Synthesis of carbon nanotubes over gold nanoparticle supported catalysts

The synthesis of carbon nanotubes (CNTs) through the catalytic decomposition of acetylene was carried out over gold nanoparticles supported on SiO₂-Al₂O₃. Monodispersed gold nanoparticles with 1.3-1.8 nm in diameter were prepared by the liquid-phase reduction method with dodecanethiol as protective agent. The carbon products formed after acetylene decomposition consist of multi-walled carbon nanotubes with layered graphene sheets, carbon nanofilaments (CNFs), and carbon nanoparticles encapsulating gold particles. The observed CNTs have outer diameters of 13-25 nm under 850 °C. The influence of several reaction parameters, such as kind of carriers, reaction temperature, gas flow rate, was investigated to search for optimum reaction conditions. The CNTs were observed at a relatively low temperature (550 °C). The silica-alumina carrier showed higher activity for the formation of CNTs than others used in the screening test. With increasing temperature, the CNTs showed cured structures having thick diameters and inside compartments. When Au content on the support was over 5 wt.%, the gold nanoparticles coagulated to form large ones >20 nm in diameter and became encapsulated with graphene layers after decomposition of acetylene.     

Gas analysis of the CVD process for high yield growth of carbon nanotubes over metal-supported catalysts

Changes in the gas composition during the methane chemical vapor deposition growth of single- and double-walled carbon nanotubes over metal-supported MgO catalysts were investigated in an attempt to increase the nanotube yield. Monitoring the gas composition by gas chromatography as a function of the reaction time provides information on the activity and lifetime of the catalyst. The degree of methane decomposition, i.e., the C-H bond dissociation, was closely related to the nanotube yield, and the Fe-Mo binary catalyst exhibited a high activity. The effects of water vapor on the catalytic nanotube growth were also studied by introducing water vapor in the inlet gas. An appropriate amount of water prolonged the lifetime of the catalyst and increased the nanotube yield by 35%.     

Complete elimination of metal catalysts from single wall carbon nanotubes

The complete removal of entrapped metallic impurities (i.e. Ni and Co) incorporated within single wall carbon nanotubes (SWNTs) has been a long-standing issue. A sonication-mediated treatment of as-obtained SWNT soot in a 1:1 mixture of aqueous hydrofluoric and nitric acids resulted in the complete elimination of these impurities as shown by energy dispersive X-ray analysis (EDAX). Contact angle measurements indicated that the wetting of SWNTs is enhanced in the presence of HF. The presence of HNO₃ and surfactant was found essential in removing the catalyst due to SWNT etching of end-caps/defects and providing better dispersion, respectively. Moreover, Raman spectroscopy indicated that the structural purity of the SWNTs is not compromised by the HF/HNO₃ purification treatment.     

The synthesis of carbon nanotubes at low temperature via carbon suboxide disproportionation

Carbon nanotubes were synthesized via a novel route using an iron catalyst at the extremely low temperature of 180 °C. In this process, carbon suboxide was used as carbon source, which changed to freshly formed free carbon clusters through disproportionation. The carbon clusters can grow into nanotubes in the presence of Fe catalyst, which was obtained by the decomposition of iron carbonyl Fe₂(CO)₉ at 250 °C under nitrogen atmosphere. The products were characterized with XRD, TEM, HRTEM and Raman spectroscopy.     

Characterization of single wall carbon nanotubes by nonane preadsorption

The preferential blocking of the interior adsorption sites of single walled carbon nanotubes (SWNTs) by n-nonane is demonstrated. Following adsorption of n-nonane and evacuation for 24 h at 323 K, it was found that interior sites with diameters less than ∼14 Å remained filled with n-nonane, blocking the physical adsorption of N₂ on these sites at 77.3 K. We demonstrate that “nonane blocking” is a very useful technique for nanotube porosity characterization.     

