Synthesis of titanium dioxide particles in supercritical CO2

172

The Journal of Supercritical Fluids, 1996, 9, 172-I 76

Synthesis of Titanium Dioxide Particles in Supercritical CO2 Maher E. Tadros,” Carol L. J. Adkins, Edward M. Russick, and Michael P. Youngman Sandia National Laboratories,

Albuquerque,

NM 87185

Received December 5, 1995; accepted in revised form June 1, 1996

Spherical particles of titanium dioxide, anatase, were prepared in a supercritical carbon dioxide medium from titanium alkoxides and water. The dissolution of the alkoxide and stabilization of water dispersions in supercritical CO2 were found to be required for the formation of spherical particles. An anionic fluorinated surfactant was used to stabilize water dispersions in supercritical CO2. This could not be realized with hydrocarbon-based surfactants. The solubility of titanium alkoxides in CO2 appears to parallel their vapor pressure which is dependent on the oligomerization of the unhydrolyzed alkoxides. The polydispersity in particle sizes is due to nucleation occurring simultaneously with particle growth, owing to changes in the degree of supersaturation during the transition from the liquid to the supercritical state. Keywords:

supercritical carbon dioxide, titanium dioxide, titanium alkoxides, aqueous dispersions, Zonyl FSJ.

INTRODUCTION Supercritical carbon dioxide is an attractive medium for the synthesis of ceramic powders because it is nontoxic, nonflammable, has a low critical temperature, a high degree of compressibility, and low cost. On the other hand, the highly nonpolar character of CO2 limits the solubility of surfactants and the ability to stabilize polar materials.’ Hydrocarbon-based surfactants have limited solubility and are not useful for the stabilization of aqueous droplets in supercritical CO;!. Surfactants with low solubility parameter moieties such as fluorocarbon or dimethyl siloxane groups are more soluble in C02.2-5 Some of these surfactants have been shown to enable formation of reverse micelles and microemulsions in supercritical CO2 .3,4 The ability to stabilize aqueous droplets in supercritical fluids provides an opportunity for a wide range of applications such as separations, chemical reactions, and synthesis of particulate materials. For example, spherical submicron particles of Al(OH)3 have been produced by reacting aqueous Al(NOs)j solutions present in the cores of reverse micelles with ammonia in supercritical propane.4 Attempts were also made to synthesize ceramic particles in supercritical fluids. Pommier et. a1.7-9 reported the preparation of the spine1 MgA1204 powders by the decomposition of the double alkoxide Mg[Al(O-secBu)4]2 in supercritical ethanol at 623 K. A small amount of water formed by thermal decomposition of ethanol is believed to be responsible for the hydrolysis-polycondensation reac0896-8446/96/0903-0172$7.50/O

tions, although thermal decomposition of the alkoxide was not ruled out. The same authorsi have also prepared Ti02 particles by the decomposition of titanium isopropoxide in supercritical ethanol at 608 K and in supercritical isopropanol at 553 K. The particles obtained by this method were spherical aggregates of primary particles (20-60 nm) with a mean diameter of about 2 pm. The crystallinity of the product, anatase, increased as the temperature of synthesis was increased from 608 to 623 K. Transformation to the rutile structure was observed at about 1173 K. The objective of this work is to define conditions for the synthesis of titanium dioxide particles in supercritical CO2 by reactions between titanium alkoxides and water. EXPERIMENTAL Materials. Reagent grade alkoxides were used without further purification. Zonyl FSJ (Du Pont) is a water soluble fluorinated anionic surfactant with the general formula (F(CF2CF2),CH,CH20),P(0)(ONH,),., x = 1 or 2, y = 2 or 1, z = l-7. It contains 15% isopropanol and 40% water. Aerosol-OT (Dow) is an anionic surfactant having the formula sodium bis (2 -ethylhexyl) sulfosuccinate. Apparatus. The reactions were carried out in a 300-mL Parr high pressure cell fitted with three 2.4 cm thick sapphire windows at right angles. Two Pyrex glass containers were placed inside the reactor. The glass con0 1996 PRA Press

The Journal of Supercritical Fluids, Vol. 9, No. 3, 1996

Synthesis of Titanium Dioxide

173

er + actants Figure

1.

