The Controllable Synthesis And Structural Studies Of Gold Nanoclusters

Gold nanoclusters with specific electronic and atomic structures possess unique optic, electronic and catalytic properties and have potential application in fields like catalysis, luminescent material, detector, biology and medicine. Recently, gold nanocluster is one of the newest nanomaterials in nanoscience and trigers great interest for fundamental research. However, before the widely use of gold nanoparticles, there are some unsolved problems blocking the development of this field. Some of these problems include:how to synthesize gold clusters with accurate number of gold atoms; how to improve the yield of gold clusters; how to determine the structure of gold clusters and how to tailor the electronic and atomic structure of gold cluster for the specfic purpose. In this dissertation, by variation chemical parameters, gold nanoparticles with different sizes and capped with differernt surfactants were prepared. With the use of X-ray absorption fine structure (XAFS), transmission electron microscope (TEM) and UV-vis technologies, the size, size distribution, electrnoic and atomic structure of gold nanoparticles were investgated. Besides, we also studied the Co-induced phase transition of BaTiO3by a combination of XAFS and X-ray diffraction (XRD) methods. This dissertation includes:1. The synthesis of gold nanoparticles with different sizesA "precursor continuous-supply" strategy was developed for controllable synthesis of0.9-3.3nm Au nanoparticles with a narrow size distribution of0.1-0.2nm, using a weak reductant to slow-down the reducing rate of AuClPPh3precursor in ethanol. Time-dependent x-ray absorption and UV-Vis absorption measurements revealed that owing to the joint use of AuClPPh3and ethanol, the remnant AuClPPh3was self-supplied and the precursor concentration was maintained at a level near to its equilibrium solubility (ca1.65mmol/L) in ethanol. Hence the nucleation duration was extended that focused the initial size distribution of the Au clusters. With reaction going on to58minutes, most of AuClPPh3with a nominal Au concentration of17.86mmol/L was converted to ethanol-soluble Au clusters with the size of about1.0nm, resulting in a high-yield synthesis.2. Gold nanoparticles capped by different surfactants Gold nanoparticles passivated by different kinds of surfactants such as triphenylphosphine (PPh3), polyvinylpyrrolidone (PVP), dodecanamine (C12H27N) and dodecanethiol (C12H26S) were prepared and the interactions of Au nanoparticles with different surfactants were also studied. TEM images showed the size of all samples was about3nm and the size distribution of gold nanoparticles could be well controlled by dodecanethiol (3.1±0.1nm). The X-ray absorption near edge spectra manifested the electron transfer between Au and S atoms. For PPh3, PVP, dodecanamine and dodecanethiol covered Au nanoparticles, the atomic structure disorder of Au-Au shell gradually increased in order ranging from0.0095to0.0152A. And dodecanamine and dodecanethiol interacted with Au nanoparticles by forming Au-N or Au-S bond.3. The surface structure of gold nanocrystalsTo compare the surface structure of gold nanocrystals, dodecanethiol passivated gold nanocrystals (3and5nm) and "bare" gold nanocrystals (3and5nm) were prepared. Using in situ X-ray absorption fine structure (XAFS), we are able to distinguish the real surface structure information of the monodispersive sphere-shape Au nanocrystals (NCs) dispersed in hexane solution, in which the NCs are free from the influences of surfactant and polar solvent. It is demonstrated that in such a solution environment the surface Au-Au bond lengths of the naked3and5nm Au NCs are very close to the bulk value, contrary to the strong surface contraction for supported, capped or dried Au NCs. Nevertheless, for the same-sized NCs capped by a strong surfactant such as dodecanethiol, a large surface contraction of-0.10A is observed, most likely due to the stress generated by Au-S interactions..4. Thiol desorption and its effects on the structure of gold nanoclustersA solvent-exchange method was proposed to remove thiol molecules from the surface of gold nanocrystals and nanoclusters, while keeping the size and size distribution of gold nanoparticles unchanged. However, the electronic and atomic structures of gold nanoparticles experienced a significant change. It is demonstrated that for dodecanethoil-protected icosahedral Au clusters of1.1nm, solvent-exchange of ethanol by hexane leads to quick desorption of the Au-thiolate protecting layers from the surface, as indicated by XAFS. The survived Au cores then undergo a much slower energy-minimization process via recombination and structural rearrangement, resulting in the formation of distorted face-centered-cubic structured clusters. In response to the dramatically changed atomic structure, the electronic structure of the Au clusters is converted from semiconducting to metallic-like characters.5. Hexagonal BaTi1-xCoxO3phase stabilized by Co dopantsThe phase transition from tetragonal to hexagonal of crystalline BaTi1-xCoxO3(0.01≤x≤0.20) powders prepared by solid state reaction was studied by x-ray diffraction and x-ray absorption fine structure. At the low Co doping level of x≤0.03, the structure of the samples is tetragonal, and it is transformed gradually to a hexagonal structure upon increasing Co content to≥0.05. The detailed analysis of Co K-edge XAFS indicated unambiguously that the doped Co ions are substantially incorporated into the BaTiO3host and likely serve as a trigger in leading the structure transition.