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Langmuir 1998, 14, 2208-2212
Preparation of Ultrafine Titanium Dioxide Particles Using Hydrolysis and Condensation Reactions in the Inner Aqueous Phase of Reversed Micelles: Effect of Alcohol Addition Hideki Sakai,*,†,‡ Hajime Kawahara,† Masuo Shimazaki,† and Masahiko Abe†,‡ Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278, Japan, and Institute of Colloid and Interface Science, Science University of Tokyo, 1-3, Kagurazaka, Shinjuku, Tokyo 156, Japan Received August 25, 1997. In Final Form: December 18, 1997 An attempt was made to prepare ultrafine titanium dioxide (TiO2) particles in the nanometer range by making use of the hydrolysis and condensation reactions of tetraisopropylorthotitanate (TTIP) in the inner aqueous phase of Aerosol OT reversed micelles formed in cyclohexane. Addition of linear alcohols (C2 to C6) to TTIP was found to affect the size of the titanium dioxide particles formed. The particle size was strongly dependent on the alkyl chain length and the amount of the alcohols. Addition of alcohols with the alkyl chain length smaller than 1-butanol increased the size of the particles over that prepared without alcohol addition, while addition of alcohols with a moderately long alkyl chain, like 1-pentanol and 1-hexanol, caused a decrease in the particle size. In particular, addition of 1-hexanol was found to yield TiO2 particles with a best dispersibility and sizes of around 10 nm.
1. Introduction It is well-known that when the size of semiconductor particles is reduced to the extent that the proportions of the surface region and bulk region of the particle are comparable, their energy band structure becomes discrete and they exhibit chemical and optical properties quite different from those of the bulk body. This effect is called the quantum size effect1 and is observed mainly with particles of diameters less than 10 nm, though it depends on their effective mass. The quantum size effect is also observed with fine titanium dioxide (TiO2) particles which have recently attracted much attention as a photocatalyst. When the size of TiO2 particles is decreased to that of the nanometer scale, their catalytic activity is enhanced because the optical band gap is widened due to the quantum size effect, combined with the increased surface area.l However, monodisperse TiO2 particles in the nanometer range are hardly prepared. This has made it difficult to correlate correctly their optical characteristics with their particle size. Hence, investigations are now being made briskly on how to prepare monodisperse TiO2 particles in the nanometer range. The reversed micelle method is one of the methods of preparing monodisperse ultrafine particles of metals and metal oxides, in which monodisperse ultrafine particles are prepared by making use of the inner aqueous phase of reversed micelles as the reaction field.2 The attempts made so far to prepare ultrafine particles by the reversed micelle method include those by Boutonnet et al. for Pt, Pd, Rh, and Ir,3 by Kon-no et al. for Fe3O4 and CaCO3,4 * To whom all correspondence should be addressed. † Faculty of Science and Technology, Science University of Tokyo. ‡ Institute of Colloid and Interface Science, Science University of Tokyo. (1) Kariyone, T.; Anpo, M.; Chiba, K.; Tomonari, M. Hyomen (Surface), 1991, 29, 156. (2) Kon-no, K. In Function and Technology of Emulsification and Dispersion; Kariyone, T., Hidaka, T., Koishi, M., Omi, S., Amano, H., Eds.; Science Forum: Tokyo, 1995; p 112.
