Effect of microstructure on detection of dilute NO2 using WO3 thin film sensors

The effects of microstructure on NO2 sensing properties were investigated for WO3 thin film sensors fabricated on SiO2/Si substrate equipped with Au comb-type microelectrodes. The effect of grain size for thin WO3 sensors was explained with the conventional model. On-the other hand, the high sensitivity NO2 sensor was obtained due to unique thickness effect. For the sensors using WO3 powders calcined at 300-400 degreesC (grain size 18-20 run), the sensitivity to dilute. NO2 had a maximum at optimal thickness. The optimal thicknesses were 7 and 25 mum for the sensors using 400 and 300 degreesC calcined powders, respectively. These sensors showed the extremely high sensitivity (Rg/Ra) of 6.5-45 to 0.02-0.05 ppm NO2 at 200 degreesC, suggesting the. possibility of environmental NO2 monitoring. Such a behavior was not observed for the sensors having large grain (calcination temperature : 600-850 degreesC). In this case, the sensitivity to NO2 monotonously decreased with increasing film thickness.