Luminescence Properties of Europium-Doped Cerium Oxide

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Luminescence Properties of Europium-Doped Cerium Oxide Nanoparticles: Role of Vacancy and Oxidation States Amit Kumar,† Suresh Babu,† Ajay Singh Karakoti,† Alfons Schulte,‡ and Sudipta Seal*,†,§ †

Advanced Materials Processing and Analysis Center (AMPAC), Department of Mechanical, Materials and Aerospace Engineering, ‡Department of Physics and §NanoScience Technology Center, University of Central Florida, 4000 Central Florida Boulevard, Orlando, Florida 32816 Received April 12, 2009. Revised Manuscript Received July 27, 2009 Enhancing the optical emission of cerium oxide nanoparticles is essential for potential biomedical applications. In the present work, we report a simple chemical precipitation technique to synthesize europium-doped cerium oxide nanostructures to enhance the emission properties. Structural and optical properties showed an acute dependence on the concentration of oxygen ion vacancy and trivalent cerium, which, in turn, could be modified by dopant concentration and the annealing temperature. Results from X-ray photoelectron spectroscopy showed an increase in tetravalent cerium concentration to 85% on annealing at 900 °C. The concentration of oxygen ion vacancy increased from 1.7  1020 cm-3 to 4.1  1020 cm-3 with the increase in dopant concentration. Maximum emission at room temperature was obtained for 15 mol % Eu-doped ceria, which improved with annealing temperature. The role of oxygen ion vacancies and trivalent cerium in modifying the emission properties is discussed.

1. Introduction Cerium oxide or ceria (CeO2), a rare earth metal oxide, is used in a wide range of applications, such as an electrolyte in solid oxide fuel cells, oxygen gas sensors, a support in automotive catalysts, a polishing agent in chemical mechanical polishing, and an ultraviolet shielding material.1-3 Recent efforts from our group have opened a new frontier for the application of rare earth oxides in biomedical applications as radical scavenging antioxidants.4 We have shown that ceria nanoparticles can mimic the radical scavenging properties analogous to naturally existing antioxidants. The nanomolar concentration of ceria nanoparticles increases cell longevity and prevents reactive oxygen species mediated cell damage in animals.5 It can also protect the healthy cells from radiation-induced damage during radiation therapy and provides neuroprotection to spinal cord neurons.6,7 Because of their nontoxic nature and excellent biocompatibility, ceria nanoparticles have the potential to be used in many biomedical applications. However, ceria shows weak emission characteristics that limit its identification in biological and cellular studies. Various approaches have been used to improve the luminescence properties of weakly emitting particles. One of the approaches is to attach a tag, such as fluorescent dye, like a shell to the surface of the nanoparticle.8 Although this approach provides reliable and consistent emission from the fluorescent tag, it suffers from the fact that the modification of the surface may lead to biological inactivity of the nanoparticle. It also makes the material more *Corresponding author. E-mail: [email protected].

(1) Mogensen, M.; Sammes, N.; Tompsett, G. Solid State Ionics 2000, 129, 63. (2) Tsunekawa, S.; Fukuda, T.; Kasuya, A. J. Appl. Phys. 2000, 87, 1318. (3) Feng, X.; Sayle, D.; Wang, Z.; Paras, M.; Santora, B.; Sutorik, A.; Sayle, T.; Yang, Y.; Ding, Y.; Wang, X.; Her, Y. Science 2006, 312, 1504. (4) Karakoti, A. S.; Monteiro-Riviere, N. A.; Aggarwal, R.; Davis, J. P.; Narayan, R. J.; Self, W. T.; McGinnis, J.; Seal, S. JOM 2008, 60, 33. (5) Chen, J.; Patil, S.; Seal, S.; McGinnis, J. F. Nat. Nanotechnol. 2006, 1, 142. (6) Tarnuzzer, R. W.; Colon, J.; Patil, S.; Seal, S. Nano Lett. 2005, 5, 2573. (7) Das, M.; Patil, S.; Bhargava, N.; Kang, J. F.; Riedel, L. M.; Seal, S.; Hickman, J. J. Biomaterials 2007, 28, 1918. (8) Patil, S.; Reshetnikov, S.; Haldar, M.; Seal, S. J. Phys. Chem. C 2007, 111, 8437.

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bulky and thus limits the penetration of the cells by the nanoparticles. A secondary approach that involves doping of the parent matrix with elements that can emit by itself upon excitation9 in the UV-visible range of the spectrum presents a better alternative. Luminescent lanthanide complexes have been widely used as biolabels and bioassays.10,11 Europium (Eu) belongs to the rare earth metal group and has a strong red emission when doped in different matrices. Eu is considered as a suitable dopant to meet the aforementioned purpose of enhancing emission in ceria for three reasons: (i) it can be excited from ultraviolet to visible light,12 (ii) the ionic radius13 of Eu3þ (0.1066 nm), being close to that of cerium (Ce) (Ce3þ, 0.1143 nm; Ce4þ, 0.097 nm), favors extensive solubility with the ceria lattice, and (iii) it increases the trivalent state of Ce, which may further enhance the biological activity of ceria. Although metal oxides, such as alkaline, transition, and lanthanide oxides and some sulfides, exhibit luminescence on doping with Eu, only few studies have been reported on doping ceria with Eu.14,15 Earlier, Wang et al.16 have shown that doping with Eu is not efficient in improving the photoluminescence (PL) properties of ceria. On the contrary, few studies have shown prominent emission upon doping Eu in ceria.17 Li et al.,18 using citrate and polyethylene glycol, synthesized 0.1-10% Eu-doped ceria and attributed the broad band (9) Li, L.; Tao, J.; Pan, H.; Chen, H.; Wu, X.; Zhu, F.; Xu, X.; Tang, R. J. Mater. Chem. 2008, 18, 5363. (10) Wen, X.; Li, M.; Wang, Y.; Zhang, J.; Fu, L.; Hao, R.; Ma, Y.; Ai, X. Langmuir 2008, 24, 6932. (11) Hemmila, I.; Mukkala, V. M.; Takalo, H. J. Alloys Compd. 1997, 249, 158. (12) Wu, J.; Wang, G. L.; Jin, D. Y.; Yuan, J. L.; Guan, Y. F.; Piper, J. Chem. Commun. 2008, 3, 365. (13) Shannon, R. D. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, A32, 751. (14) Smet, P. F.; Van Haecke, J. E.; Loncke, F.; Vrielinck, H.; Callens, F.; Poelman, D. Phys. Rev. B 2006, 74, 035207. (15) Nogami, M.; Abe, Y. J. Non-Cryst. Solids 1996, 197, 73. (16) Wang, Z.; Quen, Z.; Lin, J. Inorg. Chem. 2007, 46, 5237. (17) Liu, X. H.; Chen, S. J.; Wang, X. D. J. Lumin. 2007, 127, 650. (18) Li, L.; Yang, H. K.; Moon, B. K.; Fu, Z.; Guo, C.; Jeong, J. H.; Yi, S. S.; Jang, K.; Lee, H. S. J. Phys. Chem. C 2009, 113, 610.

