Graphene & 2D Systems Research @ IIT Madras: Highlights

Research Theme-2: Structure and dynamics of confined water

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* Tailoring Water Transport in Graphene Oxide - Bulk water is an interesting and a not-well-understood system in its own right. The confinement of water further drastically modifies its physical properties. A notable feature is the ability of water confined to nano-capillaries in graphene oxide (GO) membranes to 'super-permeate,' even as all other molecules and ions are completely blocked. GO is thus a prototypical system to investigate biological capillaries, natural systems that transport water molecules without friction. The typical graphene oxide membrane is electrically insulating. While GO is reduced, its water permeation abilities are lost, and at the same time electrical conduction is restored. In our work, we demonstrate a new form of GO which is simultaneously water inhibiting and electrically insulating, with the origin of this tailor-made property relying on its unique dual interlayer spacing. For details read our published article: Kavitha et al 2018 Nanotechnology 29 325706.

* The pathway for water transports through graphene oxide membranes - nanochannels or voids? - Graphene oxide is believed to super-permeate water (introduced as vapor) through a percolating network of sp2 rich nanochannels of spacing 1 nm. Some critics have argued that water mostly permeates through voids and there is no super-permeation. We dwell into this question, by looking at the water permeation through GO when its microstructural disorder is substantially modulated using prolonged humidity exposure. For details read our published article: Pranav et al 2019 Journal of Applied Physics 125 024303.

* The pathway for ion transport through graphene oxide membranes - nanochannels or voids? - Graphene oxide membranes have been proposed in literature as effective ionic and molecular sieve. The permeation of hydrated ions through graphene oxide is believed to happen through the same transport pathways that allow permeation of water. The space needed to accomodate the ions is though a bit larger than that for monolayer of water molecules. The flow of ions through supported, annealed graphene oxide membranes can shed light on relative contributions of two pathways - the ones through nanocapillaries and the other through voids. For details read our published article: Ravishankar et al 2019 Journal of Applied Physics 125 144304.

* Water in confinement - an ellipsometric study - A previous study from our group had focused on the optical properties of graphene oxide in the presence of water intercalation (see Ghosh et al 2015 Applied Physics Letters 106 241902). We demonstrated that the expulsion of interfacial water resulted in abrupt changes of the refractive index of GO across a narrow annealing temperature interval. Is the density of confined water different than that of bulk, is a fascinating question we partly dwelt on.

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