Growth of large-diameter (∼4 nm) single-wall carbon nanotubes in the nanospace of mesoporous material SBA-15

Single-wall carbon nanotubes (SWCNTs) have been synthesized by supported-catalyst chemical vapor deposition (CCVD) using one-dimensional (1D) channels of mesoporous silica (SBA-15; mean channel diameter, 6.0 nm) functionalized with carboxyl groups where Co and Fe complexes are encapsulated. The synthesized SWCNTs have much larger diameters than the SWCNTs synthesized by conventional CCVD. Transmission electron microscope observations reveal that large-diameter SWCNTs (<4.2 nm) are grown in 1D channels of SBA-15. Large metal particles formed in the channels should play an important role in the growth of the SWCNTs with larger diameters.     

Impact of nanometal catalysts on the laser vaporization synthesis of single wall carbon nanotubes

The effects of catalyst particle size on the purity, yield, and purification efficiency of single wall carbon nanotubes (SWCNTs) synthesized via pulsed laser vaporization were investigated. The purity of as-produced SWCNT material synthesized using Ni and Co nanometal (∼13 nm diameter) catalyst particles was compared to material synthesized using conventional micronmetal (2-3 μm diameter) particles. The SWCNT material from nanometal catalysts demonstrated a 50% increase in SWCNT purity as assessed by optical absorption spectroscopy and thermogravimetric analysis (TGA). A change in the thermal oxidation properties was also observed with the nanometal-SWCNTs exhibiting a suppression of the exothermic oxidation of post-synthesis catalyst. Statistical analysis of the TGA residue yielded mean post-synthesis catalyst particle diameters of 18 ± 6 nm and 3 ± 1 nm for the micronmetal and nanometal produced material, respectively. When a thermal oxidation profile was performed, the micronmetal-produced material showed the typical decrease in SWCNT purity with increasing oxidation temperature while the nanometal-produced material showed increasing SWCNT purity with increasing temperature. Overall, the use of nanometal catalysts significantly increases synthesis yield and offers novel thermal oxidation procedures to thermally remove carbonaceous impurities without the aid of acid treatments for the development of potential large-scale purification processing.     

Improving the synthesis of single-walled carbon nanotubes by pulsed arc discharge in air by preheating the catalysts

Single-walled carbon nanotubes (SWCNTs) were synthesized in reduced pressure air using pulsed arc discharge after preheating the catalyst. Our experimental results revealed that preheating the catalysts can assist the synthesis of SWCNTs in air under a pressure of 5-10 kPa. The SWCNTs have a diameter of 1.5-2 nm and length can reach several micrometers. The consumption rate of the anode and the production rate of CNTs and SWCNTs in air are lower than in helium atmosphere at the same pressure, respectively. Further experiment demonstrates that 600 °C is optimum temperature for preheating the catalysts to synthesize SWCNTs in air.     

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.     

Temperature and pressure dependence of molecular adsorption on single wall carbon nanotubes and the existence of an “adsorption/desorption pressure gap”

The interaction of acetone with single wall carbon nanotubes (SWCNTs) was studied by temperature programmed desorption with mass spectrometry (TPD-MS), after reflux, sonication, or exposure to 7.6 Torr of acetone vapors at room temperature. Acetone molecules adsorb strongly on SWCNTs desorbing at ∼400-900 K, corresponding to desorption energies of ∼100-225 kJ/mol, as intact molecules. Exchange of intact adsorbed molecules with gas phase species was observed in successive dosing of hydrogenated and deuterated acetone molecules. The desorption energies reported here are in stark contrast to the desorption energies (∼75 kJ/mol) reported earlier for SWCNTs interacting with acetone under high vacuum at cryogenic temperatures. This result suggests activated adsorption/desorption, and is also observed for adsorption of ethanol, methane, n-butane and 1,3-butadiene on SWCNTs and on carbon black. Quantum chemical calculations suggest that adsorption in interstitial channels of bundles formed of large-diameter SWCNTs is possible and can account for high desorption barriers, a result of strong dispersion interactions between neighboring SWCNTs.     

