Nd³⁺-Sensitized Upconversion Nanophpsphor@TiO₂ Core/Shell Nanostructures:Controllable Synthesis And Their NIR-responsive Photocatalytic Activity

Rare-earth ion-doped upconverting materials have anti-Stokes luminescence properties and emit high energy light when excited by low-energy light.The excitation wavelength is usually in the near-infrared region,and low-energy near-infrared light can be converted into high-energy ultraviolet-visible-near infrared light by the anti-Stokes emission process.Because the rare earth fluoride matrix low phonon energy,the loss of no radiative transition is small and the upconversion efficiency is relatively high,so NaYF4 is used more as a matrix.In order to increase the upconversion efficiency,a sensitizer is incorporated in the matrix.At present,the commonly used sensitizer is Yb3+,but in recent years,researchers began to focus on the role of Nd3+ ions as a sensitizer.Nd3+ has a large absorption cross section near 800 nm,but the luminescence intensity of Nd3+ sensitized upconversion nanomaterial excited by 808 nm is still low compared with the emission intensity of Yb3+ sensitized upconversion nanomaterial at 980 nm excitation.Nd3+ as the main sensitizer to construct the active core/active shell partition structure can inhibit the surface quenching effect and improve the upconversion efficiency.It can also avoid the overheating effect caused by water molecules'absor:ption of infrared light at 980 nm,photothermal damage of biological tissues,and effectively affect the fluorescence resonance energy transfer affected(FRET).TiO2 is currently the most widely used semiconductor photocatalytic material,combined with the upconversion material,can effectively absorb ultraviolet upconversion luminescence,to further expand the photocatalytic response to the excitation light range.In this dissertation,Nd3+ sensitized upconverting material is designed to compound with TiO2 semiconductor and constructing the NaYF4@NaYF4 homogeneously active core-shell structure upconverting nanocrystalline @ TiO2 heterostructure.At the same time,noble metal Ag is introduced to improve the upconversion efficiency.The composite system exhibits excellent photocatalytic activity under 808 nm light irradiation,which can significantly degrade organic pollutants and inhibit bacterial growth.The specific research contents of this paper are as follows:(1)Controlled synthesis of NaYF4:Yb,Er,Nd@NaYF4:Nd@Ti1-xMnxO2 nanoparticles with improved photocatalytic degradation and antibacterial activityThe NaYF4:Yb,Er,Nd nanoparticles were prepared by pyrolysis method.The effects of reaction time,temperature and doping concentration of rare earth ions on the morphology,phase and up-conversion fluorescence intensity were investigated.Using the method of epitaxial growth,NaYF4:Yb,Er,Nd@NaYF4:Nd(UCNPs)was continuously coated on the surface of NaYF4:Nd shell,and the different doping concentration of rare earth ions in the active shell was investigated.The effect of the conversion fluorescence intensity was determined to determine the optimal doping ratio of rare earth ions.Subsequently,the surfaces of the UCNPs were converted from hydrophobic to hydrophilic by surface modification.As a template,the surface of the UCNPs was directly coated with manganese-doped titanium dioxide(Ti1-xMnxO2)nanoparticles by an improved sol-gel method.Finally,NaYF4:Yb,Er,Nd@@NaYF4:Nd@Ti1-xMnxO2 core-shell-shell composite were obtained.The effect of the change of Mn2+ doping concentration on the activity of the composite was investigated by adjusting the amount of manganese acetate,and the optimum doping ratio of Mn2+ was determined.The results show that by doping Mn2+ into the crystal lattice,the light response of TiO2 can be broadened to the visible region.Under the irradiation of 808 nm,the visible light emitted from UCNPs can be absorbed by Ti1-xMnxO2 and produce electron hole pairs,and further react with oxygen and water in the microenvironment to produce strong oxidative ·O2-and · OH,which shows good NIR degradation and antibacterial properties.The experimental results show that the active species that plays a major role in the photocatalytic antibacterial reaction is ·O2-.MTT colorimetric test results show that the composite has good biocompatibility.(2)Synthesis of NaYF4:Nd,Tm,Yb@NaYF4:Nd,Yb@SiO2@Ag@TiO2 nanocomposites and their photocatalytic propertiesFirstly,NaYF4:Nd,Tm,Yb@NaYF4:Nd,Yb active core-active shell nanoparticles(UCNPs)were synthesized by high temperature pyrolysis and seed epitaxial growth.The incorporation of Nd3+ and Yb3+ in the NaYF4 shell increased the upconversion fluorescence intensity.The influence of Yb3+ doping in the shell and concentration regulation on the fluorescence intensity of UCNPs up-conversion was investigated experimentally,and the optimum doping ratio of rare earth ions in the active shell was determined.Secondly,NaYF4:Nd,Tm,Yb@NaYF4:Nd,Yb@SiO2 nanoparticles were prepared by reversed micelles using UCNPs as templates,and the morphology and up-conversion fluorescence intensity of UCNPs@SiO2 were influenced by adjusting the amount of TEOS.Thirdly,using UCNPs@SiO2 as a template,Ag nanoparticles were deposited on the surface by high temperature reflow method.The effect of changing AgNO3 addition on the morphology and upconversion fluorescence intensity of UCNPs@SiO2@Ag was investigated.It was found that the introduction of the noble metal Ag,can effectively improve the light emission intensity by SPR effect.Finally,a layer of TiO2 was coated on the surface by sol-gel method.The influence of the amount of TBOT added on the morphology of multilayer core-shell structure and the fluorescence intensity of upconversion was investigated.The experimental results show that with the change of the thickness of the coated TiO2,the fluorescence intensity shows a decreasing trend.And the photocatalytic performance of the composite material also changes,first increases and then decreases,there is an optimal thickness.UCNPs@SiO2@Ag@TiO2 nanocomposites showed good photocatalytic effect in the degradation of RhB and in the antibacterial experiments of Bacillus subtilis under 808 nm infrared light.The photocatalytic mechanism was also studied by using the corresponding trapping agentand the experimental results show that the active species which play the main role in the photocatalytic process are ·O2-.MTT assay showed that the composite had low cytotoxicity and good biocompatibility.(3)Synthesis of NaYF4:Yb,Tm,Nd@NaYF4:Yb,Nd@SiO2@TiO2/Ag nanocomposites and their photocatalytic propertiesUsing NaYF4:Nd,Tm,Yb@NaYF4:Nd,Yb@SiO2 as template,one-step coating of TiO2/Ag shell on the surface.The experiment not only explored the influence of reaction time on the morphology and fluorescence intensity of the products,but also adjusted the thickness of TiO2 and the deposition density of Ag by changing the amount of TBOT and AgNO3.The effects of thickness and deposition on the photocatalytic degradation of RhB organic pollutants and the antibacterial activity of Bacillus subtilis under 808 nm irradiation.The results show that,with the increase of the thickness of TiO2 on the surface of UCNPs@SiO2,the catalytic and antibacterial activity of the system first increases and then decreases.In the preparation of UCNPs@SiO2@TiO2/Ag composite system,when the addition amounts of TBOT and AgNO3 were 10?L and 3 mL respectively,the photocatalytic degradation activity of the obtained system was the best.