Epitaxial nucleation model for chiral-selective growth of single-walled carbon nanotubes on bimetallic catalyst surfaces

An epitaxial nucleation model for single-walled carbon nanotube (SWCNT) growth on bimetallic catalysts surfaces is reported in support of experimental observations of chiral enrichment. We model the bimetallic catalyst surfaces as a 2D (1 1 1) surface consisting of Ni or a combination of Ni and Fe atoms, with varying average bond length between nearest neighbor atoms which corresponds to the crystal structure of the alloys. The energies associated with nanotube cap formation on these various surfaces are calculated using density functional theory (DFT). We find that certain cap chiralities, such as (8, 4), are more stably bound to a surface that resembles a Ni_0.27Fe_0.73 bimetallic catalyst, whereas other chiralities, such as (9, 4), are more stable on a pure Ni surface. These results help explain the predominance of certain chiralities on specific bimetallic catalysts and provide a potential route to controlling the chirality of as-grown SWCNTs.     

Effect of catalyst combination on growth of single-walled carbon nanotubes

A new effective catalysts combination of iron — nickel for alcohol CVD technique was found. This catalyst catalyzed well as well as the typical catalyst of iron — cobalt catalysts, but gave a different diameter distribution. Calculating their electrical density of states under the assumption of their solid lattice structures, the result was fairly consistent with experimental results. The number of electrical states near Fermi level that is considered to be important for catalytic reaction is enough and the DOS of iron – nickel catalyst was quite similar to that of cobalt unlike manganese – copper catalyst. Consequently, a blend of catalysts that has a similar DOS to cobalt and has enough states near the Fermi level can be a good catalyst for alcohol CVD.     

Controlled cutting of single-walled carbon nanotubes and low temperature annealing

Many applications in nanotechnology require short and unentangled single-walled carbon nanotubes (SWCNT). Liquid-phase oxidative cutting gives nonuniform short tubes and causes significant material loss. Mechanical cutting is good for shortening SWCNT, but it leaves collapsed tube ends and might not be favorable for further manipulation. Solid-state reaction cutting is better for multi-walled carbon nanotubes than for SWCNT. Herein, we present a method combining mechanical and oxidative cutting. The SWCNT sample was first ground with a Wig-L-Bug grinding mill for 30 min, introducing structural defects into the side walls of SWCNT. The treated SWCNT were then soaked in a Piranha solution, in which the oxidants attack the existing side wall defects and give a complete cut. According to statistical analysis from transmission electron microscopy, most of the shortened SWCNT fall in the range of 50–200 nm. The material loss is 12.2 wt%. The functional groups on the tube surface introduced by shortening were removed by refluxing in a soda lime/water suspension. Then, the carbon nanotubes were further annealed by sonicating in ethanol. After annealing, the defect level of shortened carbon nanotubes was reduced significantly, as determined by Raman spectroscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis.     

Effective catalyst on SiO2 in ethanol CVD for growth of single-walled carbon nanotubes

We have compared catalytic activity of Co and Fe in a growth process of single-walled carbon nanotube (SWNT) by chemical vapor deposition using ethanol as a carbon source and SiO₂ as a catalyst-supporting material. Changes of the catalyst precursors (Co- and Fe acetate) in the growth process were carefully observed at three different stages: (i) after oxidation in air at 400 °C but before heating to the growth temperature (800 °C), (ii) after heating to the growth temperature in flowing Ar and H₂ but before starting the nanotube growth and (iii) after the growth process is over. During the growth of SWNT, the Co catalyst took the form of β-Co, resulting in a high yield growth. On the contrary, the Fe catalyst formed a silicate, Fe₂SiO₄, showing a poor catalytic ability. Our result shows that chemical reactions between the catalyst precursors and their supporting materials sensitively affect the catalytic ability.     

