Journal of Nanoparticle Research

, Volume 11, Issue 5, pp 1043–1051

Morphology and magnetic properties of FexCo1−x/CoyFe3−yO4 nanocomposites prepared by surfactants-assisted-hydrothermal process

Authors

  • Qin Wang
    • College of ChemistryJilin University
    • College of ChemistryJilin University
Research Paper

DOI: 10.1007/s11051-008-9495-8

Cite this article as:
Wang, Q. & Yang, H. J Nanopart Res (2009) 11: 1043. doi:10.1007/s11051-008-9495-8

Abstract

Recently, we have demonstrated the successful synthesis of FexCo1−x/CoyFe3−yO4 nanocomposites with various alkaline solutions by using surfactants-assisted-hydrothermal (SAH) process. In this article, the synthesis of FexCo1−x/CoyFe3−yO4 nanocomposites with their sizes varying between 20 nm and 2 μm was reported. X-ray powder diffraction (XRD) analyses showed that the surfactants, pH, precipitator, and temperature of the system play important roles in the nucleation and growth processes. The magnetic properties tested by vibrating sample magnetometer (VSM) at room temperature exhibit ferromagnetic behavior of the nanocomposites. These FexCo1−x/CoyFe3−yO4 nanocomposites may have a potential application as magnetic carriers for drug targeting because of their excellent soft-magnetic properties.

Keywords

StructureMagnetic propertiesMorphologyDisproportionationHydrothermalNanocompositesColloids

Introduction

Fe–Co/Fe3O4 nanocomposite has attracted considerable attention due to its novel physical and chemical properties, potential application in catalytic behaviors (Tihay et al. 2002), and magnetic properties (Tihay et al. 2000; Caillof et al. 2004). Various methods of synthesis such as high energy ball milling, reduction or decomposition of metallic precursors in polymers, oxides, or gels have been used for the synthesis of nanocomposites (Tyan et al. 1999; Fröba et al. 1999; Osso et al. 1998; Estournes et al. 1994). Pourroy et al have developed a method to synthesize Fe–Co/Fe3O4 composites based on the disproportionation of Fe(OH)2 in inorganic alkaline solutions (Läkamp and Pourroy 1996).

The magnetic nanoparticles synthesized in inorganic alkaline aqueous solutions are liable to aggregate, which would directly affect the magnetic properties of the samples (Li and Kaner 2006). Aggregation makes it especially difficult to explore the properties and applications of magnetic materials. In many synthetic processes for magnetic nanoparticles, aggregation occurs immediately as nanoparticles are generated. In conventional studies, aggregation has been simply ascribed to the direct mutual attraction between particles via van der Waals forces or chemical bonding. The strategies for preventing aggregation mainly come from conventional colloid science in which nanoparticles are coated with foreign capping agents or the surface charges are tailored to separate them via electrostatic repulsions (Hunter 1987).

In this article, we synthesized the nanocomposites (FexCo1−x/CoyFe3−yO4) in an organic alkaline solution that assisted the surfactants condition by hydrothermal method, and avoided aggregation effectively. It is well known that the hydrothermal reaction conditions are important in determining the structure of the magnetic material (Zhao et al. 2007). However, it is not clearly known how the reaction conditions affect the structure of the prepared samples during the hydrothermal process. Hence, hydrothermal conditions such as the addition of surfactant, pH, precipitator, and temperature were adjusted carefully.

Experimental section

Chemicals

Cobalt chloride hexahydrate (CoCl2 · 6H2O), ferrous chloride tetrahydrate (FeCl2 · 4H2O), 25 wt% tetramethyl ammonium hydroxide (N(CH3)4 · OH), trimethylamine solution ((CH3)3N), potassium hydroxide (KOH), hydrazine hydrate (N2H4 · H2O), sodium oleate (C18H33NaO2), cetyltrimethyl ammonium bromide (CTAB), oleic acid (CH3(CH2)7CHCH(CH2)7COOH), and polyethylene glycol 400 (PEG-400) were used for the synthesis of FexCo1−x/CoyFe3−yO4 nanocomposites. The chemicals were all of analytical reagent grade, and were used without further purification.