Nitrogen-doped carbon nanotubes synthesized with carbon nanotubes as catalyst

Nitrogen-doped carbon (CN_x) nanotubes were synthesized with carbon nanotubes (CNTs) as catalyst by detonation-assisted chemical vapor deposition. CN_x nanotubes exhibited compartmentalized bamboo-like structure. Electron energy loss spectroscopy and elemental mapping studies indicated that the synthesized tubes contained high concentration of nitrogen (ca. 17.3 at.%), inhomogeneously distributed with an enrichment of nitrogen within the compartments. X-ray photoelectron spectroscopy analysis revealed the presence of pyridine-like N and graphitic N incorporated into the graphitic network. The catalytic activity of CNTs for CN_x nanotube growth was ascribed to the nanocurvature and opening edges of CNT tips, which adsorbed C_n/CN species and assembled them into CN_x nanotubes.     

Synthesis of carbon nanotubes on carbon fibers by means of two-step thermochemical vapor deposition

The present study aimed at development of a method for synthesizing multi-walled carbon nanotubes (CNTs) on carbon paper substrates (CP) at densities as high as those so far reported for CNTs formed on quartz substrates. Applying conditions optimized for CNTs synthesis on quartz substrates, in which CP was heated at 1073 K, being placed parallel to the flow of m-xylene/ferrocene vapor, resulted in formation of extremely few deposits on CP. Forced vapor flow through the CP greatly improved the frequency and homogeneity of deposition of the Fe-bearing nanoparticles, but these became encapsulated by carbon and deactivated. The addition of H₂S to the vapor further enhanced nanoparticle deposition. Moreover, it enabled the subsequent formation of CNTs at densities as high as 2-6 × 10⁹ cm⁻². In order to realize such high population densities, it was found essential to perform CVD in a two-stage sequence commencing with nanoparticles deposition at 1073 K followed by the formation and growth of CNTs at 1273 K, with the H₂S concentration in the vapor phase optimized throughout within a range of 0.014-0.034 vol%.     

Hydrogen adsorption studies on single wall carbon nanotubes

Hydrogen adsorption data on as-grown and heat-treated single walled carbon nanotubes (SWNTs) obtained by a volumetric procedure using a Quantachrome Autosorb-1 equipment are presented. The amounts of hydrogen adsorbed at atmospheric pressure reach approximately 0.01 wt.% at 298 K and 1 wt.% at 77 K. The isosteric heat of adsorption has been calculated for both samples from H₂ equilibrium adsorption data at three temperatures, having initial values of 7.42 and 7.75 kJ mol⁻¹. Studies in porous structure by N₂ adsorption and density measurements in helium pycnometer are reported.     

Single-source precursor route to carbon nanotubes at mild temperature

Carbon nanotubes were synthesized via a single-source precursor route at 500 °C, using iron carbonyl both as carbon source and catalyst. The X-ray power diffraction pattern indicates that the products are hexagonal graphite. Transmission electron microscope (TEM) images of the sample reveal carbon nanotubes with an average inner (outer) diameter of 30 nm (60 nm). High-resolution TEM indicates that fabrication of the carbon nanotube walls was composed of ca. 40 graphene layers. The Raman spectrum shows two strong peaks at 1587 and 1346 cm⁻¹, corresponding to the typical Raman peaks of graphitized carbon nanotubes. This method avoids the separation of raw material from solvent and simplifies the operation process. At the same time, the research provides a new route to large-scale synthesis of carbon nanotubes.     

A simple combinatorial method to discover Co-Mo binary catalysts that grow vertically aligned single-walled carbon nanotubes

A simple yet versatile combinatorial method to discover binary metal nanoparticle catalysts was developed. In this method, the nominal thickness of component metals can be independently screened for a wide range by simply setting a mask with a slit above a substrate during sputter-deposition. Using this method, we prepared a catalyst library with Mo (0.2-4 nm) and Co (0.2-8 nm) thickness profiles on a SiO₂/Si wafer and discovered active catalysts that grow vertically aligned single-walled carbon nanotubes by alcohol catalytic chemical vapor deposition.     

