Photocatalytic Activity of Anatase-Type TaON with a Narrow Bandgap Energy of 2.1 eV Synthesized from Ta ₂ O ₅ Particles with a Nitridation–Oxidation Cycle Treatment

Tantalum oxynitride (TaON) is a promising material for the photocatalytic water splitting reaction. However, the bandgap of its stable β-TaON form is too high for efficient visible-light absorption, while the anatase-type δ-TaON phase with a smaller gap of 2.1 eV is metastable in the bulk. In this study, anatase-type TaON was successfully synthesized via nitridation of Ta 2 O 5 oxide particles with a nitridation–oxidation cycle treatment to achieve increased surface area and stabilize δ-TaON. Photocatalysis experiments demonstrated for the first time that δ-TaON, with an absorption edge at approximately 600 nm, shows clear photocatalytic activity for both hydrogen and oxygen evolution half-reactions with sacrificial reagents. Moreover, δ-TaON was an active Z -scheme oxygen evolution photocatalyst in the water splitting reaction. Our approach suggests a new method for surface area control and expands the potential range of applications of oxynitrides, including TaON.