Design,Electrospinning Construction And Performance Of Exceptive-Structured Rare Earth Luminescent Nanomaterials

Due to the important and widespread applications of rare earth luminescent materials in display,lighting,detection and other fields,it is an important research topic to explore simple and universal synthetic technology for preparing rare-earth luminescent materials to meet more needs.Because rare-earth luminescent materials with special structures are easier to improve the luminescent properties and obtain ideal white light,it is of great scientific significance and application value to design and construct various rare earth luminescent nanomaterials with special structures to meet different requirements.However,owing to the complexity of the operation steps and the harsh synthetic conditions,the successful preparation of rare earth luminescent nanomaterials with special structures is largely limited.In this thesis,rare-earth doped alkaline earth metal fluoride nanofibers,hollow nanofibers and nanobelts are successfully constructed by the combination of an electrospinning technology with a dual-crucible fluorination technology,and their up-/down-conversion luminescence properties are studied.In addition,a simple and universal single-axis electrospinning technology combined with the unique dual-crucible fluorination technology is innovatively designed and used for the first time to directly fabricate rare-earth doped alkaline earth metal fluoride yolk-shell nanofibers and yolk-shell nanobelts by changing the composition of the precursor solution.By applying the improved technology,Janus yolk-shell nanofibers are prepared by a parallel electrospinning technology instead of the single-axis electrospinning technology,and white-light emission is realized by constructing two zones in the Janus nanofiber.In order to prove the universal applicability of this method,the target sample is expanded from alkaline-earth-metal fluoride to rare-earth fluoride.By changing the concentration of precursor solution and relevant parameters,rare-earth fluoride yolk-shell nanospheres are creatively constructed by the combination of an electrospray ionization technology with the dual-crucible fluorination technology,and their luminescent properties and water stability are studied.The main contents of this thesis are as follows:1.MF₂:Yb³⁺,Er³⁺（M=Ba,Sr）up-conversion luminescence（UCL）one-dimensional（1D）nanostructures have been designed and constructed via the mono-axis electrospinning united with the dual-crucible fluorating technology.Controllable fabrications of MF₂:Yb³⁺,Er³⁺（M=Ba,Sr）nanofibers,nanobelts and hollow nanofibers 1D nanostructures are successfully and facilely realized via simply regulating the compositions and ratios of the electrospinning solutions by applying the above technique.The effects of morphology,calcination temperature and molar ratios of Yb³⁺to Er³⁺on the UCL characteristics of the samples are systematically investigated,and color-tuned up-conversion luminescence is achieved.It is found that as-obtained pure cubic-phase MF₂:Yb³⁺,Er³⁺（M=Ba,Sr）1D nanostructures display excellent green UCL properties under the 980-nm laser excitation.Further,the MF₂:Yb³⁺,Er³⁺（M=Ba,Sr）nanofibers have the strongest fluorescence intensity of the three kinds of morphological samples,realizing modulation of luminous intensity via regulation of morphologies.More importantly,the proposed design concept and preparation technique can be extended to manufacture other metallic fluorides 1D nanomaterials.2.A series of multicolor luminescent BaF₂:Eu^2+/3+nanofibres as well as BaF₂:Eu^2+/3+,Tb³⁺nanofibres have been devised and constructed by using the electrospinning united with the dual-crucible fluorination technology.The partial reduction of Eu³⁺is realized while the formation of Eu-doped BaF₂ one-dimensional nanofibers,realizing the co-existence of Eu²⁺and Eu³⁺and laying the foundation for multicolor luminescence.Simultaneously,multicolor luminescence and white light emission are achieved through the valence state regulation of Eu(Eu²⁺and Eu³⁺)and the energy transfer between Eu^2+/3+and Tb³⁺.In the emission spectra of BaF₂:Eu^2+/3+nanofibres,the broad peak between 350-500 nm（centered at 377nm）belongs to the 5d→4f energy level transition of Eu²⁺ion and the narrow peaks