We describe a scalable method for producing continuous graphene networks by tape casting surfactant-stabilized aqueous suspensions of functionalized graphene sheets. Similar to all other highly connected graphene-containing networks, the degree of overlap between the sheets controls the tapes' electrical and mechanical properties. However, unlike other graphene-containing networks, the specific surface area of the cast tapes remains high (>400 m(2)·g(-1)). Exhibiting apparent densities between 0.15 and 0.51 g·cm(-3), with electrical conductivities up to 24 kS·m(-1) and tensile strengths over 10 MPa, these tapes exhibit the best combination of properties with respect to density heretofore observed for carbon-based papers, membranes, or films.

Note: Micro-cantilevers with AlN actuators and PtSi tips for multi-frequency atomic force microscopy Rev. Sci. Instrum. 83, 096107 (2012) Extraction of temperature dependent electrical resistivity and thermal conductivity from silicon microwires selfheated to melting temperature J. Appl. Phys. 112, 063527 (2012) Approaching intrinsic performance in ultra-thin silicon nitride drum resonators J. Appl. Phys. 112, 064323 (2012) Thermally activated control of microfluidic friction Appl. Phys. Lett. 101, 134101 (2012) Direct actuation of cantilever in aqueous solutions by electrostatic force using high-frequency electric fields

We use colloidal gels of graphene oxide in a water-ethanol-ionic liquid solution to assemble graphene-ionic liquid laminated structures for use as electrodes in electrochemical double layer capacitors. Our process involves evaporation of water and ethanol yielding a graphene oxide/ionic liquid composite, followed by thermal reduction of the graphene oxide to electrically conducting functionalized graphene. This yields an electrode in which the ionic liquid serves not only as the working electrolyte but also as a spacer to separate the graphene sheets and to increase their electrolyte-accessible surface area. Using this approach, we achieve an outstanding energy density of 17.5 Wh/kg at a gravimetric capacitance of 156 F/g and 3 V operating voltage, due to a high effective density of the active electrode material of 0.46 g/cm 2. By increasing the ionic liquid content and the degree of thermal reduction, we obtain electrodes that retain >90% of their capacitance at a scan rate of 500 mV/s, illustrating that we can tailor the electrodes toward higher power density if energy density is not the primary goal. The elimination of the electrolyte infiltration step from manufacturing makes this bottom-up assembly approach scalable and well-suited for combinations of potentially any graphene material with ionic liquid electrolytes.

The colloidal stability of functionalized graphene sheets (FGSs) in aqueous sodium dodecyl sulfate (SDS) solutions of different concentrations was studied by optical microscopy and ultraviolet-visible light absorption after first dispersing the FGSs ultrasonically. In up to ∼10 μM SDS solutions, FGSs reaggregated within a few minutes, forming ramified structures in the absence of SDS and increasingly compact structures as the amount of SDS increased. Above ∼10 μM, the rate of reaggregation decreased with increasing SDS concentration; above ∼40 μM, the suspensions were colloidally stable for over a year. The concentration of ∼40 μM SDS lies 2 orders of magnitude below the critical surface aggregation concentration of ∼1.8 mM SDS on FGSs but above the concentration (∼18 μM) at which SDS begins to form a monolayer on FGSs. Neither surface micelle nor dense monolayer coverage is therefore required to obtain stable aqueous FGS dispersions. We support our experimental results by calculating the van der Waals and electrostatic interaction energies between FGSs as a function of SDS concentration and show that the experimentally observed transition from an unstable to a stable dispersion correlates with a transition from negative to positive interaction energies between FGSs in the aggregated state. Furthermore, our calculations support experimental evidence that aggregates tend to develop a compact structure over time.