J. Phys. Chem. C 2008, 112, 18445–18454
18445
Formation Mechanism of Amorphous TiO2 Spheres in Organic Solvents 3. Effects of Water, Temperature, and Solvent Composition Takashi Kojima† and Tadao Sugimoto*,‡ Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan ReceiVed: April 5, 2008; ReVised Manuscript ReceiVed: September 30, 2008
In a sol-gel system for the formation of spherical hydrous titania particles by hydrolysis of titanium butoxide (TBO) in a mixed solvent of n-butanol and acetonitrile with ammonia at 25 °C, effects of water content, temperature, and solvent composition have been studied. Water and ammonia were admixed with TBO in ca. 1 s, and the hydrolysis of TBO and about 80% of the subsequent precipitation of the hydrolysis product were completed earlier than 2.5 s in a standard homogeneous system with a butanol/acetonitrile (vol. ratio 1:1) solvent, as found in the preceding study of this series. In the standard system, dramatic size reduction was observed with increase in the content of water. The marked size reduction was explained in terms of enhanced nucleation of the hydrolysis product by the increase in supersaturation ratio due to the drop of the solubility of the hydrated hydrolysis product with the excess water. But, even though the temperature was raised up to 80 °C from 25 °C, the final mean particle size in the standard system (ca. 0.45 µm) was virtually left unchanged. From this striking result, it was concluded that the nucleation of the hydrolysis product started after the complete hydrolysis of TBO in the homogeneous system. The homogeneous system turned to a heterogeneous TBO emulsion system under agitation, when the temperature was lowered to 5 °C, the volume fraction of acetonitrile in the mixed solvent was raised to 0.6 or higher, or butanol was replaced by a higher alcohol such as hexanol or octanol. In the emulsion systems, titania particles were found to be formed in each solvent phase by dissolution of the TBO droplets, but their size distribution was broad due to constant nucleation near the surfaces of the TBO droplets. Also, dramatic reduction of the final particle size with the increasing fraction of acetonitrile was observed as well, as explained in terms of the constant nucleation near the surfaces of TBO droplets in addition to the lowered growth rate of the generated nuclei with the reduction of the solubility of the hydrolysis product due to the low affinity of acetonitrile to the butanol-solvated hydroxide monomer. A possibility of direct conversion of the alkoxide droplets to titania particles was also discussed. Introduction series,1,2
we have In the preceding Parts 1 and 2 of this reported the effects of ammonia on the particle formation of hydrous titania spheres and their formation mechanism in a homogeneous sol-gel system by hydrolysis of titanium butoxide (TBO) in a mixed solvent of butanol/acetonitrile (vol. ratio 1:1) at 25 °C. In this Part 3, we will deal with effects of water, temperature, and solvent composition in this sol-gel system, but specifically focusing on the particle formation in heterogeneous emulsion systems of TBO that are known to be created by control of the solvent composition. As to the precipitation of amorphous metal oxide spheres by hydrolysis of metal alkoxides in the heterogeneous systems, Hardy et al.3,4 developed a new method for fabrication of ceramic powders such as TiO2, ZrO2, and Al2O3 as amorphous spheres by hydrolysis of corresponding metal alkoxide emulsified in an organic solvent such as acetonitrile (AN). In this procedure, water was added immediately after emulsification of alkoxides by ultrasonication, and then the emulsion system * To whom correspondence should be addressed. E-mail: tdosugimoto@ pop06.odn.ne.jp. † Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan. ‡ Manazuru Institute for Superfine Particle Science, Manazuru 1912-4, Manazuru-machi, Kanagawa 259-0201, Japan.
with water was left undisturbed. Here, the added water was assumed to partly hydrolyze each surface molecule of the alkoxide droplets, followed by diffusion of the partly hydrolyzed alkoxide molecules into the interior of each droplet and the subsequent solidification of the alkoxide droplets through condensation of the partly hydrolyzed alkoxide molecules with unhydrolyzed internal ones. Since it is necessary to mostly complete the reaction before the emulsion returns to the original two separate bulk phases, the entire reaction is likely to have finished at latest within several minutes. They also prepared amorphous powders of TiO2, ZrO2, Al2O3, and bimetallic Al-Ti, Al-Zr, and Zr-Y hydrous oxides in formamide, acetonitrile, or propylene carbonate in a similar manner.4 The emulsification of alkoxides and the precipitation of hydrous metal oxides in the form of dispersion of spherical particles can be achieved at the same time by using a relatively poor solvent for both alkoxides and their hydrolysis products. They invented this method with the expectation that these particles would be produced through direct hydrolysis of the alkoxide droplets by added water, so that the excessively rapid hydrolysis of metal alkoxides would be reduced to a controllable level and a uniform composition in each composite particle would be achieved even if prepared from a mixture of alkoxides entirely different in hydrolysis rate. Actually, the size distributions of all these particles were very broad, as expected from the broad size distributions of the emulsified alkoxide droplets and, at least,
10.1021/jp802957e CCC: $40.75 2008 American Chemical Society Published on Web 11/04/2008
18446 J. Phys. Chem. C, Vol. 112, No. 47, 2008 the mean size and size distribution of some product particles appeared consistent with those predicted from direct hydrolysis of the alkoxide droplets. On the other hand, Hardy et al.3 found no precipitation of hydrous oxide particles by hydrolysis of composite alkoxide SrTi(OBu)6 (0.025 M) dissolved in butanol (BuOH) with 0.2 M H2O, but they observed definite precipitation of uniform SrTiO3 particles of ca. 0.1 µm in mean diameter when 25 wt % of BuOH solvent was replaced by AN in this system. This is obviously due to the reduction of solubility of the hydroxide monomer in BuOH by AN. But the starting solution of 0.025 M SrTi(OBu)6 in the mixed solvent of BuOH/AN (wt ratio 3:1) is a homogeneous solution in which SrTi(OBu)6 is completely dissolved. Nevertheless, if the proportion of AN in the mixed solvent of BuOH/AN is more increased, an alkoxide phase is expected to be finally separated to form a heterogeneous system, due to the reduction of the solubility of alkoxide in BuOH by AN as another effect of AN. Using such heterogeneous emulsion systems of alkoxide with mixed solvents of