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J. Phys. Chem. C 2007, 111, 11223-11230

11223

Synthesis and Luminescence Properties of Monodisperse Spherical Y2O3:Eu3+@SiO2 Particles with Core-shell Structure Huan Wang,†,‡ Min Yu,†,‡ Cuikun Lin,† Xiaoming Liu,† and Jun Lin*,† Key laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and Department of Chemistry, Northeast Normal UniVersity, Changchun 130024, People’s Republic of China ReceiVed: April 1, 2007; In Final Form: May 10, 2007

Y2O3:Eu3+ phosphor layers were deposited on monodisperse SiO2 particles with different sizes (300, 500, 900, and 1200 nm) via a sol-gel process, resulting in the formation of Y2O3:Eu3+@SiO2 core-shell particles. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), time-resolved photoluminescence (PL) spectra, and lifetimes were employed to characterize the Y2O3:Eu3+@SiO2 core-shell samples. The results of XRD indicated that the Y2O3:Eu3+ layers began to crystallize on the silica surfaces at 600 °C and the crystallinity increased with the elevation of annealing temperature until 900 °C. The obtained core-shell particles have perfect spherical shape with narrow size distribution and non-agglomeration. The thickness of the shells could be easily controlled by changing the number of deposition cycles (60 nm for three deposition cycles). Under the excitation of ultraviolet (250 nm), the Eu3+ ion mainly shows its characteristic red (611 nm, 5D0-7F2) emissions in the core-shell particles from Y2O3:Eu3+ shells. The emission intensity Eu3+ can be tuned by the annealing temperature, SiO2 core size, the number of coating cycles, and polyethylene glycol (PEG) concentration in the precursor solution, respectively.

I. Introduction Recently, much effort has been devoted to the design and controlled fabrication of nanostructured materials with functional properties. Among them the core-shell structured materials have been attracting much attention due to the ability to fine-tune their properties.1-10 Core-shell materials consist of a core structural domain covered by a shell domain, both of which may be composed of a variety of materials, including polymers, inorganic solids, and metals. In general, the preparation of coreshell structures involves surface modification of particles, often accomplished by coating or encapsulating them with a different material having the desired properties.1 So far, considerable kinds of core/shell structures have been designed and fabricated, including semiconductor/semiconductor,11 semiconductor/ dielectric,12 metal/dielectric,13 metal/ semiconductor,14 inorganic particle/ polymer,1 polymer or inorganic particle/biomolecule,1 etc. The structure, size, and composition of these particles can be easily altered in a controllable way to tailor their magnetic, optical, mechanical, thermal, electrical, electro-optical, and catalytic properties. A variety of approaches have been employed for the manufacture of the core-shell structured materials, such as coprecipitation,10,15 layer-by-layer self-assembly,1,6,8,16 surface reaction,17 sol-gel process,5,18 MOCVD,19 etc. Each of these procedures has its own advantages, disadvantages and applicability. The controlled coating of particles with homogeneous and organized layers without causing aggregation remains a challenge for the scientists.1 The newly developed displaying technologies such as plasma display panels (PDP) and field emission displays (FED) have * Author to whom all correspondence should be addressed. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Northeast Normal University.

brought forward new requirements for the morphology of phosphor materials.20 The ideal morphology of phosphor particles includes a perfect spherical shape, narrow size distribution (0.5-2 µm), and non-agglomeration. Spherical morphology of the phosphors is good for high brightness and high resolution. Additionally, high packing densities and low scattering of light can also be obtained by using spherical phosphors.21 So far, many synthetic routes have been developed to control the size and distribution of phosphor particles, such as spray pyrolysis22 and urea homogeneous precipitation.21a However, the obtained phosphor particles are still far from the ideally monodisperse spherical morphology. Silica can be fabricated controllably into spherical morphology from nano- to micrometer size,23 and it is frequently used in core-shell structured materials, either as a core or a shell.8,9,13,18 If the silica spheres are coated with phosphor layers, a kind of core-shell phosphor materials with spherical morphology will be obtained, and the size for the phosphor particles can be controlled by the silica cores and the number of coating cycles. Furthermore, because silica is cheaper than most of the phosphor materials (which often employ the expensive rare earth elements as the activators and/or host components), the coreshell phosphor materials will be cheaper than the pure phosphor materials. Due to its high efficiency ,Y2O3:Eu3+ phosphor is widely used in fluorescent lamps and flat panel display devices.21a,22d,24 Recently, nanocrystalline Y2O3:Eu3+ showed higher emission efficiency than the bulk Y2O3:Eu3+ under low voltage (1-4kv) excitation.25 However, the fine control for the morphology of Y2O3 based phosphors is still a challenge for chemists and physicists. Here in this work, we developed a large-scale and facile method to obtain monodisperse and spherical core-shell structured Y2O3:Eu3+@SiO2 phosphors by functionalization of

