Graphene-related Materials Research Articles

Morphology and properties evolution upon ring-opening polymerization during extrusion of cyclic butylene terephthalate and graphene-related-materials into thermally conductive nanocomposites

In this work, the study of thermal conductivity before and after in-situ ring-opening polymerization of cyclic butylene terephthalate into poly (butylene terephthalate) in presence of graphene-related materials (GRM) is addressed, to gain insight in the modification of nanocomposites morphology upon polymerization. Five types of GRM were used: one type of graphite nanoplatelets, two different grades of reduced graphene oxide (rGO) and the same rGO grades after thermal annealing for 1h at 1700°C under vacuum to reduce their defectiveness. Polymerization of CBT into pCBT, morphology and nanoparticle organization were investigated by means of differential scanning calorimetry, electron microscopy and rheology. Electrical and thermal properties were investigated by means of volumetric resistivity and bulk thermal conductivity measurement. In particular, the reduction of nanoflake aspect ratio during ring-opening polymerization was found to have a detrimental effect on both electrical and thermal conductivities in nanocomposites.

Effect of processing conditions on the thermal and electrical conductivity of poly (butylene terephthalate) nanocomposites prepared via ring-opening polymerization

Successful preparation of polymer nanocomposites, exploiting graphene-related materials, via melt mixing technology requires precise design, optimization and control of processing. In the present work, the effect of different processing parameters during the preparation of poly (butylene terephthalate) nanocomposites, through ring-opening polymerization of cyclic butylene terephthalate in presence of graphite nanoplatelets (GNP), was thoroughly addressed. Processing temperature (240°C or 260°C), extrusion time (5 or 10min) and shear rate (50 or 100rpm) were varied by means of a full factorial design of experiment approach, leading to the preparation of polybutylene terephthalate/GNP nanocomposite in 8 different processing conditions. Morphology and quality of GNP were investigated by means of electron microscopy, X-ray photoelectron spectroscopy, thermogravimetry and Raman spectroscopy. Molecular weight of the polymer matrix in nanocomposites and nanoflake dispersion were experimentally determined as a function of the different processing conditions. The effect of transformation parameters on electrical and thermal properties was studied by means of electrical and thermal conductivity measurement. Heat and charge transport performance evidenced a clear correlation with the dispersion and fragmentation of the GNP nanoflakes; in particular, gentle processing conditions (low shear rate, short mixing time) turned out to be the most favourable condition to obtain high conductivity values.

DFT comparison of structural and electronic properties of graphene and germanene: monolayer and bilayer systems

Two-dimensional honeycomb systems such as graphene and graphene-related materials have attracted enormous technological interest due to the wide range of applications from composites to conductive coatings. Based on density functional calculations we present a comparison of structural and electronic properties of monolayer (and bilayer) graphene and germanene, particularly, germanene have been synthesized on MoS2 with the Dirac cone properties. Our results show that germanene prefers a buckled structural conformation, which is slight more stable respect to its planar conformation. The well-known structure of graphene is compared with the buckled and planar structures of germanene at level of local density approximation. Graphene and germanene, in monolayer pristine conformations, exhibit massless Dirac fermions close to the K point. However, in germanene the top valence and bottom conduction bands are found at lower energies respect to graphene corresponding bands. The Fermi velocity in germanene is around 30% lower compared to graphene. Bilayer graphene show an anisotropic Dirac cone, while in bilayer germanene a small band gap at K point of ∼0.05 eV is detected. Our results are found to be in agreement with previous theoretical reports, and these pretend to give a compressible description from fundamental point of view.

Identification of slow relaxing spin components by pulse EPR techniques in graphene-related materials

Electron Paramagnetic Resonance (EPR) is a powerful technique that is suitable to study graphene-related materials. The challenging ability requested to the spectroscopy is its capability to resolve the variety of structures, relatively similar, that are obtained in materials produced through different methods, but that also coexist inside a single sample. In general, because of the intrinsic inhomogeneity of the samples, the EPR spectra are therefore a superposition of spectra coming from different structures. We show that by pulse EPR techniques (echo-detected EPR, ESEEM and Mims ENDOR) we can identify and characterize species with slow spin relaxing properties. These species are generally called molecular states, and are likely small pieces of graphenic structures of limited dimensions, thus conveniently described by a molecular approach. We have studied commercial reduced graphene oxide and chemically exfoliated graphite, which are characterized by different EPR spectra. Hyperfine spectroscopies enabled us to characterize the molecular components of the different materials, especially in terms of the interaction of the unpaired electrons with protons (number of protons and hyperfine coupling constants). We also obtained useful precious information about extent of delocalization of the molecular states.

