Monitoring Gas Sensors at Work: Operando Raman-FTIR Study of Ethanol Detection By Indium Oxide
Our results for ethanol gas sensing show a correlation between the sensor signal, the nature of the adsorbates, the indium oxide oxidation state and the surface hydroxyl group intensity depending on gas environment and temperature (see Fig. 1) . For example at 190°C, in the presence of ethanol (250 ppm, EtOH/N2), the hydroxyl band at 3659 cm‑1 disappears and that at 3643 cm‑1 band becomes weaker due to their reaction with ethanol forming acetate groups (937 cm-1, 2937 cm‑1). In the course of the redox reaction indium oxide is reduced as characterized by the appearance of bands at 325 cm-1 and 407 cm‑1. These changes are accompanied by a strong decrease in sensor resistance. Thus our results show that ethanol is oxidized to adsorbed acetate by indium oxide releasing electrons to the conduction band consequently decreasing the resistance.
Summarizing, our study demonstrates that detailed spectroscopic analysis under working conditions of the gas sensor is essential to unravel its mode of operation.