18760
J. Phys. Chem. C 2008, 112, 18760–18771
Formation Mechanism of Amorphous TiO2 Spheres in Organic Solvents. 1. Roles of Ammonia Tadao Sugimoto*,† and Takashi Kojima‡ 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
The roles of ammonia in the formation of monodisperse amorphous TiO2 spheres through hydrolysis of titanium butoxide in its homogeneous solution of a mixed solvent of butanol/acetonitrile with ammonia have been investigated. The hydrolysis of titanium butoxide was originally so fast as to finish almost instantly within a few seconds even in the absence of ammonia that the effect of ammonia as an accelerator of the hydrolysis of titanium butoxide was not observed. Instead, ammonia was found to play key roles as an accelerator of the precipitation of the hydrolysis product, as an inhibitor of the coagulation of the growing hydroxide particles, and as a promoter for production of highly spherical particles. All of these effects are deemed to be mainly due to the reduced affinity between butanol and the hydroxide monomer joined with ammonia through hydrogen bonding. In addition, ammonia was revealed to work as a powerful accelerator of the condensation of the hydroxide monomer, which proceeds not in the solution phase but only in the individual particles during aging after precipitation. The accelerated condensation by ammonia was explained in terms of the promoted release of hydroxide ion of the monomer in each particle by the nucleophilic coordination of ammonia to its titanium ion. These mechanisms widely applicable to other sol-gel systems of transition metal oxides as well have been discussed in comparison with silica particle systems. Introduction Monodisperse micro to nanoparticles uniform in size and shape have widely been used not only as ideal models for fundamental studies in colloid and material sciences, but also as high-quality industrial products, and a number of procedures for the preparation of uniform particles are continued to be developed.1,2 Above all, on the basis of the pioneering work of Kolbe3 for the preparation of uniform spherical SiO2 particles by hydrolysis of tetraethyl orthosilicate (TEOS) in alcoholic solvents in the presence of certain bases, Sto¨ber et al.4 systematically developed this method and established the procedure as a practical general method for the synthesis of monodisperse SiO2 spheres based on the hydrolysis of alkyl silicates in alcoholic solvents in the presence of ammonia. These authors found that ammonia worked in particular as a morphological controller to yield spherical SiO2 particles. Since then, the synthetic procedure of monodisperse particles through the hydrolysis of metal alkoxides in a homogeneous organic solution, so-called sol-gel process, has been one of the most popular processes for the preparation of monodisperse amorphous metal oxide particles, such as TiO2,5-18 ZrO2,8,16,19-22 Y-doped ZrO2,8,19,23 Ta2O5,24 SrTiO3,25 and Pb(Zr0.5Ti0.5)O3.26 However, despite the remarkable development of nanotechnology and numerous studies on sol-gel processes, their underlying mechanisms are not necessarily clear yet, presumably because these studies have mainly been focused on the application of the well-defined ideal powders to practical purposes or * To whom correspondence should be addressed. E-mail: tdosugimoto@ pop06.odn.ne.jp. † Manazuru Institute for Superfine Particle Science, Manazuru 1912-4, Manazuru-machi, Kanagawa 259-0201, Japan. ‡ Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan.
on search for optimum conditions of their synthesis in individual systems. Even in such a situation, it seems worthwhile to trace back the progress of the practical studies, focusing on the synthesis of some specific product such as TiO2 particles. For example, Barringer and Bowen5-7 obtained fairly uniform TiO2 particles of ca. 0.5 µm in mean diameter by hydrolysis of titanium ethoxide in ethanol at room temperature. The precipitation of TiO2 particles occurred in 2-90 s depending on the concentrations of Ti(OC2H5)4 from 0.1 to 0.2 M and of H2O from 0.3 to 1.5 M. Since the hydrolysis of alkyl silicates or transition metal alkoxides in pure water usually results in gellike precipitates of silicic acid or metal hydroxides, not in the form of dispersed particles, we commonly use alcoholic media in this procedure in order to have the hydrolysis products precipitated in the form of sol or dispersed particles. Since the first report of Barringer and Bowen,5 TiO2 particles have been prepared mostly from titanium ethoxide in ethanol, mainly due to the better uniformity of the product in size and spheroidal shape than with other combinations.6-13 However, even with the best combination, the resulting TiO2 particles are not necessarily fully spherical, but somewhat distorted and bound by relatively rough surfaces. In addition, a considerable part of them are not free from coagulation during their growth, and this partly spoils the sphericity of the individual particles. The rather aggregative trend of thus-prepared titania particles was found considerably restrained by the aid of an anticoagulant, hydroxypropyl cellulose (HPC).12 But, even with HPC, it was not possible to improve the sphericity of the particle shape and completely inhibit the coagulation. Interestingly, ammonia, as a key component for the preparation of SiO2 particles, has never been used in the typical systems with titanium ethoxide in ethanol for the synthesis of uniform TiO2 particles. In this context, Ikemoto et al.9 obtained only ill-defined aggregates of extremely small particles in a system with ammonia. However,
10.1021/jp8029506 CCC: $40.75 2008 American Chemical Society Published on Web 11/06/2008
Formation Mechanism of Amorphous TiO2 Spheres since a large content of water as high as ca. 9 M in the final concentration was involved at the same time in this experiment, this result seems mainly due to the excessive concentration of water, and thus essential effects of ammonia in the system with titanium ethoxide in ethanol still remain unknown. On the other hand, Hardy et al.25 prepared uniform SrTiO3 spheres by the hydrolysis of a mixed alkoxide, SrTi(OC4H9)6, homogeneously dissolved in butanol containing 25 wt% acetonitrile. Since no precipitation was observed in pure butanol under otherwise the same conditions, acetonitrile was likely to cause the precipitation of the SrTiO3 particles by lowering the solubility of the mixed hydroxide solid as the hydrolysis product of SrTi(OC4H9)6. Similarly, Shida et al.14 prepared well-dispersed highly spherical uniform TiO2 particles by hydrolysis of titanium butoxide in its homogeneous solution of a mixed butanol/acetonitrile solvent in the presence of ammonia. Unfortunately, however, the detailed mechanisms of the performance of acetonitrile and ammonia in this system were not analyzed, and thus their individual roles in the particle formation are left unresolved. In the meantime, it has been believed that the sol-gel process in general proceeds through a series of elemental steps; that is, the hydrolysis of alkoxide, the polycondensation of the hydroxide monomer with release of water and/or alcohol (tMsOH + ROsMt f tMsOsMt + ROH; M ) metal or Si; R ) H or alkyl), and the precipitation of thus produced oxolated oligomers consisting of lyophobic oxo-bridges (∼M-OM∼).4,6,10,22 However, the correlations among these elemental steps have never been resolved with clear evidence in individual systems. The objective of the present serial study is to shed light on the underlying mechanism of particle formation in general homogeneous and emulsified heterogeneous sol-gel systems, focusing on the long-term essential issues, such as the role of ammonia, the correlation among the elemental steps, and effects of temperature, water and solvents, on the basis of physicochemical analyses on a system for the formation of monodisperse TiO2 spheres through hydrolysis of titanium butoxide in a mixed alcohol/acetonitrile solvent. We will deal with the role of ammonia in part 1 (this paper), kinetics in part 2, and effects of water, temperature, and solvent composition in part 3. Experimental Materials. Reagent-grade titanium (IV) tetra-n-butoxide (TBO) (Across, Ltd.); aqueous ammonia, sodium hydroxide, sodium perchlorate, and triethanolamine (TEOA) and ascorbic acid and diantipyrylmethane (Wako Pure Chemical Industries) were used as received without further purification. Reagentgrade ethanol (EtOH), butanol (BuOH), 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 TiO2 particles were as follows. A mixed solvent of BuOH and AN in the volume ratio of 1:1, BuOH/ AN(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 ammonia and 1.0 M water was prepared in open air using the reagent-grade aqueous ammonia (solution B). These two solutions were preheated to
