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Materials Chemistry and Physics 124 (2010) 134–139

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

ovel method to control the size of well-crystalline ceria particles byydrothermal method

young-Hwan Oha,∗, Jun-Seok Nhob, Seung-Beom Chob, Jae-Seok Leea, Rajiv K. Singha

Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USALG Chem. Ltd/Research Park, I&E materials, 104-1 Moonji-dong, Yuseong-gu, Daejeon, 305-380, Republic of Korea

r t i c l e i n f o

rticle history:eceived 27 May 2009

a b s t r a c t

Well-crystalline ceria particles were synthesized by heating peptized ceria sol as precursor underhydrothermal conditions. The morphology and the crystallites size of hydrothermal ceria particles were

eceived in revised form 21 January 2010ccepted 5 June 2010

eywords:eramicshemical synthesisucleation

controlled by varying the dielectric property of solvent used in preparation of the ceria precursor. Thesynthesized particles exhibit cubic fluorite structure with size ranged from 20 to 400 nm without theformation of hard aggregates. In this work, the relationships between dielectric property of the solventand particle size were investigated in terms of the supersaturation of solute. In addition, the influencesof precipitation participating anions (OH−) and acidic hydrothermal medium on crystallites size of ceriaparticles were studied.

xides

. Introduction

The ceria (CeO2) particles have been considered as an usefulaterial owing to its various applications, such as glass polish-

ng materials [1], solid electrolytes in solid-oxide fuel cells (SOFCs)2,3] and abrasives in chemical mechanical planarization (CMP)lurry in semiconductor fabrication [4,5]. Crystallites size, sizeistribution and morphology of the ceria particles have been iden-ified as the important parameters that influence the performancef the materials [6,7]. Therefore, many approaches to controlhese properties of ceria particles have been extensively investi-ated.

The usual methods for synthesizing ceria particles are liquidhase processes, such as precipitation method [8], hydrothermalethod [9–11], sol–gel method [12] and electrochemical method

13]. Among these approaches, hydrothermal synthesis is theost attractive method since particles with the desired size andorphology can be produced by carefully manipulating param-

ters such as solution pH, concentration, reaction temperature,ime, and the type of solvent [9–11]. Besides, this process canirectly synthesize well-crystallized particles without post-heat

reatment. However, the size of ceria particles synthesized by pre-ious hydrothermal methods was limited to less than ∼50 nm.nother method for production of the ceria particles is thermalecomposition of the cerium salt such as carbonate, oxide hydrate

∗ Corresponding author. Tel.: +1 352 846 2496; fax: +1 352 392 3771.E-mail address: mhplusmy@ufl.edu (M.-H. Oh).

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.06.004

© 2010 Elsevier B.V. All rights reserved.

and hydroxide. This method leads to very porous particles withhigh surface area. However, the size and morphology of the mate-rial are very limited in that particle growth is difficult to control[8,14,15].

Over the past years, several researches using the mixedsolvent system showed the possibility to increase the control-lability of ceramic powder morphology. Park et al. introducedthe alcohol/water mixed solvent to thermally hydrolyze titaniumtetrachloride (TiCl4) in the preparation of spherical titania (TiO2)particles. The DLVO (Derjaguin Landau Verwey Overbeek) theorywas proposed to explain the effect of the alcohol/water mixedsolvent on the morphology of the particles [16]. Fang and Chenfound that the morphology of titania particles was controlled bythe dielectric constant of the solvents, which can be regulatedby changing the volume ratio of n-propanol to water [17]. Hu etal. employed the alcohol/water mixed solutions for synthesizingzirconia particles. They found that the dielectric property of themixture affects the nucleation and growth of the zirconia particles[18]. However, these reports about the mixed solvent system werefocused on formation of metal oxide through precipitation reac-tion. In addition, synthesis of single crystalline ceria particles ofmore than 50 nm under solution phase has not been reported yet.

Therefore, the purpose of the present work is to control themorphology of the ceria crystallites under hydrothermal condi-

tions using a new type of ceria precursor obtained by precipitationmethod and to investigate the effects of the dielectric propertyof solvent on formation of ceria particles. In addition, the influ-ences of the hydrothermal temperature and an acidic hydrothermalmedium on particle characteristics have been studied. This study

M.-H. Oh et al. / Materials Chemistry a

Fde

wc

2

2

(

ig. 1. XRD patterns of ceria particles synthesized from the mixture of water andifferent alcohols; (a) ethylene glycol, (b) methanol, (c) 1,4-buthylene glycol and (d)thanol.

ill provide the new approach to modulate the particle size of ceriarystallites using hydrothermal method.

