Edges Of Graphene Layers Research Articles

Facile Edge Functionalization of Graphene Layers with a Biosourced 2-Pyrone

Edge functionalization of graphene layers is of great interest in the field of materials chemistry: the properties of graphene are substantially unaltered and its compatibility and chemical reactivity with various environments can be tuned. In this work, edge functionalization of graphene layers was performed with a 2-pyrone, ethyl 3-hydroxy-2-oxo-2H-pyran-6-carboxylate (Pyr-COOEt). 2-Pyrones are C-6 unsaturated heterocyclic sugar derivatives and are intriguing building blocks for the preparation of innovative chemical structures. Sodium 3-acetoxy-2-oxo-2H-pyran-6-carboxylate was prepared starting from mucic acid, in a one-pot synthesis with a yield of about 74%, and was then transformed into the acid and then into ethyl ester derivatives. The adduct of Pyr-COOEt with a high surface area graphite (HSAG) was obtained by simply mixing and donating energy, either thermal or mechanical. The functionalization yield was estimated from thermogravimetric analysis (TGA) data and was found to be up to 91%. The adducts were characterized by Fourier transform infrared and Raman spectroscopies and wide-angle X-ray diffraction. The presence of pyrone in the adduct was clearly detected in the IR spectra, and the bulk structure of the graphitic substrate was found to be substantially unaltered by the functionalization reaction. The experimental findings suggest that the edge functionalization of the graphene layers occurred. Stable water dispersions of HSAG/Pyr adducts were prepared and studied through ultraviolet–visible analysis. Aggregates of few-layer graphene were obtained by mild sonication and centrifugation, as revealed by high-resolution transmission electron microscopy. This paper shows that a biobased molecule can be used for selectively decorating the edges of graphene layers, with oxygenated functional groups having a defined chemical structure and avoiding the use of oil-based, dangerous, and even noxious ingredients. The most plausible interpretation for the formation of the HSAG adduct with 2-pyrone seems to be the cycloaddition reaction between the edges of the graphitic substrate and the unsaturated biomolecule. Such a functionalization appears to be suitable for a scale up and paves the way for the preparation of a variety of derivatives.

FLG/silver nanoparticles: Nanocomposite by green synthesis

This study presents a green synthesis method for the exfoliation of FLG (few-layer graphene) and their functionalization with silver nanoparticles using the extract from the plant Jatropha Cordata. This is a competitive synthesis method, given that it uses graphite as the precursor material of graphene, and avoids using highly toxic compounds in the exfoliation, stabilization and functionalization processes. The relation between the intensity of the bands D and G, as well as the diverse contributions that make up band 2D suggest the exfoliation of graphene. The structural studies were performed using FFT (Fast Fourier Transform), which allowed the identification of crystalline planes on graphene and silver nanostructures. The vibrational study through FTIR (Fourier-transform Infrared Spectroscopy) allowed the identification of functional groups (hydroxyl, carbonyl and carboxyl) in the molecules of the extract. The silver nanostructures showed affinity towards the interaction of the edge of graphene layers.

The impact of adsorption on the localization of spins in graphene oxide and reduced graphene oxide, observed with electron paramagnetic resonance

We report the observations of electronic properties of graphene oxide and reduced graphene oxide, performed with electron paramagnetic resonance technique in a broad temperature range. Both materials were examined in pure form and saturated with air, helium, and heavy water molecules. We show that spin localization strongly depends on the type and amount of molecules adsorbed at the graphene layer edges (and possible in-plane defects). Physical and chemical states of edges play crucial role in electrical transport within graphene-based materials, with hopping as the leading mechanism of charge carrier transport. Presented results are a good basis to understand the electronic properties of other carbon structures made of graphene-like building blocks. Most active carbons show some degree of functionalization and are known of having good adsorptive properties; thus, controlling both phenomena is important for many applications. Sample treatment with temperature, vacuum, and various adsorbents allowed for the observation of a possible metal-insulator transition and sorption pumping effects. The influence of adsorption on the localization phenomena in graphene would be very important if to consider the graphene-based material as possible candidates for the future spintronics that works in ambient conditions.

