The preparation and characterization of composite bismuth tungsten oxide with enhanced visible light photocatalytic activity

Article abstract of DOI:10.1039/c3ce41036f

A composite photocatalyst (Bi_3.84W_0.16O _6.24-Bi₂WO₆) containing Bi, W and O elements was prepared by a facile hydrothermal method. Various characterization methods such as X-ray powder diffraction, UV-vis diffuse reflectance spectroscopy, scanning electron microscopy and transmission electron microscopy were employed to investigate the structure and optical properties. The activities of the samples were evaluated by the photocatalytic degradation of methylene blue under visible light irradiation. The results showed that when the pH of the precursor solution is 12.3, and a hydrothermal treatment at 140 °C for 20 h was used, the prepared sample shows the mixed phases of Bi_3.84W _0.16O_6.24 and Bi₂WO₆. The Bi _3.84W_0.16O_6.24-Bi₂WO₆ composite exhibited an enhanced photocatalytic activity compared with single Bi₂WO₆ or Bi_3.84W_0.16O _6.24. The rate constant of Bi_3.84W_0.16O _6.24-Bi₂WO₆ is about 5 times that of Bi ₂WO₆. It is proposed that the increased photocatalytic activity may be attributed to the formation of a heterojunction between Bi _3.84W_0.16O_6.24 and Bi₂WO ₆, which suppresses the recombination of photoexcited electron-hole pairs.

Full text of DOI:10.1039/c3ce41036f

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The preparation and characterization of composite
bismuth tungsten oxide with enhanced visible light
photocatalytic activity
Cite this: CrystEngComm, 2013, 15,
7943
Shifu Chen,*ab Wenming Tang,a Yingfei Hua and Xianliang Fua
A composite photocatalyst (Bi3.84W0.16O6.24–Bi2WO6) containing Bi, W and O elements was prepared by a
facile hydrothermal method. Various characterization methods such as X-ray powder diffraction, UV-vis
diffuse reflectance spectroscopy, scanning electron microscopy and transmission electron microscopy were
employed to investigate the structure and optical properties. The activities of the samples were evaluated
by the photocatalytic degradation of methylene blue under visible light irradiation. The results showed
that when the pH of the precursor solution is 12.3, and a hydrothermal treatment at 140 uC for 20 h was
used, the prepared sample shows the mixed phases of Bi3.84W0.16O6.24 and Bi2WO6. The Bi3.84W0.16O6.24
–
Bi2WO6 composite exhibited an enhanced photocatalytic activity compared with single Bi2WO6 or
Bi3.84W0.16O6.24. The rate constant of Bi3.84W0.16O6.24–Bi2WO6 is about 5 times that of Bi2WO6. It is
proposed that the increased photocatalytic activity may be attributed to the formation of a heterojunction
between Bi3.84W0.16O6.24 and Bi2WO6, which suppresses the recombination of photoexcited electron–hole
pairs.
Received 6th June 2013,
Accepted 6th August 2013
DOI: 10.1039/c3ce41036f
www.rsc.org/crystengcomm
adopts hydrothermal reactions. The hydrothermal method is
not only a mild and facile process but also offers a high degree
1. Introduction
Recently, heterostructure photocatalysts, which contain two or
multiple semiconductors with matching band-gaps, have
received a great deal of attention.1–6 This is because the
photoexcited carriers of the photocatalysts can be effectively
transferred from one semiconductor to another, which
suppresses the recombination of photoexcited electron–hole
pairs, and exhibits an enhanced photocatalytic activity.
Bismuth tungstate (Bi2WO6) is an aurivillius type oxide,
which has a layered structure with perovskite-like slabs of WO6
and [Bi2O2]2+ layers, and has a number of important physical
and chemical properties.7–9 In recent years, its photocatalytic
activity has been investigated extensively. The results showed
that Bi2WO6 is a promising visible light-driven photocatalyst
which can be used for the decomposition of organic
pollutants.8–13 In order to further enhance the photocatalytic
activity of Bi2WO6, coupling photocatalysts such as C3N4–
Bi2WO6, AgBr–Ag–Bi2WO6, Bi2WO6–TiO2, etc. have been pre-
pared.14–16 The above coupling photocatalysts can efficiently
separate the photogenerated electron–hole pairs and increase
the photocatalytic quantum efficiency of the reaction system.
