Effect of texture and annealing treatments in SnO 2 and Pd/SnO 2 gas sensor materials

Abstract

The chemical reactivity of a series of SnO 2 materials with different textures has been studied by XPS. A nanocrystalline SnO 2 (n-SnO 2) powder, a powder sample with a crystal size of several microns and a very compact thin film have been chosen for this comparative study. It is found that the degree of reduction upon heating in H 2 (i.e., percentage of Sn 2+/Sn 0) decreases with the degree of compaction of the samples. Thus, while in an n-SnO 2 powder a 29% reduction is obtained after heating in H 2 at 773 K, a negligible reduction was found for the compact SnO 2 thin film prepared by ion beam-induced CVD (IBICVD). This latter material has been chosen as a suitable substrate to characterize the state and function of palladium in Pd/SnO 2 sensors. Palladium was added onto the SnO 2 surface in the form of a PdCl 2 salt. Then, the sample was subjected to different thermal activation processes. Conductivity measurements at different temperatures and under several pressures of H 2 and O 2 have been carried out with the samples. Texture and previous treatments have been found to be critical factors determining the gas sensing properties of SnO 2 and Pd/SnO 2 samples. A correlation is intended between the characteristics of the Pd 3d XPS signal after the different thermal treatments and the conductance response of the Pd/SnO 2 system towards the exposure to H 2/O 2 cycles.