Sulfate modified g-C3N4 with enhanced photocatalytic activity towards hydrogen evolution: the role of sulfate in photocatalysis

Sulfate modified graphitic carbon nitride (g-C3N4) was prepared by simple co-pyrolysis of dicyandiamide and ammonium sulfate, and shows seven times higher photocatalytic activity towards hydrogen production than pristine g-C3N4. The origin of its improved photocatalytic activity was comprehensively investigated, and it was found that there are two kinds of sulfate (strongly adsorbed sulfate and a weakly adsorbed one) in the modified sample, both of which play important but slightly different roles in the photocatalysis. Compared to the strongly adsorbed one, the weakly adsorbed sulfate is more beneficial for charge separation and thus promotes more electrons to participate in the photocatalytic reaction. By applying the above synthesis method, most sulfate in our best photocatalyst exists as weakly adsorbed species, which is confirmed by advanced characterization techniques as well as DFT calculations. The increased number of electrons and improved charge separation, which are induced by the weakly adsorbed sulfate, are key to boosting the photocatalytic activity of g-C3N4. Hence, this work provides comprehensive insights into the effect of sulfate on the photocatalytic activity of g-C3N4, which help in the design of more efficient photocatalysts by suitable surface modification.