Sample preparation

According to the composition of the above formula (FexCo1x/CoyFe3−yO4), stoichiometric mole ratios of starting materials (Co2+/Fe2+ mole ratio = 0.5) were dissolved in 30 ml of distilled water and mixed surfactants to form a homogeneous solution. Then, certain amount of alkaline solution was added into mixed chloride solutions at room temperature by intensive stirring (30 min) in a nitrogen gas protective atmosphere. The mixture was then transferred into a 50 mL Teflon-lined stainless steel autoclave, sealed and maintained at 120–210 °C for 3 h. After completion of the reaction, the black solid products were collected by magnetic separation and washed several times with water and ethanol. The final products were dried in a vacuum oven for 6 h at 40 °C.

Sample characterization

X-ray diffraction (XRD) patterns were performed by a Tokyo X-ray diffractometer with Cu-Kα radiation (λ = 0.15405 nm). The morphologies and structures of the as-synthesized composite products were observed with a field scanning electron microscope (FE-SEM) and a transmission electron microscope (TEM). All the samples for the FE-SEM and TEM characterization were prepared by directly transferring the suspended products to the ITO glass slide and standard copper grid coated with an amorphous carbon film, respectively. Before the samples were withdrawn, the composites-dispersed ethanol solutions were sonicated for 30 min to obtain better particles dispersion on the copper grid. Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM) (Digital Measurement System JDM-13) with a maximum magnetic field of 10,000 Oe.

Results and discussions

Structural analysis

Figures 14 show the XRD patterns of FexCo1−x/CoyFe3−yO4 nanocomposites obtained under different conditions. Various synthetic conditions for the fabrication of FexCo1−x/CoyFe3−yO4 nanocomposites are listed in Table 2, and the relations between synthetic conditions and structure of nanocomposites are commented in the following paragraphs. From Figs. 14, the presented XRD pattern features seven Bragg diffraction peaks at 25–65°, which can be easily indexed as (220), (311), (222), (400), (422), (511), and (440) planes. All peaks can be indexed as face-centered cubic Fe3O4 with lattice constant = 8.394 Å, which is very close to the reported data (JCPDS 85–1436, = 8.393 Å). In addition, we can see (110) face which represents the bcc structure of the Fe-based alloy from the XRD patterns. The bcc and fcc are classical structures for iron and cobalt-based alloy which is isomorphous to α-Fe, and the oxide is a cobalt magnetite.
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Fig. 1

XRD patterns of FexCo1−x/CoyFe3−yO4 composites (Co/Fe = 0.5) obtained with different surfactants at 180 °C for 3 h: (a) PEG-400, (b) sodium oleate, (c) oleic acid, and (d) CTAB

In our experiments, we tried to use various surfactants as protective reagents to prepare the nanocomposites of Fe–Co alloy and cobalt-substituted magnetite under similar conditions. The size of the as-synthesized composite prepared in the absence of surfactants is about 2–3 μm (Fig. 6d), and the uniformity is much worse than the samples synthesized in the presence of surfactants. In addition, the size of the nanocomposites synthesized with various surfactants is different, which can be calculated from the XRD patterns (Fig. 1). Interestingly, when we used sodium oleate as the protective agent in our experiments, spherical monodisperse nanoparticles with a size of about 20 nm were prepared. Furthermore, we can observe that there appears a bcc structure of the Fe–Co alloy from the XRD pattern (Fig. 1b). These experimental results strongly indicated that sodium oleate could act as a protective reagent which can efficiently prevent the alloy structure from oxidation. We believe such an improvement in the protecting ability of the surfactant is due to the formation of the composite micelle. When the anionic surfactant (sodium oleate) is added to the mixed solution, it is dissociated as an alkyl anion and a sodium ion. The Fe2+ is prone to assemble into the micelle and the hydroxyl ion (OH) is adsorbed on the exterior of the micelle. Then, the surfactant is adsorbed on the surface of the micelle to form a composite micelle. The adsorption probably results not only from the hydrophilic chain of the polymer approaching the cations, but also from the interaction between the cations and the lone electron pair of oxygen atom in the surfactant. When compared with other surfactants, the protecting ability of sodium oleate is improved dramatically. Therefore, to prepare small and uniform nanocomposites, we chose sodium oleate as the protective agent in our experiment. The different surfactants affect the observed XRD patterns mainly due to the differences in the size of the nanocomposites, which we synthesized in our experiments.

