Curved Graphene Layers Research Articles

Intervention-Free Graphitization of Carbon Microspheres from a Non-Graphitizing Polymer at Low Temperature: Nanopores as Dynamic Nanoreactors.

Graphitizability of organic precursors is the topic of numerous investigations due to the wide applications of graphitic materials in the industry and emerging technologies of supercapacitors, batteries, etc. Most polymers, such as polydivinyl benzene (PDVB) are classified as non-graphitizings that do not convert to Graphite even after heating to 3000℃. Here, for the first time, the development of graphitic structure in the hierarchal porous sulfonated-PDVB microspheres without employing specific equipment or additives like metal catalysts, organic ingredients, or graphite particles, at 1100°C is reported. The abnormal additive-free graphitic structure formation is confirmed by Raman spectroscopy (ID/IG = 0.87), high-resolution transmission electron microscopy (HRTEM), and selected area diffraction patterns (SAED), as well as x-ray diffraction patterns (XRD), while preservation of aromatic compounds from the carbonization is detected by Fourier transform infrared (FTIR) analysis. Polymer evolution from room temperature to 1100°C is also studied by FTIR, Raman spectroscopy, and XRDtechniques. Based on the obtained results, it is suggested that the hierarchal and complicated ink-bottle pore network with a high surface area besides super micropores in the sulfonated-PDVB microspheres has served as nano-sized reaction media. These pores, hereafter referred as "dynamic nanoreactors", are expected to have confined the in-situ produced thermal decomposition products containing broken bond benzene rings, while changing dimensionally and structurally during the designed carbonization regime. This confinement has led to the benzene rings fusion at 250°C, a remarkable extension of them at 450°C, their growth to graphene sheets at 900°C and finally, the stacking of curved graphene layers at 1100°C. The results of this research put stress on the capability of nanopores as nanoreactors to facilitate reactions of decomposition products at low temperatures and ambient pressures to form stacked layers of graphene; A transformation that normally requires catalysts and very high pressures for only specific polyaromatic hydrocarbons.

High-density MoCx nanoclusters anchored on nanodiamond-derived nanocarbon as a robust CO2 reduction catalyst for syngas production

Molybdenum carbide (MoCx) has been considered as a promising catalyst for CO2 reduction at medium temperature (i.e. >400 °C) because of its excellent ability to dissociate CO2 and H2. However, MoCx catalysts are mainly synthesized at high carburization temperature (i.e., >650 °C), which renders its low active sites exposed and the generation of CH4 by-product, especially at a high H2/CO2 molar ratio. Herein, the core–shell structure nanodiamond @ graphene (ND@G) hybrid with enriched electron density was applied for anchoring MoCx nanoparticles. The as-synthesized MoCx/ND@G catalyst consists of ultra-high exposed MoCx nanoparticles/nanoclusters, exhibiting an extremely high and stable reverse water gas shift (RWGS) catalytic performances (211.5 μmolCO2 gMoCx−1 s−1 at 400 °C and 358.0 μmolCO2 gMoCx−1 s−1 at 600 °C, respectively) with CO selectivity up to 99.5% at H2/CO2 ratio of 4. Remarkably, the MoCx/ND@G catalyst facilitates the CO2 activation at a relatively low temperature compared to MoCx nanoparticles supported by other typical carriers (i.e., activated carbon and γ-Al2O3) and bulk β-Mo2C due to the strong interaction between MoCx species and curved graphene layers on the surface of ND@G. This study paves a practical strategy for developing a highly efficient molybdenum carbide-based catalyst for CO2 activation.

Accurate Time-Domain Modeling of Arbitrarily Shaped Graphene Layers Utilizing Unstructured Triangular Grids

The accurate modeling of curved graphene layers for time-domain electromagnetic simulations is discussed in the present work. Initially, the advanced properties of graphene are presented, focusing on the propagation of strongly confined surface plasmon polariton waves at the far-infrared regime. Then, the implementation of an unstructured triangular grid was examined, based on the Delaunay triangulation method. The electric-field components were placed at the edges of the triangles, while two different techniques were proposed for the sampling of the magnetic ones. Specifically, the first one suggests that the magnetic component is placed at the triangle’s circumcenter providing more accurate results, although instability may occur for nonacute triangles. On the other hand, the magnetic field was sampled at the triangle’s centroid, considering the second technique, ensuring the algorithm’s stability, but further approximations were required, leading to a slight accuracy reduction. Moreover, the updating equations in the time-domain were extracted via an appropriate approximation of Maxwell equations in their integral form. Finally, graphene was introduced in the computational domain as an equivalent surface current density, whose location matches the corresponding electric components. The validity of our methodology was successfully performed via the comparison of graphene surface wave propagation properties to their theoretical values, whereas the global error determination indicates the minimal triangle dimensions. Additionally, an instructive setup comprising a circular graphene scatterer was analyzed thoroughly, to reveal our technique’s advantages compared to the conventional staircase discretization.

