Two-dimensional Carbon Allotrope Research Articles

Layer Control of Tubular Graphene for Corrosion Inhibition of Nickel Wires.

Corrosion protection of complex surface is an active area of research due to its importance to commercial applications such as electrochemical fabrication. However, conventional coatings exhibit limited conductivity, thermal stability, and durability and are thus not suitable. Recent work has shown the potential of graphene, a two-dimensional carbon allotrope, for corrosion protection. The studies, however, limited themselves to simple planar geometries that provide limited insight in the applicability to relevant morphologies such as mesh electrodes and roughened surfaces. We here study the corrosion protection ability of tubular graphene (TG) on Ni-wires as a model system for such complex geometries. TG-covered Ni wires of approximately 50 μm diameters were produced by the annealing of cellulose acetate (CA) on Ni. The high quality of the TG coating was confirmed by Raman spectroscopy, scanning electron microscopy, and electrical measurements. We show that the graphene layer number could be controlled by adjusting the CA membrane quantity. We found a direct relation between the degree of corrosion inhibition with the variation of graphene layer number. The increase of graphene layers on a Ni surface could enhance its corrosion inhibition in acidic, basic, and marine environments, which shows the potential of our approach for future applications.

QPHT-graphene: A new two-dimensional metallic carbon allotrope

A new two-dimensional metallic carbon allotrope (QPHT-graphene) composed by quadrangular, pentagonal, hexagonal rings, and large tetradecagonal pores, has been predicted based on the first-principles calculations. The total energy, phonon spectra, and elastic constants calculations as well as MD simulations prove that QPHT-graphene is a metastable carbon phase and can exist at room temperature. The calculations on the mechanical properties show that QPHT-graphene possesses an anisotropic mechanical behavior, and is much softer than graphene. Both GGA-PBE and HSE06 calculations confirm that QPHT-graphene is a metallic carbon allotrope with a much higher electronic density of states of ∼0.15 eV/states per atom at the Fermi level due to the emergence of flat bands. The current prediction further broadens the list of planar metastable carbon allotropes.

Chemical Functionalization of Pentagermanene Leads to Stabilization and Tunable Electronic Properties by External Tensile Strain.

Inspired by the unique geometry and novel properties of a newly proposed two-dimensional (2D) carbon allotrope called pentagraphene, we have performed first-principles calculations to study the structural stability and electronic properties of pentagermanene (pGe) modulated by chemical functionalization and biaxial tensile strain. It is observed that the 2D pGe is energetically unfavorable. However, the 2D pentagonal nanosheets can be stabilized by both hydrogenation and fluorination. Phonon dispersion spectrum and ab initio molecular dynamics simulations demonstrated that the dynamic and thermal stabilities of the two functionalized pGe nanostructures can be maintained even under a high temperature of 500 K. Our calculations revealed that both hydrogenated and fluorinated-pentagonal germanenes are semiconductors with indirect band gaps of 1.92 and 1.39 eV (2.60 and 2.09 eV by the hybrid functional), respectively. The electronic structures of the functionalized pGes can be effectively modulated by biaxial tensile strain, and an indirect to direct gap transition can be achieved for the hydrogenated pGe sheet by 6% biaxial strain. Moreover, the band gap of the hydrogenated pGe could be further tailored from 0.71 to 3.46 eV (1.16–4.35 eV by the hybrid functional) by heteroatom doping (C/Si/Sn/Pb), suggesting the semiconductor–insulator transition for differently doped nanostructures. As a result, the functionalized pGes are expected to have promising applications in nanoelectronics and nanomechanics.