. Experimental procedure

.1. Preparation of sol-type ceria precursor

Cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O) and potassium hydroxideKOH) were used as the starting materials for the ceria precursor. Cerium(III)

Fig. 2. FETEM photomicrogr

nd Physics 124 (2010) 134–139 135

nitrate hexahydrate and potassium hydroxide were separately dissolved in dis-tilled water with a desired concentration and then mixed with the various kindsof alcohols. The volume ratio of alcohol to water was kept at 2:3. Alcohols, includingmethanol (CH3OH), ethanol (C2H5OH), ethylene glycol (C2H6O2) and 1,4-butyleneglycol (C4H10O2) were used to investigate the influence of the dielectric constantof the mixed solvents. The reaction was carried out at a temperature of 50 ◦C withstirring rate of 100 rpm for 12 h. Air was bubbled into the precipitation reactor withpassage through a gas distributor as an oxidizer. The precipitated ceria precursorwas separated via centrifugation and then redispersed in distilled water under con-tinuous stirring. The weight ratio of distilled water to a precipitated ceria precursorwas kept 5:1. The pH of the suspension solution was adjusted at pH 0.5–4.0 by addingconcentrated nitric acid (HNO3). After reaction, light yellow solution was obtained.

2.2. Hydrothermal synthesis of ceria particles

The sol-type ceria precursor was put into an autoclave with a reaction chamberof 100 cm3. Three quarters of the volume of the chamber was filled with the lightyellow solution. The hydrothermal reactions were carried out at 150–230 ◦C for6 h, corresponding to a pressure range from 200 to 800 psi. After the hydrothermalreaction, the synthesized particles were washed with distilled water three timesand were subsequently dried at 90 ◦C.

2.3. Characterization

The crystalline structure of the synthesized particles was identified through

x-ray diffraction (XRD) using CuK� radiation and High Resolution TransmissionElectron Microscope (HRTEM). The crystallites size of resulting ceria particles wasestimated according to the Scherrer equation, D = 0.9�/(ˇ cos �), where D is the grainsize, � is the wavelength of x-rays, ˇ is the half-width of the diffraction peaks, and �is the diffraction angle. The broadening of the reflection from the (1 1 1) plane wasused to calculate the crystalline size of particles. The morphology and size of par-

aphs of ceria particles.

136 M.-H. Oh et al. / Materials Chemistry and Physics 124 (2010) 134–139

F d diffg

t(f

3

3

Ftvsatbmottostgsidpchroasw

between the concentration of a saturated solution in equilibriumand dielectric constant can be expressed as following [20].

Xl = exp

(−��i

kT

)≈ exp

[− z+z−e2

4�ε0εkT(a+ + a−)

](1)

ig. 3. FESEM photographs of ceria particles prepared from the mixture of water anlycol.

icles were examined by HRTEM and Field Emission Scanning Electric MicroscopeFESEM). The average particle size was calculated by measuring ca. 100 particlesrom FESEM micrographs.

. Results and discussion

.1. Influence of solvent type on ceria particle characteristics

The XRD patterns of the synthesized particles are shown inig. 1. The 0.5 M of cerium(III) nitrate hexahydrate solution andhe 0.5 M of potassium hydroxide solution were mixed with thearious kinds of alcohols in precipitation reaction. The synthe-ized ceria precursors were hydrothermally treated at 230 ◦C afterdjusting to pH 3.0. The dielectric constant of alcohols decreases inhe following order: ethylene glycol (41.4) > methanol (33.0) > 1,4-utylene glycol (31.9) > ethanol (25.3) [19]. As shown in Fig. 1, theajor reflections associated with fluorite structure of ceria can be

bserved on all specimens regardless of the kinds of alcohols usedo prepare the ceria precursors. Fig. 2 shows the HRTEM image ofhe ceria particles which were synthesized from the mixed solventf water and ethylene glycol. They appear to be single crystallinetructure based on the fact that the lattice fringes correspondingo reflections are clearly observed. Fig. 3 shows the FESEM micro-raphs of ceria particles prepared in different mixed solvents. Theynthesized particles were prepared without hard aggregates. Its interesting to note that the ceria particles prepared using higherielectric constants show bigger morphology in spite of using samerocessing parameters and steps. The average particle size of theeria particles increased in proportion to dielectric constant of alco-ol used in the precipitation reaction as shown in Fig. 3. These

