Differences between carbon nanofibers produced using Fe and Ni catalysts in a floating catalyst reactor

Carbon nanofibers were produced by the catalytic CVD process by the floating catalyst method, in semi-industrial systems at temperatures above 1350 K. Iron-derived carbon nanofibers were produced from natural gas and xylene, using ferrocene as catalyst source, yielding a thickened submicron vapor grown carbon fibers with a core of multi-wall nanotubes. For the production of Ni derived nanofibers, natural gas was used as the carbon feedstock, and the Ni was added in a nickel compound solution. When no sulfur is used, only soot was obtained, but when sulfur is added to the reactive feedstock, a highly graphitic and very nice stacked-cup-type nanofibers with no free-CVD thickened layer were produced. TEM-EDS analysis confirms that this type of stacked-cup carbon nanofiber is produced only with a partially molten catalyst and methane as hydrocarbon source. In fact, very few fibers have either a particle tip at the end or trapped metal particle inside the wide hollow core of this type of produced carbon material.     

Local chemical vapor deposition of carbon nanofibers from photoresist

The addition of nanofeatures to carbon microelectromechanical system (C-MEMS) structures would greatly increase surface area and enhance their performance in miniature batteries, super-capacitors, electrochemical and biological sensors. Negative photoresist posts were patterned on a Au/Ti contact layer by photolithography. After pyrolyzing the photoresist patterns to carbon patterns, graphitic nanofibers were observed near the contact layer. The incorporation of carbon nanofibers in C-MEMS structures via a simple pyrolysis of modified photoresist was investigated. Both experimental results considered to consist of a local chemical vapor deposition mechanism. The method represents a novel, elegant and inexpensive way to equip carbon microfeatures with nanostructures, in a process that could possibly be scaled up to the mass production of many electronic and biological devices.     

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.     

Local field emission from individual vertical carbon nanofibers grown on tungsten filament

Individual high-aspect-ratio carbon nanofibers (CNFs) were grown on tungsten filament substrates by plasma-enhanced hot filament chemical vapor deposition. They are ∼100 nm in diameter and 6-30 μm in length with a density less than 10⁶/cm². The field emission property of single as-grown carbon nanofibers was measured in a scanning electron microscope equipped with a moveable nanoscale probe tip. The measurement results showed that the threshold field of single carbon nanofibers with different lengths was in the range of 4-5 V/μm with a corresponding emission current density of 20 μA/cm², but an evident difference in the enhancement of emitted current between nanofibers of different lengths could be found when the applied field was increased continuously. This indicates that the field emission property of single carbon nanofibers depends mainly upon their length, which is essentially attributed to the change of field enhancement factor of single carbon nanofibers. In addition, field emission of the different positions on the wall of a single carbon nanofiber was studied.     

Structural characterization of carbon nanofibers formed from different carbon-containing gases

Catalytically grown carbon nanofibers, a novel mesoporous carbon material for catalysis, were synthesized by the decomposition of carbon-containing gases (CH₄, C₂H₄ or CO) over supported nickel-iron alloy and unsupported iron. It was shown that the structures of as-synthesized and modified CNFs, including the arrangement of the graphenes in CNF, and the crystallinity and texture of CNF depended on the catalyst composition and the type of carbon-containing gas. Three types of CNFs with different microstructures were obtained: platelet CNF (Fe–CO), fishbone CNF (supported Ni–Fe alloy-CH₄, C₂H₄ or CO) and tubular CNF (supported Ni–CO). All the CNFs were mesoporous carbon materials possessing relatively high surface areas (86.6–204.7 m²/g) and were highly graphitic. Purification with acid-base treatments or high temperature treatment removed the catalyst residue without changing the basic structures of the CNFs. However, annealing significantly decreased their surface areas through the formation of loop-shaped ends on the CNF surfaces. Oxidative modification in the gas and liquid phases changed the structures only slightly, except for oxidation in air at 700 °C. The structures and textures were studied using SEM, TEM, XRD, BET and TGA.     

