Protic ionic liquid triggered self-assembly structural transition of CTAB for inducing silica spheres with radially oriented mesochannels

Mesoporous silica nanospheres with radially oriented channels have been obtained by adjusting the interaction between cetyltrimethyl ammonium bromide (CTAB) and protic ionic liquid (tripropylamine acetate) through changing the PIL content. The properties of the samples were characterized by N₂ adsorption/desorption techniques, scanning electron microscope and transmission electron microscopy. The radially oriented mesochannels, wrinkled surface morphology and particle sizes of the obtained samples were varied with different mass ratios of CTAB to PIL. Compared with silica spheres prepared without PIL, the radial mesochannels slightly bent and degenerated, meanwhile, the particle sizes were decreased from 550 to 350 nm accordingly. Moreover, the formation mechanism of the silica spheres with radially oriented channels and hierarchical porous structure which based on the interaction between PIL and CTAB was tentatively elucidated.     

Carbon nanotube core–polymer shell nanofibers

Single‐walled carbon nanotube (SWNT)/poly(methyl methacrylate) and SWNT/polyacrylonitrile composite nanofibers were electrospun with SWNT bundles as the cores and the polymers as the shells. This was a novel approach for processing core (carbon nanotube)–shell (polymer) nanofibers. Raman spectroscopy results show strain‐induced intensity variations in the SWNT radial breathing mode and an upshift in the tangential (G) and overtone of the disorder (G′) bands, suggesting compressive forces on the SWNTs in the electrospun composite fibers. Such fibers may find applications as conducting nanowires and as atomic force microscopy tips. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1992–1995, 2005     

Carbon nano-rod as a structural unit of carbon nanofibers

The structure of carbon nanofiber (CNF) with platelet texture was found to be constructed by a primary structural unit named carbon nano-rod by the authors through comprehensive examination of X-ray diffraction (XRD), high-resolution scanning electron microscopy (HR-SEM), high-resolution transmission electron microscopy (HR-TEM), and scanning tunneling microscopy (STM). Single carbon nano-rod resembles multi-walled carbon nanotube with 4–5 graphene sheets nested (ca. 2.5 nm diameters), but appears to have a hypothetical transverse of hexagonal shape with the inner diameter corresponding to the interlayer spacing. Three-dimensional model of CNF was suggested based on the carbon nano-rods, which are densely stacked perpendicular to the fiber axis to form a typical platelet CNF. Such a structural concept of CNF gives novel views on the correlation between structure and properties of CNFs for potential applications.     

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.     

Carbon nanofibers supported Pt-Ru electrocatalysts for direct methanol fuel cells

Oxidized and reduced carbon nanofibers (OCNF and RCNF) were used as supports to prepare highly dispersed PtRu catalysts for the direct methanol fuel cells (DMFC). The structural and surface features and electrocatalytic properties of bimetallic PtRu/OCNF and PtRu/RCNF were extensively investigated. FT-IR spectra show that carboxyl groups exist on the surface of the OCNF, which greatly influence the morphology and crystallinity of the electrocatalysts. Transmission electron microscopy and X-ray diffraction consistently show that PtRu/RCNF has a smaller particle size and more uniform distribution than PtRu/OCNF. However, both catalysts have very similar methanol oxidation peak current densities that are significantly lower than commercial catalyst based on current-voltage (CV) results. These two catalysts also give very similar single cell performance except for some difference in the resistance polarization region. The OCNF supported catalysts give better performance than commercial catalysts when current density is higher than 50 mA cm⁻² in spite of low methanol oxidation peak current density. These results can be ascribed to the specific surface and structural properties of carbon nanofibers.     

