Morphology Control in Gold Nanoparticle Synthesis

• Hammed A. Salami

Introduction

One of the most significant current discussions in the field of nanotechnology is the development of novel nanomaterials. When materials are reduced from bulk to the nanometer-scale dimension, they begin to exhibit unusual physical and chemical properties ^{[1, 2]}. Recently, researchers have shown an increased interest in the elucidation of the structure-function relationship of these novel nanomaterials ^{[3, 4]}. The availability of imaging techniques with nanometer resolution, such as electron microscopy has not only helped in visualizing the individual nanoparticles, but also, it has facilitated an understanding of some of the emerging properties of noble metal nanoparticles such as spectroscopic enhancement and localized surface plasmon resonance (LSPR) ^{[5, 6]}.

For noble-metal nanoparticles, these structure-function relationships have attracted significant research interests. This is because, unlike in bulk metal materials, the control of the chemical and physical properties of noble-metal nanoparticles is possible with a modification of their size and shape, and by varying the material composition ^{[1, 6]}. As a result of the unique roles played by size and shape in influencing the properties of noble-metal nanoparticles, researchers have continually focused on ways to reproducibly tailor these parameters in other to adapt the nanoparticles for optimal use in a wide range of applications, including biology^[4], energy^[7], sensing, spectroscopic enhancement^[8-10] and catalysis ^{[7, 11]}. The size of nanoparticles influences their optical properties while the shape and crystallographic facets are the major factors that determine their catalytic and surface activities ^[12].

Nanoparticles with non-spherical structures are referred to as anisotropic nanoparticles. Examples include nanocubes, nanoprisms, nanorods, etc. ^[13]. They show pronounced shape-dependent properties and functionalities, therefore a great deal of research effort has been paid at developing synthetic strategies to get a high yield of anisotropic noble metal nanoparticles having uniform structures and controlled shape and size^[5].

The deliberate control of shape has however proven to be the most challenging, despite being one of the useful parameters for optimizing the properties of noble metal nanoparticles. This is particularly more pronounced in gold nanoparticles synthesis ^{[3, 14-16]}.

Of the many shapes of gold nanoparticles, gold nanorods have continued to attract the most attention ^[2]. This is largely due to the large number of synthetic methods available, the possibility of high monodispersity and the control over the aspect ratio, which accounts for the change in their optical properties ^[17]. When molecules are adsorbed on the surface of gold nanoparticles, they undergo surface-enhanced Raman scattering (SERS) effects. This is due to the coupling effect of the plasmon band of the irradiated metal with the molecules electronic states ^{[18, 19]}. For gold nanorods, two Plasmon bands are prominent. They are the longitudinal plasmon band and the transverse plasmon band. These bands correspond to light absorption and scattering along the long and short axis of the particle respectively ^[20-22]. While the longitudinal surface plasmon resonance increases with larger aspect ratios (length/diameter), the transverse surface plasmon resonance is usually on the same wavelength as that of nanospheres, with no dependence on the aspect ratio^[23].

The current high dependence on non-renewable feedstocks can be minimized with the production of fine chemicals, petrol-derived commodities and polymer precursors from biomass^[24]. Supported gold nanoparticles have been found to be very active catalysts for a number of biomass transformation and many researchers have focused their attention in searching for the best supports, reaction conditions and mechanistic studies to improve their selectivity^{[25, 26]}.

Most catalytic studies in literature involving noble metal nanoparticles, either as mono- or bimetallic catalyst, are done with spherical nanoparticles ^[25-27]. The spherical nanoparticles used are usually immobilized onto suitable supports to form impregnated catalysts and in some cases they are preformed before immobilization ^[27]. To achieve this, methods such as wet impregnation, sol immobilization etc. are often used ^{[28, 29]}. These methods however, do not allow the control of morphology of the nanoparticles. There is therefore the need to develop an understanding of morphology control in the synthesis of anisotropic noble metal nanoparticles with high yield. It would also be interesting to explore the correlation between these controlled morphologies and catalytic activities.

Project Aims

This project will therefore aim at synthesising various morphologies of mono and bimetallic noble metal nanoparticles, with optimum control of the morphology during the synthesis. Starting with gold, we will also explore the use of colloidal methods in immobilizing the preformed nanoparticles with selected morphologies and narrow particle size distribution e.g. gold nanorods, onto suitable supports to form heterogeneous catalysts. Since the rods expose certain crystallographic planes more than most other morphologies and also have comparatively low coordination sites, they can be potentially more selective for reactions that preferably occur on low coordination sites. As a starting point we will therefore, explore their use as supported heterogeneous catalysts in selective oxidation and hydrogenation reactions for biomass transformation.

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