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Role of Surface Coating in ZrO2/Eu3+ Nanocrystals Pushpal Ghosh and Amitava Patra* Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed February 20, 2006. In Final Form: May 8, 2006 The sol-emulsion-gel method is used for the preparation of Eu3+ ion-doped and coated ZrO2 nanocrystals. Here, we report the role of surface coating, dopant concentration, and temperature of heating in the modification of their crystal structure and photoluminescence properties. It is found that the volume fraction of the tetragonal phase increases from 28.08 to 91.56% because of surface coating. This is a significant modification of the crystal phase in ZrO2 nanocrystals due to surface coating by Eu2O3. It is found that the photoluminescence properties are sensitive to the crystal structure, which is again controlled by surface coating, concentration, and heating temperature. It is found that the decay time (τ) of Eu-doped ZrO2 nanocrystals increases with increasing the concentration of dopant and with increasing the temperature of heating because of changes in their crystal phase. The emission intensity of the peak at 611-617 nm (5D0 f 7F2) of the Eu3+ ion-activated ZrO2 nanocrystals (doped and coated) is also found to be sensitive to the nanoenvironment. The average decay times are 770 and 488 µs for 1100 °C-heated 1.0 mol % Eu2O3-doped and coated ZrO2 nanocrystals, respectively. Our analysis suggests that the site symmetry of the ions plays the most important role in the modifications of the radiative and nonradiative relaxation mechanisms as a result of the overall photoluminescence properties.

1. Introduction Recently, great attention has been given to transition (TM)/ rare-earth (RE) ion-doped nanoparticles to discover their potential applications in photonic and biophotonic fields. 1-8 Because these potential applications are still very much in the design phase, further fundamental research in the field remains a challenge. This is very important because doping the nanocrystals with TM and RE ions has proven to be very difficult and has been the subject of debate in recent years. The important issue that needs to be addressed is whether dopant ions are really being incorporated into nanocrystalline or adsorbed at the nanoparticle surface, and their effect on optical properties. Very recently, Norris et al. described the important role of the crystal phase in the efficiency of TM-doped semiconducting nanoparticles.2 From a fundamental point of view, the physical understanding of the photoluminescence properties of RE ions in oxide nanocrystals and the way it changes with size, crystal phase, and concentration is now well understood.9-11 It is well established that, in the luminescence of RE ions, the highest phonon frequencies of the host lattice are responsible for nonradiative relaxations.12 In accordance with energy law, the presence of a large gap between emitting and terminal levels * To whom correspondence should be addressed. E-mail: [email protected]. Fax: (91)-33-2473-2805. (1) Lehmann, O.; Kompe, K.; Haase, M. J. Am. Chem. Soc. 2004, 126, 14935. (2) Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Nature 2005, 43, 91. (3) Guo, H.; Dong, N.; Yin, M.; Zhang, W.; Lou, L.; Xia, S. J. Phys. Chem. B 2004, 108, 19205. (4) Sivakumar, S.; Veggel, F. C. J. M. V.; Raudseep, M. J. Am. Chem. Soc. 2005, 127, 12464. (5) De la Rosa-Cruz, E.; Diaz-Torres, L. A.; Rodriguez-Rojas, R. A.; MenesesNava, M. A.; Salas, P. Appl. Phys. Lett. 2003, 83, 4903. (6) You, H.; Nogami, M. J. Phys. Chem. B 2004, 108, 12003. (7) Chen, L.; Liu, Y.; Li, Y. J. Alloys Compd. 2004, 381, 266. (8) Patra, A. Chem. Phys. Lett. 2004, 387, 35. (9) Patra, A.; Friend, C. S.; Kapoor, R.; Prasad, P. N. Appl. Phys. Lett. 2003, 83, 284. (10) Patra, A.; Friend, C. S.; Kapoor, R.; Prasad, P. N. J. Phys. Chem. B. 2002, 106, 1909. (11) Saha, S.; Chowdhury, P. S.; Patra, A. J. Phys. Chem. B 2005, 109, 2699. (12) Patra, A.; Sominski, E.; Ramesh, R.; Koltypin, Yu.; Zhong, Z.; Gedanken, A.; Minti, H.; Reisfeld, R. J. Phys. Chem. B 1999, 103, 3361.