Carbon nanotube‐reinforced polyacrylonitrile nanofibers by vibration‐electrospinning

Carbon nanotubes (CNTs) are thought to be perfect enhancive materials for composites. Multi‐wall carbon nanotubes were directly electrospun into polyacrylonitrile (PAN) nanofibers via both traditional electrospinning and vibration‐electrospinning. The fibers obtained were examined by scanning electron microscopy and X‐ray diffraction. CNTs were aggregated heavily in the fibers obtained by traditional electrospinning while CNTs were well distributed and aligned in PAN fibers obtained by vibration‐electrospinning. Copyright © 2007 Society of Chemical Industry     

Carbon nanonodules fewer than ten graphenes thick grown on aligned amorphous carbon nanofibers

A novel structure of carbon nanonodules containing fewer than 10 layers graphene has grown on amorphous carbon nanofibers by carbonization-induced self-assembly. It is found that a successive processes containing pre-oxidation in air at 220 °C and carbonization in a high vacuum (1 × 10⁻⁴ Pa) at 750 °C are necessary for the fabrication of the carbon nanonodules. Possible mechanism for the evolution of amorphous nanofibers to carbon nanonodules is presented. It is also found that the temperature of the collector during electrospinning of the fiber and the pressure of carbonization are critical factors for growth of the nanonodules. With these mechanisms, carbon nanonodules can be selectively grown on the prepared amorphous carbon nanofibers using pre-oxidation and carbonization of an electrospun glycerol–polyacrylonitrile fiber.     

Fabrication of aligned and molecularly oriented electrospun polyacrylonitrile nanofibers and the mechanical behavior of their twisted yarns

The fiber spinning technique of electrospinning was optimized in order to prepare unidirectional aligned, structurally oriented, and mechanically useful carbon precursor fibers with diameters in the nanoscale range. The fiber spinning velocity and fiber draw ratio was measured to be between 140 and 160 m/s and 1:300,000, respectively, for fibers spun from 10 wt% polyacrylonitrile (PAN) solutions with dimethylformamide (DMF). A high-speed, rotating target was used to collect unidirectional tows of PAN fibers. Aligned and (+) birefringent fibers with diameters between 0.27 and 0.29 μm (FESEM) were collected from electrospinning 15 wt% PAN in DMF solutions at 16 kV onto a target rotating with a surface velocity between 3.5 and 12.3 m/s. Dichroism measurements (Polarized FTIR) of the nitrile-stretching vibration show an increase in the molecular orientation with take-up speed. Wide angle X-ray diffraction patterns (WAXD) show equatorial arcs from the reflection at and (1120) reflection at A maximum chain orientation parameter of 0.23 was determined for fibers collected between 8.1 and 9.8 m/s. Twisted yarns of highly aligned PAN nanofibers with twist angles between 1.1 and 16.8° were prepared. The ultimate strength and modulus of the twisted yarns increase with increasing angle of twist to a maximum of 162±8.5 MPa and 5.9±0.3 GPa, respectively, at an angle of 9.3°.     

Carbon induced phase transformation in electrospun TiO2/multiwall carbon nanotube nanofibers

Nanofibers of titania and composite nanofibers of titania and multiwall carbon nanotubes were synthesized by electrospinning using a sol-gel process combined with activated carbon nanotubes. The relationships of treatment temperature, carbon nanotube content on the crystal phase, fiber morphology, and electric properties are reported. It is found that the rutile phase becomes more prominent at low heat treatment temperatures with an increase of carbon content in nanofibers, be it for higher amount of carbon due to reducing atmosphere or due to an increase in MWCNT. Atmospheric control and lower heat treatment temperatures enable crystalline nanocomposite fibers of anatase where the level of rutile is below the detection limit of XRD or Raman spectroscopy. This work provides a new path to fabricate electrospun TiO₂/MWCNT nanocomposite nanofibers with limited C-induced rutile phase.     

