Study On Preparation And Gas Sensing, Photocatalytic Properties Of Nanometer Zno

Zinc oxide (ZnO) is an important II-VI direct band gap semiconductor material, which has the hexagonal wurzite structure, and lattice constant a= 0.3250 nm,c= 0.5206 nm. Nanosized ZnO has recently attracted much attention due to its distinguished performance in electrical, optical, gas sensing, photocatalytic oxidation, making it suitable for many applications such as solar cells, light emitting diodes, transparent electrode, surface acoustic wave device, ultraviolet photodetector, gas sensor, photocatalytic degradation and so on. For example, ZnO nanostructure can adsorb diferent kinds of gas, and the adsorpted gas can change the particular surface states, energy band and conductivity of the ZnO. So it has been paid much attention as a gas sensing material. Besides, ZnO nanostructure as a well-known photocatalyst for its high efficiency, non-toxic nature and low cost, has been paid much attention in the degradation and complete mineralization of environmental pollutants. It is of great importance to investigate the gas sensing, photocatalytic properties and explore the relationship between microstructure and the gas sensing, photocatalytic properties, which are also the research topic of the improvement of the gas sensing, photocatalytic properties.In this thesis, we prepared ZnO thin film/Si heterojunction by radio-frequency magnetron sputtering; synthesized ZnO nanopower with special morphologies via a facile hydrothermal process at low temperature; assembled and designed gas sensor based on ZnO nanorods; prepared silver modified ZnO (Ag/ZnO) and supported ZnO nano-crystalline by hydrothermal method. We systematically characterized the microstructure of the above samples, studied their electrical, gas sensing and photocatalytic degradation properties. The details of our work are as follows:(1) Study on the electrical transport and ethanol gas sensing properties of ZnO thin film/Si heterojunctionWe prepared polycrystalline ZnO thin films on Si(100) substrates by radio-frequency magnetron sputtering; studied the effect of sputtering parameters (sputtering pressure, sputtering power, sputtering time, depositing temperature and so on) and Si substrates with different resistivity on the electrical transport and gas sensing properties of the ZnO thin films/Si heterojunctions; investigated the gas sensing properties of the junctions for alcohols with different chain lengths at room temperature. The gas sensing mechanism of ZnO thin films/Si heterojunctions had been established based on semiconductor theory, solid band theory and gas sensor theory. And the relationship between the structure of ZnO thin films/Si heterojunction and gas sensing property was explored. Experimental results show that the junction demonstrates the best gas sensing property when the thickness of ZnO thin film is about 41 nm, average crystallite dimension about 8 nm. It is found that gas sensing properties of the junctions depends strongly on the resistivity of Si substrate. The junctions with n-Si substrate of 2.5 ?cm or p-Si substrate of 15.6 ?cm exhibit the better ethanol gas sensing property. Compared with methanol and ethanol gases, the sensitivity of the ZnO thin film/p-Si heterojunction (p-Si resistivity: 15.6Ωcm) to isopropanol gas is bigger at the same concentration.(2) Study on assembly and ethanol gas sensing properties of gas sensor based on ZnO nanorodsAn ethanol gas sensor based on ZnO nanorods was fabricated by hydrothermal synthesis. The electrical resistance, capacitance and impedance of the sensor were investigated at room temperature when exposed to air and ethanol gas of different concentrations. We studied the changing law of each electrical parameter of the sensor in ethanol gas of different concentrations. The mechanism for the changes in the capacitance and impedance as a function of frequency was presented.(3) Study on preparation and photocatalysis properties of the nanopowders of ZnO, silver modified ZnO (Ag/ZnO) and supported ZnO nano-crystallineThe nanopowders of ZnO and Ag/ZnO were prepared respectively by hydrothermal method and photosynthesis method. The supported ZnO nano-crystalline was prepared on quartz glass by the radio-frequency sputtering as ZnO seed layers and then by hydrothermal method. The samples of crystal structure, crystal quality and surface morphology were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). We investigated the photocatalytic activity of the samples using methylene blue as the model reaction under the irradiation with ultraviolet light. It is found that the photocatalytic activity of Ag/ZnO depends on modification with a suitable Ag loading and the mechanism of photocatalytic activity is analyzed.