Photocatalytic activity for water decomposition of indates with octahedrally coordinated d10 configuration.: II.: Roles of geometric and electronic structures

The effects of electronic and local structures on the photocatalytic activities for water decomposition of different kinds of indates [MIn2O4 (M = Ca, Sr), Sr0.93Ba0.07In2O4, and AlnO(2) (A = Li, Na)] were studied. Raman spectroscopic measurements were performed to characterize their structures. The geometric structures of InO6 octahedral units were compared among the indates, and it was shown that the photocatalytically active indates possessed distorted InO6 octahedra with dipole moment, and there was a correlation between the photocatalytic activities and the dipole moment. A plane-wave density function theory (DFT) was applied to calculate the density of state and band energy diagram for SrIn2O4. The valence band was composed of the O 2p orbital, whereas the conduction band consisted of the hybridized In 5s5p orbitals with large dispersion, indicative of large mobility of photoexcited electrons in the conduction band. The electronic feature of SrIn2O4 was compared with that of a representative transition metal oxide of BaTi4O9 with an octahedrally coordinated d(0) metal ion. On the basis of the electronic and geometric consideration, a mechanism is proposed: internal fields due to the dipole moment promote the charge separation, while the broad sp conduction bands with large dispersion permit photoexcited electrons to move to RuO2 particles dispersed as a promoter.