Structure Of Graphene Layers Research Articles

The Bipolar Mode of One‐Step Plasma Electrochemical Synthesis of Few Layer Graphene Structures Decorated with Transition Metal Oxides

AbstractWe present the method of conjugated bipolar plasma electrochemical synthesis of aqueous suspensions of composites of few‐layer graphene structures with Co3O4 and Mn3O4 oxides and creation of mixed electrocatalysts based on them. As shown by the rotating disk electrode method, the best catalytic activity towards oxygen reduction reaction exceeding that for each individual component is demonstrated by the composition containing equal amounts of these composites.

Modeling the structure and interlayer interactions of twisted bilayer graphene

In this study, we have analyzed twist angles between layers at which the bilayer graphene (BLG) layer structure is periodic. In addition, the dependence of interlayer binding energies on the BLG layer’s twist angles was calculated using the atom-atom potential method. It was established that at small twist angles (below 4°) the interlayer binding energy changes monotonically, free of local energy minima and stable structural states. At large twist angles, the local energy minima positions weakly correlate with angles at which periodic Moiré superlattices arise.

Investigation of optical and electrical properties of GA/GZ/PbI2 and GA/GZ/PbI2/B nanocomposites

To introduce a suitable functional composite for electronic and optical applications, different structures of Graphene, Lead iodine and Borophene layers were studied using density functional theory (DFT). Band structures, the density of state, the projected density of state, the joint density of state, extinction index, normal incidence reflectivity, refractive index, and Electron Energy Loss Spectroscopy were calculated for these structures. Based on these the results, improving the optical absorption in the visible region and changing the population of free charge carriers obtained make made Graphene/Graphene/Lead Iodine (GA/GZ/PbI2), and Graphene/Graphene/Lead Iodine/Borophene (GA/GZ/PbI2/B) nanocomposites a suitable promising new composites for manufacturing and designing new novel light-sensitive nanoscale devices.

Role of precursor microstructure in the development of graphene quantum dots from biomass

Renewable, green and cheap biomass could meet the urgent need of cost-effective graphene quantum dots (GQDs) if microstructure and quality can be precisely controlled. Herein, for the first time, we investigate the effect of precursor microstructure on the growth of GQDs from biomass. A novel process is developed which combines carbonisation, oxidation and nitration with controlled hydrothermal fragmentation to form GQDs. The results indicate that aromatic structure of carbon material as a precursor is the key to obtain high-quality GQDs. The as-prepared GQDs possess a 3–9 layer graphene structure with an average size of 11.6 ± 1.8 nm and exhibit a moderate quantum yield of 17.5%. These GQDs are used to develop a highly selective and sensitive sensor to detect ferric ions with a detection limit as low as 26 ± 0.4 nM. This study highlights the development of high quality GQDs from biomass for real-world sensing, photocatalytic and biomedical applications.

Direct microscopic evidence of shear induced graphitization of ultrananocrystalline diamond films

The origin of ultralow friction and high wear resistance in ultrananocrystalline diamond (UNCD) films is still under active debate because of the perplexed tribochemistry at the sliding interface. Herein, we report a comparative study on surface topography and nanoscale friction of tribofilms, in wear tracks of two sets of UNCD films having different structural characteristics. Despite both the films display ultralow coefficient of friction, the UNCD films grown under Ar atmosphere (UNCDAr) exhibit a high wear resistance while the wear rate is higher for the films grown in N2 (UNCDN). Frictional force microscopic (FFM) investigations clearly reveal the manifestation of shear induced graphitization on both the films. However, the wear track of UNCDAr films have a large network of a few layer graphene (FLG) structures over the amorphous carbon tribofilms while only isolated clusters of FLG structures are present in the wear track of UNCDN films. Here, we demonstrate the direct micro-/nanoscopic evidence for the formation of large network of ~ 0.8 - 6 nm thick FLG structures, as a consequence of shear induced graphitization and discuss their decisive role in ultralow friction and wear.

