Hindawi Publishing Corporation
Journal of Chemistry
Volume 2016, Article ID 7604748, 9 pages
http://dx.doi.org/10.1155/2016/7604748
Research Article
Preparation of Magnetic Nanoparticles via a Chemically
Induced Transition: Presence/Absence of Magnetic Transition on
the Treatment Solution Used
Yanshuang Chen, Qin Chen, Hong Mao, Yueqiang Lin, and Jian Li
School of Physical Science and Technology, Southwest University, Chongqing 400715, China
Correspondence should be addressed to Jian Li; [email protected]
Received 26 November 2015; Accepted 13 December 2015
Academic Editor: José L. A. Mediano
Copyright © 2016 Yanshuang Chen et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
The dependence of magnetic transition on the treatment solution used in the preparation of magnetic nanoparticles was investigated
using as-prepared products from paramagnetic FeOOH/Mg(OH)2 via a chemically induced transition. Treatment using FeCl3 and
CuCl solutions led to a product that showed no magnetic transition, whereas the product after treatment with FeSO4 or FeCl2
solutions showed ferromagnetism. Experiments revealed that the magnetism was caused by the ferrimagnetic 𝛾-Fe2 O3 phase
in the nanoparticles, which had a coating of ferric compound. This observation suggests that Fe2+ in the treatment solution
underwent oxidation to Fe3+ , thereby inducing the magnetic transition. The magnetic nanoparticles prepared via treatment
with an FeSO4 solution contained a larger amount of the nonmagnetic phase. This resulted in weaker magnetization even
though these nanoparticles were larger than those prepared by treatment with an FeCl2 solution. The magnetic transition of the
precursor (FeOOH/Mg(OH)2 ) was dependent upon treatment solutions and was essentially induced by the oxidation of Fe2+ and
simultaneous dehydration of FeOOH phase. The transition was independent of the acid radical ions in the treatment solution, but
the coating on the magnetic crystallites varied with changes in the acid radical ion.
1. Introduction
Nanotechnology involves the understanding and control of
matter with dimensions of roughly 1–100 nm [1]. Magnetic
nanoparticles have attracted increasing interest because their
size range enables the investigation of the fundamental
aspects of magnetic-ordering phenomena in materials with
reduced dimensions, and this has the potential to lead to
new technological applications [2, 3]. Studies on magnetic
nanoparticles have focused on the development of simple
and effective methods for the fabrication of nanoparticles
with controlled size, morphology, and properties [4, 5]. Iron
oxide nanoparticles including magnetite (Fe3 O4 ), hematite
(𝛼-Fe2 O3 ), maghemite (𝛾-Fe2 O3 ), goethite (𝛼-FeOOH), and
akaganeite (𝛽-FeOOH) nanoparticles have attracted enormous attention due to their interesting properties [6–10].
Many methods have been developed for the preparation of
𝛾-Fe2 O3 magnetic nanoparticles, including coprecipitation
[11], a gas-phase reaction technique [12], direct thermal
decomposition [13], thermal decomposition-oxidation [14],
sonochemical synthesis [15], a microemulsion technique [16],
hydrothermal synthesis [17], a vaporization-condensation
method [18], and a sol-gel approach [19].
Reactions for the synthesis of oxide nanoparticles using
the coprecipitation method can be grouped into two categories. In the first category, the oxide is produced directly; in
the second category, a precursor is produced initially and then
subjected to further processing (drying, calcination, and subsequent steps) [20]. During the chemical reaction, which is
followed by calcination or annealing, a new phase is formed.
