Probing the Polaronic Landscape in Bi₄TaO₈X Perovskite Oxyhalides Photocatalysts

Journal article, 2023

Bismuth perovskite oxyhalides Bi4TaO8X (X = Cl, Br) are a promising class of photocatalysts due to their resistance to self-decomposition, a property often lacking in other photocatalysts. In order for them to efficiently carry out photocatalytic reactions, it is essential that the diffusion of photogenerated excess charges is not disrupted, such as by the formation of polarons, during transport to the surface, where the reactions take place. We here use a Koopmans-compliant hybrid functional to investigate the behavior of the photogenerated holes and electrons. We first demonstrate that electron polarons are unstable in these materials. Excess holes, on the other hand, localize and alter the atomic structure locally, leading to the formation of various polaronic configurations. Our results show that hole polarons are highly stable at the perovskite block [MO4] and possess features that are similar to those found for holes in NaTaO3. Furthermore, we find that the presence of two holes results in the occurrence of bipolaronic states, which are accompanied by the formation of O-O dimers. Finally, we show that holes do not localize within the halide block [X], in contrast to oxyhalides BiOX (X = Cl, Br), suggesting that Bi4TaO8X are more resistant to self-oxidation of X-, in accordance with the higher stability reported in experimental studies.