Optical Materials 33 (2011) 1226–1233

Contents lists available at ScienceDirect

Optical Materials

journal homepage: www.elsevier .com/locate /optmat

Determination of the local site occupancy of Eu3+ ions in ZnAl2O4

nanocrystalline powders

Da Silva, Alison Abreu, M.R. Davolos, Marian Rosaly ⇑Unesp – Univ Estadual Paulista, Instituto de Química, Luminescent Materials Laboratory – LML, 14800-900 Araraquara-SP, Brazil

a r t i c l e i n f o

Article history:Received 15 September 2010Received in revised form 25 January 2011Accepted 13 February 2011Available online 23 March 2011

Keywords:LuminescenceSpinel structurePhosphorsSpectroscopic probes

0925-3467/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.optmat.2011.02.014

⇑ Corresponding author. Tel.: +55 16 3301 6634; faE-mail address: davolos@iq.unesp.br (M. Rosaly).

a b s t r a c t

Once lanthanides-doped ZnAl2O4 have attracted attention for highly efficient phosphors and due to thecomplexity of this system, this work is focused in the understanding of the local site occupancy of thedoped ions in the spinel structure by using Eu3+ as a spectroscopic probe. Europium(III)-doped ZnAl2O4

nanocrystalline powder samples were prepared by the Pechini method. Different heat treatment temper-atures and doping levels were investigated. No impurities of residual Eu2O3 were observed for the sam-ples with Eu3+ doping levels up to 10 at.%, indicating that the doping ions are diluted into the host. Theluminescence spectroscopy from the Eu3+ ions revealed the Eu3+ ions might occupy at least two non-cen-tro-symmetric sites and that the occupation ratio might be dependent on the heat treatment or dopinglevel. It is observed that one site is related to a high covalent environment while at the other the ioniccharacter prevails. This behavior is in agreement with the luminescence lifetime results. The decay curveswere fitted according to double first-order decay model and it was confirmed the covalence differencebetween the two sites and also the population variation with the doping level. There are strong evidencesthat the europium ions substitute for the aluminum ones in the normal spinel structure. It cannot be dis-regarded that the dopant ions may be present on the surface of the particles.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

The development of advanced flat-panel displays and lightingtechnology such as field-emission displays (FEDs) and plasma dis-play panels (PDPs) requires phosphor materials endowed with ahighly efficient emission at low excitation voltages, high resistanceto current saturation, high chemical and thermal stability alongwith a long lifetime at high current densities [1]. Semiconductorphosphors doped with rare earth ions have been exhaustivelystudied in the last years. These materials show interesting opticalproperties, with potential applications in the design of optoelec-tronic materials and as efficient phosphors for flat-panel displays[2]. Among the semiconductors, metal oxide phosphors have at-tracted attention for FEDs and PDP applications especially due tothe higher chemical stability compared to the conventional sulfidephosphors (ZnS:Cu, ZnS:Ag and Y2O2S:Eu) [3,4].

Recently, zinc aluminate (ZnAl2O4) has attracted interest as animportant phosphor host for lanthanide ions. ZnAl2O4 lends itselfa wealth of applications, ranging from catalytic and ceramicmaterials to ultraviolet photoelectronic devices [5–7]. Occurringin nature as the mineral gahnite, it is a member of the zinc-based

ll rights reserved.

x: +55 16 3301 6692.

spinels, which have wide band gaps and the general formula AB2O4

[8]. In this formula A represents Zn2+ ions and B may be Al3+. Zincaluminate has the normal spinel crystal structure with the spatialgroup Fd3m in which Zn2+ ions occupy tetrahedral sites (ionicradius = 74 pm) and Al3+ ions the octahedral ones (ionicradius = 67 pm). ZnAl2O4 is endowed with chemical and structuralstability and could be highly efficient on light emission whendoped with lanthanide ions (Ln). Many researchers have reportedon luminescent studies of zinc aluminate containing Ln activatorsand it is clear the complexity of this system [2]. It is difficult todetermine if the Ln will replace the zinc ions at the tetrahedralsites or the aluminum ions at the octahedral ones. On one handthe ionic radii of Ln ions are more similar to Zn2+ radius and enableits substitution at the host matrix. On another hand the coordina-tion number of Al3+ in the octahedral sites can induce a betteraccommodation and indicates that it might be replaced by Ln.Moreover, it is possible that the Ln ions can be localized on the sur-face of the particles mainly if nanoparticles are involved. Due to thehigh surface area it may be thermodynamically favorable occupysites onto the nanoparticle surface instead into the bulk.

