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 deg. 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 deg. C, but the sensor response fluctuates widely with the operating temperature ranging from 80 to 300 deg. 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.