Large-scale and low-cost synthesis of porous carbon on the surface of commercial chlorinated polymers under the action of an intense electron beam of microsecond duration

The formation of a layer of porous carbon on the surface of low-cost commercial chlorinated polyvinyl chloride (CPVC) with the addition of ferrocene under the action of an intense electron beam of microsecond duration was investigated. The polymer was irradiated using a pulsed fore-vacuum plasma source of a large-diameter electron beam (cross-sectional area ∼100 cm2), which allows minimizes polymer charging during irradiation. The methods of scanning and transmission electron microscopy, EDX spectroscopy, Raman and X-ray photoelectron spectroscopy were used to study the carbon layers. We found that the carbon layer consists mainly of thin (∼10 nm) sheets of amorphous carbon containing curved graphene layers up to 4 nm in length. According to the XPS data analysis, the relative fraction of graphitized carbon (sp2-hybridized carbon) reaches 80% in the surface layer with a thickness of up to 3 nm. Possible mechanisms for the formation of a porous carbon layer on the CPVC surface were discussed. The porous carbon layer has a specific capacitance of 14 mF/cm2 at a scan rate of 5 mV/s. In many applications, it can be an alternative to the laser-induced graphene formed on the surface of expensive high-temperature polyimide.

Pore structure engineering of wood-derived hard carbon enables their high-capacity and cycle-stable sodium storage properties

The pore structural designability of wood-derived hard carbons (WDHCs) lays an optimal foundation for acquiring a cost-efficient and green anode material for sodium-ion batteries. Here, a WDHCs material was reported for high-capacity and cycling-stable sodium storage through a pore forming-opening strategy. Associated with the natural holey texture of wood, the WDHCs displays a hierarchical pore structure by a precise chemical treatment and a high-temperature carbonization, creating the straight channels and plentiful closed nanopores. This well-constructed channel structure is conducive to the penetration of electrolyte and the transport of sodium ions. The closed nanopores constructed by the curved graphene layers are gradually unfolded upon cycling, thus providing higher reversible capacity in the platform zone. The surface-controlled pseudocapacitive reaction favored ultrafast ion transport mainly contributes to sodium storage capacity. As a result, the developed optimal anodes harvest a high sodium storage capacity of 439 mAh g−1 at 0.1 A g−1, and excellent long-term cycling stability of 248 mA h g−1 at 0.1 A g−1 over 500 cycles as well as the capacity retention close to 100%. The galvanostatic intermittent titration technique combined with electrochemical impedance spectroscopy proved that the WDHCs anode has fast charge transport kinetics, thus exhibiting excellent rate performance.

Cross‐sectional structural characterization of the surface of exfoliated HOPG using HRTEM‐EELS

We analyzed the surface atomic structure of highly oriented pyrolytic graphite (HOPG) substrate exfoliated with adhesive tape, using high‐resolution transmission electron microscopy and scanning transmission electron microscopy‐electron energy‐loss spectroscopy (STEM‐EELS). The surface step height of the exfoliated HOPG substrate was determined using high‐angle annular dark‐field‐scanning transmission electron microscopy (HAADF‐STEM) images and the depth profiles of the EELS spectra of a cross‐sectioned thin foil specimen prepared via focused ion beam milling. The exfoliated surface of the HOPG substrate presented disordered and curved graphene layers. The STEM‐EELS measurements indicated that upon exfoliation, the surface of the HOPG substrate reacted with atmospheric water and oxygen molecules.