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18761 25 °C in an oil bath, and the reaction was started by injecting a 5 cm3 of the solution B in ca. 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 (A + B) were nominally 0.050 M TBO, 0.10 M NH3, and 0.50 M H2O. For a test to see the effect of higher concentrations of ammonia up to 1.0 M at 0.50 M H2O, we used a stock solution of ammonia in BuOH/AN(1:1), prepared by bubbling with gaseous ammonia. Finally, the dispersion was centrifuged, and the supernatant was removed. The remaining precipitate was washed 4 times with ethanol and then 4 times with pure water by centrifugation, and freeze-dried. Characterization of Particles. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) on the product particles were carried out with a S-4100 L scanning electron microscope (Hitachi) and a JEM-1200EX II (JEOL) transmission electron microscope, respectively. For the analysis of as-prepared or calcined solid structures, X-ray diffractometry (XRD) was conducted using a RINT 2000 XRD system with Cu KR ray (Rigaku). Time-Resolved Observation of Particles by SEM. A total of 3 cm3 of each sample withdrawn at times after mixing solutions A and B was immediately percolated by suction with a PTFE (polytetrafluoroethylene) membrane filter of a pore size 0.5 µm, where the filtration was completed within 5 s. The particles left on the filter were observed by SEM. Here we observed no appreciable difference in the results, when we used at the same time a filter of pore size 0.2 µm, which took ca. 30 s for one filtration but supposedly captured all particles without loss. Determination of the Yield of Precipitate as a Function of Aging Time. Immediately after the filtration of each sample in the above experiment for the time-resolved SEM observation, 1 cm3 of an aliquot was withdrawn from the filtrate and mixed with 4 cm3 of 0.05 M TEOA (triethanolamine) in ethanol to form a soluble stable Ti-TEOA complex, followed by dilution of this complex solution to 1/100 in concentration with ethanol. The concentration of titanium ion contained in the original filtrate was determined from the ultraviolet-visible spectroscopy (Shimadzu, UV-2500PC) on the UV absorption spectrum of the Ti-TEOA complex at 225 nm in the dilute ethanol solution. Here, we utilized the nature of TEOA which instantly halts the hydrolysis of TBO and even dissolves fresh precipitate of Ti(OH)4 by forming a stable Ti-TEOA complex in organic solvents.27 Also, since the yield of precipitate in the standard system was found to be kept constant at 100% after 10 min, the experimental error due to the contamination of filtrates by particles which have passed through the filter of a pore size 0.5 µm was regarded negligible. Automatic Recording of Electric Conductivity as a Function of Aging Time. The electric conductivity of a reaction system was followed automatically from the start of the reaction using a Toa electic conductometer (WM-50EG, DKK-Toa). Measurement of the Solubility of Ti(OH)4 Gel in Different Solvents. A Ti(OH)4 gel powder was prepared by hydrolyzing titanium butoxide in water under agitation, thoroughly washed with water by centrifugation, and freeze-dried. About 0.1 g of it was dispersed by agitation in a 10 cm3 of one of different solvents, such as EtOH, BuOH, AN, BuOH/AN(1:1), BuOH/ AN(1:1) + 0.5 M H2O, and BuOH/AN(1:1) + 0.5 M H2O + 0.1 M NH3, and aged for 24 h under magnetic stirring at 25 °C. Then the supernatants were retrieved by centrifugation. The solubility of Ti(OH)4 gel in each solvent was determined with
18762 J. Phys. Chem. C, Vol. 112, No. 48, 2008 each supernatant solution colorimetrically as follows.28 Two cm3 of a supernatant solution was withdrawn into a brown-glass 10 cm3-volumetric flask, previously filled in part with 1 cm3 of 10 w/v% ascorbic acid, 4 cm3 of 1 wt% diantipyrylmethane in 1 N H2SO4 and 1.6 cm3 of 6 N HCl, and finally the total volume was made up to 10 cm3 with water. Only for the sample in butanol, 0.5 cm3 of its supernatant was used instead of 2 cm3, because of the solubility limit of butanol in the aqueous solution. After 45 min for the stabilization in darkness, the concentration of titanium ion in the final solution was determined from the optical density at 385 nm and that for a reference solution of the same composition but without titanium ion, using an ultraviolet-visible spectrophotometer (Hitachi U-2000A). Tracing of Condensation in Progress. In order to trace the development of polycondensation of the partly and/or fully hydrolyzed monomer with aging, 1 cm3 of a sample was withdrawn at times after the start of the reaction and immediately mixed with 4 cm3 of 0.05 M TEOA in ethanol to halt the reaction instantly. Then it was left quiescent for 30 min or 2 weeks to dissolve a part of the precipitate free from polycondensation. After centrifugation, the concentration of the supernatant titanium ion was measured by ultraviolet-visible spectroscopy in the same way as above for the determination of yields of precipitates. The extent of condensation of a sample was evaluated from the portion left undissolved. In this procedure, the molar ratios of TEOA to excess water () [H2O]0-4[TBO]0) and TEOA to Ti(OH)4 must be set no less than 2/3 and 2, respectively, because a minimum concentration of TEOA is required for replacing the hydroxyl group of the monomer to completely dissolve condensation-free monomer. TG-DTA. Thermogravimetry (TG) and differential thermal analysis (DTA) were conducted with a TAS 2000 system (TG8120, Rigaku) by raising temperature at a rate of 5 °C/min in air from room temperature up to 1000 °C for titania powders prepared with or without ammonia to measure the content of the occluded solvent and for the standard sample to estimate the degree of condensation. Solid Phase Transition by Calcination. In order to find the correlation between TG-DTA profiles and the solid phase transition, some sample powders were heated up to prescribed temperatures at a rate of 5 °C/min in the furnace of the TGDTA analyzer (TG8120, Rigaku). On reaching one of the temperatures, the power of the heater was shut off to cool each sample down to room temperature, and the solid structure was characterized by XRD. Results and Discussion 1. Morphology of TiO2 Particles Prepared with or without Ammonia. Figure 1 shows SEMs of titania particles aged for different times in the parentheses after the addition of Solution B under the standard conditions with 0.10 M NH3 (a-c), without NH3 (d and e), or with 5.7 × 10-4 M NaOH instead of NH3 (f), where 3 cm3 of each suspension was withdrawn from the reactor to a PTFE filter, immediately filtered in 5 s by suction, and dried on the filter before the SEM observation. The initial concentration of H2O was 0.50 M in all cases. Here, the choice of 5.7 × 10-4 M as the concentration of NaOH is based on a background that 5.7 × 10-4 mol of NaOH and 0.10 mol of NH3 give the same pH 12.8 when dissolved in 0.50 mol of H2O. Highly spherical particles free from coagulation were obtained after aging for 5 s, 30 s, and 2 h in the standard system. It is of interest that the individual particles aged for 5 and 30 s bear some dimple-like dents. The dimples seem to have been created by the initial hardening of the surface layer of each particle
Sugimoto and Kojima
Figure 1. SEM images of titania particles prepared under the standard conditions by aging for 5 s (a), 30 s (b), 2 h (c); without NH3 for 5 s (d) and 2 h (e); with 5.7 × 10-4 M NaOH instead of NH3 for 2 h (f).
