WATER TRANSPORT MECHANISM IN STACKED-GRAPHENE NANOSHEETS: ROLE OF STRONG COHESIVE INTERACTION AMONG WATER MOLECULES

Abstract

Graphene, a carbon-based 2-dimensional substance, has been considered as an ideal membrane material for purifying water because of its atomic thickness and extraordinary mechanical strength. Based on this outlook, two concepts for graphene-based membranes (nanoporous graphene and stacked graphene membranes), have recently been suggested. To date, computational simulations and actual experiments have proven the excellent performance and potential for graphene-based membranes. However, due to several technological and economic challenges, it has been also shown that the practical application of graphene-based membranes is not straightforward. This review provides highlights of the current status of graphene-based membranes, and then identifies a number of key challenges that need to be resolved prior to the practical implementation of graphene into water treatment systems. Stacked-graphene nanosheets have attracted a great attention as new type of membrane due to its outstanding performance with distinct physicochemical characteristics. Many studies have indicated that size exclusion dominates mass transport in stacked-graphene membrane, but ultrafast water transport behavior has not been completely explained. In this work, diffusive and size-exclusive transport in graphene oxide (GO) membrane were confirmed by testing water and several solvents by hindered diffusion mechanism. More importantly, by observing water transport at varying thickness of GO membrane, we show that collective water transport resulting from the exceptionally high cohesive interaction amongst water molecules is essential for the ultrafast transport of water. The results indicate that, in addition to slip on the smooth graphitic surfaces, physicochemical properties of the permeant molecules that enable strong cohesion are significantly influential in governing the superior performance of stacked-graphene membranes. Nanostructured graphene oxide (GO) membranes offer outstanding mass transport performances such as ultrahigh water flux and precise molecular sieving and thus have a great potential as a novel filtration platform for energy efficient molecular or ionic separation. However, the structural stability of the GO membranes in aqueous environments remains a challenging problem, which limits its practical application perspective. We, here, report novel GO composite membrane composed of GO and asymmetrically functionalized Janus GO (JGO) sheets. Single-side functionalized JGO was made by amidation of dodecylamine. Incorporation of JGO in GO film exhibited extraordinary stability in water at broad pH values even under agitation. Moreover, JGO-doped GO composite membrane showed robust integrity in acid or base solutions over months. It exhibits high molecular retention above 95 % for uncharged and charged dye molecules, rhodamine B and brilliant blue G while maintaining water permeability comparable with previously reported GO-based membranes under osmotic pressure. This work significantly expand possibilities for new way to enhance structural stability in graphene-based or 2D-material membrane and surface-selective functionalization in membrane fabrication for many applications.