In addition to the transition from amorphous to crystalline,
the particle size increases, and the crystallites aggregate as
the calcination temperature increases [21]. The conventional
aqueous synthesis of 𝛾-Fe2 O3 particles involves three or
more steps [22], but we recently found a new route for the
production of 𝛾-Fe2 O3 magnetic nanoparticles that requires
2
Journal of Chemistry
only two steps. This method involves the preparation of
the paramagnetic FeOOH/Mg(OH)2 precursor via a coprecipitation method, followed by treatment of the hydroxide
precursor in the liquid phase using a ferrous chloride (FeCl2 )
solution [23]. During this treatment, Mg(OH)2 dissolves,
and the paramagnetic FeOOH precursor transforms into
ferrimagnetic 𝛾-Fe2 O3 nanoparticles via dehydration:
Mg (OH)2 󳨀→ Mg2+ + 2OH−
2FeOOH (paramagnetic)
󳨀→ 𝛾-Fe2 O3 (ferrimagnetic) + H2 O
(1)
(2)
This method is referred to as a chemically induced
transition [24, 25]. However, it remains unclear whether such
a magnetic transition can be produced using other solutions.
In the present work, we used aqueous solutions of ferric
chloride (FeCl3 ), cuprous chloride (CuCl), ferrous sulfate
(FeSO4 ), and ferrous chloride (FeCl2 ) as treatment solutions
to investigate the dependence of the magnetic transition on
the treatment solution used and to attempt to explain the
mechanism of the transition in the liquid phase.
2. Materials and Methods
2.1. Chemicals. FeCl3 , Mg(NO3 )2 , NaOH, CuCl, FeSO4 , and
FeCl2 of analytical grade and all other chemicals were used
as received without further purification. Distilled water was
used throughout the experiments.
2.2. Preparation. The precursor was synthesized via coprecipitation. An aqueous mixture of FeCl3 (1 M, 40 mL) and
Mg(NO3 )2 (2 M, with 0.05 mol HCl; 10 mL) was added to
an aqueous NaOH solution (0.7 M, 500 mL). The resulting
solution was then heated to boiling for 5 min under stirring.
The red-brown precursor precipitated gradually during this
process. The precursor was collected and washed with dilute
HNO3 solution (0.01 M) until the pH of the supernatant
liquid was 7-8.
The as-prepared precursor was mixed with the treatment
solution (0.25 M, 400 mL), and the resulting mixture was
allowed to boil for 30 min. The products were then dehydrated with acetone and allowed to air-dry. Treatment with
FeCl3 , CuCl, and FeSO4 solutions produced samples (1), (2),
and (3), respectively. For comparison, treatment was also
performed in an FeCl2 solution to produce sample (0).
2.3. Characterization. Crystal structures and specific magnetization curves were obtained for samples (0)–(3) using
X-ray diffraction (XRD; XRD-7000, Shimadzu, Japan) and
vibrating-sample magnetometry (VSM; HH-15, Nju-yq,
China) at room temperature, respectively. The bulk chemical
species, surface chemical composition, and morphology of
samples (0) and (3) were determined using energy-dispersive
X-ray spectroscopy (EDS; Quanta-200, Genesis, USA), X-ray
photoelectron spectroscopy (XPS; XSAM800, Krator, UK),
and transmission electron microscopy (TEM; Tecnai G20 ST,
FEI, USA), respectively.
Table 1: Atomic percentages (𝑎𝑖 ) determined from EDS measurements performed on samples (0) and (3).
Sample (0)
Sample (3)
𝑎O
60.48
62.35
𝑎Fe
37.93
32.35
𝑎Cl
1.59
𝑎S
𝑎Mg
3.93
1.37
3. Results and Discussion
3.1. Results. XRD spectra are shown in Figure 1 for all of the
samples. The crystal structure of samples (1) and (2) was
different from that of sample (0), whereas the main structure
of sample (3) was identical to that of sample (0), which
corresponded to ferrite-like 𝛾-Fe2 O3 . The results showed that
sample (3) contained MgSO4 ⋅5H2 O and Fe2 (SO4 )3 ⋅11H2 O.
