11194 J. Phys. Chem. B, Vol. 110, No. 23, 2006
Yao and Ye
of O 2p orbitals with a little contribution of the Bi 6s orbitals.
The position of the valence band, in other words the potential
of the valence band, should be dependent on the ratio of the Bi
6s to O 2p orbitals in the valence band, which should be an
important factor for the increase of the photocatalytic activities
of the solid solutions. Furthermore, as previously stated, a wide
density-of-states distribution in the conduction band is presumed
to be formed on the Ca1-xBixVxMo1-xO4 solid solutions, which
may be helpful for the mobility of the excited electrons and
may result in higher photocatalytic activities. On the other hand,
the increase of the band gap, which means that the number of
available photons decreased with the same light source, is a
possible reason for the decrease of the photocatalytic activity
of the solid solution when x values are smaller than 0.7. It is
noteworthy that the present photocatalysts prepared by the solid-
state method had a quite small surface area (<1.5 m2/g). It is
reliable to conclude that a significant increase of the photo-
catalytic activities can be obtained by increasing the surface
areas of the solid solutions as that observed in BiVO4 catalysts.
Figure 11. Composition dependence of the O2 evolved rate over
Ca1-xBixVxMo1-xO4 solid solutions under visible-light irradiation (>420
nm).
Mo0.5O4 solid solutions, indicating the present reaction is driven
by a visible-light absorption. The light-irradiation dependence
of the reaction for O2 evolutions was examined. No additional
O2 evolution was detected when the Xe lamp was turned off,
also suggesting the O2 evolution on the solid solutions should
be driven by the visible-light irradiation. It is reliable to conclude
that the O2 evolution from aqueous AgNO3 solution over the
Ca1-xBixVxMo1-xO4 solid-solution compounds is a photocata-
lytic reaction.
Conclusions
Ca1-xBixVxMo1-xO4 solid solutions were found as active
photocatalysts for O2 evolution from aqueous AgNO3 solution
under visible-light irradiation (>420 nm). The photocatalytic
activity of the solid solutions was found to be strongly dependent
on the composition, and Ca0.3Bi0.7V0.7Mo0.3O4 with a 2.42 eV
band gap showed the highest activity for O2 evolution. Photo-
physical properties and DFT calculations indicated that the band
gap of the Ca1-xBixVxMo1-xO4 solid solutions was controllable
by a change in composition. The present results suggested that
band engineering is an efficient method to develop novel
photocatalysts with visible-light response. Therefore, the finding
of novel photocatalysts based on solid solutions is encouraging.
Further research on other solid-solutions photocatalysts is also
necessary for revealing the essential factors of active solid-
solution photocatalysts.
The composition dependence of the photocatalytic activities
for O2 evolution on the solid solutions was detected. As shown
in Figure 11, the prepared solid solutions all exhibited activities
for O2 evolution from aqueous AgNO3 solutions under visible-
light irradiation, suggesting the potential of the valence band
of the solid solution should be more positive than the oxidation
potential of H2O to form O2. The two end compounds of the
solid solutions, BiVO4 and CaMoO4 synthesized under similar
conditions, showed lower activities for O2 evolution under
visible-light irradiation. The CaMoO4 did not show activities
for O2 evolution under visible-light irradiation, because of the
large band gap (3.64 eV). The activity for O2 evolution over
the monoclinic BiVO4 photocatalyst prepared by the solid-state
method is about 33.8 µmol/h, which is consistent with that
reported by other groups.12 In contrast, Ca1-xBixVxMo1-xO4 solid
solutions showed higher photocatalytic activities for O2 evolu-
tion under visible-light irradiation. Photocatalytic activities of
BiVO4 powders that had been reported strongly depended on
the crystalline phase. The scheelite monoclinic structural BiVO4
crystals exhibit a high activity for O2 evolution from aqueous
AgNO3 solution under visible-light irradiation, whereas the
photocatalytic activities of the tetragonal sheelite and zircon
structural BiVO4 for O2 evolution were reported negligible under
visible-light irradiation,12 indicating that the observed higher
photocatalytic activities of the prepared Ca1-xBixVxMo1-xO4
solid solutions cannot be attributed to the generated tetragonal
BiVO4 impurities if there are any present in the solid solutions.
The rates of O2 evolution over the solid-solution photocatalysts
are listed in Table 1. It is noteworthy that all the solid solutions
show high activities for O2 evolution, which are nearly 2-fold
higher than that of BiVO4, even when the ratio of Bi ions to
the solid solution decreases to 0.3. The highest activities were
obtained for the solid solutions when x values reach to 0.7,
whereas further decrease of the x values will reduce the
photocatalytic activities for O2 evolution. The mechanism of
the observed composition dependence of the solid-solution
photocatalytic activities is not clear at the present stage. It may
be caused by the changes in the band structure of the solid
solutions. As indicated by the DFT results (Figures 4 and 6),
the valence bands of the solid solutions are mainly composed
Acknowledgment. The authors would like to thank Ms.
Liqun Yang for her help in the experiments and thank Dr.
Weifeng Zhang for the help in the examination of the Raman
spectra. W.F. Yao thanks the Japan Society for the Promotion
of Science (JSPS) fellowship for financial support. This work
was partially supported by a Grant-in-Aid for Scientific Research
on Priority Areas (417) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) of the Japanese
Government.
Supporting Information Available: The effect of treating
temperatures on the XRD patterns, Raman spectra, and detected
photocatalytic properties of one typical solid solution Ca0.2
-
Bi0.8V0.8Mo0.2O4. This material is available free of charge via
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