Iron sites on defective BiOBr nanosheets: Tailoring the molecular oxygen activation for enhanced photocatalytic organic synthesis

Sunlight-driven activation of molecular oxygen (O₂) for organic oxidation reactions offers an appealing strategy to cut down the reliance on fossil fuels in chemical industry, yet it remains a great challenge to simultaneously tailor the charge kinetics and promote reactant chemisorption on semiconductor catalysts for enhanced photocatalytic performance. Herein, we report iron sites immobilized on defective BiOBr nanosheets as an efficient and selective photocatalyst for activation of O₂ to singlet oxygen (¹O₂). These Fe³⁺ species anchored by oxygen vacancies can not only facilitate the separation and migration of photogenerated charge carrier, but also serve as active sites for effective adsorption of O₂. Moreover, low-temperature phosphorescence spectra combined with X-ray photoelectron spectroscopy (XPS) and electronic paramagnetic resonance (EPR) spectra under illumination reveal that the Fe species can boost the quantum yield of excited triplet state and accelerate the energy transfer from excited triplet state to adsorbed O₂ via a chemical loop of Fe³⁺/Fe²⁺, thereby achieving highly efficient and selective generation of ¹O₂. As a result, the versatile iron sites on defective BiOBr nanosheets contributes to near-unity conversion rate and selectivity in both aerobic oxidative coupling of amines to imines and sulfoxidation of organic sulfides. This work highlights the significant role of metal sites anchored on semiconductors in regulating the charge/energy transfer during the heterogeneous photocatalytic process, and provides a new angle for designing high-performance photocatalysts.