Figure 2 shows the XRD patterns of samples obtained with different pH at 180 °C for 3 h. As shown in Fig.2, there are around 32 and 39  in γ-Fe2O3 diffraction peaks when the range of pH is 7–9. The spinel structure of magnetite and bcc structure of alloy appear when the range of pH is 10–13. In addition, we speculate a possible formation mechanism for the Fe–Co alloy and cobalt-substituted magnetite nanocomposite, i.e., they grow in the reverse microemulsion system with FeO serving as crystal seeds. The details are as follows: when the Fe2+ substrate is immersed in the solution, usually as ferrous ions (Fe2+), under an oxygen-deficient environment, ferrous hydroxide (Fe(OH)2) is formed. The unstable dark-green ferrous hydroxide sample is transformed to FeO when immersed in distilled water. During the hydrothermal process, the microreactors do not change their shape while the reversible disproportionation of FeO leads to nucleation and growth. In high temperature range, the FeO is in unstable phase and it is transformed to Fe3O4 and Fe phases. Further, the confinement of the reverse microemulsion favors the small and uniform nanocomposites, and thus the nanocomposites are completely formed. The proposed sequences of reactions are as follows:
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Fig. 2

XRD patterns of samples (Co/Fe = 0.5) obtained with different pH at 180 °C for 3 h: (a) pH = 7, (b) pH = 9, (c) pH = 10, (d) pH = 12, and (e) pH = 13

$$ {\text{Fe}}^{2 + } + 2{\text{OH}}^{ - } \to {\text{Fe(OH}})_{2} $$
(1)
$$ {\text{Fe(OH)}}_{2} \to {\text{FeO}} + {\text{H}}_{2} {\text{O}} $$
(2)
$$ 4 {\text{FeO}} \to {\text{Fe}} + {\text{Fe}}_{3} {\text{O}}_{4}, \quad \Updelta {\text{G}}^{0} = 12.2\;{\text{kJ/mol}} $$
(3)

With the increase in pH, the intensity of bcc structure of the Fe–Co alloy is increased (Fig. 2). That is to say, the pH in the system plays an important role in the synthesis of Fe–Co alloy and cobalt-substituted magnetite nanocomposite. The reasons are as follows: from the dynamics point of view, higher concentration of the initial reagents (OH) favors the above reaction. In addition, the influence of the chemical potential on the structure and shape evolution has been elucidated by Peng and Peng (Peng and Peng 2001, 2002). In the case of crystal growth, it is beneficial to have a higher chemical potential, which is mainly determined by the concentration of the alkaline solution. The Fe–Co alloy with a high quality and crystallinity will be obtained in concentrated alkaline solutions because a higher OH ion concentration and a higher chemical potential in the solution favor the growth of bcc structures over other possible iron oxide crystal forms. Therefore, with the increase in the concentration of the OH ion or chemical potential, the content of the Fe–Co alloy is increased, which can be reflected by the XRD patterns (Fig. 2). Based on the above discussion, we come to the conclusion that since the content of the alloy or the intensity of the bcc structure increases, structures, morphologies, and magnetic properties of the FexCo1−x/CoyFe3−yO4 nanocomposites are inevitably changed.

Figure 3 shows the XRD patterns of FexCo1−x/CoyFe3−yO4 nanocomposites obtained with different alkaline solutions at 180 °C for 3 h. As can be seen from Fig. 3, when we chose hydrazine hydrate (N2H4 · H2O) as a precipitator (see Fig. 3d), it was found that the intensity of (110) the diffraction peak representing the bcc structure of the Fe–Co alloy was the strongest. That is to say, among the four types of alkaline solutions, when we use hydrazine hydrate as the alkaline solution, the content of the alloy in the nanocomposite is the highest. The reasons are as follows: the disproportionation reaction takes place when we chose hydrazine hydrate as the precipitator. As a standard electrode, we can see that hydrazine hydrate is a strong reducing agent in alkaline environment, which can partly reduce Fe2+ and Co2+ to Fe–Co alloy. It causes a competition between the disproportionation and the reduction reactions. The dynamics of the whole reaction would be affected, and the nucleation, and thus the growth, of nanocomposites could be adjusted, and the standard electrode potentials are −0.23 and −1.16 V, respectively. The reduction scheme should be expressed as follows:
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Fig. 3