The valence 4f electrons of trivalent lanthanide ions, Ln(III), arewell shielded from the environment by the outer core 5s and 5pelectrons and are minimally involved in bonding. Because of thisshielding, the atomic properties of these ions are typicallyretained after complexation. The absorption and emission spectra

D. Silva et al. / Optical Materials 33 (2011) 1226–1233 1227

of Ln(III) ions consist of sharp, narrow bands corresponding to thef–f transitions of the metal ion. The number of bands depends onthe particular lanthanide ion and its arrangement of electronicstates. The europium ion is the most studied between the Ln(III)mainly due to the non-degenerated levels involved in electronictransitions. The Eu(III) ion is therefore a quite sensitive lumines-cent probe responding to any changes in its neighboring and thisproperty plays an important hole in the materials science.

Besides the traditional solid-state route [8,9], a myriad of meth-ods have recently been applied to the preparation of ZnAl2O4, suchas the hydrothermal synthesis [10–12] the sol–gel route [13–16]and combustion synthesis [17].

When the objective is to obtain doped materials, the bestchoices for preparing are the solution methods. These ones facili-tate the mixing at a molecular level improving the diluted condi-tion of the doped ion into the host. The solution methods thatare dependent on precursors precipitations may not produce thecorrect doping level, once some doping species could remain insolution due to differences in the solubility product. The Pechinimethod is an example of solution method in which its precursoris heat treated (decomposed) to give rise to nanostructured mate-rials with excellent stoichiometric maintenance.

This work presents structural and spectroscopic studies of Eu3+-doped ZnAl2O4 prepared by the Pechini method in order to deter-mine the site occupancy of rare earth ions into the spinel crystalstructure.

2. Experimental

Nitrate solutions of zinc, aluminum, and europium were used asthe metal sources. Citric acid and monoethylene glycol were addedto the metal nitrate solution in deionized water. The Pechini solu-tion obeys the molar ratio 1:3:16 of metals ions: citric acid: mono-ethylene glycol. The pH of the resulting solution was adjusted at5.0 by using ammonium hydroxide. A clear solution was achievedafter heating the solution around 100 �C with a significant increasein its viscosity until the formation of a resin. The resin was calcinedin air at 700, 900 or 1100 �C for 4 h. Samples doped with differentatomic levels of Eu3+ were prepared. The powder samples werecharacterized by X-ray diffraction, XRD, using Rigaku� RINT2000diffractometer (42 kV � 120 mA) with Cu Ka radiation monochro-matized by a curves graphite crystal (kka1 = 1.5405 Å,kka2 = 1.5443 Å, Ika1/Ika2 = 0.5), 2h range between 20� and 80�, stepsize of 0.02�(2h), 10s per point, divergence slit = 0.5 mm, receivingslit = 0.3 mm. Luminescence spectra and the lifetime measure-ments were carried out in a Fluorolog SPEX F212I fluorescencespectrophotometer equipped with an R928 Hamamatsu photomul-tiplier. The morphological properties of the synthesized samples atdifferent temperatures and containing different doping levels wereinvestigated by transmission electron microscopy (TEM), selectedarea electron diffraction (SAED) and also by energy dispersiveX-ray (EDS).

Fig. 1. (a) X-ray powder diffraction patterns of ZnAl2O4 samples with different Eu3+

doping levels, prepared by the Pechini method at 900 �C and (b) FWHM (0 0 1)dependence on the percentage of Eu3+.

