Powder,Paper and Foam of Few‐Layer Graphene Prepared in High Yield by Electrochemical Intercalation Exfoliation of Expanded Graphite

A facile and high‐yield approach to the preparation of few‐layer graphene (FLG) by electrochemical intercalation exfoliation (EIE) of expanded graphite in sulfuric acid electrolyte is reported. Stage‐1 H₂SO₄‐graphite intercalation compound is used as a key intermediate in EIE to realize the efficient exfoliation. The yield of the FLG sheets (<7 layers) with large lateral sizes (tens of microns) is more than 75% relative to the total amount of starting expanded graphite. A low degree of oxygen functionalization existing in the prepared FLG flakes enables them to disperse effectively, which contributes to the film‐forming characteristics of the FLG flakes. These electrochemically exfoliated FLG flakes are integrated into several kinds of macroscopic graphene structures. Flexible and freestanding graphene papers made of the FLG flakes retain excellent conductivity (≈24 500 S m^?1). Three‐dimensional (3D) graphene foams with light weight are fabricated from the FLG flakes by the use of Ni foams as self‐sacrifice templates. Furthermore, 3D graphene/Ni foams without any binders, which are used as supercapacitor electrodes in aqueous electrolyte, provide the specific capacitance of 113.2 F g^?1 at a current density of 0.5 A g^?1, retaining 90% capacitance after 1000 cycles.     

Synthesis of few-layer graphene via microwave plasma-enhanced chemical vapour deposition

If graphene is ever going to live up to the promises of future nanoelectronic devices, an easy and cheap route for mass production is an essential requirement. A way to extend the capabilities of plasma-enhanced chemical vapour deposition to the synthesis of freestanding few-layer graphene is presented. Micrometre-wide flakes consisting of four to six atomic layers of stacked graphene sheets have been synthesized by controlled recombination of carbon radicals in a microwave plasma. A simple and highly reproducible technique is essential, since the resulting flakes can be synthesized without the need for a catalyst on the surface of any substrate that withstands elevated temperatures up to 700?°C. A thorough structural analysis of the flakes is performed with electron microscopy, x-ray diffraction, Raman spectroscopy and scanning tunnelling microscopy. The resulting graphene flakes are aligned vertically to the substrate surface and grow according to a three-step process, as revealed by the combined analysis of electron microscopy and x-ray photoelectron spectroscopy.     

Highly Conductive and Transparent Large‐Area Bilayer Graphene Realized by MoCl5 Intercalation

Bilayer graphene (BLG) comprises a 2D nanospace sandwiched by two parallel graphene sheets that can be used to intercalate molecules or ions for attaining novel functionalities. However, intercalation is mostly demonstrated with small, exfoliated graphene flakes. This study demonstrates intercalation of molybdenum chloride (MoCl₅) into a large‐area, uniform BLG sheet, which is grown by chemical vapor deposition (CVD). This study reveals that the degree of MoCl₅ intercalation strongly depends on the stacking order of the graphene; twist‐stacked graphene shows a much higher degree of intercalation than AB‐stacked. Density functional theory calculations suggest that weak interlayer coupling in the twist‐stacked graphene contributes to the effective intercalation. By selectively synthesizing twist‐rich BLG films through control of the CVD conditions, low sheet resistance (83 Ω ?^?1) is realized after MoCl₅ intercalation, while maintaining high optical transmittance (≈95%). The low sheet resistance state is relatively stable in air for more than three months. Furthermore, the intercalated BLG film is applied to organic solar cells, realizing a high power conversion efficiency.     

Synthesis of mono layer graphene oxide from sonicated graphite flakes and their Hall effect measurements

Graphene, a single atom thick sheet is considered a key candidate for the future nanotechnology, due to its unique extraordinary properties. Researchers are trying to synthesize bulk graphene via chemical route from graphene oxide precursor. In the present work, we investigated a safe and efficient way of monolayer graphene oxide synthesis. To get a high degree of oxidation, we sonicated the graphite flakes before oxidation. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) results confirmed graphene oxide formation and high degree of oxidation. Raman spectroscopy and atomic force microscopy (AFM) results revealed a monolayer of graphene oxide (GO) flakes. The sheet like morphology of the GO flakes was further confirmed by scanning electron microscopy (SEM). The Hall effect measurements were performed on the GO film on a silica substrate to investigate its electrical properties. The results obtained, revealed that the GO film is perfectly insulating, having electrical resistivity up to 8.4 × 10⁸ (Ω·cm) at room temperature.     

