Effect of Surface Cations on Photoelectric Conversion Property of

May 1, 2009 - Cubic-phase zirconia nanoparticles modified with univalent surface cations have been prepared through a low-temperature solvothermal met...
52 downloads 12 Views 5MB Size
Download PDF
9114

J. Phys. Chem. C 2009, 113, 9114–9120

Effect of Surface Cations on Photoelectric Conversion Property of Nanosized Zirconia Tong-Shun Wu,†,‡ Kai-Xue Wang,† Lu-Yi Zou,‡ Xin-Hao Li,‡ Ping Wang,‡ De-Jun Wang,‡ and Jie-Sheng Chen*,† School of Chemistry and Chemical Engineering, Shanghai Jiao Tong UniVersity, Shanghai 200240, People’s Republic of China, and State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: December 11, 2008; ReVised Manuscript ReceiVed: April 12, 2009

Cubic-phase zirconia nanoparticles modified with univalent surface cations have been prepared through a low-temperature solvothermal method. The obtained products were characterized by powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The surface of the nanoparticles has also been modified by divalent and trivalent cations, such as Mg2+ and Al3+, via ion exchange. The surface photovoltage (SPV) spectroscopy and surface photocurrent (SPC) spectroscopy measurements show that the surface cations on nanoscaled ZrO2 exert significant influence on the generation of charge carriers and the transportation of the generated charge carriers between nanoparticles. The effect of surface cations on the photoelectric conversion properties of the zirconia nanoparticles has been elucidated on the basis of theoretical calculation. Introduction Solar energy, as a clean and almost inexhaustible energy source, has aroused tremendous attention during the past decades.1 The conversion of solar light into electricity, the photodecomposition of noxious organic compounds, and the photolysis of water into hydrogen and oxygen are the main approaches for the exploitation of solar energy. In this context, nanocrystalline semiconductor particles exhibit better performances than their bulk counterparts owing to increase in effective surface area and strong perturbation of crystal lattice,2 whereas the enhancement of photoelectric conversion efficiency has remained to be one of the key challenges for solar energy exploitation. Although the high active surface at nanoscale can generate charge carriers much easier, the carrier transportation between adjacent particles needs to break the intergranular potential barrier3 which exists as grain boundary resistance between nanoparticles and is detrimental to the generation of photocurrent. Therefore, in order to enhance the photoelectric conversion performance of solar cell materials, it is necessary to minimize the boundary barriers for transportation of photogenerated charge carriers, while in the meantime maximizing the surface of the nanocrystalline particles. As indicated previously, surface modification is an effective approach to improve the optical4,5 and magnetic6 properties of nanomaterials. In this paper, we demonstrate that cation modification on the surface of semiconductor nanoparticles can effectively enhance the surface photocurrent of the relevant nanomaterial (ZrO2 in our case). Moreover, the application of semiconductor nanoparticles to solar energy exploitation largely depends on how much we know about the electronic structure and charge-transfer behavior at the surface or interface. Therefore, a wide range of relevant studies have been focused on the surface of nanoparticles. For instance, influence of surface states on radiative transitions,7 * To whom correspondence should be addressed. Tel.: (+86)-2154743266. Fax: (+86)-21-54741297. E-mail: [email protected]. † Shanghai Jiao Tong University. ‡ Jilin University.

effect of adsorbed oxygen on electron transition and recombination,8 and the processes of electron injection on the interface of dye-sensitized materials9 have been extensively investigated. Nonetheless, the effect of surface cations on surface states and on electron transfer at the interface has seldom been addressed in the literature.10 Zirconia is a multifunctional compound, and this material has found widespread applications in structure ceramics,11 catalysis,12,13 fuel cell manufacture,14,15 photocatalysis,16 and sensing.17 It also shows great application potential in electronics and optics.18-20 However, this excellent semiconductor material has rarely been used in photoelectric conversion, probably because of its low conversion efficiency. In this work, we report the preparation of a cubic phase of nanosized zirconia without stabilization cations through a solvothermal route for the first time. We found that surface cation modification of the nanoparticles, which reduces the grain boundary resistance, effectively enhances the surface photocurrent of the zirconia nanocrystals and consequently the photoelectric conversion efficiency. The mechanism of photocurrent enhancement has been elucidated through theoretical calculation. Experimental Section Chemicals. NaOH, KOH, HCl, magnesium nitrate, aluminum nitrate, sodium linoleate, linoleic acid, and oleic acid were purchased from Beijing Chemical Factory. Benzyl alcohol and Y2O3 were purchased from Tianjin No. 1 Chemical Reagent Factory. ZrOCl2 · 8H2O, in which the total content of Ti, Ce, Th, and Y was less than 1%, was purchased from Shanghai Chemical Reagent Factory. All chemicals were used as received without further purification. Deionized water (PURELAB Plus, PALL) was used in all of the experiments. Sample Preparation. A. Synthesis of Mn+-Modified Zirconia Nanoparticles (Mn+-ZrO2). In a typical synthesis of M+ (M ) Na and K)-modified zirconia nanoparticles, 1.600 g (40.0 mmol) of NaOH or 2.244 g of KOH, 1.611 g (5.0 mmol) of ZrOCl2 · 8H2O were dispersed in 30 mL of benzyl alcohol under continuous stirring until homogeneous. Then 4 mL of oleic acid

