Combustion synthesis and photoluminescence of Tb3+ doped LaAlO3 nanophosphors

Optical Materials 35 (2013) 1184–1188

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Combustion synthesis and photoluminescence of Tb3+ doped LaAlO3 nanophosphors A. Dhahri a, K. Horchani-Naifer a,⇑, A. Benedetti b, F. Enrichi c, M. Férid a, P. Riello b a Laboratoire de Physico-chimie des Matériaux Minéraux et leurs Applications, Centre National des Recherches en Sciences des Matériaux, Technopole de Borj Cedria, B.P. 73, 8027 Soliman, Tunisia b Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, Dorsoduro 2137, 30123 Venezia, Italy c CIVEN (Coordinamento Interuniversitario Veneto per le Nanotecnologie), via delle Industrie 5, 30175 Marghera (VE), Italy

a r t i c l e

i n f o

Article history: Received 15 September 2012 Received in revised form 7 January 2013 Accepted 10 January 2013 Available online 13 February 2013 Keywords: Combustion process Lanthanum aluminate Terbium Nanoparticles Photoluminescence

a b s t r a c t Terbium doped lanthanum aluminate (LaAlO3) nanophosphors were successfully synthesized by a combustion process using concentrated solution of lanthanum nitrates and aluminate as oxidiser, and glycine acid as fuel. The powders were characterized by infrared spectroscopy (IR), X-ray diffraction (XRD), Rietveld refinement, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and fluorescence spectroscopy. Pure LaAlO3 phase was obtained at 800 °C heated for 4 h, without formation of any intermediate phase, with an average crystal size, as determined by TEM, of 60 nm. Intense green emission is reported at 542 nm, from the 5 D4 level, which intensity depends on Tb concentration. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Lanthanum aluminate (LaAlO3) with a perovskite-type structure presents good thermal stability with high melting point at 2180 °C, which can minimize interfacial dislocations [1]. Typically, LaAlO3 has been prepared by conventional solid-state reaction of Al2O3 and La2O3 in the temperature range of 1500–1700 °C [2,3,4]. But this method suffers from many inherent shortcomings, such as the high-temperature heat treatment which have a detrimental effect of the grain size, limited chemical homogeneity and low sintering temperature. Recently LaAlO3 have been successfully prepared by microwave irradiation [5]. Moreover, various wet and soft chemical methods including polymerized complex method using citric acid and ethylene glycol route have been reported [6]. Several low temperature (750–900 °C) chemical routes are used for preparing finer and homogeneous powders of LaAlO3 like Poly Vinyl Alcohol (PVA) with metal nitrate synthesis [7], sol–gel process [8–10], EDTA gel route [11,12], co-precipitation method [13,14], pyrolysis using triethanolamine [15] and combustion synthesis with urea and hydrazine as fuel [16–19]. This paper presents the synthesis and characterization of LaAlO3:Tb3+ phosphors, prepared by combustion synthesis [20], which has the advantage of being simple, fast and economical in doping. The structural details and optical properties of the synthe⇑ Corresponding author. E-mail address: [email protected] (K. Horchani-Naifer). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.01.013

sized phosphor have been investigated by X-ray diffraction (XRD), Rietveld refinement, transmission electron microscopy (TEM), scanning electron microscopy (SEM), infrared spectrometry (IR) studies, and fluorescence spectroscopy. 2. Experimental procedure The starting materials were lanthanum nitrate hexahydrate [La(NO3)36H2O] (98%), aluminum nitrate nonahydrate [Al(NO3)39H2O] (99%), europium(III) nitrate pentahydrate [Tb(NO3)35H2O], and glycine [H2NCH2COOH] (99%). La(NO3)36H2O and Al(NO3)39H2O Tb(NO3)35H2O and H2NCH2COOH were dissolved in distilled water. Tb3+ ions doped Lanthanum aluminate with general formula (La1x Tbx) AlO3 were prepared with different concentration of Tb (x = 2%, 5%, 10%, 15%, 20%). During the process, the molar ratio of glycine to total metal cations concentration G/M was 2 and the cation ratio of La:Al was 1:1. Glycine was used as a fuel. The resulting solution was magnetically stirred at 85 °C to get a clear and uniform solution. The solution slowly turned viscous on continued heating for about 1.5 h, it turned into a highly viscous gel. Throughout the process, no signs of precipitation or turbidity were observed. The gel was put into a vacuum oven and kept at 110 °C for 24 h, undergoing rapid dehydration and foaming followed by decomposition and generating combustible gases. These volatile combustible gases ignite and burn with a flame yielding voluminous solid. Finally this solid was ground to fine powders and was calcined at 600 °C, 700 °C and finally at 800 °C for 4 h to obtain pure LaAlO3.