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observed in the excitation spectrum to the charge transfer (CT) transition from O2- to Ce4þ. The role of defect concentration, such as oxygen ion vacancies, is a topic of debate in open literature; for example, in systems such as zinc oxide (ZnO), it has been shown that the defects act as the emitting center, whereas in other systems such as ceria, it can act as the PL quenching center. In our earlier report, we have shown that the emission of 5 wt % Eu-doped ceria can be influenced by the defects.19 Herein, we report the synthesis of nanocrystalline Eu-doped ceria prepared through a room-temperature (RT) chemical precipitation technique. We quantitatively demonstrate the role of oxygen ion vacancies as PL quenching centers in Eu-doped ceria systems. The PL properties of Eu-doped ceria systems are investigated with respect to dopant concentration and annealing temperature. The mechanism of efficient energy transfer has been correlated with the surface and defect structure. Ce3þ aids in the PL properties of Eu, whereas oxygen vacancy concentration adversely affects the PL of Eu by seizing the radiative route of emission. The effects of Eu ion concentration on the emission and symmetry are vital as it can tune the oxygen ion vacancy concentration.

2. Experimental Section Cerium nitrate (99%, Aldrich), europium nitrate (99.9%, Aldrich), and ammonium hydroxide (30 wt %, Alfa Aesar) were used as received. Deionized water (18.0 mΩ) purified in a Barnstead system was used in all the experiments. Eu-doped ceria nanoparticles were synthesized via a coprecipitation technique. Cerium nitrate and a stoichiometric amount of europium nitrate were dissolved in 500 mL of deionized water and hydrolyzed with ammonium hydroxide to maintain a pH of 9. The resultant solution was stirred and allowed to settle overnight. The precipitates were washed with water multiple times to remove any weakly adhered ions on the surface and dried at 120 °C. Using the above procedure, samples with varying mole fractions of Eu were prepared and the samples were coded as CEX, where X = 1, 5, 10, 15, 20, 25, and 30 mol %. A similar procedure was adopted to prepare nanoceria without any dopant (CE0). To evaluate the structure, symmetry, and luminescence modifications, the CE15 sample (selected based on the maximum emission among the various dopant concentration, as discussed in section 3.3) was annealed at different temperatures. The annealed samples were designated as CE15-T, where T is the annealing temperature (300, 500, 700, and 900 °C). To determine the size, lattice parameter, and strain in the samples, the as-prepared nanopowders were characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The X-ray diffraction patterns were collected using a Rigaku D/MAX XRD with Cu KR source (λ = 0.1541 nm) at a scan rate of 0.01°/step over a 20-70°(2θ) range. The HRTEM images were taken on a FEI Tecnai F30 operated at 300 kV. X-ray photoelectron spectroscopy (XPS) experiments were carried out using a Physical Electronics (PHI5400 ESCA) spectrometer with a monochromatic Al KR X-ray source operated at 300 W and a base pressure of 5 10-8 Torr. The acquisition time of the sample was kept low to minimize any surface oxidation state changes under X-ray irradiation. Raman spectra were collected using a Horiba JobinYvon LabRam infrared (IR) microRaman system using a 633 nm helium-neon laser with a spatial resolution of 2 μm to obtain the electronic structure. Ultravioletvisible (UV-vis) spectra were recorded using a Lambda 750S UV/ vis spectrophotometer to evaluate the optical properties of the samples. Fourier transform infrared (FTIR) spectra were collected using a PerkinElmer Spectrum One to study the variation in surface features with temperature, and the PL properties were investigated using a Hitachi-7000 spectrophotometer. (19) Babu, S.; Schulte, A.; Seal, S. Appl. Phys. Lett. 2008, 92, 123112.

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Figure 1. (a) XRD patterns measured at room temperature for the ceria sample without doping and ceria samples doped with Eu at 1, 5, 10, 15, 20, and 30 mol % depicting the broad peaks for (111), (200), (220), and (311) (at a 2θ value of 28.784, 33.485, 47.621, and 56.271, respectively, for undoped ceria), which confirms the nanocrystalline nature of the samples, and the peak shift is observed toward a lower angle with increasing dopant concentration. (b) XRD patterns measured for 15 mol % Eu-doped ceria heattreated at temperatures of 300, 500, 700, and 900 °C indicate that the fluorite structure becomes more crystalline at higher annealing temperatures as the FWHM of the (111) plane varies from 1.343 at 500 °C to 0.291 at 900 °C.

