Pure Graphene Sheet Research Articles

Pure Graphene Acts as an "Entropic Surfactant" at the Octanol-Water Interface.

Graphene oxide (GO) has proved itself as a nanomaterial capable of acting as a surfactant by lowering the interfacial tension of the oil-water interface due to its polar oxygen groups. However, the surfactant behavior of the pure graphene sheet─since prevention of edge oxidation in experimental setups is nontrivial─is still an unresolved issue in graphene research despite significant progress in the field in recent years. Here, we conduct both atomistic and coarse-grained simulations to demonstrate that─surprisingly─even pristine graphene, which only consists of hydrophobic carbon atoms, is attracted to the octanol-water interface and consequently reduces its surface tension by 2.3 kBT/nm2 or about 10 mN/m. Interestingly, the location of the free energy minimum is not precisely at the oil-water interface itself but is rather buried about two octanol layers into the octanol phase, being about 0.9 nm from the water phase. We demonstrate that the observed surfactant behavior is purely entropically driven and can be attributed to the unfavorable lipid-like structuring of octanol molecules at the free octanol-water interface. In essence, graphene enhances the inherent lipid-like behavior of octanol at the water interface rather than directly acting as a surfactant. Importantly, graphene does not display surfactant-like behavior in corresponding Martini coarse-grained simulations of the octanol-water system since the free liquid-liquid interface loses essential structure at the lower coarse-grained resolution. However, a similar surfactant behavior is recovered in coarse-grained simulations of longer alcohols such as dodecan-1-ol and hexadecan-1-ol. The observed discrepancies at different model resolutions enable us to construct a comprehensive model of the surfactant behavior of graphene at the octanol-water interface. The here-gained insights may facilitate the broader utilization of graphene in numerous domains of nanotechnology. Furthermore, since a drug's octanol-water partition coefficient is a crucial physicochemical parameter in rational drug discovery, we also believe that the universality of the here-illustrated entropic surfactant behavior of planar molecules deserves special attention in the drug design and development field.

A DFT study of gas molecules adsorption on intrinsic and Cu-doped graphene gas nanosensors

In this study, first-principles calculations are performed to investigate the sensitivity of intrinsic graphene sheet (GS) and Cu-doped graphene sheet (Cu-GS) gas nanosensors for adsorbing CO, H2, and H2S gas molecules using QUANTUM ESPRESSO package. The density of states (DOS), net charge transfer, adsorption energy, partial density of states (PDOS), and the most stable adsorption configuration of these molecules on GS and Cu-GS are studied. The results show the weak physical adsorption of the three gas molecules on GS. The strength of interaction between the Cu-GS system and adsorbed gas molecules is higher due to the Cu doping. It is expected that the significant increase in charge transfer and adsorption energy leads to fundamental improvement in the electrical conductivity of the Cu-GS system. The results indicate that the introduction of Cu impurity can improve the gas sensing properties of graphene-based gas nanosensors. Therefore, Cu-GS is more appropriate for detecting gas molecules compared to pure GS. The results in this study are useful for developing the design of gas nanosensors.

The Effect of Nitrogen on the Structural and Electronic Properties of Graphene Sheet using Density Functional Theory

Abstract The present research focuses on a theoretical study of structural and electronic properties of pure graphene sheet and then adding different number of N2 atoms. The calculations are carried out using the density functional theory (DFT) with hybrid functional B3LYP/6-31G level to investigate the proposed structures. Gauss View 5.0.8 program is used to design the structures of pure and doped graphene sheets. These structures are relaxed by employing the PM6 semi-empirical method with the hybrid functional B3LYPDFT at Gaussian 09 package. The results of the structural properties of the studied graphene sheets showed that good relaxation of the structures, the constant bonds values in the pure graphene sheets in the same ranges of the carbon rings structures. We calculate the total energy, High Occupied Molecular Orbital (HOMO) and Low Unoccupied Molecular Orbital (LUMO) energies and forbidden energy gap. The result of the total energy of that doping graphene sheets is result of the binding energy of each structure and indicates that these structures have relaxation, and the effect of adding N2 atoms in pure graphene sheet on the total energy of the molecule is effective. All doping graphene sheets have small forbidden energy gap, but it vibrates depending on the length and number of each sheet and the position of N2 atoms in the sheets.

