Formation Pathways of Magnetite Nanoparticles by Coprecipitation

Feb 23, 2012 - ... Engineering (BK21 Graduate Program), Korea Advanced Institute of Science and. Technology, Daejeon 305-701, Republic of Korea. ‡. ...
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Formation Pathways of Magnetite Nanoparticles by Coprecipitation Method Taebin Ahn,† Jong Hun Kim,† Hee-Man Yang,† Jeong Woo Lee,† and Jong-Duk Kim*,†,‡ †

Department of Chemical and Biomolecular Engineering (BK21 Graduate Program), Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea ‡ Center for Energy and Environment Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: Magnetite nanoparticles for biomedical applications are typically prepared using the coprecipitation technique, which is the most convenient method. However, the reaction pathways leading to the production of the magnetite phase in the coprecipitation reaction are not fully understood, despite the fact that the reaction path may be of significant importance in controlling the crystal structure, morphology, and particle size of the magnetite nanoparticles. In the present study, we identified the reaction pathways in the coprecipitation of magnetite; when base was slowly added to an iron chloride solution, akaganeite nucleated and transformed through goethite to magnetite. At high addition rates, an additional pathway in which ferrous hydroxide nucleated and transformed through lepidocrocite to magnetite competed with the former pathway. This difference was due to the pH inhomogeneity in the reaction medium that was present before homogeneous mixing. In most coprecipitation reactions, these magnetite formation pathways coexist, but the dominant process is the topotactic transformation of goethite to magnetite, mediated by arrow-shaped nanoparticles. The morphology of the arrow-shaped nanoparticles was explained on the basis of specific crystallographic relationships among the iron oxide phases. The proposed reaction scheme for magnetite coprecipitation could assist in devising a more detailed study of the reaction mechanism.

1. INTRODUCTION Processes for synthesizing magnetite (Fe3O4) nanoparticles have been extensively investigated over the past several decades, owing to the biocompatibility and high saturation magnetization of such particles, which make them suitable for diverse biomedical applications.1−5 The synthesis of iron oxides can be complicated, as there are as many as 16 distinct species in the form of oxides, hydroxides, and oxyhydroxides. However, magnetite, with an inverse spinel structure, can be synthesized by well-defined processes, such as the thermal decomposition,6 microemulsion,7 hydrothermal,8 and coprecipitation9−12 methods. Among the various techniques for magnetite synthesis, the coprecipitation method is a convenient way to synthesize magnetite nanoparticles from an aqueous iron salt (Fe2+ + Fe3+) solution; a base is simply added under an inert atmosphere at room temperature. The coprecipitation process does not produce or use any toxic intermediates or solvents, does not require precursor complexes, and proceeds at temperatures under 100 °C. This process has been recognized for its industrial importance because of its ability to be scaled up, its reproducibility, and its eco-friendly reaction conditions.4,5 However, it yields particles with a broad size distribution,1,4,5 probably because of the complicated set of pathways that lead to the formation of magnetite. © 2012 American Chemical Society

In general, the magnetite particles produced in the coprecipitation process are crystallized in a quasi-immediate process at room temperature, via a rather complex mechanism.13 It is, therefore, of significant interest to increase our understanding of the underlying mechanism, and to improve the size distribution and crystallinity of the magnetite nanoparticles produced. After early pioneering work by Massart,9 many research groups reported various phase transformations between iron oxides. Cornell and Giovanoli investigated the phase transformation of akaganeite into goethite and/or hematite in alkaline media.14 Similar results were also observed by other research groups.15−18 Gualtieri and Venturelli used in situ synchrotron X-ray powder diffraction to study the transformation of goethite to hematite.19 Hematite− magnetite20−24 and lepidocrocite−maghemite25,26 transformations were also reported. Abou-Hassan et al. recently studied the kinetics of the transformation using a coaxial flow microreactor and demonstrated the importance of pH gradients for superparamagnetic nanoparticle synthesis.27 They also separated the nucleation and growth process of ferrihydrite nanoparticles.28 However, all these studies covered only a small Received: December 8, 2011 Revised: February 23, 2012 Published: February 23, 2012 6069

