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 sensi tivity 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 Spec troscopy (XAS) to give a schematic description of the metallic particle eff ect on the electrical properties of the films. It is demonstrated that the lower the concentration, the smaller are the metallic particles. The evolut ion 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 concen tration variations and by the gas in contact with the film surface. The mod ifications are discussed on the basis of competing bulk and surface contrib utions to CO reduction processes, the latter being dominant in smaller part icles. It is inferred that smaller particles contribute as a whole to the e lectrical behavior whereas in bigger particles, a surface and a bulk contri bution have to be considered. (C) 1999 Elsevier Science S.A. All rights res erved.