Carbon nanotubes and onions from carbon monoxide using Ni(acac)2 and Cu(acac)2 as catalyst precursors

New catalyst precursors (copper and nickel acetylacetonates) have been used successfully for the synthesis of carbon nanotubes and onion particles from carbon monoxide. Catalyst nanoparticles and carbon products were produced by metal-organic precursor vapour decomposition and catalytic disproportionation of carbon monoxide in a laminar flow reactor at temperatures between 705 and 1216 °C. Carbon nanotubes (CNTs) were formed in the presence of nickel particles at 923-1216 °C. The CNTs were single-walled, 1-3 nm in diameter and up to 90 nm long. Hollow carbon onion particles (COPs) were produced in the presence of copper particles at 1216 °C. The COPs were from 5 to 30 nm in diameter and consisted of several concentric carbon layers surrounding a hollow core. The results of computational fluid dynamics calculations to determine the temperature and velocity profiles and mixing conditions of the species in the reactor are presented. The mechanisms for the formation of both CNTs and COPs are discussed on the basis of the experimental and computational results.     

Synthesis of carbon nanocapsules and carbon nanotubes by an acetylene flame method

The fabrication of carbon nanocapsules and carbon nanotubes (CNTs) using an acetylene flame method was investigated. Carbon nanocapsules, a graphitic structure of nanoparticles with a hollow core, were synthesized using catalyst-free acetylene flames while CNTs were formed with the presence of cobalt-based catalysts in addition to acetylene flames. When the synthesis of these materials was carried out, the results showed that a massive amount of high-purity carbon nanocapsules with a particle size in the range of 15-30 nm can be produced with the acetylene flame method. The CNTs produced were multi-walled carbon nanotubes measuring a few micrometers in length and 20-30 nm in diameter. The acetylene flame method holds great potential for the cost-effective production of CNTs as well as carbon nanocapsules.     

The control of diameter of carbon nanotube over Co-loaded Zeolite Y

This study examined carbon nanotubes (CNTs) with various outer diameters produced by the catalytic decomposition of acetylene over Co-loaded zeolite Y. The CNTs were grown at differed reaction temperatures, reaction times, and acetylene concentrations. In addition, the effect of the amount of Co dispersed over zeolite Y used as a support was determined. The shape and diameter of the synthesized CNT were identified by SEM and TEM analyzers. As a result, CNTs with various outer diameters were synthesized successfully. The average outer diameter of the synthesized CNTs increased with increasing amount of Co dispersed over zeolite Y regardless of the reaction temperature and reaction time. The outer diameter did not change with acetylene concentration, and the acetylene concentration was fixed to 10 cm³/min. Most of the CNT had large surface areas, >400 m²/g. The surface area increased with increasing outer diameter of the CNT until the outer diameter reached 60 nm but decreased with further increases in outer diameter.     

The effect of catalyst calcination temperature on the diameter of carbon nanotubes synthesized by the decomposition of methane

The effect of catalyst calcination temperature on the uniformity of carbon nanotubes (CNTs) diameter synthesized by the decomposition of methane was studied. The catalysts used were CoO-MoO/Al₂O₃ without prior reduction in hydrogen. The results show that the catalyst calcination temperature greatly affects the uniformity of the diameter. The CNTs obtained from CoO-MoO/Al₂O₃ catalysts, calcined at 300 °C, 450 °C, 600 °C, and 700 °C had diameters of 13.4 ± 8.4, 12.6 ± 5.1, 10.7 ± 3.2, and 9.0 ± 1.4 nm, respectively, showing that an increase in catalyst calcination temperature produces a smaller diameter and narrower diameter distribution. The catalyst calcined at 750 °C was inactive in methane decomposition. Transmission electron microscopy (TEM) studies showed that CNTs grown on the catalyst calcined at 700 °C were of uniform diameter and formed a dense interwoven covering. High-resolution TEM shows that these CNTs had walls of highly graphitized parallel graphenes.     

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.     

