Controllable synthesis and magnetic property of Fe/Fe3O4

polyhedron synthesized by solvothermal method

Qin Wang • Wenjing Jia • Jing Guo •

Jun Zhang

Received: 18 October 2011 / Accepted: 10 January 2012 / Published online: 17 January 2012

� Springer Science+Business Media, LLC 2012

Abstract Fe/Fe3O4 nano-cubes and nano-octahedrons

have been successfully synthesized by employing a facile

solvothermal method at 180 �C in the presence of ethylene

glycol (EG). Well-defined assembly of uniform Fe/Fe3O4

with an average size of 400 nm could be obtained without a

size-selection process. X-ray diffraction, X-ray photoelec-

tron spectroscopy, scanning electron microscopy and

transmission electron microscopy were used to characterize

the structure and morphology of the products. The mag-

netic properties of Fe/Fe3O4 nanocomposite were measured

by using a vibrating sample magnetometer. The result of

magnetic characterization reveals that the magnetic poly-

hedrons exhibit a ferromagnetic behavior and possess high

saturation magnetization. It is expected that these magnetic

polyhedron with uniform size would have potential appli-

cations in recording media and electrode materials.

1 Introduction

Magnetic materials with controllable size and shape have

attracted considerable attention because of their novel

morphology-dependent properties and their relevant

applications including biosensing, catalysis, and data stor-

age [1–5]. Much of the existing research has paid more

attention to the synthesis and morphological organization

of phase-pure magnetic materials [6–10]. Among magnetic

materials, Fe/Fe3O4 composite system has attracted much

attention due to its favorable magnetoelectric and transport

properties [11–15].

Considerable efforts have been expanded in the gener-

ation of nanoscale structures of magnetic composite, using

a variety of techniques such as sol–gel techniques, chem-

ical precipitation, hydrothermal approaches, solid-state

reaction, and forced hydrolysis [16–19]. However, it is still

a great challenge to develop simple and reliable synthetic

methods for the magnetic composite materials with defined

sizes, chemical components and controlled morphologies,

which strongly affect their magnetic properties.

In the experiments reported herein, we demonstrate a

simple method to synthesize Fe/Fe3O4 polyhedron in the

presence of EG via a solvothermal pathway to control their

morphologies. The results prove that the solvothermal

treatment is an effective way for the synthesis of high

quality nano-cubes and nano-octahedrons. This method

may not only provide a potential method for the control

over nanostructures, morphologies, and particle size, but

also contribute to the development of functional magnetic

materials, in particular biocompatible magnetic materials

for applications in biology and medicine as diagnostic and

therapeutic tools.

2 Experimental section

In a typical synthesis, 1 g FeCl2�4H2O was dissolved under

N2 atmosphere in 35 ml of ethylene glycol with vigorous

stirring. Then 10 ml of KOH was added dropwise. The

mixture was stirred continually for another 30 min and

then transferred into a 50 ml Teflon lined stainless steel

autoclave, sealed and maintained at 180 �C for 3 h. After

completion of the reaction, the black solid products were

Q. Wang (&) � W. Jia � J. Guo � J. Zhang (&)

College of Chemistry and Chemical Engineering,

Inner Mongolia University, Hohhot 010021,

Peoples Republic of China

e-mail: qinwang86@sina.com

J. Zhang

e-mail: cejzhang@imu.edu.cn

123

J Mater Sci: Mater Electron (2012) 23:1527–1532

DOI 10.1007/s10854-012-0623-y

collected by magnetic separation and washed several times

with water and ethanol. The final products were dried in a

vacuum oven at 50 �C for 8 h.

The crystalline structure of the composite was identified

using X-ray diffractometer (XRD, Cu-Ka, k = 0.15405

nm, Tokyo). X-Ray photoelectron spectroscopy (XPS)

measurements were performed in a VG Scientific ESCA-

LAB Mark II spectrometer equipped with two ultrahigh-

vacuum (UHV) chambers. The morphologies and structures

of the as synthesized composites products were observed

with a scanning electron microscopy (SEM) and a trans-

mission electron microscope (TEM) (JEOL-2010, 200 kV).

