Polymer-Free Emulsion-Templated Graphene-Based Sponges for Contaminant Removal

An emulsion-templated porous material can be formed by polymerizing the continuous phase of high internal phase Pickering emulsions (HIPEs). Although polymerization is a key step to maintain the pore size and integrity of the final sponge, it lowers the effective specific surface area of the final sponge as the continuous phase makes up at least half of the HIPE's volume. Hence, eliminating the need of polymerization not only eases the material processing but also leads to a greater specific surface area. Here, we report a novel strategy in which none of the emulsion phases require polymerization and is therefore a versatile methodology. For this purpose, several oil-in-water Pickering emulsions were prepared using graphene oxide (GO) and cellulose nanocrystals (CNCs) as the stabilizing agents. GO nanosheets are then reduced by mixing the emulsions with an adequate amount of vitamin C as a green reducing agent. Removal of the oil phase via multiple washing and boiling steps results in the formation of the ultimate reduced graphene oxide (rGO)/CNC sponge. The integrity of the structure remains intact and results in the formation of pores that are comparable in size to the droplets because of (i) the strong adhesion of GO and CNC at the oil/water interface in the initial Pickering emulsions and (ii) the strong intermolecular interactions between GO and CNC particles within the water phase. The sponge was then evaluated for its contaminant removal applicability using methylene blue and found to be effective in different water chemistries and outperform previously reported poly(HIPEs) and granular activated carbon. This is the first report on the formation of a polymer-free emulsion-templated sponge, and we believe that this novel nanomaterial paves the road for the fabrication of other emulsion-templated sponges. Although the proposed application in this work is contaminant removal, it could also be utilized in forming electronic devices and sensors because of the incorporation of rGO as a conductive component.