Hexagonal Microsphere/Cubic Particle ZnIn2S4 Heterojunctions: A Robust Photocatalyst for Visible-Light-Driven Conversion of 5-Hydroxymethylfurfural to 2,5-Diformylfuran Under Ambient Air Conditions

In recent years, biomass utilization has attracted extensive attention. Herein, hexagonal/cubic ZnIn2S4 (ZIS) heterojunction catalysts were synthesized via a solvothermal method for the selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF). The results demonstrated that the constructed heterojunctions effectively promoted carrier separation. The optimal catalyst achieved an HMF conversion rate of 88.8% and a DFF yield of 86.6% within 1 h in the open air. X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) characterizations confirmed the successful fabrication of the composite phase structure and revealed a porous spherical morphology. Equivalent circuit fitting of electrochemical impedance spectroscopy (EIS) data indicated that the hexagonal/cubic heterojunctions possessed the lowest charge transfer resistance (Rct = 5825 Ω), which effectively reduced interfacial charge transfer resistance and accelerated the transport of photoinduced carriers. Radical quenching experiments and electron paramagnetic resonance (EPR) spectroscopy identified superoxide radicals (·O2−) as the primary reactive species. Meanwhile, density functional theory (DFT) calculations elucidated the formation of the built-in electric field and the charge transfer mechanism. This work’s construction of Type-II ZIS heterojunctions effectively addressed the issue of rapid carrier recombination in pristine ZIS materials, providing a feasible strategy for biomass valorization.