Synthesis of superparamagnetic δ-FeOOH nanoparticles by a chemical method

Applied Surface Science 387 (2016) 996–1001

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Synthesis of superparamagnetic ␦-FeOOH nanoparticles by a chemical method Naoki Nishida a,∗ , Shota Amagasa a , Yoshio Kobayashi b,c , Yasuhiro Yamada a a b c

Department of Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Department of Engineering Science, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan Nishina Center for Accelerator-Based Science, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

a r t i c l e

i n f o

Article history: Received 10 March 2016 Received in revised form 28 June 2016 Accepted 28 June 2016 Available online 7 July 2016 Keywords: ␦-FeOOH nanoparticle Mössbauer study Superparamagnetic behavior Chemical method Gelatin Oxidation

a b s t r a c t ␦-FeOOH nanoparticles were synthesized via the oxidation of precipitates obtained from the reaction of FeCl2 and N2 H4 in the presence of sodium tartrate and gelatin in an alkaline condition. These ␦-FeOOH particles were subsequently examined using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), Mössbauer spectroscopy, and superconducting quantum interference device (SQUID) assessment. The average size of the ␦-FeOOH nanoparticles was below 10 nm, and these particles exhibited superparamagnetic behavior as a result of this small size. The precursors of the ␦-FeOOH nanoparticles were also characterized as a means of elucidating the reaction mechanism. Precipitates prior to oxidation upon rinsing with water and ethanol were analyzed by obtaining XRD patterns and Mössbauer spectra of wet and frozen samples, respectively. The precipitates obtained by the reaction of FeCl2 and N2 H4 were found to consist of a mixture of Fe3 O4 and Fe(OH)2 , and it is believed that these species then rapidly oxidized into ␦-FeOOH nanoparticles. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Nanoparticles of iron compounds are of significant interest because they have unique physical and chemical properties derived from size effects, including unusual crystal structures and magnetic behaviors [1–3]. These nanoparticles have potential applications in a wide variety of areas, such as nanoelectronics and biomedicinerelated fields involving drug delivery, hyperthermia, magnetic resonance imaging (MRI), and biosensing. Iron(III) oxide-hydroxide (FeOOH) has four crystal structures: ␣FeOOH (goethite), ␤-FeOOH (akaganeite), ␥-FeOOH (lepidocrocite), and ␦-FeOOH (feroxyhyte). The ␦-FeOOH form, however, is the only ferric oxyhydroxide to exhibit significant magnetization at room temperature [4]. Nanosized ␦-FeOOH is also intriguing because it shows unique magnetic properties based on size or shape effects [5–7]. Nanoparticles of ␦-FeOOH are typically prepared by the rapid oxidation of Fe(OH)2 with H2 O2 , and are formed as large plates or needles in the absence of a surface protective reagent. Goodilin et al. prepared anisotropic ␦-FeOOH nanoflakes that were 2–5 nm thick and had planar sizes of 20 × 20 nm using humic substances. In addition, ␦-FeOOH nanoparticles have been applied as catalysts

∗ Corresponding author. E-mail address: [email protected] (N. Nishida). http://dx.doi.org/10.1016/j.apsusc.2016.06.179 0169-4332/© 2016 Elsevier B.V. All rights reserved.

[8,9] and as an adsorbent for As(V) [10]. Because smaller particles are expected to act as more efficient catalysts or adsorbents, due to their higher specific surface areas, it is desirable to obtain especially small nanoparticles for such applications. Although various methods have been reported for the synthesis of nanoparticles of iron compounds, these methods suffer from disadvantages such as the requirement for high temperatures, the use of toxic organic solvents, or the need to employ inert gases. Research has also shown that polypeptides, in particular a native polypeptide gelatin, can serve as efficient stabilizing reagents for various nanoparticles [11,12]. In the present study, we examined the preparation of especially small spherical ␦-FeOOH nanoparticles (below 10 nm in diameter) stabilized with gelatin under an ambient atmosphere at room temperature. The structures and magnetic properties of these products were analyzed by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), superconducting quantum interference device (SQUID), and Mössbauer spectroscopy. Based on the results of these analyses, the precursors and the formation mechanism of these nanoparticles are discussed.

