Solvent Effects in the Deaggregation of Titania N Anoparticles +

Solvent Effects in the Deaggregation of Titania N anoparticles+

Danijela Vorkapic Air Products and Chemicals, Inc.* Themis Matsoukas Department of Chemical Engineering, Pennsylvania State University**

Abstract

We report on the effect of alcohols in the acid peptization of aggregated titania nanoparticle pro­ duced from alkoxides. Peptization was studied in the presence of each one of the following alcohols: methanol, ethanol, propanol and isopropanol. We find that the final particle size is correlated to the dielectric constant of the peptizing medium. Kinetic measurements reveal that the rate ofdeaggrega­ tion is not affected by the presence of alcohol; however, the tendency for reaggregation of the peptized colloid increases significantly. We conclude that alcohols prevent the full redispersion of the aggre­ gates by decreasing the colloidal stability of the suspension. This conclusion is supported by the mea­ sured zeta potential of the peptized particles, which is found to decrease when alcohol is present.

tion mixture to provide uniform mixing of the alkoxide I. Introduction and water, which react rapidly upon contact. Even if The novel optical, electronic, chemical, and struc­ no alcohol is added during synthesis, some amount is tural properties of materials fabricated from nanopar­ unavoidably present as a product of the hydrolysis ticle precursors have motivated a substantial research reaction. The effect of alcohols in titania precipitation effort in the synthesis of ultrafine particles [1-5]. is summarized in fig. 1 which shows that the final A common problem is that such particles are often particle size increases substantially as the concentra­ obtained in highly aggregated form, primarily due to tion of the alcohol is increased. This effect is stronger the difficulty in stabilizing nanometer size particles in isopropanol, weaker in butanol, and intermediate in against aggregation. With some systems it is possible propanol (only two experimental points are given for to reverse the effect of aggregation and redisperse the aggregates by peptization in a suitable chemical environment. A characteristic example is the forma­ 80 isopropoxide tion of titania nanocolloids from the hydrolysis and in iPrOH polycondensation of titanium alkoxides. This reaction produces large aggregates composed of ultrafine pri­ 60 mary particles (3 to 5 nm) which can be redispersed E' through acid peptization. The degree to which redis­ 5 .... persion is achieved varies widely. The size of the pep­ .2 tized particles is reported in the literature to range gE"' 40 from about 15 nm [6] to more than 100 nm [7], which -;;; implies that the degree of redispersion is highly vari­ .s;,.. able depending on the experimental conditions. 20 In our previous studies we have shown that the degree of redispersion has a strong dependence on ~ butoxide the type and amount of alcohol present in the peptiza­ in BuOH tion medium [6]. Often, alcohol is added to the reac- 0 0 2 3 4 5 6 Alcohol Concentration (M) * 7201 Hamilton Boulevard, Allentown, PA 18195-1501 **University Park, PA 16802 Fig. 1 Size of peptized aggregates as a function of the concentra­ + Received: May 16, 2000 tion of alcohol in the peptizing medium.

