Photocatalytic Activity for Overall Water Splitting of RuO2

The photocatalytic properties for the overall splitting of water for a single-phase solid solution of In2O3 with Y2O3, such as YxIn2-xO3, were studied...
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J. Phys. Chem. C 2008, 112, 5000-5005

Photocatalytic Activity for Overall Water Splitting of RuO2-Loaded YxIn2-xO3 (x ) 0.9-1.5) N. Arai,† N. Saito,† H. Nishiyama,† Y. Shimodaira,‡ H. Kobayashi,‡ Y. Inoue,* and K. Sato§ Department of Chemistry, Nagaoka UniVersity of Technology, Nagaoka 940-2188, Japan, Department of CiVil and EnVironmental Engineering, Nagaoka UniVersity of Technology, Nagaoka 940-2188, Japan, and Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan ReceiVed: October 2, 2007; In Final Form: December 18, 2007

The photocatalytic properties for the overall splitting of water for a single-phase solid solution of In2O3 with Y2O3, such as YxIn2-xO3, were studied in the x range of 0.9-1.5. The photocatalytic activity of YxIn2-xO3 combined with RuO2 as a promoter increased with increasing x, reached a maximum at x ) 1.3 and decreased with further increase in x. The highest activity was 2.6-fold larger than that reported for x ) 1.0. The lattice constant for x ) 1.0 increased by 1.2% for x ) 1.5 by the replacement of an In3+ ion with a Y3+ ion. The primary absorption band for YxIn2-xO3 shifted from 350 nm for x ) 1.0 to 302 nm for x ) 1.5. The Raman spectra of YxIn2-xO3 showed that the strong peak at 309 cm-1 for x ) 0 was weakened with increasing x, followed by the appearance of broad peak at around 386 cm-1, suggesting that the octahedral structures of both InO6 and YO6 were deformed in the solid solution. The DFT calculation for YxIn2-xO3 showed that the top of the valence band (HOMO) was composed of the O 2p orbital, whereas the bottom of the conduction band (LUMO) was formed by the In 5s + O 2p orbitals. The photocatalytic activity of YxIn2-xO3 as a function of x is discussed on the basis of changes in the geometric and electronic structures.

Introduction A series of p-block metal oxides consisting of various kinds of indates (MIn2O4 (M ) Ca,Sr), NaInO2, LaInO3), zinc gallate (ZnGa2O4), zinc germanate (Zn2GeO4), strontium stannate (Sr2SnO4) and various antimonates (M2Sb2O7 (M ) Ca,Sr), CaSb2O6, NaSbO3),1-8 have been determined to be photocatalytically active for water decomposition under UV illumination when RuO2 was loaded as a promoter. The p-block metal ions of Ga3+, In3+, Ge4+, Sn4+, and Sb5+, being core elements, possess d10 electronic configuration. The DFT calculations showed that the conduction bands of d10 metal oxides were formed by sp bands with large dispersion, which permitted the generation of photoexcited electrons with high mobility at the conduction bands. In this regard, the properties of conduction bands differed from those of conventional d0 transition metal oxides, which are known to have small band dispersion.9-22 Furthermore, a composite metal oxide, LiInGeO4, with the d10d10 electronic confliguration showed high photocatalytic performance, for which the distortion effects of the InO6 octahedron/ GeO4 tetrahedron and the hybridization effects of the sp orbitals of In3+ and Ge4+ metal ions at conduction bands were proposed.23 Among indium oxides with d10 configuration, In2O3 is photocatalytically inactive for the water splitting reaction, even when loading RuO2 as a promoter. This is thought to be because of the lower conduction band level than the H+/H energy level. Recently, we found that the partial replacement of an In atom of In2O3 with a Y atom activates In2O3 to undergo water splitting * Corresponding author. † Department of Chemistry, Nagaoka University of Technology. ‡ Kyoto Institute of Technolgy. § Department of Civil and Environmental Engineering, Nagaoka University of Technology.

