Oxysulfides Ln₂Ti₂S₂O₅ as Stable Photocatalysts for Water Oxidation and Reduction under Visible-Light Irradiation

Abstract

The series Ln2Ti2S2O5 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho and Er) is demonstrated to evolve H2 or O2 from aqueous solutions under visible-light (440 nm ≤ λ ≤ 650 nm) irradiation in the presence of a sacrificial electron donor (Na2S−Na2SO3) or acceptor (Ag+) without noticeable degradation. Ln2Ti2S2O5 is synthesized by sulfurization under H2S flow, and the Sm2Ti2S2O5 form is found to have the highest activity for O2 evolution. X-ray Rietveld refinements reveal that the Ln2Ti2S2O5 framework of the Pr, Nd, and Er forms is distorted from the ideal perovskite structure. The calculations of the electronic band structures of Ln2Ti2S2O5 based on plane-wave based density functional theory indicated that the top of the valence band of [Gd−Er]2Ti2S2O5 is made up of hybridized O2p, S3p, and Ln4f orbitals, whereas Ln4f orbitals are localized in other [Pr−Sm]2Ti2S2O5. In addition, the conduction band of [Gd−Er]2Ti2S2O5 consists of S3p+Ln4f and Ti3d orbitals. The photocatalytic activity is discussed on the basis of the electronic band structure and bulk material structure.