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Facile fabrication of a dual hierarchical TiO2 nanostructure
Materials Letters 68 (2012) 290–292
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile fabrication of a dual hierarchical TiO2 nanostructure Lingjie Li a, Zhuqing Zhou a, Jinglei Lei a,⁎, Jianxin He b, Shengmao Wu a, Fusheng Pan b a b
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, PR China, 400044 College of Materials Science and Engineering, Chongqing University, Chongqing, 400044 PR China
a r t i c l e
i n f o
Article history: Received 10 April 2011 Accepted 30 October 2011 Available online 4 November 2011 Keywords: Titanium oxide Hierarchical nanostructure Electrochemical anodization Electron microscopy X-ray techniques
a b s t r a c t A dual hierarchical TiO2 nanostructure is directly fabricated by electrochemically anodizing of an electropolished Ti substrate in an ethylene glycol electrolyte containing small amounts of fluoride with a two-step process, which is carried out at room temperature in a two-electrode electrochemical cell under 30 V for 3 h plus 10 min. The formed nanostructure consists of a nanoporous top layer (~ 60 nm thickness) and a nanotubular underneath layer (~ 600 nm thickness). The highly regular nanopores (~ 100 nm diameter) in hexagonal shape distribute over the top layer and the underneath layer is arranged with the well-aligned nanotubes (~ 50 nm diameter). The dual parts of the hierarchical nanostructure are both composed of TiO2, which exhibits anatase phase after being annealed at 450 °C under ambient air for 3 h. The facile fabrication of this dual hierarchical TiO2 nanostructure inspires to prepare other nanoscale materials with alike complex geometries by the similar approach. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The nanoscale TiO2 materials have been widely studied due to their fundamental properties and promising applications in sensors [1–3], photocatalytic [4–7] and photovoltaic devices [8,9], energy storage [10,11] and biomaterials [12–14]. Recently particular interests have been dedicated to the fabrication of highly ordered TiO2 nanoporous or nanotubular structures because of their dramatically enhanced properties compared with TiO2 nanoparticulate films [4,15,16]. The well-aligned TiO2 nanotubes and nanopores have been achieved by two-step or three-step electrochemical anodization processes [17–21]. As for the dual hierarchical TiO2 nanostructure consisting of both highly regular nanopores and nanotubes, which is desirable in applications where the nano-structural duality is important, e.g., chemical sensing, bioactive coating and nano-template, their facile fabrication remains a challenge and their detailed morphology, composition and crystal structure are rarely known [18–20]. The present work dedicates to develop a facile approach to prepare a dual hierarchical TiO2 nanostructure, which consists of highly regular nanopores in hexagonal shape and well aligned nanotubes. The approach demonstrated here avoids the fussy and timeconsuming processes in the present anodizing fabrication of highly ordered TiO2 nanostructures, e.g., three-step anodization [18], threeelectrode configuration [17,18], high voltage [17–20], low temperature [20], long electrolyte-aging duration [19] and long anodization
⁎ Corresponding author. Tel.: + 86 13983064116; fax: + 86 23 65112328. E-mail address: [email protected] (J. Lei). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.10.104
duration [17–20]. Moreover, the detailed morphology, composition and crystal structure of this dual hierarchical architecture are studied.
2. Experimental The titanium (Ti) foils (99.4% purity; 0.2 mm thickness) used in this work were obtained from Yunjie Metal Co., Ltd. (Baoji, PR China). Prior to anodization, they were first mechanically polished to a plane mirror finish with 200#, 400#, 800# abrasive papers and rinsed with deionized water. Then they were electropolished at 20 V for 5 min with constant stirring in a mixed solution of perchloric acid, butanol and ethanol (4:11:16 in mass) at −20 °C. After that, they were ultrasonically cleaned in acetone and deionized water respectively for 5 min. The potentiostatic anodization experiments were carried out at room temperature in a two-electrode electrochemical cell, with the pre-treated Ti foil as the working electrode and a platinum plate as the counter electrode. The anodization electrolyte was a mixed solution of ethylene glycol (100 ml) and 17.5 wt.% NH4F aqueous electrolyte (2 ml), which was prepared from analytical grade chemicals and deionized water. The voltage was applied using a DC power supply (DH1719A-5, Beijing Dahua Electronic Co., PR China). The Ti foil was first anodized at 30 V for 3 h. After that, the anodized sample was ultrasonically soaked in ethanol for 10 min and then the formed anodic layer was removed by an adhesion tape. Thus the well-textured underlying Ti surface was exposed. Subsequently, the second-step anodization was performed for 10 min under conditions identical to those in the first-step anodization. After the second-step anodization, the sample was rinsed with deionized water and then dried in air.
