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

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

Electrocatalytic properties of Pt/carbon composite nanofibers

Pt/carbon composite nanofibers were prepared by electrodepositing Pt nanoparticles directly onto electrospun carbon nanofibers. The morphology and size of Pt nanoparticles were controlled by the electrodeposition time. The resulting Pt/carbon composite nanofibers were characterized by running cyclic voltammograms in 0.20 M H₂SO₄ and 5.0 mM K₄[Fe(CN)₆] + 0.10 M KCl solutions. The electrocatalytic activities of Pt/carbon composite nanofibers were measured by the oxidation of methanol. Results show that Pt/carbon composite nanofibers possess the properties of high active surface area and fast electron transfer rate, which lead to a good performance towards the electrocatalytic oxidation of methanol. It is also found that the Pt/carbon nanofiber electrode with a Pt loading of 0.170 mg cm⁻² has the highest activity.     

Synthesis and characterization of double-walled carbon nanotubes from multi-walled carbon nanotubes by hydrogen-arc discharge

Double-walled carbon nanotubes (DWNTs) were synthesized in a large scale by a hydrogen arc discharge method using graphite powders or multi-walled carbon nanotubes/carbon nanofibers (MWNTs/CNFs) as carbon feedstock. The yield of DWNTs reached about 4 g/h. We found that the DWNT product synthesized from MWNTs/CNFs has higher purity than that from graphite powders. The results from high-resolution transmission electron microscopy observations revealed that more than 80% of the carbon nanotubes were DWNTs and the rest were single-walled carbon nanotubes (SWNTs), and their outer and inner diameters ranged from 1.75 to 4.87 nm and 1.06 to 3.93 nm, respectively. It was observed that the ends of the isolated DWNTs were uncapped and it was also found that cobalt as the dominant composition of the catalyst played a vital role in the growth of DWNTs by this method. In addition, the pore structures of the DWNTs obtained were investigated by cryogenic nitrogen adsorption measurements.     

Structural and mechanical characterization of platelet graphite nanofibers

Platelet graphite nanofibers have been characterized by scanning electron microscopy, transmission electron microscopy, electron diffraction, X-ray photoemission spectroscopy, and atomic force microscopy. The results show that the graphene sheets are stacked parallel to each other and are perpendicular to the fiber axis; the interlayer spacing is 0.34 nm. A small fraction of carbon atoms are bonded to oxygen. Solid-state nuclear magnetic resonance shows that hydrogenated carbons are under the detection limit (<5%) and that the nanofibers are dominated by sp²-bonded carbons. Mechanical measurements were made on individual nanofibers by nanoindentation.     

Preparation of highly crystalline carbon nanofibers from pitch/polymer blend

High crystalline carbon nanofibers were prepared by using polymer blend technique. Naphthalene-based mesophase pitch (AR pitch) was dispersed finely in polymethylpentene matrix, spun by using a melt-blown spinning machine, stabilized at 160 °C in an oxygen atmosphere and carbonized at 900 °C in a nitrogen atmosphere. Bundles of the carbon nanofibers with ca. 100 nm in diameter were obtained after removal of polymethylpentene at the carbonization process. No impurity carbon was observed. The carbon nanofibers consisted of fine carbon crystallites with preferred orientation along the fiber axis. After heating to 3000 °C, the carbon crystallites grew drastically to have an interlayer spacing of 0.3367 nm and a crystallite thickness of 56.9 nm, respectively, with remarkable improvement of the preferred orientation of the crystallites. Advantages and disadvantages of the present method were discussed briefly.     

Wet-etched Ni foils as active catalysts towards carbon nanofiber growth

A study of wet-chemical etching treatments for nickel foils and the growth parameters for carbon nanostructures on them using hot-filament chemical vapour deposition (CVD) is described. Catalytically-active protrusions were produced on the Ni foils with sizes and densities in the range ∼24-42 nm and ∼202-314 P μm⁻², respectively. These protrusions are found to have a key role in the growth process as they determine the yield and morphology of the carbon deposits. It is shown that well-shaped, nano-sized protrusions are required to achieve high yield growth of hollow-herringbone carbon nanofibers (CNFs) with an in-plane crystallite size of ∼23 nm. Good correlation was seen between the statistical distributions of the protrusion size (sp) and the CNF diameters (df) depicting sp/df ratios close to unity. This work sheds light on the mechanisms behind CVD growth on metal foils.     

