Investigation Of Photomediated Synthesis Of Silver Nanoparticles And The Mechanism Of Shape Evolution

In recent years,silver nanoparticles draw widespread attention because of their excellent physical and chemical properties. Localized Surface Plasmon Resonance?LSPR?of silver nanoparticles are affected by their morphology, size and other factors, which, as a result, impact its application in production. Controllable morphology and size of silver nanoparticles is a hot research field. A variety of silver particles of prism, decahedra,bipyramid, cube, octahedra, icosahedra, wire, rod, and disk have been prepared and applied in the fields of optical devices, catalysts, biosensors, drug delivery and surface-enhanced Raman scattering. The primary method for the synthesis of silver nanoparticles is photo-mediated and thermal synthesis. Mirkin's group synthesized anisotropy silver nanoparticles for the first time through the photo-mediated method, which has been proved efficient after ten years research and development.Researchers have done a lot of work in the control of the morphology evolution of nanoparticles by adjusting the wavelength, temperature, capping agents, reducing agent and pH value in photomediated synthesis. For example, Mirkin and coworkers synthesized silver right triangular bipyramids and prisms by adjusting pH value and BSPP concentration of the solution. Our group obtained the evolution of silver triangular nanoprisms and decahedron structure by adjusting the irradiating wavelength and temperature. Mechanism of morphological evolution of silver nanoparticles is ascribed to the apparent phenomena such as reaction rate, kinetics and thermodynamics for most cases. The deeper inherent mechanism is left untouched and a universal explanation is still demanded. Recently, supersaturation theory was proved to be effective in explaining the formation mechanism of ionic crystals, molecular crystals and metal nanocrystals in solutions. In this theory, apparent kinetic factors and corresponding results are combined together to study the growth mechanism of nanoparticles, which will help us in the synthesis of silver nanoparticles of different morphologies. Considering above research background, the main contents and results of this thesis are summarized as the following:1. The effects of temperature and pH value on product morphology was studied by using a wavelength of 500 nm light-emitting diode?LED? as a light source. We found that low temperature and low pH value is necessary to synthesize nanodecahedra. Silver nanoprisms were prepared under high temperature and low pH values, and triangular bipyramids were synthesized under high temperature and high pH values. We concluded that the product morphologies were influenced by temperature and pH values of the solution through their effect on the supersaturation of the solution. Within a certain range, the degree of supersaturation of the solution increased when the reaction temperature and pH value increase, so the morphology evolution of the nanoparticles occurred as the surface-energy increased when it grows in supersaturated condition and no nucleation was generated.2. In the reaction system above, under the conditions of high temperature and high pH value, we added small amounts of strong reductant NaBH₄ in the solution, and study the effects of NaBH₄ on product morphology.?111? and?110?-faceted nanoprisms were obtained with the solution containing NaBH₄.?100?-faceted triangular bipyramids were synthesize in the absence of NaBH₄. It is concluded that NaBH₄ greatly increased the supersaturation of the solution, resulting in a large number of nucleation was generated. Nanoparticles grew up in the oxidation of new nucleus provides the source of silver?Ostwald ripening mechanism?, resulting in the reaction rate decreased in the growth process of the nanoparticles. Then the supersaturation was reduced to generate the silver nanoprisms with lower surface energy. It is very important that there is no new crystal nucleus was generated to fit the rules of high energy facets of crystallites are favorited in high supersaturation condition.