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Photoluminescent properties of nanostructured Y2O3:Eu3+ powders obtainedthrough aerosol synthesis

K. Marinkovic a, L. Mancic a, L.S. Gomez b, M.E. Rabanal b, M. Dramicanin c, O. Milosevic a,*

a Institute of Technical Sciences of the Serbian Academy of Science and Arts, Belgrade, Serbiab University Carlos III, Leganes, Madrid, Spainc Institute of Nuclear Sciences ‘‘Vinca”, University of Belgrade, Belgrade, Serbia

a r t i c l e i n f o

Article history:Received 14 January 2010Received in revised form 14 May 2010Accepted 25 May 2010Available online 25 June 2010

Keywords:Yttrium oxideSpray pyrolysisNanoparticlesLuminescence

a b s t r a c t

Red emitting Y2O3:Eu3+ (5 and 10 at.%) submicronic particles were synthesized through ultrasonic spraypyrolysis method from the pure nitrate solutions at 900 �C. The employed synthesis conditions (gradualincrease of temperature within triple zone reactor and extended residence time) assured formation ofspherical, dense, non-agglomerated particles that are nanostructured (crystallite size �20 nm). The as-prepared powders were additionally thermally treated at temperatures up to 1200 �C. A bcc Ia-3 cubicphase presence and exceptional powder morphological features were maintained with heating and arefollowed with particle structural changes (crystallite growth up to 130 nm). Emission spectra were stud-ied after excitation with 393 nm wavelength and together with the decay lifetimes for Eu3+ ion 5D0 and5D1 levels revealed the effect of powder nanocrystalline nature on its luminescent properties. The emis-sion spectra showed typical Eu3+ 5D0 ?

7Fi (i = 0, 1, 2, 3, 4) transitions with dominant red emission at611 nm, while the lifetime measurements revealed the quenching effect with the rise of dopant concen-tration and its more consistent distribution into host lattice due to the thermal treatment.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Y2O3:Eu3+ is a well known red commercial phosphor used inlighting and cathode ray tubes, display materials, in tricolor fluo-rescent lamps, field emission displays and laser devices. Higherluminescence efficiency and long-term stability of this materialare mostly associated with investigating optimum doping concen-tration, nanostructuring and obtaining overall morphology control.In comparison to phosphor particles obtained by conventionalmethods that have micron-sized grains and irregular morpholo-gies, the use of fine submicronic spherical particles increases thescreen brightness and improves the resolution because of lowerscattering of light and higher packing densities [1,2]. Additionally,a densely packed layer of small-sized particles are superior for age-ing in comparison with loose packed screens. On the other hand, itwas shown that particles used in display devices should not besmaller than a certain critical size due to the fact that lumines-cence efficiency decreases because of light re-absorption and lumi-nescence quenching by the surface layer [1,3,4]. With regard to theabove mentioned, it is evident that the control over (1) morphol-ogy and size; (2) stoichiometry and composition; and (3) surfacecharacteristics must be established during synthesis process in or-der to obtain the desirable objectives of improved powder phos-

phors [5]. Among novel synthesis methods that tend to fulfillthese objectives, spray pyrolysis can provide control over theseparameters since it represents continuous aerosol decompositionprocess which engages all the advantages of wet chemical methodsand confines it further within the droplet. Single droplet acts as amicro-reactor insuring the formation of fine, spherical particleswith good compositional homogeneity and size that can be variedby controlling of specific processing parameters [6,7]. Some ofthem are already discussed in the literature when the synthesisof advanced phosphor materials is concerned [3,5,7], but for get-ting the general survey more of the specific issues should be clar-ified. One of the major disadvantages in applying spray pyrolysis inproduction of phosphor powders is that it often yields hollow par-ticles which contribute to bad thermal and mechanical stability [8].When Y2O3:Eu3+ phosphors are concerned one of the ways to over-come this obstacle is through alteration of precursor chemistry byintroducing a polymeric additive [3,8]. Also, flux addition is re-ported to be effective for elimination of particle surface defects,hence improving the luminescent properties [3,9]. However, forhigher phosphor productivity solely usage of inorganic precursoris preferable. Furthermore, the optimal doping concentration andpreferred dopant distribution are still under debate, regardless ofthe applied synthesis method [10,11]. Wide range of europiumcontent (0.5–20 at.%) is investigated in order to define its optimalconcentration in Y2O3 [12–14]. Recently, it was shown that in com-parison to micro-scaled powders where concentration quenching

0925-3467/$ - see front matter � 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.optmat.2010.05.023

* Corresponding author.E-mail address: olivera.milosevic@itn.sanu.ac.rs (O. Milosevic).

