Ultrasonic-assisted synthesis of visible-light-induced Bi2MO6 (M = W, Mo) photocatalysts

Article abstract of DOI:10.1016/j.molcata.2006.12.026

Visible-light-induced photocatalysts Bi₂MO₆ (M = W, Mo) nanocrystals have been successfully synthesized via a new ultrasonic-assisted method. Laminar structured Bi₂WO₆ nanoplates with size of ca. 100 nm and Bi₂MoO₆ nanoparticles with size of ca. 150 nm were prepared via ultrasonic pretreatment followed by calcination at 500 °C. The photocatalytic activities of as-prepared Bi₂MO₆ were about 4-6 times higher than that of the products prepared by traditional solid-state reaction (SSR), which were evaluated by the degradation of RhB dye in water under visible light irradiation (λ > 400 nm). It was found that ultrasonic irradiation played an important role in the formation of the nanomaterials with a smaller crystal size and larger surface area, which was beneficial to their photocatalytic activities. The present study suggested a promising method, as an effective improvement of traditional SSR-process, for the synthesis of other photocatalysts.

Full text of DOI:10.1016/j.molcata.2006.12.026

Journal of Molecular Catalysis A: Chemical 268 (2007) 195–200
Ultrasonic-assisted synthesis of visible-light-induced
Bi2MO6 (M = W, Mo) photocatalysts
Lin Zhou, Wenzhong Wang∗, Lisha Zhang
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,
Chinese Academy of Sciences, 1295 DingXi Road, Shanghai 200050, PR China
Received 31 October 2006; received in revised form 7 December 2006; accepted 10 December 2006
Available online 21 December 2006
Abstract
Visible-light-induced photocatalysts Bi2MO6 (M = W, Mo) nanocrystals have been successfully synthesized via a new ultrasonic-assisted method.
Laminar structured Bi2WO6 nanoplates with size of ca. 100 nm and Bi2MoO6 nanoparticles with size of ca. 150 nm were prepared via ultrasonic
pretreatment followed by calcination at 500 ◦C. The photocatalytic activities of as-prepared Bi2MO6 were about 4–6 times higher than that of
the products prepared by traditional solid-state reaction (SSR), which were evaluated by the degradation of RhB dye in water under visible light
irradiation (λ > 400 nm). It was found that ultrasonic irradiation played an important role in the formation of the nanomaterials with a smaller
crystal size and larger surface area, which was beneficial to their photocatalytic activities. The present study suggested a promising method, as an
effective improvement of traditional SSR-process, for the synthesis of other photocatalysts.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Bi2MO6 (M = W, Mo); Photocatalysis; Ultrasonic; Visible light; RhB
2−
sandwiched between (Bi2O2)2+ layers.
1. Introduction
of (An−1BnO3n+1
)
Among them, Bi2WO6 and Bi2MoO6 have been reported by
Kudo and his co-workers [13,14] as photocatalysts for O2 evo-
lution from AgNO3 solution. Lately, Bi2WO6 was demonstrated
to show activity for degradation of organic pollutants [15,16].
Even so, it is still of great importance and challenge to explore
bismuth mixed oxides photocatalysts with high efficiency and
visible light responsibility.
Traditionally, photocatalysts were prepared by solid-state-
reaction (SSR) method which is relatively easy to operate and
suitable for large-scale production. However, photocatalytic
activity of SSR-products is not high due to the large crystal
size and small surface area. Recently, the study of physical and
chemical effect of ultrasound irradiation is a rapidly growing
research area [17–21]. And the ultrasound gradually becomes an
important tool for its applications in the synthesis and modifica-
tion of nanosized functional inorganic materials. In our previous
reports, BiVO4 and Bi2O3 nanocrystals were successfully syn-
thesized by a sonochemical route which showed the enhanced
photocatalytic activities under visible light for the decomposi-
tion of pollutant [22,23].
