Evolution of Nanoparticles in the Gas Phase during the Floating Chemical Vapor Deposition Synthesis of Carbon Nanotubes

The evolution of nanoparticles (NPs) in the gas phase during the floating chemical vapor deposition synthesis of carbon nanotubes (CNTs) was investigated. NPs were detected in the gas phase along the reactor axis, and their nature and size were characterized by high-resolution transmission electron microscopy equipped with energy-dispersive spectroscopy analysis. Their roles during the CNTs growth were also discussed by correlating the information about the morphology of the CNTs synthesized on the substrate along the reactor axis. It is found that iron NPs form and grow quickly in the front of the reactor at a temperature above 750 degrees C, whereas the NP growth finally terminate when they are encapsulated by the carbon layers during the downstream region of the reactor. Because of the carbon encapsulation, iron NPs could not directly contribute to the CNT growth on the substrate. These results demonstrate that homogeneous nucleation is not the prerequisite for the CNT growth on the substrate.