Index to Volume 31

Carbon nanofibers (diameter range, 3–100 nm; length range, 0.1–1000 µm) have been known for a long time as a nuisance that often emerges during catalytic conversion of carbon-containing gases. The recent outburst of interest in these graphitic materials originates from their potential for unique applications as well as their chemical similarity to fullerenes and carbon nanotubes. In this review, we focus on the growth of nanofibers using metallic particles as a catalyst to precipitate the graphitic carbon. First, we summarize some of the earlier literature that has contributed greatly to understand the nucleation and growth of carbon nanofibers and nanotubes. Thereafter, we describe in detail recent progress to control the fiber surface structure, texture, and growth into mechanically strong agglomerates. It is argued that carbon nanofibers are unique high-surface-area materials (?200 m²/g) that can expose exclusively either basal graphite planes or edge planes. Subsequently, we will present the recently explored applications of carbon nanofibers: polymer additives, gas storage materials, and catalyst supports. The latter application is described in detail. It is shown that the graphite surface structure and the lyophilicity play a crucial role during metal emplacement and catalytic use in liquid-phase catalysis. A case in point is fiber-supported Pd catalysts for nitrobenzene hydrogenation. Finally, we summarize issues with respect to the large-scale production of carbon nanofibers, including production cost estimates and research items to be dealt with in future work.     

静电纺丝法制备碳纳米纤维及其应用

碳纳米纤维由于因其比表面积大、导电和导热性好,被广泛用于催化剂载体、吸附和储能材料。静电纺丝是制备一维纳米纤维直接、有效的方法,在介绍静电纺丝的基本原理和工艺影响因素的基础上,综述了电纺碳纳米纤维的特性及其应用。     

Carbon Nanofibers: Catalytic Synthesis and Applications

Carbon nanofibers (diameter range, 3-100 nm; length range, 0.1-1000 µm) have been known for a long time as a nuisance that often emerges during catalytic conversion of carbon-containing gases. The recent outburst of interest in these graphitic materials originates from their potential for unique applications as well as their chemical similarity to fullerenes and carbon nanotubes. In this review, we focus on the growth of nanofibers using metallic particles as a catalyst to precipitate the graphitic carbon. First, we summarize some of the earlier literature that has contributed greatly to understand the nucleation and growth of carbon nanofibers and nanotubes. Thereafter, we describe in detail recent progress to control the fiber surface structure, texture, and growth into mechanically strong agglomerates. It is argued that carbon nanofibers are unique high-surface-area materials (~200 m²/g) that can expose exclusively either basal graphite planes or edge planes. Subsequently, we will present the recently explored applications of carbon nanofibers: polymer additives, gas storage materials, and catalyst supports. The latter application is described in detail. It is shown that the graphite surface structure and the lyophilicity play a crucial role during metal emplacement and catalytic use in liquid-phase catalysis. A case in point is fiber-supported Pd catalysts for nitrobenzene hydrogenation. Finally, we summarize issues with respect to the large-scale production of carbon nanofibers, including production cost estimates and research items to be dealt with in future work.     

Structural analysis of conical carbon nanofibers

Conical carbon nanofibers are a relatively new type of carbon nanomaterial that has received considerable scientific and commercial interest due to its physical properties. However, its structure and growth mechanism have still not been determined conclusively. In this study the structure of these materials was investigated employing molecular models and structural analyses and compared with reported experimental observations, principally of cone apex angles. The results showed that stacked cone models could not explain the wide variety of apex angles observed for these nanofibers and related structures. Cone-helix models, originally proposed for other carbon conical structures, allow a variety of apex angle structures and were found to be applicable for nanofibers as well. An equation was developed that allows for prediction of cone-helix structures with good graphitic alignment. Such structures were also shown to be more compatible with the physical properties and growth mechanism of nanofibers than stacked cone structures. From these results a cone-helix structure, and a new cone-helical growth mechanism for the nanofibers based on heterogeneous nucleation on conical catalyst particles, are proposed.     

Carbon nanofiber-supported palladium nanoparticles as potential recyclable catalysts for the Heck reaction

5 wt% Pd catalysts supported on platelet carbon nanofibers has been prepared by incipient wetness impregnation. Both the calcination and the reduction temperature have a significant effect on the dispersion of palladium and it was found that about 3 nm sized Pd nanoparticles can be obtained at a calcination and reduction temperature of 250 °C and 150 °C, respectively. Pd catalysts have been applied to catalyze Heck reactions of various activated and non-activated aryl substrates. The activity increased exponentially with a decrease in Pd particle size. The high surface area, mesoporous structure of carbon nanofiber and highly dispersed palladium species on carbon nanofibers makes up one of the most active and reusable heterogeneous catalysts for Heck coupling reactions. Pd nanoparticles supported on platelet CNFs appear to be an excellent catalyst due to high activity, low sensitivity towards oxygen, almost no or low issues with leaching and high stability in multi-cycles.     

