Growth window and possible mechanism of millimeter-thick single-walled carbon nanotube forests

Our group recently reproduced the water-assisted growth method, so-called "SuperGrowth," of millimeter-thick single-walled carbon nanotube (SWNT) forests by using C ₂H ₄/H ₂/H ₂O/Ar reactant gas and Fe/Al ₂O ₃ catalyst. In this current work, a parametric study was carried out on both reaction and catalyst conditions. Results revealed that a thin Fe catalyst layer (about 0.5 nm) yielded rapid growth of SWNTs only when supported on Al ₂O ₃, and that Al ₂O ₃ support enhanced the activity of Fe, Co, and Ni catalysts. The growth window for the rapid SWNT growth was narrow, however. Optimum amount of added H ₂O increased the SWNT growth rate but further addition of H ₂O degraded both the SWNT growth rate and quality. Addition of H ₂ was also essential for rapid SWNT growth, but again, further addition decreased both the SWNT growth rate and quality. Because Al ₂O ₃ catalyzes hydrocarbon reforming, Al ₂O ₃ support possibly enhances the SWNT growth rate by supplying the carbon source to the catalyst nanoparticles. The origin of the narrow window for rapid SWNT growth is also discussed.