Two-Dimensional MoS2 Nanosheets Derived from Cathodic Exfoliation for Lithium Storage Applications.

The preparation of 2H-phase MoS2 thin nanosheets by electrochemical delamination remains a challenge, despite numerous efforts in this direction. In this work, by choosing appropriate intercalating cations for cathodic delamination, the insertion process was facilitated, leading to a higher degree of exfoliation while maintaining the original 2H-phase of the starting bulk MoS2 material. Specifically, trimethylalkylammonium cations were tested as electrolytes, outperforming their bulkier tetraalkylammonium counterparts, which have been the focus of past studies. The performance of novel electrochemically derived 2H-phase MoS2 nanosheets as electrode material for electrochemical energy storage in lithium-ion batteries was investigated. The lower thickness and thus higher flexibility of cathodically exfoliated MoS2 promoted better electrochemical performance compared to liquid-phase and ultrasonically assisted exfoliated MoS2, both in terms of capacity (447 vs. 371 mA·h·g-1 at 0.2 A·g-1) and rate capability (30% vs. 8% capacity retained when the current density was increased from 0.2 A·g-1 to 5 A·g-1), as well as cycle life (44% vs. 17% capacity retention at 0.2 A·g-1 after 580 cycles). Overall, the present work provides a convenient route for obtaining MoS2 thin nanosheets for their advantageous use as anode material for lithium storage.

NbSe2 Nanosheets/Nanorolls Obtained via Fast and Direct Aqueous Electrochemical Exfoliation for High-Capacity Lithium Storage.

Layered transition-metal dichalcogenides (LTMDs) in two-dimensional (2D) form are attractive for electrochemical energy storage, but research efforts in this realm have so far largely focused on the best-known members of such a family of materials, mainly MoS2, MoSe2, and WS2. To exploit the potential of further, currently less-studied 2D LTMDs, targeted methods for their production, preferably by cost-effective and sustainable means, as well as control over their nanomorphology, are highly desirable. Here, we report a quick and straightforward route for the preparation of 2D NbSe2 and other metallic 2D LTMDs that relies on delaminating their bulk parent solid under aqueous cathodic conditions. Unlike typical electrochemical exfoliation methods for 2D materials, which generally require an additional processing step (e.g., sonication) to complete delamination, the present electrolytic strategy yielded directly exfoliated nano-objects in a very short time (1-2 min) and with significant yields (∼16 wt %). Moreover, the dominant morphology of the exfoliated 2D NbSe2 products could be tuned between rolled-up nanosheets (nanorolls) and unfolded nanosheets, depending on the solvent where the nano-objects were dispersed (water or isopropanol). This rather unusual delamination behavior of NbSe2 was explored and concluded to occur via a redox mechanism that involves some degree of hydrolytic oxidation of the material triggered by the cathodic treatment. The delamination strategy could be extended to other metallic LTMDs, such as NbS2 and VSe2. When tested toward electrochemical lithium storage, electrodes based on the exfoliated NbSe2 products delivered very high capacity values, up to 750-800 mA h g-1 at 0.5 A g-1, where the positive effect of the nanoroll morphology, associated to increased accessibility of the lithium storage sites, was made apparent. Overall, these results are expected to expand the availability of fit-for-purpose 2D LTMDs by resorting to simple and expeditious production strategies of low environmental impact.

Influence of the properties of different graphene-based nanomaterials dispersed in polycaprolactone membranes on astrocytic differentiation.

Composites of polymer and graphene-based nanomaterials (GBNs) combine easy processing onto porous 3D membrane geometries due to the polymer and cellular differentiation stimuli due to GBNs fillers. Aiming to step forward to the clinical application of polymer/GBNs composites, this study performs a systematic and detailed comparative analysis of the influence of the properties of four different GBNs: (i) graphene oxide obtained from graphite chemically processes (GO); (ii) reduced graphene oxide (rGO); (iii) multilayered graphene produced by mechanical exfoliation method (Gmec); and (iv) low-oxidized graphene via anodic exfoliation (Ganodic); dispersed in polycaprolactone (PCL) porous membranes to induce astrocytic differentiation. PCL/GBN flat membranes were fabricated by phase inversion technique and broadly characterized in morphology and topography, chemical structure, hydrophilicity, protein adsorption, and electrical properties. Cellular assays with rat C6 glioma cells, as model for cell-specific astrocytes, were performed. Remarkably, low GBN loading (0.67 wt%) caused an important difference in the response of the C6 differentiation among PCL/GBN membranes. PCL/rGO and PCL/GO membranes presented the highest biomolecule markers for astrocyte differentiation. Our results pointed to the chemical structural defects in rGO and GO nanomaterials and the protein adsorption mechanisms as the most plausible cause conferring distinctive properties to PCL/GBN membranes for the promotion of astrocytic differentiation. Overall, our systematic comparative study provides generalizable conclusions and new evidences to discern the role of GBNs features for future research on 3D PCL/graphene composite hollow fiber membranes for in vitro neural models.