3. Results and Discussion 3.1. Size and Structural Properties. Samples were analyzed by XRD to understand any structural and size modifications. For the as-prepared particles (Figure 1a), the diffraction peaks correspond to the (111), (200), (220), and (311) planes of the cubic fluorite ceria (CeO2) structure (JCPDS no. 88-2326) having Ce in the 4þ oxidation state. Diffraction peaks corresponding to either europium oxide or hydroxide were not observed, indicating the formation of solid solution in the entire composition range. Upon doping, the peak position (2θ) shifts toward a lower angle in comparison to pure nanoceria (CE0). Ce ion can exist in two different oxidation states of 3þ and 4þ having an ionic radii of 0.1143 and 0.097 nm, respectively, whereas Eu can exist in 3þ (0.1066 nm) and 2þ (0.125 nm).13 Since XRD pattern indicates the presence of Ce in predominantly 4þ oxidation states, doping higher ionic radii Eu ions can induce a corresponding increase in lattice parameter, introducing internal tensile stress in the ceria matrix. With annealing temperature, the peaks DOI: 10.1021/la901298q

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become sharper and increase in intensity due to the growth of nanoparticles (see Figure 1b). Even upon high-temperature annealing, the resultant peaks could be indexed to pure ceria, without Eu phase separation, suggesting that the Eu ions are doped in the lattice of ceria. The lattice parameter (a) was calculated by fitting the (311) peaks with Gaussian, and the full width at half-maximum (FWHM) was obtained from the fit. To compensate for the broadening in peaks due to instrument, the correction in FWHM was done as Δ2θhkl ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi FWHM2hkl - FWHM2inst

ð1Þ

where FWHMhkl is the full width at half-maximum intensity of the reflection (hkl) plane and FWHMinst is the instrumental broadening. The instrumental broadening in the pattern of the sample was corrected using a standard quartz sample. An FWHM curve (FWHMpeak as a function of 2θ) was obtained using the scan pattern from the standard reference material. The standard sample was run under identical instrumental conditions as those of the Eu-doped and the undoped ceria samples so that the broadening of the standard is exactly the instrumental broadening in the pattern of the sample. The lattice parameter for the (hkl) plane was calculated using the equation a ¼d

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h2 þ k2 þ l 2

ð2Þ

The calculated lattice parameters are shown in Figure 2 as a function of dopant concentration and annealing temperature for CE15. The lattice parameter of CE0 was found to be higher than that of the bulk ceria (0.541 nm). With the increase in dopant concentration, there is an increase in the lattice parameter,20 which can be attributed to the presence of Eu ions in the ceria lattice. The lattice parameter increases linearly from 0.5412 nm in CE0 to 0.546 nm in CE30. The width of the peaks is influenced by the size and strain of the particles, and the two factors were separated by using a standard Williamson-Hall plot given by eq 3.21 β ¼ βsize þ βstrain ¼

0:9λ 4ðΔdÞsin θ þ t cos θ d cos θ

ð3Þ

where β is the FWHM of the diffraction peaks after correcting for instrumental broadening, λ the wavelength of the incident X-ray, θ the diffraction angle, t the average crystal size, and Δd the difference of the d spacing corresponding to a typical peak. Size and strain in the sample were obtained from the y intercept and the slope of the straight line fitted from the plot between β cos θ and sin θ (in the Supporting Information, Figure SI 1 shows a representative plot for CE15). Size and strain obtained for different dopant concentrations and annealing temperatures are reported in Table 1. The as-prepared nanoparticles were in the size range of 5-7 nm, but, on annealing at 900 °C, the size increases up to 35 nm for CE15-900. To evaluate the size and morphology, HRTEM characterization was carried out. The average crystallite size of CE15 from the TEM image in the as-synthesized condition was 5 nm (Figure SI 2a, Supporting Information). The HRTEM images of CE15 annealed at 300, 700, and 900 °C are shown in Figure SI 2b-d in the Supporting Information. As evident from TEM and XRD results, annealing the samples at higher temperature lead to (20) Li, G. R.; Dawa, C. R.; Lu, X. H.; Yu, X. L.; Tong, Y.-X. Langmuir 2009, 25, 2378. (21) Chung, F. H.; Smith, D. K. Industrial Applications of X-ray Diffraction; Marcel Dekker, Inc.: NY, 2000.

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Figure 2. Lattice parameter shown along the ordinate, obtained by fitting the Gaussian curve to the spectra of (i) ceria doped with Eu at 0, 1, 5, 10, 15, 20, and 30 mol % and (ii) 15 mol % Eu-doped ceria heat-treated at temperatures of 300, 500, 700, and 900 °C. The lattice parameter increases linearly with increasing dopant concentration, which indicates that the stress generated due to the larger ionic size of europium substitution in the lattice position of cerium increases with the increase in dopant concentration. The decrease in the lattice parameter value with temperature suggests that stress in the doped samples is released with higher annealing temperature.

increase in particle size due to grain growth in order to reduce the surface energy. From EDX quantification (Figure SI 2e, Supporting Information), the calculated Eu concentration was found to be within (5% from the anticipated value. To understand the associated structural modification with heat treatment, FTIR spectra of the powders were recorded (Figure SI 3, Supporting Information). In the as-prepared condition, FTIR spectra showed the presence of adsorbed hydroxyl species on the surface of the samples and those species could be responsible for quenching the emission through nonradiative routes. The concentration of adsorbed hydroxyl groups decreases with increase in annealing temperature as the hydroxyl groups detach from the surface.22 3.2. Surface and Symmetry Modifications. The addition of Eu ion in the ceria lattice can influence the dynamics of Ce3þ to Ce4þ. XPS analysis23 was carried out to understand the changes in the valence chemistry and binding energy of constituent elements. The recorded XPS spectra were charge-corrected with respect to the C(1s) peak at 284.6 eV. Figure 3 shows the XPS spectra for CE15, the fitted curve, and the corresponding deconvoluted peaks. The peaks denoted by v0, v 0 , u0, and u 0 are characteristic peaks of Ce3þ ions, whereas those marked by v, v 0 0 , v 0 0 0 , u, u 0 0 , and u 0 0 0 are of Ce4þ ions. The deconvoluted Ce(3d) spectrum is relatively complex due to the presence of Ce in 3þ and 4þ oxidation states as well as multiple d-splitting. The peaks in the spectrum of Ce were deconvoluted using the PeakFit (4.0) software. The spin-orbit doublet 3d3/2 (880.31 and 898.25 eV) and 3d5/2 (898.81 and 916.7 eV) is clearly observed for both valence states of Ce. XPS results indicate that Ce is in mixed valence states of 3þ (880.40, 885.5, 898.81, 903.7 ( 0.7 eV) and 4þ (882.7, 888.96, 898.2, 901.3, 907, 916.7 ( 0.7 eV). The integrated area under the curve of each deconvoluted peak was used to (22) Mena-Duran, C. J.; Kou, M. R. S.; Lopez, T.; Azamar-Barrios, J. A.; Aguilar, D. H.; Domı´ nguez, M. I.; Odriozola, J. A.; Quintana, P. Appl. Surf. Sci. 2007, 253, 5762. (23) Deshpande, S.; Patil, S.; Kuchibhatla, S.; Seal, S. Appl. Phys. Lett. 2005, 87, 133113.

