Probing Sulfur Deposition onto Carbon Nanomaterials from Aqueous, Elemental Sulfur Sols for Lithium-Sulfur Batteries.

Sulfur cathodes for lithium-sulfur batteries often rely on integrating sulfur with high surface area carbonaceous materials. Nanoscale mixing is typically achieved by a lengthy, high-temperature melt imbibition approach that employs carbon nanomaterials in an aggregated solid form. In this work, we present a simple strategy to coat carbon nanomaterials with sulfur in a cost-effective, room-temperature process using inexpensive elemental sulfur. Our results are based on hydrophobic sulfur sols, which have rarely been examined for use in the preparation of sulfur cathodes. We study the deposition mechanism on different carbon materials and find that sulfur dissolves from the sol into the aqueous phase and coats the surface of reduced graphene oxide (rGO) by heterogeneous nucleation and growth, but that this mechanism is not favored for carbon materials such as Ketjen black (KB) and graphene oxide (GO), for which undesirable homogeneous nucleation of micron-sized, insulating sulfur crystals is observed. High loading (3-4 mgsulfur/cm2) rGO-based cathodes prepared using this approach achieve discharge capacities of 1300 mAh/gsulfur (∼4.8 mAh/cm2) at 0.1C and achieve capacities 7-fold higher than cells prepared via traditional melt imbibition approaches at higher rates of 0.8C and 1C. Cells prepared without the need for added binder or conductive additive achieve projected full cell energy densities of 468 Wh/kg at 0.1C when taking into account all inactive components and assuming no lithium metal degradation, indicating that the deposition of sulfur from hydrophobic sols onto carbon nanomaterials can serve as a simple, aqueous-based, one-step process to prepare high sulfur loading cathodes with high projected energy densities.

Influence of atmospheric species on the electrical properties of functionalized graphene sheets.

We report on the time-dependent influence of atmospheric species on the electrical properties of functionalized graphene sheets (FGSs). When exposed to laboratory air, FGSs exhibit a significant, irreversible decrease in electrical conductance with time, strongly depending on the oxygen content of the FGSs. To separate the roles of charge carrier density and mobility in this aging process, we performed electron transport measurements using a back-gate field-effect transistor architecture. Investigating the position of the Dirac point under different atmospheres, we found that adsorbed atmospheric species result in pronounced p-doping, which - on a short time scale - can be reversed under nitrogen atmosphere. However, on a time scale of several days, the resistance increases irreversibly, while the Dirac point voltage remains constant. From these experiments, we conclude that the aging of FGSs is related to the chemisorption of atmospheric species leading to enhanced carrier scattering due to an increasing amount of sp3- regions and thus to a reduced charge carrier mobility.

Intrinsic Catalytic Activity of Graphene Defects for the Co(II/III)(bpy)3 Dye-Sensitized Solar Cell Redox Mediator.

We demonstrate that functionalized graphene, rich with lattice defects but lean with oxygen sites, catalyzes the reduction of Co(III)(bpy)3 as well as platinum does, exhibiting a rate of heterogeneous electron transfer, k0, of ∼6 × 10(-3) cm/s. We show this rate to be an order of magnitude higher than on oxygen-site-rich graphene oxide, and over 2 orders of magnitude higher than on the basal plane of graphite (as a surrogate for pristine graphene). Furthermore, dye-sensitized solar cells using defect-rich graphene monolayers perform similarly to those using platinum nanoparticles as the catalyst.

Buckling of dielectric elastomeric plates for soft, electrically active microfluidic pumps.

Elastic instabilities, when properly implemented within soft, mechanical structures, can generate advanced functionality. In this work, we use the voltage-induced buckling of thin, flexible plates to pump fluids within a microfluidic channel. The soft electrodes that enable electrical actuation are compatible with fluids, and undergo large, reversible deformations. We quantified the onset of voltage-induced buckling, and measured the flow rate within the microchannel. This embeddable, flexible microfluidic pump will aid in the generation of new stand-alone microfluidic devices that require a tunable flow rate.

Dispersion stability of functionalized graphene in aqueous sodium dodecyl sulfate solutions.

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.

Electrochemical sensing of nitric oxide with functionalized graphene electrodes.

The intrinsic electrocatalytic properties of functionalized graphene sheets (FGSs) in nitric oxide (NO) sensing are determined by cyclic voltammetry with FGS monolayer electrodes. The degrees of reduction and defectiveness of the FGSs are varied by employing different heat treatments during their fabrication. FGSs with intermediate degrees of reduction and high Raman ID to IG peak ratios exhibit an NO oxidation peak potential of 794 mV (vs 1 M Ag/AgCl), closely matching values obtained with a platinized Pt control (791 mV) as well as recent results from the literature on porous or biofunctionalized electrodes. We show that the peak potential obtained with FGS electrodes can be further reduced to 764 mV by incorporation of electrode porosity using a drop-casting approach, indicating a stronger apparent electrocatalytic effect on porous FGS electrodes as compared to platinized Pt. Taking into consideration effects of electrode morphology, we thereby demonstrate that FGSs are intrinsically as catalytic toward NO oxidation as platinum. The lowered peak potential of porous FGS electrodes is accompanied by a significant increase in peak current, which we attribute either to pore depletion effects or an amplification effect due to subsequent electrooxidation reactions. Our results suggest that the development of sensor electrodes with higher sensitivity and lower detection limits should be feasible with FGSs.