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.     

Flexible and freestanding manganese/iron oxide carbon nanofibers for supercapacitor electrodes

In this study, freestanding carbon nanofibers embedded with bimetallic manganese-iron oxide are fabricated for flexible supercapacitor applications via electrospinning. A polyacrylonitrile solution is used as the carbon source, and poly(methyl methacrylate) functions as a sacrificial template to produce microporous carbon nanofibers. The concentrations of manganese acetate and iron acetylacetonate are varied to determine the fabrication conditions for maximal electrochemical performance. The optimized supercapacitor cell exhibits a capacitance of 467 F g^?1 at a current density of 1 A g^?1 and a capacitance retention of 94% at the end of N = 10,000 cycles. The higher mechanical durability of the electrode is confirmed by the absence of electrochemical deterioration at the end of performing 500 bending cycles. Overall, the sample comprising carbon nanofibers, in which the optimal amount of manganese-iron oxide is embedded, has the required pseudocapacitive characteristics and is an interesting choice for high-energy-storage supercapacitor electrodes.     

Carbon nanofibers as potential materials for catalysts support,a mini-review on recent advances and future perspective

The element carbon has been used as an active catalyst as well as a catalyst support. This dual nature of carbon has been attributed to its characteristics such as high porosity, large surface area, excellent electron conductivity and chemical inert nature. Besides, the availability of different forms of carbon like graphene, activated carbon, carbon nanotubes and carbon nanofibers have provided carbon a versatile material to be used for different applications. Carbon has been widely used in different applications like electrical, bio-electrochemical, dry cells, electrodes and as a lubricant. However, in the last decades, the catalytic applications of carbon materials especially carbon nanotubes and carbon nanofibers have gained tremendous attention of the researchers worldwide. Carbon nanofibers, in particular due to thier excellent catalytic support profile like, high surface area, thermal stability and its 3D access to the reacting molecules, have been utilized for different chemical reactions. Metal supported on carbon nanofibers have been observed with better activities as compared to the traditional supported counterparts for the several reactions. This mini-review attempts to document the role of carbon nanofibers and their catalytic support profile for the some common chemical processes. The mini-review also suggests about the future innovations and research work for carbon nanofibers as potential future catalysts support.     

A novel efficient RhB absorbent of Mo2N/MoO2 composite nanofibers for wastewater treatment

Mo₂N/MoO₂ composite nanofibers have been prepared via an electrospinning and controlled nitridation process. The composite nanofibers exhibit a highly efficient Rhodamine B (RhB) absorption behavior with a rate constant of 0.153 g min⁻¹ mg⁻¹, which is about 20 times of the commercial-activated carbon material. Furthermore, the nanofibers show stable absorption activity after recycled by an environmental friendly procedure for four times. The excellent absorption performance of Mo₂N/MoO₂ composite nanofibers demonstrates a promising application of Mo₂N-related materials as an absorbent for wastewater treatment.     

Selective area synthesis of aligned carbon nanofibers by laser-assisted catalytic chemical vapor deposition

Arrays of freestanding bamboo-type carbon nanofibers were grown on the surface of a porous alumina substrate by laser-assisted catalytic chemical vapor deposition. A continuous wave argon ion laser operated at a wavelength of 488 nm was used to thermally decompose pure ethylene over nickel catalysts. Two different catalyst preparation methods were used and are compared with respect to the synthesis of aligned nanofibers. First, a thin nickel film (50 nm) was evaporated on the substrate and was subsequently laser annealed into nanoparticles. This preparation produced non-aligned nanofiber films. Second, a 50 nm thick catalyst layer was electrochemically deposited within the pores of an alumina substrate. This preparation produced an array of vertically aligned nanofibers. A growth rate dependence on radial position within the irradiated area was observed. Average linear growth rates ranging from 554 nm/s to 25 μm/s are reported. The nanofibers were examined by scanning electron microscopy and Raman spectroscopy. Fiber texture and nanotexture were determined by lattice fringe analysis from high resolution transmission electron microscopy images. The alignment mechanism is also discussed.     

Kinetically controlled synthesis of carbon nanofibers with different morphologies by catalytic CO disproportionation over iron catalyst

Carbon nanofibers (CNFs) were synthesized by CO disproportionation on iron catalyst at CO concentration between 58.3% and 75.0%, H₂ concentration between 8.3% and 25.0% and reaction temperature between 833 and 913 K. The time-depending rate of CNFs growth as a function of time was determined by an on-line mass spectrometer and the morphologies of all CNFs products were observed by electronic microscopy. Not only the CNFs growth rate but also the morphology of the grown CNFs were shown to vary with the three operating variables. SEM and TEM images showed that the three-dimensional morphologies of the CNFs were twist, helical or straight and an interesting relationship between the maximal growth rate and the morphology was observed. When the growth rate was between 0.8 and 0.9 mmol/(s gcat), the CNFs were twist. As the growth rate increased to 1.0 mmol/(s gcat), more helical nanofibers appeared. Straight nanofibers were produced when the growth rate reached the level of 1.2 mmol/(s gcat). Finally when the rate of CNFs growth was high at 1.8 mmol/(s gcat), the absolute majority of the solid products was amorphous carbon coexisting with some short and thick nanofibers. Under different operating conditions, the crystal faces of the catalysts had different anisotropy properties for carbon deposition, thus producing CNFs with different morphologies.     