Self-magnetism induced large magnetoresistance at room temperature region in graphene nanocrystallited carbon film

We report large positive magnetoresistance (MR) of over 12% at 273 K in graphene nanocrystallited pure carbon film. MR behaviors at different temperatures implied that low temperature MR was from carrier diffusive scattering and room temperature MR was from spin arrangement effect. Temperature dependences of the film resistance and magnetization recognized that as temperature decreased from 300 to 200 K, transitions occurred on the electrical transporting process from conductive mode to semi-conductive mode, and the nanocrystallited structure showed competition of ferromagnetic and antiferromagnetic interactions. The large room temperature MR was ascribed to the ferromagnetic order of spin magnetic moment arrangement at the of graphene layer edges.

Nitrogen species in few-layer graphene produced by thermal exfoliation of fluorinated graphite intercalation compounds

Few-layer graphene materials have been produced by decomposition of fluorinated graphite intercalated with acetonitrile (CH3CN) or isopropylamine ((CH3)2CHNH2) in the conditions of thermal shock. Electronic structure of the samples before and after exfoliation was studied using near-edge X-ray absorption fine structure and X-ray photoelectron spectroscopy. Analysis of the spectroscopic data revealed the same set of nitrogen species in both exfoliation products that indicates decomposition of the intercalated molecules to the atomic state. Most of the nitrogen atoms were incorporated at the edges of graphene layers in pyridinic and protonated nitrogen forms. A difference in the electrochemical performance of the graphene materials was related with the exfoliation degree and the defect density in the graphene layers. Schematic representation of a process of thermal exfoliation of acetonitrile-intercalated fluorinated graphite.

Insight into the role of the oxidized graphite precursor on the properties of copper-based MOF/graphite oxide composites

Graphites of different origin were oxidized using Brodie method and then used as composite components with copper-based metal–organic frameworks, MOF. The degree of oxidation, crystallite size and chemistry of oxygen-containing groups on the surface of graphite oxides were investigated. The parent materials, MOF and graphite oxides, and the composites were characterized by X-ray diffraction, adsorption of nitrogen, X-ray photoelectron spectroscopy, elemental analysis, scanning electron microscopy in combination with energy dispersive X-ray spectroscopy, transmission electron microscopy, thermal analysis and Fourier transform infrared spectroscopy. The results show that not only epoxy groups but also carboxylic groups at the edges of graphene layers, when present in a predominant quantity, are engaged in the formation of metal–organic framework units. The contribution of the latter mechanism of the composite formation results in a more pronounced synergistic effect, demonstrated in the formation of new pores, and the distorted MOF units at the edges of the graphene layers. When carboxylic groups are absent or present in a small quantity, epoxy, phenol and carbonyl groups are the main reactive sites for the composite formation. The disruption of the MOF crystallites, governed by the geometry and chemistry of graphite oxides, also leads to the formation of copper salts at the edges of the graphene crystals. The morphology of the composite materials and their apparent chemistry depends on the chemistry of graphite oxide, its crystallite size and its amount in the composite.

Flowing nitrogen assisted-arc discharge synthesis of nitrogen-doped single-walled carbon nanohorns

Nitrogen-doped single-walled carbon nanohorns (N-SWCNHs) have been synthesized by a flowing nitrogen assisted arc discharge method at atmospheric pressure in a tubular reactor. X-ray diffraction and thermogravimetric analysis have revealed their high quality. Scanning electron microscopy and transmission electron microscopy examinations have shown that N-SWCNHs have typical spherical structure with a diameter of 40–80nm. Oxidation treatment suggests the opening of cone-shaped caps of N-SWCNHs. The FT-IR and X-ray photoelectron spectroscopy analysis indicate that most of the nitrogen atoms are in N-6, N-5, and triple-bonded CN bonding configuration present at the defect sites or the edges of graphene layers.

Effect of soot microstructure on its ozonization reactivity

Large uncertainty among the measured uptake coefficients of O(3) on soot highlights the importance of the sources and chemical structures of soot samples in this reaction. Soot samples with different microstructures were prepared by combusting n-hexane under controlled conditions. Their reactivities to O(3) were further investigated using in situ Raman spectroscopy. The fuel/oxygen ratio in the combustion experiments not only affected the diameter of the primary particles, but also influenced the micro-chemical structure of soot. Average diameters of soot particles decreased with the decreasing fuel/oxygen ratio. Compared to the "fuel-rich" flame soot, the "fuel-lean" flame soot showed lower structural uniformity with higher disordered carbon content at the graphene layer edges (D1 band) and the surface graphene layers (D2 band) and the amorphous carbon content (D3 band). This disordered carbon was identified as the reactive component for the ozonization of both the "fuel-rich" and "fuel-lean" flame soot samples. The kinetics study demonstrated that the disordered carbon at the surface graphene layers was more active than that at the graphene layer edges in one sample, and the reactivity of these two microstructures types to O(3) in the "fuel-rich" flame soot was higher than that in the "fuel-lean" flame soot.

