Coiled carbon nanotubes growth via reduced-pressure catalytic chemical vapor deposition

Coiled carbon nanotubes were prepared by catalytic chemical vapor deposition (CCVD) on finely divided Co nano-particles supported on silica gel under reduced pressure and relatively low gas flow rates. The morphology and the graphitization of the coil tube, coil bend, and coil node of the coiled carbon nanotubes were examined by transmission electron microscope (TEM). The influence of pH value, reaction pressure, and flow rate of C₂H₂ on the growth of the coiled carbon nanotubes were also discussed. With the drastic reduction in the consumption of C₂H₂ and lower required pressure with the modified CCVD approach, the amount of amorphous carbon coated on the carbon nanotubes was shown to be greatly reduced. Most importantly, this method offers a preferable alternative for the efficient, environment-friendly and safer growth of coiled carbon nanotubes.     

Local chemical vapor deposition of carbon nanofibers from photoresist

The addition of nanofeatures to carbon microelectromechanical system (C-MEMS) structures would greatly increase surface area and enhance their performance in miniature batteries, super-capacitors, electrochemical and biological sensors. Negative photoresist posts were patterned on a Au/Ti contact layer by photolithography. After pyrolyzing the photoresist patterns to carbon patterns, graphitic nanofibers were observed near the contact layer. The incorporation of carbon nanofibers in C-MEMS structures via a simple pyrolysis of modified photoresist was investigated. Both experimental results considered to consist of a local chemical vapor deposition mechanism. The method represents a novel, elegant and inexpensive way to equip carbon microfeatures with nanostructures, in a process that could possibly be scaled up to the mass production of many electronic and biological devices.     

Synthesis of oriented nanotube films by chemical vapor deposition

Oriented nanotube films (20-35 μm thick) were synthesised on flat silicon substrates by chemical vapor deposition (CVD) of a gas mixture of acetylene and nitrogen. For the CVD we used metal oxide clusters formed by spin coating an iron(III) nitrate ethanol solution onto a silicon substrate and subsequent heating. The cluster density and its effects on the nanotube density were investigated as a function of the iron(III) nitrate concentration and the synthesis temperature. A high nanotube density was achieved with a high density of iron oxide clusters as nucleation centres for the growth of nanotubes. The cluster density was controlled by the iron(III) concentration of the ethanolic coating solution and by the synthesis temperature. The perpendicular orientation of the nanotubes with respect to the substrate surface is attributed to a high density of nanotubes.     

Microstructures of PAN-ACF modified by catalytic benzene deposition

Benzene pyrolysis was successfully introduced to modify the pore size distributions (PSD) of polyacrylonitrile based activated carbon fibers (PAN-ACF) into a sharp distribution, at the presence of nickel catalyst. The microstructures of samples were studied by means of nitrogen adsorption, XRD, and SEM. The nitrogen isotherms were analyzed in detail using the routine BET method, α_s plot, DR equation, Horvath-Kawazoe (HK) equation, and regularized density functional theory (DFT), by which the surface area, micropore volume, and PSD were obtained. The results showed that the pore size of PAN-ACF can be effectively narrowed by catalytic benzene deposition and the PSD showed a unimodal nature, exhibiting potential behavior as a molecular sieve. A fraction residue of catalyst located in the ultramicropores can be washed by acid, resulting in increased BET surface area and micropore volume, which can also be confirmed by XRD and SEM measurements.     

Formation of fine Fe-Ni particles for the non-supported catalytic synthesis of uniform carbon nanofibers

The morphological changes of Fe-Ni catalyst for the preparation of carbon nanofiber (CNF) were examined at 5 steps; (1) the precipitation of Fe-Ni carbonate from Fe-Ni nitrate solution, (2) the calcination of Fe-Ni carbonate into Fe-Ni oxide, (3) the reduction of Fe-Ni oxide, (4) the second reduction of Fe-Ni metal before the growth of CNF, and (5) the reaction with CO/H₂ for the growth of CNF. The Fe-Ni fine particle was formed from the Fe-Ni aggregate through the second reduction and successive CNF growth from CO/H₂. The temperature of these two steps is the most important factor which determines the size and shape of the Fe-Ni fine particle as a catalyst for CNF growth. The lower temperature of 580 °C provided hexagonal particles with very smooth surface sized around 100-200 nm which allowed the growth of platelet CNFs of the same diameter and cross-sectional shape of the formed catalyst particle. At the higher temperature of 630 °C, the Fe-Ni aggregate was found to give the very fine Fe-Ni particles by the two steps; the first step did the Fe-Ni particle sized around 100-500 nm which was successively degraded into smaller particles sized around 20-40 nm, thinner tubular CNFs growing with the contact of CO/H₂. Such smaller particles definitely originated from as-precipitated Fe-Ni carbonate through the steps. The metal particle on the top of CNF was almost exclusively composed of Fe although the catalyst particle before the growth of CNFs carried around 65% of iron and 35% of nickel. The preferential activity of Fe to CO gas may cause such the selectivity. The major role of Ni in the present reaction should be limited to provide the uniform particle of Fe. Controlling the size of the Fe-Ni particle through the reduction and reaction steps was proved to be a key factor to determine the dimension and structure of resultant CNF.     

