PerspectiveMaterials Science

Graphene Oxide Membranes for Ionic and Molecular Sieving

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Science  14 Feb 2014:
Vol. 343, Issue 6172, pp. 740-742
DOI: 10.1126/science.1250247

Ionic and molecular sieving membranes that enable fast solute separations from aqueous solutions are essential for processes such as water purification and desalination, sensing, and energy production (13). The two-dimensional structure and tunable physicochemical properties of graphene oxide (GO) offer an exciting opportunity to make a fundamentally new class of sieving membranes by stacking GO nanosheets (46). In the layered GO membrane, water molecules permeate through the interconnected nanochannels formed between GO nanosheets and follow a tortuous path primarily over the hydrophobic nonoxidized surface rather than the hydrophilic oxidized region of GO (7). The nearly frictionless surface of the non-oxidized GO facilitates the extremely fast flow of water molecules (5). On page 752 of this issue, Joshi et al. (8) further report that ions smaller in size than the GO nanochannel can permeate in the GO membrane at a speed orders of magnitude faster than would occur through simple diffusion. Size exclusion appears to be the dominant sieving mechanism.

When dry, GO membranes made by vacuum filtration can be so tightly packed (with a void spacing of ∼0.3 nm between GO nanosheets) that only water vapor aligned in a monolayer can permeate through the nanochannel (5). Joshi et al. found that when such a GO membrane was immersed in an ionic solution, hydration increased the GO spacing to ∼0.9 nm (8). Any ion or molecule with a hydrated radius of 0.45 nm or less could enter the nanochannel, but all larger-sized species were blocked (see the figure).

Such a sharp size cutoff by the GO membrane has important implications in a myriad of separation applications. By adjusting the GO spacing through sandwiching appropriately sized spacers between GO nanosheets, a broad spectrum of GO membranes could be made, each being able to precisely separate target ions and molecules within a specific size range from bulk solution. Compared with the typically wide pore-size distribution of commonly used polymeric membranes, the narrow channel-size distribution of GO membranes is truly advantageous for precise sieving.

The hydration of GO in aqueous solution, however, makes it more challenging to manipulate the GO spacing within a subnanometer range than to enlarge it. For example, desalination requires that the GO spacing should be less than 0.7 nm to sieve the hydrated Na+ (with a hydrated radius of 0.36 nm) from water. Such small spacing could be obtained by partially reducing GO to decrease the size of hydrated functional groups or by covalently bonding the stacked GO nanosheets with small-sized molecules to overcome the hydration force.

In contrast, an enlarged GO spacing (1 to 2 nm) can be conveniently achieved by inserting large, rigid chemical groups (6) or soft polymer chains (e.g., polyelectrolytes) between GO nanosheets, resulting in GO membranes ideal for applications in water purification, wastewater reuse, and pharmaceutical and fuel separation. If even larger-sized nanoparticles or nanofibers are used as spacers, GO membranes with more than 2-nm spacing may be produced for possible use in biomedical applications (e.g., artificial kidneys and dialysis) that require precise separation of large biomolecules and small waste molecules.

GO membranes.

(A) Water and small-sized ions and molecules (compared with the void spacing between stacked GO nanosheets) permeate superfast in the GO membrane, but larger species are blocked. (B) The separation capability of the GO membrane is tunable by adjusting the nanochannel size. (C) Several methods for the synthesis of GO membranes have been reported or are envisioned; GO nanosheets can be physically packed by vacuum filtration (options 1 to 3), or they can be stabilized by covalent bonds, electrostatic forces, or both (options 4 to 6) during layer-by-layer assembly.

GO membranes can be synthesized either by vacuum filtration or by layer-by-layer (LbL) assembly, both of which are conducted in aqueous solution without any organic solvent involved and, hence, are more environmentally friendly. The GO membranes prepared by vacuum filtration, either from a pure GO solution or a mixture of GO and spacers, might lack sufficient bonding between GO nanosheets. Because of the high hydrophilicity of GO, these membranes are likely to disperse in water, especially under cross-flow conditions typically encountered in membrane operations. In contrast, the LbL method is ideal for introducing an interlayer stabilizing force via covalent bonding (6), electrostatic interaction, or both effects during layer deposition.

The GO membrane thickness can be readily controlled by varying the number of LbL deposition cycles. Theoretically, as few as two stacked GO layers would be needed to create a sieving channel. In reality, however, deposition of additional GO layers is warranted to counteract the detrimental effects of possible defects and nonuniform deposition of GO nanosheets on the membrane's sieving capability. Finally, the LbL synthesis of GO membranes is highly scalable and cost-effective, unlike the challenging synthesis of monolayer graphene membranes, which requires the manufacturing of large-sized graphene sheets and the punching of nanopores with a narrow size distribution (9).

Indeed, the GO membrane represents a next generation of ultrathin, high-flux, and energy-efficient membranes for precise ionic and molecular sieving in aqueous solution, with applications in numerous important fields. Future research is needed to understand thoroughly the transport of water and solutes in the GO membrane, especially to fundamentally elucidate other potential separation mechanisms (e.g., charge and adsorption effects) in addition to size exclusion. More research is also needed to address specific issues concerning various exciting yet challenging applications in desalination, hydrofracking water treatment, and energy production, as well as in biomedical and pharmaceutical fields. Other largely unexplored areas include making multifunctional GO membranes with exceptional antifouling, adsorptive, antimicrobial, and photocatalytic properties.

References and Notes

  1. Acknowledgments: Supported by NSF Awards CBET 1154572 and 1158601.

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