4488
Langmuir 2002, 18, 4488-4494
Kinetics and Particle Formation Mechanism of Zinc Oxide Particles in Polymer-Controlled Precipitation from Aqueous Solution Andreas Taubert,†,* Gunnar Glasser, and Dennis Palms Max-Planck-Institute for Polymer Research, D-55128 Mainz, Germany Received December 13, 2001. In Final Form: February 26, 2002 The morphogenesis, particle size, size distribution, and phase evolution of zinc oxide precipitated in the presence of water-soluble poly(ethylene oxide-block-methacrylic acid) (P(EO-b-MAA)) and poly(ethylene oxide-block-styrene sulfonic acid) (P(EO-b-SSH)) diblock copolymers is reported. Without a polymeric additive, spindlelike particles with a central grain boundary form along with multiply twinned particles. After ∼30 min, the multiple twins are gone, small, needlelike crystals appear, and the sample becomes more polydisperse. With P(EO-b-MAA) copolymers, initially more rounded particles with the same central grain boundary and a narrow size distribution form. They preferentially grow along the crystallographic c-axis and eventually adopt a hexagonal prismatic shape. With P(EO-b-SSH) copolymers, a lamellar intermediate precipitates first. It eventually dissolves and hexagonal prismatic crystals form; in a subsequent growth process unique to these polymeric additives, the crystals grow along the crystallographic a-axis and transform to another morphology termed the “stack of pancakes” shape. Both in the absence of polymer and with P(EO-b-MAA) copolymers, multiple particle generations precipitate. With P(EO-b-SSH) copolymers, no second generation is observed. Nucleation is delayed by the P(EO-b-MAA) copolymers, while P(EOb-SSH) copolymers favor the rapid nucleation of the highly ordered lamellar intermediate.
1. Introduction Polymer-mediated mineralization of inorganic materials has been the subject of intense research because polymers and supramolecular assemblies have been found to dramatically influence the characteristics of an inorganic precipitate. The ability to influence the morphology and phase of an inorganic precipitate has important technological implications, because it may lead to improved scaling inhibitors or allow the construction of synthetic composite materials that mimic the structure of human bone. Many authors1-32 have investigated the role of synthetic14-16,26-34 and biopolymers,1-5,22,23,35 supramolecular assemblies,6-8,34 thin films,36,37 and gels38 on the mineralization process. Several studies found that * To whom correspondence should be addressed. Fax: +1 215 573 2128. E-mail:
[email protected]. † Present address: Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA. (1) Berman, A.; Hanson, J.; Leiserowitz, L.; Koetzle, T. F.; Weiner, S.; Addadi, L. Science 1993, 259, 776. (2) Berman, A.; Hanson, J.; Leiserowitz, L.; Koetzle, T. F.; Weiner, S.; Addadi, L. J. Phys. Chem. 1993, 97, 5162. (3) Aizenberg, J.; Hanson, J.; Koetzle, T. F.; Weiner, S.; Addadi, L. Chem.sEur. J. 1995, 1, 414. (4) Weiner, S.; Addadi, L. J. Mater. Chem. 1997, 7, 689. (5) Bouropoulos, N.; Weiner, S.; Addadi, L. Chem.sEur. J. 2001, 7, 1881. (6) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6, 9. (7) Mann, S. J. Mater. Chem. 1995, 5, 935. (8) Mann, S.; Burkett, S. L.; Davis, S. A.; Fowler, C. E.; Mendelson, N. H.; Sims, S. D.; Walsh, D.; Whilton, N. T. Chem. Mater. 1997, 9, 2300. (9) Falini, G.; Gazzano, M.; Ripamonti, A. J. Cryst. Growth 1994, 137, 577. (10) Falini, G.; Gazzano, M.; Ripamonti, A. Adv. Mater. 1994, 6, 46. (11) Falini, G.; Albeck, S.; Weiner, S.; Addadi, L. Science 1996, 271, 5245. (12) Falini, G.; Fermani, S.; Gazzano, M.; Ripamonti, A. Chem.s Eur. J. 1997, 3, 1807. (13) Bertoni, E.; Bigi, A.; Falini, G.; Panzavolta, S.; Roveri, N. J. Mater. Chem. 1999, 9, 779. (14) Go¨ltner, C. G.; Antonietti, M. Adv. Mater. 1997, 9, 431. (15) Co¨lfen, H.; Antonietti, M. Langmuir 1998, 14, 582.
