Boron Nitride Nanotubes—Building Blocks for Composites
Rice University researchers have discovered a way to enhance boron nitride nanotubes for next-generation composites.
Boron nitride nanotubes are primed to become effective building blocks for next-generation composites and polymer materials based on two discoveries at Rice University. Scientists at Rice have found a way to enhance a unique class of nanotubes using a chemical process pioneered at the university. The Rice lab of chemist Angel Martí took advantage of the Billups-Birch reaction process to enhance boron nitride nanotubes. The work is described in the American Chemical Society journal ACS Applied Nano Materials.
Boron nitride nanotubes, like their carbon cousins, are rolled sheets of hexagonal arrays. Unlike carbon nanotubes, however, boron nitride nanotubes are electrically insulating hybrids made of alternating boron and nitrogen atoms. Insulating nanotubes that can be functionalized will be a valuable building block for nano-engineering projects, according to Martí. “Carbon nanotubes have outstanding properties, but you can only get them in semiconducting or metallic conducting types,” he said. “Boron nitride nanotubes are complementary materials that can fill that gap.”
Until now, boron nitride nanotubes have steadfastly resisted functionalization, the “decorating” of structures with chemical additives that allows them to be customized for various applications. The very properties that give boron nitride nanotubes strength and stability, especially at high temperatures, also make them hard to modify for use in the production of advanced materials.
Developed by Edward Billups, Rice professor emeritus of chemistry, the Billups-Birch reaction frees electrons to bind with other atoms. In this case, the reaction 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 functionalization with other small molecules, including aliphatic carbon chains.
“Functionalizing the nanotubes modifies or tunes their properties,” Martí said. “When they’re pristine, they are dispersible in water, but once we attach these alkyl chains, they are extremely hydrophobic (water-avoiding). Then, if you put them in very hydrophobic solvents like those with long-chain hydrocarbons, they are more dispersible than their pristine form. This allows us to tune the properties of the nanotubes and will make it easier to take the next step toward composites. For that, the materials need to be compatible.”
After he discovered the phenomenon, de los Reyes spent months trying to reproduce it reliably. “There was a period where I had to do a reaction every day to achieve reproducibility,” he said. But that turned out to be an advantage, as the process only required about a day from start to finish. “That’s the advantage over other processes to functionalize carbon nanotubes,” de los Reyes explained. “There are some that are very effective, but they may take a few days.”
The process begins with adding pure ammonia gas to the nanotubes and cooling it to -70°C (-94°F). “When it combines with sodium, lithium or potassium—we use lithium—it creates a sea of electrons,” Martí said. “When the lithium dissolves in the ammonia, it expels the electrons.”
The freed electrons quickly bind with the nanotubes and provide hooks for other molecules. De los Reyes enhanced Billups-Birch when he found that adding the alkyl chains slowly, rather than all at once, improved their ability to bind.
The researchers also discovered that 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°C (1,112°F) stripped them of the added molecules and returned them to their nearly pristine state.
“We call it defunctionalization,” Martí said. “You can functionalize 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.”
Co-authors of the paper are Rice University research assistant Kendahl Walz Mitra; graduate students Ashleigh Smith and Axel Loredo; postdoctoral researcher Sadegh Yazdi and undergraduate student Frank Frankovsky; Emilie Ringe, assistant professor of materials science and nanoengineering and of chemistry; and Matteo Pasquali, chair of the department of chemistry and a professor of chemical and biomolecular engineering and of materials science and nanoengineering. Martí is an associate professor of chemistry, of bioengineering and of material science and nanoengineering. The National Science Foundation, the Air Force Office of Scientific Research and the Welch Foundation supported the research. For more information, visit www.rice.edu.