Electro-optical Characteristics of Aqueous Graphene Oxide Dispersion Depending on Ion Concentration

Facile alignment control of graphene oxide (GO) particles in aqueous dispersions is a highly useful technique for various potential applications. Aqueous GO dispersions were recently reported to have an extremely large Kerr coefficient, which may provide a valuable pathway to the alignment control of GO particles. We investigated the electro-optic responses of GO dispersions in various ionic solutes and with varying ionic concentrations. We found that the addition of NaOH actually improved the electro-optic sensitivity of the GO dispersion, while other ionic additives resulted in desensitization. We experimentally and theoretically elucidated the underlying mechanism of the phenomena. The mechanism is closely related to the acidic nature of the GO dispersion, which is neutralized by the addition of NaOH. The addition of ionic solutes caused only a mild change in the surface conductivity of GO particles, but it brought about a large variation in the bulk solvent conductivity. The electro-optical sensitivity agreed well with the variation in solvent conductivity. Thus, the electro-optic response of GO dispersion was influenced more by the electric properties of solvent rather than by those of the GO particle itself. We also found that the cation-exchange capacity for H+ ions in the electrical double layer is quite high; i.e., H+ ions are not likely to be replaced by other ions.