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Fig. 1. Photographic images of (a) the reaction solution immediately after preparation, and (b) the solution after rinsing with ethanol.

Fig. 2. (a) X-ray diffraction pattern, (b) TEM image, (c) histogram of the diameters and (d) HR-TEM image of the ␦-FeOOH nanoparticles.

2. Experimental Particles were prepared by a modified hydrazine reduction system under ambient atmosphere at room temperature [12]. In this synthesis, 2 g of FeCl2 ·2H2 O (iron(II) chloride), 2.3 g of Na2 C4 H4 O6 ·2H2 O (sodium tartrate) and 400 mg of gelatin were dis-

solved in 50 mL of water. The pH of this solution was subsequently adjusted by the addition of 3 g of NaOH (sodium hydroxide). The pH value changed from 4 to 11. The solution was left to stand for 10 min in order to dissolve the reagents completely, after which 15 mL of a 10 M aqueous N2 H4 ·H2 O (hydrazine) solution was added slowly dropwise while ultrasonicating (100 W, 42 kHz) the mix-

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ture. This irradiation was continued for a further 1.5 h to complete the reaction. The resulting black precipitate was collected by filtration through a membrane (Millipore, Omnipore 0.45 ␮m) and then rinsed twice with water (5 mL) and ethanol (5 mL). The surface of the precipitate turned reddish-brown within a few seconds of initiating the rinse process. As a final step, the precipitate was dried in a vacuum desiccator. The product samples were investigated using TEM (JEOL, JEM-2100, operated at 200 kV), XRD (Rigaku, RINT2500, operated at 40 kV and 100 mA), and Mössbauer spectroscopy (57 Co/Rh source). The magnetic hysteresis loops of the samples were obtained using a SQUID (Quantum Design MPMS-XL). The Zero-field-cooling (ZFC) magnetization measurements were performed using a SQUID from 5 to 300 K at 100 Oe. The mass of ␦FeOOH in various samples exclusive of the gelatin was determined by dry ashing. In these measurements, the sample was placed in a crucible and heated to decompose and evaporate all organic components, after which the mass of the residual Fe2 O3 was obtained.

Fig. 3. The X-ray diffraction patterns obtained from ␦-FeOOH nanoparticles (a) immediately after preparation, (b) after storage in a vacuum desiccator for one month and (c) after storage in the laboratory under air for two months.

3. Results and discussion The precipitate prior to rinsing with ethanol, and containing small amounts of the original aqueous solvent, was black (Fig. 1a). Following rinsing with water and ethanol, however, the color rapidly changed to a reddish-brown (Fig. 1b), demonstrating a chemical reaction. In addition, as clearly seen in these figures, the application of a magnetic field caused both samples to acquire a net magnetic moment such that they were attracted to the magnet. Thus, both samples exhibited magnetic characteristics. In other words, both samples were not antiferromagnetic materials. As shown in Fig. 2a, the XRD pattern of the reddish-brown precipitate exhibits characteristic peaks of ␦-FeOOH corresponding to the (100), (002), (102), and (110) planes (JCPDS Card No. 13–87), and these broad peaks indicate the presence of small particles. Using the Scherrer equation, the particle size was estimated to be 7 nm based on the width of the (002) peak. A TEM image showing the morphological features of the ␦-FeOOH sample is presented in Fig. 2b, in which spherical nanoparticles of less than 10 nm in diameter are observed, in agreement with the particle sizes estimated from the XRD patterns. Fig. 2c indicates a lattice fringe, ascribed to the d spacing of the ␦FeOOH [7]. In general, ␦-FeOOH particles produced by conventional methods have ellipsoidal or needle-like shapes, because ␦-FeOOH tends to grow in a specific crystal plane. In contrast, the nanoparticles synthesized in this study were spherical, since the gelatin stabilized the crystal surface and restricted the growth process. In the absence of hydrazine, the large ␦-FeOOH crystals were observed (Fig. S1). Furthermore, XRD pattern of the sample produced in the absence of sodium tartrate exhibited not only ␦-FeOOH but also Fe3 O4 and ␣-FeOOH (Fig. S2). The stability of these particles was subsequently examined. A sample kept in a vacuum desiccator for a period of one month did not exhibit any changes in its XRD pattern. In contrast, a sample stored under ambient air for two months generated increasingly sharp XRD peaks, indicating the growth of the ␦-FeOOH crystals (Fig. 3). The ␦-FeOOH nanoparticles protected with gelatin were evidently stable at room temperature, but it appears that atmospheric moisture may lead to further crystal growth. Mössbauer spectra of the ␦-FeOOH nanoparticles were acquired at 293 and 9 K (Fig. 4), with the parameters summarized in Table 1. The spectrum acquired at 293 K could be fitted using a combination of a doublet, a relaxation component and a sextet. The sextet and the relaxation component were fitted assuming that they had the same Mössbauer parameters (ı, EQ , and H). The relaxation spectrum was fitted using Wickman’s formula described in the literature [13] assuming it had a single relaxation time. As bulk

