PerspectiveIonic Transport

Two-dimensional nanofluidics

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Science  25 Mar 2016:
Vol. 351, Issue 6280, pp. 1395-1396
DOI: 10.1126/science.aaf5289

The remarkable electronic properties of graphene and related two-dimensional (2D) materials result from the confinement of electrons within the material. Similarly, the interstitial space between 2D materials can enable the 2D confinement of ions and electrolytes and alter their transport. Many different 2D sheets can be obtained by exfoliation of natural layered materials (1), and an exfoliation-reconstruction strategy can convert powders of layered materials into continuous, robust bulk forms in which lamellar nanochannels occupy a substantial volume fraction (up to several tens of percent). Nanofluidics, which enables the manipulation of confined ions and electrolytes, has applications in electrochemical energy conversion and storage, biosensing, and water purification.

Confining ion flow.

(A) Lamellar f lm with massive arrays of 2D nanofluidic channels can be made by the exfoliation-reconstruction approach, as illustrated with models of graphene oxide (GO) sheets that are terminated with negatively charged carboxyl groups. (B) Debye layers of neighboring sheets overlap to create unipolar 2D ion channels with greatly enhanced cation conductivity.

ILLUSTRATION: P. HUEY/SCIENCE

Electrolytes exhibit drastically different properties when confined in nanochannels. For example, in bulk solution, cations and anions simultaneously move along opposite directions to generate ionic current. However, in channels narrower than the Debye length of the electrolyte (a measure of how far electrostatic effects can persist), the surface charges on the inner walls repel ions of the same charge and attract the counterions, making them the dominating charge carriers (2). Such unipolar ionic transport can enhance ionic conductivity up to several orders of magnitude. Nanochannels that enable such transport can be fabricated in bulk materials, but such “topdown” methods are rather expensive and difficult to scale up. The construction of nanofluidic channels with the 2D sheets, a “bottom-up” approach, can be done simply by casting or filtration of nanosheets dispersed in solution.

The surface properties and the spacing of the 2D nanochannels can be conveniently controlled by modifying the starting sheets. No matter how electrolytes pass through the film, horizontally or vertically, they flow through the same set of 2D channels, with the only difference being flux (see the figure, panel A). To create a robust film with uniform lamellar spacing, the 2D building blocks should have uniform thickness and high aspect ratios.

A number of 2D materials are already available for this purpose. For example, filtration of graphene oxide (GO) sheets (graphene functionalized with oxygenated groups, such as carboxylates) leads to films with interlayer spacing of ∼1 nm that can be tuned by changing the degree of hydration. Anions are excluded by the negatively charged sheets, enabling cation transport controlled by surface charge (see the figure, panel B) (3). Guo et al. later demonstrated that mechanically pumping electrolyte through such 2D channels resulted in unipolar flow of cations, generating electrical current on the order of 0.1 A m−2 bar−1 (4). Two-dimensional nanochannels constructed with vermiculite clay walls supported high proton conductivities approaching those of proton-transport polymeric membranes. The vermiculite channels have extraordinary thermal stability and retain their functions even after baking at 500°C in air (5).

Packing defects such as voids and dislocations are to be expected in the lamellar films made of irregularly shaped sheets with polydisperse sizes (see the figure, panel A). The effective total volume of the 2D nanochannels can be estimated by comparing the value of ionic conductance to that of a bulk channel. Recent work by Cheng et al. aims to quantify the impact of these defects on ionic transport (6). Their continuum ion diffusion simulations treat a GO film as a stack of sheets with uniform size and few-nanometer-sized pinholes. This simple model of the nanofluidic channel walls manages to reproduce the unipolar transport observed in experiments, as well as the influence of the membrane's physical parameters on ionic transport. Experimentally, solvent exchange allows the interlayer spacing of the film to be continuously tuned, and results from the corresponding ionic conductivity measurements converged on the model's prediction. Interlayer spacing critically influences the behaviors of confined ions and can be affected by local stacking defects, so it will be useful to adopt a scanning type of x-ray diffraction technique to map the spacing over entire films.

Enhanced ionic conductivity through 2D nanofluidic membranes can be used to create electrochemical devices, especially for those with in-plane geometry (7). It is also very attractive for designing new ion-selective membranes, potentially allowing new applications under unprecedentedly extreme conditions (5). Here, the remaining challenge is to significantly increase the cross-membrane flux, perhaps by realigning the horizontal nanochannels toward the vertical direction without losing the film's structural integrity. New assembly strategies will be needed to create robust bulk lamellar materials with tunable channel orientations.

References and Notes

Acknowledgments: Seed support from Northwestern MRSEC (NSF DMR-1121262) and the Office of Naval Research (ONRN000141310556).
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