Surface Ferroelectric Effect in Oxygen Deficient (111) Strontium Titanate Single Crystals Controls Photoelectrochemical Water Oxidation

Ferroelectric materials such as BaTiO₃ have a permanent electric polarization that can be controlled with an external applied electric field. Here we demonstrate a surface ferroelectric effect in oxygen deficient (111) SrTiO_3-x and its application to improve photoelectrochemical water oxidation for the first time. After hydrogen-annealed (111) SrTiO_3-xsingle crystals are polarized with an electric field of 11 kV cm^-1under argon flow, the anodic water oxidation photocurrent increases from 0.99 to 2.22 mA cm^-2 at 1.23 V RHE (60 mWcm^-2, UV illumination) or decreases to 0.50 mA cm^-2, for the opposite field orientation. The polarization also modifies the surface photovoltage signal of the material and its flat band potential, based on Mott-Schottky measurements. This is attributed to an electric dipole near the (111) SrTiO_3-x surface, which controls the potential drop across the depletion layer and charge transfer at the solid-liquid junction. Based on DFT calculations the electric polarization results from the migration of oxygen vacancies between SrTiO_3-x surface and sub-surface regions. The use of the surface ferroelectric effect to modify the junction and photoelectrochemical properties of a non-ferroelectric material is expected to be of interest for solar energy conversion and information technology applications.