NO₂ Adsorption Properties on Various WO₃ Crystals and Relation with Their Sensing Properties

Four types of WO₃ particles, spherical, disk, cuboid, and hexagonal, were prepared and WO₃ film sensors were deposited on Au comb-type microelectrodes by dropping suspension using the various WO₃ powders obtained. The sensing properties to dilute NO₂ revealed that the cuboid WO₃ film sensor had the best response-recovery characteristics with the 90% response and 90% recovery times of 1.3 and 0.9 min to 0.05 ppm NO₂ at 200°C, respectively. On the other hand, the hexagonal WO₃ film sensor showed extremely high sensor response of 41.1 even to 0.01 ppm NO₂ at 200°C, but the sensor response fluctuates widely with the operating temperature ranging from 80 to 300 °C. The adsorption-desorption behaviours of NO₂ on various surfaces of WO₃ particles with different morphologies were investigated using temperature programmed desorption (TPD) technique, which suggesting that NO₂ molecules were adsorbed easily on the WO₃ surface. The desorbed amount of NO decreased in the order of hexagonal, cuboid, disk and spherical, which agreed with the respective sensor response of the four WO₃ film sensors.