Synthesis and Film Formation of Magnetic FeCo Nanoparticles with

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Synthesis and Film Formation of Magnetic FeCo Nanoparticles with Graphitic Carbon Shells Mami Yamada,*,†,‡ Shin-ji Okumura,† and Kohta Takahashi† †

Department of Applied Chemistry, Tokyo University of Agriculture and Technology (TAT), 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan, and ‡Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency (PRESTO-JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan

ABSTRACT FeCo nanoparticles (NPs) are promising soft magnetic nanomaterials for use in many applications; however, the present problem is how to prevent oxidation. The most powerful substance for protection of FeCo NPs is graphitic carbon (GC). Here, we present a novel and simple technique for the large-scale synthesis of GC-coated FeCo NPs with varied Fe/Co ratios based on the reductive thermal conversion of nanometric Prussian blue analogues (PBAs). The key point is that surfaces of PBA NP precursors are coordinately stabilized by an organic layer of oleylamine, which serves as a carbon source for the GC shells of FeCo NPs during the thermolysis of PBA NPs. We have also succeeded in the facile formation of a stable FeCo NP film with GC shells that displays magneto-optical properties. The technique could be adapted to other transition metals and would be used in versatile fields such as spintronics and biotechnology. SECTION Nanoparticles and Nanostructures

eCo nanoparticles (NPs) are the most promising soft magnetic nanomaterial with a high saturation magnetization Ms and a high Curie temperature Tc (>900 °C),1 expected for applications such as magnetic resonance imaging (MRI),2 perpendicular magnetic recording,3 and magnetic composite devices.4 The problem that urgently needs to be solved is how to prevent their oxidation. In order to improve their material stability, surface protection around the FeCo nanocore is essential. Among several methods for covering their surface, such as polymers,5 alkyl ligands,6 and metal oxides,7 graphitic carbon (GC) is the most suitable option from the standpoint of thermal stability, cost performance, and chemical inertness.2,8,9 However, the synthesis of FeCo NPs with GC shells generally requires large-scale instruments for arc plasma sputtering9 or chemical vapor deposition (CVD)2 or uses the precise synthesis involving toxic Fe(CO)5 and a unique Co complex.8 The development of a more simple and practical fabrication method has been a vital challenge. In this Letter, we present a novel technique for the synthesis of GC-coated FeCo NPs with varied Fe/Co ratios based on the thermal conversion of Prussian blue analogues (PBA NPs) (Scheme 1). PBAs with the general formula M1II1.5[M2III(CN)6] (defected crystal form, see Supporting Information, SI, Figure S1b), are one of the most widely known metal-organic frameworks which can be synthesized just by mixing starting materials in air. Here, we move forward in three important aspects. First, the precursor PBAs are already nanometersized in large quantities without the use of complicated techniques. Second, elemental control of the metals in the PBA NPs can be easily achieved by controlling the composition of the initial PBA. Third, the PBA NP surfaces are

coordinately stabilized with oleylamine (OA), which serves as a carbon source for the GC shells. For the first time, we have also succeeded in the facile fabrication of an FeCo NP film with GC shells that displays magneto-optical properties. We performed a large-scale synthesis of OA-stabilized PBA NPs of CozFe1.5-z[Fe(CN)6] [Fe/Co(z) = 1.0(1.25), 1.5(1), 2.0(0.83), 2.5(0.71)] using a so-called “etching reaction” by extracting PBA bulk crystals in water to the toluene organic phase containing OA (Figure 1, inset. For detailed experimental details and measurements, see SI). X-ray diffraction (XRD) patterns for the PBA NPs purified from the organic phase confirmed crystalline PBA with a face-centered cubic (fcc) structure (Figure S1a, SI).10 The PBA NP powders easily redispersed in less polar solvents such as toluene and chloroform to give a transparent solution (Figure 1b). This behavior suggests that during extraction to the organic phase, the NH2 group in OA coordinates with the Co ions on the surface of the bulk PBA,11 which are then etched into NPs. Transmission electron microscopy (TEM) revealed PBA NPs to have an average diameter of dav = ∼9-11 nm (Figure 2). UV-vis absorption intensities due to the intervalence transfer band of FeII-CN-FeIII (λmax = ∼730 nm)12 increase almost linearly with increasing Fe/Co ratio (Figure 1a). Energy dispersive X-ray (EDX) analysis confirmed that the Fe/Co ratios for the PBA NPs reflect ratios in the reaction mixtures for bulk PBAs (Table S1, SI). The conversion reaction of the PBA NPs was carried out under a N2/H2 atmosphere at 500 °C, which induced thermal

F

r 2010 American Chemical Society

Received Date: May 11, 2010 Accepted Date: June 14, 2010 Published on Web Date: June 16, 2010

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DOI: 10.1021/jz100604c |J. Phys. Chem. Lett. 2010, 1, 2042–2045

pubs.acs.org/JPCL

Figure 1. PBA NPs (0.02 g in 10 mL toluene): (a) UV-vis spectra; the inset shows photographs (left) before and (right) after extraction of PBA NPs to the toluene organic phase by oleylamine. (b) Photographs at various set Fe/Co ratios. The numbers in the figures refer to the Fe/Co ratio.

