Do We Need “Ionosorbed” Oxygen Species? (Or, “A Surface Conductivity Model of Gas Sensitivity in Metal Oxides Based on Variable Surface Oxygen Vacancy Concentration”)

Abstract

The author provides an opinion on direct experimental evidence available to support the "ionosorption theory" often employed to interpret "electrophysical" measurements made during a gas sensing experiment. This article then aims to provide an alternative framework of a "surface conductivity" model based on recent advances in theoretical and experimental investigations in solid state physics, and to use this framework as a guide toward design rules for future improvement of gas sensor performance.

Direct in situ spectroscopic evidence of the crucial role played by surface oxygen vacancies in the O2-sensing mechanism of SnO2

“…4 and discussion below). Referring to the recently described surface vacancy description of gas sensor response, 16 a potential explanation for this lack of resistance change is that the number of ionised near-surface oxygen vacancies increases with increasing temperature; 28 only ionised V O are expected to contribute to resistance change (non-ionised neutral V O already localise electrons at the vacancy site). This would also account for the lack of band bending observed even though the vacancy concentration appears to vary significantly.…”

“…4 , suggest that adsorbate dissociation does not localise electrons onto non-ionised vacancies. 16 Additionally, given that the low temperature prevents vacancy diffusion from the bulk towards the surface, any vacancies healed here must be limited to the surface-most atomic layer, which is reflected in the significant difference between ‘shallow’ and ‘deep’ O/Sn variance. Finally, the link between the observed decrease in vacancy density and baseline resistance increase through ‘gradual’ response is consistent with other studies, in which changing oxygen vacancy concentration was correlated with sensor drift.…”

“…7 ) as a function of O 2 pressure. 16 Application of ‘Occam's Razor’ therefore leads us to conclude that variable surface vacancy concentration is responsible for the O 2 sensing mechanism of SnO 2 , not extrinsic charged oxygen species. A similar conclusion was reached by Andrae et al who used synchrotron NAP XPS to examine the interaction of O 2 with donor-doped SrTiO 3 to interpret (non-simultaneous) resistance measurements.…”

“…This challenges the current unproven conceptions in the field of gas sensing 8 but is fully consistent with the recent work in the field of solid-state physics on vacancy-induced surface conductivity layers in metal oxides. 16 …”

“…Using ex situ X-ray photoelectron spectroscopy measurements (XPS), Semancik et al have previously inferred a link between decreasing surface V O concentration and increased resistance in SnO 2 , 15 and have also shown that the formation of surface V O results in a surface conductivity layer, and additionally an alternative gas sensing mechanism employing a description of ionised surface oxygen vacancies has recently been proposed. 16 Further, Elger and Hess recently published a study correlating changes in the ambient gas composition with surface adsorbed species, inferring changes in oxygen vacancy concentration from UV-vis reection spectra. 17 However, to directly link variation in vacancy concentration with change in materials resistance as a function of gas ambient requires a joint macroscopic and spectroscopic picture of a sensor working dynamically under typical temperature and pressure operating conditions.…”