One-pot preparation of Bi/Bi2WO6/reduced graphene oxide as a plasmonic photocatalyst with improved activity under visible light

Article abstract of DOI:10.1039/c6ra20316g

A novel nanocomposite, Bi nanorods and Bi₂WO₆ nanosheets supported on reduced graphene oxide (Bi/Bi₂WO₆/rGO), was synthesized via a facile one-pot solvothermal method. In the reaction process, Bi₂WO₆ nanosheets and Bi nanorods were grown in situ on the rGO sheets, which were simultaneously achieved by the reduction of GO. Such a synthetic strategy can form effective close interfacial contacts and strong interactions among Bi₂WO₆, Bi and rGO, leading to efficient separation and transfer of photogenerated electron-hole pairs. As a result, the ternary plasmonic photocatalyst exhibits a much higher photocatalytic activity than pure Bi₂WO₆ and the binary composites in the photocatalytic degradation of rhodamine B and p-chlorophenol under visible light irradiation, which could be ascribed to the synergic effects of the improved electron-hole pair separation efficiency, enhanced visible-light harvesting and the good adsorptive capacity toward dye molecules.

Full text of DOI:10.1039/c6ra20316g

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One-pot preparation of Bi/Bi WO /reduced graphene oxide as a
2
6
plasmonic photocatalyst with improved activity under visible
†
light
Yan Zhou, Sushan Ren, Qimei Dong, Yingying Li, Hanming Ding*
School of Chemistry and Molecular Engineering, East China Normal University, 500
Dongchuan Road, Shanghai 200241, China
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Abstract
A novel nanocomposite, Bi nanorods and Bi
graphene oxide (Bi/Bi WO /rGO), was synthesized via a facile oneꢀpot solvothermal
method. In the reaction process, Bi WO nanosheets and Bi nanorods were in situ
2 6
WO nanosheets supported on reduced
1
1
1
1
1
1
1
1
1
1
2
2
2
6
2
6
grown on the rGO sheets, which were simultaneously achieved by the reduction of
GO. Such a synthetic strategy can form effective close interfacial contacts and strong
interactions among Bi
of photogenerated electronꢀhole pairs. As a result, the ternary plasmonic photocatalyst
exhibited a much higher photocatalytic activity than pure Bi WO and the binary
2 6
WO , Bi and rGO, leading to efficient separation and transfer
2
6
composites in the photocatalytic degradation of rhodamine B and pꢀchlorophenol
under visible light irradiation, which could be ascribed to the synergic effects of the
improved electronꢀhole pair separation efficiency, enhanced visibleꢀlight harvesting
and the good adsorptive capacity to dye molecules.
†
Electronic supplementary information (ESI) available.
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Introduction
Semiconductorꢀbased photocatalysis is a promising technology to solve the current
1
ꢀ3
energy shortage and environmental pollution using the abundant solar light.
Undoubtedly, the semiconductor TiO is known as one of the most excellent
2
4
, 5
photocatalysts for the redox decomposition of a range of organic pollutants.
However, TiO has a wide band gap, which has limitation in visible light
2
6
ꢀ8
absorption. Therefore，it is important to develop visibleꢀlightꢀdriven photocatalysts
to efficiently utilize the solar light in the visible region.
As a new visible lightꢀresponding nonꢀtitaniaꢀbased photocatalyst, Bi WO with
2
6
9
ꢀ11
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1
1
1
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1
1
1
1
1
2
2
2
a unique layered structure has attracted increasing interest recently. However, there
are two main drawbacks that limit the application of Bi WO photocatalyst. At first,
pure Bi WO can only absorb light with wavelengths shorter than 420 nm.
Secondly, the high recombination of photogenerated charges after light absorption
2
6
1
2, 13
2
6
1
4, 15
seriously confines the light energy conversion efficiency.
It was suggested that if
a high optical absorption could be maintained and meanwhile the charge
recombination could be significantly suppressed, a good photocatalytic activity will
be anticipated. To solve these problems, one way is to fabricate heterojunction
composite photocatalysts by coupling of another material with appropriate Fermi
energy. The recombination of photogenerated electronꢀhole pairs thus can be
effectively suppressed, resulting in increased probability of the photogenerated
1
6, 17
charges to be available for photocatalytic reactions.
Reduced graphene oxide
(rGO) has recently attracted tremendous attention owing to its fascinating electrical
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and chemical properties.
When rGO is hybridized with a photocatalyst, the
excellent electronic conductivity of rGO will promote the charge transfer through its
conjugated structure, inhibiting the recombination of the photoexcited electronꢀhole
20, 21
pairs
Furthermore, rGO nanosheets can act as adsorptive centers due to their large
πꢀconjugation system and 2D planar structure, which make dye molecules adsorb
easily onto their surface via strong πꢀπ interactions between rGO and dye
22
molecules.
Lately, bismuth (Bi) have been observed to exhibit surface plasmon resonance
23
(SPR) properties. Compared to the general noble metals such as Au and Ag, Bi is
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much cheap and easily prepared. It is documented that several Biꢀmodified
photocatalysts, such as Bi/gꢀC
photocatalytic performance, which is ascribed to the SPR effect of Bi.
Considering the salient characteristics of Bi, Bi WO6, and rGO, it prompts us to
consider whether we can use a facile method to prepare a Bi/Bi WO /rGO triple
3 4 2 2 3
N and Bi/Bi O CO exhibited highly promoted
24, 25
2
2
6
nanocomposite, in which their synergic effects could improve the photocatalytic
activity.