acetonitrile and different alcohols, Ogihara et al. and Mizutani et al. fabricated many kinds of fairly uniform hydrous metal oxide spheres such as Al2O3,5-9 ZrO2,5 TiO2,5 Nb2O5,5 Ta2O5,5 Fe2O3,10 SnO2,11 BaTiO3,12,13 SrTiO3,13 PbTiO3,13 Y2O3-Al2O3,14 ZrO2-Al2O3,14 and Al2O3-SiO2.15 These uniform particles were obtained in alkoxide emulsions with mixed solvents of alcohol/acetonitrile at some specific mixing ratios. Although the size distributions of these particles were much narrower than those of Hardy et al. prepared in emulsion systems, the formation of all of these uniform particles was also explained by direct conversion of the alkoxide droplets by hydrolysis based on permeation of water into each alkoxide droplet. However, if the fairly uniform particle were produced by the same direct conversion mechanism, some rational reasoning may be needed for the great discrepancies in size distribution from the product particles of Hardy et al. or even from the other polydisperse ones of Ogihara et al. themselves prepared in different solvents by the same emulsion method, because coefficients of variation ()relative standard deviation) in the size distribution of emulsion droplets, prepared by a mechanically random process, are similarly very large, regardless of the solvent and alkoxide species. In the meantime, Ogihara et al.15,16 followed the growth process of composite hydrous oxide 3Al2O3 · 2SiO2 (mullite) by He-Ne laser photo scattering, where the hydrolysis of composite Al-Si alkloxide, 3Al(sec-OBu)3 · Si(OEt)4, proceeded in its emulsion with a mixed octanol/acetonitrile solvent (vol ratio 3:2) left quiescent at 40 °C after initial mixing with water by stirring for 60 s, where initially [Al-Si alkoxide] ) 0.05 M and [H2O] ) 0.5 M. They found rapid formation of the oxide particles of ca. 0.5 µm immediately after the start of the initial mixing in 5 s, and the mean particle size was progressively rather reduced to ca. 0.4 µm by 900 s, while the mean diameter of composite Al-Si alkloxide droplets was continuously decreased from ca. 1.5 µm to zero at the same time. An analogous trend was also observed for hydrous Al2O3 particles grown in a similar emulsion system of Al(sec-OBu)3.11 From the rather decreasing mean particle size, they concluded that the oxide particles were formed by direct conversion of the alkoxide droplets through hydrolysis by water diffusing into the alkoxide droplets and the subsequent nucleation and growth of the hydrolysis product in each interior. However, since the molar volume ratio of the mullite particles as hydroxide to the corresponding alkoxide is ca. 0.12, the final mean size of the mullite particles must be ca. 0.74 µm ()1.5 × 0.121/3) if we assume the direct conversion of the alkoxide droplets. Meanwhile, their actual final size was
Kojima and Sugimoto ca. 0.4 µm, and even if we take into account the yield of the precipitate ∼70% and compensate the loss of 30%, it is calibrated to be at most ca. 0.45 µm. It means that the number of the mullite particles increased to ca. 4.4 times as much as the number of the alkoxide droplets ()(0.74/0.45)3). Actually, we can observe in the laser scattering data that the reduction of the mean size is due to the increasing number density of smaller particles by their constant generation and limited growth, while the number of the alkoxide droplets continues to decrease with the elapse of time, until finally they disappear after 15-30 min. As a consequence, the laser scattering data may not necessarily support the direct conversion of the alkoxide droplets. The objective of the present Part 3 of this series is to investigate effects of water, temperature and solvent composition on the final particle size, size distribution, and sphericity of the particles, leading to the information on the underlying mechanisms of the particle formation and of the size and shape control in homogeneous and heterogeneous systems. In particular, the present report lays emphasis on the formation mechanism of hydrous titania particles in heterogeneous emulsion systems of titanium butoxide, which will be created simply by controlling the temperature or the solvent composition of homogeneous systems. As to the effects of solvent composition, we will vary the volume fraction of acetonitrile as a component of the mixed solvent or the alcohol species, including ethanol, hexanol, and octanol, in place of butanol. Experimental Section Materials. Reagent-grade titanium(IV) tetra-n-butoxide (TBO) (Across, Ltd.) and aqueous ammonia were used as received. Reagent-grade ethanol (EtOH), butanol (BuOH), hexanol (HeOH), octanol (OcOH), and acetonitrile (AN) (Wako Pure Chemical Industries) were dehydrated and purified with synthesized zeolite and distilled before use. Water deionized and distilled was used as a reactant of hydrolysis and for washing the product. Preparation of TiO2 Particles. The standard conditions for the preparation of hydrous TiO2 particles in a homogeneous system were the same as in Parts 1 and 2 as follows. A mixed solvent of BuOH/AN (volume ratio 1:1) was prepared, and TBO was dissolved in the mixed solvent to make a 0.10 M TBO homogeneous solution (Solution A), 5 cm3 of which was transferred into a 25-cm3 Duran (borosilicate glass) vial containing a magnetic stirrer and sealed with a screw cap. All of these procedures were performed in a glovebox filled with dry air. On the other hand, another BuOH/AN (1:1) solution, containing 0.20 M in ammonia and 1.0 M in water, was prepared in open air (Solution B). These two solutions were preheated to 25 °C in an oil bath, and the reaction was started by injecting a 5 cm3 of Solution B in 1 s with a micropipette into the same volume of Solution A agitated with the magnetic stirrer, followed by aging for 2 h at 25 °C under constant agitation. Therefore, the initial concentrations of the solutes in the standard final solution were nominally 0.050 M TBO, 0.10 M NH3, and 0.50 M H2O. Also, heterogeneous emulsion systems were fabricated by lowering the temperature to 5 °C, by increasing the content of AN in the mixed medium of BuOH/AN to 60 vol % or higher, or by replacing the BuOH in the mixed medium with a higher alcohol such as HeOH or OcOH. Finally, each suspension was centrifuged, and the supernatant was removed. The remaining precipitate was washed four times with ethanol and then four times with pure water by centrifugation, and freeze-dried. Electron Microscopy. Transmission electron microscopy (TEM) on the product particles was carried out with a JEM1200EX II (JEOL) transmission electron microscope.
Amorphous TiO2 Spheres in Organic Solvents
Figure 1. Microtrough on a slide for microscopic in situ observation of the particle formation near the alkoxide/solvent interface.
