Atomic Graphene Research Articles

Raman spectroscopy as a probe for the electronic structure of graphene at electrified interfaces

As all the atoms in monolayer graphene are on its surface, it is strongly influenced by its environment. Surface-charge transfer of graphene changes its properties including the electrical conductivity, carrier mobility, capacitance, and reactivity. The charge transfer occurs between the graphene and its substrate, and can be controlled by the electrode potential in electrochemical cells. The charge of graphene is related to its Fermi level and Fermi energy, which determines the rates of the electrochemical and photoelectrochemical reactions occurring. The simplest method to measure the charge density of graphene is Raman spectroscopy. Here, we review Raman spectroscopy measurement of graphene charge, especially that in electrochemical environments, with emphasis on its potential application in electrochemical and photoelectrochemical reactions for energy conversion.

Sulfur-Coordinated Transition Metal Atom in Graphene for Electrocatalytic Nitrogen Reduction with an Electronic Descriptor.

The adjacent chemical microenvironment of single metal atoms in heterogeneous catalysis is crucial to their chemical activity for various catalytic processes. Here, based on first-principles calculations, 25 single transition metal atom catalysts coordinated to sulfur species embedded in graphene (TM-S4-G-SACs) are reported for nitrogen reduction under ambient condition. It shows that nine TM-S4-G-SACs (TM = Mo, Sc, Cr, V, W, Ti, Nb, Mn, and Re) are promising nitrogen reduction catalysts with an optimal potential of -0.425 V. Meanwhile, 18 TM-S4-G-SACs have better catalytic activity than those with nitrogen coordination. Particularly, the catalytic activity of TM-S4-G-SACs and the adsorption energy of intermediate NH2* conform to a volcano-type correlation, which can be described by a universal electronic descriptor φ, defined by the electronegativity of the metal, adjacent coordinated atoms, and the valence electron occupancy. The above findings suggest the potential of sulfur-coordinated single metal atoms as electrocatalytic nitrogen reduction catalysts and an applicable descriptor to achieve optimal performance.

Electronic Structure and Adsorption Behavior of Non-Metallic Atoms and Aluminum Atoms on the Surface of Graphene: DFT Theoretical Calculation

Graphene has been widely studied in many fields due to its excellent physical and chemical properties. In this paper, we focus on the electronic structure and adsorption behavior of Al atoms and X ([Formula: see text], N, Si, S) atoms co-adsorption on the surface of pure graphene and Al atom adsorption on the surface of X ([Formula: see text], N, Si, S) atoms-doped graphene systematically. The adsorption energy, adsorption distance, bond length, bond angle, and electronic structure were studied using density functional theory (DFT) theoretical calculation. The results show that the adsorption capacity of (Al, X) atoms co-adsorbed on the graphene surface is significantly improved compared to the Al atoms on the pure graphene surface. The adsorption energy of Al and B atoms co-adsorbed on the graphene surface is basically the same as that of Al atoms on the B-doped graphene surface. The calculation results also show that the adsorption capacity of Al atoms and X ([Formula: see text], Si, S) atoms co-adsorbed on the surface of graphene is better than that of Al atoms on the surface of graphene doped with X (X = N, Si, S) atoms. Compared with N atom and S atom-doped systems, the Si-doped graphene surface has a stronger adsorption capacity for Al atoms than pure graphene. However, due to the doping of N and S atoms, the adsorption capacity of Al atoms on the graphene surface is somewhat reduced. Our calculation results provide a theoretical basis for further selection of Al-based graphene composite materials.

(Invited) The Oxygen Reduction Reaction Kinetics on a Platinum /Graphene Surface

The oxygen reduction reaction (ORR) is rate-limiting in polymer electrolyte fuel cells over a wide range of overpotentials. To improve our understanding of the underlying physics and chemistry of ORR, we created a model system with atomically-ordered interfaces that allows us to distinguish between bulk and electronic effects on catalytic behavior. Specifically, we developed a facile way of growing thin, epitaxial Pt nanostructures on graphene, which serves as both growth substrate and electrode. Rotating disc electrode measurements showed a high specific activity relative to platinum on glassy carbon. The enhanced ORR activity was observed despite consistent measurements that show a small electrochemically-active surface area arising from the fact that virtually all carbon atoms in monolayer graphene are surface atoms. This talk will address the reasons for the increase of ORR activity in view of the new ground state of atomically thin platinum on graphene.

