Stacked Graphene Sheets Research Articles

Highly conductive graphite nanoparticle based enzyme biosensor for electrochemical glucose detection

Graphite nanoparticle (GN), consisting of several stacked graphene sheets, shows homogenous spherical shape, high conductivity, and large surface area for biomolecular conjugation. We utilized these unique characteristics of GN for constructing an enzyme biosensor to detect glucose in real samples. To immobilize the glucose oxidase (GOx) on the GN, the surface of the GN was first modified by 1-pyrenebutyric acid N-hydroxysuccinimide ester (Py-NHS ester) by π–π stacking interaction between the pyrene moiety and the GN to produce the GN-Py-NHS ester conjugates, and then the GOx was linked with GN-Py-NHS ester through amide bond. The resultant GN-Py-GOx composite was mixed with a Nafion solution and deposited on the glassy carbon electrode as a thin film, and used for direct electron transfer-based glucose sensing. Amperometric response linearly increased in proportional to the glucose concentration up to 2.2mM, and the detection limit was measured as 50μM with sensitivity of 7.29×10−2nAmM−1cm−2. The spiked glucose in a real urine sample was also successfully analyzed, and the GN-Py-GOx biosensor performance was maintained for one month without loss of activity and sensitivity.

Graphene-supported iron-based nanoparticles encapsulated in nitrogen-doped carbon as a synergistic catalyst for hydrogen evolution and oxygen reduction reactions.

Electrolyzers and fuel cells have been extensively investigated as promising solutions for renewable energy storage and conversion. Hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are important electrocatalytic processes in electrolyzers and fuel cells. Exploring efficient non-precious metal catalysts for HER and ORR in acidic medium remains a great challenge. Herein, we report that graphene-supported iron-based nanoparticles encapsulated in a nitrogen-doped carbon (Fe@N-C) hybrid material acts as an efficient HER and ORR catalyst. The hybrid material was synthesized by pyrolysis of graphene oxide and ammonia ferric citrate followed by acid-leaching. During the pyrolysis, nitrogen was doped into a graphene lattice, and the carbon nanoshell grown on graphene effectively suppressed the stacking of graphene sheets, exposing more active sites to reactants. The hybrid material showed higher electrocatalytic activities than graphene sheets or Fe@N-C alone, which is probably attributed to the synergetic role of nitrogen-doped graphene and Fe@N-C towards the electrocatalytic reactions.

Nano-scale morphological analysis of graphene–rubber composites using 3D transmission electron microscopy

In this work three-dimensional transmission electron microscopy (3D-TEM) is exploited to characterize a soft graphene based nano-composites structure and the constituted morphology in a qualitative way. The reconstruction of the two dimensional slides into a three dimensional tomographic image is a powerful tool, when the images of the nano-object are reflected into a quasi-distinguishable object due to superposition effect. By using this technique it is possible to mark the contour area of the nano-sized object inside the soft rubber matrix. To extract information about the filler network, the clustering process of the fillers or the existence of single or multiple graphene sheets, a solution polymerised styrene butadiene rubber was selected as a soft matrix which was filled with carbon black (CB) and graphene nano-platelets (GnP). The dispersion/exfoliation of the stacked graphene sheets into individual single sheets was facilitated by the presence of carbon black in the system as understood from TEM, X-ray diffraction and Raman spectroscopic studies. The existence of oligo-layer graphene sheets was detected by this 3D-TEM, especially when the rubber matrix exists in a complex morphology arisen from filler–filler networks in all spatial dimensions.

Graphene thickness-controlled photocatalysis and surface enhanced Raman scattering.