In this study, we have developed a oneꢀpot hydrothermal method to prepare a
2 6 2 6
Bi/Bi WO /rGO nanocomposite, in which the formation of Bi WO , reduction of
3+
graphene oxide (GO) into rGO, and reduction of Bi ions into metal Bi are
simultaneously accomplished in the same synthetic system. Notably, besides being a
solvent, ethylene glycol (EG) also plays as a mild reductant for the reduction of GO
3
+
and Bi
ions, and therefore no additional reductants are required. The
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2 6
Bi/Bi WO /rGO nanocomposite exhibit a high photocatalytic activity to degrade
rhodamine B (RhB) solutions under visible light irradiation. And the photocatalytic
mechanism has been proposed as well.
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Experimental section
Synthesis of graphene oxide (GO) and Bi/Bi
GO was synthesized via a modified Hummer's method (see Supporting Information
for experimental details). For the synthesis of Bi/Bi WO /rGO, an optimal molar ratio
of Bi(NO ) to Na WO was adopted as 3.5:1. Typically, 3 mg of GO was dispersed
2 6
WO /rGO composites
2
6
3
3
2
4
1
1
1
1
1
1
1
1
1
1
2
2
2
in 20 mL of ethylene glycol followed by ultrasonication for 1 h. Then, 1 g of PVP
(polyvinylpyrrolidone Kꢀ30) and 3.5mmol of Bi(NO ·5H O were added under
stirring. Complete dissolution was required to obtain a uniform mixture. Subsequently,
0.5 mL of Na WO solution (2 mol/L) was added into the mixture and stirred for 30
3
)
3
2
2
4
min, getting a homogeneous suspension. After that, the suspension was transferred to
o
a 25 mL Teflonꢀlined stainless steel autoclave. The autoclave was heated to 180 C
and kept at this temperature for 24 h. After the autoclave was cooled naturally to room
temperature, the product was separated, washed with absolute ethanol and distilled
o
water for several cycles, and dried at 60 C in vacuum. According to the added dosage
of GO in the synthesize process, four Bi/Bi WO /rGO samples were synthesized and
2
6
denoted as Bi/Bi
Bi/Bi WO
comparison, pure Bi
2
WO
6
/rGOꢀ1, Bi/Bi
2
6 2
WO /rGOꢀ2, Bi/Bi WO
6
/rGOꢀ3 and
2
6
/rGOꢀ4 corresponding to the adding weight of GO of 1, 2, 3 and 4 mg. For
2
WO6, Bi
2
WO
6
2 6
/rGO and Bi/Bi WO were prepared using similar
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methods (see Supporting Information for experimental details).
Characterization
Xꢀray powder diffraction (XRD) analysis was conducted using a Bruker D8 Advance
Xꢀray powder diffractometer with Cu Kα radiation. The morphology was observed
with a JEOL JEMꢀ100C II transmission electron microscope (TEM) with an
accelerating voltage of 200 kV. Xꢀray photoelectron spectra (XPS) were carried out
using a Kratos AXIS Ultra DLD XPS system with an Al Kα (1486.6 eV) line at 150
W. UVꢀvis diffused reflectance spectra (DRS) were recorded using a LAMBDA 950
UV/Vis/NIR spectrophotometer. Fluorescence (FL) spectra were determined by a
Hitachi Fꢀ4500 FL spectrophotometer. Raman spectra were obtained on a Thermo
Scientific DXR Raman microscope at room temperature with 532 nm laser excitation.
Nitrogen adsorptionꢀdesorption isotherms and the BrunauerꢀEmmettꢀTeller (BET)
surface areas were collected at 77 K using ASAP 2020 system (Quantachrome
Instruments). Photoꢀelectrochemical measurements were performed in a constructed
three electrode quartz cell system. Pt sheet was used as a counter electrode and
Ag/AgCl was used as reference electrodes, while the thin film on fluorine doped tin
oxide (FTO) was used as the working electrode for investigation. The
photoelectrochemical experimental results were obtained with a CHI 760C
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1
1
2
2
2
electrochemical system. An aqueous solution of 0.2 M Na
electrolyte.
2 4
SO was used as the
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Photocatalytic experiments
The photocatalytic activities of the asꢀprepared samples were evaluated by the
degradation of rhodamine B (RhB) or colorless pꢀchlorophenol (4ꢀCP) under visible
light irradiation using a 500 W metal halide lamp with a 420 nm cutoff filter as the
light source. In a typical process, 20 mg of the photocatalyst was dispersed into 50 mL
ꢀ1
ꢀ1
RhB (10 mg L ) or 4ꢀCP (10 mg L ) solution under magnetic stirring. Prior to
irradiation, the suspension was agitated in darkness for 30 min to ensure
adsorptionꢀdesorption equilibrium. Subsequently, the suspension was kept
magnetically stirred under visible light irradiation, and 2 mL of the sample solution
was taken from the reaction system at a certain time interval. After the catalyst was
isolated by centrifugation, the supernatant was analyzed using a UVꢀvis spectrometer
1
1
1
1
1
1
1
1
1
1
2
2
2
0
(UVꢀ8000). The degradation efficiency was calculated in accordance with C/C ,
where C is the concentration of the remaining pollutant solution at each time interval,
while C is the initial concentration.