in the presence of a RuO2 promoter. The photocatalytic activity generation is related to the interactions of two different metal ions in a unit cell. Therefore, for a better understanding of the activation mechanism, it is interesting to investigate the photocatalytic properties of a single-phase solid solution of In2O3 and Y2O3, in which InO6 and YO6 are randomly distributed to the octahedral sites. Thus, in the present study, the photocatalytic activity for the overall water splitting of RuO2-loaded YxIn2-xO3 was investigated in the x range of 0.9-1.5. The morphology and crystal structures of YxIn2-xO3 were analyzed by SEM, XRD, and Raman spectroscopy, and the band structures of YxIn2-xO3 (x ) 1.0 and 1.25) were calculated using DFT. Experimental Section A solid solution metal oxide of YxIn2-xO3 (x ) 0.2 - 1.5) was prepared by a solid-state reaction at high temperatures. Various molar mixtures of Y2O3 (Nacalai tesque, GR grade) with In2O3 (Nacalai tesque, EP grade) were mechanically ground in an agar tar and calcined in air for 16 h at 1673 K. The compositions of Y and In atoms in the prepared samples were analyzed with electron probe microanalysis. YxIn2-xO3 (x ) 1.0, 1.2, 1.3, and 1.5) in mixing of the precursors were determined to be YxIn2-xO3 (x ) 1.03, 1.25, 1.33, and 1.49), respectively. The observed values were in agreement with calculated ones within experimental errors, indicating that the mixing ratios of Y to In atoms were maintained in high-temperature calcination. To load RuO2 as a promoter, the obtained metal oxides were impregnated with ruthenium carbonyl complex, Ru3(CO)12, in THF, dried at 353 K, and oxidized in air at 673 K for 5 h in order to convert the loaded Ru complex into RuO2 particles. The amount of RuO2 metal base was 1 wt %, unless otherwise described.4,6,8

10.1021/jp709629t CCC: $40.75 © 2008 American Chemical Society Published on Web 03/08/2008

Photocatalytic Activity of RuO2-Loaded YxIn2-xO3

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Details of the procedure for the photocatalytic water splitting reaction have been reported elsewhere.3,4 The RuO2-loaded powder (250 mg) was placed in distilled, ion-exchanged water (30 mL) in a quartz reaction cell with a closed gas-circulating apparatus. The photocatalyst powder was dispersed in the water by continuous bubbling of Ar gas (13.3 kPa) during photocatalytic reaction and irradiated with a 200 W Hg-Xe lamp (Hamamatsu L566-02) whose light wavelength extended up to 230 nm. The amounts of H2 and O2 produced in the gas phase were analyzed using an on-line gas chromatograph. The X-ray diffraction patterns of the metal oxides prepared were obtained using an X-ray diffractometer (Rigaku RAD III). Raman spectra and UV diffuse reflectance spectra were recorded on a Raman spectrometer (JASCO MR-1100) and a UV-vis spectrometer (JASCO V-570), respectively. Scanning electron microscopy (SEM) images were obtained using a Shimazu EPMA 1600. The band calculation was carried out using the plane-wave DFT program package Castep.24 For the calculation of YInO3, such as YxIn2-xO3 (x ) 1), the optimized Y16In16O48 structure was used as a unit cell. For YxIn2-xO3 (x ) 1.25), Y20In12O48 was used where four Y atoms in Y16In16O48 were replaced with In atoms. By using the ultra soft core potentials,25 the valence atomic configurations for Y, In, and O atoms were 4s24p65s24d1 (11 electrons), 5s24d10 5p1 (13 electrons), and 2s22p4 (6 electrons), respectively. The numbers of electrons and occupied bands were 672 and 336, and 664 and 332 respectively for Y16In16O48 and Y20In12O48. The kinetic energy cutoff was set to 300 eV. Results Figure 1a,b shows the X-ray diffraction patterns of YxIn2-xO3 in the wide and narrow ranges of diffraction angles, respectively. The wide range patterns were composed of a single-phase pattern of cubic YInO3 and remained nearly unchanged in the range from x ) 0.9 to x ) 1.5. However, as shown in the narrow angle range, each peak shifted to a lower angle nearly in proportion to x: for example, the main peak due to the (222) plane appeared at 2θ ) 29.86° for x ) 1.0 and 29.50° for x ) 1.5. Furthermore, no significant changes in the peak width were observed. Figure 2 shows the UV diffuse reflectance spectra of YxIn2-xO3 with different values for x. For x ) 1.0, such as YInO3, light absorption started at around 350 nm and reached a maximum level at around 320 nm. For x ) 0.9 (Y0.9In1.1O3), the absorption curve shifted by 10 nm to a longer wavelength. On the other hand, as x increased from x ) 1.0, the shift occurred in the opposite direction. The threshold absorption wavelength was 330, 316, and 302 nm, whereas the wavelength at the maximum absorption level shifted to 296, 282, and 254 nm, for x ) 1.1, 1.3, and 1.5, respectively. Figure 3 shows H2 and O2 production from water on 1.0 wt % RuO2-loaded YxIn2-xO3 (x ) 1.0 and 1.3) under Hg-Xe lamp irradiation. For x ) 1.0, both H2 and O2 were produced with the onset of the reaction at constant rates. In the second and third run, nearly the same production was observed. For x ) 1.3, the behavior of H2 and O2 evolutions were nearly the same throughout the reaction run. The reaction proceeded with constant reaction rates, indicative of the stability of the photocatalyst. The main difference between x ) 1.0 and x ) 1.3 was the larger H2 and O2 production for the latter. Figure 4 shows the photocatalytic activity of 1 wt % RuO2loaded YxIn2-xO3 as a function of x. The activity increased

