Plato, the greek philosopher (427-347 B.C.E.), believed that
matter consists of
very small, perfectly regularly shaped
particles. The surface of such "platonic
solids" is made up of polyhedra of equal size, equal sides, and equal angles. At
each corner, an equal number of surfaces come together. For geometric reasons,
five such solids are possible (which Plato assigned to the "elements", fire,
earth, air,
water, and ether). Researchers in Berkeley have now been able to
produce
gold crystals/order_t/'>nanocrystals with shapes reminiscent of four of the platonic
solids: tetrahedra (made of four triangles), cubes (made of six squares),
octahedra (made of eight triangles), and icosahedra (made of twenty triangles).
Like the natural scientists of Plato's age, those of today are fascinated by the
beautiful
symmetry and incredibly simple structures of the platonic solids.
However, growing
nanocrystals with these shapes is not just an academic
exercise. Their perfect symmetry is ideal for densely packing the particles into
highly ordered, two- or three-dimensional structures. This would make accessible
novel materials with tailored optical, electronic, or catalytic
properties-properties that depend not only on the size, but also the shape of
the particles.
The researchers, led by Peidong Yang, start their
production of gold
nanocrystals with a
solution of a gold-containing salt, which is injected into
boiling
ethylene glycol in the presence of a special surface-active polymer. The
ethylene glycol acts as solvent as well as reducing the gold ions to elemental
gold. The polymer stabilizes the resulting
gold nanoparticles and influences
their form. At the initially chosen
concentrations, very finely divided, gold
particles of uniform dimension form within a few minutes. Under an electron
microscope, these look like tetrahedra with flattened corners. If the experiment
is carried out at a somewhat lower gold concentration, however, 90% icosahedral
gold crystals are formed. The remaining 10% of product contains octahedral
particles. The addition of a trace amount of a
silver salt to the reaction
mixture allowed the researchers to harvest 95% cubic gold nanocrystals.
The
Formation of the different shapes is dependent on the speed of crystal
growth along the different crystallographic axes, which is clearly influenced by
the polymer and
silver ions, as well as by the concentration gradients during
generation of the
crystallization nuclei.