Using graphite–the black flaky stuff employed in pencils–researchers at Northwestern University have created a strong, flexible, and lightweight paperlike material. It could be used as electrolytes or hydrogen storage materials in fuel cells, electrodes in supercapacitors and batteries, and super-thin chemical filters. It could also be mixed with polymers or metals to make materials for use in aircraft fuselages, cars, and buildings.
The new material is made of overlapping layers of graphene, one-atom-thick sheets of carbon atoms arranged in honeycomb-like hexagons. In contrast, graphite, which becomes powdery under pressure, is made of graphene sheets stacked one on top of the other.
Rodney Ruoff, a Northwestern nanoengineering professor who led the work, published in Nature this week, says that the methods behind making the novel graphene paper could lead to even stronger versions. Right now, water molecules hold together the individual 10-nanometer-thick graphene flakes to create the micrometers-thick graphene paper. By using other chemicals as glues, the researchers could make ultrastrong paperlike materials with various properties. “The future is particularly bright because the system is very flexible … The chemistry is almost infinite,” Ruoff says.
Individual sheets of graphene were not known to exist until three years ago, when Andre Geim, a professor of physics at the University of Manchester, in the UK, used adhesive tape to get a few flakes of graphene from a graphite crystal. Researchers still don’t understand all of graphene’s properties, but they know that it can conduct electrons extremely well and is known to be exceptionally strong. “Graphene is the toughest material in the world–tougher than diamond,” Geim says. But in graphite, the graphene sheets are assembled in such a way that they do not bind strongly to each other. So they simply flake off under friction, creating a pencil’s black marks.
Ruoff’s idea was to “disassemble graphite into individual layers and reassemble them in a different way than they are in graphite.” The goal was to find a way to glue the graphene platelets together while reassembling them, which would create a tough and flexible material.
Since it’s hard to separate the graphene sheets in graphite, the researchers first used an acid to oxidize graphite and make graphite oxide. Then they put the graphite oxide in water. Individual graphene-oxide sheets easily separated in water.
When the researchers filtered the suspension, the graphene-oxide flakes settled down on the filter, randomly overlapping with each other. Water glued the flakes together; its hydrogen atoms bonded with the carbon atoms in adjacent flakes. The result was a dark-brown, thin, flexible graphene-oxide paper. By adjusting the concentration of graphite oxide in the water, the researchers changed the thickness of the paper, ranging from 1 to 100 micrometers.
In an effort to develop superstrong lightweight materials, others have used carbon nanotubes. And the new graphene-oxide paper is not as strong as carbon-nanotube films, Geim says. “The advantage of materials made from carbon nanotubes is they’re much tougher, because they entangle like spaghetti,” he says. “When you’re dealing with flat sheets, they entangle very little and are breakable.”
But the graphene-oxide paper has other key advantages. Graphite is a cheap raw material, and the filtration method is simple and leads to lots of graphene. Most important, the Northwestern researchers’ work opens up a way to manipulate graphene sheets and make paperlike materials with different properties.
When Ruoff and his colleagues oxidize graphene into graphene oxide, for instance, the carbon-based material goes from being an electrical conductor to being an insulator. Ruoff says that he can alter graphene’s chemistry in other ways to change its electrical properties and make it an insulator, a conductor, or even a semiconductor.
That electrical versatility combines with an ultrastrong material has some observers excited. “They haven’t used any tough glue between the [graphene platelets],” Geim says. “I expect very, very tough materials if a proper glue between graphene is used.”