S. Villar-Rodil

Graphene Oxide Dispersions in Organic Solvents

The dispersion behavior of graphene oxide in different organic solvents has been investigated. As-prepared graphite oxide could be dispersed in N, N-dimethylformamide, N-methyl-2-pyrrolidone, tetrahydrofuran, and ethylene glycol. In all of these solvents, full exfoliation of the graphite oxide material into individual, single-layer graphene oxide sheets was achieved by sonication. The graphene oxide dispersions exhibited long-term stability and were made of sheets between a few hundred nanometers and a few micrometers large, similar to the case of graphene oxide dispersions in water. These results should facilitate the manipulation and processing of graphene-based materials for different applications.

Atomic Force and Scanning Tunneling Microscopy Imaging of Graphene Nanosheets Derived from Graphite Oxide

Graphene nanosheets produced in the form of stable aqueous dispersions by chemical reduction of graphene oxide and deposited onto graphite substrates have been investigated by atomic force and scanning tunneling microscopy (AFM/STM). The chemically reduced graphene oxide nanosheets were hardly distinguishable from their unreduced counterparts in the topographic AFM images. However, they could be readily discriminated through phase imaging in the attractive regime of tapping-mode AFM, probably because of differences in hydrophilicity arising from their distinct oxygen contents. The chemically reduced nanosheets displayed a smoothly undulated, globular morphology on the nanometer scale, with typical vertical variations in the subnanometer range and lateral feature sizes of approximately 5-10 nm. Such morphology was attributed to be the result of significant structural disorder in the carbon skeleton, which originates during the strong oxidation that leads to graphene oxide and remains after chemical reduction. Direct evidence of structural disorder was provided by atomic-scale STM imaging, which revealed an absence of long-range periodicity in the graphene nanosheets. Only structured domains a few nanometers large were observed instead. Likewise, the nanosheet edges appeared atomically rough and ill-defined, though smooth on the nanometer scale. The unreduced graphene oxide nanosheets could only be imaged by STM at very low tunneling currents (approximately 1 pA), being visualized in some cases with inverted contrast relative to the graphite substrate, a result that was attributed to their extremely low conductivity. Complementary characterization of the unreduced and chemically reduced nanosheets was carried out by thermogravimetric analysis as well as UV-visible absorption and X-ray photoelectron and Raman spectroscopies. In particular, the somewhat puzzling Raman results were interpreted to be the result of an amorphous character of the graphene oxide material.

Preparation of graphene dispersions and graphene-polymer composites in organic media

Graphene nanosheets in the form of chemically reduced graphene oxide have been prepared in organic media without the need to chemically functionalise the starting graphene oxide nanosheets. The preparation procedure is simple and similar to that previously used for the production of stable aqueous dispersions of graphene nanosheets. The resulting organic dispersions are homogeneous, exhibit long-term stability and are made up of graphene sheets a few hundred nanometres large. The ability to prepare graphene dispersions in organic media facilitates their combination with polymers, such as polyacrylonitrile and poly(methyl methacrylate), to yield homogeneous composites.

Environmentally friendly approaches toward the mass production of processable graphene from graphite oxide

Graphene has attracted a great deal of scientific interest in latter years owing to its unique properties, with many prospective applications being actively investigated at present. However, the actual implementation of graphene in technological uses will depend critically on the development of appropriate methodologies for its mass production. In this regard, one of the most promising approaches is based on the exfoliation and reduction of graphite oxide. Graphenes derived from graphite oxide can be prepared at low cost and high throughput, can be further processed in a number of solvents, and are chemically versatile, among other attractive features. In an environment-conscious world, the availability of green approaches toward graphene production would also constitute an added advantage. During the last year, different environmentally friendly methods for the production of graphene from graphite oxide have emerged, which we highlight here. These are based on solvothermal and electrochemical processes, as well as on the use of green reductants. Several open questions and possible future directions for this research topic are also discussed.

Towards full repair of defects in reduced graphene oxide films by two-step graphitization

The complete restoration of a perfect carbon lattice has been a central issue in the research on graphene derived from graphite oxide since this preparation route was first proposed several years ago, but such a goal has so far remained elusive. Here, we demonstrate that the highly defective structure of reduced graphene oxide sheets assembled into free-standing, paper-like films can be fully repaired by means of high temperature annealing (graphitization). Characterization of the films by X-ray photoelectron and Raman spectroscopy, X-ray diffraction and scanning tunneling microscopy indicated that the main stages in the transformation of the films were (i) complete removal of oxygen functional groups and generation of atomic vacancies (up to 1,500 °C), and (ii) vacancy annihilation and coalescence of adjacent overlapping sheets to yield continuous polycrystalline layers (1,800–2,700 °C) similar to those of highly oriented graphites. The prevailing type of defect in the polycrystalline layers were the grain boundaries separating neighboring domains, which were typically a few hundred nanometers in lateral size, exhibited long-range graphitic order and were virtually free of even atomic-sized defects. The electrical conductivity of the annealed films was as high as 577,000 S·m−1, which is by far the largest value reported to date for any material derived from graphene oxide, and strategies for further improvement without the need to resort to higher annealing temperatures are suggested. Overall, this work opens the prospect of truly achieving a complete restoration of the carbon lattice in graphene oxide materials.Graphical abstract

