NO2 gas sensing characteristics of WO3 thin films synthesised by the thermal oxidation of DC sputtered W films: Effect of oxidation temperature

Gas sensors are indispensable for tracking environmental pollution, ensuring human health and safety, and detecting harmful environmental gases. Metal oxide thin films are preferred in sensor applications due to their exceptional properties. This study delves into the effect of thermal oxidation temperature of WO3 thin films on NO2 gas detection. These films are synthesized through the thermal oxidation of 200 nm tungsten metal films deposited via DC magnetron sputtering in the temperature range of 400-700 degrees C. As the oxidation temperature increases, a transition in the film morphology and crystallinity is observed using field-emission electron microscopy and grazing incidence X-ray diffraction. With the increasing oxidation temperature, WO3 thin films demonstrate a noticeable enhancement in the XRD peak intensity and peak width reduction which symbolizes the improvement of crystallisation. Also, the rise in oxidation temperature generates the cracking of thin film grains and the formation of finer grains and surface porosity. A systematic study of gas sensing characteristics across various operating temperatures ranging from room temperature to 200 degrees C was carried out which revealed an optimized operating temperature of 100 degrees C. The WO3 films oxidized at 600 degrees C exhibited the highest response of 687 % towards 10 ppm of NO2 gas concentration at 100 degrees C. The films are capable of detecting NO2 gas down to 100 ppb. The sensor demonstrates notable sensing abilities, including strong selectivity, rapid response, and good repeatability. X-ray photoelectron spectroscopy indicates the presence of a higher surface oxygen adsorption on WO3 thin film synthesised at 600 degrees C. Our study underscores the pivotal role of WO3-based gas sensors in environmental protection, showcasing their high responsiveness even at trace levels.