Shear-directed assembly of graphene oxide in aqueous dispersions into ordered arrays

A wide variety of new carbon-based materials are being developed from graphene oxide (GO) precursor sheets, whose assembly in aqueous phases determines the form, structure, and properties of the resultant carbon. Here we show that graphene oxide forms ordered linear arrays of aggregates when aqueous suspensions are subjected to shear flow in the presence of soluble salts. These linear arrays align along the vorticity direction, normal to the direction of flow. We propose that salt addition screens electrostatic repulsion and allows formation of fractal-like GO sheet aggregates by hydrophobic forces. Fluid shear in a confined gap then guides the assembly of these primary aggregates into optically visible, ordered linear arrays or "superaggregates" whose characteristics are a function of GO concentration, salt valency, salt concentration, and gap confinement. This is the first reported observation of vorticity banding in graphene oxide suspensions and the first reported observation of such banding based on salt-induced interactions. We also demonstrate that simple isometric nanoparticles of carbon or gold do not form such linear superaggregate arrays but can be assembled into such arrays using graphene oxide as a two-dimensional colloidal template.