Cascade Electric Field in ZnIn₂S₄/CuCo₂O₄ Photocatalyst for the Selective Oxidation of Biomass‐Derived 5‐Hydroxymethylfurfural in Aqueous Solutions

The photocatalytic conversion of biomass feedstock represents a promising and environmentally friendly strategy for achieving selective transformation and value addition. The slow charge dynamics and sluggish hole transfer in the oxidation reactions severely limit the photocatalytic activity. Here, the heterojunction is fabricated by synthesizing ultra thin ZnIn₂S₄ nanoflower with spinel CuCo₂O₄. The internal and interfacial electric fields are successfully constructed, which shows superior 5-hydroxymethylfurfural (HMF) valorization. HMF undergoes severe mineralization when ZnIn₂S₄ is used as the catalyst, resulting in 0.9% 2,5-diformylfuran (DFF) yield in water, while the ZnIn₂S₄/CuCo₂O₄ heterojunction catalyst exhibits 77% DFF selectivity with 88.6% HMF conversion, The cascaded bulk and internal electric fields greatly reduce the oxidation potential of holes and enhance the charge separation efficiency, thus give a remarkable 70-fold increase in DFF yield. This work overcomes the limitations of ZnIn₂S₄ application for HMF and similar alcohol oxidation reactions that typically require organic solvents, achieving a high DFF evolution rate of 724.9 µmol·g^-1·h^-1 in water within the first hour of the reaction, surpassing most reports of photocatalytic HMF selective oxidation.