Carbon nanotubes produced by aerosol pyrolysis: growth mechanisms and post-annealing effects
Résumé
Since the first observation of carbon nanotubes in 1991, their synthesis by different techniques has been extensively investigated. Recently, CVD-based methods have attracted much attention because of both their versatility and the quality of the nanotubes obtained. This work is focused on the aerosol pyrolysis process which is a catalytic CVD-based method involving pyrolysis of mixed liquid aerosols composed of both liquid hydrocarbon and catalyst precursor. This process generates clean and aligned multiwalled carbon nanotubes. To analyse and further understand the first nucleation and growth stages of nanotubes in the aerosol pyrolysis process, several samples composed of nanotubes starting to grow were prepared by flowing the aerosol in the reactor for short duration. Samples were analysed by SEM. First results indicate that the first stage involves the formation of a layer of catalyst particles in the [10–100] nm range on top of which nanotubes start to grow. An important issue for nanotube application is the control of their crystalline structure. Therefore, post-annealing treatments under flowing argon were performed on the as-synthesised multiwalled carbon nanotubes. The effects of such treatments on the structure of nanotubes are determined by a comparative HRTEM analysis and a thermogravimetric analysis under air. Results demonstrate that annealing treatment between 1700 and 2000 °C is an efficient way to improve the structure of nanotubes and thus to make them more resistant against oxidative atmosphere.