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.     

Graphene oxide: the mechanisms of oxidation and exfoliation

Graphene oxides (GOs) with large sheets and more perfect aromatic structure were prepared by a novel modified Hummers method. We demonstrated that the graphite did not need to be oxidized to such a deep degree as described in Hummers method because the space distance increased little when the oxidation proceeded to a certain extent and the obtained graphite oxides (GTOs) could be fully exfoliated to single layers with high thermal stability. The oxidation mechanism and chemical structure model of GO were proposed by analyzing the evolution of the functional groups with oxidation proceeded based on thermogravimetric analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The layer spacing calculated by molecular dynamics simulations coincided with the X-ray diffraction results. Furthermore, the size distribution and thickness of GOs were also studied. The results confirmed that the GOs prepared by the modified method were fully exfoliated to uniform single layers, and this method may be important for efficient exfoliation of GTO to GO and large-scale production of graphene.     

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.     

Effects of stage, intercalant species and expansion technique on exfoliation of graphite intercalation compound into graphene sheets

Graphite is composed of a series of stacked parallel graphene layers bonded by weak van der Waals forces. Although the weak interactions that hold the graphene sheets together allow them to slide readily over each other, the numerous weak bonds make it difficult to separate the sheets. A graphene sheet is a two-dimensional platelet consisting of a few graphene layers with an overall thickness in nanometer scale. Graphene sheets can be obtained from intercalation and subsequent exfoliation of graphite. To realize the expansion and exfoliation behaviors of graphite, graphite intercalation compound (GIC) is produced using an electrochemical method and three important factors, namely stage structure of GIC, intercalant species and expansion techniques, are taken into account. Graphene sheets produced from a lower stage FeCl3-GIC display the best exfoliation behavior in terms of specific surface area, total pore volume and expansion volume. Microwave irradiation gives rise to a more explosive expansion than heating in a furnace.     

In situ tuning of graphene oxide morphology by electrochemical exfoliation

Journal of Materials Science - This paper illustrates the electrochemical exfoliation of graphite for the synthesis of graphene oxide (GO) with few layers. Innovative design and arrangement of...     

In situ thermal preparation of polyimide nanocomposite films containing functionalized graphene sheets

Graphene oxides (GO) were exfoliated in N,N-dimethylformamide by simple sonication treatment of the as-prepared high quality graphite oxides. By high-speed mixing of the pristine poly(amic acid) (PAA) solution with graphene oxide suspension, PAA solutions containing uniformly dispersed GO can be obtained. Polyimide (PI) nanocomposite films with different loadings of functionalized graphene sheets (FGS) can be prepared by in situ partial reduction and imidization of the as-prepared GO/PAA composites. Transmission electron microscopy observations showed that the FGS were well exfoliated and uniformly dispersed in the PI matrix. It is interesting to find that the FGS were highly aligned along the surface direction for the nanocomposite film with 2 wt % FGS. Tensile tests indicated that the mechanical properties of polyimide were significantly enhanced by the incorporation of FGS, due to the fine dispersion of high specific surface area of functionalized graphene nanosheets and the good adhesion and interlocking between the FGS and the matrix.     

Synthesis of graphene soluble in organic solvents by simultaneous ether-functionalization with octadecane groups and reduction

Octadecane-functionalized graphene (OD-G) soluble in organic solvents was produced by combining the Hummers process for graphite oxidation and a simultaneous ether-functionalization and reduction approach with 1-bromooctadecane in pyridine and dimethylformamide (DMF). The exfoliated OD-Gs were testified to be monolayer sheets by transmission electron microscope (TEM) and atomic force microscopy (AFM). The functionalization with octadecane (OD) groups and the effective deoxygenation of graphene oxide (GO) were confirmed by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). It is proved that the effective reduction and functionalization of GO could be simultaneously completed during the refluxing process. The functionalization with OD groups can effectively prevent the aggregation of GO during the reduction.     