The Bi‐DOPO derivative functionalized graphene oxide: Preparation and its flame‐retardation on epoxy resin

AbstractA novel functionalized graphene oxide (F‐GO) decorated with a double‐phosphaphenanthrene structure was fabricated using a covalent modification method. The F‐GO with Bi‐DOPO groups then was introduced into the epoxy matrix as a highly effective flame retardant. The Flame retardant epoxy resins (FREP) containing F‐GO exhibited superior fire‐resistant performance with extremely low loading. When the content of F‐GO was 1 wt%, the limiting oxygen index value of the FREP was increased from 19.9% to 30.8%, and the total heat release and the peak heat release rate was decreased by 44.0% and 55.5%, respectively. Meanwhile, the tensile and impact tests results indicated that the mechanical properties of the epoxy resins were improved with the addition of F‐GO. Furthermore, the thermogravimetric analysis showed that F‐GO greatly increased the char‐yield of the cured FREP. Based on the char analyses of SEM images, F‐GO was conducive to form a more protective char barrier for FREP during combustion. F‐GO developed a synergistic flame‐retardant effect of the Bi‐DOPO derivative and graphene layer structures on epoxy resins.

Layered Ag-graphene films synthesized by Gamma ray irradiation for stable lithium metal anodes in carbonate-based electrolytes

Lithium metal batteries are considered as high energy density battery systems with very promising prospects and have been widely studied. However, The uncontrollable plating/stripping behavior, infinite volume change and dendrites formation of lithium metal anode restrict the application. The uncontrolled nucleation of lithium caused by the nonuniform multi-physical field distributions, can lead to the undesirable lithium deposition. Herein, a graphene composite uniformly loaded with Ag nano-particles (Ag NPs) is prepared through a facile Gamma ray irradiation method and assembled into self-supported film with layered structure (Ag-rGO film). When such film is used as a lithium metal anode host, the uncontrolled deposition is converted into a highly nucleation-induced process. On one hand, the Ag NPs distributed between the interlayers of graphene can preferentially induce lithium nucleation and enable uniform deposition morphology of lithium between interlayers. On the other hand, the stable layered graphene structure can accommodate volume change, stabilize the interface between anode and electrolyte and inhibit dendrites formation. Therefore, the layered Ag-rGO film as anode host can reach a high Coulombic efficiency over 93.3% for 200 cycle (786 h) at a current density of 1 mA cm−2 for 2 mAh cm−2 in carbonate-based electrolyte. This work proposes a facile Gamma ray irradiation method to prepare metal/3D-skeleton structure as lithium anode host and demonstrates the potential to regulate the lithium metal deposition behaviors via manipulating the distribution of lithiophilic metal (e.g. Ag) in 3D frameworks. This may offer a practicable thinking for the subsequent design of the lithium metal anode.

Geometric effects on the electronic structure of curved nanotubes and curved graphene: the case of the helix, catenary, helicoid, and catenoid

Since electrons in a ballistic regime perceive a carbon nanotube or a graphene layer structure as a continuous medium, we can use the study of the quantum dynamics of one electron constrained to a curve or surface to obtain a qualitative description of the conduction electrons' behavior. The confinement process of a quantum particle to a curve or surface leads us, in the so-called "confining potential formalism" (CPF), to a geometry-induced potential (GIP) in the effective Schr\"odinger equation. With these considerations, this work aims to study in detail the consequences of constraining a quantum particle to a helix, catenary, helicoid, or catenoid, exploring the relations between these curves and surfaces using differential geometry. Initially, we use the variational method to estimate the energy of the particle in its ground state, and thus, we obtain better approximations with the use of the confluent Heun function through numerical calculations. Thus, we conclude that a quantum particle constrained to an infinite helix has its angular momentum quantized due to the geometry of the curve, while in the cases of the catenary, helicoid, and catenoid the particle can be found either in a single bound state or in excited states which constitute a continuous energy band. Additionally, we propose measurements of physical observables capable of discriminating the topologies of the studied surfaces, in the context of topological metrology.

The use of graphene and its composites to suppress the shuttle effect in lithium-sulfur batteries

The lithium sulfur battery has a high theoretical capacity of 1675 mAh g−1, and is cheap and environmentally friendly, which make it a very promising secondary battery. However, its cycling stability cannot meet the requirements of industrialization due to the shuttle effect caused by the dissolution of polysulfides in the discharge process, the insulating nature of sulfur and the volume expansion of the sulfur cathode. Graphene has an excellent electrical conductivity, an extremely large specific surface area, good mechanical flexibility, and thermal and chemical stability, making it and its derivatives promising candidates to modify both the electrodes of an all-solid-state lithium-sulfur battery and the separator. The mechanisms, by which graphene and its derivatives inhibit the shuttle effect are summarized. The graphene network is very favorable for improving electron transfer rate, limiting volume expansion and facilitating lithium ion migration in the sulfur cathode of all-solid-state lithium-sulfur batteries. As modifiers of the separator, the hexagonal layer structure of graphene and its derivatives forms channels for lithium-ion transport and sulfur capture. Development strategies for using graphene and its derivatives in lithium-sulfur batteries are proposed.

