The United States of America as represented by the Secretary of the Navy (Washington, DC) earned U.S. Patent 7,781,635 for surfactant-based purification of carbon nanotubes.
Inventors Thomas E. Sutto; (Fredericksburg, VA), and Karen A. McGrady; (Fredericksburg, VA) developed a mixture and method o for purifying carbon nanotubes. A substituted imidazolium cation is utilized to suspend carbon nanotubes in a nonpolar liquid. A polar solvent immiscible with the nonpolar liquid is mixed in to remove soot from the suspension, allowing recovery of the nanotubes. The relative gentleness of the separation provides nanotubes that are undamaged and unoxidized. The components of the mixture are economically advantageous for this use and the method is simple compared to other nanotube purification methods.
It has been found that, while the nanotubes will stay suspended in the nonpolar liquid by virtue of the action of the cation, substantially all of the soot will become suspended or dissolved in the polar solvent. Conventional separation of the two immiscible solvents, e.g., by decanting, effects separation, that is, purification, of the nanotubes from the soot. The nanotubes may be further refined, if necessary, by repeating the foregoing. The nanotubes can be recovered from the nonpolar solvent by any convenient method depending on the nature of the solvent. Many nonpolar solvents can simply be evaporated while others can be subjected to microfiltration techniques to recover the nanotubes.
For many purposes in both research and commercial applications it is desired to have a plentiful source of undamaged nanotubes. Such nanotubes are sometimes referred to as native state nanotubes because, other than the purification, they have not been chemically or mechanically altered after being created by whatever method was utilized to create the nanotubes. Such native state nanotubes may have, whether by intent or not, certain defects in structure.
The nanotubes may also, again whether by intent or not, chemically or mechanically contain atoms or molecules other than carbon. In any case, what is needed in the art is a simple method of purifying carbon nanotubes without damaging the nanotubes and without using reagents that are harmful to the nanotubes, the person practicing the method, or the environment.