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Tough foam from tiny sheets

Tue, 07/29/2014 - 12:59pm
Mike Williams, Rice Univ.

An electron microscope image of foam created at Rice Univ. shows layers of graphene oxide stacked to form a 3-D structure with the help of hexagonal boron nitride linkers. Image: Ajayan Group/Rice Univ.Tough, ultra-light foam of atom-thick sheets can be made to any size and shape through a chemical process invented at Rice Univ.

In microscopic images, the foam dubbed “GO-0.5BN” looks like a nanoscale building, with floors and walls that reinforce each other. The structure consists of a pair of 2-D materials: floors and walls of graphene oxide that self-assemble with the assistance of hexagonal boron nitride platelets.

The researchers say the foam could find use in structural components, as supercapacitor and battery electrodes and for gas absorption, among other applications.

The research by an international collaboration led by the Rice laboratory of materials scientist Pulickel Ajayan is detailed online in Nature Communications.

Graphene oxide (GO) is a variant of graphene, the hexagonal lattice of carbon atoms known for its superior strength and conductivity. It can be produced in bulk by chemically exfoliating oxidized graphite. Hexagonal boron nitride (h-BN) looks like GO, with the same “chicken wire” array of atoms. An insulator known as “white graphene,” h-BN has an ability to form seamless interfaces with graphene that has led to the creation of interesting hybrid materials at Rice and elsewhere.

Soumya Vinod, the Rice graduate student who co-led the project, said she and her colleagues expected adding h-BN to graphene oxide would toughen the resulting foam, but “the ordered, layered structure was not entirely expected.”

“Once we observed the structure, we knew it was very different from the other nanoengineered foams reported and could lead to very interesting properties,” she said.

Those properties include the ability to handle a great deal of strain and still bounce back to its native form. This is remarkable, Vinod said, for a material so light that a stray breath in the lab would send the small samples flying.

Both components of the new material start as cheap, plentiful powders. Atom-thick layers of graphene oxide and h-BN are chemically exfoliated from the powders, mixed in the proper proportion with a few chemical catalysts and freeze-dried. The resulting foam takes the shape of the container and is 400 times less dense than graphite.

For testing, Vinod and her colleagues made foams of pure graphene oxide and foams with h-BN at 25 and 50% by weight. The 50% h-BN version was found to be the most mechanically stable, though she expects to optimize the mix—and increase the size—with further experimentation. “We found that more concentration of h-BN leads to low structural integrity, but we’ve yet to optimize the right amount,” she said.

A close-up look at the foam revealed the floors as self-assembled sheets of overlapping GO flakes. Cross-linking platelets of h-BN were uniformly distributed throughout the material and held the sheets together.

Samples the size of a pencil’s eraser were compressed with one or two pennies to see how well they would bounce back.

The h-BN platelets connect to graphene oxide and absorb stress from compression and stretching, preventing the GO floors from crumbling and significantly enhancing the material’s thermal stability, Vinod said. The platelets also prevented the propagation of cracks that destroyed samples with less or no h-BN.

Source: Rice Univ.

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