Fabrication of a metal oxide layer on a multi-walled carbon nanotube surface by in-flight coating using gas-phase deposition

Understanding the mechanism of the formation of a coating layer on the surface of multi-walled carbon nanotubes (MWCNTs) is a critical issue in coating technology applications. By understanding this mechanism, the structure of the coating layer can be easily controlled. In the present study, the objective is to investigate the effect of alkoxy groups on the formation of a coating layer on MWCNT surfaces. Six types of alkoxy groups based on the metal precursors were used. Metal oxide layers of SiO_x, TiO_x, and AlO_x coated on MWCNTs have been investigated by in-flight coating, i.e., by using a combination of aerosolization of MWCNTs and plasma-enhanced chemical vapor deposition. Electron microscopy images showed that titanium oxide tended to form nanoparticulates in the case of long alkoxy precursors, while silica oxide formed a smoother surface. A longer alkoxy group would generate a rougher film morphology than a short alkoxy group. Furthermore, the entire MWCNT surfaces were successfully coated by the silica-based oxide precursors. The particles adhered to the surface in the case of the aluminum-based precursor. The morphologies of the layer structures were significantly influenced by the length of the alkoxy groups and the metal precursors. The difference in the coating structures was ascribed to the different dissociation rates among them after bombardment by plasma fields.