Electronic band structures and photovoltaic properties of MWO4 (M=Zn, Mg, Ca, Sr) compounds

Article abstract of DOI:10.1016/j.jssc.2011.06.005

Divalent metal tungstates, MWO₄, with wolframite (M=Zn and Mg) and scheelite (M=Ca and Sr) structures were prepared using a conventional solid state reaction method. Their electronic band structures were investigated by a combination of electronic band structure calculations and electrochemical measurements. From these investigations, it was found that the band structures (i.e. band positions and band gaps) of the divalent metal tungstates were significantly influenced by their crystal structural environments, such as the WO bond length. Their photovoltaic properties were evaluated by applying to the working electrodes for dye-sensitized solar cells. The dye-sensitized solar cells employing the wolframite-structured metal tungstates (ZnWO₄ and MgWO₄) exhibited better performance than those using the scheelite-structured metal tungstates (CaWO₄ and SrWO₄), which was attributed to their enhanced electron transfer resulting from their appropriate band positions.

Full text of DOI:10.1016/j.jssc.2011.06.005

Journal of Solid State Chemistry 184 (2011) 2103–2107
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Electronic band structures and photovoltaic properties
of MWO (M¼Zn, Mg, Ca, Sr) compounds
4
a,b
c
a
a,b
a
a
Dong Wook Kim , In-Sun Cho , Seong Sik Shin , Sangwook Lee , Tae Hoon Noh , Dong Hoe Kim ,
d
a,b,n
Hyun Suk Jung , Kug Sun Hong
a
Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, South Korea
Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 151-744, South Korea
Department of Mechanical Engineering, Stanford University, Stanford, California 94305, USA
School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, South Korea
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Divalent metal tungstates, MWO , with wolframite (M¼Zn and Mg) and scheelite (M¼Ca and Sr)
4
Received 18 December 2010
Accepted 5 June 2011
Available online 14 June 2011
structures were prepared using a conventional solid state reaction method. Their electronic band
structures were investigated by a combination of electronic band structure calculations and electro-
chemical measurements. From these investigations, it was found that the band structures (i.e. band
positions and band gaps) of the divalent metal tungstates were significantly influenced by their crystal
structural environments, such as the W–O bond length. Their photovoltaic properties were evaluated
by applying to the working electrodes for dye-sensitized solar cells. The dye-sensitized solar cells
Keywords:
Divalent metal tungstate
Electronic band structure
Dye-sensitized solar cell
employing the wolframite-structured metal tungstates (ZnWO
4
and MgWO
4
) exhibited better perfor-
and SrWO ), which was
mance than those using the scheelite-structured metal tungstates (CaWO
4
4
attributed to their enhanced electron transfer resulting from their appropriate band positions.
2011 Elsevier Inc. All rights reserved.
&
1
. Introduction
crystal structure and other fundamental properties. From this point
of view, i.e. in order to search for efficient photoelectrode materials,
it is important to understand not only the various fundamental
properties of semiconducting photoelectrode materials, but also
their relationships with the other basic properties.
Semiconductor photoelectrode materials used in photoelectronic
devices, such as photocatalytic [1,2] and photovoltaic systems [3],
have attracted considerable attention because the energy conver-
sion efficiency of electronic devices is mainly affected by the
electrode materials. For example, the wide band gap semicon-
Divalent metal tungstates (MWO
4
, M¼Ni, Zn, Mg, Ca, Sr and
Ba) are known as wide band gap semiconductors with two major
crystal structures: wolframite and scheelite. Smaller divalent
cations like Mg, Zn and Ni in tungsten compounds favor the
construction of the wolframite structure with octahedron coordi-
nation, whereas scheelite structures with the tungsten atoms in
tetrahedral coordination are formed by larger divalent cations
such as Ca, Sr, Ba and Pb.
2
ductor, TiO , has been intensively studied and shown to exhibit
high efficiency as the photoelectrode in dye-sensitized solar cells
and/or photocatalytic systems, because of its superior optical,
photochemical and photoelectronic properties, as well as its
chemical stability [3,4]. Recently, however, the development
of novel materials [4–7] and the modification of well-known
materials [8] for using as efficient photoelectrode materials in
photoelectronic devices has been the subject of considerable
research for the purpose of further improving the energy conversion
efficiency of photoelectronic devices.