10.1021/jp810955f CCC: $40.75  2009 American Chemical Society Published on Web 05/01/2009

Effect of Surface Cations on ZrO2 Nanoparticles was added to the above solution dropwise. The final mixture was sealed in a 50 mL PTFE-lined stainless steel autoclave and heated at 160 °C for 24 h followed by slow cooling to room temperature. The resulting product was thoroughly washed with deionized water and separated by centrifugation. It was infeasible to prepare zirconia nanocrystals modified with cations other than Na+ and K+ directly. Mn+ (M ) Mg, Al)-modified zirconia nanocrystals were prepared by the post-treatment of the assynthesized Na+- or K+-modified counterparts through an ionexchange method. Typically, 100 mg of dry Na+-ZrO2 was dispersed in a 20 mL of 0.5 mol/L Mn+ (M ) Mg, Al) solution with a pH value of 3 under vigorous stirring. After stirring for 10 h, the ion-changed product was washed and isolated via similar treatment for the Na+-modified zirconia nanoparticles. B. Synthesis of Monoclinic Nanoparticles (m-ZrO2). An amount of 1.611 g (5.0 mmol) of ZrOCl2 · 8H2O was dissolved in 30 mL of benzyl alcohol and stirred vigorously until homogeneous, and to the solution was added 4 mL of oleic acid dropwise. The mixture was sealed in a 50 mL PTFE-lined stainless steel autoclave and heated at 160 °C for 24 h followed by slow cooling to room temperature. The resulting product was thoroughly washed with absolute ethanol to remove all residual reagents and separated by centrifugation. The dry sample was modified by Al3+ as well through ion exchange. C. Synthesis of Tetragonal ZrO2 Nanoparticles (t-ZrO2). This nanomaterial was prepared by following the procedure described in the literature.21 Thus, 20 mL of 0.1 mol/L ZrOCl2 aqueous solution, 1.6 g of sodium linoleate, 10 mL of ethanol, and 2 mL of linoleic acid were added to a 40 mL PTFE-lined stainless steel autoclave which was then heated at 160 °C for 20 h. The dry sample collected by centrifugation was subject to ion exchange with Al3+ to obtain the Al3+-modified zirconia nanoparticles. D. Synthesis of Y-Doped ZrO2 (Y-c-ZrO2). An amount of 54 mg (0.25 mmol) of Y2O3 was dissolved in 30 mL of 1.0 mol/L HCl solution, followed by the addition of 0.81 g (2.5 mmol) of ZrOCl2 · 8H2O. A coprecipitate was formed after the addition of ammonia solution to the above mixture, and the precipitate was washed by deionized water. The preliminary product was dried at 373 K followed by calcination at 1173 K for 1 h. The calcined sample was then surface-modified with Al3+ cations through ion exchange. Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2550 X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å) operated at 200 mA and 40 kV. The high-resolution transmission electron microscope (HRTEM) images of the obtained zirconia nanocrystals with the selected area electron diffraction (SAED) were observed on a JEOL JEM-3010 TEM microscope. The contents of the cations in the as-synthesized and the ion-exchanged nano-ZrO2 materials were analyzed through inductively coupled plasma (ICP) spectroscopy on a Perkin-Elmer Optima 3300DV spectrometer. UV-vis diffuse reflectance spectroscopy measurements were conducted on a Perkin-Elmer Lamda 20 UV-vis spectrometer. Surface Photovoltage and Surface Photocurrent Measurements. The surface photovoltage (SPV) measurements were performed on a homemade SPV spectrometer. A 500 W xenon lamp (CHF XQ500W, Global xenon lamp power, made in China) was used as the light source, and monochromatic light was obtained by a double-prism monochromator (Hilger and Watts, D300, made in England). The slit width of the entrance and exit is 1 mm. A lock-in amplifier (SR830-DSP, made in U.S.A.), synchronized with a light chopper (SR540, made in

