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Application of ultrasonic wave to clean the surface of the TiO2 nanotubes prepared by the electrochemical anodization
Applied Surface Science 257 (2011) 8478–8480
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Application of ultrasonic wave to clean the surface of the TiO2 nanotubes prepared by the electrochemical anodization Hao Xu a,b , Qian Zhang a , Chunli Zheng a , Wei Yan a,∗ , Wei Chu b a b
Department of Environmental Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
a r t i c l e
i n f o
Article history: Received 22 February 2011 Received in revised form 30 April 2011 Accepted 30 April 2011 Available online 7 May 2011 Keywords: TiO2 nanotubes Electrochemical anodization Ultrasonic wave Surfaces Microstructure
a b s t r a c t In this study, the TiO2 nanotubes were fabricated by electrochemical anodization in a NH4 F/Na2 SO4 /PEG400/H2 O electrolyte system. Ultrasonic wave (80 W, 40 kHz) was used to clean the surface of TiO2 nanotube arrays in the medium of water after the completion of the anodization. Surface morphology (FESEM) and X-ray diffraction spectrum of the nanotubes treated by sonication at 0 min, 9 min, 40 min and 60 min were compared. The experimental results showed that the precipitate on the surface of the nanotube arrays could be removed by the ultrasonic wave. The treating time had an influence on the precipitate removal and 9 min (corresponding to 12 Wh) is the suitable time for surface cleaning of the TiO2 nanotubes on this experimental condition. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction TiO2 nanotubes have attracted more and more attention due to their excellent electronic, photonic, catalytic, and gas-sensitive properties. It has a great potential in various applications such as photocatalysis [1,2], solar energy cell [3], environmental purification [4,5], and gas sensors [6]. TiO2 nanotubes have been successfully fabricated by templates based on nanoporous alumina [7], hydrothermal techniques [8], sol–gel transcription using organogelators [9], seed growth method [10] and electrochemical anodic oxidation. Among these techniques, electrochemical anodic oxidation is the simplest and likely the cheapest one. It is a self-assembling process, in which both the localized chemical dissolution and the field assisted oxidation and dissolution lead to the formation of orderly nanotubes [11,12]. However, during the anodization process, high pH levels of the electrolyte [13] and the presence of ethylene glycol [14] can result in the deposit of unexpected precipitates on the top surface of the nanotubes, which hinders the filling of the nanotubes with other functional materials. Thus, it is necessary to develop a method to effectively remove the precipitates for gaining clean surface nanotubes, which facilitates the further modification of nanotubes arrays. Some researchers [14–16] reported that ultrasonic agitation could be used to remove the precipitates on the nanotube arrays. However, there is little
∗ Corresponding author. Tel.: +86 29 82664731. E-mail address: [email protected] (W. Yan).
information on the process parameters (such as treatment time and corresponding ultrasonic power) in the published papers. In this study, ultrasonic wave was used for removing the precipitates from the surface of TiO2 nanotube arrays which were prepared by electrochemical anodization in a NH4 F/Na2 SO4 /PEG400/H2 O system. It should be noted that the whole ultrasonic treatment process was performed in the medium of water. Surface morphology and X-ray diffraction spectrum of the nanotubes treated by ultra wave at different times were compared. The purpose of this work is to investigate and optimize the application of ultrasonic wave in the surface cleaning of TiO2 nanotubes. 2. Material and methods High purity of titanium (Ti) foils (99.6%) with 0.5 mm-thickness was used (BaoTi Co. Ltd., China) in this study. All the water was deionized water produced from an EPET-40TF system (EPET Co. Ltd., Nanjing, China). Prior to the anodization, Ti foil was degreased by immersing it into aqua regia for 24 h, then rinsed with deionized water and air-dried. The electrochemical anodization was performed in a two-electrode system with the Ti foil as the anode and the platinum sheet as the counter electrode. The distance between the cathodic and anodic electrodes was approximately 1.5 cm. The electrolyte (pH 6) consisted of 0.8 wt% NH4 F (99.5%), 1.6 wt% Na2 SO4 (99.5%), and 10 wt% PEG400. The anodization was conducted at 20 V for 3 h under magnetic agitation at room temperature (20 ◦ C). The purpose is to reduce the thickness of the double layer at the
0169-4332/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.04.135
H. Xu et al. / Applied Surface Science 257 (2011) 8478–8480
8479
Fig. 1. FESEM images of the TiO2 nanotubes treated with the ultrasonic wave cleaner by various time: (a), (b) 0 min; (c), (d) 9 min; (e), (f) 40 min; (g) 60 min.
