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Fabrication of high aspect ratio zirconia nanotube arrays by anodization of zirconium foils
Materials Letters 62 (2008) 4428–4430
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Fabrication of high aspect ratio zirconia nanotube arrays by anodization of zirconium foils Jianling Zhao a,⁎, Xixin Wang b, Rongqing Xu a, Fanbin Meng a, Limin Guo a, Yangxian Li a a b
School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, PR China School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, PR China
a r t i c l e
i n f o
Article history: Received 20 February 2008 Accepted 23 July 2008 Available online 3 August 2008 Keywords: Anodization Crystallization Zirconia nanotube
a b s t r a c t The zirconia nanotube arrays with diameter of about 130 nm, a length of up to 190 μm and aspect ratios of more than 1400 were prepared by anodizing a zirconium foil in mixture of formamide and glycerol (volume ratio = 1:1) containing 1 wt.% NH4F and 3 wt.% H2O. The as-prepared nanotube arrays were amorphous zirconia. Monoclinic and tetragonal zirconia coexisted when annealed at 400 °C and 600 °C, while monoclinic zirconia was obtained at 800 °C. The ZrO2 nanotubes retained their shape after heating up to 800 °C. The lower dissolving rate of zirconia in organic electrolytes might be the main reason for fabrication of zirconia nanotube arrays with high aspect ratio. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Zirconia (ZrO2) nanotubes have many potential applications in the fields of optical, electronic, magnetic and heterogeneous catalysis, etc [1–4]. Nanotubes of zirconia have been fabricated by hydrothermal treatment, template-assistant deposition and anodization methods [5–8]. Anodic oxidation has been extensively studied for the preparation of different microstructures of valve metals [9–13]. Nanoporous structures of zirconia have been fabricated by anodization in inorganic electrolytes containing H2SO4 and small amount of HF, NaF or NH4F [14–16]. Recently Tsuchiya et al. successfully prepared zirconia nanotubes with a length up to 17 μm by electrochemical oxidation in F−-containing inorganic electrolytes [17]. Fabrication of zirconia nanotubes by anodization method in aqueous solutions has been studied in detail [14–20]. However, to the best of our knowledge, preparation of zirconia nanotubes by anodization in formamide and glycerol electrolytes has not been reported. In this article, we will discuss the fabrication of zirconia nanotubes in organic electrolytes containing formamide, glycerol and NH4F via constant-voltage experiments. Zirconia nanotube arrays with a length of up to 190 μm and an aspect ratio of more than 1400 were prepared. The fabrication of high aspect ratio zirconia nanotube arrays on the Zr foils might broaden the application scope of zirconia, improve its properties and find new uses in many fields. For example, it may be used as templates to prepare nanowires with high aspect ratio, which
⁎ Corresponding author. Tel.: +86 22 60204694; fax: +86 22 60204129. E-mail address: [email protected] (J. Zhao). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.07.054
provides us an alternative to the traditional anodic aluminum oxide porous templates. 2. Experimental details The zirconium foils (99.7% purity, 10 × 10 × 0.5 mm3) used in this study were obtained from the General Research Institute for Nonferrous Metals (Beijing, China). They were polished mechanically and washed in twice-distilled water and acetone by ultrasonic washing before use. Electrochemical experiments were carried out using a program-controlled DC source (Dahua Coop., Beijing, China). Zirconium foils were used as anodic electrode while platinum (20 × 20 × 0.1 mm3) was used as cathodic electrode. The distance between anodic and cathodic electrodes was 20 mm. Electrolytes in this process were mixture of formamide and glycerol (volume ratio = 1:1) containing 1 wt.% NH4F and 3 wt.% H2O. All anodization experiments were carried out at room temperature. During the experiments, the solutions were stirred using a magnetic stirrer. After the anodization, the samples were rinsed in deionized water, air dried and characterized. Electrochemical measurements were conducted using LK2005 electrochemical analyzer (Lanlika Instruments Inc., Tianjin, China). X-ray diffraction measurements were performed on D/maxRB diffractometer (Rigaku, Rotafles) using Cu Kα radiation (0.15416 nm). The microstructures were observed on field emission scanning electron microscopes (JSM-6301, JEOL Inc., Japan). The cross-section photographs were obtained by observing mechanically fractured samples. The transmission electron microscopy structures were obtained in TEM-200CX electron microscope (JEOL, Japan). The as-prepared samples and the samples annealed at different
J. Zhao et al. / Materials Letters 62 (2008) 4428–4430
4429
Fig. 1. SEM images of the as-prepared samples after anodization of Zr at 50 V for 24 h. a: surface structure (inset is TEM image of the top part) b: surface structure after being rinsed ultrasonically c: cross-section d: cross-section of higher magnification (inset is TEM image of zirconia nanotube).
