largely on tem-plates, whose size, structure, and physicochemical properties arepredefined. Templated synthesis provides robust ways ofprecise control over the size, shape and configuration, andgrowth direction and place of otherwise unattainable nano-structured materials. More importantly, it may also offeropportunities for the in situ assembly of discrete nanostructures(i.e., building blocks) into a well-defined hierarchicalarchitecture for practical device applications. In principle,almost all existing synthetic approaches and their combinationcan be applied to the templated synthesis of nanostructuredfunctional materials. Templates can be any substance withnanostructured features, including DNA, protein, viruses, livingorganisms, colloidal nanoparticles, nanowires, nanotubes, blockcopolymers, porous materials, etc.; the present authors refer

interested readers to excellent recent review articles given inrefs313 and 336, that extensively cover the templated synthesisand assembly of low-dimensional nanostructures and theirapplications. As discussed in previous sections, self-orderedporous AAOs with tightly controlled pore size, density, andintervals can be obtained by anodizing aluminum substrateunder proper conditions. This provides many uniqueopportunities in the templated synthesis of low-dimensionalfunctional nanostructures, allowing simple and cost-effectivepreparations of extended arrays of structurally well-defined andidentical nanostructures and also overcoming many of thedrawbacks of conventional state-of-the-art lithographic techni-ques. During the last two decades, by taking advantage of itshighly ordered structural feature, porous AAO has intensivelyutilized as a template, stencil mask, or scaffold for functionalnanostructures or nanodevices. The approaches employed fordeveloping functional nanostructures include electrochemicaldeposition (ECD), atomic layer deposition (ALD), chemicalvapor deposition (CVD), physical vapor deposition (PVD),sol−gel deposition, surface modifications with technologicallyimportant (bio)molecules, polymers or nanoparticles, meltimpregnation of materials, reactive ion etching (RIE), etc. Avast variety of nanostructures including carbon nanotubes(CNTs), metal, semiconductor, or polymer nanowires/nano-tubes, and ordered arrays of nanodots/nanoholes on varioussubstrates have been successfully fabricated by utilizing porousAAO as a template or stencil mask. Their structure−propertiesand functionalizations have been intensively investigated in anattempt to utilize them for practical applications in safety,energy, information, and biomedicine. In this section, wediscuss AAO template-based synthesis of low-dimensionalnanomaterials, their functionalizations, and applications.

9.1. Electrochemical Deposition (ECD)

Possin337 implemented first the electrochemical deposition(ECD) of metal into a track-etched mica template to obtain 40-nm-thick and 15-μm-long Sn, In, and Zn nanowires. Thetemplate approach was drawn a renewed attention by the worksof Martin and co-workers and has been popularly employed asa nanofabrication strategy.338−342 The electrochemical deposi-tion of materials into the pores of AAO template providesmarked advantages over other preparation methods ofnanowires or nanotubes. In comparison to other depositiontechniques, such as chemical vapor deposition (CVD), atomic

Figure 49. General scheme for electrochemical deposition (ECD) of materials into porous AAO.




layer deposition (ALD), and physical vapor deposition (PVD),the electrochemical deposition into porous templates is simpleand inexpensive, and can be conducted without any specialequipment.In general, to synthesize nanowires, the same general

procedure can be applied irrespective of the materials to bedeposited (Figure 49). First, a thin Ag layer is deposited ontoone face of an AAO membrane. This Ag layer serves as theworking electrode in the deposition of desired materials. Next, athin layer of sacrificial Ag (or Ni) is electrochemically depositedinto the pores. This process is recommended to avoid a so-called “puddling effect” that typically causes nailhead shaping ofone end of the deposited nanowire due to the geometric detailsof the pore mouth.342 After that, the desired material iselectrochemically deposited. The resulting nanowires/AAOcomposite sample then is dipped into HNO3 solution toremove the Ag working electrode layer and the sacrificial layer(Ag or Ni). The nanowires can be collected by dissolving theAAO template using an appropriate AAO etchant (typically,KOH or H3PO4). The choice of oxide etchant depends on thematerial deposited; the etchant solution should not react withthe nanowire material. The diameter of the resulting nanowiresis determined by the pore size of the porous AAO template,while their length is proportional to the total amount of chargepassed during the electrochemical deposition. Various metalnanowires (e.g., Au,264,343,344 Ag,344,345 Pt,343,346 Ni,343,347

Pb,348 Cu,348 Zn,349 Co,350−352 Sb353) have been synthesizedin porous AAO templates. These single component 1D metallicnanowires have been used as model systems for systematicallyinvestigating various research issues in chemistry and physics,for example, the catalytic, magnetic, thermoelectric, andplasmonic properties of 1D nanostructures. Although singlecomponent metallic nanostructures (e.g., nanowire or nano-tube) are useful for studying structure−property relations, we

focus our discussion on multisegmented metallic nanowiresconsisting of two or more different materials because of theirversatilities in terms of degree of freedom in functionalizationfor various applications.Multisegmented metallic nanowires can conveniently be

prepared by sequentially changing the electrolytic solutionduring the electrodeposition. Judicious control of the length ofeach segment allows one to obtain submicrometer barcodes,which can be used as platforms for multiplexed bio-assays.251,344,354,355 The differential reflectivity of adjacentmetal segments and the selective self-assembly of appropriatemolecules on specific metal segments enable the identificationof striping patterns by conventional optical microscopes. Morerecently, multisegmented nanowires have been utilized fordeveloping nanogap devices. Qin et al.356 developed a genericapproach to lithographically process 1D nanowires. Theprocedure, termed “on-wire lithography (OWL)”, involvesselective removal of one of the components comprising amultisegmented nanowire to create gaps. Before gap creationvia the wet-chemical etching of targeted segment(s), one side ofthe multisegmented nanowires is subjected to the deposition ofa thin layer of insulting SiO2, which acts as a support for thenanogap device. In this way, the authors could createnanometer-sized gaps (5 to several hundred nm) on nanowires.Further, by employing dip-pen nanolithography, they depositednanoscopic amounts of conducting polymer within the creatednanogaps to investigate the transport properties of nanogaps.More recently, Chen et al.357 demonstrated heterometallicnanogaps for molecular transport junctions (MTJs). Thiol-terminated molecules were assembled into heterometallicnanogaps (i.e., Pt/2-nm gap/Au) to observe their moleculardiode behavior. The on-wire lithography (OWL) technique wasfurther extended to develop a new encoding system by thesame group. By taking advantage of the facile control of the

Figure 50. Scheme of the nanodisk code method. (a) Synthesis and functionalization. (b) Thirteen possible 5-disk-pair nanodisk codes with thecorresponding binary codes. (c) SEM images of Au/Ni multisegmented nanorods before (top) and after (bottom) deposition of the backing layerand Ni etching. (d) Two-dimensional (top) and three-dimensional (middle) scanning Raman microscopy images of a 11111 nanodisk code.Representative Raman spectrum of methylene blue (bottom) taken from the center of the hot spot generated in the middle disk pair shown in theRaman maps above. Panels (a,b,d) were adapted from ref 358 with permission. Copyright 2007 The American Chemical Society. Panel (c)reproduced with permission from ref 359. Copyright 2009Macmillan Publishers Ltd.: Nature Protocols.




length of segments in the nanowire, dispersible 1D objectscontaining arrays of nanodisks were prepared and function-alized with Raman active chromophores.358,359 This allowedencoding of individual nanodisks both physically andspectroscopically (Figure 50). As proof-of-concept, the authorsdemonstrated multiplexed DNA detection at target concen-trations as low as 100 fM.Component materials in the multisegmented nanowires do

not need to be metals. They could be semiconductors orconducting polymers. Park and co-workers.360,361 have foundthat two-component nanorods consisting of metal/conductingpolymer layer can behave as mesoscopic amphiphiles. Theysynthesized segmented Au/polypyrrol (Ppy) nanorods byelectrochemical deposition of Au into porous AAO template,followed by electrochemical polymerization of pyrrol.360 Thelength of each segment could be conveniently tuned bycontrolling the deposition time (i.e., the total amount ofcharge). Au/Ppy nanorods released from the template exhibitedamphiphilic characteristics that originated from hydrophilic Ausegments and hydrophobic Ppy blocks. Individual Au/Ppynanorods self-assembled into microscale 2D sheets or 3Dbundles or tubes depending on the Au/Ppy segment lengthratio (Figure 51).

As a more convenient approach, pulsed electrochemicaldeposition can be used for the preparation of multisegmentednanowires with prescribed periodicity. Yahalom and Zadok362

utilized pulsed electrochemical deposition technique tofabricate Cu/Ni superlattice films by using a single electrolytecontaining Cu and Ni salts. Pulsed electrochemical depositionhas been extended by other researchers to prepare multilayeredfilms of magnetic/nonmagnetic metal couple to investigate thegiant magnetoresistance (GMR) effect.363−366 The techniquehas also been implemented in track-etched polycarbonatemembrane or porous AAO membrane to synthesize periodi-cally multisegmented metal nanowires with sharp interface

(Figure 52).367−373 In pulsed electrochemical deposition usinga two-component electrolytic solution, the cathodic potential is

alternately pulsed between values above and below thereduction potential of the less noble metal component.362

When the less negative potential is pulsed, more noble metal isexclusively deposited. However, when depositing a less noblemetal by pulsing the more negative potential, the more noblemetal can be co-deposited. Therefore, the concentration of thenobler metal ions should be held low enough as compared tothat of the less noble metal ions, so that the deposition of thenobler metal ions during the pulsing of more negative potentialis limited by ion diffusion.342,362 Recently, Liu and co-workers374 have demonstrated fabrication of tailor-madeinorganic nanopeapods (i.e., Co@CoAl2O4) nanowires bypulsed electrochemical deposition of Co/Pt multilayers intoporous AAO template, followed by a solid-state reactionbetween Co and Al2O3 at a high temperature.Fabrication of multisegmented metal nanotubes has also

been demonstrated by Lee et al.375 Their method was based onthe preferential electrochemical deposition of metal along thesurface of pore walls decorated with metallic nanoparticles. Theauthors immobilized Ag nanoparticles (AgNPs) on the porewall surfaces of an AAO template by utilizing spontaneousreduction of Ag+ by Sn2+ as the following reaction:

+ → ++ + +2Ag Sn 2Ag Snaq surface2

surface0

surface4

(70)

The process for AgNP immobilization is a simple variation ofthe well-established sensitization-pre-activation protocol ap-plied to AAO376,377 or track-etched polycarbonate tem-plate338,378 prior to the electroless deposition of metals (seesection 9.2). Sn2+ was first chemisorbed on the pore wallsurfaces by dipping the AAO membrane into an aqueousmixture solution of 0.02 M SnCl2 and 0.01 M HCl for 2 min.After thorough rinsing with water and drying, the resultingAAO template was immersed into 0.02 M AgNO3 solution.This cycle, usually repeated about six times, resulted in uniformdeposition of AgNPs on the oxide surfaces. With the AgNP-immobilized AAO template, metallic nanotubes, embedded inthe AAO, were synthesized by electrochemical deposition. Byperiodically changing the electrolytic solution during electro-chemical deposition of the metal, the authors could synthesizemutisegmented Au/Ni nanotubes (Figure 53).

Figure 51. SEM images of “open” superstructures formed bykinetically controlled, shape-directed assembly of Au/polypyrol(Ppy) nanorods: (a) a superstructure that has not fully closed, (b) afully closed superstructure, and (c) a physically cleaved area of thesuperstructure showing the individual rods that have been assembledinto the superstructure. Scale bars for (a,b) = 100 μm. Scale bar for (c)= 50 μm. Reproduced with permission from ref 361. Copyright 2008Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 52. TEM analysis of a freestanding Fe/Au barcode nanowire:(a) elemental line scanning of Fe and Au composition along thenanowire (Au, red; Fe, green), (b) elemental mapping of Au, and (c)elemental mapping of Fe. Reprinted with permission from ref 643.Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.





9.2. Electroless Deposition (ELD)

Electroless deposition of metals into porous templates waspioneered by Martin and co-workers.378−381 Most of the earlierworks were devoted to electroless deposition of gold within thepores of track-etched polycarbonate membranes. The typicaldeposition process is composed of the following three generalsteps:378,382,383

(1) Sensitization: This is accomplished by immersing theporous polymeric template into an aqueous solution of SnCl2to deposit Sn2+ onto the surfaces of the membrane.(2) Activation: This is accomplished by dipping the Sn2+-

sensitized polymeric membrane into a AgNO3 solution, whichyields metallic AgNPs on the membrane surfaces through asurface redox reaction (see reaction 70).(3) Electroless deposition: This entails immersing the

activated polymeric membrane into a gold plating solution, inwhich the surface-bound AgNPs act as catalyst for thereduction of Au+ to yield AuNPs on the membrane surfacethrough the following reaction.

+ → ++ +Au Ag Ag Agaq surface0

surface0

aq (71)

The surface-bound AuNPs act as autocatalysts for the reductionof Au+ to metallic Au0 in the presence of a reducing agent (e.g.,formaldehyde). Au deposition starts at the entire surface ofmembrane (i.e., pore walls and membrane faces). As a result,the surfaces of pore walls and the faces of the membrane can becoated with a thin Au layer, yielding Au nanotubes within thepores and continuous Au films on the faces of the membrane.The resulting gold nanotube/polymer composite membraneshave been successfully used for diverse applications, forexample, for selective ion-transport,379,382−387 (bio)moleculeseparations,380,388,389 biosensing, and electroanalysis.390−393

Kohli et al.381 noted that the above electroless depositionmethod applied to polycarbonate membranes does not work inporous AAO templates because of insufficient binding sites forthe Sn2+ on the pore wall surfaces. The authors solved thisproblem by modifying the surfaces of porous AAO with 2-(succinic anhydride)propyl trimethoxysilane (SAPT), whichforms covalent Al−O−Si bonds with −OH functionalities onthe oxide surfaces. The anhydride in the SAPT moleculehydroyzes to dicarboxylic acid, which chelates Sn2+ ions duringthe sensitization process. As an alternative approach to increasethe density of oxygen groups on the oxide surface, Yu et al.394

incubated porous AAO membrane in 35% hydrogen peroxidesolution prior to Sn2+-sensitization. The rate of electroless Audeposition was reported to increase with the pH of the platingbath.381 Uniform Au nanotubes within pores of AAO templateshave been obtained by performing electroless deposition at thepH < 10 and at temperature below 4 °C.381,394 At pH > 10 or ata high temperature, either porous AAO dissolves and/or thepores of the AAO template become closed due to the higherrate of metal deposition relative to the rate of mass transfer ofAu(I) and formaldehyde down to the pores. Yu et al.394

reported that Au nanotubes formed by electroless depositionare characterized by nanoclustered morphology, in which thesize of the gold nanoclusters increases with the pH of theplating bath. They showed that the size of the gold nanoclustersdetermines the catalytic properties of Au nanotubes embeddedin AAO membrane. Further, they showed that Au nanotubes/AAO composite membranes can be reused in the catalyticconversion of 4-nitrophenol into 4-aminophenol in thepresence of NaBH4 as a reductant.Activation of the porous AAO membranes can also be

accomplished by immobilizing PdNPs on the pore wallsurfaces.395−397 Fabrications of arrays of Co,398 Ni,398

Cu,398,399 Pd,400 Pt,400 binary398 or ternary401 metal alloynanotubes, nanowires,402 and nanocones403 have beendemonstrated by electroless deposition of metals into PdNP-immobilized porous AAO membranes.

9.3. Sol−Gel Deposition

Sol−gel processing has developed into a versatile protocol forthe stoichiometric synthesis of diverse nanocrystalline materials.In general, sol−gel chemistry involves the hydrolysis ofprecursor molecules under an acidic condition to prepare asuspension of colloidal particles (i.e., sol) and the subsequentcondensation of the sol particles to obtain a gel. The resultinggel is then calcined to obtain the desired material. Metalalkoxides in organic media or inorganic salts in aqueous mediacan be used as precursors. Martin and co-workers pioneered thesol−gel porous AAO templating method for the synthesis ofvarious semiconductor or insulator oxide nanostructures(nanowires and nanotubes) including TiO2, ZnO, WO3,V2O5, MnO2, Co3O4, and SiO2.

404−406 Since then, manyinvestigators have employed the sol−gel deposition method toprepare a vast variety of oxide nanostructures to investigate, forexample, photovoltaic (TiO2),

407,408 gas sensing (SnO2,ZnO),409,410 ferromagnetic (CoFe2O4),

411,412 ferroelectric(BiFeO3, SrBi2Ta2O9, PbZr0.52Ti0.48O3),

413−415 superconduct-ing (YBa2Cu3O7-δ, Bi2Sr2CaCu2Oy),

416,417 and luminescence(Y2O3:Eu, TiO2, ZnO:Dy) properties.

418−420

Typically, when depositing sol−gel into oxide nanopores, anAAO is directly dipped into a solution containing sol particlesfor a given period of time. After thermal treatment, eithernanowires or nanotubes of inorganic oxide are formed within

Figure 53. SEM images of multisegmented metal nanotubes with astacking configuration of Au/Ni/Au/Ni/Au along the nanotube axis:(a) false-colored cross-sectional SEM image of as-prepared metalnanotube-AAO composite. (b,c) SEM images of mutisegmented metalnanotubes after removal of the AAO template. Adapted withpermission from ref 375. Copyright 2005 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.




the pores. In general, immersion of porous AAO template intoa sol solution for long periods of time yields nanowires, whileimmersion for short periods of times yields nanotubes. Theformation of nanotubes indicates that the sol particles areadsorbed onto the pore wall surfaces due to electrostaticinteraction between the negatively charged pore wall surfacesand the positively charged sol particles.404 Lakshmi et al.404

reported that the gelation occurs at a faster rate within thenanopores than in the bulk reservoir, due possibly to localincrease in sol concentration at the pore wall surfaces. In theirSiO2 nanotube synthesis, Zhang et al.421 found that theviscosity of the sol solution is a key factor determining themorphology of the resulting nanotubes. Viscosity of solsolution increases with aging time. They obtained nanowiresfrom a porous AAO templated dipped for 1 min in a solsolution aged at room temperature for 2 days. From a solutionaged for 30 days, they obtained SiO2 nanotubes connected withnanowires. They also found that the formation of thenanotubes depends strongly on the temperature of the solsolution. For a short dipping time less than 1 min, bamboo-likenanowires were prepared from a sol solution at 50 °C. On theother hand, perfect nanotubes with sharp walls weresynthesized from a solution at a lower temperature (5 °C).As the temperature of the sol solution decreased, the inside wallof SiO2 nanotube became smoother.Limmer et al.422 pointed out that filling of sol particles into

oxide nanopores is mainly driven by the capillary force. Heat-treatment of the sol-coated AAO templates often results in 1Dporous nanostructures or hollow tubes due to insufficientpacking of sol particles with the oxide nanopores. Limmer andco-workers422,423 developed an electrophoretic sol−gel processto resolve the problems associated with low particle packingdensity. In their approach, the electrophoretic motion of thepositively charged sol particles under an electric field is utilized.The authors demonstrated successful syntheses of technolog-ically important oxide nanowires (e.g., TiO2, BaTiO3, SiO2,SrNb2O6, Pb(Zr,Ti)O3; diameter = 70−200 nm) by electro-phoretic deposition of sol particles into nanopores of track-etched polycarbonate membranes followed by heat-treat-ments.422,423

Although the electrophoretic sol deposition process hasdistinct merit, it is rather difficult to apply the process to poroustemplates with smaller pore diameters (<50 nm).422 Theproblem may arise due to the decreased diffusivity of solparticle in narrower pores or the size of the sol particles(typically, 10−100 nm). To resolve this issue, Maio et al.424

employed a cathodically driven sol−gel process and preparedsingle crystalline TiO2 nanowires. The authors employed a Tiprecursor solution containing NO3

− ions. The local increase inpH at the surface of electrode under a cathodic bias gives rise tothe hydrolyzation of the Ti precursor through the followingreactions:424

+ + → +− − −NO 6H O 8e NH 9OH3 2 3 (72)

+ →+ −TiO 2OH TiO(OH) (sol)22 (73)

The resulting TiO(OH)2 sol particles are converted into a 3Dnetwork of titanium oxyhydroxide gel within the nanopores ofAAO template via the following electrochemical reaction:

− → + −xTiO(OH) (sol) H O TiO (OH) (gel)x x2 2 1 2 2 (74)

As such, both the sol formation and the subsequent gelationoccur within the nanopores. By taking advantage of this fact,

the authors were able to synthesize very thin single-crystallineTiO2 nanowires (diameter ≈ 10 nm) by performing heat-treatment and subsequently removing the AAO template.As a surface modification protocol, the sol−gel template

technique offers a large degree of freedom in nanotechnologyapplications of porous AAO. Lee et al.425 demonstrated thatsol−gel-derived silica nanotubes/AAO composite can be usedas a synthetic bio-nanotube membrane for separating twoenantiomers of a chiral drug, in which individual silicananotubes act as nanometer-sized chromatography columnsin parallel. They deposited thin-walled (<3 nm) silicananotubes within porous AAO by adopting sol−gel templatesynthesis approach. Subsequently, the inner wall surfaces of thesilica nanotubes were functionalized with aldehyde terminatedsilane. This surface functionalization allowed the conjugation ofan antibody that selectively binds to one enantiomer of a drug4-[3-(4-fluorophenyl-2-hydroxyl-1-[1,2,4]triazol-1-yl-propyl]-benzonitrile, which has two chiral centers and four stereo-isomers: RR, SS, SR, and RS. The silica nanotubes can beindividually released from the porous AAO template to obtainsilica nano test tubes.426−428 In another study, the inner andouter surfaces of silica nano test tubes were selectivelyderivatized with different functional groups using silanechemistry.426 Silica nano test tubes with hydrophilic outerand hydrophobic inner surfaces were demonstrated to be idealfor extracting lipophilic molecules from aqueous solution.426 Inaddition, they can be capped with either polystyrene latexnanoparticles429 or gold nanoparticles427 to create nano-capsules. Their facile surface functionalization, loadable largehollow interior, low level of cytotoxicity, and ease of dispersionmake the silica nano test tubes an effective drug/gene deliverysystem.428,430

Yamaguchi et al.431 demonstrated that a porous AAOtemplate can used to fabricate a silica−surfactant hybridmembrane containing nanochannels oriented in parallel withrespect to the pore axis of AAO. They used a precursor solutioncontaining tetraethoxylsilane (TEOS) as the silica source andcationic cetyltrimethylammonium bromide (CTAB) as astructure guiding agent. By applying moderate aspiration, theauthors were able to introduce precursor solution into thepores of the AAO. Plastschek et al.432,433 demonstrated that theorientation of mesoporous silica structure in porous AAO canbe systematically adjusted through combination of sol−gelprocess and evaporation-induced self-assembly. Systematicvariations of the type and amount of structure guiding agentand also the amount of added inorganic salts allowed theauthors to investigate systematically the confinement effect ofthe interfacial interaction on the orientation of silica nano-channels. Their experiments revealed that ionic CTABproduces a columnar hexagonal 2D structure parallel to thevertical nanochannels of the porous AAO membrane, whereasnonionic surfactants, such as Pluronic P123 [poly(ethyleneoxide)20-poly(propylene oxide)70-poly(ethylene oxide)20] andBrij 56 [decaethylene glycol hexadecyl ether], prefer theformation of either a circular hexagonal 2D structure normal tothe nanopores of AAO or a structure with phase mixtures ofcircular and columnar orientations (Figure 54). Platschek et al.found that the moisture content of the deposited sample andthe relative humidity during the drying process influence themicroscopic separation.433

The cylindrical pore morphology of AAO, and the pore’sdimensional tuneability, make porous AAO an ideal modelsystem for systematically investigating the confinement effect



on molecular organization. We et al.434 synthesized silica−P123composite mesostructures within cylindrical nanochannels ofAAO by employing a sol−gel method. It is well-known that thisprecursor forms hexagonal silica mesostructures (i.e., SBA-15),in which the long axes of the mesochannels are aligned parallelto the surface of flat substrates. However, the authors observed

a rather complex evolution of mesochannels with the geometricconfinements. Figure 55 summarizes the evolution of theconfined mesostructures as a function of the channel diameterof AAO template. When the AAO nanochannel diameter was inthe range 55−73 nm, the confined silica−surfactant compositemesophase consisted of three coaxial layers: a straight innermesochannel and two outer concentric mesochannels withdiverse morphologies of stacked doughnuts, single helix (S-helix), or double helix (D-helix). For the confinementdimension of 49−54 nm, a coaxial double-layer helixmesostructure was evolved; the inner core = S-helix and theouter layer = S-helix or D-helix. On reducing the confinementdimension down to 34−45 nm, the inner core became astraight mesochannel (diameter = 13−16 nm), and the outerlayer consisted of stacked doughnuts, S-helix, or D-helix. Forfurther reduced confinement dimension of 31 nm, a single-layered D-helix mesostructure was observed. For confinementdimension below 30 nm, inverted peapod-like mesoporousmorphology, rather than helical mesostructures, was observed.For the confinement dimension below 18 nm, a line of alignedmesocages was formed. The authors pointed out that themesoporous silica frameworks in their composite mesophasescan be preserved even after the surfactant removal, which ismarkedly different from that case of confined polymers.

