Tungsten-oxide thin films as novel materials with high sensitivity and selectivity to NO2, O3 and H2S. Part I: Preparation and microstructural characterization of the tungsten-oxide thin films

Tungsten-oxide thin films are promising materials for use in highly effective gas-sensing devices for NO^sub 2^, ozone and H^sub 2^S detection in ambient air. In this work tungsten-oxide 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, SEM and AFM. The changes of phase composition and the microstructure in dependence of the annealing conditions are described in detail. The direction of the phase transformations for different annealing conditions is influenced by the very high macrostresses that appear as an additional, independent thermodynamic factor. During annealing at 350-400 °C for 1-3 h and at 800 °C for 1 h predominantly semiconductor phases are formed, whereas the thin films annealed at 500-600 °C for 1-3 h and 800 °C for 2 h consist mainly of phases with more pronounced metallic properties. The processes of realignment of crystal structures during solid-phase transformation lead not only to the growth of new crystallites with a preferential orientation but also to a change in the direction of preferred growth with increasing annealing temperature and time. The films can be divided into two main groups: compact (as-deposited and annealed at 350-500 °C for 1-3 h) and porous (annealed at 600-800 °C for 1-3 h) layers. The gas-sensing properties of these films and the correlation between microstructure and sensing properties will be described in the second part of this paper.[PUBLICATION ABSTRACT]