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"While we are in the early stages of research, we
see the possibility of manipulating the nanoscale structure of
platinum so that we can have control over the size, porosity,
composition, surface species, solubility, stability, and other
functional properties of these metal nanostructures," says John
Shelnutt, the Sandia scientist leading the research effort. "Such
control means that the redesigned platinum could be used in many new
applications, including catalysis, sensors, and optoelectronic and
magnetic devices."
He adds that while research groups have reported a
few platinum nanostructures - including nanoparticles, nanowires,
nanosheets, and others - the addition of new types of nanostructures
is "highly desirable and potentially technologically important."
Working with Shelnutt in the research are Frank van
Swol from Sandia, UNM graduate student Yujiang Song, and Eulalia
Pereira from the University of Porto in Portugal.
The new method of manipulating platinum was
detailed in a paper in the Journal of American Chemical Society
published in late December.
The idea for the technique is similar to
photosynthesis, in which plants use the energy from sunlight to
produce sugar. But instead of manufacturing sugar, the new method
changes a platinum ion to the neutral metal atoms. The photosynthetic
protein mimicks this repeatedly, allowing metal to be deposited as
desired at the nanoscale.
The method involves putting porphyrins - the active
part of photosynthetic proteins - along with the platinum salt in an
aqueous solution of ascorbic acid at room temperature. The porphyrins
are placed in specific locations in the solution where it is intended
that metal should be deposited. For example, the porphyrins may be
confined to micelles or liposomes. Micelles are spherical assemblies
of detergent molecules in which the heads are exposed to the water and
the tails stick together in the interior. Liposomes are similar
structures but they are larger and have water on the inside and
outside separated by a closed membrane - sort of like a cell. The
membrane is composed of two layers of detergent molecules, with the
heads on the inner and outer surface facing the water and the tails
forming the interior of the membrane.
When light is shined on the porphyrins located in
these detergent structures, the porphyrins excite, becoming catalysts
for platinum reduction and deposition. As this occurs, the metal grows
onto the surfaces of the surfactant structures as a thin sheet or in
other ways. In the case of micelles, the platinum grows into balls
that look like the common toy "Koosh(tm)" ball. The ball size can be
controlled by the amount of porphyrins and platinum in the solution,
the amount of light illuminating the solution, and the amount of time
the light is on.
For the metals platinum and palladium that form
these nanostructures, it is enough for the porphyrin molecule to grow
only a small metal "seed" particle composed of about 500 atoms. When
it reaches this size, the seed starts to catalyze its own rapid growth
(by oxidation of ascorbic acid), budding off arms in all directions
and creating the Koosh-ball-like nanostructures. The porphyrin
provides a convenient method of making these seeds at the location and
time desired, leading to a uniform and selectable nanostructure size.
The platinum nanostructures take on a different
form when they are prepared under different conditions. When the
porphyrin is in a micelle, the platinum nanostructures produced look
like Koosh balls. When the porphyrin is in the bilayer membrane of a
liposome, the platinum grows in 2-nanometer thick sheet or platinum
lace on the outer surface of the membrane, giving circular sheets -
sort of like two-dimensional Koosh balls.
Under solution conditions for which the liposomes
aggregate, growth can occur along the interfaces of the liposomes to
give platinum foamlike materials and foam nanoballs. The type of
nanostructure is mainly determined by the type of surfactant assembly
upon which the platinum grows and the extent of growth from the
individual seed nanoparticles.
Since the porphyrin remains attached to the
platinum nanostructure and active in the presence of light, it can
also perform other functions besides growing itself. For example when
illuminated with light, the platinum nanostructure evolves hydrogen
from water. This reaction is similar to one of interest to car
manufacturers looking to new ways to build automobiles powered by
hydrogen fuel cells.
Shelnutt says that in addition to structuring the
platinum, the process also happens very fast. A few minutes in light
will create many seeds, which then grow into the mature nanostructures
in tens of minutes. And the process is easy to do.
"It's so simple it's amazing," Shelnutt says. |