Effect of high temperature treatment on the microstructure and elastoplastic properties of polyacrylonitrile-based carbon fibers

The effects of high temperature treatment (2000–2800 °C) on the microstructure evolution and elastoplastic properties of polyacrylonitrile-based carbon fibers were investigated. It is found that the uneven shrinkage of the carbon fiber surfaces leads to formation of bulges whose length-to-width ratios increase with increasing heat treatment temperature. Because of the shrinkage stress, a closely stacked and curved graphene layer can be found in the sub-surface region of the carbon fibers. In the longitudinal section, the continuity of the graphite-like ribbons in the sub-surface region is worse than that in the skin region and the cross-linking regions between the graphite-like ribbons still contain a lot of defects after heat treatments. As the indentation depth increases, the axial and transverse elastic modulus of the carbon fibers decrease, which are related to the needle-like pores and the ordered graphite-like crystallites. The improvement of the slip between the graphene layers and the breakage of the graphite-like crystallites and the cross-linking regions are responsible for the decrease in axial elastic modulus with increasing heat treatment temperature. By contrast, the transverse elastic modulus is low and the excellent elasticity in the transverse direction is due to the curved graphene layers and the needle-like pores.

Nonlocal Plasmonic Modes and Plasmonic Band Structures in Cylindrically Curved Graphene

In this paper, hydrodynamic model has been analytically solved to investigate the nonlocal plasmons in cylindrically curved graphene layers. Within the quasi-static approximation, the dispersion relations for both local and nonlocal cases have been derived; the nonlocal effect is found to shift the dispersion relations upwards. High-order azimuthal modes possess different cutoff frequencies due to such nonlocality, which may occur even in large-scale highly doped structures. In periodically doped cases, the nonlocal effect can modify the corresponding plasmonic band structures, i.e., moving the locations of the bandgaps. The periodicity has made the material more sensitive to the plasmon nonlocality. Our investigations may lead to more attentions to the nonlocal plasmonic responses in graphene which are important for graphene-plasmon-based photonic devices.

Salt melt synthesis of curved nitrogen-doped carbon nanostructures: ORR kinetics boost

Implementing metal-free electrocatalysts for the oxygen reduction reaction (ORR) and revealing crucial chemical or topographical parameters driving their activity are vital for the development of power cells. The carbon-based catalysts are very often synthesized through carbonization of biopolymers, in particular, those one containing nitrogen groups such as chitosan. Unfortunately, the resulting carbonaceous materials usually lack specific porosity and exhibit low catalytic activity.Here, we demonstrate that pyrolysis of chitosan in a ZnCl2 melt assisted by the presence of LiCl results not only in a highly porous activated carbon material with a specific surface area of 1317.97 m2/g and the total nitrogen content of 6.5%, but also induces unexpected curvature in the grown graphitic layers. This is the first work that shows curved graphene layers obtained from a biopolymer precursor by its pyrolytic decomposition in the melted salt media.On the other hand, a carbonaceous material obtained from chitosan but without the salts has very low specific surface area of 7.8 m2/g, possesses no specific structural features, and contains 4.7% of nitrogen.The electrochemical studies show, that the former material is highly active towards four-electron pathway of the ORR in terms of an onset potential (0.89 V vs RHE) and the turnover frequency (TOFmax = 0.095 e site−1 s−1).We attribute this high catalytic performance to the presence of the pyridinic and pyrrolic sites in the structure. The ORR kinetics is probably further promoted by curvature in the graphitic layers.

Growth stages and hexagonal-rhombohedral structural arrangements in spheroidal graphite observed in ductile iron

Crystallographic structures of spheroidal graphite particles (graphite nodules) were examined using transmission electron microscopy (TEM). Structures of graphite nodules were investigated relative to different stages of nodule growth in ductile iron. Curved graphene layers were observed during the early growth of the graphite nodules. Thin layered stacking faults give rise to streaking in the basal reflections, which give rise to curvature of the nodule and growth steps on the surface. Columnar substructures consisting of parallel peripheral subgrains were found in the outside region of graphite nodules that were formed during the intermediate growth stage. Straight planar graphene layers were observed in the columnar substructures. Crystallographic orientation of graphite showed little change through the subgrain and the c-axes of multiple subgrains in a single columnar substructure were parallel. A method for characterizing the crystal structures of graphite based on the selected area diffraction pattern was introduced. Both hexagonal structure and rhombohedral structure were found in the spheroidal graphite particles. Possible crystallographic defects associated with hexagonal-rhombohedral structure transition were discussed.