Structural and electronic properties of T-graphene and its derivatives

Recent years there has been aroused a growing interest in designing two-dimensional (2D) structures of carbon allotropes, owing to the great success in graphene. The T-graphene is a newly proposed 2D carbon allotrope possessing tetragonal symmetry other than hexagonal symmetry of graphene. Also, the energetic and dynamical stabilities of T-graphene have been revealed. So motivated, we investigate the structural stabilities and electronic properties of T-graphene and especially its derivatives-n(n=1-5) by using the first-principle calculation based on the density function theory. By changing the atomic number (n) of the linear carbon chains connecting the two tetragon rings of T-graphene, a series of sp-sp2 hybrid structures can be formed, which is named T-graphene derivatives-n. The calculation results show that the structural stabilities, chemical bond types and electronic structures of these materials depend greatly on the parity of n. Owing to a strong π-bond formed by eight carbon atoms in T-graphene, it becomes the one with the lowest energy in all these materials studied in this work. An interesting phenomenon is found that the T-graphene derivatives-n with even n are dynamically stable as witnessed by the calculated phonon spectra without imaginary modes, while those with odd n are dynamically unstable. The metallic behaviors are present in the T-graphene derivatives-n with even carbon atoms in the linear carbon chains, showing an alternating single and triple C–C bonds. Besides, we observe that the metallicity of the T-graphene derivatives-n with even n becomes stronger as n increases. On the other hand, the linear carbon chains with odd carbon atoms are comprised of continuous C=C double bonds. These T-graphene derivatives-n with odd n also show metallic behaviors, but turn into magnetic materials (except for n=1), the magnetic moments are about 0.961μB (n=3) and 0.863μB (n=5) respectively, and ferromagnetic ordering is the only possibility for the magnetism, which rarely occurs in carbon material. Our first-principle studies indicate that the introducing carbon chains between the tetragonal carbon rings of T-graphene constitute an efficient method to obtain new two-dimensional carbon allotrope. With different numbers (even or odd) of carbon atoms on the chains, the constructed 2D carbon allotropes could show contrasting dynamical and magnetic properties. These findings provide a theoretical basis for designing two-dimensional carbon materials and carbon-based nanoelectronic devices.

Penta-graphene: A Promising Anode Material as the Li/Na-Ion Battery with Both Extremely High Theoretical Capacity and Fast Charge/Discharge Rate.

Recently, a new two-dimensional (2D) carbon allotrope named penta-graphene was theoretically proposed ( Zhang , S. ; et al. Proc. Natl. Acad. Sci. U.S.A. 2015 , 112 , 2372 ) and has been predicted to be the promising candidate for broad applications due to its intriguing properties. In this work, by using first-principles simulation, we have further extended the potential application of penta-graphene as the anode material for a Li/Na-ion battery. Our results show that the theoretical capacity of Li/Na ions on penta-graphene reaches up to 1489 mAh·g-1, which is much higher than that of most of the previously reported 2D anode materials. Meanwhile, the calculated low open-circuit voltages (from 0.24 to 0.60 V), in combination with the low diffusion barriers (≤0.33 eV) and the high electronic conductivity during the whole Li/Na ions intercalation processes, further show the advantages of penta-graphene as the anode material. Particularly, molecular dynamics simulation (300 K) reveals that Li ion could freely diffuse on the surface of penta-graphene, and thus the ultrafast Li ion diffusivity is expected. Superior performance of penta-graphene is further confirmed by comparing with the other 2D anode materials. The light weight and unique atomic arrangement (with isotropic furrow paths on the surface) of penta-graphene are found to be mainly responsible for the high Li/Na ions storage capacity and fast diffusivity. In this regard, except penta-graphene, many other recently proposed 2D metal-free materials with pentagonal Cairo-tiled structures may be the potential candidates as the Li/Na-ion battery anodes.

Thermoelectric properties of γ-graphyne from first-principles calculations

The two-dimensional graphene-like carbon allotrope, graphyne, has been recently fabricated and exhibits many interesting electronic properties. In this work, we investigate the thermoelectric properties of γ-graphyne by performing first-principles calculations combined with Boltzmann transport theory for both electron and phonon. The carrier relaxation time is accurately evaluated from the ultra-dense electron-phonon coupling matrix elements calculated by adopting the density functional perturbation theory and Wannier interpolation, rather than the generally used deformation potential theory which only considers the electron-acoustic phonon scattering. It is found that the thermoelectric performance of γ-graphyne exhibits a strong dependence on the temperature and carrier type. At an intermediate temperature of 600 K, a maximum ZT value of 0.77 can be achieved for the n-type system and can be further enhanced to 1.4 by considering isotopic effect.

Pretreatment of human hair for the determination of trace metals by ICP–MS

In recent years, the new two-dimensional planar carbon allotropes, graphyne and graphdiyne, have received significant research attention. Accordingly, the present study is devoted to investigate how the electronic structures of graphyne and graphdiyne nanoflakes are modified upon the edge oxidizing as well as Li-decoration. Different oxygen containing groups such as carbonyl, ketone, hydroxyl, carboxylic, formyl, and epoxide are considered for oxygenation of these carbon surfaces. The stability of oxidized surfaces with respect to the bare sheets has been discussed in terms of Gibbs free energy of formation. We showed that carbon surfaces terminated with carboxylic group are more stable than other oxidized surfaces. It is also found that the band gap in considered surfaces can be tuned (0.53–1.51 eV) by changing the type of oxygen containing group. Moreover, we report the binding energies for the Li adsorption at hollow sites on the basal plane and on the oxygen of functional group. Our calculations reveal that the interaction between oxygen group and carbon atoms at the edge increases the reactivity of considered sheets toward Li adsorption. The band gap of fuctionalized surfaces can be engineered by selective lithiation over a wide range between 0.01 and 1.31 eV. We found that the oxygen species plays the key role on the Li adsorption on the edge oxidized surfaces leading to either semiconducting or metallic behaviors. However, this study offers critical information for further theoretical and experimental studies on oxidized carbon materials for their possible applications in lithium ion batteries as well as hydrogen storage materials.

