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Epitaxial Assembly in Aged Colloids R. Lee Penn,*,† Gerko Oskam,‡ Timothy J. Strathmann,§ Peter C. Searson,‡ Alan T. Stone,§ and David R. Veblen† Department of Earth and Planetary Sciences, the Department of Materials Science and Engineering, and the Department of Geography and EnVironmental Engineering, Johns Hopkins UniVersity, Baltimore, Maryland 21218 ReceiVed: September 29, 2000; In Final Form: NoVember 30, 2000
Self-assembly of nanoparticles is an important mechanism of particle growth in the solution-phase synthesis of oxides and oxyhydroxides. In this work, particle growth in aqueous colloidal suspensions of anatase (TiO2), hematite (Fe2O3), feroxyhite (FeOOH), and heterogenite (CoOOH) was observed to occur by two primary mechanisms: coarsening and growth by topotactic assembly. Coarsening is governed by the growth of larger particles at the expense of smaller particles, and topotactic assembly results in single crystals of unique morphology. The hematite nanocrystals are nominally equidimensional crystals that are usually constructed from more than 10 primary building blocks. The heterogenite particles are hexagonal plates that are, on average, 0.7 µm across and 20-30 nm thick. These plates are porous and are assemblies of hundreds of oriented nanocrystalline building blocks. The feroxyhite nanocrystals attach to form ∼30 nm porous flakes that are several nanometers thick. The anatase nanocrystals assemble to form elongated, bent, or nominally equidimensional single crystals with ultimate morphologies that frequently violate crystal-symmetry rules. Kinetic experiments, using anatase particles, show that the number of isolated primary particles decreases with time and that the assembly order, which reflects the average number of primary particles per secondary particle, increases with time. Growth by oriented aggregation is highly dependent on solution chemistry and may provide a means by which intricate assemblies can be achieved without the use of organic additives.
Introduction Solution-phase methods have become widely used for the synthesis of crystalline nanoparticles and may provide models for understanding the precipitation and growth of nanophase materials in the environment. In most cases, these techniques involve precipitation from homogeneous solution.1 This approach has been used to synthesize semiconductor nanoparticles of II-VI compounds (e.g., CdS,2 CdSe,3 and PbS4), III-V compounds (e.g., InAs,3,5 GaAs6), metal oxides (TiO2,7,8 ZnO,9 Fe2O3,10,8 and Fe3O411), and metal oxyhydroxides (e.g., CoOOH,12 R-FeOOH,13 and manganese hydrous oxides14). Nanophase materials are found in a wide range of environments (e.g., mineral and rock weathering, biological systems) and are used in many applications (e.g., catalysts, sensors, magnetic carriers, energy conversion, and power sources). In general, when synthesizing nanoparticles from solution, nucleation is very fast, and subsequent growth occurs by two primary mechanisms (Figure 1): coarsening (also known as Ostwald ripening)15 and growth involving aggregation.16-20 Coarsening processes involve the growth of larger crystals at the expense of smaller crystals. Because the chemical potential of a particle increases with decreasing particle size, the equilibrium solute concentration near a small particle is higher than near a large particle, as described by the Gibbs-Thompson equation. The resulting concentration gradients lead to the transport of solute from small particles to larger particles.21 This * Corresponding author. E-mail:
[email protected]. † Department of Earth and Planetary Sciences. ‡ Department of Materials Science and Engineering. § Department of Geography and Environmental Engineering.
Figure 1. Two-dimensional representation of end-member growth mechanisms: (upper) growth of large crystals at the expense of small crystals and (lower) growth by assembly.
