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Electrochimica Acta 91 (2013) 90– 95

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

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nfluence of anodization parameters on the expansion factor of TiO2 nanotubes

ergiu P. Albu, Patrik Schmuki ∗

epartment of Materials Science, Institute for Surface Science and Corrosion (LKO), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Martensstraße 7, D-91058 Erlangen,ermany

r t i c l e i n f o

rticle history:eceived 3 September 2012eceived in revised form8 December 2012ccepted 23 December 2012vailable online 31 December 2012

a b s t r a c t

Growth of titania nanotubes was carried out in fluoride containing electrolytes using photolithograph-ically defined thin film patterns to determine the amount of expansion when the metal is converted tooxide. This expansion of TiO2 nanotubes is studied for a large set of electrochemical conditions. We showthat this parameter strongly depends on the applied anodization potential and the water content in theelectrolyte whereas the fluoride content is only of minor influence. Expansion factors were found to varybetween 1.3 and 2.8 depending on the anodizing parameters. This variation is explained in terms of effi-

eywords:iO2 nanotubesilling–Bedworth rationodizationhotolithography

ciency of oxide growth, as well as of chemical composition, density, and porosity of the TiO2 nanotubulararray.

© 2012 Elsevier Ltd. All rights reserved.

rganic electrolyte

. Introduction

TiO2 nanotube layers formed by self-ordering anodization havettracted considerable interest in science and technology owingo the high number of potential applications enabled by the tubesonic, electric and biomedical properties – for an overview see forxample Refs. [1,2].

Since the first report by Zwilling et al. [3] on the observationf the growth of TiO2 nanotube layers, not only control over mor-hology (diameter, length, smoothness) but also the mechanisticnderstanding of crucial factors for growth and self-organizationas strongly been increased. One interesting point is the observedunusual” length expansion of the tube layers [4]. That is, lengthxpansion beyond the expected expansion of oxide layers corre-ponding to the Pilling–Bedworth ratio (˚). Typically the reportedxpansion factors, when converting Ti to TiO2 nanotubes, wereetermined to be in the range of 2.7–3.1 [5], which is clearly morehan the Pilling–Bedworth ratios reported for titanium dioxide of1.93 for anatase, ∼1.76 for rutile, and ∼2.4 for an amorphoushase [6–8]. Nevertheless, the reported expansion factors wereainly determined for a very limited set of electrochemical con-

itions – this is particularly critical as the molar volume of the

xide changes with the density and the chemical composition ofhe oxide (for example, the degree of incorporated impurities mayhange with electrochemical growth conditions).

∗ Corresponding author. Tel.: +49 9131 852 7575; fax: +49 9131 852 7582.E-mail address: schmuki@ww.uni-erlangen.de (P. Schmuki).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.12.094

Within this paper we show that indeed the expansion of theanodically formed oxide during tube growth is very dependent onthe electrochemical anodization parameters.

2. Experimental

In order to determine expansion factors for oxide growth weused a photolithographic approach previously described [5]. Inthe present case we slightly modify it and used titanium layersof ∼1 �m thickness that were deposited on Fluorinated Tin Oxide(FTO, SnO2:F) glass (TCO22-15, Solaronix) substrates using elec-tron beam evaporation with a deposition rate of 0.6 nm min−1 at5 × 10−7–2 × 10−6 mbar. Samples were patterned lithographicallywith a positive photoresist (MicropositTM S1813TM G2). Initially,the photoresist was spun with a spin-coater BLE Delta 10 at1000 rpm for 10 s, followed by spinning at 4000 rpm for 30 s. Pre-baking was carried out at 110 ◦C for 60 s on a SCHOTT SLK 6 heatplate. A stainless steel mesh with square openings was used as amask during ultraviolet exposure of the photoresist film. The irradi-ated parts of the film were removed with Microposit developer andthen the samples were cleaned with water and dried under a nitro-gen stream. In order to make the film resistant to electrochemicalconditions, post-baking was done at ∼250 ◦C for ∼60 s.

