Electrical and spectroscopic characterization of SnO2 and Pd-SnO2 thick films studied as CO gas sensors

The responses of semiconductor gas sensors to the reducing gases are affected by the changes in charge carrier density arising from addition of dopants or catalysts. The aim of our work has been to obtain a better understanding of the influence of Pd on the electrical and electronic properties of thick SnO2 films. We investigated the responses of SnO2 and Pd/SnO2 films to CO in dry and wet (50% RH) air at temperatures increasing from 250 degrees C up to 450 degrees C. The samples were obtained from commercial and laboratory powders. FT-IR spectroscopy has been employed here to provide information on the changes of the electronic properties of the differently prepared samples, and the results have been compared to d.c, electrical measurements. Films prepared with pure tin oxide, both from commercial and laboratory powders, show a maximum of electrical response at 350 degrees C, and show lower responses in wet air than in dry air at temperature lower than 400 degrees C, while the responses is higher in wet air at 400-450 degrees C. Pd always decreases the temperature of the maximum response to CO, both in dry and wet air. Pd addition decreases the response to CO of the films prepared from commercial powders at temperature greater than or equal to 300 degrees C, both in dry and wet air, while at 250 degrees C there is a moderate increase of the response. In the case of films prepared from laboratory powder, Pd addition does not show a clear trend. Some comparisons with the responses to CH, analyzed in a previous paper are also reported. (C) 1998 Elsevier Science S.A. All rights reserved.