One-Step Formation of Crystalline TiO2 Nanotubular Arrays with

ARTICLE pubs.acs.org/JPCC

One-Step Formation of Crystalline TiO2 Nanotubular Arrays with Intrinsic p
n Junctions Jie Zhang, Xinhu Tang, and Dongyang Li* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2V4 ABSTRACT: It is highly desired to fabricate crystalline TiO2 nanotubes by anodization without subsequent annealing that may result in cracking and the formation of an interfacial barrier between the nanotubular arrays and the substrate, thus improving the structural integrity and corresponding photoelectric properties. In this work, an attempt was made to fabricate crystalline TiO2 nanotubular arrays through anodization in a NH4F-containing glycerol electrolyte at room temperature (RT) and 100 °C. It was demonstrated that crystalline nanotubes were successfully fabricated at both RT and 100 °C. Photocurrent and surface photovoltage spectra of the nanotubes were measured to investigate their photoinduced charge separation efficiency and semiconductor characteristics. It was interesting to observe that the asfabricated TiO2 nanotubes at RT consisted of both rutile and anatase, confirmed by SAD of TEM, which showed apparent p-type semiconductor characteristics. The mechanism for the formation of crystallites at RT and the possibility to develop intrinsic nanoscale p
n junctions were discussed. However, the nanotubes anodized at 100 °C were in the state of anatase, exhibiting n-type semiconductor characteristics.

’ INTRODUCTION Nanoscale architectures of TiO2 strongly affect its photocatalytic performance. Highly ordered crystalline TiO2 nanotubular arrays synthesized by anodization of titanium have demonstrated their effectiveness in various applications, for example, hydrogen generation,1,2 gas sensing, 3,4 dye-sensitized solar cells,5,6 photocatalysis,7
9 and biomedical applications.10 However, the as-fabricated TiO2 nanotubular arrays are generally amorphous, and annealing at elevated temperatures (>400 °C) is needed to turn the amorphous nanotubes into crystalline ones, which may, however, result in the formation of a thick barrier layer between the nanotubular film and the metal substrate.11 This barrier has been determined to be composed mainly of rutile with some nonstoichiometric oxides that could act as carrier traps,12 which negatively affect the structural integrity and deteriorate the nanotubular arrays with, for example, a reduced overall water spitting efficiency.13 The high-temperature heat treatment also limits their applications when temperature-sensitive materials, such as polymers, are incorporated to make, for example, nanotube
polymer composites. In addition, microcracking, which interferes with the charge carrier transport, could not be avoided during heat treatment even via a very fine-tuned procedure. Therefore, one-step or fabrication of crystalline TiO2 nanotubular arrays is highly desired. Some attempts, though very limited, were made to directly synthesize crystalline TiO2 at room temperature by altering the electrolyte of anodization. For instance, Allam and co-workers observed that nanotubes formed in diethylene glycol at 80 V and room temperature showed signals of both (101) and (004) crystallographic reflections of anatase;14 however, the as-anodized nanotubular arrays r 2011 American Chemical Society

exhibited low photoconversion efficiency due to their very poor crystallinity, which still needs to be improved by annealing. The main objective of this work is to fabricate crystalline TiO2 nanotubular arrays, as well as to investigate the outcomes of this one-step fabrication at room temperature (RT) and at elevated temperature. We have successfully achieved the onestep synthesis of crystalline TiO2 nanotubular arrays by anodization of titanium foils at RT and 100 °C in a glycerol-based electrolyte. Interestingly, the crystalline TiO2 nanotubes fabricated at RT contain both rutile and anatase and show apparent p-type semiconductor characteristics. It is known that anatase TiO2 is an n-type semiconductor,15 whereas rutile TiO2, according to theoretical studies16 and experimental observations,17,18 is a p-type semiconductor. However, rutile TiO2 has been reported to be an n-type semiconductor as well.19 The discrepancy in reported studies could be ascribed to the sensitivity of the semiconductor to both intrinsic and extrinsic defects. For instance, oxygen vacancies may change rutile from p-type to n-type.16 Impurity atoms also affect the semiconductor type. As an example, doping ions, for example, Fe3+,20 or nitrogen, may change the n-type of TiO2 semiconductors to p-type.21 In the present study, the doping effect is not involved. With the coexistence of rutile and anatase, nanoscale p
n junctions may form at the crystallites’ interface. These intrinsic p
n junctions may generate new opportunities for TiO2 nanotubes, for example, for applications in p
n heterojunction devices and alternative Received: April 12, 2011 Revised: October 3, 2011 Published: October 04, 2011 21529

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Figure 1. SEM top-view and cross-sectional images of TiO2 nanotubular arrays prepared by anodizing Ti foil at 50 V for 1 h at (A) RT and (B) 100 °C.

utilizations of TiO2 nanotubes. The TiO2 nanotubular arrays made at 100 °C were mainly in the state of anatase with exhibited n-type semiconductor characteristics.