9.4. Surface Modification

Coupling appropriate molecules to the pore wall surface ofAAO can greatly expand the application capability of porousAAO for the synthesis of new functional nanomaterials, sensing,recognition, and the controlled release of biologically relevantmolecules. Surface functionalization with appropriate moleculesor particles provides an opportunity for tuning of the surfaceproperties of porous AAO, such as hydrophilicity, surfacecharge, reflectivity, membrane selectivity, antifouling resistance,etc. The surfaces of porous AAO can be functionalized with awide variety of molecules, such as n-alkanoic or fluorinatedorganic acid,435−437 organosilanes,438,439 and phosphonicacids.438,440 On the other hand, most studies on surfacemodifications of porous AAO are based on silanizationchemistry. Organosilane compounds can be covalently boundon the surfaces porous AAO membranes, by taking advantageof the rich hydroxide groups on the oxide surfaces. Organo-

Figure 54. Plane view TEM images of (a) the columnar orientedhexagonal mesostructure templated with CTAB, (b) the sampletemplated with Brij at 60% humidity, and (c) the sample templatedwith P123 at 60% humidity. Reproduced with permission from ref 432.Copyright 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 55. Summary of the experimentally observed evolution of mesostructures confined within AAO nanochannels of varying diameters.Reproduced with permission from ref 434. Copyright 2009 Macmillan Publishers Ltd.: Nature Materials.





silanes on AAO surfaces can act as linkers to immobilizenanoparticles, polymers, proteins, DNA, and other moleculesfor additional functionalities. Among a variety of organosilanecompounds, aminosilanes (e.g., 3-aminoproyl trimethoxysilane(APTMS) or 3-aminopropyl triethosysilane (APTES)) havemost frequently been employed as coupling agents.Losic and co-workers441 have recently reported a new

approach for controlling the surface architecture of porousAAO membranes, which generates layered, silane-based surfacechemistries and yields distinctly different functionalities on thepore openings and the internal pore surface. The method wasbased on the remarkable stability of the silanized surface ofAAO during anodization of aluminum. After a short-term firstanodization of aluminum and removal of most of the resultingporous oxide, the surface of the remaining thin oxide layer wasderivatized with ATPES. The resulting silanized sample wassubsequently reanodized for a longer period of time. Thesurfaces of the pore wall formed by the second-step anodizationwere functionalized with hydrophobic pentafluorophenyldime-thylchlorosilane (PFPES) with distinctly different propertiesfrom the APTES present in the pore opening. By extending thework, they have also demonstrated that different functionalitiesand wettabilities can be imparted to the pore channels of AAO.Multilayered surface modifications of porous AAOs wereachieved by repeating anodization and subsequent silanizationof the pore wall oxide with different silane molecules, such asAPTES, PFPTES, and N-triethoxysilylpropyl-O-poly(ethyleneoxide) urethane (PEGS) (Figure 56).442 The thickness ofindividual functional layers could be conveniently varied bycontrolling the anodization time. Deliberate combination ofsilane molecules with different chemical properties allowedselective membrane transports of small molecular compounds.Decoration of inorganic nanoparticles on the pore wall

surface of AAO often imparts new properties to porous AAO.This decoration can be facilitated by surface modification of thepore wall surface with an organic monolayer or polymer,443−447

but can also be achieved directly on the AAO.375,448,449 Ko andTsukruk443 immobilized AuNPs on pore walls modified withpoly(diallydimethylammonium chloride) (PDDA) polyelectro-lyte by passing cetyltrimethylammonium bromide (CTAB)-stabilized AuNP solution through the AAO membrane. On an

explosive molecule, 2,4-dinitrotoluene, the AuNP-decoratedAAOs with 3D nanochannel arrays exhibited Raman enhance-ment with a factor of about 105, as compared to that observedon a AuNP monolayer with identical surface densities ofAuNPs. The authors suggested that the AAO membrane itselfcould contribute extra enhancement of Raman signals, not onlyproviding additional sites for molecule adsorption, but alsoguiding the light by the vertically aligned nanopores. A similarline of experimental results was also reported for AgNP-decorated AAOs.449,450

Rubinstein et al.446 prepared nanoparticle-decorated nano-tubes (NPNTs) by passing a citrate-stabilized AuNP solutionthrough the pores of the AAO membrane, the surface of whichwas modified with APTMS (or APTES). The formation of AuNPNTs was assumed to involve the aggregation of surface-confined AuNPs accompanied by spontaneous room-temper-ature coalescence upon drying. With the same approach,multiwalled bimetallic Au/Pd NPNTs were also prepared.451 Inanother study, Wang et al.398 utilized the terminal amine groupsof surface-attached APTES to complex with Sn2+-sensitizer inelectroless depositions of Co, Ni, and Cu nanotubes withinpores of AAO template. The surface-bound Sn2+ reduced Pd2+

into metallic PdNPs, which acted as catalyst during theelectroless deposition of metals. Recently, Tanvir et al.452

immobilized covalently recombinant human cytochrome(CYP2E1) and glucose-6-phosphate dehydrogenase (G6PD)on the pore wall of APTES-functionalized AAO membranesthrough N-succinimidyl-3-maleimidoproprionate and glutaral-dehyde cross-linker, respectively. The authors showed that theimmobilized enzymes can retain 100% of the activity of freeenzyme. A miniaturized heterogeneous membrane reactor wasrealized by stacking two independently modified membranesand placing them in a fluidic device, in which the firstmembrane regenerates the cofactor NADPH from NADP+ andglucose-6-phosphate and the second one utilizes it for CYP2E1-based catalysis. Likewise, a cell adhesion peptide has also beengrafted on APTMS-derivatized porous AAOs to improveosteoblast adhesion.453 Swan et al.453 reacted the APTMS-functionalized AAO membranes with N-succinimidyl-3-mal-eimidopropionate and subsequently with a cellular adhesionpeptide, arginine-glycine-aspartic acid-cysteine (RGDC).

Figure 56. Schematic procedure for fabricating an AAO membrane with three different layers displaying distinct chemical functionalities. Adaptedwith permission from ref 442. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.




Polymer brushes can also be grafted on the pore wall surfacesof porous AAOs. Polymer-grafted porous AAOs exhibit animproved binding capacity, stability, selectivity, biocompati-bility, and lubricating characteristics as compared to bare

counterparts.143 Polymers grafted on AAOs can be alsofunctionalized with biologically important molecules, such asproteins or DNAs. Recently, Li et al.454 prepared thermo-responsive gating membranes with tuneable length and density

Figure 57. (Left) Derivatization of PHEMA with NTA-Ni2+ prior to protein adsorption. (Right) Binding of his-tagged protein to an NTA-Ni2+

derivatized PHEMA brush inside pores of AAO. Adapted with permission from ref 456. Copyright 2007 The American Chemical Society.

Figure 58. Schematic diagram showing (a) the formation of thin polymer films using layer-by-layer (LbL) deposition of polyelectrolytes, (b) ananofiltration membrane prepared by the deposition of a multilayer polyelectrolyte film on AAO, (c) a membrane for protein purification created bygrowing polymer brushes within pores of AAO, and (d) a catalytic membrane prepared by LbL deposition of polyelectrolytes and charged metalnanoparticles. Adapted with permission from ref 462. Copyright 2008 The American Chemical Society.





of poly(N-isopropylacrylamide) (PNIPAM). The authorsreacted an APTMS-functionalized AAO membrane with theinitiator 2-bromoisobutyryl bromide (BBIB), and then theresulting membrane with −Br groups was reacted with N-isopropylacrylamide (NIPAM) monomers during atom-transferradical polymerization (ATRP) to yield PNIPAM-grafted AAOmembrane (PNIPAM-g-AAO). The density of PNIPAMgrafted on the pore wall surfaces could be controlled bychanging the number density of −Br groups (i.e., ATRPinitiator). The themo-responsive characteristics of thePNIPAM-g-AAO membranes were investigated by trackingthe diffusional permeation of vitamin B12 at temperaturesbelow and above the lower critical solution temperature(LCST). The results indicated that thermo-responsivecharacteristics are heavily affected by both the length and thedensity of grafted PNIPAM chains in the pores of AAO, andthe effect of the length of grafted PNIPAM chains is moresignificant than that of the density. Purifications of proteinsbased on reusable metal affinity membranes have beendemonstrated by the Bruening group. The use of ATRP togrow poly(2-hydroxyethyl methacrylate) (PHEMA) brushes inthe pores of AAO, followed by functionalization of the PHEMAwith nitrilotriacetate-M2+ (NTA-M2+; M = Cu or Ni)complexes, yielded AAO membranes that adsorbed proteinsvia coordination of M2+ to his-tagged proteins (Figures 57,58c).455,456 In another study by the same group,457 two types ofultrathin (∼50 nm) polymer brushes, linear PHEMA and cross-linked poly(ethylene glycol dimethacrylate) (PEGDMA), onthe pore wall surfaces of AAOs were synthesized using theATRP method. Gas permeation studies showed that thePEGDMA-AAO membrane had a CO2/CH4 selectivity of ∼20and an O2/N2 selectivity of ∼2, whereas uncross-linkedPHEMA-AAO membrane showed very low selectivity.However, the CO2/CH4 selectivity of PHEMA improved to∼8 after esterification of −OH groups of PHEMA withpentadecafluorooctanoyl chloride. Further derivatization ofPHEMA-grafted AAO membranes with octyl, hexadecyl, orpentadecafluorooctyl side chains made the membranes hydro-phobic, allowing selective removal of volatile organiccompounds (VOCs) from water via pervaporation.458 Inanother study by Bruening’s group,459 poly(ethylene glycol)(PEG)-grafted AAO (PEG-g-AAO) membranes were preparedthrough ATRP of poly(ethylene glycol methyl ether meth-acrylate) with different lengths of PEG chain from initiator-modified porous AAO. The resulting PEG-g-AAO membranescontained a mixture of short and long PEG side chains, inwhich the shorter PEG chain (8−9 ethylene oxide units)prevented crystallization, while the longer side chains (23−24ethylene oxide units) allowed the membranes to maintain aCO2/H2 selectivity of 12 at room temperature.459 Shi et al.460

prepared metal affinity membranes with uniform diameter anddistribution of pores for protein separation and purification.Chitosan(CS)-AAO membranes were prepared by activatingthe hydroxylated AAO with 3-glycidoxypropyl trimethoxysilane(GPTMS), followed by CS grafting.460 Cu2+-attached affinitymembranes were then obtained by immobilizing Cu2+ on theCS-AAO, and were utilized for the separation and purificationof hemoglobin from red cell lysate. Song et al.461 have reportedpreparation of pH-responsive poly(acrylic acid)-grafted porousAAO membranes. Silica-AAO composite membrane was firstprepared by depositing silica onto the AAO membrane throughthe sol−gel method. The pH-responsive poly(arcrylic acid)(PAA) was grafted onto the silica-AAO composite membranes

by activating with GPTMS. The pH-valve effect was observedat pH between 3 and 5. Electrochemical impedance spectros-copy (EIS) showed that the membrane resistance increasedfrom 4.3 × 105 Ω cm2 at pH 2 to 1.3 × 106 Ω cm2 at pH 6.The layer-by-layer (LbL) deposition method, which involves

the alternate adsorption of differently charged polyelectrolytes,can be applied to the substrate surfaces that allow theadsorption of an initial polymer layer (Figure 58a).462−466

The resulting multilayer films can be further functionalized withnanoparticles or biomolecules. The most used polyelectrolytesare commercially available ones, such as poly(ethylenimine)(PEI), poly(allylamine hydrochloride) (PAH), poly-(diallydimethylammonium chloride) (PDADMAC), poly-(styrenesulfonate) (PSS), poly(vinylsulfate) (PVS), and poly-(acrylic acid) (PAA). LbL deposition of polyelectrolytemultilayer films on the surfaces of porous AAO membraneswas first demonstrated by the Bruening group (Figure 58b−d).467 LbL deposition of PAH and Cu2+-complexed PAA on thesurfaces of AAO membranes and subsequent controlledremoval of Cu2+ and deprotonation allowed control of thefixed charge density within PAA/PAH multilayer films (Figure58b). The resulting Cu2+-templated PAA/PAH-AAO mem-branes exhibited a 4-fold increase in Cl−/SO4

2− transportselectivity in comparison to Cu2+-free PAA/PAH-AAOmembranes deposited under similar condition.467 Cross-linkingof Cu2+-templated PAA/PAH-AAO membranes further in-creased Cl−/SO4

2− selectivity as high as 610. It was suggestedthat this selectivity might be due to both Donnan exclusion anddiffusivity differences among ions. Size-selective transport ofneutral solute through PSS/PAH has also been reported by thesame group.468 The number of bilayers combined with differentcompositions of multilayered films turned out to significantlyinfluence the rejection, flux, and selectivity of charged solutesthrough the membranes.468−470 In addition, depositionconditions such as pH, ionic strength of the polyelectrolytesolutions, and the charge of the outermost polyelectrolyte layerwere also found to influence the molecular separation ofcations.470 Polyelectrolyte films composed of PSS/PAH orPSS/PDADMAC+PSS/PAH films were reported to be effectivein the selective removal of Mg2+ from aqueous solutionscontaining NaCl and MgCl2 salts (i.e., water softening), whilepure PSS/PDADMAC films and commercial NF270 mem-branes showed only relatively low rejections of Mg2+.470 ForPAH-terminated multilayer films, Mg2+ rejection and Na+/Mg2+

selectivity increased with the increasing charge near the surfaceof the polyelectrolyte multilayer films, by increasing the ionicstrength of the PAH deposition solution. LbL film-basedcatalytic AAO membranes were also demonstrated byBruening’s group.444 By utilizing the charge on the LbL films,citrate-stabilized AuNPs were immobilized within the pores ofAAO. Au-immobilized LbL films within the pores of AAOmembranes exhibited a similar rate constant for Au-catalyzed 4-nitrophenol reduction in solution and in membranes (Figure58d). Covalent immobilization of antibodies on LbL films (i.e.,[PAA/PAH]3PAH) in the pores of AAO membranes has alsobeen realized via carbodiimide coupling between −COOHgroups of PAA amine groups of antibodies.471 Detection limitsin the analysis of Cy5-labeled IgG were reported to be 0.02 ngmL−1 because of the high surface area of the porous AAO.471 Inaddition, resistance against nonspecific protein adsorption forthese LbL film-AAO composites has also been reported.Combination of the LbL technique and the AAO templatingapproach has been demonstrated its versatility in fabricating



composite tubular nanostructures with various functionalproperties. Non-electrostatic interactions, such as hydrogen orcovalent bonding and hybridization, have been utilized toassemble quantum dots (QDs), nanoparticles, polymers, andbiologically relevant molecules or materials into functionalnanotubes.445,462,472−481

Lipid bilayers are novel mimics of biological cellmembranes.143 Free-standing bilayer lipid membranes recon-stituted with ion-channel proteins provide an excellent systemnot only for drug screening,482,483 but also for designing highlysensitive biosensors.484,485 Conventional solid supportedmembranes (SSMs) methods utilize gold-thiol, silanization ofOH-groups, and electrostatic interactions to immobilize lipidbilayers on solid supports.486 The immobilized lipid bilayers

exhibit long-term stability. However, the close proximity(typically 0.2−2 nm) of a lipid bilayer to the surface of solidsupport limits lateral lipid mobility, insertions of largetransmembrane proteins to the lipid bilayer, or the generationof electrochemical gradient across lipid bilayer membranes,which is required for selective transport of specific ions, forinstance, in ion pumps and ligand- or voltage-gated ionchannels.486 Several approaches to decouple the lipid bilayermembrane from its underlying support have been developed toincrease the lateral mobility. These include utilization of lipidswith long hydrophilic spacers,487 water or polymer cushionsbetween lipid membrane and solid support,488−491 and surfacepatterns with different thiol-components.492 On the other hand,free-standing bilayer lipid membranes (also known as black

Figure 59. (a) Schematic illustration of a lipid bilayer formed by 1,2-diphytanoyl-sn-glycero-3-phosphocholin (DPhPC) onto a self-assembled 1,2-diphytano-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE) monolayer chemisorbed on a gold-coated surface of a porous AAO substrate. (b)Impedance analysis of a lipid bilayer bathed in 0.1 M Na2SO4 recorded in a frequency range of ω = 10−1−106 Hz before (■) and after (○)gramicidin addition. The membrane resistance was dropped by >3 × 106 Ω from Rm = 8.73 × 106 Ω to Rm = 5.45 × 106 Ω. The solid line is the resultof a fitting routine using the equivalent circuit shown in inset. Adapted with permission from ref 496. Copyright 2004 The Biophysical Society.

Figure 60. Separation of attoliter-sized compartments employing pore spanning lipid bilayers. (A) composite z-stack image (total z-distance: 15 μm)of pore-spanning lipid bilayer preventing avidin entrance into the underlying pores (black area). Main image: Lipid membrane is located at theinterface between the AAO and the bulk solution, showing both red fluorescence from the membrane and green avidin fluorescence in the overlayer.The total image size is 67 μm × 67 μm. Top and right side images are line profiles along the z-direction, manifesting that the membrane prevents thefluorescent labeled proteins from entering the pores. (B) A z-stack image of the membrane encapsulating pyranine dye molecules. The image size is34 μm × 34 μm. Adapted with permission from ref 500. Copyright 2011 The American Chemical Society.





lipid membranes, BLMs) have been formed across micrometer-sized apertures in an attempt to eliminate the steric congestionin SSMs.485 Yet the fragility of the bilayer lipid membranes onmicrometer-sized apertures often prevents their practicalapplications. The mechanical stability of BLMs can be increasedby reducing the aperture size.493,494 However, the increasedstability comes at the expense of reduced effective membranearea to which proteins can be incorporated.Pioneering works by Steinem et al.486 have led to the

development of methods for suspending lipid bilayers on thepores of porous AAO, bridging the technical gap betweenconventional SSMs and BLMs. First, the authors selectivelyfunctionalized the top surface of porous AAO (Dp = 60 ± 10nm) by chemisorbing 3-mercaptopropionic acid (MPA) onto10-nm-thick Au layer that was pre-deposited onto the AAO.Next, the MPA-modified surface was negatively charged bytreating with 10 mM tris(hydroxymethyl)aminomethane)solution (pH 8.0). Finally, a pore-spanning lipid bilayer wasobtained by adsorbing positively charged vesicles of N,N-dimethyl-N,N-dioctadecylammonium bromide (DODAB) ontothe negatively charged MPA monolayer. Pores of AAO spannedby bilayer lipid membranes are tunable in a nanometer range.Moreover, because the pore channels of AAO are separated bya semipermeable lipid membrane from the outer solution, theycan serve as nanocontainers for bio-reaction and for establish-ment of electrochemical gradients.495 Extending their previousworks, Steinem et al.496−499 showed that pore-spanning lipidbilayers on porous AAO substrates are electrically insulating,and also demonstrated that the ion-channel proteins recon-stituted in bilayer membranes are fully functional (Figure 59).More recently, the same group of authors employed pore-spanning lipid bilayers formed by spreading a giant unilamellarvesicle (GUV) on porous AAO substrates to create attoliter-sized compartments, which were used not only for entrapmentbut also for exclusion of materials. By taking advantage of theoptically transparent nature of porous AAO substrates,fluorescent molecules inside the compartments (i.e., AAOpores) as well as the lipid membranes on top of the pores couldbe visualized by using confocal laser scanning fluorescencemicroscopy (Figure 60).500 This technique will becomepowerful in biosensing applications, if it is combined with therecently developed label-free nanoporous optical waveguidesensor by Hotta et al., in which changes in reflection spectra ofthe AAO/Al multilayer films were measured in theKretschmann configuration, similarly to the conventionalsurface plasmon resonance (SPR) sensor.501

Smirmov and Poulektov502 demonstrated that phospholipidbilayers can self-assemble to form nanotubes within the poresof AAO membrane by exposing one side of membrane to anaqueous dispersion of phospholipid. The formation of lipidnanotubes was been confirmed by spin-labeling electronparamagnetic resonance (EPR),502 solid-state NMR spectros-copy,503 and fluorescence microscopy.504 In another study,Deme and Marchal505 prepared polymer-cushioned lipidbilayers within the pores of AAO. Their preparation approachconsisted of direct fusion of lipid vesicles into the poly(ethyleneglycol) (PEG)-derivatized pores of AAO.

