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J. Phys. Chem. C 2009, 113, 610–617
Photoluminescence Properties of CeO2:Eu3+ Nanoparticles Synthesized by a Sol-Gel Method Ling Li,† Hyun Kyoung Yang,† Byung Kee Moon,† Zuoling Fu,† Chongfeng Guo,† Jung Hyun Jeong,*,† Soung Soo Yi,‡ Kiwan Jang,§ and Ho Sueb Lee§ Department of Physics, Pukyong National UniVersity, Pusan 608-737, South Korea, Department of Physics, Silla UniVersity, Pusan 617-736, Korea, and Department of Physics, Changwon National UniVersity, Changwon, 641-773, Korea ReceiVed: October 1, 2008; ReVised Manuscript ReceiVed: NoVember 4, 2008
Nanocrystalline Eu3+-doped CeO2, CeO2, Sm3+-doped CeO2, and Li+, Eu3+-codoped CeO2 samples were prepared through a sol-gel process. The structure and the optical properties of the samples were characterized by X-ray diffraction, diffuse reflection spectra, and photoluminescence spectra. No luminescence was observed for nanocrystalline CeO2. The systematic investigation shows that the broad band in the excitation spectrum of CeO2:Eu3+ comes from the charge transfer (CT) transition from O2- to Ce4+, not from the oxygen vacancy, or from the CT of O2- to Eu3+. Upon increasing the fired temperature from 600 to 800 °C, the excitation spectrum shifts to lower energy. With increasing concentrations of Eu3+ up to 1% in CeO2, red shifts of the excitation spectra are observed; however, when the concentration of Eu3+ increases to 5% and 10%, blue shifts occur. The emission spectrum shows that the symmetry of the Eu3+ site becomes lower with increasing Eu3+. Based on the dielectric theory of complex crystals, the environmental factor (he) and the dielectric definition of average energy gap around the centers of Eu3+ are calculated. The reasons for the shifts of the excitation spectra are discussed in detail. 1. Introduction An unusual blue phosphor Sr2CeO4 has been developed using the combinatorial techniques of Danielson et al.1 The emission was assigned to a ligands-to-metal charge transfer (CT) transition of Ce4+.2-5An efficient energy transfer can occur from the Ce4+-O2- CT state to the trivalent RE in Sr2CeO4:RE3+ (RE stands for rare earth element).5 Among the compounds containing Ce4+, cerium oxide has been widely investigated due to high chemical stability, good dielectric strength, high refractive index, and high efficiency for absorbing UV radiation.6 Although numerous past efforts have been devoted to measuring and understanding the Raman, X-ray,7 XPS8 of CexLn1-xO1-y, optical reflectivity,9 and electronic structures3 of CeO2, there is lack of an extensive investigation on the photoluminescence properties of CeO2:RE3+, especially a clear clarification of the luminescent mechanisms of CeO2:RE3+. Furthermore, the luminescent properties of CeO2 or CeO2:RE seem to depend on the preparation method. For example, the charge transfer emission of bulk and nanocrystalline CeO2 has not been reported except for that of the nanorods.10 The photoluminescence spectra of CeO2 nanorods show unusual light emission at 370 nm and were assigned to the charge transfer transition from the localized Ce 4f state to the O 2p valence band.10 However, if the CT emission can occur in CeO2 nanorods at room temperature, the CT emission would be observed in both bulk and nanocrystalline CeO2 because the charge transfer emission is independent of the morphology of the sample.11,12 Sr2CeO4 is a classic example of this as the charge transfer emission can be observed in both bulk13 and nanocrystalline Sr2CeO4.4,14-16 It is well-known that certain oxides, like SnO2, ZrO2, CaO, or silica, show efficient * Corresponding author. E-mail:
[email protected]. † Pukyong National University. ‡ Silla University. § Changwon National University.
