One-step hydrothermal synthesis and photocatalytic performance of ZnWO 4/Bi2WO6 composite photocatalysts for efficient degradation of acetaldehyde under UV light irradiation

Article abstract of DOI:10.1016/j.apcata.2013.03.014

Bi₂WO₆ with different Bi³⁺ ion concentrations (0-30 mol%) is incorporated with the ZnWO₄ photocatalyst to improve the photocatalytic efficiency by forming a ZnWO ₄/Bi₂WO₆ composite photocatalyst with hierarchical heterostructure via a one-step hydrothermal method. X-ray diffraction and Raman spectroscopy analyses confirm the presence of ZnWO ₄ and Bi₂WO₆ as main phases in the composite photocatalyst. Scanning electron microscopy and transmission electron microscopy observations reveal that the rice- and plate-like Bi₂WO₆ nanoparticles were either separate or attached to the surface of quasi-spherical ZnWO₄ particles. A gradual increase in the Bi₂WO ₆ content of the composite photocatalyst results in a monotonic shift of the absorption edge from ca. 355 nm to longer wavelengths up to ca. 450 nm. The photocatalytic performance of the ZnWO₄/Bi₂WO ₆ composite photocatalyst was evaluated by investigating the degradation of gaseous acetaldehyde (AcH) under UV light irradiation. Only the composite photocatalyst synthesized with 30 mol% Bi³⁺ exhibits higher photocatalytic activity under UV light irradiation compared with both individual ZnWO₄ and Bi₂WO₆, and with a mechanically mixed ZnWO₄/Bi₂WO₆ composite photocatalyst. The enhanced photocatalytic activity is attributed to the n-n isotype junction formed between the two semiconductors and the charge separation of each semiconductor.

Full text of DOI:10.1016/j.apcata.2013.03.014

Applied Catalysis A: General 457 (2013) 12–20
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
One-step hydrothermal synthesis and photocatalytic performance of
ZnWO4/Bi2WO6 composite photocatalysts for efficient degradation of
acetaldehyde under UV light irradiation
Mirabbos Hojamberdieva,∗, Ken-ichi Katsumataa, Koji Moritab, Sara Aldabe Bilmesc,
Nobuhiro Matsushitaa, Kiyoshi Okadaa
a Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, 226-8503 Kanagawa, Japan
b Advanced Materials Processing Unit, Advanced Ceramics Group, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, 305-0047 Ibaraki, Japan
c Instituto de Química Física de los Materiales, Medio Ambiente y Energía (INQUIMAE), Facultad de Ciencias Exactas y Naturales, Universidad de Buenos
Aires, Pabellón II, Ciudad Universitaria, C1428EHA-Buenos Aires, Argentina
a r t i c l e i n f o
a b s t r a c t
Bi2WO6 with different Bi3+ ion concentrations (0–30 mol%) is incorporated with the ZnWO4 photocatalyst
to improve the photocatalytic efficiency by forming a ZnWO4/Bi2WO6 composite photocatalyst with hier-
archical heterostructure via a one-step hydrothermal method. X-ray diffraction and Raman spectroscopy
analyses confirm the presence of ZnWO4 and Bi2WO6 as main phases in the composite photocatalyst.
Scanning electron microscopy and transmission electron microscopy observations reveal that the rice-
and plate-like Bi2WO6 nanoparticles were either separate or attached to the surface of quasi-spherical
ZnWO4 particles. A gradual increase in the Bi2WO6 content of the composite photocatalyst results in a
monotonic shift of the absorption edge from ca. 355 nm to longer wavelengths up to ca. 450 nm. The pho-
tocatalytic performance of the ZnWO4/Bi2WO6 composite photocatalyst was evaluated by investigating
the degradation of gaseous acetaldehyde (AcH) under UV light irradiation. Only the composite photo-
catalyst synthesized with 30 mol% Bi3+ exhibits higher photocatalytic activity under UV light irradiation
compared with both individual ZnWO4 and Bi2WO6, and with a mechanically mixed ZnWO4/Bi2WO6
composite photocatalyst. The enhanced photocatalytic activity is attributed to the n-n isotype junction
formed between the two semiconductors and the charge separation of each semiconductor.
© 2013 Elsevier B.V. All rights reserved.
Article history:
Received 23 November 2012
Received in revised form 9 February 2013
Accepted 12 March 2013
Available online xxx
Keywords:
Bismuth tungstate
Composite photocatalyst
Heterostructure
Hydrothermal synthesis
UV light irradiation
Zinc tungstate
1. Introduction
high light yield, short decay time, and long afterglow to lumines-
cence [3,4]. There have been a number of recent reports concerning
Semiconductors exhibit several properties that are strongly
sensitive to their structures, and are considered to be important
materials for environmental applications, such as air purification,
water disinfection, hazardous waste remediation, and water purifi-
cation [1,2]. Among them, metal tungstates-based semiconductors
have been intensively investigated as potential candidates for pho-
tocatalytic processes. As a member of the metal tungstate family,
zinc tungstate (ZnWO4) crystallizes in the monoclinic wolframite
structure and has a high application potential in various fields as
an X-ray, ␥-scintillator, microwave system, solid-state laser host,
gas and humidity sensors, acoustic and optical fibers, and magnetic
material, due to characteristics, such as high chemical stability,
high average refractive index, high X-ray absorption coefficient,
improvement of the optical properties of ZnWO4 by the introduc-
tion of rare earth ions [5–8]; excitation of the tungstate group may
effectively transfer energy to the rare earth element and become
potential phosphors.