’ EXPERIMENTAL SECTION Highly ordered TiO2 nanotubular arrays were fabricated through potentiostatic anodization in a conventional two-electrode electrochemical cell at RT and 100 °C. A 0.2 mm thick titanium foil of 20 mm  20 mm (Alfa Aesar, 99%) was used as the anode, and a platinum foil of 10 mm  10 mm served as the cathode. The distance between the anode and the cathode was ∼20 mm. The anodization was carried out under 50 V using a DC power supply (1715A, B&K Precision Corp.). A Fisher Scientific high-temperature bath was used to control the temperature of the electrolyte. Prior to anodization, titanium foils were soaked in a solution of HF, HNO3, and H2O (1:3:10) for 20 s in order to remove the oxide layer on the surface. The electrolyte was composed of 0.05 wt % NH4F, 2 vol % H2O, and glycerol (99.5%, Fisher Scientific). FE-SEM observations were carried out under a JEOL JSM6301FXV scanning electron microscope with a field emission electron source running at 5 and 10 kV. Electron diffraction analysis and lattice imaging were carried out on a JEOL 2010 transmission electron microscope equipped with a Noran UTW X-ray detector. Photocurrents were measured using a commercial electrochemical system (PC4-750, Gamry Instruments Inc., USA). A standard three-electrode system with a platinum foil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode was used in this study. The electrolyte was 0.5 mol/L Na2SO4 aqueous solution, and the measuring potential was set as 0.1 V vs SCE. A lamp power supply (LPS-220B , Photon Technology International, NJ, USA) was used as the light source for photocurrent measurement. A scanning UV
vis spectrometer (U-3010, Hitachi, Japan) equipped with an integrating sphere of 60 mm in diameter was

Figure 2. TEM images and diffraction patterns of TiO2 nanotubes prepared at RT (A, B) and 100 °C (E, F) in the NH4F-containing electrolyte. The “near” single-crystal diffraction pattern of anatase as the inset image in (A) was occasionally observed. Images (C) and (D) are high-resolution TEM of nanotubes fabricated at RT. It is confirmed that the nanotubes fabricated at RT consist of both anatase and rutile phases. The nanotubes fabricated at 100 °C are mainly in the anatase state. Some obscure and weak rings are present in the diffraction pattern of nanotubes fabricated at 100 °C, which come from a small amount of residual rutile phase.

used to determine the reflectance spectra of TiO2 nanotubular arrays over a range of 200
700 nm at a scan speed of 300 nm/min with a reflectance reference of BaSO4. Surface photovoltage (SPV) was measured using a Kelvin Probe (SKP 5050, KP Technology, UK) equipped with a Surface Photovoltage Spectroscopy (add-on module of SPS040, KP Technology, UK).

’ RESULTS AND DISCUSSION Figure 1 illustrates cross-sectional and top views of the asfabricated nanotubular arrays at both RT and 100 °C. At RT, 21530

