Growing the speediest, shiniest nanotubes

The whizz-kids of chemistry – carbon nanotubes – have properties that make them a very versatile and promising material.

They’re making waves in medicine, pollution clean-up, solar power and even paint.

But carbon nanotubes can be chaotic to make. It’s particularly difficult to control aspects of their symmetry – or chirality.

The chirality of nanotubes decides how easily they can conduct electricity: whether they become metallic and conduct electricity with ease, or if they’re more refined semiconductors. For more refined applications of the material, chirality needs to be tightly controlled.

New theoretical research has outlined a way that the nanotubes might be made more ordered.

The researchers, who have published their idea in Science Advances, propose a way to make carbon nanotubes that selects out the ones they’re after.

Carbon nanotubes with different chiralities grow at different speeds. This means that slower-growing tubes can be separated out and removed.

This can be done with nozzles that pipe feedstock – in the form of a hot, carbon-based gas – into an area, along with catalysts to provoke the reaction.

If these nozzles and catalysts are moving, then only the faster-growing tubes will form.

“The catalyst particles are moving as the nanotubes grow, and that’s principally important,” says lead author Dr Ksenia Bets, a researcher at the George R. Brown School of Engineering in Rice University, US.

“If your feedstock keeps moving away, you get a moving window where you’re feeding some tubes and not the others.”

Once the speed is controlled to get rid of the too-short nanotubes, a technique called chemical etching can be used to take care of the too-long nanotubes.

“There are three or four laboratory studies that show nanotube growth can be reversed, and we also know it can be restarted after etching,” says Bets.

“So all the parts of our idea already exist, even if some of them are tricky.”

The researchers, with their focus on theory, haven’t tested their idea out experimentally – but they’re keen to see other labs try it.

“I’m pretty sure every single one of our reviewers were experimentalists, and they didn’t see any contradictions to it working,” says Bets.

“Their only complaint, of course, was that they would like experimental results right now, but that’s not what we do.”

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