A standard kitchen microwave has proven effective as part of a two-step process invented at Rice and Swansea universities to clean carbon nanotubes. Basic nanotubes are good for many things, such as forming into microelectronic components or electrically conductive fibers and composites. For more sensitive uses, including drug delivery and solar panels, they need to be as pristine as possible.

Nanotubes form from metal catalysts in the presence of heated gas. But catalyst residues (usually iron) sometimes remain stuck on and inside the tubes. The catalyst remnants can be difficult to remove by physical or chemical means because the same carbon-laden gas used to make the tubes allows carbon atoms to form encapsulating layers around the remaining iron.

In the new process, treating the tubes in open air in a microwave burns off the amorphous carbon. The nanotubes then can be treated with high-temperature chlorine to eliminate almost all of the extraneous particles.

Treating the nanotubes in a household microwave burns off the amorphous carbon. The labs of chemists Andrew Barron, Robert Hauge and Charles Dunnill led the study. Barron is a professor at Rice in Houston and at Swansea University in the UK. Rice's Hauge is a pioneer in nanotube growth techniques. Dunnill is a senior lecturer at the Energy Safety Research Institute at Swansea.

There are many ways to purify nanotubes, but at a cost, Barron says. "The chlorine method developed by Hauge has the advantage of not damaging the nanotubes, unlike other methods. Unfortunately, many of the residual catalyst particles are surrounded by a carbon layer that stops the chlorine from reacting, and this is a problem for making high-purity carbon nanotubes."

The researchers gathered microscope images and spectroscopy data on batches of single-walled and multi-walled nanotubes before and after microwaving them in a 1,000-watt oven, and again after bathing them in an oxidizing bath of chlorine gas under high heat and pressure. They found that once the iron particles were exposed to the microwave, it was easier to get them to react with chlorine. The resulting volatile iron chloride was removed.

Eliminating iron particles lodged inside large multi-walled nanotubes proved to be harder. However, transmission electron microscope images showed their numbers, especially in single-walled tubes, to be greatly diminished.

"We would like to remove all the iron, but for many applications residue within these tubes is less of an issue than if it were on the surface," Barron says. "The presence of residual catalyst on the surface of carbon nanotubes can limit their use in biological or medical applications."