Different Types Of Graphene Research Articles

Preparation of different graphene nanostructures for hydrogen adsorption

In this study, different types of graphene were synthesized to investigate hydrogen adsorption capacity at different pressures (0–34 bar) at room temperature (298 K). Graphene and nanoporous graphene were prepared by Chemical Vapor Deposition (CVD) method, using methane as a carbon source at a temperature of 900 °C over copper plates and nickel oxide nanocatalyst. The nickel oxide nanocatalyst was prepared by sol–gel method, whereas graphene oxide was prepared through modified Hummer's method. The products were characterized by X-ray diffraction, field emission-scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller and Raman spectroscopy. The adsorption of hydrogen was done by volumetric method. High adsorption capacity was achieved in nanoporous graphene because of its high pore volume (2.11 cm3/g) and large specific surface area (850 m2/g). Hydrogen adsorption values for nanoporous graphene, graphene and graphene oxide were determined as 2.56, 1.70 and 0.74 wt%, respectively. In addition, the hydrogen adsorption of graphene nanostructures fitted nicely to the selected two-parameter and three-parameter adsorption isotherm models. The adsorption isotherm model coefficients have been found for a 0–34 bar pressure range. The parameter values for all adsorbents showed proper conformity to the model and experimental data. Copyright © 2016 John Wiley & Sons, Ltd.

Toxicity of different forms of graphene in a chicken embryo model.

In the present work, the toxicity of three forms of graphene: pristine graphene (pG), graphene oxide (GO), and reduced graphene oxide (rGO) was investigated using a chicken embryo model. Fertilized chicken eggs were divided into the control group and groups administered with pG, GO, and rGO, in concentrations of 50, 500, and 5000μg/ml. The experimental solutions were injected in ovo into the eggs, and at day 18 of incubation, the embryo survival, body and organ weights, the ultrastructure of liver samples, and the concentration of 8-hydroxy-2'-deoxyguanosine (8-OHdG) in the livers were measured. Survival of embryos decreased significantly after treatment with all types of graphene, but not in a dose-dependent manner. The body weights were only slightly affected by the highest doses of graphene, while the organ weights were not different among treatment groups. In all experimental groups, atypical hepatocyte ultrastructure and mitochondrial damage were observed. The concentration of the marker of DNA damage 8-OHdG in the liver significantly decreased after pG and rGO treatments. Further in vivo studies with different animal models are necessary to clarify the level of toxicity of different types of graphene and to estimate the concentrations appropriate to evaluate their biomedical applications and environmental hazard.

Formulation and micro-extrusion of high concentration graphene slurries

In this study we have investigated the feasibility of preparing high concentration graphene slurries and their rheological properties as a function of various parameters including the sonicating time, solid loading, surfactant concentration, different types of graphene, aqueous and non-aqueous solvents. The feasibility of micro-extruding slurries with high graphene loading has also been studied. We demonstrate that slurries with graphene loading at 13wt% (equivalent to 6.3vol%) can be prepared to have pseudoplastic (shear thinning) properties. The 6.3vol% graphene is the highest graphene loading ever reported in the open literature. An unusual phenomenon of increasing graphene loading leading to reduced viscosity after graphene loading has reached a critical value has been observed for the first time. This unusual phenomenon is attributed to the forced alignment of graphene flakes during shearing when graphene loading is high enough and has been utilized to prepare slurries with high graphene loading and pseudoplastic properties. Micro-extrusion of these high concentration graphene slurries reveals the capability of the extrudate to maintain its shape right after extrusion of the slurries. Thus, it is expected that these high concentration graphene slurries can be used to fabricate three-dimensional objects through layer-by-layer fabrication methods with high fabrication rates in the near future.

Comparison of high-order-harmonic generation on single-layer graphene flakes with armchair and zigzag types in an intense laser field

The high-order-harmonic generation (HHG) of graphene in an intense laser field is investigated using the strong-field approximation method. The initial wave function is presented by gaussian and gamess software. The molecular structure along the $x$ and $y$ axes represents different types of graphene: armchair and zigzag, respectively. The results show that the HHG intensity of the armchair type of graphene is two magnitudes higher than that of the zigzag type in the plateau area. The ionization yield and electron density distribution are also presented to further explain this difference. Finally, by superposing a properly selected range of harmonics, a main pulse with the duration of 91 and 99 attoseconds accompanied by weak satellite pulses will be generated for the case of armchair and zigzag graphene, respectively, and the corresponding intensity from armchair graphene is much higher than that from zigzag graphene.