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Article Table 1. Size, Strain, %Ce3þ, Defect Concentration, and Band Gap for Different Samples

sample CE0 CE1 CE5 CE10 CE15 CE20 CE30 CE15-300 CE15-500 CE15-700 CE15-900

dopant concentration (mol %) 0 1 5 10 15 20 30 15 15 15 15

annealing temperature (°C)

size (nm)

strain (10-3)

band gap (eV)

% Ce3þ

defect concentration (1020 cm-3)

300 500 700 900

5-7 5-7 5-7 5-7 5-7 5-7 5-7 6.9 7.7 12.6 34.7

0.87 1.2 1.8 1.8 4.6 4.8 6.0 4.0 2.0 0.4 0.1

3.30 3.24 3.24 3.23 3.29 3.24 3.23 3.27 3.27 3.32 3.36

18.3 18.9 19.6 21.2 22.0 23.5 19.0 18.8 16.5 14.0

1.32 1.61 1.9 2.25 2.97 3.39 4.03 2.32 2.06 1.53 1.17

Figure 3. XPS spectrum of Ce(3d) for the 15 mol % Eu-doped ceria sample was deconvoluted to give the individual spin-orbit doublet of 3d3/2 and 3d5/2, and the sum of the deconvoluted peaks was used to produce the fit to the actual data.

calculate the concentration of Ce3þ ions as24 ½Ce3þ  ¼

Av0 þAv 0 þAu0 þAu 0 P Ai

ð4Þ

where Ai is the integrated area for peak “i”. From the analysis of the XPS spectra, it was found that the concentration of Ce3þ increases from 18.3% for CE1 to 23.5% for CE30 and decreased with annealing temperature. Although Eu ion can exist in 2þ or 3þ oxidation states, in the present case, Eu predominantly exist in the trivalent state, as indicated by the presence of characteristic peaks at 1134 ( 0.7 eV and 1163.5 ( 0.7 eV.25 The peak positions of Eu in 3þ and 2þ states differ by a large binding energy difference of about 9 eV, and no peak was observed in the region of 1124 ( 1 eV (Eu2þ 3d5/2) and 1153 ( 1 eV (Eu2þ 3d3/2), corresponding to the 2þ oxidation state of Eu. Both peaks of Eu3þ showed a shift by 0.7 eV toward lower binding energy with increasing dopant concentration, as shown in Figure 4. The decrease in binding energy can be attributed to increase in valence electron density.26 The FWHM of the Eu 3d5/2 decreases from 5.23 to 4.99 eV, respectively, as the dopant concentration increases from 1 to 30 mol %. For CE15, the Ce3þ concentration was found to be 21.2%, which decreases to 14.0% upon annealing at 900 °C. Although XPS identified the (24) Force, C.; Roman, E.; Guil, J. M.; Sanz, J. Langmuir 2007, 23, 4569. (25) Mercier, F.; Alliot, C.; Bion, L.; Thromat, N.; Toulhoat, P. J. Electron Spectrosc. Relat. Phenom. 2006, 150, 21. (26) Fujian, Z.; Honglei, M. A.; Chengshan, S. X.; Huizhao, Z.; Xijian, Z.; Jin, M.; Feng, J.; Hongdi, X. Sci. China, Ser. G: Phys., Mech. Astron. 2005, 48, 201.

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Figure 4. High-resolution XPS spectra of ceria samples doped with Eu at 1, 5, 10, 15, 20, and 30 mol %, taken at 15 kV and 300W using an Al X-ray source with a pass energy of 22.4 eV, showing the Eu 3d3/2 and Eu 3d5/2 peaks, both of which shift toward lower binding energy by 0.6 and 0.7 eV, respectively, with the increase in dopant concentration.

oxidation states of Ce and Eu, the modifications in crystal symmetry upon doping with Eu can be understood by following O(1s) spectra. Analyzing the spectra of the O(1s) peak of Eudoped ceria samples at room temperature shows increasing asymmetry with dopant concentration, as shown in Figure 5a, which indicates the distortion of the crystal structure. In XPS, the energy provided by the absorbed photon causes the excitation of the electronic and atomic structure of the specimen surface. Hence, the ejected electron from the sample appears with kinetic energy dependent on atomic oscillations associated with the type of bonds within the lattice. Therefore, the observed asymmetry in the O(1s) peak is due to the O2- ions located in different chemical environments, such as oxygen attached to Ce3þ, Ce4þ, Eu3þ, or Hþ, within the sample, leading to alternative energy transfer mechanisms. The effect of heat treatment on the O 1s spectra is depicted in Figure 5b. With increasing annealing temperature, the asymmetry in the oxygen peak decreases due to the reordering or removal of chemical species, such as H2O and OH. To understand the associated local environmental modifications around oxygen, the asymmetric O(1s) peak of the CE15 sample was fitted into four Gaussian-Lorentzian peaks, namely, Oa, Ob, Oc, and Od, centered at 529.0 ( 0.2 eV, 529.6 ( 0.2 eV, 531.4 ( 0.2 eV, and 533.5 ( 0.2 eV, respectively, as shown in Figure 6a. The binding energy of O(1s) for Ce3þ-O2- is reported to be higher than that of Ce4þ-O2-. For example, Praline et.al have reported the binding energy of Ce2O3 (530.3 eV) to be 0.7 eV DOI: 10.1021/la901298q

