Photophysical properties and pbotocatalytic activities of bismuth molybdates under visible light irradiation

Article abstract of DOI:10.1021/jp0622482

Aurivillius structure Bi₂MoO₆ (BG: 2.70 eV) that is a low-temperature phase showed an intense absorption band in the visible light region and photocatalytic activity for O₂ evolution from an aqueous silver nitrate solution under visible light irradiation, among various bismuth molybdates (Bi₂MoO₆, Bi₂Mo₂O ₉, and Bi₂Mo₃O₁₂) synthesized by solid-state and reflux reactions. Bi₂Mo₃O₁₂ (BG: 2.88 eV) also showed photocatalytic activity for O₂ evolution under full-arc irradiation of a Xe lamp (λ > 300 nm). The photocatalytic activity of the Aurivillius structure Bi₂MoO ₆ prepared by the reflux method was dependent on the annealing temperature after the preparation. The crystallinity was the important factor for the activity. Calculation by the density functional method indicated that the conduction band of Aurivillius structure Bi₂MoO₆ was made up of Mo 4d orbitals. It turned out that the visible-light absorption of this photocatalyst was due to the transition from the valence band consisting of O 2p orbitals to the conduction band. The corner-sharing structure of the MoO₆ octahedra contributed to the visible light response and the photocatalytic performance because excitation energy and/or photogenerated electron and hole pairs began to migrate easily in the Aurivillius structure.

Full text of DOI:10.1021/jp0622482

17790
J. Phys. Chem. B 2006, 110, 17790-17797
Photophysical Properties and Photocatalytic Activities of Bismuth Molybdates under Visible
Light Irradiation
†
†
‡
,†,§
Yoshiki Shimodaira, Hideki Kato, Hisayoshi Kobayashi, and Akihiko Kudo*
Department of Applied Chemistry, Faculty of Science, Science UniVersity of Tokyo, 1-3 Kagurazaka,
Shinjyuku-ku, Tokyo 162-8601, Japan, Department of Chemistry and Materials Technology, Kyoto Institute of
Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan, and Core Research for EVolutional Science and
Technology, Japan Science and Technology Agency (CREST, JST), 4-1-8 Honcho, Kawaguchi-shi,
Saitama 332-0012, Japan
ReceiVed: April 11, 2006; In Final Form: July 21, 2006
Aurivillius structure Bi
band in the visible light region and photocatalytic activity for O
solution under visible light irradiation, among various bismuth molybdates (Bi
Mo 12) synthesized by solid-state and reflux reactions. Bi Mo 12 (BG: 2.88 eV) also showed photocatalytic
activity for O evolution under full-arc irradiation of a Xe lamp (λ > 300 nm). The photocatalytic activity of
the Aurivillius structure Bi MoO prepared by the reflux method was dependent on the annealing temperature
after the preparation. The crystallinity was the important factor for the activity. Calculation by the density
functional method indicated that the conduction band of Aurivillius structure Bi MoO was made up of Mo
d orbitals. It turned out that the visible-light absorption of this photocatalyst was due to the transition from
the valence band consisting of O 2p orbitals to the conduction band. The corner-sharing structure of the
MoO octahedra contributed to the visible light response and the photocatalytic performance because excitation
energy and/or photogenerated electron and hole pairs began to migrate easily in the Aurivillius structure.
2
MoO
6
(BG: 2.70 eV) that is a low-temperature phase showed an intense absorption
evolution from an aqueous silver nitrate
MoO , Bi Mo , and Bi
2
2
6
2
2
O
9
2
-
3
O
2
3
O
2
2
6
2
6
4
6
1. Introduction
The photocatalytic water splitting into H2 and O2 which is
an ideal way to get H2 has been an important reaction from the
1
-3
viewpoint of global energy and environmental issues.
