Effect of centrifugation on the purity of single-walled carbon nanotubes from MCM-41 containing cobalt

Centrifugation is applied to purify single-walled carbon nanotubes (SWCNTs) grown from MCM-41 containing cobalt (Co-MCM-41) catalysts by chemical vapor deposition. We demonstrated that an optimized centrifugation force can lead to analytically pure SWCNTs. Four criteria in terms of physical and chemical properties of Co-MCM-41 were proposed for quality control of catalysts. After centrifugation at 120 000 g, SWCNT purity index based on absorption spectra of sodium cholate (SC) dispersed tubes is 0.268, approaching the previously estimated value (̃0.325) for analytically pure SWCNTs. It is a significant improvement compared to previously reported high-purity samples (e.g., super purified HIPCO (Unidym) with purity index = 0.087, and SWeNT SG 65 (South West Nano Technologies) with purity index = 0.194). In photoluminance emission spectra of SC dispersed SWCNTs, (1) line width for E₁₁ emission peaks at E₂₂ excitation energy, (2) intensities of three sidebands, and (3) intensities of E₁₁ emission peaks at E₃₃ excitation energy are found to be related to SWCNT purity. Theoretical models predict that SC molecules adsorbed on the surface of SWCNTs account for about 40-60 wt % of tube samples. Thermogravimetric analyses (TGA) confirm that SC cannot be easily washed away by common solvents. However, after removal of metallic residues (0.15 wt % by TGA), absorbed SC molecules can be eliminated by heating in air at 350 °C for 30 min. The purification protocol may prove useful in evaluating health and environmental effects of SWCNT material and improving separation efficiencies of nanotube species according to electronic type, diameter, length, and chiral handedness.