Published by Elsevier
Print ISSN: 0008-6223
Micro/mesoporous activated carbons containing oxygen and phosphorus heteroatoms were modified by incorporation of nitrogen using melamine and urea precursors. The surface chemistry was analyzed by the means of elemental analysis, XPS, and (31)P MAS NMR. The results indicate that upon the incorporation of nitrogen at high temperatures not only new species involving carbon/nitrogen/oxygen are formed but also the phosphorous environment is significantly altered. Both urea and melamine precursors have similar effects on formation of P-N and P-C bonds. These compounds, although present in small but measurable quantities seem to affect the performance of carbons in electrochemical capacitors. With an increase in the heterogeneity of phosphorus containing species and with a decrease in the content pyrophosphates the capacitance increases and the retention ratio of the capacitor is improved.
A set of carbon materials was treated by a choice of common oxidizers to investigate the mercury capture capacities at varying temperature conditions. It was found that ozone treatment dramatically increases the mercury capture capacity of carbon surfaces by factors up to 134, but the activity is easily destroyed by exposure to the atmosphere, to water vapor, or by mild heating. Freshly ozone-treated carbon surfaces are shown to oxidize iodide to iodine in solution and this ability fades with aging. FTIR analysis shows broad C-O stretch features from 950 to 1300 cm(-1), which decay upon atmospheric exposure and are similar to the C-O-C asymmetric stretch features of ethylene secondary ozonide. The combined results suggest that the ultra-high mercury capture efficiency is due to a subset of labile C-O functional groups with residual oxidizing power that are likely epoxides or (epoxide-containing) secondary ozonides. The results open the possibility for in situ ozonolysis to create high-performance carbon-based Hg sorbents.
Maghemite (γ-Fe(2)O(3))/multi-walled carbon nanotubes (MWCNTs) hybrid-materials were synthesized and their anisotropic electrical conductivities as a result of their alignment in a polymer matrix under an external magnetic field were investigated. The tethering of γ-Fe(2)O(3) nanoparticles on the surface of MWCNT was achieved by a modified sol-gel reaction, where sodium dodecylbenzene sulfonate (NaDDBS) was used in order to inhibit the formation of a 3D iron oxide gel. These hybrid-materials, specifically, magnetized multi-walled carbon nanotubes (m-MWCNTs) were readily aligned parallel to the direction of a magnetic field even when using a relatively weak magnetic field. The conductivity of the epoxy composites formed in this manner increased with increasing m-MWCNT mass fraction in the polymer matrix. Furthermore, the conductivities parallel to the direction of magnetic field were higher than those in the perpendicular direction, indicating that the alignment of the m-MWCNT contributed to the enhancement of the anisotropic electrical properties of the composites in the direction of alignment.
This work investigates the physical interactions between carbon nanomaterials and tocopheryl polyethylene glycol succinate (TPGS). TPGS is a synthetic amphiphile that undergoes enzymatic cleavage to deliver the lipophilic antioxidant, alpha-tocopherol (vitamin E) to cell membranes, and is FDA approved as a water-soluble vitamin E nutritional supplement and drug delivery vehicle. Here we show that TPGS 1000 is capable of dispersing multi-wall and single-wall carbon nanotubes in aqueous media, and for multiwall tubes is more effective than the commonly used non-ionic surfactant Triton X-100. TPGS is also capable of solubilizing C(60) in aqueous phases by dissolving fullerene in the core of its spherical micelles. Drying of these solutions leads to fullerene/TPGS phase separation and the self-assembly of highly ordered asymmetric nanoparticles, with fullerene nanocrystals attached to the hydrophobic end of crystalline TPGS nanobrushes. The article discusses surface charge, colloidal stability, and the potential applications of TPGS as a safe surfactant for "green" processing of carbon nanomaterials.
Precise arrangement of nanoscale elements within larger systems, is essential to controlling higher order functionality and tailoring nanophase material properties. Here, we present findings on growth conditions for vertically aligned carbon nanofibers that enable synthesis of high density arrays and individual rows of nanofibers, which could be used to form barriers for restricting molecular transport, that have regular spacings and few defects. Growth through plasma-enhanced chemical vapor deposition was initiated from precisely formed nickel catalyst dots of varying diameter and spacing that were patterned through electron beam lithography. Nanofiber growth conditions, including power, precursor gas ratio, growth temperature and pressure were varied to optimize fiber uniformity and minimize defects that result from formation and migration of catalyst particles prior to growth. It was determined that both catalyst dot diameter and initial plasma power have a considerable influence on the number and severity of defects, while growth temperature, gas ratio (C(2)H(2):NH(3)) and pressure can be varied within a considerable range to fine-tune nanofiber morphology.
The benzyne functionalization of chemical vapor deposition grown large area graphene and graphite was performed using a mixture of o-trimethylsilylphenyl triflate and cesium fluoride that react with the carbon surface. The reaction requires at least 2 days of treatment before the appearance of Raman and energy-dispersive X-ray spectral signatures that verify modification. Raman spectra of modified graphene and graphite show a rich structure of lines corresponding to C=C-C, C-H, and low frequency modes of surface-attached benzyne rings.
