Reduction Self-Assembly of Three-Dimensional Graphene Hydrogels: Implication as Adsorbents

We have prepared a three-dimensional (3D) graphene hydrogel with a specific surface area that is comparable to the theoretical specific surface area of two-dimensional graphene. Graphene oxide was reduced in a glass vial using aqueous sodium bisulfite in the presence of ammonia, which regulates the rate of reduction. The kinetics of reduction was monitored by the rate of consumption of sodium bisulfite, whereas the extent of self-assembly of the reduced graphene sheets was captured through reduction in the volume of the 3D graphene with time. Dynamics of self-assembly shows two distinct regimes, namely, induction and shrinkage. During the induction period, no noticeable shrinkage in the volume occurs, although there is a significant reduction of graphene oxide. During the shrinkage period, the volume shrinks continuously with time until it reaches a constant value. The onset of shrinkage requires a critical extent of reduction of graphene oxide. We hypothesize that during the induction period, the overlapping portions of the sheets undergo lateral stacking and the reduced graphene oxide sheets undergo enlargement. With the increase in ammonia concentration, the rate of reduction is lower, the induction period is longer, and the extent of reduction at the onset of shrinkage is greater. This causes progressively greater enlargement of sheets as observed from the scanning electron micrographs of the lyophilized 3D aerogel. On further reduction, the larger sheets shrink slowly and to a lesser extent. The extent of stacking of larger sheets is also reduced significantly as seen from X-ray diffraction.