Comprehensive Understanding of CO Gas Sensing by Combining Transient and Steady-State Spectroscopies: A Mechanistic Study on SnO ₂ and Gold-Loaded SnO ₂

Despite widespread use of chemoresistive gas sensors, their underlying sensor mechanism is often poorly understood on a molecular level. This work provides significantly new insight into the sensor mechanism of SnO2 and Au/SnO2 during CO gas sensing by combining modulation excitation IR spectroscopy with steady-state spectroscopy (UV–vis, IR). We demonstrate that surface lattice oxygen sites and hydroxyl groups are active sites in the CO oxidation reaction leading to the primary sensor response. Correlation of the temporal course of characteristic spectroscopic bands of surface oxygen species with the DC resistance reveals that the sensor response and reversibility cannot be solely explained by a pure reduction–reoxidation mechanism. Carbonate-related species also need to be considered, as they are only formed after the initial oxidation reaction and significantly define the total sensor response and reversibility by hindering surface reoxidation and restructuring. Direct spectroscopic evidence for CO adsorption and activation on gold nanoparticles was obtained, involving different CO–Aun+ species. Our results provide valuable new insights into the sensor mechanism of loaded metal oxide gas sensors, which is often more complex than assumed based on steady-state methods. As illustrated in this study, some key aspects of the sensor mechanism may only be accessible by applying transient spectroscopic methods.