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.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.     

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.     

Study on SiC layers synthesized with carbon ion beam at low substrate temperature

Carbon implantation into silicon, at beam energies from 30 to 60 keV and at ion doses of 3.0×10¹⁷ to 1.6×10¹⁸cm⁻² with a metal vapor vacuum arc ion, was performed to form SiC layers at substrate temperatures below 400°C. It was found that the substrate temperature for the conversion of amorphous phase SiC (a-SiC) into cubic phase SiC (β-SiC) during the carbon implantation, is much lower than the conversion temperature (approx. 900°C) of a-SiC into β-SiC induced by the post-annealing. The feature of the low substrate temperature of the metal vapor vacuum arc (MEVVA) ion source is thought to be due to the ion-beam-induced crystallization (IBIC) effect. The profile of the carbon content, which is of Gaussian shape similar to the data of TRIM-90 calculation, is associated well with the distribution of the ratio of [relative amount of β-SiC/relative amount of a-SiC] in the SiC layers. Moreover, in the carbon rich region the higher degree of crystallization is attributed to the higher β-SiC fraction.     

Effects of carbon nanotubes on rheological behavior in cellulose solution dissolved at low temperature

Multi-walled carbon nanotubes (MWNTs) were dispersed, for the first time, in cellulose solution in 9.5 wt% NaOH/4.5 wt% thiourea aqueous system pre-cooled to −5 °C. Dynamic light scattering and transmission electron microscopy results revealed a relatively strong interaction existed between MWNTs and the cellulose macromolecules, leading to a good dispersion of MWNTs in the cellulose solution. Their rheological behaviors, especially the sol-gel transition were investigated by using the advanced rheological expanded system on the basis of Winter and Chambon theory. The gel point and gel concentration of the cellulose/MWNTs solution system were determined, indicating a regularly rheological behavior. The data of loss tangent and relaxation exponent (n) indicated an enhancement in the viscoelasticity of the MWNTs/cellulose system. The results from scaling law before and beyond the sol-gel transition in the MWNTs/cellulose system confirmed that the cluster formation and alteration of the gelation structure occurred at the gel point. Interestingly, the n values calculated by both the Winter and Chambon theory and scaling law were coincident only at relatively low temperature. The predicted gel strength values of the MWNTs/cellulose system were significantly larger than the pure cellulose solution, suggesting a relatively high strength, supported by the mechanical strength of the cellulose/MWNTs material.     

Thionine-mediated chemistry of carbon nanotubes

Thionine can be employed as a kind of useful functional molecule for the non-covalent functionalization of carbon nanotubes, as it shows a strong interaction with either SWNTs or MWNTs. Attachment of thionine molecules onto the sidewalls of carbon nanotubes would improve the solubility and lower the thermal stability of original carbon nanotubes. More importantly, it may functionalize the surface of carbon nanotubes with rich NH₂ groups and therefore open up more opportunities for the surface chemistry of carbon nanotubes. It has been proved that through the modification of small thionine molecules, other kinds of species such as cytochrome C and TiO₂ nanoparticles could be easily and selectively introduced onto the surface of carbon nanotubes. With this approach, SWNTs or MWNTs can be tailored with desired functional structures and properties.     

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.     

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.     

Novel plasma chemical vapor deposition method of carbon nanotubes at low temperature for field emission display application

We developed a novel growth method of aligned carbon nanotubes. A high density plasma chemical vapor deposition (PECVD) has been employed to grow high-quality carbon nanotubes (CNTs) at low temperatures. High-density, aligned CNTs can be grown on Si and glass substrates. The CNTs were selectively-deposited on the patterned Ni catalyst layer, which was sputtered on Si. The CNTs exhibited a turn-on field of 0.9 V/μm and an emission current of 480 μA/cm² at a field of 3 V/μm.     

Efficient synthesis of carbon nanotubes over rare earth zeolites by thermal chemical vapor deposition at low temperature

Highly efficient synthesis of carbon nanotubes, by thermal chemical vapor deposition (TCVD) below 450 °C, was achieved for the first time. Besides the surface fluidization of the catalyst nanoparticles themselves, assistance of the rare earth oxides embedded in zeolite supports is supposed to be responsible for high activity and selectivity of the Ni/Mo catalyst over which carbon source C₂H₂ successfully decomposes. The enhancement of the catalysis thus makes the carbon deposition and diffusion efficient at low temperatures. The possible mechanism of catalysis process for the growth of carbon nanotubes at low temperature is discussed.     