TiO2 nanotubes were grown via anodization on both patternedand non-patterned Ti layers using a potentiostat (Jaissle IMP 88

PC) in a three-electrode configuration with reference and counterelectrodes made of platinum. The anodized area was ∼0.91 cm2.Anodization was carried out at room temperature (∼22–24 ◦C)in a potentiostatic mode using ethylene glycol (EG) (Fluka, assay

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99.5%, H2O ≤0.1%) with addition of NH4F (Fluka) in the rangef 0.01–0.2 mol dm−3 and different water contents from 0.5 wt%o 20 wt%. The applied potential was varied in the range 5–70 V.he Ti layers were typically fully anodized and converted to TiO2anotubes as discussed later in this paper. After this, the samplesere removed from the electrochemical cell, cleaned with ethanol,

mmersed in a beaker with ethanol for about 1 h and finally, let dryn air.

Some samples were annealed at 450 ◦C to elucidate the double-alled structure of TiO2 nanotubes as discussed in previous work

9].The morphology of the obtained TiO2 nanotubular layers was

valuated with a Field-Emission Scanning Electron MicroscopeHitachi FE-SEM S4800).

. Results and discussion

Fig. 1 illustrates the growth of nanotube layers on the patternedubstrate and the determination of the volume expansion fromEM cross-sections. Fig. 1a shows the morphology of initial Ti layern FTO glass. The evaporated titanium has a columnar structureFig. 1a, inset) and a preferential orientation of the film in the

0 2 direction as determined from XRD spectra (not shown here). series of comparative experiments with bulk Ti-sheets revealed

hat, the grain size and orientation seem not to influence the nano-ube growth and geometry; tubes were found to be similar to tubesrown on bulk titanium.

Patterned and non-patterned surfaces were anodized under aide variety of electrochemical conditions that were chosen to

llow for anodization of the entire layer thickness. Fig. 1b showsn SEM top view of the patterned surface after the anodizationrocess. The squares contain nanotubes whereas the surroundingrame is titanium metal protected during anodization by the pho-oresist film. In the anodized areas, the presence of the initiationayer (Fig. 1c and e) on top of nanotubes represents an excellent

arker for an intact tube top, and this ensures that the entireength of TiO2 nanotubes is present (i.e. no partial tube shorteningy etching of the tube tops occurs). Cross-sections of partially andully anodized layers are represented in Fig. 1d and f. These imageshow clearly the multilayered structure of the samples, that is thelass with an FTO layer (bright film with thickness of ∼350 nm),he evaporated titanium, and the expansion of the nanotubes inhe direction of growth.

The conversion of Ti into TiO2 nanotubes can be monitoredy current–time curves such as shown in Fig. 2a. Gas evolutionakes place when the electrolyte reaches the FTO layer, and furthernodization leads to high currents and lift-off of the nanotubu-ar layer. In order to obtain intact layers, the experiments weretopped right after the current plateau, i.e. before the current startso increase rapidly to a 3–6 times higher value. This ensures entirenodization of the layer, but prevents its loss in the electrolyte. Fullxidation of the metal makes the sample transparent to visible light.owever, SEM investigations at different sample locations showed

hat occasionally between the TiO2 nanotubes and the FTO layer aery thin film of titanium might be present (few nanometers thick).n the view of the micrometer thickness of the nanotube length andhe goal of the present work, this error can be neglected. Stages ofhe TiO2 nanotube growth are the same as for nanotubes grownrom bulk titanium [10]. In the beginning, the current drasticallyrops due to the fast surface oxidation and formation of a compactxide (called also initiation layer). In the next stages, the initiation

ayer is penetrated by fluorides and then growth of TiO2 nanotubesnderneath the initiation layer takes place. It should be noticed that

nitiation sites are influenced by the surface roughness and defects,nd these pits are aligned along boundaries (compare Fig. 1a, c and

mica Acta 91 (2013) 90– 95 91

e). In the latest stage of the nanotube growth, the current is slowlydecreasing with time, until the Ti/FTO interface is reached.

