Two-Step Growth of Goethite from Ferrihydrite - Langmuir (ACS

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Langmuir 2006, 22, 402-409

Two-Step Growth of Goethite from Ferrihydrite David J. Burleson and R. Lee Penn* Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455 ReceiVed July 11, 2005. In Final Form: October 27, 2005 Goethite (R-FeOOH) is an antiferromagnetic iron oxyhydroxide that is often synthesized by precipitation from homogeneous, aqueous solution followed by aging. This paper addresses goethite growth by phase transformation of six-line ferrihydrite nanoparticles to goethite followed by oriented aggregation of the goethite primary particles. Data tracking goethite nanocrystal growth as a function of pH, temperature, and time is presented. In general, goethite growth by oriented aggregation is faster at higher pH and at higher temperature even as growth by coarsening becomes increasingly important as pH increases. In addition, particle size measurements demonstrate that the primary nanoparticles grow by Ostwald ripening even as they are being consumed by oriented aggregation. Finally, the use of a microwave anneal step in the preparation of the precursor six-line ferrihydrite nanoparticles substantially improves the homogeneity of the final goethite product. Final goethite nanoparticles are unaggregated, acicular crystals in the tens of nanometers size range. These particles may be ideal for mineral liquid crystal and magnetic-recording media applications.

Introduction Goethite (R-FeOOH) is an antiferromagnetic iron oxyhydroxide that is often synthesized by precipitation from homogeneous solution followed by aging. This iron oxyhydroxide is ubiquitous at and near the Earth’s surface. Furthermore, accessible surface area at the Earth’s surface may be dominated by goethite mineral surfaces.1 The size and shape of natural goethite particles range from 3 to 5 nm, nominally spherical nanoparticles to submicrometer, acicular particles. Synthetic goethite nanoparticles are typically acicular and are often aggregated into bundles or rafts of oriented crystallites. Acicular magnetic particles suitable for magnetic-recording media are often produced via transformation from goethite nanoparticles to iron or iron oxide pseudomorph nanoparticles.2,3 Ideally, goethite nanoparticles for use in magnetic-recording applications or for elucidating the link between physical properties and chemical behavior are monodisperse and unaggregated. Proposed goethite growth mechanisms range from dissolution of precursor ferrihydrite nanoparticles followed by precipitation of goethite4 to oriented aggregation accompanied by phase transformation.5 Oriented aggregation has been highlighted in a number of recent works involving the growth of nanoparticles5-27 and may provide a route by which nanoparticle size, shape, and * To whom correspondence should be addressed. Telephone: 612-6264680. E-mail: [email protected]. (1) Schwertmann, U.; Taylor, R. M. Iron oxides. In Minerals in Soil EnVironments; Soil Science Society of America: Madison, WI, 1989; p 379. (2) Kurokawa, H.; Senna, M. Property control of acicular γ-Fe2O3 particles by synthesis conditions of starting goethite. Funtai Kogaku Kaishi 2000, 37, 788. (3) Varanda, L. C.; Morales, M. P.; Jafelicci, M.; Serna, C. J. Monodispersed spindle-type goethite nanoparticles from FeIII solutions. J. Mater. Chem. 2002, 12, 3649. (4) Schwertmann, U.; Murad, E. Effect of pH on the formation of goethite and hematite from ferrihydrite. Clays Clay Miner. 1983, 31, 277. (5) Guyodo, Y.; Mostrom, A.; Penn, R. L.; Banerjee, S. K. From nanodots to nanorods: Oriented aggregation and magnetic evolution of nanocrystalline goethite. Geophys. Res. Lett. 2003, 30, doi: 10.1029/2003GL017021. (6) Chemseddine, A.; Moritz, T. Nanostructuring titania. Control over nanocrystal structure, size, shape, and organization. Eur. J. Inorg. Chem. 1999, 1999, 235. (7) Shen, P.; Fahn, Y. Y.; Su, A. C. Imperfect oriented attachment: Accretion and defect generation of hexagonal inorganic-surfactant nanoparticles. Nano Lett. 2001, 1, 299. (8) Shen, P.; Lee, W. H. (111)-Specific coalescence twinning and martensitic transformation of tetragonal ZrO2 condensates. Nano Lett. 2001, 1, 707. (9) Scolan, E.; Sanchez, C. Synthesis and characterization of surface-protected nanocrystalline titania particles. Chem. Mater. 1998, 10, 3217.

