Performance degradation mechanism of the light-activated room temperature NO2 gas sensor based on Ag-ZnO nanoparticles

The chemiresistor-type sensors based on Ag-ZnO nanostructures have shown excellent sensing properties to NO2 under visible light irradiation, however, they cannot escape from the destiny of the performances degraded by long-term operation. So, aiming at improving the long-term stability, the revelation of the essence of performance degradation is crucial and also a great challenge. Here, we comparatively investigated the change rules of NO2 sensing properties after running the as-fabricated Ag-ZnO nanoparticles-based sensor multiple times during 60 days under UV and visible light irradiation. It is demonstrated that the room temperature light-activated (especially UV light) NO2 sensing performances (response and sensitivity) of the sensor degrade after running multiple times. The degraded performances can be attributed to the deteriorated crystal quality and increased oxygen vacancy defects of the sensing materials, and oxidization of more Ag-0 into Ag+ on the ZnO surfaces, which greatly increase the free charge carrier concentration in the dark and decrease photogenerated charge carrier concentration under light irradiation. Correspondingly, the photoconductivity of the device declines and its output resistance signal weakens. This work offers not only a fundamental understanding of the performance degradation of the light-activated room temperature NO2 gas sensor, but also a strategy for the study on the performance degradation mechanism of other devices.