Rolling up a Graphene Sheet

Carbon Nanotubes, CNTs, have been described as rolled‐up graphene layers. Matching this concept to experiments has been a great experimental challenge for it requires a method to exfoliate graphite, generate ordered and stable dangling carbon bonds, and roll up the layer without affecting the unpaired electrons of the dangling bonds that finally have to zip up in an orderly fashion: A tall order for any synthetic strategy. The combined use of ultrasonication of graphite in dimethylformamide and addition of ferrocene aldehyde just does it!     

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.     

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.     

Electrochemical synthesis and the functionalization of few layer graphene in ionic liquid and redox ionic liquid

Electrochemical reductive exfoliation of graphite to few layered graphene(FLG) in presence of 1-ethyl-2,3-dimethyl imidazolium bis(trifluoromethylsulfonyl) imide ionic liquid and redox ionic liquid based ferrocene has been investigated. Thus, by applying a mild negative potential(-2.7 V vs. Fc/Fc~+) to carbon electrode in ionic liquid graphene flakes could be generated. The generated materials have been characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, high resolution transmission electron microscopy and atomic force microscopy. XPS and Raman analysis show that the electrochemical reductive exfoliation provides the formation of FLG. The thickness of the resulting FLG was found to be ranged between 4 and1 nm. HR-TEM images reveal the formation of few graphene layers and in some cases single graphene layer was observed.Moreover, this electrochemical route conduces to the formation of ionic liquid functionalized FLG. Finally, the reductive exfoliation was further investigated in the presence of redox ionic liquid. XPS and electrochemical measurements confirm the presence of ferrocene.     

改进液相氧化还原法制备高性能氢气吸附用石墨烯

以液相氧化还原法为基础,并在分散剂十二烷基苯磺酸钠(SDBS)作用下制备得到高质量石墨烯,有效避免了在此过程中石墨烯大量团聚的现象.采用X射线衍射(XRD)、拉曼光谱(RS)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和原子力显微镜(AFM)等分析手段对石墨烯样品进行了表征.XRD结果体现了石墨、氧化石墨和石墨烯晶型结构的区别;SEM和TEM结果显示石墨烯呈网格状,表面平整,缺陷少;AFM分析表明样品中单层石墨烯厚度约为1.3 nm,同时也存在少许双层结构.BET测试法得到石墨烯的比表面积高达1206 m2·g-1,考察了石墨烯在高压条件下对H2的吸附性能.通过对方法改进前后所制备的石墨烯样品进行比较,结果表明,十二烷基苯磺酸钠的加入有效地减小了石墨烯的大量团聚,且得到了高质量的石墨烯.在25和55℃条件下,高质量石墨烯对氢气的吸附量分别达到1.7％(w)和1.1％(w),比之前研究结果有了很大提高.     

The effect of particle shape on the structure and rheological properties of carbon-based particle suspensions

The structure and rheological properties of carbon-based particle suspensions, i.e., carbon black(CB), multi-wall carbon nanotube(MWNT), graphene and hollow carbon sphere(HCS) suspended in polydimethylsiloxane(PDMS), are investigated. In order to study the effect of particle shape on the structure and rheological properties of suspensions, the content of surface oxygen-containing functional groups of carbon-based particles is controlled to be similar. Original spherical-like CB(fractal filler), rod-like MWNT and sheet-like graphene form large agglomerates in PDMS, while spherical HCS particles disperse relatively well in PDMS. The dispersion state of carbon-based particles affects the critical concentration of forming a rheological percolation network. Under weak shear, negative normal stress differences(ΔN) are observed in CB, MWNT and graphene suspensions, while ΔN is nearly zero for HCS suspensions. It is concluded that the vorticity alignment of CB, MWNT and graphene agglomerates under shear results in the negative ΔN. However, no obvious structural change is observed in HCS suspension under weak shear, and accordingly, the ΔN is almost zero.     

Selective etching of graphene edges by hydrogen plasma

We devised a controlled hydrogen plasma reaction at 300 °C to etch graphene and graphene nanoribbons (GNRs) selectively at the edges over the basal plane. Atomic force microscope imaging showed that the etching rates for single-layer and few-layer (≥2 layers) graphene are 0.27 ± 0.05 nm/min and 0.10 ± 0.03 nm/min, respectively. Meanwhile, Raman spectroscopic mapping revealed no D band in the planes of single-layer or few-layer graphene after the plasma reaction, suggesting selective etching at the graphene edges without introducing defects in the basal plane. We found that hydrogen plasma at lower temperature (room temperature) or a higher temperature (500 °C) could hydrogenate the basal plane or introduce defects in the basal plane. Using the hydrogen plasma reaction at the intermediate temperature (300 °C), we obtained narrow, presumably hydrogen terminated GNRs (sub-5 nm) by etching of wide GNRs derived from unzipping of multiwalled carbon nanotubes. Such GNRs exhibited semiconducting characteristics with high on/off ratios (～1000) in GNR field effect transistor devices at room temperature.     

