ABSTRACT
Luminescent, heterometallic terbium(III)-lutetium(III) terephthalate metal-organic frameworks (MOFs) were synthesized via direct reaction between aqueous solutions of disodium terephthalate and nitrates of corresponding lanthanides by using two methods: synthesis from diluted and concentrated solutions. For (TbxLu1-x)2bdc3·nH2O MOFs (bdc = 1,4-benzenedicarboxylate) containing more than 30 at. % of Tb3+, only one crystalline phase was formed: Ln2bdc3·4H2O. At lower Tb3+ concentrations, MOFs crystallized as the mixture of Ln2bdc3·4H2O and Ln2bdc3·10H2O (diluted solutions) or Ln2bdc3 (concentrated solutions). All synthesized samples that contained Tb3+ ions demonstrated bright green luminescence upon excitation into the 1ππ* excited state of terephthalate ions. The photoluminescence quantum yields (PLQY) of the compounds corresponding to the Ln2bdc3 crystalline phase were significantly larger than for Ln2bdc3·4H2O and Ln2bdc3·10H2O phases due to absence of quenching from water molecules possessing high-energy O-H vibrational modes. One of the synthesized materials, namely, (Tb0.1Lu0.9)2bdc3·1.4H2O, had one of the highest PLQY among Tb-based MOFs, 95%.
ABSTRACT
The series of luminescent NaYF4:Sm3+ nano- and microcrystalline materials co-doped by La3+, Gd3+, and Lu3+ ions were synthesized by hydrothermal method using rare earth chlorides as the precursors and citric acid as a stabilizing agent. The phase composition of synthesized compounds was studied by PXRD. All synthesized materials except ones with high La3+ content (where LaF3 is formed) have a ß-NaYF4 crystalline phase. SEM images demonstrate that all particles have shape of hexagonal prisms. The type and content of doping REE significantly effect on the particle size. Upon 400 nm excitation, phosphors exhibit distinct emission peaks in visible part of the spectrum attributed to 4G5/2â6HJ transitions (J = 5/2-11/2) of Sm3+ ion. Increasing the samarium (III) content results in concentration quenching by dipole-dipole interactions, the optimum Sm3+concentration is found to be of 2%. Co-doping by non-luminescent La3+, Gd3+ and Lu3+ ions leads to an increase in emission intensity. This effect was explained from the Sm3+ local symmetry point of view.
ABSTRACT
Two series of ß-NaYF4:Ln3+ nanoparticles (Ln = La-Nd, Sm-Lu) containing 20 at. % and 40 at. % of Ln3+ with well-defined morphology and size were synthesized via a facile citric-acid-assisted hydrothermal method using rare-earth chlorides as the precursors. The materials were composed from the particles that have a shape of uniform hexagonal prisms with an approximate size of 80-1100 nm. The mean diameter of NaYF4:Ln3+ crystals non-monotonically depended on the lanthanide atomic number and the minimum size was observed for Gd3+-doped materials. At the same time, the unit cell parameters decreased from La to Lu according to XRD data analysis. The diameter-to-length ratio increased from La to Lu in both studied series. The effect of the doping lanthanide(III) ion nature on particle size and shape was explained in terms of crystal growth dynamics. This study reports the correlation between the nanoparticle morphologies and the type and content of doping lanthanide ions. The obtained results shed light on the understanding of intrinsic factors' effect on structural features of the nanocrystalline materials.
ABSTRACT
A new series of luminescent heterometallic europium(III)-lutetium(III) terephthalate metal-organic frameworks, namely (EuxLu1-x)2bdc3·nH2O, was synthesized using a direct reaction in a water solution. At the Eu3+ concentration of 1-40 at %, the MOFs were formed as a binary mixture of the (EuxLu1-x)2bdc3 and (EuxLu1-x)2bdc3·4H2O crystalline phases, where the Ln2bdc3·4H2O crystalline phase was enriched by europium(III) ions. At an Eu3+ concentration of more than 40 at %, only one crystalline phase was formed: (EuxLu1-x)2bdc3·4H2O. All MOFs containing Eu3+ exhibited sensitization of bright Eu3+-centered luminescence upon the 280 nm excitation into a 1ππ* excited state of the terephthalate ion. The fine structure of the emission spectra of Eu3+ 5D0-7FJ (J = 0-4) significantly depended on the Eu3+ concentration. The luminescence quantum yield of Eu3+ was significantly larger for Eu-Lu terephthalates containing a low concentration of Eu3+ due to the absence of Eu-Eu energy migration and the presence of the Ln2bdc3 crystalline phase with a significantly smaller nonradiative decay rate compared to the Ln2bdc3·4H2O.
ABSTRACT
The luminescent coarse-, micro- and nanocrystalline europium(III) terephthalate tetrahydrate (Eu2bdc3·4H2O) metal-organic frameworks were synthesized by the ultrasound-assisted wet-chemical method. Electron micrographs show that the europium(III) terephthalate microparticles are 7 µm long leaf-like plates. According to the dynamic light scattering technique, the average size of the Eu2bdc3·4H2O nanoparticles is equal to about 8 ± 2 nm. Thereby, the reported Eu2bdc3·4H2O nanoparticles are the smallest nanosized rare-earth-based MOF crystals, to the best of our knowledge. The synthesized materials demonstrate red emission due to the 5D0-7FJ transitions of Eu3+ upon 250 nm excitation into 1ππ* state of the terephthalate ion. Size reduction results in broadened emission bands, an increase in the non-radiative rate constants and a decrease in both the quantum efficiency of the 5D0 level and Eu3+ and the luminescence quantum yields. Cu2+, Cr3+, and Fe3+ ions efficiently and selectively quench the luminescence of nanocrystalline europium(III) terephthalate, which makes it a prospective material for luminescent probes to monitor these ions in waste and drinking water.
