A dynamic light scattering study of photochemically reduced colloidal graphene oxide

Evolution of the hydrodynamic size d _HD of colloidal graphene oxide (GO) sheets in water induced by the photochemical GO reduction by UV light was studied by the dynamic light scattering spectroscopy. An increase of d _HD at an initial stage of the photoreduction is caused by partial photoelimination of the functional groups of GO and breaking of the hydrogen bonds between the fragments of the GO particles resulting in the sheets' unfolding. At deeper photoreduction, a decrease of d _HD is observed as a result of ππ stacking interaction between the aromatic areas of GO particles and crumpling of the GO sheets. Variations of pH affect strongly the d _HD of the photoreduced GO. At pH lowering from 6–7 to 2–3, a drastic growth of d _HD and the sheets' aggregation are observed, caused by the protonation of the carboxyl groups. At pH elevation to 11–12, on the contrary, a decrease of d _HD is observed, which is more pronounced the longer the photochemical GO reduction. The observation reflects crumpling/scrolling of the the GO sheets to minimize the contact area between the aromatic components of the photoreduced GO particles and the dispersive medium. The changes of the photoreduced GO particle size in basic media were found to be reversible.