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Electrochimica Acta 55 (2010) 4359–4367

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

abrication of nanoporous TiO2 by electrochemical anodization

rzegorz D. Sulka ∗, Joanna Kapusta-Kołodziej, Agnieszka Brzózka, Marian Jaskułaepartment of Physical Chemistry & Electrochemistry, Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30060 Krakow, Poland

r t i c l e i n f o

rticle history:eceived 24 July 2009eceived in revised form1 December 2009ccepted 12 December 2009vailable online 11 January 2010

a b s t r a c t

The formation of self-organized porous titania is achieved by electrochemical anodization under a poten-tiostatic regime. Anodic titanium oxide (ATO) was fabricated by a three-step self-organized anodizationof the Ti foil in an ethylene glycol electrolyte containing 0.38 wt% of NH4F and 1.79 wt% of H2O. Anodizingwas carried out at the constant cell potential ranging from 30 to 70 V at the temperature of 20 ◦C. It wasfound that nanoporous TiO2 arrays can be obtain only after a short duration of the third step (10 min).The influence of anodizing potential on the structural parameters of porous anodic titania including

eywords:nodizationorous titaniaanostructuresanopores

pore diameter, interpore distance, wall thickness, porosity and pore density was extensively studied. Thelinear dependencies between interpore distance, pore diameter and wall thickness upon the anodizingpotential were found. The regularity of pore arrangement was monitored qualitatively by fast Fouriertransforms (FFTs) of top-view FE-SEM images. It was found that the best arrangement of nanopores isobserved at 40 V. This finding was confirmed by the analysis of pore circularity. The highest circularity

ce ag

elf-organization of pores was observed on

. Introduction

Today, nanotechnology encompasses a wide range of materi-ls and techniques employed to the design and study of modernevices suitable for various possible commercial applications. Theurrent international trend towards fabrication, characterizationnd investigation of nanosize materials has also embraced titaniumioxide [1–4]. This is particularly due to the fact that TiO2 possessesrange of unique functional physical and chemical properties suchs: a high oxidative power, photostability, and nontoxicity [1].owever, the properties of TiO2 can be considerably improvedhen the oxide architecture is controlled on a nanoscale level and

rder arrays of nanopores, nanopillars and nanotubes are formed.mong various techniques used for the synthesis of nanostruc-

ured metal oxides, a self-organized anodization of valve metalss a simple, high-throughput and low-cost method of fabricatinganoporous or nanotubular structures [5].

Due to the enormous oxidation power and relatively good chem-cal stability, anodic titanium oxide (ATO) is a versatile materialeing used recently as an excellent catalyst for water photoly-is and hydrogen generation [6–9], decomposition of unwanted

rganic compounds [6,10–13], electrocatalysis of methanol oxi-ation [14,15] and for an inactivation of Escherichia coli bacteria16]. Moreover, anodic titanium oxide is a promising photovoltaic

aterial used for photoconversion in dye-sensitized solar cells

∗ Corresponding author. Tel.: +48 12 663 22 66; fax: +48 12 634 05 15.E-mail address: Sulka@chemia.uj.edu.pl (G.D. Sulka).

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.12.053

ain at 40 V.© 2010 Elsevier Ltd. All rights reserved.

[7,10,17–21] and in energy storage applications as a nanostructuredanode material for lithium-ion batteries [22–24]. Other poten-tial technological applications of porous anodic titanium oxideincludes structural ceramics [25], humidity [26], gas [18,27,28]and biochemical sensors [29,30] as well as biocompatible mate-rials for bone implants [31–33]. Similarly to porous anodicalumina, anodic titania has been recently utilized as a tem-plate for fabrication of Co [34], Cu [35], Ni [36–38], Pt [39],Sn [40] and CdS [38] nanowire arrays by electrodeposition,multi-walled carbon nanotubes by CVD method [41] and conduct-ing polymers including polypyrrole and poly(3-hexylthiophene)nanowire and nanotube arrays by electrochemical deposition[38].

