Anodization Fabrication of Highly Ordered TiO2 Nanotubes

Shiqi Li, Gengmin Zhang,* Dengzhu Guo, Ligang Yu, and Wei ZhangKey Laboratory for the Physics and Chemistry of NanodeVices and Department of Electronics,Peking UniVersity, Beijing 100871, China

ReceiVed: April 2, 2009; ReVised Manuscript ReceiVed: May 7, 2009

This Article focuses on the fabrication of highly ordered nanotubes and some novel nanostructures of titania(TiO2) with a two-step anodization method. The first-step anodization was actually a pretreatment of the Tifoil surface and provided well-ordered imprints that served as a template for the further growth of nanotubes.As a result, the TiO2 nanotubes growing in the second-step anodization appreciably outperformed thosefabricated with the conventional one-step Ti anodization in terms of size uniformity and arrangement orderliness.The parameters of the anodization were then modulated to obtain more complex structures. When the voltagein the second-step anodization was lower than that in the first-step anodization, a lotus root-shaped TiO2

nanostructure, in which each imprint contained several smaller nanopores, was achieved. When the secondanodization was further divided into two stages, double-layered nanotube arrays were synthesized. Theycontained two distinctly separated parts, i.e., the bamboo-shaped upper one and the smooth-walled lowerone. These results have demonstrated the effectiveness and controllability of the two-step anodization methodin producing high-quality TiO2 nanotubes, which are believed to have potential applications in such fields assolar cells, photonic crystals, and hydrogen storage.

1. Introduction

Titania (TiO2) has been widely investigated due to itsapplication prospects in such areas as gas sensors,1-5 photoca-talysis,6 and photovoltaic cells.7-9 It is well-known that proper-ties and performance of TiO2 are dependent partly on itscrystallinity and morphology. Especially, TiO2 nanotubes usuallyhave advantages over TiO2 films because the former can providelarge surface-to-volume ratio and unidirectional electricalchannel.10-18 In this context, the anodization route has beendeveloped for the preparation of the TiO2 nanotubes. In theirpioneering works, Gong et al. obtained TiO2 nanotubes by theanodization of a pure Ti sheet in an aqueous solution ofhydrofluoric (HF) acid.19 From then on, significant progress hasbeen further made in this field.20,21

In many applications of the TiO2 nanotube arrays, e.g., dye-sensitized solar cells (DSSCs) and hydrogen storage, smoothtopography and orderly arrangement are desired.22-24 Moreinterestingly, the possibility of photon manipulating with theTiO2 photonic crystal has also been suggested due to the highrefractive index (2.5-2.9) and minimal absorption in the visiblespectrum of TiO2.25-27 The demand for a strict periodicity ofthe photonic crystal entails very good uniformity of the TiO2

nanotubes in an array. In this sense, the morphology of the TiO2

nanotube arrays, e.g., the smoothness of the layer top and theorderliness of the nanotubes, still remains to be further improved.Though highly ordered Al2O3 nanopores are routinely obtainableby the anodization of Al,28 the anodization of Ti usually givesrise to TiO2 nanotube arrays with rough top surfaces and pooralignment. So far, several effective approaches to fabricatinghighly ordered TiO2 nanostructures have been developed, mainlyincluding ion track lithography,29,30 atomic layer deposition

(ALD),31 and self-organization.32,33 Among them, the two-stepanodization of a Ti foil is the most convenient and economicalmethod.32,34

The work described in this paper is largely devoted to furtherimproving the morphology of the TiO2 nanotube arrays on thebasis of the two-step anodization method. Herein, a well-texturedTi surface was obtained after the removal of the nanotube layergenerated in the first-step anodization. Then, the Ti foil thathad experienced this pretreatment was anodized again for theeventual growth of highly ordered TiO2 nanotubes. It is worthemphasizing that in this work the detachment of the firstnanotube layer from the Ti foil was achieved by an ultrasonictreatment instead of using an adhesion tape.34,35 This modifica-tion has helped avoid possible mechanical damage to the Tisurface and also greatly improved the uniformity and alignmentof the TiO2 nanotubes. As a result, high-quality arrays of TiO2

nanotubes have been achieved.Moreover, some novel structures of TiO2 based on its highly

ordered nanotube arrays were also achieved. On the one hand,the bamboo-type nanotube-based DSSCs show a significantlyhigher efficiency than those based on smooth-walled tubes dueto substantial increase in dye loading achieved by the bamboorings;36 on the other hand, however, reducing the dimensionalityof transport and recombination in DSSCs can also increase theefficiency.22 Herein, in an effort to integrate both the above twoadvantages to the samples, double-layered TiO2 nanotube arrayswith two distinctly separated parts were synthesized. The upperparts of the samples assumed a bamboolike shape and the lowerparts had smooth walls.

