Article pubs.acs.org/crystal
Nonpolar Solvothermal Fabrication and Electromagnetic Properties of Magnetic Fe3O4 Encapsulated Semimetal Bi Nanocomposites Jiaheng Wang, Han Wang, Jingjing Jiang, Wenjie Gong, Da Li, Qiang Zhang, Xinguo Zhao, Song Ma, and Zhidong Zhang* Shenyang National Laboratory for Material Science, Institute of Metal Research, and International Centre for Material Physics, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China
ABSTRACT: Magnetic Fe3O4 nanoparticles (NPs) encapsulated semimetal Bi nanocomposites were fabricated by a solvothermal method based on the nonpolar 1-octadecene. Bi and Fe ions were introduced into the oil phase by the complex with sodium oleate in the as-prepared solution. The nanocomposites were oil soluble and dispersed well in any nonpolar solution with the surface modification of oleic acid. It is argued that hydroxyl groups as the reductant induce the formation of Bi microspheres and prevent Fe3O4 NPs from oxidation. The frequency dependence of complex permittivity and permeability indicates that Bi and interface polarization of Bi/Fe3O4 nanocomposites are responsible for the dielectric resonances, and superparamagnetic Fe3O4 NPs weaken the natural resonance.
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INTRODUCTION Functional nanocomposites have attracted much attention for decades. Because of various properties of multiple materials and the interaction between them, hybrid composites exhibit many excellent performances and have extensive applications in solar cell, light emitting diodes, photocatalysis, microwave absorption, biomedical applications, etc.1−5 To obtain versatile and enhanced effects of devices, many kinds of combinations are adopted in different scales, such as metal/insulator, metal/ semiconductor, semiconductor/semiconductor, magnetic/dielectric, inorganic/organic, and so on.6−10 In our previous reports,4,11−13 several designed nanocomposites were fabricated, which generated some interesting appearances. In this paper, a novel assemblage was obtained with semimetal bismuth (Bi) microspheres encapsulated in a matrix formed by magnetic Fe3O4 nanoparticles (NPs). As well-known, Fe3O4 is a ferromagnetic oxide, while Bi has diamagnetism. However, as a typical semimetal, Bi has unusual electronic properties that result from its highly anisotropic Fermi surface, low carrier concentrations, and small effective carrier masses.14 The nanocomposites with Fe3O4 and Bi may have potential applications in the relative electromagnetic area. So, investigation on the electromagnetic properties was performed for the Fe3O4/Bi nanocomposites. © 2012 American Chemical Society
Encapsulated nanocomposites can be fabricated by arcdischarge, chemical vapor deposition, pyrolysis, hydrothermal, sol−gel techniques, etc.4,15−18 Structures grown in the liquid circumstance have advantages in repeatability, morphology control and dispersity. So, 1-octadecene as kind of oil phase was chosen as the solvent for the growth of the semimetal/ferrite nanocomposites by a solvothermal route here. As complex with oleate, water-soluble polar metal ions were transferred into the nonpolar oil solvent. It is suggested that the nanocomposites can be well modified with nonpolar groups and well dispersed in any nonpolar solvent, which is better for applications with a homogeneous coating status on some surfaces.
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EXPERIMENTS
Materials. Bismuth nitrate (Bi(NO3)3·5H2O), ferric chloride (FeCl3·6H2O), sodium oleate, oleic acid, n-hexane, glycerin, and anhydrous ethanol were purchased from Shanghai Sinopharm Chemical Reagent Co. Ltd. 1-Octadecene (ODE) was obtained from Aladdin Reagent Co. Ltd. All the reactants were of analytical reagent grade and used as received without any further purification. All of the reactions were operated under high-purity nitrogen (N2) protection. Received: February 9, 2012 Revised: May 8, 2012 Published: May 23, 2012 3499
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Preparation of Bismuth−Iron−Oleate Complex. In a typical synthesis, 10 mmol FeCl3 and 30 mmol sodium oleate were dissolved in a solution composed by 20 mL of ethanol, 30 mL of H2O, and 50 mL of n-hexane. The mixture was continuously stirred at 60 °C for 4 h with reflux, until the upper organic layer turned from transparent to brown. Then, the Fe-oleate complex dissolved in the upper n-hexane was washed and collected by a separating funnel. Bi-oleate was obtained similarly as the process above. First, Bi(NO3)3 was added into 50 mL of glycerin and slowly stirred at 60 °C for 1 h, until the powder dissolved and all the reddish brown NOx gas released completely. Then, 40 mL of ODE and sodium oleate were added. The molar ratio of [Bi3+]/[oleate] is 1:3. The mixture was heated at 100 °C under stirring, and kept at that temperature for 4 h. The light yellow upper layer was the Bi-oleate ODE solution. The process of washing and separating are still necessary here. The Fe-oleate and Bi-oleate of different ratios were redissolved into ODE together, and kept stirring at 100 °C for another 30 min. Finally, the as-prepared solution was obtained with Bi−Fe−oleate complex in ODE. In this paper, the nanocomposites were fabricated with the [Bi]:[Fe] ions molar ratio of 0:1, 0.3:1, 0.5:1, 