The crystal phase of samples (1) and (2) is difficult to be
distinguished from the XRD spectra, but it can be determined
that both did not contain ferrite-like phase. Specific magnetization curves measured at room temperature are shown for
all samples in Figure 2. The curves for samples (1) and (2)
exhibited similar, apparently paramagnetic, behavior. Similar
to sample (0), sample (3) appeared to be ferromagnetic
having coercivity (see the windows in Figure 2), but its
magnetization was weaker than that of sample (0).
According to the XRD and VSM results, samples (1) and
(2) showed no magnetic transition; in contrast, sample (3)
exhibited a transition similar to that shown by sample (0). The
specific saturation magnetization values (𝜎𝑠 ) for samples (0)
and (3) were determined from a plot of 𝜎 versus 1/𝐻 (where
𝐻 is the strength of magnetic field) in the high-field region,
which was linear for strong magnetic fields [26]. 𝜎 versus 1/𝐻
(𝐻 ≥ 7.5 kOe) plots were represented as inserts of Figure 2,
and the 𝜎𝑠 values were determined by extrapolating plots to
𝜎-axis to be 59.20 and 43.36 emu/g for samples (0) and (3),
respectively.
Figure 3 shows EDS spectra for samples (0) and (3).
Sample (0) contained Fe, O, and Cl, and sample (3) contained
S and Mg, in addition to Fe and O. The quantitative results
are listed in Table 1. XPS measurements revealed the same
chemical species that were identified by the EDS analysis
in samples (0) and (3). The XPS spectra are shown in
Figure 4. The XRD and EDS results suggested that the 𝛾Fe2 O3 phase was dominant in samples (0) and (3). There
was also a Cl-containing phase in sample (0), and two Scontaining phases in sample (3). It is possible that the Clcontaining phase in sample (0) was FeCl3 ⋅6H2 O [25], but
that the content of FeCl3 ⋅6H2 O may be so low that it did
not produce clear diffraction peaks in the XRD spectrum
(see Figure 1). The XRD results also implied that the Scontaining phases in sample (3) were MgSO4 ⋅5H2 O and
Fe2 (SO4 )3 ⋅11H2 O. Accordingly, the Fe2p3/2 lines and O1s lines
observed for the two samples and the S2p3/2 line observed
for sample (3) may be associated with two or more species.
Detailed data on binding energies are listed in Table 2. We
deduced from the experimental results that the binding
energy of Mg1s in MgSO4 ⋅5H2 O was approximately 1304.9 eV.
In addition, it is noticed that the ferrite-like spinel structure,
𝛾-Fe2 O3 and Fe3 O4 , is difficult to discriminate by XRD
3
20
30
40
2𝜃 (deg.)
50
60
70
Intensity (a.u.)
30
40
2𝜃 (deg.)
50
60
50
60
FeCl3 · 6H2 O
30
40
2𝜃 (deg.)
50
60
30
70
40
2𝜃 (deg.)
50
60
70
MgSO4 · 5H2 O
(PDF#25-0532)
20
30
70
40
50
60
70
2𝜃 (deg.)
70
(PDF#33-0645)
20
20
Intensity (a.u.)
40
2𝜃 (deg.)
Intensity (a.u.)
30
60
𝛾-Fe2 O3
10
20
(440)
(422)
(511)
(400)
(202) Cl
(220)
(311)Cl
(311)
50
70
70
𝛾-Fe2 O3
10
40
2𝜃 (deg.)
Intensity (a.u.)
20
(PDF#39-1346)
10
30
60
(PDF#39-1346)
10
Intensity (a.u.)
10
20
50
Intensity (a.u.)
(2)
10
40
2𝜃 (deg.)
(440)
(021) Fe
(130)Fe (111)
Intensity (a.u.)
Intensity (a.u.)
10
30
(151)Mg
(422)
(511)
(3)
(1)
20
(400)
10
70
(212)Mg (311)
60
(111)
Intensity (a.u.)
(440)
(511)
50
(220)
40
2𝜃 (deg.)