9.5. Template Wetting

Pores of AAO template can be utilized as nanoreactors for thesynthesis of a variety of nanotubular materials. The template-based chemical syntheses of polymer nanotubes, pioneered byC. R. Martin, typically involve either the immersion of a porous

template into a solution containing the desired monomers andthe polymerization initiator or the electropolymerization withinthe cylindrical nanopores.338,406,506−509 Electropolymerizationhas been utilized to synthesize conducting polymer nanotubeswithin the pores of AAO or track-etched polycarbonatemembranes. Electrically insulating polymer nanotubes havealso been prepared chemically. For example, polyacrylonitril(PAN) nanotubes with controlled inner diameters weresynthesized by dipping a porous template into a solutioncontaining acrylonitrile and a polymerization reagent, in whichpolymerization time determined the inner diameter ofnanotubes.510

With an alternative approach, Steinhart et al.511 developed anarguably simple, yet highly versatile technique for thefabrication of functional polymer nanotubes. The method isbased on the spontaneous spreading (or wetting) of a polymermelt or its solution on a surface with high surface energy toform the so-called “precursor film”.512−514 Surface wettingbehavior of a droplet of a liquid is described by the spreadingparameter S:513

γ γ γ= − −S sg sl lg (75)

where γsg, γsl, and γlg, respectively, are the solid−gas, solid−liquid, and liquid−gas interfacial tensions. If S is negative, aliquid drop on the substrate adopts an equilibrium shapecorresponding to a finite contact angle θe defined by Young’scondition cos θe = (γsl − γsg)/γlg. If S is positive, spontaneousspreading of the liquid occurs because the adhesion forcesbetween the liquid and the solid substrate dominate thecohesive forces within the wetting liquid, and the equilibriumsituation corresponds to complete coverage of the substrate bythe precursor film with a thickness ranging from severalangstroms to tens of nanometers (i.e., θe = 0).513,515

Accordingly, direct contact of porous inorganic templates(i.e., high-surface-energy materials) with polymer melts orsolutions (low-surface-energy materials) results in the immedi-ate wetting of pore wall surfaces with thin precursor film(Figure 61a). Steinhart et al.515 suggested that pore wall wettingis kinetically stable (but thermodynamically unstable), when thestrong adhesive forces between the liquid and solid areneutralized upon complete surface wetting, due to the finitesurface area of an individual pore of the template. To attain athermodynamically stable state (i.e., complete pore filling), thecohesive force should dominate the viscous forces of thewetting liquid. Thus, pore wall wetting and complete fillingoccur on different time scales; the latter, if it would happen atall, will require several months or even several years oftime.515,516 Therefore, polymer nanotubes can be obtained bysolidifying the wetting liquid before complete pore filling(Figure 61b−d). According to Steinhart et al.516 complete porefilling occurs when the pore diameter of the porous template issmaller than the thickness of wetting layer (10−30 nm). Thewetting layer thickness may depend on the nature of polymerand the surface state of wetting solid. It was reported thatinjection of polystyrene (PS) melt into the pores of AAO (Dp =40 nm) results in arrays of PS nanowires, rather thannanotubes.517

On the basis of the experimental observations ofspontaneous surface spreading of highly viscous melts ofpolymers with ultrahigh molecular weights (typically, 106 and107 g mol−1), Steinhart et al.516 suggested that wetting ofpolymer melt or solution would occur through surfacediffusion. Therefore, individual polymer chains should align



on the wetting surface. The cylindrical geometry of pores inAAO imposes a spatial confinement with respect to themovement of a polymer chain, in which the length of the porechannel can be regarded as infinite, while the circumference andthickness of the wetting polymer are finite with a curvature.Poly(vinylidene difluoride) (PVDF) nanotubes prepared bymelt-wetting of porous AAO (Dp = 400 nm) exhibited acurvature-directed crystallization to form α-PVDF phase, inwhich the crystallographic b-axis of α-PVDF is parallel to thelong axis of the nanotubes, the only direction to minimizecurvature constriction.518 The effects of geometric confinementand the pore wall on the mesophase formation of liquidcrystalline materials have also been reported.519,520

Template wetting is versatile in terms of the facile variationof tube wall materials. Almost all polymer solutions with a lowsurface energy can be used to synthesize functional orcomposite nanotubes. Preparations of polymer nanotubeswith high technical importance (e.g., polyether ether ketone(PEEK) or polytetrafluoroethylene (PTFE)), which are verydifficult or practically impossible to process by conventionalmethods (e.g., extrusion or injection molding of polymer), havebeen demonstrated by wetting of polymer melt impregnationinto porous templates.516 Multicomponent composite nano-tubes have also been utilized to prepare inorganic nanotubes. Inthis case, the polymers are mixed with inorganic precursors andact as a carrier in the wetting process. Chemical transformationof the inorganic precursors within the composite nanotubewalls yields inorganic nanotubes. For example, Pd nanotubeswere prepared by the wetting of porous AAO templates with

mixture solutions containing Pd(OAc)2 and poly(D,L-lactide)(PDLLA) at 350 °C, in which the PDLLA acts as a reducingagent for PdII at 160 °C and pyrolyzes at a higher 350 °C.521

The method was extended to prepare Pt nanotubes fromPt(acac)2/PDLL composite nanotubes.522

9.6. Mask Techniques

Two-dimensional (2D) periodic arrays of nanostructures (e.g.,semiconductors, metal nanoparticles, or complex oxides arrays)have attracted tremendous research attention because of theirmany unique properties and promising applications inelectronic/optoelectronics, bio-sensing, and high-density datastorage media. In 1996, Masuda and Satoh6 reported for thefirst time the fabrication of arrays of gold nanodots on siliconsubstrate by using ultrathin AAO as a stencil mask fordepositing gold. Since then, thin AAO-based surface patterninghas been popularized. The interested reader is referred toreview articles on surface nanopatterning using ultrathin AAOmasks given in refs 523 and 524. AAO-based surface patterningmethods are highly attractive in terms of throughput,accessibility, cost, and compatibility with high-temperaturenanofabrication processes, in comparison to other surfacenanopatterning techniques (for example, electron-beam directwriting (EBDW), focused ion beam (FIB) milling, nano-imprinting lithography (NIL), and scanning probe microscope(SPM)-based writing).Ultrathin AAO membranes can be transferred onto desired

substrates by supporting AAO membranes with appropriatepolymers (typically, poly(methyl methacrylate) (PMMA) orPS).6,523 In this method, a thin polymer layer is coated on oneface of the ultrathin porous AAO film. Afterward, the aluminumsubstrate and the barrier layer at the bottom of the pores areremoved. Next, the resulting AAO/polymer composite film istransferred onto a substrate of choice. Finally, the polymer isremoved by immersing the sample into an appropriate solventor by performing oxygen plasma treatment. However, thismethod often suffers from the difficulty in transferring a large-scale ultrathin AAO membrane onto a substrate withoutintroducing structural damages caused by folding, cracking, orripping of the ultrathin AAO. Recently, Meng et al.525

demonstrated the successful transfer of wafer-scale ultrathinAAO membranes (thickness = 200 nm) onto hydrophilizedsubstrates. The method involved selective etching of theunderlying aluminum of a two-step anodized AAO film to leavea rigid aluminum frame surrounding the AAO film, followed bytransfer of the resulting aluminum-frame-supported AAO filmonto a desired substrate (Figure 62). The aluminum framestabilized the ultrathin AAO film during transfer. Thehydrophilic surface of the substrate enabled a conformalcontact between the ultrathin AAO film and substrate viadropping water on the substrate. After removal of thesurrounding aluminum frame, the barrier oxide layer could beremoved by conducting argon milling to obtain a through-holeAAO membrane.A variety of materials have been fabricated into ordered

arrays of nanostructures on substrates of choice, includingarrays of semiconductor,526−535 metal,536−539 and oxidenanodots540−546 or arrays of nanoholes,547−562 nanorings,563

nanopillars,548 and nanowires.525,564 Figure 63a−d schemati-cally shows the general procedures for the fabrication of 2Dextended arrays of (a) nanoholes, (b) nanodots, (c) nanopillars,and (d) nanowires by using ultrathin AAO membranes asmasks. Ordered arrays of nanoholes have been fabricated on

Figure 61. (a) Wetting of porous templates with polymer melts orsolutions: (i) The fluid comes into contact with the template. (ii)Within a second, the pore walls are covered with a thin film of theliquid. By freezing this stage, nanotubes are preserved. (iii) A completefilling of the pore space was not observed. (b−e) SEM images ofpolymer nanotubes obtained by melt-wetting: (b) polystyrene (PS)nanotubes embedded in porous AAO, (c) PS nanotubes after removalof AAO template, (d) array of polytetrafluoroethylene (PTFE)nanobues, (e) poly(methyl methacrylate) (PMMA) nanotubes. Panel(a) was reproduced with permission from ref 515. Copyright 2003,Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Panels (b−e)reprinted with permission from ref 511. Copyright 2002 AAAS.




various substrates by conventional dry processes, such asplasma etching, reactive ion etching (RIE), and ion milling(Figure 63a). Because of high nanohole regularity and density,the resulting nanostructures exhibited many interesting proper-ties that can be exploited in the development of effectiveantireflection structures,555,556 high performance electrodes orcapacitors,559,560 waveguides and photonic crystals,561 andoptical or lasing devices.549,562 On the other hand, arrays ofnanodots can simply be obtained by depositing a desiredmaterial through an ultrathin AAO membrane (Figure 63b), inwhich a 2D array pattern, and size and interdistance of thenanodots are defined by those of the ultrathin AAO mask. Mostphysical vapor deposition (PVD) methods can be employed tocreate surface nanostructures, including e-beam evapora-tion,6,533,534,539,548 thermal evaporation,527,528,540 sputter depo-sition,535 pulsed laser deposition (PLD),541−546 and molecular-beam epitaxy (MBE).526,530−532 The generated nanodot arrayshave been used as model systems for investigating thestructure−property relations of materials (e.g., magnetism,plasmonics, photoluminescence, cathode luminescence, ferroe-lectricity) at nanometer-length scales. Arrays of nanodots have

also been used either as etching masks in dry etching processesto generate ordered arrays of nanopillars (Figure 63c)548,553 oras catalysts in the chemical vapor deposition (CVD) growth ofarrays of carbon nanotubes (CNTs),565 the MBE growth ofGaAs nanowires,566 and the vapor−liquid−solid (VLS) growthof Si,564 MgO,525 and ZnO567 nanowires (Figure 63d).Thick porous AAO membranes have been utilized as

replication masters for preparing metallic stencil masks. Leeet al.568 demonstrated that Au nanotube membranes replicatedfrom porous AAO can be utilized as deposition masks forfabricating 2D periodic arrays of catalyst nanodots for site-specific VLS growth of semiconductor nanowires (see Figure64)569,570 or ordered arrays of ferroelectric nanostruc-tures.568,571,572 Their fabrication of Au nanotube membranesinvolved the sputter deposition of thin Au film on the pore wallsurfaces of top part of AAO membrane, as well as on the topmembrane surface, followed by electrochemical deposition ofAu, to further thicken the metal layer. Subsequent removal ofthe AAO membrane by floating the sample on 30 wt % H3PO4solution resulted in a free-standing Au nanotube membrane.Finally, Au nanotube membrane was transferred onto desiredsubstrate. The replicated Au nanotube membranes exhibited ahigh degree of flexibility, allowing conformal contact even withcurved surfaces. In the process, the inner diameter of the Aunanotubes can be easily tuned by controlling the electro-deposition time. Therefore, nanodots with sizes smaller thanthe pore diameter of the starting AAO could be fabricated.568

With a similar approach, replications of ultrathin (thickness ≈20 nm) metal meshes with arrays of ordered nanoholes havealso been reported. The replicated metal meshes have beenutilized not only as stencil masks for the patterning of polymersubstrates,573 but also as catalysts for the metal-assistedchemical etching of silicon wafers to prepare arrays of siliconnanowires with controlled size, surface morphology, andcrystallographic orientations (Figure 65).573−578

9.7. Chemical Vapor Deposition (CVD)

A major challenge facing the chemical vapor deposition (CVD)of materials into the pores of AAO involves achieving a uniformdeposition of materials on the entire surface of the pore walls. Afast rate of deposition may cause blockage of pores before theprecursor chemical vapor traverses pores of high aspect

Figure 62. (a) Schematic procedure for the fabrication of ultrathinAAO membrane on a substrate. Photographs of 200-nm-thick AAOmembrane (b) supported by Al frame and (c) transferred onto a 3-in.silicon wafer. Reproduced with permission from ref 525. Copyright2012 The Royal Society of Chemistry.

Figure 63. Schematic procedure for the fabrication of 2D extended arrays of (a) nanoholes, (b) nanodots, (c) nanopillars, and (d) nanowires byusing ultrathin AAO membranes as mask.





ratio.579,580 The uniform deposition of carbon was first reportedby Kyotani et al.,581 who performed the thermal decompositionof propylene at 800 °C. Pyrolytic carbon deposition frompropylene resulted in carbon tubes within the pores of AAO, inwhich the wall thickness of the carbon tubes was dependent onthe deposition period. The carbon tubes obtained by themethod exhibited low crystalline quality. In another study, Chuet al.582 employed the pyrolytic carbon deposition method forthe preparation of carbon tubes, in which porous AAOmembrane was exposed to either ethylene or pyrene gasstream at 900 °C for 10 min. They showed that thetemperature needed for CVD of carbonaceous materials canbe lowered to 500 °C by decorating pore wall surfaces of AAOtemplate with Ni-catalyst nanoparticles.582 Apart from loweringthe CVD temperature, it was also found that the metalliccatalysts improve the crystalline quality of the resulting carbonnanostructures. Multiwalled carbon nanotubes (CNTs) embed-ded in porous AAO have also been obtained by CVD of agaseous precursor (typically, acetylene) in the temperaturerange of 550−650 °C in the presence of electrodeposited Cocatalyst at the bottom of AAO pores (Figure 66).287,583−587

The ordered arrays of CNTs have been utilized as field

emitters,585 electrodes for supercapacitors,588 and platforms forintracellular delivery.589

Porous AAO has also been utilized as a structure-guidingtemplate in CVD growth of nanowires. It has been wellestablished for the VLS growth of SiNWs that the nanowiresgrow preferentially along the ⟨111⟩, ⟨112⟩, or ⟨110⟩ directionsdepending on the diameter.590,591 At the same time, verticallyaligned [100] SiNWs on Si(100) substrate are highly desiredfor most practical applications in current complementary-metal-oxide-semiconductor (CMOS) technology. Moreover, tightcontrol over the diameter and spacing of SiNWs in conven-tional VLS growth process is difficult to achieve withoutemploying electron beam lithography to define the size andlocation of the catalyst nanoparticles. Shimizu et al.592

demonstrated homoepitaxial growth of SiNWs on Si(100)substrate by utilizing porous AAO as an orientation guidingtemplate. The method entailed the combination of electrolessdeposition of Au catalyst into AAO nanopores and VLS growthof Si (Figure 67). This approach has recently been extended byGorisse et al.593 to prepare extended arrays of vertically aligned[100] SiNWs with tightly controlled diameters and separations.Fabrications of other semiconductor nanostructures by CVD

techniques have also been reported. Cheng et al.594 obtainedsingle crystalline GaN NWs in porous AAO templates byreacting Ga2O3 vapor with NH3 gas at 1000 °C. The authorssuggested that the defects in the pore walls act as GaNnucleation centers under a saturated reaction atmosphere, andthe multiplication of screw dislocations is responsible for theunidirectional growth and formation of GaN NWs. Later, thismethod was modified by Zhang et al.,595 who employed indiumnanoparticles electrodeposited in AAO pores as catalyst, toprepare ordered arrays of GaN nanowires. VLS growth ofhexagonal wurtzite GaN nanowires with very miscible dropletsof In−Ga−N was also suggested. Jung et al.596 reportedfabrication of GaN nanotubes by metallorganic chemical vapordeposition (MOCVD) using trimethylgallium (Ga(CH3)3) andammonia (NH3) as precursors. Because of the randomnucleation of GaN nanoparticles on the surfaces of the innerpore wall of AAO, the fabricated GaN nanotubes consisted ofGaN nanoparticles with sizes of 15−30 nm. In another study, Liet al.597 reported the controlled growth of Ge nanowires ornanotubes inside a porous AAO template by low-temperatureCVD assisted by an electrodeposited metal nanorod catalyst.Depending on the types of catalytic metals (Au, Ni, Cu, Co)and germane (GeH4) concentration during CVD, Ge nano-wires or nanotubes could be synthesized at 310−370 °C. Theauthors also demonstrated the synthesis of multi-segmentednanowire junctions of Au1−xGex and Ge inside the nano-channels of porous AAO template by Au nanorod-catalyzedVLS growth process from GeH4 precursor.598 PolycrystallineCdS,599 In2O3,

600 and ZnS601 nanotube arrays have also beensynthesized within the pores of AAO membranes by low-temperature CVD methods using, respectively, cadmiumbis(diethyldithiocarbamate) [Cd(S2CNEt2)2] at 400 °C,indium acetlacetonate [In(acac)3] at 350 °C, and zincbis(diethyldithiocarbamate) [Zn(S2(NEt2)2] at 200 °C. Metal-catalyzed VLS growths of single crystalline CdS602 or ZnO603

nanowires within the pores of AAO membrane have recentlybeen reported.

9.8. Atomic Layer Deposition (ALD)

Atomic layer deposition (ALD) has drawn much attention as aversatile methodology for thin film deposition due to conformal

Figure 64. (a) SEM image of Au catalyst nanodots deposited using anAu nanotube membrane as a shadow mask. (b) Top-view SEM imageof VLS-grown ZnO nanorod arrays using Au catalyst nanodots shownin (a). Reproduced with permission from ref 569. Copyright 2006Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.




and uniform deposition of thin films on substrates withcomplicated 3D morphology.143,604−609 In ALD, a thin film of adesired material is grown in a layer-by-layer manner byrepeating a unit cycle composed of four consecutive steps: (i)precursor exposure, (ii) purging (or evacuation) of unreactedexcess gas molecules and reaction byproducts, (iii) reactantexposure, and (iv) subsequent purging (Figure 68). Because ofthe alternating exposure of gaseous precursor and reactant atthe separate steps, the film growth process is self-limiting in its

nature, avoiding CVD-like film growth. By virtue of such a self-terminating film growth process, the conformal and uniformdeposition of film with atomic scale precision is achievable overa large area.Ott et al.610 first performed ALD of Al2O3 on porous AAO

membrane to narrow the pores, and demonstrated the gasseparation capability of the pore-modulated AAO membrane.In another study, Chen et al.611 reported a strategy forfabricating a single-molecule (i.e., DNA molecule) sensorthrough surface modification of the pore walls of AAO andprecise control of the pore diameter at the single angstromscale. Since then, ALD on porous AAO has been intensivelyutilized as a facile method not only for surface modification ofoxide nanopores,612 but also for fabricating 1D functionalnanostructures, such as nanowires,613 nanorods,614,615 andnanotubes.616−625

Single-614,616,624−628 or multi-layered617,621 nanotubes com-posed of different materials can conveniently be fabricatedthrough a deliberate choice of precursor molecules during ALDon the pore wall surfaces of AAO. The ability to preciselycontrol film thickness is beneficial in tuning the thickness oftube walls and the gap between each tube. Fabrications ofmulti-segmented nanotubes have also been demonstratedrecently through the combination of 3D site-selective ALDand anodization of aluminum. Bae et al.618 have shown thatcylindrical nanopores of AAO can be continuously anodizedupon coating with thin organic and/or inorganic layers such asoctadecyltrichlorosilane (OTS)-self-assembled monolayers andALD-grown TiO2, ZnO, and ZrO2. Nanowires or nanorods canalso be obtained through the so-called “super-filling” of thepores of AAO by virtue of the superior capability of ALDtechniques for conformal film deposition over other conven-tional thin film deposition techniques.613,615 3D devicearchitectures can be realized by the ALD of appropriatematerials within pores of AAO, and the approach may bebeneficial to energy storage and energy conversion systems

Figure 65. Metal-assisted chemical etching of (100)-oriented silicon wafer by using ultrathin metal mesh for the preparation of SiNWs withcontrolled axial orientation and morphology. Reproduced with permission from refs 575,576. Copyright 2011 The American Chemical Society.