Production of graphene nanosheets by supercritical CO2 process coupled with micro-jet exfoliation

A facile and cost-effective method which combines supercritical CO₂ and micro-jet exfoliation has been developed for producing graphene nanosheets with high-quality. CO₂ molecules can intercalate into the interlayer of graphite because of their high diffusivity and small molecule size in supercritical operation. The tensile stress induced by graphite interfacial reflection of compressive waves exert on the graphite flakes, which lead to further exfoliation of graphite. Scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscopy (AFM), Raman spectrum and X-ray diffraction (XRD) are used to identify morphology and quality of the exfoliated graphene nanosheets, which reveal that the graphite was successfully exfoliated into graphene and more than 88% of graphene nanosheets are less than three layers. The yield of graphene nanosheets is about 28 wt% under optimum conditions, which can be greatly improved by repeated exfoliation of the graphene sediment. The pure graphene film has a high conductivity of 2.1 × 10⁵ S/m.     

Electrochemical preparation of few layer-graphene nanosheets via reduction of oriented exfoliated graphene oxide thin films in acetamide-urea-ammonium nitrate melt under ambient conditions

Electrochemical reduction of exfoliated graphene oxide, prepared from pre-exfoliated graphite, in acetamide-urea-ammonium nitrate ternary eutectic melt results in few layer-graphene thin films. Negatively charged exfoliated graphene oxide is attached to positively charged cystamine monolyer self-assembled on a gold surface. Electrochemical reduction of the oriented graphene oxide film is carried out in a room temperature, ternary molten electrolyte. The reduced film is characterized by atomic force microscopy (AFM), conductive AFM, Fourier-transform infrared spectroscopy and Raman spectroscopy. Ternary eutectic melt is found to be a suitable medium for the regulated reduction of graphene oxide to reduced graphene oxide-based sheets on conducting surfaces.     

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.     

Managing the degree of exfoliation and number of graphene layers using probe sonication approach

We have demonstrated a fast, versatile, and scalable approach to synthesize high-quality few layer graphene sheets with low defect ratio and high crystallinity produced from exfoliation of graphite flakes in DMF by using probe sonication. The effect of sonication time on degree of exfoliation and number of graphene layers has been fully investigated. The degree of exfoliation of graphene sheets as a function of sonication time has been successfully analyzed by XRD, UV-Vis spectroscopy, TEM, and BET studies. The morphological changes at different sonication times have also been observed by SEM. A structural and defect characterization of graphene sheets has been discussed in detail by Raman spectroscopic technique. The shift in position of 2D Raman band and its de-convolution provided information about formation of multi to few layer graphene sheets with sonication. Moreover, Raman results are highly consistent with TEM studies as per number of graphene layers is concerned.     

Preparation and characterization of graphene nanoplatelets from natural graphite via intercalation and exfoliation with tetraalkylammoniumbromide

A new method for the preparation of graphene nanoplatelets (GNP) from graphite intercalation compounds (GICs) was investigated. Donor-type ternary GICs of natural graphites, lithium ions and tetrahydrofurane (NG-Li-THF) were synthesized via a solution process, with the lithium ions in the GICs then exchanged with different tetra alkyl ammonium cations to expand the interlayer distance (d-spacing) of these GICs. Microwave irradiation of these GICs resulted in the exfoliation of GICs, forming so-called 'worm-like exfoliated graphites.' Sonication of the worm-like exfoliated graphites in acetone resulted in GNPs with different aspect ratios. Powder X-ray diffractometry, scanning electron microscopy and transmission electron microscopy were employed to characterize the GICs and GNPs. It was found that the ion-exchange of NG-Li-THF increased the volume expansion ratios, and the molecular structure of the tetra alkyl ammonium cations affected the aspect ratios of the GNPs after exfoliation.     

