Defects In Graphite Research Articles

Correlation between electrical resistance and defect concentration in graphite under non-hydrostatic stress

Combination of Raman spectroscopy and electrical resistance measurements carried out under non-hydrostatic stress reveals a direct correlation between the transport properties and the concentration of structural defects in graphite, as measured by the relative intensity of the D and D′ bands. We have found that the piezoelectric behavior of highly defected specimens can be modulated through changes in the microstructure, essentially by affecting the crystallite size. The observed correlation suggests some interesting applications based on the use of Raman spectroscopy as a technique for estimating the relative electrical behavior of microstructurally controlled graphite samples under quasi-uniaxial strain conditions.

Highly defective graphite for scalable synthesis of nitrogen doped holey graphene with high volumetric capacitance

Manipulating basal plane structure of graphene for advanced energy conversion materials design has been research frontier in recent years. By extending size of defects in the basal plane of graphene from atomic scale to nanoscale, graphene with in-plane holes can be synthesized by multiple steps oxidation and reduction of defective graphene oxide at low concentration. These complicated and low yield synthetic methods largely limited research and applications of holey graphene based high performance energy conversion materials. Inspired by graphene in-plane holes formation mechanism, an easy and scalable synthetic approach has been proposed in this work. By oxidizing widely available defective graphite mineral under high concentration, holey graphene oxide has been scalable synthesized. Through simple reduction of holey graphene oxide, nitrogen doped holey graphene with high volumetric capacitance of 439 F/cm3 was obtained. We believe this breakthrough can provide a feasible synthetic approach for further exploring the properties and performance of holey graphene based materials in variety of fields.

Visualizing subsurface defects in graphite by acoustic atomic force microscopy.

We describe a versatile platform, which combines atomic force acoustic microscopy, ultrasonic atomic force microscopy and heterodyne force microscopy. The AFM system can enable in-situ switching among these operation modes flexibly and thus benefit the discrimination of differences in mechanical properties and buried subsurface nanostructures. We demonstrate the potential of this platform for visualizing the subsurface defects of graphite. Our results show that tiny topographic edges are enhanced in acoustic oscillation signals whilst embedded defects and inhomogeneous in mechanical properties are made clearly distinguishable. The possibility of detecting subsurface defects in few-layer graphene is further discussed with first-principles calculations. Microsc. Res. Tech. 80:66-74, 2017. © 2016 Wiley Periodicals, Inc.

Nitrogen-Deficient ORR Active Sites Formation by Iron-Assisted Water Vapor Activation of Electrospun Carbon Nanofibers

Fe- and N-modified carbon nanofibers (Fe–CNF) were synthesized via electrospinning and pyrolysis as electrocatalysts for oxygen reduction reaction (ORR). In order to increase the exposed surface area with the active sites buried inside Fe–CNF, we attempted water vapor activation for Fe–CNF and observed a substantial improvement of ORR activity up to the comparable level with Pt/C. Unlike what was expected, however, water vapor activation did not significantly increase the specific surface area of Fe–CNF; instead, it induced a depletion of surface N content, which makes it difficult to explain the improved ORR activity with the increase of surface area with N-based active sites. In water vapor activation, the chemical phase of embedded particles is changed from Fe3C to Fe3O4 and nitrogen-free Fe- and C-based ORR active sites were exposed, which seemed to be related with hierarchical macro/mesopore structure and graphitic edge defects. This study demonstrates a facile activation method for better ORR activi...

Scalable and Environmentally Friendly Synthesis of Hierarchical Magnetic Carbon Nanosheet Assemblies and Their Application in Water Treatment

Large-scale assembling of graphitic carbon nanosheets to a three-dimensional hierarchical structure is a great challenge. Herein we report a facile synthesis of hierarchical magnetic carbon nanosheet assemblies (MCNSAs) via an ambient-pressure chemical vapor deposition method. To explore the formation mechanism, the as-prepared MCNSAs as well as the intermediates of synthesis were extensively characterized. It was revealed that two different carbon deposition processes, i.e., the dissolution–precipitation process and graphitic defects triggered catalytic decomposition of methane, were involved in the formation of MCNSAs. The disclosed method is simple and environmentally friendly, which is favorable for large-scale production. The resulting MCNSAs possess large surface areas, bimodal pore structures, abundant defective sites, excellent chemical stability, and sufficient magnetism. Such features afford significant advantages for application in water cleaning. As a proof of concept, the sorption performance...

Graphene-Alumina Nanocomposites with Improved Mechanical Properties for Biomedical Applications.

