FULL PAPER DOI: 10.1002/asia.201301590
Preparation of Monodisperse Ferrite Nanocrystals with Tunable Morphology and Magnetic Properties Ruizheng Liang, Rui Tian, Zhihui Liu, Dongpeng Yan,* and Min Wei[a]
Abstract: The synthesis of monodisperse magnetic ferrite nanomaterials plays an important role in several scientific and technological areas. In this work, dibasic spinel MFe2O4 (M = Mg, Ni, Co, Fe, Mn) and polybasic spinel ferrite MCoFeO4 (M = Mg, Ni, Mn, MgNi) nanocrystals were prepared by the calcination of layered double hydroxide (LDH) precursors at 900 8C, which was confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Scanning electron microscopy (SEM) and trans-
mission electron microscopy (TEM) images demonstrate that the as-obtained spinel ferrites present a singlecrystalline nature with uniform particle size and good dispersibility. The composition, morphology, and particle size can be effectively tuned by changing the metal ratio, basicity, reaction time, Keywords: iron · layered compounds · magnetic properties · monodispersity · nanostructures · spinel phases
Introduction
former can be easily removed by an ordinary external magnetic device. Therefore, effective preparation methods to produce magnetic NPs with tunable size and composition are highly desirable for future large-scale applications. Recently, several approaches have been explored to prepare superparamagnetic NPs (below 20 nm), and the corresponding synthesis mechanisms are also well established.[14–16] The preparation of magnetic NPs in the range of 20–200 nm through various methods (e.g., the sol–gel method, micelle synthesis, hydrothermal synthesis, thermal decomposition) has also been reported.[17–20] Organic thermal decomposition synthesis of magnetic NPs has been developed with particle sizes ranging from 8 nm to hundreds of nanometers.[20] However, it is still difficult to obtain uniform and highly dispersive magnetic NPs with high saturation magnetization (Ms). In addition, these conventional methods generally suffer from toxic organic reagents and/or laborious procedures. Recently, ternary and polybasic spinels have also attracted increasing attention because they can be applied in catalysis and hyperthermia field with enriched type and magnetic properties, although polybasic examples are still rare relative to their dibasic counterparts.[21–23] Therefore, the development of environmentally friendly and economical methods for the preparation of magnetic NPs with controlled size, shape, and composition would significantly promote their further application. Layered double hydroxides (LDHs), members of a family of anionic clay with flexibility in chemical composition and structural morphology, have exhibited a wide variety of applications in biology and medicine, catalysis, and environ-
The synthesis of inorganic nanomaterials (such as metals and semiconductors) with desirable functionality has been of immense scientific and technological interest.[1–3] Among these nanomaterials, magnetic nanoparticles (NPs) have been extensively studied and applied in numerous fields, such as magnetic recording media, sensors, spintronics, and many biological applications including cell recognition, drug delivery, targeted therapeutics, and intracellular imaging.[4–8] It has been known that the production of monodisperse NPs with controllable composition and size plays a key role in certain magnetic applications. For example, in magnetic resonance imaging (MRI), MnFe2O4 and Fe3O4 NPs are preferable to CoFe2O4 and NiFe2O4 owing to their high magnetic saturation values;[9] additionally, the size of MnFe2O4 and Fe3O4 NPs below 20 nm is desirable to facilitate their practical applications.[10, 11] In drug targeting and magnetic separation, however,[12, 13] magnetic NPs above 100 nm are commonly chosen over their smaller counterparts, because the
[a] R. Liang, R. Tian, Z. Liu, Dr. D. Yan, Prof. M. Wei State Key Laboratory of Chemical Resource Engineering Beijing University of Chemical Technology Beijing 100029 (P.R. China) Fax: (+ 86) 10-64425385 E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201301590.
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and temperature of the LDH precursors. In addition, these spinel ferrites show high magnetic saturation values in the range 21.7–84.3 emu g 1, which maintain a higher level than the previously reported magnetic nanoparticles. Therefore, this work provides a facile approach for the design and fabrication of spinel ferrites with controllable nanostructure and improved magnetism, which could potentially be used in magnetic and biological fields, such as recording media, sensors, drug delivery, and intracellular imaging.
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mental remediation.[24–28] It has been reported that pure MIIFeIII2O4 spinel ferrites (MII = Mg, Co, and Ni) were obtained by the calcination of MII-FeII-FeIII LDH precursors, which display enhanced magnetic properties relative to those obtained by conventional ceramic and wet chemical routes.[29, 30] However, the composition, particle size, and morphology of spinel ferrites cannot be successfully tuned, and the size distribution as well as dispersity of the NPs also need to be further improved. This motivated us to explore the possibility of synthesizing a variety of magnetic spinel ferrite NPs with finely controlled composition, particle size, and morphology based on the phase-transformation process of LDH materials. In the present work, we report the preparation of dibasic spinel ferrite MFe2O4 (M = Mg, Ni, Co, Fe, Mn) and polybasic spinel ferrite MCoFeO4 (M = Mg, Ni, Mn, MgNi) NPs by the calcination of LDH precursors. The resulting samples show high crystallinity, good dispersibility, as well as high magnetic saturation values. A phase transformation from LDH to spinel ferrites occurs successfully upon calcination at 900 8C, which was confirmed by XRD and IR spectroscopy. SEM and TEM demonstrate that the spinel ferrites possess a single-crystalline nature with uniform particle size and tunable morphology. By taking MnFe2O4 as an example, the particle size can be controlled in the range of 28–168 nm with morphologies from sphere to cube. Moreover, the Ms values of spinel ferrite NPs synthesized by this method are rather high, for example, 84.3 emu g 1 for Fe3O4, which stays at a higher level than previously reported magnetic NPs. Therefore, this work provides a facile approach for the design and fabrication of various spinel ferrite NPs with high dispersity and improved magnetism.
