VCU chemists spin nanoparticle fibers in vapor

By Lorraine Cichowski and M. Samy El-Shall

RICHMOND, Va. – Scientists at Virginia Commonwealth University have assembled a new class of metallic and related types of nanoparticle fibers and filaments by suspending nano-sized pieces of metals and other materials in vacuum chambers filled with electrically charged vapor.

The discovery, which will be presented March 23 at the 225th Annual Meeting of the American Chemical Society in New Orleans, offers a new, simplified method of assembling metallic, alloy, silicon and carbon nanoparticles without many of the quality and contamination problems associated with other nanoparticle assembly methods, particularly those using liquids or solutions.

“Nanoparticles often exhibit novel properties that are different from the properties of the material in bulk,” says M. Samy El-Shall, Ph.D., professor of physical chemistry at VCU. “That’s a key reason that scientists are targeting nanoparticles as building blocks for the next generation of stronger, lighter materials and tiny sensors. But sometimes you lose some of those special properties of nanoparticles when you assemble them.

“Using a process that produces ultra-pure nanoparticles with laser vaporization and controlled condensation and then applying an electric field, we have shown that several classes of metallic and semiconductor nanoparticles can be assembled into chains and filaments that retain their unique properties. This holds great promise for the development of novel functional materials and for the engineering of a wide variety of nanodevices and sensors.”

Nanoparticles, which can be as small as one-billionth of a meter, are pieces of matter at the scale of several atoms and molecules that give scientists the opportunity to create new materials from the atom up. Producing nanoparticles, however, involves more than just crushing iron or aluminum into tiny bits. Nanoparticles typically are produced using gas or liquid processes that first separate the atoms or molecules from the bulk material and then reunite them into small clusters.

At VCU, El-Shall and his team have been perfecting a process they developed about 10 years to produce nanoparticles called laser vaporization controlled condensation (LVCC) that uses a high-powered pulse laser to vaporize metallic and other matter in a temperature-controlled, inert gas-filled chamber, creating a mixture of atoms and ions that then are synthesized into nanoparticles of different sizes.

In their latest research, the researchers applied electric fields of different strengths during the nanoparticle synthesis process. In the presence of an electric field, the nanoparticles assembled into chain filaments and tree-like fibers, stacked end-to-end, which displayed unique stretch and contraction properties and, in some cases, exceeded several centimeters in length.

Carbon filaments grew the fastest, followed by filaments of metal alloys such as iron-aluminum and titanium-aluminum. Nanoparticles of intermetallic alloys are of particular interest to materials engineers because they create low-density, high-strength, high-performance materials such as corrosion-resistant coatings and heating elements for engines.

El-Shall said that the discovery, which was posted online March 5 and will publish in the April 3 issue of the Journal of Physical Chemistry B, could result in development of stronger plastics that incorporate the nanoparticle filaments within polymer chains. His team is studying how the nanoparticles can be used as catalysts to remove carbon monoxide from air. Other applications could involve the generation of "smart dust" to detect chemical and biological warfare agents and such environmental pollutants as sulfur and nitrogen oxides.

In addition, new ceramic materials using the nanoparticles could be developed that are strong and flexible, retaining the heat-resistant properties of normal ceramics without their characteristic brittleness. Making stronger heat shields is of particular interest to the National Aeronautics and Space Administration, which continues to look at possible failure of ceramic thermal tiles as part of its investigation into the cause of February’s explosion of the space shuttle Columbia.

El-Shall’s research was funded by grants from NASA, the National Science Foundation, Philip Morris USA and Chrysalis Technologies Inc. of Richmond.