Preparation of carbon nanofibers/carbon foam monolithic composite from coal liquefaction residue

Carbon nanofibers/carbon foam composites that are made by growing carbon nanofibers (CNFs) on the surface of a carbon foam (CF) have been prepared from coal liquefaction residues (CLR) by a procedure involving supercritical foaming, oxidization, carbonization, and catalytic chemical vapour deposition (CCVD) treatment. These new carbon/carbon composites were examined using SEM, TEM and XRD. The results show that the as-made CF has a structure with cell sizes of 300-600 μm. X-ray diffraction studies show that iron-containing contaminates are present in the CLR. However, these species may act as a catalyst in the CCVD process as established in the literature. After the CCVD treatment, the cell walls of CF are covered by highly compacted CNFs that have external diameters of about 100 nm and lengths of several tens of micrometers. This work may open a new way for direct and effective utilization of the CLR.     

Bamboo-shaped carbon nanofibers obtained in pyrolytic carbon by thermal gradient chemical vapor deposition

Bamboo-shaped carbon nanofibers were obtained in pyrolytic carbon fabricated by thermal gradient chemical vapor deposition and their micro-and nanostructure were examined by transmission and scanning electron microscopy. The results showed that, bamboo-shaped nanofibers with diameters from a few tens to about 250 nm were distributed homogeneously in the pyrolytic carbon. The nanofibers could be pulled out of the pyrolytic carbon when they were fractured.     

Preparation and electrochemical studies of electrospun TiO2 nanofibers and molten salt method nanoparticles

The TiO₂ nanofibers and nanoparticles are prepared by electrospinning and molten salt method, respectively. The materials are characterized by X-ray diffraction scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and a thermal analysis. The SEM and TEM studies showed that fibers were of average diameter ∼100 nm and composed of nanocrystallites of size 10-20 nm. Electrochemical properties of the materials are evaluated using cyclic voltammetry, galvanostatic cycling and electrochemical impedance spectroscopy. Cyclic voltammetric studies show a hysteresis (ΔV) between the cathodic and the anodic peak potentials for TiO₂ nanofibers and nanoparticles (sizes ∼15-30 nm) are in the range, 0.23-0.30 V and a redox couple Ti^4+/3+ around ∼1.74/2.0 V. Electrochemical cycling results revealed that the TiO₂ nanofibers have lower capacity fading compared to that of the nanoparticles. The capacity fading for 2-50 cycles was ∼23% for nanofibers, which was nearly one-third of that of corresponding nanoparticles (∼63%). We discussed the effect of particle size on hysteresis and cycling performance of TiO₂ nanoparticles. Impedance analysis of TiO₂ nanofibers and nanoparticles during first discharge cycle is analyzed and interpreted.     

Synthesis and optimization of barium manganate nanofibers by electrospinning

Barium manganate nanofibers were successfully synthesized for the first time after heat treatment of composite nanofibers of polyvinyl pyrrolidone (PVP), barium acetate and manganese acetate using electrospinning technique. Different PVP concentrations were used and the results show that PVP concentration had played important role in the formation, uniformity, homogeneity and particularly in the reduction of nanofibers diameter. Crystal structure, microstructure, elemental analysis and surface morphology were studied using X-ray diffraction analysis, scanning electron microscopy, energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. X-ray diffraction results show that at low temperature there is no crystallinity in the fibers sample and at ∼400 °C formations of barium manganate crystalline phase starts and finally at 700 °C all the nanofibers became single phase. The first two high intensity peaks (1 0 1) and (1 1 0) give an average crystallite size of about 20 nm. The scanning electron micrographs show that the morphology of the fibers is smooth and uniform at low temperature and become slightly porous at intermediate temperature and finally at high temperature of 700 °C the fibers become highly porous, shrank and their average diameter reduced from ∼400 nm to about 100 nm. These fibers are made of grains with sizes ranging from 15 to 30 nm. Energy dispersive X-ray spectroscopy and Fourier transform infra-red results are also in good agreement with XRD and SEM results.     

