Multi-pathway photoelectron migration in globular flower-like In2O3/AgBr/Bi2WO6 synthesized by microwave-assisted method with enhanced photocatalytic activity

Article abstract of DOI:10.1016/j.molcata.2015.12.023

The photocatalyst In₂O₃/AgBr/Bi₂WO₆ with visible light response was synthesized using microwave-assisted hydrothermal method. The crystal structure, optical properties and surface morphologies of the products were characterized by XRD, UV-vis/DRS, SEM, TEM, HRTEM, XPS and N₂ adsorption-desorption tests. The results indicate that indium exists in the form of oxide and inhibits the growth of Bi₂WO₆ at (1 3 3) crystal plane. The In₂O₃/AgBr/Bi₂WO₆ composite consists of the particles piled up into flower-like spherical structures. In₂O₃/AgBr/Bi₂WO₆ exhibits high photocatalytic activity for the Rhodamin B (RhB) degradation under ultraviolet and visible light, and, also, shows steady photocatalytic performance after four times of recycling. Besides, the potential photocatalytic mechanism of In₂O₃/AgBr/Bi₂WO₆ was proposed according to the energy band structure of the composite, and the vital role of multi-pathway photogenerated electron migration during photocatalysis process was explained.

Full text of DOI:10.1016/j.molcata.2015.12.023

Journal of Molecular Catalysis A: Chemical 414 (2016) 27–36
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Multi-pathway photoelectron migration in globular flower-like
In O /AgBr/Bi WO synthesized by microwave-assisted method with
2
3
2
6
enhanced photocatalytic activity
Xi Chena,b, Li Lia,b,c,∗, Wenzhi Zhang , Yixuan Li , Qiang Song , Jianqi Zhang , Di Liu
b
b
b
b
a
a
College of Materials Science and Engineering, Qiqihar University, Qiqihar 161006, China
College of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar 161006, China
College of Heilongjang Province Key Laboratory of Fine Chemicals, Qiqihar University, Qiqihar 161006, China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 November 2015
Received in revised form
2 3 2 6
The photocatalyst In O /AgBr/Bi WO with visible light response was synthesized using microwave-
assisted hydrothermal method. The crystal structure, optical properties and surface morphologies of the
products were characterized by XRD, UV–vis/DRS, SEM, TEM, HRTEM, XPS and N2 adsorption–desorption
tests. The results indicate that indium exists in the form of oxide and inhibits the growth of Bi2WO6 at
2
4 December 2015
Accepted 27 December 2015
Available online 31 December 2015
(
1 3 3) crystal plane. The In2O3/AgBr/Bi2WO6 composite consists of the particles piled up into flower-
like spherical structures. In2O3/AgBr/Bi2WO6 exhibits high photocatalytic activity for the Rhodamin B
RhB) degradation under ultraviolet and visible light, and, also, shows steady photocatalytic performance
(
Keywords:
after four times of recycling. Besides, the potential photocatalytic mechanism of In2O3/AgBr/Bi2WO6 was
proposed according to the energy band structure of the composite, and the vital role of multi-pathway
photogenerated electron migration during photocatalysis process was explained.
© 2015 Elsevier B.V. All rights reserved.
Microwave-assisted hydrothermal method
In2O3/AgBr/Bi2WO6
Visible light response
Photocatalysis
Rhodamin B
1
. Introduction
To date, semiconductor photocatalytic technology has become
In the current study, semiconductor compound is a kind of good
modification methods. By adding different types of semiconduc-
tor photocatalysts, such as AgBr, Bi O3 and others [13–15], the
2
a novel type of eco-friendly treatment for pollutants because pho-
tocatalysts can degrade and mineralize organic pollutants into
inorganic ions, CO , and H O ultimately under the light [1–3].
recombination of photoinduced electron–hole pairs was signifi-
cantly influenced, and, consequently, the photocatalytic activity
was improved.
2
2
Bi WO , a novel catalyst with visible light response, has received
To realize the semiconductor composite fabrication, some
excellent synthesis technologies are required. Microwave-assisted
synthesis, as a new type of green synthesis technology, has
attracted a considerable amount of attention [16,17]. Compared
to conventional heating, microwave irradiation seems to be an
attractive alternative due to its significant advantages, such as fast
heating, short reaction time and homogeneous volumetric heat-
ing [18,19]. Thus, more and more researchers begin to use the
microwave-assisted method for sample synthesis [20–22].