2. Experimental Section

The principal component of the electrolyte used in the Tianodization was ethylene glycol (C2H6O2). Importantly, thesolution also contained 0.25% (in mass) NH4F and 1% (involume) H2O. The electrolyte was aged under a 60 V voltagefor 60 h before being formally used in anodization. A Ti foil,

* Corresponding author. Phone: 86-10-62751773. Fax: 86-10-62762999.E-mail: zgmin@pku.edu.cn.

J. Phys. Chem. C 2009, 113, 12759–12765 12759

10.1021/jp903037f CCC: $40.75 2009 American Chemical SocietyPublished on Web 06/09/2009

0.1 mm in thickness and 99.7% in purity, was cleanedultrasonically in turn in acetone, deionized water, and ethanol.Then, the foil was bound to the electrolytic cell with an O-ringand used as the anode. The cathode was a piece of graphite,whose area was about four times that of the Ti foil. The anodeand the cathode were separated by approximately 3 cm. Thereaction was driven by a dc power source. All the experimentswere done at room temperature.

The fabrication of the TiO2 nanotubes presented here featuredan anodization that included two steps. In the first step, whichwas actually a pretreatment, a Ti foil was anodized at 60 V for24 h, and a layer of nanotubes grew on the foil surface. Then,the nanotube layer was removed ultrasonically in deionizedwater, and the glossy underlying Ti was exposed. As will beelaborated in the next section, a pattern was left on the foilsurface after the removal of the nanotube layer, and it wouldplay a key role in the further growth of well-aligned nanotubes.In the second step of the anodization, the pretreated Ti foil wasused as the anode again, and the voltage applied to it was either60 or 30 V in different experiments. As will be illustrated inthe next section, nanotubes resulted from the 60 V voltage andlotus-root-like nanostructures from the 30 V voltage. After theconclusion of the two-step anodization, the sample was cleanedultrasonically in ethanol and finally rinsed in turn in deionizedwater and ethanol.

Such means as scanning electron microscopy (SEM), trans-mission electron microscopy (TEM), and energy dispersiveX-ray spectroscopy (EDX) were used to characterize thesamples. The FEI XL-30 scanning electron microscope (SEM),which was also attached with an EDX spectroscope, and theH-9000NAR transmission electron microscope (TEM) workedunder the 15 and 300 kV accelerating voltages, respectively.

3. Results and Discussion

3.1. Preparatory Anodization. In most of the experimentsin this field, nanotubes were directly grown on Ti foils.37,38 Sincethe surface of a Ti foil is corrugated, the nanotubes on it areusually in a disordered array. The result of the first-stepanodization in this work is given in Figure 1, which confirmsthat indeed a one-step anodization is insufficient for obtainingwell-aligned and uniform nanotubes. Several Ti foils weresubmitted to the anodization, and some of the resulting nanotubelayers were ultrasonically peeled off. Figure 1a, which is an

SEM image of the Ti surface outside the anodization region,shows that the Ti surface was very rough and irregular. Incontrast, Figure 1b shows the regularly ordered Ti surface insidethe anodization region exposed after the removal of the nanotubelayer. This surface with regular hexagon-shaped imprintsconduced to further growth of nanotubes. As shown in the insetof Figure 1a, the EDX of a Ti foil before any pretreatmentcontains the O peak. This is in agreement with the commonknowledge that a Ti foil is generally covered with an oxidelayer before receiving any treatment.39 As shown in the insetof Figure 1b, no major O peak can be found in the EDX of theimprints; thus, their component was mainly Ti instead of itsoxides. That is, the oxygen on the Ti surface was largelyremoved in the first-step anodization. Figure 1c gives the SEMimages of the nanotube arrays that grew on the rough Ti surfacein the first-step anodization. The nanotubes had a remarkabledisparity in length, and the array surface was considerablyundulated. Figure 1d is the SEM image of the bottom side ofthe nanotube layer that was peeled off from the Ti foil. As seenfrom the direction of the incident electron beam, the bottomside of the nanotube layer shown in Figure 1d has an ap-proximate 6-fold symmetry. First, the bottoms of the nanotubesall assume a rough hexagonal shape. Second, each nanotubeis surrounded by 6 closest neighboring ones. The outer diametersof these nanotubes are on average similar to those of the imprintsshown in Figure 1b. That is, the two patterns in Figure 1b,d arein commensuration.

