Self-assembled cones of aligned carbon nanofibers grown on wet-etched Cu foils

Hot-filament chemical vapour deposition (HF-CVD) was used to grow aligned carbon nanofibers (CNFs) directly on Cu foils. Fast wet-chemical etching procedures based on hydrogen peroxide (H₂O₂) were found to have a key role on the formation of selective active substrates for the growth process. Here, a comprehensive mechanism is presented. Additionally, it is shown that nano-sized protrusions ∼8 nm with round shape and high density ∼1.7 × 10³ μm⁻² were decisive for the growth of aligned hollow-herringbone CNFs following the base-growth model. CNFs with heights of ∼1 μm and diameters around ∼8 nm show narrow diameter size distribution with remarkable correlation to the protrusion size distribution. The fibers were organised in cone-shape configurations with a cone density of ∼22 μm⁻² and a cone angle of 90°. An activation energy for the CNF growth of E_act ∼ 0.90 ± 0.16 eV was extracted from the Arrhenius plot showing that the process kinetics is governed by C diffusion in bulk Cu.     

The production of porous carbon nanofibers from cross-linked polyphosphazene nanofibers

Uniform porous carbon nanofibers with an average diameter of 90 nm were fabricated by forming polyphosphazene nanofibers and carbonizing them, without the need for any activation step. The structure and morphology of the carbon nanofibers were characterized by SEM, TEM, EDX, XRD, Raman spectrum and N₂ adsorption. Results showed that the carbon nanofibers have a BET surface area of about 540 m² g⁻¹, a total pore volume of about 0.37 m³ g⁻¹, and a narrow pore size distribution in the micropore range.     

Influence of hydrogen on the formation of a thin layer of carbon nanofibers on Ni foam

Carbon nanofibers (CNFs) were catalytically grown on Ni foam by decomposing ethylene in the presence of hydrogen. Variation of hydrogen concentration during CNF growth resulted in significant manipulation of the properties of a thin layer of CNFs. Addition of hydrogen retards carbon deposition and increases the surface area of the CNF layer because of formation of thinner fibers. The thickness of CNF layer shows an optimum at intermediate hydrogen concentrations. These effects contribute to the competitive adsorption of hydrogen and ethylene, influencing the availability of carbon on the Ni surface, which is necessary for both the formation of small Ni particles by fragmentation of polycrystalline Ni, as well as for CNF growth after formation of small particles. Furthermore, decreasing the carbon supply via adding hydrogen also delays deactivation by encapsulation of Ni particles. The thickness of the micro-porous C-layer between the Ni surface and the CNF layer decreases with hydrogen addition, at the expense of a slight loss in the attachment of the CNFs to the foam, supporting the proposition that CNFs are attached by roots in the C-layer. The addition of hydrogen after the initial CNF formation in ethylene only causes fragmentation of the C-layer, inducing significant loss of CNFs.     

The production of carbon nanofibers and thin films on palladium catalysts from ethylene-oxygen mixtures

The characteristics of carbonaceous materials deposited in fuel rich ethylene-oxygen mixtures on three types of palladium: foil, sputtered film, and nanopowder, are reported. It was found that the form of palladium has a dramatic influence on the morphology of the deposited carbon. In particular, on sputtered film and powder, tight ‘weaves’ of sub-micron filaments formed quickly. In contrast, on foils under identical conditions, the dominant morphology is carbon thin films with basal planes oriented parallel to the substrate surface. Temperature, gas flow rate, reactant flow ratio (C₂H₄:O₂), and residence time (position) were found to influence both growth rate and type for all three forms of Pd. X-ray diffraction, high resolution transmission electron microscopy, temperature-programmed oxidation, and Raman spectroscopy were used to assess the crystallinity of the as-deposited carbon, and it was determined that transmission electron microscopy and X-ray diffraction were the most reliable methods for determining crystallinity. The dependence of growth on reactor position, and the fact that no growth was observed in the absence of oxygen support the postulate that the carbon deposition proceeds by combustion generated radical species.     

