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Synthesis、Characterization and Applications of Au Clusters and Nanoparticles

Author: WenLi
Tutor: LinZhongHua；ZhouJianZhang
School: Xiamen University
Course: Physical and chemical
Keywords: Au clusters Au nanoparticles Au catalyst Interfacial structure CO oxidation
CLC: TB383.1
Type: PhD thesis
Year: 2008
Downloads: 780
Quote: 1
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Abstract


In the past two decades, Au nanoparticles have potential applications in many fields, such as catalysis area,chemical sense, biosense, and electronic nanodevices/nanocircuits, owing to their unique chemical, optical, and electronic/electrical properties. Various particle morphologies of Au have been routinely synthesized by chemical and physical methods. As far as the synthesis of nanoparticles is concerned, there is a great and growing need to develop clean, nontoxic, and environmentally friendly synthetic procedures. Biologicalmethods have recently been considered as possible environmentally friendly nanofactories. Fabrication of ordered arrays of nanoparticles and the study of their electronic and electron transport properties are significant themes of current researches. At the same time, the method for preparation of Au catalysts, which involves the deposition of the monolayer-protected Au nanoparticles （Au MPCs） on supports, is a preparation routewith extensive application prospects for creating highly active catalysts.In this paper, Au MPCs are synthesized by a two-phase protocol and, in particular, biological methods. It is described that the size, shape and monodispersity of gold nanoparticles which are synthesized by using microorganisms can be easily controlled by the modulation of reaction time and the use of protecting agents. MPCs/electrode interfacial structure and electrochemical properties of nanometer-scale electrode interface are studied by CV, DPV and EIS. Meanwhile, we report a modified colloidal deposition route-"step-by-step soakage" method for the preparation of Au catalysts for CO oxidation using prefabricated alkanethiolate self-assembled monolayer （SAM）-protected Au nanoparticles in hexane colloidal solution as precursor. The topography and structure of Au catalysts prepared are studied by TEM, XPS and FTIR. Catalytic activity measurements are also carried out. The main results from those studies are summarized as follows. 1. The synthesis of SAM-protected Au clusters /nanoparticles and the construction of self-assembled monolayers of clusters /nanoparticles on a gold electrode surface（1）.Monodispersed SAM-protected Au nanoparticles are biosynthesized extracellularly by an efficient, simple and environmentally friendly procedure using Bacillus megatherium D01 and Bacillus licheniformis R08 as the reducing agent and using dodecanethiol and mercaptoacetic acid sodium salt as the capping ligand at 26℃.It has been shown that reaction time and the protecting agent are important parameters in controlling the morphology of Au nanoparticles formed by reduction of aqueous AuCl4- ions with the biomass. A significant improvement on size, shape, and monodispersity has been achieved by the modulation of reaction time and the use of the protecting agent.（2）. Octanethiolate SAM-protected Au clusters （C8Au MPC） and dodecanethiolate SAM-protected Au clusters (C12Au MPC) are synthesized by a two-phase protocol. The construction of self-assembled monolayers of Au clusters is obtained on a gold electrode surface by self-assembly.2. Studies on quantized capacitance charging of Au MPCs and MPCs-electrode interfacial electrochemical properties（1）.Quantized capacitance charging is observed for C8Au MPCs and C12Au MPCs self-assembled on a gold electrode surface in CH2Cl2 containing 0.1mol·L-1 TBAP. The results from differential pulse voltammetry of C8Au MPCs and C12Au MPCs with average core diameter of 1.6 nm show four and five entries well-defined quantized capacitance charging peaks within the potential range of -0.8 to+0.8 V, respectively. The change trend of MPCs double-layer capacitances （Cmpcs） is that the value of CMPC is smallest near zero charge potential and increases with potential positive or negative transfer. The double-layer structure comprises both a compact double layer and a diffuse double layer. In addition, the effect of Au core size on quantized capacitance charging of C12Au MPCs is studied. The results show that the value of CMpc increases with increasing Au core size.（2）.Electrochemical impedance spectra （EiS） of the MPC modified electrode show that the electrode interfaces with adsorbed MPCs consist of two components, namely, the electrode-MPCs interface and the MPCs-solution interface. The interfacial capacitances of the electrode-MPCs interface (CDL1 and the MPCs-solution interface (CDL2) almost have no change near the potential of zero charge （ca.-0.2 V）, then CDL1 and CDL2 both change along with increasing or decreasing the electrode potentials. Quantized capacitance charging of MPCs is also theoretically analyzed by using the theory of single electron tunneling and Coulomb blockade of metal islands and semiconductor quantum dots, confirming that EIS method is an effective method for studying quantized capacitance charging of MPCs.（3） The effect of the concentration of supporting electrolyte on quantized capacitance charging of CgAu MPCs by differential pulse voltammetry. The results show that the concentration of supporting electrolyte affects the peak current and the background current of the quantized charging peaks of self-assembled MPCs. Meanwhile, the concentration of supporting electrolyte evidently affects the interfacial capacitance between the electrode surface without adsorbed MPCs and solution, but has almost no influence on the double-layer capacitance of self-assembled MPCs.（4）. The effect of the electroactive species-ferrocene on quantized capacitance charging of C8Au MPCs is studied by cyclic voltammetry and differential pulse voltammetry. The results show that the electroactive species in the solution make a little contribution to the quantized charging of the MPCs-solution interface, indicating that capacitance charging current of the interface derives from the electron transfer between the nanoparticles.3.Nano-Au/γ-Al2O3 catalysts for low-temperature oxidation of CO（1）.In this work, we report a modified colloidal deposition route——step-by-stepsoakage" method for the preparation of a series of monodispersed gold catalysts using prefabricated SAM-protected Au nanoparticles of alkanethiol with different chain length in hexane colloidal solution as precursor.（2）. For the as-prepared C8Au MPCs /γ-Al2O3 (or C12Au MPCs /γ-Al2O3) catalysts, the majority of the Au nanoparticles deposited on the support are in a narrow size range of 2-3 nm.Even after 450 h （or 600 h） of reaction, the Au particles are still in the size range of 2-4 nm. XPS results show that the valence states of Au comprise both the metallic and oxidized states.（3）. The vacuum dryness temperature affects significantly on the particle size and the catalytic activity of the Au catalysts. The particle size of the Au nanoparticles increases with raising the vacuum dryness temperature. The catalyst dried at 25℃under vacuum exhibits considerably higher activity as compared to catalysts treated at 40℃and 60℃.（4）. The thermal treatment temperature affects significantly the catalytic activity of the catalysts in the processing steps. The catalyst treated in the temperature range of 180℃-190℃exhibits considerably higher activity as compared to catalysts treated at 165℃and 250℃.（5）. It is found from the evaluation results of catalytic performances that the 2.0wt% nano-Au/###-Al2P3 completely converts CO to CO2 at -16℃and maintains the catalytic activity at nearly 100% CO oxidation for at least 800 h at 15℃,at least 600 h at 0℃,and even longer than 450 h at -5℃;The 4.0wt% nano-Au/γ-Al2O3 maintains the catalytic activity at nearly 100% conversion of CO for at least 2000 h in the presence of the reaction gas containing H2O at 15℃.Evidently, the catalyst has higher activity, good long-term stability and strong anti-moisture performance. The effect of possible factors on the catalytic activity of Au/γ-Al2O3 is studied based on the above results.

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