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Fabrication of nanoporous TiO2 by electrochemical anodization
Electrochimica Acta 55 (2010) 4359–4367
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Fabrication of nanoporous TiO2 by electrochemical anodization Grzegorz D. Sulka ∗ , Joanna Kapusta-Kołodziej, Agnieszka Brzózka, Marian Jaskuła Department of Physical Chemistry & Electrochemistry, Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30060 Krakow, Poland
a r t i c l e
i n f o
Article history: Received 24 July 2009 Received in revised form 11 December 2009 Accepted 12 December 2009 Available online 11 January 2010 Keywords: Anodization Porous titania Nanostructures Nanopores Self-organization
a b s t r a c t The formation of self-organized porous titania is achieved by electrochemical anodization under a potentiostatic regime. Anodic titanium oxide (ATO) was fabricated by a three-step self-organized anodization of the Ti foil in an ethylene glycol electrolyte containing 0.38 wt% of NH4 F and 1.79 wt% of H2 O. Anodizing was carried out at the constant cell potential ranging from 30 to 70 V at the temperature of 20 ◦ C. It was found 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 pore diameter, interpore distance, wall thickness, porosity and pore density was extensively studied. The linear dependencies between interpore distance, pore diameter and wall thickness upon the anodizing potential were found. The regularity of pore arrangement was monitored qualitatively by fast Fourier transforms (FFTs) of top-view FE-SEM images. It was found that the best arrangement of nanopores is observed at 40 V. This finding was confirmed by the analysis of pore circularity. The highest circularity of pores was observed once again at 40 V. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Today, nanotechnology encompasses a wide range of materials and techniques employed to the design and study of modern devices suitable for various possible commercial applications. The current international trend towards fabrication, characterization and investigation of nanosize materials has also embraced titanium dioxide [1–4]. This is particularly due to the fact that TiO2 possesses a range of unique functional physical and chemical properties such as: a high oxidative power, photostability, and nontoxicity [1]. However, the properties of TiO2 can be considerably improved when the oxide architecture is controlled on a nanoscale level and order arrays of nanopores, nanopillars and nanotubes are formed. Among various techniques used for the synthesis of nanostructured metal oxides, a self-organized anodization of valve metals is a simple, high-throughput and low-cost method of fabricating nanoporous or nanotubular structures [5]. Due to the enormous oxidation power and relatively good chemical stability, anodic titanium oxide (ATO) is a versatile material being used recently as an excellent catalyst for water photolysis and hydrogen generation [6–9], decomposition of unwanted organic compounds [6,10–13], electrocatalysis of methanol oxidation [14,15] and for an inactivation of Escherichia coli bacteria [16]. Moreover, anodic titanium oxide is a promising photovoltaic material 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: [email protected] (G.D. Sulka). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.12.053
[7,10,17–21] and in energy storage applications as a nanostructured anode material for lithium-ion batteries [22–24]. Other potential technological applications of porous anodic titanium oxide includes structural ceramics [25], humidity [26], gas [18,27,28] and biochemical sensors [29,30] as well as biocompatible materials for bone implants [31–33]. Similarly to porous anodic alumina, anodic titania has been recently utilized as a template 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 conducting 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 a potentiostatic regime in water-based or non-aqueous electrolytes containing fluoride ions [6,7,9–13,16,18,24,42]. Studies show that self-organized anodizing of titanium result usually in a vertically aligned, ordered TiO2 nanotube arrays over a macroscopic surface area of substrate [9,20]. By tailoring the operating conditions such as: type of electrolytes, electrolyte composition and pH, anodizing potential and temperature of the electrolyte, the diameter of nanotubes can vary in the range of 20–300 nm [6,18,42–45] while the length 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 number of reports on well-ordered arrays of nanopores synthesized using this electrochemical approach [35,47–49]. In order to obtain high-ordered arrangement of nanopores in the anodic titanium oxide layer, similarly to a self-organized anodiz-
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ing of aluminum [50–53], a multi-step anodizing procedure can be employed. For anodic titania, this approach is not very often represented in the literature due to a problem with an effective chemical removal of the oxide layer without disturbance in a titanium metal base. A multi-step anodizing procedure consists of one or two cycles of initial anodizing at the constant cell voltage and a subsequent chemical/mechanical removal of the grown oxide layer. After the removal of oxide layer, periodic concave triangular features formed on the metal surface serve as nucleation sites for the formation of nanopores during the final anodization. It is worth pointing out, that 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-organized formation of ordered arrays of nanopores by a three-step anodizing of titanium in the electrolyte based on ethylene glycol. In particular, the effect of anodizing potential on structural features of anodic
titania (pore diameter, interpore distance, porosity, pore density) and pore arrangement regularity is extensively discussed. Unlike the formation of ordered TiO2 nanotubes reported broadly in the literature, we present fabrication of TiO2 nanopore arrays and analysis of regularity of pore arrangement in anodic titania. As far as we know, very little attention has been paid to the analysis of arrangement of nanopores/nanotubes in ordered arrays of TiO2 synthesized by anodization. 2. Experimental A titanium foil (99.5% purity) 0.25 mm thick from Alfa Aesar was pre-cut in coupons (0.5 cm × 2.5 cm) with a selected working area of 0.5 cm2 . The samples were degreased in acetone than in ethanol. The back surface and the edges of the sample were insulated by an acid resistant paint layer.
Fig. 1. Idealized structure of porous anodic titanium oxide (ATO) (A) and a cross-sectional view of the anodized layer with the structural features of ATO (B).
