Showing papers by "Rodney S. Ruoff published in 2008"

Graphene-Based Ultracapacitors

Abstract: The surface area of a single graphene sheet is 2630 m2/g, substantially higher than values derived from BET surface area measurements of activated carbons used in current electrochemical double layer capacitors. Our group has pioneered a new carbon material that we call chemically modified graphene (CMG). CMG materials are made from 1-atom thick sheets of carbon, functionalized as needed, and here we demonstrate in an ultracapacitor cell their performance. Specific capacitances of 135 and 99 F/g in aqueous and organic electrolytes, respectively, have been measured. In addition, high electrical conductivity gives these materials consistently good performance over a wide range of voltage scan rates. These encouraging results illustrate the exciting potential for high performance, electrical energy storage devices based on this new class of carbon material.

Functionalized graphene sheets for polymer nanocomposites

Abstract: Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications. Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content. However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer-particle interactions. An unprecedented shift in glass transition temperature of over 40 degrees C is obtained for poly(acrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly(methyl methacrylate) there is an improvement of nearly 30 degrees C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet- poly(methyl methacrylate) rivaling those for single-walled carbon nanotube-poly(methyl methacrylate) composites.

Graphene oxide papers modified by divalent ions-enhancing mechanical properties via chemical cross-linking.

Abstract: Significant enhancement in mechanical stiffness (10–200%) and fracture strength (∼50%) of graphene oxide paper, a novel paperlike material made from individual graphene oxide sheets, can be achieved upon modification with a small amount (less than 1 wt %) of Mg2+ and Ca2+. These results can be readily rationalized in terms of the chemical interactions between the functional groups of the graphene oxide sheets and the divalent metals ions. While oxygen functional groups on the basal planes of the sheets and the carboxylate groups on the edges can both bond to Mg2+ and Ca2+, the main contribution to mechanical enhancement of the paper comes from the latter.

Synthesis and solid-state NMR structural characterization of 13C-labeled graphite oxide.

Abstract: The detailed chemical structure of graphite oxide (GO), a layered material prepared from graphite almost 150 years ago and a precursor to chemically modified graphenes, has not been previously resolved because of the pseudo-random chemical functionalization of each layer, as well as variations in exact composition. Carbon-13 (13C) solid-state nuclear magnetic resonance (SSNMR) spectra of GO for natural abundance 13C have poor signal-to-noise ratios. Approximately 100% 13C-labeled graphite was made and converted to 13C-labeled GO, and 13C SSNMR was used to reveal details of the chemical bonding network, including the chemical groups and their connections. Carbon-13–labeled graphite can be used to prepare chemically modified graphenes for 13C SSNMR analysis with enhanced sensitivity and for fundamental studies of 13C-labeled graphite and graphene.

Evidence of Graphitic AB Stacking Order of Graphite Oxides

Abstract: Graphite oxide (GO) samples were prepared by a simplified Brodie method. Hydroxyl, epoxide, carboxyl, and some alkyl functional groups are present in the GO, as identified by solid-state 13C NMR, Fourier-transform infrared spectroscopy, and X-ray photoemission spectroscopy. Starting with pyrolytic graphite (interlayer separation 3.36 A), the average interlayer distance after 1 h of reaction, as determined by X-ray diffraction, increased to 5.62 A and then increased with further oxidation to 7.37 A after 24 h. A smaller signal in 13C CPMAS NMR compared to that in 13C NMR suggests that carboxyl and alkyl groups are at the edges of the flakes of graphite oxide. Other aspects of the chemical bonding were assessed from the NMR and XPS data and are discussed. AB stacking of the layers in the GO was inferred from an electron diffraction study. The elemental composition of GO prepared using this simplified Brodie method is further discussed.

Aqueous Suspension and Characterization of Chemically Modified Graphene Sheets

Abstract: We report the production of aqueous suspensions of chemically modified graphene sheets, and electrically conductive “paperlike” material made from filtering such suspensions.

Tunable electrical conductivity of individual graphene oxide sheets reduced at "low" temperatures.

Abstract: Step-by-step controllable thermal reduction of individual graphene oxide sheets, incorporated into multiterminal field effect devices, was carried out at low temperatures (125−240 °C) with simultaneous electrical measurements. Symmetric hysteresis-free ambipolar (electron- and hole-type) gate dependences were observed as soon as the first measurable resistance was reached. The conductivity of each of the fabricated devices depended on the level of reduction (was increased more than 106 times as reduction progressed), strength of the external electrical field, density of the transport current, and temperature.

