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Tuning TiO2 Photoelectrochemical Properties by Nanoring/Nanotube Combined Structure Fang Wang,†,‡,§ Yang Liu,‡ Wen Dong,† Mingrong Shen,*,† and Zhenhui Kang*,‡ †

Jiangsu Key Laboratory of Thin Films and Department of Physics, Soochow University, Suzhou, China Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou, China § Department of Physics, Jiangsu University of Science and Technology, Zhenjiang, China ‡

bS Supporting Information ABSTRACT: The TiO2 nanoring/nanotube (R/T) combined structure was obtained by a simple two-step anodization. The as-prepared samples were characterized by X-ray diffraction, Raman, scanning electron microscopy, photoluminescence spectra, and UV vis absorption spectroscopy. The photoelectrochemical measurements demonstrate that the combined structure shows high photo-to-current conversion efficiency, fast charge transfer speed, and surface dominant photoelectrochemical response. The behavior of photoelctrochemical activity in TiO2 R/T combined structure and pure nanotubes (NTs) has been investigated through electrochemical impedance spectroscopy and photo-to-current conversion efficiency measurement. It is demonstrated that the formation of a heterojuction structure in R/T composite plays an important role in the kinetic behaviors (including charge separation efficiency and transport capability) of photogenerated charges. The TiO2 R/T combined structure greatly increases charge separation efficiency and photoconversion capability. We discuss the mechanism for the enhancement of TiO2 R/T combined structure photoelectrochemical activity and support our conclusions with ultraviolet photoelectron spectroscopy results.

1. INTRODUCTION Titanium dioxide (TiO2) is one of the widely investigated materials for solar cells, photonic crystals, photocatalysis, gas sensors, batteries, UV blockers, ion exchange, and electrochromic and self-cleaning devices because of its unique physical and chemical properties.1 4 The major obstacles to their effective utilization lie in poor photogenerated charge carrier separation and inefficient use of sunlight. To date, a variety of strategies have been utilized to improve TiO2 photoelectrochemical performance, such as coupling with secondary semiconductors, photosensitization of dyes, modification by metal and carbon nanoparticles, and doping with transition metals (Au, Pd, Pt, Rh, etc.) or nonmetal elements (N, S, I, F, etc.).5 However, most of them still suffer from the unstable nature and lack of further modification and functionalization abilities. Owing to the small dimensions of the constituent phases, nanoscaled materials possess intrinsically a large fraction of atoms (up to 40 50%) situated at surfaces or interfaces. The properties of nanoscaled materials are not only determined by those of the bulk phase but also to a considerable degree by surface and interface states. Notablely, for nanostructures with sizes not yet reaching quantum confinement region but small enough to exhibit enhanced surface effects, the large surface area makes them extremely active to their chemical environments and causes them to be particularly attractive in fields such as sensors, catalysis, and electrochemistry.6 r 2011 American Chemical Society

On the basis of the above discussion, it would be of interest to control or tune the TiO2 photoelectrochemical properties via dimensional control instead of modification and doping by heterogeneous species. Herein we present a TiO2 combined nanostructure of small-sized nanorings and nanotubes (R/T combined structure), which shows high photo-to-current conversion efficiency, fast charge transfer speed, and surface dominant photoelectrochemical response (see Scheme 1).

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Before electrochemical anodization, Ti foils (0.1 mm, 99.6% purity) were degreased by acetone, ethanol, and deionized water with ultrasonication for each 10 min and then dried in air. Synthesis of Pure TiO2 NTs. We prepared pure TiO2 NTs using one-step anodization (see Scheme S1, Supporting Information). The self-organized TiO2 nanotubes were fabricated by anodization of Ti foils in a mixed electrolyte of ethylene glycol and water (100:1 vol. %) + 1 wt % NH4F. Anodization was performed for 60 min at room temperature (20 °C) in a two-electrode configuration, which connected to a 60 V dc power supply with Received: April 19, 2011 Revised: June 13, 2011 Published: July 05, 2011 14635

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Scheme 1. Schematic Diagram for the Fabrication of TiO2 NTs and TiO2 R/T Combined Structurea

a

Tuning photoelectrochemical properties can be realized through the formation of TiO2 R/T combined structure.

