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.
Holey graphene nanosheets are potentially useful in several relevant technological applications, including electrochemical energy storage and molecular separation. Access to this material is mostly ...accomplished by resorting to standard graphene oxides obtained by common routes (e.g., the Hummers method). However, such a type of highly oxidized graphenes may not be the best option as a precursor to holey graphene on account of their chemical/structural heterogeneity and harsh synthesis conditions. Here, we report the use of highly oxidized graphene nanosheets derived by an electrochemical exfoliation/oxidation strategy as an alternative precursor to holey graphene. Compared to a standard graphene oxide with the same extent of oxidation, the electrochemically derived precursor exhibited larger aromatic domains, which provided a structural basis for its higher electrical conductivity, as well as smaller and denser oxidized regions, associated to a higher chemical homogeneity and lability of its oxygen-containing functional groups. Through selective chemical etching of the oxidized domains, the latter feature was exploited to afford holey graphene nanosheets having smaller and more uniform holes. When used as an electrode material for electrochemical charge storage, the electrochemically derived holey graphene outperformed its standard graphene oxide-based counterpart in terms of capacity and energy density. Overall, boasting distinct structural and chemical characteristics, highly oxidized graphene obtained by electrochemical means can be regarded as a prospective advantageous precursor to many graphene-based materials whose preparation has traditionally relied on the processing of graphene oxides.
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•Electrochemically-derived highly oxidized graphene is a proper precursor to holey graphene.•Anodic graphene offers advantages as precursor for holey graphene over graphene oxide.•Electrodes based on this new type of holey graphene exhibit improved capacity and energy density.
Electrolytic--usually referred to as electrochemical--exfoliation of graphite in water under anodic potential holds enormous promise as a simple, green and high-yield method for the mass production ...of graphene, but currently suffers from several drawbacks that hinder its widespread adoption, one of the most critical being the oxidation and subsequent structural degradation of the carbon lattice that is usually associated with such a production process. To overcome this and other limitations, we introduce and implement the concept of multifunctional electrolytes. The latter are amphiphilic anions (mostly polyaromatic hydrocarbons appended with sulfonate groups) that play different relevant roles as (1) an intercalating electrolyte to trigger exfoliation of graphite into graphene flakes, (2) a dispersant to afford stable aqueous colloidal suspensions of the flakes suitable for further use, (3) a sacrificial agent to prevent graphene oxidation during exfoliation and (4) a linker to promote nanoparticle anchoring on the graphene flakes, yielding functional hybrids. The implementation of this strategy with some selected amphiphiles even furnishes anodically exfoliated graphenes of a quality similar to that of flakes produced by direct, ultrasound- or shear-induced exfoliation of graphite in the liquid phase (i.e., almost oxide- and defect-free). These high quality materials were used for the preparation of catalytically efficient graphene-Pt nanoparticle hybrids, as demonstrated by model reactions (reduction of nitroarenes). The multifunctional performance of these electrolytes is also discussed and rationalized, and a mechanistic picture of their oxidation-preventing ability is proposed. Overall, the present results open the prospect of anodic exfoliation as a competitive method for the production of very high quality graphene flakes.
The liquid-phase exfoliation of graphitic carbon nitride (g-C3N4) to afford colloidal dispersions of two-dimensional flakes constitutes an attractive route to facilitate the processing and ...implementation of this novel material toward different technological applications, but quantitative knowledge about its dispersibility in solvents is lacking. Here, we investigate the dispersion behavior of exfoliated g-C3N4 in a wide range of solvents and evaluate the obtained results on the basis of solvent surface energy and Hildebrand/Hansen solubility parameters. Estimates of the three Hansen parameters for exfoliated g-C3N4 from the experimentally derived data yielded δD ≈ 17.8 MPa1/2, δP ≈ 10.8 MPa1/2, and δH ≈ 15.4 MPa1/2. The relatively high δH value suggested that, contrary to the case of other two-dimensional materials (e.g., graphene or transition metal dichalcogenides), hydrogen-bonding plays a substantial role in the efficient interaction, and thus dispersibility, of exfoliated g-C3N4 with solvents. Such an outcome was attributed to a high density of primary and/or secondary amines in the material, the presence of which was associated with incomplete condensation of the structure. Furthermore, cell proliferation tests carried out on thin films of exfoliated g-C3N4 using murine fibroblasts suggested that this material is highly biocompatible and noncytotoxic. Finally, the exfoliated g-C3N4 flakes were used as supports in the synthesis of Pd nanoparticles, and the resulting hybrids exhibited an exceptional catalytic activity in the reduction of nitroarenes.
