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

Pure SnO, and Pd/SnO, (0.4 wt.%) pastes have been prepared starting from SnO* 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, 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 SnOZ 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 01.5-0.18 and 0.45-0.50 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