Combinatorial catalyst approach for high-density growth of horizontally aligned single-walled carbon nanotubes on sapphire

We studied effects of metal catalyst and gas composition on the chemical vapor deposition (CVD) growth of horizontally aligned single-walled carbon nanotubes (SWCNTs) on r-plane sapphire substrates. The SWCNTs are sitting on the substrate and aligned along [10] direction of the sapphire surface. A combinatorial metal deposition method was applied for single and binary metal catalysts to systematically investigate the thickness and the composition dependence. The horizontally-aligned SWCNTs grown from stripe-patterned catalysts enable the direct comparison of the catalytic activity based on nanotube density. We found that the SWCNT density strongly depends on the metal catalyst in the order Fe > Co ? Ni ≈ Cu, while no nanotubes were grown over Mo. In addition, the methane concentration during CVD strongly influenced the nanotube density, and the optimal concentration varied depending on the metal species and its thickness. The study on the binary metal catalysts revealed that Fe–Co combination increases the SWCNT density (7–9 tubes/μm) about twice of the original metal film. The Co–Cu binary catalyst also showed the high density (8–10 tubes/μm) under a limited methane concentration. Different catalytic activity of each metal is discussed.     

Synthesis of single-walled carbon nanotubes using hemoglobin-based iron catalyst

Hemoglobin (Hb) was used as a catalyst for the growth of single-walled carbon nanotubes (SWCNTs). Hb was deposited onto a hydrophilic treated substrate by spin coating method. After oxidation at 800 °C, protein chains were decomposed and iron oxide nanoparticles remained with an average diameter of 2.29 nm. High quality SWCNTs were synthesized with an average diameter of 1.22 nm. The protein chains prevent iron atoms aggregation and so the size of the nanoparticles is smaller than that from ferritin-like proteins.     

The influence of bimetallic catalyst composition on single-walled carbon nanotube yield

We show that the yield of single-walled carbon nanotubes (SWCNTs) grown with bimetallic catalysts is a strong function of their atomic-scale composition. A series of compositionally-tuned Ni_xFe_1?x bimetallic catalysts with a constant mean diameter of 2.0 nm are used to catalyze the growth of nanotubes via a floating catalyst method. Increasing the Fe content in the catalysts is found to lower the fraction of SWCNTs in the collected as-grown product. Based on a simple surface-to-volume model, these results are explained by the higher carbon solubility of Fe compared to Ni which results in a larger amount of carbon precipitation and the formation of multi-walled tubes when the nanotubes are nucleated from catalysts with high Fe content. Overall, our study demonstrates that the size and composition of bimetallic catalysts must be precisely controlled to obtain high yields of SWCNTs for large-scale production.     

Effects of catalyst pre-treatment on the growth of single-walled carbon nanotubes by microwave CVD

Layers of carbon nanotubes were deposited by microwave CVD on oxidized silicon substrates coated with Al-Fe-Mo catalyst films. To achieve a tube growth at about 973 K, the ion bombardment of the catalyst surface has to be avoided. The appropriate pre-treatment of the substrates is essential for the deposition of single-walled carbon nanotubes. Annealing in air is preferable to the frequently used reducing pre-treatment prior to the deposition as a higher area density of the tubes and a better reproducibility of deposition can be obtained. To figure out this finding, selected samples were investigated by analytical transmission electron microscopy and Raman spectroscopy. It is shown that the pre-treatment has a strong effect on the size and distribution of the catalyst particles.     

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.     

Effects of bimetallic catalyst composition and growth parameters on the growth density and diameter of carbon nanotubes

Multi-wall carbon nanotubes (MWNTs) were synthesized by catalytic decomposition of acetylene over Fe, Ni and Fe-Ni bimetallic catalysts supported on alumina under various controlled conditions. The growth density and diameter of CNTs were markedly dependent on the activation time of catalysts in H₂ atmosphere, reaction time, reaction temperature, flow rate of acetylene, and catalyst composition. Bimetallic catalysts were apt to produce narrower diameter of CNTs than single metal catalysts. For the growth of CNTs at 600 ‡C under 10/100 seem flow of C₂H₂/H₂ mixture, the narrowest diameter about 20 nm was observed at the reaction time of 1 h for 20Fe : 20Ni : 60Al₂O₃ catalyst, but at that of 1.5 h for 10Fe : 30Ni : 60Al₂O₃ catalyst. It was considered that the diameter and density of CNTs decreased with the increase of the growth time mainly due to hydrogen etching. The growth of CNTs followed the tip growth mode.     

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.     