A typical anodizing process of titanium is carried out in apotentiostatic regime in water-based or non-aqueous electrolytescontaining fluoride ions [6,7,9–13,16,18,24,42]. Studies show thatself-organized anodizing of titanium result usually in a verticallyaligned, ordered TiO2 nanotube arrays over a macroscopic surfacearea of substrate [9,20]. By tailoring the operating conditions suchas: type of electrolytes, electrolyte composition and pH, anodizingpotential and temperature of the electrolyte, the diameter of nan-otubes can vary in the range of 20–300 nm [6,18,42–45] while thelength of nanotubes (layer thickness) can be up to 1 mm [7,8,46].Whereas, nanotubular structures of anodic titania are commonly

fabricated by anodizing of titanium, there are a very limited numberof reports on well-ordered arrays of nanopores synthesized usingthis electrochemical approach [35,47–49].

In order to obtain high-ordered arrangement of nanopores in theanodic titanium oxide layer, similarly to a self-organized anodiz-

4 imica A

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360 G.D. Sulka et al. / Electroch

ng of aluminum [50–53], a multi-step anodizing procedure can bemployed. For anodic titania, this approach is not very often repre-ented in the literature due to a problem with an effective chemicalemoval of the oxide layer without disturbance in a titanium metalase. A multi-step anodizing procedure consists of one or two cyclesf initial anodizing at the constant cell voltage and a subsequenthemical/mechanical removal of the grown oxide layer. After theemoval of oxide layer, periodic concave triangular features formedn the metal surface serve as nucleation sites for the formation ofanopores during the final anodization. It is worth pointing out,hat the process does not guarantee the surface area free of defects

non-hexagonally arranged nanopores) [50–53].

The aim of the present study is to investigate the self-organizedormation of ordered arrays of nanopores by a three-step anodizingf titanium in the electrolyte based on ethylene glycol. In particu-ar, the effect of anodizing potential on structural features of anodic

Fig. 1. Idealized structure of porous anodic titanium oxide (ATO) (A) and a cross-

ig. 2. Top-, bottom-, and side-views of anodic TiO2 with an EDX spectrum of the ATOontaining NH4F (0.38 wt%) and H2O (1.79 wt%) at 40 V and 20 ◦C. The duration of the thir

cta 55 (2010) 4359–4367

titania (pore diameter, interpore distance, porosity, pore density)and pore arrangement regularity is extensively discussed. Unlikethe formation of ordered TiO2 nanotubes reported broadly in theliterature, we present fabrication of TiO2 nanopore arrays and anal-ysis of regularity of pore arrangement in anodic titania. As far as weknow, very little attention has been paid to the analysis of arrange-ment of nanopores/nanotubes in ordered arrays of TiO2 synthesizedby anodization.

2. Experimental

A titanium foil (99.5% purity) 0.25 mm thick from Alfa Aesar waspre-cut in coupons (0.5 cm × 2.5 cm) with a selected working areaof 0.5 cm2. The samples were degreased in acetone than in ethanol.The back surface and the edges of the sample were insulated by anacid resistant paint layer.

sectional view of the anodized layer with the structural features of ATO (B).

surface. A three-step anodization was performed in an ethylene glycol solutiond step was 10 min and the analyzed surface area was 1.83 �m2.

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passive layer on titanium and its subsequent transformation intothe porous oxide layer. After reaching the maximum, the currentdensity slightly decreases with time and a steady-state formationof porous oxide is observed. The presence of the maximum in

