Reversible Carrier-Type Transitions in Gas-Sensing Oxides and Nanostructures

Despite many important applications of alpha-Fe2O3 and Fe doped SnO2 in semiconductors, catalysis, sensors, clinical diagnosis and treatments, one fundamental issue that is crucial to these applications remains theoretically equivocal-the reversible carrier-type transition between n- and p-type conductivities during gas-sensing operations. Herein, we present an unambiguous and rigorous theoretical analysis in order to explain why and how the oxygen vacancies affect the n-type semiconductors alpha-Fe2O3 and Fe-doped SnO2, in which they are both electronically and chemically transformed into a p-type semi-conductor. Furthermore, this reversible transition also occurs on the oxide surfaces during gas-sensing operation due to physisorbed gas molecules (without any chemical reaction). We make use of the ionization energy theory and its renormalized ionic displacement polarizability functional to reclassify, generalize and explain the concept of carrier-type transition in solids, and during gas-sensing operation. The origin of such a transition is associated with the change in ionic polarizability and the valence states of cations in the presence of oxygen vacancies and physisorped gas molecules.