temperatures were characterized by Fourier transform infrared spectroscopy (FTIR, WQF-410, China). 3. Results and discussion Fig. 1 shows the SEM images of the as-prepared samples after anodization in mixture of formamide and glycerol (volume ratio = 1:1) containing 1 wt.% NH4F and 3 wt.% H2O at 50 V for 24 h. Fig. 1a gives the surface structure of the nanotube arrays. The inset is TEM image of the top part. It is clear that the surface was not smooth and partially covered by loose solid matter. Ultrasonic rinse after anodization can dismantle most of the solids covering the mouth of nanotubes and reveal the nanotube structure (Fig. 1b). Cross-sectional microscopy indicates that nanotube arrays fabricated at this condition had a length of 190 μm (Fig. 1c). The highest magnification disclosed in Fig. 1d
Fig. 2. XRD patterns of zirconia nanotubes annealed at different temperatures in air.
verifies the nanotubes were well aligned. The inset TEM image of Fig. 1d shows that outer diameter of the zirconia nanotube was about 130 nm while the inner diameter was 66 nm and wall thickness was 32 nm. Energy dispersive spectrum confirmed that the nanotube arrays were zirconium oxide. The properties and potential applications of zirconia depend on the crystallinity and the microstructures. In order to investigate the crystallization transformation of the nanotube arrays, the as-prepared samples were annealed in air for 3 h. The XRD patterns of the as-prepared sample and the annealed samples are shown in Fig. 2. The as-prepared sample was amorphous. At 400 °C most of the diffraction peaks could be indexed as monoclinic phase (pdf Card No.37-1484). Main peak of tetragonal zirconia at 2θ = 30.12° also appeared. At 600 °C, the peaks' strength for both of the monoclinic and tetragonal phase (pdf Card No.50-1089) increased, which might be
Fig. 3. FTIR spectra of the as-prepared sample and annealed samples.
4430
J. Zhao et al. / Materials Letters 62 (2008) 4428–4430 decrease of current densities. When the nanotubes was rather long, on the one hand the growth rate of nanotubes decreased due to higher mass transfer resistance, on the other hand dissolving rate of zirconia increased owing to the larger surface area of nanotubes layer. Consequently, the current density became relatively stable. Solubility of zirconia in the mixture of formamide and glycerol organic electrolytes containing F− is lower than that in F−-containing aqueous solutions. The zirconia formed during the anodization in organic electrolytes dissolved slowly and the growth rate of zirconia nanotube arrays increased relatively, which resulted in the fabrication of zirconia nanotube arrays with high aspect ratio.
4. Conclusions
Fig. 4. I–t curve recorded during the anodization process.
attributed to the more complete crystallization. At 800 °C, the diffraction peaks of tetragonal zirconia disappeared, the zirconia nanotube arrays were of monoclinic phase (pdf Card No.37-1484). The morphology of the annealed zirconia nanotubes did not change with calcination. Fourier transform infrared (FTIR) absorption spectra of the as-prepared sample and the annealed samples are shown in Fig. 3. The absorption peaks between 400 cm− 1 and 800 cm− 1 correspond to vibration of Zr–O bond in the zirconia. The peaks between 900 cm− 1 and 1500 cm− 1 correspond to the absorption of hydroxyl. Along with the increase of temperature, the amount of hydroxyl decreased and crystallization of zirconia improved gradually, which is consistent with the XRD results. Fig. 4 shows the current transients (I–t curve) recorded during the anodization process. At the first stage of oxidation current densities decreased drastically and then decreased gradually, and remained relatively stable. The initial drastic current drop was due to the formation of oxide film at the beginning stage which elevated the resistance and reduced current densities. The formation of zirconium oxide can be depicted as follows [21–23]: ZrðmÞYZr4þ ðaqÞ þ 4e−
ð1Þ
Zr4þ ðaqÞ þ 4H2 OYZrðOHÞ4 ðaqÞ þ 4Hþ
ð2Þ
−2H2 O
ZrðOHÞ4 Y ZrO2
ð3Þ
