Anisotropic Charge Migration on Perovskite Oxysulfide for Boosting Photocatalytic Overall Water Splitting

The synthesis of photocatalysts with both broad light absorption and efficient charge separation is significant for a high solar energy conversion, which still remains to be a challenge. Herein, a narrow-bandgap Y₂Ti₂O₅S₂ (YTOS) oxysulfide nanosheet coexposed with defined {101} and {001} facets synthesized by a flux-assisted solid-state reaction was revealed to display the character of an anisotropic charge migration. The selective photodeposition of cocatalysts demonstrated that the {101} and {001} surfaces of YTOS nanosheets were the reduction and oxidation regions during photocatalysis, respectively. Density functional theory (DFT) calculations indicated a band energy level difference between the {101} and {001} facets of YTOS, which contributes to the anisotropic charge migration between them. The exposed Ti atoms on the {101} surface and S atoms on the {001} surface were identified, respectively, as reducing and oxidizing centers of YTOS nanosheets. This anisotropic charge migration generated a built-in electric field between these two facets, quantified by spatially resolved surface photovoltage microscopy, the intensity of which was found to be highly correlated with photocatalytic H₂ production activity of YTOS, especially exhibiting a high apparent quantum yield of 18.2% (420 nm) after on-site modification of a Pt@Au cocatalyst assisted by Na₂S-Na₂SO₃ hole scavengers. In conjunction with an oxygen-production photocatalyst and a [Co(bpy)₃]^2+/3+ redox shuttle, the YTOS nanosheets achieved a solar-to-hydrogen conversion efficiency of 0.15% via a Z-scheme overall water splitting. Our work is the first to confirm anisotropic charge migration in a perovskite oxysulfide photocatalyst, which is crucial for enhancing charge separation and surface catalytic efficiency in this material.