THE JOURNAL OF CHEMICAL PHYSICS 142, 184701 (2015)
Influence of grain size on optical properties of Sr2CeO4 nanocrystals M. Stefanski, L. Marciniak,a) D. Hreniak, and W. Strek Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50–422 Wroclaw, Poland
(Received 7 February 2015; accepted 27 April 2015; published online 8 May 2015) The absorption, excitation, and emission spectra of the Sr2CeO4 nanocrystals prepared by the modified sol–gel method were investigated. The impact of the average grain size of Sr2CeO4 nanocrystals on their optical properties was investigated. It was observed that with increasing the average grain size of Sr2CeO4 nanocrystals, the emission decay times decreased significantly. A similar behavior was observed for the emission quantum efficiencies and the Huang–Rhys factors. The grain size dependence of optical parameters of Sr2CeO4 nanocrystals was found well fitted by functions of the reciprocal of the grain diameter. It was shown that this dependence may be rationalized assuming that the correction for electric local field associated with effective refractive index affecting the spherical nanoparticle is governed by its shell. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4919880] I. INTRODUCTION
The Sr2CeO4 phosphor is characterized by good chemical stability and high luminescent efficiency, which gives a possibility of various industrial applications such as the xray detectors, blue phosphors in the lightening industry, or field emission displays (FED). The synthesis of Sr2CeO4 blue phosphor was presented for the first time by Danielson et al. using a combinatorial method.1 The phosphor crystallized in the orthorhombic space system in Pbma group with one-dimensional chains of edge-sharing CeO6 octahedrals which are linked by Sr2+ ions. The efficient blue luminescence in Sr2CeO4 originates from ligand-metal Ce4+ charge transfer. Over the last few years, Sr2CeO4 phosphor was widely investigated. Jiang et al. reported morphology and cathodoluminescent properties of Sr2CeO4 synthesized by chemical coprecipitation method.2 Various other techniques were developed for the preparation of the Sr2CeO4 phosphor such as Pechini’s method,3,4 citrate-gel method,5 microwaveassisted solvothermal method,6 spray pyrolysis,7 and emulsion liquid membrane system.8 The spectroscopic properties of Sr2CeO4 powder obtained by using the solid state method were investigated by Park et al.9 The morphology and optical properties of pure and Eu3+ doped Sr2CeO4 phosphors were reported by Li et al.10 Raman spectra of the Sr2CeO4 phosphor were investigated by Zhang et al.11 The luminescence properties were reported for Sr2CeO4 single-doped with Yb3+,12 Er3+,13 Eu3+,14–16 Sm3+,14 Dy3+17 and double-doped with Eu3+ and Dy3+18 and Eu3+ and Gd3+.19 In a course of studies of optical properties of Sr2CeO4 phosphors, it was observed that they are strongly affected on average particle size. It is commonly accepted that the Sr2CeO4 fine powders demonstrate a tendency to decreasing emission efficiency with reduction of grain size. The first attempt to compare the spectroscopic properties of Sr2CeO4 microcrystals with different grain sizes was performed by Mata)Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0021-9606/2015/142(18)/184701/7/$30.00
sui et al.20 Ling Li et al. have investigated the optical behavior of excitation and emission bands in bulk and nanocrystalline phosphors where they have observed a significant red shift of both the bands in nanocrystalline powders in comparison to the bulk.21 In the present work, we report the synthesis and optical studies of Sr2CeO4 nanocrystals. In particular, the impact of nanocrystal grain sizes on their optical characteristics was investigated.
II. EXPERIMENTAL
The Sr2CeO4 nanocrystals were obtained by Pechini’s method. Strontium nitrate—Sr(NO3)2 (Acros Organics, 99+%) and cerium nitrate—Ce(NO3)3·6H2O (SERVA, pure) were used as raw materials. Appropriate amounts of the reagents were dissolved in deionized water. Then citric acid was added (in molar ratio equal to 1:5 with respect to cations) to the solution under intense stirring and heating. Ethylene glycol was added in molar ratio equal to 1:1 as compared to citric acid several hours later. The obtained solution was kept in a laboratory dryer for two days at 90 ◦C. Subsequently, the sample was placed in an oven and annealed at a temperature selected from 750, 800, 850, 900, 1000, and 1050 ◦C. X-ray diffraction (XRD) spectra were measured with an X’Pert PRO powder diffractometer (PANalytical, The Netherlands) equipped with a linear PIXcel detector and using Cu Kα radiation (λ = 1.540 56 Å). The transmission electron microscopy (TEM) analysis was performed using an FEI Tecnai G2 20 X-TWIN. The absorption spectra were measured in the back scattering mode using a Cary Varian 5E UVVis-NIR spectrometer. Two types of lamp were used as the excitation source: a deuterium lamp for region 185–350 nm and a halogen lamp for region 350–3300 nm. An R928 photomultiplier tube was used for detection in ultraviolet and visible (UV–VIS) range of spectrum and a cooled PbS detector for near infra-red region (NIR). The spectrometer was supplied with two gratings: 1200 lines/mm with blaze at 250 nm for UV–VIS range and 300 lines/mm with blaze at
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FIG. 1. Visualization of orthorhombic Sr2CeO4 structure (a) and immediate environment of the Ce4+ ion (b).