Control of diameter distribution of single-walled carbon nanotubes using the zeolite-CCVD method at atmospheric pressure

Single-walled carbon nanotubes (SWCNTs) have been synthesized on zeolite powder with Fe/Co catalysts by a catalytic chemical alcohol-vapor deposition (CCAVD). We have first used a cold wall reactor at the atmospheric pressure, the system having been modified for the zeolite-CCAVD specifications by the use of radio-frequency heating. The G/D ratio (∼25), estimated by analysis of Raman spectroscopy, obtained here is equivalent to that by the conventional CCAVD method under reduced pressure, indicating the high purity of the present specimen. The estimated diameter distributions of the SWCNTs obtained at synthesis temperatures of 900, 1000 °C and constant ethanol temperature of 0 °C are 0.9-1.8 and 1.2-2.2 nm, respectively, whereas that of synthesized at synthesis temperature of 900 °C and ethanol temperatures of 40 °C ranges form 0.8 to 1.4 nm. The diameter distribution shifts towards larger diameters as the synthesis temperature is increased and the carbon supply rate (ethanol temperature) decreases, from which we suggest a selective growth model due to a competition between deposition and etching of carbon atoms.     

Production of single-walled carbon nanotubes with narrow diameter distribution using iron nanoparticles derived from DNA-binding proteins from starved cells

Iron nanoparticles derived from DNA-binding proteins from starved cells (Dps) were used to grow single-walled carbon nanotubes (SWCNTs) with narrow diameter distribution. An atomic force microscopy, Raman spectroscopy, and photoluminescence were used for evaluation of diameter or chirality distribution of the SWCNTs. We found that thin SWCNTs (1.1 nm diameter) were grown from the large Dps-derived nanoparticles (2.4 nm diameter) on and above the substrates. From the size comparison with ferritins and Co-filled apoferritins, we also found that SWCNTs become thinner as the catalyst becomes smaller. The synthesis of smaller catalysts (ca. 1 nm diameter) and their use for growth becomes crucial for the control of SWCNT diameter.     

A study of thermal stability and methane tolerance of Cu-based SOFC anodes with electrodeposited Co

Cu-based, solid oxide fuel cell (SOFC) electrodes were modified by electrodeposition of Co. The addition of only 5 vol% Co by electrodeposition significantly improved the thermal stability compared to either Cu-ceria-YSZ, Cu-Co-ceria-YSZ, or Co-ceria-YSZ electrodes prepared only by impregnation with much higher metal loadings, demonstrating that electrodeposited metal layers form metal films with better connectivity. In the absence of Co, SEM showed structural changes in the impregnated Cu after heating to 1173 K in humidified H₂ and these changes caused large increases in the ohmic resistance of fuel cells, as measured by impedance spectroscopy. In contrast, the ohmic resistance of a cell with 13 vol% Cu, 9 vol% ceria, and 5 vol% Co increased only slightly after 48 h at 1173 K in humidified H₂. While a Co-ceria-YSZ composite was found to form large amounts of carbon upon exposure to dry CH₄ at 1073 K for 3 h, the Co-Cu-ceria-YSZ composites did not form measurable amounts of carbon for the same conditions. XPS results for a Cu foil with a 250-nm Co film demonstrated that Cu migrates to the surface of the Co upon heating above 873 K, forming a stable Cu layer that appears to be approximately one monolayer thick. The implication of these results for the development of practical SOFC electrodes for the direct utilization of hydrocarbons is discussed.     

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.     

