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.     

Enhancement of electrosorption capacity of activated carbon fibers by grafting with carbon nanofibers

The composite films of activated carbon fibers (ACFs) and carbon nanofibers (CNFs) are prepared via chemical vapor deposition of CNFs onto ACFs in different times from 0.5 to 2 h and their electrosorption behaviors in NaCl solution are investigated. The morphology, structure, porous and electrochemical properties are characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, N₂ adsorption at 77 K, contact angle goniometer and electrochemical workstation, respectively. The results show that CNFs have been hierarchically grown on the surface of ACFs and the as grown ACF/CNF composite films have less defects, higher specific capacitances, more suitable mesoporous structure and more hydrophilic surface than the pristine ACFs, which is beneficial to their electrosorption performance. The ACFs/CNFs with CNFs deposited in 1 h exhibit an optimized NaCl removal ratio of 80%, 55% higher than that of ACFs and the NaCl electrosorption follows a Langmuir isotherm with a maximum electrosorption capacity of 17.19 mg/g.     

Electrochemically deposited Fe2O3 nanorods on carbon nanofibers for free-standing anodes of lithium-ion batteries

Fe₂O₃ nanorod/carbon nanofiber (CNF) composites were prepared by the electrochemical deposition of Fe₂O₃ on a web of CNFs, which was then used as a free-standing anode. The conductive, three-dimensional structure of the CNF web allowed for the electrodeposition of the Fe₂O₃ nanorods, while its high conductivity made it possible to use the composite as a free-standing electrode in lithium-ion batteries. In addition, it was easy and cheap to fabricate by a simplification of a process of cell preparation. The nanorod-like Fe₂O₃ structures could only be electrodeposited on the CNFs; flake-like Fe₂O₃ was formed on flat conductive glass substrates. It can be attributed to the different growth mechanism of Fe₂O₃ on the CNFs because of the large number of reaction sites on the CNFs, differences in the precursor concentration and diffusivity within the CNF web. The formation of aggregates of the Fe₂O₃ particles on thicker CNFs also indicated that the CNFs had determined the Fe₂O₃ growth mechanism. The synthesised Fe₂O₃/CNF composite electrode exhibited stable rate capacities at different current densities. This suggested that CNF-based composite did not exhibit the intrinsic disadvantages of Fe₂O₃. Finally, carbon coatings were deposited on the Fe₂O₃/CNF composites to further improve their electronic conductivity and rate capability.     

How Carbon-Nano-Fibers attach to Ni foam

A stable Carbon-Nano-Fiber (CNF) layer was catalytically grown on Ni foam by decomposing ethylene. Scanning electron microscopy of the cross-section of the deposited layer on Ni foam revealed the presence of two distinct carbon layers; an apparently dense layer (‘C-layer’) at the carbon-Ni interface and a CNF layer on top of that. Variation of the growth time demonstrated that both layers develop in parallel. Characterization using temperature programmed gasification in H₂, Raman spectroscopy and transmission electron microscopy confirmed that both layers consists of graphene planes, which are better ordered in CNFs as compared to C-layer. The nickel surface and the attached carbon layer have similar morphological features. This may be the reason for strong adhesion of the C-layer to Ni. CNFs are strongly attached to the C-layer via roots that penetrate into the C-layer. The interconnections of the Ni surface, C-layer and CNFs induce mechanical stability. The C-layer grows continuously with time, whereas CNF growth needs typically 20 min initiation because of the need to form small Ni particles that allow CNF formation. The continuing formation of the C-layer, also after initiation of CNF growth, is thought to be responsible for the formation of CNF roots in the C-layer.     