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is achieved at around 5 at.% [13] with the decrease of particle sizethrough nanostructuring, same phenomenon occurs at highereuropium concentration of 9 at.% [14].

Here, Y2O3:Eu3+ phosphors with advanced morphological fea-tures (highly spherical, nanostructural and dense particles) wereobtained through spray pyrolysis from pure nitrate solution. Effectof different doping concentration (5 and 10 at.%) was followed byluminescence measurement and obtained data were correlatedwith particle morphological and structural characteristics. Ob-tained luminescent properties are considered to be adequate for

Fig. 1. X-ray diffraction patterns of (Y0.90Eu0.10)2O3 system (a) and Rietveld refinedXRD pattern for (Y0.95Eu0.05)2O3/1000 �C (b).

Table 1Powders characteristics: stereological parameters and microstructural data obtained through Rietveld refinement.

Sample Stereological X-ray diffraction

Dmax% 0.3–0.8 lm fL gof rbrag a, Å cs, nm ms, %

(Y0.95Eu0.05)2O3ap 85.09 0.93 1.08 1.209 10.620 ± 0.0005 19.14 ± 0.31 0.432 ± 0.018(Y0.95Eu0.05)2O3/1000 �C 76.66 0.94 1.19 3.368 10.616 ± 0.0004 40.55 ± 1.31 0.189 ± 0.015(Y0.95Eu0.05)2O3/1100 �C 92.09 0.95 1.18 2.346 10.616 ± 0.0003 60.06 ± 2.64 0.0607 ± 0.011(Y0.95Eu0.05)2O3/1200 �C 74.12 0.83 1.13 4.024 10.616 ± 0.0005 93.14 ± 15.18 0.0963 ± 0.014(Y0.90Eu0.10)2O3ap 89.6 0.93 1.09 2.629 10.632 ± 0.0009 20.11 ± 0.48 0.529 ± 0.029(Y0.90Eu0.10)2O3/1000 �C 76.75 0.87 1.09 1.095 10.628 ± 0.0002 40.94 ± 0.65 0.197 ± 0.0074(Y0.90Eu0.10)2O3/1100 �C 84.23 0.95 1.16 2.844 10.623 ± 0.0003 66.99 ± 2.93 0.0794 ± 0.011(Y0.90Eu0.10)2O3/1200 �C 80.49 0.95 1.15 1.352 10.628 ± 0.0001 132.89 ± 4.51 0.0402 ± 0.0048

Fig. 2. SEM micrograph of (Y0.90Eu0.10)2O3 powder annealed at 1100 �C with atypical semi-quantitative EDS analysis in inset (a); distribution histogram ofparameter Dmax with corresponding cumulative curve obtained through stereolo-gical analysis (b).

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potential applications in luminescence displays, which will be theobjective of further developments.

2. Experimental

Y2O3:Eu3+ doped with 5 and 10 at.% of Eu3+ was synthesizedthrough spray pyrolysis method from 0.1 M corresponding precur-sor solutions containing stoichiometric amounts of Y(NO3)3�6H2Oand Eu(NO3)3�5H2O. An ultrasonic aerosol generator (RBI, France)with a frequency of 1.3 MHz was used to atomize previously pre-pared precursor solution having the following characteristics: pH4.8 (Orion), density q = 1.02217 g/cm3 (AP-PAAR, DMA 55), surfacetension c = 65.8 mN/m (K10T Kruss). Based on these values theaverage droplet size [15] and the mean particle size [16] were pre-dicted to be 3.35 lm and 555 nm, respectively. Using air as a car-rier gas (flow rate 1.0 l/min) the obtained aerosol was thenintroduced into triple zone tubular flow reactor with temperatureprofile 200–700–900 �C. Passing through the reactor zone thedroplets underwent the consequent processes of drying, precipita-tion and chemical reaction and the residence time of aerosol with-in reaction zone was 68 s. As-prepared (ap) powder samples werecollected at the exhaust and subjected to the post thermal treat-ments carried out at 1000, 1100 and 1200 �C for 12 h. The phase

