The effect of reaction temperature on the diameter distribution of carbon nanotubes grown from ethylene decomposition over a Co-La-O catalyst

The production of carbon nanotubes (CNTs) by the chemical catalytic vapor deposition (CCVD) process was examined over a Co-La-O catalyst prepared by combustion synthesis method. Some important advantages of preparing nanotubes are obvious with Co-La-O as catalyst, i.e., the product yield is higher (up to 412.6%) and the reaction temperature is lower (<750 °C) compared with other methods. The effects of reaction temperature on the carbon yield, diameter distribution and quality of CNTs were investigated. Transmission electron microscopy (TEM) studies show that an increase in reaction temperature produces CNTs with larger average diameter and broader diameter distribution. This is basically due to the agglomeration of metallic cobalt at higher reaction temperatures to form larger and non-uniform metallic clusters which are responsible for the growth of CNTs. Raman spectra further elucidate that better graphitized CNTs are produced at higher reaction temperatures. After a single-step purification by hydrochloric acid, the purity of CNTs can reach up to 96%.