New Research Could Spur Broader Use of 2D Materials

They’re considered some of the strongest substances on the planet, but tapping that strength has proved to be a challenge.

2D materials, thinner than the most delicate onion skin paper, have attracted intense interest because of their incredible mechanical properties. Those properties, however, dissipate when they’re stacked in multiple layers, thus limiting their usefulness.

“Think of a graphite pencil,” says Teng Li, Keystone Professor in the Department of Mechanical Engineering. “Graphite is composed of many layers of graphene, which has been found to be the world’s toughest material. Yet a graphite pencil isn’t strong at all—in fact, graphite is even used as a lubricant.”

Now, Li and collaborators at Rice University and the University of Houston have found a way to overcome this barrier, by carefully tweaking the molecular structure of 2D polymers known as known as covalent organic frameworks (COFs) to create stronger interactions between the layers of material. The findings are detailed in a new study published in Proceedings of the National Academy of Sciences.

Using molecular-level simulations, the researchers designed two COFs with minute differences in structure. The first, like most 2D materials, showed only a weak interaction among layers, but the second COF, “exhibits strong interlayer interaction and retains its good mechanical properties even as multiple layers are added,” said Rice University doctoral student Qiyi Fang, a co-lead author of the PNAS paper.

The difference is most likely due to hydrogen bonding. “We found from our simulations that the strong interlayer interactions in the second type of COF result from the significantly enhanced hydrogen bonding among its special functional groups,” said co-lead author Zhengqian Pang, a UMD post-doctoral researcher and a member of Li’s research group.

Applying their findings, the research team produced a lightweight material that not only is several times stronger than steel, but preserves its 2D properties even when stacked into multiple layers.

The potential applications are many. “COFs could make excellent filtration membranes, for instance,” said materials science and nanoengineering Professor Jun Lou, who led Rice’s team. “For a filtration system, the functional group structure at the pore will be very important. ... Now we have a way to design very strong, very fracture-resistant, multilayer 2D polymers that could be very good candidates for membrane filtration applications.”

Another potential application is for upgrading batteries: Replacing the graphite anode with a silicon one would greatly increase the storage capacity of current lithium-ion battery technologies, Lou said.
Insights from the research could also lead to advances in designing a broad range of materials, including ceramics and metals, said Li. Ceramics, for instance, depend on ionic bonding that forms at very high temperatures, which is why a broken coffee mug can’t be easily fixed. Metals, likewise, require forging at high temperatures. With the molecular tweaking being explored by the researchers, similar products could conceivably be manufactured and repaired without turning up the heat.

“Although the immediate context is 2D materials, more generally we’re pioneering ways to exploit the advantageous properties of materials without the constraints these materials present,” Li said.