Hollow nitrogen-containing core/shell fibrous carbon nanomaterials as support to platinum nanocatalysts and their TEM tomography study

Core/shell nanostructured carbon materials with carbon nanofiber (CNF) as the core and a nitrogen (N)-doped graphitic layer as the shell were synthesized by pyrolysis of CNF/polyaniline (CNF/PANI) composites prepared by in situ polymerization of aniline on CNFs. High-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared and Raman analyses indicated that the PANI shell was carbonized at 900°C. Platinum (Pt) nanoparticles were reduced by formic acid with catalyst supports. Compared to the untreated CNF/PANI composites, the carbonized composites were proven to be better supporting materials for the Pt nanocatalysts and showed superior performance as catalyst supports for methanol electrochemical oxidation. The current density of methanol oxidation on the catalyst with the core/shell nanostructured carbon materials is approximately seven times of that on the catalyst with CNF/PANI support. TEM tomography revealed that some Pt nanoparticles were embedded in the PANI shells of the CNF/PANI composites, which might decrease the electrocatalyst activity. TEM-energy dispersive spectroscopy mapping confirmed that the Pt nanoparticles in the inner tube of N-doped hollow CNFs could be accessed by the Nafion ionomer electrolyte, contributing to the catalytic oxidation of methanol.     

Nanomechanical characterization of chemical interaction between gold nanoparticles and chemical functional groups

Core/shell nanostructured carbon materials with carbon nanofiber (CNF) as the core and a nitrogen (N)-doped graphitic layer as the shell were synthesized by pyrolysis of CNF/polyaniline (CNF/PANI) composites prepared by in situ polymerization of aniline on CNFs. High-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared and Raman analyses indicated that the PANI shell was carbonized at 900°C. Platinum (Pt) nanoparticles were reduced by formic acid with catalyst supports. Compared to the untreated CNF/PANI composites, the carbonized composites were proven to be better supporting materials for the Pt nanocatalysts and showed superior performance as catalyst supports for methanol electrochemical oxidation. The current density of methanol oxidation on the catalyst with the core/shell nanostructured carbon materials is approximately seven times of that on the catalyst with CNF/PANI support. TEM tomography revealed that some Pt nanoparticles were embedded in the PANI shells of the CNF/PANI composites, which might decrease the electrocatalyst activity. TEM-energy dispersive spectroscopy mapping confirmed that the Pt nanoparticles in the inner tube of N-doped hollow CNFs could be accessed by the Nafion ionomer electrolyte, contributing to the catalytic oxidation of methanol.     

Desirable electrical and mechanical properties of continuous hybrid nano-scale carbon fibers containing highly aligned multi-walled carbon nanotubes

The continuous highly aligned hybrid carbon nanofibers (CNFs) with different content of acid-oxidized multi-walled carbon nanotubes (MWCNTs) were fabricated through electrospinning of polyacrylonitrile (PAN) followed by a series of heat treatments under tensile force. The effects of MWCNTs on the micro-morphology, the degree of orientation and ordered crystalline structure of the resulting nanofibers were analyzed quantitatively by diversified structural characterization techniques. The orientation of PAN molecule chains and the graphitization degree in carbonized nanofibers were distinctly improved through the addition of MWCNTs. The electrical conductivity of the hybrid CNFs with 3 wt% MWCNTs reached 26 S/cm along the fiber direction due to the ordered alignment of MWCNTs and nanofibers. The reinforcing effect of hybrid CNFs in epoxy composites was also revealed. An enhancement of 46.3% in Young’s modulus of epoxy composites was manifested by adding 5 wt% hybrid CNFs mentioned above. At the same time, the storage modulus of hybrid CNF/epoxy composites was significantly higher than that of pristine epoxy and CNF/epoxy composites not containing MWCNTs, and the performance gap became greater under the high temperature regions. It is believed that such a continuous hybrid CNF can be used as effective multifunctional reinforcement in polymer matrix composites.     

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.     

