Polycrystalline and amorphous sol-gel derived WO3 thin films and their gas sensing properties

Amorphous and polycrystalline tungsten trioxide (WO₃) thin films were prepared using the sol-gel process. Tungsten ethoxide was used as precursor material. The WO₃ thin films were spun onto the sapphire and silicon substrates at 2500 rpm for 30 s. The X-ray diffraction (XRD) results revealed that the films annealed at low temperature of 420°C for 1 hr. are amorphous while those annealed at as high as 500°C are polycrystalline. The scanning electronic microscope (SEM) images revealed that the film annealed at 360°C was amorphous and the surface was smooth. The film showed good responses to 100 ppm, 1000 ppm and 1% O₂ at an operating temperature of 170°C. At high operating temperatures the films showed unsaturated and sluggish responses to oxygen gas. The amorphous WO₃ thin films showed no response to ozone. The crystallized thin films showed very stable response to oxygen.