Surface-specific overgrowth of platinum on shaped gold nanocrystals.

Controlling the shape and composition of metal nanocrystals can be beneficial for tuning the optical or catalytic properties in a variety of applications. In this study, surface-specific overgrowth of platinum was found on shaped gold nanocrystals of cubes, octahedra and spheres. Platinum overgrowth was observed on the planar faces of gold cubes, while the overgrowth occurred at the vertices of gold octahedra, indicating that platinum was selectively reduced on the Au (100) surface for each gold nanocrystal shape. The platinum nuclei covered the entire surface for gold spheres, which don't have well-defined surfaces. As the Pt/Au ratio increased, a full platinum shell was formed. Solution-based UV-Vis absorption spectra of the composite nanocrystals showed that the absorption peak was red-shifted with increasing platinum coverage. The optical response of a single composite nanocrystal was measured by dark field microscope, which also demonstrated a red shift in the scattering spectrum with increasing platinum coverage. The extent of a red shift depended on the shape and composition of the composite nanocrystals.