Enhanced Eu³⁺ Emission in Aqueous Phosphotungstate Colloidal Systems: Stabilization of Polyoxometalate Nanostructures

Abstract

Luminescent Eu(3+)-containing polyphosphate-tungstate aqueous colloidal systems were prepared and studied as a function of the relative polyphosphate-tungstate content. In polyphosphate-rich solutions, Eu(3+) ions occupy cagelike sites composed of phosphate groups from the metaphosphate chains. In these sites, an average number of 0.5 water molecule coordinates to an Eu(3+) ion and the (5)D(0) emission quantum efficiency is 0.22. Tungstate addition leads to important modifications in neighboring Eu(3+) leading to coordination sites in the aqueous medium where metal ions are completely hidden from interactions with solvent molecules. Transmission electron microscopy results clearly show W-rich nanoparticles with sizes between 5 and 10 nm for all tungstate relative concentrations. For high tungstate relative contents (above 30 mol %), spectroscopic results suggest the presence of Eu(3+) in polyoxometalate (POM)-like sites by comparison with the well-known decatungstoeuropate [EuW(10)O(36)](9-) structure. These new aqueous colloids display surprisingly high (5)D(0) emission quantum efficiencies of ca 80% because of the strong ligand field provided by tungstate POM ligands and the complete absence of water molecules from the Eu(3+) first coordination shell.

Self-assembled three-dimensional NaY(WO4)2:Ln3+ architectures: Hydrothermal synthesis, growth mechanism and luminescence properties

“…6A, right). The blue broad band with a maximum at about 455 nm should be caused by the O-W charge transfer, and the other sharp lines centered at 581, 595 and 615 nm are the characteristic transitions of Eu 3+ , which can be ascribed to the 5 D 0 → 7 F J (J = 0, 1, 2) transitions of the Eu 3+ ions, respectively [43][44][45][46]. On the basis of the above results, it is clear that the blue emission of WO 4 2− groups and red emission of the doped Eu 3+ occur simultaneously and yield a white light emission.…”

Recent advances in lanthanide spectroscopy in Brazil

“…Ribeiro et al [53] have reported the synthesis of europium sodium-phosphate-tungstate colloidal systems in which the Eu 3 þ ions occupy cagelike sites composed of phosphate groups from the metaphosphate chains. The tungstate addition has led to the formation of W-rich nanoparticles (5-10 nm in diameter) and removal of water molecules from the first coordination sphere of Eu 3 þ .…”

Lanthanide Polyoxometalate Based Water-Jet Film with Reversible Luminescent Switching for Rewritable Security Printing

“…Among the well-known lanthanide polyoxometalates, the decatungstoeuropate anion ([EuW 10 O 39 ] 9– ) exhibits outstanding luminescence performance. In its structure, the Eu 3+ ions are coordinated by two [W 5 O 18 ] 6– ligands, as a result of the efficient energy transfer from O → W ligand-to-metal charge transfer (LMCT) states to the Eu 3+ ions’ excited states. − However, the poor processing capacity of polyoxometalates usually limits their practical applications. One of the most effective methods is the introduction of suitable hosts to improve the processing capacity of polyoxometalates, such as polymers, zeolites, and ionic liquids. − Among the numerous hosts, the polymers with a high film-forming ability have captured great attentions, and the resulting film with flexible property has a potential application as a paper medium to replace conventional paper.…”

“…Mechanism Analysis. It is known that Eu-containing polyoxometalates exhibit strong red emission due to the intramolecular energy transfer,[31][32][33]51 which includes two steps. First, photoexcitation into the O → W LMCT would cause the hopping of the d 1 electron and is accompanied by the released energy via the deactivated recombination between d 1 electrons and holes.…”