Fig. 4. Mössbauer spectra of ␦-FeOOH nanoparticles obtained at various temperatures.

␦-FeOOH is ferromagnetic [4], the doublet (ı = 0.36 mm/s) could result from the superparamagnetic behavior of small nanoparticles or from the surface portions of nanoparticles influenced by thermal lattice vibrations. As noted, a broad absorption assigned to a relaxation component (relaxation time ␶ = 1.3 ns) was observed. This relaxation component was attributed to the nanoparticle cores, which were impervious to the effects of thermal fluctuations. In addition, a sextet with large  was observed which attributed to larger ␦-FeOOH particles, and the Mössbauer parameters were in agrees with the previous report [5]. The sextet had broad absorptions because of the size distribution, but the yields of the larger particles were very small (11%).

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Fig. 5. (a) Magnetic hysteresis plots obtained from ␦-FeOOH nanoparticles. (b) ZFC magnetization (H = 100 Oe) for the ␦-FeOOH nanoparticles.

Fig. 6. (a) TEM image, (b) magnetic hysteresis diagrams and (c) Mössbauer spectrum of the ␦-FeOOH nanoparticles stored in air in the laboratory. Table 1 Mössbauer parameters of ␦-FeOOH nanoparticles. ı /mms−1

EQ (2␧) /mms−1

␦-FeOOH (i) ␦-FeOOH (ii) ␦-FeOOH (iii)

0.36 (0) 0.38 (2) 0.38 (2)

␦-FeOOH (i) ␦-FeOOH (ii)

0.48 (0) 0.49 (0)

Component 293 K

9K a

H /kOe

 /mms−1

Yields /%

0.71 (1) −0.07 (4) −0.07 (4)

374 (2)a 374 (2)

0.58 (1) 1.31 (12) 1.22 (14)

42 47 11

−0.01 (1) 0.08 (1)

487 (1) 519 (0)

0.57 (2) 0.56 (2)

44 56

Relaxation time ␶ = 1.3 ns.

The Mössbauer spectrum at 9 K of the same sample shows two sets of sextets while the doublet is absent because the superparamagnetism of the specimen is no longer present at this low temperature. The spectral component having a smaller hyperfine magnetic field (i) corresponds to the surface atoms of the particles, in which the exchange interactions of the d orbital with adjacent Fe atoms are decreased. On the other hand, the spectral component with the larger hyperfine magnetic field (ii) corresponds to the core Fe atoms of the particles. The results are in agreement with the previous work in the literature [5]. The hysteresis loops of the ␦-FeOOH nanoparticles obtained using the SQUID are shown in Fig. 5a. In our case, the mass of ␦-FeOOH in the particle was not clear because the particle was covered with gelatin. In these measurements, the particle mass was reduced to equal that of the ␦-FeOOH component excluding the gelatin, based on a 3% gelatin content as estimated from dry ashing tests. The coercivity and the remanence were found to be almost zero at 300 K (remanence of 0.5 emu g−1 and coercivity of 10 Oe), demonstrating the superparamagnetic behavior of the ␦-FeOOH nanoparticles. Furthermore, the magnetization value at 50,000 Oe was approximately 13.4 emu g−1 . This magnetization intensity is higher than that reported for bulk ␦-FeOOH [5,14], and almost equal

