In situ grown heterojunction of Bi2WO6/BiOCl for efficient photoelectrocatalytic CO2 reduction

Article abstract of DOI:10.1016/j.jcat.2019.06.007

CO₂ reduction is a very attractive research field in environmental, material, and chemical science in light of the energy crisis and the greenhouse effect. In this study, Bi₂WO₆/BiOCl heterojunctions were fabricated onto the F-SnO₂ transparent conductive glass in situ by a hydrothermal method. These new photocathodes, named BCW-X, were characterized by such methods as scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscope, and UV-vis spectra. Their morphology is a good 2D layered/3D flower structure. The exposed crystal plane of BiOCl was changed from (1 0 1) of the pristine form to (1 1 2) in the heterojunction. The photoelectrocatalytic reduction of CO₂ was carried out in BCW-X|KHCO₃|BiVO₄ under irradiation by an Xe lamp and external voltage from a Si solar cell (?0.6 to ?1.1 V). A PEC cell of BCW-6|KHCO₃|BiVO₄ produces ethanol at a rate of 11.4 μM h^?1 cm^?2 (600 μmol h^?1 g^?1) with 80.0% selectivity under ?1.0 V. The apparent quantum efficiency is up to 0.63%, about three times that of composite BiOCl–Bi₂WO₆ as photocathode. These phenomena can be attributed to the better photogenic electron–hole separation and high charge separation efficiency of heterojunctions.

Full text of DOI:10.1016/j.jcat.2019.06.007

Journal of Catalysis 377 (2019) 209–217
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
In situ grown heterojunction of Bi WO /BiOCl for efficient
2
6
photoelectrocatalytic CO reduction
2
a
a
a
a
b
a,b,
⇑
Jixian Wang , Yan Wei , Bingjie Yang , Bing Wang , Jiazang Chen , Huanwang Jing
a
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, Gansu, China
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
CO2 reduction is a very attractive research field in environmental, material, and chemical science in light
Received 23 April 2019
Revised 3 June 2019
Accepted 4 June 2019
of the energy crisis and the greenhouse effect. In this study, Bi
2 6
WO /BiOCl heterojunctions were fabri-
cated onto the F-SnO transparent conductive glass in situ by a hydrothermal method. These new photo-
2
cathodes, named BCW-X, were characterized by such methods as scanning electron microscopy,
transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscope, and UV-vis spec-
tra. Their morphology is a good 2D layered/3D flower structure. The exposed crystal plane of BiOCl was
changed from (1 0 1) of the pristine form to (1 1 2) in the heterojunction. The photoelectrocatalytic
Keywords:
CO reduction
2
Heterojunction
Photoelectrocatalysis
Charge separation efficiency
reduction of CO
2
was carried out in BCW-X|KHCO
3
|BiVO
4
under irradiation by an Xe lamp and external
|BiVO produces ethanol at a rate
) with 80.0% selectivity under ꢀ1.0 V. The apparent quantum effi-
ciency is up to 0.63%, about three times that of composite BiOCl–Bi WO as photocathode. These phe-
voltage from a Si solar cell (ꢀ0.6 to ꢀ1.1 V). A PEC cell of BCW-6|KHCO
3
4
ꢀ
1
ꢀ2
ꢀ1 ꢀ1
of 11.4
l
M h cm (600
l
mol h
g
2
6
nomena can be attributed to the better photogenic electron–hole separation and high charge
separation efficiency of heterojunctions.
Ó 2019 Elsevier Inc. All rights reserved.
