New insights in the adsorption of oxygen molecules on single walled carbon nanotubes

We study the adsorption of molecular oxygen, both singlet and triplet forms on single-walled arm chair and zig-zag carbon nanotubes (CNTs) using density functional calculations. It is found that singlet oxygen has more potential of chemisorption than the ground state oxygen on the walls of the pristine CNTs and that the adsorption behaviour is different for semiconducting and metallic nanotubes. Molecular oxygen in its singlet state is found to adsorb strongly on semiconducting pristine nanosurfaces with appreciable binding energies and is found at bonding distances, ∼1.47 Å from the side walls. Ground state oxygen is found to be only weakly physisorbed with distances in the range of 3.3–3.5 Å from the tube confirming the previous investigations. The effect of substitutional doping on the adsorption behaviour is studied and the adsorption is found to be enhanced with ground state oxygen coming closer at distances of ∼1.5 Å, with appreciable charge transfer, on semiconducting nanotubes doped with boron, in contrast to the weak physisorption of triplet oxygen on the intrinsic tubes. This is an interesting finding, and as substitutional doping offers a permanent solution for possible use as sensor, CNTs doped with boron can find application as a potential molecular oxygen sensor. On doping the metallic nanotubes with boron, the adsorption of singlet oxygen is found to be enhanced with appreciable charge transfer compared to adsorption on the pristine form. We also find that singlet oxygen molecule dissociate near the semiconducting nanotube walls and chemically adsorb onto the surface to form a four membered dioxetane like ring. All the cases of relevant adsorption are accompanied by significant amount of charge transfer from the nanotube to the molecule.