Spectroscopic Insight into the Role of Surface Oxygen Vacancies in the Detection of NO ₂ in SnO ₂ -Based Chemoresistive Gas Sensors

Despite widespread use of SnO₂-based conductometric gas sensors, the sensing of NO₂ remains poorly described on the atomic scale, limiting the design of next-generation sensors. Here, near-ambient-pressure X-ray photoelectron spectroscopy combined with in situ resistance measurement was used to investigate the interaction between NO₂ and a SnO₂-based sensor at room temperature and 300 °C. Through stepwise exposure and evacuation cycles, we tracked real-time changes in the O/Sn atomic ratio and electronic structure alongside the macroscopic resistance response. Exposure to NO₂ consistently increased the O/Sn ratio, indicating the healing of surface oxygen vacancies, and this effect directly correlated with an increase in resistance. At room temperature, the response was cumulative and irreversible, while at high temperatures, it was rapid, reversible, and saturated at lower gas concentrations. These findings directly support vacancy-modulated "surface conductivity" and provide experimental validation that NO₂ sensing in SnO₂ occurs via modulation of shallow donor concentrations, rather than through the classical description of ionosorption of charged oxygen species. The results contribute to an emerging unified model of gas sensing and offer insight into how dynamic equilibrium between vacancy healing and regeneration underpins temperature-dependent sensor behavior.