The molecular and electronic versatility, reactivity, and stability
of ZnWO4 have also made it a candidate environmental material for
the photodegradation of organic water pollutants and water split-
ting [9–12]. The difference in the photocatalytic activities of ZnWO4
with different morphologies, including nanorods [9], porous films
[11], and nanoparticles [13], is mainly due to differences in crys-
tallinity and specific surface area. Moreover, the photocatalytic
activity of ZnWO4 could also be influenced by the crystal growth
orientation [14], nanorod aspect ratio [15], OH− defects [16], ion
doping [17–19], and the porous structure [11]. However, ZnWO4
with a wide band gap of 3.75 eV [20] exhibits low photocatalytic
activity, but the effective combination of photon absorption, bulk
diffusion, and surface transfer of photoinduced charge-carriers
could lead to enhancement of its photocatalytic activity.
∗
Corresponding author. Tel.: +81 45 924 5369; fax: +81 45 924 5358.
E-mail addresses: hmirabbos@gmail.com, mhojamberdiev.m.aa@m.titech.ac.jp,
hmirabbos@hotmail.com (M. Hojamberdiev).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.03.014

M. Hojamberdiev et al. / Applied Catalysis A: General 457 (2013) 12–20
13
Bismuth tungstate (Bi2WO6) is one of the simplest members of
the Aurivillius oxide family of layered perovskites with the general
formula Bi2An−1BnO3n+3 (A = Ca, Sr, Ba, Pb, Na, K; B = Ti, Nb, Ta, Mo,
30 min, the white-colored suspension obtained was transferred
into a 40 mL Teflon-lined stainless steel autoclave. The autoclave
was sealed and maintained at 160 ◦C for 12 h. After hydrothermal
treatment, the autoclave was allowed to cool to room temperature.
The resulting precipitate was collected by centrifugation, washed
with deionized water several times and dried at 80 ◦C for 8 h.
W, Fe; and n = number of perovskite-like layers (An−1BnO3n+1)
2−),
which are structurally composed of alternating perovskite-like and
fluorite-like blocks. Bi2WO6 exhibits several important physical
properties, such as ferroelectric piezoelectricity, pyroelectricity,
catalytic behavior, oxide anion conductivity, nonlinear dielectric
susceptibility, and luminescence. Bi2WO6, which has a band gap of
2.80 eV, has excellent photocatalytic activity for O2 evolution and
for the mineralization of CHCl3 and CH3CHO into CO2 under visible
light irradiation [21,22].
2.2. Characterization
The crystalline phases formed in the as-synthesized composite
photocatalyst samples were identified by X-ray powder diffrac-
tion (XRD) using an RINT-2100 diffractometer (Rigaku, Japan)
˚
with monochromated Cu K␣ radiation (ꢁ = 1.5405 A) at 40 kV
Recently, composite photocatalysts have received much atten-
tion due to enhanced photocatalytic activity that results from the
formation of heterojunction structures. The combination of two
semiconductors in contact that have different redox energy levels
of their corresponding conduction and valence bands can be consid-
ered to be one of the most promising methods to improve charge
separation, increase the lifetime of charge carriers, and enhance
the efficiency of the interfacial charge transfer to the adsorbed
substrate [23]. To date, there has been much effort to enhance
the photodegradation efficiency of Bi2WO6 by coupling with other
semiconductors, such as TiO2 [24], ZnO [25], Bi2O3 [26], Co2O3 [27],
Bi2S3 [28], graphene [29], g-C3N4 [30], BiOI [31], and CeVO4 [32].
Very recently, the Bi2WO6/ZnWO4 composite photocatalyst, which
is composed of Bi2WO6 nanoparticles grown on primary ZnWO4
nanorods, was obtained by a simple hydrothermal method [33].