metal/electrolyte interface, thus ensures uniform local current density and temperature over the Ti electrode surface [17]. After the anodization was finished, the anodized Ti foil was cleaned with deionized water and subjected to ultrasonic wave (80 W, 40 kHz, KQ2200DB Sonication Cleaner, Kunshan Ultra Co. Ltd., Jiangsu, China). The cleaned TiO2 nanotube arrays in an amorphous state were then air-dried and crystallized in an oven (CMF1100, Hefei Kejin Co. Ltd., China), where the annealing was conducted in air at 500 ◦ C for 120 min. Both the heating and cooling rates were 1 ◦ C/min. Morphology of the TiO2 nanotubes was characterized by the field-emission scanning electron microscope (FESEM, JEOL, and JSM-6700F). Structure of the TiO2 nanotubes after annealing was
identified by the Rigaku D/MAX-2400X X-Ray Diffractometer with Cu-Ka radiation (0.15416 nm).
3. Results and discussion FESEM images of the TiO2 nanotubes treated with ultrasonic wave at different times were displayed in Fig. 1(a–g). Fig. 1a and b show that the top surface of nanotubes without ultrasonic wave treatment was covered by debris because hydrous titanium oxides were generated as the precipitates at a high hydrolysis rate during the anodization process [12,13]. Since the hydrous titanium oxides only physically deposits on the nanotube surface, after 9 min
8480
H. Xu et al. / Applied Surface Science 257 (2011) 8478–8480
est intensity of these diffraction peaks, suggesting under these circumstances, the well-aligned and uniform TiO2 nanotubes dominated the titanium base. On the contrary, the intensity of these titanium metal peaks on the 60-min-ultrasonic-treated TiO2 nanotubes was magnified, because little or no nanotubes were left on the titanium base. 4. Conclusions The ultrasonic wave was used to remove the precipitates from the surface of TiO2 nanotubes prepared by electrochemical anodization. Both of the FESEM and XRD images revealed that under the condition of 80 W and 40 kHz, the top surface of TiO2 nanotubes was cleaned effectively after 9 min of ultrasonic treatment (corresponding to 12 Wh) and the well-aligned and uniform TiO2 nanotubes appeared. When the treating time was extended to 40 min a small part of nanotubes broken and no nanotubes were left on the titanium base if the sonication further increased to 60 min. Fig. 2. XRD patterns of the TiO2 nanotubes treated with the ultrasonic wave cleaner by various time: 0 min; 9 min; 40 min; more than 1 h. A for TiO2 anatase; T for titanium.
