  Ab initio calculations within the density-functional theory formalism are
performed to investigate the ground state, electric charge doping, and
electronic properties of titanium and titanium dioxide monolayers adsorbed on a
graphene surface. A new ground state structrure of Ti monolayer adsorbed on
graphene is reported which is shown to be stable up to T = 500 K. Effects due
to lower and higher Ti adatoms coverage are studied. We find that the adsorbed
Ti provides a strong n-type doping which supports recent experimental
observations. On the other hand, TiO_2 can induce both p- and n-type doping in
the carbon monolayer depending on whether oxygen or titanium atoms are closest
to the substrate. We identify the structures which are responsible for the
experimentally observed autocompensation mechanism that leads to the reversion
of adsorbate effects after oxidation of the adsorbed Ti.
