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

Tungsten oxides (WO_x) 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 WO_x 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.