Carlos Drummond

Deconstructing Graphite: Graphenide Solutions

Growing interest in graphene over past few years has prompted researchers to find new routes for producing this material other than mechanical exfoliation or growth from silicon carbide. Chemical vapor deposition on metallic substrates now allows researchers to produce continuous graphene films over large areas. In parallel, researchers will need liquid, large scale, formulations of graphene to produce functional graphene materials that take advantage of graphenes mechanical, electrical, and barrier properties. In this Account, we describe methods for creating graphene solutions from graphite. Graphite provides a cheap source of carbon, but graphite is insoluble. With extensive sonication, it can be dispersed in organic solvents or water with adequate additives. Nevertheless, this process usually creates cracks and defects in the graphite. On the other hand, graphite intercalation compounds (GICs) provide a means to dissolve rather than disperse graphite. GICS can be obtained through the reaction of alkali metals with graphite. These compounds are a source of graphenide salts and also serve as an excellent electronic model of graphene due to the decoupling between graphene layers. The graphenide macroions, negatively charged graphene sheets, form supple two-dimensional polyelectrolytes that spontaneously dissolve in some organic solvents. The entropic gain from the dissolution of counterions and the increased degrees of freedom of graphene in solution drives this process. Notably, we can obtain graphenide solutions in easily processable solvents with low boiling points such as tetrahydrofuran or cyclopentylmethylether. We performed a statistical analysis of high resolution transmission electronic micrographs of graphene sheets deposited on grids from GICs solution to show that the dissolved material has been fully exfoliated. The thickness distribution peaks with single layers and includes a few double- or triple-layer objects. Light scattering analysis of the solutions shows the presence of two-dimensional objects. The typical size of the dissolved flakes can be determined by either static or dynamic light scattering (DLS) using models available in the literature for disk-shape objects. A mean lateral size of ca. 1 μm is typically observed. We also used DLS to monitor the reaggregation that occurs as these sensitive solutions are exposed to air. The graphenide solutions reported in this Account can be used to deposit random arrays of graphene flakes and large single flakes of a lateral size of tens of micrometers onto different substrates. Using the graphenide solutions described in this Account, we foresee the large-scale production of graphene-based printings, coatings, and composites.

Surfactant-free single-layer graphene in water

Dispersing graphite in water to obtain true (single-layer) graphene in bulk quantity in a liquid has been an unreachable goal for materials scientists in the past decade. Similarly, a diagnostic tool to identify solubilized graphene in situ has been long awaited. Here we show that homogeneous stable dispersions of single-layer graphene (SLG) in water can be obtained by mixing graphenide (negatively charged graphene) solutions in tetrahydrofuran with degassed water and evaporating the organic solvent. In situ Raman spectroscopy of these aqueous dispersions shows all the expected characteristics of SLG. Transmission electron and atomic force microscopies on deposits confirm the single-layer character. The resulting additive-free stable water dispersions contain 400 m2 l-1 of developed graphene surface. Films prepared from these dispersions exhibit a conductivity of up to 32 kS m-1.

Portrait of carbon nanotube salts as soluble polyelectrolytes

Soluble single-walled carbon nanotube (SWNT) salts have been synthesized with varying negative charges, and their saturation solubilities in DMSO measured. A thermodynamic model of the Gibbs free energy of dissolution, widely used for polyelectrolytes, reproduces the experimental results with striking agreement. SWNT salts appear as a paradigm of stiff polyelectrolytes.

Solutions of fully exfoliated individual graphene flakes in low boiling point solvents

Graphenide solutions (solutions of negatively charged graphene flakes) have been prepared in low boiling point solvents such as tetrahydrofuran (THF) by dissolution of the graphite intercalation compound (GIC) KC8. The presence of two-dimensional objects in solution, with an average lateral size of over one micron, is evidenced by light scattering analysis. High resolution transmission electron microscopy analysis shows that the solubilized graphene flakes are exclusively single and double layers with no evidence for thicker species. Molecular dynamics simulations support the graphene folding, observed in TEM, and suggest it is triggered by solvent nanodrops.

Water-ions induced nanostructuration of hydrophobic polymer surfaces.

When hydrophobic surfaces are in contact with water in ambient conditions a layer of reduced density is present at the interface, preventing the intimate contact between the two phases. Reducing the extent of this layer by degassing the water can have remarkable implications for the interaction between the two phases. The enhanced proximity between a hydrophobic polymer film and an aqueous solution can induce a self-assembled nanostructure on the solid surface through the development of an electro-hydrodynamic instability, due to the adsorption of the water-ions (hydronium and hydroxyl) at the interface. The self-assembled structure spontaneously relaxes back to the original flat morphology after few weeks at room temperature. This instability and the self-assembled structure are controlled by the hydrophobic surface charge, which is determined by the pH of the aqueous phase, and by the amount of gas dissolved. This effect can be easily adjusted to modify different hydrophobic polymeric substrates at the submicrometer level, opening pathways for producing controlled patterns at the nanoscale in a single simple waterborne step.