10.1021/jp072541h CCC: $37.00 © 2007 American Chemical Society Published on Web 06/29/2007

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TABLE 1: Synthesis Conditions of SiO2 Particles with Different Sizes (nm), where C is the Concentration (mol/L), N is the Number of Reaction, and t is Reaction Time (h) SiO2 300 nm 500 nm 900 nm 1200 nm

C[TEOS] C[H2O] C[NH3] 0.17 0.20 0.20 0.20

7.5 5.0 5.0 6.0

1.0 7.0 7.0 2.0

seeds

500 nm 500 nm

SCHEME 1: Formation Process of Y2O3:Eu3+@SiO2 Core-Shell Particles

VTEOS/VH2O N t

1/2 1/2

2 3 5 3

silica spheres with Y2O3:Eu3+ layers via sol-gel process, and characterize the structure, morphology, photoluminescent properties of the resulting samples in detail. II. Experimental Section The starting materials used in the experiments were tetraethoxysilane (TEOS, 99%, Beijing Beihua Chemicals Co., Ltd), Y2O3 (99.99%, Shanghai Yuelong Nonferrous Metals Ltd.), Eu2O3 (99.99%, Shanghai Yuelong Nonferrous Metals Ltd.), NH4OH (25 wt. %, analytical reagent ) A. R., Beijing Beihua Chemicals Co., Ltd.), HNO3 (A. R., Beijing Beihua Chemicals Co., Ltd.), polyethylene glycol (PEG, molecular weight ) 10000, A. R., Beijing Beihua Chemicals Co., Ltd.), citric acid (A. R., Beijing Beihua Chemicals Co., Ltd.), and ethanol (A. R., Beijing Beihua Chemicals Co., Ltd.). Synthesis of Silica Cores. Monodisperse silica spheres with different sizes (300, 500, 900 and 1200 nm) were prepared by hydrolysis of TEOS in an alcohol medium in the presence of water and ammonia by a modified procedure of the well-known Sto¨ber process.23 This process yielded the colloidal solution of silica particles with a narrow size distribution in submicrometer range, and the particle size of silica depended on relative concentration of the reactants. In a typical experiment, the mixture containing TEOS, H2O, NH4OH, and C2H5OH was stirred at room temperature for 4 h, leading to the formation of white silica colloidal suspension. The silica particles were centrifugally separated from the suspension and washed with ethanol four times. Monosized silica particles larger than 1 µm could not be obtained directly through the Sto¨ber process, so a seeded growth process was employed.26 In the seeded growth experiment, certain amounts of smaller prepared silica particles were added into the NH3-H2O-C2H5OH solution before the TEOS-containing ethanol was added to the reactor. The experimental procedure is similar to that of the Sto¨ber process. The experimental conditions for obtaining the SiO2 particles with different size (300, 500, 900, and 1200 nm) were listed Table 1. Coating of SiO2 Cores with Y2O3:Eu3+ Shells. The Y2O3: Eu3+@SiO2 core-shell particles were prepared by a Pechini sol-gel process.27 The doping concentration of Eu3+ is 5 mol % that of Y3+ in Y2O3 host, which has been optimized previously.27b Stoichiometric amounts of Y2O3:Eu2O3 were dissolved in nitric acid HNO3, then mixed with a water-ethanol (V/V ) 1:7) solution containing citric acid, which acted as chelating agent for the metal ions. The molar ratio of metal ions to citric acid was 1: 2. Then polyethylene glycol (PEG, molecular weight ) 10 000, A. R.) was added with a final concentration ranging from 0.04 to 0.16 g/ mL. The solution was stirred for 2 h to form sols, and then the above silica particles were added under stirring. After stirring for another 5 h, the suspension was separated by centrifugation. The particles were dried at 100 °C immediately and then annealed to the desired temperature (from 500 to 900 °C) with a heating rate of 120 °C h-1 and held for 2 h. (Here it should be mentioned that if annealed at 1000 °C or higher a reaction between the Y2O3 shell and SiO2 core will occur, resulting in the formation