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nanoparticles without changing the pH, even though ammonia was added such that the R value changed from 2.0 to 2.5. 3.1.1. Akaganeite Dissolves near R = 2. The evolution of the morphology over the course of the synthetic process is shown in Figure 1. The TEM images in Figure 1, XRD patterns

part of the entire transformation involving the coprecipitation reaction of magnetite. Here, we report the phase transformation of the intermediates of goethite (α-FeOOH), akaganeite (βFeOOH), and lepidocrocite (γ-FeOOH). A reaction scheme is proposed in which Fe2+ and Fe3+ have separate, but interrelated, pathways toward magnetite nanoparticles. We also report the topotactic transformation of goethite to magnetite mediated by arrow-shaped nanoparticles.

2. EXPERIMENTAL SECTION 2.1. Materials. Ferric chloride hexahydrate (FeCl3·6H2O), ferrous chloride tetrahydrate (FeCl2·4H2O), and ammonia solution (NH3(aq)) were purchased from Sigma-Aldrich. All reagents were used as received without further purification. Deionized water was used throughout the experiments. 2.2. Synthesis. Experiments were carried out at 25 °C, in a 1 L jacketed glass reactor equipped with a mechanical stirrer, a temperature sensor, a gas inflow port (for N2(g)), an exit gas tube with a water-cooled condenser, and a port for the addition of base solution.29 The iron salt solution (500 g, containing 60 mmol of FeCl3·6H2O and 30 mmol of FeCl2·4H2O) was transferred to an oxygen-free reactor, and a 0.8 M NH3 solution was added to the iron salt solution. This addition was performed either continuously, using a peristaltic pump at a constant speed (1.88 mL/min), or abruptly, where the entire NH3 solution volume was added at once. Hence, the molar ratio of ammonia to iron ions (R = [NH3]/[Fe2+ + Fe3+]) was varied with sampling intervals of 0.5 or 0.1 in R. In the abrupt addition case, samples were removed 60 min after the addition of the base solution (preliminary experimental results indicated 60 min was enough to complete the reaction). Samples were centrifuged, repeatedly washed with deionized water, and then dried at room temperature under vacuum. 2.3. Characterization. Transmission electron microscopy (TEM) images and fast Fourier transform (FFT) patterns were recorded using a Tecnai G2 F30 (FEI) at 300 kV, or a Tecnai F20 (FEI) at 200 kV. A drop of the colloidal solution in methanol was deposited onto a thin carbon film supported by a copper TEM grid, and the solvent was then allowed to evaporate. Field emission scanning electron microscopy (FESEM) images were obtained using an S-4800 instrument (Hitachi). Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were measured using an FTS 3000 instrument (Bio-Rad). Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-RB diffractometer with Cu Kα radiation (λ = 1.5406 Å), at 40 kV and 100 mA (see Table S1, Supporting Information, for the crystal structures referred to in this study).

Figure 1. TEM images of samples at R values of (a) 1.5, (b) 2.0, (c) 2.5, and (d) 3.0 (continuous addition of base). The black arrow in (c) indicates a detached arrowhead part.

Figure 2. XRD patterns of samples at R values of (a) 1.5, (b) 2.0, (c) 2.5, and (d) 3.0 (continuous addition of base): (pink cross) akaganeite (JCPDS 34-1266), (blue square) goethite (JCPDS 29-0713), (black diamond) magnetite (JCPDS 19-0629). Subscript: G, goethite.

3. RESULTS AND DISCUSSION 3.1. Continuous Addition of Base. An NH3 solution was added continuously to an iron salt solution (1:2 mol ratio mixture of Fe2+ and Fe3+) over the range of R = 0−4.0; the pH of the solution varied from 1.5 to 9.0. The color of the iron solution changed slowly from light brown, through dark brown, and finally to black, indicating the formation of magnetite nanoparticles (Figure S1, Supporting Information). Two pH plateaus (which indicated that the added base was consumed by the precipitation reaction) were observed near pH 1.5 and 5.0, corresponding to the hydroxylations of Fe3+ and Fe2+, respectively (Figure S2, Supporting Information).30,31 Near pH 5, the mixed iron salt solutions crystallized the magnetite

in Figure 2, and FTIR spectra in Figure S3 (Supporting Information) indicated that akaganeite and goethite were intermediates for magnetite. At R = 1.5, rod-shaped akaganeite particles were precipitated with low crystallinity, as indicated by the broad XRD peaks. With an increase in R to 2.0, the akaganeite particles dissolved and became rounded and smaller (∼2−3 nm), consistent with the broader XRD peaks. At R = 2.5, these small akaganeite nanoparticles evolved into a mixture of rod-shaped goethite nanoparticles (minor component) and 6070

