Synthesis of Fe-doped NiO nanofibers using electrospinning method and their ferromagnetic properties

Article abstract of DOI:10.1016/j.jmmm.2012.02.017

To make p-type diluted magnetic semiconductor (DMS), Ni _1-xFe_xO nanofibers with different Fe doping concentrations have been successfully synthesized by electrospinning method using polyvinyl alcohol (PVA) and Ni(CH₃COO)

Full text of DOI:10.1016/j.jmmm.2012.02.017

Journal of Magnetism and Magnetic Materials 324 (2012) 2070–2074
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Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Synthesis of Fe-doped NiO nanofibers using electrospinning
method and their ferromagnetic properties
Shaohui Liu a, Jianfeng Jia a,n, Jiao Wang a, Shijiang Liu b, Xinchang Wang a, Hongzhang Song a, Xing Hu a
a School of Physical Engineering and Laboratory of Material Physics, Zhengzhou University, Zhengzhou 450052, PR China
b College of Physics and Electric Information, Luoyang Normal University, Luoyang 471022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
To make p-type diluted magnetic semiconductor (DMS), Ni1ꢀxFexO nanofibers with different Fe doping
concentrations have been successfully synthesized by electrospinning method using polyvinyl alcohol
(PVA) and Ni(CH3COO)2 ꢁ 4H2O as starting materials. The nanofibers were characterized by scanning
electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, superconductivity quantum
interference device (SQUID) and X-ray photoelectron spectroscopy (XPS) test. The results show that Fe
doping has no influence on the diameter and surface morphology of NiO nanofibers, and the nanofibers
are polycrystalline with NaCl structure. All Fe-doped samples show obvious ferromagnetic properties
and the saturation magnetization is enhanced with increase of the doping concentration of Fe, which
indicates that the doped Fe has been incorporated into the NiO host and results in room-temperature
ferromagnetism in the Ni1ꢀxFexO nanofibers.
Received 30 May 2011
Received in revised form
26 December 2011
Available online 22 February 2012
Keywords:
Diluted magnetic semiconductor (DMS)
Electrospinning
Ni1ꢀxFexO
Ferromagnetism
& 2012 Elsevier B.V. All rights reserved.
1. Introduction
One-dimensional nanostructured materials have gained increas-
ing attention by virtue of their potential applications [9] due to
Diluted magnetic semiconductor (DMS) is a kind of novel semi-
conductors, which is formed using magnetic transition metal ions or
rare earth metal ions to randomly replace the non-magnetic cations
in semiconductors and to make the semiconductor show magnetic
properties [1]. Such semiconductors are potential candidates for
applications of spin-controlled devices. Unfortunately, most of the
natural DMSs have relatively low Curie temperatures (TC), which
hinder their wide applications. Since Dietl et al. [2] used average field
theory to predict that adding a magnetic ion in wide band gap oxide
semiconductor can cause it to become room temperature diluted
magnetic semiconductor, the room temperature ferromagnetism in
wide band gap semiconductors such as TiO2 [3], ZnO [4], SnO2 [5] by
substituting magnetic transition metal (3d) ions have been discov-
ered. The above mentioned DMS materials are known all as n-type
DMS materials. For potential applications of spin-controlled devices,
the p-type DMS systems are also needed. However, there are few
reports about the detailed characteristics of p-type DMS.
their remarkable magnetic, optical, electric, and chemical properties.
One-dimensional nanostructured magnetic material often shows a
high degree of magnetic anisotropy due to its special size and
structure. Therefore, compared with general ferromagnetic materials,
the one-dimensional nanostructured magnetic materials have great
value of spontaneous magnetization and coercivity. In case of dilute
magnetic semiconductor, current researches are centered on film
and bulk materials, the reports on one-dimensional nano-dilute
magnetic semiconductor were few. Nanoparticles, nanorods, and
films structure of NiO had been prepared by sol–gel techniques [10],
hydrothermal route [11] and PLD [12]. Compared with other
methods, electrospinning is a simple, versatile and convenient
approach to manufacture one-dimensional nanomaterials with the
characteristic of easy control and low cost [13].
In this paper, we report the successful fabrication of p-type DMS
Fe-doped NiO nanofibers with diameters about 100 nm via electro-
spinning technique. In order to clarify the origin of ferromagnetism
in these Fe-doped NiO nanofibers, the effects of Fe doping on the
crystal structure, morphology and detailed temperature and field-
dependent magnetization of the nanofibers were also investigated.
Stoichiometric Nickel oxide NiO is known to show an anti-
ferromagnetism that remains even at rather high temperatures
TN¼523 K [6]. The measured band gap of NiO is about 4.0 eV,
clearly indicating that it behaves as an insulator with room-
temperature conductivity less than 10ꢀ13 S/cm [7]. Although
stoichiometric NiO is
a Mott–Hubbard insulator at room
2. Experimental
temperature, after introduction of Ni2þ vacancies or doping with
Liþ, which can become a p-type semiconductor [8].
2.1. Synthesis of NiO nanofibers
n Corresponding author.
Pure NiO and Fe-doped NiO nanofibers with different doping
concentrations were synthesized by electrospinning. In a typical
E-mail address: Jiajf@zzu.edu.cn (J. Jia).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2012.02.017