Carbon nanofibers supported Ru catalyst for sorbitol hydrogenolysis to glycols: Effect of calcination

Carbon nanofiber (CNFs) supported Ru catalysts for sorbitol hydrogenolysis to ethylene glycol and propylene glycol were prepared by incipient wetness impregnation, calcination and reduction. The effect of calcination on catalyst properties was investigated using thermal gravimetry analysis, temperature-programmed reduction, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and N₂ physisorption. The results indicated that calcination introduced a great amount of surface oxygen-containing groups (SOCGs) onto CNF surface and induced the phase transformation of Ru species, but slightly changed the texture of Ru/CNFs. The catalytic performance in sorbitol hydrogenolysis showed that Ru/CNFs catalyst calcined at 240 °C presented the highest glycol selectivities and reasonable glycol yields. It was believed that the inhibition and confinement effect of SOCGs around Ru particles as well as the high dispersion of Ru particles was the key factor for the catalytic activity.     

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.     

Electrospun nanofibers with application in nanocomposites

The use of fine fiber has become an important design tool for filter media. Nanofibers-based filter media have some advantages such as lower energy consumption, longer filter life, high filtration capacity, easier maintenance, low weight rather than other filter media. The nanofibers-based filter media made up of fibers of diameter ranging from 100 to 1,000 nm can be conveniently produce by electrospinning technique. Common filter media have been prepared with a layer of fine fiber on typically forming the upstream or intake side of the media structure. The fine fiber increases the efficiency of filtration by trapping small particles, which increases the overall particulate filtration efficiency of the structure. Improved fine fiber structures have been developed in this study in which a controlled amount of fine fiber is placed on both sides of the media to result in an improvement in filter efficiency and a substantial improvement in lifetime. In the first part of this study, the production of electrospun nanofibers is investigated. In the second part, a different case studyis presented to show how they can be laminated for application as filter media. Response surface methodology (RSM) was used to obtain a quantitative relationship between selected electrospinning parameters and average fiber diameter and its distribution.     

Structural studies of oriented carbon nanotubes in alumina channels using high energy X-ray diffraction

Structural studies of multi-wall carbon nanotubes prepared by template pyrolytic carbon deposition from thermal decomposition of propylene at 800 °C inside channels of an alumina membrane have been performed using X-ray diffraction. The two-dimensional diffraction pattern of the deposited carbon nanotubes, recorded directly within the alumina template using an image plate detector, exhibits two dark arcs corresponding to the (0 0 2) graphitic reflection. The anisotropic scattering distribution indicates alignment of the nanotubes. The diffracted intensity was measured for the powdered samples after removing the alumina membrane using a point detector. A maximum scattering vector of K_max = 20 Å⁻¹ yielded the radial distribution function, providing evidence that the investigated nanotubes form a distorted hexagonal network that implies the presence of five-membered rings.     

Consolidation of carbon nanofibers with pyrolytic carbon

A strategy of industrial-scale manufacture for a wide range of carbon materials based on carbon nanofibers is proposed. It was shown that porous materials with a high sorption capacity can be obtained with the use of carbon nanofibers by means of conventional manufacturing operations. The results of studying of consolidation of carbon nanofibers with pyrolytic carbon are reported. It was found that the nature of carbon material has a substantial effect on the rate of deposition of pyrolytic carbon. The most appropriate temperature range in which carbon nanofibers should be consolidated for the preparation of materials with a high catalytic activity was determined.     

Amorphous SiCNO hollow nanofibers with mesoporous walls

In this work, for the first time, we reported the fabrication of polymer-derived amorphous SiCNO ceramics via electrospinning of tetraethoxysilane (TBOS), polyvinylpyrrolidone (PVP), cetrimonium bromide (CTAB), and urea combined with subsequent air calcination. The resultant products exhibited a well-defined one-dimensional (1D) hollow fiber nanostructure with mesoporous walls. The BET surface area of SiCNO hollow nanofibers is ~95.6 m²/g, and the average pore diameter is sized in ~9 nm. The movement of urea within the core of the as-spun polymeric fibers accounted for the formation of hollow nanofibers, and the thermal decomposition of polymers such as PVP, TEOS, and urea responded for the formation of mesopores within the walls. In addition, it was found that the contents of urea within the raw materials played a critically important role in the formation of SiCNO mesoporous hollow nanofibers, making their growth in a controlled manner.     

