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 of deaggrega­ 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.