S. Liu et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2070–2074
2071
procedure, the aqueous polyvinyl alcohol (PVA, Mw 80,000)
solution (10 wt%) was prepared by dissolving 10 g PVA powder
in 90 ml distilled water first. Then, 2 g Ni(CH3COO)2 ꢁ 4H2O and
the suitable amount of Fe(NO3)3 ꢁ 9H2O with a different atomic
ratios (Ni:Fe¼100:0, 100:1, 50:1 and 20:1) respectively were
added slowly into the aqueous PVA solution under vigorous
stirring at 40 1C for 2 h, and then, 5 ml alcohol was dropped
slowly into the solution with stirring. Ultimately, a lucid and
viscous sol solution was obtained to prepare for electrospinning.
The precursor sol solution was loaded into a 20 ml plastic
syringe with a syringe needle of which the internal diameter is
0.5 mm. The needle was connected to a DC high-voltage power
supply. In our experiment, a voltage of 10 kV was applied
between the cooper plate collector and the syringe needle with
a distance of 12 cm. The PVA/Ni(CH3COO)2 ꢁ 4H2O composite
nanofibers were collected on the cooper plate during electrospin-
ning processes. Pure NiO and Fe-doped NiO nanofibers were
finally obtained by calcination at 650 1C for 3 h in air to remove
PVA, completely.
spectroscopy (XPS) and magnetic properties of the samples was
measured by superconductivity quantum interference device
(SQUID).
To distinguish electric conduction type of the as-prepared
samples, a Hot-Probe experiment was executed [14]. A certain
amount of Ni0.98Fe0.02O nanofiber was molded by powder com-
pression machine into the block of 1.0 mm ꢂ 5 mm ꢂ 5 mm and
calcined at 1000 1C for 6 h, and then the electrically conductive
Silver glue was brushed on the two opposite sides. A couple of
cold probe and hot probe were attached to the two sides of the
sample, the hot probe is connected to the positive terminal of the
meter, while the cold probe is connected to the negative terminal.
3. Results and discussion
From the Hot-Probe experiment, a negative voltage is obtained
for the as-prepared sample, so we can draw a conclusion that the
Fe doped NiO nanofibers is a p-type semiconductor. The electrical
resistivity of Ni0.98Fe0.02O is ꢃ8 ꢂ 103
O m obtained by a conven-
2.2. Characterization methods
tional ohmmeter.
Fig. 1 shows the SEM images of the nanofibers before and after
calcination. Fig. 1(a) and (c) shows the SEM images of undoped
NiO/PVA and Fe doped NiO/PVA composite nanofibers before
calcination. It can be seen that the surface of composite fibers
before calcination were smooth. They are several millimeters long
X-ray diffraction (XRD) was employed to investigate the
crystal structure of nanofibers using Cu/K
a radiation. And the
morphologies of nanofibers were characterized by scanning
electron microscope (SEM). Besides, Raman scattering spectro-
scopy with the Arþ (514.5 nm) laser lines as the excitation
sources was used to examine the vibrational modes of the
nanofibers to identify the compositions in the nanofibers. The
valence of the dopants was analyzed by X-ray photoelectron
with
a diameter of approximately 300 nm. Furthermore, in
contrast to undoped NiO/PVA composite nanofibers, Fe doped in
NiO does not influence the morphologies of doped samples. After
calcinations at 650 1C, due to the decomposition of PVA and the
Fig. 1. SEM images of the undoped NiO/PVA and Fe doped NiO/PVA composite nanofibers before and after calcinations.