2
6
great attention [4–6]. As the simplest member of Aurivillius-
type compounds, Bi WO exhibits layered structure consisting
2
6
2
+
2−
of alternating (Bi O ) layers and octahedral (WO4) layers. As
2
2
compared to TiO , Bi WO possesses narrower band gap, and its
2
2
6
light response range can reach >440 nm which implies a better vis-
ible light response. But its narrow band gap also causes the high
recombination efficiency of photoinduced electron–hole pairs dur-
ing the photocatalytic process, and decreases the photo-quantum
yield of Bi WO to some extent. Therefore, many methods, such as
In this study, we report a novel In O /AgBr/Bi WO visible-
2
6
2 3 2 6
precious metal deposition [7,8], semiconductor composite fabrica-
tion [9,10] and so on [11,12], have been used to modify Bi WO
light photocatalyst fabricated through the three-step route.
In O /AgBr/Bi WO composite is expected to increase the migra-
2
6
2
3
2
6
and decrease the photoinduced electron–hole pair recombination.
tory routes of photoinduced electrons during photocatalytic
process, and effectively improve the visible region absorption. In
addition, besides the crystal structures and physicochemical prop-
erties of pure Bi WO , AgBr/Bi WO and In O /AgBr/Bi WO pho-
tocatalysts, their photocatalytic activities under ultraviolet-visible
light, microwave-assisted and sunlight-simulated photocatalysis
2
6
2
6
2
3
2
6
∗ Corresponding author at: College of Materials Science and Engineering, Qiqihar
University, Qiqihar 161006, China.
E-mail addresses: qqhrll@163.com, qqhrlili@126.com (L. Li).
http://dx.doi.org/10.1016/j.molcata.2015.12.023
1
381-1169/© 2015 Elsevier B.V. All rights reserved.

2
8
X. Chen et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 27–36
were also investigated. Furthermore, the possible photodegrada-
tion mechanism of RhB and the major reason for the enhancement
of photocatalytic activity were discussed, respectively.
2.2.2. Synthesis of AgBr/Bi WO
2 6
AgBr/Bi WO6 was synthesized by a deposition–precipitation
2
method. 0.45 g of as-prepared Bi WO6 was added into 10 mL dis-
2
tilled water, and, then, the ultrasonic treatment was applied for
5
min to obtain white suspension. 0.225 g of CTAB was added into
t
h
e
B
i
W
O
s
u
s
p
e
n
s
i
o
n
.
A
f
t
e
r
s
t
i
r
r
i
n
g
f
o
r
3
0
m
i
n
,
0
.
0
5
3
g
o
f
A
g
N
O
2
. Experiment
2
6
3
was dissolved in 1 mL of ammonia (25–28%wt) to form a clear solu-
tion. The AgNO3 solution was added dropwise into the mixture
suspensions, and the resulting suspensions were stirred at room
temperature for 24 h in dark. After the reaction, the precipitate was
washed with deionized water and absolute ethanol several times,
2
.1. Materials and instruments
Bismuth nitrate (Bi(NO ) ·5H O, 99.0%) and cetyl trimethyl
3
3
2
ammonium bromide (CTAB, 99.0%) were purchased from Guangfu
Fine Chemical Research Institute, Tianjin, China. Sodium tungstate
◦
and dried at 60 C for 12 h. Finally, the powder was calcined in air
◦
at 500 C for 3 h.
(
Na WO ·2H O, 99.5%) and ammonium (NH OH, 25–28%wt) were
2
4
2
4
purchased from Kaitong Chemical Reagent Co., Ltd., Tianjin,
China. Silver nitrate (AgNO , 99.8%) was obtained from Ying-
2.2.3. Synthesis of In O /AgBr/Bi WO
2 3 2 6
3
daxigui Chemical Reagent factory, Tianjin, China. Indium nitrate
In O /AgBr/Bi WO was synthesized by a microwave-assisted
2 3 2 6
(
In(NO ) ·4.5H O, 99.5%) was purchased from Sinopharm Chemi-
hydrothermal method. 0.2 g of as-prepared AgBr/Bi WO6 was
3
3
2
2
cal Reagent Co., Ltd., Shanghai, China. Glacial acetic acid (CH COOH,
9
added into 30 mL distilled water, and, then, the ultrasonic treat-
ment was applied for 5 min to obtain pale yellow suspension.
1 %wt In(NO ) ·4.5H O was added into AgBr/Bi WO suspension,
3
9.5%) was obtained from Kemiou Chemical Reagent Co., Ltd.,
Tianjin, China. Analytical grade Rhodamin B (RhB) and ethyl alco-
hol (EtOH) were used as received without further purification.
The double-distilled water was used in the experiments. MDS–8 G
microwave reactor was purchased from Xin Yi Co., Shanghai, China.
BL-GHX-V photochemical reactions instrument was purchased
from Bilang Biotechnology Co., Xian, China.
