  Diamond and diamond-like carbon (DLC) are used as coating materials for
numerous applications, ranging from biomedicine to tribology. Recently, it has
been shown that the hydrophilicity of the carbon films can be enhanced by
silicon doping, which highly improves their biocompatibility and frictional
performances. Despite the relevance of these properties for applications, a
microscopic understanding on the effects of silicon is still lacking. Here we
apply ab initio calculations to study the interaction of water molecules with
Si-incorporated C(001) surfaces. We find that the presence of Si dopants
considerably increases the energy gain for water chemisorption and decreases
the energy barrier for water dissociation by more than 50%. We provide a
physical rational for the phenomenon by analysing the electronic charge
displacements occuring upon adsorption. We also show that once hydroxylated,
the surface is able to bind further water molecules much strongly than the
clean surface via hydrogen-bond networks. This two-step process is consistent
with and can explain the enhanced hydrophilic character observed in
carbon-based films doped by silicon.
