Monitoring Gas Sensors at Work: Operando Raman-FTIR Study of Ethanol Detection By Indium Oxide

Metal-oxide semiconductors such as In₂O₃ have been used widely as gas sensing materials because of their high sensitivity to target gases and their simple fabrication. Their operation is based on changes of the materials electrical conductivity caused by the adsorption of gas molecules on the semiconductors surface. Despite progress in the field, a detailed mechanistic understanding of the gas sensing process is still missing. The knowledge-based development of better gas sensors crucially depends on the development of experimental approaches allowing for a correlation of the sensor response with spectroscopic information obtained under working conditions of the gas sensor (operando approach). In this contribution we demonstrate the potential of this approach using a new operando Raman-FTIR experiment enabling the simultaneous measurement of the sensor response, adsorbed species (Raman), changes of the metal-oxide material (Raman) and gas-phase composition (FTIR).[1]

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) [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.

References:

[1] S. Sänze, A. Gurlo, C. Hess, Angew. Chem. Int. Ed. 2013, 52, 1.