Graphite Layers Research Articles

Carbon fibers from polypropylene: Structural evolution of precursor fiber during stabilization and carbonization

Utilizing polypropylene (PP) fibers as the precursor material, Polypropylene-based carbon fibers (CFPP) were obtained by carbonization after sulfonation cross-linking with sulfuric acid. First, the mechanism of structural evolution of sulfonated PP fiber (SFPP) during carbonization was investigated at temperatures between 500 °C-1300 °C. According to the results, the chemical structure, as well as the carbon structure transformation of SFPP, depending on the carbonization temperature, i.e., the removal of organic functional groups, the fusion to small aromatic structures, and the transformation to ordered carbon structures. The carbon content of the carbon fibers obtained at 1300 °C was as high as 93.4 %. Subsequently, various PP fibers with different diameters and crystalline structures were prepared by melt spinning and subsequent thermal-drawing treatment, and the relationship between their structures and the properties of CFPPs was investigated. The sulfonation reaction rate of PP fibers increased as the sulfonation reaction proceeded and was synergistically regulated by the crystalline structure and fiber diameter. The CFPP with a smaller diameter exhibited a more homogeneous skin-core structure, together with higher overall graphitization, smaller graphite layer spacing, and better graphite microcrystalline structure, and thus its conductivity and mechanical properties of the carbon fiber were significantly improved. The mechanical properties and volume resistivity of the prepared CFPP reached 338.6 MPa and 4.91×10−5 Ω·m, respectively.

Catalytic and Capacitive Properties of Hierarchical Carbon–Nickel Nanocomposites

Hierarchically graphitic carbon that contained nickel nanoparticles (HGC-Ni (1), (2), and (3)) were prepared by the pyrolysis of three metal complexes as follows: nickel 2,2′-biyridine dichloride, nickel terephthalate 2,2′-bipyridine, and nickel phenanthroline diaqua sulfate, respectively, in the presence of anthracene or pyrene. SEM indicated that the structure of the HGC-Ni samples consisted of nickel nanoparticles with a diameter of 20–500 nm embedded in a thin layer of a hierarchical graphitic carbon layer. The EDAX of HGC-Ni indicated the presence of nickel, carbon, and nitrogen. Chlorine, oxygen, and sulfur were present in (1), (2), and (3), respectively, due to the differences in their complex precursor type. XRD indicated that the nanoparticles consisted of Ni(0) atoms. The turnover frequency (TOF) for the reduction of p-nitrophenol (PNP) increased for catalysts HGC-Ni (3), (2), and (1) and were 0.0074, 0.0094, and 0.0098 mg PNP/mg catalyst/min, respectively. The TOF for the reduction of methyl orange (MO) increased for catalysts (3), (1), and (2) and were 0.0332, 0.0347, and 0.0385 mg MO/mg catalyst/min, respectively. Thus, nickel nano-catalysts (1) and (2) provided the highest performance compared to the nano-catalysts for the reduction of PNP and MO, respectively. The first-order rate constant (min−1) of HGC-Ni (3), with respect to the reduction of PNP, was 0.173 min−1, while the first-order rate constant (min−1) for the reduction of MO by HGC-Ni (1) was 0.404 min−1. HGC-Ni (3) had the highest number of cycles with respect to PNP (17.9 cycles) and MO (22.8 cycles). The catalysts were regenerated efficiently. HGC-Ni exhibited remarkable electrochemical capacitance characteristics in the present study. This material achieved a notable specific capacitance value of 320.0 F/g when measured at a current density of 2 A/g. Furthermore, its resilience was highlighted by its ability to maintain approximately 86.8% of its initial capacitance after being subjected to 2500 charge and discharge cycles. This finding suggests that this HGC-Ni composite stands out not only for its high capacitive performance but also for its durability, making it an attractive and potentially economical choice for energy-storage solutions in various technological applications.

