Morphology and microstructure of spring-like carbon micro-coils/nano-coils prepared by catalytic pyrolysis of acetylene using Fe-containing alloy catalysts

Three-dimensional (3D) spring-like carbon nano-coils were obtained in high purity (nearly 100%) and high yield (20%) by the catalytic pyrolysis of acetylene at 750-790 °C using an Fe-based catalyst; 54Fe-38Cr-4Mn-4Mo or 71Fe-18Cr-8Ni-3Mo (SUS513). The morphologies and microstructure were examined in detail, and the growth mechanism is also discussed. The diameter of carbon fiber, from which the carbon nano-coils was formed, was 50-200 nm, the coil diameter 100-1000 nm, and the coil pitch 10-500 nm. The nano-coils were generally grown by a mono-directional growth mode, and laces with various morphologies were very commonly observed on the surface. It was observed that the spring-like carbon nano-coils are actually composed of two fused nano-coils.     

Preparation of carbon micro-coils with the application of outer and inner electromagnetic fields and bias voltage

The carbon micro-coils were prepared by the catalytic pyrolysis of acetylene at 770-775 °C using Ni, Nb, Ta and their oxides with the superimposed application of an electromagnetic field (AC, DC) from both the outer and inner sides of the reaction tube as well as with application of a bias voltage to the substrate. The effect of the electromagnetic field on the coil yield and morphology were examined. The coil yield increased by 1.1-1.2 times with the superimposed application of the AC or DC electromagnetic field. It was found that the superimposed application of the outer and inner electric magnetic fields resulted in the strong influence on the morphology of the grown carbon coils. The effect of the non-inductive zero magnetic field obtained with a counter magnetic field on the growth of the carbon coils was also examined. The maximum coil yield of 30-32.5 mg/cm²-substrate was obtained with the application of both the outer and inner DC-EM fields or non-inductive EM field.     

Prevention of Si-contaminated nanocone formation during plasma enhanced CVD growth of carbon nanotubes

We investigated the growth behavior and morphology of vertically aligned carbon nanotubes (CNTs) on silicon (Si) substrates by direct current (DC) plasma enhanced chemical vapor deposition (PECVD). We found that plasma etching and precipitation of the Si substrate material significantly modified the morphology and chemistry of the synthesized CNTs, often resulting in the formation of tapered-diameter nanocones containing Si. Either low bias voltage (∼500 V) or deposition of a protective layer (tungsten or titanium film with 10-200 nm thickness) on the Si surface suppressed the unwanted Si etching during growth and enabled us to obtain cylindrical CNTs with minimal Si-related defects. We also demonstrated that a gate electrode, surrounding a CNT in a traditional field emitter structure, could be utilized as a protection layer to allow growth of a CNT with desirable high aspect ratio by preventing the nanocone formation.     

Co-Mo catalyzed growth of multi-wall carbon nanotubes from CO decomposition

We investigated the growth of multi-wall carbon nanotubes (CNTs) catalyzed by SiO₂-supported Co-Mo bi-metallic catalyst in flowing CO at 700 °C. We found that both Co and Mo are present in catalytic particles at the tips of CNTs, but their compositions vary from one catalytic particle to another and significantly deviate from the initial mixing composition. The Co concentration and distribution in the catalytic particle of a CNT largely determines the length of the CNT. The CNT growth process is carbon adsorption on exposed area of a catalytic particle and subsequent precipitation at the CNT-catalyst interface or open CNT wall edges. The encapsulation of a catalytic particle was found to occur by the growth of the open-edged graphene walls around the particle. Two types of long CNTs were observed: one with their CNT walls ended at the CNT-particle interface, and the other with their CNT walls open to the environment. The former have diameters similar to their catalytic particle size while the latter have larger diameters.     

Activated carbon membrane with filamentous carbon for water treatment

A novel composite membrane consisting of an activated carbon membrane with filamentous carbon, applicable to water treatment, was fabricated by a combination of conventional carbonization and thermal deposition. The carbonization was performed after dipping a ceramic support in a latex polymer followed by the application of ferric sulfate [Fe₂(SO₄)₃·nH₂O] catalyst. This was followed by CVD from methane at 1050–1100 °C. The resulting membrane therefore consisted of a filamentous carbon layer grown with a combination of the catalyst and deposited hydrocarbon, and an activated carbon layer on a ceramic support. These structural characteristics were confirmed by means of the cut-off of dextran molecules, the pure water permeability, SEM, and the adsorption of phenol. Water treatment experiments using phenol and poly(methyl methacrylate) as model pollutants indicated that this membrane was able to remove dissolved organics of low molecular weight and suspended solids. Also, the filamentous carbon layer successfully prevented the fine particles from sticking on the external surface of the membrane by frequent back washing.     

Synthesis of vertically aligned carbon nanotubes on metal deposited quartz plates

Without plasma aid, we have successfully synthesized vertically aligned carbon nanotubes (CNTs) on iron-, cobalt- or nickel-deposited quartz plates by chemical vapor deposition with ethylenediamine as a precursor. The amine serves as both etching reagent for the formation of metal nanoparticles and carbon source for the growth of aligned carbon nanotubes. The carbon nanotubes were vertically aligned in high density on a large area of the plain silica substrates. The density and diameter of CNTs is determined by the thickness of the deposited metal film and the length of the tubes can be controlled by varying the reaction time. High-resolution transmission electron microscopy analysis reveals that the synthesized CNTs are multiwalled with a bamboo-like structure. Energy dispersive X-ray spectra demonstrate that the CNTs are formed as tip growths. Raman spectrum provides definite evidence that the prepared CNTs are multiwalled graphitic structure.     

