High yield multiwall carbon nanotube synthesis in supercritical fluids

Multiwall carbon nanotubes (MWNTs) with outer diameters of 10 - 50 nm and wall thicknesses of 5 - 20 nm were synthesized in supercritical toluene at temperatures ranging from 600 to 645 degrees C at 8.3 MPa. Nanotube formation was catalyzed by metallocenes such as cobaltocene, nickelocene, and ferrocene or cobalt or iron nanocrystals; toluene served as both the solvent and the carbon source for nanotube growth. Supplemental carbon sources, either hexane or ethanol (similar to 30 vol%), increased the yield of the carbon nanotubes relative to pure toluene, and catalytic amounts of water (0.75 vol%) minimized the formation of carbon filaments and amorphous carbon. Cobaltocene, with ethanol as a supplemental carbon source, gave the highest percentage of nanotubes in the product (similar to 70%) and the highest conversion of toluene to MWNTs (similar to 4%). The MWNTs tended to exhibit bamboo morphology and appear to grow by a folded-growth mechanism with graphitic sheets wrapped around the seed metal particles. Cobaltocene was also found to catalyze coiled nanotube formation, with the appearance of springs, hairpins, lassos, and coiled ropes.