Selective Deposition of Cobalt and Copper Oxides on BiVO4 Facets for Enhancement of CO2 Photocatalytic Reduction to Hydrocarbons

Article abstract of DOI:10.1002/cctc.201901115

The nanostructure of a semiconductor is a crucial parameter for efficient photocatalytic performance. Hereby, we have synthesized monoclinic bismuth vanadate (BiVO₄) crystals with controlled ratio of {010} and {110} facets. The selective deposition of CuO_x and CoO_x over {010} and {110} facets has been performed using photo-reduction and photo-oxidation methods, respectively. It resulted in a highly efficient charge separation of photogenerated electrons and holes and enhancement of the photocatalytic reduction of CO₂ by H₂O into hydrocarbons, including CH₄, C₂H₆ and C₃H₈. The controlled co-catalyst deposition over different semiconductor facets provides a strategy for the synthesis of hydrocarbons from CO₂ and H₂O by efficient charge separation.

Full text of DOI:10.1002/cctc.201901115

Accepted Article
Title: Selective deposition of cobalt and copper oxides on BiVO4
facets for enhancement of CO2 photocatalytic reduction to
hydrocarbons
Authors: Xiang Yu, Vitaly Ordomsky, and Andrei Khodakov
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Selective deposition of cobalt and copper oxides on BiVO4 facets for
enhancement of CO2 photocatalytic reduction to hydrocarbons
Dr. Xiang Yua, Dr. Vitaly V. Ordomskya, b* and Dr. Andrei Y. Khodakova*
aUniv. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité de
Catalyse et Chimie du Solide, F-59000 Lille, France
bEco-Efficient Products and Processes Laboratory (E2P2L), UMI 3464, CNRS-Solvay,
201108 Shanghai, People’s Republic of China
Corresponding authors: vitaly.ordomsky@univ-lille.fr, andrei.khodakov@univ-lille.fr
Abstract
The nanostructure of a semiconductor is a crucial parameter for efficient photocatalytic
performance. Hereby, we have synthesized monoclinic bismuth vanadate (BiVO4) crystals
with controlled ratio of {010} and {110} facets. The selective deposition of CuOx and CoOx
over {010} and {110} facets has been performed using photo-reduction and photo-oxidation
methods, respectively. It resulted in a highly efficient charge separation of photogenerated
electrons and holes and enhancement of the photocatalytic reduction of CO2 by H2O into
hydrocarbons, including CH4, C2H6 and C3H8. The controlled co-catalyst deposition over
different semiconductor facets provides a strategy for the synthesis of hydrocarbons from CO2
and H2O by efficient charge separation.
1. Introduction
Carbon dioxide is widely recognized as a major greenhouse gas that causes global
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environmental problems[1]. Worldwide consumption of limited fossil fuels not only accelerates
their depletion but also results in production of excessive atmospheric CO2. In this regard,
transformation of CO2 to the fuels is a promising method for simultaneous solution of energy
and environmental problems[2]. In 1979, Japanese researcher Fujishima and Honda reported
[3]
for the first time the photocatalytic reduction of CO2 into organic compounds using
semiconductor catalysts such as SiC, GaP, and TiO2. Since then, numerous works have
focused on the development of new semiconductor materials for reduction of CO2 [4-7]. Various
semiconductors were proven to be effective in photocatalytic reduction of CO2, for example:
TiO2 [8, 9], Ga2O3 [10, 11], Zn2GeO4 [12, 13], WO3 [14], Bi2WO6 [15, 16] and so on. The main products
of photocatalytic reduction of CO2 by water are usually methane, higher hydrocarbons,
methanol, ethanol, formaldehyde, formic acid or CO [7].
Although a lot of progress has been made in this field, the activity of photocatalytic
reduction of CO2 is still insufficient due to poor charge separation and low quantum efficiency.
In addition, photocatalytic transformation of CO2 is limited due to low adsorption over the
catalyst and difficulties in the activation of the stable CO2 molecule.
In the photocatalytic reaction, the difference in the morphology of the catalyst often has
a significant impact on the photocatalytic performance [17]. The difference in catalyst
morphology is mainly reflected by the exposure of different crystal facets, or the proportion
of exposed crystal facets. In recent years, more and more reports have addressed the effects
of exposed crystals on photocatalytic properties. It is generally believed that photocatalysts
exhibit better catalytic properties when more active crystal facets are exposed to reacting
molecules [18, 19]. In many classical photocatalytic semiconductors, it has been shown that
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controlling the exposure of a suitable crystal facets is an effective strategy to improve
[27]
photocatalytic performance, for example: TiO2 [20, 21], BiVO4 [22-24], Cu2O [25, 26] and CeO2
.