The time required for full conversion of titanium film into nano-tubes is directly proportional with the passed charge density, i.e.with the current density. The latter is influenced by the electro-chemical conditions. For example, at 50 V the full conversion takesplace in about 7 min at a mean current density of ∼9 mA cm−2, whileat 10 V about 4 h of anodization is necessary at a mean current den-sity of ∼0.25 mA cm−2 (Fig. 2a). The anodization time also increasesin electrolytes with higher amount of water or/and lower fluoridecontent. It should be noticed that the current–time characteristicsare plotted for non-patterned surfaces in order to avoid the sealingeffect of photoresist on the extracted values of the average steady-state current density. However, the expansion factors measuredfrom fully anodized non-patterned and patterned samples are sim-ilar and in the range of average error specific to electrochemicalconditions.

TiO2 nanotubes grow perpendicular on the Ti substrate and thelateral expansion of the oxide layer can be considered negligible(Fig. 1f). Thus, the expansion factor of the oxide (Fexp) can be definedas the length of the nanotubes obtained from a certain thickness oftitanium consumed.

Fexp = LNTs

LiTi − Lr

Ti

= LNTs

�LTi(1)

where LNTs is the thickness of TiO2 nanotube layers and LiTi, Lr

Tiare the initial and remaining thicknesses of the Ti film. Expan-sion factors were then determined for a number of variations inthe experimental conditions. This simple equation shows how toextract the expansion factor of the nanotubes directly from scan-ning electron images even when a partial anodization is carried outas shown in Fig. 1d.

Fig. 2b shows the influence of the applied potential on theexpansion factor. For low water content electrolytes, the expansionfactor increases drastically with the applied potential from ∼1.3 to∼2.8. This factor seems to saturate at potentials higher than 50 V.

The expansion becomes less potential-dependent in electrolyteswith higher water content, and even minor when H2O concentra-tion is higher than 10 wt% (Fig. 2b). It should be noted that higherwater content limits drastically the growth speed of the nano-tubes and their maximum length. This does not allow for a fullconversion of 1 �m Ti layer into TiO2 nanotubes at lower appliedpotentials, narrowing the range of establishing electrochemicalself-organizing conditions. On the other side, at higher potentialsbreakdown holes in the tubular layers are formed [10], which rep-resent the upper limit for self-organization.

The influence of the water content on the expansion factor isrepresented in Fig. 2c. In this case the lower limit is given by thefact that for lower water concentrations formation of pores or dis-ordered structures occurs [10–12].

In contrast to the high impact of water and applied potentialdifference on the expansion, the fluoride concentration has a weakor no influence (Fig. 2d). The slight drop of the expansion at thelowest fluoride content can be attributed to the potential drop inthe electrolyte, owing to a low electrolyte conductivity.

Most recently Thompson et al. connected the expansion factorof porous anodic structures with the Pilling–Bedworth ratio of theamorphous anodic titania (˚am) by following approach [4]

Fexp = ε

1 − �k˚am (2)

The equation is derived for unidirectional expansion of the oxide

with certain degree of porosity (�). It should be noted that theporosity component does not assume loss of titanium, but itdescribes the part of volume occupied by the oxide in a block ofa TiO2 nanotube array. The loss of titanium in the electrolyte via

92 S.P. Albu, P. Schmuki / Electrochimica Acta 91 (2013) 90– 95

F n of thp ly anoa H2O.

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ig. 1. (a) SEM top view of evaporated Ti on FTO glass. The inset shows cross-sectiorocess. SEM images of top view (c) and cross-section (d) of TiO2 nanotubes partialt 50 V. The electrolyte used for anodization was EG + 0.1 mol dm−3 NH4F + 1.6 wt%

issolution (chemically or field-assisted dissolution) is describedy efficiency of the oxide growth (ε) [13]. The impurity coefficientk) in the equation accounts for the influence of incorporated con-aminant species in the growing oxide. In this regard, the product˚am can be considered the Pilling–Bedworth ratio of the com-ound (˚c) consisting of the contaminated titanium dioxide such asi1OxFyCzHuNw where x, y, z, u and w are the real numbers denotinghe concentration of elements in one volume molar of the obtainedompound.