titanium foil as the working electrode, and graphite rod as the counter electrode. Synthesis of TiO2 R/T Combined Structure. The two-step anodization was carried out for the synthesis of TiO2 R/T combined structure (see Scheme S2, Supporting Information). First, the self-organized TiO2 NTs were fabricated by anodization of Ti foils in a mixed electrolyte of ethylene glycol and water (100:1 vol. %) + 1 wt % NH4F. The anodization time and voltage were 24 h and 60 V, respectively. After the first anodic oxidation step, the as-prepared sample was rinsed with deionized water, and the textured Ti surface with rings pattern (TiO2 nanorings, TiO2 NRs) was obtained by removing the preformed TiO2 NTs layer by ultrasonic treatment. Then it was dried in an oven at 80 °C. Subsequently, the second anodization was performed using the obtained Ti foil with textured TiO2 NRs surface as anode for a short time (10 min, 20 min, etc.) under conditions identical to those in the first anodization step. Then the anodization will further take place on the Ti foil to form TiO2 NTs under the textured TiO2 NRs surface, leading to the TiO2 R/T combined structure on the surface of Ti foil. For the post-treatments, the resulting amorphous TiO2 samples (NTs and R/T combined structure) were finally calcined at 450 °C for 2 h at a heating rate of 2 °C/min in air for crystillization. 2.2. Ethylenediamine Reflux Experiment. First, two 10 mm wide 10 mm long  0.1 mm thick samples (TiO2 NTs and TiO2 R/T combined structure) were rinsed with deionized water, and followed by drying in air for several minutes. After that, they were added to 50 mL of ethylenediamine, and then heated to reflux under stirring. After the samples were allowed to reflux for 12 h, the nitrogen-modified TiO2 NTs and R/T combined structure were obtained, followed by washing with water and dried in air for further usage. 2.3. Characterization. The size and morphology of the samples were examined with a Philips XL30 FEG scanning electron microscope (SEM) at an accelerating voltage of 20 kV. X-ray diffraction (XRD) patterns, obtained on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) using Cu KR radiation at a scan rate (2θ) of 0.05° s 1, were used to characterize the sample crystalline phases. The accelerating voltage and the applied current were 40 kV and 80 mA, respectively. The Raman spectra were obtained with a Renishaw Raman system model 1000 spectrometer with a 20 mW air-cooled argon ion laser (514.5 nm) as the exciting source

Figure 1. (a) The SEM image of the NTs layer generated in the first step anodization. (b) The Ti surface after the removal of the NTs layer. (c) Top view of hexagon-shaped nanostructure obtained in the second anodization step. (d) The partial cross-section view of (c). (e) Amplified image of (c). (f) A schematic illustration of the inner structure of (e) indicating the hexagonal order of anodic TiO2 NT arrays (average ring diameter and pore diameter are 100 and 30 nm, respectively).

(the laser power at the sample position was typically 400 μW with an average spot size of 1 μm in diameter). The photoluminescence (PL) spectra were obtained at room temperature employing a Horiba JOBIN YVON luminescence spectrometer FluoroMax-4. UV visible diffuse reflectance spectra were obtained on a UV visible spectrophotometer (Lambda 750) equipped with an integrating sphere assembly. Fine BaSO4 powders were used as a reflectance standard. Ultraviolet photoelectron spectra (UPS) were obtained using a He lamp (21.2 eV). 2.4. Photoelectrochemical Measurements. Photoelectrochemical measurements were carried out using a conventional two-electrode, single-compartment glass cell fitted with a synthesized quartz window. The electrolytic cell was filled with 0.1 M Na2SO4. The TiO2 NTs or R/T combined nanostructure electrode served as the working electrode. The counter electrode was a platinum wire. For easy measurement, the Ti substrate of TiO2 NTs and R/T combined-nanostructure electrode was connected with a copper wire through high-purity silver conducting paint. To prevent photocurrent leakage, the copper wire and the Ti substrate were sealed by a nonconducting and nontransparent epoxy resin, excluding an area of 0.25 cm2 at the center of the substrate surface for absorbing the light. The electrochemical impedance spectroscopy measurement was performed using an IM6 electrochemical workstation in the frequency range from 1 Hz to 100 MHz. A 0.1 M Na2SO4 aqueous solution was used as the electrolyte.

