Characterization of Tungsten Oxide Thin Films Produced by Spark Ablation for NO2 Gas Sensing.

Tungsten oxides (WOx) thin films are currently used in electro-chromic devices, solar-cells and gas sensors as a result of their versatile and unique characteristics. In this study, we produce nanoparticulate WOx films by spark ablation and focused inertial deposition, and demonstrate their application for NO2 sensing. The primary particles in the as-deposited film samples are amorphous with sizes ranging from 10 to 15 nm. To crystallize the samples, the as-deposited films are annealed at 500 °C in air. This also caused the primary particles to grow to 30-50 nm by sintering. The morphologies and crystal structures of the resulting materials are studied using scanning and transmission electron microscopy and X-ray diffraction, whereas information on composition and oxidation states are determined by X-ray photoemission spectroscopy. The observed sensitivity of the resistance of the annealed films is ∼100 when exposed to 1 ppm of NO2 in air at 200 °C, which provides a considerable margin for employing them in gas sensors for measuring even lower concentrations. The films show a stable and repeatable response pattern. Considering the numerous advantages of spark ablation for fabricating nanoparticulate thin films, the results reported here provide a promising first step toward the production of high sensitivity and high accuracy sensors.