0.7:1, and 1:1, which were separately named BF1, BF2, BF3, BF4, and BF5 for short. Preparation of Bi/Fe3O4 Nanocomposites. The oleic acid of 2 mL was added into the as-prepared solution and mixed up enough. Then, the solution was transferred into a 100 mL stainless steel autoclave with Teflon lining and heated in an oven at 200 °C for 8 h. After this, the nanocomposite powder was precipitated and washed by absolute ethanol and distilled water for three times each. Finally, the powder was collected and dried in an oven at 40 °C for 48 h. Characterization. The structures were investigated by X-ray diffraction (XRD) θ-2θ scan on Rigaku D/Max-2400 diffractometer with Cu Kα radiation (λ = 1.54 Å). The composition of ferrite was confirmed by X-ray photoelectron spectra (XPS) on Thermo VG ESCALAB 250 using Al Kα (1486.6 eV) as the excitation source. The morphology and components were characterized by the field-emission scanning electron microscopy (FESEM) on FEI Inspect F50 operated at 30 kV, and equipped with energy dispersive X-ray analyzer spectrum (EDX). Transmission electron microscopy (TEM), high resolution TEM (HRTEM) and selected area electron diffraction (SAED) was taken on FEI Tecnai G2 F20 microscope at 200 kV. The magnetization measurements were performed using a superconducting quantum interference device (SQUID) magnetometer on Quantum Design MPMS-7. The complex relative permittivity and permeability of the nanocomposites from 1 to 18 GHz were obtained on Agilent 8722ES network analyzer. The nanocomposites were uniformly sealed in the paraffin wax with the weight fraction of 30%.
Figure 1. (a) X-ray diffraction θ−2θ scans of the composites fabricated with [Bi]/[Fe] molar ratio of 0:1(BF1), 0.3:1(BF2), 0.5:1(BF3), 0.7:1(BF4), 1:1(BF5). The attached JCPDS cards numbered 44-1246, 65-3107, and 33-0664 are the standard diffractions of Bi, Fe3O4, and αFe2O3, respectively. (b) the Fe2p XPS scan of the composite BF3.
performed, to confirm the ferrite composition, as shown in Figure 1b. In our case, the peaks of Fe 2p3/2 and 2p1/2 at 712.8 and 726.6 eV respectively indicate that the binding energy is higher than that of γ-Fe2O3.20 No obvious satellite peak shows around 719 eV.21 It suggests that the ferrite here in the composites BF2−5 is Fe3O4. Figure 2a shows the SEM image of the composite BF3 with the [Bi]/[Fe] molar ratio of 0.5:1. It is inferred that many spheres of hundreds nanometers are embedded in the matrix. Most of the spheres are dispersed well with less aggregation. However, the size of the spheres is not unified. As the magnified image of the white box in Figure 2a, it seems that several tiny spheres distribute around the bigger one, as shown in Figure 2b. It indicates the uninterrupted crystallization and combination from the metal-oleate complex during the solvothermal process. The EDX analysis is taken on one of the spheres, as shown in Figure 2c. Excluding the peaks of Si substrate for the sample support and Au for the electrons transport, stronger peaks of Bi and weak ones of Fe suggest that the powder has a structure with Bi microspheres embedded in the Fe3O4 matrix. Besides, no obvious morphology can be seen in the Fe3O4 matrix, which infers the small particle size of Fe3O4. This is coincident with the XRD results in details. Figure 3a presents a typical TEM image of the composite BF1. It can be seen that the powder is composed by many monodisperse NPs with the size between 5 and 20 nm. Most of the Fe3O4 NPs are shown in the shape of cubes, and others are hexagonal columns. The well monodisperse status of NPs should be attributed to the electrostatic repulsion of oleic acid complex on its surface. A Bi sphere with the diameter of 1 μm
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RESULTS AND DISCUSSIONS The XRD θ−2θ scans of the nanocomposites with [Bi]:[Fe] molar ratio from 0:1 to 1:1 are shown Figure 1a. The BF1 curve indicates that the sample without adding Bi component reveals a main phase of α-Fe2O3 (JCPDS 33-0664). However, the small peaks at 2θ = 30°, 43°, and 56°, and the asymmetry of the peaks at 2θ =35 and 62° infer the existence of Fe3O4 (JCPDS 65-3107). Besides, the averaged crystal size calculated by Scherrer equation19 is less than 20 nm according to the wide FWHM. As the proportion of bismuth salt increases in the asprepared solution, all the nanocomposites reveal the existence of mixture composed by Fe3O4 and Bi (JCPDS 44-1246) as shown in the curves from BF2 to BF4. Although its percentage is not dominated, the peaks’ intensity of Bi is much stronger than that of Fe3O4 phase. Even the representative peaks of Fe3O4 seem to sink into the background as compared with Bi. The calculated crystal sizes of Fe3O4 for BF2−4 samples are around 15 nm. On the contrary, the sharp and strong peaks of Bi suggest the bigger crystal size. Due to the same cubic inverse spinel structure and nearly identical lattice parameters of Fe3O4 and γ-Fe2O3, the Fe2p XPS scan of the composite BF3 is 3500
dx.doi.org/10.1021/cg300198r | Cryst. Growth Des. 2012, 12, 3499−3504
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Figure 2. FESEM images and the EDX analysis of the sample BF3. Panel b is the magnified image of the white box in a. Panel c is for the EDX patterns of the position on a sphere, as the head point of the arrow.