3. RESULTS AND DISCUSSION 3.1. General Characterization. The morphologies of the TiO2 NTs and R/T combined structure were characterized using SEM. In our experiments, by first step anodization, the TiO2 NTs 14636

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Figure 2. (a) X-ray diffraction patterns of (a) TiO2 NTs and (b) TiO2 R/T combined structure (A, anatase; T, titanium). (b) Raman spectra of (a) TiO2 NTs and (b) TiO2 R/T combined structure.

can be easily formed on the Ti foil.7 As shown in Figure 1a, the TiO2 NTs have a diameter of approximately 100 nm, and a wall thickness of about 20 nm. After the TiO2 NTs are peeled off by ultrasonic treatment in deionized water, the concave cells with the same size and typical hexagonal shape are left on the Ti foil (shown in Figure 1b). Such hexagonal ordered imprints are composed of Ti oxides having higher electrical resistance than other locations in the Ti foil surface, which will strongly affect the further anodization behavior of Ti foil.7c After second anodization, the TiO2 R/T combined structure can be obtained (shown in Figure 1c) on the surface of Ti foil. The sizes of the hexagonal nanoring are in good agreement with those of the imprints in Figure 1b, suggesting the NRs obtained by the second-step anodization were directly developed from the imprint pattern. From the cross section view of the sample (Figure 1d), it is obvious that the combined structure is an assembly array of NTs covered by NRs. Typical TiO2 NRs are highlighted by a hexagon in Figure 1e, and the schematic structure details of the TiO2 R/T combined structure are shown in Figure 1f. Close observation shows that the edge thickness of TiO2 hexagonal NRs (with size about 100 nm) is about 10 nm, while the average diameter TiO2 NTs is about 30 nm, which is a continuous second-level nanostructure. XRD was used to identify the phase structure of the samples. Figure 2a shows the XRD pattern for TiO2 NTs (a) and TiO2 R/T combined structure (b). Quantitative analyses of these patterns show that all peaks can be indexed to the TiO2 anatase phase and Ti metal phase, respectively. The peaks of the anatase phase are from the TiO2 NTs and NRs, and those of the metal Ti phase are from the Ti substrate. Figure 2b shows the Raman spectra of TiO2 NTs in comparison with that of TiO2 R/T combined-structure in the 100 1000 cm 1 region. According to the symmetry group analysis, anatase TiO2 has 15 optical modes in Raman. Modes A1g (519 cm 1), B1g (399 and 519 cm 1), and Eg (144, 197, and 639 cm 1) are Raman-active modes of anatase and thus six fundamental transitions are expected in the Raman spectrum of anatase.8 Meanwhile, the four modes A1g (612 cm 1), B1g (143 cm 1), B2g (826 cm 1), and Eg (447 cm 1) are the Raman-active modes of rutile.9 Peaks observed in the Raman spectra of the TiO2 R/T combined structure are similar to pure TiO2 NTs and are corresponding to the TiO2 anatase phase. No characteristic rutile peaks were observed in the spectra. 3.2. Optical Properties. Figure 3a depicts the UV vis absorption spectra of the pure TiO2 NTs and R/T combined structure. The direct band gap energy can be estimated from the intercept of the tangents to the plots of (Rhν)2 vs photon energy,