Graphene/ β‐ MnO 2 composites—synthesis and its electroanalytical properties study in the Mg storage battery

Graphene/β-MnO2 composites were synthesized via ultrasonication method, followed by calcination at 400°C for 4 hours. The structural properties of synthesized graphene and graphene/β-MnO2 composites were systematically studied using different instruments, such as XRD, FESEM, TEM, EDX, XPS and FT-IR spectroscopy. Furthermore, magnesium storage performance was scientifically studied using various electroanalytical characterizations like galvanostatic charge/discharge, electrochemical impedance spectra and cyclic voltammetry. The GNS/β-MnO2-20% composite displays the discharge capacity of 110 mAh/g within 60 cycles at a current density of 25 mA/g, which is an excellent rate capability, and magnesium storage performance around 100 cycles. There is a significant improvement in electrochemical properties of GNS/β-MnO2-20% composite materials as cathode for magnesium storage system due to the synergetic interaction between a pure β-MnO2 particle and graphene sheet.

The Effect of Substituting an X (B, N, O, and F) Atom on the Structural, Magnetic and Optical Properties of Graphene Sheets

The effect of a dopant X atom (X = B, N, O, and F) on the structural, magnetic and optical properties of graphene sheets was investigated through an ab initio study. The induced dipole moment was found to be directed toward atom B in the graphene–B sheet and toward atom C (of graphene) in other dopant sheets due to the charge migration between atoms. This migration arises from the covalent bond between C and X. Both the calculated spin-polarization (P = 8.85%) and the inverse magnetic susceptibility predict that the graphene–O sheet is a weak ferromagnetic material. The electronic specific heat (CV) is computed as a function of temperature. The calculated CV of the graphene–N sheet offers better thermodynamic properties and higher Debye temperature than the other sheets studied. The optical properties of graphene–X sheets were investigated as a function of energy up to 30 eV. These properties, which prevent the electric field from penetrating the doped graphene sheet, are enhanced upon doping. The graphene–F sheet is found to have the lowest potential to absorb a photon among the sheets, in the region around 11.7 eV. The optical properties of the dopant sheets are improved compared with the pure graphene sheet, suggesting the potential for use in many applications such as antireflective coatings, optical lens coating, optical fiber core, encapsulation of various optical components, and optical adhesives.

Improvement of hydrogen storage capacity on the palladium-decorated N-doped graphene sheets as a novel adsorbent: A hybrid MD-GCMC simulation study

Abstract A hybrid molecular simulation, as a combination of molecular dynamics and Grand Canonical Monte Carlo simulation, is performed to investigate the storage capacity of hydrogen in carbon nanostructure adsorbents. Pure graphene sheet, nitrogen-doped graphene sheet, palladium-decorated graphene sheet and nitrogen-doped graphene sheet decorated with palladium atoms are selected for this purpose. Palladium is added to the structure in atomic and nanoparticle forms. Initially, all selected systems are optimized using density functional theory (DFT). The atomic charges of various structures are incorporated in the hybrid simulation. Then, hybrid simulations of hydrogen adsorption in different structures are performed at a temperature of 300 K in the pressure range of 1–40 bar. Simulation results show that among various structures, the simultaneous doping of graphene sheet with nitrogen atom and decoration of sheets by palladium atoms could increase the storage capacity by about 437% in comparison to pure graphene. In addition, the atomic form of palladium is more efficient than its nanoparticle form. Finally, comparing the adsorption capacity of the proposed structure with the target set by the US Department of Energy for 2020 indicates that proposed nanostructure can improve this target for hydrogen storage in comparison with previous carbon structure materials.

The Hydrogen Passivated Graphene Cluster and its Stability - First Principle DFT (B3LYP) Levels of Approximation with the Basis Set 3-21G

First-principles DFT (B3LYP) levels of calculations with the basis set 3-21G have been carried out in order to study the geometric stability and electronic properties of hydrogen passivated graphene (H-graphene) clusters(CN) (where N = 6, 10, 13, 16, 22, 24, 27, 30, 35, 37, 40, 42, 45, 47, 48, 50, 52, 54, 70 and 96) and perform the DOS spectrum on H-graphene (C16H10, C24H12, C30H14, C48H18, C70H22 and C96H24) using Mulliken population analysis by the Gaussian 03 W set of programs. The variations of ground state energy of graphene clusters are observed on sizes and corresponding number of carbon atoms. The binding energy per carbon atom is the function of carbon atoms for the number of carbon atoms less than 30 and saturated at carbon’s number 30 and more in the DFT (B3LYP) levels of approximation with the basis set 3-21G. The binding energy per carbon atom of a pure graphene sheet C32 is 8.03 eV/atom in the DFT (B3LYP) level of approximation with the choice of the basis set 3-21G, which is acceptable with previous reported data 7.91 eV/atom. The HOMO-LUMO gap in NBO is studied for some H-grapheneclustors C16H10, C24H12, C30H14, C48H18, C70H22 and C96H24.