microstructure can be controlled and unique morphologies can be produced.15 It is a special case of aggregation, whereby secondary particles that are new single crystals or pseudocrystals are formed.15-18,21,22,28 In the model presented by Penn,15 primary particles reversibly combine to form complexes that are analogous to outer-sphere molecular complexes. The formation of these (10) Lou, X. W.; Zeng, H. C. Complex R-MoO3 nanostructures with external bonding capacity for self-assembly. J. Am. Chem. Soc. 2003, 125, 2704. (11) Lee, W.-H.; Shen, P. On the coalescence and twinning of cubo-octahedral CeO2 condensates. J. Cryst. Growth 1999, 205, 169. (12) Pacholski, C.; Kornowski, A.; Weller, H. Self-assembly of ZnO: From nanodots to nanorods. Angew. Chem. Int. Ed. 2002, 41, 1188. (13) Cozzoli, P. D.; Curri, M. L.; Agostiano, A.; Leo, G.; Lomascolo, M. ZnO nanocrystals by a non-hydrolytic route: Synthesis and characterization. J. Phys. Chem. B 2003, 107, 4756. (14) Niederberger, M.; Krumeich, F.; Hegetschweiler, K.; Nesper, R. An iron polyolate complex as a precursor for the controlled synthesis of monodispersed iron oxide colloids. Chem. Mater. 2002, 14, 78. (15) Penn, R. L. Kinetics of oriented aggregation. J. Phys. Chem. B 2004, 108, 12707. (16) Penn, R. L.; Banfield, J. F. Imperfect oriented attachment: Dislocation generation in defect-free nanocrystals. Science 1998, 281, 969. (17) Penn, R. L.; Banfield, J. F. Oriented attachment and growth, twinning, polytypism, and formation of metastable phases: Insights from nanocrystalline TiO2. Am. Mineral. 1998, 83, 1077. (18) Penn, R. L.; Oskam, G.; Strathmann, T. J.; Searson, P. C.; Stone, A. T.; Veblen, D. R. Epitaxial assembly in aged colloids. J. Phys. Chem. B 2001, 105, 2177. (19) Nesterova, M.; Moreau, J.; Banfield, J. F. Model biomimetic studies of templated growth and assembly of nanocrystalline FeOOH. Geochim. Cosmochim. Acta 2003, 67, 1177. (20) Audinet, L.; Ricolleau, C.; Gandais, M.; Gacoin, T.; Boilot, J. P.; Buffat, P. A. Structural properties of coated nanoparticles: The CdS/ZnS nanostructure. Philos. Mag. A 1999, 79, 2379. (21) Huang, F.; Zhang, H. Z.; Banfield, J. F. Two-stage crystal-growth kinetics observed during hydrothermal coarsening of nanocrystalline ZnS. Nano Lett. 2003, 3, 373. (22) Huang, F.; Zhang, H. Z.; Banfield, J. F. The role of oriented attachment crystal growth in hydrothermal coarsening of nanocrystalline ZnS. J. Phys. Chem. B 2003, 107, 10470. (23) Ricolleau, C.; Audinet, L.; Gandais, M.; Gacoin, T. Structural transformations in II-VI semiconductor nanocrystals. Eur. Phys. J. D 1999, 9, 565. (24) Sampanthar, J. T.; Zeng, H. C. Arresting butterfly-like intermediate nanocrystals of β-Co(OH)2 via ethylenediamine-mediated synthesis. J. Am. Chem. Soc. 2002, 124, 6668. (25) Nikolakis, V. Theoretical and experimental studies of zeolite nanocrystal growth. 2001. (26) De Moor, P.-P. E. A.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; van Santen, R. A. Imaging the assembly process of the organic-mediated synthesis of a zeolite. Chem.sEur. J. 1999, 5, 2083. (27) Kuo, L. Y.; Shen, P. Shape dependent coalescence and preferred orientation of CeO2 nanocrystallites. Mater. Sci. Eng., A 2000, 277, 258. (28) Banfield, J. F.; Welch, S. A.; Zhang, H. Z.; Ebert, T. T.; Penn, R. L. Aggregation-based crystal growth and microstructure development in natural iron oxyhydroxide biomineralization products. Science 2000, 289, 751.

10.1021/la051883g CCC: $33.50 © 2006 American Chemical Society Published on Web 11/25/2005