1
. Introduction
2 2 3
Semiconductors such as TiO [13], CdS [14], Ga O [15], ZnO
[
16], Si [17], and g-C [18] have been used as catalysts in CO
3
N
4
2
Along with fast development of human society, the total con-
reduction. Recently, much attention has been paid to Bi-based
semiconductors in photocatalysis [19–23]. 2D layered semicon-
ductors of bismuth oxyhalides (BiOX, X = Cl, Br, and I) have been
applied to the degradation of hazardous pollutants [24], water
sumption of fossil fuels has been climbing to an unbelievable level,
resulting in climate change and environment issues [1]. Therefore,
it is of great significance to harvest and store solar energy [2] in
chemical fuels by using abundant CO
2
2
and H O via photocatalysis
2
splitting [25], and CO conversion [26,27]. However, the wide band
[
3–5] and electrocatalysis [6–8]. Apart from the photocatalytic
gaps of BiOX preclude their application under visible light [28]. Dif-
ferent strategies have been developed to overcome this drawback
[29], including hierarchical nanostructures [30] and crystal-facet
control [31]. On the other hand, various composites with
heterostructures were constructed from BiOCl and other materials
to augment their catalytic activity, such as graphene [32], metal
oxides [33], metals [34,35], and salts [36].
As is well known, semiconductor heterojunctions can enhance
the light absorption, the separation efficiency of electrons and
holes, and the lifetime of photogenic carriers in hybrid materials
2
and electrocatalytic reduction of CO , photoelectrocatalytic (PEC)
CO
of CO
2
reduction [9–11] is also a promising method for the reduction
into fuels and chemicals. PEC CO reduction can integrate
2
2
and optimize the advantages of photocatalysis and electrocatalysis
to enhance their efficiency. The energetic photoelectrons could
overcome the high overpotential of the electrochemical process
and electrons from the circuit could prohibit the combination of
photoelectrons and holes. To accelerate the sluggish kinetics of
multielectron reactions, some co-catalysts, such as Au, Ag, Pt, Pd,
Ru, Rh, Ir, and Re [12], were deposited on the surfaces of
semiconductors.
[37]. The Zeng group has designed a TiO
enhances the separation efficiency of photogenic charges [38].
The Yu group has built a 2D/2D heterojunction of Ti /Bi WO
to improve photocatalytic CO reduction [39]. Our group has
reported Ti /TiO [40], g-C /Ti [41], and TiO /ZnO [42]
2
/GaP p–n junction that
3
C
2
2
6
2
⇑
Corresponding author at: State Key Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou
7
C
3 2
2
3
N
4
3
C
2
2
heterojunctions, showing evident improvements in PEC CO
reduction in water.
2
30000, Gansu, China.
E-mail address: hwjing@lzu.edu.cn (H. Jing).
https://doi.org/10.1016/j.jcat.2019.06.007
021-9517/Ó 2019 Elsevier Inc. All rights reserved.
0

2
10
J. Wang et al. / Journal of Catalysis 377 (2019) 209–217
Here, a new heterojunction of Bi
onto an F-SnO transparent conductive glass (FTO) surface and
employed as a photocathode in the photoelectrocatalytic reduction
of CO . This PEC cell was a two-electrode system of BCW-X|KHCO
BiVO and performed CO reduction under simulated sunlight irra-
diation and a tiny external voltage supplied by a Si solar cell.
2
WO
6
/BiOCl was grown in situ
Cu Ka radiation. The solid-state UV–vis absorption spectra of sam-
2
ples were collected on a UV–vis spectrophotometer (UV-2600,
Shimadzu). The X-ray photoelectron spectroscopy (XPS) of BiOCl
and BCW-4 was recorded on an XPS instrument (ESCALAB 210,
2
3
|
4
2
VG Scientific) with an Mg Ka X-ray resource. The spectra of various
elements were calibrated according to C1s = 284.6 eV.
2.3. Photoelectrochemical measurement
2
. Experimental
The linear sweep voltammetry (LSV) curves of the photoelectro-
2.1. Preparation of the photocathode
catalytic cell were measured in a traditional three-electrode sys-
tem on an electrochemical workstation (CHI660E). The prepared
electrode served as a working electrode, BiVO was employed as
4
a counter electrode, and a saturated calomel electrode (SCE) was
used as a reference electrode. The LSV experiments were con-
ducted at a scan rate of 50 mV/s. The electrochemical impedance
spectra (EIS) and the Mott–Schottky plots were both illustrated
by the electrochemical workstation under EC conditions. On the
other hand, the transient photocurrent responses were detected
at ꢀ0.6 V with a two-electrode system.
