CdS Quantum Dots-Sensitized TiO2 Nanorod Array on Transparent

Sep 7, 2010 - In the as-prepared structure (Scheme 1), the TiO2 nanorod array was grown .... The enhanced (002) peak indicates that the nanorods are w...
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J. Phys. Chem. C 2010, 114, 16451–16455

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CdS Quantum Dots-Sensitized TiO2 Nanorod Array on Transparent Conductive Glass Photoelectrodes Hua Wang,† Yusong Bai,† Hao Zhang,‡ Zhonghao Zhang,† Jinghong Li,*,‡ and Lin Guo*,† School of Chemistry and EnVironment, Beijing UniVersity of Aeronautics and Astronautics, Beijing 100191, People’s Republic of China, and Department of Chemistry, Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: May 9, 2010; ReVised Manuscript ReceiVed: August 20, 2010

An oriented single-crystalline TiO2 nanorod or wire array on transparent conductive substrates would be the most desirable nanostructure in preparing photoelectrochemical solar cells because of its efficient charge separation and transport properties as well as superior light harvesting efficiency. In this study, a TiO2 nanorod array film grown directly on transparent conductive glass (FTO) was prepared by a simple hydrothermal method. The formation of CdS quantum dots (QDs) on the vertically aligned TiO2 nanorods photoelectrode was carried out by chemical bath deposition. The as-prepared materials were characterized by scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, and X-ray diffraction. The results indicate that CdS QDs with a diameter smaller than 10 nm are uniformly covered on the surface of the single-crystalline TiO2 nanorods. Under AM 1.5 G illumination, the photoelectrode was found with a photocurrent intensity of 5.778 mA/cm2 at a potential of 0 V versus Ag/AgCl and an open-circuit photovoltage of 1.292 V versus Ag/AgCl. The photocurrent is 28.6 times higher than that of a bare TiO2 nanorod array, and the photoelectrochemical properties are comparable to those of a CdS QDs-sensitized TiO2 nanotube array, suggesting that the CdS QDs-sensitized TiO2 nanorod array on FTO photoelectrodes has a potential application in solar cells. 1. Introduction TiO2 is one of the most important wide gap semiconductors and is widely investigated for use in water photoelectrolysis,1 photocatalysis,2,3 gas sensing, and photoelectrochemical cells.4-10 The one-dimensional (1D) TiO2 nanostructure has attracted much attention in preparing photoelectrochemical cells such as quantum dots (QDs)-sensitized solar cells because of its efficient charge separation and transport properties.11-16 With the same weight of TiO2, the photoelectrochemical properties of a CdS QDs-sensitized TiO2 nanotube array are better than those of a CdS QDs-sensitized TiO2 nanoparticles film.16 Most current research of QDs-sensitized 1D TiO2 nanostructure photoelectrochemical cells focuses on a polycrystalline TiO2 nanotube array or single-crystalline TiO2 nanorod/nanowire array on metal titanium foils.11-17 For a polycrystalline TiO2 nanotube array or single-crystalline nanorod/nanowire array grown on metal titanium foils, their ability to improve the performance of photoelectrochemical solar cell is suppressed by the low light harvesting efficiency, because the metal titanium foils are nontransparent, and the photoelectrodes can only be illuminated from the front side. Furthermore, a polycrystalline TiO2 nanotube still has electron scattering and trapping at grain boundaries and shows only a marginally improved electron transport rate.18,19 So, a single-crystalline TiO2 nanorod/nanowire array grown directly on a transparent conductive substrate is considered one of the most desirable nanostructures for solar cells, but its application is confined by the difficulty in actual * To whom correspondence should be addressed. Tel/Fax: +86-1082338162. E-mail: [email protected] (J.L.) or [email protected] (L.G.). † Beijing University of Aeronautics and Astronautics. ‡ Tsinghua University.

