Electrical properties evolution under reducing gaseous mixtures (H-2, H2S,CO) of SnO2 thin films doped with Pd/Pt aggregates and used as polluting gas sensors

An analysis of the electrical properties evolution of a series of SnO2 thin films doped with small amounts of Pd or Pt under pure air and air with 300 ppm CO is presented. As several types of chemical reactions are clearly in volved in the solid-gas interactions at the film surface, it has been neces sary to simplify the system by favouring interactions in absence of oxygen. Films were consequently also put in contact with N-2 or Ar mixed with smal l amounts of H-2, CO, H2S to favour behaviours in absence of oxygen gas. As under air + CO at 300 degrees C, important features of Pd/Pt-doped film co nductance closely resemble CO2 production rates of CO oxidation at the surf ace of Pd aggregates reported in the literature, an attempt of interpretati on of the conductance evolution has been made along this line. Under CO and H-2 mixed with neutral gas, dynamic (kinetic) electrical conductance measu rements show that the dispersions of metallic elements induce a two-step ti me dependent behaviour. The first step is associated with a reduction of th e oxygen molecules adsorbed at the SnO2 grain surface and an increase of el ectron density in the SnO2 depletion zone. The second step with a sharp con ductance increase implies a reduction of the metallic aggregates and an ele ctron transfer from the aggregates to the SnO2 grain conduction band. For H 2S the conductance increases smoothly. (C) 2000 Elsevier Science S.A. All r ights reserved.