0
Results and discussion
The crystalline structure of the products was investigated by the XRD method. The
results shown in Fig. 1 display that the diffraction peaks of pure Bi
2 6
WO and Bi are in
good agreement with the orthorhombic phase of Bi WO (JCPDS card no. 39ꢀ0256)
2
6
and the hexagonal phase of Bi (JCPDS card no. 44ꢀ1246), respectively. As for the
Bi/Bi WO composite, all the diffraction peaks can be indexed to the orthorhombic
phase of Bi
2
6
2
WO
6
and the hexagonal phase of Bi respectively. Furthermore, the
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2 6
diffraction peak intensities of Bi in the Bi/Bi WO composites strengthened gradually
when increasing Bi(NO
)
3 3
in the precursor solutions (Fig. S1), suggesting that the Bi
content in the Bi/Bi WO
2
6
composite can be controlled in the synthesize process. The
XRD pattern of the Bi/Bi WO /rGOꢀ3 composite is comparable to that of Bi/Bi WO
6
2
6
2
(Fig. 1f). However, the full width at half maximum (FWHM) of the diffraction peaks
of Bi and Bi WO is broadened for the Bi/Bi WO /rGO composites (Fig. S2), which
is attributed to the existence of rGO restricts the degree of crystallization in the
2
6
2
6
26
synthesize process. In addition, no characteristic diffraction peaks for rGO are
observed in the pattern because of the low amount and relatively low diffraction
27
1
1
intensity. However, the existence of rGO in the nanocomposite can be confirmed by
the Raman spectra.
∇
•
•
∇
∇
•
•
f
e
d
c
b
a
20
40
50
60
80
2
Theta (degree)
1
1
2
3
Fig. 1. XRD patterns of (a) GO, (b) Bi, (c) Bi WO , (d) Bi/Bi WO , (e) Bi WO /rGO, and (f)
2
6
2 6 2 6
1
1
1
4
5
6
Bi/Bi WO /rGOꢀ3. Triangle and solid dot refer to Bi and Bi WO respectively.
2 6 2 6
ꢀ1
As shown in Fig. 2, the Raman peak at 303 cm is assigned to the translation
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+
6ꢀ 28
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modes involving simultaneous motions of Bi and WO6 . These two peaks at 763
ꢀ1
and 817 cm are attributed to antisymmetric and symmetric A
g
modes of the OꢀWꢀO
, the intensity of Raman peaks of
Bi/Bi WO composites are slightly enhanced. This phenomenon could be attributed to
29
2 6
group terminus, respectively. Compared to Bi WO
2
6
30
the surface enhanced Raman scattering effect of metal Bi. As for rGO, there are two
ꢀ1
ꢀ1
typical Raman bands at ~1611 cm (G band) and at ~1359 cm (D band) for its
3
1
graphitized structure. In Raman spectrum of Bi/Bi
characteristic bands were also observed. In comparison with GO (I
increased D/G intensity ratio (I /I = 1.03) was observed, which revealed that the GO
2
WO
6
/rGOꢀ3, these two
D
/I = 0.98), an
G
D
G
32
1
1
1
1
has been partially reduced after the solvothermal reaction. Furthermore, the
positions of D band and G band in the ternary composites were slightly shifted (inset
2 6
of the Fig. 2), due to the reduction of GO and the combination of Bi and Bi WO with
33, 34
rGO.
1
356
1
610
1
359
1611
1400
1600
d
c
b
a
500
1000
1500
2000
-
1
Raman shift (cm )
1
1
4
5
Fig. 2 Raman spectra of (a) GO, (b) Bi WO , (c) Bi/Bi WO and (d) Bi/Bi WO /rGOꢀ3. Inset: the
2
6
2
6
2
6
1
6
enlarged Raman spectra of GO and Bi/Bi
2
WO
6
/rGOꢀ3.
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The chemical states and the surface composition of Bi/Bi WO /rGOꢀ3 sample
has been analyzed by XPS (Fig. 3). In the full XPS spectrum of Fig. 3A, the different
binding energies were assigned to W 4f, Bi 4f, W 4d, Bi 4d, C 1s, O 1s, and Bi 4p
states emission peaks, respectively. These peaks at 34.9 and 35.1 eV in
Bi/Bi
which can be assigned to the W oxidation state. The O 1s peaks for
Bi/Bi WO /rGOꢀ3 (Fig. 3C) can be deconvoluted into two bands at 529.8 and 530.7
eV, which are associated with OH hydroxyl groups and oxygen species in the lattice
2 6
WO /rGOꢀ3 correspond to W 4f5/2 and W 4f7/2 (Fig. 3B), respectively, both of
6
+
35
2
6
36
1
1
1
1
1
1
1
1
1
1
oxygen, respectively. Two main peaks with binding energies of 158.8 eV and 164.1
3+
3+
eV can be observed in Fig.3D, which can be ascribed to the Bi 4f 7/2 and Bi 4f 5/2
3
7
binding energies, respectively. Moreover, apart from the two peaks discussed above,
there are two peaks at 162.5 eV and 157.2 eV with low intensity, which can be
38
ascribed to metallic Bi. As shown in Fig. 3E, the peak at 288.7 eV for the C=O
bonding exists but its peak intensity is very low. To illustrate the reduction degree of
the GO sheets in the Bi/Bi
2
WO
6
/rGOꢀ3, the peak area ratios of C=O bonds to the total
proving the
Therefore, the results confirm the successful
area are calculated to be 0.16 by the XPS C 1s peak area analysis
,
39, 40
transformation from GO to rGO.
3+
reduction of Bi and GO into metal Bi and rGO via the facile hydrothermal process.