defect luminescence.17-20 Then the emission may come from the deficiency of oxygen in CeO2 nanorods. The excitation spectrum of CeO2:Eu lies at 373 nm using solid state reaction, while it occurs at 340 nm in CeO2:Eu prepared by the Pechini sol-gel process.21,22 In all cases, no detailed mechanism was reported. It arouses our great interest to prepare nanocrystalline CeO2 and CeO2:Eu3+ and investigate their luminescence properties together with their mechanisms. Eu3+ was introduced into CeO2 for optically active centers.23 The effect of different doping concentrations of Eu3+ on the luminescence center Eu3+ in CeO2 may be twofold: one is a change of the Coulomb field which acts on Eu3+ ions, and the other is the oxygen vacancies, which are introduced into the crystal to compensate for the effective negative charge associated with the trivalent dopant. Different doping concentrations of Eu3+ will influence the photoluminescence properties of CeO2:Eu. In this paper, nanocrystalline Eu3+-doped CeO2 and CeO2 were prepared through a sol-gel process. Sm3+-doped CeO2, and Li+, Eu3+-codoped CeO2 samples were prepared to analyze the excitation mechanism of CeO2:Eu. The photoluminescence properties of CeO2:Eu3+ were investigated under different fired temperatures and different Eu3+ doping concentration. The excitation mechanism of CeO2:Eu3+ was discussed systematically. The shifts of the excitation spectra of CeO2:Eu3+ with the fired temperature and the concentration of Eu3+ were observed. The effects of doping concentration of Eu3+ on photoluminescence were investigated. The reasons for the shifts of the excitation spectra were discussed in detail. 2. Experimental Section 2.1. Preparation. The CeO2:1% Eu3+ samples were prepared by a Pechini-type sol-gel process.24 The starting materials were cerium nitrate hexahydrate Ce(NO3)3 · 6H2O (A.R.) and europium nitrate pentahydrate Eu(NO3)3 · 5H2O (A.R.). Stoichio-
10.1021/jp808688w CCC: $40.75 2009 American Chemical Society Published on Web 12/11/2008
Properties of CeO2:Eu3+ Nanoparticles
J. Phys. Chem. C, Vol. 113, No. 2, 2009 611 TABLE 1: Unit Cell Parameters and Crystal Size for CeO2:1% Eu3+ Nanocrystalline Samples Annealed at Different Temperatures temp crystal sample (deg) size (nm) a b c
Figure 1. XRD patterns of CeO2:1% Eu3+ sample annealed at different temperatures. The standard JCPDS card data for cubic CeO2 is provided as a reference.
metric amounts of the starting materials were dissolved in water and mixed under stirring. Then citric acid monohydrate (citric acid/metal ion ) 2:1) and polyethylene glycol (PEG, molecular weight ) 8000) were added and dissolved in the above solution. The resultant mixtures were stirred for 1 h and then heated at 75 °C in a water bath until homogeneous gels formed. After being dried, the gels were ground and prefired at 400 °C for 4 h, and then fully ground and fired at 700, 800, and 900 °C, respectively. The CeO2, CeO2:x% Eu3+ (x ) 0.1, 0.5, 1, 5, 10), CeO2:1% Sm3+, and CeO2:5% Eu3+ 5% Li+ samples were prepared under thesameconditionsasabove.ThestartingmaterialswereCe(NO3)3 · 6H2O (A.R.), Eu(NO3)3 · 5H2O (A.R.), Sm(NO3)3 · 6H2O (A.R.), and Li(C5H8O2). The final obtained samples were fired at 700 °C for 4 h. 2.2. Characterization. The structural characteristics of nanocrystalline samples were examined from X-ray diffraction (XRD) patterns using a Philips XPert/MPD diffraction system with Cu KR (λ ) 0.154 05 nm) radiation. The UV-visible diffuse reflectance spectra were recorded with a model V630 spectrophotometer. The excitation and emission spectra were measured at room temperature using time resolved fluorescence meter (PTI) with a 150 W Xe lamp as an excitation source. 3. Results and Discussion 3.1. Crystallization Behavior. The CeO2:1% Eu3+ samples were prepared by a Pechini-type sol-gel (SG) process. Figure 1 shows XRD patterns of CeO2:1% Eu3+ samples annealed at different temperatures from 600 to 800 °C at intervals of 100 °C. The diffraction peaks correspond to (111), (200), (220), (311), (222), (400), (331), and (420). They can be indexed as a pure cubic fluorite CeO2 structure, which coincides well with the standard data of CeO2 (JCPDS no. 65-2975). These diffraction peaks increase in intensity and decrease in width with increase of the annealing temperature due to improvement of crystallization and increase of the nanocrystalline size. The nanocrystallite size can be estimated from the Scherer formula16
Dhkl )
kλ β cos θ
(1)
where λ is the X-ray wavelength (0.154 05 nm), β is the full width at half-maximum (in radian), θ is the diffraction angle, k is a constant (0.89), and Dhkl represents the size along (hkl) direction.