3
3
2
2
6
and it was stirred for 30 min in darkness. Then, the suspensions
were transferred into 100 mL Teflon-lined autoclave and heated by
◦
microwave at 100 C during 30 min. After the reaction, the precipi-
tate was washed with deionized water and absolute ethanol several
◦
times, and dried at 60 C for 12 h. The final product was calcined in
◦
air at 500 C for 3 h.
2
.2. Preparation of photocatalyst
.2.1. Synthesis of Bi WO
2
.3. Characterization of photocatalyst
2
2
6
The phase composition of the as-prepared samples was deter-
Globular flower-like Bi WO6 was synthesized by a microwave-
2
mined by X-ray diffraction (XRD) analysis (Bruker-AXS(D8)). The
assisted method. Typically, 1.455 g of Bi(NO ) ·5H O was dissolved
◦
◦
3
3
2
diffraction patterns were recorded in the range of 2ꢀ = 20 − 80
in 3 mL of glacial acetic acid. 0.485 g of Na WO ·2H O was dis-
2
4
2
using Cu K␣ radiation (ꢁ = 0.15406 nm). The UV–vis absorp-
tion spectra were recorded on an UV–vis spectrophotometer
(Model TU-1901) in the wavelength range of 200–800 nm, and
BaSO4 was used as the reference. The morphology and parti-
cle size were examined by scanning electron microscopy (SEM)
solved in 27 mL distilled water. Then, the Bi(NO ) solution was
3
3
added dropwise into the Na WO4 solution under vigorous stirring
2
to form a white suspension. After stirring for 30 min, the precursor
was transferred into a 100 mL Teflon-lined autoclave. The volume
of mixture was less than 40 mL. The microwave hydrothermal reac-
tions were carried out at 140 C for 2 h. Finally, the precipitate was
washed with deionized water and absolute ethanol several times,
(HitachiS-4300). The particle microstructure was investigated
◦
by transmission electron microscopy (TEM) (HitachiH-7650) and
high-resolution transmission electron microscopy (HRTEM) (JEM-
2
◦
◦
and dried at 60 C for 12 h. The final product was calcined at 300 C
for 4 h to obtain globular flower-like Bi WO , as shown in Scheme 1.
100F). X-ray photo-electron spectroscopy (XPS) was performed
2
6
using an ESCALAB 250Xi spectrometer equipped with Al K␣ radi-
Scheme 1. Possible growth mechanism of the synthesized products.

X. Chen et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 27–36
29
a
b
Fig. 1. XRD patterns of the products (a) and variations of crystallite sizes at various crystal planes (b).
ation at 300 W. The specific surface areas of the samples were
measured by a Brunauer–Emmett–Teller specific surface area
instrument (Beishide Instrumentation Technologies (Beijing) Ltd.,
Model 3H-2000PS2) with nitrogen adsorption at 77 K.
as-prepared samples. The diffraction peaks well correspond to the
crystal planes (1 3 1), (2 0 0), (2 0 2), (1 3 3), (2 6 2) and (1 9 3) of
orthorhombic Bi WO phase (JCPD standard card No.39-0256) [26].
Moreover, the diffraction peaks are high and sharp that indicates
good crystallinity. In addition, the diffraction peaks labeled by ꢀ
correspond to the crystal planes (2 0 0) and (2 2 0) of cubic AgBr
2
6
2.4. Photocatalytic experiment
(JCPD standard card No.06-0438) [27]. From Fig. 1a, the diffrac-
The photocatalytic test was processed in a specially made
tion peak intensity increased after composite formation, but the
dominant phase of samples does not changed. It is noted that the
diffraction peak of indium oxide can not be observed in the XRD pat-
terns. This result is attributed to the low content of indium oxide
which can not be detected.
double-wall beaker made of quartz with different optical sources
in it. The UV light source was a 125 W Hg-lamp with the maxi-
mum emission at 313.2 nm. The visible light source was a 400 W
Xe-lamp with the maximum emission over 410 nm (used specially
made glass to filter UV light). A 1000 W Xe lamp was used as the
simulated sunlight source, which made the distance at 8.5 cm away
from the reaction solution.
The crystallite size was estimated for different crystal planes
according to Scherrer equation: [28]:
Photocatalytic activities of samples were determined by the
−
1
Kꢁ
decolorization of Rhodamine B (RhB) aqueous solution (50 mg L ).
