Carbon Nanotubes Functionalized by NO₂:  Coexistence of Charge Transfer and Radical Transfer

The adsorption of NO₂ molecules on a series of zigzag (n,0) single-walled carbon nanotubes (SWNTs) (n = 6−12) have been investigated by first-principles methods. The results indicate that the tube diameter and the concentration of NO₂ gas determine the interactions between NO₂ molecules and the tube. The chemisorption of a single NO₂ is only possible for the tube with a small diameter (n < 10), while the second NO₂ molecule can be chemisorbed for all tubes studied here. The additions of more NO₂ molecules in different patterns have also been considered for a (8,0) tube, and the NO₂ groups prefer the pair arrangement with stronger binding energy. According to the results of band structure calculations, overall, the SWNT exhibits a p-type response upon exposure to NO₂ gas and the electron charge transfer is an important reason for explaining the enhancement of conductivity of the tube. Moreover, it is interesting that, accompanied by charge transfer, the chemisorption of NO₂ also leads to the radical transfer from the NO₂ group to the carbon atoms at the ortho and para sites of the six-membered ring. As a result, the SWNT possesses the radical characteristic, which facilitates the further functionalization of the tube wall.