Adsorption of small gas molecules on pure and Al-doped graphene sheet: a quantum mechanical study

The interaction of small gas molecules ( $$\hbox {CCl}_{4}$$ , $$\hbox {CH}_{4}$$ , $$\hbox {NH}_{3}$$ , $$\hbox {CO}_{2}$$ , $$\hbox {N}_{2}$$ , CO, $$\hbox {NO}_{2}, \hbox {CCl}_{2}\hbox {F}_{2}$$ , $$\hbox {SO}_{2}$$ , $$\hbox {CF}_{4}$$ , $$\hbox {H}_{2}$$ ) on pure and aluminium-doped graphene were investigated by using the density functional theory to explore their potential applications as sensors. It has been found that all gas molecules show much stronger adsorption on the Al-doped graphene than that of pure graphene (PG). The Al-doped graphene shows the highest adsorption energy with $$\hbox {NO}_{2}$$ , $$\hbox {NH}_{3}$$ and $$\hbox {CO}_{2}$$ molecules, whereas the PG binds strongly with $$\hbox {NO}_{2}$$ . Therefore, the strong interactions between the adsorbed gas molecules and the Al-doped graphene induce dramatic changes to graphene’s electronic properties. These results reveal that the sensitivity of graphene-based gas sensor could be drastically improved by introducing the appropriate dopant or defect. It also carried out the highest occupied molecular orbital–lowest unoccupied molecular orbital energy gap of the complex molecular structure that has been explored by M06/6-31++G** method. These results indicate that the energy gap fine tuning of the pure and Al-doped graphene can be affected through the binding of small gas molecules.

First-principles study on silicon atom doped monolayer graphene

This paper illustrates the structural, electronic and optical properties of individual silicon (Si) atom-doped single layer graphene using density functional theory method. Si atom forms tight bonding with graphene layer. The effect of doping has been investigated by varying the concentration of Si atoms from 3.125% to 9.37% (i.e. From one to three Si atoms in 4 × 4 pure graphene supercell containing 32 carbon atoms), respectively. Electronic structure, partial density of states (PDOS) and optical properties of pure and Si atom-doped graphene sheet were calculated using VASP (Vienna ab-initio Simulation Package). The calculated results for pure graphene sheet were then compared with Si atom doped graphene. It is revealed that upon Si doping in graphene, a finite band gap appears at the high symmetric K-point, thereby making graphene a direct band gap semiconductor. Moreover, the band gap value is directly proportional to the concentration of impurity Si atoms present in graphene lattice. Upon analyzing the optical properties of Si atom-doped graphene structures, it is found that, there is significant change in the refractive index of the graphene after Si atom substitution in graphene. In addition, the overall absorption spectrum of graphene is decreased after Si atom doping. Although a significant red shift in absorption is found to occur towards visible range of radiation when Si atom is substituted in its lattice. The reflectivity of graphene improves in low energy region after Si atom substitution in graphene. These results can be useful for tuning the electronic structure and to manipulate the optical properties of graphene layer in the visible region.

Blue and red shifted temperature dependence of implicit phonon shifts in graphene

We have calculated the implicit shift for various modes of frequency in a pure graphene sheet. Thermal expansion and Grüneisen parameter which are required for implicit shift calculation have already been studied and reported. For this calculation, phonon frequencies are obtained using force constants derived from dynamical matrix calculated using VASP code where the density functional perturbation theory (DFPT) is used in interface with phonopy software. The implicit phonon shift shows an unusual behavior as compared to the bulk materials. The frequency shift is large negative (red shift) for ZA and ZO modes and the value of negative shift increases with increase in temperature. On the other hand, blue shift arises for all other longitudinal and transverse modes with a similar trend of increase with increase in temperature. The q dependence of phonon shifts has also been studied. Such simultaneous red and blue shifts in transverse or out plane modes and surface modes, respectively leads to speculation of surface softening in out of plane direction in preference to surface melting.