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(a)

D ¼ 0:9k=b cos h;

LaAlO3obs

(012) (202)

(024) (122) (300) (220) (312) (134)

Intensity (au)

The X-ray powder diffraction (XRD) patterns of all samples were recorded on a Philips X’Pert system (PW3020 vertical goniometer and PW3710 MPD control unit) with Cu Ka 1, 2 radiation (k1 = 1.54059 Å and k2 = 1.54442 Å). In order to improve the signal to noise ratio, at least three runs (collected with 10 s/step and 0.05°/step) were measured. The quantitative phase analysis for samples composed of one crystal and one amorphous phase (CGXXC series) was performed on XRPD data by applying the Rietveld method (DBWS9600 computer program written by Sakthivel and Young modified by Riello et al. (1995, 1998), using a-Al2O3 as the internal standard (IS). Infrared spectra were recorded on a NICOLET 560 spectrometer using KBr pellets in the region of 4000–400 cm1. The scanning electron images of samples were recorded with scanning electron microscope (SEM) JEOL JSM-5600LV, operated at 20 kV equipped with an Oxford Instruments ISIS series 300 EDS detector. The morphology of products was characterized by transmission electronic microscopy (TEM) (Tecnai G2 ultra Twin). TEM images were taken at 300 kV with a JEOL JEM-3010 instrument, with an ultra-high resolution pole-piece (0.17 nm point resolution), equipped with a Gatan multi-scan CCD camera (Mod. 794) and an oxford EDS microanalysis detector. The powdered samples were dispersed in ethanol by sonication for approximately 5 min and deposited onto a holey carbon film grid. Photoluminescence measurements were performed with a Fluorolog3-21 system (Horiba Jobin Yvon). A 450 W xenon arc lamp was used as a broadband excitation source and a double Czerny– Turner monochromator was used to select the excitation wavelength for photoluminescence excitation. The analysis of the emitted luminescence signal from the samples was obtained by using a iHR320 single grating monochromator and a R928 Hamamatsu photomultiplier tube detector. Time resolved characterization was obtained by exciting the sample with a SpectraLED-03 laser diode, providing 377 nm excitation with 12 nm spectral bandwidth. The excitation pulse duration was set at 5 ms and the photoluminescence decay was acquired for about 20 ms, which was sufficient to allow the signal going to zero. These measurements were obtained by working in multi channel single photon counting (MCSPC) mode. All emission spectra are obtained using the same amount of powder, measured at room temperature and recorded under the same conditions. The particle size was estimated from the X-ray line broadening of the (1 1 0) diffraction peak using the Scherrer formula [21].

(110)

LaAlO3cal

20

40

2θ (°)

60

(b)

80

LaAlO3 :Tb

3+

Tb 20%

Intensity (au)

3. Experimental

3+

800°C

3+

Tb 15%

3+

Tb 10%

3+

Tb 5%

3+

Tb 2%

20

40

2θ (°)

60

80

Fig. 1. X-ray diffraction patterns of LaAlO3:Tb3+ (a) observed and calculated, and (b) doping concentration variation.

c = 13.10 Å. Which are close to the reported values (JCPDS no. 01082-0478). The atomic coordinates of LaAlO3 obtained from the Rietveld analysis are given in Table 1. Fig. 1b shows that the doping concentration does not influence the crystalline phase formation. So all diffraction peaks in these XRD patterns could be attributed to the rhombohedral perovskite crystal structure of LaAlO3. The particles size of LaAlO3 powder calcined at 800 °C for 4 h calculated from the Scherrer formula is about 60 nm.

ð1Þ 4.2. IR spectra

where D is the particle size in nm, k the radiation wavelength, h the diffraction peak angle and b is the corrected line width at half-peak intensity. The correction for instrumental peak broadening was 2 2 made using the Warren formula: b ¼ ðbobs  b Þ1=2 , where bobs is the line width at half-peak intensity related to LaAlO3 powder and b is the line width of the (1 1 0) diffraction peak of the LaAlO3.

