Graphene can literally be made in a flash by using electricity to zap nearly anything that contains carbon, including discarded food and plastic, a new study finds.
Graphene is made of flexible, transparent sheets each just one carbon atom thick. It’s 200 times stronger than steel, lighter than paper, and more electrically and thermally conductive than copper. Currently the most common way to make graphene in bulk is via exfoliation. It works a bit like how you might exfoliate your skin, and involves sloughing layers of graphene off a block of graphite.
However, chemical exfoliation uses lots of acid and is very expensive, while exfoliation using sound energy or fast-flowing fluid pries off platelets of graphene that are often more than 20 layers thick. Scientists can also produce graphene by depositing it from a vapor onto a surface, but this only makes tiny amounts.
Study lead author Duy Xuan Luong, a physicist at Rice University in Houston, came up with a new way to create graphene while experimenting with a technique called flash Joule heating, which previous work used to create metal nanoparticles. By using a capacitor bank to zap carbon powder with high-voltage electricity and heating it to more than 2,725 degrees C in less than 100 milliseconds, his team produced high-quality graphene low in defects. They detailed their findings online on 27 January in the journal Nature.
This new method requires no furnace, solvents, reactive gases, or purification steps. It also generates very little excess heat, with nearly all of its energy channeled into its target. All elements besides carbon evaporate away.
In addition, the process generates so-called "turbostratic" graphene, where the sheets are positioned like a disorderly, loose pile of cards, as opposed to so-called "A-B stacked" graphene, where the sheets are arranged like an orderly stacked deck of cards. The misaligned nature of turbostratic graphene layers makes them easy to separate when suspended in water or other solvents, whereas the A-B stacked graphene layers created through exfoliation techniques are difficult to pull apart. "The short flash does not allow graphene layers to reorganize to A-B," Luong says.
The scientists note they can make flash graphene from nearly anything with carbon in it, such as discarded food, plastic garbage, wood clippings, rubber tires, biochar, coal, and petroleum coke from oil refining. They even made flash graphene from coffee grounds. "The more carbon content in the sample, the easier it is to convert into graphene," Luong says.
All in all, it only takes 7.2 kilojoules of electrical energy to make a gram of flash graphene. The scientists hope to produce a kilogram of flash graphene per day within two years, beginning with a project recently funded by the U.S. Department of Energy to convert U.S. coal.
In a flash, carbon black turns into graphene through a technique developed by Rice University scientists. The scalable process promises to quickly turn carbon from any source into bulk graphene.Photo: Jeff Fitlow/Rice University
The researchers hope flash Joule heating may greatly reduce the cost of graphene. The present commercial price of graphene ranges from US $67,000 to $200,000 per ton, notes study senior author James Tour, a chemist at Rice University in Houston. In contrast, with their method, currently "the electricity cost is $100 per ton" of graphene produced, Luong says.
The ease and cost of making flash graphene suggests it might find use in bulk composites with concrete, plastic, metals, plywood, and other building materials, the researchers say. For example, they found that adding just 0.05 percent by weight of flash graphene to cement boosted its compressive strength by roughly 25 percent; adding 0.1 percent by weight of flash graphene to silicone rubber enhanced its compressive strength by roughly 250 percent.
The scientists note a concentration of as little as 0.1 percent graphene in cement could reduce the amount of cement needed for construction by 30 percent, slashing its environmental impacts. "The cement industry adds 5 percent of greenhouse gases to the atmosphere annually," Luong says.
The researchers founded a startup company, Universal Matter, to commercialize their discovery. "We are working both on scaling up [production] and applications for flash graphene," Luong says.
Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.