68 Copyright © 2006 The Society of Chemical Engineers, Japan

Journal of Chemical Engineering of Japan, Vol. 39, No. 1, pp. 68–76, 2006 Research Paper

Preparation of Submicron- and Nanometer-Sized Particles ofY2O3:Eu3+ by Flame Spray Pyrolysis Using Ultrasonic and Two-FluidAtomizers

Agus PURWANTO1, I. Wuled LENGGORO1,Hankwon CHANG2 and Kikuo OKUYAMA1

1Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 4-1, Kagamiyama 1, Higashi-Hiroshima-shi, Hiroshima 739-8527, Japan2Nano-Materials Group, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 305-350, Korea

Keywords: Flame Spray Pyrolysis , Ultrasonic Nebulizer, Two-Fluid Nozzle, Yttrium Oxide, Europium

Submicron- and nanometer-sized red luminescent particles (Y2O3:Eu3+) were successfully generatedby flame spray pyrolysis (FSP) and the effect of the type of the atomizer, i.e., the ultrasonic nebulizer(UN) and the two-fluid nozzle (TFN) sprayer, on the particle characteristics was investigated. The pre-pared particles were characterized by field emission-scanning electron microscopy (FE-SEM), an X-raydiffractometry (XRD) analysis and spectrophotometry. UN-FSP produced non aggregated particles witha mean diameter of 754 nm and the relative photoluminescence (PL) intensity (ratio of emission intensityof generated particles to that of commercial particles) varied from 0.36 to 1.01. The Y2O3:Eu3+ particlesgenerated by TFN-FSP were softly agglomerated with a mean diameter of 24 nm and the relative PLintensity ranged from 0.21 to 0.24. Regardless of the type of the atomizer used, the generated particleswere dense and spherical in the cubic phase and highly crystalline. These results show that TFN-FSP is aversatile method of producing Y2O3:Eu3+ nanometer-sized particles compared with UN-FSP.

Introduction

Flame reactors are attractive for synthesizing va-riety materials because of their low production cost andhigh production rate. Commercially, flame reactors areapplied to produce carbon black, fumed silica and pig-mentary titania (Stark and Pratsinis, 2002). In the flamereactor, particles reside in the flame zone for a shorttime and, as a consequence, a high production rate isachieved (Mueller et al., 2004). Flame spray pyrolysis(FSP) is a flame method that utilizes a liquid as a pre-cursor (Kammler et al., 2001). By utilization of a liq-uid precursor, mixed metal oxide particles can be gen-erated easily. The use of a liquid precursor is also drivenby the low cost of metal salts (such as nitrates and ac-etates) and the high availability and also high solubil-ity of metal salts in water or organic solvent.

To atomize the liquid precursor, an atomizer isrequired in the flame reactor system. Two types of at-omizers, the ultrasonic nebulizer (UN) and two-fluidnozzle (TFN) sprayer, are commonly employed in FSP.It seems that these two different types of atomizers gen-erate particles with different particle diameters. TFN-

Received on May 23, 2005. Correspondence concerning thisarticle should be addressed to K. Okuyama1 (E-mail address:okuyama@hiroshima-u.ac.jp).

FSP produces particles mostly on the nanometer or-der. This can be found in the synthesis of materialssuch as ZnO (Tani et al., 2002), SiO

2 (Mueller et al.,

2003), ZrO2 (Mueller et al., 2004) and SnO

2 (Sahm et

al., 2004). By UN-FSP, the produced particles aremostly on the submicron order such as in the prepara-tion of Y

2O

3:Eu3+ (Kang et al., 2001; Chang et al., 2004)

and SrTiO3:Pr,Al (Kang et al., 2002). In spite of this,

nanometer-sized particles have been produced by UN-FSP in LiCoO

2 synthesis (Jang et al., 2004). Unfortu-

nately, there is as yet no systematic investigation toevaluate the effect of the type of the atomizer on theparticle characteristics in the synthesis of the same ma-terial.