as shown in Figure 3b. The optical band gaps, obtained by extrapolating a line from the maximum slope of the curve to the x axis, are about 3.3 and 3.2 eV for R/T and NTs, respectively. Although we cannot exactly get the band gap value of the R/T combined structure by this way, the present blue shift in the value of R/T to NTs indicates a small size effect in the NR structure where the thickness is about 10 nm, corresponding to the finding by Choi’s group.10 Figure 4a shows the PL spectra recorded at room temperature for the two samples. The black and red lines are the PL spectra of TiO2 NTs and R/T combined structure, respectively, by excitation at 260 nm. The peak varies from 400 nm for NTs to 350 nm for R/T. This variation also arises from the small-size effect of the NRs, which reveals that the energy gap between the conduction and valence bands exhibits a blue shift with decreasing size.11 Further experiments show that the obtained TiO2 R/T combined structure possesses high photo-to-current conversion efficiency. To evaluate their photoelectrochemical properties, the TiO2 NT array and R/T combined structure (samples shown in panels a and c of Figure 1) were employed as photoanodes in a two-arm photoelectrochemical cell with 0.1 M Na2SO4 as the electrolyte and Pt wire as the counter electrode. The TiO2 NTs array and R/T combined-structure anodes were illuminated by monochromatic light, respectively, and the incident photon to charge carrier efficiency (IPCE) spectra of the two TiO2 electrodes are shown in Figure 4b (black and red lines for TiO2 NTs array and R/T combined structure, respectively). The red line shows two onset wavelengths, while only one onset wavelength (around 400 nm) is shown for the black line. After a detailed data analysis by peak fitting with Gaussian Lorentz functions (see green lines), the red line indicates that the two onset wavelengths are at around 350 and 400 nm, respectively. The onset wavelength at around 400 nm (for both red and black lines) is attributed to the band gap energy of TiO2, corresponding to the NT structure in TiO2 NT array and R/T combined structure. The other onset wavelength at around 350 nm (see red line and green lines) should be attributed to the NR structure in TiO2 R/T combined structure, which is consistent with that of the PL peak observed in Figure 4a. Significantly, the maximum IPCE of the TiO2 R/T combined structure was about 12% at 350 nm, while only 7% IPCE was obtained for the TiO2 NTs array electrode at 400 nm. The result indicates that the photoresponse activity of the TiO2 combined structure is found to be directly related to its surface nanorings with higher surface area than nanotubes. This approximately factor of 2 enhancement of the photocurrent conversion in the range of 300 400 nm indicates the feasibility of efficient utilization 14637

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Figure 3. (a) UV vis absorption spectra of TiO2 NTs (black line) and R/T combined-structure (red line). (b) The corresponding (Rhν)2 vs hν curve of TiO2 NTs (black line) and R/T combined structure (red line).

Figure 4. (a) PL spectra of TiO2 NTs (black line) and R/T combined structure (red line). (b) IPCE spectra of TiO2 NTs (black line) and R/T combined structure (red line) recorded under zero applied external bias. The green lines are the data analysis of red line by peak fitting with Gaussian Lorentz functions. The arrows denote characteristic peaks of NRs and NTs.

Figure 5. (a, b) EIS spectra of TiO2 R/T combined structure and NTs, respectively.

of sunlight based on another promising strategy: narrow spectra wavelength range with high conversion efficiency. 3.3. Photoelectrochemical Properties. In the followed experiments, this kind of TiO2 R/T combined structure also shows fast charge transfer speed. A detailed EIS measurement was carried out by using different TiO2 nanostructures (TiO2 NTs or R/T combined structure) as the working electrodes in solution, and the typical EIS responses are shown in panels a and b of Figure 5, respectively. The radius of the arc on the EIS Nyquist plot manifests the reaction rate occurring at the surface of electrode. The arc radius on the EIS Nyquist plot of TiO2 R/T heterostructure (Figure 5a) is smaller than that of the TiO2 NTs array (Figure 5b), indicating an effective separation of photogenerated electron hole pairs, and fast interfacial charge transfer

to the electron donor/electron acceptor occurs, as suggested by Leng et al.12 Both IPCE spectra and EIS responses reveal that the TiO2 R/T combined structure significantly increases the cross section of light harvesting and maintains a better contact with electrolytes when compared with those of the TiO2 NT array.13 These phenomena imply that the TiO2 R/T combined structure has great potential applications in solar cells for efficiency improvement. 3.4. Modification of TiO2 R/T Combined Structure Properties. As expected, the small-sized TiO2 R/T combined structure also shows an enhanced surface effect, i.e., surface dominant photoelectrochemical response. To change the surface chemical state, we further subject the obtained TiO2 R/T combined structure to a simple reflux treatment with ethylenediamine. 14638

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Figure 6. (a, b) IPCE spectra of TiO2 R/T combined structure electrode and pure TiO2 NTs electrode before (black line) and after (red line) ethylenediamine reflux recorded under no applied external bias. The inset is the corresponding PL spectra of TiO2 R/T combined structure electrode and pure TiO2 NTs electrode before (black line) and after (red line) ethylenediamine reflux. The 10 nm red shift corresponds to the change of NR surface.

