Tunable Luminescence of Sr2CeO4:M2+ (M = Ca, Mg, Ba, Zn) and

Sr2CeO4:Ln3+ (Ln = Eu, Dy, Tm) Nanophosphors

Tomasz Grzyb,† Agata Szczeszak,† Justyna Rozowska,† Janina Legendziewicz,‡ and Stefan Lis*,†

†Department of Rare Earths, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland‡Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland

*S Supporting Information

ABSTRACT: Nanophosphors based on cerium−strontium oxide(Sr2CeO4), doped with M2+ or Ln3+ (M = Ca, Mg, Ba, Zn; Ln = Eu,Dy, Tm) were successfully prepared using a modified Pechini method. Thestructure of prepared materials has been confirmed and characterized usingseveral techniques such as X-ray powder diffraction (XRD), scanning andtransmission electron microscopy (SEM and TEM). The obtained materialswere composed of nanocrystals with an average size around ∼50 nm. Theluminescence properties of synthesized nanophosphors were charac-terized by excitation or emission spectra and luminescence lifetimes. Thechromaticity coordinates were also calculated to demonstrate changes of theluminescence color.

■ INTRODUCTIONOver the past few years, much attention has been applied to thesynthesis and spectroscopic properties of luminescent materialscontaining rare earth ions. Their exceptional electronic andoptical properties result from the properties of the 4f shell ofthese ions.1−6 Production of the various red or green phosphorshas been considerably developed, but there is still a lack of com-parable efficient blue emitting materials. This deficiency existsbecause blue-light-emitting materials degrade rapidly when ex-posed to critical conditions, especially under high electric field.One of the ways of solving this problem is choosing inorganicmaterials without complex structure and complicated sub-stitutions. In particular, oxides have the required properties andhigh thermal and chemical stability.7,8

Cerium−strontium oxide is a relatively novel inorganic oxidecompound, discovered by Earl Danielson and his co-workers in1998.9,10 This material crystallizes in an orthorhombic crystalsystem with the space group Pbam (No. 55) and cell param-eters: a = 6.11897(9) Å, b = 10.3495(2) Å, and c = 3.5970(1) Å.The structure of this oxide exhibits strong anisotropy, whichis governed by a set of one-dimensional chains of trans edge-sharing CeO6 octahedrons linked by interchain Sr2+ ions.9,10

Sr2CeO4 is a very promising blue-white-emitting oxide-basedphosphor characterized by its high thermal and chemical stab-ility and excellent optical properties.11 Furthermore, Sr2CeO4 isan efficient vacuum ultraviolet excited phosphor. Uncom-plicated synthesis and the possibility of modifications makeSr2CeO4 an interesting alternative for other blue-emitting phos-phors, such as those of BAM type (BaMgAl10O17:Eu

2+), whichare known to be unsatisfactory as they lose radiation efficiencyand color, which shift during thermal treatment.12 The blue-white emission of Sr2CeO4 is assigned to O2‑−Ce4+ charge

transfer (CT). It displays a broad excitation band in the UVrange, which enables the possibility of tunable excitation. Thiscompound can be also a host for the emitting Ln3+ ions.13

Moreover, the broad absorption band in the range of 200−400 nm indicates the possibility of using Sr2CeO4, for example,in ultraviolet light-emitting diodes (UV-LEDs).Sr2CeO4 phosphor can be obtained using different methods

such as solid-state reaction, hydrothermal method, combustionprocess, and microwave-assisted synthesis.13−15 Cerium−strontium oxide is still under investigation due to its potentialapplications and improvements of its luminescent properties.This compound belongs to the group of materials whose lumi-nescent properties also depend on the dopants. An appropriateluminescence activator, chosen from a series of Ln3+ ions, cantune the emission of a Sr2CeO4 matrix through the energytransfer (ET) from the host to the Ln3+ ion.16 Therefore, in thispaper we report the results of investigations under successfullysynthesized Sr2CeO4 doped with a series of ions (Eu3+, Dy3+,Tm3+; Ca2+, Mg2+, Ba2+, Zn2+) by a modified Pechini method.