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A
Bi 4f
O 1s
Bi 4d
Bi 4p
C1s
W 4d
W 4f
0
200
400
600
800
1000
1200
Binding energy (eV)
1
B
W 4f7/2
W 4f
W 4f5/2
34
36
38
40
Binding energy (eV)
2
C
O 1s
O 1s
O-H
526
528
530
532
534
536
Binding energy (eV)
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Bi 4f
156
158
160
162
164
166
168
Binding energy (eV)
1
E
C 1s
C-C
C-O
C=O
280.0
282.5
285.0
287.5
290.0
292.5
295.0
Binding energy (eV)
2
3
2 6
Fig. 3 XPS spectra of the Bi/Bi WO /rGOꢀ3 sample: (A) full spectrum, (B) W 4f, (C) O 1s, (D) Bi 4f,
4
5
6
7
8
9
0
1
and (E) C 1s.
TEM images showing the morphology and microstructure of the asꢀprepared
Bi/Bi WO /rGOꢀ3 are presented in Fig. 4. It can be seen that the rGO sheets exhibit a
typical rippled and crumpled structure and the Bi/Bi WO composites overspread on
2
6
2
6
the rGO surface. (Fig. 4a). The apparent contrast between the darker nanorods and
brighter nanosheets as illustrated in Fig. 4b suggests the formation of Bi nanorods and
1
1
2 6
Bi WO nanosheets on the surface of rGO. The Bi nanorods appear distinctly darker
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than the Bi
2 6
WO due to the higher electron density, which could be further confirmed
41ꢀ43
by HRTEM (Fig. 4c).
The Bi nanorods with an average length of 50 nm have
been observed in the TEM images (Figs. 4b and S6a). As can be observed in Fig. 4c,
the fringe interval of 0.315 nm is in accordance with the interplanar spacing of (131)
plane of Bi WO , whereas the periodic fringe spacing of 0.325 nm can be indexed as
2
6
the (102) plane of the hexagonal Bi. To further obtain information about the Bi
structure, the special region of Bi nanorods was analyzed (Fig. 4d). The spacing of
0.227 nm between neighboring lattice fringes corresponds to the distance between two
4
4
(110) planes, indicating [100] as the growth direction for the Bi nanorods. These
1
1
1
1
results agree well with the XRD results. Moreover, the stacking width is calculated to
4
5,
be 0.37 nm for rGO layers corresponding to the spacing of the (002) lattice planes.
4
6
This spacing is very close to pristine graphite, indicating that GO has been well
reduced to graphene (rGO).
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2 6
Fig. 4 TEM images (a, b) and HRTEM image (c, d) of Bi/Bi WO /rGOꢀ3.
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Fig. 5 shows the nitrogen adsorptionꢀdesorption isotherms for samples of
Bi WO , Bi/Bi WO6，Bi WO /rGO, Bi/Bi WO /rGOꢀ3. The typical IV isotherms and
2
6
2
2
6
2
6
3
7, 38
hysteresis loops of the sample are characteristic of mesoporous materials.
The
BET surface areas of the asꢀprepared Bi WO , Bi/Bi WO , Bi WO /rGO, and
2
6
2
6
2
6
2
2
Bi/Bi WO
6
/rGOꢀ3 are 27.87, 39.12, 67.42, and 67.67 m /g, respectively. Notably, the
higher surface area of Bi/Bi
2
WO /rGOꢀ3 is attributed to the presence of rGO
6
nanosheets in the nanocomposite. Moreover, the pore volume determined by the
BarrettꢀJoynerꢀHalenda (BJH) model indicated broad distributions with a pore volume
3
of 24.65, 35.14, 68.43, and 69.26 cm /g for Bi WO , Bi/Bi WO , Bi WO /rGO, and
2
6
2
6
2
6
1
1
1
1
1
2 6 2 6
Bi/Bi WO /rGOꢀ3 respectively. The pore size distribution of Bi/Bi WO /rGOꢀ3 is
shown in the inset in Fig. 5, which shows an average pore diameter of 20 nm. Owing
to its high surface area and large pore volume, the mesoporous photocatalyst will
provide efficient adsorption sites and enhance transportation of charge carriers, which
would lead to an improvement of the photocatalytic activity.
1
1
1
40
20
00
Bi WO6
0.0144
0.0096
0.0048
0.0000
2
Bi/Bi WO6
Bi/Bi WO /rGO-3
2
2
6
Bi WO /rGO
2
6
Bi/Bi WO /rGO
2
6
8
6
4
2
0
0
0
0
0
0
50
100
150
200
250
300
Pore diamiter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
1
1
5
6
Fig. 5 Nitrogen adsorptionꢀdesorption isotherms of various samples. Inset: the pore size distribution of
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Bi/Bi WO /rGOꢀ3.