and by Nagy et al. for Ni2B and Co2B,5 all of which gave fine particles of diameters in the nanometer range and good monodispersity. Although this method has also been tested to see if it can yield monodisperse ultrafine TiO2 particles,6,7 it has failed to give monodisperse particles unless the alkoxide concentration is low because of a very fast rate of the reaction of titanium alkoxide with water. These circumstances have prompted us to prepare ultrafine TiO2 particles of a good monodispersity in high concentrations by the reversed micelle method with some modifications. Thus, the effect of alcohol addition was studied in detail on the size of TiO2 particles prepared in the present work. 2. Experimental Section 2.1. Materials. 2.1.1. Surfactant. Sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT, abbreviated hereafter to AOT; Aldrich) was used as supplied. 2.1.2. Oil Phase. The oil phase used was cyclohexane (Tokyo Kasei Kogyo) dehydrated with a molecular sieve (Wako Pure Chemical Industries). 2.1.3. Aqueous Phase. Water (Japanese Pharmacopoeia distilled water for injection, Ohtsuka Pharmaceutical Co.) was used as purchased as the aqueous phase. 2.1.4. Titanium Alkoxide. Tetraisopropylorthotitanate (TTIP; Tokyo Kasei Kogyo) or titanium(IV) 2-ethyl-1-hexanolate (Wako) was used as supplied as the titanium alkoxide. 2.1.5. Alcohols. The alcohols with different chain lengths used were 2-propanol (Tokyo Kasei Kogyo), 1-butanol (Tokyo Kasei Kogyo), 1-pentanol (Tokyo Kasei Kogyo), 1-hexanol (Tokyo Kasei (3) Boutonnet, M.; Kizling, J.; Stenius, P.; Maire, G. Colloids Surf. 1982, 5, 209. (4) (a) Gobe, M.; Kon-no, K.; Kandori, K.; Kitahara, A. J. Colloid Interface Sci. 1983, 93, 293. (b) Kandori, K.; Kon-no, K.; Kitahara, A. J. Colloid Interface Sci. 1988, 122, 78. (5) (a) Nagy, J. B.; Gourgue, A.; Derouane, E. G. Stud. Surf. Sci. Catal. 1983, 16, 193. (b) Ravet, I.; Nagy, J. B.; Derouane, E. G. Stud. Surf. Sci. Catal. 1987, 31, 505. (6) Hirai, T.; Sato, H.; Komasawa, I. Ind. Eng. Chem. Res. 1993, 32, 3014. (7) Arriagada, F. J.; Osseo-Asare, K. In Refractory Metals: Extraction, Processing and Applications; Liddell, K., Sadway, D. R., Bautista, R. G., Eds.; TMS-AIME: Warrendale, PA, 1991; pp 259-269.
S0743-7463(97)00952-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/28/1998
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Scheme 1. Schematic Diagram for Preparing TiO2 Ultrafine Particles Utilizing Reversed Micelles
Table 2. Change in Size of TiO2 Particles When TTIP Premixed with 1-Hexanol Is Added to Reversed Micellar Solution (h ) 5) a ([1-hexanol]/ [TTIP]) 0 3 6 12 a
Table 1. Change in Size of TiO2 Particles Obtained as a Function of AOT Concentration and h ([H2O]/[Alkoxide]) change per given concn of AOT/(wt %) h
1
3
5
10
1 5 10
precipitation precipitation precipitation
precipitation 800 nm 31 nm
precipitation 170 nm 20 nm
precipitation 17 nm 16 nm
Kogyo), and 2-ethylhexanol (Tokyo Kasei Kogyo). All of these alcohols were dehydrated with a molecular sieve (Wako) before use. 2.2. Experimental Methods. 2.2.1. Preparation of Ultrafine TiO2 Particles. The procedures for preparing ultrafine TiO2 particles are shown in Scheme 1. First, AOT and water were added to cyclohexane to make the AOT concentration in the system 3, 5, and 10 wt %, keeping the amount of water constant (when the AOT concentration was 1 wt %, R ) [moles of water]/[moles of AOT] ) 5). The mixed solution was left standing for 1-2 days at 30 °C after being stirred with a vortex mixer to allow formation of reversed micelles of AOT. To this reversed micellar solution was added each of the mixed solutions of titanium alkoxide and alcohol at given mixing ratios (h ) [moles of water]/[moles of alkoxide] ) 5, a ) [moles of alcohol]/[moles of alkoxide] ) 0, 12, and 24) prepared in nitrogen atmosphere. Ultrafine TiO2 particles were obtained after the mixture was stirred with the vortex mixer. 2.2.2. Measurement of Particle Size and Structure. The size of ultrafine TiO2 particles was measured at 30 °C by the dynamic light scattering method using a light scattering measuring apparatus (Malvern Instruments, System 4700C submicron particle analyzer) and with an atomic force microscope (AFM, SEIKO Instruments, SPI3800). The light source in light scattering measurements was an argon laser (Coherent; maximum output power, 5 W; wavelength, 488 nm). The samples for AFM observations were prepared as follows. A few drops of suspension of ultrafine TiO2 particles were placed on a glass substrate and air-dried. The crystal structure of the obtained TiO2 particles before and after heat treatment was analyzed by using the X-ray diffraction method. 2.2.3. Verification of Interchange Reaction between Titanium Alkoxide and Alcohol. The interchange reaction between the alkyl group of titanium alkoxide and that of alcohol was pursued with an NMR spectrometer (JEOL, JNM-PMX60si). TTIP and 1-hexanol were used as the alkoxide and alcohol, respectively, in this experiment.