L. Li et al. / Materials Letters 68 (2012) 290–292
291
Fig. 1. FE-SEM micrographs of (a) the first-step anodized sample. (b) The imprints at Ti surface after removal of the first-step anodic layer. (inset: Fourier transforms pattern of the FE-SEM image indicating the hexagonal order of the imprints). (c) The top view of the dual hierarchical nanostructure (inset: the top view at a higher magnification). (d) The crosssectional view of the dual hierarchical nanostructure (the section between two arrows denotes the top nanoporous layer).
The morphologies of the samples were characterized using a fieldemission scanning electron microscope (FESEM, FEI Nova 400, Holland). The chemical composition of the samples was analyzed by X-ray photoelectron spectroscopy (XPS, Thermoelectron ESCALAB 250, USA). The phase composition (crystal structure) of the samples was identified by X-ray diffractometer (XRD, Philips X'pert Pro, Holland).
distributed by the highly regular nanopores in hexagonal shape with an average diameter of 100 nm. The underneath layer is arranged with well-aligned nanotubes. Through the nanopores of the top layer, the mouths of the underneath nanotubes can be observed, which indicates that the nanotubes have an average diameter of 50 nm. From the cross-sectional view presented in Fig. 1d, the thickness of the
3. Results and discussion Fig. 1a illustrates the FE-SEM photograph of the first-step anodized layer on an electropolished Ti foil. From the view of the intact part of the layer, regular nanopores distribute over the anodized surface. The cross-section exposed from the cracked part exhibits tubelike feature, which indicates that the first-step anodized layer indeed consists of nanotubes though they are somewhat interconnected. This layer was then removed and the corresponding imprints on the Ti surface after removal are displayed in Fig. 1b, which shows an ordered hexagonally packed pattern with an average diameter of 100 nm. The arrangement regularity of the imprints is further derived in virtue of Fourier transforms. As shown in the inset of Fig. 1b, six distinct spots uniformly distribute on each edge of the hexagon, which confirms the hexagonal order of the imprints. The highly regular imprints can act as good geometrical guides for the further growth of well-aligned nanostructure in the subsequent anodization, which play a crucial role in the fabrication process [18–22]. Fig. 1c and d respectively shows the top and cross-sectional views of the dual hierarchical nanostructure formed in the second-step anodization. The nanostructure consists of two layers. The top layer is
Fig. 2. Schematic illustration of the dual hierarchical nanostructure.
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L. Li et al. / Materials Letters 68 (2012) 290–292
As compared with the dual porous hierarchical TiO2 structure reported [26,27], which has a top layer consisting of large (~1–20 μm) pores on the microscale, this dual hierarchical TiO2 nanostructure as-prepared is more fascinating because of its importance for the fundamental study of nanostructures as well as for the promising applications in nanodevices especially where the nano-structural duality is important. 4. Conclusions
Fig. 3. XPS survey spectrum (inset: the high resolution spectrum of Ti2p).
The present work demonstrates a facile approach to attain a dual hierarchical TiO2 nanostructure. The fabricated structure consists of a highly ordered nanoporous top layer in hexagonal shape and a well-aligned nanotubular underneath layer. The dual parts of the hierarchical nanostructure are composed of TiO2, which exhibits anatase phase after being annealed. The facile fabrication approach of this dual hierarchical TiO2 nanostructure can be easily extended to prepare other complicated nanostructure. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (20803097, 20603049), the Natural Science Foundation of CQ CSTC (2009BA4023), the Fundamental Research Funds for the Central Universities (CDJZR11220002) and the sharing fund of Chongqing University's Large-scale Equipment. References
Fig. 4. XRD patterns of the annealed sample.