Easy synthesis of a nanostructured hybrid array consisting of gold nanoparticles and carbon nanotubes

A nanostructured hybrid consisting of a high-density and uniform assembly of gold nanoparticles (AuNPs) on carbon nanotubes (CNTs) was prepared using easy methods. The pyrolysis of iron(II) phthalocyanine (FePc) on a Si substrate under an atmosphere of hydrogen/argon was used to produce multiwalled carbon nanotubes (MWCNTs) with 12 nm in diameter and 4 μm in length. Then, Au nanocolloid solution, which contained dodecanethiol-capped Au nanoparticles synthesized by solution chemical method, was deposited on the synthesized CNT array and heated at 300 °C for 1 h under Ar. The synthesis temperature of CNT governs the AuNP-CNT hybrid structure and surface nitrogen concentration from decomposition of FePC. CNTs synthesized at 800 °C exhibit the finest particle size and most homogeneous dispersity of assembled AuNPs in comparison to hybrids whose CNTs are synthesized at other temperatures. These features are considered to correlate with the surface nature of the grown CNT; good dispersity of AuNPs on CNT results from interaction between the thiolate molecules capped on the AuNPs and the N atoms doped into the grown CNT. Assembling AuNPs to CNT contributes the electrical conductivity enhancement of the CNT hybrid array.     

Formation of carbon nanotubes through ethylene decomposition over supported Pt catalysts and silica-coated Pt catalysts

Ethylene decomposition was performed over supported Pt catalysts to fabricate composites of Pt metal nanoparticles and carbon nanotubes (CNTs). All supported Pt catalysts (Pt/carbon black, Pt/CNT, Pt/MgO, Pt/Al₂O₃ and Pt/SiO₂) showed catalytic activity for ethylene decomposition at 973 K to form CNTs. Pt metal particles were found at tips of CNTs. These results indicate that Pt metal particles have catalytic activity for growth of CNTs through hydrocarbon decomposition. A broad range (5-50 nm) of CNT diameters were formed from the use of supported Pt metal catalysts although Pt metal particles in the catalysts before ethylene decomposition were relatively uniform in size (2-5 nm). These results imply that Pt metal particles in the catalysts aggregated during ethylene decomposition at 973 K. Aggregation of Pt metal particles in catalysts during ethylene decomposition could be suppressed by covering catalysts with silica layers that were a few nanometers thick. Silica-coated Pt catalysts showed high activity for ethylene decomposition to form CNTs with uniform diameters (8-10 nm) despite the uniform coverage of Pt metal particles with silica layers.     

Growth evolution of rapid grown aligned carbon nanotube forests without water vapor on Fe/Al2O3/SiO2/Si substrate

This paper presents the growth evolutions in terms of the structure, growth direction and density of rapid grown carbon nanotube (CNT) forests observed by scanning and transmission electron microcopies (SEM/TEM). A thermal CVD system at around 700 °C was used with a catalyst of Fe films deposited on thin alumina (Al₂O₃) supporting layers, a very fast raising time to the growth temperature below 25 °C/s, and a carbon source gas of acetylene diluted with hydrogen and nitrogen without water vapor. Activity of Fe catalyst nanoparticles was maintained for 5 min during CVD process, and it results in CNT forests with heights up to 0.6 mm. SEM images suggest that the disorder in CNT alignment at the initial stage of CNTs plays a critical role in the formation of continuous CNT growth. Also, the prolonged heating process leads to increased disorder in CNT alignment that may be due to the oxidation process occurring at the Fe nanoparticles. TEM images revealed that both double- and few-walled CNTs with diameters of 5-7 nm were obtained and the CNT density was controlled by thickness of Fe catalytic layer. The number of experiments at the same conditions showed a very good repeatability and reproducibility of rapid grown CNT forests.     