0
Recently, new photocatalysts consisting of metal ions with d
1
0
and d electron configurations have been developed for
photocatalytic H2 and O2 evolution from water under UV
irradiation, one after another.1
-5
Titanium of the fourth group and niobium and tantalum of
0
the fifth group are the main components of d oxide photocata-
lysts.6 WO3 with the corner-sharing structure of WO6 octahedra
is well-known to show high photocatalytic activity for O2
evolution from aqueous solutions containing electron acceptors
under visible light irradiation as a representative photocatalyst
of the sixth groups’ oxides.8-10 PbWO4 has recently been
,7
2 6 3
Figure 1. Structures of (a) Aurivillius-type Bi MoO and (b) WO .
) Ti, Nb, W, Mo, etc.) possess unique layered structures in
1
1
2-
reported to be active for water splitting under UV irradiation.
which perovskite slabs of (A B O3n+1) are sandwiched
n-1
n
2
+
15-20
In contrast, although the oxides of molybdenum, which is from
the same group as tungsten, have been used for many industrial
between (Bi O ) layers as shown in Figure 1.
structure of the (An-1B O3n+1) layer is similar to that of a
The
2
2
2
-
n
12,13
reactions and reported as fixed catalysts for partial oxidation,
WO photocatalyst because they have corner-shared octahedra
3
21-23
24,25
26
there is no report that molybdenum is used as the main
composition element of photocatalysts for water splitting into
H2 and O2, except for some molybdates with a Scheelite
structure which have activity for O2 evolution from an aqueous
silver nitrate solution.14
structures. Dielectric,
ion conductive,
luminescent, and
catalytic properties of these materials family have attracted
2
7
attention. The authors have reported that some tungstates and
titanates with Aurivillius structure are new photocatalysts for
28
H2 or O2 evolution. Moreover, BiVO4 was found to be a
Bismuth mixed oxides with the Aurivillius structure repre-
sented by (Bi2O2) (An-1BnO3n+1) (A ) Ba, Bi, Pb, etc., B
photocatalyst which is active for O evolution from an aqueous
silver nitrate solution under visible light irradiation. Some
2
2+
2-
29
bismuth oxides such as Bi2WO6 have recently been reported as
30
*
To whom correspondence should be addressed. E-mail: a-kudo@
photocatalysts for degradation of organic pollutants. Therefore,
the photocatalytic properties of the bismuth mixed oxides with
Aurivillius structure and the related compounds are also of
interest.
rs.kagu.tus.ac.jp. Fax: +81-33235-2214.
†
Science University of Tokyo.
Kyoto Institute of Technology.
Japan Science and Technology Agency.
‡
§
1
0.1021/jp0622482 CCC: $33.50 © 2006 American Chemical Society
Published on Web 08/19/2006

Bismuth Molybdate Photocatalysts
J. Phys. Chem. B, Vol. 110, No. 36, 2006 17791
In this paper, photophysical and photocatalytic properties of
3
+
2
6+
oxides consisting of a Bi ion with 6s configuration and Mo
0
ion with 4d configration were studied to develop photocatalysts
with visible-light response. The effects of the synthetic condi-
tions and phase transition on the photocatalytic activities were
examined. The energy structures were also discussed by the
plane-wave-based density functional method calculation.
2. Experimental Section
Bi2MoO6 was synthesized by a reflux method and a solid-
31
state reaction. Bi2Mo2O9 and Bi2Mo3O12 were synthesized only
by solid-state reactions.32 The starting materials used for the
reflux reaction were Bi(NO3)3‚5H2O (Kanto Chemical; purity
>99.5%) and H2MoO4 (Kanto Chemical). The starting materials
used for the solid-state reaction were Bi2O3 (Kanto Chemical;
purity >99.9%) and MoO3 (Kanto Chemical; purity >99.5%).
In the case of a solid-state reaction, the starting materials were
mixed on an alumina mortar and the mixtures were calcined at
Figure 2. Diffuse reflectance spectra of (a) AS-Bi
Bi MoO , (c) Bi Mo , and (d) Bi Mo
2 6
MoO , (b) HTP-
2
6
2
O
2 9
2
3 12
O .
