Effect of different carbon sources on the growth of single-walled carbon nanotube from MCM-41 containing nickel

Chemical vapor deposition growth of single-walled carbon nanotubes (SWCNTs) was studied using three representative carbon source sources: CO, ethanol, and methane, and a catalyst of Ni ions incorporated in MCM-41. The resulting SWCNTs were compared for similar reaction conditions. Carbon deposits were analyzed by multi-excitation wavelength Raman, TGA, TEM and AFM. Catalytic particles in the Ni-MCM-41 catalysts were characterized by TEM and synchrotron light source X-ray absorption spectroscopy. Under similar synthesis conditions, SWCNTs produced from CO had a relatively smaller diameter, while those from ethanol had a larger diameter. Methane could not produce SWCNTs on Ni-MCM-41 under the conditions used in this research. These results demonstrate that three carbon sources affect the dynamic balances between metallic cluster formation and carbon deposition/precipitation on the metallic cluster surface. Controlling SWCNT diameter relies on precisely regulating this dynamic process. Using different carbon sources we are able to shift this dynamic balance and produce SWCNTs with different mean diameters.     

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.     

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.     

Selective synthesis of single-walled carbon nanotubes on Fe–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of single-walled carbon nanotubes (SWCNTs) with narrow chirality and diameter distribution by methane decomposition over Fe–MgO catalyst is reported. The catalyst was examined by nitrogen physisorption, X-ray diffraction, temperature programmed reduction, X-ray photoelectron spectroscopy, and UV–Vis diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. High resolution electron microscopy, Raman and optical absorption spectroscopy, temperature programmed oxidation, energy dispersive X-ray spectroscopy and nitrogen physisorption were used to probe reaction selectivity, SWCNT chirality and diameter distribution, carbon yield and effectiveness of purification protocols. The yield of carbon increased with an increase in temperature, although SWCNTs selectivity decreased above the optimum synthesis temperature. Results established a clear link between the degree of dispersion of iron oxide species inside the MgO lattice and the catalyst selectivity for SWCNT growth.     

A step towards controlled-diameter single walled carbon nanotubes

This research treats a particularly challenging undertaking in carbon nanoscience: controlling the diameter distribution of single walled carbon nanotubes (SWCNTs) grown using CVD. To do so requires the fabrication of well-separated and monodispersed catalytic metal nanoparticles which can be uniformly distributed on a substrate. We propose a new strategy involving the preparation of a monolayer of chemisorbed nickel acetylacetonate which is shown by infrared spectroscopy and coupled thermogravimetric–mass spectrometry analyses to be anchored to the surface silanol groups of the substrate. After decomposition of this precursor, the nickel remains bonded with these surface silanol groups which upon further heating results in good dispersion of the nanoparticles over the silica surface. Using this catalyst, SWCNTs with a very discrete chiral preference and a narrow diameter distribution were synthesized at moderate temperature. This work thus opens new perspectives for the fabrication of uniform-diameter carbon nanotubes.     

Parametric analysis of chirality families and diameter distributions in single-wall carbon nanotube production by the floating catalyst method

We report a detailed parametric analysis of the production of single-wall carbon nanotubes (SWCNTs) by the floating catalyst method in a vertical furnace using an alcohol precursor. From a combined study of Raman and absorption spectroscopies and transmission electron microscopy, we have developed a semi-quantitative way of estimating the diameter and chiral family distribution of the SWCNTs produced. This approach shows that increased residence times and higher concentrations of metal catalyst in the precursor solution (i.e. ferrocene in ethanol) increase the diameter of the tubes produced. Increasing growth temperatures result in the formation of both larger and small diameter SWCNTs, in contrast to previous reports of increasing diameter with temperature. Varying these parameters could be effective in tailoring the size distribution of the SWCNTs to meet the needs of specific applications. Also evidence is presented to show the absence of red shifting of the Raman radial breathing modes, expected due to bundling, of the metallic tubes.     

Fabrication of single-walled carbon nanotubes dotted with Au nanocrystals: Potential DNA delivery nanocarriers

Single-walled carbon nanotubes (SWCNTs) dotted with Au nanocrystals (Au-SWCNTs) were fabricated by using a two-phase reduction of hydrogen tetrachloroaurate in the presence of thiol groups anchored to SWCNTs for their potential applications in DNA (deoxyribonucleic acid) delivery. To allow a surface reaction on SWCNTs during the metal nucleation and growth processes, Au nanocrystals were grown using a two-phase system. Raman, XPS and transmission electron microscopy results show that the Au nanocrystals were grafted primarily to the sidewalls of the SWCNTs. DNA probes were immobilized on Au-SWCNTs by the conjugation of DNA functionalized at the 3′ end with a thiol group with Au dots of SWCNTs, followed by hybridizion of complementary oligonucleotides, as verified by fluorescence-based measurements. To investigate whether the target DNA hybridized to DNA probes immobilized on Au-SWCNTs, 618-base-pair fragments of amplified DNA were prepared by polymerase chain reaction using plasmid pET-22b as a template. Atomic force micrograph (AFM) images show that the nanorod-bound DNA is recognizable with excellent specificity, indicating the potential use of such material as a versatile gene delivery carrier in gene-based disease therapy.     