In this paper, the effect of the type of the atomizeron phosphor particle preparation by FSP is presented.The red luminescent material Y

2O

3:Eu3+ was chosen as

a model for the synthesized material. This is becauseY

2O

3:Eu3+ is a phosphorous material applied widely in

display devices as well as lighting devices. Displaydevices such as flat-panel displays (FPDs), thin-filmelectroluminescence (TFEL) panels and field-emissiondisplay (FEDs) require materials with controlled char-acteristics (Sohn et al., 2002). A small size and a nar-row size distribution is required to attain a high reso-lution and a high efficiency for display devices.

69

1. Experimental

1.1 Experimental setupTo investigate the effect of atomizer type on the

characteristics of as-prepared particles by FSP, experi-mental setups were arranged as described elsewhere(Chang et al., 2004) for UN-FSP and as shown in Fig-ure 1(a) for TFN-FSP. The gas flow arrangement isshown in Figure 1(b). Figure 1(c) shows a schematicdiagram and the dimension of a two-fluid nozzle setup.The nozzle was inserted in the center of the burner tospray droplets into the flame zone. The complete proc-ess condition is shown in Table 1.1.2 Precursor preparation

The precursor solution was prepared by dissolv-ing the precursor chemicals in ultra-pure water. Thechemicals are yttrium nitrate (Y(NO

3)

3·6H

2O, 99.99%,

Kanto Chemical Co., Inc.) and europium nitrate(Eu(NO

3)

3·6H

2O, 99.95%, Kanto Chemical Co., Inc.).

For all the experiments, the molar concentration ofthe precursor was 0.3 M and the mol fraction ofEu(NO

3)

3·6 H

2O was 12%. To produce a homogenous

solution, a mixture was stirred for 2 h using an elec-tromagnetic stirrer.

Table 1 The adiabatic temperature and the predicted resi-dence time for all process conditions

CH4 [L/min] O2 [L/min] N2 [L/min] TAdiabatic [K] τ [s]

1.5 3.8 2 2825 0.02212.5 6.3 2 2900 0.03452.5 6.3 4 2772 0.03212.5 6.3 5 2719 0.02852.5 6.3 6 2675 0.02732.5 6.3 7 2618 0.0254

CH4 [L/min] O2 [L/min] N2 [L/min] TAdiabatic [K] τ [s]

1.5 3.8 5 2409 0.00902.5 6.3 4 2694 0.00942.5 6.3 5 2651 0.00792.5 6.3 6 2609 0.00632.5 6.3 7 2566 0.0037

(b) TFN-FSP

(a) UN-FSP

Fig. 1 (a) Schematic diagram of the experimental setup, (b) gas flow arrangement diagram, (c) schematic diagram with thedimension of nozzle

1.3 Adiabatic temperature calculationAdiabatic flame temperatures for all process con-

ditions were calculated using the STANJAN code

70 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN

(Reynolds, 1986). This code calculates the chemicalequilibrium state of the ideal gas mixture based onminimizing Gibbs free energy. CH

4, O

2, N

2 and H

2O

were supplied as reactants and C, CO, CO2, H, OH,

H2O, H

2, N, NO, NO

2, N

2, O

2, O and CH

4 were used as

reaction products. The calculation was conducted at astarting temperature of 298 K, and the calculationinvariables were constant pressure (1 atm) and con-stant enthalpy. H

2O was input as a reactant in order to

add the precursor effect on adiabatic flame tempera-ture.

The precursor solution was replaced with H2O

because precursor chemicals data are not available onthe STANJAN database. Based on the fact that themolar ratio of water in the precursor solution is 99.39%,the use of water to replace the precursor solution inthe calculation is reasonable. The complete result ofthe calculation of adiabatic temperature is shown inTable 1.1.4 Particle characterization

X-ray diffractometry (RINT 1000, Rigaku Corp.)with a CuKα target operated at 40 kV and 30 mA wasused to observe the crystallinity and phase purity ofsynthesized particles. The crystallite size is measuredby Scherer’s equation with the measured full-width athalf maximum (FWHM) values of (222) peaks. Theparticle’s morphology was observed by FESEM(S5000, Hitachi High Technologies Corp.). Particleswere sputtered for 30 s by ion sputtering (E 1010,Hitachi High Technologies Corp.) and then observedfor 20 kV operations. To study the optical propertiesof prepared Y

