Applied Surface Science 254 (2008) 6972–6975
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Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Electronic structure studies of the spinel CoFe2O4 by X-ray photoelectron
spectroscopy
Zhongpo Zhou a, Yue Zhang a, Ziyu Wang a, Wei Wei a, Wufeng Tang a, Jing Shi a,b, Rui Xiong a,c,*
a
Department of Physics and Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan 430072, China
International Center for Materials Physics, Shen Yang 110015, China
c
Hubei Key Laboratory on Organic and Polymeric Opto-electronic Materials, Wuhan 430072, China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 20 December 2007
Received in revised form 6 April 2008
Accepted 1 May 2008
Available online 8 May 2008
The spinel CoFe2O4 has been synthesized by combustion reaction technique. X-ray photoelectron
spectroscopy shows that samples are near-stoichiometric, and that the specimen surface both in the
powder and bulk sample is most typically represented by the formula (Co0.4Fe0.6)[Co0.6Fe1.4]O4, where
cations in parentheses occupy tetrahedral sites and those within square brackets in octahedral sites. The
results demonstrate that most of the iron ions are trivalent, but some Fe2+ may be present in the powder
sample. The Co 2p3/2 peak in powder sample composed three peaks with relative intensity of 45%, 40%
and 15%, attributes to Co2+ in octahedral sites, tetrahedral sites and Co3+ in octahedral sites. The O 1s
spectrum of the bulk sample is composed of two peaks: the main lattice peak at 529.90 eV, and a
component at 531.53 eV, which is believed to be intrinsic to the sample surface. However, the vanishing
of the O 1s shoulder peak of the powder specimen shows significant signs of decomposition.
ß 2008 Elsevier B.V. All rights reserved.
Keywords:
CoFe2O4
Electronic structure
XPS
Chemical state
1. Introduction
The cobalt ferrite CoFe2O4 is a very important magnetic
materials, which has covered a wide range of applications
including electronic devices, ferrofluids, magnetic delivery microwave devices and high density information storage due to its
wealth of magnetic and electronic properties, such as cubic
magnetocrystalline anisotropy, high coercivity, moderate saturation magnetization, high Curie temperature TC, photomagnetism,
magnetostriction, high chemical stability, wear resistance and
electrical insulation, etc. [1–12].
In structure, the spinel cobalt ferrite CoFe2O4 crystallizes in a
face-centered cubic structure with a large unit cell containing eight
formula units. There are two kinds of lattices for cation occupancy,
A and B sites have tetrahedral and octahedral coordination,
respectively. In the normal spinel structure Co is a divalent atom,
occupying tetrahedral A sites, while Fe is a trivalent atom, sitting
on the octahedral B sites. When A sites being Fe3+ ions, while B sites
equally populated by Co2+ and Fe3+ ions, the spinel structure is
referred to as the inverse kind [13,14]. Commonly, the CoFe2O4
material is considered to be mostly an inverse spinel compound
with most divalent Co ions occupying octahedral sites [7,15,16]. It
* Corresponding author.
E-mail address: [email protected] (R. Xiong).
0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2008.05.067
means that the Co and Fe cations distribute at both sites. Since the
FeA3+–FeB3+superexchange interaction is normally different from
the CoA2+–FeB3+ interaction, variation of the cation distribution
over the A and B sites in the spinel leads to different magnetic
properties of these oxides even though the chemical composition
of the compound does not change [14].
The spinel cobalt ferrite CoFe2O4 materials have been synthesized by different methods [17–19]. In most cases, variation of the
magnetic properties was obtained due to the different distribution
of the Co and Fe cations over the A and B sites. Thus, investigations
on the distribution of the Co and Fe cations over the A and B sites in
the spinel cobalt ferrite CoFe2O4 are important [4,7,13,14].
Quantitative X-ray photoelectron spectroscopy (XPS) gives not
only the chemical composition, but also information on the
chemical bonding and chemical state. This will help us to
understand the distribution of the Co and Fe cations in the spinel
cobalt ferrite CoFe2O4. Thus, in this paper, homogeneous CoFe2O4
powder and bulk were synthesized by combustion technique, and
the XPS was taken to study on the valence of the elements and
electronic configuration.