Ultrathin graphitic carbon nitride interface layer regulated nickel-iron oxyhydroxide stabilized on nickel foam for efficient oxygen evolution reaction

Regulating interfacial properties of electrocatalysts to enhance the performance of oxygen evolution reaction (OER) possesses practical significance for water splitting. Herein, we report a novel strategy to form a graphitic carbon nitride (g-C3N4) interface layer on nickel foam (NF), followed by the growth of nickel-iron (oxy)hydroxides (NiFe-OH). The NiFe-OH nanosheets are tightly anchored on the g-C3N4 layer, exhibiting enhanced surface and structural characteristics. Therefore, the as-prepared NF/g-C3N4/NiFe-OH demonstrates superior OER activity with low overpotentials of 272, 342, and 476 mV at the current densities of 50, 100, and 200 mA cm−2, respectively. The reduced Tafel slope and raised TOF values of the electrode compared to NF/NiFe-OH reveal the promoting roles of g-C3N4, which provides more active sites, induces charge redistribution, and facilitates electron transfer. Furthermore, the electrode presents reliable stability and durability without observable morphological damage, alteration in the surface chemical state, and significant depletion of active components.

Oxidation Mechanism and Surface Characterization of Pyrolytic Graphite in Simulated Air for Pyrochemical Reprocessing Application

Accidental air ingress into the containment chamber of pyrochemical reprocessing of spent nuclear fuels can cause severe oxidation of graphite components. Pyrolytic graphite (PyG) coating on conventional high-density graphite (HDG) is proposed for protection against oxidation, molten salt, and molten metal corrosion. The aim of the present study is to evaluate the thermal stability and kinetic parameters and propose a suitable oxidation mechanism of PyG under simulated air using thermogravimetry analysis (TGA). In dynamic oxidation, a linear increase in the oxidation onset, peak, and burnout temperature is observed for PyG synthesized at increasing pyrolysis temperature from 1200 to 2400°C except for 1400°C. The isothermal oxidation study showed a similar trend i.e., a decrease in the linear oxidation rate constant value for any given isotherm with increasing pyrolysis. The Arrhenius plot showed two-stage oxidation regimes operating between 600 to 1000°C. In regime-I, between 600 to ∼800°C controlled by surface chemical reaction kinetics, the activation energy is measured between 100-200 kJ.mol−1. The graphite-oxygen reaction initiates and propagates preferentially along the prismatic planes at the sample edges, where the dangling bonds of stacked graphite layer structures are exposed. In regime-II operating above ∼800°C, the surface reaction rate is infinitely faster, and the diffusional mass transport becomes a rate-limiting factor. In this regime, the graphite-oxygen initiates by pore formation on defective reactive sites on the bulk surface and propagates by pore-widening and pore-coalescence mechanism. The observed differences in the oxidation rates for different pyrolysis PyG attributed to its intrinsic material properties are investigated. This work provides new insights into the oxidation mechanism of the PyG during accidental air ingress in a pyrochemical reprocessing plant.

Surface modification by CO2 plasma boosting core shells structural Fe/Fe3C/FeN @ graphite carbon nanoparticles toward high performance microwave absorber

The surface modification of three-dimensional (3D) materials is an efficient method for adjusting their interfacial defect concentration, electronic conductivity and content of functional groups with extensive applications in catalysis, electrode materials and bioengineering. In this work, a multiphase iron nanocrystals consisting of Fe3C, Fe and FeN nanoparticles encapsulated in hierarchical structure of graphite carbon (denoted as Fe/Fe3C/FeN@GC) is synthesized for the first time by a novel high temperature plasma method. Meanwhile, more defects and functional groups are introduced by surface modification of graphite carbon layer of Fe/Fe3C/FeN@GC with controllable CO2 (low temperature) plasma. Benefiting from the advantages of multiple heterogenous interface and the abundant interfacial polarization relaxation that represent strong electromagnetic (EM) wave dissipation as well as an applicable impedance matching, the optimized Fe/Fe3C/FeN@GC demonstrate superior microwave absorption (MA) properties. The minimum reflection loss (RL) achieves −54.4 dB (more than 99.9% MA) at 17.6 GHz with a thin thickness of 1.8 mm, and the maximum effective absorption bandwidth (EAB, RL < −10 dB) is up to 6.2 GHz (11.8–18.0 GHz) at 2.0 mm. The above results reveal that the optimized Fe/Fe3C/FeN@GC composites with strong absorption, broad EAB, light mass (only filling content of 30 wt%) and ultrathin thickness are prospective candidate for high performance EM wave absorbers.

