Exploration of Gold Nanoplatelets: Synthesis, Structures, and Plasmonic Properties

Abstract

In Chapter 1, a general introduction is given on the synthesis, morphologies, surface modifications, and optical properties of metal nanocrystals (NCs). In Chapter 2, however, a single-step coating method is presented for synthesizing Au NT@hollow@mSiO2 yolk-shell nanoparticles (NPs) with tunable diameters, hollow space between core and shell, and opportunities for scale-up synthesis. In Chapter 3, a study on the thermal stability of Au NTs with different surface coatings is presented. For the Au NTs without surface coatings, noticeable deformations occur at 200 ℃ for one-hour heating. The deformation took place via surface diffusion of Au atoms and started from corners and edges where the atoms` coordination numbers are lower than those at other positions, as these atoms have lower energy barriers for diffusion. Upon raising the heating temperature, Au NTs tended to become more sphere-like particles via surface diffusion of Au atoms to reduce the surface energy of the overall particle. Surprisingly, we found that Au NT@mSiO2 YS NPs with a yolk-shell morphology, where not all sides of the Au NP were covered by silica, were more stable than Au NT@mSiO2 core-shell (CS) NPs with a core-shell morphology where all surface Au was covered by the silica coating. In Chapter 4, the mSiO2 coated Au NTs core-shell particles were reshaped by various means; by femtosecond (fs-) laser exposure, oven heating, and oxidative chemical etching. Moreover, the relationships between the LSPR bands and geometrical shape parameters were revealed which allowed to derive expressions for estimating the LSPR peak wavelengths of the Au NTs and Au NDs. In Chapter 5, we studied the symmetric and asymmetric growth of Ag, Pd, and Pt onto Au NTs in aqueous solution using L-ascorbic acid (AA) and/or salicylic acid (SA) as reductants. In Chapter 6, alloyed Au-M (Ag, Pd, and Pt) NPLs with well-defined triangular shapes were successfully obtained by heating the mSiO2 shell coated Au NT-M core-shell NPLs to a proper temperature. For Au NT-Pd and Au NT-Pt core-shell NPLs, the uneven or even spiky Pd and Pt shells tuned into smooth shells covering the Au NT cores when the particles were heated to 500 ℃, and, interestingly, phase segregation was found to occur in these two systems at 1100 ℃, leading to the formation of Au-Pt Janus NPLs, wherein the case of Pt for some particles even a sharp triangular shape was retained. The dynamic process, monitored by in-situ scanning transmission electron microscopy (STEM) imaging, showed that the overall structural evolution started with a core-shell morphology that changed into an alloyed configuration and subsequently changed to Janus-like NPLs. The solid solution alloying is driven by entropy changes that become more important at elevated temperature, while the subsequent phase separation at even higher temperature is a rather surprising result, since this is entropically unfavorable and must be driven by temperature-dependent changes to the cohesive, surface, and interface energies of the Au-M NPLs, where it should also be realized that all these changes take place within the confinement by the silica shell. In Chapter 7 a diverse strategy combining the seed-mediated method, co-reduction, and Galvanic replacement to fabricate Au NT-Ag-Pt ternary nanoparticles with various well-designed structures. The structure of trimetallic NCs can be manipulated from yolk-shell to core-shell by varying the concentration of AgNO3 or Au NTs. Furthermore, by means of time-resolved UV-VIS spectra and STEM images, it was found that Au NT-Ag-Pt yolk-shell NCs were formed via a growth-Galvanic replacement synergistic mechanism.