of impurity like Y2SiO5 phase in the samples. So we limit the annealing temperature between 500 and 900 °C in this work.) The above process was repeated several times to increase the thickness of the Y2O3:Eu3+ shells on the SiO2 surface. In this way core-shell structured Y2O3:Eu3+@SiO2 particles were obtained. The whole experimental process is shown in Scheme 1. For the purpose of comparison, the coating sols were evaporated to form gels, which were annealed in a similar process to produce the pure Y2O3:Eu3+ powder phosphors. Characterizations. The X-ray diffraction (XRD) of the samples was examined on a Rigaku-Dmax 2500 diffractometer using Cu KR radiation (λ ) 0.15405 nm). FT-IR spectra were measured with a Perkin-Elmer 580B spectrophotometer with the KBr pellet technique. The sample morphologies were inspected using a field emission scanning electron microscope (FESEM, XL30, Philips) equipped with an energy-dispersive X-ray spectrum (EDS, JEOL JXA-840) and transmission electron microscope (TEM) and high-resolution TEM (HRTEM) (FEI Tecnai G2 S-Twin transmission electron microscope) with a field emission gun operating at 200 kV. The photoluminescence (PL) spectra were taken on a Hitachi F-4500 spectrofluorimeter equipped with a 150 W xenon lamp as the excitation source. Time-resolved photoluminescence spectra and decay curves were obtained from a Lecroy Wave Runner 6100 digital Oscilloscope (1GHz) using a 250 nm laser (pulse width ) 4 ns, gate ) 50 ns) as the excitation source (Continuum Sunlite OPO).

Monodisperse Spherical Y2O3:Eu3+@SiO2 Particles

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Figure 1. X-ray diffraction patterns for the Y2O3:Eu3+@SiO2 coreshell particles annealed form 500 to 900 °C. Miller indices of diffracting lattice planes are also shown.

TABLE 2: Crystallite Sizes of Y2O3:Eu3+ in Y2O3:Eu3+@SiO2 Core-Shell Particles Annealed from 600 to 900 °C temperature (°C)

600

700

800

900

crystallite size (nm)

13

16

19

21

III. Results and Discussion 3.1. Formation and Morphology of the Core-Shell Particles. The formation and morphological properties of the coreshell structured Y2O3:Eu3+@SiO2 samples were characterized by XRD, FT-IR, SEM with EDX (energy-dispersive X-ray spectrum) and TEM (including HRTEM) techniques, respectively. XRD. Figure 1 shows the XRD patterns of Y2O3:Eu3+@SiO2 samples annealed from 500 to 900 °C. The XRD results in Figure 1 indicated that the sample remained amorphous after layers being annealed at 500 °C, began to crystallize in cubic Y2O3 phase at 600 °C, and the crystallinity increased with raising the annealing temperature until 900 °C without the reaction occurring between the SiO2 cores and Y2O3 shells. All the diffraction peaks at 2θ ) 29.2° (222, strongest), 20.5° (211), 28.5° (400), 39.9° (332), 43.5° (134), 48.5° (440), 53.2° (611), and 57.3° (622) in the samples can be well indexed to the JCPDS Card 88-1040 for cubic Y2O3. The broad band peaking at 2θ ) 22° is from the amorphous SiO2 cores (JCPDS 29-0085). No second phase was detected. In general, the nanocrystallite size can be estimated from the Scherrer formula: Dhkl ) Kλ/(β cos θ), where λ is the X-ray wavelength (0.15405 nm), β is the full-width at half-maximum, θ is the diffraction angle, K is a constant (0.89), and Dhkl means the size along (hkl) direction.28 Here we take diffraction data along the (222) plane at 2θ ) 29.2° to calculate the size of the nanocrystallites, and the estimated average crystallite sizes of Y2O3:Eu3+ are collected in Table 2. Obviously, the crystallite size increases with the increase of annealing temperature (from 13 nm for 600 °C to 21 nm for 900 °C). The formation of Y2O3:Eu3+ layer is due to the reaction of Y3+, Eu3+ ions on silica surface at hightemperature (600-900 °C), as shown in Scheme 1. In the Pechini process, the citric acid first formed chelate complexes with Y3+ and Eu3+; then the left carboxylic acid groups in the citric acid reacted with polyethylene glycol to form polyester with a suitable viscosity. The Sto¨ber process-derived silica particles contained large amount of free hydroxyl groups (-OH) and silanol groups (Si-OH) on their surface (evidenced by FTIR spectra, see next section). By stirring silica particles in the

