Preparation of Y2SiO5: Ln3+ (Ln= Eu, Tb, Sm) and Gd9. 33 (SiO4

Preparation of Y2SiO5:Ln3+ (Ln = Eu, Tb, Sm) and Gd9.33(SiO4)6O2:Ln3+ (Ln = Eu, Tb) Phosphor Fine Particles Using an Emulsion Liquid Membrane System...
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J. Phys. Chem. C 2007, 111, 168-174

Preparation of Y2SiO5:Ln3+ (Ln ) Eu, Tb, Sm) and Gd9.33(SiO4)6O2:Ln3+ (Ln ) Eu, Tb) Phosphor Fine Particles Using an Emulsion Liquid Membrane System Takayuki Hirai* and Yuki Kondo Research Center for Solar Energy Chemistry, and DiVision of Chemical Engineering, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka, Osaka 560-8531, Japan ReceiVed: July 22, 2006; In Final Form: September 29, 2006

Y2SiO5:Eu3+ and Gd9.33(SiO4)6O2:Eu3+ red phosphor particles were prepared using an emulsion liquid membrane (ELM, water-in-oil-in-water (W/O/W) emulsion) system, which could be utilized as a microreactor for precipitation reaction of rare earth oxalate and hydrolysis-condensation reaction of Si alkoxide, to obtain size- and morphology-controlled precursor particles. Y3+ (or Gd3+) and Eu3+ ions were extracted from the external water phase of the ELM system by extractant (cation carrier, 2-methyl-2-ethylheptanoic acid) and were stripped into the internal water phase, consisting of oxalic acid and tetramethyl orthosilicate (TMOS) as the Si source, to make composite Y-Eu-Si (or Gd-Eu-Si) oxalate particles. The precursor particles were 20-60 nm in size, containing small amounts of flat particles of 300 nm in size. By calcination of the precursor oxalate particles obtained in the ELM system, submicrometer-sized X1-phase Y2SiO5:Eu3+ (or Gd9.33(SiO4)6O2: Eu3+) particles were produced, which were smaller than those prepared by the conventional sol-gel method. The resulting phosphor particles demonstrated a photoluminescence around 600 nm (λex) 254 nm). Photoluminescence properties of Tb3+- or Sm3+-doped Y2SiO5 and Gd9.33(SiO4)6O2:Tb3+ particles were also investigated, and the characteristic photoluminescence corresponding to doped rare earth ions was observed.

Introduction Recently, new types of displays, such as plasma display panel (PDP), field emission display (FED), and electroluminescence (EL) panel, have attracted considerable attention. To realize those displays with high resolution, high brightness, and long operating time, researchers have extensively investigated various inorganic luminescence materials, such as YPO4:Tb3+,1 YBO3: Eu3+,2 and BaMgAl10O17:Eu2+.3 Y2SiO5 with chemical and thermal stability is known as an excellent host material of cathode luminescent phosphor, and many studies have been reported on Y2SiO5:Tb3+ 4 and Y2SiO5:Ce3+.5 In recent years, Y2SiO5:Eu3+ was found to be a promising candidate for coherent time-domain optical memory (CTDOM) applications for highdensity and high-speed data storage and has received much attention.6 Y2SiO5 is able to form two different monoclinic crystal structures arising from calcination temperature.7 The lowtemperature phase with the space group P21/c is called the X1 phase, whereas the high-temperature phase with the space group B2/b is called the X2 phase. The transformation from the X1 phase to the X2 phase appeared above 1473 K, and therefore calcination below that temperature is necessary to obtain X1Y2SiO5.8 The solid-state reaction, which is a convenient procedure for Y2SiO5 preparation, needs high-temperature calcination to achieve good crystallization of products; therefore, the X1 phase cannot be obtained. To our best knowledge, the available synthetic methods of X1-Y2SiO5 phosphor are limited, and also studies on luminescence property of that material are few.9,10 An emulsion liquid membrane (ELM, water-in-oil-in-water (W/O/W) emulsion) system has been studied for the separation of metals. In this technique, metal ions are extracted from the * To whom correspondence should be addressed. Telephone: +81-66850-6270. Fax: +81-6-6850-6273. E-mail: [email protected].