A Simple Road for the Transformation of Few-Layer Graphene into MWNTs

We report the direct formation of multiwalled carbon nanotubes (MWNT) by ultrasonication of graphite in dimethylformamide (DMF) upon addition of ferrocene aldehyde (Fc-CHO). The tubular structures appear exclusively at the edges of graphene layers and contain Fe clusters. Fc in conjunction with benzyl aldehyde, or other Fc derivatives, does not induce formation of NT. Higher amounts of Fc-CHO added to the dispersion do not increase significantly MWNT formation. Increasing the temperature reduces the amount of formation of MWNTs and shows the key role of ultrasound-induced cavitation energy. It is concluded that Fc-CHO first reduces the concentration of radical reactive species that slice graphene into small moieties, localizes itself at the edges of graphene, templates the rolling up of a sheet to form a nanoscroll, where it remains trapped, and finally accepts and donates unpaired electron to the graphene edges and converts the less stable scroll into a MWNT. This new methodology matches the long held notion that CNTs are rolled up graphene layers. The proposed mechanism is general and will lead to control the production of carbon nanostructures by simple ultrasonication treatments.

Influence of Combustion Conditions on Hydrophilic Properties and Microstructure of Flame Soot

Previous studies suggest that structure and reactivity of soot depend on combustion conditions like the fuel/oxygen ratio and nature of fuels. However, the essence of how combustion conditions affect physical and chemical properties of soot is still an open question. In this study, soot samples were prepared by combusting toluene, n-hexane, and decane under controlled conditions, and their hydrophilic properties, morphology, microstructure, content of volatile organic compounds, and functional groups were characterized. The hydrophilicity of n-hexane and decane flame soot increased with decreasing fuel/oxygen ratio, while it almost did not change for toluene flame soot. Fuel/oxygen ratio had little effect on the morphology of aggregates and the graphite crystallite size. The primary particle size and the content of volatile organic compounds on soot decreased with decreasing fuel/oxygen ratio. Less hydrophobic groups (C-H) and more hydrophilic groups (C═O) were observed on lean n-hexane and decane flame soot than that on the corresponding rich flame soot. Volatile organic compounds had little effect on the hydrophilicity of soot while the hydrophilicity correlated linearly with the ratio of C═O content to C-H content. The hydrophilic functional groups were found to be mainly located at graphene layer edges and on surface graphene layers in soot.

Surface chemical modifications induced on high surface area graphite and carbon nanofibers using different oxidation and functionalization treatments

Two graphitic carbon materials with different edge to basal plane ratio, high surface area graphite (HSAG) and graphitized carbon nanofibers (CNFs), were oxidized by two methods, aqueous-HNO 3 wet oxidation and oxygen plasma oxidation. Characterization of the materials by temperature-programmed desorption, thermogravimetry and X-ray photoelectron and Raman spectroscopies indicated that the amount and nature of oxygen surface groups introduced depended on the oxidation method and on the structure of the original material. While surface sites within the layers were only oxidized by oxygen plasma, surface sites at the edges of graphene layers were oxidized by both treatments being the wet oxidation more effective. Modification of the oxidized materials with a diamine or a triamine molecule resulted in the formation of ammonium carboxylate salt species on the carbon surface.

Effect of silver nanoparticles deposited on micro/mesoporous activated carbons on retention of NOx at room temperature

Wood-based activated carbon was modified by deposition of silver using Tollens method. Adsorbents with various contents of silver were used to study NO(2) and NO (the product of NO(2) reduction by carbon) retention. The surface of the initial and exhausted materials was characterized using adsorption of nitrogen, XRD, SEM/EDX, FTIR and TA. The results indicated that with an increasing content of silver on the surface the capacities to retain NO(2) and NO increase until the plateau is reached. The performance depends on the dispersion of nanoparticles and their chemistry. Highly dispersed small silver metal particles promote formation of chelates with NO(2) and/or with NO. An excess of Tollens reagent results in formation of larger silver crystals and silver oxide nanoparticles. If sufficiently dispersed, they also enhance the retention of NO(2) via formation of nitrates deposited in the pore system. The surface of the carbon matrix is also active in NO(2) retention, providing the small pores and edges of graphene layers, where the reductions of NO(2)/oxidation of carbon take place.