From the viewpoint of solving global energy and envi-
ronment issues, using solar energy and semiconductors as
catalysts for photocatalytic degradation of harmful pollutants
and water splitting have attracted great interest in recent
years [1–3]. Considering the relatively slow reaction rate and
poor solar efficiency of TiO2 which hindered its application,
many studies have been carried out to exploit new visible-
light-driven photocatalysts. Zou et al. [4] have firstly reported
water splitting for H2 and O2 evolution in a stoichiomet-
ric amount over the NiOx/In0.9Ni0.1TaO4 photocatalyst under
visible light irradiation. Since then, some other new visible-
light-induced photocatalysts have also been developed [5–10],
besides TiO2 based photocatalysts. Bismuth mixed oxides
2−
(Bi2O2)2+ (An−1BnO3n+1
)
(A = Ba, Bi, Pb, etc., B = Ti, Nb,
W, Mo, etc.) with Aurivillius structure are an important set of
visible-light active photocatalysts for environmental purifica-
tion and/or water splitting [11,12]. These bismuth containing
oxides possess unique layered structures by the perovskite slabs
∗
Herein, we report a new ultrasonic-assisted method for the
preparation of Bi2MO6 (M = W, Mo) photocatalysts. It was
Corresponding author. Tel.: +86 21 5241 5295; fax: +86 21 5241 3122.
E-mail address: wzwang@mail.sic.ac.cn (W. Wang).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.12.026

196
L. Zhou et al. / Journal of Molecular Catalysis A: Chemical 268 (2007) 195–200
found that ultrasonic irradiation played an important role in the
formation of the nanomaterials and final photocatalytic activi-
ties. The composition, microstructure, photophysical property
and photocatalytic activity of as-prepared products were inves-
tigated.
2. Experimental
2.1. Synthesis
All the reagents used in our experiments were of analyti-
cal purity and were used as received from Shanghai Chemical
Company.
2.1.1. Bi2WO6 nanoplates
Aqueous solutions of Bi(NO3)3·5H2O and Na2WO4·2H2O
were mixed together in 1:1 molar ratio, then the mixture was
stirred for 30 min at room temperature. Afterward, the mixture
was exposed to high-intensity ultrasound (6 mm diameter Ti-
horn, 600 W, 20 kHz) irradiation for 60 min. The sonication was
conducted without cooling, thus the temperature of the reac-
tant mixture increased gradually and the temperature reached
∼70 ◦C at the end of the reaction. The precipitates were cen-
trifuged, washed with de-ionized water, and then dried at 353 K
in air for 10 h. The obtained powders were then calcined at
500 ◦C for 2 h to produce crystallinity.
Fig. 1. XRD patterns of as-prepared (a) Bi2WO6 and (b) Bi2MoO6 powder
samples at room temperature, respectively.
2.3. Photocatalytic test
Photocatalytic activities of Bi2MO6 samples were evaluated
by photocatalytic decolorization of rhodamine-B (RhB) under
visible light. A 500 W Xe lamp was used as a light source with
a 400 nm cutoff filter to provide visible light irradiation. The
experiments were performed at room temperature as follows:
0.1 g of photocatalyst was added into 100 mL RhB solution
(10−5 mol/L). Before illumination, photocatalyst was stirred for
30 min in the dark to disperse in RhB solution. Then the solution
was exposed to visible light irradiation under magnetic stirring.
At given time intervals, 3 mL suspensions were sampled and
centrifugated to remove the particles. The filtrates of the RhB
were monitored by checking the absorbance at 553 nm during
the photo-degradation process using Hitachi U-3010 UV–vis
spectrophotometer.
2.1.2. Bi2MoO6 nanoparticles
All steps were the same as in the synthesis of Bi2WO6
nanoplates, except that (NH4)6Mo7O24·4H2O was used instead
of Na2WO4·2H2O.
The samples were also prepared without ultrasonic pretreat-
ment (named as NUS-Bi2MO6) while other steps were the
same as above. Bulk Bi2WO6 and Bi2MoO6 were synthesized
via traditional solid-state reaction according to Refs. [24,12],
respectively.
