氧化石墨烯剥离方法的研究进展

用石墨制备氧化石墨是大规模合成石墨烯的起点,也是实现石墨烯大量制备的有效方法之一。目前。制备氧化石墨的工艺相对成熟,而有效剥离出不同尺寸的石墨烯以满足不同的需求,还存在诸多技术难点需要突破。本文着重介绍了氧化石墨烯剥离方法的研究进展,包括热膨胀剥离、超声剥离、机械剥离、静电剥离、低温剥离等方法;最后展望了氧化石墨烯研究工作的发展前景。     

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)。     

A note on using expanded graphite for achieving energy- and time-efficient production of graphene nanoplatelets via liquid phase exfoliation

Although easily scalable, the production of graphene nanoplatelets (GNP) by the means of liquid-phase exfoliation of graphite flakes (GF) remains an energy- and time-intensive process. In this work, we demonstrate that significant time and energy can be saved in GNP production when employing expanded graphite (EG) in a surfactant-assisted liquid phase exfoliation process. Owing to its increased interlayer distance, the exfoliation of EG can be accomplished in a much shorter time (<30 min) compared to GF (approximately 7 h in the present case). Moreover, the energy required for the EG exfoliation is close to 80-fold lower than that for GF exfoliation. Monitoring of the mean lateral dimension, specific surface area, and graphite flake-to-GNP transition during exfoliation was performed experimentally using several analytical techniques. The EG-derived GNPs are produced much faster and require less energy for exfoliation compared to GF, thus making it a more efficient alternative technique.     

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。     

Overpotentials and solid electrolyte interphase formation at porous graphite electrodes in mixed ethylene carbonate-propylene carbonate electrolyte systems

The first electrochemical lithium insertion was characterized for several graphite materials with high degree of crystallinity, different particle size distributions and surface morphologies in an ethylene carbonate (EC)/propylene carbonate (PC) electrolyte. For coarser graphite materials and graphites with a low superficial defect concentration, an irreversible process was observed which correlated with the electrochemical exfoliation of graphite. Different natural and synthetic graphites with similar particle size distribution and active surface area showed differences in the passivation behavior during the first electrochemical reduction. The fraction of graphite particles exfoliating during the first galvanostatic lithium insertion linearly increased with length of the irreversible plateau, which concomitantly moved to more positive potentials. This behavior can be rationalized when considering, besides the surface structure, local overpotentials for the solid electrolyte interphase formation process, and especially the overpotential distribution through the graphite electrode. These overpotentials cause a distribution of the local current density attributed to the passivation process. Optimizing the particle contacts in the electrode by applying mechanical pressure or by selecting the proper binder decreased the overpotentials and suppressed the graphite exfoliation in the EC/PC electrolyte. Therefore, both graphite surface structure and electrode engineering aspects have to be considered for successful passivation against exfoliation.     

Synthesis of a pyrene-functionalized hyperbranched polyethylene ternary copolymer for efficient graphite exfoliation in chloroform and formation of ethylene-vinyl acetate/graphene nanocomposites

A strategy for preparing ethylene-vinyl acetate (EVA)/graphene nanocomposites from liquid-phase exfoliated graphene has been explored with the use of a pyrene-functionalized hyperbranched polyethylene (HBPE) ternary copolymer, HBPE@Py@PMA, as stabilizer for graphite exfoliation. The HBPE@Py@PMA was synthesized by combining the Pd-diimine-catalyzed chain walking ternary ethylene copolymerization and atomic transfer radical polymerization techniques and has been confirmed to possess a HBPE core simultaneously bearing pyrene terminal groups and polymethacrylate side chains. As stabilizer, it is found to effectively promote graphite exfoliation in CHCl₃ to render high-quality few-layer graphene with an efficiency as high as 43%. Meanwhile, it can be steadily adsorbed on the exfoliated graphene surface to concurrently render functionalized graphene well dispersible in EVA matrix with strong interfacial interactions. This allows us to obtain EVA/graphene nanocomposites from resulting graphene dispersion through simple solution mixing process. By adding only 0.5 wt% of graphene, the dielectric constant of resulting composite increases by 55% compared to pure EVA, with a dielectric loss only 0.012. The role mechanism of the HBPE@Py@PMA for promoting graphite exfoliation in CHCl₃ and the formation of EVA/graphene nanocomposites from the resulting graphene has been proposed.     