功能化石墨烯片的表面性能调控

以氧化石墨为前驱体,采用真空辅助热膨胀法在低温下即获得功能化石墨烯片。将所得石墨烯在不同温度下热处理,制备了表面化学结构不同的石墨烯片,并用透射电子显微镜、X射线衍射、X射线光电子能谱、傅里叶变换红外光谱等方法对样品进行分析表征。结果表明,还原氧化石墨烯片中含氧官能团的种类和数量均随热还原温度的升高而减小。     

Layer-by-layer growth of graphene layers on graphene substrates by chemical vapor deposition

We demonstrate a synthesis of graphene layers on graphene templates prepared by the mechanical exfoliation of graphite crystals using a developed chemical vapor deposition (CVD) apparatus that has a furnace with three temperature zones and can regulate the temperatures separately in each zone. This results in individual control over the decomposition reaction of the carbon feedstock and the growth of graphene layers by activated carbon species. CVD growth using multi-temperature zones provides wider temperature windows appropriate to grow graphene layers. We observed that graphene layers proceed by a layer-by-layer growth mode using an optical microscopy, an atomic force microscopy, and Raman spectroscopy. This result suggests that a graphene growth technique using the CVD apparatus is a potential approach for making graphene sheets with precise control of the layer numbers.     

Preparation and properties of polystyrene nanocomposites with graphite oxide and graphene as flame retardants

In this study, graphite oxides (GOs) with different oxidation degrees and graphene nanosheets were prepared by a modified Hummers method and thermal exfoliation of the prepared GO, respectively. Polystyrene (PS)/GO and PS/graphene nanocomposites were prepared via melt blending. X-ray diffraction results showed that GOs and graphene were exfoliated in the PS composites. It could be observed from the scanning electron microscope images that GOs and graphene were well dispersed throughout the matrix without obvious aggregates. Dynamic mechanical thermal analysis suggested that the storage modulus for the PS/GO1 and PS/graphene nanocomposites was efficiently improved due to the low oxygen content of GO1 and the elimination of the oxygen groups from GO. The flammability of nanocomposites was evaluated by thermal gravimetric analysis and cone calorimetry. The results suggested that both the thermal stability and the reduction in peak heat release rate (PHRR) decreased with the increasing of the oxygen groups in GOs or graphene. The optimal flammability was obtained with the graphene (5 wt%), in which case the reduction in the PHRR is almost 50 % as compared to PS.     

Hyperstage Graphite: Electrochemical Synthesis and Spontaneous Reactive Exfoliation

Covalent modification of the π‐electron basal planes of graphene enables the formation of new materials with enhanced functionality. An electrochemical method is reported for the formation of what is referred to as a Hyperstage‐1 graphite intercalation compound (GIC), which has a very large interlayer spacing d₀₀₁ > 15.3 Å and contains disordered interstitial molecules/ions. This material is highly activated and undergoes spontaneous exfoliation when reacted with diazonium ions to produce soluble graphenes with high functionalization densities of one pendant aromatic ring for every 12 graphene carbons. Critical to achieving high functionalization density is the Hyperstage‐1 GIC state, a weakening of the van der Waals coupling between adjacent graphene layers, and the ability of reactants to diffuse into the disordered intercalate phase between the layers. Graphene functionalization with 3,5‐dinitrophenyl groups provides for exceptional dispersibility (0.24 mg mL^?1) in N,N‐dimethylformamide and for conjugation with amines.     

Hierarchical architecture of ultrashort carbon nanotubes/polyaniline nanocables coated on graphene sheets for advanced supercapacitors

