Microstructural and surface characterization of solid state sensor based on LaFeO3−σ oxide for detection of NO2

Abstract

NO₂ adsorption on non-stoichiometric perovskite-type LaFeO_3-σ thick films increases the density of charge carriers (holes), thus facilitating the detection of NO₂ by conduction measurements. In this work, the microstructures and induced defects that control the sensing behaviour of LaFeO_3-σ were investigated in depth. LaFeO_3-σ powders were prepared both by thermal decomposition of a La[Fe(CN)₆]·nH₂O complex at 700 °C and by using a sol–gel method at 500 °C. Scanning electron microscopy (SEM) revealed crystallites varying in size from 100 to 300 nm depending on the decomposition temperature. Surface analysis by X-ray photoelectron spectroscopy (XPS) confirmed that sintering in nitrogen is much more effective for reducing the amount of surface oxygen than is sintering in air. At low operating temperature, there is a drift of the output response associated with the ongoing process of bulk equilibration, but it is much less at 350 °C. It is obvious that a variation of [O_ad]/[O_lattice] with sintering conditions represents the relative gas adsorption capability.