Tunable Luminescence of Sr - American Chemical Society

Article pubs.acs.org/JPCC

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. The structure of prepared materials has been confirmed and characterized using several techniques such as X-ray powder diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM). The obtained materials were composed of nanocrystals with an average size around ∼50 nm. The luminescence properties of synthesized nanophosphors were characterized by excitation or emission spectra and luminescence lifetimes. The chromaticity coordinates were also calculated to demonstrate changes of the luminescence color.

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INTRODUCTION Over the past few years, much attention has been applied to the synthesis and spectroscopic properties of luminescent materials containing rare earth ions. Their exceptional electronic and optical properties result from the properties of the 4f shell of these ions.1−6 Production of the various red or green phosphors has been considerably developed, but there is still a lack of comparable efficient blue emitting materials. This deficiency exists because blue-light-emitting materials degrade rapidly when exposed to critical conditions, especially under high electric field. One of the ways of solving this problem is choosing inorganic materials without complex structure and complicated substitutions. In particular, oxides have the required properties and high thermal and chemical stability.7,8 Cerium−strontium oxide is a relatively novel inorganic oxide compound, discovered by Earl Danielson and his co-workers in 1998.9,10 This material crystallizes in an orthorhombic crystal system with the space group Pbam (No. 55) and cell parameters: a = 6.11897(9) Å, b = 10.3495(2) Å, and c = 3.5970(1) Å. The structure of this oxide exhibits strong anisotropy, which is governed by a set of one-dimensional chains of trans edgesharing CeO6 octahedrons linked by interchain Sr2+ ions.9,10 Sr2CeO4 is a very promising blue-white-emitting oxide-based phosphor characterized by its high thermal and chemical stability and excellent optical properties.11 Furthermore, Sr2CeO4 is an efficient vacuum ultraviolet excited phosphor. Uncomplicated synthesis and the possibility of modifications make Sr2CeO4 an interesting alternative for other blue-emitting phosphors, such as those of BAM type (BaMgAl10O17:Eu2+), which are known to be unsatisfactory as they lose radiation efficiency and color, which shift during thermal treatment.12 The bluewhite emission of Sr2CeO4 is assigned to O2‑−Ce4+ charge © 2012 American Chemical Society

transfer (CT). It displays a broad excitation band in the UV range, which enables the possibility of tunable excitation. This compound 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, combustion process, and microwave-assisted synthesis.13−15 Cerium− strontium oxide is still under investigation due to its potential applications and improvements of its luminescent properties. This compound belongs to the group of materials whose luminescent properties also depend on the dopants. An appropriate luminescence activator, chosen from a series of Ln3+ ions, can tune the emission of a Sr2CeO4 matrix through the energy transfer (ET) from the host to the Ln3+ ion.16 Therefore, in this paper we report the results of investigations under successfully synthesized Sr2CeO4 doped with a series of ions (Eu3+, Dy3+, Tm3+; Ca2+, Mg2+, Ba2+, Zn2+) by a modified Pechini method.

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EXPERIMENTAL SECTION To prepare Sr2CeO4:M2+ 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 stoichiometric amounts, taking into account the amount of dopants, replacing Sr2+ ions in the structure, were dissolved in distilled water. For preparation of different ion-doped samples, certain volumes of solution containing one of the following salts (1 M): Ba(NO3)2 Received: August 19, 2011 Revised: January 7, 2012 Published: January 8, 2012 3219

