Oxysulfide Sm2Ti2S2O5 as a Stable Photocatalyst for Water Oxidation and Reduction under Visible Light Irradiation (λ ≤ 650 nm)

Abstract

A Ti-based oxysulfide, Sm₂Ti₂S₂O₅, was studied as a visible light-driven photocatalyst. Under visible light (440 nm ≤ λ ≤ 650 nm) irradiation, Sm₂Ti₂S₂O₅ with a band gap of ∼2 eV evolved H₂ or O₂ from aqueous solutions containing a sacrificial electron donor (Na₂S−Na₂SO₃ or methanol) or acceptor (Ag⁺) without any noticeable degradation. This oxysulfide is, therefore, a stable photocatalyst with strong reduction and oxidation abilities under visible-light irradiation. The electronic band structure of Sm₂Ti₂S₂O₅ was calculated using the plane-wave-based density functional theory (DFT) program. It was elucidated that the S3p orbitals constitute the upper part of the valence band and these orbitals make an essential contribution to the small band gap energy. The conduction and valence bands' positions of Sm₂Ti₂S₂O₅ were also determined by electrochemical measurements. It indicated that conduction and valence bands were found to have satisfactory potentials for the reduction of H⁺ to H₂ and the oxidation of H₂O to O₂ at pH = 8. This is consistent with the results of the photocatalytic reactions.