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Figure 5. (a) Oxygen 1s spectra of nanoceria samples doped with 1, 5, 10, 15, 20, and 30 mol % of Eu at room temperature show that the asymmetric behavior changes with dopant concentration and depict peak broadening at higher doping concentration. (b) Oxygen 1s spectra of 15 mol % Eu-doped ceria at room temperature and heat-treated at 300, 500, 700, and 900 °C show that the asymmetry in the peaks decreases with increasing temperature treatment.

higher than that of CeO2 (529.6 eV).27 Europium oxide (Eu2O3) has a higher binding energy of 531.40 eV compared with that of Ce-O bonds;28 thus, Eu3þ-O2- will have a higher binding energy as compared with that of Ce3þ-O2- based on its higher electronegativity29 and oxygen affinity relative to those of Ce3þ ions. Figure 6b shows the schematic lattice model of ceria with different chemical environments around the lattice oxygen ion. The Oa peak on the lower binding energy side of the O(1s) spectrum is attributed to O2- ions surrounded by Ce4þ ions, which corresponds to the Ce-O bond in CeO2. The deconvoluted Ob peak can be ascribed to the O2- ions in the Ce-O bond where Ce is present in the 3þ state. The Oc peak is associated with the binding energy of O2- ions that are specific to the Eu3þ-O2configuration.28 The deconvoluted Od peak is related to either the O2- ions that are in oxygen-deficient regions within the Eu-doped ceria lattice or the chemisorbed OH species on the surface, as (27) Praline, G.; Koel, B. E.; Hance, R. L.; Lee, H. I.; White, J. M. J. Electron Spectrosc. Relat. Phenom. 1979, 21, 17. (28) Wagner, C. D.; Zatko, D. A.; Raymond, R. H. Anal. Chem. 1980, 52, 1445. (29) Keyan Li, D. X. Phys. Status Solidi B 2007, 244, 1982.

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suggested by the IR spectra (Figure SI 3, Supporting Information) of the heat-treated samples. Similar analysis of the O 1s peak was carried out for otherPdoped samples. The integrated peak area ratio (IPARi = AOi/ AO) for Oa, Ob, Oc, and Od is representative of the relative amount of different species Oa (Ce4þ-O2-), Ob (Ce3þ-O2-), Oc (Eu3þ-O2-), and Od (O2--Hþ). For the room-temperature samples, it was observed that, with increasing dopant concentration, the IPAR of Oc and Ob increases, whereas that of Oa decreases, as shown in Figure 7a. This implies that the increase in Eu3þ is accompanied by an increase in Ce3þ concentration and a simultaneous decrease in the concentration of Ce4þ. Thus, it is clear that the addition of Eu in the ceria lattice replaces the Ce4þ ion in the lattice and creates oxygen vacancies. The IPAR for Od changes marginally from 0.045 for CE1 to 0.056 for CE30. The CE15 sample annealed at temperatures from 300 to 900 °C revealed that the IPAR for Oa increases from 0.56 to 0.72 and the IPAR for Ob decreases from 0.13 to 0.09 with increasing annealing temperature (Figure 7b). The IPAR for the Oc peak in the 15 mol % Eu-doped ceria sample decreases rapidly from 0.24 at room temperature to 0.147 at 500 °C and then decreased marginally to a value of 0.142 at 900 °C. The IPAR of Od for the 15 mol % doped sample decreases from 0.05 at room temperature to 0.03 at 900 °C, which suggests that, with heat treatment, the chemisorbed -OH species (as detected in FTIR spectra, Figure SI 3, Supporting Information) on the surface is released and this is expected to enhance the luminescence by seizing the nonradiative routes of energy transfer. It must be noted that the peak separation in Figure 5b is actually due to the reduction in hydroxide content. At room temperature, the contribution from OH at higher binding energy is enough to merge with the cumulative peak of Eu3þ-O2-, Ce3þ-O2-, and Ce4þ-O2-. As the sample is heat-treated, the OH content in the sample reduces and the peak intensity diminishes, which causes a dip in spectra between the binding energy corresponding to the positions of O2--Hþ and Eu3þ-O2-. Thus, the OH binding energy peak that was initially merged in the overall spectra due to higher contribution from OH, separates out in the case of the heat-treated sample as the intensity of the peak decreases with higher annealing temperatures. Also, from the analysis of the O 1s peak of the doped samples, it was found that, at room temperature, the binding energy corresponding to Oa and Oc peaks decreased by 0.8 and 0.49 eV, respectively, as the dopant concentration was increased from 1 to 30 mol %. The XPS results indicate that the chemistry of the doped sample and thus the luminescence can be controlled by appropriate amount of doping, followed by annealing at a suitable temperature. Further investigation of electronic structure of the doped samples was carried out using Raman spectroscopy. Figure 8a, b shows Raman spectra for the as-prepared as well as annealed samples. On doping ceria with Eu, the Ce4þ ions are partly replaced by Eu3þ ions, which create oxygen ion vacancies to compensate for the reduced positive charge of the cations. These oxygen ion vacancies change the valency of Ce from 4þ to 3þ, thereby increasing the Ce3þ concentration in the lattice, as supported by the XPS results. The introduction of O2- vacancies in Eu-doped ceria samples distorts the fluorite lattice. In the presence of impurities or lattice disorder, the phonon function is spatially confined, which results in wavenumber shifts and band broadening. Figure 8a shows the Raman shift of 5.84 cm-1 toward lower wavenumber for CE5, CE10, and CE15 with respect to the CE1 sample. A shift of 0.17 cm-1 for CE20 and a shift of 0.11 cm-1 for CE30 with respect to CE1 is observed. In comparison to CE15, a shift of 8.55 cm-1, 12.31 cm-1, 14.18 cm-1, and 15.12 cm-1 toward higher wavenumber is observed on annealing CE15 at 300, 500, 700, and 900 °C, respectively. In a perfect Langmuir 2009, 25(18), 10998–11007

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Figure 6. (a) Typical asymmetry in the high-resolution O 1s spectrum of 15 mol % Eu-doped ceria was resolved and attributed to the presence of oxygen in different chemical environments as the deconvolution of the experimental spectra results in peaks corresponding to the binding energy of O2- ions associated with the chemical environments of CeO2, Ce2O3, Eu2O3, and OH/O2. Fitted and actual spectra are shown. (b) Schematic lattice model showing the different chemical environments of oxygen within the crystal structure of nanoceria.

