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Photoluminescence Optimization of Luminescent Nanocomposites Fabricated by Spray Pyrolysis of a Colloid-Solution Precursor Wei-Ning Wang, a W. Widiyastuti, a I. Wuled Lenggoro, b Tae Oh Kim, c and Kikuo Okuyama a,z a Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Higashi Hiroshima 739-8527, Japan b Institute of Symbiotic Science and Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan c School of Civil and Environmental Engineering, Kumoh National University of Technology, Kyungbuk 730-710, Korea Spherical Gd 2 O 3 :Eu 3+ /silica luminescent nanocomposite particles have been synthesized by spray pyrolysis of the precursor with colloidal silica nanoparticles and gadolinium and europium nitrate solutions. The as-prepared particles were characterized using field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, photoluminescence PL spec- tra, as well as Fourier transform infrared spectroscopy. The results showed that the content of silica nanoparticles has a strong effect on the size, morphology, crystallinity, and PL properties of the composite particles. By adding colloidal silica nanoparticles into the precursors, dense luminescent particles with improved characteristics were produced because the silica nanoparticles as seeds promote heterogeneous nucleation during the pyrolysis of Gd 2 O 3 :Eu 3+ . The effect of the size of silica nanoparticles on the characteristics of the composites was investigated as well. The PL intensity and crystallinity increased with increasing silica nanoparticle size and reached optimum conditions using silica nanoparticles with an average size of 50 nm for a similar volume percentage of silica. In this case, the number and the surface area of silica nanoparticles play roles in determining the characteristics. © 2007 The Electochemical Society. DOI: 10.1149/1.2435698 All rights reserved. Manuscript submitted October 10, 2006; revised manuscript received November 30, 2006. Available electronically February 12, 2007. Luminescent particles phosphors for technological applications have been widely used in lighting and displays. A variety of oxides have good luminescent properties when doped with rare-earth ions such Eu 3+ . 1-5 However, most rare-earth oxides are very expensive. One method to reduce the cost of materials is by adding or mixing the rare-earth oxide with inexpensive materials without reducing the luminescence. Silica nanoparticles are good candidate materials for the host matrix in the rare-earth oxides, because SiO 2 is not only cheap but also transparent in the visible region and has almost no effect on the photoluminescence PL intensity. Furthermore, PL spectra can be maintained without peak shifting due to the trapping effect of oxide particles on SiO 2 nanoparticles. 6-8 The synthesis of luminescent oxide particles by silica nanopar- ticle addition has been carried out using a solid-state method 8 and wet chemical or sol-gel methods. 9-12 The solid-state method needs at least 5 h of preparation, grinding, washing, and drying. The wet chemical/sol-gel method also involves various steps and long times for preparation, such as sol-gel formation, washing, and drying. Other work has been carried out using zinc oxide of about 5 nm in size in silica-nanostructured powders by Mikrajuddin et al. using an ultrasonic spray-drying method combined with a sol-gel preparation. 6,13 All these methods are batch processes, and this may be inconve- nient for industrial applications. A one-step method is needed for industrial applications, in which particles with spherical morphology should be produced in a short operation time and at low cost. Spherical morphology can decrease the light scattering that is needed to obtain high brightness, resolution, and packing densities. 3 Using a spray pyrolysis method, spherical submicrometer particles with a monodispersed size distribution can easily be obtained, and this method has been applied to synthesize various phosphor particles. 2,3,14-17 However, by adding colloidal silica nanoparticles as an additional phase, the effect of the silica nanoparticle size, content, and the conditions of synthesis on the morphology, crystallinity, and photoluminescence characteristics of phosphor particles made by the spray pyrolysis method has not been reported extensively in previ- ous papers. In this study, we investigate the optimization and con- trollability of the properties of luminescent Gd 2 O 3 :Eu 3+ /SiO 2 com- posite particles formed by the spray pyrolysis method. Experimental Experimental procedures.— The experimental apparatus is shown schematically in Fig. 1. The apparatus mainly consists of an ultrasonic nebulizer 1.7 MHz, NE-U17, Omron Healthcare Co., Ltd., Tokyo, Japan for droplet generation, a ceramic tubular reactor with an inner diameter of 13 mm and a length of 1.3 m, and an electrostatic precipitator for collecting particles. The temperature of the precursor was maintained at 25°C using a cooling water bath BU150P, Yamato Scientific Co., Ltd., Tokyo, Japan. To keep the liquid surface at the same level, the precursor was supplied and z E-mail: [email protected] Figure 1. Schematic diagram of the experimental setup. Journal of The Electrochemical Society, 154 4 J121-J128 2007 0013-4651/2007/1544/J121/8/$20.00 © The Electochemical Society J121

