Preparation And Performance Study Of Fluoride(Oxygen)-based Upconversion Materials Composite Bismuth-based Photocatalysts

With the increasing environmental pollution and energy shortage,solar-driven photocatalysis has been widely studied and applied as a green,environmentally friendly and sustainable pollutant degradation technology.It is well known that the solar energy reaching the ground consists of ultraviolet（UV）（5%）,visible（Vis）（43%）and near-infrared（NIR）（52%）light.Traditional photocatalysts,such as TiO₂,ZnO,etc.,can only utilize UV light.Semiconductors with narrow band-gaps,such as BiOCl,BiOI,can be excited by UV-Vis light simultaneously.However,NIR light,which accounts for 52%of the solar spectrum,still cannot be used effectively.This leads to a huge waste of energy.To achieve effective utilization of the solar full-spectrum,rational design of photocatalysts with efficient NIR-response is imperative.Rare-earth ions doped upconversion luminescent materials can convert low-energy NIR light to high-energy UV-Vis light via anti-Stokes luminescence.Therefore,compounding upconversion luminescent materials with narrow band-gap photocatalysts is an effective strategy to achieve full-spectrum utilization of the solar energy.Meanwhile,in order to achieve efficient photocatalysis under NIR light,the following two aspects need to be focused on design considerations:（ⅰ）enhancing the proportion of UV-Blue light emission and luminescence intensity of upconversion luminescent materials;（ⅱ）improving the energy transfer efficiency between upconversion luminescent materials and photocatalysts.In summary,the main research and conclusions of this thesis are as follows:（1）A novel flower bud-like deep UV-emitting LuF₃:Yb³⁺/Tm³⁺upconversion nanoparticles were synthesized by a two-step hydrothermal method.Under the excitation of 980 nm laser,they exhibited deep UV emission at 254 and 288 nm,UV emission at 346 and 362 nm,and blue emission at 452 and 478 nm,respectively.The BiOCl shell was coated on LuF₃:Yb³⁺/Tm³⁺by a solvothermal method,and the LuF₃:Yb³⁺/Tm³⁺@BiOCl upconversion photocatalyst with UV-Vis-NIR response was successfully prepared.The excellent core-shell structure of LuF₃:Yb³⁺/Tm³⁺@BiOCl was confirmed by a series of test characterizations,which provided the necessary conditions for efficient energy transfer between LuF₃:Yb³⁺/Tm³⁺and BiOCl.The fluorescence lifetime decay of the ¹G₄ level of Tm³⁺in LuF₃:Yb³⁺/Tm³⁺@BiOCl further demonstrated the existence of an efficient F（?）rster resonance energy transfer（FRET）channel between LuF₃:Yb³⁺/Tm³⁺and BiOCl.Photocurrent and EIS tests illustrated LuF₃:Yb³⁺/Tm³⁺（10）@BiOCl has greater photocurrent response and smaller interface transfer resistance.Under the light irradiation ofλ>800 nm andλ>400 nm（300 W Xenon lamp with a filter）,the photocatalytic performance of LuF₃:Yb³⁺/Tm³⁺@BiOCl was always higher than pure BiOCl.A great enhancement of 42%and 64%on the MB degradation efficiency was achieved in LuF₃:Yb³⁺,Tm³⁺（10）@BiOCl compared with pure BiOCl,respectively.（2）The Lu₆O₅F₈:Yb³⁺/Tm³⁺shell layer was grown in situ on LuF₃:Yb³⁺/Tm³⁺by high-temperature calcination,which enhanced the emission intensity of upconversion nanoparticles.Among them,the intensity of blue light emission was increased by 2.36times.BiOI shell layer with a narrower band-gap was coated on LuF₃:Yb³⁺/Tm³⁺@Lu₆O₅F₈:Yb³⁺/Tm³⁺by solvothermal method to form the LuF₃:Yb³⁺/Tm³⁺@Lu₆O₅F₈:Yb³⁺/Tm³⁺@BiOI upconversion photocatalyst,which realized a greater utilization of solar energy.The tight core-shell wrapping formed by the solvothermal method achieved an efficient F（?）rster resonance energy transfer between LuF₃:Yb³⁺/Tm³⁺@Lu₆O₅F₈:Yb³⁺/Tm³⁺and BiOI.Meanwhile,the formation of heterogeneous structure also accelerated the separation and transfer of photogenerated electron-hole pairs.As a result,under NIR and Vis-NIR irradiation,the degradation rates of LuF₃:Yb³⁺/Tm³⁺@Lu₆O₅F₈:Yb³⁺/Tm³⁺（20）@BiOI for BPA were 1.24 and 1.11times of LuF₃:Yb³⁺/Tm³⁺（20）@BiOI,and 1.57 and 1.30 times of BiOI,respectively.Meanwhile,under Vis-NIR light irradiation,LuF₃:Yb³⁺/Tm³⁺@Lu₆O₅F₈:Yb³⁺/Tm³⁺（20）@BiOI basically achieved complete degradation of MB（20 mg/L）within45 min.Band-gap analysis,free radical capture experiments,and EPR tests indicate that the elimination of contaminants is attributed to h⁺and·O₂^-.The findings of this paper will provide a useful reference for the rational design of NIR-responsive photocatalysts and the efficient degradation of organic pollutants.