Abstract
Nanoscale materials are a new generation of advanced materials that are expected to exhibit unusual chemical and physical properties different from those of either the bulk materials or molecules. The unique properties of nanoscale materials are largely determined by their atomic scale structures, particularly the structures and property-modifying species on the surfaces. Thus the ability to controllably grow nanoparticles of a particular size and shape offers the opportunity to observe novel physical properties and can extend our understanding of the size and shape effects on the properties of nanoscale materials. This thesis work was directed at the synthesis of highly faceted gold nanoparticles with well-controlled particle size distribution and the formation of branched Au nanocrystals. In both cases, a seeding growth approach was adopted. The seeding growth procedure involves using small Au nanoparticles (~2.5 nm in diameter) as seeds to grow into larger Au nanoparticles by mixing the seed solution with a growth solution (a solution containing the capping surfactant sodium dodecyl sulfate and the Au source HAuCl4) in the presence of a reductant ascorbic acid. Micelles formed by the aggregation of sodium dodecyl sulfate molecules were used as soft templates to control the crystal growth. Highly faceted particles exhibiting pentagonal- and hexagonal-shaped structures with controllable diameters ranging from 5 to 50 nm have been obtained. The formation of highly faceted gold nanoparticles may be facilitated by the possibly ineffective capping interaction between the lamellar micellar structures formed by the SDS moclecules and the gold nanoparticles. Using a similar seeding growth method, branched gold nanocrystals with highly faceted faces were synthesized (~40 nm in length). The branched nanocrystals showed bipod, tripod, tetrapod, and pentapod structures. To further investigate the structures and the growth mechanisms leading to these unusual Au nanoparticles, analysis of these samples were performed by using UV-Vis absorption spectroscopy, X-ray diffraction, TEM and HRTEM characterization methods. According to both TEM and XRD results, the crystal structure of the highly faceted particles was found to consist of mostly {111} surfaces as particle size increases. Similarly, arms of the branched nanocrystals are also composed of {111} lattice planes. The multipods appear to grow along the twin boundaries of the highly faceted gold nanoparticles, as the twin boundaries on the pods originate from the centers of the nanoparticles. In the determination of the factors contributing to the formation of branched nanocrystals, the concentration of ascorbate ion was found to play a key role than that of hydrogen ions. These branched nanocrystals are stable to storage at 4 ˚C, but slowly evolve into the highly faceted crystal structures when kept at a high temperature (i.e. at 30 ˚C for 10 days). These novel and highly structured gold nanoparticles are expected to lead to new applications as they possess interesting colors and tunable light absorption properties.