Interfacial Oxygen Vacancies as Electron Trapping Centers and Kinetic Mediators of Interfacial Charge Transfer in Bi2O3/BiOBr Heterojunctions: In Situ Spectroscopic Evidence and Enhanced Photocatalysis

Abstract The controversial charge transfer mechanism in Bi2O3/BiOBr heterojunctions and the lack of direct in situ evidence for the role of interfacial oxygen vacancies remain critical challenges. An in situ HBr acid etching strategy was employed to construct Bi2O3/BiOBr heterostructures, in which BiOBr nanosheets grew vertically on the porous Bi2O3 nanocubes. This topological transformation generated abundant interfacial oxygen vacancies through the synergistic effects of the lattice mismatch and the nanoscale Kirkendall effect. Band structure analysis, in situ XPS, and EPR spectroscopy provided direct evidence that interfacial oxygen vacancies serve as electron-trapping centers. Under illumination, the photogenerated electrons from both Bi2O3 and BiOBr migrate to and accumulate at the interfacial oxygen vacancy sites, as directly evidenced by the reversible redshifts of Bi 4f, O 1s, and Br 3d peaks and the decreased intensity of the defect-related O 1s component. The trapped electrons reduce adsorbed O2 to •O2−, which is the dominant active species for both Cr(VI) reduction and tetracycline degradation. The optimized heterojunction (S3) exhibited a Cr (VI) (100 mg/L) reduction rate constant of 0.179 min−1 (8.5 times that of pure Bi2O3) and completely degraded tetracycline (TC, 50 mg/L) within 20 min. Free-radical trapping experiments revealed a distinctive kinetic fingerprint: hole scavengers significantly inhibited Cr(VI) reduction but barely affected TC degradation, which is consistent with the electron-trapping mechanism where holes are not the dominant active species for TC degradation. This work provides direct in situ spectroscopic evidence that interfacial oxygen vacancies act as electron traps and kinetic mediators of interfacial charge transfer, establishing a defect-engineering foundation for the rational design of high-efficiency heterojunction photocatalysts.