to the value reported for ␦-FeOOH nanoparticles by Pereira [6,10]. A hysteresis loop was acquired at 5 K, and gave a remanence of 5.6 emu g−1 and a coercivity of 1200 Oe, indicating ferromagnetic behavior at low temperature. These results are in good agreement with the earlier Mössbauer data. Fig. 5b shows the ZFC curves for the ␦-FeOOH nanoparticles. The ZFC curve had a broad cusp around 180 K indicative of a characteristic blocking temperature (TB) for superparamagnetic particles [15], and the Curie temperature of the sample was found to be far over 300 K. As noted, storage of a sample in ambient air resulted in growth of the ␦-FeOOH nanoparticles (Fig. 3c). These large ␦-FeOOH particles were assessed and Fig. 6 presents a TEM image, a hysteresis loop at 300 K and a Mössbauer spectrum at 293 K of the aged ␦FeOOH sample. The TEM image shows needle-like particles 200 nm in length. Magnetic hysteresis is observed in the hysteresis loop, with a remanence of 0.5 emu g−1 and a coercivity of 40 Oe. Furthermore, the Mössbauer spectrum of the larger particles was similar to that of the small nanoparticles (Fig. 4, top), although the intensity of the doublet component slightly decreased and the sextet component increased because of the reduced surface area (Table 2). These results indicate that the superparamagnetic contribution was reduced by the growth of the ␦-FeOOH nanoparticles.

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Table 2 Mössbauer parameters of large ␦-FeOOH particles. Component 293 K

a

␦-FeOOH (i) ␦-FeOOH (ii) ␦-FeOOH (iii)

ı /mms−1

EQ /mms−1

0.39 (0) 0.39 (3) 0.39 (3)

0.69 (1) 0.21 (7) 0.21 (7)

H /kOe

 /mms−1

Yields /%

407 (3)a 407 (3)

0.64 (3) 1.31 (70) 1.22 (20)

39 47 14

Relaxation time ␶ = 1.2 ns.

Table 3 Mössbauer parameters of nanoparticle samples prior to filtration. Component

ı /mms−1

EQ (2␧) /mms−1

H /kOe

 /mms−1

Yields /%

60 K

␦-FeOOH (i) ␦-FeOOH (ii) Fe3 O4 (i) Fe3 O4 (ii) Fe(OH)2

0.48 0.49 0.88 0.60 1.27 (1)

−0.01 0.08 −1.9 −0.2 2.95 (1)

488 519 470 510

0.99 (13) 0.99 (13) 0.99 (13) 0.99 (13) 0.48 (1)

17 11 7 15 50

9K

␦-FeOOH (i) ␦-FeOOH (ii) Fe3 O4 (i) Fe3 O4 (ii) Fe(OH)2

0.48 0.49 0.88 0.60 1.27 (5)

−0.01 0.08 −1.9 −0.2 2.39 (32)

488 519 470 510 205 (5)

0.63 (9) 0.63 (9) 0.63 (9) 0.63 (9)

14 10 9 15 52

Fig. 7. X-ray diffraction pattern of a nanoparticle sample prior to filtration.

In order to characterize the black precipitate obtained prior to the rinsing process, which was stable only in the presence of the original solvent, we acquired the XRD pattern of a sample of this black material while it still held a small amount of solvent (that is, before it was completely dry). As shown in Fig. 7, the XRD pattern of the black sample exhibits the characteristic peaks of ␦FeOOH and Fe3 O4 (JCPDS Card No. 19-629). This material was also readily attracted to a magnet (Fig. 1a), due to the presence of magnetic Fe3 O4 . Fe(OH)2 was expected to be present because of the highly alkaline conditions in the black sample. However, a Fe(OH)2 signal was not observed, presumably because the Fe(OH)2 was amorphous. To further characterize the sample before rinsing with ethanol, we acquired the Mössbauer spectrum of a frozen sample of the black material (Fig. 8). In preparation for this measurement, a quantity of the black precipitate, still containing a small amount of the original solvent, was encased in a shielded vessel made of plastic and placed in a cryostat. This was done because the Mössbauer spectrum could not be obtained at room temperature as a result of the overly small recoil-free fractions, so the spectrum had to be acquired at low temperature. The associated Mössbauer parameters are summarized in Table 3. The spectrum acquired at 60 K could be fitted by a combination of a doublet and four sets of sextets. The doublet was assigned to Fe(OH)2 and the sextets were assigned to ␦-FeOOH and Fe3 O4 . The Mössbauer spectrum at 9 K shows four peaks in addition to the sextets from ␦-FeOOH and Fe3 O4 . These

