Photocatalytic Activity of the RuO2-Dispersed Composite p-Block

Nov 12, 2005 - The ruthenium oxide-loaded composite p-block metal oxide LiInGeO4 with d10-d10 configuration exhibited high photocatalytic activity for...
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J. Phys. Chem. B 2005, 109, 22995-23000

22995

Photocatalytic Activity of the RuO2-Dispersed Composite p-Block Metal Oxide LiInGeO4 with d10-d10 Configuration for Water Decomposition H. Kadowaki, J. Sato, H. Kobayashi,† N. Saito, H. Nishiyama, Y. Simodaira,‡ and Y. Inoue* Department of Chemistry, Nagaoka UniVersity of Technology, Nagaoka 940-2188, Japan ReceiVed: August 10, 2005; In Final Form: September 18, 2005

The ruthenium oxide-loaded composite p-block metal oxide LiInGeO4 with d10-d10 configuration exhibited high photocatalytic activity for the overall splitting of water to produce H2 and O2 under UV irradiation. Changes in the photocatalytic activity with the calcination temperature of LiInGeO4, the amount of RuO2 loaded, and the states of RuO2 indicated that the combination of highly crystallized LiInGeO4 and a high dispersion of RuO2 particles resulted in high photocatalytic activity. Structurally, LiInGeO4 contained heavily distorted InO6 octahedra and GeO4 tetrahedra, generating a dipole moment inside. The high photocatalytic performance of RuO2-loaded LiInGeO4 supports the existing view that the photocatalytic activity correlates with the dipole moment. The DFT calculation showed that the top of the valence band (HOMO) was composed of the O 2p orbital while the bottom of the conduction band (LUMO) was formed by the hybridized In 5s5p + Ge 4s4p + O 2p orbitals. The highly dispersed conduction band, indicative of a high mobility of photoexcited electrons, was responsible for the high photocatalytic performance.

Introduction We have previously demonstrated that a series of p-block metal oxides consisting of different 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 become photocatalytically active for water decomposition under UV illumination upon RuO2 loading. Interestingly, the p-block metal ions of Ga3+, In3+, Ge4+, Sn4+, and Sb5+, being core elements, possess d10 electronic configuration. The DFT calculations revealed that the conduction band of photocatalysts with this d10 configuration was formed by the sp orbitals, whose electronic structures differed from those of the d orbitals of a conventional photocatalyst group consisting of transition-metal ions (Ti4+, Zr4+, Nb5+, Ta5+) with d0 configuration.9-22 It would be interesting to use composite metal oxides involving two kinds of the p-block metal ions, i.e., metal oxides with d10-d10 configuration, for the development of photocatalysts for water decomposition since it is thought that hybridization of the sp orbitals of two p-block metal ions may have a significant effect on the density of states and energy dispersion in the conduction bands. Furthermore, we have shown that most photocatalytically active p-block metal oxides are formed by distorted metaloxygen octahedra and tetrahedra. The presence of two different metal ions in the unit cell leads to structural effects that cause large distortion in the tetrahedral and octahedral units of the metal oxides. The resultant geometric and electronic effects suggest that composite p-block metal oxides with d10-d10 configuration would be effective photocatalysts. The present study centered around a p-block composite metal oxide containing In3+ and Ge4+ as metal ions of d10 configuration, namely, LiInGeO4, whose photocatalytic activity for † Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan. ‡ Department of Applied Chemistry, Faculty of Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan.

