PROMISING PACKAGES

Big things are in store for tiny bundles of carbon tubes whose walls are only one atom thick
 

Atomic-scale carbon tubes are among the smallest solid structures in nature. But researchers hope that they may transform industry in a major way—by storing fuel for hydrogen-powered vehicles and serving as filters to purify gases, among other uses.

Physicist Aldo Migone is one of the scientists exploring the properties of these nanotubes, as they're called. Nanoscale materials are measured in nanometers, which is getting down to the molecular or even atomic level (a nanometer is one-millionth of a millimeter). The type of nanotube Migone studies is basically a one-atom-thick layer of graphite rolled into a seamless, hollow tube. A dome of carbon atoms plugs the tube at each end, like a medicine capsule. 

These tubes can be hundreds or thousands of nanometers long, but they are almost inconceivably skinny: 1.4 nanometers in diameter, or just a few atoms' width across. And they usually occur not singly, but in bundles—typically up to 200 nanotubes together, like a bundle of pipes. 

Despite the nanotubes' small size, scientists can measure what happens to matter interacting with them. The fact that each tube is a single layer of atoms forming a closed structure gives these materials interesting properties. 

"No tool allows you to slice anything one atom thick," says Migone, professor and chair of physics at SIUC. "So there's strong interest in these from a fundamental standpoint—you can study behavior that is difficult or impossible to observe in any other system. 

"There's also strong interest from a practical standpoint, because one can envision situations in which nanotubes can help revolutionize different areas of current technology."

Nanotubes were discovered in 1991 by a Japanese researcher, Sumio Iijima. He was producing a newly discovered type of carbon—soccer-ball-shaped molecules called fullerenes—by arcing electricity between two graphite electrodes. The resulting soot contains a certain amount of fullerenes. But when Iijima took a look through a powerful electron microscope, he found that some of the soot was in the form of something that had never been seen before: nanotubes.

The tubes that Iijima discovered had multiple walls, or several layers. "They're like Russian dolls—capsules within capsules," Migone says. By 1993, however, researchers found that if they added certain metals to the graphite electrodes, the arc discharge process would yield single-walled nanotubes, self-organized into bundles. 

If nanotube bundles can eventually be produced inexpensively in bulk—and promising work elsewhere is being devoted to this goal—the potential applications are intriguing. For example, because the atomic bonds in the nanotube walls are extremely strong, the tubes might be used to reinforce other materials. "Per unit weight, you can't get anything stronger," says Migone. Other researchers want to use them to make nanosized electronic components. 

Migone is investigating the nanotubes' ability to adsorb and desorb (capture and release) gas atoms. For practical purposes, that means their potential for storing gases such as hydrogen or neon, or for separating one type of gas from a mixture. Research Corporation (a private foundation that supports science), the Petroleum Research Fund, and the National Science Foundation have funded his work.

"If you take any material as a substrate, cool it down in a vacuum chamber, and add pure gas, some of the gas will sit on—stick to—the substrate," Migone says. In the case of carbon, this adsorption is due to electrical interaction. "It's rather weak binding compared to chemical binding," he says. "That why cooling down the material is important." The lower the temperature, the more readily the gas atoms will stick, because they have less energy to bounce away.

Carbon nanotubes offer the possibility of storing "pretty high densities of gas without needing very complicated machinery to do it," says Migone. Per unit mass, the thin-walled tubes offer much more surface area for gas molecules to latch onto than a flat substrate would. The physical properties of the tubes also allow the gas atoms to pack in tightly. The atoms can reach densities similar to those in a liquid form of the gas, but at more moderate temperatures and pressures than what's normally required to keep a gas like hydrogen, say, in liquid form. It's no wonder that industry hopes to exploit nanotubes for energy systems.

Migone and his students have measured the binding energies of different gases—how strong the attachment is between the tube and the gas—at various temperatures and pressures. That allows them to know under what conditions the gas atoms are bound and released, critical details if nanotubes are someday to be used in devices to store or filter gases. They've also determined how much surface area of the nanotubes is available for adsorption. 

When gas atoms stick to nanotube bundles, Migone's lab has found, they first settle in the grooves formed where one tube abuts another. The atoms form a long chain (imagine a line of BBs in a narrow channel). When the grooves in a nanotube bundle fill up, gas atoms begin adsorbing as a film on the outer walls of the tubes themselves and can build up to two layers thick.