Pt-Co supported on single-walled carbon nanotubes as an anode catalyst for direct methanol fuel cells

Catalyst of Pt-Co supported on single-walled carbon nanotubes (SWCNTs) is prepared using mixed reducing agents. The SWCNTs were pretreated in a microwave oven to enable surface modification. Pt-Co nanoparticles with narrow particle size distribution around 5.4 nm were uniformly deposited onto the SWCNTs. Under same Pt loading mass and experimental conditions, the SWCNTs-Pt-Co catalyst shows higher electrocatalytic activity and improved resistance to CO poisoning than the SWCNTs-Pt catalyst.     

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).     

Physical and electrochemical properties of synthesized carbon nanotubes [CNTs] on a metal substrate by thermal chemical vapor deposition

Multi-walled carbon nanotubes were synthesized on a Ni/Au/Ti substrate using a thermal chemical vapor deposition process. A Ni layer was used as a catalyst, and an Au layer was applied as a barrier in order to prevent diffusion between Ni and Ti within the substrate during the growth of carbon nanotubes. The results showed that vertically aligned multi-walled carbon nanotubes could be uniformly grown on the Ti substrate (i.e., metal substrate), thus indicating that the Au buffer layer effectively prevented interdiffusion of the catalyst and metal substrate. Synthesized carbon nanotubes on the Ti substrate have the diameter of about 80 to 120 nm and the length of about 5 to 10 μm. The Ti substrate, with carbon nanotubes, was prepared as an electrode for a lithium rechargeable battery, and its electrochemical properties were investigated. In a Li/CNT cell with carbon nanotubes on a 60-nm Au buffer layer, the first discharge capacity and discharge capacity after the 50th cycle were 210 and 80 μAh/cm², respectively.     

Controlled assembly of carbon nanotubes encapsulated with amphiphilic block copolymer

An amphiphilic diblock copolymer (PEtOz-PCL) based on hydrophilic poly(2-ethyl-2-oxazoline) (PEtOz) and hydrophobic poly(ε-caprolactone) (PCL) was adsorbed in aqueous phase on the surface of single-wall carbon nanotube to produce PEtOz-PCL-encapsulated SWCNTs (PEtOz-PCL/SWCNT) with the diameter about 30 nm. The Raman spectroscopy analysis indicated that the nanotubes were physically encapsulated by the block copolymer without chemical denaturation of the nanotube. PEtOz-PCL/SWCNTs exhibited pH-responsive reversible complexation with poly(acrylic acid) or poly(methacrylic acid) in aqueous phase due to the pH-dependent hydrogen bonding between the PEtOz outer shell of PEtOz-PCL/SWCNTs with carboxyl groups. In addition, by using PEtOz as a template for the formation of metal nanoparticles, Au and Pd nanoparticles were successfully hybridized with PEtOz-PCL/SWCNTs.     

The effect of electric current on the synthesis of single-walled carbon nanotubes by temperature controlled arc discharge

The effect of discharge current on the synthesis of single-walled carbon nanotubes (SWCNTs) was studied under controlled atmosphere at 500 °C by electric arc discharge. It was shown that the production rate of collected soot was increased but the purity of SWCNTs decreased with increasing discharge current. With a current of 100 A, the SWCNT was very uniform in diameter and a high purity rate of 55% was achieved, as shown by TEM and Raman spectra. Then the influence of electric force, discharge current and catalyst distribution on the formation of SWCNTs was also discussed.     

On-substrate growth of single-walled carbon nanotube networks by an “all-laser” processing route

We report on the process latitude of an “all-laser” approach for the controlled growth of single-walled-carbon-nanotube (SWCNT) mats at predefined locations on silicon substrates. Unlike the conventional laser ablation methods where the SWCNTs are produced in the soot form, from the concomitant ablation of a graphite target loaded with metal catalyst, the “all-laser” process proceeds in two consecutive and independent steps. Indeed, the same KrF pulsed laser is first used to deposit at room temperature, the Co/Ni catalyst nanoparticles (NPs) onto the substrates – of which size and surface density can be controlled by adjusting the number of laser ablation pulses – and subsequently to grow SWCNTs onto the Co/Ni NPs sites, from the laser ablation of a pure graphite target. The grown SWCNT networks are shown to be fairly controllable by choosing the appropriate ratio of “graphite to Co/Ni-NPs” laser ablation pulses. The Co/Ni NPs and the grown SWCNTs were systematically characterized by atomic force microscopy, scanning/tunnelling electron microscopy, Raman spectroscopy and thermogravimetry analysis, and their potential as an active material in thin film transistor was evaluated. Obtained characterization data have led to identify key growth parameters of this novel approach, and to propose growth mechanism models that best describe our observations.     