reduces the probability of nonradiative decay. Lower host phonon energy has a greater number of phonons connecting the emitting level with the next lower level. The more phonons needed to gap the energy will decrease the nonradiative relaxation probability and increase the quantum yield of luminescence. To overcome the phonon decay problem, it is necessary to choose a lattice that has much lower phonon energy. The zirconia matrix seems to be an ideal medium for the preparation of highly luminescent materials because it is chemically and photochemically stable and has a high refractive index and a low phonon energy.10 Nanoparticles, on the other hand, have recently been recognized as possessing tremendous potential in the area of photonic applications.13 Combining the promising optical properties of RE ions and nanoparticles, the study of the photoluminescence properties of europium in zirconia nanoparticles is important. Because the electronic f-f transitions of the RE ions are localized in the atomic orbital of the ions, no size-dependent quantization effect from the confinement of delocalized electrons is found in these transitions. The spontaneous emission probability of optical transitions (luminescence lifetime) from RE ions doped in nanoparticles may be significantly different from their bulk counterparts.14,15 From a fundamental point of view, the physical understanding of the crystal structure and luminescence properties of RE ions in nanocrystals and the way they change with temperature of heating, concentration, and surface coating is very important. Recently, core-shell particles have been attracting a great deal of interest due to the tailoring of their properties.1,12,16,17 We have found that sol-emulsion-gel synthesis is an excellent method for preparing nanoparticles.3,10,11 Here, the sol-emulsion-gel method is used for the preparation of Eu3+-doped ZrO2 nanoparticles. Nanoparticles tend to become agglomerated into (13) Prasad, P. N. Nanophotonics; John Wiley & Sons: New York, 2004. (14) Meltzer, R. S.; Hong, K. S. Phys. ReV. B 2000, 61, 3396. (15) Meltzer, R. S.; Feofilov, S. P.; Tissue, B.; Yuan, H. B. Phys. ReV. B. 1999, 60, R14012. (16) Gedanken, A.; Reisfeld, R.; Sominski, L.; Zhong, Z.; Koltypin, Yu.; Panczer, G.; Gaft, M.; Minti, H. Appl. Phys. Lett. 2000, 77, 945. (17) Liu, G.; Hong, G.; Sun, D. J. Colloid Interface Sci. 2004, 278, 133.

10.1021/la0604883 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/06/2006

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larger assemblies because they possess high surface energies. To overcome these limitations, many researchers have turned to emulsions as a means to control the size, morphology, and heterogeneity of nanoparticles. Generally, the reverse micelles formed in water-in-oil (W/O)-type emulsions act as the microor nanoreactors in which reactions are carried out.18,19 It is known that a minimum critical micelle concentration (cmc) is needed for the surfactant molecules to self-aggregate, giving rise to “reverse micelles” in organic solvents. The hydrophilic-lipophilic balance (HLB) of nonionic surfactants relates inversely with the final particle size.19 In the present system, reverse micelles formed from the nonionic, low HLB (4.3) surfactant sorbitan monooleate (Span 80) were employed within a cyclohexane continuous phase as templates to guide the synthesis of the nanophosphor particles. The surfactant-stabilized nanocavities provide a cage-like confinement that, in principle, may be exploited to provide control over particle nucleation, growth, and agglomeration. Here, we address the role of surface coating, dopant concentration, and temperature of heating in the modification of the crystal structure and photophysical properties of ZrO2/Eu3+ nanocrystals derived from the sol-emulsion-gel technique.

and monoclinic (χm) phases were estimated from the integrated peak intensity of the (101)t plane of the tetragonal phase (It) and the (111)m and (1h11)m planes of the monoclinic phase (Im) using the following equations:20

2. Experimental Procedure

χt ) It(101)/[It(101) + Im(111) + Im(1h11)]

(2)

