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

The electrical resistivity response to CO gas exposure versus temperature has been measured for different types of SnOx-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 techniques, 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 degrees 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.