The formation mechanisms of multi-wall carbon nanotubes over the Ni modified MCM-41 catalysts

Multi-wall carbon nanotubes (MWCNTs) were grown by thermal chemical vapor deposition (thermal CVD) of CH₄ by using Ni-MCM-41 as the catalyst. Methane pyrolysis has been performed in a quartz tube reactor over the catalyst surface to form carbon atoms via dehydrogenation process. The migration and rearrangement of the surface carbon atoms result in the formation of MWCNTs. Transmission electron microscope (TEM) and scanning electron microscope (SEM) were used to determine the morphologies and structures of CNTs, and Raman spectroscopy was exploited to analyze their purity with the relative intensity between the D-band (Disorder band) in the vicinity of 1,350 cm⁻¹ which is characteristic of the sp³ structure and G-band (Graphitic band) in vicinity of 1,580 cm⁻¹ which is characteristic of the sp² structure. In addition, the controlling factors of methane pyrolysis such as the catalyst composition; the reaction temperature, and the methane flow rate on the formation of MWCNTs were investigated to optimize the structure and yield of MWCNTs. SEM/TEM results indicate that the yield of the CNTs increases with increasing Ni concentration in the catalyst. The optimized reaction temperature to grow CNT is located between 640 and 670 °C. The uniform and narrow diameter MWCNTs form at lower flow rate of methane (∼30 sccm), and non-uniform in diameter and disorder structure of MWCNTs are observed at higher flow rate of methane. This is consistent with Raman analysis that the relative intensity of I _D/I _G increases with increasing methane flow rate. The formation mechanisms of the MWCNTs on the Ni-MCM-41 catalyst have been determined to be a Tip-Growth mode with a nanoscale catalyst particle capsulated in the tip of the CNT.     

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.     

Synthesis of single-walled carbon nanotubes by an arc-discharge method using selenium as a promoter

Single-walled carbon nanotubes (SWCNTs) with high purity and very narrow diameter distribution have been synthesized using the dc arc-discharge method with Y–Ni alloy as catalyst and selenium (Se) as promoter. The SWCNTs show a very narrow diameter distribution mainly at about 1.5 nm, and can further be readily purified up to >99% purity with traditional purification including HNO₃ reflux and air oxidation. The key factor of the wetting effect of Se in the SWCNTs growth improvement process is proposed and discussed. Moreover, a new less-destructive purification method including electrolysis, air-oxidation and centrifugation has been introduced, and SWCNTs with semiconducting content up to 94% have been produced through density gradient ultracentrifugation method.     

Influence of the growth conditions on the defect density of single-walled carbon nanotubes

The influence of the temperature and precursor pressure on the defect density of single-walled carbon nanotubes (SWCNTs) grown by catalytic chemical vapor deposition was studied for several catalyst–precursor couples. The SWCNT defect density was assessed by studying the Raman D band. In situ Raman monitoring was used to determine experimental conditions allowing the preparation of samples free of pyrolytic carbon and not altered by air exposure. The most striking feature is that the Arrhenius plots of the I_G/I_D ratio systematically display a convex shape, i.e. the apparent activation energy decreases with increasing temperature. From HRTEM observations and oxidation experiments, this evolution of the D band features is ascribed to the catalytic growth of long SWCNTs with few defects at high temperature and of short and defective SWCNTs and carbon structures at low temperature. The convex Arrhenius behavior is well accounted by two kinetic models: (i) a model considering a change of intermediate states as a function of the temperature (for instance due to a phase transition of the catalyst particle or a change of intermediate carbon species) and (ii) a model considering a high-temperature process of defect creation (for instance by reaction with reactive gas species).     

Continuous production of single-walled carbon nanotubes using a supported floating catalyst

Using catalytic decomposition, a technique for the production of singe-walled carbon nanotubes (SWCNTs) is reported with a production rate up to 6 g h⁻¹ after purification, and scaling capability up to 220 g h⁻¹. This is achieved by injection of pre-prepared alumina supported catalyst powder into a modified vertical floating reactor. The product is collected in several cyclones connected in series. Wide range Raman studies (laser excitations from λ = 1064 to 488 nm) and temperature programmed oxidation measurements of the samples collected from the different cyclones show that SWCNTs were separated in situ by tube diameter. This is attributed to the different residual times of the catalyst in the reaction zone depending on particle diameter. A series of computational fluid dynamics calculations of the flow and heat transfer in the reactor, as well as modeling of catalyst particle transport reveals the parametrical dependence of the process.     

Spray coating as a simple method to prepare catalyst for growth of diameter-tunable single-walled carbon nanotubes

Spray coating is proposed as an optional wet method for preparing nano-sized particles suitable for the growth of single-walled carbon nanotubes (SWCNTs). The obtained SWCNT films are characterized by Raman spectroscopy and electron microscopy, and are confirmed to be comparable to SWCNTs produced by the conventional dip-coating process in terms of crystallinity, tube diameter and carbon yield. The mean diameter of SWCNTs can be effectively reduced from 1.85 to 1.35 nm by prolonging the deposition of Mo. In addition, spray coating allows catalyst preparation on supports other than flat wafers, as demonstrated by the synthesis of high-quality SWCNTs on Al₂O₃ fiber and quartz wool supports.