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.     

Micromechanical testing of carbon fibers deposited by low-pressure laser-assisted chemical vapor deposition

Carbon fibers were deposited directly from ethylene by laser-assisted chemical vapor deposition. The precursor gas pressures and the incident laser powers were varied. Micro-mechanical testing was carried out using a high-precision micro-manipulator. During three-point bend testing the fibers showed an elastic response, with no residual strain upon unloading, until fracture. The fibers’ strength and Young’s modulus are reported. A model for fiber fracture is proposed based on fiber cross-section analysis. Scanning electron microscopy was used to study the fiber cross-sections and the fiber surface morphology. The mechanical properties are related to the characteristic fiber microstructure investigated by Raman spectroscopy.     

Chemical vapor deposition of pyrolytic carbon on carbon nanotubes: Part 1. Synthesis and morphology

The chemical vapor deposition of the pyrocarbon from a CH₄+H₂ mixture is investigated using nanofilamentous substrates. The process consists of growing carbon nanotubes via a catalytic process, which then are thickened by pyrolytic carbon deposition to reach diameters in the nanometer to micrometer range. A key characteristic of the experimental reactor used was the long length of its isothermal zone, preceded (and followed) by a low thermal gradient zone. This allowed us to investigate the role of the variation of the local gas phase composition, which depends on the post-cracking secondary reactions, and on the quantity and quality of the deposited carbon. The ‘time of flight’ of the reactive species was found to be a leading parameter in the pyrolytic carbon deposition process. Various nanometric and micrometric morphologies, several of which are new, were synthesised and found constituted with an association of different sub-morphologies. The various morphologies, that can be sorted following a factor of morphological complexity, were investigated by scanning electron microscopy.     

Growth of carbon nanotubes by open-air laser-induced chemical vapor deposition

Carbon nanotubes have remarkable mechanical, electronic and electrochemical properties, but the full potential for application will be realized only if the growth of high quantity and quality carbon nanotubes can be optimized and well controlled. In this study, carbon nanotubes have been successfully grown on fused quartz rods by a novel open-air laser-induced chemical vapor deposition (LCVD) technique with gold palladium nanoparticles as catalyst material. In this LCVD technique, a curtain of inert nitrogen gas was used to shield the deposition zone from the surrounding environment and allows the growth of the nanotubes to occur under open-air conditions. A 35-W continuous CO₂ laser was used as a heat source to induce a local temperature rise on the substrate surface covered with metal nanoparticles, subsequently resulting in deposition of multi-wall carbon nanotubes. The carbon nanotubes deposited in this study are derived from a precursor mixture that consists of propane and hydrogen, and are in tangled form with different diameters (10-250 nm) and structures. Raman spectroscopy, transmission and scanning electron microscopy are used to investigate the microstructure and composition of the carbon nanotubes.     

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.     

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.     

Possible tactics to improve the growth of single-walled carbon nanotubes by chemical vapor deposition

The growth time, growth mode and the method of preparing the supported catalysts play an important role in the growth of single-walled nanotubes (SWNTs). Their effects on the chemical vapor deposition (CVD) growth of SWNTs with MgO-supported catalysts were investigated in this study. It is shown that the growth rate of SWNTs was large during the initial few minutes of growth, however the quality of the tubes was low owing to the formation of many defects. Long term growth may favor the formation of tubes with high quality and high yield, but the introduction of other forms of carbon (impurities) is also unavoidable. There was a balance between the increase in yield and quality and sacrifice of the purity during growth of SWNTs. MgO-supported catalysts prepared by the co-precipitation method were found to be more effective for the synthesis of SWNTs than those prepared by the widely used impregnation method. The size and dispersion state of the catalyst were found to be crucial in enhancing the growth of SWNTs. In addition, growth on the surface of SWNTs over nanosized catalyst films was shown to be more favorable for the synthesis of tube products with higher quality, yield and purity.     

Open-air carbon coatings on fused quartz by laser-induced chemical vapor deposition

Carbon films are deposited on fused quartz substrates by CO₂ laser-induced chemical vapor deposition (LCVD) using a novel open-air coating system. The hydrocarbon precursor gases are methane, propane and butane. The deposition rates of the three hydrocarbon gases are determined by measuring the mass of the carbon film deposited at constant temperature, and validated by film thickness measurements obtained using an environmental scanning electron microscope. The results indicate that butane and propane have significant deposition at 1375-1500 K, while deposition starts at 1550 K for methane. All three hydrocarbons have an exponential increase in deposition rate. Deposition rates obtained with butane and propane show a much stronger influence of temperature compared to methane. Raman spectra of deposited carbon films indicate that the surface consists of glass-like or nanocrystalline carbon. The ratio of D-peak to G-peak Raman band intensities decreases as the deposition temperature is increased, which is possibly due to self-annealing and additional surface reaction during high temperature deposition. This finding indicates that the carbon film exhibits higher structural order at high deposition temperatures, which can significantly enhance the film’s hermeticity and mechanical properties for use as an optical fiber coating.     

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.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.