Computational Optimization of Sandwich Silicone Rubber Composite for Improved Thermal Conductivity and Electrical Insulation.

The efficient dissipation of heat has emerged as a crucial concern for modern electronic devices, given the continuous increase in their power density and consumption. Thus, the utilization of thermally conductive but electrically insulating silicone rubber composites as a thermal interface material has garnered significant interest. In this study, the effects of the filler volume fraction, filler orientation, layer volume fractions, layer configuration, and a number of layers on the thermal conductivity and electrical resistivity of silicone rubber composites were examined using a multiscale finite element modeling strategy. The results demonstrated that modification of the filler orientation can change the thermal conductivity by 28 and 21 times in the in-plane and through-thickness directions, respectively. The in-plane thermal conductivities of silicone rubber/boron nitride and silicone rubber/expanded graphite layers exhibit a percolation phenomenon at filler volume fractions of 35% and 30%, respectively. The electrical resistivity of the composite increases exponentially with a decrease in the number of layers.

Thermal annealing induced graphite/diamond structure processed by high-voltage hydroxide ion treatments

Ohmic contacts with low specific contact resistance are necessary for fabrication of diamond electronic devices. This work reports that graphite/diamond ohmic contacts can be obtained by thermal annealing of diamond, patterned by hydroxide ion treatment. Surface graphitization of diamond is achieved through rapid thermal annealing at 1050 °C in vacuum. The thickness of graphite layer is found to be dependent on the duration of annealing with an approximate rate of 3.3 ∼ 3.6 nm/minute. The alteration in surface structure is analyzed using Raman spectra, showing a continuously enhanced G peak with increasing annealing time. Early stages of graphitization are discovered by X-ray photoelectron spectroscopy, after annealing diamond samples for merely 0.5 ∼ 2 min. Additionally, a distinct atomic interface between graphite and diamond is observed via cross-sectional transmission electron microscopy. Further, the surface graphite is selectively etched by hydroxide ion treatments, protected by gold patterns. In the end, the specific contact resistance of the graphite/diamond contact is determined to be 2.53 × 10-4 Ω cm2 employing the transmission line model.

Preparation of SnO2@graphite Anode based on the Oriented Deposition Methodology with Superior Electrode Behavior for Li Ion Battery

AbstractSnO2‐based anode materials are deemed to be one of the most prospective anode materials for Li ion batteries (LIBs) with higher theoretical capacity as compared to conventional graphite. However, the weak electrical conduction and large volume expansion limit its application in eVeryday life. Here, a simple and low‐cost oriented deposition methodology is developed to prepare SnO2@graphite composite, which is obtained based on the redox reaction mechanism between SnSO4 and graphite. SnO2 nanoparticles are uniformly anchored on the carbon layer of graphite. The close anchoring of SnO2 in graphite effectively inhibits the volume expansion of SnO2 during lithiation/delithiation processes, and good combination state between them guarantees the excellent electrical conductivity of the composite. As anode for LIBs, SnO2@graphite electrode delivers superior reversible capacity and provides an improved rate capability compared to graphite electrode. Benefiting from the tight combination of SnO2 and graphite, the Li ion diffusion coefficient of SnO2@graphite electrode is almost twice that of graphite electrode. Pairing with LiFePO4 cathode, the SnO2@graphite/LiFePO4 full cell exhibits higher energy/power densities by comparison with graphite/LiFePO4. Besides, this advanced SnO2@graphite composite material with superior anode behavior can be mass produced by the reported methodology.

