Cobalt hexacyanoferrate as an effective cocatalyst boosting water oxidation on oxynitride TaON photocatalyst under visible light

[Display omitted] •Cobalt hexacyanoferrate (CoHCF) functioned as an effective water oxidation cocatalyst on TaON photocatalysts.•CoHCF promoted the water oxidation from a basic (pH 8) aqueous solution containing Ag+ as an irreversible electron acceptor.•CoHCF functioned as a water oxidation cocatalyst even at pH 3, where conventional Co-based compounds does not work.•CoHCF improved the water oxidation even in the presence of reversible electron acceptors under the acidic conditions. Developing efficient cocatalysts that improve water oxidation on photocatalyst particles is highly desirable because water oxidation is the key process in artificial photosynthesis systems and represents the rate-determining step. Prussian blue analogues (PBAs) have recently attracted considerable attention as functional materials, including water oxidation (electro)catalysts, owing to their high controllability in various properties via cation and/or ligand substitutions. However, there are still only a few successful studies on the enhancement of water oxidation on particulate photocatalysts by modification with PBAs. Herein, we report the use of cobalt hexacyanoferrate (KxCo[Fe(CN)6]y·nH2O, denoted as CoHCF) as an effective cocatalyst on a visible-light-responsive oxynitride (TaON) photocatalyst for improving O2 evolution in the presence of various electron acceptors. The loading of CoHCF nanoparticles on the TaON surface via a simple impregnation method clearly enhanced the O2 evolution rate from a basic (pH 8) aqueous solution containing Ag+ as a sacrificial electron donor; the enhancement was comparable to that of the widely-studied cobalt oxyhydroxide (CoOx(OH)y). Importantly, it was revealed that the CoHCF nanoparticles loaded on TaON are chemically stable even under acidic conditions with pH below 3, whereas conventional Co-based cocatalysts such as oxide (CoOx) and oxyhydroxide (CoOx(OH)y) are unstable in these conditions due to their dissolution and thereby lose their functions. Thus, the TaON modified with CoHCF cocatalysts exhibited higher O2 evolution rates than unloaded one in acidic aqueous solutions in the presence of not only sacrificial electron acceptor (Ag+) but also reversible ones (Fe3+, IO3–) under visible light irradiation.