Two-Step Growth of Goethite from Ferrihydrite

complexes is governed by various interactions, including hydrogen bonding, van der Waals, and electrostatics. A stability constant for the concentration of complexed particles can be estimated using a combination of the DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory, which is based on electrostatics, van der Waals forces, and the Fuoss equation. The stability constant is expected to decrease with an increasing particle charge, and thus, the rate of growth by oriented aggregation is expected to slow with an increasing particle charge. The complexed primary particles can reorient with respect to one another, through Brownian motion, to achieve structural compatibility at the interface. Finally, through the removal of the solvent and/or surfactant molecules, an oriented aggregate is irreversibly formed.15 This paper addresses goethite growth by the phase transformation of six-line ferrihydrite nanoparticles to goethite nanoparticles followed by oriented aggregation to produce goethite nanorods. Data tracking goethite nanorod growth as a function of pH, temperature, and time are presented, and the contribution to overall growth by oriented aggregation and coarsening is specifically addressed. Finally, the impact of using a microwave anneal step in the preparation of the ferrihydrite nanoparticles on the final goethite particle size and size distribution is examined. Final goethite nanorods are unaggregated (in contrast to the rafts or books often observed in goethite produced by other methods), acicular crystals in the tens of nanometers size range. Experimental Procedures Prior to use, all glassware and plasticware was acid-washed with 4 M nitric acid, rinsed multiple times with distilled water, and then rinsed multiple times with Milli-Q water (Millipore Milli-Q system; 18.2 MΩ cm resistance). Milli-Q water was used to prepare all solutions and for all dialysis procedures. Precursor nanoparticle suspensions were prepared by two methods. Method 1. First, equal volumes of 0.40 M Fe(NO3)4 (Fisher) and 0.48 M NaHCO3 (Fisher) were prepared. Using a peristaltic pump, the NaHCO3 solution was added to a 0.40 M Fe(NO3)4 solution over 136 min. During hydrolysis, the suspension was vigorously mixed using a magnetic stirrer. The color changed from orange to a deep blood red color. Once the addition of NaHCO3 was complete, the resulting nanoparticle suspension was heated in a plastic bottle using a 950 W microwave (Samsung) just to boiling. During heating, the bottled suspensions were removed every 40 s and shaken to ensure more uniform heating. Immediately after heating, the suspension containers were tightly capped and plunged into an ice bath. Finally, suspensions were dialyzed (Spectra/Por 7 dialysis tubing, MWCO 2000) at room temperature against Milli-Q water for 3 days, with the dialysis water changed 3 times per day. Method 2. As in method 1, equal volumes of 0.40 M Fe(NO3)4 (Fisher) and 0.48 M NaHCO3 (Fisher) were prepared. Using a peristaltic pump, the NaHCO3 solution was added to a 0.40 M Fe(NO3)4 solution over 47 min. During hydrolysis, the suspension was vigorously mixed using a magnetic stirrer. The color changed from orange to a deep blood red color. The resulting suspension was dialyzed (Spectra/Por 7 dialysis tubing, MWCO 2000) at 4 °C against Milli-Q water for 5 days, with the dialysis water changed 3 times per day. In contrast to particles produced by method 1, no microwave anneal step was performed. After dialysis, the pH of both suspensions was ∼3.5. For aging experiments, suspension pH was adjusted using ∼1 M NaOH at room temperature. Then, suspensions were poured into plastic bottles, which were placed into a constant temperature oven. At specific time intervals, 5 mL aliquots of suspension were removed after briefly removing bottles from the oven and shaking to resuspend settled material. The nanoparticle suspensions synthesized by method 1 were used in aging experiments aimed at exploring growth as a function of suspension pH (4, 6, 8, 10, and 12). Nanoparticle suspensions

Langmuir, Vol. 22, No. 1, 2006 403 synthesized by method 2 were used to determine the effect of the microwave annealing step and in experiments aimed at exploring growth as a function of the temperature. The suspensions were adjusted to a pH of 6 and aged at 60, 90, and 120 °C. Samples for transmission electron microscopy (TEM) were prepared by first diluting a small amount of suspension by ∼1400 times using Milli-Q water and placing a single drop of the resulting suspension onto a 200 mesh holey carbon-coated copper grid (SPI), which was then allowed to dry in air. TEM images were collected using either an FEI CM30 operated at 300 kV, an FEI Tecnai T12 TEM operated at 120 kV, or an FEI Tecnai G2 30 operated at 300 kV. It is noteworthy that the particles in this study were beamsensitive. Particles appeared stable for several tens of seconds under low-intensity conditions. All images were collected using a chargecouple device (CCD) camera, and high-resolution images were collected using the lowest intensity conditions possible. The remainder of the undiluted 5 mL aliquot of suspension was allowed to dry and used for analysis by X-ray diffraction (XRD). XRD was performed using a PANalytical X’Pert PRO X-ray diffractometer equipped with an X’Celerator detector and cobalt source. Images were analyzed using Gatan Digital Micrograph 3.3.1. Particle sizes were measured, and particles were classified as primary particles or as rods based on the size and aspect ratio. At least 500 nanoparticles were counted and sized for each nanoparticle sample. Goethite nanoparticle dimensions were determined by measuring length and width (wTEM) from images taken at known magnification. Because TEM images are two-dimensional projections of threedimensional particles, wTEM was converted to the width along the a axis (w100) based on the assumption that particles will lie flat (inset of Figure 1). This assumption is consistent with atomic force microscopy (AFM) images of goethite nanoparticles (unpublished data) and with asymmetric lengths observed at goethite nanocrystal tips. Furthermore, lattice spacings were measured from highresolution images (obtained using the FEI Tecnai G2 30) containing goethite nanorods oriented with [100] parallel to the beam. This enabled the use of the goethite nanorod as an internal standard. Finally, an XRD pattern was simulated using DIFFaX29 (version 1.807) for 3.5 nm goethite particles for qualitative comparison purposes. DIFFaX is a Fortran program that considers a crystalline solid as planes of atoms, generated by unit-cell dimensions and atomic fractional coordinates. These planes are then stacked and diffraction intensities are integrated recursively. Crystallite size peakbroadening is included by setting each layer to a finite diameter and limiting the total number of layers. The simulations were produced using a Lorentzian instrumental broadening profile with a full width at half-maximum (fwhm) of 0.09° 2θ (consistent with the observed instrumental broadening of the diffractometer used to collect patterns in this work). The Co KR1 and KR2 contributions were generated separately and summed to produce the final simulations.

