Structural and luminescence properties of Eu3+ doped BaxSr1−xTiO3 (BST) nanocrystalline powders prepared by different methods

Optical Materials 28 (2006) 1284–1288 www.elsevier.com/locate/optmat

Structural and luminescence properties of Eu doped BaxSr1xTiO3 (BST) nanocrystalline powders prepared by different methods 3+

R. Pa˛zik a, D. Hreniak a, W. Stre˛k a, A. Speghini a

b,*

, M. Bettinelli

b

Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Oko´lna 2, 50-422 Wrocław, Poland b Dipartimento Scientifico e Tecnologico, University of Verona and INSTM, UdR Verona, Ca’ Vignal, Strada Le Grazie 15, 37134 Verona, Italy Received 12 August 2005; received in revised form 26 January 2006; accepted 30 January 2006 Available online 4 April 2006

Abstract Eu3+ doped BaxSr1xTiO3 (BST) nanocrystalline powders have been prepared using molten salts, sol–gel and hydrothermal techniques in order to compare their structural and luminescence properties. The sizes of the particles are around 50 nm for the BST powders obtained by the sol–gel and hydrothermal methods while they are around 400 nm for the samples prepared by the molten salts method. The Eu3+ doped BST samples show strong emission in the visible range. The luminescence spectra of the Eu3+ ions for the investigated samples depend strongly upon the preparation method. The luminescence bands are affected by a notable amount of inhomogeneous broadening. The broadening of the emission bands and the non-exponential behavior of the emission decay curves point to the presence of significant structural disorder. Ó 2006 Elsevier B.V. All rights reserved.

1. Introduction In the past few years, many research activities have been devoted to the study of ferroelectric materials [1,2]. In particular, ferroelectric titanates (such as BaTiO3) and niobates (such as strontium barium niobates, SBN) are interesting for electrical and optical technological applications due to their high dielectric constants and remarkable non-linear optical properties [3–5]. Solid solutions of BaTiO3 and SrTiO3, BaxSr1xTiO3 (BST), have also been recently studied for their applications in microwave devices and gas sensors [6,7]. Much of the research efforts on BST materials have been focused on bulk samples and films [8,9], but only a limited number of papers on nanocrystalline BST has been published [10,11]. Moreover, lanthanide doped ferroelectric titanates such as BaTiO3 or Ba0.5Sr0.5TiO3 have been studied recently for their possible applications in *

Corresponding author. Tel.: +39 045 8027900; fax: +39 045 8027929. E-mail address: [email protected] (A. Speghini).

0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.01.022

self-frequency-doubling devices [12,13]. In this work we report on the structural, morphological and spectroscopic properties of Eu3+ doped BST nanocrystalline or submicrometer powders synthesized with three different methods. 2. Experimental 2.1. Preparation of the samples Samples of Eu3+ doped nanocrystalline BaxSr1xTiO3 (x = 0.90 and 0.85) were prepared by using molten salts (mBST), sol–gel (sBST) and hydrothermal (hBST) techniques. All the BST samples were doped with 1 mol% of europium ions with respect to the alkali-earth metal ions. The BaxSr1xTiO3 samples will be labelled in the following as x Æ 100/(1  x) Æ 100 BST. 2.1.1. Molten salts technique This method was described in details in a previous paper [14]. The mBST samples were prepared using a molten NaCl

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flux. Briefly, appropriate quantities of Ba(NO3)2, Sr(NO3)2, TiO2, Eu2O3 and NaCl powders were mixed thoroughly in a platinum crucible and heated at 950 °C for 24 h. After the heat treatment, the flux was dissolved in distilled water and the BST powders were dried at 240 °C for 24 h.

2.1.3. Hydrothermal technique Differently from the molten salts and sol–gel preparation techniques, the hydrothermal technique does not require high temperature treatments, avoiding agglomeration and sintering of the grains. The starting reagents were barium, strontium and europium acetate, sodium hydroxide and titanium butoxide. Water solution of barium, strontium and europium acetates at appropriate molar ratios were added under constant stirring into a NaOH solution (pH > 10). Simultaneously, titanium butoxide was also added. The coprecipitated metal hydroxides were placed in microwave reactor (ERTEC MV 02-02) in a teflon vessel. After hydrothermal processing at 300 °C for 1 h, the powders were washed with water and dried at 100 °C for 1 h.

