Carbon nanotube production

Hocaoğlu, Caner
Carbon nanotubes (CNTs), allotropes of carbon with a cylindrical nanostructure, are one of the most attractive research subjects for scientists and industry because of their extraordinary chemical, electrical, optical, mechanical and thermal properties, and their wide range of potential application areas. Mainly, there are two types of carbon nanotubes: single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). The most commonly used methods for carbon nanotube production are arc discharge, laser ablation, and chemical vapor deposition (CVD). In the CVD method, CNTs are produced from thermal decomposition of the carbon-containing molecules on a suitable transition metal catalyst. CVD method enables large scale production of high-quality CNTs with low cost compared to other methods. The growth and morphology of CNTs can be controlled by adjusting the reaction parameters. In this study, Co and Mo impregnated CaCO3 catalysts were synthesized at different Co:Mo weight ratios and calcined at different temperatures. XRD results showed that there was mainly CaCO3 compound in the catalysts calcined at 500ºC whereas the catalysts calcined at 700 and 750 ºC were mainly composed of CaO and Ca(OH)2 compounds. In addition to these, CaMoO4, CoO, CoMoO4 and Mo2C were the other solid phases mainly observed in all catalysts. The production of CNTs was performed by chemical vapor deposition of acetylene at a temperature range of 500-700˚C using Co-Mo/CaCO3 catalysts. The synthesized nanotubes were purified with a single-step purification process in diluted nitric acid solution. SEM and TEM results showed that the synthesized materials were multi-walled carbon nanotubes with outer diameter ranging from 13 to 138 nm. MWNTs were mostly closed-end. The CNT yield was increased with an increase in the catalyst calcination and reaction temperatures. The rise in the Co:Mo weight ratio also resulted in higher CNT yields. The highest CNT yield was obtained at a reaction temperature of 700˚C using the catalyst with a Co:Mo weight ratio of 6 and a calcination temperature of 750ºC. An increase in nanotube diameters was observed with an increase in synthesis temperature. Thermal analyses revealed that the oxidation temperatures of MWNTs were around 700ºC and the purity of MWNTs was generally higher than 96%. On the other hand, Raman spectroscopy results showed the presence of structural defects in CNTs. Purified MWNTs showed Type II isotherms for nitrogen adsorption. Multi-point BET surface areas of purified nanotubes were in the range of 24.8-89.9 m2/g.