Ultra-fast construction of CuBi2O4 films supported Bi2O3 with dominant (0 2 0) facets for efficient CO2 photoreduction in water vapor

Article abstract of DOI:10.1016/j.jallcom.2021.161919

CuBi₂O₄/Bi₂O₃ thin film was synthesized on the commercial glass by a spray pyrolysis-calcination method. The monoclinic phase Bi₂O₃ with dominant (0 2 0) facets was grown on the surface of tetragonal phase CuBi₂O₄ by the temperature control of spraying process. Photocatalytic activities of the synthesized materials for CO₂ reduction were measured in the presence of water vapor under visible light irradiation (λ > 400 nm). The CO, CH₄ and O₂ yields of the optimal composite film reached 247.62, 119.27 and 418.00 μmol/m² after 12 h of irradiation. The composite film resisted physical damage and showed good photocatalytic activity in the cycling tests. Moreover, it was found that the types of main products changed with the light intensity and their yields varied with the light wavelength. The exposed (0 2 0) facets efficiently improved the adsorbed ability for H₂O molecules. Meanwhile, the hydrophobicity of the film surface ensured that the adsorbed sites of CO₂ were unoccupied by abundant H₂O molecules. The S-scheme charge transfer mechanism was further confirmed by the interlaced band alignment of the CuBi₂O₄/Bi₂O₃ heterostructure and the controlled experiment with different light conditions. The results gained in this report may open up an avenue to design advanced S-scheme heterostructures with suitable transitional-metal oxides for photoreduction CO₂ to solar fuels.

Full text of DOI:10.1016/j.jallcom.2021.161919

Journal of Alloys and Compounds 890 (2021) 161919
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Ultra-fast construction of CuBi2O4 films supported Bi2O3 with dominant
(0 2 0) facets for efficient CO2 photoreduction in water vapor
Weina Shia,c, Ji-Chao Wangb,⁎, Xiaowei Guoa,c, Hong-L⁎ing Tianb, Wanqing Zhangb,
Huiling Gaob, Huijuan Hanb, Renlong Lib, Yuxia Houb,
a School of Chemistry and Materials Engineering, Xinxiang University, Xinxiang 453000, China
b College of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453000, China
c Henan Photoelectrocatalytic Material and Micro-Nano Application Technology Academician Workstation, Xinxiang University, Xinxiang 450003, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 May 2021
Received in revised form 6 August 2021
Accepted 6 September 2021
Available online 11 September 2021
CuBi2O4/Bi2O3 thin film was synthesized on the commercial glass by a spray pyrolysis-calcination method.
The monoclinic phase Bi2O3 with dominant (0 2 0) facets was grown on the surface of tetragonal phase
CuBi2O4 by the temperature control of spraying process. Photocatalytic activities of the synthesized ma-
terials for CO2 reduction were measured in the presence of water vapor under visible light irradiation
(λ
> 400 nm). The CO, CH4 and O2 yields of the optimal composite film reached 247.62, 119.27 and
418.00 μmol/m2 after 12 h of irradiation. The composite film resisted physical damage and showed good
photocatalytic activity in the cycling tests. Moreover, it was found that the types of main products changed
with the light intensity and their yields varied with the light wavelength. The exposed (0 2 0) facets effi-
ciently improved the adsorbed ability for H2O molecules. Meanwhile, the hydrophobicity of the film surface
ensured that the adsorbed sites of CO2 were unoccupied by abundant H2O molecules. The S-scheme charge
transfer mechanism was further confirmed by the interlaced band alignment of the CuBi2O4/Bi2O3 het-
erostructure and the controlled experiment with different light conditions. The results gained in this report
may open up an avenue to design advanced S-scheme heterostructures with suitable transitional-metal
oxides for photoreduction CO2 to solar fuels.
