Mechanistic investigations of single-walled carbon nanotube synthesis by ferrocene vapor decomposition in carbon monoxide

The single-walled carbon nanotubes (SWCNTs) were synthesized by the carbon monoxide disproportionation reaction on Fe catalyst particles formed by ferrocene vapor decomposition in a laminar flow aerosol (floating catalyst) reactor. On the basis of in situ sampling of the product collected at different locations in the reactor, kinetics of the SWCNT growth and catalyst particle crystallinity were studied. Catalyst particles captured before SWCNT nucleation as well as inactive particles were determined to have cementite (Fe₃C) phase, while particles with γ- and α-Fe phases were found to be embedded in the SCWNTs. The growth rate in the temperature range from 804 to 915 °C was respectively varied from 0.67 to 2.7 μm/s. The growth rate constant can be described by an Arrhenius dependence with an activation energy of E_a = 1.39 eV, which was attributed to the carbon diffusion in solid iron particles. CNT growth termination was explained by solid-liquid phase transition in the catalyst particles. A high temperature gradient in the reactor was found to not have any effect on the diameter during the SWCNT growth and as a result on the chirality of the growing SWCNTs.     

The growth of single-walled carbon nanotubes on a silica substrate without using a metal catalyst

Single-walled carbon nanotubes (SWCNTs) have been directly grown on a SiO₂ substrate using the chemical vapor deposition (CVD) of ethanol without a catalyst. Care was taken to exclude the possibility that the SWCNT growth was induced by conventional metal catalysts such as Fe, Co and Ni resulting from the contamination. Pretreatment of the SiO₂ at 950 °C or a higher temperature in H₂ before CVD was critical for the synthesis of SWCNTs. After CVD process, nano-scale carbon particles were produced besides SWCNTs. Based on these results, we propose that the annealing of SiO₂ substrates in H₂ at high temperature generates defects on their surfaces, and these defects provide nucleation sites for the formation of carbon nanoparticles and assist the formation of carbon nanocaps, thus leading to the SWCNT growth.     

Narrow-chirality distributed single-walled carbon nanotube synthesis by remote plasma enhanced ethanol deposition on cobalt incorporated MCM-41 catalyst

We demonstrated the synthesis of single-walled carbon nanotubes (SWCNTs) with narrow chiral distribution on cobalt incorporated MCM-41 catalyst using ethanol as carbon feedstock by remote plasma enhanced chemical vapor deposition. The use of remote plasma enables the decomposition of the nontoxic ethanol to occur away from the site of SWCNT growth. This allows separate manipulation of carbon radical generation and SWCNT growth. A series of syntheses were carried out at plasma power from 0 to 250 W and growth temperature from 625 to 875 °C. Results have revealed that the plasma power and growth temperature affect carbon radical generation and recombination, as well as the reduction and nucleation of cobalt species. In addition, the remote ethanol plasma etching does not damage the grown SWCNTs. The chirality of resulting SWCNTs shows minor changes under different plasma power. On the other hand, the diameter of SWCNTs can be adjusted (from 0.7 to 1 nm) by changing the growth temperature. At the optimum condition of 200 W plasma power and 775 °C growth temperature, (7,5) and (8,4) nanotubes account for more than 50% of all semiconducting nanotube species. These results demonstrate the potential of utilizing plasma process in chiral selective growth of SWCNTs.     

Synthesis of radially aligned single-walled carbon nanotubes on a SiO2/Si substrate by introducing sodium chloride

Radially aligned single-walled carbon nanotubes (SWCNTs) were synthesized on a SiO₂/Si substrate with thermal chemical vapor deposition by introducing sodium chloride (NaCl) onto the substrate surface. The growth of such SWCNTs was sensitive to the thickness of the SiO₂ layer on the Si substrate and the speed of the reactive gas flow. Cristobalite crystals were found to be formed on the substrate after the SWCNT growth process and were significant for the growth of radially aligned SWCNTs. The SWCNTs were assumed to be directed by the cristobalite crystals along a certain crystal direction on the (1 0 1) crystal face.     