Figure 66. (a) Schematic procedure for the fabrication of carbonnanotubes (CNTs). (b) SEM image of the ordered arrays of CNTfabricated by the method in (a). Reprinted with permission from ref583. Copyright 1999 AIP Publishing LLC.





such as Li batteries629−631 and solar cells.407,626 Banergee etal.632 have recently demonstrated fabrication of densely packed3D nanocapacitors by performing the sequential ALD of a TiNbottom electrode (BE)/dielectric Al2O3/TiN top electrode(TE) within the pores of AAO (Figure 69). Because of the largeinterfacial area of TiN/Al2O3 inside the open volume of porousAAO, their 3D nanocapacitors exhibited considerably highercapacitance as compared to equivalent 2D planar capacitors:∼100 μF cm−2 for 3D nanocapacitors formed in 10-μm-thickAAO.ALD-assisted modification of AAO surfaces has also been

considered a promising approach to molecule and gassensing.619,633,634 Two different kinds of sensor configurations

have been independently reported by Comstock et al.619 andLee et al.635 Comstock et al.619 reported a fabrication strategyfor a nanotubular glucose sensor by using ALD-grown Pt thinfilms within the pores of AAO. Lee et al.635 realized fastresponse H2 sensors by transferring TiO2-coated AAO onto aglass substrate followed by selective removal of the AAO.Nanotube arrays synthesized by a similar approach have beenused as an ultraviolet (UV) sensor,636 membranes for gas andmolecule separations,612,637,638 catalytic membranes,639,640 andphotocatalysts.641,642

10. CLOSING REMARKS AND OUTLOOK

The anodization of aluminum and the resulting porous anodicaluminum oxide (AAO) are currently the subject of extensiveresearch with nearly a thousand papers published annually inthe field. In this Review, we have presented fundamentalelectrochemical processes associated with the porous-typeanodization of aluminum, the recent progress on aluminumanodization for the fabrication of self-ordered porous AAO, andapplications of porous AAO to templated synthesis offunctional nanostructures for current nanotechnology research.Over the past near 100 years, many electrochemical aspects

of aluminum anodization have been disclosed. Ionic migrationswithin the oxide have been explained in the framework of high-field conduction theory. The type of acid electrolytes andanodizing potential have direct relevance not only on thestructural parameters (e.g., pore diameter, interpore distance,and the barrier layer thickness) of the porous AAO, but also onthe incorporation of anionic species and their distribution inthe anodic oxide. Empirical relations between the parametersdefining the geometric structure of porous AAO and theanodizing potential have also been established. Pore formation

Figure 67. (a−d) Schematic cross-sections, showing the growth of SiNWs in porous AAO using Au catalyst: (a) after pre-annealing at 900 °C, (b)after HF etching to remove SiO2, (c) electroless deposition of Au catalyst, and (d) VLS growth of SiNWs. (e) Cross-sectional TEM image of a SiNWin AAO template along the [011] zone axis. The dashed line shows the interface between the original Si(100) substrate and SiNW. The magnifiedTEM image of the area marked with a white rectangle is shown in (f), which demonstrates homoepitaxial growth of SiNW. Reprinted withpermission from ref 592. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 68. Schematic illustration of a typical atomic layer deposition(ALD) process of Al2O3 ALD from trimethyl aluminum (TMA) andwater (H2O).





has long been attributed to the field-assisted dissolution ofanodic alumina. This dissolution-based pore formation modelappears to be operative for the initial stage of pore formation.However, for steady-state pore formation, there is a growingnumber of results from recent experiments and theoreticalmodelings, disproving the field-assisted oxide dissolutionmodel. These include, for instance, the direct ejection of Al3+

ions from the metal/oxide interface through the barrier oxide tothe anodizing electrolyte (from 18O isotope studies) and theviscous flow of oxide materials from the pore base toward thecell boundary (from W-tracer studies and finite-elementalanalysis). However, more systematic experimental investiga-tions using various electrolytes/tracer elements and theoreticalmodelings considering the mobility of tracer cations arerequired to fully verify the oxide flow model. Furthermore,recent studies have revealed that the pore initiation and self-organized formation of porous AAO need to be understood interms of mechano-electrochemistry. Studies have indicated thatstresses and their gradients within anodic oxide have profoundimplications on ionic migration within the anodic oxide, onmorphological instability associated with pore initiation at theearly stage of anodization, breakdown of growing oxide, as wellas the viscous flow of oxide materials and self-organization ofpores during the steady-state anodic oxidation. Although someprogress in this direction has recently been made, satisfactorycorrelation between stresses and all experimental observationshas yet to be achieved. Internal stresses of growing anodic oxideneed to be systematically evaluated in situ under controlledanodization conditions. In addition, the effect of externalstresses on the anodization kinetics as well as on the self-ordering behavior of pores needs to be further investigated.Studies on these issues would do much to advance ourknowledge on the mechanism governing the self-organizedformation of pores during anodization of aluminum. Thoroughunderstanding of the mechano-electrochemical processes mayprovide a solid foundation for exploring new electrolytesystems and novel porous architectures as well as fordeveloping ordered porous structures from other valve-metals,such as Mg, Zr, Nb, Sn, Hf, Ta, W, Bi, etc.Aluminum anodizing under properly controlled conditions

produces highly ordered porous AAOs. On the basis of recentdevelopments of various anodization methods (e.g., mild, hard,

pulse, cyclic, and guided anodization), the diameter, density,

and aspect ratio of pores, and even internal pore structures can

be tightly controlled by appropriate selection of the anodizing

conditions. These capabilities may offer large degrees of

freedom for the templated syntheses of low-dimensional

functional nanostructures, and also in the development of

AAO-based advanced devices, allowing simple and cost-

effective non-lithographic fabrication of extended arrays of

structurally well-defined and identical nanostructures. Indeed,

over the last two decades, we have witnessed fascinating

applications of porous AAO membranes as templates for the

synthesis of various nanowires and nanotubes, as masks for

extended arrays of structurally well-defined surface nano-

patterns, and also as platform materials for (bio) molecule

separations, catalysts, drug delivery, photonic, and energy

storage devices. All of these applications have been achieved

through deliberate control over the dimensions of pores and

the thickness of AAO membranes, and also through the

appropriate engineering of surface properties of porous AAO. It

is very likely that evolving experimental techniques for

engineering of the internal pore structures and for programmed

functionalizing of the surface properties of porous AAO will

further expand the application field. Accordingly, the future

prospects for nanotechnology applications of porous AAO are

very promising.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Notes

The authors declare no competing financial interest.

Figure 69. Arrays of supercapacitors fabricated by sequential ALD of TiN, Al2O3, and TiN within pores of AAO: (a) cross-sectional SEM images ofthe capacitors at the pore mouth (upper panel) and at the pore bottom (lower panel) and (b) schematic drawing of a unit cell of the capacitor.Reprinted with permission from ref 632. Copyright 2009 Macmillan Publishers Ltd.: Nature Nanotechnology.



mailto:[email protected]


Biographies

Woo Lee is a principal researcher at Korea Research Institute ofStandards and Science (KRISS) and a professor of the Department ofNano Science, University of Science and Technology (UST), Korea.He received his Ph.D. from Seoul National University (2003). Heworked with the late Prof. Ulrich Gosele as a postdoctoral researchfellow and later a group leader at Max-Planck-Institut furMikrostrukturphysik in Halle, Germany, until he joined KRISS in2008. He brought a renewed attention to the academic research onpulsed anodization as well as hard anodization of aluminum byestablishing new self-ordering regimes and also by implementing themfor the structural engineering of porous AAO. His research interestsfocus on the anodization of aluminum and template-based synthesis oflow-dimensional functional nanostructures for memory and energyharvesting applications.

Sang-Joon Park did his graduate work in Materials Science &Engineering at the Pohang University of Science and Technology(POSTECH) under the co-supervision of Woo Lee and Prof. SunggiBaik. He earned his Ph.D. with a main focus on resistive switching ofcapacitors with TiO2 active layer, grown by atomic layer deposition(ALD) in 2014. He then joined Woo Lee’s group at KRISS as apostdoctoral researcher. His research interest is atomic layerdeposition (ALD) for functional nanostructures.

ACKNOWLEDGMENTS

Support from KRISS project “Anodization Research Laboratory(KRISS-2013-13011082)” and in part from the “Future-basedTechnology Development Program (Nano Fields)” through theNational Research Foundation of Korea (NRF) funded by theMinistry of Science, ICT, and Future Planning (Grant no.2010-0029332) is greatly acknowledged. This Review isdedicated to the memory of the late Professor Ulrich Gosele,

the former director of Max-Planck-Institut fur Mikrostruktur-physik in Halle, Germany.

ABBREVIATIONSAAO anodic aluminum oxideU anodizing potentialj current densityE electric fieldE* threshold electric fieldt+ cation transport numbert− anion transport numberηj current efficiency (i.e., oxide formation

efficiency)PBR Pilling−Bedworth ratioΔU potential droptb barrier layer thicknessν hopping attempt frequencyρ density of mobile charge in C cm−3

a hopping interdistanceW hopping activation energy at zero fieldα parameter describing the asymmetry of the

activation barrier at non-zero fieldz valence of the mobile ionsF Faraday’s constantΦm/o potential drop at the metal/oxide interfaceΦo/e potential drop at the oxide/electrolyte

interfaceRBS Rutherford backscattering spectrometryTEM transmission electron microscopyAR anodizing ratio (in nm V−1)P porosity of porous AAOμi ionic chemical potentialJi flux of ion iCi concentration of ion iuio pre-exponential velocity of ion i

a migration jump distance in the oxideσ mean stressϕ electrical potentialuio standard chemical potential of ion i

zi charge number of ion iΩi molar volume of ion iUB breakdown potentialρe electrolyte resistivityCA− anion concentrationEB electric breakdownMB mechanical breakdownUEB electric breakdown potentialUMB mechanical breakdown potentialCB conduction bandtox oxide thicknessje electronic currentx travel distance of electronsα(E) the impact ionization coefficient at the

electric field Eψi the threshold energy for impact ionizationr recombination constant (r < 1)tox,B critical oxide thickness at the moment of

breakdownT temperaturejt total current densityj1 oxidation currentj2 current density consumed by the incorpo-

rated electrolyte species





γ the ratio j2/j1ρox oxide densityM1 molecular weight of the oxideM2 molecular weight of the species incorporated

into the oxidex1, x2 anion valencesy1, y2 cation valencesK unitary rate of anodization for oxide without

electrolyte incorporationφ ratio of the equivalent weight of the

incorporated species to that of oxide, thatis, φ = (M2/x2y2)/(M1/x1y1)

C electrolyte concentrationsΔP stress accumulated in the oxideε oxide permittivityγ surface tensionΓA− anion density at the oxide surface at the

breakdown potential (UB)Qc total chargeDint interpore distanceDp pore diametertw pore wall thicknessρp pore densityζp ratio of the pore diameter (Dp) to the

anodization potential (=Dp/U)MA mild anodizationHA hard anodizationPA pulse anodizationCA cyclic anodizationζMA proportionality constant between Dint and

mild anodization (MA) potential (UMA)ARMA anodizing ratio for mild anodization (MA)ARHA anodizing ratio for hard anodization (HA)PL photoluminescencetetch pore wall etching timeEXAFS extended X-ray absorption fine structureNMR nuclear magnetic resonanceXRD X-ray diffractionTG thermogravimetryDSC differential scanning calorimetryMS mass spectrometryVox molar volume of oxide grown during the

anodizationVm molar volume of metal consumed during the

anodizationn number of atoms of metal per one formula

of the oxideρm density of metalρox density of oxideρAAO density of anodic aluminum oxide (AAO)kV thickness ratio of the AAO formed during

anodization to the aluminum consumedduring anodization

hAAO vertical height of the AAOhAl vertical height of the aluminum consumed

during anodizationkv volume expansion factorσ⊥ compressive stress normal to the oxide

surfaceσES electrostatic stressε0 vacuum permittivityY Yong’s modulusΔk radius of curvature

EM elastic modulus of metal stripνM Poisson ratio of metal stripL metal strip lengthjlimit limiting currentΔK the substrate curvature in km−1

PSD power spectral density1D one dimension2D two dimension3D three dimensionR radius curvature of the metal/oxide interfacew angle subtended from the center of curvature

to the pore basesjtot total ionic current densityjox current density linked with the number of

oxygen in porous AAOjloss current density due to the loss of Al3+ ions

into the electrolytejAl current density caused by direct ejection of

Al3+ ionsjdec current density stemming from the outward

movement of Al3+ ions produced bydecomposition inside the barrier oxidetoward the electrolyte

λc critical wavelengthγ surface energyM biaxial modulus of solidUMA mild anodization potentialUHA hard anodization potentialΔU UHA − UMAMA-AAO AAO formed by mild anodization (MA)HA-AAO AAO formed by hard anodization (HA)Dint

MA interpore distance of MA-AAODint

MA interpore distance of HA-AAODp

MA pore diameter of MA-AAOtbMA barrier layer thickness of MA-AAOtbHA barrier layer thickness of HA-AAOPMA porosity of MA-AAOPHA porosity of HA-AAOζHA proportionality constant between Dint and

hard anodization (HA) potential (U)τHA pulse duration for hard anodization (HA)Q Joule’s heatRb resistance of the barrier layerTA transition between mild and hard anodiza-

tionITO indium tin oxideCNT carbon nanotubeEDS energy dispersive X-ray spectroscopyEBL electron beam lithographyLIL laser interference lithographyFIB focused-ion-beamSPM scanning probe microscopeNSL nanosphere lithographyPDMS poly-dimethylsiloxaneNIL nanoimprint lithographyRD-WETS reaction-diffusion wet-stampingSFIL step and flash imprint lithographyECD electrochemical depositionALD atomic layer depositionCVD chemical vapor depositionPVD physical vapor depositionRIE reactive ion etchingMTJ molecular transport junction



OWL on-wire lithographyPpy polypyroleGMR giant magnetoresistanceNP nanoparticleSAPT 2-(succinic anhydride)propyl trimethoxysi-

laneCTAB cetyltrimethylammonium bromideTEOS tetraethoxylsilanePluronic P123 poly(ethylene oxide)20-poly(propylene

oxide)70-poly(ethylene oxide)20Brij 56 decaethylene glycol hexadecyl etherS-helix single helixD-helix double helixAPTMS 3-aminoproyl trimethoxysilaneAPTES 3-aminopropyl triethosysilanePFPES pentafluorophenyldimethylchlorosilanePFPTES pentafluorophenyldimethylpropylchlorosi-

lanePEGS N-triethoxysilylpropyl-O-poly(ethylene

oxide) urethanePDDA poly(diallydimethylammonium chloride)NPNT nanoparticle-decorate nanotubesCYP2E1 cytochrome P450 2E1G6PD glucose-6-phosphate dehydrogenaseNADPH nicotinamide adenine dinucleotide phos-

phateNADP+ 2-oxoaldehyde dehydrogenasRGDC arginine-glycine-aspartic acid-cysteinePNIPAM poly(N-isopropylacrylamide)BBIB 2-bromoisobutyryl bromideNIPAM N-isopropylacrylamideATRP atom-transfer radical polymerizationPNIPAM-g-AAO PNIPAM-grafted AAO membraneLCST lower critical solution temperaturePHEMA poly(2-hydroxyethyl methacrylate)NTA-M2+ nitrilotriacetate-M2+, where M is Cu or NiPEGDMA poly(ethylene glycol dimetrhacrylate)VOC volatile organic compoundPEG poly(ethylene glycol)PEG-g-AAO poly(ethylene glycol) (PEG) grafted AAOCS chitosanGPTMS 3-glycidoxypropyl trimethoxysilanePAA poly(arcrylic acid)EIS electrochemical impedance spectroscopyLbL layer-by-layerPEI poly(ethylenimine)PAH poly(allylamine hydrochloride)PDADMAC poly(diallydimethylammonium chloride)PSS poly(styrenesulfonate)PVS poly(vinylsulfate)QD quantum dotSSM solid supported membranesBLM black lipid membranesMPA 3-mercaptopropionic acidDODAB N,N-dimethyl-N,N dioctadecylammonium

bromideDPhPC 1,2-diphytanoyl-sn-glycero-3-phosphocholinDPPTE 1,2-diphytano-dipalmitoyl-sn-glycero-s-phos-

phothioethanolGUV giant unilamellar vesicleSPR surface plasmon resonanceEPR electron paramagnetic resonancePAN polyacrylonitril

S spreading parameterγsg solid−gas interfacial tensionsγsl solid−liquid interfacial tensionsγlg liquid−gas interfacial tensionsθe finite contact anglePVDF poly(vinylidene difluoride)PEEK polyether ether ketonePTFE polytetrafluoroethylenePDLLA poly(D,L-lactide)EBDW electron-beam direct writingPMMA poly(methyl methacrylate)PS polystyrenePLD pulsed laser depositionMBE molecular-beam epitaxyVLS vapor−liquid−solidSiNW Si nanowireMOCVD metallorganic chemical vapor depositionTMA trimethyl aluminumOTS octadecyltrichlorosilaneBE bottom electrodeTE top electrodeUV ultraviolet

REFERENCES(1) Buff, H. Liebigs Ann. Chem. 1857, 3, 265.(2) Bengough, G. D.; Stuart, J. M. Improved process of protectingsurfaces of aluminium of aluminium alloys. U.K. Patent 223,994,August 2, 1923.(3) Diggle, J. W.; Downie, T. C.; Goulding, C. W. Chem. Rev. 1969,69, 365.(4) Sheasby, P. G.; Pinner, R. The Surface Treatment and Finishing ofAluminum and Its Alloys, 6th ed.; Finishing Publications Ltd. & ASMInternational: Materials Park, OH, and Stevenage, UK, 2001.(5) Masuda, H.; Fukuda, K. Science 1995, 268, 1466.(6) Masuda, H.; Satoh, M. Jpn. J. Appl. Phys. 1996, 35, L126.(7) Keller, F.; Hunter, M. S.; Robinson, D. L. J. Electrochem. Soc.1953, 100, 411.(8) Despic, A. R. J. Electroanal. Chem. 1985, 191, 417.(9) Takahashi, H.; Saito, Y.; Nagayama, M. J. Surf. Finish. Soc. Jpn.1982, 33, 225.(10) Thompson, G. E. Thin Solid Films 1997, 297, 192.(11) Nishinaga, O.; Kikuchi, T.; Natsui, S.; Suzuki, R. O. Sci. Rep.2013, 3, 2748.(12) Kape, J. M. Electroplat. Met. Finish. 1961, 14, 407.(13) Furneaux, R. C.; Rigby, W. R.; Davidson, A. P. Nature 1989,337, 147.(14) Thompson, G. E.; Wood, G. C. Nature 1981, 290, 230.(15) Kape, J. M. Metallugia 1959, 60, 181.(16) Lee, W.; Nielsch, K.; Gosele, U. Nanotechnology 2007, 18,475713.(17) Ono, S.; Saito, M.; Asoh, H. Electrochim. Acta 2005, 51, 827.(18) Chu, S. Z.; Wada, K.; Inoue, S.; Isogai, M.; Katsuta, Y.;Yasumori, A. J. Electrochem. Soc. 2006, 153, B384.(19) Mozalev, A.; Surganov, A.; Magaino, S. Electrochim. Acta 1999,44, 3891.(20) Mozalev, A.; Mozaleva, I.; Sakairi, M.; Takahashi, H. Electrochim.Acta 2005, 50, 5065.(21) Guntherschulz, A.; Betz, H. Z. Phys. 1934, 92, 367.(22) Verwey, J. W. Physica 1935, 2, 1059.(23) Mott, N. F. Trans. Faraday Soc. 1947, 43, 429.(24) Cabrera, N.; Mott, N. F. Rep. Prog. Phys. 1948, 12, 163.(25) Lohrengel, M. M. Mater. Sci. Eng., R 1993, 11, 243.(26) Davies, J. A.; Domeij, B.; Pringle, J. P. S.; Brown, F. J.Electrochem. Soc. 1965, 112, 675.(27) Davies, J. A.; Pringle, J. P. S.; Graham, R. L.; Brown, F. J.Electrochem. Soc. 1962, 109, 999.



http://pubs.acs.org/action/showLinks?crossref=10.1016%2Fj.electacta.2005.05.058&coi=1%3ACAS%3A528%3ADC%252BD2MXhtFGhtrvJ

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http://pubs.acs.org/action/showLinks?crossref=10.1149%2F1.2423662&coi=1%3ACAS%3A528%3ADyaF2MXkt1ais7k%253D

http://pubs.acs.org/action/showLinks?pmid=17843666&crossref=10.1126%2Fscience.268.5216.1466&coi=1%3ACAS%3A528%3ADyaK2MXmtFKktbc%253D

http://pubs.acs.org/action/showLinks?crossref=10.1002%2Fjlac.18571020302

http://pubs.acs.org/action/showLinks?crossref=10.1007%2FBF01340820

http://pubs.acs.org/action/showLinks?crossref=10.1016%2FS0040-6090%2896%2909440-0&coi=1%3ACAS%3A528%3ADyaK2sXktVGit7k%253D

http://pubs.acs.org/action/showLinks?crossref=10.1016%2F0927-796X%2893%2990005-N

http://pubs.acs.org/action/showLinks?crossref=10.1149%2F1.2218822&coi=1%3ACAS%3A528%3ADC%252BD28Xotl2hsbc%253D

http://pubs.acs.org/action/showLinks?crossref=10.1088%2F0034-4885%2F12%2F1%2F308

http://pubs.acs.org/action/showLinks?crossref=10.1149%2F1.2425225&coi=1%3ACAS%3A528%3ADyaF38XkslGjtb8%253D

http://pubs.acs.org/action/showLinks?coi=1%3ACAS%3A528%3ADyaF38Xms1SqsQ%253D%253D

http://pubs.acs.org/action/showLinks?crossref=10.1038%2F290230a0&coi=1%3ACAS%3A528%3ADyaL3MXktFCqsrY%253D

http://pubs.acs.org/action/showLinks?crossref=10.1039%2Ftf9474300429&coi=1%3ACAS%3A528%3ADyaH1cXms1yi

http://pubs.acs.org/action/showLinks?crossref=10.1143%2FJJAP.35.L126&coi=1%3ACAS%3A528%3ADyaK28XptlyqtA%253D%253D

http://pubs.acs.org/action/showLinks?crossref=10.1016%2FS0022-0728%2885%2980034-6&coi=1%3ACAS%3A528%3ADyaL2MXlsV2mtrs%253D