Effects of Ni-catalyst characteristics on the growth of carbon nanowires

Carbon nanowires (CNWs) were grown on amorphous Ni thin film catalysts in a microwave plasma-enhanced chemical vapor deposition system under a methane and hydrogen gas mixture. The resulting CNWs were found to be polycrystalline but not amorphous as commonly found elsewhere. In general, a catalyst could be seen at the tip of each CNW. However, multiple CNWs grown from a single catalyst were also regularly observed. The use of amorphous Ni thin film catalyst also resulted in the formation of an interlayer between the CNWs and the substrate. The appearance of this interlayer, however, depends on the Ni film thickness. The carbon nanowires obtained were found to exhibit an unusual microstructure in that the basal planes were perpendicular to the wire axial direction. The Raman signatures of the CNWs consist of two peaks near 1322 cm⁻¹ (D-band) and 1578 cm⁻¹ (G-band), similar to that of carbon nanotubes. The broadening of the G-band peak was found to be greater than 60 cm⁻¹ and the I_D/I_G ratio was found to decrease with FWHM as in a-C:H.     

Nanotubes and nanofilaments from carbon monoxide disproportionation over Co/MgO catalysts: I. Growth versus catalyst state

A catalyst prepared from the pyrolysis of Co and Mg nitrates and citric acid after their co-dissolution in water was used for carbon deposition from CO. Good yields of nanotubes or nanofilaments were obtained over catalysts which had been reduced by H₂ without preliminary treatment at high temperature. Nanotubes with 10 or more cylindrical carbon layers were obtained from pure CO or from CO+CO₂ mixtures. Nanofilaments with truncated conical layers were obtained from CO+H₂ mixtures in the 500–600 °C range. In both cases, high shape selectivity was obtained and almost all MgO could be eliminated by HCl treatment. The only significant impurities were embedded cobalt particles. This process is therefore suitable for preparing nanotubes or nanofilaments with good shape selectivity and 98 wt% purity. Lowering the Co content of the catalyst produces thinner nanotubes but reduces the yield.     

Differences between carbon nanofibers produced using Fe and Ni catalysts in a floating catalyst reactor

Carbon nanofibers were produced by the catalytic CVD process by the floating catalyst method, in semi-industrial systems at temperatures above 1350 K. Iron-derived carbon nanofibers were produced from natural gas and xylene, using ferrocene as catalyst source, yielding a thickened submicron vapor grown carbon fibers with a core of multi-wall nanotubes. For the production of Ni derived nanofibers, natural gas was used as the carbon feedstock, and the Ni was added in a nickel compound solution. When no sulfur is used, only soot was obtained, but when sulfur is added to the reactive feedstock, a highly graphitic and very nice stacked-cup-type nanofibers with no free-CVD thickened layer were produced. TEM-EDS analysis confirms that this type of stacked-cup carbon nanofiber is produced only with a partially molten catalyst and methane as hydrocarbon source. In fact, very few fibers have either a particle tip at the end or trapped metal particle inside the wide hollow core of this type of produced carbon material.     

Control of diameter distribution of single-walled carbon nanotubes using the zeolite-CCVD method at atmospheric pressure

Single-walled carbon nanotubes (SWCNTs) have been synthesized on zeolite powder with Fe/Co catalysts by a catalytic chemical alcohol-vapor deposition (CCAVD). We have first used a cold wall reactor at the atmospheric pressure, the system having been modified for the zeolite-CCAVD specifications by the use of radio-frequency heating. The G/D ratio (∼25), estimated by analysis of Raman spectroscopy, obtained here is equivalent to that by the conventional CCAVD method under reduced pressure, indicating the high purity of the present specimen. The estimated diameter distributions of the SWCNTs obtained at synthesis temperatures of 900, 1000 °C and constant ethanol temperature of 0 °C are 0.9-1.8 and 1.2-2.2 nm, respectively, whereas that of synthesized at synthesis temperature of 900 °C and ethanol temperatures of 40 °C ranges form 0.8 to 1.4 nm. The diameter distribution shifts towards larger diameters as the synthesis temperature is increased and the carbon supply rate (ethanol temperature) decreases, from which we suggest a selective growth model due to a competition between deposition and etching of carbon atoms.     

Growing pillars of densely packed carbon nanotubes on Ni-coated silica

We report the growth of pillar-like cylindrical structures consisting of densely packed and vertically aligned multiwalled carbon nanotubes by exposing Ni-coated oxidized-Si (001) substrates to a xylene-ferrocene mixture. The nanotube pillars have a diameter between 10 and 100 μm, and lengths of several tens of micrometres. Formation of circular microcracks in the film allows ferrocene and xylene molecules to reach the underlying SiO₂ layer where pillars nucleate and grow out of the plane of the film surface. The nanotube pillars are attractive for applications such as energy storage, electrodes, and composite reinforcements.