Before illumination, the solutions were stirred for 30 min in dark-
ness in order to reach the adsorption–desorption equilibrium. At
given time intervals, 3 mL suspension was collected and centrifuged
to remove the photocatalyst particles. The concentration of RhB
was then determined by measuring the absorbance at ꢁmax, using
an UV–vis spectrophotometer (Model TU-1901). The experimen-
tal facilities used by our group and studied photocatalytic reaction
processes can be seen elsewhere [23–25].
d = ꢀ
ꢁ
Bcosꢀ
where d is the crystallite size, K represents the shape factor (taken as
0.89), ꢁ represents the X-ray wavelength, B is the width of the X-ray
diffraction peak (full width at half-maximum), and ꢀ represents the
Bragg diffraction angle. When ꢀ is the Bragg diffraction angle of the
highest diffraction peak of sample, d becomes the average crystal-
lite size. From Fig. 1b, the crystallite sizes of AgBr/Bi WO at (1 3 1),
2
6
3
. Results and discussion
(2 0 0), (2 0 2), and (1 3 3) crystal planes are larger than those of pure
Bi WO . Moreover, as compared with AgBr/Bi WO composite, the
2
6
2
6
3.1. Crystal structure
crystallite sizes of In O /AgBr/Bi WO at (1 3 1), (2 0 0), and (2 0 2)
2 3 2 6
crystal planes are slightly increased, while crystallite size at (1 3 3)
crystal plane decreased to a certain extent. The results indicate that
To study the crystal structure of samples, X-ray diffraction anal-
yses were carried out. Fig. 1a shows the XRD patterns of the
indium ions suppress the growth of Bi WO6 (1 3 3) crystal plane.
2
Fig. 2. Williamson–Hall plots for the products.

3
0
X. Chen et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 27–36
Table 1
als shift to longer wavelengths, as compared with pure Bi WO .
2
6
The lattice constants (Å), average crystallite sizes (nm) and energy band gaps (eV)
of different synthesized products.
Importantly, there is no obvious difference of the AgBr/Bi WO6
2
and In O /AgBr/Bi WO band gaps. In consequence, indium does
2
3
2
6
Sample
Crystal parameters
d (nm)
Eg (eV)
not produce a corresponding impurity level, so that the band
gap of In O /AgBr/Bi WO does not decrease. It turned out that
a/Å
b/Å
c/Å
2
3
2
6
indium ion is not into the crystal lattice of the AgBr/Bi WO ,
but exists in the form of oxide. (This is consistent with the XRD
result). The surface optical property of In2O3/AgBr/Bi2WO6 com-
2
6
Bi2WO6
AgBr/Bi2WO6
In2O3/AgBr/Bi2WO6
5.449
5.446
5.449
16.416
16.353
16.344
5.443
5.453
5.452
11.4
23.6
24.0
2.76
2.46
2.53
posite was affected by In O3 which has a wide band gap, and
2
this led to the slight decrease of visible light absorption and
the significant increase of ultraviolet light absorption. Even so,
To estimate the lattice strain, Williamson and Hall equation was
used [29]:
In O /AgBr/Bi WO still performs the highest photocatalytic activ-
2
3
2
6
ity under visible light irradiation (according to Fig. 8 b). The result
indicates that light absorption property is not the decisive factor
on photocatalytic activity.
Bcosꢀ
ꢁ
ꢂsinꢀ
ꢁ + b
=
where B is the width of the X-ray diffraction peak (full width at half-
maximum), ꢀ represents the Bragg diffraction angle, ꢁ represents
the X-ray wavelength, ꢂ represents lattice strain. A curve that plots
Bcosꢀ/ꢁ on the y axis and sinꢀ/ꢁ on the x axis is drawn. b is inter-
cept, and the slopes of the plots of Bcosꢀ/ꢁ versus sinꢀ/ꢁ represent
the estimation of strain. Because the lattice constants of AgBr and
Bi WO are different, the lattice strain is generated at the interface
3.3. Surface morphology and microstructure analysis
To evaluate the surface morphologies of different samples, scan-
ning electron microscope (SEM) analysis was carried out and the
results are shown in Fig. 4. As one can see from Fig. 4, pure Bi WO6
2
2
6
is composed of hundreds platelet-like particles piled into regu-
lar globular flower-like spheres, and the sphere surface is sharp
between AgBr and Bi WO , and the crystal lattice of composite was
distorted, thus, made the lattice constant c of composite larger than
that of lattice constant a (Table 1).
Meanwhile, this lattice mismatch phenomenon led to the lattice
strain of Bi WO from negative to positive, as shown in Fig. 2. In
addition, from Table 1, the lattice constants of AgBr/Bi WO and
2
6
(a–c). When composited with AgBr, the platelet-like particles on
the surface of the Bi WO6 spheres became smooth and they grad-
2
ually dissolved into smaller block-like particles. However, it still
maintained the original spherical structure (d–f). Finally, the block-
like particles were partly scattered by microwave radiation on the
2
6
2
6
In O /AgBr/Bi WO have no obvious differences. In other words,
2
3
2
6
surface of AgBr/Bi WO6 spheres covered with In O , and a large
2
2
3
there is no corresponding lattice shrinkage, expansion or distortion
in In O /AgBr/Bi WO . Therefore, indium ions are not incorporated
number of spherical particles are aggregated densely (g–i).
