Temperature dependencies of sensitivity and surface chemical composition of SnOx gas sensors

Abstract The electrical resistivity response to CO gas exposure versus temperature has been measured for different types of SnO x -based gas sensors. The chemical composition of the sensor surfaces and the electronic structure of the valence band are investigated by X-ray photoelectron spectroscopy and scanning Auger microscopy technique, with the aim of explaining resistivity changes in terms of the surface oxidation/reduction mechanism. The samples are treated by Ar + sputtering, thermal annealing in UHV and oxygen at various temperatures up to 400°C. An ultrathin Pt overlayer, which enhances the gas sensitivity in a low operating temperature range, is found to be very porous. The band-gap states induced by oxygen vacancies and adsorbed hydroxyl groups are revealed by valence-band spectra. The resistivity changes of the sensors due to exposure to reducing or oxidizing gases are caused more by the changes of the surface-defect density than by the variation of excess surface charge due to oxygen adsorption.