A Simple and Expeditious Route to Phosphate-Functionalized, Water-Processable Graphene for Capacitive Energy Storage.

Phosphate-functionalized carbon-based nanomaterials have attracted significant attention in recent years owing to their outstanding behavior in electrochemical energy-storage devices. In this work, we report a simple approach to obtain phosphate-functionalized graphene (PFG) via anodic exfoliation of graphite at room temperature with a high yield. The graphene nanosheets were obtained via anodic exfoliation of graphite foil using aqueous solutions of H3PO4 or Na3PO4 in the dual role of phosphate sources and electrolytes, and the underlying exfoliation/functionalization mechanisms are proposed. The effect of electrolyte concentration was studied, as low concentrations do not lead to a favorable graphite exfoliation and high concentrations produce fast graphite expansion but poor layer-by-layer delamination. The optimal concentrations are 0.25 M H3PO4 and 0.05 M Na3PO4, which also exhibited the highest phosphorus contents of 2.2 and 1.4 at. %, respectively. Furthermore, when PFG-acid at 0.25 M and PFG-salt at 0.05 M were tested as an electrode material for capacitive energy storage in a three-electrode cell, they achieved a competitive performance of ∼375 F/g (540 F/cm3) and 356 F/g (500 F/cm3), respectively. Finally, devices made up of symmetric electrode cells obtained using PFG-acid at 0.25 M possess energy and power densities up to 17.6 Wh·kg-1 (25.3 Wh·L-1) and 10,200 W/kg; meanwhile, PFG-salt at 0.05 M achieved values of 14.9 Wh·kg-1 (21.3 Wh·L-1) and 9400 W/kg, with 98 and 99% of capacitance retention after 10,000 cycles, respectively. The methodology proposed here also promotes a circular-synthesis process to successfully achieve a more sustainable and greener energy-storage device.

High Performance Na-O2 Batteries and Printed Microsupercapacitors Based on Water-Processable, Biomolecule-Assisted Anodic Graphene.

Integrated approaches that expedite the production and processing of graphene into useful structures and devices, particularly through simple and environmentally friendly strategies, are highly desirable in the efforts to implement this two-dimensional material in state-of-the-art electrochemical energy storage technologies. Here, we introduce natural nucleotides (e.g., adenosine monophosphate) as bifunctional agents for the electrochemical exfoliation and dispersion of graphene nanosheets in water. Acting both as exfoliating electrolytes and colloidal stabilizers, these biomolecules facilitated access to aqueous graphene bio-inks that could be readily processed into aerogels and inkjet-printed interdigitated patterns. Na-O2 batteries assembled with the graphene-derived aerogels as the cathode and a glyme-based electrolyte exhibited a full discharge capacity of ∼3.8 mAh cm-2 at a current density of 0.2 mA cm-2. Moreover, shallow cycling experiments (0.5 mAh cm-2) boasted a capacity retention of 94% after 50 cycles, which outperformed the cycle life of prior graphene-based cathodes for this type of battery. The positive effect of the nucleotide-adsorbed nanosheets on the battery performance is discussed and related to the presence of the phosphate group in these biomolecules. Microsupercapacitors made from the interdigitated graphene patterns as the electrodes also displayed a competitive performance, affording areal and volumetric energy densities of 0.03 µWh cm-2 and 1.2 mWh cm-3 at power densities of 0.003 mW cm-2 and 0.1 W cm-3, respectively. Taken together, by offering a green and straightforward route to different types of functional graphene-based materials, the present results are expected to ease the development of novel energy storage technologies that exploit the attractions of graphene.

Macrophage inflammatory and metabolic responses to graphene-based nanomaterials differing in size and functionalization.