2
.1.1. Preparation of BiOCl@FTO film electrodes
BiOCl electrodes were prepared according to the literature [43].
Bi(NO O (0.4815 g) and Triton X-100 (0.5 ml) were dissolved
)
3 3
ꢁ5H
2
in 20 ml ethylene glycol to form a solution A. NaCl (0.117 g) was
dissolved in 15 ml methanol to generate a solution B. Then the
solutions A and B were mixed together and stirred for 2 h to get
a clear solution C. FTO substrates (Nippon Sheet Glass) were ultra-
sonically cleaned in acetone, ethanol, and distilled water for
3
0 min each and immersed in a solution C in a 100-ml Teflon-
lined stainless steel autoclave with the conductive side facing up.
Then it was closed and solvothermal synthesis was conducted at
2
2.4. Experiments on photocatalytic CO reduction
1
60 °C for 10 h. After the completion of the reaction, the FTO sub-
strate was taken out, rinsed extensively with ultrapure water, and
allowed to dry in ambient air.
PEC CO
quartz reactor of our own design equipped with a BiOCl or
BCW-X photocathode and a BiVO photoanode and powered by
an external Si solar cell (ꢀ0.6 to ꢀ1.1 V). The electrolyte was an
aqueous solution of KHCO (0.1 M, 60 ml) containing Eosin Y as a
photosensitizer (1 mM). The reactor was bubbled by CO
2
reduction experiments were carried out in a closed
4
2.1.2. Preparation of Bi
2 6
WO /BiOCl electrodes
The Bi WO /BiOCl heterojunction plates were prepared in situ
2
6
3
by a hydrothermal method and weighed 2.0 mg for each plate.
The as-prepared BiOCl precursors were immersed in 1 mmol
2
(
(
99.998%) for 30 min and then irradiated by a 300-W Xenon lamp
Perfect Light Co.) for 2 h. After completion of experiments, the H
1
Na
autoclave. The hydrothermal synthesis took place at 180 °C for 4,
, 8, and 10 h for BCW-4, BCW-6, BCW-8, and BCW-10, respec-
2 4
WO aqueous solution in a 100-ml Teflon-lined stainless steel
NMR spectra of liquid products were acquired by a NMR instru-
ment (JNM-ECS400, JOEL) using dimethyl sulfoxide (DMSO,
6
1
0 mM) and m-trihydroxybenzene (MTB, 50 mM) as internal stan-
tively. After completion of the reaction, the electrodes were taken
out of the autoclave, rinsed with ultrapure water, and calcined for
dards and the water suppression technique. Gas chromatography
was employed to detect gas-phase products with a CP-3380 Varian
instrument, calibrated with standard curves.
2
h at 500 °C in a muffle furnace.
2.2. Characterization of morphology and structure
3
. Results and discussion
A HitachI S-4800 scanning electron microscope (SEM) was used
to observe the morphology of samples. Transmission electron
microscopy (TEM) and high-resolution transmission electron
microscopy (HRTEM) of samples were performed on an FEI-
Talos-F200s electron microscope. Their X-ray diffraction (XRD)
patterns were collected on an X’Pert PRO diffractometer using
3.1. Characterization of photocathodes
XRD patterns of BiOCl and BCW-4 are displayed in Fig. 1a. There
are four characteristic peaks around 11.9°, 25.9°, 32.2°, and 33.5°
that are attributed to diffraction of the crystal planes (0 0 1),
Fig. 1. XRD patterns of (a) BiOCl and BCW-4 and (b) BCW-X samples.