In photocatalytic reactions, the high recombination rate of photogenerated electrons and holes
is a major factor limiting its photocatalytic performance. The recombination rate of
photogenerated electrons and holes can be greatly reduced with increase of activity by
deposition of co-catalysts over semiconductors. In general, the role of co-catalyst can be
relevant to the following processes: (1) better charge separation, (2) higher activity and
selectivity of CO2 reduction by suppression of side reactions and (3) enhanced stability of
photocatalyst [28]
.
Bismuth vanadate (BiVO4) is a promising visible light-driven semiconductor
photocatalyst with low production cost, low toxicity, high stability and narrow band gap (2.4
eV) with good response to visible light excitation. BiVO4 has demonstrated as efficient
catalyst for the reduction of CO2 into methanol and ethanol [29, 30]. However, the photocatalytic
efficiency of BiVO4 is low because of relatively low visible-light utilization and inefficient
charge separation. The formation of Z‐scheme photocatalytic systems that mimic natural
photosynthesis with photocatalytic systems and electron mediator is a promising strategy to
improve charge separation [31]. According to a few reports, the photocatalytic efficiency can
be improved by Z-scheme on the BiVO4 catalyst [32-34]
.
Herein, the separation of electrons and holes in bismuth vanadate (BiVO4) is improved
by control of the semiconductor morphology. The photocatalytic reduction of CO2 under
irradiation is significantly enhanced by selective photo-deposition of Cu and Co co-catalysts
over different facets providing Z-scheme charge flow.
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2. Experimental
2.1 Synthesis of monoclinic BiVO4 crystal with controllable exposed facets
Monoclinic BiVO4 crystal was synthesized by a hydrothermal method. In brief, the
precursors NH4VO3 (0.015 mol) and Bi(NO3)3·5H2O (0.015 mol) were dissolved in an
aqueous nitric acid solutions (2 M), and the pH value of the solution was adjusted to 2.0 with
ammonia solution (14.84 M) under continuous stirring. An orange precipitate appears over
time. After that, NaCl was added into the suspension and stirring was continued for 30 minutes.
After about 5 h aging, the suspension was transferred to a Teflon-lined stainless steel autoclave
with a capacity of 100 mL and treated under hydrothermal conditions at 473 K for 24 h. After
the autoclave was cooled down to room temperature, the yellow powdery sample was
separated by filtration, washed with deionized water for several times, and then dried at 353
K in air for overnight. Finally, the sample was calcined at 773 K in air for 2 h. The samples
were denoted as BiVO4-x M, and x represents the concentration of NaCl in the suspension.
2.2 Synthesis of monoclinic BiVO4 crystal with predominantly exposed {010} and {110} facets
Synthesis of BiVO4-010
The precursor Bi(NO3)3·5H2O (0.015 mol) was dissolved into an aqueous nitric acid
solutions (4 M). And the precursors NH4VO3 (0.015 mol) was dissolved into another aqueous
sodium hydroxide solution (2 M). Afterwards, sodium dodecyl benzene sulfonate (0.002 mol)
was added to the above solutions separately. After stirring for 30 minutes, the above two
solutions were mixed under continuous stirring. The pH value of mixed solutions was adjusted
to 7.0 with sodium hydroxide solution (2 M). After about 1 h stirring, the mixed solution was
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transferred into Teflon-lined stainless steel autoclave and hydrothermally treated at 473 K for
1.5 h. The yellow powdery was recovered by filtration, washed with deionized water for
several times, and then dried at 353 K in air for overnight. This sample was denoted as BiVO4-
010.
Synthesis of BiVO4-110
The BiVO4-110 sample was synthesized using solid-liquid state reaction by mixing equal
amount of Bi2O3 (0.005 mol) and V2O5 (0.005 mol) in an aqueous nitric acid solution (1 M).
The suspension was stirred for four days at room temperature. The yellow powdery was
separated by filtration, washed with deionized water for several times and dried at 353 K in
air for overnight. This sample was denoted as BiVO4-110.
2.3 Selective photo-deposition of metal oxides in monoclinic BiVO4 crystal
Photo-reduction and photo-oxidation procedures have been used for deposition of co-
catalysts. 1.0 g BiVO4 powder and a calculated amount of metal precursors were added in 40
mL deionized water containing a sacrificial agent. After about 20 minutes ultrasonic
dispersion, the aqueous suspension under irradiation with a 300 W Xe lamp for 5 h under
continuous stirring with the protection of nitrogen. Then the suspension was filtered, washed
with deionized water for several times and dried at 353 K in air for overnight.
5 wt% of metal in the form of metal oxides (WOx, FeOx, ZnOx, CuOx and CoOx) were
deposited over BiVO4-0.20M by photo-reduction procedure using 20 wt% of methanol as
sacrificial agent and (NH4)10(H2W12O42)·4H2O, Fe(NO3)3·9H2O, Zn(NO3)2·6H2O and
Cu(NO3)2·3H2O as precursors, respectively. The obtained samples were denoted as
WOx/BiVO4, FeOx/BiVO4, ZnOx/BiVO4 and CuOx/BiVO4.
