[7] K. Besocke, S. Berger, Rev. Sci. Instrum. 1976, 47, 840.[8] S. Bastide, R. Butruille, D. Cahen, A. Dutta, J. Libman, A. Shanzer, L.

Sun, A. Vilan, J. Phys. Chem. B 1997, 101, 2678.[9] R. Cohen, S. Bastide, D. Cahen, J. Libman, A. Shanzer, Y. Rosenwaks,

Adv. Mater. 1997, 9, 746.[10] D. Gal, E. Sone, R. Cohen, G. Hodes, J. Libman, A. Shanzer, H.-W.

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6195.[13] U. Bach, D. Lupo, P. Compte, J.-E. Moser, F. Weissörtel, J. Salbeck, H.

Spreitzer, M. Grätzel, Nature 1998, 395, 583.[14] L. Kavan, M. Grätzel, Electrochim. Acta 1995, 40, 643.[15] F. Weissörtel, Diplomarbeit, Frankfurt University 1996.

Preparation of Y2O3:Eu Phosphor Particlesof Filled Morphology at High PrecursorConcentrations by Spray Pyrolysis

By Yun Chan Kang, Hyun Sook Roh,and Seung Bin Park*

For good luminescent characteristics, phosphor materialsmust have fine size, narrow size distribution, non-aggrega-tion, and spherical morphology.[1±4] Recently, various tech-niques were applied to control the morphology of oxidephosphor particles. Spray pyrolysis is known to be one ofthe feasible processes for the preparation of phosphor par-ticles of improved morphology[5±7] because prepared in thisway they exhibit all the features for good luminescence.However, the fatal problem of the spray pyrolysis is the for-mation of hollow particles when the concentration of theprecursor salt is increased in order to increase productivity.The hollowness causes reduction in brightness and long-term stability of phosphor particles.

Eu-doped Y2O3 phosphor is a well-known red phosphorthat is used in displays such as plasma display panels(PDP), field emission displays (FED), and cathode-raytubes (CRT). Recently, spray pyrolysis using an ultrasonicresonator was applied to the preparation of Y2O3:Eu phos-phor particles,[8] and showed that the Y2O3:Eu particleswere spherical in shape and showed non-aggregation char-acteristics. However, the particles had a hollow and porousstructure. The high precipitation rate of salt at the surfaceof a droplet caused the hollow morphology of particles andeven if the stock solution concentration is only 0.2 M theparticles were still hollow and porous. As the solution con-centration increases above 0.2 M, the particles becomemore hollow and porous than those prepared from a lowconcentration solution.

In this work, morphology control of Eu-doped Y2O3 par-ticles prepared from high concentration solution (1 M) wasattempted to produce solid phosphor particles by colloidal

seed-assisted spray pyrolysis. Yttrium (Y) and gadolinium(Gd) hydroxy carbonate sols, obtained by the aqueous so-lution method, were used as nucleation seeds of Y andeuropium nitrate salts in the spray pyrolysis. The particlesprepared from a 1 M aqueous solution exhibited a hollowand porous structure before and after post-treatment. Onthe other hand, the particles prepared from colloidal solu-tions that contain a small amount of Y hydroxy carbonatesol or Gd hydroxy carbonate sol as seeds showed sphericaland filled morphology before and after post-treatment.

The colloidal solution used as the nucleation seed wasprepared by the liquid-phase reaction method using urea,as proposed by Matijevic and Hsu,[9] and Sordelet andAkinc.[10] Spherical and mono-sized Gd and Y hydroxy car-bonate sols were obtained by homogeneous precipitationin aqueous solutions by reaction with the thermal decom-position products of urea. For the Gd hydroxy carbonatesol solution, the concentration of Gd nitrate was fixed at0.005 M. The mean sizes of Gd hydroxy carbonate sol were70 and 250 nm when the concentration of urea was 2 and0.25 M, respectively. The Y hydroxy carbonate sol was alsoprepared by the same method as the Gd compound sol,however in the former case the concentration of urea wasfixed at 0.27 M. The mean size of Y hydroxy carbonate solparticles was 200 nm when the concentrations of Y nitratewere 0.005 (low conc. Y sol) and 0.025 M (high conc. Ysol). The difference in solution concentration of Y nitratechanged the sol number concentration of solutions. The solthus prepared was stable and phase separation did not oc-cur during the spray process.

