  Energy barrier of oxygen molecule dissociation on carbon nanotube or graphene
with different types of nitrogen doping is investigated using density
functional theory. The results show that the energy barriers can be reduced
efficiently by all types of nitrogen doping in both carbon nanotubes and
graphene. Graphite-like nitrogen and Stone-Wales defect nitrogen decrease the
energy barrier more efficiently than pyridine-like nitrogen, and a dissociation
barrier lower than 0.2 eV can be obtained. Higher nitrogen concentration
reduces the energy barrier much more efficiently for graphite-like nitrogen.
These observations are closely related to partial occupation of {\pi}* orbitals
and change of work functions. Our results thus provide useful insights into the
oxygen reduction reactions.
