Tungsten-oxide thin films as novel materials with high sensitivity and selectivity to NO2, O3, and H2S -: Part II:: Application as gas sensors

The electrical response of tungsten-oxide thin films as-deposited by electron-beam deposition and annealed (at 350-800degreesC for 1-3 h in O-2) to NO2, O-3 and H2S was studied both experimentally and theoretically. In order to interpret the kinetic characteristics of tungsten-oxide thin films on exposure to different gases, a model based on surface adsorption/desorption processes coupled with bulk diffusion was used. A link between the geometrical and chemical heterogeneities of the tungsten-oxide film surfaces and their performance characteristics as gas sensors was established. It was shown that the nature and amount of surface-adsorption sites in the different nonstoichiometric phases (WnO3n-2 or WnO3n-1) and WO3 as well as their conduction mechanisms are defined from the phase composition of the film, the crystallographic and electronic structures of the phases, the orientation of the crystallites within the film and the geometrical shape and dimensions of the crystallites. All tungsten-oxide thin films investigated in this work are suitable for detection of very low concentrations of NO2 (0.05-0.5 ppm in N-2 and synthetic air), ozone (25-90 ppb) and H2S (315 ppm in N-2 and synthetic air) at very low working temperatures (80-160 degreesC). The films annealed at 400 degreesC for 1-2 h are very selective to ozone at 120-160 degreesC; the films annealed at 400 degreesC for 1-3 h and at 800 degreesC for 1 h are very sensitive to NO2 (in N-2) (C) 2004 Kluwer Academic Publishers.