Effect of Oxidation on Electronic and Geometric Properties of Carbon Nanotubes

We have studied an oxidation effect of multiwalled carbon nanotubes (MWCNTs) grown by thermal chemical vapor deposition (CVD) using ultraviolet photoelectron spectroscopy (UPS) and transmission electron microscopy (TEM). With an increasing oxygen exposure, the carbon 2 p -π states at ∼3 eV below the Fermi level in the UPS spectra almost disappear, whereas the 2 p -σ states around 6 eV are significantly increased. Annealing above 1000 K results in an increase of the density of states (DOSs) near the Fermi level, although a complete recovery of the 2 p -π electron states is not observed. We also observe a mode around 23 eV that is associated with the collective σ+π plasmon excitation and chemisorbed C–O bonds, which is confirmed by the density functional calculations.