Sub-micron porous Si-C/graphite anode with interpenetrated 3D conductive networks towards high-performance lithium-ion batteries

Silicon (Si) has been envisaged as the most promising anode candidate for future lithium-ion batteries with high energy density and large power owing to its high theoretical capacity and abundance. However, the inherent drawbacks of Si anodes such as low electrical conductivity and volume variation seriously hinder their commercial scalable application. Here we demonstrate a magnesiothermic reduction process for converting marigold structured silica precursor into sub-micron silicon replicas. The complicated structured silica was converted into co-continuous composites of silicon, graphite and amorphous carbon layer by in-situ generation. This electrode delivers excellent rate capability with a discharge specific capacity of 631.7 mAh g−1 at 3 A g−1 and high cycling stability of 1114.9 mAh g−1 after 200 cycles at 0.1 A g−1. Moreover, the coulombic efficiency increased rapidly from the initial 62–97.3% after only 10 cycles and 98.8% can be obtained with negligible fading after 200 cycles. Remarkably, some byproduct partially decomposed from surfactant can be in-situ derived into amorphous carbon layer deposited in the gap of wrinkled silicon spheres, which was effectively improves the complete conductive contact and the interface compatibility between active materials. This work provides a new design strategy for preparing Si-graphite anodes suitable for commercial applications.

Real-Time Observation of Nanoscale Kink Band Mediated Plasticity in Ion-Irradiated Graphite: An In Situ TEM Study.

Graphite IG-110 is a synthetic polycrystalline material used as a neutron moderator in reactors. Graphite is inherently brittle and is known to exhibit a further increase in brittleness due to radiation damage at room temperature. To understand the irradiation effects on pre-existing defects and their overall influence on external load, micropillar compression tests were performed using in situ nanoindentation in the Transmission Electron Microscopy (TEM) for both pristine and ion-irradiated samples. While pristine specimens showed brittle and subsequent catastrophic failure, the 2.8 MeV Au2+ ion (fluence of 4.378 × 1014 cm-2) irradiated specimens sustained extensive plasticity at room temperature without failure. In situ TEM characterization showed nucleation of nanoscale kink band structures at numerous sites, where the localized plasticity appeared to close the defects and cracks while allowing large average strain. We propose that compressive mechanical stress due to dimensional change during ion irradiation transforms buckled basal layers in graphite into kink bands. The externally applied load during the micropillar tests proliferates the nucleation and motion of kink bands to accommodate the large plastic strain. The inherent non-uniformity of graphite microstructure promotes such strain localization, making kink bands the predominant mechanism behind unprecedented toughness in an otherwise brittle material.

Core-Shell Adsorbent Granules Fabricated from Sand/Graphene Oxide Nano-Composite for Adsorptive Removal of Pb (II)

Core-shell absorbent granules are produced by coating commercial sand gravels with graphite oxide (few-layer oxidized graphene sheets). Graphite oxide (GO) is synthesized chemically from vein graphite, a rare form of high purity natural graphite. Repeated coatings of graphite oxide on sand followed by low temperature (1200 C) thermal pyrolysis produce core-shell granules with a hierarchical structure in which sand gravels are covered by graphite oxide layers. The intention of this study is to assessment of feasibility of the mitigation of Pb (II) form the water by sand/graphene oxide nano-composite and study the dynamic behavior of the adsorption mechanism using the mathematical modeling. Individual materials and the resulting nanocomposite are characterized using a range of spectroscopic (FTIR, XRD, XPS) and microscopic (SEM/EDXA) techniques to elucidate the process of transforming individual materials into nanocomposite and to assess its suitability for removing Pb (II) from water. Stable sand/graphite oxide nanocomposites were obtained upon five times coating of GO on the sand surface and characterization shows that functionalized and a porous coating developed on the sand. The Pb (II) removal efficiency increases from high-acidic to neutral pH and decreases after basic (pH-8). The adsorption process takes 150 minutes to reach the equilibrium. After 150 minutes, the Pb (II) ion adsorption rate was constant. The maximum dosage of the nanocomposite needed to reach the equilibrium was 0.06g. Under the optimum conditions, 96.3% of Pb (II) removal efficiency was recorded from simulated Pb (II) containing water. According to the mathematical modeling, adsorption follows the Langmuir isotherm and fits well with the pseudo 2nd order kinetics model, indicating a homogeneous adsorption process via a monolayer process and Pb adsorbed onto the surface of the sand/GO nanocomposite by chemisorption. Sand/GO nanocomposite is an excellent candidate for removing Pb (II) from contaminated water.&#x0D; Keywords: Adsorption, Graphene oxide, Sand, Water treatment

