Response of semiconducting metal oxides to water vapor as a result of water molecules chemical transformations on catalytically active surfaces

The sensitivity (response) of six semiconducting metal oxide (SMO) structures on the basis of SnO₂ to water vapor was studied in dry and humid air over the H₂O vapor concentration range 0–2.9 vol % (0–100% RH) as the temperature of the samples changed from 200 to 600°C. The temperature dependence of the conductivity and response of SMOs to H₂O vapor had extrema at 300–400 and 250–400°C, respectively, and the sensitivity to water vapor did not decrease below 1.7–1.9 up to 600°C. Data on changes in the conductivity of SMOs and the number of OH⁻ groups in the oxygen-hydroxyl layer on the surface of SMOs depending on temperature and air humidity were obtained. A sharp change in conductivity as air humidity increased from 0 to 10% RH could be related to either a sharp change in the number of OH⁻ groups in the oxygen-hydroxyl layer because of the high polarity of water molecules or a decrease in the intergrain energy barrier. The phenomena observed and the behavior of SMO conductivity in dry and humid air were interpreted. The experimental data were used to suggest a mechanism of the formation of hydroxyl groups on the surface of SMOs. The paper contains practical recommendations concerning the temperature conditions under which the influence of humidity on changes in the conductivity of SMOs is weakest.