Synthesis And Performance Study Of Three Upconversion Nanoparticles/MOFs Composite Photocatalysts

Photocatalytic technology has attracted extensive attention due to the mild reaction conditions,simple operation,and low energy consumption.More importantly,it can convert solar energy into chemical energy,and is widely used to treat water pollution,generate hydrogen,and reduce carbon dioxide,which is in line with the requirements of sustainable development.Photocatalyst is the most important factor that determines the performance of photocatalysis.Inorganic semiconductor materials or metal organic frameworks（MOFs）are two most popular photocatalysts.However,both of them have the large band gap and the narrow light absorption range（generally in the ultraviolet region and limited visible region）resulting in the inferior photocatalytic effect.One solution is to develop composite photocatalysts combining upconversion nanoparticles （UCNPs）with semiconductors or MOFs.Because of the anti-Stokes shift properties,UCNPs can absorb near-infrared light and emit ultraviolet or visible light.These emitted lights are absorbed by the semiconductors or MOFs to enlarge the light harvesting range leading to the enhanced photocatalytic performance.However,the upconversion efficiency is still not satisfied.At present,they can only degrade some organic dyes,not antibiotics.In view of the above problems,three UCNPs/MOFs are firstly synthesized by doping rare earth ions,synthesizing nanoparticles with a core-shell structure,and refining the lattice structure of the nanoparticles with the aim to enhance the upconversion efficiency of nanoparticles and improve the photocatalytic performance.They can not only degrade the rhodamine B but also remove the antibiotics and Cr（VI）ions.NaYF₄:Yb,Tm,Er,0.5%Nd@NaYF₄:Nd（x）/NH₂-MIL-101（Fe）and NaYF₄:Yb,Tm,Er@NaYF₄:Eu（x）/NH₂-MIL-101（Fe）can simultaneously degrade several pollutants including antibiotics.The main research contents are as follows:（1）SynthesisandphotocatalyticperformanceofNaYF₄:Yb,Tm,Gd（x）/NH₂-MIL-101（Cr）（NYTG（x）/NMC）A series of NYTG（x）/NMC composite photocatalysts were synthesized for the first time.The effects of doping content of Gd³⁺and the ratio of NYTG（3%）to NMC on the photocatalytic performance were studied.The photocatalytic perforamnce was estimated under the ultraviolet-visible,near-infrared and simulated sunlight.When the doping amount of Gd³⁺is 3%and the ratio of NYTG（3%）to NMC was 1:3,NYTG（3%）/NMC showed the best photocatalytic performance.Compared with NMC and NYT/NMC,the photocatalytic performance of NYTG（3%）/NMC was increased by 22%and 6%,respectively.The doping Gd³⁺ions can increase the number of emission peaks.Moreover the emission intensity was enhanced because the lattice defects are refined by the replacement of partial Y³⁺ions in NaYF₄.This work introduces a method to enhance the upconversion fluorescence intensity by refining the lattice of UCNPs.Although NYTG（3%）/NMC can degrade organic dyes,it is still difficult to degrade antibiotics,and even more difficult to degrade multiple pollutants.（2）Synthesis and photocatalytic performance of NaYF₄:Yb,Tm,Er,0.5% Nd@NaYF₄:Nd（x）/NH₂-MIL-101（Fe）（Tm Er Nd@Nd（x）/NMF）In order to develop photocatalysts that can degrade antibiotics and other pollutants,Tm Er Nd@Nd（x）/NMF composite photocatalysts were synthesized for the first time and used to degrade ofloxacin（OFL）,tetracycline（TC）and Cr（VI）ions.By controlling the doping amount of Nd³⁺ions,the energy transfer between Nd³⁺ions and Tm³⁺ions and Er³⁺ions was suppressed,and energy loss was reduced.The intensity of fluorescence emission and quantity of absorption peaks were increased,and the photocatalytic performance was improved.Under the simulated sunlight,Tm Er Nd@Nd（20%）/NMF can degrade ofloxacin,tetracycline and Cr（VI）ions,with the degradation of 88%,65%and 91%,respectively.The excellent photocatalytic activity of Tm Er Nd@Nd（20%）/NMF is mainly attributed to two factors:first,the doped Nd³⁺ions can generate multiple ultraviolet-visible emissions,which can be absorbed by NMF,thereby improving catalytic activity;second,although Yb³⁺ions are the most commonly used sensitizer because of high upconversion efficiency,they absorb 980 nm light,which overlaps with the absorption peak of water resulting in the poor degradation ability of photocatalysts in water.Different from Yb³⁺ions,Nd³⁺ions have multiple emission peaks in the visible and near-infrared regions,which can not only absorb near-infrared light at about 800 nm,but also have little overlap with the absorption peaks of water,thus improving the light absorption capacity of UCNPs and enhancing the photocatalytic activity.（3）SynthesisandphotocatalyticperformanceofNaYF₄:Yb,Tm,Er@NaYF₄:Eu（x）/NH₂-MIL-101（Fe）（Tm Er@Eu（x）/NMF）By constructing core-shell structure UCNPs,surface fluorescence quenching can be reduced leading to the enhanced fluorescence emission intensity.However,the fluorescence quenching caused by internal lattice defects still exists.In order to solve this problem,it is usually adopted to dope energy trapping centre ions in the core.However,the energy trapping effect is limited due to the small amount of doping.In this chapter,Tm Er@Eu（x）/NMF composite photocatalysts were synthesized for the first time by doping energy trapping centre ions in the shell,and used to degrade amoxicillin and norfloxacin.Under the simulated sunlight,Tm Er@Eu（20%）/NMF can degrade amoxicillin and norfloxacin,with degradation of 90%and 91%,respectively.The excellent photocatalytic performance of Tm Er@Eu（20%）/NMF is mainly attributed to three points:first,core-shell structure of the nanoparticles reduces the surface fluorescence quenching;second,the doping of Eu³⁺ions increases the fluorescence emission peak;third,the Eu³⁺ions doped in the shell can inhibit the migration of excitation energy to lattice defects and reduce the cross-relaxation between activator ions.