  We study the binding of chlorine atoms to carbon nanotubes and graphene at a
Stone-Wales defect and to the sidewalls of pristine nanotubes. We show using ab
initio thermodynamics that if the environment is chlorine-rich enough, the
unzipping of carbon nanotubes into graphene nanoribbons with chlorinated edges
may be thermodynamically feasible. By controlling the Cl chemical potential
through temperature and pressure, opening selectively tubes below a threshold
diameter might be possible. Additionally, we find increased binding energies
for chlorine atoms bound to Stone-Wales defects as compared to defect-free
graphene and nanotubes, indicating that defects might act as nucleation sites
for tube unzipping. On graphene, no more than a single Cl atom may be bound to
the defect in ambient conditions, limiting possible Cl-induced changes in the
resulting nanoribbons.
