Combustion synthesis and photoluminescence propertiesof LaAlO3 nanophosphors doped with Yb3þ ions

A. Dhahri a, K. Horchani-Naifer a,n, A. Benedetti b, F. Enrichi c, M. Ferid a

a Laboratoire de Physico-chimie des Matériaux Minéraux et leurs Applications, Centre National des Recherches en Sciences des Matériaux, Technopole de BorjCedria, B.P. 73, Soliman 8027, Tunisiab Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, Dorsoduro 2137, 30123 Venezia, Italyc CIVEN – Coordinamento Interuniversitario Veneto per le Nanotecnologie, Via Delle Industrie 5, Marghera, Venice 30175, Italy

a r t i c l e i n f o

Article history:Received 8 February 2014Received in revised form25 March 2014Accepted 26 March 2014Available online 3 April 2014

Keywords:Combustion processLanthanum aluminateYtterbiumNanoparticlePhotoluminescence

a b s t r a c t

Ytterbium doped lanthanum aluminate (LaAlO3) nanophosphors have been prepared by a combustionprocess with glycine as a fuel. The structures of the powders were determined by X-ray diffraction (XRD),the morphology of the annealed materials was observed using scanning electron microscopy (SEM), theaverage crystalline grain sizes have been determined by transmission electron microscopy (TEM) andphotoluminescence properties using fluorescence spectroscopy. Pure LaAlO3 phase was obtained at800 1C heated for 4 h, with an average crystal size, as determined by TEM, of 60 nm. Emission spectraand decay times of main luminescence transitions were measured at room temperature. A strongemission is reported at 986 nm from the (2F5=2-

2F7=2 ) transition, whose intensity depends on Ybconcentration.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, LaAlO3 doped with rare earth ions arepromising materials for color light emission [1–4]. For instance,LaAlO3 doped with Yb3þ ions is of significant interest sinceytterbium-activated phosphors are well known for high quantumefficiency and have a low quantum defect. Yb3þ ion has only twomanifolds, the 2F7/2 ground state and the 2F5/2 excited state,therefore such processes as cross relaxation, up-conversion orexcited-state absorption [5]. LaAlO3 perovskite has at roomtemperature a rhombohedral structure with R3c symmetry anda cubic structure with space group Pm3m at temperatures above527 1C [6].

Typically, lanthanum aluminate (LaAlO3) has been prepared byconventional solid-state reactions of Al2O3 and La2O3 in thetemperature range of 1500–1700 1C [7,8]. Moreover, several lowtemperature (750–900 1C) chemical routes are used for preparingfiner and homogeneous powders of LaAlO3 like sol–gel process [9–11], EDTA gel route [12,13], co-precipitation method [14,15],pyrolysis using triethanolamine [16] and combustion synthesiswith urea and hydrazine as fuels [17–20]. Recently LaAlO3 hasbeen successfully prepared by microwave irradiation [21] and anovel two-step process [22].

In this work we present the synthesis and characterization ofLaAlO3:Yb3þ phosphors prepared by combustion synthesis [23–25]. Structural details and optical properties of the synthesizedphosphors have been investigated by X-ray diffraction (XRD),transmission electron microscopy (TEM), scanning electron micro-scopy (SEM) and fluorescence spectroscopy. Photoluminescenceemission and excitation as well as decay times were measured.

2. Experimental procedure

Samples were prepared by using the combustion method, asthe starting substrates were lanthanum nitrate hexahydrate [La(NO3)3 �6H2O] (98%), aluminum nitrate nonahydrate [Al(NO3)3 �9H2O] (99%), ytterbium(III) nitrate pentahydrate [Yb(NO3)3 �5H2O],and glycine [H2NCH2COOH] (99%). La(NO3)3 �6H2O, Al(NO3)3 �9H2O Yb(NO3)3 �5H2O and H2NCH2COOH were dissolved in dis-tilled water. Yb3þ ions doped lanthanum aluminate with generalformula (La1�x Ybx) AlO3 were prepared with different concentra-tions of Yb (x¼2%, 5% and 10%). During the process, the molar ratioof glycine to total metal cations concentration G/M was 2 and thecation ratio of La:Al was 1:1. Glycine was used as a fuel. Theresulting solution was magnetically stirred at 85 1C to get a clearand uniform solution. The solution was continuously heated forabout 1.5 h and turned into a highly viscous gel. Throughout theprocess, no signs of precipitation or turbidity were observed. The

