Invariance of Water Permeance through Size-Differentiated Graphene Oxide Laminates.

Abstract

Laminates made of graphene oxide nanosheets have been shown to exhibit high water permeance and salt rejection and, therefore, have generated immense interest from the scientific community due to their potential in separation applications. However, there is no clear consensus on the water-transport pathways through such laminates. In this study, we synthesized chemically identical graphene oxide nanosheets with 2 orders of magnitude difference in lateral sizes and measured water permeance through laminates of different thicknesses fabricated by pressure-assisted deposition of these nanosheets. Our results reveal that water permeance through these laminates is nearly the same despite such massive difference in lateral sheet size. Furthermore, we simulated fluid flow through laminates using an interconnected nanochannel network model for comparison with experiments. The simulations in combination with the experimental data show that it is unlikely that the dominant fluid transport pathway is a circuitous, lateral pathway around individual sheets, as has been proposed in some studies. Rather, nonideal factors including trans-sheet flow through pinhole defects in sheet interiors and/or flow-through regions arising from imperfect stacking in the laminates can significantly affect the fluid transport pathways. The presence of such nonidealities is also supported by thickness- and time-dependent measurements of permeance and by infrared spectroscopy, which indicates that water predominantly adopts a bulk-like structure in the laminates. These analyses are significant steps toward understanding water transport through graphene oxide laminates and provide further insight toward the structure of water inside these materials, which could have immense potential in next-generation separation applications.