J. Wang et al. / Journal of Catalysis 377 (2019) 209–217
211
(
0 1 1), (1 1 0), and (1 0 2) of tetragonal BiOCl, respectively. This is
in situ from the matrix of BiOCl, the morphology of BCW-6
(Fig. 2b) still keeps the layered structure and the thickness of the
heterojunction increases to 15 lm (Figs. S1b, S2).
in good concordance with JCPDS card No. 6-249 [43]. The pattern of
the BCW-4 heterojunction shows clearly two phases of BiOCl and
Bi
2
WO
6
in which the peaks at 28.3° and 55.8° can be assigned to
WO . When
The HRTEM (Fig. 2c) image of pure BiOCl illustrates multiple
directions of the crystal plane (1 0 1) with d101 = 0.341 nm. When
the heterojunction of BCW-6 was formed, the crystal plane
the diffraction of the (1 1 3) and (2 2 0) planes of Bi
in situ growth time for the heterojunction is prolonged, the diffrac-
tion lines of BiOCl decrease, accompanied by an increase of diffrac-
2
6
2 6
(1 1 3) of Bi WO and crystal plane (1 1 2) of BiOCl are obviously
tion lines for Bi
2
WO
6
(Fig. 1b).
seen from the image of HRTEM (Fig. 2d). We can see that the
exposed phase of BiOCl was changed from (1 0 1) to (1 1 2) due
to good compatibility between the crystal planes (1 1 2) of BiOCl
The SEM images of BiOCl (Fig. 2a) show obviously good flower-
like microspheres aggregated as nanosheets [43]. The thickness of
BiOCl grown onto the FTO is about 7.5
Supporting Information). When the Bi
l
m (see Fig. S1a in the
and (1 1 3) of Bi
2 6
WO . To further prove the formation and structure
2
WO phase germinates
6
of heterojunctions, their EDS mappings were obtained and
Fig. 2. SEM images of the photocathode, (a) BiOCl and (b) BCW-6, and HRTEM images of the photocathode, (c) BiOCl and (d) BCW-6.
2 6
Fig. 3. Fine XPS spectra of (a) Bi4f of BiOCl, Bi WO and BCW-6 and (b) Cl2p of BiOCl and BCW-6.

2
12
J. Wang et al. / Journal of Catalysis 377 (2019) 209–217
revealed that W, Bi, Cl, and O are well distributed on the surface of
the electrode BCW-6 (Fig. S3) in comparison with Bi, Cl, and O on
BiOCl (Fig. S4).
The full XPS of heterojunction BCW-6 was investigated and
depicted in Fig. S5. The fine XPS of Bi4f for BCW-6 has four peaks
(162.7 eV 4f5/2 and 157.2 eV 4f7/2), they have obvious red shifts.
On the other hand, the peaks 195.9 eV (Cl2p3/2) and 197.5 eV
(Cl2p1/2) appearing in BiOCl [45] could not be observed in hetero-
junctions due to the lower concentration of Cl in heterojunction
BCW-6 (Fig. 3b). The concentrations of Bi, W, O, and Cl elements
in heterojunction BCW-6 from EDX are shown in Table S1.
Solid UV–vis absorption spectra of photocathodes are shown in
Fig. 4a. The spectra of the BiOCl electrode appear in the range 300–
385 nm. When the heterojunction forms in the samples of BCW-X,
(
Fig. 3a) that were assigned to the Bi
and 159.0 eV 4f7/2) [44] and the BiOCl phase (161.3 eV 4f5/2 and
56.1 eV 4f7/2) in the heterojunction. Compared with the Bi4f orbi-
tals in pure Bi WO (164.5 eV 4f5/2 and 159.2 eV 4f7/2) and BiOCl
2 6
WO phase (164.1 eV 4f5/2
1
2
6
Fig. 4. (a) Normalized solid-state UV–vis absorption spectra of pure BiOCl and heterojunction electrodes. (b) The band gaps of BCW-6 and BiOCl electrodes.
Fig. 5. (a) Photocurrent densities of PEC cells. (b) LSV curves of BiOCl and BCW-6 electrodes under EC and PEC conditions. Mott–Schottky plot of electrodes: (c) BiOCl, (d)
BCW-6.