All chemicals were used as received. Zirconia propoxide (Fluka) was used as the starting material for ZrO2 sol preparation. First, 3 mL of glacial acetic acid was slowly added to 10 mL of zirconia propoxide and stirred for 30 min. Then 20 mL of n-propanol was added to the solution, which was further stirred for 15 min at room temperature. 4 mL of 50% aqueous acetic acid was slowly added to the above solution under stirring, which resulted in a clear transparent ZrO2 sol. Then, a stoichiometry amount of europium nitrate was added to this sol. The emulsified sol droplets were obtained through W/O-type emulsions with cyclohexane and sorbitan monooleate (Span 80, Fluka) as the organic liquid (oil phase) and nonionic surfactant, respectively. The principle of the process involves the dispersion of an aqueous sol containing the desired constituents under agitation into a water-immiscible organic liquid for a low dielectric constant. Gelation of the emulsified sol droplets resulted from the controlled addition of a base. Gel particles were collected by centrifugation (6000 rpm, 30 min), and then the particles were washed twice with acetone and twice with methanol. After preliminary drying at 60 °C for 12 h in a vacuum oven, annealing of the asprepared gel particles was carried out at 900, 1000, and 1100 °C in air for 1 h. To prepare the Eu2O3 (1.0 mol %)-coated nanoparticles, the required amount of dried undoped ZrO2 gel particles was taken in a beaker with 30 mL of water, and the required europium nitrate solution was added to it. The sonication was carried out for 1 h, employing a direct immersion titanium horn (Vibracell, 20 kHz, 100 W/cm2) in an open beaker kept in an ice bath. Five milliliters of triethylamine was added in drops during the sonication. The resulting product after sonication was washed extensively with water, centrifuged, and dried under vacuum. Transmission electron microscopy (TEM; JEOL model 200) was used to study the morphology and particle size of the resulting powders. The crystalline phases of the annealed powders were identified by X-ray diffraction (XRD) using a Siemens model D500. The crystalline phases of the sintered powders were identified by XRD. The crystallite sizes of the nanocrystals were calculated following Scherrer’s equation:

χm ) 1 - χt

(3)

D ) Kλ/β cos Θ

(1)

where K ) 0.9, D represents the crystallite size (Å), λ is the wavelength of Cu KR radiation, and β is the corrected half width of the diffraction peak. The volume fraction of the tetragonal (χt) (18) Chatterjee, M.; Patra, A. J. Am. Ceram. Soc. 2001, 84, 1439. (19) Chatterjee, M.; Naskar, M. K.; Siladitya, B.; Ganguli, D. J. Mater. Res. 2000, 15, 176.

Figure 1. X-ray powder diffraction patterns of 0.05 mol % Eudoped ZrO2 nanocrystals heated at different temperatures for 1 h.

The excitation and emission spectra were recorded on a PerkinElmer LS55 luminescence spectrometer, using a solid sample holder at room temperature. We pressed the particles to form a smooth, opaque flat disk for optical study. For lifetime measurements of the Eu ions, the multichannel scaling (MCS) set up was used. The samples were excited at 372 nm using an IBH Nanoled-372 at a repetition rate of 100 Hz, a time per channel of 5 µs, and a channel per sweep of 1000. The decay curves were analyzed by IBH software. The goodness of the fit was checked by evaluating χ2 from a plot of weighted residuals and an autocorrelation function.

3. Results and Discussion 3.1. Structural Investigations. Figure 1 shows the XRD patterns of 0.05 mol % Eu3+-doped ZrO2 nanocrystals obtained after heating at 900, 1000, and 1100 °C. The phase composition and average crystallite size as a function of the dopant concentration, temperature of heating, and surface coating are summarized in Table 1. Both monoclinic and tetragonal phases are present here. Two lines are at 28.2° (111h) and 31.4° (111) for the monoclinic phase (m) (JCPDS 37 1484), and the line is at 30.17° (111) for the metastable tetragonal phase (JCPDS 17 0923). The volume fractions are 90.62% and 9.38% for monoclinic and tetragonal phases, respectively, for 900 °C-heated samples. The volume fraction of the monoclinic phase is increased from 90.62 to 98.84% upon increasing the temperature of heating from 900 to 1000 °C. Finally, the monoclinic phase reaches 100% at 1100 °C. It is clearly seen that the volume fraction of the monoclinic phase increases upon increasing the temperature of heating. The small shifting of the lines with changing the temperature of heating indicates the structural change due to the substitution of Zr4+ ions by the Eu3+ ions. The calculated crystallite sizes from the (111h) peak are 31.49, 42, and 42.06 nm for 900, 1000, and 1100 °C, respectively. Therefore, crystallite size increases upon increasing the temperature of heating. Figure 2a presents the XRD patterns of different concentrations of Eu-doped ZrO2 nanocrystals heated at 1000 °C. The change in the crystal phase upon changing the dopant concentration is observed at a fixed temperature. The volume fraction of the (20) Chang, S. M.; Doong, R.-a. Chem. Mater. 2005, 17, 4837.

