Preparation of highly crystalline nanofibers on Fe and Fe-Ni catalysts with a variety of graphene plane alignments

The present study confirmed that highly crystalline nanofibers with controlled structure may be prepared over Fe and Fe-Ni alloy catalysts. The degree of graphitization of various carbon nanofibers (CNFs) was analyzed by using C(0 0 2) peaks from the XRD profiles. The C(0 0 2) peaks of CNFs over Fe catalyst shifted to higher angle and became narrower as the preparation temperature increased from 560 to 620 °C. Tubular CNFs prepared at temperature higher than 630 °C showed lower 2θ angles compared to those of platelet fibers. CNFs prepared over Fe-Ni catalysts tended to resemble those prepared over Fe catalysts. The degree of graphitization of platelet CNFs resembled natural graphite, while d_0 0 2 of the tubular CNFs showed values below the 3.39 Å reported as a theoretical minimum for a cylindrical alignment. Lc_0 0 2 of platelet and tubular CNFs increased by heat treatment at 2000 and 2800 °C though d_0 0 2 changed little. A transverse section of platelet and tubular CNFs had a hexagonal shape, not a round shape. The hexagonal column allows AB stacking of hexagonal planes that can give perfect hexagonal alignment.     

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 d₀₀₂ 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).     

Novel carbon nanofibers of high graphitization as anodic materials for lithium ion secondary batteries

Carbon nanofibers (CNFs) of high graphitization degree were prepared by a CVD process at 550-700 °C. They showed different structures according to catalyst and preparation temperatures. The structure of CNF prepared from CO/H₂ over an iron catalyst was controlled from platelet (P) to tubular (T) by raising the decomposition temperature from 550 to 700 °C. The CNFs prepared over a copper-nickel catalyst from C₂H₄/H₂ showed the typical herringbone (HB) structure regardless of the reaction temperatures. The CNFs prepared over Fe showed d₀₀₂ of 0.3363-0.3381 nm, similar to that of graphite, indicating very high graphitization degree in spite of the low preparation temperature. Such CNFs of high graphitization degree showed high capacity of 297-431 mA h/g, especially in the low potential region. However, low first cycle coulombic efficiency of ≈60% is a problem to be solved. The graphitization of the CNF preserved the platelet texture, however, and formed the loops to connect the edges of the graphene sheets. Higher graphitization temperatures made the loop more definite. The graphitized CNF showed high capacity (367 mA h/g); however, its coulombic efficiency was not so large despite its modified edges by graphitization, indicating that the graphene edges were not so influential for the irreversible reaction of Li ion battery.     

The production of a homogeneous and well-attached layer of carbon nanofibers on metal foils

Carbon nanofibers (CNFs) were deposited on metal foils including nickel (Ni), iron (Fe), cobalt (Co), stainless steel (Fe:Ni; 70:11 wt.%) and mumetal (Ni:Fe; 77:14 wt.%) by the decomposition of C₂H₄ at 600 °C. The effect of pretreatment and the addition of H₂ on the rate of carbon formation, as well the morphology and attachment of the resulting carbon layer were explored. Ni and mumetal show higher carbon deposition rates than the other metals, with stainless steel and Fe the least active. Pretreatment including an oxidation step normally leads to higher deposition rates, especially for Ni and mumetal. Enhanced formation of small Ni particles by in situ reduction of NiO, compared to formation using a Ni carbide, is probably responsible for higher carbon deposition rates after oxidation pretreatment. The addition of H₂ during the CNF growth leads to higher carbon deposition rates, especially for oxidized Ni and mumetal, thus enhancing the effect of the reduction of NiO. The diameters of CNFs grown on metal alloys are generally larger compared to those grown on pure metals. Homogenously deposited and well-attached layers of nanotubes are formed when the carbon deposition rate is as low as 0.1-1 mg cm⁻² h⁻¹, as mainly occurs on stainless steel.     

