Soft-x-ray photoemission spectroscopy and ab initio studies on the adsorption of NO2 molecules on defective multiwalled carbon nanotubes

The adsorption of NO2 molecules on defective multiwalled carbon nanotubes has been studied by soft-x-ray photoemission. The valence band and carbon core-level spectra have been acquired before, during, and after NO2 exposure. The spectra show a reversible decrease of the density of states at the top of the valence band when NO2 molecules are adsorbed on the (carbon nanotubes) CNTs. No shift of the C 1s spectra has been observed. Theoretical calculations, using density-functional theory, have been performed on the CNT+NO2 system, considering semiconducting nanotubes with different diameters and introducing a Stone-Wales [Chem. Phys. Lett. 128, 501 (1986)] defect. The calculation confirms the decrease of the density of states at the top of the valence band in the CNT+NO2 system, while close to the adsorption site new states appear very close to the Fermi level.