Influence of the Total Gas Flow at Different Reaction Times for CVD-Graphene Synthesis on Polycrystalline Nickel

Optimization of the total gas flow (CH4+H2) during the reaction step for different reaction times for CVD-graphene synthesis on polycrystalline nickel foil using an atmospheric pressure set-up is reported. Athickness valuerelated to number of graphene layers in each of the synthesized samples was determined using an Excel-VBA application. This method assigned athickness valuebetween 1 and 1000 and provided information on the percentage of each type of graphene (monolayer, bilayer, and multilayer) deposited onto the polycrystalline nickel sheet. The influence of the total gas flow during the reaction step and the reaction time was studied in detail. Optical microscopy showed that samples were covered with different types of graphene, such as multilayer, few-layer, bilayer, and monolayer graphene. The synthesis variables were optimized according to thethickness valueand the results were verified by Raman spectroscopy. The best conditions were obtained with a reaction temperature of 980°C, a CH4/H2flow rate ratio of 0.07 v/v, a reaction time of 1 minute, and a total gas flow of 80 NmL/min. In the sample obtained under the optimized conditions, 80% of the area was covered with monolayer graphene and less than 1% with multilayer graphene.

Comparison of catalytic performance of different types of graphene in Li–O2 batteries

We report comparative catalytic studies of two types of commercial graphene nanoplatelets as electrode materials with different physical properties for Li–O2 batteries. The graphene nanoplatelets were characterized by field-emission scanning electron microscopy, transmission electron microscopy, N2 physisorption, X-ray photoelectron spectroscopy, and Raman spectroscopy. The electrochemical performance of the graphene nanoplatelets, which have large pore volume density, large pore diameter, and high population of defect sites, was evaluated using galvanostatic discharge–charge cycling and a depth-of-discharge test. Higher specific capacity of the Li–O2 battery was achieved when graphene nanoplatelets were used as an electrode material; this specific capacity could be associated with pore volume density, pore diameter, and number of defects in the graphene nanoplatelets.

Thermal treatment and chemical doping of semi-transparent graphene films

Graphene transparent conductive films have been proposed as indium tin oxide (ITO) modifiers for application in optoelectronic devices. Here thermal and chemical treatments of graphene-based transparent films are systematically investigated with a view to increasing their conductivity and tuning their work function. We look at two different types of graphene, obtained either via spin-coating of graphene oxide (GO) aqueous dispersions, or vacuum-filtration of liquid-phase exfoliated (LPE) graphene in highly volatile, non-toxic solvents such as isopropanol and ethanol. As-deposited films are relatively resistive, but we are able to lower their sheet resistance by up to three orders of magnitude for both LPE and GO, so as to reach values of ∼105Ω/□ via thermal treatments at temperatures of ∼350/400°C at pressures of ∼10−6/10−4 mbar. Most importantly, the physisorption of the molecule (CF3SO2)2NH (trifluoromethanesulfonimide) results in an increase of the films work function by up to 0.5eV, to yield a value of ∼5.3eV. This is comparable or slightly better than what can be achieved with poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS), depending on PSS concentration, thereby confirming the potentially beneficial role of chemical doping of liquid dispersions of graphene-derivatives for application to organic electronics.

Large-scale graphene-based composite films for flexible transparent electrodes fabricated by electrospray deposition

Large-scale transparent conducting electrodes were fabricated using the electrospray method on a glass wafer and polyethylene terephthalate film using chemically reduced graphene oxide and poly (3,4-ethylenedioxythiophene) (PEDOT). Graphene oxide (GO) is prepared by the modified Hummers method, and reduced GO (RG) is prepared at low temperature. By varying the concentration of RG and PEDOT of the composite material on the substrate, the electrical conductivity and transmittance of the electrode was controlled. The optical transmittance values of the graphene-based electrode at a wavelength of 550 nm were between 81 and 95% and had sheet resistances from 370 to 5400 Ω sq−1. After 1000 cycles of a bending test, the sheet resistances of the graphene-based composite films were unchanged. Different types of graphene and graphene-based electrodes were characterized by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, high-resolution Raman spectroscopy, x-ray photoelectron spectroscopy, x-ray diffraction, transmittance, and electrical conductivity measurements.