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Figure 1. XRD patterns of A) Mg2Fe LDH, MgFe LDH, MgFe2 LDH, and B) Mg2FeO4, MgFeO4, MgFe2O4.
tion of the hydroxide groups in the LDH sheets and the interlayer water molecules (nOH); the band at approximately 1380 cm 1 is assigned to the symmetric and asymmetric C= O stretching modes of the carbonate groups. After calcination, the FTIR spectra of the samples exhibit a strong band in the range of 575–580 cm 1 that is attributed to metal oxide stretching vibrations of the magnetite lattice (Figure 2), thereby further indicating the formation of spinel ferrites. To better study the surface properties of the LDHs
Results and Discussion Morphology and Structural Characterization of Dibasic Spinel Ferrite NPs Figure 1A shows typical XRD patterns of the obtained MgFe LDHs with various Mg/Fe ratios, in which a series of (00l) characteristic reflections at 2q = 11.8, 23.7, and 34.18 were observed. In addition, the intensity of characteristic reflections clearly decreases from Mg2Fe LDHs to MgFe2 LDHs, thus indicating the decrease in crystallinity as Fe content increases. After calcination at 900 8C for 2 h, a typical cubic ferrite diffraction pattern can be observed for all of the samples (Figure 1B), thus confirming the phase transformation from LDHs to spinels. The characteristic reflections of MgO appear at approximately 37.08 (111) for the samples Mg2FeO4 and MgFeO4 in Figure 1B, which suggests that the ferrite (MgFe2O4) will be mixed with MgO upon calcination of the Mg2Fe LDH sample. In addition, the reflections of MgO at 43.28 (200) and 62.78 (220) usually overlap with the ferrite (MgFe2O4) (400) and (440) reflections.[29] The Supporting Information displays the FTIR spectra of these LDH materials, in which the broad band centered at approximately 3450 cm 1 corresponds to the stretching vibra-
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Figure 2. FTIR spectra of MFe2O4 (M = Mg, Ni, Co, Mn, Fe) and MCoFeO4 (M = Mg, Ni, Mn, MgNi).
and spinel ferrites, the z potential of the as-prepared samples was measured (Table 1 and in the Supporting Information). The results show that the surface of the LDH materials is positively charged, and the surface potential increases with the increase of trivalent metal content when the surface charge of the spinel ferrites samples is below 23.7 mV, which could provide electrostatic repulsion to enhance colloidal stability. The transformation of the z potential from
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Table 1. The z-potential values of MFe2O4 (M = Mg, Ni, Co, Mn, Fe) and MCoFeO4 (M = Mg, Ni, Mn, MgNi). Type Mg2Fe LDH MgFe LDH MgFe2 LDH Mg2FeO4 MgFeO4 MgFe2O4 NiFe2O4 MnFe2O4 FeFe2O4 CoFe2O4 MgCOFeO4 MnCoFeO4 NiCoFeO4 MgCoNiFeO4
z [emV] 23.1 44.2 57.4 25.7 38.4 43.1 23.7 28.4 31.5 35.7 27.6 34.3 41.9 59.7 Figure 4. SEM images of A) Mg2Fe LDH, B) MgFe LDH, C) MgFe2 LDH, D) Mg2FeO4, E) MgFeO4, and F) MgFe2O4. TEM images of G) Mg2FeO4, H) MgFeO4, and I) MgFe2O4. Scale bars, 200 nm.
positive to negative might be attributed to the generation of defects during calcinations of the LDHs, which increases the electron/hole of the spinel ferrite NPs.[31] Moreover, in aqueous solution the ferrite NPs display a well-defined Tyndall effect (Figure 3), which demonstrates that they can be well dispersed owing to their large surface potential. The chemi-
Figure 3. Tyndall effect of MFe2O4 (M = Mg, Ni, Co, Mn, Fe) and MCoFeO4 (M = Mg, Ni, Mn, MgNi).
cal compositions of the obtained products described above are listed in the Supporting Information; their experimental metal ratios are close to the initial nominal ones. The SEM images (Figure 4A–C) reveal that the MgFe LDH NPs prepared by the hydrothermal method are nearly spherical, with narrow size distribution and an average particle size of 20 nm. After calcination at 900 8C for 2 h, the morphology and particle size of the resulting MgFe spinels are nearly unchanged, whereas the dispersity is clearly improved, which can be viewed in the SEM (Figure 4D–F) and TEM images (Figure 4G–I). Energy-dispersive X-ray spectroscopy (EDX) mapping analysis of MgFe2O4 (see the Supporting Information) shows the existence of Mg and Fe elements. In addition, MgFe2 LDHs and MgFe2O4 NPs with a large particle size of 100 nm can be further prepared by increasing the amount of NaOH, which can be seen in the Supporting Information. We further prepared the CoFe2O4, NiFe2O4, MnFe2O4, and FeFe2O4 ferrites by the LDH precursor method. XRD patterns confirm that the structural transformation occurs from LDHs (Figure 5A) into ferrites (Figure 5B). All the reflections can be indexed to a magnetite phase: CoFe2O4 (JCPDS 22-1068), NiFe2O4 (JCPDS 22-1012), MnFe2O4
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Figure 5. A) Optical XRD patterns of MFe2 LDH (M = Ni, Co, Mn, Fe) and B) MFe2O4 (M = Ni, Co, Mn, Fe).