KOH activation of pitch-derived carbonaceous materials—Effect of carbonization degree

Two series of mesophase pitches and semi-cokes of different carbonization degree were produced by heat treatment of anthracene oil derived pitches P1 and P4 in the temperature range of 460–700 °C. These carbonaceous materials were activated with potassium hydroxide at 700 °C using 1:3 reagents ratio to assess the effects of the precursor optical texture and carbonization degree on the activation behavior. The results show that the increase in the pitch pretreatment temperature suppresses propensity to the pore generation while enhancing particle breaking. The effect can be illustrated by decreases in the BET surface area S BET from ~ 2700 to ~ 1500 m 2 g −1 and the micropore volume V DR from ~ 0.85 to ~ 0.45 cm 3 g −1. These parameters are inversely related with the H/C atomic ratio of precursor. In contrast, the anisotropic development of pitch coke, varying from flow type to mosaics, has a slight effect on the activation behaviour. The mechanism of porosity generation, that is proposed, stresses the role of hydrogen occurring at the edges of graphene layers and potassium metal insertion/deinsertion on the porosity development and particle disintegration during KOH activation of pitch-derived carbons.

Surface structural disordering in graphite upon lithium intercalation/deintercalation

We report on the origin of the surface structural disordering in graphite anodes induced by lithium intercalation and deintercalation processes. Average Raman spectra of graphitic anodes reveal that cycling at potentials that correspond to low lithium concentrations in Li x C (0 ≤ x < 0.16) is responsible for most of the structural damage observed at the graphite surface. The extent of surface structural disorder in graphite is significantly reduced for the anodes that were cycled at potentials where stage-1 and stage-2 compounds ( x > 0.33) are present. Electrochemical impedance spectra show larger interfacial impedance for the electrodes that were fully delithiated during cycling as compared to electrodes that were cycled at lower potentials ( U < 0.15 V vs. Li/Li +). Steep Li + surface-bulk concentration gradients at the surface of graphite during early stages of intercalation processes, and the inherent increase of the Li x C d-spacing tend to induce local stresses at the edges of graphene layers, and lead to the breakage of C–C bonds. The exposed graphite edge sites react with the electrolyte to (re)form the SEI layer, which leads to gradual degradation of the graphite anode, and causes reversible capacity loss in a lithium-ion battery.

Anode performance of boron-doped graphites prepared from shot and sponge cokes

The structures and anode performances of graphitized pristine and boron-doped shot and sponge cokes have been comparatively studied by means of scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and galvanostatic measurement. The results show that high degree of graphitization can be obtained by the substituted boron atom in the carbon lattice, and boron in the resultant boron-doped graphites mainly exist in the form of boron carbide and boron substituted in the carbon lattice. Both of boron-doped graphites from shot and sponge cokes obtain discharge capacity of 350 mAh g −1 and coulombic efficiency above 90%. Apart from commonly observed discharge plateau for graphite, boron-doped samples in this study also show a small plateau at ca. 0.06 V. This phenomenon can be explained that Li ion stores in the site to be void-like spaces that are produced by “molecular bridging” between the edge sites of graphene layer stack with a release of boron atoms substituted at the edge of graphene layer. The effect of the amount of boron dopant and graphitization temperature on the anode performance of boron-doped graphite are also investigated in this paper.

Open and Closed Edges of Graphene Layers

Edge structures of thermally treated graphite have been studied by means of atomically resolved high-resolution TEM. The method for the determination of a monolayer or more than one layer graphene sheets is established. A series of tilting experiments proves that the zigzag and armchair edges are mostly closed between adjacent graphene layers, and the number of dangling bonds is therefore minimized. Surprisingly bilayer graphene often exhibits AA stacking and is very hard to distinguish from a single graphene layer. Open edge structures with carbon dangling bonds can be found only in a local area where the closed (folding) edge is partially broken.