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Page 1: 717 Widiyastuti Chem Eng 2007

Journal of The Electrochemical Society, 154 �4� J121-J128 �2007� J121

Photoluminescence Optimization of LuminescentNanocomposites Fabricated by Spray Pyrolysisof a Colloid-Solution PrecursorWei-Ning Wang,a W. Widiyastuti,a I. Wuled Lenggoro,b Tae Oh Kim,c andKikuo Okuyamaa,z

aDepartment of Chemical Engineering, Graduate School of Engineering, Hiroshima University,Higashi Hiroshima 739-8527, JapanbInstitute of Symbiotic Science and Technology, Tokyo University of Agriculture and Technology, Koganei,Tokyo 184-8588, JapancSchool of Civil and Environmental Engineering, Kumoh National University of Technology,Kyungbuk 730-710, Korea

Spherical Gd2O3:Eu3+/silica luminescent nanocomposite particles have been synthesized by spray pyrolysis of the precursor withcolloidal silica nanoparticles and gadolinium and europium nitrate solutions. The as-prepared particles were characterized usingfield-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, photoluminescence �PL� spec-tra, as well as Fourier transform infrared spectroscopy. The results showed that the content of silica nanoparticles has a strongeffect on the size, morphology, crystallinity, and PL properties of the composite particles. By adding colloidal silica nanoparticlesinto the precursors, dense luminescent particles with improved characteristics were produced because the silica nanoparticles asseeds promote heterogeneous nucleation during the pyrolysis of Gd2O3:Eu3+. The effect of the size of silica nanoparticles on thecharacteristics of the composites was investigated as well. The PL intensity and crystallinity increased with increasing silicananoparticle size and reached optimum conditions using silica nanoparticles with an average size of 50 nm for a similar volumepercentage of silica. In this case, the number and the surface area of silica nanoparticles play roles in determining thecharacteristics.© 2007 The Electochemical Society. �DOI: 10.1149/1.2435698� All rights reserved.

Manuscript submitted October 10, 2006; revised manuscript received November 30, 2006.Available electronically February 12, 2007.

0013-4651/2007/154�4�/J121/8/$20.00 © The Electochemical Society

Luminescent particles �phosphors� for technological applicationshave been widely used in lighting and displays. A variety of oxideshave good luminescent properties when doped with rare-earth ionssuch Eu3+.1-5 However, most rare-earth oxides are very expensive.One method to reduce the cost of materials is by adding or mixingthe rare-earth oxide with inexpensive materials without reducing theluminescence. Silica nanoparticles are good candidate materials forthe host matrix in the rare-earth oxides, because SiO2 is not onlycheap but also transparent in the visible region and has almost noeffect on the photoluminescence �PL� intensity. Furthermore, PLspectra can be maintained without peak shifting due to the trappingeffect of oxide particles on SiO2 nanoparticles.6-8

The synthesis of luminescent oxide particles by silica nanopar-ticle addition has been carried out using a solid-state method8 andwet chemical or sol-gel methods.9-12 The solid-state method needs atleast 5 h of preparation, grinding, washing, and drying. The wetchemical/sol-gel method also involves various steps and long timesfor preparation, such as sol-gel formation, washing, and drying.Other work has been carried out using zinc oxide of about 5 nm insize in silica-nanostructured powders by Mikrajuddin et al.using an ultrasonic spray-drying method combined with a sol-gelpreparation.6,13

All these methods are batch processes, and this may be inconve-nient for industrial applications. A one-step method is needed forindustrial applications, in which particles with spherical morphologyshould be produced in a short operation time and at low cost.Spherical morphology can decrease the light scattering that isneeded to obtain high brightness, resolution, and packing densities.3

Using a spray pyrolysis method, spherical submicrometer particleswith a monodispersed size distribution can easily be obtained, andthis method has been applied to synthesize various phosphorparticles.2,3,14-17 However, by adding colloidal silica nanoparticles asan additional phase, the effect of the silica nanoparticle size, content,and the conditions of synthesis on the morphology, crystallinity, andphotoluminescence characteristics of phosphor particles made by thespray pyrolysis method has not been reported extensively in previ-

z E-mail: [email protected]

ous papers. In this study, we investigate the optimization and con-trollability of the properties of luminescent Gd2O3:Eu3+/SiO2 com-posite particles formed by the spray pyrolysis method.

