Engineering Single-Atom Sn-doped ZnIn₂S₄ for Enhanced Aqueous-Phase Selective Photooxidation of 5-Hydroxymethylfurfural

ZnIn₂S₄-based photocatalysts have attracted considerable attention in biomass valorization owing to their tunable band structures and abundant surface reaction sites. However, the aqueous-phase selective photooxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) remains highly challenging due to the insufficient O₂ activation, substrate mineralization, and competitive coupling side reactions. Herein, a single-atom Sn-doped ZnIn₂S₄ photocatalyst (Sn-ZIS) was constructed. Systematic characterizations confirm that Sn species are atomically dispersed and incorporated into the ZnIn₂S₄ lattice through Sn-S coordination. Under aqueous-phase reaction conditions, Sn-ZIS achieves 88.7% HMF conversion, 80.5% DFF selectivity, and 93.3% C equilibrium, higher than those of ZnIn₂S₄ and many reported aqueous-phase photocatalytic systems. The better performance of Sn-ZIS is mainly attributed to the synergistic regulation of the band structure and local electronic structure induced by Sn incorporation. The optimized band structure moderately weakens the oxidative ability of photogenerated holes, thereby reducing HMF mineralization to CO₂. While the regulation of the local electronic structure promotes O₂ adsorption and activation to generate superoxide radicals, it simultaneously facilitates DFF desorption from the catalyst surface. Synergy between the two functions drives the selective conversion of HMF into DFF. This work demonstrates the potential of single-atom doping for enhancing aqueous-phase biomass photooxidation and provides a rational strategy for designing efficient ZnIn₂S₄-based photocatalysts.