Results and Discussion Two types of nanoparticles were observed in TEM images of the unaged particles synthesized by method 1: nanorods and nanodots (see upper image of Figure 1). The classification of particles as nanorods or nanodots was based on the size and aspect ratio of each particle examined by TEM. Of the 2235 particles imaged, 99.8% (by number) of the nanoparticles were classified as dots. An average diameter of 3.6 nm was determined by measuring the size of 507 dots from a randomly selected subset of the TEM images collected. The remaining 0.2% (by number, a total of five nanorods were observed) were classified as nanorods with an average size of 5 × 25 nm. Figure 1 shows representative TEM images of samples aged at 90 °C at a pH of 8.0. In general, nanodots are consumed as nanorods are produced. Figure 2 shows XRD patterns as a function (29) Treacy, M.; Newsam, J.; Deem, M. A general recursive method for calculating diffracted intensities from crystals containing planar faults. Proc. R. Soc. London, Ser. A 1991, 433, 499.

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Figure 2. XRD patterns for nanoparticles aged at pH 8 and 90 °C for 0, 24, 96, 144, 196, 279, and 543 h. Primary particles were synthesized by method 1. For a comparison, the powder diffraction pattern of goethite is shown at the bottom (ICDD card 29-0713). The table inset details the fwhm of the goethite (110) peak at 24.70° 2θ for the aged samples. The sharp diffraction peak observed at 34.3° 2θ in several of the diffraction patterns is due to the presence of NaNO3.

Figure 1. TEM images of nanoparticles aged at pH 8 and 90 °C for 0, 24, 144, and 543 h. Primary particles were synthesized by method 1. In the 24 h image, the inset schematic shows how the TEM-measured width was converted to the width along the a direction based on the assumption that the nanorods lie flat on {011}.

of aging time. The pattern labeled “0 hrs” demonstrates that the unaged material is primarily six-line ferrihydrite (Fe5HO8‚4H2O), and patterns labeled with aging times show that aged material is predominantly goethite. Particle size analysis and classification using TEM images show that the goethite nanorods grow at the expense of the primary particles and demonstrate that both oriented aggregation, which is dominant, and coarsening are important growth mechanisms in this system (discussed below). Particle size analysis of the unaggregated nanodots suggests that these precursor particles grow by coarsening (Figure 3). Coarsening, also known as Ostwald ripening, is a mechanism driven by the fact that the chemical potential of a particle increases with decreasing particle size, which is described by the GibbsThompson equation. The work of Lifshitz, Slyozov, and

Figure 3. Plot of the average primary nanoparticle radius cubed (r3) versus aging time for particles aged at 90 °C for pH 4, 6, 8, 10, and 12. Primary particles were synthesized by method 1. Solid lines represent linear fits, which were determined by the method of least squares.

Wagner30,31 derived a rate law for this process by combining the Gibbs-Thompson equation and Fick’s first law

jr3 - jro3 ) kt

(1)

where jr is the average particle size at time t, jro is the average initial particle size, and k is the rate constant. This rate law is commonly used to describe diffusion-limited particle growth in the solid state and in liquids. Thus, larger particles grow at the (30) Lifshitz, I. M.; Slyozov, V. V. The kinetics of precipitation from supersaturated solid solutions. J. Phys. Chem. Solids 1961, 19, 35.