Figure 2. FT-IR spectra of the as-formed SiO2 (a), 800 °C annealed Y2O3:Eu3+@SiO2 core-shell sample, (b) and pure Y2O3:Eu3+ powder (c).

solution, a lot of Y3+ and Eu3+ were absorbed onto the silica particles by physical and chemical interactions. After drying and annealing process, Y2O3:Eu3+@SiO2 core-shell particles are formed. FT-IR. The FT-IR spectra were recorded for the as-formed SiO2, 800 °C annealed pure Y2O3:Eu3+ powder, and Y2O3: Eu3+@SiO2 core-shell samples are shown in parts a-c of Figure 2, respectively. For the sample of as-formed SiO2, the FT-IR spectrum several broad absorption bands of the OH (3429 cm-1), H2O (1630 cm-1), Si-O-Si (1109 cm-1, 802 cm-1), Si-OH (951 cm-1), and Si-O (471 cm-1) bands are observed. This fact indicates that the as-formed SiO2 particles contain a large amount of OH groups and H2O on their surfaces.27c The surface Si-OH groups play an important role in bonding the metal ions (Y3+, Eu3+) to form the Y2O3 :Eu3+ shells on the SiO2 surface in the annealing process (Scheme 1). In Figure 2b for Y2O3:Eu3+@SiO2 core-shell sample, the characteristic absorption peaks of the Y-O bonds (566 cm-1) from Y2O3: Eu3+ shell (also present for pure Y2O3:Eu3+ powder in Figure 2c) and the Si-O-Si bond (1109 cm-1, 802 cm-1) for amorphous SiO2 (Figure 2a) have been observed clearly. These results are consistent with those of XRD and further confirm the formation of crystalline Y2O3:Eu3+ coatings on the silica surface via the sol-gel deposition and the annealing process. FESEM and TEM. The FESEM micrographs for the asformed silica particles (500 nm), 800 °C annealed pure Y2O3: Eu3+ powder and the SiO2 particles coated by one to four-layer of Y2O3:Eu3+ as well as the EDX are shown in Figure 3a-g, respectively. Obviously, the as-formed SiO2 sample consists of well separated spherical particles with an average size of 500 nm and a narrow size distribution (Figure 3a), while the pure Y2O3:Eu3+ sample contains aggregated fine particles from 60 to 100 nm (Figure 3b). After being coated by one (Figure 3c), two (Figure 3d), three (Figure 3e) and four (Figure 3f) layer of Y2O3:Eu3+, the resulted Y2O3:Eu3+@SiO2samples still keep the morphological properties of the silica particles; i.e., these particles are still spherical and nonaggregated with rough surfaces and slightly larger than pure silica particles due to the additional layers of Y2O3:Eu3+ on them. The same situation holds for other sized SiO2 particles (300, 900, 1200 nm) coated with Y2O3:Eu3+ layers (Figure 4a,b,c). The energy-dispersive X-ray (EDX) analysis confirms the presence of Y, O (from the Y2O3 shells) and Si (from the silica cores), as shown in Figure 4d. The Eu element was not detected due to its low concentration

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Figure 3. FESEM micrographs of the as-formed SiO2 particles (a), 800 °C, annealed Y2O3:Eu3+ powder (b), and the SiO2 particles coated with, one (c) two (d), three (e), and four (f) layers of Y2O3:Eu3+, respectively.

(but it can be detected clearly from the luminescence spectra, see next part). In order to see the core-shell structure for Y2O3:Eu3+@SiO2particles, TEM measurements were representatively performed on 500 nm silica particles coated by three times of Y2O3:Eu3+ layer, as shown in Figure 5. In Figure 5a for the general TEM image, the core-shell structure can be observed clearly due to