3. Results and discussion

Once the octahedral sites are more favorable to be occupied bythe Eu3+ ions, the samples were prepared assuming that thelanthanide ions would substitute for the aluminum ions. In theZnAl2O4 normal spinel structure the Zn2+ ions are located intetrahedral sites while the Al3+ ions may occupy only the octahe-dral ones. Despite the similar ionic radii of Zn2+ and Eu3+, thelanthanide ions hardly accept small coordination numbers, sincethe excess charge would not be easily stabilized by the crystal field.On the another hand the Eu3+ substitution for aluminum ions ismore favorable in view of the coordination number of octahedral

sites and the charge stabilization even when considering the largedifference in their ionic radii.

The powder X-ray diffraction patterns of pristine ZnAl2O4 andEu-doped ZnAl2O4 prepared by the Pechini method and heat trea-ted at 900 �C are shown in Fig. 1.

Single-phase zinc aluminate with the spinel crystal structurewas observed (JCPDS 82-1036) for all samples, along with no impu-rities from any residual ZnO, Al2O3 or Eu2O3, as usually observed byother preparation methods [17,18].

Only the ZnAl2O4 crystalline phase reflections are present, withno dependence on the Eu3+ content (up to 10 at.%). This result indi-cates that the Eu3+ ions may be diluted into the host.

Fig. 2. TEM images of ZnAl2O4 powder samples with: (a) 1% of Eu3+, (b) 5% of Eu3+

and (c) 10% of Eu3+ prepared by the Pechini method at 900 �C.

1228 D. Silva et al. / Optical Materials 33 (2011) 1226–1233

Another interesting observation is related to the profiles fromthe X-ray powder diffraction patterns. The reflections are enlargedas the europium concentration increases, while the signal to noiseratio (or resolution) seems to decrease. Such loss in resolutionmight be related to a smaller degree of crystallinity or to a smallerparticle size. The full width at a half maximum (FWHM) from the(0 0 1) reflection was calculated in order to address this issue.Fig. 1b shows the dependence of FWHM on the Eu3+ content.

As shown in Fig. 1b, the FWHM values present an exponential-like growth behavior up to 7 at.% of Eu3+ doping, indicating a de-crease in microcrystallite size. At high lanthanide concentration,the FWHM value seems to be almost constant and so the micro-crystallite size might not change appreciable. The lanthanide con-centration effects on the microcrystallite size indicate that theeuropium ions are probably acting on the surface of the particles,somehow hindering grain growth.

In order to detect and address the Eu3+ insertion effects on theparticle size, some samples were analyzed by TEM. Fig. 2 shows theTEM images from the ZnAl2O4 powder samples containing 1, 5 and10% at. of europium. The SAED pattern is shown in Fig. 2 as aninset.

By analyzing the SAED patterns it is possible to observe that allsamples present high crystallinity independent on the europiumcontent. This result confirms that the X-ray diffraction peakenlargement is not due to a loss in crystallinity.

The particle size might decrease upon increasing the lanthanidedoping level. For 1% at. of doping, the particle diameter is around35 nm while with 5% at. the size decreases to about 15 nm. At10% at. of europium the particle diameter is around 10 nm. Theseresults are in agreement with previous discussions from the pow-der XRD analysis and provide additional evidence that the lantha-nide doping in ZnAl2O4 has direct effects on the particle surfacehindering grain growth and promoting the agglomeration of theparticles. Further EDS measurements (not shown here) were car-ried out for these three samples in order to analyze different parti-cles or regions. Signals of O, Zn, Al and Eu were detected in all thesemeasurements, which indicates the rare earth ions are diluted ontothe ZnAl2O4 surface in a homogeneous distribution.

