Oxygen vacancy-induced ferroelectric effect in (111) strontium titanate single crystals controls photoelectrochemical water oxidation

Abstract

Ferroelectric materials, such as tetragonal BaTiO₃, have a permanent electric polarization that can be controlled with an external electric field, however, a ferroelectric polarization in cubic SrTiO₃ is forbidden by the higher symmetry of the lattice. Here we demonstrate that hydrogen annealed SrTiO_3−x single crystals can be polarized electrically, and that the polarization controls the activity for photoelectrochemical water oxidation, a pathway to solar hydrogen fuel. Specifically, it is observed that the anodic water oxidation photocurrent increases from 0.99 to 2.22 mA cm⁻² at 1.23 V RHE (60 mW cm⁻², UV illumination) or decreases to 0.50 mA cm⁻² after electric polarization of hydrogen-annealed (111) SrTiO_3−x single crystals in forward or reverse direction. The polarization also modifies the surface photovoltage signal of the material and its flat band potential, based on Mott–Schottky measurements. These observations are attributed to the formation of an electric dipole at the (111) SrTiO_3−x surface, which alters the potential drop across the depletion layer at the solid–liquid junction, and with it the electron transfer barrier. Density functional theory calculations confirm that an electric dipole can result from the movement of oxygen vacancies between the surface or sub-surface layers of SrTiO_3−x. The filling of these surface oxygen vacancies is the probable cause for the observed disappearance of the electric polarization after 24 h storage in air and 48 h in argon. Overall, this work establishes a new surface-based ferroelectric effect in SrTiO_3−x and its use for solar energy conversion during photoelectrochemical water oxidation. Because oxygen vacancy defects are common, similar electric polarization effects are to be expected in other metal oxides.