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

A two-step synthesis via the polymerized complex method (2PC) was developed to improve the photocatalytic activity of a Sillen-Aurivillius oxychloride Bi4TaO8Cl and related oxychlorides for O-2 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 O-2 evolution (e.g., 20 mu mol h(-1) for 2PC_973 vs. 4 mu mol h(-1) for 1SSR_973). Overall water splitting into stoichiometric H-2 and O-2 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 Sillen-Aurivillius type oxychlorides, Bi4NbO8Cl, Bi6NbWO14Cl and Sr2Bi3Ta2O11Cl, all of which exhibited superior water splitting activity compared to those prepared through the 1SSR.