There is substantial evidence for toxicity and/or carcinogenicity upon inhalation of pure transition metals in fine particulate form. Carbon nanotube catalyst residues may trigger similar metal-mediated toxicity, but only if the metal is bioavailable and not fully encapsulated within fluid-protective carbon shells. Recent studies have documented the presence of bioavailable iron and nickel in a variety of commercial as-produced and vendor "purified" nanotubes, and the present article examines techniques to avoid or remove this bioavailable metal. First, data are presented on the mechanisms potentially responsible for free metal in "purified" samples, including kinetic limitations during metal dissolution, the re-deposition or adsorption of metal on nanotube outer surfaces, and carbon shell damage during last-step oxidation or one-pot purification. Optimized acid treatment protocols are presented for targeting the free metal, considering the effects of acid strength, composition, time, and conditions for post-treatment water washing. Finally, after optimized acid treatment, it is shown that the remaining, non-bioavailable (encapsulated) metal persists in a stable and biologically unavailable form up to two months in an in vitro biopersistence assay, suggesting that simple removal of bioavailable (free) metal is a promising strategy for reducing nanotube health risks.
Recent research has led to increased concern about the potential adverse human health impacts of carbon nanotubes, and further work is needed to better characterize those risks and develop risk management strategies. One of the most important determinants of the chronic pathogenic potential of a respirable fiber is its biological durability, which affects the long-term dose retained in the lungs, or biopersistence. The present article characterizes the biodurability of single-walled carbon nanotubes using an in vitro assay simulating the phagolysosome. Biodurability is observed to depend on the chemistry of nanotube surface functionalization. Single-walled nanotubes with carboxylated surfaces are unique in their ability to undergo 90-day degradation in a phagolysosomal simulant leading to length reduction and accumulation of ultrafine solid carbonaceous debris. Unmodified, ozone-treated, and aryl-sulfonated tubes do not degrade under these conditions. We attribute the difference to the unique chemistry of acid carboxylation, which not only introduces COOH surface groups, but also causes collateral damage to the tubular graphenic backbone in the form of neighboring active sites that provide points of attack for further oxidative degradation. These results suggest the strategic use of surface carboxylation in nanotube applications where biodegradation may improve safety or add function.
There is a pressing need to develop rapid whole animal-based testing assays to assess the potential toxicity of engineered nanomaterials. To meet this challenge, the embryonic zebrafish model was employed to determine the toxicity of fullerenes. Embryonic zebrafish were exposed to graded concentrations of fullerenes [C(60), C(70), and C(60)(OH)(24)] during early embryogenesis and the resulting morphological and cellular responses were defined. Exposure to 200 μg/L C(60) and C(70) induced a significant increased in malformations, pericardial edema, and mortality; while the response to C(60)(OH)(24) exposure was less pronounced at concentrations an order of magnitude higher. Exposure to C(60) induced both necrotic and apoptotic cellular death throughout the embryo. While C(60)(OH)(24) induced an increase in embryonic cellular death, it did not induce apoptosis. Our findings concur with results obtained in other models indicating that C(60)(OH)(24) is significantly less toxic than C(60). These studies also suggest that that the embryonic zebrafish model is well-suited for the rapid assessment of nanomaterial toxicity.
A nanoassembly of single-walled carbon nanotubes coated by a thin layer of silica followed by quantum dots was prepared. That the quantum dots retained their photoluminescent properties after deposition onto the silylated carbon nanotubes suggests that the thin layer of silica prevented the quenching of the fluorescence by the nanotubes. This fluorescent nanoassembly represents an excellent building block for photoelectric and optical devices and biological nanoprobes.
Better understanding of electron transfer (ET) taking place at the nano-bio interface can guide design of more effective functional materials used in fuel cells, biosensors, and medical devices. Single-walled carbon nanotube (SWCNT) coupled with biological enzymes serves as a model system for studying the ET mechanism, as demonstrated in the present study. SWCNT enhanced the activity of horseradish peroxidase (HRP) in the solution-based redox reaction by binding to HRP at a site proximate to the enzyme's activity center and participating in the ET process. ET to and from SWCNT was clearly observable using near-infrared spectroscopy. The capability of SWCNT in receiving electrons and the direct attachment of HRP to the surface of SWCNT strongly affected the enzyme activity due to the direct involvement of SWCNT in ET.
Large-area mono- and bilayer graphene films were synthesized on Cu foil (~ 1 inch(2)) in about 1 min by a simple ethanol-chemical vapor deposition (CVD) technique. Raman spectroscopy and high resolution transmission electron microscopy revealed the synthesized graphene films to have polycrystalline structures with 2-5 nm individual crystallite size which is a function of temperature up to 1000°C. X-ray photoelectron spectroscopy investigations showed about 3 atomic% carboxylic (COOH) functional groups were formed during growth. The field-effect transistor devices fabricated using polycrystalline graphene as conducting channel (L(c)=10 μm; W(c)=50 μm) demonstrated a p-type semiconducting behavior with high drive current and Dirac point at ~35 V. This simple one-step method of growing large area polycrystalline graphene films with semiconductor properties and easily functionalizable groups should assist in the realization of potential of polycrystalline graphene for nanoelectronics, sensors and energy storage devices.
