Growth of double-walled carbon nanotubes from silicon oxide nanoparticles

Double-walled carbon nanotubes (DWCNTs) were synthesized by a metal-catalyst-free chemical vapor deposition method using silicon oxide nanoparticles as a catalyst. The diameters and lengths of the DWCNTs are in the ranges of 3–5 nm and 1–5 μm, respectively. The amount of DWCNTs produced is about 70%, while the remainder is single-walled carbon nanotubes. A heat treatment of the SiO2/Si substrate used was found to be crucial for controlling the size of the catalyst nanoparticles, and hence for the growth of the DWCNTs. Flat or cone-shaped caps were observed for the DWCNTs, indicating that the growth of the DWCNTs from the non-metal catalyst follows a vapor–solid–solid mechanism. Energy dispersive spectroscopy and X-ray photoelectron spectroscopy characterizations confirmed that no metal impurity exists in the obtained DWCNT samples.