Carbon nanotubes coated by carbon nanoparticles of turbostratic stacked graphenes

Carbon nanotubes (CNTs) decorated by a high density of carbon nanoparticles of turbostratic graphene stacks have been fabricated by low energy hydrocarbon ion deposition at 700 °C. Transmission and scanning electron microscopy show that the carbon particles of turbostratic graphene stacks extend from the nanotube surface. The diameter of CNTs decreases with the increasing percentage of hydrogen in the gas phase. Raman spectroscopy indicates that the formation of carbon nanoparticles of turbostratic graphene stacks results from the high temperature used in the experiment. Meanwhile, Raman spectroscopy and high resolution transmission electron microscopy indicate that the carbon nanoparticle degree of crystallinity is lower with increasing hydrogen content in the gas phase due to the etching effect of hydrogen ions.     

Diameter control of carbon nanotubes by changing the concentration of catalytic metal ion solutions

We have developed a simple new method to control the diameter of carbon nanotubes (CNTs) using catalytic nanoparticle arrays fabricated by filling the pores of well-ordered porous anodic aluminum oxide (AAO) templates with a metal ion solution. Fe ion solution was used to fill the pores in which Co had been deposited electrochemically, and then the template was dried naturally on a magnet. After this process, the pores were widened in NaOH solution. Well-graphitized multi-walled CNTs were grown from almost all the pores and were very long in length and homogeneous in diameter. We were able to control the diameter of CNTs, simply, by changing the concentration of iron ion solution. For example, the average outer diameters of the CNTs are 7 ± 1.5, 13 ± 1, and 17 ± 1 nm when the concentrations of Fe ion in their mother solutions were 1.0 × 10⁻³, 3.0 × 10⁻³, and 6.0 × 10⁻³ M, respectively. The inner diameters of these CNTs corresponded to the calculated diameters of Fe nanoparticles by assuming that all Fe ions contained in each pore are reduced to a single nanoparticle. This means that homogeneous nanoparticles are made in each pore. Our new method could be used to fabricate homogeneous nanoparticles from most metal ion solutions.     

Effects of bimetallic catalyst composition and growth parameters on the growth density and diameter of carbon nanotubes

Multi-wall carbon nanotubes (MWNTs) were synthesized by catalytic decomposition of acetylene over Fe, Ni and Fe-Ni bimetallic catalysts supported on alumina under various controlled conditions. The growth density and diameter of CNTs were markedly dependent on the activation time of catalysts in H₂ atmosphere, reaction time, reaction temperature, flow rate of acetylene, and catalyst composition. Bimetallic catalysts were apt to produce narrower diameter of CNTs than single metal catalysts. For the growth of CNTs at 600 ‡C under 10/100 seem flow of C₂H₂/H₂ mixture, the narrowest diameter about 20 nm was observed at the reaction time of 1 h for 20Fe : 20Ni : 60Al₂O₃ catalyst, but at that of 1.5 h for 10Fe : 30Ni : 60Al₂O₃ catalyst. It was considered that the diameter and density of CNTs decreased with the increase of the growth time mainly due to hydrogen etching. The growth of CNTs followed the tip growth mode.     

Synthesis of helical carbon nanotubes, worm-like carbon nanotubes and nanocoils at 450 °C and their magnetic properties

High purity (99.21 wt.%) helical carbon nanotubes (HCNTs) were synthesized in large quantity over Fe nanoparticles (fabricated using a coprecipitation/hydrogen reduction method) by acetylene decomposition at 450 °C. Field-emission and transmission electron microscope images reveal that the selectivity to HCNTs (with two or three coiled nanotubes connected to a catalyst nanoparticle) is up to ca. 93%. The yield of HCNTs (as defined by the equation: ) is ca. 7474% in a run of 6 h, higher than any of those reported in the literature. If hydrogen was introduced during acetylene decomposition for ca. 30 min, the HCNTs mainly consisted of two coiled tubes connected to a catalyst nanoparticle, and carbon nanocoils (CNCs) of different structures were generated. If hydrogen was present throughout acetylene decomposition, worm-like carbon nanotubes (CNTs) as well as CNCs were produced in large quantities. Because the HCNTs and worm-like CNTs are attached to Fe nanoparticles, the nanomaterials are high in magnetization.     