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Journal of Luminescence

http://dx.doi.org/10.1016/j.jlumin.2014.03.0640022-2313/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author.E-mail address: karima_horchani@yahoo.com (K. Horchani-Naifer).

Journal of Luminescence 153 (2014) 408–411

gel was put into a vacuum oven and kept at 110 1C for24 h, undergoing rapid dehydration and foaming followed bydecomposition and generating a solid precursor. Finally this solidprecursor was ground to fine powders and was calcined at 800 1Cfor 4 h to obtain pure LaAlO3.

3. Experimental

X-ray powder diffraction (XRD) patterns of all samples wererecorded on a Philips X'Pert system (PW3020 vertical goniometerand PW3710 MPD control unit) with CuKα 1,2 radiation(λ1¼1.54059 Å and λ2¼1.54442 Å). In order to improve the signalto noise ratio, at least three runs (collected with 10 s/step and0.051/step) were measured.

Scanning electron images of samples were recorded with ascanning electron microscope (SEM) JEOL JSM-5600LV, operated at20 kV equipped with an Oxford Instruments ISIS series 300 EDSdetector.

Morphologies of products were characterized by transmissionelectronic microscopy (TEM) (Tecnai G2 ultra Twin). TEM imageswere taken at 300 kV with a JEOL JEM-3010 instrument, with anultra-high resolution pole-piece (0.17 nm point resolution),equipped with a Gatan multi-scan CCD camera (Mod. 794) andan Oxford EDS microanalysis detector. The powdered sampleswere dispersed in ethanol by sonication for approximately 5 minand deposited onto a holey carbon film grid.

Photoluminescence measurements were performed with aFluorolog 3-21 system (Horiba Jobin Yvon). A 450 W xenon arclamp was used as a broadband excitation source and a doubleCzerny–Turner monochromator was used to select the excitationwavelength for photoluminescence excitation.

The analysis of the emitted luminescence signal from thesamples was obtained by using an iHR320 single grating mono-chromator and a R928 Hamamatsu photomultiplier tube detector.Time resolved characterization was obtained by exciting thesample with a SpectraLED-03 laser diode, providing 377 nmexcitation with 12 nm spectral bandwidth. The excitation pulseduration was set at 5 ms and the photoluminescence decay wasacquired for about 20 ms, which was sufficient to allow the signalreach zero. These measurements were obtained by workingin multi-channel single photon counting (MCSPC) mode. Allemission spectra were obtained using the same amount of powder,measured at room temperature and recorded under the sameconditions.

4. Results and discussion

4.1. X-ray diffraction

The X-ray diffraction patterns of Yb3þ doped LaAlO3 powdersobtained at 800 1C are shown in Fig. 1. So all diffraction peaks inthese XRD patterns could be attributed to the rhombohedralperovskite structure of LaAlO3 (JCPDS No. 01-082-0478) with spacegroup R-3c (No.167).

4.2. TEM and SEM analyses

Fig. 2a presents TEM micrographs of LaAlO3 powders annealedat 800 1C, showing that the powders are composed of monocrys-talline nanoparticles and exhibit the formation of aggregatesamong them. The nanoparticles have a polyhedral morphologywith particle size about 60 nm.

Fig. 2b shows high resolution TEM images (HRTEM), pointingout that the samples are perfectly crystalline, as can be seen by theclear and uniform distribution of lattice planes.

Fig. 3 illustrates SEM image of LaAlO3:Yb (5%) annealed at800 1C. The nanocrystallites are agglomerated in bigger polyhedralgrains.

