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DOI: 10.1002/adfm.200701119
Shape control of new FexO–Fe3O4 and Fe1–yMnyO–Fe3–zMnzO4 nanostructures** By Cristina Hofmann, Irene Rusakova, Teyeb Ould-Ely, Darı´o Prieto-Centurio´n, Keith B. Hartman, Anna T. Kelly, Andreas L¨uttge, and Kenton H. Whitmire*
New nanoparticle shapes of iron oxide (FexO–Fe3O4, where 0.8 < x < 1) and iron-manganese oxide (Fe1–yMnyO–Fe3–zMnzO4, where 0 < y < 1, and 0 < z < 3) were synthesized by decomposition of the corresponding metal formates in tri-n-octylamine/oleic acid mixtures at elevated temperatures (ca. 370 8C), under an inert atmosphere. Details of the syntheses leading to the various shapes of nanoparticles are provided as a function of the reactions parameters, that is, precursor type and concentration, surfactant concentration, water concentration, reaction time, and temperature. Different electron microscopy techniques were used to characterize the crystal phases and the novel shapes of these nanostructures. Nanoparticles of FexO–Fe3O4 were produced with different shapes, that is spheres, hexagons, and cubes, depending on the reaction conditions. By tuning the conditions, iron oxide nanocubes with concave faces were produced exclusively. Electron and X-ray diffraction data reveal these nanocubes to be single-crystal FexO (wu¨stite) with small amounts of Fe3O4 (magnetite). For the mixed metal system, solid solutions of Fe1–yMnyO with very small amounts of Fe3–zMnzO4 were observed, in which the produced oxide had a larger Fe:Mn ratio than present in the starting reagents. Adjusting the iron to manganese ratio in the mixed-metal nanoparticles resulted in different shapes. Nanoparticles with ca. 1:1 (Fe:Mn) ratios displayed a ‘dog-bone-like’ morphology, which can be considered a shape in between a pure FexO–Fe3O4 nanocube and the rod-like nanostructures previously reported for the manganese oxide system. In general, higher Fe:Mn ratios (e.g., 9:1) in the product resulted in nanostructures with cubic shapes, while lower Fe:Mn values (e.g., 2:8) resulted in long (ca. 200 nm) rod-like nanostructures with flared ends. All of the nanostructures reported here exhibit internal structures that suggest a growth mechanism with etching on negatively curved rough crystal faces. Oxidation of the nanoparticles occurred with retention of their original shape.
1. Introduction Iron and manganese oxides have many technological applications that result from their magnetic and catalytic properties.[1] Nanoparticles (NPs) of these oxides have potential applications in information storage, medical imaging, drug delivery, and water remediation.[2–5] In general, the physical properties of NPs can vary with size and shape, but the production of NPs with specific sizes and shapes is still a significant challenge.[6–11] The literature has several examples of NPs with a variety of shapes, but in general these are [*] Prof. K. H. Whitmire, C. Hofmann, Dr. T. Ould-Ely ´n, K. B. Hartman, A. T. Kelly, Prof. A. Lu ¨ttge D. Prieto-Centurio Department of Chemistry, MS60, Rice University 6100 Main Street, Houston, TX 77005-1892 (USA) E-mail:
[email protected] Dr. I. Rusakova Texas Center for Superconductivity, HSC Bldg., University of Houston, Houston, TX 77204-5002 (USA) [**] The authors would like to thank (C-0976) and the National Science for Biological and Environmental 0647452). Supporting Information InterScience.
