Inlay of Bi2O2CO3 nanoparticles onto Bi2WO6 nanosheets to build heterostructured photocatalysts

Article abstract of DOI:10.1039/c3dt53325e

Surface-dispersive-type Bi₂O₂CO₃/Bi ₂WO₆ heterostructured nanosheets were successfully prepared via anion exchange in a hydrothermal process with the graphitic carbon nitride (g-C₃N₄) as a precursor of CO₃ ^2-. The Bi₂O₂CO₃ nanoparticles (with diameters about 5 nm) were highly homogeneously dispersed and inlaid in the single-crystalline Bi₂WO₆ nanosheets. The composites with intimate interfacial contacts between Bi₂O₂CO₃ and Bi₂WO₆ exhibited superior visible light photocatalytic activity towards the degradation of rhodamine B (RhB). The composite nanosheets containing 7.86 wt% Bi₂O₂CO₃ showed the best performance and the degradation rate of RhB was 6 times faster than that with the bare Bi₂WO₆. The dramatic enhancement of the photocatalytic activity of the Bi₂O₂CO₃/Bi ₂WO₆ photocatalysts can be attributed to the hetero-interfaces between Bi₂O₂CO₃ and Bi ₂WO₆, their intrinsically layered structure, two-dimensional morphology and the effective separation of the photoinduced carriers at the interfaces and in the semiconductors. This method can be used to design and prepare other Aurivillius heterostructured semiconductors for efficient light harvesting and energy conversion applications.

Full text of DOI:10.1039/c3dt53325e

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Inlay of Bi O CO nanoparticles onto Bi WO
2
2
3
2
6
nanosheets to build heterostructured
photocatalysts†
Cite this: Dalton Trans., 2014, 43,
3660
Yang-Sen Xu, Ze-Jun Zhang and Wei-De Zhang*
Surface-dispersive-type Bi
anion exchange in a hydrothermal process with the graphitic carbon nitride (g-C
CO CO nanoparticles (with diameters about 5 nm) were highly homogeneously dispersed
2−. The Bi
and inlaid in the single-crystalline Bi WO nanosheets. The composites with intimate interfacial contacts
between Bi CO and Bi WO exhibited superior visible light photocatalytic activity towards the de-
gradation of rhodamine B (RhB). The composite nanosheets containing 7.86 wt% Bi CO showed
the best performance and the degradation rate of RhB was 6 times faster than that with the bare Bi WO
The dramatic enhancement of the photocatalytic activity of the Bi CO /Bi WO photocatalysts can be
attributed to the hetero-interfaces between Bi O CO and Bi WO , their intrinsically layered structure,
2
O
2
CO
3
/Bi
2
WO
6
heterostructured nanosheets were successfully prepared via
3 4
N
) as a precursor of
3
2
O
2
3
2
6
2
O
2
3
2
6
2
O
2
3
2
6
.
2
O
2
3
2
6
Received 26th November 2013,
Accepted 12th December 2013
2
2
3
2
6
two-dimensional morphology and the effective separation of the photoinduced carriers at the interfaces
and in the semiconductors. This method can be used to design and prepare other Aurivillius hetero-
structured semiconductors for efficient light harvesting and energy conversion applications.
DOI: 10.1039/c3dt53325e
www.rsc.org/dalton
heterostructure between the narrow and/or wide bandgap
semiconductor provides significant advantages both for light
Photocatalytic degradation of organic pollutants has attracted absorption and charge separation. Owing to these benefits,
1
. Introduction
1
2
intense interest because it is a promising and cost-effective several types of heterostructures, such as noble metal/metal
1
13
14
technology to address environmental issues. To better utilize oxide,
graphene/semiconductor,
and hybrid metal
1
5,16
visible light that accounts for 43% of solar energy, efforts have oxide,
have attracted much interest. It is reported that a
been made to exploit visible-light-responsive photocatalysts. 28-fold enhancement of the photocatalytic activity was
2
,3
New strategies such as ion exchange, surface plasmon reso- achieved by Ag/AgBr/g-C N hybrid since it efficiently reduced
3
4
4
5
nance, and band engineering have been employed to design the recombination of photoinduced electron–hole pairs and
semiconductors with high activity, such as Ag- and Bi-contain- strong absorbance in the visible region and near-infrared light
ing multiple-metal oxides. Importantly, these photocatalysts for the surface plasmon resonance absorption of Ag nanocrys-
always have large surface areas or certain exposed faces with tals. Recently, the Aurivillius-related oxide family (Bi-based
high activity or have special morphologies. However, the materials) with a typical layered structure has been success-
1
7
6
,7
8,9
pragmatic photocatalytic efficiency of metal oxides remains fully introduced into photocatalysis to efficiently degrade
1
8–20
low because of the bottleneck caused by the poor quantum organic dyes under visible light.
yield, which results from the rapid recombination of photo- cally nanostructured Bi /Bi WO
induced electrons and holes.