The as-prepared In O /AgBr/Bi WO was investigated by trans-
2
3
2
6
2
3
2
6
into the crystal lattice, but they exist in the form of indium oxide.
mission electron microscopy (TEM) and high-resolution TEM
HRTEM), and the results are shown in Fig. 5. As seen, the flower-
(
3.2. Optical properties
like spheres were composed of a large number of particles, and
there were some cracking because of nonuniform accumulation.
The HRTEM image of In O /AgBr/Bi WO composite is shown in
To investigate the optical absorption properties of pure
2
3
2
6
Bi WO , AgBr/Bi WO and In O /AgBr/Bi WO samples, the dif-
2
6
2
6
2
3
2
6
Fig. 5b. After fast Fourier transform (FFT) over different selected
areas, Fig. 5c, e and g were obtained. Then, dealt with Fig. 5c, e and
g through inverse fast Fourier transform (IFFT) to obtain Fig. 5d, f
and h, respectively. In Fig. 5d, f and h, the measured lattice spac-
ing of 0.288 nm corresponds to the (2 0 0) lattice plane of cubic AgBr
[30], the lattice spacing of 0.292 nm corresponds to the (2 2 2) lattice
fuse reflectance spectra of as-prepared samples were recorded and
they are shown in Fig. 3.
According to the equation, the band gaps of different samples
were estimated and the values are shown in Table 1.
2
˛
hꢃ = A(hꢃ − Eg)
plane of cubic In O3 [31], and the lattice spacing of 0.315 nm cor-
2
where ˛, hꢃ, A, and Eg are the optical absorption coefficient, pho-
tonic energy, proportionality constant, and band gap, respectively
responding to the (1 3 1) plane of orthorhombic Bi WO were also
2
6
observed [26], respectively. These results are in good agreement
with the XRD and XPS analyses.
[
30]. Fig. 3 shows that the absorption edges of composite materi-
3.4. XPS analysis
The chemical states of elements on the surface of
In O /AgBr/Bi WO were further studied by X-ray photoelec-
2
3
2
6
tron spectroscopy (XPS), and the results are shown in Fig. 6. The
survey XPS spectrum of In O /AgBr/Bi WO indicates presence
2
3
2
6
of Bi, W, Ag, Br, In, O and C elements, as shown in Fig. 6a. The
C1s peak at around 284.6 eV can be attributed to the signal from
carbon contamination on the sample surface, and the line was used
for calibration [32]. From Fig. 6b, the energy difference of 5.3 eV
between Bi4f7/2 and Bi4f5/2 components at 159.1 and 164.4 eV
3+
confirms the bismuth valence state Bi in the In O /AgBr/Bi WO
2
3
2
6
composite [26]. The element levels shown in Fig. 6c, d and e reveal
that W, Ag and Br species in In O /AgBr/Bi WO are in the states
2
3
2
6
of W+6, Ag and Br , respectively [33,34]. The XPS signals for
+1
−1
3+
In3d3/2 at a binding energy of 452.4 eV corresponds to the In
cations (Fig. 6f) [35], and oxygen is in the state of O
−2
Fig. 3. UV–vis/DRS spectra and plot of (ahv)1/2 versus energy (hv) of different sam-
(Fig. 6g)
ples (inset).
[25]. The quantitative analysis of In2O3/AgBr/Bi2WO6 is displayed

X. Chen et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 27–36
31
Fig. 4. SEM images of the pure Bi2WO6 (a–c); AgBr/Bi2WO6 (d–f); In/AgBr/Bi2WO6 (g–i).
in Fig. 6h. The result shows that the mole ratio of In O3 to AgBr to
3.6. Photocatalytic performance
2
Bi WO is about 1:20:123.
2
6
To assess the photocatalytic performance of synthesized prod-
ucts, the UV light, visible light, microwave-assisted, and simulated
sunlight photocatalytic degradation RhB tests were carried out,
3.5. N2 adsorption–desorption isotherms
and the results are shown in Fig. 8a–d. Orthorhombic Bi WO6
2
presents a certain photocatalytic activity under different photo-
catalytic conditions. The AgBr/Bi WO6 and In O /AgBr/Bi WO
The surface areas and pore structures of pure Bi WO ,
2
6
2
2
3
2
6
AgBr/Bi WO and In O /AgBr/Bi WO were detected by N2
2
6
2
3
2
6
composites both show higher photocatalytic activities than that
of pure Bi WO , and In O /AgBr/Bi WO composite presents the
adsorption–desorption analysis, as shown in Fig. 7. According to the
classification of IUPAC, the N2 adsorption–desorption isotherms of
the as-prepared samples all exhibit the type of IV isotherm with
H3 hysteresis loop. This type of isotherm and hysteresis loop was
attributed to the cohesion of capillaries and the aggregation of par-
ticles, respectively [25].