Figure 1. X-ray diffraction patterns for YxIn2-xO3 (x ) 0.9-1.5) measured over (a) a wide range and (b) a narrow range.

Figure 2. UV diffuse reflectance spectra of YxIn2-xO3(x ) 0.9-1.5) with different x values.

gradually in the range x ) 0.9-1.3 with increasing x, reached a maximum at x ) 1.3, and significantly decreased at x ) 1.5. Figure 5 shows the SEM micrographs of YxIn2-xO3 (x ) 1.0, 1.3, 1.5). Irregular-shaped particles with uneven surfaces were observed for YInO3 (x ) 1.0). The morphology of the particles for x ) 1.3 and 1.5 was similar to that observed for YInO3. Furthermore, the morphological features remained unchanged, irrespective of changes in x.

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Figure 3. Repeated run of water splitting on 1.0 wt % RuO2-dispersed YxIn2-xO3 ((a) x ) 1.0 and (b) x ) 1.3) under UV irradiation.

Figure 5. SEM images of YxIn2-xO3 for (a) x ) 1.0, (b) x ) 1.3, and (c) x ) 1.5.

Figure 4. Photocatalytic activity of 1.0 wt % RuO2-dispersed YxIn2-xO3 (x ) 0.9-1.5) for H2 and O2 production as a function of x.

Figure 6 shows the Raman spectra for YxIn2-xO3(x ) 0.02.0). For In2O3 (x ) 0.0 in YxIn2-xO3), the peaks appeared at 112, 118, 121, 134, 156, 172, 215, 309, 370, 393, 499, and 631 cm-1. Two strong peaks were observed at 134 and 309 cm-1. Upon increasing x from 0.2 to 1.0 in YxIn2-xO3 (x ) 0.0-2.0), the broadening and decreased intensity of the peaks at 134 and 309 cm-1 was observed. The peak at 309 cm-1 almost disappeared at x ) 0.7. For YInO3 (x ) 1.0), an extremely broad peak was observed at 386 cm-1 and became sharper with increasing x. At x ) 1.5-1.7, the spectra showed characteristic