102 KONA No.18 (2000) the butoxide/butanol system because of the limited tion. All measurements and theoretical analyses in solubility of butanol in water). This trend can be attrib­ this paper are for this slow part of the process. uted to two possible effects: the alcohol may inhibit Particle sizes were analyzed by withdrawing sam­ the peptization of the aggregated colloid, or/and it ples from the peptization medium, diluting them in may enhance the reaggregation of the dispersed par­ water, and measuring the hydrodynamic diameter by ticles. Both mechanisms would result in larger final light scattering (2030AT Brookhaven model using a particles. We have recently proposed a kinetic model He-Ne laser operating at A-=632.8 nm). The reported of peptization based on the idea that redispersion is sizes represent the average of 3 measurements. Zeta the result of competition between the peptization of potential measurements were performed in a Zeta aggregated particles and the reaggregation of the PALSE model by Brookhaven Instruments. peptized colloid, and have shown that the model pro­ vides a quantitative description of the peptization of III. Peptization model titania [8]. This model allows us to measure experi­ mentally the rate constants for peptization and reag­ Our interpretation of the peptization experiments gregation and to correlate them to the process para­ is based on a reversible aggregation/ deaggregation meters. The goal of the present study is to elucidate model. The model has been described in detail else­ the mechanism by which alcohols inhibits the full where [8] and its salient features are summarized peptization of titania nanocolloids by measuring the here. The basic premise is that while deaggregation peptization and reaggregation rate constants in the produces smaller particles (fragments) from a cluster presence of various low-molecular weight alcohols. of primary particles but the fragments are subject to reaggregation, as shown schematically in fig. 2. In this picture a "particle" is a cluster of aggregated II. Experimental primary particles and the size of the cluster is deter­ Titania nanoparticles were synthesized by reaction mined by the competition between deaggregation and between titanium isopropoxide and water in the pres­ reaggregation. Steady-state is reached when the rates ence of nitric acid. The concentration of the alkoxide of the two processes are balanced. This condition was 0.23 M and the amount of acid corresponds to defines the final size and the degree of redispersion [H +]I [Til molar ratio of 0.5. A specified amount of that can be achieved. This model provides a simple nitric acid (J.T. Baker) was mixed with distilled water interpretation of the observed final particle size with­ in a glass bottle and the solution was placed in a tem­ out explicit reference to the mechanisms responsible perature controller bath maintained at 50°C. Titanium for the peptization of the aggregates, and allows the isopropoxide (supplied by Aldrich) was added drop­ calculation of the rate constants from kinetic experi­ wise as the solutions were constantly stirred at 300 ments. Treating the deaggregation rate as a first-order RPM. Titania is formed according to the reaction process in the concentration of particles, the aggrega­ tion rate as second-order process, and equating the Ti(iPr0) +2H 0 -tTi0 +4iPrOH 4 2 2 rate of the two processes we obtain the following The precipitation of particles is immediately mani­ expression for the cluster size, Dx, at steady state [8]: fested by the formation of a highly turbid suspension. This suspension was divided into 5 equal samples and Dx _ ( 3C Ka )1dt (1) D - , 1rpD~ Kd was let to stand for five minutes. The samples were 0 then mixed with a specified volume of an aqueous where Do is the size of the primary particles, C is the solution containing either methanol, ethanol, propanol, mass concentration of titania, pis the material density, isopropanol, or water only (no alcohol). Within a few hours a white-blue solution was observed indicating the progress of peptization. After 6 h of continuous stirring, intermittent stirring was applied (20 s of stir­ ring followed by 10 s of rest) to minimize shear-induced aggregation. After 10 h, the stirring was turned off and the temperature was set to 25°C. Under these conditions peptization continues for several days as indicated by the decrease of the measured particle Fig. 2 Schematic representation of the deaggregation/reaggre­ size and by the increased transparency of the solu- gation model of peptization.

KONA No.18 (2000) 103 Ka, Kd, are the aggregation and deaggregation (pepti­ same concentrations of titania and acid but differ in zation) rate constants, respectively, and d1 is the frac­ the type of alcohol that is present. tal dimension (d1=3 for compact particles, d1 <3 for The kinetic experiments are summarized in fig. 3 fractal clusters). If we neglect the dependence of the which shows the size (hydrodynamic diameter) of the rate constants on size, we find that the approach to peptizing aggregates as a function of time over a period the final steady-state size is given by [8] of one week. In support of our previous findings, the presence of alcohol results in larger final sizes, thus (2) lower degree of redispersion of the aggregated nanoparticles. The alcohol effect is most pronounced where A is constant. Equations (1) and (2) provide at the early stages of peptization. For example, after the basis for interpreting the peptization experiments. one day of peptization the size in water/isopropanol is According to Eq. (2), the deaggregation rate constant about 90 nm compared 30 nm in water. The differ­ can be obtained from the slope of a semilog plot of ence among various alcohols decreases with peptiza­ D!Dx -1 versus time. Once Kd is known, the aggrega­ tion time but even so the final size clearly reflects the tion rate constant, Ka, is calculated from Eq. (1). In environment in which peptization took place. The this manner we can obtain the rate constants for deag­ ranking of the solvents in terms of dispersion effi­ gregation and reaggregation from measurements of ciency is: water>methanol>ethanol>propanol>iso­ the size as function of time. Detailed tests and discus­ propanol. The dielectric constant of the correspond­ sion of the validity of this model for the peptization of ing liquids at 25°C is 78.5, 32.6, 24.3, 20.1 and 18.1, for titania can be found in Ref. [8]. water, methanol, ethanol, propanol and isopropanol, respectively [9]. Thus, the quality of the solvent in terms of dispersion efficiency is in the order of IV. Results and Discussion increasing dielectric constant. This order is preserved In our previous work [6], the alcohol was added even after the dielectric constant of the medium is along with all the other reactants and thus was pre­ adjusted for the amount of water that is present (see sent during precipitation as well as during peptization. table 1). To remove any possible effect of the alcohol on the From these experiments we extract the rate con­ size of the primary particles and the structure and stants for deaggregation and reaggregation. The deag­ cohesiveness of the precipitates, in all of the experi­ gregation rate constant was obtained by fitting Eq. (2) ments reported here the alcohol was added after the to the data of figure 3 and the solid lines in that fig­ precipitation of titania. This ensures that the precipi­ ure represent these fits. The reaggregation Ka is cal­ 3 tate is formed under identical conditions and that any culated from Eq. (1) with d1=1.72 and p=3.84 g/cm subsequent differences are solely due to effects dur­ ing peptization. The particles were prepared by react­ 100~---L--~--~----L_ __l_ __ ~--~

