Undesirable Defects of Graphene Oxide Provide Remarkable Mechanical Properties

Researchers at Northwestern University have discovered that the inherent defects of graphene oxide provide the material surprising mechanical properties.

“Graphene is so perfect,” said Northwestern Engineering’s Jiaxing Huang. “And graphene oxide is more defective, so it’s like the weaker, less exciting version of graphene.”

Graphene has many extraordinary properties, which have caught the attention of engineers and scientists. However, graphene oxide has been considered an inferior version of graphene till now.

The research team was led by Horacio Espinosa, the James N. and Nancy J. Farley Professor in Manufacturing and Entrepreneurship at Northwestern’s McCormick School of Engineering. They employed a novel modeling and experimentation method to study graphene oxide at the atomic level, and the discoveries they made could pave the way for scaling up the utilization of graphene oxide.

“Our team discovered that graphene oxide exhibits remarkable plastic deformation before breaking,” said Espinosa. “Graphene is very strong, but it can break suddenly. We found that graphene oxide, however, will deform first before eventually breaking.”

This difference in the properties of these two materials could be compared to other common objects. “Ceramic is strong,” he says, “but if you break it, it will shatter. Now if a plastic cup is squeezed, it will bend before it snaps.”

An unusual mechanochemical reaction was found to be the cause of the plasticity in graphene oxide. Graphene oxide is in the form of an epoxide that is made up of one oxygen atom and two carbon atoms. This can be considered to be in the form of a triangle that is made up of two carbon atoms at the base, having a single oxygen atom at the top. When the bonds of the epoxide are broken chemically, the carbon-carbon bond remains intact, while the carbon-oxygen bonds break. However, application of a mechanical force to the graphene oxide makes the carbon-carbon bond to break, while the carbon-oxygen bond remains in its state.

“We uncovered this surprise on the atomic scale,” said SonBinh T. Nguyen, professor of chemistry in the Weinberg College of Arts and Sciences. “This is completely different than what occurs in other materials and a very unusual property for the graphene oxide sheet.”

The graphene oxide’s material properties could be tuned when the functionality of the material is known at the atomic scale. In order to scale up the material, the researchers are expanding their study to understand the mechanical properties of the interfaces of graphene oxide-polymers.

“Our studies imply that the answers to scaling up graphene oxide may lie, in part, to the chemistry at the atomic level,” Espinosa said. “With more information obtained at different length scales as well as advances in synthesis methods, we will eventually piece the puzzle together.”

This study paper has been published in Nature Communications. The National Science Foundation’s Designing Materials to Revolutionize and Engineer Our Future program, and the Army Research Office have provided support for this study.

Espinosa, Jiaxing Huang, and Nguyen are the co-principal investigators of the study paper. The co-first authors are Xiaoding Wei, a postdoctoral fellow; and Rafael A. Soler-Crespo and Lily Mao, graduate students.