The Nature of Graphene Surfaces As Determined from the Wettability Studies of Basal and Edge Planes

Abstract

Electrochemical studies of two-atom thick graphene sheet have attracted significant attention from the research community due to the attractive properties of its basal and edge surfaces. The graphene edge surface contains dangling bonds, unsaturated valencies, and can favorably attach to a plethora of functional groups. It has been demonstrated that the graphene edge surface has several orders of magnitude higher specific capacitance, rapid electron transfer rates and much stronger electrocatalytic activity than those of graphene basal surfaces. The graphene basal plane has been known to possess a high conductivity. Coupling these properties of the graphene basal and edge surfaces presents graphene or high quality single crystal graphite as potential electronic material. But the utilization of this versatile material in aquatic conditions is still limited due to unknown key properties of the basal and edge surfaces such as interaction of these surfaces with water and the isoelectric point. In this paper, we have tried to throw some light on the wettability characteristics of graphite basal and edge surfaces using carbon nanoparticles (fullerene and fullerol) and other characterization techniques. Highly oriented pyrolytic graphite (HOPG) is used as a model surface for fundamental studies of the wetting characteristics of graphite surfaces. HOPG is a highly anisotropic material, exhibiting different properties on its face and edge surfaces. The essential aim of this work was to provide surface chemistry information for HOPG surfaces. Fundamental wetting studies were accomplished through contact angle measurements of the as-received HOPG single crystal, which was characterized by its distinct face and edge surfaces. To study the effect of functional groups, contact angle measurements were repeated on the HOPG face and edge surfaces oxidized with different concentrations of hydrogen peroxide. Further, Raman spectroscopic studies were performed on the as-received and oxidized HOPG basal and edge surfaces. Atomic force microscopy (AFM) was also used a tool to image the HOPG surfaces and for surface force measurements to determine the iso-electric point (IEP) of the face and edge surface of HOPG. Zeta potential measurements of graphite powder in solutions of varying pH were also done to determine its iso-electric point and compare to the values reported in literature for graphitic carbon materials. Further AFM was used to study the interaction of the fullerene and fullerol nanoparticles on the graphite face and edge surfaces. The zeta potential measurements of the above mentioned carbon nanoparticles were also carried out to confirm that the interaction was due to hydrophobic or hydrophilic forces and not due to electrostatic interactions. In addition molecular dynamics simulations were also carried out to verify the interactions of the carbon nanoparticles with the graphite surfaces. Results indicate interesting charge properties of the graphite basal and edge surfaces and that the basal surface is highly hydrophobic whereas the edge is less hydrophobic (not completely hydrophilic), which makes it a promising electronic material for aquatic use.