Temperature gradient chemical vapor deposition of vertically aligned carbon nanotubes

We present temperature gradient chemical vapor deposition (TG CVD) for producing vertically aligned (VA-) carbon nanotubes (CNTs). Independent heaters on the gas inlet and catalyst substrate sides of a cold-wall, vertical CVD reactor can modulate the gas temperature gradient to lead to controlled thermal histories of acetylene precursor. Our growth results reveal that such a precursor thermal history can play a significant role in the growth and structural features of the resultant VA-CNTs. We find several gas thermal zones particularly important to the VA-CNT growth by evaluating the precursor dwell time in different zones. Thermal treatment of the acetylene precursor at 600–700 °C is found crucial for the synthesis of VA-CNTs. When this thermal zone is conjoined in particular with a zone >700 °C, efficient growths of single-walled and double-walled VA-CNTs can be achieved. These gas thermal zones can contribute to VA-CNT growths by mixing various secondary hydrocarbons with acetylene, corroborated by the results of our reacting flow simulation. Our findings emphasize the influence of gas-phase reactions on the VA-CNT growth and suggest that our TG CVD approach can be practically utilized to modulate complex gas-phase phenomena for the controlled growth of VA-CNTs.     

Tailoring carbon nanotube/matrix interface to optimize mechanical properties of multiscale composites

Pyrolytic carbon (PyC) was deposited on surfaces of carbon nanotubes (CNT) which were grown on carbon fibers to optimize the interfacial bonding between CNT/Matrix. The PyC protected CNT effectively and weakened CNT/Matrix interfacial strength, leading to long pull-out of CNT compared to brittle fracture of uncoated CNT. The well-protected CNT have more effective contributions to the improvement of mechanical properties. A “fiber-PyC/SiC-(CNT + PyC)-(CNT + SiC)” structure was formed using this process.     

A study of the deactivation of low loading Ni/Al2O3 steam reforming catalyst by tetrahydrothiophene

The steam reforming (SR) of ethanol/phenol mixture (168 g_TOT/N m³, ethanol:phenol 2:1 mol, GHSV = 54,000 h^− 1), assumed as a model for tar mixtures, has been studied over a 5% Ni/Al₂O₃ catalyst (155 m²/g), in the presence and in the absence of 210 ppm tetrahydrothiophene (THT) as a sulphur containing contaminant. The sulphidation of the catalyst by THT has been studied by IR spectroscopy. Infrared spectra of CO adsorbed at low temperature over the oxidized, the reduced and the sulphurized catalyst have also been recorded. The catalyst acts as a bifunctional one, with the behaviour attributed to the uncovered support (alumina modified by nickel ions) at 773 K (dehydration of ethanol to ethylene, dehydrogenation to acetaldehyde and alkylation of phenol with ethanol) that fully disappears at 973 K when steam reforming occurs very selectively. By lowering back the reaction temperature, the support behaviour reappears. THT poisons selectively the Ni component, thus causing the appearance of the support behaviour also at 973 K. IR experiments show that THT deposes sulphur at the catalyst surface with the production of gas-phase 1,3-butadiene, thus converting the catalyst into a “sulphided” SR-inactive state. The steam reforming activity of the poisoned catalyst progressively reappears upon feeding back S-free feed at 973 K. IR study suggests that steam “cleans” the catalyst surface by sulphur, generating a “disordered” surface with dispersed Ni^2 + and Ni⁰ species, that could slowly re-approach the initial active state.     

Alcohol-assisted rapid growth of vertically aligned carbon nanotube arrays

Millimeter-to-centimeter scale vertically aligned carbon nanotube (VACNT) arrays are widely studied because of their immense potential in a range of applications. Catalyst control during chemical vapor deposition (CVD) is key to maintain the sustained growth of VACNT arrays. Herein, we achieved ultrafast growth of VACNT arrays using Fe/Al₂O₃ catalysts by ethanol-assisted two-zone CVD. One zone was set at temperatures above 850 °C to pyrolyze the carbon source and the other zone was set at 760 °C for VACNT deposition. By tuning synthesis parameters, up to 7 mm long VACNT arrays could be grown within 45 min, with a maximal growth rate of ∼280 μm/min. Our study indicates that the introduction of alcohol vapor and separation of growth zones from the carbon decomposition zone help reduce catalyst particle deactivation and accelerate the carbon source pyrolysis, leading to the promotion of VACNT array growth. We also observed that the catalyst film thickness did not significantly affect the CNT growth rate and microstructures under the conditions of our study. Additionally, the ultralong CNTs showed better processability with less structural deformation when exposed to solvent and polymer solutions. Our results demonstrate significant progress towards commercial production and application of VACNT arrays.     