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

The Journal of Physical Chemistry C

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(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. Aqueous solutions of lanthanides ions, Ln3+, used in the synthesis, were prepared from their oxides Dy2O3 (Stanford Materials, 99.99%), Eu 2 O 3 (Stanford Materials, 99.99%), Tm 2 O 3 (Stanford Materials, 99.99%), which were dissolved in distilled water with an appropriate amount of nitric acid, HNO3 (POCh, ultrapure). The excess amount of acid was removed by evaporating 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 polymerizing agents, respectively (e.g., for pure Sr2CeO4, reactants were mixed in molar proportion 2Sr2+:1Ce3+:24CA:14EG, or for the 5% Eu3+ doped sample: 1.9Sr2+:1Ce3+:0.1Eu3+:24CA:14EG). The dopant concentrations in all samples were varied within the 0.5−5 wt % range. The final mixture was heated and stirred on an electric stove for about 30 min. Afterward, solutions were concentrated by slow evaporation (about 8 h) in a drying oven, hence transparent brown or yellow gels were obtained. Precursors were fired in a range from 900 to 1100 °C for 3 h in a muffle furnace in an air atmosphere. Obtained compounds were ground 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 Advance diffractometer with Cu Kα1 (λ = 1.541874 Å) radiation in the 2θ range from 10° to 60°. Cell parameters were estimated using Rietveld analysis with the help of Maud 2.0 software.17,18 The transmission (TEM) and scanning (SEM) electron micrographs of the selected specimens were taken using a JEM 1200 EXII transmission electron microscope (accelerating voltage of 80 kV) and a CAIS EVO 40 scanning electron microscope (accelerating voltage of 19−20 kV), respectively. The excitation and emission spectra of all samples were recorded on a HITACHI F-7000 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Excitation and emission spectra were corrected for the instrumental 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 Mickiewicz University, in Poznan, Poland), a tunable dye laser, and an M12 FCV 51 photomultiplier. The luminescence decay curves were recorded on a Tektronix TDS 2022B oscilloscope. All of the measurements were performed at room temperature.

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

20.6°, 29°, and 42° 2θ assigned to SrCO3 as an impurity. No distortion of the oxide crystal structure occurs in connection with introducing different impurities into the lattice. Generally, it is really difficult to obtain pure oxide even when different methods are used.19−21 In order to determine orthorhombic structure and cell parameters of the synthesized materials, Rietveld analysis has been used. Crystalline parameters of Sr2CeO4 are consistent with 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 the reference data, and this effect is also observed in the chromaticity diagram and luminescence lifetimes. The biggest cell volume was calculated for the structure containing Ba2+ ions. It is an effect of the larger size of Ba2+ ions relative to the replaced Sr2+. The crystal cell parameters are collected in Table 1. The average crystallite sizes were estimated from the Scherrer equation:22

D=

0.9λ cos θ β2 − β20

(1)

where D is an average grain size, the factor 0.9 is the Scherrer constant, λ denotes the X-ray radiation wavelength, β and θ are the full-width at half-maximum of an observed diffraction peak located at θ, and β0 represents a scan aperture of the diffractometer. Obtained nanopowders were composed of nanocrystallites with an average size of ∼50 nm. All of the calculated values are summarized in Table 1. Cerium−strontium oxide was also doped with chosen lanthanide ions. Distortion of the oxide crystal structure did not occur, although the Ln3+ ions have a larger charge than Sr2+ ions. The XRD patterns of the samples containing different concentrations of the Tm3+, Dy3+, and Eu3+ ions, presented in Figure 2, were homogeneous and consistent with the standard data. The average crystallite sizes of the Sr2CeO4:Ln3+ were also ∼50 nm (Table 2). TEM and SEM images of the Sr2CeO4 nanophosphors are shown in Figure 3. The presented pictures show the morphology of the synthesized materials, demonstrating the irregular crystallite size distribution and highly agglomerated grains. There are some small (not more than a few nanometers)

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RESULTS AND DISCUSSION Sr2CeO4 phosphor was doped by selected M2+ ions in order to modify the structural properties and, as a consequence, the luminescence characteristics through replacing Sr2+ ions by Zn2+, Ba2+, Ca2+, or Mg2+ ions in the structure. This operation changed the size of crystallographic cell and the characteristics of the CeO6 octahedrons. The XRD patterns of the prepared Sr2CeO4 doped with 2% Zn2+, Ba2+, Ca2+, and Mg2+ ions and calcined at 1000 °C are illustrated in Figure 1. On the basis of the presented X-ray analysis and in comparison with the reported data, it has been verified that in all samples the major phase comprises the orthorhombic Sr2CeO4. However, in each case, some reflection peaks were observed at 28.5° and 33° 2θ related to the presence of small amounts of CeO2. Additionally, for Sr2CeO4 doped with 2% Zn2+ and Mg2+, there were diffraction peaks at 3220



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Table 1. Selected Structure Parameters and the Average Crystallite Sizes of Sr2CeO4 Doped with 2% Mg2+, Ca2+, Ba2+, and Zn2+ Ions sample Sr2CeO4a Sr2CeO4 Sr2CeO4:Mg2+ 2% Sr2CeO4:Ca2+ 2% Sr2CeO4:Ba2+ 2% Sr2CeO4:Zn2+ 2% a

a (Å)

b (Å)

c (Å)