2O

3:Eu3+, RF-5300PC (Shimadzu Corp.)

with a spectral resolution of 0.2 nm at room tempera-ture was used. For the excitation of phosphor in theUV region, a xenon lamp was used. The photolumi-nescence (PL) intensity of particle excitation at 254nm was measured. Relative PL intensity is calculatedto compare the brightness of the as-prepared particleswith that of commercial particles as reported by Changet al. (2004). The calculation was conducted as the ra-tio of the peak emission intensity of the as-preparedparticles to that of commercial particles. The Y

2O

3:Eu3+

commercial particle (lot no. 8XE047) is supplied byKasei Optonix Ltd.1.5 Residence time calculation

To calculate the residence time of particles in theflame zone, the volume of the flame was estimated.The relation of the residence time to the total gas flowrate, the measured volume of the flame, working tem-perature and working pressure is (Ténégal et al., 2003):

τ = ( )V

D

T

T

P

P0

0

0

1

Because the experiment was carried out under at-mospheric conditions and the adiabatic temperature

calculation is applied, the pressure term in Eq. (1) canbe eliminated to form

τ = ( )V

D

T

T0

0 2

The complete calculation of the predicted residencetime under various operating conditions is shown inTable 1.

2. Results and Discussion

2.1 Flame patternThe flame patterns of UN-FSP and TFNFSP are

shown in Figure 2. By increasing the methane flowrate from 1.5 to 2.5 L/min, the flame height of UN-FSP was increased from 4.2 to 7.2 cm (Figures 2(a)and (b)). The flame height of TFN-FSP was increasedfrom 7.5 to 10.1 cm when the same methane flow rateincrease was applied (Figures 2(d) and (e)). The flameheight was determined as the distance of the burnerface to the end of the luminous flame zone. The in-crease in the flame height is easily understood as theeffect of fuel adding to the flame. The effect of fuelflow rate on the flame height in FSP was also reportedby Mueller et al. (2003). They reported that the flameheight increased from 17 to 44 cm when the liquid feed,which contained the precursor and ethanol fuel, wasincreased from 11.1 to 33.3 mL/min. Increasing nitro-gen flow rate demonstrated an opposite effect on theflame height. The flame height of UN-FSP was reducedfrom 7.2 to 5.6 cm when the nitrogen flow rate wasincreased from 2 to 7 L/min (Figures 2(b) and (c)).Figures 2(e) and (f) show that the flame height of TFN-FSP was reduced from 10.1 to 5.6 cm when the nitro-gen flow rate was increased from 5 to 7 L/min.

The flame shape of UN-FSP is cone-like and thewidth of the flame peak was increased by increasingthe nitrogen flow rate as shown in Figures 2(a), (b)and (c). The flame of TFNFSP is divided into 2 partsof different shapes (Figures 2(d), (e) and (f)). Close tothe burner face, the flame shape is dome-like. In theupper part of flame, the flame shape is cylindrical witha small diameter. This shape is caused by carrier gasflow characteristics in the flame zone. Nitrogen gasenters the flame zone at a high velocity because of sizereduction at the tip of the nozzle. Subsequently, thepressure of the space around the nitrogen gas stream islow and thus the methane and oxygen gas flows ap-proach the nitrogen stream.2.2 Particle’s characteristics

Figure 3 shows the SEM images of commercialand as-prepared Y

2O

3:Eu3+ particles. The commercial

particles were not spherical and imperfect-shaped, andhad particle diameters on the micron order. Figures3(a), (c) and (e) show the SEM images of particles

VOL. 39 NO. 1 2006 71

prepared by UN-FSP. The as-prepared particles weredense, spherical and non agglomerated. A small numberof nanoparticles were observed due to condensation ofevaporated particles at a high temperature. This obser-vation was also reported by Kang et al. (2001) whentheir adiabatic flame temperature was approximately3900°C. In our results, the observed nanoparticles werenot as many as they reported. This is because a loweradiabatic temperature (2900 K) was applied in our in-vestigation.