XRD patterns of the FexCo1−x/CoyFe3−yO4 (Co/Fe = 0.5) composites synthesized by hydrothermal method under various alkaline solutions: (a) KOH, (b) N(CH3)3, (c) N(CH3)4OH, and (d) N2H4 · H2O

$$ 2 {\text{Fe}}^{2 + } + {\text{N}}_{2} {\text{H}}_{4} + 4{\text{OH}}^{ - } \to 2{\text{Fe}} + {\text{N}}_{2} + 4{\text{H}}_{2} {\text{O}} $$
(4)
$$ {\text{Co}}^{2 + } + {\text{N}}_{2} {\text{H}}_{4} + 4{\text{OH}}^{ - } \to 2{\text{Co}} + {\text{N}}_{2} + 4{\text{H}}_{2} {\text{O}} $$
(5)
$$ {\text{N}}_{2} {\text{H}}_{4} \leftrightarrow {\text{N}}_{2} + 4{\text{H}}^{ + } + 4{\text{e}}^{ - } $$
(6)
$$ {\text{N}}_{2} {\text{H}}_{4} + 4{\text{OH}}^{ - } \leftrightarrow {\text{N}}_{2} + 4{\text{H}}_{2} {\text{O}} + 4{\text{e}}^{ - } $$
(7)

From the above discussion, we found that Fe2+ is prone to assemble into the micelle and the hydroxyl ion (OH) is adsorbed on the exterior of the micelle. It is well known that the addition of complexing agents can affect the nucleation and growth of the particles, which can consequently modify the particle morphology and size. Organic alkaline solution is a strong complexing agent for metallic irons. The uniformity of the solution will provide a good environment for the growth of high quality nanocomposite. The differences in the complexing abilities of the alkaline solution on the exterior of the micelle maybe the main reason that the choice of the alkaline solution impacts the bcc content of the alloy and the magnetic properties of the composite.

In addition, the alkaline solution we selected is less polar than water; the addition of alkaline solution to water will result in a less polar solvent. Due to the chemical structure of the alkaline solution molecules, the addition of even a few percent of alkaline solution to water can also sharply reduce the surface tension of water, where alkaline solution behaves like a surfactant or a complexing agent. As verified above, the particle diameter could be controlled by the alkaline solution content. We explain this effect to be due to the reduced polarity of water by the alkaline solution. When alkaline solution is mixed into water, the mixed solvent will have a polarity that is less than pure water. In such a less polar medium, the kinetic properties of metallic irons M2+ (Fe2+, Co2+) as well as the nucleation kinetics for the formation of alloy nanoparticles may be different from those in pure water. The nucleation for nanoparticle formation is expected to be easier in a less polar medium than in a polar medium (water), and maybe due to the differences between the polarities and the complexing abilities of the four types of alkaline solutions; the size, structure, morphology and magnetic properties of the nanocomposites are different.

In addition, the mean particle diameters were calculated from the XRD patterns according to the linewidth of the (311) plane refraction peak using Scherrer Eq. 8. However, the particle diameters from TEM measurements are larger than the observed crystal sizes from XRD, due to the presence of noncrystalline surface layers (Cornell and Schwertmann 1996; Voit et al. 2001). The values obtained are shown in Table 1
$$ D = \;\frac{{K{{\uplambda}}}}{{b\;\cos {{\uptheta}}}} $$
(8)
where λ is the wavelength of the incident X-ray (1.54056 Å), θ is the diffraction angle, and β is the full-width at half maximum. The lattice parameter a and the interplanar spacing dhkl were determined by Eq. 10 and Bragg’s law (Eq. 9) (Cullity and Stock 2001).
Table 1

Types of alkaline solutions, lattice parameters, interplanar spacing, and diameter determined using X-ray VD0 diffraction and TEM

Types of precipitators

a (Å)

d311

DX-ray (nm)

DTM (nm)

Spinel

bcc

Spinel

bcc

N(CH3)3

8.388

2.529

32.2

39.9

80

20

N(CH3)4OH

8.394

2.531

24.9

19.4

20

20

KOH

8.393

2.530

43.2

18.7

2,000

500

N2H4 · H2O

8.392

2.530

34.0

25.7

80

20

Table 2

Synthesis conditions for the preparation of FexCo1−x/CoyFe3−yO4 composites (Co/Fe = 0.5)

Sample

Surfactant

pH

Precipitator

Temperature (°C)

