Abstract
Historically, in gas sensing literature, the focus on “mechanisms” has been on oxygen species chemisorbed (ionosorbed) from the ambient atmosphere, but what these species actually represent and the location of the adsorption site on the surface of the solid are typically not well described. Recent advances in computational modelling and experimental surface science provide insights on the likely mechanism by which oxygen and other species interact with the surface of SnO2, providing insight into future directions for materials design and optimisation. This article reviews the proposed models of adsorption and reaction of oxygen on SnO2, including a summary of conventional evidence for oxygen ionosorption and recent operando spectroscopy studies of the atomistic interactions on the surface. The analysis is extended to include common target and interfering reducing gases, such as CO and H2, cross-interactions with H2O vapour, and NO2 as an example of an oxidising gas. We emphasise the importance of the surface oxygen vacancies as both the preferred adsorption site of many gases and in the self-doping mechanism of SnO2.
Highlights
In gas sensing literature, the focus on “mechanisms” has been on oxygen species chemisorbed from the ambient atmosphere, but what these species represent and the location of the adsorption site on the surface of the solid are typically not well described
The formation of vacancies in SnO2 introduces shallow electron donors, which may become thermally ionised and cause downward band bending. This would manifest itself as a shift of peaks to higher binding energy (BE) in photoelectron spectroscopy, which was observed in both UPS and X-ray photoelectron spectroscopy (XPS) on reduced and oxidised SnO2 samples [53]
The insight provided by recent molecular modelling studies, backed by emerging operando spectroscopy, offers a new perspective on the gas-sensitive phenomena of SnO2 based gas sensors, and our understanding of the sensing mechanisms needs to be re-evaluated
Summary
SnO2 , is among the most common sensitive materials used in CGS. It is abundant, inexpensive, non-toxic and shows excellent stability to reducing conditions. Large SnO2 single crystals, which are notoriously challenging to grow, preferentially expose the (110), (101) and (100) surfaces [26] Of these three, the (110) was shown to have the lowest energy and is the subject of most computational adsorption studies [17,18,27,28,29,30,31], though some results are available for the (101) surface [32,33]. The formation energies of these vacancies mutually depend on the defects’ densities, as indicated by computational studies [34] It means that while the initial reduction of the stoichiometric surface will proceed by mainly removing Obr , at a relative Vbr density of 0.5, the formation energy difference between bridging and in-plane defects is much smaller, so both should coexist at the surface. The role of vacancies in sensor operation mechanisms is two-fold; as a self-doping mechanism, which provides intrinsic conductivity in SnO2 , and as the preferred adsorption site for oxygen, which does not adsorb onto stoichiometric surfaces of SnO2 , as indicated by many computational studies [17,28,32] and explained
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
