On the kinetics of carbon nanotube growth by thermal CVD method

The role of ammonia (NH3) on obtaining good quality vertically aligned multi-walled carbon nanotubes (CNTs) in thermal chemical vapor deposition (CVD) method has been widely studied. It was generally agreed that NH3 helps to maintain catalyst metal surface active by reacting with amorphous carbon. In this article, a systematic study in varying the temperature and mixing ratio of gases was conducted in order to clarify the role of NH3 and revealed a criterion for optimized condition window in the growth processes. In addition, this study has also carried out a statistical analysis through intensive TEM observations on the tube diameters, bamboo spacing, and the formation rate of each diaphragm under various temperatures and carbon source/NH3 ratios. While the formation of the separation diaphragms were indeed a result of bulk diffusion of carbon atoms from bottom of the Ni nanoparticle following thermal dehydrogenization to the top of the Ni nanoparticle, there were other carbon atoms diffusing presumably via surface diffusion to the CNT-metal interface and contributed to the growth of tube wall; in other words, the CNTs growth is simultaneous renucleation and growth processes, instead of a continuous renucleation and growth process. This kinetics-based mechanism in combination with the proposed role of NH3 could not only successfully explain the effects of the process parameters including temperature and the mixing gas ratio, but also could be used for pursuing the goal of lower growth temperature for thermal CVD method which is very important for many applications of CNTs.