water decomposition was assessed after RuO2-loading. The optimal conditions for high photocatalytic performance were determined experimentally. The photocatalytic activity was compared with that of a metal oxide, LiInO2, with In3+ as its only metal ion,5 and the advantages of the d10-d10 configuration were discussed on the basis of the local geometric structure and band structures calculated by the DFT method. Experimental Section For the preparation of LiInGeO4, an equimolar mixture of Li2CO3 (Nacalai tesque, EP grade), In2O3 (Nacalai tesque, EP grade), and GeO2 (Nacalai tesque, GR grade) was calcined in air for 16 h in the temperature range 1123-1523 K. For RuO2 loading, LiInGeO4 was impregnated with the ruthenium carbonyl complex Ru3(CO)12 in THF, followed by oxidation in air at 673 K for 5 h to convert the loaded Ru complex into dispersed RuO2 particles. Details of the procedure for photocatalytic decomposition of water have been reported elsewhere.3,4 The RuO2-loaded LiInGeO4 powder (250 mg) was placed in distilled, ion-exchanged water (30 mL) in the quartz reaction cell of a closed gascirculating apparatus. The photocatalyst powder was dispersed in water through continuous bubbling Ar gas (13.3 kPa) circulation during the photocatalytic reaction and irradiated with a 200 W Hg-Xe lamp (Hamamatsu L566-02). The amounts of H2 and O2 produced in the gas phase were analyzed by an online gas chromatograph. The X-ray diffraction patterns of the LiInGeO4 prepared were obtained with an X-ray diffractometer (Rigaku RAD III). UV diffuse reflectance spectra were recorded on a UV-vis spectrometer (JASCO V-570). Scanning electron microscopy (SEM) images were obtained using a Shimadzu EPMA 1600. The surface area was measured by the BET method (Yuasa Chem BET3000). The band calculation of LiInGeO4 was carried out using the plane-wave DFT program package Castep.23 According to the

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Figure 1. Production of H2 and O2 from water on 1.0 wt % RuO2dispersed LiInGeO4 under UV irradiation. LiInGeO4 was prepared by calcination at 1473 K. Figure 3. XRD patterns of LiInGeO4 calcined at 1173 K (a), 1273 K (b), 1373 K (c), and 1473 K (d).

Figure 2. Photocatalytic activity of RuO2-dispersed LiInGeO4 for H2 and O2 production as a function of the calcination temperature of LiInGeO4.

ultrasoft core potential scheme,24 the valence atomic configurations of LiInGeO4 were 1s22s1 for Li, 5s24d105p1 for In, 4s24p2 for Ge, and 2s22p4 for O. The numbers of electrons and occupied bands for LiInGeO4 were 176 and 88, respectively. The kinetic energy cutoff was set to 330 eV. Results Figure 1 shows water decomposition on 1.0 wt % RuO2dispersed LiInGeO4 under Hg-Xe lamp irradiation. Both H2 and O2 were produced starting with the initial stage of reaction. The O2 production was low and increased with elapsed time, leveling off after 2 h of irradiation. In the second run, H2 and O2 were produced at constant rates. The production of H2 and O2 gradually decreased in the initial stages and became constant after a prolonged run (e.g., eighth run). The concentration of Li ions dissolved in the water before reaching constant activity was measured, which was 0.07% of the total Li ion in LiInGeO4 (this amount roughly corresponded to Li ions located in 12 layers from the surface). Thus, only a small part of Li is removed, which is considered to be associated with the initial activity decrease. Figure 2 shows the photocatalytic activity of 1.0 wt % RuO2-dispersed LiInGeO4 in H2 and O2 production as a function of the calcination temperature used during LiInGeO4 preparation. The activity increased with increasing temperature from 1123 K and reached a maximum at 1273 K, beyond which it rapidly decreased. The H2/O2 ratio was 3.2, 2.0, 2.0, and 2.8

