SrTiO3 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 SrTiO3 and other perovskite oxides ideal platforms to study the atomistic mechanisms of photocatalytic water oxidation. PbTiO3 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. Our results show that in SrTiO3 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 TiO2 edges.<br/>We hypothesize that these results are consistent with a small spatial extent of the photo generated electron-hole pairs coupled to the localization of the photogenerated hole on the SrO surface. To test this hypothesis, we have theoretically evaluated the dissociation and overpotential properties of a series of alcohols in these surfaces. We also performed the same experimental photocatalytic procedure on aqueous solutions of these alcohols. Our results show that silver is still only deposited in the vicinity of SrO surfaces and that there is a direct correlation between the amount of silver deposited and the overpotential of the de-protonated Oxygen bound to the SrO surface.<br/>In contrast, in PbTiO3 ferroelectric polarization allows for full separation of electron and holes, i.e, dissociation of the exciton. In this case, 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 TiO2 terminated polar-up domains, independently of their size.

Evaluating photocatalytic active sites as function of polarization, level alignment and spontaneous dissociation of hydroxyl groups at perovskite oxide surfaces

256th National Meeting (2018)

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