2
6
2
3
2
6
highest photocatalytic activity of all under different conditions,
even far superior to commercial P25. In addition, as shown in
the inset of Fig. 8d, the strongest absorption peak of RhB solution
decreases gradually with an absorption band shifts from 553 to
5
41 nm after 5 h irradiation, indicates the degradation of RhB [36].
As seen from pore size distributions, the samples all have meso-
porous distributions. Moreover, the pore size distribution of pure
Bi WO is broad with a certain number of macroporous (>50 nm).
Table 2
2
6
BET surface areas, average pore diameters and pore volumes of different synthesized
products.
However, AgBr/Bi WO6 and In O /AgBr/Bi WO composites both
2
2
3
2
6
present the tightly packed structure, and their bore diameters are
uniform. Compared to pure Bi WO , because of this tightly packed
2
3
Sample
SBET (m /g)
D (nm)
Vtotal (cm /g)
2
6
structure, the average pore diameters and pore volumes of compos-
ites has decreased obviously, and the BET surface areas of samples
also present a similar regularity (Table 2).
Pure Bi2WO6
AgBr/Bi2WO6
In2O3/AgBr/Bi2WO6
35.35
11.29
8.33
23.32
13.25
12.53
0.206
0.037
0.026

3
2
X. Chen et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 27–36
Fig. 5. TEM and HRTEM images of the In/AgBr/Bi2WO6 (a, b); FFT (fast Fourier transition) patterns and inverse FFT patterns of the fringes of AgBr (c, d), In2O3 (e, f), and
Bi2WO6 (g, h), respectively.
To discuss the reaction kinetics of the RhB degradation, the anal-
ysis of kinetic experiments is shown in Fig. 8e. According to the
experiment, the kinetic equation can be expressed as follows [26]:
be enhanced during photocatalytic process. Therefore, salicylic
acid, one of the organic pollutants except dye, was designed as
an extra model molecule to test the photocatalytic activity of
In O /AgBr/Bi WO , and the dependence of the degradation rate
2
3
2
6
ꢂ
ꢃ
of salicylic acid under UV irradiation by synthesized products was
plotted in Fig. 8g. After exposure to UV light for 120 min, it can
Ct
−ln
= kt + b
C0
be seen that In O /AgBr/Bi WO performed a favorable photocat-
2
3
2
6
where Ct represents the concentration at reaction time t (mg L−1),
alytic activity, even far superior than P25. The result proved that
In O /AgBr/Bi WO also has good photocatalytic activity for the
−
1
1
C0 is the initial RhB concentration (mg L ) stirred for 30 min
in darkness, k is the rate constant (min ), and b is the inter-
2
3
2
6
−
degradation of non-dye organic pollutants.
cept. From Fig. 8e, the parameter −ln(Ct/C ) is proportional to
It is well known that the specific surface area of the photocata-
lyst has an influence on its photoactivity to a certain extent. Usually,
larger specific surface area can increase the contact between pho-
tocatalyst and organic pollutant molecules and then improves the
0
reaction time t basically, and it indicates that the degradation
of dye RhB follows the pseudo-first-order reaction kinetics. The
calculated rate constants of direct photolysis, P25, pure Bi WO ,
2
6
AgBr/Bi WO and In O /AgBr/Bi WO under simulated sunlight
photocatalytic activity. However, In O /AgBr/Bi WO performs the
2
6
2
3
2
6
−4
2 3 2 6
−5
−3
−3
irradiation are 6.83 × 10 , 5.85 × 10 , 1.14 × 10 , 1.25 × 10
highest photocatalytic activity of all though it has the smallest spe-
cific surface area. This result suggests that the BET surface area of
In O /AgBr/Bi WO is not the major reason for the enhancement
−
3
−1
and 1.71 × 10 min , respectively. Thus, the photocatalytic activ-
ities of various samples under simulated sunlight irradiation are:
In O /AgBr/Bi WO > AgBr/Bi WO > pure Bi WO > P25 > direct
2
3
2
6
2
3
2
6
2
6
2
6
of photocatalytic activity.