■ EXPERIMENTAL SECTIONTo prepare Sr2CeO4:M

2+ or Sr2CeO4:Ln3+, solutions of

Sr(NO3)2 (POCh, p.a.) and CeCl3·6H2O (ROTH, 99.9%)were used as starting materials (1 M). Their stoichiometricamounts, taking into account the amount of dopants, replacingSr2+ ions in the structure, were dissolved in distilled water. Forpreparation of different ion-doped samples, certain volumes ofsolution containing one of the following salts (1 M): Ba(NO3)2

Received: August 19, 2011Revised: January 7, 2012Published: January 8, 2012

Article

pubs.acs.org/JPCC

© 2012 American Chemical Society 3219 dx.doi.org/10.1021/jp208015z | J. Phys. Chem. C 2012, 116, 3219−3226

(Aldrich, 99+ %), Ca(NO3)2·4H2O (POCh, p.a.), Mg-(NO3)2·6H2O (POCh, p.a.), Zn(NO3)2·6H2O (POCh, p.a.),Dy(NO3)3, Eu(NO3)3, and Tm(NO3)3 were added. Aque-ous solutions of lanthanides ions, Ln3+, used in the synthesis,were prepared from their oxides Dy2O3 (Stanford Materials,99.99%), Eu2O3 (Stanford Materials, 99.99%), Tm2O3(Stanford Materials, 99.99%), which were dissolved in distilledwater with an appropriate amount of nitric acid, HNO3 (POCh,ultrapure). The excess amount of acid was removed by evapo-rating the obtained solutions several times.To the solutions mixed in stoichiometric proportions, citric

acid monohydrate (CA; CHEMPUR, p.a.) and ethylene glycol(EG; CHEMPUR, p.a.) were added as chelating and polymer-izing agents, respectively (e.g., for pure Sr2CeO4, reactants weremixed in molar proportion 2Sr2+:1Ce3+:24CA:14EG, or for the5% Eu3+ doped sample: 1.9Sr2+:1Ce3+:0.1Eu3+:24CA:14EG).The dopant concentrations in all samples were varied withinthe 0.5−5 wt % range. The final mixture was heated and stirredon an electric stove for about 30 min. Afterward, solutions wereconcentrated by slow evaporation (about 8 h) in a drying oven,hence transparent brown or yellow gels were obtained. Pre-cursors were fired in a range from 900 to 1100 °C for 3 h in amuffle furnace in an air atmosphere. Obtained compounds wereground with an agate mortar and pestle into fine powders.The X-ray powder diffraction (XRD) pattern measurements

were carried out with the use of a Bruker AXS D8 Advancediffractometer with Cu Kα1 (λ = 1.541874 Å) radiation in the2θ range from 10° to 60°. Cell parameters were estimated usingRietveld analysis with the help of Maud 2.0 software.17,18

The transmission (TEM) and scanning (SEM) electronmicrographs of the selected specimens were taken using a JEM1200 EXII transmission electron microscope (acceleratingvoltage of 80 kV) and a CAIS EVO 40 scanning electron micro-scope (accelerating voltage of 19−20 kV), respectively.The excitation and emission spectra of all samples were

recorded on a HITACHI F-7000 fluorescence spectrophotom-eter equipped with a 150 W xenon lamp as the excitationsource. Excitation and emission spectra were corrected for theinstrumental response.The luminescence lifetimes of obtained nanopowders were

measured using a detection system consisting of a nitrogen laser(constructed at the Faculty of Chemistry, Adam MickiewiczUniversity, in Poznan, Poland), a tunable dye laser, and an M12FCV 51 photomultiplier. The luminescence decay curves wererecorded on a Tektronix TDS 2022B oscilloscope. All of themeasurements were performed at room temperature.

