Structure Of Carbon Nanofibers Research Articles

Temperature-mediated control of the growth of an entangled carbon nanofiber layer on stainless steel micro-structured reactors

A well-adhered layer of carbon nanofibers (CNFs) was grown on stainless steel microreactors by decomposition of a hydrocarbon over microreactors previously coated with Ni dispersed on alumina. In a previous work, we reported that the growth temperature and hydrocarbon (methane or ethane) modulated the morphology and size of the grown carbon species. Using ethane, the carbon yield increased dramatically for temperatures exceeding 898K. At these temperatures some carbon protrusions arise, which plug microreactor channels and render the microreactor unsuitable for use. For methane at all the tested temperatures or for ethane at the lowest temperatures (853 and 873K), the microreactor channels were covered completely by a uniform mat of entangled CNFs ready for catalytic use. Here, we show that the growth temperature and hydrocarbon can also control the primary structure of CNFs, either rolled graphitic planes parallel to the axis (multi-wall carbon nanotubes) or graphitic planes forming an angle with respect to the axis (fishbone type).

Functionalization of carbon nanofibers through electron beam irradiation

Carbon nanofibers were oxidized in air at 350°C under the influence of a 3MeV electron beam at doses of 1000 and 3500kGy. XPS analysis showed that oxygen was readily incorporated on the surface: the ratio O 1s/C 1s increased approximately by a factor of three when the carbon nanofibers were irradiated at 3500kGy. The oxidized nanofibers exhibited better dispersion than as-received nanofibers when mixed with water/methanol (50% v/v). Raman spectroscopy revealed that the ID/IG ratios were statistically unchanged for all samples because the damage on the nanofiber surface was highly localized and did not lead to modifications on the bulk carbon nanofiber structure. On the other hand, SEM, TEM, and AFM images illustrated that cutting, welding, and collapse may occur on the structure of irradiated nanofibers. BET analysis showed that the surface area of irradiated samples did not significantly increase, probably because the radiation process may lead to micro-porosity formation. The temperature during irradiation had no effect on the nanofibers’ surface and the thermal stability of the irradiated samples was determined to be lower.

Influence of carbon nanofiber structure on properties of linear low density polyethylene composites

AbstractThree types of carbon nanofibers (MJ, Pyrograf®III PR‐19 and PR‐24) were incorporated into linear low density polyethylene (LLDPE) using intensive mixing. The electrical volume resistivity of composites decreased with the addition of CNFs from over 1012 Ω cm for pure LLDPE to less than 104 Ω cm for carbon nanofibers (CNF) contents of 15 wt% or more. Tensile modulus increased from 110 MPa for pure LLDPE to 200 MPa and 300 MPa for 15 wt% MJ and 15 wt% PR composites, respectively. However, the tensile strength remained fairly unchanged at about 20 MPa. Strain‐to‐failure decreased from 690% for pure LLDPE to 460% and 120% for 15 wt% MJ and 15 wt% PR composites, respectively. It was inferred that the interfacial interactions of LLDPE matrix with MJ fibers is better than that with PR fibers, resulting from the rougher surface of MJ fibers. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers

Effect of nitrogen in the reaction atmosphere on the microstructure of carbon nanofibers grown by thermal chemical vapor deposition

Carbon nanofibers (CNFs) with different microstructures were synthesized by thermal chemical vapor deposition using different growth temperatures and methane/nitrogen gas mixtures. High-resolution transmission electron microscopy images revealed that bamboolike structure could be formed both by increasing the growth temperature and by increasing the nitrogen content in the reaction atmosphere at a lower growth temperature. Elemental analysis results indicated that no significant change in the nitrogen concentration was found regardless of the increase of nitrogen flow in the feed gas. The formation of bamboolike structure of CNFs and the effect of nitrogen gas on the microstructure change of CNFs were discussed.