2 3 2 6
Relative to single semiconductors, composite photocatalysts Degussa P25. The Bi S /Bi WO composite photocatalyst pos-
For instance, hierarchi-
2
O
3
2
6
can improve the visible
light efficiency in removing aqueous RhB compared with the
2
1
take advantage of each component to counteract their individ- sesses a wide photoabsorption and exhibits significantly
ual shortcomings to show synergistic effects, thus exhibiting enhanced photocatalytic activity for phenol degradation under
1
0,11
22
much higher photocatalytic activity.
2 2 3
Construction of a visible light irradiation. Analogously, the Bi O CO /BiOI het-
erojunction shows enhanced visible light photocatalytic
activity in photo cleaning of wastewater containing RhB,
methylene blue (MB) and crystal violet. In addition, anion
School of Chemistry and Chemical Engineering, South China University of
Technology, 381 Wushan Road, Guangzhou 510640, People’s Republic of China.
E-mail: zhangwd@scut.edu.cn; Fax: +86-20-8711 4099; Tel: +86-20-8711 4099
23
exchange reaction can easily be used to control the volume
fractions of heterojunctions by selecting appropriate reaction
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c3dt53325e
2
3
1
temperature and/or reaction time. Cheng et al. used the
3660 | Dalton Trans., 2014, 43, 3660–3668
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anion exchange strategy to synthesize Bi
2
WO
6
hollow micro- synthesized via a hydrothermal process according to our pre-
2
4
spheres from BiOBr based on the microscale Kirkendall effect, vious report.
and the as-prepared Bi WO displayed a high CO adsorption
capacity and visible light photocatalytic conversion efficiency
of CO2 to methanol. Moreover, heterostructured sesame- The crystal structures and phase compositions of the prepared
biscuit-like Bi CO /Bi MoO nanoplatelets constructed by samples were determined by powder X-ray diffractometer
the anion exchange approach exhibited a 64-fold faster rate (X’Pert PRO MPD). The morphologies of the samples were
2
6
2
2.2. Characterization
2
O
2
3
2
6
2
4
than single Bi MoO in degradation of RhB.
observed using a scanning electron microscope (FESEM, LEO
2
6
To the best of our knowledge, no attempt has been made to 1530 VP) and a transmission electron microscope (TecnaiG220,
systematically investigate the morphology and component of FEI). UV-vis diffuse reflection spectra (DRS) of the samples
heterostructured Bi O CO /Bi WO . Additionally, BiOCl, were recorded using a UV-vis spectrophotometer (UV-3010,
2
6
2
3
2
6
Bi
2
O
2
CO
3
and Bi
2
WO
belong to the layered Aurivillius-related Hitachi, Japan) with BaSO as a reference. The BET specific
4
2
+
family, consisting of [Bi
2
O
2
]
layers sandwiched between two surface area and N adsorption–desorption isotherms of the
2
−
2−
slabs of X ions, atoms or groups (where X = Cl , CO and samples were measured using a Quantochrome NOVA 1200e
). The anions of these compounds can be easily instrument at 77 K.
3
2
−
WO
4
adjusted by the ion exchange process. Herein, we report the
one pot hydrothermal synthesis of surface-dispersive-type
2.3. Measurement of photocatalytic activity
2 2 3 2 6
Bi O CO /Bi WO heterostructured photocatalysts from BiOCl The photocatalytic performance of the as-prepared samples
via anion exchange. Both the evolution of the structure of the was evaluated by degradation of RhB under visible light
catalysts and the enhancement mechanism of the photo- irradiation at room temperature. A 300 W tungsten halide
catalytic activity are extensively discussed. This method may be lamp (Foshan Electric Light Ltd) was used as a light source,
further extended to the designed synthesis of novel and highly equipped with an ultraviolet cutoff filter to provide visible
efficient visible-light-driven Bi O CO -based heterojunction light (λ ≥ 420 nm). The distance between the liquid surface of
2
2
3
photocatalysts.
