How plasma treatment of a graphene membrane has potential for water desalination

Membranes at the limit

Researchers in the US show how plasma treatment of a graphene membrane has potential for water desalination.

Plasma treatment feature article graphene membrane has potential for water desalination

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plasma treatment of a graphene membrane

Membranes at the limit

Water desalination membranes can be created by etching nanometre-sized pores in a single layer of graphene.

Dong-Yeun Koh and Ryan P. Lively, courtesy of Nature Nanotechnology

The transport of molecules across a nanoporous membrane is the foundation of numerous separation and purification processes. The membranes must be selective by allowing the transport of some molecules, but not others, and the separation process can be improved by reducing the thickness of the membrane. The ultimate limit of this refinement is, of course, a one-atom-thick layer.

Graphene potentially offers such a membrane and the development of scalable methods for synthesizing the material has increased interest in this application. However, a pristine graphene monolayer is impermeable to all atoms and molecules due to its two-dimensional array of tightly packed carbon atoms. To  generate permeability and molecular selectivity, nanoscale defects need to be created in the material. Removing one carbon atom from a graphene lattice (a mono-vacancy) will, for example, form a pore with an area of 2.6 Å2 (ref. 2). By fabricating pores of specific sizes, and with sufficient areal density, it should be possible to develop membranes that offer exquisite molecular sieving properties and ultrahigh molecular fluxes. However, eliminating carbon atoms from graphene in a highly controlled manner is extremely challenging.

Recently, several researchers have begun to explore the potential of graphene membranes with nano- and atomic-scale defects. Selective gas transport through single and multilayered graphene membranes has, for example, been demonstrated. Furthermore, it has been shown that pore sizes in a free-standing graphene layer can be tailored between 1 μm and less than 10 nm (ref. 4). Writing in Nature Nanotechnology, Ivan Vlassiouk, Shannon Mahurin and colleagues now show that nanoporous graphene membranes can be controllably created using a plasma-etching process and the resulting membranes used to desalinate water.

Nanoporous graphene membranes
 Image courtesy of Nature Nanotechnology
 The researchers — who are based at Oak Ridge National Laboratory, New Mexico State University and the University of Tennessee — first synthesized a single layer of graphene using chemical vapour deposition on a copper catalyst. The graphene was then transferred to a silicon nitride wafer with a single 5-μm channel, and nanometre-sized pores were created in the suspended graphene by exposing it to short bursts of oxygen plasma (Fig.1). Finally, the graphene–wafer composite was placed as a barrier between a salt solution and either air or water, and the water transport rates and salt rejection of the membranes measured. With an optimal plasma exposure time, a salt rejection of nearly 100%, combined with a high water flux, could be achieved.

Conventional polymeric membranes for water desalination operate based on a solution–diffusion mechanism in which water molecules sorb into the polymer and then diffuse through vacancies in the polymer network. This solution–diffusion model has also proved effective in describing nanoporous membranes fabricated out of zeolites, metal–organic frameworks and carbon molecular sieves. However, it is not clear how to mechanistically describe permeation in atomically thick nanoporous membranes, as many of the continuum-level assumptions used to describe solution–diffusion  permeation are no longer valid at this thickness limit.

Mahurin and colleagues show that the water permeability of their single-layer graphene membranes is astoundingly high (10–9 mol m m–2 s–1 Pa–1) when water vapour is on the downstream side of the membrane (similar to a pervaporative separation process). The ultrahigh fluxes in this case can most likely be attributed to subnanometre-sized droplet evaporation at the vapour/liquid interface. In the experiments with liquid on the downstream side (similar to standard osmotic processes), the researchers observed water permeabilities (10–14 mol m m–2  s–1  Pa–1) comparable to those of commercial seawater reverse osmosis membranes (1.6 × 10–14 mol m m–2  s–1  Pa–1), while still maintaining high salt rejection. It is also important to note that single-sheet graphene membranes are 250 times thinner than the separating layer in commercial reverse osmosis membranes, so an improvement in water flux of at least 250 times could be expected under the same driving force.

Although research into nanoporous graphene membranes is still in its early stages, the proof-of-concept experiments of Mahurin and colleagues highlight the need for more insight into how to scale such two-dimensional membranes into devices that can offer meaningful water desalination productivities.

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