synthesization. Inspiringly, Grimes and Aydil have recently synthesized single-crystalline TiO2 nanowires and nanorods on FTO by a facile hydrothermal method, respectively,20,21 and further research of the nanostructures is being taken by the followers,22-24 but QDs sensitization of this kind of nanostructure has rarely been investigated.24 QDs-sensitized TiO2 can extend the light absorbance to the visible light region and then improve the photoelectrochemical efficiency, so the investigation of this novel QDs-sensitized TiO2 nanostructure should be significant. In this work, we prepared a CdS QDs-sensitized TiO2 nanorod array on FTO photoelectrodes by a simple chemical method and investigated their photoelectrochemical properties. In the as-prepared structure (Scheme 1), the TiO2 nanorod array was grown directly on FTO, and CdS QDs were deposited on TiO2 nanorods. The matching of band edges between CdS and TiO2 was important to form a type II heterojunction. For the transparence of FTO, more CdS QDs can be illuminated by incident light from the back side of the photoelectrode, and single-crystalline TiO2 nanorods have superior electron separation and transport properties, resulting in high photoelectrochemical properties achievement. 2. Experimental Section 2.1. TiO2 Nanorod Array on FTO Substrate. The TiO2 nanorod array was prepared using a hydrothermal synthesis reported previously.21 Briefly, 12 mL of deionized water was mixed with 12 mL of concentrated hydrochloric acid (mass fraction 36.5-38%). The mixture was stirred at ambient conditions for 5 min before 0.4 mL of titanium butoxide was added (Beijing Chemical Co.). After it was stirred for another 5 min, the mixture was placed in a Teflon-lined stainless steel autoclave of 45 mL volume. Then, one piece of FTO substrate

10.1021/jp104208z  2010 American Chemical Society Published on Web 09/07/2010

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SCHEME 1: Schematic of the CdS QDs Deposited on TiO2 Nanorod Array Electrode, Energy Band of CdS-TiO2 Coupled Semiconductor, and Photoinduced Charge Separation and Transport in the Composite Structuresa

a In the as-prepared structure, TiO2 forms a type II heterojuction with CdS. Once photoexcited, an electron in the CdS QD lies above the conduction band edge of the TiO2 and then decreases its energy by transferring in the TiO2 nanorod to the collected FTO substrate.

(F:SnO2, 14 Ω/square, Nippon Sheet Glass Group, Japan), ultrasonically cleaned for 60 min in a mixed solution of deionized water, acetone, and 2-propanol (volume rations of 1:1:1), was placed at an angle against the wall of the Teflon liner with the conductive side facing down. The hydrothermal synthesis was conducted at 150 °C for 20 h in an electric oven. After synthesis, the FTO substrate was taken out, rinsed extensively with deionized water, and allowed to dry in ambient air. 2.2. CdS QDs Deposition on TiO2 Nanorod Array. CdS QDs were deposited on TiO2 nanorods through a sequential chemical bath deposition (CBD) method.25 Typically, the TiO2 nanorod array substrate was immersed in a solution of Cd(NO3)2 (0.05 M) for 2 min, then rinsed with deionized water, and immersed in a Na2S (0.05 M) solution for another 2 min followed by another rinsing with deionized water. Such an immersion cycle was repeated several times until the desired deposition of CdS QDs was achieved. 2.3. Characterization. The morphology of the sample was studied by a field-emission gun scanning electron microscope (Hitachi S-4800, 5 KV). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) investigations were carried out by a JEOL JEM-2100F microscope. The TiO2 sample was detached from the FTO substrate, then dispersed in ethanol, and dropped onto a carbon film supported on a copper grid. Energy dispersive spectroscopy (EDS) equipped on TEM was used to analyze the composition of the structure. The X-ray diffraction (XRD) spectra of the samples were recorded by a Rigaku Dmax 2200 X-ray diffractometer with Cu KR radiation (λ ) 1.5416 Å). Diffuse reflectance absorption spectra of bare TiO2 nanorod array and TiO2 nanorod array deposited with CdS QDs for different cycles were recorded in the range from 350 to 650 nm using a Hitachi U-3010 spectroscopy with BaSO4 as a reference. The photoluminescence (PL) spectra were collected at room temperature by a LabRAM HR800 (HORIBA Jobin Yvon) confocal Raman Spectrometer, with an excited wavelength at 325 nm using He-Cd Laser. 2.4. Photoelectrochemical Measurements. The photocurrent intensity versus measured potential (I-V curve) measurements were performed in a standard three-electrode configuration. The as-prepared CdS QDs-sensitized TiO2 nanorod array on FTO electrode was used as the working electrode, a platinum wire as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode. One M Na2S aqueous solution was used as the electrolyte. The working electrode was illuminated from