G.D. Sulka et al. / Electroch

Prior to anodization, Ti samples were electrochemically pol-shed in a mixed solution of acetic acid (99.5%), sulfuric acid98%), and hydrofluoric acid (40%) (60:15:25 in volume) at con-tant current density of 70 mA cm−2 and 20 ◦C for 1 min, followedy chemical polishing in a mixture of HF (40%) and nitric acid (65%)1:3 in volume) for 10 s until a mirror finish was exposed. Thenodization was performed using a two-electrode cell with theitanium foil as the working electrode and a platinum foil as theounter electrode. The distance between Ti samples and the counterlectrode was 3 cm. The anodic oxide layer was formed by a three-tep procedure under a constant cell voltage ranging between 30nd 70 V in an ethylene glycol solution containing NH4F (0.38 wt%)nd H2O (1.79 wt%) at 20 ◦C. The duration of the first and secondnodizing steps was 3 h. After both anodizing steps, an adhesiveape was used for removal of the grown oxide layer. Immedi-tely after the oxide removal, the titanium sample was re-anodizednder conditions identical to those in previous anodizationteps. Finally, the third anodization was usually performedor 10 min.

The morphologies of anodic porous titania nanostructures andDX analysis were performed with a field emission scanning elec-ron microscope (FE-SEM/EDS, Hitachi S-4700 with a Noran System). The thickness of porous anodic titania was estimated fromE-SEM cross-sectional views of anodized specimens for variousnodizing potentials. The scanning probe image processor WSxM.0 Develop 7.6 [54] was employed to calculate and analyze two-imensional fast Fourier transforms (FFTs) of FE-SEM images. The

mageJ 1.37v software [55] was used for estimation of circularityoefficients of ATO films.

. Results and discussion

A porous anodic titanium oxide layer can grow by self-organizednodizing of titanium under well-controlled conditions. The idealtructure of the highly ordered TiO2 layer is represented by alosed-packed array of hexagonally arranged cells containing poresn each cell center (Fig. 1). The morphology of anodic titania cane described by characteristic parameters of porous layer includ-

ng: interpore distance (Dc), pore diameter (Dp), wall thickness (W),orosity and pore density (n). Similarly to anodization of aluminum,he structural features of porous titania can be easily controllably byltering the anodizing conditions. During the porous oxide growth,thin and compact barrier layer (B) at the pore bottom/electrolyte

nterface is continuously dissolved by locally increased field, andnew barrier layer at the metal/oxide interface is rebuilt. For a

teady-state film growth, there is a dynamic equilibrium betweenhe rate of film growth and its field-assisted dissolution. As a resultf the steady-state growth of oxide, the cylindrical in section poresre formed (Fig. 1). The thickness of the oxide layer (depth of pores)epends on the anodizing potential, process duration and type ofnodizing electrolyte (possible chemical etching of formed oxide).

Fig. 2 shows typical FE-SEM top-, bottom- and side-viewsmages of anodic titania layer obtained by the three-step anodiza-ion performed in an ethylene glycol solution containing NH4F0.38 wt%) and H2O (1.79 wt%) at 40 V and 20 ◦C. The duration ofhe third step was 10 min. As can be seen from FE-SEM images,nodizing of titanium in an ethylene glycol-based electrolyte canive a periodic lattice of nanopores. The dimensions of pores andistances between them are uniform but the pore arrangement

s far from the ideal hexagonal arrangement. The triangular lat-

ice of cells is often disturbed and there are only small domainsith hexagonally arranged pores over the analyzed surface area of

.83 �m2. In cross-section, the oxide coating is a relatively thick asor 10 min of anodization. The EDX spectra (Fig. 2) reveal that thelose-packed cells of titania contain a little amount of embedded

cta 55 (2010) 4359–4367 4361

fluorine. The fluoride ions are incorporated into the porous struc-ture of oxide as a direct result of the migration of electrolyte species(F−) towards the Ti anode during the anodizing process. A typicalcontent of incorporated fluoride ions was found to be between 3.4and 5.6 wt%.

