  Motivated by the recent photochlorination experiment [B. Li et al., ACS Nano
5, 5957 (2011)], we study theoretically the interaction of chlorine with
graphene. In previous theoretical studies, covalent binding between chlorine
and carbon atoms has been elusive upon adsorption to the graphene basal plane.
Interestingly, in their recent experiment, Li et al. interpreted their data in
terms of chemical bonding of chlorine on top of the graphene plane, associated
with a change from sp2 to sp3 in carbon hybridization and formation of graphene
nanodomains. We study the hypothesis that these domains are actually fractured
graphene with chlorinated edges, and compare the energetics of
chlorine-containing graphene edge terminations, both in zigzag and armchair
directions, to chlorine adsorption onto infinite graphene. Our results indicate
that edge chlorination is favored over adsorption in the experimental
conditions with radical atomic chlorine and that edge chlorination with
sp3-hybridized edge carbons is stable also in ambient conditions. An ab initio
thermodynamical analysis shows that the presence of chlorine is able to break
the pristine graphene layer. Finally, we discuss the possible effects of the
silicon dioxide substrate on the chlorination of graphene.
