Large scale production of spherical WO3 powder with ultrasonic spray pyrolysis assisted by sol–gel method for hydrogen detection

The spherical WO3 powder was prepared by a facile ultrasonic spray pyrolysis process assisted by a sol–gel method. The thermal analysis of tungsten oxide precipitate indicated that there is 10% weight loss in the temperature range of 25–350 °C with two distinct steps, which correspond to the loss of the interlayer water molecules and the coordinated water molecules. The crystal structure and morphology have been investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicate that the XRD peaks for the obtained sample can be indexed to the orthorhombic WO3·H2O and triclinic WO3 with the crystal water being eliminated after heat treatment. The particle size distribution was characterized by laser particle size analyzer and the result shows a double distribution; 50% of the total volume is composed of particles which are smaller than 0.74 μm. The d.c. electrical response to hydrogen of the spin-coating thick film sensor has been measured in the temperature range from 200 °C to 300 °C. The results indicate that the sensor performs as a n-type semiconductor, and it shows the maximum and minimum responses at 200 °C and 300 °C, respectively, with nearly linear dependence on the hydrogen gas concentration.