external water phase into the membrane phase, and then extracted and concentrated into the internal water phase. It has been found that the internal water phase can be utilized for the preparation of size- and morphology-controlled fine particles, because of a restricted reaction area of the micrometer-sized internal water droplet. The particles, such as submicrometersized spherical rare earth oxalate particles11 and calcium carbonate particles,12 have been prepared due to the reactions of oxalic acid or sodium carbonate fed into the internal water phase with the metal ions transported from the external water phase. This technique can also be applied to the preparation of composite oxalate particles by feeding several kinds of metal ions into the external aqueous phase. For example, oxide particles such as SrPbO3 and Sr2PbO4,13 Co and Ni ferrites,14 Y2O3:Eu3+,15 Y2O3:Yb3+, Er3+,16 and Sr2CeO4 and rare earthdoped Sr2CeO417 have then been obtained by calcination of the composite oxalate particles. In this work, we prepared X1-Y2SiO5:Eu3+ fine particles using an ELM system. Y-Eu-Si composite oxalate particles were prepared in the internal water phase, via the precipitation reactions of Y3+ and Eu3+ with oxalic acid and the hydrolysiscondensation reaction of tetramethyl orthosilicate (TMOS). The resulting composite Y-Eu-Si oxalate particles were calcined to obtain X1-Y2SiO5:Eu3+, and the size, morphology, and luminescence properties of the resulting particles were investigated. Gd9.33(SiO4)6O2:Eu3+ phosphor fine particles were also prepared via a similar method. In addition, we examined the luminescence properties of Tb3+- or Sm3+-doped Y2SiO5 and Gd9.33(SiO4)6O2:Tb3+ phosphors. Experimental Section 2-Methyl-2-ethylheptanoic acid (VA-10, supplied by Shell Chemical Co.) was used as the extractant. Sorbitan sesquioleate

10.1021/jp064655j CCC: $37.00 © 2007 American Chemical Society Published on Web 11/24/2006

Y2SiO5:Ln3+ and Gd9.33(SiO4)6O2:Ln3+ Phosphor

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Figure 1. SEM images for composite Y-Eu-Si oxalate particles prepared at {Eu/(Y+Eu)}f ) 0.05 (a) in the ELM system at (Si/Y)f ) 1.25 and (b) by the sol-gel method (dried gel).

Figure 2. X-ray diffraction patterns for composite Y-Eu-Si oxide particles prepared at {Eu/(Y+Eu)}f ) 0.05 in the ELM systems at (a) (Si/Y)f ) 1.0, (b) (Si/Y)f ) 1.25, and (c) (Si/Y)f ) 1.5, and (d) by the sol-gel method. Calcination condition: (a-c) 1273 K for 3 h; (d) 1323 K for 2 h.

Figure 3. Effect of calcination temperature on X-ray diffraction patterns for Y2SiO5:Eu3+ particles obtained by calcination of composite oxalate particles prepared in the ELM system at (Si/Y)f ) 1.25 and {Eu/(Eu+Y)}f ) 0.05.

(Span 83) used as the surfactant and tetramethyl orthosilicate (TMOS) were supplied by Tokyo Kasei Kogyo Co., Ltd. Yttrium chloride (YCl3), europium chloride (EuCl3), gadolinium chloride (GdCl3), sodium acetate, and oxalic acid were supplied by Wako Pure Chemical Industries, Ltd. To prepare the composite Y-Eu-Si oxalate and Gd-EuSi oxalate particles in the ELM system, the previously reported procedure13-17 was modified. For the internal phase, oxalic acid aqueous solution (0.3 mol/L) and the appropriate quantity of TMOS were mixed and heated at 323 K for 30 min with stirring. The quantity of TMOS fed into the internal water phase was described as the molar ratio of TMOS to the rare earth ions (Y3++Eu3+ or Gd3++Eu3+) fed into the external water phase, as (Si/Y)f or (Si/Gd)f. The internal water phase and the organic membrane phase (kerosene containing 0.5 mol/L of VA-10 and 10 wt % Span 83) were mixed at a volume ratio of 1:1 and emulsified by means of mechanical homogenizer (12 000 rpm). The resulting W/O emulsion (10 mL) was then added to the external water phase (50 mL) containing YCl3 (or GdCl3), EuCl3, and 0.02 mol/L sodium acetate, and stirred vigorously with a magnetic stirrer, to form the W/O/W emulsion. The total rare earth concentration of the feed external water phase was always fixed at 4 mmol/L, and the molar ratio of feed concentrations in the solution, {Eu/(Y+Eu)}f or {Eu/(Gd+Eu)}f,