Examination of Spheroidal Graphite Growth and Austenite Solidification in Ductile Iron

Microstructures of a ductile iron alloy at different solidification stages were captured in quenching experiments. Etched microstructures showed that spheroidal graphite particles and austenite dendrites nucleated independently to a significant extent. Growth of the austenite dendrite engulfed the spheroidal graphite particles after first contacting the nodule and then by forming an austenite shell around the spheroidal graphite particle. Statistical analysis of the graphite size distribution was used to determine the nodule diameter when the austenite shell was completed. In addition, multiple graphite nucleation events were discerned from the graphite particle distributions. Majority of graphite growth occurred when the graphite was in contact with the austenite. Circumferential growth of curved graphene layers appeared as faceted growth fronts sweeping around the entire surface of a spheroidal graphite particle which was at the early growth stage. Mismatches between competing graphene growth fronts created gaps, which divided the spheroidal graphite particle into radially oriented conical substructures. Graphene layers continued growing in each conical substructure to further extend the size of the spheroidal graphite particle.

Influence of waste cooking oil biodiesel on the nanostructure and volatility of particles emitted by a direct-injection diesel engine

ABSTRACTTo reduce air pollution and the reliance on fossil fuel, biodiesel has been widely investigated as an alternative fuel for diesel engines. The purpose of this study is to investigate the influence of waste cooking oil (WCO) biodiesel on the physical properties and the oxidation reactivity of the particles emitted by a diesel engine operating on WCO biodiesel as the main fuel. Experiments were conducted on a direct-injection diesel engine fueled with biodiesel, B75 (75% biodiesel and 25% diesel on volume basis, v/v), B50, B20, and diesel fuel, at five engine loads and at an engine speed of 1920 rev/min. Particulate samples were collected to analyze the particulate nanostructure, volatility, and oxidation characteristics. Biodiesel or low-load operation leads to smaller primary particles and more disordered nanostructures having shorter and more curved graphene layers. It can be found that particles from biodiesel, blended fuels, or low-load operation have higher volatile mass fractions and faster o...

Low temperature CVD growth of ultrathin carbon films

We demonstrate the low temperature, large area growth of ultrathin carbon films by chemical vapor deposition under atmospheric pressure on various substrates. In particularly, uniform and continuous carbon films with the thickness of 2-5 nm were successfully grown at a temperature as low as 500 oC on copper foils, as well as glass substrates coated with a 100 nm thick copper layer. The characterizations revealed that the low-temperature-grown carbon films consist on few short, curved graphene layers and thin amorphous carbon films. Particularly, the low-temperature grown samples exhibited over 90% transmittance at a wavelength range of 400-750 nm and comparable sheet resistance in contrast with the 1000oC-grown one. This low-temperature growth method may offer a facile way to directly prepare visible ultrathin carbon films on various substrate surfaces that are compatible with temperatures (500-600oC) used in several device processing technologies.

SiC-assisted growth of tubular graphenic cones with carbon nanotube tips

SiC-assisted growth of tubular graphenic cones (TGCs) with carbon nanotube tip is achieved with high yield in the microwave plasma chemical vapor deposition process. No pre-existing metal or semiconductor catalyst particles are required on the substrate prior to the deposition process. Instead, the in situ grown SiC crystallites serve as the catalyst for the growth of TGCs. Thanks to the easy formation of SiC on various substrates, the process is compatible with a wide range of substrates, i.e. Si, diamond, 2H–SiC, GaN, and SiO2, but not only limited to them. In addition to developing the approach, the mechanism for the formation of TGCs is also proposed. When Si is used as the substrate, the 3C–SiC crystallites grow epitaxially on the surface, which further initiate the epitaxial growth of graphene with its basal plane parallel to the {111} planes of 3C–SiC. The further expansion of graphene is constrained by the 3C–SiC crystallites, leading to the formation of curved graphene layers, i.e. onion-like or bowl-shaped graphene-based carbon. These curved graphene layers are believed to be the possible nucleation sites for the growth of TGCs. Inclusion of boron in the gas phase promotes the growth rates and the final yields of TGCs.

