Burstein-Moss band engineering of amine-functionalised In₂S₃ for visible-light photocatalytic degradation of PFAS

Per- and polyfluoroalkyl substances (PFAS) are persistent, mobile, and toxic pollutants whose remediation in water remains a major environmental challenge, and most reported photocatalytic systems still rely on ultraviolet (UV) irradiation, chemical reductants, or sacrificial reagents. Here we report amine-functionalised indium sulfide (Am-In₂S₃) nanoplates in which the formation of In-N surface bonds induces a Burstein-Moss shift of the conduction band and generates indium vacancies that trap photogenerated charges, together enabling efficient visible-light activity in a sulfide host. The same surface chemistry raises the isoelectric point from 2.33 to 8.91 and increases adsorption of sodium p-perfluorous nonenoxybenzenesulfonate (OBS), a representative aromatic PFAS, by approximately fivefold, coupling electrostatic capture and photocatalytic turnover at the same active sites. Under visible-light irradiation (>420 nm), Am-In₂S₃ achieves 98.2% OBS removal within 90 min and 81.9% total organic carbon (TOC) removal within 2 h without UV, peroxide, or sacrificial agents; defluorination reaches 21.7% over 8 h, indicating substantial but incomplete mineralisation. Density functional theory calculations, electron paramagnetic resonance spectroscopy, and radical-quenching experiments support a frontier-molecular-orbital-directed dual-site mechanism: electrophilic photogenerated holes attack the electron-rich HOMO localised on the benzenesulfonate head group, while nucleophilic electrons and superoxide radicals attack the electron-poor LUMO on the perfluoroalkyl chain, driving concerted H/F exchange and carbon-chain shortening. Am-In₂S₃ retains activity across diverse water matrices and in a floating-sponge continuous-flow reactor (400 cm²) under natural sunlight, sustaining >96% OBS removal at solar irradiances of 0.56-0.75 kW m^-2. The work positions Burstein-Moss band engineering of sulfide photocatalysts as a route to solar-driven degradation of aromatic PFAS and informs the development of materials for sustainable treatment of persistent organic pollutants.