Synthesis of nitrogen-containing carbon nanofibers by catalytic decomposition of ethylene/ammonia mixture

The formation of carbon nanofibers (CNFs) doped with nitrogen was investigated during decomposition of C₂H₄/NH₃ mixtures at 450-675 °C over metal catalysts: 90Ni-Al₂O₃, 82Ni-8Cu-Al₂O₃, 65Ni-25Cu-Al₂O₃, 45Ni-45Cu-Al₂O₃, 90Fe-Al₂O₃, 85Fe-5Co-Al₂O₃, 62Fe-8Co-Al₂O₃, 62Fe-8Ni-Al₂O₃. It was found that the yield of CNFs, their structural and textural properties, as well as nitrogen content in CNFs are strongly dependent on the synthesis conditions such as: catalyst used, feed composition, temperature and duration. The 65Ni-25Cu-Al₂O₃ was proved to be the most efficient catalyst for the production of nitrogen-containing carbon nanofibers (N-CNFs) with nitrogen content up to 7 wt.%. Ammonia concentration in the feed equal 75 vol.%, temperature 550 °C and duration 1 h were found to be the optimum reaction parameters to reach the maximum nitrogen content in N-CNFs. TEM studies revealed that the nanofibers have a helical morphology and a “herringbone” structure composed of graphite sheets. According to the XPS data, the nitrogen incorporation in the N-CNF structure leads to the formation of two types of nitrogen coordination: pyridinic and quaternary, and their abundance depends on the reaction conditions.     

Growth mechanisms of carbon nanofilaments on Ni-loaded diamond catalyst

Nickel-loaded oxidized powdered diamond (Ni/O-Dia) was used for the synthesis of carbon nanofibers (CNFs).CNFs grown at 400 °C, from both CH₄ and C₂H₆ over Ni/O-Dia, had herring-bone type graphene sheets and those grown at 600 °C had tube type graphene sheets. The amount of CNFs was greatly affected by the growth temperature in both CH₄ and C₂H₆. In all the cases, Ni particle was found on the tip end of the grown CNFs.No carbon formation was observed at 650 °C or higher temperature from both CH₄ and C₂H₆, because Ni particles on O-Dia were covered with graphene sheets.Termination of the growth of CNFs was ascribed to the rapid decomposition of hydrocarbons on active Ni surface as compared to the dissolution and diffusion of the carbon on Ni surface into bulk Ni particle to give CNFs on the opposite side of active Ni surface.     

Synthesis of graphene on silicon carbide substrates at low temperature

A method for the synthesis of millimeter-scaled graphene films on silicon carbide substrates at low temperatures (750 °C) is presented herein. Ni thin films were coated on a silicon carbide substrate and used to extract the substrate’s carbon atoms under rapid heating. During the cooling stage, the carbon atoms precipitated on the free surface of the Ni and formed single-layer or few-layer graphene. The result shows that the number of graphene layers might be further controlled by appropriate process conditions. In contrast to the epitaxial graphene synthesis on single crystal silicon carbide, the graphene prepared here are continuous over the entire Ni-coated area, and can be stripped from the substrate much more easily for further characterization. The large-scaled, low temperature and transferable features of our method suggest the potential for future graphene-based applications.     

Production of Carbon Nanotube by Ethylene Decomposition over Silica-Coated Metal Catalysts

Formation of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) through the decomposition of ethylene at 973 K was achieved using various metal catalysts covered with silica layers. CNFs of various diameters were formed by ethylene decomposition over a Co metal catalyst supported on the outer surface of the silica. In contrast, silica-coated Co catalysts formed CNTs with uniform diameters by ethylene decomposition. Silica-coated Ni/SiO₂ and Pt/carbon black also formed CNTs with uniform diameters, while CNFs and CNTs with various diameters were formed over Ni/SiO₂ and Pt/carbon black without a silica coating. These results indicate that silica layers that envelop metal particles prevent sintering of the metal particles during ethylene decomposition. This results in the preferential formation of CNTs with a uniform diameter.     

