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.
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