Journal of Alloys and Compounds 493 (2010) 340–345
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Hydrothermal synthesis and electronic properties of FeWO4 and CoWO4
nanostructures
S. Rajagopal a , D. Nataraj a,∗ , O. Yu. Khyzhun b , Yahia Djaoued c , J. Robichaud c , D. Mangalaraj d
a
Thin Film & Nanomaterials Laboratory, Department of Physics, Bharathiar University, Coimbatore 641 046, India
Department of Structural Chemistry of Solids, Frantsevych Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanivsky Street,
UA-03142 Kyiv, Ukraine
c
Laboratoire de Micro-spectroscopies Raman et FTIR, Université de Moncton-Campus de Shippagan, 218, boul. J.-D. Gauthier, Shippagan, NB, Canada E8S 1P6
d
Department of Nanoscience and Technology, Bharathiar University, Coimbatore 641 046, India
b
a r t i c l e
i n f o
Article history:
Received 23 November 2009
Received in revised form
16 December 2009
Accepted 18 December 2009
Available online 28 December 2009
Keywords:
Iron tungstate
FeWO4
Cobalt tungstate
CoWO4
Electronic structure
X-ray photoelectron spectroscopy
X-ray emission spectroscopy
X-ray absorption spectroscopy
a b s t r a c t
Iron tungstate (FeWO4 ) and cobalt tungstate (CoWO4 ) nanostructures were prepared by the hydrothermal method using sodium tungstate (Na2 WO4 ·2H2 O), ferrous ammonium sulfate [(NH4 )2 Fe(SO4 )2 ·6H2 O]
and cobalt chloride (CoCl2 ·6H2 O) solutions as precursors. The crystal structure and morphology of
the as-prepared tungstates were characterized by X-ray diffraction analysis and transmission electron
microscopy. The above characterizations render that the products obtained belong to the monoclinic
crystal system and P2/a space group, with average sizes of nanoparticles of about 150 nm and 70 nm in
the case of FeWO4 and CoWO4 , respectively. Electronic properties of the FeWO4 and CoWO4 tungstates
were studied using several X-ray spectroscopy methods, mainly X-ray photoelectron spectroscopy (XPS),
X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS). The XPS valence-band and
core-level spectra, the XAS W LIII edges (unoccupied W d-like states) as well as the XES bands reflecting
the energy distribution of the valence W d- and O p-like states were recorded. The present XPS and XAS
results reveal that the as-prepared FeWO4 and CoWO4 tungstates are in a composition close to a stoichiometric one. The XES results render that the W 5d- and O 2p-like states contribute throughout the
whole valence-band region of the FeWO4 and CoWO4 tungstates, however maximum contributions of
the O 2p-like states occur in the upper, whilst the W 5d-like states in the lower portions of the valence
band, respectively.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Metal tungstates with the general formula MWO4 (M denotes
a bivalent cation) belong to an important family of inorganic functional materials [1] and their crystal structures are controlled by
cationic radii [2]. Tungstates with large radii of bivalent cations
(e.g., Ca, Ba, Pb, and Sr) tend to a scheelite-type tetragonal structure, whilst small cationic radii (e.g., Fe, Mn, Co, Ni, Mg, and Zn) are
in favor of forming a wolframite-type monoclinic structure [2]. The
main difference among the two above structures is that, every tungsten atom is surrounded by four oxygen atoms in a scheelite-type
structure, whilst six oxygen atoms surround every tungsten atom
in MWO4 tungstate crystallizing in a wolframite-type structure [2].
The tungstates continue attracting considerable attention because
they are found to be a fascinating family of materials possessing
various technological applications such as microwave, scintilla-
∗ Corresponding author. Tel.: +91 422 2428444.
E-mail address: [email protected] (D. Nataraj).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.12.099
tion, optical modulation, magnetic and writing-reading-creasing
devices, humidity sensors, optical fibers and photoluminescence
materials [3,4]. Therefore, many tungstates such as MWO4 (M = Mn,
Fe, Co, Ni and Cu) [5], MnWO4 [6,7,8], FeWO4 [1,3,7], CoWO4 [2,9],
NiWO4 [7,10,11], CuWO4 [12,13], ZnWO4 [7,11,14] and CdWO4
[15] with a wolframite-type structure as well as CaWO4 [7,16],
AWO4 (A = Ca, Sr) [17], SrWO4 [7,18,19], SnWO4 [20,21], BaWO4
[7,22,23] and PbWO4 [4,24–27] with a scheelite-type structure
have been synthesized employing various novel methods, which
allow to obtain well-defined and high-pure materials, mainly nanosize tungstates.
