Key Engineering Materials
ISSN: 1662-9795, Vol. 633, pp 11-16
doi:10.4028/www.scientific.net/KEM.633.11
© 2015 Trans Tech Publications, Switzerland
Online: 2014-11-07
Characterization of Co-modified γ-Fe2O3 Based
Composite Nanoparticles
Junming Lia, Jian Lib, Longlong Chen, Xiaomin Gong and Hong Mao
School of Physical Science and Technology, Southwest University, Chongqing, 400715, P. R. China
a
[email protected], [email protected] (corresponding author)
Key words: chemical synthesis; γ-Fe2O3 nanoparticles; Co-modification; magnetism
Abstract. During the synthesis of γ-Fe2O3 nanoparticles using a chemically-induced transition in a
FeCl2 solution, Co-surface modification was attempted by adding Co(NO3)2 and NaOH to the
solution. The magnetization behaviors, morphologies, crystal structure, and chemical compositions of
the as-prepared samples were characterized using vibrating sample magnetometry, transmission
electron microscopy, X-ray diffractometry, energy dispersive X-ray spectroscopy, and X-ray
photoelectron spectroscopy. The as-prepared particles consisted of γ-Fe2O3/CoFe2O4 composite
crystallite and a CoCl2·6H2O coating. The molar, mass and volume ratios of the phases were
estimated from the characterization results for each sample. The Co-modified γ-Fe2O3 nanoparticles’
anisotropic constant is approximately 1.48×10-1 J/cm3. Their coercivity depends on the size of
composite crystallites, which is based on the γ-Fe2O3/CoFe2O4 content rather than the Co content.
Introduction
Magnetic nanoparticles of diameter less than 100 nm represent an important class of artificial
nanostructural materials, and have attracted increasing interest in fundamental science and in
technological applications [1,2]. A nanocomposite is a material composed of two or more phases, in
which at least one phase is on the scale of nanometers. The combination of different physical or
chemical properties may lead to completely novel materials [3]. For instance, for surface Co-modified
γ-Fe2O3 particles used as a recording media, the Co-containing surface layer will induce a coercivity
enhancement without the loss of saturation magnetization in the γ-Fe2O3 core [4]. In addition,
magnetic nanocomposites have applications that range from ferrofluids to separation science and
technology [5].
Magnetization (moment per unit volume) M is an important physical parameter used to
characterize magnetic materials. In practice, the volume of a particle Vp is too difficult to measure
directly, so the magnetization is obtained usually from M=σ·ρ, where σ is the specific magnetization
(moment per unit mass) and ρ is the known density of the material [6]. The volume fraction of
particles in ferrofluids, as described by φv= Vp/(Vp+Vc), where Vp is the volume of the particles and
Vc is the volume of the carrier liquid, is also an important characteristic parameter to which the
behavior of ferrofluids is related [7]. Therefore, the volume of the particles is an important feature for
magnetic nanoparticles and is generally obtained from an accurate measure of the mass m and the
known density ρ, i. e. Vp=m/ρ. However, for composite nanoparticles, the density of the particles is
no longer uniform, so that their volume cannot be determined by measuring just their mass.
Generally, Co surface modifications of γ-Fe2O3 particles are accomplished in a post-synthesis step
[8]. Recently, we developed a chemical-induced transition to synthesize γ-Fe2O3 nanoparticles [9].
Using this method, Zn-surface modification of γ-Fe2O3 nanoparticles were accomplished during
synthesis [10]. In the present work, Co-surface modification of γ-Fe2O3 nanoparticles were attempted.
The features of as-prepared products were studied with characterization techniques.
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications, www.ttp.net. (#69816828, Pennsylvania State University, University Park, USA-18/09/16,18:37:50)
12
Testing and Evaluation of Inorganic Materials V
Fig. 1 Specific magnetization curves at room temperature for samples (1) and (2).
Fig. 2 Typical TEM images of samples (1) and (2). The insets are HRTEM images.