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5 wt% of metal in the form of metal oxides MnOx, PbO2 and CoOx have been deposited
over CuOx/BiVO4 by photo-oxidation technique using NaIO3 (0.1 M) as the electron acceptor
and Mn(NO3)2·xH2O, Pd(NO3)2·xH2O and Co(NO3)3·6H2O as precursor, respectively. The
obtained samples were denoted as MnOx/CuOx/BiVO4, PbO2/CuOx/BiVO4 and
CoOx/CuOx/BiVO4.
For comparison, the reference sample CoOx/CuOx/BiVO4(imp) has been synthesized by
impregnation of Cu(NO3)2·3H2O and Co(NO3)3·6H2O over BiVO4-0.20M with subsequent
calcination at 673 K.
2.4 Characterization.
The X-ray powder diffraction (XRD) experiments were conducted using a Bruker AXS
D8 diffractometer with Cu Kα radiation (λ = 0.1538 nm). The XRD patterns were collected
in the 5−80° (2θ) range. The diffuse reflectance UV-visible spectra of the catalysts were
recorded on a Perkin-Elmer Lambda 650 S UV/VIS spectrometer equipped with an integrating
sphere covered with BaSO4 as a reference.
The textural properties of the samples were studied by N2 physisorption on a
Micromeritics Tristar 3020 apparatus.
The photoluminescence spectroscopy measurements were performed on a LabRam HR
(Horiba scientific). For excitation, 532 nm radiation from a diode-pumped solid-state 300 µW
laser was used. The spectrophotometer has an entrance slit of 100 µm, and is equipped with a
300 lines/mm-1 grating that permits to achieve a spectral resolution of 3.8 cm-1 per pixel. The
luminescence light was detected with a CCD camera operating at 138 K.
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The SEM measurements were carried out using JEOL scanning electron microscope with
30 Kv accelerating voltage.
The XPS spectra were registered using a Kratos Axis spectrometer, equipped with an
aluminum monochromater for a 1486.6 eV source working at 120 W. All spectra were
recorded under a vacuum of 10-8 Torr and recalibrated afterwards with the binding energy of
the Al 2p at 74.6 eV.
2.5 Photocatalytic tests
The photocatalytic reduction of CO2 in water was carried out in a homemade stainless-
steel batch reactor (~250 mL) with a quartz window on the top of the reactor. The light source
was 400 W Xe lamp (Newport). The catalyst was placed on a quartz glass holder in the middle
of reactor (Figure S1, SI). 15 ml of liquid water was added at the bottom of reactor. The
reaction has been conducted at 323 K and 0.2 MPa of CO2. Before start of the photocatalytic
reaction, the reactor was kept in the dark for 1 h to ensure an adsorption-desorption
equilibrium between the photocatalyst and the reactants. Subsequently, the reactor was
irradiated by a 400 W Xe lamp. The spectral range of the irradiation was selected using
Hamamatsu optical filter. The photocatalytic reaction time was typically 7 h.
The gas phase reaction products (CO, CH4, C2H6 and C3H8) were analyzed by on-line
gas chromatography (GC, PerkinElmer Clarus® 580). Liquid products such as HCHO and
CH3OH were analyzed by off-line GC analysis. The GC was equipped with a PoraBOND Q
and ShinCarbon ST 100/120 columns with detection by flame ionization detector (FID) and a
thermal conductivity detector (TCD).
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3. Results and discussion
3.1 Monoclinic BiVO4 crystal with designable facets
Uniform truncated tetragonal bipyramidal morphology BiVO4 with designable exposed
facets has been prepared using hydrothermal method with variation of NaCl content [24]. XRD
measurements indicate that these samples exhibit a monoclinic structure [23, 35] (Figure 1). The
intensity of the (040) diffraction peak at 30.5 2θ depends on the concentration of NaCl.
Increase of NaCl concentration from 0.04 to 0.5 M leads to increase of intensity with
subsequent rapid decrease when the concentration reaches 2 M, indicating that NaCl affects
the morphology of BiVO4.
The SEM images show that the BiVO4 crystals were composed of uniformed single
crystal with truncated tetragonal bipyramidal shape (Figure 2b-e) when the concentration of
NaCl is between 0.04 - 0.50 M. However, the BiVO4 crystals has an irregular morphology
(Figure 2a,f) when NaCl was not included or if the concentration is too high. Interestingly,
the morphology of monoclinic BiVO4 crystals changed gradually with the concentration of
NaCl. Crystal model of monoclinic BiVO4 crystals with truncated tetragonal bipyramidal
shape contains two {010} facets on the top and bottom and eight {110} facets on the edges
(Figure 3). The fraction of {010} facets has been calculated by the crystal model equation.
