The effects of catalyst structure and morphology on the growth of carbon nanotubes

In order to control the morphologies of the carbon nanotubes and carbon capsules, it is crucial to investigate the microstructure and properties of catalysts. Fe/Co/SiO_2 areogel catalysts prepared by a sol-gel process followed by supercritical drying were used for the synthesis of carbon nanotubes and nano-capsules. Crystal transformations and morphologies of the catalysts treated at various temperatures were examined by N_2 physisorption, XRD, TEM, HRTEM, EDS, and SAED analysis. The relationship between the carbon weight-gain and treatment temperatures for the catalysts was also investigated. Results show that with the increase of heat-treatment temperature from 600～1?000?℃, the BET specific surface area and the carbon weight-gain of the catalysts decrease from 312.4?m~2/g to 79.6?m~2/g and from 2.548 to 0.415, respectively and an amorphous catalyst with particle size of 5?nm transforms into crystal particles with size of about 60?nm. The product made with the catalyst treated at low temperatures mainly consists of carbon nanotubes, whereas, it is composed of carbon encapsulated Fe/Co nanoparticles if the catalyst is treated at high temperatures.