Ethanol Gas Sensing by Indium Oxide: An Operando Spectroscopic Raman-FTIR Study

The ethanol gas-sensing mechanism of indium oxide has been investigated in detail by Raman spectroscopy in combination with resistance measurements of the indium oxide sensor material and Fourier transform infrared (FTIR) gas-phase analysis. The observed surface species depend on the gas environment and sensor temperature. Raman spectra taken at lower operating temperatures of the sensor (190 °C) during ethanol gas sensing reveal the presence of surface acetate and reduced indium oxide. Addition of oxygen to the feed leads to the formation of surface formate-like species besides acetate. At elevated temperatures of 325 °C an increase in the amount of the gas-phase products acetaldehyde, ethene, carbon oxide, and water is observed. Under these conditions the sensor surface is characterized by carbon species if oxygen is absent. The sensor signal is correlated with the nature of the adsorbates, the presence of surface hydroxyl groups, and the indium oxide oxidation state. The proposed gas-sensing mechanism is corroborated by detailed analysis of the spectroscopic and resistance response of indium oxide during exposure to acetaldehyde and ethene. Our results demonstrate the importance of detailed spectroscopic studies under working conditions to unravel the mode of operation of gas sensors.