ing titanium isopropoxide and as a result, 0.92 M of 0 isopropanol -water isopropanol is present in all of our samples during D propanol- water peptization (assuming complete hydrolysis of the D. ethanol water 80 0 methanol- water alkoxide). This amount is in addition to 3 M of the 'V water selected alcohol that is mixed after precipitation and 6 is the same in all samples. The following alcohols 5 .... were included in this study: methanol, ethanol, .2 60 propanol and isopropanol. Butanol is not part of this ge list because it has limited solubility in water (approxi­ mately 0.5 M at room temperature). In addition, one sample was peptized without adding alcohol. An 40 equivalent volume of water was added to this sample to bring the concentration of titania to the same level as that of the alcohol-containing samples. This adjust­ ment is necessary in order to obtain the same concen­ 0 2 3 4 5 6 7 tration of titania in all samples (recall that according Time (days) to Eq. (1) the final size is also function of the concen­ Fig. 3 The hydrodynamic diameter of peptizing particles as tration of titania). Therefore, all samples contain the function of time. The lines are fits based on Eq. (2).

104 KONA No.18 (2000) Table I Summary of results Both the zeta potential and the dielectric constant of

water/ water/ water/ water/ the medium are important for stability against aggre- water MeOH EtOH PrOH iPrOH gation. In a simplified picture, the stability factor, W, D(nm) 21.5 24.5 27 32 34 of the suspension can be expressed as [12]

E of solution (25°C) 78.54 74 70.3 67.4 66.8 W""' exp(Vn,ax/kBT)/KR (3) zeta potential (m V) 20 15 9.5 8.0 7.5 where Vmax is the maximum repulsive potential, ap- 21 Ka X 10 (em' Is) 3.90 5.00 7.05 8.72 10.5 proximately equal to the electrostatic repulsion be- Kd (1/day) 0.890 0.905 1.07 0.976 1.05 tween the zeta potentials of two particles,

(4) 12 In the above, K is the inverse screening length, T is the temperature, kB is Boltzmann's constant, c. is the 9 0 Kax 1021 (cm'1/s) 10 0 Kd (1/days) dielectric constant of the solvent, c.0 is the permittivity of free space, R is the particle radius and ll's is the zeta potential. In the presence of alcohols the zeta 8 c potential is found to be lower by as much as a factor 19

ders by the thermal hydrolysis of Zr0Cb·8H20 in an [8]. The results are shown in fig. 4 where we plot the alcohol/water solution [13]. These authors showed rate constants as a function of the dielectric constant that the presence of alcohols greatly influences the of the solvent. The aggregation rate constant decreases morphology and the size of final zirconia particles. significantly, by a factor of more than 2, as we go Particles obtained in ethanol/water solution have soft from the low-dielectric constant solvent (isopropanol/ aggregates made of ultrafine primary particles. In water) to the high-dielectric constant solvent (water). case of the tert-BuOH/water solution, the particles By contrast, the deaggregation rate constant remains were large and spherical with a broad size distribu­ unaffected. tion whereas the particles obtained from propanol/ The correlation between rate of aggregation and water and isopropanol/water solutions were small dielectric constant is strong evidence that the observed and spherical with a narrow size distribution. Similar behavior is due to reduced colloidal stability. To ex­ observations were reported for titania synthesized plore this hypothesis we performed measurements of under low water-to-titanium ratios [10, 14, 15] and for the zeta potential of the peptized particles and the silica precipitated from alkoxides [16, 17]. In all of results are reported in table 1. The measured poten­ these systems, spherical, or almost spherical, parti­ tial follows the same trend as the aggregation rate cles grow by accretion of precursor units - oligomers constant, namely, it decreases with decreasing dielec­ or small particles - to final sizes considerably larger tric constant of the solvent. The decrease of the sur­ than the ones obtained in the present study. Despite face potential is analogous to that observed in other differences between these systems and the peptizing colloids and is attributed to the adsorption of the alco­ environment of our study, our results provide direct hol on the oxide surface and the subsequent reduc­ evidence that the rate of aggregation is indeed higher tion in the number of the ionized surface sites [10,11]. in the presence of alcohols and that the size increase

