Modulation of band alignment with water redox potentials by biaxial strain on orthorhombic NaTaO3 thin films.

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Photocatalysis-assisted water splitting using semiconductor materials greatly depends on the bandgap size and the alignment of band edges relative to the reaction potentials. We used ab initio computational methods to show that the biaxial strain on [100]-oriented orthorhombic NaTaO3 thin films grants the modulation of surface states, favoring either the hydrogen evolution reaction (HER) or the oxygen evolution reaction (OER), which basically rules the perovskite photocatalytic performance. Under compression, the outermost TaO6 and TaO4 polyhedra become more distorted, and electrostatic repulsion increases the energy of Ta 5d surface states. As they overcome the O2/H2O potential, they cease to contribute to the OER. At the same time, the H+/H2 remains below the conduction band, leveraging the HER over the OER. The tensile strain lowers the outermost polyhedra distortions, stabilizing both Ta 5d surface and conduction band states, and increasing the charge centered around surface Ta atoms. Consequently, the bands are better aligned with O2/H2O and H+/H2 potentials, which benefits the overall water splitting photocatalysis. Our results evidence that combining facet and strain engineering is an effective way of altering the photocatalytic activity of orthorhombic [100] NaTaO3 thin films.