(422)
(400)
(220)
30
(111)Mg
20
(0)
(131)Fe
(021) Mg
10
(311)Cl
(111)
Intensity (a.u.)
(0)
(202) Cl
(311)
Journal of Chemistry
10
Fe2 (SO4 )3 · 11H2 O
(PDF#28-0496)
20
30
40
2𝜃 (deg.)
50
60
70
Figure 1: XRD patterns of all samples, with (hkl), (hkl)Cl , (hkl)Mg , and (hkl)Fe corresponding to 𝛾-Fe2 O3 , FeCl3 ⋅6H2 O, MgSO4 ⋅5H2 O, and
Fe2 (SO4 )3 ⋅11H2 O, respectively.
due to peak broadening [27] and by XPS because the data
are very close (see Table 2). However, Fe3 O4 is not very stable
and is sensitive to oxidation [28]. It was found that Fe3 O4
nanocrystallites transformed into 𝛾-Fe2 O3 nanocrystallites
using ferric nitrate treatment [29]. Therefore, it is judged that
the magnetic phase for samples (0) and (3) is 𝛾-Fe2 O3 , rather
than Fe3 O4 or mixed phase of both 𝛾-Fe2 O3 and Fe3 O4 .
TEM observations revealed that sample (3) was identical
to sample (0) and that it consisted of nearly spherical
nanoparticles. Typical TEM images for both samples are
shown in Figure 5. Statistical analysis [30] indicated that the
diameter of the nanoparticles fitted a lognormal distribution.
The median diameters (𝑑𝑔 ) for samples (0) and (3) were
10.55 and 13.16 nm, respectively, and the associated standard
4
Journal of Chemistry
(0)
(1)
0.0
−0.5
−0.05
6
0.00
0.05
H (kOe)
0
𝜎 (emu/g)
60
−30
−60
−10
−5
58
56
54
0.00
0
0
(2)
−6
0.07
0.14
1/H (kOe)
5
−12
−10
10
−5
0
H (kOe)
H (kOe)
5
10
0.5
(3)
0.0
−0.5
−0.05
0.00
0.05
H (kOe)
0
44
𝜎 (emu/g)
𝜎 (emu/g)
25
𝜎 (emu/g)
50
−25
42
0.000
−50
−10
−5
0
H (kOe)
0.125 0.250
1/H (kOe)
5
10
Figure 2: Specific magnetization curves measured at room temperature for all samples.
Fe
(0)
(3)
O
Counts (a.u.)
Fe
Counts (a.u.)
𝜎 (emu/g)
30
12
0.5
𝜎 (emu/g)
𝜎 (emu/g)
60
S
O
Mg
Cl
2
4
6
8
2
Energy (keV)
Figure 3: EDS spectra for samples (0) and (3).
4
Energy (keV)
6
8
Journal of Chemistry
5
(0)
Count (a.u.)
Count (a.u.)
Fe2p3/2
(3)
O1s
Mg1s
Fe2p3/2
O1s
Cl2p3/2
1200 1000 800 600 400
Binding energy (eV)
(0)
720
1200 1000 800 600 400
Binding energy (eV)
0
(0)
716
712
Binding energy (eV)
536
708
534
532
530
528
Binding energy (eV)
716
712
Binding energy (eV)
708
536
(0)
Cl
526
204
202
534
P1
532
530
528
Binding energy (eV)
(0)
526
204
202
200
(0)
Mg
170
168
166
Binding energy (eV)
164
1308
1306
1304
1302
Binding energy (eV)
1300
(3)
Counts (a.u.)
Counts (a.u.)
172
170
168
166
Binding energy (eV)
164
1308
1306
1304
1302
Binding energy (eV)
Figure 4: XPS spectra for samples (0) and (3).
198
Binding energy (eV)
Counts (a.u.)
Counts (a.u.)
172
194
(3)
(3)
174
200
198
196
Binding energy (eV)
Counts (a.u.)
P2
S
174
0
(3)
Counts (a.u.)