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3.1.2. Oriented Aggregation to Goethite. Rod-shaped akaganeite nanoparticles grown along the [001] direction are produced by the hydrolysis of FeCl3 solution at pH ∼ 1−2 ([OH]/[Fe3+] < 2.7).32,33 The akaganeite nanoparticles take up protons in acidic solution. The akaganeite structure is stabilized by the incorporation of a stoichiometric amount of Cl− ions, but these are easily replaced by OH− ions at higher pH. Such ion replacement may induce the phase transformation of akaganeite to goethite, which can be described as a dissolution−recrystallization process.14−16 Recent studies indicate that, when the structure of akaganeite nanoparticles collapses and their particle size becomes comparable to a few unit cells, akaganeite fragments may have a goethite-like crystalline structure.17,18 Our results also demonstrate that, as chloride ions were replaced by hydroxide ions and the akaganeite dissolved, the crystalline structure of the small (∼2−3 nm) fragments of akaganeite became nearly indistinguishable from that of goethite (Figure 5a,b, Table S2

rounded magnetite nanoparticles (main component). At R > 3.0, pure rounded magnetite nanoparticles were obtained. Near pH 5 (2.0 ≤ R ≤ 2.5), our results indicated that akaganeite dissolved, followed by the crystallization of goethite and magnetite. Because such a dramatic and complex transition occurred between R = 2.0 and 2.5, we took samples at narrow intervals (intervals of 0.1 R) in this range and characterized the crystal structures (Figures 3, 4, and S3, spectrum b (Supporting

Figure 3. TEM images of samples at R values of (a) 2.1, (b) 2.2, (c) 2.3, and (d) 2.4 (continuous addition of base). The black arrow in (d) indicates a detached arrowhead part.

Figure 5. TEM and HRTEM images of samples at R values of (a, b) 2.0 and (c, d) 2.1 (continuous addition of base). (b) [1̅11]A zone axis HRTEM image obtained from the rectangular area in (a). (c) The lattice fringe spacings of 0.31 nm corresponded to 2 times the (160)G and (1̅60)G planes. (d) The lattice fringe spacings of 0.27 nm corresponded to the (130)G plane (black arrows indicate junctions). Subscripts: A, akaganeite; G, goethite.

(Supporting Information)). The HRTEM image and the corresponding FFT pattern (Figure 5b) of the fragment show the akaganeite structure, confirming the presence of (211) and (33̅ 0̅ ) planes, which are very similar to the (021̅ ) and (040) planes of goethite, respectively (Table S2, Supporting Information). Neighboring akaganeite fragments may have experienced an oriented aggregation due to a thermodynamic driving force working to reduce the surface energy (Figure 5c,d).34−36 Akaganeite fragments with high OH− replacement could, therefore, be used as building blocks for goethite structures with only slight rearrangements; thus, the species dissolved at high pH may also have recrystallized on the blocks, making the oriented aggregates more perfect. This crystal growth by oriented aggregation often produced small misorientations at the interface between the blocks in the goethite nanorods (Figure 5c,d).37 Therefore, the evolution of akaganeite to goethite proceeded via a “fragmentation by

Figure 4. XRD patterns of samples at R values of (a) 2.1, (b) 2.2, (c) 2.3, and (d) 2.4 (continuous addition of base): (blue square) goethite, (black diamond) magnetite.