Although promising results have been obtained with this composite
nanocatalyst, the efficiency is limited due to the rapid recombi-
nation of photogenerated charge carriers and the back reaction of
intermediate chemical species [34]. Nanostructured powders do
not support internal space charges; therefore, the photogenera-
ted electrons and holes are not efficiently separated [35]. Another
composite photocatalyst was also synthesized using a two-step
hydrothermal method and the photocatalytic activity was evalu-
ated for the degradation of Rhodamine B under UV light irradiation
[36].
and 40 mA. The XRD patterns of the samples were measured
at a scanning rate of 2◦/min in the 2ꢂ range of 10–70◦. The
crystalline phases present in the as-synthesized composite photo-
catalysts were also confirmed using a T64000 Raman spectroscopy
(Horiba Jobin Yvon S.A.S., France) with an Ar laser (514.5 nm)
operated at 50 mW. The morphologies and particle sizes of the
as-synthesized composite photocatalysts were examined using an
S-4500 ultra high resolution scanning electron microscope (SEM)
(Hitachi, Japan), operated at an acceleration voltage of 15 kV. The
SEM was also equipped with an X-ray energy dispersive spec-
troscopy (EDS) attachment for elemental analysis. Transmission
electron microscopy (TEM) observations were performed on pow-
der samples using a JEM-2010UHR electron microscope (JEOL,
Japan), operated at an acceleration voltage of 200 kV to distinguish
the crystal structures and particle sizes. For TEM observation, a
powder sample was dispersed in ethanol under ultrasonication, and
a small amount of the suspension was dropped on the holey carbon
grid. UV–vis diffuse reflectance spectra of the composite photo-
catalysts were recorded at room temperature with a Lambda 950
UV/vis/NIR spectrophotometer (PerkinElmer, USA) in the wave-
length range of 200–600 nm, using BaSO4 as a reference.
2.3. Photodegradation experiments
Acetaldehyde (AcH), which is a typical indoor air pollutant, was
selected as a probe molecule to evaluate the photocatalytic activ-
ity of the as-synthesized composite photocatalyst under UV light
irradiation. Photodegradation of AcH was conducted in a batch-
type reactor over the as-synthesized composite photocatalyst at
room temperature (25 ◦C). The photocatalyst (50 mg) was placed
in the 500 mL Pyrex® glass reaction vessel. Pure air (Taiyo Nippon
Sanso Corp., Japan) was blown through the reaction vessel at room
temperature to remove any air contaminants. A certain amount
of AcH was then introduced into the reaction vessel using a 2 mL
Pressure-Lok® glass syringe until the concentration of AcH in the
reactor reached ca. 250 ppm. After equilibration of adsorption onto
the composite photocatalyst in the dark for 24 h, the reactor was
placed under an FL10BLB black-light lamps (Toshiba, Japan) with a
wavelength range of 290–420 nm with a peak at 352 nm. UV light
was emitted with an irradiance of ca. 0.4 mW/cm2, which was mea-
sured with a UV 340 UV light meter (Lutron Electronic Enterprise
Co., Ltd., Taiwan). The decrease and increase in the respective con-
centrations of AcH and CO2 were monitored using a GC-2014 gas
chromatograph (Shimadzu, Japan) equipped with a 2 m Porapak-
Q column, methanizer and a flame ionization detector, with N2
employed as the carrier gas.
We propose
a one-step strategy for the preparation of
the ZnWO4/Bi2WO6 composite photocatalyst by hydrothermal
processing, and the photocatalytic activity of the resultant compos-
ite photocatalyst is evaluated for the degradation of acetaldehyde
(AcH) under UV light irradiation. The ZnWO4/Bi2WO6 ratio in the
composite photocatalyst was also varied to determine the optimal
composition of the photocatalyst for higher photocatalytic effi-
ciency under UV light irradiation.
2. Experimental
2.1. Synthesis
The ZnWO4/Bi2WO6 composite photocatalyst was prepared
using a one-step hydrothermal method. The experimental proce-
dures were as follows: first, 2.5 mmol of Zn(NO3)2·6H2O (Wako
Pure Chemical Industries, Ltd., Japan) was dissolved in 15 mL
of deionized water (Millipore Milli-Q Plus purification system,
18.2 Mꢀ cm), 2.5 mmol of Bi(NO3)3·5H2O (Wako Pure Chemical
Industries, Ltd., Japan) was dissolved in 15 mL of diluted nitric acid,
and 2.5 mmol of Na2WO4 (Wako Pure Chemical Industries, Ltd.,
Japan) was dissolved in 15 mL of deionized water. 15 mL of bismuth
nitrate solution with different mole percentages (0–30 mol%) was
introduced into the zinc nitrate solution, and then 15 mL of sodium
tungstate solution was added under vigorous stirring. The pH of
the suspension was adjusted by the addition of aqueous ammo-
nia (Wako Pure Chemical Industries, Ltd., Japan) to determine the
optimal synthesis conditions. After being well homogenized for
3. Results and discussion
3.1. Effect of the pH of synthesis solution
The hydrothermal method is mainly based on low-temperature
solution processes where the pH of the synthesis solution (or

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M. Hojamberdiev et al. / Applied Catalysis A: General 457 (2013) 12–20
phases: ZnWO4 (JCPDS card no. 15-0774) with the monoclinic
wolframite structure and WO3 (JCPDS card no. 33-1387) with the
hexagonal structure, which is an undesirable phase in the compos-
ite photocatalyst. As the pH of synthesis suspension was increased
to 5, the WO3 phase completely disappeared, while the ortho-
rhombic Bi2WO6 (JCPDS card no. 39-0256) phase began to form,
which indicated that a synthesis suspension pH of 3 was more
favorable for the formation of the WO3 phase rather than the
Bi2WO6 phase. All reflections in the XRD pattern of the powders
that were hydrothermally synthesized at pH 7 could be readily
assigned to the ZnWO4 and Bi2WO6 phases. No XRD reflections
corresponding to impurity phases were detected in these com-
posite photocatalysts. Further increase in the pH of the synthesis
suspension to 9 led to the simultaneous precipitation of ZnWO4
and Bi2WO6 as major phases, with the inclusion of another unde-
sired phase, ZnO (JCPDS card no. 65-3411). Comparison of all the
XRD data for the products hydrothermally synthesized at different
pH indicated that the only pH 7 was favorable for the formation of
both the ZnWO4 and Bi2WO6 phases without any impurity phases
under the current experimental conditions.
