New insights into the synthesis of Sillén–Aurivillius oxyhalides: molten salts induce interlayer halogen competing reaction

Abstract

Numerous studies disclosed that molten salts provide a special liquid reaction environment, accelerate the diffusion rate of active elements, and promote the formation of high-purity materials in the molten-salt synthesis method. Herein, the interlayer halogen-competing reactions between molten salts and halogen layers during the formation of BaBi₄TiNbO₁₁X (X = Cl, Br) oxyhalides were discovered for the first time. The liquid environment provided by molten salts can facilitate the full migration of halogen atoms between molten salts and halogen layers and form an even distribution of halogen atoms, which is ascribed to the weak van der Waals forces of the halogen layers between [BaBiO₂]⁺ and [Bi₂O₂]²⁺ blocks. By employing this mechanism, BaBi₄TiNbO₁₁(Cl_1−xBr_x) with different x contents could be easily obtained. DFT calculations showed that the mixed halogen oxyhalides, such as BaBi₄TiNbO₁₁(Cl_0.5Br_0.5), introduce new chemical bonds between the O and Bi atoms, which increase their contribution to lowest occupied/unoccupied orbitals, thus regulating the energy band and accelerating the separation and transfer of photogenerated charge carriers. Consequently, the optimized BaBi₄TiNbO₁₁(Cl_0.5Br_0.5) sample showed a superior degradation rate of RhB compared to the traditional P25 photocatalyst, and its O₂ evolution performance was also enhanced compared with other blank samples.