title: 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.
authors.affiliation: Institute of Chemistry, São Paulo State University-UNESP, CP 355, Araraquara-SP 14801-970, Brazil.
authors.forename: A J
authors.initials: AJ
authors.lastname: Barbosa
authors.forename: L J Q
authors.initials: LJ
authors.lastname: Maia
authors.forename: B
authors.initials: B
authors.lastname: Montanari
authors.forename: R R
authors.initials: RR
authors.lastname: Gonçalves
authors.forename: Y
authors.initials: Y
authors.lastname: Messaddeq
authors.forename: R A S
authors.initials: RA
authors.lastname: Ferreira
authors.forename: L D
authors.initials: LD
authors.lastname: Carlos
authors.forename: S J L
authors.initials: SJ
authors.lastname: Ribeiro