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

La(5)Ti(2)MS(5)O(7), 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 La(5)Ti(2)MS(5)O(7) (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 La(5)Ti(2)CuS(5)O(7) while S 3p orbitals alone for La(5)Ti(2)AgS(5)O(7). On the other hand, Ti 3d orbitals were the major components of the conduction band edges of both La(5)Ti(2)CuS(5)O(7) and La(5)Ti(2)AgS(5)O(7). Importantly, it was found that the paths of photoexcited electrons and holes in La(5)Ti(2)MS(5)O(7) 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 La(5)Ti(2)AgS(5)O(7) for H(2) evolution under visible light irradiation (at λ = 420 ± 10 nm) reached 1.2%. Moreover, La(5)Ti(2)MS(5)O(7) loaded with IrO(2) were active for photocatalytic O(2) evolution although the valence band maximum was composed of S 3p orbitals. These results suggest that La(5)Ti(2)MS(5)O(7) modified with appropriate cocatalysts are promising photocatalysts for water splitting under visible light irradiation.