composition of the obtained powders was analyzed by X-ray dif-fraction (X’Pert Philips diffractometer) and Rietveld refinementby Topas Academic [17] software gave a more detailed insight intopowders microstructural properties. Morphological features andparticle chemical homogeneity were investigated by means ofscanning electron microscopy (Philips SEM XL30/EDS Dx4) andtransmission electron microscopy (TEM JEOL JEM 4000 EX). Basedon SEM micrographs, the semi-automatic image analysis was pre-formed on 200–300 particles (Lecia Q500MC) and in this work thefollowing stereological parameters were considered: diameterDmax (lm) and form parameter fL (for an ideal sphere equals 1).The photoluminescence spectra and lifetime measurements ofthe phosphors were recorded at room temperature, on the Fluoro-log-3 Model FL3-221 spectrofluorometer system (HORIBA Jobin–Yvon). Emission spectra were measured utilizing 450 W Xenonlamp as a excitation source, while 150 W pulsed Xenon lamp withTBX detector was used for life time measurements.

3. Results and discussion

All synthesized powders possess cubic crystal structure thatcorresponds to (Y0.95Eu0.05)2O3 (JCPDS 25-1011, space group Ia-3,a = 10.604 Å). Fig. 1a represents X-ray diffraction patterns of the

Fig. 3. TEM micrographs of as-prepared (Y0.95Eu0.05)2O3 system (a) and annealed for 12 h at: 1000 �C (b), 1100 �C (c), 1200 �C (d).

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samples with 10 at.% of Eu3+. It is evident that reflection peaks ofthermally treated samples are narrower and have higher intensi-ties in comparison to as-prepared ones, indicating their highercrystallinity. Fig. 1b illustrates Rietveld refinement of the XRD pat-terns related to the sample of (Y0.95Eu0.05)2O3 thermally treated at1000 �C/12 h, while Table 1 summarizes the calculated microstruc-tural (crystallite size and microstrain) and crystal lattice parame-ters for all investigated samples. Calculated data implies that as-prepared powders have crystallite size around 20 nm and thathigher energy supply in the case of thermally treated samplesled to further crystallite growth, and to the decrease of microstraindue to further crystal lattice ordering. Crystal lattice parameterswere not affected by thermal treatment and for all samples the va-lue is slightly higher in comparison to pure Y2O3 (JCPDS 41-1105,space group Ia-3, a = 10.60 Å). The increase in this value is expectedand is explained by substitution of Y3+ ions with Eu3+, bearing inmind the difference in atomic radius of these two ions (0.90 and0.95 Å, respectively) [7].

SEM analysis for Y2O3:Eu3+ system presented in Fig. 2a revealedthat spray pyrolysis led to the formation of the spherical, un-agglomerated particles. The chemical purity of the synthesizedpowders was confirmed through EDS analysis presented as an insetin SEM micrograph (Fig. 1a). The powders stereological analysis,expressed through statistical review of stereological parameters:diameter – Dmax and form perimeter – fL (Fig. 2b and Table 1) im-plied that obtained particles are quite uniform in their size andshape. Namely, based on cumulative distribution of parameterDmax it is shown that majority of the particles (74.12–92.09%) havesize in the range of 300–800 nm and that they are highly spherical(fL �0.9).

TEM analysis (Fig. 3) revealed that synthesized particles possessfilled morphology. As mentioned before, formation of dense parti-cles is, to an extent, a challenge for the applied synthesis method[3,8,9]. In our case, employed conditions (precursor solution con-centration, increased residence time and gradual increase of tem-perature within triple zone reactor) have led to the formation ofpreferred filled morphology. Presented TEM images shown that ap-plied thermal treatment increases the surface roughness due topromoted crystal growth. The particles remain un-agglomeratedand spherical in shape up to temperature of 1200 �C where inter-particle sintering occurs. Formation of sintering necks evident fromFig. 3d is reflected through stereological analysis which showedsignificant drop of fL factor for this sample (0.83).

Together, TEM and XRD analysis revealed another typical fea-ture of particles prepared through spray pyrolysis. Namely, fromTEM images in Fig. 3 it can be seen that spherical particles are com-posed of smaller primary particles. HRTEM images, Fig. 4, showthat the size of these primary entities are in agreement with thecrystallite size obtained from Rietveld refinement (Table 1) con-firming the nanostructural nature of the investigated powders. Ascan be seen from Fig. 4 and Table 1, better correlation for crystallitesize is found in the as prepared particles, while higher degree ofdiscrepancy is a characteristic of powders treated at high temper-ature. This is expectable, since the thermally promoted growth ofcrystallites is accompanied with the alteration in crystallite shape(from spherical – Fig. 4a towards polygonal – Fig. 4b), which is notrecognized by the performed Rietveld refinement. Additionally,HRTEM images indicate the presence of defects in the grain bound-ary region.