3. Results and Discussion 3.1. Effects of Concentrations of AOT and Alkoxide on the Size of TiO2 Particles. Table 1 shows the size of TiO2 particles obtained as a function of AOT concentration and the molar ratio of water to TTIP, h,
change per given concn of AOT/(wt %) 1 3 5 10 precipitation precipitation precipitation precipitation
800 nm 950 nm 180 nm 34 nm
170 nm 160 nm 23 nm 12 nm
17 nm 34 nm a a
Limit of detection.
when the alkoxide is added to each of the reversed micellar solutions of AOT in cyclohexane (1, 3, 5, and 10 wt %) to give the molar ratios (h) of 1, 5, and 10. Here, the amount of water is kept constant (for example, when the AOT concentration was 1 wt %, the molar ratio of water to AOT, R, was 5. X-ray diffraction measurements show that as prepared particles are amorphous, while the particles calcined at 500 °C for 1 h are anatase form. When AOT concentration was low (1 wt %) and the alkoxide concentration was high (h ) 1), the flocculation rate of TiO2 particles formed was high and white precipitates were observed. In the other conditions, the solution became bluish white or transparent, indicating the formation of particles with high dispersibility. As seen from the table, the size of TiO2 particles decreases with increasing AOT concentration. When h ) 10, for instance, the particle size was 31 nm at an AOT concentration of 3 wt % while it was reduced to 20 nm at a higher AOT concentration of 5 wt %. The molar ratio R became smaller when the AOT concentration rose since the amount of water was kept constant in all cases in this work. Hence, the size of reversed micelles, the reaction field, also became smaller as the AOT concentration rose, presumably resulting in a decrease in the size of TiO2 particles formed in the reversed micelles. Water molecules in the inner aqueous phase of reversed micelles are different from those in the bulk phase and all are bound strongly to the hydrophilic groups of the AOT molecules when R is lower than 5.8 This probably makes the rate of reaction of water with TTIP slower than that of bulk water, thereby suppressing the growth of TiO2 particles because of the retarded reaction rate of the alkoxide. The table also shows that the growth of TiO2 particles can be suppressed by raising h. At an AOT concentration of 5 wt %, for example, the particle size was 170 nm when h ) 5, whereas the size decreased to 20 nm if h ) 10. This would be caused by a decreased number of TiO2 particles due to an increase in h and hence a decrease in TTIP concentration. This in turn lowers the probability of particle collision to repress particle flocculation and coalescence. 3.2. Effect of Alcohol Addition on the Size of TiO2 Particles. The effect of alcohol addition was examined on the size of TiO2 particles prepared by the reversed micelle method. Alcohol (2-propanol) is formed also by hydrolysis and condensation reaction of TTIP. In this study, the amount and alkyl chain length of added alcohol were varied and the effect on the size of produced TiO2 particles was studied. TiO2 particles were prepared in the following manner. At a fixed value of 5 for h, 1-hexanol was added to TTIP in the proper amounts to make a ()[moles of alcohol]/[moles of TTIP]) of 0, 3, 6, and 12, and these mixtures were added to reversed micellar solutions of various concentrations to initiate particle formation. The results obtained are shown in Table 2. Although no decrease in the particle size was observed when a was (8) Kawai, T. Hyomen 1996, 34, 578.