Preparation of short and water-dispersible carbon nanotubes by solid-state cutting

A new approach is reported to cut conventionally long (>10 μm) and entangled carbon nanotubes (CNTs) to those with short lengths (∼300 nm) and excellent dispersion in water and ethanol. This was achieved by depositing Fe₂O₃ nanoparticles on CNTs first and then inducing a chemical reaction between them at a temperature of 850 °C. The consumption of carbon during the reduction of Fe₂O₃ with CNTs was responsible for the cutting. Fourier transform infrared, X-ray photoelectron and Raman spectroscopies demonstrated that the cutting had induced little impact on the intrinsic graphitic structure. The present cutting approach based on the localized reaction in solid-state has advantages of producing short CNTs with a narrow length distribution, high dispersion, and a low material loss over previous ones based on gaseous or liquid-state reactions, and would have wide applications.     

Catalytic synthesis of carbon nanotubes using Ni- and Co-doped calcium tartrates

Calcium tartrate doped with Ni and/or Co has been used as a catalyst source in the chemical vapor deposition synthesis of carbon nanotubes (CNTs). Thermolysis of doped calcium tartrate in an inert atmosphere was shown to yield Ni, Co or Ni-Co nanoparticles ∼6 nm in diameter dispersed in a calcium oxide matrix. The CNT synthesis was carried out by ethanol vapor decomposition at 800 °C. The structure of the products was characterized by transmission electron microscopy and Raman spectroscopy. It was found that Ni nanoparticles embedded in CaO provide the narrowest diameter distribution of CNTs, while the bimetallic Ni-Co catalyst allows the formation of the thinnest CNTs with the outer diameter of ∼2 nm. This type of CNT is more likely to be responsible for the lowest value of the turn-on field (∼1.8 V/μm) for the emission current detected for the latter sample.     

Analysis of effluent gases during the CCVD growth of multi-wall carbon nanotubes from acetylene

Catalytic chemical vapor deposition was used to grow multi-walled carbon nanotubes on a Fe:Co:CaCO₃ catalyst from acetylene. The influent and effluent gases were analyzed by gas chromatography and mass spectrometry at different time intervals during the nanotubes growth process in order to better understand and optimize the overall reaction. A large number of byproducts were identified and it was found that the number and the level for some of the carbon byproducts significantly increased over time. The CaCO₃ catalytic support thermally decomposed into CaO and CO₂ resulting in a mixture of two catalysts for growing the nanotubes, which were found to have outer diameters belonging to two main groups 8-35 nm and 40-60 nm, respectively.     

Electroreduction of oxygen on Pt nanoparticle/carbon nanotube nanocomposites in acid and alkaline solutions

The kinetics of O₂ reduction on novel electrocatalyst materials deposited on carbon substrates were studied in 0.5 M H₂SO₄ and in 0.1 M NaOH solutions using the rotating disk electrode (RDE) technique. Pt nanoparticles (PtNP) supported on single-walled (PtNP/SWCNT) and multi-walled carbon nanotubes (PtNP/MWCNT) were prepared using two different synthetic routes. Before use, the CNTs were cleaned to minimize the presence of metal impurities coming from the catalyst used in the synthesis of this material, which can interfere in the electrochemical response of the supported Pt nanoparticles. The composite catalyst samples were characterised by transmission electron microscopy (TEM) showing a good dispersion of the particles at the surface of the carbon support and an average Pt particle size of 2.4 ± 0.7 nm in the case of Pt/CNTs prepared in the presence of citrate and of 3.8 ± 1.1 nm for Pt/CNTs prepared in microemulsion. The values of specific activity (SA) and other kinetic parameters were determined from the Tafel plots taking into account the real electroactive area of each electrode. The electrodes exhibited a relatively high electrocatalytic activity for the four-electron oxygen reduction reaction to water.     