The current–time transients recorded during the third anodiz-ing step performed under cell voltage ranging between 30 and70 V are shown in Fig. 3A. Fig. 3B shows the current–time curvesrecorded for successive anodizing steps performed at 40 V. The cur-rent density evolutions are typical for anodization of valve metalswith formation of the porous oxide layer [5]. For anodizing poten-tials lower than 50 V, the current–time transient hits a minimumfor a very early stage of the process duration (about 30 s) followedby a flattened maximum at about 90–120 s. These changes in thecurrent density are results of the growing compact, high-resistant,

Fig. 3. Current density vs. anodizing time recorded for the third anodizing step per-formed at various anodizing potentials (A) and for different anodizing steps at 40 V(B). The anodization was carried out in ethylene glycol containing NH4F (0.38 wt%)and H2O (1.79 wt%) at 20 ◦C.

4 imica A

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362 G.D. Sulka et al. / Electroch

he current–time curve is usually ascribed to the pore rearrange-ent process occurring on the surface and resulting in a network

lose-packed pores [5]. For anodizing potentials higher than 40 V,he current–time transients show a decrease as a function of timeFig. 3A). Such a behavior is typical for the hard-anodization exper-ments where the flowing current is a relatively high and the oxideayer grows rapidly [5]. With the increasing thickness of the oxideayer, diffusion path along the nanopores towards the oxide/metalnterface for oxygen-containing species extends significantly. Inonsequence, ionic current reflected by recorded current decreasesith time. The evolution of the current density with anodizing time

uring successive anodizing steps at 40 V are shown in Fig. 3B. Asan be seen, the shape of the current–time curves for a first anodiz-ng step is a bit different from those recorded for the second andhird anodizing step. The minimum and maximum of current den-

ig. 4. Dependence of the steady-state current density, I (A) and ln I (B) on anodiz-ng potential. The anodization was carried out in ethylene glycol containing NH4F0.38 wt%) and H2O (1.79 wt%) at 20 ◦C. The estimated regression equations andorrelation coefficients (R) are: (A) y = 0.2201 exp(0.0511x), R2 = 0.9877 and (B)= 0.0511x − 1.5137, R2 = 0.9877.

cta 55 (2010) 4359–4367

sity are not so evidently resolved for the first step curve as we canobserved for transients recorded for the second and third steps. Themaximum of current density is stretched to about 400 s while forthe second and third steps appears usually below 100 s. The occur-rence of a minimum current density is seen in each step but, thetime at which this minimum occurs decreases with the increasingnumber of anodizing step. Consequently for successive anodizingsteps, the steady-state conditions for the formation of pores areachieved within a shorter time period and recorded current densi-ties are higher. This indicates that the nucleation of pores is easieron pre-textured titanium surface. Notice importantly that after

removal of the oxide layer grown during anodizing step, the regulararray of concaves is present on the titanium surface. This concavesact as nucleation sites for pore growth in the next anodizing step.

Fig. 5. Effect of anodizing potential on the thickness (A) and aspect ratio (B) ofporous oxide layer grown after the third anodization carried out in an ethylene gly-col solution containing NH4F (0.38 wt%) and H2O (1.79 wt%) at 20 ◦C. The duration ofthe third step was 10 min. The estimated regression equations and correlation coef-ficients (R) are: (A) y = 0.1556x − 3.5636, R2 = 0.9961 and (B) y = 2.0612x − 40.137,R2 = 0.9937.

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SEM top-view images. For each FFT pattern, 3 main profiles alongthe FFT intensity were constructed. In this way, the average value ofthe main distance of the lattice in the inverse space was estimatedon the basis of 27 measurements for each studied anodizing poten-

G.D. Sulka et al. / Electroch

During the steady-state growth of TiO2 layer under a potentio-tatic regime current density remains almost unchanged. Fig. 4Ahows the average values of the current density recorded duringteady-state growth of porous anodic titania in an ethylene glycololution containing NH4F (0.38 wt%) and H2O (1.79 wt%) at variousnodizing potentials. As can be seen from Fig. 4A, an exponentialurve can be fitted through the experimental data points. Accordingo the high-field conduction theory, the current density I is relatedo the voltage drop across the barrier layer, as follows:

= ˛ exp(

ˇ · U

B

)(1)

here ˛ and ˇ are electrolyte- and material-dependent constantst a given temperature, and U/B is the effective electric field acrosshe barrier layer with thickness B. After logarithmic transformation,linear dependence between ln I and U is obtained

n I = ln ˛ +(

ˇ

B· U

)(2)

rom a linear relationship presented in Fig. 4B, the calculated valuesf ˛ and ˇ/B are 0.9985 A cm−2 and 5.11 × 10−5 V, respectively.