1192 nm for NIR range. The beam was chopped at 30 Hz repetition. The luminescence decay profiles and emission and excitation spectra were measured using an FLS980 Fluorescence Spectrometer from Edinburgh Instruments. The emission and excitation spectra were corrected with respect to the detector sensitivity and the lamp characteristics.
III. RESULTS AND DISCUSSION A. Crystal structure and morphology
Sr2CeO4 crystallizes in orthorhombic space system in Pbam group. Fig. 1 presents the Sr2CeO4 crystal structure and close environment of the Ce4+ ion. One can see that the structure of Sr2CeO4 consists of polyhedrons. Their centers are occupied by Ce4+ while corners by O2− ions. Ce4+ ions are 6-fold coordinated at only one crystallographic position. There occur two different distances between Ce4+ and O2− ions which are equal to 2.201 Å and 2.308 Å. In consequence, two sites of cerium are observed. The unit cell parameters of Sr2CeO4 were determined to be a = 6.1190 Å, b = 10.3495 Å, and c = 3.5970 Å. The Sr2CeO4 nanocrystals were annealed at different temperatures: 750, 800, 850, 900, 1000, and 1050 ◦C. The corresponding X-ray diffraction patterns are presented in Fig. 2(a). One can see that the sample annealed at 750 ◦C was amorphous. Pure orthorhombic phase of Sr2CeO4 was obtained above this temperature. The absence of additional peaks in XRD patterns confirms phase purity of the obtained nanocrystals. Moreover, with increase of annealing temperature, narrowing of diffraction peaks was observed,
which is associated with increase of crystallites size at higher temperatures. Using Rietveld refinement method, the grain sizes were determined. The average crystallite sizes calculated for annealing temperatures 800, 850, 900, 1000, and 1050 ◦C were equal to 55, 68, 73, 140, and 217 nm, respectively. Fig. 2(b) presents the dependence of grain sizes on the annealing temperature. It was found that the grain size shows a significant nonlinear increase with the calcination temperature, from 60 nm at 800 ◦C to 220 nm at 1050 ◦C. The crystal cell parameters of Sr2CeO4 nanocrystals annealed at different temperatures were determined by Rietveld refinement. Results of the calculations are presented in Table I. TEM pictures of Sr2CeO4 nanocrystals are presented in Fig. 3. It can be seen that the grains of the samples were well structured but with a tendency to agglomerate. The average size of the grains was determined to be about 50 and 200 nm. The sizes obtained from the TEM images are comparable to the values calculated using Rietveld refinement. B. Optical properties
Fig. 4 presents the scheme of the electronic relaxations in Sr2CeO4 host. After excitation in UV, an electron is transferred from the valence band to the conduction band associated in absorption spectra with a low spin singlet excited state (LS) being a spin-allowed transition ∆S = 0 and with the high spin triplet excited state (HS) being the spin forbidden transition ∆S = 1. Subsequently, the nonradiative relaxation from the conduction band to the emitting levels of cerium occurs. As the result, emission from two energy levels of metal-ligand charge transfer (MLCTterm and MLCTeq) can be observed,
FIG. 2. The X-ray diffraction patterns (a) and the dependence of grain size of Sr2CeO4 nanocrystals on annealing temperature (b).
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TABLE I. Cell parameters of Sr2CeO4 nanocrystals annealed at different temperatures. Cell parameters Sample
a (Å)
b (Å)
c (Å)
β (Å)
V (Å3)
Grain size (nm)
Rp (%)
Reference (ICSD no. 50-0115) 800 ◦C 850 ◦C 900 ◦C 1000 ◦C 1050 ◦C
6.12 6.12 6.12 6.12 6.12 6.12
10.35 10.35 10.34 10.34 10.35 10.35
3.60 3.60 3.60 3.60 3.60 3.60
90 90 90 90 90 90
228 228 228 228 228 228
... 55 68 73 140 217
... 2.05 1.91 2.52 2.02 2.13
with the energy distances between the band maxima equal to the energy differences between the emitting levels of Ce4+ ions.10 Fig. 5 presents the absorption, excitation, and emission spectra of Sr2CeO4 nanocrystalline powders characterized by different average grain sizes. The origin of both MLCT bands is associated with the LS and HS transitions of Ce4+ ion, respectively.22 The Ce4+ ions are coordinated to two different oxygen atoms forming two different Ce4+ sites. Such a behavior is evidenced clearly in excitation spectra for the HS absorption band at 320–360 nm related to the Ce–O2 site. It is interesting also to see that with increasing grain size, the intensity of the HS absorption band in the excitation spectra increases. The dependence of HS absorption band intensity on the average grain size is shown in Fig. 5(e). The excitation spectra of Sr2CeO4 nanocrystals were recorded at λem = 580 nm at room temperature. The spectra consist of inhomogeneously broadened absorption bands with the maxima at 275 nm and 340 nm. One can see that with increasing grain size, there is a slight red shift of absorption bands both in the absorption and excitation spectra. The relationship between the energies of absorption bands and the size of spherical type nanocrystals is described in terms of 1/R,2 where R is the average radius within the effective mass approximation based on quantum confinement.23 Another approach to size effect based on classical physics properties of dielectric sphere predicts 1/R dependence.24 Since the elemental charges on the surface of small spherical capacitors contribute to the total electric potential, it can be considered as size dependent capacitance of nanocrystalline sphere. The size dependence of red shift of absorption and excitation bands observed in our measurements is shown in Fig. 5(d).
