Europium incorporated into titanium oxide by the sol-gel method

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Europium incorporated into titanium oxide by the sol-gel method

Lucas Alonso Rocha; Lilian Rodrigues Avila; Bruno Leonardo Caetano; Eduardo Ferreira Molina; Hérica Cristina Sacco; Katia Jorge Ciuffi; Paulo Sergio Calefi; Eduardo José Nassar^*

Universidade de Franca, Av. Dr. Armando Salles de Oliveira, 201, C.P. 82, 14404-600 Franca - SP, Brazil

ABSTRACT

In this work titanium sol was prepared from tetraethylorthotitanate (TEOT) in ethanol, stabilized with beta-diketonate 2,4 pentanedione in molar ratio 1:1 homogenized by magnetic stirring, europium ion was add as structural probe. The xerogels were heat treated at 500, 750 and 1000 °C and the characterization was realized by x-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA/DSC) and photoluminescence (PL). The excitation spectra of Eu (III) ion present maximum in 394 nm correspondent to ⁵L₆ level and emission spectra present bands characteristic transitions arising from the ⁵ D₀^{® 7}F_J (J = 0, 1, 2, 3, 4) manifolds to samples treat at 500 and 750 °C. The Eu (III) emission disappear, when heated at 1000 °C, probably due to phase transition anatase to rutile and migrations of ions to the external surface that was proved by x-ray diffraction, transmission electronic microscopy and the thermogravimetric analyses of xerogels.

Keywords: europium (III), xerogels, luminescence, TEM

1. Introduction

Titanium oxide has been investigated widely in the form of bulk, powders, films and membranes^1-3 and can be applied in several fields such as electronics, gas and humidity sensors, photocatalytic properties and others. Nanocrystalline titania was synthesized chemically by the sol-gel route. Titania undergoes a transformation phase from the low-temperature anatase phase to rutile above 450 °C, and has been found to extend up to 1000 °C, depending on the method of preparation and on the presence of suitable dopant oxides¹.

The advantages of sol-gel powders over conventional powders is that their size and shape, molecular scale homogeneity, and enhanced reactivity (lower processing temperatures)⁴ can be controlled.

Excitation of the lanthanide in such schemes occurs with the absorption of light by a coordinating ligand and the subsequent transfer of its electronic energy to the emissive excited state manifold of the lanthanide ion⁵. The electronic energy states of lanthanide 4 f ⁿ configurations are only minimally affected by their surroundings owing to the effective shielding of the 4 f electrons from the external field by the 5 s² 5 p⁶ arrangement. Therefore, these energy states remain practically invariable for a given ion in all its compounds and in different environments. These absorption and emission spectra of lanthanide (III) ions give sharp, spectrally narrow bands⁶.

From the technological viewpoint, a prospective way to synthesize luminescent materials is by the sol-gel method, whose main advantages are its low cost, simplicity, flexibility and absence of induced radiation defects^7-9.

In this work, we studied the synthesis and characterization of xerogels (powder) of titanium doped with europium III and prepared by the conventional sol-gel process. After heat treatment at various temperatures, the samples were studied by transmission electron microscopy (TEM), thermogravimetric analysis (TGA/DTA/DSC), photoluminescence (PL) and x-ray diffraction.

2. Experimental Sections

Titanium sol was prepared from tetraethylorthotitanate (TEOT from Aldrich) in ethanol (EtOH), and the metal alkoxide reaction was controlled by beta-diketone 2,4 pentanedione (acac) in a molar ratio of 1:1. 1.0 mmol of acac was added to 10 mL of EtOH under magnetic stirring. After 5 minutes the 1.0 mmol of TEOT was added to the mixture. The ethanolic europium chloride solution (EuCl₃) was added to the sol on molar percentages of 0.1, 0.2 and 0.3% and the sol was homogenized by magnetic stirring for 30 minutes.

These solutions were dried at room temperature and the resulting xerogels (powders) were heat-treated at 500, 750 and 1000 °C in porcelain crucibles.

Luminescence data were obtained under both continuous (450W) and pulsed (5 J/pulse, 3 µs bandwidth) Xe lamp excitation with a SPEX FLUOROLOG F2121 spectrofluorimeter at room temperature. All the spectra were corrected by spectrometer optics, lamp output and detector response.

X-ray diffractograms (XRD) were obtained using a Siemens (D 5005) x-ray diffractometer and Cu Ka radiation.

A thermogravimetric analysis (TGA/DTA/DSC) was carried out (Instruments SDT Q600 – Simultaneous DSC-DTA-TGA) in air at a heating rate of 20 °C min^-1, from 25 to 1500 °C.

The morphology of the systems was investigated by transmission electron microscopy (TEM) of a drop of power suspension deposited on a copper grid. The TEM analysis was performed with a 200 kV Philips CM 200 microscope.

3. Results and Discussion

The synthesis of TiO₂ networks by the sol-gel process consists of forming an inorganic network through the hydrolysis and polymerization of the required titanium alkoxide in ethanolic solution, followed by gelation to form titanium xerogel doped Eu (III) ions.

Figure 1 depicts the excitation and emission spectra of a sample doped with a molar percentage of 0.3% of europium.

The photoluminescence data of Eu (III) ions displayed a similar behavior. In the excitation spectra of the samples heated at 500 and 700 °C, the maximum, which appeared at 394 nm, was ascribed to the ⁵L₆ level of Eu (III). However, this band was not observed in the sample heated at 1000 °C. The emission spectra presented transitions arising from ⁵D₀ to ⁷F_j (J = 0, 1, 2, 3 and 4) manifolds excited at their maximum¹⁰ in the samples heated at 500 and 700 °C, but the sample heated at 1000 °C showed no Eu (III) luminescence.

