Study On Preparation And Characterization Of Niobate Luminescent Nanomaterials

Nanoscaled luminescent materials have been the hot research because of their peculiar characteristics compared to bulk counterparts. Niobates have excellent thermal and chemical stability. They are an important kind of photoactived host materials. At present, solid state reaction method has been used to prepare niobates, while wet chemistry method is less reported, In this dissertation, niobate luminescent nanomaterials prepared by citrate sol-gel combustion method have been reported, and their photoluminescent properties were investigated systemically, some new luminescent phenomena were detected. In addition, mesoporous Nb-2O₅ nanoparticles and Nb₂O₅ nanorods were prepared and characterized.In Chapter 1, we briefly introduced the theory of luminescence, the luminescent mechanism, preparation methods, characterization mains and properties of lanthanide luminescent materials. The research progress in the field of niobate luminescenct materials was also summarized.In Chapter 2, we studied the preparation and luminescent properties of undoped and Ln³⁺ (Ln = Er, Sm, Tb and Tm)-doped YNbO₄ nanocrystals, and proposed the energy transfer mechanism between host and the Ln³⁺ ions. All the YNbO₄ samples crystallized in both monoclinic and tetragonal phase at the investigated temperature range of 950-1050℃, and the monoclinic phase dominated in the mixed phases. The excitation band of YNbO₄ shows a red-shift with the decrease of the tetragonal phase. The emission spectrum of each YNbO₄ sample contains a broad band with a maximum at 400 nm. The luminescence of YNbO₄ is ascribed to the metal-to-ligand charge-transfer transition (MLCT) in NbO₄ groups. The influence of the heat treatment conditions on the shape and position of the emission band is imperceptible. Lanthanide ions doped in YNbO₄ can generate a variety of colors because of their abundant electronic energy levels. The YNbO₄:Ln³⁺ samples also crystallized in both monoclinic and tetrahedral phase, then the mixed crystal fields can increase the emission intensity and the extent of peak splitting of Ln³⁺. The excitation spectrum of YNbO₄:Ln³⁺ contains two parts: the strong band is caused by the O^2-→Nb⁵⁺ charge-transfer transition, and some weak peaks at longer wavelength are corresponding to f-f transitions within Ln³⁺. The appearance of the niobate band in the excitation spectrum shows that energy transfer possibly occurs from niobate groups to Ln³⁺. Excitation into the niobate groups at 245 ran yields the characteristic transitions of the Ln³⁺ ions. The Ln³⁺ emissions in YNbO₄ crystal lattice cover blue, green and orange-red color due to their characteristic f-f transitions. The luminescent quenching of the YNbO₄ host further confirms the energy transfer from NbO₄ to Ln³⁺. Obvious splitting of the Ln³⁺ emission peaks was observed clearly, for example, the main emission peaks of Er³⁺ at 519, 524 and 533 nm are associated with the ²H_11/2→⁴I_15/2 transition, and the other two strong ones at 542 and 554 nm with the ⁴S_3/2→⁴I_15/2 transition. The strong splitting observed in each transition is caused by the strong crystal fields of both monoclinic and tetragonal phase of YNbO₄. The emission intensity of Ln³⁺ is strongly affected by doping concentration. The optimum concentrations for Er³⁺, Sm³⁺, Tb³⁺ and Tm³⁺ are 1, 2, 5 and 2 mol %, respectively. The mechanism of the energy transfer between host and the Ln³⁺ ions was given by analyzing the luminescent relationship between them. It is that the energy transfer occurs from NbO₄ groups to every intra-4f transition state of the doped Ln³⁺ ions, which is higher than the MLCT state of YNbO₄; after the nonradiative transition between the intra-4f transition states to populate the emitting levels, the Ln³⁺ ions emit their characteristic luminescence though the f-f transitions.In Chapter 3, we studied the