peak patterns close to Y2O3, in which the Raman peaks appeared at 118, 132, 165, 184, 197, 320, 333, 381, 435 473, 569, and 596 cm-1, with a remarkably strong peak at 381 cm-1. In the band structure and density of states (DOS) of YxIn2-xO3 (x ) 1.0), the occupied bands in the low level region were the Y4s band at approximately -40 eV and the Y4p band at approximately -20 eV. The O 2s band was observed at around -15 and -17 eV, and the In 4d band appeared at -12.3 eV as a single peak without mixing with other atomic orbitals (AOs; data not shown). Figure 7 shows the valence and conduction bands. The DOS breaks down into the projected DOS (PDOS), which is represented in terms of the angular momentum of AOs by different colored of lines. Figure 8 shows the AO and atom decomposed PDOS for a wider energy range. The valence band was primarily formed by the O 2p band. The bottom of the unoccupied level (conduction band) consisted of the In 5s + O 2p AOs. Furthermore, the contribution from the In 5p and Y 4d AOs increased in the upper region of the conduction band.

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Figure 8. Angular momentum and AO decomposed projected DOS for Y, In, and O atoms along with that of all atoms for occupied and unoccupied bands of YxIn2-xO3 (x ) 1.0).

Figure 6. Raman spectra of YxIn2-xO3(x ) 0.0-2.0). Excitation wavelength: 514.54 nm.

Figure 9. Electron density contour map for the top of the valence band (HOMO) of YxIn2-xO3 (x ) 1.0). Y: light blue. In: brown. O: red.

the In 5s5p AOs increased slightly, reflecting an increase in the Y atom ratio. The estimated band gaps increased to 2.42 eV. Discussion Figure 7. Band dispersion and angular momentum decomposed projected density of states (PDOS) for YxIn2-xO3 (x ) 1.0).

The band dispersion was remarkably large for the bottom of conduction band. The band gap related to photoexcitation was calculated to be 2.19 eV. The electron density contour maps for the HOMO and LUMO levels are shown in Figures 9 and 10, respectively. For the HOMO, the electron density is solely localized on the O atoms, whereas for the LUMO, it is largely localized on In atoms with a small portion on the O atoms. Figures 11 and 12 show the band structure and PDOS of YxIn2-xO3 (x ) 1.25). The band structure and PDOS were similar to those for x ) 1.0, but the contribution of the Y 4d to

In2O3 and Y2O3 have cubic structures with a lattice constant of a ) b ) c ) 1011 and 1060 pm, respectively.26,27 The larger lattice constant for Y2O3 is attributed to the larger ionic radii of Y3+ (103pm) compared with In3+ (97 pm). The two cubic oxides form a single-phase solid solution, in which a melt of the InO6 and YO6 octahedron is randomly distributed. As shown in the X-ray diffraction pattern (Figure 1), a diffraction pattern was characteristic of the cubic crystal structure in the x range of 0.9-1.5; however, each peak shifted to a lower angle in proportion to increasing x. This is a result of an increase in the lattice constant: a 50% increase in Y atoms brought about a 1.2% enhancement in the lattice constant. These results clearly indicate that the prepared YxIn2-xO3 (x ) 0.9-1.5) was

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Figure 12. Angular momentum and AO decomposed projected DOS for Y, In, and O atoms along with that of all atoms for occupied and unoccupied bands for YxIn2-xO3 (x ) 1.25).

Figure 10. Electron density contour map for the bottom of the conduction band (LUMO) of YxIn2-xO3 (x ) 1.0). See Figure 9 for the color coding.

