Thin films of SiO2-WO3 composites with various SiO2 contents (0-20 wt%) were prepared by means of sol-gel method. The grain size of WO3 decreased with increasing SiO2 content, resulting in an increase in NO2 sensitivity. On the other hand, the response transient to NO2 was brought to be the quickest at 5% SiO2. The thin film of SiO2(5%)-WO3 was far more sensitive to NO2 than the thin film or sintered block of pure WO3. It could detect dilute NO2 (0.1-2 ppm) in air at 350°C sensitively and fairly selectively. The NO2 sensing characteristics were stable over the whole test period of 10 days.
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