Fritz Haber Institute of Max Planck Society , Faradayweg 4-6 , 14195 Berlin , Germany.
Scientific Center for Optical and Electron Microscopy , ETH Zurich , Otto-Stern-Weg 3 , 8093 Zurich , Switzerland.
Nano Lett. 2019 Aug 14;19(8):5380-5387. doi: 10.1021/acs.nanolett.9b01888. Epub 2019 Aug 6.
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 FeC, based on lattice fringes that can be imaged during growth. However, the oscillatory growth behavior of the CNT and the structural dynamics of the apex area strongly indicate that the carbon concentration in the catalyst particle is fluctuating during the course of CNT growth. Extended observations further reveal that the catalyst splitting can occur: whereas the majority of the catalyst stays at the base and continues catalyzing CNT growth, a small portion of it gets trapped inside of the growing nanotube. The catalyst splitting can take place multiple times, leading to shrinkage of both, catalyst size and diameter of CNT, and finally the growth termination of CNT due to the full coverage of the catalyst by carbon layers. Additionally, in situ observations show two more scenarios for the growth termination, that is, out-migration of the catalyst from the growing nanotube induced by (i) Oswald ripening and (ii) weakened adhesion strength between the catalyst and CNT.
自发现以来，人们一直致力于研究碳纳米管（CNTs）的生长机制。随着原位技术在 CNT 生长研究中的应用，人们对 CNT 生长机制有了重要的认识，这极大地提高了我们的理解水平。然而，以前的原位实验主要在远低于实际条件的低压下进行。在相关条件下关于生长动力学的直接信息仍然缺乏，因此非常需要这些信息。在这项工作中，我们通过原位透射电子显微镜报告了在近常压下观察多壁碳纳米管（MWCNT）生长和终止的原子级观测结果。基于实时成像，我们能够揭示出在催化 CNT 生长过程中，工作催化剂的尖端在不断地重塑，而在基底处，催化剂仍然呈多面体形，几乎没有任何形态变化。基于生长过程中可以成像的晶格条纹，我们确定活性催化剂为结晶态 FeC。然而，CNT 的振荡生长行为和尖端区域的结构动力学强烈表明，在 CNT 生长过程中，催化剂颗粒中的碳浓度在不断波动。进一步的扩展观察揭示了催化剂分裂的发生：尽管大部分催化剂仍留在基底处并继续催化 CNT 生长，但一小部分催化剂会被困在生长中的纳米管内。催化剂分裂可能会多次发生，导致催化剂尺寸和 CNT 直径缩小，最终由于碳层完全覆盖催化剂而导致 CNT 生长终止。此外，原位观察还显示了两种更常见的生长终止情景，即（i）奥斯特熟化和（ii）催化剂与 CNT 之间的结合力减弱导致催化剂从生长中的纳米管中向外迁移。