Wang et al. the different electron penetrability for cores and shells. The cores are black spheres with an average size of 500 nm (in diameter), and the shells have gray color with an average thickness of 60 nm (as labeled in the Figure 5a). From the HRTEM image performed on the surface of Y2O3:Eu3+@SiO2 sample (Figure 5b), we can see crystalline phase (Y2O3) with well resolved lattice fringes. The measured distance between the adjacent lattice fringes is 0.294 nm, just corresponding to the interplanar distance of Y2O3 (222) planes, agreeing basically with the d (222) spacing of the literature value (0.306 nm) within the experimental error.25 These results further confirm the presence of crystalline Y2O3:Eu3+ on the surface of SiO2 particles, agreeing well with the XRD results. It should be mentioned that the SiO2 cores are amorphous without obvious lattice fringes being detected, and minor amount of uncoated Y2O3:Eu3+ flocks can be observed in Figure 5a. 3.2. Luminescent and Kinetic Properties of Y2O3: Eu3+@SiO2 Particles. The Y2O3:Eu3+@SiO2 particles exhibit a strong red emission under short UV irradiation. Figure 6 shows excitation (a) and emission (b) spectra for Y2O3:Eu3+@SiO2 particles annealed at 900 °C, respectively. The excitation spectrum was performed by monitoring the emission of Eu3+ 5D -7F transition at 611 nm. It can be seen clearly that the 0 2 excitation spectrum consists of a broad band with maximums at 250 nm, which can be attributed to the charge-transfer band (CTB) between O2- and Eu3+. In the longer wavelength region, f-f transition lines of Eu3+ can be observed with very weak intensity with respect to the strong CTB of Eu3+-O2-. Upon excitation into the CTB at 250 nm, the obtained emission spectrum is composed of 5D0,1-7FJ (J ) 0, 1, 2, 3, 4) emission lines of Eu3+ (Figure 6b), dominated by the hypersensitive red emission 5D0-7F2 transition of Eu3+ at 611 nm. The other emission peaks located at 535 (5D1-7F1), 579 (5D0-7F0), 585,

Figure 4. FESEM micrographs of the two layer Y2O3:Eu3+-coated SiO2 particles of (a) 300, (b) 900, (c) 1200 nm, and EDX of the core-shell structured Y2O3:Eu3+@SiO2 particles (d).

Monodisperse Spherical Y2O3:Eu3+@SiO2 Particles

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Figure 7. The decay curves for the 5D1-7F1 (a) and 5D0-7F2 (b) emission of Eu3+ in Y2O3:Eu3+@SiO2 sample annealed at 800 °C (λex ) 250 nm laser). Figure 5. TEM micrographs for 800 °C, annealed two layer Y2O3: Eu3+-coated SiO2 particles (a) and high-resolution TEM (b) for the sample.

Figure 6. Excitation (a) and emission (b) spectra for Y2O3:Eu3+@SiO2 core-shell particles.

591, 598 (5D0-7F1), 651(5D0-7F3), and 709 nm (5D0-7F4) are labeled in Figure 6b. In cubic Y2O3:Eu3+, there are two crystallographic sites for Eu3+: one is with C2 symmetry and another with S6 symmetry. 27b, 29 Eu3+ at C2 site contributes 5D0-7F0, 1, 2 transitions to the main part of visual luminescence of Y2O3:Eu3+ (580-640 nm). However, the 5D0-7F1 transition lines are allowed for both C2 and S6 sites, are expected to arise

from Eu3+ (C2) and Eu3+ (S6) sites simultaneously. 27b, 29 The crystal field splitting of Eu3+ 5D0-7F1, 2 transitions can be seen clearly, indicating that the Y2O3:Eu3+ layer is well crystallized on the surface of SiO2 particles (agreeing well with the XRD and HRTEM results in last section). The presence of emission lines from higher excited states of Eu3+ (5D1) is attributed to the low vibration frequency of Y-O bond (566 cm-1). The multiphonon relaxation by Y-O vibration is not able to bridge the gaps between the higher energy levels (5D1) and 5D0 level of Eu3+ (the energy difference between 5D1 and 5D0 is 1570 cm-1) completely, resulting in the presence of the emission from the 5D1 higher level. 24a On the other hand, in view of the high doping concentration (5 mol %) of Eu3+, the higher 5D1 emission might be quenched by cross-relaxation occurring between two neighboring Eu3+ ions, such as Eu3+ (5D1) + Eu3+ (7F0) f Eu3+ (5D0) + Eu3+(7F3), resulting in the very weak intensity of 5D17F emission [the intensity ratio (R) of 5D -7F to 5D -7F is 1 0 2 1 1 46]. 24a The kinetic properties for the luminescence of Eu3+ in SiO2@Y2O3:Eu3+ particles were investigated by PL decay and time-resolved emission spectra. The PL decay curves of for the luminescence of Eu3+ (5D1-7F1 and 5D0-7F2) in SiO2@Y2O3: Eu3+ particles are shown in Figure 7. These curves can be well fitted by a single-exponential function as I(t) ) I0 exp(-t/τ) (I0 is the initial emission intensity at t ) 0 and τ is the 1/e lifetime of the emission center), and the lifetimes for 5D0 (detected at 611 nm), 5D1 (detected at 535 nm) of Eu3+ were determined to be 1.637 ms (1637 µs) and 42.77 µs, respectively, basically agreeing with the reported lifetime values for Eu3+-doped Y2O3.27b, 29 The time-resolved emission spectra of Eu3+ were recorded at room temperature by exciting into the CTB (Eu3+-O2-) using

