Electronic band structure, optical properties, and photocatalytic hydrogen production of barium niobium phosphate compounds (BaO-Nb₂O ₅-P₂O₅)

Barium niobium phosphate compounds (BaNb₂P₂O ₁₁ and Ba₃Nb₂P₄O₁₈), with corner-sharing NbO₆ octahedra as well as PO₄ tetrahedra, were prepared by a conventional solid-state reaction method, and their optical properties, electronic band structure, and photocatalytic activity were investigated. The powders were characterized by X-ray powder diffraction (XRD), field-emission SEM (FESEM), high-resolution TEM (HRTEM), and UV/Vis and fluorescence spectroscopy. Both compounds exhibit similar optical band gaps, 3.55 eV for BaNb₂P₂O₁₁ and 3.58 eV for Ba ₃Nb₂P₄O₁₈. However, photoluminescence spectra revealed that a radiative recombination process of charge carriers is dominant in Ba₃Nb₂P₄O ₁₈ (strong blue-white emission at 300 K) under UV irradiation, whereas no obvious emission was observed from BaNb₂P ₂O₁₁. The photocatalytic activity for the evolution of H₂ from the splitting of pure water was evaluated. BaNb ₂P₂O₁₁, which has a smaller photon emission at room temperature, exhibited a much higher photocatalytic activity than Ba ₃Nb₂P₄O₁₈. The difference in the photocatalytic activity of the two compounds is attributed to their different electronic band structures, resulting from their different crystal-structure environments.