Shape-Controlled Synthesis, Self-Assembly And SERS Properties Of Noble Metal Nanoparticles

This dissertation is about shape-controlled synthesis, self-assembly and surface-enhanced Raman scattering (SERS) properties of noble metal nanoparticles. There are five chapters in this dissertation:the first chapter is an introduction of fundamental concepts and some recent advances on solution-phase synthesis, self-assembly, surface plasmon resonance (SPR) and SERS properties of noble metal nanoparticles; the second chapter is shape-controlled synthesis of Palladium nanoparticles in aqueous solution using cetyltrimethylammonium bromide (CTAB) as a capping agent; the third chapter is three-dimensional self-assembly of Pd nanocubes through solution vaporizing; the fourth chapter is synthesis and SERS properties of flower-like Ag nanospheres with different surface morphologies; the fifth chapter is synthesis and growth mechanism of one-dimensional Ag heteronanostructures. The main content of chapter two to five is given below.1. In chapter two, morphological evolution of Pd nanoparticles was studied in a solution phase synthesis using CTAB and sodium ascorbate as a capping agent and a reducing agent respectively. In this study, we developed a method for high yield synthesis of Pd nanocues (-95%) and Pd star-shaped icosahedra (-65%). The role of capping agents in controlling the morphology of Pd nanoparticles was studied, considering the effects of the capping agents on the nucleation and growth process. In the nucleation stage, capping agents influence the distribution of single-crystal and multiple-twinned seeds; in the growth stage, capping agents selectively enlarge one set of crystallographic facets. We also studied the SPR and SERS properties of the Pd nanoparticles.2. In chapter three, we developed a method for three-dimensional self-assembly of Pd nanocubes. Through slowly vaporizing the solution containing CTAB-coated Pd nanocubes and excess CTAB, we obtained simple-cubic microcubes assembled by Pd nanocubes. The size of the microcubes could be slightly controlled by changing the evaporation temperature. The method described here is independent of the property of Pd, so it is expected to be applicable to CTAB-coated nanocubes of other materials. For example, we also achieved three-dimensional self-assembly of silver nanocubes using this method.3. In chapter four, we report a high-yield synthesis of Ag nanospheres with complex surface morphology which are formed by the agglomeration of small Ag nanoparticles. The size of the silver nanosphers can be controlled by changing the concentration of poly(vinylpyrrolidone) (PVP) which acts as a stabilizer. In addition, the surface morphology of the nanospheres can be well controlled through controlling the shape of primary Ag nanoparticles by introducing a small quantity of ionic capping agents into the solution. Through changing the concentration of the two types of capping agents separately, Ag nanospheres with well controlled size and morphology were obtained. We find that the flower-like nanospheres with sharp tips on their surface exhibit much better SERS properties than non-agglomerated spherical Ag nanoparticles.4. In Chapter five, we demonstrate that we can fabricate one-dimensional Ag rod-needle heteronanostructures (RNHSs) and plate-belt heteronanostructures (PBHSs). The formation mechanism was also studied. For RNHSs, the stacking faults (including twins, local hcp phase, and so on) arising from fast growth promote the growth along<111> direction, which is a new mechanism for formation of 1D noble metal nanostructures. For PBHSs, the twins lead to the formation of Ag nanoplate, while 4H crystal structures are responsible for the formation of Ag nanobelts. Condition-sensitive coexisting phase preferences can be applied as a new way of controlling the shap and thus the properties of Ag nanostructures.