Wang et al. the back side within an area about 0.2 cm2 with a solar-simulated light source (AM 1.5 G filtered, 100 mW/cm2, 69911, Oriel). The photoelectrochemical response of the sample was measured by an electrochemical analyzer (CHI 660C Instruments, United States) under the irradiation. Photocurrent action spectra and incident photo to current conversion efficiency (IPCE) measurements were performed with a 500 W xenon lamp with a monochromator. The photoelectrodes were illuminated from the back side within an area about 0.12 cm2 by incident light. The generated photocurrent signals were collected by using a lock-in amplifier (standford instrument SR830 DSP) synchronized with a light chopper (standford instrument SR540). The monochromatic illuminating light intensity was about 15 µW/cm2 at 475 nm estimated with a radiometer (Photoelectronic Instrument Co. IPAS). 3. Results and Discussion 3.1. Characterization of CdS QDs-Sensitized TiO2 Nanorod Array. The highly ordered, single-crystalline rutile TiO2 nanorod array on FTO substrates (denoted as FTO/TiO2) was synthesized through a simple hydrothermal method. CdS QDs were deposited on the TiO2 nanorods (denoted as FTO/TiO2/ CdS) through a sequential CBD method. Morphologies of FTO/ TiO2 and FTO/TiO2/CdS electrodes are shown in Figure 1. Figure 1A is a typical SEM image of the FTO/TiO2 electrode. The top facets of rods are square, which show the expected growth habit of the tetragonal crystal. The top facets of the nanorods appear to contain many step edges, which are the substrates for further growth of the nanorod, while the side facets are smooth. Figure 1B is a low magnification of the FTO/TiO2 electrode, which reveals that the entire surface of the FTO substrate is covered uniformly with TiO2 nanorods. Figure 1C is a SEM image of a cross-sectional view of the same sample, and the inset represents a higher magnification of such an array, showing that the nanorods are nearly perpendicular to the FTO substrate and about 3 µm in length, and the average diameter of nanorods is 250 nm at the bottom and 100 nm on the top, which decreases gradually from bottom to top. After deposition with CdS QDs for 30 cycles, the ordered TiO2 nanorod array structure was retained, as shown in Figure 1D. Figure S1 in the Supporting Information shows a TiO2 nanorod array deposited with CdS QDs for different cycles from 5, 10, and 20 to 30. Closer observation reveals that an increased amount of CdS QDs adsorbed on TiO2 nanorods accompanied the increase in CBD cycles. After 30 cycles of deposition, the entire surface of the nanorods is covered with CdS crystallites. Figure 1E,F shows a HRTEM image and the corresponding selected area electron diffraction (SAED) pattern of a bare TiO2 nanorod array, respectively, confirming that the nanorods are single-crystalline. Lattice fringes with interplanar spacing d110 ) 3.2 Å and d001 ) 2.9 Å are consistent with the tetragonal rutile phase (JCPDS card no. 21-1276). The nanorods grow along the (110) crystal plane with a preferred (001) orientation. Figure 1G is the typical TEM image of a TiO2 nanorod array deposited with CdS for 30 CBD cycles, showing that a large amount of CdS nanocrystallites have been deposited on the TiO2 nanorods. The diameter of CdS nanocrystallites is smaller than 15 nm, and each nanocrystallite consists of a number of QDs with a diameter smaller than 10 nm. Figure 1H is the HRTEM image of the sample. The observed lattice spacing of 0.319 nm in the central part of the image corresponds to the (110) plane of tetragonal rutile TiO2. The observed 0.206 and 0.336 nm fringes of the QDs on the nanorod correspond to the (111) and

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Figure 2. XRD of FTO/TiO2 (a) and FTO/TiO2/CdS (b) electrodes.

Figure 3. Diffuse reflectance absorption spectra of FTO/TiO2 electrode and FTO/TiO2/CdS electrodes with CdS QDs deposition for different cycles by CBD method.

Figure 1. Morphologies of FTO/TiO2 and FTO/TiO2/CdS electrodes: typical top view SEM images of FTO/TiO2 electrode at (A) high and (B) low magnifications; (C) a cross-sectional view of the well-aligned TiO2 nanorod array, the inset represents a higher magnification of such array; (D) a typical SEM image of FTO/TiO2/CdS electrode; panels E and F are a HRTEM image and the corresponding selected area electron diffraction (SAED) pattern of bare TiO2 nanorod array, respectively, and the inset in panel E is a typical TEM image of the single bare TiO2 nanorod; and panels G and H are a TEM image and the corresponding HRTEM image of a single TiO2 nanorod deposited with CdS QDs for 30 cycles, respectively.