(28) Shimizu, K.; Thompson, G. E.; Wood, G. C.; Xu, Y. Thin SolidFilms 1982, 88, 255.(29) Cherki, C.; Siejka, J. J. Electrochem. Soc. 1973, 120, 784.(30) Siejka, J.; Ortega, C. J. Electrochem. Soc. 1977, 124, 883.(31) Wu, Z.; Richter, C.; Menon, L. J. Electrochem. Soc. 2007, 154, E8.(32) Skeldon, P.; Thompson, G. E.; Garcia-Vergara, S. J.; Iglesias-Bubianes, L.; Blanco-Pinzon, C. E. Electrochem. Solid-State Lett. 2006,9, B47.(33) Shimizu, K.; Kobayashi, K.; Thompson, G. E.; Wood, G. C.Philos. Mag. A 1992, 66, 643.(34) Shimizu, K.; Kobayahi, K.; Thompson, G. E.; Wood, G. C. J.Surf. Finish. Soc. Jpn. 1991, 42, 645.(35) Guntherschulz, A.; Betz, H. Z. Phys. 1932, 68, 145.(36) Dewald, J. F. Acta Metall. 1954, 2, 340.(37) Dewald, J. F. J. Electrochem. Soc. 1955, 102, 1.(38) Chao, C. Y.; Lin, L. F.; Macdonald, D. D. J. Electrochem. Soc.1981, 128, 1187.(39) Olsson, C.-O. A.; Hamm, D.; Landolt, D. J. Electrochem. Soc.2000, 147, 4093.(40) Brown, F.; Mackintosh, W. D. J. Electrochem. Soc. 1973, 120,1096.(41) Su, Z.; Buhl, M.; Zhou, W. J. Am. Chem. Soc. 2009, 131, 8697.(42) Davies, J. A.; Domeij, B. J. Electrochem. Soc. 1963, 110, 849.(43) Khalil, N.; Leach, J. S. L. Electrochim. Acta 1986, 31, 1279.(44) Thompson, G. E.; Xu, Y.; Skeldon, P.; Shimizu, K.; Han, S. H.;Wood, G. C. Philos. Mag. B 1987, 55, 651.(45) Dekker, A.; Middelhoek, A. J. Electrochem. Soc. 1970, 117, 440.(46) Takahashi, H.; Nagayama, M. Corros. Sci. 1978, 18, 911.(47) Houser, J. E.; Hebert, K. R. Phys. Status Solidi A 2008, 205,2396.(48) Hebert, K. R.; Houser, J. E. J. Electrochem. Soc. 2009, 156, C275.(49) Battaglia, V.; Newman, J. J. Electrochem. Soc. 1995, 142, 1423.(50) Bradhurst, D. H.; Leach, J. S. L. J. Electrochem. Soc. 1966, 113,1245.(51) Tajima, S.; Shimizu, K.; Baba, N.; Matsuzawa, S. Electrochim.Acta 1977, 22, 845.(52) Tajima, S. Electrochim. Acta 1977, 22, 995.(53) Shimizu, K.; Tajima, S. Electrochim. Acta 1980, 25, 259.(54) Kasalica, B.; Belca, I.; Stojadinovic, S.; Sarvan, M.; Peric, M.;Zekovic, L. J. Phys. Chem. C 2007, 111, 12315.(55) Stojadinovic, S.; Belca, I.; Kasalica, B.; Zekovic, L.; Tadic, M.Electrochem. Commun. 2006, 8, 1621.(56) Ikonopisov, S. Electrochim. Acta 1975, 20, 783.(57) Kasalica, B.; Stojadinovic, S.; Zekovic, L.; Belca, I.; Nikolic, D.Electrochem. Commun. 2005, 7, 735.(58) Stojadinovic, S.; Belca, I.; Zekovic, L.; Kasalica, B.; Nikolic, D.Electrochem. Commun. 2004, 6, 1016.(59) Stojadinovic, S.; Zekovic, L.; Belca, I.; Kasalica, B. Electrochem.Commun. 2004, 6, 427.(60) Vermilyea, D. A. J. Electrochem. Soc. 1957, 104, 542.(61) Jackson, N. F. J. Appl. Electrochem. 1973, 3, 91.(62) Habazaki, H.; Ogasawara, T.; Konno, H.; Shimizu, K.; Nagata,S.; Skeldon, P.; Thompson, G. E. Corros. Sci. 2007, 49, 580.(63) Habazaki, H.; Uozumi, M.; Konno, H.; Shimizu, K.; Skeldon, P.;Thompson, G. E. Corros. Sci. 2003, 45, 2063.(64) Yahalom, J.; Zahavi, J. Electrochim. Acta 1970, 15, 1429.(65) Leach, J. S. L.; Pearson, B. R. Corros. Sci. 1988, 28, 43.(66) Li, Y.; Shimada, H.; Sakairi, M.; Shigyo, K.; Takahashi, H.; Seo,M. J. Electrochem. Soc. 1997, 144, 866.(67) Proost, J.; Vanhumbeeck, J.-F.; Van Overmeere, Q. Electrochim.Acta 2009, 55, 350.(68) Matykina, E.; Arrabal, R.; Skeldon, P.; Thompson, G. E.;Habazaki, H. Thin Solid Films 2008, 516, 2296.(69) Ikonopisov, S.; Girginov, A.; Machkova, M. Electrochim. Acta1979, 24, 451.(70) Santway, R. W.; Alwitt, R. S. J. Electrochem. Soc. 1970, 117, 1282.(71) Burger, F. J.; Wu, J. C. J. Electrochem. Soc. 1971, 118, 2039.(72) Wood, G. C.; Pearson, C. Corros. Sci. 1967, 7, 119.(73) Alwitt, R. S.; Vijh, A. K. J. Electrochem. Soc. 1969, 116, 388.

(74) Di Quarto, F.; Piazza, S.; Sunseri, C. J. Electroanal. Chem. 1988,248, 99.(75) Di Quarto, F.; Piazza, S.; Sunseri, C. J. Electrochem. Soc. 1984,131, 2901.(76) Yahalom, J.; Hoar, T. P. Electrochim. Acta 1970, 15, 877.(77) Kato, M.; Uchida, E.; Kudo, T. J. Met. Finish. Soc. Jpn. 1984, 35,475.(78) Arifuku, F.; Yoneyama, H.; Tamura, H. Electrochim. Acta 1980,25, 863.(79) Albella, J. M.; Montero, I.; Martínez-Duart, J. M. J. Electrochem.Soc. 1984, 131, 1101.(80) Albella, J. M.; Montero, I.; Martínez-Duart, J. M. Thin SolidFilms 1985, 125, 57.(81) Albella, J. M.; Montero, I.; Martínez-Duart, J. M. Electrochim.Acta 1987, 32, 255.(82) Zahavi, J.; Yahalom, J. Electrochim. Acta 1971, 16, 89.(83) Di Quarto, F.; Piazza, S.; Sunseri, C. Corros. Sci. 1986, 26, 213.(84) Dyer, C. K.; Leach, J. S. L. J. Electrochem. Soc. 1978, 125, 1032.(85) Jouve, G.; Derradji, N. E. J. Less-Common Met. 1982, 86, 161.(86) Vermilyea, D. A. Acta Metall. 1954, 2, 476.(87) Albella, J. M.; Montero, I.; Martínez-Duart, J. M.; Parkhutik, V.J. Mater. Sci. 1991, 26, 3422.(88) Yahalom, J.; Zahavi, J. Electrochim. Acta 1971, 16, 603.(89) Ikonopisov, S.; Girginov, A.; Machkova, M. Electrochim. Acta1977, 22, 1283.(90) Ikonopisov, S. Electrochim. Acta 1977, 22, 1077.(91) Nigam, R. K.; Singh, K. C.; Maken, S. Thin Solid Films 1987,155, 115.(92) Ikonopisov, S. Electrochim. Acta 1969, 14, 761.(93) Ikonopisov, S.; Elenkov, N. J. Electroanal. Chem. 1978, 86, 105.(94) Shimizu, K. Electrochim. Acta 1981, 26, 1691.(95) Albella, J. M.; Montero, I.; Fernandez, M.; Gomez-Aleixandre,C.; Martínez-Duart, J. M. Electrochim. Acta 1985, 30, 1361.(96) Montero, I.; Albella, J. M.; Martínez-Duart, J. M. J. Electrochem.Soc. 1985, 132, 814.(97) Sato, N. Electrochim. Acta 1971, 16, 1683.(98) Garcia-Vergara, S. J.; Skeldon, P.; Thompson, G. E.; Habazaki,H. Electrochim. Acta 2006, 52, 681.(99) Sulka, G. D. Highly ordered anodic porous alumina formationby self-organized anodizing. In Nanostructured Materials in Electro-chemistry; Eftekhari, A., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA:Weinheim, 2008; pp 8−20.(100) O’Sullivan, J. P.; Wood, G. C. Proc. R. Soc. London, Ser. A 1970,317, 511.(101) Parkhutik, V. P.; Shershulsky, V. I. J. Phys. D: Appl. Phys. 1992,25, 1258.(102) Sulka, G. D.; Parkoła, K. G. Electrochim. Acta 2007, 52, 1880.(103) Lee, W.; Kim, J.-C.; Gosele, U. Adv. Funct. Mater. 2010, 20, 21.(104) Ebihara, K.; Takahashi, H.; Nagayama, M. J. Met. Finish. Soc.Jpn. 1983, 34, 548.(105) Ebihara, K.; Takahashi, H.; Nagayama, M. J. Met. Finish. Soc.Jpn. 1982, 33, 156.(106) Li, A. P.; Muller, F.; Birner, A.; Nielsch, K.; Gosele, U. J. Appl.Phys. 1998, 84, 6023.(107) Hwang, S.-K.; Jeong, S.-H.; Hwang, H.-Y.; Lee, O.-J.; Lee, K.-H. Korean J. Chem. Eng. 2002, 19, 467.(108) Ono, S.; Masuko, N. Surf. Coat. Technol. 2003, 169−170, 139.(109) Nielsch, K.; Choy, J.; Schwirn, K.; Wehrspohn, R. B.; Gosele,U. Nano Lett. 2002, 2, 677.(110) Chu, S.-Z.; Wada, K.; Inoue, S.; Isogai, M.; Yasumori, A. Adv.Mater. 2005, 17, 2115.(111) Lee, W.; Ji, R.; Gosele, U.; Nielsch, K. Nat. Mater. 2006, 5, 741.(112) Lee, W.; Schwirn, K.; Steinhart, M.; Pippel, E.; Scholz, R.;Gosele, U. Nat. Nanotechnol. 2008, 3, 234.(113) Schwirn, K.; Lee, W.; Hillebrand, R.; Steinhart, M.; Nielsch, K.;Gosele, U. ACS Nano 2008, 2, 302.(114) Hunter, M. S.; Fowle, P. J. Electrochem. Soc. 1954, 101, 481.(115) Vrublevsky, I.; Parkoun, V.; Schreckenbach, J. Appl. Surf. Sci.2005, 242, 333.



http://pubs.acs.org/action/showLinks?crossref=10.1016%2F0013-4686%2871%2985130-7&coi=1%3ACAS%3A528%3ADyaE3MXhtlequ7g%253D

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(116) Vrublevsky, I.; Parkoun, V.; Schreckenbach, J.; Marx, G. Appl.Surf. Sci. 2004, 227, 282.(117) Vrublevsky, I.; Parkoun, V.; Sokol, V.; Schreckenbach, J. Appl.Surf. Sci. 2005, 252, 227.(118) Li, Y.; Zheng, M.; Ma, L.; Shen, W. Nanotechnology 2006, 17,5101.(119) Schwirn, K. Harte Anodisation von Aluminium mit VerdunnterSchwefelaure. Ph.D. Thesis, Martin-Luther-Universitat Halle-Witten-berg, Halle (Saale), 2008.(120) Yamamoto, Y.; Baba, N.; Tajima, S. Nature 1981, 289, 572.(121) Xu, W. L.; Zheng, M. J.; Wu, S.; Sheng, W. Z. Appl. Phys. Lett.2004, 85, 4364.(122) Li, Y. B.; Zheng, M. J.; Ma, L. Appl. Phys. Lett. 2007, 91,073109.(123) Vojkuvka, L.; Santos, A.; Pallares, J.; Ferre-Borrull, J.; Marsal,L. F.; Celis, J. P. Surf. Coat. Technol. 2012, 206, 2115.(124) Roa, J. J.; Gaston-García, B.; García-Lecina, E.; Muller, C.Ceram. Int. 2012, 38, 1627.(125) Thompson, G. E.; Wood, G. C. Anodic Films on Aluminium.In Treatise on Materials Science and Technology, Corrosion: AqueousProcess and Passivation; Scully, J. C., Ed.; Academic Press: New York,1983; pp 286−287.(126) Ono, S.; Ichinose, H.; Masuko, N. Corros. Sci. 1992, 33, 841.(127) Mason, R. B. J. Electrochem. Soc. 1955, 102, 671.(128) Wood, G. C.; Skeldon, P.; Thompson, G. E.; Shimizu, K. J.Electrochem. Soc. 1996, 143, 74.(129) Han, H.; Park, S.-J.; Jang, J. S.; Ryu, H.; Kim, K. J.; Baik, S. ACSAppl. Mater. Interfaces 2013, 5, 3441.(130) Thompson, G. E.; Furneaux, R. C.; Wood, G. C. Corros. Sci.1978, 18, 481.(131) Ono, S.; Masuko, N. Corros. Sci. 1992, 33, 503.(132) Ono, S.; Ichinose, H.; Kawaguchi, T.; Masuko, N. Corros. Sci.1990, 31, 249.(133) Thompson, G. E.; Furneaux, R. C.; Wood, G. C. J. Electrochem.Soc. 1978, 125, 1480.(134) Thornton, M. C.; Furneaux, R. C. J. Mater. Sci. Lett. 1991, 10,622.(135) Le Coz, F.; Arurault, L.; Data, L. Mater. Charact. 2010, 61, 283.(136) Oka, Y.; Takahashi, T.; Okada, K.; Iwai, S.-i. J. Non-Cryst. Solids1979, 30, 349.(137) El-Mashri, S. M.; Jones, R. G.; Forty, A. J. Philos. Mag. A 1983,48, 665.(138) Farnan, I.; Dupree, R.; Forty, A. J.; Jeong, Y. S.; Thompson, G.E.; Wood, G. C. Philos. Mag. Lett. 1989, 59, 189.(139) Brown, I. W. M.; Bowden, M. E.; Kemmitt, T.; MacKenzie, K.J. D. Curr. Appl. Phys. 2006, 6, 557.(140) Kirchner, A.; MacKenzie, K. J. D.; Brown, I. W. M.; Kemmitt,T.; Bowden, M. E. J. Membr. Sci. 2007, 287, 264.(141) Farnan, I.; Dupree, R.; Jeong, Y.; Thompson, G. E.; Wood, G.C.; Forty, A. J. Thin Solid Films 1989, 173, 209.(142) Ingham, C. J.; ter Maat, J.; de Vos, W. M. Biotechnol. Adv. 2012,30, 1089.(143) Jani, A. M. M.; Losic, D.; Voelcker, N. H. Prog. Mater. Sci.2013, 58, 636.(144) Mardilovich, P. P.; Govyadinov, A. N.; Mukhurov, N. I.;Rzhevskii, A. M.; Paterson, R. J. Membr. Sci. 1995, 98, 131.(145) Levin, I.; Brandon, D. J. Am. Ceram. Soc. 1998, 81, 1995.(146) Burgos, N.; Paulis, M.; Montes, M. J. Mater. Chem. 2003, 13,1458.(147) Le Coz, F.; Arurault, L.; Fontorbes, S.; Vilar, V.; Datas, L.;Winterton, P. Surf. Interface Anal. 2010, 42, 227.(148) Ozao, R.; Ochiai, M.; Ichimura, N.; Takahashi, H.; Inada, T.Thermochim. Acta 2000, 353−354, 91.(149) Bocchetta, P.; Sunseri, C.; Chiavarotti, G.; Quarto, F. D.Electrochim. Acta 2003, 48, 3175.(150) Mata-Zamora, M. E.; Saniger, J. M. Rev. Mex. Fis. 2005, 51,502.(151) McQuaig, M. K., Jr.; Toro, A.; Van Geertruyden, W.; Misiolek,W. Z. J. Mater. Sci. 2011, 46, 243.

(152) Chang, Y.; Ling, Z.; Liu, Y.; Hu, X.; Li, Y. J. Mater. Chem. 2012,22, 7445.(153) Pilling, N. B.; Bedworth, R. E. J. Inst. Met. 1923, 29, 529.(154) Phull, B. Evaluating stress-corrosion cracking. In ASMHandbook, Corrosion: Fundametals, Testing, and Protection; Cramer, S.D., Covino, B. S., Jr., Eds.; ASM International: Materials Park, OH,2003; pp 575−616.(155) Burgers, W. G.; Claasen, A.; Zernike, J. Z. Phys. 1932, 74, 593.(156) Bernard, W. J.; Cook, J. W. J. Electrochem. Soc. 1959, 106, 643.(157) Dorsely, G. A., Jr. J. Electrochem. Soc. 1969, 116, 466.(158) Garcia-Vergara, S. J.; Habazaki, H.; Skeldon, P.; Thompson, G.E. Nanotechnology 2007, 18, 415605.(159) Garcia-Vergara, S. J.; Skeldon, P.; Thompson, G. E.; Habazaki,H. Corros. Sci. 2007, 49, 3696.(160) Mercier, D.; Van Overmeere, Q.; Santoro, R.; Proost, J.Electrochim. Acta 2011, 56, 1329.(161) Baron-Wiechec, A.; Ganem, J. J.; Garcia-Vergara, S. J.; Skeldon,P.; Thompson, G. E.; Vickridge, I. C. J. Electrochem. Soc. 2010, 157,C399.(162) Arurault, L. Trans. Inst. Met. Finish. 2008, 86, 51.(163) Vrublevsky, I.; Parkoun, V.; Schreckenbach, J.; Marx, G. Appl.Surf. Sci. 2003, 220, 51.(164) Zhou, F.; Al-Zenati, A. K. M.; Baron-Wiechec, A.; Curioni, M.;Garcia-Vergara, S. J.; Habazaki, H.; Skeldon, P.; Thompson, G. E. J.Electrochem. Soc. 2011, 158, C202.(165) Jessensky, O.; Muller, F.; Gosele, U. Appl. Phys. Lett. 1998, 72,1173.(166) Surganov, V. F.; Mozalev, A. M.; Mozaleva, I. I. Russ. J. Appl.Chem. 1997, 70, 254.(167) Vrublevsky, I.; Parkoun, V.; Sokol, V.; Schreckenbach, J.; Marx,G. Appl. Surf. Sci. 2004, 222, 215.(168) Li, F.; Zhang, L.; Metzger, R. M. Chem. Mater. 1998, 10, 2470.(169) Nelson, J. C.; Oriani, R. A. Corros. Sci. 1993, 34, 307.(170) Vanhumbeeck, J.-F.; Proost, J. Electrochim. Acta 2008, 53,6165.(171) Stoney, G. G. Proc. R. Soc. London, Ser. A 1909, 82, 172.(172) Vermilyea, D. A. J. Electrochem. Soc. 1963, 110, 345.(173) Moon, S.-M.; Pyun, S.-I. J. Solid State Electrochem. 1998, 2, 156.(174) Moon, S.-M.; Pyun, S.-I. Electrochim. Acta 1998, 43, 3117.(175) Van Overmeere, Q.; Blaffart, F.; Proost, J. Electrochem.Commun. 2010, 12, 1174.(176) Capraz, O. O.; Hebert, K. R.; Shrotriya, P. J. Electrochem. Soc.2013, 160, D501.(177) Van Overmeere, Q.; Proost, J. Electrochim. Acta 2010, 55, 4653.(178) Van Overmeere, Q.; Proost, J. Electrochim. Acta 2011, 56,10507.(179) Van Overmeere, Q.; Vanhumbeeck, J.-F.; Proost, J. Rev. Sci.Instrum. 2010, 81, 045106.(180) Sulka, G. D.; Stroobants, S.; Moshchalkov, V. V.; Borghs, G.;Celis, J.-P. J. Electrochem. Soc. 2004, 151, B260.(181) Park, S.-H.; Lee, Y.; Lee, J.-K.; Kim, K.-B. Electrochem. Solid-State Lett. 2006, 9, D31.(182) Cojocaru, C. S.; Padovani, J. M.; Wade, T.; Mandoli, C.;Jaskierowicz, G.; Wegrowe, J. E.; i Morral, A. F.; Pribat, D. Nano Lett.2005, 5, 675.(183) Xiang, Y.; Lee, W.; Nielsch, K.; Abstreiter, G.; i Morral, A. F.Phys. Status Solidi RRL 2008, 2, 59.(184) Masuda, H.; Nishio, K.; Baba, N. Appl. Phys. Lett. 1993, 63,3155.(185) Gowtham, M.; Eude, L.; Cojocaru, C. S.; Marquardt, B.; Jeong,H. J.; Legagneux, P.; Song, K. K.; Pribat, D. Nanotechnology 2008, 19,035303.(186) Oh, J.; Thompson, C. V. J. Electrochem. Soc. 2011, 158, C71.(187) Hoar, T. P.; Yahalom, J. J. Electrochem. Soc. 1963, 110, 614.(188) Albella, J. M.; Montero, I.; Jimenez, M. C.; Martínez-Duart, J.M. Electrochim. Acta 1991, 36, 739.(189) Van Overmeere, Q.; Mercier, D.; Santoro, R.; Proost, J.Electrochem. Solid-State Lett. 2012, 15, C1.