6
9
73-973 K for 5 h in air using an alumina crucible (purity:
gaps of AS-Bi2MoO6, HTP-Bi2MoO6, Bi2Mo2O9, and Bi2-
Mo3O12 were estimated to be 2.70, 3.02, 3.10, and 2.88 eV from
the onsets of absorption edges, respectively. AS-Bi2MoO6 of a
low-temperature phase with a corner-sharing structure of MoO6
9.7%). In the case of a reflux reaction, the starting materials
were soaked in water in a stoichiometric ratio, refluxed on a
mantle heater for 24 h. The products were washed with water
and filtered. The obtained precursors were dried at 320 K for
1
8
octahedra possessed the onset of the absorption edge at the
longest wavelength among them. HTP-Bi2MoO6 and Bi2Mo2O9
15 h in an oven and calcined at several temperatures (Bi2MoO6,
573-973 K for 5 h; Bi2Mo2O9, 823 K for 5 h; Bi2Mo3O12, 773-
973 K for 5 h) in air using an alumina crucible. We have
1
8,32
with isolated MoO4 tetrahedra
showed the onsets at the
shortest wavelength. Bi2Mo3O12 with both MoO4 tetrahedra and
especially paid attention to the phase transition temperature for
the calcination of Bi2MoO6. The purity of the obtained powders
were confirmed by X-ray diffraction (Rigaku: RINT1400,
MiniFlex).
Photocatalytic O2 evolution from an aqueous silver nitrate
solution and H2 evolution from an aqueous methanol solution
were carried out in a gas-closed circulation system. Catalyst
powder (0.3 g) was dispersed by a magnetic stirrer in a reactant
solution (0.05 mol/L AgNO3(aq) or 10 vol % MeOH(aq), 150
mL) in a cell with a side-window made of Pyrex. The light
source was a 300-W Xe lamp (ILC technology; Cermax LX-
3
2
MoO6 octahedra showed just the onset of the intermediate
region. The less the coordination number of oxygen ions to a
molybdenum ion was, the wider the band gap was. This is
explained by the interaction of oxygen and molybdenum orbitals.
The less the coordination number is, the stronger the interaction
between oxygen ions and a molybdenum ion is. The strong
interaction makes the energy gap between a conduction band
consisting of Mo 4d and a valence band consisting of O 2p
wide. This results in the band gaps of the samples consisting
of MoO4 tetrahedra being wider than those consisting of MoO6
octahedra. Moreover, the widths of the conduction bands of the
samples consisting of MoO4 tetrahedra would be narrower than
those consisting of MoO6 octahedra because the d-d splitting
affecting the conduction bandwidth in MoO4 tetrahedra is
smaller than that in MoO6 octahedra judging from crystal field
theory. This effect also makes the band gap wider if the valence
band positions are similar to each other. Therefore, we concluded
that the band gap was mainly influenced by the coordination
number of oxygen ions to a molybdenum ion.
3
00F). Cutoff filters (Hoya) were employed for controlling the
wavelength of incident light. Amounts of evolved gases were
determined using gas chromatography (Shimazu, GC-8A, MS-
5
using a UV-vis-NIR spectrometer (Jasco; UbestV-570) and
were converted from reflection to absorbance by the Kubelka-
Munk method. Surface areas were determined by BET measure-
ments (Coulter; SA3100). Photocatalyst powders were observed
by a scanning electron microscope (Hitachi; S-5000). Thermo-
gravimetric analysis and phase transition were examined by a
thermogravimetry differential thermal analyzer (Ulvac; TGD-
A, TCD, Ar carrier). Diffuse reflection spectra were obtained
3.2. Energy Structure. The band structures of low-temper-
ature phase AS-Bi2MoO6, HTP-Bi2MoO6, Bi2Mo2O9, and Bi2-
Mo O were studied by the plane-wave-based density functional
9
600).
