Growth kinetics of MWCNTs synthesized by a continuous-feed CVD method

Unlike two-step chemical vapor deposition (CVD) methods using pre-deposited catalyst particles, in a continuous-feed CVD process, the liquid feed (consisting of catalytic precursor and hydrocarbon source) is continuously supplied into the reactor causing catalyst particle formation, nucleation of carbon nanotubes (CNTs) and CNT growth to occur simultaneously throughout the reaction period. In order to observe these processes, CVD experiments were conducted for different durations (30 s to 3 h) and the product multiwalled carbon nanotubes (MWCNTs) were characterized using scanning electron microscopy. It was found that the nanotubes did not grow in the vapor phase and that substrates played an important role in the growth by providing a place for them to anchor before growth took place. Based on transmission electron microscopy images, it has been suggested that MWCNTs grew by root-growth mechanism from the catalyst particles that were deposited on the substrate during the early stages. At long process times, continuously supplied feed gas produced additional catalyst particles which were deposited mostly on the growing nanotube mat. Due to weak catalyst-mat interaction, the additional nanotubes grew by tip growth. A comprehensive MWCNT growth model has been presented for the continuous-feed CVD.