Fig. 2. Top-, bottom-, and side-views of anodic TiO2 with an EDX spectrum of the ATO surface. A three-step anodization was performed in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 40 V and 20 ◦ C. The duration of the third step was 10 min and the analyzed surface area was 1.83 m2 .
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Prior to anodization, Ti samples were electrochemically polished in a mixed solution of acetic acid (99.5%), sulfuric acid (98%), and hydrofluoric acid (40%) (60:15:25 in volume) at constant current density of 70 mA cm−2 and 20 ◦ C for 1 min, followed by 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. The anodization was performed using a two-electrode cell with the titanium foil as the working electrode and a platinum foil as the counter electrode. The distance between Ti samples and the counter electrode was 3 cm. The anodic oxide layer was formed by a threestep procedure under a constant cell voltage ranging between 30 and 70 V in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C. The duration of the first and second anodizing steps was 3 h. After both anodizing steps, an adhesive tape was used for removal of the grown oxide layer. Immediately after the oxide removal, the titanium sample was re-anodized under conditions identical to those in previous anodization steps. Finally, the third anodization was usually performed for 10 min. The morphologies of anodic porous titania nanostructures and EDX analysis were performed with a field emission scanning electron microscope (FE-SEM/EDS, Hitachi S-4700 with a Noran System 7). The thickness of porous anodic titania was estimated from FE-SEM cross-sectional views of anodized specimens for various anodizing potentials. The scanning probe image processor WSxM 4.0 Develop 7.6 [54] was employed to calculate and analyze twodimensional fast Fourier transforms (FFTs) of FE-SEM images. The ImageJ 1.37v software [55] was used for estimation of circularity coefficients of ATO films.
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fluorine. The fluoride ions are incorporated into the porous structure of oxide as a direct result of the migration of electrolyte species (F− ) towards the Ti anode during the anodizing process. A typical content of incorporated fluoride ions was found to be between 3.4 and 5.6 wt%. The current–time transients recorded during the third anodizing step performed under cell voltage ranging between 30 and 70 V are shown in Fig. 3A. Fig. 3B shows the current–time curves recorded for successive anodizing steps performed at 40 V. The current density evolutions are typical for anodization of valve metals with formation of the porous oxide layer [5]. For anodizing potentials lower than 50 V, the current–time transient hits a minimum for a very early stage of the process duration (about 30 s) followed by a flattened maximum at about 90–120 s. These changes in the current density are results of the growing compact, high-resistant, passive layer on titanium and its subsequent transformation into the porous oxide layer. After reaching the maximum, the current density slightly decreases with time and a steady-state formation of porous oxide is observed. The presence of the maximum in
3. Results and discussion A porous anodic titanium oxide layer can grow by self-organized anodizing of titanium under well-controlled conditions. The ideal structure of the highly ordered TiO2 layer is represented by a closed-packed array of hexagonally arranged cells containing pores in each cell center (Fig. 1). The morphology of anodic titania can be described by characteristic parameters of porous layer including: interpore distance (Dc ), pore diameter (Dp ), wall thickness (W), porosity and pore density (n). Similarly to anodization of aluminum, the structural features of porous titania can be easily controllably by altering the anodizing conditions. During the porous oxide growth, a thin and compact barrier layer (B) at the pore bottom/electrolyte interface is continuously dissolved by locally increased field, and a new barrier layer at the metal/oxide interface is rebuilt. For a steady-state film growth, there is a dynamic equilibrium between the rate of film growth and its field-assisted dissolution. As a result of the steady-state growth of oxide, the cylindrical in section pores are formed (Fig. 1). The thickness of the oxide layer (depth of pores) depends on the anodizing potential, process duration and type of anodizing electrolyte (possible chemical etching of formed oxide). Fig. 2 shows typical FE-SEM top-, bottom- and side-views images of anodic titania layer obtained by the three-step anodization performed in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 40 V and 20 ◦ C. The duration of the third step was 10 min. As can be seen from FE-SEM images, anodizing of titanium in an ethylene glycol-based electrolyte can give a periodic lattice of nanopores. The dimensions of pores and distances between them are uniform but the pore arrangement is far from the ideal hexagonal arrangement. The triangular lattice of cells is often disturbed and there are only small domains with hexagonally arranged pores over the analyzed surface area of 1.83 m2 . In cross-section, the oxide coating is a relatively thick as for 10 min of anodization. The EDX spectra (Fig. 2) reveal that the close-packed cells of titania contain a little amount of embedded
Fig. 3. Current density vs. anodizing time recorded for the third anodizing step performed at various anodizing potentials (A) and for different anodizing steps at 40 V (B). The anodization was carried out in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C.
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the current–time curve is usually ascribed to the pore rearrangement process occurring on the surface and resulting in a network close-packed pores [5]. For anodizing potentials higher than 40 V, the current–time transients show a decrease as a function of time (Fig. 3A). Such a behavior is typical for the hard-anodization experiments where the flowing current is a relatively high and the oxide layer grows rapidly [5]. With the increasing thickness of the oxide layer, diffusion path along the nanopores towards the oxide/metal interface for oxygen-containing species extends significantly. In consequence, ionic current reflected by recorded current decreases with time. The evolution of the current density with anodizing time during successive anodizing steps at 40 V are shown in Fig. 3B. As can be seen, the shape of the current–time curves for a first anodizing step is a bit different from those recorded for the second and third anodizing step. The minimum and maximum of current den-
Fig. 4. Dependence of the steady-state current density, I (A) and ln I (B) on anodizing potential. The anodization was carried out in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C. The estimated regression equations and correlation coefficients (R) are: (A) y = 0.2201 exp(0.0511x), R2 = 0.9877 and (B) y = 0.0511x − 1.5137, R2 = 0.9877.