The formation mechanism of Bi/Bi
Based on the above experimental results and analysis, a possible synthesis process of
Bi/Bi WO /rGO is proposed. The in situ oneꢀstep synthetic route of Bi/Bi WO /rGO
nanocomposites is illustrated in Scheme 1. The reactions involved in the formation of
Bi/Bi WO /rGO structures can be summarized as follows:
2 6
WO /rGO nanocomposites
2
6
2
6
2
6
3
+
ꢀ
Bi(NO
3
)
3
→
→
Bi + 3NO
3
(1)
(2)
(3)
+
2ꢀ
1
1
Na WO
2
6
2Na + WO
4
3+
2ꢀ
4
2Bi + WO
→
2 6
Bi WO
3+
+
1
2
Bi + HOCH
2
CH
2
OH
→
+ 2H
(4)
2
ꢀ
1
1
3
4
2 Bi
2HOCH CHO
6CH CHO + 2Bi → 3CH COCOCH + Bi + 6H
+ WO
4
→
2
WO
6
+ HOCH
2
CH
2
OH
(5)
2
CH
2
OH 2CH
→
3
(6)
(7)
3+
+
1
1
1
1
1
2
5
6
7
8
9
0
3
3
3
The overall synthesis can be divided into four consecutive stages. At the early
stage of the reaction process, when Bi(NO) is dissolved in the GO solution (Eq. 1),
3
3
+
the positively charged Bi could be uniformly adsorbed onto the surface of the GO
sheets, which is negatively charged due to the existence of OH and C=O functional
15

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3
+
3+
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
groups (Scheme 1AꢀB). After Bi ions have been adsorbed, the reaction between Bi
3
+
and EG takes place on the surface of GO, forming EGꢀBi complexes via the
3
+
47
coordination between Bi ions and OH groups (Eq. 4, Scheme 1C). Then, in the
following solvothermal treatment at high operating temperature, the complex is
3+
2ꢀ
gradually decomposed to release the Bi ions, which are then reacted with WO
4
48
ions to form Bi
Meanwhile, metal Bi and rGO can be formed simultaneously (Eqs. 6 and 7, Scheme
1D) during the hydrothermal treatment. When Bi(NO is excessive in the reaction,
there are still many residual Bi ions absorbed on the surface of GO after the
2 6
WO nanosheets on the surface of the graphene components (Eq. 5).
3 3
)
3
+
3+
1
1
1
1
1
1
1
1
1
1
2
2
2
2 6
formation of Bi WO . These residual Bi ions can be reduced into Bi nanorods by the
reductant of EG in the presence of PVP. Therefore, the content of Bi nanorods can be
easily tuned just by varying the precursor ratios. It has been reported that only
urchinꢀlike bismuth structures were obtained by selfꢀassemble of Bi nanorods in pure
4
9
EG solution, in which a typical coordinative reduction course was suggested. It is
believed that the anisotropic effects of crystal and the face inhibitor function of PVP
50
play crucially important roles in the formation of 1D nanostructures.
In the
solvothermal reaction process, PVP molecules could preferentially adsorb onto the
primary surfaces of Bi crystals through OꢀBi bonding as reported in the previous
44
work. The polymer molecules adsorbed on some surfaces of the Bi crystals could
5
1
significantly decrease their growth rates and lead to highly anisotropic growth.
Therefore, the isolated Bi nanorods were formed in the presence of PVP under
appropriate reaction conditions.
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1
2
2 6
Scheme 1. Schematic illustration of the synthesis procedure of Bi/Bi WO /rGO composites.
3
4
5
6
7
8
9
0
1
2
3
UVꢀvis diffuse reflectance spectra (DRS) of the asꢀprepared samples are shown
in Fig. 6. Compared with that of pure Bi WO , the lightꢀharvesting ability of
Bi/Bi WO is apparently enhanced, ranging from UV to visible light region. This
enhancement is attributed to the SPR effect of plasmonic Bi. Additionally, the
absorption of Bi/Bi WO is gradually intensified with increasing the content of Bi
(Fig. S3). For Bi/Bi WO /rGO, the continuous background absorption from 400ꢀ800
2
6
2
6
5
2
2
6
2
6
53
1
1
1
1
nm is enhanced due to the black body effect of graphene. In addition, this
observation also clearly indicates that more visible light can be absorbed in the
2 6
asꢀprepared Bi/Bi WO /rGO nanocomposite to generate more electronꢀhole pairs, and
thus its photocatalytic activity will be improved under visible light irradiation. This
17

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1
2
3
4
5
6
hypothesis was confirmed by measuring the degradation of RhB over the obtained
samples under the same conditions. With increasing the amount of rGO, the
2 6
Bi/Bi WO /rGO composites show a continuously enhanced visibleꢀlight absorption in
54
the range of 400ꢀ800 nm, which is in agreement with the previous report. Therefore,
the incorporation of Bi nanorods and rGO sheets increased the light absorption of the
ternary composites in the visible region.
g
a
300
400
500
600
700
800
Wavelength (nm)
Fig. 6 UVꢀvis diffuse reflection spectra of (a) Bi WO , (b) Bi/Bi
7
8
2
6
2
WO
6
, (c) Bi
2
WO
6
/rGO, (d)
9
0
1
2
3
4
5
Bi/Bi
2
WO
6
/rGOꢀ1, (e) Bi/Bi
2
WO
6
/rGOꢀ2, (f) Bi/Bi
2
WO
6
/rGOꢀ3, and (g) Bi/Bi
2
WO
6
/rGOꢀ4.
