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.