sity are not so evidently resolved for the first step curve as we can observed for transients recorded for the second and third steps. The maximum of current density is stretched to about 400 s while for the second and third steps appears usually below 100 s. The occurrence of a minimum current density is seen in each step but, the time at which this minimum occurs decreases with the increasing number of anodizing step. Consequently for successive anodizing steps, the steady-state conditions for the formation of pores are achieved within a shorter time period and recorded current densities are higher. This indicates that the nucleation of pores is easier on pre-textured titanium surface. Notice importantly that after removal of the oxide layer grown during anodizing step, the regular array of concaves is present on the titanium surface. This concaves act 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) of porous oxide layer grown after the third anodization carried out in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C. The duration of the third step was 10 min. The estimated regression equations and correlation coefficients (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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During the steady-state growth of TiO2 layer under a potentiostatic regime current density remains almost unchanged. Fig. 4A shows the average values of the current density recorded during steady-state growth of porous anodic titania in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at various anodizing potentials. As can be seen from Fig. 4A, an exponential curve can be fitted through the experimental data points. According to the high-field conduction theory, the current density I is related to the voltage drop across the barrier layer, as follows:
I = ˛ exp ˇ ·
U B
(1)
where ˛ and ˇ are electrolyte- and material-dependent constants at a given temperature, and U/B is the effective electric field across the barrier layer with thickness B. After logarithmic transformation, a linear dependence between ln I and U is obtained
ln I = ln ˛ +
ˇ ·U B
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nia varies between 20 and 100 in the range of anodizing potentials from 30 to 70 V. From Fig. 5A can be seen also that the growth rate of porous anodic titania depends linearly on the anodizing potential and can change from about 7.6 to 44.0 m h−1 . For the highest studied potential, the rate of oxide growth is close to a high-speed rate of oxide growth (even 50 m h−1 ) observed in a hard anodizing of aluminum [5]. The characteristic parameters of porous anodic titania including interpore distance (Dc ) pore diameter (Dp ), and wall thickness (W) are controllable by operating conditions of anodization. For the given anodizing potential, the average interpore distance of the ATO lattice was calculated from fast Fourier transforms of 9 different FESEM top-view images. For each FFT pattern, 3 main profiles along the FFT intensity were constructed. In this way, the average value of the main distance of the lattice in the inverse space was estimated on the basis of 27 measurements for each studied anodizing poten-
(2)
From a linear relationship presented in Fig. 4B, the calculated values of ˛ 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) and aspect ratio (Fig. 5B) of porous oxide layer grown during anodizing in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) was studied at 20 ◦ C. The duration of the third step was 10 min. The thickness of the oxide layer grown during anodizing at various cell potentials was estimated from low magnification FE-SEM images. The aspect ratio, defined as a ratio between the length of pores and their diameter was also calculated. The data presented in Fig. 5 show that the thickness and aspect ratio of porous TiO2 increase linearly with increasing anodizing potential. In contrast to these results, the current density increases exponentially (compare with Fig. 4A). Therefore, evidently increasing anodizing potential decreases gradually a Faradaic current efficiency in electrochemical oxidation of titanium. The estimated oxide thickness was about 1.3 and 7.3 m for the lowest and highest studied potentials, respectively. The calculated aspect ratio of anodic tita-
Fig. 6. Anodizing potential influence on the average interpore distance, pore diameter and wall thickness of porous anodic titania. The three-step anodizations were performed in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C. The estimated regression equations and correlation coefficients (R) are: (1) 2.1044x + 23.208, R2 = 0.9891, (2) y = 0.3373x + 47.906, R2 = 0.9699 and (3) y = 0.8835x − 12.349, R2 = 0.9905 for interpore distance, pore diameter and wall thickness, respectively.
Fig. 7. Anodizing potential influence on porosity (A) and pore density (B) of anodic titania formed by three-step anodizing performed in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C. The estimated regression equations and correlation coefficients (R) are: (A) y = 1545.8x − 1.0741, R2 = 0.993 and (B) y = 38.718 exp(−0.0331x), R2 = 0.9999.
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tial. The Dc values calculated from the FFT profiles are presented in Fig. 6. The experimental data points shows that the interpore distance is linearly dependent upon the anodization voltages and varies from about 90 to 175 nm in the range of anodizing potential from 30 to 70 V. For all anodizing potentials, the average pore diameter was calculated from 9 different FE-SEM top-view images with a ImageJ image analysis software [55]. In pore diameter calculations, pore shapes were assumed to be perfect circles. The average value of pore diameter calculated from the software are presented in Fig. 6. A linear relationship between pore diameter and anodizing potential is observed. As can be seen, the pore diameter increases from about 59 to 72 nm when anodizing potential increases from 30 to 70 V. In general, the wall thickness of porous anodic titania
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 various anodizing potentials. The calculated W values are linearly dependent 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 be estimated from the following expressions [5]: P=
√ · 2 3
D 2 p
Dc
(4)
Fig. 8. FE-SEM top-views together with their 2D fast Fourier transforms (FFTs) and 3D representations of FE-SEM images for porous titania formed at various anodizing potentials by the three-step anodization performed in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C. The duration of the third step was 10 min for all anodizing voltages and the analyzed surface area was 1.72 m2 .
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Fig. 9. FFT image of anodic porous titania and its profile analysis of the FFT intensity along the marked solid line.