Counts (a.u.)
(3)
720
200
Counts (a.u.)
O
Counts (a.u.)
Counts (a.u.)
Fe
200
S2p3/2
1300
196
194
6
Journal of Chemistry
(3)
(0)
100 nm
100 nm
Figure 5: Typical TEM images for samples (0) and (3).
Table 2: Binding energies (eV) of elements, determined from XPS measurements performed on samples (0) and (3).
Sample (0)
Fe2 O3
Fe3 O4
FeCl3
Sample (3)
Fe2 O3
Fe3 O4
Fe2 (SO4 )3
MgSO4
Fe2p3/2
710.81
710.9
710.8
711.3
Fe2p3/2
711.41
710.9
710.8
711.05
O1s
530.19
530.2
530.1
Cl2p3/2
198.84
199
O1s
530.40 (P1);
530.2
530.1
531.65 (P2)
531.6
?
S2p3/2
168.99
Mg1s
1304.92
169.1
168.6
?
Note: the standard data are from the NIST online database for XPS at http://www.nist.gov; there are no O1s and Mg1s binding energies for MgSO4 .
deviation values (ln 𝜎𝑔 , where 𝜎𝑔 is the geometry deviation)
were 0.37 and 0.31, respectively.
3.2. Discussion. Experimental XRD and magnetization curve
measurements indicated that the products after treatment
with an FeCl3 solution were not ferromagnetic. The product
after treatment with an FeSO4 solution exhibited a magnetic
transition, similar to that observed for the products generated
after treatment with an FeCl2 solution. These products mainly
consisted of a ferrimagnetic 𝛾-Fe2 O3 phase. These results
show that the magnetic transition was induced by Fe2+ ,
rather than by the acid radical ions. Additionally, samples
(0) and (3) contained FeCl3 ⋅6H2 O and Fe2 (SO4 )3 ⋅11H2 O,
which, respectively, correspond to the ferrous salts FeCl2 and
FeSO4 treatment solution. This observation suggests that the
ferrous salts induced the transformation of FeOOH in the
precursor into 𝛾-Fe2 O3 via dehydration, and this resulted in
the simultaneous oxidation of Fe2+ to Fe3+ . The results for
sample (1) show that the ferric salt did not induce a transition,
suggesting that the oxidization of the ferrous ions in the
liquid phase was necessary for the dehydration of amorphous
FeOOH to form magnetic phase. The experimental results for
sample (2) show that the cuprous ions (Cu+ ), which may not
have had the tendency to oxidize to cupric ions (Cu2+ ) in the
liquid phase [31], did not cause FeOOH to form magnetic
phase. This implies that the dehydrating action of the cuprous
ions was weaker than that of the ferrous ions in the chemically
induced transition in the liquid phase.
The experimental results also indicated that the products
after treatment with an FeSO4 solution contained MgSO4 ,
but the products after treatment with an FeCl2 solution
had no corresponding Mg-containing constituent. This result
suggests that FeSO4 in the treatment solution not only
induced the transformation of FeOOH into 𝛾-Fe2 O3 via
dehydration and then underwent oxidation into Fe2 (SO4 )3 ,
but also reacted with dissolved Mg(OH)2 , producing MgSO4 :
Mg2+ + 4 (Fe3+ + SO4 2− )
󳨀→ MgSO4 + Fe2 (SO4 )3 + 2Fe3+
(3)
According to (3), MgSO4 first adsorbed onto the 𝛾-Fe2 O3
crystallites, forming MgSO4 ⋅5H2 O. Some Fe3+ and SO4 2−
ions also adsorbed, forming Fe2 (SO4 )3 ⋅11H2 O on the outermost layer. Thus, the solutions of ferrous salts with different
acid radicals (e.g., FeCl2 and FeSO4 ) could induce the transformation of the FeOOH/Mg(OH)2 precursor into a 𝛾-Fe2 O3
Journal of Chemistry
7
Table 3: Molar and mass percentages of phases in samples (0) and (3).
