| Metal oxide Semiconductors are the best promising candidates for gas sensing applications because of their high sensitivity towards many target gases in conjunction with easy fabrication methods and low cost. To date, studying on gas-sensing materials have been the major breakthrough for improving the gas-sensing properties, mainly including the type of materials, morphology and structure, exposed facets and surface modification. From the viewpoint of device structure, gas sensor prototypes were usually synthesized in the form of thin films, in which the powders are screen printed on prefabricated electrodes ceramics tubes or plates followed by annealing at an appropriate temperature. Nevertheless, only a small fraction of the species adsorbed near the grain boundaries is active in modifying the electrical transport properties. Recently, nanoarray materials based gas sensors are of particular interest because of having not only great miniaturization potential, but also open space to allow for easy contact and diffusion of the gas molecular into the inner region. There is thus still an evident need for detailed investigations regarding the device structure.In this thesis, we carried out the research from the high performance metal oxide semiconductor based gas sensor, and hydrothermally synthesized three novel nanoarrays materials for gas-sensing application. By comparing and characterizing the precursors, we analytically obtained the growth mechanism. In addition, we measured the gas-sensing tests for all the nanoarray materials. Finally, we proposed a concept of self-powered gas-sensing system. The main contents in this thesis include:1. The rhombus-shaped ZnO nanorod arrays were synthesized by a facile fluorine-mediated hydrothermal method. In the formation process, F- play an essential role in controlling the morphorogy. The ZnO nanorod arrays grown on the pre-deposition of seeds directly can be used as the gas sensor prototype without the complicated film formation process. The sensor annealed at 450? showed the best performance, and the response to 100 ppm ethanol reached ?11.8 at the working temperature of 300?. The sensor also exhibited good response/recovery speed (4 s and 7 s), excellent selectivity, repeatability and stability. The rhombus-shaped ZnO nanorod arrays based gas sensor demonstrates a typical surface-controlled n-type semiconductor gas-sensing character. Compared to traditional ZnO gas-sensing materials, the rhombus-shaped ones exhibit entire advantages of open space, porosity good crystallinity and large amounts of point defects on the surface.2. The rhombus-shaped Co3O4 nanorod arrays were synthesized via the similar fluorine-mediated hydrothermal route. The hydrothermal temperature is critical to the morphorogy of the nanostructures and the affinity to Co2+ of different anions is found in the order of CO32->F->OH-.Direct growth on the substate further simplifies the fabrication process. The synthesized sensor annealed at 450? showed the highest sensitivity to ethanol. The gas response to 100 ppm ethanol reached ?31.4 and the optimal working temperature was as low as 160?. Meanwhile, the sensor exhibited good response/recovery kinetics (61s and 54 s), outstanding selectivity and stability. The gas-sensing properties of Co3O4 can be explained by a typical surface-controlled p-type behavior. We consider that the better ethanol gas sensing performance than rhombus-shaped ZnO nano arrays can be attributed to the special exposed facets.3. According to the affinity to Co2+ of different anions, we replaced HMT by urea and obtained the needle-like Co3O4 nanowire arrays. The Co3O4 nanoneedle arrays based gas sensor annealed at 350? showed high performance in ethanol detection. The highest sensitivity reached-89.6 for 100 ppm ethanol vapor and the optimal working temperature was 130?. Although having excellent selectivity and stability, the response and recovery time are only 449 s and 646 s, respectively. It can be attributed to different energy binding sites between negatively charged chemisorbed oxygen and Co2+ or Co3+. We consider the special exposed crystal facets of Co3O4 affect the gas-sensing properties significantly.4. Finally, we proposed a blow-driven triboelectric nanogenerator (TENG) based self-powered gas-sensing system. When an external resistive sensor is applied, the voltage drop applied over the load by the TENG increases with increasing the load resistance of a sensor. Here we choose the rhombus-shaped Co3O4 nanorod arrays based gas sensor to connect with the TENG. When the TENG was blew by a tester after alcohol drinking, the breathed-out alcohol vapor will dramatically increase the resistance of the Co3O4 sensor, which trigger the siren. This work not only presents a new blow-driven self-powered alcohol detector, but also greatly advances a concept of TENG based self-powered electronic nose system. |
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