Low cost growth route for single-walled carbon nanotubes from decomposition of acetylene over magnesia supported Fe-Mo catalyst

A large amount of single wall carbon nanotubes (SWNTs) was successfully produced by thermal decomposition of C₂H, at 800 °C over magnesia supported Fe-Mo bimetallic catalysts in a tubular flow reactor under an atmosphere of hydrogen flow. The growth density of SWNTs increased with increasing the weight percent of the catalyst metals (wt% ratio of two metals: 50 : 50) supported on magnesia (MgO) from 5 to 30 wt%. The yield of SWNTs reached 144.3% over 30 wt% metal-loaded catalyst. Raman measurements showed the growth of bundle type SWNTs with diameters ranging from 0.81 to 1.96 nm. The growth of SWNTs was also identified by thermal gravimetric analysis (TGA) and Raman spectroscopy.     

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.     

Modelling gas-phase synthesis of single-walled carbon nanotubes on iron catalyst particles

A simple model for the gas-phase synthesis of carbon nanotubes on iron catalyst particles has been developed. It includes a growth model for the catalyst particles and describes nanotube growth processes through carbon monoxide disproportionation and hydrogenation. Models for particle-particle interactions and sintering are also included. When carbon arrives at a catalyst particle it can either dissolve in the particle until a saturation limit is reached, or form a graphene layer on the particle, or go on to form a nanotube. Two models for incipient nanotube growth are considered. The first allows nanotubes to form once a catalyst particle reaches the saturation condition. The second only allows nanotubes to form on the collision of two saturated particles. The particle system is solved using a multivariate stochastic solver coupled to the gas-phase iron chemistry using an operator splitting algorithm. Comparison with experimental data gives a good prediction of the nanotube length, and reasonable values of catalyst particle diameter and nanotube diameter. A parametric study is presented in which the carbon monoxide reaction rate constants are varied, as is the fraction of carbon allowed to form nanotubes relative to surface layers. The assumptions of the coagulation and sintering models are also discussed.     

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.     

Chemistry of single-walled carbon nanotubes

In this Account we highlight the experimental evidence in favor of our view that carbon nanotubes should be considered as a new macromolecular form of carbon with unique properties and with great potential for practical applications. We show that carbon nanotubes may take on properties that are normally associated with molecular species, such as solubility in organic solvents, solution-based chemical transformations, chromatography, and spectroscopy. It is already clear that the nascent field of nanotube chemistry will rival that of the fullerenes.     

Formylation of single-walled carbon nanotubes

Formyl or aldehyde groups are transferred to the surface of single-walled carbon nanotubes (SWCNTs) by reaction of reduced carbon nanotubes with N-formylpiperidine. This could open the way for more versatile chemical modification reactions of carbon nanotubes than is currently possible using functionalization methods reported to date. The formylated SWCNTs were characterized by thermogravimetric analysis-mass spectrometry and Raman, UV-vis-NIR and FTIR spectroscopy. The location and distribution of the functional groups was determined by AFM using electrostatic interactions with gold nanoparticles. The formylated SWCNTs were further derivatized with a fluorescent dye and studied using fluorescence spectroscopy.     

Chiral-selective growth of single-walled carbon nanotubes on stainless steel wires

Single-walled carbon nanotubes (SWCNTs) were successfully grown on calcined stainless steel wires at 700 °C using CO as the carbon source. By contrast, the raw stainless steel wires produced only necklace-like multi-walled carbon nanotubes. Photoluminescence spectroscopy studies showed that SWCNTs grown from calcined stainless steel have a narrow diameter distribution and a high chiral selectivity of (6,5) nanotubes. The pre-growth heat treatment of the stainless steel leads to formation of iron and chromium oxides. The reduction of iron oxide results in formation of Fe nanoparticles which, anchored by chromium oxide, account for the chiral-selective growth of SWCNTs.     

Promotion of cell growth with magnetically enhanced single-walled carbon nanotubes

We demonstrate that purified and functionalized single walled carbon nanotubes (SWNTs) promote the growth of NIH3T3 mouse fibroblast cells under a magnetic field. The SWNTs are functionalized in acidic solutions by attaching carboxyl groups (–COOH) on their surfaces. Functionalized SWNTs (fSWNTs) exhibit a ferromagnetic property when dispersed in water. Cytotoxicity after the delivery of the fSWNTs into the cells is significantly reduced due to the complete removal of toxic metallic impurities during the functionalization process. The efficient uptake of the fSWNTs by the cells is confirmed through transmission electron microscopy (TEM). It is discovered that the growth of the NIH3T3 cells treated with the fSWNTs is enhanced by up to 25% than control cells when an external magnetic field is applied. Our findings may lead to the non-invasive and non-toxic drug delivery as well as targeted cell therapy with fSWNTs.