Elsevier

Optical Materials

Volume 27, Issue 2, November 2004, Pages 249-255
Optical Materials

Synthesis and luminescence properties of ZrO2 and ZrO2/SiO2 composites incorporating Eu(III)–phenanthroline complex prepared by a catalyst-free sol–gel process

https://doi.org/10.1016/j.optmat.2004.05.001Get rights and content

Abstract

Class I organic–inorganic hybrid materials consisting of zirconia and zirconia/silica matrices doped with europium(III)–1,10-phenanthroline complex were prepared by a catalyst-free sol–gel route. Two different experimental procedures were developed to incorporate the coordination compound into the matrix in order to study the possible influence of the preparation method on the optical properties of the rare earth ion. Besides, the effects of the annealing temperature and the composition of the matrix were investigated. Infrared spectra, Eu(III) luminescence lifetimes, relative emission intensities, excitation and emission spectra are reported. The products yielded under UV excitation strong red emission similar to the one of the neat coordination compound. The relative emission intensities and lifetimes show a maxima when determined as a function of annealing temperature, preparation procedure and matrix composition.

Introduction

Rare earth (RE) ion based compounds have been extensively studied due to their good luminescence properties deriving from electronic transitions among 4f levels. These transitions are peculiar because in the majority of cases they give rise to atomic-like spectral lines. The emission spectrum of the Eu(III) ion consists of five narrow bands corresponding to transitions from the excited state 5D0 to the 7Fj (j=0, 1, 2, 3, 4) levels of the 4f6 configuration. Among them the hypersensitive 5D0  7F2 one, located at about 611 nm, is the characteristic europium red emission largely used in cathode-ray tube [1].

Rare earth ions form with a great variety of heterocyclic ligands stable complexes which exhibit excellent luminescence properties attributed to the “antenna effect” [2], [3]. This term, coined by J.-M. Lehn, denotes an efficient energy transfer from the triplet state of an absorbing coordinated organic molecule (the sensitiser) to a chelated RE ion (the activator). In such a way the so-called light converting molecular devices (LCMD's) can overcome the very small absorption cross-section within the UV-visible spectral region of the RE ion and allow a good conversion of the excitation energy into its characteristic emission [1], [4], [5], [6]. Much effort has been done not only to prepare different classes of organic ligands in order to improve the quantum yield of the device, but also to find suitable host matrices for the organometallic compounds to increase light output and to improve thermal-, photo- and mechanical stabilities necessary for their potential applications as lasers, phosphors and so on [7], [8], [9], [10].

The sol–gel route, a mild condition process mainly based on controlled hydrolysis and polycondensation of metal alkoxides, has proved to be the suitable approach for the introduction of organic molecules into an inorganic material. Recently, some lanthanide complexes such as [Eu(phen)2]Cl3 or [Tb(bpy)2]Cl3, which are susceptible to high temperature degradation, have been incorporated into organically modified silicate (ORMOSIL) [11], organically modified ceramic (ORMOCER) [8] or SiO2-MxOy (M=Zr and Ta) [12] matrices via the sol–gel process. Moreover, Reisfield et al. have shown that zirconia, as compared with silica, proves to be particularly interesting as a host material for phosphors containing Eu(III), Tb(III) and Sm(III) thanks to its maximum phonon energy lower than silica [13]. In fact, a lower phonon cut-off depress the multiphonon relaxation processes of the RE ion levels and therefore enhances the intensity of the emitted light. Moreover, for the same reason the amount of water in the composite prepared by the sol–gel method should be kept as low as possible, due to the strong coupling of the OH- oscillators with the energy levels of the rare earth ion.

In this paper organic–inorganic hybrid composites were prepared by a catalyst-free sol–gel route; zirconia and zirconia-silica sols were doped in two different ways: the “one-step procedure” consists of directly doping the sol with the Eu(phen)2(NO3)3 complex, while in the “two-step procedure” ion and ligand are separately added.

For the reasons mentioned above, we found it interesting to study the structure and the luminescence properties of some Eu(III) doped-hybrid composites as a function of (i) composition of the matrix (ZrO2 or ZrO2/SiO2), (ii) preparation method (one- or two-step procedure) and (iii) annealing temperature.

Section snippets

Experimental section

Zirconium n-propoxide (ZP, ∼70% solution in 1-propanol, Fluka), tetraethoxysilan (TEOS, 98% GC, Aldrich), 1,10-phenanthroline (phen, 99%, Aldrich), and Eu(NO3)3 · 5H2O (99.9%, Aldrich) were used as received, chloroform stabilised with 1% ethanol (analytical grade, RDH), ethanol (analytical grade, BDH) and N,N-dimethylformamide (DMF, ACS, Fluka) were used for the preparation of the solutions. Eu(phen)2(NO3)3 was conventionally prepared [14] by addition of the appropriate amount of phen ligand to

Results and discussion

As for the thermal analysis, the TG and DSC curves of the composites (not shown) exhibit an exothermic peak accompanied by loss of mass at about 350 °C, ascribed to the oxidation of the organic ligand, and an exothermic peak without loss of mass at about 500 °C, ascribed to the crystallisation of the matrix. The maximum annealing temperature used for the prepared sol–gel samples was 250 °C, thus far from both the temperatures of oxidation of the organic ligand and crystallisation of the

Conclusion

ZrO2 and ZrO2/SiO2 composites doped with Eu(phen)23+ salt have been prepared through a modified sol–gel procedure characterised by the absence of addition of both catalyst and water generally used to promote the hydration and policondensation reactions of gels. The luminescence spectra of the materials under investigation appear to be typical of a disordered system. An intense red light emission of the composite materials is observed under UV light irradiation, indicating an efficient energy

Acknowledgements

The authors thank Erica Viviani (Universita' di Verona) for the expert technical assistance.

The financial support from MURST (Project COFIN-2001) and MIUR is gratefully acknowledged.

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