Smalley scientists show astounding nanotube growth with flying carpets, ‘odako’ kites

By MIKE WILLIAMS
Rice News staff

With products that range from carpets to kites, you’d think Rice chemist Bob Hauge was running a department store.

What he’s really running is a revolution in the world of carbon nanotechnology.

	This photo of microscopic bundles of “odako” grown at Rice University shows single-walled nanotubes lifting iron and aluminum oxide “kites” as they grow while remaining firmly rooted in a carbon base.

In newly published research, Hauge’s Rice University team describes a method for making ”odako,” bundles of single-walled carbon nanotubes (SWNT) named for the traditional Japanese kites they resemble. It may lead to a way to produce meter-long strands of nanotubes, which by themselves are no wider than a piece of DNA.

In the paper published this month in Nano Research, Hauge and graduate students Cary Pint and Noe Alvarez explained that the odako after which the bundles are named are gigantic kites that take many hands to fly, hence the many lines that trail from them.

In this case, the lines are nanotubes, hollow cylinders of pure carbon. Individually, they’re thousands of times smaller than a living cell, but Hauge’s new method creates bundles of SWNTs that are sometimes measured in centimeters, and, he said, the process could eventually yield tubes of unlimited length.

Large-scale production of nanotube threads and cables would be a godsend for engineers in almost every field. They could be used in lightweight, superefficient power-transmission lines for next-generation electrical grids, for example, and in ultra-strong and lightning-resistant versions of carbon-fiber materials found in airplanes. Hauge, a distinguished faculty fellow in chemistry at Rice’s Richard E. Smalley Institute for Nanoscale Science and Technology, said the SWNT bundles may also prove useful in batteries, fuel cells and microelectronics.

Odako grow from carbon fibers treated with iron and an aluminum oxide catalyst. The bare fibers at left were covered during the catalyst deposition process.

To understand how Hauge makes these nanokites, it helps to have a little background on flying carpets.

Last year, Hauge and colleagues found they could make compact bundles of nanotubes starting with the same machinery the U.S. Treasury uses to embed paper money with unique markings that make the currency difficult to counterfeit.

Hauge and his team — which included senior research fellow Howard Schmidt and Professor Matteo Pasquali, both of Rice’s Department of Chemical and Biomolecular Engineering; graduate students Pint and Sean Pheasant; and Kent Coulter of San Antonio’s Southwest Research Institute — used this printing process to deposit elements onto a long Mylar roll. The middle layer consisted of tiny iron particles that cause nanotubes to grow under proper conditions. Over that was a layer of aluminum oxide, and on the bottom was a release layer the team could activate with a solvent to loosen the aluminum oxide and iron, which was ground into flakes with a mortar and pestle.

Here’s where the process took off. In a mesh cage placed into a furnace, the flakes would lift off and ”fly” in the flowing chemical vapor while arrays of nanotubes grew vertically in tight, forest-like formations atop the iron particles. When done cooking and viewed under a microscope, the bundles of tubes looked remarkably like the pile of a carpet.

Single-walled nanotubes grow from iron “islands” deposited between a carbon substrate and the aluminum oxide catalyst.

While other methods used to grow SWNTs had yielded a paltry 0.5 percent ratio of nanotubes to substrate materials, Hauge’s technique brought the yield up to an incredible 400 percent. The process will likely facilitate large-scale SWNT growth, Pint said.

In the latest research, the team replaced the Mylar with pure carbon. In this setup, the growing nanotubes literally raise the roof, lifting up the iron and aluminum oxide from which they’re sprouting while the other ends of the tubes stay firmly attached to the carbon. As the bundle of tubes grows higher, the catalyst becomes like a kite, flying in the hydrogen and acetylene breeze that flows through the production chamber.

The team discovered that odako grow not only on flat layers of Grafoil, a flexible graphite material, but also on carbon fiber, even when that fiber is woven into a material. Photos show the odako follow the rounded form of the fibers even while growing to great lengths, though the researchers note shorter may be better for the manufacture of composite materials. The SWNTs remain bound to the carbon fiber while the free ends act like hooks in Velcro, grabbing onto the epoxy used in composites.

They noted odako growth may be possible on such other materials as quartz fibers and a variety of metals.

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Hauge and his team now seek the holy grail of nanotube growth: a catalyst that will not die, enabling furnaces that churn out continuous threads of material. The surprising answer, as revealed in another paper co-authored by Hauge, Pint and former Rice graduate student Laura McJilton, published by Nano Letters last December, may involve water, which has been found to dramatically increase the life and activity of catalysts.

”If we could get these growing so they never stop — so that, at some point, you pull one end out of the furnace while the other end is still inside growing — then you should be able to grow meter-long material and start weaving it. You’ve got to somehow make that catalyst stay alive forever,” he said.

”That’s a very difficult thing to do, but it’s not an impossible task.”