Functionalized graphene sheets for polymer nanocomposites

Mechanical properties of methyl functionalized graphene: a molecular dynamics study

Molecular dynamics simulations have been performed to study the mechanical properties of methyl (CH(3)) functionalized graphene. It is found that the mechanical properties of functionalized graphene greatly depend on the location, distribution and coverage of CH(3) radicals on graphene. Surface functionalization exhibits a much stronger influence on the mechanical properties than edge functionalization. For patterned functionalization on graphene surfaces, the radicals arranged in lines perpendicular to the tensile direction lead to larger strength deterioration than those parallel to the tensile direction. For random functionalization, the elastic modulus of graphene decreases gradually with increasing CH(3) coverage, while both the strength and fracture strain show a sharp drop at low coverage. When CH(3) coverage reaches saturation, the elastic modulus, strength and fracture strain of graphene drop by as much as 18%, 43% and 47%, respectively.

Modeling of thermal transport in pillared-graphene architectures

Carbon nanotubes (CNT) and graphene are considered as potential future candidates for many nano/microscale integrated devices due to their superior thermal properties. Both systems, however, exhibit significant anisotropy in their thermal conduction, limiting their performance as three-dimensional thermal transport materials. From thermal management perspective, one way to tailor this anisotropy is to consider designing alternative carbon-based architectures. This paper investigates the thermal transport in one such novel architecture-a pillared-graphene (PG) network nanostructure which combines graphene sheets and carbon nanotubes to create a three-dimensional network. Nonequilibrium molecular dynamics simulations have been carried out using the AIREBO potential to calculate the thermal conductivity of pillared-graphene structures along parallel (in-plane) as well as perpendicular (out-of-plane) directions with respect to the graphene plane. The resulting thermal conductivity values for PG systems are discussed and compared with simulated values for pure CNT and graphite. Our results show that in these PG structures, the thermal transport is governed by the minimum interpillar distance and the CNT-pillar length. This is primarily attributed to scattering of phonons occurring at the CNT-graphene junctions in these nanostructures. We foresee that such architecture could potentially be used as a template for designing future structurally stable microscale systems with tailorable in-plane and out-of-plane thermal transport.

Toward ubiquitous environmental gas sensors-capitalizing on the promise of graphene

Atomically thin sheets of carbon known as "graphene" have captured the imagination of much of the scientific world during the past few years. Although these single sheets of graphite were under our noses for years-within technologies ranging from the humble pencil, which has been around since at least 1565 (Petroski, H. The Pencil: A History of Design and Circumstance; Alfred A. Knopf: New York, 1993), to modern nuclear reactors-graphene was merely considered as part of graphite's crystal structure until 2004, when Novoselov, Geim, and colleagues (Science 2004, 306, 666-669) first presented some of the surprising electrical properties of graphene layers they had isolated by mechanically peeling sheets off graphite crystals. Today, graphene's unique electronic structures and properties, bolstered by other intriguing properties discovered in the intervening years, threaten the dominance of carbon nanotubes, a more mature allotrope of carbon, in potential applications from electronics to sensors. In this review, we will consider the promise of graphene for producing small-scale gas sensors for environmental monitoring.

A scalable, solution-phase processing route to graphene oxide and graphene ultralarge sheets

High yield production of graphene oxide and graphene sheets with an ultralarge size (up to approximately 200 microm) was realized using a modified solution-phase method.