J. Phys. Chem. C, Vol. 113, No. 21, 2009 9115 TABLE 1: Cation Molar Ratios of the As-Synthesized and the Ion-Exchanged Nano-ZrO2 Materials Obtained by ICP Analysis sample element molar ratio

Na+-ZrO2 K+-ZrO2 Mg2+-ZrO2

Al3+-ZrO2

Zr/Na

Zr/K

Zr/Na/Mg

Zr/Na/Al

1.0:0.28

1.0:0.16

1.0:0.25:0.1

1.0:0.24:0.1

U.S.A.), was employed to amplify the photovoltage signal. The range of modulating frequency was from 20 to 70 Hz, and the spectral resolution was 1 nm. The raw SPV data were normalized using an illuminometer (Zolix UOM-1S, made in China).22 For the SPV measurements, 20 mg of the sample was spread in a 0.8 mm × 0.8 mm shallow pit of an indium tin oxide (ITO) electrode and then pressed with another piece of ITO electrode to construct a sandwich-structured photovoltaic cell (Supporting Information Figure S1). The sandwich structure ensured that the light-penetrating depth is much less than the powder layer thickness. The contact between the samples and the ITO electrode is not ohmic.22 The surface photocurrent (SPC) was recorded on the same spectrometer, except that the samples were loaded in the comblike electrode with an external bias (9.65 V) on the two sides (Supporting Information Figure S2). On-off experiment of photocurrent of the samples was conducted on an electrochemical workstation (CHI630b, Shanghai Chen Hua Instrumental Co. Ltd.). The samples were placed between comblike electrodes with a 10 V bias to record the data. All the samples were degassed under vacuum before the SPV and SPC measurement. Computational Method. The FT-IR spectra (not shown) indicate that no organic species were present in the obtained nanoparticle samples, and therefore, only the effects of the surface cations were considered in the calculation process. It was assumed that all of the modified cations were bonded with the surface dangling oxygen. A part of the zirconia crystal surface containing 2 × 2 super cell and eight layers of atoms was chosen for simulation in the periodic crystal and as an ideal model for calculation. The thickness of vacuum slab was 10.00 Å, and the slab position was at 5.00 Å from bottom. According to the atomic ratios of cations over Zr (see Table 1), four cations were introduced into this system and were set on the (111) crystal face. The geometrical optimization was obtained at PBE/ DND level in DMol3 of Material Studio package, and the electron structures were calculated by employing PBE/DND in CASTEP of Material Studio package. Results and Discussion Characterization of the Samples. Figure 1 shows the HRTEM images of the obtained zirconia nanocrystals and the corresponding SAED and fast Fourier transform (FFT) patterns. The mean size of the ZrO2 nanocrystals is approximately 5 nm, and the corresponding lattice spacings, 0.2963, 0.2566, and 0.1831 nm, can be assigned to the (111), (200), and (220) planes of cubic ZrO2, respectively. The cubic structure of the nanoZrO2 material is further confirmed by the powder XRD pattern as shown in Figure 2. For comparison, the XRD patterns of the monoclinic zirconia (m-ZrO2), the tetragonal zirconia (t-ZrO2), and the yttrium-stabilized cubic zirconia (Y-c-ZrO2) materials are also presented in Figure 2. The XRD pattern of the obtained ZrO2 sample appears different from those of m-ZrO2 and t-ZrO2 samples but well corresponds to that of the Y-c-ZrO2 material, indicating that the ZrO2 sample we obtained belongs to the cubic phase. Nevertheless, the diffraction peaks of the ZrO2 sample are broader than those of the Y-c-ZrO2 material, suggestive of smaller particle size for the as-prepared ZrO2.

9116

J. Phys. Chem. C, Vol. 113, No. 21, 2009

Figure 1. HRTEM image (a) of ZrO2 nanoparticles. The inset in (b) is the SAED pattern, whereas that in (c) is the FFT of the selected square.