At present, the preparation of Bi2WO6 photocatalysts usually
of crystallinity and controllable particle size of the products.
Many studies about the effects of the crystallinity, size and
morphology on the photocatalytic properties of Bi2WO6
synthesized by the hydrothermal process have been carried
out. The results showed that the solution pH values of the
hydrothermal reaction strongly influence the composition of
the products. When the pH of the precursor solution is lower
than 11, the XRD analysis showed that the crystal product is
Bi2WO6. However, when the pH exceeds 11, the crystal product
is Bi3.84W0.16O6.24, which is another complex oxide of bismuth
tungsten oxide.17–19 It is known that Bi3.84W0.16O6.24 is a cubic
crystalline phase and Bi2WO6 is an orthorhombic crystalline
phase. To date, there have been a few investigations concern-
ing Bi3.84W0.16O6.24 as a photocatalyst.17,18,20 However, to the
best of our knowledge, the heterostructured photocatalyst
Bi3.84W0.16O6.24–Bi2WO6 containing the same elements has not
been reported.
In this paper a composite photocatalyst Bi3.84W0.16O6.24
–
Bi2WO6 containing Bi, W and O elements was prepared by a
hydrothermal method with different pH values of the
precursor solution. The structure and optical properties of
the photocatalyst were characterized by X-ray power diffraction
(XRD), UV-vis diffuse reflectance spectroscopy (DRS), scanning
electron microscopy (SEM) and transmission electron micro-
scopy (TEM). The photocatalytic activities of the samples were
investigated through the photocatalytic degradation of methy-
aDepartment of Chemistry, Huaibei Normal University, Huaibei, Anhui, 235000,
People’s Republic of China. E-mail: chshifu@chnu.edu.cn; Fax: +86-561-3090518;
Tel: +86-561-3806611
bDepartment of Chemistry, Anhui Science and Technology University, Fengyang,
Anhui, 233100, People’s Republic of China
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taken from the reaction suspension, centrifuged at 4000 rpm
for 20 min and filtered through a 0.2 mm millipore filter to
remove the particles. The filtrate was then analyzed. In order
to determine the reproducibility of the results, at least
duplicated runs were carried out for each condition for
averaging the results.
lene blue (MB) under visible light irradiation. The
Bi3.84W0.16O6.24–Bi2WO6 composite exhibited a higher photo-
catalytic performance compared with single Bi2WO6 or
Bi3.84W0.16O6.24. Mechanisms for the increased photocatalytic
activity and the formation of the photocatalyst were also
proposed.
2.4. Characterization
In order to determine the crystal phase composition and the
crystallite size of the photocatalysts, X-ray diffraction measure-
ments were carried out at room temperature using a BRUKER
D8 ADVANCE X-ray powder diffractometer with Cu Ka
2. Experimental section
2.1. Materials
radiation (l = 1.5406 Å) and a scanning speed of 3u min21
.
Bismuth nitrate (Bi(NO3)3?5H2O), ammonium tungstate
(H40N10O41W12), nitric acid (HNO3), sodium hydroxide
(NaOH), MB and other chemicals used in the experiments
are of analytical reagent grade and were supplied from
Shanghai Chemical Reagent Factory of China without further
purification. Distilled water was used in all of our experiments.
The accelerating voltage and emission current were 40 kV and
30 mA, respectively. UV-vis diffuse reflectance spectroscopy
measurements were carried out using a UV 3600 (SHIMADZU,
Japan) UV-VIS-NIR spectrophotometer equipped with an
integrating sphere attachment. The analysis range was from
200 to 700 nm, and BaSO4 was used as a reflectance standard.
