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Wrinkle a piece of paper, probably destined to be thrown into the trash can; However, new research shows that repeatedly wrinkling the nano-material graphene can enhance some of its characteristics. In some cases, even the more wrinkled the better.

The engineering research of Brown University shows that the hydrophobic property of graphene will be significantly improved after repeated folding-this property is very helpful for manufacturing self-cleaning surfaces. Folded graphene also has enhanced electrochemical performance, which may be more beneficial to the electrodes of batteries and fuel cells.

The research results were published in the magazine.

Folding age

Robert Hurt and Ian Wong, engineering researchers at Brown University, laid a good foundation for this research. Previously, the team said that by introducing wrinkles into graphene, they could transform the substrate used to cultivate cells into a complex environment more like the body. For this latest achievement, Professor Chen Boyan, the head of the postdoctoral research team, said that he hoped to build a complex structure combining folds. "I want to see if it is possible to create a higher-level structure," said Professor Chen.

In order to do this, the researchers deposited graphene oxide on the shrink film-taking advantage of the fact that polymer films shrink when heated. When the film shrinks, the graphene at the top will be compressed, making it wrinkle. In order to observe what kind of fold structure will be produced, the researchers repeatedly compressed the same graphene. After the first shrinkage, the membrane dissolves, and then graphene is put into another new membrane to continue.

In the continuous contraction, the researchers tried some different configurations. For example, sometimes they hold both ends of the film and let them shrink along only one axis. On the other hand, the periodic graphene layer produced by the fixed film will have substantially parallel folds on its entire surface. The film that is not clamped will shrink in two dimensions, that is, XY direction, resulting in random wrinkling of graphene surface.

In several successive testing methods, the team also used different shrinkage modes for testing. For example, they may shrink the same piece of graphene, clamping the film first, then not clamping, and finally clamping. Or not clamping, clamping and then releasing. They also rotate graphene between contractions in different configurations, sometimes making it perpendicular to the original direction.

The research team found that this continuous shrinkage can greatly compress the graphene sheet, reducing it to 40 times. They also said that this method can make some interesting patterns along the surface, such as folds can be superimposed on each other.

Professor Robert Hurt, a professor at Brown University School of Engineering and one of the authors of the paper, said: "When you study deeply, you will find a larger corrugated structure, which contains the small corrugated structure left by the original. 」

This contraction is first clamped, loosened and then clamped; The other is not clamping, then clamping and then releasing, and the two look obviously different. Professor Wang, the correspondent of another paper, said: "The order of operation will determine the final structure." Unlike two times three equals three times two, materials have a' memory', and we will get different results with different shrinkage sorting. 」

The researchers proposed a structural classification from different contraction configurations. Then, they tested several structures to see how they changed the characteristics of graphene layers.

Enhanced characteristics

The researchers found that the wrinkled graphene surface will become super hydrophobic-it can prevent moisture from adhering to the surface. When water touches a hydrophobic surface, it will form water droplets and slide down (when the contact angle between these water droplets and the surface exceeds 160 degrees, which means that very few water droplets can touch the material, the material is considered to be super-hydrophobic). The researchers found that they can make superhydrophobic graphene from three loose shrinkage folds.

The team also found that folding can improve the electrochemical performance of graphene, which is beneficial to the next generation of energy storage and capacity equipment. The results show that the electrochemical current density will increase by 400% if the folded graphene layer is used as the battery electrode compared with the flat graphene layer. The increase of current density will create a more efficient battery.

"You don't need a new material," said Professor Chen. "You just need to wrinkle the graphene. 」

In addition to batteries and waterproof coatings, this compressed graphene may also be suitable for stretchable electronic devices-wearable sensors.

The team plans to continue to use different production methods to fabricate structures on graphene and other nanomaterials.

Professor Wang said: "Not only graphene, but also many new two-dimensional nanomaterials have some interesting properties." "So other materials or composite materials can also form special structures with unexpected functions. 」

This research was supported by the Seed Fund of Brown University. Chen Baoyan was funded by Hibbit Engineering Researcher Program, and they supported excellent postdoctoral researchers to smoothly transition to independent enterprises. Jaskiranjit Soddy, Yang Qiu, Thomas M. Valentine, Ruben Shpits Steinberg and Dr. Wang Zhongying are the co-authors of this paper.