Luminescence and energy transfer of the europium (III) tungstate obtained via the Pechini method

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Abstract

This work presents the preparation of the europium tungstate compound, Eu2(WO4)3, through the use of a polymeric precursor according to the Pechini method. In the preparation of this precursor ammonium tungstate, europium nitrate, edta, citric acid and ethylene glycol were mixed and heated at 700°C for 4 h, leading to a luminescent compound. X-ray diffraction, infrared spectroscopy, thermogravimetric analysis and diffuse reflectance spectroscopy were used to characterize the luminescent material. The photoluminescent properties of this compound were studied based on the f6-intraconfigurational transitions in the spectral range from 250 to 720 nm. When the photoexcitation is monitored into the O→W (260 nm) and O→Eu3+ (310 nm) ligand-to-metal charge-transfer states (LMCT) and 7F05L6 transition of the Eu3+ ion (393 nm) the emission spectra present transitions from the 5D0 emitting level at 77 and 298 K. The emission spectra at room temperature, with excitation at the O→W and O→Eu3+ LMCT states, do not present the emission bands from the 5D3, 5D2 and 5D1 emitting levels. On the other hand, the emission spectrum under excitation at the 7F05L6 transition of the Eu3+ ion shows narrow emission bands arising from the 5DJ levels (J=1, 2 and 3). One of the main points of the present study is that this phenomenon is interpreted on the basis of a resonance crossover between the LMCT states and the 5DJ levels (J=1, 2 and 3). The emission quantum efficiency of the Eu3+ ion in this system is also discussed.

Introduction

The research on the luminescent materials containing trivalent rare earth ions (RE3+) has increased considerably in the last three decades [1], [2], [3]. Major applications are in emissive displays and fluorescent lamps. In addition, some X-ray detector systems are based on luminescent materials as well. Quite a few of these materials also found their way into applications. In many cases, rare earth phosphors noticeably improved the performance of the devices.

The photoluminescent properties of the Eu3+ and Tb3+ ions make them potential candidates for use as luminescent materials [4], [5], [6], [7], [8]. However, the europium ion provides additional facilities in the interpretation of the spectral data as compared to the terbium ion. The Eu3+ ion has a great advantage because it has non-degenerate ground and emitting states and the 5D07F0 transition gives information about the impurity or if this ion occupies more than one site symmetry, particularly of the type Cnv, Cn, or Cs. The intensity of the 5D07F1 transition (allowed by magnetic dipole) is formally insensitive to the crystal field environment and consequently can be used as a reference transition.

Inorganic luminescent materials containing rare earth ions usually present very intense absorption bands in the ultraviolet region consistent with allowed interconfigurational transitions, 4fN→4fN−15d, and with ligand-to-metal charge-transfer states (LMCT), that may mask the forbidden narrow intraconfigurational 4fN→4fN transitions [1], [9]. The LMCT states depend on: (a) the distance between the metal ion and ligands—this transition shifts toward lower energy when the bond length increases, (b) the optical electronegativity (χopt)—the electronegativity of a ligand can alter the position of the LMCT states, becoming very helpful to predict the energy of this transition in different chemical environments and (c) the electroaffinity of the rare earth ion where the LMCT state corresponds to a reduction 4fN→4fN+1L−1, whereas the RE3+ ions gain one electron, for example: the Eu3+ ions (4f6) tend to reduce in order to obtain the half-filled stable shell configuration [9].

McDonald et al. [10] were the first to report the europium tungstate preparation from Eu2O3 at 1000°C and to study its luminescent properties. Borchardt [11] prepared the europium tungstate with similar purposes and Templeton and Zalkin [12] studied the crystal structure of Eu2O3·3WO3. Since then, some different ways were used to synthesize the europium tungstate, but always involving high temperatures and/or a long time of heating. In the last decade, several low temperature preparation techniques have been used to prepare fine particle systems such as co-precipitation [13], sol–gel method [14] and hydrothermal synthesis [15].

In this paper, we have used the Pechini method [16], [17], [18], [19], [20] to prepare the Eu2(WO4)3 compound. This technique known due to the low cost and versatility is a low temperature synthetic method that uses the dissolution of cations in an aqueous citric acid (CA) solution. Ethylene glycol (EG) addition promotes polymerization (esterification). After polymerization the segregation of cations during thermal decomposition is minimal, owing to the formation of high viscosity polyester. Besides the preparation of the Eu2(WO4)3, we also investigated its photoluminescent properties, which have not been reported in the literature. An emission quenching phenomenon observed for the 5D1,2,3 manifolds, under excitation at the LMCT states, is here discussed and interpreted as a resonance crossover between the LMCT states and the 5D1,2,3 levels. The relatively low emission quantum efficiency (19%) for the 5D0 level, as compared to the K5Eu2(WO4)5.5 compound, is interpreted as a partial quenching also due to the LMCT states.

Section snippets

Preparation and measurements

The RE2(WO4)3 compounds (RE3+=Eu and Gd) were prepared by the Pechini method [16], [17], [18], [19], [20]. The starting materials were ammonium tungstate (99.999%, Acros), europium and gadolinium nitrates—synthesized from RE2O3 (99.9%, Aldrich), ethylene glycol (99.5%, Merck) and citric acid (99.5%, Merck). First, the ammonium tungstate was dissolved in heated aqueous solution (∼60°C) adjusting the pH at ∼7.0 with ammonium hydroxide and nitric acid. Second, aqueous solutions of the rare earth

Characterization

The XRD pattern of the powder Eu2(WO4)3 obtained after heating the precursor at 700°C showed characteristic lines of standard compound with a monoclinic (pseudo-orthorhombic) lattice [12], according to the JCPDS card #22–287 (omitted figure). It is noted the absence of the peaks assigned to the europium oxide and WO3 group, which indicates that the Eu2(WO4)3 compound was obtained in a pure form.

Fig. 1 shows the TG/DTG curves of the precursor powder that were obtained after heating the polymeric

Concluding remarks

The Eu2(WO4)3 compound was prepared using the Pechini method that produces a phase-pure at reduced temperature in contrast to the conventional solid-state preparation. The diffuse reflectance data shows the absorption of the O→W LMCT state and the sharp lines corresponding to the Eu3+ ion. The infrared and Raman spectra indicated only one type of WO4 tetrahedron. The 5D07F0 transition showed one peak assigned and a mono-exponential for the decay curve of emitter 5D0 level indicate the presence

Acknowledgements

We thank the Brazilian agencies FAPESP and CNPq (RENAMI) for financial support. We also thank Dr. A.C.V. Coelho and V.F.J. Kozievitch from the Departamento de Engenharia Quı́mica da Escola Politécnica (USP) for XRD pattern recording, Dr. D.L.A. Faria from the Laboratório de Espectroscopia Molecular (IQ-USP) for the Raman spectrum and Dra. D.F. Parra for help with the manuscript and fruitful discussions.

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