It is interesting to note that there are no red shifts for the emission bands. They do not change with the grain size. The emission of charge transfer (CT) transition to the singlet ground state of Sr2CeO4 Ce4+–(4f 0)–O2−(2p6) is the spin forbidden process that occurs with the spin change, and it may be responsible for a negligible small energy red shift with the size. A similar observation was also reported for Sr2CeO4 bulk and nanocrystals.20,25 The characteristic energy features associated with absorption, excitation, and emission are listed in Table II. To get an insight into the nature of electron-phonon interaction, we have determined the Huang–Rhys parameter S 26–28 ∆Es = (2S − 1)~ω,
(1)
where ∆Es describes the Stokes shift and ~ω is the effective phonon energy. Taking the effective phonon energy of Sr2CeO4 crystal to be ~ω = 560 cm−1,11 the Huang–Rhys factor S could be determined. The magnitudes of Stokes shift and Huang–Rhys parameter were determined from the emission and excitation spectra for the Sr2CeO4 nanocrystals with different grain sizes using the data listed in Table III for the respective LS and HS transitions. One can see that the Stokes shift and Huang–Rhys parameters decrease with increasing the grain size. They are much larger for the MLCT absorption bands associated with the LS transition than with the HS transitions. It is reasonable to assume that the emission intensity is more efficient when the Stokes shift is larger because there is no overlap between the absorption and emission transitions restricting the nonradiative interion transitions. It is the reason why there is no correspondence between absorption and excitation spectra because the largest Stokes shift is associated with more energetic LS absorption transitions.
FIG. 3. TEM images of Sr2CeO4 nanocrystals annealed at 800 (a) and 1050 ◦C (b).
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grains. For the smallest grain sizes, it becomes lower by almost 50%. D. Luminescence kinetics
FIG. 4. Diagram of electronic relaxations in Sr2CeO4 nanocrystals.
C. Luminescence quantum efficiency (QE)
The luminescence QE for the Sr2CeO4 nanocrystals with different grain sizes was measured at room temperature. It increased from 9.43% for the smallest grains up to 15.70% for the largest ones. A correlation between the quantum efficiency and the grain sizes is presented in Fig. 6. It was found that the QE increased significantly according to the following relation: QE = QE L 1 − D1 , where D means the average grain size of Sr2CeO4 nanocrystals and QE L is a proportionality parameter. One can conclude that the quantum efficiency increases with the grain size reaching a maximum of 15.5% for largest
Fig. 7 presents the luminescence decay profiles of Sr2CeO4 nanocrystals characterized by different average grain sizes recorded at room and liquid nitrogen temperatures. One can see that the decay profiles were almost perfectly exponential and the decay times shortened with increasing the grain size. The observed exponential decays suggest that only one Ce4+ site contributes in dominant way to the luminescence decay and that there is no cooperative interactions between Ce4+ ions that could be responsible for nonexponential decay profiles of MLCT emission transitions. Another important feature is that the luminescence decay times measured at room temperature were significantly shorter, by more than one order of magnitude, than those measured at 77 K. It means that nonradiative relaxation is associated predominantly with multiphonon transitions. E. Discussion of grain size effect
The decay rate k of MLCT emission is the sum of radiative krad and nonradiative k nrad transitions k = krad + knrad .
(2)
FIG. 5. The absorption (a), excitation (monitored at 21 277 cm−1) (b), emission (excited at 36 630 cm−1) (c) spectra for Sr2CeO4 nanocrystals with different grain sizes recorded at 300 K, and the grain size dependence of LS peak position (d) and HS intensity (e) of Sr2CeO4 nanocrystals.
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TABLE II. The energy features observed in absorption, excitation, and emission spectra for different grain size Sr2CeO4 nanocrystals.
Grain size (nm) 55 68 73 140 217
Absorption spectra (cm−1)
Excitation spectra (cm−1)
LS
HS
LS
HS
38 892 38 692 38 594 38 000 37 275
29 801 29 546 29 709 29 702 28 602
37 353 36 871 36 552 35 614 35 372
31 457 30 316 29 776 28 987 28 956
Emission spectra (cm−1)
20 945
It is well known that rates of radiative transitions are weakly affected by temperature. So a main contribution to the temperature dependence of decay rate should be associated with nonradiative transitions. The observed decay time significantly decreased with the grain size, extremely for smaller ones. One can note that the influence of grain size was more pronounced at low temperature measurements— with increasing temperature, the decay time decreased. The correlation between the decay rates and grain sizes of the investigated samples is shown in Fig. 7(c). One can see that the decay rates, both at 77 and 300 K, increased with increasing the grain sizes of Sr2CeO4 nanocrystals. The enhancement of decay rate may be associated both with radiative and nonradiative multiphonon transitions. There appears a question how the grain size affects these relaxation channels. The influence of size on rate constant of electronic relaxation processes is associated with the local electric field E loc in dielectric medium. In general case, it is introduced in the description of transition probabilities as the correction factor for the local ( loc ) 2 field within the Lorentz model f L (n) = EE ,29–31 where E is the strength of macroscopic electric field. The refractive index of the medium n is replaced by effective refractive index ne . It must be taken into account in any formula describing the rate of radiative and nonradiative transitions. The effective refractive index is dependent on frequency of a photon participating in the relaxation process (photon dispersion). It is well known that n increases extremely strongly for high energy photons in UV and visible ranges. It remains practically constant for low frequency quanta (infrared region). So it means that the multiphonon processes in an isolated nanocrystal are not sensitive to changes of effective refractive index and in consequence to decreasing the grain size.32 TABLE III. The parameters of Stokes shift and Huang–Rhys factor with the effective phonon energy for Sr2CeO4 nanocrystals with different grain sizes. Stokes shift ∆E s (cm−1) Grain size (nm) 55 68 73 140 217
Huang–Rhys factor S
LS
HS
LS
HS
15 712 15 712 15 444 14 408 13 932
10 512 9 371 8 831 8 042 8 011
14.5 14.5 14.3 13.4 12.9
9.9 8.9 8.4 7.7 7.6
Effective phonon energy ~ω (cm−1)
560
FIG. 6. The dependence of luminescence quantum yield on the average grain size of Sr2CeO4 nanocrystals.