esults indicate that the alcohol affects the physical propertiesf the reaction medium without changing the reaction paths andrrangements of the crystal structure. This can be quantitativelyeen from Fig. 4. The curve (a) depicts the average particle sizehereas curve (b) shows the crystallites size with respect to the

erent alcohols; (a) ethanol, (b) 1,4-buthylene glycol, (c) methanol and (d) ethylene

dielectric constant of alcohols used in the mixed solvent. It is foundthat the size and the crystallites size of ceria particle increasedwith increase in the dielectric constant. These results indicate thatthe solvent type may affect properties of the synthesized particlesbecause different alcohols show different dielectric constant anddifferent affinity forwards water.

The dielectric constant of a solvent is the quantitative mea-sure of its ability to decrease attraction between two oppositelycharged ions. The dielectric constant is defined by the free energyfor the coulomb interaction between two charges. The relationship

Fig. 4. (a) Average particle size and (b) crystallites sizes of ceria particles synthesizedwith different dielectric constants of alcohols.

M.-H. Oh et al. / Materials Chemistry and Physics 124 (2010) 134–139 137

conce

wscac(isdc

sst

J

wtissi

o[

l

wosuMs

Fig. 5. FESEM photographs of ceria particles prepared with different

here the Xl is identified with the solubility of solute in anyolvent. ��i is the difference in energy when going from the asso-iated state to the dissociated state of two systems. a+ and a−re ionic radii of ions charged z+ and z− and ε is the dielectriconstant of the medium and e represents the elementary chargee = 1.602 × 10−19 C). As can be seen from the Eq. (1), the solubil-ty of the solute is proportional to the dielectric constant of theolution. This infers that the solubility is larger as increasing theielectric constant and can be adjusted by changing the dielectriconstant of solution.

According to the classical nucleation theory [21], the particleize is dependent on the supersaturation of solute, which affectstrongly the nucleation rate. This can be expressed in the form ofhe Arrhenius velocity equation [21].

= A exp

[− 16��32

3k3T3(ln S)2

](2)

here J is the nucleation rate, A is the rate constant, � is interfacialension between the solute and the solution, is the number ofon formula units, k is the Boltzmann constant and S is the super-aturation of solute. This equation indicates that the increase inupersaturation induces a great number of nuclei due to the rapidncrease of the nucleation rate.

Additionally, the relationship between solubility and the radiusf nuclei can be expressed by the basic Gibbs–Thomson equation21].

n[

X

Xl

]= ln S =

[2Vm�

RTr

](3)

here X is the concentration of solution, S is the supersaturation

f solute, is the density of the solid, Vm is the molar mass of theolid in the solution and R is the gas constant. With increasing sol-bility of the solute, the supersaturation of the solute is decreased.oreover, the nuclei radius is proportional to the solubility of the

olute. From (1)–(3) equations, it can be found that the dielectric

ntrations of potassium hydroxide; (a) 0.5 M, (b) 1.0 M and (c) 1.5 M.

constant of solution affects nucleation rate and the radius of nuclei.Moreover, the size of crystallites can be modulated by varying thesolvent type.

In this work, the nucleation rate has been modulated by chang-ing of the dielectric constant of mixed solvent. A number of nucleiare decreased with increasing the dielectric constant of alcoholsused in precipitation reaction. Therefore, the crystallites size andthe morphology of the ceria particles increased with the increaseof dielectric constant of the solvent.

3.2. Effect of the precipitation participating anions on nucleationand growth

Fig. 5 shows FESEM micrographs of the ceria particles synthe-sized from different ceria precursors which were precipitated inthe mixed solvent of ethylene glycol and water using different con-centrations of potassium hydroxide. The hydrothermal reactionswere carried out at 230 ◦C for 6 h. As shown in Fig. 5, it is foundthat the size of ceria particles was strongly dependant on the potas-sium hydroxide concentration. Particle size was decreased with theincrease in potassium hydroxide concentration.