Structure of the carbon nanofilaments formed by liquid phase carbonization in porous anodic alumina template

Platelet structure carbon nanofilaments of ∼30 nm in diameter have been prepared by heating a mixture of porous anodic alumina template and poly(vinyl)chloride (PVC) powders in an argon atmosphere, and the change in their structure and morphology with heat treatment temperature, ranging from 600 to 2800 °C, has been examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen gas adsorption measurements. The diameter of the carbon nanofilaments formed does not change with heat treatment temperature, being in agreement with the pore diameter of the template, while their length is reduced with the temperature. The platelet-type orientation of graphene layers is evident even at 600 °C with the layer structure further developing with increasing heat treatment temperature. The carbon nanofilaments formed at lower temperatures have micropores, while those formed at higher temperatures do not have porosity. Highly graphitized carbon nanofilaments have been obtained after heat treatment at 2800 °C, with another characteristic structural feature being presence of loops at the edge of graphene layers formed at 2800 °C.

High rate capability of carbon nanofilaments with platelet structure as anode materials for lithium ion batteries

Carbon nanofilaments (CNFs) with platelet structure have been prepared by liquid phase carbonization using porous anodic alumina template, and their lithium ion insertion/extraction properties have been examined as a function of heat treatment temperature and filament diameter. The CNFs heat-treated at 1000 °C reveal higher capacitance and higher rate capability compared with those heat-treated at higher temperatures. Further, it is found that higher reversible capacity is obtained for the CNFs with reduced diameter. The reversible capacity of highly graphitized CNFs formed at 2800 °C is less than 200 mA h g −1 at a current density of 50 mA g −1, being far lower than the theoretical capacity (372 mA h g −1) of graphite. A probable reason is the presence of loop at the edge of graphene layers.

Catalysts for the Oxygen Reduction from Heat-Treated Iron(III) Tetramethoxyphenylporphyrin Chloride: Structure and Stability of Active Sites

Structure and stability of an iron-based catalyst for the oxygen reduction reaction, prepared by heat treatment of carbon-supported iron(III) tetramethoxyphenylporphyrin chloride (FeTMPP−Cl), were investigated. The oxygen reduction in acid electrolyte was examined with the rotating (ring) disk electrode. The measurements confirmed that H2O2 is generated as a byproduct of the oxygen reduction. The structural elucidation of the catalyst showed that the porphyrin decomposes during heat treatment. Nitrogen atoms of the heat-treated porphyrin become bonded at the edge of graphene layers as pyridine- and pyrrole-type nitrogen. Two Fe3+ components as well as metallic, carbidic and oxidic iron were detected by Mössbauer spectroscopy. An electrochemical longevity test and two degradation experiments with sulfuric acid and H2O2 showed that H2O2 causes the degradation of active sites. A 6-fold coordinated Fe3+ compound seems to be responsible for the catalytic activity. Only 8% of the primary iron content is present in the active iron component.

Infrared Spectroscopy of Carbon Materials: A Quantum Chemical Study of Model Compounds

The present work reports a theoretical study of the infrared spectra of chemical structures that are suitable to the description of the surface chemistry of carbon materials. Prior to any consideration, the computational approach was tested and adapted by comparing the predicted IR spectra to those obtained experimentally for various reference compounds. Several models were considered, subsequently accounting for the most relevant functional groups that have been postulated to decorate the edges of graphene layers on carbon materials (i.e., anhydrides, carboxyls, lactones, phenolic, quinones, and pyrones). For each of the previous functional groups, different structures involving a different number of fused rings were considered. This strategy allowed us to establish the effect of conjugation on the shift of the IR frequencies corresponding to a given functional group. Cooperative effects between different functional groups (phenol−carboxyl, phenol−lactone, and so on) were another aspect that revealed itself to be an interesting issue when assigning frequencies in the IR spectra of highly oxidized carbon materials. Thus, it was found that the frequencies of the CO bonds present in acid functional groups were systematically lowered when phenolic groups were close enough to establish hydrogen bonds. Special attention was also paid to the elucidation of the origin of the 1600-cm-1 band of carbons. It was found that, in the case of acid carbons, this band can be assigned to CC stretching of carbon rings decorated mainly with phenolic groups. Cyclic ethers in basic carbons would also promote absorption in the 1600-cm-1 region of the IR spectrum. Finally, the predicted assignments are employed to interpret the IR spectra obtained experimentally for several activated carbons.