Lamellar Lyotropic Liquid Crystal System In The Preparation Of Gold And Silver Nanomaterials

Synthesis of noble metal nanostructures has been an active research area for many decades, because of the importance of these materials for catalysis, photography, electronics, photonics, information storage, optoelectronics, biological labeling, imaging and sensing. A lot of efforts have been focused on controlling of the shape, size, crystallinity and structures. Particular emphasis has recently been placed on the control of shape, because in many cases it allows one to finely tune the properties with a greater versatility than can be achieved otherwise. In this dissertation lyotropic liquid crystals are used as templates to obtain noble metal nanostructures with controllable shape and size. The experimental research includes two parts.In the first part, using LLC lamellar phase made of lecithin and water to form mimic biomembrane, we synthesize gold and silver nanoparticles. During this progress, the vitamin E in lecithin acts as the reducing agent.The first step of our experiments is to define Vitamin E's reducibility with FT-IR, then POM and SAXS are used to determine the LLC phase structure. At last, The effects of LLC composition, capping agent, reaction temperature, and pH value are studied by TEM and UV-vis, etc.Templated by AuCV-containing lecithin lamellar phases, a mixture of Au nanoplates and nanoparticles is prepared. The selective adsorption of lecithin headgroups on metal surfaces plays an important role in the plate-like nanoparticle formation, but the adsorption is such a relatively weak force that the nanoplates are formed to irregular shape. Due to the rapid reduction rate, plenty of spherical nanoparticles are formed. Adding CTAB to LLC as capping agent, we got regular and larger plates, which show hexagonal and triangular geometries with smooth edges. The reaction rate decreased due to the formation of complex compound between CTAB and AuCl₄^-. It is helpful to the formation of consequent growth of plates. The strong adsorption of CTAB on surface of Au nuclei results in regular and larger plate-like products. For AgNO₃-containing system, irregular silver nanoplates and nanoparticles are also obtained, which are smaller than Au products obtained from AuCl₄^--containing system. This may be due to the slower reaction rate, which makes the templating effect more important. The formation of irregular silver plates also suggests the headgroup adsorption weakly on silver.Our effort for using biological LLC as template to synthesize anisotropic noble metal nanoparticles will be helpful to understand the formation mechanism of nanoparticles with controllable shape and size in biomimic system.In the second part of dissertation, anisotropic gold and silver nanostructures are prepared from LLC lamellar phase templates made of nonionic surfactant C₁₂E₄ and water. Single-crystalline gold nanoplates, with triangular or hexagonal shapes, are successfully synthesized for the first time with a large-scale amount from lamellar LLC templates. And spherical silver nanoparticles are also obtained. POM (polarized optical microscope), SAXS (small angle X ray scattering), TEM (transmission electron microscope), SEM (Scanning electron microscope ) and UV-vis spectrum measurements are used to investigate effect of different conditions on the product, including LLC composition, reaction temperature, pH value, concentration of capping agents poly (N-vinyl-2- pyrrolidone) (PVP) and metal salts.In HAuCl₄-containing lamellar phase, the morphology of obtained nanoparticles can be tuned by changing the LLC composition, and the yield of the triangular or hexagonal gold nanoplates is high. And gold nanoplates are single crystals with a preferential growth direction along the Au(111) plane. The longest edge is up to more than ten microns in size. During the reaction process, the lamellar structure of LLC restricts the reactant and nuclei in water domain. This is very important for the following single-crystal growth of nanoplates. By adding capping agent PVP, the reaction rate can be increased and the size of nanoplate be tuned. In AgNO₃-containing LLC, we get spherical Ag nanoparticles by reduction.Comparing the Au nanoplates and Ag nanoparticles obtained in the same LLC system, we think the difference between products is caused by the positive electrical charge on the EO groups. In the AgNO₃-containing system, the Ag⁺ is repeled by the positive electrical charge and restrained in the middle of water domain and this leads to the formation of nanoparticles. And in the HAuCl₄-containing system, the positive electrical charge attracts the AuCl₄^- to the surface of EO groups and this leads to the formation of Au nanoplates. The thickness of nanoplates can be varied by changing the repeated distance. And this indicates that the LLC template have an obvious effect on the products.Obtained results suggest a novel, simple and effective way to produce particles with controllable shape and size, as well as a clue to understand the mechanism for crystal growth.