Al³⁺ Modification of Graphene Oxide Membranes: Effect of Al Source

Graphene oxide (GO) membranes are promising materials for water filtration applications due to abundant nanochannels in the membrane structure. Because GO membranes are unstable in water, metal cations such as Al³⁺ are often introduced to the membrane structure to promote cross-linking between individual GO sheets. Here, we describe a simple yet versatile method to incorporate Al³⁺ into GO membranes formed via a slow self-assembly process. Specifically, we directly added aluminum to acidic GO sheet solutions from a variety of sources: Al₂O₃, AlCl₃ and Al foil. Each species reacts differently with water, which can affect the GO solution pH and thus the density of carboxylate groups on the sheet edges available for cross-linking to the Al³⁺ cations. We demonstrate through characterization of the GO sheet solutions as well as the as-formed membranes' morphologies, hydrophobicities, and structures that the extent to which the Al³⁺ cross-links to the GO sheet edges vs. the GO sheet basal planes is dependent on the Al source. Our results indicate that greatest enhancements in the membrane stability occur when electrostatic and coordination interactions between Al³⁺ and the carboxylate groups on the GO sheet edges are more extensive than Al³⁺-π interactions between basal planes.