pH=9
pH=7
pH=5
pH=3
Bi2WO6
ZnWO4
WO3
ZnO
10
20
30
40
50
60
70
Cu K
α
2θ [°]
Fig. 2 shows the microstructural features of powders hydrother-
mally synthesized at 160 ◦C for 12 h with 10 mol% Bi3+ at pH
3–9. The powders hydrothermally synthesized at pH 3 have large
irregular particles mixed with submicron rod- and plate-like parti-
cles. EDS results (not shown here) suggest that the large irregular
particles are ZnWO4, whereas the rod- and plate-like particles
are representative of the WO3 phase. The powders hydrother-
mally synthesized at pH 5 are mostly agglomerated quasi-spheres
constructed from the aggregation of ZnWO4 nanorods. A few plate-
like particles are also visible, which indicates the presence of the
Bi2WO6 phase. These results are consistent with the XRD data
shown in Fig. 1. Hydrothermal synthesis at pH 7 has a pronounced
effect on the morphology of the resultant powders; the majority
of particles shown in Fig. 2 are submicron-sized ZnWO4 rod-like
Fig. 1. XRD patterns of ZnWO4/Bi2WO6 composite photocatalyst hydrothermally
synthesized with 10 mol% Bi3+ at 160 ◦C for 12 h with various pH of synthesizing
suspensions.
suspension) is one of the key factors that influences the precipi-
tation of a desired phase. Therefore, preliminary experiments were
first performed with various hydrothermal conditions (different
pH) for synthesis of the ZnWO4/Bi2WO6 composite photocatalyst.
The effect of the synthesis solution pH on the formation of the
ZnWO4/Bi2WO6 composite photocatalyst was studied using XRD.
Fig. 1 shows the XRD patterns of powders hydrothermally syn-
thesized at 160 ◦C for 12 h with 10 mol% Bi3+ at pH 3–9. When
the pH of the synthesis suspension was adjusted to 3, the as-
synthesized powders predominantly consisted of two crystalline
Fig. 2. SEM micrographs of ZnWO4/Bi2WO6 composite photocatalyst hydrothermally synthesized with 10 mol% Bi3+ at 160 ◦C for 12 h with various pH of synthesizing
suspensions.

M. Hojamberdiev et al. / Applied Catalysis A: General 457 (2013) 12–20
15
Fig. 4 shows the SEM micrographs of samples hydrothermally
synthesized at 160 ◦C for 12 h with various Bi3+ ion concentrations
at pH 7. The pure ZnWO4 powders without Bi3+ have larger irregular
particles formed by the aggregation of nano- and submicron-sized
particles. With the addition of 5 mol% Bi3+ to the synthesis sys-
tem, the ZnWO4/Bi2WO6 composite photocatalyst was obtained
with irregular morphology, and the particle size was significantly
diminished in comparison to that of pure ZnWO4. The morpholo-
gies of the ZnWO4/Bi2WO6 composite photocatalysts obtained
with 10–30 mol% Bi3+ shown in Fig. 4 are principally comprised of
three types of distinctive particles: large micrometer-sized quasi-
spheres, and nanometer-sized rice- and plate-like particles. These
particles become larger and more distinct with the increase in Bi3+
from 10 to 30 mol%, which suggests that the incorporation of Bi3+
ions is essential for alteration of the overall morphology of the
composite photocatalyst.
30 mol%
25 mol%
20 mol%
15 mol%
10 mol%
5 mol%
0 mol%
Bi2WO6
ZnWO4
10
20
30
40
50
60
70
To discriminate the particles and gain insight into the crystal
structures, TEM analysis was conducted on the ZnWO4/Bi2WO6
composite photocatalyst that was hydrothermally synthesized at
160 ◦C for 12 h with 30 mol% Bi3+ at pH 7. Fig. 5 shows the TEM
micrographs (a, c, d), EDS spectra (b), selected area diffraction
(SAD) patterns (insets), and HRTEM micrographs (e, f) of the
ZnWO4/Bi2WO6 composite photocatalyst. The TEM image of the
composite photocatalyst in Fig. 5a indicates the coexistence of three
different particles in the composite photocatalyst, which is con-
sistent with the SEM observations. Elemental analysis results of
these three different particles using EDS are shown in Fig. 5b. The
quasi-spherical particles (0.8–1.3 ␮m) correspond to the ZnWO4
phase and the rice (400–600 nm long and 80–150 nm wide)- and
plate (300–500 nm diameter)-like particles to the Bi2WO6 phase.