photolysis. In addition, 4 cycles of photocatalytic degradation
of RhB were carried out by using In O /AgBr/Bi WO under
2
3
2
6
illumination of UV light. As shown in Fig. 8f, after 4 runs, the
photocatalytic activity of In O /AgBr/Bi WO still exhibits a high
3.7. Possible photocatalytic mechanism
2
3
2
6
degradation of RhB. In addition, due to the sensitization effect from
dyes, the photocatalytic performances of photocatalysts would
To find out the active species of In O /AgBr/Bi WO during pho-
2
3
2
6
tocatalytic reaction, the capture experiment of In O /AgBr/Bi WO
2
3
2
6

X. Chen et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 27–36
33
Fig. 6. XPS spectra of In2O3/AgBr/Bi2WO6 (a), Bi 4f (b), W 4f (c), Ag 3d (d), Br 3d (e), In 3d (f), O 1s (g), and quantitative analysis diagram of In2O3/AgBr/Bi2WO6 (h).
has been carried out. Trapping of radicals and holes in experiments
involves the utilization of isopropanol (IPA), benzoquinone (BQ),
and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) as
completely. The degradation of RhB has little variation through
adding 1 mM IPA as quencher of OH. However, the RhB degradation
was suppressed by the BQ and EDTA-2Na addition indicating that
superoxide radicals and holes play major roles in the RhB degrada-
tion under UV light irradiation.
•
•
•
−
+
hydroxyl radicals ( OH), superoxide radicals ( O ), and holes (h )
2
scavengers, respectively [37]. As shown in Fig. 9, after illumi-
nated without scavenger for 60 min, RhB was almost degradated
3
0
20
0
0
.003
.002
0
0
.006
.004
0
0
.009
.006
a
25
b
c
120
15
2
0
5
8
0
0
0
0.003
0.001
0.002
1
1
0
5
0
0
.000
10 0.000
5
0
.000
4
0
20
40
60
80
100
0
20
40
60
80
0
20
40
60
80
100
D/nm
D/nm
D/nm
AgBr/Bi WO
In/AgBr/Bi WO6
Bi WO
2
6
2
2
6
0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
0
.0
0.2
0.4
0.6
0.8
1.0
P/P0
P/P0
P/P0
Fig. 7. Nitrogen adsorption–desorption isotherms and BJH pore size distributions (insets a–c) of pure Bi2WO6 (a), AgBr/Bi2WO6 (b) and In2O3/AgBr/Bi2WO6 (c).

3
4
X. Chen et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 27–36
1
.0
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
b
a
0.8
0.6
0.4
0.2
0.0
Direct photolysis
P25
Direct photolysis
P25
Bi WO
2
6
Bi WO
2
6
AgBr/Bi WO
2 6
AgBr/Bi WO
2
6
In O /AgBr/Bi WO
2
3
2
6
In O /AgBr/Bi WO
2
3
2
6
0
15
30
45
60
0
30
60
90
120
150
180
t/min
t/min
100%
1.0
c
d0.8
8
6
4
2
0%
In O /AgBr/Bi WO
6
AgBr/Bi WO6
2
3
2
Bi WO6
2
P25
2
0%
0.6
0
0
.8
.6
0
h
h
0% Direct photolysis
0.4
Direct photolysis
P25
5
0.4
0.2
2
Bi WO6
0%
0
0.2
AgBr/Bi WO6
2
0
.0
In O /AgBr/Bi WO
2
3
2
6
4
00
450
500
550
600
650
Wavelength(nm)
0.0
Ultraviolet light (after irradiating 30 min)
0
60
120
180
240
300
t/min
1
.0
Direct photolysis
P25
st
nd
rd
th
e
1
Run
2
Run
3
Run
4 Run
f 0.8
Bi WO
0
0
0
.4
.2
.0
2
6
AgBr/Bi WO
2
6
In O /AgBr/Bi WO
2
3
2
6
0.6
0
0
0
.4
.2
.0
0
60
120
180
240
300
0
60
120
180
240
t/min
t/min
1
0
0
0
0
0
.0
g
.8
.6
.4
.2
.0
0.3
0
h
Direct photolysis 0.2
P25
2 h
2
Bi WO6
0.1
AgBr/Bi WO6
2
In O /AgBr/Bi WO
2
3
2
6
0
.0
2
50
300
350
400
Wavelenth(nm)
0
30
60
t/min
90
120
Fig. 8. UV photocatalytic degradation RhB profiles with different synthesized products (a); visible-light photocatalytic degradation RhB profiles with different samples (b);
microwave-assisted photocatalytic degradation RhB profiles with different samples (c); simulated sunlight photocatalytic degradation RhB profiles with different samples
(
d), and the absorption spectra of the RhB solution with In2O3/AgBr/Bi2WO6 under simulated sunlight (inset); simulated sunlight photocatalytic degradation RhB kinetics
with different samples (e); the recycling use results of In2O3/AgBr/Bi2WO6 under UV photocatalytic degradation (f); UV photocatalytic degradation salicylic acid profiles with
−
1
different synthesized products (g), and the absorption spectra of the salicylic acid solution (50 mg L ) with In2O3/AgBr/Bi2WO6 under UV light (inset).