■ RESULTS AND DISCUSSIONSr2CeO4 phosphor was doped by selected M2+ ions in orderto modify the structural properties and, as a consequence,the luminescence characteristics through replacing Sr2+ ions byZn2+, Ba2+, Ca2+, or Mg2+ ions in the structure. This operationchanged the size of crystallographic cell and the characteristicsof the CeO6 octahedrons.The XRD patterns of the prepared Sr2CeO4 doped with 2%

Zn2+, Ba2+, Ca2+, and Mg2+ ions and calcined at 1000 °C areillustrated in Figure 1. On the basis of the presented X-rayanalysis and in comparison with the reported data, it has beenverified that in all samples the major phase comprises the ortho-rhombic Sr2CeO4. However, in each case, some reflection peakswere observed at 28.5° and 33° 2θ related to the pres-ence of small amounts of CeO2. Additionally, for Sr2CeO4doped with 2% Zn2+ and Mg2+, there were diffraction peaks at

20.6°, 29°, and 42° 2θ assigned to SrCO3 as an impurity. Nodistortion of the oxide crystal structure occurs in connectionwith introducing different impurities into the lattice. Generally,it is really difficult to obtain pure oxide even when differentmethods are used.19−21

In order to determine orthorhombic structure and cellparameters of the synthesized materials, Rietveld analysis hasbeen used. Crystalline parameters of Sr2CeO4 are consistentwith given reference data (JCPDS 50-0115). Doping by Mg2+,Ca2+, and Zn2+ resulted in volume reduction of the unit cell.Samples containing Ca2+ ions differ the most from thereference data, and this effect is also observed in the chro-maticity diagram and luminescence lifetimes. The biggest cellvolume was calculated for the structure containing Ba2+ ions.It is an effect of the larger size of Ba2+ ions relative tothe replaced Sr2+. The crystal cell parameters are collected inTable 1.The average crystallite sizes were estimated from the

Scherrer equation:22

= λ

θ β − βD

0.9

cos 202

(1)

where D is an average grain size, the factor 0.9 is the Scherrerconstant, λ denotes the X-ray radiation wavelength, β and θ arethe full-width at half-maximum of an observed diffraction peaklocated at θ, and β0 represents a scan aperture of the diffracto-meter. Obtained nanopowders were composed of nano-crystallites with an average size of ∼50 nm. All of the calculatedvalues are summarized in Table 1.Cerium−strontium oxide was also doped with chosen

lanthanide ions. Distortion of the oxide crystal structure didnot occur, although the Ln3+ ions have a larger charge than Sr2+

ions. The XRD patterns of the samples containing differentconcentrations of the Tm3+, Dy3+, and Eu3+ ions, presented inFigure 2, were homogeneous and consistent with the standarddata. The average crystallite sizes of the Sr2CeO4:Ln

3+ were also∼50 nm (Table 2).TEM and SEM images of the Sr2CeO4 nanophosphors are

shown in Figure 3. The presented pictures show the morphol-ogy of the synthesized materials, demonstrating the irregularcrystallite size distribution and highly agglomerated grains.There are some small (not more than a few nanometers)

Figure 1. XRD patterns of Sr2CeO4 nanopowders doped with 2%Zn2+, Ba2+, Ca2+, or Mg2+ ions calcined at 1000 °C.

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particles observed, but some larger grains are also present. Thisresulted in an inaccuracy of the Scherrer calculations and a largedistribution of values obtained by this method.Characteristic emission of the blue phosphor Sr2CeO4

is connected with the CT phenomena from orbitals of O2−

ions to the empty 4f shell of Ce4+ ions. Under ultravioletradiation, the excitation of the ground state to one of theexcited states, t1u−f or t1g−f, associated with two groups of O2−

ions (equatorial and terminal) occurs. Due to spin-forbiddentransition t1u−f, the related absorption or excitation band is lessintense compared to the band connected with the t1g−ftransition.14 Emission in the range of 400−600 nm is connectedwith the radiative relaxation process from the excited CT stateof the CeO6 complex.9

Figure 4a presents the excitation and emission spectra ofSr2CeO4 without any dopant and integrated intensities of thehost doped by various M2+ ions. The excitation spectrum showsa broad asymmetric band, composed of three peaks at 243, 296,and 330 nm. Li et al. explained the presence of the peak at 296nm as a superposition of the two remaining peaks.23 The higherenergy band (243 nm) originates from O1→Ce4+ transi-tion, where O1 is the terminal oxygen ion in the structure of

