Phase Transition,photocatalytic Hydrogen Production And Ion-doped Luminescence Properties Of Bi₉M₂O₁₈Cl（M=P,V）

With the development of industrialization,energy depletion and environmental pollution make it urgent to develop new and clean energy sources.The extensive and abundant solar energy can be converted into hydrogen,a new green and clean energy source,through a series of redox reactions by photocatalysts.So more and more scientists are working on the study of hydrogen production by photocatalytic water-splitting.In addition,the new solid-state lighting method of white light emitting diodes is attracting attention in the field of luminescence for its excellent luminous efficiency,low power consumption and environmental friendliness.In the pursuit of higher luminous efficiency and better performance of white light emitting diodes,rare earth doped phosphors are of great significance to their development.Both photocatalytic and luminescent properties require the development and research of semiconductor materials with great properties.Among the many semiconductor materials,bismuth-based Aurivillius compounds stand out due to their unique layered structure and excellent photocatalytic properties.Inorganic phosphates as well as vanadates have become hotspots for substrate materials in luminescent materials due to their lower cost,great physical and chemical stability,and their strong ability to bind groups to structural units.Based on these two points,this paper investigates two oxychloride phosphate/vanadate with[Bi₂O₂]²⁺layers resembling bismuth-based Aurivillius compounds.The crystal structure,photocatalytic water-splitting performance and Eu³⁺and Y³⁺ion doped luminescence of Bi₉P₂O₁₈Cl were investigated,as well as the luminescence performance of Bi₉V₂O₁₈Cl doped with Eu³⁺and Y³⁺ions.The crystal structure and photocatalytic water-splitting performance of Bi₉P₂O₁₈Cl were studied.（1）The structure of Bi₉P₂O₁₈Cl was first solved by single-crystal XRD diffraction.At room temperature,Bi₉P₂O₁₈Cl crystallizes in monoclinic space group P2₁/m while at low temperatures the compound crystallizes in monoclinic space group P2₁/n.The microscopic morphology was characterised by scanning electron microscope（SEM）tests and the excellent thermal stability of Bi₉P₂O₁₈Cl was illustrated by the results of thermal stability tests.The Eu³⁺and Y³⁺ion-doped Bi₉P₂O₁₈Cl were synthesized by solid state method and luminescence properties were investigated by fluorescence spectrometry.The optimum doping concentration of Eu³⁺is 0.1 and the CIE chromaticity coordinate point is（x=0.4914,y=0.4302）,in the orange-red light region.The optimum doping concentration of Y³⁺is 0.1under 384 nm excitation,and the results showed that Y³⁺enhanced the luminescence of Bi₉P₂O₁₈Cl.DFT calculations and UV diffuse reflectance spectroscopy have led to the conclusion that Bi₉P₂O₁₈Cl is an indirect band gap semiconductor with a band gap of approximately 2.62 e V at room temperature.The positions of the conduction and valence bands were determined by Mott-Schottky images at 800 Hz and 1000 Hz and combined with its transient photocurrent response to theoretically showed that Bi₉P₂O₁₈Cl has a strong photocatalytic water-splitting performance.The hydrogen production by photocatalytic water-splitting of Bi₉P₂O₁₈Cl pure phase powder and small Bi₉P₂O₁₈Cl-JM crystals grown along a certain crystal plane are investigated experimentally.Under the irradiation of visible light,the hydrogen production rate of Bi₉P₂O₁₈Cl pure phase powder is 33.69μmol·g^-1·h^-1,and that of Bi₉P₂O₁₈Cl-JM crystals is 11.26μmol·g^-1·h^-1.（2）The pure phase Bi₉V₂O₁₈Cl powder was prepared using a high temperature solid phase method and characterised by SEM combined with EDS energy spectroscopy for its microscopic morphology and constituent elemental composition.Its excellent thermal stability was obtained using thermogravimetric-differential thermal analysis.The band gap value of the indirect band gap semiconductor Bi₉V₂O₁₈Cl was obtained as 2.63 e V at room temperature by UV-Vis diffuse reflectance spectroscopic characterisation and calculations of the energy band structure and the density of electronic states.Similarly,the luminescence properties of Eu³⁺and Y³⁺ion-doped Bi₉V₂O₁₈Cl were synthesized by solid phase method and luminescence properties were investigated by fluorescence spectrometry.The optimum doping concentration of Eu³⁺is 0.1 at an excitation wavelength of 350 nm,with a CIE chromaticity coordinate point of（x=0.5561,y=0.4302）and an emission spectrum in the red region.With an optimum doping concentration of 1 for Y³⁺at 340 nm excitation wavelength,the overall emission intensity of Bi₉V₂O₁₈Cl:Y³⁺is enhanced by nearly 143 times relative to Bi₉V₂O₁₈Cl.And its CIE chromaticity coordinate point is（x=0.3802,y=0.4904）,which indicates that it is a yellow fluorescent luminescent material.