Experimental

Experimental procedures.— The experimental apparatus isshown schematically in Fig. 1. The apparatus mainly consists of anultrasonic nebulizer �1.7 MHz, NE-U17, Omron Healthcare Co.,Ltd., Tokyo, Japan� for droplet generation, a ceramic tubular reactorwith an inner diameter of 13 mm and a length of 1.3 m, and anelectrostatic precipitator for collecting particles. The temperature ofthe precursor was maintained at 25°C using a cooling water bath�BU150P, Yamato Scientific Co., Ltd., Tokyo, Japan�. To keep theliquid surface at the same level, the precursor was supplied and

Figure 1. Schematic diagram of the experimental setup.

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cycled using a peristaltic pump �PST-350, Iwaki Asahi Techno GlassCo., Tokyo, Japan�. The reactor wall temperature was kept at1000°C. Nitrogen was selected as the carrier gas with a constantflow rate of 2 L/min.

Precursor preparation.— Gadolinium and europium nitratehexahydrates �purity of 99.95%, supplied by Kanto Chemical Co.,Inc., Tokyo, Japan� were selected as precursors and were dissolvedin ultrapure water. Europium with a doping concentration of 10atom % of Gd�NO3�3 was selected; this is the optimum dopingconcentration observed by Kang et al. for Gd2O3:Eu3+ synthesis.2

Colloidal silica nanoparticles with average sizes of 5 nm �Snowtec-XS�, 15 nm �Snowtec-20�, 50 nm �Snowtec-XL�, 85 nm �Snowtec-ZL�, and 300 nm �MP-3040� were used to investigate the effect ofsilica nanoparticle size and concentration on the characteristics ofGd2O3,Eu3+/silica composite particles.

Characterization and analysis.— The droplet size distribution ofprecursors was measured using laser diffraction technology �Spray-tec, Malvern Instruments, Ltd., Worcestershire, UK�. One typicalexample of droplet size and size distribution is shown in Fig. 2 for a0.1 M Gd�NO3�3 + Eu�NO3�3 aqueous solution with a carrier gasflow rate of 2 L/min. The volume mean diameter of 4.38 �m isused for all the subsequent calculations, assuming that the dropletsize does not change significantly when the precursor concentrationsare increased slightly. The real experimental results of droplet sizemeasurements showed that the difference between the droplet sizesof precursors with 0.01 M and 1.0 M was below 5.0%. It may bebecause there is no big difference in physicochemical properties ofprecursors with various concentrations.

The crystal characteristics of product particles were analyzed byan X-ray diffractometer �XRD, RINT 2200V, Rigaku-Denki Corp.,Tokyo, Japan� using Cu K� radiation �wavelength 1.54 � operatedat 40 kV and 30 mA. The crystal size was calculated using theScherrer equation from the full width at half maximum �fwhm� ofthe principal peak �222� in the corresponding XRD pattern. Themorphology was checked using a field-emission scanning electronmicroscope �FESEM, S-5000, Hitachi Corp., Tokyo, Japan� oper-ated at 20 kV. For FESEM analysis, the sample was first suspendedin ethanol by ultrasonication for approximately 10 min. A drop ofthe suspension was then deposited onto the silicon wafer, dried, andsputter-coated with Au in vacuum for 30 s �E-1010, Hitachi High-Technologies Co., Tokyo, Japan�. The coating rate of the ion sputteris 10 nm/min, and the coated Au layer on the particle surface isapproximately 5 nm in thickness, which has no noticeable effects onthe submicrometer particle size measurement from FESEM images.The geometric mean diameter �Feret diameter, Dp� and geometricstandard deviation �GSD, �� were determined by randomly sam-pling about 200 particles from FESEM photographs. Assuming that

Figure 2. A typical example of droplet size and size distribution of aqueoussolutions of gadolinium and europium nitrates containing colloidal silicananoparticles generated by an ultrasonic nebulizer.