4.3. Photoluminescence studies

Recently optical properties of Yb3þ doped LaAlO3 were inves-tigated by Lemanski and Deren [5]. The excitation spectrumrecorded at 986 nm of Yb3þ doped LaAlO3 with different concen-trations (2%, 5% and 10%) is shown in Fig. 4. A series of sharp linesis in the region 260–360 nm.The broadband at 295 nm can beassigned to charger transfer band (CTB). The emission spectra

Fig. 1. XRD diffractograms of LaAlO3:Yb3þ obtained at 800 1C.

Fig. 2. (a) TEM and (b) HRTEM images of LaAlO3:Yb (5%) obtained at 800 1C.

A. Dhahri et al. / Journal of Luminescence 153 (2014) 408–411 409

under 295 nm excitation of Yb3þ doped LaAlO3 with differentconcentrations (2%, 5% and 10%) are shown in Fig. 5. The transition(2F5=2-

2F7=2 ) located at 986 nm is the most intense.

The photoluminescence emission intensity increases withincreasing Yb3þ concentration from 2% to 5%, reaching the max-imum when the concentration of Yb3þ is 5% and then slightlydecreasing for higher concentrations, indicating the occurrence ofconcentration quenching. The reason for concentration quenchingis that the interaction of Yb3þ–Yb3þ also increases with increasingYb3þ concentration, so an excessive doping of rare earth ionsshould be avoided because it is detrimental for the phosphorefficiency.

Fig. 6 illustrates that the emission intensity of the transition(2F5=2-

2F7=2 ) at 986 nm firstly increases up to 5% for Yb3þ

concentration, and then decreases when Yb3þ concentrationcontinuously increases.

Fig. 7 illustrates the decay time of (2F5=2-2F7=2 ) at 986 nm.

These curves are not decreasing in a single exponential way,so they were fitted by a double exponential function. For thisreason the values of lifetimes were obtained by an averagecalculation as [26]

τavgffiA1τ21þA2τ22A1τ1þA2τ2

The values of lifetimes of Yb3þ doped LaAlO3 with differentconcentrations (2%, 5% and 10%) confirm that the decay timedepends on Yb3þ concentration [5]; we can see a distinct decreasein lifetime for 10% of Yb3þ concentration which equals 6.1 μscompared to the sample containing 5% which equals 10.85 μs. This

Fig. 4. Excitation spectra of LaAlO3:Yb (2%; 5%; 10%).

Fig. 5. Emission spectra of LaAlO3:Yb (2%; 5%; 10%) obtained at 800 1C.

2 4 6 8 10

Inte

nsity

(au)

Yb concentration (%)

2F5/22F7/2

Fig. 6. Dependence of the relative emission intensity of (2F5=2-2F7=2 ) transition on

Yb concentration.

40200

6.1 µs

10.85 µs

21.5 µs

exc 295 nm

Time(µs)

Yb 2% Yb 5% Yb 10%

Fig. 7. The lifetime decay curves of LaAlO3:Yb (2%; 5% and 10%).

Fig. 3. SEM image of LaAlO3:Yb (5%) obtained at 800 1C.

A. Dhahri et al. / Journal of Luminescence 153 (2014) 408–411410

is due to energy migration processes among Yb3þ ions which leadto the luminescence quenching.

5. Conclusion

Ytterbium doped LaAlO3 nanophosphors were obtained at800 1C using a combustion method with a perovskite structure.The TEM image shows that the main crystallite size of our samplesis 60 nm. The most intense emission of Yb3þ in LaAlO3 wasregistered for the transition (2F5=2-

2F7=2 ) at 986 nm. The photo-luminescence intensity increases with increasing of Yb3þ concen-tration from 2% to 5%, reaching the maximum when theconcentration of Yb3þ is 5% up to 5% the intensity decreasebecause of concentration quenching.

Acknowledgment

Project supported by the Ministry of Higher Education, Scien-tific Research and Technology of Tunisia.

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