the suspension and the light source was 20 cm. For each run,
0
(
.10 g photocatalyst was added into 100 mL RhB solution
5.0 mg L− ). Prior to irradiation, the suspension was stirred in
1
2
. Experimental section
the dark for 30 min to ensure the absorption–desorption equi-
librium of the dye on the catalyst. During the absorption and
irradiation process, approximately 4 mL of the suspension
was collected, and then centrifuged at 12 000 rpm twice
and analyzed using a Hitachi U-3900H spectrophotometer at
λ = 554 nm.
2
.1. Preparation of the photocatalysts
All chemicals were used without further purification. g-C N
was prepared and refined according to our previous report.
Bi
3
4
7
1
2 2 3 2 6
O CO /Bi WO composites were synthesized by a hydrother-
mal process. In brief, a certain amount of g-C N was dis-
3
4
persed in 20 mL H
ultrasonication, then 1.6 mmol Bi(NO
the above suspension. The pH was adjusted by 1.0 M NaOH.
2
O and 1.0 mL 3.0 M HCl with the aid of
3
)
3
·5H O was added into
2
3
. Results and discussion
3.1. Crystal structure and morphology
0
6
2 4 2
.80 mmol Na WO ·2H O was added and then stirred for
0 min. After that, the mixture was transferred to a Teflon- The Bi CO /Bi WO nanosheets were prepared through an
2
O
2
3
2
6
lined autoclave up to 80% of the total volume. The autoclave anion exchange reaction starting from BiOCl platelets under a
was sealed and heated at 180 °C for 12 h. After cooling down hydrothermal process at 180 °C (to overcome the kinetic
2
−
to room temperature naturally, the products were collected by barrier), as shown in Scheme 1. In such a process, CO
3
was
filtration and washed by deionized water, and then dried at generated from the decomposition of g-C N . The evolution of
3
4
6
0 °C for 24 h. For comparison, Na
g-C to synthesize Bi CO /Bi
Bi(NO ) ·5H O and Na WO ·2H O, while Bi O CO plates were obtained g-C N is presented in Fig. 1Aa. The tetragonal BiOCl
2
CO
3
was used instead of the phase structure of the obtained products was characterized
3
N
4
2
O
2
3
2
WO
6
composites from by XRD, as revealed in Fig. 1A. The XRD pattern of the
3
3
2
2
4
2
2
2
3
3 4
Scheme 1 Schematic illustration of the synthetic procedure (anion exchange strategy).
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41-1488) appeared and no BiOCl phase was detected by the
XRD, which means that the Bi O CO /Bi WO composite was
2
2
3
2
6
−
obtained. These results obviously indicate that Cl in BiOCl
2
−
2−
indeed exchanged with CO
3
and WO
4
at 180 °C under the
hydrothermal conditions. Fig. 1B is the Rietveld refinement of
sample d in Fig. 1A, which reveals that the weight ratio of
Bi
2
O
2
CO
3
to Bi
2
WO
6
is 7.86 : 92.14. Therefore, the formula of
(Bi WO ) or
the obtained composite is (Bi O CO )
2
2
3 0.0786
2
6 0.9214
2 0.9214 0.0786 5.9214
Bi W C O .