1 · 1014 n= √ 3 · Dc2
(5)
The effects of anodizing potentials on the porosity of ATO films formed by a self-organized anodizing performed in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) are shown in Fig. 7A. As can be seen in Fig. 7A, the porosity of anodic titania decreases from about 39.3% to 15.6% with increasing anodizing potential from 30 to 70 V. Hence, the pore density is inversly proportional to the square value of the interpore distance the exponential decrease in pore density is expected with increasing anodizing potential. Our experimental results show indeed, that pore density decreases exponentially with increasing anodizing potential (Fig. 7B). For example, when anodizing potential increases from 30 to 70 V, the pore density of ATO film decreases from about 1.4 × 1010 to 3.8 × 109 pores cm−2 . Typical FE-SEM top-view images with their 2D fast Fourier transforms and 3D representations of FE-SEM micrographs of porous anodic titania formed at various anodizing potentials after a third anodizing step performed in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) are shown in Fig. 8. The duration of the third step was 10 min for all anodizing potentials. Additionally, a typical average intensity profile along the given 2D FFT image is also collected in Fig. 8 for all studied anodizing potentials. The fast Fourier transform of the periodic lattice provides information about periodicity of the structure in the inverse scale. For a triangular lattice with a hexagonal pore arrangement, a FFT pattern consists of six distinct spots on the edges of a hexagon. In case of the disturbed pore arrangement, the FFT pattern is characterized by a ring-shape or disc-shape form as we can see in Fig. 8. The narrow ring-shape form indicates a preserved short-distance periodicity of the lattice and slightly disturbed long-range order in the nanopore network. On the other hand, for the FFT pattern with six distinct spots (high pore arrangement) the average profile along the FFT width should show two very smooth and high peaks. Fig. 8 shows that anodizing potential influences considerably the nanopore arrangement. The best hexagonal arrangement of nanopores is observed for the anodizing potential of 40 and 60 V where relatively narrow ring shapes are observed. This fact is reflected also by the highest intensity profiles of FFT images for these both potentials. The qualitative analysis of pore arrangement regularity, using FFT images and intensities of FFT profiles is difficult to decide definitely which of anodizing potentials gives better results. In order to get a deeper insight into the regularity of pore arrangement, the quantitative analysis was performed. A regularity ratio (R), defined as a ratio of the maximum intensity of the 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) of anodic porous titania formed by three-step anodizing carried out in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C.
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Fig. 11. FE-SEM top-view images of anodic titania fabricated by three-step anodizing of titanium in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 60 V and 20 ◦ C. The duration of the third step was 30 min (A), 45 min (B), 60 min (C) and 180 min (D).
half-maximum was calculated from the average FFT profile for various anodizing potentials (Fig. 9). Notice importantly, FFT images were constructed from the FF-SEM micrographs taken at the same magnification for samples anodized at various potentials. With increasing anodizing potential increases significantly interpore distances and consequently decreases number of pores present on the same surface area (the same magnification FE-SEM images). As a result of that, different numbers of hexagonally arranged nanopores were taken for FFT calculations. In order to avoid this influence of pore number on regularity analysis, the average regularity ratio (R) calculated from the FFT profiles were multiplied by pore density (n). Fig. 10A shows the dependence between n·R and anodizing potentials. Below and above the anodizing potential of 40 V, a significant decrease in the regularity of pore order is observed. The uniformity of the porous structure can be investigated by the analysis of pore shapes and calculating a pore circularity [55]. The value of circularity equals to 1.0 indicates a perfect circle. As the value approaches 0.0, it indicates an increasingly elongated polygon. For the given anodizing potential, the average pore circularity was calculated from 9 different FE-SEM top-view images. As can be seen in Fig. 10B, a significant improvement in a circular shape of pores is observed at 40 V. This result is consistent with the analysis of pore arrangement regularity. The structure of anodic TiO2 depends considerably on the duration of the third anodizing step (Fig. 11). For the ethylene glycol-based electrolyte, the porous structure is formed only for short period of anodizing time not exceeding 45 min. However, further increase of the anodizing time to 3 h results in fibrous-like structure with nanotubes underneath (Fig. 11D). This is attributed to the chemical dissolution of the newly formed anodic oxide by F− ions present in the electrolyte according to the given equation: TiO2 + 6F− + 4H+ → TiF2− + 2H2 O 6
1. The three-step anodizing of titanium in an ethylene glycol solution containing NH4F (0.38 wt%) and H2 O (1.79 wt%) can result in well-arranged honeycomb structures. The duration of the third step significantly affects the morphology of the film. The porous structure of TiO2 can be obtained only for short durations. For longer periods of anodizing time TiO2 nanowires/nanotubes are formed on the Ti surface. 2. The oxide thickness formed after 10 min of the third step increases linearly with increasing anodizing potential. The current efficiency of anodic oxidation of Ti decreases gradually with increasing applied anodic potential. 3. The interpore distance, pore diameter and wall thickness of porous anodic titania are linearly dependent upon the anodization. 4. The porosity and pore density of ATO layers formed under the studied range of potentials decrease exponentially with increasing anodizing potential. 5. In the range of anodizing potentials from 30 to 70 V, the best hexagonal arrangement of pores and highest pore circularity is observed at 40 V.
Acknowledgments This research was partially supported by the European COST action D41 “Inorganic oxide surfaces and interfaces”, financed by the Polish Ministry of Science and High Education (DWM/N112/ COST/2008). The SEM imaging was performed in the Laboratory of Field Emission Scanning Electron Microscopy and Microanalysis at the Institute of Geological Sciences, Jagiellonian University, Poland.
