We use 2D sheets as a material platform to study how ions behave under confinement, and how they can be manipulated by coupling other properties of the sheets.
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, the surface charges on the inner walls repel ions of the same charge and attract the counterions, making them the dominating charge carriers. Such unipolar ionic transport can enhance ionic conductivity up to several orders of magnitudes. Nanofluidic materials and devices provide a platform for the manipulation of confined ions and electrolytes. Such studies provide insights for applications including electrochemical energy conversion and storage, biosensing, and water purification.
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, such as those based on graphene materials or clays, can be obtained by exfoliation of from their bulk layered forms, 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).
Compared to other types of nanochannels, these 2D nanofluidic materials have a unique structural feature: Their channel height is uniform throughout the entire volume of the materials, regardless of the size and shape of the macroscopic form of the materials. This provides uniform ionic confinement throughout the body of materials. 2D nanofluidic materials are also easy to fabricate, scale up, and they provide a material platform to study how the optoelectrical responses of the “channel walls” (i.e., the 2D sheets) interact with confined ions.
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