Selective Ion Transport through Functionalized Graphene Membranes Based on Delicate Ion-Graphene Interactions

Recently, graphene oxide (GO) membranes have been reported with the ability to separate different solutes in aqueous suspensions by a molecular sieving effect. On the other hand, we propose that the chemical interactions between ions and GO membranes might also take effect in selective ion transmembrane transportation. In this paper, on the basis of the permeation of Cu2+ and Mg2+ sources through hydroxyl-, carboxyl-, and amino-functionalized graphene membranes, the delicate ion graphene interactions which might be responsible for the selective ion permeation are investigated. We demonstrate experimentally that the coordination between transition-metal cations and carboxyl fiinctionalities and the cation-pi interactions between main-group cations and sp(2) regions are responsible for the selective transport of small ions through graphene-based membranes, which is beyond the scope of molecular sieving effect proposed previously. Notably, by grafting amino groups onto the graphene basal planes, the permeations of Cu2+ and Mg2+ cations are both weakened. These results not only throw light upon the mechanism for the selective ion permeation through graphene-based membranes but also lay a foundation for the separation of target ions by grafting specific functionalities.