Wu et al.

Figure 3. Powder X-ray diffraction patterns of the K+-ZrO2 nanoparticles and the Na+-ZrO2 samples after ion-exchange surface modification by Mg2+ and Al3+.

Figure 2. Powder X-ray diffraction patterns of the monoclinic m-ZrO2, the tetragonal t-ZrO2, the cubic Y-c-ZrO2, and the as-synthesized Na+-ZrO2.

Pure fluorite-type cubic zirconia is not considered to be stable at temperatures lower than 2730 °C, and incorporation of cations (Ce4+, Y3+) other than Zr4+ in the crystal lattice of ZrO2 is usually indispensible to the stabilization of the cubic zirconia structure at ambient temperature. However, it was reported that for powders with very small critical crystallite size in the range from 5 to 10 nm, pure cubic ZrO2 would be retained at ambient temperature without introduction of dopants.23,24 The obtained cubic zirconia from the solvothermal system in our work can be attributed to both the small average crystal size and the surface cations which can prevent the transition from cubic phase to energetically favorable phases by reducing the surface energy of nano-ZrO2. The successful preparation of cubic ZrO2 nanocrystals lays the foundation for elucidation of physicochemical properties of cubic nanozirconia without lattice-incorporated cations. The ICP spectroscopy indicates that the as-synthesized zirconia sample contains Na+ cations which can be replaced with other cations such as Mg2+ and Al3+ through simple ionexchange reactions. Nanocrystalline K+-ZrO2 can also be prepared if KOH instead of NaOH is used in the synthetic system. For the successful incorporation of cations into the lattice, the difference of the radii among the cations and the lattice atoms should not exceed 12%.25,26 The radii of Zr4+, Na+, and K+ are 0.79, 0.95, and 1.33 Å, respectively. Thus, the Na+ and K+ cations in the as-synthesized zirconia samples are expected to exist on the surface rather than in the lattice of the ZrO2 nanocrystals. The fact that the Na+ and K+ cations can be replaced by other cations through ion exchange further confirms that the cations are located on the nanocrystal surface. The powder XRD patterns of the as-synthesized samples and those modified with various cations (see Table 1) through ion

Figure 4. SPV (a) and the SPC (b) spectra of cation-modified cubic nano-ZrO2.

exchange are illustrated in Figure 3. From the XRD results it is concluded that all the ZrO2 samples are pure phases, and the crystal structures of the particles in the samples are stable during the surface modification processes. The broadened peaks in the XRD patterns also indicate that the sizes of the ZrO2 particles are at nanometer scale, in agreement with the HRTEM observation (Figure 1). Analysis of SPV and SPC. To elucidate the effect of surface cations on the photoelectric conversion properties of the cubic ZrO2 nanocrystal materials, SPV and SPC spectra of the samples have been recorded (Figure 4). Both the SPV and the SPC spectra indicate that the red edges of photoelectric responses are at about 380 nm (3.26 eV). Besides the band gap absorption of ZrO2 at wavelengths below 250 nm, the cubic ZrO2 nanocrystals exhibit additional absorptions, albeit weak, between 250 and 400 nm as shown in the UV-vis spectra (Figure 5). Because the band gap (4.96 eV) of ZrO2 distinctly exceeds the energy of the light with a wavelength longer than 250 nm, the observed photoelectric responses are not attributed to the ZrO2 band gap absorptions but rather to the additional sub-band-gap absorptions. These sub-band-gap absorptions may arise from surface states of the nano-ZrO2 materials. According to Kronik

Effect of Surface Cations on ZrO2 Nanoparticles

Figure 5. UV-vis diffuse reflectance spectra of the cation-modified zirconia nanoparticle samples. The K/M stands for Kubelka-Munk units.

