Preparation And Luminescent Properties Of Nanoscale Pyrochlore Type Rare Earth Stannates Luminescent Materials

With the development of nanotechnology, nanoscale luminescent materials have gradually been considered to be the next generation novel luminescent materials due to the numerous unique properties they possess. Among the nanoscale luminescent materials, rare earth ions doped pyrochlore type complex oxides are believed to be the ideal luminescent materials owing to their excellent chemical and thermal stability. Pyrochlore type complex oxides are becoming increasingly important, mainly because of their actual and potential applications in the areas of information display, light engineering, optical communication, and so on. Preparation of micro/nanomaterials with well-defined size, morphology, dimensionality and diversity through novel synthesis approaches and investigation of their formation mechanism should be a key precondition to reveal the relation between the structures and properties.The synthesis of pyrochlore type La2Sn207, Y2Sn2O7 and Gd2Sn2O7 micro/nanocrystals doped by Eu3+, Ce3+ and Tb3+, respectively, were studied in the present dissertation. The as-synthesized micro/nanocrystals were investigated by XRD, SEM, TEM, EDS, SAED, XPS, FT-IR, Raman, TG-DSC and PL. And the optical properties of rare earth ions doped pyrochlore type rare earth stannates micro/nanocrystals were also discussed. Some new and interesting results were obtained and listed as follows.A new fractional precipitation combined with hydrothermal route was developed to synthesize a novel 3D octahedral La2Sn207:Eu3+ microcrystals with pyrochlore structure in water solution. The effects of the composition of starting reagents, ratio of starting reagents, kind of mineralizers, pH of precursor solution, concentrations of starting reagents, reaction temperature, time and Eu3+ doping content on the phases and morphologies of the as-synthesized samples were investigated. The results reveal that the pH of the precursor solution, ratio of Ln:Sn, precursor concentration and reaction temperature not only plays an important role in determining the phase of the as-synthesized products, but also has a significant influence on the morphologies of the samples. High-quality and uniform octahedrons with an average size of about 700 nm could be easily obtained at the pH value of 12. The possible reaction mechanism of pyrochlore type crystals and formation mechanism of octahedral La2Sn207:Eu3+microcrystals were briefly proposed. The photoluminescence spectra show a series orange-red emission lines around 587,602,615 and 630 nm attributing to 5D0-7F1 and 5D0-7F2 transitions of Eu3+for La2Sn2O7:Eu3+micro/nanocrystals. Meanwhile, the concentration quenching phenomenon can be observed if the Eu3+doping content exceeds 18 at%in La2Sn207:Eu3+nanocrystals. The photoluminescence spectra reveal that 3D octahedral La2Sn207:Eu3+ sample display improved luminescence compared with the samples with other shapes. And high temperature heat treatment is helpful to enhance the intensities of PL spectra.Phase-pure Ce3+/Tb3+ doped and co-doped lanthanum stannates (La2Sn207) nanocrystals were synthesized by a co-precipitation process combined with hydrothermal techniques assisted by ascorbic acid. The as-synthesized samples were single-phase cubic pyrochlore nanocrystals with a typical size of 10-20 nm. The influences of the pH of precursor solution, reaction temperature, time and Ce3+/Tb3+doping and co-doping content on the shape and phase structure of the as-synthesized products were discussed. The second octahedral-like micro-structure La2Sn207:Tb3+which is assembled with the first octahedral-like nanocrystals could easily synthesize via a co-precipitation process combined with hydrothermal techniques assisted by CTAB. The formation mechanism of second octahedral-like micro-structure La2Sn207:Tb3+was proposed. PL spectra show a dominating green-emitting line around 544 nm attributing to 5D4-7F5 magnetic dipole transition for Tb3+doped and Ce3+/Tb3+co-doped La2Sn207 nanocrystals. Meanwhile, the concentration quenching phenomenon was observed in both Tb3+doped and Ce3+/Tb3+co-doped La2Sn207 nanocrystals. Furthermore, an interesting enhancement of the energy transfer induced green emission was observed in the as-synthesized Ce3+/Tb3+co-doped La2Sn2O7 nanocrystals. The dynamic process of Tb3+luminescence in Tb3+ doped and Ce3+/Tb3+ co-doped La2Sn2O7 nanocrystals and the energy transfer process from Ce3+to Tb3+were investigated. The results show that the ascorbic acid in hydrothermal process plays a key role in the high photoluminescence properties of Ce3+/Tb3+co-doped La2Sn207 nanocrystals. And the rapidly decrease in the photoluminescence intensity of Tb3+ions for Ce3+/Tb3+co-doped La2Sn207 nanocrystals was observed when the sample was heat treated in air, attributing to the Ce3+ions have been oxidated into Ce4+ions.A simple co-precipitation process combined with hydrothermal method at low temperature was adopted to synthesize the phase-pure Y2Sn207:Eu3+nanocrystals with pyrochlore structure. The results show that the Y2Sn207:Eu3+nanocrystals can be easily obtained under the comparatively wide synthetic conditions to compare with that of La2Sn207:Eu3+nanocrystals. The PL spectra show a series of orange-red narrow emission lines due to 5D0-7F1 and 5D0-7F2 transitions of Eu3+for Y2Sn2O7:Eu3+nanocrystals. And the concentration quenching phenomenon was observed when the Eu3+doping content was above 9 at %in Y2Sn207 lattice. The PL results reveal that the Eu3+ions located at a D3d site in the Y2Sn207 matrix. The high heat treatment temperature is helpful to enhance the intensities of PL spectra of Y2Sn207:Eu3+ nanocrystals.Novel tube-like Y2Sn2O7:Tb3+crystals with pyrochlore structure have been successfully synthesized via a hydrothermal route. The experimental results show that ammonia plays a key role in the fabrication of Y2Sn207:Tb3+with tube-like morphology. The photoluminescence spectra indicate that the tube-like Y2Sn2O7:Tb3+ crystals emit strong green light at 543 nm under UV excitation. The growth mechanism of tube-like Y2Sn2O7:Tb3+crystals is proposed.A simple co-precipitation process combined with hydrothermal method at low temperature was adopted to synthesize the phase-pure pyrochlore type Gd2Sn207:Eu3+nanocrystals. The experimental results show that the Gd2Sn207:Eu3+nanocrystals can be easily obtained under the more general synthetic conditions to compare with that of La2Sn207:Eu3+and Y2Sn2O7:Eu3+nanocrystals. The PL spectra reveal that the Gd2Sn207:Eu3+ nanocrystals display strong orange-red light under UV excitation, attributing to 5D0-7F1 and 5D0-7F2 transitions of Eu3+ions in Gd2Sn2O7:Eu3+ nanocrystals. The PL spectra show the highest intensity of photoluminescent can be observed in the Gd2Sn207:Eu3+(7 at %) nanocrystals. And the Eu3+ ions located at a D3d site in the Gd2Sn207 system. The heat treatment is helpful to enhance the intensities of PL spectra of Gd2Sn207:Eu3+ samples.