Molar percentage (𝑦)
𝛾-Fe2 O3 /FeCl3 𝛾-Fe2 O3 /MgSO4 /Fe2 (SO4 )3
Sample (0)
Sample (3)
Mass percentage (𝑧)
𝛾-Fe2 O3 /FeCl3 ⋅6H2 O
𝛾-Fe2 O3 /MgSO4 ⋅5H2 O/Fe2 (SO4 )3 ⋅11H2 O
97.24/2.76
95.42/4.58
87.34/7.81/4.85
75.44/8.87/15.69
percentages of 𝛾-Fe2 O3 , MgSO4 ⋅5H2 O, and Fe2 (SO4 )3 ⋅11H2 O
(𝑦𝛾 , 𝑦Mg , and 𝑦S , resp.) could be described as follows:
FeCl3 · 6H2 O
eCl 2
In F
Mg(OH)2
𝛾-Fe2 O3 + Mg2+ + OH−
𝑦Mg =
FeOOH
In F
eSO
4
(𝑎Fe − 2 (𝑎S − 𝑎Mg ) /3) /2
𝑎Fe /2 + 𝑎Mg
𝑎Mg
𝑎Fe /2 + 𝑎Mg
× 100,
(5)
× 100,
𝑦S = 100 − 𝑦𝛾 − 𝑦Mg ,
𝛾-Fe2 O3
Fe2 (SO4 )3 · 11H2 O
+ Fe3+ + OH−
MgSO4 · 5H2 O
Figure 6: Schematic diagram of the formation of magnetic nanoparticles using FeCl2 and FeSO4 solutions.
magnetic phase via dehydration. However, the coating on the
𝛾-Fe2 O3 crystallites varied when the treatment solution was
changed. The results revealed that sample (0) contained 𝛾Fe2 O3 and FeCl3 ⋅6H2 O, and sample (3) contained 𝛾-Fe2 O3 ,
MgSO4 ⋅5H2 O, and Fe2 (SO4 )3 ⋅11H2 O. A schematic diagram
illustrating the formation of the nanoparticles using FeCl2
and FeSO4 solutions is shown in Figure 6. As the magnetic
nanoparticles have surface heterolayers that are different
from the magnetic core, the magnetic core spins close to
surface can be pinned by the layer, and the pinning would
cause an unusually large coercivity [32, 33]. So, such 𝛾-Fe2 O3
based magnetic nanoparticles appeared to be apparently
ferromagnetic due to surface pinning rather than shape
effects [34].
Because the metal salts were paramagnetic, the difference in the apparent magnetization of samples (0) and (3)
depended on the 𝛾-Fe2 O3 content. For sample (0), the molar
percentages of 𝛾-Fe2 O3 and FeCl3 ⋅6H2 O (𝑦𝛾 and 𝑦Cl , resp.)
could be described as follows:
𝑦𝛾 =
𝑦𝛾 =
(𝑎Fe − 𝑎Cl /3) /2
× 100,
(𝑎Fe − 𝑎Cl /3) /2 + 𝑎Cl /3
(4)
𝑦Cl = 100 − 𝑦𝛾 ,
where 𝑎Fe and 𝑎Cl are the atomic percentages of Fe and
Cl in sample (0), respectively. For sample (3), the molar
where 𝑎Fe , 𝑎Mg , and 𝑎S are the atomic percentages of Fe, Mg,
and S in sample (3), respectively. Thus, the molar percentages
of each phase (𝑦𝑖 ) in samples (0) and (3) could be calculated
from the 𝑎𝑖 values, which were measured using EDS, and are
listed in Table 3. The mass percentages of each phase (𝑧𝑖 ) in a
sample were deduced using the following expression:
𝑧𝑖 =
𝑦𝑖 𝐴 𝑖
× 100,
∑ 𝑦𝑖 𝐴 𝑖
(6)
where 𝐴 𝑖 is the molar weight of the 𝑖th phase. Accordingly, the
mass percentage of each phase was calculated for samples (0)
and (3) from the molar percentages (𝑦𝑖 ) and molar weights of
𝛾-Fe2 O3 , FeCl3 ⋅6H2 O, MgSO4 ⋅5H2 O, and Fe2 (SO4 )3 ⋅11H2 O.
