When associated with other molecules, the sheets of carbon atoms that give form to graphene can attain properties that are even more surprising. A team of researchers at Rice University, in collaboration with physicists at the University of Campinas (Unicamp), has developed a type of sponge that is extremely light, tough, and malleable, through a chemical reaction that bonds graphene oxide (GO), a variant of graphene, to the hexagonal form of boron nitride (BN), a synthetic compound used as a lubricant and additive in cosmetics. When small samples of this sponge were compressed with one or two pennies, they easily bounced back to their original shape. The nanometric structure of the new material, called GO-0.5BN, is similar to the framework of a building under construction, with floors and walls that self-assemble from a base of graphene oxide sheets reinforced with boron nitride platelets. GO-0.5BN is 400 times less dense than graphite.
Composed solely of boron and nitrogen atoms, boron nitride arranges its molecules in a hexagonal configuration similar to that of graphene. The two compounds combine seamlessly, producing a tougher material with greater mechanical malleability. “The new material is chemically and thermally stable and can be used in energy-storing systems, such as supercapacitors and battery electrodes, and it can also absorb gases,” says Douglas Galvão from the Gleb Wataghin Physics Institute at Unicamp, who participated in the study. “Boron nitride reinforces the structure of graphene oxide, which has a few gaps and can become brittle in certain points,” explains theoretical physicist Pedro Alves da Silva Autreto, who is conducting post-doctoral research at Unicamp on a FAPESP scholarship and who spent some time at Rice, where he ran computer simulations to predict the characteristics of GO-0.5BN. The process used to obtain the sponge and its properties was presented in a scientific paper published on July 29, 2014 in Nature Communications.
Graphene oxide has practically the same properties as pure graphene, but is simpler and cheaper to produce. This explains why researchers prefer to use this variant of graphene in their experiments. It can be produced in large amounts by chemically exfoliating oxidized graphite. The presence of oxygen atoms in the hexagonal lattice of graphene carbons gives the compound an additional advantage: it is easier to pile up graphene oxide sheets – and thus create layers that are both extremely tough and extremely thin – than to use pure graphene. “We expected that adding boron nitride to graphene oxide would generate a new structure, but not exactly one that had the ordered layered structure that we ended up with,” says electrical engineer Soumya Vinod from Rice University, first author of the paper in Nature Communications.
The hexagonal boron nitride platelets are uniformly distributed across the walls and floors that make up the internal structure of the sponge. They bind together the graphene oxide sheets that serve as a kind of skeleton for GO-0.5BN. According to Vinod, the platelets absorb stress from compression and stretching and prevent the graphene oxide floors from crumbling or becoming cracked, in addition to increasing the compound’s thermal stability.
Before discovering the chemical formulation of the sponge described in the paper, the researchers tested other versions of the new material that contained different percentages of the two ingredients. While the group at Rice University combined different quantities of powdered graphene oxide and boron nitride, Autreto ran computer simulations trying to predict the properties of the material under development to provide parameters that his colleagues could use to refine their experiments. “I was the only theoretical physicist among 50 experimental researchers in Professor Pulickel Ajayan’s group,” says Autreto, referring to his stint at the American university. The most stable version of the sponge was the one that had boron nitride accounting for half of its final weight. Graphene oxide interacts with boron nitride due to the action of chemical catalysts. The spongy material produced by the reaction is freeze-dried, losing its moisture by sublimation. The resulting foam takes the shape of its container. “Once we had the necessary amounts of graphene oxide and hexagonal boron nitride in hand, we took two or three days to produce the foam,” Vinod explains.
The nanostructured sponge that retains its shape and can be used to store energy or absorb gas has not yet been protected by a commercial patent. The partnership between Unicamp and Rice is expected to continue and generate new projects. “Two post-doctoral researchers from our group will join Professor Ajayan’s team in order to continue the collaboration,” says Galvão, who advised Autreto in his master’s and doctorate and is now supervising his post-doctoral research.
Structural, mechanical and transport properties of graphene and related structures (No. 11/13259-7); Grant mechanism Post-doctoral research grant; Principal investigator Douglas Soares Galvão (IFGW/Unicamp); Grant recipient Pedro Alves da Silva Autreto; Investment R$139,310.43 (FAPESP).
VINOD, S. et al. Low-density three-dimensional foam usings elf-reinforced hybrid two-dimensional atomic layers. Nature Communications. 29 jul. 2014.