Keywords:
Photocatalysis
CO2 reduction
CuBi2O4/Bi2O3
Exposed facets
S-scheme
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
photocatalytic activity for CO2 reduction over semiconductor cata-
lyst can be promoted by controlling photocatalytic structures for
Artificial photosynthesis of converting CO2 and H2O into hydro-
carbon fuel is an eminently promising strategy to cope with fleetly
increased energy demand and greenhouse gas (mainly CO2) emis-
sions [1,2]. Yet, the CO2 hydrogenation process needs a bond dis-
sociation energy of more than 120 kcal/mol, resulting in the high
activation energy in conversion process [3–5]. The CO2 photo-
reduction using semiconductor catalysts is a renewable and pro-
mising technique to produce valuable carbon derivatives [6,7].
However, in the conventional photocatalytic system (liquid-solid
two-phases system), only CO2 dissolved in the water was utilized
because the hydrogen source for the CO2 conversion should be
ideally supplied by water molecules [8]. Due to the poor solubility
and slow diffusion rate of CO2 in water, the competing hydrogen
evolution reaction of water reduction was dominant. Hence, the
direct delivery of gas phase CO2 and H2O to the photocatalytic re-
action interface.
Since the adsorbed CO2 molecules on the catalyst surface can be
rapidly depleted under strong reaction driving force, the supply of
CO2 molecules is usually critical for efficient CO2 photoreduction
[9,10]. Meanwhile, the H2O adsorption and activation on the catalyst
surface can not be omitted in CO2 conversion process, because of the
high overpotential for H2O oxidation reaction [10]. Although porous
nanomaterials usually possess large surface-to-volume ratios, the
surface adsorption sites for H2O molecules are still insufficient,
leading to low concentration and slow diffusivity of H2O molecules
at the catalyst surface [11]. Hence, enhancing adsorption capacity of
reactant molecules on catalyst is conducive to increase the con-
centration and transfer rate of CO2 reduction and H2O oxidation at
the catalyst surface, further boosting CO2 reduction and suppressing
the hydrogen evolution reaction. The influence of exposed crystal
facets of semiconductor catalyst is never ignored [12,13]. Chen et al.
synthesized shell like TiO2 with exposed (001) facets for enhanced
⁎
Corresponding authors.
E-mail addresses: wangjichao@hist.edu.cn (J.-C. Wang), yxhou@163.com (Y. Hou).
https://doi.org/10.1016/j.jallcom.2021.161919
0925-8388/© 2021 Elsevier B.V. All rights reserved.

W. Shi, J.-C. Wang, X. Guo et al.
Journal of Alloys and Compounds 890 (2021) 161919
photocatalytic CO2 reduction in gas-solid system, and proved that
{001} and {101} facets acted as reduction sites and oxidation sites,
respectively [14]. Moreover, the rapid recombination of photo-
induced electron-hole pairs and wide band gap for a single com-
ponent semiconductor limit the application of CO2 photoreduction
[15–17]. The heterojunction construction not only promotes the se-
paration of photo-generated carriers, but also adjusts distribution of
oxidation and reduction sites on the photocatalyst surface [18,19]. In
particular, Step scheme (S-scheme) heterojunction photocatalytic
system is highly recommended due to the promoted separation of
photo-induced electrons and holes without sacrificing their redox
abilities [20–24]. Therefore, the controlled formation of S-scheme
composite with specific exposed facets could realize the adsorption
ability and selectivity for H2O or CO2 molecules of the reaction site
on photocatalyst surface. Additionally, it is always difficult to process
the complete separation of powdered catalyst from catalytic system,
causing second pollution for heterogeneous photocatalytic system
[25,26]. Hence, thin film photocatalyst has gradually been paid great
attention due to its larger surface area and easier recycling
[25,27,28].
CuBi2O4 has recently received scientific interest as a promising
candidate due to its suitable bandgap of 1.5–1.8 eV and negative
conduction band location [29–31], enabling thermodynamically
feasible for photocatalytic CO2 reduction. Nonetheless, photo-
catalytic CO2 reduction with H2O was not achievable for mono-
component CuBi2O4 due to its higher VB potential than the oxidation
potential of O2/H2O. On the basis, heterojunction construction has
been proposed to mitigate the limitation of single CuBi2O4 [32–34].
