Low temperature response of nanostructured tungsten oxide thin films toward hydrogen and ethanol

Semiconducting metal oxide based gas sensors usually operate in the temperature range 200-500 degrees C. In this paper, we present a new WO3 thin film based gas sensor for H-2 and C-2 H5OH, operating at 150 degrees C. Nanostructured WO3 thin films were synthesized by thermal evaporation method. The properties of the as-deposited films were modified by annealing in air at 300 degrees C and 400 degrees C. Various analytical techniques such as AFM, TEM, XPS, XRD and Raman spectroscopy have been employed to characterize their properties. A clear indication from TEM and XRD analysis is that the as-deposited WO3 films are highly amorphous and no improvement is observed in the crystallinity of the films after annealing at 300 degrees C. Annealing at 400 degrees C significantly improved the crystalline properties of the films with the formation of about 5 nm grains. The films annealed at 300 degrees C show no response to C2H5OH (ethanol) and a little response to H-2, with maximum response obtained at 280 degrees C. The films annealed at 400 degrees C show a very good response to H-2 and a moderate response to C2H5OH (ethanol) at 150 degrees C. XPS analysis revealed that annealing of the WO3 thin films at 400 C produces a significant change in stoichiometry, increasing the number of oxygen vacancies in the film, which is highly beneficial for gas sensing. Our results demonstrate that gas sensors with significant performance at low operating temperatures can be obtained by annealing the WO3 films at 400 degrees C and optimizing the crystallinity and nanostructure of the as-deposited films. (C) 2012 Elsevier B.V. All rights reserved.