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4. Conclusions The major findings of this study can be summarized as follows:
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Fabrication of nanoporous TiO2 by electrochemical anodization Grzegorz D. Sulka ∗ , Joanna Kapusta-Kołodziej, Agnieszka Brzózka, Marian Jaskuła Department of Physical Chemistry & Electrochemistry, Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30060 Krakow, Poland
a r t i c l e
i n f o
Article history: Received 24 July 2009 Received in revised form 11 December 2009 Accepted 12 December 2009 Available online 11 January 2010 Keywords: Anodization Porous titania Nanostructures Nanopores Self-organization
a b s t r a c t The formation of self-organized porous titania is achieved by electrochemical anodization under a potentiostatic regime. Anodic titanium oxide (ATO) was fabricated by a three-step self-organized anodization of the Ti foil in an ethylene glycol electrolyte containing 0.38 wt% of NH4 F and 1.79 wt% of H2 O. Anodizing was carried out at the constant cell potential ranging from 30 to 70 V at the temperature of 20 ◦ C. It was found 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 pore diameter, interpore distance, wall thickness, porosity and pore density was extensively studied. The linear dependencies between interpore distance, pore diameter and wall thickness upon the anodizing potential were found. The regularity of pore arrangement was monitored qualitatively by fast Fourier transforms (FFTs) of top-view FE-SEM images. It was found that the best arrangement of nanopores is observed at 40 V. This finding was confirmed by the analysis of pore circularity. The highest circularity of pores was observed once again at 40 V. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Today, nanotechnology encompasses a wide range of materials and techniques employed to the design and study of modern devices suitable for various possible commercial applications. The current international trend towards fabrication, characterization and investigation of nanosize materials has also embraced titanium dioxide [1–4]. This is particularly due to the fact that TiO2 possesses a range of unique functional physical and chemical properties such as: a high oxidative power, photostability, and nontoxicity [1]. However, the properties of TiO2 can be considerably improved when the oxide architecture is controlled on a nanoscale level and order arrays of nanopores, nanopillars and nanotubes are formed. Among various techniques used for the synthesis of nanostructured metal oxides, a self-organized anodization of valve metals is a simple, high-throughput and low-cost method of fabricating nanoporous or nanotubular structures [5]. Due to the enormous oxidation power and relatively good chemical stability, anodic titanium oxide (ATO) is a versatile material being used recently as an excellent catalyst for water photolysis and hydrogen generation [6–9], decomposition of unwanted organic compounds [6,10–13], electrocatalysis of methanol oxidation [14,15] and for an inactivation of Escherichia coli bacteria [16]. Moreover, anodic titanium oxide is a promising photovoltaic material 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: [email protected] (G.D. Sulka). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.12.053
[7,10,17–21] and in energy storage applications as a nanostructured anode material for lithium-ion batteries [22–24]. Other potential technological applications of porous anodic titanium oxide includes structural ceramics [25], humidity [26], gas [18,27,28] and biochemical sensors [29,30] as well as biocompatible materials for bone implants [31–33]. Similarly to porous anodic alumina, anodic titania has been recently utilized as a template 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 conducting 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 a potentiostatic regime in water-based or non-aqueous electrolytes containing fluoride ions [6,7,9–13,16,18,24,42]. Studies show that self-organized anodizing of titanium result usually in a vertically aligned, ordered TiO2 nanotube arrays over a macroscopic surface area of substrate [9,20]. By tailoring the operating conditions such as: type of electrolytes, electrolyte composition and pH, anodizing potential and temperature of the electrolyte, the diameter of nanotubes can vary in the range of 20–300 nm [6,18,42–45] while the length 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 number of reports on well-ordered arrays of nanopores synthesized using this electrochemical approach [35,47–49]. In order to obtain high-ordered arrangement of nanopores in the anodic titanium oxide layer, similarly to a self-organized anodiz-
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ing of aluminum [50–53], a multi-step anodizing procedure can be employed. For anodic titania, this approach is not very often represented in the literature due to a problem with an effective chemical removal of the oxide layer without disturbance in a titanium metal base. A multi-step anodizing procedure consists of one or two cycles of initial anodizing at the constant cell voltage and a subsequent chemical/mechanical removal of the grown oxide layer. After the removal of oxide layer, periodic concave triangular features formed on the metal surface serve as nucleation sites for the formation of nanopores during the final anodization. It is worth pointing out, that 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-organized formation of ordered arrays of nanopores by a three-step anodizing of titanium in the electrolyte based on ethylene glycol. In particular, the effect of anodizing potential on structural features of anodic
titania (pore diameter, interpore distance, porosity, pore density) and pore arrangement regularity is extensively discussed. Unlike the formation of ordered TiO2 nanotubes reported broadly in the literature, we present fabrication of TiO2 nanopore arrays and analysis of regularity of pore arrangement in anodic titania. As far as we know, very little attention has been paid to the analysis of arrangement of nanopores/nanotubes in ordered arrays of TiO2 synthesized by anodization. 2. Experimental A titanium foil (99.5% purity) 0.25 mm thick from Alfa Aesar was pre-cut in coupons (0.5 cm × 2.5 cm) with a selected working area of 0.5 cm2 . The samples were degreased in acetone than in ethanol. The back surface and the edges of the sample were insulated by an acid resistant paint layer.
Fig. 1. Idealized structure of porous anodic titanium oxide (ATO) (A) and a cross-sectional view of the anodized layer with the structural features of ATO (B).
Fig. 2. Top-, bottom-, and side-views of anodic TiO2 with an EDX spectrum of the ATO surface. A three-step anodization was performed in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 40 V and 20 ◦ C. The duration of the third step was 10 min and the analyzed surface area was 1.83 m2 .