Fig. 8. Mössbauer spectra of a nanoparticle sample prior to filtration at different temperatures.

results are in good agreement with previous reports that the Mössbauer spectrum of Fe(OH)2 at 4.2 K has four peaks due to the overlap of transitions [16]. In general, the Mössbauer spectrum of Fe3 O4 has multiple components below the Verwey transition, and the spectrum becomes complex due to the overlap of absorptions [17]. The corresponding absorptions in the Mössbauer spectra were too weak to be resolved into each component. Therefore, we fitted ␦-FeOOH and Fe3 O4 fixing the Mössbauer parameters (ı, EQ , and H); the parameters of ␦-FeOOH were fixed at the parameters shown in Table 1, and the parameters of two sets of sextets were fixed at the

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values reported in the literature [17]. The fitting of the Mössbauer spectra was performed assuming that the half-width of the peaks had the same value. It was demonstrated that the magnetic components in the Mössbauer spectra might be attributed to the mixture of ␦-FeOOH and Fe3 O4 . Typically, ␦-FeOOH is formed through the rapid, direct oxidation of Fe(OH)2 [4–6]. However, in our case, Fe3 O4 nanoparticles were derived from the black precipitate after adding N2 H4 in solution. Although the precipitate was in a reducing environment, 42% of the total Fe atoms were oxidized to Fe3+ based on estimations from the Mössbauer data. Contact with atmospheric oxygen while handling the material could have oxidized 42% of the Fe2+ to Fe3+ , while the remainder of the Fe2+ (58%) was unoxidized due to the presence of the N2 H4 and the gelatin. The subsequent rinsing process oxidized both the Fe3 O4 and Fe(OH)2 to ␦-FeOOH nanoparticles. In general, Fe3 O4 are stable in air or oxidize slightly only to a mixture of ␥-Fe2 O3 /nonstoichiometric Fe3 O4 . Meanwhile, Reˇcnik et al. demonstrated that a few atomic layers of ␦-FeOOH are formed on the ␥-Fe2 O3 octahedral surfaces [18]. In our case, it is expected the synthesized nanoparticle have high reactivity due to the small size. Therefore, Fe3 O4 then rapidly oxidized into ␦-FeOOH nanoparticles. 4. Conclusions ␦-FeOOH nanoparticles were successfully synthesized via a room temperature chemical reaction of FeCl2 in the presence of gelatin. The resulting nanoparticles were spherical and less than 10 nm in diameter. Mössbauer spectra and hysteresis loops of the ␦-FeOOH nanoparticles showed superparamagnetic behavior due to the small particle sizes. These particles were found to have increased magnetization intensity relative to that of bulk ␦-FeOOH. The precursors of the ␦-FeOOH nanoparticles were characterized by obtaining the XRD pattern of a wet sample and the Mössbauer spectrum of a frozen sample prior to rinsing with water and ethanol. The precipitate obtained by adding N2 H4 to the FeCl2 solution was determined to be a mixture of Fe3 O4 and Fe(OH)2 . Subsequently, the Fe3 O4 and Fe(OH)2 were oxidized to ␦-FeOOH nanoparticles by rinsing under the ambient atmosphere. While the spherical ␦FeOOH nanoparticles protected by gelatin were stable when kept in a vacuum, the nanoparticles formed needle-like particles when aged in the ambient atmosphere. Acknowledgment The authors wish to thank Dr. M. Enomoto (Tokyo University of Science) for technical support in connection with the SQUID measurements.

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