Defect-free Graphene Sheets Research Articles

Modeling of Inert Gas Sensors Using First Principles Methods

The detection of inert gases is difficult due to their inactive nature what makes preparation of the applicable gas sensor a challenging task. This work reports comprehensive first principles investigations to design inert gas sensors. Graphene (Gr) sheets decorated with palladium clusters Pd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> (n = 2-6) were optimized after which adsorption of inert gases He, Ne, Ar, Kr, Xe and Rn was carried out using density functional theory (DFT) based formalism. The reactivity of the sensors comprising of Pd cluster-decorated-defected-graphene is significantly higher than that of the sensor with bare and defect free graphene sheet. The adsorption caused redistribution of the electronic structure which provides basis for sensing of the adsorbate. The gas sensor Pd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -Gr exhibited good sensitivity towards neon and xenon while Pd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -Gr appeared more effective in the detection of krypton gas. Helium is appropriately detected by Pd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -Gr sensor and Pd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> -Gr sensor is found capable of sensing radon and argon gases. Pd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> -Gr sensor is not a favorable sensor for sensing of the inert gases. The findings of this work are beneficial for fabrication of inert gas sensors for small and industrial scale applications.

Adsorption of yttrium bisphthalocyanine on pristine and defect-contaning graphene models: A DFT study

We studied the noncovalent interactions of yttrium bisphthalocyanine YPc2 with graphene sheet models with and without topological defects. The density functional theory results obtained for the cluster models containing Stone-Wales defect (SW), isolated pentagon (5) and pyracylene unit (5665) were compared to those for defect-free graphene sheet (G). The optimized geometry for adsorption complexes, bonding strength and some electronic parameters were analyzed. In all cases YPc2 molecule suffers considerable distortion, whose direction and degree depend on a particular model and site of adsorption (‘exo’ and ‘endo’, in the case of conical 5 and 5665 models); also, it is more pronounced for Pc ligand directly contacting graphene surface. In the case of 5_exo and 5665_exo complexes, the bending follows the direction of already existing curvature of Pc ligand, whereas for 5_endo and 5665_endo its direction changed. For G and SW models, the changes in curvature result in almost flat geometry of contacting Pc. The interactions are strong for all the noncovalent complexes analyzed, increasing in the following order: 5665_endo <5665_exo < 5_exo < 5_endo < G < SW. The plots of HOMO, LUMO and spin density are analyzed, as well as the changes in HOMO-LUMO gap energy.

Preparation of ultrathin defect-free graphene sheets from graphite via fluidic delamination for solid-contact ion-to-electron transducers in potentiometric sensors

In this study, ultrathin and defect-free graphene (Gr) sheets were prepared through a fluid dynamics–induced shear exfoliation method using graphite. The high hydrophobicity and surface area of Gr make it attractive as a solid-contact ion-to-electron transducer for potentiometric K+ sensors, in which the electrodes are fabricated through a screen-printing process. The electrochemical characterization demonstrates that Gr solid contact results in a high double-layer capacitance, potential stability, and strong resistance against water layer, gases, and light. The Gr-based K+ sensors showed a Nernstian slope of 53.53 mV/log[K+] within a linear concentration range of 10−1–10−4 M, a low detection limit of 10−4.28 M, a fast response time of ~8 s, good repeatability, and excellent long-term stability. Moreover, the Gr-based K+ sensors provided accurate ion concentrations in actual samples of human sweat and sports drinks.

High-Throughput Production of Heterogeneous RuO2/Graphene Catalyst in a Hydrodynamic Reactor for Selective Alcohol Oxidation

We report on the high-throughput production of heterogeneous catalysts of RuO2-deposited graphene using a hydrodynamic process for selective alcohol oxidation. The fluid mechanics of a hydrodynamic process based on a Taylor–Couette flow provide a high shear stress field and fast mixing process. The unique fluidic behavior efficiently exfoliates graphite into defect-free graphene sheets dispersed in water solution, in which ionic liquid is used as the stabilizing reagent to prevent the restacking of the graphene sheets. The deposition of RuO2 on a graphene surface is performed using a hydrodynamic process, resulting in the uniform coating of RuO2 nanoparticles. The as synthesized RuO2/IL–graphene catalyst has been applied efficiently for the oxidation of a wide variety of alcohol substrates, including biomass-derived 5-hydroxymethylfurfural (HMF) under environmentally benign conditions. The catalyst is mechanically stable and recyclable, confirming its heterogeneous nature.