Computer Modelling of Metal Nanoparticles Adsorbed on Graphene

The influence of deposited Al, Pd, Co, Au, Ni atoms on a single-layer graphene substrate is investigated using computer simulations. The computer modelling of spraying nanoparticles on the basis of molecular dynamics method is implemented using the NVIDIA®CUDATM technique. According to the results of model calculations, the general behaviour of the system is investigated. The experiments are performed to study the sputtering of atoms of different metals under different initial conditions of the system. Based on these sprays, the behaviour at the beginning of the interaction of the deposited atoms with the substrate is analysed. The time dependences of the lateral position of nanoparticles’ centre of mass and the substrate force throughout the experiment for different sprayed materials are compared. The behaviour of total and potential energies, temperature and momentum of the system is studied. As shown, there is a jump in total energy and temperature as well as a change in the behaviour of the momentum and the substrate force when carbon atoms of graphene are reached for all depositions.

Catalytic performances of fluorine-doped graphene/Pb composite for thermal decomposition of ammonium perchlorate

Fluorine-doped graphene supported PbO (F-rGO/PbO) composite was prepared through a facile water bath method to study an improvement strategy for its catalytic performance in thermal decomposition of ammonium perchlorate (AP). The characterization results showed that the PbO with the size of 20 ∼ 30 nm are highly dispersed on the surface of F-doped graphene (F-rGO). Compared with other samples, the F-rGO/PbO exhibited superior catalytic performance, as 3 wt% of catalyst added in AP, the peak temperature for HTD reduced by 124°C, and the peak temperature for LTD increased by 8°C. The released apparent specific heat of the thermal decomposition of AP catalyzed by F-rGO/PbO (3603.7 J/g) is five times of that of pure AP (603.5 J/g). The doping of F atoms in graphene increased BET surface areas of the F-rGO carrier and enhanced the content of hydroxyl groups on the surface of graphene, which improved the thermal decomposition rate of AP. While the highly dispersed PbO NPs further increased BET surface areas of F-rGO/PbO to provide more active sites during the thermal decomposition of AP.

Uniform cobalt nanoparticles embedded in nitrogen-doped graphene with abundant defects as high-performance bifunctional electrocatalyst in overall water splitting

Co nanoparticles with uniform size (about 5 nm) embedded in N-doped graphene (Co-NG) were explored in this work. The introduction of a second carbon source of citric acid during synthesis prevented the Co atoms from growing up, thus regulating the size of the cobalt nanoparticles. N atoms in N-doped graphene had more lone-pair electrons, making it easier to capture electrons from hydrated ions, and facilitating the dynamics procedure of HER. Furthermore, N dopant rendered larger positive charge density on the adjacent carbon atoms, which was conducive to OER and HER. At 10 mA cm−2 of the current density, the Co-NG/CC catalyst's overvoltage of HER was 78 mV, approaching that of 20% Pt/C (59 mV), an efficient precious metal electrocatalyst for HER, while its overvoltage of OER was about 225 mV, 12.5% lower than that of RuO2 (257 mV, a common precious metal oxide OER electrocatalyst). In addition, this Co-NG/CC composite bifunctional catalyst displayed good electrochemical stability in alkali solution and might be designed as a dual-function catalyst in the application of overall water splitting. The cell voltage of Co-NG/CC//Co-NG/CC was only 1.66 V, approaching to that of full precious metal cell of Pt/C//RuO2 (1.52 V), and revealing the good commercial application prospects of this composite bifunctional catalyst.

Enhanced Magnetoresistance Effect in Graphene Coupled to a Ferromagnetic Oxide with Charge Orbital Ordering

In this paper, we fabricated graphene/Fe3O4 heterostructure devices by stacking monolayer graphene on magnetite (Fe3O4) substrate and investigated their magneto-transport properties. Interestingly, graphene/Fe3O4 heterostructure devices exhibit a giant magnetoresistance (MR) of 70% at a low magnetic field of 0.65[Formula: see text]T and at 11[Formula: see text]K, which is three times greater than that of graphene on SiO2. Based on standard two-fluid model and LDA[Formula: see text]U simulation, we showed that the observed enhanced MR effect is due to the increased disorder in graphene induced through the charge polarization via the alignment of C atoms of graphene over the charge ordered B-site cations of Fe3O4. Our results demonstrate a potential way to enhance graphene MR effect through coupling graphene with a suitable substrate with charge orbital ordering.

