Ozone Oxidation of Single Walled Carbon Nanotubes from Density Functional Theory

Ozone is known to react with single-walled nanotubes (SWNTs) to form oxide species on the nanotubes and, upon annealing, to etch the SWNTs. However, the mechanism of ozone attack is not known. We use gradient-corrected density functional theory to compute the potential energy surfaces for O-3 dissociation on the side-wall of a pristine (8,8) SWNT. Two decomposition pathways were considered; the first involves the formation of a Criegee intermediate, with a barrier of 17 kcal/mol, followed by transformations leading to lactone, quinone, and carbonyl functional groups. The activation barriers for these transformations are below 23 kcal/mol. The cleavage of the lactone group, evolving CO and CO2, have barrier heights of 39.4 and 49.3 kcal/mol, respectively. This agrees well with experimental findings that the evolution of CO2 and CO occur at 600 K. The second decomposition pathway involves the direct cleavage of the ozonide, forming a singlet O-2 and an ether or epoxide group on the SWNT. This pathway competes with the Criegee mechanism; the barrier for forming singlet O(2)is 7.9 kcal/mol, which is 9.1 kcal/mol lower than the barrier to formation of the Criegee intermediate, indicating that formation of ether or epoxide groups is kinetically favored. However, formation of ester and carbonyl groups could proceed by subsequent addition of O-3 on newly generated defect sites. Vibrational frequency calculations were carried out on cluster models in order to predict infrared absorption signals of local structures. The calculated results for C=O stretching frequencies agree well with experiments. Analysis of the calculated frequencies indicates that the unassigned experimental band at 1380 cm(-1) is due to ester and ether groups, while the unassigned band at 925 cm(-1) is due to epoxide groups. The vibrational frequency of the O+-O- stretch in the Criegee intermediate is in the range 1055-1096 cm(-1).