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S. Liu et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2070–2074
Fig. 2. The XRD patterns of Ni1ꢀxFexO(x¼0, 0.01, 0.02, 0.05) nanofibers after
sintering at 650 1C.
Fig. 3. The room temperature Raman spectra of NixFe1ꢀxO nanofibers.
transformation from metal salts into metal oxides, the diameters
of undoped and Fe-doped NiO nanofibers shrank drastically to
50–100 nm, as shown in Fig. 1(b) and (d).
Fig. 2 shows the XRD patterns of Ni1ꢀxFexO(x¼0, 0.01, 0.02,
0.05) nanofibers. It can be seen that five diffraction patterns at
hysteresis loop of the samples increased with the concentration of
dopant Fe indicated that the maximum values of magnetization
and coercive field are increased, which is an identification of the
increase of proportion of ferromagnetic phase with the concen-
tration of dopant Fe. The presence of the hysteresis loops of
NixFe1ꢀxO at room temperature indicates that the Curie tempera-
ture is above room temperature.
y
¼37.21, 43.41, 62.91, 75.21 and 79.41 correspond to (111), (200),
(220), (311) and (222) characteristic peaks of cubic crystalline
NiO. No impurity phase was observed with the sensitivity of XRD
measurement, and the lattice constants of Fe doped NiO nanofi-
bers hardly shifted with the increase of Fe content since the ionic
In order to explore the magnetized behavior of the samples,
the magnetization curves of field cooling (FC) and zero field
cooling (ZFC) were tested. As shown in Fig. 5, when extrinsic
magnetic field H¼100 Oe is applied, whether on undoped samples
or on Fe doped samples, their field cooling (FC) and zero field
cooling (ZFC) curves were bifurcate obviously at lower tempera-
ture range, while they overlapped nearby 300 K. As indicated in
Fig. 5, the FC magnetization of Ni0.98Fe0.02O nanofibers rapidly
increases with the decreasing temperature and becomes quite
larger than the ZFC magnetization. Oppositely, for the ZFC–FC
magnetization curves of undoped NiO nanofibers, the difference
between FC magnetization and ZFC magnetization was tiny, and
the value of FC magnetization or ZFC magnetization of these
nanofibers was far lower than that of Ni0.98Fe0.02O nanofibers. The
phenomenon indicates that Fe dopant NiO exhibits obviously
ferromagnetic character in spite of the antiferromagnetism for
undoped NiO. In the following we discuss the behavior of
magnetic moments during FC or ZFC process. The ZFC magnetiza-
tion curve is typically obtained by cooling in zero fields from a
high temperature and measuring the magnetization at stepwise
increasing temperatures in a small extrinsic magnetic field. The
FC magnetization curve is typically obtained by measuring at
stepwise-decreasing temperatures in the same small extrinsic
magnetic field at each temperature. When the sample is cooled at
non-zero field (FC), the spin moments are ordered almost parallel
to the direction of applied field, the sample shows non-zero
magnetization even if the extrinsic magnetic field decreases to
zero. Thus, MFC (T) behavior is similar to the behavior of
spontaneous magnetization, which can be easily explained by
mean-field theory. When the sample is cooled at zero-field (ZFC),
the spin moments are totally disordered. The sample shows zero-
magnetization when the extrinsic magnetic field is zero. If a small
extrinsic magnetic field is applied to the sample at low tempera-
ture, the spin moments rotate towards the extrinsic magnetic
field direction, and the sample shows net magnetization. A large
field and a large spontaneous magnetization (or MFC) give a large
radii of Ni2þ(0.69 A), Fe (0.74 A), and Fe (0.64 A) are quite
2þ
3þ
˚
˚
˚
close.
In order to further identify whether there is any trace amount
of second phase in the NixFe1ꢀxO nanofibers, the room tempera-
ture Raman scattering of samples were examined. The room
temperature Raman spectra of NixFe1ꢀxO nanofibers is shown in
Fig. 3. For all samples with or without Fe doping, the Raman
spectra are almost the same. Two Raman peaks are located at
about 570 and 1100 cmꢀ1 in the range of 200–2000 cmꢀ1. The
former peak can correspond to the one-phonon (1P) longitudinal
optical (LO) phonon modes of NiO, and the latter peak can be
assigned to two-phonon (2P) 2LO modes [15]. Stoichiometric NiO
is in NaCl structure and is expected not to show first order Raman
scattering. Therefore, the first-order Raman scattering peak cen-
tered at 570 cmꢀ1 is caused by nickel vacancies that break down
selection rules, as in oxygen-rich or ‘‘black’’ NiO [16]. The first-
order Raman scattering peak of 570 cmꢀ1 shown in the spectra
was enhanced with the increases of Fe concentration, implying a
high nickel vacancy concentration. Raman measurements show
that both the undoped and Fe-doped NiO nanofibers have a good
crystal quality with cubic crystalline NiO, and since a small
amount of Fe doping will not change the structure of NiO. Fe
doping neither caused a structural change nor induced a secondary
phase. This is consistent with the XRD patterns.
Fig.
4 shows the hysteresis loops of undoped NiO and
NixFe1ꢀxO samples measured at room temperature. For the
undoped NiO samples, as shown in Fig. 4a, it can be seen that
the extrinsic magnetic field dependence of magnetic moment is
almost linear, which is a typical antiferromagnetic-like behavior,
whereas all the NixFe1ꢀxO samples show ferromagnetic-like
behaviors as shown in Fig. 4b. The magnetization of samples do
not saturate up to 30,000 Oe, which indicates the coexistence of
ferromagnetic phase and antiferromagnetic phase. The area of