Biomimetic synthesis and characterization of carbon nanofiber/hydroxyapatite composite scaffolds

Three dimensional electrospun carbon nanofiber (CNF)/hydroxyapatite (HAp) composites were biomimetically synthesized in simulated body fluid (SBF). The CNFs with diameter of ∼250 nm were first fabricated from electrospun polyacrylonitrile precursor nanofibers by stabilization at 280 °C for 2 h, followed by carbonization at 1200 °C. The morphology, structure and water contact angle (WCA) of the CNFs and CNF/HAp composites were characterized. The pristine CNFs were hydrophobic with a WCA of 139.6°, resulting in the HAp growth only on the very outer layer fibers of the CNF mat. Treatment in NaOH aq. solutions introduced carboxylic groups onto the CNFs surfaces, and hence making the CNFs hydrophilic. In the SBF, the surface activated CNFs bonded with Ca²⁺ to form nuclei, which then easily induced the growth of HAp crystals on the CNFs throughout the CNF mat. The fracture strength of the CNF/HAp composite with a CNF content of 41.3% reached 67.3 MPa. Such CNF/HAp composites with strong interfacial bondings and high mechanical strength can be potentially useful in the field of bone tissue engineering.     

Optical transmittance and electrical conductivity of silica glass with biserial and hierarchical network structures made of carbon nanofibers

Silica glass composites, with biserial and hierarchical percolative network made of carbon nanofibers (CNFs), was fabricated using a layer-by-layer technique and spark plasma sintering to obtain high optical transmittance and electrical conductivity. Owing to the network, the critical volume fraction, V_c, for the CNF percolation in the silica glass-matrix composite (0.5–0.7 vol%), when the electrical conductivity of the composite drastically increased with change from insulator (～10^?10 S/m) to conductor (～10^?1 S/m), is smaller than theoretical V_c predicted for the three-dimensional random orientation of CNFs (2.6 vol% for the CNF aspect ratio of 30). The conductivity of the composite with above the V_c of CNFs (～10 S/m) is higher than that reported for the polymer-matrix composite (～10^?5–～10^?3 S/m). Furthermore, high optical transmittance was observed for the electrically conductive composite with V_c of CNFs.     

Carbon nanoflake growth from carbon nanotubes by hot filament chemical vapor deposition

We report the growth of carbon nanoflakes (CNFs) on Si substrate by the hot filament chemical vapor deposition without the substrate bias or the catalyst. CNFs were grown using the single wall carbon nanotubes and the multiwall carbon nanotubes as the nucleation center, in the Ar-rich CH₄–H₂–Ar precursor gas mixture with 1% CH₄, at the chamber pressure and the substrate temperature of 7.5 Torr and 840 °C, respectively. In the H₂-rich condition, CNF synthesis failed due to severe etch-removal of carbon nanotubes (CNTs) while it was successful at the optimized Ar-rich condition. Other forms of carbon such as nano-diamond or mesoporous carbon failed to serve as the nucleation centers for the CNF growth. We proposed a mechanism of the CNF synthesis from the CNTs, which involved the initial unzipping of CNTs by atomic hydrogen and subsequent nucleation and growth of CNFs from the unzipped portion of the graphene layers.     

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.     

Hot pressed and spark plasma sintered zirconia/carbon nanofiber composites

Zirconia/carbon nanofiber composites were prepared by hot pressing and spark plasma sintering with 2.0 and 3.3 vol.% of carbon nanofibers (CNFs). The effects of the sintering route and the carbon nanofiber additions on the microstructure, fracture/mechanical and electrical properties of the CNF/3Y-TZP composites were investigated. The microstructure of the ZrO₂ and ZrO₂–CNF composites consisted of a small grain sized matrix (approximately 120 nm), with relatively well dispersed carbon nanofibers in the composite. All of the composites showed significantly higher electrical conductivity (from 391 to 985 S/m) compared to the monolithic zirconia (approximately 1 × 10⁻¹⁰ S/m). The spark plasma sintered composites exhibited higher densities, hardness and indentation toughness but lower electrical conductivity compared to the hot pressed composites. The improved electrical conductivity of the composites is caused by CNFs network and by thin disordered graphite layers at the ZrO₂/ZrO₂ boundaries.     

Use of facile mechanochemical method to functionalize carbon nanofibers with nanostructured polyaniline and their electrochemical capacitance

A facile approach to functionalize carbon nanofibers [CNFs] with nanostructured polyaniline was developed via in situ mechanochemical polymerization of polyaniline in the presence of chemically treated CNFs. The nanostructured polyaniline grafting on the CNF was mainly in a form of branched nanofibers as well as rough nanolayers. The good dispersibility and processability of the hybrid nanocomposite could be attributed to its overall nanostructure which enhanced its accessibility to the electrolyte. The mechanochemical oxidation polymerization was believed to be related to the strong Lewis acid characteristic of FeCl₃ and the Lewis base characteristic of aniline. The growth mechanism of the hierarchical structured nanofibers was also discussed. After functionalization with the nanostructured polyaniline, the hybrid polyaniline/CNF composite showed an enhanced specific capacitance, which might be related to its hierarchical nanostructure and the interaction between the aromatic polyaniline molecules and the CNFs.     

