Hydrothermal Synthesis And Luminescent Properties Of β-Li₂TiO₃ Phosphors Doped With Lanthanide Ions

The titanate tri-phosphors doped with lanthanide ions have been serving as a focus for the white light phosphors in the last few years. Excited by the near ultraviolet light or the blue light, a broader band excitation spectrum is achieved and thus yielding better color rendition properties. In the supercell structure of β-Li2 Ti O3, there are three sorts of Li+ ions, of which the Li1+ ions occupy the asymmetric lattice points, while Li2+ and Li3+ ions occupy the symmetric lattice points. Enhanced photoluminescent properties can be obtained by efficient electron transitions between varied energy levels withing the proper lanthanide ions, when doping in β-Li2 Ti O3 to substitute Li+ ions at varied lattice points under reasonable synthesis conditions.In this thesis, the β-Li2 Ti O3: Eu3+ phosphors, the β-Li2 Ti O3: Sm3+ phosphors, the β-Li2 Ti O3: Tm3+ phosphors, the β-Li2 Ti O3: Ce4+ phosphors, and the β-Li2 Ti O3: Eu3+, Dy3+, Tb3+ phosphors were synthesized, respectively by the hydrothermal method. Better photoluminescent properties, smaller particle size, more uniform particle size distribution, easier substitution of Li+ ions with the lanthanide ions, and potential better coating processing were achieved by optimization of the hydrothermal synthesis parameters. The photoluminescent properties and mechanisms were investigated. Effect of the governing supercell structure on the photoluminescent properties was discussed as well. The thesis may be beneficial to the potential insight investigation of the co-related β-Li2 Ti O3 phosphors and exploration of the commercial applications.Initially, the hydrothermal processing parameters of β-Li2 Ti O3 matrix powders are optimized by the orthogonal experimental design. The significance order for better development of the supercell is as, hydrothermal time > Li+ concentration > hydrothermal temperature > packing ratio. The optimized parameters for β-Li2 Ti O3 matrix precursor are determined as, the hydrothermal time for 2 h; the Li+ concentration of 0.5 mol/L; the hydrothermal temperature of 120 °C; and the packing ratio of 70 %. By which the calculated(002)( 133)I/I of the β-Li2 Ti O3 matrix is 1.17, and the calculated mean grain size is 37 nm.The β-Li2 Ti O3: Eu3+ phosphors with higher color purity are synthesized with the optimized processing parameters. A combined analysis of the Raman spectra and the emission spectra shows that most of the Eu3+ ions substitute the Li1+ ions, and the residual ones substitute the Li3+ ions, yielding the stronger red emission at lem ~ 618 nm by the electric dipole allowed 5D0 â†' 7F2 transition within Eu3+ when excited at lex ~ 394 nm. The luminescent properties can be enhanced by control of the calcination time and the Eu3+ doping concentration. Optimized color rendition properties of the β-Li2 Ti O3: 0.5 mol%Eu3+ phosphors calcined at 600 °C for 36 hours are obtained with CIE color coordinate of(x = 0.63, y = 0.37), the color temperature of 4667 K, the color rendering index of 81.7, the color purity of 99.7 %, the Judd–Ofelt intensity parameters ?2 of 8.01′10-20 cm2, the ?4 of 0.62′10-20 cm2, the branching ratio b2 of 82.9 %, and the quantum efficiency h of 91 %.In contrast, the β-Li2 Ti O3: Sm3+ phosphors with lower color temperature are synthesized by substituting most of the Sm3+ ions of Li3+ ions, which occupy the symmetric lattice points. The red-orange emission at lemi ~ 594 nm and lem ~ 618 nm by the electric dipole allowed 4D5/2 â†' 6H7/2 transition within Sm3+ is obtained by the near ultraviolet excitation at lex ~ 394 nm. The photoluminescent properties can also be enhanced by control of the calcination time and the Sm3+ doping concentration. Optimized color rendition properties of theβ-Li2 Ti O3: 0.25 mol%Sm3+ phosphors calcined at 600 °C for 36 hours are obtained with CIE color coordinate of(x = 0.57, y = 0.41), the color temperature of 1876 K, the color rendering index of 72.6, and the color purity of 94.7 %.For the synthesized β-Li2 Ti O3: Tm3+ phosphors with excellent monochromatic properties, near equivalent substitution of the Tm3+ ions of Li1+ ions and Li3+ ions yields the stronger blue emission at lem ~ 470 nm by the 1G4 â†' 3H6 transition within Tm3+ when excited at lex ~ 230 nm. Optimized color rendition properties of the β-Li2 Ti O3: 0.5 mol%Tm3+ phosphors calcined at 600 °C for 24 hours are obtained with CIE color coordinate of(x = 0.15, y = 0.10), the color temperature of 10571 K, the color rendering index of 71.1, and the color purity of 90.3 %.As for the white-light β-Li2 Ti O3: Ce4+ phosphors, they have superior higher color rendering index. Emissions at 465 nm, 563 nm, 575 nm, and 620 nm originates from the transition between d- f electron configurations within Ce3+ and the charge-transfer absorption of Ce4+- O2-. The color rendition properties can be enhanced by control of the calcination time and the Ce4+ doping concentration. CIE calculation shows that the optimized processing at 600 °C for 36 hours for β-Li2 Ti O3: 4 mol%Ce4+ phosphors are beneficial to providing the white emission(x = 0.32,y = 0.33) that is very close to the standard white(x = 0.33,y = 0.33) with color temperature of 4775 K, color rendering index of 86.7.For β-Li2 Ti O3 co-doped with Eu3+, Dy3+, Tb3+ ions, when excited at 394 nm, white light emission at lem ~ 438 nm to lem ~ 618 nm is provided. In the co-doping system, energy transfer is exhibited between Eu3+, Dy3+, and Tb3+ ions, a reasonable way of which is proposed as Eu3+ â†' Dy3+ â†' Tb3+. A combination of 1 mol%Eu3+, 1 mol%Dy3+, and 0.25 mol%Tb3+ co-doping provides the white emission(x = 0.35,y = 0.33) that is very close to the standard white as well, with color temperature of 4735 K, and color rendering index of 79.3.