Sr2CeO4, and the peak at 330 nm results from the CTtransition between the equatorial oxygen ion and the Ce4+ ion(O2→Ce4+).23 The maximum of the excitation peak of the bulksamples should be at around 298 nm, while nanometricSr2CeO4 shows excitation with a maximum at 272 nm.23 Oursynthesized samples had a maximum of excitation at 293 nm,thus a small shift was also observed, however, smaller thanexpected. Because this maximum is a superposition of twoexcitation peaks connected with O1→Ce4+ and O2→Ce4+ CT,proportions between these two bands could also shift themaximum.The emission spectra monitored at 296 nm excitation

wavelength display a broad emission band peaking at 465 nm,which was assigned to the CT transition. A slight shift of theemission band toward low wavelengths, compared to previouslypublished values for the bulk material (485 or 472 nm) isassociated with a reduction of the crystallites’ sizes.9,11,23 Itdirectly influenced the energy gap of Sr2CeO4, enlarging thedistance between the ground and excited CT states.Doping with M2+ ions caused slight changes in the

luminescence intensity. Introduction of Ca2+ ions into theSr2CeO4 structure decreased the most effective cell volumefrom all used M2+ dopants. In the case of smaller ions such asMg2+ and Zn2+, changes are smaller than expected if it is causedonly by dopant ions smaller than Sr2+. We have also observedchanges in the color of emission (Figure 6), from which wehave concluded that the distance between O2+ and Ce4+ ions ischanged by Ca2+ doping. Luminescence properties of speci-mens containing other s-block and d-block ions as dopants donot change in a systematic way. For Sr2CeO4:Mg2+ andSr2CeO4:Zn

2+, the concentration of Mg2+ and Zn2+ ionsoptimal for the matrix luminescence intensity is 0.5%. On theother hand, the addition of Ba2+ systematically increases theemission intensity.Electron transfer from O2− to Ce4+ can occur without mod-

ifying electron spin, resulting in a singlet excited state.However, when the electron spin is changing, the tripletexcited state is formed. Relatively long luminescence decaytimes (∼50 μs) can be attributed to the long-lived triplet state,from which the emissions is generated.14 Excitation of theelectron from the O2− ligand into the empty 4f levels of theCe4+ cation results in the formation of the Ce3+(4f1)−O−(2p5)excited state. According to the Hund’s rule, the obtained twounpaired electrons take parallel spins, forming a lowerenergetic, triplet state. Radiative relaxation to the groundstate of Sr2CeO4, Ce

4+−(4f0)−O2‑(2p6), which is a singlet state,occurs with the changing spin of the electron, which is apartially forbidden process and is responsible for observed longlifetimes.The M2+ ions, excluding Ca2+, have no significant influence

on the luminescence lifetime of Sr2CeO4 (Figure 5a) calculated

Table 1. Selected Structure Parameters and the Average Crystallite Sizes of Sr2CeO4 Doped with 2% Mg2+, Ca2+, Ba2+, and Zn2+

Ions

sample a (Å) b (Å) c (Å) V (Å3) Rwb (%) calculated crystallite size (nm)

Sr2CeO4a 6.11897 10.3495 3.597 227.79

Sr2CeO4 6.1163(8) 10.3456(8) 3.5962(5) 227.56(4) 9.01 44 ± 2Sr2CeO4:Mg2+ 2% 6.1141(7) 10.3402(3) 3.5946(0) 227.25(8) 6.31 51 ± 6Sr2CeO4:Ca

2+ 2% 6.1044(7) 10.3323(0) 3.5930(1) 226.62(3) 8.18 58 ± 8Sr2CeO4:Ba

2+ 2% 6.1266(2) 10.3619(7) 3.6012(9) 228.62(4) 8.82 51 ± 6Sr2CeO4:Zn

2+ 2% 6.1138(1) 10.3390(6) 3.5945(7) 227.21(7) 9.53 48 ± 2aReference data, JCPDS 50-0115. bAgreement factor, quality of the Rietveld refinement.

