Trace Cu⁺-dominated band structure engineering in Cu_xIn_0.25ZnS_y for promoting photocatalytic H₂ evolution

As an attractive semiconductor photocatalyst, (CuInS₂)_x-(ZnS)_y has been intensively studied in photocatalysis, due to its unique layered structure and stability. Here, we synthesized a series of Cu_xIn_0.25ZnS_y photocatalysts with different trace Cu⁺-dominated ratios. The results show that doping with Cu⁺ ions leads to an increase in the valence state of In and the formation of a distorted S structure, simultaneously inducing a decrease in the semiconductor bandgap. When the doping amount of Cu⁺ ions is 0.04 atomic ratio to Zn, the optimized Cu_0.04In_0.25ZnS_y photocatalyst with a bandgap of 2.16 eV shows the highest catalytic hydrogen evolution activity (191.4 μmol.h^-1). Subsequently, among the common cocatalysts, Rh loaded Cu_0.04In_0.25ZnS_y gives the highest activity of 1189.8 μmol·h^-1, corresponding to an apparent quantum efficiency of 49.11 % at 420 nm. Moreover, the internal mechanism of photogenerated carrier transfer between semiconductors and different cocatalysts is analyzed by the band bending phenomenon.