Shape-Controlled Synthesis of Monodisperse Gold Nanocrystals and Gold-Based Hybrid Nanocrystals

Abstract

At the nanoscale, the physicochemical properties of metals are strongly shape-dependent, a finding that has generated tremendous interest and considerable efforts in the shape-controlled synthesis of nanometals. This thesis study is an attempt to develop synthesis strategies capable of producing noble metal nanocrystals (NCs) with programmable diversity and complexity. This will be demonstrated by the synthesis of two particular types of nanostructures: polyhedral NCs bound by high-index facets and heterogeneous metal NCs (HMNCs) with programmable architecture.

High quality high-index NCs with customizable particle attributes such as size, crystallinity and exposed facets were synthesized. Specifically monodisperse concave trisoctahedral (TOH) gold NCs with high-index {hhl} facets and in various sizes were produced by seed-mediated growth under kinetically controlled conditions. Through a slight modification of the preparation details, a new Au nanostructure - shield-like Au NCs with a single twin plane and high-index {hhl} facets could also be grown. These NCs were then used as templates to guide the evolution of high-index facets on a different metal by heteroepitaxial growth. This was demonstrated by the epitaxial growth of a Pd shell on concave TOH Au NC seeds under carefully controlled growth conditions. Polyhedral Au@Pd NCs with three different classes of high-index facets, namely concave TOH NCs with {hhl} facets, concave hexoctahedral NCs with {hkl} facets and tetrahexahedral NCs with {hk0} facets; could be synthesized in high yield. The miller indices of NCs were also modifiable.

We have developed strategies capable of creating architectural diversity of HMNCs through rational and independent engineering of every architectural determining element of a HMNC, namely the spatial arrangement and the shape and size of its component NCs. This was exemplified by HMNCs consisting of monometallic Au NCs in the centre surrounded by bimetallic AgPd satellite NCs. The distribution of the satellite NCs on the central NCs could be varied, i.e. they could be located selectively on the corners or along the edges of the central NCs. The central NC was enclosed by {111} or {100} facets (or both) upon which satellite NCs with exposed {111} or {100} facets (or both) were deposited.