Figure 11. Band dispersion and angular momentum decomposed projected density of states (PDOS) for YxIn2-xO3 (x ) 1.25).

composed of a randomly distributed YO6 and InO6 octahedron in a single-phase solid solution. The photocatalytic activity of 1 wt % RuO2-loaded In2O3 was negligible for the water splitting reaction under Hg-Xe lamp illumination. For 1.0 wt % RuO2-loaded YxIn2-xO3 (x ) 0.5), a small amount of hydrogen was evolved, but the activity was small. Significant activity was observed for x ) 0.9. Furthermore, the photocatalysts produced H2 and O2 for the reaction run, as observed for 1.0 wt % RuO2-loaded YxIn2-xO3 (x ) 1.0 and 1.3). Thus, it is clear that the addition of Y to In2O3 activates

the oxide, making it photocatalytically active. Figure 4 shows that the activity increased with increasing x, passed through a maximum at x ) 1.3, and decreased considerably at x ) 1.5. The maximum in the absorption bands of YxIn2-xO3 shifted to a short wavelength of 250 nm at x ) 1.5 as shown in Figure 2, but the Hg-Xe lamp used for photocatalytic run provided light of the wavelength up to 220 nm, indicating that the decreases in the activity above x > 1.3 were not due to the shortage of light intensity. The SEM image showed that YxIn2-xO3 (x ) 0.0, 1.0, and 1.5) had irregular-shaped particles, with similar morphologies. Thus, the macro-crystal structures are not responsible for the activity changes. The Raman spectra of Y2O3 have 4Eg + 4Ag + 14Fg modes, in which eight modes attributed to Fg have the same frequencies as those for Eg and Ag.28 Figure 6 shows Raman spectra in which, the Fg + Ag modes appeared at 596, 473, 381, and 165 cm-1, and the Eg + Ag modes at 569, 435, 333, 320, and 197 cm-1. The peaks at 118, 132, and 184 cm-1 were associated with Fg mode (the other weak peaks attributed to Fg at 399 and 526 cm-1 were missing in this spectra). The strongest peak at 381 cm-1 for Y2O3 and 309 cm-1 for In2O3 deviated from a relation29 of metal-oxygen bond length versus Raman wavenumber. This indicates that the strongest peak was not due to the stretching vibration only, as demonstrated by the assignment of the Ag + Fg mode for Y2O3. The strongest peak for Y2O3 became broader upon addition of an In atom, and shifted to higher wavenumber by 5 cm-1. The random distributions of InO6 and YO6 in YxIn2-xO3 yielded edge connections sharing two oxygen atoms between two octahedra. Significant changes in the vibrational modes of the InO6 and YO6 octahedra with x of YxIn2-xO3 suggest that the structures of both the InO6 and the YO6 octahedra are considerably deformed. A correlation between the photocatalytic activity and the distorted oxygen-metal octahedron has been demonstrated in a series of p-block metal oxide photocatalysts.7 For example, photocatalytically active SrIn2O4 and Sr0.93Ba0.07In2O4 have two kinds of distorted octahedral InO6, whereas inactive LiInO2 is composed of undistorted InO6 octahedron (RuO2 is loaded as a promoter). A model that the unsymmetrical fields of distorted octahedra promote electron-hole separation has been proposed.30-33 Thus, one of the factors to consider for activity enhancement with increasing x in the range x ) 1.0-1.3 is the distortion effects on the InO6 octahedron. A decrease in the activity above x > 1.3 is due to the decrease in the concentration of InO6. Furthermore, it may be possible that the excess Y promotes the crystallization of YxIn2-xO3, resulting in better performance of photocatalysts. This may be because Y eliminates impurities