all particles are ideal spheroids, the geometric diameter can be trans-formed into the volume mean diameter, Dv, using the number fre-quency distribution equation, Dv = ����NDp

3�/N�1/3, where N is thenumber of particles. Transmission electron microscopy �TEM, JEM-3000F, JEOL, Tokyo, Japan� operated at 300 kV was used to char-acterize the inner structure of the composite particles. The PL spec-tra were measured using a luminescence spectrometer �RF-5300PC,Shimadzu Corp., Kyoto, Japan� with a xenon lamp at an excitationwavelength of 254 nm. During the measurement, phosphor particlesweighing about 0.5 g were inserted into a cylindrical aluminumholder with a diameter of 4.5 mm and a height of 1.5 mm. Theoptical spectrum was measured at a resolution of 0.2 nm at roomtemperature. The chemical properties of the resulting powder sur-face were analyzed using a Fourier transform infrared �FTIR� spec-trophotometer �IRPrestige-21, Shimadzu Corp., Kyoto, Japan�. TheFTIR measurements were carried out at room temperature with aresolution of 2 cm−1 using the conventional KBr pellet technique.

Results and Discussion

Evaluation of one droplet to one particle mechanism.— Onedroplet to one particle �ODOP�, a typical particle formation mecha-nism in conventional spray pyrolysis derived from mass balance,was first evaluated using precursors without silica nanoparticle ad-dition. Assuming that the as-prepared particles are solid with spheri-cal morphology, Eq. 1 can be used to predict the final particle sizefrom the measured droplet size

Dv,p = Dv,d�CM

��1/3

�1�

The terms Dv,p, Dv,d, C, M, and � are the volume mean diameter ofparticles ��m�, the volume diameter of droplets ��m�, the precursorconcentration �mol/L�, the product particles �Gd2O3:Eu3+� molecu-lar weight �g/mol�, and the density of the product particles �g/L�,respectively. Using Eq. 1 the particle diameter can be controlled byvarying the concentrations of the solution.

To investigate the effect of precursor concentration onGd2O3:Eu3+ synthesis without silica nanoparticle addition, the tem-perature and the carrier gas flow rate were maintained constant at1000°C and 2 L/min, respectively. The precursor concentrationswere varied from 0.005 to 1.0 M. Figure 3 shows a comparison ofthe particle size between the predicted data using Eq. 1 and themeasured values from FESEM images. Some examples of FESEMimages and the corresponding measured particle size distributionsare shown in Fig. 4. From Fig. 3, the particle size has a linearrelationship with the precursor concentration on the logarithmicform, i.e., the particle size increases with increasing precursorconcentration. It follows Eq. 1 with its logarithmic form:

Figure 3. Comparison of particle diameters between the theoretical valuesand the measured data from the corresponding FESEM images �for pureGd2O3:Eu3+�.

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logDv,p = 1/3 logC + log�Dv,d�M /��1/3�, where the slope is 1/3 andthe intercept is log�Dv,dM /�1/3�. It should be noted that the interceptis constant, since Dv,d, M, and � have constant values for a certainproduct material. However, for solution concentrations greater than0.1 M, the particle sizes are slightly larger than those predicted bytheoretical calculations because of the formation of hollow particles,as shown in the FESEM image in Fig. 4c. Hollow particles aregenerated because the solute diffusion rate within a droplet is fasterthan the vapor evaporation rate. Therefore, the concentration in thecenter of the evaporating droplet is less than the equilibrium satura-tion concentration when the crust is formed on the droplet surface;these are the “hollow particle generation criteria”.15 For concentra-tions below 0.1 M, dense particles can be formed, because the solutediffusion into the center of a droplet can reach the equilibrium satu-ration concentration of the solute before the droplet surface reachesthe critical supersaturation. Similar experimental and numerical re-sults were obtained by Lenggoro et al.18 Larger particles were ob-tained because the solute concentration in the droplet surface ex-ceeded the critical supersaturation before the inner dropletconcentration reached equilibrium saturation. The shrinkage of thedroplet stops owing to crust formation on the droplet surface. Theevaporation still occurs through the crust, which leads to the forma-tion of crust particles with high porosity.19

Using similar assumptions with the ODOP mechanism in Eq. 1that is for single-component particles, the calculation was applied topredict the size of Gd2O3:Eu3+/SiO2 composite particles usingEq. 2, which was modified from Ohshima et al.20,21

Dv,p = Dv,d�MSiO2CSiO2

�SiO2

+MGd2O3:EuCGd2O3:Eu

�Gd2O3:Eu�1/3

�2�

where MSiO2, MGd2O3:Eu, CSiO2

, CGd2O3:Eu, �SiO2and �Gd2O3:Eu are the

molecular weights of silica and europium-doped gadolinium oxide�g/mol�, the silica nanoparticle concentration �g/L�, the precursorconcentration �g/L�, the silica density �g/L�, and the europium-dopedgadolinium oxide density �g/L�, respectively.