and the subsequent internal contraction by evaporation of the occluded solvent after filtration. But they became less definite with aging until they finally disappeared from particles aged for 20 min or more, probably because the internal tissue became uniformly rigid with progressive solidification in each particle and release of the occluded solvent during aging. As will be shown afterward, a condensation process in each particle may, at least, partly be involved in this solidification. In the absence of ammonia, the particles were drastically coagulated and appeared to be much softer and stickier than those obtained in the standard system with ammonia. Figure 1d reveals that the coagulation occurred within a few seconds, but in the late growth stage, since the original particles in each aggregate are comparable in size to the standard ones. The extremely soft and fluidal tissue initially observed in Figure 1d appeared to become more or less rigid from their surfaces with aging, as inferred from Figure 1e showing their bodies unevenly collapsed by escape of solvent while drying. Figure 1f shows that when 5.7 × 10-4 M NaOH was used in place of ammonia, the particles were somewhat coagulated, but much less than those prepared without both of NaOH and ammonia, suggesting an effect of sodium hydroxide to inhibit the coagulation to some extent, presumably owing to the repulsive forces between particles based on the negative electric potential at a high pH above the isoelectric point () 5.5 in water from ref 7). The wrinkled surfaces leave the evidence of internal contraction of these soft particles by escape of a relatively large amount of interior solvent after filtration. From these results, the particles prepared with ammonia appear to involve a less amount of solvent. Also, a basic medium may serve to prevent coagulation to some extent, but the dramatic roles of ammonia as an anticoagulant, as a shape controller to yield highly spherical particles and as a promoter of the solidification are not likely to be explicable only from the nature of ammonia as a kind of base. 2. Precipitation Rate and Solubility. Figure 2 shows the changes in the yield of precipitate with time on 5-min and 180min scales in the standard system with 0.10 M NH3, without NH3, with 5.7 × 10-4 M NaOH instead of NH3, and in pure BuOH without AN and NH3. In the standard system, the yield of precipitate sharply increased to ca. 90% within 10 s, followed by much slower increase up to 100% by 10 min, as revealed in Figures 2a and b. From this characteristic behavior of precipita-
Formation Mechanism of Amorphous TiO2 Spheres
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18763 TABLE 1: Solubility of Ti(OH)4 Gel in Different Solvents at 25 °C solvent
solubility (M)
H2Oa C2H5OH C4H9OH CH3CN C4H9OH/CH3CN(1:1) C4H9OH/CH3CN(1:1) + 0.5 M H2O C4H9OH/CH3CN(1:1) + 0.5 M H2O + 0.1 M NH3
3.2 × 10-6 5.08 × 10-5 1.34 × 10-4 4.1 × 10-6 3.9 × 10-6 µAθ (BuOH)
(8)
µA0
if is assumed to be kept constant. On the other hand, the stability of hydrogen bonding of solvent with the hydroxide monomer may decrease in the order of H2O, EtOH, and BuOH, θ and thus the corresponding µAL n may increase in this order as θ θ θ µAL (H2O) < µAL (EtOH) < µAL (BuOH) n n n
From eqs 7-9, the order of
µ0L
(9)
for these solvents must be
µL0(H2O) < µL0(EtOH) < µL0(BuOH)
(10)
If we consider the polarity of these solvents, this order of µ0L is reasonable. Now, one may readily understand the reason for the low solubility of the hydroxide gel (or the low xAβ ) in bulk water in Table 1 despite the strong affinity of water to the hydroxide monomer, if one considers the low overall affinity of water to the hydroxide monomer (or the high µAθ ) due to the exceptionally strong affinity between water molecules (or the low µ0L), even with the strong intrinsic affinity of water to θ ). On the other hand, the high the monomer (or the low µAL n solubility in butanol seems to be mainly due to the low affinity between butanol molecules, rather than its high intrinsic affinity to the hydroxide monomer. The low solubility in acetonitrile may be explained by its small overall affinity to the monomer, due to the minor hydrogen-bond interaction with the monomer, and the strong affinity between acetonitrile molecules owing to its own high polarity. It is also noteworthy that the solubility in butanol for the hydroxide gel prepared in water and freeze-dried in Table 1 is much lower than the solubility of the hydrolysis product asprepared in butanol (g5 × 10-2 M) in Figure 2 by a factor of more than 2 orders of magnitude. This means that hydroxide monomer previously solvated by BuOH is prevented from bonding together necessary for precipitation. In other words, µA0 in this butanol system is extremely high. Conversely, the µ0A for the precipitate prepared once in bulk water, or by addition of a large amount of excess water, is much lower than prepared directly in butanol, suggesting that hydration to the hydroxide monomer may promote or reinforce, rather than prevent, the hydrogen bonding between hydroxide monomer units of the gel network, e.g., by hydrogen-bond bridging of the adsorbed water. Hence, not only the low µ0L but also the low µA0 in eq 6 may decisively contribute to the low solubility of the hydroxide gel θ , the solubility of in water. Since kT ln xAβ ) µA0 + nµ0L - µAL n θ . A is eventually a function of µA0, µ0L, and µAL n The structure models of Ti(OH)4 solid precipitated in water, pure butanol, and butanol with ammonia, and the effects of these
Formation Mechanism of Amorphous TiO2 Spheres
Figure 3. Equilibria of Ti(OH)4 monomer between liquid phase (top) and solid phase (bottom) in (a) water, (b) BuOH, and (c) BuOH + NH3, where kp and ks are the rates of deposition and dissolution, respectively; the superscripts W, B, and A denote water, butanol, and ammonia, respectively.