Effect of Water Vapor on Electrical Properties of Individual Reduced Graphene Oxide Sheets

Abstract: The electrical conductivity and gas-sensing characteristics of individual sheets of partially reduced graphene oxide are studied, and the results display a strong dependence on the chosen reduction method Three reduction procedures are considered here: thermal, chemical, and a combined chemical/thermal approach Samples treated by chemical/thermal reduction display the highest conductivity whereas thermally reduced samples display the fastest gas-sensing response times The chemo-resistive response to water vapor adsorption is well fit by a linear driving force model The conductivity upon exposure to water vapor and measured as a function of the gated electric field displays significant hysteresis These results illustrate how the chemical structure of graphene oxide may be tailored to optimize specific properties for applications such as field effect devices and gas sensors

Polymer−Graphite Nanocomposites: Effective Dispersion and Major Property Enhancement via Solid-State Shear Pulverization

Abstract: Recei Ved July 28, 2007 ReVised Manuscript Recei Ved NoVember 2, 2007 Introduction. Polymer nanocomposites are of scientific and commercial interest because of their potential for enhanced properties compared to neat polymer. 1-18 For example, improvements in mechanical properties are expected when highaspect-ratio nanofillers are well-dispersed or exfoliated in polymer;7,8 prototypical nanofillers include layered silicates (clay)9-14 and carbon nanotubes. 15-18 A carbon-based material of intense, recent focus in nanotechnology is graphite. 19 Despite its natural abundance and use since the Middle Ages, 19 graphite and its derivatives have only recently emerged as a nanomaterial of choice, as exceptional mechanical and electrical properties are observed when the sp 2-hybridized carbon layers termed “graphene sheets” are isolated or in “paper” form. 20-23 Chemically similar to carbon nanotubes and structurally analogous to layered silicates, graphite has the potential to be an outstanding nanofiller in the form of individual graphene layers or nanoscale layered stacks. Despite potential advantages, there are relatively few reports of graphite-based polymer nanocomposites. 23-37 This is because effective dispersion or exfoliation of graphite is practically impossible with melt processing. Most polymer -graphite hybrids are made from chemically or thermally pretreated graphite, e.g., graphite oxide, expanded graphite, or thermally exfoliated graphite oxide. 23-36 Even with pretreatment, nanocomposite production by conventional processing is challenging due to thermodynamic and/or kinetic limitations, sometimes leading to limited property enhancement. Here we employ solid-state shear pulverization (SSSP) to produce polymer -graphite nanocomposites that are not subject to the thermodynamic/kinetic limitations associated with conventional processes. With SSSP, a modified twin-screw extruder applies shear and compressive forces to solid-state materials; this process has previously yielded blend compatibilization and nanoscale dispersion in polymer blends and organoclay-based nanocomposites. 38-44 We now demonstrate that the continuous, scalable SSSP process can result in well-dispersed unmodified, as-recei Ved graphitein polypropylene (PP), leading to a 100% increase in Young’s modulus and a ∼60% increase in yield strength in comparison with neat PP.

Characterization of Thermally Reduced Graphene Oxide by Imaging Ellipsometry

Abstract: The dispersion functions for the refractive index and the extinction coefficient of single- and multiple-layer graphene oxide samples were measured by imaging spectroscopic ellipsometry in the wavelength range of 350−1000 nm and were compared to previously reported results measured by confocal microscopy. The dispersion functions for thin platelets were also compared to those obtained by standard spectroscopic ellipsometry on a deposit consisting of many overlapping graphene oxide layers. Changes were observed in both the thickness of the deposits and the values of the dispersion parameters following heating. A model is proposed to explain these observations, based on the removal of water between the graphene-oxide layers upon thermal treatment.

Incorporation of the electrode electrolyte interface into finite-element models of metal microelectrodes

Abstract: An accurate description of the electrode–electrolyte interfacial impedance is critical to the development of computational models of neural recording and stimulation that aim to improve understanding of neuro–electric interfaces and to expedite electrode design. This work examines the effect that the electrode–electrolyte interfacial impedance has upon the solutions generated from time-harmonic finite-element models of cone- and disk-shaped platinum microelectrodes submerged in physiological saline. A thin-layer approximation is utilized to incorporate a platinum–saline interfacial impedance into the finite-element models. This approximation is easy to implement and is not computationally costly. Using an iterative nonlinear solver, solutions were obtained for systems in which the electrode was driven at ac potentials with amplitudes from 10 mV to 500 mV and frequencies from 100 Hz to 100 kHz. The results of these simulations indicate that, under certain conditions, incorporation of the interface may strongly affect the solutions obtained. This effect, however, is dependent upon the amplitude of the driving potential and, to a lesser extent, its frequency. The solutions are most strongly affected at low amplitudes where the impedance of the interface is large. Here, the current density distribution that is calculated from models incorporating the interface is much more uniform than the current density distribution generated by models that neglect the interface. At higher potential amplitudes, however, the impedance of the interface decreases, and its effect on the solutions obtained is attenuated.

Patch clamp technique: Review of the current state of the art and potential contributions from nanoengineering:

Abstract: The patch clamp technique permits high-resolution recording of the ionic currents flowing through a cell's plasma membrane. In different configurations, this technique has allowed experimenters to ...