1a

PEG-400

12

N(CH3)4OH

180

1b

Sodium oleate

12

N(CH3)4OH

180

1c

Oleic acid

12

N(CH3)4OH

180

1d

CTAB

12

N(CH3)4OH

180

2a

Sodium oleate

7

N(CH3)4OH

180

2b

Sodium oleate

9

N(CH3)4OH

180

2c

Sodium oleate

10

N(CH3)4OH

180

2d

Sodium oleate

12

N(CH3)4OH

180

2e

Sodium oleate

13

N(CH3)4OH

180

3a

12

KOH

180

3b

Sodium oleate

12

N(CH3)3

180

3c

Sodium oleate

12

N(CH3)4OH

180

3d

Sodium oleate

12

N2H4 · H2O

180

4a

Sodium oleate

12

N(CH3)4OH

120

4b

Sodium oleate

12

N(CH3)4OH

150

4c

Sodium oleate

12

N(CH3)4OH

180

4d

Sodium oleate

12

N(CH3)4OH

210

$$ d_{\text{hkl}} = \frac{\lambda }{{2{\text{Sin}}\theta }} $$
(9)
$$ d_{\text{hkl}} \; = \;\frac{a}{{\sqrt {h^{2} + k^{2} + l^{2} } }} $$
(10)
Figure 4 shows the XRD patterns of samples synthesized at different temperatures for 3 h. As it can be seen from Fig. 4, when the temperature of system is 120 °C or 150 °C, we synthesized the metastable γ-Fe2O3 which is observed from the color of the final products and the XRD standard cards. With the increase in temperature, the intensity of alloy in the bcc structure also increased. Based on the above discussion, we can come to the conclusion that the surfactants, pH, the types of precipitators, and hydrothermal temperature have important impact on the structure of the FexCo1−x/CoyFe3−yO4 nanocomposites.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9495-8/MediaObjects/11051_2008_9495_Fig4_HTML.gif
Fig. 4

XRD patterns of the samples synthesized by hydrothermal method at pH = 12 with different temperatures (a) 120 °C, (b) 150 °C, (c) 180 °C, and (d) 210 °C

Magnetic properties

The FexCo1−x/CoyFe3−yO4 nanocomposites are composed of Fe–Co alloy and cobalt-magnetite because of the following reactions: Fe0 + Co(II) → Fe(II) + Co0 and x Co + (1 − x) Fe → CoxFe1−x. The Fe–Co alloy is a bcc structure for 25% or more Fe; and it is a fcc structure for a concentration of Fe less than 10%. Therefore, the magnetic properties of composites are mainly decided by two aspects: one is the contribution of magnetic properties arising from Fe3+ on B-sites of magnetite; and the other is the percentage of Fe–Co alloy in the composite. It should be apparent from the mechanism of the composite formation that the system is consisted of ferromagnetic alloys FexCo1−x and CoyFe3−yO4. This means that there would be an exchange coupling between the FexCo1−x alloy and CoyFe3−yO4. Therefore, the content of the Fe–Co alloy which is decided by the type of alkaline solutions has very important impact on the magnetic properties of the composite.

Figure 5 shows the magnetic hysteresis curves measured at room temperature for FexCo1−x/CoyFe3−yO4 composites synthesized with various alkaline solutions. As it can be seen from Fig. 5, the saturation magnetization of samples 5a and 5b are obviously smaller than that of the sample 5c. This is mainly because the size of the samples 5a and 5b are less than that of the sample 5c. This point can be confirmed by the XRD and TEM images. However, it can be observed that the sample synthesized with hydrazine hydrate (N2H4 · H2O) exhibits a saturation magnetization of 180 emu/g which is higher than that of the samples 5a, 5b, and 5c. This indicates that the content of alloy is the key factor to determine the magnetic properties of the FexCo1−x/CoyFe3−yO4 composites. Based on the above discussion, we found that the Fe–Co alloy has very important impact on the magnetic properties of the FexCo1−x/CoyFe3−yO4 nanocomposites. When we chose hydrazine hydrate as the precipitator, we found that the intensity of the Fe–Co alloy is the strongest one, which is far more than other samples. In our synthesis of nanocomposites, the organic surfactant (sodium oleate) has been used to control the nanoparticles size and prevent them from aggregation. The surfactant molecules form an organic shell on each single nanoparticle, which prevents the coalescence of the nanocrystals and the growth of bigger nanoparticles by reducing the surface tension. In this sense, the nanoparticle size and surface effects should also be taken into account in order to understand the magnetic behaviour of these nanocomposites. It is reported that the saturation magnetization of nanoparticles was much lower than that of the correspondent bulk sample, and decreased with the reduction of the particle size (Martinez et al. 1998; Berkowitz et al. 1968; Parker et al. 1993). In our experiments, the saturation magnetization of Fe–Co alloy and cobalt-substituted magnetite nanocomposite synthesized with trimethylamine solution and tetramethyl ammonium hydroxide at room temperature was 63.9 and 83.5 emu/g, which are smaller than that of the composite prepared with potassium hydroxide (91.7 emu/g). The magnetic behavior of the nanocomposite is very sensitive to the crystallinity and nanoparticle sizes (Lopez-Quintela and Rivas 1993). The smaller particle size may result in the decrease of saturation magnetization, as the presence of shape anisotropy can significantly enhance the magnetic properties (Pelecky and Rieke 1996). The other samples have the sample laws. It may be elucidated that the magnetic domains of the nanocomposites increased with the growth of grains, because the growth of nanoparticles is faster in inorganic alkaline solutions. The magnetic parameters of nanocomposite variation with different alkaline solutions are listed in Table 3.
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Fig. 5