The rate of electric dipole transitions of metal ion in nanocrystalline grain immersed in vacuum is described by the following formula:33,34 2 1 (3) k ∝ f (ED) (ne + 2) ne λ 0−2, 3 where f (ED) is the oscillator strength of electric dipole transition, λ 0 is wavelength in vacuum, and ne is the effective refractive index. It is usually calculated by using the volume mixing rule29–31 and may be expressed as the volume-mean refractive index, ne = f i ni , (4) i
where ni is the partial refractive index of the ith component and f i is the volume fraction. For two components, nanocrystal and vacuum, it may be expressed as ne = xn + (1 − x) no ,
(5)
where x is the fraction of the volume occupied by nanosized grains.29–31,35–40 Since the refractive index of vacuum is no = 1, the effective refractive index is given by ne = x (n − 1) + 1. This expression is valid assuming a uniform distribution of refractive index in the whole volume of nanocrystal. In reality, in such a system, the majority of ions are located at the surface of a nanocrystal. It is well known that the refractive index increases with the mass density. It means that the effective refractive index at the nanocrystal shell is much smaller than the refractive index of the nanocrystal core. Lü et al. have shown, using the assumption of spherical particle model, that a major influence to the optical relaxations is associated with the surface of nanoparticles (see Fig. 8).38 The density of the shell formed by Ln ions at the nanoparticle increases with its diameter D = 2r. It is reasonable to assume that the volume of the nanocrystal outside the shell composed from rare earth ions is simply related to the fraction of volume, 4/3πR3 − 4/3π(R − d)3 2d 3 = 1 − (1 − ), (6) D 4/3πR3
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FIG. 7. The luminescence decay profiles of Sr2CeO4 nanocrystals measured at 300 K (a) and 77 K (b). The size dependence of decay rate measured at 77 and 300 K (c).
where R is the nanocrystal radius and d is the shell thickness. After simple manipulation, this equation may be approximated by its leading component 6d D giving a major contribution to the volume fraction. It allows one to conclude that the oscillator strength is proportional to the reciprocal value of grain diameter, k ∝ D −1. In the course of fitting of optical dependences, it was found that they were well described by a simple formula D−1 or (1 − D −1), where D means the average diameter of nanocrystalline grain assuming its spherical shape. The presented above discussion rationalizes in a simple way the observed dependences of optical parameters of Sr2CeO4 nanocrystals on their average grain diameters. The role of the nanocrystal size was discussed earlier by several authors in terms of surface-to-volume ratio that quantitatively describes the linear relationship for luminescence parameters of colloidal solutions of Er3+ and Yb3+ codoped upconversion nanocrystals.39–42
IV. SUMMARY
The Sr2CeO4 nanocrystals were successfully prepared using the Pechini’s method and annealed at different temperatures. The XRD patterns confirmed clear orthorhombic phase of the nanocrystals. Only the Sr2CeO4 nanocrystalline powders calcinated at 750 ◦C were amorphous. The average grain sizes of Sr2CeO4 nanocrystals obtained at different annealing temperatures were determined using Rietveld refinement. It was observed that with increase of annealing temperature, the grain sizes increased significantly. Elucidating the impact of the average grain size on the optical characteristics was a main purpose of our investigations. It was observed that the optical properties associated with the absorption, excitation, and emission spectra of Sr2CeO4 nanocrystals were strongly affected by the average grain size. The quantum efficiencies for investigated samples were determined. It was found that the quantum efficiencies increase with increasing grain size. Moreover, the Huang–Rhys factors and Stokes shift significantly increased with grain size. Extremely high values of these factors are characteristic for the metalligand charge transfer transition bands. The luminescence decay profiles were well fitted by exponential curves which indicated limited participation of energy transfer between different Ce4+ ions. An increase of the average grain size decreases the Sr2CeO4 luminescence lifetimes. The size effect on optical characteristics was discussed in terms of effective refractive index assuming a leading role of the surface shell of nanocrystal grains.
ACKNOWLEDGMENTS
The work was supported by Wroclaw Research Centre EIT+ within the project “The Application of Nanotechnology in Advanced Materials”—NanoMat (POIG.01.01.02-02002/08) co-financed by the European Regional Development Fund (Operational Programme Innovative Economy, 1.1.2). 1E. Danielson, M. Devenney, D. M. Giaquinta, J. H. Golden, R. C. Haushalter,
FIG. 8. The model used in the explanation of the changes of refractive index with the grain size of the nanocrystals.