The graphitization of carbon nanofibers produced by the catalytic decomposition of natural gas

Fishbone carbon nanofibers (CNFs) were produced by methane decomposition in a fluidized bed reactor using nickel-copper based catalysts that were prepared with different promoters (SiO₂, Al₂O₃, TiO₂, MgO). The CNFs were subjected to heat treatment (HT) in the temperature range 2400-2800 °C to explore their ability to graphitize. The influence of treatment temperature and CNF metal content on the structural and textural parameters of the resulting heat treated carbon nanofibers was studied. More-ordering was achieved in CNFs containing Si and Ti because of the catalytic effect of these metals. Since titanium carbide appeared after the HT, the formation of graphitic material by carbide decomposition seems to be a plausible mechanism to explain the catalytic graphitization of the CNFs. A parallel evolution of the structural and textural properties of the nanofibers during HT was found, suggesting that a decrease of the specific surface area is caused by the removal of structural defects and an increase of crystallite size.     

Carbon nanofibers: Synthesis, characterization, and electrochemical properties

Carbon nanofibers (CNFs) have been synthesized by co-catalyst deoxidization process by a reaction between C₂H₅OC₂H₅, Zn and Fe powder at 650 °C for 10 h. These nanofibers exhibit diameters of ∼80 nm and lengths ranging from several micrometers to tens of micrometers. X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy indicate that as-prepared CNFs possess low graphitic crystallinity. The resultant CNFs as electrode shows capacity of ∼220 mAh/g and high reversibility with little hysteresis in the insertion/deintercalation reactions of lithium-ion. In addition, the possible growth of CNFs is discussed.     

Synthesis of carbon nanotubes over gold nanoparticle supported catalysts

The synthesis of carbon nanotubes (CNTs) through the catalytic decomposition of acetylene was carried out over gold nanoparticles supported on SiO₂-Al₂O₃. Monodispersed gold nanoparticles with 1.3-1.8 nm in diameter were prepared by the liquid-phase reduction method with dodecanethiol as protective agent. The carbon products formed after acetylene decomposition consist of multi-walled carbon nanotubes with layered graphene sheets, carbon nanofilaments (CNFs), and carbon nanoparticles encapsulating gold particles. The observed CNTs have outer diameters of 13-25 nm under 850 °C. The influence of several reaction parameters, such as kind of carriers, reaction temperature, gas flow rate, was investigated to search for optimum reaction conditions. The CNTs were observed at a relatively low temperature (550 °C). The silica-alumina carrier showed higher activity for the formation of CNTs than others used in the screening test. With increasing temperature, the CNTs showed cured structures having thick diameters and inside compartments. When Au content on the support was over 5 wt.%, the gold nanoparticles coagulated to form large ones >20 nm in diameter and became encapsulated with graphene layers after decomposition of acetylene.     

Preparation and characterization of pillared carbons obtained by pyrolysis of silylated graphite oxides

Pillared carbons were prepared by pyrolyzing various graphite oxides silylated by 3-aminopropylmethyldiethoxysilane. They were formed when silylated graphite oxides with silicon contents of 12.6% or higher were pyrolyzed in vacuo at 500-600 °C. Their interlayer spacings were 1.23-1.31 nm. When silylated graphite oxide was prepared at 90 °C, the reductive decomposition of graphite oxide by amino groups of 3-aminopropylmethyldiethoxysilane was suppressed and pillared carbon with higher crystallinity was obtained. At higher temperatures of pyrolysis, silylated graphite oxide decomposed to residual carbon without pillars. The pillars between the carbon layers contained methyl groups originating from the 3-aminopropylmethyldiethoxysilane. Based on the interlayer spacing and elemental analysis data, a structure model for the pillar is proposed. Pillared carbons showed type IV nitrogen adsorption isotherms and they contained both mesopores and a small volume of micropores. The BET surface area of the pillared carbon reached a maximum value of 236 m²/g, when it was prepared from graphite oxide silylated at 105 °C for 20 days.     

