Anion Ordering in CaTaO2N: Structural Impact on the Photocatalytic Activity. Insights from First-Principles

In view of its use in photocatalysis, we have here investigated several anion ordering models systems with the same chemical composition but different anionic arrangements of CaTaO2N and compared their chemical and physical features. We have found that the valence band structure varies as a function of the anion ordering, with a significant impact on the photocatalytic related properties, such as bandgaps, carrier effective masses, and band edge positions. In detail, from the comparison with two-dimensional ones, three-dimensional anion orderings have more positive valence band maximum positions, a more suitable condition for oxygen evolution reaction. By analyzing the electronic structures, we found that the difference in valence band structure among the anion orderings stems from the different orbital overlap between the Ta 5d and N 2p bands (d-p pi interactions): in two-dimensional anion orderings they cannot overlap due to their symmetry mismatch, while the lowered symmetry of N 2p bands in three-dimensional anion orderings allows the overlap with Ta 5d bands, inducing a net stabilization of the valence band.