Sensing properties of atmospheric plasma-sprayed WO3 coating for sub-ppm NO2 detection

Tungsten oxide (WO3) sensitive layers for NO2 sensors were fabricated by atmospheric plasma spraying technique followed by a heat-treatment. The phase structure and microstructure of the as-sprayed and heat-treated coatings were characterized by X-ray diffraction and scanning electron microscope. The sensing characteristics of these sensors were obtained by measuring the response towards NO2 in moist air with a concentration in the range of 0–450 ppb and at a working temperature in the range of 95–240 °C. The results showed that the WO3 coating prepared with low temperature plasma (Ar plasma) had a better sensitivity than the one deposited with high temperature plasma (Ar–H2 plasma) due to a more porous structure. The obtained WO3 sensors were found to exhibit different sensing behaviors depending on the working temperature and NO2 concentration. The electrical resistance of WO3 sensor increased at high NO2 concentration in moist air and at a low working temperature. Whereas, it was found that the resistance decreased when the NO2 concentration was lower than 93 ppb and the working temperature was higher than 130 °C.