The observed lanthanide effects on the particles surface is inagreement with some authors that refers to the term ‘‘surface dop-ing’’ when nanoparticles are involved. Even if was tried to include avery high concentration of dopant into such a nanoparticle, the lo-cal energy of the dopant is larger than in a larger chunk of thesemiconductor. It may be thermodynamically unfavorable for thedopant to remain as a dopant in the nanoparticle and it may be ex-cluded from the semiconductor ‘bulk’ onto the particle surface[19]. For example, Mn can be incorporated into ZnS nanocrystalsgiving the characteristic orange luminescence of Mn–doped ZnS.However, if the Mn is surface-adsorbed on the ZnS nanocrystal sur-face, the luminescence shifts to the ultraviolet [20]. Recently, theability to dope nanocrystals has been linked with the ability ofthe growing nanocrystal surface to adsorb the dopant species [21].

Another useful tool to study the lanthanide occupancy is theluminescence spectroscopy. In light of the non-degenerated levelsinvolved in the europium transitions, the excitation and emissionspectra may be used to unveil the ion neighboring. The room-tem-perature photoluminescence excitation and emission spectra of thenanostructured spinels doped with 1% of Eu3+ are shown in Fig. 3.

In the ZnAl2O4:Eu3+ emission spectrum, the set of bands peakingat 570–720 nm is assigned to the characteristic 4f ? 4f sub-shelltransitions from 5D0 ?

7FJ (J = 0–4) of Eu3+, as detailed in Fig. 3.Among the emissions, the bands around 590 nm (5D0 ?

7F1) areassociated to magnetic dipole transitions and hardly change withthe crystal field strength around the ion. The band at 615 nm(5D0 ?

7F2) is related to an electric dipole transition within the 4fshell highly sensitive to structural changes and environmental

effects in the vicinity of the Eu3+ ion. The luminescence intensityratio of 5D0 ?

7F2 to 5D0 ?7F1 transitions, usually known as the

asymmetry ratio, gives a measure of the degree of distortion atthe local environment of the Eu3+ ion in the matrix. Therefore, alarge asymmetry ratio value indicates strong electric fields of lowsymmetry at the Eu3+ ions. In addition, the existence of the5D0 ?

7F0 transition located around 579 nm is also an indicationof very asymmetrical sites. In an ideal normal spinel, the Zn2+ cat-ions occupy tetrahedral sites (Td symmetry), whereas the Al3+ cat-ions sites are octahedral (Oh symmetry). However, in real spinelsamples, particularly these nanostructures, a somewhat disordereddistribution of the cations among these sites exists. Thus the crystallattice is rather imperfect. On the other hand, the substitution ofEu3+ into the Zn2+ or Al3+ sites may results in lattice distortiondue to the significant difference in the ionic radii between them.

Fig. 3. Excitation and emission spectra, measured at room temperature, of 1% Eu-doped ZnAl2O4 samples prepared by the Pechini method at 900 �C.

Fig. 4. Excitation spectra (kem = 619 nm), measured at room temperature, of theZnAl2O4:Eu3+ samples prepared by the Pechini method at 900 �C with different REdoping levels.

D. Silva et al. / Optical Materials 33 (2011) 1226–1233 1229

In addition, Eu3+ ions can be distributed on the surface of thenanoparticles (as evidenced by XRD and TEM analysis in this work)due to the porosity of the spinel structure. As a result, Eu-centers inZnAl2O4 spinel are complex, and the electric dipole transitions willbecome allowed due to the more disordered surroundings and low-er symmetry of the cation location. The full width at half a maxi-mum (FWHM) of the 5D0 ?

7F2 transition line reaches �15 nm. Ingeneral, the transition is composed of several sub bands, split bycrystal field perturbations. The strongly inhomogeneous broaden-ing and the fine structure obscured in the emission band providefurther evidence for the formation of multi-site distribution dueto the presence of Eu3+ ions in rather disordered surroundings.

The excitation spectrum of ZnAl2O4:Eu3+ consists of some sharplines in the range 350–500 nm, corresponding to the characteristicexcitation bands of Eu3+ from the direct transitions within the 4f6

shell (Fig. 3) and a broad band in the ultraviolet region around260 nm. The broad band arises from the charge-transfer transition(O2�? Eu3+) from surrounding anions to Eu3+.