A detailed analysis by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) of nitroxide-functionalized graphene oxide layers (GOFT) dispersed in Nylon 6 nanofibers is reported herein. The functionalization and exfoliation process of graphite oxide to GOFT was confirmed by TEM using electron diffraction patterns (EDP), wherein 1 to 4 graphene layers of GOFT were observed. The distribution and alignment of GOFT layers within a sample of Nylon 6 nanofiber reveals that GOFT platelets are mainly within the fiber, but some were partially protruding from it. Furthermore, Nylon 6 nanofibers exhibit an average diameter of 225 nm with several microns in length. GOFT platelets embedded into the fiber, the pristine fiber, and amorphous carbon were analyzed by EELS where each spectra [corresponding to the carbon edge (C-K)] exhibited changes in the fine structure, allowing a clear distinction between: i) GOFT single-layers, ii) Nylon-6 nanofibers, and iii) the carbon substrate. EELS analysis is presented here for the first time as a powerful tool to identify functionalized graphene single-layers (< 4 layers of GOFT) into a Nylon 6 nanofiber composite.
A facile method for preparing functionalized graphene oxide single layers with nitroxide groups is reported herein. Highly oxidized graphite oxide (GO=90.6%) was obtained, slightly modifying an improved Hummer's method. Oxoammonium salts (OS) were investigated to introduce nitroxide groups to GO, resulting in a one-step functionalization and exfoliation. The mechanisms of functionalization/exfoliation are proposed, where the oxidation of aromatic alcohols to ketone groups, and the formation of alkoxyamine species are suggested. Two kinds of functionalized graphene oxide layers (GOFT1 and GOFT2) were obtained by controlling the amount of OS added. GOFT1 and GOFT2 exhibited a high interlayer spacing (d0001 = 1.12nm), which was determined by X-ray diffraction. The presence of new chemical bonds C-N (~9.5 %) and O-O (~4.3 %) from nitroxide attached onto graphene layers were observed by X-ray photoelectron spectroscopy. Single-layers of GOFT1 were observed by HRTEM, exhibiting amorphous and crystalline zones at a 50:50 ratio; in contrast, layers of GOFT2 exhibited a fully amorphous surface. Fingerprint of GOFT1 single layers was obtained by electron diffraction at several tilts. Finally, the potential use of these materials within Nylon 6 matrices was investigated, where an unusual simultaneous increase in tensile stress, tensile strain and Young's modulus was observed.
Carbonaceous materials are amenable to microwave heating to varying degrees. The primary indicator of susceptibility is the complex permittivity (ϵ*), of which the real component correlates with polarization and the imaginary term represents dielectric loss. For a given material, the complex permittivity is dependent upon both frequency and temperature. Here we report the complex permittivities of three activated carbons of diverse origin over the frequency range from 0.2 to 26 GHz. Dielectric polarization–relaxation phenomena for these materials are also characterized. Measurements were made using a coaxial dielectric probe and vector network analyzer based system across the temperature region between 22 and 190 °C.
Structured carbon nitride (CNx), thin solid films, also known as fullerene-like, consist of, upon nitrogen substitution, bent and cross-linked graphene planes. They were synthesized by unbalanced reactive magnetron sputtering and analyzed by high-resolution transmission electron microscopy (HRTEM) in combination with X-ray photoelectron spectroscopy (XPS). The microstructure evolution in terms of plane alignment, extension and cross-linking can be controlled by adjusting the synthesis conditions, such as growth temperature, N2 fraction in the discharge gas and ion energy. HRTEM on plan-view samples was used to examine the structural changes depending on growth temperature and N2 fraction. The problem of projection artifacts for imaging the structural features was partially overcome by selected area electron diffraction analysis, where it is shown that diffraction corresponding to 3.5 Å is associated with the formation of structured CNx. The incorporation of N is crucial for the evolution of heavily bent and frequently cross-linked basal planes, since it triggers pentagon formation and cross-linking at much lower energies compared to pure carbon films. Therefore, the two spectral features in the nitrogen 1s core electron spectra as examined by XPS were correlated to the microstructure evolution.
A detailed synthesis process of ultra-small single-walled carbon nanotubes (SWNTs) aligned in the channels of AlPO4-5 zeolite single crystals is reported. The structure of such ultra-small SWNTs was analyzed by means of transmission electron microscopy (TEM) and polarized Raman scattering. TEM images showed that the diameter of the SWNTs is as small as 0.4 nm, which is the size of three possible structures: the zigzag (5,0), armchair (3,3) and chiral (4,2). The polarization dependence of the Raman lines indicates that the structures of the nanotubes are dominated by the zigzag (5,0) form. Line-shape analysis of the tangential A1g Raman mode shows that the SWNTs are metallic with a finite electronic density-of-state near the Fermi energy level, which is in agreement with the band structural calculation using local-density function approximation (LDA) as well as the electric transport measurement.
We have observed the energy structure in the density of occupied states of graphene grown on n-type 6H-SiC (0001). The structure revealed with photoelectron spectroscopy is described by creation of the quantum well states whose number and the energy position (E1 = 0.3 eV, E2 = 1.2 eV, E3 = 2.6 eV ) coincide with the calculated ones for deep (V = 2.9 eV) and narrow (d = 2.15 A) quantum well formed by potential relief of the valence bands in the structure graphene/n-SiC. We believe that the quantum well states should be formed also in graphene on dielectric and in suspended graphene.