Exceptional photocatalytic enhancement of graphene-semiconductor composites has been widely reported, but our understanding of the role that graphene plays in this enhancement remains limited, which arises from the difficulty of precisely controlling graphene hybridization. Here we present a general platform of a graphene-semiconductor hybrid panel (GHP) system wherein a precise number of layers of graphene are hybridized with photoactive semiconductors (e.g. TiO2, ZnO) to study systematically how graphene affects the photocatalysis. The results show that the graphene enhancement of the photocatalysis depends on the number of graphene layers, with the maximum performance observed at 3 layers. Photodeposited indicators of gold particles further reveal that graphene thickness governs the density of photocatalytic sites and charge transfer efficiency at the graphene-semiconductor interfaces. We suggest that quantized energy levels caused by different numbers of stacked graphene sheets along the vector normal to the graphene basal plane affect the charge transfer routes and lead to the graphene thickness-controlled photocatalysis. GHP substrates deposited with gold particles are promising, uniform substrates for surface enhanced Raman scattering (SERS) applications with the enhancement factor as high as ∼10(8) on 3-layer graphene.

Dissipative plasmon‐solitons in multilayer graphene

AbstractNonlinear properties of a multi‐layer stack of graphene sheets are studied. It is predicted that such a structure may support dissipative plasmon‐solitons generated and supported by an external laser radiation. Novel nonlinear equations describing spatial dynamics of the nonlinear plasmons driven by a plane wave in the Otto configuration are derived and the existence of single and multi‐hump dissipative solitons in the graphene structure is predicted.

Unusual reflection of electromagnetic radiation from a stack of graphene layers at oblique incidence

We study the interaction of electromagnetic (EM) radiation with single-layer graphene and a stack of parallel graphene sheets at arbitrary angles of incidence. It is found that the behavior is qualitatively different for transverse magnetic (or p-polarized) and transverse electric (or s-polarized) waves. In particular, the absorbance of single-layer graphene attains a minimum (maximum) for the p (s)-polarization at the angle of total internal reflection when the light comes from a medium with a higher dielectric constant. In the case of equal dielectric constants of the media above and beneath graphene, for grazing incidence graphene is almost 100% transparent to p-polarized waves and acts as a tunable mirror for the s-polarization. These effects are enhanced for a stack of graphene sheets, so the system can work as a broad band polarizer. It is shown further that a periodic stack of graphene layers has the properties of a one-dimensional photonic crystal, with gaps (or stop bands) at certain frequencies. When an incident EM wave is reflected from this photonic crystal, the tunability of the graphene conductivity renders the possibility of controlling the gaps, and the structure can operate as a tunable spectral-selective mirror.

High performance field emission properties of graphite nanoplatelet field emitters

Graphite nanoplatelet (GNP) that consists of several tens of layers of graphene sheets is a promising candidate for electron field emission. The GNP emitter shows good field emission properties with a high emission current and a robust long-term stability because of the sharp edges, the high aspect ratio, and the stacked graphene sheets. Most of the electrons are emitted from the sharp edges of GNPs. The GNP emitters fabricated by the screen printing method reveal the turn-on electric field of 1.77 V/μm, the threshold electric field of 4.47 V/μm, the maximum emission current density of 39 mA/cm2, and the good emission stability for 20 h with little degradation rate of the emission current. The GNP field emitters can be a good candidate for several field emission applications such as flat lamps, field emission displays, and x-ray sources.

Highly Disordered Carbon as a Superior Anode Material for Room‐Temperature Sodium‐Ion Batteries

Highly Disordered Carbon as a Superior Anode Material for Room‐Temperature Sodium‐Ion Batteries

A stable and active iron catalyst supported on graphene nano-flakes for the oxygen reduction reaction in polymer electrolyte membrane fuel cells

Two of the major challenges for the commercialization of automotive fuel cells are cost and durability. Cost estimations indicate that at a large production rate, the majority of the stack component cost comes from the catalyst ink. Decreasing this cost depends on finding a durable cost-effective platinum-free electrocatalyst. In this work, pure graphene nano-flake (GNFs) powders are produced by plasma decomposition. The GNFs are composed of 5–20 layers of stacked graphene sheets, a structure that appears to be closely associated to catalyst stability and durability. Additionally, the large number of attachment sites for nitrogen and atomic iron functionalization provide an avenue for improving activity. The GNFs were nitrogen functionalized and then used to support atomic iron to create the active sites for a non-noble catalyst. Iron was successfully incorporated at a value of 0.28at% on the surface of the catalyst structure. The catalyst was used on the cathode side of a polymer electrolyte membrane fuel cell (PEMFC) and showed stability over 100h. The performance of the catalyst demonstrates, to our knowledge, the first proof of a stable strictly iron-based fuel cell catalyst.