It is noteworthy to mention that the Eu3+concentration has di-rect effects on the excitation and emission spectra. Fig. 4 showsthe excitation spectra of ZnAl2O4 containing different Eu3+ dopinglevels.

A dependence of the ratio between the charge transfer band(O2�? Eu3+) and the f–f transitions with europium concentrationis observed in Fig. 4. The intensity of the f–f transitions becomeshigher than the intensity of the broad band as the doping levelincreases, which is a characteristic behavior of increasing cova-lence character in the system, once the shell mixing promotes arelaxation of the Laporte selection rule.

In the emission spectra it is also possible to observe somedifferences in the PL profiles as the europium concentration in-creases. The emission spectra of ZnAl2O4:Eu3+ samples under dif-ferent doping levels are shown in Fig. 5a.

While at lower concentrations the emission peaks are enlargeddue to the disorder at the ion environment, under high dopinglevels (as 10% at.) the PL spectrum profile is a little bit differentwhen compared to the others. The emission lines are narrower,characterizing ordered site occupation or sites with a definedsymmetry. Furthermore, it can also be observed some Eu3+

concentration dependence in the 5D0 ?7F0 transitions located

within the 570–585 nm (Fig. 5b). As such transitions occur be-tween non-degenerated levels, it is well known that each signal as-signed to a particular 5D0 ?

7F0 transition may be understood as adifferent occupied site without an inversion center. Therefore, it ispossible to state that in the normal ZnAl2O4 spinel prepared in thiswork the Eu3+ ions are occupying two different sites, designatedhere as A and B (the observed maximum peaks are 576.0 and579.5 nm, respectively).

Once the signal intensity is proportional to the site population,it is possible to observe that the site A prevails over B by increasingthe europium content. In Fig. 5b it is possible to see that under 0.1%at. of doping the site A is not visible and at 10% at. its populationseems to be close to that from the B site.

In order to separate the contribution from A and B sites, the PLexcitation spectra were recorded by fixing the emission wave-length at special positions (excitation spectrum under selectiveemission). The kem were fixed at 576.0 and 579.5 nm to differenti-ate the contributions from each site (A or B). Fig. 6 shows thespectra.

The main difference between the two excitation spectra is re-lated to the band position of the charge-transfer. For the spectrumwith the fixed emission wavelength at 576.0 nm (related to the siteA), the broad excitation band is shifted to a lower energy position.This indicates that the site A is endowed with a more covalentenvironment than site B. Based on this behavior, it is possible toconclude that the increase in covalence with europium contentattributed on the Fig. 4 results is an effect of population growthfrom A site.

Notwithstanding, it is possible to obtain the emission spectrawith a higher contribution from site A or B by selecting the excita-tion wavelength. In Fig. 7 it is possible to see the emission spectraof ZnAl2O4:Eu3+ under different kex.

The profile of the emission spectra and the emission intensity ofeach site at the 5D0 ?

7F0 transitions are dependent on the excita-tion wavelength. When site A prevails (fixing the excitation at296 nm), the spectrum is composed of narrow emission lines(Fig. 7a). This behavior indicates ordered site occupancy, whilefor site B (fixing the excitation at 394 nm) the major contribution

Fig. 5. (a) PL emission spectra (kex = 260 nm), measured at room temperature, ofthe ZnAl2O4:Eu3+ powder samples prepared by the Pechini method at 900 �C withdifferent doping levels and (b) zoomed region associated to the 5D0 ?

7F0

transitions.

Fig. 6. PL excitation spectra, measured at room temperature, of the ZnAl2O4:Eu3+

0.1% samples prepared by the Pechini method at 900 �C. Both excitation spectrawere measured by fixing the excitation wavelength at the different 5D0 ?

7F0.

1230 D. Silva et al. / Optical Materials 33 (2011) 1226–1233

from the emission lines is enlarged due to the disorderedenvironment.

Fig. 7b shows that it is very difficult to completely separate thecontribution from each site. However, it is possible to concludethat the site A is endowed with an ordered and covalent environ-ment, while the site B the ionic character prevails with a multi-sitedistribution due to disordered surroundings.