V (Å3)

Rwb (%)

6.11897 6.1163(8) 6.1141(7) 6.1044(7) 6.1266(2) 6.1138(1)

10.3495 10.3456(8) 10.3402(3) 10.3323(0) 10.3619(7) 10.3390(6)

3.597 3.5962(5) 3.5946(0) 3.5930(1) 3.6012(9) 3.5945(7)

227.79 227.56(4) 227.25(8) 226.62(3) 228.62(4) 227.21(7)

9.01 6.31 8.18 8.82 9.53

calculated crystallite size (nm) 44 51 58 51 48

± ± ± ± ±

2 6 8 6 2

Reference data, JCPDS 50-0115. bAgreement factor, quality of the Rietveld refinement.

Sr2CeO4, and the peak at 330 nm results from the CT transition between the equatorial oxygen ion and the Ce4+ ion (O2→Ce4+).23 The maximum of the excitation peak of the bulk samples should be at around 298 nm, while nanometric Sr2CeO4 shows excitation with a maximum at 272 nm.23 Our synthesized samples had a maximum of excitation at 293 nm, thus a small shift was also observed, however, smaller than expected. Because this maximum is a superposition of two excitation peaks connected with O1→Ce4+ and O2→Ce4+ CT, proportions between these two bands could also shift the maximum. 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 the emission band toward low wavelengths, compared to previously published values for the bulk material (485 or 472 nm) is associated with a reduction of the crystallites’ sizes.9,11,23 It directly influenced the energy gap of Sr2CeO4, enlarging the distance between the ground and excited CT states. Doping with M2+ ions caused slight changes in the luminescence intensity. Introduction of Ca2+ ions into the Sr2CeO4 structure decreased the most effective cell volume from all used M2+ dopants. In the case of smaller ions such as Mg2+ and Zn2+, changes are smaller than expected if it is caused only by dopant ions smaller than Sr2+. We have also observed changes in the color of emission (Figure 6), from which we have concluded that the distance between O2+ and Ce4+ ions is changed by Ca2+ doping. Luminescence properties of specimens containing other s-block and d-block ions as dopants do not change in a systematic way. For Sr2CeO4:Mg2+ and Sr2CeO4:Zn2+, the concentration of Mg2+ and Zn2+ ions optimal for the matrix luminescence intensity is 0.5%. On the other hand, the addition of Ba2+ systematically increases the emission intensity. Electron transfer from O2− to Ce4+ can occur without modifying electron spin, resulting in a singlet excited state. However, when the electron spin is changing, the triplet excited state is formed. Relatively long luminescence decay times (∼50 μs) can be attributed to the long-lived triplet state, from which the emissions is generated.14 Excitation of the electron from the O2− ligand into the empty 4f levels of the Ce4+ cation results in the formation of the Ce3+(4f1)−O−(2p5) excited state. According to the Hund’s rule, the obtained two unpaired electrons take parallel spins, forming a lower energetic, triplet state. Radiative relaxation to the ground state of Sr2CeO4, Ce4+−(4f0)−O2‑(2p6), which is a singlet state, occurs with the changing spin of the electron, which is a partially forbidden process and is responsible for observed long lifetimes. The M2+ ions, excluding Ca2+, have no significant influence on the luminescence lifetime of Sr2CeO4 (Figure 5a) calculated

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 Doped with Tm3+, Dy3+, and Eu3+ Ions Calcined at 1000 °C calculated crystallite size (nm) sample

