Abstract
To investigate the effect of aging at 580 °C in wet air (humid aging) on the oxygen adsorption on the surface of SnO2 particles, the electric properties and the sensor response to hydrogen in dry and humid atmospheres for SnO2 resistive-type gas sensors were evaluated. The electric resistance in dry and wet atmospheres at 350 °C was strongly increased by humid aging. From the results of oxygen partial pressure dependence of the electric resistance, the oxygen adsorption equilibrium constants (K1; for O− adsorption, K2; for O2− adsorption) were estimated on the basis of the theoretical model of oxygen adsorption. The K1 and K2 in dry and wet atmospheres at 350 °C were increased by humid aging at 580 °C, indicating an increase in the adsorption amount of both O− and O2−. These results suggest that hydroxyl poisoning on the oxygen adsorption is suppressed by humid aging. The sensor response to hydrogen in dry and wet atmosphere at 350 °C was clearly improved by humid aging. Such an improvement of the sensor response seems to be caused by increasing the oxygen adsorption amount. Thus, the humid aging offers an effective way to improve the sensor response of SnO2 resistive-type gas sensors in dry and wet atmospheres.
Highlights
Resistive-type semiconductor gas sensors using SnO2 have been well investigated and successfully applied to commercial gas alarm systems, air quality sensors, odor sensors, and alcohol monitors for human breath since their invention 65 years ago [1,2,3,4]
We investigated the effect of aging at 580 ◦ C in wet air on the oxygen adsorption and gas sensing properties for SnO2 resistive-type gas sensors
We found that humid aging slightly increased the crystallite size, leading to a slight decrease in the specific surface area of SnO2 nanoparticles
Summary
Resistive-type semiconductor gas sensors using SnO2 have been well investigated and successfully applied to commercial gas alarm systems, air quality sensors, odor sensors, and alcohol monitors for human breath since their invention 65 years ago [1,2,3,4]. SnO2 gas sensors detect combustible gases such as hydrogen, carbon monoxide, and ethanol by a change in the electrical resistance. The electrical resistance change is derived from the interaction of adsorbed oxygen with combustible gases at approximately 250–350 ◦ C. The gas detection mechanism of SnO2 gas sensors is based on the reaction of adsorbed oxygen with combustible gases on the surface [5,6,7,8,9,10,11]. E− is the carrier electron in SnO2 , and O− ad and O2− ad are the adsorbed oxygen that trapped (captured) electrons The occurrence of such oxygen adsorption was experimentally elucidated by temperature programmed desorption (TPD)-, [8] electrical resistance-, [9] and electron spin resonance (ESR)-measurements [12]. When combustible gases are present in the gas phase, adsorbed oxygen disappears from the surface by a combustion reaction with gases and trapped electrons are transferred back to
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