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Table 1. Phase Composition and Crystallite Sizes of ZrO2/Eu Nanocrystalsa

composition 0.05 mol % Eu2O3

temperature of heating (°C) 900

undoped ZrO2

1000

0.05 mol % Eu2O3

1000

0.2 mol % Eu2O3

1000

1.0 mol % Eu2O3

1000

undoped ZrO2 0.05 mol % Eu2O3 0.2 mol % Eu2O3

1100 1100 1100

1.0 mol % Eu2O3

1100

1.0 mol % Eu2O3 coated 1.0 mol % Eu2O3 coated

1100 1000

a

phase composition (%)

crystallite size (nm)

m (90.62) t (9.378) m (98.99) t (1.08) m (98.84) t (1.16) m (94.7) t (5.3) m (71.92) t (28.08) m (100) m (100) m (95.7) t (4.3) m (88.67) t (11.33) m (100) m (8.43) t (91.56)

31.49 ND 41.57 ND 42.06 ND 30.72 28.39 42.28 29.66 41.9 42 34.82 42.7 35.65 24.51 27.90 28.56

ND: not detectable; m: monoclinic phase; t: tetragonal phase

Figure 2. X-ray powder diffraction patterns of different concentrations of Eu-doped ZrO2 nanocrystals prepared at 1000 °C (a) and 1100 °C (b) for 1 h.

monoclinic phase is 98.99% for the undoped ZrO2 sample. However, the volume fraction of the monoclinic phase decreases from 98.8 to 71.92% upon increasing the dopant concentration from 0.05 to 1.0 mol % Eu. On the other hand, the volume fraction of the tetragonal phase increases with increasing the dopant concentration. At low dopant concentrations, the monoclinic structure of zirconia is observed with a space group of P21/c, where each Zr atom is in a 7-fold coordination with oxygen atoms.21 The crystal structure of tetragonal ZrO2 is a body-centered

lattice with the space group P42/nmc. The symmetry of lattice for the monoclinic structure is 2/m (C2h), and symmetry lattice for the tetragonal structure is 4/mmm (D4h). It is well-known that doping zirconia with trivalent dopant has been the traditional approach for metastable tetragonal phase stabilization.22 The calculated crystallite sizes from the (111h) peak are 41.57, 42.06, 30.72, and 42.28 nm for undoped, 0.05, 0.2, and 1.0 mol % Eu-doped ZrO2 nanocrystals at 1000 °C, respectively. The tetragonal content increases from 1.2 to 28.08% for 0.05-1.0 mol % Eu3+ ion concentrations, respectively. However, the crystallite size is not dependent on dopant concentration. Only the monoclinic phase (Figure 2b) is obtained for undoped and 0.05 mol % Eu-doped ZrO2 samples when the temperature of heating is 1100 °C. The volume fractions of the monoclinic phase are 95.7 and 88.67% for 0.2 and 1.0 mol % Eu-doped ZrO2 samples, respectively. The sizes are 41.9, 42, 34.8, and 42.7 nm for undoped, 0.05, 0.2 and 1.0 mol % Eu-doped ZrO2 nanocrystals prepared at 1100 °C, respectively. The crystallite sizes have no significant change relative to those of the 1000 °C-heated samples. Here, we have seen the change in the crystal phase due to a change in dopant concentration but not their crystal size. The effect of increasing the tetragonal phase is due to the removal of oxygen vacancies. The oxygen vacancies are considered to be responsible for the formation of the tetragonal phase instead of the thermodynamically stable monoclinic phase upon crystallization. Usually, oxygen vacancies are introduced by the incorporation of allovalent ions, including RE ions, into the ZrO2 lattice.23 The monoclinic phase is stable when the composition is close to the stoichiometry value. Figure 3a,b shows the XRD pattern of 1.0 mol % Eu-doped and 1.0 mol % Eu-coated ZrO2 nanocrystals heated at 1000 and 1100 °C temperatures, respectively. The volume fractions of tetragonal phase are 91.56 and 28.08% for 1000 °C heat-treated coated and doped samples, respectively. This is a drastic change in their crystal phase due to coating on the surface of ZrO2 nanoparticles. Again, the peak at 29.32° (401) confirms the presence of Eu2O3. We think this is the most significant effect due to surface coating. This result clearly indicates that surface coating plays an important role in the modification of the crystal structure of ZrO2. To the best of our knowledge, the modification of the crystal structure of ZrO2 nanocrystals by surface coating has not been reported in the literature. It is known that synthesizing ZrO2 particles with metastable tetragonal crystal structures is very important for technological purposes. Garvie24 showed the existence of a critical size of ∼30 nm, below which the metastable tetragonal phase is stable in nanocrystalline ZrO2 and above which the monoclinic phase is stable. This phase transformation is a surface phenomenon, and several studies have proposed that surface OH groups diffuse into the inner ZrO2 and consequently occupy oxygen vacancies to promote tetragonal-to-monoclinic phase transformation.20,25 In the case of a coated sample, the diffusion of the surface OH groups is restricted, and that will hinder the transformation to the monoclinic phase. This is a new and unprecedented result in this study. However, a 100% monoclinic phase is present in 1100 °C-heated coated samples (Figure 3b). This clearly shows that the temperature of heating also plays an important role. The peaks at 28.83 (202), 29.68 (401), and 30.54° (003) confirm the presence of Eu2O3 oxide coating on the surface (21) Lopez, E. F.; Escribano, V. S.; Panizza, M.; Carnasciali, M. M.; Busca, G. J. Mater. Chem. 2001, 11, 1891. (22) Ping, L.; Chen, I.-W; Penner-Hahn, J. E. J. Am. Ceram. Soc. 1994, 77, 118. (23) Zhang, Y. W.; Yang, Y.; Tian, S. J.; Liao, C. S.; Yan, C. H. J. Mater. Chem. 2002, 12, 219. (24) Garvie, R. C. J. Phys. Chem. 1965, 69, 1238. (25) Gao, X. J. Phys. Chem. Solids 1999, 60, 539.