Yoon et al. [12] reported on the relationships between the crystal
structure and the microwave dielectric properties for various diva-
lent metal tungstates. Shan et al. [9] investigated the structure-
dependent photocatalytic activity of MWO
4
(M¼Ca, Sr and Ba). In
Generally, it is believed that the photoelectronic properties of a
material are strongly influenced by its crystal structure, and there
have been a lot of reports [9–11] on the relationships between the
addition, divalent metal tungstates have been studied in a variety of
applications, such as dielectrics [12,13], humidity sensors [14],
catalysts [15], photocatalysts [9,16,17] and laser materials [18,19].
Only a few metal tungstates have been investigated as photoelec-
trochemical devices [20,21]. Zhao et al. [20] investigated the photo-
n
current response and photocatalytic activity of porous ZnWO
and Pandey et al. [21] recently reported interesting photoelectro-
chemical properties using CuWO films. However, to the best of our
4
films,
Corresponding author. Fax: þ82 2 886 4156.
E-mail addresses: dong0414@snu.ac.kr (D.W. Kim),
kshongss@plaza.snu.ac.kr (K.S. Hong).
4
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.06.005

2
104
D.W. Kim et al. / Journal of Solid State Chemistry 184 (2011) 2103–2107
knowledge, their application as the photoanode materials for
dye-sensitized solar cells has not been reported thus far.
In the present study, divalent metal tungstates, MWO
4
(M¼Zn,
Mg, Ca and Sr), with two different crystal structure groups were
prepared using a solid-state reaction method, and their electronic
band structures were investigated using Density Functional
Theory (DFT) calculations and electrochemical measurement
methods. Moreover, the dye-sensitized solar cell performance
was also evaluated and, thereby, the effect of the electronic band
structure on the photovoltaic properties was demonstrated.
2
. Experimental section
2.1. Materials preparation
Fig. 1. Crystal structures of MWO
b) scheelite structure (M¼Ca, Sr).
4
: (a) wolframite structure (M¼Zn, Mg) and
MWO
conventional solid-state reaction method. The starting materials
of ZnO, MgO, CaCO , SrCO and WO with 99.9% purity were
4
(M¼Zn, Mg, Ca and Sr) powders were prepared using a
(
3
3
3
purchased from High Purity Chemical (Osaka, Japan). Mixtures of
the starting materials were homogenized by ball milling in
ethanol for 24 h. The mixed slurries were dried in an oven
crystal structures are reconstructed from the previous data of the
unit cell parameters and atomic positions [9,12]. Although they
have a similar chemical formula, their structural environments are
quite different. In metal tungstates with the wolframite structure,
the tungsten (W) atom is octahedrally surrounded by six oxygen
(90 1C) and then calcined at 900 1C for 2 h in air.
2.2. Materials characterization
(
O) atoms to form WO
a scheelite structure exhibit a tetragonal structure consisting of
WO tetrahedra.
6
octahedra, whereas metal tungstates with
The crystal structures and phase purities of the calcined powders
were examined by X-ray diffraction (XRD, M18XHF-SRA, Mac-
Science, Yokohama, Japan). The powder morphology and size dis-
tributions were observed by field emission scanning electron
microscopy (FESEM, JSM-6330F, JEOL, Tokyo, Japan), and the specific
surface areas were measured using the BrunauerEmmettTeller
method (BET, BELSORP-mini II, Bel, Osaka, Japan). The electroche-
mical measurements were performed using a potentiostat (CHI
4
In order to predict the effects of the crystal structure on the
electronic band structure in divalent metal tungstates, density
functional theory (DFT) calculations were conducted. Fig. 2 shows
the calculated electronic band structures and density of states
(DOS) of the divalent metal tungstates. Upon the formation of the
conduction band/valence band (CB/VB), the contribution of the
elements was similar, i.e. the bottom of the CB mainly consisted
of the W 5d orbitals and the top of the VB was predominantly
composed of the O 2p orbitals. Although the contribution of each
608C, CH Instruments, Austin, TX, USA) in a conventional three
electrode system with a platinum counter electrode and a saturated
calomel reference electrode (SCE). In addition, the electronic band
structures were calculated using the standard Cambridge serial total
energy package (CASTEP) based on plane wave density functional
theory (DFT). The kinetic energy cut-off was taken to be 300.0 eV,
and the type of the exchange-correlation potential chosen was the
generalized gradient approximation (GGA) [22] using the Perdew–
Bruke–Ernzerhof (PBE) functional [23].
