Electronic Structure and Surface Properties of Non-Stoichiometric Fe2O3-δ (α and γ) and Its Application in Gas Sensing

Crystalline nanoparticles of γ-Fe2O3 and α-Fe2O3 as candidates for gas sensing were synthesized by flame spray process (FSS). γ-Fe2O3 was obtained in a one step process, while α-Fe2O3 was obtained after heat-treatment of as-prepared γ-Fe2O3 nanoparticles. With a combination of diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), advanced near edge X-ray absorption fine structure (NEXAFS) spectroscopy, we probed the electronic structure and described the mechanisms of reducing gas sensing at the molecular scale and correlated the sensing property with charge transfer and valence band characteristics. The surface composition was determined by the XPS and resonance RBS (Rutherford backscattering spectrometry). The core level and valence band spectra of mild-oxidized and mild-reduced samples revealed the non-stoichiometric character of Fe2O3 and contribution of electronic surface defects. These two polymorphic forms of Fe2O3 exhibit significant differences of the chemical surface composition, electronic structure and resulting sensing properties.