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Shaping Nanoparticles
By Amanda Crowell
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 Transmission Electron Microscope (TEM) image of sample 1, showing the size and shape distribution of the cubic particles. (B) High-resolution lattice image of a cubic platinum particle. (Inset) The Fourier transform of the lattice image gives the optical diffractogram of the particle. (Photo courtesy of Science.) | Fig. 2: (A) Low-magnification TEM image of sample 2, indicating the abundance of tetrahedra. (B) High-resolution image of a tetrahedral particle. (Inset) The Fourier transform of the lattice image gives the optical diffractogram of the particle. (Photo courtesy of 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)
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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