On the origin of the stability of graphene oxide membranes in water

Graphene oxide (GO) films are known to be highly stable in water and this property has made their use in membrane applications in solution possible. However, this state of affairs is somewhat counterintuitive because GO sheets become negatively charged on hydration and the membrane should disintegrate owing to electrostatic repulsion. We have now discovered a long-overlooked reason behind this apparent contradiction. Our findings show that neat GO membranes do, indeed, readily disintegrate in water, but the films become stable if they are crosslinked by multivalent cationic metal contaminants. Such metal contaminants can be introduced unintentionally during the synthesis and processing of GO, most notably on filtration with anodized aluminium oxide filter discs that corrode to release significant amounts of aluminium ions. This finding has wide implications in interpreting the processing-structure-property relationships of GO and other lamellar membranes. We also discuss strategies to avoid and mitigate metal contamination and demonstrate that this effect can be exploited to synthesize new membrane materials.