The effect of anodizing potential on the thickness (Fig. 5A) andspect ratio (Fig. 5B) of porous oxide layer grown during anodizingn an ethylene glycol solution containing NH4F (0.38 wt%) and H2O1.79 wt%) was studied at 20 ◦C. The duration of the third step was0 min. The thickness of the oxide layer grown during anodizingt various cell potentials was estimated from low magnificationE-SEM images. The aspect ratio, defined as a ratio between theength of pores and their diameter was also calculated. The data pre-ented in Fig. 5 show that the thickness and aspect ratio of porousiO2 increase linearly with increasing anodizing potential. In con-rast to these results, the current density increases exponentially

compare with Fig. 4A). Therefore, evidently increasing anodiz-ng potential decreases gradually a Faradaic current efficiency inlectrochemical oxidation of titanium. The estimated oxide thick-ess was about 1.3 and 7.3 �m for the lowest and highest studiedotentials, respectively. The calculated aspect ratio of anodic tita-

ig. 6. Anodizing potential influence on the average interpore distance, poreiameter and wall thickness of porous anodic titania. The three-step anodiza-ions were performed in ethylene glycol containing NH4F (0.38 wt%) and H2O1.79 wt%) at 20 ◦C. The estimated regression equations and correlation coefficientsR) are: (1) 2.1044x + 23.208, R2 = 0.9891, (2) y = 0.3373x + 47.906, R2 = 0.9699 and3) y = 0.8835x − 12.349, R2 = 0.9905 for interpore distance, pore diameter and wallhickness, respectively.

cta 55 (2010) 4359–4367 4363

nia varies between 20 and 100 in the range of anodizing potentialsfrom 30 to 70 V. From Fig. 5A can be seen also that the growth rate ofporous anodic titania depends linearly on the anodizing potentialand can change from about 7.6 to 44.0 �m h−1. For the highest stud-ied potential, the rate of oxide growth is close to a high-speed rateof oxide growth (even 50 �m h−1) observed in a hard anodizing ofaluminum [5].

The characteristic parameters of porous anodic titania includ-ing interpore distance (Dc) pore diameter (Dp), and wall thickness(W) are controllable by operating conditions of anodization. For thegiven anodizing potential, the average interpore distance of the ATOlattice was calculated from fast Fourier transforms of 9 different FE-

Fig. 7. Anodizing potential influence on porosity (A) and pore density (B) of anodictitania formed by three-step anodizing performed in ethylene glycol containingNH4F (0.38 wt%) and H2O (1.79 wt%) at 20 ◦C. The estimated regression equationsand correlation coefficients (R) are: (A) y = 1545.8x − 1.0741, R2 = 0.993 and (B)y = 38.718 exp(−0.0331x), R2 = 0.9999.

4 imica A

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364 G.D. Sulka et al. / Electroch

ial. The Dc values calculated from the FFT profiles are presentedn Fig. 6. The experimental data points shows that the interporeistance is linearly dependent upon the anodization voltages andaries from about 90 to 175 nm in the range of anodizing potentialrom 30 to 70 V. For all anodizing potentials, the average pore diam-ter was calculated from 9 different FE-SEM top-view images withImageJ image analysis software [55]. In pore diameter calcula-

ions, pore shapes were assumed to be perfect circles. The averagealue of pore diameter calculated from the software are presented

n Fig. 6. A linear relationship between pore diameter and anodizingotential is observed. As can be seen, the pore diameter increasesrom about 59 to 72 nm when anodizing potential increases from0 to 70 V. In general, the wall thickness of porous anodic titania

ig. 8. FE-SEM top-views together with their 2D fast Fourier transforms (FFTs) and 3Dotentials by the three-step anodization performed in ethylene glycol containing NH4F (0ll anodizing voltages and the analyzed surface area was 1.72 �m2.

