Researchers based at the National Center for Nanoscience and Technology, China, are exploring the strategy to prepare PVP-capped Au@Ag core-shell nanostructures with various shapes. The Au cores with well defined shapes were found to direct the overgrowth of the corresponding Ag shells.

The group used a scanning transmission electron microscope (STEM) in a high-angle angular dark field (HAADF) configuration to distinguish Au cores and Ag shells. The images reveal that the Ag shells with a cubic shape can be formed from octahedral, truncated-octahedral and cubic Au cores. Single-twinned bi-triangular or bi-hexagonal Au cores predominated mainly by {111}-type facets can epitaxially evolve into the single-twinned inverted bi-triangular Ag shells predominated by {100}-type facets. Decahedral Ag shells are epitaxially overgrown from the decahedral Au cores and Ag shell nanorods with a five-twinned cross section from the Au nanorod cores with the five-twinned cross section. The observation suggests that in most cases the {111} facet-dominated Au cores tend to overgrow into the {100} facet-dominated Ag shells in the present reaction system.

In general, FCC-type crystals can exhibit different morphologies due to an intrinsic growth habit partly determined by thermodynamic factors. From the viewpoint of thermodynamics, the FCC crystal surfaces with the lower-energy planes are expected to predominate in the product morphologies. The change in the surface energies for various FCC crystalline planes obeys the order of γ{111} γly; γ{100} < γ{110}. However, the transition of the crystal planes from {111}-dominated Au cores to {100}-dominated Ag shells in the study implies the significance of the adsorption kinetics of the surface adsorbors such as surfactant and ions, besides thermodynamic factors. The difference of selective absorbance of the adsorbors on different crystal planes can effectively passivate certain crystal planes and facilitate the growth of other crystal planes.

The researchers presented their work in Nanotechnology