The morphologies of the products were characterized by
SEM, TEM, and HRTEM. As displayed in Fig. 2A, one can
obviously observe that heterogeneous BiOCl platelets with the
length less than 1 μm are obtained. The pure Bi
rectangular nanosheets with length about several hundred
nanometers. Interestingly, the Bi CO /Bi WO obtained in
the hydrothermal process with addition of 11 wt% g-C also
2 6
WO shows
2
O
2
3
2
6
3 4
N
demonstrates nanosheet features, as shown in Fig. 3C. The
close-up view of the nanosheets presented in Fig. 3D indicates
that the sheets are ca. 30–40 nm thick and 200 nm to 1 μm
long. Furthermore, the layered characteristic of BiOCl,
Bi
2
O
2
CO
3
and Bi
2
WO
6
benefits the anion exchange and the for-
WO nanosheets. Actually, the layered
mation of Bi
2
O
2
CO
3
/Bi
2
6
nature favors the self-assembly of the units to form the charac-
teristic sheet-like nanostructure of the materials. For compari-
2 2 3 3 4
son, the prepared Bi O CO nanoplates and g-C N particles
are shown in Fig. S1.†
HRTEM images provided more detailed structural infor-
mation on the as-synthesized composite. The superstructure of
the Bi O CO /Bi WO nanosheets with addition of 11 wt%
2
2
3
2
6
3 4
g-C N as a precursor was characterized. The semitransparent
and wrinkled features in Fig. 3A and 3B vividly illustrate the
well-defined sheet-shaped nanostructure of the Bi O CO /
2
2
3
Bi
nanosheets are rectangles with length ranging from 500 nm to
μm and width from 100 to 400 nm. This is consistent with
the SEM observation in Fig. 2D. Higher magnified images
Fig. 3C) provide the detailed structures of Bi CO /Bi WO
heterojunction (sample d in Fig. 1A) (refined results: revealing that many quasi-spherical nanoparticles are homo-
6
2 6
WO composite, which further show that most of the
1
3 4 2 6
Fig. 1 (A) XRD patterns of the g-C N (a), BiOCl (b), Bi WO (c) and
Bi
process with 11 wt% g-C
Bi CO /Bi WO
wp = 0.1081, R = 0.0833, Rblnk = 0.0857, and χ = 1.988).
2
O
2
CO
3
/Bi
2
WO
6
composite synthesized at 180 °C by a hydrothermal
(
2
O
2
3
2
6
,
3 4
N
(d). (B) Rietveld refinement of the
2
O
2
3
2
2
R
p
geneously dispersed and inlaid in the nanosheets. The inset in
Fig. 3C clearly shows that the diameter of the nanoparticles is
about 5 nm. The typical HRTEM, as shown in Fig. 3D, further
platelets (JCPDS 06-0249) were prepared at room temperature demonstrates the detailed heterostructure. Obviously, the
by stirring Bi(NO ·5H O in 0.02 M HCl (Fig. 1Ab). According lattice fringes and the interface of Bi O CO and Bi WO can
3
)
3
2
2
2
3
2
6
to its XRD pattern, the (001), (002) and (003) diffraction peaks be observed. The measured d-spacing of the crystallographic
are sharper and stronger, while (110) is relatively weaker. This planes is about 0.295 and 0.228 nm, which corresponds well to
means that the BiOCl crystals favor growth along the c-axis the (013) and (006) plane of tetragonal Bi O CO respectively.
2
2
3
2
5
+
(
[001] orientations). This orientated growth arises from H in Importantly, the exposed facet of the Bi WO can be identified
2 6
2
6,27
the solution preferentially absorbed on the (001) facet.
as the (060) plane with the measured spacing between the
Pure orthorhombic structure of Bi
2 6
WO (a = 5.457 Å, b = adjacent fringes being 0.274 nm. The above observation
1
6.435 Å, c = 5.438 Å, JPCDS 39-0256) was also prepared by this suggests that the nanoparticles are Bi O CO interlaid in the
2
2
3
process without g-C N , as illustrated in Fig. 1Ac. With Bi WO nanosheets, revealing that a surface-dispersive-type
3
4
2
6
addition of NaOH to control the pH of the solution, and then structure of the Bi O CO /Bi WO nanocomposite was success-
2
2
3
2
6
by hydrothermal treatment at 180 °C for 12 h, a well crystal- fully obtained. This also illustrates a fact that the ion exchange
lized orthorhombic Bi WO was formed. After the addition of reaction is superior in terms of the selective formation of a
2
6
2
8
g-C
3 4
N , as vividly shown in Fig. 1Ad, some diffraction peaks of tetra- dimer structure with an epitaxial heterojunction. The crystal
gonal Bi O CO (a = 3.865 Å, b = 3.865 Å, c = 13.675 Å, JPCDS structures of Bi WO and Bi O CO in Fig. 3D are clearly
2
2
3
2
6
2
2
3
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2 6 2 2 3 2 6 3 4
Fig. 2 FESEM images of the as-obtained BiOCl (A), Bi WO (B), Bi O CO /Bi WO (11 wt% g-C N ) with low (C) and high magnification (D).
displayed in Fig. 3E and 3G. Fig. 3F represents the heterojunc- solution became transparent again, which further illustrated
2
−
tion models between Bi
2
WO
6
and Bi
2
O
2
CO
3
.