Synthesis of curved graphene layers on metallic dendrites

Due to strong in-plane covalent bonds and weak out-of-plane van der Waals forces, graphene layers can bend easily to form curved structures. In this research, curved graphene films were grown on metallic dendrites as the molten alloys of Ni–Cu and Ni–Al solidified inside a graphitic crucible. During the melting process, due to solidification shrinkage the liquid in between the growing dendrites were decanted and left behind exposed dendrites wetted by a thin molten blanket. The blanket eventually froze and rejected carbon which crystallized as curved graphene layers conforming to dendrites. The curved films were studied with detailed Raman spectroscopy and scanning electron microscopy. The examined layers possessed high degree of perfection and were as thin as a few layers. They also consisted of faceted out-of-plane wrinkles due to thermal expansion coefficient mismatch with the metallic substrate.

The influence of deposition temperature on the microstructure of isotropic pyrocarbon obtained by hot-wall chemical vapor deposition

Bulk isotropic pyrocarbon was successfully prepared by hot-wall chemical vapor deposition. The density of the products was measured by Archimedes method in ethanol. The structural parameters of pyrocarbon were derived from X-ray diffraction (XRD) and Raman spectroscopy. The morphology and texture of pyrocarbon was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the density of pyrocarbon increases from 1.53 to 1.73 g/cm3 with the increase of deposition temperature. The d002 spacing decreases gradually from 0.3487 to 0.3449 nm as temperature rises from 950 to 1100 °C. Lc increases greatly from 6.5 to 10.5 nm, whereas La slightly rises from 4.0 to 3.6 nm. Under TEM, the fringe lattice images are composed of curved graphene layers, and the length of graphene layers is greatly improved with the increasing temperature. The selected area electron diffraction patterns of the products are complete rings, indicating their isotropic feature.

Structural Units and Their Periodicity in Carbon Nanotubes

It is widely accepted that carbon nanotubes (CNTs) consist of coaxially curved graphene layers. However, some CNTs, synthesized at relatively low temperature, are self-organized into rod- or plate-shaped mesostructural units with periodic structure. These findings give another point of view on the formation mechanism of CNTs.

Probing the Electronic Effect of Carbon Nanotubes in Catalysis: NH3 Synthesis with Ru Nanoparticles

Carbon nanotubes (CNTs) have been shown to modify some properties of nanomaterials and to modify chemical reactions confined inside their channels, which are formed by curved graphene layers. Here we studied ammonia synthesis over Ru as a probe reaction to understand the effect of the electron structure of CNTs on the confined metal particles and their catalytic activity. The catalyst with Ru nanoparticles dispersed almost exclusively on the exterior nanotube surface exhibits a higher activity than the CNT-confined Ru, although both have a similar metal particle size. Characterization with TEM, N(2) physisorption, H(2) chemisorption, temperature-programmed reduction, CO adsorption microcalorimetry, and first-principles calculations suggests that the outside Ru exhibits a higher electron density than the inside Ru. As a result, the dissociative adsorption of N(2), which is an electrophilic process and the rate-determining step of ammonia synthesis, is more facile over the outside Ru than that over the inside one.

Formation process of graphene on SiC (0 0 0 1)

High-resolution transmission electron microscopy has revealed the formation process of graphene layers on SiC (0 0 0 1). Initially, nucleation occurs at SiC steps, covering them with a few layers of graphene. These curved graphene layers stand almost perpendicularly on the lower terrace. Graphene subsequently grows over the terrace region. The growth is often pinned by lattice defects of the SiC substrate.

The effect of graphitization temperature on the structure of helical-ribbon carbon nanofibers

Structural rearrangement of helical-ribbon carbon nanofibers (CNFs) was studied as a function of graphitization temperature. The as-produced nanofibers are composed of a helical ribbon of graphene spiralled about and angled to the fiber axis. The discrete layers of graphene ribbon overlap each other forming the helical-ribbon in contrast to the discontinuous cones of the more common stacked-cup CNF morphology. After heat treatment to 2400 °C and above, the CNFs were completely free of residual metal catalyst inclusions, principally nickel used in their synthesis, and other functionalities. The formation of loops at the graphene edges was also observed. Heat treatment through the temperature range 1500–2800 °C resulted in a relatively minor contraction in interlayer spacing d 002 from 0.3381 to 0.3363 nm. This was attributed to the highly graphitic character of the as-produced CNFs. However, there were significant increases in the crystallite thickness L c through this temperature range. In addition, heat treatment above 2400 °C induced a marked change of the nanofiber morphology from circular to faceted polygonal cross-section resulting from the re-ordering of the turbostratic, curved graphene layers to regions of planar graphene layers with 3-dimensional graphitic structure (AB stacking).