3
12
The plane-wave-based density functional method calculation
method to clarify which orbitals contribute to valence band and
conduction band formation.
was carried out for the low-temperature phase Bi2MoO6 with
Aurivillius structure (AS-Bi2MoO6), high-temperature phase Bi2-
MoO6 (HTP-Bi2MoO6), Bi2Mo2O9, and Bi2Mo3O12 by employ-
Figure 3a,b shows the band structure, the density of states
(DOS), and the density contour maps for the LUMO and HOMO
33
ing a CASTEP program. The core electrons were replaced
of low-temperature phase AS-Bi2MoO6. It was estimated that
the valence band (#A25-#A60) was mainly derived from the O
with ultrasoft core potentials,34 and the valence electronic
2
3
2
6
configurations for Bi, Mo, and O atoms were 6s 6p , 4s 4p -
2
p orbitals and the conduction band (#A61-#A70) was derived
1
5
2
4
5
s 4d , and 2s 2p , respectively. The calculations were carried
from the Mo 4d orbitals and the Bi 6p orbitals. The contribution
out using the conventional unit cells of [Bi2MoO6]2 for AS-
Bi2MoO6, [Bi2MoO6]16 for HTP-Bi2MoO6, [Bi2Mo2O9]16, and
Bi2Mo3O12]4. The total numbers of the electrons were 120, 960,
36, and 496, and the numbers of the occupied orbitals were
0, 480, 368, and 248, respectively.
of the Bi 6s orbitals to the valence band formation, as seen in
29,30
BiVO4 and Bi2WO6, was not observed.
The O 2p orbitals
[
7
6
along the direction of the layer of MoO6 octahedra mainly
contributed to the formation of the top of the valence band
(HOMO) rather than that in the vertical direction. Moreover,
3
. Results and Discussion
the contribution of O 2p orbitals of the Bi2O2 layer was not
observed for the HOMO. The Bi 6p orbitals for the LUMO
were not detected when the isosurface in the density of states
3
.1. Diffuse Reflectance Spectrum. Diffuse reflectance
spectra of bismuth molybdates are shown in Figure 2. The band

17792 J. Phys. Chem. B, Vol. 110, No. 36, 2006
Shimodaira et al.
2 6
Figure 3. (a) Band structure and total density of state and (b) density contour maps for the LUMO and HOMO for AS-Bi MoO calculated by the
density functional method.
was 0.3. They were observed when the isosurface in the density
of states was 0.1. Hence, the contribution of each orbital was
not equivalent, although the LUMO consisted of hybrid orbitals
of Mo 4d and Bi 6p orbitals. The Mo 4d orbitals mainly
contributed to the LUMO rather than the Bi 6p orbital. The
main orbitals consisting of the conduction band were Mo 4d
orbitals. Visible light absorption of AS-Bi2MoO6 was revealed
to be due to the transition from the valence band consisting of
the O 2p orbitals to the conduction band derived from the
primary Mo 4d orbitals in MoO6 octahedra and the secondary
Bi 6p orbitals.
orbitals contributed to raising their valence bands resulting in a
narrowing of their band gaps.
Especially paying attention to the Mo 4d orbitals forming
the conduction band (#D249-#D270) of Bi2Mo3O12, which
contains both MoO4 tetrahedra and MoO6 octahedra, by analyz-
ing density contour maps, one finds the Mo 4d orbitals of MoO6
octahedra widely contributed to the whole conduction band (#D-
249-#D270), whereas the MoO4 tetrahedra contributed to only
the middle of the conduction band (#D257-#D263). This means
that the Mo 4d orbitals of the MoO6 octahedra generate wide
conduction bands. It is also one of the reasons that the band
gap of a material containing the MoO6 octahedra was smaller
than that containing the MoO4 tetrahedra. The calculated band
gaps of AS-Bi2MoO6, HTP-Bi2MoO6, Bi2Mo2O9, and Bi2-
Mo3O12 were 0.68, 2.86, 3.07, and 2.42 eV, respectively. The
order in the calculated band gaps corresponded to the values
experimentally determined from the diffuse reflectance spec-
trum. Distribution of the band structure gives information on
All valence bands of HTP-Bi2MoO6, Bi2Mo2O9, and Bi2-
Mo3O12 mainly consisted O 2p orbitals as well as occurred for
AS-Bi2MoO6 as shown in Figures 4-6. However, the tops of
the valence bands including the HOMOs of HTP-Bi2MoO6 (#B-
4
2
77-#B480), Bi2Mo2O9 (#C366-#C368), and Bi2Mo3O12 (#D-
47-#D248) consisted not only of the O 2p orbitals but also of
Bi 6s orbitals partly. All conduction bands of the three materials
consisted of Mo 4d orbitals. In contrast to AS-Bi2MoO6, Bi 6s
3
5
the mobility of excitation energy. The distribution of AS-Bi2-

Bismuth Molybdate Photocatalysts
J. Phys. Chem. B, Vol. 110, No. 36, 2006 17793
2 6
Figure 4. (a) Band structure and total density of state and (b) density contour maps for the LUMO and HOMO for HTP-Bi MoO calculated by
the density functional method.