The effect of the amount of silica colloids, with an average sizeof 50 nm, added to the precursor on the Gd2O3:Eu3+/SiO2 compos-ite size is shown in Fig. 5 on the basis of theoretical calculation andFESEM analysis. The Gd2O3:Eu3+ precursor concentrations selectedwere 0.1 and 0.025 M, whereas the concentration of colloidal silicananoparticles in the precursor was varied from 0.005 to 1.0 M. Thetheoretical calculation is in good agreement with the FESEM analy-sis, which suggests that the composite particles prepared by silicananoparticle addition are all dense particles. The morphologies of

some Gd2O3:Eu3+/SiO2 samples and their particle distributions areshown in Fig. 6. Spherical morphology of composite particles wasachieved in all cases.

From these results, we suggest that dense particles can be pro-duced by adding colloidal silica nanoparticles into solution precur-sors. Similar results were found by Roh et al.22 They reported thatparticles prepared from an aqueous solution were hollow and po-rous. Nevertheless, dense spherical particles were produced fromcolloidal precursors. In this case, the heat transfer inside the dropletwas improved because the added colloidal nanoparticles acted asconductors within the gadolinium nitrate aqueous solution. The so-lution precipitates and crystallizes amid colloidal silica nanopar-ticles during subsequent steps. Another reason for dense particleformation is that the silica nanoparticles function as seeds forGd2O3:Eu3+ particle formation by a heterogeneous nucleationmechanism. Without addition of colloidal particles into the precur-sors, the crystallites grow rapidly on the surface of a droplet, whichtends to generate particles porous on the surface but hollow inside,as has been explained previously. The morphology and the forma-tion mechanisms of composite particles as well as pure Gd2O3:Eu3+

particles are schematically shown in Fig. 7.

Effect of SiO2 colloidal nanoparticle content.— To investigatethe effect of SiO2 content, the composite particle size was main-tained constant at 500 nm. This size was selected because the pre-

Figure 4. FESEM images and the corre-sponding size and size distribution ofGd2O3:Eu3+ generated from various pre-cursor concentrations: �a� 0.005, �b� 0.30,and �c� 1.0 M.

Figure 5. Comparison of particle diameters between the theoretical valuesand the measured data from the corresponding FESEM images �forGd O :Eu3+/SiO �.

2 3 2
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ferred spherical phosphor particle size to achieve 100% screen lighttransmission coverage is usually less than 1 �m.23 From the massbalance and calculation using Eq. 2, the relative number of moles ofGd and Eu atoms in one droplet decreases when the SiO2 contentincreases if the product particle size remains constant. The silicacontent in Gd2O3:Eu3+ was varied from 0 to 75 vol %, which isequivalent to a molar ratio of SiO2/Gd2O3:Eu3+ ranging from 0 to5.4. Other parameters, i.e., furnace temperature, flow rate of carriergas, and the size of silica nanoparticles, were also kept constant at1000°C, 2 L/min, and 50 nm, respectively.

The morphology of composite particles at different volume per-centages of colloidal silica nanoparticles is shown in Fig. 8. Forparticles without silica nanoparticle addition �zero volume ratio�, theparticles generated were porous and larger than the dense particlespredicted from perfect droplet shrinkage using Eq. 1, as shown inFig. 8a. The reason was explained in the previous section. In thecases of 5–50 vol % silica nanoparticle addition, the nucleatedGd2O3:Eu3+ embedded well in silica particles to form a smoothsurface of composite particles, and the morphology was similar forall cases, as shown in Fig. 8b and c. In the case of 75 vol % silicananoparticle addition, the number of silica nanoparticles �primarycolloidal silica� increased significantly. The silica nanoparticles pre-dominated the composite particles and resulted in a rough surface,

as shown in Fig. 8d. This occurs because the number of primarysilica particles in a droplet increased significantly and insufficientGd2O3:Eu3+ particles occupied the empty space between silicananoparticles when evaporation occurred.