Information)). At R = 2.1, nearly pure goethite nanorods were observed as an intermediate phase. As R was increased to 2.5, these goethite nanoparticles gradually disappeared, and the proportion of magnetite nanoparticles simultaneously increased. These results indicated that the overall variation of the crystal structures during the continuous addition of the base solution could be expressed as “akaganeite → goethite → magnetite”. 6071

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dissolution of akaganeiteoriented aggregation to goethite recrystallization” process. 3.1.3. Topotactic Transformation. TEM and SEM images taken during the transformation from goethite to magnetite (2.1 ≤ R ≤ 2.5) show the diverse crystal habits (Figures 6 and

Figure 7. HRTEM images obtained from the rectangular area in each inset. (a) The lattice fringe spacing of 0.42 nm corresponded to 2 times the (400)Mt plane (R = 2.2). (b) The lattice fringe spacings of 0.225 and 0.97 nm corresponded to the (121)G and 2 times the (111)Mt planes, respectively (R = 2.2). (c) The lattice fringe spacing of 0.225 nm corresponded to the (121)G plane (R = 2.1). (d) The lattice fringe spacings of 0.495 and 0.254 nm corresponded to the (020)G and (101)G planes, respectively (R = 2.3).

5c,d) and hematite grew on the surface of the tips of the goethite nanorods. Magnetite has ccp arrays of oxygen anions along the [111]Mt, which is parallel to [001]H, and that [11̅0]Mt is parallel to [110]H in the case of the topotactic transformation of hematite to magnetite (Figure S6, Supporting Information).20,21 In that case, the hcp oxygen packing of hematite can play a role as a template for the ccp oxygen packing of magnetite.22,23,40 Thus, magnetite can also develop arrowheads similar to the hematite shape from two imaginary octahedra of magnetite (Figure 6c). TEM and HRTEM images show the arrow-shaped nanoparticles, which have magnetite lattice planes in the arrowhead region and goethite lattice planes in the arrow shaft region (Figures 6 and 7). Therefore, the transformations of goethite through hematite to magnetite proceeded via a topotactic process based on the above structural relationships. The topotactic transformation of goethite to hematite involves dehydration and local rearrangement processes,19,41,42 which are accelerated by the presence of a small amount of Fe2+, which enables electron hopping between Fe2+ and Fe3+.43−46 As the transformation proceeds, the destruction of the crystal structure of goethite nanorods progresses gradually due to the dehydration and rearrangement processes. Conversely, the crystal structure of magnetite forms in the arrowhead region. As a result, the arrow-shaped nanoparticles, which show magnetite lattice fringes, have an obscure goethite crystalline structure, while the arrow-shaped nanoparticles, which display obvious goethite lattice fringes, possess ambiguous arrowhead shapes and an unclear magnetite crystalline structure (Figures 6 and 7). This destruction of the goethite structure explains the absence of double-arrowshaped nanoparticles. 3.1.4. Adsorption of Fe2+ Ions on Maghemite. Although hematite is more stable than maghemite in the bulk state, the thermodynamic stability of maghemite becomes comparable to

Figure 6. (a) Crystallographic relationship between goethite and hematite. (b) Schematic drawing of an arrow-shaped nanoparticle consisting of a nanorod of goethite and a rhombohedron of hematite with a [110]H zone axis. (c) Schematic drawing of an arrow-shaped nanoparticle consisting of a nanorod of goethite and two halves of two imaginary octahedra of magnetite with a [11̅0]Mt zone axis. (d) TEM image of an arrow-shaped nanoparticle at R = 2.2. (e) [11̅0]Mt zone axis HRTEM image obtained from the rectangular area in (d). The lattice fringe spacings of 0.42 and 0.18 nm corresponded to 2 times the (004̅)Mt and (664)Mt planes, respectively. Blue, goethite; red, hematite (rhombohedral); black, hematite (hexagonal); and green, magnetite. Subscripts: G, goethite; H, hematite; Mt, magnetite.