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http://pubs.acs.org/action/showLinks?coi=1%3ACAS%3A528%3ADC%252BC3MXhsFGqs7bE

(190) Van Overmeere, Q.; Nysten, B.; Proost, J. Appl. Phys. Lett.2009, 94, 074103.(191) Garcia-Vergara, S. J.; Skeldon, P.; Thompson, G. E.; Habazaki,H. Corros. Sci. 2007, 49, 3772.(192) Singh, G. K.; Golovin, A. A.; Aranson, I. S. Phys. Rev. B 2006,73, 205422.(193) Hoar, T. P.; Mott, N. F. J. Phys. Chem. Solids 1959, 9, 97.(194) Hebert, K. R.; Albu, S. P.; Paramasivam, I.; Schmuki, P. Nat.Mater. 2012, 11, 162.(195) Raja, K. S.; Misra, M.; Paramguru, K. Electrochim. Acta 2005,51, 154.(196) Sample, C.; Golovin, A. A. Phys. Rev. E 2006, 74, 041606.(197) Stanton, L. G.; Golvin, A. A. Phys. Rev. B 2009, 79, 035414.(198) Thamida, S. K.; Chang, H.-C. Chaos 2002, 12, 240.(199) Houser, J. E.; Hebert, K. R. J. Electrochem. Soc. 2006, 153,B566.(200) Houser, J. E.; Hebert, K. R. Nat. Mater. 2009, 8, 415.(201) Hunter, M. S.; Fowle, P. J. Electrochem. Soc. 1954, 101, 514.(202) Leach, J. S. L.; Neufeld, P. Corros. Sci. 1969, 9, 413.(203) Nagayama, M.; Tamura, K. Electrochim. Acta 1968, 13, 1773.(204) Chowdhury, P.; Thomas, A. N.; Sharma, M.; Barshilia, H. C.Electrochim. Acta 2014, 115, 657.(205) Singh, G. K.; Golovin, A. A.; Aranson, I. S.; Vinokur, V. M.Europhys. Lett. 2005, 70, 836.(206) Friedman, A. L.; Brittain, D.; Menon, L. J. Chem. Phys. 2007,127, 154717.(207) Oh, J.; Thompson, C. V. Electrochim. Acta 2011, 56, 4044.(208) Baron-Wiechec, A.; Burke, M. G.; Hashimoto, T.; Liu, H.;Skeldon, P.; Thompson, G. E.; Habazaki, H.; Ganem, J.-J.; Vickridge, I.C. Electrochim. Acta 2013, 113, 302.(209) Garcia-Vergara, S. J.; Clere, D. L.; Hashimoto, T.; Habazaki,H.; Skeldon, P.; Thompson, G. E. Electrochim. Acta 2009, 54, 6403.(210) Garcia-Vergara, S. J.; Skeldon, P.; Thompson, G. E.; Habazaki,H. Appl. Surf. Sci. 2007, 254, 1534.(211) Habazaki, H.; Shimizu, K.; Skeldon, P.; Thompson, G. E.;Wood, G. C. J. Electrochem. Soc. 1996, 143, 2465.(212) Habazaki, H.; Shimizu, K.; Skeldon, P.; Thompson, G. E.;Wood, G. C.; Zhou, X. Trans. Inst. Met. Finish. 1997, 75, 18.(213) Garcia-Vergara, S. J.; Hashimoto, T.; Skeldon, P.; Thompson,G. E.; Habazaki, H. Electrochim. Acta 2009, 54, 3662.(214) Iglesias-Rubianes, L.; Skeldon, P.; Thompson, G. E.; Habazaki,H.; Shimizu, K. J. Electrochem. Soc. 2002, 149, B23.(215) Pringle, J. P. S. Electrochim. Acta 1980, 25, 1423.(216) Garcia-Vergara, S. J.; Skeldon, P.; Thompson, G. E.; Habazaki,H. Surf. Interface Anal. 2007, 39, 860.(217) Bolger, C. T.; Fois, G.; Petkov, N.; Sassiat, N.; Burke, M.;Quinn, A. J.; Cross, G. L. W.; Holmes, J. D. Nanotechnology 2012, 23,175602.(218) Garcia-Vergara, S. J.; Skeldon, P.; Thompson, G. E.; Habazaki,H. Corros. Sci. 2008, 50, 3179.(219) Forgazza, M.; Santamaria, M.; Di Quarto, F.; Garcia-Vergara, S.J.; Molchan, I.; Skeldon, P.; Thompson, G. E.; Habazaki, H.Electrochim. Acta 2009, 54, 1070.(220) Herrera-Erazo, A. E.; Habazaki, H.; Shimizu, K.; Skeldon, P.;Thompson, G. E. Corros. Sci. 2000, 42, 1823.(221) Csokan, P. Metalloberfla che 1961, 15, B49.(222) Csokan, V. P.; Hollo, M. Werkst. Korros. 1961, 12, 288.(223) Csokan, P.; Sc., C. C. Electroplat. Met. Finish. 1962, 15, 75.(224) Lee, W. JOM 2010, 62, 57.(225) Schwartz, G. C.; Platter, V. J. Electrochem. Soc. 1975, 122, 1508.(226) Hillebrand, R.; Muller, F.; Schwirn, K.; Lee, W.; Steinhart, M.ACS Nano 2008, 2, 913.(227) Masuda, H.; Hasegawa, E.; Ono, S. J. Electrochem. Soc. 1997,144, L127.(228) Zhang, L.; Cho, H. S.; Li, F.; Metzger, R. M.; Doyle, W. D. J.Mater. Sci. Lett. 1998, 17, 291.(229) Masuda, H.; Yada, K.; Osaka, A. Jpn. J. Appl. Phys. 1998, 37,L1340.

(230) Shingubara, S.; Morimoto, K.; Sakaue, H.; Takahagi, T.Electrochem. Solid-State Lett. 2004, 7, E15.(231) Sun, C.; Luo, J.; Wu, L.; Zhang, J. ACS Appl. Mater. Interfaces2010, 2, 1299.(232) Ono, S.; Saito, M.; Ishiguro, M.; Asoh, H. J. Electrochem. Soc.2004, 151, B473.(233) Chen, W.; Wu, J.-S.; Xia, X.-H. ACS Nano 2008, 2, 959.(234) Martín, J.; Manzano, C. V.; Caballero-Calero, O.; Martín-Gonzalez, M. ACS Appl. Mater. Interfaces 2013, 5, 72.(235) Su, Z.; Hahner, G.; Zhou, W. J. Mater. Chem. 2008, 18, 5787.(236) Su, Z.; Zhou, W. Adv. Mater. 2008, 20, 3663.(237) Su, Z.; Zhou, W. J. Mater. Chem. 2011, 21, 357.(238) Hoar, T. P. Corros. Sci. 1967, 7, 341.(239) Tu, G. C.; Chen, I. T.; Shimizu, K. J. Jpn. Inst. Light Met. 1990,40, 382.(240) Ono, S.; Saito, M.; Asoh, H. Electrochem. Solid-State Lett. 2004,7, B21.(241) Csokan, P. Trans. Inst. Met. Finish. 1964, 41, 51.(242) Arrowsmith, D. J.; Clifford, A. W.; Moth, D. A. J. Mater. Sci.Lett. 1986, 5, 921.(243) Wada, K.; Shimohira, T.; Yamada, M.; Baba, N. J. Mater. Sci.1986, 21, 3810.(244) Kashi, M. A.; Ramazani, A.; Noormohammadi, M.; Zarei, M.;Marashi, P. J. Phys. D: Appl. Phys. 2007, 40, 7032.(245) Li, Y.; Ling, Z. Y.; Chen, S. S.; Wang, J. C. Nanotechnology2008, 19, 225604.(246) Srolovitz, D. J. Acta Metall. 1989, 37, 621.(247) Li, F. Nanostructure of anodic porous alumina films of interestin magnetic recording. Ph.D. Thesis, The University of Alabama, AL,1998.(248) Gill, V.; Guduru, P. R.; Sheldon, B. W. Int. J. Solids Struct. 2008,45, 943.(249) Matthias, S.; Schilling, J.; Nielsch, K.; Muller, F.; Wehrspohn,R. B.; Gosele, U. Adv. Mater. 2002, 14, 1618.(250) He, B.; Son, S. J.; Lee, S. B. Langmuir 2006, 22, 8263.(251) Brunker, S. E.; Cederquist, K. B.; Keating, C. D. Nanomedicine2007, 2, 695.(252) Matthias, S.; Muller, F. Nature 2003, 424, 53.(253) Casanova, F.; Chiang, C. E.; Li, C.-P.; Schuller, I. K. Appl. Phys.Lett. 2007, 91, 243103.(254) Bruschi, L.; Mistura, G.; Liu, L.; Lee, W.; Gosele, U.; Coasne,B. Langmuir 2010, 26, 11894.(255) Miller, M. A. Anodizing process and system. U.S. Patent2,920,018, January 5, 1960.(256) Schaedel, F. C. Aluminum anodizing method. U.S. Patent3,418,222, December 24, 1968.(257) Yokoyama, K.; Konno, H.; Takahashi, H.; Nagayama, M. Plat.Surf. Finish. 1982, 7, 62.(258) Tu, G. C.; Huang, L. Y. Trans. Inst. Met. Finish. 1987, 65, 60.(259) Raj, V.; Rajaram, M. P.; Balasubramanian, G.; Vincent, S.;Kanagaraj, D. Trans. Inst. Met. Finish. 2003, 81, 114.(260) Murphy, J. F.; Michelson, C. E. A Theory for the Formation ofAnodic Oxide Coatings on Aluminium. Proceedings of AnodizingAluminium; Nottingham, England, 1961; pp 83−95.(261) Takahashi, H.; Nagayama, M.; Akahori, H.; Kitahara, A. J.Electron Microsc. 1973, 22, 149.(262) Rasmussen, J. Plat. Surf. Finish. 2001, 88, 42.(263) Lee, W.; Scholz, R.; Gosele, U. Nano Lett. 2008, 8, 2155.(264) Lee, W.; Kim, J.-C. Nanotechnology 2010, 21, 485304.(265) Losic, D.; Lillo, M.; Losic, D., Jr. Small 2009, 5, 1392.(266) Losic, D.; Losic, D., Jr. Langmuir 2009, 25, 5426.(267) Wang, B.; Xu, G.; Cui, P. Mater. Lett. 2013, 93, 336.(268) Yang, C.-J.; Liang, S.-W.; Wu, P.-W.; Chen, C.; Shieh, J.-M.Electrochem. Solid-State Lett. 2007, 10, C69.(269) Miney, P. G.; Colavita, P. E.; Schiza, M. V.; Priore, R. J.;Haibach, F. G.; Myrick, M. L. Electrochem. Solid-State Lett. 2003, 6,B42.(270) Rabin, O.; Herz, P. R.; Lin, Y.-M.; Akinwande, A. I.; Cronin, S.B.; Dresselhaus, M. S. Adv. Funct. Mater. 2003, 13, 631.



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http://pubs.acs.org/action/showLinks?system=10.1021%2Fam3020718

(271) Crouse, D.; Lo, Y.-H.; Miller, A. E.; Crouse, M. Appl. Phys. Lett.2000, 76, 49.(272) Asoh, H.; Matsuo, M.; Yoshihama, M.; Ono, S. Appl. Phys. Lett.2003, 83, 4408.(273) Shiraki, H.; Kimura, Y.; Ishii, H.; Ono, S.; Itaya, K.; Niwano, M.Appl. Surf. Sci. 2004, 237, 369.(274) Tian, M.; Xu, S.; Wang, J.; Kumar, N.; Wertz, E.; Li, Q.;Campbell, P. M.; Chan, M. H. W.; Mallouk, T. E. Nano Lett. 2005, 5,697.(275) Kokonou, M.; Nassiopoulou, A. G.; Giannakopoulos, K. P.Nanotechnology 2005, 16, 103.(276) Seo, H.-S.; Jung, Y.-G.; Jee, S.-W.; Yang, J. M.; Lee, J.-H. Scr.Mater. 2007, 57, 968.(277) Chen, P.-L.; Kuo, C.-T.; Tsai, T.-G.; Wu, B.-W.; Hsu, C.-C.;Pan, F.-M. Appl. Phys. Lett. 2003, 82, 2796.(278) Sander, M. S.; Tan, L.-S. Adv. Funct. Mater. 2003, 13, 393.(279) Choi, J.; Sauer, G.; Goring, P.; Nielsch, K.; Wehrspohn, R. B.;Gosele, U. J. Mater. Chem. 2003, 13, 1100.(280) Zhao, X.; Lee, U.-J.; Seo, S.-K.; Lee, K.-H. Mater. Sci. Eng., C2009, 29, 1156.(281) Yang, Y.; Chen, H.; Mei, Y.; Chen, J.; Wu, X.; Bao, X. SolidState Commun. 2002, 123, 279.(282) Yasui, N.; Imada, A.; Den, T. Appl. Phys. Lett. 2003, 83, 3347.(283) Crouse, M. M.; Miller, A. E.; Crouse, D. T.; Ikram, A. A. J.Electrochem. Soc. 2005, 152, D167.(284) Vorobyova, A. I.; Outkina, E. A. Thin Solid Films 1998, 324, 1.(285) Mozalev, A.; Surganov, A.; Magaino, S. Electrochim. Acta 1999,44, 3891.(286) Mozalev, A.; Khatko, V.; Bittencourt, C.; Hassel, A. W.;Gorokh, G.; Llobet, E.; Correig, X. Chem. Mater. 2008, 20, 6482.(287) Iwasaki, T.; Motoi, T.; Den, T. Appl. Phys. Lett. 1999, 75, 2044.(288) Chu, S.-Z.; Wada, K.; Inoue, S.; Segawa, H. J. Electrochem. Soc.2011, 158, C148.(289) Oh, J.; Thompson, C. V. Adv. Mater. 2008, 20, 1368.(290) Oh, J.; Shin, Y. C.; Thompson, C. V. J. Electrochem. Soc. 2011,158, K11.(291) Chu, S. Z.; Wada, K.; Inoue, S.; Todoroki, S. J. Electrochem. Soc.2002, 149, B321.(292) Chu, S. Z.; Wada, K.; Inoue, S.; Todoroki, S. Electrochim. Acta2003, 48, 3147.(293) Inoue, S.; Todoroki, S.; Suehara, S.; Konishi, T.; Chu, S. Z.;Wada, K.; Kikkojin, T.; Isogai, M.; Katsuta, Y.; Sakamoto, T.;Yasumori, A. J. Non-Cryst. Solids 2006, 352, 632.(294) Musselman, K. P.; Mulholland, G. J.; Robinson, A. P.; Schmidt-Mende, L.; MacManus-Driscoll, J. L. Adv. Mater. 2008, 20, 4470.(295) Foong, T. R. B.; Sellinger, A.; Hu, X. ACS Nano 2008, 2, 2250.(296) Schierhorn, M.; Boettcher, S. W.; Kraemer, S.; Stucky, G. D.;Moskovits, M. Nano Lett. 2009, 9, 3262.(297) Hill, J. J.; Haller, K.; Ziegler, K. J. J. Electrochem. Soc. 2011, 158,E1.(298) Chu, S.-Z.; Wada, K.; Inoue, S.; Todoroki, S.-i. Chem. Mater.2002, 14, 4595.(299) Masuda, H.; Yasui, K.; Sakamoto, Y.; Nakao, M.; Tamamura,T.; Nishio, K. Jpn. J. Appl. Phys. 2001, 40, L1267.(300) Kimura, Y.; Shiraki, H.; Ishibashi, K.-i.; Ishii, H.; Itaya, K.;Niwanoa, M. J. Electrochem. Soc. 2006, 153, C296.(301) Holubowitch, N.; Nagle, L. C.; Rohan, J. F. Solid State Ionics2012, 216, 110.(302) Shimizu, K.; Habazaki, H.; Skeldon, P.; Thompson, G. E.;Wood, G. C. J. Surf. Finish. Soc. Jpn. 1999, 50, 2.(303) Skeldon, P.; Shimizu, K.; Thompson, G. E.; Wood, G. C.Philos. Mag. B 1990, 61, 927.(304) Habazaki, H.; Skeldon, P.; Shimizu, K.; Thompson, G. E.;Wood, G. C. Corros. Sci. 1995, 37, 1497.(305) Mozalev, A.; Sakairi, M.; Saeki, I.; Takahashi, H. Electrochim.Acta 2003, 48, 3155.(306) Tatarenko, N. I.; Mozalev, A. M. Solid-State Electron. 2001, 45,1009.

(307) Mozalev, A.; Sakairi, M.; Takahashi, H. J. Electrochem. Soc.2004, 151, F257.(308) Masuda, H.; Yamada, H.; Satoh, M.; Asoh, H.; Nakao, M.;Tamamura, T. Appl. Phys. Lett. 1997, 71, 2770.(309) Masuda, H.; Asoh, H.; Watanabe, M.; Nishio, K.; Nakao, M.;Tamamura, T. Adv. Mater. 2001, 13, 189.(310) Asoh, H.; Nishio, K.; Nakao, M.; Yokoo, A.; Tamamura, T.;Masuda, H. J. Vac. Sci. Technol., B 2001, 19, 569.(311) Masuda, H.; Yotsuya, M.; Asano, M.; Nishio, K.; Nakao, M.;Yokoo, A.; Tamamura, T. Appl. Phys. Lett. 2001, 78, 826.(312) Smith, J. T.; Huang, Q.; Franklin, A. D.; Janes, D. B.; Sands, T.D. Appl. Phys. Lett. 2008, 93, 043108.(313) Jones, M. R.; Osberg, K. D.; Macfarlane, R. J.; Langille, M. R.;Mirkin, C. A. Chem. Rev. 2011, 111, 3736.(314) Mikulskas, I.; Juodkazis, S.; Tomasiunas, R.; Dumas, J. G. Adv.Mater. 2001, 13, 1574.(315) Choi, J.; Nielsch, K.; Reiche, M.; Wehrspohn, R. B.; Gosele, U.J. Vac. Sci. Technol., B 2003, 21, 763.(316) Fournier-Bidoz, S.; Kitaev, V.; Routkevitch, D.; Manners, I.;Ozin, G. A. Adv. Mater. 2004, 16, 2193.(317) Lee, W.; Ji, R.; Ross, C. A.; Gosele, U.; Nielsch, K. Small 2006,2, 978.(318) Matsui, Y.; Nishio, K.; Masuda, H. Small 2006, 2, 522.(319) Liu, C. Y.; Datta, A.; Wang, Y. L. Appl. Phys. Lett. 2001, 78,120.(320) Masuda, H.; Kanezawa, K.; Nishio, K. Chem. Lett. 2002, 31,1218.(321) Sun, Z.; Kim, H. K. Appl. Phys. Lett. 2010, 81, 3458.(322) Kim, B.; Park, S.; McCarthy, T. J.; Russell, T. P. Small 2007, 3,1869.(323) Lipson, A. L.; Comstock, D. J.; Hersam, M. C. Small 2009, 5,2807.(324) Lai, K.-L.; Hon, M.-H.; Leu, I.-C. Nanoscale Res. Lett. 2011, 6,157.(325) Oshima, H.; Kikuchi, H.; Nakao, H.; Itoh, K.-i.; Kamimura, T.;Morikawa, T.; Matsumoto, K.; Umada, T.; Tamura, H.; Nishio, K.;Masuda, H. Appl. Phys. Lett. 2007, 91, 022508.(326) Kustandi, T. S.; Loh, W. W.; Gao, H.; Low, H. Y. ACS Nano2010, 4, 2561.(327) Masuda, H.; Matsui, Y.; Yotsuya, M.; Matsumoto, F.; Nishio,K. Chem. Lett. 2004, 33, 584.(328) Liu, N. W.; Datta, A.; Liu, C. Y.; Wang, Y. L. Appl. Phys. Lett.2003, 82, 1281.(329) Chen, B.; Lu, K.; Tian, Z. Electrochim. Acta 2010, 56, 435.(330) Chen, B.; Lu, K.; Tian, Z. Langmuir 2011, 27, 800.(331) Shingubara, S.; Murakami, Y.; Morimoto, K.; Takahagi, T. Surf.Sci. 2003, 532−535, 317.(332) Nasir, M. E.; Allsopp, D. W. E.; Bowen, C. R.; Hubbard, G.;Parsons, K. P. Nanotechnology 2010, 21, 105303.(333) Xia, Y.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153.(334) Guo, L. J. Adv. Mater. 2007, 19, 495.(335) Asoh, H.; Nishio, K.; Nakao, M.; Tamamura, T.; Masuda, H. J.Electrochem. Soc. 2001, 148, B152.(336) Liu, Y.; Goebl, J.; Yin, Y. Chem. Soc. Rev. 2013, 42, 2610.(337) Possin, G. E. Rev. Sci. Instrum. 1970, 41, 772.(338) Martin, C. R. Science 1994, 266, 1961.(339) Martin, C. R. Adv. Mater. 1991, 3, 457.(340) Martin, C. R. Chem. Mater. 1996, 8, 1739.(341) Penner, R. M.; Martin, C. R. Anal. Chem. 1987, 59, 2625.(342) Hurst, S. J.; Payne, E. K.; Qin, L.; Mirkin, C. A. Angew. Chem.,Int. Ed. 2006, 45, 2672.(343) Kovtyukhova, N. I.; Mallouk, T. E. Chem.Eur. J. 2002, 8,4355.(344) Nicewarner-Pena, S. R.; Freeman, R. G.; Reiss, B. D.; He, L.;Pena, D. J.; Walton, I. D.; Cromer, R.; Keating, C. D.; Nata, M. J.Science 2001, 294, 137.(345) Choi, J.; Sauer, G.; Nielsch, K.; Wehrspohn, R. B.; Gosele, U.Chem. Mater. 2003, 15, 776.



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http://pubs.acs.org/action/showLinks?crossref=10.1116%2F1.1556397&coi=1%3ACAS%3A528%3ADC%252BD3sXisVyqurc%253D

(346) Su, Y.-K.; Qin, D.-H.; Zhang, H.-L.; Li, H.; Li, H.-L. Chem.Phys. Lett. 2004, 388, 406.(347) Nielsch, K.; Muller, F.; Li, A. P.; Gosele, U. Adv. Mater. 2000,12, 582.(348) de Menten de Horne, F.; Piraux, L.; Michottea, S. Appl. Phys.Lett. 2005, 86, 152510.(349) Wang, J.-G.; Tian, M.-L.; Kumar, N.; Mallouk, T. E. Nano Lett.2005, 5, 1247.(350) Chaure, N. B.; Stamenov, P.; Rhen, F. M. F.; Coey, J. M. D. J.Magn. Magn. Mater. 2005, 290−291, 1210.(351) Yasui, K.; Morikawa, T.; Nishio, K.; Masuda, H. Jpn. J. Appl.Phys. 2005, 44, L469.(352) Xu, X.; Xu, Y. Mater. Lett. 2006, 60, 2069.(353) Zhang, Y.; Li, G.; Wu, Y.; Zhang, B.; Song, W.; Zhang, L. Adv.Mater. 2002, 14, 1227.(354) Tok, J. B.-H.; Chuang, F. Y. S.; Kao, M. C.; Rose, K. A.; Pannu,S. S.; Sha, M. Y.; Chakarova, G.; Penn, S. G.; Dougherty, G. M. Angew.Chem., Int. Ed. 2006, 45, 6900.(355) Walton, I. D.; Norton, S. M.; Balasingham, A.; He, L.;Dominador, F.; Oviso, J.; Gupta, D.; Raju, P. A.; Natan, M. J.;Freeman, R. G. Anal. Chem. 2002, 74, 2240.(356) Qin, L.; Park, S.; Huang, L.; Mirkin, C. A. Science 2005, 309,113.(357) Chen, X.; Yeganeh, S.; Qin, L.; Li, S.; Xue, C.; Braunschweig,A. B.; Schartz, G. C.; Ratner, M. A.; Mirkin, C. A. Nano Lett. 2009, 9,3974.(358) Qin, L.; Banholzer, M. J.; Millstone, J. E.; Mirkin, C. A. NanoLett. 2007, 7, 3849.(359) Banholzer, M. J.; Qin, L.; Millstone, J. E.; Osberg, K. D.;Mirkin, C. A. Nat. Protoc. 2009, 4, 838.(360) Park, S.; Lim, J.-H.; Chung, S.-W.; Mirkin, C. A. Science 2004,303, 348.(361) Ciszek, J. W.; Huang, L.; Wang, Y.; Mirkin, C. A. Small 2008,4, 206.(362) Yahalom, J.; Zadok, O. J. Mater. Sci. 1987, 22, 499.(363) Cohen-Hyams, T.; Plitzko, J. M.; Hetherington, C. J. D.;Hutchison, J. L.; Yahalom, J.; Kaplan, W. D. J. Mater. Sci. 2004, 39,5701.(364) Uhlemann, M.; Gebert, A.; Herrich, M.; Krause, A.; Cziraki, A.;Schultz, L. Electrochim. Acta 2003, 48, 3005.(365) Marsza, M.; Jaworski, J.; Michalik, A.; Prokop, J.; Stachura, Z.;Voznyi, V.; Bolling, O.; Sulkio-Cleff, B. J. Magn. Magn. Mater. 2001,226−230, 1735.(366) Urbaniak, M.; Lucin ski, T.; Stobiecki, F. J. Magn. Magn. Mater.1998, 190, 187.(367) Piraux, L.; George, J. M.; Despres, J. F.; Leroy, C.; Ferain, E.;Legras, R.; Ounadjela, K.; Fert, A. Appl. Phys. Lett. 1994, 65, 2484.(368) Blondel, A.; Meier, J. P.; Doudin, B.; Ansermet, J.-P. Appl. Phys.Lett. 1994, 65, 3019.(369) Liu, K.; Nagodawithana, K.; Searson, P. C.; Chien, C. L. Phys.Rev. B 1995, 51, 7381.(370) Tourillon, G.; Pontonnier, L.; Levy, J. P.; Langlais, V.Electrochem. Solid-State Lett. 2000, 3, 20.(371) Guo, Y.-G.; Wan, J.-J.; Zhu, C.-F.; Yang, D.-L.; Chen, D.-M.;Bai, C.-L. Chem. Mater. 2003, 15, 664.(372) Chen, M.; Searson, P. C.; Chien, C. L. J. Appl. Phys. 2003, 93,8253.(373) Dubois, S.; Marchal, C.; Beuken, J. M.; Piraux, L.; Duvail, J. L.;Fert, A.; George, J. M.; Maurice, J. L. Appl. Phys. Lett. 1999, 70, 396.(374) Liu, L.; Lee, W.; Scholz, R.; Pippel, E.; Gosele, U. Angew.Chem., Int. Ed. 2008, 47, 7004.(375) Lee, W.; Scholz, R.; Nielsch, K.; Gosele, U. Angew. Chem., Int.Ed. 2005, 44, 6050.(376) Yuan, X. Y.; Wu, G. S.; Xie, T.; Geng, B. Y.; Lin, Y.; Meng, G.W.; Zhanga, L. D. Solid State Sci. 2004, 6, 735.(377) Yuan, X. Y.; Wu, G. S.; Xie, T.; Lin, Y.; Zhang, L. D.Nanotechnology 2004, 15, 59.(378) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920.

