Studies On Preparation And Properties Of Nanosized Vanadate Luminescence Materials And Bismuth Titanate Series Photo-catalytic Thin Films

This dissertation includes two parts. The first part involves the studies on preparation, characterization and photoluminescence performance of Vanadate Nanomaterials. This is the main content of this dissertation. The second part is focused on the studies on preparation, characterization and photo-catalytic performance of bismuth titanate series thin films.Luminescence materials have found wide applications in many fields, such as display, illumination and photo-electronic devices. With the rapid development of nanotechnology, nanoscaled luminescence materials have drawn much attention and been the hot topic because these materials have many miraculous properties compared with the bulk materials. Vanadates have excellent thermal and chemical stability. Their luminescence properties, however, have not drawn enough attention. In order to explore novel luminescence systems with high efficiency, in this thesis, we synthesized vanadate nanomaterials via different methods and studied their luminescence properties systemically.In Chapter 1, we briefly illustrated the corresponding luminescence theory of solids, the preparation methods and the unique properties of nanomaterials and nanosized luminescence materials. The research progress in the field of nanosized vanadate luminescence materials was also summarized.In Chapter 2, we studied the synthesis and characterization of nanosized rare earth vanadate (YVO₄ and LaVO₄) luminescence materials. (1)YVO₄: Eu³⁺ nanocrystals with different sizes have been synthesized via a citric acid sol-gel combustion method using citric acid as a fuel/reductant and nitrates as oxidants. Their PL properties have been also systematically studied. The influence of processing conditions, such as the calcination temperature, the citric acid/nitrates molar ratio and the amounts of lithium nitrate (LiNO₃) additive, on the powder characteristics and fluorescence properties of the resultant yttrium orthvanadate doped with 5mol% Eu³⁺ have been investigated. From the results, we can conclude: The emission intensity of Eu³⁺ in YVO₄: Eu³⁺ increases with the increasing of the calcination temperature (T), which shows that high calcination temperature is favorable for high luminescence intensity of the samples within nanometer scale. The citric acid/nitrates ratio (9) is a predominant factor for the growth of YVO₄: Eu³⁺ nanocrystals with different particles sizes, which affects their PL emission intensity greatly. The optimumφis 5/2 for the highest luminescence intensity. The luminescence intensity of this compound is enhanced remarkably by the incorporation of Li⁺ ions. The luminescence intensity of YVO₄: Eu³⁺ with addition of 5mol% Li⁺ ions increases as much as 11 times compared with the Li⁺ ions-free YVO₄: Eu³⁺ sample. (2)YVO₄: Er³⁺ and YVO₄: Er³⁺, Mn²⁺ nanocrystals have been synthesized by precipitation means and the luminescence properties have been investigated systematically. The following conclusions can be obtained from our study: The emission intensity of Er³⁺in the YVO₄ host at 553 nm can reach the optimum value when the concentration of Er³⁺is 0.3 mol%, which is a result of concentration quenching. The crystallinity of YVO₄ particles improves gradually with increasing the annealing temperature, which leads to the increasing of the PL intensity of YVO₄: Er³⁺o.oo3 samples. As the energy transfer from Mn²⁺ to Er³⁺ exists, the PL intensity of YVO₄: Er³⁺_0.003, Mn²⁺_0.001 is about 1.5 times of that of YVO₄:Er³⁺_0.003 (3) LaVO₄:Eu³⁺ nanocrystals with different sizes and morphologies have been synthesized via a precipitation method. The PL properties have also been systematically studied. From the results, we can conclude: The reaction pH is a predominant influencing factor for the growth of LaVO₄: Eu³⁺ nanocrystals with different sizes and morphologies, and their sizes and morphologies affect their PL emission intensities evidently. The emission intensity is highly dependent on the dopant concentration and the optimum concentration of dopant is 3mol%. The emission intensity of Eu³⁺ in LaVO₄ matrix is lowered after co-doped with Dy³⁺ due to the energy transfer from Eu³⁺ to Dy³⁺.In Chapter 3, we studied the combustion synthesis and characterization of nanosized alkaline earth vanadate (Ca₃(VO₄)₂ and Sr₃(VO₄)₂) luminescence materials. Ca₃(VO₄)₂:Eu³⁺, Mn²⁺ powders with different sizes and morphologies have been synthesized via a citric acid sol-gel