was varied. After being stirred for 30 min, the W/O emulsion was separated from the external aqueous solution and then stirred gently at 333 K for 90 min, to enhance the condensation reaction. The W/O emulsion was then demulsified by adding 50 mL of acetone. The particles formed in the water droplets were separated by centrifuge, washed with acetone twice, and dried at room temperature. The composite oxalate particles were finally calcined in air at the required temperature for the required time, to obtain X1-Y2SiO5:Eu3+ or Gd9.33(SiO4)6O2:Eu3+ particles. X1-Y2SiO5:Eu3+ particles were also prepared by the sol-gel method,18 for comparison purposes. Y2O3 and Eu2O3 were dissolved in HNO3 solution and mixed with ethanol containing tetraethyl orthosilicate (TEOS). Dilute NH4OH was added to the solution to adjust pH, and the solution was stirred until a gel appeared. The resulting gel was dried, calcined at 1323 K for 3 h, and ground to obtain X1-Y2SiO5:Eu3+ particles. Gd9.33(SiO4)6O2:Eu3+ particles were prepared via a similar sol-gel method but calcined at 1423 K for 3 h. Both the composite oxalate and the composite oxide particles were characterized by scanning electron microscopy (SEM, Hitachi S-5000 or S-2250N) and powder X-ray diffraction (XRD, Philips PW-3050). To determine the metal concentrations in each phase of the ELM system, the separated W/O emulsion

170 J. Phys. Chem. C, Vol. 111, No. 1, 2007

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Figure 4. SEM images for Y2SiO5:Eu3+ particles prepared at {Eu/(Y+Eu)}f ) 0.05 (a) using the ELM system at (Si/Y)f ) 1.25 and (b) by the sol-gel method. Calcination condition: 1323 K for 3 h.

Figure 5. (a) Excitation and (b) emission spectra for Y2SiO5:Eu3+ particles prepared at {Eu/(Y+Eu)}f ) 0.05 (A) using the ELM system at (Si/Y)f ) 1.25 and (B) by the sol-gel method. Calcination condition: 1323 K for 3 h.

Figure 7. Emission spectra for (a) Y2SiO5:Tb3+ and (b) Y2SiO5:Sm3+ prepared using the ELM system at {Ln/(Y+Ln)}f ) 0.05 (Ln ) Tb, Sm). (a) (Si/Y)f ) 1.25 and calcined at 1473 K for 2 h; (b) (Si/Y)f ) 1.1 and calcined at 1423 K for 3 h.

Figure 8. SEM image for composite Gd-Eu-Si oxalate particles prepared in the ELM system at (Si/Gd)f ) 1.4 and {Eu/(Gd+Eu)}f ) 0.05.

Figure 6. Effect of {Eu/(Y+Eu)}f on the emission intensity of Y2SiO5:Eu3+ particles prepared using the ELM system at (Si/Y)f ) 1.25 and by the sol-gel method. λex ) 254 nm.

Results and Discussion

was demulsified electrically and the organic membrane phase was then stripped with 1 mol/L of HCl. The metal concentrations both in the external aqueous phase and in the resulting stripping solutions were determined via an inductively coupled argon plasma atomic emission spectrometer (ICP-AES, Nippon JarrellAsh ICAP-575 Mark II). The metal concentration of the internal water phase was calculated by mass balance. The photoluminescence spectra of the particles, packed in the powder cell with a quartz window, were measured by a spectrofluorometer (Hitachi F-4500, Xe lamp).