The adsorption of molecular and atomic hydrogen as well as other molecules in the atmosphere on vacancies in the (0 0 0 1) graphite surface are investigated using density functional theory. Atomic hydrogen adsorbs with energies ranging from 4.7 to 2.3 eV. The validity of the model is confirmed by the good agreement between calculated vibrational spectra and those of high-resolution electron energy loss spectroscopy. It is shown that molecular hydrogen dissociates with a barrier of 1.1 eV on this model system. Water and oxygen also dissociate with respective barriers of 1.6 and 0.2 eV. Carbon dioxide and nitrogen have no interaction with the defect whereas carbon monoxide is incorporated into the vacancy with an activation energy of 1.5 eV. A comparison is made with the reactivity of graphene edges, both zigzag and armchair.
We measure the concentration of carbon adatoms on the Ru(0 0 0 1) surface that are in equilibrium with C atoms in the crystal’s bulk by monitoring the electron reflectivity of the surface while imaging. During cooling from high temperature, C atoms segregate to the Ru surface, causing graphene islands to nucleate. Using low-energy electron microscopy (LEEM), we measure the growth rate of individual graphene islands and, simultaneously, the local concentration of C adatoms on the surface. We find that graphene growth is fed by the supersaturated, two-dimensional gas of C adatoms rather than by direct exchange between the bulk C and the graphene. At long times, the rate at which C diffuses from the bulk to the surface controls the graphene growth rate. The competition among C in three states – dissolved in Ru, as an adatom, and in graphene – is quantified and discussed. The adatom segregation enthalpy determined by applying the simple Langmuir–McLean model to the temperature-dependent equilibrium concentration seriously disagrees with the value calculated from first-principles. This discrepancy suggests that the assumption in the model of non-interacting C is not valid.
Adsorption of thermal (2000 K) D atoms on (0 0 0 1) surfaces of various highly oriented pyrolytic graphite (HOPG) and natural graphite substrates was studied under ultra high vacuum (UHV) conditions with thermal desorption spectroscopy (TDS). D chemisorption on terrace and terrace edge sites of graphite (0 0 0 1) surfaces was identified. Recombinative desorption of D adsorbed on terraces was observed between 400 and 600 K. The analysis of TD spectra from various graphite surfaces reveals the existence of three desorption states intrinsic to graphite (0 0 0 1), and proposed as being due to adsorbate structures composed of one (monomer) and two neighbouring (dimer) chemisorbed D atoms, and aggregates thereof (mixed). The dimer structure is supposed to exhibit higher stability than the monomer. Reaction of D with terrace edges leads to the formation of CD, CD2 and CD3-groups at edge C atoms. These groups decompose during heating between 790 and 1300 K via release of gaseous D2 and CDx, C2Dx and C3Dx-hydrocarbons.
Oriented growth of carbon nanotubes lying on a single-crystal MgO(0 0 1) surface is reported. The nanotubes were grown at 900 °C by chemical vapour deposition using a mixture of carbon monoxide and hydrogen, with Co catalyst nanoparticles formed by self-assembling on MgO under ultra high vacuum conditions. Field emission scanning electron microscopy measurements reveal that the nanotubes grow parallel to the MgO(0 0 1) plane and preferentially along the two perpendicular directions, [1 1 0] and . The interactions between oxygen atoms of the MgO substrate and carbon nanotubes would drive the directional growth. The Raman spectra support that most nanotubes with a diameter ranging from 1.5 to 4 nm, observed by transmission electron microscopy and atomic force microscopy, are single-walled. The observed directional growth requires a very good surface quality of the MgO substrate. Interestingly, rough areas damaged by water vapour prevent the formation of nanoparticles necessary to the nucleation of nanotubes.
Among the many anticipated applications of graphene, some - such as transistors for Si microelectronics - would greatly benefit from the possibility to deposit graphene directly on a semiconductor grown on a Si wafer. We report that Ge(001) layers on Si(001) wafers can be uniformly covered with graphene at temperatures between 800{\deg}C and the melting temperature of Ge. The graphene is closed, with sheet resistivity strongly decreasing with growth temperature, weakly decreasing with the amount of deposited C, and reaching down to 2 kOhm/sq. Activation energy of surface roughness is low (about 0.66 eV) and constant throughout the range of temperatures in which graphene is formed. Density functional theory calculations indicate that the major physical processes affecting the growth are: (1) substitution of Ge in surface dimers by C, (2) interaction between C clusters and Ge monomers, and (3) formation of chemical bonds between graphene edge and Ge(001), and that the processes 1 and 2 are surpassed by CH$_{2}$ surface diffusion when the C atoms are delivered from CH$_{4}$. The results of this study indicate that graphene can be produced directly at the active region of the transistor in a process compatible with the Si technology.
We have used low-energy electron microscopy and diffraction to examine the significance of lattice orientation in graphene growth on Cu(001). Individual graphene domains undergo anisotropic growth on the Cu surface, and develop into lens shapes with their long axes roughly aligned with the Cu<100> in-plane directions. The long axis of a lens-shaped domain is only rarely oriented along a C<11> direction, suggesting that carbon attachment at "zigzag" graphene island edges is unfavorable. A kink-mediated adatom attachment process is consistent with the behavior observed here and reported in the literature. The details of the ridged moire pattern formed by the superposition of the graphene lattice on the (001) Cu surface also evolve with the graphene lattice orientation, and are predicted well by a simple geometric model. Managing the kink-mediated growth mode of graphene on Cu(001) will be necessary for the continued improvement of this graphene synthesis technique.