Radio-frequency plasma system to selectively grow vertical field-aligned carbon nanofibers from a solid carbon source

Vertical field-aligned carbon nanofibers (CNFs), exhibiting a “herring-bone” and a “bamboo-like” structure, were grown at 560 °C using nickel (Ni) as a catalyst and an innovative radio-frequency (RF) plasma-enhanced chemical vapor deposition system. To limit the carbon supply, thereby providing a highly selective growth process with no detrimental parasitic carbon layer formation, a solid graphite sample-holder, RF-polarized, was used as a single carbon source in combination with a pure H₂ feed gas. The morphology and the dimensions of the obtained CNFs are investigated with respect to the growth duration. High-resolution transmission electron microscopy analyses typically display a Ni particle at the fiber tip, but this particle is not encapsulated by graphene layers, allowing its easy removal with a chemical acid treatment. Moreover, the particle’s upper surface consists of a peculiar polycrystalline area, assumed to be essential for the growth mechanisms and possibly made of nickel carbide. The crucial role played by the average vertical electric field, naturally created in the plasma sheath and responsible for sample-holder and substrate bombardment by cationic species, is highlighted to understand the growth mechanisms of these as-grown oriented CNFs and their progressive base destruction by etching phenomena.     

First-principles study of carbon diffusion in bulk nickel during the growth of fishbone-type carbon nanofibers

Ab initio plane wave density functional theory calculations are performed to investigate the carbon diffusion in bulk nickel during the growth of fishbone-type carbon nanofibers (CNFs). Results indicate that the octahedral interstitial sites are preferred for C dissolution relative to the tetrahedral sites. And the heat of solution of C in paramagnetic (PM) Ni is larger than that in ferromagnetic (FM) Ni because the induced C atom quenches the magnetic moments of neighboring Ni atoms. The bulk diffusion has been successfully described under two different C concentrations. At the initial CNF growth stage, the C concentration in bulk Ni is low and the calculated energy barriers for the diffusion of an isolated C atom are 1.641 eV and 1.678 eV in the Ni FM and PM state, respectively. When the C content is increased to 20 at.%, two models are established. In one case, it is assumed that all C atoms hop in the same direction at the same time, and the calculated activation energies are 1.137 eV and 1.126 eV. In the other case, only one C atom is permitted to move with the neighboring C atoms fixed at the octahedral sites and the corresponding barriers are decreased to 0.972 eV in the Ni FM state. Through these calculations, it is concluded that the magnetic state has a minor effect on the diffusion energy barrier which can be substantially lowered by the increase of C concentration in bulk Ni. Comparing the activation energy for bulk diffusion with the surface diffusion results, the reason for the formation of different CNF morphologies has been revealed.     

Flame synthesis of carbon nanofibers on carbon paper: Physicochemical characterization and application as catalyst support for methanol oxidation

We developed a simple, rapid and highly efficient flame synthesis method for direct growing carbon nanofibers (CNFs) on carbon paper (CP) using a common laboratory ethanol flame as both heat and carbon sources. High density CNFs with tangled solid-cored structure were uniformly formed over the Ni-plated CP surface in ∼20 s. The morphologies of the CNFs were characterized by scanning electron microscopy and transmission electron microscopy. X-ray diffraction study revealed the graphitic nature of the CNFs. Raman spectroscopy analysis confirmed that the CNFs are disordered graphitic nanocrystallites with high degree of exposed edges. Electrochemical impedance spectroscopy and cyclic voltammetry were used to show that growing CNFs directly on CP facilitates electron transfer with concomitant increase in double-layer capacitance. The CNF/CP was used as support for Pt nanoparticles to study their supporting effect on the catalyst performance. The as prepared Pt/CNF electrocatalyst exhibited much improved electrocatalytic activity for methanol oxidation compared to Pt/CP and commercial Pt/C on CP. High electronic conductivity and improved electrochemical behavior of the CNF/CPs, resulted from direct contact of the nanofibers with CP, combined with unique properties of CNFs, make the synthesized CNF/CPs promising for fuel cell applications.     