Preparation of Y-Eu-Si Composite Oxalate Particles Prepared in the ELM System. Figure 1 shows SEM images of Y-Eu-Si composite oxalate particles prepared in the ELM system and dried gel prepared by the sol-gel method, at {Eu/ (Y+Eu)}f ) 0.05. Fine particles of 20-60 nm in diameter were obtained in the ELM system at (Si/Y)f ) 1.25, together with small amounts of flat particles about 300 nm in size (Figure 1a). On the other hand, the dried gel prepared by the sol-gel method was centimeter-sized blocks, as shown in Figure 1b. Because the resulting gel had a highly hygroscopic property, conversion of the bulky gel into powder by grinding under air

Y2SiO5:Ln3+ and Gd9.33(SiO4)6O2:Ln3+ Phosphor

Figure 9. X-ray diffraction patterns for composite Gd-Eu-Si oxide particles prepared at {Eu/(Gd+Eu)}f ) 0.05 using the ELM systems at (a) (Si/Gd)f ) 0.95, (b) (Si/Gd)f ) 1.0, and (c) (Si/Gd)f ) 1.4, and (d) by the sol-gel method. Calcination condition: 1423 K for 3 h.

was fairly difficult. The ELM system is therefore effective in preparing fine particles. The time-course variations for the mole fractions of the metal ions in the external water phase, organic membrane phase, and internal water phase of the ELM system were examined (Supporting Information, Figure S1). After 30 min of stirring, the organic membrane phase contained few amounts of Y3+ and Eu3+ for all of the ELM systems, indicating that the metal ions extracted from the external water phase were effectively transported to the internal water phase, although complete transportation of the metal ions into the internal water phase was not achieved. The time-course variations of the ELM systems at (Si/Y)f ) 0-1.2 show similar curves for both Y3+ and Eu3+, and the external water phase maintained about 30% of Y3+ and 15% of Eu3+ after 30 min of stirring, whereas at (Si/Y)f ) 2.5, about 45% of Y3+ and 35% of Eu3+ remained in the external water phase, and the transport rate into the internal water phase decreased. These results indicate that TMOS fed into the internal water phase had no serious effects on the transport of Y3+ and Eu3+ from the external water phase to the internal water phase when (Si/Y)f ) 1.2 and below. Eu3+ is preferentially transported into the internal water phase as compared to Y3+, which is a similar tendency in the system without TMOS as described in the previous paper.15 Calcination of Composite Y-Eu-Si Oxalate Particles. Figure 2 shows XRD patterns for the composite Y-Eu-Si oxide