A JEOL JSM-6610LV scanning electron microscope with 20 kV
scanning voltages was employed to observe the morphologies
of as-prepared catalysts. Transmission electron microscopy
and high-resolution transmission electron microscopy
(HRTEM) images were obtained using a JEOL-2010 transmis-
sion electron microscope, using an accelerating voltage of 200
kV. The Brunauer–Emmett–Teller (BET) surface areas were
measured using a Micromeritics ASAP 2020 N2-physisorption
method at 77 K.
2.2. The preparation of the photocatalysts
The preparation of the samples was carried out by a facile
hydrothermal method. In
a typical synthesis, 4.171 g
Bi(NO3)3?5H2O was dissolved in 400 mL of nitric acid (4.0
mol L21) to get a transparent solution (named as solution A);
1.095 g H40N10O41W12?H2O was dissolved in 400 mL of nitric
acid (0.2 mol L21) to get a transparent solution (named as
solution B). Then solution A was added dropwise into B, and
stirred constantly for 3 h. The pH value of the precursor
solution was adjusted with 2.0 mol L21 of an NaOH solution
and 0.2 mol L21 of nitric acid. The samples were prepared at
pH values of 6, 11, 12.3 and 12.5, respectively. The precursor
formed was loaded into a 80 mL Teflon-lined stainless steel
autoclave up to 80% of the total volume and kept under a
series of experiment conditions. Subsequently, the autoclave
was cooled to room temperature naturally. The samples were
prepared at 120, 140 and 160 uC for 20 h with a pH value of
12.3, respectively. Another series of samples were prepared at
140 uC for different times (16, 20 and 24 h) with a pH value of
12.3 and at 140 uC for 20 h with different pH values of 6, 11,
12.3 and 12.5, respectively. The samples were collected,
washed with deionized water and dried at 80 uC in air.
2.5. Analysis
The concentration of MB in the solution was determined
spectrophotometrically. The photooxidation efficiency of MB
was calculated from the following expression:
g = [(C0 2 Ct)/C0] 6 100%
(1)
where g is the photocatalytic efficiency; C0 is the concentration
of the reactant before illumination; Ct is the concentration of
the reactant after illumination time (t).
3. Results and discussion
2.3. Photoreaction apparatus and procedure
Experiments were carried out in photoreaction apparatus.21,22
A 500 W Xenon lamp (Institute of Electric Light Source,
Beijing) with a maximum emission at about 470 nm was used
as a visible light source. The wavelength of the visible light is
controlled through a cutoff filter (l . 420 nm, Instrument
Company of Nantong, China). The 300 mL reaction solution
containing the reactants and photocatalyst was put in the
sealed beakers, and a magnetic stirring device was used to stir
the reaction solution. In the experiments, the initial pH of the
reaction solution is about 7.0, the initial concentration of MB
is 5.0 6 1026 mol L21 and the amount of photocatalyst is 2.0 g
3.1. Characterization of the Bi3.84W0.16O6.24–Bi2WO6
photocatalyst
3.1.1. XRD analysis. XRD was used to investigate the phase
structure of the photocatalysts. The hydrothermal conditions
strongly influence the composition and crystalline nature of
the prepared samples. The XRD patterns of the samples
prepared at 140 uC for 20 h with different pH values are shown
in Fig. 1.
It is clear that the pH value of the precursor solution is a key
factor that influences the composition and crystalline nature
of the prepared samples. When the pH of the precursor
solution is 6, the prepared sample is amorphous, and when
the pH is 11, the sample exhibits the standard crystal structure
of Bi2WO6 (JCPDS No. 39-0256). This means that when the pH
of the precursor solution is lower than 11, it is favorable for the
L
21. In order to disperse the photocatalyst powder, the
suspensions were ultrasonically vibrated for 20 min prior to
irradiation. Prior to illumination, the suspension was magne-
tically stirred in the dark for 30 min to reach an adsorption–
desorption equilibrium. After illumination, the samples were
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Fig. 1 The XRD patterns of the samples prepared at 140 uC for 20 h.
Fig. 2 The XRD patterns of the samples prepared at 120 uC, 140 uC and 160 uC
for 20 h.
formation of the orthorhombic crystalline phase of Bi2WO6.