Preparation of in situ grown silicon carbide nanofibers radially onto carbon fibers and their effects on the microstructure and flexural properties of carbon/carbon composites

Silicon carbide nanofibers (SiCNFs) used as the second reinforcements of carbon/carbon composites were grown radially on the carbon fiber surface. The microstructure of SiCNFs and their effects on the microstructure and flexural properties of C/C composites were investigated. Results show that there are many defects such as twin crystals and stacking faults in SiCNFs which were grown by catalytic chemical vapor deposition. During the same process, the skin region of carbon fiber has changed. Several SiC layers are formed and the arrangement of the graphite layers around SiC layers is more orderly. In the next chemical vapor infiltration, due to the induction of SiCNFs, the middle textural pyrocarbon were formed firstly and then is the high textural pyrocarbon. The existence of SiCNFs also contributes to the three-phase interface between pyrocarbon, SiCNFs and carbon fibers, resulting in a good bond between carbon fiber and matrix. Those structural changes lead the better flexural properties of SiCNF–C/C composites compared with C/C composites.     

Synthesis of radially aligned polyaniline dendrites

Radially aligned polyaniline dendrites with nanotube junctions have been synthesized using tartaric acid as the dopant without the aid of any surfactants. These dendritic nanotubes with 80-400 nm in outer diameter, 30-50 nm in wall thickness, and several micrometers in length can self assemble into urchin-like nanostructures. The geometrical shape of the individual branch is a cone. The nanotube junctions may provide potential applications in nanoelectronic devices. Furthermore, the influences of other organic acid, such as citric acid, oxalic acid, and acrylic acid, on the morphologies on polyaniline nanostructures have been investigated.     

Carbon nanofibers grown over graphite supported Ni catalyst: relationship between octopus-like growth mechanism and macro-shaping

Carbon nanofibers (CNFs) with a uniform diameter of ca. 30 nm have been grown via catalytic decomposition of C₂H₆/H₂ mixture over a nickel (1 wt.%) catalyst supported on graphite microfibers which constitutes the macroscopic shape of the final C/C composite. The productivity reached 50 g of CNFs / g of Ni / h on stream and is among the highest reported to date. The resulting composite consisting in a web-like network of CNFs covering the starting catalyst was characterized by SEM and TEM in order to get more insight on the relationship between the starting nickel catalyst particles and the as-grown CNFs. Apparently the CNFs growth proceeds from different mechanisms: base-growth mechanism involving especially the large nickel particles, tip-growth mechanism involving the smaller nickel particles and tip/octopus-growth mechanism, the most frequent involving all particles. The restructuration of the nickel particle from a globular to a more faceted structure seems to be the key step to produce an extremely large quantity of CNF with yields up to 100 wt.% after 2 h of synthesis.     

Modeling of fishbone-type carbon nanofibers with cone-helix structures

Proposing a plausible model for fishbone-type carbon nanofiber (f-CNF) proves to be a challenge. A practical approach to the construction of a cone-helix model is suggested on the basis of early experimental observations and from a geometric perspective. With the introduction of disclination angle and overlap angle into graphene sheets, the resultant helical cones have variable morphologies which rationalize the broad distribution of the f-CNF apex angles. A fraction of overlap angles can produce energetically favorable cone-helix models with high densities of coincident lattice points. Once the nearest coincident points to the apex is identified, both the overlap angle and the degree of graphitic alignment can be obtained. Periodic boundary conditions are imposed on the cone-helix models to depict the f-CNF morphologies along the principal axes. The lattice strain induced by the multi-layered model is found to have a negative effect on the structural stability. After the central parts of f-CNFs are removed, the lattice strain around the cone tips is eliminated, and the cone-helix model of more graphite layers is energetically more favorable. X-ray diffraction simulations are finally conducted to evaluate the reliability of the proposed models and to reveal the identity of the reflections at the diffraction angle of 44.5°.