Two-dimensional Carbon Structures Research Articles

TPO12-graphene: A new two-dimensional metallic carbon with 4–5 ring for Lithium ion battery

For many decades, the stability of two-dimensional carbon structures involving connected 4–5 ring remained speculative. Recently, some of the planar carbon structures with connected 4–5 ring have showcased possible stability. In this work we present a new 4–5 ring carbon structure, belonging to Pmmm space group, having additional octagonal and dodecagonal rings to fill the space. The stability of this structure has been ensured in terms of its dynamical, thermal and mechanical stability. The electronic structure indicates that it is a non-magnetic metallic material. Moreover, the Fermi surface points to distinct electron and hole pockets in the first Brillouin zone. The quantum capacitance of the material suggests its use as anode in asymmetric supercapacitors. Besides its low optical absorption in visible range, its potential as an anode material for lithium-ion batteries is worth exploring. The storage capacity results of 558 mAh/g for the new material may be encouraging for its application as a lithium-ion battery anode material.

Tribological and mechanical characterization of carbon-nanostructures based PEEK nanocomposites under extreme conditions for advanced bearings: A molecular dynamics study

Polyetheretherketone (PEEK) carbon nanomaterial-based nanocomposites, incorporating zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) carbon structures have gained prominence as advanced materials with the potential to enhance tribological properties paving the way for a new era in the development of advanced bearings for high-performance machinery applications. This study delves into the forefront of friction and wear analysis, employing all atom molecular dynamics (aa-MD) simulations to engineer PEEK polymer nanocomposites with precision under elevated temperature and higher velocity. The research presents optimization of mechanical strength and tribological properties crucial to meet the demanding requirements of industrial machinery. The addition of C60 fullerenes reduced COF by 16.59% and wear rates by 6.52% due to its spherical shape and lubrication effects. Introduction of carbon-nanotubes further decreased COF by 54.54% and wear rates by 32.5%, offering resistance to abrasive and adhesive wear. S-graphene showed notable performance, reducing COF by 69.40% and wear rates by 75.03%, making it promising for sliding and wear applications. Carbon-nanocones reduced COF by 60.6% and wear rates by 72.06%, slightly higher than S-Gr due to abrasive effects and increased surface contact. These findings underscore the potential of reinforcement dimensionality in enhancing the tribological properties of PEEK based nanocomposites.

Superpentagraphene C34: A two-dimensional network structure of C20 fullerene

The well-known smallest icosahedral fullerene C20 is comprising five-membered carbon-rings in all sp2 bonding states. Here we identify by ab initio calculations a new quasi two-dimensional (2D) carbon allotrope in sp2-sp3 bonding states, comprising the unusual five-membered C20 fullerene as the basic structural units, with a 34-atom hexagonal cell in P6/mmm(D6h1) symmetry, and term it as spg-C34 superpentagraphene. Total energy calculations show that it is energetically more stable than the previously reported 2D carbon forms such as pentagraphene and ph-graphene. Its structural stability has been verified by phonon mode analysis and ab initio molecular dynamics simulations. Electronic band calculations reveal that this 2D spg-C34 carbon is a semiconductor with a sizable indirect band gap of 3.31 eV. Moreover, we show that such 2D spg-C34 carbon structure has a larger layer modulus (281 N/m), a larger in-plane stiffness (524 N/m), and a smaller Poisson's ratio (0.069) than the values for graphene. These results establish a new exotic quasi-2D carbon phase and offer insights into its outstanding electronic and mechanical properties of two-dimensional carbon structure.

Configuration stability and physical properties of new diamondene structure

The structure of monolayer diamond or diamondene based on bi-layer graphene through pressure conversion, combining characteristics of both graphene and diamond, has been confirmed theoretically and experimentally. In this work, by extending diamondene structure to the minimum repeating unit of bulk diamond along different crystallographic orientations, the configuration stability and physical properties of 21 kinds of diamondene were systematically investigated based on first-principles methods, and effects of hydrogen saturation were also presented. It was found that diamondene formed by compressing layered graphene would graphitize, and surface termination could prevent the graphitization. The electronic properties of diamondene can be adjusted by the concentration of H atoms with the bandgap varying from 0 eV to 3.45 eV, and some configurations exhibit ferromagnetism or anti-ferromagnetism. Moreover, diamondene has a high Young's modulus comparable to bulk diamond. Particularly, the configuration containing dumbbell units is twice as hard as diamond. The formation energies of all stable diamondene configurations were illustrated, and it was found that it is easier to synthesize diamondene by compressing few-layer graphene. Furthermore, the instability law of two-dimensional carbon structure was systematically discussed, which is important for designing new carbon allotropes.