DFT and tight binding Monte Carlo calculations related to single-walled carbon nanotube nucleation and growth

Density-functional theory (DFT) calculations for idealized nucleation processes of (5, 5) and (10, 0) single-walled carbon nanotubes (SWCNTs) on a 55 atom nickel cluster (Ni₅₅) showed that it requires a larger chemical potential to grow a carbon island (which is the simplest structure that can lead to formation of the SWCNTs) on the cluster than to extend the island into a SWCNT or to have the carbon atoms dispersed on the cluster surface. Hence, in the thermodynamic limit the island will only form once the (surface of the) cluster is saturated with carbon, and the island will spontaneously form a SWCNT at the chemical potentials required to create the island. The DFT (zero Kelvin) and tight binding Monte Carlo (1000 K) also show that there is a minimum cluster size required to support SWCNT growth, and that this cluster size can be used to control the diameter, but probably not the chirality, of the SWCNT at temperatures relevant to carbon nanotube growth. It also imposes a minimum size of clusters that are used for SWCNT regrowth.     

Efficiency of C60 incorporation in and release from single-wall carbon nanotubes depending on their diameters

We show that the efficiency of incorporating C₆₀ in single-wall carbon nanotubes (SWCNTs) and that of the incorporated C₆₀’s release from the SWCNTs depend on the SWCNT diameter. Through transmission electron microscopy, we found that the C₆₀ incorporation efficiency reached its maximum at diameters of 1-2 nm, while the efficiency of C₆₀ release from SWCNTs in toluene was maximized at 3-5 nm. The difficulty of C₆₀ release from SWCNTs with diameters of 5-6 nm might reflect either the effective packing of C₆₀ inside SWCNTs or a flattened SWCNT structure. We occasionally observed C₆₀ molecules arranged in a line along the sidewall inside SWCNTs with large diameters/width (>7 nm), indicating that large diameter SWCNTs were sometimes flattened.     

Synthesis of single-walled carbon nanotubes produced using a three layer Al/Fe/Mo metal catalyst and their field emission properties

Single-walled carbon nanotubes (SWCNTs) were synthesized on SiO₂/Si substrates by thermal chemical vapor deposition using an Al/Fe/Mo triple layer catalyst, methane (CH₄) as the carbon source, and a mixture of Ar/H₂ (10% H₂) as the carrier gas. The effects of volume ratio of CH₄ to Ar/H₂ (10% H₂), pretreatment time, growth temperature, and Al underlayer thickness on SWCNT growth were studied. The pretreatment time in Ar/H₂ and Al underlayer thickness were found to be crucial for a high-yield of high-purity SWCNTs, since they both governed the size of the catalyst nanoparticles. The optimum growth conditions were found to be a pretreatment time of 20 min, growth time of 10 min, growth temperature of 900 °C, and CH₄/Ar/H₂ flow rates of 50/900/100 sccm, with a catalyst composed of Al (2 nm)/Fe (1 nm)/Mo (0.5 nm). The SWCNTs grown under these conditions have excellent field emission characteristics with low turn-on and threshold fields of 2.4 and 4.3 V/μm, respectively, and a current density of 38.5 mA/cm² at 5 V/μm.     

Surfactant-free assembling of functionalized single-walled carbon nanotube buckypapers

The electrical and textural properties of single-walled carbon nanotube buckypapers were tunned through chemical functionalization processes. Single-walled carbon nanotubes (SWCNTs) were covalently functionalized with three different chemical groups: Carboxylic acids (-COOH), benzylamine (-Ph-CH₂-NH₂), and perfluorooctylaniline (-Ph-(CF₂)₇-CF₃). Functionalized SWCNTs were dispersed in water or dimethylformamide (DMF) by sonication treatments without the addition of surfactants or polymers. Carbon nanotube sheets (buckypapers) were prepared by vacuum filtration of the functionalized SWCNT dispersions. The electrical conductivity, textural properties, and processability of the functionalized buckypapers were studied in terms of SWCNT purity, functionalization, and assembling conditions. Carboxylated buckypapers demonstrated very low specific surface areas (< 1 m²/g) and roughness factor (R_a = 14 nm), while aminated and fluorinated buckypapers exhibited roughness factors of around 70 nm and specific surface areas of 160-180 m²/g. Electrical conductivity for carboxylated buckypapers was higher than for as-grown SWCNTs, but for aminated and fluorinated SWCNTs it was lower than for as-grown SWCNTs. This could be interpreted as a chemical inhibition of metallic SWCNTs due to the specificity of the diazonium salts reaction used to prepare the aminated and fluorinated SWCNTs. The utilization of high purity as-grown SWCNTs positively influenced the mechanical characteristics and the electrical conductivity of functionalized buckypapers.     

