The Rheological and Structural Properties of Aqueous Graphene Oxide Dispersions: Concentration and pH Dependence.

A modified Hummer's method was used to synthesize aqueous dispersions of graphene oxide (GO). The morphology, chemical structure, and exfoliation state of GO were analyzed by combining scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The structural and rheological properties of the GO dispersions were studied as a function of GO concentration and pH. Increasing the concentration of GO revealed shorter interparticle distances between GO sheets. This induced a transition from fluid to nematic gel-like structures, as observed by polarized optical microscopy (POM). The Herschel-Bulkley model was used to fit the shear thinning curves and to demonstrate the viscoelastic behavior. Both the yield stress and viscoelastic moduli in the linear viscoelastic regime increased. As pH increases, the color of the aqueous GO dispersions becomes darker, the negative values of the zeta potential increase, the distances between the GO sheets decrease as observed by a slight shift of the correlation peak toward higher values of the scattering vector modulus in small-angle X-ray scattering (SAXS), and the rheological properties (yield stress and viscoelastic moduli in the linear viscoelastic region) decrease. These results can be explained by a change in the morphology of GO related to their hydrophilicity. This work presents relationships between rheological and structural properties of GO sheet dispersions, with particular emphasis on the effects of concentration and pH.