Two-Dimensional Tri-Hex Carbon: A Promising, Efficient, and Acid-Resistant Photocatalyst for Water Splitting

Photocatalytic hydrogen production via water splitting has been a promising method to produce clean energy and effectively reduce environmental pollution. Herein, a specific bandgap-oriented structure search was performed. A two-dimensional (2D) carbon structure comprising triatomic and hexatomic carbon rings, named 2D tri-hex carbon, was proposed to possess suitable bandgap and band edge positions for photocatalytic water splitting. Our results show that 2D tri-hex carbon has a high absorption coefficient under sunlight and high photocatalytic water-splitting efficiency under acidic conditions. The study provides insight into exploring a promising candidate in 2D carbon materials for acid-corrosion-resistant photocatalytic water-splitting applications.

Strain engineering of electronic structure and mechanical switch device for edge modified Net-Y nanoribbons

Net-Y is a new two-dimensional carbon structure, which has attracted research interest recently. Here, we study the relevant AB-type ribbons with edge modification, focusing on their strain controlling effects on their electronic structure and device characteristics. Intrinsic ribbons are metallic, but hydrogen or oxygen termination can transform them into semiconductors. Applying strain can effectively control the band gap size, resulting in a transition from an indirect band gap to a smaller direct band gap under appropriate strain, favorably to light absorbing. Strain can also change the work function of ribbons, especially for compressive strains, the work function is lowered significantly, which is beneficial to the improving of the field emission behaviors of ribbons. The analysis demonstrates that the change in band gap size is closely related to the variation of bonding and non-bonding composition between atoms with strain, while the change of work function is due to the variation of the attraction force and repulsion force between atoms upon strain. More interestingly, the strain can significantly regulates the <i>I</i>-<i>V</i> characteristic of device based on related ribbons. Therefore, a strain-gated mechanical switch with a very high current switching ratio <i>I</i><sub>on</sub>/<i>I</i><sub>off</sub> can be obtained by making it reversibly work between the “on” and “off” states with stretching and compressing ribbons, which is of great significance in developing the logic circuits for flexible wearable electronic devices.

Topology-dependent conjugation effects in graphdiyne molecular fragments

Graphdiynes (GDYs) as two-dimensional carbon structures based on sp2 hybridized aromatic rings connected by sp-hybridized acetylenic linear links are gathering an increasing popularity, both for their peculiar properties and for the promising applications. In these materials, structural features affect the degree of π-electron conjugation resulting in different electronic and vibrational properties. Indeed, how topology, connectivity between sp and sp2 domains and system size are related with the final properties is fundamental to understand structure-property relationships and to tailor the properties by proper structure design.By using a computational approach based on density functional theory calculations, we here investigate structure-property relations in a class of 1D and 2D GDY molecular fragments as building block models of extended structures. By analysing how the structure can modulate the π-electron conjugation in these systems, HOMO-LUMO gap is found to depend on the peculiar topology and connections between linear sp domains and aromatic units. A topological indicator is computed, showing a trend with the gap and with the frequency of the main vibrational mode occurring in Raman spectra. Our findings can contribute to guide the molecular design of new GDY-based sp-sp2 carbon materials, aiming at tuning their properties by precise control of the structure.

Mechanical Properties of Two-Dimensional sp2-Carbon Nanomaterials

Graphene is a two-dimensional crystal in which sp2-hybridized carbon atoms have valence bonds with three neighbors. Theoretically, other two-dimensional carbon structures were predicted, in which each carbon atom has valence bonds with three neighbors. In this paper, the molecular dynamics method is used to analyze the mechanical properties and structural transformations of such materials under uniaxial and biaxial stretching. The dependences of the tensile membrane forces on the applied tensile strain are constructed, the limiting values of the membrane forces and strains are determined. The three structures studied differ in their density, and it could be expected that the strength of the structures should decrease with decreasing density. However, it turned out that such a correlation did not manifest itself in all cases: a less dense structure may turn out to be stronger due to the fact that all interatomic bonds in it turn out to be loaded more uniformly. The results can be useful in analyzing the potentialities of application of sp2-carbon membranes in various technologies.

