Influences of moisture on the response of a neat tin oxide gas sensor to oxygen at 623 K were investigated. Two types of moisture effects were easily recognized, that is, moisture acted as a inhibitor to the oxygen adsorption in form of O- (type I sites) and O2- (type II sites) by being adsorbed competitively (short term effect), whereas the moisture admitted at elevated temperature acted as a promoter to increase the population of type II sites or the adsorptive strength of type I sites (long term effect). A tremendous increase of the type II site population as well as the existence of threshold pressure for oxygen adsorption on the same sites suggests the formation of a sort of surface hydrate, dehydration of which seems to leave type II sites behind. The response to oxygen could be reproduced satisfactorily by using adsorption constants of oxygen and water, threshold pressure of oxygen and semiconductor properties of tin oxide.
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