0.5%

1%

2%

5%

Sr2CeO4:Tm3+ Sr2CeO4:Dy3+ Sr2CeO4:Eu3+

50 ± 6 49 ± 8 49 ± 5

52 ± 5 48 ± 7 57 ± 9

51 ± 8 56 ± 9 53 ± 8

53 ± 4 43 ± 18 56 ± 7

particles observed, but some larger grains are also present. This resulted in an inaccuracy of the Scherrer calculations and a large distribution 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 ultraviolet radiation, the excitation of the ground state to one of the excited states, t1u−f or t1g−f, associated with two groups of O2− ions (equatorial and terminal) occurs. Due to spin-forbidden transition t1u−f, the related absorption or excitation band is less intense compared to the band connected with the t1g−f transition.14 Emission in the range of 400−600 nm is connected with the radiative relaxation process from the excited CT state of the CeO6 complex.9 Figure 4a presents the excitation and emission spectra of Sr2CeO4 without any dopant and integrated intensities of the host doped by various M2+ ions. The excitation spectrum shows a broad asymmetric band, composed of three peaks at 243, 296, and 330 nm. Li et al. explained the presence of the peak at 296 nm as a superposition of the two remaining peaks.23 The higher energy band (243 nm) originates from O1→Ce4+ transition, where O1 is the terminal oxygen ion in the structure of 3221



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Figure 3. SEM images of prepared nanopowders annealed at 1000 °C: (a) Sr2CeO4, (b) Sr2CeO4:Tm3+ 2%, and (c) Sr2CeO4:Dy3+ 2%. TEM images: (d) Sr2CeO4:Tm3+ 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+ or Zn2+ ions, λex = 296 nm, λem = 465 nm; (b) Sr2CeO4:Tm3+ (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. 3222



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Figure 5. Luminescence decay times of (a) Sr2CeO4:2%M2+, λex = 337 nm, λem = 465 nm; (b) Sr2CeO4:Tm3+ (0.5−5%), λex = 290 nm, λem = 464 nm; (c) Sr2CeO4:Dy3+ (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.

distance between chains of octahedrons CeO6. Distortions of the anisotropic structure due to the destruction end defects of CeO6 chains could result in a changed phonon energy of the matrix and increased quenching throughout the multiphonon relaxation. Also, the mechanism of the Sr2CeO4 luminescence,16 based on donor−donor ET along the chains, could be affected by Ca2+ doping and therefore the resulting chains’ distortions. Doping Sr 2CeO4 with different lanthanide ions may change the emission color of the matrix. In the emission spectrum of Sr2CeO4:Tm3+, presented in Figure 4b, the additional peaks due to f−f transitions of the Tm 3+ 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 the same shape as those registered for pure Sr 2CeO4. Both the excitation and emission spectra show that the increase in the Tm3+ concentration causes a gradual decrease in the intensity of the matrix luminescence. This is a result of the ET from the matrix to Tm3+ ions.20 Decreased intensity of interion Tm3+ emission is also observed with the increase in concentration, which is caused by the different crossrelaxation processes. Figure 5b presents luminescence lifetimes of Sr2CeO4:Tm3+ (0.5−5%) measured at λem = 476 nm. The presented decays show biexponential character, and to obtain the lifetime values

using a single-exponential equation (eq 2): −t / τ

I = I0 + Ae

(2) 2+

The addition of Ca ions causes the progressive reduction of cerium−strontium oxide emission (Supporting Information, Figure S1), until the luminescence decay time is nearly 4 times shorter than the luminescence lifetime of the matrix (Table 3). Table 3. Luminescence Lifetimes for Different Ion-Doped Sr2CeO4 Samples Calcined at 1000°C luminescence decay times (μs)a Sr2CeO4

a

49.92

Sr2CeO4:M2+

0.5%

1%

2%

5%

Ba2+ Ca2+ Mg2+ Zn2+

43.05 37.50 50.26 49.19

47.46 29.76 52.31 49.51

51.39 21.58 48.21 48.90

51.10 13.39 52.04 49.33

Maximum error Δτ = 0.09 μs.

Less efficient and shorter luminescence of Sr2CeO4:Ca 2+ may correspond to the distortion of the anisotropic structure of the matrix. The Ca2+ ions affect the volume of the crystallographic cell, associated with a decrease in the value of parameter a (Table 1), which is connected with the 3223



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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:Tm3+ 2%, (c) Sr2CeO4:Dy3+ 5%, (d) Sr2CeO4:Dy3+ 1%, (e) Sr2CeO4:Dy3+ 0.5%, (f) Sr2CeO4:Eu3+ 2%.