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tubes grew up to 560 nm on average within 1 h under an anodization voltage of 50 V. The average outer diameter of the nanotubes was 120 nm. When the temperature of the electrolyte was elevated to 100 °C, the average tube length increased to 5.3 μm but the average outer diameter decreased to 50 nm, and meanwhile, the wall thickness decreased to 9 nm, compared with 18 nm of those made at RT. Such changes were consistent with TEM images, as shown in Figure 2. During the early stage of the self-organized formation of nanotubes, pores form first as a result of the following reaction with local chemical dissolution of the oxide scale:22 TiO2 þ 6F
þ 4Hþ f TiF6 2
þ 2H2 O The reaction rate tends to increase with the bath temperature, and more pores would form during anodization at higher temperatures. In addition, at the elevated temperature, both the electrochemical oxidation of the Ti foil and the chemical dissolution of the oxide layer would speed up, leading to a faster growth of nanotubes. The accelerated dissolution limits the wall thickness, for example, to around 9 nm in contrast with 18 nm of nanotubes made at RT. Along with the smaller wall thickness, the elevated temperature resulted in longer nanotubes with a smaller diameter. However, if the temperature is too high, nanotubes cannot form. Anodization of Ti was tried at 150 °C. In this case, the reaction appeared to be too fast and no nanotubes were observed. Figure 2A,B illustrates TEM images and corresponding electron diffraction patterns of TiO2 nanotubes fabricated at RT. The nanotubes are polycrystalline, consisting of both rutile and anatase phases, confirmed by generally observed discrete diffraction rings (Figure 2B) and an occasionally observed “near” single-crystal diffraction pattern illustrated in the insert of Figure 2A. The SAD size used for the TEM examination was in the range of 300
500 nm. In the diffraction rings of the rutile phase (Figure 2B), there are some weaker spots belonging to anatase. Similar diffraction ring patterns were observed in different locations. The observed crystallites were generally very small, in the range of 5
10 nm, estimated with HRTEM. Some crystallites occasionally grew up with larger sizes, evidenced by the “near” single-crystal diffraction pattern of anatase illustrated in the inset in Figure 2A, which could come from a few overlapped larger crystallites that might be oriented in similar or close crystallographic orientations. This suggests the existence of occasionally oversized crystallites in the nanotubes. The anatase and rutile phases are randomly distributed in the nanotubes as their electron diffraction patterns were observed in randomly selected areas. This has been further confirmed with high-resolution transmission electron microscopy (HRTEM). Figure 2C,D illustrates lattice images of rutile and anatase domains in the nanotubes made at room temperature, further confirming the one-step formation of crystalline nanotubes and the coexistence of rutile and anatase in the nanotubes. It should be mentioned that, though we have confirmed the existence of crystalline anatase and rutile phases based on the electron diffraction patterns and lattice images, it is difficult to precisely determine the geometry of these nanoscale crystallites in the nanotubes due to limited capability of the present TEM. More studies are needed in order to further characterize the nanotubes in terms of the crystallite size, phase distribution, and the ratio of anatase to rutile. Figure 2E,F illustrates a TEM image and corresponding electron diffraction pattern of TiO2 nanotubes fabricated at

Figure 3. Diffuse reflectance absorption spectra of TiO2 nanotubular arrays fabricated at RT (cyan) and 100 °C (sparse). The inset is a plot of the transformed Kubelka
Munk function vs the energy of light.

100 °C, which are mainly in the anatase state (confirmed by the diffraction rings in Figure 2F). There are some obscure and weak rings present in the diffraction pattern of nanotubes fabricated at 100 °C, which may possibly come from a small amount of residual rutile phase. On the basis of the structural characterization, the anodization temperature is an important factor that can vary the ratio of anatase to rutile in the nanotubes fabricated under the current condition, which leads to an energy band gap shift and the change in semiconductor type, as described in the following section. The success in the fabrication of crystalline TiO2 in the glycerol-based electrolyte could be attributed to a limited anodization rate when the oxidation occurred in this viscous electrolyte in which diffusion was slow. The viscosity of the electrolyte could play a crucial role in the formation of crystalline nanotubes by anodization. At lower temperatures, the viscosity of the electrolyte increased, which further slowed down the electrochemical oxidation of Ti foil as well as the chemical dissolution of the oxide layer. This may provide sufficient time for ions to migrate to the equilibrium positions or the crystalline lattice sites that correspond to the minimum free energy, thus facilitating the formation of crystallites. Photocatalytic properties of the fabricated TiO2 nanotubes were investigated. Figure 3 illustrates diffuse reflectance absorption spectra of as-anodized TiO2 nanotubes fabricated at both RT and 100 °C. As shown, the absorption edge of the nanotubes fabricated at 100 °C is about 362 nm, whereas the sample fabricated at RT shows a slight shift of its absorption edge in the direction of visible light. The inset is a plot of the transformed Kubelka
Munk function versus the energy of light. The point of intersection of the as-plotted line and x-axis indicates the band gaps of the as-prepared samples. As demonstrated, the nanotubes fabricated at 100 °C with a smaller diameter exhibit a larger band gap of 3.43 eV, compared with 3.27 eV of the nanotubes fabricated at RT. The determined band gaps explain the slight shift of the absorption edge of TiO2 fabricated at RT. It is known that rutile TiO2 has a narrower band gap than the anatase one; the existence of rutile in the TiO2 nanotubes fabricated at RT could be mainly responsible for the “red” shift of the absorption 21531