The mass percentage values are listed in Table 3. Table 3
indicates that the main species in samples (0) and (3) was 𝛾Fe2 O3 . The 𝜎𝑠 values for samples (0) and (3) could therefore
be expressed as
𝜎𝑠 ≈ 𝑧𝛾 ⋅
𝜎𝛾,𝑠
100
,
(7)
where 𝑧𝛾 is the mass percentage of the 𝛾-Fe2 O3 phase and
𝜎𝛾,𝑠 is the specific saturation magnetization of the 𝛾-Fe2 O3
phase. For samples (3) and (0), their 𝜎𝛾,𝑠 can be regarded as
same since the 𝛾-Fe2 O3 phase resulted from the same FeOOH
phase. Thus, the ratio of the 𝜎𝑠 values for samples (3) and (0)
was proportional to the mass fraction 𝑧𝛾 of 𝛾-Fe2 O3 . From
the experimental results for the magnetization, we found that
the ratio of the 𝜎𝑠 values for samples (3) and (0) was 0.73,
which agreed with the ratio of the mass percentages 𝑧𝛾 of the
𝛾-Fe2 O3 phase (0.79). As a consequence, the mass percentage
was available.
The volume percentage of each phase of the samples (Φ𝑖 )
could be obtained from the 𝑧𝑖 value:
Φ𝑖 =
𝑧𝑖 /𝜌𝑖
× 100,
∑ 𝑧𝑖 /𝜌𝑖
(8)
where 𝜌𝑖 is the density of the 𝑖 phase. The 𝜌𝑖 values for
𝛾-Fe2 O3 , FeCl3 ⋅6H2 O, MgSO4 ⋅5H2 O, and Fe2 (SO4 )3 ⋅11H2 O
8
Journal of Chemistry
Table 4: Volume percentages of phases (Φ𝑖 ) in samples (0) and (3).
𝛾-Fe2 O3 /FeCl3 ⋅6H2 O
88.69/11.31
Sample (0)
Sample (3)
𝛾-Fe2 O3 /MgSO4 ⋅5H2 O/Fe2 (SO4 )3 ⋅11H2 O
56.18/17.07/26.75
were 4.90, 1.844, 1.896, and 2.14 g/cm3 , respectively. The
values calculated for Φ𝑖 are listed in Table 4.
Because the particles in samples (0) and (3) were nearly
spherical, the ratio of the average particle volume in sample
(0) (⟨𝜐⟩(0) ) to the average particle volume in sample (3)
(⟨𝜐⟩(3) ) could be described as follows:
3
⟨𝜐⟩(0) ⟨𝑑⟩𝜐,(0)
=
,
⟨𝜐⟩(3) ⟨𝑑⟩3𝜐,(3)
(9)
where ⟨𝑑⟩𝜐,(0) and ⟨𝑑⟩𝜐,(3) are the diameters corresponding
to the average volumes of the particles in samples (0) and
(3), respectively. ⟨𝑑⟩𝜐 could be calculated directly from the
size distribution parameters obtained using TEM by using the
equation ⟨𝑑⟩𝜐 = exp(ln 𝑑𝑔 + 1.5 ln2 𝜎𝑔 ) [30]. Accordingly, a
⟨𝑑⟩3𝜐,(0) /⟨𝑑⟩3𝜐,(3) value of 0.62 was obtained. Additionally, the
average volume of the 𝛾-Fe2 O3 based nanoparticles, which
contained many phases, (⟨𝜐⟩), can be expressed as ⟨𝜐⟩ =
⟨𝜐𝛾 ⟩ ∑ Φ𝑖 /Φ𝛾 , where ⟨𝜐𝛾 ⟩ is the average volume of the 𝛾Fe2 O3 phase and Φ𝛾 is the volume percentage of the 𝛾Fe2 O3 phase. Because the 𝛾-Fe2 O3 phase was produced from
FeOOH, the ⟨𝜐𝛾 ⟩ values for samples (0) and (3) were the
same. Therefore, the ratio of ⟨𝜐⟩(0) to ⟨𝜐⟩(3) could be described
by Φ𝛾,(3) /Φ𝛾,(0) ; that is,
⟨𝜐⟩(0) Φ𝛾,(3)
=
,
⟨𝜐⟩(3) Φ𝛾,(0)
(10)
where Φ𝛾,(0) and Φ𝛾,(3) are the volume percentages of the 𝛾Fe2 O3 phase in samples (0) and (3), respectively. Thus, the
⟨𝜐⟩(0) /⟨𝜐⟩(3) value obtained from the volume percentages was
0.63, which agreed with the value of ⟨𝑑⟩3𝜐,(0) /⟨𝑑⟩3𝜐,(3) (0.62)