2. Experimental
CoFe2O4 powder and bulk samples in our experiment were
synthesized by combustion technique. In brief description,
analytical reagent cobalt (II) nitrate hexahydrate, iron (III) nitrate
Z. Zhou et al. / Applied Surface Science 254 (2008) 6972–6975
nonahydrate and urea were used. Stoichiometric amounts of the
cobalt and iron nitrates were weighed out under dry conditions,
intimately mixed in a widemouth vitreous silica basin and heated
on a hot blanket inside a fume cupboard, under ventilation. With a
rise in temperature, melting occurred and a dark liquid was
produced. Soon after the thickened liquid began frothing, ignition
took place, leading to rapid increase that propagated in swift
ripples to walls of the basin. The reaction produced dry, very fragile
foam, which transformed into powder [19]. The powders were
ground in an agate mortar and pestle to a fine powder, and then,
some powders were cold pressed into disks with diameter of
15 mm and thickness of about 1.4 mm at room temperature under
a pressure of 12 MPa. In the final, the compacted powders and the
disks were directly put into silica boats separately, and sintered at
1200 8C for 4 h in argon atmosphere.
X-ray diffraction (XRD) patterns were recorded on Bruker D8
ADVANCE powder diffractometer using Cu Ka radiation. The
working voltage V, current I and time constant t were 40 kV, 40 mA
and 0.2 s, respectively. X-ray Photoelectron Spectroscopy studies
were performed using a KRATOS XSAM-800 ESCA/SIMS/ISS spectrometer with monochromatic Mg Ka (1253.6 eV) radiation, the
binding energies of samples has been calibrated by taking the
carbon 1s peak as reference (285.0 eV).
3. Results and discussion
Fig. 1 is the X-ray powder diffraction patterns of the CoFe2O4
powder and bulk. It shows that the powder and bulk samples all
have a single spinel phase and all peaks could be indexed according
to the standard card of the spinel cobalt ferrite CoFe2O4 [JCPPS card
No. 22-1086].
Fig. 2 gives the wide-scan XPS spectra of the CoFe2O4 bulk
(curve 1) and powder (curve 2) samples in the binding energy of 0–
1000 eV. As demonstrated in Fig. 2, the crystals contain Fe, Co, and
O elements, and no other impurity element was detected in the
spectrum up to 1000 eV except carbon. The carbon on the surfaces
of the specimens is probably due to contamination caused by
handling or pumping oil, since the samples have been undergone a
1200 8C high-temperature calcinations procedure.
Stoichiometric information can be obtained from core photoemission intensity data. The element composition can be
quantified by use of X-ray photoelectron intensity values (In)
and appropriate sensitivity factors (Sn): rn = In/Sn. SFe, SCo and SO
values of 3.8, 4.5 and 0.68 were found by procedures similar to that
of Ref. [20]. Because the iron, cobalt and oxygen X-ray photoelectron peaks overlap significantly, the spectrum must first be
Fig. 1. XRD patterns of the CoFe2O4, (a) bulk and (b) powder.
6973
Fig. 2. Wide-Scan XPS spectra of the CoFe2O4, bulk (curve 1) and powder (curve 2).
integrated to obtain the N(E) vs. X-ray photoelectron spectrum and
the individual components separated to obtain the integrated XPS
intensity for each element independently [21]. Then, rCo:rFe:rO = 1:1.8:4.1 and 1:1.9:3.7 to bulk sample and powder sample
were obtained separately.
Regions in which detailed core spectra were collected including
the 2p photoelectron regions of the metallic constituents of the
oxides, as well as the oxygen 1s core photoelectron regions. The
high-resolution narrow-scan XPS spectra of Fe 2p, Co 2p, and O 1s
peaks of the CoFe2O4 specimen are shown in Figs. 3–5, respectively.
Fig. 3 shows the Fe 2p core-electron spectrum of CoFe2O4
powder and bulk samples. The peak shape for the two samples is
similar, and has an asymmetric shape. The spectrum apparently
reveals the presence of two nonequivalent bonds of Fe ions in
CoFe2O4 compounds, which is consistent with that there are two
kinds of lattice sites for Fe ions occupancy in CoFe2O4 compounds.
We attempt to resolve the data into two components to represent
these two sites. For the CoFe2O4 powder, it yields Fe 2p3/2 binding
energies of 710.65 and 713.26 eV, and Fe 2p1/2 binding energies of
724.23 and 725.36 eV. For the CoFe2O4 bulk, it yields Fe 2p3/2
binding energies of 710.48 and 713.00 eV, and Fe 2p1/2 binding
energies of 723.60 and 725.70 eV. The doublets in powder and bulk
samples can be ascribed to Fe3+ ions in octahedral sites and Fe3+
Fig. 3. Fe 2p XPS for bulk (a) and powder (b) samples. (The arrow indicates the
approximate position of the satellite characteristic of octahedral Fe2+.)