P

Figure 7. Integrated peak area ratio (IPARi = AOi/

AO) obtained from XPS analysis of O 1s spectra of doped ceria samples shows how, with doping and heat treatment, the oxygen atmosphere changes (a) at RT for 1, 5, 10, 15, 20, and 30 mol % Eu doping and (b) for CE15 heattreated at 300, 500, 700, and 900 °C.

confinement and residual strain in the sample.33 As seen from the XRD results (Table1), the strain in the sample increases with dopant concentration and so does the broadening in the Raman line (Figure 8a). The shape change in the Raman line and the lattice disorder induced by the dopant are explored by interpreting the data using the spatial correlation model.34 According to this model, the Raman line intensity I(w) at frequency w is given as ! Z 1 -q2 L2 d3q exp ð5Þ I ¼ 4 ½w - wðqÞ2 þ ðΓ0 =2Þ2 0

crystal of cerium oxide, each cerium atom is surrounded by eight oxygen ions and symmetrical stretching vibrations of the CeO8 vibrational unit results in a first-order allowed Raman active triply degenerate F2 g mode30 at 465 cm-1. As reported in earlier studies31 on Raman spectra of ceria nanoparticles, the F2 g mode shifts toward lower energies with increasing line width while the line shape becomes asymmetric with decreasing particle size. A similar shift has been observed in the case of Eu-doped ceria samples with respect to bulk ceria, and on increasing the dopant concentration above 15 mol %, the degree of shift is reduced. With increasing annealing temperature, this peak shifts toward higher energy. The peak shift can be directly correlated to oxygen ion vacancies in doped ceria samples. The concentration of oxygen ion vacancies increases with the amount of dopant,32 and with annealing, these vacancies tend to migrate and get annihilated at the surface, leading to improved phonon lifetime that would, in turn, broaden and increase (for Eu-doped ceria) the intensity of the peaks in the PL measurement. The evolution of the Raman line can be attributed to the combined effect of phonon

where q is the wave vector expressed in units of 2π/a (a is a lattice constant). The term exp(-q2L2/4) represents the Gaussian spatial correlation function, where L is the correlation length representing the average extension of the material homogeneity region. Γ0 is the FWHM of the Raman line for a perfect crystal (9.2 cm-1). The intensity from the model was calculated by taking the sum over three equally weighted branches of phonon dispersion, as done previously,35 and the defect concentration (N) was calculated

(30) Palmqvist, A. E. C.; Wirde, M.; Gelius, U.; Muhammed, M. Nanostruct. Mater. 1999, 11, 995. (31) Keramidas, V. G.; White, W. B. J. Chem. Phys. 1973, 59, 1561. (32) Spanier, J. E.; Robinson, R. D.; Zheng, F.; Chan, S. W.; Herman, I. P. Phys. Rev. B 2001, 64, 245407.

(33) Radovic, M.; Dohcevic-Mitrovic, Z.; Scepanovic, M.; Grujic-Brojcin, M.; Matovic, B.; Boskovic, S.; Popvic, Z. V. Sci. Sintering 2007, 39, 281. (34) Parayanthal, P.; Pollak, F. H. Phys. Rev. Lett. 1984, 52, 1822. (35) Patil, S.; Seal, S.; Guo, Y.; Schulte, A.; Norwood, J. Appl. Phys. Lett. 2006, 88, 243110.

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Figure 8. (a) Raman spectra for room-temperature ceria samples doped with 1, 5, 10, 15, 20, and 30 mol % of Eu show a shift toward decreasing wavenumber and increasing broadening as the amount of dopant is increased. (b) Raman spectra for 15 mol % Eu-doped ceria heat-treated at 300, 500, 700, and 900 °C shift slightly toward higher energy with annealing temperature.

using the equation N ¼

3 4πL3

ð6Þ

The value of the correlation length obtained from the model, as shown in Figure 9a, varies from 11.4 to 8.4 at RT. Figure 9b compares the amount of Ce3þ obtained from XPS data and the defect concentration calculated using the model for Raman data. With increasing dopant concentration from CE1 to CE30, the Ce3þ concentration increases from 0.183 to 0.235, but the increase in defect concentration is steeper (from 1.61  1020 cm-3 to 4.03 x1020 cm-3) (Figure 9b). Analyzing the effect of the two phenomena (increase in Ce3þconcentration and increase in defect concentration) on luminescence, it is expected that the former will enhance the luminescence, whereas the latter will quench the luminescence properties. Therefore, it is predicted that the increase in luminescence by Eu doping in the ceria lattice will be quenched after reaching an optimal point where the negative effect of dopant concentration will overcome the luminescence intensification by increasing Ce3þconcentration. 11004 DOI: 10.1021/la901298q

Figure 9. (a) Correlation length for the as-prepared samples with variation in dopant concentration and annealing temperature. (b) A comparison between the ratio of Ce3þ (calculated from XPS spectra) and defect concentration (calculated from Raman spectra) with respect to dopant concentration.

3.3. Optical and Luminescence Properties. To understand the modification of optical properties upon doping, absorption and luminescence spectra were recorded. On annealing, the particle size increases with a reduction in Ce3þ concentration.36 The optical transmittance spectra (Figure SI 4, Supporting Information) indicated a blue shift of the band gap due to the valence transition of Ce3þ to Ce4þ induced by thermal annealing. The decrease of Ce3þ concentration will eliminate some localized states within the band gap due to the corresponding decrease of defects (vacancies). Thus, the blue shift of Eg among the Eu-doped ceria samples is due to the decrease in Ce3þ content, with increasing grain size induced by heat treatment and not due to the quantum size confinement. A similar trend in the blue shift has been observed for CeO2 films on sapphire36 and Eu-doped ceria prepared by sol-gel,18 when Eu concentration increases to 5% and 10%. Figure 10a shows the excitation spectra for emission at 611 nm in the as-prepared condition, whereas Figure 10b depicts the spectra of 15 mol % Eu-doped ceria with annealing temperature. (36) Chen, M. Y.; Zu, X. T.; Xiang, X.; Zhang, H. L. Physica B (Amsterdam, Neth.) 2007, 389, 263.