Activated porous carbon nanofibers using Sn segregation for high-performance electrochemical capacitors

Activated porous carbon nanofibers (CNFs) with three different types of porous structures, which were controlled to contain 1, 4, and 8 wt% of Sn–poly(vinylpyrrolidone) (PVP) precursors in the core region and 7 wt% polyaniline (PAN)–PVP precursors in the shell region during electrospinning, were synthesized using a co-electrospinning technique with H₂-reduction. The formation mechanisms of activated porous CNF electrodes with the three different types of samples were demonstrated. The activated porous CNFs, for use as electrodes in high-performance electrochemical capacitors, have excellent capacitances (289.0 F/g at 10 mV/s), superior cycling stability, and high energy densities; these values are much better than those of the conventional CNFs. The improved capacitances of the activated porous CNFs are explained by the synergistic effect of the improved porous structures in the CNF electrodes and the formation of activated states on the CNF surfaces.     

The preparation of a novel Si–CNF composite as an effective anodic material for lithium–ion batteries

The suggested pyrolytic carbon (PC) coated Si-carbon nanofiber (CNF) composites can be a solution to provide higher discharge capacity and cycle-ability facilitating the 1st cycle coulombic efficiency of cheap metallic Si particles as an appropriate anodic material for Li-ion battery. The CNFs on the surface of Si particle can provide flexible space to relieve volumetric expansion during charge. Well-controlled PC coating on the surface of Si particle can improve the coherence of CNFs on the Si particle, thereby to enhance the role of CNF as all the more effective electrode material. The additional PC coating on the Si–CNF composite can accomplish the lower surface area and afford the improvement of the 1st cycle coulombic efficiency. Ingenious combinations of PC coating and CNF compositeness successfully made the novel type Si–CNF composite to achieve a remarkable discharge capacity (1115 mA h g⁻¹), an excellent cycle-ability (77% retention rate after 20th cycle), and a good 1st cycle coulombic efficiency (79%) for the effective application as an anode material.     

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.     

Characterization of surface oxygen complexes on carbon nanofibers by TPD, XPS and FT-IR

A fishbone type carbon nanofiber (CNF) is functionalized by different chemical and thermal treatments, and characterized by TPD, FT-IR and XPS. TPD is proved to be an effective technique to characterize surface oxygen complexes on carbon nanofibers, a novel type of mesoporous and highly graphitic carbon material. TPD spectra are analyzed by a modified deconvolution method with a multiple Gaussian function, allowing for more precise determination of each of the oxygen complexes on the surface than those reported in the literature. The surface properties of these modified CNFs measured by FT-IR and XPS are in good agreement with the TPD results. All the CNF surfaces possess more CO-producing oxygen complexes than CO₂-producing ones. Different functionalization methods result in different types and distributions of oxygen complexes on the CNF surface. The gas phase oxidation of the CNF mainly increases the number and concentration of carbonyl groups, while the oxidation in the liquid phase increases those of both carboxyl and anhydride groups. Moreover, thermal annealing of CNF in an inert gas at 1700 °C strongly decreases the amount of surface oxygen complexes though CNF subsequently undergoes gas oxidation.     