Affinity Test of Solvents to Ti(OH)4 Gel. Ti(OH)4 gel powder was prepared by adding 4 cm3 of TBO to 40 cm3 of water under agitation, followed by washing four times with water by centrifugation and suctional filtration. About 0.1 g of the gel powder was dispersed by agitation in a 50 cm3 of one of different solvents, such as water, ethanol, butanol, octanol, and acetonitrile, and aged for 1 h under magnetic stirring at 25 °C. Then, the solvents were removed by suctional filtration. Each powder was redispersed in the same kind of fresh solvent, aged for 1 h again, and filtered by suction. The content of occluded or adsorbed solvent for each powder was soon measured by TG-DTA with a TAS 2000 system (TG8120, Rigaku) through raising temperature at a rate of 5 °C/min in air from room temperature up to 1000 °C. Visual Observation of the Particle Formation. In order to specify in which phase of a heterogeneous emulsion system the titania particles are actually formed, the hydrolysis of TBO was carried out in a test tube with an excess bulk TBO in contact with the BuOH/AN (1:1) solvent containing H2O and NH3. Namely, 0.01 mol of TBO (∼3.4 g) was added to 5 cm3 of a mixed solvent BuOH/AN(1:1) in a 15-cm3 polycarbonate test tube, and then 5 cm3 of solvent B, containing 1.0 M H2O and 0.20 M NH3, was added thereto, followed by vigorous agitation for 60 s with a magnetic stirrer. Thus-prepared emulsion of TBO was then left quiescent to allow the phase separation of a bulk TBO phase, and visually check the position of the dispersed titania particles in the heterogeneous system separated into the upper solvent phase and the lower bulk TBO phase. In-situ Optical Microscopy of the Particle Formation. Insitu optical microscopy on the reaction for the formation of titania particles was carried out with a specifically designed setup to directly observe an area including the alkoxide/solvent interface, as illustrated in Figure 1. The rectangular microtrough was fabricated by pasting a plastic-film frame on the slide glass. At the outset, TBO was poured into an end of the microtrough. Then, a mixed solvent of BuOH/AN (1:1) containing 0.50 M H2O and 0.10 M NH3 was slowly introduced from the other end to lead it into contact with TBO at nearly the center of the trough. Immediately after the contact, the trough was covered with a cover glass, and the progress of the particle formation in the interfacial zone was directly observed through an optical microscope. Results and Discussion 1. Effects of Water. Figure 2 shows TEM images of titania particles prepared in the standard homogeneous system with 0.50 M H2O and those with 2.0 or 5.0 M H2O, where the mean sizes of the products were 0.45, 0.17, and 0.08 µm, respectively. With the increase in water content, the final particle size was
J. Phys. Chem. C, Vol. 112, No. 47, 2008 18447 drastically reduced, and the size distribution was dramatically broadened. If we assume that the hydrolysis of TBO is completed before the start of the nucleation of the hydrolysis product in the standard system with 0.50 M H2O, as suggested in Part 2 of this series,2 the hydrolysis of TBO with a higher concentration of water such as 2.0 or 5.0 M H2O must have finished still earlier than the start of the nucleation. Thus, if the role of water was only a hydrolysis reagent, we would obtain uniform particles of almost the same mean size as in the standard system (∼0.45 µm). Conversely, the drastic reduction of the final particle size with 2.0 and 5.0 M H2O reveals that water works not only as a hydrolysis reactant. As has been shown in Part 1 of this series, the solubility C∞ of the hydroxide monomer is significantly reduced by the presence of excess water mainly for two reasons.1 One is the reduced affinity of the hydrated monomer to butanol of the mixed BuOH/AN solvent, and the other is the stabilization of the hydroxide precipitate through the reinforced hydrogen bonding among the monomer units of the precipitate by hydration. The solubility of the hydroxide monomer produced by the preceding hydrolysis process must drop much faster with a greater amount of excess water added at a higher speed, as in the systems with 2.0 or 5.0 M H2O. Thus, in a system with a high concentration of excess water, the supersaturation ratio S ()C/C∞), as the main determinant of the nucleation rate, will soar with the drop of C∞ and will soon reach the critical level for nucleation S*, followed by the nucleation stage in a quasi-steady state of the nucleation in balance with the decreasing solubility. Since the supersaturation ratio S is kept nearly constant at a level slightly above the critical supersaturation ratio S* in this quasi-steady state, C and C∞ are both reduced until finally S falls below S* to terminate the nucleation stage. When the excess concentration of water is high, C and C∞ at the end of the nucleation stage are both so low that the particle growth thereafter is quite limited due to the extremely low supersaturation ()C - C∞) for it. At the same time, the particles in a system with a high concentration of excess water must be subjected to incessant coagulation because of their highly adhesive surfaces by hydration. The dramatic reduction of the final particle size, the broad size distribution, and the notable evidence of coagulation with increasing water content in Figure 2b and c may readily be understood on these mechanisms. 2. Effects of Temperature. Figure 3 shows TEM images of TiO2 particles prepared in the standard system but at different temperatures. At 5 °C, phase separation of TBO occurred due to the reduction of the solubility of TBO in the mixed solvent of BuOH/AN (1:1), and thus the system turned to a heterogeneous emulsion system from the originally homogeneous one at 25 °C. If we assume that the titania particles are formed in the solvent phase through the dissolution of the alkoxide droplets and not by the direct conversion of the alkoxide droplets, as suggested in the introduction section, the broad size distribution in the heterogeneous system may be ascribed to the constant nucleation near the surfaces of the TBO droplets, where the local concentration of the hydrolysis product is high. Also, the somewhat large mean size seems to be explained by lowered dissolution rate of TBO droplets at the low temperature, resulting in the smaller particle number.17,18 On the other hand, above 25 °C, the system was homogeneous and the product particles were kept uniform in size with increasing temperature. But we found no appreciable difference in the mean size within 0.45 ( 0.03 µm even up to 80 °C, as seen in Figure 3. If the nucleation started during the hydrolysis of TBO, the final particle number would be proportional to the
18448 J. Phys. Chem. C, Vol. 112, No. 47, 2008
Kojima and Sugimoto
Figure 2. TEMs of hydrous TiO2 particles prepared in the standard homogeneous system with 0.50, 2.0, or 5.0 M H2O.