of ultrasonic wave treatment (80 W, 40 kHz) the precipitates were removed. After the surface was cleaned, the well-aligned and uniform TiO2 nanotubes reappeared as shown in Fig. 1c and d. The top structures of these nanotubes are opened in either a circular or oval shape and the average diameter is ranged from 70 to 90 nm with wall thickness of 10–20 nm. As the treatment time increased to 40 min, the surface was still clean except that a small part of nanotubes was broken (Fig. 1e and f). It is likely that 9 min of ultrasonic treatment provided enough energy (12 Wh) to remove the precipitates out of the surface of nanotubes, while extended exposure of nanotubes to ultrasound made them absorbing too much (i.e. unnecessary) energy and consequently damaged the tube structure. To verify this, the ultrasonic treatment was further extended to more than 60 min, and some nanotubes were seriously damaged and detached from the titanium base completely (the area of dark color), whereas some nanotube residues remained on the titanium base with shortened lengths as shown in Fig. 1g. These results indicated that under the tested circumstances, the optimal duration for ultrasonic cleaning of the as-prepared TiO2 nanotubes was 9 min, and the extension of the ultrasonic interval resulted in the rupture of the nanotubes. The anodized nanotube was found to be amorphous, so a high temperature annealing was applied to obtain the crystalline phase. Fig. 2 shows the XRD images of the TiO2 nanotubes after annealing treatment. The primary crystal phase was anatase. The diffraction peaks of 2 at 25.5◦ , 35.1◦ , 48.3◦ , and 55.3◦ corresponded to the crystal face (1 0 1), (0 0 4), (2 0 0) and (2 1 1) of anatase TiO2 respectively [5,18]. Anatase crystalline TiO2 is generally accepted to have significant photocatalytic activity. The TiO2 nanotubes without ultrasonic treatment showed the largest diffraction peak intensity of (1 0 1) crystal face, then followed by the order of 9-min-ultrasonic-treated TiO2 nanotubes and 40-min-ultrasonic-treated TiO2 nanotubes, whereas for the 60-min-ultrasonic-treated TiO2 nanotubes such a diffraction peak was not detectable. These results suggested that more TiO2 nanotubes were destroyed with the prolonged ultrasonic treatment, which accorded with the previous findings in FESEM (Fig. 1c–g). The reason of the TiO2 nanotubes without ultrasonic treatment having the strongest peak at 2 = 25.5◦ is likely due to the precipitates, which contain additional titanium dioxide in anatase phase and contribute to the peak intensity. It is known that the diffraction peaks of 2 at 38.5◦ , 40.5◦ , 53.5◦ , 63.2◦ and 76.5◦ were attributed to titanium metal [18]. From Fig. 2, it was noted that the 9-min-ultrasonic-treated TiO2 nanotubes demonstrated the weak-
Acknowledgments The authors gratefully acknowledge the financial supports from the National High Technology Research and Development Program of China (2008AA10Z307), President Research Fund of Xi’an Jiaotong University (08140016), and the Hong Kong Polytechnic University (GU-784). References [1] O.K. Varghese, M. Paulose, T.A. Latempa, C.A. Grimes, High-rate solar photocatalytic conversion of CO2 and water vapor to hydrocarbon fuels, Nano Lett. 9 (2009) 731–737. [2] Y.S. Chen, J.C. Crittenden, S. Hackney, et al., Preparation of a novel TiO2 -based pn junction nanotube photocatalyst, Environ. Sci. Technol. 39 (2005) 1201–1208. [3] J.H. Park, S. Kim, A.J. Bard, Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting, Nano Lett. 6 (2006) 24–28. [4] G.H. Zhao, X. Cui, M.H. Liu, et al., Electrochemical degradation of refractory pollutant using a novel microstructured TiO2 nanotubes/Sb-doped SnO2 electrode, Environ. Sci. Technol. 43 (2009) 1480–1486. [5] Z.Y. Liu, X.T. Zhang, S. Nishimoto, et al., Efficient photocatalytic degradation of gaseous acetaldehyde by highly ordered TiO2 nanotube arrays, Environ. Sci. Technol. 42 (2008) 8547–8551. [6] Y.Y. Zhang, W.Y. Fu, H.B. Yang, et al., Synthesis and characterization of TiO2 nanotubes for humidity sensing, Appl. Surf. Sci. 254 (2008) 5545–5547. [7] A. Michailowski, D. Aimawlwai, G.S. Cheng, M. Moskovits, Highly regular anatase nanotube arrays fabricated in porous anodic templates, Chem. Phys. Lett. 349 (2001) 1–5. [8] B.D. Yao, Y.F. Chan, X.Y. Zhang, W.F. Zhang, Z.Y. Yang, N. Wang, Formation mechanism of TiO2 nanotubes, Appl. Phys. Lett. 82 (2003) 281–283. [9] J.H. Jung, H. Kobayashi, K.J.C. van Bommel, S. Shinkai, T. Shimizu, Creation of novel helical ribbon and double-layered nanotube TiO2 structures using an organogel template, Chem. Mater. 