pubs.acs.org/Langmuir © 2010 American Chemical Society
Enhanced Eu3þ Emission in Aqueous Phosphotungstate Colloidal Systems: Stabilization of Polyoxometalate Nanostructures )
)
A. J. Barbosa,† L. J. Q. Maia,†,‡ B. Montanari,† R. R. Gonc-alves,§ Y. Messaddeq,† R. A. S. Ferreira, L. D. Carlos, and S. J. L. Ribeiro*,† † Institute of Chemistry, S~ ao Paulo State University-UNESP, CP 355, Araraquara-SP 14801-970, Brazil, Institute of Physics, Federal University of Goi as-CP 131, Goi^ ania 74001-970-Go, Brazil, §Departamento de Quı´mica, Faculdade de Filosofia, Ci^ encias e Letras de Ribeir~ ao Preto, USP, Ribeir~ ao Preto-SP 14040-901, Brazil, and Departamento de Fı´sica, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal )
‡
Received February 4, 2010. Revised Manuscript Received July 7, 2010 Luminescent Eu3þ-containing polyphosphate-tungstate aqueous colloidal systems were prepared and studied as a function of the relative polyphosphate-tungstate content. In polyphosphate-rich solutions, Eu3þ 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 Eu3þ ion and the 5D0 emission quantum efficiency is 0.22. Tungstate addition leads to important modifications in neighboring Eu3þ 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 Eu3þ in polyoxometalate (POM)-like sites by comparison with the well-known decatungstoeuropate [EuW10O36]9- structure. These new aqueous colloids display surprisingly high 5D0 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 Eu3þ first coordination shell.
Introduction Sodium polyphosphate (NaPO3), commercially known as Graham’s salt, is the only water-soluble inorganic phosphate obtained in general by NaH2PO4 3 H2O melting and quenching. The obtained glasslike material displays a linear metaphosphatebased structure with the number of tetrahedral [PO4] units ranging from a few units to 300, depending on the melting time and temperature.1 The interesting aspects of the solution chemistry of polyphosphate chains in water have led to many different applications ranging from water softening to the stabilization of mineral suspensions.1-3 In fact, the flexibility offered by the long chain, allowing metal ions to be fitted into the structure, and the coordinating properties of the middle PO3 units make polyphosphates strong complexing agents. Eu3þ-containing polyphosphate colloidal systems have been studied by some of us.4 Eu3þ-containing colloidal systems display two-fold interest either from the point of view of the preparation of new nanostructured luminescent systems or from the structural study point of view because spectroscopic properties are very sensitive to the ligand field occupied by the metal ion. The reason for the observed spectroscopic variations as a function of the matrix chemical nature allows the utilization of Eu3þ ions as structural probes.5 In this sense, a structural model was described for the interaction between metal ions and the polyphosphate chains in water.4 For a relatively high P/Eu molar ratio (P/Eu > 6),
the encapsulation of the metal ions in polyphosphate-based cagelike local sites was proposed. In aqueous solutions and in the absence of strongly coordinating anions, Eu3þ is known to be coordinated to eight to nine water molecules that efficiently quench the emission from the Eu3þ 5D0 excited state. As a consequence, a low value of 0.04 is observed for the 5D0 emission quantum efficiency (η), η = kr/kT = kr/(kr þ knr) where kr, kT, and knr are the radiative, total, and nonradiative decay rates, respectively. In the polyphosphate cagelike sites, Eu3þ ions were observed to be coordinated to an average of 0.5 water molecule in addition to ∼6 oxygen atoms from coordinating phosphate groups. In those sites, the 5D0 quantum efficiency was observed to increase to 0.22.4 When the concentration of Eu3þ increases and the saturation of cagelike sites is achieved (P/Eu
