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 420degreesC for 1 hr. are amorphous while annealed at as high as 500degreesC are polycrystalline. The Scanning Electronic Microscope (SEM) images revealed that the film annealed at 360degreesC was amorphous and the surface was smooth. The film showed good responses to 100 ppm, 1000 ppm and 1% O-2 at an operating temperature of 170degreesC. 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.