Effects of crystal and electronic structures of ANb 2O 6 (A=Ca, Sr, Ba) metaniobate compounds on their photocatalytic H 2 evolution from pure water

Abstract

Alkali-earth metaniobate compounds, ANb 2O 6 (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 m 2/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 CaNb 2O 6 > SrNb 2O 6 > BaNb 2O 6. 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 SrNb 2O 6 calculated from the electronegativity data was higher than those of CaNb 2O 6 and BaNb 2O 6. The photoluminescence spectra revealed that CaNb 2O 6 exhibited a strong blue luminescence emission (at 300K), while no obvious emissions were observed in either SrNb 2O 6 or BaNb 2O 6. 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 H 2 from pure water. SrNb 2O 6, which has a higher conduction-band edge potential than the other compounds, exhibited higher photocatalytic activity.