This paper presents a study on graphene platelet (GPL)-reinforced alumina (Al2O3) ceramic composites and the relationships between the loading of GPL and both mechanical properties and in vitro biocompatibility. Al2O3 powders with different GPL contents were prepared and sintered using a gas protected pressure-less furnace. The examination of the results shows the density of the composites varying from 99.2% to 95.6% with the loading of GPL from 0.75 to 1.48 vol %. Raman studies show that moderate agglomerations of GPLs occur during the ball milling process and graphitic defects were produced during the high temperature processing. Mechanical properties of the Al2O3 matrix are significantly improved by adding GPLs. A maximum increase of approximately 60% in flexural strength and 70% in fracture toughness are achieved by introducing 0.75 vol % GPLs. In the biocompatibility tests, it was found that cells directly seeding on top of GPL/Al2O3 samples showed better initial attachment (3 h after seeding) and viability (3 days after incubation) than the monolithic Al2O3, indicating that the GPL/Al2O3 composites have comparable or more favorable biocompatibility. The excellent mechanical and biomedical properties of the GPL/Al2O3 composites may enable them to be applied to a wide range of engineering and biomedical applications.

Thermal annealing of C ion irradiation defects in nuclear graphite studied by positron annihilation

In order to investigate the thermal behaviour of radiation induced point defects in nuclear graphite, ETU10 graphite was implanted with 350 keV C+ ion to doses of 1015 and 1016 cm-2. The point defects introduced by the implantation were characterized by Positron Annihilation Doppler Broadening (PADB) and their thermal behaviour was studied during “in situ” annealing at Delft Variable Energy Positron beam (VEP). The annealing was performed for 5 minutes at temperatures ranging from 300 K (as implanted) to 1500 K in steps of 100 K. For both doses, an annealing stage at around 450 K is observed followed by a second stage around 700 K. For the high dose implantation vacancy complexes are found which are stable up to a temperature around 1400K.

Molecular dynamics simulations of irradiation defects in graphite: Single crystal mechanical and thermal properties

Molecular dynamics simulations of single crystal graphite have been performed, incorporating point and extended defect structures formed as a result of thermally annealed radiation damage. These AIREBO potential calculations have simulated the changes to crystal dimensions, elastic moduli and thermal expansion due to the formation of different representative morphologies of extended irradiation-induced defects (vacancy and interstitial aggregates) and over a range of atomic displacement fractions. We find that, in addition to the simulation method reproducing the important mechanical and thermal properties of virgin graphite, the property changes caused by the formation of extended defects in the graphite crystal structure qualitatively agree with experimental observations at different irradiation/annealing temperature regimes. The results of these calculations provide a direct insight into how the underlying atomic scale defects and dislocations created by radiation damage can lead to material property changes, and demonstrate how this computationally efficient simulation method can be employed to reproduce crystallite changes in large-scale models of polygranular graphite.

Encapsulation of iron atoms between fragments of graphene planes

In this paper, we apply results of analysis of the Raman spectrum from an amorphous carbon film modified with iron (a − C :Fe) which leads to construction of clusters containing fragments of graphene and a layer of iron atoms. Such clusters may be candidates for the role of microwave radiation absorbers. For the Raman experiments we performed deposition of a thin film of a − C : Fe. Details of the film deposition process, together with the corresponding Raman experiments are presented in this paper. Comparison with a literature model was performed for the intensity ratio of the maxima for specific Raman bands, using the letter D to represent “disorder” and the letter G to represent “graphite”. Comparison gives evidence for the existence of nanosize fragments of graphene embedded in an amorphous matrix of the a − C :Fe film. The diameter of the fragments is predicted to attain a value of about 1.2 nm. Moreover, in a comparison with experimental data for defective graphite, the position of the maximum of the D band for a−C :Fe appeared to be red shifted. This could support the proposition that damping of Raman-active oscillations occurs by an electron gas in fragments of graphene after introduction of iron, e.g. after intercalation. On the basis of these estimates, we modeled a symmetrical polycyclic aromatic hydrocarbon having a similar size and converted it into fragment of a graphene plane by removal of hydrogen atoms occupying the edge states. To preserve symmetry, we placed atoms of iron on top of the fragment, situating them exactly above the centers of hexagons at a certain distance and placed another fragment of graphene on the top of the iron atoms, symmetrically. For a distance of 2.52 ˚ the centers of the hexagon and the iron atom, distortions of carbon-carbon valence bonds and angles were found to be minimal, as shown through optimization of the system’s geometry using the Avogadro molecular editor. This result supports an hypothesis of graphene fragments during the growth of an amorphous carbon film modified by simultaneous addition of a dopant metal such as iron. This example may illustrate the stability of a two-dimensional electron gas confined between the fragments.

