Two-step synthesis of Sillén–Aurivillius type oxychlorides to enhance their photocatalytic activity for visible-light-induced water splitting

Abstract

A two-step synthesis via the polymerized complex method (2PC) was developed to improve the photocatalytic activity of a Sillén–Aurivillius oxychloride Bi₄TaO₈Cl and related oxychlorides for O₂ evolution (i.e., water oxidation) in Z-scheme water splitting under visible light. This method uses the polymerized complex reaction to prepare a precursor oxide (e.g., Bi₃TaO₇), which is subsequently calcined with BiOCl to yield a pure Bi₄TaO₈Cl phase with smaller particle sizes than those obtained via a conventional single-step solid-state reaction (1SSR). Furthermore, time-resolved microwave conductivity (TRMC) measurements revealed that the Bi₄TaO₈Cl sample prepared by the 2PC method at 973 K (2PC_973) achieved more than five times longer-lived charge separation than that by the 1SSR at 973 K (1SSR_973), which probably arises from lower numbers of charge-recombination centers produced in the 2PC synthesis. Thus, the synthesized Bi₄TaO₈Cl samples exhibited a higher rate of O₂ evolution (e.g., 20 μmol h⁻¹ for 2PC_973 vs. 4 μmol h⁻¹ for 1SSR_973). Overall water splitting into stoichiometric H₂ and O₂ was demonstrated by constructing a Z-scheme photocatalytic system consisting of 2PC_973, Ru-modified SrTiO₃:Rh, and an Fe³⁺/Fe²⁺ shuttle redox mediator, with an external quantum efficiency of 0.9% at 420 nm, which was much higher than that using the sample derived from the optimized 1SSR method at 1173 K (0.4%). The 2PC synthesis was successfully extended to other Sillén–Aurivillius type oxychlorides, Bi₄NbO₈Cl, Bi₆NbWO₁₄Cl and Sr₂Bi₃Ta₂O₁₁Cl, all of which exhibited superior water splitting activity compared to those prepared through the 1SSR.