Synthetically Enforced Cation Migration in Sillén–Aurivillius Hybrid Perovskites Boosts Photocatalytic Hydrogen Evolution

Sillén–Aurivillius (S–A) hybrid layered perovskites constitute an important class of intergrowth compounds that have been recently demonstrated as high-performing semiconductor photocatalysts. The present study reports the synthesis of a series of three-layer S–A perovskites (A3X1 hybrids), Bi₄AA′Ti₂NbO₁₄Cl (A, A′ = Sr and Ba), by an innovative approach involving interchange of Sr and Ba between the starting Sillén and Aurivillius blocks to examine the cation redistribution in the resulting intergrowth phases. Rietveld structure refinements reveal the preferred occupation of Sr in the perovskite block, while the larger Ba is grounded in the Sillén block. Due to cation migration between the fluorite-like [Bi₂O₂] layer and the perovskite block during intergrowth formation, the projected composition Bi₄Ba_[P]Sr_[S]Ti₂NbO₁₄Cl (where [P] indicates the perovskite block, while [S] indicates the fluorite block) evolves into the phase with a mixed cation distribution, Bi₄Ba_0.1[P]Sr_0.9[P]Ba_0.9[S]Sr_0.1[S]Ti₂NbO₁₄Cl. The cation migration appears to improve the packing by simultaneously reducing the height of the perovskite block and decreasing the divergence in the Bi–O bond lengths of the fluorite block simultaneously. This leads to greater mixing of Ti-3d, Nb-4d, and Bi6p states contributing near the conduction band minima. The cation-migrated S–A hybrid shows enhanced photocatalytic hydrogen evolution (PHE) as compared to the hybrid perovskites with nonmigrated or unmixed cation distribution. The present investigation discusses the innovative synthesis, cation migration, site disorder, and first-principles electronic structure calculations to unveil their role in enhanced PHE.