The influence of the local current density on the electrochemical exfoliation of graphite in lithium-ion battery negative electrodes

The local current density related to the exfoliation process of graphite negative electrodes in mixed ethylene carbonate/propylene carbonate electrolytes was controlled by a variation of the current applied to lithium half-cells containing either single type graphite electrodes or electrodes consisting of mixtures of an exfoliating and a non-exfoliating graphite. The partial local current density attributed to graphite passivation and its distribution within the volume of the electrode was found to be a key parameter to explain differences in the exfoliation behaviour observed for graphite electrodes. The local current density is related with a local overpotential which may suppress one of several possible electrochemical processes. In a negative electrode consisting of a mixture of a non-exfoliating and an exfoliating graphite component, the exfoliation of the exfoliation-sensitive graphite component can be completely suppressed when increasing the partial local current density attributed to the graphite exfoliation process above a certain threshold, by either decreasing the amount of exfoliating graphite particles in the electrode or by increasing the total current density, i.e., the specific current. The consideration of the local current density distribution for the electrochemical processes throughout the electrode leads to a more general concept for the graphite passivation behaviour during the first lithium insertion in lithium-ion batteries.     

High-rate production of few-layer graphene by high-power probe sonication

Graphene can be obtained via sonication-assisted liquid-phase exfoliation, but the production rate of few-layer graphene (FLG) is still low. A high-power probe sonicator was used in this study to improve the production rate of FLG, and its performance was optimized by controlling processing parameters such as the initial graphite concentration, surfactant concentration and liquid volume. By optimizing processing parameters, production rate of FLG dispersions in N-methyl-2-pyrrolidone (NMP) were greater than 1 g/h, which is the best value achieved in the sonication-assisted exfoliation process. In the case of liquid exfoliation in water/surfactant solution, production rate of FLG was achieved approximately 0.28 g/h. Our work here demonstrates that graphene concentration in a probe-sonication process does not depend on the shape of vessel, and it is predictable by power law models.     

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.     

Enhancements of clay exfoliation in polymer nanocomposites using a chemical blowing agent

A novel approach, via the use of a blowing agent (BA), to enhance clay exfoliation in polymer nanocomposites is reported. A commercial organoclay was treated with a common low‐cost BA, which was then melt‐processed with polymers in an extruder. The characterization results revealed that the degrees of exfoliation of the clay were increased when using BA. Consequently, the resultant nanocomposites exhibited improvements in thermal degradation temperature as compared to their corresponding polymers and nanocomposites without the involvement of BA. They also presented simultaneous improvements in stiffness, strength, ductility and toughness. These property enhancements were mainly attributed to the presence of BA in clay interlayers which degraded during melt compounding, generated gases and pushed clay layers apart, facilitating clay exfoliation under the shear forces. This work demonstrates the potential of using a BA to produce high‐performance polymer/clay nanocomposites by melt processing. © 2014 Society of Chemical Industry     

A comparative study on different aqueous-phase graphite exfoliation methods for few-layer graphene production and its application in alumina matrix composites

Four different methods (attrition milling, shear mixing, low-power bath sonication and tip sonication) used for the aqueous-phase, surfactant-assisted exfoliation of graphite were compared. Few-layer graphene (FLG) concentration, yield and production rate were measured for each method at different production times and the quality of the as produced FLG was determined using Raman spectroscopy and X-ray diffraction. It was inferred from the results that a combined method comprising tip sonication and shear exfoliation would offer the best balance between quality and quantity of FLG for relatively short processing times (<6 h). FLG dispersions produced with this method were used to fabricate 1 wt.% FLG/Al₂O₃ nanocomposites by ball milling and extrusion, followed by pressure-less sintering. The influence of the FLG addition on the microstructure and mechanical properties was studied, with observed increases of 26.4% and 67.6% in flexural strength and fracture toughness,respectively, and a 25.3% decrease in average grain size.     

