Rice University Professors Yuri Mackeyev and Lon J. Wilson have developed a fast and “facile” method of removing metal impurities from carbon nanotubes with distilled liquid bromine at room temperature. According to U.S. Patent Application 20100021797, the technique involves treating carbon nanotubes with distilled bromine in a substantially oxygen- and water-free atmosphere and then removing the distilled bromine from the carbon nanotubes. Purified carbon nanotubes are produced with an iron content from about 2.5 to about 3.5 percent by weight and are substantially free of derivatization at the ends and defect sites are made available via this method.
The Mackeyev and Wilson's purification reaction may be carried out under a dry nitrogen or argon atmosphere or other inert gas which has been appropriately been dried and determined to be substantially oxygen free. The purification by reaction with bromine may be conveniently carried out at room temperature. The distilled bromine is removed from the carbon nanotubes after a period of time, typically about thirty minutes.
The purified carbon nanotubes may include single-wall carbon nanotubes (SWCNTs), multi-wall carbon nanotubes, and double wall carbon nanotubes. Additionally, the method may also be carried out on mixtures of these carbon nanotube types. As SWCNTs are relatively more reactive than the multi-wall carbon nanotube counterparts, a procedure developed for SWCNTs would likely be applicable to the higher multi-wall carbon nanotube types, says the inventors
Advantageously, the bromine method of purifying carbon nanotubes uses Br2(l) as an oxidant to produce SWCNTs of high purity (ca. 1-4% by weight iron) without elevated temperatures and pressures and with minimal sidewall damage. The SWCNTs are substantially free of derivatization at the ends or defect sites of the SWCNTs. The method presents a convenient laboratory procedure, is commercially viable, since Br2(l) is relatively inexpensive and can be recycled. In addition, the energy cost of the process is relatively low, and SWCNT purity and integrity have been judged to be generally as good or better than can be obtained by alternative oxidative purification methods.
FIG. 2 a) FIG. 2a shows TEM image of raw SWCNTs and b) shows Br2(l)-purified SWCNTs purified to a residual iron content of ca. 3% by weight.