Chapter 4: Recent Advances in (Oxy)sulfide Photocatalysts for Solar-driven Water Splitting and CO₂ Reduction

Metal sulfides are an attractive material group as photocatalysts in terms of utilizing visible light in sunlight. However, S²⁻ in metal sulfides are oxidized by photogenerated holes. Hence, the metal sulfides rarely oxidize water to O₂ under photoirradiation. To achieve water splitting and CO₂ reduction as artificial photosynthetic reactions in which water is used as a sole electron donor, application of metal sulfides to photoelectrode and Z-scheme systems is one useful strategy. In both systems, metal sulfides are used as a reduction part that is photocathode and photocatalyst for the reduction of water and CO₂. Although metal sulfides only contain S²⁻ as the anion, metal oxysulfides contain S²⁻ and O²⁻. The valence band maximum formed by the O 2p and S 3p hybridized orbitals of oxysulfides stabilizes S²⁻ ions when compared to those in the sulfides. Such metal oxysulfides are active for photocatalytic water oxidation to O₂.