Figure 2. TEM images of PBA NPs at various set Fe/Co ratios and average diameters dav. The numbers in the figures refer to the Fe/Co ratio. Scale bar: 50 nm.

Scheme 1. Summary of the Chemical Reactions of This Method Table 1. Properties of the Prepared GC-Coated FeCo NP Fe/Coa

a [Å]b

dav-TEM [nm]c

dav-XRD [nm]d

Ms [emu g-1]e

1.0

2.854

17

18

172

1.5

2.852

16

17

170

2.0

2.849

13

14

158

2.5

2.845

12

11

146

a

Fe/Co ratio of the initial PBA NPs. b Lattice constant calculated from XRD. c Average core diameter determined from TEM images. d Average core diameter determined from XRD patterns and Scherrer's equation. e Saturation magnetization per FeCo determined from the field dependence of magnetization curves at 300 K.

removal of the CN bridging ligand (Figure S2, SI). XRD patterns for the heat-treated samples exhibited the main peaks due to the base-centered cubic (bcc) crystal phase of FeCo (Figure S3, SI; for example, JCPDS 44-1433 for FeCo). The calculated lattice dimension a decreased with increasing Fe/Co ratio of the initial PBA NPs (Table 1), indicating that a single-phase FeCo alloy was produced based on Vegard's law. We should mention that several broad peaks in the XRD patterns indicated the contamination of iron carbide (Fe3C: JCPDS 77-0255) produced during the calcination process. TEM images of the PBA NPs after thermal conversion revealed uniform spherical NPs (dav = 12-17 nm) (Figures 3a and S4, SI). EDX elemental nanomapping showed that both Fe and Co atoms are homogeneously distributed throughout the NPs (Figure 3a, bottom). The core composition was confirmed by the measured lattice spacing of dspacing = 2.0 Å, which corresponds to the distance between FeCo(110) planes (Figure 3b, inset). Note that the XPS spectra showed the peaks at 707.1 (Fe2p) and 779.4 eV (Co2p) after the thermal conversion reaction, which proves that the valence of Fe and


Figure 3. PBA NPs (Fe/Co=1.0) after thermal conversion: (a) (top) TEM, (bottom) EDX elemental nanomappings; (b) HRTEM images.

Co atoms in the calcined material is neutral (Figure S5, SI).13 TEM (Figure 3a, top, inset) and high-resolution TEM (HRTEM; Figure 3b) images showed a distinct core-shell nanostructure. FeCo cores were surrounded by layered shells with a dspacing of ∼3.3 Å, which can be assigned to the spacing of

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C(002) planes.14 Each particle appeared to be coated with approximately seven curved graphitic sheets. Raman spectroscopy revealed a graphitic carbon G peak at 1590 cm-1 and a disordered D peak at 1340 cm-1, providing evidence for the E2g mode in the basal plane of the graphite shells (Figure S6, SI).15 The mechanism of conversion from PBA NPs to GC-coated FeCo NPs is similar to the growth of multiwalled carbon nanotubes on magnetic NPs.16 The OA on PBA NPs is first decomposed into small carbon species, followed by the thermal removal of the CN-bridging ligands of the PBAs, generating metallic FeCo. As the FeCo gathers into NPs, there is a simultaneous dissolution of carbon into the FeCo NPs. When dissolution reaches saturation, precipitation and reconstruction of the carbon around the FeCo NPs results in the formation of protective graphitic shells.17 Superconducting quantum interference device (SQUID) measurements confirmed that GC-coated FeCo NPs exhibited excellent magnetic behavior (Figure S7, SI). Their saturation magnetization (Ms) was in the high range of 146-172 emu g-1 (Figure S7a (SI), Table 1) with small coercivity of 80-130 Oe (Figure S7c, SI). Their high magnetism thus enables fast correction from a suspension by an applied magnetic field (Figure S7b, SI), suggesting their effectiveness for use as magnetic beads. However, their Ms is lower than that for the bulk FeCo alloy (e.g., 250 emu g-1 for Fe/Co = 1)2 due to the contamination of nonmagnetic iron carbide (see above) and structural disorder on the particle surface. This surface contribution works effectively for smaller particles, which explains the decrease of Ms with increasing Fe/Co ratio. Note that the GC-coated FeCo NPs possessed high stability against oxidation in air without weight increase up to ∼150 °C (Figure S8, SI). Actually, the XRD pattern due to the oxidized metal species was not detected after a month. Finally, this method was extended to the easy fabrication of a magneto-optical film comprised of FeCo NPs with GC shells. A green spin-coated film of PBA NP (Fe/Co = 1.0) precursors on a glass substrate was converted to a transparent gray film via themolysis (Figure S9, SI). The XRD pattern for the resulting film confirmed the presence of a FeCo phase with a bcc crystal structure (Figure S10, SI). Cross-sectional scanning electron microscopy (SEM) images showed that the FeCo cores with ∼10 nm were assembled in 3D with a film thickness of ∼150 nm (Figure 4a). Atomic force microscopy (AFM) was used to probe the film surface, revealing particle diameters dav of ∼20-25 nm, which almost agrees with the total diameter of the FeCo core and GC shells (Figure 4b). The Faraday rotation (FR) spectrum of the film showed magnetooptical dispersion with both a negative and positive peak at 540 and 670 nm, respectively (Figure 4c); this is characteristic of the FeCo alloy.18 The hysteresis curve measured at 670 nm showed FR angles of -5.1 mdeg (Figure 4c, inset). In summary, we introduced a novel method for the largescale synthesis of GC-coated FeCo NPs using a thermal conversion reaction of OA-stabilized PBA NPs. The surface ligand of OA served as a carbon source, and carbon reconstruction was catalyzed by FeCo during thermolysis. The resulting GC-coated FeCo NPs had excellent magnetic properties and high stability against oxidation. We developed this system for the facile formation of a stable magneto-optical


