Combined operando IR/Raman spectroscopy applied to loaded metal-oxide gas sensors

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Metal oxide semiconductors have been commonly used as gas sensor materials due to their high sensitivity to target gases and their easy fabrication. Their mode of operation is based on changes of the electrical conductivity resulting from the adsorption of gas molecules on the surface of the semiconductor. Despite considerable progress in the field, a detailed mechanistic understanding of the gas sensing process is still missing. The rational development of gas sensors with increased selectivity and sensitivity will crucially depend on a thorough understanding of their mode of operation. To this end, the development and application of new experimental approaches is needed. As has been shown in the first funding period, operando Raman spectra allow for new mechanistic insight into the mode of operation of metal oxide semiconductors during operation, inter alia, by combination with operando UV-Vis spectra. It has been found, however, that this approach is limited regarding a more profound analysis of technically relevant metal oxide gas sensors, i.e., materials loaded with additives (e.g. noble metal).This follow-up project aims at elucidating the mode of operation of additives in metal oxide gas sensors and developing an integrated mechanistic view, including the metal oxide, the additive, as well as their mutual interplay. The focus is on Au- and Cu-loaded indium oxide and cerium oxide gas sensors and their application towards ethanol (EtOH) and CO detection. To this end, operando Raman spectroscopy will be combined with operando IR spectroscopy within one experimental setup for the first time in the context of gas sensors, besides exploring the potential of transient vibrational spectroscopic methods and surface plasmon-based Raman enhancements. These mechanistic studies will be supported by results from UV-Vis und photoelectron spectroscopy as well as by assignments of vibrational bands by DFT calculations.Emphasis will be put on the correlation of the sensor response with the spectroscopic results, i.e., the type of adsorbates, the state of the additive, as well as the oxidation state of the metal oxide. The mechanism is expected to depend strongly on the gas environment and temperature as both have an influence on the surface species. To this end, detailed temperature-dependent studies between 100°C and 400°C will be conducted in different gas environments, i.e., N2, O2, N2/EtOH and O2/EtOH, as well as N2/CO und O2/CO. Realistic conditions will be simulated by the presence of H2O and CO2. By using different metal oxides (In2O3, CeO2), additives (Au, Cu), and analytes (EtOH, CO) we will explore to which extent the results obtained can be generalised.