Graphene Oxide Liquid Crystal Domains: Quantification and Role in Tailoring Viscoelastic Behavior.

Graphene oxide liquid crystals (GOLCs) were exfoliated in a wide variety of solvents (water, ethylene glycol (EG), N-methyl-2-pyrrolidone (NMP), and dimethylformamide (DMF)) by high-speed shearing of graphite oxide. Quantitative polarized light imaging of the equilibrium nematic phases of the lyotropic GOLCs gives insights into the extent of aggregation and quantifiable textural features such as domain size, d. Large nematic domains >100 µm with a high overall degree of order were obtained in water and ethylene glycol, in contrast to ∼5-50 µm domains in NMP and DMF at comparable volume fractions. Comprehensive rheological studies of these GOLCs indicate that larger domains correlate with higher viscosity and higher elasticity, and scaling analysis shows a power-law dependence of the Ericksen number (Er) with domain size (Er ∝ d3.09). The improved understanding of the relationship between the microstructure and flow properties of GOLCs leads us to an approach of mixed solvent-based GOLCs as a means to tune viscoelastic properties. We demonstrate this approach for the formation of shear-aligned GOLC films for advanced flexible electronic applications such as all-carbon conductive films and thermal heaters.

Solvent Transport Behavior of Shear Aligned Graphene Oxide Membranes and Implications in Organic Solvent Nanofiltration.

Solvent transport in membranes composed of stacked sheets of graphene oxide (GO) with molecular scale channels and a complex arrangement of hydrophobic and hydrophilic domains is not well understood. Here, we observe that the interlayer space between GO sheets expands in different solvents without disturbing the membrane integrity and is typically larger in aqueous media compared to nonaqueous media. However, the membranes have a tighter molecule sieving feature in aqueous media as demonstrated by lower permeance and higher solute rejection arising from interfacial water layers "sticking" to charged polar groups. As a result of this polar interaction, the permeance of polar solvents in GO membrane scales inversely to the polarity of the solvent, which is contrary to other polymeric and ceramic hydrophilic membranes and also scales inversely to the viscosity of solvents as per continuum expectations. We highlight the extended solvent-handling space of GO membranes, such as in polar protic, polar aprotic, and nonpolar solvents, demonstrating versatility over a commercial nanofiltration membrane, and we predict exciting new applications in advanced separation engineering.

The inside-out supercapacitor: induced charge storage in reduced graphene oxide.

Iontronic circuits are built using components which are analogous to those used in electronic circuits, however they involve the movement of ions in an electrolyte rather than electrons in a metal or semiconductor. Developments in these circuits' performance have led to applications in biological sensing, interfacing and drug delivery. While transistors, diodes and elementary logic circuits have been demonstrated for ionic circuits if more complex circuits are to be realized, the precident set by electrical circuits suggests that a component which is analogous to an electrical capacitor is required. Herein, an ionic supercapacitor is reported, our experiments show that charge may be stored in a conductive porous reduced graphene oxide film that is contacted by two isolated aqueous solutions and that this concept extends to an arbitrary polarizable sample. Parametric studies indicate that the conductivity and porosity of this film play important roles in the resultant device's performance. This ionic capacitor has a specific capacitance of 8.6 F cm-3 at 1 mV s-1 and demonstrates the ability to filter and smooth signals in an electrolyte at a variety of low frequencies. The device has the same interfaces as a supercapacitor but their arrangement is changed, hence the name inside-out supercapacitor.

Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide.

Graphene-based membranes demonstrating ultrafast water transport, precise molecular sieving of gas and solvated molecules shows great promise as novel separation platforms; however, scale-up of these membranes to large-areas remains an unresolved problem. Here we demonstrate that the discotic nematic phase of graphene oxide (GO) can be shear aligned to form highly ordered, continuous, thin films of multi-layered GO on a support membrane by an industrially adaptable method to produce large-area membranes (13 × 14 cm(2)) in <5 s. Pressure driven transport data demonstrate high retention (>90%) for charged and uncharged organic probe molecules with a hydrated radius above 5 Å as well as modest (30-40%) retention of monovalent and divalent salts. The highly ordered graphene sheets in the plane of the membrane make organized channels and enhance the permeability (71 ± 5 l m(-2) hr(-1) bar(-1) for 150 ± 15 nm thick membranes).