Growth of single-walled carbon nanotubes with controlled diameters and lengths by an aerosol method

On the basis of combined study of the transmission electron microscopy, scanning electron microscopy, Raman spectroscopy and ultraviolet–visible–near infrared absorption spectroscopy, the properties of the single-walled carbon nanotubes (SWCNTs), synthesized by aerosol (floating catalyst) chemical vapor deposition method by ferrocene vapor decomposition in the presence of carbon monoxide, are studied in details. The results show that increasing the temperature gives rise to the formation of high quality and large diameter SWCNTs. By monitoring the water-cooled probe position, both the bundle length and the diameter of the SWCNTs are effectively tuned due to the variation of the residence time and temperature profile in the reactor. An introduction of a small amount of CO₂ suppresses the growth of small diameter nanotubes and enlarges the mean diameter of SWCNT samples. The mean diameter of SWCNTs could be easily altered in a broad range from 1.1 to 1.9 nm during growth, which is essential for the SWCNT applications in optical and electronic devices.     

A Temperature Window for the Synthesis of Single-Walled Carbon Nanotubes by Catalytic Chemical Vapor Deposition of CH4 over Mo2-Fe10/MgO Catalyst

A temperature window for the synthesis of single-walled carbon nanotubes by catalytic chemical vapor deposition of CH₄ over Mo₂-Fe₁₀/MgO catalyst has been studied by Raman spectroscopy. The results showed that when the temperature is lower than 750 °C, there were few SWCNTs formed, and when the temperature is higher than 950 °C, mass amorphous carbons were formed in the SWCNTs bundles due to the self-decomposition of CH₄. The temperature window of SWCNTs efficient growth is between 800 and 950 °C, and the optimum growth temperature is about 900 °C. These results were supported by transmission electron microscope images of samples formed under different temperatures. The temperature window is important for large-scale production of SWCNTs by catalytic chemical vapor deposition method.     

Polyaniline/single-wall carbon nanotube (PANI/SWCNT) composites for high performance supercapacitors

PANI/SWCNT composites were prepared by electrochemical polymerisation of polyaniline onto SWCNTs and their capacitive performance was evaluated by means of cyclic voltammetry and charge-discharge cycling in 1 M H₂SO₄ electrolyte. The PANI/SWCNT composites single electrode showed much higher specific capacitance, specific energy and specific power than pure PANI and SWCNTs. The highest specific capacitance, specific power and specific energy values of 485 F/g, 228 W h/kg and 2250 W/kg were observed for 73 wt.% PANI deposited onto SWCNTs. PANI/SWCNT composites also showed long cyclic stability. Based upon the variations in the surface morphologies and specific capacitance of the composite, a mechanism is proposed to explain enhancement in the capacitive characteristics. The PANI/SWCNT composites have demonstrated the potential as excellent electrode materials for application in high performance supercapacitors.     

Assembly of untreated single-walled carbon nanotubes at a liquid-liquid interface

Untreated single-walled carbon nanotubes (SWCNTs) were assembled at a liquid-liquid interface to form an ultrathin film. The SWCNTs were dispersed into water using sodium dodecyl sulfate (SDS) as a solubilizing agent. Then, hexane was added to the dispersion to create a liquid-liquid interface. The SWCNTs were assembled at the interface to form a smooth ultrathin film when ethanol was added to the SWCNT water dispersion/hexane solution. The assembly mechanism was considered to be caused by the decreased wettability of SDS-coated SWCNT during the addition of ethanol because of desorption of SDS from the SWCNT surface. The assembly was remarkably robust and easily transferable to substrates. An AFM image of the film transferred onto a silicon substrate shows a closely packed uniform film of 3-8 nm thickness. The SWCNT ultrathin film showed high transparency of ca. 97% with an electrical conductivity of 71.4 S/cm. Fabrication processing was carried out in ambient conditions, thereby making it an attractive application for use in flexible electric devices.     

Variations in the microstructure and electrical resistance of the SWCNT films under consecutive photoflash exposures

A film of unpurified single-walled carbon nanotubes (SWCNTs) synthesized by the floating catalyst method using ferrocene as the catalyst precursor was subjected to different numbers of flashes and the products were studied. In addition to the remaining SWCNTs, Fe₂SiO₄ particles covered with amorphous carbon were found to attach on the SWCNTs, and the size increased with flash numbers. Fe₂SiO₄ arose from the oxidation of Fe₃C, a ferrocene-induced catalyst particle embedded in the SWCNTs, where Si provided by SiO₂ released from the mullite tube at 1200 °C during SWCNT growth. The amorphous carbon coating was attributed to insufficient time of the precipitated carbon to crystallize during rapid cooling after the flash. Variation of the Raman I_D/I_G ratio from an initial value of 0.035 to 0.025 after 100 flashes was due to competition between the removal of carbon from the nanotubes and the formation of amorphous carbon on the Fe₂SiO₄ particle surface. The electrical resistance of the SWCNT film increased with the number of flashes but the change became progressively smaller, with the increment decreasing from 17.5% to 0.2%. Similar experiments using purified SWCNTs were performed, and no such particles were observed.     

