The strength of 2D materials can prove challenging to harness, despite being among the strongest materials in the world.
Due to its excellent mechanical characteristics, the 2D material which is thinner than the finest onion skin paper has attracted much attention. However, when the materials are stacked in multiple layers, those characteristics disappear, which limits their applicability.
Think graphite pencil. Its core is made of graphite, and graphite is made up of many layers of graphene, which has been found to be the heaviest material in the world. But graphite pencils aren’t strong at all—in fact, graphite is used as a lubricant.
Teng Li, Keystone Professor, Department of Mechanical Engineering, University of Maryland
By meticulously adjusting the molecular structures of 2D polymers known as covalent organic frameworks (COFs), Li and colleagues at Rice University and the University of Houston have devised a method to overcome this obstacle. A recent study released on Proceedings of the National Academy of Sciences detailing the results.
This is a very interesting starting point.
Jun Luo, Professor, Materials Science and NanoEngineering, University of Maryland
The researchers created two COFs with slight structural variations after studying different functional groups—that is, the arrangement of molecular elements—using molecular-level simulations. The behavior of the multilayered COF was then investigated. It turns out that small structural variations lead to very different results.
Like most 2D materials, initial COF displayed only weak interactions between layers, and as more layers were added, strength and flexibility decreased.
The second COF exhibits strong interlayer interactions and retains its good mechanical properties even when multiple layers are added.
Qiyi Fang, Study Co-Lead Author and Doctoral Student, Rice University
The researchers concluded that hydrogen bonding is most likely responsible for this phenomenon.
Study co-author Zhengqian Pang, a UMD post-doctoral researcher and member of Li’s research group, added, “We find from our simulations that the strong interlayer interactions in the second type of COF result from the significantly enhanced hydrogen bonding between its special functional groups.”
The study team then used their results to create a lightweight material that not only has greater strength than steel, but also retains its 2D properties even when stacked into layers.
There are many potential applications.
Lou added, “COF can make excellent filtration membranes, for example. For a filtration system, the structure of the functional groups in the pores will be very important. As you have, say, dirty water flowing through a COF membrane, the functional groups in the pore will just catch the dirt and let the desired molecules through.”
“In this process, the mechanical integrity of the membrane will be very important. Now we have a way to design very strong, highly crack-resistant, multilayer 2D polymers that could be very good candidates for membrane filtration applications.,” he has stated.
He further pointed out, “Another potential application is in upgrading batteries: Replacing the graphite anode with silicon would greatly increase the storage capacity of current lithium-ion battery technology.”
Li said that the research insights could further improve the construction of various materials, such as metals and ceramics. For example, ionic bonds, which develop in ceramics at very high temperatures, make a broken coffee cup difficult to repair.
Similarly, forming metals at high temperatures is required. Similar products could theoretically be produced and refined without increasing heat, thanks to molecular adjustments being investigated by researchers.
Li concluded, “Although the immediate context is 2D materials, more generally we are pioneering ways to exploit the advantageous properties of materials without the constraints present in these materials.”
This study was funded by the Welch Foundation (C-1716), Maryland Advanced Research Computing Center, and the Army Research Laboratory Cooperation Agreement (W911NF-18-2-0062).