polymeric additives control not only the particle shape, size, and size distribution of the precipitate15,16,22,23,25,27-29,31-33,39-45 but also the crystal phase.25,38,46-48 Thus, by simply adding a polymer to a reaction solution, monodisperse and uniform inorganic particles with a well-defined crystal phase may be obtained in many cases. Despite the many published studies on how polymer composition and polymer concentration affect precipitation and crystallization, there is still a lack of understanding of how the polymers interact with the growing crystals on a molecular scale, especially at short reaction times and with the progression of the reaction. Few studies5,21,30,35,49,50 have addressed the question of the crystal growth mech(16) Co¨lfen, H.; Limin, Q. Chem.sEur. J. 2000, 7, 106. (17) Fendler, J. H. Supramol. Chem. 1995, 6, 209. (18) Fendler, J. H. Curr. Opin. Solid State Mater. Sci. 1997, 2, 365. (19) Schwarz, K.; Epple, M. Chem.sEur. J. 1998, 4, 1898. (20) Hasse, B.; Ehrenberg, H.; Marxen, J. C.; Becker, W.; Epple, M. Chem.sEur. J. 2000, 6, 3679. (21) Busch, S.; Dolhaine, H.; DuChesne, A.; Heinz, S.; Hochrein, O.; Laeri, F.; Podebrad, O.; Vietze, U.; Weiland, T.; Kniep, R. Eur. J. Inorg. Chem. 1999, 10, 1643. (22) Kro¨ger, N.; Deutzmann, R.; Sumper, M. Science 1999, 286, 1129. (23) Kro¨ger, N.; Lehmann, G.; Rachel, R.; Sumper, M. Eur. J. Biochem. 1997, 250, 99. (24) Calvert, P.; Rieke, P. Chem. Mater. 1996, 8, 1715. (25) Kawaguchi, H.; Hirai, H.; Sakai, K.; Sera, S.; Nakajima, T.; Ebisawa, Y.; Koyama, K. Colloid Polym. Sci. 1992, 270, 1176. (26) Donners, J. J. J. M.; Heywood, B. R.; Meijer, E. W.; Nolte, R. J. M.; Roman, C.; Schenning, A. P. H. J.; Sommerdijk, N. A. J. M. Chem. Commun. 2000, 1937. (27) Marentette, J. M.; Norwig, J.; Sto¨ckelmann, E.; Meyer, W. H.; Wegner, G. Adv. Mater. 1997, 9, 647. (28) Qi, L. M.; Co¨lfen, H.; Antonietti, M. Chem. Mater. 2000, 12, 2392. (29) O ¨ ner, M.; Norwig, J.; Meyer, W. H.; Wegner, G. Chem. Mater. 1998, 10, 460. (30) Rieger, J.; Thieme, J.; Schmidt, C. Langmuir 2000, 16, 8300. (31) Colfen, H. Macromol. Rapid Commun. 2001, 22, 587. (32) Qi, L.; Colfen, H.; Antonietti, M.; Li, M.; Hopwood, J. D.; Ashley, A. J.; Mann, S. Chem.sEur. J. 2001, 7, 3526. (33) Bigi, A.; Boanini, E.; Cojazzi, G.; Falini, G.; Panzavolta, S. Cryst. Growth Des. 2001, 1, 239. (34) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Science 2001, 294, 1684.