14 (2002) 1445–1447. [10] Z.R.R. Tian, J.A. Voigt, J. Liu, B. Mckenzie, H.F. Xu, Biommetic arrays of oriented helical ZnO nanorods and columns, J. Am. Chem. Soc. 125 (2003) 12384–12385. [11] A. Ghicov, P. Schmuki, Self-ordering electrochemistry: a review on growth and functionality of TiO2 nanotubes and other self-aligned MOx structures, Chem. Commun. 22 (2009) 2791–2808. [12] S. Rani, S.C. Roy, M. Paulose, et al., Synthesis and applications of electrochemically self-assembled titanic nanotube arrays, Phys. Chem. Chem. Phys. 12 (2010) 2780–2800. [13] Q. Cai, M. Paulose, O.K. Varghese, C.A. Grimes, The effect of electrolyte composition on the fabrication of self-organized titanium oxide nanotube arrays by anodic oxidation, J. Mater. Res. 20 (2005) 230–236. [14] M. Paulose, K. Shankar, S. Yoriya, et al., Anodic growth of highly ordered TiO2 nanotube arrays to 134 m in length, J. Phys. Chem. B 110 (2006) 16179 –16184. [15] C.A. Grimes, G.K. Mor, TiO2 Nanotube Arrays: Synthesis, Properties and Applications, Springer, New York, 2009. [16] M. Paulose, H.E. Prakasam, O.K. Varghese, et al., TiO2 nanotube arrays of 1000 m length by anodization of titanium foil: phenol red diffusion, J. Phys. Chem. C 111 (2007) 14992–14997. [17] G. Patermarakis, K. Moussoutzanis, Mathematical models for the anodization conditions and structural features of porous anodic Al2 O3 films on aluminum, J. Electrochem. Soc. 142 (1995) 737–743. [18] G.K. Mor, O.K. Varghese, M. Paulose, et al., A review on highly ordered, vertically oriented TiO2 nanotube arrays: fabrication, material properties, and solar energy applications, Sol. Energy Mater. Sol. Cells 90 (2006) 2011–2075.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Application of ultrasonic wave to clean the surface of the TiO2 nanotubes prepared by the electrochemical anodization Hao Xu a,b , Qian Zhang a , Chunli Zheng a , Wei Yan a,∗ , Wei Chu b a b
Department of Environmental Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
a r t i c l e
i n f o
Article history: Received 22 February 2011 Received in revised form 30 April 2011 Accepted 30 April 2011 Available online 7 May 2011 Keywords: TiO2 nanotubes Electrochemical anodization Ultrasonic wave Surfaces Microstructure
a b s t r a c t In this study, the TiO2 nanotubes were fabricated by electrochemical anodization in a NH4 F/Na2 SO4 /PEG400/H2 O electrolyte system. Ultrasonic wave (80 W, 40 kHz) was used to clean the surface of TiO2 nanotube arrays in the medium of water after the completion of the anodization. Surface morphology (FESEM) and X-ray diffraction spectrum of the nanotubes treated by sonication at 0 min, 9 min, 40 min and 60 min were compared. The experimental results showed that the precipitate on the surface of the nanotube arrays could be removed by the ultrasonic wave. The treating time had an influence on the precipitate removal and 9 min (corresponding to 12 Wh) is the suitable time for surface cleaning of the TiO2 nanotubes on this experimental condition. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction TiO2 nanotubes have attracted more and more attention due to their excellent electronic, photonic, catalytic, and gas-sensitive properties. It has a great potential in various applications such as photocatalysis [1,2], solar energy cell [3], environmental purification [4,5], and gas sensors [6]. TiO2 nanotubes have been successfully fabricated by templates based on nanoporous alumina [7], hydrothermal techniques [8], sol–gel transcription using organogelators [9], seed growth method [10] and electrochemical anodic oxidation. Among these techniques, electrochemical anodic oxidation is the simplest and likely the cheapest one. It is a self-assembling process, in which both the localized chemical dissolution and the field assisted oxidation and dissolution lead to the formation of orderly nanotubes [11,12]. However, during the anodization process, high pH levels of the electrolyte [13] and the presence of ethylene glycol [14] can result in the deposit of unexpected precipitates on the top surface of the nanotubes, which hinders the filling of the nanotubes with other functional materials. Thus, it is necessary to develop a method to effectively remove the precipitates for gaining clean surface nanotubes, which facilitates the further modification of nanotubes arrays. Some researchers [14–16] reported that ultrasonic agitation could be used to remove the precipitates on the nanotube arrays. However, there is little
∗ Corresponding author. Tel.: +86 29 82664731. E-mail address: [email protected] (W. Yan).
