Tetracyanoethylene oxide-functionalized graphene and graphite characterized by Raman and Auger spectroscopy

The tetracyanoethylene oxide (TCNEO) functionalization of chemical vapor deposition grown large area graphene and graphite was performed using reaction of TCNEO with carbon surface in chlorobenzene. The successful functionalization has been confirmed by Raman and Auger spectroscopy, and by numerical modeling of the structure and vibrational modes of TCNEO-functionalized graphene. Raman spectra of TCNEO-functionalized graphene and graphite show several groups of lines corresponding to vibrations of attached carbonyl ylide. One of key signatures of TCNEO attachment is the high intensity Raman band at ∼1450 cm⁻¹, which represents the C–CC in plane vibrations in functionalization-distorted graphene. Raman spectra indicate the existence of central (pristine) attachment of TCNEO to graphene surface.     

Theoretical characterization of reduction dynamics for graphene oxide by alkaline-earth metals

First-principles calculation identifies elementary processes in the thermal reduction of graphene oxide (GO) and reveals the effects of alkaline-earth metals (AEMs) in recovering the graphene. These metals are highly effective in removing residual oxygen groups resistive to thermal reduction, as well as healing the defects formed during the reduction, such as the carbonyl groups. In the AEM-assisted reduction, the AEMs serve as an electron reservoir of high chemical potential that forces electron transfer to the GO, whereas pristine carbon regions on the GO serve as a “bridge” to facilitate the electron transfer directly to oxidized carbon. This enables fast kinetics for the breaking of both C–O and CO bonds. Complete reduction is observed in our simulation at T ≤ 600 K within 32 ps for a 28%-oxygen-coverage GO model.     

Influence of N-doping on the structural and photoluminescence properties of graphene oxide films

Nitrogen (N) was doped into graphene oxide (GO) films at temperatures of 600–900 °C under the flow of a mixture of NH₃ and Ar. The N (atomic) concentration was varied in the range of 3.63–7.45%. XPS and FTIR spectra show that there are mainly single C–N and double CN bonds in the GO sheet. Raman spectra indicate that the G band becomes closer to the position of the G band of graphite with increasing doping temperature, and thus reveal that N doping produces a blue-shift of the G-band. In room-temperature photoluminescence (PL) spectra, N-doping produces an increase not only in the overall PL intensity, but also in the wavelength of the peak maxima. The shift of the induced PL of N-doped graphene is attributed mainly to the increased number of graphitic (or quaternary) N.     

High strength measurement of monolayer graphene oxide

In this study, the strength of monolayer graphene oxide membranes was experimentally characterized. The monolayer GO membranes were found to have a high carbon-to-oxygen ratio (∼4:1) and an average strength of 17.3 N/m (24.7 GPa). This measured strength is orders of magnitude higher than previously reported values for graphene oxide paper and is approximately 50% of the 2D intrinsic strength of pristine graphene. In order to corroborate strength measurements, experimental values were compared to theoretical first-principles calculations. Using a supercell constructed from experimental measurements of monolayer graphene oxide chemistry and functional structure, density functional theory calculations predicted a theoretical strength of 21.9 N/m (31.3 GPa) under equibiaxial tension, in good agreement with the experimental data. Furthermore, computational simulations were used to understand the underlying fracture mechanism, in which bond cleavage occurred along a path connecting oxygenated carbon atoms in the basal plane. This work shows that monolayer graphene oxide possesses near-theoretical strength reaching tens of GPa.     

Ion beam irradiation of few-layer graphene and its application to liquid crystal cells

Few-layer graphene was irradiated with a low-energy Ar⁺ ion beam and applied to liquid crystal (LC) cells as both a transparent electrode and a homogeneous alignment enhancer. The optimum conditions for ion-beam-irradiated few-layer graphene (I-G) are 80 eV of beam energy and 1 s of irradiation time, which is confirmed by the decrease of the sheet resistance and increase of water contact angle with similar transmittance to pristine few-layer graphene. It is shown that the ion beam treatment of few-layer graphene under optimized conditions removes the surface contaminants and flattens the surface without damages. LC cells on I-G show similar device performance to that of LC cells based on indium tin oxide (ITO) with rubbing-treated polyimide. These results are very encouraging for replacing ITO electrodes with few-layer graphene in LC display applications.     