KONA No.l8 (2000) 105 can be attributed entirely to colloidal destabilization References brought about the alcohol. A second important finding of this study is that 1) Lafait and S. Berthier, in Nanophase Materials: Synthe­ alcohols have no effect in the deaggregation process sis-Properties-Applications (NATO ASI Series, Kluwer itself, as demonstrated by the constant value of Kd in all Scientific, Boston, 1993), p. 449. 2) L. E. Brus, in Nanophase Materials: Synthesis-Properties­ samples. Therefore, neither the type nor the amount Applications (NATO ASI Series, Kluwer Scientific, of alcohol interferes with the breakage of aggregates. Boston, 1993), p. 433. This result supports the notion that the mechanism of 3) 0. Koper and K. ]. Klabunde, in Nanophase Materials: deaggregation is not electrostatic but rather chemical Synthesis-Properties-Applications (NATO ASI Series, in nature, involving the breakage of chemical bonds Kluwer Scientific, Boston, 1993), p. 789. between adjacent primary particles [8]. 4) A. Tschope and]. Y. Ying, in Nanophase Materials: Syn­ thesis-Properties-Applications (NATO ASI Series, Kluwer Scientific, Boston, 1993), p. 781. V. Conclusions 5) R. W. Siegel and G. E. Fougere, in Nanoparticles in Solids and Solutions, edited by]. Fender and I. Dekany (NATO The degree of redispersion of titania precipitates ASI Series, Kluwer Scientific, Boston, 1996), p. 233. formed by the aggregation of nanometer-size primary 6) D. Vorkapic and T. Matsoukas,]. Am. Cer. Soc. 81, 2818 particles is affected by the presence of alcohols in the (1998). peptizing solution. Alcohols hinder the dispersion of 7) C. Lijzenga, V. T Zaspalis, K. Keizer and A. ]. Burrgraaf, the aggregated particles and result in larger particle Key Engineering Materials 61-62, 379 (1991). sizes. This effect can be attributed entirely on the 8) D. Vorkapic and T Matsoukas, ]. Colloid Interface Sci. 214,283 (1999). reduced colloidal stability of the suspension which 9) Handbook of Chemistry and Physics, edited by R. C. Weast enhances the reaggregation of the peptized aggre­ (CRC Press, Boca Raton, 1974). gates. The peptization mechanism itself is not affected 10) H. K. Park, D. K. Kim, and C. H. Kim, ]. Am. Ceram. by the presence of the alcohol and its rate constant Soc. 80, 743 (1997). remains the same regardless of the amount or type of 11) P Hesleitner, N. Kallay, and E. Matijevic, Langmuir 7, alcohol present. 178 (1991). 12) R. ]. Hunter, Zeta Potential in Colloid Science (Acade­ mic Press, New York, 1981). Acknowledgements 13) Y. T Moon, H. K. Park, D. K. Kim, and C. H. Kim, ]. Am. Ceram. Soc. 78, 2690 (1995). This work was supported by the National Science 14) M. T Harris and C. H. Byers, J. of Non-Crystalline Foundation under grant CTS#9702653. Solids 103, 49 (1988). 15) M. T Harris, 0. A. Basaran, and C. H. Byers, Mat. Res. Soc. Symp. Proc. 271, 291 (1992). 16) W. Stober and A. Fink, ]. Colloid Interface Sci. 26, 62 (1968). 17) T Matsoukas and E. Gulari, ]. Colloid Interface Sci. 124, 252 (1988).

106 KONA No.18 (2000) I Author's short biography I

Danijela Vorkapic Danijela Vorkapic was born in Yugoslavia and received her B.S. degree in chemical engineering from the University of Belgrade. During her undergraduate studies she joined North Carolina State University, Department of Material Science, for three months and worked as an undergraduate research assistant while helping design and conduct experiments on the atomic layer epitaxy of Si, SiC, and GaN. After receiving her B.S. degree, she worked in "Duga", a paints and varnishes com­ pany in Belgrade, as a junior engineer of process development optimizing proc­ esses of coatings technology. Later, she joined Penn State University to pursue her Ph.D. under the guidance of Dr. Themis Matsoukas. For her Ph.D. dissertation she conducted studies on the kinetics and colloidal stability of nanosize titania for­ mation for the qualitative and quantitative understanding of process mechanisms. After completing her Ph.D. she joined Air Products and Chemicals Inc where she is now a senior research engineer.

Themis Matsoukas Themis Matsoukas received his undergraduate degree in Chemical Engineering from the National Technical University in Athens, Greece, and his Ph.D. from the University of Michigan. He was a postdoctoral researcher at UCLA and later joined the Pennsylvania State University where he is now Associate Professor of Chemical Engineering. He has conducted experimental and theoretical research in particulate systems, including synthesis and characterization of nanocolloids, modeling of particle growth in liquid and gas-phase media, the sol-gel synthesis of silica and titania colloids and gels, numerical methods for the solution of population balances, and the plasma processing of particulate materials.

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