The x-ray diffraction line profiles of the 002 reflections of graphite samples ground in a ball mill for periods up to 90 hours were studied using newly developed peak analysis methods that permitted separation of overlapping peaks having different peak positions, intensities, and line-breadths. For 3.354A ≤ d002 ≤ 3.375A a continuous range of single phases characterized by symmetrical peaks of increasing breadths was observed, but peaks whose centroids fell between 3.375A and 3.55A were asymmetrical and composed of a superposition of symmetrical peaks of increasing breadths characterized by 3.375Å, 3.40Å, 3.44Å, and 3.55Å spacings. There is some indication of a phase characterized by a 3.49Å interlayer spacing. These results confirm previous reports of discrete 002 spacings in carbons and disordered graphite and suggest that elemental carbon(s) are composed of mixtures of these metastable graphite phases.
A technique is presented for semi-quantitative digital analysis of 002 lattice fringe images of carbon materials. The automated technique yields statistics on fringe length and tortuosity, as well as new quantitative parameters describing the mode, degree, and length scale of orientational order among graphene layers. The technique is applied to combustion-derived carbons with special emphasis on the nanostructural characterization of a variety of solid fuel chars as a function of residence time. During the combustion of pulverized coal and biomass (120 ms at 1700–1900 K), the main features of char nanostructure are established early, near the end of the rapid, in-flame pyrolysis. Only for the high-volatile bituminous coals is the nanostructural order in the young chars significantly altered by the subsequent char combustion process. A noteworthy general observation is the presence of two distinct length scales for orientational order in most chars. At short length scales (<30 Å) a high degree of orientational order is observed reflecting the presence of small molecular orientation domains. At larger length scale (>30 Å), a lesser but significant degree of orientational order is still present among the distinct domains. Quantitative analysis of this low-grade long-range order using Maier–Saupe theory indicates that it cannot arise by a liquid crystal mechanism. Long-range orientational order in these samples can thus be classified as ‘mesophasic’, exhibiting a high degree of order arising through liquid crystal formation in the fluid phase of pyrolysis (or retention of anisotropy in very high rank coals), and ‘statistical’, perhaps arising from elongational strain during carbonization.
High-temperature pyrolysis of two fullerene precursors – 1,2′-binaphthyl and 1,3-oligonaphthyl – has been investigated. An oligomer of naphthalene with the appropriate orientation of fragments, which contains all 60 carbon atoms, 12 of 20 six-membered rings and 71 of 90 carbon–carbon bonds required to form the C60 fullerene cage was synthesized in a three-step synthesis from naphthalene. The formation of fullerene during pyrolysis was confirmed by MALDI-TOF and HPLC analysis of the toluene extract obtained from the raw soot. It was found that the toluene extract contains free C60 fullerene but the main share of fullerenes exists in the form of their derivatives. The yield of free C60 was estimated as 0.1% by HPLC but the overall yield of C60 seems to be higher and was estimated as ≈1%.
The adsorption at low concentration (zero surface coverage) of n-alkanes (from n-C2 to n-C5) benzene, trichloroethylene (TCE) and 1,2-dichloroethane (DCE) on active carbons has been measured by inverse gas–solid chromatography (IGC). The results offer new unexpected insights on the nature of the adsorption mechanisms of these systems under conditions very close to those found in the real applications for the elimination of atmospheric pollutants. In particular, the adsorbate–adsorbent interactions are governed mainly by dispersive forces even for those adsorbates with strong dipolar moment (TCE and DCE) which are capable of specific interactions. Although the introduction of large amounts of oxygen groups on the surface of the adsorbent increases the specific component of the adsorption, still the dispersive component is clearly dominant. In addition, the introduction of oxygen groups produces a decrease in the adsorption capacity. Nevertheless, it is shown that the presence of pores with appropriate sizes (close to the molecular dimension) has a more important effect on the retention of the adsorbates than chemical surface groups. Therefore, it is more desirable to have active carbons with accessible pores of the appropriate size than oxygen groups to get a good performance in the adsorption of these molecules.
Pyrocarbon deposition from ethylene, acetylene and 1,3-butadiene was studied with a vertical hot-wall reactor at ambient pressure and 1000 °C; initial partial pressures of the hydrocarbons and residence time were varied. Steady-state pyrocarbon deposition rates and corresponding compositions of the gas-phase were determined. Reaction models describing homogeneous gas-phase and heterogeneous pyrocarbon deposition reactions were derived and applied for simulation of pyrocarbon deposition rates and the inhibiting effect of hydrogen. This latter effect is ascribed to a blocking of active sites at the growing pyrocarbon surface.
The present work investigated the evolution of catalytic activity of activated carbon in 1,3,6-naphthalenetrisulphonic acid (NTS) ozonation in aqueous phase. Activated carbons pre-treated with ozone showed a reduction in NTS oxidation rate and efficacy of TOC removal that increased with ozone exposure time. The ozone treatment increased the number of surface oxygenated (electron-withdrawing) groups on the carbon, therefore reducing its basic character and its reductive properties. This effect reduced the reactivity of the activated carbon to ozone and, therefore, the extent of the ozone decomposition into highly oxidative species.