in inorganic matrices.28,29 The luminescence decays of the Dy3+ ions can be monoexponential or, much more often, nonexponential. Monoexponential decays are obtained when Dy3+ ions are strongly quenched by cross relaxation.30 Also, quenching by the host could change the decay character or, as also possible, result in occupation by Dy3+ ions sites near the surface of nanocrystals.31 Throughout the addition of Eu3+ ions into the Sr2CeO4 structure, red emission connected with f−f transitions of dopant can be observed, instead of blue matrix luminescence. Registered luminescence spectra are presented in Figure 4d. The excitation spectra show a wide band characteristic for theO2‑→Ce4+ CT with maxima at 290 and 335 nm. In contrast to other spectra for previously described Ln3+ ions, the spectra obtained for Sr2CeO4:Eu3+ are clearly characterized by a narrow band associated with electron f−f transitions of Eu3+ ions. Moreover, the bands in the region 200−450 nm differ in shape and intensity from that in earlier presented systems. Since Eu3+ ions in oxide exhibit their own CT states, they can take part in the ET also. Their role in the currently presented system is complex and depends on the energies of both CT states. Our earlier calculations of the role of CT states on ET demonstrated this clearly.32 The emission spectra consist of characteristic lines assigned to Eu3+ ions, originated from transitions from 5D2, 5D1, and 5D0 levels into one of the components belonging to the ground state 7FJ. Transitions from the higher levels 5D2 and 5D1 are the most intensive when the concentration of Eu3+ ions is relatively low. However, the intensities of measured 5D2→7F0 (466.9 nm), 5 D 2→7F2 (491.2 nm), 5 D2→ 7 F3 (510.7 nm), 5 D 1→ 7 F1 (535.5 nm), and 5D1→7F2 (556 nm) bands are significantly reduced with increasing concentration of Eu3+ ions. This is the result of concentration quenching and therefore nonradiative relaxation of higher excited levels of Eu3+ ions. The changes in the intensities of the other transitions 5D0→7F0 (580.9 nm), 5 D0→7F1 (593.9 nm), 5D0→7F2 (617.2 nm), and 5D0→7F3 (655.4 nm) are relatively smaller. The ratio between 5D0→7F1 and 5D0→7F2 bands also change as a result of the increasing

we used an appropriate equation:

I = I0 + A1e−t / τ1 + A2e−t / τ2

(3)

Calculated luminescence lifetimes indicate the regularity of decreasing values with the increase of the doped Tm3+ ions concentration. This could be an effect of the interaction between Tm3+ ions and therefore quenching of their excited states. Luminescence lifetimes of Tm3+ ions are short compared with 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 consistent with known examples in the literature.26,27 The origin of the second component is not sufficiently explained. The known long-lived states of Tm3+ ions are 3H4 and 3F4 multiplets, which can be fed by cross-relaxation processes. Emission from these levels 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 in Figure 4c. The excitation spectra are a simple broad band assigned to CT transition. Peaks associated with f−f transitions of Dy3+ ions were not observed because of their low intensity in comparison 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 of Dy3+ in the host. It is due to the cross-relaxation between Dy3+ ions. The CT band also gradually disappears as a result of ET from the matrix to the Dy3+ ions. Performed luminescence kinetic measurements are shown in Figure 5c. The lifetimes were extracted by a single exponential fitting of the decay curves (using eq 2). The luminescence lifetimes decrease with increasing concentration of the Dy3+ ions. This confirms the concentration quenching in the obtained nanomaterials. The values of luminescence lifetimes are in the range of 155−249 μs, and that is typical for Dy3+ ions 3224