Photocatalytic Activity of RuO2-Loaded YxIn2-xO3 and structural imperfections that frequently work as traps for charge recombination. However, this is unlikely, because no significant narrowing was observed for the X-ray diffraction peaks. In the DFT calculation for the electronic structure of YInO3, the Y4s, Y4p O2s, and In4d bands appeared in a lower energy region. The In4d band was not mixed with other AOs. As shown in Figures 7 and 8, the main component in the valence band was the O 2p AOs. However, the lower region of the valence band was slightly hybridized with the In 5s AO, and the middle portion was slightly hybridized with the In 5p and Y 4d AOs. This type of mixing occurs in the in-phase (bonding). Further, the overlap between the In 5s and 5p AOs was small in the valence band. The upper part of the valence band consisted of the O 2p AOs, slightly mixed with the Y 4d AO’s, but the HOMO levels were solely localized on the O 2p AOs, as shown in the contour map of the HOMO (Figure 9). On the other hand, the conduction band was composed of the In 5s, O 2p, In 5p, and Y 4d AOs, in which the two latter bands appeared in the higher energy region. The LUMO level consisted of the In 5s and O 2p AOs and had a large dispersion. This suggests that the LUMO covered an energy range of approximately 2 eV from the bottom of the conduction band rather than only the bottom. Electron transfer occurs upon illumination, in which a large dispersion in the conduction bands is able to generate photoexcited electrons with large mobility. This is thought to be associated with the photocatalytic performance of RuO2-loaded YInO3. In a comparative DFT calculation of the electronic structures of In2O3 and Y2O3, the valence band of In2O3 was determined to be composed of the O 2p AOs with a small contribution from the In 5s5p AOs in the lower part of the valence band. On the other hand, the conduction band had the In 5s5p AOs. The valence band of Y2O3 was primarily formed by the O 2p AOs, whereas the Y 4d AOs were also involved over the entire range of the valence band. The conduction band was composed of the Y 4d and Y 5p AOs. A comparison of YInO3 with each of the single metal oxides of Y2O3 and In2O3 suggested that the valence and conduction bands of YInO3 consist of the Y 4d AOs hybridized with the O 2p and In 5s AO’s, respectively. The valence and conduction band structures for YxIn2-xO3 (x ) 1.25) were similar to those of YInO3. In the AO PDOS for the Y, In, and O atoms of YxIn2-xO3 (x ) 1.25) (Figure 12), however, the degree of hybridization of the Y 4d in the valence and conduction band was higher for YxIn2-xO3 (x ) 1.25), which is consistent with the atomic ratio of the unit cells. The band gap was estimated to be 2.19 and 2.42 eV for YInO3 and YxIn2-xO3 (x ) 1.25), respectively. The contribution of the Y 4d AOs for the lower region of the conduction band appeared to increase the LUMO level slightly, and the magnitude was larger for YxIn2-xO3 (x ) 1.25). This result is in good agreement with the shift of the primary absorption bands to shorter wavelengths with increasing Y content in the UV diffuse reflectance spectra of YxIn2-xO3. The shift of the LUMO to a higher energy level due to the contribution of increased Y content is advantageous for the production of high-energy electrons and therefore for the photocatalytic reaction. Conclusion A single-phase solid solution of YxIn2-xO3 was photocatalytically active for water splitting when combined with RuO2 as a promoter in the x range of 0.9-1.5. The highest activity