10.1021/la011799a CCC: $22.00 © 2002 American Chemical Society Published on Web 05/03/2002
Mechanism of Zinc Oxide Particle Formation
anism on a basis other than by investigating the morphologies of the final product. Understanding the molecular and supramolecular processes that lead to a specific product, however, is the crucial piece of information to enable the fabrication of tailored materials for, for example, scaling inhibitors. The present study seeks to determine how different water-soluble polymers affect particle formation and growth over time and to thereby improve the understanding of polymer-controlled precipitation. Various authors15,16,27-29,31,32,45 have previously shown that water-soluble diblock copolymers are effective crystal modifiers in aqueous precipitation reactions. Here, we investigated the influence of water-soluble poly(ethylene oxide-block-methacrylic acid) (P(EO-b-MAA)) and poly(ethylene oxide-block-styrene sulfonic acid) (P(EO-b-SSH)) diblock copolymers on the morphologies, sizes, and size distributions of zinc oxide during precipitation. With only one crystalline phase,51 zinc oxide serves as a model system for controlled crystallization processes without phase transitions complicating the study. 2. Experimental Section 2.1. Materials. The detailed procedure for polymer synthesis is described elsewhere.52 Poly(tert-butyl methacrylate-blockethylene oxide) (P(tBMA-b-EO)) precursor polymers were prepared via sequential anionic polymerization or a combination of radical and anionic polymerization of tert-butyl methacrylate (tBMA) and ethylene oxide (EO). The free methacrylic acid was obtained by treating the P(tBMA-b-EO) precursor polymer with an HCl/dioxane mixture at 80 °C for several hours. The polymers obtained via the combined radical-anionic synthesis contain a sulfur atom between the two blocks due to the incorporation of the R,ω-mercaptoethanol initiator. Poly(ethylene oxide-blockstyrene) precursor polymers were obtained via sequential anionic polymerization of styrene and ethylene oxide.52 The poly(styrene) blocks were sulfonated with concentrated H2SO4 and neutralized with NaOH, yielding poly(ethylene oxide-block-styrene sulfonic acid) block copolymers after ion exchange chromatography. In the following, MAA refers to a methacrylic acid, SSH to a styrene sulfonic acid, and EO to an ethylene oxide monomer unit. All subscripts denote the respective degrees of polymerization, and C12 is a C12H25 alkyl chain that is part of one polymer. All other chemicals were of analytical grade and were used without further purification. (35) Banfield, J. F.; Welch, S. A.; Zhang, H.; Thomsen Ebert, T.; Penn, R. L. Science 2000, 289, 751. (36) Kuether, J.; Bartz, M.; Seshadri, R.; Vaughan, G. B. M.; Tremel, W. J. Mater. Chem. 2001, 11, 503. (37) Ngankam, P. A.; Lavalle, P.; Voegel, J. C.; Szyk, L.; Decher, G.; Schaaf, P.; Cuisinier, F. J. G. J. Am. Chem. Soc. 2000, 122, 8998. (38) Falini, G.; Fermani, S.; Gazzano, M.; Ripamonti, A. J. Chem. Soc., Dalton Trans. 2000, 3983. (39) Adair, J. H.; Suvaci, E. Curr. Opin. Colloid Interface Sci. 2000, 5, 160. (40) Matijevic, E. Acc. Chem. Res. 1981, 14, 22. (41) Matijevic, E. Langmuir 1986, 2, 12. (42) Matijevic, E. Chem. Mater. 1993, 5, 412. (43) Matijevic, E. Curr. Opin. Colloid Interface Sci. 1996, 1, 176. (44) Sastry, M.; Kumar, A.; Damle, C.; Sainkar, S. R.; Baghwat, M.; Ramasvamy, V. CrystEngComm 2001, 21, 1. (45) Sedlak, M.; Colfen, H. Macromol. Chem. Phys. 2001, 202, 587. (46) Thompson, J. B.; Paloczi, G. T.; Kindt, J. H.; Michenfelder, M.; Smith, B. L.; Stucky, G. D.; Morse, D. E.; Hansma, P. K. Biophys. J. 2000, 79, 3307. (47) Zaremba, C. M.; Morse, D. E.; Mann, S.; Hansma, P. K.; Stucky, G. D. Chem. Mater. 1998, 10, 3814. (48) Zaremba, C. M.; Belcher, A. M.; Fritz, M.; Li, Y.; Mann, S.; Hansma, P. K.; Morse, D. E.; Speck, J. S.; Stucky, G. D. Chem. Mater. 1996, 8, 679. (49) Alivisatos, A. P. Science 2000, 289, 736. (50) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (51) West, A. R. Grundlagen der Festko¨ rperchemie; VCH: Weinheim, 1992. (52) Seitz, C. Ph.D. Thesis, University of Mainz, Mainz, Germany, 1999.
