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박성규, 김기도, 김희택

한양대학교 화학공학과

Synthesis of monodispersed SiO2 and TiO2 nanopaticles

using a semibatch reactor and the effect of parameters

on particle size and size distribution

Sung-Kyoo Park, Ki-Do Kim, Hee-Taik Kim

Department of Chemical Engineering, Hanyang University

INTRODUCTION

Nowadays, monodispersed nano-sized inorganic particles have received considerable attention not only in the scientific field, but also in various industrial applications because their optical, electrical and densification properties. The methods known for the synthesis of those particles are aerosol method and microemulsion method. However aerosol method needs high-cost production facility and has low production yield. And also microemulsion method needs to remove surfactants after finishing reaction but it is impossible to take surfactants off perfectly from produced material.

To overcome these problems, we used sol-gel method to synthesis nano-sized inorganic particles. The advantages of this methods are; (1) production of high-purity material, (2) low-cost installation for production facility, (3) low-temperature synthesis.

SiO2 and TiO2 have widely examined as a model studies to produce monodispersed particles having sizes ranging from several nanometer to several micrometers by sol-gel method. The synthesis mechanism of SiO2 and TiO2 particles can be considered through the hydrolysis and condensation of TEOS, TEOT in ethanol media respectively. The each reaction is written in abbreviated form as;

(1) Silica

[eqn: rmSi(OR)_4 + 4H_2 O~ -> ~Si(OH)_4 + 4ROH]

[eqn: n rmSi(OH)_4 ~ -> ~itn rmSiO_2 + 2it n rmH_ 2 O]

(2) Titania

[eqn: rmTi(OR )_4 + 4H_2 O~ -> ~Ti(OH)_4 + 4ROH]

[eqn: rmTi(OH)_4 ~ -> rmTiO_2 CDOT rmxH_2 O + (2-rmx) rmH_ 2 O]

In this work, we examine the effects of each parameter on the produced particles. The parameters, R=[H2O]/[TEOS], ammonia(NH3), feed rate of reactant, and reaction temperature considered on the formation of SiO2 nanoparticles. The effects of R=[H2O]/[TEOT], steric stabilizer hydroxypropycellulose (HPC), feed rate of reactant, and reaction temperature on the formation of TiO2 nanoparticles. In addition, we used a statistical experimental method to establish the optimal conditions and to find the main parameters affecting the properties of SiO2 and TiO2 nanoparticles.

EXPERIMENTAL

Materials

SiO2  In the synthesis of SiO2 particles, TEOS(99.9%, supplied by Aldrich Chemical Co.), ethanol(99.9%, supplied by Sigma Chemical Co.) solution, and water-ethanol solution with ammonium hydroxide (28% in NH3, Yukuri Pure Chemicals Co. LTD.). Ammonia was used as a catalyst. The reactants were used as prepared without any purification.

TiO2  In the synthesis of TiO2 particles, TEOT (99.9%, supplied by Aldrich Chemical Co.), ethanol(99.9%, supplied by Sigma Chemical Co.) solution with hydroxypropylcelluose (HPC, molecular weight 1000,000, supplied by Aldrich Chemical Co.), and water-ethanol solution were used. HPC was used as a dispersant. The reactants were used as prepared without any purification.

Synthesis of Nanoparticles

Monodispersed spherical SiO2 and TiO2 nanoparticles were prepared by the hydrolysis of alkoxide in a semi-batch reactor. The apparatus consists of a reservoir of starting solutions, a micro feed pump with a constant flow rate feeding the starting solution A(TEOS／ethanol or TEOT／HPC／ethanol) into the reactor with another solution B(water／ethanol／ammonia or water／ ethanol) and the experiment was conducted with dry nitrogen.

Design of Statistical Experimental Method

We used a statistical experimental method to find the optimal conditions for satisfying the two properties at the same. We carried out twelve experiments and took three levels for each factor.

Measurement

Particle size, particle size distribution, and shape of the powders were determined by a Field Emission Scanning Electron Microscope (FE-SEM, JEOL, JSM-6330F, Japan), Transmission Electron Microscope (TEM, JEOL, JEM-2010, Japan), and a Laser Particle Size Analyzer (Otsuka Electronics, LPA-3000, 3100, Japan).