Diglyme as a Promoter for the Electrochemical Formation of Quaternary Graphite Intercalation Compounds Containing two Different Types of Solvents

AbstractCo‐intercalation using ether‐based electrolytes renders graphite as a promising anode material in sodium‐ion batteries (SIBs). While most research on electrochemical solvent co‐intercalation in graphite has focused on linear ethers such as mono‐, di‐, tri‐, tetra‐, and penta‐glyme, we herein investigate the possibility of reversible electrochemical co‐intercalation with alternative solvents, especially cyclic ethers tetrahydrofuran (THF) and 1,3‐dioxolane (DOL), which show no signs of co‐intercalation on their own. We demonstrate, however, that this reaction becomes feasible when incorporating diethylene glycol dimethyl ether (2G, diglyme) as an additive. Operando X‐ray diffraction and ex‐situ ss‐NMR techniques are employed comprehensively to understand the co‐intercalation reaction of these cyclic ethers, along with Na+‐glymes, into graphite during cycling. When using these mixed electrolytes i. e., THF/2G and DOL/2G, the voltage profiles changes compared to the pure glyme‐based electrolyte, while showing comparable specific capacities and good long‐term durability. Overall, we propose that even trace amounts of diglyme prompt the co‐intercalation of THF and DOL into graphite layers. This leads to the formation of quaternary graphite intercalation compounds (q‐GICs), expanding beyond the realm of ternary graphite intercalation compounds (t‐GICs).

Role of the codeposited C and W element on the tribological performance of WS2 coating

WS2 is a widely used solid lubricating material that exhibits applications in various fields, including automotive components, precision instruments, and key parts that require antiwear properties. However, WS2 is highly susceptible to humidity, which significantly limits its practical utility. In order to investigate and enhance the tribological and mechanical properties of WS2 coatings, a method involving codeposition of carbon (C) and tungsten (W) with WS2 was employed using magnetron sputtering, resulting in the successful preparation of W-diamondlike carbon (DLC)/WS2 composite coatings. Subsequent investigations revealed a synergistic effect of C and W through various methods. The addition of W noticeably improved both the nanohardness and Young’s modulus of the W-DLC coatings, thereby further enhancing the mechanical properties of the W-DLC/WS2 composite coatings and improving their wear resistance. However, it was observed that excessive W tended to oxidize, intensifying the abrasive wear of the composite coatings during friction. Moreover, Raman spectroscopy analysis indicated the presence of carbon in the composite coatings in the form of DLC, which served as a lubrication phase. The presence of DLC facilitated the formation of transfer films composed of graphite. Additionally, it was discovered that the number of graphite layers in the transfer films had an impact on the tribological properties of the coatings.

Fabrication of graphene from graphite using high-powered ultrasonic vibrators

This paper utilizes an efficient and environmentally friendly method for synthesizing graphene from graphite, namely liquid-phase exfoliation. High-power density ultrasonic vibrators were used to separate graphite layers into graphene in a liquid medium. During layer separation, ultrasonic waves provided mechanical energy to break the Van der Walls bonds and separate graphite layers into graphene. In our study, graphene was synthesized by ultrasonicating graphite in Tween 80 for 1 to 5 h, followed by magnetic stirring and surfactant removal. The FESEM and Raman measurements demonstrated that high-frequency ultrasound waves were effective at breaking the Van der Waals bonding force between adjacent graphite layers. Average flake sizes (lateral) were reduced with increasing ultrasonication time, reaching a minimum value of 317 nm with 5 h of ultrasonic treatment. These results show that liquid-phase exfoliation is a cost-effective method to obtain low-defect few-layer graphene.

