Synergy of Built-In Electric Field and Oxygen Vacancies in Bi ₃ O ₄ Br@NiO _<i>x</i> for Enhanced Low-Temperature H ₂ S Gas Sensing

This study demonstrates the synergistic effect between the built-in electric field (IEF) and oxygen vacancies for dramatically enhancing low-temperature H 2 S sensing. A series of Bi 3 O 4 Br@NiO x heterojunctions were synthesized. The formation of an intrinsic IEF in Bi 3 O 4 Br and an interfacial IEF at the heterojunction is confirmed by surface photovoltage measurements, energy band analysis and theoretical calculations, while oxygen vacancies are quantified by X-ray photoelectron spectroscopy and electron paramagnetic resonance measurements. The optimal sensor delivers a response of 16.3 toward 10 ppm of H 2 S at 90 °C and 30% RH, eight times that of pure NiO x, along with excellent selectivity and humidity stability. The enhanced performance arises from the synergistic effect of oxygen vacancies, the intrinsic IEF of Bi 3 O 4 Br and the interfacial IEF, which together promote gas adsorption, charge separation, and carrier transport, as supported by quenched PL and reduced charge transfer resistance. This work highlights the collective role of IEF and oxygen vacancies in achieving high-performance, low-temperature gas sensors.