Cobalt encapsulated in N‑doped graphene sheet for one-pot reductive amination to synthesize secondary amines

To develop an efficient base-metal reductive amination catalyst for synthesis of secondary amines is still a major challenge. In this study, an efficient N-doped graphene sheet-coated cobalt catalyst ([email protected]) was developed through a simple pyrolysis process, which could gave 99.5 % yield of N-benzylaniline by one-pot reductive amination of nitrobenzene with benzaldehyde during at least 5 cycles. Catalyst characterization and control experiments confirmed that the robust catalytic performance of the catalyst is probably due to the synergy effect of in situ generated Co-Nx encapsulated in N‑doped graphene layer and appropriate meso-pore structure. Additionally, The substrate adaptability of the catalyst was proved since a variety of corresponding secondary amines were smoothly obtained under relatively mild conditions, which makes the secondary amine synthesis strategy based on [email protected] shows excellent application prospect.

Laser generation on weak modes in graphene structure with asymmetric unit cell

Problem formulating. Lasing on strong («radiative») plasmon resonance mode in graphene structure with dual grating gate with asymmetric unit cell requires strong gain. It is possible to achieve lasing in a weak ("non-radiative") mode at a lower gain rate. Goal. Theoretical study of laser generation on weak plasmonic resonance mode in single layer graphene structure screened by dual grating gate with asymmetric unit cell. Result. Laser generation on weak plasmonic resonance mode in single layer graphene structure screened by dual grating gate with asymmetric unit cell is reached. Excitation of a weak plasmon resonance mode requires less gain than excitation of a radiative one. Practical meaning. Results can be used to create sources of terahertz waves.

Porous Graphene Composite Polymer Fibres.

Since the isolation of graphene, there have been boundless pursuits to exploit the many superior properties that this material possesses; nearing the two-decade mark, progress has been made, but more is yet to be done for it to be truly exploited at a commercial scale. Porous graphene (PG) has recently been explored as a promising membrane material for polymer composite fibres. However, controlling the incorporation of high surface area PG into polymer fibres remain largely unexplored. Additionally, most polymer-graphene composites suffer from low production rates and yields. In this paper, graphene-loaded microfibres, which can be produced at a very high rate and yield have been formed with a carrier polymer, polycaprolactone. For the first time, PG has been incorporated into polymer matrices produced by a high-output manufacturing process and analysed via multiple techniques; scanning electron microscopy (SEM), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Raman spectra showed that single layer graphene structures were achieved, evidence for which was also backed up by the other techniques. Fibres with an average diameter ranging from 3–8 μm were produced with 3–5 wt% PG. Here, we show how PG can be easily processed into polymeric fibres, allowing for widespread use in electrical and ultrafiltration systems

Barrier function of graphene for suppressing the smoke toxicity of polymer/black phosphorous nanocomposites with mechanism change

Recently, black phosphorous (BP) nanosheets as an emerging nanomaterial have presented significant fire safety improvement in polymer nanocomposites. However, as elemental phosphorus, fire safety improvement effect of BP nanosheets on polymer composites builds on the conversion of gaseous pyrolysis products into smoke particles, which inevitably promotes the formation and release of smoke particles. From the perspective of overall fire safety improvement, it is vital to simultaneously suppress the heat release and smoke production of polymer/BP composites. Herein, melamine-mediated graphene/black phosphorous nanohybrids (GNS/MA/BP) were fabricated through electrostatic-driving self-assembly process and introduced into polyether thermoplastic polyurethane (TPU). During combustion, the barrier function provided by thermally stable layered structure of graphene (GNS) enables more pyrolysis products of BP nanosheets to be kept within condensed phase and react with polymer matrix. Compared to pure TPU, the incorporated hierarchical nanostructure (GNS/MA/BP-2) decreases PHRR, THR, and total CO2 release of TPU composite by 54.7%, 23.5%, and 32.5%, respectively. Beside, in contrast to TPU-BP composite, the release rate of toxic smoke and CO gas of TPU-GNS/MA/BP-2 composite are reduced by 46.7% and 49.4%. With barrier function of graphene, the heat and smoke release behavior of polymer/BP nanocomposites is effectively suppressed.

