一、 以植晶法水相合成立方體銀奈米晶體及其形貌變化 二、 製備系統性形狀演繹的金銀核殼結構及其光學性質探討

Abstract

CHAPTER 1 Seed-Mediated Growth of Silver Nanocubes and Their Morphological Transformation Silver nanoparticles are often synthesized in organic solvents with the use of high reaction temperatures. If nanoparticles can be synthesized in aqueous solution, the method would be energy-saving and environmentally friendly. In the literature, long reaction time and high temperatures are still need to synthesize silver nanocubes. Here we present a facile and low temperature approach to prepare silver nanocubes in aqueous solution and investigat how the reaction rate controls the final product morphology. In this study, we have developed a seed-mediated growth method to synthesize Ag nanocrystals in aqueous solution. The method involves the addition of a small volume of a seed solution to an aqueous solution of silver nitrate (AgNO3), cetyltrimethylammonium chloriode (CTAC), and ascorbic acid (AA). We utilized AgNO3 as silver source, CTAC as surfactant, and AA as reducing agent. Silver nanocubes were generated in 2 hours at 60 oC. Transmission electron microscopy (TEM), powder X–ray diffraction (PXRD) pattern, and scanning electron microscopy (SEM) have been employed to characterize the nanocubes enclosed by {100} facets. The edge length of cubes can also be tuned from 46 to 55 nm. Here, we also present the effects of NH3 solution on morphological transformation. The acceleration of the reaction rate by introducing NH3 solution promotes the formation of the {111} facets. The solution color at different time points during synthesis also proved that the reaction rate controlled the final particle morphology. CHAPTER 2 Synthesis of Au–Ag Core–Shell Heterostructures with Systematic Shape Evolution and Their Optical Properties In this study, we have utilized rhombic dodecahedral gold nanocrystals as the structure-directing cores for the growth of Ag shells in aqueous solution. Au–Ag core–shell heterostructures with different morphologies can be directly synthesized. The reagents we used are silver nitrate (AgNO3), cetyltrimethylammonium chloriode (CTAC), ascorbic acid (AA), and sodium hydroxide (NaOH). By simply varying the concentration of reducing agent or silver source, shape evolution from cubes, truncated cubes, cuboctahedra, truncated octahedra and octahedra were obtained. The reaction was finished within 50 minutes at 30 oC. This is a time- and energy saving method. These monodisperse nanocrystals can readily form self-assembled structures. By monitoring the solution color at different time points during synthesis or changing the temperature, particle growth rates was found to be fastest for octahedra covered by {111} facets. On the other hand, a slower reaction rate favors the generation of cubes enclosed by {100} facets. The nanocube and nanooctahedra size can also be tuned within a range. UV–vis spectra were used to investigate their unique optical property and suggested that their optical responses are closely related to silver shell thickness and gold core size. Both spectral blue-shifts and red-shifts of the Au–Ag nanocrytals compared to Au cores have been observed. With very thin shell thickness, spectral blue-shift was recorded. As particle size increases, red-shift occur.