Characterization of thin multi-walled carbon nanotubes synthesized by catalytic chemical vapor deposition

Thin multi-walled carbon nanotubes (t-MWCNTs) synthesized by catalytic chemical vapor deposition were characterized by transmission electron microscopy (TEM), Raman spectroscopy, and gas adsorption. TEM observations showed that the innermost diameter ranges from less than 1 to 5 nm and the outermost diameter ranges from 4 to 12 nm, which are much smaller than conventional thick MWCNTs. We observed clear radial breathing modes in Raman spectra, which are attributed to the existence of inner nanotubes in small diameters less than 3 nm. We also performed N2 adsorption at 77 K after the tip opening of t-MWCNTs by nitric acid in order to analyze the tube inner diameters. The pores with sizes less than 4 nm corresponding to the inner empty space of t-MWCNTs were largely developed, in good agreement with TEM and Raman observations.