The factors of preparation of carbon nanotubes by catalytic decomposition of methane

Using Fe-MgO, Co-MgO and Ni-MgO catalysts preparated by citric acid method, carbon nanotubes (CNTs) were synthesized from decomposition of methane at 873 K, 973 K and 1013 K, respectively, and were characterized by TEM and Raman spectra. The effects of catalysts, preparation temperature and reaction time on the shape, yield and inter - structure of carbon nanotubes were discussed. TEM results and Raman spectra show that the shapes and inter-structures of carbon nanotubes produced on different catalysts at the same temperature are quite different. Carbon nanotubes synthesized on Fe-MgO incline to be rods, with very thick wall, and the diameter varied greatly from 10 nm to 80 nm. However, the diameter of carbon nanotubes produced on Co-MgO is about 35 nm, and about 20 nm on Ni-MgO. As can see from the Raman spectra, the G peaks of carbon nanotubes produced on the three catalysts shift slightly, which indicate the difference of the inter-structure of nanotubes. The experiments show that the yield of carbon nanotubes varied with decomposition temperature. Using Fe-MgO catalysts, yield of carbon nanotubes grows with the decomposition temperature. However, using Ni-MgO catalysts, the yield drops when the temperature increases. The reaction temperature with the largest yield of carbon nanotubes is higher than 1013 K on Fe-MgO catalysts, about 973 K on Co-MgO catalysts, and lower than 873 K on Ni-MgO catalysts, respectively. The results also show that the yield of carbon nanotubes does not increase with the decomposition time. There is a saturation time of the carbon nanotube growth. The carbon nanotubes do not grow any more when the saturation time reaches. For example, on Ni-MgO catalysts, the carbon nanotubes stop growing after decomposition of CH4 for 2 h.