Raman Characterization of Single-Walled Nanotubes of Various Diameters Obtained by Catalytic Disproportionation of CO

Single-walled carbon nanotubes prepared by disproportionation of CO over Co-Mo/SiO₂ catalysts have been characterized by Raman spectroscopy, using several excitation energies. By varying the reaction temperature, different ranges of nanotube diameter were obtained. The average diameter of a single-walled nanotube produced at 750 °C was 0.9 nm, while it increased up to about 1.5 nm when the synthesis was conducted at 950 °C. The analysis of the Raman spectra obtained with a range of laser excitation energies not only gives a definite description of the single-walled nanotubes diameters but also helps differentiate the metallic or semiconducting character of the samples. This analysis can be done by comparing the experimental data with calculated gap energies as a function of nanotube diameter as well as comparing the relative intensity of bands centered at 50–60 cm^-1 lower than the tangential G mode. The analysis of this feature, which can be fitted with a Breit–Wigner–Fano line, offers a method for distinguishing between metallic and semiconducting single-walled carbon nanotubes.