Investigation of Space Charge Distribution of Low-Density Polyethylene/GO-GNF (Graphene Oxide from Graphite Nanofiber) Nanocomposite for HVDC Application

This paper reported a research on space charge distribution in low-density polyethylene (LDPE) nanocomposites with different types of graphene and graphene oxide (GO) at low filler content (0.05 wt%) under high DC electric field. Effect of addition of graphene oxide or graphene, its dispersion in LDPE polymer matrix on the ability to suppress space charge generation will be investigated and compared with MgO/LDPE nanocomposite at the same filler concentration. At an applied electric field of 80 kV/mm, a positive packet-like charge was observed in both neat LDPE, MgO/LDPE, and graphene/LDPE nanocomposites, whereas only little homogenous space charge was observed in GO/LDPE nanocomposites, especially with GO synthesized from graphite nano fiber (GNF) which is only -100 nm in diameter. Our research also suggests that dispersion of graphene oxide particles on the polymer matrix plays a significant role to the performance of nanocomposites on suppressing packet-like space charge. From these results, it is expected that nano-sized GO synthesized from GNF can be a promising filler material to LDPE composite for HVDC applications.

DFT modeling of the covalent functionalization of graphene: from ideal to realistic models

The production of multiple types of graphene, such as free standing, epitaxial graphene on silicon carbide and metals, graphene in solution, chemically grown graphene-like molecules, various graphene nanoribbons, and graphene oxide with different levels of reduction and various chemical composition, demonstrate the need for additional investigation beyond the basic principles of graphene functionalization for avoidance of occasional contradictions between the predictions from first-principles simulations and experimental results. Herein, we discuss the current state of modeling of the different types of graphene using density functional theory (DFT) methods. We focus on the static (substrate, shape, curvature, strain and doping) and dynamic (starting point of functionalization, migration barriers and stability of configurations) aspects that provide a more correct and selective modeling of the chemisorption of various chemical species on the graphene scaffold. Based on the recent modeling of experimentally realized functionalization of different types of graphene we can conclude that the formation of uniform one- or two-sided functionalized graphene discussed in earlier studies is an exception to the typical scenarios of graphene chemistry. The presence of different substrates, defects and lattice distortions, such as ripples and strain, results in the formation of clusters or lines from the functional groups. Several configurations of the chemical species on the graphene substrate have been found to exist with ideal models but are only stable for graphene functionalized under special conditions. The role of the technical parameters, such as the functionals and computational methods employed, and the important guidelines for the modeling of the chemical properties of various types of graphene are also discussed.

Large-Area Synthesis of Graphene on Palladium and Their Raman Spectroscopy

We present a detailed investigation of the nucleation sites, growth, and morphology of large-area graphene samples synthesized via chemical vapor deposition (CVD) on bulk palladium substrates. The CVD chamber was systematically controlled over a large range of growth temperatures and durations, and the nature of graphene growth under these conditions was thoroughly investigated using a combination of scanning electron microscopy and a statistical analysis of >500 Raman spectra. Graphene growth was found to initiate at ∼825 °C, above which the growth rate increased rapidly. At T = 1000 °C, defect-free high-quality graphene was found to grow at an unprecedented rate of tens of micrometers per second, orders of magnitude faster than past reports on Cu- or Ni-based growth, thus leading to macroscopic coverage of the substrate within seconds of growth initiation. By arresting the growth at lower temperatures, we found that graphene nanoislands preferred to nucleate at very specific positions close to terrace edges and step inner edges. Evidence of both epitaxial and self-limiting growth was found. Along with monolayer graphene, both Bernal and turbostratic multilayer graphene could be obtained. A detailed evolution of the different types of graphene, as a function of both growth temperature and duration, has been presented. From these, optimal growth conditions for any chosen type of graphene sample can be inferred.

Graphene, the new nanocarbon

Graphene is a fascinating new nanocarbon possessing, single-, bi- or few- (≤ ten) layers of carbon atoms forming six-membered rings. Different types of graphene have been investigated by X-ray diffraction, atomic force microscopy, transmission electron microscopy, scanning tunneling microscopy and Raman spectroscopy. The extraordinary electronic properties of single-and bi-layer graphenes are indeed most unique and unexpected. Other properties of graphene such as gas adsorption characteristics, magnetic and electrochemical properties and the effects of doping by electrons and holes are equally noteworthy. Interestingly, molecular charge-transfer also markedly affects the electronic structure and properties of graphene. Many aspects of graphene are yet to be explored, including synthetic strategies which can yield sufficient quantities of graphene with the desired number of layers.