Seedless synthesis of monodisperse single-crystalline cuboctahedral and spherical gold nanoparticles with tunable sizes

Abstract

Gold nanoparticles (Au NPs) are of great current interest because of their unique optical and chemical prop-erties, which make them attractive for wide range of applications, such as surface-enhanced Raman scattering, colorimetric sensing, catalysis, and drug delivery. Because the optical and chemical properties of Au NPs are strongly dependent on their shape and size, substantial research efforts have been made to synthesize Au NPs of different shapes and sizes. Recently, these efforts have focused on the synthesis of polyhedral Au NPs because of their unique properties induced by their well-defined facets and corners. The most widely used method to prepare polyhedral Au NPs is the seed-mediated synthesis which has led to the fabrication of a wide range of nanostructures including cubes, octahedra, rhombic dodecahedra, cuboctahedra, icosahedra, plate and prisms, and more exotic shapes with high-index surface facets. Among polyhedral Au NPs, cubes, octahedra and cuboctahedra can be grown from single-crystal seeds containing {111} and {100} facets, and the shapes of the synthesized particles are determined by the relative growth rates of the {100} and {111} facets. Unlike cubes and octahedra that contain only one type of facet, cuboctahedra containing both {111} and {100} facets require a subtler balance between the growth rates of the two facets. Although Au cuboctahedra of a few different sizes have been previously reported as an intermediate morphology in the transition between cubes and octahedra, in most of cases a rather broad size distribution or a limited size control has been the main issues. Thus, synthesis of monodisperse Au cuboctahedra with controlled sizes has remained challenging.

In the addition to the issues in the synthesis of cuboctahedral Au NPs, the general problem to obtain mono-disperse polyhedral Au NPs in high morphological yield in the seed-mediated synthesis relies on the uniformity and crystallinity of the initial seeds. Attempts have been done to prepare the single-crystalline spherical Au NPs which were used as a simple model for seeds in the seed-mediated growth method at which the shape evolution of the nanocrystals can be conveniently monitored. The successful work for the preparation for the single-crystalline spherical Au NPs was based on chemical etching of single-crystalline Au nanorods, and then they were used as seeds to produce various shapes and sizes of polyhedral Au NPs. Despite the excellent uniformity in terms of both size and shapes, this method involves multiple protocols, such as the synthesis of Au nanorods and chemical etch-ing. In addition to that, the single-crystalline spherical Au NPs obtained from nanorods etching have the size above 20 nm which limits their use as seeds to obtain small polyhedral Au NPs (below than 40 nm). As a result, synthesis of single-crystalline spherical Au NPs still remains a challenging task. Thus, the present works focused in the synthesis of single-crystalline cuboctahedral and spherical Au NPs with tunable sizes using a “simple seedless synthesis” which are described as follows:

In chapter 2, monodisperse Au cuboctahedra with tunable sizes ranging from 40 to 80 nm have been syn-thesized using cetyltrimethylammonium 4-vinylbenzoate (CTAVB) as a selective capping and mild reducing agent in the presence of a high concentration of HCl in aqueous solution. The HCl provides strong oxidative etching power to remove structural defects, resulting in single-crystal seeds, and significantly reduces the particle growth rate. This slow particle growth provides an easy and reliable way of tuning the particle size by stopping the reaction at different times and allowing sufficient time for the surface self-diffusion of Au atoms. Combined with the selective capping of {100} facets with $CTA^+$ , the surface self-diffusion of Au atoms from {111} to {100} facets is considered to be the key mechanism for the formation of Au cuboctahedra and their stable growth without morphological deformation. To the best of my knowledge, this is the first report on the seedless synthesis of monodisperse cuboctahedral Au NPs with tunable sizes.

In chapter 3, monodisperse single-crystalline spherical Au NPs with tunable sizes ranging from 8 to 12 nm has been achieved using CTAVB as a selective capping and mild reducing agent in the presence of a high concen-tration of HCl, and $AgNO_3$ in aqueous solution. In this method, the formation of single crystalline Au NPs with a spherical shape was induced by HCl and $Ag^+$ . HCl provides strong oxidative etching power to remove structural defects, resulting in single-crystal seeds, and significantly reduces the particle growth rate. Meanwhile, depending on its concentration, $Ag^+$ has selective adsorption on the {111} or both the {111} and {100} facets of single-crystal seeds and also reduces the growth rate on those facets. The selective capping of {100} facets with $CTA^+$ combined with selective deposition of $Ag^+$ on the crystal facets of single-crystal seeds, and etching induced by HCl, are considered to be the key mechanism for the formation of spherical Au NPs. This slow particle growth rate also provides an efficient way to finely control the particle size by stopping the reaction at different times. The highly monodisperse single-crystalline Au NPs were shown to be excellent seeds to obtain monodisperse octahedral Au NPs with tunable sizes.

The approach reported here, which synergistically combines the uses of mild reducing agents and a high concentration of HCl to form single-crystal seeds and to enhance the contribution of atomic surface self-diffusion and/or with the addition of additive, such as $AgNO_3$ in directing the particle shape, might be applicable for the synthesis of single-crystalline cuboctahedal and spherical NPs of other metals, such as platinum and silver, and possibly also for synthesizing metallic NPs of other shapes. The highly monodisperse single-crystalline spherical Au NPs obtained in this work could be used as ideal seeds to produce various kinds of polyhedral Au NPs, making the availability for fabricating polyhedral Au NPs superlattice using self-assembly and also to study their plasmonic phenomena.