Grain structures of nitrogen-doped graphene synthesized by solid source-based chemical vapor deposition

Doping of a foreign element in sp2 hybridized graphene lattice is of significant importance to tune the electrical and chemical properties. Here, we report on the grain structures of substitutional nitrogen-doped graphene synthesized by an atmospheric pressure (AP) solid source-based chemical vapor deposition (CVD) technique. Nitrogen-doped graphene was synthesized by mixing solid camphor and melamine as carbon and nitrogen source, respectively. The precursor materials quantity significantly affects the graphene growth on Cu foil and thereby the nitrogen doping and content. Transmission electron microscopy (TEM) analysis was performed to determine the nitrogen substitutional sites in the graphene. Dark-field (DF) TEM analysis was carried out to evaluate the graphene grain structure grown with introduction of nitrogen dopant. We obtained different grain orientations, where an individual grain size is more than 5 μm. Our findings show that graphitic nitrogen defects can be introduced in the large individual graphene grain by the developed solid source-based CVD technique.

Characterizing various types of defects in nuclear graphite using Raman scattering: Heat treatment, ion irradiation and polishing

Raman spectroscopy has proved to be an appropriate technique to probe defects in carbon-based materials owing to its high sensitivity, most often focused on the commonly used ID/IG parameter. However, this ratio may be activated by various types of defects and in a completely independent manner. Therefore, discriminating between defects is challenging. The central idea of the present work is to provide a better understanding of the Raman response to the various types of defects that may appear in nuclear graphite (carbon–carbon composite) during its manufacturing process, its operation in the nuclear reactor, or even during its preparation process such as polishing which is usually used prior to Raman characterization. This work also demonstrates the discrimination of the defect types using the combination of the ID/IG and FWHM(G), two structural disorder indicators evolving differently according to the type and the concentration of the introduced defects into the carbon network. The ion-beam irradiation was used here as an effective way for creating defects that could be similar to those created by neutrons in the nuclear reactor.

Dynamic friction-force microscopy using fractional-resonance excitation: Image contrast of graphite surface defects

Dynamic friction-force microscopy using fractional-resonance excitation: Image contrast of graphite surface defects

Theoretical study of the interaction between metallic fission products and defective graphite

The adsorption and diffusion behaviors of the metallic fission products of Cs, Sr, Ag on the defective graphite surface with double vacancy and Stone–Wales defect are theoretically studied by using density functional theory. Our results show that the Cs, Sr and Ag adatoms are adsorbed preferably at the defect areas of graphite surface. Electronic structure analysis indicates that the defects enhance the adsorption of metallic fission products. Further study on the diffusion properties also proves that the adatoms prefer to diffuse toward the defective area. These processes can readily occur due to zero or ignorable diffusion energy barriers of different kinds of point defects, indicating that point defects can serve as the trap sites for the metallic fission products.

Influence of Carbon Surface Defects on Double Layer Charge Storage and Ion Dynamics in Electrochemical Capacitors

Supercapacitors, which rely on electrosorption of ions in a porous carbon electrode, can supplement or even replace traditional batteries in energy harvesting and storage applications. While supercapacitors offer > 10 kW/kg power densities, their < 5 Wh/kg energy density levels are insufficient for many automotive and grid storage applications. Most prior efforts have focused on novel high-performing ionic liquid electrolytes and porous carbons with tunable pore diameters and high accessible surface areas. However, we still lack fundamental understanding of the influence of electrode surface properties, such as graphitic defects and functional groups, on capacitance limitations, charge/discharge dynamics, and electrochemical stability of the electrolyte under high potentials. These surface properties significantly impact quantum capacitance contributions and must be investigated to optimize ion screening, charge mobilities, and operating voltage windows. Our efforts focused on the influence of surface functional groups and structural ordering on electrochemical behavior of room-temperature ionic liquid electrolytes [1]. We synthesized carbide-derived carbon via halogen etching of TiC precursor at 800 °C, yielding powder with 0.67 nm pores. Using 700-1800 °C vacuum annealing, 800 °C H2, 600 °C NH3, or 400 °C air treatments, we, respectively, defunctionalized, aminated, hydrogenated, and oxidized pore surfaces. Room-temperature 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonimide) was subsequently confined in the pores of functionalized CDCs. By maintaining a consistent pore structure, we decoupled the key properties that affect double layer charge storage and ion dynamics. We demonstrated the structural changes and resulting changes in surface properties, such as hydrophobicity and electrical conductivity, and correlated them with capacitance and ionic resistance. In particular, we confirmed the detrimental influence of defect removal on capacitance but found its positive influence on rate handling and electrochemical stability. Quasi-elastic neutron scattering showed a logarithmic decay relationship between relaxation time and scattering intensity, as well as surface chemistry-dependent coefficients of elastic scattering. Electrochemical results correlated neutron scattering behavior with ionic resistance and rate handling and underscored the significance of intermolecular interactions between electrolyte ions and functional groups on pore surface. Capacitance and cyclability differences between functionalized CDCs underscored the selective effects of surface porous carbons with similarly modified non-porous ssamples including graphene, carbon onions, and carbon black. We demonstrate divergent electrochemical behaviors that depend on ion confinement in similarly sized pores, shedding fundamental insights into key surface properties that govern electrochemical capacitor performance. Reference: 1. Dyatkin, B.; Gogotsi, Y., Effects of Structural Disorder and Surface Chemistry on Electric Conductivity and Capacitance of Porous Carbon Electrodes. Faraday Discussions 2014, 172 (1), 139 - 162. Figure 1