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Figure 3. XRD patterns of 1.0 mol % Eu3+-doped and coated ZrO2 nanocrystals prepared at 1000 °C (a) and 1100 °C (b) for 1 h.

of ZrO2 nanocrystals. In the case of a doped sample, tetragonal (11.33%) and monoclinic (88.67%) phases are observed. It reveals that surface coating plays an important role in the modification of the crystal phase. Results obtained in this study clearly demonstrate that the evolution of the crystalline phase of ZrO2 varies upon changing the temperature of heating, dopant concentration, and surface coating. Figure 4 shows TEM images of 1.0 mol % Eu3+-doped ZrO2 nanocrystals prepared at 1100 °C. The average size of the particles is ∼42 nm for Eu-doped nanocrystals, in agreement with the results obtained from XRD studies. Figure 4b shows a selected area of the electron diffraction pattern. The diffraction spots are observed in the selected area electron diffraction (SAED) pattern, and this diffraction pattern confirms the (002) plane of the monoclinic crystal phase of ZrO2. Figure 4c shows the highresolution image with the corresponding fast Fourier transformation (FFT) pattern of 1.0 mol % Eu-doped ZrO2 nanocrystals. The measured lattice spacing in the high-resolution TEM (HRTEM) image is 2.85 Å, which corresponds to the (111) plane of the monoclinic ZrO2, and this result further confirms the XRD data. Figure 5 shows TEM images of 1.0 mol % Eu3+-coated ZrO2 nanocrystals prepared at 1100 °C. Figure 5a clearly represents the surface coating of nanocrystals. The thickness of the surface coating of Eu2O3 is around 2-3 nm (Figure 5b). Figure 5c shows the high-resolution image with the corresponding FFT pattern of Eu2O3 oxide coating. The measured lattice spacing in the HRTEM image is 2.94 Å, which corresponds to the (4h02) plane of the monoclinic phase of Eu2O3. The diffraction spots (Figure 5d) are observed in the SAED pattern, and this diffraction pattern confirms the (4h02) plane of the monoclinic phase of Eu2O3. Results indicate that chemical synthesis through the solemulsion-gel method is an excellent route for the preparation of Eu-doped and Eu-coated ZrO2 nanocrystals.