2.2. Characterization
The powder X-ray diffraction (XRD) patterns of the as-
synthesized samples were measured on a D/MAX 2250 V
diffractometer (Rigaku, Japan) using monochromatized Cu K␣
(λ = 0.15418 nm) radiation under 40 kV and 100 mA and scan-
ning with the 2θ ranging from 10◦ to 70◦. The morphologies
and microstructures of as-prepared samples were examined
with transmission electron microscopy (TEM) and high resolu-
tion TEM (HRTEM) (JEOL JEM-2100F; accelerating voltage:
200 kV). UV–vis diffuse reflectance spectra of the samples were
obtained on an UV–vis spectrophotometer (Hitachi U-3010)
using BaSO4 as reference. Nitrogen adsorption–desorption mea-
surements were conducted at 77.35 K on a Micromeritics Tristar
3000 analyzer after samples were degassed at 200 ◦C for 6 h.
The Brunauer–Emmett–Teller (BET) surface area was estimated
using adsorption data in a relative pressure range from 0.05 to
0.3.
Fig. 2. The schematic crystal structures of orthorhombic phase Bi2MO6 (M = W,
Mo) composed of MO6 octahedral layers and Bi–O–Bi layers.

L. Zhou et al. / Journal of Molecular Catalysis A: Chemical 268 (2007) 195–200
197
3. Results and discussion
photocatalytic performance of photocatalysts because it makes
excitation energy and/or photogenerated electron and hole pairs
began to migrate easily [12].
3.1. Crystal structure
The phase and composition of the products were investigated
using XRD measurement. Fig. 1 shows the XRD patterns of
the Bi2MO6 (M = W, Mo) powder samples. All the diffraction
peaks can be indexed to orthorhombic Bi2MO6 according to
the JCPDS card no. 39-0256 for M = W and no. 21-0102 for
M = Mo, respectively. Afterrefinement, thecrystallatticeparam-
3.2. Morphology and microstructure
The morphology and microstructure of Bi2MO6 (M = W, Mo)
samples were revealed by TEM. Fig. 3a and b are the TEM
imagesofBi2WO6 sample. ThepanoramicviewshowninFig. 3a
indicates that the as-synthesized products are composed of lam-
inar structure with size of ca. 100 nm. Close observation on the
samples revealed by high magnification TEM image (Fig. 3b)
shows most of these Bi2WO6 nanoplates are square-plate-like.
The selective area electron diffraction (SAED) pattern of an
individual nanoplate (Fig. 3c) shows well-defined ED spots
indicating the single-crystal nature of the nanoplate, and the
ED pattern can be indexed to the diffraction pattern of [0 1 0]
zone axis indicating a preferred (0 1 0) surface orientation of
the nanoplates. Fig. 3d shows a typical HRTEM image of a
single Bi2WO6 nanoplate, and the inset of this figure shows the
˚
˚
eters were calculated as follows: a = 5.456 A, b = 16.438 A, and
˚
˚
˚
˚
c = 5.441 A for Bi2WO6, and a = 5.495 A, b = 16.187 A, and
c = 5.486 A for Bi2MoO6.
Shown in Fig. 2 is schematic crystal structure of Bi2MO6. It
is a layered structure composed of MO6 octahedral layers and
Bi–O–Bi layers. The samples were crystallized in an orthorhom-
bic system with the space group Pcba and Pna21 for Bi2WO6
and Bi2MoO6, respectively. The MO6 octahedral are connected
to each other by corner-sharing. It is found that this corner-
sharing structure contributed to the visible-light response and the
Fig. 3. (a) Low-magnification TEM image of as-synthesized Bi2WO6 nanoplates. (b) High-magnification TEM image of a Bi2WO6 nanoplate. (c) The corresponding
SAED pattern of an individual nanoplate. (d) A HRTEM image near the edge of a single nanoplate (inset).

198
L. Zhou et al. / Journal of Molecular Catalysis A: Chemical 268 (2007) 195–200
low-resolution TEM image. The clear lattice fringe indicates the
single-crystalline nature of the nanoplate. The observed lattice
spacings of 0.272 and 0.273 nm correspond to (0 0 2) and (2 0 0)
planes of orthorhombic Bi2WO6. This result is in well accor-
dance with that of XRD pattern in Fig. 1a, where the (0 0 2) and
(2 0 0) peaks are much stronger compared with those of stan-
dard patterns. In addition, under 200 kV electron exposure, the
explicit edges gradually became rough and many tiny particles
with size of ∼5 nm grew from it (Fig. 3b). The phenomenon
was also observed by Zhu et al. when they synthesized Bi2WO6
nanoplates via a hydrothermal process [11]. The electron beam
may create some crater on the basal plane and lead to further
stochastic growth on the edge.
