A stable, porous graphene nano-membrane thinner than a nanometre has been produced by researchers at ETH Zurich. That’s 100,000 times thinner than a human hair.
The new membrane opens the door to a new generation of waterproof clothing, and also to ultra-rapid filtration.
Made of two layers of graphene, a two-dimensional film made of carbon atoms, on which is etched tiny pores of a precisely defined size, it is permeable to tiny molecules, but larger molecules or particles can pass only slowly or not at all.
“With a thickness of just two carbon atoms, this is the thinnest porous membrane that is technologically possible to make,”
says co-lead author Jakob Buchheim.
Potential Uses of Graphene Nano-membrane
“Our membrane is not only very light and flexible, but it is also a thousandfold more breathable than Goretex,”
says Kemal Celebi, one of the lead authors of the study.
The membrane could also potentially be used to separate gaseous mixtures into their constituent parts or to filter impurities from fluids. Researchers were able to show for the first time that graphene membranes could be suitable for water filtration.
They also envision a potential use for the membrane in devices used for accurate measurement of gas and fluid flow rates that are crucial to unveiling the physics around mass transfer at nanoscales and separation of chemical mixtures.
Focused Ion Beam Milling
Not only did the researchers produce the starting material, a double-layer graphene film with a high level of purity, but they also mastered a technique called focused ion beam milling to etch pores into the graphene film.
In focused ion beam milling, which is also used in the production of semiconductors, a beam of helium or gallium ions is controlled with a high level of precision in order to etch away material.
The researchers were able to etch pores of a specified number and size into the graphene with unprecedented precision. This process, which could easily take days to complete, took only a few hours in the current work.
“This is a breakthrough that enables the nanofabrication of the porous graphene membranes,”
explains Empa scientist Ivan Shorubalko, who also contributed to the study.
High Precision Fabrication
In order to accomplish this level of precision, the researchers had to work with double-layer graphene.
“It wouldn’t have been possible for this method to create such a membrane with only one layer because graphene in practice isn’t perfect,”
says team leader Hyung Gyu Park.
The material can exhibit certain irregularities in the honeycomb structure of the carbon atoms. Now and again, individual atoms are missing from the structure, which not only impairs the stability of the material but also makes it impossible to etch a high-precision pore onto such a defect.
The researchers solved this problem by laying two graphene layers on top of each other. The probability of two defects settling directly above one another is extremely low, said Park.
Maximal Permeation
One major advantage of the minuscule dimensions involved is that the thinner a membrane, the lower its permeation resistance. The lower the resistance, the higher the energy efficiency of the filtration process.
“With such atomically thin membranes we can reach maximal permeation for a membrane of a given pore size and we believe that they allow the fastest feasible rate of permeation,”
says Celebi.
Before they are ready for use on an industrial scale or for the production of functional waterproof clothing, the manufacturing process needs to be further developed.
To examine the fundamental science, the researchers worked with tiny pieces of the membrane with a surface area of less than one-hundredth of a square millimeter. Objectives going forward will be to produce larger membrane surfaces and impose various filtering mechanisms.
Reference: K. Celebi, J. Buchheim, R. M. Wyss, A. Droudian, P. Gasser, I. Shorubalko, J.-I. Kye, C. Lee, H. G. Park. Ultimate Permeation Across Atomically Thin Porous Graphene. Science, 2014; 344 (6181): 289 DOI: 10.1126/science.1249097