Ordered mesoporous carbon (D-COU-1) films were synthesized by an organic–organic self assembly method using 1,5-dihydroxynaphthalene as carbon source. Channel-like pores with a well-ordered hexagonal structure were observed by TEM observations. The thermal stability of the D-COU-1 film was compared with the COU-1 film prepared using resorcinol (R-COU-1). The XRD results indicate that the structure of D-COU-1 film was less shrinkable under carbonization compared to that of the R-COU-1 film.
The density functional theory (DFT) has been used to simultaneously investigate physi-/chemi-sorption properties of hydrogen on the (10,0) single-walled carbon nanotube (SWCNT) walls. Physisorption of H2 outside the CNT with a vertical orientation to the tube axis above the center of a hexagon surface is the most stable state of physisorption and its binding energy is very weak, −0.792 kcal/mol. In the chemisorption of two hydrogen atoms the most stable state is above two adjacent carbon atoms of a hexagon with a C–H bond length of 1.10 Å and one C–H bond energy of −45.761 kcal/mol. Based on these results, we have also investigated the transition state and the reaction pathway from physisorption to chemisorption of hydrogen on the CNT. The energy barrier of the reaction from physisorption to chemisorption is about 78.837 kcal/mol and the reaction is not spontaneous at 0 K. Through the calculations of the Gibbs free energy change from physisorption to chemisorption with temperatures, we learned that it is not easy for the reaction to occur, which is a major obstacle for the practical use of the CNT as a hydrogen storage medium.
The effects of 100 ppb ozone exposure on the adsorption of 1 ppm toluene on activated carbon are presented for dry (less than 5% RH) and humid (55% RH) air. In dry air, the 10% toluene breakthrough times of granular carbon beds exposed to ozone for 208 days are 17% less than those of unexposed carbon beds. At 55% RH, the corresponding reduction in toluene breakthrough time is 78%. For a humid environment with 100 ppb ozone, filter life would be reduced by more than half compared to the expected life based on tests in the absence of ozone. This degradation is attributed to changes in carbon surface chemistry, surface area, and pore volume that occur with relatively brief exposure to the ozone.
Intercalation of Ni atoms under a two-dimensional graphene film (2DGF) on (111)Ir was studied in ultra high vacuum using high resolution Auger spectroscopy. Ni atoms were shown to be intercalated effectively at 900 to 1500 K, and a polylayer nickel film was shown to grow under the 2DGF. A relationship between a proportion κ of Ni atoms intercalated under 2DGF and temperature was determined and it was shown that κ decreases from ∼30% at 900 K to ∼3% at 1500 K.
Graphene on copper is a system of high technological relevance, as Cu is one of the most widely used substrates for the CVD growth of graphene. However, very little is known about the details of their interaction. One approach to gain such information is studying the superlattices emerging due to the mismatch of the two crystal lattices. However, graphene on copper is a low-corrugated system making both their experimental and theoretical study highly challenging. Here, we report the observation of a new rotational Moire superlattice of CVD graphene on Cu (111), characterized by a periodicity of $1.5 \pm 0.05$ nm and corrugation of $0.15 \pm 0.05$ $\hbox{\AA}$ , as measured by Scanning Tunneling Microscopy. To understand the observed superlattice we have developed a newly parameterized Tersoff-potential for the graphene/Cu (111) interface fitted to nonlocal van der Waals density functional theory (DFT) calculations. The interfacial force field with time-lapsed CMD provides superlattices in good quantitative agreement with the experimental results, for a misorientation angle of $10.4 \pm 0.5,^{\circ}$ without any further parameter adjustment. Furthermore, the CMD simulations predict the existence of two non-equivalent high-symmetry directions of the Moir\'e pattern that could also be identified in the experimental STM images.
Using First-principle calculations, substrate effect of O-terminated (rt3 x rt3) MgO (111) on graphene was investigated for spintronics application. Surprisingly, the graphene can be turned into a spin-polarized semiconductor, which implies that the totally spin-polarized current can be generated and its on/off switching can be also controlled. The origin of the spin-polarized band structure is spin-ordering due to alternative sp2-sp3 covalent bondings induced by the MgO (111) substrate. The results indicate that the tailored pattern of the chemisorption can be highly efficient or introducing totally spin-polarized current to the graphene.
(a) The scheme and (b) geometry of EELS experiments in the present work. (For discussion of details, please see the text.)  
The EELS spectra of (a) the clean Ni(111) surface and (b) the graphene/Ni(111) system measured at different primary electron energies and at q ≈ 0 ˚ A −1 .
The angle-resolved EELS spectra (E p = 100 eV) of (a) clean Ni(111) and (b) the graphene/Ni(111) system taken with an energy step of 1 @BULLET with respect to the specular reflection (shown by the thick solid line) in the Γ−K direction of the first BZ of graphene. (c) The dispersion, E loss (q ) of the higher (squared green symbols) and the lower (round brown symbols) energy components of the doublet structure in EELS spectra of the graphene/Ni(111) system. The parabolic fit to the dispersion of the higher energy component in the region of |q | ≤ 1.0 ˚ A −1 is shown by the solid blue line.  