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Figure 5. Plot of the rod width (open symbols), length (dotted symbols), and aspect ratio (filled symbols) as a function of the aging time and suspension pH. Primary particles were synthesized by method 1. The relative standard deviation of rod width, length, and aspect ratio was 35%.

Figure 4. Plot of percent rods (by number) for particles aged at 90 °C at pH 4, 6, 8, 10, and 12. Aging times, in hours, are indicated to the right of each bar. Primary particles were synthesized by method 1.

expense of smaller particles by diffusion of molecular-scale species through solution.32,33 Figure 3 shows the evolution of r3 versus time for nanodots from samples aged at each pH, and the linear fits are reasonable. As pH increases, the rate constant for growth by coarsening increases, which is consistent with an increase in the iron oxide solubility with an increasing pH. A similar analysis plotting the average volume of the goethite nanorods versus time yields unacceptably poor linear fits, which confirms that coarsening is not the dominant mechanism by which the goethite nanorods grow. The rates at which nanodots are consumed and nanorods are produced significantly accelerate with increasing pH (Figure 4). A complete conversion from nanodots to goethite nanorods is not observed in suspensions aged at or below pH 6 even after 3 weeks of aging, while complete conversion is observed for samples aged at pH 10 and 12 within 48 h. Figure 5 shows the evolution of particle width, length, and aspect ratio as a function of time. The initial average nanorod size for unaged nanoparticle samples (0 h) is ∼5 × 25 nm, and the average rod size increases to 10 × 50 nm after aging. In general, the nanorod length and width stay constant over the entire aging period, suggesting that, once the nanorods reach a threshold average size, existing particles do not continue to grow. Time-resolved TEM images (Figure 1) show that the goethite nanorods grow at the expense of the nanodots. High-resolution TEM images (e.g., Figure 6) demonstrate that the texture and morphology of the goethite nanorods is consistent with growth by aggregation of primary particles. Figure 6 shows three highresolution TEM images of individual goethite nanorods collected using low-intensity conditions. In the upper image, 2.5 Å lattice (31) Wagner, C. Theorie der Alterung von Niederschla¨gen durch Umlo¨sen. Z. Elektrochem. 1961, 65, 581. (32) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies. Annu. ReV. Mater. Sci. 2000, 30, 545. (33) Oskam, G.; Hu, Z.; Penn, R. L.; Pesika, N.; Searson, P. C. Coarsening of metal oxide nanoparticles. Phys. ReV. E 2002, 66, 011403/1.

Figure 6. High-resolution TEM images of nanorods from suspensions of method 1 synthesized particles aged at pH 6 and 90 °C for 196 h. Lattice fringe spacings are consistent with goethite. The translucent outlines serve to highlight the morphology and texture of the particles. In all three cases, the scale bars are 5 nm.

fringes running parallel to the length of the particle are visible. This spacing is consistent with both goethite and ferrihydrite. In the two lower images, the lattice fringe spacings and their angular relationship are consistent with goethite oriented with [100] parallel to the beam, and the lattice fringes running parallel to the length of the crystal are (040). Lattice fringes span the length of all three particles, demonstrating that they are single crystals. In the upper micrograph, the lattice spacing running parallel to the length of the particle matches that of the (040) lattice fringes in the lower two images. Thus, we conclude that the nanorod shown in the upper image is most likely goethite. Viewing the upper micrograph at a low angle reveals the presence of defects and slight misorientations between neighboring regions of crystalline material, which is highlighted by transparent lines