(220) planes of the cubic phase of CdS, respectively (JCPDS no. 80-0019). Energy dispersive spectroscopy (EDS) was applied to determine the composition of the nanostructure. In the spectrum (Figure S2 in the Supporting Information), Ti and O peaks result from the TiO2 nanorod array, and the 1:1 molar ratio of Cd to S in different samples tested confirms the stoichiometric formation of CdS. Figure 2 shows XRD patterns of the FTO/TiO2 and FTO/ TiO2/CdS electrodes. XRD patterns (Figure 2, curve a) of FTO/ TiO2 electrode reveal that the TiO2 nanorods have a tetragonal rutile structure. The enhanced (002) peak indicates that the nanorods are well crystallized and grow in [001] direction with the growth axis parallel to the FTO substrate. SnO2 glass substrates have rutile structure (JCPDS no. 41-1445). As compared with curve a, additional peaks appear in curve b, which may be attributed to cubic CdS phase. The XRD peaks corresponding to the Cubic CdS phase are broad, suggesting that the size of the deposited CdS crystallites on the surface of the TiO2 nanorods is very small. 3.2. Optical and Photoelectrochemical Properties of CdS QDs-Sensitized TiO2 Nanorod Array. Diffuse reflectance absorption spectra of FTO/TiO2 electrode and FTO/TiO2/CdS

Figure 4. I-V curves of FTO/TiO2 (a) photoelectrode, FTO/CdS (b), and FTO/TiO2/CdS (c) photoelectrodes with CdS QDs deposition for 30 cycles. The photoelectrodes were measured versus Ag/AgCl under simulated sunlight with an illumination intensity of 100 mW/cm2 in 1 M Na2S aqueous solution.

electrodes with CdS QDs deposition for different cycles are shown in Figure 3. The FTO/TiO2 electrode can absorb only ultraviolet light with wavelength smaller than 410 nm. For FTO/ TiO2/CdS electrodes, the light absorbance extends to visible light region, and the absorbance of the spectra increases with an increase in CBD cycles, indicating an increased adsorption amount of CdS. The smaller CdS crystallites grow and aggregate into larger crystallites accompanied with the increased adsorption amount of CdS, but excessive growth and aggregation of CdS crystallites will increase the chance of the recombination of the photoelectrons and holes, which will result in the decrease of the photoelectrochemical properties. Figure 4 shows the I-V curves of FTO/TiO2 photoelectrode, FTO deposited with CdS QDs (denoted as FTO/CdS), and FTO/ TiO2/CdS photoelectrodes with CdS QDs deposition for 30 cycles, respectively. For the FTO/TiO2 photoelectrode, the opencircuit photovoltage is 0.654 V versus Ag/AgCl, while those of FTO/CdS and FTO/TiO2/CdS photoelectrodes are 0.942 and 1.292 V, respectively. The open-circuit photovoltage of the FTO/

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Wang et al. photocurrent intensity and incident light intensity at different illuminating wavelengths using the equation:

IPCE (%) )

Figure 5. Photocurrent action spectra (A) and incident photon to current conversion efficiency (IPCE) spectra (B) of FTO/TiO2 (a) photoelectrode, FTO/CdS (b), and FTO/TiO2/CdS (c) photoelectrodes with CdS QDs deposition for 30 cycles, respectively. The monochromatic illuminating light intensity was about 15 µW/cm2 at 475 nm. The illumination area of the electrodes was about 0.12 cm2.