Low temperature growth mechanisms of vertically aligned carbon nanofibers and nanotubes by radio frequency-plasma enhanced chemical vapor deposition

Using radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD), carbon nanofibers (CNFs) and carbon nanotubes (CNTs) were synthesized at low temperature. Base growth vertical turbostratic CNFs were grown using a sputtered 8 nm Ni thin film catalyst on Si substrates at 140 °C. Tip growth vertical platelet nanofibers were grown using a Ni nanocatalyst in 8 nm Ni films on TiN/Si at 180 °C. Using a Ni catalyst on glass substrate at 180 °C a transformation of the structure from CNFs to CNTs was observed. By adding hydrogen, tip growth vertical multi-walled carbon nanotubes were produced at 180 °C using FeNi nanocatalyst in 8 nm FeNi films on glass substrates. Compared to the most widely used thermal CVD method, in which the synthesis temperature was 400–850 °C, RF-PECVD had a huge advantage in low temperature growth and control of other deposition parameters. Despite significant progress in CNT synthesis by PECVD, the low temperature growth mechanisms are not clearly understood. Here, low temperature growth mechanisms of CNFs and CNTs in RF-PECVD are discussed based on plasma physics and chemistry, catalyst, substrate characteristics, temperature, and type of gas.     

Epitaxial growth of large-area single-layer graphene over Cu(1 1 1)/sapphire by atmospheric pressure CVD

We report the atmospheric pressure chemical vapor deposition (CVD) growth of single-layer graphene over a crystalline Cu(1 1 1) film heteroepitaxially deposited on c-plane sapphire. Orientation-controlled, epitaxial single-layer graphene is achieved over the Cu(1 1 1) film on sapphire, while a polycrystalline Cu film deposited on a Si wafer gives non-uniform graphene with multi-layer flakes. Moreover, the CVD temperature is found to affect the quality and orientation of graphene grown on the Cu/sapphire substrates. The CVD growth at 1000 °C gives high-quality epitaxial single-layer graphene whose orientation of hexagonal lattice matches with the Cu(1 1 1) lattice which is determined by the sapphire’s crystallographic direction. At lower CVD temperature of 900 °C, low-quality graphene with enhanced Raman D band is obtained, and it showed two different orientations of the hexagonal lattice; one matches with the Cu lattice and another rotated by 30°. Carbon isotope-labeling experiment indicates rapid exchange of the surface-adsorbed and gas-supplied carbon atoms at the higher temperature, resulting in the highly crystallized graphene with energetically most stable orientation consistent with the underlying Cu(1 1 1) lattice.     

Effects of reduction process and carbon nanotube content on the supercapacitive performance of flexible graphene oxide papers

The effects of the reduction process and carbon nanotube (CNT) content on the supercapacitive behavior of electrodes made from flexible, binder-free thick graphene oxide (GO) papers are studied. It is found that the supercapacitive performance depends on several factors, including the presence of oxygenated functional groups after reduction, the interlayer spacing of the GO papers and their wettability with electrolyte. A moderate reduction of GO papers using hydrazine or annealing at a low temperature of 220 °C in air is proven to be more beneficial to achieve a high capacitance than the heavy reduction using a hydrazine vapor or a high temperature thermal treatment. The addition of a small amount of CNT, typically 12.5 wt.%, to form thick GO/CNT sandwich papers gives rise to an excellent specific capacitance of 151 F g^?1 at a current density of 0.5 A g^?1, as well as a retention ratio of 86% of the initial value after 6000 charge/discharge cycles at 5 A g^?1. These improvements arise from the synergistic effects of the increased electronic conductivity and effective surface area associated with large electrochemical active sites due to the presence of intercalated CNT.     