information on the process parameters (such as treatment time and corresponding ultrasonic power) in the published papers. In this study, ultrasonic wave was used for removing the precipitates from the surface of TiO2 nanotube arrays which were prepared by electrochemical anodization in a NH4 F/Na2 SO4 /PEG400/H2 O system. It should be noted that the whole ultrasonic treatment process was performed in the medium of water. Surface morphology and X-ray diffraction spectrum of the nanotubes treated by ultra wave at different times were compared. The purpose of this work is to investigate and optimize the application of ultrasonic wave in the surface cleaning of TiO2 nanotubes. 2. Material and methods High purity of titanium (Ti) foils (99.6%) with 0.5 mm-thickness was used (BaoTi Co. Ltd., China) in this study. All the water was deionized water produced from an EPET-40TF system (EPET Co. Ltd., Nanjing, China). Prior to the anodization, Ti foil was degreased by immersing it into aqua regia for 24 h, then rinsed with deionized water and air-dried. The electrochemical anodization was performed in a two-electrode system with the Ti foil as the anode and the platinum sheet as the counter electrode. The distance between the cathodic and anodic electrodes was approximately 1.5 cm. The electrolyte (pH 6) consisted of 0.8 wt% NH4 F (99.5%), 1.6 wt% Na2 SO4 (99.5%), and 10 wt% PEG400. The anodization was conducted at 20 V for 3 h under magnetic agitation at room temperature (20 ◦ C). The purpose is to reduce the thickness of the double layer at the
0169-4332/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.04.135
H. Xu et al. / Applied Surface Science 257 (2011) 8478–8480
8479
Fig. 1. FESEM images of the TiO2 nanotubes treated with the ultrasonic wave cleaner by various time: (a), (b) 0 min; (c), (d) 9 min; (e), (f) 40 min; (g) 60 min.
metal/electrolyte interface, thus ensures uniform local current density and temperature over the Ti electrode surface [17]. After the anodization was finished, the anodized Ti foil was cleaned with deionized water and subjected to ultrasonic wave (80 W, 40 kHz, KQ2200DB Sonication Cleaner, Kunshan Ultra Co. Ltd., Jiangsu, China). The cleaned TiO2 nanotube arrays in an amorphous state were then air-dried and crystallized in an oven (CMF1100, Hefei Kejin Co. Ltd., China), where the annealing was conducted in air at 500 ◦ C for 120 min. Both the heating and cooling rates were 1 ◦ C/min. Morphology of the TiO2 nanotubes was characterized by the field-emission scanning electron microscope (FESEM, JEOL, and JSM-6700F). Structure of the TiO2 nanotubes after annealing was
identified by the Rigaku D/MAX-2400X X-Ray Diffractometer with Cu-Ka radiation (0.15416 nm).