Diameter control of multiwalled carbon nanotubes using experimental strategies

Multiwalled carbon nanotubes (MWNTs) were synthesized using a chemical vapor deposition floating feed method in a vertical reactor. Effects of the preparation variables on the average diameter of carbon nanotubes were systematically examined using the fractional factorial design (FFD), path of the steepest ascent, and central composite design (CCD) coupled with the response surface methodology. From the FFD study, the main and interactive effects of reaction temperature, methane flow rate, and chamber pressure were concluded to be the key factors influencing the diameter of MWNTs. Two empirical models, representing the dependence of the diameter of carbon nanotubes at the vicinities around maximum (420 nm) and minimum (15 nm) on the reaction temperature and methane flow rate, were constructed in two independent CCD studies. These models, shown as contour diagrams, indicated that the diameter of carbon nanotubes generally increased with increasing reaction temperature and methane flow rate. Based on both models, the diameter of MWNTs from 15 to 420 nm can be controlled precisely by using a continuous CVD fabrication method.     

Selective growth of single-walled carbon nanotubes over Co–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of SWCNTs with narrow chirality and diameter distribution by methane decomposition over a Co–MgO catalyst is reported. Raman spectroscopy, temperature programmed oxidation (TPO), UV–Vis–NIR absorption spectroscopy, and nitrogen physisorption were used to probe SWCNTs morphology, reaction selectivity, SWCNTs chirality and diameter distribution, and carbon yield. The catalyst was examined by nitrogen physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), and UV–Vis-diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. The results established a clear link between the degree of dispersion of Co species inside the MgO lattice and the catalyst activity and selectivity for SWCNT growth. High dispersion and stabilization of Co species influenced catalytic activity for methane decomposition and the high SWCNT selectivity. The yield of carbon and SWCNT selectivity increased with an increase in temperature, however, SWCNTs diameter distribution shifts to larger diameter tubes as synthesis temperature was increased.     

Gadolinium and europium catalyzed growth of single-walled carbon nanotubes

The inner transition metals, gadolinium (Gd) and europium (Eu) have been shown to catalyze the growth of single-walled carbon nanotubes (SWCNTs) using chemical vapor deposition. The Gd and Eu nanocatalysts, prepared using a diblock copolymer templating method and characterized by atomic force microscopy, were uniformly spaced over a large deposition area with an average diameter of 1.9 nm and narrow size distribution. Characterization by transmission electron microscopy and Raman spectroscopy confirms the presence of SWCNTs catalyzed by Gd and Eu with an average diameter of 2.05 nm.     

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.     

Parametric study of synthesis conditions in plasma-enhanced CVD of high-quality single-walled carbon nanotubes

High-quality single-walled carbon nanotubes (SWCNTs) have been synthesized from H₂-CH₄ mixtures on a MgO-supported bimetallic Mo/Co catalyst using microwave plasma-enhanced chemical vapor deposition (PECVD). Reaction parameters including temperature, H₂:CH₄ ratio, plasma power, and synthesis time have been examined to assess their influence on SWCNT synthesis. Raman spectroscopy and high-resolution field emission scanning electron microscopy reveal that the quality, selectivity, density and predominant diameter of SWCNTs depend on the varied synthesis parameters. Results of this study can be used to optimize SWCNT synthesis conditions and products and to improve understanding of the growth of SWCNTs by PECVD.     

Thermally activated model for tensile yielding of pristine single-walled carbon nanotubes with nonlinear elastic deformation

A thermally activated model for evaluating the tensile yield strain of pristine single-walled carbon nanotubes (SWCNTs) is established. Using a parabolic function to accurately describe the dependence of stress on strain, we derive a yield function relating the yield strain to temperature, strain rate and tube length. The activation energy and activation area are then determined from the MD results of the tensile yield strain as a function of temperature. We find that the activation energies for armchair SWCNTs range from 7.18 to 11.94 eV, depending on the radius of SWCNTs. Analyses of activation area and MD results reveal that the nucleation of a critical defect, which leads to the failure of SWCNTs, grows from a single bond size at 300 K to almost twice the size at 2100 K. On the basis of activation parameters, our model can be used to predict the yield strain of SWCNTs under experimental conditions.     

