Exfoliation Of Graphite Intercalation Compounds Research Articles

Strength dependence of epoxy composites on the average filler size of non-oxidized graphene flake

Enhancing the filler-to-matrix contact area by incorporating large graphene flakes (GFs) is considered a viable approach for improving the mechanical properties of polymer composites. Graphene oxide (GO) flakes have been initially pursued for this application owing to the advantage of producing large-sized GFs. However, the defective nature of GO makes it technically challenging to precisely assess their dimensional effect on resulting mechanical properties, limiting their versatility as reinforcing materials. Therefore, it is highly desired to fabricate GFs with minimum defects and large lateral size. In this paper, we report the fabrication of high-quality edge functionalized non-oxidized graphene flakes (f-NOGFs) by the liquid exfoliation of graphite intercalation compounds. As-prepared f-NOGFs are over 1 μm in lateral size and contain less than 6.25% of impurity oxygen. By using size-sorted f-NOGFs, we systematically studied the relationship between mechanical properties and lateral sizes in f-NOGFs-incorporated epoxies. We identify that epoxy composites containing 0.6 wt% of f-NOGFs with a lateral size of 1 μm present outstanding mechanical properties; elastic modulus of 3.65 GPa, ultimate tensile strength of 95.74 GPa and toughness of 2.52 MJ m−3. The study presented in this paper could provide better understanding for optimization of the mechanical reinforcement of graphene-polymer composites.

Factors Affecting the Exfoliation of Graphite Intercalation Compounds for Graphene Synthesis

We investigate the mechanism of the intercalation-aided exfoliation of graphite using van der Waals force-corrected density functional theory (DFT) calculations. From a comparative study on various intercalation systems, we find that, depending on the intercalant species, the exfoliation energies vary significantly, not only due to the size of intercalants but also due to interactions with the host graphite. While it is generally perceived that an expanded interlayer distance with intercalants weakens the binding between graphene layers, as the van der Waals forces decrease, the calculations reveal that the intercalation of electronegative or electropositive intercalants (e.g., Li, K, F, Cl, and Br) result in a 1.5–5-fold higher exfoliation energy than pristine graphite due to additional binding forces from charge transfer between intercalants and graphene layers. Furthermore, we demonstrate that this additional binding force could be manipulated with cointercalation or neutral intercalants, which hints at effective exfoliation strategies with graphite intercalation compounds. This theoretical study broadens our understanding of the mechanism underlying graphite exfoliation and will facilitate development of more effective exfoliation strategies for other related layered materials.

Graphene Made by Mechanical Exfoliation of Graphite Intercalation Compound

Graphene Made by Mechanical Exfoliation of Graphite Intercalation Compound

Graphene Made by Mechanical Exfoliation of Graphite Intercalation Compound

We report a method of making few-layer graphene flakes by mechanically exfoliating SbCl5–graphite intercalation compounds (GICs). The number of layers of exfoliated graphene flakes had a particular distribution relevant to the stage structure of the GICs. The carrier doping of the few-layer graphene flakes was about two orders of magnitude smaller than that expected from the stoichiometry of the GICs. The measured electric mobility was comparable to that made from pristine graphite.

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.

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.

Graphene: preparation and structural perfection

Three basic processes for the production of monolayer graphene are reviewed: cleavage of graphite crystals, exfoliation of graphite intercalation compounds and chemical vapor deposition. The relationship of the structural perfection of the obtained thin flakes to the preparation method is discussed.