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the Robert Welch Foundation Foundation through the Center Nanotechnology (CBEN, EECis available online from Wiley
thermodynamically-favored forms (i.e., spheres, rods, wires, belts, and disks). Non-thermodynamic (anisotropic and multibranched) shapes, however, may have enhanced physical properties owing to their intrinsic anisotropy. Also, these tend to have a larger surface area than a traditionally shaped NP of the same size. However, these anisotropic and multibranched forms, such as tetrapods, hexapods, tripods, stars, and dumbbells, are more difficult to produce. Even though the synthesis and growth mechanisms of traditionally shaped NPs have been widely investigated,[12–16] the synthesis and the kinetically controlled growth mechanisms of anisotropic and multibranched NPs are not completely understood. The need to understand NP shape formation has prompted investigations of shape-control syntheses and the development of new growth-mechanism theories.[15,17–24] Understanding the growth mechanisms of anisotropic and multibranched NPs is essential in order to control their production for technological applications. We report herein an investigation on shape control of new shapes of iron and iron-manganese oxide NPs as a function of reaction parameters: precursor type and concentration, surfactant concentration, water concentration, reaction time, and temperature. While some anisotropically shaped iron oxide and ironmanganese oxide NPs have been reported using other
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precursors and conditions,[23,25] the shapes presented here, including the solid solutions of iron-manganese oxide NPs have not been previously reported. Moreover, studies on the factors controlling the shapes are in short supply. Once the original shape of both iron and iron-manganese oxide NPs is formed, the NPs seem to be very robust and can be oxidized under mild conditions while retaining their original shape.[23] In the case of the iron-manganese oxide NPs, the presence of the second metal provides opportunities for further shape variation.
2. Results and Discussion 2.1. Oxide phase identification NPs of iron oxide (FeOX) were produced from decomposition of FeII(HCOO)2 2H2O in a tri-n-octylamine:oleic acid (TOA:OA) mixture, under inert atmosphere at around 370 8C. X-ray diffraction (XRD), and transmission electron microscopy (TEM) selected area electron diffraction (SAED) patterns, recorded from single nanocrystals and from polycrystalline samples, confirmed the NPs to be wu¨stite (facecentered cubic (fcc) FexO (0.8 < x < 1); Fm3m; no. 225; ˚ ) with small amounts of magnetite (Fe3O4; Fd3m; a ¼ 4.326 A ˚ ) (Fig. 1). Since magnetite and maghemite no. 227; a ¼ 8.399 A (g-Fe2O3) have unit cells with only about 1% difference ˚ ), Raman spectroscopy (maghemite: P4332; no. 212; a ¼ 8.346 A was used along with TEM SAED, and results indicated that
Figure 1. a) TEM SAED pattern from a polycrystalline sample of FeOX NPs. Rings with s subscripts are from the spinel Fe3O4 (magnetite) and the other ¨stite). b) XRD pattern from a polycrystalline FeOX rings are from FexO (wu sample.
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magnetite, and not maghemite, was present (see Supporting Information, SI).[26,27] Based on Raman spectroscopy, maghemite was only found in samples that were reacted for more than 30 min after oxide formation, which indicates extensive NP oxidation during extended reaction periods. TEM SAED patterns on single FeOX NPs give unequivocal evidence that the NPs are single crystals composed of wu¨stite (FexO) and magnetite (Fe3O4) (Fig. 2a–d; see enlarged image in SI).[26] The dark-field (DF) TEM images reveal that Fe3O4 in a single FeOX NP is concentrated on the edges and corners, indicating that the core of the NP is composed of wu¨stite (Fig. 2e–f). This suggests that the NPs are originally formed as wu¨stite and that magnetite forms upon mild oxidation of the outermost layer, which is exposed to air after decomposition. This is one of the few examples of wu¨stite NPs.[23,25,28–30] In fact, wu¨stite is a metastable phase only obtained above 567 8C using solid state methods.[1] In order to obtain wu¨stite below 567 8C the process must be a kinetic one. Therefore the phase and the non-thermodynamic (anisotropic) shape suggest that NP production is kinetically driven.[23]
Figure 2. a) Experimental and b) simulated TEM SAED patterns of a single crystal FeOX NP along the [001] zone axis; (FexO (black) and Fe3O4 (white)). Bright Field (BF) TEM images of c) a nonetched and d) an etched (obtained with addition of water; see discussion below) FeOX NP, with SAED pattern insets (enlarged SAED patterns are shown in SI Fig. S2). DF TEM images of FeOX NPs recorded using gFe3 O4 ¼ 220 showing bright areas for the distribution of Fe3O4 in the e) nonetched and f) etched NPs.