1
1
1
1
1
1
The degree of charge recombination can be evaluated by photoluminescence (PL)
spectroscopy (Fig. 7). The lower PL intensity reflects the lower recombination rate of
5
5
photogenerated electrons and holes. Obviously, Bi/Bi
diminished PL intensity in comparison with pure Bi WO
deposition of Bi results in a remarkable decline in the recombination rate of
2
WO
6
shows a significantly
2
6
, which suggests that the
18

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2
5
1
2
3
4
5
6
7
8
9
0
photogenerated electronꢀhole pairs. In Bi
be transferred efficiently from Bi WO to rGO due to the high charge carrier mobility
of the rGO sheets, leading to a low charge recombination rate. The PL results
demonstrated that the rGO layers with a twoꢀdimensional ꢀconjugated structure
could serve as an effective electronꢀaccepting material. A more efficient separation
of electronꢀhole pairs was observed in Bi/Bi WO /rGOꢀ3, which may be due to the
charge transfer between Bi/Bi WO and rGO. These results clearly indicate that the
2 6
WO /rGO, photogenerated electrons could
2
6
π
56
2
6
2
6
recombination of photogenerated charge carriers of Bi
2
WO
6
is greatly restrained by
the coupling of Bi and rGO.
1
Bi WO
2
6
Bi/Bi WO
2
6
Bi WO /rGO
2
6
Bi/Bi WO /rGO-3
2
6
300
400
500
600
700
Wavelength (nm)
1
1
1
2
Fig. 7 Photoluminescence spectra of various photocatalysts.
1
1
1
3
4
5
Fig. 8 shows the transient photocurrent responses of the obtained samples. The
photocurrents were stable and reversible at lightꢀon and lightꢀoff for all samples. In
19

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1
2
3
4
5
comparison with Bi
composite exhibited an increased current density, about 1.6, 1.2, and 1.15 times than
that of Bi WO , Bi/Bi WO , and Bi WO /rGO, respectively. The improvement of
2 6 2 6 2 6 2 6
WO , Bi/Bi WO , and Bi WO /rGO, Bi/Bi WO /rGOꢀ3
2
6
2
6
2
6
photocurrent indicated an enhanced photogenerated electronꢀhole separation
efficiency, which is in good agreement with the PL measurements.
0.12
0.08
0.04
0.00
on
off
d
c
b
a
0
30
60
90
120
150
Irradiation times (s)
6 2 6
WO , (b) Bi/Bi WO , (c) Bi WO /rGO, and (d)
6
7
Fig. 8 Photocurrent responses of (a) Bi
2
6
2
8
9
0
1
2
3
4
5
6
7
2 6
Bi/Bi WO /rGOꢀ3.
1
1
1
1
1
1
1
1
The photocatalytic activities of the samples were evaluated by the degradation of
RhB under visible light irradiation (λ>420nm), as shown in Fig. 9. A blank control
test (without photocatalyst) confirmed that the selfꢀdegradation of RhB can be
negligible. The Bi/Bi
2 6
WO composite has a higher photocatalytic activity than that of
Bi WO , which is due to the SPR effect of Bi. In addition, the photocatalytic activity
2
6
was further enhanced by increasing the Bi amount in the composites (Fig. S4), which
is consistent with the results of DRS. Compared with pure Bi WO and Bi/Bi WO ,
2
6
2
6
2 6
the Bi/Bi WO /rGO photocatalysts exhibited much higher photocatalytic activity (Fig.
20

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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
9A). About 99.9% of the RhB molecules was decomposed in 120 min when
Bi/Bi WO /rGOꢀ3 was used as the photocatalyst, while only 49% and 83% of the
RhB molecules were degraded when pure Bi WO and Bi/Bi WO were employed,
respectively. Moreover, the photocatalytic activity of the Bi/Bi WO /rGO composites
2
6
2
6
2
6
2
6
increases with increasing the rGO content, and Bi/Bi
degradation efficiency. However, when more GO was added, the photocatalytic
activity of the Bi/Bi WO /rGO photocatalyst is decreased. It is probably because the
dense black layers of excessive rGO shield the light irradiation on Bi and Bi WO
2 6
WO /rGOꢀ3 shows the highest
2
6
2
6
,
which could weaken the photocatalytic activity of the composite. Furthermore,
excessive rGO also can act as a recombination center, which then decreases the
1
1
1
1
1
1
1
1
1
1
2
2
57
efficiency of charge separation.
In order to directly shown the photocatalytic activity enhancement of
Bi/Bi WO /rGO, LangmuirꢀHinshelwood kinetics model was applied to calculate the
apparent pseudoꢀfirst order rate constant
constants corresponding to, Bi/Bi WO
Bi/Bi WO /rGOꢀ2, Bi/Bi WO /rGOꢀ3 and Bi/Bi
0.0111, 0.0099, 0.0115, 0.0203,0.0355 and 0.0147 min , which are about 2.27, 2.02,
2
6
5
8
k
.
As can be seen in Fig. 9B, the rate
Bi WO /rGO, Bi/Bi WO /rGOꢀ1,
WO /rGOꢀ4 were estimated to be
2
6
,
2
6
2
6
2
6
2
6
2
6
ꢀ1
ꢀ1
2.35, 4.14 ,7.24 and 3 times as much as that over Bi
These results clearly indicated that introducing appropriate amount of Bi and rGO into
the Bi WO photocatalyst system can greatly enhance its photocatalytic activity.
2 6
WO (0.0049 min ), respectively.