Bi2O3 has been studied as a promising photocatalyst due to its strong
oxidizability and appropriate band gap for visible light absorption
[32,35,36], enabling hybridization with CuBi2O4 to construct het-
erojunction composites with increased light harvesting property,
prolonged lifetime of carriers and enhanced catalytic performance
[37,38]. Particularly, the CuBi2O4/Bi2O3 composite was reported to
exhibit superior performance for photocatalytic degradation [39].
Nevertheless, the research of CuBi2O4/Bi2O3 film is scarcely focused
on photocatalytic CO2 reduction up till now.
precursors. Additionally, Bi2O3 nanoparticles were obtained from the
bare Bi2O3 film by ultrasound treatment. The physically mixed CBO/
BO-M film was prepared by spraying the alcoholic suspension, which
contained the Bi2O3 and CuBi2O4 particles though scraped off the
glass substrates. In addition, CBO/BO-020-P film was obtained by
physical damage treatment of the CBO/BO-020 sample, and the ex-
periment details were illustrated in Fig. S1.
The CuBi2O4 film supported Bi2O3 without dominant (0 2 0) fa-
cets, abbreviated as CBO/BO, was obtained by the spray pyrolysis
method similar to CBO/BO-020, except that the spray-drying tem-
perature was 150 °C. The contents of metal ions for all of the ob-
tained samples were detected by ICP-AES measurements, and the
real Bi/Cu ratios (Table S1) in the composites were basically con-
sistent with initial inventory ratio.
2.2. Characterization
The crystal structures and phases of the obtained samples were
studied by the D8 Advance powder X-ray diffractometer (Bruker
Technology Co., Ltd.) with Cu Kα radiation (λ = 1.5418 Å). The che-
mical components and valence states were analyzed using the X-ray
photoelectron spectrometer (XPS, PHI5000 VersaProbe, ULVAC-PHI
Inc., Japan). The material morphology was measured by field emis-
sion scanning electron microscope (FE-SEM, NANO SEM430, FEI,
USA). The images of transmission electron microscopy (TEM) were
recorded by the JEM-2100 electron microscope (JEOL Ltd., Japan).
The UV–vis diffuse reflectance spectra (DRS) were obtained using
the UV–vis-NIR spectrometer (Cary 5000, Agilent Technology Co.,
Ltd., USA).
2.3. Photocatalytic performance tests
The photocatalytic performance for CO2 reduction was in-
vestigated in the gas-solid phase catalytic system with 0.16 MPa
pressure. The reactant gas was composed of high purity CO2 gas
(99.999%) and water vapor. A 300 W xenon arc lamp (PLS-SXE300,
Beijing China Eduction Au-Light Co., Ltd., China) with a UV cutoff
filter was adopted as the light source in the photocatalytic reactions.
The gaseous and liquid products were detected by gas chromato-
graph (GC-7890II, Beijing China Eduction Au-Light Co., Ltd., China)
and GC-MS (7890A-5975C, Agilent Technology Co., Ltd., USA), re-
spectively. The isotopes tracer experiments were performed in a
sealed reactor. The 13CO2 or H218O reactants were added into the
sealed reactor. After UV light irradiation of 18 h, the gas mixture in
container was measured by GC-MS (7890A-5975C and MAT 271).
The experiment process was detailed in Supporting Information S1.
Photoelectrochemical test and electrochemical impedance spectro-
scopy (EIS) measurement were performed in a conventional three-
electrode system using the electrochemical workstation (CHI 660E,
Shanghai Chenhua Co., Ltd., China). The H2O2 capture tests for
photocatalyst were also detailed in Supporting Information S2.
In this study, we synthesized a series of CuBi2O4/Bi2O3 composite
films on the glass substrates by the spray pyrolysis method. The
influence of the spraying temperature on the growth of Bi2O3 na-
noparticle in composite was studied. Photocatalytic activities of CO2
reduction over the CuBi2O4/Bi2O3 films were investigated in the
presence of water vapor. Compared to the bare materials, the as-
prepared CuBi2O4/Bi2O3 composites were found to exhibit highly
improved photocatalytic performance, and the selectivity was able
to be tuned by modulating the light intensity. The enhanced catalytic
activity was attributed to efficient charge separation originated from
the staggered band structure, and a S-scheme charge transfer me-
chanism was further proposed and confirmed.