Dendritic Carrier Based on PEG: Design and Degradation of Acid-sensitive Dendrimer-like Poly(ethylene oxide)s

Degradable dendrimer-like PEOs were designed using an original ABC-type branching agent featuring a cleavable ketal group, following an iterative divergent approach based on the anionic ring opening polymerization (AROP) of ethylene oxide and arborization of PEO chain ends. A seventh generation dendrimer-like PEO carrying 192 peripheral hydroxyls and exhibiting a molar mass of 446 kg · mol(-1) was obtained in this way. The chemical degradation of these dendritic scaffolds was next successfully accomplished under acidic conditions, forming linear PEO chains of low molar mass (≈2 kg · mol(-1)), as monitored by (1)H NMR, SEC, and MALDI-TOF mass spectrometry as well as by AFM.

Boundary Lubricant Polymer Films: Effect of Cross-Linking

We have studied the adsorption and lubricant properties of a multifunctional triblock copolymer poly(L-lysine)-b-poly(acrylic acid)-b-poly(L-lysine). In particular, we investigated the nature of the layer adsorbed under different conditions of polymer and salt concentration and the lubricant properties of the polymer layer before and after its chemical cross-linking by bridging the poly(acrylic acid) blocks. We found that the amount of polymer adsorbed is controlled by the ionic strength and the polymer concentration in the solution. In all cases, the self-assembled polymer layer is a poor lubricant before cross-linking, but the cohesion and load-carrying ability of the layer are substantially improved by this reaction. However, the chemically cross-linked coating has a limited deformation capacity as a consequence of its permanent network nature, and irreversible damage is observed after excessive strain of the film.

Boundary lubricant films under shear: effect of roughness and adhesion

The normal interaction and the behavior under shear of mica surfaces covered by two different triblock copolymers of polylysine-polydimethysiloxane-polylysine were studied by combining the capabilities of the surface forces apparatus and the atomic force microscopy. At low pH values these copolymers spontaneously adsorb on the negatively charged mica surfaces from aqueous solutions as a consequence of the positive charge of the polylysine moieties. The morphology of the adsorbed layer is determined by the molecular structure of the particular copolymer investigated. This morphology plays a fundamental role on the behavior of the adsorbed layers under shear and compression. While nonadhesive smooth layers oppose an extremely small resistance to sliding, the presence of asperities even at the nanometric scale originates a frictional resistance to the motion. The behavior of uniform nonadhesive nanorough surfaces under shear can be quantitatively understood in terms of a simple multistable thermally activated junction model. The electric charge of the adsorbed copolymer molecules and hence the adhesion energy between the coated surfaces can be modified by varying the pH of the surrounding media. In the presence of an adhesive interaction between the surfaces the behavior under shear is strongly modified. Time-dependent mechanisms of energy dissipation have to be evoked in order to explain the changes observed.

Triblock copolymer lubricant films under shear : Effect of molecular Cross-Linking

The normal interaction and the behavior under shear of mica surfaces covered by a triblock copolymer of poly(L-lysine)-b-polydimethysiloxane-b-poly(L-lysine) (Plys40-b-PDMS40-b-Plys40) before and after cross-linking reaction with two dicarboxylic acids were studied, combining the capabilities of the surface forces apparatus and atomic force microscopy (AFM). At low pH values, this copolymer spontaneously adsorbed on the negatively charged mica surfaces from aqueous solutions as a consequence of the positive charge of the polylysine moieties, forming an extremely smooth boundary layer. This smooth layer displays a very small resistance to shear under small loads, exhibiting outstanding lubrication properties. Nevertheless, the fusion of two contacting layers can be induced by compression at a certain pressure, a process that causes a marked increase in the friction forces. Cross-linking of the adsorbed polymer molecules by covalent bond formation with dicarboxylic acids increases the mechanical stability of the adsorbed layers and hinders the fusion of the boundary layers under pressure but impairs its lubrication properties to a certain extent.

On the conformational state of molecules in molecularly thin shearing films

In their recent PNAS article, Rosenhek-Goldian et al. (1) write: “Intermittent sliding (stick–slip motion) between solids … even in the presence of lubricants, is believed to occur by shear-induced fluidization of the lubricant film (slip), followed by its resolidification (stick).” The authors describe their results on the stick–slip of a commonly studied liquid, octmethylcyclotetrasiloxane (OMCTS), whose soft, quasi-spherical molecules have a diameter of ∼1 nm, and measured no dilatency at the ∼0.1-nm resolution level during rapid slips, and conclude “… that, in contrast to accepted and long-standing belief, shear melting (fluidization) of thin lubricant layers does not occur in stick–slip sliding … ” and “… that other mechanisms, such as intralayer slip within the lubricant film, or at its interface with the confining surfaces, may be the dominant dissipation modes” (1). Rosenhek-Goldian et al. suggest that these two alternative mechanisms may be the ones that occur in general with other liquid lubricant systems.