600 700 800
15.2 27.8 52.8
cell parameter (Å) 5.385 75 ( 0.003405 5.392 51 ( 0.000860 5.401 81 ( 0.001313
R/O ) I(5D0 f F2)/ I(5D0 f 7F1)
7
0.0812 0.1306 0.2217
In addition, the refined crystallographic unit cell parameters of the calcined products were calculated using the software Jade 5.0.The calculated nanocyrstallite size and the unit cell parameters of the samples are listed in Table 1. The crystallite size of CeO2:1% Eu3+ increased with annealing temperature. They are 15.2, 27.8, and 52.8 nm for the samples fired at 600 °C (a), 700 °C (b), and 800 °C (c), respectively. This can be ascribed to the sintering effect of freestanding particles at higher temperature.25CeO2:x% Eu3+ (x ) 0.1, 0.5, 1, 5, 10) were prepared by a Pechini-type sol-gel (SG) process. Figure 2 shows the XRD patterns of the samples. The diffraction peaks can be indexed as a pure cubic fluorite CeO2 structure, which coincides well with the standard data of CeO2 (JCPDS no. 652975) The nanocrystallite sizes and the unit cell parameters of the samples annealed at 700 °C are calculated and listed in Table 2. Their sizes range from 22.8 to 28 nm. They are very close to each other probably due to the same fired conditions. However, with increase of the doping concentration of Eu3+ to 1%, the unit cell parameters of the compounds increase. This is perhaps because the radius of Eu3+ (r ) 0.1066 nm) is larger than the radius of Ce4+ (r ) 0.097 nm), and more Eu3+ occupies the Ce4+ site with increase of Eu3+ doping concentration. Strangely, when the doping concentration of Eu3+ is up to 5% and 10%, it becomes disordered. This is perhaps due to the existence of the many oxygen vacancies.
Figure 2. XRD patterns of CeO2:x% Eu3+(x ) 0.1, 0.5, 1, 5, 10) samples and the standard data for CeO2 (JCPDS no. 65-2975) as a reference.
TABLE 2: Unit Cell Parameters and Crystal Sizes for CeO2:x% Eu3+ (x ) 0.1, 0.5, 1, 5, 10) Nanocrystalline Samples Annealed at 700 °C sample 3+
CeO2:0.1% Eu CeO2:0.5% Eu3+ CeO2:1.0% Eu3+ CeO2:5.0% Eu3+ CeO2:10% Eu3+
crystal size (nm)
cell parameter (Å)
23.0 22.8 28.0 27.5 23.6
5.383 0 ( 0.001 318 5.394 85 ( 0.001 261 5.403 54 ( 0.001 215 5.398 60 ( 0.002 291 5.401 81 ( 0.002 461
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Figure 3. Photoluminescence spectra (λex ) 348 nm) of CeO2:1% Eu3+ annealed at 600 °C (a), 700 °C (b), and 800 °C (c).