Cementation of colloidal particles on electrodes in a galvanic microreactor.

We have studied the processes leading to the cementation of colloidal particles during their autonomous assembly on corroding copper electrodes within a Cu-Au galvanic microreactor. We determined the onset of particle immobilization through particle tracking, monitored the dissolution of copper as well as the deposition of insoluble products of the corrosion reactions in situ, and showed that particle immobilization initiated after reaction products (RPs) began to deposit on the electrode substrate. We further demonstrated that the time and the extent of RP precipitation and thus the strength of the particle-substrate bond could be tuned by varying the amount of copper in the system and the microreactor pH. The ability to cement colloidal particles at locations undergoing corrosion illustrates that the studied colloidal assembly approach holds potential for applications in dynamic material property adaptation.

Adsorption of sodium dodecyl sulfate on functionalized graphene measured by conductometric titration.

We report on the adsorption of sodium dodecyl sulfate (SDS) onto functionalized graphene sheets (FGSs) in an aqueous system, measured at broad SDS and FGS concentration ranges by conductometric surfactant titration. At dilute SDS concentrations (<12 µM in bulk solution), there is evidence of a counterion exchange between hydronium ions (from the dissociation of acidic chemical functionalities on FGS) and sodium ions coadsorbing with dodecyl sulfate monomers onto FGSs. We find that, for FGS with a carbon-to-oxygen ratio of ~18, monolayer adsorption of SDS on FGS reaches full surface coverage by ~12 µM SDS. Additionally, the critical surface aggregation concentration (csac) for surface micelle formation on FGS is measured to be ~1.5 mM SDS. The transition from monolayer adsorption to surface micelle formation appears to occur at a similar SDS concentration on FGSs as on graphite, suggesting there is little difference in the surfactant adsorption behavior on both materials. We estimate that the FGS surface area available for SDS adsorption is ~600 m(2)/g, which is significantly less than expected for FGSs in suspension and indicates the presence of regions on FGS on which SDS adsorption does not occur.

Directed motion of colloidal particles in a galvanic microreactor.

The mechanisms leading to the deposition of colloidal particles in a copper-gold galvanic microreactor are investigated. Using in situ current density measurements and particle velocimetry, we establish correlations between the spatial arrangement and the geometry of the electrodes, current density distribution, and particle aggregation behavior. Ionic transport phenomena are responsible for the occurrence of strongly localized high current density at the edges and corners of the copper electrodes at large electrode separation, leading to a preferential aggregation of colloidal particles at the electrode edges. Preferential aggregation appears to be the result of a combination of electrophoretic effects and changes in bulk electrolyte flow patterns. We demonstrate that electrolyte flow is most likely driven by electrochemical potential gradients of reaction products formed during the inhomogeneous copper dissolution.

Local voltage drop in a single functionalized graphene sheet characterized by Kelvin probe force microscopy.

We studied the local voltage drop in functionalized graphene sheets of subµm size under external bias conditions by Kelvin probe force microscopy. Using this noninvasive experimental approach, we measured ohmic current-voltage characteristics and an intrinsic conductivity of about 3.7 × 10(5) S/m corresponding to a sheet resistance of 2.7 kΩ/sq under ambient conditions for graphene produced via thermal reduction of graphite oxide. The contact resistivity between functionalized graphene and metal electrode was found to be <6.3 × 10(-7) Ωcm(2).

Tuning of structural color using a dielectric actuator and multifunctional compliant electrodes.

We have developed electrically conducting silicone elastomer nanocomposites that serve both as compliant electrodes in an electrostatic actuator and, at the same time, as optically active elements creating structural color. We demonstrate the capabilities of our setup by actuating an elastomeric diffraction grating and colloidal-crystal-based photonic structures.

Functionalized graphene as a catalytic counter electrode in dye-sensitized solar cells.

When applied on the counter electrode of a dye-sensitized solar cell, functionalized graphene sheets with oxygen-containing sites perform comparably to platinum (conversion efficiencies of 5.0 and 5.5%, respectively, at 100 mW cm(-2) AM1.5G simulated light). To interpret the catalytic activity of functionalized graphene sheets toward the reduction of triiodide, we propose a new electrochemical impedance spectroscopy equivalent circuit that matches the observed spectra features to the appropriate phenomena. Using cyclic voltammetry, we also show that tuning our material by increasing the amount of oxygen-containing functional groups can improve its apparent catalytic activity. Furthermore, we demonstrate that a functionalized graphene sheet based ink can serve as a catalytic, flexible, electrically conductive counter electrode material.