It is almost common knowledge that the electrolyte needs anaging process before it can be used for an anodization fabricationof TiO2 nanotubes,40,41 though the exact mechanism of this agingprocess is still poorly understood. As introduced in section 2,this practice was also followed in this work. For confirmationof its role in the anodization, an electrolyte that had onlyexperienced a 10-h aging process, as against the aging processas long as 60 h for other samples in this work, was intentionallyused in the anodization, and the result is given in Figure 1e,which shows the Ti foil surface exposed after the ultrasonicremoval of a nanotube layer. Compared with the result shownin Figure 1b, the whole pattern in Figure 1e is quite irregular.The boundaries between the hexagonal imprints in Figure 1eare not well developed, either. Thus, it has been made clearthat a thorough aging of the electrolyte is also indispensable toobtaining a regular imprint pattern.

Figure 1. Results of the first-step anodization: (a) the SEM image of the Ti surface outside the anodization region (inset is the EDX); (b) the Tisurface after the removal of the nanotube layer (inset is the EDX); (c) the SEM image of the nanotube layer generated in the first-step anodization(inset was obtained when the sample was titled by 45° from the incident electron beam); (d) the bottom side of the nanotube layer that shed fromthe Ti foil; (e) the Ti foil surface exposed after the ultrasonic removal of a nanotube layer fabricated with an insufficiently aged electrolyte.

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3.2. Second-Step Anodization. After the first-step anodiza-tion and the removal of the nanotube layer, the Ti foil wasassembled back to the electrolytic cell again as the anode forthe second-step anodization. Figure 2 shows the porous layergenerated under a 60 V anodizing voltage in the second-stepanodization. The nanotube array shown in Figure 2a,b is muchmore uniform in alignment and length than that shown in Figure1c, confirming the necessity of the first-step anodization as apretreatment. Well-ordered porous layers of anodized aluminumoxide (AAO) are readily available.42,43 Nonetheless, within theauthors’ knowledge, reports on arrays of TiO2 nanotubes withsuch good alignment and length uniformity are still rare. Justlike the sample after the first-step anodization, the nanotube layershown in Figure 2a,b was also peeled off from the Ti foilultrasonically. The bottom side of the nanotube layer and theimprint pattern left on the Ti surface are shown in Figure 2c,d,respectively. As indicated in the inset of Figure 2a, the nanotubeshave a circular internal transverse section. Nonetheless, as eachnanotube is surrounded by 6 nearest neighbors, its outer sectionand the pattern it leaves on the Ti foil surface both assume ahexagonal shape, as shown in Figure 2d and the inset of Figure2c. In Figure 2b, each nanotube has 6 protrusions at the fringeof its top end. The sizes of the hexagons formed by theseprotrusions are in good agreement with those of the imprints inFigure 1b, suggesting that the nanotubes in the second-stepanodization directly developed from the imprint pattern left onthe Ti surface. It is also worth pointing out that the imprintpatterns left on the Ti foil surface after the first-step and thesecond-step anodizations, respectively shown in Figures 1b and2d, do not show obvious disparity in uniformity and orderliness.Therefore, as is argued in the preceding paragraph, aging theelectrolyte was more effective in improving the quality of theimprint pattern than performing more anodization.

