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[Display omitted] • The regulation of trace Cu+ ions on the structures and band of Cu x In 0.25 ZnS y solid solution are studied. • The mechanism of energy band bending and matching between Cu x In 0.25 ZnS y and cocatalysts are revealed. • The optimized Cu x In 0.25 ZnS y @Rh photocatalyst exhibits a high catalytic HER activity under the visible light irradiation. As an attractive semiconductor photocatalyst, (CuInS 2) x -(ZnS) y has been intensively studied in photocatalysis, due to its unique layered structure and stability. Here, we synthesized a series of Cu x In 0.25 ZnS 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 0.04 In 0.25 ZnS 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 0.04 In 0.25 ZnS 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. [ABSTRACT FROM AUTHOR]