As mentioned earlier, it is expected that the europium ionsmust substitute for aluminum ions at the ZnAl2O4 particles surface.

In a previous work [22] we studied the aluminum octahedral sitesin zinc aluminate by means of 27Al solid-state MAS NMR and FT-IRspectroscopy. It was possible to conclude the occurrence of twodistinct AlO6 sites with different symmetries and their ratio wereassociated to some changes arising from heat treatment tempera-ture. One site presented a higher symmetry while the other onewas characterized by the lower environment symmetry. In thesame work it was evidenced that the ordered site prevailed at hightemperatures or the disordered ones would be reorganized withthe heat treatment.

In the present work two different crystallographic sites for Eu3+

were found. One of them is under ordered environment conditions.This behavior is near to the aluminum site occupancy characteriza-tion and indicates a Eu3+ substitution for Al3+. Furthermore, asshown in Fig. 8, the europium population sites are dependent onthe heat treatment as well and the site A (the ordered one) prevailsat high temperatures or the site B reorganizes to achieve an or-dered arrangement.

Fig. 8a and b shows the emission spectra of europium-dopedzinc aluminate prepared at different heat treatment temperatures.The doping level was fixed to study the heat treatment effects.

When the powder sample is prepared at 1100 �C, the spectrumprofile is composed of narrow lines, in excellent agreement withthe characteristics of site A. For the samples prepared at 700 or900 �C no considerable changes are observed in the ratio betweenthe A and B sites. This result is in good agreement to that observedfor Al3+ site characterization [21]. Furthermore, higher the temper-ature of treatment higher is the probability to the europium ions todiffuse from the surface into the particles bulk. In this way, the siteB can be considered as the surface site occupation of Eu3+ ions andsite A as the ordered one into the bulk. On one hand, the fact of 700or 900 �C of heat treatment not provide enough energy to diffusethe doping ions to particles bulk is a good explanation for the samepopulation results observed in the spectra of the Fig. 8b. Onanother hand when the heat treatment is achieved under 1100 �C

Fig. 7. (a) Emission spectra, measured at room temperature, of the ZnAl2O4:Eu3+ 5%sample prepared by the Pechini method at 900 �C and (b) Emission spectrameasured at 77 K under different excitation wavelengths for the 5D0 ?

7F0

transition.Fig. 8. (a) Emission spectra (kex = 260 nm), measured at room temperature, of theZnAl2O4:Eu3+ 5% sample prepared by the Pechini method at different heat treatmenttemperatures and (b) zoomed region corresponding to the 5D0 ?

7F0 transition.

D. Silva et al. / Optical Materials 33 (2011) 1226–1233 1231

the diffusion process might occur and site A population willprevails once the cations will migrates to the particle bulk.

Wiglusz et al. [23] tried to understand Eu3+ occupancy inMgAl2O4 by means on the Judd–Ofelt parameters calculation.These parameters contain contributions from the forced electric di-pole and dynamic coupling mechanism giving rise to importantinformations about the ion neighboring. They obtained highJudd–Ofelt parameters values, which indicates that in nanocrystal-line hosts a high fraction of the Eu3+ ions is located on the surfaceof the particles where the influence of crystal field is different from

sites inside nanocrystals. High value of the X2 parameter indicateshighest hypersensitive behavior of the 5D0 ?

7F2 transitions. Sam-ples doped with different amounts of Eu3+ ions showed significantdifferences in luminescence intensities and thus, also in Judd–Ofeltparameters. Although the spinel structure is formed, the heattemperature affect the embedding of the Eu3+ ions inside of thecrystallites, and consequently, the fraction of Eu3+ on the surface.

Fig. 9. Emission decay curves, measured at room temperature, of zinc aluminatesamples prepared by the Pechini method at 900 �C with different Eu3+ doping levels.

Table 1Decay parameters calculated from the curves in Fig. 9 of the ZnAl2O4 samples dopedwith Eu3+ ions at different doping levels.