Figure 2. XRD patterns of Sr2CeO4 doped with Tm3+, Dy3+, and Eu3+

ions calcined at 1000 °C.

Table 2. The Average Crystallite Sizes of the Sr2CeO4 Dopedwith Tm3+, Dy3+, and Eu3+ Ions Calcined at 1000 °C

calculated crystallite size (nm)

sample 0.5% 1% 2% 5%

Sr2CeO4:Tm3+ 50 ± 6 52 ± 5 51 ± 8 53 ± 4

Sr2CeO4:Dy3+ 49 ± 8 48 ± 7 56 ± 9 43 ± 18

Sr2CeO4:Eu3+ 49 ± 5 57 ± 9 53 ± 8 56 ± 7

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Figure 3. SEM images of prepared nanopowders annealed at 1000 °C: (a) Sr2CeO4, (b) Sr2CeO4:Tm3+ 2%, and (c) Sr2CeO4:Dy

3+ 2%. TEM images:(d) Sr2CeO4:Tm

3+ 2% and (e) Sr2CeO4:Eu3+ 2%.

Figure 4. Excitation and emission spectra of (a) Sr2CeO4 and comparison of the luminescence intensities of samples doped with Mg2+, Ca2+, Ba2+ orZn2+ ions, λex = 296 nm, λem = 465 nm; (b) Sr2CeO4:Tm

3+ (0.5−5%), λex = 290 nm, λem = 464 nm; (C) Sr2CeO4:Dy3+ (0.5−5%), λex = 290 nm,

λem = 574 nm; (d) Sr2CeO4:Eu3+ (0.5−5%), λex = 290 nm, λem= 616 nm, calcined at 1000 °C.

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using a single-exponential equation (eq 2):

= + − τI I Ae t0

/(2)

The addition of Ca2+ ions causes the progressive reduction ofcerium−strontium oxide emission (Supporting Information,Figure S1), until the luminescence decay time is nearly 4 timesshorter than the luminescence lifetime of the matrix (Table 3).

Less efficient and shorter luminescence of Sr2CeO4:Ca2+

may correspond to the distortion of the anisotropicstructure of the matrix. The Ca2+ ions affect the volume ofthe crystallographic cell, associated with a decrease in thevalue of parameter a (Table 1), which is connected with the

distance between chains of octahedrons CeO6. Distortionsof the anisotropic structure due to the destruction enddefects of CeO6 chains could result in a changed phononenergy of the matrix and increased quenching throughoutthe multiphonon relaxation. Also, the mechanism of theSr2CeO4 luminescence,16 based on donor−donor ET alongthe chains, could be affected by Ca2+ doping and thereforethe resulting chains’ distortions.Doping Sr2CeO4 with different lanthanide ions may

change the emission color of the matrix. In the emissionspectrum of Sr2CeO4:Tm

3+, presented in Figure 4b, theadditional peaks due to f−f transitions of the Tm3+

ions appears. These bands can be assigned to 1G4→3H6

(476.7 nm) and 1G4→3F4 (654.6 nm) transitions of the

Tm3+ ions.24 However, the excitation spectra present thesame shape as those registered for pure Sr2CeO4. Both theexcitation and emission spectra show that the increase inthe Tm3+ concentration causes a gradual decrease in theintensity of the matrix luminescence. This is a result ofthe ET from the matrix to Tm3+ ions.20 Decreased intensityof interion Tm3+ emission is also observed with the increasein concentration, which is caused by the different cross-relaxation processes.Figure 5b presents luminescence lifetimes of Sr2CeO4:Tm

3+

(0.5−5%) measured at λem = 476 nm. The presented decaysshow biexponential character, and to obtain the lifetime values

Figure 5. Luminescence decay times of (a) Sr2CeO4:2%M2+, λex = 337 nm, λem = 465 nm; (b) Sr2CeO4:Tm

3+ (0.5−5%), λex = 290 nm, λem = 464 nm;(c) Sr2CeO4:Dy

3+ (0.5−5%), λex = 337 nm, λem = 574 nm; (d) Sr2CeO4:Eu3+ (0.5−5%), λex = 337 nm, λem = 616 nm, calcined at 1000 °C.