The thickness of the oxide film was different due to microheterogeneity of the metal surface. Thicker oxide layer formed at the convex part, while thinner oxide layer formed at the concave part. Higher electric strength and reaction rate existed at the concave part owing to the lower mass transfer resistance, resulting in the deep-etch and formation of the original nanopores. Volume changes accompanying the oxidization of zirconium led to the internal stress in oxide between the nanopores. Microcracks emerged at the position where the internal stress concentrated and was high enough. Dissolving of the oxide at the fracture surface in F−-containing electrolytes Eq. (4) induced the formation of nanotubes. ZrO2 þ 6F− þ 4Hþ Y½ZrF6 2− þ 2H2 O
ð4Þ
Along with the extension of reaction, the nanotubes' length grew swiftly, at the same time, zirconia dissolved in F−-containing electrolytes slowly. Higher formation rate of zirconia nanotube layer and lower dissolving rate of zirconia resulted in the slow
Zirconia nanotube arrays were prepared using direct anodization of zirconium in a mixture of formamide and glycerol (volume ratio = 1:1) containing 1 wt.% NH4F and 3 wt.% H2O. The diameter of the nanotubes was about 130 nm with a length of up to 190 μm and an aspect ratio of more than 1400 when anodization took place at 50 V for 24 h. X-ray diffraction showed that the as-prepared nanotubes were amorphous zirconia. Monoclinic and tetragonal zirconia coexisted when annealed at 400 °C and 600 °C, while monoclinic zirconia was obtained at 800 °C. The morphology of the annealed zirconia nanotubes did not change with calcination. Fabrication of zirconia nanotube arrays with higher aspect ratio than that in inorganic electrolytes might be due to the lower dissolving rate of zirconia in organic electrolytes. Acknowledgements This work is supported by the Key Project of Chinese Ministry of Education (No.208013), Tianjin Natural Science Foundation (07JCYBJC03300) and Natural Science Foundation of Hebei Province of China (E2007000044). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Liu X, Lu GQ, Yan Z. Appl Catal A-Gen 2005;279:241–5. Yamamoto N, Sato S, Takahashi R, Inui K. J Mol Catal A-Chem 2005;243:52–9. Park HJ, Choi GM. Solid State Ionics 2006;177:2261–7. Liu X, Huang A, Ding C, Chu PK. Biomaterials 2006;27:3904–11. Shin H, Jeong D, Lee J, Sung MM, Kim J. Adv Mater 2004;16:1197–200. Bao J, Tie C, Xu Z, Ma Q, Hong J, Sang H, et al. Adv Mater 2002;14:44–7. Rao CNR, Satishkumar B, Govindaraj A. Chem Commu 1997;16:1581–2. Yin Y, Lu Y, Chen S. Mod Chem Indus 2005;5:38–9. Zhao JL, Wang XH, Sun TY, Li LT. Nanotechnology 2005;16:2450–4. Sieber I, Kannan B, Schmuki P. Electrochem Solid ST 2005;8:J10–2. Choi J, Lim JH, Lee SC, Chang JH, Kim KJ, Cho MA. Electrochim Acta 2006;51:5502–7. Zhao JL, Wang XH, Chen RZ, Li LT. Solid State Commun 2005;134:705–10. Mor GK, Varghese OK, Paulose M, Shankar K, Grimes CA. Sol Energ Mat Sol C 2006;90:2011–75. Tsuchiya H, Macak JM, Sieber I, Schmuki P. Small 2005;1:722–5. Tsuchiya H, Schmuki P. Electrochem Commun 2004;6:1131–4. Lee WJ, Smyrl WH. Electrochem Solid ST 2005;8:B7–9. Tsuchiya H, Macak JM, Taveira L, Schmuki P. Chem Phys Lett 2005;410:188–91. Tsuchiya H, Macak JM, Sieber I, Schmuki P. Mater Sci Forum 2006;512:205–10. Lee WJ, Smyrl WH. Curr Appl Phys 2005, doi:10.1016/j.cap.2007.04.036. Tsuchiya H, Macak JM, Ghicov A, Taveira L, Schmuki P. Corros Sci 2005;47:3324–35. Zhang L, Macdonald DD, Sikora E, Sikorat J. J Electrochom Soc 1998;145:898–905. Tsukada T, Venigalla S, Adair JH, Am J. Ceram Soc 1997;80:3187–92. Zhao JL, Xu RQ, Wang XX, Li YX. Corros Sci 2008;50:1593–7.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Fabrication of high aspect ratio zirconia nanotube arrays by anodization of zirconium foils Jianling Zhao a,⁎, Xixin Wang b, Rongqing Xu a, Fanbin Meng a, Limin Guo a, Yangxian Li a a b
School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, PR China School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, PR China
a r t i c l e
i n f o
Article history: Received 20 February 2008 Accepted 23 July 2008 Available online 3 August 2008 Keywords: Anodization Crystallization Zirconia nanotube