J. Wang et al. / Journal of Catalysis 377 (2019) 209–217
213
the absorption bands enlarge to 470 nm, which is superior to both
BiOCl (385 nm) and Bi WO (425 nm) due to the interface of
heterojunction. Therefore, the band gaps of heterojunctions BCW-
X are narrowed. The band gap of BCW-6 is 2.64 eV (Fig. 4b). Other
data of samples are presented in Fig. S6.
potentials [46]. Compared with a lower CB potential (vs. NHE) of
2
6
pure BiOCl (ꢀ0.155 V), two CB potentials of photocathode BCW-6
appear at ꢀ0.628 V (Bi
2
WO
6
) and ꢀ0.447 V (BiOCl) that ensure
appropriate redox potentials to produce ethanol and methanol
[42].
The transient photocurrent curves of the photocathode were
recorded at a constant bias of ꢀ0.6 V under chopped light irradia-
tion and are depicted in Figs. S8a and 6a. Their photocurrent den-
sities of BCW-X are higher than that of the precursor BiOCl due to
the formation of heterojunctions. Generally, the spike formation of
transient photocurrent is attributed to electron–hole recombina-
tion [47]. We can clearly see from Fig. 6a that there are no transient
spikes in the transient photocurrent curves of heterojunction BCW-
6. This phenomenon is ascribed to the high charge separation effi-
ciency in heterojunctions.
3
.2. Photoelectrochemical properties of photocathodes
To understand the photoelectrocatalytic properties of photo-
cathodes, their linear sweep voltammetry (LSV) curves were deter-
mined by a CHI660E electrochemical workstation with a three-
electrode system under both EC and PEC conditions in the range
of bias potential from 0 to ꢀ1.2 V (Fig. 5a, b). The photocurrent
densities of BCW-X heterojunctions increase obviously with
increasing bias potential from ꢀ0.6 V, compared with an almost a
straight line near zero of BiOCl (Fig. 5a). This can be attributed to
the contribution of heterojunction structure. On the other hand,
the PEC current densities of heterojunctions are higher than their
EC current densities owing to the contribution of photoelectrons
To further understand the working mechanism of new hetero-
junctions in photoelectrocatalytic CO
charge separation ( sep) and charge transfer (
mined and calculated based on the current density of the PEC cell
2
reduction, the efficiency of
g
g
trans) were deter-
(Figs. 5b, S7a).
in the presence of Na
2
SO
3
(JNa2SO3) [48]. The data were calculated
To better understand the properties of semiconductors, their
according to the equations
Mott-Schottky plots were measured and are shown in Figs. 5c, d,
and S7. A positive slope for the BiOCl electrode suggests an n-
type semiconductor (Fig. 5c). We can see from Figs. 5d and S7 that
all heterojunctions of BCW-X have two positive slopes, indicating
that these new heterojunctions are n–n homo-type heterojunctions
gsep ¼ JNa2SO3=Jabs
;
ð1Þ
ð2Þ
gtrans ¼ JKHCO3=JNa2SO3
:
The Jabs can be calculated from a standard AM1.5G solar spec-
[
40]. Since the flat band potential of n-type semiconductor is close
trum and experimental UV–vis spectra (Figs. 6b, c, S8). The detailed
to the bottom edge of the conduction band, the intercepts collected
in MS plots can be generally considered as conduction band (CB)
calculations can be found in the Supporting Information. The
can be estimated by dividing the photocurrent density JNa2SO3
g
sep
Fig. 6. (a) The transient photocurrent curves of BiOCl and BCW-6 electrodes. (b) The Jabs spectra of BiOCl and (c) BCW-6 electrodes. (d) The charge separation efficiency of
BiOCl and BCW-X photocathodes.

2
14
J. Wang et al. / Journal of Catalysis 377 (2019) 209–217
To investigate the interfacial charge transfer between these
photocathodes and the electrolyte, electrochemical impedance
spectroscopy (EIS) was performed at ꢀ0.6 V under dark conditions
(Fig. 7). The Rct values of heterojunctions are much lower than
BiOCl, indicating higher mobility of electrons. Smaller cycles of
BCW-4 and BCW-6 imply weak resistance that is beneficial for
charge transfer [40].