Graphene oxide-TiO2 and reduced graphene oxide-TiO2 nanocomposites: Insight in charge-carrier lifetime measurements

A hybrid nanocomposites containing nanocrystalline TiO2 and graphene related materials (graphene oxide and reduced graphene oxide) were successfully prepared elevated pressure using hydrothermal method. The structural and textural properties as well as the presence of different carbonaceous structures on the surface of obtained nanocomposites have been characterized by means of XRD, FTIR/DRS and Raman spectroscopy. FTIR/DRS analysis allowed to find characteristic peaks of graphene oxide: epoxy stretching at 1229cm−1 and alkoxy stretching vibration at 1059cm−1. In the spectrum of nanocomposites containing rGO, all the bands decreased or even disappear. It can be indicated that GO was successfully reduced to rGO. SEM studies showed difference between graphene oxide (aggregated, crumpled, non-transparent) and reduced graphene oxide (thin and transparent) flakes. The Time Resolved Microwave Conductivity analysis have been utilized to investigate the excess of charge-carrier lifetimes in TiO2-graphene oxide and TiO2-reduced graphene oxide hybrid nanocomposites. It was generally concluded that modification of starting TiO2 with carbonaceous precursors leads to the slight increase of intensity of the TRMC signals, indicating that more electrons are induced in the conduction band of hybrid nanocomposites. It is also worth mentioning that the decay of studied signals of modified powders depends on added carbon precursor, indicating that the type of deposited species may affect the slight or significant suppression of recombination or charge-carriers trapping.

Studying lowest energy structures of carbon clusters by bond-order empirical potentials

A very recently developed optimization algorithm for carbon clusters (C n s) (Yen and Lai J Chem Phys 142:084313, 2015) is combined separately with different empirical bond-order potentials which were proposed also for carbon materials, and they are applied to calculate the lowest energy structures of C n s studying their structural changes at different size n. Based on predicted structures, we evaluate the practicality of four analytic bond-order empirical potentials, namely the Tersoff, Tersoff–Erhart–Albe, first-generation Brenner and second-generation Brenner (SGB) potentials. Generally, we found that the cluster C n (n = 3–60) obtained by the SGB potential undergoes a series of dramatic structural transitions, i.e., from a linear → a single ring → a multi-ring/quasi-two-dimensional bowl-like → three-dimensional fullerene-like shape; such variability of structural forms was not seen in the other three potentials. On closer examination of the C n s calculated using this potential and further comparing them with those obtained by the semiempirical density functional tight-binding theory calculations, we found that these C n are more realistic than similar works reported in the literature. In this respect, due to its potential applications in the study of chemically complex systems of different atoms especially chemical reactions (Che et al. Theor Chem Acc 102:346, 1999), the SGB potential can, moreover, be used to investigate larger size C n , and calculated structural results by this potential are naturally input configurations for higher-level density functional theory calculations. Another most remarkable finding in the present work is the C n results calculated by Tersoff–Erhart–Albe empirical potential. It predicts a two-dimensional development of graphene structure, exhibiting always a zigzag edge in the optimized clusters. This empirical potential can thus be applied to study graphene-related materials such as that shown in a recent paper (Yoon et al. J Chem Phys 139:204702, 2013).