2. 2. Experimental section
2.1. Synthesis of CuBi2O4, Bi2O3 and CuBi2O4/Bi2O3 films
3. Results and discussion
The commercial glass substrates were cleaned by ethanol and
deionized water before use. CuBi2O4, Bi2O3 and CuBi2O4/Bi2O3 films
were deposited on the substrates by the spray pyrolysis method. In
the typical synthesis of CuBi2O4 films supported Bi2O3 with domi-
nant (0 2 0) facets, two precursor solutions were prepared by dis-
solving 0.4484 g Bi(NO3)3·5H2O in 2 mL acetic acid and 0.0960 g Cu
(NO3)2·3H2O in 38 mL ethanol, respectively. The solutions were then
mixed and sprayed onto the glass substrates at 260 °C, and the final
sample was annealed at 500 °C for 2 h. The obtained composite film
was abbreviated as CBO/BO-020.
3.1. Structure, morphology and composition
Fig. 1 showed the XRD patterns of the obtained samples. The
distinct peaks of CuBi2O4 sample at 20.8°, 27.9°, 30.7°, 33.2°, 37.4°,
46.6° and 52.9° were corresponding to (2 0 0), (2 1 1), (0 0 2), (3 1 0),
(2 0 2), (4 1 1) and (2 1 3) facets of tetragonal phase CuBi2O4 (JCPDs
No: 48-1886), respectively. Besides, the diffraction peaks of bare
Bi2O3 sample indicated the formation of monoclinic phase (JCPDs
No: 14-0699). In addition to the peaks originated from CuBi2O4 in
the composites, the other characteristic peaks at 25.8°, 26.9°, 27.4°,
33.1° and 33.2° were indexed to (0 2 0), (1 1 1), (−1 2 0), (1 2 1) and
(2 0 0) crystal facets of Bi2O3. The peak intensity at 21.7° in the CBO/
For comparison, bare CuBi2O4 and Bi2O3 films were obtained by
the similar spray pyrolysis procedure using the corresponding
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W. Shi, J.-C. Wang, X. Guo et al.
Journal of Alloys and Compounds 890 (2021) 161919
(0 2 0) crystal facet of Bi2O3 grew well in the presence of CuBi2O4.
For further exploring the effect of the growth of (0 2 0) facets, a
series of composite samples with different spray-drying tempera-
tures were obtained by the similar method. According to the XRD
results (Fig. S2), with increasing the spray-drying temperature, the
peak intensity of (0 2 0) facets in the composites gradually increased.
Therefore, the spray-drying temperature influenced facet growth of
the precursor, leading to the development of (0 2 0) facet of Bi2O3 in
the composites.
The composition and valance states of the obtained samples
were studied by XPS measurements. The high-resolution spec-
trum of Cu 2p was shown in Fig. 2a, and the two distinct peaks
centered at 934.1 eV and 954.1 eV were corresponding to Cu 2p3/2
and Cu 2p1/2 of Cu(II), respectively [38,40]. Besides, the satellite
peaks were distinctly observed due to a d9 configuration in the
ground state of Cu(II)-based oxides [41]. The deconvolution result
of Bi 4f core level spectrum was shown in Fig. 2b. Two obvious
peaks at binding energies of 159.4 eV and 163.6 eV were assigned
to Bi 4f7/2 and Bi 4f5/2, which were in consistent with those of Bi
(III)-based oxides [33,41,42]. Additionally, the deconvolution
peaks of O 1s spectrum (Fig. 2c) were resolved into two compo-
nents originated from lattice oxygen and surface oxygen defects
[29,38,43]. It was noteworthy that the peaks of bare Bi2O3 and
CuBi2O4 samples were slightly shifted compared with those of the
CBO/BO-020 composite, which indicated the successful formation
of CBO/BO-020 heterojunction [38].