Figure 3. TEMs of hydrous TiO2 particles prepared in the standard system, but at different temperatures.
hydrolysis rate,17-19 and thus the final particle size would be reduced to a great extent with increasing temperature up to an extreme level such as 80 °C, since normally the hydrolysis of metal alkoxide is dramatically accelerated with increasing temperature, compared to the acceleration of the particle growth. At least, there would be almost no possibility of the constant particle size with the increase in temperature up to 80 °C. Hence, the constant particle size up to 80 °C seems to verify our earlier prediction in Part 2 of this series that the hydrolysis of TBO is completed before the start of the nucleation of the hydrolysis product even in the standard system at 25 °C. If the nucleation of the hydrolysis product starts after the hydrolysis process like this case, the nucleation stage must be finished by the production of some fixed number of stable nuclei sufficient to reduce the supersaturation below the critical level for the start of nucleation, irrespective of the temperature, so that the final number of the uniform particles is expected to be almost the same, independently of the temperature. As a result, the final particle size becomes almost the same regardless of the temperature. In this case, there is no mass balance between the generation rate of hydroxide monomer by hydrolysis of TBO and its consumption rate for the nucleation and the growth of the generated nuclei, but the uniformity of the product particles is secured by a mechanism close to the LaMer model in terms of the separation between the nucleation and growth stages by an automatic reduction of supersaturation with the nucleation process.19 3. Effects of Acetonitrile in the Mixed Solvent of BuOH/ AN. Figure 4 shows the transmission electron micrographs of titania particles prepared in a mixed solvent of BuOH/AN different in the volume fraction of AN under otherwise the
standard conditions. Although the reaction time was taken for 2 h at 25 °C in all cases, the precipitation of hydrous TiO2 particles was completed with 100% yield, at latest, by 30 min, owing to the effect of ammonia. Here, TBO was found to be homogeneously dissolved in Solution A, insofar as the volume fraction of AN was equal to 0.5 or less, including the standard system. However, when the volume fraction of AN became equal to 0.6 or greater, a TBO phase was found to be separated from the mixed solvent, and the amount of the separated TBO was increased with increasing volume fraction of AN of the mixed solvent. This trend shows that the solubility of TBO is reduced with increasing proportion of AN in the mixed solvent, owing to the low affinity of AN to the alkyl group of the metal alkoxide. In addition, the titania particles are coagulated extensively when prepared without AN, but become less aggregative and more spherical in shape with increasing volume fraction of AN. These facts are readily understood if one considers the less adhesive surfaces and the increased interfacial energy of the hydrous titania particles, due to the decreasing affinity of the mixed solvent to titanium hydroxide monomer with increasing fraction of AN, as evident from the dramatic reduction of the solubility of Ti(OH)4 with the increase of AN in the mixed solvent of BuOH/AN (see Part 1).1 As a consequence, the size distribution of the product became narrower with increasing fraction of AN up to 0.5, owing to the decreasing probability of coagulation. Here, it is noteworthy that the solubilities of both TBO and its hydrolysis product are very low in AN, suggesting low affinity of AN not only to alkyl group, but also to the hydroxide monomer. On the other hand, the affinity of butanol to hydroxide monomer is high, as obvious from the high solubility of the hydroxide monomer produced
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J. Phys. Chem. C, Vol. 112, No. 47, 2008 18449
Figure 4. TEMs of hydrous TiO2 particles prepared under the standard conditions but in mixed solvents of BuOH/AN different in the volume fraction of AN.
in butanol.1 Thus, as discussed in Part 1 of this series,1 the reduction of the solubility of the hydroxide monomer with increasing AN in the mixed solvent of BuOH/AN seems to be due to the low affinity of AN to the hydroxide monomer solvated by BuOH. However, as the volume fraction of AN exceeded 0.5, the size distribution was suddenly broadened in accord with the transition from a homogeneous system to a heterogeneous system. At the same time, the mean size of the product particles was dramatically reduced with increasing fraction of AN in the heterogeneous system. The reduction of the mean size of oxide particles with increasing content of AN in a mixed solvent of alcohol/AN was also reported by Ogihara et al.5 in an emulsion system of Zr(OBu)4 for production of hydrous ZrO2 particles in a mixed solvent of OcOH/AN without ammonia, and thus this fact may not be limited only to our emulsion system. On the other hand, the mean size of the alkoxide droplets are known to sharply increase with increasing fraction of AN due to the reduction of the solubility of alkoxide,9,16 The completely opposite trends in mean size for the alkoxide droplets and the product particles clearly indicate that the hydrous oxide particles in these heterogeneous systems were not formed by direct conversion of the alkoxide droplets. In addition, they found that the final mean particle sizes of hydrous Al2O3 and ZrO2 particles prepared from Al(sec-OBu)3 and Zr(OBu)4, whose initial droplet sizes were both 0.9 µm, were 0.21 and 0.31 µm, respectively, in a system of a mixed solvent OcOH/AN (volume ratio 3:2) aged for 1 h at 25 °C with [alkoxide] ) 0.05 M and [H2O] ) 1.0 M as initial concentrations.5 If we take into account the yield of these particles ∼70%, the final sizes of Al2O3 and ZrO2 on assumption of 100% yield are calibrated to be 0.24 and 0.35 µm, respectively. However, if we used the data of density for hydrous Al2O3 ()2.42 g cm-3)20 and ZrO2 ()3.1 g cm-3)21 and for alkoxides ()0.9-1.0 g cm-3), the molar volume ratios
of Al2O3 and ZrO2 as hydroxides to the corresponding alkoxides are ca. 0.12 and 0.14, respectively, so that the final particle sizes of the hydrous Al2O3 and ZrO2 particles must be ca. 0.44 and 0.47 µm, respectively, if we assume the direct conversion of the alkoxide droplets. In other words, the numbers of the Al2O3 and ZrO2 particles actually produced were ca. 6.2 and 2.4 times as much as the initial numbers of the respective alkoxide droplets. A similar result is also obtained with mullite particles,15,16 as referred to in the introduction section. As a consequence, there seems to be no doubt that these particles are not formed by the direct conversion mechanism. If the particles prepared in the present heterogeneous system with a mixed solvent of BuOH/AN (AN g 60 vol %) are not the products of the direct conversion of the alkoxide droplets, the particles must be formed in the solution phase ()solvent phase) basically in a similar manner to the particle formation in a homogeneous system, but with dispersed alkoxide droplets. Namely, in a heterogeneous system, the low concentration of alkoxide molecules in the continuous solution phase in Solution A may react with water and form hydroxide particles through nucleation and the subsequent growth immediate after mixing with Solution B containing water and ammonia. In response to the consumption of the alkoxide in the solution phase, the alkoxide droplets will start to be dissolved to make up for the depleted alkoxide, and the replenished alkoxide will soon be hydrolyzed and precipitated for the growth of the particles in the solution phase. Thus, the alkoxide droplets may act as reservoirs of alkoxide for furnishing metal ion like the monomer droplets in an emulsion polymerization system for furnishing organic monomer to produce monodisperse polymer particles in aqueous media.17 However, in view of the broad size distribution of the product particles, the interfacial zones of the alkoxide droplets may also act as nucleation sites, where new nuclei are constantly generated under the locally high super-
