초임계수에서의 수열합성법을 이용한 산화아연 미세입자 연속 합성

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

이은용, 임종성, 김재덕, 이윤우

한국과학기술연구원 초임계유체연구실

Preparation of ultra-fine zinc oxide particles by hydrothermal synthesis in Supercrital Water with a Continuous Flow Reactor

Eun young Lee, Jong Sung Lim, Jae-Duck Kim, Youn-Woo Lee

Supercritical Fluid Research Lab., Korea Institute of Science and Technology

Introduction

화학공학의 이론과 응용 제8권 제1호 2002년

Zinc oxide is one of the important ceramic materials, and has been found to have several applications in electronic devices such as gas sensors, varistors, and transducers.[1] Zinc oxide nano particles were applied in cloth, fiber, paper and cosmetics due to their unique characteristic of antibiosis, preventing ultraviolet rays and protecting skins. Therefore , there has been many attempts to make nano-size particulates of ZnO owing to their wild applications. Ultra-fine zinc oxide particles can be manufactured by sol-gel process, solid state method, spray pyrolysis or hydrothermal method etc.[5] In the solid-state method for preparation metal oxide, it adapted mass production however required long reaction time. In sol-gel method, reaction time was shorter than that of solid-state method. However, this method is producted to noxious gas which was resulted in environmental problems and spent on much energy. In the hydothermal method, water was used as a reaction medium so that it becomes simple and environmentally friendly process whereas it needed in much energy. In the hydrothermal synthesis at supercritical conditions, there is the possibility of continuous production of zinc oxide fine particles having high crystallinity in a short period of time.

Supercritical conditions of water are above 374℃ and 221bar. In this region, water has particular properties such as reduced local molecular ordering with less effective hydrogen bonding, decreased dielectric constant as well as ionic dissociation constant. In addition, the solubility of inorganics is greatly decreased because of a low ionic dissociation. The low viscosity and the high diffusivity of supercritical water lead to efficient transport of solute into solid-liquid interface and growth of good quality single crystals. From these properties described above, supercritical water has been investigated in synthesis of fine metal oxides.[2,3]

The objective of this study is to evaluate the effect of process parameters (temperature, residence time, concentration of the aqueous solution) on the particle size and morphology.

Experimental

Figure 1 shows a continuous hydrothermal reaction under supercritical conditions apparatus for synthesis of fine zinc oxide particles in this study. Zinc nitrate (Kanto chemical,96+%)  solution, dissolved in distilled water, was used as a precusor and it's concentrations were varied at 0.01, 0.05 and 0.1M. Potassium hydroxide (KOH) aqueous solution was used to adjusted the solution PH. So KOH, mineralizer, enhance the formation of zinc hydroxide and thus increasd the formation rate of zinc oxide. The alkali molar ratio (R) , which is defined as the ratio of KOH concentration to zinc nitrate concentration, was changed between 0.5 and 8. The alkali molar ratio (R) is defined as follows.

화학공학의 이론과 응용 제8권 제1호 2002년

[eqn: R ={ [KOH]} over {[Zn(NO sub 3 ) sub 2 ] }]

Then the solution was pressurized to 300bar and mixed at a mixing point (MP1) with a large amount of supercritical water(included KOH) heated by an electric furnace and pressurized by a high - pressure mini-pump (Milton Roy, USA). Because a large amount of the water in supercritical conditions was brought into to heat the stream of the precusor, the temperature of mixed solution at MP1 is rapidly heating up to the desired temperature.

1. Reactor

2. Furnace

3. High-pressure Pumps

4. Pre-heaters

5. Cooler

6. In - Line Filters

7. Back-Pressure Regulator

8. Filtrate Reservoir

9. Zinc precursor (Zn(NO3)2)

10. Distilled water including KOH

Figure 1. Supercritical water synthesis flow apparatus.

The pressure of the supercritical water in the reactor were maintained at 300 bar, while temperature in the reactor was varied from 350 to 450℃. The inside volume of the reactor was 22 cm3 and the corresponding residence time was 60 and 30 sec, which means that the metal oxide preparation process was completed within 1 min. This residence time is much shorter than that for any other preparation methods. The residence time in a flow type reactor, □, was evaluated from following;[7]

[eqn: □={V} over {{F}×{□_25℃}over{□_x℃}}]

where, V is reactor volume and F is the total flow rate; □25℃ and □x℃ are the densities of pure water at room temperature and x℃, respectively. The effluent from the flow type reactor was cooled to room temperature by a cooler. After cooling, the formed particles were collected by Tee-type filter (pore diameter:0.5mm, TF series, Nupre Co., U.S.A) disposed in front of the back-pressure regulator (P/N:26-1721-24, Tescom Co., MN, U.S.A), which controlled the system pressure. Morphology and size of the particles obtained were observed using scanning electron microscope (SEM) analysis and Scherrer eqn. The reaction products were analyzed by X-ray diffraction (XRD) with  Cu K□ radiation between 5°and 90°.

results and discussion

The reactions for synthesis of zinc oxide can be expressed as follows:

Hydrolysis : [eqn: Zn(NO sub 3 ) sub 2] + 2H2O [eqn: <->] Zn(OH)2 + [eqn: 2HNO sub 3]

Dehydration : Zn(OH)2 [eqn: <->] [eqn: ZnO] + H2O

Overall Reaction Chemistry : [eqn: Zn(NO sub 3 ) sub 2] + H2O [eqn: <->] [eqn: ZnO] + [eqn: 2HNO sub 3]

(a)	(b)
(c)

Figure 2. SEM micrographs of zinc oxide particle at various conditions

Figure 2 shows the SEM images of some ZnO particles by hydrothermal method under supercritical conditions at 400℃, 300bar and 0.05M of zinc nitrate solution. As shown figure 2, the morphology of zinc oxide particles can be prepared from rod-like shape to spherical shape. However In figure 2 (b) and (c), particles are cohered as  groups.

Figure 3. The X-ray diffractions of ZnO nano-particles prepared

Also figure 3 illustrated XRD patterns of obtained zinc oxide particles under various conditions. All diffraction peaks are assigned to ZnO as reported in JCPDS file.[6]

Conclusion

The synthesis of ultra fine zinc oxide particles in the continuous supercritical water system

was investigated. The hydrothermal technique is proved to be one of the sucessful methods to

prepare crystalline zinc oxide particles. In our preliminary, it is found that ZnO particles can

be prepared with different morphologies and further experiments on various conditions is on

going.

화학공학의 이론과 응용 제8권 제1호 2002년

Reference

[1] Chung-Hsin Lu, Chi-Hsien Yeh, ceram. int., 26, 351-357(2000)

[2] K.H. Song, S.H Do, J.S. Lim, Y-W Lee, J-D. Kim, Y.Y. Lee, Theories and Applications of Chemical Engineering, 6(1), 2085-2088(2000).

[3] Chemical Reviews, 1999, Vol. 99, No 2, 515-518.

[4] Andrej Degen, Marija Kosec, Journal of the European Ceramic Society, 20, 667-673(2000).

[5] A.J.C.Fiddes, K.Durose, A.W.Brinkman, J.Woods, P.D.Coates, A.J.Banister, Journal of Crystal Growth, 159, 210-213(1996).

[6] Joint Committee on Powder Diffraction Standards, Powder Diffraction File, Card No. 36-1451, Swarthmore,PA.

[7] Yukiya Hakuta, Tadafumi Adschiri, Toshiyuki Suzuki, Toshihiro Chida, Kazuei Seino, and Kunio Arai, J. Am. Ceram. Soc., 81 [9] 2461 (1998)