Construction homojunction and co-catalyst in ZnIn2S4 photoelectrode by Co ion doping for efficient photoelectrochemical water splitting

•The Mg:ZnIn2S4/Co:ZnIn2S4 homojunction was synthesized by two-step hydrothermal method.•The deeply doped Co2+ can change the gradient energy level to form a homojunction with the Mg-doped ZnIn2S4.•The Co2+ on the surface can be used as reaction sites for water oxidation to promote the charge utilization.•The dual effect of Co2+ doping makes the ZnIn2S4 achieve an excellent photocurrent density.