When the pH of the precursor solution is 12.5, the XRD
pattern of the sample exhibits the standard crystal structure of
Bi3.84W0.16O6.24 (JCPDS No. 43-0447). This conclusion is in
accordance with the previous report.17 However, when the pH
of the precursor solution is 12.3, the prepared sample shows
mixed phases of Bi3.84W0.16O6.24 and Bi2WO6. Based on the
results, it is concluded that the alkalescence condition is
favorable for the formation of Bi2WO6, and the stronger
alkaline medium favors the formation of Bi3.84W0.16O6.24. The
Bi3.84W0.16O6.24–Bi2WO6 composite can only be obtained when
the pH value is appropriate. The phase change is due to the
high solubility of WO422 in alkaline media where [Bi2O2]2+ and
Bi3.84W0.16O6.24–Bi2WO6. The peaks of Bi2WO6 or
Bi3.84W0.16O6.24 change with different treatment temperatures
and times. From the calculation by the relative intensities of
the diffraction peaks of the two phases it is known that when
the hydrothermal conditions are set at 140 uC for 20 h, the
contents of Bi3.84W0.16O6.24 and Bi2WO6 in the sample are 88%
and 12%, respectively.23
In order to distinguish the difference between the samples,
the enlarged XRD patterns of the Bi2O3, Bi3.84W0.16O6.24
,
Bi3.84W0.16O6.24–Bi2WO6 and Bi2WO6 samples in the region
22
WO4 can precipitate rapidly.17–19
When the pH value of the precursor solution is 12.3, the
effects of the hydrothermal temperature and time on the
crystalline phase of the samples are shown in Fig. 2 and 3,
respectively. From Fig. 2, it can be seen that, when the
hydrothermal temperature is 120 uC, the sample displays the
crystal structure of Bi3.84W0.16O6.24, and when the hydrother-
mal temperature is 140 uC, the prepared sample shows mixed
phases of Bi3.84W0.16O6.24 and Bi2WO6. However, when the
hydrothermal temperature is increased to 160 uC, the crystal-
line phase of the sample prepared changes into Bi2O3. From
Fig. 3, it is clear that the hydrothermal time also influences the
composition of the prepared samples. When the hydrothermal
conditions are at 140 uC for 16 h, 20 h and 24 h, the
composition of the prepared samples are Bi3.84W0.16O6.24
,
Bi3.84W0.16O6.24–Bi2WO6 and Bi2O3–Bi3.84W0.16O6.24 respec-
,
tively. From the above results, it is clear that it is only when
the hydrothermal conditions are set at 140 uC for 20 h that the
Bi3.84W0.16O6.24–Bi2WO6 photocatalyst can be formed. When
the hydrothermal temperature changes from 120 uC to 160 uC,
the peaks change from cubic Bi3.84W0.16O6.24 to Bi2O3.
Apparently, the pH value and the hydrothermal temperature
and time all play important roles in the formation of
Fig. 3 The XRD patterns of the samples prepared at 140 uC for 16 h, 20 h and
24 h.
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Fig. 6 TEM and HR-TEM images of the Bi3.84W0.16O6.24–Bi2WO6 sample.
3.1.2. SEM and TEM analysis. The microstructures and
morphologies of the Bi2WO6, Bi3.84W0.16O6.24 and
Bi3.84W0.16O6.24–Bi2WO6 samples were characterized by SEM.