and confirmed the validity of both the coating model of the
nanoparticle structure and the volume percentage.
4. Conclusions
Metal ions in the treatment solution play a key role in
the magnetic transition during the preparation of magnetic nanoparticles via the chemically induced transition of
FeOOH/Mg(OH)2 . No magnetic transition occurred when
FeCl3 and CuCl were used as treatment solutions; in contrast,
the product obtained using treatment with an FeSO4 solution,
which is similar to that obtained using an FeCl2 solution,
showed ferromagnetic behavior. The magnetic phase of the
two products was 𝛾-Fe2 O3 . Experimental results revealed
FeCl3 and Fe2 (SO4 )3 in the products after treatment with
FeCl2 and FeSO4 solutions, respectively. Thus, Fe2+ , which
likely has a stronger dehydrating ability than the cuprous
ion Cu+ , might have undergone oxidation to Fe3+ , thereby
inducing a magnetic transition via dehydration. Treatment
with an FeSO4 solution resulted in the formation of MgSO4
on the 𝛾-Fe2 O3 crystallites (the binding energy of Mg1s is
∼1304.9 eV), whereas no Mg-containing compound formed
in the product of the treatment using an FeCl2 solution. This
shows that the nonmagnetic coating on the magnetic 𝛾-Fe2 O3
crystallites varied with changes in the acid radical ions in the
treatment solution. Treatment with an FeSO4 solution led to
a higher content of the nonmagnetic phase in the product,
and thus weaker magnetization, despite the fact that these
particles were larger than those formed after treatment with
an FeCl2 solution. Due to the surface pinned effect, such 𝛾Fe2 O3 based magnetic nanoparticles, which were produced
via the chemically induced transition, exhibited apparently
ferromagnetism.
Here, we demonstrated the use of an FeOOH/Mg(OH)2
precursor and a ferrous salt treatment solution (as FeCl2
solution or FeSO4 solution) as a simple and effective method
for the preparation of 𝛾-Fe2 O3 -based magnetic nanoparticles.
Obviously, the alkali-oxide FeOOH dehydrating into 𝛾-Fe2 O3
was induced with Fe2+ in ferrous salt solution transforming
into Fe3+ . For the preparation of magnetic nanoparticles via
chemically induced transition (CIT) method, the relation
between acting energy of the alkali-oxide dehydrating and
oxidation of the ferrous ions is interesting, and this mechanism will be further investigated in future work.
Conflict of Interests
The authors declare that there is no conflict of interests
regarding the publication of this paper.
Acknowledgments
Financial support for this work was provided by the Innovation Foundation Project of Southwest University, China (no.
1318001), and the National Science Foundation of China (no.
11074205).
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