Langmuir, Vol. 18, No. 11, 2002 4489 2.2. Crystallizations. In a typical crystallization, 446 mg of Zn(NO3)‚6H2O (Fluka) and 12 mg of the respective polymer were dissolved in 100 mL of deionized water. The solution was heated to 90 °C, and a solution of 210 mg of hexamethylene tetramine (Fluka) in 2 mL of deionized water was added to the slowly stirred solution. The reaction was allowed to proceed for various times at 90 °C and quenched by cooling the reaction flask in an ice bath. The white precipitate was separated by centrifugation, washed with water and ethanol, and dried in a vacuum oven at 60 °C for 2 days. 2.3. Scanning Electron Microscopy. Scanning electron microscopy (SEM) was performed on dry powders with a LEO 1530 SEM with a field emission gun and without sample sputtering. The acceleration voltage was 1 kV, the working distance was 5 mm, and the aperture size was 30 µm. 2.4. Particle Size Distributions. Particle sizes and distributions were determined from SEM images using Micrografx Designer software. At least 100 crystals were measured per sample. To avoid errors due to projections of the threedimensional crystal shape to a two-dimensional image, only crystals that were clearly visible and lying flat on the substrate were considered for measurements. The so-obtained histograms were fitted using Gauss distributions.53 Some size distributions could only be reasonably fitted with multipeak distributions. To obtain a simpler, though less accurate, relation between the polymer composition and particle size distributions, all distributions were also fitted with a single Gauss curve, thus allowing the identification of very general trends in the data. 2.5. Particle Numbers. Particle numbers N were obtained from size distribution histograms and the yield of the precipitate at the corresponding reaction time. Particle numbers were calculated using eq 1.
∑ x
π
IxY
() Wx 2
)
2
LxF
Fx
∑M ) ∑N
x
x
x
)N
x ) 1, 2, 3, 4, ... (1)
x
For every column x in the length distribution histogram, the width wx corresponding to the length lx was obtained from the aspect ratio R ) L/W, where L is the mean length and W is the mean width of the crystals in the sample. The volume Vx of one crystal in the column x was calculated assuming the crystals may be described as cylinders with length Lx and radius Wx/2. The weight Mx of one crystal in the column x was calculated via the bulk density F of zincite (F ) 5.7 g cm-1).51 The mass Fx of all crystals in one column x was calculated from the total yield Y and the relative intensity Ix of the respective column x. The number Nx of crystals in one column x was Fx/Mx. The sum of N1 + N2 + N3 ... was the total particle number N in the sample. This method for particle number determination is based on several assumptions including a cylindrical crystal shape, a perfect morphology, and a fixed aspect ratio. These data are thus subject to a rather large experimental error. Yet secondary crystallizations could be identified and distinguished. 2.6. Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) experiments were made on a Philips PW1820 equipped with a graphite monochromator and a proportional counter. The radiation used was Cu KR (λ ) 1.5418 Å), patterns were recorded from 5° to 90°, and the powders were mounted on an aluminum sample holder. 2.7. Transmission Electron Microscopy. Powders were dispersed in 2 mL of ethanol and ultrasonicated for 5 min. Several drops of the suspension were placed on a carbon-coated copper grid and allowed to dry in air. Experiments were made on a Philips CM12 transmission electron microscope equipped with a LaB6 cathode operated at 120 kV under low dose conditions. Bright and dark field images and diffraction patterns were recorded with 35 mm film (Ilford PanF) and digitalized with a negative scanner (UMAX5000). Image analysis was performed with commercially available software packages (SISanalySIS and AdobePhotoshop 5). (53) Hunter, R. J. Foundations of Colloid Science; Oxford Science Publications: Oxford, 1986; Vol. 1 + 2.
4490
Langmuir, Vol. 18, No. 11, 2002
Taubert et al.
Figure 1. Morphogenesis of (a) a control sample, (b) a sample crystallized with 120 mg/L of EO68-b-MAA8-C12, and (c) a sample crystallized with 120 mg/L of EO144-b-MAA9. 2.8. Small-Angle X-ray Scattering. Small-angle X-ray scattering experiments were done on a Laue camera with a sample to detector distance of 60 cm. Samples were loaded in glass capillaries, and the experiments were performed under vacuum. Data were acquired on photographic film, and acquisition times varied from 48 to 72 h. 2.9. Induction Periods. Induction periods, that is, the time between base addition and the first detectable precipitation, were measured on a custom-built device consisting of a helium-neon laser (543 nm, 4 mW), a temperature bath, a quartz glass sample cell, and a photodiode attached to a computer. The sample cell with 100 mL of a polymer-containing 15 mM zinc nitrate solution was immersed in the temperature bath, and the laser was passed through both bath and cell. The transmission of the solution at 90 °C was used as the baseline. A solution of 210 mg of hexamethylene tetramine in 2 mL of deionized water was rapidly added to the slowly stirred solution via a syringe. The rapid decrease in the laser intensity due to bubble formation upon base addition was used to calibrate the start of the reaction. The end of the induction period was characterized by a decrease in the transmitted intensity due to increasing scattering by the precipitating particles, as detected by the diode. The induction periods were determined by the intersection of two tangents applied to the intensity-time curves. 2.10. pH Measurements. pH measurements were made using a Mettler Digital pH-Meter with a Mettler Toledo InLab 424 glass electrode for high-temperature experiments. The pH-Meter was calibrated at pH 4, 7, and 10 with Merck buffer solutions.