A planar carbon allotrope with linear bipentagon-octagon and hexagon arrangement

A two-dimensional (2D) metallic carbon allotrope is proposed, which consists of linearly aligned bipentagon-octagon and hexagon rings in a planar sheet. The relatively high percentage of hexagon and the regular arrangement of the polygons make it energetically more favorable than most of other predicted 2D carbon allotropes. Phonon dispersions without negative frequencies also indicate its stability. Electronic structure calculations show that its metallic nature is mainly due to the atoms shared by the pentagon, hexagon and octagon. Its lattice thermal conductivity is only about one fifth of that of graphene. Armchair- and zigzag-edged nanoribbons of this structure are also studied. The former is metallic while the latter has a small band gap due to the spin-polarized edge states. The appropriate band gap and the significantly reduced thermal conductivity suggest potential applications in thermoelectricity.

Porous Graphene Layers on Pt Catalyst for Long-Term Stability of Fuel Cell Electrode

With the development of various energy technologies, much interest is focused on the feasibility and efficiency study of energy devices such as fuel cells, batteries, supercapacitors and water electrolysis systems. Among them, polymer electrolyte fuel cell (PEFC), which convert hydrogen into electric energy with zero emission of pollutants, is one of the most promising environmentally friendly technologies. Despite innumerable studies for more than half a century, degradation of the key component, membrane electrode assembly (MEA), is still a big obstacle for the commercialization of PEFC system. For the high performance and long-term stability of Pt/C electrode catalyst, the agglomeration of Pt particles and dissolution or detachment of Pt particles from carbon support have to be improved. For this purpose, we adopted a new shape of nano-carbon material for the surface modification of Pt catalyst.Graphene has been an attractive two-dimensional carbon allotrope having large surface area and electronic conductivity. Graphene also shows high flexibility and mechanical strength so that it can be used for large number of applications such as flexible display or printable electronics, etc [1-2]. In most cases, people need large-area graphene films with little or no defect for high thermal and electronic conductivity. Also, people need to transfer the as-prepared graphene film for each application. All of these processes are not easy or simple, and they are also labor-intensive processes. However, in the case of catalysis, porous graphene films can be synthesized via very simple one-step process and can be used as effective protective layers for Pt catalysts. In this study, we developed porous graphene films in order to improve the long-term stability of Pt catalysts maintaining the high performance of them. The graphene films were synthesized by single-step vaporization process, where the number of graphene layers and the defects in their structure are manipulated by temperature and composition of the precursors. In this process, the amounts of structural defects, pyridine was simultaneously introduced to the vaporization process, which is much easier and cost-effective compared to the conventional NH3-treatment at high temp [3]. Consequently, our Pt/C catalysts coated with porous graphene films showed similar initial activity compared with the commercial catalysts (Pt 40wt%, Johnson Matthey) showing more than 150% higher long-term stability [4]. [1] A.K. Geim and K. S. Novoselov, Nature Mater. 6 (2007) 183.[2] X. L. Li, G. Y. Zhang, X. D. Bai, X. M. Sun, X. R. Wang, E. Wang and H. J. Dai, Nature Nanotech. 3 (2008) 538.[3] Y. Wang, Y. Shao, D. W. Matson, J. Li and Y. Lin, ACS nano 4(4) (2010) 1790.[4] H. Kim, A. Robertson, S. O. Kim, J. M. Kim and J. H. Warner, ACS nano 9(6) (2015) 5947.

Progress and prospect of two dimensional carbon graphdiyne

New materials have been consented as one of the important keys to break through the bottleneck of science and technology. Motivated by this, lots of researchers make efforts to discover or develop new materials in unconventional architectures. In the past two decades, tremendous efforts have been devoted to finding new carbon allotropes such as fullerene, carbon nanotube, and graphene. Different carbon allotropes have been demonstrated by altering the periodic binding motif consisted of sp, sp2 and sp3 hybridized carbon atoms. These carbon materials with great advantages of structure and electronic structure, the practical experiments, displaying many applications in the fields of catalyst, rechargeable battery, solar cell, information technology, gas separation, water purification and electronic device. The theoretical prediction and practical advances in graphdiyne (GDY) discussed here indicate that these new two-dimensional (2D) carbon allotropes are being introduced as next-generation materials in the carbon family.