Figure 7. (a) The UPS spectra of TiO2 NTs and TiO2 R/T combined structure. The inset shows the low kinetic-energy cutoff, from which the work functions for NTs and R/T are calculated to be 3.97 and 4.02 eV, respectively. (b) Schematic energy-level diagram for the TiO2 NRs and NTs.

The corresponding IPCE and PL spectra of the TiO2 R/T combined structure before and after ethylenediamine refulx treatment are shown in Figure 6a (the black and red lines for before and after reflux treatment, respectively). Interestingly, for the TiO2 R/T combined structure after reflux treatment, there is a 10 nm red shift in both PL (from 350 to 360 nm) and IPCE (for peak at about 310 nm corresponding to NR structure in TiO2 R/T combined structure, from 310 to 320 nm) spectra peaks, comparing to the sample before reflux treatment. This red shift phenomenon simultaneously observed in the IPCE and PL spectra was ascribed to the surface state change of the NRs by the ethylenediamine modification, leading to the band gap change. In contrast, the IPCE peak at about 360 nm does not change even after the refulx treatments with ethylenediamine. According to the above discussion (see Figure 4b), the IPCE peak at 360 nm is attributed to the spectra response of the TiO2 NTs. To further confirm whether the surface modifations have effect on the IPCE peak of the TiO2 NTs structure, we also compared the IPCE spectra of the TiO2 NTs array before and after ethylenediamine refulx treatment (see Figure 6b) and found no obvious change for such a peak. Then it can be concluded that the surface-dominant photoelectrochemical response of TiO2 R/T combined structure mainly originated from the surface modification of the NRs in the TiO2 R/T combined structure.14 The results also show that this kind of TiO2 R/T combined structure is a promising candidate with tunable capability for photocatalyst and the UV light detector design.

To further test whether the TiO2 R/T combined structure prepared by the two-step anodization method has improved photocatalytic activity for degradation of pollutants, we have also investigated the performance of the as-prepared TiO2 R/T combined structure for degradation of methyl orange. We also find that the TiO2 R/T combined structure shows better performance than the pure TiO2 NTs. The photocatalytic performance observation is consistent with the findings of Zhang’s group.7b The presence of small sized surface NRs with higher surface area than NTs is able to control the as-formed TiO2 NTs photoelectrochemical properties, which increases the cross section of light harvesting and leads to enhanced photocatalytic activity. 3.5. Mechanism of Improved Photoelectrochemical Performance. The transport mechanism of the TiO2 R/T combined structure can be understood from an energy-level diagram, which is shown in Figure 7b. We measured UPS spectra of pure TiO2 NTs and TiO2 R/T combined structure, respectively, to determine the corresponding work functions of the two samples. Figure 7a displays UPS spectra of pure TiO2 NTs and TiO2 R/T combined structure. The valence bands of both samples are approximately 3 9 eV below the Fermi level. The large structure between 10 and 16 eV is assigned to secondary electron emission. From the UPS data, the optical band gap can be estimated from the energy difference between the Fermi level and valence band maximum. The work functions of both samples can be derived from the low kinetic energy cut-offs in the secondary emission 14639

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The Journal of Physical Chemistry C features. Work functions of 3.97 and 4.02 eV were determined for NTs and R/T, respectively. Although we cannot exactly get the work function of the NR structure by this way, it can be seen that the offset (∼0.05 eV) of the work function between the NRs and NTs is sufficient for charge separation. Therefore, the NRs structure serves as the electron transporter for effective charge separation and rapid transport of the photogenerated electrons, which is demonstrated in Figure 7b.