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Figure 11. Luminescence emission spectra (excitation at 466 nm)

Figure 10. Excitation spectra with emission at 611 nm for (a) asprepared and (b) annealed samples. No emission is observed for pure ceria. Among the room-temperature samples, the PL first increases up to 15 mol % Eu doping in ceria and then decreases, whereas the PL increases with the increase in annealing temperature for the CE15 sample.

Although pure ceria (CE0) does not show any peak on following the emission at 611 nm, a sharp peak at 466 nm characteristic to Eu 4f-4f transitions11 was observed in Eu-doped ceria samples. Because the O2--Eu3þ CT lies in a much shorter wavelength region and CeO2 has a band gap around 3.3 eV (∼370 nm), the broad peak is related to mainly Ce4þ-O2- CT37 with some overlapping with the intraconfigurational Eu 4f-4f transitions. As evident from Figure 10a, with increasing amount of Eu doping, the intra4f6 transition bands of Eu3þ become stronger. The intensity was found to increase with increase in dopant concentration, but beyond 15 mol %, the intensity decreases due to the concentration quenching effect. Figure 11a,b shows the emission spectra of various doped and annealed samples on exciting at 466 nm. With 466 nm as the excitation wavelength, the observed effect of variation in emission can be correlated to the dopant concentration. We have shown previously19 that, through charge transfer, the energy will not be transferred to Eu efficiently. With increasing Eu3þ concentration, there is an increase in intensity up to 15 mol %, a further increase in dopant concentration decreases the Eu3þ-Eu3þ distance, leading to an effective energy transfer between the neighboring ions. Hence, the (37) Li, L.; Zhou, S.; Zhang, S. Chem. Phys. Lett. 2008, 453, 283.

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for (a) as-prepared samples show that the PL increases with dopant concentration and saturates at 15 mol % Eu and for (b) annealed samples show that the maximum PL at room temperature increases with the increase in annealing temperature. The first two peaks (580 nm, 595 nm) originate from magnetic dipole transition, and the other peaks are electric dipole induced emissions. To compare the spectra, the peaks were normalized in the lower wavelength region (500 nm) and the relative change has been presented.

excited state moves to the quenching sites, dissipating the energy nonradiatively. As a result of the concentration quenching, the emission intensity decreases. The critical concentration for observing the concentration quenching depends on the solubility of dopants in the matrix and varies with the type of dopants and the matrix. In the case of phosphors, the very low solid solubility of dopants results in the segregation of the dopant material, leading to concentration quenching even at lower concentrations. For example, in the Y2O3 and Gd2O3 matrices, the concentration quenching from Eu was observed at 3.5% and 5%, respectively.38,39 Both Eu and Ce belong to the lanthanide series and demonstrate higher solid solubility, and thus, Eu can be dispersed evenly throughout the ceria matrix even at relatively higher Eu concentration.40 Because of higher solid solubility, the concentration quenching was observed at a higher Eu dopant concentration (15 mol %) in the present experiments. To understand the structural modification and emission characteristics, CE15, which showed maximum intensity in the as-prepared conditions, was (38) Flores-Gonzalez, M. A.; Lebbou, K.; Bazzi, R.; Louis, C.; Perriat, P.; Tillement, O. J. Cryst. Growth 2005, 277, 502. (39) Kim, E. J.; Kang, Y. C.; Park, H. D.; Ryu, S. K. Mater. Res. Bull. 2003, 38, 515. (40) Gschneidner, K. A. Bull. Alloy Phase Diagrams 1989, 3, 183.

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Figure 12. Asymmetry ratio (intensity ratio of ED to MD allowed transition) with dopant concentration for as-prepared and 900 °C annealed samples shows that the asymmetric position of Eu in the ceria lattice (Ce4þ-O-Eu3þ) is favored on increasing dopant concentration in both RT and annealed samples.

annealed at different temperatures. The emission intensity was found to increase with the increase in annealing temperature. The PL shows the characteristic Eu3þ emission that can be assigned to various transitions 5D0-7FJ (J = 0, 1, 2, etc),19 and these emissions can reveal much about the local environment of the Eu3þ ion. The multiple peaks in the spectra are due to the splitting of the Eu3þ 4f shell. The Judd-Ofelt (J-O)41,42 theory is the most useful and popular method in the analysis of spectroscopic studies of rare earth ions in different hosts. As per J-O theory, the emission lines are a cumulative effect of magnetic dipole (MD) transition and electric dipole (ED) transition, depending on the specific environment of Eu3þ in any matrix. According to the J-O theory, ED transition (5Do-7F2) centered at about 611 and 629 nm, is only allowed in the absence of inversion symmetry and is hypersensitive to the local electric field. On the other hand, the MD transition (5Do-7F1) with emission at 591 nm is a magnetic dipole allowed transition, which is insensitive to the crystal environment. Ceria has a fluorite structure with every Ce ion surrounded by eight equatorial oxygen ions in Oh symmetry. The emission intensity of Eu3þ is very critical to its location in the lattice, that is, the type of environment around Eu3þ ions.43 When Ce4þ is replaced with Eu3þ, the symmetry can be either lowered or improved, depending on the site occupied by the dopant. In the present case, the ED transition intensity was found to be higher, indicating that Eu3þ mainly occupies the lattice sites that reduce the Oh inversion symmetry. The intensity ratio of the ED-allowed transition (5D0-7F2) and the MD-allowed transition (5D0-7F1), known as the intensity or asymmetry ratio, is a measure of the degree of distortion from the inversion symmetry of the local environment of the Eu3þ ion in the lattice. The influence of doping concentration on the asymmetry ratio was also observed for Eu-doped SnO2.44 Figure 12 shows that the intensity ratio increases for as-prepared samples with respect to dopant concentration and samples annealed at 900 °C. This suggests that an asymmetric position of Eu in the CeO2 lattice is favored with increasing dopant concentration, not only at the annealed temperature but also at RT. On excitation (41) Judd, B. R. Phys. Rev. 1962, 127, 750. (42) Ofelt, G. S. J. Chem. Phys. 1962, 37, 511. (43) Andresen, A.; Bahar, A. N.; Conradi, D.; Oprea, I.; Pankrath, R.; Voelker, U.; Betzler, K.; Wohlecke, M.; Caldino, U.; Martin, E.; Jaque, D.; Sole, J. G. Phys. Rev. B 2008, 77, 214102. (44) Jiangtao, C.; Jun, W.; Fei, Z.; De, Y.; Guangan, Z.; Renfu, Z.; Pengxun, Y. J. Phys. D: Appl. Phys. 2008, 10, 105306.