MoO6 was larger than those of HTP-Bi2MoO6, Bi2Mo2O9, and
Bi2Mo3O12. This property is advantageous for photocatalytic
performance.
It is considered that the reason AS-Bi2MoO6 showed higher
activity than Bi2Mo3O12 is the crystal structure. AS-Bi2MoO6
possesses perovskite-like slabs consisting of MoO6 octahedra
of which the structure is often advantageous for photocatalysts36
because excitation energy can easily migrate in the structure as
suggested from the distribution of the band structure as shown
in Figure 2a. Another reason was the difference in the number
of absorbed photon because of the different band gaps. On the
other hand, Bi2Mo3O12 has a certain connection between MoO4
tetrahedra and MoO6 octahedra while the degree of isolation of
MoO4 tetrahedra is large in HTP-Bi2MoO6 and Bi2Mo2O9.
Therefore, the degree of the localization of photogenerated
3
.3. Photocatalytic Activities. The photocatalytic activities
of bismuth molybdates are summarized in Table 1. Low-
temperature phase AS-Bi2MoO6 and Bi2Mo3O12 showed reason-
able activities for O2 evolution from an aqueous silver nitrate
solution under visible light irradiation. If this is expressed in
another way, only these materials containing MoO6 octahedra
in the structure showed the photocatalytic activity whereas those
containing MoO4 tetrahedra showed negligible photocatalytic
activities among the bismuth molybdates tested.

17794 J. Phys. Chem. B, Vol. 110, No. 36, 2006
Shimodaira et al.
2 2 9
Figure 5. (a) Band structure and total density of state and (b) density contour maps for the LUMO and HOMO for Bi Mo O calculated by the
density functional method.
electron-hole pairs in HTP-Bi2MoO6 and Bi2Mo2O9 is larger
than that in Bi2Mo3O12. It results in that photocatalytic activities
of HTP-Bi2MoO6 and Bi2Mo2O9 were negligible.
construction of a Z-scheme for overall water splitting under
visible light irradiation.
3
8
Figure 7 shows the X-ray diffraction patterns of Bi2MoO6
powders obtained by calcining the precursor prepared by a reflux
method. A single low-temperature phase AS-Bi2MoO6 was
obtained even as prepared. The Aurivillius phase was observed
by calcination at wide range temperatures (373-823 K). The
phase transition from the Aurivillius phase to the high-
temperature phase, HTP-Bi2MoO6, was observed at 873 K. The
single high-temperature form was obtained above 923 K. This
result agreed with the TG-DTA measurement reported by
No bismuth molybdates were active for H2 evolution from
an aqueous methanol solution. From the band calculation, the
bottom-up effect of the valence band by Bi 6s orbitals as seen
in the BiVO4 photocatalyst material29 was not observed for AS-
Bi2MoO6, indicating that the top of the valence band was formed
by only O 2p orbitals. Therefore, the conduction band level of
AS-Bi2MoO6 does not seem to be enough for water reduction
to form H2, judging from the valence band character and the
0
37
39
band gap of a d metal oxide. We believe that the development
of new photocatalysts with activities even for a half reaction of
water splitting, H2 or O2 evolution in the presence of sacrificial
regents, is important. This material actually contributes to the
Kodama et al. In the phase transition of Bi2MoO6, there is an
irreversible second-order phase transition from ferroelectrics to
paraelectrics with anomalous specific heat thereby transforming
the layer structure containing MoO6 octahedra into a bulk

Bismuth Molybdate Photocatalysts
J. Phys. Chem. B, Vol. 110, No. 36, 2006 17795
Figure 6. (a) Band structure and total density of state and (b) density contour maps for the LUMO and HOMO for Bi
2
Mo
3
O12 calculated by the
density functional method.