Figure 6. On-off photocurrent responses of as-synthesized Na+-ZrO2 (bottom) and K+-ZrO2 (top) nanoparticles in the UV-vis region (λ > 200 nm, 500 W xenon lamp). The ordinate unit is microamps. The responses are prompt and reproducible when on-off cycles of the UV-vis illumination were repeated. The intensity of the short-circuit photocurrent of the Na+-ZrO2 sample was 1.0 µA, about twice that of the K+-ZrO2 material.

and Shapira’s explanation,27,28 surface states are surface-localized electronic states within the semiconductor band gap, involving complex species such as dangling bonds, defects, and atoms adsorbed on the surface. Because ZrO2 is an n-type semiconductor29,30 as revealed in Figure 6, the sub-band-gap photoelectric responses should be originated from the electron transitions from a surface state situated at an energy Et into the conduction band (Ec) driven by the illumination by photons with energy hν g Ec - Et.27

J. Phys. Chem. C, Vol. 113, No. 21, 2009 9117 The SPV signal intensity (Figure 4a) varies to a less extent from sample to sample, with that of the Al3+-ZrO2 being slightly stronger than that of Na+-ZrO2. This result indicates that the amounts of photogenerated charge carriers accumulated on the electrodes are comparable for all the samples under investigation. In other words, the type of the surface cations has no marked influence on the SPV intensity of the nanozirconia material because, for SPV measurement, the photogenerated charge carriers, which may be accumulated on the electrodes through dipole-dipole induction just as in a capacitor, are not necessarily to be transported between particles. In contrast to the SPV signals, the SPC intensities of the ZrO2 nanoparticles modified with various cations differ to a great extent. As shown in Figure 4b, the SPC intensity of the Al3+modified sample is 1 order of magnitude stronger than those of the alkali-metal-modified ones. The effect of different cations on SPC signals is also demonstrated by on-off experiment of photocurrent as shown in Figure 6. According to energy band theory, electrons would be excited from a surface state into the conduction band and move in one particular direction under an external electric field to form photocurrent. However, how the electrons are transported between two adjacent ZrO2 nanoparticles, and how the surface cations affect the electron transportation has not been theoretically addressed yet. In this work, the density functional theory (DFT) was applied to the cubic zirconia system to elucidate the effects of surface cations on the photocurrent generation of the nanozirconia materials. Two processes including the photoexcitation to generate the charge carriers and the following transportation of electrons/ holes should be involved in the generation of SPC. Without surface cations, many dangling oxygen atoms exist on the surface of ZrO2 nanoparticles, leading to the negatively charged surfaces. In the photoexcitation process, the photogenerated electrons would be pushed into the bulk of the particles by the static electric field,27 which makes it difficult for these electrons to be transported from particle to particle. However, for the cation-modified samples, the photogenerated electrons would be attracted by the surface cations and accumulate on the nanoparticle surface, ready for transportation between particles. In the subsequent electron transportation process, the surface cations play a significant role. As shown in Figure 7, the electron clouds of Mg2+-modified/Al3+-modified zirconia on the surface are delocalized. This delocalized electron cloud structure is

Figure 7. Electron clouds of cubic ZrO2 (a), Na+-ZrO2 (b), Mg2+-ZrO2 (c), and Al3+-ZrO2 (d) obtained by employing PBE/DND in CASTEP of Material Studio package through further calculations on the results of optimized surface geometrical structure.

9118

J. Phys. Chem. C, Vol. 113, No. 21, 2009

Wu et al.

Figure 8. Optimized surface geometrical structure of cubic nano-ZrO2 (a), Na+-ZrO2 (b), Mg2+-ZrO2 (c), and Al3+-ZrO2 (d). The upper graphs are three-dimensional, whereas the nether ones are the top views of the geometrical structures (red, O; light blue, Zr; purple, Na; green, Mg; pink, Al).

Figure 9. Density of states for cubic ZrO2 (a), Na+-ZrO2 (b), Mg2+-ZrO2 (c), and Al3+-ZrO2 (d). CB: conduction band.

contributed by all the atoms (including zirconium and oxygen) on the surface, and it can accommodate and transfer electrons effectively, rendering the transportation of electrons on the surface easier. When two nanoparticles get close enough, their delocalized electron clouds overlap, resulting in the connection of the electron clouds of the two nanoparticles. Upon photoexcitation, the photogenerated electrons tend to get delocalized and transport smoothly from one particle to another. Therefore, the surface cation modification accumulates photogenerated electrons on the surface on one hand, whereas reduces the grain boundary resistance on the other, and consequently enhances the SPC signal. Although electron cloud structure affects the SPC intensity to a certain degree, the difference between the SPC signals of Al3+-ZrO2 and Mg2+-ZrO2 suggests that band structure is also important in determining the SPC signal intensity. As shown in the geometric optimization (Figure 8, parts c and d), the surfaces of Al3+-ZrO2 and Mg2+-ZrO2 are reconstructed, and the corresponding crystal lattices are distorted. Lattice variation

