Construction of N-doped g-C₃N₄/NH₂-MIL-125(Ti) S-scheme heterojunction for enhanced photocatalytic degradation of organic pollutants: DFT calculation and mechanism study

Considerable attentions are currently paid to water pollution, which seriously threats human health. Semiconductor catalysts with step-like (S-scheme) heterojunction are often used for degrading organic pollutants in water bodies due to their high carrier separation efficiency, high redox capacity, and broad light absorption capacity. In this paper, a novel S-scheme N-doped g-C₃N₄/NH₂-MIL-125(Ti) heterojunction composite is synthesized for photodegradation of organic pollutants. The structural measurements indicate that the NH₂-MIL-125(Ti) nanoparticles are intimately anchored on the surface of porous N-doped g-C₃N₄ nanosheets. The experimental characterizations as well as the DFT theoretical calculation for samples demonstrate that the N-doped g-C₃N₄/NH₂-MIL-125(Ti) composites not only bear larger specific surface area providing abundant reactive sites, but also improve the separation efficiency of photogenerated carriers during photodegradation. In addition, the degradation rate of RhB solution catalyzed by 25 wt% N-doped g-C₃N₄/NH₂-MIL-125(Ti) (25 wt% NCM) attains the highest degradation rate (96.4%) under visible light irradiation. Moreover, the mineralization of RhB solution and intermediates produced in the degradation process are analyzed by total organic carbon (TOC) and high- performance liquid chromatography (HPLC) tests, respectively. Electron paramagnetic resonance (EPR) with free radical capture measurements show that the photodegradation of RhB is mainly ascribed to the superoxide radicals (·O₂^-) and hole (h⁺). This study provides an effective method to construct S-scheme heterojunction photocatalysts and apply them to wastewater treatment.