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.     

Image analysis characterization of multi-walled carbon nanotubes

An original image analysis method is presented to characterize multi-walled carbon nanotubes from transmission electron microscopy images. The analysis is performed in three steps: (i) image preprocessing in order to isolate the nanotubes from the background, (ii) image segmentation, aiming at keeping only the measurable sections of nanotubes, and finally (iii) tube characteristics measurement. The measurement is based on a Lambert-like electron absorption law and is performed on the original gray level image itself. Two geometrical and one physical characteristics are determined for each tube, namely, its outer and inner radius and a linear electron absorption coefficient. The method is illustrated by comparing a pristine and an annealed carbon nanotube samples. The compaction of the tube walls during annealing is shown to result from a lowering of the external radius while the inner radius is left unchanged.     

The Fe(CO)5 catalyzed pyrolysis of pentane: carbon nanotube and carbon nanoball formation

The Fe(CO)₅ catalyzed pyrolysis of pentane was investigated. The study, performed in a quartz tube, revealed that a range of carbonaceous materials could be formed with products determined by the temperature profile in the tube, the Fe(CO)₅ content in pentane, the carrier gas flow rate, the pyrolysis temperature and the pyrolysis time. The distribution of carbonaceous products also depends on the competition of the pyrolysis of Fe(CO)₅ and pentane in the different regions of the reactor. Carbonaceous materials produced included graphite film, carbon nanotubes and carbon nanoballs. The formation of carbon nanotubes takes place in a region in the quartz tube where both the pyrolysis of both Fe(CO)₅ and pentane occur, with carbon nanotubes formed by the pyrolysis of pentane. Alignment of carbon nanotubes was found when a high Fe(CO)₅ concentration was used.     

Structure changes of single-wall carbon nanotubes and single-wall carbon nanohorns caused by heat treatment

Raman spectra and transmission electron microscope images showed that diameter enlargement of HiPco, a kind of single-wall carbon nanotube, accompanied by tube-wall corrugation was caused by heat treatment (HT) at 1000 to 1700 °C. Further enlargement accompanied by straightening of the tube walls and incorporation of carbon fragments within the tubes became obvious after HT at 1800 to 1900 °C. The transformation of some single-wall carbon nanotubes into multi-wall nanotubes was observed after HT at 2000 °C, and most single-wall tubes were transformed into multi-wall ones by HT at 2400 °C. What influence the Fe contained in the HiPco tubes had on these structure changes was unclear; similar changes were observed in single-wall carbon nanohorns that did not contain any metal. This indicates that thermally induced changes in the structure of single-wall carbon nanotubes can occur without a metal catalyst. Heat treatment increased the integrity of the nanotube-papers, and this increase may have been due to tube-tube interconnections created by HT.     

New continuous gas-phase synthesis of high purity carbon nanotubes by a thermal plasma jet

We have developed a new gas-phase synthesis technique to produce carbon nanotubes (CNTs) with a continuous process and at high temperature, by using a thermal plasma jet. A thermal plasma jet was generated by applying a direct current of 100-300 A, using Ar as the plasma gas with a flow rate of ∼6 ksccm. The temperature of the thermal plasma jet was very high (∼10⁴ K) and the velocity was very fast (∼100 m/s). Fe(CO)₅ and CO were used as a catalyst precursor and carbon source, respectively. The yield of CNTs was dramatically increased by attaching a helical extension reactor at the end of the plasma nozzle. High purity (∼80%) CNTs were produced with a continuous process by using a thermal plasma jet with helical extension reactor equipment. The number of CNT walls produced was critically affected by the hydrogen gas injected as an auxiliary plasma gas. Without hydrogen gas, single-walled carbon nanotubes whose diameter was about 1 nm were mostly produced while with hydrogen gas double-walled carbon nanotubes (about 4 nm in diameter) were predominantly produced, with small amount of 3- and 4-walled carbon nanotubes.     