Dissolution dynamics of thin films measured by optical reflectance.

Measuring the dissolution dynamics of thin films in situ both with spatial and temporal resolution can be a challenging task. Available methods such as scanning electrochemical microscopy rely on scanning the specimen and are intrinsically slow. We developed a characterization technique employing only an optical microscope, a digital charge coupled device camera, and a computer for image processing. It is capable of detecting dissolution rates of the order of nm/min and has a spatial and temporal resolution which is limited by the imaging and recording setup. We demonstrate the capabilities of our method by analyzing the electrochemical dissolution of copper thin films on gold substrates in a mild hydrochloric acid solution. Due to its simplicity, our technique can be implemented in any laboratory and can be applied to a variety of systems such as thin film sensors or passive coatings.

Inhibition and promotion of copper corrosion by CTAB in a microreactor system.

We report on an optical microscopy technique for the analysis of corrosion kinetics of metal thin films in microreactor systems and use it to study the role of cetyltrimethylammonium bromide surfactant as a corrosion inhibitor in a copper-gold galvanic coplanar microsystem. A minimum in the dissolution rate of copper is observed when the surfactant concentration is approximately 0.8 mM. To explain why the inhibitory role of the surfactant does not extend to higher concentrations, we use zero resistance ammetry with separated half cells and show that while the surfactant inhibits cathodic reactions on gold, it also promotes the corrosion of copper because of the catalytic action of bromide counterions. These two competing processes lead to the observed minimum in the dissolution rate.

Pattern formation in 4:1 resonance of the periodically forced CO oxidation on Pt(110).

Periodically forced oscillatory reaction-diffusion systems may show complex spatiotemporal patterns. At high-frequency resonant forcing, multiple-phase patterns can be found. In the present work, the dynamics of turbulent CO oxidation on Pt(110), forced with the fourth harmonic of the system's natural frequency, is investigated. Experiments result in subharmonic entrainment, where the system locks to a quarter of the forcing frequency. Cluster patterns are observed, where different parts of the pattern show a defined phase difference. The experimental results are compared with numerical simulations using the realistic Krischer-Eiswirth-Ertl model for catalytic CO oxidation. Using the fourth harmonic of an uncoupled surface element's natural frequency, we find 3:1 entrainment with three-phase cluster patterns in a wide parameter range of forcing amplitudes and frequency detuning. Numerical analysis of the spatially extended, turbulent system reveals a remarkable upshift of the mean oscillation frequency compared to homogeneous oscillations. Using the fourth harmonic of the most prominent frequency found in the turbulent system results in four-phase patterns with partial or full 4:1 entrainment, depending on the forcing parameters chosen.

Suppression of spatiotemporal chaos in the oscillatory CO oxidation on Pt(110) by focused laser light.

Chemical turbulence in the oscillatory catalytic CO oxidation on Pt(110) is suppressed by means of focused laser light. The laser locally heats the platinum surface which leads to a local increase of the oscillation frequency, and to the formation of a pacemaker which emits target waves. These waves slowly entrain the medium and suppress the spatiotemporal chaos present in the absence of laser light. Our experimental results are confirmed by a detailed numerical analysis of one- and two-dimensional media using the Krischer-Eiswirth-Ertl model for CO oxidation on Pt(110). Different control regimes are identified and the dispersion relation of the system is determined using the pacemaker as an externally tunable wave source.

Control of spatiotemporal chaos in catalytic CO oxidation by laser-induced pacemakers.

Control of spatiotemporal chaos is achieved in the catalytic oxidation of CO on Pt(110) by localized modification of the kinetic properties of the surface chemical reaction. In the experiment, a small temperature heterogeneity is created on the surface by a focused laser beam. This heterogeneity constitutes a pacemaker and starts to emit target waves. These waves slowly entrain the medium and suppress the spatiotemporal chaos that is present in the absence of control. We compare this experimental result with a numerical study of the Krischer-Eiswirth-Ertl model for CO oxidation on Pt(110). We confirm the experimental findings and identify regimes where complete and partial controls are possible.

Optical imaging of pattern formation: reflection anisotropy microscopy applied to globally coupled oscillatory CO-oxidation.

A reflection anisotropy microscope (RAM) creates the contrast in its images from a change in the polarization orientation of reflected light from a surface. This may stem from local variations of the reconstruction of a surface, being initiated by changes in an adsorption layer. The advantages and disadvantages of a recently improved RAM versus other optical imaging techniques are discussed. We demonstrate the unique features of RAM and present the first experimental findings of so called 2pi phase kinks in the globally coupled CO-oxidation on Pt(110).