Fig. 4a–c are the TEM images of Bi2MoO6 sample showing
that the product contains a large quantity of nanoparticles with
size of ca. 150 nm, which is in good agreement with that evalu-
ated using Scherrer equation based on XRD patterns. The SAED
pattern (Fig. 4d) recorded on an individual Bi2MoO6 nanopar-
ticle reveals the single-crystalline nature of the sample. Unlike
Bi2WO6 nanoplates mentioned above, Bi2MoO6 nanoparticles
are stable under 200 kV electron exposure, no visible damage
was observed.
The Brunauer–Emmett–Teller (BET) surface areas of
Bi2MO6 samples were also estimated using N2 adsorption
data. The BET surface areas of samples were estimated to be
ca. 8.38 m2/g for Bi2WO6 and ca. 2.33 m2/g for Bi2MoO6,
which was much higher than of the reference SSR-samples
of ca. 0.64 m2/g [24] for Bi2WO6 and ca. 0.9 m2/g [12] for
Bi2MoO6. These results demonstrated that Bi2MO6 nanocrys-
tals with relatively small crystal size and large surface area
could be synthesized, via ultrasonic pretreatment and followed
by calcination, as compared with the products of traditional
SSR-process.
3.3. UV–vis diffuse reflectance spectra and band gap
The UV–vis diffuse reflectance spectra of the Bi2MO6
(M = W, Mo) samples are shown in Fig. 5. The solid-style curve
Fig. 4. Low-magnification (a, b) and high-magnification (c) TEM images of as-synthesized Bi2MoO6 nanoparticles. (d) The corresponding SAED pattern of an
individual nanoparticle.

L. Zhou et al. / Journal of Molecular Catalysis A: Chemical 268 (2007) 195–200
199
Fig. 6. Photocatalytic degradation of RhB using Bi2WO6 nanoplates, SSR-
Bi2WO6 and the sample without ultrasonic treatment (NUS-Bi2WO6) as catalyst
(λ > 400 nm).
Fig. 5. UV–vis diffuse reflectance spectra of Bi2WO6 nanoplates and Bi2MoO6
nanoparticles.
represented Bi2MoO6 nanoparticles, and the dashed-dotted style
curve represented Bi2WO6 nanoplate. Both of the samples
showed absorption in visible light region in addition to that in
the UV light region, which implying the possibility of photocat-
alytic activity over these materials under visible light irradiation.
The steep absorption edge indicating that their absorption rel-
evant to the band gap is due to the intrinsic transition of these
semiconductors [25–27]. For a crystalline semiconductor, it is
known that the optical absorption near the band edge follows the
formula αhν = A(hν − Eg)n/2 [28], where α, h, ν, Eg, and A are
absorption coefficient, Planck constant, light frequency, band
gap, and a constant, respectively. Among them, n decides the
characteristics of the transition in a semiconductor. For Bi2MO6,
the value of n is 1. The energy of the band gap could be thus
estimated to be ∼2.77 eV for Bi2WO6 nanoplates and ∼2.63 eV
for Bi2MoO6 nanoparticles.
The electronic structure of Bi2MO6 have been reported
[12,15] based on the DFT calculations. It is revealed that the
visible-light response for Bi2WO6 was due to the transition from
the valence band formed by the hybrid orbitals of Bi 6s and O 2p
to the conduction band of W 5d. This special electronic struc-
ture makes the VB largely dispersed, which favors the mobility
of photo-excitated holes and thus is beneficial to photocatalytic
oxidation of organic pollutants [5]. While for Bi2MoO6, valence
band is only composed of O 2p orbitals. Moreover, the corner-
sharing structure of both WO6 and MoO6 octahedra contributed
to the visible-light response and the photocatalytic performance
[12].
RhB/Bi2MO6 suspension decreases and the color changes light
gradually, indicating the degradation of the dye and removal
of ethyl groups [29]. A similar experiment on an aqueous
RhB solution in the absence of Bi2MO6 photocatalysts was
carried out under the same experimental conditions, but the
absorption intensity and absorption wavelength hardly changed
with increasing irradiation time, as reported previously [30].