at a calcination temperature of 1123, 1173, 1273, and 1473 K, respectively. Figure 3 shows the X-ray diffraction patterns of LiInGeO4 calcined at different temperatures. In the case of the sample calcined at 1173 K, most of the peaks were attributed to LiInGeO4, but relatively strong peaks due to In2O3 and Li2GeO3 were observed as well. Upon calcination at 1273 K, the In2O3 and Li2GeO3 peaks were all significantly weaker. Calcination at 1373 K yielded a diffraction pattern due entirely to the single-phase LiInGeO4. Nearly the same pattern was obtained by calcination at 1473 K. With increasing temperature from 1173 to 1473 K, the major peaks became narrower and their intensity increased. Figure 4 shows SEM micrographs of LiInGeO4 calcined at different temperatures. Calcination at 1173 K yielded irregularly shaped fine particles. At 1273 K, the particles retained their morphology, but were considerably enlarged, and above 1373 K, marked growth due to agglomeration occurred. The surface area was 0.8-1.0 m2 g-1 for calcination below 1273 K and decreased to 0.5 m2 g-1 above 1373 K. Figure 5a shows the variation in photocatalytic activity with the amount of RuO2 loading. The activity increased with the amount of loading, reached a maximum at 1 wt %, and decreased upon further loading. Figure 5b shows the influence of the oxidation temperature of the Ru complex on the activity. The highest activity was obtained at a medium oxidation temperature between 473 and 773 K, namely, 673 K. The ratio of H2 to O2 was 4.1, 2.0, and 4.4 for an oxidation temperature of 473, 673, and 773 K, respectively, indicating that the highest activity was obtained with the stoichiometric ratio of H2 to O2. Figure 6 shows the UV diffuse reflectance spectra of LiInGeO4 calcined in the 1173-1373 K range. Light absorption started at around 440 nm irrespective of the calcination temperature. For LiInGeO4 calcined at 1173 K, the absorption increased, passed through a maximum at around 330 nm, and leveled off at 250 nm. The absorption at 330 nm attenuated with increasing calcination temperature and was dramatically reduced upon calcination at 1373 K. The main absorption occurred at 250 nm. Nearly the same spectrum was obtained for LiInGeO4 calcined at 1473 K. Figure 7 shows the band structure and density of states (DOS) of LiInGeO4. (The DOS breaks down into the angular momentum of atomic orbitals (AOs) represented by different line shapes.) The energy at the top of the valence band is taken as zero. The occupied bands are the Li 1s (nos. 1-4), O 2s + Ge

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Figure 4. SEM images of LiInGeO4 calcined at 1173 K (a), 1273 K (b), 1373 K (c), and 1523 K (e).

Figure 6. UV diffuse reflectance spectra of LiInGeO4 calcined at 1173 K (a), 1273 K (b), and 1373 K (c).

Figure 5. Variation in the photocatalytic activity of RuO2-dispersed LiInGeO4 for H2 and O2 production with the amount of RuO2 (a) (oxidized at 673 K) and oxidation temperature of 1 wt % loaded Ru species (b).

4s4p + In 5p (nos. 5-20), In 4d (nos. 21-40), and O 2p + Ge 4s4p (nos. 41-88) bands in increasing order of energy, where the last is the valence band. The bottom of the unoccupied level (conduction band) is composed of the Ge 4s, In 5s, and O 2p AOs. The Ge 4p, In 5p, and Li 2s2p AOs, in order of increasing energy, form the next energy level. Thus, it is revealed that LiInGeO4 has the characteristic electronic structure of a d10 semiconductor. To obtain more exact information on the atom-specific character of each band, the DOS is further broken into AOs, i.e., the projected DOS (PDOS) in terms of atomic and angular momentum contributions. Figures 8 and 9 show the total DOS

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Figure 7. Band dispersion and DOS for (LiInGeO4)4.

and AO PDOS for Li, In, Ge, and O atoms for the higher energy region (from -10 to 0 eV) of occupied bands and the lower energy region (from 0 to 15 eV) of unoccupied bands, respectively. The density spread above 0 eV is an artifact of the band shape smearing technique and has no physical meaning. The band gap is estimated to be 3.17 eV as the energy difference between the top of the valence band and the bottom of the conduction band. The electron density contour maps of HOMO and LUMO levels related to photoexcitation are shown in Figures 10 and 11, respectively. Discussion LiInGeO4 with its d10-d10 configuration made a strong photocatalyst for producing H2 and O2 from water under HgXe lamp illumination when RuO2 was loaded onto its surface. The dependence of the photocatalytic activity on the calcination temperature of LiInGeO4 showed trends similar to those observed in other p-block metal oxides such as RuO2-loaded MIn2O4 (M ) Ca, Sr),2,6 NaInO2,5 ZnGa2O44, and Zn2GeO4:8 with increasing calcination temperature, the activity increased significantly, passed through a maximum, and then sharply decreased. In the low-temperature regime where the activity increased, the SEM images (Figure 4) showed a considerable increase in the size of LiInGeO4 particles and a narrowing of the X-ray diffraction peaks. These results demonstrate that the crystallization of LiInGeO4 is responsible for activity enhancement, since it eliminates impurities and structural imperfections that frequently work as traps for charge recombination. In the calcination temperature regime above 1473 K where the activity decreased, an exaggerated growth of LiInGeO4 particles occurred, resulting in a marked decrease in the surface area of LiInGeO4. The activity drop is directly related to the reduction of the surface area of LiInGeO4. Furthermore, the relationship

