Chemical vapour deposition of pyrolytic carbon on carbon nanotubes: Part 3: Growth mechanisms

As a first step to identify the growth mechanism of various pyrolytic carbon deposit morphologies onto multiwall carbon nanotubes (MWNTs) presented in earlier papers, we determined their growth chronology by carrying-out synthesis experiments involving a large time range. We propose that the formation of any of the deposit morphologies is the consequence of the primary formation of hydrocarbon liquid droplets in the gas phase and their subsequent deposition onto the MWNTs. This makes the formation mechanisms of the various deposit morphologies depend on physical phenomena related to the wetting of nanotube surfaces by the droplets, where the [droplet diameter]/[nanotube diameter] ratio plays an important role. The droplets are the result of the recombination of species issued from the cracking of the gaseous precursor (methane), and their characteristics (number, size, and aromaticity) depend on experimental parameters such as temperature, time of flight, and gas phase composition. The results bring a new light to the currently admitted hypotheses for the mechanisms of pyrolytic carbon deposition, and revitalise the liquid droplet theory formerly proposed by Grisdale in the 1950s, at least in the range of conditions investigated.