In-situ self-assembly of ZnIn2S4/CuO heterojunctions for efficient photocatalytic oxidation of HMF to DFF in aqueous media

Photocatalytic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to high-value 2,5-diformylfuran (DFF) represents a promising strategy for practicing green chemistry. Currently, most catalysts, such as ZnIn2S4 (ZIS), only function effectively in organic solvents like acetonitrile but result in almost complete HMF mineralization in aqueous media due to the strong oxidative power of holes. To address the issue, this study proposes the formation of n-p heterojunctions using a series of p-type semiconductors (CuO) with n-type semiconductor (ZIS) to lower the valence band position, thereby inhibiting mineralization and selectively oxidizing HMF to DFF. The interface-induced p-n heterojunction of ZIS/CuO-180 facilitates the separation of photogenerated charge carriers. A series of CuO particles with varying sizes, band positions and crystal planes were synthesized by adjusting the hydrothermal temperature. Notably, it is discovered that the conventional complex steps for heterojunction formation can be bypassed. By adding individual ZIS and CuO into the reaction solution, they can immediately self-assemble and convert HMF to DFF. The catalytic performance of this in-situ self-assembled system is comparable to that of pre-assembled heterojunctions, offering a low-cost and convenient approach with broad applicability in catalyst ratio optimization and future photocatalytic reactions. The kinetics of HMF oxidation in the first 10 min reached 0.036 min−1, with almost complete conversion achieved within 3 h in aqueous solution, surpassing the rates of most HMF oxidation processes. Mechanistic insights were gained through trapping experiments and electron spin resonance (ESR) characterization, which identified singlet oxygen (1O2) as the decisive reactive species among various active entities (electrons, holes, and radicals). This innovative strategy holds significant potential for advancing the field of photocatalytic heterojunction catalysts.