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Figure 3. XRD pattern for a) a polycrystalline sample of FeMnOX NPs. ¨stite) and c) MnO (manganosite) are Calculated patterns for b) FexO (wu presented for reference; the dotted line (matched against the crystallographic database) indicates the NPs to be a Fe0.7Mn0.3O solid solution.
The NPs produced from mixtures of FeII(HCOO)2 2H2O and MnII(HCOO)2 2H2O (FeMnOX) were confirmed by XRD to be solid solutions of fcc Fe1–yMnyO.[31] Figure 3 shows the XRD pattern of the decomposition product from a 1:1 homogeneous FeII(HCOO)2 2H2O:MnII(HCOO)2 nH2O mixture that resulted in Fe0.7Mn0.3O NPs. Previously reported iron-manganese oxide NPs are either MnxFeyO4 (x þ y ¼ 3) or MnxFeyO3 (x þ y ¼ 2), and unlike the ones reported here, do not have a defined NP shape.[32,33] SAED on a single FeMnOX NP (Fig. 4) confirms the NP to be a single crystal of Fe1–yMnyO with a small amount of the magnetite analogous phase (Fe3–zMnzO4) (insert in Fig. 4a). Translational moire´ fringes are observed in the bright-field (BF) TEM owing to the presence of a small amount of the spinel (oxidized) magnetite; these results are analogous to those reported for manganese oxide NPs (Fig. 4).[17] The lattice spacing was determined to be ˚ for the ˚ for the Fe1–xMnxO phase and 8.402 A 4.383 A Fe3–zMnzO4 phase by powder X-ray diffraction. An expanded view on a point on the FeMnOX NP clearly shows the atomic planes (Fig. 4b). 2.2. Nanoparticle morphology and shaping process The morphology of the pure iron oxide NPs, FeOX, as determined by TEM, Scanning Electron Microscopy (SEM),
Figure 4. a) Bright-field (BF) TEM image of an FeMnOX NP with SAED pattern along the [001] zone axis (inset). b) High resolution (HR) TEM image recorded from a small selected area (square in image a), showing atomic planes.
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Figure 5. NTCs images of FeOX: a) BF TEM, b) SEM (inset: sketch of a tetracube), c) AFM, and d) AFM measurement of a concave face with a vertical distance of 15.84 nm.
and Atomic Force Microscopy (AFM) (Fig. 5), can be described as cubes with concave faces; which is consistent with wu¨stite’s cubic crystal habit.[1] The geometric figure that most resembles the morphology of these NPs is known as a tetracube (Fig. 5b inset), and thus the NPs with this geometrical form will be referred to as nanotetracubes (NTCs). While the TEM images suggest the morphology of a four-pointed star (Fig. 5a),[23,25] closer examination of the BF TEM images of the NTCs reveals thickness fringes that indicate a difference in height between the edges and the center of the FeOX NTCs; therefore, a flat morphology (i.e., plate) is not likely. SEM images confirm the three-dimensionality of the NTCs and provide a more clear view of the shapes (Fig. 5b). AFM was used to confirm the concavity of the NTCs and to approximate the depth of the cavity on the faces, which was approximately 16 nm deep (Fig. 5c–d).[26] The nonetched NTCs (Figs. 2c and 6a) exhibit thickness fringes corresponding to the incipient star-like shape of the
Figure 6. BF TEM images of a) nonetched and b) etched FeOX NTCs. Thickness fringes with ‘star-like’ shape are observed in the nonetched NPs. Arrows show where etching takes place (on the {100} faces).