Enhanced mechanical properties of nanocomposites at low graphene content

In this study, the mechanical properties of epoxy nanocomposites with graphene platelets, single-walled carbon nanotubes, and multi-walled carbon nanotube additives were compared at a nanofiller weight fraction of 0.1 +/- 0.002%. The mechanical properties measured were the Young's modulus, ultimate tensile strength, fracture toughness, fracture energy, and the material's resistance to fatigue crack propagation. The results indicate that graphene platelets significantly out-perform carbon nanotube additives. The Young's modulus of the graphene nanocomposite was approximately 31% greater than the pristine epoxy as compared to approximately 3% increase for single-walled carbon nanotubes. The tensile strength of the baseline epoxy was enhanced by approximately 40% with graphene platelets compared to approximately 14% improvement for multi-walled carbon nanotubes. The mode I fracture toughness of the nanocomposite with graphene platelets showed approximately 53% increase over the epoxy compared to approximately 20% improvement for multi-walled carbon nanotubes. The fatigue resistance results also showed significantly different trends. While the fatigue suppression response of nanotube/epoxy composites degrades dramatically as the stress intensity factor amplitude is increased, the reverse effect is seen for graphene-based nanocomposites. The superiority of graphene platelets over carbon nanotubes in terms of mechanical properties enhancement may be related to their high specific surface area, enhanced nanofiller-matrix adhesion/interlocking arising from their wrinkled (rough) surface, as well as the two-dimensional (planar) geometry of graphene platelets.

Functionalized graphene sheet colloids for enhanced fuel/propellant combustion

We have compared the combustion of the monopropellant nitromethane with that of nitromethane containing colloidal particles of functionalized graphene sheets or metal hydroxides. The linear steady-state burning rates of the monopropellant and colloidal suspensions were determined at room temperature, under a range of pressures (3.35-14.4 MPa) using argon as a pressurizing fluid. The ignition temperatures were lowered and burning rates increased for the colloidal suspensions compared to those of the liquid monopropellant alone, with the graphene sheet suspension having significantly greater burning rates (i.e., greater than 175%). The relative change in burning rate from neat nitromethane increased with increasing concentrations of fuel additives and decreased with increasing pressure until at high pressures no enhancement was found.

Processing of graphene for electrochemical application: noncovalently functionalize graphene sheets with water-soluble electroactive methylene green

To explore graphene applications in various fields, the processability of graphene becomes one of the important key issues, particularly with the increasing availability of synthetic graphene approaches, because the direct dispersion of hydrophobic graphene in water is prone to forming agglomerates irreversibly. Here, a facile method is proposed to increase the dispersity of graphene through noncovalent functionalization graphene with a water-soluble aromatic electroactive dye, methylene green (MG), during chemical reduction of graphene oxide (GO) with hydrazine. Atomic force microscopic and UV-vis spectrophotometric results demonstrate that chemically reduced graphene (CRG) functionalized with MG (CRG-MG) is well-dispersed into water through the coulomb repulsion between MG-adsorbed CRG sheets. The electrochemical properties of the formed CRG-MG are investigated, and the results demonstrate that CRG-MG confined onto a glassy carbon (GC) electrode has lower charge-transfer resistance and better electrocatalytic activity toward the oxidation of NADH, in relation to pristine CRG (i.e., without MG functionalization). This method not only offers a facile approach to dispersing graphene in water but also is envisaged to be useful for investigations on graphene-based electrochemistry.

Spontaneous formation of nanostructures in graphene

We report in-depth studies of nanostructures formed in graphene on soft substrates. Periodic buckles with amplitude of nanometer scale spontaneously appear at edges of single-layer membranes after cooling of samples from above the substrate's glass-transition temperature. Stress modulation at step-edges between single- and few-layer further induces penetrating nanobuckles into the few-layer. The evolvement of single-layer folding into double and triple-layer stacks at elevated temperature was also probed in detail, and we show that the developed interfaces are clear of polymer contamination. Our results underscore the possibility to construct diverse nanostructures and to design novel devices based on graphene.

Graphene: the new two-dimensional nanomaterial

Every few years, a new material with unique properties emerges and fascinates the scientific community, typical recent examples being high-temperature superconductors and carbon nanotubes. Graphene is the latest sensation with unusual properties, such as half-integer quantum Hall effect and ballistic electron transport. This two-dimensional material which is the parent of all graphitic carbon forms is strictly expected to comprise a single layer, but there is considerable interest in investigating two-layer and few-layer graphenes as well. Synthesis and characterization of graphenes pose challenges, but there has been considerable progress in the last year or so. Herein, we present the status of graphene research which includes aspects related to synthesis, characterization, structure, and properties.