2
6
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1.0
0.8
0.6
0.4
0.2
0.0
A
Blank
Bi WO
2
6
Bi/Bi WO
2
6
Bi WO /rGO
2
6
Bi/Bi WO /rGO-1
2
6
Bi/Bi WO /rGO-2
2
6
Bi/Bi WO /rGO-3
2
6
Bi/Bi WO /rGO-4
2
6
-
40
-20
0
20
40
60
80
100
120
Time (min)
1
B
Blank
Bi WO6
2
Bi/Bi WO
3.9
2.6
1.3
0.0
2
6
Bi WO /rGO
2
6
Bi/Bi WO /rGO-1
2
6
Bi/Bi WO /rGO-2
2
6
Bi/Bi WO /rGO-3
2
6
Bi/Bi WO /rGO-4
2
6
0
20
40
60
80
100
120
Time (min)
2
3
Fig. 9 (A) Photocatalytic degradation curves of RhB as a function of irradiation time over various
photocatalysts under visible light irradiation (the time before 0 min means in the dark). (B) Kinetic
curves of photocatalytic degradation of RhB fitted from the data of Figure 9A.
4
5
6
7
8
9
It is usually accepted that the degrading RhB over the photocatalyst would occur
partially via photosensitization pathway. In order to eliminate the photosensitization
effect, colorless
pꢀchlorophenol (4ꢀCP) was used as the model pollutant. The
22

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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
photocatalytic degradation results are shown in Fig. 10. In contrast to Bi
Bi/Bi WO , and Bi WO /rGO, an enhanced photocatalytic activity was observed for
the Bi/Bi WO /rGOꢀ3 composite. As shown in Fig. 10A, a similar trend to the
photocatalytic degradation of RhB was observed under visible light irradiation. The
Bi/Bi WO /rGOꢀ3 composite exhibits the highest photocatalytic performance among
all the tested samples for RhB and 4ꢀCP removal. Fig. S5 shows the evolution of
UVꢀvis spectra for RhB and 4ꢀCP solution in the presence of Bi/Bi WO /rGOꢀ3 under
2 6
WO ,
2
6
2
6
2
6
2
6
2
6
visible light irradiation. The UVꢀvis peak intensity of RhB at 553 nm decreased
gradually with irradiation time, while its position remained unchanged, suggesting
that the RhB molecules were directly destroyed through the cleavage of the
1
1
1
1
1
1
5
9
chromophore ring structure rather than a deꢀethylation process. The UVꢀvis intensity
of 4ꢀCP also decreased with irradiation time, but slowly compared with that of RhB.
Only 52% 4ꢀCP was removed, while almost 99% RhB was degraded after 120 min.
Thus, the degradation efficiency for 4ꢀCP is lower than that for RhB (Fig. 10B),
which implies that photosensitization plays a role in the RhB degradation.
1.0
0.9
0.8
0.7
0.6
0.5
0.4
A
Bi WO6
2
Bi/Bi WO6
2
Bi WO /rGO
2
6
Bi/Bi WO /rGO-3
2
6
-20
0
20
40
60
80
100
120
Time (min)
1
6
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B 1
.0
.8
.6
.4
.2
.0
4
RhB
-CP
0
0
0
0
0
-3
O
G
O6
O
/r
G
O
W
W
6
/r
O6
W
i
i
i
O
6
W
B 2
B 2
i/
B
i
B 2
i/B 2
B
1
2
Fig. 10 (A) Photocatalytic degradation of 4ꢀCP as a function of irradiation time over the different four
3
4
5
6
7
8
9
0
1
2
3
4
5
2 6 2 6
samples. (B) The comparison of removal efficiencies for RhB and 4ꢀCP over Bi WO , Bi/Bi WO ,
Bi
2
WO
6
/rGO and Bi/Bi
2
WO
6
/rGOꢀ3 after 120ꢀmin visible light irradiation.
ꢀ
+
As is known, various active species (•OH, •O
2
, and h ) generated in the
photocatalytic processes played key roles. Therefore, the trapping experiments were
carried out to explore the main active species. Isopropyl alcohol (IPA),
p
ꢀbenzoquinone (BQ), and triethanolamine (TEOA) were adopted as the traps for •
ꢀ
+
60
1
1
1
1
1
1
OH, •O
2
, and h , respectively. Fig. 11 shows the effect of different scavengers on
WO /rGOꢀ3. It can be easily
seen that the addition of IPA does not cause deactivation of the Bi/Bi WO /rGOꢀ3
the photocatalytic degradation RhB and 4ꢀCP over Bi/Bi
2
6
2
6
photocatalyst in both cases, indicating that
involved in the photocatalytic processes. However, the photocatalytic performance of
Bi/Bi WO /rGOꢀ3 significantly decreases by the addition of TEOA and BQ. Thus,
•OH was not the major active species
2
6
24

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ꢀ
+
1
2
3
4
5
6
7
8
9
0
1
•O
2
radicals and h were proved to be the dominant active species in the
photocatalytic degradation of RhB and 4ꢀCP over the Bi/Bi
2
WO /rGOꢀ3 composite
6
photocatalyst under visible light irradiation. According to the previous report, the CB
electrons of Bi WO possess a poor reduction power owing to its more positive
2
6
+
ꢀ
2
potential (+0.3 V, vs. SHE) than the oneꢀelectron oxygen reduction (O + H + e =
61
HO
2
, ꢀ0.046 V vs. SHE). It seem plausible that the conduction band potential of
ꢀ
Bi WO
2
6
is incapable to reduce the dissolved O
2
to •O
2
radicals. However, with the
WO
addition of rGO, the electronic interaction and charge equilibrium between Bi
2
6
and rGO lead to the negative shift of the Fermi level and the change in energy levels
of the conduction band and valence band, indicating that the photocatalytic
1
1
6
2, 63
performance of reduction reactions is enhanced.