J. Phys. Chem. C, Vol. 111, No. 1, 2007 171 particles obtained by calcination of the composite oxalate particles prepared in the ELM system at (Si/Y)f ) 1.0, 1.25, and 1.5, together with that of composite oxide particles prepared by the sol-gel method. The precursors prepared in the ELM and by the sol-gel method were calcined at 1273 K for 2 h and at 1323 K for 3 h, respectively. The XRD patterns revealed that the main crystal structure of the oxide particles obtained is controlled by (Si/Y)f, such as (a) Y2O3 at (Si/Y)f ) 1.0, (b) X1-Y2SiO5 at (Si/Y)f ) 1.25, and (c) Y4.67(SiO4)3O at (Si/Y)f ) 1.5. It is thus possible to prepare various compositions of composite Y-Si oxide particles in the ELM system by controlling the quantity of TMOS fed in the internal water phase. X1-Y2SiO5 is successfully prepared at (Si/Y)f ) 1.25, although a minor phase of Y2O3 is slightly incorporated. The oxide particles prepared by the sol-gel method show a well-defined XRD pattern attributable to X1-Y2SiO5 (Figure 2d). To obtain a more pure X1-Y2SiO5 phase, a detailed investigation was carried out on the calcination condition of the precursor particles. Figure 4 shows the XRD patterns for Y2SiO5:Eu3+ particles prepared using the ELM system at (Si/Y)f ) 1.25 and calcined at various temperatures. After calcination at 1173 K for 4 h, the resulting oxide particles were converted to X1Y2SiO5 with low peak intensity. The peak intensity is improved with rising calcination temperature, but is still weak after calcination at 1273 K for 3 h. Calcination at 1323 K for 3 h gives the XRD pattern of greater peak intensity and attributable to X1-Y2SiO5, with slight peaks attributable to the Y2O3 phase, although the peak for the minor phase was smaller than that for X1-Y2SiO5 obtained at 1273 K. Calcination at 1423 K for 2 h brings about an increase of the Y2O3 phase, which may be accompanied by the slight transformation from the X1 phase to the X2 phase. Figure 4 shows the SEM images for the X1-Y2SiO5:Eu3+ particles prepared at {Eu/(Y+Eu)}f ) 0.05 using the ELM system and by the sol-gel method and calcined at 1323 K for 3 h. Although the particles prepared using the ELM system were slightly sintered, most particles maintained submicrometer size smaller than 200 nm (Figure 4a). The particle size is thus small enough as compared to the particles prepared by the sol-gel method, which show irregular shape and are larger than 10 µm in size (Figure 4b). Photoluminescence Properties of X1-Y2SiO5:Eu3+ Particles. Figure 5 shows the excitation and emission spectra for X1-Y2SiO5:Eu3+ particles prepared at {Eu/(Y+Eu)}f ) 0.05 using the ELM system and by the sol-gel method and calcined at 1323 K for 3 h. The excitation spectra are composed of a strong broadband with a maximum at 252 nm and some sharp lines in the wavelength region longer than 350 nm (Figure 5a).

Figure 10. SEM images for Gd9.33(SiO4)6O2:Eu3+ particles prepared at {Eu/(Gd+Eu)}f ) 0.05 (a) using the ELM system at (Si/Gd)f ) 1.4 and (b) by the sol-gel method. Calcination condition: 1423 K for 3 h.