the fact that the CO
3
do exist in the solution and was gener-
ated from the decomposition of g-C N . A possible reaction is
3
4
3
.2. Formation mechanism of the composites
listed in eqn (4). This result can be indirectly proved by the
2
−
In the hydrothermal process, it is found that the g-C N , like experiment that using Na
2
CO
3
as a CO
3
source, the XRD
3
4
mesoporous silica SBA-15, suffers from collapse of its structure pattern (Fig. S3†) of the obtained sample is the same as the
2
9
in water at high temperature. Under hydrothermal treatment, one prepared by using g-C
3
N
4
. Moreover, after the hydrother-
, the pH value of the solution
. Such a phenomenon hap- changed from 6.3 to 8.4, which can be explained by reaction (3).
g-C N decomposed to CO and NH , which reacted with water mal treatment of the pure g-C
3
N
4
3
4
2
3
+
2−
and produced NH
4
and CO
3
2
−
exchanged with Cl− in BiOCl to form
. Based on the experimental results about the evolution
, the same prepared process of morphology and structure, a possible formation process of
to replace C
. The ZnO such heterostructured Bi CO /Bi WO nanosheets was sche-
nanorods and nanowires were obtained, as shown in Fig. S2,† matically illustrated. Accordingly, a chemical transformation
pened when methenamine (C
6 12 4
H N
) was used for the syn- The generated CO
3
3
0
thesis of ZnO nanorods and nanowires in aqueous solutions.
2 2 3
Bi O CO
+
To testify the production of NH
was conducted using g-C
4
N
4
6
H
12
N
4
2
O
2
3
2
6
3
+
which clearly demonstrated that the NH
4
was produced in process can be expressed as the chemical reaction equations:
this hydrothermal process. The possible reactions are listed in
Bi3 þ Cl þ H O ! BiOCl # þ 2H
þ
ꢀ
þ
ð1Þ
ð2Þ
ð3Þ
ð4Þ
2
−
2
eqn (2) and (3). The generation of CO3 from the decompo-
sition of g-C was further proved by an experiment as
follows. After hydrothermal treatment of the pure g-C
under the same experimental conditions, the up-layer clear
solution was added dropwise into the fresh CaCl solution.
Immediately, the transparent CaCl solution became turbid.
3 4
N
g-C3N4 þ H2O ! CO2 þ NH3 ðslowÞ
3 4
N
NH3 þ H2O ! NH4þ þ OHꢀ
2
CO2 þ 2OHꢀ ! CO32ꢀ þ H2O
2
Many small needle-like crystals (CaCO ) can be obviously
3
4BiOCl þ WO4 þ CO32ꢀ ! Bi2O2CO3 þ Bi2WO6 þ 4Clꢀ ð5Þ
2ꢀ
observed. Additionally, after adding 0.02 M HCl, the turbid
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2 2 3 2 6 2 6 2 2 3
Fig. 3 TEM (A, B) and HRTEM (C, D) images of the Bi O CO /Bi WO composites. (E, F and G) are the crystal structures of Bi WO , Bi O CO /
2 6 2 2 3
Bi WO and Bi O CO , respectively. The solid box indicates the unit cell.
Because BiOCl, Bi O CO and Bi WO all belong to the Bi O CO diffraction peaks appeared, which apparently con-
2
2
3
2
6
2
2
3
Aurivillius-family layered compounds, and are composed of firms that such an anion exchange reaction is ongoing at a
2
+
[
Bi O ] layers sandwiched between two slabs of X ions, atoms high pH value in the solution. After addition of 6.0 mL NaOH,
2 2
−
2−
2−
or groups (where X = Cl , CO
and WO4 ), which provide a hybrid composed of Bi WO and Bi O CO was found,
2 6 2 2 3
3
the possible transfer conditions from BiOCl to the Bi
2
O
2
CO
3
/
although BiOCl still existed in the composite, as illustrated in
Bi WO composite. In BiOCl, the double X layers contact to Fig. S4b.† This indicates that Bi O CO can also be obtained
2
6
2
2
3
2
+
−
the [Bi O ] layers, which benefits the anion exchange of Cl
by CO
via anion exchange reaction from BiOCl at high pH. With the
presence of more NaOH (>6.0 mL), all the peaks can be per-
2
2
−
2
2−
3
and WO
4
. In addition, the stability of Bi
2
O
2
CO
3
and Bi WO is higher than that of BiOCl. The transformation fectly indexed to the Bi WO and Bi O CO , without any other
2
6
2
6
2
2
3
2
−
of BiOCl to Bi O CO and Bi WO in the presence of CO
impurities (Fig. S4c and d†). The relationship between the
Therefore, under composition of the products and the pH value is listed in
the hydrothermal process, the Bi O CO /Bi WO composites Table 1. The results reveal that the g-C N can be decomposed
2
2
3
2
6
3
2
−
3,17
and WO
4
is thermodynamically favored.
