Alain Pénicaud

Singling out the Electrochemistry of Individual Single-Walled Carbon Nanotubes in Solution

Bandgap fluorescence spectroscopy of aqueous, micelle-like suspensions of SWNTs has given access to the electronic energies of individual semiconducting SWNTs, while substantially lower is the success achieved in the determination of the redox properties of SWNTs as individual entities. Here we report an extensive voltammetric and vis-NIR spectroelectrochemical investigation of true solutions of unfunctionalized SWNTs and determine the standard electrochemical potentials of reduction and oxidation as a function of the tube diameter of a large number of semiconducting SWNTs. We also establish the Fermi energy and the exciton binding energy for individual tubes in solution. The linear correlation found between the potentials and the optical transition energies is quantified in two simple equations that allow one to calculate the redox potentials of SWNTs that are insufficiently abundant or absent in the samples.

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.

Stoichiometric control of single walled carbon nanotubes functionalization

Covalent modifications of single-walled carbon nanotubes, while being useful for their manipulation and functionalization, alter the electronic properties through disruption of the nanotube π electronic system. To avoid such negative impact, we demonstrate here that carbon nanotubes can be alkylated in a controlled manner by first preparing and isolating reduced nanotube salts with varied charge/C ratios. The reduction reaction, in the present work performed in THF with a K/naphthalene salt, is almost quantitative and the KCx salt can be isolated with a wide range of x values ranging from 10 to 370, while remaining soluble in DMSO. The reaction of these salt solutions with two alkyl bromide reagents yields functionalized SWCNTs, as demonstrated by combined IR, UV-vis-NIR and Raman spectroscopies, XPS and thermogravimetric measurements. In particular, quantification of the number of functional groups grafted shows a direct correlation with the charge/C ratio of the initial salts. Detailed analysis of Raman spectra confirms this control over the extent of covalent functionalization and shows it is not selective towards any type of SWCNT. This latter observation is ascribed to the isolation of the intermediate salts as solids, resulting in homogenization of charge densities on the reduced CNTs.

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.

Transparent Carbon Nanotube Network for Efficient Electrochemiluminescence Devices

A carbon nanotube-based electrode that combines transparency and good conductivity was used for the first time to develop an electrochemiluminescence (ECL) device. It resulted in an excellent material for ECL applications thanks to the very favorable overpotential of amine oxidation that represents the rate-determining step for the signal generation in both research systems and commercial instrumentation. The use of carbon nanotubes resulted in a ten times higher emission efficiency compared with commercial transparent indium tin oxide (ITO) electrodes. Moreover, application of this material for proof-of-principle ECL imaging was demonstrated, in which micro-beads were used to mimic a real biological sample in order to prove the possibility of obtaining single cell visualization.

Mixed-valence bis(ethylenedithio)tetrathiafulvalenium (BEDT-TTF) monolayers sandwiched between extended close-packed Keggin-type molecular metal oxide cluster arrays: synthesis, unprecedented acentric structure, and preliminary conducting and e.s.r. properties of (BEDT-TTF)8SiW12O40

The synthesis, acentric sandwich structure, and preliminary electronic properties of conducting plate-like single-crystals of a 3,4;3′,4-bis(ethylenedithio)-2,2′,5,5′-tetrathiafulvalene (BEDT-TTF) salt of the tetravalent 12-tungstosilicate molecular anion SiW12O404– are described as well as evidence for a correlation between the non-centrosymmetrical character of the bi-dimensional solid and that of the discrete molecular cluster anion.

High Yield Synthesis of Aspect Ratio Controlled Graphenic Materials from Anthracite Coal in Supercritical Fluids

This paper rationalizes the green and scalable synthesis of graphenic materials of different aspect ratios using anthracite coal as a single source material under different supercritical environments. Single layer, monodisperse graphene oxide quantum dots (GQDs) are obtained at high yield (55 wt %) from anthracite coal in supercritical water. The obtained GQDs are ∼3 nm in lateral size and display a high fluorescence quantum yield of 28%. They show high cell viability and are readily used for imaging cancer cells. In an analogous experiment, high aspect ratio graphenic materials with ribbon-like morphology (GRs) are synthesized from the same source material in supercritical ethanol at a yield of 6.4 wt %. A thin film of GRs with 68% transparency shows a surface resistance of 9.3 kΩ/sq. This is apparently the demonstration of anthracite coal as a source for electrically conductive graphenic materials.

First cation radical mixed-valence hybrid salts of the paramagnetic octahedral cluster Nb6Cl183–. Preparation, crystal structures, and conducting and magnetic properties of pentakis(2,3,6,7-tetramethyl-1,4,5,8-tetra-selena- and -thia-fulvalenium) hexachloro(dodeca-µ2-chloro-octahedro-hexaniobate)

Electrically conducting single crystals of (D)5Nb6Cl18(CH2Cl2)0.5[D = 2,3,6,7-tetramethyl-1,4,5,8-tetra-selena- and -thia-fulvalene (TMTTF) and (TMTTF) in (1) and (2) respectively] have been grown electrochemically; their unique crystal structure shows a one-dimensional stack with a periodicity twice that of the Bechgaard salts; the observed Curie–Weiss behaviour of the magnetic susceptibility demonstrates the presence of one non-interacting spin per unit-cell; this is not purely a cluster spin as indicated by e.s.r. spectroscopy.