3. Results and Discussion 3.1. Morphogenesis. In the control sample (Figure 1a), most crystals initially adopt a spindlelike morphology and some are highly aggregated. After ∼20 min, the spindlelike crystals grow and their faces form. Equilibrium face formation starts at the side faces, followed by edge formation and basal planes. Meanwhile, the aggregated structures are gone. After ∼45 min, many small needlelike particles appear. The central grain boundary discussed in a previous paper54 is already visible after 2 min. With P(EO-b-MAA) copolymers, crystal growth follows the same general progression as in the control sample but the particles are monodisperse and have a uniform shape at short reaction times regardless of the polymer. No aggregates are found. After ∼10 min, the samples start to differentiate. With EO68-b-MAA8-C12 (Figure 1b) uniform hexagonal prismatic particles and with EO144b-MAA9 (Figure 1c) hexagonal prisms and particles with (54) Taubert, A.; Palms, D.; Weiss, O ¨ .; Piccini, M.-T.; Batchelder, D. N. Chem. Mater., in press.
Figure 2. (a) Overview over a large particle from the first precipitate, (b) large particle containing small hexagonal prisms, and (c) morphogenesis of a sample crystallized with 120 mg/L of EO68-b-SSH25.
irregular shapes and different sizes form. This implies that the nucleation process must be nearly independent of the overall molecular weight and block length ratio of the P(EO-b-MAA) diblock copolymers. This independence is not true of the crystal growth process. Here, polymer composition has a strong influence on product formation. It was argued earlier29,54 that the molecular characteristics affect the steric shielding and adsorption density of the polymers on the crystal surfaces, which then affects the formation of the product. With all P(EO-b-SSH) copolymers, the precipitation involves two steps. First, very large particles without any structure precipitate (Figure 2a). After ∼4 min, small, slightly hexagonal prisms appear within this matrix (Figure 2b). These particles exhibit the central grain boundary found in all other samples and rapidly grow
Mechanism of Zinc Oxide Particle Formation
Langmuir, Vol. 18, No. 11, 2002 4491
Figure 3. Time-yield curves for some crystallizations.
Figure 4. Size distribution histograms of a control sample with reaction time.
within the matrix material. Only after ∼10-15 min do the large particles disappear and the remaining crystals start to adopt the “stack of pancakes” morphology discussed in a previous publication (Figure 2c).54 The lamellae (the single pancakes) formed at this stage are roughly 30 nm thick, as determined via transmission electron microscopy (TEM). We conclude that the initial precipitate is an intermediate that eventually dissolves while the stack of pancakes shaped zincite crystals form and the intermediate serves as a reservoir for crystal growth. 3.2. Time-Yield Curves. Time-yield curves (Figure 3 and Supporting Information) of the control samples and the samples crystallized with P(EO-b-MAA) copolymers reach a plateau at ∼40-50% yield. With P(EO-b-SSH) copolymers, the time-yield behavior is approximately linear without a plateau. Yields below ∼20 min are about tripled for the P(EO-b-SSH) copolymer compared to the control sample or the P(EO-b-MMA) samples. Extrapolating a linear fit for the P(EO-b-SSH)-containing samples to a reaction time t ) 0 results in a yield of ∼17%. Since this is physically impossible, we expect a fast first precipitation step that yields 17% product in 2 min. This indicates that P(EO-b-SSH) copolymers are very effective nucleating agents for the first precipitate. 3.3. Particle Size and Preferred Growth Directions. The length and width distributions of the control sample are broad even at short reaction times (Figure 4). After ∼8 min, the sample becomes more polydisperse. After ∼45 min, the most intense maximum in the distributions is shifted toward smaller crystal dimensions. With 120 mg/L of EO68-b-MAA8-C12, the crystals are smaller and have a narrower size distribution than in the control sample at all reaction times (Figure 5a). The crystal
Figure 5. Size distribution histograms of a sample crystallized with 120 mg/L of (a) EO68-b-MAA8-C12 and (b) EO68-b-SSH25 with reaction time.