at 592and 613 nm belong to the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ energy levels transition of Eu³⁺ion,respectively.Concurrently,luminescence colors of BaF₂:Eu^2+/3+nanofibres directly range from blue to yellow region through different wavelengths（286,297,318,361,381 and 393nm）excitation.Therefore,by doping green-light emitting Tb³⁺into BaF₂:Eu^2+/3+nanofibres to acquire BaF₂:Eu^2+/3+,Tb³⁺nanofibres,white light emission can be easily realized.Meaningfully,the luminous color of BaF₂:Eu^2+/3+,Tb³⁺nanofibres can be adjusted from blue,white,and yellow to orange by using multi-channel（250,264,286,297,318,361,381 and 393 nm）excitation,which greatly improves the practicability.Further,the mechanism of multicolor luminescence and white-light emission is systematically studied,and Eu²⁺→Tb³⁺→Eu³⁺energy transfer process in BaF₂:Eu^2+/3+,Tb³⁺nanofibres is further advanced and clarified.The new findings indicate that BaF₂:Eu^2+/3+,Tb³⁺nanofibres will have emerging application in the fields of color lighting,backlight displays,white-light-emitting device and other fields.3.A simple and universal technique of the combination of the uniaxial electrospinning with the subsequent peculiar dual-crucible fluorination technology is innovatively designed and used to construct rare-earth doped alkaline earth metal fluoride yolk-shell nanofibers（YSNFs）and yolk-shell nanobelts（YSNBs）directly for the first time.Yb³⁺/Er³⁺co-doped alkaline earth metal fluoride（BaF₂,SrF₂ and Ca F₂）as core layer and SiO₂ as shell layer are applied to construct the yolk-shell one-dimensional nanostructures.By HF acid treatment of the samples,it is not only proved that the shell layer is silica,but also independent core-layer MF₂:Yb³⁺,Er³⁺（M=Ba,Sr,Ca）nanofibers and nanobelts with good size uniformity and excellent up-conversion luminescence are obtained.Especially,only fluorination process can form such exceptive YSNFs and YSNBs one-dimensional nanostructures,and the formation mechanisms of YSNFs and YSNBs are proposed and verified,and further the novel technique is established.The design philosophy and the construction technique for the exceptional yolk-shell one-dimensional nanostructures afford a facile approach for the fabrication of other functional metal fluorides yolk-shell one-dimensional nanostructures.4.The[SrF₂:Eu³⁺@SiO₂]//[SrF₂:Tb³⁺@SiO₂]Janus yolk-shell nanofibers with two zones are innovatively designed and constructed by the combination of a parallel electrospinning technology with the dual-crucible fluorination technology.The concentration of Eu³⁺ions in SrF₂:Eu³⁺@SiO₂ side is fixed at 9%,and the effect of different contents of Tb³⁺in SrF₂:Tb³⁺@SiO₂ side on the fluorescence performance of the Janus yolk-shell nanofibers is investigated.The results show that with the increase of Tb³⁺doping concentration,the emission peak of Tb³⁺at 545 nm increases,while the emission intensity of Eu³⁺at 592 nm does not obviously change,which proves that no obvious energy transfer between Tb³⁺and Eu³⁺exists.Because the unique structure of Janus yolk-shell nanofibers effectively inhibit the energy transfer between the two luminescent ions by limiting the two ions to their own zones,white light emission is successfully achieved.The research results provide a new idea for the design and construction of novel white light emitting materials.5.A series of LaF₃:Pr³⁺@SiO₂ yolk-shell nanospheres（YSNS）with tunable multicolor luminescence are constructed by the innovative design and efficient combination of an electrospray ionization technology with the dual-crucible fluorination technology.Tunable luminescence,especially white-light emission,is successfully obtained by adjusting the Pr³⁺ion concentration.Noticeably,the water resistance of LaF₃:Pr³⁺@SiO₂ YSNSs is greatly improved by coating with silica shell compared with LaF₃:Pr³⁺nanospheres without SiO₂ shell.The water stability of SiO₂-coated LaF₃:Pr³⁺@SiO₂ YSNSs is improved by 15%compared with LaF₃:Pr³⁺nanospheres after the samples are immersed in water for 72 hours,and the fluorescence intensity can be maintained at 86%of the initial intensity.The formation mechanism of yolk-shell nanospheres is proposed in detail,and further the novel preparation technique is established.The prepared LaF₃:Pr³⁺@SiO₂ YSNSs have important application prospects.