4 4 4
element is similar, ZnWO (ZW), MgWO (MW), CaWO (CW) and
SrWO (SW) showed a wide range of band gap energies, viz. 2.95,
4
3.48, 4.25 and 4.58 eV, respectively, which were similar to the
average values taken from previous studies [9,16,26–30]. These
differences in the band gaps were attributed to the variations of
the crystal field strength resulting from the different interactions
between the orbitals of the W and O elements. The wolframite-
structured metal tungstates (ZW and MW) appeared to have a
2.3. Photovoltaic performance
˚
˚
longer W–O bond length (ZW¼1.951 A and MW¼1.942 A) in the
WO octahedra compared with those of the scheelite-structured
6
The sandwich-typed DSSCs were fabricated using a previously
˚
˚
metal tungstates (CW¼1.782 A and SW¼1.779 A), as summar-
ized in Table 1. According to previous studies [7,10,11], the band
gap in metal oxide compounds is strongly influenced by the
crystal structure (e.g. metal–oxygen bond length), i.e. a longer
bond length induces a weaker metal–oxygen orbital interaction
and, thus, leads to a lower band gap. Therefore, the lower band
gaps of the wolframite-structured metal tungstates were due to
the weak band splitting, which originated from the longer bond
reported procedure [24]. The thicknesses of the metal tungstate films
were measured by FESEM and the amounts of the dye adsorbed were
quantified by UV–vis spectroscopy (Lambda 35, Perkin-Elmer,
Waltham, MA, USA). The photovoltaic properties were examined
using a potentiostat under an illumination of AM 1.5 with a solar
ꢀ
2
simulator (Peccell Technologies; intensity: 100 mW cm ). Addition-
ally, an electrochemical impedance analysis of the fabricated cells
was also conducted using a potentiostat under illumination and by
applying an open circuit voltage (VOC) as the bias, in order to obtain
information on the electric transport at the various interfaces [25].
6
length in the WO octahedra than that in the scheelite structure.
On the other hand, another difference between the wolframite-
and scheelite-structured metal tungstates was the configuration at
the bottom of the CB and the top of the VB. Both ZW and MW
showed relatively dispersive band structures compared with those
of CW and SW. This indicates that the photogenerated charge
carriers of the wolframite-structured metal tungstates have a
smaller effective mass, leading to higher mobility [11]. As a result,
it was confirmed that the electronic band structures in divalent
metal tungstates are intimately related to their crystal structural
environments.
3
. Results and discussion
3.1. Crystal and electronic band structure
The schematic crystal structures of the divalent metal tung-
states, MWO
4
, with the wolframite structure (M¼Zn and Mg) and
scheelite structure (M¼Ca and Sr) are presented in Fig. 1. The

D.W. Kim et al. / Journal of Solid State Chemistry 184 (2011) 2103–2107
2105
4 4 4 4
Fig. 2. Electronic band structures and density of states (DOS): (a) ZnWO , (b) MgWO , (c) CaWO and (d) SrWO .
Table 1
Crystal structures and electronic band structures of divalent metal tungstates.
Compounds
Crystal structure
˚
Band gap (eV)
Calculated
Flat band potential
V vs. SCE at pH 8)
W–O bond length (A)
(
Literature [9,16,26–30]
ZnWO
MgWO
CaWO
SrWO
4
(ZW)
(MW)
(CW)
Wolframite
Scheelite
1.951
1.942
1.782
1.779
2.95
3.48
4.25
4.58
3.65
3.92
4.54
4.49
ꢀ1.10
ꢀ1.02
ꢀ1.16
ꢀ1.24
4
4
4
(SW)
Metal tungstate powders were practically synthesized and
characterized in order to support the above results obtained by
theoretical calculations. Fig. 3 shows the XRD patterns of the
metal tungstate powders prepared by a solid-state reaction
method. Each powder exhibited only a single phase, and all of
the peaks were assigned to ZW, MW, CW and SW, as indexed in
JCPDS Nos. 88-0251, 27-0789, 41-1431 and 08-0490, respectively.
Additionally, the morphologies of the obtained powders were
irregular, but their particle sizes were similar to each other, viz.
approximately 1–3
mm (not shown here). There was also little
difference in the BET surface area of the metal tungstate powders
2
ꢀ1
(
ꢁ1 m g ).
Generally, the flat band potential (Vfb) is the potential required
to flatten the band bending caused by the semiconductor–
electrolyte junction, and it is a very useful parameter for the
study of the electronic band structure. Hence, Vfb was measured
by the electrochemical method for practical verification.
Fig. 4 shows the Mott–Schottky plots of the metal tungstate
thick film electrodes in 1.0 M KCl (pH 8) electrolyte solution.