Figure 4. TEM micrographs (a), SAED (b), and FFT (c) of 1.0 mol % Eu-doped ZrO2 nanocrystals annealed at 1100 °C.

3.2. Luminescence Properties. 3.2.1. Static Photoluminescence. Figure 6 shows the excitation and emission spectra of 1.0 mol % Eu-doped ZrO2 nanocrystals prepared at 900 °C. The peak at 240 nm is the excitation band due to the charge transfer (CT) state for europium-oxygen interactions. The electronic transition from the 2p orbital of O2- to the 4f orbital of Eu3+ produces an excited CT band whose energy depends on the potential field of the ions surrounding O2-. We used 240 nm as the excitation wavelength for all samples. The prominent emission bands were observed at 607 and 592 nm. Generally, the emission spectra are attributed to the 5D0 f 7FJ (J ) 0-2) transition of the Eu3+ ions; that is, the band at 560-570 nm is assigned to 5D f 7F , while the band at 570-603 nm and the band at 0 0 603-640 nm are related to the transitions of 5D0 f 7F1 and 5D0 f 7F2, respectively. Figure 7 shows the emission spectra of 0.05 mol % Eu2O3doped ZrO2 nanocrystals prepared at different temperatures. All spectra were taken under identical conditions. It is clearly seen from this figure that the intensity of the emission bands of Eu increases upon changing the temperature of heating from 900 to 1100 °C. In europium, the 5D0 f 7F1 (592 nm) transition is

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Figure 7. The emission spectra of 0.05 mol % Eu2O3-doped ZrO2 nanocrystals heated at different temperaturess900, 1000, and 1100 °Csfor 1 h. [detector voltage ) 650 V, λex ) 240 nm, slit Ex ) 5, and slit Em ) 5].

Figure 8. The emission spectra of different concentrations (mol %) of Eu2O3-doped ZrO2 nanocrystals annealed at 1100 °C for 1 h. [detector voltage ) 650 V, λex ) 240 nm, slit Ex ) 5, and slit Em ) 5].

Figure 5. TEM micrographs (a,b), FFT (c), and SAED (d) of 1.0 mol % Eu-coated ZrO2 nanocrystals annealed at 1100 °C.

Figure 6. The excitation and emission spectra of 900 °C heattreated 1.0 mol % doped ZrO2 nanocrystals. [detector voltage ) 650 V, slit Ex ) 5, and slit Em ) 5].

mainly magnetically allowed (a magnetic-dipole transition), while 5D0 f 7F2 (617 nm) is a hypersensitive forced electricdipole transition being allowed only at low symmetries with no inversion center.12 Thus, the intensity ratio I(5D0 f 7F2)/I(5D0 f 7F1) serves as an effective spectroscopic probe of the site

symmetry in which europium is situated, that is, the higher the ratio, the lower the site symmetry. The ratio increases upon increasing the temperature of heating from 900 to 1100 °C, indicating that the dopant ions are in lower site symmetry upon increasing the temperature of heating. It is also seen from the XRD study that the volume fraction of the monoclinic phase increases from 90.62 to 100% upon increasing the temperature of heating, indicating the asymmetric environment increases with increasing the temperature of heating. The enhancement of the emission intensities related to the temperature of heating is ascribed to two other factors as well: one is the growth of the ZrO2 particles, and the second is the loss in the amount of acetate and hydroxyl groups, which lowers the multiphonon relaxation rate. Here, we believe that the loss of hydroxyl groups is the dominant factor to increase the emission intensity with increasing the temperature of heating. Figure 8 shows the emission spectra of different concentrations of Eu (0.05, 0.2, and 1.0 mol %)-doped ZrO2 nanocrystals prepared at 1000 °C. It is clearly seen from this figure that the intensity of these emission bands of Eu3+ ions increases upon increasing the concentration of dopant ions from 0.05 to 1.0 mol % (Figure 8). From the XRD study, it is seen that the volume fraction of the monoclinic phase decreases and the volume fraction of tetragonal phase increases upon increasing the dopant concentration from 0.05 to 1.0 mol % Eu. As a result, the different site symmetries of the dopant ion produces a large inhomogeneous broadening of the electronic forced dipole transitions (EFDPs). EFDPs of RE ions are strongly affected by the local field symmetry and coupling strength with the host matrix. The value of the

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Figure 10. Photoluminescence decay curves of different concentrations of Eu-doped ZrO2 nanocrystals prepared at 1100 °C.

