image: (a) Schematic of the r-GO hydrogel capillary shrinkage process; (b) capillary force exerted on the r-GO sheets during solvent evaporation.
Materials with both a high density and a large surface area are required in many applications, typically for energy storage under a limited space. However, they are hard to obtain by using conventional strategies. In the previous study, Quan-Hong Yang et al. reported that graphene oxide (GO) can be used to produce a porous carbon material with a high density of 1.58 g cm-3 from hydrogel by evaporation-induced drying. However, the shrinkage of hydrogels is not yet clearly illustrated and there is still no full understanding of how the capillary forces work.
Recently, the same group from Tianjin University, China explored the capillary shrinkage of graphene oxide hydrogels in Science China Materials (DOI: 10.1007/s40843-019-1227-7) based on the different surface tension of the trapped solvent.
They chose water and 1,4-dioxane which have a sole difference in surface tension to investigate the mechanism of such a network shrinkage in r-GO hydrogel, and found the surface tension of the evaporating solvent and the associated capillary force regulated by the interfacial interaction between the r-GO sheets and the solvent determined the capillary forces in the nanochannels. Solvents with higher surface tensions generate stronger capillary forces during evaporation, which can compact the r-GO framework into a dense yet porous material. More promisingly, by using solvents with different surface tensions, the microstructure of the resulting materials can be precisely manipulated and densified, realizing an excellent balance of the density and porosity in materials not limited to carbon materials. This work provides a reliable methodology of controlled shrinkage of flexible graphene network and has great potential for high volumetric performance in practical devices.