7, and S4 and S5 (Supporting Information)). Of particular note were the arrow shapes, which formed parallelograms with identical angles. For these arrow-shaped nanoparticles, a specific crystallographic relationship would exist between goethite (orthorhombic) and hematite (hexagonal), or between hematite and magnetite (cubic). The formation of arrow-shaped nanoparticles can be explained by the following possible mechanisms. The structure of goethite and hematite can be described as consisting of slightly distorted hcp arrays of anions (O2−, OH−) stacked along the [100]G direction and the [001]H direction, for goethite and hematite, respectively.38 In addition, the b axis of goethite is 2 times the a axis of hematite and the c axis of hematite is 3 times the a axis of goethite (Table S1, Supporting Information).16,19 Since the structure of hematite can also be expressed using rhombohedral axes,39 the crystallographic relationships among the three kinds of crystal axes can be depicted schematically, as shown in Figure 6a (see Figure S6, Supporting Information, for detailed crystal models). On the basis of these relationships, the formation of the arrow-shaped nanoparticles shown in Figure 6b would be possible if the goethite nanorods grew along the [100]G direction (Figure 6072

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arrowheads acted as seeds for magnetite growth and underwent oriented aggregation, in a similar fashion as in the phase transformation of akaganeite to goethite. Both a slight misorientation between the primary particles and an overall parallelism of the lattice fringes (which are usually found in crystals grown via the oriented aggregation mechanism)37 were observed in the magnetite nanoparticles at R = 3.0 (Figure 9).

that of hematite at the nanoscale, due to the high surface energy of hematite. Consequently, hematite nanoparticles may alter their structure and may possess a maghemite-like structure (inverse spinel structure) near the surface; this tendency is amplified as the particle size decreases.18,40,47 The structure of magnetite resembles that of maghemite closely enough that they can form a continuous solid solution with each other,18 and the adsorption of Fe2+ ions on the maghemite surface induces the conversion of maghemite to magnetite by raising R.13,24 In fact, several driving forces may have combined to cause phase transformations at this stage (pH ∼ 5). Increases in R, the thermodynamic stability of hematite and maghemite, and the adsorption of Fe2+ gave rise to the transformations from goethite to hematite,15,48 hematite to maghemite, and maghemite to magnetite, respectively. Moreover, the presence of Fe2+ in the reaction medium catalyzed all these phase transformations.49 The conversion of goethite to magnetite should pass through the hematite and maghemite phases from a crystallographic viewpoint. Therefore, hematite and maghemite may be transient phases in the transformation of goethite into magnetite. The angles of the arrowheads shown in Figures S4 and S5 (Supporting Information) were more similar to those of hematite than magnetite (Figure 6b,c), but the lattice planes were similar to those of magnetite rather than hematite (albeit imperfectly similar) (Figures 6e and 7). This suggests that the arrowhead structure was an intermediate phase between goethite and magnetite that was not observed as distinct peaks of hematite or maghemite in the XRD patterns (Figure 4). The transformation to magnetite occurred almost simultaneously with the generation of hematite on the surface of the goethite nanoparticles. We used ex situ characterization methods in this study, which may have hindered the ability to detect the intermediate phases. Further studies of phase transformations during coprecipitation reactions by in situ analyses will be implemented for stronger support of the proposed formation pathways. 3.1.5. Magnetite Seed. The intermediate character of the arrowhead regions implies that the arrowhead sections detached from the goethite before, or soon after, it formed the magnetite crystal structures. Other work has suggested that dehydration and iron migration may induce void structures in goethite, which subsequently collapse at the end of the reaction, during the topotactic transformation to hematite.19,50 The topotactic transformations between hematite and magnetite may trigger dissolution and fracturing along the interphase boundaries, due to the induced internal stress resulting from the crystal structural difference between two phases, which finally leads to separation.22,23 These voids and fractures also occurred in our experiments, and the magnetite-like structures eventually detached from the goethite nanoparticles as nuclei (Figures 1c, 3d, and 8). Once detached, the separated

Figure 9. [111]Mt zone axis HRTEM images of oriented aggregates of magnetite nanoparticles. The lattice fringe spacings of 0.3 nm corresponded to {220}Mt planes.