Effect of morphology and defectiveness of graphene-related materials on the electrical and thermal conductivity of their polymer nanocomposites

In this work, electrically and thermally conductive poly (butylene terephthalate) nanocomposites were prepared by in-situ ring-opening polymerization of cyclic butylene terephthalate (CBT) in presence of a tin-based catalyst. One type of graphite nanoplatelets (GNP) and two different grades of reduced graphene oxide (rGO) were used. Furthermore, high temperature annealing treatment under vacuum at 1700{\deg}C was carried out on both RGO to reduce their defectiveness and study the correlation between the electrical/thermal properties of the nanocomposites and the nanoflakes structure/defectiveness. The morphology and quality of the nanomaterials were investigated by means of electron microscopy, x-ray photoelectron spectroscopy, thermogravimetry and Raman spectroscopy. Thermal, mechanical and electrical properties of the nanocomposites were investigated by means of rheology, dynamic mechanical thermal analysis, volumetric resistivity and thermal conductivity measurements. Physical properties of nanocomposites were correlated with the structure and defectiveness of nanoflakes, evidencing a strong dependence of properties on nanoflakes structure and defectiveness. In particular, a significant enhancement of both thermal and electrical conductivities was demonstrated upon the reduction of nanoflakes defectiveness.

Non-Precious Metal Electrocatalysts for the Oxygen Reduction Reaction Based on Nitrogen Doped Graphene: Catalysts Development and Electrode Structure Design

The design of new functional nanomaterials for developing new electrocatalysts for oxygen reduction reaction (ORR) in acid and alkaline media. The research has been focused on developing carbon nanostructures especially graphene/graphene oxide materials with superior structural and functional properties for fuel cell applications. Electrocatalysts based on graphene and graphene-oxide (GO) are more homogeneous and possess properties such as excellent conductivity, good chemical stability and can be functionalized in a controlled manner. To facilitate fabrication, immobilization and distribution of non-precious metal nanoparticles, various types of graphene (e.g. reduced graphene oxide, RGO, and CFx graphene with carbon black) have been modified with the different transition metals (e.g. Co, Ni, Ag, Au, Cu, Mn) analogue of polynuclear Prussian Blue, namely with ultra-thin Co, and Ni hexacyanoferrate layers. Following the heat-treatment step at higher temperatures, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic sites are generated. Their formation and distribution are facilitated by the voltammetric potential cycling in KOH electrolyte. The most promising electrocatalytic results with respect to the reduction of oxygen (the highest currents and the most positive electroreduction potentials) have been obtained when graphene nanostructures are combined or intermixed with Vulcan XC-72R nanoparticles. What is even more important that, due to the presence of the polynuclear cyanoferrate modifier or linker, the amounts of the undesirable hydrogen peroxide intermediate are significantly decreased. An electrocatalytic system, that utilizes metal hexacyanometallates nanoparticles modified graphene and graphene related materials, is developed and characterized here using transmission electron microscopy and such electrochemical diagnostic techniques as cyclic volammetry and rotating ring-disk voltammetry in a 0.5 M H2SO4 electrolyte and in a 0.1 M KOH electrolyte and upon introduction (as cathode) to the low-temperature hydrogen-oxygen fuel cell. Comparative measurements have been performed against the model noble metal (Vulcan-supported platinum nanoparticles) catalyst.

Effect of thermal annealing on the heat transfer properties of reduced graphite oxide flakes: A nanoscale characterization via scanning thermal microscopy

This paper reports on the thermal properties of reduced graphite oxide (RGO) flakes, studied by means of scanning thermal microscopy (SThM). This technique was demonstrated to allow thermal characterization of the flakes with a spatial resolution of the order of a few tens of nanometers, while recording nanoscale topography at the same time. Several individual RGO flakes were analyzed by SThM, both as obtained after conventional thermal reduction and after a subsequent annealing at 1700 °C. Significant differences in the thermal maps were observed between pristine and annealed flakes, reflecting higher heat dissipation on annealed RGO flakes compared with pristine ones. This result was correlated with the reduction of RGO structure defectiveness. In particular, a substantial reduction of oxidized groups and sp3 carbons upon annealing was proven by X-ray photoelectron and Raman spectroscopies, while the increase of crystalline order was demonstrated by X-ray diffraction, in terms of higher correlation lengths both along and perpendicular to the graphene planes. Results presented in this paper provide experimental evidence for the qualitative correlation between the defectiveness of graphene-related materials and their thermal conductivity, which is clearly crucial for the exploitation of these materials into thermally conductive nanocomposites.

Brianyoungite/Graphene Oxide Coordination Composites for High-Performance Cu(2+) Adsorption and Tunable Deep-Red Photoluminescence.

