Synthesis of sulfur doped g-C₃N₄ with enhanced photocatalytic activity in molten salt

In this paper, sulfur doped g-C₃N₄ (S-g-C₃N₄) was successfully prepared at 500 ℃ for 3 h via a modified molten salt method using dicyandiamide as the main raw material, trithiocyanuric acid as the sulfur source and LiBr-KCl as the reaction medium. The as-prepared S-CN5.0% sample (the mass ratio of trithiocyanuric acid to dicyandiamide was 5.0%) composed of irregular flakes showed a band gap of 1.83 eV, which was narrower than that (2.55 eV) of pristine g-C₃N₄. The S-CN5.0% sample also exhibited an outstanding absorption capacity of visible light. Moreover, the photodegradation rate toward methylene blue and tetracycline were respectively 10 and 20 times as high as that of bulk g-C₃N₄ prepared by conventional heating methods, confirming its superior photocatalytic performance. These results can be attributed to that the replacement of lattice nitrogen with sulfur atom tuned the electronic structure of g-C₃N₄, improved the absorption of visible light, optimized the separation of photogenerated electron-hole pairs, and consequently enhanced the photocatalytic activity of g-C₃N₄. Moreover, the trapping experiments implied that hole (h⁺) and superoxide radical (·O²⁻) were the main active species in the process of photodegradation.