Experimental
The preparation of the nanoparticles using chemically-induced transitions can be divided into two
steps. First, the precursor was synthesized using coprecipitation of both Fe3+ and Mg2+. The detailed
preparation process and composition of the precursor was described previously [9]. In the second
step, the precursor was added to the FeCl2 solution (400 mL, 0.25 M), which was added at the boiling
point and the solution was boiled for 20 minutes. Then, the Co(NO3)2 solution (50 mL, 1M) was
added to the boiling FeCl2 solution, and the mixture solution was boiled for 10 min. Finally, the 0.7 M
NaOH solution was added and the solution was boiled for
another 10 min. Samples (1) and (2) were prepared by using
20 mL and 40 mL of the NaOH solution, respectively.
The specific magnetization curves of the samples were
measured with a vibrating sample magnetometer (VSM,
HH-15), with alternating magnetic field cycles at room
temperature. The morphologies, crystal structures, and bulk
and surface compositions were characterized using transition
electron microscopy (TEM, Tecnai G20 ST), X-ray
diffractometry (XRD, D/Max-RC), energy dispersive X-ray
spectroscopy (EDS, Quarta-200), and X-ray photoelectron
spectroscopy (XPS, Thermo ESCA 250).
Results and analysis
Specific magnetization curves of the samples are shown in
Fig. 1. It can be seen that these samples are ferromagnetic.
The specific saturation magnetization σs is estimated from a
Fig. 3 XRD spectra of samples
(1) and (2).
Key Engineering Materials Vol. 633
13
Table. 1 Atomic percentage ai from EDS
Sample
Fe
Co
Cl
Co/Cl
1
96.57
1.36
2.07
0.66
2
93.45
3.50
3.05
1.15
plot of σ vs. 1/H in the high field region (H≥8.5 KOe)
[11]. The values of σs are 46.7 and 55.8 emu/g for
samples (1) and (2), respectively.
The TEM observation indicated that all samples
consisted of nearly spherical nanoparticles, as shown
in Fig. 2. HRTEM revealed these particles to be single
crystallites, as shown in the inset of Fig. 2.
Fig. 3 shows XRD spectra, which show that these
samples possess ferrite-like spinel structures with
Fig. 4 XPS spectra of samples (1) and (2).
γ-Fe2O3 (PDF # 24-0081) and CoFe2O4 (PDF #
22-1086) features with trace CoCl2·6H2O (PDF # 29-0466). For spinel ferrites, the average grain size
dc can be estimated from the (311) peaks using Scherer’s formula [12]: dc= kλ/βcosθ, where k is a
constant (k=0.89), λ is the wavelength of the X-ray (Cu-kα=0.1542 nm), θ is the Bragg diffraction
angle of the (311) plane, and β is the full-width at half-maximum of the (311) peak. For the two
samples, their dc are about 11.02 nm and 11.05 nm, respectively.
EDS measurements revealed that the samples were composed of O, Fe, Co, and Cl. The atomic
percentages of Fe, Co, and Cl, ai, are listed in Table 1. It can be seen that the amount of Co in sample
(2) is more than that in sample (1).
Fig. 5 XPS results of O1s (a) and Co2p3/2 (b).
Table. 2 Binding energy data from XPS (eV)
O1s
Co2p3/2
Fe2p3/2
Cl2p
P1
P2
P1
P2
532.8
530.5
711.0
783.1
779.7
198.31
sample (1)
532.2
529.9
710.3
783.2
779.9
198.49
sample (2)
529.5
710.9
γ-Fe2O3
529.8
710.8
779.9
CoFe2O4
CoCl2·6H2O
782.9
?
H2O
532.6
Note: Standard data for Fe2O3, CoFe2O4, CoCl2·6H2O, and H2O from the NIST online database for
X-ray photoelectron spectroscopy at www.nist.gov, but no data for Cl2p in CoCl2·6H2O is given.
14
Testing and Evaluation of Inorganic Materials V
XPS measurements confirm the existence of the same chemical elements for each sample as was
determined by EDS. The XPS spectra are shown in Fig. 4. By resolving the Co2P3/2 lines, it can be
determined from the binding energy data that the samples are CoFe2O4 and CoCl2. In addition, the
O1s data show that there is molecular H2O in the samples. The molecular H2O may correspond to the
H2O in CoCl2·6H2O and / or adsorbed water on the samples. Co2P3/2 and O1s lines are shown in Fig.