Figure 9 shows the corresponding XRD patterns ofGd2O3:Eu3+/SiO2 composite particles with various silica volumepercentages. The results show that the XRD patterns are in accor-dance with the cubic Gd2O3 reference �JCPDS card 12-797� withoutSiO2 interference. However, in the case of 50 vol % silica addition,a trace of Gd2SiO5 �JCPDS 40-287� was found in the XRD patternsdue to the reaction between SiO2 and Gd�NO3�3. An amorphousphase was observed in the case of 75 vol % silica added, whichsuggests that the space in the composite particles was mostly occu-pied by silica nanoparticles.

The cubic structure of Gd2O3 contains two crystallographicallydifferent cation sites with C2 and S6 symmetry, respectively, whichis a structure similar to that of Y2O3.23 The crystalline size �Dc� ofGd2O3 calculated using the Scherrer equation at 2� = 28.82° �thestrongest scattering peak �222� of cubic crystalline Gd2O3� in theXRD patterns shows that the largest crystallite size is obtained in thecase of 5 vol % of silica nanoparticles addition, followed by10 vol % silica nanoparticle addition. In the case of no silica addi-

Figure 6. FESEM images and the corre-sponding size and size distribution ofGd2O3, Eu3+/SiO2 generated from 0.1 MGd�NO3�3 and Eu�NO3�3 mixed solutionswith various SiO2 concentrations: �a�0.002, �b� 0.01, and �c� 0.1 M.

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tion, the crystallite size is lower compared with those obtained from5 and 10 vol % silica addition. Therefore, the concentration of silicananoparticles in a droplet plays an important role in Gd2O3:Eu3+

crystalline growth. Silica addition promotes heterogeneous nucle-ation and crystallite growth subsequently up to at a certain value.However, increasing the silica content to more than the optimumvalue tends to suppress crystallite growth because not enough spaceis available for crystal growth, especially for the case of 75 vol %silica nanoparticles addition.

The PL properties of composite particles with various silica vol-ume percentages are shown in Fig. 10. The excitation wavelengthwas selected as 254 nm, which is the characteristic of a Gd2O3:Eu3+

particle. From Fig. 10, the characteristic luminescence peak of Eu3+

at approximately 612 nm was observed in all cases due to the red5D0 → 7F2 transition at C2 sites within europium.24,25 However,another peak at approximately 594 nm due to the 5D0 → 7F1 tran-sition of europium was also prominent in SiO2:Eu3+ particles underexcitation at 394 nm.26,27 Therefore, PL analysis of theGd2O3:Eu3+/SiO2 composites was also carried out under the sameexcitation wavelength �394 nm� to check the possibility of the ex-istence of SiO2:Eu3+. Only one prominent peak at approximately612 nm � 5D0 → 7F2� was detected, but no peak at approximately

594 nm � 5D0 → 7F1� was detected, indicating that Eu3+ ions in thecomposites do not exist in the SiO2 site but rather in the Gd2O3 site.Note that the characteristic red peak at 612 nm for Eu3+ is stableunder different excitation wavelengths in the UV region.28 The rela-tive PL intensity at 612 nm of the composites was found to have thesame tendency as that of crystallite size calculated from the corre-sponding XRD patterns. The maximum PL intensity was achieved inthe case of 5 vol % silica addition, followed by 10 vol %, and15 vol %. All these PL intensities are higher than that in the case ofpure Gd2O3:Eu3+. The PL intensities of the composites tend to de-crease when added silica nanoparticles exceed 15 vol %. In the caseof 75 vol % silica addition, very weak PL intensity was found. Thetendency of PL intensity correlates well with that of crystallite sizesof the particles as shown by XRD data �Fig. 9�, suggesting thatcrystallite size has a strong effect on the PL intensity because itkeeps the phosphor particles similar in size at around 500 nm �seethe insert in Fig. 10�.

FTIR analysis was carried out to check the surface chemistry ofthe Gd2O3:Eu3+/SiO2 composites and pure Gd2O3:Eu3+. The resultsare shown in Fig. 11. From Fig. 11, in the case of pure Gd2O3:Eu3+,a broad peak at approximately 3400 cm−1 was observed because ofthe stretching of OH groups on the particle surface, which was also

Figure 7. Schematic diagram of particleformation mechanisms from different pre-cursors in the conventional spray process.