Synthesis of carbon-coated graphene electrodes and their electrochemical performance

In this work, C-graphene composed of core graphene and carbon shells was prepared to obtain a new type of carbon electrode materials. Carbon shells containing nitrogen groups were prepared by coating polyaniline (PANI) onto graphene by in situ polymerization and subsequent carbonization at 850 °C. After carbonization, the C-graphene contained 6.5% nitrogen and showed a 2D plate structure and crystallinity like that of pristine graphene. In addition, the C-graphene exhibited electrochemical performance superior to that of pristine graphene, and the highest specific capacitance (170 F/g) of the C-graphene was obtained at a scan rate of 0.1 A/g, as compared to 138 F/g for pristine graphene. This superior performance was attributed to the synergistic effect of porous carbon layer and the graphene and the pseudocapacitive effect by the nitrogen groups formed on the carbon electrode after carbonization.     

Selective cell adhesion on an ion implanted poly(bisphenol A carbonate) film

Conventional ion implantation has been shown to modify the surface properties of polymers such as their hardness, conductivity, and biocompatibility. In this study, poly(bisphenol A carbonate) (PC) films were implanted by Ar⁺ ions with an energy level of 100 keV in order to improve their biocompatibility. The results of the surface analyses revealed an increasing intensity of the functional groups on the ion implanted PC surfaces such as OH, CO, and CO after implantation, which contributed to an improvement of their hydrophilicity. The in vitro cell culture test revealed that the HaCaT cells were selectively adhered to the ion implanted regions of the PC.     

Interaction energies, structure, and dynamics at functionalized graphitic structure-liquid phase interfaces in an aqueous calcium sulfate solution by molecular dynamics simulation

Molecular dynamics simulations were performed to study the effect of surface functionalization of graphitic structures on the molecular-scale energetic, structure, and dynamics of water and ions at graphite surface-liquid phase interfaces. Three types of carbon surface structures were investigated: a pristine graphite plane and two graphite planes functionalized with hydroxyl (-OH) and carboxylate (-COO⁻, deprotonated carboxyl) groups. A divalent salt, calcium sulfate, was combined with water to simulate an electrolyte liquid phase. Results highlighted the ordering of H₂O molecules that occurs near graphite surfaces and revealed a subtle effect on the position of the H₂O layers associated with ions and functional group type. Surface functionalization of the graphitic structures affected the H-bond network and the orientation of near-surface H₂O molecules, decreased the ion hydration, and significantly restrained the mobility of near-surface H₂O molecules and bulk Ca²⁺ and ions.     

Reduced graphene oxide with tunable C/O ratio and its activity towards vanadium redox pairs for an all vanadium redox flow battery

Electrochemically reduced graphene oxides (ERGO) are obtained under various reducing potentials in the phosphate buffer solution (PBS). Different characterization methods are used to analyse the changes of structure and surface chemical condition for graphene oxide (GO). The results show that GO could be reduced controllably to certain degree and its electrochemical activity towards VO₂⁺/VO²⁺ and V³⁺/V²⁺ redox couples is also tunable using this environmentally friendly method. The catalytic mechanism of the ERGO is discussed in detail, the CO functional groups other than the C–O functional groups on the surface of ERGO more likely provide reactive sites for those redox couples, leading to a more comprehensive understanding about the catalytic process than previous relevant researches. This controllable modification method and the ERGO as electrode reaction catalyst with enhanced battery performance are supposed to have promising applications in the all vanadium redox flow battery.     

