Effect of texture and annealing treatments in SnO2 and Pd/SnO2 gas sensor materials

The chemical reactivity of a series of SnO2 materials with different textures has been studied by XPS. A nanocrystalline SnO2 (n-SnO2) 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 Sn2+/Sn-0) decreases with the degree of compaction of the samples. Thus, while in an n-SnO2 powder a 298 reduction is obtained after heating in H-2 at 773 K, a negligible reduction was found for the compact SnO2 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/SnO2 sensors. Palladium was added onto the SnO2 surface in the form of a PdCl2 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 SnO2 and Pd/SnO2 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/SnO2 system towards the exposure to H-2/O-2 cycles. (C) 1999 Elsevier Science S.A. All rights reserved.