Firstly, the measured Vfb values were ꢀ1.10, ꢀ1.02, ꢀ1.16 and
ꢀ
1.24 V vs. SCE for ZW, MW, CW and SW, respectively, all of
Fig. 3. X-ray diffraction patterns of MWO
4
(M¼Zn, Mg, Ca and Sr) powders
which were negative. These negative values indicated that all of
prepared by a solid-state reaction method.

2
106
D.W. Kim et al. / Journal of Solid State Chemistry 184 (2011) 2103–2107
SW4CW4ZW4MW (lowest). This indicates that MW exhibited
the highest potential difference ( ) between the CB edge level and
D
the LUMO level of N719 and, thus, a relatively higher driving force
for electron injection from N719 to MW. On the contrary, the
driving force for electron transfer from N719 to SW was the
lowest. This is noteworthy for understanding the photoelectro-
chemical properties of these divalent metal tungstates.
3.2. Photovoltaic performance
DSSCs were fabricated using the prepared metal tungstate
powders as the photoelectrodes and their photovoltaic properties
were measured, in order to investigate the relationship between the
electronic band structure and the photovoltaic/photoelectrochemical
properties. Fig. 6 shows the photocurrent density–voltage (J–V)
curves of the fabricated DSSCs. As summarized in Table 2, although
all of the fabricated cells exhibited quite low photovoltaic perfor-
mances, they were sufficient to investigate the difference in their
photovoltaic properties. Moreover, these efficiency values could be
further improved using nano-sized powders with a higher surface
area. The photon energy conversion efficiency was observed to be in
the order of ZW4MW4CW4SW. It was found that the DSSCs
employing the metal tungstates with the wolframite structure (ZW
and MW) exhibited better performance than those with the scheelite
structure (CW and SW). Basically, the role of the semiconductor layer
in the DSSC is to assist the electron transport from the dye molecules
to the transparent conducting oxide (TCO), i.e. the electron injection
and collection process, and thus the electronic band structure is an
important factor for achieving a higher cell performance. Therefore,
the cell efficiency is bound to be affected by the above factors, for
example the CB edge level and the dispersive band structure.
The measured cell efficiency showed somewhat different results
compared with the trend of the CB edge level. Accordingly, an
Fig. 4. Mott–Schottky plots of MWO
4
(M¼Zn, Mg, Ca and Sr) electrodes in 1 M KCl
electrolyte (pH 8) at a frequency of 1 kHz.
Fig. 5. Suggested electronic band structures of MWO
4
(M¼Zn, Mg, Ca and Sr)
compounds.
the synthesized divalent metal tungstates exhibited n-type semi-
conducting behavior. Furthermore, to facilitate the understanding
of these divalent metal tungstates and for the purpose of
comparison, the potential value vs. SCE should be converted to
the potential vs. normal hydrogen electrode (NHE) by adding
0
0
.242 V. Also, the linear pH dependence of Vfb with a slope of
.059 V/pH was reported in previous studies [31] for most metal
oxide semiconductors, which was attributed to the degree of
protonation at the electrode surface. Using these relations, the Vfb
values of ZW, MW, CW and SW were found to be ꢀ0.386, ꢀ0.306,
ꢀ
0.446 and ꢀ0.526 V vs. NHE at pH 0, respectively.
Assuming that the difference between the flat band potential
and the CB edge potential is negligible for n-type semiconductors,
the CB edge levels of the divalent metal tungstates can be
obtained. On the basis of these Vfb values and the band gaps
obtained from the DFT calculation, the overall band structures of
the metal tungstates are described in Fig. 5. The lowest unoccu-
pied molecular orbital (LUMO) and the highest occupied mole-
cular orbital (HOMO) levels of the N719 dye molecule are also
indicated [32], as shown in the figure; all of the CB levels of these
compounds were observed to be positioned within the gap
between the LUMO and the HOMO levels of the N719 dye
molecule. Among them, the metal tungstates with the wolframite
structure showed a relatively lower CB edge level than those with
the scheelite structure, i.e. the CB edge levels were in the order of
Fig. 6. J–V characteristics of the DSSCs employing the synthesized divalent metal
tungstate photoelectrodes.
Table 2
Photovoltaic parameters of DSSCs employing MWO
4
(M¼Zn, Mg, Ca and Sr)
photoelectrodes.