Influence of the textual properties of activated carbon nanofibers on the performance of electric double-layer capacitors

To investigate the relationship between textural properties and electrochemical properties, activated carbon nanofibers were manufactured using an electrospinning process followed by chemical activation using KOH or NaOH. The specific surface area of the KOH-activated carbon nanofibers was higher than that of NaOH-activated carbon nanofibers; however, the total pore volume and mesopore volume of the NaOH-activated carbon nanofibers were greater than those of the KOH-activated carbon nanofibers when the same number of moles of KOH and NaOH were used. The specific capacitances increased as the specific surface area and pore volume of the activated carbon nanofibers were increased. However, the specific capacitance obtained at a high scan rate (50 mV/s) and the retained capacitance of the activated carbon nanofibers increased with increasing total pore and mesopore volume, especially for mesopores with diameters of 2–4 nm.     

Synthesis of carbon–silica core–shell nanofibers from a dispersion of fluorinated carbon nanofibers in solvated polysiloxane

Carbon–silica core–shell fibers (which unusually consist of carbon nanofibers coated with silica) were synthesized using a two-step process. First, fluorination of carbon nanofibers (CNFs) allows their homogenous dispersion into a polysiloxane matrix. A longlife dispersion of nanofibers in solvated polysiloxane has been prepared. Second, the polysiloxane/fluorinated carbon was thermally treated in air until 700 °C. Defluorination and conversion of polysiloxane into silica occur and result in carbon–silica core–shell fibers. The thermal treatment of the polysiloxane/carbon and the resulting silica/carbon–silica core–shell nanostructures were investigated using solid state nuclear magnetic resonance using ¹⁹F, ¹³C ¹H, and ²⁹Si nuclei, X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopies.     

Carbon Nitride-Related Nanomaterials from Chemical Vapor Deposition: Structure and Properties

Nanoscale-sized carbon nitride-related materials exhibit a wealth of interesting structural, electronic, and optical property behaviors. Chemical vapor deposition technology allows almost unlimited freedom to produce films with compositions and structures approaching the nanometer scale among light elements. Aligned polymerized carbon nitride (CN) nanobells have been grown on a large scale and provide excellent field electron emission properties, as described by a side-emission mechanism. Separation of single CN nanobells and fabrication of heterojunctions between CN nanobells and pure carbon nanotubes have been achieved. Boron carbonitride (BCN) nanofibers with controlled orientation and composition have been synthesized; these nanofibers show strong blue-violet photoluminescense at room temperature. Recent progress also has been made on nitrogen-containing diamond, CN, and BCN films. The purpose of this paper is to survey the work that has been conducted and to detail the level of understanding that has been attained in the research on nitride-related materials.     

Electrochemical properties of double wall carbon nanotube electrodes

Electrochemical properties of double wall carbon nanotubes (DWNT) were assessed and compared to their single wall (SWNT) counterparts. The double and single wall carbon nanotube materials were characterized by Raman spectroscopy, scanning and transmission electron microscopy and electrochemistry. The electrochemical behavior of DWNT film electrodes was characterized by using cyclic voltammetry of ferricyanide and NADH. It is shown that while both DWNT and SWNT were significantly functionalized with oxygen containing groups, double wall carbon nanotube film electrodes show a fast electron transfer and substantial decrease of overpotential of NADH when compared to the same way treated single wall carbon nanotubes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Cobalt/copper-decorated carbon nanofibers as novel non-precious electrocatalyst for methanol electrooxidation

In this study, Co/Cu-decorated carbon nanofibers are introduced as novel electrocatalyst for methanol oxidation. The introduced nanofibers have been prepared based on graphitization of poly(vinyl alcohol) which has high carbon content compared to many polymer precursors for carbon nanofiber synthesis. Typically, calcination in argon atmosphere of electrospun nanofibers composed of cobalt acetate tetrahydrate, copper acetate monohydrate, and poly(vinyl alcohol) leads to form carbon nanofibers decorated by CoCu nanoparticles. The graphitization of the poly(vinyl alcohol) has been enhanced due to presence of cobalt which acts as effective catalyst. The physicochemical characterization affirmed that the metallic nanoparticles are sheathed by thin crystalline graphite layer. Investigation of the electrocatalytic activity of the introduced nanofibers toward methanol oxidation indicates good performance, as the corresponding onset potential was small compared to many reported materials; 310 mV (vs. Ag/AgCl electrode) and a current density of 12 mA/cm² was obtained. Moreover, due to the graphite shield, good stability was observed. Overall, the introduced study opens new avenue for cheap and stable transition metals-based nanostructures as non-precious catalysts for fuel cell applications.     

