A titanium-based oxysulfide photocatalyst: La5Ti2MS5O7 (M = Ag, Cu) for water reduction and oxidation

La5Ti2MS5O7, which absorb visible light of up to 570 nm (M = Ag) and up to 650 nm (M = Cu), respectively, have photocatalytic activities for both water reduction and oxidation. In this study, structural and optical properties, electronic state distributions, and photocatalytic activity for water reduction and oxidation on La5Ti2MS5O7 (M = Ag, Cu) were investigated. Density functional theory (DFT) calculations of the electronic band structures and charge densities indicated that hybrid orbitals of Cu 3d and S 3p formed the valence band edge of La5Ti2CuS5O7 while S 3p orbitals alone for La5Ti2AgS5O7. On the other hand, Ti 3d orbitals were the major components of the conduction band edges of both La5Ti2CuS5O7 and La5Ti2AgS5O7. Importantly, it was found that the paths of photoexcited electrons and holes in La5Ti2MS5O7 bulk were disassociated, which could be favorable for efficient charge separation. In fact, the activity for H-2 evolution after loading of Pt was significantly high among existing Ti-based oxysulfide photocatalysts. The apparent quantum efficiency of Pt-loaded La5Ti2AgS5O7 for H-2 evolution under visible light irradiation (at lambda = 420 +/- 10 nm) reached 1.2%. Moreover, La5Ti2MS5O7 loaded with IrO2 were active for photocatalytic O-2 evolution although the valence band maximum was composed of S 3p orbitals. These results suggest that La5Ti2MS5O7 modified with appropriate cocatalysts are promising photocatalysts for water splitting under visible light irradiation.