The goethite nanorods of the arrow-shaped nanoparticles finally collapsed, as explained above, and then provided the Fe3+ ions for the growth reaction of the oriented aggregates with the magnetite-like structure. Meanwhile, the Fe2+ ions, which were initially added to the reaction medium but had not precipitated, were used for the growth reaction. Therefore, the transformation of goethite to magnetite occurred through a “topotactic transformationdetachment produced by internal stressoriented aggregationgrowth of the oriented aggregates” process. 3.2. Abrupt Addition of Base (R < 2.67). It was determined that the transformation (taking place with changes from low pH to high pH) occurred as “akaganeite → goethite → (hematite → maghemite) → magnetite” when the base was added continuously. In conventional batch-operation coprecipitation reactions, however, large amounts of base solution are abruptly (all at once) added to the iron salt solution. The intermediates produced in the course of the reaction could be different when using the two different methods, because the abrupt addition of base solution can intensify the inhomogeneity of the pH in the reaction medium before complete mixing and can alter the reaction rate and reaction pathway. It is, therefore, important to control the process of mixing the iron salt solution and the base solution. 3.2.1. Metastable Lepidocrocite from Ferrous Hydroxide. In the range of R ≈ 2.0−2.2, we observed lepidocrocite, consisting of layered iron(III) oxide octahedra bonded by hydrogen bonding via hydroxide layers. Interestingly, no lepidocrocite was observed in the continuous addition experiments. A large proportion of the particles were tabular lepidocrocite, with small amounts of goethite at R = 2.0. As R increased to 2.2, the proportion of lepidocrocite decreased continuously, and the proportion of goethite and magnetite increased. Finally, at R = 2.6, lepidocrocite totally disappeared and magnetite became the main phase, accompanied by goethite as a minor phase (Figures 10, 11, and S3, spectrum c (Supporting Information)). Lepidocrocite is produced by the oxidation of ferrous hydroxide (Fe(OH)2) under slightly acidic conditions (pH ∼ 5−7), and ferrous hydroxide is formed at pH > ∼6−7.13,51 The abrupt addition of base, as indicated previously, may have

Figure 8. (a, b) TEM images of goethite showing the void morphology. (c) TEM image showing the detached arrowhead parts. 6073

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structure of lepidocrocite consists of ccp arrays of anions stacked along the [150] direction, which corresponds to the [111] direction of inverse spinel structures (maghemite and magnetite).52 There were large quantities (half the amount of Fe3+) of Fe2+ in the reaction solution, enough for lepidocrocite to transform into magnetite through a topotactic process.11 Magnetite with a sheet structure was observed at R = 2.1, which may have originated from lepidocrocite (Figure 10d). We now consider the possible reasons for the observation that lepidocrocite was present and maghemite was absent at R = 2.0. The presence of lepidocrocite implied that some regions of the reaction medium had a pH high enough to form ferrous hydroxide, where Fe3+ naturally precipitates. Since we dissolved Fe2+ and Fe3+ in a 1:2 molar ratio, magnetite would form in that high-pH region if the direct reaction between Fe2+ and Fe3+ occurred in the aqueous phase. The magnetite phase should then have transformed into maghemite as the pH decreased with further solution mixing.13 At R = 2.0, however, large amounts of lepidocrocite and an absence of maghemite were observed. Therefore, magnetite did not form by the direct reaction of Fe2+ and Fe3+ in the aqueous phase, but via the phase transformation of iron oxyhydroxides (goethite and lepidocrocite). 3.3. Conventional Coprecipitation Reaction (Abrupt Addition of Base, R ≫ 2.67). From the results described above, we propose a complete reaction scheme for the coprecipitation of magnetite (see Figure 12). In the conventional coprecipitation process, the iron salt solution and the base solution are mixed abruptly and quickly form a contact

Figure 10. TEM images of samples at R values of (a) 2.0, (b, d) 2.1, and (c) 2.2 (abrupt addition of base). (d) [11̅ 2]Mt zone axis HRTEM image obtained from the rectangular area in (b).