3.2. Photoluminescent Properties. Under 348 nm Xe-lamp excitation, pure nanocrystalline CeO2 did not exhibit any luminescence, and the CeO2:1% Eu samples showed orange luminescence. Figure 3 shows the emission spectra of CeO2: 1% Eu3+ fired at 600 °C (a), 700 °C (b), and 800 °C (c) (named CeO600, CeO700, and CeO800). All the spectra show the typical emission of Eu3+ ions and can be assigned to the 5D0 f 5 7 7FJ transitions (J ) 0-4). The D0 f F1 transition located at 591 nm dominates. It originates from the parity-allowed magnetic dipole transition. The intensity of the 5D0 f 7F2 transition located at 611 and 630 nm increased with increasing temperature. They originate from the electronic dipole transition. CeO2 crystallizes in a cubic fluorite-type structure with space group Fm3m.26 The shapes of the unit cell of CeO2 containing four molecules, Ce-centered cube and skeleton construction with eight equatorial atoms O in CeO2, are shown below in Figure 9c. In this lattice, the point-group symmetry of Ce sites in the fluorite CeO2 structure is ideally Oh. It is well-known that the 5 D0 f 7F2 transition is hypersensitive, while the 5D0 f 7F1 transition is insensitive to the crystal field environment.27-29 In a site with an inversion symmetry, the magnetic dipole transition 5 D0 f 7F1 is dominant, while in a site without inversion symmetry, the 5D0 f 7F2 electronic transition becomes the strongest one.30,31 Therefore, the intensity ratio of the transitions 5 D0 f 7F2 to 5D0 f 7F1 (eq 2) is a good measure of the symmetry of the Eu3+ site. The R/O values for the CeO2:1% Eu3+ fired at 600 °C (a), 700 °C (b), and 800 °C (c) are listed in Table 1. The values increase with increasing fired temperature. It indicates that the Eu3+ mainly occupies the site Oh with inversion symmetry, and the local symmetry of Eu3+ ions becomes lower with increasing temperature. It may be due to the increase of the distorted lattices and the oxygen deficiencies in the nanocrystalline CeO2:Eu3+.32 5 7 R I( D0f F2) ) 5 O I( D f7F ) 0
(2)
1
Figure 4 shows the excitation spectra of CeO2:1% Eu3+ fired at 600 °C (a), 700 °C (b), and 800 °C (c) by monitoring the 5 D0 f 7F1 transition of Eu3+. They were very broad bands from 250 to 375nm with maximum at 340, 350, and 364 nm, respectively. In general, the charge transfer transition from the ligands to the Eu3+ ions can easily occur in Eu3+-doped compounds. The width of the Eu-O CT band was about 0.91 ( 0.26 eV. 33The
Figure 4. Excitation spectra of CeO2:1% Eu3+ samples annealed at 600 °C (a), 700 °C (b), and 800 °C (c) (λem) 591nm).
widths of the excitation spectra of CeO2:1% Eu3+ were about 0.67 eV, which falls within the range of CT transitions of Eu3+. Some authors excluded the possibility of Eu-O CT because it has been regarded that the O2--Eu3+ CT states lie at much higher energy levels at around 36 500-33 700 cm-1 (corresponding to 274-296 nm).34 However, the CT energies of some Eu3+-doped oxides, such as La26O27(BO3)8:Eu3+ (300 nm),35 LaAlO3:Eu3+ (310 nm),36,37 SrLa2BeO5:Eu3+ (320 nm),38 etc., are out of the range 274-296 nm. And also the CT energy of Eu3+ in the compounds has a quantitative relationship with the environmental factor (he).39,40 It can be expressed according to the following equation:
ECT ) 2.804 + 6.924 × exp(-1.256 × he)
(3)
With increase in the environmental factor, the CT energy decreases.40 Taking the sample CeO2:1% Eu3+(a) as an example, the lattice constant is 5.385 75 ( 0.003 405 Å and the refractive index of CeO2 is about 2.54.3 According to the dielectric theory of complex chemical bonds,40-42 the covalency and the polarizability can be calculated. They are 0.2032 and 0.7496, respectively. Then the environmental factor (he) can be obtained (he) 2.2077) using the relative equation40
he )
(∑ fcµRµbQBµ2) ⁄ 1
2
(4)
µ
where fcµ is the covalency for any individual bond µ surrounding the center ion in a multibond crystal, Rbµ is the polarizability of the ligand bond volume in the type-µ bond (in Å3), and QµB stands for the presented charge of the ligands in the chemical bond of the type-µ. The charge transfer energy from O2- to Eu3+ in the sample CeO2:1% Eu3+ (a) can be calculated using eq 3.The predicted charge transfer is 3.24 eV (382.7 nm). Considering the error of prediction, the CT energy from O2- to Eu3+ is about 3.24 ( 0.13 eV (368-397 nm).40 It is very close to the excitation spectra of CeO2:1% Eu3+. Therefore, further experiments are necessary to analyze whether the broad excitation spectrum originates from the CT energy of Eu3+-O2-. CeO2:Sm3+ was selected to solve the question. This is because the CT energy of Sm3+ ions is always higher by 1.21 eV than the CT energy of trivalent Eu in the same compound. 33,43 The relationship between the CT energy of Eu3+ and Sm3+ in the same host lattice can be written as
