ZnO@g-C3N4 photocatalyst with switchable carrier transfer mechanism between type-Ⅱ and S-scheme through S doping

g-C3N4 nanosheets were synthesized using ball milling, followed by sulfur (S)-doping via the one-pot method. Subsequently, a S-doped ZnO@g-C3N4 composite photocatalyst was produced by calcining ZnO sol precursor–coated S-doped g-C3N4 nanosheets. This study investigated the S-doping mechanism in the g-C3N4 component and its impact on the structure and photoelectric properties of the composite photocatalyst. The carrier transfer mechanism in the ZnO@g-C3N4 photocatalyst shifted from type-II to S-scheme due to S-doping. Moreover, S-doping enhanced the photocatalytic performance of the composite. In terms of precursor quantification, the composite photocatalyst with a thiourea doping source amount of 0.65 mol exhibited optimal performance. In the degradation process, the dominant active radicals produced by the photocatalyst sample without S-doping were identified as •OH radicals, whereas those produced by the S-doped photocatalyst sample were identified as •O2− and •OH radicals. Consequently, the composite photocatalyst demonstrated superior performance in methylene blue (MB) solution at a pH of 11 compared to its performance in the MB solutions at pH values of 3 and 7.