Growth mechanism of vapor phase CVD-grown multi-walled carbon nanotubes

The formation mechanisms involved in the growth of carbon nanotubes (CNTs) by spray pyrolysis was studied. Both iron and nickel were used as catalysts for growth, and nanotubes were also produced using thermal chemical vapor deposition for comparison. Transmission electron microscopy was used to analyze the encapsulated metal catalyst particles found within the tubes, and the dimensions and location of these particles was recorded. CNTs grown by spray pyrolysis were found to have encapsulated particles in both the middle and end of tubes, with large length to diameter ratios. As a result of these observations, it is concluded that nanotubes grown using spray pyrolysis are formed via an open-ended, root growth mechanism. Additionally, the presence of multiple, high aspect ratio particles within single tubes is explained by an additional growth theory. During the continued growth of these CNTs, metal atoms or nanoscale metal catalyst particles deposit in the open ends of growing tubes, forming new particles and helping to prevent tube closure. CNTs grown with thermal CVD did not contain similar elongated particles or particles along the middle of the tubes, indicating that this new growth mechanism is only applicable in the case of tubes grown via spray pyrolysis or other vapor phase CVD growth methods.     

Microstructure and microscopic deposition mechanism of twist-shaped carbon nanocoils based on the observation of helical nanoparticles on the growth tips

Carbon nanocoils possessing a twisting form were prepared from an Fe-based alloy (SUS410) catalyst, the crystallographic properties of the catalyst surface, which was present on the growth tip, were examined, and a novel growth mechanism of the nanocoils is presented. Single-helix twisted carbon nanocoils with a coil diameter of 300-400 nm were obtained at 700-800 °C with a yield of 60% and a purity of nearly 100%. The catalyst structure observed on the growth tip was a Fe₅C₂ (monoclinic) or Fe₇C₃ (orthorhombic) single crystal, in which some of the Fe atoms were substituted by Cr. Carbon nanoparticles with a helical structure were observed on the surface of catalyst particle. It is considered that the carbon nanoparticles are a helical carbon supply source. That is, the catalyst particle supplies microscopic carbon nanoparticles by helical deposition patterns on the catalyst surface to form macroscopic helical patterns of carbon nanocoils.     

Influence of electric field and emission current on the configuration of nanotubes in carbon nanotube layers

The influence of applied electric field (E_av) and emission current (I_FE) on the configuration of conical layers carbon nanotubes (CLNTs) grown by CVD on the edge of Ni foil has been investigated. TEM profile imaging revealed a high concentration of nanotubes near the foil edge surface, whereas on the nanotube layers’ outer surfaces single, non-oriented nanotubes with open ends free of catalytic particles, were observed. After sufficient electric field application many nanotubes became oriented towards the anode, but one or two of them were found to be always a few microns more extended. In situ SEM investigation showed that below E_av = 3.2–3.9 V/μm, emission was achieved at the expense of originally existing free nanotube ends. Configuration changes began at larger electric fields. On the observed foil edge length (14.6–17.8 μm, with an edge thickness of 200 μm) one or two nanotubes extended towards the anode and probably became the main emitters. Upon further increasing the field to E_av = 5.7–8 V/μm and at an emission current I_FE = 2 × 10⁻⁵ A these tubes disappeared (or essentially shortened). At E_av = 8 V/μm and higher and at an exposure time up to 40 min, several tens of extended nanotubes appeared, with one or two extended well beyond the others. This nanotube configuration pattern is connected with electrostatic screening between the nanotubes. Our interpretation of the data suggests that in the investigated range of E_av and I_FE, a limited number of nanotubes are emitting and these nanotubes are constantly changing as E_av, I_FE and exposure time increase.     

Diameter control of carbon nanotubes by changing the concentration of catalytic metal ion solutions

We have developed a simple new method to control the diameter of carbon nanotubes (CNTs) using catalytic nanoparticle arrays fabricated by filling the pores of well-ordered porous anodic aluminum oxide (AAO) templates with a metal ion solution. Fe ion solution was used to fill the pores in which Co had been deposited electrochemically, and then the template was dried naturally on a magnet. After this process, the pores were widened in NaOH solution. Well-graphitized multi-walled CNTs were grown from almost all the pores and were very long in length and homogeneous in diameter. We were able to control the diameter of CNTs, simply, by changing the concentration of iron ion solution. For example, the average outer diameters of the CNTs are 7 ± 1.5, 13 ± 1, and 17 ± 1 nm when the concentrations of Fe ion in their mother solutions were 1.0 × 10⁻³, 3.0 × 10⁻³, and 6.0 × 10⁻³ M, respectively. The inner diameters of these CNTs corresponded to the calculated diameters of Fe nanoparticles by assuming that all Fe ions contained in each pore are reduced to a single nanoparticle. This means that homogeneous nanoparticles are made in each pore. Our new method could be used to fabricate homogeneous nanoparticles from most metal ion solutions.     

