Trivalent ion adsorption onto electrified graphene*

A complete understanding of ion behavior and water structure near graphene interfaces is critical toward developing promising graphene technologies, including graphene-based membranes for heavy metal separation efforts. While graphene-water interfaces are interesting to both fundamental and applied efforts, molecular-scale information is generally lacking, as these interfacial regions are difficult to probe. Here, we study yttrium ion adsorption onto electrified graphene surfaces using X-ray crystal truncation rod (CTR) and resonant anomalous X-ray reflectivity (RAXR). Both approaches only consider the liquid-graphene interface, and together provide molecular-level, elementally-specific information about yttrium and water organization near graphene interface.

From our CTR data, we find water is structured near graphene surfaces without an applied potential. This structure changes when yttrium ions are also present but still occurs. When we apply negative potentials to the graphene, yttrium ions readily adsorb, as evidenced by our RAXR data. A model-dependent analysis of our RAXR data shows trivalent yttrium ions overcharge the graphene surface. This overcharging effect has been observed in other systems using yttrium and likely stems from enhanced screening effects. Taken together, these data provide critical molecular-level interfacial information about ion and water behavior near electrified graphene interfaces.