| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 98 === Through the photocatalysis of titanium dioxide (TiO2), a novel method is developed in this study for fabricating metallic nanowires (NWs) without using seeds, surfactants or templates. It is verified that this method can simplify the manufacturing procedures and the nanowires thus produced exhibit excellent electrical conductivity. The heat treatment of this process can be divided into two steps: the first step is the annealing treatment for the TiO2 substrate, which can provide the heat energy for the crystallization of anatase TiO2, and also generate a small amount of UV sources to excite TiO2 by blackbody radiation. This activates electrons and holes on the surface of TiO2 films and enables the reduction of metallic ions from the solution, and subsequently forms the nucleation sites of nanowires. The second step is the post heat treatment for the growth of nanowires. In the post heat treatment, thermal-electrons in TiO2, an n-type semiconductor, can further reduce the metallic ions and thus the metal atoms accumulate and stack on the pre-formed nucleation sites along a certain preferred orientation to form one dimensional nanostructure. This thesis builds a brand new model for the synthesis of metallic nanowires, of which the nucleation and growth mechanisms are much different from the others in literatures. Accordingly, this unique method is named “Thermally Assisted Photoreduction, TAP” process.
Several experimental parameters are varied to clarify the relationship between the substrate and the yield of metallic nanowires, such as the degree of crystallization, surface roughness, photocatalytic ability and surface orientation. Atomic Force Microscope (AFM) and X-ray diffraction (XRD) results suggest that a better crystallinity, along with a rugged surface of anatase TiO2, gives rise to a higher yield of the nanowires. This means that the crystallinity and surface morphology of the anatase TiO2 substrate significantly influence the nucleation of nanowires. The super-hydrophilicity of TiO2, which means full excitation of TiO2, is harmful for the formation of nanowires. Conversely, a suitable number of photo-electrons on the surface of TiO2 under insufficiently excited conditions are required for forming nanowires. According to the results of Photoluminescence (PL) and Time Resolved Photoluminescence (TRPL), (101) oriented films which is capable of obtaining large quantity of nanowires exhibit a more complex surface structure, longer life time for photo-exited electrons and consequently a greater photocatalytic activity. The finding can be supported by high resolution transmission electron microscopy (HRTEM).
Furthermore, through the adjustment of the conditions of post heat treatment (time and temperature), the yield and aspect ratio, as well as the uniformity of the metallic NWs can be controlled. The electrical behavior investigated using a nanomanipulation device indicates that the metallic nanowires by the TAP process possess excellent electrical conductivity, which is believed to have a great potential for the application in nano-electronics packaging.
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