| Upconversion photoluminescence (UCPL) nanomaterials, being capable of converting pump photons with low energy into visible light, are potential functional materials, and owing to their unique characteristics, they have found promising applications in the areas of biological medicine, three-dimensional displays, solar cells, optical sensors, data storage, photoswitching and so forth. Y2Ti2O7, one of the typical pyrochlore-type complex oxides, is the ideal candidate matrix for UCPL materials due to its excellent properties such as low phonon energy, high physico-chemical stability and easy doping with rare earth ions. Taking these into considerations, the sol-gel method and solution combustion approach were developed to synthesize phase-pure and well-crystillized Y2Ti207nanoparticles with good dispersity, and key conditions during the preparation process were optimized with theoretical analysis and experiments, and the phase structures, morphologies and UCPL properties of doped Y2Ti207nanocrystals were investigated systemically.A new methology to prepare Y2Ti207nanocrystals by citric acid sol-gel technique was developed. Based on thermodynamical equilibria constants, the theoretical calculation about solubility isotherms demonstrated that the appropriate pH value for forming the stable Y3+-Ti4+-citrate complex was6.4-7.0. And also, the key conditions such as the pH value of the precursor solution and the amount of citric acid were optimized by experiments. Under the optimum conditions (the pH value of the precursor solution is6.5, the molar ratio of citric acid to the total cations is2.5), the as-prepared Y2Ti2O7nanoparticles with good dispersity and narrow size distribution were nearly sphere-like, the average particle size was about20～30nm. The experiment results were in good agreement with the theoretical analysis of solubility isotherms, which provided the theoretical foundation and experimental method to synthesize pyrochlore-type titanate nanocrysals with good dispersity.A series of novel UCPL nanocrystals Y2Ti2O7:Er3+,Yb3+and Y2Ti2O7:Er3+,Yb3+,Li+were prepared via citric acid sol-gel approach. For all the samples under investigation, bright visible upconversion emission can be clearly observed with naked eyes under the relatively weak980nm excitation. The upconversion green (-525and545nm) and red (-660nm) emissions were obtained, which are ascribed to the (2H11/2,4S3/2)â†'4I15/2and4F9/2â†'4I15/2radiation transitions of Er3+ions, respectively. UCPL intensity was strongly dependent on the calcining temperatures, Yb3+and Li+ion concentrations. As for (Y0.99-xEr0.01Ybx)2Ti207(x=0-12.5mol%) samples, the intensity was increased greatly with the temperature rise (600～1000℃), the optimum concentration for Yb3+ion is7.5mol%; As for (Y0.915-xEr0.01Yb0.075Lix)2Ti2O7(x=0-15.0mol%) samples, the intensity reached the maximum at the calcining temperature of800℃and Li+ion concentration of10.0mol%. Compared with Li+-free samples, the upconversion red and green emission intensity was drastically increased by a factor of18.6and8.3, respectively, which is the result of the reduction of the local symmetry of Er3+ions and the number of hydroxyl groups. The pump power dependence of the emission intensity suggested that the two photon process was responsible for UCPL of Y2Ti207:Er3+,Yb3+and Y2Ti207:Er3+,Yb3+,Li+nanocrystals. Detailed proof showed Er3+/Yb3+/Li+doped Y2Ti2O7nanocrystals are efficient upconversion phosphors.A facile and novel solution combustion approach with glycine was developed to prepare pure-phase Y2Ti2O7nanocrystals with good dispersity. The influence of the amount of glycine, the pH value of precursor solution and the annealing temperature on the structure, crystallinity, morphology and particle size was discussed. The results indicated that as-obtained products were pure-phase pyrochlore Y2Ti207nanoparticle with good crystallinity, dispersity, narrow size distribution and the average size of20～30nm under the appropriate conditions (the molar ratio of glycine to nitrate is2.0, the pH value of precusor solution is2.0and the calcining temperature is800℃). Compared with the conventional solid state method, this novel systhesis strategy had a great many advantages such as more facile process, shorter reaction time and erergy saving. A series of novel UCPL nanocrystals Y2Ti2O7:Ho3+,Yb3+and Y2Ti2O7:Ho3+,Yb3+,Li+were synthesized by the solution combustion strategy with glycine. The results showed that, for all the samples under investigation, bright visible upconversion red (-660nm) and green(-550nm) emissions were observed, which are attributed to the5F5â†'5I8and (5F4,5S2)â†'5I8transitions of Ho3+ions, respectively. The calcining temperature, Yb3+and Li+ion concentration had a considerable influence on UCPL intensity. As for (Y0.99-xHo0.01Ybx)2TiO7(x=0-12.5mol%) samples, the intensity reached the maxim at the calcining temperature of800℃and the Yb3+ion concentration of7.5mol%. Compared with Yb3+-free (Yo.99Ho0.01)2Ti207samples, the UCPL intensity of (Y0.915Ho0.01Yb0.075)2Ti207nanocrystals was increased by a factor of32. As for (Yo.915-xHo0.01Yb0.075Lix)2Ti207samples (x=0-15.0mol%) samples, the emission reached the maximum at the calcining temperature of800℃, when incorporating12.5mol%Li+ion, the UCPL green and red emission intensity was about10.2and4.8times, respectively. The reasons for this enhancement were revealed:one is the modification of the local symmetry of Ho3+ions, and the other is the reduction of the possibility of non-radiation transitions. The relationship between UCPL intensity and pump power indicated that the two photon process accounted for UCPL of Y2Ti2O7:Ho3+,Yb3+and Y2Ti2O7:Ho3+,Yb3+,Li+nanocrystals.Shell-core structure Y2Ti2O7:Er3+,Yb3+,Li+@SiO2and Y2Ti2O7:Ho3+,Yb3+,Li+@SiO2upconversion composites were fabricated successfully by a two-step strategy. The detailed results demonstrated the pH value of the precursor solution had a strong influence on the coating reaction on the SiO2microspheres via sol-gel technique, and Y2Ti2O7-based UCPL nanocrystals could be coated on the SiO2microspheres completely and homogeneously at the pH value of7.0. The shell-core structure exhibited spherical morphology and monodispersity, and the shell layer is～20nm. And also, the shell-core structure unconversion composites showed the same UCPL intensity as the corresponding uncoatd upconversion nanoparticles under the excitiation of980nm. The research findings in the thesis, therefore, are of great significance for the research and development of novel, high efficient and cost-effective Y2Ti207-based upconversion nanomaterials. |
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