It is generally accepted that the formation mechanism of theTiO2 nanotubes resembles that of the AAO nanopores.44,45 Asintroduced in section 2, the electrolyte contained a trace amount

of water. The water was electrolyzed at the Ti anode, and alayer of compact anodic oxide resulted:46

Then, chemical dissolution of the oxide as soluble fluoridecomplexes began to compete with the anodic oxidation, and adirect complexation of the high-field transported Ti cations alsooccurred at the oxide/electrolyte interface:46,47

The current-time (I-t) curve acquired during the growth ofthe nanotubes, shown in Figure 2e, is consistent with the abovereactions. The rapid drop in the initial stage of the curve resultedfrom the generation of the barrier layer described by reaction1. Then, the current increased slightly when reaction 2 domi-nated momentarily, because the ion transport became possibleagain as the oxide layer was thinned. While both field-assisteddissolution and chemical dissolution made a contribution to thedissolution of the oxide layer, it is the field-assisted dissolutionthat dominated this stage due to the relatively large electric fieldacross the thin oxide layer.45,48 Finally, when a balance wasreached among reactions 1-3, the current stabilized at a certainvalue.47

Actually, in the first-step anodization, the nanotube formation,which depended remarkably on local electric field and solutiondiffusion rate, did not begin uniformly across the Ti foil surface.First, further growth of the oxide layer generated by theelectrochemical oxidation in reaction 1 was controlled by field-assisted transport of Ti4+ cations and O2- anions through thelayer;47 second, local acidification of the solution, which wasindispensable to the chemical dissolution of TiO2 in reaction 2,was largely determined by the establishment of a pH gradient.49

Therefore, the nanotube growth took place preferentially at somelocations that can simultaneously provide sufficiently high localelectric field and a narrow channel-shaped morphology. Theformer was necessary for the oxidation of Ti and the generationof H+ cations as well, and the latter was favorable to theaccumulation of the generated H+ cations. As shown in Figure1a, the morphology of the Ti surface before the pretreatmentwas irregular; thus, the nanotube growth on it commenced witha rather random spatial distribution. In the anodization of theTi substrate into TiO2, the formation of the relatively orderedarray was the result of a competition between the initiallyexistent nanotubes. During an autocatalysis sequence, only thenanotubes with optimal starting conditions, such as a large depthand a good orientation at the beginning, were allowed to growcontinuously and finally evolve into members in the array.50

Though this self-organization mechanism was effective inmodulating the nanotube growth, the eventual result was stillnot satisfactory, as shown in Figure 1c. The random initialgrowth still had a negative influence on the orderliness anduniformity of the array. Moreover, the bending of some poresduring the autocatalysis was also a concern.50

In contrast, when the pretreated Ti foil, shown in Figure 1b,was used as the anode, both the electric field and the morphologyhad a regular distribution across the surface at the verybeginning. Actually, the ordered imprints played the role oftemplate for the growth of the nanotubes, and the initial

Figure 2. Nanotube array generated in the second-step anodizationunder a 60 V voltage: (a) top view; (b) observed from a 45° angle; (c)the bottom side of the nanotube layer after its removal from the Tifoil; (d) the exposed Ti surface after the removal of the nanotube layer.(e) The I-t curve during the nanotube formation.

Ti + 2H2O - 4e- f TiO2 + 4H+ (1)

TiO2 + 6F- + 4H+ f [TiF6]2- + 2H2O (2)

Ti4+ + 6F- f [TiF6]2- (3)

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randomness was effectively avoided. As a result, the uniformityand orderliness of the nanotube arrays were greatly improvedin the second-step anodization.

For a better understanding of the role of the imprints leftafter the first-step anodization, it is worth pointing out that avery flat surface of a Ti foil is not necessarily conducive toorderly growth of a nanotube layer. A specially processed Tifoil was purchased and used as the anode in a conventionalone-step anodization. It had been mechanically polished so thatits surface was very flat and smooth.23,51 Figure 3 shows thenanopores that grew on the flat surface of this Ti foil. Theirdiameter and depth are not so uniform compared with those ofthe nanotubes shown in Figure 2. Some pores are actually veryshallow, indicating the lack of strong field necessary for keepingthe anodization progressing at some locations. This phenomenonis easily explainable. Since the surface was very flat, the electricfield across it was nearly equal everywhere, and the anodizationcould start only at some random locations. Thus, the nanoporeswith poor uniformity resulted. While on a surface with regularimprints, e.g., the one shown in Figure 1b, the strongest electricfield occurred at the bottoms of the scallop-shaped hemisphericalimprints and further growth of the nanotubes naturally startedthere.44,49