Temperature effect on the formation of catalysts for growth of carbon nanofibers

Turbostratic carbon nanofibers (CNFs), platelet graphite nanofibers (PGNFs) and tubular graphite nanofibers (TGNFs, also called multi-walled carbon nanotubes) were synthesized using thermal decomposition from a mixture of poly(ethylene glycol) and NiCl₂. A detailed study found that the synthesis temperature dramatically affected the morphology and topography of the catalysts, which play an important role in the synthesis of the various CNFs. At the temperature of 600 °C, irregular shape nanocatalysts with very rough surfaces were formed for the synthesis of turbostratic CNFs. Cubic-like nanocatalysts were formed at 750 °C for PGNFs and truncated cone-like nanocatalysts were formed at 850 °C for TGNFs. The surface roughness and the shape of the catalysts determined the stacking order of the graphene layers so that different types of CNF were formed. The growth direction of the graphene layers was from the Ni(1 1 1) plane for PGNFs and from the Ni(1 1 0) plane for TGNFs. Characterizations and field emission properties of these materials were also studied and compared.     

Characterisation of hydrophobic carbon nanofiber-silica composite film electrodes for redox liquid immobilisation

Carbon (50-150 nm diameter) nanofibers were embedded into easy to prepare thin films of a hydrophobic sol-gel material and cast onto tin-doped indium oxide substrate electrodes. They promote electron transport and allow efficient electrochemical reactions at solid|liquid and at liquid|liquid interfaces. In order to prevent aggregation of carbon nanofibers silica nanoparticles of 7 nm diameter were added into the sol-gel mixture as a “surfactant” and homogeneous high surface area films were obtained. Scanning electron microscopy reveals the presence of carbon nanofibers at the electrode surface. The results of voltammetric experiments performed in redox probe—ferrocenedimethanol solution in aqueous electrolyte solution indicate that in the absence of organic phase, incomplete wetting within the hydrophobic film of carbon nanofibers can cause hemispherical diffusion regime typical for ultramicroelectrode like behaviour.The hydrophobic film electrode was modified with two types of redox liquids: pure tert-butylferrocene or dissolved in 2-nitrophenyloctylether as a water-insoluble solvent and immersed in aqueous electrolyte solution. With a nanomole deposit of pure redox liquid, stable voltammetric responses are obtained. The presence of carbon nanofibers embedded in the mesoporous matrix substantially increases the efficiency of the electrode process and stability under voltammetric conditions. Also well-defined response for diluted redox liquids is obtained. From measurements in a range of different aqueous electrolyte media a gradual transition from anion transfer dominated to cation transfer dominated processes is inferred depending on the hydrophilicity of the transferring anion or cation.     

Development of carbon nanofibers from aligned electrospun polyacrylonitrile nanofiber bundles and characterization of their microstructural, electrical, and mechanical properties

Carbon nanofibers with diameters of 200-300 nm were developed through stabilization and carbonization of aligned electrospun polyacrylonitrile (PAN) nanofiber bundles. Prior to the oxidative stabilization in air, the electrospun PAN nanofiber bundle was tightly wrapped onto a glass rod, so that tension existed during the stabilization. We also investigated several carbonization procedures by varying final carbonization temperatures in the range from 1000 to 2200 °C. The study revealed that: (1) with increase of the final carbonization temperature, the carbon nanofibers became more graphitic and structurally ordered; (2) the carbon nanofiber bundles possessed anisotropic electrical conductivities, and the differences between the parallel and perpendicular directions to the bundle axes were over 20 times; and (3) the tensile strengths and Young's moduli of the prepared carbon nanofiber bundles were in the ranges of 300-600 MPa and 40-60 GPa, respectively.     