(379) Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268,700.(380) Jirage, K. B.; Hulteen, J. C.; Martin, C. R. Science 1997, 278,655.(381) Kohli, P.; Wharton, J. E.; Braide, O.; Martin, C. R. J. Nanosci.Nanotechnol. 2004, 4, 605.(382) Jirage, K. B.; Hulteen, J. C.; Martin, C. R. Anal. Chem. 1999,71, 4913.(383) Martin, C. R.; Nishizawa, M.; Jirage, K.; Kang, M. J. Phys.Chem. B 2001, 105, 1925.(384) Martin, C. R.; Nishizawa, M.; Jirage, K.; Kang, M.; Lee, S. B.Adv. Mater. 2001, 13, 1351.(385) Lee, S. B.; Martin, C. R. Chem. Mater. 2001, 13, 3236.(386) Lee, S. B.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11850.(387) Siwy, Z.; Heins, E.; Harrell, C. C.; Kohli, P.; Martin, C. R. J.Am. Chem. Soc. 2004, 126, 10850.(388) Yu, S.; Lee, S. B.; Kang, M.; Martin, C. R. Nano Lett. 2001, 1,495.(389) Wirtz, M.; Yu, S.; Martin, C. R. Analyst 2002, 127, 871.(390) Kobayashi, Y.; Martin, C. R. Anal. Chem. 1999, 71, 3665.(391) Wirtz, M.; Parker, M.; Kobayashi, Y.; Martin, C. R. Chem. Rec.2002, 2, 259.(392) Steinle, E. D.; Mitchell, D. T.; Wirtz, M.; Lee, S. B.; Young, V.Y.; Martin, C. R. Anal. Chem. 2002, 74, 2416.(393) Kohli, P.; Wirtz, M.; Martin, C. R. Electroanalysis 2004, 16, 9.(394) Yu, Y.; Kant, K.; Shapter, J. G.; Addai-Mensah, J.; Losic, D.Microporous Mesoporous Mater. 2012, 153, 131.(395) Volpe, M.; Inguanta, R.; Piazza, S.; Sunseri, C. Surf. Coat.Technol. 2006, 200, 5800.(396) Inguanta, R.; Amodeo, M.; D’Agostino, F.; Volpe, M.; Piazza,S.; Sunseri, C. J. Electrochem. Soc. 2007, 154, D188.(397) Wu, J. P.; Brown, I. W. M.; Bowden, M. E.; Kemmitt, T. SolidState Sci. 2010, 12, 1912.(398) Wang, W.; Li, N.; Li, X.; Geng, W.; Qiu, S. Mater. Res. Bull.2006, 41, 1417.(399) Li, N.; Li, X.; Yin, X.; Wang, W.; Qiu, S. Solid State Commun.2004, 132, 841.(400) Chu, S. Z.; Kawamura, H.; Mori, M. J. Electrochem. Soc. 2008,155, D414.(401) Azizi, A.; Mohammadi, M.; Sadrnezhaad, S. K. Mater. Lett.2011, 65, 289.(402) Yuan, X.; Du, C.; Sun, G.; Pan, N. Appl. Surf. Sci. 2007, 253,4546.(403) Nagaura, T.; Takeuchi, F.; Yamauchi, Y.; Wada, K.; Inoue, S.Electrochem. Commun. 2008, 10, 681.(404) Lakshmi, B. B.; Dorhout, P. K.; Martin, C. R. Chem. Mater.1997, 9, 857.(405) Lakshmi, B. B.; Patrissi, C. J.; Martin, C. R. Chem. Mater. 1997,9, 2544.(406) Hulteen, J. C.; Martin, C. R. J. Mater. Chem. 1997, 7, 1075.(407) Foong, T. R. B.; Shen, Y.; Hu, X.; Sellinger, A. Adv. Funct.Mater. 2010, 20, 1390.(408) Kang, T.-S.; Smith, A. P.; Taylor, B. E.; Durstock, M. F. NanoLett. 2009, 9, 601.(409) Wang, G. X.; Park, J. S.; Park, M. S.; Gou, X. L. Sens. Actuators,B 2008, 131, 313.(410) Ozturk, S.; Kılınc, N.; TasAltin, N.; Ozturk, Z. Z. Thin SolidFilms 2011, 520, 932.(411) Ji, G.; Tang, S.; Xu, B.; Gu, B.; Du, Y. Chem. Phys. Lett. 2003,379, 484.(412) Wu, Z.; Niu, P.; Yang, B.; Yu, R. Appl. Phys. A: Mater. Sci.Process. 2011, 105, 177.(413) Xu, X.; Qian, T.; Zhang, G.; Zhang, T.; Li, G.; Wang, W.; Li, X.Chem. Lett. 2007, 36, 112.(414) Feng, M.; Wang, W.; Zhou, Y.; Jia, D. J. Sol-Gel Sci. Technol.2009, 52, 120.(415) Min, H.-S.; Lee, J.-K. Ferroelectrics 2006, 336, 231.(416) Zhang, G.; Lu, X.; Zhang, T.; Qu, J.; Wang, W.; Li, X.; Yu, S.Nanotechnology 2006, 17, 4252.



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(417) Lai, S. H.; Hsu, Y. C.; Lan, M. D. Solid State Commun. 2008,148, 452.(418) Wu, G. S.; Lin, Y.; Yuan, X. Y.; Xie, T.; Cheng, B. C.; Zhang, L.D. Nanotechnology 2004, 15, 568.(419) Lei, Y.; Zhang, L. D.; Meng, G. W.; Li, G. H.; Zhang, X. Y.;Liang, C. H.; Chen, W.; Wang, S. X. Appl. Phys. Lett. 2001, 78, 1125.(420) Wu, G. S.; Zhuang, Y. L.; Lin, Z. Q.; Yuan, X. Y.; Xie, T.;Zhang, L. D. Physica E 2006, 31, 5.(421) Zhang, M.; Bando, Y.; Wada, K. J. Mater. Res. 2000, 15, 387.(422) Limmer, S. J.; Seraji, S.; Forbess, M. J.; Wu, Y.; Chou, T. P.;Nguyen, C.; Cao, G. Adv. Mater. 2001, 13, 1269.(423) Limmer, S. J.; Seraji, S.; Wu, Y.; Chou, T. P.; Nguyen, C.; Cao,G. Adv. Funct. Mater. 2002, 12, 59.(424) Miao, Z.; Xu, D.; Ouyang, J.; Guo, G.; Zhao, X.; Tang, Y. NanoLett. 2002, 2, 717.(425) Lee, S. B.; Mitchell, D. T.; Trofin, L.; Nevanen, T. K.;Soderlund, H.; Martin, C. R. Science 2002, 296, 2198.(426) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N.; Nevanen, T. K.;Soderlund, H.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11864.(427) Son, S. J.; Lee, S. B. J. Am. Chem. Soc. 2006, 128, 15974.(428) Buyukserin, F.; Martin, C. R. Appl. Surf. Sci. 2010, 256, 7700.(429) Hillebrenner, H.; Buyukseri, F.; Kang, M.; Mota, M. O.;Stewart, J. D.; Martin, C. R. J. Am. Chem. Soc. 2006, 128, 4236.(430) Chen, C.-C.; Liu, Y.-C.; Wu, C.-H.; Yeh, C.-C.; Su, M.-T.; Wu,Y.-C. Adv. Mater. 2005, 17, 404.(431) Yamacuchi, A.; Uejo, F.; Yoda, T.; Uchida, T.; Tanamura, Y.;Yamashita, T.; Teramae, N. Nat. Mater. 2004, 3, 337.(432) Platschek, B.; Petkov, N.; Bein, T. Angew. Chem., Int. Ed. 2006,45, 1134.(433) Platschek, B.; Petkov, N.; Himsl, D.; Zimdars, S.; Li, Z.; Kohn,R.; Bein, T. J. Am. Chem. Soc. 2008, 130, 17362.(434) Wu, Y.; Cheng, G.; Katsov, K.; Sides, S. W.; Wang, J.; Tang, J.;Fredrickson, G. H.; Moskovits, M.; Stucky, G. D. Nat. Mater. 2004, 3,816.(435) Chang, C.-S.; Suen, S.-Y. J. Membr. Sci. 2006, 275, 70.(436) Cheow, P.-S.; Liu, L.; Toh, C.-S. Surf. Interface Anal. 2007, 39,601.(437) Oberg, K.; Persson, P.; Shchukarev, A.; Eliassona, B. Thin SolidFilms 2001, 397, 102.(438) Liakos, I. L.; Newman, R. C.; McAlpine, E.; Alexander, M. R.Surf. Interface Anal. 2004, 36, 37.(439) Chen, Y.-F.; Hu, Y.-H.; Chou, Y.-I.; Lai, S.-M.; Wang, C.-C.Sens. Actuators, B 2010, 145, 575.(440) Yildirim, O.; Gang, T.; Kinge, S.; Reinhoudt, D. N.; Blank, D.H. A.; Van der Wiel, W. G.; Rijnders, G.; Huskens, J. Int. J. Mol. Sci.2010, 11, 1162.(441) Jani, A. M. M.; Anglin, E. J.; McInnes, S. J. P.; Losic, D.;Shapter, J. G.; Voelcker, N. H. Chem. Commun. 2009, 3062.(442) Jani, A. M. M.; Kempson, I. M.; Losic, D.; Voelcker, N. H.Angew. Chem., Int. Ed. 2010, 49, 7933.(443) Ko, H.; Tsukruk, V. V. Small 2008, 4, 1980.(444) Dotzauer, D. M.; Dai, J.; Sun, L.; Bruening, M. L. Nano Lett.2006, 6, 2268.(445) He, Q.; Cui, Y.; Ai, S.; Tian, Y.; Li, J. Curr. Opin. ColloidInterface Sci. 2009, 14, 115.(446) Lahav, M.; Sehayek, T.; Vaskevich, A.; Rubinstein, I. Angew.Chem., Int. Ed. 2003, 42, 5576.(447) Hobler, C.; Bakowsky, U.; Keusgen, M. Phys. Status Solidi A2010, 207, 872.(448) Thormann, A.; Teuscher, N.; Pfannmcller, M.; Rothe, U.;Heilmann, A. Small 2007, 3, 1032.(449) Lu, Z.; Ruan, W.; Yang, J.; Xu, W.; Zhao, C.; Zhao, B. J. RamanSpectrosc. 2008, 40, 112.(450) Ji, N.; Ruan, W.; Wang, C.; Lu, Z.; Zhao, B. Langmuir 2009, 25,11869.(451) Sehayek, T.; Lahav, M.; Popovitz-Biro, R.; Vaskevich, A.;Rubinsein, I. Chem. Mater. 2005, 17, 3743.(452) Tanvir, S.; Pantigny, J.; Boulnois, P.; Pulvin, S. J. Membr. Sci.2009, 329, 85.

(453) Swan, E. E. L.; Popat, K. C.; Desai, T. A. Biomaterials 2005, 26,1969.(454) Li, P.-F.; Xie, R.; Jiang, J.-C.; Meng, T.; Yang, M.; Ju, X.-J.;Yang, L.; Chu, L.-Y. J. Membr. Sci. 2009, 337, 310.(455) Sun, L.; Dai, J.; Baker, G. L.; Bruening, M. L. Chem. Mater.2006, 18, 4033.(456) Jain, P.; Sun, L.; Dai, J.; Baker, G. L.; Bruening, M. L.Biomacromolecules 2007, 8, 3102.(457) Balachandra, A. M.; Baker, G. L.; Bruening, M. L. J. Membr. Sci.2003, 227, 1.(458) Sun, L.; Baker, G. L.; Bruening, M. L. Macromolecules 2005, 38,2307.(459) Grajales, S. T.; Dong, X.; Zheng, Y.; Baker, G. L.; Bruening, M.L. Chem. Mater. 2010, 22, 4026.(460) Shi, W.; Shen, Y.; Ge, D.; Xue, M.; Cao, H.; Huang, S.; Wang,J.; Zhang, G.; Zhang, F. J. Membr. Sci. 2008, 325, 801.(461) Song, C.; Shi, W.; Jiang, H.; Tu, J.; Ge, D. J. Membr. Sci. 2011,372, 340.(462) Bruening, M. L.; Dotzauer, D. M.; Jain, P.; Ouyang, L.; Baker,G. L. Langmuir 2008, 24, 7663.(463) Decher, G. Science 1997, 277, 1232.(464) Jin, W.; Toutianoush, A.; Tieke, B. Langmuir 2003, 19, 2550.(465) Toutianoush, A.; Tieke, B. Mater. Sci. Eng., C 2002, 22, 135.(466) Peyratout, C. S.; Dahne, L. Angew. Chem., Int. Ed. 2004, 43,3762.(467) Balachandra, A. M.; Dai, J.; Bruening, M. L. Macromolecules2002, 35, 3171.(468) Liu, X.; Bruening, M. L. Chem. Mater. 2004, 16, 351.(469) Hong, S. U.; Ouyang, L.; Bruening, M. L. J. Membr. Sci. 2009,327, 2.(470) Ouyang, L.; Malaisamy, R.; Bruening, M. L. J. Membr. Sci.2008, 310, 76.(471) Dai, J.; Baker, G. L.; Brening, M. L. Anal. Chem. 2006, 78, 135.(472) Liang, Z.; Susha, A. S.; Yu, A.; Caruso, F. Adv. Mater. 2003, 15,1849.(473) Lee, D.; Nolte, A. J.; Kunz, A. L.; Rubner, M. F.; Cohen, R. E. J.Am. Chem. Soc. 2006, 128, 8521.(474) Lee, D.; Cohen, R. E.; Rubner, M. F. Langmuir 2007, 23, 123.(475) Ai, S.; Lu, G.; He, Q.; Li, J. J. Am. Chem. Soc. 2003, 125, 11140.(476) Feng, C.-L.; Zhong, X.; Steinhart, M.; Caminade, A.-M.;Majoral, J.-P.; Knoll, W. Adv. Mater. 2007, 19, 1993.(477) Tian, Y.; He, Q.; Tao, C.; Li, J. Langmuir 2006, 22, 360.(478) Alem, H.; Blondeau, F.; Glinel, K.; Demoustier-Champagne, S.;Jonas, A. M. Macromolecules 2007, 40, 3366.(479) Cho, Y.; Lee, W.; Jhon, Y. K.; Genzer, J.; Char, K. Small 2010,6, 2683.(480) Hou, S.; Harrell, C. C.; Trofin, L.; Kohli, P.; Martin, C. R. J.Am. Chem. Soc. 2004, 126, 5674.(481) Quinn, J. F.; Johnston, A. P. R.; Such, G. K.; Zelikin, A. N.;Caruso, F. Chem. Soc. Rev. 2007, 36, 707.(482) Kongsuphol, P.; Fang, K. B.; Ding, Z. Sens. Actuators, B 2013,185, 530.(483) Hirano-Iwata, A.; Taira, T.; Oshima, A.; Kimura, Y.; Niwano,M. Appl. Phys. Lett. 2010, 96, 213706.(484) Bally, M.; Bailey, K.; Sugihara, K.; Grieshaber, D.; Voros, J.;Stadler, a. B. Small 2010, 6, 2481.(485) Hirano-Iwata, A.; Niwano, M.; Sugawara, M. Trends Anal.Chem. 2008, 27, 512.(486) Hennesthal, C.; Steinem, C. J. Am. Chem. Soc. 2000, 122, 8085.(487) Naumann, R.; Jonczyk, A.; Kopp, R.; van Esch, J.; Ringsdorf,H.; Knoll, W.; Griiber, P. Angew. Chem., Int. Ed. 1995, 34, 2056.(488) Cremer, P. S.; Boxer, S. G. J. Phys. Chem. B 1999, 103, 2554.(489) Radler, J.; Strey, H.; Sackmann, E. Langmuir 1995, 11, 4539.(490) Sackmann, E. Science 1995, 271, 43.(491) Sackmann, E.; Tanaka, M. Trends Biotechnol. 2000, 18, 58.(492) Heyse, S.; Ernst, O. P.; Dienes, Z.; Hofmann, K. P.; Vogel, A.H. Biochemistry 1998, 37, 507.(493) Mayer, M.; Kriebel, J. K.; Tosteson, M. T.; Whitesides, G. M.Biophys. J. 2003, 85, 2684.



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(494) Simon, A.; Girard-Egrot, A.; Sauter, F.; Pudda, C.; D’Hahan, N.P.; Blum, L.; Chatelain, F.; Fuchs, A. J. Colloid Interface Sci. 2007, 308,337.(495) Hennesthal, C.; Drexler, J.; Steinem, C. ChemPhysChem 2002,10, 885.(496) Romer, W.; Steinem, C. Biophys. J. 2004, 86, 955.(497) Schmitt, E. K.; Vrouenraets, M.; Steinem, C. Biophys. J. 2006,91, 2163.(498) Schmitt, E. K.; Nurnabi, M.; Bushby, R. J.; Steinem, C. SoftMatter 2008, 4, 250.(499) Schmitt, E. K.; Weichbrodt, C.; Steinem, C. Soft Matter 2009,5, 3347.(500) Lazzara, T. D.; Carnarius, C.; Kocun, M.; Janshoff, A.; Steinem,C. ACS Nano 2011, 5, 6935.(501) Hotta, K.; Yamaguchi, A.; Teramae, N. ACS Nano 2013, 6,1541.(502) Smirnov, A. I.; Poluektov, O. G. J. Am. Chem. Soc. 2003, 125,8434.(503) Chekmenev, E. Y.; Gor’kov, P. L.; Cross, T. A.; Alaouie, A. M.;Smirnov, A. I. Biophys. J. 2006, 91, 3076.(504) Li, R.-Q.; Marek, A.; Smirnov, A. I.; Grebel, H. J. Chem. Phys.2008, 129, 095102.(505) Deme, B.; Marchal, D. Eur. Biophys. J. 2005, 34, 170.(506) Parthasarathy, R. V.; Martin, C. R. Nature 1994, 369, 298.(507) Cai, Z.; Lei, J.; Liang, W.; Menon, V.; Martin, C. R. Chem.Mater. 1991, 3, 960.(508) Parthasarathy, R. V.; Martin, C. R. Chem. Mater. 1994, 6, 1627.(509) Martin, C. R. Acc. Chem. Res. 1995, 28, 61.(510) Parthasarathy, R. V.; Phani, K. L. N.; Martin, C. R. Adv. Mater.1995, 7, 896.(511) Steinhart, M.; Wendorff, J. H.; Greiner, A.; Wehrspohn, R. B.;Nielsch, K.; Schilling, J.; Choi, J.; Gosele, U. Science 2002, 296, 1997.(512) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827.(513) Ausserre, D.; Picard, A. M.; Leger, L. Phys. Rev. Lett. 1986, 57,2671.(514) Heslot, F.; Cazabat, A. M.; Levinson, P. Phys. Rev. Lett. 1989,62, 1286.(515) Steinhart, M.; Wendorff, J. H.; Wehrspohn, R. B.ChemPhysChem 2003, 4, 1171.(516) Steinhart, M.; Wehrspohn, R. B.; Gosele, U.; Wendorff, J. H.Angew. Chem., Int. Ed. 2004, 43, 1334.(517) Lee, W.; Jin, M.-K.; Yoo, W.-C.; Lee, J.-L. Langmuir 2004, 20,7665.(518) Steinhart, M.; Senz, S.; Wehrsphon, R. B.; Gosele, U.;Wendorff, J. H. Macromolecules 2003, 36, 3646.(519) Steinhart, M.; Zimmermann, S.; Goring, P.; Schaper, A. K.;Gosele, U.; Weder, C.; Wndorff, J. H. Nano Lett. 2005, 5, 429.(520) Steinhart, M.; Murano, S.; Shaper, A. K.; Ogawa, T.; Tsuji, M.;Gosele, U.; Weder, C.; Wendorff, J. H. Adv. Funct. Mater. 2005, 15,1656.(521) Steinhart, M.; Jia, Z.; Schaper, A. K.; Wehrspohn, R. B.; Gosele,U.; Wendorff, J. H. Adv. Mater. 2003, 15, 706.(522) Luo, Y.; Lee, S. K.; Hofmeister, H.; Steinhart, M.; Gosele, U.Nano Lett. 2004, 4, 143.(523) Lei, Y.; Cai, W.; Wilde, G. Prog. Mater. Sci. 2007, 52, 465.(524) Sulka, G. D.; Zaraska, L.; Stępniowski, W. J. Anodic PorousAlumina as a Template for Nanofabrication. In Encylopedia ofNanoscience and Nanotechnology, 2nd ed.; Nalwa, H. S., Ed.; AmericanScientific Publishers: Califonia, 2011; Vol. 11, pp 261−349.(525) Meng, G.; Yanagida, T.; Nagashima, K.; Yanagishita, T.; Kanai,M.; Oka, K.; Klamchuen, A.; Rohong, S.; Horprathum, M.; Xu, B.;Zhuge, F.; He, Y.; Masuda, H.; Kawai, T. RSC Adv. 2012, 2, 10618.(526) Mei, X.; Kim, D.; Ruda, H. E.; Guo, Q. X. Appl. Phys. Lett.2003, 81, 361.(527) Lei, Y.; Chim, W.-K. Chem. Mater. 2005, 17, 580.(528) Lei, Y.; Chim, W. K.; Sun, H. P.; Wilde, G. Appl. Phys. Lett.2005, 86, 103106.(529) Cloutier, S. G.; Guico, R. S.; Xu, J. M. Appl. Phys. Lett. 2005,87, 222104.

