| Carbon nanotubes(CNTs)have shown great potential for use in many areas due to their unique one-dimensional tubular structure and excellent physicochemical properties.The unique electrical properties,i.e.,semiconducting and metallic characteristic of CNTs,are closely related to their chiral structures.To realize the appealing application of CNTs in electronic devices,it is essential to finely control their chirality and electrical properties.In recent years,cobalt(Co)-based high melting point alloy(e.g.Co-W)catalysts,have shown great promise in the chirality-controlled growth of CNTs,yielding single chirality CNTs with a high content of 97%.However,the fundamental growth mechanism of CNTs from such catalysts remains unclear,such as the active phase of the catalyst,the interfacial structures between the catalysts and growing CNTs and growth kinetics of CNTs.In this dissertation,we focus on the growth mechanism of CNTs from Co-based catalysts and directly investigate the growth thermodynamics and kinetics at(near)atmospheric pressure using a closed-cell environmental transmission electron microscope(E-cell).The main results obtained are as follows:(1)Precise identification of the active phase of Co catalyst for CNT growth and its catalytic growth mechanism.To clarify the controversy about the active phase of Co catalyst for CNT growth and the unclear carbon diffusion mechanism,we in situ investigated the structural evolution of Co catalyst nanoparticles(NPs)during the incubation,nucleation,and growth of CNTs under near-atmospheric pressure using an E-cell.Based on statistics results and strict error analysis,a precise phase identification method was established and the active phase of Co catalyst was identified as orthocrystalline Co3C phase,which remained unchanged during the whole CNT growth process.Density functional theory(DFT)calculations show that Co3C is the thermodynamically preferred phase during CNT growth,and the supply of carbon atoms is realized by diffusion through the catalyst surface and NP-CNT interface.(2)Active phase of Co-W and Co-Mo alloy catalysts and CNT-catalyst interfacial structures.To investigate the growth mechanism of CNTs from Co-W and Co-Mo alloy catalysts,we in situ grew CNTs from Co-W and Co-Mo alloy catalysts under atmospheric pressure using an E-cell.The active phases of the catalysts and CNTcatalyst interface structures during CNT growth were characterized and analyzed.Based on the established precise phase identification method,the active phases of the Co-W and Co-Mo catalyst were identified to be a single phase of cubic M6C type ηcarbide.In addition,independent rotation of catalyst NPs during the growth of CNTs was observed,implying a slippery interface between CNTs and catalyst NPs due to a weak interfacial interaction and undefined orientation dependence.(3)Growth kinetics of CNTs from Co and Co based alloy catalysts.To explore the influence of pressure gap and catalyst composition on the growth kinetics of CNTs,the growth kinetics of CNTs from Co catalyst at different temperatures and gas pressures and the differences in the growth kinetics of CNTs from Co and Co-W/CoMo alloy catalysts were in situ investigated.It was found that pressure was the main factor affecting the growth rates of CNTs on Co catalyst within a certain temperature range and the growth modes of CNTs had different tendencies at different growth rates,i.e.,CNTs grew faster following the tip growth mode at high pressures,while the growth rates of CNTs decreased sharply and tended to grow following the base growth mode at low pressures.By comparing the growth kinetics of CNTs from Co and Co-W/CoMo alloy catalysts at atmospheric pressures,it was found that the growth rates of CNTs from Co-W and Co-Mo alloy catalysts were two orders of magnitude slower than that of pure Co catalyst.Theoretical calculations showed that the carbon supply for CNT growth from Co-based alloy catalysts was achieved by diffusion through the catalyst surface and CNT-catalyst interface,and the 5-6 orders of magnitude difference in the carbon diffusion rates on Co and Co-based alloy catalysts influences the growth rates of CNTs. |
PDF Full Text Request