Graphene oxide sheet laminate and method

Abstract: A macroscale sheet laminate includes individual graphene oxide sheets layered one on another in a manner to form a self-supporting paper-like laminated product. The product can be fabricated by making a suspension of individual graphene oxide sheets and assembling the graphene oxide sheets as a laminate on a fluid-permeable support by flow-directed assembly. The laminate is dried and released from the membrane filter as a self-supporting sheet laminate.

Synthesis and Characterization of Single-Crystal Strontium Hexaboride Nanowires

Abstract: Catalyst-assisted growth of single-crystal strontium hexaboride (SrB6) nanowires was achieved by pyrolysis of diborane (B2H6) over SrO powders at 760−800 °C and 400 mTorr in a quartz tube furnace. Raman spectra demonstrate that the nanowires are SrB6, and transmission electron microscopy along with selected area diffraction indicate that the nanowires consist of single crystals with a preferred [001] growth direction. Electron energy loss data combined with the TEM images indicate that the nanowires consist of crystalline SrB6 cores with a thin (1 to 2 nm) amorphous oxide shell. The nanowires have diameters of 10−50 nm and lengths of 1−10 μm.

Ceramic composite thin films

Abstract: A ceramic composite thin film or layer includes individual graphene oxide and/or electrically conductive graphene sheets dispersed in a ceramic (e.g. silica) matrix. The thin film or layer can be electrically conductive film or layer depending the amount of graphene sheets present. The composite films or layers are transparent, chemically inert and compatible with both glass and hydrophilic SiOx/silicon substrates. The composite film or layer can be produced by making a suspension of graphene oxide sheet fragments, introducing a silica-precursor or silica to the suspension to form a sol, depositing the sol on a substrate as thin film or layer, at least partially reducing the graphene oxide sheets to conductive graphene sheets, and thermally consolidating the thin film or layer to form a silica matrix in which the graphene oxide and/or graphene sheets are dispersed.

Apparent Enhanced Solubility of Single-Wall Carbon Nanotubes in a Deuterated Acid Mixture

Abstract: An apparent enhanced solubility of single-wall carbon nanotubes (SWNTs) in the deuterated form of the standard 3 : 1 sulfuric (H2SO4) to nitric (HNO3) acid mixture treatment is reported and attributed to the stronger interaction of deuterium bonds with the single-wall carbon nanotube surface. UV-Visible spectroscopy was used to characterize the apparent enhanced solubility of the SWNTs treated in deuterated forms of the acid mixture in comparison to the standard acid mix, while FTIR was used to analyze the nature of the functional groups generated on the SWNTs as a result of the different acid treatments. The apparent enhanced solubility reported here is consistent with the limited number of computational and experimental results published in the literature regarding the interaction of carbon nanotubes with deuterated solvents; however, a detailed understanding of the underlying mechanism responsible for this observation is currently lacking. The apparent increased solubility observed here could potentially be utilized in many applications where carbon nanotube dispersion is required.

Multiscale simulation of nanostructures based on spatial secant model: a discrete hyperelastic approach

Abstract: The main objective of this paper is to present a coarse-grained material model for the simulation of three-dimensional nanostructures. The developed model is motivated by the recent progress in establishing continuum models for nanomaterials and nanostructures. As there are conceptual differences between the continuum field defined in the classical sense and the nanomaterials consisting of discrete, space-filling atoms, existing continuum measures cannot be directly applied for mapping the nanostructures due to the discreteness at small length scale. In view of the fundamental difficulties associated with the direct application of the continuum approach, we introduce a unique discrete deformation measure called spatial secant and have developed a new hyperelastic model based on this measure. We show that the spatial secant-based model is consistently linked to the underlying atomistic model and provides a geometric exact mapping in the discrete sense. In addition, we outline the corresponding computational framework using the finite element and/or meshfree method. The implementation is within the context of finite deformation. Finally we illustrate the application of the model in studying the mechanics of low-dimensional carbon nanostructures such as carbon nanotubes (CNT). By comparing with full-scale molecular mechanics simulations, we show that the proposed coarse-grained model is robust in that it accurately captures the non-linear mechanical responses of the CNT structures.

Quantized fracture mechanics and related applications for predicting the strength of defective nanotubes

Abstract: A new energy-based theory, Quantized Fracture Mechanics (QFM), is presented that modifies continuum-based fracture mechanics. The differentials in Griffith’s criterion are substituted with finite differences; the implications are remarkable. Fracture of tiny systems with a given geometry and type of loading occurs at quantized stresses that are well predicted by QFM. QFM is self-consistent, agreeing to first-order with linear elastic fracture mechanics (LEFM), and to second-order with non-linear fracture mechanics (NLFM): the equation of the R-curve is consequently derived. For vanishing crack length QFM predicts a finite ideal strength in agreement with Orowan’s prediction. The different fracture Modes (opening I, sliding II and tearing III), and the stability of the fracture propagations, are treated in a simple way. In contrast to LEFM, QFM has no restrictions on treating defect size and shape. As an example, strengths predicted by QFM are compared with experimental and numerical results on carbon nanotubes containing defects of different size and shape.