Studies on pyrolysis of Nomex polyaramid fibers

Abstract The pyrolysis behavior of Nomex [poly( m -phenylene isophtalamide)] fibers under argon was studied using thermoanalytical and infrared spectroscopic methods to get direct information on the progressive changes undergone by the solid material and its carbon fiber residues. Simultaneous thermogravimetry (TG)–differential thermal analysis (DTA) measurements were carried out in a thermobalance in order to establish the different steps in the thermal degradation of Nomex. Samples pyrolyzed to various carefully controlled extents were prepared in the same thermobalance by heating under the same conditions to different temperatures selected as a function of the TG/DTA results. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of all samples were obtained in order to follow changes in chemical composition along the pyrolysis process. Rupture of hydrogen bonds takes place in the 300–400°C interval, being followed by a complex thermal event involving two weight loss steps and three endothermal DTA peaks. An intermediate aryl nitrile forms from heterolytic decomposition of Nomex above 433°C and decomposes totally at 698°C. From 473°C onwards, the DRIFTS spectra evidence a drop in intensity of amide bands in parallel with increases in primary amine and carbonyl groups. Total disappearance of amide bands (607°C) is followed by a DTA exotherm at 627°C attributed to condensation reactions. Accordingly, CC skeletal vibrations in polyaromatic networks develop confirming the occurrence of a progressive loss of heteroatoms and an increase in aromaticity.

Production of aqueous dispersions of inorganic graphene analogues by exfoliation and stabilization with non-ionic surfactants

The production of stable aqueous suspensions of several inorganic graphene analogues [MoS2, WS2 and hexagonal BN (h-BN)] by exfoliation of the corresponding bulk layered materials via sonication has been investigated, with a particular focus on the use and efficacy of non-ionic surfactants as dispersing agents. For the two metal dichalcogenides, some non-ionic surfactants afforded highly concentrated dispersions (up to several milligrams per milliliter), outperforming dispersions produced with an ionic surfactant or in water–alcohol mixtures in the absence of surfactant, which were taken as reference systems. Furthermore, suspensions with metal dichalcogenide to surfactant concentration ratios as high as 2.4–3.5 could be attained through appropriate processing of the as-prepared suspensions, which should be advantageous for the preparation of materials and devices with minimal interference from the surfactant. In the case of h-BN, all surfactants failed to yield suspensions with concentration significantly above that achieved in water alone, which was attributed to the chemical peculiarities of h-BN platelets exfoliated in water via sonication. The suspensions produced with the most successful non-ionic surfactants exhibited long-term stability (months) and were made up of platelets with lateral dimensions from 50 up to a few hundred nanometers and thicknesses of a few to several nanometers. Raman spectroscopy analysis suggested that edge effects dominate the detailed spectral features for the MoS2 and WS2 platelets, in particular the position of their Raman bands. Such results indicate that extreme caution must be exercised when using this technique to gauge the thickness of small-sized MoS2/WS2 platelets, such as those typically produced by liquid-phase exfoliation approaches. Overall, the present results should facilitate the manipulation and use of these two-dimensional materials in several prospective application areas, such as biomedicine or photocatalysis.

Achieving Extremely Concentrated Aqueous Dispersions of Graphene Flakes and Catalytically Efficient Graphene-Metal Nanoparticle Hybrids with Flavin Mononucleotide as a High-Performance Stabilizer

The stable dispersion of graphene flakes in an aqueous medium is highly desirable for the development of materials based on this two-dimensional carbon structure, but current production protocols that make use of a number of surfactants typically suffer from limitations regarding graphene concentration or the amount of surfactant required to colloidally stabilize the sheets. Here, we demonstrate that an innocuous and readily available derivative of vitamin B2, namely the sodium salt of flavin mononucleotide (FMNS), is a highly efficient dispersant in the preparation of aqueous dispersions of defect-free, few-layer graphene flakes. Most notably, graphene concentrations in water as high as ∼50 mg mL(-1) using low amounts of FMNS (FMNS/graphene mass ratios of about 0.04) could be attained, which facilitated the formation of free-standing graphene films displaying high electrical conductivity (∼52000 S m(-1)) without the need of carrying out thermal annealing or other types of post-treatment. The excellent performance of FMNS as a graphene dispersant could be attributed to the combined effect of strong adsorption on the sheets through the isoalloxazine moiety of the molecule and efficient colloidal stabilization provided by its negatively charged phosphate group. The FMNS-stabilized graphene sheets could be decorated with nanoparticles of several noble metals (Ag, Pd, and Pt), and the resulting hybrids exhibited a high catalytic activity in the reduction of nitroarenes and electroreduction of oxygen. Overall, the present results should expedite the processing and implementation of graphene in, e.g., conductive inks, composites, and hybrid materials with practical utility in a wide range of applications.

Chemically Exfoliated MoS2 Nanosheets as an Efficient Catalyst for Reduction Reactions in the Aqueous Phase

Chemically exfoliated MoS2 (ce-MoS2) nanosheets that incorporate a large fraction of metallic 1T phase have been recently shown to possess a high electrocatalytic activity in the hydrogen evolution reaction, but the potential of this two-dimensional material as a catalyst has otherwise remained mostly uncharted. Here, we demonstrate that ce-MoS2 nanosheets are efficient catalysts for a number of model reduction reactions (namely, those of 4-nitrophenol, 4-nitroaniline, methyl orange, and [Fe(CN)6](3-)) carried out in aqueous medium using NaBH4 as a reductant. The performance of the nanosheets in these reactions is found to be comparable to that of many noble metal-based catalysts. The possible reaction pathways involving ce-MoS2 as a catalyst are also discussed and investigated. Overall, the present results expand the scope of this two-dimensional material as a competitive, inexpensive, and earth-abundant catalyst.

Biomolecule-assisted exfoliation and dispersion of graphene and other two-dimensional materials: a review of recent progress and applications

Financial support from the Spanish MINECO and the European Regional Development Fund (ERDF) through project MAT2015-69844-R as well as from “Plan de Ciencia, Tecnologia e Innovacion 2013-2017 del Principado de Asturias” and ERDF (project GRUPIN14-056) is gratefully acknowledged.