For many researchers, graphene is ideal for use in filtration membranes. A single sheet of graphene resembles atomically thin chicken wire, and is composed of carbon atoms joined in a pattern that makes the material extremely tough and impervious to even the smallest atom: helium.
Researchers have developed techniques to fabricate graphene membranes and precisely riddle them with tiny holes, or nanopores, the size of which can be tailored to filter out specific molecules. For the most part, scientists synthesize graphene through a process called chemical vapor deposition, in which they first heat a sample of copper foil and then deposit onto it a combination of carbon and other gases.
Graphene-based membranes have mostly been made in small batches in the laboratory, where researchers can carefully control the material's growth conditions; however, if graphene membranes are ever to be used commercially, they will have to be produced in large quantities, at high rates, and with reliable performance.
An industrial, scalable, continuous manufacturing process has been developed that produces long strips of high-quality graphene that is tailored for use in membranes that filter a variety of molecules, including salts, larger ions, proteins, or nanoparticles. Such membranes should be useful for desalination, biological separation, and other applications.
The setup combines a roll-to-roll approach — a common industrial approach for continuous processing of thin foils — with the common graphene-fabrication technique of chemical vapor deposition to manufacture high-quality graphene in large quantities and at a high rate. The system consists of two spools connected by a conveyor belt that runs through a small furnace. The first spool unfurls a long strip of copper foil less than 1 centimeter wide. When it enters the furnace, the foil is fed through first one tube and then another, in a “split-zone” design.
While the foil rolls through the first tube, it heats up to a certain ideal temperature, at which point it is ready to roll through the second tube, where a specified ratio of methane and hydrogen gas is pumped and deposited onto the heated foil to produce graphene.
As the graphene exits the furnace, it is rolled onto the second spool. The foil can be fed continuously through the system, producing high-quality graphene at a rate of 5 centimeters per minute. The longest run lasted almost four hours, during which about 10 meters of continuous graphene was produced. Once the graphene was produced using the roll-to-roll method, the foil was unwound from the second spool and small samples were cut out. The samples were cast with a polymer mesh, or support, and subsequently the underlying copper was etched away.