One of the thinnest membranes in the world is picky about what molecules it lets through.
Researchers were able to build a "graphene oxide membrane less than [two] nanometers thick with high permeation selectivity between hydrogen and carbon dioxide gas molecules," a University of South Carolina press release reported.
The membrane's distinguishes between different molecules by determining their size. Hydrogen and helium may pass freely through the ultra-thin membrane, but "carbon dioxide, oxygen, nitrogen, carbon monoxide and methane," move through much more slowly.
"The hydrogen kinetic diameter is 0.289 nm, and carbon dioxide is 0.33 nm. The difference in size is very small, only 0.04 nm, but the difference in permeation is quite large" Miao Yu, a chemical engineer in USC's College of Engineering and Computing who led the research team, said. "The membrane behaves like a sieve. Bigger molecules cannot go through, but smaller molecules can."
The membrane is composed of an "overlapping mosaic of graphene oxide flakes." The researchers said it was similar to covering a table with playing cards, but on a molecular scale. This feat is extremely difficult because if any of the "table" (which is actually a porous aluminum oxide support) is left uncovered it could allow "leaks."
Preventing leaks is why past membranes have been considerably thicker. "At least 20 nm, and usually thicker," Miao Yu said.
The research team found a way to solve the problem of "inter-flake" leaks.
"They dispersed graphene oxide flakes, which are highly heterogeneous mixtures when prepared with current methods, in water and used sonication and centrifugation techniques to prepare a dilute, homogeneous slurry. These flakes were then laid down on the support by simple filtration," the press release reported.
Through this method, the team was able to create a membrane that was only 1.8-nm-thick that would only allow gas molecules to pass into the graphene.
"They found by atomic force microscopy that a single graphene oxide flake had a thickness of approximately 0.7 nm. Thus, the 1.8-nm-thick membrane on aluminum oxide is only a few molecular layers thick, with molecular defects within the graphene oxide that are essentially uniform and just a little too small to let carbon dioxide through easily," the team reported.
The finding could be used to separate climate-change-causing carbon dioxide from other gases, which could greatly benefit environmental research.
