Graphene oxide is a promising, emerging separation material, as it is durable, easily dispersible in water, and has shown impressive adsorption potential in bulk studies. Currently, there is limited information about specific graphene oxide-metal ion interactions at the interface, because it is experimentally challenging to observe interfacial phenomena without being overwhelmed by the surrounding bulk. However, a mechanistic understanding of ion adsorption at the interface is critical toward developing groundbreaking separation advances, including improved sorption efficiency and membrane regeneration.<br/><br/>We address this information deficit using interfacial-specific x-ray reflectivity and fluorescence to probe graphene oxide-ion interactions, and, for the first time, nonlinear sum frequency generation vibrational spectroscopy to the consider local water structure near graphene oxide monolayers at the air/water interface. These techniques specifically consider the nanometer-thick interfacial region, which shows dynamics and properties greatly different from those of the bulk. Via reflectivity and fluorescence, we find that trivalent yttrium ions preferentially adsorb to graphene oxide compared to divalent strontium and monovalent cesium ions. These trivalent ions affect the graphene oxide monolayer structure as well. Our sum frequency generation vibrational spectroscopy data show that yttrium disrupts the hydrogen bonding network formed between the functional groups of graphene oxide, namely carboxylic acids, and water. Taken together, we posit yttrium preferentially interacts with the hydrophilic edges of the graphene oxide flakes within the monolayer. Thus, ion adsorption onto graphene oxide is more complex than simple electrostatics. These results pave the way toward improved graphene oxide membranes and applications and provide fundamental information about one ion adsorption mechanism at the interface.

Ion adsorption on graphene oxide monolayers at the air/water interface

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.