Long-term-durable anti-icing superhydrophobic composite coatings

We prepared carbon-based superhydrophobic composite coatings through a quick technique, merging multiwalled carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) to obtain hierarchical nanostructures on fiber-reinforced polymer (FRP) sheets; this was followed by supercritical fluid (SCF) processing and physical mixing (PM). The prepared SCF–MWCNT–CNF and PM–MWCNT–CNF composite coatings showed high water contact angles of 171.6 and 160°. The surface morphologies of the composite coatings revealed a lot of even nanostructures and folding at high magnifications. A high number of CNFs were added to the MWCNTs to initiate different nanoroughnesses in the composite coatings. The as-prepared superhydrophobic composite coatings showed excellent anti-icing properties, as indicated by the supercooled water droplet (-20°C) test under environmental conditions. Also, the surface of the SCF–MWCNT–CNF superhydrophobic composite coating showed excellent antifouling properties. We studied the surface wettability increasing different temperatures (30–180°C) in the SCF–MWCNT–CNF composite; this exposed the fact that the FRP sheets were thermally stable up to 100°C, and a while later, they changed from a superhydrophobic state to a superhydrophilic state at 180°C. This study revealed an economically workable method for the preparation of MWCNT–CNF composites with SCF techniques. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47059.     

Synthesis of composite materials of carbon nanofibres and ceramic monoliths with uniform and tuneable nanofibre layer thickness

Composite materials consisting of ceramic monoliths and carbon nanofibres (CNFs) have been synthesized by catalytic growth of CNFs on the γ-alumina washcoating layer covering the walls of a ceramic monolith. The composites possess a relatively uniform mesoporous layer of CNFs of relatively small diameter. The thin alumina washcoating (ca. 0.1 μm) prevents the CNFs from being trapped inside the alumina pores and hence the CNFs grow freely throughout the washcoating layer to form a uniform layer of CNFs that completely covers the surface of the monolith walls. The growth temperature is found to control the thickness of the CNF layer (2-4 μm), the growth rate of the nanofibres, and the mechanical strength of the resulting CNF-monolith composite. At ideal conditions, a complete adhesion of the CNF layer and higher mechanical strength than the original cordierite monolith can be obtained. The CNF layer has an average pore size of 17 nm with absence of microporosity which renders these monoliths promising candidates for the use as catalyst supports, especially for liquid phase reactions. The CNFs have small diameters (5-30 nm) due to the high dispersion of Ni particles in the growth catalyst and the CNFs exhibit an unusual branched structure.     

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.     

Electrospun mesoporous carbon nanofibers produced from phenolic resin and their use in the adsorption of large dye molecules

Mesoporous carbon nanofibers (CNFs) were prepared by a sol–gel/electrospinning process using phenolic resin precursor as carbon source and triblock copolymer Pluronic F127 as template. The final CNFs were obtained after carbonization of as-spun nanofibers and removal of SiO₂. Three samples (C-1, C-2, and C-3) with different pore textures were synthesized. The CNF structures were characterized by scanning and transmission electron microscopy, and N₂ adsorption–desorption measurements, demonstrating that the samples consisted of nanofibers with mesopores and the mesopore volumes depended on the amount of tetraethyl orthosilicate in the spinnable sols. According to thermogravimetric analysis, the CNF yields of 2.57%, 2.78%, and 2.13% from the spinnable sols for sample C-1, C-2, and C-3 were obtained, respectively. The mesoporous CNFs were used as highly efficient adsorbents for large dye molecules. The relationship between the pore textures and adsorption properties was studied. It is suggested that the adsorption of different dyes depend on an appropriate pore size distribution in addition to surface area. However, the adsorption capacity of the regenerated adsorbents gradually decreased with the number of regeneration cycles. The adsorption of acid red 1 could reach 186 mg g^?1 for C-3 after seven regeneration cycles. Furthermore, the mechanical strength of CNFs needs improvement.     

Synthesis of a carbon nanofiber/carbon foam composite from coal liquefaction residue for the separation of oil and water

A carbon nanofiber (CNF)/carbon foam composite was fabricated from coal liquefaction residue (CLR) through a procedure involving template synthesis of carbon foam and catalytic chemical vapor deposition (CCVD) treatment. The high solubility and high pyrolysis yield make CLR a promising carbon precursor for the synthesis of carbon materials using the template method. The carbon foam has cell size of about 500 μm and a porosity as high as 95 vol.%. Fe species naturally present in the CLR disperse homogeneously on the surface of the carbon foam acting as a catalyst in the CCVD process. After the CCVD treatment, the whole surface of the carbon foam is covered by entangled CNFs with external diameters of 20–100 nm and lengths of several tens of micrometers. The obtained CNF/carbon foam composites are effective selective adsorbents in the separation of oil and water, through a combination of hydrophobicity and capillary action.