Influence of microstructure of tungsten oxide thin films on their general performance as ozone and NOx gas sensor

Tungsten oxides thin films were obtained by electron beam deposition and annealed in the temperature range 350-800°C for 1-3 h. The structure, morphology and phase composition of the as-deposited and annealed films were characterized by X-ray diffraction and AFM. The electrical response towards NO2 and O3 was studied both experimentally and theoretically. In order to interprete the kinetic characteristics of tungsten oxide thin films upon 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 heterogeneties of the tungsten oxide film surfaces and their performance characteristics as gas sensors was established. It was shown that the nature and the amount of the surface adsorption sites of the different non-stoichiometric phases (WnO3n-2 or WnO3n-1) or WO3, and their conductive mechanism are defined from: the phase composition of the film, the crystallographic and electronic structure of the phases, the orientation of the crystallites within the film, and the geometrical form and dimensions of the crystallites. All tungsten oxide thin films investigated in this work are suitable to detect very low concentrations of NO2 (0.05-0.5 ppm in N2 and synthetic air) and ozone (25-90 ppb) at very low working temperatures (80-160°C). The films annealed at 400°C for 1-2 h are very selective to ozone and the films annealed at 400°C for 3 h and at 800°C for 1 h are very selective to NO2