Table 3. Luminescence Lifetimes for Different Ion-DopedSr2CeO4 Samples Calcined at 1000°C

luminescence decay times (μs)a

Sr2CeO4 49.92

Sr2CeO4:M2+ 0.5% 1% 2% 5%

Ba2+ 43.05 47.46 51.39 51.10Ca2+ 37.50 29.76 21.58 13.39Mg2+ 50.26 52.31 48.21 52.04Zn2+ 49.19 49.51 48.90 49.33

aMaximum error Δτ = 0.09 μs.

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we used an appropriate equation:

= + +− τ − τI I A Ae et t0 1

/2

/1 2 (3)

Calculated luminescence lifetimes indicate the regularity ofdecreasing values with the increase of the doped Tm3+ ionsconcentration. This could be an effect of the interactionbetween Tm3+ ions and therefore quenching of their excitedstates. Luminescence lifetimes of Tm3+ ions are short comparedwith the values characteristic for the other lanthanide ions;often they are no longer than 50 μs.25 In the presented results,shorter components do not exceed 1 ms, and that is consistentwith known examples in the literature.26,27 The origin of thesecond component is not sufficiently explained. The knownlong-lived states of Tm3+ ions are 3H4 and

3F4 multiplets, whichcan be fed by cross-relaxation processes. Emission from theselevels occurs in the near- and far-infrared.Due to the possibility of ET from Sr2CeO4 to Ln3+ ions,

cerium−strontium oxide was also used as the matrix for Dy3+

ions. The excitation and emission spectra are presented inFigure 4c. The excitation spectra are a simple broad bandassigned to CT transition. Peaks associated with f−f transitionsof Dy3+ ions were not observed because of their low intensity incomparison to the CT band.The emission spectra display a broad band assigned to

emission of Sr2CeO4 and lines typical for f−f transitions of Dy3+ions. Luminescence intensity of the observed 4F9/2→

6H15/2(481.5 nm), 4F9/2→

6H13/2 (573.1 nm), and 4F9/2→6H11/2

(668.8 nm) transitions decrease with rising concentration ofDy3+ in the host. It is due to the cross-relaxation between Dy3+

ions. The CT band also gradually disappears as a result of ETfrom the matrix to the Dy3+ ions.Performed luminescence kinetic measurements are shown in

Figure 5c. The lifetimes were extracted by a single exponentialfitting of the decay curves (using eq 2). The luminescencelifetimes decrease with increasing concentration of the Dy3+

ions. This confirms the concentration quenching in theobtained nanomaterials. The values of luminescence lifetimesare in the range of 155−249 μs, and that is typical for Dy3+ ions

in inorganic matrices.28,29 The luminescence decays of the Dy3+

ions can be monoexponential or, much more often,nonexponential. Monoexponential decays are obtained whenDy3+ ions are strongly quenched by cross relaxation.30 Also,quenching by the host could change the decay character or, asalso possible, result in occupation by Dy3+ ions sites near thesurface of nanocrystals.31

Throughout the addition of Eu3+ ions into the Sr2CeO4structure, red emission connected with f−f transitions ofdopant can be observed, instead of blue matrix luminescence.Registered luminescence spectra are presented in Figure 4d.The excitation spectra show a wide band characteristic fortheO2‑→Ce4+ CT with maxima at 290 and 335 nm. In contrastto other spectra for previously described Ln3+ ions, the spectraobtained for Sr2CeO4:Eu

3+ are clearly characterized by a narrowband associated with electron f−f transitions of Eu3+ ions.Moreover, the bands in the region 200−450 nm differ in shapeand intensity from that in earlier presented systems. Since Eu3+

ions in oxide exhibit their own CT states, they can take part inthe ET also. Their role in the currently presented system iscomplex and depends on the energies of both CT states. Ourearlier calculations of the role of CT states on ET demonstratedthis clearly.32