(a,b) The EELS spectra measured at the specular reflection angle for (1) clean Ni(111), (2) graphene/Ni(111), (3) the 1 ML Au/graphene/Ni(111) and (4) the graphene/Au/Ni(111) systems with the primary electron energy of 150 eV. The spectra were arbitrary normalized for ease of comparison. The shift to the lower loss energies of the π plasmon of the graphene layer after intercalation of gold and the disappearing of the interface interband excitations at about 3.3 eV are observed. (a) and (b) show wide energy-loss scan and its zoom in the energy-loss range of the π plasmon, respectively.  
We have performed electron energy-loss spectroscopy (EELS) studies of Ni(111), graphene/Ni(111), and the graphene/Au/Ni(111) intercalation-like system at different primary electron energies. A reduced parabolic dispersion of the \pi plasmon excitation for the graphene/Ni(111) system is observed compared to that for bulk pristine and intercalated graphite and to linear for free graphene, reflecting the strong changes in the electronic structure of graphene on Ni(111) relative to free-standing graphene. We have also found that intercalation of gold underneath a graphene layer on Ni(111) leads to the disappearance of the EELS spectral features which are characteristic of the graphene/Ni(111) interface. At the same time the shift of the \pi plasmon to the lower loss-energies is observed, indicating the transition of initial system of strongly bonded graphene on Ni(111) to a quasi free-standing-like graphene state.
Pyrolytic carbons deposited in fluidized beds at 1200 to 1400°C from various hydrocarbon-helium mixtures using otherwise identical process parameters were compared. The trends of microstructure or density with increasing deposition rate were very similar at a given temperature. For each hydrocarbon investigated at 1400°C there was a minimum in the density vs. deposition rate curve. Pyrolytic carbons deposited at 1200°C at similar initial rates near 10 μ/hr from seven hydrocarbon-helium mixtures (methane, ethane, ethylene, acetylene, propane, cyclohexane and benzene) had very similar microstructures, densities, preferred orientations and apparent crystallite sizes. The results support the view that pyrolysis of hydrocarbons in fluidized beds involves dissociation into small fragments which then take place in polymerization-dehydrogenation reactions and indicate that the general mechanisms of pyrolytic carbon deposition in fluidized beds are independent of the hydrocarbon. The various microstructures were explained in terms of the extent of incorporation of gas-borne droplets or soot particles into the deposits or the variations of coating environment within the fluidized beds. The systematic variations of the properties of the pyrolytic carbons deposited from methanehelium mixtures at 1400, 1300 and 1200°C were investigated in detail and found to be consistent with a published model of pyrolytic carbon deposition in fluidized beds.
Carbon materials produced through the thermal decomposition of biomass (rice hulls) and a polymer (polyvinyl chloride) were investigated by 13C high-resolution solid-state NMR under two different magnetic fields (2.0 and 9.4 T). The details revealed by the high-field NMR spectra provide important information about the chemical changes in the initial stages of pyrolysis: These are shown to be directly related to the original structure of the precursors and the results complement well some conclusions existent in the literature. From a heat treatment temperature of about 600°C upwards, the general shape of the 13C NMR spectra, attained with low applied magnetic field, is very similar for both chars, with a strong resonance line near 125 ppm from TMS (carbon nuclei in aromatic planes). The analysis of the evolution of the main parameters associated with this resonance line shows a behavior typical of heat-treated carbon materials, which is interpreted on the basis of the structural evolution of both chars. We show that the results are well understood when a comparison is made with the features of the 13C NMR spectrum of polycrystalline graphite.
The influence of structural characteristics, in particular heteroatom content, on the ammoniatreatment of carbon-13 materials has been studied. The resulting carbon-13 materials were investigated for their temperature-programmed combustion characteristics by thermogravimetric analysis-mass spectrometry (TG-MS). The use of ca 99% isotopically pure carbon-13 allowed the analysis of N2O and N2 in the presence of CO2 and CO respectively. Bimodal peaks for N2, NO and N2O were detected in temperature-programmed combustion, suggesting two types of nitrogen functionality in the carbon. The relative yields of the three products were strongly influenced by the heteroatom content of the precursor sample. The sample which had a high heteroatom content gave N2 as the major product rather than N2O. Oxygen, present as an impurity in the carbons, is an important factor in the incorporation of nitrogen by treatment with ammonia.
The process of adsorption and desorption of sulphur dioxide on a low-ash active carbon is studied, in the temperature range between 130°C and 170°C, using two different gaseous mixtures containing SO2. Oxygen, present at the adsorption stage, plays a very important role in the variation of the amont of SO2 adsorbed during some adsorption-desorption cycles; this effect is closely related to the variation of certain important characteristics of the carbon.
The purpose of this paper is to present the results of performance analysis of a heat driven continuous vapor adsorption refrigerator with activated carbon as the adsorbent and 1,1,1,2-tetrafluoroethane (HFC-134a) as the refrigerant. A set of four adsorption cells takes on the role of the mechanical compressor in the conventional vapor compression refrigeration (VCR) system. Three specimens of activated charcoal under various packing densities were investigated. A parametric analysis was carried out with several evaporating, condensing/adsorbing and desorbing temperatures which are typical operating conditions catered to by HFC-134a. A new integrated relative performance evaluation scheme is proposed. It uses the maximum cycle uptake difference as a factor against which the coefficient of performance (COP) and exergetic efficiency are evaluated. It is shown that there is an optimal set of operating conditions wherein the exergetic efficiency is the maximum. A major part of the thermal energy input is for sensible heating of the compressor body.