Moreover, after the Ti foil was cut using scissors, the sideprofile of the TiO2 layer generated in the second step anodizationwas exposed, and the results of SEM and TEM observation aregiven in Figure 4. Unlike AAO, the TiO2 layer in Figure 4a isnot a continuous oxide layer with nanopores embedded in it.Instead, the layer is actually an assembly of nanotubes that packclosely to each other. Moreover, as can be clearly seen in theinset of Figure 4a, small gaps exist between the abuttingnanotubes.52 The generation of these gaps is ascribed mainly tothe mechanical stress in the oxide tubes.50 Also, the nanotubelayer in Figure 4a is covered with a porous film; thus, it appearsas a nanopore array in the top view. In some laboratories, TiO2

nanotube arrays covered with porous films were fabricated byanodizing mechanically polished Ti substrates. The films caneffectively prevent tube tops from being attacked by chemicaldissolution.23,51 The length of a nanotube does not increase whenthe rate of oxidation at the Ti/oxide interface at the bottomequals the rate of dissolution at the oxide/electrolyte interfaceat the top. Hence, preparation of long nanotubes is possible onlywhen the tube tops are well protected. The TiO2 nanotube arraysprepared by the two-step anodization in this work were alsocovered with porous film and thus had the potential of beingfurther prolonged in future work.

Figure 4b shows that the nanotubes are closed at the oxide/Ti interface. It is argued that there exists a thin dense interfacebetween the porous AAO and the underlying Al substrate.53

Similarly, Figure 4b indicates that a barrier layer also occurredbetween the TiO2 nanotubes and the metallic Ti. This observa-tion has confirmed Albu et al.’s argument that the growth ofthe nanotubes is preceded by the growth of a compact anodicoxide.41 The compact oxide layer moves further into the metalwith thickness unchanged when the rate of oxide growth at the

metal-oxide interface equals that of oxide dissolution at theoxide-electrolyte interface.45

Figure 4a,b respectively show that the nanotubes are openat the top and closed at the bottom. From now on, the openend of a nanotube is referred to as the “mouth”. The twofigures also disclose that the nanotubes have smaller cavitiesnear the bottom than the top. The inner diameter at the mouthshown in the inset of Figure 4a is around 100 nm while thatat the bottom shown in Figure 4b is around 50 nm. Duringthe formation of the hollow nanotubes, as the dissolutionproceeded from the top to the bottom, the upper parts of thenanotubes were exposed to the solution for a longer timethan the lower parts.54 Hence the walls of the nanotubes atthe mouths were apparently thinner than those at the bottom.As shown in Figures 2a and 4b, the wall thickness of thenanotubes is 17 and 50 nm at the mouth and the bottom,respectively.

It is also noticed that the side walls of the nanotubes haveobvious thickness variation, often referred to as ripples;55 oneexample is given in Figure 4c. This is a common phenomenonin the fabrication of nanotubes by anodization.52 So far, theripples are ascribed to the periodic oscillations of the current inanodization,47 and it has been reported that the bamboo-shapednanotube, which is actually a nanostructure with more drasticripples along the side walls, is available when the dc voltage isreplaced by an ac voltage.41 The registered voltage in ananodization under a galvanostatic mode has a tendency tooscillate, which is attributed to sequential growth and lift-offof TiO2 nanotube layers.56 The steady-state current density ofthe anodization is diffusion limited.57 Thus, growth of nanotubesis steadier in electrolytes with large viscidities than in an aqueouselectrolyte.37 Herein, the current density-time curve under thepotentiostatic mode was also recorded. As shown in Figure 4d,the current oscillation is small. Accordingly, the ripples shownin Figure 4c are also small. The rate of the nanotube growth islargely determined by that of the chemical dissolution of theTiO2 at the nanotube bottom, which, as is clear in eq 2, furtherdepends on the local pH value there. According to eq 1, thelocal acidification of the solution in the vicinity of the nanotubebottom results from the anodization of Ti driven by the externalcurrent. Therefore, when the current of the anodization droppedto a relatively low value during the oscillation, H+ ions becameless available and the dissolution rate of the TiO2 was accord-ingly lowered. Thus, the growth of the nanotubes slowed downconsiderably, and the ripples resulted along the sidewalls of thenanotubes.41 At the mouth, the diffusion of the H+ ions wasmuch easier than that inside the nanotubes. Hence, the pHgradient was not well established at the beginning, and the initialdissolution of the TiO2 should be relatively slow but quiteuniform. Consequently, as shown in Figure 4a, no ripple is foundnear the nanotube mouths.

