Preparation And Photoluminescence Of RE-doped Layered Double Hydroxides

Layered double hydroxides have received extensive attention because of their uniquestructure and properties. Attributed to the replaceable metal ions in the layers and theexchangeable interlayer anions, layered double hydroxides have derived from a large classof functional materials which have been widely used in medicine, adsorption, catalysis,electrochemistry and other fields. The resulted rare earth doped layered double hydroxideshave shown a potential application in many fields. However, the rare-earth luminescenceLDHs materials generally have weak luminescence properties in the present study. How toimprove their luminescent properties became the key to solve the practical application.Based on the unique structure and properties of layered double hydroxides materials, andexcellent luminescence properties of rare earth ions, we have used the hydrothermal methodand urea homogeneous co-precipitation method to prepare new types of rare-earthluminescent LDHs materials, we also studied the factors which affect the luminescence ofrare earth ions and explore the ways to enhance their luminescence properties.Use ammonia as the alkali source, we prepared nitrate Eu³⁺-doped Mg-Al LDHsprecursors by hydrothermal method, and then introduced the MoO42-to interlayer region byion-exchange method to prepare a new type of rare earth luminescent LDHs materials.Results reveal that Eu³⁺ions were likely incorporated into the hydrotalcite lattice andMoO42-anions were successfully intercalated into interlayer region of the LDHs with theMo/Al molar ratio close to0.40. The basal spacing increases from8.9to9.3afterion-exchange. Infrared and Raman results show that the molybdenum species mainly existsin the interlayer region in the form of molybdate, a small amount of molybdate formeddimers Mo₂O₇^2-in ion-exchange process. Photoluminescence characterization resultsshowed that the luminescent properties of RE-doped LDHs increased with Eu contentincreasing. But when Eu³⁺/(Eu³⁺+Al³⁺) molar ratio rises to0.2, the resulted luminescentLDHs materials present fluorescence quenching. Under UV excitation, the molybdatetransfer the absorption energy to europium ions in layers, greatly enhancing thecharacteristic luminescence of europium ion.Use ammonia as the alkali source, we prepared nitrate Tb3+-doped Mg-Al LDHsprecursors by hydrothermal method, and introduced the WO42-to interlayer region by ion-exchange method to prepare a new type of rare earth luminescent LDHs materials. XRD,FT-IR, EDS test results show that the terbium ions replaced with the crystal position oftrivalent aluminum ions, while the tungsten radical ions were successfully intercalated intointerlayer region of the LDHs, The basal spacing increases from8.9to10.2afterion-exchange. TEM results show that nitrate terbium-doped Mg-Al LDHs precursors havethe flake shape and good regularity, the Particle size is about100nm. After ion exchange,the samples maintain the flake shape, but the particle size tends to decrease to about30nm.Photoluminescence characterization results show that tungsten anion transfer the absorptionenergy to europium ions in layers, greatly enhancing the characteristic luminescence ofterbium ion under UV excitation.We prepared different terbium content doped LDHs by urea homogeneouscoprecipitation method. Results reveal that the content of terbium ions have a great impacton the morphology of the samples in the synthesis process, with the increasing dopedconcentration of terbium, the hexagonal sheet structure of samples gradually disappearedand layers gradually thinning and particle size decreases. In addition, the presence ofterbium ions, LDHs grain gradually tend to embedded together. Photoluminescencecharacterization results showed that with the increasing doped concentration of terbium, theemission intensity of terbium ion gradually increased. We also studied the affect of thermaltreatment on the structure and properties of the samples. With rise of thermal treatmenttemperature, the terbium-doped LDHs maintained the layered structure when the crystallinewater layer off, and then layered structure decompose to form a composite metal oxide, andgenerate oxide spinel structure ultimately. Luminescence properties of the samples firstdecreased with the rise of thermal treatment temperature and then gradually increased. Thereason for this phenomenon is that luminescence properties have a great relationship withthe degree of crystallinity. There are more crystal defects with the crystallinity decreased,and the crystal defects increased the concentration of non-active center, resulting in thefluorescence quenching and not conducive to the characteristics luminescence of terbiumion.We prepared Li+/Tb3+co-doped LDHs using urea Homogeneous co-precipitationmethod. XRD results show that the samples have high crystallinity. The co-doping of Li+ion change the lattice parameters of the LDHs, while ICP results further confirmed that the co-doping of Li+ion in the synthesis process. SEM and TEM results show that co-doping ofLi+ion did not have a visible impact on the morphology of the samples. In thephotoluminescence performance, the characteristics of terbium ion luminescence emissionintensity markedly improved when co-doping a small amount of lithium ions, but when thedoping concentration of lithium ions continue to increase, the Li+lead to the terbium ionfluorescence quenching.