Swelling of Graphene Oxide Membranes in Aqueous Solution: Characterization of Interlayer Spacing and Insight into Water Transport Mechanisms

Abstract

Graphene oxide (GO) has recently emerged as a promising 2D nanomaterial to make high-performance membranes for important applications. However, the aqueous-phase separation capability of a layer-stacked GO membrane can be significantly limited by its natural tendency to swell, that is, absorb water into the GO channel and form an enlarged interlayer spacing (d-spacing). In this study, the d-spacing of a GO membrane in an aqueous environment was experimentally characterized using an integrated quartz crystal microbalance with dissipation and ellipsometry. This method can accurately quantify a d-spacing in liquid and well beyond the typical measurement limit of ∼2 nm. Molecular simulations were conducted to fundamentally understand the structure and mobility of water in the GO channel, and a theoretical model was developed to predict the d-spacing. It was found that, as a dry GO membrane was soaked in water, it initially maintained a d-spacing of 0.76 nm, and water molecules in the GO channel formed a semiordered network with a density 30% higher than that of bulk water but 20% lower than that of the rhombus-shaped water network formed in a graphene channel. The corresponding mobility of water in the GO channel was much lower than in the graphene channel, where water exhibited almost the same mobility as in the bulk. As the GO membrane remained in water, its d-spacing increased and reached 6 to 7 nm at equilibrium. In comparison, the d-spacing of a GO membrane in NaCl and NaSO solutions decreased as the ionic strength increased and was ∼2 nm at 100 mM.

2D Nanosheets and Their Composite Membranes for Water, Gas, and Ion Separation

“…These studies focused on the mechanical properties of the GO materials and showed that the integration improved the connectivity of the GO nanosheets, which led to an increase in both tensile strength and toughness.C ovalent crosslinking by esterification using dicarboxylic acids could control the physical properties of the nanosheet membranes ( Figure 2a). [14] Covalent crosslinking through crosslinkers containing hydroxyl groups resulted in ar egular interspacing of 0.76 nm, with less swelling tendency,d epending on the type of crosslinker.T he crosslinking of GO nanosheets also increased the elastic moduli from 0.03 to 3.49 GPa. Thei nterlayer spacing of pristine GO is about 0.69 nm but is increased to 6-7nminthe aqueous state due to its high swelling tendency.…”

“…[1] Nanosheet membranes were first used in ion separation applications for water or liquid treatment processes such as nanofiltration and ultrafiltration. [14] Preventing their swelling during the separation process and maintaining high membrane performance are the key issues for graphene-based membranes. [14] Preventing their swelling during the separation process and maintaining high membrane performance are the key issues for graphene-based membranes.…”

Precisely Controlling Nanochannels of Graphene Oxide Membranes through Lignin‐Based Cation Decoration for Dehydration of Biofuels

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Lignin-based cations introduced into graphene oxide (GO) have been found to bring about stabilization of the nanostructure and the active sites and to give rise to various interactions for subsequent modification with polyelectrolyte and nanospacers,w ith av iew to precisely controlling the nanochannels of the GO-based membranes.T he resulting membranes exhibited excellent performance in biofuel dehydration with water flux of 4000-6000 gm À2 h À1 ,w hich exceeds that of the stateof-the-art polymericand GO-based membranes.

Graphene-based membranes have emergedo ver the past few years as promising materials for diverse separation applications, such as water purification, solvent dehydration, and gas separation. [4] However, GO membranes can swell or even crack when immersed in aqueous solutions, [5] whereupon the stacked nanochannels are disrupted and the practical performance is significantly compromised. [2] Grapheneo xide (GO) has been the most frequently employed derivativeo fg raphene for developing such membranes, owing to its abundant oxygenated groups, [3] which enable its dispersion in water and greatly improvet he possibilities for assembly of membrane nanostructures, including laminates and hybrids.

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“…[1] Atomically thin graphene nanosheets endow membranes with at hinner active layer and sub-nanometersized pores, affording greater permeation flux and more-selective moleculart ransport, respectively. [4] However, GO membranes can swell or even crack when immersed in aqueous solutions, [5] whereupon the stacked nanochannels are disrupted and the practical performance is significantly compromised. [4] However, GO membranes can swell or even crack when immersed in aqueous solutions, [5] whereupon the stacked nanochannels are disrupted and the practical performance is significantly compromised.…”

Factors affecting the separation performance of graphene oxide membranes: mechanical support, properties of graphene oxide, and exotic species

“…Benefiting from the extraordinary physicochemical properties of GO nanosheets, the GO membrane has advantages in terms of its tunable separation performance, potentially ultrafast water transport, and high chemical activities to be functionalized . Unlike the tortuous cylindrical pores in the traditional polymeric membranes, separation by the GO membranes primarily relies on the ideally horizontal 2D nanocapillaries formed by the tightly stacked GO nanosheets . The protruding oxygen‐containing functional groups keep apart the adjacent GO nanosheets to form nanocapillaries suitable for separation at the NF level .…”