The 3D ‘monoliths’ — grown between forming ice crystals — add elasticity to the super-strength and conductivity of graphene sheets.
It can support 50,000 times its own weight, springs back into shape after being compressed by up to 80% and has a density much lower than most comparable metal-based materials. A new superelastic, three-dimensional form of graphene can even conduct electricity, paving the way for flexible electronics, researchers say.
The team, led by Dan Li, a materials engineer at Monash University in Clayton, Australia, coaxed 1-centimetre-high graphene blocks or 'monoliths' from tiny flakes of graphene oxide, using ice crystals as templates. The work is published today in Nature Communications1.
Graphene, a two-dimensional form of carbon that was first isolated less than a decade ago, has exceptional mechanical strength and electrical conductivity, but making use of these properties means first finding ways to scale up from nano-sized flakes (see ‘Graphene spun into metre-long fibres’).
Li and his colleagues adapted an industrial technique called freeze casting to do just that. This involves growing layers of an oxygen-coated, soluble version of graphene called graphene oxide between forming ice crystals. On cooling the aqueous solution of graphene oxide flakes, a thin layer of the nanomaterial becomes trapped between the growing crystals, forming a continuous network that retains its structure once the ice is thawed.
Researchers have used this method before2, but the resulting material had poor mechanical strength — a property that Li attributes to the oxygen layer that coats each flake, which weakens bonding between neighbouring flakes in the network.
In the latest study, researchers show that by partially stripping the oxygen coating before freeze casting, they could enhance the bonding between adjacent flakes in the network, producing much stronger materials.
After freeze casting, the honeycomb-like network held its shape as the ice was removed. The researchers could then chemically convert the graphene oxide into graphene, strengthening inter-sheet bonding, and so the material itself, still further.
Fill the void
Li attributes the new graphene's properties to its structure: the individual graphene sheets are neatly aligned, forming an ordered network of hexagonal pores.
Rodney Ruoff, a researcher in graphene assemblies at the University of Texas at Austin, says that the material “is very interesting for the extremely low density that the researchers achieve, as well as its exceptional mechanics”. He adds that the structure could be used as a scaffold for flexible battery electrodes, or form the basis of many composite materials. “It would be interesting to fill the pores with rubber materials, for example,” he says. “There is a great interest in making rubber thermally or electrically conductive without harming its elastic properties.”
Li says that the superelastic graphene has potential for use in biomedical applications. “Biomaterials people are very interested in this structure because the pore sizes match existing tissue scaffolds very well,” he says.
Qiu, L., Liu, J. Z., Chang, S. L. Y., Wu, Y. & Li, D. Nat. Commun. 3, 1241 (2012).
Bai, H., Li, C., Wang, X. & Shi, G. J. Phys. Chem. C 115, 5545–5551 (2011).
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Mitchell Crow, J. Graphene towers promise 'flexi-electronics'. Nature (2012). https://doi.org/10.1038/nature.2012.11930