Theoretical Study of Carrier Mobility in Two-Dimensional Tetragonal Carbon Allotrope from Porous Graphene**Supported by the Fundamental Research Funds for the Central Universities, the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the National Natural Science Foundation of China under Grant No 11204123.

The carrier mobility of two-dimensional tetragonal carbon allotrope (T-CA) from porous graphene is investigated by first-principles calculations. T-CA can be constructed from divacancy and Stone–Thrower–Wales defects from graphene. T-CA is a direct semiconductor with a band gap of 0.4 eV at Γ point. T-CA possesses a high carrier mobility of the order of 104 cm2 V–1 s−1. As our study demonstrates, T-CA has potential applications for next-generation electronic materials.

The Progress on Graphene-based Catalysis

Graphene is an atomically thin, two-dimensional allotrope of carbon with excellent chemical, mechanical, and electronic properties. Recent studies have revealed that using graphene or modified graphene as a catalyst support or metal-free catalyst can result in significant improvement in the catalytic activity, selectivity, reusability and tolerance due to the unique structure of graphene. This review summarized the recent progress on the preparation of the graphene-based catalysts and their application in traditional chemical reactions, electrochemical reactions and photochemical catalysis. The perspectives of achieving graphene-based catalysts with high catalytic performance have also been included. Keywords: Catalyst support, graphene-based catalyst, graphene oxide, heterogeneous catalyst, homogeneous catalyst, organic reaction.

Possible Formation of Graphyne on Transition Metal Surfaces: A Competition with Graphene from the Chemical Potential Point of View

Graphyne (GY), a two-dimensional (2D) allotrope of carbon with mixed sp/sp2 hybridization, is predicted to exist in many stable phases and has recently received great attention. However, it is energetically less stable than graphene and remains difficult to be synthesized in experiment to date. In this report, the possible environments for synthesis of graphyne on Ru(0001), Rh(111), and Pd(111) substrates are investigated by considering three typical phases of GY (α, β, and γ). Their structures, interactions with metal substrates, as well as thermodynamic stability are calculated using first-principles calculations. The chemical potential phase diagram of GYs and graphene on metal substrates is constructed. For all these substrates, the α phase of GY can form in the carbon-poor environment, while formation of graphene dominates in the carbon-rich condition.

Heterogeneous Epitaxy: Designed Peptides Scale Graphene’s Surface

To the capable climber, a sheer mountain face on close inspection presents tenuous finger- and toe-holds that allow scaling of its vertical surface. Just as assumptions about scale influence our perceptions of everyday objects, similar questions arise regarding appropriate length scales for describing molecular recognition or self-assembly processes. Such is the case with targeting graphene—on one scale, this two-dimensional (2D) allotrope of carbon is only a few Angstroms thick and exceptionally flat in the facial dimension.

Disparate Strain Dependent Thermal Conductivity of Two-dimensional Penta-Structures.

Two-dimensional (2D) carbon allotrope called penta-graphene was recently proposed from first-principles calculations and various similar penta-structures emerged. Despite significant effort having been dedicated to electronic structures and mechanical properties, little research has been focused on thermal transport in penta-structures. Motivated by this, we performed a comparative study of thermal transport properties of three representative pentagonal structures, namely penta-graphene, penta-SiC2, and penta-SiN2, by solving the phonon Boltzmann transport equation with interatomic force constants extracted from first-principles calculations. Unexpectedly, the thermal conductivity of the three penta-structures exhibits diverse strain dependence, despite their very similar geometry structures. While the thermal conductivity of penta-graphene exhibits standard monotonic reduction by stretching, penta-SiC2 possesses an unusual nonmonotonic up-and-down behavior. More interestingly, the thermal conductivity of penta-SiN2 has 1 order of magnitude enhancement due to the strain induced buckled to planar structure transition. The mechanism governing the diverse strain dependence is identified as the competition between the change of phonon group velocity and phonon lifetime of acoustic phonon modes with combined effect from the unique structure transition for penta-SiN2. The disparate thermal transport behavior is further correlated to the fundamentally different bonding nature in the atomic structures with solid evidence from the distribution of deformation charge density and more in-depth molecular orbital analysis. The reported giant and robust tunability of thermal conductivity may inspire intensive research on other derivatives of penta-structures as potential materials for emerging nanoelectronic devices. The fundamental physics understood from this study also solidifies the strategy to engineer thermal transport properties of broad 2D materials by simple mechanical strain.

