CoMoCAT® Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes have a diameter of about 1nm and the length is nearly million times longer.The wrapping of a single atom thickness graphite layer into a seamless cylinder forms a (Single-walled Carbon Nanotubes) SWCNT.

Overview

Aldrich Materials Science, in collaboration with SouthWest Nanotechnologies, offers two high-purity SWCNTs as shown in Table 1 produced by the CoMoCAT catalytic chemical vapor deposition method.

Table 1. High-Purity Single-walled Carbon Nanotubes

Synthesis of SWNTs with Controlled Structures using the CoMoCAT Process

For large-scale production of nanotubes, the use of a particulate, high-surface area catalyst is very advantageous. In a typical supported catalyst, the active species (e.g. a metal cluster) is stabilized in a high state of dispersion over the surface of a refractory support such as alumina, silica or magnesia. This catalyst type is similar to those used in the chemical and petrochemical industry in the production of polymers, fuels, solvents, etc. One of the key advantages of using supported catalysts is that the engineering aspects of the possible reactor designs (fluidized bed, fixed bed, transport bed, rotary kiln, etc.) are well-known in industry and scaling-up is a mature technology.

It is widely recognized that in an unrestricted state (e.g. during laser ablation) the growth rate of single-walled carbon nanotubes is at least higher than several microns-per-second. By contrast, when the growth occurs via catalytic decomposition of carbon-containing molecules on high surface-area catalysts, the overall growth process continues in a scale of minutes to hours. It is clear that while the amount of carbon deposits slowly increases with time, this does not necessarily mean that the growth of a given nanotube is that slow. That is, the slow rate observed for the overall rate of carbon deposition comprises an induction period followed by a fast nanotube growth rate. Accordingly, new nucleation sites will appear on a high-surface area material and each site will give rise to a nanotube that grows relatively fast. The nanotubes that grow later will be constricted by the presence of those grown earlier.

To have a high selectivity towards SWCNT, nucleation of the nanotube embryo needs to occur before the metal particle sinters. Several approaches have been followed to avoid rapid sintering. The strategy used in the CoMoCAT method is to keep the active Cobalt species (Co) stabilized in a non-metallic state by interaction with Molybdenum oxide (MoO₃) before it is reduced by the carbon-containing compound (CO). When exposed to carbon monoxide, the Co-Mo dual oxide is carburized, producing Molybdenum carbide and small metallic Co clusters, which remain in a high state of dispersion and result in high selectivity towards SWNT of very small diameter. Lower temperature syntheses and stabilization of small metal clusters yield a CoMoCAT nanotube product with a smaller average diameter and a narrower distribution of structures compared to other synthesis methods. The CoMoCAT process utilizes fluidized bed reactors as shown in Figure 1 to maintain precise control of the temperature and flow rates, resulting in high (n,m) selectivity.

Figure 1. An illustration of a fluidized bed reactor, which is able to scale up the production of SWNTs using the CoMoCAT® process

This information has been sourced, reviewed and adapted from materials provided by Sigma Aldrich.

For more information on this source, please visit Sigma Aldrich.