The apparatus used in this work was an ultrasonic spraygenerator with a 1.7 MHz resonator. Spray solutions wereobtained by adding Y and Eu nitrate precursors into the as-prepared colloidal solution. The overall solution concentra-tion was 1.0 M. The doping concentration of Eu was fixedat 6 at.-% of Y concentration. The Y2O3:Eu particles weredirectly prepared from colloidal and aqueous solutions byspray pyrolysis at 1300 �C. The flow rate of air used as car-rier gas was 2 L/min. The residence time in the tubular re-actor was 3 s. The particles directly prepared by spray py-rolysis were post-treated at 1300 �C for 1 h for furthercrystallization and activation of particles.

In commercial processes, a high solution concentration isrequired for a high production rate of phosphor particles.The high solution concentration is also recommended forproduction of micrometer-sized phosphor particles. Forthese reasons, all the particles were prepared at higher so-lution concentration (>1 M) than those of the previouswork.[8]

Figure 1 shows the SEM photograph of Y2O3:Eu parti-cles prepared from the conventional aqueous solution afterpost-treatment. The as-prepared particles at 1300 �C werepost-treated at 1300 �C for 1 h. The as-prepared particlesexhibited a spherical shape, smooth surface, and hollowstructure and the high evaporation rate of the dropletcaused the hollowness of the particles. The hollowness of

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[*] Prof. S. B. Park, Dr. Y. C. Kang, H. S. RohDepartment of Chemical EngineeringKorea Advanced Institute of Science and Technology373-1, Kusong-dong, Yusong-gu, Taejon, 305-701 (Korea)

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as-prepared particles was also obtained at low reactor tem-peratures (900 �C). After post-treatment, all the particlesbecame hollow and porous. The crystallization process ofparticles caused the formation of a porous shell with arough surface. To overcome this problem in the spray py-rolysis, colloidal seed-assisted spray pyrolysis was intro-duced for the preparation of phosphor particles of sphericaland filled structure even after post-treatment at high tem-peratures.

The Gd component used as the nucleation seed in thepreparation of Y2O3:Eu phosphor particles, did not serve asimpurity because Eu-doped Y2O3 phosphor was frequentlyused as the complex form of (YGd)2O3:Eu for high bright-ness in displays. Figure 2 shows the SEM photograph of par-ticles prepared from Gd hydroxy carbonate sol solution. Theas-prepared particles at 1300 �C were post-treated at1300 �C for 1 h. The particles had more filled and non-por-ous structure than those (Fig. 1a) prepared from the conven-tional aqueous solution. The fine size of particles preparedfrom the colloidal solution compared to those of the aque-

ous solution also reveal that the particles prepared fromcolloidal solution are much less hollow. Additionally, theparticles prepared from the Gd hydroxy carbonate sol solu-tion maintained sphericity and non-aggregation characteris-tics of as-prepared particles even after post-treatment. Incolloidal seed-assisted spray pyrolysis, the salts precipitateon the surface of the nanometer size sol particles dispersedin droplets by heterogeneous precipitation. Therefore theparticles prepared from colloidal solutions have more filledand non-porous structure than those of aqueous solution be-cause precipitation of salts occurs uniformly inside the drop-let in the case of colloidal seed-assisted spray pyrolysis.

The size of the sols used as nucleation seeds affected themorphology of the particles prepared by spray pyrolysis.The particles prepared from a colloidal solution of 70 nmGd hydroxy carbonate sol showed hollow and porous mor-phology. On the other hand, the particles prepared from250 nm Gd hydroxy carbonate sol exhibited a filled andnon-porous structure even after post-treatment at 1300 �C.The particles prepared from Y hydroxy carbonate sol solu-tion also had a high thermal stability at post-treatment con-ditions. The Y2O3:Eu particles prepared from Y hydroxycarbonate sol solution had more filled and less porousstructure than those prepared from the conventional aque-ous solution. The sol concentration of Y hydroxy carbonatedid not influence the morphology of prepared particles.

