Improvement of SO2 sensing properties of WO3 by noble metal loading

SO2 sensing properties of several semiconductor metal oxides have been investigated in the temperature range of 100–800°C. Each oxide exhibited complex temperature- and time-dependent SO2 response. Among the oxides tested, WO3 exhibited the highest SO2 sensitivity at 400°C, accompanied by a resistance increase, but its resistance decreased in SO2 at temperatures higher than 500°C. To improve the SO2 sensitivity of WO3, effect of a metal addition was also tested. Among the metals tested, the addition of 1.0 wt.% Ag was most effective for improving the sensitivity at 450°C. In the case of the 1.0Ag/WO3, however, the sensor resistance decreased in SO2 over the whole temperature range studied. Thus, the addition of Ag led to changes in both the surface states of SO2-related adsorbates and the electronic interactions between the adsorbates and WO3. The resistance increase of WO3 upon exposure to SO2 at 400°C was suggested to arise from the formation of SO2− at sites different from those for oxygen adsorbates, while the resistance increase of 1.0Ag/WO3 at 450°C from the formation of SO42− on the Ag loaded due to the reaction of a gaseous SO2 molecule and two Oad2− ions on the Ag. Interference from NO2 to the SO2 sensitivity was found to be more significant in the case of 1.0Ag/WO3.