Figure 3. TEM images of the nanocomposites BF1 (a) and BF3 (b and c). The inset in b is the SAED pattern. (d) HRTEM image of the interface between Bi sphere and Fe3O4 NPs in BF3.
In previous reports,22 Fe3O4 NPs have been fabricated by pyrolysis method starting with Fe-oleate complex. In this paper, the heating process at ambient pressure has been replaced by a high pressure solvothermal route in a nonpolar phase. However, for the composite BF1, the product is a mixture of α-Fe2O3 and Fe3O4. This should be attributed to the oxygen sealed in the clave, which directly oxidizes a big part of Fe3O4 to α-Fe2O3 as the temperature arising. This reaction does not happen in the fabrication of other samples. It is believed that
in the composite BF3 is shown in Figure 3b. Obviously, many Fe3O4 NPs less than 20 nm wrap around the Bi spherical core. Figure 3c shows the detailed surface of the sphere. It seems that the tiny NPs adsorb on the surface of Bi microsphere and form a compact layer. However, it is suggested that some distance still exists between these particles. This adsorptive compact state of the interface between Bi sphere and Fe3O4 NPs can be seen clearly in the HRTEM image as shown in Figure 3d. 3501
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hydroxyl groups from glycerin perform an important role as a reductant in the formation of the Bi/Fe3O4 nanocomposites. While Bi(NO3)3 dissolved into glycerin at 60 °C, parts of nitrate radicals would collapse and bismuth subnitrate (4BiNO3(OH)2·BiO(OH)) formed. Then, as the complex between Bi ions and the carboxyl groups of sodium oleate, plenty of hydroxyl groups are introduced into the new polymers in the ODE, as shown in Figure 4. As the polar
Figure 4. Status sketch of the metal-oleate clusters in the ODE phase.
radicals in the nonpolar ODE solution, the most stable way for the hydroxyl groups is to penetrate themselves into the polymer clusters, while the hydrocarbon radicals stick out and wrap around the whole clusters. This status can also make the clusters well disperse in the oil. Furthermore, the metal ions and hydroxyl radicals become much closer for reduction. Since Bi is a kind of semimetal with weak metallicity, the reductant reaction can be easily induced to build Bi as the existence of hydroxyl, while the temperature and pressure are appropriate.23 So, the oxidation of Fe3O4 is prevented at the same time. Moreover, oleic acid can limit the crystal growth of Fe3O4, but it does not work for Bi obviously. So many residual oleic acid molecules enrich on the surface of NPs. As the nonpolar of hydrocarbon radicals, some of the long chain hydrocarbon radicals are inclined to absorb and overlap with each other, and form the encapsulated nanocomposites with Fe3O4 NPs layer on the Bi microspheres surface. Figure 5a shows the temperature dependence of magnetization curves for the nanocomposites BF3 in the cases of zerofield-cooled (ZFC) and field cooled (FC) with the field of 100 Oe. An obvious transition around 100 K in the ZFC curve should indicate the blocking temperature (TB) of superparamagnetic nature. It is coincident with the TEM images since the single domain size for Fe3O4 is less than 20 nm.24 Besides, the broad transition is believed to be the effect of the wide size distribution of these NPs. The M−H loops of the nanocomposites BF1-5 at 200 K are shown in Figure 5b. It reveals that all of the samples are superparamagnetic at 200 K. Since the antiferromagnetism of α-Fe2O3 at this temperature and the γ-Fe2O3 phase can be excluded in the nanocomposites BF2−5 according to the XPS analysis above, the signal of ferromagnetic Fe3O4 NPs should be predominant in the M−H
Figure 5. (a) ZFC−FC magnetization curves of the nanocomposites BF3 with the field of 100 Oe. M−H loops of the nanocomposites BF15 at 200 (a) and 5 K (b), respectively. The inset is the M−H loops at low field.
