Moisture effects on pure and Pd-doped SnO 2 thick films analysed by FTIR spectroscopy and conductance measurements

Abstract

Pure SnO 2 and Pd/SnO 2 (0.4 wt.%) pastes have been prepared starting from SnO 2 powder obtained with the procedures generally used for gas-sensor materials. The pastes are printed on 96% alumina substrates for electrical measurements and the layers detached from the support are used for Fourier-transform infrared (FTIR) investigations. Conductance measurements in dry and wet air are presented together with the FTIR spectra for temperatures ranging from 100 to 450°C. By alternating wet and dry air, it has been observed that the conductance of pure SnO 2 samples changes in a reversible way only for temperatures over approximately 200°C, while the samples catalytically treated with Pd do not present any significant irreversible component. The IR analysis on pure SnO 2 samples treated in wet air at temperatures above 200°C shows the formation of a very broad absorption of electronic nature almost completely destroyed by a subsequent dry-air treatment. The absorption may be due to two families of donor levels at 01.5−0.50 eV, respectively, from the bottom of the conduction band. Changes of the electronic absorption shape, intensity and reversibility to dry-air contact are observed on the same samples treated in wet air below 200°C. These results are compared with those obtained for Pd catalytically modified materials. The conductance and impedance measurements in the presence of methane are presented for both the wet- and dry-air treated pure and Pd-doped thick films.