| Rare earth (RE) luminescent materials form an important class of functional materials for various applications, such as solid-state lighting, fluorescence detecting, photo-electronic displays, and X-ray imaging. In recent years, there has been a widespread interest in the investigation of new and novel families of high-performance luminescent materials doped with RE ions. Titanoniobate compounds are a valuable class of luminescent host materials because of their excellent physical and chemical properties and exciting optical characteristics. Up to now, Titanoniobates have been always prepared by the traditional solid state sintering method, while the other methods are less reported. In this dissertation, a series of pure and different ions doped titanoniobate luminescent materials with micro-nanometer particles were fabricated by a sol-gel combustion method followed by a heat treatment process. The crystal structure and microstructure of samples were characterized, and their photoluminescent properties were investigated systemically, as well as some related luminescence mechanisms were proposed.The main contents of this dissertation are summarized as follows:In Chapter 1, we briefly introduced the basic theories of luminescent materials, including the definition and classification of luminescent materials, the luminescence theories and properties of rare earth luminescent nanomaterials, and the preparation methods and current investigation of rare earth luminescent materials.In Chapter 2, the pure and different RE3+ (RE=Eu, Dy, Tb, Ho, Ce, Er) doped aeschynite-type LaNbTiO6 luminescent micro-nanomaterials have been successfully prepared by a sol-gel combustion method and post-annealing process for the first time. We studied the photoluminescence properties and mechanisms of all the LaNbTiO6-based phosphors systemically. It is found that LaNbTiO6:Eu3+ phosphors have potential applications on the luminescence and display devices. LaNbTiO6:Ce3+ phosphors are a new class of full-color luminescent materials. There is a hope of white-light emitting from changing the doping content of Ce3+ in LaNbTiO6:Ce3+ samples. XRD results show that the pure aeschynite-type LaNbTiO6 samples can be obtained when the annealing temperature is higher than 1000℃. UV-DRS results reveals the direct band gap of LaNbTiO6 with large grains (>200nm) is calculated to be 3.27eV, while the optical absorption edge of the small particles shows an obvious blue-shift. The luminescence of LaNbTiO6 samples show a strong blue emission band centered at 440nm, which can be ascribed to the Nb5+ (Ti4+)→O2- metal-to-ligand charge-transfer (MLCT) transitions in the Nb(Ti)O6 groups. All the diffraction peaks of LaNbTiO6:RE3+ (RE=Eu, Dy, Tb, Ho, Ce, Er) can be assigned to the standard data of orthorhombic LaNbTiO6 powder. LaNbTiO6:RE3+ samples have different optical absorption edges due to the effect of various doping ions. LaNbTiO6:RE3+ samples contain the excitation peaks ascribed to the characteristic absorption of f-d or f-f transitions. Under the corresponding ultraviolet (UV) or near ultraviolet (near-UV) light excitation, LaNbTiO6:RE3+ phosphors exhibit some emission peaks with different colors in the visible light region due to the characteristic d-f or f-f transitions of RE3+. We can find that the post-annealing temperature and doping concentration have no influence on the position and shape of the emission peaks. RE3+ substitutions for La3+ component can occupy the low symmetric lattice location in the host, so the symmetry-sensitive electric dipole transitions including the 5D0→7F2 transition of Eu3+ (614nm) and the 4F9/2→6H13/2 transition of Dy3+ (571nm) have more intense emission. The 5D4→7F5 transition of Tb3+ (545nm), 5F4+5S2→5I8 transition of Ho3+ (545nm), and 4S3/2→4I15/2 transition of Er3+ (548nm) dominate the luminescence and produce strong green emission in the LaNbTiO6 host, respectively. The luminescence of LaNbTiO6:Ce3+ sample is composed of two parts:the intrinsic emission of LaNbTiO6 and the characteristic d-f transitions of Ce3+, producing three different color light:blue (440nm), green (560nm), and red (610nm). The luminescence of Er3+, Bi3+ co-activated LaNbTiO6 sample indicates that Bi3+ plays an important role of sensitizer of luminescence and transfers the absorbed energy to the surrounding Er3+, improving the luminescence of Er3+. When the concentration of Bi3+ is up to 2mol%, the luminescence of Er3+ is increased in 5.3 times.In Chapter 3, the euxenite-type YNbTiO6 micro-nanomaterials have been prepared by a sol-gel combustion method and post-annealing process. We studied the photoluminescence processes and mechanisms of