cta 55 (2010) 4359–4367

can be given by the following equation [5]:

W = Dc − Dp

2(3)

On the basis of Eq. (3) the wall thickness was calculated for variousanodizing potentials. The calculated W values are linearly depen-dent on the anodizing potential (Fig. 6).

For the ideal hexagonal arrangement of pores in the ATO layer,the porosity of the lattice (P) and pore density (n), defined as a

total number of pores occupying the surface area of 1 cm2, can beestimated from the following expressions [5]:

P = �

2√

3·(

Dp

Dc

)2

(4)

representations of FE-SEM images for porous titania formed at various anodizing.38 wt%) and H2O (1.79 wt%) at 20 ◦C. The duration of the third step was 10 min for

G.D. Sulka et al. / Electrochimica Acta 55 (2010) 4359–4367 4365

le analysis of the FFT intensity along the marked solid line.

n

Tfgaaiitippf1

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= 1 · 1014

√3 · D2

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he effects of anodizing potentials on the porosity of ATO filmsormed by a self-organized anodizing performed in an ethylenelycol solution containing NH4F (0.38 wt%) and H2O (1.79 wt%)re shown in Fig. 7A. As can be seen in Fig. 7A, the porosity ofnodic titania decreases from about 39.3% to 15.6% with increas-ng anodizing potential from 30 to 70 V. Hence, the pore density isnversly proportional to the square value of the interpore distancehe exponential decrease in pore density is expected with increas-ng anodizing potential. Our experimental results show indeed, thatore density decreases exponentially with increasing anodizingotential (Fig. 7B). For example, when anodizing potential increasesrom 30 to 70 V, the pore density of ATO film decreases from about.4 × 1010 to 3.8 × 109 pores cm−2.

Typical FE-SEM top-view images with their 2D fast Fourierransforms and 3D representations of FE-SEM micrographs oforous anodic titania formed at various anodizing potentials after ahird anodizing step performed in an ethylene glycol solution con-aining NH4F (0.38 wt%) and H2O (1.79 wt%) are shown in Fig. 8.he duration of the third step was 10 min for all anodizing poten-ials. Additionally, a typical average intensity profile along the givenD FFT image is also collected in Fig. 8 for all studied anodizingotentials. The fast Fourier transform of the periodic lattice pro-ides information about periodicity of the structure in the inversecale. For a triangular lattice with a hexagonal pore arrangement, aFT pattern consists of six distinct spots on the edges of a hexagon.n case of the disturbed pore arrangement, the FFT pattern is char-cterized by a ring-shape or disc-shape form as we can see in Fig. 8.he narrow ring-shape form indicates a preserved short-distanceeriodicity of the lattice and slightly disturbed long-range order

n the nanopore network. On the other hand, for the FFT patternith six distinct spots (high pore arrangement) the average pro-le along the FFT width should show two very smooth and higheaks. Fig. 8 shows that anodizing potential influences consider-bly the nanopore arrangement. The best hexagonal arrangementf nanopores is observed for the anodizing potential of 40 and0 V where relatively narrow ring shapes are observed. This fact

s reflected also by the highest intensity profiles of FFT images forhese both potentials. The qualitative analysis of pore arrangementegularity, using FFT images and intensities of FFT profiles is diffi-ult to decide definitely which of anodizing potentials gives better

esults.