Reactor schematics.

tainers were sufficiently tall to ensure that the rim of the containers was higher than the top portion of the viewing windows as shown in Figure 1. This configuration ensure that the contents of the two glass containers did not mix with each other upon filling the reactor with liquid CO2 to within the viewing level. The general procedure employed was as follows. An alkoxide was placed in one of the containers and an aqueous solution of surfactant was placed in the other container. The reactor was then flushed and filled with CO;? at 288 K to a level corresponding to the top portion of the viewing windows (110 mL). The contents of the containers were stirred by magnetic bars and a magnetic stirrer. The reactor was then heated to the desired temperature for a given time and then cooled to 293 K, vented, and opened. Particles were collected and examined by SEM, X-ray diffraction, and thermal analyses. The solubility of alkoxides in supercritical CO2 were measured in a high pressure view cell. A high pressure “syringe” was used to vary the volume and pressure. A sapphire window allows viewing by a fiber optic boroscope. The entire system was placed in a thermostated chamber. All solubility experiments were carried out at 3 13.0 + 0.1 K. Given amounts of an alkoxide and CO2 were weighed into the cell which was then heated and stirred. The pressure was increased until a homogeneous phase was observed. The pressure was then decreased slowly until a “dew point” was observed corresponding to conditions for phase separation of the alkoxide. CONTROL EXPERIMENTS The following control experiments were carried out to verify the ability of selected surfactants to stabilize aqueous dispersions in supercritical CO;!. An aqueous solution of AgNOs and a surfactant was placed in one of the glass containers and an aqueous solution of NaCl and the same surfactant was placed in the other. The solutions were stirred and the reactor was charged with CO2 and heated to 323 K, the pressure at this temperature was 9.9 MPa. This sequence was followed in all of the control experiments. The results indicated that when no surfactant or Aerosol AOT was used, no AgCl precipitate was observed, that is, no mixing occurred between the two solutions, that is, the solutions remained in their respective beakers. On the other hand, when the

Figure 2. Silver chloride particles prepared from aqueous solutions of AgNO? and NaCl and Zonyl FSJ in supercritical

CO*. Magnification 1,386 K (I cm = 7.2 nm). fluorinated surfactant Zonyl FSJ was used, fine particles of AgCl were found in the bottom of the reactor as well as in the two beakers, Figure 2. These particles are similar to particles prepared by the direct mixing of two microemulsion solutions, one containing AgNOs and the other containing NaC1.i i Dyes have been used in the past to ascertain the formation of stable micelles and microemulsions in supercritical fluids.‘,i2 The precipitation method adopted here is preferred over the dye solubility method because of the difficulties associated with observations through the reactor and potential interactions between the dye and carbonic acid, for example, thymol blue is bleached in the presence of CO2 and water. Another control experiment was carried out in which water was placed In one of the containers and titanium(IV) isopropoxide, Ti[OCH(CHs)&, in the other container. No surfactant was used. The reactor was charged with CO2 and heated to 323 K for 4 h. No particles were observed on the walls of the reactor, although a small amount of precipitate was observed in the water container due to diffusion of some titanium(IV) isopropoxide. RESULTS In a typical experiment, 2.0 g of titanium(IV) isopropoxide are placed in one of the glass containers, 1 .O g of water and 2.0 g of Zonyl FSJ were mixed and placed in the other container. The reaction procedure described in the experimental section was followed. The reactor was

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et al.

Figure 3. Scanning electron micrograph of titanium dioxide particles prepared from Ti[OCH(CHJ& and aqueous solution of Zonyl FSJ in supercritical CO,. Magnification 10 K(1 cm= I pm).

The Journal

of Supercritical

Fluids,

Figure 5. X-ray diffraction powder prepared in supercritical

pattern CO,.

Figure 6. TGA/DTA prepared in supercritical

Vol. 9, No. 3, 1996

of titanium

dioxide

results of titanium dioxide CO,. I O”/min in air.

powder

Figure 4. High resolution field emission scanning electron micrograph of particles shown in Figure 3. Magnification 200 K (1 cm = 50 nm). X-ray diffraction showed the particles to be poorly crystalline anatase, Figure

5. heated to 323 K for 4 h. The pressure increased during heating and reached at value of 9.9 MPa at 323 K. A

white powder was observed covering the walls of the reactor and the glass containers. No hydrolyzable liquid was found in the reactor indicating a complete reaction. SEM analysis indicate the formation of polydispersed spherical particles in the 0.1-2 pm range, Figure 3. The particles were also examined by a high resolution field emission SEM. Figure 4 taken at 200 K magnification shows that the particles exhibit a relatively smoother surface, compared to the particles shown in ref. 10. It is possible, however, that these particle could have been formed by agglomeration of very fine primary particles. X-ray diffraction analysis of this product showed the presence of poorly crystalline anatase phase (Figure 5).