Controlling the yield and structure of carbon nanofibers grown on a nickel/activated carbon catalyst

Carbon nanofibers (CNFs) were grown via the chemical vapor deposition of C 2H 4 on an activated carbon (AC)-supported Ni catalyst. The texture of the CNF/AC composites can be tuned by varying the growth temperature and by treatment in reducing atmosphere prior to C 2H 4/H 2 exposure. The Ni-catalyzed gasification of the AC support increases the microporosity of the composite and shown to be dominant throughout the composite synthesis especially during reduction, subsequent treatment in reducing atmosphere, and CNF growth at low temperatures. N 2 isotherm and scanning electron microscope were used to characterize the texture and morphology of the composites. Subsequent treatment in reducing atmosphere were shown to increase the Ni catalyst activity to grow CNFs. High resolution transmission electron microscope however did not reveal any microstructural difference for Ni catalyst with and without the subsequent reduction treatment. We propose in this paper that the carbon dissolutions during treatment of the catalyst might have an implication on the CNF growth.

Study of the Surface Chemistry of Modified Carbon Nanofibers by Oxidation Treatments in Liquid Phase

Carbon nanofibers (CNF) were grown by thermocatalytic decomposition of methane. Their texture and surface chemistry were modified by different oxidation treatments with HNO3 at different concentrations or a mixture of HNO3-H2SO4 to optimise their ability of dispersing active metal particles, because this material will be used as electrocatalytic support for polymeric electrolyte fuel cells. The effect of liquid phase oxidation on the surface chemistry and the textural properties of the CNF was studied by temperature programmed desorption (TPD), scanning electron microscopy (SEM) and N2-physisorption. Moreover, their thermal stability was studied by temperature programmed oxidation (TPO). During oxidation treatments functional groups were created and their number was function of the oxidation treatment conditions. Results indicated that an increase in severity of the oxidation treatment produces an increase in the number of surface oxygen groups and in the thermal stability. However, a very severe treatment can destroy partially the structure of carbon nanofibers.

Direct Synthesis and Structural Analysis of Nitrogen-Doped Carbon Nanofibers

Carbon nanofibers containing a range of nitrogen contents of 1-10 atom % were directly synthesized by catalytic chemical vapor deposition over nickel-based catalysts at 350-600 degrees C using acetonitrile and acrylonitrile. The nitrogen content was controlled by careful choice of the reaction conditions. The N-doped carbon nanofibers showed herringbone structure with 20-60 nm diameter. X-ray photoelectron spectroscopy was applied to examine the chemical state of nitrogen in carbon nanofibers. Structural features of N-doped carbon nanofibers were examined in X-ray diffraction and electron microscopy. The mechanism for nitrogen including the structure of carbon nanofibers through the catalysis was discussed on the basis of the results.

The effect of graphitization temperature on the structure of helical-ribbon carbon nanofibers

Structural rearrangement of helical-ribbon carbon nanofibers (CNFs) was studied as a function of graphitization temperature. The as-produced nanofibers are composed of a helical ribbon of graphene spiralled about and angled to the fiber axis. The discrete layers of graphene ribbon overlap each other forming the helical-ribbon in contrast to the discontinuous cones of the more common stacked-cup CNF morphology. After heat treatment to 2400 °C and above, the CNFs were completely free of residual metal catalyst inclusions, principally nickel used in their synthesis, and other functionalities. The formation of loops at the graphene edges was also observed. Heat treatment through the temperature range 1500–2800 °C resulted in a relatively minor contraction in interlayer spacing d 002 from 0.3381 to 0.3363 nm. This was attributed to the highly graphitic character of the as-produced CNFs. However, there were significant increases in the crystallite thickness L c through this temperature range. In addition, heat treatment above 2400 °C induced a marked change of the nanofiber morphology from circular to faceted polygonal cross-section resulting from the re-ordering of the turbostratic, curved graphene layers to regions of planar graphene layers with 3-dimensional graphitic structure (AB stacking).

