The production of chemically converted graphenes from graphite fluoride

The liquid-phase extraction of graphene fluoride by exfoliation of graphite fluoride in dimethylformamide and subsequent reduction with triethylsilane or zinc particles yields the corresponding chemically converted graphene, namely reduced graphene fluoride. The formation of either graphene fluoride (fluorographene) or graphene monolayers in the course of the reaction was demonstrated by several microscopy techniques. Fluorine elimination after reduction was verified by elemental analysis and infrared/Raman spectroscopy.     

Two-dimensional transparent hydrophobic coating based on liquid-phase exfoliated graphene fluoride

Transparent hydrophobic coatings were prepared with graphene fluoride (GF), which was fabricated by a mild and controllable approach of liquid-phase ultrasonic exfoliation of graphite fluoride (GiF) with a selected solvent of ethanol. The structure of the resulting GF sheets with nanoscale thickness was confirmed by transmission electron microscope (TEM), atomic force microscopy (AFM), X-ray diffraction and Raman spectroscopy. Fourier transform infrared spectroscopy and energy dispersive analysis show that C–F bonds from GiF are well preserved in the resulting GF by the method of liquid-phase ultrasonic exfoliation, which are responsible for its excellent hydrophobicity and light transmission. The obtained GF coating has a contact angle of 123° and light transmittance of up to 92% when GF mass density is 0.6 μg cm⁻². Water erosion experiments and ultraviolet aging tests suggest these coatings may possess extended service life outdoors.     

Utilization of multiple graphene layers in fuel cells. 1. An improved technique for the exfoliation of graphene-based nanosheets from graphite

An improved, safer and mild method was proposed for the exfoliation of graphene like sheets from graphite to be used in fuel cells. The major aim in the proposed method is to reduce the number of layers in the graphite material and to produce large quantities of graphene bundles to be used as catalyst support in polymer electrolyte membrane fuel cells. Graphite oxide was prepared using potassium dichromate/sulfuric acid as oxidant and acetic anhydride as intercalating agent. The oxidation process seemed to create expanded and leafy structures of graphite oxide layers. Heat treatment of samples led to the thermal decomposition of acetic anhydride into carbondioxide and water vapor which further swelled the layered graphitic structure. Sonication of graphite oxide samples created more separated structures. Morphology of the sonicated graphite oxide samples exhibited expanded the layer structures and formed some tulle-like translucent and crumpled graphite oxide sheets. The mild procedure applied was capable of reducing the average number of graphene sheets from 86 in the raw graphite to nine in graphene-based nanosheets. Raman spectroscopy analysis showed the significant reduction in size of the in-plane sp² domains of graphene nanosheets obtained after the reduction of graphite oxide.     

RESS制备石墨烯纳米片及防止其再团聚的新方法

本文以天然鳞片石墨为原料,利用超临界状态下二氧化碳的快速膨胀（RESS）来剥离石墨产生石墨烯纳米片。电子显微镜（SEM）表征证实RESS可有效地实现石墨的剥离,并产生了一些石墨烯纳米片层。同时,为了解决再团聚难题,提出利用碳纳米管在产生石墨烯纳米片间穿层的方法和利用小分子包覆法来防止其再团聚,实验证实都起到良好的效果。     

Graphene nanosheets production using liquid-phase exfoliation of pre-milled graphite in dimethylformamide and structural defects evaluation

The non-oxidation-based procedure is proposed for the production of high-quality graphene nanosheets using graphite as the raw materials. This research demonstrated a hybrid two-step production method by liquid-phase exfoliation (LPE) of Premilled graphite in Dimethylformamide (DMF) and compared it with the purely milled and just sonicated samples. However, a simple physical separation procedure composed of two centrifuge processes also designed for the separation of the products in each step. By this process, the exfoliated graphite, less-exfoliated ones and produced nanoparticles are separated, and the less-exfoliated ones are reused again in moderate sonication process. Two grades of graphene nanosheets and a grade of graphitic nanoparticles result at the end. The quality and the nature of defects in all graphene samples produced from LPE, wet milling of graphite and a combination of both, was investigated and discussed by Raman spectroscopy related indices. Raman spectra analysis indicates the adverse effect of sonication power on the in-plane defects formation in the graphene nanosheets which could be hindered by the reduction in power of sonication along with the pre-milling of the graphite. Also inductively-coupled plasma (ICP) and field emission scanning electron microscopy (FE-SEM) analysis used for further characterization of the milled-sonicated sample.     

