Synthesis of Ultrafine TiO2 Particles from Hydrolysis of Ti(OiPr)4 with PEO-b-PFOMA Reverse Micelles in CO2

Studies in Surface Science and Catalysis 153 S.-E. Park, J.-S. Chang and K.-W. Lee (Editors) © 2004 Elsevier B.V. All rights reserved.

569

Synthesis of Ultrafine TiO2 Particles from Hydrolysis of Ti(O'Pr)4 with PEO-6-PFOMA Reverse Micelles in CO2 Kwon Taek Lima*, Ha Soo Hwanga, Seong-Soo Hongb, Chan Parkc, Won Ryood, and Keith P. Johnston a

Division of Image Sci. and Eng., Pukyong National University, Pusan 608-739, Korea. 'Division of Applied Chemical Eng., Pukyong National University, Pusan 608-739, Korea. c Division of Materials Sci. and Eng., Pukyong National University, Pusan 608-739, Korea. d Department of Chemical Eng., The University of Texas at Austin, Austin, TX 78712, USA Ultrafine titanium dioxide particles were produced by the controlled hydrolysis of titanium tetraisopropoxide (TTIP) in hydrated reverse micelles formed in CO2 using poly (ethylene oxide-WocA> 1H,1H perfluorooctyl methacrylate) (PEO-6-PFOMA) as a surfactant. The size of the particles and the stability of dispersions in CO2 were affected by the molar ratio of water-to-surfactant head group (w0), concentration of reactants, and the composition of the polymeric surfactant. 1. INTRODUCTION There has been much interest in procedures to prepare and stabilize uniform nanoparticles from microemulsions [1-3]. Previously we have reported the use of w/c microemulsions stabilized by PFPECOO"+NH4 and PDMAEMA-6-PFOMA to synthesize nanosized titanium dioxide which has attracted much attention as photocatalyst [4,5]. Stable colloids were formed from low-water content reverse micelles, and PFOMA provided greater steric stabilization against particle aggregation in CO2. A major advantage of colloids in CO2 is the ability to break the colloidal solution simply by reducing the pressure, allowing for products. The present work describes the controlled formation of TiO2 nanoparticles from the reaction of titanium tetraisopropoxide (TTIP) in hydrated reverse micelles in CO2 using nonionic type surfactant PEO-&-PFOMA. The formation of particles is monitored visually and with DLS and TEM to investigate the effects of surfactant concentration and w0 on the particle size and the stability of the colloidal solutions. 2. EXPERIMENTAL 2.1. Materials

570

TTIP (Aldrich) and research grade CO2 (Daeyoung Co., 99.99 %) were used as received. Poly (ethylene oxide-block- 1H,1H perfluorooctyl methacrylate) (PEO-6-PFOMA) (Fig. 1) was prepared by atom transfer radical polymerisation [6]. 2.2. Preparation of TiO2 In a typical nanoparticle preparation, 0.12g of PEOo.55K-6-PFOMA15K and 8.6mg of water were placed in 28ml-variable volume view cell [4], and 6.62g of liquid CO2 was introduced using a syringe pump (ISCO, 260D). The vessel was pressurized to 280 bar by backside pressure of the moving piston in the reactor. After the formation of reverse micelles with magnetic stirring, 0.0355ml of TTIP charged in a sample loop of a 6-port valve was slowly added into the cell. After stirring the reaction mixture for 24 h, CO2 was vented, and the product was collected. The prepared nanoparticles were washed with ethanol, dried at 105 °C for 1 day, and calcined at 500 °C for 3 h. 2.3. Measurements The sizes of reverse micelles and TiC<2 particles in CO2 were measured by DLS with a remote cell connected to the view cell reactor. The particle size and morphology were observed with transmission electron microscopy (TEM, JEOL, JEM-2020) for a Fig. 1. Structure of PEO-6-PFOMA

200 kV accelerating voltage.