Fig. 1. XRD patterns of the obtained bare and composite samples.
BO-020 sample was obviously stronger than those of CBO/BO and
bare Bi2O3 samples. By measuring the respective peak areas of the
(0 2 0), (−1 2 0), (1 2 1) and (2 0 0) facets, the fraction of (0 2 0)
facets from XRD patterns were estimated by the following equation
(Eq. (1)):[12].
w(0 0) = [ A(0
/
A(0
+
A(
+
A(1
+
A(2
]
0)
2
2
0)
2
0)
1
2 0)
2
0)
0
The cross sectional images of the CuBi2O4, Bi2O3 and CBO/BO-020
samples were shown in Fig. 3a–c. The height of the thin film was
about 1.1 µm, and the Cu, Bi, and O elements were homogenously
distributed as indicated in the EDS mapping images. Moreover, the
(1)
The calculated percentage of (0 2 0) facets for CBO/BO and CBO/
BO-020 were about 2.2% and 36.8%, respectively. It indicated that the
Fig. 2. XPS spectra (a. Cu 2p, b. Bi 3p and c. O 2p) of the CuBi2O4, Bi2O3 and CBO/BO-020 samples.
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W. Shi, J.-C. Wang, X. Guo et al.
Journal of Alloys and Compounds 890 (2021) 161919
Fig. 3. SEM images of (a, d) Bi2O3, (b, e) CuBi2O4 and (c, f) CBO/BO-020 samples and TEM image (g, h) of CBO/BO-020 sample.
morphology of pore structure was observed for all the samples, and
the surface area of CBO/BO-020 was slightly higher than those of
Bi2O3 and CuBi2O4 (Table S2). The morphology and composition of
the obtained samples were shown in Fig. 3d–h. All the films were
composed of uniformly dispersed nanoparticles, and the particle size
of Bi2O3 (Fig. 3d) was smaller compared with those of CuBi2O4
(Fig. 3e) and CBO/BO-020 (Fig. 3f). The TEM image of CBO/BO-020 in
Fig. 3g indicated that the interplanar spacings of 0.165 and 0.240 nm
corresponded to (3 3 2) and (2 0 2) facets of tetragonal phase
CuBi2O4, and the interplanar spacings of 0.253, 0.243 and 0.407 nm
were assigned to (0 3 1), (1 3 0) and (0 2 0) facets of monoclinic
phase Bi2O3. Fast-Fourier Transform (FFT) diffraction pattern (inset
of Fig. 3g) of the selected dotted bordered rectangle region con-
firmed the existence of tetragonal CuBi2O4 and monoclinic Bi2O3,
which were consistent with XRD results. Hence, the TEM observa-
tions and FFT pattern of the CBO/BO-020 composite revealed that
both CuBi2O4 and Bi2O3 were present and fused at the interfaces. The
HAADF-STEM image and corresponding EDX mapping of CBO/BO-
020 in Fig. 3h presented uniform distribution of Bi and O elements in
the composite. Cu element only appeared on the surface of larger
particles, and it was concluded that Bi2O3 was successfully deposited
on the CuBi2O4 surface. Combined with the SEM and XRD results, it
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W. Shi, J.-C. Wang, X. Guo et al.
Journal of Alloys and Compounds 890 (2021) 161919
Fig. 4. DRS spectra (a) and plots of (αhν)2 versus hν (b) of the obtained samples.
was concluded that smaller Bi2O3 particles were successfully de-
posited on the CuBi2O4 surface. The rough surface of the composite
film was formed by two kinds of nanoparticles, which further leaded
to specific hydrophilia ability of the composite photocatalyst.