length increases until a mean length of ∼1 µm at ∼30 min is reached. Length and width distributions subsequently broaden with progressing crystallization, and a second peak appears at smaller crystal dimensions. The appearance of peaks at smaller crystal dimensions is indicative for higher crystal generations as will be shown below. The lower intensity of the peaks at smaller particle sizes in Figure 5a compared to Figure 4 indicates a smaller population of the second generation. The broadening of the size distributions is indicative for overlapping nucleation and growth periods. The particle length distributions with 120 mg/L of EO68b-SSH25 are broader than with 120 mg/L of EO68-b-MAA8C12 but not as broad as in the control sample (Figure 5b). The width distributions remain narrow over the whole reaction time. The crystals have a mean length of ∼1.1 µm already after 2 min. While the length remains approximately constant, the particles grow in width, which is unique to these polymers. The ratio R of length to width (aspect ratio) allows for the detection of preferred growth directions. It is plotted versus yield in Figure 6. The abscissa plots yield instead of reaction time in order to correlate crystal growth to the
4492
Langmuir, Vol. 18, No. 11, 2002
Taubert et al.
Figure 6. Variation of the aspect ratios for a control sample, a sample crystallized with 120 mg/L of EO68-b-MAA8-C12, and a sample crystallized with 120 mg/L of EO68-b-SSH25.
rate of crystallization. The particles in the control sample grow along the long axis until at ∼30% yield a sudden reduction in the mean length is observed. This change arises from the appearance of the smaller crystals seen in Figure 1. With EO68-b-MAA8-C12, R steadily increases with yield. This indicates a preferential, though slow, crystal growth along the long crystal axis without the formation of new particles. The deviation from the linear behavior at ∼50% yield is due to formation of a second crystal generation with a somewhat higher aspect ratio. The slower crystal growth compared to that of the control sample is consistent with earlier data29,54 suggesting that the polymer preferentially adsorbs on the basal planes and restricts crystal growth along the long crystal axis. With P(EO-b-SSH) copolymers, the aspect ratio decreases. Crystal growth predominantly occurs along the long axis leading to a length of ∼1.1 µm at ∼4 min, followed by enhanced growth along the wide axis. Because the mean crystal length is shorter than for the large crystals in the control sample, we conclude that the polymers adsorb on the basal planes, thereby restricting the largest achievable crystal length. Adsorption of the polymers on the basal planes, however, cannot explain the formation of the stack of pancakes morphology. The formation of this morphology can be explained only if the polymers also adsorb on the side faces at later reaction stages and prevent the formation of the hexagonal prismatic particle morphology. Alternatively, P(EO-b-SSH) copolymers favor heterogeneous nucleation on the side faces, which would also lead to the observed morphology. 3.4. Particle Numbers. Particle numbers were estimated via eq 1. Figure 7 clearly demonstrates that higher crystal generations are formed in the control sample and to a lesser extent also in the sample precipitated with EO68-b-MAA8-C12. With the copolymer, secondary nucleation is less intrusive and occurs later. This is due to a presumed stabilization effect of the polymer and thus delayed formation of the second crystal generation. In addition, the slower particle number increase at the beginning of the precipitation with EO68-b-MAA8-C12 may be due to the stabilization of subcritical nuclei by the polymeric additive. With 120 mg/L of EO68-b-SSH25, no second crystal generation was found. Particle numbers remain constant within the experimental error and over the period investigated. 3.5. Phase Evolution. PXRD patterns of the control sample exhibited an amorphous halo after 2 min, which is gone later (Figure 8a). With 120 mg/L of EO68-b-MAA8C12, no such halo is observed. Already after 2 min, only zincite reflections are found (Figure 8b). We have also performed experiments with 700 mg/L of EO68-b-MAA8C12 to assess the effect of polymer concentration on the
Figure 7. Calculated particle numbers for a control sample, a sample crystallized with 120 mg/L EO68-b-MAA8-C12, and a sample crystallized with 120 mg/L EO68-b-SSH25.