Due to the fact that many physical and chemical properties of
solids can be understood and explained based on their electronic
structures, electronic properties of various tungstates were analyzed from experimental and/or theoretical points of view, mainly
in NiWO4 [10], CaWO4 [16,28], ZnWO4 [14,30], SrWO4 [17,32],
SnWO4 [21], PbWO4 [28,29,31], BaWO4 [29], CdWO4 [29,33], and
CuWO4 [12,34]. Regarding FeWO4 and CoWO4 tungstates, to the
best of our knowledge, their electronic structure has not yet been
investigated employing X-ray spectroscopy methods. Ejima et al.
S. Rajagopal et al. / Journal of Alloys and Compounds 493 (2010) 340–345
341
[5] have performed microscopic optical (absorption and reflection)
and ultraviolet spectroscopy (UPS) measurements on single microcrystals of iron tungstate, FeWO4 . These measurements suggest
that in FeWO4 the O 2p- and Fe 3d-like states hybridize and spread
wide in the valence-band region. Additionally, the experimental
data by Ejima et al. [5] suggest that the bottom of the conduction
band of FeWO4 is attributed to the empty Fe 4s-like states, whilst
the contribution of the W 5d-like states in the conduction band is
located in the higher energy side.
In the present paper we aim at an experimental study of the
electronic structure of hydrothermally prepared iron tungstate
(FeWO4 ) and cobalt tungstate (CoWO4 ) nanopowders. With this
aim, we have used possibilities of the X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES) and X-ray
absorption spectroscopy (XAS) methods in order to measure the
XPS valence-band and core-level spectra, the XAS W LIII edges
(unoccupied W d-like states) as well as the XES bands reflecting
the energy distribution of the occupied W 5d- and O 2p-like states.
2. Experimental details
2.1. Synthesis of tungsten oxide nanostructures
All the chemicals were of analytical grade and taken without further purification or modification. Sodium tungstate (Na2 WO4 ·2H2 O) was used as the starting
material. Ferrous ammonium sulfate [(NH4 )2 Fe(SO4 )2 ·6H2 O] and cobalt chloride
(CoCl2 ·6H2 O) solutions were the other chemicals used for the growth. In order to
adjust the pH, we used a sodium hydroxide (NaOH) solution.
Two different reactions were made for the preparation of tungstate nanostructures. In the first experiment, sodium tungstate (6.6 g, 0.2 mol) and ferrous
ammonium sulfate (0.4 g, 0.1 mol, 10 ml DDW) were dissolved in DD water separately, and mixed under vigorous stirring. A dark brown color mixture was obtained.
By adding sodium hydroxide solution to this mixture, its pH was adjusted to 8. In the
second experiment, sodium tungstate (6.6 g, 0.2 mol) and cobalt chloride (1.786 g,
0.5 mol, 15 ml in DDW) solutions were prepared separately and mixed with constant
stirring. In this case, a thick red color solution was obtained and by adding sodium
hydroxide solution its pH was set to be at 8. These two solutions were transferred
separately into 40 ml Teflon lined stainless steel autoclave and maintained at 180 ◦ C
for 24 h to get the final product. The as-obtained products were washed several
times both in water and ethanol and finally dried at 100 ◦ C for 2 h.
2.2. Characterization techniques
The surface morphology and phase state of the as-formed nanostructures were
characterized using transmission electron microscopy (TEM) and X-ray diffraction
(XRD) analysis. For studies of TEM images, a standard technique and a 2011 JEOL
STEM apparatus were used. A PANalytical X-ray diffractometer (XPERT PRO) was
employed for XRD analysis of the tungstates under consideration. The XRD measurements were carried out using monochromatic Cu K␣ radiation. XRD patterns
in the range 2 = 10–80◦ were derived using step-by-step scanning. In the present
study the step size was 0.05◦ , with counting time of 5 s per data point.