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asymmetry of the Eu3+ environment due to the growing amounts of defects caused by the exchange of Sr2+ ions by higher charged Eu3+ ions. This leads to distortion of the matrix structure because no compensation for the lacking charge has been used.33 Generally, the high ratio between 5D0→7F2 and 5 D0→7F1 provides the conclusion that Eu3+ ions (and also other Ln3+ ions) occupy sites with a low symmetry and without an inversion center, most probably Sr2+ sites with C1 symmetry. Observed gradual reduction of the host emission intensity is a confirmation 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 relatively short luminescence lifetimes (0.79−0.88 ms) make the cerium−strontium oxide applicable in optoelectronics and similar areas producing various types of displays or lightening. The calculated lifetimes do not change significantly with rising amounts of Eu3+ ions in the matrix. Single-exponential decay curves confirm that Eu3+ ions occupy one type of sites: Sr2+ sites. Summarizing the spectroscopic characteristics of the prepared materials, Sr2CeO4 is an appropriate host for luminescent Tm3+, Dy3+, and Eu3+ ions. The presented emission spectra show intense emission bands under UV radiation, assigned to f−f transitions of Ln3+ ions. Luminescence of the Ln3+ dopants, randomly distributed within the crystal volume, could be attributed to the Förster nonradiative ET mechanism under excitation migration conditions.16,34 For the pure Förster mechanism, the quenching rates are independent of temperature because they are governed by the spectral overlap integral, which is proportional to the overlap in the emission spectrum of the donor and the absorption spectrum of the acceptor. Because in the Sr2CeO4 host, CeO6 octahedrons are ordered along chains, electronic excitation energy is delocalized and migrates through the donor−donor ET before it finally encounters the acceptor, Ln3+ ions. This causes the dependence of the transfer rates on the temperature. The characteristics of the excitation spectra, the observed emission, as well as the luminescence decay times are the effect of ET from CT state of the matrix to the Ln3+ ions. The absence of luminescence peaks associated with levels higher than 1G4, 4F9/2, and 5D2, respectively, for transitions of Tm3+, Dy3+, and Eu3+ ions, suggested that the ET occurs to the Ln3+ level, which has energy slightly lower than the CT state of Sr2CeO4, but the energy gap between them is large enough to eliminate the back ET.20,32 Energy values of the highest observed 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 good accordance with the observed energy of CT transition of pure matrix (465 nm). On the basis of the luminescence spectra, we can conclude that ET occurs from the CT state of the matrix, with energy lying around 21 500 cm−1, to the 1G4 energy level of the Tm3+ ion, the 4F9/2 level of the Dy3+ ion, and the 5D2 level of the Eu3+ ion. Therefore, emission from these and lower energetic 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, were characterized by describing the effective color of luminescence. The chromaticity coordinates, CIE 1931, were calculated and presented 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 with Mg2+, Ba2+, and Zn2+, coinciding with the color of luminescence of pure matrix. Change of color occurred only for Ca2+ ions, which have the greatest impact on the structure and therefore the luminescence properties or emission decay kinetics of Sr2CeO4. Significant tunabilty was observed in the case of materials doped with Dy3+ and Eu3+ ions. Slightly smaller shift of CIE values was caused by doping with Tm3+ ions. Modification of the chromaticity coordinates of Sr2CeO4:Tm3+ consists of the emission color shifts toward shorter blue wavelengths when the concentration of doped ions is 0.5 and 1%, and in the direction of white-blue color when the concentration of Tm3+ ion is 2 and 5%. The important factor is the change in intensity luminescence, which for the human eye is seen as a deepening of 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 on the amounts of Dy3+ ions in the matrix. Use of Eu3+ ions allows for 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 chromaticity coordinates.

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CONCLUSIONS Cerium−strontium oxides (Sr2CeO4) doped with Mg2+, Ca2+, Ba2+, Zn2+, Tm3+, Dy3+, or Eu3+ were successfully synthesized via a modified Pechini method. Addition of Ca2+ ions causes the greatest change in the structural properties among the series of chosen M2+ ions. Color and luminescence emission lifetimes were also modified. Introduction of the lanthanide ions (especially dopants Dy3+ and Eu3+ ions) into the Sr2CeO4 structure tuned the color of matrix emission. The emission color could be easily tuned by changing the dopant concentration. The chromaticity coordinates of the obtained samples were calculated and presented on the CIE diagram. These parameters demonstrated that change of luminescence color of prepared materials depends on the type and concentration of the dopant. The measured excitation and emission spectra allowed us to determine the mechanism of ET from the matrix to Tm3+, Dy3+, or Eu3+ ions.

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ASSOCIATED CONTENT

S Supporting Information *

The changes of excitation and emission spectra of Sr2CeO4 samples calcined at 1000 °C and doped with 0.5−5% Ca2+ ions are presented in Figure S1. Quenching of the Sr2CeO4 CT emission with increasing concentration of Ca2+ dopant ions is clearly visible, which is in accordance with the luminescence lifetimes shown above (Table 3). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +48 61 8291345; e-mail: [email protected].

ACKNOWLEDGMENTS T. G. gratefully acknowledges the financial support in the form of the VENTURES project operated by the Foundation for Polish Science and financed by the EU European Regional Development Fund, Ventures/2009-4/2. 3225



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