Surface Of Exfoliated Graphite Research Articles

Preparation of magnetic composite sorbent based on exfoliated graphite with metallic iron, cobalt and nickel using melamine as a reducing agent

Known preparation methods of the magnetic sorbents based on exfoliated graphite (EG) with metallic phase take several hours and requires the use of a large amount of reducing gas at a high temperature, which is not technologically advanced. The aim of the work is to obtain EG modified with iron, cobalt, and nickel using new simple method. This method includes the thermal treatment in nitrogen atmosphere of the mixture of expandable graphite, Mn+ nitrates (Mn+ = Fe3+, Co2+, Ni2+) and melamine. The thermal decomposition of melamine leads to formation of ammonia, which reduces the products of the Mn+ nitrates decomposition (α-Fe2O3, Co3O4/CoO, NiO) to metallic iron, cobalt and nickel. The use of different amount of melamine as reducing agent leads to the formation of mixtures with different compositions containing iron oxides Fe3O4, FeO, metallic iron α-Fe, Fe-C alloy γ-(Fe,C) and iron carbide Fe3C, which were confirmed by Mossbauer spectroscopy. Metallic Co and Ni on the EG surface is formed in nitrogen atmosphere by reduction with carbon even without melamine. Obtained EG/Fe has the highest saturation magnetization of 54.7 emu/g. EG/Co and EG/Ni have lower saturation magnetization of 40.1 and 12.0 emu/g, which is due to lower saturation magnetization of the individual metals.

Visible light responsive MoS2/Ag@WO3/EG photoanode with highly stable Z-scheme induced circular electron motion pioneered by Exfoliated graphite for bisphenol a photoelectrodegradation

The alarming issue of endocrine disrupting chemicals increase in aquatic environment has heightened concerns about their potential impact on human health and ecosystem. Taking cognizance of this, the current investigation focused on fabrication of effective and efficient photoanode (MoS2/Ag@WO3/EG) from molybdenum disulphide/silver@tungsten trioxide (MoS2/Ag@WO3) photocatalyst and exfoliated graphite (EG) for the photoelectrocatalytic degradation of bisphenol A (BPA). The characterization results and Density Functional Theory (DFT) calculations together affirmed a strong interfacial contact and charge transfer direction at the interface of the materials for substantial promotion of charge separation and increased photoelectrocatalytic performance. The charge separation was further increased by EG’s “give-accept or redistribution” strategy of the charge at the interface that allowed circular motion of electrons in the formed Z-scheme heterostructure. The MoS2/Ag@WO3/EG photoanode exhibited the highest photoelectrocatalytic activity for BPA degradation with efficiency of 100% at optimum experimental conditions. The high degradation efficiency was attributed to uplifted visible light absorption emanating from EG surface plasmon resonance (SPR) contribution, formation of Z-scheme for efficient charge separation, and high stability of the photoanode, which possessed renewable surface. Moreover, scavenger experiments confirmed the participation of all the radicals (●O2– >●OH > h+) in BPA decomposition. Based on XPS, DFT, and radical experiments, the Z-scheme mechanism was proposed, and this work shed light on the production of highly effective house-made photoanode for photoelectrocatalytic processes, thus this can be a good fit in the industrial setup for degradation of other pollutants like microbials.

CoS nanoparticle encapsulated S-doped carbon nanofiber/nanotube hybrid grown on exfoliated graphite for long-lifespan and high-rate potassium ion batteries

Exfoliated graphite (EG) as important matrix material for potassium-ion batteries (PIBs) suffers from its inherent low surface activity, which results in poor interactions with active matter (e.g. metal sulfides) and undesired initial capacity decay of the EG-based electrodes. To address these issues, in this study, an attempt for EG’ surface engineering has been carried out through a stepwise gas-phase strategy. A S-doped carbon nanofiber/nanotube (CNF/CNT) hybrid was in-situ introduced on the surface of EG to form a novel micro-/nano- multiscale matrix structure. CoS nanoparticles are encapsulated in the inner of CNFs and CNTs. The special core–shell configuration not only prevents the aggregation of CoS during the sulfidation process, but also confines the active nanoparticles to avoid pulverization and maintains stable integrated electrode structure. Moreover, the couple effect between the active site-rich nature of CNFs and the high conductivity of CNTs as well as the formation of 3D porous network contribute to substantially elevated reversible capacity (411 mAh g−1), excellent initial cycling stability and rate capability (184 mAh g−1 at 2.0 A g−1). The effective surface modification strategy dominated by rational hybridization of CNFs and CNTs provides a new avenue for the development of high-performance electrode materials in PIB applications.