XPS valence-band and core-level spectra of the samples under study were measured using the UHV-Analysis-System assembled by SPECS (Germany). The system is
equipped with a PHOIBOS 150 hemispherical analyzer. The base pressure in the sublimation ion-pumped chamber of the system was less than 8 × 10−10 mbar during
the present experiments. The Al K␣ radiation (E = 1486.6 eV) was used as a source of
XPS spectra excitation. The XPS spectra were measured at the constant pass energy
of 25 eV. The energy scale of the spectrometer was calibrated by setting the measured Au 4f7/2 and Cu 2p3/2 binding energies to 84.00 ± 0.05 eV and 932.66 ± 0.05 eV,
respectively, with respect to the Fermi energy, EF . Energy drift due to charging effects
was calibrated, taking the XPS C 1s (284.6 eV) core-level spectrum of hydrocarbons.
The ultrasoft X-ray emission O K␣ bands (K → LII,III transition), reflecting the
energy distribution of the O 2p-like states, in the tungstates under consideration
were obtained using an RSM-500 spectrometer equipped with a diffraction grating
possessing 600 lines/mm and a radius of curvature of R ≈ 6 m. The detector was a
secondary electron multiplier VEU-6 with a CsI photocathode. Operating conditions
of an electron gun in the present experiments were the following: accelerating voltage, Ua = 5 kV; anode current, Ia = 3.5 mA. The spectrometer energy resolution was
about 0.3 eV in the energy region corresponding to the position of the O K␣ band. The
fluorescent X-ray emission W L␤5 (LIII → OIV,V transition) bands, reflecting primarily
the energy distributions of the W 5d-like states, were obtained using a Johanntype DRS-2M spectrograph. As a disperse element, a quartz crystal with the (0 0 0 1)
reflection plane was used when recording the W L␤5 bands. An X-ray BHV-7 tube
with a gold anode operating at Ua = 45 kV and Ia = 75 mA was used as a source of
spectra excitation. The spectrograph energy resolution was estimated to be 0.35 eV
when measuring the XES W L␤5 spectra.
Fig. 1. X-ray diffraction patterns of FeWO4 (upper panel) and CoWO4 (bottom panel)
nanostructures.
The XAS W LIII edges, reflecting the energy distribution of the unoccupied W dlike states, were obtained with a KRUS-1 spectrometer using scintillation recording
of the X-ray radiation intensity and employing Dobrovolsky’s method of ‘a variable
field of absorption’ [35]. The technique was analogous to that used earlier when
studying the edges in substoichiometric tungsten oxides [36]. As a disperse element, a quartz crystal with the (1 3 4̄ 0) reflecting plane and a radius of curvature of
R = 872 mm was used. The XAS W LIII spectra were excited by a BSV-23 X-ray tube
operating at Ua = 14 kV and Ia = 40 mA. The absorber covering half of the rotating sample cell was made in the form of a thin film of the tungstates under consideration
with a wax binder.
3. Results and discussion
Fig. 1 shows the XRD patterns for the as-prepared FeWO4 and
CoWO4 tungstates. The tungstates follow a wolframite-like monoclinic crystal structure. Their lattice parameters are as follows:
a = 4.73 Å, b = 5.70 Å, c = 4.95 Å, ˇ = 90.0◦ for FeWO4 and a = 4.947 Å,
b = 5.682 Å, c = 4.66 Å, ˇ = 90.0◦ for CoWO4 . The lattice parameters of the FeWO4 and CoWO4 samples under consideration are
consistent with the values reported in Refs. [3,9] and tabulated
by Joint Committee on Powder Diffraction Standards (JCPDS);
files Nos. (21-0436) and (15-0867) for iron and cobalt tungstate,
respectively. As Fig. 1 reveals, no characteristic peaks from nonreacting starting materials are detected on the XRD patterns of
the as-prepared FeWO4 and CoWO4 tungstates, indicating that the
products obtained are single phase materials. The relatively broad
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S. Rajagopal et al. / Journal of Alloys and Compounds 493 (2010) 340–345
Fig. 2. TEM images of different magnifications of the as-prepared FeWO4 (upper patterns) and CoWO4 (bottom patterns) nanostructures.
peaks on the XRD patterns presented in Fig. 1 probably result from
small sizes of the as-prepared FeWO4 and CoWO4 tungstates as will
be discussed just below.