3. Results and discussion FESEM images of the TiO2 nanotubes treated with ultrasonic wave at different times were displayed in Fig. 1(a–g). Fig. 1a and b show that the top surface of nanotubes without ultrasonic wave treatment was covered by debris because hydrous titanium oxides were generated as the precipitates at a high hydrolysis rate during the anodization process [12,13]. Since the hydrous titanium oxides only physically deposits on the nanotube surface, after 9 min
8480
H. Xu et al. / Applied Surface Science 257 (2011) 8478–8480
est intensity of these diffraction peaks, suggesting under these circumstances, the well-aligned and uniform TiO2 nanotubes dominated the titanium base. On the contrary, the intensity of these titanium metal peaks on the 60-min-ultrasonic-treated TiO2 nanotubes was magnified, because little or no nanotubes were left on the titanium base. 4. Conclusions The ultrasonic wave was used to remove the precipitates from the surface of TiO2 nanotubes prepared by electrochemical anodization. Both of the FESEM and XRD images revealed that under the condition of 80 W and 40 kHz, the top surface of TiO2 nanotubes was cleaned effectively after 9 min of ultrasonic treatment (corresponding to 12 Wh) and the well-aligned and uniform TiO2 nanotubes appeared. When the treating time was extended to 40 min a small part of nanotubes broken and no nanotubes were left on the titanium base if the sonication further increased to 60 min. Fig. 2. XRD patterns of the TiO2 nanotubes treated with the ultrasonic wave cleaner by various time: 0 min; 9 min; 40 min; more than 1 h. A for TiO2 anatase; T for titanium.
of ultrasonic wave treatment (80 W, 40 kHz) the precipitates were removed. After the surface was cleaned, the well-aligned and uniform TiO2 nanotubes reappeared as shown in Fig. 1c and d. The top structures of these nanotubes are opened in either a circular or oval shape and the average diameter is ranged from 70 to 90 nm with wall thickness of 10–20 nm. As the treatment time increased to 40 min, the surface was still clean except that a small part of nanotubes was broken (Fig. 1e and f). It is likely that 9 min of ultrasonic treatment provided enough energy (12 Wh) to remove the precipitates out of the surface of nanotubes, while extended exposure of nanotubes to ultrasound made them absorbing too much (i.e. unnecessary) energy and consequently damaged the tube structure. To verify this, the ultrasonic treatment was further extended to more than 60 min, and some nanotubes were seriously damaged and detached from the titanium base completely (the area of dark color), whereas some nanotube residues remained on the titanium base with shortened lengths as shown in Fig. 1g. These results indicated that under the tested circumstances, the optimal duration for ultrasonic cleaning of the as-prepared TiO2 nanotubes was 9 min, and the extension of the ultrasonic interval resulted in the rupture of the nanotubes. The anodized nanotube was found to be amorphous, so a high temperature annealing was applied to obtain the crystalline phase. Fig. 2 shows the XRD images of the TiO2 nanotubes after annealing treatment. The primary crystal phase was anatase. The diffraction peaks of 2 at 25.5◦ , 35.1◦ , 48.3◦ , and 55.3◦ corresponded to the crystal face (1 0 1), (0 0 4), (2 0 0) and (2 1 1) of anatase TiO2 respectively [5,18]. Anatase crystalline TiO2 is generally accepted to have significant photocatalytic activity. The TiO2 nanotubes without ultrasonic treatment showed the largest diffraction peak intensity of (1 0 1) crystal face, then followed by the order of 9-min-ultrasonic-treated TiO2 nanotubes and 40-min-ultrasonic-treated TiO2 nanotubes, whereas for the 60-min-ultrasonic-treated TiO2 nanotubes such a diffraction peak was not detectable. These results suggested that more TiO2 nanotubes were destroyed with the prolonged ultrasonic treatment, which accorded with the previous findings in FESEM (Fig. 1c–g). The reason of the TiO2 nanotubes without ultrasonic treatment having the strongest peak at 2 = 25.5◦ is likely due to the precipitates, which contain additional titanium dioxide in anatase phase and contribute to the peak intensity. It is known that the diffraction peaks of 2 at 38.5◦ , 40.5◦ , 53.5◦ , 63.2◦ and 76.5◦ were attributed to titanium metal [18]. From Fig. 2, it was noted that the 9-min-ultrasonic-treated TiO2 nanotubes demonstrated the weak-
