Known to the ancient Greeks, there are five Platonic solids that can be constructed by selecting a regular convex polygon and having the same number of them meet at each corner: tetrahedron, octahedron, hexahedron (cube), icosahedron, dodecahedron. The beauty in their symmetry and their apparent simplicity continue to inspire generations of mathematicians and scientists. In nature, certain viruses and radiolaria also routinely take the form of these polyhedral shapes. Recently, the concept of shape control has started to revitalize the centuries-old metal colloidal synthesis. Nanoparticles of various shapes, rods, wires, prisms, cubes, particularly those of silver and platinum, have been prepared using a variety of different methodologies. We recently demonstrated a systematic shape-evolution of metal nanocrystals with sizes of 5-300 nm in a modified polyol process. By adding surface-regulating polymer and foreign ions, we can readily access the distinct shapes of tetrahedron, cube, octahedron, and icosahedron with high yield and good uniformity. These nanocrystals have the perfect symmetry for 2- and 3-dimensional packing and therefore could enable the rational tuning of their optical (surface plasmon) and catalytic properties.


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