We've written a lot about the potential of using graphene in electronics and materials science, but there are challenges when it comes to producing and utilizing these one-atom-thick sheets of carbon on a large scale. While a lack of an internal structure provides graphene with an abundance of surface area, sheets of the material tend to stick together like a stack of paper, resulting in a reduction in surface area and effectiveness. Now, taking inspiration from a trashcan of crumpled-up papers, Northwestern University researchers have developed a new form of graphene that can't be stacked.
Common steps in materials processing, such as heating, solvent washing, compression, and mixing with other materials can see graphene sheets banding together due to a strong interaction between the sheets called the "Van der Waals attraction." Not only does this reduce the amount of original surface area available, stacked graphene sheets also become rigid and lose their processability. Some previous efforts have involved trying to physically keep the sheets apart by inserting non-carbon "spacers", but this changes the chemical composition of the material.
By crumpling graphene sheets into balls, researchers have created a form of graphene that allows the material to remain pure, retaining the same electrical properties and surface area as separate flat sheets, but is more useful for applications that require large amounts of the material.
Lead researcher Jiaxing Huang and his team made the crumpled graphene balls by creating freely suspended water droplets containing graphene-based sheets. Using a carrier gas to blow the aerosol droplets through a furnace, he water quickly evaporated and the thin graphene sheets were compressed by capillary force into near-spherical particles.
"If you imagine a trash can filled with paper crumples, you really get the idea," says Huang. "The balls can stack up into a tight structure. You can crumple them as hard as you want, but their surface area won't be eliminated, unlike face-to-face stacking."
Huang says the resultant crumpled graphene particles are also remarkably stable against mechanical deformation because the ridges formed in the crumpling process make them harder the harder you compress them.
"We expect this to serve as a new graphene platform to investigate application in energy storage and energy conversion," Huang said.