Figure 9. The emission spectra of 1.0 mol % Eu2O3-doped and coated ZrO2 nanocrystals prepared at (a) 1000 and (b) 1100 °C. [detector voltage ) 650 V, λex ) 240 nm, slit Ex ) 5, and slit Em ) 5].

intensity ratio I(5D0 f 7F2)/I(5D0 f 7F1) increases with increasing the concentration of ions from 0.05 to 1.0 mol %. This result indicates that the dopant ions are in a lower site symmetry at higher concentration. It is well-known that the f-f transitions arising from the forced electric dipole are partially forbidden and become partially allowed when the ion is situated at a lowsymmetry site. Such a situation allows the intermixing of the f states with higher electronic configurations,12 and, as a result, the optical transition probability increases, that is, the radiative emission rate increases. It may be the reason for the increase in the luminescence intensity with increasing the concentration of ions. Under the same experimental conditions, namely, the same wavelength excitation with the same bandwidth, we compared the luminescent intensities of Eu3+-doped nanocrystals with those of Eu3+-coated nanocrystals. Figure 9a depicts the emission spectra of 1.0 mol % Eu2O3-doped ZrO2 nanocrystals and 1.0 mol % Eu2O3-coated ZrO2 nanocrystals prepared at 1000 °C. In the case of Eu-doped ZrO2 nanocrystals, peaks at 617 nm (5D0 f 7F2) and 596 nm (5D0 f 7F1) are observed. However, in the case of the coated sample, peaks at 610 nm (5D0 f 7F2) and 594 nm (5D0 f 7F1) are observed. It is known that an emission band shifts to higher energy if the crystal field is weak because the amount of covalence is low. The relation between the Eu-O bond distance and the emission intensity can be explained by a configurational-coordinate model.26 According to this model, when the equilibrium distance (∆r) between the excited state of the ion and that of the ground state decreases, it is easier for the luminescence center to return from the excited state to the ground state through a radiative transition. That indicates that a smaller (26) Blasse, G.; Bril, A. Philips Technol. ReV. 1970, 31, 314.

value of ∆r corresponds to a stronger emission. This result reveals that ∆r is smaller in a doped sample than a coated sample. It indicates that the Eu-O bond distance is shorter in a doped sample than a coated sample. In the case of 1100 °C-heated samples, peaks at 617 nm (5D0 f 7F2) and 596 nm (5D0 f 7F1) are observed in both the samples (Figure 9b). From the XRD study, it is confirmed that the monoclinic phase is obtained in both samples. Results indicate that the local structure of RE ions in a doped sample is different from that in coated nanocrystals. Recently, Lehmann et al.1 reported that Eu3+ ions in the interior of LaPO4 nanoparticles occupy the same lattice sites as those in the bulk materials, and the spectroscopic properties of Eu3+ ions in surface sites differ significantly from those in the interior of the nanoparticles because of their different crystal fields. Here, we believe that the change in emission intensity is due to modification of the local environment and the coordination number of RE ions. More systematic analysis is required to understand this phenomenon. 3.2.2. Time-ResolVed Photoluminescence. Figure 10 shows the photoluminescence decays for 1100 °C-heated Eu-doped ZrO2 nanocrystals, monitored at the 5D0 f 7F2 transition (617 nm). The decay times are 472 and 625 µs for 0.05 and 0.2 mol % Eu3+-doped ZrO2 nanoparticles prepared at 1100 °C, respectively. Here we observed single-component decay. However, at higher concentrations, biexponential decay is observed. For 1.0 mol % Eu3+-doped ZrO2 nanoparticles prepared at 1100 °C, the decay components are τf ) 137 µs (20.5%) and τs ) 798 µs (79.5%), and the average decay time is 770 µs. This result clearly indicates that the decay time arises from two different sources at higher concentrations, which are due to radiative and nonradiative relaxation pathways. The decay times are 355, 365, and 472 µs for 0.05 mol % Eu3+-doped ZrO2 nanocrystals prepared at 900, 1000, and 1100 °C, respectively. This result also agrees with the emission spectra. Figure 11 depicts the photoluminescence decay curve of 1100 °C-heated 1.0 mol % Eu-doped and 1.0 mol % Eu-coated ZrO2 nanocrystals, monitored at the 5D0 f 7F2 transition (617 nm). The decay components are τf ) 137 µs (20.5%) and τs ) 798 µs (79.5%) and the average decay time is 770 µs for 1.0 mol % Eu3+-doped ZrO2 nanoparticles prepared at 1100 °C. However, the decay components are τf ) 42 µs (48%) and τs ) 522 µs (52%) and average decay time is 488 µs for 1.0 mol % Eu3+-coated ZrO2 nanoparticles prepared at 1100 °C. The plausible explanation for the decreasing decay times of the coated sample is the probability of energy transfer between neighboring Eu ions. In the present study, we observed a decrease in decay time from both components (fast and slow) in the coated sample. Therefore, we believe that an energy transfer between ion-ion interactions, that is, a cross-relaxation nonradiative process, takes

