(A) study on the behavior of fe catalysts in the growth of carbon nanotubes

Abstract

The behavior of Fe catalyst in the growth of carbon nanotubes and following growth mechanism are studied in this thesis. In order to investigate the nucleation and growth mechanism of carbon nanotubes, it is essential to obtain information about intermediate phases such as a carbide. Many research groups have reported the presence of iron carbides during CNTs synthesis. However no researchers performed the characterization of catalytic particle inside before CNTs synthesis. For the investigation of the behavior of catalytic particles at the initial stage before CNTs was grown, the specimens were prepared which are synthesized for the very short time (0sec, 20sec, 40sec, 1min) at 850℃. The SEM analysis was carried out to show the morphology of catalytic particles and CNTs. The TEM (conventional and in-situ) analysis was used to identify the microstructure of them. The Fe thin film was deposited on Si substrate by DC-sputtering, which was identified as $Fe_3O_4$ polycrystllines with 7-8nm thickness. After heating at 850℃ and NH3 treatment, the $Fe_3O_4$ thin layer changed into $Fe_3O_4$ nanoparticles with the diameter in the range 20-50nm. Fe3O4 nanoparticles acted as nucleation seeds of the growth of CNTs. Most of nanoparticles were polycrystallines, while some of them were single crystals having low index facet planes. These single crystals did not play a role in the growth of CNTs due to low reaction rate of the decomposition of hydrocarbon gas($C_2H_2$) and reduction of $Fe_3O_4$ phase. After flowing reaction gases for 20sec, there wasn’t any remarkable change in the $Fe_3O_4$ nanoparticles, indicating that the decompositon of $C_2H_2$ gas and reduction reaction of $Fe_3O_4$ were not occurred yet. In the case of infusing acetylene gases for 40sec, a significant change was observed. The reduction of $Fe_3O_4$ particles(the presence of FeO) and intermediate phases (γ-Fe, $Fe_3C and $Fe_2C) caused by carbon bulk diffusion were observed through selective area d...