The photoluminescent investigations presented in Fig. 5ashowed typical emission spectra of Eu3+ ion incorporated in Y2O3

host lattice and are represented for the case of (Y0.95Eu0.05)2O3 sys-tems. The emission spectra were obtained at room temperaturethrough 393 nm excitation of Eu3+ ion into 5L6 absorption level.Five characteristic lines are seen and ascribed to 5D0 ?

7Fi (i = 0,1, 2, 3, 4) f –f spin forbidden transitions [18]. The red emission peak

of the highest intensity corresponds to 5D0 ?7F2 transition and is

positioned at 611 nm wavelength. Also, 5D1 ?7F2 (kem = 533 nm)

transitions are captured within these spectra. The characteristicsof emission spectra for systems with 5 and 10 at.% of Eu3+ are sum-marized in Table 2.

Sharp emission lines represent a characteristic of local environ-ment and serve as sensitive microprobes of the crystallographicsites that dopant occupies in the host. In Y2O3 a total of 75% sitesare noncentrosymmetric having C2 symmetry and the remaining25% are centrosymmetric having S6 symmetry. Owing to the pres-ence of the center of symmetry, only magnetic transitions (selec-tion rule DJ = 0, ±1, J = 0 ? J = 0 forbidden) are allowed for theEu3+ (S6) ion [19]. Fig. 5b denotes wavelengths ascribed to Eu3+

in these sites: peak at 587.1 nm belongs to 5D0 ?7F1a (C2) and at

582.1 nm to 5D0 ?7F1a (S6) transition, respectively. Similar peak

positions are reported in Eu3+ doped nanocrystalline Y2O3

[13,20]. The ratio of these two emission peaks exhibits the symme-try ratio (sr) of the C2 sublattice to the S6 sublattice. A ratio ofapproximately 3 indicates that there is no preferential occupationby Eu3+ of the C2 and S6 sites [20,21]. Here, this ratio is obtainedonly in the case of (Y0.95Eu0.05)2O3 as-prepared sample, while inall others this value is higher (�6) implying preferential occupa-tion of C2 site. The observed differences are consequence either

Fig. 4. HRTEM images of as-prepared (Y0.95Eu0.05)2O3 particles (a) and annealed at1100 �C/12 h (b).

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of applied thermal treatment or higher europium content. Namely,it is known that the ratio of the emission intensity of C2 site to theemission intensity of S6 site increases with increase of europiumconcentration in the sample due to strong energy transfer thattakes place from S6 to C2 site in the case of higher europium con-tent [21,22]. Maximum Stark splitting (DE) of the 7F1 manifold,that occurred under the influence of the crystal field, are alsomarked in Fig. 5b and values from the spectra presented in Table 2are in the good agreement with theoretical values for Y2O3:Eu3+

(DE(Y2O3) = 355 cm�1) [23].Fig. 6a represents the decay curves of the 5D0 ?

7F2 emission le-vel at 393 nm excitation wavelength (kem = 611 nm) for the case ofas-prepared and thermally treated (1000–1200 �C) (Y1.90Eu0.10)2O3

powders. The decay curves can be extrapolated with a single-expo-nential function and the obtained lifetimes for both systems((Y0.95Eu0.05)2O3 and (Y0.90Eu0.10)2O3) are presented within Table2. Based on the fluorescence decay curves of the 5D0 emitting levelit was concluded that applied synthesis method (spray pyrolysis)led to the formation of nanostructured powders having longerlifetimes in comparison to Y2O3:Eu3+ in its bulk form(s(5D0 ?

7F2)theor = 1.0 ms) [24]. The value of �1.5 ms for(Y0.95Eu0.05)2O3 sample is slightly higher than values reported inthe literature for the powders obtained through spray pyrolysis[13]. Nanocrystalline nature and dense particle morphology areprobably associated with the enhanced lifetime values. When com-paring the 5D0 lifetimes for the phosphors with 5 and 10 at.% it isevident that concentration quenching occurs for the samples with

higher doping concentration. Higher doping concentration leads tonon-radiative transfer within host lattice, hence leading to loweremission lifetimes [25]. The lower values of lifetimes for thermallytreated samples in comparison to as-prepared ones could be asso-ciated to the higher values of symmetry ratio sr (Table 2), whichcorrespond to the relative occupation of two sites since shorterlifetimes are expected for the main occupancy of C2 site (0.98 msC2 site and 7.7 ms for S6 site) [26]. Increase of the particle surfaceroughness, recognized by TEM (Fig. 3) can also be related withthe decrease of emission lifetimes.

