Gas-chromism in ultrasonic spray pyrolyzed tungsten oxide thin films

A simple and inexpensive ultrasonic spray pyrolysis (USP) technique has been employed to deposit tungsten oxide (WO₃) thin films by spraying 2.0 mM aqueous ammonium metatungstate solution onto the amorphous glass substrates kept at 250°C. The films were further annealed at 400°C for 4 h in air. X-ray diffraction (XRD) technique was used to determine the crystallinity and to identify the WO₃ phases. It was found that the films were sub-stoichiometric, WO_3-z. To study gas-induced properties, a catalyzing layer of platinum (Pt) was sputtered onto it. The gas-induced electrical and optical properties of Pt/WO₃/glass samples were studied and results reported. It was found that electrical resistivity decreased by a factor of 10 within 2 min and stabilized after 15 min, after H₂ gas exposure. Similarly the optical transmittance of the samples attenuated from 55% to 10% within 15–20 min. The reversible changes in electrical resistivity and optical transmittance were observed when the samples were exposed to oxygen. The response times and sensitivity of the samples were estimated.