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Z. Zhou et al. / Applied Surface Science 254 (2008) 6972–6975
Fig. 4. Co 2p3/2 XPS for bulk (a) and powder (b) specimens. (The arrow indicates the
approximate position of the satellite characteristic of octahedral Co2+.)
ions in tetrahedral sites, respectively. In CoFe2O4 powder, the
doublets of Fe 2p3/2 binding energies at 710.65 eV and Fe 2p1/2
binding energies at 724.23 eV are due to the contributions from
Fe3+ ions in octahedral sites, while the doublets of Fe 2p3/2 binding
energies at 713.26 eV and Fe 2p1/2 binding energies at 725.36 eV
are due to the contributions from Fe3+ ions in tetrahedral sites. In
CoFe2O4 bulk, the doublets of Fe 2p3/2 binding energies at
710.48 eV and Fe 2p1/2 binding energies at 723.60 eV are due to
the contributions from Fe3+ ions in octahedral sites, while the
doublets of Fe 2p3/2 binding energies at 713.00 eV and Fe 2p1/2
binding energies at 725.70 eV are due to the contributions from
Fe3+ ions in tetrahedral sites. The relative contributions to the
overall intensity of Fe3+ ions in octahedral sites and tetrahedral
sites are 70% and 30% for both the powder and the bulk sample,
which means that the Fe3+ ions occupy 60% tetrahedral sites and
70% octahedral sites in both the powder and the bulk sample. The
result is consistent with that was reported by Nakagomi et al. [22],
in which Co2+ was found to occupy both the tetrahedral and
octahedral sites in spinel CoxFe3–xO4 prepared by combustion
reaction.
The 2p3/2 to 2p1/2 separation and satellite structure are useful in
characterizing the iron chemical environment. The iron 2p
spectrum of the powder sample is different from that of the bulk
samples, and shows developed satellite structure characteristic of
Fig. 5. The XPS spectrum of O 1s peak for the CoFe2O4 surface, bulk (a) and powder
(b).
high spin octahedral cations, as that exhibited in the Fe2+ metal
monoxides [23]. Thus, some Fe2+ may be present in the powder
sample. This spectrum of the powder sample also broadens,
although in this case the broadening of the iron is predominately to
the higher binding energy side of the 2p transitions.
The XPS spectrum of Co 2p3/2 in CoFe2O4 shown in Fig. 4 also
exhibits asymmetric. The quantitative peak fitting procedure for
Co 2p3/2 is rather complicated owing to various physics effects
including core and valence band interactions. Inspection of the
measured Co 2p3/2 peak of the bulk sample (a) shows that it
composes of two main doublets with peak position at 779.82 and
781.40 eV, and with relative contributions to the overall Co
intensity of 60% and 40%, respectively. The two peaks with binding
energy of 779.82 and 781.40 eV are ascribed to Co2+ ions in
octahedral sites and Co2+ ions in tetrahedral sites, respectively.
However, the Co 2p3/2 peak in powder sample (b) composes of
three peaks positioned at 779.70, 781.00 and 782.80 eV, with
relative contributions to the overall Co intensity of 45%, 40% and
15%, ascribes to Co2+ in octahedral sites, tetrahedral sites and Co3+
in octahedral sites.
Compared with spectrum of the powder, the spectrum of the
octahedral Co2+ cations in bulk sample has a very intense,
characteristic satellite at 786 eV, which is some 4–6 eV higher
in binding energy than the Co 2p3/2 signal. Therefore, majority high
spin Co2+ cations occupy octahedral sites in the CoFe2O4 spinel
lattice [21,15]. The intense satellite structure found at the high
binding energy side of the Co 2p3/2 and Co 2p1/2 (not shown)
transition is believed to be a direct consequence of the band
structure associated with octahedral Co2+ in the oxide lattice,
which allows for admixture of oxygen 2p character and leads two
possible final states in the photoemission process:
7
7
2p6 3d þ hn ! 2p5 3d þ e
or
8
2p5 3d V þ e ;
where V represents an electron vacancy in the 2p band of the
neighboring O2 lattice anions.
Because of the extreme sensitivity of the charge-transfer
process to the overlap between adjacent Co2+ and O2 orbital,
the satellite to main peak intensity is highly dependent upon the
geometry and defect nature of the compound [21].
The possibilities of the oxidation of Co2+ to Co3+ is not entirely
impossible if the oxidation of cobalt is compensated by a number
of iron cations reduced from Fe3+ to Fe2+, or the migration of Co2+ to
tetrahedral sites. The low spin Co3+ atom gives rise to much weaker
satellite feature than do high spin Co2+, owing to the presence of
unpaired valence electrons in the Co3+ orbital [15]. The exact form
of this defect-cobalt structure cannot be unequivocally determined
with XPS. However, the cobalt 2p spectrum corresponding to the
powder sample has no significant difference compared with the
bulk specimen, although the spectral features broaden slightly,
occurring at the higher binding energy side of the 2p peak in the
intensity of the 781.00 eV peak.