Diamond synthesis by plasma chemical vapor deposition in liquid and gas

The characteristics of diamond synthesis by 2.45 GHz microwave plasma chemical vapor deposition (CVD) under pressures greater than atmospheric pressure were investigated. The deposits on Si substrates were identified by scanning electron microscopy and Raman spectroscopy. The growth rate of diamond was found to be 250 μm/h at 300 kPa, which is ten times greater than that of the conventional low-pressure CVD method. In order to make high-speed deposition of diamond effective, the diamond growth rates for gas-phase microwave plasma CVD were compared to those from the in-liquid plasma CVD method. The growth rate was found to increase as system pressure increased, displaying the same tendency of that in-liquid plasma CVD. The amounts of input microwave energy per unit volume of diamond in the gas-phase and in-liquid plasma CVD methods were also compared. The amount of input microwave energy per unit volume of diamond was found to be 0.6 to 1 kWh/mm³.     

Self-organization of nitrogen-doped carbon nanotubes into double-helix structures

Self-organization of nitrogen-doped carbon nanotube (N-CNT) double helices was achieved by chemical vapor deposition (CVD) with Fe–Mg–Al layered double hydroxides (LDHs) as the catalyst precursor. The as-obtained N-CNT double helix exhibited a closely packed nanostructure with a catalyst flake on the tip, which connected the two CNT strands on both sides of the flake. A mechanism for the self-organization of N-CNTs into double-helix structures with a moving catalyst head is proposed. Effective carbon/nitrogen sources, high-density active catalyst nanoparticles, space confinement, and the precise chiral match between the two CNT strands are found to be crucial for the N-CNT double helix formation. The morphologies of N-CNTs can be well tuned between bamboo-like and cup-stacked structures, and a CNT/N-CNT heterojunction can be constructed by changing the carbon feedstock from C₂H₄ to CH₃CN during CVD growth. N-CNT double helices with a length of 10–36 μm, a screw pitch of 1–2 μm, a CNT diameter of 6–10 nm, and a N-content of 2.59 at.% can be synthesized on the LDH catalysts by the efficient CVD growth.     

Characterization of the near-surface gas-phase chemical environment in atmospheric-pressure plasma chemical vapor deposition of diamond

A numerical model was developed and used to study the near-surface gas-phase chemistry during atmospheric-pressure radio-frequency (RF) plasma diamond chemical vapor deposition (CVD). Model predictions of the mole fractions of CH₄, C₂H₂, C₂H₄ and C₂H₆ agree well with gas chromatograph measurements of those species over a broad range of operating conditions. The numerical model includes a two-dimensional analysis of the sampling disturbance in the thin boundary layer above the substrate, accounts for chemistry in the gas chromatography sampling line, and utilizes a reaction mechanism that is significantly revised from a previously reported version. The model is used to predict the concentrations of H, CH₃, C₂H₂ and C at the diamond growth surface. It is suggested that methyl, acetylene and atomic carbon may all contribute significantly to film deposition during atmospheric-pressure RF plasma diamond CVD. The growth mechanism used in the model is shown to predict growth rates well at moderate substrate temperatures (∼1100 to 1230 K) but less well for lower (∼1000 K) and higher (∼1300 K) temperatures. The near-surface gas-phase chemical environment in atmospheric-pressure RF plasma diamond CVD is compared with several other diamond CVD environments. Compared with these other methods the thermal plasma is predicted to produce substantially higher concentration ratios at the surface of both H/CH₃ and C₂H₂/CH₃.     

Thermodynamics and regeneration studies of CO2 adsorption on multiwalled carbon nanotubes

Multiwalled carbon nanotubes (CNTs) were fabricated and modified by 3-aminopropyl-triethoxysilane (APTS) solutions to study thermodynamics and regeneration of CO₂ adsorption from gas streams. The CO₂ adsorption capacities of CNTs and CNT(APTS) decreased with temperature indicating the exothermic nature of adsorption process while the thermodynamic analysis gave low isosteric heats of adsorption, which are typical for physical adsorption. The cyclic CO₂ adsorption on CNT(APTS) showed that the adsorbed CO₂ could be effectively desorbed via thermal treatment at 120 °C for 25 min while the adsorbed CO₂ due to physical interaction could be effectively desorbed via vacuum suction at 0.145 atm for 30 min. If a combination of thermal and vacuum desorption was conducted at 120 °C and 0.145 atm, the time for effectively desorbing CO₂ could be further shortened to 5 min. The adsorption capacities and the physicochemical properties of CNT(APTS) were preserved during 20 cycles of adsorption and regeneration. These results suggest that the CNT(APTS) can be stably employed in prolonged cyclic operation and they are thus possibly cost-effective sorbents for CO₂ capture from flue gases.     

