Nature of the Catalyst Particles in CCVD Synthesis of Multiwalled Carbon Nanotubes Revealed by the Cooling Step Study

Abstract

The true chemical nature and physical state of the catalyst particles in Catalytic Chemical Vapor Deposition (CCVD) synthesis of carbon nanotubes are the subject of intense discussions, as it is one of the keys to understand their growth mechanisms. The CCVD method considered in this article involves pyrolysis of mixed liquid aerosols and leads to the synthesis of large carpets of multiwalled nanotubes (MWNTs) partially filled with iron-based materials. The experimental approach consists in studying the influence of the cooling procedure applied at the end of the synthesis. Both slow standard cooling or quenching were performed, and the structure and chemical state of the iron-based particles were compared through complementary local and global investigations involving X-ray diffraction, electron microscopy, electron diffraction, as well as electron energy loss spectroscopy. We clearly demonstrate that iron-based catalyst particles are carbon-rich and oxygen-free in quenched samples, and that they oxidize during the slow cooling step. It is inferred that they are very probably molten supersaturated carbon-metal particles during the NT growth.