Atomistic Understanding of Two-dimensional Electrocatalysts from First Principles.

Two-dimensional electrocatalysts have attracted great interest in recent years for renewable energy applications. However, the atomistic mechanisms are still under debate. Here we review the first-principles studies of the atomistic mechanisms of common 2D electrocatalysts. We first introduce the first-principles models for studying heterogeneous electrocatalysis then discuss the common 2D electrocatalysts with a focus on N doped graphene, single metal atoms in graphene, and transition metal dichalcogenides. The reactions include hydrogen evolution, oxygen evolution, oxygen reduction, and carbon dioxide reduction. Finally, we discuss the challenges and the future directions to improve the fundamental understanding of the 2D electrocatalyst at atomic level.

Graphene on silicon: Effects of the silicon surface orientation on the work function and carrier density of graphene

Density functional theory has been employed to study graphene on the (111), (100), and (110) surfaces of bare silicon (Si) substrates, which provide three different densities of surface atoms. There are several interesting findings. First, carbon atoms in graphene can form covalent bonds with Si atoms, when placed close enough on Si (111) and (100) surfaces, but not on the (110) surface. The Si (111) surface shifts the Fermi level of graphene into its conduction band, resulting in an increase of the electron density by three orders of magnitude. The work function of graphene is increased by 0.29 eV on the (111) surface, likely due to the surface dipole from the redistribution of $\ensuremath{\pi}$ orbitals. The change in the number of available states below the Fermi level of graphene due to its interaction with the Si surface, is the main cause for the unconventional doping reported in this paper. The electron density can also be increased by eighty times on a Si (100) substrate without the shift of Fermi level, which is another clear example of the proposed doping mechanism. These striking effects that different orientations of a silicon substrate can have on the properties of graphene are related to the surface atom density of the substrate. These results provide valuable guidance to the growth of graphene on Si for various purposes for electronic devices.

The sensing mechanism of HCHO gas sensor based on transition metal doped graphene: Insights from DFT study

Formaldehyde (HCHO), as primary chemical feedstock, is widely used in various fields. Unfortunately, it can cause harmful impacts on human health due to its high toxicity. Therefore, effective monitoring and detection of HCHO is significant. HCHO gas sensor based on doped graphene was extensively investigated in recent years. However, the relationship between gas-sensing performance and electronegativity of doped atoms has not been clarified. In this research, Ni, Pd and Pt are used as dopants, and the adsorption behaviors, energies, electronic and optical properties of HCHO molecule on doped graphene were systematically investigated by density functional theory (DFT) method to explore the effect of electronegativity of doped atoms on the sensing property. The calculated results show that graphene doped with higher electronegativity atoms is beneficial to cause a recordable electric signal due to larger change of energy gap. The adsorption energy increases steadily with an enhancement in the electronegativity of doped atoms. It is clear that there is an obvious linear relationship between gas-sensing performance and electronegativity of doped atoms. In other words, specific sensitivities (e.g., adsorption energy, electrical conductivity, recovery time, chemical reactivity parameters and so on) of gas sensor can be adjusted by embedding different electronegativity atoms in graphene, which provide meaningful information to design highly sensitive graphene-based gas sensors. Besides, the adsorption and desorption performance of HCHO can be controlled by applying positive or negative electric field.