Direct transfer of the CVD-grown graphene on copper foils on SiO2 substrate under supercritical CO2 assisted-cleaning technique

The transfer of CVD-grown graphene sheets onto arbitrary substrates is important for the development of practical applications. Unfortunately, designing a low cost and highly efficient graphene transfer technique to achieve defect-free graphene sheets with low contact resistance onto various substrates still remains a challenge. In this paper, a CVD grown monolayer graphene sheet was directly transferred on SiO2/Si substrate. We found that a combination of floating copper with graphene films on ammonium persulfate solution with an original method of supercritical CO2 fluid can effectively produce clean and dry samples without damaging the crystalline quality of graphene. This method does not require any polymeric material to be desposited on the graphene films at any stage. Samples are analyzed by optical microscopy, Raman spectroscopy, scanning electron microscopy and atomic force microscopy. This method is very promising for cleaning graphene samples for electronic device fabrication.

Graphene–Solvent Interactions in Nonaqueous Dispersions: 2D ROESY NMR Measurements and Molecular Dynamics Simulations

The liquid phase exfoliation of graphite by sonication in nonaqueous solvents like N-methyl-2-pyrrolidone (NMP) provides a simple and scalable route to dispersions of defect-free graphene sheets. The role of the solvent is crucial to the process; it is the interactions of the solvent with the graphene sheets that prevent agglomeration and stabilize the dispersion. Here, we show that the two-dimensional solution nuclear magnetic resonance (NMR) technique, rotating frame Overhauser effect spectroscopy (ROESY), provides a molecular signature of these interactions in graphene–NMP dispersions. Significant differences are observed in the spectra of the dispersions as compared to the pure solvent. Using classical molecular dynamics simulations, we show that these differences arise because of the induced layering of solvent molecules with reduced rotational mobility in the vicinity of the graphene sheets. The reduced mobility of solvent molecules in the dispersion as compared to the bulk solvent are reflected as differences in their two-dimensional ROESY NMR.

Carbon Precursor Structures and Graphene on Palladium Nanoparticles

Due to their stable catalytic properties graphene-encapsulated metal nanoparticles (NPs) have come into the focus of heterogeneous catalysis. Research on graphene’s atomic structure is a prerequisite for understanding and controlling catalytic processes; however, it is still at its very beginning considering metal NPs. This atomic-scale scanning tunnelling microscopy study documents preliminary growth stages and the structural properties of graphene on highly oriented pyrolytic graphite (HOPG) supported palladium NPs. It is shown that an annealing of the NPs at ∼650 °C in a few tens of Langmuir of ethylene (ethene) leads first to a carbon precursor structure covering the NP’s (111) facets. At higher ethylene exposures, the precursor structure turns into single-layer graphene encapsulating the NPs. Graphene-encapsulated palladium NPs with a size larger than ∼30 nm exhibit mostly one single and an almost defect-free graphene sheet on a facet. In most of the cases, such a perfect growth does not take place o...

Facile and Scalable Synthesis Method for High-Quality Few-Layer Graphene through Solution-Based Exfoliation of Graphite

Here we describe a facile and scalable method for preparing defect-free graphene sheets exfoliated from graphite using the positively charged polyelectrolyte precursor poly(p-phenylenevinylene) (PPV-pre) as a stabilizer in an aqueous solution. The graphene exfoliated by PPV-pre was apparently stabilized in the solution as a form of graphene/PPV-pre (denoted to GPPV-pre), which remains in a homogeneous dispersion over a year. The thickness values of 300 selected 76% GPPV-pre flakes ranged from 1 to 10 nm, corresponding to between one and a few layers of graphene in the lateral dimensions of 1 to 2 μm. Furthermore, this approach was expected to yield a marked decrease in the density of defects in the electronic conjugation of graphene compared to that of graphene oxide (GO) obtained by Hummers' method. The positively charged GPPV-pre was employed to fabricate a poly(ethylene terephthalate) (PET) electrode layer-by-layer with negatively charged GO, yielding (GPPV-pre/GO)n film electrode. The PPV-pre and GO in the (GPPV-pre/GO)n films were simultaneously converted using hydroiodic acid vapor to fully conjugated PPV and reduced graphene oxide (RGO), respectively. The electrical conductivity of (GPPV/RGO)23 multilayer films was 483 S/cm, about three times greater than that of the (PPV/RGO)23 multilayer films (166 S/cm) comprising RGO (prepared by Hummers method). Furthermore, the superior electrical properties of GPPV were made evident, when comparing the capacitive performances of two supercapacitor systems; (polyaniline PANi/RGO)30/(GPPV/RGO)23/PET (volumetric capacitance = 216 F/cm3; energy density = 19 mWh/cm3; maximum power density = 498 W/cm3) and (PANi/RGO)30/(PPV/RGO)23/PET (152 F/cm3; 9 mWh/cm3; 80 W/cm3).

