초임계 수열합성법에 의한 Cerium(IV) Oxide 나노입자의 합성

전창석, 송규호, 임종성, 김재덕, 이윤용, 이해원*, 이윤우

한국과학기술연구원 국가지정 초임계유체연구실, 세라믹공정연구센터*

Hydrothermal Synthesis of

Cerium(IV) Oxide Nano Particles in Supercritical Water

Chang Seok Jeon, Kyu Ho Song, Jong Sung Lim,

Jae-Duck Kim, Youn Yong Lee, Hai Won Lee*, Youn-Woo Lee

NRL for Supercritical Fluid, Ceramic Processing Research Center*,

Korea Institute of Science and Technology

39-1 Haweolkok-dong, Sungbuk-ku, Seoul, 136-791 Korea

Introduction

Cerium(IV) Oxide (Ceria, CeO2) is currently being used as a chemical mechanical polishing medium, catalytic supports and an oxygen ion conductor in solid oxide fuel cells (SOFCs). Typical methods for producing ceria fine particles are solid-state, sol-gel, precipitation, thermal decomposition, flash combustion, and hydrothermal synthesis method. Hydrothermal synthesis using supercritical water is known to be superior to the other methods from the view point of energy conservation because this method is carried out at comparatively low temperature (350~450℃) and have a short reaction time. Moreover this method does not require calcination and the process is simple, so scale-up to the continuous commercial production is easy. In recent work, Y. Hakuta et al.[2] prepared ceria nano particles by a hydrothermal synthesis method at very short residence time and reported that mean particle sizes became much smaller at supercritical conditions. They also reported that the formation of nano-size CeO2 particles was attributed to a high degree of supersaturation due to the high rate of hydrolysis of Ce(NO3)3 as well as the low solubility of CeO2 in supercritical water.

In this work, ultrafine ceria particles were synthesized under supercritical hydrothermal conditions in continuous flow reactor. The effect of a solution concentration on the crystal properties of ceria was investigated. The formed products were characterized by X-ray diffractometry (XRD, VG Scientific, England, 2803-S), Scanning electron microscopy (SEM, Hitachi, Japan S-4200) and Zeta potential analyzer (ZetaPlus).

Experimental

Cerium(IV) Nitrate Hexahydrate (Ce(NO3)3□6H2O, reagent grade; Kanto Chemical Co., Inc) was used as the starting material for preparation of ceria.  The apparatus used is schematically shown in Fig. 1. The reactor employed was a stainless-steel vertical-tube reactor (3/4 inch ID, 3/8 inch OD, 310 mm long and 22.08 cm3 volume). Cerium nitrate aqueous solution (0.1 mol/□, 0.05 mol/□, 0.01 mol/□) was fed by a high-pressure pump with a flow rate of 9.4 □/min. Distilled water was pumped into a preheater with a flow rate of 21.9 □/min. Then preheated water was mixed with the cerium nitrate solution at mixing point2 (MP2).  Temperature of the mixed flow was maintained at 400℃ from the reactor inlet to the reactor exit and temperature deviation was 5℃. Reaction pressure was controlled by back pressure regulator (Tescom Co., U.S.A) at 300bar.

1. Reactor, 2. Heater, 3. High-pressure pump, 4. Preheater, 5. Cooler, 6. Filter, 7. Back-pressure regulator, 8. Temperature controller, 9. Temperature indicator, 10. Reservoir, 11. Cerium nitrate solution, 12. Distilled water

Fig. 1. Apparatus for continuous hydrothermal synthesis in supercritical condition.

Ceria was recovered at a in-line filter (TF series Tee-type, 0.5□ element) and at bottle which was in the system exit. Then products were washed by distilled water and filtered with a membrane filter (Cellulose acetate, 0.2□, Advantec MFS, Inc.). After filtration, ceria particles were dried at 85℃ for 10min.

Results and Discussion

In this work, we could prepare highly crystalline ceria fine particles with supercritical water hydrothermal synthesis method continuously. The results of the experiment concerning the effect of initial cerium nitrate solution concentration on the supercritical hydrothermal products can be seen that the crystallite size increases with the initial concentration. SEM micrographs of the particles in Fig. 2. show fine particles with size (40~60 nm), which coincides with the XRD-determined grain size (55 nm) as shown in Fig. 4. As shown in Fig. 2 and Fig. 3, our synthesized ceria is a octahedral shape with a highly feceted interface which is represented of crystallinity degree. But it seems that  there are some secondary particles, of which the size is bigger than 100nm and it needs more study for improvement of the agglomeration.

Especially, in Chemical Mechanical Polishing (CMP) process, CMP slurry demand fully dispersed condition with the maximum electrical repulsive force between the particles.  For this reason, it is better to have the large zeta potential (in absolute value) at the desired pH of CMP slurry. In this work, we found that the prepared ceria particles had relatively large value (-40mV) of zeta potential in the wide range of solution pH (pH 6 ~ pH 12), as shown in Fig. 5. The fact that these particles had no isoelectric point (IEP) or very small pH value (pH 2, in case of extrapolation) might indicate that there occurred little hydration on the particle surface (IEP increases as hydration increases)[3]. However, secondary particle size was changed from 2□m to 200nm with pH value, which is quite comparative to or slightly larger than the value measured in commercial CMP slurries.

In conclusion, we prepared good crystalline cerium(IV) oxide nano particles by hydrothermal synthesis in supercritical conditions. These particles had a octahedral fluorite structure and high zeta potential value (-40mV) in the wide range (pH 6 ~ pH 12). The size of secondary particles with pH changes was comparatively small over large range (pH 6 ~ pH 12). Absence of the IEP was supposed that there was little hydration on the particle surface. The effect of the initial concentration of cerium nitrate could be clearly seen in fact that the particle size increased with increasing the initial concentration.

Acknowledgement

This work has been supported by the National Research Laboratory Program for Supercritical Fluid and the authors would like to the Ministry of Science and Technology.

Reference

[1] Youn-Woo Lee, Theories and Application of Chemical Engineering, 6(2),            3085-3088 (2000)

[2] Y. Hakuta, S. Onai, H. Terayama, T. Adshiri, K. Arai, Journal of Materials

Science Letters, 17, 1211-1213 (1998)

[3] M. Hirano, Y. Fukuda, H. Iwata, Y. Hotta, M. Inagaki, J. Am. Ceram. Soc.,

83(5), 1287-1289 (2000)

[4] M. Hirano, E. Kato, Journal of Materials Science Letters 15, 1249-1250 (1996)

Fig. 2. SEM image of ceria prepared at 0.05mol/□ Ce(NO3)3, 400℃ 300bar	Fig. 3. SEM image of ceria prepared at 0.10mol/□ Ce(NO3)3, 400℃ 300bar

Fig. 4. XRD patterns of the ceria prepared at 0.05 mol/□, Ce(NO3)3 400℃, 300bar

Fig. 5.  Zeta potential as a function of pH for Ceria particles prepared

from 0.05 mol/□ Ce(NO3)3 solutions. 400℃, 300bar.