ZIF-8 induced N, P double doped porous molybdenum phosphide hydrogen evolution catalyst

Molybdenum based phosphides have gradually become the investigation frontier of hydrogen evolution catalysts due to the platinum like electronic structure. In this paper, N, P double-doped graphite carbon layers wrapped with MoP nanoparticles (P–MoP@NPC), is directly compounded by one-step high-temperature carboning PMo12-APP@ZIF-8 precursor. P–MoP@NPC possesses splendid hydrogen production reactive, in two kinds of electrolytes, when the overpotentials of P–MoP@NPC are 132 mV and 161 mV, the slope of current density reaching 10 mA cm−2, Tafel slope are 67 and 79 mV dec−1, respectively. In the bargain, P–MoP@NPC displays satisfactory durability in acid and alkaline mediums and can continue to work for more than 24 h. The wonderful hydrogen production ability of P–MoP@NPC is ascribed to the synergistic effect of MoP nanoparticles, hierarchical porous structure as well as N, P double-doped carbon layers. The confinement effect of ZIF-8 makes most of the synthesized MoP nanoparticles evenly distributed. The hierarchical porous structure caused by the volatilization of Zn atoms with high temperature can expose more active centers. The N, P double-doped graphitic carbon layers not only protect the catalyst but also adjust the electronic structure of MoP, promoting the hydrogen production capacity. This work may provide a general way to study molybdenum phosphide catalysts synthesized by APP as a green phosphorus source for energy conversion.

Graphite as a Skin and Epithelium Dosimeter at mammographic energies

Use is made of 50 μm high purity graphite foils, investigating the potential of these in evaluation of dose in dermal and epithelial layers, measuring the thermoluminescence yield obtained as a result of irradiation at mammographic energies. In particular the effort supports the ICRU recommendation that skin dose be assessed at a skin depth 0.07 mm, equating to the uniquely mitotically active dermal layer responsible for the replenishment of skin cells. Support for the results of empirical investigation have been sought from Monte Carlo simulations using the TOPAS code. In study of x-rays generated in the 25- to 30 kVp range, the energetic electrons produced in interaction with the graphite have an approximate range of ∼7 μm, so that with a stacked graphite layer arrangement dose assessment is provided at 7 μm, 57 μm, 107 μm etc. TL yield results for the first three layers of graphite, covering nominal depths from the surface through to 150 μm, obtained on a 50 μm spatial resolution scale, show deposited dose values to be approximately flat to within ±5% throughout this depth. The indication is that for irradiations made using x-ray energies of a few tens of keV use of the present graphite layer arrangement can provide estimation of dermal and epithelial dose.

Microwave-induced biomass pyrolysis coupled with hydrothermal char composites catalysis to selectively prepare phenols-rich liquid products

In this study, three types of hydrothermal char-supported metal nanoparticle composites (HCM-Fe, HCM-Co, and HCM-Ni) were prepared using pinewood as the raw material via an ion hydrothermal pretreatment followed by microwave heating method, which were then served as both catalysts and microwave absorbers to enhance the rapid pyrolysis of pine wood particles for the selective preparation of phenols-rich bio-oil. The combined effects of hydrothermal carbonization and microwave heating enriched the pore structure of the composite surface, yielding ordered carbon structures such as carbon microspheres, carbon nanotubes, and graphite layers. Also, metal nanoparticles were generated and uniformly anchored on the hydrothermal char matrix by the graphite layers. The three types of hydrothermal carbon-based catalysts exhibit excellent microwave absorption performance, facilitating rapid heating of pine particles at a rate as fast as 5.49 °C/s. Among these catalysts, HCM-Co demonstrates the best catalytic performance, efficiently promoting the cleavage of the β-O-4 bond in lignin and achieving a liquid product yield of 38.4 % at 500 °C. The resulting bio-oil is characterized by its richness in phenolic compounds, accounting for 54.9 % of its composition, and partial mitigation of acidity.