5. The binding energy data for all elements are listed in Table 2.
Fig. 6 The formation process of Co-modified γ-Fe2O3 nanoparticles.
Discussion
The experimental results and analysis indicate that, for Co-modification during the synthesis with the
addition of Co(NO3)2-NaOH, γ-Fe2O3 crystallites were formed first, which is attributed to the
precursor transforming in the FeCl2 solution [9]. When Co(NO3)2-NaOH was added to the solution,
CoFe2O4 grew epitaxially on the γ-Fe2O3 core to produce γ-Fe2O3/CoFe2O4 composite crystallite with
a spinel structure. Such composite crystallite is difficult to distinguish from γ-Fe2O3 or CoFe2O4 by
XRD, but can be revealed by also using XPS. FeCl3 was then adsorbed on the composite crystallites.
Accordingly, the formation process of Co-modified γ-Fe2O3 nanoparticles can be depicted
schematically, as shown in Fig. 6.
From measured atomic percentages of Fe, Co, and Cl by EDS (respectively represented by aFe, aCo,
and aCl), the molar percentages of γ-Fe2O3, CoFe2O4, and CoCl2·6H2O phases (respectively
represented by yγ, yCo, and yCl) can be estimated by
[a Fe − (aCo − aCl 2) × 2] 2
[a Fe − (aCo − aCl 2) × 2] 2 + (aCo − aCl 2) + aCl
yγ =
yCo =
yCl =
2
× 100
[aFe − (aCo − aCl
aCo − aCl 2
×100
2) × 2] 2 + (aCo − aCl 2) + aCl 2
[aFe − (aCo − aCl
aCl 2
× 100
2) × 2] 2 + (aCo − aCl 2) + aCl 2
(1)
From the molar percentage results of each phase, yi, the mass fraction and volume fraction of the
phase, respectively represented by zi and φi, can be deduced from
zi =
yA
× 100
∑y A
(2)
z ρ
∑z ρ
(3)
i
i
i
i
φi =
i
i
i
×100
i
where Ai is the molecular weight and ρi is the density for the i phase. Accordingly, the molar, mass,
and volume percentages of the γ-Fe2O3, CoFe2O4, and CoCl2·6H2O phases in the samples are
calculated, as listed in Table 3. Furthermore, the average density of each sample <ρ> can be obtained
by
Key Engineering Materials Vol. 633
15
Table. 3 Molar, mass, and volume percentages of the phases
Sample (1)
Sample (2)
γ-Fe2O3 CoFe2O4
CoCl2·6H2O γ-Fe2O3 CoFe2O4 CoCl2·6H2O
Molar percentage (%) y
97.24
0.65
2.11
92.75
4.08
3.17
Mass percentage (%) z
94.96
0.94
3.10
89.64
5.80
4.56
94.58
0.84
7.58
84.07
4.98
10.95
Volume percentage (%) φ
Table. 4 Average density, aturation magnetization, and anisotropy constant for as-prepared samples.
Ms [kA/m]
Ka [10-1 J/cm3]
<ρ> [g/cm3]
Sample (1)
4.64
216.83
1.49
Sample (2)
4.56
254.40
1.48
ρ = ∑ φi ρ i 100
(4)
Consequently, saturation magnetization of each sample Ms (= <ρ>σs) can be obtained. The <ρ> and
Ms values are listed in Table 4.
For magnetic nanoparticles, the dependence of coercivity Hc on anistropic constant Ka can be
described as
1

k T  2
H c = 2 K a 1 − 5 B  

 K aV  

 

µ0 M s
(5)
where kB is the Boltzman constant, T is the absolute temperature, V is the volume of the particle, and
µ 0 is the vacuum permeability [13,14]. Assuming the particles are spherical, and its magnetic
diameter is dc and T=300K, the anisotropy constant Ka for each sample can be determined, as listed in
Table 4. The calculated Ka value of sample (2) is slightly smaller than that of sample (1). This
difference could be the result from the error in V (=πdc3/6) from dc. As a consequence, it was
concluded that, for the Co-modified γ-Fe2O3 nanoparticles, their coercivity depends on the size of the
spinel structure composite crystallites rather than the Co content, which is similar to the finding of
Engle and Mallinso [15].