For the purpose of probing the mechanism behind the rippleformation, here the constant applied voltage was replaced by aperiodic square-waved voltage in the second-step anodization,whose value was alternately 30 and 60 V. The structure shownin Figure 4e was obtained when both the 30 and 60 V voltageslasted 10 s in one period. In this case, the application of a square-waved voltage did not make obvious difference from theconstant voltage that yielded Figure 4c. When the duration ofthe 30 V voltage was prolonged to 90 s, a bamboo-shapedstructure, shown in Figure 4f, was attained. It is believed thatthe formation of the nanotube, i.e., the dissolution of the TiO2,almost stopped under a voltage as low as 30 V. The “bamboojoints” in Figure 4f were developed shortly after the transition

Figure 3. Nanopores fabricated on a mechanically polished Ti foil.

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to the 60 V voltage. Therefore, the results presented in Figure4e,f have confirmed that indeed the periodic oscillation of theanodizing current was responsible for the ripples.

It is worth pointing out that bamboo-shaped TiO2 nanotubesare also readily attainable in aqueous electrolyte.58 Since theaqueous electrolyte has small viscidity, the bamboo-shapednanotubes can result from the oscillation of current even underconstant anodizing voltage. In contrast, as in this work, whenan organic electrolyte with a relatively large viscidity is em-ployed, a constant voltage usually gives rise to ripples alongthe nanotube walls, and bamboo-shaped structures are obtainableonly when the voltage is subjected to a dramatic periodic change.

3.3. Lotus-Root-Shaped Nanostructures and Double-Layered Nanostructures. In other experiments, the anodizingvoltage was varied, and different nanostructures were obtained.For example, Figure 5 gives the result of the second-stepanodization under a 30 V anodizing voltage. The first-stepanodization was similar to that employed in the above work.That is, it was performed under a 60 V voltage, and the nanotubelayer generated in it was ultrasonically removed. The nano-structure in it has two levels and resembles a lotus root in shape.The first level consists of cells less than 0.2 µm in size, andone of them is highlighted by a hexagon in the inset of Figure

5a. The pores with smaller diameters, one of them highlightedby a circle in the inset of Figure 5a, inside the cells constitutethe second-level structure. It is noticed that no such nanoporesextend across any neighboring cells. That is, the nanopores allevolved in the interior of the cells. On average, the size of thecells is similar to that of the imprints shown in Figure 1b.Therefore, it is reasonable to believe that the cells in Figure 5correspond to the imprints in Figure 1b. It is known from Figure5b that the nanostructure is also an assembly of nanotubes, andthe cells of the first-level structure indeed correspond to theimprints shown in Figure 1b. It merits pointing out that thislotus-root-shaped nanostructure was available only when theanodizing voltage in the second-step anodization was lowenough. As shown in Figure 2a, it was not obtained when theanodizing voltage was 60 V in the second-step anodization.Since the anodizing voltage in the second-step anodization, 30V, was lower than that in the first-step anodization, 60 V, thenanotubes generated in the second-step anodization were thinnerthan those generated in the first-step anodization.20,44 Therefore,several nanotubes simultaneously developed inside one imprint,and the lotus-root-shaped nanostructure resulted.

Under appropriate conditions, TiO2 nanotube arrays withdifferent structures along the longitudinal direction of the tubeshave been attained by previous researchers. For example, Yanget al. and Macak et al., respectively, synthesized double-layeredTiO2 nanotube arrays by a two-step anodization, in which thefirst-step anodization was performed in an aqueous electrolyteand the second-step anodization in a nonaqueous electrolyte.In their nanotube arrays, rough and large-diameter nanotubesconstituted the upper parts while smooth and small-diameterones constituted the lower parts.57,59 For another example,Yasuda and Schmuki obtained multilayer zirconium titanatenanotube arrays by alternately applying and cutting off theanodizing voltage. Interestingly, they claimed that the formation

Figure 4. Side view of the TiO2 nanotubes generated in the second-step anodization: (a) the side view of the nanotubes; (b) the bottom of thenanotubes and the barrier layer; (c) the ripples; (d) the current-time curve under a potentiostatic mode; (e) the bamboo-shaped tubes generatedunder a square-waved anodizing voltage (both the 30 V and the 60 V voltages lasted 10 s in one period); (f) the bamboo-shaped tubes generatedunder another square-waved anodizing voltage (the 30 V voltage lasted 90 s and the 60 V voltage still lasted 10 s).

