Oxygen and Bromine Vacancies Synergistically Induce Local Polarization Electric Field for Enhanced Photocatalytic Nitrogen Fixation on BiOBr

The low efficiency of photogenerated charge separation significantly hinders the photocatalytic nitrogen (N₂) fixation. Local polarization electric field (LPEF) induced by defects has been known to enhance charge separation, yet the synergistic effects and mechanisms related to different types of defects in pure phases remain poorly understood. In this study, defect-free bismuth oxybromide (BiOBr; BOB), together with single vacancy (BOB-V_Br and BOB-V_O) and dual vacancy (BOB-V_BrO) analogues, were successfully synthesized, and the presence of these specific vacancies was comprehensively characterized. Notably, the dual vacancy BOB-V_BrO exhibited the highest photocatalytic NH₃ generation rate of 266 μmol g^–1 h^–1 in a liquid–solid biphasic system, which was 6.1, 1.5, and 1.4 times higher than those of BOB, BOB-V_Br, and BOB-V_O, respectively. Furthermore, the NH₃ generation capacity of BOB-V_BrO reached an impressive rate of 978 μmol g^–1 h^–1 in a gas–liquid–solid triphasic system. Photoelectrochemical tests revealed that BOB-V_BrO demonstrated the highest light conversion efficiency, followed by BOB-V_O, BOB-V_Br, and BOB. The relative intensity of the internal electric field in BOB-V_BrO was also significantly high, being 1.8, 2.4, and 3.9 times greater than those of BOB-V_O, BOB-V_Br, and BOB, respectively. The Br and O vacancies synergistically induced LPEF between the [O]/[Br] and [Bi] layers. In situ irradiation X-ray photoelectron spectroscopy indicated that O and Br vacancies of the oligomers could synergistically enhance the LPEF, thereby facilitating the transfer of photogenerated electrons from O/Br to Bi. Additionally, the practical feasibility of BOB-V_BrO in photocatalytic N₂ fixation was validated to produce liquid nitrogenous fertilizer for plant growth, revealing its potential application in agricultural production.