18450 J. Phys. Chem. C, Vol. 112, No. 47, 2008 saturation of the hydroxide monomer as a result of steady hydrolysis of the concentrated alkoxide in the interfacial zones and/or direct attack of water on the superficial alkoxide molecules. If we reinterpret the constant generation of hydrous oxide particles in the laser scattering experiment by Ogihara et al.11,15,16 by the constant generation of nuclei at the alkoxide/ solvent interfaces, the progressive reduction of the mean particles size in their data may readily be understood. Thus, if the alkoxide emulsion system is constantly agitated by a stirrer, we will not be able to obtain uniform particles, because the nuclei constantly generated at the alkoxide/solvent interfaces at different times start to keep growing in the uniformly supersaturated solution phase. Presumably, this is the main reason for the precipitation of polydisperse particles in our agitated emulsion systems in Figures 3a and 4d-f, in contrast to the quiescent emulsion systems of Ogihara et al., in which particles will cease to grow when they are separated away from the highly supersaturated interfacial zone into the bulk solution phase scarcely supersaturated with hydroxide monomer, through their own migration and/or the regression of the alkoxide/ solution interfaces of the diminishing alkoxide droplets. As it takes about 15-30 min to use up the alkoxide droplets in quiescent emulsion systems,11,15,16 in contrast to our standard homogeneous system, in which the fast hydrolysis of TBO and ca. 80% of the subsequent precipitation of the hydrolysis product are finished by 2.5 s,2 the extremely slow hydrolysis in heterogeneous systems must be due to the low dissolution rate of the alkoxide droplets. Now it is obvious that the hydrolysis rate of alkoxide and the following precipitation rate of the hydrolysis product in the heterogeneous system are both limited by the dissolution rate of the alkoxide droplets, while the hydrolysis of alkoxide in the standard homogeneous system finishes almost instantly, followed by the precipitation of the hydrolysis product independent of the preceding hydrolysis process. The reduction of the particle size, or the increase in the final particle number with increasing proportion of AN in the mixed solvent is basically due to the reduction of the solubility of the hydroxide monomer with increasing AN. Here, note that in this case the solubility of the hydrolysis product is predetermined by the solvent composition, in contrast to the progressive reduction of solubility with addition of excess water. If the solubility C∞ is lowered with a higher volume fraction of AN in the mixed BuOH/AN solvent, the nucleation may start at a lower critical concentration C*, because the nucleation practically starts when the supersaturation ratio S reaches a common critical level S* ()C*/C∞). In addition, the nucleation rate is greatly accelerated even with a slight increase of the supersaturation above the critical concentration C* and soon balanced with the generation rate of the hydroxide monomer by the hydrolysis of alkoxide, so that the upper limit of the steady concentration of hydroxide monomer, C, is kept low but slightly above the critical concentration C*. In contrast to the nucleation rate mainly governed by supersaturation ratio ()C/C∞), the particle growth rate is a function of supersaturation ()C - C∞), e.g., the volume growth rate of the generated nuclei, V˙ , is given by
V˙ ) 4πr20kpVm(C - C∞) p ≈ 4πr20kpVmC∞p (S * -1) p where r0 is the radius of the generated nuclei, kp is the linear growth rate constant, Vm is the molar volume of the precipitate, and p is a constant g1. Since S* is a constant, V˙ is proportional to C∞p , e.g., p ) 3 in the present standard system,2 and thus V˙ must remain at a low level in a system with a low C∞. If the
Kojima and Sugimoto
Figure 5. TEMs of hydrous TiO2 particles prepared under the standard conditions but in mixed solvents of alcohol/AN different in the alcohol species.
growth rate of the nuclei is low, the number of the nuclei is increased instead of increasing each volume of the nuclei, so that the final particle size is reduced. In addition, the constant nucleation at the alkoxide/solvent interfaces, continued until the alkoxide droplets disappear, causes not only the size distribution broadening, but also further reduction of the final particle size. All of these factors are reflected on the results of Figure 4d-f. The most extreme case is the system with 100% acetonitrile in Figure 4f. But in this case, the small particles are not adhesive unlike those prepared with a high concentration of water, and thus no appreciable evidence of coagulation is observed. 4. Effects of Alcohol Species in the Mixed Solvent of Alcohol/AN. Figure 5 shows TEMs of hydrous TiO2 particles prepared in mixed solvents of alcohol/AN (vol ratio 1:1) with different alcohol species under otherwise the standard conditions. With the transition from ethanol to the higher alcohols, the mean particle size increases, and the size distribution was drastically broadened when the alcohol species was altered from butanol to hexanol or octanol. Meanwhile, the alteration of alcohol species from butanol to hexanol or octanol caused the phase separation of TBO in Solution A, so that the homogeneous system turned to a heterogeneous emulsion system under constant stirring. Thus, the dramatic size distribution broadening can be explained in terms of the constant nucleation at the alkoxide/solvent interfaces, followed by the continuous growth of the generated nuclei in the solution phase of the emulsion systems. The phase separation observed with the change of alcohol species from butanol to the still higher alcohols is caused by the reduction of the solubility of TBO with increasing oleophilicity of these alcohols. But, in contrast to the case with increasing proportion of AN, the final size of the product particles was not appreciably decreased with the alteration of the alcohol species to higher alcohols. In particular, the particle size was rather increased with replacement of ethanol by butanol. It may correspond to the increase in the solubility of the hydroxide monomer with the change of the alcohol species from ethanol to butanol, as suggested from the solubility of Ti(OH)4 gel in butanol, more than twice as high as in ethanol (see Table 1 in Part 1).1 Thus, if butanol is replaced by ethanol, the
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Figure 6. TG-DTA diagrams of Ti(OH)4 gel immediately after aging in agitated different solvents, including ethanol, butanol, octanol, acetonitrile, and butanol/acetonitrile (vol ratio 1:1) for a total of 2 h at 25 °C and removal of the solvents by suction filtration.