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Influence of grain size on optical properties of Sr2CeO4 nanocrystals M. Stefanski, L. Marciniak,a) D. Hreniak, and W. Strek Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50–422 Wroclaw, Poland
(Received 7 February 2015; accepted 27 April 2015; published online 8 May 2015) The absorption, excitation, and emission spectra of the Sr2CeO4 nanocrystals prepared by the modified sol–gel method were investigated. The impact of the average grain size of Sr2CeO4 nanocrystals on their optical properties was investigated. It was observed that with increasing the average grain size of Sr2CeO4 nanocrystals, the emission decay times decreased significantly. A similar behavior was observed for the emission quantum efficiencies and the Huang–Rhys factors. The grain size dependence of optical parameters of Sr2CeO4 nanocrystals was found well fitted by functions of the reciprocal of the grain diameter. It was shown that this dependence may be rationalized assuming that the correction for electric local field associated with effective refractive index affecting the spherical nanoparticle is governed by its shell. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4919880] I. INTRODUCTION
The Sr2CeO4 phosphor is characterized by good chemical stability and high luminescent efficiency, which gives a possibility of various industrial applications such as the xray detectors, blue phosphors in the lightening industry, or field emission displays (FED). The synthesis of Sr2CeO4 blue phosphor was presented for the first time by Danielson et al. using a combinatorial method.1 The phosphor crystallized in the orthorhombic space system in Pbma group with one-dimensional chains of edge-sharing CeO6 octahedrals which are linked by Sr2+ ions. The efficient blue luminescence in Sr2CeO4 originates from ligand-metal Ce4+ charge transfer. Over the last few years, Sr2CeO4 phosphor was widely investigated. Jiang et al. reported morphology and cathodoluminescent properties of Sr2CeO4 synthesized by chemical coprecipitation method.2 Various other techniques were developed for the preparation of the Sr2CeO4 phosphor such as Pechini’s method,3,4 citrate-gel method,5 microwaveassisted solvothermal method,6 spray pyrolysis,7 and emulsion liquid membrane system.8 The spectroscopic properties of Sr2CeO4 powder obtained by using the solid state method were investigated by Park et al.9 The morphology and optical properties of pure and Eu3+ doped Sr2CeO4 phosphors were reported by Li et al.10 Raman spectra of the Sr2CeO4 phosphor were investigated by Zhang et al.11 The luminescence properties were reported for Sr2CeO4 single-doped with Yb3+,12 Er3+,13 Eu3+,14–16 Sm3+,14 Dy3+17 and double-doped with Eu3+ and Dy3+18 and Eu3+ and Gd3+.19 In a course of studies of optical properties of Sr2CeO4 phosphors, it was observed that they are strongly affected on average particle size. It is commonly accepted that the Sr2CeO4 fine powders demonstrate a tendency to decreasing emission efficiency with reduction of grain size. The first attempt to compare the spectroscopic properties of Sr2CeO4 microcrystals with different grain sizes was performed by Mata)Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0021-9606/2015/142(18)/184701/7/$30.00
sui et al.20 Ling Li et al. have investigated the optical behavior of excitation and emission bands in bulk and nanocrystalline phosphors where they have observed a significant red shift of both the bands in nanocrystalline powders in comparison to the bulk.21 In the present work, we report the synthesis and optical studies of Sr2CeO4 nanocrystals. In particular, the impact of nanocrystal grain sizes on their optical characteristics was investigated.
II. EXPERIMENTAL
The Sr2CeO4 nanocrystals were obtained by Pechini’s method. Strontium nitrate—Sr(NO3)2 (Acros Organics, 99+%) and cerium nitrate—Ce(NO3)3·6H2O (SERVA, pure) were used as raw materials. Appropriate amounts of the reagents were dissolved in deionized water. Then citric acid was added (in molar ratio equal to 1:5 with respect to cations) to the solution under intense stirring and heating. Ethylene glycol was added in molar ratio equal to 1:1 as compared to citric acid several hours later. The obtained solution was kept in a laboratory dryer for two days at 90 ◦C. Subsequently, the sample was placed in an oven and annealed at a temperature selected from 750, 800, 850, 900, 1000, and 1050 ◦C. X-ray diffraction (XRD) spectra were measured with an X’Pert PRO powder diffractometer (PANalytical, The Netherlands) equipped with a linear PIXcel detector and using Cu Kα radiation (λ = 1.540 56 Å). The transmission electron microscopy (TEM) analysis was performed using an FEI Tecnai G2 20 X-TWIN. The absorption spectra were measured in the back scattering mode using a Cary Varian 5E UVVis-NIR spectrometer. Two types of lamp were used as the excitation source: a deuterium lamp for region 185–350 nm and a halogen lamp for region 350–3300 nm. An R928 photomultiplier tube was used for detection in ultraviolet and visible (UV–VIS) range of spectrum and a cooled PbS detector for near infra-red region (NIR). The spectrometer was supplied with two gratings: 1200 lines/mm with blaze at 250 nm for UV–VIS range and 300 lines/mm with blaze at
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FIG. 1. Visualization of orthorhombic Sr2CeO4 structure (a) and immediate environment of the Ce4+ ion (b).