Electrical Properties of Electrochemically Exfoliated Graphite with Copper Addition

In this study, the electrical properties of exfoliated graphite (EG) with copper addition is reported. Firstly, EG samples were synthesized using electrochemical exfoliation of the pre-treated graphite sheet then combined with various copper concentrations. X-Ray Diffraction (XRD) characterization indicated that the composites of EG, Cu, and Cu2O were formed. A higher Cu2O amount obtained when the higher concentration of copper precursor was added in the EG. Scanning Electron Microscopy (SEM) images showed that more particles with larger sizes were anchored on the EG surface as higher copper concentration. The electrical properties of the samples were measured using the four-point probe method. The highest electrical conductivity of 37.86 S/cm was produced in the EG sample with 1 wt.% of copper addition. The result reveals that the optimum copper addition can increase the electrical properties of EG.

Oil removing properties of exfoliated graphite in actual produced water treatment

Removal of oil from actual produced water (composed of mainly oil and water emulsion) by exfoliated graphite (EG) was investigated. Laboratory scale sorption columns of EG could reduce the oil concentration in produced water from 278 and 66mg/L to 1.2mg/L, and undetectable, respectively. A separate experiment with TOC (total organic carbon) demonstrated that TOC decreased from 566mg/L in the feed solution to 1.5mg/L in the effluent. Both results (oil densitometer and TOC) showed excellent ability of EG for oil removal from produced water. SEM and FT-IR observations confirmed the deposition of oily material on the surface of EG during the water treatment process. TG-DTA of oil-sorbed EG showed a gradual weight decrease in the temperature range between 100–600°C, being considered to be the decomposition of sorbed oil. Raman spectroscopy and XRD measurements of EG samples, before and after the treatment, indicated no structural change of EG. Molecular dynamics simulations revealed that octadecane molecules are selectively adsorbed on the EG surface. Effect of isomer type of octadecane molecules on the sorption energy was also discussed.

Interaction of the Helium, Hydrogen, Air, Argon, and Nitrogen Bubbles with Graphite Surface in Water.

The interaction of the confined gas with solid surface immersed in water is a common theme of many important fields such as self-cleaning surface, gas storage, and sensing. For that reason, we investigated the gas-graphite interaction in the water medium. The graphite surface was prepared by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG). The surface chemistry and morphology were studied by X-ray photoelectron spectroscopy, profilometry, and atomic force microscopy. The surface energy of HOPG was estimated by contact angle measurements using the Owens-Wendt method. The interaction of gases (Ar, He, H2, N2, and air) with graphite was studied by a captive bubble method, in which the gas bubble was in contact with the exfoliated graphite surface in water media. The experimental data were corroborated by molecular dynamics simulations and density functional theory calculations. The surface energy of HOPG equaled to 52.8 mJ/m2 and more of 95% of the surface energy was attributed to dispersion interactions. The results on gas-surface interaction indicated that HOPG surface had gasphilic behavior for helium and hydrogen, while gasphobic behavior for argon and nitrogen. The results showed that the variation of the gas contact angle was related to the balance between the gas-surface and gas-gas interaction potentials. For helium and hydrogen the gas-surface interaction was particularly high compared to gas-gas interaction and this promoted the favorable interaction with graphite surface.

One-step microwave preparation of a Mn3O4 nanoparticles/exfoliated graphite composite as superior anode materials for Li-ion batteries

The fabrication of exfoliated graphite (EG) is highly polluting due to the discharge of large amount of manganese-contained wastewater. Here, a facile and green chemistry route is developed to prepare a Mn3O4 nanoparticles (NPs)/EG composite by artfully tuning the traditional fabrication process of EG. During this treatment, Mn3O4-NPs with high crystallinity and uniform dimension of ∼7nm are found to be homogeneously and firmly anchored on the surface of EG. The composite as an anode material of Li-ion batteries exhibits favorable electrochemical performances, such as decay-free charge capacity of 655mAhg−1 extending to 120 cycles and excellent rate capability.

Electrochemical properties of exfoliated graphite affected by its two-step modification

Exfoliated graphite (EG) was modified by a two-stage process consisting of electrochemical oxidation followed by the thermal treatment. Within the former one, the process of re-intercalation of H2SO4 into the EG by linear sweep voltammetry was carried out. Thus, obtained re-intercalated EG underwent heat treatment at 800 °C in order to synthesize re-exfoliated EG (re-EG). The electrochemical features of the re-EG were examined in the model process of phenol electrooxidation carried out by cyclic voltammetry technique in alkaline solution with and without phenol addition. Taking into account the anodic charges as a main criterion of electrochemical activity, it was found that the modification of EG caused over twofold improvement of its electrochemical activity. This behavior is related with the changes within the chemical composition of modified EG surface and on much smaller scale with the modification of its structure. The degree of electrochemical activity improvement depends on the conditions under which the processes of re-intercalation and re-exfoliation were performed. The results of Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis supported by the data of the Brunauer-Emmett-Teller (BET) surface area and scanning electron microscope (SEM) observations allow an understanding of the physicochemical properties of re-exfoliated EG and enhancement of its electrochemical activity.