solvents on the solubility of each precipitate are illustrated in Figure 3 to explain the causes of the low solubility of the precipitate in water, the high solubility in butanol, and the reduction of the high solubility in butanol by ammonia. Although the distinctive two-step precipitation process, consisting of an extremely rapid first step and a much slower subsequent step, is commonly observed regardless of the presence or absence of ammonia, the extremely slow second step without ammonia seems to be caused by the gradual reduction of the solubility of the precipitate associated with its own internal stabilization equivalent to the reduction of µA0. As the solubility of hydroxide gel is known to be gradually lowered due to the stabilization of the gel network with the development of hydrogen bonding among the monomer units by aging,27,31 it is no wonder that µA0 is more or less lowered as a result of reactions in solid such as hydrogen bonding, condensation, desolvation, crystallization, etc. On the other hand, in the presence of ammonia, the second step much faster than in its absence may proceed under a supersaturation created immediately after the rapid drop of the solubility of the precipitate by ammonia. But, even in this case, the second step is much slower than its first step. Since the first step is virtually unaffected by the presence of ammonia, the effective solubility of the hydrolysis product in the initial stage appears commonly close to ca. 0.025 M from Figure 2, and thus the initial supersaturation, C - C∞, in the first step with C ≈ 0.050 M is ca. 0.025 M even in the presence of ammonia. But, since the initial supersaturation in the second step with ammonia, where C ≈ 0.025 M and C∞ ≈ 0, seems to be also close to 0.025 M, the slower second step than the first one is likely to be due to the lower kinetic constant for the deposition of monomer bound by ammonia onto the rigid and less adhesive particle surfaces with ammonia. In this sense, the precipitation mechanism of the second step with ammonia essentially differs from that in its absence, in contrast to the first step common to the systems with or without ammonia. Figure 4 exhibits the change of electric conductivity with time in the standard system with or without ammonia, or with NaOH instead of ammonia, on 180-min scale. The high-leveled electric conductivity in the system with 5.7 × 10-4 M NaOH instead of 0.10 M NH3 reveals stronger basicity and a much higher electrolyte concentration in the solution phase, including free ions of Na+ and OH- and negatively charged titanium hydroxide
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18765
Figure 4. Electric conductivity as a function of aging time on 180min scale under the standard conditions, without NH3, and with 5.7 × 10-4 M NaOH instead of NH3.
Figure 5. TG-DTA diagrams of titania powders only superficially dried immediately after their preparation under the standard conditions in the presence (a) or absence (b) of NH3.
complexes, than in the standard system with 0.10 M NH3, probably due to the adsorption loss of ammonia and its limited dissociation in the modestly polar medium in the latter. The negatively charged titanium complexes may contribute to the supernatant concentration of titanium somewhat higher than in the system without both NaOH and NH3 in Figure 2. Thus, NaOH rather increases the solubility of the hydroxide precipitate in contrast to the effect of NH3. Also, the surface negative charge of the hydroxide particles with deprotonated hydroxyl groups may reduce the possibility of their coagulation to some extent, as actually observed with the particles in Figure 1f fairly free from coagulation, compared to those in Figure 1, panels d and e, prepared in the absence of both ammonia and NaOH. But, in view of the much greater anticoagulation power of ammonia with the less surface negative charge, its main power source as an anticoagulant may not be the electric repulsive force. 3. Effect of Ammonia on the Affinity of the Hydrolysis Product to the Solvent. Figure 5 shows results of TG-DTA analysis on the TiO2 powders, which were prepared by aging at 25 °C for 2 h in the standard system with or without ammonia, and the supernatant was removed by suctional filtration. Each powder was then superficially dried without washing through passing air by additional suction on the filter for 10 min and
18766 J. Phys. Chem. C, Vol. 112, No. 48, 2008
Sugimoto and Kojima
Figure 6. TEM images of titania particles prepared under the standard conditions ([NH3] ) 0.1 M) (a); with 0.4 M NH3 (b); with 0.01 M NaClO4 added to the standard system (c).
pressing between blotters prior to the TG-DTA measurement. The gradual weight loss of 28.9 or 29.2 wt% before the first exothermic peak for each powder is mainly due to the dehydration of Ti(OH)4 to TiO2, where the theoretical weight loss is 31.1 wt%. The exothermic peaks at 250 and 261 °C for the respective powders are due to the combustion of occluded organic matters (mostly butanol as will be shown in part 3 of this series), since corresponding peaks in this temperature range are not observed in nitrogen atmosphere.32 The exothermic peaks at 479 °C (a) and 471 °C (b) correspond to the crystallization from amorphous state to anatase, as confirmed by XRD, and the associated weight loss seems to be due to the escape of remaining butanol, kept unexposed to the air in the internal texture until the transition to anatase. No DTA peak with the transition from anatase to rutile was observed for these powders. It is noteworthy that the weight loss of 6.0% at 250 °C for the powder prepared in the standard system with ammonia is significantly smaller than 11.0% at 261 °C for the powder prepared in the absence of ammonia. That is, the occlusion of butanol through the precipitation of the solvated hydroxide is much less in the particles prepared with ammonia. This result supports that the solvation of butanol to the hydroxide species is considerably blocked by ammonia. Now it seems obvious that ammonia reduces the affinity of hydroxide monomer to butanol and thus lowers the solubility of the hydroxide precipitate. This effect of ammonia leads not only to the acceleration of precipitation, but also to the achievement of the anticoagulation and the high sphericity of particles, as to be discussed afterward. 4. Effects of Ammonia as an Electrolyte. TEMs in Figure 6 show the effects of a higher concentration of ammonia and the addition of an inert electrolyte, NaClO4, to the standard system on coagulation. Obviously, a high concentration of ammonia, such as 0.4 M, or addition of 0.01 M NaClO4 is enough to cause coagulation. The sign of coagulation was already observed even with 0.2 M NH3 or 0.001 M NaClO4. Since the coagulation usually lasts throughout the precipitation of particles for nucleation and growth, the resulting size distribution is generally broadened, as clearly observed in Figure 6, panels b and c. In this case, if the coagulation in the nucleation stage is particularly enhanced, the final mean size is remarkably increased, as observed in Figure 6c. These effects may be explained in terms of the colloidal instability induced by electrolytes shielding the electrostatic repulsive force between particles.33 Conversely, it is confirmed that the repulsive force based on the electric surface charge is effective for colloidal stabilization in the organic medium as well, insofar as the electrolyte concentration is sufficiently low. Thus, an adequate concentration of ammonia may serve as a kind of base to induce negative charge on the particle surfaces, and make some contribution to anticoagulation.
Figure 7. Undissolved percentage in the TEOA treatment of the precipitates as a function of aging time in the preparation system under the standard conditions, without NH3, and with 5.7 × 10-4 M NaOH instead of NH3. The solid and dotted lines indicate the TEOA treatments for 30 min and 2 weeks, respectively.