Figs. 5c and d show that a number of rice- and plate-like Bi2WO6
particles are attached to the surface of the quasi-spherical ZnWO4
particles, and the rice-like Bi2WO6 particles are constructed with
a significant amount of tiny nanorods. Separately located particles
were most probably detached from the composite photocatalyst
by strong utrasonication used during the TEM sample preparation.
The ring-like SAD patterns shown in the insets of Figs. 5c and d
reveal the overall polycrystallinity of the quasi-spherical ZnWO4
particles and the rice-like Bi2WO6 particles. The HRTEM images
shown in Figs. 5e and f confirm uniform lattice fringes with an
interval of 0.362 nm that correspond to the d spacing between
adjacent (110) crystallographic planes of monoclinic ZnWO4, while
the lattice fringes of d = 0.320 are consistent with the (131) crys-
tallographic planes of orthorhombic Bi2WO6. The SEM and TEM
observations indicate that the as-synthesized composite photo-
catalyst has a contacting junction structure between ZnWO4 and
Bi2WO6 crystals.
Although time-dependent experiments were not performed
in this study, based on previous reports, we briefly present the
formation processes for the ZnWO4/Bi2WO6 composite photocat-
alyst, without a strong intention to provide an exact mechanism.
It is supposed that in the competitive precipitation process, the
formation of tiny crystalline nuclei of ZnWO4 in the supersat-
urated medium occurs first. While the crystal growth process
of ZnWO4 probably occurs at the expense of smaller crystals,
according to the Gibbs-Thompson theory, tiny crystalline nuclei
of Bi2WO6 begin to form from Bi3+ and remaining [WO4]2–ions.
Under hydrothermal treatment, ZnWO4 and Bi2WO6 nanorods
form from tiny nuclei, due to their anisotropic nature, and are
further self-assembled into quasi-spheres and rice- and plate-
like particles, respectively, by an oriented attachment process,
of which the driving force is a reduction of the surface energy
of crystal planes [41,42]. Consequently, a number of rice- and
plate-like particles of Bi2WO6 are attached to the surface of quasi-
spherical particles of ZnWO4, where a moderate junction structure
is formed.
Cu K
α
2θ [°]
Fig. 3. XRD patterns of ZnWO4/Bi2WO6 composite photocatalyst hydrothermally
synthesized at 160 ◦C for 12 h with various Bi3+ ion concentrations at pH 7.
particles, and the number of plate-like Bi2WO6 particles has also
increased with the increase in the pH. With further increase of the
synthesis suspension pH to 9, the sizes of the rod- and plate-like
particles are decreased, along with the formation of small irregular
particles of ZnO. The SEM results demonstrate that the morphology
of the composite photocatalyst can easily be tailored by adjustment
of the pH. We have also investigated the effect of the synthesis sus-
pension pH on the morphologies of ZnWO4 [37], YVO4:Eu3+ [38,39],
and PbI2 [40] under hydrothermal conditions.
3.2. Effect of Bi3+ ion concentration
The Bi3+ ion concentration was varied from 0 to 30 mol% in the
synthesis suspension to obtain various mole fractions of Bi2WO6
in the composite photocatalyst and investigate the contribution
of Bi2WO6 to the degradation of AcH under UV light irradia-
tion. Fig. 3 shows the XRD patterns of composite photocatalyst
hydrothermally synthesized at 160 ◦C for 12 h with various Bi3+
ion concentrations at pH 7. As expected, the sample hydrother-
mally synthesized without Bi3+ ions contains only the ZnWO4
phase. In contrast, the introduction of 5–30 mol% Bi3+ into the
synthesis suspension resulted in composite photocatalysts com-
posed of both the ZnWO4 and Bi2WO6 phases. No XRD reflections
belonging to secondary phases were evident, even in the composite
photocatalyst synthesized with the highest Bi3+ ion concentra-
tion (30 mol%). The intensity of the reflections corresponding to
the Bi2WO6 phase gradually increased with the Bi3+ ion concen-
tration. It should be noted that no peak shift was observed in
the XRD patterns for both phases, which provides evidence that
the current experimental conditions were not adequate for an ion
doping process. This was also confirmed by comparison of the cal-
culated lattice parameters for the phases present in the samples.