nanoporous top layer and the nanotubular underneath layer can be evaluated, which is approximately 60 nm and 600 nm, respectively. Moreover, it can be clearly seen that the nanotubes are well-oriented and have smooth tube walls, which are different from those formed in the first-step anodization. The illustration shown in Fig. 2 schematically represents this dual hierarchical nanostructure. The chemical composition of the formed hierarchical nanostructure is investigated by XPS. The survey spectrum for the nanoporous top layer is shown in Fig. 3. The existence of elements Ti, O, and C is confirmed. C is contamination induced from the nonaqueous electrolyte and during storage of the sample in air. The Ti 2p1/2 and 2p3/2 peaks (as shown in the inset of Fig. 3) have binding energies of 464.7 eV and 458.9 eV, respectively, very close to those of bulk titania [23,24], which indicate that the chemical composition of the nanoporous top layer might be TiO2. The XPS result for the underneath nanotubular layer (The spectrum was recorded after the two-step anodized sample was sputtered with Ar+ ion bombardment for 60 min to penetrate through the nanoporous top layer.) is similar to that of the nanoporous top layer shown in Fig. 3, which suggests that the dual parts of the hierarchical TiO2 nanostructure have the same chemical composition. Fig. 4 shows the XRD pattern of the two-step anodized sample after being annealed at 450 °C under ambient air for 3 h. The strong diffraction peaks at about 2θ = 34.99°, 38.36°, 40.11°, 52.93°, 62.89°, 70.78° and 76.15° are related to the metallic Ti substrate [4,25]. The diffraction peaks at about 2θ = 25.22° and 37.72° are believed to originate from the anatase titania of the formed nanostructure [15,25], which confirms that the dual hierarchical nanostructure is TiO2.
[1] Paulose M, Varghese OK, Mor GK, Grimes CA, Ong KG. Nanotechnology 2006;17: 398–402. [2] Du XY, Wang Y, Mu YY, Gui LL, Wang P, Tang YQ. Chem Mater 2002;14:3953–7. [3] Al-Homoudi IA, Thakur JS, Naik R, Auner GW, Newaz G. Appl Surf Sci 2007;253: 8607–14. [4] Macak JM, Zlamal M, Krysa J, Schmuki P. Small 2007;3:300–4. [5] Kang XW, Chen SW. J Mater Sci 2010;45:2696–702. [6] Yun HJ, Lee H, Joo JB, Kim ND, Yi J. Electrochem Commun 2010;12:769–72. [7] Shrestha NK, Yang M, Nah Y-C, Paramasivam I, Schmuki P. Electrochem Commun 2010;12:254–7. [8] Shim H-S, Na S-I, Nam SH, Ahn H-J, Kim HJ, Kim D-Y, et al. Appl Phys Lett 2008;92: 183107. [9] Foong TRB, Shen YD, Hu X, Sellinger A. Adv Funct Mater 2010;20:1390–6. [10] Zhang WK, Wang L, Huang H, Gan YP, Wang CT, Tao XY. Electrochim Acta 2009;54:4760–3. [11] Choi D, Wang D, Viswanathan VV, Bae I-T, Wang W, Nie Z, et al. Electrochem Commun 2010;12:378–81. [12] Das K, Bandyopadhyay A, Bose S. J Am Ceram Soc 2008;91:2808–14. [13] Kodama A, Bauer S, Komatsu A, Asoh H, Ono S, Schmuki P. Acta Biomater 2009;5: 2322–30. [14] Ma QQ, Li MH, Hu ZY, Chen Q, Hu WY. Mater Lett 2008;62:3035–8. [15] Liu ZY, Zhang XT, Nishimoto S, Jin M, Tryk DA, Murakami T, et al. J Phys Chem C 2008;112:253–9. [16] Ghicov A, Albu SP, Macak JM, Schmuki P. Small 2008;4:1063–6. [17] Macak JM, Albu SP, Schmuki P. Phys Status Solidi RRL 2007;1:181–3. [18] Zhang GG, Huang HT, Zhang YH, Chan HLW, Zhou LM. Electrochem Commun 2007;9:2854–8. [19] Li SQ, Zhang GM, Guo DZ, Yu LG, Zhang W. J Phys Chem C 2009;113:12759–65. [20] Wang DA, Yu B, Wang CW, Zhou F, Liu WM. Adv Mater 2009;21:1964–7. [21] Shin Y, Lee S. Nano Lett 2008;8:3171–3. [22] Li LJ, Yan DX, Lei JL, He JX, Wu SM, Pan FS. Mater Lett 2011;65:1434–7. [23] Moulder F, Stickle WF, Sobol PE, Bomben KD. Handbook of X-Ray Photoelectron Spectroscopy. Eden Prairie, MN: Perkin-Elmer; 1991. [24] Li G, Liu ZQ, Lu J, Wang L, Zhang Z. Appl Surf Sci 2009;255:7323–8. [25] Xie YB, Zhou LM, Lu J. J Mater Sci 2009;44:2907–15. [26] Crawford GA, Chawla N. J Mater Res 2009;24:1683–7. [27] Crawford GA, Chawla N. Acta Mater 2009;57:854–67.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile fabrication of a dual hierarchical TiO2 nanostructure Lingjie Li a, Zhuqing Zhou a, Jinglei Lei a,⁎, Jianxin He b, Shengmao Wu a, Fusheng Pan b a b
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, PR China, 400044 College of Materials Science and Engineering, Chongqing University, Chongqing, 400044 PR China
a r t i c l e
i n f o
Article history: Received 10 April 2011 Accepted 30 October 2011 Available online 4 November 2011 Keywords: Titanium oxide Hierarchical nanostructure Electrochemical anodization Electron microscopy X-ray techniques