RESULTS AND DISCUSSION

1. Effect of reaction time TiO2 particle size was decreased as reaction proceeded, while SiO2 particle size was increased. Same behavior was observed for all experiments. It seems that the difference of hydrolysis rate of between TEOS and TEOT caused these results. In the synthesis of SiO2, hydrolysis of TEOS is a very slow reaction, even if an acid or a base is used as a catalyst. However, TEOT reacts so fast with even a little of water and makes a number of small particle, then it agglomerates each other to form a large particle.

2.  Effect of water concentration  In the synthesis of TiO2, there was a reduction in mean diameter with increasing water concentration (R) while standard deviation of the particle size has a tendency  to increase. if high water concentration is applied, a higher nucleation rate occurred and, therefore, smaller particles are prepared. Moreover, the hydrogen bond of TiO2 nanoparticles under the condition of a high ratio of R is stronger than that of a low ration of R. Therefore agglomeration occurred strongly and size distribution becomes very broad. For the particles made without HPC at higher water concentration (R=155), particle size was about 60 nm and size distribution was not good. The reduction in particle size and standard deviation with water concentration in the reaction mixture at a fixed polymer (HPC) and TEOT concentration can be due to the effect of water concentration on the degree of hydrogen bond between the particles and the kinetics of hydrolysis and condensation reactions.

In the synthesis of SiO2, there was increase in mean diameter with increasing water concentration (R) and ammonia concentration. To minimize the particle size, the value of R and ammonia concentration must be decreased. The hydrolysis is a very slow reaction even if an acid or a base is used as a catalyst. It is apparent that the higher ammonia and water concentration must be attributed to the higher hydrolysis rate. Arguments based on a simple monomer addition model show that the final particle depends on the ration of the hydrolysis rate.

T. Matsoukas and Erdogan Gulari reported that the effect of ammonia concentration is to promote hydrolysis, but also to promote the polymerization (condensation) rate, resulting in faster kinetics, and larger particle sizes. In many other studies, the same result was observed.  However, they reported that the increase water concentration leads to smaller particles. On the contrary, the large particle size was obtained in the higher water concentration by our experiments. We can explain this as follow. If high water concentration is applied, a high nucleation rate occurred, so a lot of small sub-particles are prepared at a short period. But, the hydrogen bond of SiO2 sub-particles under the condition of a higher water concentration is stronger than that of a lower water concentration because of excess water. Therefore, sub-particles in the higher water concentration agglomerate each other and grow into a large particle. And also, it is sure that high concentration of water and ammonia attributes to broaden the particle size distribution.

3. Effect of feed rate of reactants  Using a semi-batch system, the higher feed rate is applied, the smaller SiO2 particles are produced while the lager TiO2 particles are obtained. It is because of the difference of hydrolysis rate, which causes different particle growth period in both synthesis system. However, by using the semi-batch system, it is possible to prepare the particles having a narrow size distribution at a low fed rate of reactant because of a relatively short nucleation period in both system.

CONCLUSION

SiO2 and TiO2 nanoparticles were formed from the hydrolysis and concentration of TEOS, TEOT respectively by using a semi-batch process. The properties of those particles were controlled by varying the molar ration R, concentration of ammonia, the amount of HPC, feed rate of reactant solution and reaction temperature. Statistical design of experiments was used to select the major factors that significantly influenced the synthesis of monodispersed SiO2, TiO2 nanoparticles.

As a result of this work, the main difference of those particle properties was caused by the hydrolysis rate of starting materials.

REFERENCES

1.	E. A. Barringer and H. K. Bowen; "Monodisperse TiO2 Powders by	Hydrolysis Titanium Tetraethoxide. 1. Synthesis and Physical Properties",	Langmuir, 1, 414-420 (1985)

2.	J. H. Jean and T. A. Ring; "Processing Monosized TiO2 Powders	Generated with HPC Dispersant", Am. Ceram. Soc Bull, 65(12), 1574-1582	(1986)

3.	A. Van Blaaderen, J. Van Geest and A. Vrij; "Monodisperse Colloidal Silica	Spheres from tetraalkoxysilanes: Particle formation and growth	mechanism", J. Colloid Interface Sci., 154, 2, 481-501 (1992)

4.	T. Matsoukas and Erdogan Gulari; "Dynamics of Growth of silica particles	from Ammonia-Catalyzed Hydrolysis of Tetra-ethyl-orthosilicate", J.	Colloid Interface Sci., 124, 1, 252-261 (1988)