(JCPDS 74-2403), and FeFe2O4 (JCPDS 75-1609). Figure 6A–C and G–I show typical SEM and TEM images of the MFe2O4 (M = Ni, Co, Fe) ferrites, which have a spherical morphology and very narrow size distribution. Electron-diffraction patterns (Figure 6J–L) obtained from one individual MFe2O4 NP reveal the single-crystalline nature of these samples. The lattice spacing of FeFe2O4 was estimated to be 0.22 nm, which is close to the standard {400} lattice spacing for the cubic spinel structured magnetite. For CoFe2O4, the lattice spacing was measured to be 0.30 nm, which agrees well with the distance between the {200} lattice planes. The lattice spacing of NiFe2O4 was 0.48 nm, which is accord with the distance between the {101} lattice planes. EDX (Fig-
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Figure 7. SEM images of A) Mn2Fe LDH and Mn2FeO4, B) MnFe LDH and MnFeO4, and C) MnFe2 LDH and MnFe2O4. D) TEM image and EDX spectrum of MnFe2O4.
Figure 6. SEM, EDX profiles, and TEM images of A, D, G) FeFe2O4, B, E, H) CoFe2O4, and C, F, I) NiFe2O4. The corresponding Fourier transform and lattice fringe images of J) FeFe2O4, K) CoFe2O4, and L) NiFe2O4.
ure 6D–F) and mapping (see the Supporting Information) reveal the chemical compositions of these ferrite samples and a high dispersion of each element. X-ray photoelectron spectra (XPS) measurements were carried out for the FeFe2O4 sample, in which two main peaks located at 711.8 and 724.8 eV are observed (see the Supporting Information), which is in good agreement with that of the Fe 2p3/2 and Fe 2p1/2 state, respectively. Preparation of MnFe Spinels with Various Chemical Composition, Shape, and Particle Size
Figure 8. SEM images and particle-size distribution histograms of MnFe2O4 spinels (the scale bar is 500 nm).
The physicochemical properties of nanomaterials have a strong relationship with the chemical composition, average particle size, and particle shape. One of the goals of this work is to obtain ferrites with tunable structural parameters. By taking MnFe ferrite as the example (Figure 7), several MnFe ferrites with different Mn/Fe ratios have been synthesized through adjusting the feeding ratio, which were verified by both the elemental analysis (Table S1) and EDX mapping results (see the Supporting Information). The particle size of MnFe2O4 can be finely controlled from approximately 28 to 168 nm by gradually increasing the amount of NaOH and reaction time in the preparation of LDH precursors (Figure 8), which is related to the nucleation rate imposed by alkalinity.[32] Moreover, different morphologies (e.g., sphere, cube, octahedron, rod) of MnFe2O4 can be obtained by changing the temperature and basicity (Figure 9); the details are described in the Experimental Section and the Supporting Information. The main reason for the trans-
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formation can be explained as follows. The excess amount of NaOH promotes the growth along the long-axis orientation of MnFe2O4 crystals while the increased temperature and crystallization time accelerate this process, which is similar to the previous report.[33] Ternary and Polybasic Spinels Recently, the ternary and polybasic ferrites that contain Fe, Co, Mn, and Ni elements have attracted increasing attention owing to their broad applications in catalysis and hyperthermia fields. To extend the type and properties of ferritebased magnetic NPs, we further explored the synthesis of polybasic ferrite MCoFeO4 (M = Mg, Mn, Ni, MgNi). XRD measurements were performed to identify the crystallographic structure and to estimate the particle size of these samples, as summarized in the Supporting Information. All
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Magnetic Properties of These Spinel NPs The magnetic properties of these ferrite NPs were further investigated with a vibrating sample magnetometer. Figure 11A shows the hysteresis loops measured at 300 K for MFe2O4 ferrites. The Ms values are 84.3, 63.5, 50.1, 24.6, and 78.2 emu g 1 for FeFe2O4, MnFe2O4, NiFe2O4, MgFe2O4, and CoFe2O4, respectively. In addition, the Ms values of MCoFeO4 (M = Mg, Ni, Mn, MgNi) are 33.4, 45.1, 61.3, and 21.7 emu g 1 at 300 K, respectively (Figure 11C), thus indicating their ferromagnetic-like behavior at room temperature. The difference in Ms values of the as-produced spinel NPs can be ascribed to the variable composition of metal elements. Generally, the magnetic moments per unit of MnFe2O4, FeFe2O4, CoFe2O4, and NiFe2O4 are estimated to be 5, 4, 3, and 2 mB, respectively, which result in the difference in the mass magnetization values for MnFe2O4, FeFe2O4, CoFe2O4, and NiFe2O4 NPs. Moreover, the Ms values of spinel NPs synthesized by this method are generalFigure 9. TEM images of different morphology of the MnFe2O4 spinels: ly higher than other approaches (56.2–81.9 emu g 1 for A) sphere, B) cube, C) octahedron, and D) club. Fe3O4),[34–36] and this can be attributed to the enhancement in crystallinity and the surface-adsorbed species that were removed by high-temperature calcination. the reflections at 18.28 (111), 30.18 (200), 35.48 (311), 43.08 For the different sizes of MnFe2O4, the Ms values of (400), 53.78 (422), 57.28 (511), and 62.68 (440) are consistent with the standard XRD data. SEM and TEM images show MnFe2O4 particles were determined to be 70.1, 63.5, 57.0, that the obtained MgCoFeO4 and NiCoFeO4 particles have 50.3, and 41.9 emu g 1 for 168, 130, 90, 50, and 28 nm partia nearly spherical shape and uniform size ( 120 nm), cles, respectively (see the Supporting