The comparative effect of two different annealing temperatures and times on the sensitivity and long-term stability of WO3 thin films for detecting NO2

We have deposited 150-nm-thick WO₃ films on Si₃N₄/Si substrates provided with platinum interdigital electrodes and annealed in static air at 300°C and 500°C temperatures for 24 h and 200 h. The morphology, crystalline phase, and chemical composition of the films have been characterized using AFM, grazing incidence XRD and high resolution XPS techniques. The sensor resistance response curve has been obtained in the 0.2 -4 ppm NO₂ gas concentration range in humid air (50% relative humidity), varying the operating temperature between 25 and 250°C. By plotting both sensor resistance and gas concentration logarithmically, the response is linear over the investigated dynamic range. Sensor sensitivities, here defined as the ratio of sensor resistance in gas to that in air (i.e., S=R_Gas/R_Air), have been compared at a given NO₂ gas concentration (0.2 ppm). The long-term stability properties have been evaluated by recording film sensitivity for 1 yr under standardized test conditions. Increasing the annealing temperature from 300 to 500°C causes the sensitivities to decrease. The 300/24h film is shown to be the most sensitive at S=233, but with poor long-term stability properties. The 300/200h film with S=32 is stable over the examined period. The 500/24 and the 500/200 films are shown to be less sensitive with S=16 and S=14, respectively. The longer the annealing time and the higher the temperature, the poorer the sensitivity, but with positive effects upon the long-term stability of the electrical response. The influence of the annealing conditions on sensitivity and long-term stability has been correlated with the concentration of surface defects, like reduced WO₃ phase (i.e., W⁴⁺), which resulted in a strong effect on the sensors´ response.