nucleation of the hydrolysis product in the homogeneous system may possibly start during the hydrolysis process of TBO with addition of Solution B before the concentration of the hydroxide monomer reaches the maximum level ()0.05 M), because of the lower solubility of the hydrolysis product. This seems to be the reason for the production of the smaller particles by the replacement of butanol with ethanol. Moreover, the final particle size in the heterogeneous systems of HeOH/AN (1:1) and OcOH/AN (1:1) is as large as in the homogeneous system of BuOH/AN (1:1), suggesting fairly high solubility of the hydrolysis product in these heterogeneous systems due to considerably strong affinity of hexanol and octanol to the hydroxide monomer as that of butanol. The relatively low solubility of the hydrolysis product in ethanol seems to be mainly due to the greater affinity between ethanol molecules than for the higher alcohols, equivalent to the lower chemical potential of pure ethanol µL0 (see eq 6 of Part 1), based on the higher polarity of ethanol. On the other hand, the solubility of the hydrolysis product in the heterogeneous systems of HeOH/AN (1:1) and OcOH/AN (1:1) is likely to remain higher than in the heterogeneous systems of BuOH/AN (AN g 60 vol %) attained by increasing content of AN. The affinity of these solvents to the hydroxide monomer may be evaluated not only from the solubility of the hydroxide gel in Table 1 in Part 1 of this series,1 but also from TG-DTA diagrams of a hydroxide gel powder after aging in individual solvents, as shown in Figure 6, whose experimental details are given in the Experimental Section. In this figure, the initial endothermic weight loss is mainly due to the evaporation of water associated with the transition from Ti(OH)4 to TiO2,
theoretically 31.1%, and the following exothermic transitions with weight loss are due to the combustion of the occluded organic solvents from 238 to 252 °C and the crystallization of the solid from amorphous to anatase ranging from 328 to 380 °C, as confirmed by XRD. In the TG diagrams, the weight loss of the gel powders above the boiling points of these solvents corresponds to the loss of the occluded or adsorbed ones. The very small weight loss of less than 4% by escape of ethanol or acetonitrile for the powders after aging in these solvents in Figure 6a and d may reflect their low affinity to the hydroxide monomer, in consistency with the solubility data and with the lowered particle size by use of ethanol instead of butanol (Figure 5a) or a large volume fraction of acetonitrile in the mixed solvent (Figure 4). On the other hand, butanol and octanol are shown to be occluded in the solid to a great extent, revealing their strong affinity to the hydroxide monomer, as expectable from the high solubility of monomer in bulk butanol and the fact that the relatively large-sized particles in Figure 5c and d obtained by use of hexanol or octanol in the mixed solvent are comparable to those in Figure 5b by use of butanol. Also, the high content of the organic matter in the solid aged in the mixed solvent of BuOH/AN (1:1) in Figure 6e, comparable to the content of butanol in the solid aged in pure butanol in Figure 6b, seems to be mostly butanol from the low affinity of AN to the hydroxide monomer and butyl group. In other words, AN does not prevent the solvation of butanol to the hydroxide monomer, but it lowers the solubility of the hydroxide monomer in BuOH/AN (1:1) because of the low affinity of AN to the butanol-solvated monomer, as predicted from the solubility data in Table 1 of Part 1. Similarly, from the high content of
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Figure 7. TEMs of hydrous TiO2 particles prepared under the standard conditions but in mixed solvents of alcohol/AN different in the alcohol species, in the absence of ammonia.
specifically oleophilic octanol in Figure 6c, the low affinity of polar solvent AN to the butanol-solvated monomer is expected to be still more enhanced with the octanol-solvated monomer. Incidentally, the relatively small weight loss with the dehydration of the hydroxide gel aged in ethanol or in BuOH/ AN (1:1) may suggest some possibility of condensation among the monomer units of the hydroxide gel during aging in these solvents, at least, in the absence of water. Figure 7 shows TEMs of hydrous TiO2 particles prepared in the same way as those in Figure 5, but without ammonia. Their mean sizes are larger than those in Figure 5 as a whole, because there is no ammonia that significantly reduces the solubility of the hydroxide monomer. Also, their aggregative trend and the spoiled sphericity can readily be explained by the absence of ammonia, as has been discussed in detail in Part 1 of this series.1 The size distribution broadening caused by the constant nucleation during the particle growth is also observed in the heterogeneous systems with hexanol or octanol. Surprisingly, however, the particles prepared in the mixed solvent of OcOH/ AN are of excellent sphericity even in the absence of ammonia. This may be explained in terms of the raised specific surface energy owing to the lowered affinity of AN of the mixed solvent OcOH/AN to the oleophilic octanol-solvated hydroxide monomer units of the particle surfaces. In addition, since ammonia was not involved in these systems, the product particles must have been left uncondensed, so that there is more or less evidence of dissolution for all particles in Figure 7 in the washing process with ethanol (see Figures 7 and 9 of Part 1 of this series1). But they appear to become more proof against ethanol with increasing oleophilicity of the occluded alcohol. 5. Visual Observation of Particle Formation with Excess TBO. In order to specify the phase, the alkoxide phase or the solvent phase, in which the titania particles are actually formed in an alkoxide emulsion system, we carried out a model experiment to directly observe the position of the generated particles by adding 5 cm3 of Solution B containing 0.20 M NH3 and 1.0 M H2O in 1 s to an emulsion of TBO, consisting of 0.01 mol of TBO (∼3.4 g) and 5 cm3 of a mixed solvent BuOH/ AN (1:1), in a 15-cm3 polycarbonate test tube under vigorous