N-Doped Graphene-CNT Cathode Composites for High-Performance Li-S Batteries

Reaching beyond the horizon of Li-ion batteries is a formidable challenge, as energy storage will be more important in the future than at any time in the past. This requires the exploration of new chemistry, especially electrochemistry, and new materials. Among the available choices, lithium-sulfur (Li-S) batteries are attractive because of their low cost, environmental friendliness, and superior energy density (theoretically 2500 Wh/kg). Despite their great promise, the commercialization of Li-S batteries is hindered by low cycle stability, fast capacity fading, and low efficiency; these problems are induced by the polysulfide shuttle reaction that occurs through dissolution into electrolyte, the loss of active sulfur material, and the electronically insulating nature of sulfur. To tackle these issues, we synthesized a novel nanostructured composite consisting of Graphene and Carbon Nanotubes (CNT), each pre-doped with Nitrogen (N), as a host material for sulfur impregnation. In this unique composite, not only do CNTs act as the spacer units between the graphene sheets, avoiding losses in performance through restacking and agglomerate formation, but they also contribute to the material activity, overall surface area/nanoporosity, and the composite conductivity over the effective doping method strategy. In the next step, a new method of sulfur impregnating into the composite structure was implemented to effectively confine the sulfur into graphitic clusters, voids, and defects in N-doped graphene–CNT composite. The carbon nanostructured framework serves not only as a conductive layer for encapsulating sulfur and polysulfides without direct contact with electrolyte, but also as a nano-electrochemical reaction chamber. Cyclic voltammetry (CV) of the sulfur-impregnated N-doped graphene-CNT in high-dielectric solvent behaved similar to low-dielectric solvent, indicating that the material was not solvent dependent. Strong interaction between N-doped composite and sulfur or polysulfides enabled us to demonstrate Li–S cells with a high reversible capacity (1005-1592 mAh/g), outstanding cycling stability, and great Coulombic efficiency (~97.5%). The results indicate that the sulfur-impregnated N-doped graphene-CNT nanocomposite is a very promising cathode material for high-performance lithium–sulfur batteries. Figure 1

Reactive Force Field Study of Li/C Systems for Electrical Energy Storage.

Graphitic carbon is still the most ubiquitously used anode material in Li-ion batteries. In spite of its ubiquity, there are few theoretical studies that fully capture the energetics and kinetics of Li in graphite and related nanostructures at experimentally relevant length, time-scales, and Li-ion concentrations. In this paper, we describe the development and application of a ReaxFF reactive force field to describe Li interactions in perfect and defective carbon-based materials using atomistic simulations. We develop force field parameters for Li-C systems using van der Waals-corrected density functional theory (DFT). Grand canonical Monte Carlo simulations of Li intercalation in perfect graphite with this new force field not only give a voltage profile in good agreement with known experimental and DFT results but also capture the in-plane Li ordering and interlayer separations for stage I and II compounds. In defective graphite, the ratio of Li/C (i.e., the capacitance increases and voltage shifts) both in proportion to the concentration of vacancy defects and metallic lithium is observed to explain the lithium plating seen in recent experiments. We also demonstrate the robustness of the force field by simulating model carbon nanostructures (i.e., both 0D and 1D structures) that can be potentially used as battery electrode materials. Whereas a 0D defective onion-like carbon facilitates fast charging/discharging rates by surface Li adsorption, a 1D defect-free carbon nanorod requires a critical density of Li for intercalation to occur at the edges. Our force field approach opens the opportunity for studying energetics and kinetics of perfect and defective Li/C structures containing thousands of atoms as a function of intercalation. This is a key step toward modeling of realistic carbon materials for energy applications.

