||Carbon nanotubes (CNTs) were grown by chemical vapor deposition (CVD) in the temperature range of 800-1000oC using the Co-Mg and Co/Mo-Mg catalysts. The effects of catalyst composition, Co/Mo ratio and growth temperature on the CNTs were investigated.
The Co-Mg and Co/Mo-Mg catalysts with 1-25 at% Co or Co/Mo content were prepared by the coprecipitation method using NaOH as precipitating agent. The XRD results indicated that Co/Mg and Co/Mo-Mg catalysts have a brucite-like layer structure, and the (001) d-spacing of Co/Mo-Mg catalyst was larger than that of Co-Mg catalyst probably due to the insertion of Mo-containing species between the layers, leading to the reduction of grain size of Co/Mo-Mg catalyst.
The effect of metal content (Co or Co/Mo) on the carbon yield is different for the Co-Mg and Co/Mo-Mg catalysts. The carbon yield decreased with increasing metal content in the former, but increased in the latter. However, the metal contents most suitable for the CNT growth were 3 at% and 10 at% for the Co-Mg and Co/Mo-Mg catalysts, respectively, based on the SEM observation. In these optimum conditions, the carbon yield of Co/Mo-Mg catalyst is twice that of Co-Mg catalyst. For the Co/Mo (1/3) catalysts, a lot of multi-walled CNTs (MWCNTs) could be found for a metal content over 15 at%, whereas the products were mostly double-walled CNTs (DWCNTs) when the metal content was below 10 at%.
For the Co/Mo catalysts with a metal content of 10 at%, the carbon yield of Co/Mo (3/1) catalyst at 900℃ was lower and the Co/Mo (1/1) and Co/Mo (1/3) catalysts had higher and similar yield and also less amorphous carbon. Some triple-walled and four-walled CNTs could be found for the Co/Mo (3/1) catalysts, whereas almost all the products were DWCNTs for the Co/Mo (1/1) and Co/Mo (1/3) catalysts. For the growth temperature higher than 900℃, the carbon yield was found to increase with increasing temperature, but the number of CNTs decreased and MWCNTs with bigger diameter and carbon nanofibers could be found in the products. The products grown at 850℃ and 900 ℃ were similar in terms of yield and morphology. For a growth temperature of 800℃, higher yield CNTs could be synthesized using Co/Mo (3/1) catalysts.