Electrical properties evolution under reducing gaseous mixtures (H2, 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 involved in the solid–gas interactions at the film surface, it has been necessary to simplify the system by favouring interactions in absence of oxygen. Films were consequently also put in contact with N2 or Ar mixed with small amounts of H2, CO, H2S to favour behaviours in absence of oxygen gas. As under air+CO at 300°C, important features of Pd/Pt-doped film conductance closely resemble CO2 production rates of CO oxidation at the surface of Pd aggregates reported in the literature, an attempt of interpretation of the conductance evolution has been made along this line. Under CO and H2 mixed with neutral gas, dynamic (kinetic) electrical conductance measurements show that the dispersions of metallic elements induce a two-step time dependent behaviour. The first step is associated with a reduction of the oxygen molecules adsorbed at the SnO2 grain surface and an increase of electron density in the SnO2 depletion zone. The second step with a sharp conductance increase implies a reduction of the metallic aggregates and an electron transfer from the aggregates to the SnO2 grain conduction band. For H2S the conductance increases smoothly.