Effect of Humid Aging on the Oxygen Adsorption in SnO2 Gas Sensors

To investigate the effect of aging at 580 degrees 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 degrees C was strongly increased by humid aging. From the results of oxygen partial pressure dependence of the electric resistance, the oxygen adsorption equilibrium constants (K-1; for O- adsorption, K-2; for O2- adsorption) were estimated on the basis of the theoretical model of oxygen adsorption. The K-1 and K-2 in dry and wet atmospheres at 350 degrees C were increased by humid aging at 580 degrees 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 degrees 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.