Modified carbon nanotubes: an effective way to selective attachment of gold nanoparticles

Through various modifications of carbon nanotubes (CNTs), gold nanoparticles were selectively attached to the nanotube and the locations of functional groups were further confirmed. Using cationic polyethyleneamine or anionic citric acid as the dispersant, the surface properties of CNTs could be changed to yield a basic or acidic surface. By electrostatic interaction, the CNTs could be successfully coated with about 10 nm gold nanoparticles. After heat treatment in NH₃, about 1–2 nm gold nanocluster-filled nanotubes were achieved. This shows that the heat treatment with NH₃ could make CNTs open-ended and generate functional basic groups on the inner wall of the nanotubes.     

Synthesis of diameter-controlled carbon nanotubes using centrifugally classified nanoparticle catalysts

Iron-based nanoparticles, centrifugally classified by size, with variation of subnanometer order, have been used for the growth of diameter-controlled carbon nanotubes (CNTs) for the first time via catalytic chemical vapor deposition. The centrifugal classification of nanoparticles is facilitated by fractional precipitations through the sequential addition of ethanol to a hexane solution containing the nanoparticles. Three different nanoparticle sizes were obtained, which have average diameters and standard deviations of 3.9 ± 0.8 nm, 3.3 ± 0.6 nm, and 2.8 ± 0.4 nm. By the classification process of nanoparticles, the standard deviation of the average diameter of the fractionated nanoparticles decreased by around one half of that of the as-synthesized nanoparticles. In addition, we demonstrate a technique for estimating the average diameter of each classified nanoparticle using conventional low-angle X-ray diffraction, without the need for time-consuming TEM observation and analysis. From the three classified nanoparticle sizes, with average diameters of 2.8, 3.3, and 3.9 nm, CNTs with average diameters of 3.1, 3.6, and 4.5 nm were obtained by changing growth temperatures, respectively. Therefore, centrifugally classified nanoparticles are one of the most promising ‘seeds’ for use in the diameter-selective growth of CNTs.     

Synthesis of narrow-diameter carbon nanotubes from acetylene decomposition over an iron-nickel catalyst supported on alumina

Carbon nanotubes (CNTs) were synthesized by the catalytic decomposition of acetylene over 40Fe:60Al₂O₃, 40Ni:60Al₂O₃ and 20Fe:20Ni:60Al₂O₃ catalysts. High density CNTs of 20 nm diameter were grown over the 20Fe:20Ni:60Al₂O₃ catalyst, whereas low growth density CNTs of 40 and 50 nm diameter were found over 40Fe:60Al₂O₃ and 40Ni:60Al₂O₃ catalysts. Smaller catalyst particles enabled the synthesis of highly dense, long and narrow-diameter CNTs. It was found that a homogeneous dispersion of the catalyst was an essential factor in achieving high growth density. The carbon yield and the quality of CNTs increased with increasing temperature. For the 20Fe:20Ni:60Al₂O₃ catalyst, the carbon yield reached 121% after 90 min at 700 °C. The CNTs were grown according to the tip growth mode. Based on reports regarding hydrocarbon adsorption and decomposition over different faces of Ni and Fe, the growth mechanism of CNTs over the 20Fe:20Ni:60Al₂O₃ catalyst are discussed.     

Combined catalyst system for preferential growth of few-walled carbon nanotubes

The effect of a combined catalyst system on the synthesis of carbon nanotubes (CNTs) from methane (CH₄) was investigated using molybdenum trioxide (MoO₃) as a conditioning catalyst and molybdenum-doped iron supported on magnesia as the main catalysts. Without the conditioning catalyst, only single-walled CNTs with diameters smaller than 2 nm were formed on the main catalyst. With the conditioning catalyst, double-walled CNTs and few-walled CNTs with larger hollow diameters than 2 nm were produced with much higher yields. The combination of the two kinds of Mo-containing catalyst would more effectively transform CH₄ into reactive species related to the early stage of nanotube formation on the main catalyst, resulting in the increase of the yield, diameter and layer number of the CNTs.