a b s t r a c t The zirconia nanotube arrays with diameter of about 130 nm, a length of up to 190 μm and aspect ratios of more than 1400 were prepared by anodizing a zirconium foil in mixture of formamide and glycerol (volume ratio = 1:1) containing 1 wt.% NH4F and 3 wt.% H2O. The as-prepared nanotube arrays were amorphous zirconia. Monoclinic and tetragonal zirconia coexisted when annealed at 400 °C and 600 °C, while monoclinic zirconia was obtained at 800 °C. The ZrO2 nanotubes retained their shape after heating up to 800 °C. The lower dissolving rate of zirconia in organic electrolytes might be the main reason for fabrication of zirconia nanotube arrays with high aspect ratio. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Zirconia (ZrO2) nanotubes have many potential applications in the fields of optical, electronic, magnetic and heterogeneous catalysis, etc [1–4]. Nanotubes of zirconia have been fabricated by hydrothermal treatment, template-assistant deposition and anodization methods [5–8]. Anodic oxidation has been extensively studied for the preparation of different microstructures of valve metals [9–13]. Nanoporous structures of zirconia have been fabricated by anodization in inorganic electrolytes containing H2SO4 and small amount of HF, NaF or NH4F [14–16]. Recently Tsuchiya et al. successfully prepared zirconia nanotubes with a length up to 17 μm by electrochemical oxidation in F−-containing inorganic electrolytes [17]. Fabrication of zirconia nanotubes by anodization method in aqueous solutions has been studied in detail [14–20]. However, to the best of our knowledge, preparation of zirconia nanotubes by anodization in formamide and glycerol electrolytes has not been reported. In this article, we will discuss the fabrication of zirconia nanotubes in organic electrolytes containing formamide, glycerol and NH4F via constant-voltage experiments. Zirconia nanotube arrays with a length of up to 190 μm and an aspect ratio of more than 1400 were prepared. The fabrication of high aspect ratio zirconia nanotube arrays on the Zr foils might broaden the application scope of zirconia, improve its properties and find new uses in many fields. For example, it may be used as templates to prepare nanowires with high aspect ratio, which
⁎ Corresponding author. Tel.: +86 22 60204694; fax: +86 22 60204129. E-mail address: [email protected] (J. Zhao). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.07.054
provides us an alternative to the traditional anodic aluminum oxide porous templates. 2. Experimental details The zirconium foils (99.7% purity, 10 × 10 × 0.5 mm3) used in this study were obtained from the General Research Institute for Nonferrous Metals (Beijing, China). They were polished mechanically and washed in twice-distilled water and acetone by ultrasonic washing before use. Electrochemical experiments were carried out using a program-controlled DC source (Dahua Coop., Beijing, China). Zirconium foils were used as anodic electrode while platinum (20 × 20 × 0.1 mm3) was used as cathodic electrode. The distance between anodic and cathodic electrodes was 20 mm. Electrolytes in this process were mixture of formamide and glycerol (volume ratio = 1:1) containing 1 wt.% NH4F and 3 wt.% H2O. All anodization experiments were carried out at room temperature. During the experiments, the solutions were stirred using a magnetic stirrer. After the anodization, the samples were rinsed in deionized water, air dried and characterized. Electrochemical measurements were conducted using LK2005 electrochemical analyzer (Lanlika Instruments Inc., Tianjin, China). X-ray diffraction measurements were performed on D/maxRB diffractometer (Rigaku, Rotafles) using Cu Kα radiation (0.15416 nm). The microstructures were observed on field emission scanning electron microscopes (JSM-6301, JEOL Inc., Japan). The cross-section photographs were obtained by observing mechanically fractured samples. The transmission electron microscopy structures were obtained in TEM-200CX electron microscope (JEOL, Japan). The as-prepared samples and the samples annealed at different
J. Zhao et al. / Materials Letters 62 (2008) 4428–4430
4429
Fig. 1. SEM images of the as-prepared samples after anodization of Zr at 50 V for 24 h. a: surface structure (inset is TEM image of the top part) b: surface structure after being rinsed ultrasonically c: cross-section d: cross-section of higher magnification (inset is TEM image of zirconia nanotube).