A comprehensive lattice-stability limit surface for graphene

The limits of reversible deformation in graphene under various loadings are examined using lattice-dynamical stability analysis. This information is then used to construct a comprehensive lattice-stability limit surface for graphene, which provides an analytical description of incipient lattice instabilities of all kinds, for arbitrary deformations, parametrized in terms of symmetry-invariants of strain/stress. Symmetry-invariants allow obtaining an accurate parametrization with a minimal number of coefficients. Based on this limit surface, we deduce a general continuum criterion for the onset of all kinds of lattice-stabilities in graphene: an instability appears when the magnitude of the deviatoric strain γ reaches a critical value γc which depends upon the mean normal strain E¯ and the directionality θ of the principal deviatoric stretch with respect to reference lattice orientation. We also distinguish between the distinct regions of the limit surface that correspond to fundamentally different mechanisms of lattice instabilities in graphene, such as structural versus material instabilities, and long-wave (elastic) versus short-wave instabilities. Utility of this limit surface is demonstrated in assessment of incipient failures in defect-free graphene via its implementation in a continuum finite elements analysis (FEA). The resulting scheme enables on-the-fly assessments of not only the macroscopic conditions (e.g., load and deflection) but also the microscopic conditions (e.g., local stress/strain, spatial location, temporal proximity, and nature of incipient lattice instability) at which an instability occurs in a defect-free graphene sheet subjected to an arbitrary loading condition.

Oligothiophene-Graphene Ensembles

The rise of graphite exfoliation pumping up graphene’s chemistry brings significant impact towards the preparation of novel hybrid materials combining the exceptional mechanical and electronic properties of graphene sheets with those of photoinduced charge transfer from organic chromophores interacting with the carbon nanostructure. Applications including charge transfer processes demand both defect-free graphene sheets and robust chemical procedures for the chemical or supramolecular grafting of the desired organic molecules onto the sp2-honeycomb lattice of graphene. Covalent chemistry of graphene leads to strong interactions between the organic material and the graphitic skeleton, however, introduces defects at the anchoring sites, thus acting as insulators. On the other hand, when supramolecular interactions are utilized to integrate organic moieties to graphene, the resulted hybrids although may suffer from the weaker π-π interactions and the subsequent release part of the organic units to the solution, the extended aromatic lattice of graphene sheets remains intact and undisrupted, as no bond formation takes place, thus allowing for ballistic transport of charges with negligible loss of energy. Oligothiophenes in general and terthiophenes in particular, are readily accessible organic chromophores with interesting electronic properties. The stiff skeleton of oligothiophenes and the presence of the sulfur atom generate a stable π-conjugated chain, which can interact through multiple π-π stacking forces with the sp2 graphene lattice. In this frame, the ease of oligothiophenes chemical manipulation allows the preparation of oligothiophene-graphene ensembles showing enhanced solubility in organic solvents. The latter is justified due to the insertion of alkyl groups at 3' and 4' position of the thiophenes ring. Herein, the preparation of oligothiophene-graphene ensembles will be presented, followed by their full characterization through Raman and IR spectroscopy, TEM imaging and TGA analysis. Notably, our first data from the newly formed materials, based on electronic absorption, photoluminescence and electrochemistry, indicate the interaction of the organic units and the carbon allotrope under light irradiation. Partial financial support from GSRT/NSRF 2007-2013 through action “Postdoctoral support” project GRAPHCELL PE5(2126) is acknowledged.

Advanced phase change composite by thermally annealed defect-free graphene for thermal energy storage.

Organic phase change materials (PCMs) have been utilized as latent heat energy storage and release media for effective thermal management. A major challenge exists for organic PCMs in which their low thermal conductivity leads to a slow transient temperature response and reduced heat transfer efficiency. In this work, 2D thermally annealed defect-free graphene sheets (GSs) can be obtained upon high temperature annealing in removing defects and oxygen functional groups. As a result of greatly reduced phonon scattering centers for thermal transport, the incorporation of ultralight weight and defect free graphene applied as nanoscale additives into a phase change composite (PCC) drastically improve thermal conductivity and meanwhile minimize the reduction of heat of fusion. A high thermal conductivity of the defect-free graphene-PCC can be achieved up to 3.55 W/(m K) at a 10 wt % graphene loading. This represents an enhancement of over 600% as compared to pristine graphene-PCC without annealing at a comparable loading, and a 16-fold enhancement than the pure PCM (1-octadecanol). The defect-free graphene-PCC displays rapid temperature response and superior heat transfer capability as compared to the pristine graphene-PCC or pure PCM, enabling transformational thermal energy storage and management.

