Luminescence Modulation through Ratiometric Quenching in BiOCl:Eu Red Phosphor under H ₂ S Gas Exposure

Ratiometric gas sensing based on luminescence intensity ratios (LIR) offers robustness against environmental fluctuations, because it relies on relative rather than absolute signal intensity. While LIR-based techniques using inorganic phosphors have been widely applied for temperature and pH sensing, their implementation in gas sensing remains limited. In this study, Eu3+-doped BiOCl phosphors were investigated for the ratiometric detection of low concentrations of hydrogen sulfide (H2S) gas. X-ray diffraction analysis confirmed the successful incorporation of Eu3+ into the BiOCl lattice, accompanied by a gradual lattice contraction by Eu3+ doping. Scanning electron microscopy revealed plate-like morphologies typical of BiOCl with homogeneous elemental distribution. Photoluminescence and photoluminescence excitation spectra showed the characteristic red emissions of Eu3+ (5D0 → 7FJ transitions) and a broad excitation band in the near-UV region, mainly attributed to the band-edge transition. Upon exposure to 1–5 ppm of H2S gas, the Eu3+ emission intensities were selectively quenched in a concentration-dependent manner. In particular, the LIR of I700/I596 and I700/I620 exhibited clear variation with H2S concentration, enabling reliable ratiometric sensing. Electron spin resonance and X-ray photoelectron spectroscopy revealed that the quenching mechanism originates from the H2S-induced partial reduction of Bi3+ to Bi2+, generating oxygen vacancies that act as nonradiative recombination centers. Judd–Ofelt analysis further indicated a moderate increase in the Ω2 parameter and a more pronounced enhancement of Ω4, suggesting an increase in Eu–O bond covalency without significant changes in local symmetry. These insights provide a deeper understanding of the luminescence modulation mechanism and demonstrate the potential of BiOCl:Eu as a robust ratiometric gas sensor material.