Furthermore, the ZnWO4 and Bi2WO6 phases in the composite pho-
tocatalyst were quantitatively analyzed by the Rietveld method
using the obtained XRD patterns. The percentage of Bi2WO6 phase
in the composite photocatalyst increases with the increase in the
Bi3+ ion concentration as follows: 0% (0 mol%) < 6% (5 mol%) < 9%
(10 mol%) < 12% (15 mol%) < 22% (20 mol%) < 29% (25 mol%) < 34%
(30 mol%). The ZnWO4 phase still prevails as the predominant frac-
tion in the composite photocatalyst. Thus, the direct relationship
between the Bi3+ ion concentration added and the mole fraction
of Bi2WO6 phase present in the composite photocatalyst was ver-
ified.

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M. Hojamberdiev et al. / Applied Catalysis A: General 457 (2013) 12–20
Fig. 4. SEM micrographs of ZnWO4/Bi2WO6 composite photocatalyst hydrothermally synthesized at 160 ◦C for 12 h with various Bi3+ ion concentrations at pH 7.
3.3. Raman spectra
even vibrations (8Ag + 10Bg) are Raman active [43]. The Raman
peaks at ca. 126, 148, 166, 191, 198, 270, 277, 316, 343, 356,
408, 516, 550, 680, 710 and 787 cm−1 are assignable to the inter-
nal vibrations of the WO6 octahedra of ZnWO4. Note that two
Raman peaks at 93 and 908 cm−1 are not shown here. The Raman
spectrum of pure Bi2WO6 with the orthorhombic Pbca(61) space
group shown in Fig. 6 is similar to that of the orthorhombic
Pca21 space group [44]; therefore, it is considered that the Raman
Fig. 6 shows the Raman spectra of ZnWO4/Bi2WO6 compos-
ite photocatalysts hydrothermally synthesized at 160 ◦C for 12 h
with various Bi3+ ion concentrations at pH 7, including that
for pure Bi2WO6. The space groups of ZnWO4 and Bi2WO6 are
P2/c = C24h and Pca21 = C25v, respectively. Group theory analysis for
wolframite-type ZnWO4 predicts 36 lattice modes, of which 18

M. Hojamberdiev et al. / Applied Catalysis A: General 457 (2013) 12–20
17
Fig. 5. TEM micrographs (a, c, d), EDS spectra (b), SAD patterns (insets), and HRTEM micrographs (e, f) of ZnWO4/Bi2WO6 composite photocatalyst hydrothermally synthesized
at 160 ◦C for 12 h with 30 mol% Bi3+ at pH 7.
peaks of the Pbca(61) phase can be assigned to those modes of
the Pca21 phase. The number of optic modes for the Pca21 phase
is 26A1 + 27A2 + 26B1 + 26B2. The A2 modes are Raman active and
the A1, B1, and B2 modes are both IR and Raman active [45].
From the Raman spectra of composite photocatalyst and Bi2WO6,
the Raman peaks at 788 and 703 cm−1 can be assigned to the
symmetric and asymmetric stretching modes of WO6 octahe-
dra that involve motions of the apical and equatorial oxygen
atoms perpendicular to and within layers, respectively. The Raman
peaks at 412, 300, 278, 256, 215 and 187 cm−1 can be attributed
to the bending modes of WO6 octahedra and the stretching
and bending modes of BiO6 polyhedra. The Raman peaks at 140
and 147 cm−1 may be assigned to the translations of tungsten and
bismuth ions [44].
As the Bi3+ ion concentration in the synthesis suspension
increases, the intensity of the Raman modes corresponding to the
monoclinic ZnWO4 phase is decreased. In addition, all the Raman
spectra we carefully examined and no considerable shift of the
Raman peaks of the ZnWO4 and Bi2WO6 phases was confirmed.
The overall intensities of the Raman spectra for the composite

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M. Hojamberdiev et al. / Applied Catalysis A: General 457 (2013) 12–20
increase in the Bi2WO6 content in the composite photocatalyst
results in a monotonic shift of the absorption edge from ca. 355 nm
to longer wavelengths up to ca. 450 nm, due to the intrinsic transi-
tions of both semiconductors. This also supports the formation of
a junction structure between ZnWO4 and Bi2WO6 in the compos-
ite photocatalyst. The band gaps of the ZnWO4/Bi2WO6 composite
photocatalysts range from ca. 3.08 to ca. 2.90 eV, in accordance with
the Bi2WO6 content. Therefore, considering the absorption edge
of the composite photocatalyst, it seems reasonable that the as-
synthesized composite photocatalyst can be excited under UV light
irradiation, which would result in higher photocatalytic activity.
30 mol%
25 mol%
20 mol%
15 mol%
10 mol%
5 mol%
3.5. Photocatalytic activity
0 mol%
Bi2WO6
The photocatalytic activity of the as-synthesized composite
photocatalysts for the photodegradation of AcH under UV light irra-
diation was evaluated and compared with pure ZnWO4, Bi2WO6,
and a mechanical mixture of ZnWO4/Bi2WO6. AcH is known as a key
indoor air pollutant and is also largely formed as an intermediate
during the photocatalytic oxidation of other organic compounds
[47]. The generation of CO2 from the photocatalytic oxidation of
AcH occurs according to the following reaction:
100
200
300
400
500
600
700
800
Raman shift [cm-1]
Fig. 6. Raman spectra of ZnWO4/Bi2WO6 composite photocatalyst hydrothermally
synthesized at 160 ◦C for 12 h with various Bi3+ ion concentrations at pH 7.