Quenching of local magnetic moment in oxygen adsorbed graphene nanoribbons

The electronic and magnetic properties of oxidized zigzag and armchair graphene nanoribbons, with hydrogen passivated edges, have been investigated from ab initio pseudopotential calculations within the density functional scheme. The oxygen molecule in its triplet state is adsorbed most stably at the edge of a zigzag nanoribbon. The Stoner metallic behavior of the ferromagnetic nanoribbons and the Slater insulating (ground state) behavior of the antiferromagnetic nanoribbons remain intact upon oxygen adsorption. The formation of a spin-paired C-O bond drastically reduces the local atomic magnetic moment of carbon at the edge of the ferromagnetic zigzag ribbon.     

Intercalation-etching of graphene on Pt(111) in H_2 and O_2 observed by in-situ low energy electron microscopy

Graphene layers are often exposed to gaseous environments in their synthesis and application processes, and interactions of graphene surfaces with molecules particularly H_2 and O_2 are of great importance in their physico-chemical properties. In this work, etching of graphene overlayers on Pt(111) in H_2 and O_2 atmospheres were investigated by in-situ low energy electron microscopy. Significant graphene etching was observed in 10~(-5) Torr H_2 above 1023 K, which occurs simultaneously at graphene island edges and interiors with a determined reaction barrier at 5.7 eV. The similar etching phenomena were found in 10.7 Torr O_2 above 973 K, while only island edges were reacted between 823 and 923 K. We suggest that etching of graphene edges is facilitated by Pt-aided hydrogenation or oxidation of edge carbon atoms while intercalation-etching is attributed to etching at the interiors at high temperatures. The different findings with etching in O_2 and H_2 depend on competitive adsorption, desorption, and diffusion processes of O and H atoms on Pt surface, as well as intercalation at the graphene/Pt interface.     

Transition-metal-catalyzed unzipping of single-walled carbon nanotubes into narrow graphene nanoribbons at low temperature

Open, sesame! Graphene nanoribbons (GNRs) with smooth edges and controllable widths are crucial for graphene electronic and spintronic applications. High-quality narrow GNRs can be synthesized from single-walled carbon nanotubes at 200-300?°C using a Cu-atom catalyst, which dramatically reduces the energy barrier of unzipping from 3.11 to 1.16?eV.     

Electrochemical oxidation of p-nitrophenol using graphene-modified electrodes, and a comparison to the performance of MWNT-based electrodes

The electrochemical oxidation of p-nitrophenol (p-NP) has been studied comparatively on a graphene modified electrode and a multiwall carbon nanotube (MWNT) electrode by using cyclic and differential pulse voltammetry. The sensors were fabricated by modifying screen-printed electrodes with graphene and MWNT nanomaterials, respectively, both dispersed in Nafion polymer. p-NP is irreversibly oxidized at +0.9?V (vs. the Ag/AgCl) in solutions of pH 7. The height and potential of the peaks depend on pH in the range from 5 to 11. In acidic media, p-NP yields a well-defined oxidation peak at +0.96?V which gradually increases in height with the concentration of the analyte. In case of differential pulse voltammetry in sulfuric acid solution, the sensitivity is practically the same for both electrodes. The modified electrodes display an unusually wide linear response (from 10???M to 0.62?mM of p-NP), with a detection limit of 0.6???M in case of the graphene electrode, and of 1.3???M in case of the MWNT electrode.