172 J. Phys. Chem. C, Vol. 111, No. 1, 2007 The former is attributable to the Eu3+-O2- charge-transfer band (CTB), and the latter is assigned to the intraconfigurational 4f4f transitions of Eu3+, which contain 7F0,1 f 5L6 transitions arising from the electric dipole transitions and 7F0,1 f 5D2 transitions arising from the magnetic dipole transitions.19 For the emission spectra excited by UV light (λex ) 254 nm), many sharp peaks appear in the longer wavelength region corresponding to red photoluminescence, which are attributed to the transitions between the excited 5D0 level and 7FJ (J ) 0, 1, 2) level of the Eu3+ (Figure 5b).19 A weak peak distinctly appears at 578 nm, arising from the strictly forbidden 5D0 f 7F0 transition. The parity-allowed 5D0 f 7F1 transition around 590 nm is caused by the magnetic dipole transition, including the most strong emission peak at 587 nm. The 5D0 f 7F2 transition, resulting from the electric dipole transition, appears around 620 nm, and a dominant peak appears at 614 nm. The peak positions of those emission spectra are exactly the same irrespective of synthetic methods, whereas the peak intensity of Y2SiO5:Eu3+ prepared using the ELM system shows about 60% of that prepared by the sol-gel method. The greater emission intensity of the particles prepared by the sol-gel method is assumed to be due to the larger particle size (>10 µm).15,16 It has been supposed that surface defects such as oxygen defects suppress emission intensity, and this is enhanced by the decrease of particle size and increase of the specific surface area. It is well known that the emission intensity is affected by the doping concentration of activator ion and most phosphors have the optimal doping concentration. For X1-Y2SiO5:Eu3+ prepared using the ELM system and by the sol-gel method, intensity variations of the emission peaks observed at 587 and 614 nm were investigated as a function of {Eu/(Y+Eu)}f. When {Eu/(Y+Eu)}f is varied, no difference in the spectral shape was observed for X1-Y2SiO5:Eu3+ irrespective of the preparation method. As shown in Figure 6, the emission intensity of Y2SiO5:Eu3+ prepared by the sol-gel method increases monotonously with the increase of {Eu/(Y+Eu)}f. In contrast, Y2SiO5:Eu3+ prepared using the ELM system shows maximum emission intensity at {Eu/(Y+Eu)}f ) 0.10 (ca. 85% of that prepared by the sol-gel method), and then the intensity decreases remarkably at {Eu/(Y+Eu)}f ) 0.15. Although it is difficult to determine the accurate particle composition, this tendency is fairly different from that previously reported for the Y2SiO5:Eu3+ particles prepared via the sol-gel technique.20 Slightly contained Y2O3 phase as a minor phase (Figure 3) may possibly affect the emission property of the Y2SiO5:Eu3+ particles prepared using the ELM system. When the smaller size of the particles is taken into consideration, the emission intensity of 85% of that from the Y2SiO5:Eu3+ particles prepared via the sol-gel technique should be reasonable. Photoluminescence Properties of X1-Y2SiO5:Tb3+ and X1-Y2SiO5:Sm3+ Particles. Figure 7 shows the emission spectra of X1-Y2SiO5:Tb3+ and X1-Y2SiO5:Sm3+, prepared using the ELM system at {Ln/(Y+Ln)}f ) 0.05 (Ln ) Tb, Sm). The spectrum of Y2SiO5:Tb3+ (λex ) 254 nm) presents the characteristic green luminescence of Tb3+, which is composed of the sharp strong peak at 543 nm and the weak peaks at 485 and 592 nm (Figure 7a). All of those peaks arise from the transitions between the excited 5D4 level and 7FJ (J ) 6, 5, 4) level of the Tb3+.18 In particular, the 5D4 f 7F5 transition at 543 nm, which is the predominant green color emitting transition, is attributed to the magnetic dipole transition.21,22 As shown in Figure 7b, the spectrum of Y2SiO5:Sm3+ (λex ) 400 nm) reveals two sharp peaks at 565 and 601 nm, in accord with 4G5/2 f 6HJ (J ) 5/2, 7/ ) transitions of Sm3+.23 2

Hirai and Kondo

Figure 11. (a) Excitation and (b) emission spectra for Gd9.33(SiO4)6O2: Eu3+ particles prepared using the ELM system at (Si/Gd)f ) 1.4 and {Eu/(Gd+Eu)}f ) 0.05.

Figure 12. Effect of {Eu/(Gd+Eu)}f on the emission intensity of Gd9.33(SiO4)6O2:Eu3+ particles prepared using the ELM system at (Si/Gd)f ) 1.4. λex ) 254 nm.

Preparation of Gd9.33(SiO4)6O2:Eu3+ Particles Using the ELM System. Gd-Eu-Si composite oxalate particles of 2060 nm in size were prepared successfully in the ELM system at (Si/Gd)f ) 1.4, as shown in Figure 8. In contrast, the dried gel prepared by the sol-gel method was a centimeter-sized block. Conversion of the bulky gel into powder by grinding under the air was fairly difficult, because the dried gel had a highly hygroscopic property, as in the case of Y-Eu-Si composite oxalate. The ELM system is therefore also effective in preparing Gd-Eu-Si composite oxalate fine particles. The time-course variations for the mole fractions of the metal ions in the external water phase, organic membrane phase, and internal water phase of the ELM system (Figure S2) show that 3-11% of the rare earth ions remain in the organic membrane phase under the presence of TMOS with (Si/Gd)f ) 1.4, and the transportation of the ions into the internal water phase is thus decreased to ca. 70% after 30 min of stirring. Figure 9 shows XRD patterns for the composite Gd-Eu-Si oxide particles obtained by calcination of the composite oxalate particles prepared in the ELM system at various (Si/Gd)f, together with that of the particles prepared by the sol-gel method. All of the precursor oxalate particles were calcined at 1423 K for 3 h. The crystal structure of the oxide particles prepared using the ELM system varies with (Si/Gd)f, such as (a) Gd2O3 with Gd2SiO5 at (Si/Gd)f ) 0.95, (b) Gd9.33(SiO4)6O2 with a small amount of Gd2SiO5 at (Si/Gd)f ) 1.0, and finally (c) Gd9.33(SiO4)6O2 without any impurity phase at (Si/Gd)f ) 1.4. It is thus possible to prepare the composite Gd-Si oxide