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drawn parallel to the local lattice fringes. In addition, this particle appears to be composed of nodules that are similar in size to the nanodots (i.e., 3-5 nm), which are consumed as growth proceeds. The translucent outline serves to highlight the morphology and texture of this particle. In lower resolution images, the nodular morphology is often not apparent (e.g., Figure 1), and at longer aging times, nodular features become less pronounced as particles become more strongly faceted and more smooth in appearance (as in the lower two images of Figure 6), although features consistent with growth by aggregation, such as dimpling, are not completely erased. We interpret this to indicate that the image shown in the upper image of Figure 6 is of a newly formed goethite nanorod and that defects and misorientations are removed via slow recrystallization and slow growth by coarsening. This interpretation is consistent with both XRD peak-broadening analysis (Figure 2), which shows that the peak width of the 110 reflection decreases as a function of time despite the observation (by TEM) that the average goethite size and shape do not change as aging continues and the observation that the nanodots grow by coarsening even as they are consumed by ongoing goethite nanorod production and growth. The variation in contrast, the nodular appearance, and the incorporation of defects and slight misorientations along the length of the particle shown in the upper image of Figure 6 are consistent with a particle composed of oriented building block nanoparticles. In addition, the size, shape, and nodular nature of nanoparticles aged in aqueous suspensions at pH 4, 6, 10, and 12 at 90 °C (TEM images not shown) were similar to those shown for the samples aged at pH 8. The combination of the high-resolution images shown in Figure 6 and the particle size analysis shown in Figure 4 clearly supports the conclusion of goethite growth by aggregation of nanodots. Ferrihydrite-Goethite Phase Transformation. There are two plausible hypotheses (upper left image in Figure 7) regarding the question of when the phase transformation from six-line ferrihydrite to goethite occurs. First, six-line ferrihydrite nanodots convert to goethite nanodots, and the resulting goethite nanodots grow by oriented aggregation to produce the nanorods. Second, six-line ferrihydrite nanodots are converted to ferrihydrite nanorods through recognition aggregation, which results in some degree of structural accord at the interface between the nanodot particles without the implied constraint on relative orientation, followed by conversion to goethite. It is plausible that phase transformation is particle size-dependent in this system, which would mean that the ferrihydrite-goethite transformation would occur at a threshold particle size similar to what is observed in the anatase/rutile system.34,35-38 In the first hypothesis, nanodot transformation from ferrihydrite to goethite would occur once the threshold particle size has been reached via growth by coarsening. In the second hypothesis, phase transformation could occur once a sufficient number of ferrihydrite nanodots aggregate to exceed the threshold particle size required for phase transformation. Subsequent nanorod growth could occur by recognition aggregation of ferrihydrite nanoparticles onto goethite nanorods followed by phase transformation initiated at the ferrihydrite-goethite interface, similar to the discussion in refs (34) Gribb, A. A.; Banfield, J. F. Particle size effects on transformation kinetics and phase stability in nanocrystalline TiO2. Am. Mineral. 1997, 82, 717. (35) Navrotsky, A. Thermochemistry of nanomaterials. ReV. Mineral. Geochem. 2001, 44, 73. (36) McHale, J. M.; Navrotsky, A.; Perrotta, A. J. Effects of increased surface area and chemisorbed H2O on the relative stability of nanocrystalline γ-Al2O3 and R-Al2O3. J. Phys. Chem. B 1997, 101, 603. (37) Ranade, M. R.; Navrotsky, A.; Zhang, H. Z.; Banfield, J. F.; Elder, S. H.; Zaban, A.; Borse, P. H.; Kulkarni, S. K.; Doran, G. S.; Whitfield, H. J. Energetics of nanocrystalline TiO2. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6476. (38) Zhang, H.; Banfield, J. F. Thermodynamic analysis of phase stability of nanocrystalline titania. J. Mater. Chem. 1998, 8, 2073.

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Figure 7. XRD patterns for a sample of method 1 ferrihydrite that was dialyzed for 3 weeks at room temperature (upper) and the fresh, method 1 ferrihydrite (middle, also shown in Figure 2). The lower, gray pattern is a simulated pattern for 3.5 nm goethite particles. The schematic in the upper left represents the two hypotheses for the transformation from ferrihydrite to goethite as discussed in the text. The dark gray ovals represent ferrihydrite, and the light gray ovals represent goethite. The upper right image is a high-resolution TEM image of method 1 material that was aged at 90 °C for 196 h. Transparent outlines are drawn around two nanoparticles with lattice spacings consistent with goethite (noted in the image). The image shown was taken from a larger image that included a goethite nanorod oriented with [001] parallel to the electron beam. This enabled the use of the goethite nanorod as an internal standard.

39 and 40. In the first hypothesis, primary particles with lattice fringe spacings consistent with goethite would be expected, and this was observed because lattice fringe images enabled the classification of a subset of nanodots as conclusively goethite (upper right image in Figure 7). Furthermore, XRD results are most consistent with the first hypothesis. The upper XRD pattern shown in Figure 7 is from a sample of method 1 ferrihydrite that was dialyzed for 3 weeks at room temperature. Peak-broadening analysis, using the Scherer equation and silicon standard, yields an average domain size of 3.5 nm for the (110) reflection. For a qualitative comparison, the XRD pattern for the fresh, method 1 ferrihydrite (also shown in Figure 2) and a simulated pattern for 3.5 nm goethite are included and serve to demonstrate that the material from the upper pattern is a mixture of six-line ferrihydrite and a relatively small amount of nanogoethite. Unfortunately, the lack of a detailed knowledge of the structure of ferrihydrite makes quantitation of the relative ferrihydrite and goethite content by XRD impossible. In the second hypothesis, ferrihydrite nanorods would be expected. However, images of nanorods with lattice fringe spacings exclusively consistent with ferrihydrite have not been observed, even when those nanorods appear to be composed of only two or three primary particles. In fact, those HRTEM images of oriented particles that appear to be “dimers” have lattice fringe spacings consistent with goethite (highlighted particle in the upper right image of Figure 7). On (39) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Aggregation-based crystal growth and microstructure development in natural iron oxyhydroxide biomineralization products. Science 2000, 289, 751. (40) Gaboriaud, F.; Ehrhardt, J.-J. Effects of different crystal faces on the surface charge of colloidal goethite (R-FeOOH) particles: An experimental and modeling study. Geochim. Cosmochim. Acta 2003, 67, 967.