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Figure 9. The PL emission intensity of Y2O3:Eu3+@SiO2 sample as function of the annealing temperature (°C) (λex ) 250 nm). Figure 8. Time-resolved emission spectra of Eu3+ in the Y2O3: Eu3+@SiO2 sample.

a 250 nm laser with delayed time (t) ranging from 8 to 1000 µs, as shown in Figure 8. We can better understand the relaxation process from 5D1 to 5D0 for the luminescence of Eu3+ by analyzing the time-resolved emission spectra. Under the excitation of 250 nm laser, an electron is excited from the ground state to the charge transfer state (CTS, Eu3+-O2-), from which the electron rapidly relaxes to higher excited 5D states (5D1, 2, 3) first, then to lowest 5D0 excited state, and emissions are produced by radiative transitions from the excited 5D states (mainly 5D0) to the ground 7FJ (J ) 0-4) states. In Figure 8 we can see that, from t ) 8 µs (Figure 8a) to t ) 100 µs (Figure 8d), both of the emissions from 5D1 (using 5D1-7F1 emission as representative) and 5D0 (using 5D0-7F2 emission as representative) excited states can be observed clearly, but the intensity ratio (R) of 5D0-7F2 to 5D1-7F1 increases from 3.3 (Figure 8a) to 24.0 (Figure 8d). This is indicative of an electron relaxation process from the 5D1 to 5D0 state. With further increase of the delayed time (t ) 400 µs for Figure 8e and t ) 1000 µs for Figure 8f), the 5D1-7F1 emission becomes undetectable with respected to the 5D0-7F2 emission. Meanwhile the 5D0-7F2 emission becomes weak in intensity due to the depopulation of 5D state. 27c These results can be further understood considering 0 the lifetime values of 5D1 (42.77 µs) and 5D0 (1637 µs) of Eu3+ and decay curves (Figure 7a: the 5D1-7F1 emission has finished the decay process after t ) 200 µs) mentioned in last paragraph. 3.3. Tuning of PL Intensity in the Y2O3:Eu3+@SiO2 Core-Shell Phosphors. The photoluminescence intensity of Y2O3:Eu3+@SiO2 core-shell phosphors can be tuned by several experimental factors, such as annealing temperature, PEG concentration in the precursor solution, the number of coatings and SiO2 core sizes. The PL intensity of Eu3+ in Y2O3: Eu3+@SiO2 core-shell phosphors have been investigated as a function of annealing temperature, as shown in Figure 9. From Figure 9, it is found clearly that the PL intensity of Eu3+ increases with annealing temperature from 600 to 900 °C. This is due to the improvement of crystallinity and the decrease of the impurities such as organic species, as indicated previously.27 Another important factor affecting the PL intensity of the sample is PEG concentration in the precursor solutions. The PL intensity of sample as function of concentration in the precursor solutions is shown in Figure 10. First, the PL intensity of Eu3+ increases with the PEG concentration from 0.04 to 0.08 g/mL, reaching a maximum at [PEG] ) 0.08 g/mL, then decreases gradually until the PEG concentration ) 0.16 g/mL. The PEG concentra-

Figure 10. The PL emission intensity of Y2O3:Eu3+@SiO2 sample as function of the PEG concentration in the precursor solutions (λex ) 250 nm).