Eu3+, Li+ co-doped YNbTiO6 sample and Mn2+, Er3+ or Dy3+ co-doped YNbTiO6 samples systematically. The researches show that the luminescent intensities of YNbTiO6:0.13Eu3+,0.03Li+ phosphor is over 4 times than that of the Y2O3:Eu3+ commercial red phosphor which is widely used at present. Compared with Er3+ or Dy3+ doped YNbTiO6 sample, the emission peaks intensities of Mn2+, Er3+ or Dy3+ co-doped YNbTiO6 phosphors have been enhanced obviously. These results indicate that both Li+ and Mn2+ are good sensitizers of the euxenite-type YNbTiO6 luminescent materials. It can be seen from the XRD results that the annealing temperature at 900℃is sufficient for the formation of pure orthorhombic YNbTiO6 nanoparticles. Such a low temperature is much lower than 1250℃that is the synthetic temperature by the traditional solid state sintering method. SEM results show that the annealing temperature takes an effect for particle size and morphology, for instance, when the temperature raises from 900℃up to 1200℃, the average particle size of samples varies from 30-40nm to 400-450nm. The excitation spectrum of YNbTiO6 sample is a broad excitation band during 200-330nm. YNbTiO6 sample has a broad emission band centered at 509nm which can be assigned to the self-trapped excitons (STEs) transition of the host under 270nm excitation. The luminescence of YNbTiO6 sample is strongly influenced by the annealing temperature. It is clear that the luminescence of YNbTiO6 sample prepared at 900℃is 9.5 times of the one obtained at 1200℃. The new YNbTiO6:Eu3+ and YNbTiO6:Eu3+, Li+ red emitting phosphors have been prepared for the first time. The introduction of Li+ into YNbTiO6:Eu3+ is able to result in significant changes of the crystallinity and particle size, and brings a clear red-shift of optical absorption edge. The emission spectra of YNbTiO6:Eu3+ and YNbTiO6:Eu3+, Li+ phosphors can be ascribed to the 5D0→7F1-4 and 5D1→7F1-2 transitions of Eu3+, including a dominant red emission peak at 611nm due to the 5D0→7F2 of Eu3+. Compared with the commercial Y2O3:Eu3+ phosphor, both YNbTiO6:Eu3+ and YNbTiO6:Eu3+, Li+ phosphors have more intense luminescence. In particular, the emission intensity of the optimal Li+-doped YNbTiO6:Eu3+ sample was examined to be over 400% of commercial Y2O3:Eu3+ phosphor. The introduction of Li+ can lead to a more reduced symmetry around Eu3+ and create some oxygen vacancies somewhere in the lattice, which act as the sensitizers for the energy transfers to increase the transition probabilities of Eu3+. The emission spectrum of YNbTiO6:Mn2+ sample contains the intrinsic emission of YNbTiO6 and the characteristic transitions of Mn2+. The 4S3/2→4I15/2 transition of Er3+ and 4F9/2→6H13/2 transition of Dy3+ dominate the luminescence of YNbTiO6:Er3+ and YNbTiO6:Dy3+ phosphors, respectively. There are effective energy transfer processes from Mn2+→Er3+ and Mn2→Dy3+ in the Mn2+ doped YNbTiO6:Er3+ and Mn2+ doped YNbTiO6:Dy3+ phosphors, increasing the luminescence of Er3+ and Dy3+, respectively.In Chapter 4, the pure Aurivillius structure Bi3NbTiO9 nanocrystals and RE3+ activated Bi3NbTiO9 luminescent nanomaterials have been successfully prepared by a sol-gel combustion method at lower temperature. It is found that the luminescent intensities of Gd3+, Eu3+ co-doped Bi3NbTiO9 phosphors are evidently higher than that of Eu3+ doped Bi3NbTiO9 samples. It can be confirmed that the Bi3NbTiO9:0.05Eu3+, 0.02Gd3+ phosphor is a novel orange-red emitting luminescent material with high brightness. XRD results indicate that the pure Bi3NbTiO9 nanoparticles can be formed at 550℃in our case. The synthetic temperature is much lower than that reported by the solid state reaction method (850℃). The photoluminescence properties of Bi3NbTiO9 nanoparticles show that a broad blue emission centered at 442nm can be ascribed to the MLCT transitions in the Nb(Ti)O6 groups, while the emission peaks centered at 526 and 630nm can be ascribed to the defects luminescence. Bi3NbTiO9:Eu3+ and Bi3NbTiO9:Eu3+, Gd3+ red phosphors have been synthesized in our experiments. The characteristic 5D0→7F0-4 transitions of Eu3+ were observed for both of them. Among these multiband emissions, the 5D0→7F2 transition band centered at 616nm is predominant. The investigation illuminates that the Eu3+ ions substitute for Bi3+ ions at A site in the pseudo-perovskite layers of host. Gd3+ doped Bi3NbTiO9:Eu3+ phosphor has an obvious increased luminescence, for instance, the luminescence of 0.02mol% Gd3+ doped Bi3NbTiO9:0.05Eu3+ sample is 3.5 times of the sample without Gd3+In Chapter 5, a concise summary of the contents was given. |
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