Exfoliation of Graphite Oxide in Water without Sonication: Bridging Length Scales from Nanosheets to Macroscopic Materials

Abstract

Dispersed graphene oxide (GO) nanosheets in water were synthesized via a new approach involving sonication-free exfoliation, as promoted by repetitive simple freeze–thaw cycles. These cycles consist of rapid freezing of an aqueous solution containing graphite oxide and subsequent thawing of the resultant solid. This approach is effective for exfoliation of graphite oxide and yields approximately 80% GO after six repetitive freeze–thaw cycles. The GO synthesized by the new method experiences minimal fragmentation during the exfoliation process and has a lateral size at least 3-fold larger than that of GO prepared by using sonication, as evidenced by atomic force microscopy and dynamic light scattering. We also demonstrate use of the resulting exfoliated GO that is synthesized using this approach as a building block for the synthesis of a low-density (≈0.02 g cm–3) macroporous monolithic material, using directional freezing, which forms ice rods within the solution that served as the template to direct the assembly of the solution-dispersed GO nanosheets into a honeycomb-like morphology with a 10 μm macropore opening and sheet-like walls. Such a unique morphology of the synthesized monolith has broad applicability for advanced functional materials that allow extremely high throughput with minimal pressure drop as well as electronic and energy storage materials.