3.3. Photoelectrocatalytic reduction of CO
2
Initially, the experiment was carried out by using BiOCl as a
photocathode in the PEC cell BiOCl|KHCO |BiVO
3
4
under ꢀ0.6 V of
external voltage (Fig. 8a), generating only a small amount of metha-
nol (MeOH). When the heterojunction BCW-X was used as a photo-
2
cathode, the PEC CO reduction was investigated under external
voltages of ꢀ0.6, ꢀ0.8, ꢀ1.0, and ꢀ1.1 V. The PEC cell BCW-10|
ꢀ
1
ꢀ2
KHCO
0.6 V (Fig. 8a). The PEC cell BCW-10|KHCO
an apparent quantum efficiency (AQE, = 0.49%) that is 12 times
higher than that of the PEC cell BiOCl|KHCO |BiVO = 0.04%,
Fig. 8c). Furthermore, the PEC cell BCW-6|KHCO |BiVO
3
|BiVO
4
yields methanol at a rate of 12.5
l
M h cm under
Fig. 7. Nyquist plots of different photocathodes.
ꢀ
3
|BiVO
4
demonstrates
g
(
g
g
Fig. S9a) by Jabs, and are presented in Fig. 6d. It can be seen that the
3
4
(g
4
sep of all heterojunction electrodes are 3–5 times higher than the
sep of BiOCl at ꢀ1.0 V. The trans of working electrodes can be
3
yields etha-
ꢀ1
ꢀ2
ꢀ1 ꢀ1
g
nol at a rate of 11.4
l
M h cm (600
lmol h
g
) with 80.0%
calculated with Eq. (2) using the data at bias potential ꢀ0.6 V,
selectivity under ꢀ1.0 V (Fig. 8b) that can be attributed to the mor-
the trans (Fig. S9b) of heterojunctions BCW-4, BCW-6, BCW-8,
g
phology control [49]. It exhibits the highest AQE [50] (0.63%) under
and BCW-10 are 24.9, 32.2, 31.2, and 30.1%, respectively, which
are higher than 21.6% for BiOCl. This indicates that the mobility
of carriers in heterojunctions is superior to that for the precursor
BiOCl semiconductor.
external voltage ꢀ1.0 V in the PEC cell BCW-6|KHCO
3
|BiVO
4
1
(
Fig. 8c). The H NMR spectra of liquid products are displayed in
Figs. S11–S14. In our new artificial photosynthesis system, O
produced as well (Fig. S15).
2
was
Fig. 8. The production rates of heterojunction PEC cells for five different photocathodes at (a) ꢀ0.6 V and (b) ꢀ1.0 V. (c) The apparent quantum efficiency of heterojunction
PEC cells at ꢀ0.6 and ꢀ1.0 V. (d) The production rates of composite PEC cells at external voltage ꢀ1.0 V.

J. Wang et al. / Journal of Catalysis 377 (2019) 209–217
215
Table 1
Physical properties of heterojunction BCW-6 compared with those of the reported heterojunctions.
Material
Bi4f (XPS, eV)
Absorption bands (Uv–vis, nm)
Flat band (Mott-Shottky, V)
EIS (
X)
Ref.
Bi
Bi
2
WO
WO
6
:BiOCl = 3:1
/BiOCl
–
–
–
–
30,000
–
–
17,500
713
350
[52]
[53]
[54]
[51]
This work
This work
2
6
164.5, 159.2
164.6, 159.3
163.8, 158.5
164.6, 159.3
164.1, 159.0
431
440
459
425
470
BWO-PMS
BiOCl-Bi
BiOCl-Bi
ꢀ0.42
2
WO
WO
6
–
2
6
(2:1) composite
ꢀ0.285
BCW-6 heterojunction
ꢀ0.447, ꢀ0.628
1
61.3, 156.1
Table 2
Performance of BCW-6 compared with the literatures of Bi-based materials.
Material
Condition
Main product
Rate (mmol/g h)
Ref.