Synergistic effect of polyvinylpyrrolidone noncovalently modified graphene and epoxy resin in anticorrosion application

In this article, polyvinylpyrrolidone (PVP) was used for the noncovalent modification on the surface of graphene. Compared with covalent modification, this method maintained the original structure of graphene layers, thereby maximizing the original properties of graphene. The π–π noncovalent bond was formed between PVP and graphene by X-ray photoelectron spectroscopy analysis, indicating that PVP successfully modified graphene. The thickness of graphene layer was measured by atomic force microscopy, which showed that the distance between graphene layers was increased by 5–6 nm, and the stability of the modified graphene in N, N-dimethylformamide was remarkably improved. The obtained composite coating by combination of the modified graphene and the epoxy resin was subjected to electrochemical impedance test to obtain the best anticorrosive effect of the coating with the graphene content of 0.3 wt%. The results showed that the addition of graphene to the epoxy resin could effectively improve the anticorrosive effect. Meanwhile, the good electrical conductivity allowed the electrons which lost from the substrate to led to air or saline rapidly, thereby reducing the combination of iron ions with oxygen and the generation of corrosion products (iron oxides).

Room Temperature Oxidation of the Surface of Highly Oriented Pyrolytic Graphite (HOPG) with Nitrogen Dioxide in the Presence of Supported Palladium Particles

The interaction of palladium nanoparticles (average size, ∼3 nm) deposited on the surface of highly oriented pyrolytic graphite (HOPG) with nitrogen dioxide at room temperature and a pressure of 10–6 or 10–5 mbar was studied by X-ray photoelectron spectroscopy (XPS). It was shown that the structure of several surface graphene layers was destroyed under these conditions due to the oxidation of carbon at the interface between Pd and HOPG. The reaction proceeded with the participation of oxygen atoms, which were formed as a result of the dissociation of NO2 molecules on the palladium surface. Palladium particles retained their metallic nature, but they penetrated deep into graphite in this case.

In Situ Dynamic Manipulation of Graphene Strain Sensor with Drastically Sensing Performance Enhancement

AbstractIt is extremely challenging to directly observe how the relative position of the nanosheet changes the charge transport in the channel. Previous work on graphene stacking strain sensors relies on nanosheet slip to detect small mechanical signals. However, the direct experimental verification evidence is still inadequate. In this work, the sliding conductive transmission of graphene nanosheets slip is directly measured through an improved in situ transmission electron microscopy observation technique. By accurately manipulating the atomic scale of graphene nanosheets to achieve nanoscale sliding, the resistance change between nanosheets in the in situ observation system can be directly measured. Besides, a mechanical sensor based on graphene layer structure is designed, which demonstrates a high gauge factor (4303 at maximum strain of 93.3%), negligible hysteresis (5–10%), and excellent stability over 3000 stretch–release cycles. Besides, the precise control of characteristics offers a practical approach of retaining high stability from device to device. This strain sensor is applied in various cases, especially the health monitor of the human cervical vertebra.

Improved photovoltaic performance of graphene-based solar cells on textured silicon substrate

Abstract Graphene has attracted much interest as an active layer in heterojunction solar cells due to its outstanding properties such as flexibility, transparency, mechanical strength and elevated carrier mobility. In this research, a new technique was presented in order to enhance the efficiency of graphene–based heterojunction solar cells by employing a textured silicon (Si) substrate. Here, two sets of devices were fabricated based on flat and pyramidal structure of Si and the photovoltaic properties of graphene/Si heterojunction solar cells were compared. Selective chemical dissolution of Si wafers was carried out in order to produce pyramidal skeleton. Reduced graphene oxide (rGO) was then transferred on pyramidal Si through electrophoretic deposition (EPD) technique. The evidence of graphene layers on Si substrates was studied using Raman spectroscopy, X–ray diffractometry (XRD) and atomic force microscopy (AFM) analysis. The morphology of samples indicated an enhancement in rGO/Si interface area when the pyramidal structure is applied. Moreover, the enhanced surface area of this sample which is due to elevated roughness of pyramidal structure and wrinkles of graphene layers promotes its antireflective behavior which was proven using reflectance spectroscopy. The average reflectance of the graphene layer on the textured Si was ∼14% in the wavelength range of 400–800 nm which is lower than that of rGO on flat Si. The improved optical properties of graphene on pyramidal silicon can broaden its potential applications in optoelectrical devices such as high-efficiency solar cells. In order to study the photovoltaic properties of rGO/Si samples, a passive layer was formed on Si substrate and a square frame of Ag was coated on it which was acted as a top contact. The current–voltage characteristics showed that the efficiency of rGO/Si heterojunction solar cells was improved when textured silicon was applied.