Plasma enhanced growth of single walled carbon nanotubes at low temperature: A reactive molecular dynamics simulation

Low-temperature growth of carbon nanotubes (CNTs) has been claimed to provide a route towards chiral-selective growth, enabling a host of applications. In this contribution, we employ reactive molecular dynamics simulations to demonstrate how plasma-based deposition allows such low-temperature growth. We first show how ion bombardment during the growth affects the carbon dissolution and precipitation process. We then continue to demonstrate how a narrow ion energy window allows CNT growth at 500 K. Finally, we also show how CNTs in contrast cannot be grown in thermal CVD at this low temperature, but only at high temperature, in agreement with experimental data.     

Highly compressible carbon nanowires synthesized by coating single-walled carbon nanotubes

Carbon nanowires with a diameter of 40–60 nm were synthesized by coating single-walled carbon nanotubes (SWCNTs) in an intermittent, one-stage DC arc discharge process. Transmission electron microscopy shows that these nanowires consist of a SWCNT with an amorphous carbon coating, whose thickness depends on the time of arc discharge. The mechanical properties of blocks of these nanowires were tested by load–unload cyclic compression and static force thermomechanical experiments. The results show that carbon nanowire blocks exhibit better compressive behavior than pure SWCNTs blocks, and carbon nanowires show a typical nonlinear strain–temperature response due to the amorphous carbon layer. A mechanism of adsorption-controlled growth of amorphous carbon on the SWCNTs in the vapor phase is proposed for the formation of the nanowires.     

The effect of catalyst calcination temperature on the diameter of carbon nanotubes synthesized by the decomposition of methane

The effect of catalyst calcination temperature on the uniformity of carbon nanotubes (CNTs) diameter synthesized by the decomposition of methane was studied. The catalysts used were CoO-MoO/Al₂O₃ without prior reduction in hydrogen. The results show that the catalyst calcination temperature greatly affects the uniformity of the diameter. The CNTs obtained from CoO-MoO/Al₂O₃ catalysts, calcined at 300 °C, 450 °C, 600 °C, and 700 °C had diameters of 13.4 ± 8.4, 12.6 ± 5.1, 10.7 ± 3.2, and 9.0 ± 1.4 nm, respectively, showing that an increase in catalyst calcination temperature produces a smaller diameter and narrower diameter distribution. The catalyst calcined at 750 °C was inactive in methane decomposition. Transmission electron microscopy (TEM) studies showed that CNTs grown on the catalyst calcined at 700 °C were of uniform diameter and formed a dense interwoven covering. High-resolution TEM shows that these CNTs had walls of highly graphitized parallel graphenes.     

High temperature rearrangement of disordered nanoporous carbon at the interface with single wall carbon nanotubes

Composites of nanoporous carbon and single wall carbon nanotubes were heat treated in vacuum at temperatures ranging from 1200 to 2000 °C. The resultant interface between the two allotropes of carbon was characterized using high resolution transmission electron microscopy and Raman spectroscopy. At the interface between the nanoporous carbon and the nanotube, the nanotube served as a template for ordering and orientation of the normally disordered nanoporous carbon along the nanotube axis during high temperature treatment. When annealed at 2000 °C, the nanoporous carbon transformed to graphitic nanoribbon which in turn crushed the nanotube to form a nanoscale carbon “bulb”. This result is interesting since at these temperatures, the native nanoporous carbon is well known to resist ordering and is therefore referred to as being a “non-graphitizing” carbon. That the nanotube should act as a template for the incipient graphitization suggests that bonding and strength for load transfer may be developed at these interfaces.     

Electron transfer chemistry of octadecylamine-functionalized single-walled carbon nanotubes

Single-walled carbon nanotubes were chemically functionalized by virtue of the interactions between the nanotube-bound carboxylic moieties and octadecylamine ligands. The electronic conductivity properties of the resulting nanotubes were probed voltammetrically. Two approaches were employed. The first entailed the fabrication of a nanotube monolayer at the air|water interface and the conductivity was measured in situ with a vertically aligned interdigitated arrays (IDAs) electrode. The overall current profiles are analogous to those of a Coulomb blockade and the conducting current pathways are found to be one-dimensional within the two-dimensional arrays of nanotubes. The second technique was taking advantage of the dispersibility of the nanotubes in a solution where conventional electrochemical methods were used. From these measurements, the nanotube bandgap energy could also be estimated, which was quite comparable to that determined by spectroscopic measurements.