mode of growth may result in the formation of faceted particles if it occurs near equilibrium and there is sufficient difference in the surface energies of different crystallographic faces. Crystal growth by aggregation can occur by a range of mechanisms, producing particle assemblies built from randomly oriented to highly oriented nanoparticles.16-18 Epitaxial aggregation is an important mode of crystal growth.19,20 Primary particles may aggregate in an oriented fashion to produce a larger single crystal, or they may aggregate randomly and reorient, recrystallize, or undergo phase transformations to produce larger single crystals. Aggregation-growth mechanisms provide a route for the incorporation of defects, such as edge and screw dislocations, in stress-free and initially defect-free nanocrsytalline materials.18 Microstructural features such as
10.1021/jp003570u CCC: $20.00 © 2001 American Chemical Society Published on Web 02/22/2001
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defects (e.g., edge dislocations), porosity, and particle morphology yield important clues to mechanisms by which primary building blocks assemble to produce larger particles. Structural and crystallographic variations in nanophase materials play a critical role in determining chemical,22 electrical, and optical behavior, highlighting the importance of a detailed understanding of the growth mechanism. In this article we report on the mechanisms of particle growth for four oxides and oxyhydroxides from homogeneous solution: hematite (Fe2O3) synthesized by aging an acidic ferric chloride solution, feroxyhite (FeOOH) by oxidation of a ferrous chloride solution, anatase (TiO2) by sol-gel synthesis and hydrothermal aging, and heterogenite (CoOOH) by oxidation of a cobalt (II) chloride solution. We demonstrate that oriented aggregation is an important growth mechanism in these systems. Experimental Section Colloid Syntheses. Hematite particles were synthesized using a method modified from Hamada and Matijevic23 and Penners.24 A 225-mL filtered solution of 1.5 M FeCl3‚6H2O and 20 mM HCl, prepared using deionized and distilled water (DDW, 18 MΩ-cm resistivity), was added at a rate of 10 mL/min to a continuously stirred polyethylene bottle containing 9.25 L of a 3.75 mM HCl solution at 100 °C. The resulting mixture was placed in a gravity convection oven at 100 °C for 48 h and then allowed to cool to room temperature. The particles that formed were allowed to settle, and the supernatant liquid was siphoned off and discarded. The hematite particles were cleaned by repeated centrifugation at 7000 rpm at 10 °C for 7 h, followed by resuspension of the particles for 16 h in 1 mM perchloric acid solution or DDW. Perchloric acid was used for the first three washes to remove amorphous coatings on the particles.24 The particles were then washed in DDW water an additional 10 times. Feroxyhite particles were prepared by first precipitating an Fe(II)-bearing phase followed by oxidation using a hydrogen peroxide solution.25 A 2-L solution of 0.10 M FeCl2 was prepared using Ar-sparged DDW, and a 5 M NaOH solution was added dropwise to the continuously stirred solution to a final pH of 8.0. The resulting suspension was placed in a chemical fume hood, and 330 mL of a 30 wt % H2O2 (2.9 mol H2O2) solution was quickly added. The suspension immediately turned reddish brown. The particles were allowed to settle, and the supernatant liquid was siphoned off and discarded. The feroxyhite particles (confirmed by X-ray diffraction) were cleaned by dialysis until a final supernatant conductivity comparable with a 5 mM NaCl solution (7.4 µΩ-1 cm-1) was obtained. Heterogenite particles were prepared using a modification of the method of Stone and Ulrich.12 A Co(II)-bearing precipitate was first produced and then oxidized using NaOCl. Starting solutions of 2.9 mM NaOH (3.0 L) and 1.2 mM CoCl2 (3.0 L) were prepared, heated to 80 °C using a constant-temperature water bath, and sparged using Ar overnight. The NaOH solution was delivered to the CoCl2 solution vessel using a peristaltic pump with a flow rate of 40 mL/min. A pale-pink suspension resulted. The reaction vessel was hand-rocked to ensure mixing and maintained at 80 °C. Eighteen minutes after the NaOH addition was complete, 1.0 L of 17.5 mM NaOCl was quickly added; the suspension immediately turned brown. Five hours later, the bottle containing the suspension was removed from the 80 °C water bath and placed in a room-temperature water bath. The heterogenite particles were cleaned by first centrifuging the suspension, then discarding the liquid supernatant, and
Figure 2. Low-resolution (upper) and high-resolution (lower) zeroloss TEM images of hematite particles prepared by aging an acidic ferric chloride solution. Black arrows highlight boundaries in which a slight misorientation between primary particles has been incorporated, and white arrows indicate boundaries in which there is no misorientation between primary particles.