Layered double hydroxides as catalysts for the efficient growth of high quality single-walled carbon nanotubes in a fluidized bed reactor

A family of layered double hydroxides (LDHs), such as Fe/Mg/Al, Co/Mg/Al, and Ni/Mg/Al LDHs, were used as catalysts for the efficient growth of single-walled carbon nanotubes (SWCNTs) in a fluidized bed reactor. The LDH flakes were agglomerated into clusters with sizes ranging from 50 to 200 μm, and they can be easily fluidized with a gas velocity ranging from 2.3 to 24 cm/s. After calcination and reduction, small metal catalyst particles formed and distributed uniformly on the flakes. At the reaction temperature, the introduction of methane realized the growth of SWCNTs with the diameter of 1-4 nm. The loose structure of LDH agglomerates afforded a yield as high as 0.95 g_CNT/(g_cat h) of SWCNTs with a surface area of 930 m²/g. Compared with Fe/Mg/Al LDH, Ni/Mg/Al and Co/Mg/Al LDHs showed a better selectivity to SWCNTs. The highest selectivity for metallic SWCNTs was obtained using Co/Mg/AI LDHs as the catalyst.     

Separation of single-walled carbon nanotubes from graphite by centrifugation in a surfactant or in polymer solutions

Electric arc single-walled carbon nanotubes (SWCNTs) can be separated from their graphitic impurities by a single centrifugation process in a surfactant or in polymer solutions. The purity of SWCNT dispersions, evaluated from near infrared (NIR) spectroscopy measurements, substantially increased after centrifugation at a moderate speed. The supernatant NIR purity was affected by the surfactant choice, following the sequence: sodium cholate ∼ Pluronic F68 > sodium dodecylbenzene sulfonate > Pluronic F127 > sodium dodecyl sulfate. NIR purity was also influenced by the centrifugation speed and the pristine SWCNT concentration in the starting dispersion, but not by the surfactant concentration. SWCNT enrichment was not observed in a pure organic solvent (N,N′-dimethylformamide) under identical centrifugation conditions. X-ray diffraction analysis demonstrated that graphitic impurities were mostly eliminated from SWCNTs during the centrifugation process in a surfactant or in polymer solutions. Thermogravimetric analysis under CO₂ showed that metallic impurities were substantially reduced during the centrifugation process.     

Fabrication of single-walled carbon nanotube Schottky diode with gold contacts modified by asymmetric thiolate molecules

We have fabricated single-walled carbon nanotube (SWCNT) Schottky diodes by asymmetrically modifying the two Au/SWCNT contacts using different thiolate molecules, methanethiol (CH₃SH) and trifluoroethanethiol (CF₃CH₂SH). Characterization has revealed that highly asymmetrical contacts with Schottky barrier heights of ∼190 and ∼40 meV (increased by over 70% and decreased by over 60%, respectively with respect to that of pristine Au/SWCNT contact of ∼110 meV) were achieved for the Au/SWCNT contacts modified by CH₃SH and CF₃CH₂SH, respectively. The performance of our SWCNT Schottky diodes is as follows: the forward and reverse current ratio (I_forward/I_reverse) higher than 10⁴, a forward current as high as ∼5 μA, a reverse leakage current as low as ∼100 pA, and a current ideality factor as low as ∼1.42. This is at least comparable to, if not better than SWCNT Schottky diodes fabricated with asymmetrical metals, where one contact is a metal with a work function lower than that of SWCNTs to yield a Schottky contact, while the other has a work function higher than that of SWCNTs to achieve an ohmic (more near ohmic) contact.     

Continuous catalyst-free production of fatty acid ethyl esters from soybean oil in microtube reactor using supercritical carbon dioxide as co-solvent

This work reports the transesterification of soybean oil in supercritical ethanol in a continuous catalyst-free process in microtube reactor using carbon dioxide as co-solvent. For this purpose it was employed two microtube reactors with different internal diameter, 0.775 and 0.571 mm. The experiments were performed in the temperature range of 523-598 K, pressure of 10 MPa and 20 MPa, oil to ethanol molar ratio of 1:20 and 1:40, and co-solvent to substrates mass ratio from 0.05:1 to 0.2:1. Results demonstrated that temperature, pressure and co-solvent to substrate mass ratio had a positive effect on fatty acid ethyl esters (FAEE) production, with appreciable yields achieved at 598 K, 20 MPa, oil to ethanol molar ratio of 1:20, using a CO₂ to substrate mass ratio of 0.2:1. The micro-reactor with the smallest inner diameter led to slightly higher FAEE yields at the specified reactions conditions compared to the micro-reactor with greater internal diameter. A semi-empirical kinetic model was proposed in attempt to represent the experimental data with satisfactory fitting results found.     

