Rice University researchers discovered how 2D materials flow in liquid, potentially enabling scientists to create macroscopic-scale materials with similar properties.
Two-dimensional nanomaterials are extremely thin ⎯ only several atoms thick ⎯ sheet-shaped materials. They behave very differently from materials we’re used to in daily life and can have really useful properties: They can withstand a lot of force, resist high temperatures and so on. To take advantage of these unique properties, we have to find ways to turn them into larger-scale materials like films and fibers.
Utana Umezaki, Study Lead Author and Graduate Student, Rice University
Sheets of 2D materials must be correctly aligned to preserve their unique features in bulk form, a procedure that frequently takes place during the solution phase. Researchers at Rice University concentrated on two materials: hexagonal boron nitride, which has a structure similar to graphene but is built of boron and nitrogen atoms, and graphene, which is composed of carbon atoms.
We were particularly interested in hexagonal boron nitride, which is sometimes called ‘white graphene’ and which, unlike graphene, doesn’t conduct electricity but has high tensile strength and is chemically resistant. One of the things that we realized is that the diffusion of hexagonal boron nitride in solution was not very well understood. In fact, when we consulted the literature, we found that the same was true for graphene. We couldn’t find an account of diffusion dynamics at the single molecule level for these materials, which is what motivated us to tackle this problem.
Angel Martí, Professor, Chemistry, Bioengineering, Materials Science and Nanoengineering, Rice University
The researchers utilized a fluorescent surfactant, such as glowing soap, to label the nanomaterial samples and make their movement apparent. Researchers could track the trajectories of the samples and discover the link between their size and how they move by watching videos of their motion.
Umezaki added, “From our observation, we found an interesting trend between the speed of their movement and their size. We could express the trend with a relatively simple equation, which means we can predict the movement mathematically.”
Graphene was shown to move slower in liquid solution, probably because its layers are thinner and more flexible than hexagonal boron nitride, resulting in greater friction. Researchers believe that the formula generated from the experiment might be used to describe how other 2D materials move in similar situations.
Martí stated, “Understanding how diffusion in a confined environment works for these materials is important because ⎯ if we want to make fibers, for example ⎯ we extrude these materials through very thin injectors or spinnerets. So this is the first step toward understanding how these materials start to assemble and behave when they are in this confined environment.”
As one of the first studies to analyze the hydrodynamics of 2D nanosheet materials, this study fills a gap in the field and could prove useful in addressing 2D material manufacturing problems.
“Our final objective with studying these building blocks is to be able to generate macroscopic materials,” Martí further stated.
The study’s corresponding authors are Anatoly Kolomeisky, a Rice professor of chemistry and chemical and biomolecular engineering, and Matteo Pasquali, the A.J. Hartsook Professor of Chemical and Biomolecular Engineering as well as a professor of chemistry, materials science, and nanotechnology.
The research was funded by the National Science Foundation (1807737, 2108838), the Air Force Office of Scientific Research (FA9550-19-1-7045), and the Welch Foundation (C-2152, C-1668, C-1559).
Umezaki, U., et. al. (2024) Brownian Diffusion of Hexagonal Boron Nitride Nanosheets and Graphene in Two Dimensions. ACS Nano. doi:10.1021/acsnano.3c11053.