TiO2/SnO2/g-C3N4 Type-II heterojunction: DFT design, PEC coupling technologies, and degradation mechanisms with active site analysis

Photoelectrocatalysis (PEC) can realize the efficient removal of organic pollutants in water. However, reasonably choosing coupling technologies to improve the PEC degradation efficiency remains a challenge. This study proposed a novel "Theory-Structure-Performance-Mechanism" experimental framework. TiO₂/SnO₂/g-C₃N₄ was designed via density functional theory (DFT) calculations and synthesized using a new time-saving method. The type-II heterojunction characteristics of TiO₂/SnO₂/g-C₃N₄ was identified, the reactive species were speculated, and two coupling technologies were designed. Compared to PEC, PEC-PMS and PEC-Self-Fenton technologies could significantly enhance the model pollutant rhodamine B (RhB) removal from 36 % to 94 % and 85 %, respectively, mainly due to the improved electron utilization. The specific mechanisms were as follows: the ^•SO₄^- generated by the activation of PMS for the former, and the ^•OH produced from H₂O₂ derived from the dual-electron reduction of O₂ for the latter. In addition, two potential RhB degradation pathways were proposed, and the results of the condensed Fukui function indicated that, f⁰ was more suitable for analysing active sites, and the C-N single-bonded structure of RhB was more likely to be attacked by ^•OH in PEC system. This novel experimental framework offers guidance for designing coupling technologies to improve the PEC degradation efficiency. At the same time, it provides a basis for exploring the corresponding attack active sites of different reactive species in the PEC system.