Hydrocarbon Sensing with LaFeO ₃ -Based Gas Sensors; In-Operando Kelvin Probe Investigation of the Transduction Function

Because of the recent industrial developments, the analytical tasks that need to be solved in order to ensure a safe and healthy environment have become increasingly complex. This highlights the need for high performance sensor systems, and one way to achieve that is by adding novel materials with exceptional sensing properties to the currently used ones. One that attracted the interest of the gas sensor research community is LaFeO3, which delivers high sensor signals and very good selectivity to hydrocarbon. Its surface chemistry has been investigated, and it was shown that the high sensor response is due to the formation of surface formate species. However, the way in which the surface effects are translated into sensor signals is not yet understood, and that is a critical knowledge gap, which our study addresses. The experimental foundation is provided by simultaneously performing in-operando work function changes and DC sensor resistance measurements in different gas mixtures. The experimental data were evaluated with the help of a conduction model that considers the necks formed between larger grains to be the current limiting factor. The model is an adaptation of an older one, which was devised for compact layers to fit the necks’ cylindrical geometry. Using it made it possible to determine the relationship between the sensor resistance and the surface band bending and, furthermore, to extract the electron affinity changes. In this way, a complete picture of the impact of various gases' exposure on surface band bending and electron affinity changes was obtained. Combined with already available spectroscopic data, our results identify the carbonates/bicarbonates as dipoles, whereas it confirms the effect of formate surface species on band bending. The success of this approach opens new avenues for both spectroscopic studies and strategies to improve sensing performance. Moreover, the model enabled the determination of key material properties, including the concentration of holes in the bulk and the position of the Fermi level.