The synthesis of graphene sheets with controlled thickness and order using surfactant-assisted electrochemical processes

An electrochemical route is reported for the production of graphene sheets using the following steps: electrochemical intercalation of sodium dodecyl sulfate (SDS) into graphite followed by electrochemical exfoliation of a SDS-intercalated graphite electrode. These electrochemical processes yield a stable colloidal graphene/SDS suspension. The potential value for SDS intercalation into graphite plays an important role in determining the structural order, size, and number of layers of synthesized graphene sheets. Raman spectroscopy and transmission electron microscopy results indicate that graphene sheets with the highest structural order and lowest number of layers can be obtained by using relatively high intercalation potentials. Average size and thickness of graphene sheets prepared at high potentials for SDS intercalation into graphite were measured to be about 500 and 1 nm, respectively, indicating presence of graphene sheets as thin as a monolayer. UV–vis spectra of graphene/SDS suspensions show that a large amount of the reduced form of graphene flakes is obtained after successive electrochemical intercalation and exfoliation processes.     

Hydrogenation and exfoliation of graphene using polyamine reagents

An efficient hydrogenation and exfoliation of graphene has been accomplished using polyamines as hydrogenation reagents. The source of graphene can be either chemical vapour deposition grown graphene, bulk graphite or highly ordered pyrolytic graphite (HOPG). Hydrogenation of graphite and HOPG is accompanied by exfoliation yielding suspensions of single-layer and few-layer hydrogenated graphene or graphane. Graphane nanoribbons with aspect ratios greater than ten are produced in abundance during the polyamine hydrogenation of pre-sonicated bulk graphite. Graphane samples have been characterized by transmission electron microscopy, scanning electron microscopy, scanning tunnelling microscopy, low-energy electron diffraction, Auger electron spectroscopy and Raman spectroscopy as well as elemental analysis.     

Incorporate boron and nitrogen into graphene to make BCN hybrid nanosheets with enhanced microwave absorbing properties

This work demonstrates the conversion of graphene oxide into BCN hybrid nanosheets by reaction with boric acid and urea at 900 °C, during which boron and nitrogen atoms are incorporated into the graphene atomic sheets. X-ray diffraction pattern and X-ray photoelectron spectroscopy reveal the existence of h-BN. High-resolution electron microscopy and Raman spectrum indicate the presence of graphene-like layers with h-BN nanodomains. The content of h-BN in the BCN nanosheets can also be tuned by further heat-treatment in an ammonia environment, which in turn affects the band gap of these nanosheets. The electromagnetic parameters suggest that these samples can be used as good microwave absorbing materials at G band (5.6–8.2 GHz) and X band (8.2–12.4 GHz). This study provides a simple route to BCN hybrid nanosheets with tunable band gap and adjustable conductivity for microwave absorbing applications.     

Graphene nanosheets prepared by low-temperature exfoliation and reduction technique toward fabrication of high-performance poly(1-butene)/graphene films

Thermal exfoliation and reduction of graphene oxide (GO) were performed to prepare graphene nanosheets at 300 °C under the ambient atmosphere without any supplementary conditions. The microstructure and morphology of the resulting graphene nanosheets were characterized with scanning electron microscopy, transmission electric microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. The composite films based on poly(1-butene) (PB) and graphene nanosheets were prepared successfully through solution blending and compression molding. The morphological investigation suggested that the graphene nanosheets with nanoscale thickness achieved a homogeneous dispersion in the PB matrix. The composite films exhibited a sharp transition from insulating state to the conducting one with a low percolation threshold, followed by a high electrical conductivity at graphene content higher than 1.6 vol %. The composite films also achieved high dielectric constant with low dielectric loss due to the effective electrical conductive path established by graphene nanosheets in a local range. Moreover, the mechanical evaluation demonstrated that a considerable reinforcement was achieved for the composite films due to the strong interfaces between the graphene nanosheets and PB matrix. The introduction of graphene nanosheets not only enhanced the nucleation capability and crystallinity of PB domain but also improved the thermal stability of the composite films. In addition, the composite films showed an increase in storage modulus and a decrease in loss factors due to the incorporation of graphene nanosheets.     

Utilization of multiple graphene nanosheets in fuel cells: 2. The effect of oxidation process on the characteristics of graphene nanosheets

Structural properties of graphene nanosheets that will be used as electrode material in fuel cells were investigated at different oxidation times. As the oxidation time was increased, the strong bonding between graphene layers in graphite was reduced and graphene layers started to exfoliate forming clusters with a few number of graphene layers. The variations in interplanar spacings, layer number and percent crystallinity indicated how stepwise chemical procedure influenced the morphology of graphite. It was possible to produce relatively flat graphene clusters with definite number of layers by controlling the oxidation time. Graphene nanosheets were characterized in detail by scanning electron microscopy, atomic force microscopy, X-ray diffraction, Raman spectroscopy, and thermal gravimetric analyzer.     

