Flux Growth of Single-Crystal Na₂Ta₄O₁₁ Particles and their Photocatalytic Hydrogen Production

Abstract

Single-crystal particles of the layered natrotantite, i.e., Na₂Ta₄O₁₁, were prepared within a K₂SO₄/Na₂SO₄ flux for flux-to-reactant molar ratios from 12:1 to 1:1 at a reaction temperature of 1000 °C for 2 h. Depending on the conditions, the flux reactions yielded crystals of Na₂Ta₄O₁₁ that ranged in size from ∼100 nm to ∼1000 nm. The highest and lowest flux amounts yielded more isolated single crystals with sharper facets and surfaces, whereas intermediate flux amounts yielded more aggregates of particles with smooth and rounded surface features. All products were characterized by UV–vis diffuse reflectance techniques and were found to exhibit an indirect bandgap size of ∼4.1–4.3 eV and a larger direct bandgap transition of ∼4.5 eV. When the crystals are suspended in aqueous solutions and irradiated by ultraviolet light, they exhibit stable photocatalytic rates for hydrogen production of ∼13.4 μmol of H₂·g^–1·h^–1 to ∼34.1 μmol of H₂·g^–1·h^–1. The higher photocatalytic rates are found for the single crystals with the highly faceted and nanoterraced surfaces. Electronic structure calculations based on density functional theory confirm the lowest-energy bandgap transition is indirect and between the Γ and M <i>k</i>-points in the valence and conduction band states, respectively. The bandgap excitation is found to result in delocalization of the excited electrons over a layer of condensed TaO₇ pentagonal bipyramids, which is a relatively unexplored structural feature for photocatalytic metal oxides