Figure 8. FESEM images ofGd2O3:Eu3+/SiO2 particles with differentSiO2/Gd2O3:Eu3+ volume percentages: �a�0, �b� 5, �c� 50, and �d� 75 vol % with anaverage colloidal silica nanoparticle sizeof 50 nm.

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observed by Söderlind et al. in the case of Gd2O3 nanocrystals pre-pared by combustion.29 The OH group in the Gd2O3:Eu3+/SiO2composites was also found in FTIR spectra, an observation whichwas confirmed by previous research.8 The characteristic peak at ap-proximately 550 cm−1 was due to the absorption of Gd�Eu�–Obonds in the case of pure Gd2O3:Eu3+, in good agreement with theprevious research.4,11 This characteristic peak intensity began to de-crease with increasing volume percentages of silica. In the case of75 vol % silica addition, no Gd�Eu�–O peak was found in the FTIRspectra because the surface was completely covered by silica nano-particles, which was also confirmed by the FESEM images shown inFig. 8d. A principal peak at approximately 1100 cm−1 was clearlyidentified in the FTIR spectra in this case and was assigned to asym-metric stretching vibration modes of the Si–O–Si bridge of the si-loxane link.9 The peak intensity decreased with decreasing volume

Figure 9. XRD patterns of Gd2O3:Eu3+/SiO2 particles with differentSiO2/Gd2O3:Eu3+ volume percentages: 0, 5, 10, 25, 50, and 75 vol % withan average colloidal silica nanoparticle size of 50 nm.

Figure 10. PL spectra of Gd2O3:Eu3+/SiO2 particles with differentSiO2/Gd2O3:Eu3+ volume percentages: 0, 5, 10, 15, 25, 50, and 75 vol %with an average colloidal silica particle size of 50 nm. Inset: Relationshipbetween the SiO2 volume percentages and the relative PL intensity at 612 nmas well as the Scherrer size.

of added silica. These FTIR results reveal that the surface chemistryof the Gd2O3:Eu3+/SiO2 composites is of great importance for PLintensity. They confirm again that the PL intensity of the compositeis high when Eu3+ is in Gd2O3 sites, i.e., in the cases in which theGd�Eu�–O peak was observed instead of the Si–O–Si peak.

Effect of SiO2 colloidal nanoparticle size.— To investigate theeffect of the size of silica nanoparticles on the morphology, crystal-linity, and PL properties of composite particles, five sizes of SiO2colloids were selected, 5, 15, 50, 85, and 300 nm. The wall tempera-ture along the furnace was maintained constant at 1000°C, and thenitrogen flow rate was 2 L/min. Other parameters maintained con-stant were 10 atom% Eu doped in Gd2O3, 5 vol % SiO2 nanopar-ticle content, and 500 nm generated particle size calculated usingEq. 2. The morphology of as-prepared particles can be seen in Fig.12, and dense particles approximately 500 nm in size can be pro-duced independent of the size of the silica nanoparticles. Hollowparticles and porous surfaces do not appear by adding silica to theparticles generated. It has been described previously that particle

Figure 11. FTIR spectra of Gd2O3:Eu3+/SiO2 particles with differentSiO2/Gd2O3:Eu3+ volume percentages: �a� 0, �b� 3, �c� 5, �d� 25, and �e�75 vol % with an average colloidal silica nanoparticle size of 50 nm.

Figure 12. FESEM images of Gd2O3:Eu3+/SiO2 particles with different SiO2sizes: �a� 5, �b� 15, �c� 50, and �d� 300 nm with 5 vol % SiO2. nanoparticlecontent.

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formation follows the heterogeneous nucleation mechanism. Hetero-geneous nucleation occurs on the surface of a silica nanoparticle asa foreign particle, in contrast with homogenous nucleation, whichtakes place in the volume of the solution.30 At low supersaturation,heterogeneous nucleation on the foreign particle has a strong inter-action and is a good structural match with the nucleating phase.31

Figure 13 shows a TEM image of the inner structure of the compos-ite particles with 15 vol % silica addition with an average colloidalsilica particle size of 5 nm. In Fig. 13, dense particles are observedwithin which the silica nanoparticles are well dispersed, which con-firms our prediction and other results from previous research.