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etched NTCs (Figs. 2d and 6b) that are obtained when water is added to the reaction mixture. This suggests that the shaping process involves dissolution of the thinner (probably more defective) areas already present in the initial cubic shapes, therefore resulting in NTCs (Figs. 2d and 6b). As previously mentioned each NP forms as wu¨stite (FexO) and the outermost layer eventually oxidizes to magnetite (Fe3O4) after decomposition and etching are completed. Wu¨stite (FexO), a rock-salt structure, is iron deficient with metal vacancies throughout, which may make it more prone to etching.[1] Etching (digestive ripening) is observed to take place preferentially on the wustite-rich {100} faces of the NP. This is probably because the dissolved material from the faces crystallizes on the edges and corners as explained in the Berg effect, forming Hopper-like nanocrystals such as the NTCs.[24] The shaping process, as seen in other similar systems, agrees with predictions of crystal growth theory from rough, negatively curved surfaces coupled with a digestive ripening mechanism.[17] Such processes lead to symmetrical shapes like stellated polyhedra or Archimedean solids such as the tetracubes obtained here. 2.3. Shape as a function of reaction parameters Of the reaction parameters studied (i.e., water concentration, time, temperature, and surfactant ratio), only water concentration and surfactant ratio were found to have a significant effect on NP size and shape. Reaction time only had a small Ostwald ripening effect, and the only effect reaction temperature had is that no NPs were obtained below 340 8C.[26] The ratio of surfactants, TOA:OA (mL), was systematically varied, and in general most of the desired shapes were obtained at relatively high TOA:OA ratios (Fig. 7). NTCs failed to be produced in the absence of OA. Metal oleates have been reported to decompose into shaped NPs under different conditions and with different surfactants. This coupled with the fact that there is no shape formation without OA, suggests that the oleate anion plays a unique role in the NP formation and shaping phenomena.[21,23,25,26,34] A probable mechanism for the decomposition reaction is that the metal formate anions undergo fast exchange with oleic acid, followed by hydrolysis of the metal oleate into a hydroxide or oxy-hydroxide, and finally into the oxide. A hydrolysis step and the etching progress would explain the need for water in the formation of these particular NPs. Additionally, of all the reaction parameters tested, water concentration had the greatest effect on the shape of the NPs. In general, it was observed that when anhydrous precursors and surfactants were used, no NTCs, or fragments thereof, were obtained.[26] On the other hand, when hydrated precursors and surfactants were employed, non-thermodynamic shaped NPs were consistently obtained.[26] The water contribution of the dihydrate metal formates is approximately 100 mL. When small amounts of water (ca. 100 mL) were deliberately added to the reaction mixture, NTCs with sharper edges and corners resulted. However, increasing the amount of water added to
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Figure 7. Surfactant ratio effect on NP shapes for FeOX. Fe:TOA:OA [TOA:OA, mL] ratios are: a) 1:6.1:0 [8:0], b) 1:5.3:1.1 [7:1], c) 1:5.0:1.6 [6.5:1.5], d) 1:4.6:2.1 [6:2], e) 1:3.8:3.2 [5:3], and f) 1:3.1:4.2 [4:4] (see larger image in SI). Reaction Conditions: 1–5 min, with hydrated precursor and surfactants, and no water added.
almost double (ca. 200 mL) caused the NPs to be less well-defined and more irregular, possibly because of increased solubility of the metal hydroxide or oxy-hydroxide intermediate. Fig. 8 illustrates the water effect on the NPs; FeOX NPs prepared with added water have more concave faces and sharper corners, while the FeMnOX NPs (with relatively higher manganese content as compared to iron) get longer when additional water is included in the reaction mixture. These findings are consistent with previous results, in which MnO arms of nanocrosses grow as a function of increasing water concentration. In general, results indicate that a very delicate balance exists between the surfactant concentration and the amount of water needed in order to obtain the right dissolution-crystallization process that will tune the shape of the NPs.[17,23,35] Shapes of FeMnOX NPs were also tuned by using different ratios of iron to manganese precursors FeII(HCOO)2:MnII(HCOO)2 (Fig. 9). In cases when a Fe0.7Mn0.3O solid solution was produced using the ‘ideal’ synthetic conditions to form NTCs (i.e., hydrated formates with 7:1 volume of TOA:OA and 100 mL of HPLC H2O) an intermediate ‘dog-bone-like’ shape between a NTC and the previously reported MnO shapes was
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Figure 8. Effect of water for FeOX and FeMnOX NPs: a) FeOX no water added, b) FeOX with 100 mL of HPLC water, c) FeMnOX no water added, and d) FeMnOX with 100 mL of HPLC water (Conditions: 1–5 min reactions of hydrated surfactants and precursors, in a 7:1 (mL) of TOA:OA).
