Interplay between CO and Surface Lattice Oxygen Ions in the Vacancy-Mediated Response Mechanism of SnO₂-Based Gas Sensors

Despite having been commercially available for more than half a century, conductometric gas sensors still lack a definite description of their operation mechanism, which hinders research into improving their characteristics. With the advent of operando spectroscopy comes the opportunity to elucidate their working principle by observing their surface during sensing. To that end, we have employed near-ambient pressure (NAP) XPS with simultaneous resistance measurements to correlate the macroscopic sensor response with atomistic changes to the sensor's surface under exposure to CO, a common target gas. Our results show a clear relationship between the sensor response and the change in surface stoichiometry of SnO₂, suggesting that near-surface oxygen vacancies play a vital role in the sensing mechanism, in support of a vacancy-modulated "surface conductivity" mechanism.