The particles prepared by UN-FSP had a meandiameter of 754 nm when the methane flow rate of2.5 L/min was supplied (Figure 3(c)). The mean di-ameter of the particles was measured on the basis of

the SEM images. Chang et al. (2004) reported thatapplying a methane flow rate of 1.5 L/min resulted inparticles with a mean particle diameter of 696 nm. Proc-ess temperature and particle residence time in the flamezone are the key parameters for determining particlecharacteristics. As shown in Table 1, adiabatic tem-perature increased from 2825 K (compared with thatobtained by the Chang et al. (2004) calculation, with-out precursor effect correction, the adiabatic tempera-ture is 2864 K) to 2900 K when the methane flow ratewas increased from 1.5 to 2.5 L/min. The predictedresidence time also increases from 0.0221 to 0.0345 swhen the methane flow rate was increased from 1.5 to2.5 L/min. Obviously, the particles enlargement is due

Fig. 2 Flame pattern of: (a)–(c) UN-FSP and (d)–(f) TFN-FSP

72 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN

to particles remaining at a higher temperature for alonger time. This can be understood because the coa-lescence and the coagulation rates increase withprocess temperature and residence time (Jang et al.,

Fig. 3 FE-SEM images of commercial and as-prepared Y2O

3:Eu3+ particles synthesized by (a), (c), (e) UN-FSP and (b), (d),

(f) TFN-FSP

2004).Figures 3(b), (d) and (f) show SEM images of

particles prepared by TFN-FSP. Y2O

3:Eu3+ particles

prepared by the methane flow rate 1.5 L/min were

VOL. 39 NO. 1 2006 73

agglomerated with an irregular shape (Figure 3(b)). Themorphology improved well when a methane flow rateof 2.5 L/min was supplied as shown in Figures 3(d)and (f). As-prepared particles were spherical and softlyagglomerated. Increasing the nitrogen flow rate did notsignificantly affect the mean size diameter of particles,which is on average approximately 24 nm. To the bestof our knowledge, Y

2O

3:Eu3+ particles on the nanometer

order have never been produced by FSP synthesis.However, Y

2O

3:Eu3+ nanoparticles have been generated

by other methods such as chemical vapor synthesis(Konrad et al., 1999) and precipitation (Wakefield etal., 2001).

The photoluminescence spectrum of the particlesprepared by UN-FSP under excitation at 254 nm isshown in Figure 4. Particles exhibit a strong red emis-sion at 611 nm, which corresponds to the hypersensi-tive transition from the 5D

0 to the 7F

2 level of Eu3+.

Surprisingly, by increasing the methane flow rateto 2.5 L/min, the relative PL intensity of the as-pre-pared particles became 1.01. The as-prepared particleswere produced by a one-step process without flux add-ing. Photoluminescence intensity is strongly affectedby crystallinity as well as morphology. By increasing

the flame temperature, particles received higher energyand subsequently, the particle crystallinity was im-proved. The high relative PL intensity of Y

2O

3:Eu3+

prepared by UN-FSP was also reported by Seo et al.(2003). However, their particles were prepared by FSPwith the addition of flux (LiCl and Gd(NO

3)

3) in the

precursor and were post-treated with annealing.The inset image in Figure 4 shows the effect of

the nitrogen flow rate on the relative PL intensity ofparticles prepared by UN-FSP. When the nitrogen flowrate was increased from 2 to 7 L/min, relative PL in-tensity decreased from 1.01 to 0.36. The increase inthe nitrogen flow rate reduces adiabatic temperature(2900–2618 K), residence time (0.034–0.025 s) (Ta-ble 1), and also particle’s crystallite diameter (42.3–34 nm) (Figure 4 inset). These facts show that the in-creasing nitrogen flow rate shortens residence time andreduces adiabatic temperature. This condition leads tothe generation of particles with a low crystallinity anda small crystallite diameter. As a result, phosphor par-ticles emit low photoluminescence intensity.

