Preparation And Properties Of Rare-earth Doped Zirconia-Based Luminescent Materials

Nanotechnology based on ceramic materials that involve zirconia has important applications in not only sophisticated optoelectronic and photonic devices but also a broad range of biomedical applications.Zirconia is promising candidate as the rare earth luminescent host for its superior mechanical,electrical,thermal and optical pproperties,namely high hardness,high dielectric constant,good thermal stability,high melting point,high refractive index,wide band gap,good chemical and photochemical stability.In addition,low phonon energy,optical transparency in visible and near infrared light regions are also made it as idea matrix for the preparation of luminescent materials.Sm3+ is one of the most interesting rare earth elements which often shows stable trivalence state and shows promising photoluminescent properties in orange red visible region.Er3+ has negligible electromagnetic interference,convert near infrared light into visible emission.It was determined that Yb3+ with much stronger absorption cross-section at 980 nm is the most commonly used as sensitizers to enhance the pumping absorption.Various routes for preparing ZrO2 nanometer materials with spherical morphology and uniform size distribution have been developed.In this paper,we employed a polymer-network gel method to prepare ZrO2:Sm3+,ZrO2:Sm3+-Sn4+ and ZrO2:Er3+-Yb3+ phosphors.Leading research contents and results are listed as follows:(1)The zirconia powders were prepared by polymer network gel method,and the optimum preparation conditions were determined by series experiments.The results indicate that the best route is as follows.Firstly,the concentration of the initial metal ions in the solution is 0.1 mol/L.Secondly,a certain of citric acid(citric acid/metal ion=1.5:1)was added to the above solution as a chelating agent for the metal ions.Subsequently,the pH value was adjusted to 4-5 using ammonium hydroxide.Then,a certain amount of acrylamide was added to the solution according to the ratio that monomer to metal ions are of 1:9.Meanwhile,proportional(monomers/crosslinking agent=8:1)N,N'-methylene diacrylamide was dissolved in the solution.At the end,the highly transparent solution with magnetic stirring was heated at 70 ? in a water bath and immediately persulfate(1%of the monomer mass)was introduced to initiate polymerization.(2)We have synthesizd a series of ZrO2:Sm3+ nanophosphors by polymer-network gel method and the composition,luminescence properties and concentration quenching mechanism were studied.It has been discovered that with the increase of Sm3+ concentration,the samples get transformed from monoclinic to tetragonal and then to cubic phase.All nanophosphors show the characteristics peaks of Sm3+ in the wavelength range 500?700 um under high-energy ultraviolet light(404 nm)excitation.As the Sm3+ concentration increases,the emission intensity increased gradually and reached a maximum at 0.5 mol%.The quenching of luminescence intensity seen at higher Sm3+ concentration is due to dipole-dipole interaction.(3)ZrO2:0.5 mol%Sm3+-x mol%Sn4+ co-doped phosphors were prepared by polymer network gel method and the composition,fluorescence emission and long afterglow were studied.It has been discovered that the ZrO2:Sm3+ nanophosphors don't show afterglow.After co-doping with Sn4+,intense afterglow luminescence is observed in ZrO2:Sm3+-Sn4+ and it can last as long as 1000 s at maximum.The afterglow decay curves of ZrO2:Sm3+-Sn4+ samples are fitted by three exponential model,in other words,the decay process consists of initial fast,intermediate and slow decay.Compared with ZrO2:Sm3+ phosphors,the emission intensity of ZrO2:Sm3+-Sn4+ increased under ultraviolet light excitation and the maximum reached at 10 mol%.But the co-doping of Sn4+ did not change the position and quantity of the emission peak.(4)We have synthesized ZrO2:0.5 mol%Er3+-x mol%Yb3+ up-conversion phosphors by polymer-network gel method and the composition,up-conversion luminescence and phosphorescent mechanism,long afterglow were studied.It has been discovered that by control of the Yb3+ concentration,the ZrO2:Er3+-xYb3+phosphors are capable of generating color tunable upconversion luminescence from green to yellow,then to orange and finally to red.The tunable luminescence emission is due to the energy back transfer from Er3+ to Yb3+ and the cross relaxation between the two neighboring Er3+ ions.Introducing Y3+ into ZrO2 host would get the cubic phase ZrO2 which is unfavourable for upconversion luminescence because of the formation of Y3+-Vo··-_Yb3+ among Y3+ ions,oxygen vacancies and Yb3+ ions which prevent the energy transfer process between Yb3+ and Er3+.