TiO2/CdS photoelectrode is obviously higher than those of the other two, demonstrating a shift in Fermi level to more negative potential as a result of coupling between TiO2 and CdS QDs in the composite system. The photocurrent intensity is 5.778 mA/ cm2 for the FTO/TiO2/CdS photoelectrode at a potential of 0 V versus Ag/AgCl and 0.202 and 0.622 mA/cm2 for FTO/TiO2 and FTO/CdS, respectively. The photocurrent intensities of the FTO/TiO2/CdS photoelectrode are 28.6 and 9.3 times higher than those of FTO/TiO2 and FTO/CdS. It is enhanced by the larger supported surface areas of the TiO2 nanorod array to adsorb CdS QDs, which can increase visible light absorption to generate a photocurrent. Second, with the transparent FTO substrate, the photoelectrode is illuminated from the back side, and the photoelectrons that transfer length to the collect substrate are short, which can effectively suppress the recombination losses;26,27 therefore, more CdS QDs can be induced to generate photoelectrons. Third, the single-crystalline structure of the nanorods allows a fast and efficient transfer of the photogenerated electrons from CdS QDs to the collected FTO substrate, which can effectively suppress the recombination of electrons and holes. This assumption could be confirmed by the PL results in Figure S3 in the Supporting Information. Photocurrent action spectra of FTO/TiO2 (curve a), FTO/CdS (curve b), and FTO/TiO2/CdS (curve c) photoelectrodes are shown in Figure 5A. The FTO/TiO2 photoelectrode shows the maximum photocurrent in the UV range corresponding to the bandgap of nanocrystalline TiO2. The FTO/CdS photoelectrode shows a broad photocurrent peak in the visible light range corresponding to the bandgap of nanocrystalline CdS. The FTO/ TiO2/CdS photoelectrode shows a broad photocurrent peak with a much higher intensity in the range from 375 to 550 nm, which are consistent with the diffuse reflectance absorption spectrum of the electrode. Notably, the photocurrent of FTO/TiO2/CdS photoelectrode shows significant improvement in the visible light region. The IPCE spectra (Figure 5B) corresponding to the photocurrent action spectra (Figure 5A) were determined from the

1240 × I (A) × 100% λ (nm) × Pi (W)

(1)

The IPCE of FTO/TiO2/CdS photoelectrode is about 20% in visible light area from 400 to 500 nm, which suggests a significantly enhanced overall efficiency. With a photocurrent intensity of 5.778 mA/cm2 at 0 V versus Ag/AgCl, open-circuit photovoltage of 1.292 V versus Ag/AgCl, and IPCE of about 20% in the visible light area from 400 to 500 nm, the photoelectrochemical properties of a CdS QDssensitized TiO2 nanorod array of 3 µm in length on FTO photoelectrode achieved here are comparable to those of a CdS QDs-sensitized TiO2 nanotube array of more than 10 µm long photoelectrode in the literature.12,28 The favorable result is attributed to two reasons. On the one hand, the TiO2 nanorod arrays grow directly on transparent FTO substrates, which allows more incident light to pass through the back side of the photoelectrode to induce more CdS QDs to generate photoelectrons. On the other hand, the single-crystalline structure of the nanorods allows a fast and efficient transfer of the photogenerated electrons from CdS QDs to the collected FTO substrates, which can effectively suppress the recombination of electrons and holes. To achieve better photoelectrochemical properties, in one way, we can develop a modified synthetic method to increase the surface area by growing vertically aligned TiO2 nanorods with a high aspect ratio and high density and to increase the coverage of CdS QDs on TiO2 nanorod array substrates. In another way, a CdS/CdSe-cosensitized TiO2 nanorod array on FTO photoelectrodes should be effective.15,29-31 4. Conclusions A CdS QDs-sensitized single-crystalline rutile TiO2 nanorod array on FTO substrate photoelectrodes were fabricated by a simple chemical method, and their photoelectrochemical properties were investigated. These photoelectrodes allow more incident light to pass through the back side of FTO substrates to induce more CdS QDs to generate photoelectrons and can efficiently separate and transfer photogenerated electrons from CdS QDs to the collected FTO substrates. On the basis of these advantages, the photoelectrodes were found with a photocurrent intensity of 5.778 mA/cm2 at a potential of 0 V versus Ag/ AgCl, an open-circuit photovoltage of 1.292 V versus Ag/AgCl, and an IPCE of about 20% in the visible light area from 400 to 500 nm. The photoelectrochemical properties achieved here are comparable to those of CdS QDs-sensitized TiO2 nanotube array photoelectrodes, suggesting that the CdS QDs-sensitized TiO2 nanorod array on FTO photoelectrodes has a potential application in solar cells. Further research of QDs sensitization of this nanostructure will be of great significance. Acknowledgment. This work was supported by the National BasicResearchProgramofChina(2010CB934700,2007CB310500, and 2006CB932301) and National Natural Science Foundation of China (50725208, 11079002 and 20973019) as well as Specialized Research Fund for the Doctoral Program of Higher Education (20091102110035). Supporting Information Available: SEM images of TiO2 nanorod array deposited with CdS QDs for different cycles,