Fabrication of oxide-free graphene suspension and transparent thin films using amide solvent and thermal treatment

High quality graphene sheets were produced from graphite by liquid phase exfoliation using N-methyl-2-pyrrolidone (NMP) and a subsequent thermal treatment to enhance the exfoliation. The exfoliation was enhanced by treatment with organic solvent and high thermal expansion producing high yields of the high-quality and defect-free graphene sheets. The graphene was successfully deposited on a flexible and transparent polymer film using the vacuum filtration method. SEM images of thin films of graphene treated at 800°C showed uniform structure with no defects commonly found in films made of graphene produced by other techniques. Thin films of graphene prepared at higher temperatures showed superior transmittance and conductivity. The sheet-resistance of the graphene film treated at 800°C was 2.8×103kΩ/□ with 80% transmittance.

Computational studies of graphene growth mechanisms

Density functional theory (DFT) and semiempirical tight-binding (TB) methods have been used to study the mechanism of graphene growth in the presence and absence of a catalytic surface. Both DFT and TB geometry optimized structures relevant to graphene growth show that the minimum energy growth mechanism is via the sequential addition of carbon hexagons at the edge of the graphene sheet. Monte Carlo (MC) simulations based on the TB model show that defect-free graphene sheets can be grown provided one has the proper combination of temperature, chemical potential, and addition rate. In this work, growth of perfect graphene structures has been simulated at the atomic level. Comparison of the growth mechanism in the absence and presence of a nickel catalyst surface shows that the catalyst (i) allows for adsorption of carbon atoms at surface and subsurface sites, (ii) enables formation of long, stable strings of carbon atoms, and (iii) stabilizes small flakes of graphene that can act as precursors to subsequent growth.

Interaction between graphene and nickel(1 1 1) surfaces with commensurate and incommensurate orientational relationships

The interaction energy between a rigid defect-free graphene sheet and a fixed Ni(111) surface with respect to rotational and translational displacements is examined using an empirical bond order potential. The Rosei (hollow site) structure, and others in which metal atoms are adjacent to hollow sites in graphene, give rise to global and local energy minima, respectively, whereas the interaction energy is almost constant for incommensurate structures. Only a few degrees rotation from the Rosei structure substantially diminishes the energy barrier for translational displacement.

Defect-free graphene metal oxide composites: formed by lithium mediated exfoliation of graphite

In this work, defect-free graphene–metal oxide nanocomposites have been synthesized via direct reaction of Li–graphite alloy with an alcohol-based metal precursor solution. The intercalated Li atoms, within the graphitic galleries, are responsible for the reduction of the alcohol, which results in hydrogen evolution for direct exfoliation of defect-free graphene from the graphite. Meanwhile, the intercalated Li atoms serve as a strong reductant as well for reduction of metal ions, and act in the subsequent deposition of metal/metal oxide nanoparticles on the graphene layers. The resulting defect-free graphene sheets, with uniformly distributed nano-sized metal oxide particles on their surfaces, have excellent physical and electrochemical properties due to the synergistic effect between the active nanoparticles and the graphene sheets.

Interaction between two graphene sheets with a turbostratic orientational relationship

The interaction of two rigid defect-free graphene sheets with various turbostratic orientational relationships is examined using a Lennard-Jones potential. When one layer is rotated from the AB stacking order, maxima and minima in the potential energy occur at angles corresponding to AA and AB stacking. The energy gap between AA and AB stacking is estimated to be much smaller than the average thermal energy at room temperature. Turbostratic orientations diminish the energy for translational displacement to zero. The presence of defects is required to explain non-zero shear strengths in pure carbon nanotube fibres with no commensurate orientation of graphene sheets.

Greatly enhanced adsorption and catalytic activity of Au and Pt clusters on defective graphene

We report an investigation on CO oxidation catalyzed by Au(8) or Pt(4) clusters on defective graphene using first-principles approach based on density functional theory. The simplest single-carbon-vacancy defect on graphene was found to play an essential role in the catalyzed chemical reaction of CO oxidation. When supported on a defect-free graphene sheet, the reaction barrier of CO oxidation catalyzed by Au(8) (Pt(4)) clusters was estimated to be around 3.0 eV (0.5 eV), and when adsorbed on defective graphene, the reaction barrier was greatly reduced to around 0.2 eV (0.13 eV).

Elastic Properties of C andBxCyNzComposite Nanotubes

We present a comparative study of the energetic, structural, and elastic properties of carbon and composite single-wall nanotubes, including BN, ${\mathrm{BC}}_{3}$, and ${\mathrm{BC}}_{2}\mathrm{N}$ nanotubes, using a nonorthogonal tight-binding formalism. Our calculations predict that carbon nanotubes have a higher Young modulus than any of the studied composite nanotubes, and of the same order as that found for defect-free graphene sheets. We obtain good agreement with the available experimental results.