temperatures were characterized by Fourier transform infrared spectroscopy (FTIR, WQF-410, China). 3. Results and discussion Fig. 1 shows the SEM images of the as-prepared samples after anodization in mixture of formamide and glycerol (volume ratio = 1:1) containing 1 wt.% NH4F and 3 wt.% H2O at 50 V for 24 h. Fig. 1a gives the surface structure of the nanotube arrays. The inset is TEM image of the top part. It is clear that the surface was not smooth and partially covered by loose solid matter. Ultrasonic rinse after anodization can dismantle most of the solids covering the mouth of nanotubes and reveal the nanotube structure (Fig. 1b). Cross-sectional microscopy indicates that nanotube arrays fabricated at this condition had a length of 190 μm (Fig. 1c). The highest magnification disclosed in Fig. 1d
Fig. 2. XRD patterns of zirconia nanotubes annealed at different temperatures in air.
verifies the nanotubes were well aligned. The inset TEM image of Fig. 1d shows that outer diameter of the zirconia nanotube was about 130 nm while the inner diameter was 66 nm and wall thickness was 32 nm. Energy dispersive spectrum confirmed that the nanotube arrays were zirconium oxide. The properties and potential applications of zirconia depend on the crystallinity and the microstructures. In order to investigate the crystallization transformation of the nanotube arrays, the as-prepared samples were annealed in air for 3 h. The XRD patterns of the as-prepared sample and the annealed samples are shown in Fig. 2. The as-prepared sample was amorphous. At 400 °C most of the diffraction peaks could be indexed as monoclinic phase (pdf Card No.37-1484). Main peak of tetragonal zirconia at 2θ = 30.12° also appeared. At 600 °C, the peaks' strength for both of the monoclinic and tetragonal phase (pdf Card No.50-1089) increased, which might be
Fig. 3. FTIR spectra of the as-prepared sample and annealed samples.
4430
J. Zhao et al. / Materials Letters 62 (2008) 4428–4430 decrease of current densities. When the nanotubes was rather long, on the one hand the growth rate of nanotubes decreased due to higher mass transfer resistance, on the other hand dissolving rate of zirconia increased owing to the larger surface area of nanotubes layer. Consequently, the current density became relatively stable. Solubility of zirconia in the mixture of formamide and glycerol organic electrolytes containing F− is lower than that in F−-containing aqueous solutions. The zirconia formed during the anodization in organic electrolytes dissolved slowly and the growth rate of zirconia nanotube arrays increased relatively, which resulted in the fabrication of zirconia nanotube arrays with high aspect ratio.
4. Conclusions
Fig. 4. I–t curve recorded during the anodization process.
attributed to the more complete crystallization. At 800 °C, the diffraction peaks of tetragonal zirconia disappeared, the zirconia nanotube arrays were of monoclinic phase (pdf Card No.37-1484). The morphology of the annealed zirconia nanotubes did not change with calcination. Fourier transform infrared (FTIR) absorption spectra of the as-prepared sample and the annealed samples are shown in Fig. 3. The absorption peaks between 400 cm− 1 and 800 cm− 1 correspond to vibration of Zr–O bond in the zirconia. The peaks between 900 cm− 1 and 1500 cm− 1 correspond to the absorption of hydroxyl. Along with the increase of temperature, the amount of hydroxyl decreased and crystallization of zirconia improved gradually, which is consistent with the XRD results. Fig. 4 shows the current transients (I–t curve) recorded during the anodization process. At the first stage of oxidation current densities decreased drastically and then decreased gradually, and remained relatively stable. The initial drastic current drop was due to the formation of oxide film at the beginning stage which elevated the resistance and reduced current densities. The formation of zirconium oxide can be depicted as follows [21–23]: ZrðmÞYZr4þ ðaqÞ þ 4e−