Chemical vapor deposition of pyrolytic carbon on carbon nanotubes. Part 2. Texture and structure

In a previous study we showed both the formation of genuine vapor grown carbon fibers (VGCFs) and of new and peculiar carbon nanotube-supported morphologies using a chemical vapor deposition process. Briefly, the latter are an association of beads (or fiber segments) with a more or less rough surface and more or less extended cone-based sub-morphologies with a smooth surface. The investigation of these materials regarding their texture, nanotexture and structure by transmission electron microscopy is reported, as a first step to understanding the formation mechanisms. It is shown that VGCFs exhibit a concentric texture, however with a variable microporous character and nanotexture quality. On the other hand, beads and related morphologies have a coarsely concentric microporous texture, as opposed to the cones and related morphologies, which exhibit a perfectly concentric and dense texture similar to that of perfect multiwall carbon nanotubes. Cross-sections performed with ultra-microtomy have revealed the spatial and textural relationship between cones and beads.     

Preparation, characterization and growth mechanism of platelet carbon nanofibers

The synthesis of platelet carbon nanofibers (PCNFs) on a silicon substrate using chemical vapor deposition method is reported. Scanning electron microscope, high-resolution transmission electron microscopy, and Raman spectroscopy were used to characterize the nanofibers. It is found that these platelet nanofibers are of the order of 10 μm long, and most have a nearly rectangular transverse section with several hundreds nm wide and several tens of nm thick. Structure analysis reveals that the carbon layers of platelet nanofibers are parallel to each other, and have a uniform (0 0 2) orientation that is perpendicular to the fiber axis. Many faults and nanodomain have been found in the nanofibers. It is suggested that the PCNF grow in tip growth mechanism by the precipitation of carbon from the side facet of catalyst flakes.     

Synthesis of multi-branched porous carbon nanofibers and their application in electrochemical double-layer capacitors

Multi-branched carbon nanofibers with a porous structure have been synthesized on a Cu catalyst doped with Li, Na, or K. The products were characterized by field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. Using this new type of nanofiber as polarized electrodes, an electrochemical double-layer capacitor with a specific capacitance of ca. 297 F/g was obtained using 6 M KOH as the electrolyte.     

Synthesis of carbon nanofibers and measurements of hydrogen storage

The synthesis of carbon nanofibers was carried out by catalytic decomposition of ethylene in presence of hydrogen. Bimetallic catalysts, e.g. Fe-Cu or Ni-Cu, were synthesized by coprecipitation, reduction-precipitation and reverse microemulsion techniques and were proven to have a strong influence on the morphology of the nanofibers. The best results in terms of synthesis homogeneity were obtained by supporting the bimetallic catalyst on a high surface area silica support by the “incipient wetness” method. The hydrogen storage capacity of carbon nanofibers was tested in a custom made Sievert apparatus operating up to 160 bar and 450 °C. Several “in situ” activation procedures were experimented, however according to our data carbon nanofibers do not seem a suitable candidate for hydrogen storage. With the purpose of promoting a “spillover” function, 2 wt.% Pd-doped nanofibers were prepared. After loading at 77 bar, a hydrogen storage of 1.38 ± 0.30 wt.% was measured at room temperature.     

A simple route for the synthesis of coral-like accretion of hollow carbon microspheres with thin walls

Coral-like accretion of hollow carbon microspheres with thin walls has been successfully synthesized through a carbon dissolution-precipitation process, where the decomposition of cyclohexane on nickel powder, at 550 °C in a sealed system, and is followed by rapid cooling. The final product is tens of microns in size with smooth and uniformly thin shell thickness of approximately 3-4 nm. XRD, FESEM, EDX, TEM, HRTEM and Raman spectra were used to characterize the products. The effects of reaction temperature, type of carbon source and cooling process on the formation of the final products have been studied, and a possible formation mechanism for the coral-like product has been proposed. This coral-like accretion of large capacity hollow carbon microspheres has potential applications as a catalyst carrier, in storage or protection of unstable organic materials and microreactors.