S. Liu et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2070–2074
2073
Fig. 6. XPS spectra of Fe 2p, for the Ni0.98Fe0.02O nanofiber.
defects (e.g., Ni vacancy) could be introduced into NiO, which are
randomly localized over the host lattice. Thus, this kind of disorder
certainly breaks the translation symmetry of the system and the
magnetic order in NiO grains would be interrupted, which causes
relatively weak coupling between the sublattices and gives rise to
ferromagnetism by interacting via a competing antiferromagnetic
superexchange and ferromagnetic double-exchange coupling [12]
through the introduced Fe ions and free charge carriers.
As discussed earlier, the content of Ni vacancy is increased with
Fe concentration, and Fe doping into NiO nanofibers neither
caused a structural change nor induced a secondary phase, we
believe that the room temperature ferromagnetism of NixFe1ꢀx
O
nanofibers is intrinsic. As the concentration of dopant Fe increases,
all Fe-doped samples can result in a significant enhanced satura-
tion magnetic moment. This indicates that the doped Fe ions play
an important role in mediating the magnetic interactions. In
addition, as Yan et al. [17] propose that the Zn vacancy can induce
the room-temperature ferromagnetism in Mn-doped ZnO by first-
principles calculations, we can also believe that the Ni vacancy can
also induce the room-temperature ferromagnetism in Fe-doped
NiO; as illustrated in Fig. 3, the Raman scattering peak of
570 cmꢀ1 is enhanced with the increases in concentration of Fe,
implying a high nickel vacancy concentration.
Fig. 4. (a) The hysteresis loops of undoped NiO nanofibers and (b) NixFe1ꢀx
(x¼0.01, 0.02, 0.05) nanofibers measured at 300 K.
O
The valence of the Fe ions in the NixFe1ꢀxO nanofiber is
verified by the XPS measurements. As shown in Fig. 6, it can be
seen that the spectra consist of Fe 2p3/2 and Fe 2p1/2 peaks, which
showed that the doped Fe in the Ni0.98Fe0.02O nanofiber exists as
Fe2þ, and no metallic Fe was observed in our samples. It can also
indicate that the origin of ferromagnetism can be ruled out by the
ferromagnetic secondary phase (e.g., metallic Fe).
4. Conclusions
Fe-doped NiO nanofibers with room-temperature ferromag-
netism have been successfully synthesized by electrospinning
method. We have studied the morphology and structures as well
as the magnetic properties for different Fe doping concentrations.
The results show that Fe doping has little influence on the
diameter and surface morphology of NiO nanofibers, and the
nanofibers exhibit the NaCl structure and no Fe-related secondary
phases are formed in the samples. We believe that the room
temperature ferromagnetism of NixFe1ꢀxO nanofibers is intrinsic.
All Fe-doped samples show obvious ferromagnetic properties and
Fig. 5. ZFC–FC magnetization curves of undoped NiO and Ni0.98Fe0.02O nanofibers.
ZFC magnetization. But the magnetic anisotropy (or coercive
field) resists the rotation to extrinsic magnetic field direction.
For the undoped NiO sample, which is antiferromagnetic
below the Neel temperature (573 K), as doped by Fe ions, the

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S. Liu et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2070–2074
as the concentration of dopant Fe increases, the saturation
magnetization (Ms) is also correspondingly enhanced. We believe
that the doped Fe has been incorporated into the NiO host, which
induced Ni vacancy and excited the room-temperature ferromag-
netism in the NixFe1ꢀxO nanofibers.
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