obtained (Figs. 4a and 8c).[17,34] The ability to tune these NPs by only changing the metal content is remarkable, plus the physical properties of the oxides should be affected by the size and shape of the NP and the metal composition. It is important to note that the amount of metal, either Fe or Mn, incorporated in the crystal structure of the FeMnOX NPs does not correspond to the initial stoichiometry of the metal precursor added to the reaction mixture (Fig. 9). Iron was preferentially incorporated into the nanostructures: approximately 90% of the iron added as compared to approximately 50% of the manganese added. The difference between the solubility products of the corresponding hydroxides, namely Fe(OH)2 (ksp ¼ 4.9 1017) and Mn(OH)2 (ksp ¼ 1.6 1013) at 25 8C, is substantial and may explain the observed metal ratios in the final products. In other words, the FeII ions are preferentially incorporated into the crystal structure due to lower solubility of the iron hydroxide. As in the previously reported manganese oxide nanocrosses and nanohexapods, the length of the arms of manganese-rich FeMnOX NPs increased with water concentration (Fig. 9). The fact that the lengthening effect with water is more noticeable in manganese-rich NPs, agrees with the higher solubility of manganese hydroxide.[17]
2.4. Nanoparticle oxidation with shape retention The NPs produced are mainly composed of FexO (wu¨stite) or the analogous Fe1–yMnyO. These phases are antiferromagnetic, and were therefore oxidized to obtain the corresponding magnetic oxides. Samples of both FeOX and FeMnOX NPs
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Figure 9. Effect of precursor ratio (FeII(HCOO)2:MnII(HCOO)2) in FeMnOX NPs: a) 3:1, b) 1:1 (NPs produced using 7:1 volume ratio of TOA:OA are shown in Fig. 8c), c) 1:3, and d) 1:9. XRD reveals the phases to be Fe1-yMnyO solid solutions.[31] Conditions: hydrated surfactants and precursors, with a 6:2 volume (TOA:OA), and 100 mL HPLC water, reacted for 5 min.
were successfully oxidized under relatively mild conditions, by heating and stirring in a high boiling point solvent (hexadecane; boiling point 287 8C) for approximately 36 h (under air).[23,35] XRD verified the FeOX and FeMnOX NPs to be either wu¨stite (FexO) or Fe0.8Mn0.2O before oxidation, and Fe3O4 (magnetite) or Fe2.4Mn0.6O4 after oxidation, for FeOX and FeMnOX respectively (Fig. 10). TEM images of the NPs before and after oxidation revealed that they retain their original shape after being oxidized. As shown in the XRD patterns, the FeMnOX NPs are not as crystalline as before the oxidation process. After oxidation the colloidal suspensions were easily manipulated with a magnet, corroborating the ferrimagnetic character of the NPs.[1] Complete oxidation of the NPs with retention of shape was not expected as this process was thought to require a fairly large rearrangement of the crystalline lattice that would lead to loss of the NP morphology. In our previous investigations with
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and stable system for the investigation of growth and shaping reaction parameters. Inherent internal structural defects in the nanocrystals and the effect surfactants and water had on the NPs’ shape confirm our previous observations regarding growth mechanisms and allow us to conclude that the shaping phenomenon is a result of a digestive ripening of rough negatively curved faces, assisted by water. Conversion to the magnetic phase was achieved by oxidation and the original shape of the NPs was retained. The NPs formed have sharp (pointed) corners that may serve as privileged site of additional growth that may lead to complex dendritic systems. The concave faces on the NTCs may serve as sites for smaller NPs or molecules to be adsorb. Magnetic studies, functionalization of the NPs and studies on applications such as water remediation are underway.
4. Experimental
Figure 10. BF TEM images and the corresponding XRD patterns for FeOX NTCs a) before (FexO) and b) after oxidation (Fe3O4), and for FeMnOX NPs c) before (Fe0.8Mn0.2O) and d) after oxidation (Fe2. 4Mn0.6O4).