Electrochemical characterization of single-walled carbon nanotubes for electrochemical double layer capacitors using non-aqueous electrolyte

Single-walled carbon nanotubes (SWCNTs) were investigated by cyclic voltammetry and electrochemical impedance spectroscopy in a non-aqueous electrolyte, 1 M Et₄NBF₄ in acetonitrile, suitable for supercapacitors. Further, in situ dilatometry and in situ conductance measurements were performed on single electrodes and the results compared to an activated carbon, YP17. Both materials show capacitive behavior characteristic of high surface area electrodes for supercapacitors, with the maximum full cell gravimetric capacitance being 34 F/g for YP17 and 20 F/g for SWCNTs at 2.5 V with respect to the total active electrode mass. The electronic resistance of SWCNTs and activated carbon decreases significantly during charging, showing similarities of the two materials during electrochemical doping. The SWCNT electrode expands irreversibly during the first electrochemical potential sweep as verified by in situ dilatometry, indicative of at least partial debundling of the SWCNTs. A reversible periodic swelling and shrinking during cycling is observed for both materials, with the magnitude of expansion depending on the type of ions forming the double layer.     

Multiple “optimum” conditions for Co-Mo catalyzed growth of vertically aligned single-walled carbon nanotube forests

Carbon nanotubes (CNTs) were grown directly on substrates by alcohol catalytic chemical vapor deposition using a Co-Mo binary catalyst. Optimum catalytic and reaction conditions were investigated using a combinatorial catalyst library. High catalytic activity areas on the substrate were identified by mapping the CNT yield against the orthogonal gradient thickness profiles of Co and Mo. The location of these areas shifted with changes in reaction temperature, ethanol pressure and ethanol flow rate. Vertically aligned single-walled CNT (SWCNT) forests grew in several areas to a maximum height of ca. 30 μm in 10 min. A pure Co catalyst yielded a vertically aligned SWCNT forest with a bimodal diameter distribution. The effects of Mo on the formation of catalyst nanoparticles and on the diameter distribution of SWCNTs are discussed and Mo as thin as a monolayer or thinner was found to suppress the broadening of SWCNT diameter distributions.     

Self-assembled nano-arrays of single-walled carbon nanotube-octa(hydroxyethylthio)phthalocyaninatoiron(II) on gold surfaces: Impacts of SWCNT and solution pH on electron transfer kinetics

The construction by sequential self-assembly process of reproducible, highly stable and pH-responsive redox-active nanostructured arrays of single-walled carbon nanotubes (SWCNTs) integrated with octa(hydroxyethylthio)phthalocyaninatoiron(II) (FeOHETPc) via ester bonds on a gold surface (Au-Cys-SWCNT-FeOHETPc) is investigated and discussed. The successful construction of this electrode is confirmed using atomic force microscopy and X-ray photoelectron spectroscopy as well as from the distinct cyclic voltammetric and electrochemical impedance spectroscopic profiles. The Au-Cys-SWCNT-FeOHETPc electrode exhibited strong dependence on the reaction of the head groups and the pH of the working electrolytes, the surface pK_a is estimated as 7.3. The high electron transfer capability of the Au-Cys-SWCNT-FeOHETPc electrode over other electrodes (Au-Cys-SWCNT or the Au-Cys-FeOHETPc or the Au-FeOHETPc) suggests that SWCNT greatly improves the electronic communication between FeOHETPc and the bare gold electrode. The electron transfer rate constant (k_app) of Au-Cys-SWCNT-FeOHETPc in pH 4.8 conditions (∼1.7 × 10⁻² cm⁻² s⁻¹) over that of the electrode obtained from SWCNT integrated with tetraaminophthalocyninatocobalt(II) (Au-Cys-SWCNT-CoTAPc) (5.1 × 10⁻³ cm⁻² s⁻¹) is attributed to the possible effect of the central metal on the phthalocyanine core and substituents on the peripheral positions of the phthalocyanine rings. We also prove that aligned SWCNT arrays exhibit much faster electron transfer kinetics to redox-active species in solutions compared to the randomly dispersed (drop-dried) SWCNTs.