2
2
3
2
6
3 4
were obtained via the anion exchange strategy.
in the hydrothermal process at 180 °C. However, no Bi O CO
2 2 3
was produced at pH 2–3, while Bi
2 2 3 2 6
O CO /Bi WO was formed
3
.3. Preparation conditions on the photocatalytic activity of
the composites
.3.1. Effect of pH of the solutions. Fig. 4A is the XRD pat- however, are significantly different towards the degradation of
terns of the obtained Bi CO /Bi WO
composites with RhB under visible light. As depicted in Fig. 4B, the highest
addition of NaOH from 6.4 to 6.8 mL. It shows that all the photocatalytic activity of the Bi CO /Bi WO occurred at pH
diffraction peaks can be indexed to Bi O CO and Bi WO . 8.4 (with addition of 6.6 mL NaOH) in its preparation.
at pH > 8. The obtained Bi O CO /Bi WO heterostructures are
2
2
3
2
6
all nanosheets (Fig. S5†). Their photocatalytic activities,
3
2
O
2
3
2
6
O
2
2
3
2
6
2
2
3
2
6
However, after adding 5.0 mL NaOH (pH value of the solution
is 2.1), a well crystallized orthorhombic Bi WO
was formed, g-C
with a small amount of BiOCl (Fig. S4a†). No g-C N and to further understand the primary growth mechanism, control
3.3.2. Effect of the amount of g-C
3
N
4
. To verify the role of
N
4
in the formation of Bi CO /Bi
2
O
2
3
2
WO nanosheets, and
6
2
6
3
3
4
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Fig. 4 XRD patterns (A) and photocatalytic activity (B) of the samples
prepared in the presence of different volumes of NaOH (1.0 M).
(
a) 6.4 mL, (b) 6.5 mL, (c) 6.6 mL, (d) 6.7 mL, and (e) 6.8 mL.
Table 1 The composition of the products obtained at different pH
values
NaOH/mL
pH value
Products
5
6
6
6
.0
2.1
Bi
2
Bi
2
Bi
2
Bi
2
6
WO , BiOCl
.0–6.3
.4–8.0
.6
7.4–8.1
8.1–9.2
8.4
O
2
O
2
O
2
CO
CO
CO
3
3
3
, Bi
, Bi
, Bi
2
WO
2
WO
2
WO
6
6
6
, BiOCl
Fig. 5 SEM images of the samples obtained with different contents of
experiments were carried out under identical conditions by
varying the amounts of g-C . Notably, the XRD result
Fig. S6†) demonstrates that Bi O CO /Bi WO composites
3 4
g-C N as a precursor. (A) 5%, (B) 11% and (C) 20%.
3 4
N
(
2
2
3
2
6
3 4
3 4
were obtained with addition of g-C N . The SEM images show their performance. As depicted in Fig. 6, the more g-C N is
the sheet structure of the composites, as illustrated in Fig. 5A– used, the higher the photocatalytic activity of the Bi O CO /
2
2
3
C and Fig. S7.† All of the as-synthesized products with loading Bi WO catalysts, and the highest photocatalytic activity
2
6
3 4
3 4
of g-C N from 0 to 50 wt% are small sheets with length less appears when 11 wt% of g-C N is used as a precursor. The
than 500 nm.
activity of the catalysts does not obviously increase upon
further increase of g-C N (from 11 to 13 wt%), and even
The degradation of RhB catalyzed by the Bi O CO /Bi WO
2
2
3
2
6
3 4
composites under visible light was investigated to evaluate decreases sharply when 20 wt% of g-C N is used. This may be
3
4
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2 2 3 2 6
Fig. 6 Photocatalytic activity of the Bi O CO /Bi WO composites
3 4
prepared with different amounts of g-C N .
Fig. 7 XRD patterns of the Bi
2
O
2
CO
3
/Bi
2
WO
6
composites attained after
reaction for (a) 8 h, (b) 12 h, (c) 16 h, and (d) 20 h.
similar to the formation of Bi O CO /Bi MoO nanoplatelets.
