Using a TiO2/ZnO double-layer film for improving the sensing performance of ZnO based NO gas sensor

NO gas sensors, based on ZnO thin film (ZnOfilm), TiO2 nanoparticulate film (TiO2NP), and TiO2NP/ZnOfilm double-layer film, were fabricated, and their sensing characteristics towards NO gas were investigated in this study. The maximal response of a ZnOfilm deposited onto a rougher Al2O3 substrate, towards NO gas, was higher than that of a ZnOfilm deposited on a smoother glass substrate. Although the sensing response of the TiO2NPs itself towards NO gas was minute, the TiO2NP/ZnOfilm double-layer film showed enhanced response as compared with TiO2NP or ZnOfilm single-layer film. In addition, the sensor response of the TiO2NP/ZnOfilm double-layer film was strongly influenced by the annealing time for the film preparation; the maximum response to NO was enhanced about 6.2 times as the annealing time was increased from 30 min to 2 h. Based on the XPS results, the increase in the transition zone between TiO2NP and ZnOfilm along with the appearance of Ti3+ state was noticed when the annealing time was increased. With the high sensitive TiO2NP/ZnOfilm/Al2O3 electrode, the limit of detection (S/N = 3) can be achieved at 8.8 ppb. The double-layer TiO2NP/ZnOfilm also showed improved selectivities with respect to NO2 and CO.