(110)

(a)

Normalized intensity (a.u.)

2.1.2. Sol–gel technique The details of the preparation method of nanocrystalline sBST powders were described in a previous paper [15]. In summary, acetic acid solutions of barium and strontium acetate and water solution of Eu(NO3)3 were added dropwise into an titanium butoxide solution in acetylacetone under constant stirring. The obtained transparent yellowish solutions were vigorously stirred at 50 °C for about 2 h. The obtained sols were dried at approximately 100 °C for 24 h to obtain a BST gel. The samples were subsequently annealed at 900 °C for 4 h. The choice of the annealing temperature was based on DTA–TGA results (not shown) from which it results that the weight loss is finished at 900 °C.

1285

(111)

(100)

(200)

(211) (210)

(b)

(c)

SrTiO3 BaTiO3

20

25

30

35

40

45

50

55

60

2θ (deg) Fig. 1. XRD patterns of the 90/10 Eu3+ doped BST samples prepared by (a) molten salt, (b) hydrothermal and (c) sol–gel techniques. Bottom: JCPDS Powder Diffraction File Cards for cubic BaTiO3 and SrTiO3.

3. Results and discussion

ray diffraction. As a representative example, the obtained patterns for the 90/10 samples, are shown in Fig. 1. From a careful comparison of the diffraction spectra with the reference standard diffraction cards for cubic BaTiO3 and SrTiO3 phases [16,17], it results that no barium or strontium titanate phases are present. In the case of hBST, a broad peak appears around 2h  24°, indicating that some impurities of BaCO3 are present in the samples [18]. This impurities were completely removed by washing the hBST samples with a 0.1 M HCl solution, as revealed by a X-ray analysis on the purified hBST samples (not shown). An analysis of the (2 0 0) peak at 2h  45° for all samples does not clearly show the characteristic splitting of the (2 0 0) reflection due to the presence of the tetragonal phase of BaTiO3 into two components ascribed to (2 0 0) and (0 0 2) reflections, suggesting that all the BST samples could be described by a cubic structure. However, since the diffraction peaks are slightly broadened due to the nanocrystalline size of the particles and/or the resolution of the X-ray data (see Fig. 1), it cannot be excluded that a certain amount of tetragonal phase is present. From the X-ray data for BST samples of different composition, it can be noted that on increasing the concentration of the Sr2+ ions, the reflections shift toward higher 2h values [19], as it is expected from the contraction of the unit cell caused by the substitution of the larger Ba2+ ions with the smaller Sr2+ ions. This result obviously confirms that the strontium incorporates into the barium titanate structure.

3.1. XRD analysis

3.2. SEM results

The Eu3+ doped 90/10 and 85/15 BST samples obtained with the three different techniques were characterized by X-

Scanning electron microscope images were taken in order to estimate the size and evaluate the morphology of

2.2. Instrumental details The X-ray data were measured using a Siemens D5000 powder diffractometer. SEM photos were collected using a Field Emission SEM LEO1530. Room temperature luminescence spectra were excited with the second harmonic radiation (k = 532 nm) of a continuous wave Nd–YAG laser. The signal was measured with a Jobin-Yvon TRW 1000 spectrophotometer and an air cooled photomultiplier (Hamamatsu R928). The emission spectra were corrected for the instrument response. Emission lifetimes were measured at room temperature with a Tektronix TDS 3052 oscilloscope (500 MHz, 5 GS/s) exciting with the second harmonic radiation (532 nm) of a pulsed Nd:YAG laser.