Graphene oxide (GO) is a good adsorbent for heavy-metal ions because the oxygen functional groups offer active adsorption sites, but a small-size GO with dense oxygen-containing groups has high water solubility causing difficulty in separation. Herein, GO is bound to large brianyoungite (BY) by Zn-O coordination via a hydrothermal reaction that produces BY-GO composites with hollow spherical and flakelike morphologies that are easy to remove. By producing abundant oxygen-containing groups on GO, the Cu(2+) adsorption capacity increases to 1724.1 mg/g, which is the highest value in graphene-related materials. The experimental and theoretical analysis clearly shows that the infrared spectral shifts toward the low-frequency side of C-O-H and O═C-O bending vibrations in the BY-GO composites stem from the Zn(2+) (or Cu(2+)) coordination with O atoms in GO. The BY-GO also exhibits tunable deep-red photoluminescence up to 750 nm with a quantum yield of about 1%, which may be useful in infrared optoelectronic devices and solar energy exploitation. The photoluminescence which is different from that previously reported from chemically derived GO can be attributed to the optical transition in the disorder-induced localized states of the carbon-oxygen functional groups.

Synthesis, toxicity, biocompatibility, and biomedical applications of graphene and graphene-related materials.

Graphene is a two-dimensional atomic crystal, and since its development it has been applied in many novel ways in both research and industry. Graphene possesses unique properties, and it has been used in many applications including sensors, batteries, fuel cells, supercapacitors, transistors, components of high-strength machinery, and display screens in mobile devices. In the past decade, the biomedical applications of graphene have attracted much interest. Graphene has been reported to have antibacterial, antiplatelet, and anticancer activities. Several salient features of graphene make it a potential candidate for biological and biomedical applications. The synthesis, toxicity, biocompatibility, and biomedical applications of graphene are fundamental issues that require thorough investigation in any kind of applications related to human welfare. Therefore, this review addresses the various methods available for the synthesis of graphene, with special reference to biological synthesis, and highlights the biological applications of graphene with a focus on cancer therapy, drug delivery, bio-imaging, and tissue engineering, together with a brief discussion of the challenges and future perspectives of graphene. We hope to provide a comprehensive review of the latest progress in research on graphene, from synthesis to applications.

Toxicity Evaluation of Graphene Oxide in Kidneys of Sprague-Dawley Rats.

Recently, graphene and graphene-related materials have attracted a great deal of attention due their unique physical, chemical, and biocompatibility properties and to their applications in biotechnology and medicine. However, the reports on the potential toxicity of graphene oxide (GO) in biological systems are very few. The present study investigated the response of kidneys in male Sprague-Dawley rats following exposure to 0, 10, 20 and 40 mg/Kg GO for five days. The results showed that administration of GOs significantly increased the activities of superoxide dismutase, catalase and glutathione peroxidase in a dose-dependent manner in the kidneys compared with control group. Serum creatinine and blood urea nitrogen levels were also significantly increased in rats intoxicated with GO compared with the control group. There was a significant elevation in the levels of hydrogen peroxide and lipid hydro peroxide in GOs-treated rats compared to control animals. Histopathological evaluation showed significant morphological alterations of kidneys in GO-treated rats compared to controls. Taken together, the results of this study demonstrate that GO is nephrotoxic and its toxicity may be mediated through oxidative stress. In the present work, however, we only provided preliminary information on toxicity of GO in rats; further experimental verification and mechanistic elucidation are required before GO widely used for biomedical applications.

Microstructure, residual stress, and intermolecular force distribution maps of graphene/polymer hybrid composites: Nanoscale morphology-promoted synergistic effects