5. Effect of Ammonia on the Condensation Rate. Figure 7 shows the percentage of solid, left undissolved by quiescent aging for 30 min at room temperature after mixing 1 cm3 of an aliquot of a suspension with 4 cm3 of 0.05 M TEOA in ethanol, as a function of aging time after the start of the reaction in the standard system with 0.10 M ammonia and in the same systems but without ammonia or with 5.7 × 10-4 M NaOH in place of ammonia. Here, TEOA was used to instantly halt the precipitation of hydroxide monomer and to leach uncondensed solid monomer. For the standard system, in particular, data after a quiescent aging for 2 weeks with TEOA are also shown. Particles prepared by aging even for 24 h in the absence of ammonia and NaOH were completely dissolved within 30 min by this TEOA treatment. This was also the case with particles aged for less than 2 min in the standard system with ammonia, but, with the progress of aging, the proportion left undissolved increased and reached 100% after 2 h. Hence one may be able to use this nature of TEOA as a criterion of the degree of condensation of the titanium hydroxide monomer. It was also found that NaOH promoted the condensation, but this effect was much smaller than with ammonia, despite the fact that the system with 5.7 × 10-4 M NaOH was actually more basic than with 0.10 M NH3 in the present organic medium, as suggested from Figure 4. In any case, it is noteworthy that the condensation process is found to occur only in the interiors of particles. On the other hand, the proportion of solid left undissolved is drastically lowered by the longer treatment with TEOA for 2 weeks; e.g., about 90% of the particles prepared in the standard system by aging for 20 min were left undissolved by the TEOA treatment for 30 min, but mostly dissolved after the treatment for 2 weeks. This means that a fairly long time is needed for complete leaching of uncondensed hydroxide molecules, maybe with partly condensed hydroxides of low molecular weights, from particles made of a well developed 3D-network of condensed polymer. If 2 weeks are assumed to be sufficient for this TEOA treatment to completely leach uncondensed hydroxide monomer, the condensation process is a much slower process than the precipitation, at least, by a factor of 3 orders of magnitude. In other words, there is almost no possibility of condensation of solute species before precipitation, and thus the condensation is not required for the precipitation of the
Formation Mechanism of Amorphous TiO2 Spheres hydrolysis product, in contrast to the conventional view in which the condensation process is a prerequisite for the precipitation of colloidal particles in sol-gel systems.4,6,10,22 The present conclusion may not be limited only to our sol-gel system of titania. For example, the condensation of hydroxide monomer may not be required for the precipitation of ordinary metal oxides such as the hydrous titania6,10 and zirconia22 particles in their sol-gel systems without ammonia in ethanol, in which the hydrolysis may finish instantly on addition of water and the precipitation of the hydrolysis products may be caused simply by their relatively low solubility in ethanol (see part 2 for more detailed discussion). In view of the extremely fast hydrolysis reaction of TBO finished almost instantly with ample water before the start of condensation, there seems to be no possibility of the alcoholreleasing condensation of remaining titanium butoxide groups (∼TiOBu + HOTi∼ f ∼Ti-O-Ti∼ + BuOH) in this system. Also, if the condensation with dehydration exclusively takes place in solid after precipitation, there seems to be no reason for limiting the dehydration process only to the condensation between different hydroxide monomer molecules. At least, we should not preclude a possibility of the self-dehydration from a single hydroxide molecule in solid; i.e., dM(OH)2 f dMO + H2O, involved in the dehydration process. Hence, the term, “condensation,” used in this paper is not limited only to the original meaning, but also involves the connotation of selfdehydration as well. Prior to the discussion on the role of ammonia in the condensation mechanism, we need to recall the fact that the condensation was much more promoted with 0.10 M NH3 than with 5.7 × 10-4 M NaOH despite the higher basicity with the latter, since it suggests a specific function of ammonia other than as a kind of base. Ammonia may possibly be coordinated to titanium ion and the hydroxyl hydrogen of the hydroxide monomer. But, the negative charge of the hydroxyl oxygen induced by coordination of ammonia to the hydroxyl hydrogen, or by deprotonation of the hydroxyl group, may have a rather minor contribution to the dehydration process, since this is an effect of ammonia only as a base much weaker than 5.7 × 10-4 M NaOH. Meanwhile, if we consider the dissociation of the much more stable Ti-OH bond but necessary for the dehydration, the coordination of ammonia to titanium ion may enhance the condensation more efficiently through donating its lonepair electrons to the titanium ion to facilitate the release of hydroxide ion from the titanium ion, leading to the reaction between the dehydroxylated titanium ion and a deprotonated hydroxide of the neighboring monomer under the basic condition. The elementary step of the condensation to form an oxobridge (∼Ti-O-Ti∼) or the self-dehydration to form an oxy double bond (∼TidO) is schematically shown in Figure 8. In the case with 5.7 × 10-4 M NaOH instead of 0.10 M NH3, the deprotonation of the hydroxyl group of Ti(OH)4 molecules may be more pronounced. But if we consider that the release of the stable hydroxyl group from Ti4+ is the rate-determining step of the condensation process, the condensation rate must be much lower than with 0.1 M NH3. This seems to be the reason for the much slower condensation with 5.7 × 10-4 M NaOH. 6. Effects of Washing Procedure. Figure 9 shows SEMs of particles collected by filtration after aging for 30 s (a) or 2 h (b) and rinsed once with ethanol by suction on the filter, followed by washing 3 times with ethanol and 4 times with water by centrifugation and freeze-drying. One may find clear evidence of dissolution with the highly porous particles damaged
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18767
Figure 8. Mechanisms of condensation and self-dehydration of titanium hydroxide monomer by the aid of ammonia.
Figure 9. SEM images of titania particles, rinsed once with ethanol by suctional filtration and washed 3 times with ethanol and 4 times with water by centrifugation, after their preparation by aging for 30 s (a) and 2 h (b).
and deformed like pumices in Figure 9a. On the other hand, perfectly spherical particles are left almost intact, but with tiny particles deposited on some of them in Figure 9b. Surprisingly, about 70% of the total weight of the particles aged for 30 s in Figure 9a was lost during the rinsing and repeated washing with ethanol, while no significant weight loss was observed for the particles aged for 2 h in Figure 9b. Since the uncondensed particles in Figure 9a are readily dissolved with ethanol, their solubility in ethanol is undoubtedly much higher than in the mixed solvent of BuOH/AN with ammonia in which the particles were precipitated (see Table 1). Also, it is obvious that their solubility in ethanol is dramatically lowered by the progressive condensation with aging for 2 h. However, on addition of water for washing the samples different in aging time after the repeated washing with ethanol, the supernatant water turned turbid for both samples, although the turbidity with the sample aged only for 30 s was much more distinct. This result reveals that the irregular shapes of the uncondensed porous particles aged only for 30 s in Figure 9a are created by the drastic dissolution with ethanol and reprecipitation of a relatively large amount of hydroxide monomer dissolved in the remaining ethanol on mixing with water as a poor solvent of the hydroxide monomer. Also, the tiny particles located on the original ones aged for 2 h in Figure 9b must have been formed by
18768 J. Phys. Chem. C, Vol. 112, No. 48, 2008
Figure 10. TG-DTA diagrams of titania particles prepared under the standard conditions, but different in final treatment: (a) suctional filtration without washing and drying on the filter by suction for 1 h and in a desiccator with silica gel for 4 days; (b) standard centrifugal washing with ethanol and water and freeze-drying.
