Graphene, the 2D form of carbon based material existing as a single layer of atoms arranged in a honeycomb lattice, has set the science and technology sectors alight with interest in the last decade in view of its astounding electrical and thermal properties, combined with its mechanical stiffness, strength and elasticity. Two distinct strategies have been undertaken for graphene production, i.e. the bottom-up and the top-down. The former relies on the generation of graphene from suitably designed molecular building blocks undergoing chemical reaction to form covalently linked 2D networks. The latter occurs via exfoliation of graphite into graphene. Bottom-up techniques, based on the organic syntheses starting from small molecular modules, when performed in liquid media, are both size limited, because macromolecules become more and more insoluble with increasing size, and suffer from the occurrence of side reactions with increasing molecular weight. Because of these reasons such a synthesis has been performed more and more on a solid (ideally catalytically active) surface. Substrate-based growth of single layers can be done also by chemical vapor deposition (CVD) or via reduction of silicon carbide, which unfortunately relies on the ability to follow a narrow thermodynamic path. Top-down approaches can be accomplished under different environmental conditions. Alongside the mechanical cleavage based on the scotch tape approach, liquid-phase exfoliation (LPE) methods are becoming more and more interesting because they are extremely versatile, potentially up-scalable, and can be used to deposit graphene in a variety of environments and on different substrates not available using mechanical cleavage or growth methods. Interestingly, LPE can be applied to produce different layered systems exhibiting different compositions such as BN, MoS2, WS2, NbSe2, and TaS2, thereby enabling the tuning of numerous physico-chemical properties of the material. Furthermore, LPE can be employed to produce graphene-based composites or films, which are key components for many applications, such as thin-film transistors, conductive transparent electrodes for indium tin oxide replacement, e.g. in light-emitting diodes, or photovoltaics. In this review, we highlight the recent progress that has led to successful production of high quality graphene by means of LPE of graphite. In particular, we discuss the mechanisms of exfoliation and methods that are employed for graphene characterization. We then describe a variety of successful liquid-phase exfoliation methods by categorizing them into two major classes, i.e. surfactant-free and surfactant-assisted LPE. Furthermore, exfoliation in aqueous and organic solutions is presented and discussed separately.

Graphene via sonication assisted liquid-phase exfoliation

A method is demonstrated to prepare graphene dispersions at high concentrations, up to 1.2 mg mL(-1), with yields of up to 4 wt% monolayers. This process relies on low-power sonication for long times, up to 460 h. Transmission electron microscopy shows the sonication to reduce the flake size, with flake dimensions scaling as t(-1/2). However, the mean flake length remains above 1 microm for all sonication times studied. Raman spectroscopy shows defects are introduced by the sonication process. However, detailed analysis suggests that predominantly edge, rather than basal-plane, defects are introduced. These dispersions are used to prepare high-quality free-standing graphene films. The dispersions can be heavily diluted by water without sedimentation or aggregation. This method facilitates graphene processing for a range of applications.

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High-concentration solvent exfoliation of graphene.

A method is presented to produce graphene dispersions, stabilized in water by the surfactant sodium cholate, at concentrations up to 0.3 mg/mL. The process uses low power sonication for long times (up to 400 h) followed by centrifugation to yield stable dispersions. The dispersed concentration increases with sonication time while the best quality dispersions are obtained for centrifugation rates between 500 and 2000 rpm. Detailed TEM analysis shows the flakes to consist of 1−10 stacked monolayers with up to 20% of flakes containing just one layer. The average flake consists of ∼4 stacked monolayers and has length and width of ∼1 μm and ∼400 nm, respectively. These dimensions are surprisingly stable under prolonged sonication. However, the mean flake length falls from ∼1 μm to ∼500 nm as the centrifugation rate is increased from 500 to 5000 rpm. Raman spectroscopy shows the flake bodies to be relatively defect-free for centrifugation rates below 2000 rpm. The dispersions can be easily cast into high-qualit...

High-concentration, surfactant-stabilized graphene dispersions.

Due to its unprecedented physical properties, graphene has generated huge interest over the last 7 years. Graphene is generally fabricated in one of two ways: as very high quality sheets produced in limited quantities by micromechanical cleavage or vapor growth or as a rather defective, graphene-like material, graphene oxide, produced in large quantities. However, a growing number of applications would profit from the availability of a method to produce high-quality graphene in large quantities.This Account describes recent work to develop such a processing route inspired by previous theoretical and experimental studies on the solvent dispersion of carbon nanotubes. That work had shown that nanotubes could be effectively dispersed in solvents whose surface energy matched that of the nanotubes. We describe the application of the same approach to the exfoliation of graphite to give graphene in a range of solvents. When graphite powder is exposed to ultrasonication in the presence of a suitable solvent, the ...

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Liquid Exfoliation of Defect-Free Graphene

Current research trends on plastic pollution and ecological impacts on the soil ecosystem: A review

Aqueous suspensions of length selected single-walled carbon nanotubes were studied by atomic force microscopy (AFM) in order to probe the influence of sonication on nanotube scission. The maximum of the tube length distribution, lM, initially exhibits a power law dependence on the sonication time, t  roughly as lM ≈ t-0.5. This and the limiting behavior observed at longer times can be rationalized to first order in terms of a continuum model deriving from polymer physics. In this picture, the strain force associated with cavitation scales with the square of the nanotube length. Scission stops when the strain force falls below the critical value for nanotube disruption.

The mechanism of cavitation-induced scission of single-walled carbon nanotubes.

Future microelectronic and microelectromechanical systems (MEMS) will require a decreased size as well as high electrical conductivity combined with high mechanical strength. Noble metal nanowires are ideal building blocks for these applications due to their excellent electrical and mechanical properties. Since the properties of metal nanowires strongly depend on their size and microstructure, developing an approach to produce and tune wire size and structure is highly relevant. Although a variety of methods such as electrodeposition guided by templates, hydrothermal reduction, colloidal methods, and vapor-phase deposition have been successfully used to synthesize gold, silver, and copper nanowires, most efforts have focused on size-controlling and sizedependent properties. Much less is known about tuning microstructure and about microstructure-dependent properties due to difficulties associated with controlling the microstructure within such small objects. However, it is well established that microscopic defects such as dislocations, stacking faults, twins, and grain boundaries play a key role in bulk-material properties. For example, nanoscale twinned copper foil exhibits a significantly improved strength (10-fold) while maintaining a high electrical conductivity.

Nanoscale twinned copper nanowire formation by direct electrodeposition.

It can be enhanced even more ifthe polymers attached to the surface carry charges. Theresulting electrosteric interaction can be understood interms of the increased osmotic pressure of the counter-ions if the polyelectrolyte chains attached to the surfacesof the particles are to share the same volume.

Thomas Koch

Papers

The mechanism of cavitation-induced scission of single-walled carbon nanotubes.

Nanoscale twinned copper nanowire formation by direct electrodeposition.

Engineering the interaction of latex spheres with charged surfaces: AFM investigation of spherical polyelectrolyte brushes on mica

Investigation of ultra fine particulate matter emission of rubber tires

DNS of momentum and heat transfer over rough surfaces based on realistic combustion chamber deposit geometries