ꢀ2
Semiconductors
VOC (V)
JSC (mA cm
)
FF
Eff (%)
3
omax in R (Hz)
ZnWO
MgWO
CaWO
SrWO
4
(ZW)
(MW)
(CW)
0.57
0.46
0.41
0.39
0.21
0.18
0.13
0.12
0.459 0.054
0.429 0.036
0.422 0.023
0.433 0.020
3.74
4.54
3.74
3.74
4
4
4
(SW)

D.W. Kim et al. / Journal of Solid State Chemistry 184 (2011) 2103–2107
2107
potential values and indicated that the metal tungstates with the
wolframite structure had lower conduction band edge levels than
those with the scheelite structure. On the basis of the electronic
band structure analysis, the metal tungstates were used as the
working electrodes for DSSCs and the resulting energy conversion
efficiencies were found to be in the order of ZnWO
CaWO 4SrWO . The relatively higher DSSC performance of the
metal tungstates with the wolframite structure, especially ZnWO
4 4
4MgWO 4
4
4
4
,
was attributed to their efficient electron transfer, i.e. higher electron
injection and lower electron recombination rates.
Acknowledgment
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korean government (MEST)
(2009-0092779) and this work was also supported by the Nano
R&D program through the National Research Foundation of Korea
funded by MEST (2009-0082659) and the research program 2009
of the Kookmin University in Korea.
Fig. 7. Electrochemical impedance spectra of MWO
4
-based DSSCs (M¼Zn, Mg, Ca
and Sr).
additional electrochemical impedance spectrum analysis was per-
formed to further understand the difference in the charge transfer
and the charge recombination rate. Fig. 7 presents the Nyquist plots
of the DSSCs employing the divalent metal tungstates. Generally, the
impedance components in a DSSC are observed in the frequency
References
[
1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995)
9–96.
[2] A. Kudo, Int. J. Hydrogen Energy 32 (2007) 2673–2678.
6
3
5
3
ranges of 10 ꢁ10 Hz (
These components are assigned to the impedances (R) at the
TCO/semiconductor ( ), counter electrode/electrolyte ( ) and
semiconductor/dye/electrolyte ( ) interfaced, and that caused by
the diffusion of the electrolyte ( ) [25]. It was observed that the
measured value of R (at ) gradually decreased in the order of SW,
CW, ZW and MW. The decrease in R was attributed to the increase
in the injected electron density resulting from the adsorbed N719
dye molecules, due to the higher between the CB edge level and
o
1
or
o
2 3 4
), 1–10 Hz (o ) and 0.1–1 Hz (o ).
[
[
3] M. Gratzel, Nature 414 (2001) 338–344.
4] S. Burnside, J.-E. Moser, K. Brooks, M. Gratzel, J. Phys. Chem. B 103 (1999)
o
1
o
2
9328–9332.
o
3
[5] B. Tan, E. Toman, Y. Li, Y. Wu, J. Am. Chem. Soc. 129 (2007) 4162–4163.
[
[
[
[
6] T. Lana-Villarreal, G. Boschloo, A. Hagfeldt, J. Phys. Chem. C 111 (2007)
549–5556.
7] I.-S. Cho, C.H. Kwak, D.W. Kim, S. Lee, K.S. Hong, J. Phys. Chem. C 113 (2009)
10647–10653.
o
4
5
3
o
3
3
8] J.Y. Kim, S.B. Choi, D.W. Kim, S. Lee, H.S. Jung, J.-K. Lee, K.S. Hong, Langmuir 24
(2008) 4316–4319.
D
9] Z. Shan, Y. Wang, H. Ding, F. Huang, J. Mol. Catal. A—Chem. 302 (2009) 54–58.
the LUMO level. Therefore, although MW seemed to be a favorable
material to be used as the photoelectrode from the viewpoint of
electron injection, the ZW-based DSSC showed higher performance
than the MW-based DSSC. These results could be additionally
explained by another factor affecting the photovoltaic performance,
[10] S. Ouyang, Z. Li, Z. Ouyang, T. Yu, J. Ye, Z. Zou, J. Phy. Chem. C 112 (2008)
3134–3141.
[
11] S. Ouyang, N. Kikugawa, D. Chen, Z. Zou, J. Ye, J. Phys. Chem. C 113 (2009)
560–1566.
1
[12] S.H. Yoon, D.-W. Kim, S.-Y. Cho, K.S. Hong, J. Eur. Ceram. Soc. 26 (2006)
2051–2054.
[
[
[
13] R.C. Pullar, S. Farrah, N.McN. Alford, J. Eur. Ceram. Soc. 27 (2007) 1059–1063.