From Fig. 5, it is clear that Bi2WO6 is comprised of irregular
nanoflakes with an approximate size of 250 nm which are
aggregated like cabbages. However, Bi3.84W0.16O6.24 is com-
prised of regular ball-like nanoparticles with a size of about 50
nm. From Fig. 5(c), it is obvious that Bi3.84W0.16O6.24–Bi2WO6
is comprised of irregular nanoflakes and regular ball-like
nanoparticles, and the ball-like nanoparticles cover the surface
of the irregular nanoflakes. In order to investigate the interface
of the sample (Bi3.84W0.16O6.24–Bi2WO6), the sample prepared
at 140 uC for 20 h was chosen for TEM and HR-TEM
characterization. From Fig. 6(a), it can be seen that Bi2WO6
shows irregular flakes and Bi3.84W0.16O6.24 exhibits darker ball
nanoparticles with a size of about 30–50 nm. The HR-TEM
images are shown in Fig. 6(b) and (c). It is clear that the crystal
lattice fringes of 0.20 nm in the darker section are in
accordance with the (220) plane of Bi3.84W0.16O6.24, while the
lattice fringes of 0.19 nm in the gray part corresponds to the
(202) crystallographic plane of Bi2WO6. Based on the above
results, it can be confirmed that a heterojunction produced by
Bi3.84W0.16O6.24 and Bi2WO6 was obtained in our experimental
conditions. It is proposed that the formation of the hetero-
junction between Bi3.84W0.16O6.24 and Bi2WO6 is favorable for
the transfer of photoexcited electrons and holes.
Fig. 4 The enlarged XRD patterns of the Bi2O3, Bi3.84W0.16O6.24
,
Bi3.84W0.16O6.24–Bi2WO6 and Bi2WO6 samples.
2h (25.0–35.0) are shown in Fig. 4, respectively. The well-
defined peaks at 2h = 28.32u and 32.77u are assigned to the
(131) and (002) planes of Bi2WO6, respectively. The peaks at 2h
= 27.94u and 32.33u correspond with the (111) and (200) planes
of Bi3.84W0.16O6.24, respectively. The peaks at 2h = 28.02u and
32.43u correspond with the (111) and (200) lattice planes of
Bi2O3, respectively.
The crystallite size was calculated by an X-ray line broad-
ening analysis using the Scherrer equation, and the crystallite
size was estimated by the following equation:23
L = lK/bcosh
(2)
where L is the average crystallite size in nanometers, l is the
average wavelength of the X-ray radiation, K is a constant taken
as 0.9, b is the line width at half-maximum height in radians,
and h is the diffracting angle. From the calculation, it is known
that the grain sizes of Bi3.84W0.16O6.24 and Bi2WO6 prepared at
140 uC for 20 h are 31.5 nm and 15.1 nm, respectively. This is
consistent with the morphology observed in the TEM images.
3.1.3. UV-vis analysis. The optical absorption of a semi-
conductor is closely related to its electronic structure. UV-vis
diffuse reflectance spectroscopy was carried out to investigate
the optical properties of the photocatalysts.
The UV-vis diffuse reflectance spectra of the samples are
shown in Fig. 7. It is clear that the samples show a strong
photo-absorption capacity in the UV-visible region from 200 to
450 nm. It is considered that the samples should exhibit
potential photocatalytic activity under visible light. The steep
absorption edges of Bi2WO6 and Bi3.84W0.16O6.24 are at about
450 nm and 460 nm, respectively. This indicates that the
visible light adsorption is due to the intrinsic band-gap
transition instead of the transition from impurity levels.8,14
From Fig. 7, it also can be seen that the Bi3.84W0.16O6.24
–
Bi2WO6 sample exhibits slightly increased photo-absorption in
the visible light region than that of pure Bi3.84W0.16O6.24 and
Bi2WO6, which may be attributed to the interactions between
Bi3.84W0.16O6.24 and Bi2WO6.
It is known that the optical absorption near the band edge of
a crystal obeys the following equation:17,22
Fig. 5 SEM images of the Bi2WO6 (a), Bi3.84W0.16O6.24 (b) and Bi3.84W0.16O6.24
–
Bi2WO6 (c) samples.
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structure and morphology of Bi2WO6 and Bi3.84W0.16O6.24 are
different. Generally speaking, bismuth tungsten oxide exists in
two different phases. Bi2WO6 is an orthorhombic crystalline
phase and Bi3.84W0.16O6.24 is a cubic crystalline phase. The
alkalescence condition is favorable for the formation of
Bi2WO6, while a stronger alkaline medium favors the forma-
tion of Bi3.84W0.16O6.24. The Bi3.84W0.16O6.24–Bi2WO6 compo-
site can only be obtained when the pH value is appropriate.