In addition, a control experiment also showed that RhB did
not degradate in the dark at the presence of photocatalysts.
These results confirmed that the degradation of RhB solution
is caused by Bi2MO6 photocatalysts under visible light
irradiation.
For comparison, the same photodegradation experiment
using traditional SSR-Bi2MO6 and the NUS-Bi2MO6 samples
were also performed. The results reveal that the photodegrada-
tion rates of Bi2WO6 nanoplates and Bi2MoO6 nanoparticles
are evidently higher than that of the corresponding SSR-samples
(4–6 times) and the NUS-samples (2–2.4 times). Ultrasonic
3.4. Photocatalytic activities
The photocatalytic activities of Bi2MO6 (M = W, Mo) was
evaluated by degradation of RhB dye in water under visible light
irradiation (λ > 400 nm). RhB shows a major absorption band at
553 nm. The temporal evolution of the spectral changes taking
place during the photodegradation of RhB over Bi2WO6 and
Bi2MoO6 samples were displayed in Figs. 6 and 7, respectively.
Under visible light irradiation (λ > 400 nm), the absorption of
Fig. 7. Photocatalytic degradation of RhB using Bi2MoO6 nanoparticles, SSR-
Bi2MoO6 and the sample without ultrasonic treatment (NUS-Bi2MoO6) as
catalyst (λ > 400 nm).

200
L. Zhou et al. / Journal of Molecular Catalysis A: Chemical 268 (2007) 195–200
irradiation may play an important role in the formation of the
nanomaterials and final photocatalytic activities. The unique
chemical effects of ultrasound arise from acoustic cavitation,
that is, the ultrasonic vibrations produce microscopic bubbles
(cavities), which expand and implode violently, creating mil-
lions of shock waves. During the acoustic cavitation process,
very high temperatures (>5000 K), pressures (>20 MPa), and
cooling rates (>1010 K/s) can be achieved upon the collapse
of the bubbles [31]. The transient high-temperature and
high-pressure field provides a favorable environment for the
anisotropic growth of nanocrystals. Cavitations and shock waves
created by ultrasound can accelerate solid particles to high
velocities leading to interparticle collisions and inducing effec-
tive fusion at the point of collision. The energy generated during
collision can induce the crystallization of the amorphous parti-
cles, responsible for the further crystallization process [32,33].
So we believe it is ultrasound that caused a higher surface area
and smaller particle size of the products. A similar result is
reported by Yu et al. when they prepared mesoporous titanium
dioxide nanocrystalline photocatalyst via ultrasonic process
[19]. As compared with that of SSR-samples, the higher photo-
catalytic activities should relate to the higher surface area of the
nanocrystals since the photocatalytic reaction area is increased
and the efficiency of the electron-hole separation is promoted
[34,35]. Further more, one can see that Bi2WO6 nanoplates
exhibited higher photocatalytic activities than Bi2MoO6
nanoparticles. The result can be ascribed to not only the
difference of electronic structure, but also their morphology and
surface area. The holes generated inside the Bi2WO6 nanoplates
had more opportunity to transfer to the surface and act with
the organic molecules owe to their thin 2D laminar structure
[11].
Acknowledgements
We acknowledge the financial support from Chinese
Academy of Sciences and Shanghai Institute of Ceramics
under the program for Recruiting Outstanding Overseas Chinese
(Hundred Talents Program), and the National Natural Science
Foundation of China (No. 50672117).
References
[1] A. Fujishima, K. Honda, Nature 238 (1972) 37.
[2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95
(1995) 69.
[3] K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen,
Nature 440 (2006) 295.
[4] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 625.
[5] J. Tang, Z. Zou, J. Ye, Angew. Chem., Int. Ed. 43 (2004) 4463.
[6] I. Tsuji, H. Kato, H. Kobayashi, A. Kudo, J. Am. Chem. Soc. 126 (2004)
13406.
[7] I. Tsuji, H. Kato, A. Kudo, Angew. Chem., Int. Ed. 44 (2005) 3565.
