L. Guardia

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.

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.

Developing green photochemical approaches towards the synthesis of carbon nanofiber- and graphene-supported silver nanoparticles and their use in the catalytic reduction of 4-nitrophenol

A green, photochemical approach for the liquid-phase synthesis of carbon nanomaterial-supported silver nanoparticles is proposed. The method is based on irradiating a colloidal dispersion containing the carbon nanomaterial, a metal precursor and an environmentally friendly reducing agent (bioreductant) with UV light at room temperature. Two representative carbon materials have been used, namely platelet-type graphite nanofibers and graphene oxide. The experimental conditions that afford the photochemical growth of the nanoparticles on each carbon support are also investigated and discussed. In addition, the resulting carbon–silver nanoparticle hybrids are demonstrated to be notably effective catalysts for the reduction of 4-nitrophenol to 4-aminophenol with NaBH4. Particularly, the graphene oxide-based samples were seen to exhibit exceptional catalytic activity towards such reaction. Finally, it is also shown that with a suitable choice of bioreductant the present UV approach can afford highly reduced graphene oxide samples comparable to those attained with well-known, efficient chemical reductants (e. g., hydrazine at ∼100 °C), thus constituting an attractive room temperature alternative to such reduction methods.

Highly efficient silver-assisted reduction of graphene oxide dispersions at room temperature: mechanism, and catalytic and electrochemical performance of the resulting hybrids

Metal-assisted reduction of graphene oxide (GO) has recently emerged as a fast, efficient and room-temperature method towards the preparation of chemically derived graphene, but according to the mechanisms of reduction that have been proposed, not all relevant metals (e.g., Ag) should be a priori effective for this purpose. Here, we show that aqueous GO dispersions can be very efficiently reduced at room temperature with NaBH4 using Ag nanoparticles (Ag NPs) as catalysts, either generated in situ from appropriate precursors (AgNO3) or added to the dispersions as pre-formed objects. We propose and investigate a reduction mechanism that involves the charging of the Ag NPs with excess electrons obtained from the oxidation of a product of the spontaneous hydrolysis of NaBH4 in the aqueous medium. These excess electrons are then transferred to the GO sheets, triggering their reduction. The catalytic and electrochemical performance of the reduced GO–Ag NP hybrids that result from this process has also been examined. In particular, the hybrids are seen to exhibit very high catalytic activity in the reduction of 4-nitrophenol to 4-aminophenol as a model reaction, and are also effective towards the electrochemical reduction of H2O2.

Impact of Covalent Functionalization on the Aqueous Processability, Catalytic Activity, and Biocompatibility of Chemically Exfoliated MoS2 Nanosheets

Chemically exfoliated MoS2 (ce-MoS2) has emerged in recent years as an attractive two-dimensional material for use in relevant technological applications, but fully exploiting its potential and versatility will most probably require the deployment of appropriate chemical modification strategies. Here, we demonstrate that extensive covalent functionalization of ce-MoS2 nanosheets with acetic acid groups (∼0.4 groups grafted per MoS2 unit) based on the organoiodide chemistry brings a number of benefits in terms of their processability and functionality. Specifically, the acetic acid-functionalized nanosheets were furnished with long-term (>6 months) colloidal stability in aqueous medium at relatively high concentrations, exhibited a markedly improved temporal retention of catalytic activity toward the reduction of nitroarenes, and could be more effectively coupled with silver nanoparticles to form hybrid nanostructures. Furthermore, in vitro cell proliferation tests carried out with murine fibroblasts suggested that the chemical derivatization had a positive effect on the biocompatibility of ce-MoS2. A hydrothermal annealing procedure was also implemented to promote the structural conversion of the functionalized nanosheets from the 1T phase that was induced during the chemical exfoliation step to the original 2H phase of the starting bulk material, while retaining at the same time the aqueous colloidal stability afforded by the presence of the acetic acid groups. Overall, by highlighting the benefits of this type of chemical derivatization, the present work should contribute to strengthen the position of ce-MoS2 as a two-dimensional material of significant practical utility.