Fig. 4a showed the UV-Vis light absorption spectra of the ob-
tained samples. All the obtained composite samples and CuBi2O4
sample exhibited broad photoresponse range of up to 650 nm. The
direct band gaps (Eg) of CuBi2O4 and Bi2O3 samples estimated by the
Kubelka-Munk function (Fig. 4b) were 1.86 and 2.87 eV, respectively,
agreeing well with previously reported values [44,45].
intensity of (0 2 0) facets for Bi2O3 exhibited inferior photocatalytic
activity, and the CO, CH4 and O2 yields of CBO/BO-020 reached
127.13, 70.10 and 231.95 μmol/m2, respectively. The yields of CBO/
BO-020 for CO2 reduction were higher than those in the previous
studies of Bi2O3 based materials (Table S3). To exclude the possibility
of the produced fuels generated from other synthesis residues, 13C
and 18O labeled isotopic tests were implemented by substituting
12CO2/H216O with 13CO2/H218O. The results of GC-MS tests for the
products were presented in Fig. 5b. Besides, the CO and CH4 yields of
CBO/BO-020-M obviously decreased compared with those of CBO/
BO-020, indicating that the loosely contacted interfaces were in-
sufficient for photocatalytic CO2 reduction. It indicated that the
formation of heterojunction promoted the photocatalytic activity for
CO2 reduction [18]. Additionally, the CO, CH4 and O2 yields of CO2
reduction over CBO/BO-020-P sample still reached 132.21, 66.05 and
201.11 μmol/m2, respectively, indicating that the CBO/BO-020 com-
posite film on the glass resisted physical damage, which was valu-
able in photocatalytic application. Hence, the CBO/BO-020
composite photocatalyst exhibited satisfactory performance for
photocatalytic CO2 reduction with water vapor. The influences of
light wavelength and intensity on photocatalytic activity and kinds
3.2. Photocatalytic performance
The photocatalytic performance for CO2 reduction with water
vapor was studied under visible light illumination. No products were
detected for CO2 reduction without photocatalyst or illumination in
the catalytic system. As shown in Fig. 5a, no product was detected
for bare Bi2O3, and a small quantity of CO was produced over
CuBi2O4 sample. Moreover, all the composites exhibited higher
photocatalytic activities of CO2 reduction than those of bare Bi2O3
and CuBi2O4. Particularly, the CBO/BO-020 sample with high
Fig. 5. Product yields over the composites for photocatalytic CO2 reduction with water vapor under visible-light irradiation (a: A. Bi2O3, B. CuBi2O4, C. CBO/BO, D. CBO/BO-020, E.
CBO/BO-020-M and F. CBO/BO-020-P) and Mass spectra of 13CO, 13CH4 and 18O2 products over the CBO/BO-020 photocatalyst for CO2 photoreduction (b).
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Journal of Alloys and Compounds 890 (2021) 161919
Fig. 6. Total yields (a) and durability test (b) of CBO/BO-020 for photocatalytic CO2 reduction with water vapor (each cycle illuminated for 7 h).
of products for CO2 reduction were also investigated. In Table S4, CO
and CH4 yields over the CBO/BO-020 catalyst changed slightly when
the light wavelength range was widened. However, with increase of
light intensity, the product yields of CO2 reduction obviously in-
creased and the molar ratio of CH4/CO was improved. CO was the
main product at 320 mW/cm2, and the total yield increased sig-
nificantly in comparison with those at 80, 230 and 230 mW/cm2.
Besides, the minor product of CH4 was also observed with the light
intensity of above 230 mW/cm2. The results suggested that the
photocatalytic properties of the photocatalyst were greatly related to
the intensity of irradiation light, which was consistent with the re-
sults in previous study [46]. Based on the above results, the product
yields were mainly dependent on light intensity for photocatalytic
CO2 reduction.
Fig. 6a showed the product yields for CO2 reduction over CBO/
BO-020 sample with visible-light illumination time prolonging. The
CO, CH4 and O2 yields of CBO/BO-020 continually increased, and the
yield rates reached 20.64, 9.94 and 34.83 μmol/m2/h under light ir-
radiation for 12 h. The photocatalytic stability of the typical CBO/BO-
020 catalyst was measured by a five-time durability test, and the
results were shown in Fig. 6b. The yields of CO and CH4 slightly
decreased and reached 167.03 and 96.44 μmol/m2 after five cycles.