Acknowledgments The authors gratefully acknowledge the financial supports from the National High Technology Research and Development Program of China (2008AA10Z307), President Research Fund of Xi’an Jiaotong University (08140016), and the Hong Kong Polytechnic University (GU-784). References [1] O.K. Varghese, M. Paulose, T.A. Latempa, C.A. Grimes, High-rate solar photocatalytic conversion of CO2 and water vapor to hydrocarbon fuels, Nano Lett. 9 (2009) 731–737. [2] Y.S. Chen, J.C. Crittenden, S. Hackney, et al., Preparation of a novel TiO2 -based pn junction nanotube photocatalyst, Environ. Sci. Technol. 39 (2005) 1201–1208. [3] J.H. Park, S. Kim, A.J. Bard, Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting, Nano Lett. 6 (2006) 24–28. [4] G.H. Zhao, X. Cui, M.H. Liu, et al., Electrochemical degradation of refractory pollutant using a novel microstructured TiO2 nanotubes/Sb-doped SnO2 electrode, Environ. Sci. Technol. 43 (2009) 1480–1486. [5] Z.Y. Liu, X.T. Zhang, S. Nishimoto, et al., Efficient photocatalytic degradation of gaseous acetaldehyde by highly ordered TiO2 nanotube arrays, Environ. Sci. Technol. 42 (2008) 8547–8551. [6] Y.Y. Zhang, W.Y. Fu, H.B. Yang, et al., Synthesis and characterization of TiO2 nanotubes for humidity sensing, Appl. Surf. Sci. 254 (2008) 5545–5547. [7] A. Michailowski, D. Aimawlwai, G.S. Cheng, M. Moskovits, Highly regular anatase nanotube arrays fabricated in porous anodic templates, Chem. Phys. Lett. 349 (2001) 1–5. [8] B.D. Yao, Y.F. Chan, X.Y. Zhang, W.F. Zhang, Z.Y. Yang, N. Wang, Formation mechanism of TiO2 nanotubes, Appl. Phys. Lett. 82 (2003) 281–283. [9] J.H. Jung, H. Kobayashi, K.J.C. van Bommel, S. Shinkai, T. Shimizu, Creation of novel helical ribbon and double-layered nanotube TiO2 structures using an organogel template, Chem. Mater. 14 (2002) 1445–1447. [10] Z.R.R. Tian, J.A. Voigt, J. Liu, B. Mckenzie, H.F. Xu, Biommetic arrays of oriented helical ZnO nanorods and columns, J. Am. Chem. Soc. 125 (2003) 12384–12385. [11] A. Ghicov, P. Schmuki, Self-ordering electrochemistry: a review on growth and functionality of TiO2 nanotubes and other self-aligned MOx structures, Chem. Commun. 22 (2009) 2791–2808. [12] S. Rani, S.C. Roy, M. Paulose, et al., Synthesis and applications of electrochemically self-assembled titanic nanotube arrays, Phys. Chem. Chem. Phys. 12 (2010) 2780–2800. [13] Q. Cai, M. Paulose, O.K. Varghese, C.A. Grimes, The effect of electrolyte composition on the fabrication of self-organized titanium oxide nanotube arrays by anodic oxidation, J. Mater. Res. 20 (2005) 230–236. [14] M. Paulose, K. Shankar, S. Yoriya, et al., Anodic growth of highly ordered TiO2 nanotube arrays to 134 m in length, J. Phys. Chem. B 110 (2006) 16179 –16184. [15] C.A. Grimes, G.K. Mor, TiO2 Nanotube Arrays: Synthesis, Properties and Applications, Springer, New York, 2009. [16] M. Paulose, H.E. Prakasam, O.K. Varghese, et al., TiO2 nanotube arrays of 1000 m length by anodization of titanium foil: phenol red diffusion, J. Phys. Chem. C 111 (2007) 14992–14997. [17] G. Patermarakis, K. Moussoutzanis, Mathematical models for the anodization conditions and structural features of porous anodic Al2 O3 films on aluminum, J. Electrochem. Soc. 142 (1995) 737–743. [18] G.K. Mor, O.K. Varghese, M. Paulose, et al., A review on highly ordered, vertically oriented TiO2 nanotube arrays: fabrication, material properties, and solar energy applications, Sol. Energy Mater. Sol. Cells 90 (2006) 2011–2075.