Figure 5. Lotus-root-shaped nanostructure obtained under a 30 Vanodizing voltage in the second-step anodization: (a) top view; (b) sideview.

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of new nanotubes in the lower part of the array started in thegaps between the existing nanotubes in the upper part.60

In this work, as shown in Figure 6, double-layered nanotubearrays with morphology different from those in the aboveexamples were achieved. Here the two-step anodization routewas still employed, and the first-step anodization was similarto that used in obtaining the nanotube arrays shown in Figure2. It is the second-step anodization that was modulated. Afterthe pretreatment, regularly distributing hexagonal imprints wereleft on the Ti foil surface. The sequential second-step anodizationconsisted of two stages. First, the anodizing voltage jumpedbetween 60 and 30 V for 24 h, with the 60 V voltage lasting10 s and the 30 V voltage 90 s in each period. The bamboo-shaped upper part of the layer in Figure 6 was generated in thisstage. Then, without being cutting off, the voltage was switchedto and continuously kept at 60 V for another 3 h, and the lowerpart of the array, which was an assembly of smooth nanotubes,resulted. Different from those in the above-mentioned works,the nanotubes in the upper part of Figure 6 do not pause at theboundary between the upper and the lower parts. Instead, theycontinuously evolve from the bamboo-shaped part to thesmooth-walled part. Moreover, the nanotubes in the two partshave a similar diameter. Therefore, it is reasonable to believethat the nanotubes in the lower part started growing directlyfrom the bottom of those in the upper part. There were two keypoints in fabricating the double-layered nanotube array shownin Figure 6. First, in order to avoid possible disruption betweenthe nanotubes of the upper part and the lower part, the anodizingvoltage should not be cut off when being switched from thefirst stage to the second one. Second, in order to avoid possibledisparity in diameter between the nanotubes in the upper partand the lower part, the anodizing voltage in the second stageshould be equal to the larger one of the two voltages in the firststage. Furthermore, as indicated by the arrow, the interfacebetween the upper and the lower parts is quite distinct and flat.This phenomenon is attributable to the pretreatment of the Tifoil, which ensured that the growth of the bamboo-shapednanotubes started at a relatively flat surface and thus stoppedalmost in the same plane.

4. Conclusion

In summary, by a two-step anodization method, whichfeatured a preparatory anodization as a pretreatment of the Tifoil surface, highly ordered TiO2 nanotube arrays have beenachieved. Two key issues were found to be crucial in guarantee-ing the orderliness and alignment of the nanotube arrays. First,after the first-step anodization, the nanotube layer should beremoved ultrasonically instead of with an adhesion tape, so thatpossible damage to the imprints left on the Ti foil could beavoided. Second, the electrolyte should be subjected to asufficiently long aging process before being used in theanodization. In the oxide layer, the nanotubes packed closelyto each other and were covered by a layer of porous film at thetop; thus, the layer appeared to be a nanopore array in the top

view. The thickness variation, viz. ripple, along the nanotubewall was observed, and its origin was experimentally verifiedto be the current fluctuation during the anodization. Furthermore,two novel TiO2 nanostructures were achieved by modulatingthe experimental parameters in the second-step anodization.Lowering the anodizing voltage in the second step to anappropriate value resulted in a lotus-root-shaped nanostructure,in which nanotubes with smaller diameters grew inside theimprints. Also, by properly dividing the second-step anodizationfurther into two stages, a double-layered nanotube array, witha bamboo-shaped upper part and a smooth-walled lower part,was generated. These newly developed TiO2 nanotube arraysare expected to have potential applications in solar cells,photonic crystals, and hydrogen storage.

Acknowledgment. This work was supported by the NationalNatural Science Foundation of China (No. 90606023) and theMOST of China (No. 2006CB932402).

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