inevitably leads to alteration in energy band structure, as demonstrated by the density of states (DOS) for the different samples (Figure 9). The DOS for Al3+-ZrO2 at the conduction band bottom edge is higher than those for Mg2+-ZrO2 and for Na+-ZrO2. In other words, Al3+-ZrO2 possesses more orbitals to accept photoexcited electrons for generation of SPC than Mg2+-ZrO2 and Na+-ZrO2, accounting for the higher SPC intensity of Al3+-ZrO2. Therefore, it is concluded that the SPC intensity is affected not only by the electron clouds but also by the DOS in the conduction band. In order to investigate the influence of cation modification on other zirconia phases, ZrO2 compounds (without any surface metal cations) with tetragonal, monoclinic, and Y-stabilized cubic structures are also modified by Al3+. The SPV and SPC spectra of the samples with or without Al3+ are shown in Figure 10. In principle, the SPV intensity is determined by two factors, that is, the generation of electron (hole) charge carriers and the separation of these charge carriers. Before Al3+ modification, the recombination of the

Effect of Surface Cations on ZrO2 Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 21, 2009 9119

Figure 10. For comparison, in panels a and c the SPV spectra for unmodified/Al3+-modified monoclinic, tetragonal, and yttrium-stabilized cubic ZrO2 materials are presented, respectively. In panels b and d the SPC spectra of the corresponding samples are presented.

photogenerated electrons and holes inside the particles is rapid,22 and hence the corresponding SPV signals are weak (Figure 10a). In contrast, after Al3+ modification, the generated electrons can be attracted by the surface cations, efficiently reducing the recombination rate of electrons and holes and increasing the amount of separated charge carriers. As a result, SPV signal intensity is distinctly enhanced, as shown in Figure 10c. On the other hand, in comparison with those of the samples without surface cations (Figure 10b), the SPC signal intensities of Al3+-modified m-ZrO2 and t-ZrO2 materials increase significantly as well (Figure 10d). This SPC intensity enhancement is attributable to the reduction of grain boundary resistance by the surface cation modification, as pointed out earlier for the cubic ZrO2. However, the effect of cation modification on SPV and SPC signals is not significant for large ZrO2 particles. For example, the large-sized Y-c-ZrO2 crystals which have a small surface/ volume ratio show no SPV and SPC responses both before and after the cation modification. Therefore, the surface cation modification is effective only for nanoparticles with a small size and a high surface area. Conclusions In order to convert light into electricity, the involved materials should be able to absorb light energy to generate charge carriers effectively on one hand and to transport the photogenerated charge carriers smoothly on the other. Our work demonstrates that surface cation modification on the ZrO2 nanocrystals plays a key role in both the generation and the transportation of charge carriers. Theoretical calculation on the surface and electron structures of the nanoparticles with/without cation modification further reveals that cation modification reduces the grain boundary resistance, alters surface states, accumulates photoexcited electrons on the surface, and therefore enhances the SPC intensity. Although it may not be feasible to use ZrO2 as a photoelectric conversion material due to its large band gap, our

cation modification strategy applied to the ZrO2 materials bears considerable significance for exploitation of other semiconductor nanoparticle systems with smaller band gaps. Acknowledgment. We thank M. Y. Guo for assistance in the HRTEM characterization and L. Z. Zhao for assistance in theoretical calculation. Financial support from the NSFC (20731003) and the MOST of China (2007CB613303) is gratefully acknowledged. Supporting Information Available: Schematic configuration for SPV measurement (Figure S1) and schematic of the structure of the photocurrent cell (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature 2007, 449, 885–890. (2) Ferna´ndez-Garcı´a, M.; Martı´nez-Arias, A.; Hanson, J. C.; Rodriguez, J. A. Chem. ReV. 2004, 104, 4063–4104. (3) Rothschild, A.; Levakov, A.; Shapira, Y.; Ashkenasy, N.; Komen, Y. Surf. Sci. 2003, 456–460. (4) Liu, D. P.; Li, G. D.; Su, Y.; Chen, J. S. Angew. Chem., Int. Ed. 2006, 45, 7370–7373. (5) Liu, D. P.; Li, G. D.; Li, J. X.; Li, X. H.; Chen, J. S. Chem. Commun. 2007, 4131–4133. (6) Li, X. H.; Zhang, D. H.; Chen, J. S. J. Am. Chem. Soc. 2006, 128, 8382–8383. (7) Dijken, A. V.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Meijerink, A. J. Phys. Chem. B 2000, 104, 1715–1723. (8) Dijken, A. V.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Meijerink, A. J. Phys. Chem. B 2000, 104, 4355–4360. (9) Lenzmann, F.; Krueger, J.; Burnside, S.; Brooks, K.; Gratzel, M.; Gal, D.; Ruhle, S.; Cahen, D. J. Phys. Chem. B 2001, 105, 6347–6352. (10) Harima, Y.; Kawabuchi, K.; Kajihara, S.; Ishii, A.; Ooyama, Y.; Takeda, K. Appl. Phys. Lett. 2007, 90, 103517. (11) Garvie, R. C.; Hannink, R. H.; Pascoe, R. T. Nature 1975, 258, 703–704. (12) Haw, J. F.; Zhang, J.; Shimizu, K.; Venkatraman, T. N.; Luigi, D.-P.; Song, W.; Barich, D. H.; Nicholas, J. B. J. Am. Chem. Soc. 2000, 122, 12561–12570.

