Synthesis,Structure And Photocatalytic Activity Of Mo-Doped Ca₂NiWO₆ Perovskite

With increasingly serious energy crisis and environmental pollution,the development of clean energy demand continues to rise.The photocatalytic water splitting to generate hydrogen and oxygen that the solar energy is transformed into chemical energy has attracted significant attention.If the efficiency of water splitting to generate hydrogen and oxygen can be improved,not only can reduce the consumption of fossil fuels,and reduce the environmental pollution.However,it is a multi-step and complex reaction,so far just only a few photoatalysts have been discovered to be active for overall water splitting into hydrogen and oxygen under visible light irradiation.So,many attempts have been done to increase the activity of the half reaction of water splitting.The double-perovskite-type compound Ca2NiWO6 is a kind of new photocatalytic materials,having a better photocatalytic activity under visible light irradiation,which has become a hotspot in the field of photocatalysis.In this thesis,a series of Ca2NiW1-xMoxO6 were synthesized by a solid-state reaction.With a combination of experimental and theoretical study,the mechanism of doping contributing to the change of band structure and thus determining the photocatalytic activity was proposed.The main research work and conclusions were summarized as below:(1)A series of Mo-doped Ca2NiW1-xMoxYO6(x = 0,0.01,0.02,0.03,0.04,0.05)were synthesized by a solid-state reaction.The physical and optical properties of these photocatalysts were characterized by X-ray diffraction(XRD),X-ray absorption fine structure(XAFS),scanning electron microscope(SEM),X-ray photoelectron spectroscopy(XPS),UV-visible diffuse reflectance spectra(UV-vis DRS).The result indicated that the doped Mo6+ ions can be substituted into the W6+ sites in the structure,and enhance the absorption of Ca2NiWO6 in the visible light region.Meanwhile it lead to the formation of oxygen vacancies,not cause significant change of the crystal morphology and structure.(2)The photocatalytic activity of oxygen evolution from water was evaluated under visible light irradiation.The Mo doping increased significantly the photocatalytic activity of Ca2NiWO6.The optimal Ca2NiW0.97Mo0.03O6 showed approximately 2 times higher oxygen evolution rate than that of the pure Ca2NiWO6.To deeply understand the effect of Mo doping on the electronic structure of Ca2NiWO6,the band structure and the density of state(DOS)of the pure and doped samples were calculated by the plane-wave-based density functional theory.The change of the electronic structure was that the top of VB remained unchanged after the Mo6+ doping,whilst the bottom of CB was shifted to more positive potentials as compared to that of the pure Ca2NiWO6.The result indicated that the Mo dopant narrowed the band gap,thus extending the absorption in visible light region.However,the high dopant level increased the extent of defects in the oxide structure,which acted as electron-hole recombination centers,with a consequent decrease of the photocatalytic reaction rate.This work exemplifies that the ions doping is one of the most important factors to engineer the electronic structure of double-perovskite-type semiconductors and in turn obtain the enhanced visible-light driven photocatalytic performance.