Improved hydrogen sorption/desorption capacity of exfoliated NiCl 2-graphite intercalation compound effected by thermal treatment

In the present work an advantageous effect of thermal exfoliation of graphite intercalation compound with nickel chloride (NiCl 2-GIC) on the kinetics of the reaction of hydrogen sorption/desorption occurring in alkaline solution in electrodes made of exfoliated material is proven. The electrochemical investigations were carried out using the cyclic voltammetry method. It was shown that the changes occurred in the crystalline and chemical structure and morphology due to the subsequent thermal exfoliation of NiCl 2-GIC primarily exfoliated by chemical method give rise to the increase of the anodic charge associated with both the recovery reaction of hydrogen stored in the electrode material during the precedent process of hydrogen adsorption/absorption and the oxidation reactions of metallic nickel. The improvement of the reversible hydrogen storage capacity is attributed to both the porous structure of graphite and the presence of nickel oxide highly dispersed in the exfoliated graphite matrix. The anodic charge corresponding to the recovery reaction of hydrogen stored in the electrode material appeared to be the highest for NiCl 2-GIC subjected to chemical exfoliation in hydrogen peroxide solution followed by heattreatment at 800 °C in air. After this process a two-fold enhancement of the hydrogen storage capacity was noted.

Preparation of exfoliated graphite by microwave irradiation

Exfoliated graphite (EG) is an important raw material for the production of flexible graphite sheets, which have been widely used as gaskets, thermal insulators, fire-resistant composites, etc. [1–3]. Recently this material was shown to have excellent capabilities for oil sorption and recovery [4,5]. The preparation of EG was usually done by rapid heating of either graphite residue or intercalation compounds, but also conducted by different heating systems, including inductively coupled plasma, laser irradiation and flame heating [1]. The exfoliation of graphite intercalation compounds by means of microwave irradiation was also reported, which gave no significant difference in resultant EG from those prepared by rapid heating [6]. The same conclusion was obtained from the analysis of pore structure inside the worm-like particles of EG prepared by microwave irradiation [7]. Exfoliation under microwave irradiation seems to be interesting process because it can be performed at room temperature. However an accumulation of the experimental data through the fundamental studies on the influences of the microwave irradiation power, the precursors of graphite and intercalate, for example, is required for the practical application of this process. In the present work, the exfoliation of two residue compounds with H2SO4 under microwave of different powers was carried out. The residue compound GIC1 was prepared through electrochemical intercalation (anodic oxidation) of H2SO4 with an electric power consumption of 7.7 A h/ kg, followed by water rinsing [8,9]. The residue compound GIC2 was prepared through washing the intercalation compound synthesized in concentrated H2SO4 with H2O2 at room temperature (chemical intercalation) [10]. SEM photos of these two residue compounds are shown in Fig. 1. The residue compound GIC2 was partially exfoliated along the c-axis of the original graphite flakes. Two precursors were exfoliated under microwave irradiation, applying different powers from 500 to 1000 W for 40 s. For each experiment, the precursor powder was placed at the bottom of the beaker in a layer of about 1 mm thickness, ensuring that all particles were in mutual contact. EG samples thus prepared were characterized by the mass loss during the exfoliation, BET surface area, bulk density, SEM observation and thermoexfoliation coefficient. Bulk density was measured from mass and volume of the samples after exfoliation. The thermoexfoliation coefficient defined as the volume ratio of exfoliated graphite to its precursor, VEG/VGIC, was calculated by measuring the change in sample volume before and after exfoliation. For comparison, two precursors were also exfoliated by a rapid heating to 1000 C. Under microwave irradiation, a marked exfoliation of the precursors was observed, accompanied by fuming and lightening. The characteristics of EGs obtained are summarized in Table 1. After microwave irradiation, a marked volume increase of the lump of both residue compounds after microwave irradiation were observed, just like in the case of rapid heating to 1000 C. EG obtained was confirmed to be composed from so-called worm-like particles as shown in Fig. 2. The pore structure in these worm-like particles was analyzed by an aid of image processing in our previous paper [7]. The apparent distance between neighbouring balloons on the worm-like particles looks different, depending on the precursor residue compound and also on exfoliation conditions. These distances in EG prepared from the precursor GIC1 under 1000 W power were not homogeneous, as shown in Fig. 2(a), but that

Electrochemical Characterization of Binderless, Recompressed Exfoliated Graphite Electrodes: Electron-Transfer Kinetics and Diffusion Characteristics