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Prior to anodization, Ti samples were electrochemically polished in a mixed solution of acetic acid (99.5%), sulfuric acid (98%), and hydrofluoric acid (40%) (60:15:25 in volume) at constant current density of 70 mA cm−2 and 20 ◦ C for 1 min, followed by 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. The anodization was performed using a two-electrode cell with the titanium foil as the working electrode and a platinum foil as the counter electrode. The distance between Ti samples and the counter electrode was 3 cm. The anodic oxide layer was formed by a threestep procedure under a constant cell voltage ranging between 30 and 70 V in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C. The duration of the first and second anodizing steps was 3 h. After both anodizing steps, an adhesive tape was used for removal of the grown oxide layer. Immediately after the oxide removal, the titanium sample was re-anodized under conditions identical to those in previous anodization steps. Finally, the third anodization was usually performed for 10 min. The morphologies of anodic porous titania nanostructures and EDX analysis were performed with a field emission scanning electron microscope (FE-SEM/EDS, Hitachi S-4700 with a Noran System 7). The thickness of porous anodic titania was estimated from FE-SEM cross-sectional views of anodized specimens for various anodizing potentials. The scanning probe image processor WSxM 4.0 Develop 7.6 [54] was employed to calculate and analyze twodimensional fast Fourier transforms (FFTs) of FE-SEM images. The ImageJ 1.37v software [55] was used for estimation of circularity coefficients of ATO films.
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fluorine. The fluoride ions are incorporated into the porous structure of oxide as a direct result of the migration of electrolyte species (F− ) towards the Ti anode during the anodizing process. A typical content of incorporated fluoride ions was found to be between 3.4 and 5.6 wt%. The current–time transients recorded during the third anodizing step performed under cell voltage ranging between 30 and 70 V are shown in Fig. 3A. Fig. 3B shows the current–time curves recorded for successive anodizing steps performed at 40 V. The current density evolutions are typical for anodization of valve metals with formation of the porous oxide layer [5]. For anodizing potentials lower than 50 V, the current–time transient hits a minimum for a very early stage of the process duration (about 30 s) followed by a flattened maximum at about 90–120 s. These changes in the current density are results of the growing compact, high-resistant, passive layer on titanium and its subsequent transformation into the porous oxide layer. After reaching the maximum, the current density slightly decreases with time and a steady-state formation of porous oxide is observed. The presence of the maximum in
3. Results and discussion A porous anodic titanium oxide layer can grow by self-organized anodizing of titanium under well-controlled conditions. The ideal structure of the highly ordered TiO2 layer is represented by a closed-packed array of hexagonally arranged cells containing pores in each cell center (Fig. 1). The morphology of anodic titania can be described by characteristic parameters of porous layer including: interpore distance (Dc ), pore diameter (Dp ), wall thickness (W), porosity and pore density (n). Similarly to anodization of aluminum, the structural features of porous titania can be easily controllably by altering the anodizing conditions. During the porous oxide growth, a thin and compact barrier layer (B) at the pore bottom/electrolyte interface is continuously dissolved by locally increased field, and a new barrier layer at the metal/oxide interface is rebuilt. For a steady-state film growth, there is a dynamic equilibrium between the rate of film growth and its field-assisted dissolution. As a result of the steady-state growth of oxide, the cylindrical in section pores are formed (Fig. 1). The thickness of the oxide layer (depth of pores) depends on the anodizing potential, process duration and type of anodizing electrolyte (possible chemical etching of formed oxide). Fig. 2 shows typical FE-SEM top-, bottom- and side-views images of anodic titania layer obtained by the three-step anodization performed in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 40 V and 20 ◦ C. The duration of the third step was 10 min. As can be seen from FE-SEM images, anodizing of titanium in an ethylene glycol-based electrolyte can give a periodic lattice of nanopores. The dimensions of pores and distances between them are uniform but the pore arrangement is far from the ideal hexagonal arrangement. The triangular lattice of cells is often disturbed and there are only small domains with hexagonally arranged pores over the analyzed surface area of 1.83 m2 . In cross-section, the oxide coating is a relatively thick as for 10 min of anodization. The EDX spectra (Fig. 2) reveal that the close-packed cells of titania contain a little amount of embedded
Fig. 3. Current density vs. anodizing time recorded for the third anodizing step performed at various anodizing potentials (A) and for different anodizing steps at 40 V (B). The anodization was carried out in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C.
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the current–time curve is usually ascribed to the pore rearrangement process occurring on the surface and resulting in a network close-packed pores [5]. For anodizing potentials higher than 40 V, the current–time transients show a decrease as a function of time (Fig. 3A). Such a behavior is typical for the hard-anodization experiments where the flowing current is a relatively high and the oxide layer grows rapidly [5]. With the increasing thickness of the oxide layer, diffusion path along the nanopores towards the oxide/metal interface for oxygen-containing species extends significantly. In consequence, ionic current reflected by recorded current decreases with time. The evolution of the current density with anodizing time during successive anodizing steps at 40 V are shown in Fig. 3B. As can be seen, the shape of the current–time curves for a first anodizing step is a bit different from those recorded for the second and third anodizing step. The minimum and maximum of current den-
Fig. 4. Dependence of the steady-state current density, I (A) and ln I (B) on anodizing potential. The anodization was carried out in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C. The estimated regression equations and correlation coefficients (R) are: (A) y = 0.2201 exp(0.0511x), R2 = 0.9877 and (B) y = 0.0511x − 1.5137, R2 = 0.9877.