BOC-OV
HSA-Bi
Photo
Electro
Photo
Photo
Photo
Photo
CO
HCOOH
16.76
–
2.6
[26]
[27]
[55]
[56]
[57]
[39]
Bi
QDh-Bi
PtO /Bi
TB2
2
WO
6
-HNRS
CH
CH
CH
CH
CH
CH
CH
4
3
4
3
4
3
3
2
WO
6
OH
OH
8.2
x
2
WO
6
108.8
0.44
1.78
600
151
BCW-6
Photoelectro
CH
OH
2
OH
This work
For comparison, four composite material samples were pre-
pared by hydrothermal methods as well. The detailed procedure
is described in the Supporting Information. The SEM images of ref-
erence samples exhibit blocky morphology (Figs. S16a–c). The
HRTEM of composite material with a ratio of 3:1 for KCl/Na
Fig. S16d) demonstrates that the interfaces between the BiOCl
and Bi WO phases are not compact. Solid UV–vis absorption spec-
tra of photocathodes are shown in Fig. S17. The composite materi-
als have absorption similar to that of pure Bi WO , which is better
than in the previous report [51]. The LSV curves show that the
composite materials with a ratio of 3:1 and 2:1 for KCl/Na WO
have higher photocurrents than BiOCl and the composite materials
with a ratio of 1:1 and 1:2 for KCl/Na WO have lower photocur-
rents than BiOCl at bias potentials from ꢀ0.9 to ꢀ1.2 V
Fig. S18a). Their Mott–Schottky plots suggest that BiOCl and Bi
WO are n-type semiconductors (Fig. S18b, d). The conduction
band position of the composite material (Fig. S18c) is between
BiOCl and Bi WO , like a simple mixture and different from a
2 4
WO
(
2
6
2
6
2
4
2
4
(
2
-
6
2
6
heterojunction. The formation rates of liquid products are depicted
in Fig. 8d and show that methanol was the major product in the
2 6
Scheme 1. A proposed mechanism for the PEC cell with Bi WO /BiOCl
heterojunction.
composite material. When composite BiOCl–Bi
a photocathode, its apparent quantum efficiency was reduced to
.24% due to higher Rct (Fig. S19) leading to lower mobility of
electrons.
2 6
WO was used as
3
.5. Proposed mechanism of photoelectrocatalytic system
0
According to our experimental results for the PEC cell, a possi-
ble mechanism for PEC CO reduction to chemical fuels is proposed
and illustrated in Scheme 1. The electrons in the ground state of
Bi WO and the dye molecules could jump to the excited state
2
3.4. Comparison with the literature
2
6
under light irradiation. The generated photoelectrons transfer to
the CB band of BiOCl through the heterojunction. Then the photo-
electrons can combine with protons to generate active hydrogen
2 6
For better understanding of the advantages of new Bi WO /
BiOCl heterojunctions, the physical properties of BCW-6 are com-
pared with the literature (Table 1). We can see that they demon-
strate excellent absorption of solar light, small Rct, and four XPS
peaks of Bi4f that clearly differ from those for the reported hetero-
junctions. To the best of our knowledge, this is the first attempt to
atoms, which can immediately reduce CO
while, hydroxyl groups (OH ) transfer to the photoanode of BiVO
2
to hydrocarbon. Mean-
ꢀ
4
2
and are oxidized to O .
apply the Bi
BCW-6 heterojunction yields ethanol at a rate of 600
and methanol at a rate of 151
2 6 2
WO /BiOCl heterojunction in CO reduction. The
ꢀ1
ꢀ1
l
mol h
g
4. Conclusions
ꢀ1
ꢀ1
l
mol h
g . The total rate of
carbon-based products is 12.4–606.8 times higher than in the liter-
ature (Table 2). This result is really superior to the reported results,
due to the excellent properties of new heterojunctions.
In summary, in situ grown Bi
designed, prepared, and applied to PEC CO
time. These new heterojunctions of BCW-X electrodes in the PEC
2
WO
6
/BiOCl heterojunctions were
2
reduction for the first

2
16
J. Wang et al. / Journal of Catalysis 377 (2019) 209–217
cell BCW-X|KHCO
vert water and CO
3
|BiVO
4
demonstrate an excellent ability to con-
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