3. RESULTS AND DISCUSSION Table 1 summarizes the results of particle formation with different reactant concentrations and w0 values. Because a certain amount of water can dissolve in bulk CO2, for example 0.14 wt% at 276 bar and 25 °C and the added water is below the solubility limit in pure CO2

571 Table 1 Ultrafine TiO2 particles 8 from hydrolysis of TTIP in W/C microemulsions stabilized by PEO-6-PDHFOMA Concentration Surfactant

[TTIP] (mM)

wob

Size by DLS (nm)

Surfactant

Water

Reverse

wt%

wt%

micelles 28.1

Size by TEM (nm)

Particle

Particle0

34.3

23

PEO2K-5-

7.2

2

0.50

0.055

PDHFOMA13K

14.4

5 2

0.40

0.11

Cloudy

1.9

0.055

Deep brown

5 5

0.76

0.055

Deep brown

17.4

1.8

0.13

Cloudy

14 17 24

7.2

5

1.00

0.13

Cloudy

21

PEOo.55K-6-

7 9

PDHFOMA15K

PDHFOMA8K a

Reaction conditions: R = 4, 280 bar, 25 °C,

b

[H2O]/[EO] in PEO block, uncorrected value,

"After calcination at 500 °C for 3 hrs. without surfactant, it may be expected that the hydrophilic surfactant head groups are partly hydrated in the core of the reverse micelles and the CO 2 -philic tails interact with carbon dioxide. The tails are sufficiently solvated by CO2 to minimize micelles interactions. The reverse micelles with 0.40 wt% of PEOnc-fr-PDHFOMAmc for w 0 = 5 took dark orangish tint, and turned cloudy after the precursor was injected. The instability became more pronounced as water content increased, which is mainly ascribed to insufficient steric stabilization. The larger w 0 produces stronger micelle-micelle interactions resulting in increased particle flocculation. The larger w 0 may also speed up the hydrolysis rate leading to faster nucleation and growth, thus greater flocculation in a less controlled manner. With reduced amount of water w 0 = 2, a clear micellar solution was formed. The solution became dark orangish even after the precursor was injected. Hence it is obvious that low-water content PEO-&-PFOMA can suppress the growth and aggregation of TiO 2 particles effectively. The particle size increased with w 0 as well as the concentration of reactants. The size distribution of the particles in the reactor was measured by DLS. The particle size of the injection of TTIP increased compared to the size of the reverse micelles, which may be ascribed to the particle growth and to reorganization of surfactants about the growing particles. The surfactant stabilizes the small ionic particles produced from hydrolysis of TTIP either by adsorption or by preferential distribution into the micellar cores, and should provide steric stabilization against colloidal aggregation. XRD measurements showed that the particles as prepared were amorphous,

while the

572

(A)

(B)

Fig. 2. TEM micrographs of TiO2 particle prepared in reverse micelles of PEO2K-6particles calcined at 500 °C for 3 hrs were identified to be in the anatase form.

Fig. 2

depicts TEM micrographs of T1O2 particles obtained from different PEO-Z>-PFOMA surfactants. It is apparent that the particles from

PEO2K-&-PFOMAI3K

are larger and more

polydisperse than those from PEO0.55K-6-PFOMA15K. A plausible explanation might be the fact that lower PFOMA block copolymer has higher hydrophilic / CCh-philic balance, thus provides less stability to hydrolysis products. 4. CONCLUSION T1O2 nanoparticles were produced by the controlled hydrolysis of TTIP in PEO-fr-PFOMA reverse micelles. For the hydrolysis of TTIP in low-water content reverse micelles in CO2, the TiO2 particles were stable and optically transparent. Greater control of particle formation was achieved by w0 and the concentration of reactants. The important parameters controlling particle size and aggregation were w0, the concentration of reactants, and the chemical composition of surfactants. REFERENCES 1. V. Pillai, D.O. Shah, C. Soalns, and H. Kunieda, (Eds.), Industrial Application of Microemulsion, Marcel Dekker, New York, (1997) p.227 2. E. Stathatos, P. Lianos, F.D. Monte, D. Levy, and D. Tsiourvas, Langmuir, 13 (1997) 4295 3. M. S. Lee, G.-D. Lee, and S.-S. Hong, J. Ind. Eng. Chem., 9 (2003) 412 4. K. T. Lim, H. S. Hwang, M. S. Lee, G. D. Lee, S. -S. Hong, and K. P. Johnston, Chem. Comm., 14(2002)1528 5. K. T. Lim, H. S. Hwang, W. Ryoo, and K. P. Johnston, Langmuir, 20 (2004) 2466 6. K. T. Lim, M. Y. Lee, M. J. Moon, G. -D. Lee, S. -S. Hong, J. L. Dickson, and K. P. Johnston, Polymer, 43 (2002) 7043