Two-Step Growth of Goethite from Ferrihydrite

the basis of the combination of the XRD and HRTEM data, we conclude that the nanorods grow by oriented aggregation of goethite nanodots. Effect of Suspension pH on the Growth Rate. The rate of primary particle consumption accelerates with increasing pH (Figure 4). We interpret this result to indicate that the concentration of primary particle complexes as in ref 15 increases with an increasing pH, which is an interpretation that is consistent with the published isoelectric points of iron oxyhydroxide particles, which is generally reported as pH 7.0 to 9.5 for iron oxides,40-42 and would accelerate growth by oriented aggregation substantially. After the analysis presented by Penn,15 the stability constant for the formation of particle complexes can be estimated. From that value, the fraction of particles in particle complexes can be estimated. Using a particle loading of 10 g/L, an ionic strength of 0.01 M, a separation distance of 0.5 nm, a Hamaker constant of 5 × 10-20 J (aqueous medium43), a temperature of 90 °C, and a unitless dielectric constant for water at 90 °C of 58.12,44 the fraction of particles in particle complexes is predicted to be approximately constant in the range of 0-35 mV surface potential (ca. 2 pH units above and below the isoelectric point). Taking pH 8 as the isoelectric point of goethite nanoparticles,40,42 this estimate would predict the growth rate at pH 8 to exceed that at pH 10 and 12. This is not observed. Rather, the rate of rod production at pH 10 is approximately 20 times faster than the growth rate at pH 8. The observation that growth accelerates rather than decelerates once the isoelectric point has been exceeded (i.e., rate of conversion at pH 12) indicates that coarsening becomes a more and more important contributor at high pH, which is consistent with the increasing solubility of ferrihydrite and goethite at higher pH. These results show that goethite growth is most likely a combination of oriented aggregation and coarsening, as was shown by Penn and Banfield for the hydrothermal coarsening of titanium dioxide.16,17 We further conclude that it is growth by oriented aggregation that controls the size and shape in this system based on the observation that the final particle size and shape is consistent over all pH ranges. Effect of Temperature and the Microwave Anneal Step on the Growth Rate. Figure 8 shows representative TEM images of samples (method 2; no microwave anneal step at pH 6) aged at 60, 90, and 120 °C. TEM characterization of unaged nanoparticles synthesized by method 2 revealed that the sample consists entirely of nanodots and the average primary particle size is 3.0 nm (data not shown). Not surprisingly, the rate of goethite rod formation increases with aging temperature (Figure 9). Particle size analysis and classification of nanoparticles aged at 90 and 120 °C reveals a bimodal rod size distribution, and size and shape data are presented in Figure 10. Rod-shaped particles were classified according to two criteria: rods with an aspect ratio less than 4 and a width greater than 5 nm were classified as low aspect ratio (LAR) rods, and the remaining rods were classified as high aspect ratio (HAR) rods. HAR rods are more abundant by number (at the longest aging times: 98% at 60 °C, 88% at 90 °C, and 74% at 120 °C) and are similar in appearance to those observed in aged samples of method 1 nanoparticles (Figures 1 and 6). The inset table in Figure 9 details the percent of LAR rods as a function of the aging time and temperature. (41) Kosmulski, M. A literature survey of the differences between the reported isoelectricnext term points and their discussion. Colloid Surf., A 2003, 222, 113. (42) Cornell, R. M.; Schwertmann, U. The Iron Oxides: Structure, Properties, Reactions, Occurrences, and Uses, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2003; p 188. (43) Stokes, R. J.; Evans, D. F. Fundamentals of Interfacial Engineering; VCH Publishers: New York, 1997; p 701. (44) Archer, D. G.; Wang, P. The dielectric constant of water and DebyeHueckel limiting law slopes. J. Phys Chem. Ref. Data 1990, 19, 371.

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Figure 8. TEM images of nanoparticles aged at 60, 90, and 120 °C and pH 6. Aging times are noted in white. Primary particles were synthesized by method 2.

Figure 9. Plot of the percent of rods versus aging time for particles aged at pH 6 and temperatures of 60, 90, and 120 °C. Primary particles were synthesized by method 2. The table inset details the percent of rods classified as low aspect ratio rods (LAR) (aspect ratio < 4, and width > 5 nm).