High‐Performance Pristine Graphene/Epoxy Composites With Enhanced Mechanical and Electrical Properties

High‐strength conductive pristine graphene/epoxy composites are prepared by two simple processing methods – freeze dry/mixing and solution processing. PVP‐stabilized graphene is aggregation‐resistant and allows for excellent dispersion in both the resin and final composite, as confirmed by optical microscopy and SEM images. The superior dispersion quality results in excellent nanofiller/matrix load transfer, with a 38% increase in strength and a 37% improvement in modulus for 0.46 vol% graphene loading. The composites have a very low electrical percolation threshold of 0.088 vol%. Despite the effectiveness of both methods, the freeze‐drying method is more promising and versatile enough to be used for graphene dispersion in a wide range of other composite precursors.

     

Exploring the effect of functionalization of graphene on the quantum capacitance by first principle study

Graphene is considered as one of the most promising electrode materials for supercapacitors. Despite many advantages of graphene-based electrodes, there remains a major limitation, which is low value of specific capacitance. This limitation is observed in graphene-based electrodes’ due to their low quantum capacitance. The present article explored the variations of quantum capacitance of graphene sheets through their functionalization with monovalent and divalent functional groups. The findings of this report demonstrate that functionalization of graphene may improve the gravimetric quantum capacitance compared to pristine graphene. The monovalent functional groups show symmetric behavior in negative and positive applied voltages. They have a significant effect on the quantum capacitance, especially at low voltages. In contrast, the divalent functional groups show asymmetric behavior in negative and positive applied voltages. They have an additive effect on the quantum capacitance, especially at higher voltages and negative bias. However, both kinds of functional groups enhance the quantum capacitance compared to pristine graphene.     

Mechanical behavior of interlayer-bonded nanostructures obtained from bilayer graphene

We report a comprehensive computational study of the mechanical behavior of two-dimensional carbon-based nanostructures generated from CC interlayer bonding through chemical functionalization in bilayer graphene, based on molecular-dynamics simulations of uniaxial tensile deformation according to a reliable interatomic bond-order potential. These nanostructures range from superlattices of two-dimensional diamond-like nanodomains embedded in twisted bilayer graphene to fully interlayer-bonded graphene bilayers that constitute two-dimensional diamond-like films. We have analyzed in detail the fracture mechanisms of the nanostructures under tension as a function of the extent of interlayer bonding through chemical functionalization. In most cases, fracture is initiated at the interface between pristine graphene and interlayer-bonded two-dimensional diamond-like domains in the composite structure and subsequently propagates across the material leading to failure through brittle cleavage. However, beyond a certain density of interlayer bonds with specific spatial distribution, there is a transition to ductile failure with a structural response that is characterized by void formation and coalescence.     

Synthesis of platinum/graphene composites by a polyol method: The role of graphite oxide precursor surface chemistry

Graphite oxide (GO) with varied oxidation degrees and hexachloroplatinic acid were used as precursors for the synthesis of Pt/graphene composites via a polyol approach. It is revealed that the epoxy-rich basal planes of GO serve as the active sites for the nucleation of Pt nanoparticles via COPt bonds. With the increased oxidation degree of GO, both the nucleation sites and the repulsive forces between the GO and hexachloroplatinic anions and between the graphene and the negatively charged Pt nanoparticles increase, and this kind of subtle balance leads to a first decrease and then an increase of Pt size and size distribution on the graphene as well as the decrease of Pt loading. Moreover, adjusting the pH in the aging stage significantly increases the Pt loading both on the basal planes and at the edges of graphene, which is attributed to the introduction of positively charged sites by π sites on the basal planes of graphene and the protonation of carbonyl and carboxyl groups at the edges.     

