Kinky nanotubes
More important, Sumpter adds, when boron is incorporated into the chicken-wire lattice of carbon atoms, it can induce an elbow – a slight bend – in the nanotube. Each nanotube could have multiple bends, the group’s calculations show, so “you get a network that winds through in a three-dimensional pattern.”
If that isn’t enough, at the elbows “the reactivity is different. If you have something close to it they’ll chemically bond across that,” so the nanotubes make sturdy connections. Things get messy, with kinked nanotubes bonding into Y-shaped and four-way junctions. This microscopic tube tangle grows into a remarkably sturdy macroscopic sponge.
Creating strong covalent chemical bonds – ones in which atoms share electrons – that weld carbon nanotubes has always been difficult, Terrones says. The secret, the researchers found, is introducing minute amounts of other elements, in this case boron, at temperatures as high as 850 Celsius.
Sumpter, Vincent Meunier (now an RPI professor) and their Oak Ridge colleagues used Jaguar, the Cray XT at the Oak Ridge Leadership Computing Facility (LCF), and other machines to computationally decode how boron creates nanotube defects. Their tool of choice is density functional theory (DFT), a quantum mechanical modeling method that calculates the arrangement and interactions of electrons in atoms and molecules.
“These are relatively large (atomic) systems” – multiple carbon structures, Sumpter says, each with odd defects that throw a wrench into the usual DFT calculations. Yet “it’s certainly doable these days, in particular with computing facilities” like the ORNL LCF.
In fact, for six years Sumpter and Meunier have calculated the topological defects induced by doping nanotubes with sulfur, nitrogen, phosphorus and other elements. “The strength of calculations is we often can do them much more efficiently” than experiments, Sumpter says. “We can ask a lot more questions about what happens, so we can probe through more elements than they can possibly probe experimentally in a reasonable amount of time.”
Using Jaguar, the researchers showed that boron atoms preferentially substitute for carbon atoms in a string along the inside of the bent tube, Terrones says. It was a surprising discovery, he adds, but researchers examined the sponges with high-tech instruments to confirm the theorists’ predictions.
Hashim was aware of the ORNL group’s early results in 2008, when he went to work with Terrones, then at Mexico’s Institute of Scientific Research and Technology, on a National Science Foundation (NSF) exchange. Hashim was studying with RPI materials scientist Pulickel Ajayan and thought boron impurities would make nanotubes better absorb lithium ions for battery applications.
To make the sponges, an aerosol of toluene (an industrial feedstock comprised of carbon and hydrogen), ferrocene (a compound of iron, carbon and hydrogen that catalyzes the reactions) and triethylborane (which provides the boron atoms) was released into a reactor chamber under high temperature.