Well-dispersed single-walled carbon nanotube/polyaniline composite films

Single-walled carbon nanotube (SWNT)/polyaniline (PANI) composite films with good uniformity and dispersion were prepared by electrochemical polymerization of aniline containing well-dissolved SWNTs. The results of atomic force microscopy (AFM) and UV-Vis adsorption spectroscopy show that aniline can be used to solubilize SWNTs via formation of donor-acceptor complexes. The electrochemical deposition of SWNT-aniline solutions have been investigated by cyclic voltammetry. The results show that SWNT-based aniline solutions exhibit a drastic increase in peak current within the potential scanning region. The doping effect of SWNTs on PANI films was investigated by electrochemistry and FTIR spectroscopy. The results indicate that the enhanced electroactivity and conductivity of the SWNT/PANI composite films may be due to the strong interaction between SWNTs and PANI, which facilitates the effective degree of electron delocalization.     

Electrochemical characterization of carbon nanotubes as electrode in electrochemical double-layer capacitors

Carbon nanotubes uniformly 50 nm in diameter were directly grown on graphite foil. Cyclic voltammetry (CV) shows that the carbon nanotube/graphite foil electrode has a high specific capacitance (115.7 F/g at a scan rate of 100 mV/s) and exhibits typical double-layer behavior. A rectangular-shaped CV curve persists even at a scan rate of 100 mV/s in 1.0 M H₂SO₄ aqueous solution, which suggests that the carbon nanotube electrode could be an excellent candidate as the electrode in electrochemical double-layer capacitors. In addition, the influence of the potential scan rate, aging, and the electrolyte solution on the specific capacitance of nanotube electrodes was also studied.     

High-throughput screening for carbon nanotube production

High-throughput screening has been used to optimise the catalyst composition for the growth of carbon nanotubes. Microarrays of candidate catalysts were formed by printing metal salt solutions onto quartz substrates. Each solution formed a spot approximately 250 μm in diameter, with a density of 384 different spots per square inch. The solutions consisted of three components: a catalyst precursor, a substrate precursor (TEOS) and a micelle agent (P85). The printed substrates were calcined and then reacted in methane and hydrogen. The reacted substrates were studied using Raman spectroscopy and electron microscopy. The effect of varying the relative amounts of the three components was investigated. Analysis showed that bamboo nanotubes were grown when the metal to silicon ratio was high and small diameter nanotubes, including some single-walled tubes, were grown when the ratio was low.     

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.     

Multi-walled carbon nanotubes on amorphous carbon films

Nanotubular structures composed of layered graphite sheets or other layered materials have been studied intensely by both scanning and transmission electron microscopy. In this paper, we will show how graphite structures, that are inherent to the production process of the amorphous carbon support films, used for both SEM and TEM studies can be easily mistaken for the actual sample structures. We will further report that these artifacts appear in both commercial as well as homemade holey carbon support films on copper grids, and suggest that to successfully study the “real” nanotubular structures only support films made from materials other than carbon should be used.     

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.     

The ruthenium catalysed synthesis of carbon nanostructures

Reaction of propene over silica-, alumina- or titania-supported Ru catalysts in a tubular quartz flow reactor at moderate temperatures (500-700 °C) and atmospheric pressure produced Ru containing carbon nanofibres/tubes. TEM studies revealed that the fibres/tubes grew away from the support and contained Ru metal particles in their tips. The results indicate that sintering of the Ru during reduction with H₂, to a critical size not less than 30 nm, is required for fibre/tube formation.     

Structural study of the 0.4-nm single-walled carbon nanotubes aligned in channels of AlPO4-5 crystal

A detailed synthesis process of ultra-small single-walled carbon nanotubes (SWNTs) aligned in the channels of AlPO₄-5 zeolite single crystals is reported. The structure of such ultra-small SWNTs was analyzed by means of transmission electron microscopy (TEM) and polarized Raman scattering. TEM images showed that the diameter of the SWNTs is as small as 0.4 nm, which is the size of three possible structures: the zigzag (5,0), armchair (3,3) and chiral (4,2). The polarization dependence of the Raman lines indicates that the structures of the nanotubes are dominated by the zigzag (5,0) form. Line-shape analysis of the tangential A_1g Raman mode shows that the SWNTs are metallic with a finite electronic density-of-state near the Fermi energy level, which is in agreement with the band structural calculation using local-density function approximation (LDA) as well as the electric transport measurement.