Thermal exfoliation of fluorinated graphite

The thermal exfoliation of graphite fluoride was investigated using two starting materials: fluorinated HOPG and powdered room temperature graphite fluoride (RTGF) post-treated in pure F₂ gas in order to adjust the relative contents of intercalated species, the carbon hybridization and the C–F bonding. Firstly, the thermal exfoliation of HOPG sample (of composition CF_0.57) at the nanoscale is highlighted using scanning tunneling microscopy (STM). Such process involves a defluorination, which is accelerated by disruption of the graphene sheets. Similar breaking occurs during the exfoliation of post-treated RTGF and evolves CF₄ and C₂F₆ gases. Moreover, the exfoliation using a thermal shock is assisted by the fast deintercalation of the catalyst species in the region where the covalence of the C–F bonds is weakened. In such way, exfoliation occurs with the quasi-total defluorination.     

Graphitic platelets prepared by electrochemical exfoliation of graphite and their application for Li energy storage

Electrochemical exfoliation of graphite in the flame-retarded electrolyte is used for the preparation of novel carbon materials for the first time. Graphitic platelets with submicron thickness are prepared by an electrochemical graphite exfoliation route in the trimethyl phosphate (TMP) based electrolyte. The morphology and size of the graphitic platelets can be controlled by adjusting the reaction temperature and current density. A possible mechanism is proposed for the interesting graphite exfoliation in the TMP-based electrolyte. Compared with common micrometer-scale graphite and graphene nanosheets, this novel graphitic material exhibits different voltage profiles but good rate capability as anode in the Li-ion batteries.     

Fabrication of a hybrid graphene/layered double hydroxide material

A hybrid graphene/Ni²⁺–Fe³⁺ layered double hydroxide material has been fabricated by the hydrothermal treatment of a mixed suspension of the exfoliated graphite oxide, Ni(NO₃)₂ 6H₂O, Fe(NO₃)₃·9H₂O, urea and trisodium citrate. The Ni²⁺–Fe³⁺ layered double hydroxide platelets are first homogeneously grown on the surface of GO nanosheets which are then reduced to graphene under a mild hydrothermal treatment. In the hybrid graphene/Ni²⁺–Fe³⁺ layered double hydroxide material, the restacking of graphene nanosheets is effectively prevented by the formation of Ni²⁺–Fe³⁺ layered double hydroxide platelets, and the graphene nanosheets exist in a complete exfoliation state.     

Electrically conductive polyethylene terephthalate/graphene nanocomposites prepared by melt compounding

Graphene nanosheets were prepared by complete oxidation of pristine graphite followed by thermal exfoliation and reduction. Polyethylene terephthalate (PET)/graphene nanocomposites were prepared by melt compounding. Transmission electron microscopy observation indicated that graphene nanosheets exhibited a uniform dispersion in PET matrix. The incorporation of graphene greatly improved the electrical conductivity of PET, resulting in a sharp transition from electrical insulator to semiconductor with a low percolation threshold of 0.47 vol.%. A high electrical conductivity of 2.11 S/m was achieved with only 3.0 vol.% of graphene. The low percolation threshold and superior electrical conductivity are attributed to the high aspect ratio, large specific surface area and uniform dispersion of the graphene nanosheets in PET matrix.     

Exfoliation of graphene nanosheets in aqueous media

Graphene has attracted much attention and holds great promise in various applications due to its extraordinary properties. To realize applications of graphene in large scale, developing a facile, green and cost-effective method for mass production of high-quality graphene is highly desired. Relative to expensive and complicated bottom-up approaches, top-down methods for graphene production are promising owing to their low cost and simplicity. Specifically, exfoliation of graphene nanosheets in liquid phase is favorable for their dispersion, functionalization and processing. Instead of highly toxic organic solvents, using water as the liquid medium makes exfoliation process eco-friendly and sustainable. In this review, recent progress on exfoliation of graphene nanosheets in water is discussed, with a particular focus on exfoliation and stabilizing mechanism in various aqueous media. Different water-based exfoliation methods, such as liquid-phase exfoliation and electrochemical exfoliation, are surveyed.     

