Photocatalytic Water Splitting For Hydrogen Generation On KNbO₃Microcubes:a Combined Experimental And Theoretical Study

Potassium niobate (KNbO3) is receiving increasing attention due to its excellent nonlinear optical, piezoeleetrie, ferroelectric, and photocatalytic properties etc. Potassium niobate (KNbO3) is also a perovskite with good availability, low toxicity and high stability under light illumination, which is benefit of the potential application as photocatalyst. Thus, in recent years, broad scale researches are producted on the field of potassium niobate acted as a photocatalyst. Potassium niobate (KNbO3) containing Nb5+cations are typical UV-responsive photocatalysts with empty d orbital in the conduction band. There are a number of crystalline phases associated with KNbO3. The structural variations associated with the phase transitions on KNbO3are expected to be physico-chemically different. However, the current researches of the potassium niobate on photocatalytic were focused on the orthorhombic phase structure, no reports about other phase structures were showed.In this thesis, potassium niobate (KNbO3) microcubes with orthorhombic and tetragonal phases were hydro thermally prepared. The compositions, morphology, crystalline structure and electronic structure of the as-synthesized KNbO3were systematically characterized by powder X-ray diffraction, nitrogen adsorption-desorption, micro-Raman spectrascopy, Fourier transform infrared spectroscopy, diffuse reflectance UV-visible spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. Further, we examined their photocatalytic property toward hydrogen generation by water splitting, and the results were compared with that of cubic KNbO3in the form of microcubes. The main results are as follows:1. The photocatalytic reactivity was shown to be phase-dependent, the rate of the hydrogen production normalized with respect to the BET surface area are showed below, cubic (318μmol·h-1·m-2), orthorhombic (234μmol·h-1·m-2), tetragonal (174μmol·h-1·m-2). 2. The nature of the phase-dependent photo catalytic property was unraveled by first-principles density functional calculations. The best photocatalytic performance of cubic KNbO3is ascribed to its highest symmetry in the bulk crystalline structure and associated unique electronic structure. And the surface electronic structure plays a key role leading to the photo reactivity discrepancy between ortho rhombic and tetragonal KNbO3.