Determination of oxygen adsorption species on SnO₂: Exact analysis of gas sensing properties using a sample gas pretreatment system

Oxygen adsorption plays key roles in resistive-type SnO₂ gas sensors that can very sensitively detect combustible gases such as CO and hydrocarbons. Thus, exact evaluation of the electric resistance of SnO ₂ in response to oxygen is important to understand the oxygen adsorption mechanism. However, infinitesimal impurities contained in even high-grade commercial oxygen cylinders impose great effect on the sensitivity of SnO₂. In this study, we designed an experimental system, which composed of gas pretreatment chambers including a Pt/Al2O3 combustion catalyst and a zeolite adsorbent, for exact analysis of gas sensing properties. Our experimental system allowed for the accurate determination of the dependence of the electrical resistance (R) on oxygen partial pressure (PO₂) by removing impurities in sample gases. According to the linear correlation between R versus PO₂^1/4, we concluded that oxygen adsorbed on the SnO₂ surface in the form of O^2- at 350 and 450°C in extremely dry conditions that was achieved using the experimental system. The competitive adsorption on the SnO₂ surface in the form of O- and O₂- was suggested at 300°C.