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.
Graphene aerogels derived from a biomolecule‐assisted aqueous electrochemical exfoliation route are explored as cathode materials in sodium–oxygen (Na–O2) batteries. To this end, the natural ...nucleotide adenosine monophosphate (AMP) is used in the multiple roles of exfoliating electrolyte, aqueous dispersant, and functionalizing agent to access high quality, electrocatalytically active graphene nanosheets in colloidal suspension (bioinks). The surface phenomena occurring on the electrochemically derived graphene cathode is thoroughly studied to understand and optimize its electrochemical performance, where a cooperative effect between the nitrogen atoms and phosphates from the AMP molecules is demonstrated. Moreover, the role of the nitrogen atoms in the adenine nucleobase of AMP and short‐chain phosphate is unraveled. Significantly, the use of such cathodes with a proper amount of AMP molecules adsorbed on the graphene nanosheets delivers a discharge capacity as high as 9.6 mAh cm−2 and performs almost 100 cycles with a considerably reduced cell overpotential and a coulombic efficiency of ≈97% at high current density (0.2 mA cm−2). This study opens a path toward the development of environmentally friendly air cathodes by the use of natural nucleotides which offers a great opportunity to explore and manufacture bioinspired cathodes for metal–oxygen batteries.
Graphene air cathode containing a natural nucleotide adenosine monophosphate (AMP) for Na–O2 batteries is reported. This molecule presents multiple roles: exfoliating electrolyte, aqueous dispersant, and functionalizing agent. Moreover, the nitrogen‐containing nucleobase and the acidic POH sites in orthophosphates from AMP work cooperatively to promote the nucleation of NaO2 during discharge while enhancing its dissolution during charge.
Graphene aerogels derived from a biomolecule-assisted aqueous electrochemical exfoliation route are explored as cathode materials in sodium-oxygen (Na-O
) batteries. To this end, the natural ...nucleotide adenosine monophosphate (AMP) is used in the multiple roles of exfoliating electrolyte, aqueous dispersant, and functionalizing agent to access high quality, electrocatalytically active graphene nanosheets in colloidal suspension (bioinks). The surface phenomena occurring on the electrochemically derived graphene cathode is thoroughly studied to understand and optimize its electrochemical performance, where a cooperative effect between the nitrogen atoms and phosphates from the AMP molecules is demonstrated. Moreover, the role of the nitrogen atoms in the adenine nucleobase of AMP and short-chain phosphate is unraveled. Significantly, the use of such cathodes with a proper amount of AMP molecules adsorbed on the graphene nanosheets delivers a discharge capacity as high as 9.6 mAh cm
and performs almost 100 cycles with a considerably reduced cell overpotential and a coulombic efficiency of ≈97% at high current density (0.2 mA cm
). This study opens a path toward the development of environmentally friendly air cathodes by the use of natural nucleotides which offers a great opportunity to explore and manufacture bioinspired cathodes for metal-oxygen batteries.
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)63–) 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.
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.
The complete repair of a defective lattice of individual graphene oxide sheets by high temperature annealing is investigated and rationalized.
Abstract
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 (G
mec
); and (iv) low-oxidized graphene via anodic exfoliation (G
anodic
); 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.
Adsorption and desorption of H2 and D2 from porous carbon materials, such as activated carbon at 77 K, are usually fully reversible with very rapid adsorption/desorption kinetics. The adsorption and ...desorption of H2 and D2 at 77 K on a carbon molecular sieve (Takeda 3A), where the kinetic selectivity was incorporated by carbon deposition, and a carbon, where the pore structure was modified by thermal annealing to give similar pore structure characteristics to the carbon molecular sieve substrate, were studied. The D2 adsorption and desorption kinetics were significantly faster (up to ×1.9) than the corresponding H2 kinetics for specific pressure increments/decrements. This represents the first experimental observation of kinetic isotope quantum molecular sieving in porous materials due to the larger zero-point energy for the lighter H2, resulting in slower adsorption/desorption kinetics compared with the heavier D2. The results are discussed in terms of the adsorption mechanism.
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.
An appealing strategy that overcomes the hydrophobicity of pristine graphene and favors its interaction with biological media is colloidal stabilization in aqueous medium with the support of a ...biomolecule, such as flavin mononucleotide (FMN), as exfoliating/dispersing agent. However, to establish FMN-stabilized graphene (PG-FMN) as suitable for use in biomedicine, its biocompatibility must be proved by a complete assessment of cytotoxicity at the cellular level. Furthermore, if PG-FMN is to be proposed as a theranostic agent, such a study should include both healthy and tumoral cells and its outcome should reveal the nanomaterial as selectively toxic to the latter. Here, we provide an in-depth comparative in vitro analysis of the response of Saos-2 human sarcoma osteoblasts (model tumor cells) and MC3T3-E1 murine preosteoblasts (undifferentiated healthy cells) upon incubation with different concentrations (10–50 μg mL–1) of PG-FMN dispersions constituted by flakes with different average lateral size (90 and 270 nm). Specifically, the impact of PG-FMN on the viability and cell proliferation, reactive oxygen species (ROS) production, and the cellular incorporation process, cell-cycle progression, and apoptosis has been evaluated. PG-FMN was found to be toxic to both types of cells by increasing ROS production and triggering cell-cycle arrest. The present results constitute a cautionary tale on the need to establish the effect of a nanomaterial not only on tumor cells but also on healthy ones before proposing it as anticancer agent.
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.
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