2CH3CHO + 5O2phot−oc→atalyst4CO2 + 4H2O.
(1)
hv
photocatalysts were significantly enhanced. The enhancement in
the intensities of the Raman spectra was primarily postulated by
Chen et al. [46] on the basis of two theories: electromagnetic,
which relies upon the excitation of localized surface plasmons, and
chemical, which rationalizes the effect through the formation of
charge-transfer complexes.
Fig. 8 shows the photodegradation of AcH over various samples
as a function of time, in which C0 and C are the concentrations
of AcH after 24 h adsorption equilibrium in the dark at time t0
and after a period of UV light irradiation t, respectively. A blank
test with 250 ppm AcH in the absence of a photocatalyst was also
run under UV light irradiation, where the concentration of AcH
decreases negligibly over time, which indicates the stability of AcH
under UV light irradiation. The same experimental conditions were
applied for all samples. The incorporation of Bi2WO6 in ZnWO4
was observed to enhance the photocatalytic activity compared to
that of pure ZnWO4. Nevertheless, only the composite photocata-
lyst with 30 mol% Bi3+ exhibited complete conversion (100%) of AcH
into CO2 after 6 h of UV light irradiation, whereas the mechanically
mixed counterpart (ZnWO4/30 mol% Bi2WO6 composite photocat-
alyst) had a slightly lower conversion (94%) of AcH into CO2.
Mechanical mixing tends to cause the formation of crystal lat-
tice defects that serve as electron-hole recombination centers for
the composite photocatalyst. However, it is expected that a one-
step synthesis of the composite photocatalyst would enable better
contact between the two semiconductors, which would facilitate
charge transfer from one particle to another. The overall conver-
sion of AcH into CO2 for the as-synthesized samples was calculated
using the formula:
3.4. UV–vis diffuse reflectance spectra
Fig. 7 shows the UV–visible diffuse reflectance spectra of
ZnWO4/Bi2WO6 composite photocatalysts hydrothermally synthe-
sized at 160 ◦C for 12 h with various Bi3+ ion concentrations at pH
7. The light absorbance of pure ZnWO4 lies in the UV region with an
absorption edge of ca. 355 nm, and the band gap is estimated to be
ca. 3.8 eV. With the introduction of Bi3+ ions in the synthesis sus-
pension, ZnWO4 becomes associated with Bi2WO6 in the composite
photocatalyst. Therefore, compared with pure ZnWO4, a gradual
ꢀ
ꢁ
[CO2]
2[CH3CHO]
Conversion (%) =
× 100
(2)
The following results were obtained: 36.2% (ZnWO4) < 44.0%
(Bi2WO6) < 52.8% (5 mol%) < 60.8% (10 mol%) < 71.2% (15 mol%)
< 79.8% (20 mol%) < 86.2% (25 mol%) < 94.0% (mechanical mixture of
ZnWO4/30 mol% Bi2WO6) < 100% (30 mol%). In addition, apparent
first-order rate constants (k) for the degradation of AcH were cal-
culated using the Langmuir-Hinshelwood kinetic model:
0 mol%
5 mol%
10 mol%
15 mol%
20 mol%
25 mol%
30 mol%
ꢀ
ꢁ
C
−ln
= kt,
(3)
C0
where C0 and C are the concentrations of AcH after 24 h adsorp-
tion equilibrium in the dark at time t0 and after a period of
250
300
350
400
450
500
550
Wavelength [nm]
UV light irradiation t, respectively.
A good linear relation-
ship was observed for the ln(C/C0) vs. time (t) plot over the
first 4 h; therefore, the apparent rate constant (k) was calcu-
lated by fitting this portion of the linear plot. The calculated
Fig. 7. UV–vis diffuse reflectance spectra of ZnWO4/Bi2WO6 composite photo-
catalyst hydrothermally synthesized at 160 ◦
concentrations at pH 7.
C
for 12 h with various Bi3+ ion

M. Hojamberdiev et al. / Applied Catalysis A: General 457 (2013) 12–20
19
in the core may occur, which would lead to lower photocatalytic
activity [34,48].
It is evident that an increase in the Bi2WO6 content leads to
higher photocatalytic activity of the composite photocatalyst, even
though some of the Bi2WO6 particles are not attached to the surface
of the quasi-spherical ZnWO4 particles. This is because the layered
structure of Bi2WO6 is favorable for charge transfer, which retards
the recombination of photogenerated holes and electrons [49,50],
and the higher specific surface area also provides more active reac-
tion sites for the photocatalytic reaction to occur on the surface of
the photocatalyst [51]. In addition, the valence band of the Bi2WO6
photocatalyst consists of O 2p and Bi 6p hybrid orbitals, and the
bottom of the conduction band is formed by the W 5d orbitals with
a small contribution from the Bi 6p orbitals. The hybridization of
the Bi 6s and O 2p levels also causes the valence band to become
largely dispersed, which favors the mobility of photoinduced holes
in the valence band, which is beneficial for photocatalytic oxidation
reactions [52].