Preparation of Fischer–Tropsch cobalt catalysts supported on carbon nanofibers and silica using homogeneous deposition-precipitation

Homogeneous deposition-precipitation on either a silica or carbon nanofiber (CNF) support of cobalt from basic solution using ammonia evaporation was studied and compared with conventional deposition from an acidic solution using urea hydrolysis. In the low-pH experiment, the interaction between precipitate and silica was too high; cobalt hydrosilicates were formed requiring a reduction temperature of 600 °C, resulting in low cobalt dispersion. Lower interaction in experiments performed in a basic environment yielded a well-dispersed Co₃O₄ phase on silica, and after reduction at only 500 °C, a catalyst with 13-nm cobalt particles was obtained. On CNF from an acidic solution, cobalt hydroxy carbonate precipitated and displayed a low interaction with the support resulting after reduction at 350 °C in a catalyst with 25-nm particles. From basic solution we obtained high dispersion of cobalt on the CNF, probably related to the greater ion adsorption. After drying, Co₃O₄ crystallites were obtained that, after reduction at 350 °C, resulted in a catalyst with 8-nm Co particles. Samples prepared in the high-pH experiment had 2–4 times higher cobalt-specific activity in the Fischer–Tropsch reaction than their low-pH counterparts. CNF support materials combined with the high-pH deposition-precipitation technique hold considerable potential for cobalt-based Fischer–Tropsch catalysis.     

Mesoporous carbon spheres grafted with carbon nanofibers for high-rate electric double layer capacitors

Carbon nanofibers were grafted onto mesoporous carbon spheres to produce “sea urchin-like” mesoporous carbon with a nanofiber content of 25 wt.%. Because of its combined features of high electronic conductivity and efficient electrolyte transport, the sea urchin-like mesoporous carbon assembled in electric double layer capacitors shows outstanding high-rate performance with a voltammetric scan rate as high as 3000 mV s⁻¹. Ac impedance analysis shows that this method of carbon nanofiber grafting promotes electronic percolation and ionic transportation in the carbon electrode, reducing the capacitive relaxation time to less than one fourth of its original value. Electrochemical oxidation in sea urchin-like mesoporous carbon produces a capacitance increase of ca. 200% while retaining high electronic and ionic conductivities in the electrode.     

Combined XPS and TPD study of oxygen-functionalized carbon nanofibers grown on sintered metal fibers

A composite material consisting of carbon nanofibers (CNFs) grown on sintered metal fiber filters was modified by H₂O₂ or plasma-generated O₃. Coupling temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) techniques in the same UHV apparatus allowed the direct correlation of the nature of the created O-functional groups and their evolution as CO and CO₂ upon heating. The two oxidative treatments yielded different distributions of O-containing groups. The relative contribution of oxidized carbon was very low in the C1s region, hence the functional groups were more robustly analyzed through the O1s region. The comparison of the released oxygen by integration of the TPD CO, CO₂ and H₂O spectra with the intensity loss of the XPS O1s spectra showed good agreement. In order to fit the data adequately, the set of O1s spectra was deconvoluted in at least four peaks for the differently activated samples. Finally, it was shown that functional groups formed by H₂O₂-treatment (mostly non-phenolic OH groups) are more thermally stable than those formed by O₃-treatment. The latter treatment increases the concentration of carboxylic functionalities, which decompose at temperatures < 800 K; O₃-activated CNFs should therefore show a more pronounced acidic behavior.     

Influence of the inherent metal species on the graphitization of methane-based carbon nanofibers

Carbon nanofibers (CNFs) containing different proportions of Ni and Si were produced from methane decomposition in a fluidized bed reactor with a nickel–copper based catalyst. They were subjected to heat treatment in the temperature interval 1800–2800 °C for the purpose of studying the influence of the inherent metal species on their ability to graphitize. The participation of Ni and Si species on the graphitization of the methane-based CNFs through the formation of a nickel silicide phase as an intermediate state which further promotes the production of silicon carbide was inferred. Moreover, since silicon carbide was observed by X-ray diffraction after the heat treatment of the CNFs at temperatures ?2400 °C, the formation of graphite at the expense of the carbide decomposition seems to be a plausible mechanism to explain the catalytic graphitization of these CNFs. Because of this effect, carbon materials with crystalline parameters in the range of synthetic graphites which are currently employed in energy applications were prepared in this work. A progressive improvement of the degree of the structural order of the materials prepared with increasing Si/Ni weight ratio in the CNFs was observed.     

