Characterization of sol-gel prepared WO3 thin films as a gas sensor

Tungsten trioxide $({\mathrm{WO}}_{\mathrm{3}})$ thin films have been prepared by the sol-gel process and annealed at different temperatures of 400, 500, 600, and 700 °C for 1 h. The morphology, microstructure, crystalline structure, and composition of the films have been analyzed using scanning electron microscopy (SEM), x-ray diffraction, Rutherford backscattering spectroscopy (RBS), and x-ray photoelectron spectroscopy (XPS) techniques. The SEM analysis showed that the films annealed at 400 °C are smooth and uniform. However, these evolved as granular at an annealing temperature of 500 °C. The films annealed at still higher temperatures have two distinct grains of different shapes and sizes. The films annealed below 400 °C are amorphous. Annealing at 500 °C resulted in the films having polycrystalline structure. RBS and XPS characterization have revealed that the films annealed at 400 °C are stoichiometric. Annealing above this temperature resulted in the films becoming off-stoichiometric. The electrical resistance of the films annealed at 500 °C increased 18 times when exposed to 175 ppb ${\mathrm{O}}_{\mathrm{3}}$ gas compared to the air exposure, with the response time being as short as 1–2 min.