In-situ/operando characterization of FeOx-based chemiresistive sensor of acetone vapours by X-ray absorption spectroscopy

Metal oxide thin films are archetypal active elements employed in chemiresistive gas sensors. Most existing metal oxide-based sensors operate at elevated temperatures; however, the sensing mechanism has generally been deduced considering the room-temperature properties of metal oxide compounds. We report an in-operando Xray absorption spectroscopy study on a chemiresistive sensor of acetone vapours; namely, the chemical structure of iron-oxide nanoparticles (NPs) exposed to trace concentrations of acetone vapours under varying temperature conditions was examined. Our results show that the iron-oxide NPs, identified as maghemite/magnetite (gamma-Fe2O3/Fe3O4) compound, partially reduced with temperature. Likewise, the iron oxide NPs were partially reduced upon exposure to trace concentrations of acetone vapour (at constant temperature). The latter observation suggests that acetone molecules are chemically adsorbed on iron oxide. The chemical changes are discussed in terms of the conductive response of a consubstantial chemiresistive sensor. Acetone chemisorption requires a revision of the commonly accepted sensing mechanism based on the physical adsorption of acetone molecules.