Fig. 2 shows IR spectra of the samples containing different doping concentrations obtained at 800 °C. All spectral profiles are identical; the two frequencies at 660 and 445 cm1 are typical for the M–O (possibly La–O and Al–O stretching frequencies) vibrations for the perovskite structure compounds [22].

4. Results and discussion

4.3. TEM and SEM analysis

4.1. X-ray diffraction

Fig. 3a presents TEM micrographs of LaAlO3 powders calcined at 800 °C, showing that the powders are composed by monocrystal-

The X-ray diffraction patterns of LaAlO3:Tb3+ are shown in Fig. 1. According to observed and calculated XRD analysis (Fig. 1a), the structural analysis of LaAlO3 obtained at 800 °C is carried by Rietveld refinement program. It crystallizes in a pure rhombohedral perovskite structure with space group R-3c (No. 167). The reliability factors obtained from the refinement are Rwp = 8.02, Rp = 5.82, Rexp = 3.58, with unit cell dimensions a = 5.37 Å and

Table 1 Atomic coordinates of LaAlO3. Element

Wyckoff position

Site occupancy

x

y

z

La Al 0

6a 6b 18e

1/6 1/6 1/2

0 0 0.528

0 0 0

0.25 0 0.25

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Tb

3+

20%

Tb

3+

15%

3+

10%

Transmittance (a.u)

Al-O 6

Tb

Tb

3+

5%

Tb

3+

2%

Al-O 6

Al-O 6

Al-O 6

Al-O 6

800

600

-1

Wavenumber (cm ) Fig. 2. FT-IR spectra of LaAlO3:Tb3+ doping concentration variation.

line nanoparticles and exhibit the formation of aggregates among them. The nanoparticles have a spherical morphology with particle size about 60 nm, which agrees very well with the results given by the Scherrer formula. Fig. 3b illustrates the high resolution TEM images (HRTEM), pointing out that the samples are perfectly crystalline, as can be seen by the uniform distribution of lattice planes. Fig. 3c shows the energy dispersive X-ray spectrum (EDS) we observe peaks assigned to Al3+, La3+, O2 and Tb3+ ions. Fig. 4 shows SEM micrographs of LaAlO3:Tb (10%) annealed at 800 °C, The micrographs clearly indicate the agglomeration of polyhedral crystallites, which have no uniform shapes and sizes. This non-uniformity of shape and size can be assigned to the non-uniform distribution of temperature and mass flow in the combustion process. The non-uniformity is not in contradiction with TEM pictures, because TEM pictures shows the individual particles which consist of aggregates. 4.4. Photoluminescence studies Terbium-activated phosphors are well known as excellent emitters of green light. The fluorescence of Tb3+ under UV excitation is mainly due to the 5 D4 ! 7 FJ (J = 6; 5, 4, 3) transitions, with the strongest emission for J = 5 at 542 nm, while the f–f transition lines from the higher level 5 D3 are not observed due to the increased concentration of Tb3+ [23]. Optical properties of Tb3+ doped LaAlO3 was investigated first by Deren et al. [24]. The excitation spectrum recorded at 542 nm of 10% Tb3+ doped LaAlO3 is shown in Fig. 5. The series of sharp lines in the region 300–500 nm. The lines belong to transitions between the energy levels of the 4f8 configuration [25]. The emission spectra under 377 nm excitation of Tb3+ doped LaAlO3 with different concentration (2%, 5%, 10%, 15% and 20%) are shown in Fig. 6. The transition (5 D4 ! 7 F5 ) located at 542– 550 nm is the most intense, and responsible for the green color. At 488–492 nm (5 D4 ! 7 F6 ) at 581–594 nm (5 D4 ! 7 F4 ), at 620– 624 nm (5 D4 ! 7 F3 ) are observed. The appearance of the emission band at 730 nm is probably due to lack of La in as-grown sample. Annealing in air induces oxygen excess in nanocrystals, and gives emission due to holes. The photoluminescence emission intensity increases with increasing Tb3+ concentration from 2% to 10%, reaching the maximum when the concentration of Tb3+ is 10% and then slightly decreases for higher concentrations, indicating the occurrence of concentration quenching. The reason for concentration quenching

Fig. 3. (a) TEM micrograph, (b) HRTEM images and (c) EDS spectrum of LaAlO3:Tb (10%) obtained at 800 °C.

is that the interaction of Tb3+–Tb3+ also increases with increasing Tb3+ concentration, so an excessive doping of rare earth ions should be avoided because it is detrimental for the phosphor efficiency.