The Exfoliation of Graphite Particles in the Vibratory Disk Mill

The exfoliation of graphite particles during the treatment in the vibratory disk mill containing commercial sugar was investigated. During the collision of the grinding ring with the grinding chamber wall an intensive stressing of graphite takes place. After treatment in the mill, the resulting dark paste was solved in water and subjected to 10 min sonication. The suspension was centrifuged to remove the not exfoliated multilayered particles. Analysis of Raman spectra shows the presence of graphite sheets in suspension. The represented method of exfoliation is environmentally friendly and scalable.     

On the Consistency of the Exfoliation Free Energy of Graphenes by Molecular Simulations

Monolayer graphene is now produced at significant yields, by liquid phase exfoliation of graphites in solvents. This has increased the interest in molecular simulation studies to give new insights in the field. We use decoupling simulations to compute the exfoliation free energy of graphenes in a liquid environment. Starting from a bilayer graphene configuration, we decouple the Van der Waals interactions of a graphene monolayer in the presence of saline water. Then, we introduce the monolayer back into water by coupling its interactions with water molecules and ions. A different approach to compute the graphene exfoliation free energy is to use umbrella sampling. We apply umbrella sampling after pulling the graphene monolayer on the shear direction up to a distance from a bilayer. We show that the decoupling and umbrella methods give highly consistent free energy results for three bilayer graphene samples with different size. This strongly suggests that the systems in both methods remain closely in equilibrium as we move between the states before and after the exfoliation. Therefore, the amount of nonequilibrium work needed to peel the two layers apart is minimized efficiently.     

Vacuum-assisted microwave reduction/exfoliation of graphite oxide and the influence of precursor graphite oxide

One-step synthesis of high quality graphene at gram-scale quantities is important for industrial applications, e.g. in electrochemistry for sensing and energy storage. Currently, thermal reduction/exfoliation of graphite oxide (GO) is a typical method of choice. However, it has the drawback of requiring specialized equipment for rapid thermal shock. A recent alternative method, microwave-assisted exfoliation, usually suffers from poor reduction of graphite oxide and thus low C/O ratios. Herein we show that vacuum-assisted microwave reduction/exfoliation of graphite oxide in a closed system leads to high C/O ratios and partial hydrogenation of graphene (2.6 at.% of H). Microwave irradiation of graphite oxide in vacuum leads to outgassing from GO and the creation of plasma which aids temperature distribution and hydrogenation. This plasma is quickly extinguished by further dramatic evolution of gases from GO and consequent pressure increase. We assess the influence of precursor graphite oxide, prepared by Hummers, Staudenmaier, and Hofmann methods, upon the materials properties of microwave exfoliated graphene. We show that microwave-exfoliated graphenes prepared from different graphite oxides show very fast heterogeneous electron transfer rates, with similar electrochemical behaviour to thermally reduced graphene oxide.     

High-yield water-phase exfoliated few-defect graphene for high performance polymer nanocomposites

Application of graphene requires a high-yield, low-cost, scalable production method, but it remains highly challenging. We here report a water-phase technique to produce few-defect graphene nanosheets (FGS) with a high exfoliation yield (92%), based on the chemically expanded graphite with ultrahigh specific surface areas, and demonstrate the application in graphene-polymer nanocomposites. The exfoliated FGS has low degree of oxidation and preserves good mechanical and electrical properties, revealing promising potential for improving comprehensive properties of polymer composites. When 0.5 wt% FGS was incorporated to poly(methyl methacrylate) (PMMA), the 5% weight loss temperature and storage modulus increase by 87°C and 21%, respectively, relative to the neat polymer. With increasing the content of FGS to 4.6 wt%, the glass transition temperature of the composite increases by 25°C. In addition, the composites show a percolation threshold as low as 0.25 vol% and excellent electrical conductivity (50 S/m for 2.7 vol% FGS-PMMA composite).     