Intrinsically conducting (or π-conjugated) polymers and graphene related materials are attractive for manufacturing hybrid nanocomposite materials and devices owing to their multifunction at the constituents' interface i.e. graphene/conducting polymer. To utilize these materials, understanding of and relating their microscopic internal structure, surface morphology, and physical properties become indispensable. In this work, we report on the electrochemically synthesized conducting polymers with electrochemically processed graphene nanosheets forming hybrid films and the enhancement of mechanical properties of polymers arising due to synergistic effects promoted by nanostructured morphology and vicinal polymeric chain ordering induced by embedded or impregnated graphene nanosheets. We investigated surface topography, chain ordering, residual stress distribution, force curves and force volume imaging using micro-Raman spectroscopy, Raman mapping, atomic force microscopy and force spectroscopy gaining insights into structural and interfacial bonding, conjugation length distribution, and intermolecular forces. Traditional force curves measure the force felt by the tip as it approaches and retracts from a point on the sample surface (i.e. tip-sample adhesion force), whereas force volume is an array of force curves over an extended sample area. Moreover, detailed structural studies are able to demonstrate that the bonding configurations and structural conformations intertwined with crystallinity and surface chemistry in both the polymers and graphene derivatives nanosheets have a strong effect on nanoscale intermolecular forces and surface elasticity maps. Meanwhile the spring constant (k) estimated from the force curves using elastic contact model was synchronously enhanced for hybrid composites with interspersed graphene sheets on polymer chains forming a kind of ordered stacked structure. Alternatively, the mechanical properties of polymers are improved following in the order: PAni/ErGO > PPy/ErGO > PAni/GO > PPy/GO > PAni ≥ PPy. The electrostatic force between the tip and residual charged polymers surface, are also discussed in terms of multilayer model consisting of three layers analogous to electrostatic double-layer. Furthermore, the information in the force volume measurement was decoupled from topographic data offering new insights into the materials' surface and mechanical properties of hybrid nanocomposites. These measurements are complemented with electron microscopy and X-ray diffraction revealing their surface morphology, intrinsic structure and crystallinity having far reaching implications and impacts on alternative renewable electrochemical energy, photovoltaic and aerospace device applications.

Interaction of graphene-related materials with human intestinal cells: an in vitro approach.

Graphene-related materials (GRM) inherit unique combinations of physicochemical properties which offer a high potential for technological as well as biomedical applications. It is not clear which physicochemical properties are the most relevant factors influencing the behavior of GRM in complex biological environments. In this study we have focused on the interaction of GRM, especially graphene oxide (GO), and Caco-2 cells in vitro. We mimiked stomach transition by acid-treatment of two representative GRM followed by analysis of their physicochemical properties. No significant changes in the material properties or cell viability of exposed Caco-2 cells in respect to untreated GRM could be detected. Furthermore, we explored the interaction of four different GO and Caco-2 cells to identify relevant physicochemical properties for the establishment of a material property-biological response relationship. Despite close interaction with the cell surface and the formation of reactive oxygen species (ROS), no acute toxicity was found for any of the applied GO (concentration range 0-80 μg ml(-1)) after 24 h and 48 h exposure. Graphene nanoplatelet aggregates led to low acute toxicity at high concentrations, indicating that aggregation, the number of layers or the C/O ratio have a more pronounced effect on the cell viability than the lateral size alone.

Nanocomposites for automotive: enhanced graphene-based polymer materials and multi-scale approach

The present initiative provides a summary and overview on graphene related materials (GRM) for automotive applications and investigates efficient methods to integrate graphene as polymer reinforcements within composite materials for energy-efficient and safe vehicles (EESVs). An approach that starts from the nanoscale through the graphene elaboration by experiments to meso/macro-scale by continuum mechanics modelling is discussed with respect to some limiting factors in terms of the large scale production, the interfacial behaviour, the amount of wrinkling and network structure. Finally, a strategy for modelling such a composite is elaborated in the framework of the Graphene Flagship to well understand such limitations for a full applicability of graphene. It is anticipated that this initiative will advance innovative lightweight graphene composites and their related modelling, designing, manufacturing, and joining capabilities suitable for automotive industry which requires unique levels of affordability, mechanical performance, green environmental impact and energy efficiency. This leads to complete understanding of the new graphene composites and their applicability in high-volume production scenarios.

Reductively PEGylated carbon nanomaterials and their use to nucleate 3D protein crystals: a comparison of dimensionality.