1192 nm for NIR range. The beam was chopped at 30 Hz repetition. The luminescence decay profiles and emission and excitation spectra were measured using an FLS980 Fluorescence Spectrometer from Edinburgh Instruments. The emission and excitation spectra were corrected with respect to the detector sensitivity and the lamp characteristics.
III. RESULTS AND DISCUSSION A. Crystal structure and morphology
Sr2CeO4 crystallizes in orthorhombic space system in Pbam group. Fig. 1 presents the Sr2CeO4 crystal structure and close environment of the Ce4+ ion. One can see that the structure of Sr2CeO4 consists of polyhedrons. Their centers are occupied by Ce4+ while corners by O2− ions. Ce4+ ions are 6-fold coordinated at only one crystallographic position. There occur two different distances between Ce4+ and O2− ions which are equal to 2.201 Å and 2.308 Å. In consequence, two sites of cerium are observed. The unit cell parameters of Sr2CeO4 were determined to be a = 6.1190 Å, b = 10.3495 Å, and c = 3.5970 Å. The Sr2CeO4 nanocrystals were annealed at different temperatures: 750, 800, 850, 900, 1000, and 1050 ◦C. The corresponding X-ray diffraction patterns are presented in Fig. 2(a). One can see that the sample annealed at 750 ◦C was amorphous. Pure orthorhombic phase of Sr2CeO4 was obtained above this temperature. The absence of additional peaks in XRD patterns confirms phase purity of the obtained nanocrystals. Moreover, with increase of annealing temperature, narrowing of diffraction peaks was observed,
which is associated with increase of crystallites size at higher temperatures. Using Rietveld refinement method, the grain sizes were determined. The average crystallite sizes calculated for annealing temperatures 800, 850, 900, 1000, and 1050 ◦C were equal to 55, 68, 73, 140, and 217 nm, respectively. Fig. 2(b) presents the dependence of grain sizes on the annealing temperature. It was found that the grain size shows a significant nonlinear increase with the calcination temperature, from 60 nm at 800 ◦C to 220 nm at 1050 ◦C. The crystal cell parameters of Sr2CeO4 nanocrystals annealed at different temperatures were determined by Rietveld refinement. Results of the calculations are presented in Table I. TEM pictures of Sr2CeO4 nanocrystals are presented in Fig. 3. It can be seen that the grains of the samples were well structured but with a tendency to agglomerate. The average size of the grains was determined to be about 50 and 200 nm. The sizes obtained from the TEM images are comparable to the values calculated using Rietveld refinement. B. Optical properties
Fig. 4 presents the scheme of the electronic relaxations in Sr2CeO4 host. After excitation in UV, an electron is transferred from the valence band to the conduction band associated in absorption spectra with a low spin singlet excited state (LS) being a spin-allowed transition ∆S = 0 and with the high spin triplet excited state (HS) being the spin forbidden transition ∆S = 1. Subsequently, the nonradiative relaxation from the conduction band to the emitting levels of cerium occurs. As the result, emission from two energy levels of metal-ligand charge transfer (MLCTterm and MLCTeq) can be observed,
FIG. 2. The X-ray diffraction patterns (a) and the dependence of grain size of Sr2CeO4 nanocrystals on annealing temperature (b).
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TABLE I. Cell parameters of Sr2CeO4 nanocrystals annealed at different temperatures. Cell parameters Sample
a (Å)
b (Å)
c (Å)
β (Å)
V (Å3)
Grain size (nm)
Rp (%)
Reference (ICSD no. 50-0115) 800 ◦C 850 ◦C 900 ◦C 1000 ◦C 1050 ◦C
6.12 6.12 6.12 6.12 6.12 6.12
10.35 10.35 10.34 10.34 10.35 10.35
3.60 3.60 3.60 3.60 3.60 3.60
90 90 90 90 90 90
228 228 228 228 228 228
... 55 68 73 140 217
... 2.05 1.91 2.52 2.02 2.13
with the energy distances between the band maxima equal to the energy differences between the emitting levels of Ce4+ ions.10 Fig. 5 presents the absorption, excitation, and emission spectra of Sr2CeO4 nanocrystalline powders characterized by different average grain sizes. The origin of both MLCT bands is associated with the LS and HS transitions of Ce4+ ion, respectively.22 The Ce4+ ions are coordinated to two different oxygen atoms forming two different Ce4+ sites. Such a behavior is evidenced clearly in excitation spectra for the HS absorption band at 320–360 nm related to the Ce–O2 site. It is interesting also to see that with increasing grain size, the intensity of the HS absorption band in the excitation spectra increases. The dependence of HS absorption band intensity on the average grain size is shown in Fig. 5(e). The excitation spectra of Sr2CeO4 nanocrystals were recorded at λem = 580 nm at room temperature. The spectra consist of inhomogeneously broadened absorption bands with the maxima at 275 nm and 340 nm. One can see that with increasing grain size, there is a slight red shift of absorption bands both in the absorption and excitation spectra. The relationship between the energies of absorption bands and the size of spherical type nanocrystals is described in terms of 1/R,2 where R is the average radius within the effective mass approximation based on quantum confinement.23 Another approach to size effect based on classical physics properties of dielectric sphere predicts 1/R dependence.24 Since the elemental charges on the surface of small spherical capacitors contribute to the total electric potential, it can be considered as size dependent capacitance of nanocrystalline sphere. The size dependence of red shift of absorption and excitation bands observed in our measurements is shown in Fig. 5(d).