All the samples for the SEM characterization were prepared

by directly transferring the suspended products to the ITO

glass slide and standard copper grid coated with an amor-

phous carbon film, respectively. Before the samples were

withdrawn, the composites dispersed ethanol solutions were

sonicated for 40 min to obtain the 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.

3 Results and discussion

The crystal structures of the samples were checked by

XRD. The XRD patterns of Fe/Fe3O4 composites derived

from different concentrations of KOH in EG are displayed

in Fig. 1. The obvious diffraction peaks at 2h = 44� can be

assigned to the bcc structure of the Fe alloy. The other

diffraction peaks can be readily indexed to a face-centered

cubic structure of magnetite with lattice parameters

a = 8.380–8.399 A that is very close to the reported data

(JCPDS 85-1436, a = 8.40 A). The lattice parameters,

interplanar spacing, and average diameter determined using

X-ray diffraction and TEM are shown in Table 1. As can

be seen from Table 1, with the increase of the concentra-

tion of KOH, the average diameter of the composites is not

changed significantly. Meanwhile, the positions and rela-

tive intensities of these reflection peaks agree well with

those of the Fe3O4 nanoparticles in the literature [20–22].

The strong and sharp diffraction peaks suggest that the as-

prepared Fe/Fe3O4 nanocomposites are well crystallized.

To further prove the composition of the products, as-

obtained Fe/Fe3O4 nanocomposites were examined by

XPS. Their spectra are shown in Fig. 2a and b, corre-

sponding to the binding energies of Fe2p and O1s. The

figure shows that the peaks located at 724.8 and 710.8 eV

correspond to Fe2p1/2 and Fe2p3/2, and the peaks of 530 eV

could be attributed to O1 s. The data are also consistent

with the reported values of Fe3O4 in the literature [23]. No

obvious iron alloy was detected in the nanocomposite due

to the high binding energies of Fe 2p electrons, which are

hardly emitted with X-ray when they are underneath the

composites.

The morphology of the products was examined by SEM

and TEM. Figure 3 are typical SEM images of the prod-

ucts, clearly showing that the Fe/Fe3O4 synthesized in this

work possesses cubic and octahedral structure. And with

the increase of KOH quantity, the transformation from

cubes to octahedrons is observed (Fig. 3c, d). The quantity

of the KOH in the precursor solution is found to be very

important for the morphology and the microstructure. We

believe that KOH behaves not only as a precipitator but

also as a surfactant in the solvothermal process. The growth

of octahedron structures should require a relatively high

chemical potential environment in the solution, that is, a

relatively high concentration of KOH [24–26].

To understand the formation of the Fe/Fe3O4 polyhe-

dron, a series of experiments had been carried out. It is

shown that the concentration of KOH in the reaction sys-

tem is an important factor in determining the morphology

of the product. The TEM images of products with different

20 30 40 50 60 70

(440)

(511)

(422)(110)(400)

(222)

(311)

(d)

(c)

(b)

Inte

nsit

y(a.

u)

(a)

(220)

Fe

Fig. 1 XRD patterns of the Fe/Fe3O4 composites derived from

different concentrations of KOH in EG synthesized at 180 �C for 3 h:

(a) 0.5 mol L-1, (b) 1 mol L-1, (c) 3 mol L-1, and (d) 5 mol L-1

Table 1 KOH quantity, lattice parameters, interplanar spacing, and

diameter determined using X-ray diffraction and TEM

KOH quantity

(mol L-1)

a (A) d311 DX-ray

(nm)

DTEM

(nm)

