Wednesday, July 25, 2007

Ultrastrong Paper from Graphene

A new paperlike material could lead to novel types of light and flexible materials.

By Prachi Patel-Predd

The right stuff: Researchers at Northwestern University have reassembled one-atom-thick graphene sheets that make up soft and flaky graphite crystals in order to create a tough, flexible, paperlike material. Credit: Dmitriy Dikin

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.