The utility of noble-metal nanocrystals in applications ranging from catalysis to biomedical applications has increased with the ability to finely tune their shapes and sizes. In particular, the catalytic activity of the nanocrystals is strongly affected by their shape, as this parameter is directly related to the atomic arrangement on the surface. Another way to alter the surface atomic arrangement is through changing the crystal structure (or phase) of the nanocrystal, a property known as polymorphism. A powerful method to achieve such control over the crystal structure is through template-directed growth to obtain metastable core-shell nanocrystals. In this dissertation, I present a number of studies delving into the mechanistic details behind template-directed phase control for Pd and Ru, alongside an evaluation of their catalytic properties.
First, the importance of particle size on successful template-directed deposition was demonstrated through deposition of Ru on 12-, 18-, 22-, and 26-nm Pd nanoplates, where small nanoplates resulted in fcc-Ru shells, while the larger ones gave hcp-Ru overgrowth. The size dependence was ascribed to a trade-off between the bulk and surface energies that changed with particle size. On small nanoplates, the high proportion of total surface area coming from the side faces makes it favorable to grow fcc-Ru, which deposits smoothly on the side facets (low surface energy) at the expense of forming a metastable phase (high bulk energy). For large nanoplates, only a small proportion of the surface area comes from the side, promoting the growth of hcp-Ru as the resulting jagged side faces (high surface energy) could be compensated by the formation of a thermodynamically stable phase (low bulk energy). To further elucidate the mechanistic details involved in phase-controlled synthesis, the influence of the template’s shape was investigated next. When Ru was deposited on 8-25 nm Pd cubic nanocrystals, the Ru shell took an fcc phase, but on 14-26 nm Pd octahedral nanocrystals, the Ru was deposited as fcc on the small templates before reverting to hcp on the larger ones. The {100} facets displayed on cubic templates forced the Ru to take the fcc phase due to a symmetry mismatch between the facets of the fcc¬-Pd template and hcp-Ru, while on octahedral templates, the displayed {111} facets could be symmetrically aligned with either hcp- or fcc-Ru, allowing for the overgrowth of either crystal structure. Thus, on octahedral templates, the crystal structure depends on particle size and it is determined by the balance of surface and bulk energies. With an improved understanding of template-directed phase control, this method could be extended to obtain hcp-Pd deposition on an hcp¬-Ru template. Under careful control of the reaction conditions, Pd could be deposited on an hcp-Ru template in either the standard fcc phase, or in a novel, metastable hcp phase. It was essential to slow down the reduction rate of the Pd precursor in order to obtain phase-controlled Pd. The ability to control the crystal structure of noble-metal nanocrystals, coupled with a mechanistic understanding of this process, will enable the development of nanostructured materials with unique properties through rational and deterministic syntheses.Ph.D