2
2
3
2
6
That is, less g-C
particles, while more g-C
3
N
4
resulted in fewer Bi
2
O
2
CO
3
nano- Bi O CO /Bi WO nanosheets was achieved when its prepa-
2
2
3
2
6
2
4
3
N
4
led to thicker nanosheets.
ration time was prolonged to 12 h. Further extension of the
Consequently, too much or too little g-C N would lead to reaction time does not increase the activity. In addition, as is
3
4
the loss of the interface between Bi
2
O
2
CO
3
2 6
and Bi WO , thus known to all, large BET surface area is good for high photo-
further deteriorating the catalytic activity of the composite catalytic activity. The surface areas of the Bi WO and
2
6
2
−1
catalysts. The apparent rate constant (k) of Bi WO -based Bi O CO /Bi WO are 2.6 and 9.3 m g , respectively.
2
6
2
2
3
2
6
photocatalysts starting from various amounts of g-C
as presented in Fig. S8A,† increases remarkably with the
3 4
N ,
3.4. Stability of the catalyst and photocatalytic mechanism
increasing amounts of g-C N and reaches the summit at Catalysts’ stability is a vital issue in terms of their applications.
3
4
−
1
1
1 wt% of g-C
much as that of the pure Bi
catalysts are also superior to the Bi O CO /Bi WO obtained After five cycles, as shown in Fig. 8, the photocatalytic activity
3
N
4
(k = 2.61 h ), which is about 6 times as To examine the stability of the Bi O CO /Bi WO photocatalyst,
2
2
3
2
6
−
1
2
6
WO (0.4548 h ). The composite the sample was collected after degradation of RhB for reuse.
2
2
2
−
3
2
6
by using Na
2
CO
3
as a source of CO
3
(Fig. S9†). The details of the Bi O CO /Bi WO catalyst remains 94.5%. In addition,
2 2 3 2 6
of the photocatalytic degradation process of RhB were the Bi O CO /Bi WO catalyst can be easily separated from an
2
2
3
2
6
investigated by UV-vis spectroscopy. As depicted in Fig. S8B,† aqueous suspension by sedimentation, while the commercial
in the presence of the Bi CO /Bi WO , the intensity of Degussa-P25 can only be separated by centrifugation. This
the absorption peak of RhB decreases gradually with the indicates that the Bi O CO /Bi WO composite possesses an
2
O
2
3
2
6
2
2
3
2
6
increase of irradiation time, which is accompanied with the
gradual blue-shift of the main absorption peak (from 554
to 498 nm), corresponding to a step-by-step de-ethylation
3
1,32
of RhB.
.3.3. Effect of the reaction time. Time-dependent experi-
3
ments were carried out at 180 °C to identify the effect of reac-
tion time on the performance of the obtained samples, as
exhibited in Fig. 7. A composite with well-crystallized Bi
and Bi CO was formed when the reaction time extended to
h (in Fig. 7a). The intensity of the diffraction peaks of the
WO increases gradually upon the increase of the reaction
2 6
WO
2
O
2
3
8
Bi
2
6
time to 12 h. However, the morphology is not changed greatly,
as shown in Fig. S10.† Moreover, it is found that the crystal
structure plays an important role in determining the final mor-
phology of the products. The layered crystals containing
2
+
[
Bi O ] slabs enable the chemical transformation from BiOCl
2 2
platelets to Bi
tain the sheet morphology. As shown in Fig. S11,† the result
of RhB degradation revealed that the highest activity of the presence of Bi
2 6 2 2 3
WO and Bi O CO with high fidelity and main-
Fig. 8 Cycling runs of the photocatalytic degradation of RhB in the
CO /Bi WO under visible light.