Application of carbon nanostructures toward SO2 and SO3 adsorption: a comparison between pristine graphene and N-doped graphene by DFT calculations

We studied the adsorption of SO_x (x?=?2,3) molecules on the surface of pristine graphene (PG) and N-doped graphene (NDG) by density functional theory (DFT) calculations at the B3LYP/6-31G(d) level. We used Mulliken and NBO charge analysis to calculate the net charge transfer of adsorbed SO_x on pristine and defected graphene systems. Our calculations reveal much higher adsorption energy and higher net charge transfer by using NDG instead of pristine graphene. Furthermore, the density of state (DOS) graphs point to major orbital hybridization between the SO_x and NDG, while there is no evidence of hybridization by using pristine graphene. Based on our results, it is found that SO₂ and SO₃ molecules can be adsorbed on the surface of NDG physically and chemically with adsorption energies (E_ads) of ?27.5 and 65.2?kJ?mol^?1 (19.6 and 51.4?kJ?mol^?1 BSSE), respectively, while low adsorption energies were calculated in the case of using pristine graphene. So we introduced NDG as a sensitive adsorbent/sensor for detection of SO₂ and SO₃.     

Electron microscopy analyses of natural and highly oriented pyrolytic graphites and the mechanically exfoliated graphenes produced from them

The microstructure of natural graphite (NG) and highly ordered pyrolytic graphite (HOPG) blocks, and their mechanically exfoliated graphenes, prepared by only one stamping on a meshed Cu grid, was investigated using dual beam scanning electron microscope/focus ion beam (SEM/FIB) and transmission electron microscopy (TEM). Channeling contrast enhanced SEM/FIB images and cross section TEM micrographs showed that HOPGs have very shallow grains, less than 30 nm in thickness and 6-30 μm in size, while NGs have grains larger than mm in size with some folded layers. In order to understand the structure of exfoliated graphene, an observation along the z-axis of lattice on suspended membranes was conducted. Through the aid of diffraction pattern (DP) simulations, demonstrating the dependency of the intensity ratio between and on the number of layers and stacking sequences, a suspended monolayer could be distinguished from a multilayer. With the observations of unfolded graphene edges, we found that the graphene edges always have a roughness below a nanometer despite their straight feature in a low magnification view and have no preferential direction. DP and HREM analyses of folded edges in NG membranes also revealed that the folding direction is arbitrary and graphite is mostly stacked with AB sequence.     

Synthesis of chemically controllable and electrically tunable graphene films by simultaneously fluorinating and reducing graphene oxide

Fluorine and oxygen co-doped graphene with controllable element coverage was effectively synthesized through simultaneously fluorinating and reducing graphene oxide by pyrolysis of fluorinated graphite. Morphology investigation indicates that the doped graphene is of few-layered thickness, and the prepared films exhibit layered structure through cross-section. Chemical composition analysis confirms that fluorine has been grafted onto graphene scaffold through CF covalent bond, and the doping level can be readily manipulated just by adjusting the reaction temperature. The structural changes of graphene induced by the controllable doping thus facilitate the tunable electrical property, which can be tuned over several orders of magnitude. These results indicate our method is not only potentially useful to tailor the chemical surface and electronic structure of graphene, but also can find applications in novel electronic devices based on graphene co-doped with different dopants.     

Thin films of carbon nanotubes and chemically reduced graphenes for electrochemical micro-capacitors

We report the making of chemically reduced graphene (CRG) sheets separated by layer-by-layer-assembled multi-walled carbon nanotubes (MWCNTs) for electrochemical micro-capacitor applications. Submicron thin films of amine-functionalized MWCNTs (MWCNT-NH₂) and CRG derived from graphene oxides, were shown to be cross-linked with amide bonds having high packing densities of ∼70%. These carbon-only electrodes were found to have large volumetric capacitance of in an acidic electrolyte (0.5 M H₂SO₄). The electrode capacitance in a neutral electrolyte (1 M KCl) was found much lower, which supported the hypothesis that the observed high capacitances in the acidic electrolyte can be attributed primarily to redox reactions between protons and surface oxygen-containing groups on carbon.     

