Influence of electrodes ageing on the responses of SnO2 sol-gel sensors

Metal oxides films are generally applied as sensing layers, in semiconductor gas sensors. These sensors based on electrical conductivity changes have numerous advantages as high sensitivity, simple design and low cost. Great efforts have been made with the aim to determine and improve the parameters that contribute to the electrical conductivity and that influence the electrical response of the sensors. Nevertheless, these devices show some well-known limitations as poor selectivity, temperature and moisture dependence and, especially, ageing and drift. The main cause of drift is, of course, changes in morphology of the sensing layer. In fact, the electrical properties of the sensing layer and the transduction function of the device depend strongly on the layer morphology. Moreover also the quality of the electrodes has to be considered as parameter that can influence the ageing of the device. In this paper, we studied the gas sensing characteristics, in particular the stability and reliability, of different pure and Ni-, Os-, Pt-, Pd- doped SnO₂ thin films based sensors prepared by the sol-gel technique and the influence of the ageing of the Au electrical contacts on the performances of the sensors. We studied the responses of different sensors towards some gases and vapours. SEM results were showed. The surface chemical composition of the samples has been studied by XPS (X-ray Photoelectron Spectroscopy) analysis.