[8] K. Maeda, T. Takata, M. Hara, N. Saito, Y. Inoue, H. Kobayashi, K. Domen,
J. Am. Chem. Soc. 127 (2005) 8286.
[9] J. Luo, P. Maggard, Adv. Mater. 18 (2006) 514.
[10] H. Tada, T. Mitsui, T. Kiyonaga, T. Akita, K. Tanaka, Nature Mater. 5
(2006) 782.
[11] C. Zhang, Y. Zhu, Chem. Mater. 17 (2005) 3537.
[12] Y. Shimodaira, H. Kato, H. Kobayashi, A. Kudo, J. Phys. Chem. B 110
(2006) 17790.
[13] A. Kudo, S. Hijii, Chem. Lett. 28 (1999) 1103.
[14] J. Yu, A. Kudo, Chem. Lett. 34 (2005) 1528.
[15] H. Fu, C. Pan, W. Yao, Y. Zhu, J. Phys. Chem. B 109 (2005) 22432.
[16] H. Fu, L. Zhang, W. Yao, Y. Zhu, Appl. Catal. B 66 (2006) 100.
[17] A. Gedanken, Ultrason. Sonochem. 11 (2004) 47.
[18] N.A. Dhas, K.S. Suslick, J. Am. Chem. Soc. 127 (2005) 2368.
[19] J.G. Yu, M.H. Zhou, B. Cheng, H.G. Yu, X.J. Zhao, J. Mol. Catal. A 227
(2005) 75.
[20] X.F. Qiu, C. Burda, R.L. Fu, L. Pu, H.Y. Chen, J.J. Zhu, J. Am. Chem. Soc.
126 (2004) 16276.
[21] W. Ho, J. Yu, J. Mol. Catal. A 247 (2006) 268.
[22] L. Zhou, W. Wang, S. Liu, L. Zhang, H. Xu, W. Zhu, J. Mol. Catal. A 252
(2006) 120.
[23] L. Zhang, W. Wang, J. Yang, Z. Chen, W. Zhang, L. Zhou, S. Liu, Appl.
Catal. A 308 (2006) 105.
[24] J. Tang, Z. Zou, J. Ye, Catal. Lett. 92 (2004) 53.
[25] S. Zhang, C. Zhang, Y. Man, Y. Zhu, J. Solid State Chem. 179 (2006) 62.
[26] J. Yin, Z. Zou, J. Ye, J. Phys. Chem. B 107 (2003) 4936.
[27] J. Tang, Z. Zou, J. Ye, J. Phys. Chem. B 107 (2003) 14265.
[28] M.A. Butler, J. Appl. Phys. 48 (1977) 1914.
[29] W. Zhao, C. Chen, X. Li, J. Zhao, J. Phys. Chem. B 106 (2002) 5022.
[30] T. Wu, G. Liu, J. Zhao, J. Phys. Chem. B 102 (1998) 5845.
[31] K.S. Suslick, S.B. Choe, A.A. Cichowlas, M.W. Grinstaff, Nature 353
(1991) 414.
4. Conclusion
A new ultrasonic-assisted method was developed for the
preparation of visible-light-induced Bi2MO6 (M = W, Mo) pho-
tocatalysts. The as-prepared Bi2MO6 nanocrystals exhibited
relatively small crystal size and large surface area as compared
with the products prepared by traditional solid-state reaction.
The Bi2MO6 photocatalysts showed much higher (4–6 times)
photocatalytic activities under visible light irradiation, as com-
pared with the corresponding sample prepared by traditional
solid-state reaction. These higher photocatalytic activities can
be ascribed to the morphology, smaller particle size and higher
surface areas. Such an ultrasonic-assisted route, as an effective
improvement of SSR-process, is worth to be extended to other
photocatalysts systems.
[32] S.J. Doktycz, K.S. Suslick, Science 247 (1990) 1067.
[33] J. Geng, J. Zhu, H. Chen, Cryst. Growth Des. 6 (2006) 321.
[34] D. Wang, J. Tang, Z. Zou, J. Ye, Chem. Mater. 17 (2005) 5177.
[35] J. Yu, J. Xiong, B. Cheng, S. Liu, Appl. Catal. B 60 (2005) 211.