The effect of types of solution on the crystallinity and lu-minescence properties of phosphor particles was also inves-tigated. Generally, the phosphor particles of higher crystal-linity had improved brightness. The XRD spectra of theY2O3:Eu particles prepared from aqueous and colloidal so-lutions by spray pyrolysis were compared. The particlesprepared at 1300 �C by spray pyrolysis were post-treated at1300 �C for 1 h. The Y2O3:Eu particles prepared fromcolloidal solutions of Y and Gd hydroxy carbonate sol ex-hibited a higher degree of crystallinity than those preparedfrom aqueous solution. The crystallite sizes of particles pre-pared from aqueous and colloidal solutions were 50 and61 nm, respectively.

Figure 3 shows the photoluminescence (PL) characteris-tics of Y2O3:Eu phosphor particles prepared from aqueous

Fig. 1. SEM of Y2O3:Eu particles prepared from the aqueous solution afterpost-treatment.

Fig. 2. SEM of particles prepared from 250 nm Gd colloidal solution afterpost-treatment.

550 600 650 700

0

10000

20000

30000

40000250nm Gd sol

70nm Gd sol

High Conc. Y sol

Low Conc. Y sol

No sol

Wavelength [nm]

PL

Inte

nsity

Fig. 3. PL spectra of Y2O3:Eu phosphor particles prepared from aqueousand colloidal solutions.

and colloidal solutions. All of the samples prepared fromcolloidal solutions exhibit higher brightness than those pre-pared from the conventional aqueous solution. The parti-cles prepared from a high concentration of Y hydroxy car-bonate sol had the highest PL intensity, which was 115 % ofthat of the particles prepared from the conventional aque-ous solution. However, the measurement of PL intensitiesof particles was carried out on the bulk powder. In order tosee the difference in luminescence of particles of hollowand filled structure, measurements should be carried outon the film form of phosphor particles at the real operatingconditions of displays.

In the case of spray pyrolysis, particles prepared fromhigh solution concentrations were had a more hollow andporous structure than those prepared from low-concentra-tion solution. The colloidal seed-assisted spray pyrolysis in-troduced in this work was found to be applicable to thecontrol of morphology of phosphor particles when thestock solution concentration was high. For the colloidalseed-assisted spray pyrolysis, the stable colloidal solutionshould be used for homogeneity of phase and morphologyof the phosphor particles. The colloidal solution of Y and

Gd hydroxy carbonate sol obtained by the liquid phase re-action method using urea was appropriate for the prepara-tion of Y2O3:Eu particles of filled and non-porous structureat high concentration of the precursor solution.

Received: August 23, 1999Final version: January 6, 2000

±[1] R. E. Siever, P. D. Milewski, C. Y. Xu, B. A.Watkins, Extended Ab-

stracts of the Third International Conference on the Science and Tech-nology of Display Phosphors, Hungington Beach, CA 1997, p. 303.

[2] R. P. Rao, J. Electrochem. Soc. 1996, 143, 189.[3] C. Xu, B. A. Watkins, R. E. Sievers, X. Jing, P. Trowga, C. S. Gibbons,

A. Vechi, Appl. Phys. Lett. 1997, 71, 1643.[4] L. E. Shea, J. McKittrick, O. A. Lopez, J. Am. Ceram. Soc. 1996,

79(12), 3257.[5] Y. C. Kang, S. B. Park, I. W. Lenggoro, K. Okuyama, J. Phys. Chem.

Solids 1999, 60(3), 379.[6] Y. C. Kang, S. B. Park, I. W. Lenggoro, K. Okuyama, J. Electrochem.

Soc, 1999, 146(3), 1227.[7] Y. C. Kang, S. B. Park, I. W. Lenggoro, K. Okuyama, Jpn. J. Appl.

Phys. 1999, 38(4A), 2013.[8] Y. C. Kang, S. B. Park, I. W. Lenggoro, K. Okuyama, J. Mater. Res.

1999, 14(6), 2611.[9] E. Matijevic, W. P. Hsu, J. Colloid Interface Sci. 1987, 118(2), 506.

[10] D. Sordelet, M. Akinc, J. Colloid Interface Sci. 1988, 122(1), 47.

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