A simple mechanical technique to obtain carbon nanoscrolls from graphite nanoplatelets

A simple approach for the bulk production of carbon nanoscrolls (CNSs) is described. This method is based on the application of shear-friction forces to convert graphite nanoplatelets into carbon nanoscrolls using a bi-axially oriented polypropylene (BOPP) surface. The combined action of shear and friction forces causes the exfoliation of graphite nanoplatelets and the simultaneous roll-up of graphite layers. Evidence of the CNS formation is given by optical microscopy, scanning electron microscopy, and transmission electron microscopy. These investigations reveal that the CNSs have a long tube-like and fusiform structure with a hollow core surrounded by few layers of graphene. Micro-Raman spectroscopy shows that the produced structures are not defect free, and optical spectroscopy reveals distinctive features due to the presence of two weak absorption bands at 224 and 324 nm.     

Fabrication of polyethylene/graphite nanocomposite from modified expanded graphite

A novel process was employed to fabricate a polymer/expanded graphite nanocomposite by modifying the conducting filler expanded graphite (EG) with unsaturated polyester resin (UPR). The modified expanded graphite (MEG) was prepared from EG in which the graphite nanosheets, already present in EG, were wrapped and isolated by the UPR during processing. The as‐prepared MEG was reduced to powder form to improve its dispersion in the matrix. MEG powders were embedded into a high‐density polyethylene (HDPE) matrix via melt‐extrusion in a single‐screw extruder to prepare the conducting composite. The as‐prepared HDPE/EG conducting composite exhibited a low percolation threshold of ～5.7 wt% due to the high aspect ratio of graphite nanosheets. Mechanical properties such as the tensile and impact strength were also studied. Scanning electron microscopy was used to characterize the microstructure of EG, MEG powder and the resulting nanocomposites. Copyright © 2006 Society of Chemical Industry     

Ammonia borane assisted solid exfoliation of graphite fluoride for facile preparation of fluorinated graphene nanosheets

Fluorinated graphene, which combines the unique properties of graphite fluoride and graphene, has attracted considerable attention in recent years. Here, we developed a facile, efficient, and scalable method for high-yield exfoliation of graphite fluoride into fluorinated graphene (fluorographene) nanosheets. The exfoliation approach consists of solid ball milling of graphite fluoride with ammonia borane and followed washing with ethanol to get rid of ammonia borane from the products. The majority of the as-synthesized fluorographene nanosheets consist of 1–6 atomic layers with grain sizes in the range of 0.3–1 μm. X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy demonstrated that fluorographene has the same structure as pristine graphite fluoride.     

Development of modified expanded graphite‐filled solution polymerized styrene butadiene rubber vulcanizates in the presence and absence of carbon black

Expanded graphite (EG) consists of a huge number of partially exfoliated graphite sheets with the thickness of most of the sheets in the nanometer range. This study analyses the effect of EG and modified EG (MEG) on the mechanical, thermal, and dynamic mechanical properties of solution styrene butadiene rubber (SSBR) with and without carbon black (CB). The surface of the EG was modified to enhance its dispersion in the SSBR matrix. In addition, oil‐extended carboxylated styrene butadiene rubber (XSBR) was used as a compatibilizer to disperse MEG in the base nonpolar SSBR matrix. XSBR/MEG nanocomposites have been prepared by solution mixing. The obtained nanocomposites were incorporated into the SSBR matrix in the presence of CB by melt blending. Morphological properties of the nanocomposites revealed intercalation of MEG sheets in the SSBR matrix. Nanocomposites containing MEG in the presence of CB show improvement in mechanical, thermal, and dynamic mechanical properties. POLYM. ENG. SCI., 54:33–41, 2014. © 2013 Society of Plastics Engineers     

液相剥离石墨烯的制备及其电化学性能

在有机溶剂中超声剥离膨胀石墨制备单层和多层石墨烯,对其微观形貌和结构进行表征,采用四探针表征方法考察了溶剂、时间和还原剂对石墨烯电导率的影响,优化了制备工艺参数,分析了石墨烯的性能. 结果表明,以丙酮为分散剂时石墨烯导电率最高,可达219.09 S/cm,石墨烯具有良好的电化学性能,比电容达18.8 F/g,在pH=7的磷酸盐缓冲溶液中,在0.1 V电压下对Pb2+有灵敏的溶出峰,检出限位达0.05 mmol/L.