Y2SiO5:Ln3+ and Gd9.33(SiO4)6O2:Ln3+ Phosphor

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Figure 13. (a) Excitation and (b) emission spectra for Gd9.33(SiO4)6O2:Tb3+ particles prepared using the ELM system at (Si/Gd)f ) 1.4 and {Tb/ (Gd+Tb)}f ) 0.05.

particles of various compositions by using the ELM system by controlling (Si/Gd)f. In contrast, the oxide particles prepared by the sol-gel method showed a characteristic XRD pattern of poor crystallinity corresponding to Gd9.33(SiO4)6O2, as shown in Figure 9d. Even when the calcination time was elongated to 24 h, the crystallinity was not improved. Calcination at higher temperature, such as 1773 K, will probably be required to obtain pure Gd9.33(SiO4)6O2 in this case.24 Figure 10 shows the SEM images for the Gd9.33(SiO4)6O2: Eu3+ particles prepared at {Eu/(Gd+Eu)}f ) 0.05 in the ELM system and by the sol-gel method, calcined at 1423 K for 3 h. Although the particles prepared by using the ELM system are slightly sintered, most particles maintained their size smaller than 600 nm (Figure 10a), whereas the oxide particles prepared by the sol-gel method, obtained via grinding of the bulk oxide, show irregular shape and are larger than 5 µm in size (Figure 10b). Photoluminescence Properties of Gd9.33(SiO4)6O2:Eu3+ and Gd9.33(SiO4)6O2:Tb3+ Particles. Figure 11 shows the excitation and emission spectra for Gd9.33(SiO4)6O2:Eu3+ prepared at {Eu/(Gd+Eu)}f ) 0.05 by using the ELM system and calcined at 1423 K for 3 h. The excitation spectrum (λem ) 614 nm) is composed of a strong band with a maximum at 275 nm attributable to the O2--Eu3+ CTB, and some sharp peaks at longer wavelength region attributable to the intraconfigurational 4f-4f transitions of Eu3+.19 The emission spectrum (λex ) 254 nm) exhibits sharp peaks corresponding to red photoluminescence, arising from the transitions between the excited 5D0 level and 7FJ (J ) 0, 1, 2) level of Eu3+ (Figure 11b),19,21 as in the case of Y2SiO5:Eu3+ (Figure 5b). The strong electric dipole transitions around 620 nm (maximum at 614 nm, 5D0 f 7F2) may be caused by the low local symmetry (C3 and/or Cs) for Eu3+ in the Gd9.33(SiO4)6O2 host lattice.25,26 Figure 12 shows the relationships between emission intensities of Gd9.33(SiO4)6O2:Eu3+ at 587 and 614 nm and {Eu/(Gd+Eu)}f. No change in the emission spectral shape for Gd9.33(SiO4)6O2: Eu3+ was observed by changing the {Eu/(Gd+Eu)}f value. The intensities are seen to increase from a value of {Eu/(Gd+Eu)}f ) 0.05 and to attain maximum at {Eu/(Gd+Eu)}f ) 0.15, and then decrease at {Eu/(Gd+Eu)}f ) 0.20.

Figure 14. Effect of {Tb/(Gd+Tb)}f on the emission intensity of Gd9.33(SiO4)6O2:Tb3+ particles prepared using the ELM system at (Si/Gd)f ) 1.4. λex ) 254 nm.