reprecipitation of a small amount of dissolved hydroxide monomer in the remaining ethanol. If we take into account the report of Fegley and Barringer8 that the BET surface area of titania particles prepared in ethanol was dramatically increased (ca. 20 times) when washed with water instead of ethanol, the hydroxide monomer dissolved in ethanol in the interiors of the particles may be leached out by water permeating in exchange for the internal ethanol and precipitated together with the other monomer remaining in the external ethanol. Thus, the great increase of the BET surface area may be explained in terms of the removal of the internal hydroxide monomer dissolved in the internal ethanol by leaching with water, leading to a highly porous structure, unlike the explanation of Fegley and Barringer8 in terms of the surface coating of ultrafine titania particles by rapid hydrolysis of residual unreacted titanium ethoxide on the particle surfaces upon contact with water for washing. Figure 10 shows TG-DTA diagrams of powders prepared under the standard conditions, but different in final treatment: one was collected by suctional filtration and dried on the filter in the stream of air by suction for 1 h and in a desiccator with silica gel for 4 days, and the other was washed with ethanol 4 times and with water 4 times by centrifugation and freeze-dried according to the standard procedure. The very little exothermic peaks of DTA at temperatures near 250 °C at 238 and 255 °C and the distinct sharp exothermic peaks near 390 °C at 391 and 387 °C for both powders are due to the combustion of organic matters and the solid phase transition from amorphous to anatase, respectively, as evident from the XRD profiles in Figure 11 of the standard powder heated up in the same way to the temperatures indicated in the furnace of the same TG-DTA apparatus. The organic matters occluded in both powders have mostly been removed, as obvious from the negligibly small weight loss with their combustion and the solid phase transition, and thus the total weight loss of 30.3% or 19.0% is mostly due to dehydration by calcination. In this case, even the transition temperature to anatase is significantly lowered to ca. 390 °C with these powders almost free of organic matters from ca. 480 °C with the sample containing plenty of organic matters in Figure 5a. But what we should mainly note is that the significant difference in the weight loss by dehydration between the two powders in Figures 10a and b. Since the weight loss of 30.3%
Sugimoto and Kojima
Figure 11. XRD profiles of titania powders prepared and then treated in the standard procedure, and finally heated up to 300, 450, and 1000 °C at a rate of 5 °C/min.
for the powder dried in a desiccator without washing is very close to the theoretical weight loss of water ()31.1%), the water released by condensation in particles is not likely to escape even by storing in a desiccator with silica gel at room temperature for 4 days. On the other hand, since the weight loss for the powder thoroughly washed with ethanol and water, and freezedried is 19.0%, corresponding to ca. 2/3 of the theoretical weight loss, and since it is known that a Ti(OH)4 gel powder prepared in bulk water is not dehydrated at all by the same freezedrying,29 at least 1/3 of the Ti(OH)4 monomer of the particles has been dehydrated by condensation during the aging under the standard conditions, and the water released by the condensation but left in the particles can partly or totally be removed by this final treatment. If the water released by condensation is totally be removed by this final treatment, the condensation must be limited only to ca. 1/3 of the total hydroxide. Since it has been shown in Figure 7 that ca. 40% of the total solid of the particles aged for 2 h was dissolved by the TEOA treatment for 2 weeks and that ca. 15% was still dissolved with particles aged even for 24 h, the condensation is undoubtedly incomplete with particles aged for 2 h and even with those aged for 24 h. In addition, if 40% of total solid is dissolved by the TEOA treatment, much more percentage of the hydroxide may remain uncondensed, because even a limited degree of condensation seems sufficient to form a fully developed three-dimensional network which completely prevents the dissolution. In this sense, the degree of condensation limited only to 1/3 even after aging for 2 h may not be surprising. However, we found in a separate experiment that the weight loss due to the dehydration in the TG analysis was always very close to 19% in the range of 18 to19% even with powders aged for 24 h at 25 or 80 °C in the standard system and processed by the standard final treatment, although the initial condensation rate at 80 °C was ca. 10 times faster than at 25 °C. This result may suggest two possibilities. One is that the hydration level finally reaches a constant value of equilibrium through rehydration of a part of the oxide of the internal surfaces of the porous particles during the washing with water. If we consider that the BET surface area of titania particles of this kind after washing with water is usually as high as 200-300 m2/g,6,8 the partial rehydration in the washing process seems possible. The other is a possibility of an upper limit in condensation itself due to a hydration equilibrium in
Formation Mechanism of Amorphous TiO2 Spheres the presence of a constant excess water in each particle, regardless of the temperature. Actually, there was no apparent difference between the precipitates prepared by aging at 25 °C for 2 h or at 80 °C for 24 h in the turbidity of washing water added to a precipitate just after washing with ethanol and in the number density of the tiny particles deposited on the original product. Probably, both of these possibilities may actually occur. As a consequence, TG-DTA data may not necessarily reflect the degree of condensation during the particle synthesis. In this context, it is noteworthy that the rehydration, or hydrolytic depolymerization, of siloxane polymers is significantly accelerated with increasing excess concentration of water in silica systems, particularly in their basic ranges,34,35 showing that the condensation reaction is basically reversible. 7. Origins of the Versatility of Ammonia. Finally, let us give insights into the sources of the versatile roles of ammonia in sol-gel systems as an accelerator of the particle formation in the hydrolysis of alkoxide and in the precipitation of the hydrolysis product, as an anticoagulant during the particle formation, as a shape controller to yield highly spherical particles, and as an accelerator of the condensation process of the hydroxide monomer. a. Hydrolysis Accelerator. Ammonia is known as an accelerator of hydrolysis of tetraethyl orthosilicate (TEOS) used for the preparation of silica particles, but as no more than a kind of plain base.36-38 On the other hand, the hydrolysis of TBO was so fast as to finish within a few seconds even in the absence of ammonia, and thus we could not find any effect of ammonia as an accelerator of the hydrolysis. b. Precipitation Accelerator. The role of ammonia as an accelerator of the precipitation of the hydrolysis product of TBO is deemed to be due to the reduction of solubility of the hydroxide monomer in the alcoholic solvent with the decreasing affinity of the hydroxide monomer to the solvent, caused by the hydrogen-bond conjugation of ammonia to the hydroxide monomer. The yield of the precipitate was dramatically raised and reached 100% in a short time owing to this effect. If this effect of ammonia is in our knowledge, it is no wonder that additional precipitation of very fine titania particles was observed by Kumazawa et al.,13 when they added ammonia to a sol-gel system of titania particles in equilibrium with residual Ti(OH)4 solute in ethanol after the ordinary precipitation. On the other hand, if the solubility of the precipitate is fully lowered by ammonia, the nucleation must occur at a low critical concentration of the hydroxide monomer during the hydrolysis, and thus the growth rate of the generated nuclei is lowered under the low supersaturation. In this case, the final particle size must be reduced.39,40 But, in our system, we could find almost no effect of ammonia on the final particle size within the tested range up to 1.0 M NH3, as partly shown in Figures 1 and 6, which means that ammonia has virtually no effect on the solubility of the precipitate in the nucleation stage. It is in accord with the kinetic behavior of precipitation in Figure 2, in which ammonia shows no effect on the first step of the precipitation. Therefore, it seems that the relatively low concentration of ammonia cannot fully join with the hydroxide monomer to lower its solubility in the nucleation stage finished within a few seconds. It is noteworthy that ammonia works as a reagent to reduce the solubility of hydroxide precipitate in alcohols, since it usually acts as a solubilizer in water by forming ammonia complexes through its coordination to the transition metal ion, which conversely increases the solubility of the precipitate and promotes the particle growth.1