P-doped hard carbon material for anode of sodium ion battery was prepared by using polyphosphoric acid modified petroleum asphalt as precursor

The practical value of hard carbon anode in sodium ion batteries has garnered significant attention. Asphalt-derived hard carbon offers lower cost and scale advantages compared to expensive resin and bio-based hard carbon materials. However, the synthesis of asphalt-derived hard carbon with significantly enhanced reversible capacity remains challenging. This study focuses on synthesizing P-doped hard carbon material using polyphosphoric acid-modified petroleum asphalt as a precursor and the pre-oxidation method. Doping P elements in hard carbon induces deformation of the molecular layer structure within the pseudo-graphite domains, expanding graphite layer spacing and increasing the number of internal closed micropores, thereby enhancing the sodium storage capacity of the matrix. The reversible capacity of P-doped hard carbon (PHC-1300) increased from 240.3 mA h g−1 to 359.9 mA h g−1 during the initial charge-discharge cycle at a rate of 0.05 C compared to undoped hard carbon (HC-1300). Moreover, phosphate exhibits higher reduction ability than adjacent carbon atoms, generating electron holes that partially compensate for limited electron conduction capability in hard carbon material. The electronic conductivity of the material increases from 26.1114 S cm−1 to 36.6418 S cm−1, demonstrating a significant enhancement compared to high graphitized soft carbon (SC-1300).Therefore, the practical significance of employing this straightforward precursor modification method to prepare P-doped hard carbon warrants attention.

Monitoring Aging Effects in Graphite Bisulfates by Means of Raman Spectroscopy

Graphite bisulfate (GBS) compounds consist of graphite layers intercalated by HSO4− ions and H2SO4 molecules. Owing to electrostatic interactions with the graphene plane, HSO4− ions cause point defects in the graphite’s crystalline structure, while H2SO4 molecules are free to move via diffusion in the spaces between the adjacent graphite sheets and segregate to form linear defects. In the present work, we report the results of our investigation using Raman spectroscopy on the temporal evolution of such defects on selected GBS samples over 84 months. Two characteristic lengths correlated with the average distance between defects have been estimated and their evolution with aging was investigated. The results show a decrease in the density of point-like defects after aging, regardless of the pristine structural configuration of the GBS samples, revealing a structural instability. This study can provide significant information for the technological development of industrial processes aimed to produce expanded graphite based on GBS precursors, where the aging of GBS is known to influence the efficiency and quality.

Ultrahigh-mobility semiconducting epitaxial graphene on silicon carbide.

Semiconducting graphene plays an important part in graphene nanoelectronics because of the lack of an intrinsic bandgap in graphene1. In the past two decades, attempts to modify the bandgap either by quantum confinement or by chemical functionalization failed to produce viable semiconducting graphene. Here we demonstrate that semiconducting epigraphene (SEG) on single-crystal silicon carbide substrates has a band gap of 0.6 eV and room temperature mobilities exceeding 5,000 cm2 V-1 s-1, which is 10 times larger than that of silicon and 20 times larger than that of the other two-dimensional semiconductors. It is well known that when silicon evaporates from silicon carbide crystal surfaces, the carbon-rich surface crystallizes to produce graphene multilayers2. The first graphitic layer to form on the silicon-terminated face of SiC is an insulating epigraphene layer that is partially covalently bonded to the SiC surface3. Spectroscopic measurements of this buffer layer4 demonstrated semiconducting signatures4, but the mobilities of this layer were limited because of disorder5. Here we demonstrate a quasi-equilibrium annealing method that produces SEG (that is, a well-ordered buffer layer) on macroscopic atomically flat terraces. The SEG lattice is aligned with the SiC substrate. It is chemically, mechanically and thermally robust and can be patterned and seamlessly connected to semimetallic epigraphene using conventional semiconductor fabrication techniques. These essential properties make SEG suitable for nanoelectronics.