Figure 13 shows the excitation and emission spectra for Gd9.33(SiO4)6O2:Tb3+ prepared using the ELM system at {Tb/(Gd+Tb)}f ) 0.05 and calcined at 1423 K for 3 h. The excitation spectrum contains two strong bands at shorter wavelength region than 270 nm and two weak peaks around 310 nm (Figure 6a). The band with a maximum at 248 nm is attributable to the spin-allowed transition between the 4f8 and 4f75d electron configuration of Tb3+.25 The sharp peaks are attributable to the intraconfigurational 4f-4f transitions of Tb3+.22,25 By the excitation with UV light, Gd9.33(SiO4)6O2:Tb3+ demonstrates the strong green luminescence, arising from the transitions between the excited 5D4 level and 7FJ (J ) 4, 5, 6) level of Tb3+ (Figure 13b), as in the case of Y2SiO5:Tb3+ (Figure 7a). Figure 14 shows the relationship between {Tb/(Gd+Tb)}f and intensities of emission peaks at 542 and 549 nm corresponding to 5D4 f 7F5 transitions. The emission spectral shape was not influenced by the {Tb/(Gd+Tb)}f value. The intensities are seen to increase from a value of {Tb/(Gd+Tb)}f ) 0.05 and to attain maximum at {Tb/(Gd+Tb)}f ) 0.20, and then decrease rapidly at {Tb/(Gd+Tb)}f ) 0.25.

174 J. Phys. Chem. C, Vol. 111, No. 1, 2007 References and Notes

Conclusion X1-Y2SiO5:Eu3+

Hirai and Kondo

Gd9.33(SiO4)6O2:Eu3+

and phosphor particles were prepared by using an emulsion liquid membrane (ELM) system. The Y-Eu-Si and Gd-Eu-Si composite oxalate particles were obtained, containing fine particles of 20-60 nm in size and submicrometer-sized flat particles. By calcination of the precursor composite oxalate particles, prepared at appropriate (Si/Y)f and (Si/Gd)f values, submicrometer-sized Y2SiO5:Eu3+ and Gd9.33(SiO4)6O2:Eu3+ particles were obtained, which were much smaller than those synthesized by the conventional sol-gel method. The phosphor particles thus obtained showed red photoluminescence corresponding to 5D0 f 7FJ (J ) 0, 1, 2) transitions of Eu3+ and showed the maximum emission intensity at the conditions of {Eu/(Y+Eu)}f ) 0.10 in the feed external aqueous solution for Y2SiO5:Eu3+ and {Eu/(Gd+Eu)}f ) 0.15 for Gd9.33(SiO4)6O2:Eu3+. Y2SiO5:Tb3+, Y2SiO5:Sm3+, and Gd9.33(SiO4)6O2:Tb3+ phosphors exhibited the characteristic photoluminescence according to doped lanthanoid ions, and, in particular, Y2SiO5:Tb3+ and Gd9.33(SiO4)6O2:Tb3+ exhibited a strong green luminescence. The present study thus describes that the combination of the precipitation reaction of rare earth oxalate and hydrolysiscondensation reaction of Si alkoxide in the ELM system is a promising candidate as a synthetic method of size- and morphology-controlled composite Ln-Si oxide particles. Acknowledgment. We are grateful to Mr. Masao Kawashima of the “Gas Hydrate Analyzing System (GHAS)”, Osaka University, for his experimental assistance in the characterization of the particles with SEM, and to the Division of Chemical Engineering for the Lend-Lease Laboratory Systems. We are also grateful to the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT), for financial support through the Grant-in-Aid for Scientific Research on Priority Areas (417) “Fundamental Science and Technology of Photofunctional Interfaces” (Nos. 15033244 and 17029037), and to the New Energy and Industrial Technology Development Organization (NEDO) for financial support through the “Nanotechnology Materials Program - Nanotechnology Particle Project” based on funds provided by the Ministry of Economy, Trade, and Industry, Japan (METI). Supporting Information Available: Time-course variations for mole fractions of the metal ions. This material is available free of charge via the Internet at http://pubs.acs.org.

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