Figure 7. X-ray analysis of phase transformations K as a function of heating-time of titanium dioxide prepared in supercritical CO*.

at 823 powder

Thermogravimetric and differential thermal analyses were used to study the weight loss and crystallization behavior of the powders. Typical results are depicted in Figure

6. The nroduct

loses about

10% of its weight

be-

The Journal of Supercritical Fluids, Vol. 9, No. 3, 1996

Figure 8. Scanning electron micrograph of titanium dioxide prepared by hydrolysis of Ti[OCH(CH3)& in a mixlure of pentanol and Zonyl FSJ. Magnification 10 K (1 cm = 1 pm).

Figure 9. Scanning electron micrograph of titanium dioxide particles prepared from titanium ethoxide and aqueous solution of Zonyl FSJ in supercritical CO*. Magnification, lOK(I cm= 1 pm).

tween 298 and 473 K. An additional 18% weight loss occurs between 473 and 573 K. Two exothermic peaks appear at about 573 and 773 K. To elucidate possible phase transformation corresponding to these exothermic peaks a hot stage kinetic X-ray experiment was carried out at 823 K. The results, Figure 7, indicate the appearance of a strong anatase peaks (at 20 = 25.29 and 37.80) within seconds of heating and a gradual appearance of the rutile phase peaks (at 28 = 27.38 and 36.08). A comparison of particles formed in the supercritical CO;! environment and particles formed by hydrolysis of titanium(IV) isopropoxide in a mixture of pentanol, Zonyl FSJ, and a small amount of water was carried out. Figure 8, shows that the particles formed in pentanol were not spherical. Several experiments were carried out in the supercritical fluid reactor in the temperature range of 323-

Synthesis of Titanium Dioxide

175

Figure 10. Scanning electron micrograph of titanium dioxide particles prepared from Ti[OCH,CH(C,H,)(CH,),CH,l, and aqueous solution of Zonyl FSJ in supercritical CO,. Magnification, 1 K (I cm = 10 pm).

368 K and pressure range of 10.3-18.6 MPa. The products obtained were similar to the one obtained at 323 K as determined by morphology, crystallinity, and thermal analyses. Experiments were also carried out with other titanium alkoxides. In the case of titanium(IV) ethoxide, Ti(OC2H5)2, the morphology of the particles obtained, Figure 9, were similar to those obtained with titanium(IV) isopropoxide, Figure 3. The reaction, however, was not complete after 4 h at 323 K, a hydrolyzable liquid was found in the beaker containing the ethoxide. Experiments with titanium(IV) butoxide, 2-ethylhexoxide, TWCHd2CH314. titanium(IV) Ti[OCH2CH(C;?Hs)(CH2)2CH3]2, or aluminum tri-sec-butoxide, Al[OCH(CH2)C2H5]3, did not lead to formation of spherical particles. Instead a mass of irregularly shaped particles was formed in the alkoxide container, Figure 10. Solubility data for these alkoxides in supercritical CO2 at 3 13.0 * 0.1 K are given in Table I. The solubility of these materials in supercritical CO2 appears to parallel the vapor pressure, decreasing as the vapor pressure decreases (indicated by the boiling points). Titanium(IV) ethoxide is known to form polymeric species, tetrameric in the solid state and trimeric in benzene solutions. On the other hand, the sterically hindered titanium(IV) isopropoxide, Ti[OCH(CH3)2]4, is monomeric. This may account for its high solubility in supercritical CO:!. Titanium(IV) butoxide and aluminum tri-see-butoxide are also known to form oligomers in the unhydrolyzed states. In the case of titanium ethoxide the reaction was not complete and only a few spherical particles of Ti02 were formed. Titanium ethoxide is more soluble than the butoxide (no spherical particles were formed) but is less soluble than the isopropoxide (all particlers were spherical). Thus, spherical particle formation in the supercritical phase under the conditions employed is clearly dependent on the solubility of the alkoxide.

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Tadros et al.

The Journal of Supercritical Fluids, Vol. 9, No. 3, 1996 TABLE Alkoxide

Ti

I

Alkoxide Weight %

f -/ 1 0

Dew Point, MPa

Boiling Point

4.25

8.2

505

K/101.0

KPa

4.19

12.1

423

K/l .3 KPa

3.05

18.6

479

K/l .3 KPa

473

W30

‘4

Ti iO-] 4

6.13

>62.0

mm

3

CONCLUSION Spherical particles of titanium dioxide can be formed in supercritical CO2 medium provided that the alkoxide is sufficiently soluble in CO* and an appropriate surfactant is used to stabilize a water dispersion in supercritical CO*.

(4)

Science (5)

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ACKNOWLEDGMENT This work was performed at Sandia National Laboratories, supported by the U.S. Department of Energy under contract number DE-A604-94AL8.5000.

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