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at 466 nm, the intensity ratio of the as-prepared samples increased from 0.5 to 1.35, indicating more disorder in the crystal lattice with dopant concentration. Because Ce can exist in 3þ and 4þ oxidation states, the coexistence of (Ce3þ-O-Eu3þ) and (Ce4þ-O-Eu3þ) arrangements in the crystalline environment is possible. As mentioned in section 3.1, the size of the Eu3þ ion is close to that of Ce3þ rather than to the size of Ce4þ, which implies that the (Ce3þ-O-Eu3þ) configuration has more symmetry as compared with the (Ce4þ-O-Eu3þ) configuration. A higher mol % of doping generates lattice distortion of the local environment around Eu3þ ions, as supported by the XRD results, and thus favors the electric dipole induced emission (5Do-7F2). On annealing, due to diffusion kinetics, the Ce4þ concentration increases, as also supported from the XPS results, and thus the asymmetric (Ce4þ-O-Eu3þ) configuration in the lattice increases, which suggests that ED transition will be more favorable than MD transition. To understand the host lattice influence on emission, the ceria matrix was excited at 370 nm and has been discussed in the Supporting Information (Figure SI 5). The luminescence properties of the doped sample strongly depend on the composition of samples. A blue shift is observed in the luminescent spectra of Eu-substituted ceria, which is probably due to the influence of the 5d electron states of Eu3þ in the crystal field because of ionic size variation causing crystal defects. Figure SI 6 (Supporting Information) shows how the variation in the peak intensity at 610 nm for the doped samples excited at 466 nm changes as a function of dopant concentration and annealing temperature. It is observed from the above two plots that the PL is highest for 15 mol % doping of Eu in ceria nanoparticles. On annealing the samples, the luminescence intensity first decreases and then becomes very efficient with increasing temperature. The increase in PL intensity upon annealing the samples can be understood by comparing the concentration of defects and trivalent cerium in the system. With annealing, the defect concentration in the lattice, calculated using the spatial correlation model on Raman data, is suppressed from 2.969  1020 cm-3 for CE15 to 1.16  1020 cm-3 for CE15-900. The XPS results indicate that annealing the CE15 decreases the Ce3þ/Ce4þ ratio from 0.21 at room temperature to 0.14 at 900 °C. On heat treatment, the decrease in the concentration of Ce3þ decreases the luminescence intensity of the 15 mol % Eu-doped ceria. Also, with increasing annealing temperature, the defect concentration that acts as the undesired sites offering a nonradiative transfer route to electrons in Eudoped ceria decreases and thus favors PL. This increase in the PL of Eu-doped ceria due to decrease in defect concentration is unlike that observed in zinc oxide (ZnO) or titania (TiO2).45 In the case of ZnO or TiO2 nanoparticles, the observed emissions are not due to band edge emission but attributed to the band edge free excitons and bound excitons. On a nanoscale regime and at low-temperature annealing, these exciton formations lead to a more enhanced luminescence of ZnO or TiO2. The observed emission in the Eu-doped ceria is due to the defect concentration and changes with annealing temperature. The present data from PL suggest that, at low-temperature annealing, the effect of decreasing Ce3þ concentration on lowering the luminescence intensity transcends the increase in luminescence by decreasing the defect concentration. With further heat treatment, the increase in luminescence intensity due to the reduction in the oxygen ion vacancy concentration is much more prominent than (45) Gu, F.; Wang, S. F.; Lu, M. K.; Zhou, G. J.; Xu, D.; Yuan, D. R. Langmuir 2004, 20, 3528.

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the effect of decreasing Ce3þ/Ce4þ ratio. The explanation is based on data from 15 mol % Eu-doped ceria; however, the same effect must hold true for all other doped samples, as depicted in Figure SI 6 (Supporting Information). Therefore, due to the cumulative effect of decreasing defect concentration and decreasing Ce3þ/ Ce4þ ratio with increasing annealing temperature, reveals a minima in emission intensity when plotted with respect to temperature and then increases sharply with temperature.

cerium and oxygen ion vacancies in affecting the PL response of Eu-doped ceria were identified. Reducing the concentration of oxygen ion vacancies by annealing at a suitable temperature resulted in an increase in the PL intensity. In the current investigation, the maximum luminescence at room temperature was obtained from CE15. The present work quantitatively shows the effect of defect concentration and trivalent cerium on the emission properties of Eu-doped ceria.

4. Conclusion Nanocrystalline ceria samples with various dopant concentrations have been prepared by a simple room-temperature chemical precipitation technique. The size, structure, surface, defect, and optical properties were characterized. Strong visible emission was obtained even from 1 mol % dopant concentration by retaining the trivalent oxidation state of Eu in nanocrystalline ceria. The PL intensity increases with dopant concentration and saturates at 15 mol % dopant. On further increase in the dopant amount, quenching of the PL was observed due to the concentration quenching effect. The defect concentration and Ce3þ/Ce4þ ratio primarily determines the Eu3þ emission. Contrary roles of trivalent

Acknowledgment. This research was supported by NSF NIRT CBET 0708172.

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Supporting Information Available: A representative plot for CE15 for calculating stress and strain in the sample, HRTEM and EDX results for CE15, IR spectra of CE15 as a function of annealing temperature, reflectance spectra for samples with respect to dopant concentration as well as annealing temperature, emission spectra of the samples with 370 nm excitation, peak intensity variation followed by 466 nm excitation and 612 nm emission. This material is available free of charge via the Internet at http://pubs.acs.org.

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