(530) Mei, X.; Blumin, M.; Kim, D.; Wu, Z.; Ruda, H. E. J. Cryst.Growth 2003, 251, 253.(531) Jung, M.; Lee, H. S.; Park, H. L.; Mho, S.-i. Curr. Appl. Phys.2006, 6S1, e187.(532) Mei, X.; Blumin, M.; Sun, M.; Kim, D.; Wu, Z. H.; Ruda, H. E.;Guo, Q. X. Appl. Phys. Lett. 2003, 82, 967.(533) Chen, Z.; Lei, Y.; Chew, H. G.; Teo, L. W.; Choi, W. K.; Chim,W. K. J. Cryst. Growth 2004, 268, 560.(534) Xu, W. L.; Zheng, M. J.; Ding, G. Q.; Shen, W. Z. Chem. Phys.Lett. 2005, 411, 37.(535) Guo, Q.; Mei, X.; Ruda, H.; Tanaka, T.; Nishio, M.; Ogawa, H.Jpn. J. Appl. Phys. 2003, 42, L508.(536) Masuda, H.; Yasui, K.; Nishio, K. Adv. Mater. 2000, 12, 1031.(537) Liu, K.; Nogues, J.; Leighton, C.; Masuda, H.; Nishio, K.;Roshchin, I. V.; Schuller, I. K. Appl. Phys. Lett. 2002, 81, 4434.(538) Nam, W.; Seo, H.; Park, S. C.; Bae, C. H.; Nam, S. H.; Park, S.M.; Ha, J. S. Jpn. J. Appl. Phys. 2004, 43, 7793.(539) Kossyrev, P. A.; Yin, A.; Cloutier, S. G.; Cardimona, D. A.;Huang, D.; Alsing, P. M.; Xu, J. M. Nano Lett. 2005, 5, 1978.(540) Lei, Y.; Chim, W.-K. J. Am. Chem. Soc. 2005, 127, 1487.(541) Gao, X.; Liu, L.; Birajdar, B.; Ziese, M.; Lee, W.; Alexe, M.;Hesse, D. Adv. Funct. Mater. 2009, 19, 3450.(542) Lee, W.; Han, H.; Lotnyk, A.; Schubert, M. A.; Senz, S.; Alexe,M.; Hesse, D.; Baik, S.; Gosele, U. Nat. Nanotechnol. 2008, 3, 402.(543) Rodriguez, B. J.; Gao, X. S.; Liu, L. F.; Lee, W.; Naumov, I. I.;Bratkovsky, A. M.; Hesse, D.; Alexe, M. Nano Lett. 2009, 9, 1127.(544) Kim, Y.; Han, H.; Lee, W.; Baik, S.; Hesse, D.; Alexe, M. NanoLett. 2010, 10, 1266.(545) Han, H.; Park, Y. J.; Baik, S.; Lee, W.; Alexe, M.; Hesse, D.;Gosele, U. J. Appl. Phys. 2010, 108, 044102.(546) Han, H.; Kim, Y.; Alexe, M.; Hesse, D.; Lee, W. Adv. Mater.2011, 23, 4599.(547) Cheng, G.; Moskovits, M. Adv. Mater. 2002, 14, 1567.(548) Liang, J.; Chik, H.; Yin, A.; Xu, J. J. Appl. Phys. 2002, 91, 2544.(549) Liang, J.; Hong, S.-K.; Kouklin, N.; Beresford, B.; Xu, J. M.Appl. Phys. Lett. 2003, 83, 1752.(550) Shingubara, S.; Okino, O.; Murakami, Y.; Sakaue, H.; Takahagi,T. J. Vac. Sci. Technol., B 2001, 19, 1901.(551) Nakao, M.; Oku, S.; Tamamura, T.; Yasui, K.; Masuda, H. Jpn.J. Appl. Phys. 1999, 38, 1052.(552) Guo, Q.; Takaka, T.; Nishio, M.; Ogawa, H.; Mei, X.; Ruda, H.Jpn. J. Appl. Phys. 2002, 41, L118.(553) Kouklin, N.; Chik, H.; Liang, J.; Tzolov, M.; Xu, J. M.; Heroux,J. B.; Wang, W. I. J. Phys. D: Appl. Phys. 2003, 36, 2634.(554) Liang, J.; Luo, H.; Beresford, R.; Xu, J. Appl. Phys. Lett. 2004,85, 5974.(555) Kanamori, Y.; Hane, K.; Sai, H.; Yugami, H. Appl. Phys. Lett.2001, 78, 142.(556) Tian, L.; Ram, K. B.; Ahmad, I.; Menon, L.; Holtz, M. J. Appl.Phys. 2005, 97, 026101.(557) Masuda, H.; Watanabe, M.; Yasui, K.; Tryk, D.; Rao, T.;Fujishima, A. Adv. Mater. 2000, 12, 444.(558) Masuda, H.; Yasui, K.; Watanabe, M.; Nishio, K.; Nakao, M.;Tamamura, T.; Rao, T. N.; Fujishima, A. Electrochem. Solid-State Lett.2001, 4, G101.(559) Honda, K.; Rao, T. N.; Tryk, D. A.; Fujishima, A.; Watanabe,M.; Yasui, K. J. Electrochem. Soc. 2001, 148, A668.(560) Honda, K.; Rao, T. N.; Tryk, D. A.; Fujishima, A.; Watanabe,M.; Yasui, K.; Masuda, H. J. Electrochem. Soc. 2000, 147, 659.(561) Nakao, M.; Oku, S.; Tanaka, H.; Shibata, Y.; Yokoo, A.;Tamamura, T.; Masuda, H. Opt. Quantum Electron. 2002, 34, 183.(562) Wang, Y. D.; Chua, S. J.; Sander, M. S.; Chen, P.; Tripathy, S.;Fonstad, C. G. Appl. Phys. Lett. 2004, 85, 816.(563) Hobbs, K. L.; Larson, P. R.; Lian, G. D.; Keay, J. C.; Johnson,M. B. Nano Lett. 2004, 4, 167.(564) Lombardi, I.; Hochbaum, A. I.; Yang, P.; Carraro, C.;Maboudian, R. Chem. Mater. 2006, 18, 988.(565) Lei, Y.; Yeong, K.-S.; Thong, J. T. L.; Chim, W.-K. Chem.Mater. 2004, 16, 2757.



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(566) Wu, Z. H.; Mei, X. Y.; Kim, D.; Blumin, M.; Ruda, H. E. Appl.Phys. Lett. 2002, 81, 5177.(567) Chik, H.; Liang, J.; Cloutier, S. G.; Kouklin, N.; Xu, J. M. Appl.Phys. Lett. 2004, 84, 3376.(568) Lee, W.; Alexe, M.; Nielsch, K.; Gosele, U. Chem. Mater. 2005,17, 3325.(569) Fan, H. J.; Lee, W.; Hauschild, R.; Alexe, M.; Le Rhun, G.;Scholz, R.; Dadgar, A.; Nielsch, K.; Kalt, H.; Krost, A.; Zacharias, M.;Gosele, U. Small 2006, 2, 561.(570) Fan, H. J.; Lee, W.; Scholz, R.; Dadgar, A.; Krost, A.; Nielsch,K.; Zacharias, M. Nanotechnology 2005, 16, 913.(571) Lee, S. K.; Lee, W.; Alexe, M.; Nielsch, K.; Hesse, D.; Gosele,U. Appl. Phys. Lett. 2005, 86, 152906.(572) Lee, S. K.; Hesse, D.; Alexe, M.; Lee, W.; Nielsch, K.; Gosele,U. J. Appl. Phys. 2005, 98, 124302.(573) Park, S.-J.; Han, H.; Rhu, H.; Baik, S.; Lee, W. J. Mater. Chem.C 2013, 1, 5330.(574) Han, H.; Kim, J.; Shin, H. S.; Song, J. Y.; Lee, W. Adv. Mater.2012, 24, 2284.(575) Kim, J.; Han, H.; Kim, Y. H.; Choi, S.-H.; Kim, J.-C.; Lee, W.ACS Nano 2011, 5, 3222.(576) Kim, J.; Kim, Y. H.; Choi, S.-H.; Lee, W. ACS Nano 2011, 5,5242.(577) Kim, J.; Rhu, H.; Lee, W. J. Mater. Chem. 2011, 21, 15889.(578) Huang, Z.; Zhang, X.; Reiche, M.; Liu, L.; Lee, W.; Shimizu, T.;Senz, S.; Gosele, U. Nano Lett. 2008, 8, 3046.(579) Hulteen, J. C.; Martin, C. R. Template synthesis ofnanoparticles in nanoporous membranes. In Nanoparticles andNanostructured Films: Preparation, Characterization and Applications;Fendler, J. H., Ed.; Wiley-VCH: New York, 1998; pp 235−262.(580) Shingubara, S. J. Nanopart. Res. 2003, 5, 17.(581) Kyotani, T.; Tsai, L.-f.; Tomita, A. Chem. Mater. 1996, 8, 2109.(582) Che, G.; Lakshmi, B. B.; Martin, C. R.; Fisher, E. R. Chem.Mater. 1998, 10, 260.(583) Li, J.; Papadopoulos, C.; Xu, J. M. Appl. Phys. Lett. 1999, 75,367.(584) Suh, J. S.; Lee, J. S. Appl. Phys. Lett. 1999, 75, 2047.(585) Jeong, S.-H.; Hwang, H.-Y.; Lee, K.-H.; Jeong, Y. Appl. Phys.Lett. 2001, 78, 2052.(586) Yanagishita, T.; Sasaki, M.; Nishio, K.; Masuda, H. Adv. Mater.2004, 16, 429.(587) Meng, G.; Jung, Y. J.; Cao, A.; Vajtai, R.; Ajayan, P. M. Proc.Natl. Acad. Sci. U.S.A. 2005, 102, 7074.(588) Chen, Q.-L.; Xue, K.-H.; Shen, W.; Tao, F.-F.; Yin, S.-Y.; Xu,W. Electrochim. Acta 2004, 49, 4157.(589) Park, S.; Kim, Y.-S.; Kim, W. B.; Jon, S. Nano Lett. 2009, 9,1325.(590) Schmidt, V.; Wittemann, J. V.; Gosele, U. Chem. Rev. 2010,110, 361.(591) Schmidt, V.; Senz, S.; Gosele, U. Nano Lett. 2005, 5, 931.(592) Shimizu, T.; Xie, T.; Nishikawa, J.; Shingubara, S.; Senz, S.;Gosele, U. Adv. Mater. 2007, 19, 917.(593) Gorisse, T.; Dupre, L.; Gentile, P.; Martin, M.; Zelsmann, M.;Buttard, D. Nanoscale Res. Lett. 2013, 8, 287.(594) Cheng, G. S.; Zhang, L. D.; Zhu, Y.; Fei, G. T.; Li, L.; Mo, C.M.; Mao, Y. Q. Appl. Phys. Lett. 1999, 75, 2455.(595) Zhang, J.; Zhang, L. D.; Wang, X. F.; Liang, C. H.; Peng, X. S.;Wang, Y. W. J. Appl. Phys. 2001, 115, 5714.(596) Jung, W.-G.; Jung, S.-H.; Kung, P.; Razeghi, M. Nanotechnology2006, 17, 54.(597) Li, X.; Meng, G.; Xu, Q.; Kong, M.; Zhu, X.; Chu, Z.; Li, A.-P.Nano Lett. 2011, 11, 1704.(598) Li, X.; Meng, G.; Qin, S.; Xu, Q.; Chu, Z.; Zhu, X.; Kong, M.;Li, A.-P. ACS Nano 2012, 6, 831.(599) Shen, X.-P.; Yuan, A.-H.; Wang, F.; Hong, J.-M.; Xu, Z. SolidState Commun. 2005, 133, 19.(600) Shen, X.-P.; Liu, J.-J.; Fan, X.; Jiang, Y.; Hong, J.-M.; Xu, Z. J.Cryst. Growth 2005, 276, 471.

(601) Shen, X.-P.; Han, M.; Hong, J.-M.; Xue, Z.; Xu, Z. Chem. Vap.Deposition 2005, 11, 250.(602) Fan, Z.; Razavi, H.; Do, J.-w.; Moriwaki, A.; Ergen, O.; Chueh,Y.-L.; Leu, P. W.; Ho, J. C.; Takahashi, T.; Reichertz, L. A.; Neale, S.;Yu, K.; Wu, M.; Ager, J. W.; Javey, A. Nat. Mater. 2009, 8, 648.(603) Fan, Z.; Dutta, D.; Chien, C.-J.; Chen, H.-Y.; Brown, E. C.Appl. Phys. Lett. 2006, 89, 213110.(604) Bae, C.; Yoo, H.; Kim, S.; Lee, K.; Kim, J.; Sung, M. M.; Shin,H. Chem. Mater. 2008, 20, 756.(605) Detavernier, C.; Dendooven, J.; Pulinthanathu Sree, S.;Ludwig, K. F.; Martens, J. A. Chem. Soc. Rev. 2011, 40, 5242.(606) George, S. M. Chem. Rev. 2009, 110, 111.(607) Knez, M.; Nielsch, K.; Niinisto, L. Adv. Mater. 2007, 19, 3425.(608) Marichy, C.; Bechelany, M.; Pinna, N. Adv. Mater. 2012, 24,1017.(609) Narayan, R.; Monteiro-Riviere, N. A.; Brigmon, R. L.; Pellin,M. J.; Elam, J. W. JOM 2009, 61, 12.(610) Ott, A. W.; Klaus, J. W.; Johnson, J. M.; George, S. M.;McCarley, K. C.; Way, J. D. Chem. Mater. 1997, 9, 707.(611) Chen, P.; Mitsui, T.; Farmer, D. B.; Golovchenko, J.; Gordon,R. G.; Branton, D. Nano Lett. 2004, 4, 1333.(612) Velleman, L.; Triani, G.; Evans, P. J.; Shapter, J. G.; Losic, D.Microporous Mesoporous Mater. 2009, 126, 87.(613) Kim, W.-H.; Park, S.-J.; Son, J.-Y.; Kim, H. Nanotechnology2008, 19, 045302.(614) Chang, Y.-H.; Wang, S.-M.; Liu, C.-M.; Chen, C. J. Electrochem.Soc. 2010, 157, K236.(615) Yang, C.-J.; Wang, S.-M.; Liang, S.-W.; Chang, Y.-H.; Chen, C.;Shieh, J.-M. Appl. Phys. Lett. 2007, 90, 033104.(616) Bachmann, J.; Jing, J.; Knez, M.; Barth, S.; Shen, H.; Mathur,S.; Gosele, U.; Nielsch, K. J. Am. Chem. Soc. 2007, 129, 9554.(617) Bae, C.; Yoon, Y.; Yoo, H.; Han, D.; Cho, J.; Lee, B. H.; Sung,M. M.; Lee, M.; Kim, J.; Shin, H. Chem. Mater. 2009, 21, 2574.(618) Bae, C.; Zierold, R.; Montero Moreno, J. M.; Kim, H.; Shin,H.; Bachmann, J.; Nielsch, K. J. Mater. Chem. C 2013, 1, 621.(619) Comstock, D. J.; Christensen, S. T.; Elam, J. W.; Pellin, M. J.;Hersam, M. C. Adv. Funct. Mater. 2010, 20, 3099.(620) Gaspard, P.; Hithesh, K. G.; Goran, S.; Wouter van der, W.;Niclas, R. Nanotechnology 2013, 24, 015602.(621) Gu, D.; Baumgart, H.; Abdel-Fattah, T. M.; Namkoong, G.ACS Nano 2010, 4, 753.(622) Marianna, K.; Emma, H.; Viljami, P.; Mikko, R.; Markku, L.Nanotechnology 2010, 21, 035301.(623) Pitzschel, K.; Moreno, J. M. M.; Escrig, J.; Albrecht, O.;Nielsch, K.; Bachmann, J. ACS Nano 2009, 3, 3463.(624) Shin, H.; Jeong, D. K.; Lee, J.; Sung, M. M.; Kim, J. Adv. Mater.2004, 16, 1197.(625) Yoon, J.; Kim, S.; No, K. Mater. Lett. 2012, 87, 124.(626) Martinson, A. B. F.; Elam, J. W.; Hupp, J. T.; Pellin, M. J. NanoLett. 2007, 7, 2183.(627) Sander, M. S.; Cote, M. J.; Gu, W.; Kile, B. M.; Tripp, C. P.Adv. Mater. 2004, 16, 2052.(628) Bachmann, J.; Zierold, R.; Chong, Y. T.; Hauert, R.; Sturm, C.;Schmidt-Grund, R.; Rheinlander, B.; Grundmann, M.; Gosele, U.;Nielsch, K. Angew. Chem., Int. Ed. 2008, 47, 6177.(629) Bae, C.; Yoon, Y.; Yoon, W.-S.; Moon, J.; Kim, J.; Shin, H. ACSAppl. Mater. Interfaces 2010, 2, 1581.(630) Chen, X.; Pomerantseva, E.; Banerjee, P.; Gregorczyk, K.;Ghodssi, R.; Rubloff, G. Chem. Mater. 2012, 24, 1255.(631) Panda, S. K.; Yoon, Y.; Jung, H. S.; Yoon, W.-S.; Shin, H. J.Power Sources 2012, 204, 162.(632) Banerjee, P.; Perez, I.; Henn-Lecordier, L.; Lee, S. B.; Rubloff,G. W. Nat. Nanotechnol. 2009, 4, 292.(633) Du, X.; George, S. M. Sens. Actuators, B 2008, 135, 152.(634) Moreno i Codinachs, L.; Birkenstock, C.; Garma, T.; Zierold,R.; Bachmann, J.; Nielsch, K.; Schoning, M. J.; Fontcuberta i Morral,A. Phys. Status Solidi A 2009, 206, 435.(635) Lee, J.; Kim, D. H.; Hong, S.-H.; Jho, J. Y. Sens. Actuators, B2011, 160, 1494.



http://pubs.acs.org/action/showLinks?system=10.1021%2Fcm202901z&coi=1%3ACAS%3A528%3ADC%252BC38XjsFegsLc%253D

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http://pubs.acs.org/action/showLinks?system=10.1021%2Fnl802324y&pmid=18698834&coi=1%3ACAS%3A528%3ADC%252BD1cXpvVars7o%253D

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http://pubs.acs.org/action/showLinks?crossref=10.1016%2Fj.snb.2008.08.015&coi=1%3ACAS%3A528%3ADC%252BD1cXhsVWlsL%252FF

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http://pubs.acs.org/action/showLinks?system=10.1021%2Fcm803112p&coi=1%3ACAS%3A528%3ADC%252BD1MXmtF2rurc%253D

http://pubs.acs.org/action/showLinks?system=10.1021%2Fcm960063%2B&coi=1%3ACAS%3A528%3ADyaK28XksFahsrk%253D

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http://pubs.acs.org/action/showLinks?crossref=10.1016%2Fj.electacta.2004.04.010&coi=1%3ACAS%3A528%3ADC%252BD2cXlsFektbc%253D

http://pubs.acs.org/action/showLinks?system=10.1021%2Fnl050462g&pmid=15884897&coi=1%3ACAS%3A528%3ADC%252BD2MXjtVWntb4%253D

(636) Liu, C.-M.; Chen, C.; Cheng, H.-E. Electrochem. Solid-State Lett.2011, 14, K33.(637) Cameron, M. A.; Gartland, I. P.; Smith, J. A.; Diaz, S. F.;George, S. M. Langmuir 2000, 16, 7435.(638) Tran, T. H. Y.; Haije, W. G.; Longo, V.; Kessels, W. M. M.;Schoonman, J. J. Membr. Sci. 2011, 378, 438.(639) Pellin, M. J.; Stair, P. C.; Xiong, G.; Elam, J. W.; Birrell, J.;Curtiss, L.; George, S. M.; Han, C. Y.; Iton, L.; Kung, H.; Kung, M.;Wang, H. H. Catal. Lett. 2005, 102, 127.(640) Stair, P. C.; Marshall, C.; Xiong, G.; Feng, H.; Pellin, M. J.;Elam, J. W.; Curtiss, L.; Iton, L.; Kung, H.; Kung, M.; Wang, H. H.Top. Catal. 2006, 39, 181.(641) Kemell, M.; Pore, V.; Tupala, J.; Ritala, M.; Leskela, M. Chem.Mater. 2007, 19, 1816.(642) Tan, L. K.; Kumar, M. K.; An, W. W.; Gao, H. ACS Appl.Mater. Interfaces 2010, 2, 498.(643) Lee, J. H.; Wu, J. H.; Liu, H. L.; Cho, J. U.; Cho, M. K.; An, B.H.; Min, J. H.; Noh, S. J.; Kim, Y. K. Angew. Chem., Int. Ed. 2007, 46,3663.



http://pubs.acs.org/action/showLinks?crossref=10.1016%2Fj.memsci.2011.05.022&coi=1%3ACAS%3A528%3ADC%252BC3MXosFamtLw%253D

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http://pubs.acs.org/action/showLinks?crossref=10.1007%2Fs11244-006-0055-0&coi=1%3ACAS%3A528%3ADC%252BD28Xht1Cmu7zL

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