To clearly explain the enhancement in the photocatalytic activ-
ity of the ZnWO4/Bi2WO6 composite photocatalysts in terms of
n-n isotype junctions, further investigation is required. ZnWO4 and
Bi2WO6 are n-type semiconductors; therefore, it is supposed that
during UV light irradiation of the n-n isotype junction, the elec-
trons from the ZnWO4 region diffuse into the Bi2WO6 region, which
creates an accumulation of negative charges in the Bi2WO6 region
and an accumulation of positive charges in the ZnWO4 region in
the vicinity of the junction. This is expected to provide an internal
electrostatic field directed from the ZnWO4 region to the Bi2WO6
region, which creates an energy barrier for the electron transfer
from ZnWO4 to Bi2WO6 [53,54]. Moreover, another mechanism
may be attributed to the state of tungsten present in both semi-
conductors. W6+ is generally a strong oxidizing agent that is readily
reduced to W5+. The band structure diagram presented in Ref. [55]
features a donor level that arises from the 5d1 configuration of W5+
,
between the valence (O2−) and conduction (W6+) bands. There-
fore, it is considered that both W6+ and W5+ are present in the
composite photocatalyst synthesized in this study. Thus, by reach-
ing a saturation level of W6+-to-W5+ conversion, the presence of
oxygen can further re-oxidize W5+ to W6+ [56], which could also
contribute to the photocatalytic enhancement of the composite
photocatalyst.
Fig. 8. Photocatalytic oxidative decomposition of AcH (250 ppm) without a photo-
catalyst (blank), over ZnWO4, Bi2WO6, a mechanicall mixture of ZnWO4/Bi2WO6
(30 mol%), and over the as-synthesized ZnWO4/Bi2WO6 composite photocatalyst
samples (50 mg) under UV light irradiation (ca. 0.4 mW/cm2). C0 and C are the
concentrations of AcH at time t0 and t, respectively.
4. Conclusions
The effect of Bi2WO6 content in the ZnWO4/Bi2WO6 composite
photocatalyst on the performance for the photocatalytic degrada-
tion of AcH under UV light irradiation was investigated in this study.
A gradual increase in the Bi2WO6 content of the composite photo-
catalyst resulted in a monotonic shift of the absorption edge from
ca. 355 nm to longer wavelengths up to ca. 450 nm. The compos-
ite photocatalyst synthesized with 30 mol% Bi3+ exhibited higher
photocatalytic activity for the conversion of AcH into CO2 under
UV light irradiation compared with that for individual ZnWO4
and Bi2WO6, and that for a mechanically mixed ZnWO4/Bi2WO6
composite photocatalyst. The enhancement of the photocatalytic
activity can be attributed to n-n isotype junctions formed between
the two semiconductors and to the charge separation of each
semiconductor, because the ZnWO4 and Bi2WO6 particles in the
composite photocatalyst are either joined or separate.
apparent rate constant increases in the following order:
0.0583 min−1 (ZnWO4) < 0.0820 min−1 (5 mol%) < 0.0930 min−1
(10 mol%) < 0.1030 min−1 (15 mol%) < 0.1132 min−1 (20 mol%) <
0.1230 min−1 (Bi2WO6) < 0.1298 min−1 (25 mol%) < 0.1573 min−1
(mechanical mixture of ZnWO4/30 mol% Bi2WO6) < 0.1840 min−1
(30 mol%). The apparent rate constant increases with the Bi2WO6
content in the composite photocatalyst. The highest apparent rate
constant was observed for the sample synthesized with 30 mol%
Bi3+, which is attributed to successful charge transfer achieved
from one semiconductor to another. Compared with the previously
reported results for the same composite system prepared by a
simple hydrothermal method [33], the addition of 30 mol% Bi3+
was much higher, and the photocatalytic activity was monotoni-
cally increased with the Bi2WO6 content. The main reason for this
difference may be related to the partial coverage of the surface
of the quasi-spherical ZnWO4 particles, as shown in Fig. 5, which
allows light absorption to occur in the ZnWO4 particles. It has been
widely reported that if a core particle is thickly covered by a shell,
then interfacial transfer may be impeded or no light absorption
Acknowledgements
MH would like to thank the Japan Society for the Promotion
of Science (JSPS) for the postdoctoral fellowship under which
the present study was carried out. The authors wish to thank

20
M. Hojamberdiev et al. / Applied Catalysis A: General 457 (2013) 12–20
Mr. Jean-Christophe Jaud and Mr. Yukihiro Komatsubara for per-
forming the Rietveld analysis and TEM measurements, respectively.
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