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 TiO(2)NPs 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 2h. 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 arid CO. (C) 2011 Elsevier B.V. All rights reserved.