2
O
2
3
2
6
3666 | Dalton Trans., 2014, 43, 3660–3668
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efficient photocatalytic activity for the degradation of organic
2 2 3 2 6
A possible pathway in the Bi O CO /Bi WO heterostruc-
pollutants under visible light irradiation and can be easily sepa- tures might be considered for the generation of charge carriers
rated for reuse. (Fig. S12†) based on the photocatalytic results. Under visible
The photocatalytic property depends on its energy band light illumination (λ > 420 nm), the Bi WO can be excited to
structures. The UV-vis DRS of the obtained samples are illus- generate electrons and holes. The RhB may also be excited and
trated in Fig. 9, which indicates that Bi CO and Bi WO can inject electrons from the excited state of the dye (RhB*) to the
absorb solar energy with a wavelength of about 366 and heterostructure,
2
6
2
O
2
3
2
6
3
4,35
while the photo-generated electrons on
52 nm, respectively. For a crystalline semiconductor, the the surface of Bi WO transfer to the conduction band of
4
2
6
2 2 3
optical absorption near the band edge follows the equation Bi O CO under the inducement action of the internal electric
n/2
αhv = A(hv − E
g
)
g
, where α, v, E and A are the absorption field. Moreover, the photoinduced holes in the valance band
coefficient, the light frequency, the bandgap and a constant, of Bi WO can easily transfer to its surface, so the recombina-
2
6
respectively. The bandgap energy is estimated to be 2.70 eV for tion of electron–hole pairs on the Bi
2 6
WO can be reduced,
Bi WO (n = 2) and 3.38 eV for Bi CO (n = 4). The conduc- thus facilitating the holes to react with RhB. Furthermore, the
2
6
2
O
2
3
tion band and valence band positions of the prepared samples nanosheet texture is beneficial for promoting the photo-
e
can be calculated by the following equation: EVB = X − E + catalytic efficiency because electrons and holes can be easily
e
0
.5E
g
, where X is the Mulliken’s electronegativity, E is the separated and transfer to the surface to participate in the
energy of a free electron on the hydrogen scale (4.50 eV), and surface reactions. All the above-mentioned resulted in the
3
3
E
g
is the bandgap. Accordingly, the tops of valance band (VB) remarkable enhancement of the photocatalytic activity.
of Bi CO and Bi WO are calculated to be 3.55 and 2.90 V.
2
O
2
3
2
6
The conduction bands are estimated to be 0.17 and 0.20 V
versus the normal hydrogen electrode (NHE) at pH 7, respect-
ively (Fig. S12†). After the migration of the photo-generated
4
. Conclusions
−
charges, the E
(e ) (+0.2 V) are more positive than In summary,
a
2 2 3 2 6
surface-dispersive-type Bi O CO /Bi WO
CB,Bi WO6
2
•
−) (−0.33 V vs. NHE),17 which means that the photo- nanosheet heterostructure has been successfully prepared by a
E(O
generated CB-electrons cannot reduce the surface chemisorbed facile template-free hydrothermal process. The reaction temp-
/ O
2 2
•
−
O2 to produce O2 . However, the photoinduced holes, erature and crystal structures play particularly important roles
VB,Bi WO6 (h ) (+2.9 V vs. NHE), are strong oxidation species and in the anion exchange reactions, thus affecting the photo-
+
E
2
can directly oxidize the adsorbed RhB, which can be demon- catalytic performance of the catalysts. Compared to the pure
strated by trapping experiments. The main oxidative species in Bi WO nanoplates, the sheet-like Bi CO /Bi WO hetero-
2
6
2
O
2
3
2
6
the photocatalytic process can be detected through the trap- junction shows dramatically enhanced visible light photo-
ping experiments of radicals and holes. As shown in Fig. S13,† catalytic activity. The formation of a heterojunction in the
under visible light irradiation, the photocatalytic degradation regions between the Bi O CO and Bi WO improves the
2 2 3 2 6
of RhB is obviously inhibited by the addition of a hole scaven- charge separation and consequently enhances the efficiency of
ger, disodiumethylenediamine tetraacetate (EDTA-2Na). The the degradation process. Additionally, the photo-generated
2 2 3 2 6
activity decreases down to 3.6% of that with EDTA-2Na, holes in the Bi O CO /Bi WO photocatalysts turn out to be the
whereas it is only slightly suppressed by addition of the dominant active species in the photocatalytic reaction. The
hydroxyl radical scavenger, isopropanol. This indicates that SEM images and the XRD patterns of the samples before and
holes are the dominant active species for the oxidation of RhB. after reaction indicate that Bi O CO /Bi WO composites have
2 2 3 2 6
good morphology and chemical stability. This strategy can be
further extended to the synthesis and study of new kinds of
Bi O CO -based nanostructures for the environment and
2
2
3
energy use.
Acknowledgements
The authors thank the National Natural Science Foundation of
China (no. 21273080 and 21043005) for the financial support.
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