Effect of graphene aerogel on thermal behavior of phase change materials for thermal management

A phase change material consisting of three-dimensional graphene aerogel (GA) and octadecanoic acid (OA) was produced. The GA was assembled from the sheets of graphene oxide in a hydrothermal reaction. The pore sizes of the network were several micrometers and the pore walls consisted of thin layers of stacked graphene sheets. OA was impregnated into GA by capillary forces, with the GA acting as the support. The GA/OA composite had a thermal conductivity about 2.635W/mK at a GA loading fraction of ∼20vol%, which was about 14 times that of the OA (0.184W/mK). The transient heating and cooling responses of the material were investigated for thermal energy storage. The GA had a low bulk density so that the weight percent of the GA in the composite was only about 15%. The composite presents a high heat storage capacity of 181.8J/g, which was very close to the value of the OA alone (186.1J/g).

Towards a highly-efficient fuel-cell catalyst: optimization of Pt particle size, supports and surface-oxygen group concentration

In the present work, methanol oxidation reaction was investigated on Pt particles of various diameters on carbon-nanofibers and carbon-black supports with different surface-oxygen concentrations, aiming for a better understanding of the relationship between the catalyst properties and the electrochemical performance. The pre-synthesized Pt nanoparticles in ethylene glycol, prepared by the polyol method without using any capping agents, were deposited on different carbon supports. Removal of oxygen-groups from the carbon supports had profound positive effects on not only the Pt dispersion but also the specific activity. The edge structures on the stacked graphene sheets in the platelet carbon-nanofibers provided a strong interaction with the Pt particles, significantly reconstructing them in the process. Such reconstruction resulted in the formation of more plated Pt particles on the CNF than on the carbon-black and exposure of more Pt atoms with relatively high co-ordination numbers, and thereby higher specific activity. Owing to the combined advantages of optimum Pt particle diameter, an oxygen-free surface and the unique properties of CNFs, Pt supported on heat-treated CNFs exhibited a higher mass activity twice of that of its commercial counterpart.

Rapid synthesis of free-standing MoO3/Graphene films by the microwave hydrothermal method as cathode for bendable lithium batteries

Highly flexible, binder-free, MoO3 nanobelt/graphene film electrode is prepared by a two-step microwave hydrothermal method. Graphene is first prepared by an ultra-fast microwave hydrothermal method and then mixed with MoO3 solution to synthesize the MoO3 nanobelt/graphene composite, which exhibits the combination of stacked graphene sheets and uniform MoO3 nanobelts with widths of 200–500 nm and lengths of 5–10 μm. The charge–discharge measurements show that the as-synthesized MoO3/graphene hybrid materials demonstrate excellent rate capability, large capacity, and good cycling stability compared to the pure MoO3 film. An initial discharge capacity of 291mAhg−1 can be obtained at 100 mA g−1, with a capacity of 172mAhg−1 retained after 100 cycles. The results show that the MoO3/graphene designed in this study can be used as a free-standing cathode material in rechargeable bendable Lithium batteries.

Role of deoxy group on the high concentration of graphene in surfactant/water media

Graphene nanosheets have been successfully dispersed via surfactant-assisted exfoliation of graphite using sodium cholate (SC) and sodium deoxycholate (SDC) surfactants in aqueous media. The concentration of SC and SDC surfactants and saturated graphite concentration have been well optimized to highly disperse graphene in water. The concentration of dispersed graphene in water has been estimated to be 0.52 and 2.58 mg mL−1 for SC and SDC respectively. The absence of an oxygen molecule at the centre aromatic ring of SDC offers a flat and hydrophobic surface that helps to form uniform bilayer micelles on graphitic surfaces and facilitates efficient exfoliation of graphene in a short duration of sonication. The discrepancy in the electrostatic interaction of SC-G over SDC-G dispersions and its re-aggregation stability were compared by zeta potential measurements. The thermal analysis of surfactant covered graphene powders exhibit weak bonding of surfactants with graphitic surfaces through electrostatic interactions. FESEM and AFM images demonstrate the successful exfoliation of few layer graphene with uniform dispersion. Crystalline quality and the stacking of graphene sheets in SC-G and SDC-G dispersions were analyzed using micro-Raman scattering and photoluminescence spectra.