Electrocatalyst nanoarchitectonics with molybdenum-cobalt bimetallic alloy encapsulated in nitrogen-doped carbon for water splitting reaction

The rational construction of highly efficient electrocatalysts comprising multiple components with distinctive bifunctionalities is still a challenge for the practical application due to their sluggish kinetics for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Here, a series of cobalt-molybdenum alloy nanorods encapsulated in N-doped carbon shells (CoMo@NC) is synthesized via an in-situ carbonization-reduction method using CoMoO4 as the precursor. The high conductivity, strain-induced effect, and synergistic interactions between N-doped carbon and bimetallic cores endow the optimized catalyst with outperforming catalytic performances for HER and OER in alkaline solution with low overpotentials (98 mV for HER and 336 mV for OER at 10 mA cm−2), as well as high durability. The overall water splitting device using the CoMo@NC sample achieved at 800 ℃ treatment (CoMo@NC-800) as bifunctional catalyst could possess a low voltage of 1.67 V to drive a current density of 10 mA cm−2 and high durability. Furthermore, density functional theory calculations reveal that the pyridinic-N atoms of graphene anchored on CoMo alloy nanoparticles can efficiently modulate the electronic structure to generate optimal free energy of hydrogen adsorption (−0.029 eV), suggesting excellent HER intrinsic activity. This work may provide a facile avenue to achieve multiple metallic alloy-based nanomaterials for boosting electrochemical water splitting performance.

Multi-Scale Modeling of Graphene/Polymer Nanocomposites-Molecular Interfacial Interactions

This paper presents a multi-scale modeling approach involving interfacial interactions to predict the elastic properties and mechanical behavior of single-layer graphene-reinforced nanocomposites under tension load. A multi-scale model was developed using the finite element method of the tripartite structure consisting of graphene in epoxy, the interfacial region and their Van Der Walls interactions. The effect of graphene chirality was investigated by proposing a methodology of graphene-Van Der Walls interactions-polymer with determined geometric dimensions. Parametric modeling was performed to model the interactions between Van Der Walls, graphene and interface material atoms using the finiteelement method with molecular mechanics approach. Numerical analysis of graphene nanoparticles by embedding them in an epoxy with their real dimensions is not an appropriate task today. In particular, it is not possible to analyze these real graphene nanoparticles as multiple by randomly dispersing them in the epoxy polymer. Therefore, in this research, a model was developed to overcome this problem and to investigate the effect of molecular interactions on loads in different axes. The results show that graphene nanocomposites in armchair geometry give higher stress values and behave more rigidly. As the volume ratio increases, the mechanical performances increase. It is seen that the graphene direction is much stronger than the thickness direction. It is clear that the volume ratio effect in the thickness direction has a slight effect on the tensile behavior.

Quasiperiodic Van der Waals Heterostructures of Graphene and Hexagonal Boron Nitride

Advancement toward opening a bandgap at the Dirac point induced by symmetry breaking paved the way to realize 2D heterostructures with graphene and hexagonal boron nitride (h‐BN). An alternate arrangement of graphene and h‐BN layers in a 3D stacking can tune the bandgaps of these composites depending on the position of B and N atoms with respect to C atoms of graphene. Herein, a unique possibility of arranging graphene and h‐BN atomic layers in a quasiperiodic Fibonacci sequence to study the possibilities of controlling the electronic properties of these heterostructures is explored. Density functional theory calculations combined with van der Waals corrections reveal that these quasiperiodic heterostructures are more stable than normal periodic stacking of monolayers of graphene and h‐BN. Moreover, for certain arrangements of atomic layers, sizeable bandgaps can be obtained.

First-Principles Study of Vacancy and Impurities Defects in Graphene

In this work, we have studied the electronic and magnetic properties of 1C atom vacancy defects in graphene (1Cv-d-G), 1N atom impurity defects in graphene (1NI-d-G) and 1O atom impurity defects in graphene (1OI-d-G) materials through first principles calculations based on spin-polarized density functional theory (DFT) method, using computational tool Quantum ESPRESSO (QE) code. From band structure and density of states (DOS) calculations, we found that supercell structure of monolayer graphene is a zero bandgap material. But, electronic bands of 1Cv-d-G, 1NI-d-G and 1OI-d-G materials split around the Fermi energy level and DOS of up & down spins states appear in the Fermi energy level. Thus, 1Cv-d-G, 1NI-d-G and 1OI-d-G materials have metallic properties. We have studied the magnetic properties of pure and defected materials by analyzing density of states (DOS) and partial density of states (PDOS) calculations. We found that graphene and 1OI-d-G materials have non-magnetic properties. On the other hand, 1C vacancy atom and 1N impurity atom induced magnetization in 1Cv-d-G & 1NI-d-G materials by the rebonding of dangling bonds and acquiring significant magnetic moments of values -0.75μB/cell & 0.05μB/cell respectively through remaining unsaturated dangling bond. Therefore, non-magnetic graphene changes to magnetic 1Cv-d-G and 1NI-d-G materials due to 1C atom vacancy defects and 1N atom impurity defects. The 2p orbital of carbon atoms has main contribution of magnetic moment in these defected structures.