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found for a Eu3+ doped BaTiO3 material prepared with the same molten salts technique [14]. 3.3. Luminescence results Room temperature excitation of the samples at 532.0 nm occurs inside the 7F1 ! 5D1 transition of Eu3+ [20]. This transition has an electric dipole nature and satisfies the usual Judd–Ofelt selection rules [21]. Since both states involved in this transition are split by the crystal field, this excitation has to be considered non-site selective. The room temperature luminescence spectra in the visible range of both the Eu3+ doped 90/10 and 85/15 BST samples under investigation show characteristic bands assigned to the 5D0 ! 7FJ (J = 0, 1, 2, 3, 4) electronic transitions. As a representative example, the emission spectra for the 85/15 BST samples are shown in Fig. 3. The spectra are dominated by an emission band at about 615 nm assigned to the 5D0 ! 7F2 hypersensitive transition, which is stronger than the 5D0 ! 7F1 magnetic dipole transition, at variance with the spectra of 80/20 and 65/35 Eu3+ doped BST ceramic materials shown by Samantary et al. [22]. From the emission spectra, it can be noted that the overall widths of the emission bands are different for samples prepared by different methods (see Fig. 3). This evidence indicates a different splitting of the Stark levels for the 7FJ multiplets of the Eu3+ ions and/or inhomogeneous broadening for differently prepared samples. This behavior can be attributed to the presence of a high concentration of structural defects (e.g., Ba2+ or Sr2+ cation vacancies) induced by various mechanisms of charge compensation due to the relatively high concentration of Eu3+ ions in the samples.

5

7

D0 7

7

F2

(a)

F1 7

F0

7

F3

F4

Fig. 2. SEM images of the 90/10 Eu doped BST samples prepared by (a) molten salts, (b) hydrothermal and (c) sol–gel techniques.

the grains of the BST samples. Fig. 2 shows pictures of the 90/10 Eu3+ doped BST. For hBST and sBST, most particles are spherical single crystals with an average size of 50 nm. On the other hand, mBST particles are much larger as the average particle size is about 400 nm. In case of the sBST grains form clusters and agglomerates consisting of many particles with clearly marked grain boundaries; this result is characteristic of materials prepared at high sintering temperature. For hBST the grains do not form clusters and agglomerates as they are mostly separated and not sintered. In the case of mBST, the SEM image shows submicrometer particles with spherical shapes and a large size distribution, ranging from about 200 to 1200 nm. The size and the morphology of the particles are similar to those

Intensity (a.u.)

3+

(b)

(c)

560

600

640

680

720

Wavelength (nm) Fig. 3. Room temperature emission spectra of the Eu3+ doped 85/15 BST samples prepared by (a) molten salts, (b) hydrothermal and (c) sol–gel techniques.

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R1 R tmax tIðtÞdt tIðtÞdt 0 ’ R0tmax sm ¼ R 1 IðtÞdt IðtÞdt 0 0

It is well known that the asymmetry ratio R¼

Ið5 D0 ! 7 F2 Þ Ið5 D0 ! 7 F1 Þ

of the integrated intensities of the 5D0 ! 7F2 and 5 D0 ! 7F1 transitions can be considered indicative of the asymmetry of the coordination polyhedron of the Eu3+ ion [23]. The R values are 2.6 ± 0.1, 5.1 ± 0.1 and 7.5 ± 0.1 for the 85/15 mBST, hBST and sBST samples, respectively, suggesting a different average local environment of the Eu3+ ions in the differently prepared samples. In particular, the obtained R values indicate that the average symmetry around the Eu3+ ions decrease and therefore the distortion of the coordination polyhedron increase on passing from the mBST to the hBST and from the hBST to the sBST samples. Moreover, the band at about 580 nm attributed to the forbidden 5D0 ! 7F0 electric dipole transition, is very weak for the mBST powders (see Fig. 3), suggesting a high symmetry of the Eu3+ environment, in agreement with the obtained low R value for this sample. The room temperature emission decay curves for the 85/ 15 BST samples obtained using a pulsed laser excitation at 532 nm are shown in Fig. 4. Different emission decay profiles are observed for the three BST samples. In fact, the emission decay profile for the mBST sample is strongly non-exponential. The non-exponential behavior is less pronounced for the hBST and sBST samples, as for the latter sample the decay profile is in practice exponential. We calculate the effective emission decay time using the equation [24]:

(a)

1E-3 1E-4 1E-5 1E-6

Intensity (a.u.)

