Among the family of wide band-gap semiconducting metal oxides, tungsten trioxide is the most promising oxide for gas
sensing. A metastable open-structured hexagonal phase of WO3 was successfully synthesized using acid precipitation
method. The oxide was characterized using SEM, TEM and XRD. The sensing property to reducing ammonia gas was
measured. The sensitivity is much higher than that of monoclinic tungsten oxides. The oxide polymorph exhibits a p-n
type transition when the temperature goes from 100 °C to 300 °C.
Selective detection of small amounts of toxic gases, such as ammonia and CO is very important to environmental monitoring as well as for medical diagnoses. MoO<sub>3</sub> and WO<sub>3</sub> have been identified as suitable materials for detecting these gases with high sensitivity. Sol-gel processed thin films of MoO<sub>3</sub>, WO<sub>3</sub> and their combination have been prepared at SUNY Stony Brook by the hydrolysis of metal alkoxide precursors followed by spin coating and were deposited on alumina heater/electrode
containing substrates that were produced by the Brescia group. Sensing tests were carried out in the state-of-the-art gas
sensor testing facilities available in Brescia, where the electrical resistance of sensor arrays was recorded as a function
of gas concentration, for various combinations of gases (including ammonia, CO, NO<sub>2</sub>, Methanol, isoprene, etc) at 10%
relative humidity and at temperatures ranging from 400-500°C. The MoO<sub>3</sub>-WO<sub>3</sub> composite system showed the best
stability at the highest testing temperature. The sensing results obtained are correlated with the structural characteristics
of the sensing films. This work has been carried out as a joint collaboration between the Advanced Materials Characterization Laboratory of SUNY Stony Brook (USA) and the Sensor Lab at the University of Brescia (Italy) and was funded by a NSF-AAAS (WISC) grant awarded to Perena Gouma.