sity are not so evidently resolved for the first step curve as we can observed for transients recorded for the second and third steps. The maximum of current density is stretched to about 400 s while for the second and third steps appears usually below 100 s. The occurrence of a minimum current density is seen in each step but, the time at which this minimum occurs decreases with the increasing number of anodizing step. Consequently for successive anodizing steps, the steady-state conditions for the formation of pores are achieved within a shorter time period and recorded current densities are higher. This indicates that the nucleation of pores is easier on pre-textured titanium surface. Notice importantly that after removal of the oxide layer grown during anodizing step, the regular array of concaves is present on the titanium surface. This concaves act 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) of porous oxide layer grown after the third anodization carried out in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C. The duration of the third step was 10 min. The estimated regression equations and correlation coefficients (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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During the steady-state growth of TiO2 layer under a potentiostatic regime current density remains almost unchanged. Fig. 4A shows the average values of the current density recorded during steady-state growth of porous anodic titania in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at various anodizing potentials. As can be seen from Fig. 4A, an exponential curve can be fitted through the experimental data points. According to the high-field conduction theory, the current density I is related to the voltage drop across the barrier layer, as follows:
I = ˛ exp ˇ ·
U B
(1)
where ˛ and ˇ are electrolyte- and material-dependent constants at a given temperature, and U/B is the effective electric field across the barrier layer with thickness B. After logarithmic transformation, a linear dependence between ln I and U is obtained
ln I = ln ˛ +
ˇ ·U B
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nia varies between 20 and 100 in the range of anodizing potentials from 30 to 70 V. From Fig. 5A can be seen also that the growth rate of porous anodic titania depends linearly on the anodizing potential and can change from about 7.6 to 44.0 m h−1 . For the highest studied potential, the rate of oxide growth is close to a high-speed rate of oxide growth (even 50 m h−1 ) observed in a hard anodizing of aluminum [5]. The characteristic parameters of porous anodic titania including interpore distance (Dc ) pore diameter (Dp ), and wall thickness (W) are controllable by operating conditions of anodization. For the given anodizing potential, the average interpore distance of the ATO lattice was calculated from fast Fourier transforms of 9 different FESEM top-view images. For each FFT pattern, 3 main profiles along the FFT intensity were constructed. In this way, the average value of the main distance of the lattice in the inverse space was estimated on the basis of 27 measurements for each studied anodizing poten-
(2)
From a linear relationship presented in Fig. 4B, the calculated values of ˛ 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) and aspect ratio (Fig. 5B) of porous oxide layer grown during anodizing in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) was studied at 20 ◦ C. The duration of the third step was 10 min. The thickness of the oxide layer grown during anodizing at various cell potentials was estimated from low magnification FE-SEM images. The aspect ratio, defined as a ratio between the length of pores and their diameter was also calculated. The data presented in Fig. 5 show that the thickness and aspect ratio of porous TiO2 increase linearly with increasing anodizing potential. In contrast to these results, the current density increases exponentially (compare with Fig. 4A). Therefore, evidently increasing anodizing potential decreases gradually a Faradaic current efficiency in electrochemical oxidation of titanium. The estimated oxide thickness was about 1.3 and 7.3 m for the lowest and highest studied potentials, respectively. The calculated aspect ratio of anodic tita-
Fig. 6. Anodizing potential influence on the average interpore distance, pore diameter and wall thickness of porous anodic titania. The three-step anodizations were performed in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C. The estimated regression equations and correlation coefficients (R) are: (1) 2.1044x + 23.208, R2 = 0.9891, (2) y = 0.3373x + 47.906, R2 = 0.9699 and (3) y = 0.8835x − 12.349, R2 = 0.9905 for interpore distance, pore diameter and wall thickness, respectively.
Fig. 7. Anodizing potential influence on porosity (A) and pore density (B) of anodic titania formed by three-step anodizing performed in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C. The estimated regression equations and correlation coefficients (R) are: (A) y = 1545.8x − 1.0741, R2 = 0.993 and (B) y = 38.718 exp(−0.0331x), R2 = 0.9999.
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tial. The Dc values calculated from the FFT profiles are presented in Fig. 6. The experimental data points shows that the interpore distance is linearly dependent upon the anodization voltages and varies from about 90 to 175 nm in the range of anodizing potential from 30 to 70 V. For all anodizing potentials, the average pore diameter was calculated from 9 different FE-SEM top-view images with a ImageJ image analysis software [55]. In pore diameter calculations, pore shapes were assumed to be perfect circles. The average value of pore diameter calculated from the software are presented in Fig. 6. A linear relationship between pore diameter and anodizing potential is observed. As can be seen, the pore diameter increases from about 59 to 72 nm when anodizing potential increases from 30 to 70 V. In general, the wall thickness of porous anodic titania
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 various anodizing potentials. The calculated W values are linearly dependent 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 be estimated from the following expressions [5]: P=
√ · 2 3
D 2 p
Dc
(4)
Fig. 8. FE-SEM top-views together with their 2D fast Fourier transforms (FFTs) and 3D representations of FE-SEM images for porous titania formed at various anodizing potentials by the three-step anodization performed in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C. The duration of the third step was 10 min for all anodizing voltages and the analyzed surface area was 1.72 m2 .
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Fig. 9. FFT image of anodic porous titania and its profile analysis of the FFT intensity along the marked solid line.