1E-7

(b)

1E-3

1287

where I(t) represents the luminescence intensity at time t corrected for the background and the integrals are evaluated on a range 0 < t < tmax where tmax  sm. The obtained sm values for the samples are 0.20 ± 0.01, 0.44 ± 0.01 and 0.57 ± 0.01 ms for the 85/15 mBST, hBST and sBST, respectively. The non-exponential shape of the luminescence decay curves, also observed for Eu3+ doped BaTiO3 samples prepared by the same sol–gel technique [25], is mainly ascribed to the disorder affecting the sites in which the Eu3+ ions are accommodated, as evidenced before by the significant inhomogeneous broadening of the emission bands. Nonetheless, due to the non-exponential behavior of the decay curves, particularly evident for the mBST sample, it is possible to suppose that a clustering of Eu3+ ions and therefore some energy transfer processes between the Eu3+ ions could be present. The values of the effective decay times are lower than those found for Eu3+ doped BaTiO3 samples prepared by the same sol–gel technique (1.9–2.2 ms) [26]. Moreover, it is worth to note that although the average site symmetry for the mBST samples is higher than those for the hBST and sBST samples, pointing to a longer radiative emission decay time, the experimental effective decay time is lower for the mBST sample. The shortening of the sm value for the mBST samples investigation is most probably due to a higher concentration of defects. This behavior was already observed for a BaTiO3 sample prepared by the same molten salt technique in which the Eu3+ emission was found to be completely quenched [14]. The same holds true for the hBST sample with respect to the sBST one, although in this case the presence of some AOH groups could also contribute to the quenching of the Eu3+ emission. In fact, the hBST samples were dried at low temperature and some residual absorbed water could still be present. Further spectroscopic measurements (FTIR, Raman) are in progress to better clarify this point; they will be published in a future paper. 4. Conclusions

1E-4 1E-5 1E-6

(c)

1E-3 1E-4 1E-5 1E-6 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time (ms) Fig. 4. Room temperature decay curves of the Eu3+ doped 85/15 BST samples prepared by (a) molten salts, (b) hydrothermal and (c) sol–gel techniques.

Nanocrystalline or submicrometer Eu3+ doped BST powders have been prepared using three different synthesis techniques. From the X-ray structural characterization it has been observed that the powders are solid solutions of barium and strontium titanates. Moreover, the size and morphology of the particles depend upon the preparation technique. The molten salts technique produces submicrometer size BST particles (about 400 nm) differently from the hydrothermal and sol–gel techniques which give rise to nanometer size BST particles (about 50 nm). All the Eu3+ doped BST samples show a strong emission in the visible range. From the asymmetry ratios, it results that the Eu3+ site symmetry decreases on passing from the mBST to the hBST samples and from the hBST to the sBST ones.

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The significant observed inhomogeneous broadening of the emission bands can be attributed to the presence of a high concentration of structural defects. This explanation is corroborated by the non-exponential behavior of the experimental decay curves and the shortening of the effective decay times, in particular for the hBST and mBST samples. Acknowledgements The authors are grateful to Erica Viviani (Universita` di Verona) for expert technical assistance, Dr. Piotr Solarz (ILTSR PAS, Wroclaw) for emission decay measurements and to Dmitry Kolesnikov (IHPP PAS, Warsaw) for collecting of SEM images. This work was partially supported by the European Union–European Social Fund. The researcher bilateral program in the framework of the Protocol for scientific and technological collaboration between the Republic of Italy and the Republic of Poland (12 CH) is acknowledged for financial support to authors. These studies were partially supported by the Polish Ministry of Science under Grant No. 3 T08A 006 29. References [1] C.H. Ahn, K.M. Rabe, J.-M. Triscone, Science 303 (2003) 488. [2] S. Zhong, S.P. Alpay, Z.-G. Ban, J.V. Mantese, Appl. Phys. Lett. 86 (2005) 1. [3] Y. Mao, S. Banerjee, S.S. Wong, J. Am. Chem. Soc. 125 (2003) 15718. [4] P. Shi, X. Yao, L. Zhang, X. Wu, M. Wang, X. Wan, Solid State Commun. 134 (2005) 589. [5] M.O. Ramı´rez, D. Jaque, L. Ivleva, L.E. Bausa´, J. Appl. Phys. 95 (2004) 6185.

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