具有SiC缓冲层的Si衬底上直接沉积碳原子生长石墨烯

石墨烯是近年发现的一种新型多功能材料.在合适的衬底上制备石墨烯成为目前材料制备的一大挑战.本文利用分子束外延(MBE)设备,在Si 衬底上生长高质量的SiC 缓冲层,然后利用直接沉积C原子的方法生长石墨烯,并通过反射式高能电子衍射(RHEED)、拉曼(Raman)光谱和近边X 射线吸收精细结构谱(NEXAFS)等实验技术对不同衬底温度(800、900、1000、1100 °C)生长的薄膜进行结构表征.实验结果表明,在以上衬底温度下都能生长出具有乱层堆垛结构的石墨烯薄膜.当衬底温度升高时,碳原子的活性增强,其成键的能力也增大,从而使形成的石墨烯结晶质量提高.衬底温度为1000 °C时结晶质量最好.其原因可能是当衬底温度较低时,碳原子活性太低不足以形成有序的六方C-sp2环.但过高的衬底温度会使SiC 缓冲层的孔洞缺陷增加,衬底的Si 原子有可能获得足够的能量穿过SiC薄膜的孔洞扩散到衬底表面,与沉积的碳原子反应生成无序的SiC,这一方面会减弱石墨烯的生长,另一方面也会使石墨烯的结晶质量变差.     

Electrochemical unzipping of multi-walled carbon nanotubes for facile synthesis of high-quality graphene nanoribbons

Here we report a remarkable transformation of carbon nanotubes (CNTs) to nanoribbons composed of a few layers of graphene by a two-step electrochemical approach. This consists of the oxidation of CNTs at controlled potential, followed by reduction to form graphene nanoribbons (GNRs) having smooth edges and fewer defects, as evidenced by multiple characterization techniques, including Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. This type of "unzipping" of CNTs (single-walled, multi-walled) in the presence of an interfacial electric field provides unique advantages with respect to the orientation of CNTs, which might make possible the production of GNRs with controlled widths and fewer defects.     

Pore wall thickness and interpore influence on adsorption of alkanes in carbons using explicit pore models

In modeling of activated carbons, the pores are often assumed to be slit-shaped formed of a constant number of graphene layers. X-ray diffraction studies show that micropores are formed between stacks of different numbers of graphene layers. In this study, we investigate, through the grand canonical Monte Carlo method, the influence on the adsorbed alkanes densities of pore walls with different graphene layers thickness and the related interpore adsorbate interaction when the pore wall has only one graphene layer. All studies of thickness and interpore interaction to date were performed using the Steele 10-4-3 potential model. Instead of Steele model, we propose explicit models made up of graphene layers of discrete carbon atoms. We also investigated the sensitivity of the system to the cut-off and solid-fluid parameter. With our explicit model we found that the influence of the number of carbon layers is not significant for n>2 as previously observed by Steele model??DFT studies. The system was also insensitive to cut-off and well deep parameter variations. A new pore model with an extra dummy graphene wall was proposed to investigate the interpore interaction. The interpore interaction study with the alkanes series C1 to C4 shows that the retention capacity of heavier alkanes is the same whether for activated carbons with few layers (stronger interpore interaction) as for carbons with two or more layers (stronger solid-fluid interaction) assuming negligible surface mediation. The explicit models proposed can be successfully used in the elaboration of virtual porous carbon models to reproduce wall thickness and interpore adsorbate interactions phenomena.     

Simulated porosity and electronic structure of nanoporous carbons

Nanoporous carbon refers to a broad class of materials characterized by nanometer-size pores, densities lower than water, large specific surface areas, and high porosities. These materials find applications in nanocatalysis and gas adsorption, among others. The porosity structure, that determines the properties and functionalities of these materials, is still not characterized in detail. Here, we reveal the detail porosity structure and the electronic properties of a type of nanoporous carbons, the so called carbide derived carbons (CDCs), through a simulation scheme that combines large simulation cells and long time scales at the empirical level with first-principles density functional calculations. We show that the carbon network consists in one layer thick nanographenes interconnected among them. The presence of specific defects in the carbon layers (heptagons and octagons) yields to open pores. These defects are not completely removed through annealing at high temperatures. We also suggest that, in contrast with graphene which is a zero-gap semiconductor, these materials would have a metallic character, since they develop an electronic band around the Fermi level. This band arises from the electronic states localized at the edges of the nanographene layers.     

Effect of carbon cathode morphology on the electrode/electrolyte interface structure

The study has focused on identifying possible factors that determine differences in electrocatalytic activity between carbon cathodes with different morphologies in Li–air batteries. The structure of the aprotic solvent dimethyl sulfoxide at carbon surfaces, such as nanotube, graphene plane, and single- and multilayer graphene edges, has been investigated using molecular dynamics simulation. It has been found that the solvent has a layered structure near the graphene plane, the graphene edge, and the nanotube. Moreover, the sharpness of the solvent layers decreases with the increasing surface curvature. The interface structure at the edge of the multilayer graphene has a qualitatively different, chessboard structure. It has been assumed that the observed differences in the interfacial solvent structure can influence the concentration distribution of the Li⁺ and O₂ reactants and in their adsorption rates, thereby affecting the kinetics of the oxygen reduction reaction.     