Information), thus inwhereas MnCoFeO4 spinel displays a nearly cubic shape dicating that the Ms value increases as the particle sizes in( 180 nm) (Figure 10A–C and E–G). Moreover, a clubbed crease. Of all the MFe2O4 spinels, CoFe2O4 shows the largest MgNiCoFeO4 spinel was prepared (Figure 10D and H; coercivity, which is caused by the existence of cobalt species for its high Curie point (Figure 11B). Similarly, MCoFeO4 (M = Mg, Ni, MgNi) also possess a large coercivity (Figure 11D). Thus, it can be concluded that the factors of composition, crystallinity, and size can influence the Ms value. Moreover, the homogeneous metal distribution within the LDH layer puts the magnetic unit in a highly dispersed state, which is beneficial to obtain the uniform ferrite NPs upon transformation from the LDHs. The Figure 10. TEM and SEM images of A, E) MgCoFeO4, B, F) NiCoFeO4, C, G) NiCoFeO4, and D, H) MgNiCosize, morphology, and composiFeO4. tion can also be tuned, which could achieve the desired properties of ferrite NPs. Therefore the ferrite NPs obtained by length 500 nm, width 100 nm). The EDX results (see the this method might render them advantageous for several apSupporting Information) further illustrate the elemental plications, such as biomedicine, storage, and sensors. composition of MCoFeO4 (M = Mg, Mn, Ni, MgNi). Therefore, it can be concluded that the single-crystalline polybasic ferrites NPs with tunable particle size, shape, and composition can also be prepared by the polybasic LDH approach. Conclusion Dibasic ferrites MFe2O4 (M = Mg, Ni, Co, Fe, Mn) and polybasic ferrites MCoFeO4 (M = Mg, Ni, Mn, MgNi) were pre-
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Sample Characterization UV-visible powder X-ray diffraction patterns of the samples were collected with a Shimadzu XRD-6000 diffractometer using a CuKa source with a scan step of 0.028 and a scan range between 3 and 708. The morphology of the samples was investigated by use of a scanning electron microscope (Zeiss SUPRA 55) with an accelerating voltage of 20 kV combined with energydispersive X-ray spectroscopy for determination of the metal composition. TEM images were recorded with Philips Tecnai 20 and JEOL JEM-2010 high-resolution transmission electron microscopes. The accelerating voltage was 200 kV in each case. The metal contents of the samples were determined by inductively coupled plasma (ICP) emission spectroscopy with a Shimadzu ICPS-7500 instrument. XPS were recorded with a Thermo Figure 11. Room-temperature (300 K) magnetic hysteresis loops of A, B) MFe2O4 (M = Mg, Ni, Co, Mn, Fe) VG Escalab 250 X-ray photoelectron and C, D) MCoFeO4 (M = Mg, Ni, Mn, MgNi). spectrometer at a pressure of about 2 10 9 Pa with AlKa X-rays as the excitation source. The magnetic properpared by calcination of LDH precursors. The synthetic stratties of the microspheres were measured with an LDJ 9600 vibrating egy allows the direct production of highly crystalline, monosample magnetometer at room temperature. The FTIR spectra were redisperse, and hydrophilic spinel NPs. The particle size (28– corded with a Vector 22 (Bruker) spectrophotometer using the KBr pellet technique in the range of 4000–400 cm 1 with 2 cm 1 resolution. 168 nm), different morphologies (e.g., sphere, cube, octaheAnalysis of z potentials was carried out using photon correlation specdron, club), metal types, and compositions of ferrite-based troscopy (Nano Granularity Analyzer Zetasizer-3000HS, Malvern InstruNPs can be effectively tailored. Moreover, the obtained ferments).
rites show high magnetic saturation values, and the Ms values of spinel NPs synthesized by this method (84.3 emu g 1 for Fe3O4) are generally higher than other asreported values (56.2–81.9 emu g 1 for Fe3O4). Therefore, this work provides a facile approach for the design and fabrication of ferrite NPs, which is expected to be an alternative synthetic method of magnetic materials for magnetic and biomedical applications.
Acknowledgements This work was supported by the 973 Program (grant no. 2014CB932102), the National Natural Science Foundation of China (NSFC), the Scientific Fund from Beijing Municipal Commission of Education (20111001002), the Fundamental Research Funds for the Central Universities, and the 111 Project (grant no. B07004).
Experimental Section
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Synthesis of LDH and Spinel NPs MFe2 LDHs (M = Mg, Ni, Mn, Co, Fe) and MCoFe LDHs (M = Mg, Ni, Mn, MgNi) were prepared by hydrothermal synthesis under alkaline conditions. By taking the MgFe2 LDH as an example, a solution (100 mL) that contained MgACHTUNGRE(NO3)2·6 H2O (0.025 mol) and FeACHTUNGRE(NO3)3·9 H2O (0.05 mol) as well as NaOH (100 mL, 0.15 mol) were simultaneously added to a colloid mill rotating at 3000 rpm and mixed for 1 min. The resulting slurry was removed from the colloid mill and was sealed into a Teflon-lined stainless-steel autoclave (90 mL) and heated at 100 8C for 1 day. The product was washed with hot distilled water and anhydrous ethanol several times until the filtrate was colorless, then it was dried under vacuum at 60 8C for 6 h. The synthesis conditions of various LDH precursors with different components, sizes, and morphologies are listed in the Supporting Information. The spinels can be obtained by calcination of the above LDHs in a muffle furnace at 900 8C for 2 h. To avoid particle aggregation, the heating rate was precisely controlled at 1 8C min 1. For the preparation of FeFe2O4 spinels, a nitrogen atmosphere was needed during the process of calcination.