ECT(Sm3+:A) ) 1.21 eV + ECT(Eu3+:A) (eV)
(5)
where A stands for the host lattice. If the excitation spectrum of CeO2:1% Eu3+ (b) (350 nm; i.e., 3.54 eV) is the CT band of
Properties of CeO2:Eu3+ Nanoparticles Eu3+, then the excitation spectrum of CeO2:1% Sm3+ should be positioned at 261 nm (4.75 eV). Figure 5 shows the excitation spectra of CeO2:1% Eu3+ and CeO2:1% Sm3+ fired at 700 °C by monitoring the 5D0 f 7F1 transition of Eu3+ (591 nm) and the 4G5/2 f 6H5/2 of Sm3+ (574 nm). The photoluminescence excitation spectra reveal that both of them have the same excitation bands peaking at 353 nm. The same phenomenon has been observed in CeO2:RE3+ (RE3+ ) Eu3+ and Sm3+) thin films.34,44 If both of the trivalent Eu3+ and Sm3+ are doped into CeO2, the above experimental phenomenon can successfully demonstrate that the broad excitation spectra do not come from the CT from the O2- to the trivalent rare earth. Figure 6 shows the photoluminescence spectra of the CeO2:1% Eu3+ and CeO2: 1% Sm3+ using 353 nm excitation. Two groups of strong emission bands of Sm3+ are observed; one is at 574 nm from the transition of 4G5/2 f 6H5/2, and the other is at 616 and 619 nm from the transition of 4G5/2 f 6H7/2. The 5D0 f 7F1 transition located at 591 nm is observed in CeO2:1% Eu3+. They show that the trivalent Eu and Sm ions have been successfully doped into the CeO2. Moreover, the efficiency of ultraviolet light excitation was promoted, and the subsequent energy was transferred to Eu3+ and Sm3+ in photoluminescence. Also, the optical diffuse reflectance spectrum of CeO2 obtained using the sol-gel method (CeO2-SG) is measured. As
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Figure 7. Optical diffuse reflectance spectrum (black) of (CeO2-SG) and excitation spectrum of CeO2:0.1% Eu3+ (red).
Figure 8. Excitation spectra of SG-CeO2:1% Eu3+ and SS-CeO2:1% Eu3+.
Figure 5. Photoluminescence excitation spectra for CeO2:1% Eu3+ and CeO2:1% Sm3+, respectively.
Figure 6. Emission spectra of CeO2:1% Eu3+ and CeO2:1% Sm3+ with 353 nm excitation.
shown in Figure 7, comparing with the excitation spectrum of CeO2:0.1% Eu3+, it is can be seen that they have the same peaks at 348 nm. From the analysis above, it can be demonstrated that the broad excitation bands in the samples CeO2:Eu3+ do not come from the CT transition from O2- to Eu3+, but they may come from the CT transition from O2- to Ce4+ or from the oxygen vacancy. In the preparation of CeO2:Eu3+ via the Pechini-type sol-gel (SG) process, more oxygen vacancies can be created compared to the method through the solid state reaction (SS). As has been reported for ZrO2, if the excitation spectra come from the oxygen vacancy, the spectra intensity of the sample via SG is stronger than that of the sample through SS.20 Usually, no luminescence can be observed for the sample via SS.20 In order to check whether the broad excitation spectra come from the oxygen occupancy, we prepared CeO2:1% Eu3+ sample through the solid state reaction (SS-CeO2:1% Eu). In this case, the SS-CeO2:1% Eu sample was prepared by direct annealing of CeO2 and Eu2O3. However, the resulting SS-CeO2:1% Eu shows a much stronger excitation spectrum than SG-CeO2:1% Eu (the excitation spectra are shown in Figure 8). From the above results, we can safely conclude that the excitation spectrum is not from the oxygen vacancy. Therefore, the excitation spectrum can be assigned to the charge transfer from the O2- to Ce4+.3,21,22,34,44 However, as compared with nanocrystalline Sr2CeO4, only one excitation peak of CeO2:1% Eu is observed (Figure 9f),
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Figure 9. Crystal structures, excitation spectra, and energy level diagrammatic sketches of nanocrystals of Sr2CeO4 and CeO2: (a) shape of the unit cell of Sr2CeO4 that contains two molecules; (b) Ce-centered octahedron and skeleton construction with four equatorial atoms O2 and two Ce-O1 in Sr2CeO4; (c) shape of the unit cell of CeO2 containing four molecules; (d) Ce-centered cube and skeleton construction with eight equatorial atoms O in CeO2 and CeO2:1% Eu3+(λem) 591 nm, temperature ) 600 °C), respectively (the O atoms are red, Ce atoms are light blue, and the Sr atoms are light gray). (g) and (h) indicate the energy level diagrams with schematic illustration of the excitation spectra bands for nanocrystals of Sr2CeO4 and CeO2:1% Eu3+, respectively. (The conduction bands and the valence bands are shown using the rectangles; Eg and ECT indicate the average energy gap of the chemical bond and the charge transfer energy, respectively).