It is interesting to note that there are no red shifts for the emission bands. They do not change with the grain size. The emission of charge transfer (CT) transition to the singlet ground state of Sr2CeO4 Ce4+–(4f 0)–O2−(2p6) is the spin forbidden process that occurs with the spin change, and it may be responsible for a negligible small energy red shift with the size. A similar observation was also reported for Sr2CeO4 bulk and nanocrystals.20,25 The characteristic energy features associated with absorption, excitation, and emission are listed in Table II. To get an insight into the nature of electron-phonon interaction, we have determined the Huang–Rhys parameter S 26–28 ∆Es = (2S − 1)~ω,
(1)
where ∆Es describes the Stokes shift and ~ω is the effective phonon energy. Taking the effective phonon energy of Sr2CeO4 crystal to be ~ω = 560 cm−1,11 the Huang–Rhys factor S could be determined. The magnitudes of Stokes shift and Huang–Rhys parameter were determined from the emission and excitation spectra for the Sr2CeO4 nanocrystals with different grain sizes using the data listed in Table III for the respective LS and HS transitions. One can see that the Stokes shift and Huang–Rhys parameters decrease with increasing the grain size. They are much larger for the MLCT absorption bands associated with the LS transition than with the HS transitions. It is reasonable to assume that the emission intensity is more efficient when the Stokes shift is larger because there is no overlap between the absorption and emission transitions restricting the nonradiative interion transitions. It is the reason why there is no correspondence between absorption and excitation spectra because the largest Stokes shift is associated with more energetic LS absorption transitions.
FIG. 3. TEM images of Sr2CeO4 nanocrystals annealed at 800 (a) and 1050 ◦C (b).
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grains. For the smallest grain sizes, it becomes lower by almost 50%. D. Luminescence kinetics
FIG. 4. Diagram of electronic relaxations in Sr2CeO4 nanocrystals.
C. Luminescence quantum efficiency (QE)
The luminescence QE for the Sr2CeO4 nanocrystals with different grain sizes was measured at room temperature. It increased from 9.43% for the smallest grains up to 15.70% for the largest ones. A correlation between the quantum efficiency and the grain sizes is presented in Fig. 6. It was found that the QE increased significantly according to the following relation: QE = QE L 1 − D1 , where D means the average grain size of Sr2CeO4 nanocrystals and QE L is a proportionality parameter. One can conclude that the quantum efficiency increases with the grain size reaching a maximum of 15.5% for largest
Fig. 7 presents the luminescence decay profiles of Sr2CeO4 nanocrystals characterized by different average grain sizes recorded at room and liquid nitrogen temperatures. One can see that the decay profiles were almost perfectly exponential and the decay times shortened with increasing the grain size. The observed exponential decays suggest that only one Ce4+ site contributes in dominant way to the luminescence decay and that there is no cooperative interactions between Ce4+ ions that could be responsible for nonexponential decay profiles of MLCT emission transitions. Another important feature is that the luminescence decay times measured at room temperature were significantly shorter, by more than one order of magnitude, than those measured at 77 K. It means that nonradiative relaxation is associated predominantly with multiphonon transitions. E. Discussion of grain size effect
The decay rate k of MLCT emission is the sum of radiative krad and nonradiative k nrad transitions k = krad + knrad .
(2)
FIG. 5. The absorption (a), excitation (monitored at 21 277 cm−1) (b), emission (excited at 36 630 cm−1) (c) spectra for Sr2CeO4 nanocrystals with different grain sizes recorded at 300 K, and the grain size dependence of LS peak position (d) and HS intensity (e) of Sr2CeO4 nanocrystals.
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TABLE II. The energy features observed in absorption, excitation, and emission spectra for different grain size Sr2CeO4 nanocrystals.
Grain size (nm) 55 68 73 140 217
Absorption spectra (cm−1)
Excitation spectra (cm−1)
LS
HS
LS
HS
38 892 38 692 38 594 38 000 37 275
29 801 29 546 29 709 29 702 28 602
37 353 36 871 36 552 35 614 35 372
31 457 30 316 29 776 28 987 28 956
Emission spectra (cm−1)
20 945
It is well known that rates of radiative transitions are weakly affected by temperature. So a main contribution to the temperature dependence of decay rate should be associated with nonradiative transitions. The observed decay time significantly decreased with the grain size, extremely for smaller ones. One can note that the influence of grain size was more pronounced at low temperature measurements— with increasing temperature, the decay time decreased. The correlation between the decay rates and grain sizes of the investigated samples is shown in Fig. 7(c). One can see that the decay rates, both at 77 and 300 K, increased with increasing the grain sizes of Sr2CeO4 nanocrystals. The enhancement of decay rate may be associated both with radiative and nonradiative multiphonon transitions. There appears a question how the grain size affects these relaxation channels. The influence of size on rate constant of electronic relaxation processes is associated with the local electric field E loc in dielectric medium. In general case, it is introduced in the description of transition probabilities as the correction factor for the local ( loc ) 2 field within the Lorentz model f L (n) = EE ,29–31 where E is the strength of macroscopic electric field. The refractive index of the medium n is replaced by effective refractive index ne . It must be taken into account in any formula describing the rate of radiative and nonradiative transitions. The effective refractive index is dependent on frequency of a photon participating in the relaxation process (photon dispersion). It is well known that n increases extremely strongly for high energy photons in UV and visible ranges. It remains practically constant for low frequency quanta (infrared region). So it means that the multiphonon processes in an isolated nanocrystal are not sensitive to changes of effective refractive index and in consequence to decreasing the grain size.32 TABLE III. The parameters of Stokes shift and Huang–Rhys factor with the effective phonon energy for Sr2CeO4 nanocrystals with different grain sizes. Stokes shift ∆E s (cm−1) Grain size (nm) 55 68 73 140 217
Huang–Rhys factor S
LS
HS
LS
HS
15 712 15 712 15 444 14 408 13 932
10 512 9 371 8 831 8 042 8 011
14.5 14.5 14.3 13.4 12.9
9.9 8.9 8.4 7.7 7.6
Effective phonon energy ~ω (cm−1)
560
FIG. 6. The dependence of luminescence quantum yield on the average grain size of Sr2CeO4 nanocrystals.