Low-Threshold Field Electron Emission from Two-Dimensional Carbon Structures

Particles of multilayer graphene obtained by the method of self-propagating high-temperature synthesis are proposed as an active cathode component for field electron emission. It is shown that this material makes it possible to implement a new technology for creating efficient field emitters with a developed surface. It has been established that the effect of low-threshold field electron emission is observed in this material. In pulsed electric fields, the possibility of obtaining high-current electron beams with currents up to hundreds of amperes has been confirmed.

Ni anchored C2N monolayers as low-cost and efficient catalysts for hydrogen production from formic acid

Noble-metal-free catalysts are highly desirable for hydrogen generation from formic acid dehydrogenation. Herein, using first-principles density functional theory calculations, we design a series of nickel-anchored nitrogenated holey two-dimensional carbon structures (Nix@C2N, x = 1–3) as formic acid dehydrogenation catalysts. For all Nix@C2N surfaces, the formic acid dehydrogenation preferably proceeds via the formate pathway. The effective barrier continuously decreases for formic acid dehydrogenation while increases for hydrogen formation from Ni1@C2N to Ni3@C2N. The side reaction producing carbon monoxide and water via the carboxyl or formyl pathway cannot occur on Ni1@C2N or Ni2@C2N and is not preferred on Ni3@C2N, and thus, the Nix@C2N catalysts possess excellent selectivity of hydrogen. Notably, the unsaturated nitrogen atom of substrate also participates in the reaction and exhibits synergetic effect with the nickel component in Ni1@C2N and Ni2@C2N. The Gibbs free energetic span analysis predicts that the order of reactivity is Ni2@C2N (0.79 eV) > Ni1@C2N (0.87 eV) > Ni3@C2N (1.23 eV), and the turnover frequency of Nix@C2N is evaluated. The results are compared with the experimental and theoretical reports of some palladium-based catalysts. The present work suggests that the Nix@C2N may be promising noble-metal-free catalysts for formic acid dehydrogenation with high performance and low cost.

Elastomer-like deformation in high-Poisson's-ratio graphene allotropes may allow tensile strengths beyond theoretical cohesive strength limits

Phenomenological theories of deformation in brittle solids generally envisage fracture under tension as the breaking of atomic bonds perpendicular to the fracture plane, thereby ignoring the role of bond rotation. Using density functional theory calculations, we found that during the early stage of the uniaxial tension of graphene allotropes with high Poisson's ratios, bond rotation effectively lessens bond stretch and increases the fracture strain. Specifically, in the deformation of an allotrope, Gr10, with a Poisson's ratio of 0.8, bond rotation results in an S-shaped stress-strain curve, akin to those of elastomers. Moreover, the tensile strength (σth) and the Young's modulus (E) of Gr10 exceed the theoretical cohesive strength limit σth≈E/10, reaching σth≈E/1.7. However, a universal relationship between bond lengths and charge density distribution along bond paths was found to be suitable for all carbon-carbon covalent bonds. Consequently, all carbon-carbon bonds obey a common shape of bond-force vs. bond-strain curve, with bond strength, S, and bond stiffness, K, following S≈K/9; hence, σth≈E/10 remains valid for the low-Poisson's-ratio graphene allotropes whose deformation is dominated by bond stretch. Overall, we suggest the trade-off between bond stretching and bond rotation can be utilized to enhance the fracture strain of two-dimensional carbon structures.

Surface-enhanced Raman spectroscopy of hexabenzobenzene, C24H12, an analogue of a graphene nanostructure

ABSTRACTWhile large scale fabrication of graphene nanoribbons remains a challenge, there exist materials which can be fabricated in quantities such as hexabenzobenzene,HBZB, (C24H12) and which have a two-dimensional (2D) carbon structure similar to graphene nanostructures. Using a 632 nm laser, no Raman spectra could be obtained from the solid material because of a strong luminescence produced by the laser. However, surface-enhanced Raman spectroscopy enabled the measurement of some of the Raman active modes. The G and D modes, which are characteristic fingerprints of a 2D graphene structure, were observed at 1331 and 1600 cm−1, respectively. Density functional theory at the B3LYP/6-31G* level was used to calculate the minimum energy structure and the Raman active vibrational frequencies of HBZB. The calculated minimum energy structure was 2D having D6h symmetry in agreement with the experimental structure in the liquid phase. The calculated frequencies were in good agreement with the measured values.

