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 desoprtion 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.