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 H₂ and exposure to CH₄ 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 CH₄ 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 CH₄ is corroborated by direct catalyst activation in the absence of H₂. Reduction begins in pure hydrocarbon around 300 °C and leads to coking activities at 450 °C comparable to those for samples pre-reduced in H₂. Skeletal metal catalysts offer distinct advantages in low-temperature natural gas conversion.     

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.     

Microstructure of carbon derived from mangrove charcoal and its application in Li-ion batteries

In this study, the microstructure of mangrove-charcoal-derived carbon (MC) was studied using XRD, STM and TEM. MC was found to consist of aligned quasi-spherical structural units with diameters of around 5-20 nm. It shows typical hard carbon characteristics, including a strongly disoriented single graphene layer and BSU, formed by two or three graphene layers stacked nearly parallel. Some curved and faceted graphene layers, especially closed carbon nanoparticles with fullerene-like, were observed in the as-prepared samples. MC was also evaluated as an anodic material for Li-ion batteries. MC carbonized at 1000 °C possessed the highest available discharge capacity (below 0.5 V) of 335 mAh g⁻¹, the high first-cycle coulombic efficiency of 73.7%, good rate and cyclic capability and PC-based electrolyte compatibility. ⁷Li nuclear magnetic resonance (NMR) spectra of fully lithiated mangrove charcoal-derived carbons indicated the co-existence of three Li species.     

Effects of oxidation and heat treatment of acetylene blacks on their electrochemical double layer capacitances

Correlations between the electrochemical double layer capacitances of various acetylene blacks modified by surface oxidation and heat treatment, and their morphologies are presented. The acetylene blacks were different from each other in primary structural unit size (equivalent to mean particle diameter). They were oxidized in air at 300 °C for 1 h to produce graphene sheets protruding from the surfaces of the spherical particles. In addition, the surfaces of the acetylene blacks were modified by heat treatments from 1000 °C to 2800 °C, which resulted in a morphological change from surfaces covered with protruding graphene sheets to ones wrapped with basal planes of graphite. Correlations between the capacitances of the acetylene blacks and the observed morphologies showed that the surface covered with protruding graphene sheets was roughly 10 times more effective in capacitive charging than the surface of graphite basal planes. Specifically, the surface specific capacitance of the edged-graphene-sheet-covering surface was 146 mF/m², while that of the basal-planes-wrapping surface was 16 mF/m². It was concluded that the capacitances of the acetylene blacks were mainly defined by surface morphology, which were in turn influenced by structural unit size and degree of oxidation.     

Direct synthesis of carbon nanofibers from South African coal fly ash

Carbon nanofibers (CNFs), cylindrical nanostructures containing graphene, were synthesized directly from South African fly ash (a waste product formed during the combustion of coal). The CNFs (as well as other carbonaceous materials like carbon nanotubes (CNTs)) were produced by the catalytic chemical vapour deposition method (CCVD) in the presence of acetylene gas at temperatures ranging from 400°C to 700°C. The fly ash and its carbonaceous products were characterized by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), laser Raman spectroscopy and Brunauer-Emmett-Teller (BET) surface area measurements. It was observed that as-received fly ash was capable of producing CNFs in high yield by CCVD, starting at a relatively low temperature of 400°C. Laser Raman spectra and TGA thermograms showed that the carbonaceous products which formed were mostly disordered. Small bundles of CNTs and CNFs observed by TEM and energy-dispersive spectroscopy (EDS) showed that the catalyst most likely responsible for CNF formation was iron in the form of cementite; X-ray diffraction (XRD) and Mössbauer spectroscopy confirmed these findings.