Fex-Co1-x bimetallic catalysts for highly efficient growth of carbon nanotubes on carbon fibers

This paper aims to develop a catalyst for highly efficient growth of carbon nanotubes (CNTs) on carbon fibers (CFs) with improved tensile strength by chemical vapor deposition (CVD). The effect of the composition of Fe_x-Co_1-x catalysts on the morphology of the synthesized CNTs and the tensile properties of the CFs at different stages of the CVD process were explored. It was found that the bimetallic catalysts possessed better catalytic effect than monometallic catalysts at low temperature. The detailed study of the tensile properties of CFs at different stages of CVD process manifested that the composition of catalysts affected the catalytic activity. When the atomic fraction of Fe was 50%, the bimetallic catalyst among Fe_x-Co_1-x family was considered to hold the highest catalytic efficiency due to the least damage to the fibers and the ability to generate more carbon atoms to repair and reinforce CFs. The corresponding tensile strength of the final CNTs-grafted CFs was 11.53% higher than that of the desized CFs.     

Comparison of preparation methods for carbon nanotubes supported iron Fischer–Tropsch catalysts

Carbon nanotubes supported iron catalysts were prepared by incipient wetness, deposition/precipitation using K₂CO₃, and deposition/precipitation using urea. The incipient wetness method and the deposition/precipitation technique using urea yielded highly dispersed Fe³⁺ on the carbon nanotubes support. The deposition/precipitation technique using K₂CO₃ also yielded larger Fe₂O₃-crystallites. After reduction the three catalysts had similar metal surface areas. Nevertheless, the activity of these catalysts in the Fischer–Tropsch synthesis differed significantly with the catalyst prepared by incipient wetness being the most active one. It is speculated that the differences in the performance of the catalysts might be attributed to the different crystallite size distributions, which would result in a variation in the amount of the different phases present in the catalyst under reaction conditions. The selectivity in the Fischer–Tropsch synthesis over the three catalysts seems to be independent of the method of preparation.     

Effect of adding nickel to iron-alumina catalysts on the morphology of as-grown carbon nanotubes

The synthesis of carbon nanotubes (CNTs) from ethylene decomposition by Fe/Al₂O₃ and Fe/Ni/Al₂O₃ catalysts (Fe:Ni=10:1) is studied. A small amount of nickel introduced into the catalyst can significantly increase the yield of CNTs, but the nanotubes change from straight tubes with concentric parallel carbon sheets to helical tubes of the fish-bone type. Raman characterization of CNTs prepared at 823 and 1023 K and CNTs annealed at 2473 K shows that CNTs deposited on the Fe/Ni/Al₂O₃ catalyst have poor crystallinity, as compared with that on the Fe/Al₂O₃ catalyst. These differences are explained by a mechanism of formation of helical tubes of the fish bone type that takes into consideration the differences in the chemical nature of the catalyst with and without nickel.     

The removal of encapsulated catalyst particles from carbon nanotubes using molten salts

A novel method, using molten salts, is described for the removal of encapsulated nickel catalyst particles from multi-walled carbon nanotubes. The multi-walled carbon nanotubes, synthesised by the decomposition of methane and hydrogen over a NiO/SiO₂ aerogel catalyst, were treated in a LiCl-KCl eutectic molten salt and subsequently by hydrochloric acid to remove the nickel catalyst particles. The influence of the molten salt treatment on the microstructure of the carbon nanotubes was investigated by XRD, SEM and TEM analyses. The molten salt treatment promoted uncapping of the carbon capsules and the formation of strip-shaped carbon fragments. It was found that the hydrochloric acid treatment could then remove the nickel particles from the broken carbon capsules which was not possible prior to the molten salt treatment. The stability of carbon nanotubes in the molten salt is closely related to their ordered structure.