Carbon nanotubes and carbon nanofibers fabricated on tubular porous Al2O3 substrates

Carbon nanotubes and carbon nanofibers were grown at different temperatures on porous ceramic Al₂O₃ substrates with single channel geometry by means of a chemical vapor deposition technique using methane as carbon source and palladium as catalyst. Time-resolved in-situ Fourier transformed infrared spectroscopy was used for the investigation of methane decomposition for characterizing the catalyst’s performance. With increasing synthesis temperature, a structural transition from carbon nanofibers to carbon nanotubes was observed. At a synthesis temperature of 700 °C, solely carbon nanofibers were found, whereas at 800 °C a mixture of two types of bamboo-shaped carbon nanofibers were obtained, suggesting a structural transition. A synthesis temperature to 850 °C results in bamboo-shaped multi-walled carbon nanofibers and multi-walled carbon nanotubes. The carbon products and the observed structural transition were characterized by means of field emission scanning electron microscopy, high-resolution transmission electron microscopy, thermal gravimetric analysis, and Raman spectroscopy.     

Synthesis of carbon nanofibers: effects of Ni crystal size during methane decomposition

The effect of the crystal size of Ni on the growth of carbon nanofibers (CNFs) was studied in a tapered oscillating element microbalance reactor. Small Ni crystals yield a low growth rate and fast deactivation and thus a low final yield of CNF. Large Ni crystals reduce the growth rate because of low surface area. An optimum growth rate and yield of carbon nanofibers can be achieved on optimally sized Ni crystals (around 34 nm). A model has been proposed for interpreting the kinetic effects of the Ni crystal size based on a detailed mechanism of the carbon nanofiber growth. The reduced coking rate on a small-sized Ni crystal is a result of increased saturation concentration of CNF and thus a low driving force for carbon diffusion through the Ni crystals. As a consequence, the surface coverage of carbon increases, which enhances the formation of encapsulating carbon and thus the deactivation. Both the low initial coking rate and the fast deactivation result in a low yield of carbon nanofibers on small-sized crystals. The results also indicate that hydrogen has a significant effect on the formation of CNF, and an optimum partial pressure of hydrogen exists for the CNF growth.     

Patterned growth and differentiation of neural cells on polymer derived carbon substrates with micro/nano structures in vitro

The growth of neuroblastoma (N2a) and Schwann cells has been explored on polymer derived carbon substrates of varying micro and nanoscale geometries: resorcinol–formaldehyde (RF) gel derived carbon films and electrospun nanofibrous (∼200 nm diameter) mat and SU-8 (a negative photoresist) derived carbon micro-patterns. MTT assay and complementary lactate dehydrogenase (LDH) assay established cytocompatibility of RF derived carbon films and fibers over a period of 6 days in culture. The role of length scale of surface patterns in eliciting lineage-specific adaptive response along, across and on the interspacing between adjacent micropatterns (i.e., “on”, “across” and “off”) has been assayed. Textural features were found to affect 3′,5′-cyclic AMP sodium salt-induced neurite outgrowth, over a wide range of length scales: from ∼200 nm (carbon fibers) to ∼60 μm (carbon patterns). Despite their innate randomness, carbon nanofibers promoted preferential differentiation of N2a cells into neuronal lineage, similar to ordered micro-patterns. Our results, for the first time, conclusively demonstrate the potential of RF-gel and SU-8 derived carbon substrates as nerve tissue engineering platforms for guided proliferation and differentiation of neural cells in vitro.     

Rational design of mesoporous LiFePO4@C nanofibers as cathode materials for energy storage

In this work, the mesoporous LiFePO₄@C nanofibers have been successfully fabricated through a facile electrospinning method. The structure, morphology, chemical composition and lithium storage performance have been systematically investigated. The results reveal that the LiFePO₄ grains with particle size of ～15 nm are uniformly dispersed in the mesoporous carbon nanofibers. The LiFePO₄@C electrode presents a high reversible capacity and excellent rate performance. It delivers a discharge capacity of 107 mAh g⁻¹ and retains 105 mAh g⁻¹ over 200 cycles at 10C. The excellent electrochemical performances are attributed to the novel nanostructure where LiFePO₄ nanoparticles are embedded in the carbon fibers. This designed structure can significantly enhance the conductivity of LiFePO₄@C, accelerate the diffusion of electrolyte, and thus facilitate the transport of electrons and Li-ions.