| The unique physical and chemical properties of nanoscale materials are dependent on their composition, size, and shape. In the case of noble metal nanostructures, the ability to selectively synthesize particles of a particular architecture enables rational control over their plasmonic, catalytic, and electronic properties. However, achieving precise control over the shape of noble metal nanocrystals remains a challenge. In this thesis, an effort is made to better understand how crystal growth can be rationally controlled in the plasmon-mediated synthesis of silver nanoparticles and the seed-mediated synthesis of gold nanostructures. A novel method that we developed to track nanostructure growth by electron microscopy is described in Chapter 3. This technique is used to unambiguously determine the growth pathway of a single crystalline seed into a 20-fold twinned icosahedron, including the identification of eight intermediate structures formed along the way, illustrating how different particle shapes are interrelated and evolve from one another. In Chapter 6 the independent and synergistic effects of silver ions and halides on the growth of gold nanoparticles in a seed-mediated synthesis are investigated. It was found that one series of nanoparticles that differ in shape can be prepared by controlling the reaction kinetics, either through the concentration of reducing reagent or though the use of halides to (1) affect the reduction potential and solubility of the gold ion species and (2) selectively passivate different facets of the gold nanoparticle surface. A second series of particle shapes can be accessed through the underpotential deposition of silver ions, which act as a facet-selective shape-directing capping agent. The silver coverage on the surface of the gold nanoparticles, and consequently their shape, can be controlled by either modulating the silver ion concentration in the growth solution or by using halides to adjust the relative strengths of the Ag +/Ag0-halide and Au+/Au0-halide interactions. Collectively, these studies have not only provided an improved understanding of nanocrystal growth but have also led to the synthesis of new and exotic nanostructures, such as heterometallic nanoparticles with asymmetric metallic distributions (Chapter 2), high-index faceted gold concave nanocubes (Chapter 4), and gold octahedra with hollow features (Chapter 5). |