Ab-initio calculation of electronic structure and optical properties of AB-stacked bilayer α-graphyne

Monolayer α-graphyne is a new two-dimensional carbon allotrope with many special features. In this work the electronic properties of AA- and AB-stacked bilayers of this material and then the optical properties are studied, using first principle plane wave method. The electronic spectrum has two Dirac cones for AA stacked bilayer α-graphyne. For AB-stacked bilayer, the interlayer interaction changes the linear bands into parabolic bands. The optical spectra of the most stable AB-stacked bilayer closely resemble to that of the monolayer, except for small shifts of peak positions and increasing of their intensity. For AB-stacked bilayer, a pronounced peak has been found at low energies under the perpendicular polarization. This peak can be clearly ascribed to the transitions at the Dirac point as a result of the small degeneracy lift in the band structure.

Two-Dimensional γ-Graphyne Suspended on Si(111): A Hybrid Device

Graphynes (GYs) are a new class of two-dimensional (2D) carbon allotrope materials that are similar to graphene but with C2 units inserted into certain bonds to produce physical properties distinct from those of graphene. In this work, atomic, electronic, and quantum transport properties of γ-GY—a particular type of graphyne—absorbed on the silicon (111) surface are investigated from atomistic first principles. γ-GY possesses an intrinsic direct band gap, and when interacting with the Si(111), interesting subgap electronic structure is induced to mediate charge transport. In particular, the transmission spectra of the γ-GY/Si(111) hybrid device have a high broad peak at the Fermi level due to hybridization. For γ-GY/Si(111) transport junctions having a finite length trench underneath the γ-GY, substantial charge conduction can still occur through the γ-GY bridge. Nonequilibrium calculations suggest that the hybrid 2D γ-GY/Si(111) transport junction is highly controllable by external voltages.

Li-decoration on the edge oxidized graphyne and graphdiyne: A first principles study

In recent years, the new two-dimensional planar carbon allotropes, graphyne and graphdiyne, have received significant research attention. Accordingly, the present study is devoted to investigate how the electronic structures of graphyne and graphdiyne nanoflakes are modified upon the edge oxidizing as well as Li-decoration. Different oxygen containing groups such as carbonyl, ketone, hydroxyl, carboxylic, formyl, and epoxide are considered for oxygenation of these carbon surfaces. The stability of oxidized surfaces with respect to the bare sheets has been discussed in terms of Gibbs free energy of formation. We showed that carbon surfaces terminated with carboxylic group are more stable than other oxidized surfaces. It is also found that the band gap in considered surfaces can be tuned (0.53–1.51eV) by changing the type of oxygen containing group. Moreover, we report the binding energies for the Li adsorption at hollow sites on the basal plane and on the oxygen of functional group. Our calculations reveal that the interaction between oxygen group and carbon atoms at the edge increases the reactivity of considered sheets toward Li adsorption. The band gap of fuctionalized surfaces can be engineered by selective lithiation over a wide range between 0.01 and 1.31eV. We found that the oxygen species plays the key role on the Li adsorption on the edge oxidized surfaces leading to either semiconducting or metallic behaviors. However, this study offers critical information for further theoretical and experimental studies on oxidized carbon materials for their possible applications in lithium ion batteries as well as hydrogen storage materials.

Prediction of two planar carbon allotropes with large meshes.

Two novel two-dimensional (2D) carbon allotropes named C(y) and C(z) with large meshes are predicted based on first-principles calculations. Their formation energies are lower than that of graphdiyne, which was recently synthesized in an experiment. Molecular dynamics simulations indicate that C(y) and C(z) are stable even when the temperature is over 1000 K. The calculated Poisson's ratios of C(y) and C(z) show their anisotropic mechanical properties. The electronic structure calculations indicate that C(y) is a metal, while C(z) behaves as a semiconductor. Moreover, C(z) shows conductive anisotropy suggesting its potential in nanoelectronic devices. Meanwhile, their well-defined mesh structures are suitable for molecular sieves.

Graphdiyne oxide as a platform for fluorescence sensing.

Graphdiyne (GD), a new kind of two-dimensional carbon allotrope consisting of a hexagonal ring and a diacetylenic linkage unit, is observed to exhibit a high fluorescence quenching ability and can be used as a new platform for fluorescence sensing, where GD oxide, the oxidized form of GD, is found to exhibit higher quenching ability than GD. As a proof-of-concept demonstration, GD oxide is used to establish a new platform for effective fluorescence sensing of DNA and thrombin with a high sensitivity and selectivity.