The emission spectra consist of characteristic lines assignedto Eu3+ ions, originated from transitions from 5D2,

5D1, and5D0

levels into one of the components belonging to the groundstate 7FJ. Transitions from the higher levels 5D2 and

5D1 are themost intensive when the concentration of Eu3+ ions is relativelylow. However, the intensities of measured 5D2→

7F0 (466.9 nm),5D2→

7F2 (491.2 nm), 5D2→7F3 (510.7 nm), 5D1→

7F1(535.5 nm), and 5D1→

7F2 (556 nm) bands are significantlyreduced with increasing concentration of Eu3+ ions. This is theresult of concentration quenching and therefore nonradiativerelaxation of higher excited levels of Eu3+ ions. The changes inthe intensities of the other transitions 5D0→

7F0 (580.9 nm),5D0→

7F1 (593.9 nm), 5D0→7F2 (617.2 nm), and 5D0→

7F3(655.4 nm) are relatively smaller. The ratio between 5D0→

7F1and 5D0→

7F2 bands also change as a result of the increasing

Figure 6. Chromaticity diagram of Sr2CeO4 samples doped with 5% M2+ or Ln3+ ions and their luminescence under UV lamp radiation (254 nm):(a) Sr2CeO4, (b) Sr2CeO4:Tm

3+ 2%, (c) Sr2CeO4:Dy3+ 5%, (d) Sr2CeO4:Dy

3+ 1%, (e) Sr2CeO4:Dy3+ 0.5%, (f) Sr2CeO4:Eu

3+ 2%.

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asymmetry of the Eu3+ environment due to the growingamounts of defects caused by the exchange of Sr2+ ions byhigher charged Eu3+ ions. This leads to distortion of the matrixstructure because no compensation for the lacking charge hasbeen used.33 Generally, the high ratio between 5D0→

7F2 and5D0→

7F1 provides the conclusion that Eu3+ ions (and also other

Ln3+ ions) occupy sites with a low symmetry and without aninversion center, most probably Sr2+ sites with C1 symmetry.Observed gradual reduction of the host emission intensity is aconfirmation of ET from the matrix to the Eu3+ ions.Figure 5d shows the luminescence decay curves of Sr2CeO4

doped with different amounts of Eu3+ ions. The relativelyshort luminescence lifetimes (0.79−0.88 ms) make thecerium−strontium oxide applicable in optoelectronics andsimilar areas producing various types of displays or lightening.The calculated lifetimes do not change significantly with risingamounts of Eu3+ ions in the matrix. Single-exponential decaycurves confirm that Eu3+ ions occupy one type of sites:Sr2+ sites.Summarizing the spectroscopic characteristics of the

prepared materials, Sr2CeO4 is an appropriate host forluminescent Tm3+, Dy3+, and Eu3+ ions. The presentedemission spectra show intense emission bands under UVradiation, assigned to f−f transitions of Ln3+ ions.Luminescence of the Ln3+ dopants, randomly distributed

within the crystal volume, could be attributed to the Forsternonradiative ET mechanism under excitation migration con-ditions.16,34 For the pure Forster mechanism, the quenching ratesare independent of temperature because they are governed by thespectral overlap integral, which is proportional to the overlap inthe emission spectrum of the donor and the absorption spectrumof the acceptor. Because in the Sr2CeO4 host, CeO6 octahedronsare ordered along chains, electronic excitation energy is delocalizedand migrates through the donor−donor ET before it finallyencounters the acceptor, Ln3+ ions. This causes the dependence ofthe transfer rates on the temperature.The characteristics of the excitation spectra, the observed

emission, as well as the luminescence decay times are the effectof ET from CT state of the matrix to the Ln3+ ions. Theabsence of luminescence peaks associated with levels higherthan 1G4,

4F9/2, and5D2, respectively, for transitions of Tm

3+,Dy3+, and Eu3+ ions, suggested that the ET occurs to the Ln3+

level, which has energy slightly lower than the CT state ofSr2CeO4, but the energy gap between them is large enough toeliminate the back ET.20,32 Energy values of the highestobserved transitions for Tm3+ ions 1G4→

