| With the improvement of living standard, the requirements of the health and living environment around are increasing day by day. Especially, it becomes more urgent for real-time monitoring of the air pollution gas concentration. Therefore, metal oxide based gas sensors which can realize this goal has aroused great attention and focused investigation. At present, researchers has focused on the controllable synthesis of gas-sensing materials while the study on the science and technique problems during the fabrication process of gas sensors from gas-sensing materials has been reported rarely. In this paper, it is important to solve the problems including aggregation, damage and decreased activity of metal oxide nanostructures during the fabrication process of gas sensors. For this purpose, the control of microstructures including morphology and surface defects of metal oxide nanomaterials on the sensor surface by using new material-device intergration techniques is developed to enhance the gas sensing properties.At first, thin film made from8-nm-diameter and exposed-{10-10}-facet ZnO nanorods which were self-aligning in a straight line on the surface of Al2O3tube-like sensor was prepared by a simple dip-coating method. On this surface, we successfully synthesized a ZnO nanorods array with exposed (0001) plane and100nm in diameter in situ by a facile solution-processing technique. More importantly, the gas-sensing properties showed an obvious improvement:a high sensitivity (3-fold prefactor Ag), fast response (less than10s) and low detection limit (1ppm) to benzene and ethanol. On basis of these results, the effect of exposed facet is dominant rather than the size effect, and the order of gas-sensing properties of ZnO crystal face is (0001)>{10-10}. Moreover, it was found that the surface structure at atomic level was a key factor in improving the oxygen adsorption and, consequently, the gas-sensing performance of a ZnO nanorods array based gas sensor.The above method is very complicated and time-consuming. To solve this problem, ZnO nanotetrapods (T-ZnO) which can be prepared on a large scale were used as gas-sensing materials. To control the morphology and surface defects of T-ZnO, the sintering temperature was varied. With an increase in the sintering temperature from350to750℃, the feet and cross of T-ZnO became gradually shorter and bigger, respectively, and subsequently tetrahedron-shaped ZnO nanoparticles were produced instead of T-ZnO at850℃. The morphological evolution was explained by a new physical model based on Thomson effect, leading to a decrease in the surface area and an obvious variation in the grain contact. Meanwhile, the surface defects were also changed:the amount of zinc interstitial (Zni) was decreased while oxygen vacancy (Vo) showed an inverse trend. Moreover, the best gas-sensing performance towards formaldehyde and methanol was obtained after sintering at450℃. This was mainly attributed to the synergetic effect between the best grain contact (meaning that more nanoparticles can make contributions to the sensor response) and more zinc interstitial as well as larger surface area (supplying more chemisorbed oxygen).However, the above coating method is usually inefficient. To fabricate gas sensor on a large scale, T-ZnO were made into thick sensing film by a facile screen-printing technique. On the surface of flat-type gas sensor, the surface defects of T-ZnO thick film including intrinsic defects and extrinsic defects can be controlled by injecting several microlitres of La ions solution and post treatment to improve gas-sensing properties. With increasing the concentration of La ions, the ratio of Lazn was increased while the relative ratio of oxygen vacancy showed an inverse trend, leading to a decrease in the optimal working temperature of ZnO sensor. The lowest optimal working temperature was250℃when the concentration of La ions was0.05mol/L. These results indicated that the increased ratio of Lazn played a dominant role in decreasing the optimal working temperature. In such fabrication of gas sensors, the morphology of metal oxide was difficult to control because metal oxide were very fragile. To solve the problem, CNTs were at first made into thick film on the coplanar gas sensors by a screen printing technique and used as a template. Then, several different metal oxide precursor solutions could be injected into the thick film. After sintering process, CNTs were removed and these metal oxide net-like films with porous structures were obtained, leading to an increased sensitivity as well as better selectivity, indicating that metal oxide microstructures and composition played an important role in the gas sensing properties. |
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