then resuspending the particles in DDW. This cleaning procedure was repeated five times. Anatase particles were prepared using the sol-gel technique.26 Titanium tetraisopropoxide (Ti-(O-iPr)4) was used as precursor, and the colloid was prepared in aqueous solution acidified with HNO3 to pH 1. The resulting colloid was aged hydrothermally at temperatures ranging from 160 °C to 220 °C. A typical procedure was as follows: 15 mL of Ti-(O-iPr)4 was added dropwise to 185 mL of DDW acidified with 1.3 mL of concentrated HNO3 under vigorous stirring at room temperature. The colloidal suspension was then heated to 85 °C with stirring in an open flask for about 12 h to remove volatile reaction products (viz. propanol). The colloid was then heated at an elevated temperature in a closed titanium pressure vessel for 16 h. The resulting colloid suspension was white and had a TiO2 concentration of ∼75 g/L. Particle Characterization. All materials were examined using a Philips CM300FEG transmission electron microscope
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Figure 3. High-resolution zero-loss TEM image of feroxyhite assemblies prepared by oxidation of a ferrous chloride solution.
(TEM) equipped with a Gatan Imaging Filter (GIF) for energyfiltered imaging. All TEM images were collected using a Gatan CCD camera and Digital Micrograph software. Zero-loss TEM images in this work are formed using an energy-filtering technique that excludes electrons that have undergone significant energy losses due to inelastic scattering events (e.g., inner-shell ionization and plasmon interactions). The electron beam passes through a magnetic prism, a part of the GIF, and the electron beam is spread as a function of kinetic energy. By using an energy window of 10 eV centered about the accelerating voltage of the microscope, only low-loss (e.g., phonon-scattering events) and zero-loss (elastic-scattering events) are included in a zeroloss image. The exclusion of higher-loss electrons improves image resolution and contrast.27 Each sample was prepared by first diluting each suspension with DDW and then placing one drop of each resulting suspension on a holey carbon TEM grid. Results and Discussion Figure 2 shows two zero-loss TEM micrographs of hematite, Fe2O3, particles. The upper portion of the figure shows a typical low-resolution image that provides a general indication of particle morphology and size. The vast majority of particles appear to be composed of many primary building block particles ranging in size from less than 5 nm to more than 20 nm. The lower portion of the figure shows a typical high-resolution image of these crystallites. Lattice fringes indicate crystalline material. Parallel fringes indicate parallel crystallographic orientation between primary building block neighbors. Slight misorientations across some interfaces are apparent when the micrograph is viewed from a low angle and reveal the incorporation of edge dislocations within these interfaces (highlighted by black arrows). The “dimples” and “creases” (highlighted by black [slight misorientations] and white arrows [no apparent misorientations]) in the morphology of these particles suggest that many are constructed of more than 10 primary building block particles. Several mechanisms can be envisioned for the production of hematite particles comprised of oriented nanoparticle building blocks. Primary iron oxyhydroxide particles may have aggregated in a random manner, undergone recrystallization to result in parallel orientation of the primary particles, and subsequently undergone dehydration to form hematite, as discussed by Matijevic and Scheiner8 and Banfield et al.19 Alternatively, primary particles may have aggregated in an
Figure 4. Low-resolution (top and middle) and high-resolution (bottom) zero-loss TEM images of heterogenite particles prepared by oxidation of a cobalt (II) chloride solution. The two bold white lines in the bottom image are parallel and highlight the misorientation between two regions of the assembled particle. The narrow white lines highlight the presence of dislocations.
oriented fashion, producing assemblies of oriented nanoparticles that subsequently underwent dehydration to form hematite. Previous work investigating the growth mechanism of ellipsoidal hematite particles revealed an oriented aggregation mechanism whereby primary particles aggregate to produce porous and elongated “monocrystals.”17 A third possibility involves the dissolution of the primary precipitate (iron oxohydroxides) and precipitation of hematite nanocrystals,28,29 which may subsequently assemble to form secondary hematite particles. Distinguishing among these three possibilities will require the application of time-resolved high-resolution TEM to observe the crystallographic relationships between reaction products as a function of time. Results presented by Blesa and Matijevic28 and by Schwertmann et al.29 strongly suggest that a dissolu-
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Figure 5. High-resolution zero-loss TEM images of anatase particles prepared by sol-gel synthesis and hydrothermal aging. The upper left-hand image shows a u-shaped crystallite, the upper center image shows an elongated anatase crystallite that contains an edge dislocation at the interface between the two primary building block particles, and the upper right-hand image shows a crystal constructed from at least four primary anatase nanocrystals. Below each of the three upper images, schematic outlines show one possible attachment scheme that could give rise to the observed crystal morphologies. The lower image shows an elongated crystal constructed of at least three primary building block crystallites. The black and white lines highlight the degree of misorientation between two regions of this particle.
