Effect of microstructure on detection of dilute NO/sub 2/ using WO/sub 3/ thin film sensors

The effects of microstructure on NO/sub 2/ sensing properties were investigated for WO/sub 3/ thin film sensors fabricated on SiO/sub 2//Si substrate equipped with Au combtype microelectrodes. The effect of grain size for thin WO/sub 3/ sensors was explained with the conventional model. On the other hand, the high sensitivity NO/sub 2/ sensor was obtained due to unique thickness effect. For the sensors using WO/sub 3/ powders calcined at 300-400/spl deg/C (grain size : 18-20 nm), the sensitivity to dilute NO/sub 2/ had a maximum at optimal thickness. The optimal thicknesses were 7 and 25 /spl mu/m for the sensors using 400 and 300/spl deg/C calcined powders, respectively. These sensors showed the extremely high sensitivity (Rg/Ra) of 6.5-45 to 0.02-0.05 ppm NO/sub 2/ at 200/spl deg/C, suggesting the possibility of environmental NO/sub 2/ monitoring. Such a behavior was not observed for the sensors having large grain (calcination temperature: 600-850/spl deg/C). In this case, the sensitivity to NO/sub 2/ monotonously decreased with increasing film thickness.