% of Eu3+ PA sA (ms) PB sB (ms) R2

1 0.30 0.48 0.72 1.72 0.999825 0.47 0.43 0.61 1.24 0.99987

10 0.70 0.34 0.38 1.08 0.99985

1232 D. Silva et al. / Optical Materials 33 (2011) 1226–1233

Also, they observed that the highest Eu3+ concentrations increasethe fraction of the ions in non-centro-symmetric sites.

Another interesting parameter to be studied is the lifetime. Thisvalue is calculated from the emission decay curve, and it is depen-dent on the activator concentration, temperature and the nature ofthe transitions.

The lifetime measurements and emission decay were recordedfor the ZnAl2O4:Eu3+ samples at different dopant concentrations.The excitation wavelength was fixed at 260 nm (the charge-trans-fer transition O2�? Eu3+) while the emission was maintained at619 nm (5D0 ?

7F2 transitions). The decay curves are shown inFig. 9.

An exponential decay was observed for all samples. The lifetime‘‘s‘‘ may be estimated from the time associated to the decrease of2/3 from the initial intensity A decrease in this parameter under

Fig. 10. Linearized emission decay curves of the zinc aluminate samples preparedby the Pechini method at 900 �C with different Eu3+ doping levels.

increasing doping level is observed. At high doping levels thenon-radiative decay between the activator centers is more likelyto take place due to the smaller distance among them. To deter-mine more exactly the lifetime, the decay curves have to be linear-ized and so the slope of each curve is used to calculate s. Thelinearized decay curves are shown in Fig. 10.

The curves in Fig. 10 are not straight lines, which indicate thatthe system does not follow a simple exponential decay. This behav-ior precludes the calculation of lifetime from the slope of thecurves. Each curve profile shows at least two different decays.One at short lifetime, up to 4 ms, and the other higher than 4 ms.These two contributions are related to the two different Eu3+ sites.

Furthermore, it is possible to observe that the decay at shortlifetimes is more pronounced under increasing the europium con-tent. As shown before, the site A population is higher than the siteB at high doping levels and so the short lifetime contribution mightbe due to the site A. Besides, the site A lifetime is expected to beshorter than the site B due to its covalent character, as evidencedby the excitation spectra.

An interesting approach could be used to determine the lifetimefor each site. However, it is important to assume that there is nointeraction between them in spite of the high dopant concentra-tion used in this work. Therefore, the system could be treated asa double-exponential first-order decay. In this way the decay curvewill be considered as the sum of two exponential curves related toeach site and so the decay curves may be fitted according to thefollowing equation:

I ¼ I0 þ PA expð�T=sAÞ þ PB expð�T=sBÞ

where sA and sB represent the emission lifetime of site A and B,respectively. The parameters PA and PB are the initial emissionintensity of each site, and so the decay curve intensity at 0 ms isthe sum of PA and PB. In this way, if the curve is normalized tothe maximum intensity of 1, the values PA and PB will be propor-tional to the population of each site.

Table 1 shows the parameters (PA, PB, sA, sB and R2) obtained forthe Eu-doped zinc aluminate samples by fitting the curves in Fig. 9according to the double-exponential model.

The results in Table 1 are very interesting. sA is smaller than sB

due to the difference in covalence character between the sites. Asexpected, for both sites the lifetime decreases with the increasingdoping level due to the non-radiative transitions.

Moreover, the PA and PB values are in good agreement to theemission profiles discussed previously, indicating that the site Apopulation increases under increasing the Eu3+ content. The sumof PA and PB might differ from 1 at high dopant concentrationdue to some interaction between the activator centers.