tion step is likely involved in the growth of these hematite particles. Because the overall morphology of the hematite particles is nominally equidimensional rather than extended along a specific crystallographic direction, we conclude that assembly does not occur preferentially across a specific set of crystal faces. Figure 3 shows a high-resolution TEM micrograph of feroxyhite particles. Low-angle misorientations between primary feroxyhite nanoparticles are visible in this image, as in Figure 2. In addition, variations in contrast reveal the incorporation of porosity and the presence of internal interfaces between primary nanoparticles. That these particles are extended normal to and thin along [001] suggests preferential attachment normal to [001]. Figure 4 shows a series of zero-loss images of heterogenite, CoOOH. The overall morphology of the large crystals is consistent with the hexagonal symmetry of heterogenite (upper). These micron-sized plates consist of oriented ∼3 nm primary building block crystallites (center). Variations in contrast reveal the internal porosity of these plates. Thickness maps (the ratio of the zero-loss image intensity to the unfiltered TEM image intensity)27 indicate that the large plates are 20-25 nm thick, on average. The detailed organization of the building block
particles is not clear, because the thickness of these plates consists of ∼7-8 primary building block particles. The bottom image shows a high-resolution zero-loss TEM image where misoriented boundaries occur between heterogenite building block particles. The bold white lines, which are parallel, highlight the angular misorientation between building block crystallites. The thinner white lines highlight two places where edge dislocations have been incorporated into the boundaries between building block crystallites. The incorporation of porosity and defects in the overall morphology of these particles suggests oriented aggregation as the primary growth mechanism. The overall hexagonal-plate morphology suggests that the vector of attachment-driven growth is contained within (001). Thus, attachment must favor crystallographic faces that are parallel or subparallel to [001]. Such crystallographic specificity during aggregation-based particle growth suggests the possibility of tailoring assembly to favor specific crystal faces with the ultimate goal of achieving over particle morphology. Figure 5 shows a series of zero-loss TEM micrographs of anatase TiO2 nanocrystals. In all four micrographs, attachment processes give rise to single crystals of unique morphology. In these cases, the assembly morphology differs from the crystallographic symmetry of anatase. The upper left-hand image
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J. Phys. Chem. B, Vol. 105, No. 11, 2001 2181 to brookite (TiO2) and rutile (TiO2), and defects such as edge and screw dislocations.18,30 Hence, controlled oriented assembly may provide a route by which phase transformations are initiated or possibly inhibited in nanocrystalline materials. Summary
Figure 6. Plot of the average assembly order (squares), which is the average number of primary particles per anatase particle, and the percentage of particles that are primary (i.e., have an assembly order of one, circles) versus time. Experiments were performed at 160 °C (solid lines) and 200 °C (dashed lines). The average assembly order and percentage of particles that are primary were determined using highresolution TEM images and reflect minimum estimates that were determined by counting “dimples” and “creases” of 103-190 particles.