2
TABLE 1: Photocatalytic O
Molybdates
2
Evolution over Bismuth
was drastically decreased from 41.1 to 0.9 m /g. Figure 8 shows
SEM images of the low-temperature phase (AS-Bi2MoO6) as
prepared and calcined at 823 K. The particle shape of noncal-
cined Bi2MoO6 was platelike, and the size was 20-100 nm.
The particle size of the sample calcined at 823 K was 1-3 µm
by sintering. The sintered particles consisted of aggregates of
primary particles.
incident
activityb
photocatalysta
AS-Bi MoO
band gap/eV
2.70
light/ nm
-1
2
O /µmol h
2
6
λ > 420
λ > 300
λ > 300
λ > 300
λ > 420
λ > 300
55
127
0.7
1.8
7.6
46
reflex method
HTP-Bi MoO
2
6
3.02
3.10
2.88
Bi
Bi
2
Mo
Mo
2
O
O
9
Shifts of absorption edges of diffuse reflectance spectra were
observed by the calcination at wide temperature ranges as shown
in Figure 9. The absorption edge was shifted to the long-
wavelength side and became steep as the calcination temperature
was increased from 373 to 823 K. The change in the DRS is
due to the crystallinity as observed by XRD. The narrow band
gap was obtained for AS-Bi2MoO6 calcined at 823 K, which
possessed high crystallinity. AS-Bi2MoO6 showed the highest
photocatalytic activity. On the other hand, the absorption edge
2
3
12
a
b
0
.5 g. Reactant solution: an aqueous silver nitrate solution (0.05
-
1
mol L ), 150 mL. Light source: 300 W Xe lamp.
structure containing MoO4 tetrahedra.18,40,41 The half-width of
a peak at 28° was decreased as the calcination temperature was
higher than 773 K as shown in Table 2. This indicated that the
crystallinity became good. At the same time, the surface area

17796 J. Phys. Chem. B, Vol. 110, No. 36, 2006
Shimodaira et al.
Figure 7. X-ray diffraction patterns of Bi
temperatures: (a) 373 K; (b) 673 K; (c) 773 K; (d) 803 K; (e) 823 K;
f) 853 K; (g) 863 K; (h) 873 K; (i) 923 K; (j) 973 K.
2 6
MoO calcinated at various
(
TABLE 2: Surface Area and Half-Width of a Peak at 28° of
Bi MoO Powder Calcined at Different Temperatures
2
6
calcination
temp/K
surf area/
m g
half-width of
a peak at 28°/deg
2
-1
3
5
6
7
8
8
73 (uncalcination)
41.1
1.2
1.1
0.304
0.259
0.246
0.188
0.169
0.162
73
73
73
03
23
0.9
2 6
Figure 8. Scanning electron microscope images of Bi MoO . Ac-
celeration voltage: 10 kV. Key: (a) only refluxed at 373 K; (b) refluxed
at 373 K and calcined at 823 K.
0.9
was blue-shifted by the phase transition around 823 K of the
calcination temperature. The absorption edge was at the shortest
wavelength when the single phase of HTP-Bi2MoO6 was
obtained.