The formation of ceria particles involves a series of chemicalreaction [22] and the whole process can be classified into four stages[23]. During these stages, hydrated cerium hydroxide complexeswere generated and dehydrated. Precipitation participating anions(OH−) were generated via the hydrolysis of potassium hydroxidewith the molecular water of cerium salt. Precipitation participat-ing anions were exhausted by hydration of tetravalent cerium ions(Ce4+) and subsequently caused the decrease in the pH. More-over, the formation of crystalline particles includes usually two

steps: nucleation and growth. In order to prepare the particles withdesigned size, both steps should be controlled. In nucleation step,the increase of nuclei induces the smaller particle size. In growthstep, secondary nucleation occurs in high supersaturation becausecrystal growth has a lower energy barrier than that of the nucle-

138 M.-H. Oh et al. / Materials Chemistry and Physics 124 (2010) 134–139

F s of n0

aasotrigtogh(

S

wtTw

3

ouptcthpio

ture [9,25]. It appeared that higher temperature in hydrothermalreaction promotes the crystal growth in the ceria particles fromthe hydrated cerium hydroxide complexes according to thedissolution–precipitation mechanism.

ig. 6. FESEM photographs of ceria particles prepared from different concentration.5 and (d) pH 0.5.

tion. In order to investigate the effect of precipitation participatingnions on the formation of ceria particles, 0.5 and 1.5 M of potas-ium hydroxide solution added in cerium salt solution and the pHf these solutions was kept 7.8 and 11.6, respectively. The pH ofhe solution was changed to 4.4 and 9.1 after precipitation for 12 h,espectively. It is attributed to the fact that precipitation participat-ng anions under basic solution are more slowly consumed duringrowth of nuclei. This result indicates that increasing precipita-ion participating anions induces continuously the large numberf nuclei and the decrease of solubility which help to decrease therowth rate. Under basic condition, the solubility product is muchigher than the solubility constant, meaning the supersaturationS) is very large [24].

= [Ce3+][OH−]Ksp

(4)

here Ksp is the solubility constant of Ce(OH)3. A high supersatura-ion induces a great number of nuclei due to secondary nucleation.herefore, the crystallites size and the particle size of ceria decreaseith increasing precipitation participating anions.

.3. Effect of hydrothermal conditions

The effect of an acidic hydrothermal medium on the formationf ceria particles is shown in Fig. 6. Ceria precursor was synthesizedsing the mixed solvent of ethylene glycol and water with 0.5 M ofotassium hydroxide solution. With decreasing the pH of the solu-ion, the size of the ceria particles increased under hydrothermalondition. This result is related to Ostwald ripening phenomena in

he liquid phase system [25]. Wu et al. found that the acidity ofydrothermal medium played a key role in dissolution of smallerarticles, which directly influences the structure. In this work, the

ncrease in hydrogen ions led to a sizable increase in the solubilityf the ceria precursor. This implies that the solute diffuses quickly

itric acid in hydrothermal conditions at 230 ◦C for 12 h; (a) pH 4, (b) pH 2.5, (c) pH

and the crystal growth is more rapid with dissolution of the ceriumprecursor in an acidic medium.

The effect of the hydrothermal treatment temperature onthe crystalline size is summarized in Fig. 7. The crystallites sizewas increased with an increase in hydrothermal treatment tem-perature. Under hydrothermal conditions, the concentration ofthe hydrogen ion (H+) is increased with increasing tempera-

Fig. 7. Crystallites size for ceria particles prepared from different pH at (a) 150 ◦C,(b) 200 ◦C and (c) 230 ◦C.

istry a

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M.-H. Oh et al. / Materials Chem

. Conclusions

Well-crystalline ceria (CeO2) particles were synthesized bysing sol-type ceria precursor under hydrothermal conditions atH 0.5–4.0. The ceria precursor was prepared by chemical precipi-ation in mixed solution of water and alcohols, including methanol,thanol, 1,4-butylene glycol and ethylene glycol (EG), separately.he resultant particles exhibit cubic fluorite structure with sizeanged from 20 to 400 nm. The particle sizes were determinedy XRD and SEM analyses. The results showed that the crystal-

ites size and the morphology of the hydrothermal ceria particlesncreased with an increase in the dielectric constant of alcoholssed in precipitation reaction and hydrothermal treatment tem-erature and a decrease in the pH of hydrothermal medium, whichffect nucleation rate and crystal growth. Consequently, the size oferia particles was easily controlled in the range from 20 to 400 nmithout post-heat treatment.

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