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Table 3. Size of the TiO2 Particles When the Alkoxide-Alcohol Premixed Solution Is Added to the AOT Reversed Micellar Solution (Water/Alkoxide/Alcohol Molar Ratio, 1:5:24) change per given concn of AOT/(wt %) alcohol
1
3
5
10
2-propanol 1-butanol without alcohol 1-pentanol 1-hexanol
precipitation precipitation precipitation precipitation precipitation
precipitation precipitation precipitation precipitation 130 nm
precipitation precipitation 590 nm 170 nm 18 nm
precipitation 820 nm 57 nm 32 nm 11 nm
Scheme 2. Interchange Reaction of Tetraisopropylorthotitanate with Alcohols
Figure 1. AFM image of fine TiO2 particles deposited onto glass substrate. The TiO2 particles were prepared with R([H2O]/ [AOT]) ) 0.45, h ([H2O]/[TTIP] ) 5, and a ([1-hexanol]/[TTIP]) ) 24.
increased from 0 to 3, further increase in a to 6 and 12 or in 1-hexanol addition produced a decrease in the particle size. For example, when the AOT concentration was 5 wt %, the particle size was 160 nm at a ) 3 while it was 23 nm at a ) 6. Investigation was then made on how the alkyl chain length of alcohol affects the particle size. Table 3 shows the sizes of TiO2 particles prepared using the hydrolysis and condensation reactions for mixtures of TTIP and alcohols. The alcohols used were 2-propanol, 1-butanol, and 1-pentanol, in addition to 1-hexanol. Each of these alcohols was mixed with TTIP to give a value of 24 for a, and each of the mixtures was added to the reversed micellar solution, the alkoxide concentration being kept at h ) 5. The table reveals that the addition of 1-butanol increases the size of TiO2 particles over that of those prepared without alcohol addition, while the addition of alcohols with a moderately long alkyl chain such as 1-pentanol or 1-hexanol yields very fine TiO2 particles. In particular, 1-hexanol addition gives TiO2 particles of a best dispersibility and with diameters of about 10 nm even at relatively high TTIP concentrations. Figure 1 is an AFM image of a sample of TiO2 particles prepared under the above conditions. The sample was made by placing a few drops of a suspension of the TiO2 particles on a glass substrate and air-drying the drops. The AFM image indicates a nearly monodisperse size distribution of the TiO2 particles around a 10 nm diameter. X-ray diffraction measurements show that as prepared TiO2
particles are amorphous, while the particles calcined at 450 °C for 2 h are anatase form. 3.3. Factors Involved in the Effect of Alcohol Addition on the Size of TiO2 Particles. 3.3.1. Effect of the Polarity of Solvent. Since the surface of TiO2 is hydrophilic, it is thought that TiO2 particles tend to flocculate in nonpolar solvents.9 That is, an increase in the polarity of the solvent caused by alcohol addition may be a reason for the decreased particle size of TiO2 brought about by the addition of alcohols such as 1-hexanol. The effect of solvent polarity was then examined on the size of TiO2 particles formed. First, TiO2 particles were prepared using mixtures in various ratios of TTIP and 2-hexanone, a nonalcoholic polar solvent. Table 4 shows the sizes of TiO2 particles in relation to the dielectric constants of the mixed solvents, indicating that 2-hexanone addition to the system has practically no effect on the particle size even though it causes an increase in the dielectric constant of the solvent, in contrast to the size decreasing effect of 1-hexanol addition. When TiO2 particles were prepared with the addition of 2-propanol, the alkyl chain length of which is the same as that of TTIP, to the system, the particle size was found to increase with the increasing amount added of the alcohol, contrary to expectation (Table 5). This would be due to an increase in the proportion of free water, resulted from a decrease in the proportion of the water strongly bound to the hydrophilic groups of reversed micelles, because 2-propanol is reported to be solubilized in reversed micelles.10 What have so far been mentioned demonstrate that the polarity of mixed solvents affects insignificantly the decrease in the size of the TiO2 particles formed. 