In situ transmission electron microscopy of temperature-dependent carbon nanofiber and carbon nanotube growth from ethanol vapor

Abstract

Carbon nanofibers (CNFs) and carbon nanotubes (CNTs), which have been known for decades, recently gained significant commercial interests for various applications. However, the controlled synthesis of CNF/CNT has been hindered by the lack of nanoscale experimental evidence on how the growth temperature affects the growth process under real growth environment. We report the in situ transmission electron microscopy (TEM) experiments to directly observe Ni catalyzed CNF/CNT growth from alcohol precursor at near atmospheric pressure using a homebuilt bubbler system for the introduction of ethanol vapor. Using real time imaging, we revealed the active state of the Ni catalyst during the temperature-dependent CNF/CNT growth (600-800 C). We observed the formation of CNFs starting from 600 C and CNTs were formed at higher temperatures. The lattice parameter measurements pointed to an expansion of the Ni lattice as the temperature was increased, which we attribute to increased carbon solubility. The as-grown CNFs and CNTs were further characterized by XPS, Raman spectroscopy, and EELS, that allowed to have a highly reliable overall view of the structure changes with temperature. Results revealed that the change in structure with temperature was caused by the combined effects of increased carbon solubility and graphitization of the walls of the growing nanostructure. This increased carbon solubility in turn affected carbon diffusion and could be the reason for the change in structure from CNF to CNT at high temperature. Using in situ TEM we clearly revealed the effect of growth temperature on the structural changes.