Oxysulfides Ln 2 Ti 2 S 2 O 5 as Stable Photocatalysts for Water Oxidation and Reduction under Visible-Light Irradiation

The series Ln 2 Ti 2 S 2 O 5 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho and Er) is demonstrated to evolve H 2 or O 2 from aqueous solutions under visible-light (440 nm ≤ λ ≤ 650 nm) irradiation in the presence of a sacrificial electron donor (Na 2 S−Na 2 SO 3 ) or acceptor (Ag + ) without noticeable degradation. Ln 2 Ti 2 S 2 O 5 is synthesized by sulfurization under H 2 S flow, and the Sm 2 Ti 2 S 2 O 5 form is found to have the highest activity for O 2 evolution. X-ray Rietveld refinements reveal that the Ln 2 Ti 2 S 2 O 5 framework of the Pr, Nd, and Er forms is distorted from the ideal perovskite structure. The calculations of the electronic band structures of Ln 2 Ti 2 S 2 O 5 based on plane-wave based density functional theory indicated that the top of the valence band of [Gd−Er] 2 Ti 2 S 2 O 5 is made up of hybridized O2p, S3p, and Ln4f orbitals, whereas Ln4f orbitals are localized in other [Pr−Sm] 2 Ti 2 S 2 O 5 . In addition, the conduction band of [Gd−Er] 2 Ti 2 S 2 O 5 consists of S3p+Ln4f and Ti3d orbitals. The photocatalytic activity is discussed on the basis of the electronic band structure and bulk material structure.