To understand the relationship between adsorbed oxygen on the surface of indium oxide (In 2 O 3 ) and its CO-sensing property in wet and dry atmospheres, the oxygen adsorption properties and sensor response to CO were investigated. In 2 O 3 particles were prepared by a precipitation method and characterized using X-ray diffraction, scanning electron microscopy, and nitrogen absorption measurements. The O− and O 2− adsorption equilibrium constants, K 1 and K 2 , respectively, were calculated from the relationship between the electrical resistance and the oxygen partial pressure (P O2 ) using a competitive adsorption model. It was found that the values of both K 1 and K 2 decreased with increasing humidity, indicating a reduction in the amount of oxygen adsorption on the In 2 O 3 surface in a wet atmosphere. The In 2 O 3 particles can detect CO at ppm level in dry and wet atmospheres, although they were greatly reduced in wet atmospheres. Moreover, we found that the sensor response to CO is closely related to the values of K 1 and K 2 .
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