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Preparation of Monodisperse Ferrite Nanocrystals with Tunable Morphology and Magnetic Properties Ruizheng Liang, Rui Tian, Zhihui Liu, Dongpeng Yan,* and Min Wei[a]
Abstract: The synthesis of monodisperse magnetic ferrite nanomaterials plays an important role in several scientific and technological areas. In this work, dibasic spinel MFe2O4 (M = Mg, Ni, Co, Fe, Mn) and polybasic spinel ferrite MCoFeO4 (M = Mg, Ni, Mn, MgNi) nanocrystals were prepared by the calcination of layered double hydroxide (LDH) precursors at 900 8C, which was confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Scanning electron microscopy (SEM) and trans-
mission electron microscopy (TEM) images demonstrate that the as-obtained spinel ferrites present a singlecrystalline nature with uniform particle size and good dispersibility. The composition, morphology, and particle size can be effectively tuned by changing the metal ratio, basicity, reaction time, Keywords: iron · layered compounds · magnetic properties · monodispersity · nanostructures · spinel phases
Introduction
former can be easily removed by an ordinary external magnetic device. Therefore, effective preparation methods to produce magnetic NPs with tunable size and composition are highly desirable for future large-scale applications. Recently, several approaches have been explored to prepare superparamagnetic NPs (below 20 nm), and the corresponding synthesis mechanisms are also well established.[14–16] The preparation of magnetic NPs in the range of 20–200 nm through various methods (e.g., the sol–gel method, micelle synthesis, hydrothermal synthesis, thermal decomposition) has also been reported.[17–20] Organic thermal decomposition synthesis of magnetic NPs has been developed with particle sizes ranging from 8 nm to hundreds of nanometers.[20] However, it is still difficult to obtain uniform and highly dispersive magnetic NPs with high saturation magnetization (Ms). In addition, these conventional methods generally suffer from toxic organic reagents and/or laborious procedures. Recently, ternary and polybasic spinels have also attracted increasing attention because they can be applied in catalysis and hyperthermia field with enriched type and magnetic properties, although polybasic examples are still rare relative to their dibasic counterparts.[21–23] Therefore, the development of environmentally friendly and economical methods for the preparation of magnetic NPs with controlled size, shape, and composition would significantly promote their further application. Layered double hydroxides (LDHs), members of a family of anionic clay with flexibility in chemical composition and structural morphology, have exhibited a wide variety of applications in biology and medicine, catalysis, and environ-
The synthesis of inorganic nanomaterials (such as metals and semiconductors) with desirable functionality has been of immense scientific and technological interest.[1–3] Among these nanomaterials, magnetic nanoparticles (NPs) have been extensively studied and applied in numerous fields, such as magnetic recording media, sensors, spintronics, and many biological applications including cell recognition, drug delivery, targeted therapeutics, and intracellular imaging.[4–8] It has been known that the production of monodisperse NPs with controllable composition and size plays a key role in certain magnetic applications. For example, in magnetic resonance imaging (MRI), MnFe2O4 and Fe3O4 NPs are preferable to CoFe2O4 and NiFe2O4 owing to their high magnetic saturation values;[9] additionally, the size of MnFe2O4 and Fe3O4 NPs below 20 nm is desirable to facilitate their practical applications.[10, 11] In drug targeting and magnetic separation, however,[12, 13] magnetic NPs above 100 nm are commonly chosen over their smaller counterparts, because the
[a] R. Liang, R. Tian, Z. Liu, Dr. D. Yan, Prof. M. Wei State Key Laboratory of Chemical Resource Engineering Beijing University of Chemical Technology Beijing 100029 (P.R. China) Fax: (+ 86) 10-64425385 E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201301590.
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and temperature of the LDH precursors. In addition, these spinel ferrites show high magnetic saturation values in the range 21.7–84.3 emu g 1, which maintain a higher level than the previously reported magnetic nanoparticles. Therefore, this work provides a facile approach for the design and fabrication of spinel ferrites with controllable nanostructure and improved magnetism, which could potentially be used in magnetic and biological fields, such as recording media, sensors, drug delivery, and intracellular imaging.
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mental remediation.[24–28] It has been reported that pure MIIFeIII2O4 spinel ferrites (MII = Mg, Co, and Ni) were obtained by the calcination of MII-FeII-FeIII LDH precursors, which display enhanced magnetic properties relative to those obtained by conventional ceramic and wet chemical routes.[29, 30] However, the composition, particle size, and morphology of spinel ferrites cannot be successfully tuned, and the size distribution as well as dispersity of the NPs also need to be further improved. This motivated us to explore the possibility of synthesizing a variety of magnetic spinel ferrite NPs with finely controlled composition, particle size, and morphology based on the phase-transformation process of LDH materials. In the present work, we report the preparation of dibasic spinel ferrite MFe2O4 (M = Mg, Ni, Co, Fe, Mn) and polybasic spinel ferrite MCoFeO4 (M = Mg, Ni, Mn, MgNi) NPs by the calcination of LDH precursors. The resulting samples show high crystallinity, good dispersibility, as well as high magnetic saturation values. A phase transformation from LDH to spinel ferrites occurs successfully upon calcination at 900 8C, which was confirmed by XRD and IR spectroscopy. SEM and TEM demonstrate that the spinel ferrites possess a single-crystalline nature with uniform particle size and tunable morphology. By taking MnFe2O4 as an example, the particle size can be controlled in the range of 28–168 nm with morphologies from sphere to cube. Moreover, the Ms values of spinel ferrite NPs synthesized by this method are rather high, for example, 84.3 emu g 1 for Fe3O4, which stays at a higher level than previously reported magnetic NPs. Therefore, this work provides a facile approach for the design and fabrication of various spinel ferrite NPs with high dispersity and improved magnetism.