Structure and electrical conductivity of nitrogen-doped carbon nanofibers

The effect of nitrogen concentration in carbon nanofibers (CNFs) on the structural and electrical properties of the carbon material was studied. CNFs with nitrogen concentration varied from 0 to 8.2 wt.% (N-CNFs) with “herringbone” structure were prepared by decomposition of ethylene and ethylene mixture with ammonia over 65Ni–25Cu–10Al 2O 3 (wt.%) catalyst at 823 K. Detailed investigation of the CNFs and N-CNFs by XPS, FTIR and Raman spectroscopy showed that the nitrogen introduction in carbon material distorts the graphite-like lattice and increases the structure defectiveness. Both effects become more significant as the nitrogen concentration in N-CNFs grows. The electrical conductivity of N-CNFs with different nitrogen concentrations is caused by the competition of the nanofiber graphite-like structure disordering after introduction of nitrogen atoms and doping of an additional electron into the delocalized π-system of the graphite-like material. As a result, the maximum electrical conductivity among the samples studied was observed at nitrogen concentration in N-CNFs equal to 3.1 wt.%.

Processing and Structure of Carbon Nanofiber Paper

A unique concept of making nanocomposites from carbon nanofiber paper was explored in this study. The essential element of this method was to design and manufacture carbon nanofiber paper with well‐controlled and optimized network structure of carbon nanofibers. In this study, carbon nanofiber paper was prepared under various processing conditions, including different types of carbon nanofibers, solvents, dispersants, and acid treatment. The morphologies of carbon nanofibers within the nanofiber paper were characterized with scanning electron microscopy (SEM). In addition, the bulk densities of carbon nanofiber papers were measured. It was found that the densities and network structures of carbon nanofiber paper correlated to the dispersion quality of carbon nanofibers within the paper, which was significantly affected by papermaking process conditions.

Carbon nanofiber-supported palladium nanoparticles as potential recyclable catalysts for the Heck reaction

Abstract 5 wt% Pd catalysts supported on platelet carbon nanofibers has been prepared by incipient wetness impregnation. Both the calcination and the reduction temperature have a significant effect on the dispersion of palladium and it was found that about 3 nm sized Pd nanoparticles can be obtained at a calcination and reduction temperature of 250 °C and 150 °C, respectively. Pd catalysts have been applied to catalyze Heck reactions of various activated and non-activated aryl substrates. The activity increased exponentially with a decrease in Pd particle size. The high surface area, mesoporous structure of carbon nanofiber and highly dispersed palladium species on carbon nanofibers makes up one of the most active and reusable heterogeneous catalysts for Heck coupling reactions. Pd nanoparticles supported on platelet CNFs appear to be an excellent catalyst due to high activity, low sensitivity towards oxygen, almost no or low issues with leaching and high stability in multi-cycles.

A peculiar composite structure of carbon nanofibers growing on a microsized tin whisker

In this work, we report a method to synthesize a peculiar composite structure of tubular carbon nanofibers (CNFs) growing on a microsized tin (Sn) whisker. The material used is a commercially available copper clad laminate (CCL). The CCL is composed of a surface copper (Cu) layer and a bottom polymer (phenol-formaldehyde resin) board, in which the polymer board is used as the carbon source. Using lithography and lift-off techniques, the Cu layer was patterned to a stripelike Cu trace. A Sn thin film was then evaporated on the polymer board near the Cu trace. To release the residue stress that resulted from the evaporation; Sn whiskers with diameters of about 2 to 5 μm were formed on the Sn thin film after evaporation. By passing an electric current through the Cu trace, the Cu trace was heated due to Joule heating and served as a heating source for the thermal decomposition of phenol-formaldehyde. After heat treatment, the CNFs grew on the surface of the Sn whiskers with tubular hollow-cored structure. The diameter of the tubular CNFs is about hundreds of nanometers and their length can reach several micrometers. The growth mechanism of the brushlike composite structure is also discussed.