Spectral properties of single-walled carbon nanotubes encapsulating fullerenes

Single-walled carbon nanotubes (SWCNTs) with diameter ranged from 1.22 to 1.6 nm filled with C₆₀, C₇₀ and C₆₀H₂₈ molecules (peapods), as well as double-walled carbon nanotubes (DWCNTs) derived from peapods, were studied by HRTEM, UV-vis-NIR and Raman spectroscopy. Suspensions with accurate concentration were used for spectroscopic studies to enable quantitative comparison of different substances. Filling of the SWCNTs with C₇₀ molecules resulted in a reduced van der Waals interaction between the tubes in a bundle. The DWCNTs have lower intensity of the van Hove bands and weaker photoluminescence. Raman spectra at 633 and 1064 nm excitation wavelengths reveal that RBM frequencies of C₆₀ and C₇₀ peapods are equally downshifted compared to empty tubes. It was found that filling of the nanotubes with C₆₀ and C₇₀ caused spectral shifts of absorption bands: thin tubes display red shifts, while thick ones show blue shifts. DWCNTs and C₆₀H₂₈@SWCNTs do not show any shifts. All the results suggest that the filling of nanotubes with fullerenes alters the average diameter of the electron cloud around SWCNT framework; namely, it increases for thin SWCNTs, and decreases for thick ones. Our attempts to structurally assign thick nanotubes using reported extrapolations from data for thin tubes were unsuccessful.     

Synthesis of high quality single-walled carbon nanotubes on natural sepiolite and their use for phenol absorption

Natural sepiolite mineral was used as a catalyst and catalyst support for the efficient growth of single-walled carbon nanotubes (SWCNTs). With the introduction of hydrogen, uniform active metal nanoparticles can be formed in the fibrous structure of sepiolite ore. High-quality SWCNTs with few defects and a large aspect ratio (over 10⁴) were synthesized. The structure of the CNTs was modified by controlling the catalyst composition with ion-exchange method and changing the growth conditions. The 1 wt.% Fe-based catalyst exhibited excellent activity for the growth of SWCNTs, while the Co/Mo-based catalyst preferred to grow small diameter SWCNTs. The reaction temperature showed a sensitive selectivity to the chirality and diameter distribution of the SWCNTs produced. Catalysts based on the sepiolite can also afford the effective growth of SWCNTs in a fluidized bed reactor. As-grown SWCNTs/calcined-sepiolite demonstrated excellent ability of phenol adsorption, with an adsorption capacity of 155.8 mg/g, which was much higher than that on natural sepiolite (12.7 mg/g).     

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.     

Selective removal of metallic single-walled carbon nanotubes by combined in situ and post-synthesis oxidation

A combined in situ and post-synthesis gas phase oxidation approach for selective removal of metallic single-walled carbon nanotubes (m-SWCNTs) is reported. The in situ oxidation is performed by introducing a small amount of oxygen during the synthesis of SWCNTs by floating catalyst chemical vapor deposition, and the post-synthesis oxidation is conducted by heat-treating the synthesized SWCNTs in air at 400 °C. A combination of characterization techniques shows that m-SWCNTs were selectively removed as a result of their higher reactive activity to oxygen compared to semiconducting SWCNTs, and the diameter distribution of the SWCNTs is narrowed to a range of 1.5-2.0 nm. The mechanism of the combined in situ and post-synthesis oxidation approach is discussed.     

Hydrogen adsorption in different carbon nanostructures

Hydrogen adsorption in different carbonaceous materials with optimized structure was investigated at room temperature and 77 K. Activated carbon, amorphous carbon nanotubes, SWCNTs and porous carbon samples all show the same adsorption properties. The fast kinetics and complete reversibility of the process indicate that the interaction between hydrogen molecules and the carbon nanostructure is due to physisorption. At 77 K the adsorption isotherm of all samples can be explained with the Langmuir model, while at room temperature the storage capacity is a linear function of the pressure. The surface area and pore size of the carbon materials were characterized by N₂ adsorption at 77 K and correlated to their hydrogen storage capacity. A linear relation between hydrogen uptake and specific surface area (SSA) is obtained for all samples independent of the nature of the carbon material. The best material with a SSA of 2560 m²/g shows a storage capacity of 4.5 wt% at 77 K.