Effects of crystal and electronic structures of ANb2O6 (A=Ca,Sr, Ba) metaniobate compounds on their photocatalytic H2 evolution from pure water

Alkali-earth metaniobate compounds, ANb₂O₆ (A = Ca, Sr, Ba), were prepared by the conventional solid-state reaction route and their electronic band structures and photocatalytic activities were investigated. The prepared powders were characterized using X-ray diffraction (XRD), field-emission electron microscopy (FE-SEM), UV–vis diffuse reflectance spectroscopy, and fluorescence spectroscopy. It was found that the particle sizes (∼1 μm) and BET surface areas (∼1 m²/g) of the metaniobate compounds were nearly identical. From the electronic band structure calculations, however, the band-gap energies of these metaniobate compounds were found to be in the order of CaNb₂O₆ > SrNb₂O₆ > BaNb₂O₆. These calculated band-gap energies were consistent with those estimated from the UV–vis diffuse reflectance spectra. Moreover, the conduction-band edge (reduction potential) of SrNb₂O₆ calculated from the electronegativity data was higher than those of CaNb₂O₆ and BaNb₂O₆. The photoluminescence spectra revealed that CaNb₂O₆ exhibited a strong blue luminescence emission (at 300K), while no obvious emissions were observed in either SrNb₂O₆ or BaNb₂O₆. The luminescence behaviors of these metaniobate compounds and their band structure variations originating from their crystal structures play an important role in their photocatalytic activity for the evolution of H₂ from pure water. SrNb₂O₆, which has a higher conduction-band edge potential than the other compounds, exhibited higher photocatalytic activity.