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.     

Studying the conversion of graphite into nanographene sheets using supercritical phase exfoliation method

Abstract

This work studies the application of supercritical-phase exfoliation method to produce 100% yield of high-quality graphene. This simple and cost-effective method utilizes supercritical carbon dioxide and ultrasonication to produce pure and defect-free mono-, few- and multi-layer graphene sheets. The process parameters such as pressure, sonication time, sonication amplitude and the amount of starting graphite were examined. The production of defect-free single-, few- and multi-layer graphene sheets was confirmed using atomic force microscopy and Raman spectroscopy. The Fourier-transform infrared spectroscopy confirms that our method does not cause any oxidation to the synthesized graphene. The conductivity of the best yield graphene sample has been tested by four-point probe method and high electrical conductivity of 8.5?×?10⁴ S/m was recorded. The synthesized graphene can be used in many applications such as supercapacitors, batteries, composites and conductive inks.     

Electrostatic deposition of graphene

Loose graphene sheets, one to a few atomic layers thick, are often observed on freshly cleaved HOPG surfaces. A straightforward technique using electrostatic attraction is demonstrated to transfer these graphene sheets to a selected substrate. Sheets from one to 22?layers thick have been transferred by this method. One sheet after initial deposition is measured by atomic force microscopy to be only an atomic layer thick (～0.35?nm). A few weeks later, this height is seen to increase to ～0.8?nm. Raman spectroscopy of a single layer sheet shows the emergence of an intense D band which dramatically decreases as the number of layers in the sheet increase. The intense D band in monolayer graphene is attributed to the graphene conforming to the roughness of the substrate. The disruption of the C-C bonds within the single graphene layer could also contribute to this intense D band as evidenced by the emergence of a new band at 1620?cm(-1).     

Large scale cathodic exfoliation of graphite using deep eutectic solvent and water mixture

A short-time and low-cost synthesis route was used to produce large lateral size (from 2 to 15 μm) from monolayers to few layers of graphene by a two-step process of electrochemical exfoliation with a deep eutectic solvent in a mixture with water that can be reused, and ultrasonic bath. The graphene was characterized by SEM, TEM, AFM, Raman and electrochemical activity. During the electrochemical exfoliation, high expanded graphene particles were obtained and these were dispersed in a mixture of water with 5%wt ethylene glycol by an ultrasonic bath in order to complete the exfoliation process. An enhancement of the electrical conductivity of these dispersions was obtained with the increase of graphene concentration, 0.38 mg/mL, which best result was achieved with 30 wt% water and a DC voltage of 10 V. It was possible to add a conductive layer to a glass substrate with the graphene obtained and Tyndall effect was observed.     

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.     

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.     

高浓度石墨烯水分散液的制备与表征

利用高压均质液相剥离法,以鳞片石墨为原料,水为介质,制备高浓度石墨烯水分散液。采用紫外可见光谱研究表明活性剂浓度、高压均质压力和循环次数对石墨烯水分散液浓度C_G的影响。通过拉曼光谱、扫描电镜、透射电镜、激光粒度仪分析水分散液中石墨烯的结构和形貌。结果表明:通过调节各工艺参数,获得了浓度为324.3mg·L^-1的石墨烯水分散液,所得浓度是超声液相剥离法的10倍;石墨烯水分散液中石墨烯缺陷少、厚度薄、片径大,具有良好的品质;将所得石墨烯分散液制备石墨烯自支撑膜,其电导率可达3.2×10~4S·m^-1。     

Synthesis and characterization of electrochemically-reduced graphene

Graphene has superior electrical conductivity than graphite and other allotropes of carbon because of its high surface area and chemical tolerance. Electrochemically processed graphene sheets were obtained through the reduction of graphene oxide from hydrazine hydrate. The prepared samples were heated to different temperatures such as 673 and 873 K. X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), transmission electron microscopy (TEM), Raman spectra and conductivity measurements were made for as-prepared and heat-treated graphene samples. XRD pattern of graphene shows a sharp and intensive peak centred at a diffraction angle (2θ) of 26·350. FTIR spectra of as-prepared and heated graphene were used to confirm the oxidation of graphite. TEM results indicated that the defect density and number of layers of graphene sheets were varied with heating temperature. The hexagonal sheet morphology and purity of as-prepared and heat treated samples were confirmed by SEM–EDX and Raman spectroscopy. The conductivity measurements revealed that the conductivity of graphene was decreased with an increase in heating temperature. The present study explains that graphene with enhanced functional properties can be achieved from the as-prepared sample.     

