Photocatalytic water splitting to produce hydrogen fuel is one possible solution to the renewable energy storage problem. SrTiO\({}_{3}\) is a high efficient photocatalyst under UV irradiation. It is also a perovskite oxide, which allows it to be epitaxially grown with well defined surfaces. This control makes of SrTiO\({}_{3}\) and other perovskite oxides ideal platforms to study the atomistic mechanisms of photocatalytic water oxidation. PbTiO\({}_{3}\) is a ferroelectric perovskite oxide that also absorbs in the UV range. In addition to controlled epitaxial growth, it is possible to control its ferroelectric polarization at the nanoscale. We apply this control to investigate the interplay between polarization and surface termination on the photocatalytic activity of these two materials using the photocatalyzed reduction of silver-nitrate solution to silver as a proxy reaction for evaluating water oxidation. We also perform ab initio molecular dynamics simulations of liquid water on all the experimentally grown perovskite oxide surfaces. Our results clearly evidence the critical role played by surface termination and polarity on water dissociation into proton and hydroxyl ions at the surfaces. These surface hydroxyl ions are the initial species in the water oxydation cycle. Our results show that in SrTiO\({}_{3}\) the oxidation and reduction reactions occur at the boundary between surface termination changes. Water oxydation occurs primarily in SrO terminated surfaces, where the overpotential for the photo-generated holes is larger. The silver reduction occurs at the TiO\({}_{2}\) edges. These results are consistent with a small spatial extent of the photo generated electron-hole pairs. In contrast, in PbTiO\({}_{3}\) ferroelectric polarization allows for full separation of electron and holes. The oxidation and reduction reactions can directly be controlled by ferroelectric domain writing with an AFM tip. Oxidation occurs at PbO terminated polar-down domains while silver is reduced at TiO\({}_{2}\) terminated polar-up domains.

less than 200 words