Highly efficient and large-scale synthesis of graphene by electrolytic exfoliation

Highly efficient and large-scale synthesis of graphene from graphite was produced by electrolytic exfoliation using poly(sodium-4-styrenesulfonate) as an effective electrolyte. Scanning and transmission electron microscopy, and atomic force microscopy confirmed the existence of monolayer graphene sheets and stacks containing a few graphene sheets. Raman spectroscopy demonstrated that the as-prepared graphene sheets have low defect content. Based on the measurement of FTIR spectra, the edge-to-face interaction (π-π interaction) between the graphene surface and aromatic rings of poly(sodium-4-styrenesulfonate) could be primarily responsible for producing exfoliation of the graphite electrode to graphene during electrolysis. In contrast to micromechanical exfoliation, electrolytic exfoliation can be scaled up for large-scale and continuous graphene production.     

电化学阴极剥离制备少层石墨烯及其微型超级电容器

以石墨为原料高效、绿色、低成本制备少层石墨烯,对石墨烯的规模化生产和应用具有非常重要的意义。电化学阴极剥离法是一种高效制备少层石墨烯的方法,但已有的报道均采用有机溶液体系,成本高且不够绿色环保。开发了一种绿色的水溶液电化学剥离方法,在6 mol·L^-1 KOH溶液中,将石墨作为阴极进行快速剥离制备出少层石墨烯。获得的少层石墨烯具有含氧量低[1.27%(质量)]、缺陷少(I_D/I_G < 0.035)、片径尺寸为5~10 μm、高电导率(大于200 S·cm^-1)以及良好溶液可加性等特点。基于此,采用叉指型掩模板辅助过滤的方法可以高效制备出图案化石墨烯基平面微电极,在硫酸-聚乙烯醇凝胶电解液中,构筑的准固态微型电容器在没有金属集流体存在的情况下,表现出高扫描速率,达到了100000 mV·s^-1,弛豫时间常数低至24 ms;以1-乙基-3甲基-咪唑双(三氟甲基磺酰基)亚胺和双(三氟甲基磺酰基)亚胺锂盐的混合液为电解液,所构建的微型超级电容器的工作电压达4.0 V,体积能量密度为113 mW·h·cm^-3,远高于目前报道的微型超级电容器的电化学性能(<50 mW·h·cm^-3)。     

Graphene nanosheets as cathode catalysts for lithium-air batteries with an enhanced electrochemical performance

Graphene nanosheets have been investigated as cathode catalysts for lithium-air batteries with alkyl carbonate electrolyte. Field emission scanning electron microscopy, transmission electron microscope and Raman spectroscopy have confirmed the high quality of the as-prepared graphene nanosheets and the surface analysis has identified the mesoporous characteristic of graphene nanosheets. The electrochemical properties of graphene nanosheets as cathode catalysts in lithium-air batteries were evaluated by a galvanostatic charge/discharge testing. The reaction products on the graphene nanosheets cathode were analyzed by X-ray diffraction and Fourier transform infrared spectroscopy. The graphene nanosheet electrodes exhibited a much better cycling stability and lower overpotential than that of the Vulcan XC-72 carbon. This work demonstrated that graphene nanosheets could be an efficient catalyst for lithium-air batteries.     

The effect of heat treatment on formation of graphene thin films from graphene oxide nanosheets

Graphene thin films with very low concentration of oxygen-containing functional groups were produced by reduction of graphene oxide nanosheets (prepared by using a chemical exfoliation) in a reducing environment and using two different heat treatment procedures (called one and two-step heat treatment procedures). The effects of heat treatment procedure and temperature on thickness variation of graphene platelets and also on reduction of the oxygen-containing functional groups of the graphene oxide nanosheets were studied by atomic force microscopy and X-ray photoelectron spectroscopy. While formation of the thin films composed of single-layer graphene nanosheets with minimum thickness of 0.37 nm and nearly without any functional group bonds was observed at the high temperature of 1000 °C in the one-step reducing procedure, similar high quality graphene thin films were obtained at the lower temperature of 500 °C in our two-step reducing temperature. The results also indicated possibility of efficient reduction of the graphene oxide thin films at even lower heat treatment temperatures (?500 °C).     

Peeling off graphene from Co nanoparticle covered graphite in a scanning tunneling microscope

We demonstrate that graphene layers can be removed from atomically flat graphite terraces that are covered with deposited Co nanoparticles (NPs) using the tip of a scanning tunneling microscope. Peeling with monolayer resolution is achieved by scanning the NP covered graphite surface in close proximity. In this exfoliation process part of the lifted upper graphene layer is cut from this layer, while the surrounding part relaxes back to the underlying graphite surface. This gives rise to the appearance of spatially varying Moiré patterns and to a modified electronic interaction with the underlying graphite, which is probed by scanning tunneling spectroscopy. NP assisted exfoliation provides a promising route for preparation of (few-layer) graphene under ultra-high vacuum conditions, ideally suited for high resolution scanning tunneling microscopy and spectroscopy based experiments.