In order to get a deeper insight into the regularity of porerrangement, the quantitative analysis was performed. A regu-arity ratio (R), defined as a ratio of the maximum intensity ofhe peak in the average FFT profile to the width of the peak at

Fig. 10. Effect of anodizing potential on regularity ratio (A) and pore circularity (B) ofanodic porous titania formed by three-step anodizing carried out in ethylene glycolcontaining NH4F (0.38 wt%) and H2O (1.79 wt%) at 20 ◦C.

4366 G.D. Sulka et al. / Electrochimica Acta 55 (2010) 4359–4367

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T

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ig. 11. FE-SEM top-view images of anodic titania fabricated by three-step anodizind 20 ◦C. The duration of the third step was 30 min (A), 45 min (B), 60 min (C) and

alf-maximum was calculated from the average FFT profile for var-ous anodizing potentials (Fig. 9). Notice importantly, FFT images

ere constructed from the FF-SEM micrographs taken at the sameagnification for samples anodized at various potentials. With

ncreasing anodizing potential increases significantly interpore dis-ances and consequently decreases number of pores present onhe same surface area (the same magnification FE-SEM images).s a result of that, different numbers of hexagonally arrangedanopores were taken for FFT calculations. In order to avoid this

nfluence of pore number on regularity analysis, the average reg-larity ratio (R) calculated from the FFT profiles were multipliedy pore density (n). Fig. 10A shows the dependence between n·Rnd anodizing potentials. Below and above the anodizing poten-ial of 40 V, a significant decrease in the regularity of pore orders observed. The uniformity of the porous structure can be inves-igated by the analysis of pore shapes and calculating a poreircularity [55]. The value of circularity equals to 1.0 indicates a per-ect circle. As the value approaches 0.0, it indicates an increasinglylongated polygon. For the given anodizing potential, the averageore circularity was calculated from 9 different FE-SEM top-view

mages. As can be seen in Fig. 10B, a significant improvement in aircular shape of pores is observed at 40 V. This result is consistentith the analysis of pore arrangement regularity.

The structure of anodic TiO2 depends considerably on theuration of the third anodizing step (Fig. 11). For the ethylenelycol-based electrolyte, the porous structure is formed only forhort period of anodizing time not exceeding 45 min. However, fur-her increase of the anodizing time to 3 h results in fibrous-liketructure with nanotubes underneath (Fig. 11D). This is attributedo the chemical dissolution of the newly formed anodic oxide by F−

ons present in the electrolyte according to the given equation:

iO2 + 6F− + 4H+ → TiF2−6 + 2H2O (6)

. Conclusions

The major findings of this study can be summarized as follows:

titanium in ethylene glycol containing NH4F (0.38 wt%) and H2O (1.79 wt%) at 60 Vin (D).

1. The three-step anodizing of titanium in an ethylene glycol solu-tion containing NH4F (0.38 wt%) and H2O (1.79 wt%) can result inwell-arranged honeycomb structures. The duration of the thirdstep significantly affects the morphology of the film. The porousstructure of TiO2 can be obtained only for short durations. Forlonger periods of anodizing time TiO2 nanowires/nanotubes areformed on the Ti surface.

2. The oxide thickness formed after 10 min of the third stepincreases linearly with increasing anodizing potential. The cur-rent efficiency of anodic oxidation of Ti decreases gradually withincreasing applied anodic potential.

3. The interpore distance, pore diameter and wall thickness ofporous anodic titania are linearly dependent upon the anodiza-tion.

4. The porosity and pore density of ATO layers formed under thestudied range of potentials decrease exponentially with increas-ing anodizing potential.

5. In the range of anodizing potentials from 30 to 70 V, the besthexagonal arrangement of pores and highest pore circularity isobserved at 40 V.

Acknowledgments

This research was partially supported by the European COSTaction D41 “Inorganic oxide surfaces and interfaces”, financed bythe Polish Ministry of Science and High Education (DWM/N112/COST/2008). The SEM imaging was performed in the Laboratory ofField Emission Scanning Electron Microscopy and Microanalysis atthe Institute of Geological Sciences, Jagiellonian University, Poland.

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