Preparation and surface structures of carbon nanofibers produced from electrospun PAN precursors

Preparation and surface structures of carbon nanofibers produced from electrospun PAN precursors

Preparation and surface structures of carbon nanofibers produced from electrospun PAN precursors

Carbon nanofibers with diameters in the range of 100 nm to 300 nm were obtained by stabilizing and carbonizing electrospun polyacrylonitrile (PAN) precursors. The morphologies and structures of the nanofibers and PAN precursors were investigated by scanning electron microscopy, scanning tunneling microscopy, and differential scanning calorimetry. The diameters of the PAN precursors and carbon nanofibers showed a log-normal distribution. The cyclization exothermic peak shifted to a lower temperature for electrospun fibers, which suggested that cyclization could be more easily initiated. Pits 10 nm in length and 5 nm in width formed on the surface of the carbon nanofibers, caused by the rough surface of the electrospun precursors and their shrinkage during heat treatment.

Surface characterization and functionalization of carbon nanofibers

Carbon nanofibers are high-aspect ratio graphitic materials that have been investigated for numerous applications due to their unique physical properties such as high strength, low density, metallic conductivity, tunable morphology, chemical and environmental stabilities, as well as compatibility with organochemical modification. Surface studies are extremely important for nanomaterials because not only is the surface structurally and chemically quite different from the bulk, but its properties tend to dominate at the nanoscale due to the drastically increased surface-to-volume ratio. This review surveys recent developments in surface analysis techniques used to characterize the surface structure and chemistry of carbon nanofibers and related carbon materials. These techniques include scanning probe microscopy, infrared and electron spectroscopies, electron microscopy, ion spectrometry, temperature-programed desorption, and atom probe analysis. In addition, this article evaluates the methods used to modify the surface of carbon nanofibers in order to enhance their functionality to perform across an exceedingly diverse application space.

FABRICATION, MORPHOLOGY, AND STRUCTURE OF ELECTROSPUN PAN-BASED CARBON NANOFIBERS

This study reports the fabrication, morphology and structure of carbon nanofibers prepared by electrospinning a precursor of polyacrylonitrile (PAN)/dimethyl formamide (DMF), followed by carbonization of the electrospun nanofibers. Effects of PAN concentration (4-12 wt.%) and applied voltage (5-15 kV) on the morphology and distribution of the as-spun nanofibers diameter were investigated. Fibers with diameter ranging from 100 nm to 1600 nm were obtained depending on the electrospinning condition. The diameters of the as-spun nanofibers increased with increasing the solution concentration. On the other hand, the diameters of the fibers decreased with increasing applied voltage. The optimal parameters for obtaining the PAN-nanofibers with narrow size distribution of 481 ± 101 nm and without beads formation along the fibers were revealed. These good nanofibers were subjected to carbonization in either argon or nitrogen atmosphere at 1000 °C for 2 h. Carbonization in nitrogen and argon resulted in the nanofibers of 249 ± 37 nm and 225 ± 31 nm in diameter with 97.2 % and 95.9 % in purity, respectively, but the thermal-resistant ability was not much different. The D and G peaks from Raman scattering were analyzed using Gaussian-Lorentzian curve-fitting, and the graphitic crytallite domain size (L a ) was estimated from the ratio of the integrated intensity of D and G peaks. The domain size of the graphite layers was 2.87 ± 0.31 nm and 3.02 ± 0.79 nm for the nanofibers carbonized under nitrogen and argon atmosphere, respectively.