a b s t r a c t A dual hierarchical TiO2 nanostructure is directly fabricated by electrochemically anodizing of an electropolished Ti substrate in an ethylene glycol electrolyte containing small amounts of fluoride with a two-step process, which is carried out at room temperature in a two-electrode electrochemical cell under 30 V for 3 h plus 10 min. The formed nanostructure consists of a nanoporous top layer (~ 60 nm thickness) and a nanotubular underneath layer (~ 600 nm thickness). The highly regular nanopores (~ 100 nm diameter) in hexagonal shape distribute over the top layer and the underneath layer is arranged with the well-aligned nanotubes (~ 50 nm diameter). The dual parts of the hierarchical nanostructure are both composed of TiO2, which exhibits anatase phase after being annealed at 450 °C under ambient air for 3 h. The facile fabrication of this dual hierarchical TiO2 nanostructure inspires to prepare other nanoscale materials with alike complex geometries by the similar approach. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The nanoscale TiO2 materials have been widely studied due to their fundamental properties and promising applications in sensors [1–3], photocatalytic [4–7] and photovoltaic devices [8,9], energy storage [10,11] and biomaterials [12–14]. Recently particular interests have been dedicated to the fabrication of highly ordered TiO2 nanoporous or nanotubular structures because of their dramatically enhanced properties compared with TiO2 nanoparticulate films [4,15,16]. The well-aligned TiO2 nanotubes and nanopores have been achieved by two-step or three-step electrochemical anodization processes [17–21]. As for the dual hierarchical TiO2 nanostructure consisting of both highly regular nanopores and nanotubes, which is desirable in applications where the nano-structural duality is important, e.g., chemical sensing, bioactive coating and nano-template, their facile fabrication remains a challenge and their detailed morphology, composition and crystal structure are rarely known [18–20]. The present work dedicates to develop a facile approach to prepare a dual hierarchical TiO2 nanostructure, which consists of highly regular nanopores in hexagonal shape and well aligned nanotubes. The approach demonstrated here avoids the fussy and timeconsuming processes in the present anodizing fabrication of highly ordered TiO2 nanostructures, e.g., three-step anodization [18], threeelectrode configuration [17,18], high voltage [17–20], low temperature [20], long electrolyte-aging duration [19] and long anodization
⁎ Corresponding author. Tel.: + 86 13983064116; fax: + 86 23 65112328. E-mail address: [email protected] (J. Lei). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.10.104
duration [17–20]. Moreover, the detailed morphology, composition and crystal structure of this dual hierarchical architecture are studied.
2. Experimental The titanium (Ti) foils (99.4% purity; 0.2 mm thickness) used in this work were obtained from Yunjie Metal Co., Ltd. (Baoji, PR China). Prior to anodization, they were first mechanically polished to a plane mirror finish with 200#, 400#, 800# abrasive papers and rinsed with deionized water. Then they were electropolished at 20 V for 5 min with constant stirring in a mixed solution of perchloric acid, butanol and ethanol (4:11:16 in mass) at −20 °C. After that, they were ultrasonically cleaned in acetone and deionized water respectively for 5 min. The potentiostatic anodization experiments were carried out at room temperature in a two-electrode electrochemical cell, with the pre-treated Ti foil as the working electrode and a platinum plate as the counter electrode. The anodization electrolyte was a mixed solution of ethylene glycol (100 ml) and 17.5 wt.% NH4F aqueous electrolyte (2 ml), which was prepared from analytical grade chemicals and deionized water. The voltage was applied using a DC power supply (DH1719A-5, Beijing Dahua Electronic Co., PR China). The Ti foil was first anodized at 30 V for 3 h. After that, the anodized sample was ultrasonically soaked in ethanol for 10 min and then the formed anodic layer was removed by an adhesion tape. Thus the well-textured underlying Ti surface was exposed. Subsequently, the second-step anodization was performed for 10 min under conditions identical to those in the first-step anodization. After the second-step anodization, the sample was rinsed with deionized water and then dried in air.
