Oxysulfides Ln2Ti2S2O5 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 less than or equal to lambda less than or equal to 650 nm) irradiation in the presence of a sacrificial electron donor (Na2S-Na2SO3) or acceptor (Ag+) without noticeable degradation. Ln(2)Ti(2)S(2)O(5) is synthesized by sulfurization under H2S flow, and the Sm2Ti2S2O5 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)Ti2S2O5 is made up of hybridized O2p, S3p, and Ln4f orbitals, whereas Ln4f orbitals are localized in other [Pr-Sm](2)Ti2S2O5. In addition, the conduction band of [Gd-Er](2)Ti2S2O5 consists of S3p+Ln4f and Ti3d orbitals. The photocatalytic activity is discussed on the basis of the electronic band structure and bulk material structure.