UV-light enhanced oxidation of carbon nanotubes

Ab initio density functional theory (DFT) calculations of the interactions between selected semiconducting and metallic single-walled carbon nanotubes (SWCNTs) (as well between single and double graphene sheets) and single oxygen molecules are carried out in order to provide a rationale for the recent experimental observations of UV-light accelerated oxidation of carbon nanotubes and the accompanying changes in the thermoelectric power. The computational results obtained show that these experimental findings can be related to UV-light excitation of oxygen molecules from their ground spin-triplet state into a higher-energy spin-singlet state. Such excitation lowers the activation energy for molecular-oxygen chemisorption to a nanotube, increases the adsorption energy and promotes charge transfer from the nanotube to the oxygen molecule. Lattice defects such as 7-5-5-7 and Stone–Wales defects are found to play a critical role in enhancing oxygen molecule/nanotube bonding and in affecting the extent of charge transfer. Contrary to this, the effects of nanotube diameter and chirality and the number of walls appear to be less significant.