Specificity of electrophysical and gas-sensitive properties of nanocomposite ZnO–TiO2 films formed by solid-phase pyrolysis

Abstract

ZnO–TiO₂ thin films containing 0.5 mol%, 1.0 mol%, and 5.0 mol% ZnO were synthesized by oxidative solid-phase pyrolysis. The materials contained anatase and rutile phases with particle size of 6–13 nm, as confirmed using X-ray phase analysis and scanning electron microscopy. When a certain number of ZnO crystallites appeared in the TiO₂ film structure in the temperature range of room temperature to 220 °C, a two-level response of the film resistance was observed, differing by approximately 10%, as obtained by electrophysical measurements. The two-level response correlates with the formation of two donor energy levels of 0.28 and 0.33 eV in the band structure of the ZnO–TiO₂ films. The donor level with a higher activation energy corresponded to the Ti vacancy (V⁻_Ti), and that with a lower activation energy corresponded to the Zn vacancy (V⁻_Zn). Two levels of gas-sensitive properties were noted for 0.5ZnO–TiO₂, 1ZnO–TiO₂, and 5ZnO–TiO₂ under the influence of 50 ppm NO₂ at 250 °C. Such two-level responses can be ascribed to the pinning of the Fermi level on ZnO and TiO₂ nanocrystallites. The mechanism of the beak-shaped and two-level responses of sensors based on composite nanomaterials when exposed to various gases was elucidated.