Raman study of multiwalled carbon nanotubes functionalized with oxygen groups

We present changes in the first and second order Raman spectra of multiwalled carbon nanotubes (MWNTs) functionalized with oxygenated groups. The oxygen groups were introduced onto the nanotube surface through two strong acid purification routes: (1) reflux in concentrated (70%) HNO3 acid for 4h at 80°C and (2) ultrasonification in 3 HNO3 (70%):1H2SO4 (98%) for 8.5h. Raman spectroscopy, using two laser excitation wavelengths (514.5 and 632.8nm), x-ray photoelectron spectroscopy, and thermal gravimetric analysis were employed to study the evolution of the products. All the techniques revealed a higher degree of functionalization for scheme 2 compared to scheme 1. Charge transfer phenomena were manifested by a shift of the C1s core level towards higher binding energies. We found that the intensity of both the D and G energy Raman modes if normalized to the second order mode D* mode follows similar trends upon acid treatments. We interpret this result together with the observed dispersion of G mode as an indication that the G mode in carbon nanotubes is defect induced in a double resonant process. Both acid schemes cause an upshift of D and G Raman modes, due to intercalation of acid molecules, exerting pressure on the sp2 structure and an electron transfer from the π states in MWNTs to the oxygen atoms.