MnO NPs the shape of the NPs was retained upon partial oxidation, although NPs did appear to have rougher surfaces after the oxidation. The fact that wu¨stite retains the shape better than manganese oxide NPs after oxidation, even though they have similar crystal lattices, might be because of the inherent iron deficiency of wu¨stite (FexO). This metal deficiency may provide a facile pathway for rearrangement of the lattice without causing too much strain on the unit cell and the NP itself.[35] In fact, rearrangement from wu¨stite to magnetite, for example, may be facilitated because both are in cubic space groups and the unit cell of the latter is almost double that of the former (vide supra). The same principles explained here apply to the mixed-metal phase.
3. Conclusions In summary, we report new NP shapes of FexO–Fe3O4 (wu¨stite–magnetite) and Fe1–yMnyO–Fe3–zMnzO4. The NPs were confirmed to be single crystals using TEM SAED. By varying the reaction parameters, the shape of the NPs obtained can be controlled. The shape and phase of the NPs obtained suggest the formation process is kinetically driven. The NP production method, which is based on the decomposition of a metal carboxylate in a mixture of TOA and OA, is a reliable
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Materials: Tri-n-octylamine (TOA) 98%, Oleic acid (OA) 90%, and manganese (II) formate dihydrate were obtained from the Aldrich Chemical Company, formic acid (88%), HPLC-grade water, and hydrogen peroxide (30%) were obtained from Fisher Scientific. Metallic iron was obtained from the Mallinckrodt Company. Organic [36] solvents were distilled following standard methods. Iron(II) formate dihydrate (FeII(HCOO)2 2H2O) was prepared by modifying a literature procedure to prepare iron (III) formate (i.e., by adding less [37] hydrogen peroxide). Product was dried under reduced pressure and resulted in a fine beige powder of FeII(HCOO)2 2H2O. Syntheses: Precursor decomposition was carried out under an inert atmosphere using standard Schlenk techniques. A general decomposition method is as follows: In a 100 mL three-neck round-bottomed flask fitted with a reflux condenser, the corresponding metal formate (3 mmol; 0.5 g total) was mixed with different volume ratios of a TOA:OA solution (8 mL total) under an inert atmosphere (Supporting Information, SI). The mixture was evacuated and purged with argon three times while stirring rapidly. The reaction mixture was stirred vigorously while the temperature was rapidly ramped (60 8C min1) to approximately 370 8C. Reflux was held until decomposition occurred, which is marked by a sudden change in color from light beige to dark brown. After decomposition the reaction was refluxed further for various periods of time (see SI). The heating mantle was then removed and the suspension was left to cool to around 100 8C (while stirring), upon which the mixture was carefully quenched with a 1 mL aliquot of dry ethanol. When time dependency was investigated, a 1 mL aliquot was removed with a syringe from the reaction flask, and quenched in dry ethanol. The product was then centrifuged, decanted, and re-dispersed in dry hexane (10 mL) by sonicating for ca. 10 min. All products were washed, sonicated, centrifuged, and decanted three times with dry hexane. To obtain a dry powder, each product was dried under reduced pressure after thorough washing with hexane. For water-free experiments the metal formates and the surfactants were dried at 110 8C under reduced pressure (102 torr) for around 4 h and were kept under an inert atmosphere until used. In some reactions the water effect was investigated by adding HPLC-grade water (100–200 mL) to the reaction mixture before heating (SI). The amount of added water was comparable to the amount of water present in the hydrated metal salts. Products were oxidized by heating in hexadecane (in air) for approximately 36 hrs, while stirring vigorously. Oxidation products were then washed and dried as described above. The decomposition method described above omits specifics (i.e., amount) on: 1. surfactant volume, 2. reaction times, 3. water added. These parameters are specified in the discussion, as they function to tune the
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NPs morphology. Tables with some conditions and TEM images for each of the products are provided in Supporting Information. Characterization: Raman spectra were collected on a Renishaw inVia Imaging Microscope, using a 633 nm He-Ne laser with 1200 mm1 grating. The laser beam was focused with a spot size of