ð1Þ
Zr4þ ðaqÞ þ 4H2 OYZrðOHÞ4 ðaqÞ þ 4Hþ
ð2Þ
−2H2 O
ZrðOHÞ4 Y ZrO2
ð3Þ
The thickness of the oxide film was different due to microheterogeneity of the metal surface. Thicker oxide layer formed at the convex part, while thinner oxide layer formed at the concave part. Higher electric strength and reaction rate existed at the concave part owing to the lower mass transfer resistance, resulting in the deep-etch and formation of the original nanopores. Volume changes accompanying the oxidization of zirconium led to the internal stress in oxide between the nanopores. Microcracks emerged at the position where the internal stress concentrated and was high enough. Dissolving of the oxide at the fracture surface in F−-containing electrolytes Eq. (4) induced the formation of nanotubes. ZrO2 þ 6F− þ 4Hþ Y½ZrF6 2− þ 2H2 O
ð4Þ
Along with the extension of reaction, the nanotubes' length grew swiftly, at the same time, zirconia dissolved in F−-containing electrolytes slowly. Higher formation rate of zirconia nanotube layer and lower dissolving rate of zirconia resulted in the slow
Zirconia nanotube arrays were prepared using direct anodization of zirconium in a mixture of formamide and glycerol (volume ratio = 1:1) containing 1 wt.% NH4F and 3 wt.% H2O. The diameter of the nanotubes was about 130 nm with a length of up to 190 μm and an aspect ratio of more than 1400 when anodization took place at 50 V for 24 h. X-ray diffraction showed that the as-prepared nanotubes were amorphous zirconia. Monoclinic and tetragonal zirconia coexisted when annealed at 400 °C and 600 °C, while monoclinic zirconia was obtained at 800 °C. The morphology of the annealed zirconia nanotubes did not change with calcination. Fabrication of zirconia nanotube arrays with higher aspect ratio than that in inorganic electrolytes might be due to the lower dissolving rate of zirconia in organic electrolytes. Acknowledgements This work is supported by the Key Project of Chinese Ministry of Education (No.208013), Tianjin Natural Science Foundation (07JCYBJC03300) and Natural Science Foundation of Hebei Province of China (E2007000044). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Liu X, Lu GQ, Yan Z. Appl Catal A-Gen 2005;279:241–5. Yamamoto N, Sato S, Takahashi R, Inui K. J Mol Catal A-Chem 2005;243:52–9. Park HJ, Choi GM. Solid State Ionics 2006;177:2261–7. Liu X, Huang A, Ding C, Chu PK. Biomaterials 2006;27:3904–11. Shin H, Jeong D, Lee J, Sung MM, Kim J. Adv Mater 2004;16:1197–200. Bao J, Tie C, Xu Z, Ma Q, Hong J, Sang H, et al. Adv Mater 2002;14:44–7. Rao CNR, Satishkumar B, Govindaraj A. Chem Commu 1997;16:1581–2. Yin Y, Lu Y, Chen S. Mod Chem Indus 2005;5:38–9. Zhao JL, Wang XH, Sun TY, Li LT. Nanotechnology 2005;16:2450–4. Sieber I, Kannan B, Schmuki P. Electrochem Solid ST 2005;8:J10–2. Choi J, Lim JH, Lee SC, Chang JH, Kim KJ, Cho MA. Electrochim Acta 2006;51:5502–7. Zhao JL, Wang XH, Chen RZ, Li LT. Solid State Commun 2005;134:705–10. Mor GK, Varghese OK, Paulose M, Shankar K, Grimes CA. Sol Energ Mat Sol C 2006;90:2011–75. Tsuchiya H, Macak JM, Sieber I, Schmuki P. Small 2005;1:722–5. Tsuchiya H, Schmuki P. Electrochem Commun 2004;6:1131–4. Lee WJ, Smyrl WH. Electrochem Solid ST 2005;8:B7–9. Tsuchiya H, Macak JM, Taveira L, Schmuki P. Chem Phys Lett 2005;410:188–91. Tsuchiya H, Macak JM, Sieber I, Schmuki P. Mater Sci Forum 2006;512:205–10. Lee WJ, Smyrl WH. Curr Appl Phys 2005, doi:10.1016/j.cap.2007.04.036. Tsuchiya H, Macak JM, Ghicov A, Taveira L, Schmuki P. Corros Sci 2005;47:3324–35. Zhang L, Macdonald DD, Sikora E, Sikorat J. J Electrochom Soc 1998;145:898–905. Tsukada T, Venigalla S, Adair JH, Am J. Ceram Soc 1997;80:3187–92. Zhao JL, Xu RQ, Wang XX, Li YX. Corros Sci 2008;50:1593–7.