Hierarchical metastable γ-TaON hollow structures for efficient visible-light water splitting

Hierarchical tantalum-based oxide and (oxy)nitride with hollow urchin-like nanostructures have been synthesized for the first time by an in situ self-assembly wet-chemical route in addition with post-thermal nitridation. Notably, a single-phase metastable γ-TaON with hollow urchin-like spheres among the tantalum (oxy)nitrides was obtained during the phase transformation process from an orthorhombic Ta₂O₅ to a typical monoclinic β-TaON, corresponding the order of phase formation: Ta₂O ₅ → γ-TaON → β-TaON → Ta₃N ₅. The combined effect of the crystal and electronic structures and hierarchical morphology on the tunable photocatalytic and photoelectrochemical performances of the tantalum-based photocatalysts was systematically investigated. Efficient photocatalytic hydrogen production as high as 381.6 μmol h^-1 with an apparent quantum efficiency of 9.5% under 420 nm irradiation (about 47.5 times higher than that of the conventional TaON) was achieved over this metastable γ-TaON architecture. Furthermore, the hierarchical γ-TaON photoanode exhibited a photocurrent density of ∼1.4 mA cm^-2 at 0.8 V vs. SCE in Na₂SO₄ solution under visible light irradiation. This excellent photocatalytic activity is ascribed to the unique urchin-like nanostructure with large specific surface area, the metastable crystal structure and appropriate electronic structure as well as the efficient charge carrier separation.