The decay curve for the ⁵D₀®⁷F₂ transition of Eu (III) in the TiO₂ matrix consists of a monoexponential function, resulting in a lifetime (ms) of about 0.45 ± 0.01 for the samples heated at 500 °C and of 0.40 ± 0.03 for those heated at 750 °C. This result indicates that Eu (III) occupies only one type of site in these materials. The lifetime was not obtained for the samples heated at 1000 °C.

The excitation energy of Eu (III) can be absorbed by the vibration of ligands, thus decreasing the lifetime¹¹. Because of the electric-di-pole character of Eu (III), the intensities of the ⁵D₀®⁷F₀ and ⁵D₀®⁷F₂ transitions are strongly dependent on the surrounding Eu (III). The corresponding band of ⁵D₀®⁷F₁ transition has a magnetic dipole nature and its intensity is not affected by its surroundings¹². In both emission spectra, the bands corresponding to the ⁵D₀®⁷F₂ transitions showed a higher intensity than the ⁵D₀®⁷F₁ transitions. This difference indicated that the Eu (III) occupies sites without an inversion center^13-15. The presence of ⁵D₀®⁷F₀ transitions indicates that the Eu (III) is located in a site with a C_nv , C_n or C_s symmetry¹⁶ . The presence of nonhomogeneous sites in the TiO₂ structure was observed based on the band-width emission¹⁷.

The luminescence of Eu (III) ions doped in xerogels and heat-treated at different temperatures (500, 750 °C) shows similar luminescence bands. However, from 1000 °C up, the emission of Eu (III) ions disappeared, probably due to a phase transition from anatase to rutile and to migrations of ions to the external surface, as confirmed, respectively, by x-ray diffraction and transmission electronic microscopy. A large quantity of Eu (III) appeared on the surface of titanium oxide heated at 1000 °C, as observed by Energy Dispersive x-ray Analysis (EDX), and quenching occured due to the high concentration of Eu (III). The transfer of emissive center energy to the level of another centre promoted a loss of energy.

X-ray powder diffraction measurements indicated that the differences between percentages of europium were insignificant. Figure 2 illustrates the x-ray diffraction patterns, showing that the material was totally amorphous at room temperature. Crystallization started at 500 °C and, at 750 °C, the sample's appearance was almost totally crystalline. After further increasing the treatment temperature to 1000 °C, the material presented a totally crystalline structure¹.

The x-ray diffraction measurements showed that the material was completely amorphous at room temperature and that the onset of crystallization in the anatase phase occurred at 500 °C, while sample's appearance was almost totally crystalline in the same phase at 750 °C¹⁸. After increasing the treatment temperature to 1000 °C, the material showed a totally crystalline structure in the rutile phase.

The xerogels' thermal stability was investigated by thermogravimetric analysis of all the samples. However, Figure 3 shows only the xerogel at 0.3%, since the results of the materials containing different percentages of europium were similar.

The TGA/DTA/DSC presented a mass loss between 50 and 250 °C, and another between 350 and 450 °C. A structural change occurred from 450 to 750 °C and from 950 to 1100 °C, with DTA revealing two exothermic peaks at 450 - 750 °C and an endothermic peak at 1000 °C.

The thermal stability of the xerogels was investigated by TGA/ DTA/DSC. The DTA curves of the materials displayed up to two exothermic peaks, one at 300 and another at 450 °C. Weight loss was observed in the same region due to pyrolysis of the organic group, dehydroxylation and collapse of the layered structure, and recrystallization of the pyrolysis product into oxides¹⁹.

The TGA in Figure 3 indicates that the first mass loss corresponded to a loss of water and solvent molecules which were only adsorbed in material (50 - 250 °C) while the second mass loss was related to the loss of organic groups attached to the material (300 - 450 °C). Another weight loss occurred between 950 - 1050 °C, corresponding to loss of oxygen molecules due to structural changes.

The TGA showed a gradual mass loss from 450 to 750 °C, relating to the structural transition from amorphous to anatase, which was confirmed by DSC. An endothermic peak relating to the transition of anatase to rutile phase appeared at 1000 °C.

Figure 4 shows a TEM image of TiO₂:Eu (III) heat-treated at different temperatures. Figure 4c depicts a sample heat-treated up to 1000 °C, showing migrations of Eu (III) ions to the material's external surface.

Figure 4a

4. Conclusions

The luminescence of Eu (III) ions in titanium oxide depends on the surroundings and the material presents a new structure, the rutile phase, when the temperature reaches 1000 °C. In this phase, Eu (III) ions cannot occupy an intermediary position in the unit cell, giving rise to migrations to the material's external surface and extinguishing the luminescence.

Our x-ray and thermal analyses confirmed the phase change and the TEM analysis confirmed the migrations of Eu (III) ions to the surface.

We conclude that the preparation of titanium oxide by the sol-gel method is more cost-effective than other process, and that the short time and rapid definition of temperature-dependent phases is very important in these materials' applications.

Acknowledgments

The authors are grateful to FAPESP and CAPES (Brazil) for the financial support of this work.

Received: March 3, 2005; Revised: June 28, 2005

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Publication Dates

• Publication in this collection
  10 Oct 2005
• Date of issue
  Sept 2005

History

• Received
  03 Mar 2005
• Accepted
  28 June 2005