preparation and luminescent properties of ANb₂O₆ (A = Zn, Ni and Sr) nanocrystals. Low temperature citrate sol-gel combustion method has been used to prepare the ANb₂O₆ samples, and the combustion method has been confirmed to be an effcient method of preparing niobate nanocrystals.The XRD results show that both ZnNb₂O₆ and SrNb₂O₆ crystallized in orthorhombic phase, whereas NiNb₂O₆ crystallized in tetragonal phase. The main emission bands of ZnNb₂O₆,NiNb₂O₆ and SrNb₂O₆ ascribed to MLCT of distorted NbO₆ groups center at 445, 442 and 442 nm, respectively. The calcination temperature has a visible effect on the emission intensity of ANb₂O₆, for example, with increasing the calcination temperature, the emission intensity of NiNb₂O₆ decreased while that of SrNb₂O₆ increased. Dy³⁺ and Eu³⁺ ions were introduced into the ZnNb₂O₆ crystal lattice. The emissions of ZnNb₂O₆:Dy³⁺ from both NbO₆ groups and the ⁴F_9/2→⁶H_15/2,13/2 transitions of the Dy³⁺ ions were detected, however, the NbO₆ luminescence enhanced with the Dy³⁺ luminescence decreasing. The emissions of ZnNb₂O₆:Eu³⁺ from both NbO₆ groups and the ⁵D₀→ 7F_1,2,3,4transitions of the Eu³⁺ ions were detected, however, the Eu³⁺ luminescence enhanced with the NbO₆ luminescence decreasing. This indicates that there is energy transfer between Dy³⁺ or Eu³⁺ and the NbO₆ groups, but the transfer manners are different for Dy³⁺ and Eu³⁺. In the ZnNb₂O₆ crystal lattice, the electric dipole transition of Dy³⁺ (⁴F_9/2→⁶H_13/2) or Eu³⁺ (⁵D₀→⁷F₂) dominates, indicating that the Dy³⁺ or Eu³⁺ ions occupy the sites with low symmetry. Dy³⁺ ions were introduced into the NiNb₂O₆ crystal lattice. No characteristic luminescence of Dy³⁺ can be observed, but the blue emission intensity of the NbO₆ groups increases with proper Dy-doping concentrations. The emission intensity of NiNb₂O₆:0.04Dy³⁺ is nearly 3.25 times stronger than that of the pure NiNb₂O₆. Mn²⁺ ions were incorporated into the SrNb₂O₆ crystal lattice; most of the Mn²⁺ ions may occupy the Nb⁵⁺ positions. However, no emission from Mn²⁺ was observed, while the blue emission intensity of the NbO₆ groups varied with the Mn-doping concentrations. The NbO₆ emission intensity of SrNb₂O₆:0.015Mn²⁺ is the highest in all of the Mn-doped samples and is about 2.2 times higher than that of the undoped SrNb₂O₆.In chapter 4, mesoporous Nb₂O₅ nanoparticles have been successfully prepared without using any templatings or surfactants. The porous structure was confirmed by the low-angle peak at around 20 = 2.9°and the hysteresis loop at P/Po = 0.3-0.9 in the N₂ adsorption-desorption isotherm. The Brunauer-Emmett-Teller (BET) surface area is 12.09 m²/g, and the average pore size is 3.4 nm. The pore formation is attributed to the loss of water molecules contained in the precursor niobic acid (Nb₂O₅·nH₂O) during the rapid calcination process. It was proposed that the rate difference between niobic acid decomposition and Nb₂O₅ crystal growth determines the formation of pores or not. Spherical and flake-like Nb₂O₅ samples were obtained by varying the experimental parameters and were characterized. Possible explanations for the formation of the Nb₂O₅ nanocrystals with different morphologies were discussed. In addition, Nb₂O₅ nanorods have been prepared using water/ethanol media, Nb₂O₅ nanorods with smaller aspect ratios can be obtained with increasing the water/ethanol ratio. The nanorods prefer to grow along the (001) crystal face. The optical bandgap Eg is calculated to be about 3.15 eV. From the photoluminescence spectrum, two emission bands at 407 and 496 nm, respectively, were observed. The origin of the luminescence was discussed. It was thought that the 407 nm luminescence is due to the near bandgap exciton transition, and the 496 nm luminescence is ascribed to the self-trapped exciton transition in the NbO₆ groups locating at the surface of Nb₂O₅.In chapter 5, a concise summary of the contents was given.