The magnetic hysteresis curves measured at room temperature for FexCo1−x/CoyFe3−yO4 composites synthesized with various alkaline solutions: (a) N(CH3)3, (b) N(CH3)4OH, (c) KOH, and (d) N2H4 · H2O

Table 3

The magnetic parameters of composite variation with different alkaline solutions

Precipitator

Ms (emu/g)

Hc (Oe)

Mr (emu/g)

N(CH3)3

63.9

1347

23.8

N(CH3)4OH

83.5

1738

32.6

KOH

91.7

1160

35.5

N2H4 · H2O

180

310

15.6

Morphology

FE-SEM images of the composites derived from different precipitators are shown in Fig. 6. From FE-SEM photos of Fig. 6c, the crystallites are composed of octahedral and small particles with crystallite sizes of ca. 500 and 80–100 nm. In an inorganic alkaline aqueous solution (Fig. 6d), quick nucleation and quick growth rate may lead directly to the size distribution in a broader scope, and good crystallization, regular and monodispersed equilateral octahedral morphology, with the crystallite sizes of 1–5 μm as can be discerned from Fig. 6d. With different alkaline solutions, the transformation from spheres to octahedra was observed. The types of alkaline solutions in the precursor solutions have been found to be very important for the structure of the composites.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9495-8/MediaObjects/11051_2008_9495_Fig6_HTML.jpg
Fig. 6

SEM images of FexCo1−x/CoyFe3−yO4 composites derived from different precipitators. (a) N(CH3)4OH, (b) N2H4 · H2O, (c) (CH3)3N, and (d) KOH

The structures of the FexCo1−x/CoyFe3−yO4 composites have been further examined by TEM. From the TEM image of Fig. 7b, we can observe hexagons with diameters of about 50–90 nm and irregular small particles with diameters of about 15 nm. The hexagons are the projections of octahedra. The projections of octahedra are parallelograms or hexagons because of the different view angles. Both of them may be spinel phase. The metal alloys could be in the inner (Fig. 7b) region of spinel particles, so that they can consist in the sample steadily. The inset in Fig. 7a, b is the electronic diffraction image, and the nature of single crystal can be revealed. It is the same as the octahedra in Fig. 7d. The sample synthesized with tetramethyl ammonium hydroxide as precipitator is basically a spherical monodisperse nanoparticle (see Fig. 7a). The above discussions prove that the precipitator has an effect on the morphologies of the FexCo1−x/CoyFe3−yO4 composites.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9495-8/MediaObjects/11051_2008_9495_Fig7_HTML.jpg
Fig. 7

TEM images of FexCo1−x/CoyFe3−yO4 composites obtained with different precipitators (a) N(CH3)4OH, (b) N2H4 · H2O, (c) (CH3)3N, and (d) KOH

Conclusions

A simple straightforward synthetic method has been developed for the preparation of FexCo1−x/CoyFe3−yO4 composite by changing the hydrothermal conditions. The transformation of the microstructural composite is determined by the types of precipitators. For single-phase FexCo1−x/CoyFe3−yO4 composites, spherical nanoparticle is favorable over other crystal forms, and the nanocomposite has high saturation magnetization (83.5 emu/g) and moderate coercivity (1,738 Oe). The magnetic properties of the nanocomposites were depended on their morphologies. All samples showed outstanding soft-magnetic properties at room temperature; the spherical particles, especially, displayed widespread application potentials in the biological field.

Acknowledgments

This work is supported by the National Natural Science Foundation of China.

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© Springer Science+Business Media B.V. 2008