Conclusion
During the synthesis of γ-Fe2O3 nanoparticles using a chemically-induced transition in a FeCl2
solution by adding Co(NO3)2 and NaOH to the solution, CoFe2O4 grew epitaxilly on the γ-Fe2O3
crystallites to yield composite spinel structure crystallites. Co2+ and Cl-1 adsorbed onto these
γ-Fe2O3/CoFe2O4 composite crystallites and formed nanoparticles with core-shell structures, which
consisted of three parts as follows: a γ-Fe2O3 core, a CoFe2O4 expitaxial layer, and an outermost
CoCl2·6H2O layer. The phase ratios can be obtained from the element content measurements, and
then the average particle density <ρ> and saturation magnetization can be determined. While the
content of Co(NO3)2 is fixed in the treating solution, the amounts of both CoFe2O4 and CoCl2·6H2O
phases will increase with additional NaOH. For the Co-modified γ-Fe2O3 nanoparticles, their
anisotropic constant Ka is about same, and the coercivity Hc depends on the size of the
γ-Fe2O3/CoFe2O4 composite crystallite rather than on the Co content.
Acknowledgement
Financial support for this work was provided by the National Science Foundation of China (grant No.
11074205)
16
Testing and Evaluation of Inorganic Materials V
References
[1] C.R. Lin, C.C. Wang, I.H. Chen, Magnetic behavior of core–shell particles, J. Magn. Magn.
Mater. 304 (2006) e34-e36.
[2] S. Sun. Recent Advances in Chemical Synthesis, Self-Assembly, and Applications of FePt
Nanoparticles, Adv. Mater. 18 (2006) 393-403.
[3] D.V. Szabό, D. Vollath, Nanocomposites from Coated Nanoparticles, Adv. Mater. 11 (1999)
1313-1316.
[4] J. Nogués, J.Sort, V. Langlais, V. Skumryev, S. Suriñach, J.S. Muñoz, M.D. Barό, Exchange bias
in nanostructures, Phys. Rep. 422 (2005) 65-117.
[5] Q.X. Liu, Z.H. Xu, J.A. Finch, R. Egerton, A Novel Two-Step Silica-Coating Process for
Engineering Magnetic Nanocomposites, Chem. Mater. 10 (1998) 3936-3940.
[6] J. Crangle, The Magnetic Properties of Solids. Grat Britain, Edward Arnold, 1977.
[7] B. Huke, M. Lücke, Magnetic properties of colloidal suspensions of interacting magnetic
particles, Rep. Prog. Phys. 67 (2004) 1731-1768.
[8] M.P. Sharrock, Particulate Magnetic Recording Media: A Review, IEEE. Trans. Magn. 25 (1989)
4374-4389.
[9] B.C. Wen, J. Li, Y.Q. Lin, X.D. Liu, J. Fu, H. Miao, Q.M. Zhang, A novel preparation method for
γ-Fe2O3 nanoparticles and their characterization, Mater. Chem. Phys. 128 (2011) 35-38.
[10] L.L. Chen, J. Li, Y.Q. Lin, X.D. Liu, L.H. Lin, D.C. Li, Surface modification and
characterization of γ-Fe2O3 nanoparticles synthesized by chemically-induced transition, Mater.
Chem. Phys. 141 (2013) 828-834.
[11] R. Arulmurugan, G.Vaidyanathan, S. Sendhilnathan, B. Jeyadevan, Co–Zn ferrite nanoparticles
for ferrofluid preparation: Study on magnetic properties, Physica. B. 363 (2005) 225-231.
[12] T. Sato, T. Iijima, M. Seki, N. Inagaki, Magnetic properties of ultrafine ferrite particles, J. Magn.
Magn. Mater. 65 (1987) 252-256.
[13] C.P. Bean and J.D. Livingston, Toward a model for Co-surface-treated Fe-oxides, J. Appl. Phys.
30 (1959) 120s-129s.
[14] F.E. Luborsky and T.O. Paine, Superparamagnetism, J. Appl. Phys. 31 (1960) 68s-70s.
[15] D.F. Eagle and J.C. Mallinson, On the Coercivity of γ-Fe2O3 Particles, J. Appl. Phys. 38 (1967)
995-997.