Characterization and Properties of Graphene Nanoplatelets/XNBR Nanocomposites

Graphene nanoplatelets (GNPs) are two-dimensional carbon structure materials with single or multilayers graphite plane which possesses attractive characteristics including high electrical conductivity, high modulus, high strength, high thermal conductivity and high specific surface area. In this study, the GNPs were modified by maleic anhydride (MA) and vinyltrimethoxysilane (VTMOS), and then reinforced carboxy NBR (XNBR) to prepare GNPs/nitrile rubber (XNBR) nanocomposites. The modified GNPs were characterized by Fourier Transform Infrared Spectrometry (FT-IR), Raman spectroscopy. The properties of GNPs/XNBR nanocomposite such as thermogravimetric analysis (TGA), thermal conductivity and mechanical properties were investigated. The experimental results show that the successful modification of GNPs can improve the properties of the nanocomposites.

An efficient and pH-universal ruthenium-based catalyst for the hydrogen evolution reaction.

The hydrogen evolution reaction (HER) is a crucial step in electrochemical water splitting and demands an efficient, durable and cheap catalyst if it is to succeed in real applications. For an energy-efficient HER, a catalyst must be able to trigger proton reduction with minimal overpotential and have fast kinetics. The most efficient catalysts in acidic media are platinum-based, as the strength of the Pt-H bond is associated with the fastest reaction rate for the HER. The use of platinum, however, raises issues linked to cost and stability in non-acidic media. Recently, non-precious-metal-based catalysts have been reported, but these are susceptible to acid corrosion and are typically much inferior to Pt-based catalysts, exhibiting higher overpotentials and lower stability. As a cheaper alternative to platinum, ruthenium possesses a similar bond strength with hydrogen (∼65 kcal mol-1), but has never been studied as a viable alternative for a HER catalyst. Here, we report a Ru-based catalyst for the HER that can operate both in acidic and alkaline media. Our catalyst is made of Ru nanoparticles dispersed within a nitrogenated holey two-dimensional carbon structure (Ru@C2N). The Ru@C2N electrocatalyst exhibits high turnover frequencies at 25 mV (0.67 H2 s-1 in 0.5 M H2SO4 solution; 0.75 H2 s-1 in 1.0 M KOH solution) and small overpotentials at 10 mA cm-2 (13.5 mV in 0.5 M H2SO4 solution; 17.0 mV in 1.0 M KOH solution) as well as superior stability in both acidic and alkaline media. These performances are comparable to, or even better than, the Pt/C catalyst for the HER.

Multi-disclination configurations in pentagonal microcrystals and two-dimensional carbon structures

A mechanism of decrease in the elastic (latent) energy of a solid containing disclination defects by introducing multi-disclination configurations of opposite sign has been considered. The relation of the proposed model with relaxation modifications of microcrystals with pentagonal symmetry, as well as with the structure of two-dimensional carbon films, has been discussed. An approach to the prediction of new carbon structures inherently containing multi-disclination configurations with screening has been demonstrated.

Environmental aging effect on interlaminar properties of graphene nanoplatelets reinforced epoxy/carbon fiber composite laminates

Graphene nanoplatelets are two-dimensional carbon structure materials with single or multilayers graphite plane which possesses attractive characteristics. In this study, the environmental aging effect on interlaminar properties of graphene nanoplatelet containing different proportions (0.25, 0.50, 0.75 wt%) reinforced epoxy/carbon fiber (carbon fiber reinforced plastic) composite laminates including interlaminar shear strength and fracture toughness were investigated. The interlaminar properties of graphene nanoplatelets/carbon fiber reinforced plastic composite laminates were improved over that of neat carbon fiber reinforced plastic composite laminates. Experimental results showed that the composite laminates containing graphene nanoplatelets possesses the appreciable improvement. The mechanisms responsible for the interlaminar enhancement were identified by studying the fracture surfaces using field emission scanning electron microscopy.