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Figure 4. Photocurrent responses of TiO2 nanotubes fabricated at 100 °C and RT.

edge. In other words, the “blue shift” of the absorption edge of the nanotubes fabricated at 100 °C is attributed to the fact that they are present mostly in the state of anatase. It is noticed that the determined band gaps of both the anatase and the rutile
anatase mixed nanotubes are larger than those of bulk TiO2:16 the band gap of anatase is about 3.2 eV for bulk and usually 3.3 eV for annealed or crystallized nanotubes, and that of rutile is about 3.0 eV. The larger band gap values determined in the present case could be attributed to the quantum size effect. It is known that the Coulombic interaction between electrons and holes cannot be neglected for nanoparticles. When the particle size decreases, the electron and hole are closer and have higher kinetic energies.17 In the case of semiconductors, the electron
hole pair can only “fit” into such a particle when the charge carriers are in a state of higher kinetic energy. This may result in splitting of energy bands into discrete quantized levels and the “band gap” increasing with decreasing the particle size.18 The quantum size effect is strong;23,24 for example, the band gaps of anatase crystallites with diameters around 21 and 12 nm are 3.14 and 3.29 eV, respectively.24 The relatively large band gaps as measured in this study are likely attributed to the size of very tiny crystallites, compared to those in annealed or well-crystallized nanotubes. Reddy and co-workers25 conducted band-gap studies on TiO2 nanoparticles. They estimated the size-dependent bandgap shift from the following equation26   p2 π 2 1 1 1:786e2
0:248E/Ry ΔE ¼ þ
mh 2R 2 me εR where p is the reduced Planck’s constant, R the radius of the crystallite, E/Ry the effective Rydberg energy, ε is the dielectric constant of anatase TiO2 = 86, and me and mh are the electron and hole masses, respectively. On the basis of this relationship, ΔEg for an average particle size of 5
10 nm is calculated to be around 0.1
0.2 eV, which is consistent with our experimental observations in this study. It should be mentioned that doping different elements, such as B, N, S, C, Sn, and Pt, also influences the band gap of TiO2 nanotubes.27,28 However, there was no element-doping effect during the present anodization processes at both RT and 100 °C,

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Figure 5. SPV spectra of as-anodized TiO2 nanotube arrays fabricated at (A) RT and (B) 100 °C.

since XPS analysis (not shown here) could not detect any other element except Ti and O in the nanotubes. Thus, the higher band gaps may mainly result from the quantum size effect, and the measured increase in the band gap with increasing the anodization temperature should be attributed to the corresponding dominance of anatase in the nanotubes made at the elevated temperature. Photocurrent responses of the fabricated nanotubes were evaluated under pulsed irradiation of UV
vis light in order to investigate the photon-induced charge separation efficiency of the nanotubes fabricated at different voltages without heat treatment. The working electrode potential was set at 0.1 V. SCE. As shown in Figure 4, the nanotubes fabricated at both 100 °C and RT responded to the UV
vis light. It was interesting to observe that the nanotubes fabricated at 100 °C showed an increase in photocurrent as the UV
vis light was turned on, which reflected the behavior of n-type semiconductors. The nanotubes made at RT, however, showed a sudden drop of photocurrent when the UV
vis light was turned on, which is a typical response of p-type semiconductors to the UV
vis light. The subsequent increase of the photocurrent after the drop may result from the recombination of electrons and holes during the measurement process. As discussed earlier, the RT-fabricated nanotubular arrays contain both anatase and rutile phases, which act as n-type and p-type semiconductors, respectively.18,29,30 Thus, the apparent p-type characteristic of the nanotube arrays synthesized at RT should be attributed to the higher fraction of rutile in the nanotubes. One may expect that nanoscale p
n junctions exist at the interface between anatase and rutile domains. It is known that p-type semiconductors have positive charge carriers, holes, which are “free” to move in the crystal lattice. n-type semiconductors have mobile negative carriers, electrons. Near a p
n junction, electrons diffuse from the n-type region across the p
n interface and combine with holes in the p-type region. As a result, the p-type region near the junction becomes positively charged while the n-type region carries localized positive charges, leading to the formation of an inner electric field at the p
n interface. For photocatalytic materials, such an inner electric field may help to separate photogenerated electron
hole pairs and reduce the recombination of electrons and holes, thus enhancing the photocatalytic efficiency.31 21532

