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Posted: Apr 23, 2014
Filters made of graphene - the thinnest feasible membrane
(Nanowerk News) Researchers led by Hyung Gyu Park of the Department of Mechanical and Process Engineering at ETH Zurich have produced a stable porous membrane that is thinner than a nanometer. This is a 100,000 times thinner than the diameter of a human hair. The membrane consists of two layers of the much exalted graphene, a two-dimensional honeycomb-like film made of carbon atoms, through which the researchers shot tiny pores of a precisely defined size. The membrane can thus permeate tiny molecules. Larger molecules or particles, on the other hand, 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 PhD student Jakob Buchheim, one of the two lead authors of the study, which has just been published in the journal Science ("Ultimate Permeation Across Atomically Thin Porous Graphene").
Separation of large molecules (white) from small molecules (blue) by a porous graphene membrane (grey). (Illustration: ETH Zurich)
Better than Goretex
The ultra-thin graphene membrane may one day be used for a range of different purposes, including waterproof clothing. “Our membrane is not only very light and flexible, but it is also a thousand times more breathable than Goretex,” says Kemal Celebi, a postdoc in Park’s laboratory and also one of the lead authors of the study. The membrane could also potentially be used to separate gaseous mixtures into their constituents or to filter impurities from fluids. This is because by perforating the graphene layer regularly and consistently the researchers were able to demonstrate for the first time ever that graphene membranes are suitable for water filtration: the thinnest feasible membrane is able to withstand pressure gradients of up to 2 atmospheres.
Especially remarkable is the large amounts of gas that can pass through the membrane. This feature is based on the effect that a molecule passing through the pore does not pass through a “tunnel” – as it would do with regular microfilters. It rather passes through a “door”. So there is less friction at the wall resulting in a much smaller “traffic jam” while passing through the pore. Nevertheless it is still possible to separate larger molecules from smaller ones, especially when the pores are tiny. This means porous graphene could be used to purify natural gas or other gaseous bulk materials from impurities.
Breakthrough in nanofabrication
The researchers not only succeeded in producing the starting material, a double-layer graphene thin film, with a high level of purity; they also mastered a technique called focused ion beam (FIB) milling to shoot pores into the graphene layer. In this process, 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 desired number and size into the graphene with unprecedented precision. This process, which would otherwise take days to complete, took only a few hours using the FIB technology. “This is a breakthrough that enables the nanofabrication of porous graphene membranes,” explains Empa scientist and FIB expert Ivan Shorubalko who also contributed to the study.
Scanning electron microscope (SEM) image of a perforated graphene membrane. (Image: Empa)
In order to achieve this level of precision, the researchers had to work with double-layered graphene. “It wouldn’t have been possible to create such a membrane with our technology from a single layer because graphene in practice isn't perfect,” says Park. The material can exhibit certain irregularities in the honeycomb structure. Now and again, individual carbon atoms are missing, which not only impairs the stability of the material but also makes it impossible to etch a high-precision pore at the site of such a defect. The researchers solved this problem by laying two layers of graphene on top of each other. The probability of two defects settling directly above one another is extremely low, explains Park.
Fastest possible filtration rate
A key advantage of the tiny dimensions is that the thinner a membrane, the lower its permeation resistance. And 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,” says Celebi. This should speed up filtration enormously, the researchers hope.
However, before these applications 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 investigate the fundamental science, the researchers worked with tiny pieces of membrane with a surface area of less than one hundredth of a square millimeter. The first goal now is to produce larger membranes and study various filtration mechanisms.