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

TLDR

Sm(2)Ti(2)S(2)O(5) is a Ti‑based oxysulfide investigated as a visible‑light photocatalyst. Its electronic band structure was computed via DFT and its conduction and valence band positions were measured electrochemically. Under 440–650 nm irradiation, Sm(2)Ti(2)S(2)O(5) with a ~2 eV band gap stably produces H₂ and O₂ from aqueous solutions containing sacrificial donors or acceptors, demonstrating strong visible‑light reduction and oxidation capabilities that align with its favorable band potentials.

Abstract

A Ti-based oxysulfide, Sm(2)Ti(2)S(2)O(5), was studied as a visible light-driven photocatalyst. Under visible light (440 nm < or = lambda < or = 650 nm) irradiation, Sm(2)Ti(2)S(2)O(5) with a band gap of approximately 2 eV evolved H(2) or O(2) from aqueous solutions containing a sacrificial electron donor (Na(2)S-Na(2)SO(3) 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 Sm(2)Ti(2)S(2)O(5) 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 Sm(2)Ti(2)S(2)O(5) 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 H(2) and the oxidation of H(2)O to O(2) at pH = 8. This is consistent with the results of the photocatalytic reactions.