Preparation of graphene by jet cavitation

Despite its bright prospects, graphene faces challenges including issues concerning mass production. Here we present a totally green approach whereby common crystal graphite can be exfoliated into graphene sheets in aqueous solution by jet cavitation. This is possible mainly because the tensile stress caused by graphite-solution interfacial reflection of compressive waves acts an intensive 'suction disk' on the graphite flakes. We confirm the presence of graphene sheets by diverse characterizations. The graphene yield by our method is estimated as ～ 4 wt%, which could potentially be improved by further processing. The method, of a mechanical nature, is powerful compared to the traditional low-throughput micromechanical cleavage. Our work here illustrates jet cavitation as a facile, low cost, timesaving and laborsaving route, which can potentially be scaled up to mass production of graphene.     

石墨纳米片对铜电子浆料导电性的影响

为改善铜浆导电性,以表面改性的金属铜粉为主要导电相,通过添加少量导电性优异的石墨纳米片作为导电增强相制备复合电子浆料,并采用四探针测试仪、扫描电子显微镜(SEM)等分析测试方法研究了石墨纳米片的参数、添加量对铜电子浆料导电性能的影响.结果表明:选用厚度为3~5 nm,片径为5μm的石墨纳米片作为导电增强相,制得石墨纳米片—铜电子浆料,在460℃烧结后导电膜层的电阻率较小;石墨纳米片与铜粉质量比为2∶98时,测得浆料电阻率为17.14 mΩ·cm,相比纯铜浆料电阻率34.43 mΩ·cm降低了50.22%.分析电子浆料导电机理并建立导电相连接几何模型,在导电膜层中,部分折断的石墨纳米片会填充到铜颗粒之间的空隙中,较长石墨纳米片则会形成"搭桥"现象,增加导电相之间的连接,形成较紧密的微观组织和良好的导电网络,从而改善复合浆料的导电性.     

A facile approach to synthesize rose-like ZnO/reduced graphene oxide composite: fluorescence and photocatalytic properties

Rose-like ZnO/reduced graphene oxide composites are synthesized by a simple, scalable, and facile route without using any surfactants. The well-defined ZnO microstructure has a hexagonal hierarchical rose-like architecture, which is composed of densely packed uniform thin flakes. The N,N-dimethylformamide/water system employed here acts as an organic solvent as well as a reagent, and two competing reactions give rise to the formation of the hexagonal hierarchical rose-like ZnO architecture. Compared with bare ZnO, the rose-like ZnO/reduced graphene oxide composite displays the fluorescence quenching property. Finally, the as-prepared products possess considerable photocatalytic property under visible light for the degradation of methylene blue.     

Extremely high response of electrostatically exfoliated few layer graphene to ammonia adsorption

Extremely high gas sensing properties of p-type few layer graphene flakes exfoliated from highly oriented pyrolytic graphite have been demonstrated. The current response to ammonia adsorption is strongly dependent on film thickness and is higher than that for graphene by 1-8 orders of magnitude. A maximal response was found for sample thickness ～ 2 nm. The effect is attributed to the formation of multiple p-n-p junctions at the grain boundaries in the polycrystalline graphene flakes exposed to ammonia-containing ambient.     

Large and flat graphene flakes produced by epoxy bonding and reverse exfoliation of highly oriented pyrolytic graphite

We present a fabrication method producing large and flat graphene flakes that have a few layers down to a single layer based on substrate bonding of a thick sample of highly oriented pyrolytic graphite (HOPG), followed by its controlled exfoliation down to the few to single graphene atomic layers. As the graphite underlayer is intimately bonded to the substrate during the exfoliation process, the obtained graphene flakes are remarkably large and flat and present very few folds and pleats. The high occurrence of single-layered graphene sheets being tens of microns wide in lateral dimensions is assessed by complementary probes including spatially resolved micro-Raman spectroscopy, atomic force microscopy and electrostatic force microscopy. This versatile method opens the way for deposition of graphene on any substrates, including flexible ones.     

Bi- and trilayer graphene solutions

Bilayer and trilayer graphene with controlled stacking is emerging as one of the most promising candidates for post-silicon nanoelectronics. However, it is not yet possible to produce large quantities of bilayer or trilayer graphene with controlled stacking, as is required for many applications. Here, we demonstrate a solution-phase technique for the production of large-area, bilayer or trilayer graphene from graphite, with controlled stacking. The ionic compounds iodine chloride (ICl) or iodine bromide (IBr) intercalate the graphite starting material at every second or third layer, creating second- or third-stage controlled graphite intercolation compounds, respectively. The resulting solution dispersions are specifically enriched with bilayer or trilayer graphene, respectively. Because the process requires only mild sonication, it produces graphene flakes with areas as large as 50 μm(2). Moreover, the electronic properties of the flakes are superior to those achieved with other solution-based methods; for example, unannealed samples have resistivities as low as ～1 kΩ and hole mobilities as high as ～400 cm(2) V(-1) s(-1). The solution-based process is expected to allow high-throughput production, functionalization, and the transfer of samples to arbitrary substrates.     