The rate of electric dipole transitions of metal ion in nanocrystalline grain immersed in vacuum is described by the following formula:33,34 2 1 (3) k ∝ f (ED) (ne + 2) ne λ 0−2, 3 where f (ED) is the oscillator strength of electric dipole transition, λ 0 is wavelength in vacuum, and ne is the effective refractive index. It is usually calculated by using the volume mixing rule29–31 and may be expressed as the volume-mean refractive index, ne = f i ni , (4) i
where ni is the partial refractive index of the ith component and f i is the volume fraction. For two components, nanocrystal and vacuum, it may be expressed as ne = xn + (1 − x) no ,
(5)
where x is the fraction of the volume occupied by nanosized grains.29–31,35–40 Since the refractive index of vacuum is no = 1, the effective refractive index is given by ne = x (n − 1) + 1. This expression is valid assuming a uniform distribution of refractive index in the whole volume of nanocrystal. In reality, in such a system, the majority of ions are located at the surface of a nanocrystal. It is well known that the refractive index increases with the mass density. It means that the effective refractive index at the nanocrystal shell is much smaller than the refractive index of the nanocrystal core. Lü et al. have shown, using the assumption of spherical particle model, that a major influence to the optical relaxations is associated with the surface of nanoparticles (see Fig. 8).38 The density of the shell formed by Ln ions at the nanoparticle increases with its diameter D = 2r. It is reasonable to assume that the volume of the nanocrystal outside the shell composed from rare earth ions is simply related to the fraction of volume, 4/3πR3 − 4/3π(R − d)3 2d 3 = 1 − (1 − ), (6) D 4/3πR3
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FIG. 7. The luminescence decay profiles of Sr2CeO4 nanocrystals measured at 300 K (a) and 77 K (b). The size dependence of decay rate measured at 77 and 300 K (c).
where R is the nanocrystal radius and d is the shell thickness. After simple manipulation, this equation may be approximated by its leading component 6d D giving a major contribution to the volume fraction. It allows one to conclude that the oscillator strength is proportional to the reciprocal value of grain diameter, k ∝ D −1. In the course of fitting of optical dependences, it was found that they were well described by a simple formula D−1 or (1 − D −1), where D means the average diameter of nanocrystalline grain assuming its spherical shape. The presented above discussion rationalizes in a simple way the observed dependences of optical parameters of Sr2CeO4 nanocrystals on their average grain diameters. The role of the nanocrystal size was discussed earlier by several authors in terms of surface-to-volume ratio that quantitatively describes the linear relationship for luminescence parameters of colloidal solutions of Er3+ and Yb3+ codoped upconversion nanocrystals.39–42
IV. SUMMARY
The Sr2CeO4 nanocrystals were successfully prepared using the Pechini’s method and annealed at different temperatures. The XRD patterns confirmed clear orthorhombic phase of the nanocrystals. Only the Sr2CeO4 nanocrystalline powders calcinated at 750 ◦C were amorphous. The average grain sizes of Sr2CeO4 nanocrystals obtained at different annealing temperatures were determined using Rietveld refinement. It was observed that with increase of annealing temperature, the grain sizes increased significantly. Elucidating the impact of the average grain size on the optical characteristics was a main purpose of our investigations. It was observed that the optical properties associated with the absorption, excitation, and emission spectra of Sr2CeO4 nanocrystals were strongly affected by the average grain size. The quantum efficiencies for investigated samples were determined. It was found that the quantum efficiencies increase with increasing grain size. Moreover, the Huang–Rhys factors and Stokes shift significantly increased with grain size. Extremely high values of these factors are characteristic for the metalligand charge transfer transition bands. The luminescence decay profiles were well fitted by exponential curves which indicated limited participation of energy transfer between different Ce4+ ions. An increase of the average grain size decreases the Sr2CeO4 luminescence lifetimes. The size effect on optical characteristics was discussed in terms of effective refractive index assuming a leading role of the surface shell of nanocrystal grains.
ACKNOWLEDGMENTS
The work was supported by Wroclaw Research Centre EIT+ within the project “The Application of Nanotechnology in Advanced Materials”—NanoMat (POIG.01.01.02-02002/08) co-financed by the European Regional Development Fund (Operational Programme Innovative Economy, 1.1.2). 1E. Danielson, M. Devenney, D. M. Giaquinta, J. H. Golden, R. C. Haushalter,
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