For a compact oxide with a growth efficiency of 100%, the expan-ion factor is the same as the Pilling–Bedworth ratio. De facto, thempurity coefficient is an explicit factor in the equation that reflectshe impact of incorporation of impurities on the molar volume

nd density of the oxide. This is the result of the assumption thatilling–Bedworth ratio for the compact amorphous titania has aefined value of ∼2.43 and oxide contamination (or stoichiometryariations) are considered by the impurity coefficient [4,5].

e multilayered sample (Glass/FTO/Ti). (b) Patterned surface of Ti after anodizationdized at 10 V. SEM images of top view (e) and cross-section (f) of nanotubes grown

The porosity of the nanotubes does not change much over a widerange of electrochemical conditions used in this paper and it is sim-ilar to the case of Al2O3 pores, i.e. for most applied conditions it is inthe range of 0.1–0.3 [14]. On the other hand, the efficiency of nano-tube growth was determined to be high (80–95%) in the beginningof experiment and then it decreases when reaching steady-stategrowth of the nanotubes [5,15].

Incorporation of species from the electrolyte changes themeasured molar volume of the oxide and ultimately apparentlyincreases the measured Pilling–Bedworth ratio and accordingly, theexpansion factor. Considering impurity incorporation measured viaEDX [9,15], RBS [16], and XPS [10] (as shown in Table 1), the cal-culated ˚c is in the range of 2.4–2.7, and the expansion factor is

calculated to be 2.8–3.2 for an efficiency of ∼95% (specific efficiencyfor the initial phase of nanotube growth at high applied potentials)[15]. In general, the oxide growth efficiency approaches 100% atthe beginning of the experiment and then decreases in time and

S.P. Albu, P. Schmuki / Electrochimica Acta 91 (2013) 90– 95 93

Fig. 2. (a) Examples of I–t curves characteristic of TiO2 nanotube growth. (b) Influence of the applied potential difference on the expansion factor. The electrolytes wereE sion faw 0 wt%a erent

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G + 0.1 mol dm−3 NH4F + different water contents. (c) Water influence on the expanere EG + 0.1 mol dm−3 NH4F with different amounts of water in the range of 0.5–1

t applied potential difference of 50 V. The electrolytes were EG + 1.6 wt% H2O + diff

ith thickness of the grown nanotubes to about 50%. However, aetailed study of the oxide and the current efficiencies are carriedut elsewhere [15].

These average expansions are fairly in line with the experimen-ally determined values (Fig. 2b). However, the calculated valuesave a high degree of uncertainty. This comes from the difficulty ofeasuring the exact composition of the nanotubes which changes

long the tube length and across the wall thickness [9,17]. Further-ore, the density of the oxide may also be a subject to change under

ifferent electrochemical conditions. In the present study, the den-ity of amorphous oxide was assumed to be constant at 3.1 g cm−3

s reported in literature [18].A wide variation in expansion factor was also observed during

he growth of alumina pores [19–22]. These values in the range.8–1.9 are lower and higher than the expansion of formation ofompact pure Al2O3 that is ∼1.28 [22]. Similar to the porous alu-ina case, higher values obtained for nanotubular TiO2 as shown

n Fig. 2 could be explained via variable porosity coefficient ormpurity incorporation. Nevertheless, the porosity of the nanotubesresented in this study changes only in a narrow range and for sim-licity it can be averaged at ∼0.2. Therefore, a high expansion factor

an be attributed to impurity incorporation and possibly to a changen the density of the oxide. These assumptions are supported by therends observed experimentally (see Fig. 3). The nanotubes grown

able 1illing–Bedworth ratio and expansion factor coefficients calculated from literature data o

Electrolyte composition Average chemical comp

H C

EG + 0.2 mol dm−3 HF + 0.12 mol dm−3 H2O2 at% – 9.2parts – 0.3

EG + 0.1 mol dm−3 NH4F + 1 mol dm−3 H2O at% – 4.1parts – 0.1

EG + 0.2 mol dm−3 NH4F + different H2O content parts 0.57 0.1

ctor at different applied potential differences (20 V and 50 V). The used electrolytes. (d) Dependence of the expansion factor on the fluoride content in the electrolytefluoride content in the range of 0.01–0.2 mol dm−3 NH4F.

in ethylene glycol show a double-wall structure that is revealedwhen nanotubes are crystallized at high temperatures [9]. The innershell that is a carbon-rich layer can reflect to a certain extent thedegree of impurity incorporation.