Having in mind that better luminescence is obtained in the caseof 5 at.% doping, for these samples additionally decay kinetics from5D1 emitting level was investigated at 393 nm (kem = 533 nm). De-cay curves are presented in Fig. 6b. and represent a complex de-excitation path that could not be presented by a single-exponentialcurve. For this reason the lifetimes of Eu3+ 5D1 emitting level werecalculated (Table 2) as the average decay time, savr, based on thefollowing dependence: savr =

RtI(t)dt/

RI(t)dt. Influence of thermally

treatment can be monitored through 5D1 decay kinetics since as-prepared powders had a lower value of this parameter comparedto thermally treated ones (11 and 20 ls, respectively). This canbe prescribed to lower crystallinity and higher microstrain in as-prepared powders determined by XRD investigation. Also, decreaseof emission intensity from 5D1 emitting level can be related to theincreased probability of cross-relaxation mechanism which occurswith an increase of Eu3+ concentration in the yttria host lattice[26]. This implies that cross-relaxation effect is stronger in the case

Table 2The summarized characteristics of emission spectra for Y2O3:Eu3+ with 5 and 10 at.% Eu3+.

(Y0.95Eu0.05)2O3 (Y0.90Eu0.10)2O3

ap 1000 �C 1100 �C 1200 �C ap 1000 �C 1100 �C 1200 �C

5D0 ?7F0 (C2), nm 580.3 580.4 580.4 580.4 580.4 580.4 580.4 580.4

5D0 ?7F1a(S6), nm 582.1 582.3 582.2 582.2 582.2 582.0 582.1 582.1

5D0 ?7F1a (C2), nm 587.1 587.2 587.1 587.2 587.1 587.2 587.2 587.2

5D0 ?7F1b (C2), nm 592.9 592.8 592.8 592.9 592.8 592.8 592.8 592.8

5D0 ?7F1c (C2), nm 599.2 599.1 599.2 599.4 599.2 599.2 599.2 599.2

DE (cm�1) 344.0 338.2 344.0 346.5 344.0 341.0 341.0 341.0sr 2.99 6.70 6.76 6.68 5.39 6.45 6.40 6.31s(5D0 ?

7F2) (ms) 1.47 1.46 1.40 1.42 1.24 1.21 1.14 1.14savr(5D1 ? 7F2) (ls) 11.33 18.97 19.74 19.78 – – – –

Fig. 5. Emission spectra for (Y0.95Eu0.05)2O3 system (a) and closer view on Stark components of the 7F1 manifold of the Eu3+ and peak corresponding to Eu3+ incorporated in C2

and S6 crystallographic site of Y2O3 host lattice (b).

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of as-prepared samples. All above stated observations indirectlydepicted more homogeneous distribution of Eu3+ ions in the caseof the annealed samples [18].

4. Conclusion

Highly spherical, submicronic, dense, non-agglomerated parti-cles having polycrystalline nature and good luminescent character-istics were synthesized. For all samples doped with 5 at.% of

europium, 5D0 decay analysis yielded lifetime values in the rangeof 1.4–1.5 ms, while 5D1 lifetime values revealed more homoge-nous distribution of Eu3+ ions in thermally treated ones. Concentra-tion quenching in samples doped with 10 at.% of europium wasevident from 5D0 decay, but even in those cases obtained valuesof lifetime are higher than reported for bulk. Based on the resultspresented one can conclude that spray pyrolysis is a feasible meth-od for synthesis of cubic Y2O3:Eu3+ nanostructural powders thatpossess favorable morphological properties for applications asred phosphor in optoelectronic devices, in particular for lumines-cent displays.

Acknowledgements

The research is financially supported through the Project#142010 of the Ministry of Science and Technological Develop-ment of Serbia and COST 539 Action. The assistance in TEM charac-terization of the Electron Microscopy Center, UniversidadComplutense de Madrid, Spain is kindly acknowledged. Authorsgreatly acknowledge contribution of Branka Jordovic related topowder stereological analysis.

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Fig. 6. Fluorescence decay curves of the 5D0 ?7F2 emission following excitation at

393 nm (kem = 611 nm) (a) and 5D1 ?7F2 emission at excitation wavelength 393 nm

(kem = 533 nm) (b).

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