Exfoliated graphite modified with pyrolytic carbon

Exfoliated graphite (EG) has been modified with pyrolytic carbon produced by 750°C carbonization of poly(vinylidene fluoride) (PVDF), tolylene diisocyanate (TDI), and polyisocyanate (PIC) applied to the EG surface from solutions. The pyrolytic carbon obtained had the form of particles ranging in size from 200 nm to 1 μm. The microstructural characteristics of the carbon-carbon materials depend on the chemical composition of the carbon precursor: S = 15.8 to 63 m2/g; nitrogen sorption from 24.7 to 59.2 cm3/g (−190°C); micropore volume, up to 7.4 mm3/g; benzene vapor absorption from 0.07 to 0.27 g/g (20°C).

Exfoliated graphite as a flexible and conductive support for Si-based Li-ion battery anodes

Exfoliated graphite (EG) was found to be a flexible and conductive support of anode materials for lithium ion batteries through the preparation of the composite of pyrolytic carbon-coated nano-sized silicon nanoparticles supported by exfoliated graphite (pC-Si-EG). Electrochemical analyses revealed that pC-Si-EG composite delivered a high capacity of 902.8mAhg−1 at a current density of 200mAg−1 and an excellent cycling stability with 98.4% capacity retention after 40 cycles. It was found that the polycrystalline silicon nanoparticles went through some very interesting changes and broke up into smaller Si nanoparticles dispersing onto the surface of EG after charging/discharging cycles. The results demonstrated that EG plays an important role in the superior electrochemical performances of pC-Si-EG anode due to its high porosity, excellent electronic conductivity and good flexibility.

Sulfur-free exfoliated graphite with large exfoliated volume: Preparation, characterization and its adsorption performance

Sulfur-free exfoliated graphite (EG) with large exfoliated volume (760mLg−1) was prepared through chemical oxidation, followed by microwave exfoliation. SEM results indicated that well-developed network pores in the worm-like structure of EG could be clearly observed. XPS results displayed that some functional groups were yielded on the surface of the as-prepared EG. Moreover, the capacity of EG was evaluated by the adsorption of rodamine B and 4-chlorophenol aqueous solution. Results showed that EG exhibited high adsorption capacity for RhB and 4-CP, which could be attributed to the functional groups and well-developed network pores in the worm-like EG.

Multiple anodic regeneration of exfoliated graphite electrodes spent in the process of phenol electrooxidation

The present paper deals with the studies concerning anodic regeneration of exfoliated graphite (EG) electrodes coated with oligomer products of incomplete phenol electrooxidation. The electrochemical activity of regenerated samples depends on the concentration of regenerating electrolyte and potential applied during the regeneration. The effective method of a single-step regeneration of spent EG electrodes was found to be anodic treatment proceeded in 6 M KOH at the potential of 1.3 V vs. Hg/HgO/0.5 M KOH. During the oxidative regeneration of EG, the physical and chemical interactions between intensively generated active oxygen and/or OH· radical and oligomer covering the EG surface take place. As has been proved by FTIR and XPS analyses, active oxidative agents cause significant modification of chemical composition of oligomer that resulted in enhanced concentration of surface functionalities mainly involving C=O bonds. The results of multiple regeneration of EG electrode, considered as promising method for practical application, showed that the highest efficiency of regeneration is attained due to potentiostatic treatment carried out in 6 M KOH at the potential of 1.2 V. The electrochemical activity of EG regenerated at this potential increases gradually with the number of regenerating loops and after the third regeneration is almost fourfold higher compared to that of the original EG.

Models for Type VI Adsorption Isotherms from a Statistical Mechanical Formulation

In this study, two theoretical expressions for the Type VI isotherm in the IUPAC classification are presented. The formulation of these new expressions is based on a rigorous statistical mechanical description. The expressions allow for the estimation of physicochemical parameters within the theoretical model. The proposed models allow a good correlation to Type VI experimental isotherms taken from the literature such as adsorption isotherms for adsorption of methane onto MgO (100) and onto an exfoliated graphite surface. The parameters intervening in the adsorption process have been deduced directly from experimental adsorption isotherms by numerical simulation. The theoretical expressions provide an understanding and interpretation of the adsorption isotherms at the microscopic level.

