Nanotube “rebar” helps graphene toughen up

Ultrathin and highly conductive, graphene shows promise for flexible electronics and energy-dense battery electrodes. But materials scientists fret that the material’s brittleness could hold it back from practical applications. Carbon nanotubes could come to the rescue, new research shows. Sensitive measurement techniques and simulations reveal that coating graphene with carbon nanotubes helps it resist fracture in much the same way that rebar makes concrete structures more resilient (ACS Nano 2018, DOI: 10.1021/acsnano.8b02311).

The individual carbon-carbon bonds in graphene are some of the strongest found in nature. “In a perfect world, it should be very strong,” says Jun Lou, a nanomechanics expert at Rice University. But zoom out, and the material is brittle, fracturing with ease at the sites of the most minor flaws in its structure. In 2014, Lou published a paper establishing that pure graphene has a fracture toughness of just 4 megapascals, making it more brittle than safety glass, which is designed to shatter upon impact (Nat. Comm. 2014 DOI: 10.1038/ncomms4782). This will be a problem in applications like flexible electronics, he says. Tears in the material could short circuit graphene devices or lead to expensive manufacturing problems. “When you get to the real, commercial realm, reliability becomes an issue,” Lou says.

One way to combat this is to try to control graphene’s crystalline structure to eliminate defects, but this is difficult. Rice University nanochemist James M. Tour took a different approach, inspired by composite building materials, using another carbon nanomaterial that also has good electrical properties: carbon nanotubes. He developed a graphene-nanotube hybrid he called rebar graphene by coating a copper surface with carbon nanotubes, using chemical vapor deposition to grow graphene on top, and then peeling the nanotube-studded graphene off (ACS Nano 2014, DOI: 10.1021/nn501132n). Tour believed the material would be resilient just like rebar-containing concrete, and he teamed up with Lou to do the extensive mechanical testing needed to prove this hunch.

Lou’s group performed repeated fracture experiments under different types of electron microscopes, using a tiny mechanical device to pull on the rebar graphene at different angles relative to the nanotubes. Experimental methods alone can’t capture what happens when graphene fractures—it happens too fast. So the team collaborated with Brown University researchers who specialize in molecular simulations. By stitching together images from many experiments and simulating the events, the team was able to get a full picture of what happens when rebar graphene tears, down to the level of individual nanotubes.

“It’s taken us a couple years to understand what’s going on,” Lou says. “In pristine graphene, the crack is unstoppable, and goes in a straight line.” In rebar graphene, the fracture moves in a zig zag, with nanotubes holding the graphene together like bridges, dragging the torn sides back together until the stress is too high and the nanotubes give. Rebar graphene has a fracture toughness of 10.7 MPa, more than twice that of pristine graphene. Though reliability in actual devices still needs to be shown, these basic materials properties are good news for those looking to make rugged electronics from carbon nanomaterials. Tour’s previous work showed that rebar graphene has a lower electrical resistance and better transparency than plain graphene.

Lou says other promising ultrathin, two-dimensional electronic materials, such as molybdenum disulfide, are also brittle and might be toughened using the same method. Ting Zhu, a mechanical engineer at Georgia Institute of Technology, is impressed by the techniques used in the study, and agrees it will have wider implications. “This paves the way for the development of next-generation rebar graphene as well as other 2-D composite materials that are strong and tough, which can be used for flexible electronics and energy storage,” he says.