The related chemical processes are proposed as follows:
solution A:
hydrolysis
{
BiðNO3Þ3zH2O /CC? BiOðNO3Þz2Hzz2NO3
(6)
hydrolysis
2BiOðNO3ÞzH2O /CC? Bi2O2ðOHÞðNO3ÞzHzzNO3{ (7)
solution B:
+
H
40N10O41W12?7H2O + 10H+ = 12H2WO4 + 10NH4
(8)
Under the alkalescence and stronger alkaline (pH . 11)
22
conditions, WO4
can independently exist in the solution.
Fig. 7 The UV-vis diffuse reflection spectra of the samples (a: Bi3.84W0.16O6.24
–
When solution A was added dropwise into B, [Bi2O2]2+ and
Bi2WO6, b: Bi2WO6, c: Bi3.84W0.16O6.24).
22
WO4 can precipitate rapidly in alkaline media.17–19 Bi2WO6
and Bi3.84W0.16O6.24 are synthesized respectively through the
hydrothermal method with the different reaction conditions:
ahn = A(hn 2 Eg)n/2
(3)
hydrothermal
Bi2O2ðOHÞðNO3ÞzH2WO4zOH{
In this equation, a, h, n, A and Eg are the absorption coefficient,
Planck constant, light frequency, proportionality constant and
band gap, respectively; n decides the type of the transition in a
semiconductor (n = 1, direct absorption; n = 4, indirect
absorption). The values of n for Bi2WO6 and Bi3.84W0.16O6.24
are all 1. By applying this equation, the band gaps of Bi2WO6
and Bi3.84W0.16O6.24 are 2.75 eV and 2.69 eV, respectively.
These results are in accordance with the previous results.17,24
The conduction band (CB) and valence band (VB) positions
of the Bi3.84W0.16O6.24 and Bi2WO6 samples are estimated by
the following equations:25,26
(9)
Bi2WO6zNO{3 z2H2O
hydrothermal
Bi2O2ðOHÞðNO3ÞzH2WO4zOH{
(10)
Bi3:84W0:16O
6:24zNO{3 zH2O
3.3. The evaluation of the photocatalytic activity
The photocatalytic activities of the samples were evaluated by
the photocatalytic oxidation of MB under visible light
irradiation (l ¢ 420 nm). In order to evaluate the photo-
catalytic activity of the photocatalysts, the blank test and the
dark absorption test were also carried out. The blank test
shows that photo-induced self-sensitized photodegradation
has little influence on the results of the experiment. At the
same time, the dark absorption test in the absence of
irradiation but with the catalysts shows that no significant
change in the substrate concentration is found.
EVB = X 2 Ee + 0.5Eg
ECB = EVB 2 Eg
(4)
(5)
where EVB and ECB are the VB and CB edge potentials, X is the
electronegativity of the semiconductor, which is the geometric
mean of the electronegativity of the constituent atoms, Ee is
the energy of the free electrons on the hydrogen scale (likely
4.5 eV), and Eg is the band gap energy of the semiconductor.
The X values of the Bi3.84W0.16O6.24 and Bi2WO6 samples
calculated are 5.96 and 6.21 eV, respectively.27,28 The EVB of
Bi3.84W0.16O6.24 and Bi2WO6 are calculated to be 2.81 and 3.08
eV, respectively. The ECB of Bi3.84W0.16O6.24 and Bi2WO6 are
estimated to be 0.12 and 0.33 eV, respectively.
It has been proved that the photocatalytic degradation of
MB was found to follow first-order kinetics.25 The kinetics
equation can be expressed as follows:
lnC0/Ct = kt
(11)
where k is the first-order rate constant. The kinetics of the MB
degradation with different photocatalysts (Bi3.84W0.16O6.24
3.2. Formation mechanism
,
Bi3.84W0.16O6.24–Bi2WO6 and Bi2WO6) is illustrated in Fig. 8.
