Shaping Nanoparticles
By Amanda Crowell
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GEORGIA TECH RESEARCHERS have succeeded in creating specific
shapes and sizes of colloidal platinum nanoparticles, a
development that could lead to advances in the field of catalysis.
"It is known that catalysis on metal surfaces depends on the face
of the metal crystal used," says Dr. Mostafa A. El-Sayed,
principal investigator for the project and the Julius Brown
Professor of Chemistry in the School of Chemistry and
Biochemistry. "When nanoparticles of certain shapes are used, it
is expected that their catalytic activities will vary from one
another -- and, most likely, from metal crystal surfaces -- as
they have edges and corners that clean-polished crystal faces do
not have.
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Fig. 1: (A) Low-magnification Fig. 2: (A)
Transmission Electron Low-magnification TEM image
Microscope (TEM) image of of sample 2, indicating the
sample 1, showing the size and abundance of tetrahedra. (B)
shape distribution of the High-resolution image of a
cubic particles. (B) tetrahedral particle.
High-resolution lattice image (Inset) The Fourier
of a cubic platinum particle. transform of the lattice
(Inset) The Fourier transform image gives the optical
of the lattice image gives the diffractogram of the
optical diffractogram of the particle. (Photo courtesy of
particle. (Photo courtesy of Science.)
Science.)
"In addition, since a good fraction of the atoms of nanoparticles
are located on the surface, where catalysis takes place,
nanoparticles are expected to be much more effective in catalysis
per gram than their crystals," he adds.
Although previous studies have explored the factors that influence
the size distribution, stability and catalytic activity of
colloidal particles, this work marks the first time researchers
have been able to control the shape and size of such particles in
colloidal aqueous solutions at room temperature.
In addition to catalysis, the results could have implications for
other fields, since colloidal metal nanoparticles are used as
photocatalysts, adsorbents, sensors and ferrofluids, as well as in
optical, electronic and magnetic devices.
The collaborative project included researchers from California and
Germany, and is funded by the U.S. Office of Naval Research. This
summer, Science magazine published a paper describing the group's
work, titled "Shape-Controlled Synthesis of Colloidal Platinum
Nanoparticles," in its June 28 issue.
El-Sayed's primary collaborators include: Dr. Zhong L. Wang, an
associate professor in Georgia Tech's School of Material Sciences
and Engineering; Temer S. Ahmadi, a graduate student registered at
the University of California, Los Angeles, but who came to Tech to
finish his Ph.D. research when El-Sayed moved from California to
Atlanta two years ago; Travis C. Green, a graduate student in
Georgia Tech's School of Chemistry and Biochemistry; and Dr. Arnim
Henglein, a professor at the Hahn-Meitner Institut in Germany.
To achieve their results, the researchers altered the ratio of the
concentration of the capping polymer material to the concentration
of the platinum cations (positively charged ions).
The capping polymer material -- in this case, sodium polyacrylate
-- wraps around the particles to stop their growth and make them
soluble in water, but does not affect their chemical reactivity.
To create colloidal samples for the study, the researchers
synthesized platinum nanoparticles in a liquid solution at room
temperature, introducing argon and hydrogen gases. The latter
served to reduce the platinum ions into neutral atoms in the
process of making the nanoclusters.
Three different samples were used, each with a different
concentration of the capping polymer. All other factors, such as
the salt and pH levels, the solvent used and the temperature, were
kept constant.
The researchers observed several different geometric shapes,
including tetrahedral, cubic, irregular- prismatic, icosahedral
and cubo-octahedral forms. The distribution of the shapes was
dependent on the ratio of capping polymer material to the platinum
cation.
The first sample, for example, had a ratio of polymer
concentration to platinum salt of 1 to 1, and contained 80 percent
cubic particles. Sample 2 had a ratio of 5 to 1 and was dominated
by tetrahedral shapes. It also had small percentages of polyhedral
and irregular-prismatic particles.
Sample 3, with a ratio of 2.5 to 1, contained a mixture of
tetrahedral, polyhedral and irregular-prismatic particles.
In all three samples, researchers were able to reproduce
tetrahedral and cubic particles three times.
But learning to control the shape and size of colloidal
nanoparticles is just the beginning. The researchers now must
explore the mechanisms of the process at the molecular level, to
understand how it works. This will include detailed studies of how
solution properties such as pH, ionic strength, viscosity and
temperature affect shape distribution.
"Once we understand how a certain shape for a nanoparticle grows,
and the type of catalysis that each shape can induce," El-Sayed
says, "we will be able to tailor the nanoparticle shape needed to
catalyze the different chemical reactions important to producing
energy, cleaning the environment or making expensive [drugs] more
economical."
Further information is available from Dr. Mostafa A. El-Sayed,
School of Chemistry and Biochemistry, Georgia Institute of
Technology, Atlanta, GA 30332-0400. (Telephone: 404/ 894-0292)
(E-mail: mostafa.el-sayed@chemistry.gatech.edu)
---------------------------------
Information published about this
project does not necessarily reflect
the position of the policy of the
U.S. Government, and no official
endorsement should be inferred.
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Last updated: 27 Nov. 1996