Cation disorder and octahedral distortion control of internal electric field, band bending and carrier lifetime in Aurivillius perovskite solid solutions for enhanced photocatalytic activity

Abstract

The overall photocatalytic activity shown by a semiconductor photocatalyst stems from a complex interplay of several critical factors that includes light absorption, carrier recombination dynamics, charge transfer resistance, lifetime of photogenerated charge carriers and surface adsorption. While most of the factors are intrinsic to a semiconductor controlled by its structure and composition, the overall activity is determined by the dominance of a single or multiple factors. The present work examines the competing roles of these factors in the photocatalytic activity of a closely related series of Aurivillius perovskite solid solutions, Bi_6−xSr_xTi_3+xFe_2−xO₁₈ (x = 0.75, 0.5, and 0.25). The cation disorder in the intergrowth structure and octahedral distortion that control the internal electric fields in these compounds alter multiple factors responsible for enhanced photocatalytic activity. The five-layer Aurivillius perovskites forming in the non-centrosymmetric F2mm space group with highly distorted octahedra at the terminal position and least distorted octahedra at the central perovskite layer are controlled by a second-order Jahn–Teller effect of Ti⁴⁺. While a greater extent of dye adsorption leads to faster photocatalytic degradation in Bi₅SrTi₄FeO₁₈ and other Aurivillius perovskites, Bi_5.5Sr_0.5Ti_3.5Fe_1.5O₁₈ (x = 0.5) shows inferior photocatalytic activity as compared to Bi_5.75Sr_0.25Ti_3.25Fe_1.75O₁₈ (x = 0.25) and Bi_5.25Sr_0.75Ti_3.75Fe_1.25O₁₈ (x = 0.75) despite having the highest dye adsorption in the series. The highest rate of photocatalytic RhB degradation in the Fe_1.75 compound is due to its longest average carrier lifetime in the series, sluggish e⁻–h⁺ recombination and lower charge transfer resistance. Furthermore, the enhanced activity observed under external bias is understood to have originated from a greater upward band bending effect, which acts as thermal barrier for e⁻–h⁺ recombination and facilitates effective charge separation and transfer. The work elucidates the delicate interplay of the competing factors affecting the photocatalytic activity and establishes the origin of non-uniform and enhanced photocatalytic activity across the homologous solid solution series of layered perovskites. The protocol will help breaking compositional limits in uncovering new solid-solution compounds with enhanced activity by appropriate substitutions and further enhancement under external bias for faster removal and degradation of pollutants.