Reversible carrier-type transition in gas-sensing oxides and nanostructures

TL;DR

This paper provides a rigorous theoretical explanation for the reversible carrier-type transition in gas-sensing oxides like a-Fe2O3 and Fe-doped SnO2, driven by oxygen vacancies and physisorbed gases, crucial for sensor applications.

Contribution

It introduces a theoretical framework using ionization energy theory to explain and generalize the carrier-type transition in semiconducting oxides during gas sensing.

Findings

Reversible n- to p-type transition explained theoretically

Oxygen vacancies and physisorbed gases alter ionic polarizability

Framework applicable to various oxide-based sensors

Abstract

Despite many important applications of a-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. Here, we give unambiguous and rigorous theoretical analysis in order to explain why and how the oxygen vacancies affect the n-type semiconductors, a-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 to explain the concept of carrier-type transition in solids, and during gas-sensing operation. The origin of such a transition is associated to the change in ionic polarizability and the valence states of cations in the presence of (a) oxygen vacancies and (b) physisorped gas molecules.