0.5 8.399 2.5324 46.9 250

1.0 8.395 2.5313 27.4 200

3.0 8.380 2.5267 69.7 500

5.0 8.399 2.5326 43.8 600

1528 J Mater Sci: Mater Electron (2012) 23:1527–1532

123

concentrations of KOH are shown in Fig. 4. As shown in

the TEM images, the solvothermal treatment based on the

ferrous ions disproportionation methods resulted in the

cubic and octahedral morphologies with average diameter

of 400 nm. And when the concentration of KOH is

0.5 mol L-1, only particles are obtained, but closer

inspection revealed that the particles have the trend to form

cubes. According to the experimental results, Fe/Fe3O4

Fig. 3 SEM images of Fe/Fe3O4 derived from different concentrations of KOH in EG: a 0.5 mol L-1, b 1 mol L-1, c 3 mol L-1, and

d 5 mol L-1

700 710 720 730 740

Fe2pFe2p1/2

Cou

nts/

s

Binding Energy (eV)

(a)Fe2p

3/2

525 530 535 540 545

Cou

nts(

s)

Binding Energy (eV)

O1s(b)

Fig. 2 XPS of the Fe/Fe3O4 nanocomposite: a expanded spectra of Fe2p and b expanded spectra of O1s

J Mater Sci: Mater Electron (2012) 23:1527–1532 1529

123

nanocubes will be formed when the concentration of KOH

range from 1 to 3 mol L-1, and the concentration of

3.0 mol L-1 is optimal for the growth of Fe/Fe3O4 nano-

cubes. With the increase of the concentration of KOH,

octahedrons and hexagons morphologies began to appear in

the products (shown in Fig. 4d). The projection of octa-

hedron is hexagons because of the different view angles

(Fig. 4d). The concentration of OH- and the growth rate

along [100] or [111] planes, plays a key role in determining

the final morphology of the particles [27]. With the

increasing of OH-, the shape of the particles undergoes an

evolution from a cube to an octahedron. That is to say the

nucleation and growth rate along [100] or [111] planes and

the final morphologies of Fe/Fe3O4 are mainly decided by

the concentration of KOH. A possible illustration of the

formation mechanism for the Fe/Fe3O4 polyhedron is

suggested as Scheme 1.

The magnetic properties of the Fe/Fe3O4 composites,

which are of importance for practical applications, were

investigated with a VSM at room temperature. Figure 5

shows magnetic hysteresis curve measured at room temper-

ature for the sample synthesized with KOH = 0.5 mol L-1.

The hysteresis loop of the Fe/Fe3O4 nanocomposites exhibits

a ferromagnetic behavior with saturation magnetization

(Ms), remanent magnetization (Mr), and coercivity (Hc)

values of ca.78, 37 emu/g and 1,300 Oe, respectively. The

saturation magnetization is higher than those of Fe3O4

nanopyramid (52.5 emu/g), nanoparticles (68.7 emu/g) and

Scheme 1 Schematic illustration of the growth process of Fe/Fe3O4

polyhedron (EG stands for ethylene glycol)

Fig. 4 TEM images of Fe/

Fe3O4 derived from different

concentrations of KOH in EG:

a 0.5 mol L-1, b 1 mol L-1,

(c) 3 mol L-1, and d 5 mol L-1

1530 J Mater Sci: Mater Electron (2012) 23:1527–1532

123

nanowires (71 emu/g) [28–30]. The additional increase in Ms

is consistent with and can be attributed to the presence of an

extra 10% amount of Fe in Fe/Fe3O4 nanocomposites. We

calculated the percentage of Fe in Fe/Fe3O4 through Moss-

bauer spectrum. Table 2 shows the Mossbauer spectra

parameters of sample measured at room temperature. It

should be apparent from the mechanism of the composite

formation that the system consists of ferromagnetic (a-Fe)

and ferromagnetic (Fe3O4) forming a composite structure.

This means that there would be exchange coupling between

the a-Fe and Fe3O4. The relative concentrations of the dif-

ferent phases were calculated from the corresponding reso-

nance areas. We can come to the conclusion that the

percentage of the Fe in Fe/Fe3O4 is about 10% by Mossbauer

spectrum calculations. In order to confirm the calculation

result, we also calculated the percentage of the Fe by TG–

DTA curves. The composites have been analyzed by using

TG measurements in air. They do not oxidize below 400 �C

in air although they contain metallic iron and Fe(II) in the

spinel phase (Fig. 1). They correspond to the transformation

of the composites into a mixture of FeO, Fe2O3 and Fe3O4.