X. Chen et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 27–36
35
vacuum (E = −4.5 eV for NHE). E signifies the conduction band
C
CB
(CB) potential, EVB refers to the valence band (VB) potential, and Eg
is the band gap of the semiconductor.
On the basis of the above results, a possible photocatalytic
mechanism of In O /AgBr/Bi WO was proposed in Fig. 10. When
2
3
2
6
In O /AgBr/Bi WO is illuminated by the light, electrons transfer
2
3
2
6
from valence band (VB) to conduction band (CB) in the semicon-
ductor, forming photoinduced electrons in conduction band, and
leaving the same number of holes in valence band at the same
time. However, there has electrostatic attraction between photo-
generated electrons in conduction and holes in valence band which
leads to the recombination of photoinduced electrons and holes
easily. It is noted that semiconductor composite material can inhibit
the recombination of electrons–holes pairs effectively in essence.
The reason is that the position of In O3 conduction band is more
2
negative than that of AgBr, and, thus, photoinduced electrons can
transfer to the AgBr conduction band efficiently. Similarly, the AgBr
conduction band is more negative than that of Bi WO , photoin-
Fig. 9. The results of reactive species trapping experiments.
2
6
duced electrons also transfer to the conduction band of Bi WO .
In consequence, type of multi-pathway photoelectron migration
increases the efficiency of photoinduced electrons and holes. Mean-
2
6
Table 3
The absolute electronegativity X (eV), energy band gaps Eg (eV), conduction band
and valence band potential E (eV) of Bi2WO6, AgBr and In2O3.
while, the holes in valence band has oxidized H O on the surface
of catalyst, leads to the generation of hydroxyl radicals ( OH), thus,
2
Semiconductor
X (eV)
Eg (eV)
ECB (eV)
EVB (eV)
•
Bi2WO6
AgBr
In2O3
6.21
5.24
5.79
2.76
2.6 [39]
3.75 [31]
0.31
−0.56
−0.59
3.07
2.04
3.16
decreases the recombination of charge carriers, and restrains the
semiconductor photocorrosion. In addition, owing to the photo-
sensitivity of AgBr, AgBr was decomposed by light into a little silver
and bromine during photocatalytic reaction [40]. Thereinto, silver
metal plays an important role in trapping electrons. On the one
hand, silver metal not only captures photoinduced electrons at the
interface of the heterostructures, but also enhances the rate of elec-
trons transfer, hence, inhibits the recombination of charge carriers.
On the other hand, the silver metal can also capture the pho-
toinduced electrons produced by AgBr, thus, prevents AgBr from
further photocorrosion, that is generates more silver metal. Then,
the photoinduced electrons from the conduction band of semi-
conductor, and the photoinduced electrons trapped by silver both
begin to react with dissolved oxygen on the surface of the catalyst,
To investigate the migration pathway of photoinduced carri-
ers in In O /AgBr/Bi WO composite, the formula (1) and (2) were
2
3
2
6
utilized to estimate [38], the results are shown in Table 3.
ECB = X + EC − 0.5Eg
(1)
(2)
EVB = ECB + Eg
where X represents the absolute electronegativity of the semi-
conductor, and it could be manifested as the geometric mean of
electronegativity of combining elements, EC refers to the scaling
factor describing the redox level of the reference electrode to the
•
−
forming the superoxide radicals ( O2 ). However, hydroxyl radi-
Fig. 10. The possible photocatalytic reaction mechanism of In2O3/AgBr/Bi2WO6 composite.

3
6
X. Chen et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 27–36
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ences on the potentials of semiconductor conduction bands cause
the multi-way migrations of photoinduced electrons which inhibit
the recombination of electron–hole pairs essentially, that being the
major reason for the enhancement of photocatalytic activity.
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. Conclusion
The high-crystallinity In O /AgBr/Bi WO photocatalyst was
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Acknowledgements
3
This study are supported by the National Natural Science
Foundation of China (21376126), Natural Science Foundation of
Heilongjiang Province, China (B201106), Scientific Research of Hei-
longjiang Province Educaton Department (12511592), Government
of Heilongjiang Province Postdoctoral Grants, China (LBH-Z11108),
Open Project of Green Chemical Technology Key Laboratory of
Heilongjiang Province College, China (2013 year), Postdoctoral
Researchers in Heilongjiang Province of China Research Initia-
tion Grant Project (LBH-Q13172), Innovation Project of Qiqihar
University Graduate Education (YJSCX2014-009X) and Qiqihar
University in 2015 College Students Academic Innovation Team
Funded Projects.
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