| Summary: | 博士 === 國立臺灣科技大學 === 應用科技研究所 === 104 === This thesis systematically studies microstructural characteristics, optical properties, growth mechanism and phase transitions of multi-dimensional core-shell nanostructures. Au-Ag alloy nanostructures with various shapes and Cu@Ag NPs were synthesized using a successive reduction method. With respect of Au-Ag, twined Ag NPs and single crystalline Ag NWs were adopted as templates respectively and alloyed with the same amount of Au+ ions. STEM HAADF images observed from different rotation angles confirm that Ag NPs turned into AuAg alloy rings. Single crystalline Ag NWs became Ag@AuAg core-shell wires instead of hollow nanostructure. It is proposed that the twin boundaries of the Ag templates were the dominating factor causing hollow alloy nanostructures. PVP-protected Cu@Ag core-shell nanoparticle deposits were also synthesized and their phase transformation upon heating in air was investigated via in situ synchrotron radiation X-ray diffraction. The core-shell structural feature can be verified by the surface plasmon resonance peaks and EDS line scans, as well as Moiré patterns raised by overlapping the Cu and Ag lattices which could be oberved in the core region. The protection of Ag can significantly reduce the oxidation of Cu NPs. The phase separation feature between Cu and Ag gives rise to the dewetting of Ag shell at high temperatures
With respect to metal/oxide structure, this study successfully developed a universal method for synthesizing ultra-long metal core-oxide shell composite nanowires. In the absence of oxide precursors, templates, inoculants and surfactants, vertically grown Ag/TiO2 and Ag/CeO2 core–shell NWs were synthesized on oxides substrates using a one step process.
Experimental results confirm the substrate (TiO2 and CeO2) is soluble into molten salt and the Ag NWs grow vertically together with the migration of dissolved Ti or Ce from the substrate to the Ag NW surface and forms a continuous and dense oxide shell. In addition, the spontaneous oxide valence and Ag surface plasmon resonance were investigated by electron energy loss spectroscopy (EELS). The peak shifting due to the presence of mono or multi-oxide layers can be seen in EELS. The shift of surface plasmon resonance is related to the material, thickness as well as the lamination of the oxide shell, mainly due to the reflectance.
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