When the metal composition is undoubtedly known, the

formulae can be calculated: (Fe0.13)/[Fe0.29Fe0.58O4]. The

calculated result indicates that the percentage of ion in the

system is about 13%. The results mentioned above two

methods matched well with each other. And according to the

following formula, we calculated the magnetization of the

composites and come to the conclusion that the magnetiza-

tion of the Fe/Fe3O4 is about 90 emu/g.

Ms ¼ WFe �Ms Fe þWFe3O4�Ms Fe3O4

where Ms is the saturation magnetization (a unit of emu/g)

and W is the percentage of every compound in the

composite.

The magnetic properties of our synthesis of nanocom-

posites 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

in the composite. It should be apparent from the mecha-

nism of the composite formation that the system is con-

sisted of ferromagnetic (Fe) and ferromagnetic (Fe3O4)

[10]. This means that there would be exchange coupling

between the Fe and Fe3O4. Therefore, the content of the Fe

has very important impact on the magnetic properties of the

Fe/Fe3O4 composite. The magnetic parameters tested by

vibrating sample magnetometer at room temperature for

Fe/Fe3O4 composites derived from different concentration

of KOH are listed in Table 3. Table 3 shows that the Fe/

Fe3O4 composite synthesized at 180 �C for 3 h with

[KOH] = 0.5 mol L-1 has the maximal coercive field of

1300 Oe corresponding to the maximal remnant magneti-

zation of 37.0 emu/g. The high symmetry of crystal will

lead to low crystalline anisotropy, which is one reason for

cubic sample to have lower coercivity relative to the

irregular sample. Although the detailed reasons are not

clear enough, the coercivity of Fe/Fe3O4 are influenced by

many factors, such as size, structure, and morphologies

et al. The different coercivity in this work may be mainly

caused by single-crystalline structure and anisotropy

including crystalline anisotropy and shape anisotropy.

Table 2 The Mossbauer

spectra parameters of sample

measured at room temperature

Sublattice IS (mm/s) QS (mm/s) Hin (Koe) HWHM (mm/s Area ratio (%)

A 0.65 ± 0.01 0 471.7 ± 1 0.66 ± 0.01 45.4

B 0.33 ± 0 0.05 ± 0.01 491.4 ± 0.5 0.31 ± 0 54.6

Table 3 Magnetic properties of the produced Fe/Fe3O4 composites

derived from different concentration of KOH

KOH quantity (mol L-1) Ms (emu/g) Hc (Oe) Mr (emu/g)

0.5 78 1,300 37

1.0 97 280 26

3.0 96 500 17

5.0 97 390 15

The contents of KOH are in the range of 0.5–5

-10000 -5000 0 5000 10000

-80

-60

-40

-20

0

20

40

60

80

100

Ms

(em

u/g)

Hc (Oe)

Fig. 5 Magnetization hysteresis curves measured at room temperature

for the Fe/Fe3O4 nanocomposites synthesized with KOH = 0.5 mol L-1

J Mater Sci: Mater Electron (2012) 23:1527–1532 1531

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4 Conclusion

In summary, a facile solvothermal method has been used to

synthesize Fe/Fe3O4 polyhedron with an average size of

400 nm in the presence of EG. The concentration of the

KOH in the precursor solution is found to be very impor-

tant for the structure, morphology and magnetic properties

of the samples. The as-prepared Fe/Fe3O4 nanocomposites

show relatively high saturation magnetization (78 emu/g),

which possibly show potential applications in recording

media and electrode materials. The present approach pro-

vides a convenient and effective way to prepare other

composites polyhedron with a high versatility for adjusting

the size and inner structure of the particle.

Acknowledgments This work is supported by Program of Higher-

lever talents of Inner Mongolia University (SPH-IMU-Z20100107),

National High Technology Research and Development Program (863

program, 2010AA03A407), National Natural Science Foundation of

China (20961005), Key Project of National Natural Science Foun-

dation of Inner Mongolia (2010Zd03).

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