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The Journal of Physical Chemistry C The respective n-type and p-type responses of the nanotubes are also supported by their surface photovoltage (SPV) spectra, as shown in Figure 5. The spectrum of the nanotubular arrays made at RT demonstrates that their surface photovoltage became more negative when the illumination photon energy was higher. However, the SPV of the nanotubular arrays fabricated at 100 °C increased with increasing the illumination photon energy. SPV is related to the charge transfer normal to the surface of a semiconductor.32 For n-type semiconductors (e.g., the nanotubular arrays fabricated at 100 °C), the Fermi level is closer to the bottom of the conduction band. When electrons from occupied surface states in the energy range of the forbidden band are excited by light and get into the conduction band (the space charge layer), the density of free electrons will increase and so will the normal of SPV. For p-type semiconductors (e.g., the nanotubular arrays fabricated at RT), their Fermi level is closer to the top of the valence band. When electrons in occupied surface states are excited by light and get into the valence band, the density of free holes will decrease and so will the normal of SPV. The measured photocurrent responses are consistent with the TEM observations that the nanotubes synthesized at both RT and high temperature are crystalline. In addition, the existence of the rutile phase, exhibiting the p-type semiconductivity,29,33 in the nanotubes fabricated at RT explains their apparent p-type semiconductor characteristics, as demonstrated in Figures 4 and 5. The successful one-step fabrication of both n-type and p-type semiconductive TiO2 nanotubular arrays makes it achievable to produce TiO2 nanotubular arrays with the coexistence of n-type and p-type semiconductive domains and nanoscale p
n interfaces or junctions through varying the electrolyte temperature during anodization. Such a combination of n-type and p-type semiconducting nanotubular arrays may have potential applications in TiO2 p
n heterojunction devices that consist of alternative layers of p-type and n-type TiO2 domains. The p
n heterojunction structure helps to reduce the recombination of photogenerated charge carriers.34 The coupling of anatase with rutile has demonstrated improved photocatalytic activity over individual anatase and rutile phases.35,36 Thus, the success in the one-step fabrication of crystalline TiO2 nanotubes and controllable combination of anatase and rutile structures would generate new opportunities for extending the application of TiO2 nanotubes with higher photocatalytic activity and efficiency.

’ CONCLUSIONS The one-step fabrication of crystalline TiO2 nanotubular arrays was successfully achieved by anodization of titanium foils in a NH4F-containing glycerol electrolyte at both RT and 100 °C under an anodization voltage of 50 V. Longer and thinner nanotubes with smaller diameter were fabricated at the elevated temperature. TEM examination showed that the nanotubes made at RT consisted of both anatase and rutile phases, whereas those made at 100 °C were mainly in the anatase state. The former exhibited apparent p-type semiconductor characteristics due to the presence of the rutile phase and the latter showed n-type semiconductor characteristics, evidenced by opposite trends of changes in photocurrent and SPV and the band-gap shift. This study has demonstrated that it is achievable to produce p
n junctions in the crystalline TiO2 nanotubes and the fractions of anatase and rutile could be adjusted by changing the anodization temperature, which would generate new opportunities

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for extended applications of TiO2 nanotubes with higher photocatalytic activity and efficiency.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +1-780-492-6750. Fax: +1-780-492-2881. E-mail: [email protected].