Multilayer super-short carbon nanotubes (SSCNTs) could be prepared by tailoring raw multiwalled carbon nanotubes (MWCNTs) with mechanical-stirring and ultrasonic oxidation-cut method. The SSCNTs/polyaniline/reduced graphene oxide (SSCNTs/PANI/RGO) ternary hybrid composite was fabricated by reducing SSCNTs/PANI/GO precursor prepared by self-assembly from the complex dispersion of graphene oxide (GO) and the as-prepared SSCNTs/PANI nanocables, followed by redoping and reoxidation of the reduced PANI to restore the conducting structure of PANI in the ternary composite. The microscope images indicated that SSCNTs/PANI nanocables could uniformly distribute in the conductive network of graphene sheets and prevent the agglomeration of graphene. Such the hierarchical structure perfectly facilitates the contact between PANI for faradaic energy storage and electrolyte ions, and efficiently utilizes the double-layer capacitance of SSCNTs and graphene sheets at the electrode–electrolyte interfaces. The maximum specific capacitance of the SSCNTs/PANI/RGO composite achieved 845 F g^?1, which was much higher than that of pure PANI and SSCNTs/PANI nanocables. Moreover, the ternary composite also showed the good cycling stability, retaining about 96% of its initial capacitance after 1000 cycles because of the synergistic effect and conductive network of SSCNTs/PANI nanocables and graphene sheets. Therefore, the combined effects between SSCNTs/PANI nanocables and graphene sheets taking advantage of both charging and faradaic processes could readily explain the excellent electrochemical performance for supercapacitors.     

Preparation, characterization, and application of electrochemically functional graphene nanocomposites by one-step liquid-phase exfoliation of natural flake graphite with methylene blue

Electrochemically functional graphene nanocomposites have been directly prepared by one-step liquid-phase exfoliation of natural flake graphite with methylene blue (MB). UV-visible spectra of the obtained aqueous dispersions of graphene-methylene blue (G-MB) nanocomposite at different exfoliation time indicate that the concentration of graphene dispersion increased markedly with increasing exfoliation time. Atomic force microscopy (AFM) and Raman spectroscopy verified that the graphene was exfoliated into single-layer or bilayer states. FT-IR spectroscopy of G-MB suggests that a ??-?? stacking interaction is involved in the structure-associated interactions between graphene and adsorbed MB molecules. A G-MB nanocomposite modified glassy carbon (GC) electrode exhibits excellent electrochemical properties and good electrochemical stability. Additionally, the G-MB/GC modified electrode shows more favorable electron transfer kinetics for potassium ferricyanide and potassium ferrocyanide probe molecules, which are important electroactive compounds, compared with reduced graphene oxide (RGO)-MB/GC, RGO/GC, bare GC and graphite/GC electrodes. Furthermore, the G-MB/GC modified electrode exhibits good electrocatalytic activity toward hydrogen peroxide (H₂O₂) and ??-nicotinamide adenine dinucleotide (NADH). The excellent electroactivity, electrochemical stability and electrocatalytic activity of the G-MB nanocomposites prepared in this work are potentially very useful for basic electrochemical studies and for the practical development of electronic devices such as biosensors and photovoltaic cells.      

交变电场剥离氧化石墨的研究

化学还原法被认为是实现大规模生产石墨烯的主要途径之一,而剥离氧化石墨为氧化石墨烯是获得高品质石墨烯的关键步骤。采用交变电场剥离氧化石墨,研究了电压U、频率f和电极间距d对剥离结果的影响。结果表明氧化石墨烯的厚度和横向尺寸随着电压的增大而减小,随着频率的增大而增大;在电压能满足剥离效果的前提下,随着电极间距的增大,氧化石墨烯的厚度和横向尺寸呈减小趋势。     

Tailor made exfoliated reduced graphene oxide nanosheets based on oxidative-exfoliation approach

This study presents a versatile and scalable strategy of ‘oxidation controlled exfoliation’ of rGO nanosheets synthesized from both Hummers and modified Hummers method. A co-relation between degree of oxidation of graphite oxide (GO) sheets and exfoliation of resulting synthesized rGO nanosheets has been successfully developed which in turn reflects in various properties of rGO sheets. The extent of exfoliation of rGO sheets has been well analyzed by XRD, SEM, BET and TEM techniques. Moreover, the quantitative analysis of degree of oxidation of GO has been estimated from FTIR spectra using quotient law method. The variations in number of rGO layers, defect density and sp² domain size have been investigated in detail by Raman spectroscopic technique. Both qualitative-quantitative analysis of rGO nanosheets have been discussed from their SAED pattern and HR-TEM images. The optical characterization of GO and corresponding rGO nanosheets has been studied in detail by UV- Vis spectroscopic technique.