9120

J. Phys. Chem. C, Vol. 113, No. 21, 2009

(13) (a) Ahn, E. S.; Gleason, N. J.; Nakahira, A.; Ying, J. Y. Nano Lett. 2001, 1, 149–153. (b) Adler, S. B. Chem. ReV. 2004, 104, 4791–4843. (14) Mamak, M.; Coombs, N.; Ozin, G. J. Am. Chem. Soc. 2000, 122, 8932–8939. (15) Virkar, A. V.; Chen, J.; Tanner, C. W.; Kim, J. W. Solid State Ionics 2000, 131, 189–198. (16) Kohno, Y.; Tanaka, T.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1998, 94, 1875–1880. (17) Bae, J. W.; Park, J. Y.; Hwang, S. W.; Yeom, G. Y.; Kim, K. D.; Cho, Y. A.; Jeon, J. S.; Choi, D. J. Electrochem. Soc. 2000, 147, 2380– 2384. (18) Wang, S. J.; Ong, C. K.; Xu, S. Y.; Chen, P.; Tjiu, W. C.; Chai, J. W.; Huan, A. C. H.; Yoo, W. J.; Lim, J. S.; Feng, W.; Choi, W. K. Appl. Phys. Lett. 2001, 78, 1604–1606. (19) Houssa, M.; Afanas’ev, V. V.; Stesmans, A.; Heyns, M. M. Appl. Phys. Lett. 2000, 77, 1885–1887. (20) Garcia, J. C.; Scolfaro, L. M. R.; Lino, A. T.; Freire, V. N.; Farias, G. A.; Silva, C. C.; Alves, H. W. L.; Rodrigues, S. C. P.; da Silva, J. E. F. J. Appl. Phys. 2006, 100, 104103–104109.

Wu et al. (21) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature 2005, 437, 121– 124. (22) Lin, Y.; Wang, D.; Zhao, Q.; Yang, M.; Zhang, Q. J. Phys. Chem. B 2004, 108, 3202–3206. (23) Lamas, D. G.; Rosso, A. M.; Suarez Anzorena, M.; Ferna´ndez, A.; Bellino, M. G.; Cabezas, M. D.; Walso¨e de Reca, N. E.; Craievich, A. F. Scr. Mater. 2006, 55, 553–556. (24) Tsunekawa, S.; Ito, S.; Kawazoe, Y.; Wang, J.-T. Nano Lett. 2003, 3, 871–875. (25) Li, P.; Chen, W. I. J. Am. Ceram. Soc. 1994, 77, 118–128. (26) Li, P.; Chen, W. I. J. Am. Ceram. Soc. 1994, 77, 1281–1288. (27) Kronik, L.; Shapira, Y. Surf. Interface Anal. 2001, 31, 954–965. (28) Kronik, L.; Shapira, Y. Surf. Sci. Rep. 1999, 37, 1–206. (29) Schneemeyer, L. F.; Wrighton, M. S. J. Am. Chem. Soc. 1979, 101, 6496–6500. (30) Chen, W.; Yuan, H. M.; Wang, J. Y.; Liu, Z. Y.; Xu, J. J.; Yang, M.; Chen, J. S. J. Am. Chem. Soc. 2003, 125, 9266–9267.

JP810955F