Oxygen reduction reaction properties of carbon nanofibers: Effect of metal purification

Carbon nanofibers (CNFs) are grown on metal catalysts and electrochemical treatment is used to remove the metal catalyst residuals from the as-grown CNFs. For comparison, the CNFs are also purified by a chemical method and a thermal method. The oxygen reduction reaction (ORR) properties of CNFs purified by these three methods are examined by cyclic voltammetry. CNFs treated by the electrochemical method have a more positive ORR onset reduction potential and peak potential compared with those treated by chemical and thermal methods, and this is because the microstructures of CNFs are less changed by electrochemical method. However, they have a lower electrochemical capacity and ORR peak current than those treated by the chemical method. Cyclic voltammetric measurements at different scan rates confirm that the oxygen reductions on CNFs treated by electrochemical and chemical methods are controlled by diffusion, while on CNFs treated by thermal method is partially influenced by diffusion.     

Growth of carbon nanofibers on activated carbon fiber fabrics

Activated carbon fiber fabrics, an excellent adsorbent, were used as catalyst supports to grow carbon nanofibers. Because of the microporous structure of the activated carbon fibers, the catalysts could be distributed uniformly on the carbon surface. Based on this concept, the carbon nanofibers can be grown directly on the activated carbon fiber fabrics. We demonstrate that carbon nanofibers with a diameter between 20 and 50 nm for most of the fibers can be synthesized uniformly and densely on activated carbon fiber fabrics, impregnated by nickel nitrate catalyst precursor, using catalytic chemical vapor deposition. Although the carbon nanofibers are not straight with a crooked morphology, they form a three-dimensional network structure. Structure characterizations by TEM and XRD indicate that the carbon nanofibers have a turbostratic graphite structure and the graphite layers are stacked with a herringbone structure.     

Consolidation of carbon nanofibers with pyrolytic carbon

A strategy of industrial-scale manufacture for a wide range of carbon materials based on carbon nanofibers is proposed. It was shown that porous materials with a high sorption capacity can be obtained with the use of carbon nanofibers by means of conventional manufacturing operations. The results of studying of consolidation of carbon nanofibers with pyrolytic carbon are reported. It was found that the nature of carbon material has a substantial effect on the rate of deposition of pyrolytic carbon. The most appropriate temperature range in which carbon nanofibers should be consolidated for the preparation of materials with a high catalytic activity was determined.     

Direct synthesis of carbon nanofibers on modified biomass-derived activated carbon

Carbon nanofibers were synthesized on activated carbons produced from agricultural waste using chemical vapor deposition. Importantly, iron already present in the ash content of the activated carbon was employed as a natural catalyst for nanofiber formation. The need for a wet chemical catalyst preparation step was avoided.     

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 C₂H₄ 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 C₂H₄/H₂ 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. N₂ 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.     

The characterization and application of p-type semiconducting mesoporous carbon nanofibers

The microstructures of mesoporous carbon nanofibers were characterized by scanning electron microscopy, transmission electron microscopy, nano-Raman, nitrogen adsorption-desorption and optical transmission. They possessed a high specific surface area 840 m² g⁻¹ and a 1.07 eV band gap. All mesoporous carbon nanofiber network can act as the channel material in p-type field-effect transistor devices with field-effect mobilities over 10 cm²/V s. Furthermore, mesoporous carbon nanofiber network exhibits better sensitivity and faster response to NO₂ gas than that of carbon nanotubes, which makes it a promising candidate as poisonous gas sensing nanodevices.     

Helical carbon nanofibers with a symmetric growth mode

Helical carbon nanofibers with a symmetric growth mode were synthesized by the decomposition of acetylene with a copper catalyst. There were always only two helical fibers symmetrically grown over a single copper nanocrystal. The two helical fibers had opposite helical senses, but had identical cycle number, coil diameter, coil length, coil pitch, cross section, and fiber diameter. The irregular tips and helical reversals of the two helical fibers further revealed the symmetric growth mode. This mirror-symmetric growth mode was induced by the shape changes in copper nanocrystals during catalyzing the decomposition of acetylene. Upon contacting the initial copper nanocrystals with irregular shapes, acetylene began to decompose to form two straight fibers (the irregular tips). At the same time, shape changes in copper nanocrystals began. Once they changed from an irregular to a regular faceted shape, the two straight fibers ceased to grow and two regular helical nanofibers with opposite helical senses began to grow. If the regular faceted nanocrystals continue to change shapes during fiber growth, the two helical fibers possibly changed helical senses at the same time, resulting in helical reversals. The shape changes were caused by the changes in surface energy resulting from the acetylene-adsorption on the copper nanocrystals.     

Formation of carbon nanofibers via carbon monoxide disproportionation

Experimental results suggesting that carbon nanofibers are formed from amorphous carbon released at several compact active sites are reported. It was shown that the sites in question are catalyst crystal lattice defects formed at the crystallite contact boundaries.