Synthesis of bamboo-like carbon nanotubes on a copper foil by catalytic chemical vapor deposition from ethanol

Bamboo-like carbon nanotubes (CNTs) were synthesized on a copper foil by catalytic chemical vapor deposition (CVD) from ethanol. The effects of temperature (700–1000 °C) and duration (5–60 min) on the growth of CNTs were investigated. Morphology and structure of the CNTs were characterized by scanning and transmission electron microscopy and Raman spectroscopy. The yield and size of the CNTs increased with temperature. Those prepared at 700 °C had a copper droplet tip and those at 800–900 °C had a copper nanoparticle inside. An amorphous carbon film consisted of a porous and non-porous layer was observed on the surface of the copper substrate, and the CNTs were really grown from this carbon film. The thickness of the carbon film increased from 187 to 900 nm when the duration increased from 5 to 60 min. It was also found that the copper foil became porous after ethanol CVD treatment. The growth mechanism of the CNTs, carbon film and motion of copper catalyst were discussed. It is proposed that a carbon film first deposited on the top surface of the copper foil while the top surface of the copper foil partially melted and migrated across the carbon film, where CNTs formed.     

Carbon nanotube and nanofiber syntheses by the decomposition of methane on group 8–10 metal-loaded MgO catalysts

We have found that several precious metal-loaded MgO catalysts are active in the formation of carbon nanotube (CNT) by the chemical vapor deposition (CVD) of methane. The catalysts were prepared with nine metals (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt) by impregnation onto a high surface area MgO. CNT synthesis was carried out in the temperature range from 600 °C to 1000 °C after reduction with H₂ at 800 °C.The amount of carbon deposited and crystallinity in the produced CNT on nine metals showed interesting tendencies: (i) The amount of carbon formed increased in the following transition series metals: first < second < third row transition elements, and (ii) the index of crystallinity I_G/I_D in Raman-bands of the CNTs decreased in the following order: 8 > 9 > 10 in the Periodic Table. Group 8 and 9 metals produced tube type fibers composed of the graphite layers arranged parallel to the fiber axis. On the other hand, carbon nanofibers (CNFs) grown on group 10 metals had herringbone type graphene sheets.     

A novel ultra-light reticulated SiC foam with hollow skeleton

In this study, ultra-light reticulated SiC foam (SF) with hollow skeleton was prepared by applying chemical vapor deposition technique to deposit SiC layer on carbon foam (CF) skeleton, followed by high temperature oxidation of CF. The microstructures of materials were examined by SEM and SF samples show higher specific surface area (349 ± 13 m²/g), initial oxidation temperature (1000 °C) and compressive stress (0.6 MPa) than CF. The compression test results show that the compressive strength of SF increased with the CVD time. While the compressive strength decreased significantly, when the CVD temperature reached 1200 °C. Keeping in view superior observed related characteristics, the prepared SF with special structures was anticipated to be suitable for catalysis, energy storage or membrane science.     

Very large cathode current and long term stability of vacuum sealed tubes with engrafted-carbon-nanotube emitters

For the future commercial applications of carbon nanotubes (CNTs) in high power vacuum microwave amplifiers or compact X-ray tubes, we have attempted to fabricate engrafted CNT field emitters on a metallic substrate using both screen printing and chemical vapor deposition. Cobalt nano-grains are doped in the printed CNT paste and act as the catalyst for the engrafted growth of CNTs by the cold wall chemical vapor deposition. Stable cathode current (~ 30 mA) from a small area (~ 1.5 mm²) of engrafted CNT emitters was measured in a vacuum-sealed diode tube. High current density (> 1.6A/cm²) has also gotten in the vacuum sealed tube in which the emitters spread about 0.78 mm² after an aging process that lasts more than 12 h in DC mode with the water cooling of the anode.     

In situ growth of carbon nanotubes in alumina–zirconia nanocomposite matrix prepared by solution combustion method

In this work successful synthesis of multiwall carbon nanotube (CNT) using solution combustion and chemical vapor deposition (CVD) methods are reported. Ceramic nanocomposite samples of (Al_2?xFe_xO₃)–(y)ZrO₂ formula with x = 0.017, 0.034 and 0.17 and y = 0.15 were initially prepared. These were then subjected to CVD process during which the in situ reduction of iron oxide to metallic iron (Fe/Fe₃C) phase/s provided the necessary catalyst for the CNT formation. The formation of long flexible filaments with a smooth and regular surface bridging between alumina–zirconia (AZ) grains could be detected. The diameters of the formed filaments were in the range of ～70 to ～320 nm and length of the order of some tens of micrometers. However, transmission electron microscope (TEM) examinations also revealed the existence of small amounts of Bamboo-like carbon along with more or less straight CNTs. This could be related to the lack of strong interactions between the metallic iron phase/s and the nanocomposite support.     