’ ACKNOWLEDGMENT This research was sponsored by the Natural Science and Engineering Research Council of Canada (NSERC). The authors thank Dr. Xinwei Cui, Dr. Huatao Wang, Mr. George Braybrook, and Ms. Deann Rollings for their help on SEM and TEM characterizations. ’ REFERENCES (1) Kim, E. Y.; Park, J. H.; Han, G. Y. J. Power Sources 2008, 184, 284. (2) Bai, J.; Li, J. H.; Liu, Y. B.; Zhou, B. X.; Cai, W. M. Appl. Catal., B 2010, 95, 408. (3) Joo, S.; Muto, I.; Hara, N. J. Electrochem. Soc. 2010, 157, 221. (4) Sennik, E.; Colak, Z.; Kilinc, N.; Ozturk, Z. Z. Int. J. Hydrogen Energy 2010, 35, 4420. (5) Xu, C. K.; Shin, P. H.; Cao, L. L.; Wu, J. M.; Gao, D. Chem. Mater. 2010, 22, 143. (6) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007, 7, 69. (7) Varghese, O. K.; Paulose, M.; LaTempa, T. J.; Grimes, C. A. Nano Lett. 2010, 10, 750. (8) Macak, J. M.; Zlamal, M.; Krysa, J.; Schmuki, P. Small 2007, 3, 300. (9) Zlamal, M.; Macak, J. M.; Schimuki, P.; Krysa, J. Electrochem. Commun. 2007, 9, 2822. (10) Macak, J. M.; Tsuchiya, H.; Taveira, L.; Ghicov, A.; Schmuki, P. J. Biomed. Mater. Res. 2005, 75A, 928. (11) Yang, Y.; Wang, X. H.; Li, L. T. J. Am. Ceram. Soc. 2008, 91, 632. (12) Varghese, O. K.; Gong, D. W.; Paulose, M.; Grimes, C. A.; Dickey, E. C. J. Mater. Res. 2003, 18, 156. (13) Allam, N. K.; Grimes, C. A. J. Phys. Chem. C 2009, 113, 7996. (14) Allam, N. K.; Grimes, C. A. Langmuir 2009, 25, 7234. (15) Nakato, Y.; Akanuma, H.; Shimizu, J.; Magari, Y. J. Electroanal. Chem. 1995, 396, 35. (16) Ferrari, A. M.; Szieberth, D.; Noel, Y. J. Mater. Chem. 2011, 21, 4568. (17) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13, 3843. (18) Weller, H. Angew. Chem., Int. Ed. 1993, 32, 41. (19) Miki, T.; Yanagi, H. Langmuir 1998, 14, 3405. (20) Liau, L. C. K.; Lin, C. C. Appl. Surf. Sci. 2007, 253, 8798. (21) Mowbray, D. J.; Martinez, J. I.; Lastra, J. M. G.; Thygesen, K. S.; Jacobsen, K. W. J. Phys. Chem. C 2009, 113, 12301. (22) Cai, Q. Y.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Mater. Res. 2005, 20, 230. (23) Lee, S.; Cho, I. S.; Noh, J. H.; Hong, K. S.; Han, G. S.; Jung, H. S.; Jeong, S.; Lee, C.; Shin, H. Phys. Status Solidi A 2010, 207, 2288. (24) Venkatachalam, N.; Palanichamy, M.; Murugesan, V. Mater. Chem. Phys. 2007, 104, 454. (25) Madhusudan Reddy, K.; Manorama, S. V.; Ramachandra Reddy, A. Mater. Chem. Phys. 2003, 78, 239. (26) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (27) Tang, X. H.; Li, D. Y. J. Phys. Chem. C 2008, 112, 5405. (28) Nam, W.; Han, G. Y. J. Chem. Eng. Jpn. 2007, 40, 266. (29) Savage, N.; Chwieroth, B.; Ginwalla, A.; Patton, B. R.; Akbar, S. A.; Dutta, P. K. Sens. Actuators, B 2001, 79, 17. (30) Lei, Y. F.; Leng, Y. X.; Yang, P.; Wan, G. J.; Huang, N. Sci. China, Ser. E 2009, 52, 2742. 21533

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(31) Lin, X. P.; Xing, J. C.; Wang, W. D.; Shan, Z. C.; Xu, F. F.; Huang, F. Q. J. Phys. Chem. C 2007, 111, 18288. (32) Kronik, L.; Shapira, Y. Surf. Sci. Rep. 1999, 37, 1. (33) Nowotny, M. K.; Bogdanoff, P.; Dittrich, T.; Fiechter, S.; Fujishima, A.; Tributsch, H. Mater. Lett. 2010, 64, 928. (34) Honda, H.; Ishizaki, A.; Soma, R.; Hashimoto, K.; Fujishima, A. J. Illum. Eng. Soc. 1998, 27, 42. (35) Kawahara, T.; Konishi, Y.; Tada, H.; Tohge, N.; Nishii, J.; Ito, S. Angew. Chem., Int. Ed. 2002, 41, 2811. (36) Liu, Z. Y.; Zhang, X. T.; Nishimoto, S.; Jin, M.; Tryk, D. A.; Murakami, T.; Fujishima, A. Langmuir 2007, 23, 10916.

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