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Figure 1. XRD patterns of A) Mg2Fe LDH, MgFe LDH, MgFe2 LDH, and B) Mg2FeO4, MgFeO4, MgFe2O4.
tion of the hydroxide groups in the LDH sheets and the interlayer water molecules (nOH); the band at approximately 1380 cm 1 is assigned to the symmetric and asymmetric C= O stretching modes of the carbonate groups. After calcination, the FTIR spectra of the samples exhibit a strong band in the range of 575–580 cm 1 that is attributed to metal oxide stretching vibrations of the magnetite lattice (Figure 2), thereby further indicating the formation of spinel ferrites. To better study the surface properties of the LDHs
Results and Discussion Morphology and Structural Characterization of Dibasic Spinel Ferrite NPs Figure 1A shows typical XRD patterns of the obtained MgFe LDHs with various Mg/Fe ratios, in which a series of (00l) characteristic reflections at 2q = 11.8, 23.7, and 34.18 were observed. In addition, the intensity of characteristic reflections clearly decreases from Mg2Fe LDHs to MgFe2 LDHs, thus indicating the decrease in crystallinity as Fe content increases. After calcination at 900 8C for 2 h, a typical cubic ferrite diffraction pattern can be observed for all of the samples (Figure 1B), thus confirming the phase transformation from LDHs to spinels. The characteristic reflections of MgO appear at approximately 37.08 (111) for the samples Mg2FeO4 and MgFeO4 in Figure 1B, which suggests that the ferrite (MgFe2O4) will be mixed with MgO upon calcination of the Mg2Fe LDH sample. In addition, the reflections of MgO at 43.28 (200) and 62.78 (220) usually overlap with the ferrite (MgFe2O4) (400) and (440) reflections.[29] The Supporting Information displays the FTIR spectra of these LDH materials, in which the broad band centered at approximately 3450 cm 1 corresponds to the stretching vibra-
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Figure 2. FTIR spectra of MFe2O4 (M = Mg, Ni, Co, Mn, Fe) and MCoFeO4 (M = Mg, Ni, Mn, MgNi).
and spinel ferrites, the z potential of the as-prepared samples was measured (Table 1 and in the Supporting Information). The results show that the surface of the LDH materials is positively charged, and the surface potential increases with the increase of trivalent metal content when the surface charge of the spinel ferrites samples is below 23.7 mV, which could provide electrostatic repulsion to enhance colloidal stability. The transformation of the z potential from
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Table 1. The z-potential values of MFe2O4 (M = Mg, Ni, Co, Mn, Fe) and MCoFeO4 (M = Mg, Ni, Mn, MgNi). Type Mg2Fe LDH MgFe LDH MgFe2 LDH Mg2FeO4 MgFeO4 MgFe2O4 NiFe2O4 MnFe2O4 FeFe2O4 CoFe2O4 MgCOFeO4 MnCoFeO4 NiCoFeO4 MgCoNiFeO4
z [emV] 23.1 44.2 57.4 25.7 38.4 43.1 23.7 28.4 31.5 35.7 27.6 34.3 41.9 59.7 Figure 4. SEM images of A) Mg2Fe LDH, B) MgFe LDH, C) MgFe2 LDH, D) Mg2FeO4, E) MgFeO4, and F) MgFe2O4. TEM images of G) Mg2FeO4, H) MgFeO4, and I) MgFe2O4. Scale bars, 200 nm.
positive to negative might be attributed to the generation of defects during calcinations of the LDHs, which increases the electron/hole of the spinel ferrite NPs.[31] Moreover, in aqueous solution the ferrite NPs display a well-defined Tyndall effect (Figure 3), which demonstrates that they can be well dispersed owing to their large surface potential. The chemi-
Figure 3. Tyndall effect of MFe2O4 (M = Mg, Ni, Co, Mn, Fe) and MCoFeO4 (M = Mg, Ni, Mn, MgNi).
cal compositions of the obtained products described above are listed in the Supporting Information; their experimental metal ratios are close to the initial nominal ones. The SEM images (Figure 4A–C) reveal that the MgFe LDH NPs prepared by the hydrothermal method are nearly spherical, with narrow size distribution and an average particle size of 20 nm. After calcination at 900 8C for 2 h, the morphology and particle size of the resulting MgFe spinels are nearly unchanged, whereas the dispersity is clearly improved, which can be viewed in the SEM (Figure 4D–F) and TEM images (Figure 4G–I). Energy-dispersive X-ray spectroscopy (EDX) mapping analysis of MgFe2O4 (see the Supporting Information) shows the existence of Mg and Fe elements. In addition, MgFe2 LDHs and MgFe2O4 NPs with a large particle size of 100 nm can be further prepared by increasing the amount of NaOH, which can be seen in the Supporting Information. We further prepared the CoFe2O4, NiFe2O4, MnFe2O4, and FeFe2O4 ferrites by the LDH precursor method. XRD patterns confirm that the structural transformation occurs from LDHs (Figure 5A) into ferrites (Figure 5B). All the reflections can be indexed to a magnetite phase: CoFe2O4 (JCPDS 22-1068), NiFe2O4 (JCPDS 22-1012), MnFe2O4
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Figure 5. A) Optical XRD patterns of MFe2 LDH (M = Ni, Co, Mn, Fe) and B) MFe2O4 (M = Ni, Co, Mn, Fe).