Surface modification of CNFs via plasma polymerization of styrene monomer and its effect on the properties of PS/CNF nanocomposites

Carbon nanofibers (CNF) were modified via plasma assisted polymerization in a specially designed reactor. The effect of the plasma reactor conditions, such as power and time, on the extent of the CNFs modification was examined. Polystyrene (PS) coated nanofibers plus PS polymer were then processed in a Brabender torque rheometer mixing chamber to obtain PS/CNF nanocomposites, with 0.5, 1.0, 3.0, and 5.0 wt % of CNF. The effect of the plasma treatment on the dispersion of the nanofibers and on the compatibility between the nanofibers and the polymer matrix was also examined. Modification of the CNFs was assessed by measuring the contact angle of water in a “bed” of nanofibers and by examining its dispersion in several solvents. The morphology of PS/CNF nanocomposites was studied through scanning electron microscopy (SEM). Contact angles decreased in all cases, indicating a change in hydrophobicity of the modified CNFs. This change was confirmed in the CNF dispersion tests in several solvents. SEM micrographs show the difference between the original and the PS coated CNF. In addition, fractured samples show the effect of this treatment, in the sense that the CNF seem to be completely embedded in the polymer matrix, which clearly indicates the high compatibility between the PS and the modified (PS coated) CNF. As a consequence, a much better dispersion of the treated CNF was observed. Finally, the tensile modulus of PS/CNF composites increased slightly with respect to PS when using untreated CNFs, but more than doubled when using plasma treated CNFs. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Poly(ϵ‐caprolactone)‐functionalized carbon nanofibers by surface‐initiated ring‐opening polymerization

Carbon nanofibers (CNFs) were covalently functionalized with biodegradable poly(?‐caprolactone) (PCL) by in situ ring‐opening polymerization (ROP) of ?‐caprolactone in the presence of stannous octoate. Surface oxidation treatment of the pristine CNFs afforded carboxylic CNFs (CNF‐COOH). Reaction of CNF‐COOH with excess thionyl chloride (SOCl₂) and glycol produced hydroxyl‐functionalized CNFs (CNF‐OH). Using CNF‐OH as macroinitiator, PCL was covalently grafted from the surfaces of CNFs by ROP, in either the presence or absence of sacrificial initiator, butanol. The grafted PCL content was achieved as high as 64.2 wt %, and can be controlled to some extent by adjusting the feed ratio of monomer to CNF‐OH. The resulting products were characterized by FTIR, NMR, Raman spectroscopy, TGA, DSC, SEM, TEM, HRTEM, and XRD. Core–shell nanostructures were observed under HRTEM for the PCL‐functionalized CNFs because of the thorough grafting. The PCL‐grafted CNFs showed different melting and crystallization behaviors from the mechanical mixture of PCL and CNF‐OH. This approach to PCL‐functionalized CNFs opens an avenue for the synthesis, modification, and application of CNF‐based nanomaterials and biomaterials. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

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.     

Anticorrosion performance of polyaniline nanostructures on mild steel

Different one-dimensional nanostructured polyanilines were synthesized in sulfuric acid solutions by conventional polymerization, interfacial polymerization and direct mixed reaction, respectively. The products were characterized with SEM, UV–vis and FTIR and the anticorrosion performance of products on mild steel were studied using electrochemical measurement in 3.5% NaCl aqueous solution. Results showed that the polyaniline nanofibers synthesized by direct mixed reaction have uniform morphology with diameters of 60–100 nm and more excellent protective properties than conventional aggregated polyaniline. Comparative studies revealed that the nanostructure and morphology of polyaniline could influence its anticorrosion performance.     

Metal Dusting as a Route to Produce Active Catalyst for Processing Chlorinated Hydrocarbons into Carbon Nanomaterials

The interaction of 1,2-dichloroethane (DCE) vapors with bulk NiCr alloy resulting in complete metal wastage with the formation of carbon nanofibers (CNFs) has been described from mechanistic point of view. The genesis of surface transformations of the bulk NiCr alloy exposed to corrosive reaction mix Ar/H₂/DCE at 550 °C was studied by AFM, SEM and TEM techniques. The mechanism of metal dusting in aggressive reaction media was suggested. The observed process has been considered as a way to produce the self-organizing Ni/CNF catalyst which is effective for further decomposition of chlorinated hydrocarbons leading to nanostructured carbon product.     

The beneficial effect of CO2 in the low temperature synthesis of high quality carbon nanofibers and thin multiwalled carbon nanotubes from CH4 over Ni catalysts

A low temperature chemical vapor deposition method is described for converting CH₄ into high-quality carbon nanofibers (CNFs) using a Ni catalyst supported on either spinel or perovskite oxides in the presence of CO₂. The addition of CO₂ has a significant influence on CNF purity and stability, while the CNF diameter distribution is significantly narrowed. Ultimately, the addition of CO₂ changes the CNF structure from fishbone fibers to thin multiwalled carbon nanotubes. A new “in situ” cooling principle taking into account dry reforming chemistry and thermodynamics is introduced to account for the structural effects of CO₂.     

