Influence of Humidity on NO2-Sensing and Selectivity of Spray-CVD Grown ZnO Thin Film above 400 °C

Thin films are being used more and more in gas sensing applications, relying on their high surface area to volume ratio. In this study, ZnO thin film was produced through a thermal aerosol spraying and chemical vapor deposition (spray-CVD) process at 500 °C using zinc acetate as a precursor. The phase identification and the morphologies of the film were investigated by XRD and SEM, respectively. Gas-sensing properties of the ZnO thin film were evaluated toward NO 2, CO, and NO at a moderate temperature range (400–500 °C) in dry and humid air (relative humidity = 2.5, 5, 7.5, and 10% RH). The obtained results show good sensor signal for both NO 2 (ΔR/R 0 = 94%) and CO (92%) and poor sensor signal to NO (52%) at an optimum temperature of 450 °C in dry air. The response and recovery times decrease with the increase of NO 2 concentration. In the presence of humidity (10% of RH), the sensor is more than twice as sensitive to NO 2 (70%) as CO (29%), and accordingly, exhibits good selectivity toward NO 2. As the amount of humidity increases from 2.5 to 10% RH, the selectivity ratio of ZnO thin film to NO 2 against CO increases from 1 to 2.4. It was also observed that the response and the recovery rates decrease with the increase of relative humidity. The significant enhancement of the selectivity of ZnO thin film toward NO 2 in the presence of humidity was attributed to the strong affinity of OH species with NO 2. View Full-Tex