SERS spectroscopy studies on the electrochemical oxidation of single-walled carbon nanotubes in sulfuric acid solutions

Surface-enhanced Raman scattering (SERS) and cyclic voltammetry (CV) were used to investigate oxidation–reduction processes of single-wall carbon nanotube (SWNT) films deposited on Au supports in 0.5 M H₂SO₄ solutions. In the potential range (0; +1000) and (0; +1500) mV versus saturated calomel electrode (SCE), the oxidation–reduction reactions of SWNT films are quasi-reversible and irreversible, respectively. Anodic polarization of SWNT films until +1000 mV versus SCE produced compounds similar to the bisulfate intercalated graphite. Regardless of excitation wavelength, i.e. 1064 or 676.4 nm, variation in the Raman spectra exhibited a decrease in the intensity of the bands associated with the radial breathing mode (RBM) situated in the 120–240 cm⁻¹ spectral range. Also an increase in the intensity of the D band is accompanied an up-shift of this band. A gradual decrease of the Breit–Wigner–Fano component was observed at λ_exc=676.4 nm. Partial restoration of the Raman spectra was achieved by a subsequent alkaline solution treatment. Potentials higher than +1000 mV versus SCE resulted in SWNTs breakage and fragments of different length were formed such as closed-shell fullerene. This was observed in the SERS spectrum by: (i) the disappearance of the RBM band, (ii) the increased D-band shifted to ca. 1330 cm⁻¹ and (iii) the appearance of a new band at 1494 cm⁻¹, frequently observed also in the Raman spectrum of fullerenes on the type C₇₀, C₈₄, C₁₁₉, as well as in its derivative compounds (e.g. C₆₀O, clathrates, etc.). Appearance and increase in the intensity of the Raman band at 1494 cm⁻¹ as result of an anodic polarization of the SWNT film in solution of H₂SO₄ 0.5 M in 1-butanol is a further evidence of the nanotubes breakage.