J. Phys. Chem. C, Vol. 112, No. 13, 2008 5005 was obtained for the excess YO6 (x ) 1.3) and 2.6-fold larger than that of YInO3 (x ) 1.0). On the basis of the absorption band shifts, changes in Raman peaks, and band structures calculated by the DFT method, the activity increases by solid solution formation were attributed to the deformation of InO6/ YO6 octahedral units and also to the upward shifts of conduction band levels. The advantage of a solid solution of metal oxide is the ease of formation of distorted metal-oxygen octahedral units and of hybridization of various orbitals with different properties, and its employment is useful for the development of photocatalysts for water splitting reaction. Acknowledgment. This work was supported by the Core Research for Evolutionary Science and Technology (CREST) of the Japan Science and Technology Corporation (JST) and by a Grant-in-Aid for Basic Research (No. 5) from The Ministry of Land, Infrastructure and Transport Government of Japan. References and Notes (1) Sato, J.; Saito, S.; Nishiyama, H.; Inoue, Y. J. Phys. Chem. 2001, 105, 6061. (2) Sato, J.; Saito, S.; Nishiyama, H.; Inoue, Y. Chem. Lett. 2001. 868. (3) 3. Sato, J.; Saito, S.; Nishiyama, H.; Inoue, Y. J. Photochem. Photobiol. A: Chem. 2002, 148, 85. (4) Ikarashi, K.; Sato, J.; Kobayashi, H.; Saito, S.; Nishiyama, H.; Inoue, Y. J. Phys. Chem. 2002, 106, 9048. (5) 5. Sato, J.; Kobayashi, H.; Saito, S.; Nishiyama, H.; Inoue, Y. J. Photochem. Photobiol. A: Chem. 2002, 158, 139. (6) Sato, J.; Saito, S.; Nishiyama, H.; Inoue, Y. J. Phys. Chem. B 2003, 107, 7965. (7) Sato. J.; Kobayashi, H.; Inoue, Y. J. Phys. Chem. B 2003, 107, 7970. (8) Sato, J.; Ikarashi, K.; Kobayashi, H.; Saito, S.; Nishiyama, H.; Inoue, Y. J. Phys. Chem. B 2004, 108, 4369. (9) Domen, K.; Kudo, A.; Onishi, T. J. Catal. 1986, 102, 92. (10) Inoue, Y.; Kubokawa, T.; Sato, K. J. Phys. Chem. 1991, 95, 4059. (11) Ogura, S.; Kohno, M.; Sato, K.; Inoue, Y. Appl. Surf. Sci. 1997, 121/123, 521. (12) Inoue, Y.; Asai, Y.; Sato, K. J. Chem. Soc., Faraday Trans. 1994, 90, 797. (13) Kohno, M.; Kaneko, T.; Ogura, S.; Sato, K.; Inoue, Y. J. Chem. Soc., Faraday Trans. 1998, 94, 89. (14) Takata, T.; Furumi, Y.; Shinohara, K.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen, K. Chem. Mater. 1997, 9, 1063. (15) Takata, T.; Shinohara, K.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen, K. J. Photochem. Photobiol. A: Chem. 1997, 106, 45. (16) Sayama, K.; Arakawa, H. J. Phys. Chem. 1993, 97, 531. (17) Kudo, A.; Tanaka, A.; Domen, K.; Maruya, K.; Aika, K.; Onishi, T. J. Catal. 1998, 111, 67. (18) Kudo, A.; Kato, H.; NakaGawa, S. J. Phys. Chem. B 2000, 104, 571. (19) Kato, H.; Kudo, A. Catal. Lett. 1999, 58, 153. (20) Ishihara, T.; Nishiguchi, H.; Fukamachi, K.; Takita, Y. J. Phys. Chem. B 1999, 103, 1. (21) Kato, H.; Kudo, A. Chem. Phys. Lett. 1998, 295, 487. (22) Kato, H.; Kudo, A. Chem. Lett. 1999, 1027. (23) Kadowaki, H.; Sato, J.; Kobayashi, H.; Saito, N.; Nishiyama, H.; Shimodaira, Y.; Inoue, Y. J. Phys. Chem. B 2005, 109, 22995. (24) Payne, M. C.; Teter, M. P.; Allan, D. C.; Arias, T. A.; Joannopoulos, J. D. ReV. Mod. Phys. 1992, 64, 1045. (25) Vanderbilt, D. Phys. ReV. 1990, B41, 7892. (26) ICSD No. 50846. (27) ICSD No. 153500. (28) Repelin, Y.; Proust, C.; Husson, E.; Beny, J. M. J. Solid State Chem. 1995, 118, 163. (29) White, W. B.; Keramidas, V. G. Spectrochim. Acta, 1972, 28A, 501. (30) Inoue, Y.; Asai, Y.; Sato, K. J. Chem. Soc., Faraday Trans. 1994, 90, 797. (31) Kohno, M.; Ogura, S.; Sato, K.; Inoue, Y. Chem. Phys. Lett. 1997, 267, 72. (32) Inoue, Y.; Kubokawa, T.; Sato, K. J. Phys. Chem. 1991, 95, 4059. (33) Ogura, S.; Kohno, M.; Sato, K.; Inoue, Y. Phys. Chem. Chem. Phys. 1999, 1, 179.