Graphene oxide: structural analysis and application as a highly transparent support for electron microscopy

We report on the structural analysis of graphene oxide (GO) by transmission electron microscopy (TEM). Electron diffraction shows that on average the underlying carbon lattice maintains the order and lattice-spacings of graphene; a structure that is clearly resolved in 80 kV aberration-corrected atomic resolution TEM images. These results also reveal that single GO sheets are highly electron transparent and stable in the electron beam, and hence ideal support films for the study of nanoparticles and macromolecules by TEM. We demonstrate this through the structural analysis of physiological ferritin, an iron-storage protein.

Electrical conductivity of graphene films with a poly(allylamine hydrochloride) supporting layer

The electrical conductivity of graphene oxide (GO) and reduced graphene oxide (RGO) films with poly(allylamine hydrochloride) (PAH) supporting layers is investigated. Graphene-PAH hybrid films were produced in a two-step procedure that consisted of vacuum filtration for GO (or RGO) dispersion to fabricate the graphene thin films on quartz substrates, followed by the deposition of PAH onto the graphene films via solution casting. Highly selective deposition of the PAH layer on the graphene sheets was confirmed through the detection of the fluorescence signals of hybridized Cy3-DNA onto the PAH-coated graphene surfaces. In this case, electrostatic interaction plays an important role in the selective deposition process. Interestingly, it was found that the electrical conductivity of RGO films was significantly enhanced by 120% after PAH treatment, whereas that of the GO films was reduced by 98% of its initial conductivity. This finding was interpreted in terms of the molecular structure and oxygen functionalities of GO and RGO films combined with the ionic conduction characteristics of hydrated PAH on the RGO film.

Confinement effects on glass transition temperature, transition breadth, and expansivity: comparison of ellipsometry and fluorescence measurements on polystyrene films

Using ellipsometry, we characterized the nanoconfinement effect on the glass transition temperature (T (g)of supported polystyrene (PS) films employing two methods: the intersection of fits to the temperature (Tdependences of rubbery- and glassy-state thicknesses, and the transition mid-point between rubbery- and glassy-state expansivities. The results demonstrate a strong effect of thickness: T(g) (bulk) - T(g)(23 nm) = 10 degrees C. The T -range needed for accurate measurement increases significantly with decreasing thickness, an effect that arises from the broadening of the transition with confinement and a region below T (g) where expansivity slowly decreases with decreasing T . As determined from expansivities, the T (g) breadth triples in going from bulk films to a 21-nm-thick film; this broadening of the transition may be a more dramatic effect of confinement than the T (g) reduction itself. In contrast, there is little effect of confinement on the rubbery- and glassy-state expansivities. Compared with ellipsometry, T (g) 's from fluorescence agree well in bulk films but yield lower values in nanoconfined films: T (g)(bulk) - T (g)(23 nm) = 15( degrees ) C via fluorescence. This small difference in the T (g) confinement effect reflects differences in how fluorescence and ellipsometry report "average T (g) " with confinement. With decreasing nanoscale thickness, fluorescence may slightly overweight the contribution of the free-surface layer while ellipsometry may evenly weight or underweight its contribution.

Chemical functionalization of graphene

Experimental and theoretical results on chemical functionalization of graphene are reviewed. Using hydrogenated graphene as a model system, general principles of the chemical functionalization are formulated and discussed. It is shown that, as a rule, 100% coverage of graphene by complex functional groups (in contrast with hydrogen and fluorine) is unreachable. A possible destruction of graphene nanoribbons by fluorine is considered. The functionalization of infinite graphene and graphene nanoribbons by oxygen and by hydrofluoric acid is simulated step by step.