Synergistic effects and mechanism of multiwalled carbon nanotubes with magnesium hydroxide in halogen-free flame retardant EVA/MH/MWNT nanocomposites

The synergistic effects and mechanism of multiwalled carbon nanotubes (MWNTs) with magnesium hydroxide (MH) in halogen-free flame retardant EVA/MH/MWNT nanocomposites have been studied by cone calorimeter test (CCT), limiting oxygen index (LOI), thermogravimetric analysis (TGA), torque test, morphological evolution experiment, and scanning electron microscopy (SEM). The data obtained from the CCT, LOI, and TGA show that suitable amount of MWNTs has synergistic effects with MH in the EVA/MH/MWNT nanocomposites. The MWNTs can considerably decrease the heat release rates and mass loss rate by about 50-60%, prolongate the combustion time to near two times, and increase the LOI values by 5% when 2 wt% MWNTs substitute for the MH in the EVA/MH/MWNT samples. The TGA data also show that the synergistic effects of MWNTs with MH apparently increase the thermal degradation temperatures and final charred residues of the EVA/MH/MWNT samples. The experimental observations from the torque, morphological evolution tests, and SEM give positive evidences that the synergistic mechanism of MWNTs with MH can be described to: (i) the increase of melt viscosity because of network structure formation of MWNTs in the EVA/MH matrix; (ii) the enhancement of thermo-oxidation stability due to the MWNTs' mechanical strength and integrity of the charred layers in the EVA/MH/MWNT nanocomposites; (iii) the formation of compact charred layers promoted by MWNTs acted as heat barrier and thermal insulation. All the above-mentioned factors efficiently enhance thermal and flame retardant properties and protect the EVA/MH/MWNT nanocomposite materials to be burning.     

Thermal and spectroscopic characterization of polypropylene-carbon nanotube composites

Thermogravimetric (TG) and varied temperature Raman spectroscopic measurements of melt-blended polypropylene composites (PP) with double wall (DWNT) and multi-wall carbon nanotubes (MWNT) revealed that the incorporation of carbon nanotubes into polymer matrix increased the thermal stability comparing to the virgin polypropylene. The characterization of reference nanotubes was also done by Raman microscopy and TG measurements. Varied temperature rheological analysis provided further information about the thermal decomposition of the composites indicating the formation of high strength char in case of MWNT and limited applicability of DWNT at high temperature. The residue of the decomposition of PP-MWNT nanocomposites consists of nanotubes of spectroscopically higher purity comparing to the original one indicating the thermally induced chemical changes in the solid phase.     

Intercalation-etching of graphene on Pt(111) in H<Subscript>2</Subscript> and O<Subscript>2</Subscript> observed by <Emphasis Type="Italic">in-situ</Emphasis> low energy electron microscopy

Graphene layers are often exposed to gaseous environments in their synthesis and application processes, and interactions of graphene surfaces with molecules particularly H₂ and O₂ are of great importance in their physico-chemical properties. In this work, etching of graphene overlayers on Pt(111) in H₂ and O₂ atmospheres were investigated by in-situ low energy electron microscopy. Significant graphene etching was observed in 10^-5 Torr H₂ above 1023 K, which occurs simultaneously at graphene island edges and interiors with a determined reaction barrier at 5.7 eV. The similar etching phenomena were found in 10^-7 Torr O₂ above 973 K, while only island edges were reacted between 823 and 923 K. We suggest that etching of graphene edges is facilitated by Pt-aided hydrogenation or oxidation of edge carbon atoms while intercalation-etching is attributed to etching at the interiors at high temperatures. The different findings with etching in O₂ and H₂ depend on competitive adsorption, desorption, and diffusion processes of O and H atoms on Pt surface, as well as intercalation at the graphene/Pt interface.     

Determination of catecolamines on electrodes modified with multiwalled carbon nanotubes

The electrochemical behavior of several biologically active catecolamines in studied on a glassy carbon electrode with the surface preliminarily modified by a composite film containing preliminarily carboxylated multi-walled carbon nanotubes (MWNT). The coatings are characterized by the methods of cyclic voltammetry and scanning microscopy. It is shown that the use of hybrid composites prepared by immobilizing MWNT and gold nanoparticles into the film of poly(isonicotinic) acid provides the high diffusion permeability of the surface layers and the efficiency of the electron transfer as regards catecolamines. The possibility of using these electrodes for selective determination of these substances in drugs widely used in the modern practical medicine is demonstrated.