Figure 8. PXRD patterns of different samples after different reaction times: (a) control sample; (b) sample crystallized with 120 mg/L of EO68-b-MAA8-C12; (c) sample crystallized with 700 mg/L of EO68-b-MAA8-C12; (d) sample crystallized with 120 mg/L of EO68-b-SSH25. Al indicates aluminum substrate reflections.
crystallization rate. Patterns obtained from these materials show a broad halo, and the only evidence for zincite at 2 min is a barely visible hump at the zincite (100) position (Figure 8c). These data imply that without polymer and at high polymer concentration some (mostly amorphous) intermediate precipitates but dissolves or recrystallizes to eventually leave pure zincite. It is currently not clear why no amorphous intermediate is observed at 120 mg/L of EO68-b-MAA8-C12. With 120 mg/L of EO68-b-SSH25, an amorphous halo and a series of distinct reflections are detected (Figure 8d). Zincite reflections appeared at ∼4 min, and after ∼20 min only zincite reflections are found. Higher concentrations of the polymer inhibited the formation of the zincite until ∼15 min and led to sharper reflections indicative of a better order within this material and a more intense halo (Figure 9). The lattice spacings calculated from the PXRD patterns were 34.2, 17.2, 11.8, 8.9, 7.1, 2.7, and 1.5 Å. This ∼1/n (with n ) 1, 2, 3, ...) sequence of the reflection position and the asymmetry of some of the reflections are
Mechanism of Zinc Oxide Particle Formation
Langmuir, Vol. 18, No. 11, 2002 4493
Figure 9. PXRD patterns of samples precipitated with 120 mg/L (a) and 400 mg/L (b) of EO68-b-SSH25 after 6 min.
characteristic for lamellar structures55 with a lamellar thickness of ∼3.4 nm. Figures 8 and 9 clearly demonstrate that zincite formation in the presence of P(EO-b-SSH) copolymers is preceded by a highly ordered lamellar intermediate, which later dissolves in favor of the thermodynamically more stable zincite. Small-angle X-ray scattering of these intermediates confirmed the indexing of the mesostructure because no reflections were found at lower q values. Rieger at al.30 recently found that precipitating CaCO3 nanoparticles were initially fixed in a polyelectrolyte network in the amorphous or vaterite form. Both are kinetically favored but thermodynamically unstable with respect to calcite. At sufficiently high polymer concentrations, however, these particles did not change further. At polymer concentrations below a critical threshold, the initial nanoparticles dissolved and crystallized as calcite. This process is conceptually identical to our results on the zincite/P(EO-b-SSH) system, with the additional feature that the intermediates observed in the current study are themselves highly ordered. We performed TEM experiments on precipitates isolated after 6 min to determine the morphology of the intermediates found in the PXRD experiments. While in samples precipitated with 120 mg/L of EO68-b-MAA8-C12 almost exclusively crystalline particles were found, samples precipitated with 700 mg/L EO68-b-MAA8-C12 contained two different species. The TEM images and selected area diffraction (SAD) patterns revealed roughly spherical and often highly aggregated amorphous particles (Figure 10a). Crystalline zincite particles were observed in the same sample (Figure 10b), which was anticipated from the PXRD data, demonstrating that high EO68-b-MAA8-C12 concentrations stabilize an amorphous intermediate. The sample precipitated with 120 mg/L EO68-b-SSH25 contained some zincite crystals after 6 min. However, a significant fraction of the particles in this sample consisted of a lamellar species, consistent with the PXRD data. These particles were of irregular shape but of uniform microstructure. The lamellar thickness obtained from TEM data was between 3.3 and 3.4 nm, which is in excellent agreement with the powder X-ray data. The amorphous contribution of the PXRD pattern is due to a fraction of (55) Reynolds, R. C. Diffraction by small and disordered crystals; Mineralogical Society of America: Washington, DC, 1989.
Figure 10. TEM bright field images of samples crystallized for 6 min. (a) Amorphous particle from a sample precipitated with 700 mg/L of EO68-b-MAA8-C12. Inset: SAD pattern. (b) Zincite particle from a sample precipitated with 700 mg/L of EO68-b-MAA8-C12. Inset: SAD pattern corresponding to the [001] zone axis of zincite. (c) Lamellar mesostructure obtained with 120 mg/L of EO68-b-SSH25. The image is slightly defocused to show an amorphous region (circle).
Figure 11. Induction periods for different concentrations of EO76-b-SSH43 and EO68-b-MAA8-C12 as determined for a 15 mM Zn(NO3)2 solution at 90 °C.
amorphous material in the lamellar intermediate (Figure 10c). 3.6. Induction Periods. The steady increase of the induction period for EO68-b-MAA8-C12 implies that higher polymer concentrations stabilize subcritical nuclei (Figure 11). Alternatively, since aqueous poly(ethylene oxide) solutions phase separate at high temperatures56 and weak polyacids irreversibly complex divalent cations and subsequently precipitate,57 nucleation may occur not in solution but on or within the phase-separated polymer containing poly(methacrylic acid) complexing Zn2+. Hartgerink et al.34 found that synthetic peptide amphiphile (PA) fibers showed increased electron density prior to precipitation of hydroxyapatite. This was interpreted as to be due to a high local ion supersaturation on the PA (56) Bekiranov, S.; Bruijnsma, R.; Pincus, P. Phys. Rev. E 1997, 55, 577. (57) Sabbagh, I.; Delsanti, M. Eur. Phys. J. E 2000, 1, 75.