L. Li et al. / Materials Letters 68 (2012) 290–292
291
Fig. 1. FE-SEM micrographs of (a) the first-step anodized sample. (b) The imprints at Ti surface after removal of the first-step anodic layer. (inset: Fourier transforms pattern of the FE-SEM image indicating the hexagonal order of the imprints). (c) The top view of the dual hierarchical nanostructure (inset: the top view at a higher magnification). (d) The crosssectional view of the dual hierarchical nanostructure (the section between two arrows denotes the top nanoporous layer).
The morphologies of the samples were characterized using a fieldemission scanning electron microscope (FESEM, FEI Nova 400, Holland). The chemical composition of the samples was analyzed by X-ray photoelectron spectroscopy (XPS, Thermoelectron ESCALAB 250, USA). The phase composition (crystal structure) of the samples was identified by X-ray diffractometer (XRD, Philips X'pert Pro, Holland).
distributed by the highly regular nanopores in hexagonal shape with an average diameter of 100 nm. The underneath layer is arranged with well-aligned nanotubes. Through the nanopores of the top layer, the mouths of the underneath nanotubes can be observed, which indicates that the nanotubes have an average diameter of 50 nm. From the cross-sectional view presented in Fig. 1d, the thickness of the
3. Results and discussion Fig. 1a illustrates the FE-SEM photograph of the first-step anodized layer on an electropolished Ti foil. From the view of the intact part of the layer, regular nanopores distribute over the anodized surface. The cross-section exposed from the cracked part exhibits tubelike feature, which indicates that the first-step anodized layer indeed consists of nanotubes though they are somewhat interconnected. This layer was then removed and the corresponding imprints on the Ti surface after removal are displayed in Fig. 1b, which shows an ordered hexagonally packed pattern with an average diameter of 100 nm. The arrangement regularity of the imprints is further derived in virtue of Fourier transforms. As shown in the inset of Fig. 1b, six distinct spots uniformly distribute on each edge of the hexagon, which confirms the hexagonal order of the imprints. The highly regular imprints can act as good geometrical guides for the further growth of well-aligned nanostructure in the subsequent anodization, which play a crucial role in the fabrication process [18–22]. Fig. 1c and d respectively shows the top and cross-sectional views of the dual hierarchical nanostructure formed in the second-step anodization. The nanostructure consists of two layers. The top layer is
Fig. 2. Schematic illustration of the dual hierarchical nanostructure.
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L. Li et al. / Materials Letters 68 (2012) 290–292
As compared with the dual porous hierarchical TiO2 structure reported [26,27], which has a top layer consisting of large (~1–20 μm) pores on the microscale, this dual hierarchical TiO2 nanostructure as-prepared is more fascinating because of its importance for the fundamental study of nanostructures as well as for the promising applications in nanodevices especially where the nano-structural duality is important. 4. Conclusions
Fig. 3. XPS survey spectrum (inset: the high resolution spectrum of Ti2p).
The present work demonstrates a facile approach to attain a dual hierarchical TiO2 nanostructure. The fabricated structure consists of a highly ordered nanoporous top layer in hexagonal shape and a well-aligned nanotubular underneath layer. The dual parts of the hierarchical nanostructure are composed of TiO2, which exhibits anatase phase after being annealed. The facile fabrication approach of this dual hierarchical TiO2 nanostructure can be easily extended to prepare other complicated nanostructure. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (20803097, 20603049), the Natural Science Foundation of CQ CSTC (2009BA4023), the Fundamental Research Funds for the Central Universities (CDJZR11220002) and the sharing fund of Chongqing University's Large-scale Equipment. References
Fig. 4. XRD patterns of the annealed sample.
