Nanostructured SnO2 thin films: effects of porosity and catalytic metals on gas-sensing sensitivity

In this paper, an overview of structural and sensing properties of undoped and Pt/Pd-doped tin oxide (SnO2) thin films obtained by the aerosol pyrolysis (pyrosol process) is provided. The detection mechanism and the role of the small metal nanoparticles (NPs) in the sensing processes have been discussed in detail. It has been specifically shown how tightly the film microstructure correlates with the catalytic metal concentration, and thus acts as a driving factor of film sensitivity evolution. The normalized evolution of the experimental sensitivity is closely fitted by the product of film porosity and NPs number density. Under CO and H2 mixed with neutral gas, dynamic electrical conductance measurements show that the dispersions of metallic elements induce a two-step time-dependent behavior. Only, when they are incorporated in a very weak quantity (which corresponds to very small particles), Pt/Pd nanoparticles change in a reversible manner, their oxidation state, according to the nature of the atmosphere. An electronic model has been proposed to explain the high sensitivity observed at low temperature. Different situation has been observed in the presence if H2S gas when the conductance increases smoothly without any induction period.