Electrical properties under polluting gas (CO) of Pt- and Pd-doped polycrystalline SnO2 thin films:: analysis of the metal aggregate size effect

In Pt- or Pd-doped SnO2 thin films prepared by using a submicronic aerosol pyrolysis method where both metal elements are co-deposited, the best sensitivity to CO is obtained from low-concentration Pd/Pt precursor solutions. TEM observations from SnO2 thin films doped with small amounts of platinum have been combined with results obtained from in situ X-ray Absorption Spectroscopy (XAS) to give a schematic description of the metallic particle effect on the electrical properties of the films. It is demonstrated that the lower the concentration, the smaller are the metallic particles. The evolution of the particle size and density as a function of concentration can be understood in terms of nucleation-growth processes. The XAS analysis shows, in particular, that the Pt local environments are modified by metal concentration variations and by the gas in contact with the film surface. The modifications are discussed on the basis of competing bulk and surface contributions to CO reduction processes, the latter being dominant in smaller particles. It is inferred that smaller particles contribute as a whole to the electrical behavior whereas in bigger particles, a surface and a bulk contribution have to be considered. (C) 1999 Elsevier Science S.A. All rights reserved.