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5

Em 542 nm

7

F5

Intensity (au)

D4

2%

5%

10%

15%

20%

Tb concentration (%) Fig. 7. Dependence of the relative emission intensity of Tb3+ on its doping concentration for Tb-doped LaAlO3.

Tb 2% Tb 5% Tb 10% Fig. 4. SEM images of LaAlO3:Tb (10%) obtained at 800 °C.

τ = 3,86ms

3+

D3

τ = 3,68ms

F6

5

L1,5,10

5

7

intensity (a.u)

Tb 10 % Exc ( λ em = 542 nm)

7

F6

5

D4

7

F

3,1 ms

0

10

20

Time (ms)

30

Fig. 8. Decay time of LaAlO3:Tb (2%, 5%, 10%).

300

350

400

450

500

λ nm

As we can seen in Fig. 7 that the emission intensities of the transition (5 D4 ! 7 F5 ) at 542 nm firstly increases up to 10% for Tb3+ concentration, and then decreases when Tb3+ concentration continuously increase. Fig. 8 reports the decay time of (5 D4 ! 7 F5 ) at 542 nm. These curves are not decreasing in a single exponential way, so they were fitted by a double exponential function. For this reason the values of lifetimes were obtained by an average calculation [26]:

Fig. 5. Excitation spectra of LaAlO3:Tb (10%).

5

D4

7

F5

λ ex=377nm

Intensity (au)

2% 5% 10% 15% 20%

5 5

D4

D4

7

F6

7

F4 5D4

savg ffi

A1 s21 þ A2 s22 A1 s1 þ A2 s2

The values of lifetimes for too concentration of Tb3+ ions (2%, 5%, 10%), confirm that the decay time depends on Tb3+ concentration [24], we can see a distinct decreasing of lifetime for 10% of Tb3+ concentration, equals 3.1 ms compared to the sample containing 2% equals 3.86 ms.

7

F3

5. Conclusion 500

550

600

λnm

650

700

Fig. 6. Emission spectra of LaAlO3:Tb (2%, 5%, 10%, 15% and 20%) obtained at 800 °C.

A pure LaAlO3 with a perovskite structure was obtained at 800 °C using a combustion method. The TEM image shows that we obtain a nanopowder with the particle size about 60 nm. The

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photoluminescence of LaAlO3 was related to 5 D4 ! 7 FJ (J = 6; 5, 4, 3) transitions of Tb3+ ion, the most intense emission of Tb3+ in LaAlO3 was registered for the transition (5 D4 ! 7 F5 ) at 542 nm, up to 10% of Tb3+ the intensity decrease because of concentration quenching. Acknowledgments This work is supported by the Ministry of Higher Education and Scientific Research in Tunisia. Mr. Tiziano Finotto, Mr. Loris Bertoldo and Mr. Davide Cristofori are gratefully acknowledged for conducting the XRD measurements and TEM images. References [1] R.K. Simon, C.E. Platt, K.P. Daly, A.E. Lee, M.K. Wager, Appl. Phys. Lett. 53 (1988) 2677. [2] S.Y. Cho, I.T. Kim, K.S. Hong, J. Mater. Res. 14 (1999) 114. [3] B. Jancar, D. Suvorov, M. Valant, G. Drazic, J. Eur. Ceram. Soc. 23 (2003) 1391. [4] I. Zvereva, Y. Smirnov, V. Gusarov, V. Popova, J. Choisnet, Solid State Sci. 5 (2003) 343. [5] J. Prado-Gonal, A.M. Arevalo-Lopez, E. Moran, Mater. Res. Bull. 46 (2011) 222– 230. [6] M. Kakihan, T. Okubo, J. Alloys Compd. 266 (1998) 129. [7] A.K. Adak, P. Pramanik, Mater. Lett. 30 (1997) 269.

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