(JCPDS 74-2403), and FeFe2O4 (JCPDS 75-1609). Figure 6A–C and G–I show typical SEM and TEM images of the MFe2O4 (M = Ni, Co, Fe) ferrites, which have a spherical morphology and very narrow size distribution. Electron-diffraction patterns (Figure 6J–L) obtained from one individual MFe2O4 NP reveal the single-crystalline nature of these samples. The lattice spacing of FeFe2O4 was estimated to be 0.22 nm, which is close to the standard {400} lattice spacing for the cubic spinel structured magnetite. For CoFe2O4, the lattice spacing was measured to be 0.30 nm, which agrees well with the distance between the {200} lattice planes. The lattice spacing of NiFe2O4 was 0.48 nm, which is accord with the distance between the {101} lattice planes. EDX (Fig-
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Figure 7. SEM images of A) Mn2Fe LDH and Mn2FeO4, B) MnFe LDH and MnFeO4, and C) MnFe2 LDH and MnFe2O4. D) TEM image and EDX spectrum of MnFe2O4.
Figure 6. SEM, EDX profiles, and TEM images of A, D, G) FeFe2O4, B, E, H) CoFe2O4, and C, F, I) NiFe2O4. The corresponding Fourier transform and lattice fringe images of J) FeFe2O4, K) CoFe2O4, and L) NiFe2O4.
ure 6D–F) and mapping (see the Supporting Information) reveal the chemical compositions of these ferrite samples and a high dispersion of each element. X-ray photoelectron spectra (XPS) measurements were carried out for the FeFe2O4 sample, in which two main peaks located at 711.8 and 724.8 eV are observed (see the Supporting Information), which is in good agreement with that of the Fe 2p3/2 and Fe 2p1/2 state, respectively. Preparation of MnFe Spinels with Various Chemical Composition, Shape, and Particle Size
Figure 8. SEM images and particle-size distribution histograms of MnFe2O4 spinels (the scale bar is 500 nm).
The physicochemical properties of nanomaterials have a strong relationship with the chemical composition, average particle size, and particle shape. One of the goals of this work is to obtain ferrites with tunable structural parameters. By taking MnFe ferrite as the example (Figure 7), several MnFe ferrites with different Mn/Fe ratios have been synthesized through adjusting the feeding ratio, which were verified by both the elemental analysis (Table S1) and EDX mapping results (see the Supporting Information). The particle size of MnFe2O4 can be finely controlled from approximately 28 to 168 nm by gradually increasing the amount of NaOH and reaction time in the preparation of LDH precursors (Figure 8), which is related to the nucleation rate imposed by alkalinity.[32] Moreover, different morphologies (e.g., sphere, cube, octahedron, rod) of MnFe2O4 can be obtained by changing the temperature and basicity (Figure 9); the details are described in the Experimental Section and the Supporting Information. The main reason for the trans-
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formation can be explained as follows. The excess amount of NaOH promotes the growth along the long-axis orientation of MnFe2O4 crystals while the increased temperature and crystallization time accelerate this process, which is similar to the previous report.[33] Ternary and Polybasic Spinels Recently, the ternary and polybasic ferrites that contain Fe, Co, Mn, and Ni elements have attracted increasing attention owing to their broad applications in catalysis and hyperthermia fields. To extend the type and properties of ferritebased magnetic NPs, we further explored the synthesis of polybasic ferrite MCoFeO4 (M = Mg, Mn, Ni, MgNi). XRD measurements were performed to identify the crystallographic structure and to estimate the particle size of these samples, as summarized in the Supporting Information. All
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Magnetic Properties of These Spinel NPs The magnetic properties of these ferrite NPs were further investigated with a vibrating sample magnetometer. Figure 11A shows the hysteresis loops measured at 300 K for MFe2O4 ferrites. The Ms values are 84.3, 63.5, 50.1, 24.6, and 78.2 emu g 1 for FeFe2O4, MnFe2O4, NiFe2O4, MgFe2O4, and CoFe2O4, respectively. In addition, the Ms values of MCoFeO4 (M = Mg, Ni, Mn, MgNi) are 33.4, 45.1, 61.3, and 21.7 emu g 1 at 300 K, respectively (Figure 11C), thus indicating their ferromagnetic-like behavior at room temperature. The difference in Ms values of the as-produced spinel NPs can be ascribed to the variable composition of metal elements. Generally, the magnetic moments per unit of MnFe2O4, FeFe2O4, CoFe2O4, and NiFe2O4 are estimated to be 5, 4, 3, and 2 mB, respectively, which result in the difference in the mass magnetization values for MnFe2O4, FeFe2O4, CoFe2O4, and NiFe2O4 NPs. Moreover, the Ms values of spinel NPs synthesized by this method are generalFigure 9. TEM images of different morphology of the MnFe2O4 spinels: ly higher than other approaches (56.2–81.9 emu g 1 for A) sphere, B) cube, C) octahedron, and D) club. Fe3O4),[34–36] and this can be attributed to the enhancement in crystallinity and the surface-adsorbed species that were removed by high-temperature calcination. the reflections at 18.28 (111), 30.18 (200), 35.48 (311), 43.08 For the different sizes of MnFe2O4, the Ms values of (400), 53.78 (422), 57.28 (511), and 62.68 (440) are consistent with the standard XRD data. SEM and TEM images show MnFe2O4 particles were determined to be 70.1, 63.5, 57.0, that the obtained MgCoFeO4 and NiCoFeO4 particles have 50.3, and 41.9 emu g 1 for 168, 130, 90, 50, and 28 nm partia nearly spherical shape and uniform size ( 120 nm), cles, respectively (see the Supporting Information), thus