  Chemisorption of hydrogen on graphene is studied using atomistic simulations
with the second generation of reactive empirical bond order Brenner
inter-atomic potential. The lowest energy adsorption sites and the most
important metastable sites are determined. The H concentration is varied from a
single H atom, to clusters of H atoms up to full coverage. We found that when
two or more H atoms are present, the most stable configurations of H
chemisorption on a single graphene layer are ortho hydrogen pairs adsorbed on
one side or on both sides of the graphene sheet. The latter has the highest
hydrogen binding energy. The next stable configuration is the ortho-para pair
combination, and then para hydrogen pairs. The structural changes of graphene
caused by chemisorbed hydrogen are discussed and are compared with existing
experimental data and other theoretical calculations. The obtained results will
be useful for nanoengineering of graphene by hydrogenation and for hydrogen
storage.