Figure 10 shows the relationship between calcination tem-
perature and photocatalytic oxygen evolution of Bi2MoO6
prepared by a reflux method. Bi2MoO6 calcined at 823 K
showed the highest photocatalytic activity for O2 evolution under
visible light irradiation. Moreover, the decrease in the photo-
catalytic activity corresponded to an irreversible phase transition
as shown in Figure 7. The improvement of the photocatalytic
activity by calcination around 823 K was due to an increase in
crystallinity of Aurivillius structure as shown in Table 2. The
half-width of the peak at 28° for the sample calcined at 803 K
was quite narrow compared with that at 773 K. Moreover, the
former was similar to that at 823 K. This indicated that an
increase in the crystallinity was almost completed at 803 K
although the crystallinity of the sample calcined at 823 K was
slightly better than that at 803 K. This is the reason the
photocatalytic activity suddenly increased around 803 K of the
calcination. This result indicated that the crystallinity was the
important factor rather than the surface area for this photocata-
lytic reaction.
Figure 9. Diffuse reflectance spectra of Bi
reflux method calcined at various temperatures: (a) 373 K; (b) 823 K;
2 6
MoO prepared by the
(c) 853 K; (d) 923 K.
There was no difference in the photocatalytic activity for the
O evolution between catalysts prepared by a solid-state reaction
2
and a reflux method. AS-Bi MoO was also prepared by only
2
6
calcination at higher than 773-823 K in the solid-state reaction.
In contrast to the solid-state reaction, it was able to be prepared
by calcinations between 373 and 823 K in the reflux method.
It is noteworthy that the reflux method made us investigate the

Bismuth Molybdate Photocatalysts
J. Phys. Chem. B, Vol. 110, No. 36, 2006 17797
the photocatalytic ability of AS-Bi2MoO6. The reflux method
made the investigation of the photocatalytic activity of AS-Bi2-
MoO6 prepared at a wide range of calcination temperature
possible. This investigation revealed the importance of crystal-
linity for the photocatalytic performance of AS-Bi2MoO6.
Acknowledgment. This work has been supported by the
Core Research for Evolutional Science and Technology (CREST)
of the Japan Science and Technology Agency (JST) and a Grant-
in-Aid (No. 14050090) for the priority Area Research (No. 417)
from the Ministry of Education, Culture, and Technology.
References and Notes
(
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Figure 10. Photocatalytic oxygen evolution under visible light
(
irradiation over Bi
2 6
MoO calcinated at several temperatures: (a) low-
temperature phase, Aurivillius structure; (b) high-temperature phase.
(
-
1
Sample (0.5 g): an aqueous silver nitrate solution (0.05 mol L ), 150
mL. Light source: 300-W Xe lamp with a cutoff filter (λ > 420 nm).
Cell: side irradiation type reaction vessel made of Pyrex.
(
(
(
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1
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(
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catalytic O
Bi MoO photocatalyst prepared by the reflex method upon irradiated
wavelength. Sample (0.5 g): an aqueous silver nitrate solution (0.05
mol L ), 150 mL. Light source: 300-W Xe lamp with cutoff filters.
Cell: side irradiation type reaction cell made of Pyrex.
2
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Figure 11 shows the diffuse reflectance spectrum and the
dependence of O2 evolution from an aqueous silver nitrate
solution over the AS-Bi2MoO6 photocatalyst prepared at optimal
condition upon cutoff wavelength using cutoff filters. The
horizontal axis indicates the cutoff wavelength in which
transmittance is almost 0%. The onset of the wavelength
dependency of the photocatalytic activity agreed with that of
the diffuse reflectance spectrum indicating that the O2 evolution
reaction photocatalytically proceeded over Bi2MoO6 by band
gap excitation.
(
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driven photocatalyst for O2 evolution from an aqueous silver
nitrate solution. This study succeeded in the discovery of molyb-
denum oxide photocatalysts for O2 evolution from aqueous
solutions under visible light irradiation. AS-Bi2MoO6 possesses
the corner-sharing structure of MoO6 octahedra as seen in the
well-known WO3 photocatalyst with visible light response. This
structure affected the energy gap between HOMO and LUMO,
the width of the conduction band, and the delocalization of
excitation energy, which contributed to the band structure and
(
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(