Pd-doped SnO2 thin films deposited by assisted ultrasonic spraying CVD for gas sensing:: selectivity and effect of annealing

Polycrystalline Pd-doped SnO2, thin films (0.25-1.75 mu m) have been deposited by spray pyrolysis on silicon nitride substrate heated between 460 and 540 degrees C. The gas sensitivity (S = (G(gas) - G(air))/G(air)) in the steady-state and in a dynamic regime have been evaluated under CO (300 ppm in air), ethanol (100 ppm in air) and methane (1000 ppm in air) as a function of temperature. The thin films synthesised at lower temperature (460-500 degrees C) are the most sensitive to CO according to their smaller crystallite size and higher resistivity. In the steady-state, sensitivities up to 4500 have been obtained for the thinnest films at low working temperature of 100 degrees C. A cross-sensitivity to ethanol and to methane was observed. Annealing under pure air stabilises the microstructure and increases the resistivity in air of the Pd-doped SnO2, thin films, which provides a sensitivity to carbon monoxide 2 to 10 times higher, since the resistance of the films under CO remains unchanged. (C) 1998 Elsevier Science S.A. All rights reserved.