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Figure 11. Photoluminescence decay curves of 1.0 mol % Eudoped and coated ZrO2 nanocrystals prepared at 1100 °C.

place in coated sample. It is well understood that the probability of ion-ion interactions is much higher in the coated sample than it is in the doped sample. This result agrees well with the emission spectra (Figure 9). This result clearly reveals the influence of surface coating on the photoluminescence properties. The total relaxation rate can be expressed as

1/τ )

∑WNR + ∑A + PCR

(4)

where ∑A is the total radiative (photon) emission rate, ∑WNR is the nonradiative (multiphonon) emission rate, and PCR is the rate of cross-relaxation between adjacent ions. The term ∑A basically depends on the crystal structure and symmetry, while the term ∑WNR basically depends on the host phonon spectrum (cutoff phonon energy) and hydroxyl groups. Therefore, the lifetime may change because of modification in the crystal structure, phonon spectrum, and hydroxyl groups. Multiphonon relaxation is related to the number of phonons:

WNR ) A exp (-Bp)

(5)

where A and B are constants, and p is the number of phonons and can be calculated using the equation given below:

p ) ∆E/hω ) 12000 cm-1/hω where ∆E ) 12000

cm-1

is the energy gap between the

(6) 5D

0

and

levels, and hω is the energy of the phonons (expressed in wavenumbers) of the crystal host. Considering the IR frequencies of tetragonal and monoclinic structures are 435 and 270 cm-1, respectively, the number of phonons is 27.6 and 44.4 for tetragonal and monoclinic structures, respectively.27 With an increasing number of phonons p, WNR decreases, and, as a result, the lifetime increases (see eq 4). Therefore, the monoclinic phase, that is, the asymmetric host, is the most favorable matrix for a highluminescence lifetime. The results clearly reveal that the emission process depends on crystal structure. Therefore, it is clear that modification of the radiative relaxation mechanism occurs as a result of the change in the symmetry by changing the crystal phase. Thus, we conclude that the changes observed in the decay time of the excited state of Eu3+ ions must be due to modifications in the radiative and nonradiative relaxation processes due to crystal structure, which is controlled by dopant concentration, temperature of heating, and surface coating.

4. Conclusions In conclusion, chemical synthesis through the sol-emulsiongel method is a promising route for the preparation of doped and coated ZrO2/Eu nanocrystals having particle diameters of ∼42 nm at 1100 °C. Our results highlight the influence of dopant concentration, temperature of heating, and surface coating on the crystal structure of the ZrO2 host and the photoluminescence properties of the Eu ions. It is found that the decay time (τ) of Eu-doped ZrO2 nanocrystals increases upon increasing the concentration of dopant and upon increasing the temperature of heating because of the change in their crystal phase. Again, surface coating has an important effect on controlling their crystal phase and their properties. Therefore, judicious choice of crystal structure, surface coating, and dopant concentration are responsible for efficient emission of Eu3+ ions in ZrO2 nanocrystals. Our analysis suggests that the site symmetry of the ions plays the most important role in the modification of radiative and nonradiative relaxation mechanisms as a result on the overall photoluminescence properties. Acknowledgment. The Department of Science and Technology (NSTI, No.SR/S5/NM-05/2003) is acknowledged for financial support. LA0604883 (27) Phillippi, C. M.; Mazdiyasni, K. S. J. Am. Ceram. Soc. 1971, 54, 254.