4494
Langmuir, Vol. 18, No. 11, 2002
fibers. In our case, the polymer would lead to a locally increased Zn2+ concentration via complexation of the metal ions but the polymers may also interact with the particles as they start appearing in the gel or the solution and initially stabilize them at very small sizes until they grow larger. As expected from time-yield curves, the induction periods for P(EO-b-SSH) copolymers are short (∼1 s) and independent of polymer concentration. Given the fact that SEM, PXRD, and TEM detect an intermediate at short reaction times, we conclude that these polymers are powerful nucleation promoters for this metastable first precipitate. Because in strong polyelectrolytes such as poly(sulfonic acids) electrostatics rather than chemical association govern the interactions between the polyelectrolytes and divalent cations,57 we may speculate that here the polymer/metal ion associate is still in solution and not present as a polymer/metal ion precipitate as in the case of P(EO-b-MAA) copolymers until the precipitating agent hexamethylene tetramine is added. This difference will strongly affect nucleation. 3.7. pH Values. All pH curves showed a steep increase after base addition. After ∼10 min, the pH reached a plateau governed by the solution composition, that is, the presence or absence of a polyacid and its strength. Because no second increase in pH was found, the formation of the second crystal generations in some samples cannot be due to a (unphysical) second supersaturation imposed on the reaction mixture. It rather originates from the mechanical interaction of the crystals with the glass vessel or other crystals followed by heterogeneous nucleation and growth on these so-generated particles. 4. Suggested Growth Model On the basis of the data presented in this and a previous study,54 we propose a simple particle formation model for the polymer-controlled zincite precipitation from aqueous solution. Without polymer, a series of fast overlapping nucleation and growth periods takes place. The initial precipitate is at least in part amorphous. Once the supersaturation is lower, the crystals grow along their long axes. With increasing reaction time, higher crystal generations form, probably via the formation of very small particles and subsequent heterogeneous nucleation on these particles. The addition of P(EO-b-MAA) copolymers modifies both nucleation and growth, probably by complex formation
Taubert et al.
with the zinc ions, by stabilizing subcritical nuclei or by stabilizing very small, presumably partly amorphous, kinetically favored particles precipitated within the phaseseparated polymer. This first precipitation is followed by the precipitation of zincite and a preferred crystal growth along the long crystal axis. Since the polymers may also irreversibly adsorb onto some surfaces (preferably the basal planes), the crystal growth process is slowed. The formation of multiple particle generations is most likely due to mechanical interactions between particles and the glass vessel or between several particles, followed by heterogeneous nucleation and growth. P(EO-b-SSH) copolymers are very effective nucleating agents for a highly ordered lamellar intermediate, because already after extremely short reaction times a high yield is found. This kinetically favored metastable intermediate eventually dissolves as the thermodynamically more stable zincite is formed. As a result, the lamellar intermediate has a double role in the precipitation process: (1) it very effectively reduces the initial supersaturation and thereby allows for highly controlled nucleation conditions for the zincite precipitation and (2) it serves as a material reservoir for the zincite formation, such that it releases the bound components slowly enough to allow for controlled crystal growth after nucleation. Contrary to the P(EO-b-MAA) copolymers, P(EO-b-SSH) copolymers also seem to adsorb onto both basal and side faces, thus leading to particles with a monodisperse size distribution and the observed “stack of pancakes” morphology. Acknowledgment. Thanks are due to Dr. C. Seitz and Dr. D. J. Valenti for polymer synthesis and G. Jentzsch for help with crystallization experiments. We also thank Prof. G. Wegner, Dr. G. Lieser, and Dr. J. Norwig for valuable discussions. Dr. M. Bockstaller and Dr. C. C. Honeker are acknowledged for insightful comments and for reviewing the manuscript. This work was funded by the Federal Ministry of Research and Technology, Grant No. 03D0045. Supporting Information Available: Listing of all polymers used in this study, time-yield curves for all experiments, and a set of typical pH curves vs reaction time. This material is available free of charge via the Internet at http://pubs.acs.org. LA011799A