The preparation of graphene-based nanomaterials (GBNs) with appropriate stability and biocompatibility is crucial for their use in biomedical applications. In this work, three GBNs differing in size and/or functionalization have been synthetized and characterized, and their in vitro biological effects were compared. Pegylated graphene oxide (GO-PEG, 200-500 nm) and flavin mononucleotide-stabilized pristine graphene with two different sizes (PG-FMN, 200-400 nm and 100-200 nm) were administered to macrophages, chosen as cellular model due to their key role in the processing of foreign materials and the regulation of inflammatory responses. The results showed that cellular uptake of GBNs was mainly influenced by their lateral size, while the inflammatory potential depended also on the type of functionalization. PG-FMN nanomaterials (both sizes) triggered significantly higher nitric oxide (NO) release, together with some intracellular metabolic changes, similar to those induced by the prototypical inflammatory stimulus LPS. NMR metabolomics revealed that macrophages incubated with smaller PG-FMN displayed increased levels of succinate, itaconate, phosphocholine and phosphocreatine, together with decreased creatine content. The latter two variations were also detected in cells incubated with larger PG-FMN nanosheets. On the other hand, GO-PEG induced a decrease in the inflammatory metabolite succinate and a few other changes distinct from those seen in LPS-stimulated macrophages. Assessment of TNF-α secretion and macrophage surface markers (CD80 and CD206) further corroborated the low inflammatory potential of GO-PEG. Overall, these findings revealed distinct phenotypic and metabolic responses of macrophages to different GBNs, which inform on their immunomodulatory activity and may contribute to guide their therapeutic applications.

Aqueous Cathodic Exfoliation Strategy toward Solution-Processable and Phase-Preserved MoS2 Nanosheets for Energy Storage and Catalytic Applications.

The production of MoS2 nanosheets by electrochemical exfoliation routes holds great promise as a means to access this two-dimensional material in large quantities for different practical applications. However, the use of electrolytes based on synthetic organic salts and solvents, as well as issues related to the unwanted oxidation and/or phase transformation of the exfoliated nanosheets, constitute significant obstacles that hinder the industrial adoption of the electrochemical approach. Here, we introduce a safe and sustainable method for the cathodic delamination of MoS2 that makes use of aqueous solutions of very simple and widely available salts, mainly KCl, as the electrolyte. Combined with an appropriate biomolecule-based solvent transfer protocol, such an electrolytic exfoliation route is shown to afford colloidally dispersed, oxide-free, and phase-preserved MoS2 nanosheets of high structural quality in considerable yields. The mechanisms behind the efficient aqueous delamination of the bulk MoS2 cathode are also discussed and rationalized on the basis of the penetration of hydrated cations from the electrolyte between its layers and the immediate reduction of the accompanying water molecules. An asymmetric supercapacitor assembled with a cathodic MoS2 nanosheet-single walled carbon nanotube hybrid as the positive electrode and activated carbon as the negative electrode delivered energy densities (e.g., 26 W h kg-1 at 750 W kg-1 in 6 M KOH) that were competitive with those of other MoS2-based asymmetric devices. When used as a catalyst for the reduction of nitroarenes, the present cathodically exfoliated nanosheets exhibited one of the highest activities reported so far with MoS2 nanostructures, the origin of which is accounted for as well. Overall, by facilitating access to this two-dimensional material through a particularly simple, efficient, and cost-effective technique, these results should expedite the practical implementation of MoS2 nanosheets in energy storage, catalysis, and beyond.

Electrospun silk fibroin scaffolds coated with reduced graphene promote neurite outgrowth of PC-12 cells under electrical stimulation.

Novel approaches to neural research require biocompatible materials capable to act as electrode structures or scaffolds for tissue engineering in order to stimulate or restore the functionality of damaged tissues. This work offers promising results that indicate the potential use of electrospun silk fibroin (SF) scaffolds coated with reduced graphene oxide (rGO) in this sense. The coated material becomes conductor and electroactive. A complete characterisation of SF/rGO scaffolds is provided in terms of electrochemistry, mechanical behaviour and chemical conformation of fibroin. The excellent biocompatibility of this novel material is proved with cultures of PC-12 cells. The coating with rGO improved the adhesion of cells in comparison with cells growing onto the surface of pure SF scaffolds. Also, the use of SF/rGO scaffolds combined with electrical stimulation promoted the differentiation into neural phenotypes reaching comparable or even superior levels to those obtained by means of the traditional treatment with neural growth factor (NGF).