The number fraction of LAR rods increases as the aging temperature and time increases. Only 4.1% LAR rods were observed in suspensions aged at 60 °C for 1080 h, while 26% were observed at 120 °C after 48 h. In general, HAR rods have a higher aspect ratio with a decreasing temperature: average sizes are 9 × 62 nm for 120 °C, 8 × 68 nm for 90 °C, and 7 × 67 nm for 60 °C. The average size of LAR rods is ∼25 × 65 nm. As in aged samples of method 1 ferrihydrite, the average size of the rods aged at 60 and 90 °C does not change over time despite the decreasing ratio of the numbers of HAR and LAR rods. However, at an elevated temperature (120 °C), the rods steadily increase in length over time. XRD patterns of pH 6 nanoparticle suspensions aged for 1080 h at 60 °C, 336 h at 90 °C, and 48 h at 120 °C are shown in Figure 11. Diffraction patterns have been normalized to the goethite (110) peak. The small peak at 34.2° 2θ is consistent with the presence of a small amount of NaNO3, and the small peak observed at ∼28.1° 2θ is consistent with the presence of hematite. Rietveld refinement (using PANalytical Highscore Plus software package) was performed to quantify the relative amount of goethite and hematite in the 60 and 90 °C samples. The hematite content of the 60 and 90 °C samples were 0 and 10 wt %, respectively (goodness of fit for each pattern was 5 nm) or high aspect ratio (HAR; all other rods), and data for both types are shown. The average relative standard deviation of rod width, length, and aspect ratio was 49%. Primary particles were synthesized by method 2.

of fit could not be achieved for the 120 °C pattern. Thus, the hematite content in the 120 °C sample was estimated to be 16 wt % based on the relative intensities of the (110) goethite peak and the (011)rh hematite peak in comparison to the 90 °C sample. The shape of the (110) peak is consistent with a bimodal goethite size distribution, which is consistent with the TEM data and explains the poor fit via Reitveld refinement. Examining lattice fringe images of both LAR and HAR rods demonstrated that a small fraction of the LAR rods were hematite pseudomorphs (i.e., LAR morphology was retained). Lattice fringe images of the HAR rods were consistent with goethite. Particle size analysis of the nanodots demonstrates that the nanodots grow by coarsening, which is consistent with the experiments discussed above. Average r3 increases with aging time for each of the temperature series, and linear fits are good

for both the 90 and 120 °C datasets (Figure 12). This suggests that the nanodots grow by Ostwald ripening at 90 and 120 °C. In contrast, the 60 °C dataset is not fit well using the Ostwald ripening model. The primary difference between method 1 and method 2 preparations is the microwave anneal step. In method 1, the precursor suspension was microwave-heated until the suspension had just begun to boil. In method 2, this step was not performed. One important difference between these materials is the average particle size. In terms of goethite produced from each of these materials, a comparison of the 90 °C and pH 6 datasets show that goethite production is considerably slower in experiments using method 1 precursor particles. Interestingly, method 2 material produced a bimodal rod distribution, whereas method 1 material did not. XRD patterns show that both materials are six-line ferrihydrite. Particle size analysis (by HRTEM) shows that the average size of the method 1 primary particles (3.6 nm) is somewhat larger than the average size of the method 2 primary particles (3.0 nm), and this size difference is apparent in the XRD patterns. However, no other difference was discernible. In fact, TEM images suggest that the method 1 material is less homogeneous because of the presence of a small number of

Two-Step Growth of Goethite from Ferrihydrite

nanorods. This result highlights the importance of developing consistent procedures for the preparation of nanocrystalline materials and suggests that the fast anneal step by microwave irradiation improves the homogeneity of the six-line ferrihydrite nanoparticles. Finally, the homogeneity of the goethite produced is superior when method 1 precursor particles are used.

Conclusions In general, goethite nanorods grow from six-line ferrihydrite through phase transformation followed by growth by oriented aggregation. Growth is faster at higher pH and higher temperatures. In addition, particle size measurements demonstrate that the nanodots grow by Ostwald ripening even as they are being consumed. Finally, the use of a microwave anneal step in the

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preparation of the precursor six-line ferrihydrite nanoparticles substantially improves the homogeneity of the final goethite product. Final nanoparticles are unaggregated, acicular crystals in the tens of nanometers size range. These particles may be ideal for mineral liquid crystal and magnetic-recording media applications. Acknowledgment. The authors acknowledge financial support from the National Science Foundation (CAREER-0346385) and the University of Minnesota. Furthermore, the authors thank J. Myers for assistance with the simulated XRD pattern for 3.5 nm goethite. LA051883G