Fig. 3a shows that the inner shell-to-wall thickness ratiodecreases with higher content of water in the electrolyte, and thistrend is similar to expansion factors shown in Fig. 2c. The SEMimages were taken near the tube bottom. The inner shell repre-sents more than 70% of the wall thickness at low water content andit decreases substantially to <20% at high water content. Decreasein the impurity concentration with higher water content may beascribed to a better compactness and less defective titania grownunder these conditions. Similarly, higher applied potentials leadto a faster oxide growth as well as to higher steady-state currentdensities and efficiencies [23]. This may allow formation of moredefective porous oxide layers and higher impurity incorporation(Fig. 3b) contributing to the expansion factor increase (Fig. 2b). Inspite of these findings, the real impact of the impurity incorporationrepresents a subject of debate. While the degree of incorporationcan change the walls thickness of the nanotubes one may doubtthat this is determining factor in the oxide expansions. A straight-

forward argument would be that the outer part of the tubes (outershell) is barely influenced by the degree of impurity incorporationand it remains dense and compact after combustion of carbon and

n average composition of TiO2 nanotubes assuming high growth efficiencies (95%).

osition of TiO2 nanotubes ˚c Fexp Tool Ref.

N O F Ti

7 – 56.40 6.55 27.78 ∼2.7 ∼3.2 EDX [9]3 – 2.03 0.24 1.00

5 – 50.43 10.24 35.18 ∼2.4 ∼2.8 XPS [10]2 – 1.43 0.29 1.00

9 0.05 1.73 0.10 1.00 ∼2.5 ∼2.9 RBS [16]

94 S.P. Albu, P. Schmuki / Electrochimica Acta 91 (2013) 90– 95

Fig. 3. (a) The inner shell (IS) to wall thickness (w) ratio (IWR) as a function of water content. The SEM insets are for 1 wt%, 10 wt% and 20 wt% H2O. The nanotubes wereg ts of wf 4F + ∼1

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rown at 50 V in ethylene glycol containing 0.1 mol kg−1 NH4F and different amounor 10 V, 30 V and 60 V. The nanotubes were grown at 50 V in EG + 0.1 mol dm−3 NH

uoride contaminants [9]. One can assume that at the same degreef the impurities the key factor in the expansion of nanotubes ofitanium oxide compound is the efficiency of the oxide growth.

For example, under different electrochemical conditions, lowerxpansion factors can be explained by the efficiency of the oxiderowth. TiO2 nanotubes grow slower at lower applied potentialsnd/or in the electrolytes with higher water content (Fig. 2a). Thisllows the system to enter much earlier in the steady-state growth,nd the efficiency of nanotube growth decreases to 50–55% owingo a higher loss of Ti ions in the electrolyte [5,15]. As a result, expan-ion factors as low as 1.2–1.5 can be expected.

. Conclusions

The present work shows that the expansion factor of TiO2 nano-ubes strongly depends on the anodization potential and the waterontent in the electrolyte while the fluoride content is not of sig-ificant influence. Pilling–Bedworth ratio and expansion factor canhange over a wide range with the electrochemical conditions. Aigher active dissolution of titanium lowers the expansion factorhile the incorporation of impurities and defects increases it. The

xpansion can be an important factor in applications of nanotubulariO2 arrays, as it can generate stress or volume difference betweenhe substrate and the oxide, influencing the design, efficiency andunctionality of devices.

When considering theoretical modeling and simulations ofiO2 nanotube growth, the present work suggests that experi-ental variations in real Pilling–Bedworth ration should be taken

nto consideration, according to specific chemical composition ofhe nanotubes, their density, porosity and the efficiency of oxiderowth.

cknowledgments

We would like to acknowledge DFG and DFG Cluster of Excel-ence EAM for financial support.

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ater. (b) IWR dependence on the applied potential difference. The SEM insets are.6 wt% H2O.

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