High-current field emission of point-type carbon nanotube emitters on Ni-coated metal wires

The fabrication and field emission characteristics are reported for point-type carbon nanotube (CNT) emitters formed by transferring a CNT film onto a Ni-coated Cu wire with a diameter of 1.24 mm. A Ni layer plays a role in enhancing the adhesion of CNTs to the substrate and improving their field emission characteristics. On firing at 400 °C, CNTs appear to directly bonded to a Ni layer. With a Ni layer introduced, a turn-on electric field of CNT emitters decreases from 1.73 to 0.81 V/μm by firing. The CNT film on the Ni-coated wire produces a high emission current density of 667 mA/cm² at quite a low electric field of 2.87 V/μm. This CNT film shows no degradation of emission current over 40 h for a current density of 60 mA/cm² at electric field of 6.7 V/μm. X-ray imaging of a printed circuit board with fine features is demonstrated by using our point-type CNT emitters.     

Long term oxidation behaviour of liquid phase pressureless sintered SiC–AlN ceramics obtained without powder bed

This paper describes the results of the investigation of the oxidation behaviour of 50 wt.% SiC–50 wt.%AlN composites obtained by means of pressureless sintering with Y₂O₃ as sintering-aid and without using a powder bed to limit the weight loss. Sintered bodies show high density (>98%T.D.) and the microstructure reveals a matrix composed by SiC–AlN solid solution with Y₁₀Al₂Si₃O₁₈N₄ as grain boundary phase. This material exhibits a parabolic oxidation kinetic in the temperature range 1200–1500 °C. Long term oxidation (200 h) induces the formation of a protective oxide scale, composed by mullite, cristobalite and yttrium disilicide, up to 1400 °C. At higher temperature (1500 °C), SiC oxidation behaviour changes from “passive” to “active” and the oxide layer consequently shows porosity due to the formation of gaseous species like SiO and CO.     

Synthesis,processing, characterization,and applications of red mud/carbon nanotube composites

In the present study, red mud/carbon nanotube (RM/CNT) composites were synthesized by decomposition of hydrocarbon gas using the chemical vapor deposition (CVD) method. Red mud, a by-product of the Bayer process of alumina production, was used as a catalyst in this process. Synthesis was done at different growth temperatures (600, 700, 750 and 850 °C). The microstructure and morphology of the synthesized nanocomposite was characterized by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), Simultaneous Thermal Analysis (STA), and Raman spectra analysis.The prepared nanocomposite, which was prepared at 750 °C, was used for Pb (II) adsorption. The adsorption characteristics of the nanocomposite for Pb (II) removal were investigated as a function of pH, adsorbent dose, and contact time. The best result was obtained with 0.05 g of adsorbent at a pH=5 with 40 min of contact time. The results proved that RM/CNT composites are a good adsorbent for lead ions in comparison with unprocessed RM or pristine CNTs.     

Synthesis of hollow carbon nano-onions and their use for electrochemical hydrogen storage

In this study, we report an efficient method for synthesis of well-graphitized hollow carbon nano-onions (CNOs). CNOs were firstly fabricated by chemical vapor deposition (CVD) method at 850 °C using an Fe–Ni alloy catalyst with diameters of 10–15 nm. Then hollow CNOs were obtained by annealing as-prepared CNOs at 1100 °C for 3 h. It is found that during the CVD growth, the presence of nickel retards the deactivation of Fe–Ni–C austenite, providing the possibility for the growth of up to two hollow CNOs from each alloy particle. The subsequent high-temperature annealing led to the escaping of the Fe–Ni alloy from the graphitic layers, and the re-catalysis of precipitation and graphitization of the carbon atoms previously dissolved in the alloy particle (Fe_0.64Ni_0.36) to form hollow CNOs. The hollow CNOs exhibit good performance as materials for electrochemical hydrogen storage, with a discharge capacity of 481.6 mAh/g under a current density of 500 mA/g, corresponding to a hydrogen storage capacity of 1.76 wt.%. Our results demonstrate that the hollow CNOs are promising materials as a storage medium for hydrogen as a fuel source.