TABLE 3: Average Energy Gaps of Different Chemical Bonds Ce-O in CeO2:1% Eu3+ Samples Annealed at 600 °C (a), 700 °C (b), and 800 °C (c) and Comparison of the Average Energy Gaps of Two Different Chemical Bonds, Ce-O1 and Ce-O2, in Sr2CeO4 sample a b c Sr2CeO4
space group Fm3jm Z)4 Pbam Z)2
cell parameters (Å)
bond type
length (Å)
coordination number (C.N.)
Eµg (eV)
µ ECT (eV)
a ) 5.385 75 ( 0.003 405 a ) 5.392 51 ( 0.000 860 a ) 5.401 81 ( 0.001 313 a ) 6.109 5 ( 0.001 553 b ) 10.337 2 ( 0.019 72 c ) 3.595 7 ( 0.000 717
Ce-O Ce-O Ce-O Ce-O1
2.3321 2.3350 2.3391 2.1979
8 8 8 2 4
10.7960 10.7641 10.7491 19.1455
3.65 3.52 3.41 5.12
Ce-O2
2.3059
13.2907
3.85
and the CT excitation energy of CeO2 (340 nm) is lower than the three CT peaks of Sr2CeO4. The reason can be given based on the dielectric theory of complex chemical bonds.41,45 It is well-known that three CT excitation peaks at 242, 279, and 322 nm (Figure 9e) are observed in nanocrystalline Sr2CeO4.4 As shown in Figure 9a, Sr2CeO4 is orthorhombic, with the space
group Pbam (no. 55).46 There are two molecules per unit cell, and each cerium atom is coordinated by six oxygen atoms, as shown in Figure 9b. The octahedron presents two trans-terminal Ce-O1 perpendicular to the plane defined by four equatorial O2 atoms, while the Ce-O1 bonds are about 0.1 Å shorter than the Ce-O2 bonds.
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Figure 10. Energy level diagrams with schematic illustration of the excitation spectra of the nanocrystals of CeO2:1% Eu3+ annealed at 600 °C (a), 700 °C (b), and 800 °C (c). Their excitation spectra are shown below the energy level diagrams (the red is the spectrum of sample annealed at 600 °C; blue, the spectrum of sample annealed at 700 °C; black, the spectrum of sample annealed at 800 °C).
TABLE 4: Excitation Spectral Positions, Intensity Ratio of the Transitions 5D0 f 7F2 to 5D0 f 7F1 for CeO2:x% Eu3+ (x ) 0.1, 0.5, 1, 5, 10) Nanocrystalline Samples Annealed at the Same Conditions
Figure 11. Excitation spectra of CeO2:x% Eu3+ (x ) 0.1, 0.5, 1, 5, 10) nanocrystalline samples.
It is known that CeO2 crystallizes in a cubic fluorite-type structure with space group Fm3m.26 Ce-centered cube and skeleton construction with eight equatorial atoms O in CeO2 are shown in Figures 9c and 9d. There is only one kind of Ce-O bond in CeO2, so there is only one broad CT excitation peak in CeO2. The average energy gaps of Ce-O in CeO2:1% Eu3+ can be predicted using the dielectric theory of complex crystals.45 The values are shown in Table 3.