4. Conclusions

ZnAl2O4 powder samples containing Eu3+ ions were successfullyprepared by the Pechini method. No impurities of any residualEu2O3 could be observed for the samples with doping levels upto 10 at.%. The Eu3+ ions seemed to be diluted into the host anddirect effects on the particle surface were observed. The lumines-cence spectroscopy of Eu3+ revealed it may occupy at least twonon-centro-symmetric sites and that their ratio might be

D. Silva et al. / Optical Materials 33 (2011) 1226–1233 1233

dependent on the heat treatment and doping level. It was observedthat one site is related to a high covalence environment while atthe other the ionic character prevails. There are strong evidencesthat the europium ions substitute for the aluminum ones in thenormal spinel structure. It cannot be disregarded that the dopantions may be present on the surface of the particles in a homoge-neous condition and the temperature of treatment and the dopinglevel can promote the dopant diffusion to ordered sites into theparticles bulk.

In order to study the lifetime features of each site, the systemwas fitted according to a double-exponential first-order decay. Thistreatment confirmed the covalence difference between the A and Bsites and also the population variation with the doping level.

Acknowledgements

The authors are indebted to CNPq and FAPESP for the financialsupport and acknowledge Dr. Agnaldo de Souza Gonçalves forrevising the manuscript. AAS thanks FAPESP for the scholarship.

References

[1] Z. Lou, J. Hao, Thin Solid Films. 450 (2004) 334–340.

[2] W. Strek, P. Deren, A. Bednarkiewicz, M. Zawadzki, J. Wrzyszcz, J. AlloysCompd. 300–301 (2000) 456–458.

[3] Z. Xu, Y. Li, Z. Liu, Z. Xiong, Mater. Sci. Eng. B 110 (2004) 302–306.[4] J. Bang, M. Abloudi, B. Abrams, P.H. Holloway, J. Lumin. 106 (2004) 177–185.[5] Y. Wu, J. Du, K.L. Choy, L.L. Hench, J. Guo, Thin Solid Films. 472 (2005) 150–156.[6] H. Grabowska, M. Zawadzki, L. Syper, Appl. Catal. A 314 (2006) 226–232.[7] L. Chen, X. Sun, Y. Liu, K. Zhou, Y. Li, J. Alloys. Compd. 376 (2004) 257–261.[8] S.K. Sampath, J.F. Cordaro, J. Am. Ceram. Soc. 81 (1998) 649–654.[9] H. Matsui, C.N. Xu, H. Tateyama, Appl. Phys. Lett. 78 (2001) 1068–1070.

[10] M. Zawadzki, Solid State Sci. 8 (2006) 14–18.[11] C.C. Yang, S.Y. Chen, S.Y. Cheng, Powder Technol. 148 (2004) 3–6.[12] X.Y. Chen, C. Ma, Opt. Mater. 32 (2010) 415–421.[13] X. Duan, D. Yuan, X. Wang, H. Xu, J. Sol–Gel Sci. Technol. 35 (2005) 221–224.[14] X. Duan, D. Yuan, Z. Sun, C. Luan, D. Pan, D. Xu, M. Lv, J. Alloys Compd. 386

(2005) 311–314.[15] V. Singh, V. Natarajan, J. Zhu, Opt. Mater. 29 (2007) 1447–1451.[16] A.A. Da Silva, A.S. Gonçalves, M.R. Davolos, J. Sol–Gel Sci. Technol. 49 (2009)

101–105.[17] X. Wei, D. Chen, Mater. Lett. 60 (2006) 823–827.[18] Y. Ivakin, M. Danchevskaya, O. Ovchinnikova, G. Muravieva, J. Mater. Sci. 41

(2006) 1377–1383.[19] G. Hodes, Adv. Mater. 19 (2007) 639–655.[20] K. Sooklal, B.S. Cullum, S.M. Angel, C.J. Murphy, J. Phys. Chem. 100 (1996)

4551–4555.[21] S.C. Erwin, L. Zu, M.I. Haftel, A.L. Efros, T.A. Kennedy, D.J. Norris, Nature 436

(2005) 91-x.[22] A.A. Da Silva, A.S. Gonçalves, M.R. Davolos, J. Nanosci. Nanotechnol. 8 (2008)

5690–5695.[23] R.J. Wiglusz, T. Grzyb, S. Lis, W. Strek, J. Lumin. 130 (2010) 434–441.