Easy synthesis and characterization of high quality graphene sheets produced from mesocarbon microbeads

An easy method of producing graphene sheets with high quality from mesocarbon microbeads (MCMBs) is demonstrated using oxidation, rapid expansion and ultrasonic treatment. Single layer graphene sheets have been successfully prepared from MCMBs through thermal exfoliation. The structure of the graphene sheets was investigated by scanning and transmission electron microscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. MCMBs expanded mainly along the c-axis and formed worm-like ellipses from the original regular spherules. Exfoliation continued with the fragmentation of the pieces and produced delamination of the graphene sheets. MCMBs can be an excellent starting material for producing high quality graphene sheets with high yields, becoming an attractive raw material for industrial up-scale.     

Direct exfoliation of graphite into graphene in aqueous solution using a novel surfactant obtained from used engine oil

Large-scale production of high-quality graphene is very critical for practical applications of graphene materials and devices. Exfoliation of graphite in an aqueous solution of surfactants is one of the most promising approaches to produce graphene. In this study, a novel anionic surfactant [sulfonated used engine oil (SUEO)], which was prepared from used engine oil, was employed to exfoliate the graphite nanoplatelets into graphene sheets in an aqueous solution under sonication to form a stable dispersion. The efficiency of SUEO for exfoliating and dispersing graphene was investigated and compared with that of traditional surfactants, such as sodium dodecyl sulfate, sodium dodecyl benzene sulfate, cetyl trimethyl ammonium bromide, and polyvinylpyrrolidone. Result showed that the graphene dispersion with excellent stability had a higher concentration (0.477 mg/mL) than others at 0.5 g/L optimal SUEO dosage in 4 h sonication time. The superior performance of SUEO can be attributed to its special molecular structures, whose hydrophobic moieties contain cycloalkanes/aromatics with different molecular weights and/or side chain –R with different lengths. Structural diversities are very helpful to the “jigsaw-puzzle” process on the graphene surface, where the total interfacial energy of the mixture system was minimized. Microscopic (SEM, TEM, and AFM) and spectroscopic (XRD, XPS, and Raman) measurements revealed that the dispersion consisted of few-layer graphene sheets with lower levels of defects or oxidation. This study presents a new class of dispersing agents for graphene that assists in the exfoliation process in water with high concentration and the stabilization of the graphene sheets against reaggregation.     

A facile approach to the synthesis of graphene nanosheets under ultra-low exfoliation temperature

High specific surface area graphene nanosheets have been obtained from graphite oxide by using an effective modified exfoliation method under vacuum, the exfoliation temperature (135 degrees C) is much lower than that conventionally applied (1050 degrees C) to obtain monolayer graphene sheets via rapid thermal shock. These products have fluffy and highly porous structure and with a lateral size typically of a few micrometers. Transmission electron microscopy (TEM) observation shows that it looks like a wrinkled transparent ultrathin film consisting of single or few-layer graphene sheets, and their Brunauer-Emmett-Teller surface area is as large as 750 m2/g. Simultaneously, X-ray photoelectron spectroscopy analysis revealed that considerable amount of oxygen-containing groups (C/O ratio, 5:1) retained on the graphene sheets after exfoliation process, which would provide convenience for further modification of the surface properties and chemistry of graphene sheets. This work offers a facile and scalable approach to fabricate graphene oxide and opens up a new vista of various potential applications electronics and composite materials.     

