NO2 sensitivity of WO3 thin film obtained by high vacuum thermal evaporation

The gas sensitivity, selectivity and stability properties of WO3 thin films for the detection of NO2 gas in the concentration range 0.2-5 ppm, have been evaluated and discussed in the light of the preparation conditions and working temperature. Thin films were obtained by evaporating high purity WO3 powder by an electrically heated crucible at about 5 x 10(-4) Pa on sapphire substrates provided with Pt interdigital type sputtered electrodes and annealed for 1 h at 400, 500 and 600 degrees C. The film morphology, crystalline phase and chemical composition were characterised through AFM, low angle XRD and XPS. The electrical response was measured by means of DC current mode. The annealed films showed crystallographic orientation belonging to the triclinic structure of WO3, while the as deposited films were found to be amorphous. The binding energies of O 1s and W 4f confirmed the existence of the WO3 phase, with a stoichiometric ratio close to the theoretical one. All the films showed the highest sensitivity to NO2 at a working temperature of 200 degrees C. The 500 degrees C annealed film was found to be the most sensitive to NO2 gas, compared to those annealed at 400 and 600 degrees C, No cross sensitivity effects were found by exposing the sensors to CO, CH4. WO3 films showed strong sensitivity to C2H5OH and H2O, Long term stability test at a working temperature of 350 degrees C, performed by cycling the films in dry air and 5 ppm NO2 revealed no substantial change in the electrical properties in terms of drift and sensitivity.