XRD and XPS measurements were further conducted to study the
stability of photocatalyst after the durability test. As shown in
Figs. S3 and S4, there were no obvious differences in the crystal
structure and chemical state between the fresh and used photo-
catalysts. However, the O2 yield obviously decreased. According to
the results of the H2O2 capture tests (Fig. S5), the H2O2 molecules
were formed and remained on the surface after illumination. It
implied that the oxidation site on the photocatalyst may be occupied
by the intermediate products and then resulted in its deactivation.
The used CBO/BO-020 catalyst was regenerated by sodium sulfite
treatment. It was notable that the products yields of regenerated
catalyst obviously increased. Hence, the decrease photocatalytic
activity was caused by the toxicity of H2O2 molecules for the active
sites on the composite surface.
Fig. 7. Transient photocurrent responses of Bi2O3, CuBi2O4, CBO/BO and CBO/BO-020
catalysts.
on-off irradiation cycles. The transient photocurrent densities of
composites were about 3 and 9 times higher than those of CuBi2O4
and Bi2O3, respectively. Hence, the formation of heterojunction
played a key role in promoting the separation of photo-generated
electrons and holes.
For exploring the band structure of CuBi2O4 and Bi2O3, the VB-
XPS measurement was investigated and the result was shown in
Fig. 8. The VB values of CuBi2O4 and Bi2O3 were calculated by the
linear intersection method and reached 1.05 and 3.02 eV, respec-
tively [46]. According to the Eg values of the obtained samples
(Fig. 4b), the CB positions of CuBi2O4 and Bi2O3 were determined to
be −0.81 and 0.15 eV, respectively. The staggered Type II band
alignment was thus formed in the CuBi2O4/Bi2O3 composite.
Based on the band energy alignment of the CuBi2O4 and Bi2O3
composite (Fig. 9), the p-n heterojunction was established by tight
contact between the Bi2O3 and CuBi2O4 components, and an inner
electrical field was formed at the interface, facilitating the transfer
of photo-induced carriers. In the case of the common double-
charge transfer mode in the heterojunction (Fig. 9a), the
3.3. Photocatalytic mechanism
As shown in Fig. 7, the transient photocurrent responses of Bi2O3,
CuBi2O4, CBO/BO and CBO/BO-020 were measured over several
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W. Shi, J.-C. Wang, X. Guo et al.
Journal of Alloys and Compounds 890 (2021) 161919
accumulated electrons in the CB of Bi2O3 with insufficient reduc-
tion ability would not drive CO2 reduction due to the more positive
redox potentials of CO2/CO (− 0.53 V) and CO2/CH4 (− 0.24 V) [47],
which was inconsistent with the experimental results. According to
the above photocatalytic results, CO was produced over CBO/BO-
020 under irradiation of 400–800 nm with light intensity of
80 mW/cm2 (Table S4). The control experiment was carried out
under the similar condition except for the light source with wa-
velength range of 635–665 nm. Since the visible light solely excited
CuBi2O4 in the composite, and the photogenerated electrons would
transfer from the CB of CuBi2O4 to that of Bi2O3 for CO2 reduction.
Yet, few CO product was detected. Based on the promoted photo-
catalytic performance, a S-scheme mechanism was proposed and
illustrated in Fig. 9b. Photo-induced electrons in the CB of Bi2O3
recombined with photo-induced holes in the VB of CuBi2O4
(Fig. 9b), and the composite retained strong reduction ability of
electrons in the CB of CuBi2O4, and CO2 can be reduced to CO and
CH4 readily. Meanwhile, the photoinduced holes in the VB of Bi2O3
easily realized H2O oxidation [48]. Consequently, the S-scheme
heterojunction was formed in the CuBi2O4/Bi2O3 composite
photocatalyst.
Fig. 8. VB-XPS spectra of the obtained CuBi2O4 and Bi2O3 samples.