Temperature and substrate dependence of structure and growth mechanism of carbon nanofiber

The carbon nanofibers were grown on Ni/Si and Ni/Ti/Si substrates in 1 atm CH 4 atmosphere at 640 °C and 700 °C by thermal chemical vapor deposition method. The carbon nanofibers were characterized by field emission scanning electron microscopy, transmission electron microscopy, and Raman spectrometry for morphology, microstructure, and crystallinity. The electron emission property of carbon nanofibers was also investigated by current–voltage ( I– V) measurement. The results showed that the solid amorphous carbon nanofibers could be grown on Ni/Si substrate at 640 °C through tip growth mechanism, the carbon nanotubes could be grown on Ni/Si substrate at 700 °C through tip growth mechanism, and the carbon nanotubes could be grown on Ni/Ti/Si substrate at 700 °C through root growth mechanism.

Control of carbon nanostructure: From nanofiber toward nanotube and back

The unique properties of carbon nanofibers (CNFs) make them attractive for numerous applications ranging from field emitters to biological probes. In particular, it is the deterministic synthesis of CNFs, which requires precise control over geometrical characteristics such as location, length, diameter, and alignment, that enables the diverse applications. Catalytic plasma enhanced chemical vapor deposition of vertically aligned CNFs is a growth method that offers substantial control over the nanofiber geometry. However, deterministic synthesis also implies control over the nanofiber’s physical and chemical properties that are defined by internal structure. Until now, true deterministic synthesis has remained elusive due to the lack of control over internal graphitic structure. Here we demonstrate that the internal structure of CNFs can be influenced by catalyst preparation and ultimately defined by growth conditions. We have found that when the growth rate is increased by 100-fold, obtained through maximized pressure, plasma power, and temperature, the resulting nanofibers have an internal structure approaching that of multiwalled nanotubes. We further show that the deliberate modulation of growth parameters results in modulation of CNF internal structure, and this property has been used to control the CNF surface along its length for site specific chemistry and electrochemistry.

Evidence for growth mechanism and helix-spiral cone structure of stacked-cup carbon nanofibers

New evidence for the structure of Ni-catalyzed stacked-cup carbon nanofibers (CNFs) has been found. This type of carbon nanofiber exhibits a wide hollow core as well as a large diameter (between 40 and 140 nm). The fibers have been produced by the floating catalyst method using natural gas as carbon feedstock, a sulfur compound, and a nickel catalyst. It was found that the catalytic particles are heterogeneous with two different parts: one composed of metallic Ni, which is the catalytically active portion of the particle, and another composed of NiS, which allows for the hollow nanofiber structure. The hollow core of the fibers has similar dimensions to the NiS volume of the particle and the graphitic layers grow from the rear nickel region of the particle. Nevertheless, the NiS component seems to be indispensable in producing the helix-spiral formation of the graphitic structure, as clearly shown by the TEM studies.

Thermogravimetric studies of carbon nanofiber formation from methane at low temperature over Ni-based skeletal catalysts and the effect of substrate pre-carburization

Using thermogravimetry (TG) under conditions that minimize inhibition by the hydrogen produced, the intrinsic catalytic rates of skeletal Ni, pure and alloyed with solute metals Fe, Co, or Cu, were evaluated in methane decomposition to carbon nanofibers. In “standard” tests, i.e., after pre-reduction in H 2 and exposure to CH 4 directly at 450 °C, several catalysts reached stable activities exceeding 4 mg C/mg cat./h, comparable with literature values obtained at 500 °C or above. TG evidence is presented for partial bulk carburization of Ni in CH 4 below 350 °C, which leads to substantially increased coking rates. TEM evidence supports the view that carburization promotes catalyst particle disintegration, thereby inducing faster and more stable nanofiber growth. Irregularities in alloy response to carburization are interpreted in terms of the stability of the respective mixed-metal carbides. TEM also shows that alloying changes the metal nanocrystallite shape (habit), with consequences for the carbon nanofiber structure. Evidence for the easy dissociation of CH 4 is corroborated by direct catalyst activation in the absence of H 2. Reduction begins in pure hydrocarbon around 300 °C and leads to coking activities at 450 °C comparable to those for samples pre-reduced in H 2. Skeletal metal catalysts offer distinct advantages in low-temperature natural gas conversion.