Fig. 2 shows TEM images of the FeWO4 and CoWO4 tungstates
under study. The morphology of the iron tungstate resembles
pentagon-shaped nanocrystals, whereas cobalt tungstate looks like
oval-shaped nanocrystals. High-magnification TEM images show
that the nanocrystals have an average size of about 150 nm in the
case of FeWO4 tungstate, whilst an average size of the CoWO4
tungstate nanoparticles is about 70 nm. Unidirectional alignment
of lattice fringes confirms the single crystalline nature of the asprepared tungstate nanostructures. Thongtem et al. [2] previously
obtained similar results when synthesizing CoWO4 nanoparticles
on glass slides at 250–450 ◦ C using the same starting reagents
as in the present work. However, CoWO4 nanorods were synthesized by the hydrothermal method in Ref. [9] (CoCl2 and
Na2 WO4 were used as reaction reagents). Additionally, hierarchical plate-like FeWO4 microcrystals were directly synthesized by
a simple solvothermal route in Ref. [1] using (FeCl2 ·6H2 O) and
(Na2 WO4 ·2H2 O) solutions as starting materials. At present, we are
not aware about the hydrothermal synthesis of FeWO4 nanostructures using [(NH4 )2 Fe(SO4 )2 ·6H2 O] and (Na2 WO4 ·2H2 O) solutions
as precursors.
Survey XPS spectra of the FeWO4 and CoWO4 samples under
study are presented in Fig. 3. For the above tungstates, all the spectral features, except the C 1s level, are attributed to the constituent
element core-levels or Auger lines. The presence of the C 1s line on
the survey XPS spectra (Fig. 3) is due to the hydrocarbons adsorbed
on the FeWO4 and CoWO4 nanostructure surfaces. However, the
intensities of the C 1s core-level lines, as the survey XPS spectra
reveal, are rather low for both tungstates studied.
The XPS W 4f and O 1s core-level spectra of the FeWO4 and
CoWO4 tungstates are shown in Fig. 4. It is evident that the XPS W
Fig. 3. Survey XPS spectra of (a) FeWO4 and (b) CoWO4 nanostructures.
S. Rajagopal et al. / Journal of Alloys and Compounds 493 (2010) 340–345
Fig. 4. XPS (A) W 4f and (B) O 1s core-level spectra of (a) FeWO4 and (b) CoWO4
nanostructures.
4f core-level spectra in the both tungstates under consideration are
simple spin-doublets with the XPS W 4f7/2 binding energies corresponding to those of tungsten in the formal valence +6 [36,37]. It
should be noted that values of the XPS W 4f and O 1s core-level
binding energies listed in Table 1 are close to those in other representative tungsten-bearing oxides, such as WO3 [36,38], several
scheelite- and wolframite-type [12,18,29] as well as KY(WO4 )2 type [39–41] tungstates. The XPS Fe 2p and Co 2p core-level spectra
of FeWO4 and CoWO4 , respectively, are presented in Fig. 5, and their
binding energies are listed in Table 1. Comparison with the XPS literature data [37] reveals that iron atoms in FeWO4 and cobalt atoms
in CoWO4 tungstates under study are in the formal valence +2.
Table 1
XPS core-level binding energies (eV) for the FeWO4 and CoWO4 nanostructure
tungstates under consideration.
Compound
FeWO4
CoWO4
XPS core-level electrons
Fe 2p3/2
Co 2p3/2
W 4f7/2
O1s
711.29
–
–
780.71
35.57
35.65
530.56
530.68
Note: uncertainty is ±0.05 eV.
343
Fig. 5. XPS (A) Fe 2p and (B) Co 2p core-level spectra of FeWO4 and CoWO4 , respectively.
It is well known that X-ray absorption spectroscopy provides
important information on the charge transfer and the energy distribution of unoccupied states in the conduction band of solids
[42]. Fig. 6 presents the XAS W LIII edges of FeWO4 and CoWO4
tungstate nanostructures. For comparison, the analogous edges of
pure metallic tungsten and the monoclinic form of WO3 [34] are
also plotted. Energy positions of the inflection points of the XAS W
LIII edges determined in Ref. [36] for tungsten atoms in the formal
valence states 0 and +6 are depicted in Fig. 6. From this figure it
is evident that the inflection points of the W LIII edge of FeWO4
and CoWO4 tungstate nanostructures reveal a high-energy shift in
comparison with its position in pure tungsten. The values of highenergy shifts of the inflection points of the W LIII edges of FeWO4
and CoWO4 tungstate nanostructures equal to that of the edge in
WO3 within the present XAS uncertainties. Studies of the XAS W LIII
edges of substoichiometric tungsten oxides, both monoclinic and
hexagonal [36,43], indicate the almost monotonous high-energy
shift of the inflection point of the edges with increasing the oxygen
content x in WOx (2 ≤ x ≤ 3). This shift is explained by increasing
the positive effective charge on tungsten atoms in such a case [36].