A range of carbon nanomaterials, with varying dimensionality, were dispersed by a non-damaging and versatile chemical reduction route, and subsequently grafted by reaction with methoxy polyethylene glycol (mPEG) monobromides. The use of carbon nanomaterials with different geometries provides both a systematic comparison of surface modification chemistry and the opportunity to study factors affecting specific applications. Multi-walled carbon nanotubes, single-walled carbon nanotubes, graphite nanoplatelets, exfoliated few layer graphite and carbon black were functionalized with mPEG-Br, yielding grafting ratios relative to the nanocarbon framework between ca. 7 and 135 wt%; the products were characterised by Raman spectroscopy, TGA-MS, and electron microscopy. The functionalized materials were tested as nucleants by subjecting them to rigorous protein crystallization studies. Sparsely functionalized flat sheet geometries proved exceptionally effective at inducing crystallization of six proteins. This new class of nucleant, based on PEG grafted graphene-related materials, can be widely applied to promote the growth of 3D crystals suitable for X-ray crystallography. The association of the protein ferritin with functionalized exfoliated few layer graphite was directly visualized by transmission electron microscopy, illustrating the formation of ordered clusters of protein molecules critical to successful nucleation.

Large-scale conversion of helical-ribbon carbon nanofibers to a variety of graphene-related materials

The control of the oxidation time of helical-ribbon carbon nanofibers enables the production of a wide variety of graphene materials.

Graphene and Graphene Composites in Electrochemical Capacitors and Li-Ion Batteries

Graphene and derived materials have recently emerged as a new promising class of materials featuring unique electrical, mechanical, magnetic and thermal properties. Electrochemical energy conversion and storage devices, in particular supercapacitors and Li-ion batteries, are widely regarded as one of main areas where graphene related materials may find application. The present work describes synthesis and structural properties of reduced graphene oxide (rGO) and rGO/Fe3O4. These materials are subsequently tested in symmetrical capacitors and hybrid capacitors with alkaline electrolyte as well as in a conventional Li-ion cell setup. It is demonstrated that certain electrochemical devices can clearly benefit form graphene materials in terms of power capability. On the other hand, particle agglomeration and increased reactivity hinders making full usage of the graphene capability.

Enhancing the saturable absorption and carrier dynamics of graphene with plasmonic nanowires

We experimentally investigated the nonlinear optical response in few-layered graphene/silver nanowires (G/NWs) hybrid films that consist of two-dimensional graphene and network of one-dimensional silver nanowires, and found an enhanced third-order optical nonlinearity based on the open-aperture femtosecond Z-scan measurement. The enhancement of the optical nonlinearity is attributed to interfacial nanowires network on graphene for large local field enhancement and exciton–plasmon–photon conversion channel. In addition, accelerated carrier relaxation times in G/NWs are also observed by optical pump–probe spectroscopy, in which nanowires might supply an additional relaxation channel for graphene. The nanowire-induced defects and the increase of either electrons or holes induced by charge transfer may also contribute to the ultrafast dynamics in G/NWs. Our findings provide a new avenue of modulating the nonlinear optical response of graphene-related materials, and further suggest that G/NWs could be developed as nonlinear optical material in novel tunable laser photonics devices, such as laser mode locker, Q switcher, etc.

Graphene for Energy: Which Promising Applications and How?

With thousands of publications per year, graphene has raised a particularly great attention these last years. It is true that with an extraordinary combination of physical and technical properties including high flexibility, lightness, electronic mobility and ability to easily change its chemical properties by surface chemistry, graphene appears as an outstanding material which characteristics can interest many fields of application. Among them, energy conversion, production or storage are of such interest that the European's biggest research initiative, the “Graphene Flagship[1]” dedicated a specific work package only to that topic. The present talk aims at reviewing some of these energy applications where graphene and graphene related materials are thought to provide disruptive performances. Actually, fulfil of these promises will depend on the control of the full value chain going from graphene production to integration into the energy devices. Moreover, to fully benefit from graphene properties, the integration strategies have to address the interfacing issue of graphene with hosting material. Further graphene functionalization steps are thus often necessary to adapt the surface graphene properties. It leads to the development of various graphene functionalization technologies without which a successful transfer of graphene into energy applications would be impossible. Various examples dealing with graphene doping, nano-structuring or nano-decorating will be discussed to illustrate the key role of these enabling technologies. [1] http://graphene-flagship.eu/