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18769 In a typical sol-gel system of silica particles prepared from tetraethyl orthosilicate (TEOS) with ammonia in ethanol, the precipitation of silica particles occurs concomitantly with the condensation of the hydroxide monomer whose rate is limited by the relatively slow hydrolysis of TEOS.36-38 Thus, the condensation of the hydroxide monomer has been thought necessary for the precipitation of silica particles,4 and this idea appears to have been extended to all other sol-gel systems. As a result, it has long been emphasized that the key role of ammonia is a catalyst for the hydrolysis of alkyl silicate and the condensation of the hydrolysis product.36-38,40 However, the hydrolysis of TEOS is even faster in acidic conditions than in basic media with ammonia or other bases, and the condensation of its hydrolysis product is always associated with hydrolysis in all pH range from acid to base.36,41,42 But, in the absence of ammonia, the precipitation of the hydrolysis product, if any, is very slow, and the precipitate is not discrete colloidal silica but only in a gel-like form.42 As a consequence, the condensation of the hydrolysis product may possibly be independent of the precipitation event, and the primary role of ammonia for precipitation of silica particles seems to be a controller of the affinity of the hydroxide monomer to alcoholic media to lower its solubility, as found in the present titania system. c. Anticoagulant. The role of ammonia as an anticoagulant is also prominent. If the lyophilicity or the affinity of hydroxide monomer to its solvent is excessively high, the solvated hydroxide monomer in particles may make their texture soft and their surfaces adhesive. In such a system, the coagulation of the particles is very fast and usually irreversible. On the contrary, if the lyophilicity of the hydroxide monomer is lowered by the presence of ammonia, the solid structure becomes more rigid with much less occluded solvent, and the particle surfaces lose its adhesiveness. In this case, the solubility of the precipitate is lowered, so that the specific surface energy () surface energy per unit area) of the particles must be increased.1,43,44 The increase of the specific surface energy may tend to promote the aggregative fusion among particles from a thermodynamic viewpoint because of its basic requirement for minimizing the total surface energy of the system. Nevertheless, the loss of adhesiveness kinetically rather retards the coagulation. In addition, if the concentration of ammonia is set at an adequate level, the coagulation of the particles may further be prevented owing to the electrostatic repulsive force of the particles with negative surface charges. But, if the concentration of ammonia is excessively high, not only the coagulation is conversely accelerated due to the high ionic strength, but also the specific surface energy is still more raised. In this case, the irreversible coagulation among particles may be rather enhanced again, but through the deposition of solute onto the contact points of the attached particles for cementing them together to reduce the total surface energy as much as possible.1 The enhanced coagulation with the thermodynamic effect of excessive ammonia appears to be involved in the coagulation of the particles with 0.4 M NH3 in Figure 6b. These adverse effects of ammonia may lead us to the elucidation of the well-known dramatic increase of the final particle size with increasing concentration of ammonia up to 7 M in the sol-gel system of silica particles.4,37 As mentioned in the preceding section, if the solubility of hydroxide monomer in the nucleation stage is simply lowered by ammonia, the final particle size must be reduced. Since the final particle size is nevertheless increased with increasing concentration of ammonia, there must be an offsetting coagulation process of the nascent nuclei enhanced by ammonia. Thus, the effect of
18770 J. Phys. Chem. C, Vol. 112, No. 48, 2008 ammonia on the final particle size of silica particles must be understood in terms of the reduction of the particle number due to the enhanced coagulation of generated nuclei by the excessive ammonia in the nucleation stage. This deductive conclusion may logically support the similar explanation of Van Blaaderen et al. inferred from their finding that the final size of silica particles was significantly increased by increase in ionic strength with LiNO3 in the early precipitation stage.38 But, this effect of ammonia was not observed in our titania system, probably because the nucleation finished earlier than the added ammonia getting effective. d. Shape Controller. The increased specific surface energy of the particles by ammonia may contribute to the achievement of the highly spherical shape of each particle, because the thermodynamic requirement for minimizing the surface area of each particle is reinforced by the increase in the specific surface energy. The inhibition of coagulation in the growth stage by ammonia may make an additional contribution to the excellent sphericity. These shape control mechanisms with ammonia may work in the sol-gel system of silica spheres as well. Generally, these effects of ammonia for production of spherical particles are available only when the surface rearrangement of the deposited solute for crystallization is strictly prevented by the hydration and/or solvation of the solute and a relatively low temperature so that the product is basically amorphous. If we perform the hydrolysis of TBO in pure water, we cannot obtain a particulate product, but only a hydroxide gel as a continuous precipitate. Since the solubility of this precipitate is very low as shown in Table 1, the heavily hydrated continuous precipitate must have a sufficiently high specific surface energy, despite the high intrinsic affinity of water to the hydroxide monomer. In this case, some supersaturation is always required for its precipitation to clear the free energy barrier for nucleation in proportion to the third power of the specific surface energy, and thus the precipitation must be associated with a nucleation process. But the generated nuclei may soon be coagulated together, resulting in a continuous gel-like precipitate, because of their extremely adhesive and soft texture with a high content of water and the high specific surface energy to minimize the total interfacial area. e. Condensation Accelerator. The role of ammonia as an accelerator of condensation has been explained in terms of the coordination of NH3 to the Ti4+ ion of Ti(OH)4 monomer within each particle, leading to its dehydroxylation. Actually, the binding energy of NH3 with titanium ion is expected to be even greater than that of H2O from a relevant theoretical calculation.43 For example, the effect of ammonia as a powerful shape controller to yield ellipsoidal anatase titania particles clearly shows its strong affinity to the surface titanium ion of the crystalline TiO2 particles grown in an aqueous solution.27 The specific role of ammonia in the condensation of titanium hydroxide may analogously apply to other general transition metal hydroxides. Also, ammonia is known to accelerate the condensation of the hydrolysis product of alkyl silicate by accelerating the hydrolysis as a kind of base.36,41 But the specific effect of ammonia on the condensation rate of silicic acid is left unknown. Apart from the role of ammonia, the hydrolysis of tetramethyl orthosilicate (TMOS) in methanol was found to be much faster than the condensation of the hydrolysis product in an acidic range in the study with 1H NMR and/or 29Si NMR by Assink and Kay.46 Since the condensation process does not seem to be needed for the prompt precipitation of silicic acid with ammonia, the condensation in a sol-gel system of TMOS with ammonia
Sugimoto and Kojima may proceed within the silica particles after their total precipitation like the present titania system. But, even in a system of TEOS with ammonia, in which the hydrolysis process is so slow that at least a part of the condensation process occurs at the same time with the hydrolysis and prompt precipitation,37,38 one may need to consider a possibility of precipitation faster than condensation proceeding mostly within silica particles. In summary, the origin of the versatility of ammonia in sol-gel systems for general oxide particles may mostly be ascribed to its hydrogen-bond interaction with the hydroxyl group of the hydrolysis product. But, in case of transition metal oxides, its nucleophilic coordination to the metal ion may work in addition to it. Conclusions 1. The roles of ammonia in the present system for the precipitation of amorphous titanium hydroxide particles may be summarized as a precipitation accelerator, anticoagulant, shape controller, and condensation accelerator. Ammonia seems to control the affinity of the hydroxide monomer to butanol, to reduce the solubility of the hydroxide monomer as a precipitation accelerator, to make the particles less adhesive as an anticoagulant, and to regulate the surface energy of the product particles as a shape controller. The repulsive force between particles based on the electrostatic negative surface charge created by ammonia also reduces the probability of coagulation. As a condensation accelerator, ammonia may coordinate to the titanium ion of hydroxide monomer and promote the release of hydroxide ion from titanium ion. 2. The condensation process is much slower than the precipitation process and proceeds only in the precipitated particles, so that it is independent of the precipitation event. 3. Ammonia may reduce the affinity of hydroxide monomer to butanol by hydrogen-bond-type linkage with the hydroxyl group of the monomer to reduce the solvation of butanol to the hydroxyl group of the monomer, unlike the reduction of the affinity of the BuOH/AN(1:1) solvent to the hydroxide monomer based on the low affinity of acetonitrile to the butanol-solvated monomer. 4. The high solubility of hydroxide gel in butanol seems to be mainly due to the low affinity between butanol molecules, rather than its high intrinsic affinity to hydroxide monomer. The solubility is specifically high when the hydrolysis product is generated in bulk butanol, because of the low affinity between the solid monomer molecules solvated by butanol. On the other hand, the low solubility of the hydroxide gel in water seems to be owing to the strong affinity between water molecules, in addition to the highly stable hydrogen bond between solid monomer units of the gel network promoted by hydration, despite the strong intrinsic affinity of water to the hydroxide monomer. 5. The above conclusions may basically be applicable to almost all sol-gel systems, but the effect of ammonia on the condensation rate in silica systems is still left unknown. References and Notes (1) Sugimoto, T. Monodispersed Particles; Elsevier: Amsterdam, 2001. (2) Sugimoto, T., Ed.; Fine Particles: Synthesis, Characterization, and Mechanisms of Growth; Marcel Dekker: New York, 2000. (3) Kolbe, G. Das Komplexchemische Verhalten der Kieselsa¨ure; Dissertation: Jena, 1956. (4) Sto¨ber, W.; Fink, A.; Bohn, J. J. Colloid Interface Sci. 1968, 26, 62. (5) Barringer, E. A.; Bowen, H. K. J. Am. Ceram. Soc. 1982, 65, C-199. (6) Barringer, E. A.; Bowen, H. K. Langmuir 1985, 1, 414. (7) Barringer, E. A.; Bowen, H. K. Langmuir 1985, 1, 420.