Figure 11. XRD patterns of samples at R values of (a) 2.0, (b) 2.1, (c) 2.2, and (d) 2.6 (abrupt addition of base): (red circle) lepidocrocite (JCPDS 08-0098), (blue square) goethite, (black diamond) magnetite. Figure 12. Formation pathways of magnetite nanoparticles by coprecipitation method. Main intermediate phases are shown in yellow areas.

created incomplete mixing locally, leading to a local pH gradient in the reaction medium before it could homogeneously mix and reach pH 5. It is likely that, in the local high-pH region, ferrous hydroxide was formed first and then (as the mixing proceeded) underwent a transformation to lepidocrocite via the substitution of Fe2+ (in the structure) with Fe3+ (in solution), or via the oxidation of Fe2+ ions. The fact that goethite was formed as a main phase at R = 2.2even under the abrupt addition conditions (Figure 11, pattern c)indicated that the “akaganeite → goethite → (hematite → maghemite) → magnetite” route observed in the continuous addition method was still being followed. The overall direction of the pH change in the abrupt addition method was also from pH 1.5 to pH 5. Consequently, as R increased, the formation of goethite increased, and the formation and size of lepidocrocite, therefore, decreased, due to the lack of available Fe3+ (which was consumed for goethite). 3.2.2. Topotactic Transformation. Magnetite can be formed not only from goethite but also from lepidocrocite. The

interface between the iron-rich solution (pH ∼ 1.5) and the base-rich solution (pH ∼ 11). In the iron-rich solution, the base diffuses across the contact interface, while iron ions diffuse in the opposite direction. Finally, the contact interface will be dispersed in a precipitation event. Although the interface will be maintained for a short period, the low-pH iron-rich side will accept the base, which will increase the pH and react with the less-stable Fe3+ ions to form akaganeite, while the high-pH base-rich side will incorporate not only Fe3+ but also Fe2+ ionic diffusion to form the ferrous hydroxide. Therefore, two different nucleation processes will be initiated as the solutions begin to mix, and both synthetic routes (the “akaganeite → goethite → (hematite → maghemite) → magnetite” route and the “ferrous hydroxide → lepidocrocite → (maghemite) → magnetite” route) will be followed to create the magnetite structure. 6074

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When a large quantity (R ≫ 2.67) of base solution is mixed abruptly with the iron solution, the pH may increase at a more rapid rate compared with continuous addition of base. The growth of akaganeite may then be suppressed, the formation of goethite may be strengthened, and the formation of lepidocrocite may be weakened, due to the lack of Fe3+. In addition, the hematite and maghemite may be part of a transient phase. Therefore, goethite and ferrous hydroxide would be the major intermediates controlling the phase transformation and particle growth.

ASSOCIATED CONTENT

S Supporting Information *

Crystal structures, color photograph of reaction solution, pH curve, FTIR spectra, further details in Figure 5b, TEM and SEM images of arrow-shaped nanoparticles, crystal models, and magnetic attraction. This material is available free of charge via the Internet at http://pubs.acs.org.

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4. CONCLUSIONS The present results showed that magnetite nanoparticles were formed in the coprecipitation process by the phase transformation of iron oxyhydroxides, rather than the direct reaction of Fe2+ and Fe3+ in the aqueous phase. This was detected using X-ray diffraction, electron microscopy, and FTIR spectroscopy. Akaganeite nucleated and transformed to goethite, which underwent a topotactic transformation to magnetite as the pH of the iron salt solution increased slowly and continuously. Simultaneously with the above reaction route, an additional reaction route was followed: ferrous hydroxide nucleated and transformed to lepidocrocite, which underwent a topotactic transformation to magnetite as the iron salt solution and the base solution mixed quickly and abruptly. Especially, the topotactic transformation of goethite to magnetite was mediated by the unprecedented arrow-shaped nanoparticles. These phase transformations were consistent with the specific crystallographic relationships among the iron oxide phases. The growth of goethite and magnetite proceeded through an oriented aggregation mechanism. In coprecipitation reactions, the formation of magnetite proceeds via the reaction pathways described above, which are complex and interrelated. This may explain the wide particle size distribution and low crystallinity of magnetite nanoparticles prepared using the coprecipitation process. The findings of this study indicate the importance of the mixing process (i.e., the mixing of the iron salt solution and the base solution) in determining the coprecipitation reaction route and provide the foundations for a more detailed study of the reaction mechanism.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program, through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2009-0076882). We thank Hyung Bin Bae in the KAIST Research Supporting Team for TEM measurements. 6075

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