NO2 sensing properties of nanostructured tungsten oxide thin films

Tungsten oxide (WO3) nanoparticles have been successfully synthesized by sol–gel method for detection of NO2 gas in the low concentration range (5–100 ppm). The sensor based on WO3 was fabricated on glass substrate using simple and cost effective drop casting method. The structure and morphology of WO3 nanoparticles were studied using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) techniques. X-ray photoelectron spectroscopy (XPS) confirms formation of WO3. The chemiresistive gas sensing performance of WO3 films were studied towards various reducing (NH3, H2S, CH3OH & C2H5OH) and oxidizing (NO2 & Cl2) gases in the 100–300 °C temperature range. The experimental results demonstrate that, WO3 films were highly sensitive and selective towards NO2 gas than other test gases. The relationship between sensor response and various concentrations of NO2 gas was systematically studied and explored. WO3 thin films exhibit maximum response of 34% (towards 100 ppm NO2) with very fast response time of 24 s and are able to detect low concentration of 5 ppm NO2 gas with response of 16%. The proposed sensing mechanism of WO3 thin films in presence of air and NO2 gas was discussed. Furthermore, the interaction of NO2 gas with WO3 film was also investigated using impedance spectroscopy.