Detailed knowledge of the dispersion behavior of reduced graphene oxide (RGO) in solvents is important for its practical applications. Such behavior is expected to be different to that observed for ...pristine graphene, as a result of the chemically heterogeneous structure of RGO (patchwork of pristine and highly oxidized graphene domains). We have investigated the dispersibility of RGO in a wide range of solvents and analyzed the results on the basis of solvent surface energies and Hansen solubility parameters. Although RGO exhibited some features that are characteristic of pristine graphene, its dispersion behavior was dominated by its oxidized graphene domains, with alcohols being commonly the most successful solvents. Estimates of the effective Hansen parameters for RGO derived from the experimental data (δD≈16.9MPa1/2, δP≈10.7MPa1/2 and δH≈14.1MPa1/2) were consistent with this view and afforded the rational design of solvent mixtures that surpassed the best single-component solvents in terms of dispersed amount of RGO. RGO–polymer composites could then be readily prepared in the best performing solvents. Overall, the present results provide a guide to the processing of RGO in the liquid phase with practical utility in the preparation of different graphene-based materials.
Due to its large surface area, high electrical conductivity as well as mechanical and thermal stability, pristine graphene has the potential to be an excellent support for metal nanoparticles (NPs), ...but the scarce amount of intrinsic chemical groups/defects in its structure that could act as anchoring sites for the NPs hinders this type of use. Here, a simple strategy based on the stabilization of pristine graphene in aqueous dispersion with the assistance of a low amount of flavin mononucleotide (FMN) is shown to yield a material that combines high electrical conductivity and abundance of extrinsic anchoring sites, so that pristine graphene–metal (Pd and Pt) NP hybrids with good dispersion and metal loading can be obtained from FMN–stabilized graphene. The activity of these hybrids towards the methanol oxidation reaction (MOR) both in acidic and alkaline media is studied by cyclic voltammetry (CV) and their stability investigated by chronoamperometry. The pristine graphene–Pt NP hybrid prepared by this simple, eco–friendly protocol is demonstrated to outperform most previously reported pristine graphene– and reduced graphene oxide–metal NP hybrids as electrocatalyst for the MOR, both in terms of catalytic activity and stability, avoiding at the same time the use of harsh chemicals or complex synthetic routes.
The molecular functionalization of two-dimensional MoS2 is of practical relevance with a view to, for example, facilitating its liquid-phase processing or enhancing its performance in target ...applications. While derivatization of metallic 1T-phase MoS2 nanosheets has been relatively well studied, progress involving their thermodynamically stable, 2H-phase counterpart has been more limited due to the lower chemical reactivity of the latter. Here, we report a simple electrolytic strategy to functionalize 2H-phase MoS2 nanosheets with molecular groups derived from organoiodides. Upon cathodic treatment of a pre-expanded MoS2 crystal in an electrolyte containing the organoiodide, water-dispersible nanosheets derivatized with acetic acid or aniline moieties (∼0.10 molecular groups inserted per surface sulfur atom) were obtained. Analysis of the functionalization process indicated it to be enabled by the external supply of electrons from the cathodic potential, although they could also be sourced from a proper reducing agent, as well as by the presence of intrinsic defects in the 2H-phase MoS2 lattice (e.g., sulfur vacancies), where the molecular groups can bind. The acetic acid-functionalized nanosheets were tested as a non-noble metal-based catalyst for nitroarene and organic dye reduction, which is of practical utility in environmental remediation and chemical synthesis, and exhibited a markedly enhanced activity, surpassing that of other (1T- or 2H-phase) MoS2 materials and most non-noble metal catalysts previously reported for this application. The reduction kinetics (reaction order) was seen to correlate with the net electric charge of the nitroarene/dye molecules, which was ascribed to the distinct abilities of the latter to diffuse to the catalyst surface. The functionalized MoS2 catalyst also worked efficiently at realistic (i.e., high) reactant concentrations, as well as with binary and ternary mixtures of the reactants, and could be immobilized on a polymeric scaffold to expedite its manipulation and reuse.
The stabilization of reduced graphene oxide (RGO) sheets in aqueous dispersion using a wide range of surfactants of anionic, non-ionic and zwitterionic type has been investigated and compared under ...different conditions of pH, surfactant and RGO concentration, or sheet size. The observed differences in the performance of the surfactants were rationalized on the basis of their chemical structure (e.g., alkylic vs. aromatic hydrophobic tail or sulfonic vs. carboxylic polar head), thus providing a reference framework in the selection of appropriate surfactants for the processing of RGO suspensions towards particular purposes. RGO-surfactant composite paper-like films were also prepared through vacuum filtration of the corresponding mixed dispersions and their main characteristics were investigated. The composite paper-like films were also electrochemically characterized. Those prepared with two specific surfactants exhibited a high capacitance in relation to their surfactant-free counterpart.
The thickness of unreduced and chemically reduced graphene oxide sheets deposited on different substrates was measured by different scanning probe microscopy (SPM) variants. Inaccurate and ...inconsistent results are obtained when thickness is derived as a sheet-to-substrate height, which is the typically employed approach to determine such a parameter. Measuring overlapped regions between different sheets leads to more realistic thickness values, which clearly reflect, for example, the removal of oxygen functionalities from graphene oxide following chemical reduction. The results underline the precautions that are required to draw valid conclusions from SPM-derived thickness data of chemically modified graphenes.
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.
The recent emergence of environmentally friendly methods for the production of graphene from graphite oxide is highlighted.
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