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

Despite many important applications of α‐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 α‐Fe2O3 and Fe‐doped SnO2, in which they are both electronically and chemically transformed into a p‐type semiconductor. 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. Reversible carrier‐type transition: Rigorous theoretical details are given to explain why and how a given gas molecule interacts via physisorption on an oxide surface to initiate the carrier‐type transition (see picture). The renormalized polarizability functional is used to prove that there is only one origin for this carrier‐type transition, even though there are three causes.