Unzipping to build the future


Schematic representation of the gradual unzipping of one wall of a carbon nanotube to form a nanoribbon. Although depicted here as an internal rather than end-origin of the initial attack, the location of initial attack may vary. (Credit: Dmitry V. Kosynkin)

RICE (US)—Scientists have found a simple way to create basic elements for aircraft, flat-screen TVs, electronics, and other products that incorporate sheets of tough, electrically conductive material. And the process begins with a zipper.

Research by the Rice University lab of James Tour has uncovered a room-temperature chemical process that splits, or unzips, carbon nanotubes to make flat nanoribbons. The technique makes it possible to produce the ultrathin ribbons in bulk quantities.

These ribbons are straight-edged sheets of graphene, the single-layer form of common graphite found in pencils. You’d have to place thousands of them side by side to equal the width of a human hair, but tests show graphene is 200 times stronger than steel.

“If you want to make conductive film, this is what you want,” says Tour, Rice’s Chao Professor of Chemistry and also a professor of mechanical engineering and materials science and computer science. “As soon as we started talking about this process, we began getting calls from manufacturers that recognized the potential.”

The process involves sulfuric acid and potassium permanganate, which have been in common use since the 1890s. This chemical one-two punch attacks single and multiwalled carbon nanotubes, reacting with the carbon framework and unzipping them in a straight line.

The unzipping action can start on the end or in the middle, but the result is the same—the tubes turn into flat, straight-edged, water-soluble ribbons of graphene. When produced in bulk, these microscopic sheets can be “painted” onto a surface or combined with a polymer to let it conduct electricity.

Nanotubes have been used for that purpose already. “But when you stack two cylinders, the area that is touching is very small,” Tour explains. “If you stack these ribbons into sheets, you have very large areas of overlap. As an additive for materials, it’s going to be very large, especially for conductive materials.”

Nearly all of the nanotubes subjected to unzipping turn into graphene ribbons, Tour says, and the basic process is the same for single or multiwalled tubes. Single-walled carbon nanotubes convert to sheets at room temperature and are good for small electronic devices because the width of the unzipped sheet is highly controllable. But the multiwalled nanotubes are much cheaper starting materials, and the resulting nanoribbons would be useful in a host of applications.

That’s why Tour is banking on bulk, made possible by processing multiwalled tubes, which unzip in one hour at 130 to 158 degrees Fahrenheit. (Until now, making such material in more than microscopic quantities has involved a chemical vapor deposition process at more than 1,500 degrees Fahrenheit.) “Multiwalled carbon nanotubes are concentric tubes, like Russian nesting dolls,” he adds. “We cut through 20 walls, one at a time, during the reaction process.”

Tour says conductive nanoribbons could replace indium tin oxide (ITO), commonly used in flat-panel displays, touch panels, electronic ink, and solar cells. “ITO is very expensive, so lots of people are looking for substitutes that will give them transparency with conductivity,” he says.

The work was funded by the Defense Advanced Research Projects Agency, the Federal Aviation Administration, and Wright Patterson Air Force Laboratory through the U.S. Air Force Office of Scientific Research.

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