Figure 8. Expanded AO partial DOS of valence bands for Li (a), In (b), Ge (c), and O (d) atoms in (LiInGeO4)4.

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Figure 11. Electron density contour map for the bottom (no. 89) of the conduction band (LUMO). See Figure 10 for the color coding.

Figure 9. Expanded AO partial DOS of conduction bands for Li (a), In (b), Ge (c), and O (d) atoms in (LiInGeO4)4.

Figure 10. Electron density contour map for the top (no. 88) of the valence band (HOMO): Li, purple; In, brown; Ge, green; O, red.

between the amount of RuO2 loading and photocatalytic activity (see Figure 5a) indicated that the excess RuO2 on the LiInGeO4 surface lowered the activity. This is due to the reduction in the density of active sites on the RuO2 particles because of the agglomeration and growth of the particles. Thus, the appearance of a maximum in the curve of the photocatalytic activity vs the calcination temperature of LiInGeO4 indicates that the combination of well-crystallized LiInGeO4 and highly dispersed RuO2 particles enhances the performance of the photocatalysts. The observation that the highest activity coincided with the oxidation of impregnated Ru species at 673 K (Figure 5b) is consistent with previous results obtained for BaTi4O9 and Na2Ti6O13. The X-ray photoelectron spectra showed that the binding energy of the Ru 3d5/2 level for Ru species oxidized at 673 K

was 280.9 eV. This was in good agreement with that of Ru4+ species, which indicated the formation of RuO2 particles as an enhancer of catalytic activity. A correlation between photocatalytic activity and the distortion of octahedral and tetrahedral metal oxygen units has been demonstrated in a series of p-block metal oxide photocatalysts with d10 configuration.7 Both SrIn2O4 and Sr0.93Ba0.07In2O4 had two kinds of distorted octahedral InO6 (the dipole moments were 2.8 and 1.1 D (debye) for the former and 1.70 and 2.58 D for the latter), whereas LiInO2 was composed of regular, undistorted InO6 octahedra, whose dipole moment was zero.25,26 Both RuO2dispersed SrIn2O4 and Sr0.93Ba0.07In2O4 were photocatalytically active for water decomposition under Hg-Xe lamp irradiation, but RuO2-loaded LiInO2 showed negligible activity under similar reaction conditions. Furthermore, Zn2GeO4 contained distorted GeO4 tetrahedra with a dipole moment of 1.6 D and was photocatalytically active for water decomposition in the presence of RuO2.8 According to a model proposed in previous studies27-30 of RuO2-dispersed BaTi4O9 and Na2Ti6O13 with d0 configuration, the internal fields due to the dipole moment of distorted TiO6 octahedra promote electron-hole separation. It is interesting to find out whether the correlation between activity and dipole moment is also observed in the case of composite metal oxides with a d10-d10 electronic configuration. Figure 12 shows the crystal structure of LiInGeO4.31 The octahedral InO6 and tetrahedral GeO4 are joined through the corner oxygen atoms. Both the InO6 octahedron and GeO4 tetrahedron in LiInGeO4 are heavily distorted. The position of the In cation in the InO6 octahedron deviates from the center of gravity of the surrounding six oxygen anions: a dipole moment of 3.0 D is present in the InO6 unit. The GeO4 tetrahedron is also distorted, and has a dipole moment of 3.4 D. Thus, the present results for the RuO2-dispersed LiInGeO4 showing high photocatalytic performance are in line with the correlation between activity and dipole moment. The advantage of d10-d10 composite metal oxides over metal oxides consisting of a single d10 metal ion is the ease of formation of distorted octahedral and tetrahedral units because of the presence of two metal ions with different sizes in the unit cell. As can be seen in Figure 7, the Li 1s band only appeared at the lowest energy region (ca. -40 eV). The O 2s band split into two peaks, which were hybridized with the Ge 4s AO and the Ge 4p and In 5p AOs, respectively. The In 4d band had a single strong peak and was not mixed with other AOs. As shown in Figure 8, the main component of the valence band was the O 2p AOs, which was divided into three parts according to the energy. The lower part is hybridized with the Ge 4s and In 5s AOs, and the middle part is hybridized with the Ge 4p and In 5p AOs. There is no overlap or mix between the Ge 4s and 4p