Figure 4. (a) Cross-sectional SEM image, (b) AFM image, and (c) the Faraday rotation spectrum of the FeCo NPs (Fe/Co = 1.0) with GC shells measured at 300 K (magnetic field = 1 T). (Inset) Hysteresis loop at 670 nm.

film of FeCo NPs with GC shells. The technique could be adapted to other transition metals and would be used in versatile fields such as spintronics to manufacture magnetic/ magneto-optical devices via chemical solution processing and biochemistry to functionalize the GC shells.

EXPERIMENTAL SECTION Preparation of PBA NPs. Ten milliliters of x M Fe(NO3)2 3 6H2O and (0.28 - x) M Co(NO3)2 3 6H2O (x = 0.07, 0.11, 0.14, 0.16) was added to 8 mL of 0.14 M (aq) K3[Fe(CN)6], and the mixture was vigorously stirred for 10 min. The precipitate was centrifuged, washed with water three times and with methanol once, and then dried in vacuum to give a bulk PBA of CozFe1.5-z[Fe(CN)6] (Fe/Co = 1.0, 1.5, 2.0, 2.5). Ten grams of bulk PBA dispersed in 20 mL of H2O was added to 150 mL of 0.14 M oleylamine, and the mixture was stirred for 1 h at room temperature. The organic phase was separated and dried over anhydrous Na2SO4. The solution was filtered through Celite to give a transparent toluene solution of PBA NPs with various Fe/Co ratios. PBA NPs in powder form were subsequently precipitated after the addition of sufficient methanol and were washed with methanol two times. The obtained powdery PBA NPs were dried in vacuum. Film Formation of PBA NPs. The toluene solution of PBA NPs (Fe/Co = 1) prepared as noted above was concentrated to ∼1 mL by solvent evaporation. Then, a thin film of PBA NPs was fabricated by a general spin-coating method using 100 μL of the condensed solution dropped onto a glass slide (2 2 cm2) with a substrate rotating program of 500 rpm 50 s þ 7000 rpm 10 s using a Mikasa IH-DX2. The obtained film was dried in vacuum. Thermolysis of PBA NPs. One gram of powdered PBA NPs was mounted in a ceramic boat, and the spin-coated film of PBA NPs was set in a furnace with a Technosystem AF2120403-HSP under a H2 atmosphere (N2/H2 = 10; the total gas flow rate was 330 mL/min), and the temperature was raised to 500 °C at a rate of 10 °C/min. The furnace was cooled down naturally to room temperature.

SUPPORTING INFORMATION AVAILABLE Experimental details and additional results for EDX, TGA, and XRD for PBA NPs

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and XRD, XPS, TEM, Raman spectroscopy, SQUID, and TGA for the FeCo NPs, as well as micrographs of PBA NPs before and after themolysis, XRD of the FeCo NP films, and information on the collection of FeCo NPs from a DMF suspension with a magnet. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author: *To whom correspondence should be addressed. E-mail: m-yamada@ cc.tuat.ac.jp. Tel and Fax: þ81-42-388-7379.

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ACKNOWLEDGMENT We thank Prof. Dr. T. Iyoda and Dr. K. Ito at

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the Tokyo Institute of Technology (TIT) for TEM and SQUID measurement. We also thank Prof. Dr. J. Onoe at TIT for XRD of the formed film. We also thank Prof. Dr. M. Tanaka at the University of Tokyo for the FR spectrum. We are very grateful to JEOL Ltd., Hitachi High-Technologies Co., and JASCO Co. for the technical support in HRTEM, SEM, EDX elemental nanomapping, and Raman spectroscopy. This work was financially supported by PRESTO-JST.

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REFERENCES

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Synthesis and Film Formation of Magnetic FeCo Nanoparticles with

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