3H6 (476.7 nm),Dy3+ ions 4F9/2→

6H15/2 (481.5 nm), and Eu3+ ions 5D2→7F0

(466.9 nm) are the confirmation of ET and are in goodaccordance with the observed energy of CT transition of purematrix (465 nm). On the basis of the luminescence spectra, wecan conclude that ET occurs from the CT state of the matrix,with energy lying around 21 500 cm−1, to the 1G4 energy levelof the Tm3+ ion, the 4F9/2 level of the Dy3+ ion, and the 5D2level of the Eu3+ ion. Therefore, emission from these and lowerenergetic manifolds, occupied through relaxation processes,could be observed.All of the synthesized materials doped with M2+ and Ln3+

ions, obtained using the Pechini method at 1000 °C, werecharacterized by describing the effective color of luminescence.The chromaticity coordinates, CIE 1931, were calculated andpresented in the chromaticity diagram (Figure 6).Synthesis of the Sr2CeO4 doped with M2+ ions was carried

out in order to modify the matrix emission color. Points,

presented in the diagram, correspond to samples doped withMg2+, Ba2+, and Zn2+, coinciding with the color of luminescenceof pure matrix. Change of color occurred only for Ca2+ ions,which have the greatest impact on the structure and there-fore the luminescence properties or emission decay kinetics ofSr2CeO4.Significant tunabilty was observed in the case of materials

doped with Dy3+ and Eu3+ ions. Slightly smaller shift of CIEvalues was caused by doping with Tm3+ ions. Modification ofthe chromaticity coordinates of Sr2CeO4:Tm

3+ consists of theemission color shifts toward shorter blue wavelengths when theconcentration of doped ions is 0.5 and 1%, and in the directionof white-blue color when the concentration of Tm3+ ion is 2and 5%. The important factor is the change in intensityluminescence, which for the human eye is seen as a deepeningof the blue color.As a result of doping with Dy3+ ions, it is possible to change

the color of luminescence from white to yellow, depending onthe amounts of Dy3+ ions in the matrix. Use of Eu3+ ions allowsfor changing the emission color from almost white to red.Chosen images of obtained phosphors are shown in Figure 6.The colors are similar to those identified by the chromaticitycoordinates.

■ CONCLUSIONSCerium−strontium oxides (Sr2CeO4) doped with Mg2+, Ca2+,Ba2+, Zn2+, Tm3+, Dy3+, or Eu3+ were successfully synthesizedvia a modified Pechini method. Addition of Ca2+ ions causesthe greatest change in the structural properties among theseries of chosen M2+ ions. Color and luminescence emissionlifetimes were also modified. Introduction of the lanthanideions (especially dopants Dy3+ and Eu3+ ions) into the Sr2CeO4structure tuned the color of matrix emission. The emissioncolor could be easily tuned by changing the dopantconcentration.The chromaticity coordinates of the obtained samples were

calculated and presented on the CIE diagram. Theseparameters demonstrated that change of luminescence colorof prepared materials depends on the type and concentration ofthe dopant. The measured excitation and emission spectraallowed us to determine the mechanism of ET from the matrixto Tm3+, Dy3+, or Eu3+ ions.

■ ASSOCIATED CONTENT*S Supporting InformationThe changes of excitation and emission spectra of Sr2CeO4samples calcined at 1000 °C and doped with 0.5−5% Ca2+ ionsare presented in Figure S1. Quenching of the Sr2CeO4 CTemission with increasing concentration of Ca2+ dopant ions isclearly visible, which is in accordance with the luminescencelifetimes shown above (Table 3). This material is available freeof charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: +48 61 8291345; e-mail: blis@amu.edu.pl.

■ ACKNOWLEDGMENTST. G. gratefully acknowledges the financial support in the formof the VENTURES project operated by the Foundation forPolish Science and financed by the EU European RegionalDevelopment Fund, Ventures/2009-4/2.

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