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 degreesC. Each oxide exhibited complex temperature- and time-dependent SO2 response. Among the oxides tested, WO3 exhibited the highest SO2 sensitivity at 400 degreesC, accompanied by a resistance increase, but its resistance decreased in SO2 at temperatures higher than 500 degreesC. 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 degreesC. 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 degreesC 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 degreesC from the formation of SO42- On the Ag loaded due to the reaction of a gaseous SO2 molecule and two O-ad(2-) ions on the Ag. Interference from NO;? to the SO2 sensitivity was found to be more significant in the case of 1.0Ag/WO3. (C) 2001 Elsevier Science B.V. All rights reserved.