Low-density carbon foam was synthesized from an aqueous acidic sucrose solution. The resin formed by heating this solution underwent foaming and set into a solid green foam which was sintered in the temperature range 573–1223 K. The green and the sintered foams were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction and scanning electron microscopy. A thermogravimetric analyzer coupled with a mass spectrometer was used to study the thermal stability of green foam under an argon atmosphere in the temperature range 303–1273 K. The suitability of this carbon foam and a commercially available reticulated vitreous carbon for trapping radioactive 137Cs was studied by a tracer technique in static sodium at 473 K. A NaI(Tl) detector was used to measure 137Cs activity trapped by these materials. Trapping efficiency for 137Cs and distribution coefficient of 137Cs between sodium and the carbon foam were found to be in the range 73–77% and 4.9–6.0 × 102, respectively.
Chemical functionalities of hydrogen and carbon in a series of cokes obtained from heavy crude oils in a Mobile continuous flow laboratory coker pilot unit are probed with high-resolution solid state NMR. The fractions of aromatic hydrogen and carbon, as determined from 1H combined rotation and multiple-pulse spectroscopy (CRAMPS), and 13C magic angle spinning (MAS) experiment with and without cross polarization (CP), varied only slightly between 0.49 and 0.65 and between 0.88 and 0.92, respectively, for the samples studied. A comparison with the results of direct excitation (13C MAS) NMR showed that CP/MAS NMR spectra taken with a contact time of 1 ms well represented relative carbon intensities. The high-resolution spectra, in combination with previously reported wideline 1H NMR data and the results of elemental analysis, are used to derive several structural parameters, including aromatic and aliphatic hydrogen to carbon ratios and the average formula per 100 carbon atoms. Finally, the “average” structures for studied cokes are proposed and discussed. Most cokes are concluded to consist of molecules having approximately 10 aromatic rings bearing only few substitutions.
A series of semi-cokes, containing from virtually 0 to 100% mesophase, were produced from the THF-soluble fraction of a soft coal tar pitch. The mesophase contents were estimated by high temperature in situ 1H NMR and optical microscopy with extremely close agreement being obtained between the two techniques. Quantitative solid state 13C NMR measurements using the single pulse excitation technique indicated that the extent of anisotropy in the pitch is a function of the condensation of the aromatic structure, where isotropic pitches contain 5–6 rings on average, assuming pericondensation, which increases to 9–10 rings for the fully converted mesophase pitches. Characterisation of the pyridine-solubles (PS) and insolubles (PI) from the pitches confirmed that development of mesophase in the coal tar pitch is limited by the loss of low molecular mass compounds, in that the PS contain 5–6 rings throughout the series while the PI, even in largely the initial isotropic state, are as condensed as in the anisotropic pitches.
The bulk chemistry of commercial carbon blacks and carbon blacks obtained by vacuum pyrolysis (CBP) of used tires was investigated by 13C-NMR spectroscopy with and without magic angle spinning of the sample. Two different kinds of carbon atoms can be distinguished: Graphite like carbon atoms in poly-condensed aromatic rings and carbon atoms in a less ordered environment. Commercial carbon blacks and CBP obtained under different pyrolysis conditions have practically the same concentrations of the different types of carbon atoms in the bulk, whereas earlier ESCA and SIMS investigations have shown that the surface chemistry of CBP is different from commercial carbon blacks and depends strongly on the pyrolysis conditions. Thus, during the pyrolysis only the carbon black surface chemistry is changed. The carbon black bulk structure was also studied by X-ray diffraction. The XRD results, including the radial distribution function (RDF) indicated, in agreement with the NMR results, that the bulk structure of commercial carbon blacks and of CBP are similar.
13C NMR spectra and spin-lattice relaxation times were measured for single-wall carbon nanotubes with 99.9 and 50.0% 13C enrichments and natural abundance (1.1% 13C) prepared by catalytic decomposition of CH4. The 13C isotropic shift is about 116 ppm from tetramethylsilane, being estimated from magic-angle-spinning (MAS) spectra. The value does not depend on the degree of the 13C enrichment. The 13C MAS NMR spectra show two additional small peaks at 171 and 152 ppm, which are ascribed to carbon species at defects or edges. The line widths of the main isotropic peak in MAS spectra are about 30 ppm, the origin of which is mostly chemical shift dispersion, reflecting a distribution of diameter and helicity. The line width in the 13C static spectra originates from chemical shift dispersion, chemical shift anisotropy and dipole–dipole interactions between 13C spins as well as between 13C and 1H spins at defects or edges. 1H NMR spectra confirm the presence of H-containing species. The 13C spin-lattice relaxation is dominated presumably by interaction with magnetic impurities.
Top-cited authors
Rodney Ruoff
  • Ulsan National Institute of Science and Technology
Richard Piner
  • Texas Research Institute Austin, Inc
Sasha Stankovich
  • Milliken & Company
Sonbinh Nguyen
  • Northwestern University
D. A. Dikin
  • Temple University