Therefore, the coincidence of magnitudes of the high-energy shifts
of the inflection points of the XAS W LIII edges of FeWO4 and CoWO4
with that in WO3 indicates that tungsten atoms in the tungstate
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S. Rajagopal et al. / Journal of Alloys and Compounds 493 (2010) 340–345
Fig. 6. The W LIII absorption spectra of FeWO4 and CoWO4 nanostructures; the spectra derived in Ref. [34] for pure metallic tungsten and the monoclinic form of WO3
are also presented for comparison (Note: the arrows depict the energy positions of
the inflection points of the W LIII edges).
nanostructures under study are in the formal valence state +6. This
finding is in agreement with the data of measurements of the XPS
W 4f core-level binding energies of FeWO4 and CoWO4 nanostructures (Table 1) as discussed above.
Fig. 7 displays for the FeWO4 and CoWO4 tungstates the X-ray
emission W L␤5 and O K␣ bands matched with the XPS valenceband spectra provided that a common energy scale is used. The
method of matching the above XES and XPS spectra on a common energy scale is analogous to that applied successfully when
studying the electronic structure of a number of tungsten-bearing
oxides and tungstates [34,43,44]. Results of studies of the X-ray
emission bands presented in Fig. 7 indicate that in the FeWO4
and CoWO4 tungstates the main contributions of the W 5d- and
O 2p-like states are observed at the bottom and near the top of
the valence band, respectively, with contributions of the mentioned states throughout other portions of the valence band of the
compounds. These experimental findings are in agreement with
theoretical first-principles band-structure calculations of a number
of MWO4 (M = Ca, Cu, Zn, Cd, and Pb) wolframite- and scheelite-type
tungstates [28,30,33,34]. These band-structure calculations clearly
demonstrate that the W 5d-like states contribute mainly near the
bottom of the valence band of MWO4 (M = Ca, Cu, Zn, Cd, Pb) compounds, whilst the O 2p-like states are the main contributors into
the top of the valence band of the above tungstates, with also significant portions of contributions of the O 2p-like states throughout
their valence-band regions. These peculiarities of MWO4 (M = Ca,
Cu, Zn, Cd, and Pb) tungstates, as Fig. 7 reveals, are also characteristic for the FeWO4 and CoWO4 nanostructures with average particle
sizes of about 150 nm and 70 nm, respectively. We would like to
mention that in nonstoichiometric tungsten oxides and tungstates
an additional near-Fermi sub-band is formed on their XPS valenceband spectra [36,44,45]. The XPS valence-band spectra of the both
Fig. 7. The X-ray emission W L␤5 and O K␣ bands matched on a common energy
scale with the XPS valence-band spectra of FeWO4 (upper panel) and CoWO4 (bottom panel) nanostructures.
tungstates do not reveal such a sub-band (Fig. 7). This fact confirms
the stoichiometry of the oxygen sublattice in the both tungstates
under consideration.
It is necessary to mention that, studies of the X-ray emission Fe
L␣ and Co L␣ bands, reflecting the energy distributions of mainly
the Fe 3d- and Co 3d-like states, respectively, are in progress now
in our group for the FeWO4 and CoWO4 tungstate nanostructures.
4. Conclusions
FeWO4 and CoWO4 monoclinic tungstates with average sizes
of nanoparticles of about 150 nm and 70 nm, respectively,
were synthesized hydrothermally in the present work using
sodium tungstate (Na2 WO4 ·2H2 O), ferrous ammonium sulfate
[(NH4 )2 Fe(SO4 )2 ·6H2 O] and cobalt chloride (CoCl2 ·6H2 O) solutions
as precursors. XPS data reveal that the as-prepared FeWO4 and
CoWO4 nanostructure tungstates are in a composition close to a
stoichiometric one. Comparison on a common energy scale of the
X-ray emission W L␤5 and O K␣ bands and the XPS valence-band
spectra indicates that, the W 5d- and O 2p-like states contribute
throughout the whole valence-band region of the FeWO4 and
CoWO4 tungstates, however maximum contributions of the O 2plike states occur in the upper, whilst the W 5d-like states in the
lower portions of the valence band, respectively.
Acknowledgement
One of the authors, S. Rajagopal would like to thank Bharathiar
University for awarding University Research Fellowship to carry
out this work.
S. Rajagopal et al. / Journal of Alloys and Compounds 493 (2010) 340–345
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