14] A.K. Bhattacharya, R.G. Biawas, A. Hartridge, J. Mater. Sci. 32 (1997) 353–356.
15] D.S. Kim, M. Ostromecki, I.E. Wachs, J. Mol. Catal. A—Chem. 106 (1996)
93–102.
namely the maximum frequency (
o
max) in R
3
is inversely propor-
tional to the electron lifetime ( ) [25,33]. The
t
omax value of the MW-
based DSSC was relatively higher (i.e. shorter electron lifetime), as
shown in Table 2, which indicates that the recombination rate of
injected electrons is higher. Consequently, the higher photovoltaic
performance for the ZW-based DSSC was considered to originate
from its electronic properties, such as the higher driving force for the
electron injection process and lower electron recombination rate.
Overall, the electronic band structure and photovoltaic properties
of DSSCs are closely related to each other and, moreover,
the photovoltaic performance can be adjusted and improved by
controlling the crystal structure and electronic band structure of the
semiconductor materials.
[
[
[
16] H. Fu, J. Lin, L. Zhang, Y. Zhu, Appl. Catal. A—Gen. 306 (2006) 58–67.
17] G. Huang, Y. Zhu, Mat. Sci. Eng. B 139 (2007) 201–208.
18] N. Faure, C. Borel, M. Couchaud, G. Basset, R. Templier, C. Wyon, Appl. Phys. B
63 (1996) 593–598.
[19] A.A. Kaminskii, H.J. Eichler, K.-I. Ueda, N.V. Klassen, B.S. Redkin, L.E. Li,
J. Findeisen, D. Jaque, J. Garcia-Sole, J. Fernandez, R. Balda, Appl. Optics 38
(1999) 4533–4547.
[20] X. Zhao, W. Yao, Y. Wu, S. Zhang, H. Yang, Y. Zhu, J. Solid State Chem. 179
(2006) 2562–2570.
[
[
[
21] P.K. Pandey, N.S. Bhave, R.B. Kharat, Mater. Lett. 59 (2005) 3149–3155.
22] J.A. White, D.M. Bird, Phys. Rev. B 50 (1994) 4954–4957.
23] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865–3868.
[24] H.S. Jung, J.-K. Lee, S. Lee, K.S. Hong, H. Shin, J. Phys. Chem. C 112 (2008)
476–8480.
25] T. Hoshikawa, M. Yamada, R. Kikuchi, K. Eguchi, J. Electrochem. Soc. 152
2005) E68–E73.
8
[
[
[
[
(
4
. Conclusion
26] R. Lacomba-Perales, J. Ruiz-Fuertes, D. Errandonea, D. Martinez-Garcia,
A. Segura, EPL 83 (2008) 37002.
27] M. Bonanni, L. Spanhel, M. Lerch, E. Fuglein, G. Muller, Chem. Mater. 10
Divalent metal tungstates with wolframite and scheelite struc-
(1998) 304–310.
tures were successfully prepared and their electronic band struc-
tures were characterized. From the electronic band calculation, the
wolframite-structured metal tungstates exhibited comparatively
28] S.K. Arora, B. Chudasama, Crys. Res. Technol. 41 (2006) 1089–1095.
[29] Y. Zhang, N.A.W. Holzwarth, R.T. Williams, Phys. Rev.
12738–12750.
B 57 (1998)
[
[
[
[
30] V.B. Mikhailik, H. Kraus, D. Wahl, M. Itoh, M. Koike, I.K. Bailiff, Phys. Rev. B 69
2004) 205110.
31] H. Gerischer, in: H. Eyring, D. Henderson, W. Jost (Eds.), Physical Chemistry:
An Advanced Treatise, vol. IXA, Academic Press, New York, 1970.
smaller band gaps of 2.95 and 3.48 eV for ZnWO
whereas the scheelite-structured metal tungstate exhibited much
4
larger band gaps of 4.25 and 4.58 eV for CaWO and SrWO ,
4 4
and MgWO ,
(
4
32] S. Gubbala, V. Chakrapani, V. Kumar, M.K. Sunkara, Adv. Funct. Mater. 18
respectively; this was attributed to their different crystal structural
environments, such as their W–O bond lengths. Also, the overall
band structures were described with the aid of the flat band
(2008) 2411–2418.
33] J. van de Lagemaat, N.-G. Park, A.J. Frank, J. Phys. Chem. B 104 (2000)
2044–2052.