Electrohydrodynamic atomization approach to graphene/zinc oxide film fabrication for application in electronic devices

Graphene-based composites represent a new class of materials with potential for many applications. Metal, semiconductor, or any polymer properties can be tuned by attaching it to graphene. Here, a new route for fabrication of graphene based composites thin films has been explored. Graphene flakes (<4 layers) and a well-known semiconductor zinc oxide (ZnO) (<50 nm particle size) have been dispersed in N-methylpyrrolidone and ethanol, respectively. Thin film of graphene flakes is deposited and decorated with ZnO nanoparticles to fabricate graphene/ZnO composite thin film on silicon substrate by electro hydrodynamic atomization technique. Graphene/ZnO composite thin film has been characterized morphologically, structurally and chemically. To investigate electronic behavior of the composite thin film, it is deployed as cathode in a diode device i.e. indium tin oxide/poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate)/polydioctylfluorene-benzothiadiazole/(graphene/ZnO). The J–V analysis of diode device has shown that at voltage of 1 V, the current density in organic structure is at low value of 4.69 × 10^?3 A/cm² and when voltage applied voltage is further increased; the device current density has increased by the order of 200 that is 1.034 A/cm² at voltage of 12 V.     

High-quality production of graphene by liquid-phase exfoliation of expanded graphite

As one of the most promising candidates, graphene exhibits a potential application in post-silicon nanoelectronics. However, it is a key issue to produce high-quality graphene in large scale. Here, a facile method is demonstrated to produce graphene dispersions by exfoliation of expanded graphite in the co-solvent with N,N-dimethylformamide (DMF) and water. We confirm that the optimal ratio of DMF to water for graphene exfoliation is 9:1 (v:v) by means of UV–Vis absorption spectra. This exfoliation results in large flakes ∼2 μm in diameter, which can potentially be improved by adjusting the sonication power. The relatively perfect hexagonal structure of graphene is confirmed by Raman spectroscopy and the as-prepared graphene nanosheet film the as-prepared graphene nanosheet film possesses good electrical conductivity (∼8.3 × 10³ S m⁻¹). DC electrical transport phenomena for the deposited film of graphene nanosheets are well described in terms of conduction models for non-crystal semiconductor. This convenient approach provides an extensive route to prepare high-quality graphene nanosheets.     

Graphene for reducing bubble defects and enhancing mechanical properties of graphene/cellulose acetate composite films

In this study, we have demonstrated a strategy by which graphene was used to reduce the bubble defects and enhance the mechanical properties in graphene/cellulose acetate (Gr/CA) composite films. Mono- and multilayer graphene flakes were successfully prepared in the water–acetone mixtures by a jet cavitation method. Moreover, outstanding enhancement of mechanical properties of Gr/CA composite films were obtained at relatively low concentration of graphene flakes. Young’s modulus of these composite films increased linearly with the graphene flakes loading, due to the significantly high surface area of graphene and strong interactions between graphene flake and CA. Furthermore, three-dimensional channel formed by graphene flakes could increase the degassing speed and reduce the negative effects of bubbles. The Gr/CA composite has excellent mechanical properties and, more importantly, it is a natural and environmentally friendly polymer composite.     

Matrix interference-free method for the analysis of small molecules by using negative ion laser desorption/ionization on graphene flakes

This work presents a new approach for the analysis of small molecules with direct negative ion laser desorption/ionization (LDI) on graphene flakes. A series of matrix interference-free mass spectra were obtained for the analysis of a wide range of small molecules including peptides, amino acids, fatty acids, as well as nucleosides and nucleotides. The mixture of analytes and graphene flakes suspension were directly pipetted onto a sample plate for LDI-time-of-flight mass spectrometry (TOFMS) analysis. Deprotonated monomeric species [M-H](-) ions were homogeneously obtained on uniform graphene flakes film when negative ion mode was applied. In positive ion mode, the analytes were detected in form of multiple adduct ions such as sodium adduct [M+Na](+), potassium adduct [M+K](+), double sodium adduct [M+2Na-H](+), double potassium adduct [M+2K-H](+), as well as sodium and potassium mixed adduct [M+Na+K-H](+). Better sensitivity and reproducibility were achieved in negative ion mode compared to positive ion mode. It is believed that the new method of matrix interference-free negative ion LDI on graphene flakes may be expanded for LDI-MS analysis of various small molecules.     

Functional microspheres of graphene quantum dots

Graphene-quantum-dot microspheres (GQDSs) have been prepared by assembly of graphene quantum dots (GQDs) via a water-in-oil (W/O) emulsion technique without the addition of any surfactants. Although made of quantum-sized graphene dots, the as-formed GQDSs are solid and remain intact after slight ultrasonication. The versatile W/O emulsion method allows the in?situ intercalation of functional nanocomponents into the GQDSs for specific applications. As exemplified by the Fe(3)O(4)-containing GQDSs, Fe(3)O(4)-GQDSs exhibit a large magnetic response. Furthermore, the embedded Fe(3)O(4) nanoparticles in GQDSs can act as the catalysts for the growth of carbon nanotubes (CNTs), which opens the opportunities for fabricating new complex structures of CNTs surrounding GQDSs by simple chemical vapor deposition.     