Degradation of methyl orange in artificial wastewater through electrochemical oxidation using exfoliated graphite electrode

Exfoliated graphite (EG), with good electronic conductivity, was used as an anode material for the electrochemical degradation of methyl orange (MO), an artificial textile wastewater. Through the electrochemical degradation, 98.6% color removal and 58.5% chemical oxygen demand reduction was achieved at a pH of 2.0, a current density of 13 mA·cm −2, an electrolyte (NaCl) concentration of 0.1 mol·dm −3 and an electrolysis time of 20 min. The degradation is based on chemical adsorption and electro-oxidation. MO molecules are first adsorbed on the EG surface by hydrogen bonds between the -OH functional groups on EG and the MO molecules, which are then degraded by electrochemical oxidation. EG can therefore be regarded as an excellent material for the treatment of textile wastewater.

Improved electrooxidation of phenol at exfoliated graphite electrodes

In the presented paper, we report on electrochemical oxidation of phenol occurring at exfoliated graphite (EG) in alkaline solution. The mechanism of the electrocatalytic reaction of phenol oxidation was modified on adding methanol to the phenol-containing electrolyte. Using the voltammetry method, the influence of methanol additive on cyclic behavior of EG electrode was examined. A particular attention has been paid to the first two cycles when an abrupt decrease in electrocatalytic activity of various electrode materials has been observed. The results obtained showed that in the presence of methanol EG, electrode preserves its electrocatalytic activity for a longer time of phenol oxidation. In the absence of methanol in a phenol/KOH electrolyte, the charge of phenol oxidation peaks decreases sharply on cycling, whereas in the presence of methanol, the observed drop is considerably inhibited. The anodic charge attained for the 15th cycle of phenol oxidation in methanol-admixed electrolyte is the same as that for the third cycle recorded in methanol-free electrolyte. The thermogravimetric analysis (TG), Fourier-transformed infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) data showed that an improved electrocatalytic activity of EG can be accounted for by new chemical composition of oligomer film built on the EG surface with the participation of methanol and/or the products of its oxidation.

Reversible Electrochemistry of Cytochrome c on Recompressed, Binderless Exfoliated Graphite Electrodes

The direct electrochemistry of cytochrome c (cyt-c) has been investigated on exfoliated graphite (EG) electrodes. The as-polished and roughened (using SiC emery sheet) EG surfaces are inactive for the direct electron transfer. However, when the EG electrode was sonicated before the experiment, a pair of redox waves were obtained for freely diffusing cyt-c in the solution phase. The formal potential was found to be 0.01 V (vs. SCE) in 0.1 M phosphate buffer at a pH of 7.1. The electrochemical response for the adsorbed cyt-c on sonicated EG electrodes, which is shown to have carbonyl functional groups on its surface, shows nearly reversible voltammograms in the same electrolyte. However, the formal potential in the adsorbed state is more negative than that observed for the solution phase cyt-c. A structure based on an open heme conformation proposed by Hildebrandt and Stockburger is probably present on the EG surface. It is suggested that the electrochemistry at the EG electrode is essentially governed by favourable electrostatic interactions.

Electrooxidation of phenol at exfoliated graphite electrode in alkaline solution

The present paper reports on exfoliated graphite (EG) used for the cyclic electrochemical process of phenol oxidation in alkaline solution. It is shown that the electrochemical activity of anode-produced EG decreases considerably in the second cycle due to the deposition of an oligomer film, composed of the products of phenol oxidation, on the EG surface. Thermal treatment of the inactive graphite anode in air at 500 °C provided a regenerated material of activity three times higher for the first cycle and 2.6 times higher for three cycles as compared to the original anode. The reason for such a behavior is assigned to a carbon film formed on the EG surface during the carbonization/oxidation processes involving the products of phenol oxidation. Comparative studies showed that electroactivity of the original EG can also be enhanced if before the process of phenol oxidation the original EG is activated by heat treatment. Unfortunately, the electrochemical activity of the product of such a treatment is higher only for the first cycle of phenol oxidation and drops dramatically in the following cycles.

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.

Renewable surface electrodes based on dopamine functionalized exfoliated graphite:: NADH oxidation and ethanol biosensing

Exfoliated graphite (EG) was modified by covalently attaching dopamine (DA) (3,4-dihydroxyphenethylamine) through amide linkages, using COOH groups introduced on the EG surface. The modified material was characterized by FT-IR spectroscopy, X-ray photoelectron spectroscopy and electrochemical techniques. Composites of DA modified EG dispersed in organically modified silicates were prepared by a sol–gel process. Electrodes were fabricated by casting the composites in glass tubes. The sol–gel based electrodes were found to be active for the electrocatalytic oxidation of NADH and biosensing of ethanol in presence of NAD + and alcohol dehydrogenase enzyme. The modified composite electrodes were found to be stable for several months. The surface of the electrode could be renewed just by mechanically polishing the electrode using emery sheets. The modified EG was also pressed and restacked in the form of a pellet and the use of this material as a binderless bulk-modified electrode was also demonstrated. The performance of sol–gel derived composite EG electrodes with binderless bulk-modified EG electrodes was compared.