Kojima and Sugimoto agitation with a magnetic stirrer. Since this amount of TBO, corresponding to ca. 0.74 M TBO in molarity in the total system, was far over the solubility of TBO in BuOH/AN (1:1), TBO was separated as a bulk phase underneath the solvent phase, as soon as the agitation was stopped. Figure 8 shows photographs of this inhomogeneous system, immediately after or 2 h after stopping additional agitation for 60 s continued after the mixing of the two solutions. Obviously, the particles are located in the upper solvent phase, revealing that the TBO does not work as either a dispersion medium of the hydrous TiO2 particles or a solvent of the hydroxide monomer. This result is rather reasonable if one consider that water molecules which instantly react with TBO will not be able to diffuse into the TBO phase, and that the hydrolysis product of the greater affinity to alcoholic solvents with hydroxyl group will diffuse into the alcoholic solvent phase rather than the TBO phase. 6. Optical Microscopy on Particle Formation near the Alkoxide/Solvent Interface. In order to confirm this conclusion, we conducted optical microscopy on the formation process near the alkoxide/solvent interfaces according to the method in the Experimental Section. As clearly shown in Figure 9, the oxide particles are densely formed in the solvent phase along the alkoxide/solvent interface and diffused deep into the bulk zone of the solvent phase. Consequently, there seems to be no doubt that the hydrous oxide particles in our heterogeneous systems are formed in the solvent phase and not inside the alkoxide droplets. 7. On the Possibility of Direct Conversion of Alkoxide Droplets. At least, in our emulsion systems created by lowering temperature, by increasing the proportion of acetonitrile in the mixed solvent of BuOH/AN, or by using higher alcohols such as hexanol and octanol instead of butanol in the mixed solvent, there is virtually no possibility of particle formation by direct conversion of the alkoxide droplets. However, the hydrous oxide particles prepared in acetonitrile in the alkoxide emulsion system of Hardy et al.3,4 are extremely large, e.g., Al2O3 (0.24 µm), ZrO2 (0.35 µm), and TiO2 (0.75 µm), compared to the TiO2 particles prepared in acetonitrile of our system (∼0.038 µm). In addition, the densities of their hydrous oxide particles, e.g., ZrO2 (2.5 g cm-3) and TiO2 (2.1 g cm-3), were much lower than 3.1 g cm-3 common to ZrO2 and TiO2 usually obtained from homogeneous sol-gel systems,21,22 and each particle of some powders was actually found dented after drying, suggesting that, at least, some of the powders originally consisted of hollow particles. These results imply a possibility of direct conversion of the alkoxide droplets, starting with rapid formation of a hydroxide shell layer on the surface of each droplet, followed by solidification with inward permeation of water with or without the solvent, or outward permeation of alkoxide, through the shell layer. Since the shell layer is supposedly made of highly hydrated hydroxide gel network to which the affinity of acetonitrile and alkoxide is very low, the most probable process is the independent permeation of water from the solvent phase in response to the consumption of the hydration water on the internal wall of the shell layer by the reaction with the core alkoxide. Along with the increase in the thickness of the shell layer, the contraction of the core/shell particle with a liquid core consisting of alkoxide and the byproduct alcohol may proceed with progressive solidification of the loose gel network and escape of a part of the byproduct alcohol by diffusion through the hydrated shell layer, until the shell layer becomes fully compact and rigid. If the contraction of a particle is terminated in this way in the midst of the conversion process, it must invariably become a hollow particle with a cavity full
Amorphous TiO2 Spheres in Organic Solvents
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Figure 8. Photos of a model heterogeneous system immediately after or 2 h after stopping additional agitation for 60 s after admixing 5 cm3 of BuOH/AN (1:1) containing 1.0 M H2O and 0.20 M NH3 with 10 mmol of TBO in 5 cm3 of BuOH/AN (1:1) in a 15-cm3 polycarbonate test tube under magnetic stirring. The separated upper and lower phases are those of the mixed solvent and TBO, respectively. For the sample after 2 h, a close-up near the interface is also shown. The generated hydrous TiO2 particles are located in the solvent phase.
Figure 9. Microscopic observation of hydrous TiO2 particles formed in the solvent phase near the solvent/alkoxide interface, where the alkoxide is TBO and the solvent is the mixed solvent of BuOH/AN (1:1) containing 0.50 M H2O and 0.10 M NH3.
of the byproduct alcohol at the end of the reaction after the final deposition of hydrolysis product of the remaining alkoxide onto the internal wall of the solid shell. In this context, as mentioned in the introduction section, Hardy et al.4 proposed a mechanism on the direct conversion of the alkoxide droplets in terms of solidification of alkoxide droplets by way of diffusion of partly hydrolyzed alkoxide from the surfaces into the interiors and the following condensation process. However, in view of the instantaneous hydrolysis of TBO into fully hydrolyzed Ti(OH)4 and the so slow condensation of titanium hydroxide solid monomer as to be virtually undetectable even after aging for 24 h at 25 °C in the absence of ammonia (see Part 1),1 it seems unlikely that the condensation process is involved in the direct conversion of metal alkoxides completed within several minutes at room temperature. Meanwhile, they also observed precipitation of extremely small particles aloof from the size distribution of coexisting main particles, and found that the relatively small particles in a mixed metal oxide powder prepared from a mixed alkoxide was different in composition from the larger ones. These facts may suggest a possibility of concurrent precipitation of the oxide particles in the solvent phase as well. Probably, the direct conversion process in a quiescent alkoxide emulsion may become dominant, when the concentration of water is extremely high as that in the systems of Hardy et al. (∼10 M excess), and the solubilities of alkoxide and its hydrolysis product are both exceedingly low in an inert solvent freely miscible with water, such as acetonitrile and propylene carbonate. Under these