Exfoliated graphite (EG) is prepared by the thermal exfoliation of graphite intercalation compounds at different temperatures. Surface and bulk physicochemical properties of EG are followed by spectroscopic and analytical methods and are observed to be a function of exfoliation temperature. EG particles can be recompressed without any binder and used as surface-renewable electrodes. Surface preparation is accomplished by either polishing or roughening the electrode surface using emery sheets. Effects of exfoliation temperature and the surface preparation on the electron-transfer kinetics and on the diffusion characteristics have been followed by electrochemical methods using several benchmark redox systems. It is found that the electron-transfer kinetics and the diffusion of K(4)[Fe(CN)(6)] are affected by the nature of the EG surface while that of iron(II)(1,10-phenanthroline)(3) and cobalt(II)(1,10-phenanthroline)(3) are not affected by the surface preparation. The redox systems are classified into different groups according to their kinetic sensitivity. Diffusion of electroactive species toward the EG electrodes is found to nonlinear. Current-time plots suggest that the recompressed EG electrodes can be modeled as fractals.

Exfoliation of Graphite Intercalation Compounds: Classification and Discussion of the Processes from New Experimental Data Relative to Graphite-Acid Compounds

Exfoliation of Graphite Intercalation Compounds: Classification and Discussion of the Processes from New Experimental Data Relative to Graphite-Acid Compounds

Exfoliation exothermique des composés d'insertion du carbone contenant de l'acide perchlorique

Exfoliation of graphite intercalation compounds (GIC) is usually carried out by an abrupt heating above a critical temperature. This method does not allow exfoliating graphite particles whose average diameter is smaller than 200 μm. In this paper, we propose using an exothermic dissociation of the intercalate for exfoliating small particles of graphite or carbons. Dilute perchloric acid is one intercalate that presents this property. Its intercalation into graphite and cokes was carried out by dipping the materials into the following liquids: • 70% HClO 4 heated at controlled temperatures (thermal process[4]) or with CrO 3 assistance (usual chemical process[1,3]); • HClO 4-HNO 3-H 2O mixtures characterized by the molecular percentage of each species (Table 2). In this latter case, the GICs based on H.O.P.G. were characterized by X-ray diffraction at different durations of intercalation; it appears that the presence of a volumic concentration of commercial grade perchloric acid lower than 75% allows synthetizing a first stage GIC, whereas a higher concentration leads only to second stage compounds (Table 2). Chemical analyses of these GICs (Table 3) indicate that the proportion of intercalated HClO 4 increases with the perchloric acid concentration in the liquid mixture in excess. Moreover, thermal analyses of the graphite perchlorate (based on Madagascar flakes) indicate an exothermic peak which is attributed to the intercalate decomposition with exfoliation of the intercalation compound (Figure 3). This behaviour led us to study the exothermic exfoliation of natural graphite particles of different diameters and of coke particles (Table 1). Characterization of the exfoliated material was performed by means of scanning electron microscopy and krypton (noted by ∗) or nitrogen physisorption. The study of a homogenous set of exfoliated samples based on Madagascar natural graphite flakes indicated that the specific surface areas increases with the HClO 4 proportion in the liquid mixture (Table 4). As concerns the carbon materials intercalated by the thermal process or by dipping into a 50/50 nitric-perchloric mixture and exfoliated (Table 5), one can observe that: • the nature of the intercalate influences the specific area of the exfoliated material: in fact, exfoliation of HClO 4-GICs prepared by the thermal process leads to a specific surface area of 153 m 2/g, whereas that of the exfoliated HClO 4-HNO 3-GIC is only 42 nr/g. • the specific surface area of the exfoliated natural graphite decreases with the average diameter of the pristine graphite (see Tables 1 and 5). • the ratio of the specific area of exfoliated coke to that of pristine coke indicates that the exfoliation quality is comparable to the one of the natural graphite. The S.E.M. observations (Figure 4) on these two families of carbon materials show the vermicular aspect characteristic of exfoliation. In the case of the exfoliated coke (HTT 1000°C), the exfoliation seems to concern only the particle edges. In conclusion, it seems to be possible to exfoliate all kinds of carbon materials that are able to intercalate dilute perchloric acid.