inwhereas MnCoFeO4 spinel displays a nearly cubic shape dicating that the Ms value increases as the particle sizes in( 180 nm) (Figure 10A–C and E–G). Moreover, a clubbed crease. Of all the MFe2O4 spinels, CoFe2O4 shows the largest MgNiCoFeO4 spinel was prepared (Figure 10D and H; coercivity, which is caused by the existence of cobalt species for its high Curie point (Figure 11B). Similarly, MCoFeO4 (M = Mg, Ni, MgNi) also possess a large coercivity (Figure 11D). Thus, it can be concluded that the factors of composition, crystallinity, and size can influence the Ms value. Moreover, the homogeneous metal distribution within the LDH layer puts the magnetic unit in a highly dispersed state, which is beneficial to obtain the uniform ferrite NPs upon transformation from the LDHs. The Figure 10. TEM and SEM images of A, E) MgCoFeO4, B, F) NiCoFeO4, C, G) NiCoFeO4, and D, H) MgNiCosize, morphology, and composiFeO4. tion can also be tuned, which could achieve the desired properties of ferrite NPs. Therefore the ferrite NPs obtained by length 500 nm, width 100 nm). The EDX results (see the this method might render them advantageous for several apSupporting Information) further illustrate the elemental plications, such as biomedicine, storage, and sensors. composition of MCoFeO4 (M = Mg, Mn, Ni, MgNi). Therefore, it can be concluded that the single-crystalline polybasic ferrites NPs with tunable particle size, shape, and composition can also be prepared by the polybasic LDH approach. Conclusion Dibasic ferrites MFe2O4 (M = Mg, Ni, Co, Fe, Mn) and polybasic ferrites MCoFeO4 (M = Mg, Ni, Mn, MgNi) were pre-
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Sample Characterization UV-visible powder X-ray diffraction patterns of the samples were collected with a Shimadzu XRD-6000 diffractometer using a CuKa source with a scan step of 0.028 and a scan range between 3 and 708. The morphology of the samples was investigated by use of a scanning electron microscope (Zeiss SUPRA 55) with an accelerating voltage of 20 kV combined with energydispersive X-ray spectroscopy for determination of the metal composition. TEM images were recorded with Philips Tecnai 20 and JEOL JEM-2010 high-resolution transmission electron microscopes. The accelerating voltage was 200 kV in each case. The metal contents of the samples were determined by inductively coupled plasma (ICP) emission spectroscopy with a Shimadzu ICPS-7500 instrument. XPS were recorded with a Thermo Figure 11. Room-temperature (300 K) magnetic hysteresis loops of A, B) MFe2O4 (M = Mg, Ni, Co, Mn, Fe) VG Escalab 250 X-ray photoelectron and C, D) MCoFeO4 (M = Mg, Ni, Mn, MgNi). spectrometer at a pressure of about 2 10 9 Pa with AlKa X-rays as the excitation source. The magnetic properpared by calcination of LDH precursors. The synthetic stratties of the microspheres were measured with an LDJ 9600 vibrating egy allows the direct production of highly crystalline, monosample magnetometer at room temperature. The FTIR spectra were redisperse, and hydrophilic spinel NPs. The particle size (28– corded with a Vector 22 (Bruker) spectrophotometer using the KBr pellet technique in the range of 4000–400 cm 1 with 2 cm 1 resolution. 168 nm), different morphologies (e.g., sphere, cube, octaheAnalysis of z potentials was carried out using photon correlation specdron, club), metal types, and compositions of ferrite-based troscopy (Nano Granularity Analyzer Zetasizer-3000HS, Malvern InstruNPs can be effectively tailored. Moreover, the obtained ferments).
rites show high magnetic saturation values, and the Ms values of spinel NPs synthesized by this method (84.3 emu g 1 for Fe3O4) are generally higher than other asreported values (56.2–81.9 emu g 1 for Fe3O4). Therefore, this work provides a facile approach for the design and fabrication of ferrite NPs, which is expected to be an alternative synthetic method of magnetic materials for magnetic and biomedical applications.
Acknowledgements This work was supported by the 973 Program (grant no. 2014CB932102), the National Natural Science Foundation of China (NSFC), the Scientific Fund from Beijing Municipal Commission of Education (20111001002), the Fundamental Research Funds for the Central Universities, and the 111 Project (grant no. B07004).
Experimental Section
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Synthesis of LDH and Spinel NPs MFe2 LDHs (M = Mg, Ni, Mn, Co, Fe) and MCoFe LDHs (M = Mg, Ni, Mn, MgNi) were prepared by hydrothermal synthesis under alkaline conditions. By taking the MgFe2 LDH as an example, a solution (100 mL) that contained MgACHTUNGRE(NO3)2·6 H2O (0.025 mol) and FeACHTUNGRE(NO3)3·9 H2O (0.05 mol) as well as NaOH (100 mL, 0.15 mol) were simultaneously added to a colloid mill rotating at 3000 rpm and mixed for 1 min. The resulting slurry was removed from the colloid mill and was sealed into a Teflon-lined stainless-steel autoclave (90 mL) and heated at 100 8C for 1 day. The product was washed with hot distilled water and anhydrous ethanol several times until the filtrate was colorless, then it was dried under vacuum at 60 8C for 6 h. The synthesis conditions of various LDH precursors with different components, sizes, and morphologies are listed in the Supporting Information. The spinels can be obtained by calcination of the above LDHs in a muffle furnace at 900 8C for 2 h. To avoid particle aggregation, the heating rate was precisely controlled at 1 8C min 1. For the preparation of FeFe2O4 spinels, a nitrogen atmosphere was needed during the process of calcination.
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