Oxidation of 5-Hydroxymethylfurfural over ZnIn₂S₄/Ti₃C₂T_x Photocatalyst Modulated by Sulfur Vacancy Defects and Schottky Heterojunctions

Conversion of biomass into chemical products with solar energy is an important avenue for technological sustainability. Constructing efficient Schottky heterojunctions with vacancy defects can be a productive approach for achieving photooxidation of 5-hydroxymethylfurfural (HMF). Herein, sulfur vacancy (Vs) defect-enriched ZnIn₂S₄ is combined with Ti₃C₂T_x MXene (Vs-ZIS/MX) containing Schottky heterojunctions via self-assembly to form photocatalysts for efficient oxidation of HMF to 2,5-diformylfuran (DFF) under air-ambient conditions. HMF conversion of 87.3% with 72.9% DFF selectivity is obtained by the Vs-ZIS/MX heterojunction photocatalyst under visible light (420 nm < λ <780 nm, 2 h). Density functional theory calculations reveal that the Schottky heterojunction and regulated S vacancies enhance the electronic coupling and lower HMF adsorption energy, establishing a directional electron transfer channel from Vs-ZIS to MXene and an enhanced built-in electric field. Extended X-ray absorption fine structure spectroscopy shows that a stable metal (Zn)O bond coordination is formed between Vs-ZIS and MXene, and the enrichment of local electrons at S vacancy defects sites and metal–O coordination makes it possible to extract the delocalized electrons from ZIS to MXene. This work demonstrates a method to modulate the electron cloud density and enhance transport with a built-in electric field on photogenerated carriers for catalytic conversion of organic alcohols.