Aqueous Exfoliation of Transition Metal Dichalcogenides Assisted by DNA/RNA Nucleotides: Catalytically Active and Biocompatible Nanosheets Stabilized by Acid-Base Interactions.

The exfoliation and colloidal stabilization of layered transition metal dichalcogenides (TMDs) in an aqueous medium using functional biomolecules as dispersing agents have a number of potential benefits toward the production and practical use of the corresponding two-dimensional materials, but such a strategy has so far remained underexplored. Here, we report that DNA and RNA nucleotides are highly efficient dispersants in the preparation of stable aqueous suspensions of MoS2 and other TMD nanosheets at significant concentrations (up to 5-10 mg mL-1). Unlike the case of common surfactants, for which adsorption on 2D materials is generally based on weak dispersive forces, the exceptional colloidal stability of the TMD flakes was shown to rely on the presence of relatively strong, specific interactions of Lewis acid-base type between the DNA/RNA nucleotide molecules and the flakes. Moreover, the nucleotide-stabilized MoS2 nanosheets were shown to be efficient catalysts in the reduction of nitroarenes (4-nitrophenol and 4-nitroaniline), thus constituting an attractive alternative to the use of expensive heterogeneous catalysts based on noble metals, and exhibited an electrocatalytic activity toward the hydrogen evolution reaction that was not impaired by the possible presence of nucleotide molecules adsorbed on their active sites. The biocompatibility of these materials was also demonstrated on the basis of cell proliferation and viability assays. Overall, the present work opens new vistas on the colloidal stabilization of 2D materials based on specific interactions that could be useful toward different practical applications.

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.

From graphene oxide to pristine graphene: revealing the inner workings of the full structural restoration.

High temperature annealing is the only method known to date that allows the complete repair of a defective lattice of graphenes derived from graphite oxide, but most of the relevant aspects of such restoration processes are poorly understood. Here, we investigate both experimentally (scanning probe microscopy) and theoretically (molecular dynamics simulations) the thermal evolution of individual graphene oxide sheets, which is rationalized on the basis of the generation and the dynamics of atomic vacancies in the carbon lattice. For unreduced and mildly reduced graphene oxide sheets, the amount of generated vacancies was so large that they disintegrated at 1773-2073 K. By contrast, highly reduced sheets survived annealing and their structure could be completely restored at 2073 K. For the latter, a minor atomic-sized defect with six-fold symmetry was observed and ascribed to a stable cluster of nitrogen dopants. The thermal behavior of the sheets was significantly altered when they were supported on a vacancy-decorated graphite substrate, as well as for the overlapped/stacked sheets. In these cases, a net transfer of carbon atoms between neighboring sheets via atomic vacancies takes place, affording an additional healing process. Direct evidence of sheet coalescence with the step edge of the graphite substrate was also gathered from experiments and theory.

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.

Adsorption of n-alkanes on plasma-oxidized high-strength carbon fibers.

The objective of this work was to characterize the degree of heterogeneity brought about by oxygen plasma treatment of carbon fibers by studying its effects on the adsorption of n-alkanes. Untreated and unsized high-strength carbon fibers were subjected to oxygen plasma treatments with different degrees of severity. A sample of the same material oxidized following a standard industrial method was also studied for comparison. Adsorption of C5-C10n-alkanes at 303-353 K was measured by inverse gas chromatography (IGC). Elution peaks were symmetrical for the fresh and industrially oxidized samples; however, a large extent of asymmetry was observed for the plasma-treated fibers. Differences in surface heterogeneity were quantified in terms of several adsorption thermodynamic magnitudes. Differential heats of adsorption exhibited values similar to those corresponding to the probe-basal plane interaction. The dispersive component of the surface tension of the solids increased clearly upon plasma oxidation, the increase being systematic according to the severity of plasma treatment. It can be concluded that plasma oxidation generates high-surface-energy sites responsible for trapping of n-alkane molecules, this effect being more marked as the chain length increases. The possibility of this effect being associated to creation of micropores was ruled out on the basis of volumetric CO2 adsorption experiments and IGC measurements at finite dilution. Scanning tunneling microscopy observations allowed us to establish a possible connection between fiber surface nanostructure and IGC results. The sites accessible to n-alkane molecules in the industrially oxidized sample seem to be highly disordered, thus leading to a weaker interaction with the adsorbate.