Effects of Ion Adsorption on Graphene Oxide Films and Interfacial Water Structure: A Molecular-Scale Description

Graphene oxide is a promising, emerging separation material, as it is durable, dispensable in water, and has naturally forming functional groups. Bulk studies using graphene oxide flakes have demonstrated impressive metal adsorption. However, little interfacial information about metal adsorption on graphene oxide is available and inferring interfacial structure from bulk experiments is usually not possible. A mechanistic understanding of ion adsorption on graphene oxide films is critical toward advanced separations, including improved sorption efficiency and membrane regeneration. In this paper, we study metal ion adsorption onto graphene oxide films formed at the air/water interface using x-ray reflectivity (XR), x-ray fluorescence near total reflection (XFNTR), and vibrational sum frequency generation spectroscopy (SFG). These interface-specific techniques provide the electron density profile normal to the interface, number of adsorbed ions, and information about the orientational ordering and hydrogen-bonding network of interfacial water, respectively. Via XFNTR and SFG, 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 film structure significantly. The SFG data show that at least two different interfacial water populations can be described, based on their hydrogen bonding strength, and the adsorbed ions affect these populations differently. We demonstrate that ion adsorption onto graphene oxide is more complex than simple electrostatics and requires thorough interfacial investigation. These results pave the way toward improved soft-scaffold graphene oxide membranes and applications and provide fundamental information about the ion adsorption mechanism at the interface.