From Fig. 8, it is clear that the photocatalytic degradation
Based on the results of the XRD, it is known that the pH value
of the precursor solution and the hydrothermal temperature
and time all play important roles in the formation of Bi2WO6,
Bi3.84W0.16O6.24 and Bi3.84W0.16O6.24–Bi2WO6. In addition the
process of MB follows
a first-order kinetics equation.
Bi3.84W0.16O6.24–Bi2WO6 exhibits the highest photocatalytic
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Fig. 9 The UV-visible spectral changes of MB for the different photocatalysts (a:
fresh solution, b: Bi2WO6, c: Bi3.84W0.16O6.24, d: Bi3.84W0.16O6.24–Bi2WO6;
illumination time = 40 min).
Fig. 8 The first-order kinetics of the photocatalytic degradation of MB a:
Bi3.84W0.16O6.24–Bi2WO6; b: Bi3.84W0.16O6.24; c: Bi2WO6.
activity compared with Bi2WO6 and Bi3.84W0.16O6.24. The
activity of Bi3.84W0.16O6.24 is higher than that of Bi2WO6. The
rate constants of the MB degradation calculated for
Bi3.84W0.16O6.24–Bi2WO6, Bi3.84W0.16O6.24 and Bi2WO6 are
0.0262 min21, 0.0065 min21 and 0.0056 min21, respectively.
It is clear that the rate constant of Bi3.84W0.16O6.24–Bi2WO6 is
about 5 times that of Bi2WO6.
The UV-visible spectral change of the MB degradation is
shown in Fig. 9. It is clear that the disappearing rates of the
characteristic absorption peaks of MB for the different
photocatalysts are different. For Bi3.84W0.16O6.24–Bi2WO6, the
disappearing rate of the characteristic absorption peak is the
3.4. A proposed reaction mechanism
It is known that the photocatalytic activity of a photocatalyst
mainly depends on two important factors. One is to increase
the amount of the photoexcited active species and the other is
to increase the separation efficiency of the photoexcited
electron–hole pairs. For the heterostructured photocatalysts,
two prerequisites for photoexcited carrier transfer are the
electron density diversity and the energy gap matching of the
semiconductors.29–31 Fig. 11 shows the band-gap structure
diagram of Bi3.84W0.16O6.24 and Bi2WO6. The CB level of
Bi3.84W0.16O6.24 lies at about 0.12 eV (vs. NHE), while the VB
fastest compared with Bi2WO6 and Bi3.84W0.16O6.24
.
The lifetime of the catalyst is an important parameter in the
practical application. The stability of the photocatalytic activity
for Bi3.84W0.16O6.24–Bi2WO6 was evaluated by cyclic experi-
ments. The reactions were carried out under the same
experiment conditions. After each reaction, the used sample
was washed and then dried at 80 uC for 3 h. Fig. 10 shows the
results of the repeated experiment. It is clear that after the
photocatalytic reaction for 5 cycles, no obvious deactivation of
the photocatalyst can be found, indicating that the
Bi3.84W0.16O6.24–Bi2WO6 photocatalyst is rather stable.
In order to determine the relationship between the BET
surface area and the photocatalytic activity, an evaluation of
the BET surface areas of the photocatalysts was carried out.
The results showed that the BET surface areas of Bi2WO6,
Bi3.84W0.16O6.24 and Bi3.84W0.16O6.24–Bi2WO6 are 17.2, 21.6 and
1 9.7 m2
g
21, respectively. It is clear that the enhanced
photocatalytic activity of Bi3.84W0.16O6.24–Bi2WO6 cannot be
attributed to the BET surface area. It is proposed that the
enhanced photocatalytic activity can only be ascribed to the
formation of a heterojunction between Bi3.84W0.16O6.24 and
Bi2WO6.
Fig. 10 The cyclic experiments of the Bi3.84W0.16O6.24–Bi2WO6 photocatalyst.
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Acknowledgements
This study was supported by the Natural Science Foundation
of China (NSFC, grant No. 20973071, 51172086, 21103060 and
51272081).
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A composite Bi3.84W0.16O6.24–Bi2WO6 photocatalyst was pre-
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