sample
excitation spectral position (nm)
R/O ) I(5D0 f 7F2)/ I(5D0 f 7F1)
CeO2:0.1% Eu3+ CeO2:0.5% Eu3+ CeO2:1.0% Eu3+ CeO2:5.0% Eu3+ CeO2:10% Eu3+
348 350 353 350 343
0.0625 0.0954 0.1655 0.6667 1.3513
The Eg of the sample CeO2:1% Eu3+ fired at 600 °C is 10.7960 eV, which is lower than the average energy gap of the Ce-O2 bond in Sr2CeO4. CT energy increases with an increase of Eg,45 and the CT energy of CeO2:1% Eu3+ (340 nm) is lower than the O22--Ce4+ CT energy of Sr2CeO4 (322 nm). It is in agreement with the experimental phenomenon (Figures 9e and 9f). Figures 9g and 9h indicate the energy level diagrams with schematic illustration of the excitation spectra bands for the nanocrystals Sr2CeO4 and CeO2:1% Eu3+, respectively. The CT energies are indicated using solid double arrows. They correspond to the energy difference between the valence band (VB) and the ground state of Ce3+(4f0), i.e., (VB f 2F5/2)43. According to the predicted values of Eg, it can be known that the valence band of Ce-O in CeO2:1% Eu3+ is much higher than the valence band of Ce-O1 or Ce-O2 in Sr2CeO4.. Therefore, the transition energy from the VB of the bond Ce-O in CeO2:1% Eu3+ to
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TABLE 5: Charge Transfer Energies (ECT) in Eu3+-Doped CeO2, Environmental Factors he, he ) (∑µ fcµrbµQBµ2)1/2, as Well as Some Chemical Bond Parameters, Such as Covalency fc, Polarizability of the Chemical Bond Volume rb, and Presented Charge of the Nearest Anion QB in the µ Type of Chemical Bonds sample 3+
CeO2:0.1% Eu CeO2:0.5% Eu3+ CeO2:1.0% Eu3+ CeO2:5.0% Eu3+ CeO2:10% Eu3+
distance (dCe-O) (Å)
fc
Rb
QB
coordination number (C.N.)
he
ECT (eV)
2.3310 2.3360 2.3398 2.3377 2.3391
0.2032 0.2031 0.2029 0.2031 0.2031
0.7486 0.7542 0.7577 0.7562 0.7579
2 2 2 2 2
8 8 8 8 8
2.2060 2.2141 2.2178 2.2170 2.2193
3.563 3.543 3.513 3.543 3.615
the ground state of Ce3+ is lower than the transition energy from the valence bands of Ce-O1 and Ce-O2 to the ground state of Ce3+ in Sr2CeO4. So the CT energy of CeO2:1% Eu3+ is lower than the CT energy of Sr2CeO4. Figure 4 shows that the CT peaks of CeO2:1% Eu3+ shift to lower energy with increase of the fired temperature. This is because the dielectric energy gaps Eg decrease in the sequence CeO600, CeO700, and CeO800 (Table 3). It is known that the CT energy increases with increase of Eg,43 so that the CT energy decreases in the following order:
ECT(CeO600) > ECT(CeO700) > ECT(CeO800) It can be explained using the energy level diagrams with schematic illustration of the excitation spectra of the nanocrystals of CeO2:1% Eu3+, as shown in Figure 10. The VB energy level of the bond Ce-O increases in the following sequence:
VB(CeO600) < VB(CeO700) < VB(CeO800)
to the increase of the oxygen vacancy when the concentration of Eu3+ is very high. Figure 12 shows the emission spectra of CeO2:x% Eu3+ (x ) 0.1, 0.5, 1, 5, 10) nanocrystalline samples. The intensity ratios of the transitions 5D0 f 7F2 to 5D0 f 7F1 measuring the symmetry of the Eu3+ site are calculated and listed in Table 4. With the increase of the concentration of Eu3+, the values of R/O increase. This indicates that the symmetry around the center ions decreases. The number of ligands coordinating to the center ions changes, which leads to the change of the environmental factor. The fact he is not the same as the ideal he (in Table 4). Then the point-group symmetry of the center ions’ Ce4+ site is not ideally Oh. 23 In order to investigate the influence of the oxygen vacancy on the charge transfer energy of CeO2:5% Eu3+, CeO2:5% Eu3+ 5% Li+ was prepared, in which oxygen vacancies are compensated by codoping of Li+ ions, as reported in ref 47. The intensity of the excitation spectrum and the emission in CeO2:
It is shown in Figure 10. The ground states of the Ce3+ in the three samples are almost the same due to the shield of 4f energy orbitals by the 5s and 5p. Therefore, the CT energy corresponding to the transition energy from the CB to the ground state of Ce3+ decreases in the following order:
ECT(CeO600) > ECT(CeO700) > ECT(CeO800) Figure 11 shows excitation spectra of CeO2:x% Eu3+ (x ) 0.1, 0.5, 1, 5, 10) nanocrystalline samples by monitoring the 5 D0 f 7F1 transition of Eu3+ (591 nm) (named Ce0.1%Eu, Ce0.5%Eu, Ce1%Eu, Ce5%Eu, and Ce10%Eu). The spectra consist of a broad band and some sharp lines. The broad bands are assigned to the CT of Ce-O, and the sharp lines are assigned to the intratransition of 4f. From Figure 11, we can find that for CeO2:x% Eu3+ powders with low concentration of Eu3+, the host excitation band is dominant and the bands shift to higher energy upon raising the concentration of Eu3+. By increasing the Eu3+ concentration, the intratransitions of the 4f shell of Eu3+, especially the one at 467 nm, become stronger; however, a blue shift of broad bands occurred. The excitation peaks of the CeO2:x% Eu3+ powders are listed in Table 4. The charge transfer energy is strongly influenced by the environmental factor (he) quantitatively.40 The unit cell parameters are listed in Table 2. Using the known refractive index of CeO2, n ) 2.54, 3 the environmental factors can be calculated.40 The values are shown in Table 5. The calculation is based on the hypothesis that the point-group symmetry of the center ions’ Ce4+ site is ideally Oh. As can be shown from the results of the environmental factors, at low concentration of Eu3+, he increases with increase of the concentration of Eu3+. It is known that with the increase of he the charge transfer energy decreases.40 Then the charge transfer energy from O2- to Ce4+ shifts to lower energy when the concentration increases from 0.1% to 1%. However, when the concentration of Eu5+ is 5% or 10%, the charge transfer energy becomes higher although the he increases. It may be due
Figure 12. (Above) Emission spectra of CeO2:x% Eu3+ (x ) 0.1, 0.5, 1, 5, 10) nanocrystalline samples (λex ) 548 nm). Below is the enlarged emission spectrum of CeO2:10%Eu3+.
Properties of CeO2:Eu3+ Nanoparticles
J. Phys. Chem. C, Vol. 113, No. 2, 2009 617 had been included. The correct version was published on December 19, 2008. References and Notes
Figure 13. Excitation spectra and emission spectra of CeO2:5% Eu3+ and CeO2:5% Eu3+ 5% Li+ obtained using the sol-gel method.
5% Eu3+ 5% Li+ increases. But the R/O decreases (0.59), thereby demonstrating that Li+ compensates for the oxygen vacancies. As shown in Figure 13, the excitation peak of CeO2: 5% Eu3+ 5% Li+ is at 356 nm, which shifts to lower energy about 5 nm compared with that of CeO2:5% Eu3+. However, in the large doping regime, the charge transfer energy increases with increase of the oxygen vacancy. Therefore, blue shifts of the CeO2:5% Eu3+ and CeO2:10% Eu3+ are observed. 4. Conclusion Nanocrystalline CeO2, Eu3+-doped CeO2, Sm3+-doped CeO2, and Li+ and Eu3+-codoped CeO2 samples have been successfully prepared through a Pechini-type sol-gel process. No emission was observed in the photoluminescence emission spectrum of undoped CeO2 at room temperature. The structure and the optical properties of the resulting samples were characterized by X-ray diffraction, diffuse reflection spectra, and photoluminescence excitation and emission spectra. These results show that the excitation spectra of CeO2:Eu3+ do not come from the oxygen vacancy or from the CT from O2- to Eu3+; however, they are attributed to the charge transfer (CT) transition from O2- to Ce4+. With increasing fired temperature in the range of 600-800 °C, the CT peak shifts to lower energy. This is due to the decrease of the average energy gap. With an increase of concentration of Eu3+ up to 1% in CeO2, red shifts of the excitation spectra are observed, and at 5% and 10% blue shifts occur. These compounds, in the low doping regime, maintain the fluorite structure of CeO2. The lattice constant increases with increasing concentration of Eu3+, which leads to an increase of the environmental factor. In the high doping regime, the sites of Eu3+ are strongly affected by the oxygen vacancy. The emission spectrum shows that the symmetry of the Eu3+ site becomes lower with increasing Eu3+. Acknowledgment. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2007-412J00902). This work was also partially supported by a grant-inaid for the National Core Research Center Program from MOST and KOSEF (no. R15-2006-022-03001-0). Note Added after ASAP Publication. This article was published ASAP on December 11, 2008 before all the revisions
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