Study On The Growth And Surface Structure Control Of Metal Nanoparticles

Naonomaterials with excellent physical and chemical properties have triggered great interest for their extensive application prospect, and have played a significant role in fields such as energy, materials, information and biology. Thereinto, metal nanoparticles with unique atomic and electronic structures have attracted broad attention for their particular optic, electronic and catalytic properties. On the one hand, studying of the metal nanoparticles growth process and the environment influence for structure and property is of great importance for controllable synthesis of new nanomaterials with particular properties. On the other side, surface modification is necessary for designing materials with special surface structure. However, detection of atomic scale is desired for metal nanoparticles growth process and surface modification. More effort should be put on the experiment and theoretic of environment factors in nanoparticles growth process and surface ligand.To the questions above, in this dissertation gold nanoparticles capped with surfactants were obtained with a simple controllable wet chemistry method. The size, size distribution of gold nanoparticles during growth process were observed by ransmission electron microscope technique (TEM). And electronic and atomic structure were investigated with the use of X-ray absorption fine structure (XAFS) as well as UV-vis technologies. The mechanism of surface thiol adsorption and desorption was consequently proposed. Besides, by combining XAFS technology and theory, we also studied the structure of copper nanocluster and the diffusion process between Cu@Pt bimetal core-shell nanoparticles. This dissertation includes:1. Solvent influence on the role of thiols in growth of thiols-capped Au nanocrystals.Gold nanoparticles with different sizes (1.2nm,2.5nm,3.4nm) and size distribution were prepared in solvent with different polarities.Using x-ray absorption spectroscopy, we demonstrate that solvent can significantly influence the adsorption of thiols on Au nanoparticles, and thereby affects their growth. It is shown that increasing the solvent polarities leads to the higher thiol coverage on the NC surface. The underlying reason for the solvent dependence is proposed to be the synergistic effects of different hydrogen-bond and hydrophobic interactions between the sulfhydryl and alkyl groups of thiols with the solvents molecules. The high coverage of thiols results in smaller nuclei, and further screens the interparticle van der Waals attraction in the growth stage, leading to a smaller size of NCs.2. Removal of alkanethiol ligands from colloidal Au nanoparticlesAn efficient method for removing alkanethiol ligands from Au nanoparticles surface was developed. Gold nanoparticles (3.3nm) capped with dodecanethiol prepared in ethanol was redispersed in tetrahydrofuran (THF). XAFS resulets indicate that the coverage of thiols gradually decrease with the increase of THF. In addition, thiols located at different Au atoms desorption in different orde. THF effectively eliminates the Au-S bonds on the Au NPs surface with the absence of new capping agent, and keeping the particle size distribution and monodispersity preserved. During this process, THF exsit as aprotic solvent, in which is clined to bimolecular nucleophilic substitution reaction. And consequently, the lone pair electron in THF attack the electron-deficient S atom to form a transient compound, and almost instantaneously, the S atom detach from surface Au atom.3. Surfactant influence structure of Cu nanoclusters.Copper nanoclusters with controlled sizes (0.6nm and1.8nm) were prepared by changing the concentration of surfactant on the base of Brust-Schiffrinstrategy. XAFS was used to investigate the shape, size, electronic and atomic structures. The large specific surface area and excess thiols brought about lots of dangling bond. For nanoparticles smaller than2.0nm, high ratio of surface Cu atom results in rather strong size-dependent atomic distance contraction. The large specific surface area and excess thiols brought about lots of dangling bond.4. In situ XAFS studies the thermal diffusion in bimetallic Cu@Pt nanoparticlesBimetallic Cu@Pt nanoparticles of-8.6nm were prepared by successive reduction. In situ XAFS demonstrated that different with the conventional interdiffusion route, a dominant unidirectional diffusion route involving the migration of the Pt shell atoms into the Cu core upon heating and forming a diffusion layer composed of substitutional CuPt-like intermetallic compounds. As Kirkendalleffect, the diffusion rate of Pt atom is greater than that of Cu atom, which consequently lead to the unidirectional diffusion of from Pt atom to Cu core.