First-principles study on hydrogen adsorption on nitrogen doped graphene

In this paper we have investigated the adsorption of Hydrogen on Nitrogen doped graphene in detail by means of first-principles calculations. A comprehensive study is performed of the structural, electronic and optical properties of hydrogen atoms adsorbed on dopant atoms sites and on carbon atoms neighboring dopant atoms. The effect of doping has been investigated by varying the concentration of doping atoms from 3.125%( one atom of nitrogen in 32 host atoms) to 6.25% ( two nitrogen atoms in 32 host atoms). Similarly the effect of adsorption has been investigated by varying the concentration of hydrogen atoms and also varying the adsorption sites. Band structure, partial density of states (PDOS) and optical properties of pure, nitrogen doped and hydrogen adsorbed graphene sheet were calculated using VASP (Vienna ab-initio Simulation Package). The calculated results for pure graphene sheet were then compared with nitrogen doped graphene and Hydrogen adsorbed graphene sheet. It is found that upon nitrogen doping the Dirac point in the graphene band structure shifts below the Fermi Energy level and energy gap appears at the high symmetric K-point. On the other hand, by adsorption of Hydrogen atom, there is further change in the band structure near the Fermi level and also the energy gap at the high symmetric K-point is increased. There is change in the dielectric function and refractive index of the graphene after H atoms adsorption on N-doped graphene. The overall absorption spectra is decreased in case of nitrogen doping and after adsorption process of Hydrogen atoms. However a significant red shift in absorption towards visible range of radiation is found to occur for hydrogen atoms adsorbed on nitrogen doped graphene sheet. The reflectivity peak of graphene increases in low energy region after H adsorption on N-doped graphene. The results can be used to tune the Fermi Energy level and to tailor the optical properties of graphene sheet in visible region.

First principle calculations of thermodynamic properties of pure graphene sheet and graphene sheets with Si, Ge, Fe, and Co impurities

The thermal properties of pure graphene and graphene–impurity (impurity = Fe, Co, Si, and Ge) sheets have been investigated at various pressures (0–7 GPa) and temperatures (0–900 K). Some basic thermodynamic quantities such as bulk modulus, coefficient of volume thermal expansion, heat capacities at constant pressure and constant volume of these sheets as a function of temperature and pressure are discussed. Furthermore, the effect of the impurity density and tensile strain on the thermodynamic properties of these sheets are investigated. All of these calculations are performed based on the density functional theory and full quasi harmonic approximation.

Effect of Fe, Co, Si and Ge impurities on optical properties of graphene sheet

The electronic and linear optical properties of pure graphene and impurity-graphene (with Fe, Co, Si and Ge impurities) sheets are investigated by using the full potential linear augmented plane wave plus local orbital (FPLAPW+lo) in the framework of the density functional theory (DFT). The calculated results are obtained within the generalized gradient approximation using the Perdew–Burke–Ernzerhof scheme in the presence of spin-orbit interaction. The band structure, partial electron density of states, dielectric function, absorption coefficient, optical conductivity, extinction index, energy loss function, reflectivity and the refraction index of these sheets for parallel and perpendicular electromagnetic wave polarization to sheet are investigated. The optical conductivity of Si-graphene and Ge-graphene sheets for the parallel electromagnetic wave polarization to the sheet starts with a gap about 0.4eV confirms that these sheets have semiconductor behavior. Also the optical spectra of these sheets are anisotropic along these two wave polarizations. The dielectric function in the static limit of pure graphene sheet for perpendicular electromagnetic wave polarization to sheet does not significant change in the presence of Si, Ge, Fe and Co impurities. The static refractive index of Fe-graphene and Co-graphene sheets for parallel electromagnetic wave polarization to sheet is much larger than the corresponding value of pure graphene sheet.

The Electronic and Magnetic Properties and the Topological Phase Graphene Sheet with Fe, Co, Si, and Ge Impurities

The structural, electronic, and magnetic properties of pure graphene sheet and graphene sheet with Fe, Co, Si, and Ge impurities are investigated. The calculated results are done within density functional theory in the presence of spin-orbit coupling using the generalized gradient approximation. Electron density of states, band order, electron charge distribution, magnetic moment of these sheets, and the effect of pressure on the band order of graphene sheet with Fe impurity are investigated.

Thermodynamic properties of pure and doped (B, N) graphene

Ab initio density functional perturbation theory (DFPT) has been employed to study the thermodynamical properties of pure and doped graphene sheet and the results have been compared with available theoretical and experimental data.