A
1.0
0.8
0.6
0.4
0.2
0.0
No scavenger
addition of IPA
addition of BQ
addition of TEOA
0
20
40
60
80
100
120
Time (min)
1
2
25

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B
1.0
0.9
0.8
0.7
0.6
0.5
No scavenger
addtion of IPA
addtion of BQ
addtion of TEOA
0
20
40
60
80
100
120
Time (min)
1
2
Fig. 11 Effects of different scavengers on (A) RhB and (B) 4ꢀCP degradation over Bi/Bi WO /rGOꢀ3
2
6
3
4
5
6
7
8
9
0
1
2
3
4
5
6
under visible light irradiation.
In addition to photocatalytic efficiency, the stability of photocatalysts is another
important issue for its practical application. To study the photocatalytic stability and
reusability of Bi/Bi
2
WO
6
/rGOꢀ3, the photocatalysis process was repeated five times
WO /rGOꢀ3 composite
under the same conditions. As shown in Fig. 12A, the Bi/Bi
2
6
had a high and stable activity for photocatalytic degradation of RhB under visible
light irradiation. After five cycling runs, the photocatalyst has no detectable loss of its
1
1
1
1
1
1
1
2 6
photocatalytic activity. XRD patterns of Bi/Bi WO /rGOꢀ3 before and after the
photocatalytic reaction were comparable in Fig. 12B. There is no apparent change
observed in the XRD patterns. TEM and XPS analysis proved that there is no
apparent change in composition and morphology for Bi/Bi
photocatalytic process (Figs. S6 and S7), which also confirmed the stability of the
structure. It can be concluded that Bi/Bi WO /rGO possesses an efficient
2 6
WO /rGOꢀ3 during the
2
6
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1
photocatalytic activity and can be easily separated for reuse.
1.0
0.8
0.6
0.4
0.2
0.0
A
3
rd
4th
5th
1
st
2nd
0
100
200
300
400
500
600
Time (min)
2
B
after
before
Bi JCPDS no. 44-1246
Bi WO JCPDS no. 39-0256
2
6
20
40
60
80
2
Theta (degree)
3
4
Fig. 12 (A) Cycling runs in the photocatalytic degradation of RhB in the presence of
5
6
7
8
9
Bi/Bi WO /rGOꢀ3. (B) XRD diffraction patterns of Bi/Bi WO /rGOꢀ3 measured before and after a
2
6
2
6
photocatalytic reaction under visible light irradiation.
The enhanced photocatalytic activities of the Bi/Bi
2 6
WO /rGO composites could
be ascribed to the synergic effects of the following factors. Firstly, the introduction of
27

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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
Bi nanorods in the composite broadened the light absorption in the visible region,
which could be ascribed to the SPR effect of metal Bi. Secondly, due to their
matching energy levels, the interfacial charge transfer could effectively enhance the
separation efficiency of photogenerated electrons and holes. Finally, the presence of
rGO provides more active adsorption sites on the surface of rGO in the photocatalytic
reaction. A proposed mechanism for the separation and transportation of
2 6
photogenerated electronꢀhole pairs at the Bi/Bi WO /rGO interface is shown in
Scheme 2. Electrons and holes could be generated from both the photoꢀexcited
Bi WO and the SPRꢀexcited Bi under visible light irradiation. Since the Fermi
2
6
1
1
1
1
1
1
1
1
1
1
2
2
2
energy level of Bi (E
(CB) of Bi WO (ECB = 0.24 eV vs. SHE).
SPRꢀexcited electrons transferring from Bi to the CB of Bi
electrons can transfer from the CB of Bi WO to the surface of rGO nanosheets, due
to their twoꢀdimensional conjugated ꢀstructure and ultrahigh charge carrier mobility.
Concomitantly, the holes transfer to the Bi WO and Bi surfaces, which corresponds
to the oxidation of dye molecules into CO . Thus, in the case of Bi/Bi WO /rGO, rGO
sheets served as an acceptor of the photogenerated electrons of Bi WO and Bi, which
f
= ꢀ0.17 eV vs. SHE) is more negative than the conduction band
64, 65
2
6
It is energetically favorable for the
WO . Subsequently, these
2
6
2
6
π
2
6
2
2
6
2
6
will suppress the recombination of photogenerated electronꢀhole pairs. The electrons
in rGO can be captured by the dissolved O molecules to generate superoxide anion
2
ꢀ
radicals (•O
2
) and then react with RhB (or 4ꢀCP). In the meantime, the holes in the
composite can directly oxidize the pollutants into CO
intermediates.
2 2
and H O, and other
28

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1
2
3
2 6
Scheme 2. Proposed mechanism degradation of RhB over Bi/Bi WO /rGO composite.
4
5
6
7
8
9
0
1
2
3
4
5
Conclusions
In summary, the excellent heterojunction structure of rGO supported Bi nanorods
and Bi WO nanosheets was obtained via a oneꢀpot solvothermal process. The
2
6
obtained composite possesses an enhanced visible light harvesting, good adsorptive
capacity and efficient charge separation of the photogenerated electronꢀhole pairs. The
ternary composite photocatalyst had a better performance for RhB photocatalytic
degradation compared with the pristine Bi WO and the binary composite of
1
1
1
1
1
1
2
6
Bi/Bi
visible light response of Bi
Bi/Bi WO /rGO heterojunctions and rGO sheets. In addition, the photocatalyst
exhibits a high stability and can be reused for five times without any degeneration in
2
WO
6
. Such excellent performance should be attributed to the SPR effect of Bi,
2
WO , vectorial charge carrier transfer in the
6
2
6
29