Few layers of graphene as transparent electrode from botanical derivative camphor

Here, we report synthesis of large area graphene sheets by control pyrolysis of solid botanical derivative camphor (C₁₀H₁₆O) and fabrication of transparent electrodes. Raman study shows highly ordered graphene sheet with minimum defects. Second order Raman spectrum shows that graphene layers are more than single layer and can be controlled with amount of camphor pyrolyzed. Transmission electron microscopic images show presence of 4 layers for thinner and 13 layers for thicker graphene sheets. Transferred graphene sheets on glass substrates show very good transparency in wide range of wavelength (0.3-2 μm). Electrical measurements of the graphene sheets show thickness dependent sheet resistance. A sheet resistance of 203 Ω/sq is obtained at a transmittance of 63.5% of the graphene sheet. The technique to fabricate few layer of graphene as transparent electrode from camphor is both viable and scalable for potential large area optoelectronic applications.     

Influence of graphite oxide drying temperature on ultra-fast microwave synthesis of graphene

Ultra-fast synthesis of graphene has been reported by microwave assisted graphene oxide reduction. In this article, the graphene oxide was initially dried above room temperature. The initial heat treatment of graphene oxide demonstrates a distinct improvement of exfoliation rate of graphene sheets. This method provides an efficient way for mass production of high quality graphene sheets. Raman spectroscopy, scanning electron microscopy, and X-ray diffraction techniques has been used to characterize reduced graphene sheets. The quality of reduced graphene was found to be affected by the initial drying temperature of graphite oxide.     

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.     

Preparation of non-covalently functionalized graphene using 9-anthracene carboxylic acid

A simple way of achieving stable aqueous dispersion of graphene by non-covalent functionalization using 9-anthracene carboxylic acid has been successfully accomplished. Unlike in chemically reduced graphene, the C-sp(2) network of the graphene remains undistorted and therefore of superior quality. The non-covalent functionalization facilitates the exfoliation of graphite layers in a polarity controlled combination of media. A detailed exfoliation mechanism is proposed based on the controlled experiment and is supported by the data from UV-vis spectroscopy, transmission electron microscopy, and x-ray diffraction studies. Formation of monolayer graphene has been confirmed from Raman spectroscopy. The graphene based ultracapacitor shows a high value of specific capacitance (148 F g(-1)).     

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.     

Raman spectroscopy as a probe of graphene and carbon nanotubes

Progress in the use of Raman spectroscopy to characterize graphene samples for the number of graphene layers and doping level they contain is briefly reviewed. Comparisons to prior studies on graphites and carbon nanotubes are used for inspiration to define future promising directions for Raman spectroscopy research on few layer graphenes.     

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.     

Production of monolayer, trilayer, and multi-layer graphene sheets by a re-expansion and exfoliation method

Mass graphene sheets were obtained by a re-expansion and exfoliation method which uses the commercially available graphite intercalation compounds as the starting materials. The as-prepared samples have been characterized by scanning electronic microscopy, transmission electron microscope, atomic force microscope, Fourier transform infrared spectrometer spectroscopies, and Raman microscope. Using high resolution transmission electron microscopy on 60 samples, it was shown that the final samples exhibit 8 % monolayer, 19 % bilayer, and 77 % less than or equal to five layers, and the sheets are largely free of defects and chemical functional groups after thermal reduction.     

The preparation and characterization of non-covalently functionalized graphene

Recently, much work has focused on the exfoliation of graphene through a combination of oxidation and sonication procedures, followed by reduction through chemical methods. We demonstrated that the individual graphene oxide sheets can be readily reduced by using phenolphthalin as both reducing agent and stabilizer. The obtained non-covalently functionalized chemically reduced graphene oxide (CRG) can be dispersed in organic solvents very well, such as alcohol, N,N-dimethylformamide, N,N-Dimethylacetamide, N-methyl-2-pyrrolidone, etc., which can give practical applications in large scale production of oil dispersible graphene and have a potential in polymer nanocomposites fabrication.