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

Abstract

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 H₂ evolution after loading of Pt was significantly high among existing Ti-based oxysulfide photocatalysts. The apparent quantum efficiency of Pt-loaded La₅Ti₂AgS₅O₇ for H₂ evolution under visible light irradiation (at λ = 420 ± 10 nm) reached 1.2%. Moreover, La₅Ti₂MS₅O₇ loaded with IrO₂ were active for photocatalytic O₂ 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.