Facile preparation of nitrogen-doped few-layer graphene via supercritical reaction

To achieve the applications of graphene, the modulation of its electrical properties is of great significance. The element doping might give a promising approach to produce fascinating properties of graphene. Herein we report a facile chemical doping method to obtain nitrogen-doped (N-doped) few-layer graphene sheets through supercritical (SC) reaction in acetonitrile at temperature as low as 310 °C, using expanded graphite as starting material. X-ray photoelectron spectroscopy analysis revealed that the level of nitrogen-doping (N-doping) increased from 1.57 to 4.56 at % when the reaction time was tuned from 2 to 24 h. Raman spectrum confirmed that the resulting N-doped few-layer graphene by SC reaction maintain high quality without any significant structural defects. Electrical measurements indicated that N-doped few-layer graphene sheets exhibit a typical n-type field-dependent behavior, suggesting the N-doping into the lattice of graphene. This work provides a convenient chemical route to the scalable production of N-doped graphene for potential applications in nanoelectronic devices.     

Hysteresis I–V nature of mechanically exfoliated graphene FET

Graphene is an attractive material for device applications due to its excellent electrical and mechanical properties. The mechanical exfoliation is an attractive method to fabricate graphene devices using mono and multilayer graphene flakes. As the graphene is very sensitive to atmosphere the occurrence of hysteresis and p-doping is common. This paper reports electrical characterization and hysteresis effect of graphene field effect transistor (FET) fabricated using mechanically exfoliated graphene flakes. Raman spectra and atomic force microscopy techniques have been used to examine the quality and thickness of the exfoliated graphene. This fabricated graphene FET has shown hysteresis nature with p-type doping. The possible reason for the observed hysteresis and p-doping has been explained.     

Soluble graphene through edge-selective functionalization

Thermally expanded graphite was functionalized with 4-bromophenyl addends using the in situ diazonium formation procedure, and after mild sonication treatment in N,N′-dimethylformamide, thin graphene layers were exfoliated from the bulk graphite. These chemically-assisted exfoliated graphene (CEG) sheets had higher solubility than pristine graphene without any stabilizer additive. More than 70% of these soluble flakes had less than 5 layers. Energy filtered transmission electron microscopy (EFTEM) elemental mapping provided evidence of the edge-selective diazonium functionalization with graphene. A majority of the Br signals came from the edges of the CEG indicating that the basal planes were not highly functionalized. The CEG was also characterized by X-ray photoelectron spectroscopy, atomic force microscopy, Raman spectroscopy, and transmission electron microscopy.     

Carbon nanotube/graphene nanocomposite as efficient counter electrodes in dye-sensitized solar cells

We demonstrated the replacement of the Pt catalyst normally used in the counter electrode of a dye-sensitized solar cell (DSSC) by a nanocomposite of dry spun carbon multi-walled nanotube (MWNT) sheets with graphene flakes (Gr-F). The effectiveness of this counter electrode on the reduction of the triiodide in the iodide/triiodide redox (I(-)/I(3)(-)) redox reaction was studied in parallel with the use of the dry spun carbon MWNT sheets alone and graphene flakes used independent of each other. This nanocomposite deposited onto fluorinated tin-oxide-coated glass showed improved catalytic behavior and power conversion efficiency (7.55%) beyond the use of the MWNTs alone (6.62%) or graphene alone (4.65%) for the triiodide reduction reaction in DSSC. We also compare the use of the carbon MWNT/Gr-F composite counter electrode with a DSSC using the standard Pt counter electrode (8.8%). The details of increased performance of graphene/MWNT composite electrodes as studied are discussed in terms of increased catalytic activity permitted by sharp atomic edges that arise from the structure of graphene flakes or the defect sites in the carbon MWNT and increased electrical conductivity between the carbon MWNT bundles by the graphene flakes.     

超声剥离制备石墨烯纳米片及其结构表征

以鳞片石墨为原料,采用插层氧化法制得可膨胀氧化石墨,然后经高温热解获得膨胀石墨,再通过超声剥离得到石墨烯纳米片,采用FTIR、XRD、SEM、TEM和Raman对所得石墨烯纳米片的微观结构进行表征。结果表明,可膨胀氧化石墨在800℃高温热解30 s得到膨胀体积最大的膨胀石墨,由80目和100目鳞片石墨制得的膨胀石墨的最大体积分别为215 mL/g和85 mL/g,且在50℃条件下超声剥离5 h分别得到30~50层和6~20层的石墨烯纳米片。