CdS Quantum Dots-Sensitized TiO2 Nanorod Array EDS, and PL spectra. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5, 191. (3) Park, J. H.; Kim, S.; Bard, A. J. Nano Lett. 2006, 6, 24. (4) Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A.; Ong, K. G. Nanotechnology 2006, 17, 398. (5) O’regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (6) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (7) Zaban, A.; Micic, O. I.; Gregg, B. A.; Nozik, A. J. Langmuir 1998, 14, 3153. (8) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V.; Yu, P. Y. J. Am. Chem. Soc. 2006, 128, 2385. (9) Zhu, K.; Norman, A. G.; Ferrere, S.; Frank, A. J.; Nozik, A. J. J. Phys. Chem. B 2006, 110, 25451. (10) Lee, H. J.; Leventis, H. C.; Moon, S. J.; Chen, P.; Ito, S.; Haque, S. A.; Torres, T.; Nuesch, F.; Geiger, T.; Zakeeruddin, S. M.; Gra¨tzel, M.; Nazeeruddin, M. K. AdV. Funct. Mater. 2009, 19, 2735. (11) Chen, S.; Paulose, M.; Ruan, C.; Mor, G. K.; Varghese, O. K.; Kouzoudis, D.; Grimes, C. A. J. Photochem. Photobiol., A 2006, 181, 177. (12) Sun, W. T.; Yu, Y.; Pan, H. Y.; Gao, X. F.; Chen, Q.; Peng, L. M. J. Am. Chem. Soc. 2008, 130, 1124. (13) Banerjee, S.; Mohapatra, S. K.; Das, P. P.; Misra, M. Chem. Mater. 2008, 20, 6784. (14) Seabold, J. A.; Shankar, K.; Wilke, R. H. T.; Paulose, M.; Varghese, O. K.; Grimes, C. A.; Choi, K. S. Chem. Mater. 2008, 20, 5266.

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16455 (15) Lee, Y. L.; Lo, Y. S. AdV. Funct. Mater. 2009, 19, 604. (16) David, R. B.; Prashant, V. K. AdV. Funct. Mater. 2009, 19, 805. (17) Lee, J. C.; Sung, Y. M. Appl. Phys. Lett. 2007, 91, 113104. (18) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (19) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007, 7, 69. (20) Feng, X. J.; Shankar, K.; Varghese, O. K.; Paulose, M.; Latempa, T. J.; Grimes, C. A. Nano Lett. 2008, 8, 3781. (21) Liu, B.; Aydil, E. S. J. Am. Chem. Soc. 2009, 131, 3985. (22) Shankar, K.; Feng, X. J.; Grimes, C. A. ACS Nano 2009, 3, 788. (23) Yang, X. F.; Zhuang, J.; Li, X. Y.; Chen, D. H.; Ouyang, G. F.; Mao, Z. Q.; Han, Y. X.; He, Z. H.; Liang, C. L.; Wu, M. M.; Yu, J. C. ACS Nano 2009, 3, 1212. (24) Hensel, J.; Wang, G. M.; Li, Y.; Zhang, J. Z. Nano Lett. 2010, 10, 478. (25) Larramona, G.; Chone, C.; Jacob, A.; Sakakura, D.; Delatouche, B.; Pere, D.; Cieren, X.; Nagino, M.; Bayon, R. Chem. Mater. 2006, 18, 1688. (26) Shen, Q.; Arae, D.; Toyoda, T. J. Photochem. Photobiol., A 2004, 164, 75. (27) Gra¨tzel, M. Inorg. Chem. 2005, 44, 6841. (28) Gao, X. F.; Sun, W. T.; Hu, Z. D.; Ai, G.; Zhang, Y. L.; Feng, S.; Li, F.; Peng, L. M. J. Phys. Chem. C 2009, 113, 20481. (29) Niitsoo, O.; Sarkar, S. K.; Pejoux, C.; Ruhle, S.; Cahen, D.; Hodes, G. J. Photochem. Photobiol., A 2006, 181, 306. (30) Lee, Y. L.; Chi, C. F.; Liau, S. Y. Chem. Mater. 2010, 22, 922. (31) Wang, G. M.; Yang, X. Y.; Qian, F.; Zhang, J. Z.; Li, Y. Nano Lett. 2010, 10, 1088.

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