shows a u-shaped crystallite in the lower center portion of the image. The upper center image shows an elongated anatase crystallite that contains an edge dislocation in the interface between the two primary building block particles. The upper right-hand image shows a crystal constructed from at least four primary anatase nanocrystals. Strong faceting of the nanocrystal building blocks and defects incorporated at building block interfaces are clearly seen in this image. Viewing this image at low angle highlights the defects incorporated at these boundaries. Below each of the three upper images, possible attachment schemes for the observed crystal morphologies are outlined. The lower image shows an elongated crystal constructed of at least three primary building block crystallites. The interface between the larger right-hand portion and large left-hand portion of this particle does not contain visible defects. However, the left-hand portion of the particle is built from two or more crystallites, and there is visible misorientation between regions of this particle (highlighted by black and white lines). If the attachment vector can be selected by changing solution and surface chemistry, then a high degree of control over ultimate morphologies and particle microstructure may be possible. Figure 6 shows the percentage of particles with an assembly order of one (circles) and the average assembly order (squares) as a function of time for anatase particles hydrothermally aged at 160 °C and 200 °C. Assembly order is a minimum estimate of the number of primary particles that comprise a secondary particle and is obtained by counting “dimples” and “creases” as observed in TEM images. The decreasing number of primary particles and the increasing average assembly order confirm that assembly occurs and that the microstructural features observed did not form during primary particle growth. These kinetic results indicate that primary anatase particles assemble in an oriented fashion to produce secondary particles with unique morphologies. The observation of microstructural features such as defects and dimples retained in the resultant single crystal indicates that recyrstallization, which would be expected to remove such structural features, is not a dominant mechanism in these systems. Defect incorporation is expected to increase the reactivity of nanophase materials, and Penn et al.22 demonstrated that preferential dissolution occurs along such boundaries of misorientation in heterogenite. In addition, an aggregation mechanism involving attachment of anatase particles across crystallographically specific surfaces was used to explain the presence of unique morphologies, structural elements common
In summary, we find that the solution-based growth of several metal oxide/oxyhydroxide particles is dominated by aggregation processes. Preferential aggregation across a specific set of crystallographic faces gives rise to single crystals that are elongated normal to those faces. When attachment does not involve a preferred set of crystal faces, large assemblies with morphologies that mirror the building block morphology result. Understanding this growth mechanism may provide a route by which microstructure, morphology, and size control can be achieved. Acknowledgment. The authors gratefully acknowledge funding from the National Science Foundation (EAR 9418090 and EAR 0073955) and the U.S. Environmental Protection Agency National Center for Environmental Research and Quality Assurance (R82-6376). T.J.S. was supported by a STAR Fellowship from the U.S. Environmental Protection Agency, National Center of Environmental Research and Quality Assurance. Finally, G.O. and P.C.S. gratefully acknowledge support from EIC Laboratories and the Department of Energy under contract DE-FG02-98ER82567. References and Notes (1) Livage, J.; Henry, M.; Sanchez, C. Prog. Solid State Chem. 1988, 18, 259-341; Henglein, A. Top. Curr. Chem. 1988, 143, 113-180; Matijevic, E. Langmuir 1986, 2, 12-20; Matijevic, E. Annu. ReV. Mater. Sci. 1985, 15, 483-516. (2) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 552-559. (3) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343-5344. (4) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. J. Chem. Phys. 1987, 87, 7315-7322; Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 83, 1406-1410. (5) Gribb, A. A.; Banfield, J. F. Am. Mineral. 1997, 82, 717-728. (6) Olshavsky, M. A.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1990, 112, 9438-9439. (7) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1988, 92, 5196-5201. (8) Matijevic, E.; Scheiner, P. J. Colloid Interface Sci. 1978, 63, 509524. (9) Bahnemann, D. W.; Kormann, C.; Hoffmann, M. R. J. Phys. Chem. 1987, 91, 3789-3798. (10) Leland, J. K.; Bard, A. J. J. Phys. Chem. 1987, 91, 5076-5083. (11) Sugimoto, T.; Matijevic, E. J. Colloid Interface Sci. 1980, 74, 227243. (12) Stone, A. T.; Ulrich, H. J. J. Colloid Interface Sci. 1989, 132, 509522. (13) Schwertmann, U.; Murad, E. Clays Clay Miner. 1983, 31, 277284. (14) Post, J. E. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 3447-3454; Manceau, A.; Gorshkov, A. I.; Drits, V. A. Am. Miner. 1992, 77, 11441157; Hem, J. D.; Roberson, C. E.; Fournier, R. B. Water Resour. Res. 1982, 18, 563-570. (15) Wong, E. M.; Bonevich, J. E.; Searson, P. C. J. Phys. Chem. B 1998, 102, 7770-7775. (16) Penn, R. L.; Banfield, J. F. Geochim. Cosmochim. Acta 1999, 63, 1549-1557; Privman, V.; Goia, D. V.; Park, J.; Matijevic, E. J. Colloid Interface Sci. 1999, 213, 36-45. (17) Ocana, M.; Morales, M. P.; Serna, C. J. J. Colloid Interface Sci. 1995, 171, 85-91. (18) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969-971. (19) Banfield, J. F.; Welch, S. A.; Zhang, H. Z.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751-754. (20) Alivisatos, A. P. Science 2000, 289, 736-737. (21) Sugimoto, T. AdV. Colloid Interface Sci. 1987, 28, 65-108. (22) Penn, R. L.; Stone, A. T.; Veblen, D. R. J. Phys. Chem. B, submitted for publication.
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