Formation of carbon nanofibers/nanotubes by chemical vapor deposition using Al2O3/KOH

Carbon nanofibers/tubes were successfully formed by catalytic cracking methane using Al2O3/KOH as the catalyst precursors. The effects of pyrolysis temperature and the molar ratio of Al2O3 to KOH on the growth of carbon nanofibers/tubes were investigated. The results showed that the precursors were transformed into potassium aluminate (K2O·Al2O3) during the heating process. Pyrolysis temperature and molar ratio have great influence on the conversion rate of methane, the morphology and the graphitization degree of the products. The optimum temperature for the growth of carbon nanofibers/tubes was 900 °C and the diameter was 50–150 nm. Clusters of carbon nanofibers/tubes were also observed in the products. Finally, the formation mechanism of carbon nanomaterials was discussed based on the morphological changes of the products at different temperatures and the existing theories. At low temperature, methane was cracked under the action of catalyst to form metastable carbon nanosheets. After increasing the temperature, the product was transformed into more stable carbon nanofibers/tubes. However, excessive temperature would increase the size of the catalysts, thereby weakening the precipitation power of carbon nanofibers/tubes, and finally forming carbon nanospheres.