A flake of functionalized hexagonal-boron nitride created at Rice University, as seen under a transmission electron microscope. Image: Angel Martí Group/Rice University.
A flake of functionalized hexagonal-boron nitride created at Rice University, as seen under a transmission electron microscope. Image: Angel Martí Group/Rice University.

Hexagonal-boron nitride (h-BN) is tough, but scientists at Rice University are making it easier to get along with.

Two-dimensional (2D) h-BN, an insulating material also known as ‘white graphene’, is four times stiffer than steel and an excellent conductor of heat, which is a benefit to composites that rely on it to enhance their properties. But those qualities also make h-BN hard to modify. Its tight hexagonal lattice of alternating boron and nitrogen atoms is highly resistant to change, unlike graphene and other 2D materials that can be easily modified – or functionalized – with other elements.

The Rice lab of chemist Angel Martí has now come up with a protocol for enhancing h-BN with carbon chains. These turn the 2D tough guy into a material that retains its strength but is more amenable to bonding with polymers or other materials in composites.

The protocol is described in a paper in the Journal of Physical Chemistry, which also suggests that h-BN can be made more dispersible in organic solvents. It involves a using a modified version of the Billups-Birch reaction process, which Martí and his team had previously used to alter boron nitride nanotubes, to attack the defenses of h-BN and covalently attach carbons.

Birch reduction, which was discovered in the 1940s and enhanced in 2004 by Rice chemistry professor Edward Billups to functionalize carbon nanotubes, frees electrons to bind with other atoms. In the Rice process, Martí and his team use the Billups-Birch reaction to control the amount of h-BN functionalization by varying the amount of lithium in the reaction.

Lithium is an alkali metal that sheds free electrons when combined with liquefied ammonia. Mixed with h-BN flakes and a carbon source – in this case, 1-Bromododecane – the reaction produces an alkyl radical, a chemical species that reacts with h-BN and makes a bond.

According to Martí, it's the best method found so far to modify h-BN, which resists change even under high temperatures.

"You take a little bit of graphite and put it in a furnace at 800°C, and it will be gone," he said. "You take hexagonal-boron nitride and do the same, and it will still be there smiling at you.

"That gives you an idea of how stable it is, and that's the problem we wanted to address. The material is good for certain applications, but to control its properties for manufacturing, you have to graft different groups onto the surface."

He said that a 20-to-1 molar ratio of lithium to h-BN optimized the process of grafting carbon chains to the surface and edges of h-BN. Furthermore, because the base h-BN remains stable under high temperatures, it can be returned to its pristine state by simply burning off the functional chains.

While h-BN is naturally hydrophilic (water-attracting), the functional carbons make them nearly superhydrophobic (water-avoiding), a good property for making protective films. But even when enhanced, the flakes remain amenable to dispersion in non-polar solvents.

Martí’s group is now exploring what other kinds of molecules can be grafted onto white graphene. "What about benzene groups? What about ethers? What about groups that will make it compatible with other materials?’ he said.

"There's a lot of interest in making composite materials between h-BN, boron nitride nanotubes and polymers. Ultimately, we'd like to graft different groups onto h-BN and build a library, kind of a toolbox, of functional groups that can be used with these materials."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.