Boron set to become king of the nanotubes

Boron nitride nanotubes are primed to become major building blocks for next-generation composite and polymer materials after a new discovery at Rice University in the US.

A team from the university have discovered a way to 'decorate' electrically insulating boron nitride nanotubes with functional groups, making them more suitable for use with polymers and composite materials. Boron nitride nanotubes, like their carbon cousins, are rolled sheets of hexagonal arrays. Unlike carbon nanotubes, they're electrically insulating hybrids made of alternating boron and nitrogen atoms.

Insulating nanotubes that can be functionalized will be a valuable building block for nanoengineering projects, Chemist Angel Marti said. "Carbon nanotubes have outstanding properties, but you can only get them in semiconducting or metallic conducting types. Boron nitride nanotubes are complementary materials that can fill that gap."

Until now, these nanotubes have steadfastly resisted functionalisation, the "decorating" of structures with chemical additives that allows them to be customised for applications. The very properties that give boron nitride nanotubes strength and stability, especially at high temperatures, also make them hard to modify for their use in the production of advanced materials. But the Billups-Birch reaction developed by Rice Professor Emeritus of Chemistry Edward Billups, which frees electrons to bind with other atoms, allowed Martí and lead author Carlos de los Reyes to give the electrically inert boron nitride nanotubes a negative charge. That, in turn, opened them up to functionalisation with other small molecules, including aliphatic carbon chains.

The researchers also discovered the process is reversible. Unlike carbon nanotubes that burn away, boron nitride nanotubes can stand the heat. Placing functionalized boron nitride tubes into a furnace at 600 degrees Celsius stripped them of the added molecules and returned them to their nearly pristine state. "We call it defunctionalisation," Martí said. "You can functionalisze them for an application and then remove the chemical groups to regain the pristine material. That's something else the material brings that is a little different."

The work is described in the American Chemical Society journal ACS Applied Nano Materials.