Single-atom-thick sheets of carbon called graphene have some amazing properties: graphene is strong, highly electrically conductive, flexible, and transparent. This makes it a promising material to make flexible touch screens and superstrong structural materials. But creating these thin carbon sheets, and then building things out of them, is difficult to do outside the lab.
Now an advance in making and processing graphene in solution may make it practical to work with the material at manufacturing scale. Researchers at Rice University have made graphene solutions 10 times more concentrated than any before. They’ve used these solutions to make transparent, conductive sheets similar to the electrodes on displays, and they’re currently developing methods for spinning the graphene solutions to generate fibers and structural materials for airplanes and other vehicles that promise to be less expensive than today’s carbon fiber.
Whatever the end product, it’s ideal to start with a high-concentration solution of graphene, but existing methods can’t achieve this, says James Tour, professor of chemistry at Rice University. Graphene isn’t very soluble, partly because of its dimensions, and partly because of its chemistry. Graphene is just one atom thick, but its surface area is huge. “If you want to work with graphene, you’re working dilute, which makes sense, because this is a huge whopping molecule,” Tour says.
Most methods for making graphene start with graphite and involve flaking off atom-thin sheets of graphene, usually using chemical means. “The key is to make single-layer graphene, to not destroy it in the process, and to do it in high volume,” says Yang Yang, professor of materials science and engineering at the University of California, Los Angeles. Some of the existing methods for making graphene from graphite and then manipulating it in solution involve adding soluble groups to the surface of the molecule, but this chemical change destroys graphene’s electrical properties.
The Rice researchers make graphene solutions using a method they initially developed for working with carbon nanotubes. About five years ago, researchers led by the late Nobel laureate Richard Smalley discovered that highly concentrated sulfuric acid, so strong it’s called a “superacid,” can bring carbon nanotubes into solution by coating their surfaces with ions. Last year, the Rice group, now led by chemist Matteo Pasquali, showed they could use superacid solutions of carbon nanotubes to make fibers hundreds of meters long; the group has contracted with a major chemical company to commercialize the process.
The Rice researchers recently demonstrated that even stronger superacids can separate graphite into sheets of graphene and bring them into solution. Unlike other methods involving chemical reactions that alter graphene, the superacid solution does not degrade the material’s properties. The group has used the solutions to make sheets of graphene with low electrical resistance and is now “full steam ahead” using these solutions to make graphene fibers, says Tour.
Tour expects the graphene processing method to have two major applications: transparent electrodes and structural materials. In both areas, it may bring down costs. Indium tin oxide, the transparent conducting material most commonly found in touch screens and solar cells, is expensive and brittle, says Benji Maruyama, senior materials research engineer at the Air Force Research Laboratory in Ohio. The U.S. Air Force is funding the Rice research. Many groups have demonstrated the advantages of graphene electrodes in terms of conductivity and flexibility; the Rice method should make it possible to manufacture them over large areas.
The process could also be used to bring down the costs of lightweight, tough structural materials made from carbon fiber. These materials have been around for decades, but they remain expensive because the processes used to manufacture them are complex and result in lost material. Instead of making pure carbon into fibers directly, as in the Rice process, the current process starts with a nitrile polymer fiber that’s heated to turn it into graphite. These fibers are then woven into mats and glued together to make a bulk material. “They’re used in aircraft, but not in automobiles, because the costs are too high,” says Tour. “If we can do this more cheaply and get as good or better properties, there is the potential for a real advance in carbon fibers.”