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Kadowaki et al. feature of the LUMO is a large dispersion. This indicates that the LUMO contains no density at the bottom of the conduction band, but rather covers an energy range of ca. 4 eV at the bottom of the conduction band. Thus, the large band dispersion is a consequence of the large overlap among the n + 1 shell AOs, characteristic of d10 semiconductors. In the electron transfer from the O2p AO to the hybridized In 5s5p + Ge 4s4p + O 2p AOs upon illumination, the large dispersion in the conduction bands generates photoexcited electrons with large mobility, which is evidently associated with the high photocatalytic performance of RuO2-loaded LiInGeO4. In conclusion, the d10-d10 configuration is useful for photocatalysis from the viewpoint of both geometric and electronic structure. Acknowledgment. This work was supported by the Core Research for Evolutional Science and Technology (CREST) and Solution Oriented Research for Science and Technology (SORST) programs of the Japan Science and Technology Corp. (JST) and by a Grant-in-Aid for Scientific Research on Priority Area (17029022) from The Ministry of Education, Science, Sports and Culture of Japan. References and Notes

Figure 12. A schematic representation of the crystal structure of LiInGeO4.

AOs over the entire energy range. The In 5s and 5p AOs mix and overlap in a wide energy region. The intensity of In 5s and 5p AOs is lower than that of Ge 4s and 4p AOs. The upper part of the valence band is composed of the O 2p AO, slightly mixed with Li 2s2p AOs, but the HOMO level is solely localized in the O 2p AO. As shown in Figure 9, the bottom of the conduction band is composed of the Ge 4s, In 5s, and O 2p AOs. With increasing energy in the conduction band, Ge 4p, In 5p, and Li 2s2p AOs appear in that order. The Ge 4s4p and In 5s5p AOs contribute to both the valence and conduction bands. Hybridization with the O 2p AO occurs in phase for the former and out of phase for the latter. Our previous calculation for LiInO2 showed that the valence band consisted of the O 2p AOs, and the conduction band was formed by the In 5s5p AOs mixed with the Li 2s2p AOs.5 Although Li 2s2p AOs appear in the conduction band of both LiInO2 and LiInGeO4, the difference is that the Li 2s2p AOs in LiInGeO4 contribute to the conduction band only in a high-energy region, while the Ge 4s4p AOs are available in a comparatively lower energy region. Thus, from a viewpoint of band structures, the contribution of Ge 4s4p to the conduction bands is responsible for the high photocatalytic activity of RuO2dispersed LiInGeO4. Figures 10 and 11 show the contour maps for the HOMO and LUMO levels, respectively. Figure 10 clearly shows that the top of the valence band is characteristic of the O 2p AO. No hybridization with the other AOs takes place. On the other hand, the bottom of the conduction band is composed mainly of the In 5s and Ge 4s AOs, exhibiting a slight mixing with the In 5p, Ge 4p, and O 2p AOs. Thus, the LUMO is composed of In 5s5p and Ge 4s4p AOs as well as the O 2p AO, and the polarization effects by the In 5p and Ge 4p AOs are well represented in Figure 11. As shown in Figure 7, the interesting

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