  Polarons are ubiquitous in many semiconductors and have been linked with
conductivity and optical response of materials for photovoltaics and
heterogeneous catalysis, yet how surface polarons influence adsorption remains
unclear. Here, by modelling the surface of rutile titania using density
functional theory, we reveal the effect of small surface polarons on water
adsorption, dissociation, and hydrogen bonding. On the one hand the presence of
such polarons significantly suppresses dissociation of water molecules that are
bonded directly to polaronic sites. On the other hand, polarons facilitate
water dissociation at certain non-polaronic sites. Furthermore, polarons
strengthen hydrogen bonds, which in turn affects water dissociation in hydrogen
bonded overlayer structures. This study reveals that polarons at the rutile
surface have complex, multi-faceted, effects on water adsorption, dissociation
and hydrogen bonding, highlighting the importance of polarons on water
structure and dynamics on such surfaces. We expect that many of the physical
properties of surface polarons identified here will apply more generally to
surfaces and interfaces that can host small polarons, beyond titania.