Longer Trapping Expected for Graphene Atom Chips

Predictions indicate that atom chips that use graphene contacts in their circuitry rather than metal ones can trap atoms for significantly longer times.

Isolated Co single atoms in nitrogen-doped graphene for aluminum-sulfur batteries with enhanced kinetic response

Al-S batteries are promising next generation energy storage devices due to their high theoretical energy density (1340 Wh kg−1), low cost, and safe operation. However, the electrochemical performance of Al-S batteries suffers poor reversibility owing to slow kinetic processes determined by the difficulty of reversible conversion between Al and S. Here, we proposed a single-atom catalysts comprising Co atoms embedded in a nitrogen-doped graphene (CoNG) as an electrochemical catalyst in the sulfur cathode that renders a reduced discharge-charge voltage hysteresis and improved sulfur utilization in the cathode. The structural and electrochemical analyses suggest that the CoNG facilitated both the formation and oxidation of AlSx during the electrochemical reactions of the sulfur species. Consequently, the CoNG-S composite can deliver a considerably reduced voltage hysteresis of 0.76 V and a reversible specific capacity of 1631 mAh g−1 at 0.2 A g−1 with a sulfur utilization of more than 97%.

Graphene and its Phononics: A Review

Graphene, a unique allotrope of carbon, has garnered a huge amount of attention amongst researchers as its unique properties and promising applications in various real-life domains like in efficient batteries, solar cells, medicinal technologies, environmental remediation, circuit boards, lighting and display, and anti-corrosion has resulted in its commercial exploitation and implementation in everyday life. Since 2004 graphene has been one of the most beautiful scientific and technological achievements. The unique electronic cloud forming the bond between different carbon atoms in graphene leads to several inquisitive questions raised in the field of quantum physics. Till now graphene has been exploited for its electronic and optical properties but new research has shown that phononic properties of graphene will lead to the development of “killer” practical applications in the future. In this review article we have explored the structure, properties and the phononics of graphene with a special reference to few-layer graphene (FLG) and graphene Thermal Interface Materials (TIMs).

Study on the tribological properties of copper coated by graphene and h-BN from the atomic scale

Copper with high thermal and electrical conductivity had been widely utilized as frictional materials. However, their wear resistance was too weak to meet the actual requirements. In this study, graphene nanosheet and hexagonal boron nitride (h-BN) with strong mechanical properties were coated onto the surface of copper to improve the wear resistance. Their nano-tribological properties were studied by molecular dynamics (MD) simulation to understand the wear mechanisms well from the nanoscale. The simulated results indicated that the average friction coefficient of copper coated by h-BN and graphene decreased by almost 50% compared with pure copper. The interactions between Cu atoms and carbon atoms in diamond counterpart and graphene were interpreted by analyzing the variations of the radius distribution function (RDF). The h-BN and graphene lubrication mechanisms were discussed by monitoring van der Waals (vdW) force, slippage, the energy of the system, and temperature variations during the process of friction. Furthermore, Von Mises stress distributions on the copper substrates and dislocation evolution were also investigated to virtually observe the change of atoms.

<PE-AT>Strain acting on adsorption of C60 onto graphene from DFTB algorithm

Density functional tight-binding calculations have been employed to study the interaction between C60 with pristine and strained single layer graphene, respectively. Differential charge density, adsorption energy, chemical potential, and Mülliken populations are used to identify the adsorption behaviors for nine configurations of C60 onto graphene. The uniaxial tension is acted along armchair or zigzag direction of graphene, where the strain is within the elastic range determined from stress-strain curves. The calculations show that the direction of the applied strain and the magnitude of the strain greatly affect the adsorption of these configurations. When the strain changes, Mülliken charge distribution presents apparent differences among the atoms in graphene and C60. These greatly affect the adsorption energy and chemical potentials of these configurations.