curve. However, the α-Fe2O3 phase in the nanocomposite BF1 can distort the M−H loop with adding a linear increase of magnetization and also a higher saturation magnetization value (as shown in the M−H loop of BF1). This does not happen in the loops of BF2-5 with Bi/Fe3O4 nanocomposites, due to the diamagnetism of Bi and the absence of γ-Fe2O3. The M−H loops of BF1-5 at 5 K show distinct hysteresis with coercivities from 270 to 590 Oe, which should be attributed to the frozen spin state at temperatures below TB. Because Bi has a very high molecular weight of 208.98, the relative proportion of Fe3O4 should be less. So it infers the similar level of magnetic components in all nanocomposites. As the Fe3O4 proportion dropping, the value of saturation magnetization decreases from BF2 to BF5. Figure 6a presents the complex permittivity (εr) of the paraffin samples from BF1 to BF5 as a function of frequency in the range between 1 and 18 GHz. As the increase of the 3502
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dielectric resonance of nanocomposites in the high frequency. The interface polarization between Bi spheres and superparamagnetic Fe3O4 NPs induced several broad resonances in the low frequency. As the increase of Bi content, these resonances are enhanced and shift toward low frequency. This process offers a novel way to fabricate encapsulated nanocomposites in nonpolar oil phase. Future investigation should be done on nanocomposites with different combinations, structures control and functional properties for application.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
Figure 6. (a) Real (ε′) and imaginary (ε″) parts of the complex relative permittivity (εr), (b) real (μ′) and imaginary (μ″) parts of the complex relative permeability (μr) of nanocomposites BF1-5 from 1 to 18 GHz, as marked in the legend.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Basic Research Program (No. 2012CB933103) of China, Ministry of Science and Technology China and National Natural Science Foundation of China with Grant number 50901078, 51171185.
frequency, the real parts of the permittivity decrease a little at the beginning, and then seem to be constant with less fluctuation. For the nanocomposite BF1 without any Bi component, a resonance peak appears obviously at 16.1 GHz. Similar resonance peaks gradually show up at 16 and 16.9 GHz in the curves of BF4 and BF5 respectively as the Bi content increases. Corresponding peaks can be found in the imaginary parts of the permittivity with the same frequency. It infers the typical nonlinear dielectric resonance.25 Due to the same proportion level of magnetic Fe3O4 content in every composite as analysis above, the distinctive resonance in BF1 should be attributed to the dielectric behavior of α-Fe2O3. Bi should also be treated as a dielectric material here and be responsible for the peaks in the BF4 and BF5 curves as the proportion lifted to a special level. It is noticed that in the imaginary parts of permittivity, the same trend appears for the composites BF1−3 in low frequency with a broad resonance around 5.4 GHz. For both BF2 and BF3, several resonances can be seen above 11 GHz. However, with the increase of Bi, resonances seem to shift toward low frequency and two peaks show up at 3.2 and 9.1 GHz respectively with higher permittivity value. It should correspond to the enhanced interfacial polarization on Fe3O4 NPs encapsulated Bi microspheres.26,27 Figure 6b is the frequency dependence of complex permeability (μr) from BF1-5. The trend of the patterns is almost the same for the real and imaginary parts of permeability in all the paraffinnanocomposite samples. The value decreases as the frequency lifts. Two resonances can be observed around 8.5 and 15 GHz, which is attributed to the exchange resonance of Fe3O4 in every sample.28 No obvious natural resonance in the low frequency shows up, which may result from the tiny crystallites of magnetic Fe3O4 NPs and the nonpolar radicals on the surface of nanocomposites.
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REFERENCES
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CONCLUSIONS Novel metal-encapsulated Bi/Fe3O4 nanocomposites were fabricated by a solvothermal route in nonpolar ODE phase. The samples are oil soluble and well dispersed in any nonpolar solution, which is better for coating in future applications. Compared with the Bi free sample, it is suggested that the hydroxyl groups introduced from bismuth subnitrate play an important role for the reduction of metallic Bi and prevent the Fe3O4 NPs from oxidation. The results of electromagnetic properties indicate that Bi is responsible for the distinctive 3503
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