Photocatalytic water splitting for hydrogen generation on cubic, orthorhombic, and tetragonal KNbO3 microcubes

Abstract

Potassium niobate (KNbO₃) microcubes with orthorhombic and tetragonal phases were hydrothermally prepared and characterized by powder X-ray diffraction, nitrogen adsorption–desorption, micro-Raman spectroscopy, Fourier transform infrared spectroscopy, diffuse reflectance UV-visible spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. The photoreactivity of the as-prepared KNbO₃ samples was evaluated regarding the hydrogen evolution from aqueous methanol under UV, and the results were compared with that of cubic KNbO₃ microcubes. The photocatalytic reactivity was shown to be phase-dependent, following the order cubic > orthorhombic > tetragonal. Insight into the phase-dependent photocatalytic properties was gained by first-principles density functional calculations. The best photocatalytic performance of cubic KNbO₃ is ascribed to it having the highest symmetry in the bulk structure and associated unique electronic structure. Further, the surface electronic structure plays a key role leading to the discrepancy in photoreactivity between orthorhombic and tetragonal KNbO₃. The results from this study are potentially applicable to a range of perovskite-type mixed metal oxides useful in water splitting as well as other areas of heterogeneous photocatalysis.