Characterization of carbon nanofiber composites synthesized by shaping process

Catalytically grown carbon nanofibers (CNFs) are shaped into pellets in desired size and configuration by a conventional molding process so as to extend the potential applications of CNFs in industrial heterogeneous catalysis. After shaping, a novel carbon nanofiber composite with sufficient mechanical strength is produced, in which isolated CNFs are connected by a carbon network formed through polymer binder carbonization. Characterization of the synthesized CNF composite is performed by using HRTEM, XRD, Raman, N₂ physisorption, TPD and TGA. A comparison of the textural and structural properties, as well as the surface chemistry is made amongst the CNFs, the CNF composite, and a commercial coal-based activated carbon, in order to attain a comprehensive understanding of the CNF composite. The results show that the CNF composite preserves the mesoporous texture of the CNFs which will be beneficial to those reactions of mass transfer control. The modification effect of oxidative treatments on physico-chemical properties of the CNF composite is also investigated. More surface oxygen-containing groups are introduced to the composite by treating the material either in boiling HNO₃ solution or in static air at 400 °C.     

Polymer crystallization and precipitation‐induced wrapping of carbon nanofibers with PBT

Carbon nanofibers (CNFs) have attracted significant interest because of their excellent mechanical, electrical, and physical properties. Recent advances in chemical functionalization strategies are anticipated to extend their utility in various applications. In this study, noncovalent methods of CNF functionalization utilizing solution crystallization and precipitation techniques were used to create hybrid nanostructures consisting of CNFs and poly(butylene terephthalate) (PBT). Key to this study is the finding that o‐chlorophenol can be used as a suitable solvent to dissolve PBT to generate these nanostructures. PBT crystallization was documented via wide‐angle X‐ray analysis and differential scanning calorimetry and was due to the nucleation effect of the CNFs. The sizes of the PBT crystals could be manipulated by altering the polymer concentration. The solution crystallization and precipitation techniques provide an alternative strategy to alter and control the nanostructure/polymer interface. The resulting nanohybrid structures may potentially find use in a broad range of applications including electronic devices, sensors, and as reinforcing agents in a polymer matrix. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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.     

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.     

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.     

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.     

Effects of surface modification,carbon nanofiber concentration,and dispersion time on the mechanical properties of carbon‐nanofiber–polycarbonate composites

The time effect of ultrasonication was investigated for dispersing carbon nanofibers (CNFs) into a polycarbonate (PC) matrix on the mechanical properties of thus‐produced composites. The effects of CNF surface modification by plasma treatment and the CNF concentration in composites on their mechanical properties were also explored. The plasma coating was characterized by HRTEM and FT‐IR. Furthermore, the plasma polymerization (10 w) treatment on the CNF enhanced the CNF dispersion in the polymer matrix. The mechanical properties of the CNF–PC composites varied with the dispersion time, at first increasing to a maximum value and then dropping down. After a long ultrasonic treatment (24 h), the properties increased again. At a high concentration, the CNF‐PC suspension became difficult to disperse. Additionally, the possible mechanisms for these behaviors are simply proposed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3792–3797, 2007     

Grafting of polystyrene on carbon nanofibers by introducing a methacrylate unit

BACKGROUND: To overcome the self‐agglomeration of carbon nanofibers (CNFs), a number of studies have dealt with surface modification of CNFs to improve their wettability. In this work, a novel, simple and practical approach based on the introduction of methacrylate units to graft polymers onto the surface of CNFs is proposed. RESULTS: The functionalized CNF samples were characterized using Fourier transform infrared and Raman spectroscopy, thermal gravimetric analysis and transmission electron microscopy. The molecular weight of grafted polystyrene (PS) is influenced by the monomer feed ratio. The stabilities of various CNF dispersions were measured using an optical analyzer. When the content of grafted PS is 34.2%, the functionalized CNFs attain a stable dispersion in xylene. CONCLUSION: In the presence of methacrylate units, PS has been successfully grafted onto the surface of CNFs via an in situ polymerization. Moreover, the introduction of methacrylate units opens the functionalization of CNFs to other routes. Copyright © 2009 Society of Chemical Industry     

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.     

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).