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 °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 (∼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 (∼70%) and the highest conversion of toluene to MWNTs (∼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.