Tungsten trioxide (WO3) thin films were deposited by Rf-magnetron sputtering onto Pt interdigital electrodes fabricated on corning glass substrates. NO2 gas sensing properties of the prepared WO3 thin films were investigated by incorporation of catalysts (Sn, Zn, and Pt) in the form of nanoclusters. The structural and optical properties of the deposited WO3 thin films have been studied by X-ray diffraction (XRD) and UV-Visible spectroscopy, respectively. The gas sensing characteristics of all the prepared sensor structures were studied towards 5？ppm of NO2 gas. The maximum sensing response of about 238 was observed for WO3 film having Sn catalyst at a comparatively lower operating temperature of 200°C. The possible sensing mechanism has been highlighted to support the obtained results. 1. Introduction Nitrogen dioxide (NO2) is one of the most harmful gases to the ecosystem and provides a major contribution to air pollution [1]. The detection of NO2 is crucial for monitoring environmental pollution resulting from combustion processes, particularly industrial emissions or vehicle exhaust [2]. Occupational Safety and Health Administration (OSHA, United States Department of Labour) declares the Permissible Exposure Limit (PEL) of NO2 gas as 5？ppm for general industries and 20？ppm as Immediately Dangerous to Life or Health Concentrations (IDLHs) [3]. NO2 gas is the main precursor for ozone layer depletion in lower atmosphere and also produces acid rain which is slowly damaging the ecosystem. Commercially, many NO2 gas sensors are available in the market but they have poor sensitivity, high operating temperature, bulky size and are very costly. Thus, there is an urgent requirement of cheap, highly sensitive and selective NO2 gas sensors which could be operated at lower operating temperature. Gas sensors based on metal oxide semiconductors are used in a wide variety of applications including gas monitoring and detection applications [4–6]. Considerable research has been carried out on the development of chemical sensors based on semiconductor metal oxides such as SnO2, ZnO, and TiO2 because of their high sensitivity towards many reducing as well as oxidizing gases [7–9]. Tungsten trioxide has attractive electrical properties and reactivity to oxidizing gases making it one of the best candidates for gas sensing applications [10–12]. The tungsten trioxide (WO3) thin films and nanostructures are seen to be an excellent candidate for NO2 gas detection [13, 14] because the W transition metal is found to be with different oxidation states (W5+, W6+) enhancing the
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