Poly(ethylene terephthalate) nanosheets prepared by CO2-laser supersonic multi-drawing

Poly(ethylene terephthalate) (PET) nanosheets were fabricated by winding nanofibers onto a spool. The nanofibers were prepared by irradiating PET fibers with radiation from a carbon dioxide laser while drawing them at supersonic velocities. A supersonic jet was generated by blowing air into a vacuum chamber through the fiber injection orifice. A new vacuum chamber was developed to produce nanosheets; it has seven fiber injection orifices and a spool to collect the nanofibers. A rectangular nanosheet that was 17 cm wide, 18 cm long, and 30 μm thick was obtained by collecting nanofibers for 10 min. The nanosheet is composed of nanofibers with an average fiber diameter of 350 nm. This technique is a novel method for producing nanosheets.     

Synthesis of novel Y-junction hollow carbon nanotrees

A simple catalyst-dwindling route without additional templates has been developed, for the first time, to synthesize novel Y-junction hollow carbon nanotrees in a high-yield of 90% by the reaction of ferrocene with 1,2-dichlorobenzene at 180 °C. The nanotrees have an average length about 5 μm, an average external trunk diameter of about 500 nm, branch diameters ranging from 70 to 100 nm, and wall thickness ranging from 24 to 30 nm. Unlike traditional carbon nanotubes, these nanotrees have close-ended branches but open-ended roots. X-ray powder diffraction, field emission scanning electron microscopy, transmission electron microscope, high-resolution transmission electron microscopy, electron diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy were used to characterize the products. It is also found that proper reaction temperatures and appropriate proportion of ferrocene and 1,2-dichlorobenzene play important roles in this route. The formation mechanism of treelike carbon nanotubes was simply proposed.     

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.     

Modeling of fishbone-type carbon nanofibers: A theoretical study

Five models for fishbone-type carbon nanofibers (f-CNFs) have been proposed and their geometries have been optimized through molecular dynamics and molecular mechanics simulations. The angle between graphene layers and fiber axis can take the values 9.71°, 19.60°, 30.03°, 41.99°, and 56.43°, which agree satisfactorily with the experimentally determined values. With reduction of the angle, the interlayer spacing of graphene sheets increases from 0.3376 nm to 0.3392 nm accordingly, due to the strain of the graphitic cones. In addition, X-ray diffraction (XRD) simulations are also implemented. With the high-resolution transmission electron microscopy (HRTEM) results taken into account, the XRD simulations verify that f-CNFs can adopt not only a regular arrangement but also a turbostratic way, and our models are good approximations for describing the bulk microstructures of f-CNFs.     

Formation of nanocarbons during activation of mesocarbon microbeads with potassium hydroxide

Carbon nanotubes (CNTs) together with carbon nanofibers (CNFs) have been produced on the surface of and inside mesocarbon microbeads containing Co nanoparticles during their activation with potassium hydroxide (KOH). The resulting CNFs consist of a number of platelet-shaped sub-units with width of about 500 nm and thickness of 50 nm. The CNTs, with diameter of about 200 nm, grow from the inside to the surface of the activated carbon beads. The results indicate that, in addition to the Co nanoparticles, the existence of KOH also plays an important role in the nanocarbon growth.     

EELS analysis of Nylon 6 nanofibers reinforced with nitroxide-functionalized graphene oxide

A detailed analysis by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) of nitroxide-functionalized graphene oxide layers (GOFT) dispersed in Nylon 6 nanofibers is reported herein. The functionalization and exfoliation process of graphite oxide to GOFT was confirmed by TEM using electron diffraction patterns (EDP), wherein 1–4 graphene layers of GOFT were observed. The distribution and alignment of GOFT layers within a sample of Nylon 6 nanofiber reveals that GOFT platelets are mainly within the fiber, but some were partially protruding from it. Furthermore, Nylon 6 nanofibers exhibited an average diameter of 225 nm with several microns in length. GOFT platelets embedded into the fiber, the pristine fiber, and amorphous carbon were analyzed by EELS where each spectra [corresponding to the carbon edge (C-K)] exhibited changes in the fine structure, allowing a clear distinction between: (i) GOFT single-layers, (ii) Nylon-6 nanofibers, and (iii) the carbon substrate. EELS analysis is presented here for the first time as a powerful tool to identify functionalized graphene single-layers (< 4 layers of GOFT) into a Nylon 6 nanofiber composite.     

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.