The crystallinity of the composites increases as the size of silicananoparticles increases up to the optimum at 50 nm and decreaseswhen the size is larger than 50 nm, as shown by XRD analysis inFig. 14. The measurement of PL intensity correlated well with the

Figure 13. TEM image of a Gd2O3:Eu3+/SiO2 particle containing 15 vol %SiO2 nanoparticles of 5 nm in diameter.

Figure 14. XRD patterns of Gd2O3:Eu3+/SiO2 particles with different SiO2sizes: 5, 15, 50, 85, and 300 nm with 5 vol % SiO nanoparticle content.

2

XRD results. The highest PL intensity was obtained using silicananoparticles of 50 nm. This was followed by sizes of 85, 300, 15,and 5 nm silica nanoparticles, as shown in Fig. 15. The correlationbetween crystallite size and PL intensity is shown in the inset in Fig.15. The number and the surface area effects of added colloidal silicananoparticles are considered the main reasons for this phenomenon.The number of SiO2 nanoparticles in one droplet increases as theSiO2 size decreases. In contrast, the surface area decreases as theSiO2 size increases. In this case, the volume of silica nanoparticlesas a matrix in this study is similar, because the volume percentage ofsilica for each size was maintained constant at 5 vol %. Increasingthe surface area promotes heterogeneous nucleation. However,smaller silica nanoparticles can move easier than larger ones. Thesesmaller nanoparticles undergo a random Brownian motion due tocollisions with small and fast-moving solvent molecules.32 This mo-tion can disrupt the heterogeneous nucleation process. Larger colloidparticles, as well as reducing the surface area, can scatter a lightbeam passing through them; this is called the Tyndall effect.32 Thiseffect can decrease the PL intensity of composite particles. Furtherstudy is still required regarding the interaction of colloidal particlesin solvent systems. From this experiment, 50 nm silica nanoparticlesare the optimum size to obtain the optimum number of particles andsurface area.

Conclusions

Gd2O3:Eu3+/silica composite particles have been synthesized byultrasonic spray pyrolysis. The effects of colloidal silica nanopar-ticle content and size on the morphology, crystallinity, and PL inten-sity of the synthesized composite particles have been investigatedsystematically. The results show that the morphology of as-preparedparticles tends to become dense in the case of colloidal silica nano-particle addition, whereas mainly hollow and porous particles wereobtained without added colloidal silica nanoparticles. The crystallin-ity and PL intensity of Gd2O3:Eu3+/silica composites dependedstrongly on silica nanoparticle content and size. The optimum char-acteristics were achieved in the case of 5 vol % silica nanoparticleaddition and an average silica particle size of 50 nm. The PL andcrystallinity characteristics of Gd2O3:Eu3+/silica composites de-creased when the silica nanoparticle content was larger than10 vol % and the average size was larger than 50 nm. The number,volume, and surface area effects of added silica nanoparticles on theheterogeneous nucleation of the composite particles are consideredpossible reasons for this behavior.

Figure 15. PL spectra of Gd2O3:Eu3+/SiO2 particles with different SiO2sizes: 5, 15, 50, 85, and 300 nm with 5 vol % SiO2 nanoparticle content.Insert: Relationship between the SiO2 particle size and the relative PL inten-sity at 612 nm as well as the Scherrer size.

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Acknowledgments

The authors want to thank Yoko Taguchi for assistance in thespray experiments and Takashi Ogi for the TEM analysis. ProfessorT. Iizawa is acknowledged for providing the FTIR experimentalsetup. We thank Nissan Chemical Industries, Ltd., Tokyo, Japan, forproviding SiO2 colloids. The Japan Society for the Promotion ofScience �JSPS� and the Ministry of Education, Culture, Sports, Sci-ence and Technology �MEXT� of Japan are acknowledged for pro-viding a postdoctoral fellowship �W.N.W.� and a doctoral scholar-ship �W.W.�, respectively. Grants-in-Aid sponsored by MEXT andJSPS are acknowledged �K.O., I.W.L.�. This work was also sup-ported in part by NEDO’s “Nanotechnology Particle Project” basedon funds provided by the Ministry of Economy, Trade, and Industry�METI�, Japan.

Hiroshima University assisted in meeting the publication costs of thisarticle.

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