3.3.2. Effect of Interchange Reaction between Titanium Alkoxide and Alcohol. The interchange reactions between titanium alkoxides and alcohols were studied. As shown in Scheme 2, the interchange reaction between an alkoxide and an alcohol is the reaction in which they interchange their alkyl groups.11 Increase in the number of carbon atoms or branches in the alkyl group of the alkoxide is known to cause, in general, a decrease in the rate of the reaction of the alkoxide with water.12 Hence, if the interchange reaction occurs between TTIP and 1-hexanol, the growth of TiO2 particles is expected to be suppressed. An attempt was then made to monitor by means of lH NMR the interchange reaction between TTIP and 1-hex(9) Kiyono, M. Titanium Dioxide; Gihoudo: Tokyo, 1991; p 224. (10) Menassa, P.; Sandorfy, C. Can. J. Chem. 1985, 63, 3367. (11) Mehrotra, R. C. J. Non-Cryst. Solids 1988, 100, 1. (12) Materials Science Society of Japan, Ed. Ultrafine Particle as Materials; Shokabo: Tokyo, 1993; p 140.
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Langmuir, Vol. 14, No. 8, 1998 2211
Table 4. Effect of 2-Hexanone Addition on the Size of TiO2 Particles When TTIP Is Used as Titanium Alkoxide change per given [2-hexanone]/[TTIP] (dielectric constant) concn of AOT/(wt %)
0 (2.05)
4.67 (2.16)
10.1 (2.28)
18.7 (2.48)
1 3 5 10
precipitation precipitation 180 nm 16 nm
precipitation 680 nm 110 nm 16 nm
precipitation 740 nm 170 nm 16 nm
precipitation 550 nm 190 nm 17 nm
Table 6. Effect of 1-Hexanol Addition on the Size of TiO2 Particles When Ti[OCH2CH(C2H5)(CH2)3CH3]4 Is Used as Alkoxide (Measured 1 Day after Addition of Alkoxide) change per given concn of AOT/(wt %) 1 3 5 10
a ([1-hexanol]/ [TTIP]) 0 12
precipitation precipitation
28 nm 950 nm
7 nm 100 nm
5 nm 8 nm
Table 7. Effect of 2-Ethylhexanol Addition on the Size of TiO2 Particles When Ti[OCH2CH(C2H5)(CH2)3CH3]4 Is Used as Alkoxide (Measured 7 days after Addition of Alkoxide) change per given concn of AOT/(wt %) 1 3 5 10
a ([1-hexanol]/ [TTIP]) 0 12 24 a
Figure 2. 1H NMR spectra of TTIP/1-hexanol mixed solution: (a, top) a ) [1-hexanol]/[TTIP] ) 1; (b, bottom) a ) 12. Table 5. Effect of 2-Propanol Addition on the Size of TiO2 Particles When TTIP Is Used as Titanium Alkoxide concn of AOT/(wt %) 1 3 5 10
change per given [2-propanol]/[TTIP] (dielectric constant) 0 (2.05) 3.76 (2.18) 7.56 (2.28) precipitation 510 nm 170 nm 14 nm
precipitation precipitation 1800 nm 75 nm
precipitation precipitation precipitation 250 nm
anol. Here, a slight difference in the peak position due to CH of the isopropyl group between the alkoxide and alcohol was used to distinguish them from each other. In Figure 2a is shown the result obtained when a (the molar ratio of the alcohol to TTIP) was 1. The existence of the CH peaks of both alkoxide and alcohol in the figure demonstrates that the interchange reaction really took place. Figure 2b shows the result obtained at a ) 12, which is identical with the value used in the preparation of TiO2 particles in this work, exhibiting only one CH peak. This should be brought about by complete replacement of the isopropyl groups of titanium alkoxide molecules by