Single-walled nanotubes (SWNTs) are tubes of carbon atoms linked by conjugated single and double bonds; they resemble a sheet of graphite rolled up to form a cylinder and stitched together with carbon-carbon bonds. Like atomic force microscope. (You may access additional information on atomic force microscopy.)

Nanotubes can be formed by evaporating cobalt and graphite in an electric arc. Each SWNT is a uniform 1.38 nm in diameter and several micrometers (10^-6 m) long. When prepared by this method, the tubes contain approximately 20% cobalt by weight. However, in the presence of nickel and yttrium catalysts, Bernier at the University of Montpelier in France and John Fischer at the University of Pennsylvania (3) report that pure carbon nanotubes can be produced in large quantities (grams). The availability of such large quantities of pure carbon nanotubes is a major boost for future research on these materials.

One intriguing property of SWNTs is their ability to absorb large amounts of gases, including hydrogen (1). When absorbed in these tubes, hydrogen takes up far less volume than in the gas phase. Hence these tubes offer the possibility of storing large quantities of hydrogen gas in a condensed form. The research team of Bekkedahl at the National Renewable Energy Laboratory in Golden, Colorado, has made improvements in the production and purification of SWNTs. In addition, by aligning the molecules to increase their packing density, they have increased the storage capacity for hydrogen per unit volume. The hydrogen storage properties of SWNTs may have important applications in transportation. The stored hydrogen could be used in electric-powered vehicles in which electrIcity is generated by fuel cells that consume the stored hydrogen to form non-polluting water. One of the problems with hydrogen-fueled vehicles is that there is currently no safe way of storing and transporting the hydrogen. SWNTs provide a way to safely store hydrogen, since at normal temperatures and pressures the danger of flammability with hydrogen is no greater than with gasoline. A major problem at this point is recovering hydrogen from the nanotubes. This and other hurdles must be overcome before this potential storage capacity becomes a commercial reality.

Another interesting property of SWNTs was discovered by Dal and coworkers at Rice University in Texas. These researchers found that SWNTs act as “quantum wires” and are able to conduct electricity (2). The researchers measured the electrical conduction of thin films of nanotubes deposited on silicon and silicon oxide. Their results confirmed theories predicting that these one-dimensional structures would be able to conduct electricity due to their extensive network of delocalized pi orbitals, which would mimic the “electron sea” found in metallic structures. Single molecules tend to be semiconductors or insulators, not conductors. Diamond, quartz, and silicon, for example, effectively exist as single giant molecules linked by directed single bonds; diamond and quartz are insulators, while silicon is a semiconductor. In this context, the conducting properties of carbon nanotubes are most unusual and are a direct result of their delocalized bonding. It is thought that in this material electrons can travel the length of a tube, then jump to another tube to continue their journey. The future of conducting nanodevices awaits exploration, but discovery of their electrical properties may open up a new era in electronics.

Much still remains to be learned about single-walled nanotubes. Their hydrogen storage and conducting properties open up the possibility that, one day, we may be able to stop by the local hydrogen station to fill up our nanotube tanks before we drive to work, or we may see nanotube conductors forming the matrix of our new laptops. These possibilities assure us that nanotubes and their applications will be subjected to much scrutiny in the future.

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