Tuning the Equilibrium of Excitons and Carriers in the (001) Plane-Exposed Bismuth Oxyhalide Enables Efficient Selective Photocatalytic Oxidation of 5-Hydroxymethylfurfural

The photocatalytic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) is a crucial reaction step in biomass conversion. However, our understanding of photocatalytic HMF conversion in the liquid phase remains insufficient, and the product yield remains unsatisfactory. This study proposes a high-selectivity catalytic mechanism based on exciton-mediated resonance energy transfer by regulating the equilibrium between excitons and charge carriers in bismuth oxybromide through iodine doping. Iodine doping narrows the energy gap between singlet and triplet excitons, significantly enhancing exciton-induced singlet oxygen (¹O₂) generation while suppressing the nonselective radical pathway dominated by carriers. Moreover, hydroxyl groups on the {001} crystal facet of the catalyst facilitate the precise activation of the C-H/O-H bonds in HMF through hydrogen bonding interactions. The synergistic effects of optimized surface chemical states and enhanced exciton resonance energy transfer endow the catalyst with a high DFF yield and selectivity of 84.0% and 91.5%, respectively. Experimental and theoretical evidence indicates that the holes drive bond cleavage in HMF, with ¹O₂ acting as a proton acceptor. Overall, this work pioneers the exploration of exciton effects in HMF conversion, establishing an activity-enhancement paradigm through exciton resonance energy transfer and providing mechanistic insights into the photosynthesis of fine chemicals.