conditions, the start of the hydrolysis reaction may be limited only to the surfaces of the alkoxide droplets, and the instantly formed hydroxide shell layers will serve to block their subsequent dissolution. Then, all the following processes, including hydrolysis and precipitation of the hydrolysis product, will proceed exclusively in the alkoxide droplets without their own dissolution. For this purpose, agitation of the system must strictly be prohibited to maintain the integrity of the fragile alkoxide droplets in the course of this conversion process. But, even if the ideal conditions for the direct conversion are apparently satisfied, it does not seem possible to completely exclude the concomitant precipitation in the solvent phase. On the other hand, if the direct conversion process is more or less involved in an alkoxide emulsion system, we cannot expect uniform particles. Nevertheless, it seems possible to obviate the direct conversion of alkoxide droplets and obtain fairly uniform products even in alkoxide emulsions, if we perform the particle synthesis in a quiescent alkoxide emulsion with relatively low concentrations of alkoxide (∼0.05 M) and water (0.1-0.5 M) in a single or mixed inert water-miscible solvent, in which alkoxide and its hydrolysis product are slightly soluble up to some proper levels on the order of 10-2 M. Thus, in this system, some post-treatment such as gradual addition of ammonia or a poor solvent to promote the precipitation of the residual hydroxide monomer may be required for achievement of a high yield close to 100%. Conclusions 1. Heterogeneous systems of alkoxide emulsions were created by reduction of the solubility of TBO from our standard homogeneous system through lowering temperature, increasing proportion of AN in the mixed BuOH/AN solvent, or replacing BuOH with still higher alcohols in the mixed solvent, in which hydrous TiO2 particles were formed by their nucleation and growth in the solvent phase using hydrolysis product of TBO molecules furnished through the dissolution of the TBO droplets. Because of the constant nucleation at the alkoxide/solvent interfaces with a high local concentration of the hydroxide monomer, the product was always polydisperse. But it was suggested possible to produce fairly uniform product in a similar heterogeneous system without agitation. 2. Final particle size of hydrous TiO2 was drastically reduced by lowering the solubility of the hydrolysis product in the standard homogeneous system with excess water over a some limited concentration, or in a heterogeneous system of TBO emulsion with a high proportion of acetonitrile in the mixed solvent of BuOH/AN. The reduction of the
18454 J. Phys. Chem. C, Vol. 112, No. 47, 2008 solubility of hydrolysis product is commonly achieved by lowering the affinity between the hydrolysis product and solvent. But, in the case with excess water, the affinity of the hydrolysis product to BuOH as a component of the mixed BuOH/AN solvent is reduced by hydration to the hydrolysis product, while in the case with AN, the affinity of the mixed solvent of BuOH/AN to the hydrolysis product is reduced by dilution of BuOH with AN whose affinity to the butanolsolvated hydroxide monomer is much lower than that of BuOH. As a result, the small particles prepared with excessive water left evidence of their extensive coagulation, because of the adhesive surfaces with the heavily hydrated surface monomers. 3. Highly spherical hydrous TiO2 particles were obtained in a mixed solvent of OcOH/AN (1:1) in its heterogeneous TBO emulsion, even in the absence of ammonia. It was explained in terms of the increased surface energy of the particles due to the low affinity of acetonitrile to the oleophilic octanol-solvated surface monomer. 4. Direct conversion of the alkoxide droplets into hydrous oxide particles may possibly become dominant in a quiescent emulsion of alkoxide droplets dispersed in an inert medium, miscible with water but extremely poor as a solvent for both the alkoxide and the hydrolysis product, in which a high concentration of water on the order of 10 M is contained. But the size distribution of the product is generally very broad, since the originally broad size distribution of alkoxide droplets is directly reflected on it. References and Notes (1) Sugimoto. Y.; Kojima, T. J. Phys. Chem. C, 2008, DOI 10.1021/ jp8029506.
Kojima and Sugimoto (2) Sugimoto. Y.; Kojima, T. J. Phys. Chem. C, 2008, DOI 10.1021/ jp802954u. (3) Hardy, A. B.; Gowda, G.; McMahon, T. J.; Riman, R. E.; Rhine, W. E.; Bowen, H. K. In Ultrastructure Processing of AdVanced Ceramics; Mackenzie, J. D., Ulrich, D. L., Eds.; Wiley Interscience: New York, 1987; p 407. (4) Hardy, A. B.; Rhine, W. E.; Bowen, H. K. J. Am. Ceram. Soc. 1993, 76, 97. (5) Ogihara, T.; Yanagawa, T.; Ogata, N.; Yoshida, K. J. Ceram. Soc. Jpn. 1993, 101, 315. (6) Ogihara, T.; Nakajima, H.; Yanagawa, T.; Ogata, N.; Yoshida, K.; Matsushita, N. J. Am. Ceram. Soc. 1991, 74, 2263. (7) Mizutani, N.; Ikeda, M.; Lee, S. K.; Shinozaki, K.; Kato, M. J. Ceram. Soc. Jpn. 1991, 99, 183. (8) Lee, S. K.; Shinozaki, K.; Mizutani, N. J. Ceram. Soc. Jpn. 1992, 100, 1140. (9) Lee, S. K.; Shinozaki, K.; Mizutani, N. J. Ceram. Soc. Jpn. 1993, 101, 470. (10) Ogihara, T.; Yabuuchi, M.; Yanagawa, T.; Ogata, N.; Yoshida, K.; Nagata, N.; Ogawa, K.; Maeda, U. J. Soc. Powder Technol. Jpn. 1994, 31, 620. (11) Ogihara, T.; Mizutani, N. Inorg. Mater. 1996, 33, 177. (12) Ikeda, M.; Lee, S. K.; Shinozaki, K.; Mizutani, N. J. Ceram. Soc. Jpn. 1992, 100, 680. (13) Ogihara, T.; Yanagawa, T.; Ogata, N.; Yoshida, K.; Iguchi, M; Nagata, N.; Ogawa, K. J. Soc. Powder Technol. Jpn. 1994, 31, 795. (14) Ogihara, T.; Wada, K; Yoshida, T.; Yanagawa, T.; Yoshida, K.; Matsushita, N. Ceram. Int. 1993, 19, 159. (15) Ogihara, T.; Yanagawa, T.; Ogata, N.; Yoshida, K.; Iguchi, M; Nagata, N.; Ogawa, K. J. Ceram. Soc. Jpn. 1994, 102, 778. (16) Ogihara, T. In Fine Particles: Synthesis, Characterization, and Mechanisms of Growth; Sugimoto, T., Ed.; Marcel Dekker: New York, 2000; p 43. (17) Sugimoto, T. Monodispersed Particles; Elsevier: Amsterdam, 2001. (18) Sugimoto, T.; Shiba, F.; Sekiguchi, T.; Itoh, H. Colloids Surf., A 2000, 164, 183. (19) Sugimoto, T. J. Colloid Interface Sci. 2007, 309, 106. (20) Ingebrethsen, B.; Matijevic´, E. J. Aerosol. Sci. 1980, 11, 271. (21) Fegley, B.; Barringer, E. Mater. Res. Soc. Symp. Proc. 1984, 32, 187. (22) Barringer, E. A.; Bowen, H. K. J. Am. Ceram. Soc. 1982, 65, C-199.
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