Z. Shan et al. / Journal of Molecular Catalysis A: Chemical 302 (2009) 54–58
57
upon photoexcitation. On the other hand, MO becomes unstable in
acidic conditions, the ꢁmax of MO, which is commonly measured
by the energy required for decomposing dye molecular structure,
increases from 464 to 506 nm with the pH value decreasing from
6.9 to 3.1. This also makes the decolorization easier.
3.3. The influencing factors
There are many factors which influence the photocatalytic prop-
erty of a semiconductor, such as phase purity, crystallinity, surface
area, morphology, band gap, band structure, crystal structure and
so forth. During the sample preparation in the present experiments,
the experiment related factors were sufficiently taken into account.
The pure phase MWO4 (M = Ca, Sr, Ba) powders synthesized by
a solid state reaction are well crystallized (see Fig. 1). Therefore,
the phase purity and crystallinity cannot explain the photocatalytic
activity difference. Surface area is widely accepted to be one of the
key factors to determine the activity of a photocatalyst, because
the parameter to some extent combinatively covers the effects of
such exterior factors as particle size, pore volume, particle morphol-
ogy, etc. Normally, a larger surface area improves the photocatalytic
activity. In the present three compounds, the BET surface areas are
in the decreasing order of CaWO4 > SrWO4 > BaWO4 (see Table 2).
From the viewpoint of surface area, the photocatalytic activities
should have the same order. However, the result of experiment
(CaWO4 < SrWO4 < BaWO4) is the opposite. Apparently, the surface
area could not account for their photocatalytic activity difference.
Also, there seems to be no correlation between optical band gap
(light absorption) and photocatalytic activity (see Table 2).
Fig. 5. Tetragonal structure of MWO4 (M = Ca, Sr, Ba).
with similar band structure (similar crystal structure or
chemistry). Besides the present result of MWO4 (M = Ca,
Sr, Ba) photocatalyst system, other examples are SrSb2O7
In the three compounds, M2+ ions are photocatalytically inac-
tive while W6+ is photocatalytically active. The conduction band
mainly consists of W 5d orbitals and the valence band is primarily
composed of O 2p orbitals. Thus, the similar band structure it is con-
sidered to result in the similar electric properties and the similar
interactions between the W6+ based catalysts and the aqueous dye
(MO) solutions. This is believed to be the main reason why efforts
were made to compare various photocatalysts with the same active
center and the similar band structure were chosen as examples to
be compared by some structure-dependent models [1–3,5–7].
The experiment-related or exterior factors mentioned above
could not account for the photocatalytic activity difference of
MWO4 (M = Ca, Sr, Ba). The photocatalytic activity discrepancy can
only be ascribed to the mobility difference of electron–hole sepa-
ration in the three materials. It is believed that the mobility for the
electron–hole separation and transportation of a semiconductor is
intrinsically initiated by the detailed crystal structural properties,
as suggested in literatures [1–11]. Generally, our proposed model
could be suitable to explain detailed properties. The same phase of
the tetragonal structure is available for CaWO4, SrWO4 and BaWO4,
with space group I41/a, as shown in Fig. 5. Each W atom is coordi-
nated to four O anions to form a [WO4] tetrahedron with the W–O
bond length of 1.78 Å in CaWO4. The W atom also has some bonding
contacts with another four O anions in four neighboring tetrahedra
WO4, and the W· · ·O bonds are about 3.0 Å in CaWO4. Such contacts
are useful for charges to transfer or exchange via the tetrahedral
(64.45%) > CaSb2O7
(63.84%) > RbTi6O13
(80.46%) > KTaO3
(65.35%), Pt/BaCrO4 (59.19%) > Pt/SrCrO4 (61.18%), SrIn0.5Nb0.5O3
(72.77%) > CaIn0.5Nb0.5O3 (73.76%) > BaIn0.5Nb0.5O3 (75.18%), and
so forth [11–25].
To sum up, this macro-structural parameter (PF) in the model
scales the structural openness of the semiconductor, and reflects
the capacity of the metal-oxygen polyhedral deformations, the spa-
tial amplitude of atomic vibrations in the crystal lattice, and the
consequent electron–hole separation and carrier transport.
(85.03%),
NaTi6O13
LiTaO3
BaTa2O6
(61.65%) > KTi6O13
(68.99%) > NaTaO3
(62.42%) > MgTa2O6
4. Conclusion
MWO4 (M = Ca, Sr, Ba) were synthesized by a solid state reaction
method. The investigation results show that their catalytic proper-
ties are in the order of BaWO4 > SrWO4 > CaWO4 under both neutral
and acidic conditions. The difference of their catalytic properties
is in close connection with their micro-structural properties. The
structure-openness model works well to account for such a differ-
ence. The model for structure–property relationship may be a new
method to estimate the properties of photocatalysts.
Acknowledgements
The research was financially supported by National 973 Pro-
gram of China Grant 2007CB936704, National Science Foundation
of China Grant 50772123, Science and Technology Commission of
Shanghai Grant 0752nm016 and Science and Technology Commis-
sion of Shanghai Grant 08JC1420200.
[W–O] network. The crystal structure suggests that the packing
∞
factor of BaWO4 is 57.50%, lower than 63.51% of SrWO4 and 67.75% of
CaWO4. Therefore the structure-openness degree is in the order of
BaWO4 > SrWO4 > CaWO4 (see Table 2). As the structure-openness
degree increases, the vibratility or movability of atoms intensify,
and the charge exchange or transfer via [W–O] network become
∞
References
more spatially available. In other words, BaWO4 shows the best
photocatalytic performance of the three compounds owing to its
highest structure-openness degree.
The structure–property relationship model works well to
reveal a photocatalytic activity difference of the photocatalysts
[1] Y. Inoue, M. Kohno, S. Ogura, K. Sato, Chem. Phys. Lett. 267 (1997) 72–76.
[2] Y. Inoue, M. Kohno, S. Ogura, K. Sato, J. Chem. Soc., Faraday Trans. 93 (1997)
2433–2437.
[3] A. Kudo, H. Kato, S. Nakagawa, J. Phys. Chem. B 104 (2000) 571–575.
[4] J. Wang, Z. Zou, J.H. Ye, J. Phys. Chem. Solids 66 (2005) 349–355.