In situ observation of the growth mechanisms of carbon nanotubes under diverse reaction conditions

We report in situ environmental transmission electron microscope observations of the nucleation and growth of multi-wall and single-wall carbon nanotubes formed by the catalytic decomposition of acetylene (C_2H2) on Ni/SiO₂ catalyst. The growth rate, structure and morphology of the carbon nanotubes formed depended upon reaction temperature and pressure. Under 20-100 mTorr of gas pressures at 480°C, serpentine-shaped or zigzag, multi-wall carbon nanotubes grew at an average rate of 35-40 nm sec ^-1. At pressures <10 mTorr at the same temperature, straight single-wall carbon nanotubes with nearly uniform diameters (∼3.5 nm) formed at average growth rates of 6-9 nm sec^-1. The growth of both straight and serpentine carbon nanotubes tends to proceed at non-uniform rates, with frequent pauses followed by growth spurts. The nanotubes frequently contained sharp bends that turned the nanotube axis by ∼60° and ∼120°. We conjecture that the bends are related to the change in growth direction that is dictated by the crystallographic orientation of the catalyst particle. The rotations of the nanometer-sized catalyst particle may be caused by transient melting-recrystallization events caused by local thermal variations. The nanotube attempts to follow the preferred growth direction, while simultaneously attempting to maintain a seamless 3-coordinated graphene wall. This latter condition is most easily satisfied by the introduction of pentagon-heptagon defect pairs dissociated to the opposite sides of the nanotube creating the 60° bend.