Atomistic Descriptions of Gas-Surface Interactions on Tin Dioxide

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.

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

“…No significant interaction between gaseous O 2 and a perfect (nondefective) (110) SnO 2 surface is found to occur in the majority of computational studies, i.e., Sn 5c is not an active site for O 2 adsorption (although it should be noted that the van der Waals interactions associated with physisorption are typically neglected in many such computational studies). It is also worth noting that even under oxygen concentrations consistent with those found in ambient atmosphere a fully oxidized surface is not expected, and therefore, if we consider adsorption on a partially reduced (defective) SnO 2‑x surface, computational modeling reveals the active site for initial O 2 adsorption on the (110) surface of SnO 2 to be a bridging oxygen vacancy, O v , (explicitly highlighted in Figure and shown in blue in Figure ).…”

Dealuminated Zeolite Y/SnO2 Nanoparticle Hybrid Sensors for Detecting Trace Levels of Propanol as a Lung Cancer Biomarker

“…The activation energy for each LCB was calculated by applying the Arrhenius equation. Since dimensions and phase of n-type SnO 2 remain the same during the temperature variation, the Arrhenius equation gives the dependence of the rate of change in resistance, K , on the absolute temperature T (in kelvin) − where A 0 is the pre-exponential factor, Δ E is the activation energy, and R is the gas constant. The resistance’s rates of change ( K ≈ d R /d t ) were evaluated from the variation in the resistance change during a period of time after the exposure to 200 ppb propanol, formaldehyde, and toluene at different operating temperatures.…”

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

“…Computational studies have been used to provide new insights on the mechanism of interaction between gaseous species and the surface of sensing materials; in general such studies predict that O 2 does not interact with stoichiometric surfaces of SnO 2 , highlighting oxygen vacancies (V O ) as essential in enabling adsorption. 10,12–16 These studies indicate that gaseous O 2 adsorbed on a metal oxide dissociates in a thermally activated process ( Fig. 1 ).…”

“…This causes upward band bending (relative to the surface in the absence of adsorbates – the ‘flat band’ condition), which is witnessed as increased sensor resistance. 9 For the prototype CGS material, SnO 2 , there is no direct evidence for O − species, 10 and whilst other adsorbates have been shown to exist on SnO 2 , for example, O 2 − , their effect on resistance is considered small. 11 …”

“…This causes upward band bending (relative to the surface in the absence of adsorbatesthe 'at band' condition), which is witnessed as increased sensor resistance. 9 For the prototype CGS material, SnO 2 , there is no direct evidence for O À species, 10 and whilst other adsorbates have been shown to exist on SnO 2 , for example, O 2 À , their effect on resistance is considered small. 11 Computational studies have been used to provide new insights on the mechanism of interaction between gaseous species and the surface of sensing materials; in general such studies predict that O 2 does not interact with stoichiometric surfaces of SnO 2 , highlighting oxygen vacancies (V O ) as essential in enabling adsorption.…”