Fig. 5 shows the core-level spectrum of O 1s in CoFe2O4. The
main peak of the bulk specimen (a) has a binding energy of
529.90 eV, which has been observed for lattice O2 at approximately this point in a number of rocksalt and spinel 3d metal
oxides, including CoO, and Co3O4 [16]. A second higher-binding
energy peak is found at 531.53 eV, and perhaps a third peak at
533.00 eV. The third peak is only present in about 7.7% of the total
O 1s intensity and may simply be an artifact of fitting asymmetric
XPS peaks with symmetric Gaussian functions [21]. Establishing a
unique assignment for the 531.53 eV O 1s peak is not straightforward. There are several possible defects and contaminants with a
comparable XP binding energy and similar peaks in O 1s spectra
taken on cobalt and iron containing oxides, those results may be
from ambient adsorption, under-coordinated lattice oxygen,
Z. Zhou et al. / Applied Surface Science 254 (2008) 6972–6975
Table 1
The analysis results of Co 2p, Fe 2p and O 1s XPS spectra for the CoFe2O4 powder and
bulk samples
Sample
Powder
Spectrum
Co 2p3/2
Fe 2p3/2
O 1s
Bulk
Co 2p3/2
Fe 2p3/2
O 1s
BE(eV)
Assignment
Atomic percentage (%)
2+
779.70
781.00
782.80
710.65
713.26
530.17
532.29
Octahedral Co
Tetrahedral Co2+
Octahedral Co3+
Octahedral Fe3+
Tetrahedral Fe3+
CoFe2O4
CoFe2O4
45
40
15
70
30
89
11
779.82
780.40
710.48
713.00
529.90
531.53
533.00
Octahedral Co2+
Tetrahedral Co2+
Octahedral Fe3+
Tetrahedral Fe3+
CoFe2O4
CoFe2O4
Artifact
60
40
70
30
60
33
7
Co2O3/Fe2O3 surface phases, or species intrinsic to the surface of
the spinel. Thus, while it is not possible to rule out low levels of
contaminants or defect species entirely, a fair percentage of oxygen
intensity in the CoFe2O4 O 1s spectrum is believed to be
representative of surface oxide intrinsic to the CoFe2O4 spinel
lattice.
In Fig. 5(b), the main peak is at 530.17 eV, and decreases slightly
compared with that in Fig. 5(a), and the O 1s shoulder vanishes
which shows significant signs of oxygen depletion. The reduction
of oxygen concentration may come from the oxidation of Co2+ to
Co3+, or may be caused by calcination procedure for the loose
compaction. Thermodynamic considerations have to be made in
cases of high temperature and low oxygen partial pressure, as
present in the surfaces when the spinel was formed. An attempt
may be made to attribute this to the relative mobility of the ions
concerned via tetrahedral and octahedral lattice sites, with the
relative preferences for the two sites affecting rates of diffusion.
The relative ion mobility may be deduced to be: Fe3+ > Fe2+ > Co2+.
The mobility can be used to explain the enhanced surface
concentration of cobalt and depletion of iron. The 532.29 eV peak
is assigned an artifact also.
Thus, the sample can be represented by (Co0.4Fe0.6)[Co0.6Fe1.4]O4 approximately, where cations in parentheses locate in
tetrahedral sites and those within square brackets in octahedral
sites. Table 1 summarizes the analysis results of the Co 2p, Fe 2p
and O 1s XPS spectra for the CoFe2O4 powder and bulk samples.
4. Conclusion
results show that the Fe3+ cations occupy 60% tetrahedral sites and
70% octahedral sites both in the bulk and powder specimens, but
some Fe2+ may be present in the powder sample. The cobalt cations
are predominantly present as Co2+ cations. For the bulk sample,
60% Co2+ cations located in octahedral sites, and 40% in tetrahedral
sites. The Co 2p3/2 peak in powder sample consisted three peaks
with relative contributions to the overall Co intensity of 45%, 40%
and 15%, ascribes to Co2+ in octahedral sites, tetrahedral sites and
Co3+ in octahedral sites. The O 1s spectrum of the bulk sample is
composed of two peaks: one at 520.90 eV, similar to that was found
for lattice O2 in the monoxides, and a second at 531.53 eV, which
we believe to be characteristic of the surface spinel. Furthermore,
the O 1s peak of the powder specimen show significant signs of
decomposition for depletion of oxygen.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (Grant No. 10674105, No. 10474074 and No.
10534030) and 973 Program (2007CB607501).
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6975
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