SO2 sensing mechanism of Ag/WO3. Surface states and electronic interactions

Abstract

SO2 sensing properties of WO3 loaded with and without a metal have been investigated in the temperature range of 200-800°C. X-ray photoelectron spectroscopic studies and temperature programmed desorption spectra from SO2-preadsorbed WO3-based sensor materials were also conducted to clarify the adsorption states of SO2 and their electronic interactions, and then to discuss the SO2 sensing mechanism. The resistance of WO3 increased upon exposure to SO2 at temperatures lower than 500°C, due to the formation of SO2-ad at adsorption sites different from those for oxygen adsorbates. In contrast, the resistance of WO3 decreased in SO2 at temperatures higher than 550°C, due to the formation of SO42-ad by the reaction of a gaseous SO2 molecule and two O2-ad adions. The largest resistance change of WO3 induced by SO2, i.e. the highest sensitivity, was observed at 400°C. Among the metals tested, the addition of 1.0 wt% Ag resulted in the most significant enhancement in sensitivity at 450°C, while the resistance decreased in SO2 over the whole temperature range studied. In the case of 1.0 Ag/WO3 the resistance was determined mainly by the surface state of Ag. It was considered that the formation of SO42-ad was facilitated even at a temperature of 450°C by the reaction of gaseous SO2 and O2-ad on the Ag surface and that this caused the resistance decrease.