Although the photocurrent density of CBO/BO was similar with
that of CBO/BO-020, the CBO/BO-020 sample exhibited higher pho-
tocatalytic performance. The exposed (0 2 0) facets of Bi2O3 may play
a key role for photocatalytic process of CO2 reduction with water
vapor. As shown in Fig. 10a, the H2O adsorption ability of CBO/BO-
020 was distinctly stronger than that of CBO/BO, because of the
exposed (0 2 0) facets of Bi2O3 composition. Combined with the
analysis of the electron transfer in heterojunction, it was thus
speculated that H2O oxidation reaction occurred on the Bi2O3 surface
in the composite. Yet, as shown in Fig. 10b and c, the wetting angles
of CBO/BO-020 and CBO/BO reached about 133° and 118°, respec-
tively, due to the porous structure on the films surface, and all
composites exhibited stronger hydrophobicity. Although abundant
H2O molecules were adsorbed on the surface of the CBO/BO-020
sample, the adsorbed sites of CO2 on the surface were unoccupied by
the stronger hydrophobicity [8]. Hence, the exposed (0 2 0) facets of
Bi2O3 in the composites improved the adsorbed property for H2O
molecules on the photocatalyst surface, leading to enhanced CO2
photoreduction performance.
Fig. 9. Schematic diagram for the common transfer mode (a) and S-scheme transfer
mode (b) of photo-induced carrier in the CuBi2O4/Bi2O3 composites.
Fig. 10. H2O adsorption curves (a) and wetting angle images (b and c) of the CBO/BO-020 and CBO/BO samples.
7

W. Shi, J.-C. Wang, X. Guo et al.
Journal of Alloys and Compounds 890 (2021) 161919
4. Conclusion
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In summary, a series of composite films containing of monoclinic
Bi2O3 with dominant (0 2 0) facets and tetragonal CuBi2O4 were
synthesized on the surface of commercial glasses by a spray pyr-
olysis-calcination method. The temperature control of spraying
process influenced the growth of Bi2O3 particle on the CuBi2O4
surface. The CO, CH4 and O2 yields of the optimal CuBi2O4/Bi2O3
composite reached 247.62, 119.27 and 418.00 μmol/m2 after 12 h of
visible-light irradiation for CO2 reduction, and the photocatalyst
showed good photocatalytic activity after 5 cycles. The type of main
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of Bi2O3 improved the adsorbed ability of H2O molecules, enhancing
photocatalytic performance for CO2 reduction in water vapor.
Meanwhile, the stronger hydrophobicity of the film surface could
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occupied by abundant H2O molecules. The S-scheme charge transfer
mode of CuBi2O4/Bi2O3 was further proposed for photocatalytic CO2
reduction. This work may provide a promising S-scheme CuBi2O4/
Bi2O3 film photocatalyst for photocatalytic CO2 reduction, and
meaningful information for hopefully stimulating more insightful
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Weina Shi: Conceptualization, Investigation, Resources,
Writing – review & editing, Project administration. Ji-Chao Wang:
Conceptualization, Methodology, Resources, Data Curation, Writing
– original draft, Supervision, Funding acquisition. Xiaowei Guo:
Methodology, Software, Formal analysis. Hong-Ling Tian:
Validation, Formal analysis. Wanqing Zhang: Software, Data
Curation, Investigation. Huiling Gao: Resources, Software. Huijuan
Han: Validation, Visualization, Funding acquisition. Renlong Li:
Writing – review & editing, Funding acquisition. Yuxia Hou: Formal
analysis, Investigation, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
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Acknowledgements
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photocatalytic materials, Chin. J. Catal. 42 (2021) 710–730, https://doi.org/10.
1016/S1872-2067(20)63698-1
This work was supported by the financial supports of National
Natural Science Foundation of China (Nos. 51802082 and 51903073),
Natural Science Foundation of Henan Province (No. 212300410221),
Program for Science & Technology Innovation Talents in Universities
of Henan Province (No. 21HATIT016), Key Scientific Research Project
of Colleges and Universities in Henan Province (No. 21A430030 and
20A150017), Key Scientific and Technological Project of Henan
Province (No. 212102210473 and 202102310595) and “Climbing”
Project of Henan Institute of Science and Technology (No.
2018CG04).
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Appendix A. Supporting information
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.jallcom.2021.161919.
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