The more wrinkles the better for graphene

Crumple a piece of paper and it's probably destined for the trash can, but new research shows that repeatedly crumpling sheets of the two-dimensional nanomaterial graphene can actually enhance some of its properties. In some cases, the more crumpled the better.

The research by engineers at Brown University shows that wrinkling and crumpling graphene in a multi-step process makes it significantly better at repelling water – a property that could be useful for novel self-cleaning surfaces. Crumpled graphene also possesses enhanced electrochemical properties that could find use in electrodes for batteries and fuel cells. The results are published in a paper in Advanced Materials.

This new research builds on previous work by Robert Hurt and Ian Wong at Brown's School of Engineering, which showed that introducing wrinkles into graphene produces substrates for culturing cells that are similar to the complex environments in which cells grow in the body. For this latest work, the researchers, led by postdoctoral fellow Po-Yen Chen, wanted to build more complex architectures incorporating both wrinkles and crumples. "I wanted to see if there was a way to create higher-generational structures," Chen said.

To do that, the researchers deposited layers of graphene oxide onto shrink films – polymer membranes that shrink when heated. As the films shrink, the graphene on top is compressed, causing it to wrinkle and crumple. To see what kind of structures they could create, the researchers compressed the same graphene sheets multiple times. After the first shrink, the film was dissolved away and the graphene was placed on a new film to be shrunk again.

What is more, the researchers sometimes applied physical constraints during the shrinking process: clamping opposite ends of the films so that they shrank along just one axis. Clamped films yielded graphene sheets with wrinkles across its surface that were periodic and essentially parallel. Unclamped films shrank in two dimensions, both length- and width-wise, creating a graphene surface that was crumpled in random shapes.

The team conducted these different modes of shrinking over three successive generations. For example, they might shrink the same graphene sheet on a clamped film, then an unclamped film, then a clamped film again; or unclamped, clamped and unclamped. They also rotated the graphene between shrinkings, sometimes placing the sheet perpendicular to its original orientation.

This multi-generational approach substantially compressed the graphene sheets, making them as small as one-fortieth of their original size. The researchers also showed that successive generations of graphene sheets would possess interesting patterns across their surface – wrinkles and crumples that were superimposed onto each other, for example.

"As you go deeper into the generations you tend to get larger wavelength structures with the original, smaller wavelength structure from earlier generations built into them," said Hurt, a professor of engineering at Brown and one of the paper's corresponding authors. A sheet that was clamped, unclamped and then clamped looked different from one that was unclamped, clamped and then unclamped.

"The sequence matters," said Wong, also a corresponding author on the paper. "It's not like multiplication, where two times three is the same as three times two. The material has a 'memory' and we get different results when we wrinkle or crumple in a different order."

The researchers generated a kind of taxonomy of structures born from different combinations of shrinking processes and then tested several of those structures to see how they altered the properties of the graphene sheets.

They found that a highly crumpled graphene surface becomes superhydrophobic – able to resist wetting by water. When water touches the surface of a hydrophobic material, it beads up and rolls off. When the contact angle of those water beads with the surface exceeds 160° – meaning very little of the water bead touches the surface – the material is said to be superhydrophobic. The researchers showed that three unclamped rounds of shrinking could make graphene superhydrophobic.

The team also showed that crumpling could enhance the electrochemical behavior of graphene, which could be useful for next-generation energy storage and generation systems. When used as a battery electrode, crumpled graphene demonstrated as much as a 400% increase in electrochemical current density over flat graphene sheets. That increase in current density could make for vastly more efficient batteries.

"You don't need a new material to do it," Chen said. "You just need to crumple the graphene." In addition to batteries and water-resistant coatings, graphene compressed in this manner might also be useful for stretchable electronics, leading to applications such as wearable sensors.

The group plans to continue experimenting with different ways of generating structures on graphene and other nanomaterials. "There are many new two-dimensional nanomaterials that have interesting properties, not just graphene," Wong said. "So other materials or combinations of materials may also organize into interesting structures with unexpected functionalities."

This story is adapted from material from Brown University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.