Abstract: Oxysulfide semiconductors are promising photocatalysts for visible light-driven water splitting. For a widely studied narrow-bandgap Sm₂Ti₂O₅S₂ (STOS), limited bulk charge separation and slow surface reaction heavily restrict its photocatalytic performance. Here, well-crystallized STOS oxysulfide nanosheets, synthesized by a flux-assisted solid-state reaction, were proved to show prominent facet-oriented charge transport property, in which photogenerated electrons migrated to {101} planes and holes to {001} planes of each particle. Hydrogen evolution cocatalysts were therefore precisely positioned on the electron-rich facets to boost the water reduction reaction. In particular, in-situ formation of a Pt_shell@Ir_core core-shell structure on the electron-rich {101} facets and an IrO₂ on the hole-accumulated {001} facets greatly assisted the sacrificial photocatalytic H₂ production over STOS, resulting in an apparent quantum yield as high as 35.9% at 420 nm. By using the highly-active STOS as H₂ evolution photocatalyst, a Mo:BiVO₄ as oxygen evolution photocatalyst, and a [Co(bpy)₃]^2+/3+ as redox shuttle, a Z-Scheme overall water splitting system was constructed to achieve a solar-to-hydrogen conversion efficiency of 0.175%. This work not only elucidates the facet-dependent charge transfer mechanism on STOS but also proposes an ideal strategy for enhancing its photocatalytic performance.

Key words: Oxysulfide, Photocatalysis, Anisotropic charge transport, Overall water splitting, Hydrogen production