Ferroelectric Materials for Photocatalytic Water-Splitting – Strained Mixed Anion Perovskites

Polarity, for example in ferroelectric materials, can significantly increase a catalyst’s performance by improving charge-carrier separation. However, polar distortions also increase the band gap as shown for epitaxially strained SrTiO₃ (1). While this band-gap increase is small for oxides, our density functional theory calculations show a much larger increase for oxynitrides: The enhanced covalency due to reduced electronegativity of nitrogen compared to oxygen results in larger strain-induced polar distortions and therefore more strongly increased band gaps by up to 1.5 eV. The reduced electronegativity, which leads to a higher valence band in oxynitrides and therefore a band gap in the visible that is attractive for photocatalysis, thus also has a detrimental effect on photo absorption when polar distortions are present. This results in a trade-off between small band gaps and polarity. We will discuss different strategies on how to overcome this trade-off with mixed anion perovskite compounds, which have not yet been considered for photocatalytic water-splitting.
(1) RF Berger et al. PRL 107.14(2011):146804