combustion method using citric acid as a fuel/reductant and nitrates as oxidants. The PL properties have been also systematically studied. The optimized annealing temperature is 750℃in the present case. The emission intensity at 613 nm of Eu³⁺ in the Ca₃(VO₄)₂ host can reach the optimum value when the concentration of Eu³⁺ is 6 mol%, because of the existence of concentration quenching. The emission intensity of Ca₃(VO₄)₂: Eu³⁺, Mn²⁺ decreases owing to the energy transfer from Eu³⁺ to Mn²⁺. Sr₃(VO₄)₂: Eu³⁺ nanocrystals with different sizes and morphologies have also been synthesized via a citric acid combustion method. The effects of processing parameters, such as the molar ratio of citric acid to nitrates (φ), calcination temperature (T) and the amounts of the additive boric acid HBO₃ (B) on the powder characteristics and fluorescence properties of the resultant Sr₃(VO₄)₂ doped with 5% Eu³⁺ have been investigated. From the results, we can conclude: The molar ratio of citric acid to nitrates (φ) is a predominant influencing factor for the growth of Sr₃(VO₄)₂: Eu³⁺ nanocrystals with different particle sizes and emission intensities evidently. The optimumφis 6/3 for the highest luminescence intensity. The emission intensity of Sr₃(VO₄)₂ : Eu³⁺ decreases with the increasing of the calcination temperature (T), owing to the falling of the specific surface areas. The optimum addition amount of boric acid is 5 mol% of the total reagent. The emission intensity of Sr₃(VO₄)₂: Eu³⁺ increases 1.5 times compared with the particles prepared without the additive.In Chapter 4, rod-like or spherical Pb₅(VO₄)₃OH nanocrystals have been successfully prepared through a precipitation process in the presence of sodium dodecylbenzenesulfonate (SDBS) or dodecyldimethylbenzylammonium bromide (DDBAB), respectively. The effect of ion-surfactants in the formation process of nanocrystals may be correlated with the charge and stereochemistry properties of the surfactants. The 601 nm emission of the Pb₅(VO₄)₃OH nanocrystals is ascribed to the ³P₁-¹S₀ transition of Pb²⁺ ions. As one of the novel pollutant treatment method, photocatalysis gains more and more attentions in the field of environmental purification. Developing new photocatalysts and the supported technique of photocatalysts have always been the hot topic of the researchers. In Chapter 5, we chose bismuth titanate (Bi₁₂TiO₂₀ Bi₄Ti₃O₁₂ and Bi₂Ti₂O₇) as the study target, prepared bismuth titanate series thin films by CSD(chemical solution decomposition) method and systematically examined their photocatalytic performance. (1)Sillenite Bi₁₂TiO₂₀ thin films with high photocatalytic activity have been firstly prepared. When annealed at 500℃for 15 minutes, the prepared sample with 4 coatings exhibits the highest photocatalytic activity. After irradiation for 2 h, the methyl orange solution is degraded by 94% on the Bi₁₂TiO₂₀ thin films annealed at 500℃for 15 minutes. Moreover, Bi₁₂TiO₂₀ thin films prepared by CSD method show a good anti-inactivation stability and adhere well on the silicate substrate after they have been used for many times. (2)Bi₄Ti₃O₁₂ and (Bi_1-xLa_x)₄Ti₃O₁₂ with bismuth-layered perovskite structure thin films were successfully prepared and their photocatalytic properties have been firstly investigated. When annealed at 550℃for 15 minutes, the prepared Bi₄Ti₃O₁₂ thin films by 4 coating cycles exhibited the highest photocatalytic activity. La³⁺ doping could improve the photocatalytic activity of Bi₄Ti₃O₁₂ thin films. An optimal La³⁺ dopant amount for (Bi_1-xLa_x)₄Ti₃O₁₂ thin films is 20 at%. After irradiation for 2h, the methyl orange solution was degraded by 83% on the (Bi_0.8La_0.2)₄Ti₃O₁₂ films. Additionally, substituted La³⁺ ions can act as a grain-growth inhibitor in perovskite (Bi_1-xLa_x)₄Ti₃O₁₂ thin films. (3)Dopant-free Bi₂Ti₂O₇(BT) thin films with pyrochlore structure and La-doped bismuth titanate (BLT) thin films were successfully prepared and their photocatalytic properties were firstly investigated. When annealed at 550℃for 30 minutes, the prepared BT thin films by 6 coatings cycles exhibited the highest photocatalytic activity. La³⁺ doping could improve the photocatalytic activity of BT thin films. An optimal La³⁺ dopant amount for BLT thin films is 5 at%. After irradiation for 2 h, the methyl orange solution was degraded by 61% on the BLT films annealed at 550℃for 30 minutes. In addition, substituted La³⁺ ions can act as a stabilizer of perovskite BLT and a grain-growth inhibitor in BLT thin films.In Chapter 6, a concise summary of our work was given.