The photoluminescence spectrum of the particlesprepared by TFN-FSP is shown in Figure 5. Whenthe methane flow rate was set at 2.5 L/min and the

Fig. 4 Photoluminescence (PL) spectra of commercial and as-prepared Y2O

3:Eu3+ particles under excitation at 254 nm pre-

pared by UN-FSP for various process conditions (Inset: Relative PL intensity and particles crystallite diameter [nm]at various process conditions)

74 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN

nitrogen flow rate was varied from 2 to 7 L/min, rela-tive PL intensity ranged from 0.21 to 0.24. The rela-tive PL intensity was 0.08 at a methane flow rate of1.5 L/min. This low photoluminescence intensity iscaused by poor particle morphology, that is irregularin particle shape, agglomeration (Figure 3(b)) and poorcrystallinity. The inset image in Figure 5 shows a cor-relation between relative PL intensity and crystallitediameter. An increase in crystallite diameter results inan increase in the PL intensity of the as-prepared par-ticles. Generally, particles prepared by TFN-FSP emita PL intensity lower than those prepared by UNFSP.These results are in good agreement with those ofShimomura and Kijima (2004) who showed that a smalldiameter tends to decrease the brightness of Y

2O

3:Eu3+

particles.Regardless of the type of the atomizer, as-prepared

particles were pure and had a cubic phase as shown bythe XRD pattern in Figure 6. The crystallite size di-ameter of particles prepared by UN-FSP was in therange of 34.1 to 42.3 nm (Figure 4 inset). Particlecrystallite diameter increased from 40.3 (Chang et al.,2004) to 42.3 nm when the methane flow rate was ad-justed from 1.5 to 2.5 L/min. This implies that increas-

ing the methane flow rate improves particle crystallin-ity. In TFN-FSP, the crystallite diameter of as-preparedparticles was in the range from 17.2 to 24.8 nm (Fig-ure 5 inset). From the SEM images, the mean particlediameter was approximately 24 nm. It denotes thatY

2O

3:Eu3+ particles by TFN-FSP were single-crystal

particles.

Conclusions

In this paper, the characteristics of Y2O

3:Eu3+ (red

luminescent material) produced by UN-FSP and TFN-FSP are systematically investigated. The particles pre-pared by UN-FSP were dense, spherical and non-ag-glomerated. For the methane flow rate of 2.5 L/minand the nitrogen flow rate of 2 L/min, the as-preparedparticles had a mean diameter of 754 nm and the rela-tive photoluminescence intensity was 1.01.Nanoparticles with a mean diameter of 24 nm wereproduced using TFN-FSP with a methane flow rate of2.5 L/min. Generally, particles prepared by TFN-FSPemitted a photoluminescence intensity lower than thatof particles prepared by UN-FSP. From the XRD pat-tern obtained, it was found that regardless of the type

Fig. 5 Photoluminescence (PL) spectra of commercial and as-prepared Y2O

3:Eu3+ particles under excitation at 254 nm pre-

pared by TFN-FSP for various process conditions (Inset: Relative PL intensity and particles crystallite diameter [nm]at various process conditions)

VOL. 39 NO. 1 2006 75

Fig. 6 X-ray diffraction patterns of commercial and as-prepared particles by UN-FSP and TFN-FSP

of atomizer used, particles had a cubic phase with ahigh crystallinity.

AcknowledgementThe authors wish to thank Mr. Toshiaki Hiramoto and Mr.

Takashi Ozaki for their assistance during experiment and Dr. NaotoKijima for providing the commercial sample. This work is partlyfunded by METI/NEDO Nanotechnology Particle Project andGrants-in-Aids (K. O., I. W. L.) and doctoral scholarship (A. P.)from the Ministry of Education, Culture, Sports, Science and Tech-nology.

NomenclatureD

0= total gas flow rate delivered by mass flow meters

at standard temperature and pressure [L/min]P = working pressure [atm]P

0= standard pressure [atm]

T = working temperature [K]T

0= standard temperature [K]

τ = residence time [s]

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