>H Advance In Shaping Nanoparticles

From: Pat Powers (mrskin@mindspring.com)
Date: Mon Dec 16 1996 - 22:36:30 MST


Here's an article on shaping nanoparticles that I thought might be of
interest to the group. It comes from "Research Horizons" a publication of
the Georgia Tech Research Institute. Other articles can be found at:
http://www.gtri.gatech.edu./res-hor/rh-welcome.html

     Shaping Nanoparticles

                              By Amanda Crowell

                                  [Image]

     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.

                  [Image] [Image]

       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.

                                  [Image]

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     Last updated: 27 Nov. 1996



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