William Marsh Rice University (Houston, TX) in U.S. Patent Application 20100008843 details a multi-step process for the purification of single-wall carbon nanotubes (SWNTs). The purification processes remove the extraneous carbon as well as metal-containing residual catalyst particles.
Known methods of single-wall carbon nanotube production result in a single-wall carbon nanotube product that contains single-wall carbon nanotubes in addition to impurities including residual metal catalyst particles and amounts of small amorphous carbon sheets that surround the catalyst particles and appear on the side of the single-wall carbon nanotubes.
The present purification processes remove the extraneous carbon as well as metal-containing residual catalyst particles, say inventors Robert H Hauge, Ya-Qiong Xu, Haiqing Peng, Richard E Smalley and Irene Morin Marek.
The method for purifying small-diameter CNTs, includes the steps of:
a) providing a quantity of unpurified small-diameter CNTs,wherein the unpurified small-diameter CNTs further comprise metal particles and non-nanotube carbon structures;
b) simultaneously exposing the unpurified small-diameter CNTs to oxygen and a fluorine-containing gas at elevated temperatures to form metal oxide/fluoride particles and partially-purified small-diameter CNTs,wherein the exposing step oxidizes and removes at least a majority of the non-nanotube carbon structures; andwherein the exposing step oxidizes the metal particles to metal oxide particles and inactivates the metal oxide particles by at least partially converting the metal oxide particles into metal fluoride particles to form the metal oxide/fluoride particles;wherein the metal oxide/fluoride particles are non-catalytic toward small-diameter CNT oxidation;
c) annealing the partially-purified small-diameter CNTs in an atmosphere comprising H2 to yield reduced partially-purified small-diameter CNTs;
d) extracting the reduced partially-purified small-diameter CNTs with an acid to yield purified small-diameter CNTs, wherein the extracting step comprises removing the metal oxide/fluoride particles.
FIG. 3 depicts TEM images of (a) raw material and (b) a purified sample treated with 20% O2 and 5% SF6.