Yttrium trihydride enhanced lithium storage in carbon materials

Metal hydrides (MHx) are gaining increasingly interest as anode materials for lithium ion batteries. The most widely accepted lithium storage mechanism for MHx is the conversion mechanism to form LiH. Our previous study suggested a different lithium storage mechanism of rare earth trihydride (REH3, RE = Y, La and Gd)-graphite composite: the reactive H released from the REH3-REH2 conversion may enhance the lithium storage capacity of graphite by forming a Li–C–H composite. In this work, this interesting phenomenon is further studied by comparing the lithium storage performance of YH3/carbon composite using different carbon materials: graphite, acetylene black, activated carbon and commercial graphene. The enhanced lithium storage capacity by YH3 addition is only observed for graphite, indicating the essential role of the intact layered graphene structure in this new lithium storage mechanism.

Study of graphene layer growth on dielectric substrate in microwave plasma torch at atmospheric pressure

The initial stage of graphene layer deposition on silicon oxide substrate by ethanol decomposition in dual-channel microwave plasma torch at atmospheric pressure was studied in dependence on precursor flow rate and delivered microwave power. Depending on ethanol flow rate and substrate temperature, horizontally or vertically aligned graphene nanosheets with various density could be prepared directly on dielectric substrate. In the regime with high microwave power, above 400 W, mixture of amorphous carbon particles and graphene sheets was deposited on the substrate. Prepared layers were analyzed by scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The microwave plasma diagnostics was carried out using optical emission spectroscopy (OES). The sample analysis showed increasing density of horizontally aligned carbon nanosheets with increasing ethanol flow rate and their delamination and transition into vertically aligned graphene sheets with increasing substrate temperature. The Raman spectroscopy analysis of layers showed presence of D (1345 cm−1), G (1585 cm−1) and 2D (2685 cm−1) peaks with 2D/G ratio of 1.59 and full width at half maximum (FWHM) of 2D peak was 42 cm−1, corresponding to few layer graphene structure. In case of amorphous nanoparticles deposition, the D* peak at 1210 cm−1 and D** at 1500 sccm−1 was observed in Raman spectra with D/G ratio of 1.19 and C1s XPS spectra of carbon contained 20.4 at.% of sp3 carbon phase in comparison to 8.3 at.% in case of graphene nanosheets layer. High D/G ratio, up to 3.5, and low intensity 2D band was characteristic for vertically aligned graphene nanosheets layers. The possibility to influence density and size of graphene nanosheets on substrate represents promising alternative for future deposition of graphene on arbitrary substrate.

Effect of graphene on tribological properties of Ni based composite coatings prepared by oxidation reduction method

In order to study the effect of graphene on the tribological properties of Ni-based composite coatings, a hydrosol graphene was synthesized by oxidation reduction and then added into an electroplating bath to form Ni-graphene composite coatings. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and Thermo-gravimetric analysis (TGA) were used to study the crystal structure, spectral characteristics, reduction degree, lamellar thickness and thermal stability of the prepared products in the reaction. During the oxidation of graphite, a large number of functional groups are bonded in the structural layer, and parts of these stable functional groups remain on the reduced surface. The diffraction peaks of graphene become weaker to a line, and the average particle size is 30nm. Raman show that the D and G peaks of graphene shift to the right and the peak width extends, and the increasing of ID/IG value indicates that the higher disorder degree. Graphene has a single layer structure, and its thermal stability is lower than that of graphite. The thickness of Ni-graphene composite coatings with graphene contents (0.1%wt, 0.2%wt and 0.4%wt) is about 40-45μm. The friction factor (0.61) and wear rate(1.9 × 10−4 mm3/N·m) of the composite coating with 0.4%wt graphene content are reduced by 31% and 65% than those of the matrix under a load of 300 g. The very thin layer structure of graphene can form deposit layer between friction contact surfaces and reduce oxidation wear and furrow at high temperature, which can effectively improve the wear resistance and friction reduction properties of the matrix.