Although scientists in the energy field had hoped that the gaps between tubes in a nanotube bundle also could store hydrogen through adsorption, one of Migone's more important findings is that this isn't possible. Although the gaps are about half a nanometer across, the action of the carbon and gas atoms' electron clouds reduces the effective opening to about .25 nanometer. Gas atoms larger than that simply won't fit inside. This rules out not only hydrogen, but most other gases as well.

Why not store hydrogen inside the tubes themselves? That can be done, but right now it doesn't look as feasible for commercial applications, Migone says. The process of chemically or mechanically "uncapping" the tubes creates a residue clogging the opening that requires high heat and a high vacuum to remove. As a result, he has studied only closed tubes so far.

In the early and mid-1990s, Migone had been doing adsorption experiments on flat substrates such as graphite layers. When reports about single-walled carbon nanotubes started coming out, he was one of the first scientists to begin doing adsorption studies with these structures.

Undergraduate students Erica Mackie and Sarah Weber contributed a lot to that early work. Since then, several other undergraduates and five graduate students have worked on the nanotube research. One of them, Saikat Talapatra, received a prestigious doctoral fellowship in 2001 from the Link Foundation, which supports energy-related research.

"The work with hydrogen adsorption was mainly driven by him," says Migone. "He did a lot of measurements of binding energies and the surface area available for adsorption. He was especially interested in the potential applications of nanotubes. With him, this [research] exploded." Talapatra is now doing postdoctoral research at Rensselaer Polytechnic Institute.

If nanotubes hold promise for hydrogen storage—say, for hydrogen-powered vehicles—a critical question is whether the gas can be stored at a reasonably high temperature.

"The ideal is to be able to pack atoms closely at or close to room temperature," says Migone. "Right now it's not looking like carbon nanotubes can do it at room temperature—you might have to use liquid nitrogen to cool it." That's still an improvement on other substrates, which would require much colder temperatures, but it may not be good enough for commercial applications.

Does that mean nanotubes won't be useful for energy systems? Not at all, says Migone. Much remains to be investigated. For example, his lab has begun comparing the adsorption properties of nanotubes produced by different means.

Most of the team's work has been done with nanotubes produced by the arc discharge process described earlier. But commercial labs can now furnish nanotubes made by two other methods: a laser process and a high-temperature chemical process. As Migone explains, "These (nanotubes) could have small structural differences, or differences in the amount of impurities present, such as various metals or different forms of carbon." 

Although suppliers use various processes to remove as many impurities as possible from the nanotubes they sell, the effectiveness of these methods varies. Furthermore, says Migone, "Some of the methods used to remove impurities are harsh—they punch holes in the nanotube." In these cases, lab data on the tubes' properties can be tough to interpret.

Migone expects that nanotubes made or purified with certain methods are likely to give clearer—and more desirable—adsorption results than those made with other methods. He is especially interested in what his team will find with the laser-produced nanotubes, which have "very low levels of impurities to begin with."

He is even more optimistic about what might be achieved with a composite combining carbon nanotubes with other materials. He is collaborating with Khalid Lafdi, formerly of SIUC and now at the University of Dayton, on this effort.

That joint research is very much directed to applications, Migone says. But much of his work is more fundamental: as a physicist, he likes the opportunity nanotubes give him to study material behaving as if it were one-dimensional. 

Atoms of gas adsorbed on a flat substrate can be compared to balls on a billiard table, he says: they're free to move around on the surface, but not to leave that surface. In some senses, the atoms behave like matter in two dimensions.

But gas atoms adsorbed in a groove between two nanotubes are constrained even more. They can switch places in the line, but they can't move up or sideways. In effect, says Migone, "They behave like matter in one dimension."

He and his students have determined the phases of various gases in this quasi one-dimensional state—that is, whether they seem to act more like a liquid or solid—and under what conditions phase transitions occur. 

The properties of one-dimensional matter may, down the road, be useful technologically. Meanwhile, learning about them increases scientific understanding.

"There's no immediate practicality (to this part of the research)," says Migone, "but it's an exciting thing to be able to do. It's not every day that people have access to material in one dimension."

—Marilyn Davis

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