tion will control the viscosity of the polymer sol solution. Low viscosity of sol will result in easy deposition of a thin layer of Y2O3:Eu3+ on the SiO2 particles, proper increase of sol viscosity can increase the thickness of Y2O3:Eu3+ layer followed by the increase of PL intensity. However, when the sol viscosity reached a critical value, it will be difficult for the sol to be coated on the SiO2 particles homogeneously and more organic impurities (such as -OH, -OR, -CH2, etc.) will be introduced on the SiO2 particles. These impurities cannot be removed completely in the following annealing process and impair the PL intensity.27 It is found from our experiment that the optimum PEG concentration is 0.08 g/mL, which yields the highest PL intensity for the resulting phosphors, agreeing well with the previous results.27c The Y2O3:Eu3+@SiO2 core-shell phosphors also show different PL intensity with number of the coatings (N). The PL intensity of Eu3+ increases with the increase of the coating number, as shown in Figure 11. Obviously this can be attributed to the increase of the thickness Y2O3:Eu3+ shells on the SiO2 cores. However, the PL intensity of four layer Y2O3:Eu3+ coated SiO2 core-shell phosphors is still not as strong as that of pure Y2O3:Eu3+ powder phosphors (it can reached about 80% that the pure Y2O3:Eu3+ powder phosphors). This can be mainly attributed to the fact that the emitting volume of Y2O3:Eu3+ in the Y2O3:Eu3+@SiO2 core-shell samples is less than that of the pure Y2O3:Eu3+ powders.27c,29a It is also shown that PL intensity of the Y2O3:Eu3+@SiO2core-shell phosphors (one

Monodisperse Spherical Y2O3:Eu3+@SiO2 Particles

J. Phys. Chem. C, Vol. 111, No. 30, 2007 11229 increase with the increase of SiO2 cores, which also can result in the enhancement of the PL intensity.33 IV. Conclusion Spherical core-shell structured Y2O3:Eu3+@SiO2 particles with uniform size distribution have been successfully prepared by sol-gel method followed by annealing at high temperature. Upon UV excitation, the luminescence properties are typical those of Y2O3:Eu3+. The PL intensity of the Y2O3:Eu3+@SiO2 core-shell particles can be tuned by the annealing temperature, PEG concentration and number of coatings, and SiO2 core size. The advantages of the phosphors prepared by this process are the easy availability of homogeneous spherical morphology in different size and its wide practicality for other phosphor materials.

Figure 11. The PL emission intensity of Y2O3:Eu3+@SiO2 sample as a function of the number of coatings (N). The photoluminescence intensity of pure Y2O3:Eu3+ powder is also given for comparison (λex ) 250 nm).

Acknowledgment. This project is financially supported by the foundation of “Bairen Jihua” of Chinese Academy of Sciences, the MOST of China (2003CB314707), and the National Natural Science Foundation of China (NSFC 50572103, 20431030, 00610227). Dr. M. Yu is grateful for the special starting research fund for the Awardees of President Prize of Chinese Academy of Sciences (2005-2007). References and Notes

Figure 12. The PL emission intensity of Y2O3:Eu3+@SiO2 sample as a function of the SiO2 core size (λex ) 250 nm).

layer Y2O3:Eu3+ coated SiO2 particles) varied with the size of the SiO2 cores, as shown in Figure 12. From Figure 12 it can be seen that the PL intensity increases with the increase of SiO2 core particle size basically. These particles were annealed simultaneously in the same furnace, and it is expected that the Y2O3:Eu3+ layer has equal crystalline quality (and crystallite size) on either small or large SiO2 spheres. Quite a few papers reported the decrease of efficiency for the luminescence of Y2O3: Eu3+ nanophosphors with the reduction of particle size due to the increase of surface defects and surface dangling bonds in smaller particles.30 For submicrometer spherical Y2O3:Eu3+ particles (250-540 nm), it is also observed that the PL intensity increases with increasing crystallite size and particle size.31 Recently, Debnatha’s group investigated the luminescence properties of a series of Y2O3:Eu3+ samples with particle sizes ranging from 50 to 300 nm as a function of aging time, and found that the loss of luminescence efficiency in small sized nanophosphors is mainly due to hydration effect, which is very effective for small particles because of their larger surface area.32 For our Y2O3:Eu3+@SiO2 core-shell samples, it is obvious that the final size of the composite particles will increase with the SiO2 cores. Thus, the PL intensity of Y2O3:Eu3+@SiO2 increases with the increase of SiO2 cores, which can be explained in a similar way as stated above.30-32 Furthermore, it is believed that the amount of emitting Eu3+ ions per SiO2 particle will

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