Non-isothermal crystallization kinetics assessment of poly(lactic acid)/graphene nanocomposites

In this work, the effects of the presence and modification of graphene nanoplatelets (GNps) on the crystallization of the poly(lactic acid) (PLA) were studied. Functionalization of GNps was accomplished by acid treatment. Nanocomposite samples were prepared by solution method containing pristine and functionalized graphene. In contrast to pristine PLA, crystallization of the samples contains nano filler initiates at higher rates that showed the role of heterogeneous nucleating effects of these particles in crystallization of the PLA. Then, the effect of nano filler functionalization was comprised. Initial slope of the crystallization (S _i) and full width at the half height maximum of crystallization peak are indicative of nucleation rate and spherulite size distribution, respectively; which upon the addition of the functionalized graphene nanoplatelets (FGNps), S _i increased and spherulites gained normal size distribution. Non-isothermal and crystallization kinetics of the samples were studied using differential scanning calorimetry at heating rates of 2, 4, 6 and 10 °C/min. Performed techniques such as furrier transform infrared, dynamic-mechanical thermal analysis and visual observation of sediments confirmed the successful modification of the graphene platelets. Also, non-isothermal analysis pinpointed the fact that crystallization temperature (T _c) of the nanocomposites has increased by 11–21 °C, compared to the neat PLA. Upon verification of Avrami’s theory, it was conducted that dominant mechanism of nucleation of the nanocomposite samples was 2D circular diffusion; wherein, Avrami’s exponent (n) was determined as 2. Moreover, it was deduced from Avrami’s equation that “n” have no discernible changes in nanocomposites containing GNps or FGNps. Electrical devices and shape memories can be the main application of these nanocomposites.     

Identifying atomic sites in N-doped pristine and defective graphene from ab initio core level binding energies

Theoretical calculations have been carried out to predict N(1s) binding energy values in N-doped graphene which take into account initial and final state effects. A simple way to carry out ΔSCF Hartree–Fock calculations is proposed, validated against experiment for a series of N-containing organic molecules and applied to realistic N-doped nanosized pristine and defective graphene models. Final state effects appear to be important and strongly suggest that only two types of N are likely to be detected on N-doped pristine graphene by X-ray Photoelectron Spectroscopy with binding energy values of 398.6 and 400.5 eV, respectively and relative to C(1s) at 285 eV in agreement to recent experiments for quasi free standing N-doped graphene. Two cases of N-doping in defective graphene have also been considered and calculated results compared with recent experimental measurements. Calculated values for C(1s) including final state effects strongly suggests that values for core level binding energy of N and other dopants will be close to their absolute values if referred to C(1s) at 290.2 eV. The proposed approach is general enough to be successfully applied to other cases of interest.     

Thermal conductivity and structure of non-covalent functionalized graphene/epoxy composites

Non-covalent functionalization was used to functionalize graphene nanosheets (GNSs) through π–π stacking of pyrene molecules with a functional segmented polymer chain, which results in a remarkable improvement in the thermal conductivity of GNS-filled polymer composites. The functional segmented poly(glycidyl methacrylate) containing localized pyrene groups (Py-PGMA) was prepared by atom transfer radical polymerization, and Py-PGMA was characterized by nuclear magnetic resonance spectroscopy. Raman spectra, X-ray photoelectron spectroscopy and thermogravimetric analysis reveal the characteristics of Py-PGMA–GNS. Differential scanning calorimetry indicated that the functional groups on Py-PGMA–GNSs can generate covalent bonds with the epoxy matrix, and further form a cross-linked structure in Py-PGMA–GNS/epoxy composites. The Py-PGMA on the GNS surface not only plays an important role to facilitate a homogeneous dispersion in the polymer matrix but also improves the GNS–polymer interaction, which results in a high contact area. Consequently, the thermal conductivity of integrated Py-PGMA–GNS/epoxy composites exhibited a remarkable improvement and is much higher than epoxy reinforced by multi-walled carbon nanotubes or GNSs. The thermal conductivity of 4 phr Py-PGMA–GNS/epoxy has about 20% (higher than that of pristine GNS/epoxy) and 267% (higher than pristine MWCNT/epoxy).