nanoporous top layer and the nanotubular underneath layer can be evaluated, which is approximately 60 nm and 600 nm, respectively. Moreover, it can be clearly seen that the nanotubes are well-oriented and have smooth tube walls, which are different from those formed in the first-step anodization. The illustration shown in Fig. 2 schematically represents this dual hierarchical nanostructure. The chemical composition of the formed hierarchical nanostructure is investigated by XPS. The survey spectrum for the nanoporous top layer is shown in Fig. 3. The existence of elements Ti, O, and C is confirmed. C is contamination induced from the nonaqueous electrolyte and during storage of the sample in air. The Ti 2p1/2 and 2p3/2 peaks (as shown in the inset of Fig. 3) have binding energies of 464.7 eV and 458.9 eV, respectively, very close to those of bulk titania [23,24], which indicate that the chemical composition of the nanoporous top layer might be TiO2. The XPS result for the underneath nanotubular layer (The spectrum was recorded after the two-step anodized sample was sputtered with Ar+ ion bombardment for 60 min to penetrate through the nanoporous top layer.) is similar to that of the nanoporous top layer shown in Fig. 3, which suggests that the dual parts of the hierarchical TiO2 nanostructure have the same chemical composition. Fig. 4 shows the XRD pattern of the two-step anodized sample after being annealed at 450 °C under ambient air for 3 h. The strong diffraction peaks at about 2θ = 34.99°, 38.36°, 40.11°, 52.93°, 62.89°, 70.78° and 76.15° are related to the metallic Ti substrate [4,25]. The diffraction peaks at about 2θ = 25.22° and 37.72° are believed to originate from the anatase titania of the formed nanostructure [15,25], which confirms that the dual hierarchical nanostructure is TiO2.
[1] Paulose M, Varghese OK, Mor GK, Grimes CA, Ong KG. Nanotechnology 2006;17: 398–402. [2] Du XY, Wang Y, Mu YY, Gui LL, Wang P, Tang YQ. Chem Mater 2002;14:3953–7. [3] Al-Homoudi IA, Thakur JS, Naik R, Auner GW, Newaz G. Appl Surf Sci 2007;253: 8607–14. [4] Macak JM, Zlamal M, Krysa J, Schmuki P. Small 2007;3:300–4. [5] Kang XW, Chen SW. J Mater Sci 2010;45:2696–702. [6] Yun HJ, Lee H, Joo JB, Kim ND, Yi J. Electrochem Commun 2010;12:769–72. [7] Shrestha NK, Yang M, Nah Y-C, Paramasivam I, Schmuki P. Electrochem Commun 2010;12:254–7. [8] Shim H-S, Na S-I, Nam SH, Ahn H-J, Kim HJ, Kim D-Y, et al. Appl Phys Lett 2008;92: 183107. [9] Foong TRB, Shen YD, Hu X, Sellinger A. Adv Funct Mater 2010;20:1390–6. [10] Zhang WK, Wang L, Huang H, Gan YP, Wang CT, Tao XY. Electrochim Acta 2009;54:4760–3. [11] Choi D, Wang D, Viswanathan VV, Bae I-T, Wang W, Nie Z, et al. Electrochem Commun 2010;12:378–81. [12] Das K, Bandyopadhyay A, Bose S. J Am Ceram Soc 2008;91:2808–14. [13] Kodama A, Bauer S, Komatsu A, Asoh H, Ono S, Schmuki P. Acta Biomater 2009;5: 2322–30. [14] Ma QQ, Li MH, Hu ZY, Chen Q, Hu WY. Mater Lett 2008;62:3035–8. [15] Liu ZY, Zhang XT, Nishimoto S, Jin M, Tryk DA, Murakami T, et al. J Phys Chem C 2008;112:253–9. [16] Ghicov A, Albu SP, Macak JM, Schmuki P. Small 2008;4:1063–6. [17] Macak JM, Albu SP, Schmuki P. Phys Status Solidi RRL 2007;1:181–3. [18] Zhang GG, Huang HT, Zhang YH, Chan HLW, Zhou LM. Electrochem Commun 2007;9:2854–8. [19] Li SQ, Zhang GM, Guo DZ, Yu LG, Zhang W. J Phys Chem C 2009;113:12759–65. [20] Wang DA, Yu B, Wang CW, Zhou F, Liu WM. Adv Mater 2009;21:1964–7. [21] Shin Y, Lee S. Nano Lett 2008;8:3171–3. [22] Li LJ, Yan DX, Lei JL, He JX, Wu SM, Pan FS. Mater Lett 2011;65:1434–7. [23] Moulder F, Stickle WF, Sobol PE, Bomben KD. Handbook of X-Ray Photoelectron Spectroscopy. Eden Prairie, MN: Perkin-Elmer; 1991. [24] Li G, Liu ZQ, Lu J, Wang L, Zhang Z. Appl Surf Sci 2009;255:7323–8. [25] Xie YB, Zhou LM, Lu J. J Mater Sci 2009;44:2907–15. [26] Crawford GA, Chawla N. J Mater Res 2009;24:1683–7. [27] Crawford GA, Chawla N. Acta Mater 2009;57:854–67.