Nanostructured arrays of stacked graphene sheets

Molecular oxygen etching of HOPG surfaces prepatterned by Ga+ focused-ion-beam irradiation (FIB) has been used to generate large-area arrays of nanometer-sized graphite blocks. AFM and SEM imaging show that structures with lateral sizes down to ∼100 nm and heights of between 30 and 55 nm can be routinely fabricated. The trenches separating the graphite blocks form in the early oxidation stages via preferential gasification (into CO and CO2) of the gridlike amorphized carbon regions written by FIB. In the later oxidative etching stages, gasification of the graphite nanoprism faces laterally terminating the graphite blocks becomes the major reaction channel. Correspondingly, graphite blocks are (further) reduced in lateral extent while the trenches in between are widened. Raman and photoionization spectroscopies indicate that the quality of the topmost nG sheet(s) covering the blocks also decreases with increasing etching time—as the size and lateral density of defect-mediated etch pits increases. nG block arrays are useful substrates with which to probe the size-dependent properties of nanographene, as they comprise large numbers of uniform sheets (ca. 4 × 1010 cm−2 for an array of 0.5 × 0.5 μm2) thus allowing for the application of area-integrating spectroscopic methods. We demonstrate this by examining the Raman features of nG block arrays which include a graphene-rim-region fingerprint mode. Individual nG sheets can be exfoliated from nG stacks by means of electron-irradiation-induced charging. We have explored a number of printing/manipulation strategies aimed at controllable electromechanical transfer of nG sheet arrays to silicon wafers.

Graphene modified Li3V2(PO4)3 as a high-performance cathode material for lithium ion batteries

In this paper, graphene-modified Li3V2(PO4)3 composites have been developed as Li-ion battery cathode materials with high specific capacity and excellent cycling stability. The composites are prepared by spray–drying process using graphene oxide nanosheet and citric acid as carbon source. X-ray diffraction patterns demonstrate that the as-prepared samples are well crystallized with orderly monoclinic structures. SEM and TEM images reveal that the 3D network graphene and Li3V2(PO4)3 primary nanoparticles interlace with each other. The citric acid-derived amorphous carbon species interrupt the stacking of graphene sheets and minimize in-plane anisotropy of electronic migration within graphene layers, which would facilitate fast electron migration and Li+ diffusion throughout the micro-sized spherical secondary particles. The discharge capacity of the composite could respectively achieve 131.4 and 181.5mAhg−1 in the voltage range 3.0–4.3V and 3.0–4.8V at 0.1C discharge rate, which almost reach the theoretical capacity of the cathode material. Both of them nearly showed no capacity decay at 0.1C charging and discharging rate after long cycles.

Synthesis, Modeling, and Experimental Characterization of Graphite Nanoplatelet-Based Composites for EMC Applications

Graphite nanoplatelets (GNPs) are bidimensional carbon nanostructures consisting of stacks of graphene sheets, having thickness in the range from one up to a few tens of nanometers, and lateral linear dimension in the micrometer range. These nanostructures represent a good candidate to replace carbon nanotubes in composites for electromagnetic applications. This paper proposes a new model based on the Maxwell-Garnett approach to compute the effective complex permittivity of GNP-filled nanocomposites. The effect of the dimensional probabilistic distribution of the nanofiller is investigated. To this purpose, an extensive experimental characterization of the morphological and physical properties of the GNPs after synthesis is performed. The proposed model is validated by comparison with the measured effective permittivity of GNP-composites with different concentrations, and it is used for the design of radar-absorbing materials in the frequency range 1-18 GHz.