1 · 1014 n= √ 3 · Dc2
(5)
The effects of anodizing potentials on the porosity of ATO films formed by a self-organized anodizing performed in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) are shown in Fig. 7A. As can be seen in Fig. 7A, the porosity of anodic titania decreases from about 39.3% to 15.6% with increasing anodizing potential from 30 to 70 V. Hence, the pore density is inversly proportional to the square value of the interpore distance the exponential decrease in pore density is expected with increasing anodizing potential. Our experimental results show indeed, that pore density decreases exponentially with increasing anodizing potential (Fig. 7B). For example, when anodizing potential increases from 30 to 70 V, the pore density of ATO film decreases from about 1.4 × 1010 to 3.8 × 109 pores cm−2 . Typical FE-SEM top-view images with their 2D fast Fourier transforms and 3D representations of FE-SEM micrographs of porous anodic titania formed at various anodizing potentials after a third anodizing step performed in an ethylene glycol solution containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) are shown in Fig. 8. The duration of the third step was 10 min for all anodizing potentials. Additionally, a typical average intensity profile along the given 2D FFT image is also collected in Fig. 8 for all studied anodizing potentials. The fast Fourier transform of the periodic lattice provides information about periodicity of the structure in the inverse scale. For a triangular lattice with a hexagonal pore arrangement, a FFT pattern consists of six distinct spots on the edges of a hexagon. In case of the disturbed pore arrangement, the FFT pattern is characterized by a ring-shape or disc-shape form as we can see in Fig. 8. The narrow ring-shape form indicates a preserved short-distance periodicity of the lattice and slightly disturbed long-range order in the nanopore network. On the other hand, for the FFT pattern with six distinct spots (high pore arrangement) the average profile along the FFT width should show two very smooth and high peaks. Fig. 8 shows that anodizing potential influences considerably the nanopore arrangement. The best hexagonal arrangement of nanopores is observed for the anodizing potential of 40 and 60 V where relatively narrow ring shapes are observed. This fact is reflected also by the highest intensity profiles of FFT images for these both potentials. The qualitative analysis of pore arrangement regularity, using FFT images and intensities of FFT profiles is difficult to decide definitely which of anodizing potentials gives better results. In order to get a deeper insight into the regularity of pore arrangement, the quantitative analysis was performed. A regularity ratio (R), defined as a ratio of the maximum intensity of the 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) of anodic porous titania formed by three-step anodizing carried out in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 20 ◦ C.
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Fig. 11. FE-SEM top-view images of anodic titania fabricated by three-step anodizing of titanium in ethylene glycol containing NH4 F (0.38 wt%) and H2 O (1.79 wt%) at 60 V and 20 ◦ C. The duration of the third step was 30 min (A), 45 min (B), 60 min (C) and 180 min (D).
half-maximum was calculated from the average FFT profile for various anodizing potentials (Fig. 9). Notice importantly, FFT images were constructed from the FF-SEM micrographs taken at the same magnification for samples anodized at various potentials. With increasing anodizing potential increases significantly interpore distances and consequently decreases number of pores present on the same surface area (the same magnification FE-SEM images). As a result of that, different numbers of hexagonally arranged nanopores were taken for FFT calculations. In order to avoid this influence of pore number on regularity analysis, the average regularity ratio (R) calculated from the FFT profiles were multiplied by pore density (n). Fig. 10A shows the dependence between n·R and anodizing potentials. Below and above the anodizing potential of 40 V, a significant decrease in the regularity of pore order is observed. The uniformity of the porous structure can be investigated by the analysis of pore shapes and calculating a pore circularity [55]. The value of circularity equals to 1.0 indicates a perfect circle. As the value approaches 0.0, it indicates an increasingly elongated polygon. For the given anodizing potential, the average pore circularity was calculated from 9 different FE-SEM top-view images. As can be seen in Fig. 10B, a significant improvement in a circular shape of pores is observed at 40 V. This result is consistent with the analysis of pore arrangement regularity. The structure of anodic TiO2 depends considerably on the duration of the third anodizing step (Fig. 11). For the ethylene glycol-based electrolyte, the porous structure is formed only for short period of anodizing time not exceeding 45 min. However, further increase of the anodizing time to 3 h results in fibrous-like structure with nanotubes underneath (Fig. 11D). This is attributed to the chemical dissolution of the newly formed anodic oxide by F− ions present in the electrolyte according to the given equation: TiO2 + 6F− + 4H+ → TiF2− + 2H2 O 6
1. The three-step anodizing of titanium in an ethylene glycol solution containing NH4F (0.38 wt%) and H2 O (1.79 wt%) can result in well-arranged honeycomb structures. The duration of the third step significantly affects the morphology of the film. The porous structure of TiO2 can be obtained only for short durations. For longer periods of anodizing time TiO2 nanowires/nanotubes are formed on the Ti surface. 2. The oxide thickness formed after 10 min of the third step increases linearly with increasing anodizing potential. The current efficiency of anodic oxidation of Ti decreases gradually with increasing applied anodic potential. 3. The interpore distance, pore diameter and wall thickness of porous anodic titania are linearly dependent upon the anodization. 4. The porosity and pore density of ATO layers formed under the studied range of potentials decrease exponentially with increasing anodizing potential. 5. In the range of anodizing potentials from 30 to 70 V, the best hexagonal arrangement of pores and highest pore circularity is observed at 40 V.
Acknowledgments This research was partially supported by the European COST action D41 “Inorganic oxide surfaces and interfaces”, financed by the Polish Ministry of Science and High Education (DWM/N112/ COST/2008). The SEM imaging was performed in the Laboratory of Field Emission Scanning Electron Microscopy and Microanalysis at the Institute of Geological Sciences, Jagiellonian University, Poland.
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4. Conclusions The major findings of this study can be summarized as follows:
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