Formation Mechanism of Amorphous TiO2 Spheres (8) Fegley, B., Jr.; Barringer, E. A. In Better Ceramics Through Chemistry; Brinker, C. J., Clark, D. E., Ulrich, D. R., Ed.; Elsevier: New York, 1984; p 187. (9) Ikemoto, T.; Uematsu, K.; Mizutani, N.; Kato, M. Yogyo-KyokaiShi 1985, 93, 261. (10) Jean, J. H.; Ring, T. A. Langmuir 1986, 2, 251. (11) Jean, J. H.; Ring, T. A. Am. Ceram. Soc. Bull. 1986, 65, 1574. (12) Mates, T. E.; Ring, T. A. Colloids Surf. 1987, 24, 299. (13) Kumazawa, H.; Otsuki, H.; Sada, E. J. Mater. Sci. Lett. 1993, 12, 839. (14) Shida, K.; Kawai, T.; Kon-no, K. Abstracts of 50th Symposium on Colloid and Interface Chemistry; Chem. Soc. Japan: Saga, 1997; p. 310. (15) Eiden-Assmann, S.; Widoniak, J.; Maret, G. Chem. Mater. 2004, 16, 6. (16) Widoniak, J.; Eiden-Assmann, S.; Maret, G. Colloids Surf. A 2005, 270, 329. (17) Mine, E.; Hirose, M.; Nagao, D.; Kobayashi, Y.; Konno, M. J. Colloid Interface Sci. 2005, 291, 162. (18) Pal, M.; Serrano, J. G.; Santiago, P.; Pal, U. J. Phys. Chem. C 2007, 111, 96. (19) Fegley, B., Jr.; White, P.; Bowen, H. K. Am. Ceram. Soc. Bull. 1985, 64, 1115. (20) Ikemoto, T.; Mizutani, N.; Kato, M.; Mitarai, Y. Yogo-Kyokai-Shi 1985, 93, 109. (21) Ogihara, T.; Mizutani, N.; Kato, M. Ceram. Intern. 1987, 13, 35. (22) Ogihara, T.; Mizutani, N.; Kato, M. J. Am. Ceram. Soc. 1989, 72, 421. (23) Uchiyama, K.; Ogihara, T.; Ikemoto, T.; Mizutani, N.; Kato, M. J. Mater. Sci. 1987, 22, 4343. (24) Ogihara, T.; Ikemoto, T.; Mizutani, N.; Kato, M.; Mitarai, Y. J. Mater. Sci. 1986, 21, 2771. (25) 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., Ed.; Wiley Interscience: New York, 1987; p 407.
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18771 (26) Ogihara, T.; Kaneko, H.; Mizutani, N.; Kato, M. J. Mater. Sci. Lett. 1988, 7, 867. (27) Sugimoto, T.; Okada, K.; Itoh, H. J. Disper. Sci. Technol. 1998, 19, 143. (28) Jeffery, P. G.; Gregory, G. R. E. C. Analyst 1965, 90, 177. (29) Sugimoto, T.; Zhou, X.; Muramatsu, A. J. Colloid Interface Sci. 2002, 252, 339. (30) The Merck Index, 7th ed.; Merck & Co.: Rahway (New Jersey), 1960. (31) Sugimoto, T.; Zhou, X.; Muramatsu, A. J. Colloid Interface Sci. 2003, 259, 43. (32) Mizutani, N. In Chobiryushi to Zairyo; Hojo, H., Ed.; Shohkabo: Tokyo, 1993; p 151. (33) Overbeek, J. Th. G. In Colloid Science; Kruyt, H. R., Ed.; Elsevier: Amsterdam, 1952; p 245. (34) Brinker, C. J. J. Non-Cryst. Solids 1988, 100, 31. (35) IlerR. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979; p 172. (36) Aelion, R.; Loebel, A.; Erich, F. J. Am. Chem. Soc. 1950, 72, 5705. (37) Matsoukas, T.; Gulari, E. J. Colloid Interface Sci. 1988, 124, 252. (38) Van Blaaderen, A.; Van Geest, J.; Vrij, A. J. Colloid Interface Sci. 1992, 154, 481. (39) Sugimoto, T. J. Colloid Interface Sci. 2007, 309, 106. (40) Sugimoto, T. In Encyclopedia of Surface and Colloid Science; Hubbard, A. T., Ed.; Marcel Dekker: New York, 2002; p 3208. (41) Kelts, L. W.; Effinger, N. J.; Melpolder, S. M. J. Non-Cryst. Solids 1986, 83, 353. (42) Brinker, C. J.; Keefer, K. D.; Schoefer, D. W.; Assink, R. A.; Kay, B. D.; Ashley, C. S. J. Non-Cryst. Solids 1984, 63, 45. (43) Sugimoto, T. J. Colloid Interface Sci. 1996, 181, 259. (44) Sugimoto, T.; Shiba, F. J. Phys. Chem. 1999, 103, 3607. (45) Magnusson, E.; Moriarty, N. W. Inorg. Chem. 1996, 35, 5711. (46) Assink, R. A.; Kay, B. D. J. Non-Cryst. Solids 1988, 99, 359.
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