Facile Preparation and Electrochemical Properties of V2O5-Graphene Composite Films as Free-Standing Cathodes for Rechargeable Lithium Batteries

Free-standing, flexible V2O5-graphene composite films were prepared by simple filtration of aqueous dispersions of V2O5 nanowires and graphene sheets. V2O5 nanowires were preferentially oriented along the plane of the film as they were sandwiched between stacked graphene sheets in the composite film. The V2O5 content in the composites was adjusted simply by varying the relative amount of the dispersions. Thermal annealing at 300°C increased the conductivity of the composite films. Due to their integrated structure and high flexibility, the composite films were directly usable as flexible electrodes. In binder-free, sandwich-type lithium battery cells, a composite film containing 75.8 wt% V2O5 delivered discharge capacities of 283 mAh g−1 and 252 mAh g−1 at a current density of 50 mA g−1 during the first and 50th cycles, respectively. Owing to its higher conductivity, a composite film containing 42.8 wt% V2O5 delivered a discharge capacity of 189 mAh g−1 at 750 mA g−1. The good performance resulted from the integrated structure of the V2O5 network embedded in stratified graphene nanosheets, with V2O5 as the Li+ host and graphene nanosheets providing the electron pathway. These composite films are promising candidates for electrical energy storage applications that require flexible electrodes.

Control of Super Hydrophobic and Super Hydrophilic Surfaces of Carbon Nanowalls Using Atmospheric Pressure Plasma Treatments

Super hydrophobic and super hydrophilic surface treatment methods for carbon nanowalls (CNWs) which are carbon nanostructures consisting of stacks of graphene sheets vertically standing on substrates and having ultra high aspect ratios of over 50, were developed using atmospheric pressure plasma, and their mechanisms were discussed on the basis of the elemental compositions of surfaces. The contact angle of water droplet on CNWs markedly decreased from 137.3 to 6.2° with the atmospheric pressure plasma treatments using Ar gas. This indicates the super hydrophobic and super hydrophilic transition of CNW surfaces. Although a morphological change was hardly found, the surface compositions of fluorine and oxygen atoms markedly changed after the plasma treatment for only 5 s. Furthermore, the treatment time dependence of the surface tension of CNWs indicates two different mechanisms at earlier and later stages of surface modification using atmospheric pressure plasma.

Model of large volumetric capacitance in graphene supercapacitors based on ion clustering

Electric double-layer supercapacitors (SCs) are promising devices for high-power energy storage based on the reversible absorption of ions into porous conducting electrodes. Graphene is a particularly good candidate for the electrode material in SCs due to its high conductivity and large surface area. In this paper, we consider SC electrodes made from a stack of graphene sheets with randomly inserted spacer molecules. We show that the large volumetric capacitances $C\ensuremath{\gtrsim}100\phantom{\rule{0.28em}{0ex}}\mathrm{F}/{\mathrm{cm}}^{3}$ observed experimentally can be understood as a result of collective intercalation of ions into the graphene stack and the accompanying nonlinear screening by graphene electrons that renormalizes the charge of the ion clusters.

Conductive-probe AFM characterization of graphene sheets bonded to gold surfaces

Conducting probe atomic force microscopy (CP-AFM) has been used to perform mechanical and electrical experiments on graphene layers bonded to polyaminophenylene (PAP) films grafted on gold substrates. This technique is a new approach for the characterization of graphene sheets and represents a complementary tool to Raman spectroscopy. The combination of friction and electrical imaging reveals that different stacked graphene sheets have been successfully distinguished from each other and from the underlying PAP films. Lateral force microscopy has shown that the friction is greatly reduced on graphene sheets in comparison with the organic coating. The electrical resistance images show very different local conduction properties which can be linked to the number of underlying graphene sheets. The resistance decreases very slowly when the normal load increases. Current–voltage curves display characteristics of metal–molecule–metal junctions.