Comparison as Effective Photocatalyst or Adsorbent of Carbon Materials of One, Two, and Three Dimensions for the Removal of Reactive Red 2 in Water

Abstract Photocatalytic degradation and removal by adsorption of the Reactive Red 2 (RR2) dye in the presence of different pristine and functionalized one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) carbon structures were investigated and compared, considering also the loading of catalyst/adsorbent. Previous to photocatalytic and adsorption evaluation, all carbon materials were characterized to verify functionalization, structural changes, and surface properties useful in the removal of this pollutant in water. Materials were analyzed by Raman and infrared spectroscopies, transmission electron microscopy, and besides, the pH of the point of zero charge (pHpzc) was determined by potentiometric titrations, and band gap by ultraviolet–visible spectroscopy through Tauc plot. Removal of RR2 increases with the load of carbon materials. One-dimensional materials showed better performance in the decolorization of RR2 dye, removing the whole color of the solution. RR2 removal values obtained...

Phagraphene: A Low-Energy Graphene Allotrope Composed of 5-6-7 Carbon Rings with Distorted Dirac Cones.

Using systematic evolutionary structure searching we propose a new carbon allotrope, phagraphene [fæ'græfi:n], standing for penta-hexa-hepta-graphene, because the structure is composed of 5-6-7 carbon rings. This two-dimensional (2D) carbon structure is lower in energy than most of the predicted 2D carbon allotropes due to its sp(2)-binding features and density of atomic packing comparable to graphene. More interestingly, the electronic structure of phagraphene has distorted Dirac cones. The direction-dependent cones are further proved to be robust against external strain with tunable Fermi velocities.

Achieving extremely concentrated aqueous dispersions of graphene flakes and catalytically efficient graphene-metal nanoparticle hybrids with flavin mononucleotide as a high-performance stabilizer.

The stable dispersion of graphene flakes in an aqueous medium is highly desirable for the development of materials based on this two-dimensional carbon structure, but current production protocols that make use of a number of surfactants typically suffer from limitations regarding graphene concentration or the amount of surfactant required to colloidally stabilize the sheets. Here, we demonstrate that an innocuous and readily available derivative of vitamin B2, namely the sodium salt of flavin mononucleotide (FMNS), is a highly efficient dispersant in the preparation of aqueous dispersions of defect-free, few-layer graphene flakes. Most notably, graphene concentrations in water as high as ∼50 mg mL(-1) using low amounts of FMNS (FMNS/graphene mass ratios of about 0.04) could be attained, which facilitated the formation of free-standing graphene films displaying high electrical conductivity (∼52000 S m(-1)) without the need of carrying out thermal annealing or other types of post-treatment. The excellent performance of FMNS as a graphene dispersant could be attributed to the combined effect of strong adsorption on the sheets through the isoalloxazine moiety of the molecule and efficient colloidal stabilization provided by its negatively charged phosphate group. The FMNS-stabilized graphene sheets could be decorated with nanoparticles of several noble metals (Ag, Pd, and Pt), and the resulting hybrids exhibited a high catalytic activity in the reduction of nitroarenes and electroreduction of oxygen. Overall, the present results should expedite the processing and implementation of graphene in, e.g., conductive inks, composites, and hybrid materials with practical utility in a wide range of applications.

Synergistic Effect of Dual-Doping of Graphene Nanosheets on the Electrochemical Performance of Lithium-Air Cells

Rechargeable lithium-air battery is considered as one of the promising next-generation energy storage systems for application in electric vehicles due to its high theoretical energy density. In general, the performance of lithium-air cells in non-aqueous electrolyte is significantly affected by the large polarization in air cathode due to the deposition of discharge products as well as the ineffective electrocatalytic activity of electrode materials during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). To address these issues, noble metals are being used as catalysts, which may increase the cost of lithium-air cells. Recently, graphene nanosheets with a two-dimensional sp2-hybridized carbon structure have been actively explored as metal-free air cathode due to their superior electronic conductivity, mechanical strength, chemical stability, high surface-to-volume ratio and structural flexibility. The edge sites and other defect sites present in the basal plane of graphene prepared by chemical method have been demonstrated as catalytic sites for ORR.1 Furthermore, doping of graphene with heteroatoms such as nitrogen, boron, sulfur and phosphorus breaks the charge neutrality of carbon atoms and acts as catalytic centers.2 In this study, we demonstrate the synergistic effect of dual-doping of graphene nanosheets on the performance of lithium-air cells. Chemical and morphological characterization of the dual-doped graphene nanosheets along with their enhanced electrochemical performance over the single-heteroatom doped graphene as metal-free air-cathode will be presented.