Highly stable Cu doped SnO₂/TiO₂ photocatalyst enables selective 3e⁻ ORR-driven •OH generation for robust GenX removal

In this study, the photocatalytic degradation efficiencies of GenX using SnO₂/TiO₂ composites and their Cu-doped counterparts were investigated. Although SnO₂ and TiO₂ separately exhibited limited GenX removal, the SnO₂/TiO₂ composite demonstrated significantly enhanced activity, which was further boosted by aeration. Cu doping at 2 wt% [Cu(2 %)-SnO₂/TiO₂] magnified the GenX (Hexafluoropropylene oxide dimer acid; HFPO-DA) removal efficiency (96.4 %) with superior kinetics (0.399 min⁻¹) and stability. Various characterization results revealed that Cu doping induced a type II heterojunction, facilitating electron transfer from TiO₂ to SnO₂ and ultimately to Cu₂O. Cu(2 %)-SnO₂/TiO₂ exhibited a unique band structure with a lower conduction band minimum and higher valence band maximum, which suppressed photocorrosion while promoting reactive oxygen species generation, particularly •OH. This was evidenced by the enhanced H₂O₂ production, increased photocurrent density, and decreased charge recombination, as confirmed by time-resolved photoluminescence analysis. Notably, Cu doping altered the electron transfer pathway, favoring a 3-electron oxygen reduction reaction for efficient •OH production. In situ surface-enhanced Raman spectroscopy analysis revealed dynamic changes in the electronic structure of Cu(2 %)-SnO₂/TiO₂ upon O₂, UV-LED (400 nm), and H₂O₂ exposure, further supporting the proposed mechanism. These findings demonstrate that Cu doping at optimal levels significantly enhances the photocatalytic activity and stability of SnO₂/TiO₂ composites for efficient GenX degradation.