the n-hexyl groups of 1-hexanol molecules. Hence, it is certain that the isopropyl groups of titanium alkoxide molecules are completely interchanged with the n-hexyl groups of 1-hexanol molecules in the preparation conditions of TiO2 particles adopted in the present work. The effect of 1-hexanol addition was also examined when titanium 2-ethyl-1-hexanolate with a long (C6) and branched alkyl group was used as the alkoxide. If interchange reaction takes place between the alkoxide and 1-hexanol in this system, the alkyl group of the alkoxide changes to an n-hexyl group that has no branch. Hence, if the interchange reaction would occur, 1-hexanol addition was expected to cause the size of TiO2 to increase. The results are shown in Table 6, where the size of the
precipitation precipitation precipitation
180 nm 230 nm 55 nm
14 nm 10 nm a
a a a
Limit of detection.
TiO2 particles increases with an increasing amount added of 1-hexanol. Consequently, it is clear that the interchange reaction between alkoxide and 1-hexanol significantly affects the size of TiO2 particles prepared by the reversed micelle method. 3.3.3. Effect of Alcohols that Penetrate into the Palisade Layer of Reversed Micelles. As mentioned before, the addition of 2-propanol to TTIP caused the size of TiO2 particles to increase over that of those prepared without alcohol addition. This may be ascribed to the solubilization of 2-propanol into reversed micelles, thus reducing the proportion of water strongly bound to the hydrophilic groups of the micelles (bound water) and hence increasing the proportion of free water which is more reactive with the alkoxide than bound water. On the other hand, since alcohols with a moderately long alkyl chain such as 1-pentanol and 1-hexanol adsorb in the vicinity of the hydrophilic groups of reversed micelles (palisade layer),13 it may also be possible that the molecular packing in reversed micelles is tightened by the alcohol to slow the molecular exchange rate between micelles and monomers, thereby reducing the rate of particle growth. Preparation of fine TiO2 particles was then conducted by using 2-ethylhexanol, which is known to penetrate into the palisade layer14 as the alcohol and titanium 2-ethyl-1hexanolate with the same alkyl chain length as that of the alcohol as the alkoxide. Table 7 indicates that the size of TiO2 particles is decreased when a ) 24, whereas no change in the size of TiO2 particles is observed when a ) 0 and 12. This suggests that alcohols that penetrate into the palisade layer have a suppressing effect on the growth of TiO2 particles. 4. Conclusions The effect of alcohol addition was examined on the size of TiO2 particles prepared by using the hydrolysis reaction of alkoxides in the inner aqueous phase of reversed surfactant micelles, and the following results were obtained. (13) D’Aprano, A.; Donato, I. D.; Pinio, F.; Liveri, V. T. J. Solution Chem. 1989, 18, 949. (14) Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 3543.
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(1) The effect of the polarity of the solvent used was insignificant on the decrease in the size of TiO2 particles. (2) Addition of an alcohol such as 1-hexanol to the solvent suppressed the growth of TiO2 particles through a reduction in the rate of alkoxide hydrolysis reaction brought about by the interchange reaction of the alkyl groups of the alcohol and the alkoxide.
Sakai et al.
(3) Alcohols that penetrate into the palisade layer of reversed surfactant micelles were suggested to suppress the growth of TiO2 particles by decreasing the rate of molecular exchange between micelles and monomers. LA970952R