Thermal resistance from irradiation defects in graphite

An atomistic level understanding of how varying types and numbers of irradiation induced defects affect thermal resistance in graphite is vital in designing accident tolerant fuels for next-generation nuclear reactors. To this end we performed equilibrium molecular dynamics simulations and computed the change to thermal conductivity due to a series of clustering and non-clustering point defects using the Green–Kubo method. In addition, we present a comprehensive discussion of several approaches to converge the integral of the heat current autocorrelation function. Our calculations show that more energetically favorable clustering defects exhibit fewer low frequency modes and increase the anisotropic nature of graphite selectively exerting a significant effect on thermal resistance along the c-axis.

Thermal structure of low-grade accreted Lower Cretaceous distal turbidites, the Central Pontides, Turkey: insights for tectonic thickening of an accretionary wedge

Abstract: Albian-Turonian subduction-accretionary complexes are exposed widely in the Central Pontides. A major portion of the accretionary complexes is made up of a metaflysch sequence consisting of slate/phyllite and metasandstone intercalation with blocks of marble, Na-amphibole bearing metabasite, and serpentinite. The metaflysch sequence represents distal parts of a large Lower Cretaceous submarine turbidite fan deposited on the Laurasian active continental margin that was subsequently accreted and metamorphosed during the Albian. Raman spectra of carbonaceous material of the metapelitic rocks revealed that the metaflysch consists of metamorphic packets with distinct peak metamorphic temperatures. The majority of the metapelites are low-temperature (ca. 330°C) slates characterized by lack of differentiation of the graphite (G) and D2 defect bands. They possibly represent offscraped distal turbidites along the toe of the Albian accretionary wedge. Other phyllites are characterized by a slightly pronounced G band with a D2 defect band occurring on its shoulder. Peak metamorphic temperatures of these phyllites are constrained to 370-385 °C. The phyllites are associated with a strip of incipient blueschist facies metabasites and are found as a sliver within the offscraped distal turbidites. We interpret the phyllites as underplated continental sediments together with oceanic crustal basalt along the basal dA©collement. Tectonic emplacement of the underplated rocks into the offscraped distal turbidites was possibly achieved by out-of-sequence thrusting causing tectonic thickening and uplift of the wedge.

In‐plane and out‐of‐plane defects of graphite bombarded by H, D and He investigated by atomic force and Raman microscopies

Graphite samples exposed to H, D and He plasma at fluencies from 1016 to 1018 cm−2 have been investigated by means of atomic force and Raman microscopies. The ion energy was varied between 40 and 800 eV, and the ion incidence was either perpendicular (Highly Oriented Pyrolitic Graphite) or parallel (carbon/carbon composite) to the basal plane. When increasing the impinging ion energy, the growth of nanometric domes at the surface has been observed by atomic force microscopy and the incident kinetic energy has been found as the parameter determining their height. Two different Raman signatures related to (1) a graphitic nano‐crystalline component similar to that of a 1014 cm−2 bombarded 1‐, 2‐ and 3‐layer graphene, and to (2) an amorphous component, have been evidenced. Polarization studies have revealed that these components are related to regions with either in‐plane or out‐of‐plane disorder, coexisting in the material. These Raman studies have also revealed that both the defect–defect distance in the first case and the aromatic domain size in the second case are typically 1 nm. When the number of vacancies created in the material increases, the number of in‐plane defects decreases to the benefit of the out‐of‐plane defects. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Powder properties of hydrogenated ball-milled graphite

Ball milling is an effective way of producing defective and nanostructured graphite. In this work, the hydrogen storage properties of graphite, ball-milled in a tungsten carbide milling pot under 3bar hydrogen for various times (0–40h), were investigated by TGA-Mass Spectrometry, XRD, SEM and laser diffraction particle size analysis. For the conditions used in this study, 10h is the optimum milling time resulting in desorption of 5.5wt% hydrogen upon heating under argon to 990°C. After milling for 40h, the graphite became significantly more disordered, and the amount of desorbed hydrogen decreased. After milling up to 10h, the BET surface area increased while particle size decreased; however, there is no apparent correlation between these parameters, and the hydrogen storage properties of the hydrogenated ball-milled graphite.