Growth and Termination Dynamics of Multiwalled CarbonNanotubes at Near Ambient Pressure: An in Situ Transmission ElectronMicroscopy Study

摘要

Understanding the growth mechanism of carbon nanotubes (CNTs) has been long pursued since its discovery. With recent integration of in situ techniques into the study of CNT growth, important insights about the growth mechanism of CNT have been generated, which have improved our understanding significantly. However, previous in situ experiments were mainly conducted at low pressures which were far from the practical conditions. Direct information about the growth dynamics under relevant conditions is still absent and thus is highly desirable. In this work, we report atomic-scale observations of multiwalled CNT (MWCNT) growth and termination at near ambient pressure by in situ transmission electron microscopy. On the basis of the real-time imaging, we are able to reveal that the working catalyst is constantly reshaping at its apex during catalyzing CNT growth, whereas at the base the catalyst remains faceted and barely shows any morphological change. The active catalyst is identified as crystalline Fe3C, based on lattice fringes that can be imaged during growth. However, the oscillatory growth behavior of the CNT and the structural dynamicsof the apex area strongly indicate that the carbon concentration inthe catalyst particle is fluctuating during the course of CNT growth.Extended observations further reveal that the catalyst splitting canoccur: whereas the majority of the catalyst stays at the base andcontinues catalyzing CNT growth, a small portion of it gets trappedinside of the growing nanotube. The catalyst splitting can take placemultiple times, leading to shrinkage of both, catalyst size and diameterof CNT, and finally the growth termination of CNT due to the fullcoverage of the catalyst by carbon layers. Additionally, in situ observationsshow two more scenarios for the growth termination, that is, out-migrationof the catalyst from the growing nanotube induced by (i) Oswald ripeningand (ii) weakened adhesion strength between the catalyst and CNT.