4. CONCLUSIONS The present work suggests that it would be of interest and possible to improve the synthetic method to obtain various smallsized TiO2 combined structures with higher photo-to-current conversion efficiency and faster charge transfer speed. However, much is required to understand and control the surface-dominant photoelectrochemical properties of this kind of TiO2 R/T combined structure. Better additive candidates (organic molecule/ ligand or inorganic clusters) may be beneficial to the present surface modification process and increasing the corresponding IPCE peak red shift of TiO2 R/T combined strucure. In summary, we have fabricated the TiO2 R/T combined structure by simple two-step anodization of a Ti foil. Such kind of small-sized combined structure exhibited excellent photoelectrochemical properties (high IPCE efficiency, fast charge transfer speed, and PL and IPCE spectra tuned by simple chemical surface modification) of TiO2. Our results show that the heterojuction from the NRs and NTs plays an important role in determining the photoelectrochemical properties of the combined structure. The present synthetic strategy is expected to be a general method to control and tune the properties of nanosized semiconductors.

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(5) (a) Li, D.; Chen, Z.; Chen, Y.; Li, W.; Huang, H.; He, Y.; Fu, X. Environ. Sci. Technol. 2008, 42, 2130. (b) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (c) Li, D.; Haneda, H.; Labhsetwar, N. K.; Hishita, S.; Ohashi, N. Chem. Phys. Lett. 2005, 401, 579. (6) (a) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (b) Liu, C. M.; Yang, S. H. ACS Nano 2009, 3, 1025. (c) Ghicov, A.; Schmuki, P. Chem. Commun. (Cambridge, U.K.) 2009, 2791. (7) (a) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nanotechnology 2007, 18, 065707. (b) Zhang, G. G.; Huang, H. T.; Zhang, Y.; Chan, H. L. W.; Zhou, L. M. Electrochem. Commun. 2007, 9, 2854. (c) Li, S.; Zhang, G.; Guo, D.; Yu, L.; Zhang, W. J. Phys. Chem. C 2009, 113, 12759. (d) Shin, Y.; Lee, S. Nano Lett. 2008, 8, 3171. (e) Meng, X.; Lee, T.-Y.; Chen, H.; Shin, D.-W.; Kwon, K.-W.; Kwon, S. J.; Yoo, J.-B. J. Nanosci. Nanotechnol. 2010, 10, 4259. (8) Oksaka, T.; Izumi, F.; Fujiki, Y. J. Raman Spectrosc. 1978, 7, 321. (9) Hara, Y.; Nicol, M. Phys. Status Solidi B 1974, 94, 317. (10) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (11) (a) Kang, Z. H.; Tsang, C. H. A.; Zhang, Z. D.; Zhang, M. L.; Wong, N. B.; Zapien, J. A.; Shan, Y. Y.; Lee, S. T. J. Am. Chem. Soc. 2007, 129, 5326. (b) Kang, Z. H.; Tsang, C. H. A.; Wong, N. B.; Zhang, Z. D.; Lee, S. T. J. Am. Chem. Soc. 2007, 129, 12090. (c) Kang, Z. H.; Liu, Y.; Tsang, C. H. A.; Ma, D. D. D.; Fan, X.; Wong, N. B.; Lee, S. T. Adv. Mater. 2009, 21, 661. (d) Li, H. T.; He, X. D.; Kang, Z. H.; Huang, H.; Liu, Y.; Liu, J. L.; Lian, S. Y.; Tsang, C. H. A.; Yang, X. B.; Lee, S. T. Angew. Chem., Int. Ed. 2010, 49, 4430. (12) Leng, W. H.; Zhang, Z.; Zhang, J. Q.; Cao, C. N. J. Phys. Chem. B 2005, 109, 15008. (13) Zhang, L. W.; Wang, Y. J.; Cheng, H. Y.; Yao, W. Q.; Zhu, Y. F. Adv. Mater. 2009, 21, 1286. (14) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Science 2011, 331, 746.

’ ASSOCIATED CONTENT

bS

Supporting Information. Flow diagrams of the one-step and two-step anodization processes for the synthesis of pure TiO2 NTs and TiO2 R/T combined structure, respectively. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT This work is supported by the National Basic Research Program of China (973 Program) (No. 2010CB934500), the National Natural Science Foundation of China (NSFC) (No. 21073127, 21071104, 20801010, 20803008, 91027041, and 11004146), the Foundation for the Author of National Excellent Doctoral Dissertation of P R China (FANEDD)(No. 200929), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). ’ REFERENCES (1) Fujishima, A.; Honda, K. Nature 1972, 37, 238. (2) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (3) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (4) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. 14640

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