From Structures to Applications—Designing Diverse Architectures of 3D Porous Graphene Oxide and Reduced Graphene Oxide Through Pickering Emulsion Templating

Graphene oxide (GO) is an oxidized graphene derivative containing hydrophilic oxygen functional groups and hydrophobic sp² graphene domains. GO sheets are stable in water; thus, they are ideal building blocks to assemble the 2D GO from aqueous suspensions into 3D architectures with large surface area and availability for functionalization. GO is an interesting starting material as one can further reduce GO and obtain reduced graphene oxide (rGO) to restore the properties of graphene, such as mechanical strength and electrical conductivity. The broad applications of graphene-based porous materials demand fabrication methods that are simple and yet can create complex architectures and compositions. However, it remains a challenge to assemble GO into porous materials mainly based on GO or rGO with controllable pore size, different interconnectivity, and tunable compositions. GO Pickering emulsion templating is a potential solution to fabricating well-defined graphene-based architectures. GO sheets can stabilize Pickering emulsions because of their amphiphilicity. By controlling the formation of emulsions, one can directly prepare porous GO templated by emulsion droplets. In this study, we used GO Pickering emulsion strategy to obtain porous GO and rGO materials with complex architectures, interconnected or closed pores, and tunable compositions tailored to different applications. We first introduced a dual-templating approach involving emulsion and ice as templates, to create GO, rGO, and composite scaffolds which have interconnected hierarchical structures with variable pore size and composition designed for bone tissue engineering. We developed stable GO emulsions in which GO flakes serve as both stabilizers and matrix builders. Upon freezing and drying these emulsions, large pores are templated by the oil droplets, and small pores by ice crystals formed in the water phase, whose sizes can be controlled by freezing temperatures. The complex architectures are tailored for bone tissue engineering, and we showed GO scaffolds are excellent substrates for mesenchymal stem cell penetration and growth. We subsequently introduced rGO/cobalt oxideporous electrodes for selective seawater electro-oxidation through GO emulsion templating. By adjusting the GO assembly on emulsion droplets, we generated closed pores templated by GO emulsions enclosing Co₃O₄ particles in the oil droplets. After reduction, the pore walls composed of rGO are permeable to water and gases while limiting the diffusion of dissolved ions like chloride to the cobalt oxide particles deposited on the internal walls of rGO pores. The closed rGO pores and the enclosure of cobalt oxide particles improved the electrode’s selectivity for water oxidation application. In summary, our study based on GO Pickering emulsions allows us to control the architecture and composition of GO, rGO and composite porous materials. With appropriate design of the GO Pickering emulsions and the architecture of porous materials, one can enhance the performance of GO and rGO based porous materials for a variety of applications.