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


he 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 material1 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 atomicscale defects. Selective gas transport through single and multilayered graphene membranes has, for example, been demonstrated3. 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 5. 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

O2 plasma

Water flux Pore generation

Salt rejection

Figure 1 | Nanoporous graphene membranes for water desalination. Pores can be created in a single layer of graphene by exposing it to oxygen plasma for short periods of time. The subnanometre pores then readily permit transmembrane water flux, while largely rejecting hydrated ions.

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


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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. To achieve this, several challenges will have to be addressed. First, maintaining the mechanical stability of the membranes when faced with the 385

news & views high shear rates that are typical in water purification devices will be an issue, and is likely to require large, continuous sheets of graphene that can be readily sealed into a membrane module. Supporting such large sheets and scalably creating nanopores in the graphene will also be challenges. Furthermore, ever-present issues in water purification processes are membrane fouling and concentration polarization, and this will be no different for graphene-

based membranes. Indeed, Mahurin and colleagues hypothesize that in their osmotic permeation experiments some of the graphene nanopores might be blocked by salt ions, which could explain the relatively low water permeabilities they observe under those conditions. Despite the many challenges facing such membranes, the work of Mahurin and colleagues suggests that practical, atomically thick membranes could, in the future, become a reality. ❐

Dong-Yeun Koh and Ryan P. Lively are at the School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332-0100, USA. e-mail: [email protected] References 1. 2. 3. 4. 5.

Li, X. et al. Science 324, 1312–1314 (2009). O’Hern, S. C. et al. Nano Lett. 14, 1234–1241 (2014). Kim, H. W. et al. Science 342, 91–95 (2013). Celebi, K. et al. Science 344, 289–292 (2014). Surwade, S. P. et al. Nature Nanotech. 10, 459–464 (2015).


Tomography for plasmonics

Two-dimensional cathodoluminescence projections are used to reconstruct the plasmonic excitation of nanoscale crescents by tomography.

Marek Malac


omography is an imaging technique that allows the reconstruction of a three-dimensional object from a collection of two-dimensional projection images. Images of almost any type can be


used as long as the relationship between the two-dimensional projections and the object properties are known and satisfy the projection theorem: the image contrast should vary linearly with the property


Polystyrene Gold


Far-field cathodoluminescence


Figure 1 | Cathodoluminescence signal collection in a scanning electron microscope. A focused electron beam (red) is stepped over a nano-crescent made of a polystyrene core and gold shell. The light (gamma) generated by the incident electron beam at each position is linked to the plasmonic excitations at that location and can be collected by a spectrometer. The nano-crescents studied by Atre and colleagues are rotationally symmetric around the z axis. The symmetry reduces the requirements for the number of projections needed to reconstruct a three-dimensional representation of the plasmonic modes. For objects where the mutual orientation of the electron beam and the object has no effect, a single projection image is sufficient to generate a virtual tilt series that can be used to reconstruct the object in three dimensions. For plasmons, the mutual orientation of the beam and the excited object needs to be taken into account, in principle requiring a large set of projections. Nevertheless, Atre and colleagues achieve a good agreement between simulations and the plasmonic excitation map obtained from only seven projections. Image of prism reproduced with permission: © dip2000/Thinkstock. 386

of interest of the sample. Because of its generality, tomography, envisioned by Johan Radon in 1917, has been widely used, from probing the internal structure of the Earth1 to imaging the internal organs of living organisms. Now, writing in Nature Nanotechnology Ashwin Atre and co-workers from Stanford University and the FOM Institute AMOLF in the Netherlands show that tomography can be utilized to image plasmons in nanoscale objects using two-dimensional cathodoluminescence projections2. Imaging plasmon modes at the nanoscale is extremely challenging because the physical dimensions of the objects are much smaller than the wavelength of the light coupling to them. Researchers, therefore, have tried to use shorter-wavelength radiation, such as electron beams as used in cathodoluminescence (CL) and electron energy-loss spectroscopy (EELS)3. In CL imaging, a small electron beam probe is placed at a known location within the object; the electron beam excites the sample and the light emanating from the object is then detected in the far field, as shown in Fig. 1. EELS, on the other hand, analyses the energy of the electrons that pass through the sample offering good spectral and spatial resolution, as well as good collection efficiency 4. Detecting the emitted light by CL has the advantage that the spectral resolution usually exceeds that achievable by analysing transmitted electrons. The fact that EELS, unlike CL, probes all excitations means that radiative (light-emitting) and nonradiative modes cannot be discriminated. Comparing CL and EELS spectra offers this


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Nanoporous graphene: Membranes at the limit.

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