The percentages of {010} facets can be calculated as follows [21, 24]
S010 = 2a2,
.
S110 = 8 * 0.5 * (a + b) * LDE, LDE = LCE – LCD, LCE = 0.5b / cosθ, LCD = 0.5a / cosθ,
S110 = 2 * (b2 – a2) / cosθ,
F010% = S110 / (S110 + S010) = 2a2 / 2 * (b2 – a2) / cosθ = cosθ / (cosθ + b2 / a2 –1)
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Here θ is the theoretical value (66.07o) for the angle between the {010} and {110} facets
of monoclinic BiVO4 crystal. Note that the fraction of {010} facets increases significantly
with the increase of NaCl concentration from 26 % to 75 % (Table 1).
(121)
(040)
(110)
(011)
NaCl conc. (mol L-1)
(020)
2.0
0.50
0.20
0.10
0.05
BiVO4-010
BiVO4-110
20
30
40
50
60
2 (degree)
Figure 1. XRD patterns of the monoclinic BiVO4 samples with different concentration of NaCl
and BiVO4 with predominately exposed {010} and {110} facets
BiVO4-010 and BiVO4-110 samples with predominantly {010} and {110} facets have
been synthesized. The SEM images confirm that the BiVO4 crystals have {010} and {110}
facets predominantly exposed (Figure 2). XRD measurements also confirm synthesis of
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BiVO4-010 and BiVO4-110 samples with predominantly {010} and {110} facets (Figure 1).
The UV-Vis diffuse reflectance spectra of the BiVO4 crystals with different concentration
of NaCl is shown in Figure 4. The samples exhibit intense absorption in the ultraviolet region
(< 400 nm) as well as visible light region (< 500 nm). The band gap energy for different
morphology samples estimated using Tauc’s plots of UV-Vis spectra is the same, being 2.3 eV
(Table 1).
Figure 2. SEM images of BiVO4 samples synthesized with different concentration of NaCl.
NaCl concentration: (a) 0 M, (b) 0.04 M, (c) 0.10 M, (d) 0.20 M, (e) 0.50 M, (f) 2.0 M, (g)
BiVO4-010, (h) BiVO4-110. Scale bar: 1.0 μm.
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Figure 3. Crystal model of monoclinic BiVO4 crystals with truncated tetragonal bipyramidal
shape.
Photoluminescence spectroscopy has been used to evaluate the recombination of holes
and electrons of different BiVO4 catalysts. A luminescence band centered at 620 nm was
observed for monoclinic BiVO4 crystals (Figure 5). It is generally accepted that
photoluminescence is produced by the recombination of photogenerated carriers on a
semiconductor, and therefore the higher intensity of the luminescent band means that the
probability of photogenerated carrier recombination is higher [36]. For the series of BiVO4
samples, the intensity of the luminescence band decreases in the order of BiVO4-0.04M >
BiVO4-0M > BiVO4-2.0M > BiVO4-0.50M > BiVO4-0.10M > BiVO4-0.20M, indicating that
BiVO4-0.20M with 64 % of {010} facet has the lowest ability of electron-hole recombination.
Note that the irregular BiVO4 crystals show a higher ability of electron-hole recombination.
In other words, truncated tetragonal bipyramidal shape is good for electron-hole separation.
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BiVO4-0M
BiVO4-0.04M
BiVO4-0.10M
BiVO4-0.20M
BiVO4-0.50M
BiVO4-2.0M
300
400
500
600
 (nm)
Figure 4. UV-Vis spectra of BiVO4 samples synthesized with different concentration of NaCl.
(a)
BiVO4-0M
BiVO4-0.20M
BiVO4-0.50M
BiVO4-2.0M
BiVO4-0.04M
BiVO4-0.10M
200
400
600
800
1000
Wavelength (nm)
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(b)
BiVO
4
CuO /BiVO
x
4
CuO /CoO /BiVO
x
x
4
200
400
600
800
1000
Wavelength (nm)
Figure 5. (a) Photoluminescence spectra of BiVO4 samples synthesized with different
concentration of NaCl. (b) Photoluminescence spectra of BiVO4-0.20M crystal with selective
photo-deposited the metal oxides.
The UV-Vis diffuse reflectance spectra (Figure 6) of CuOx/CoOx /BiVO4-0.20M
nanocomposite and reference compounds are displayed in Figure. It is noted that each sample
shows different absorption profile in visible light region except for that of strong absorption
in the UV-light region. The presence of Co3O4 results in an increase in the ability of visible
light absorption. Co3O4 has absorption in nearly all of the visible light range and induces an
extension of the light absorption spectrum of the composite semiconductor even at low cobalt
contents. The band gap energy for different nanocomposites was estimated using Tauc’s plots
of UV-Vis spectra. The direct band gaps of BiVO4, CuOx/ BiVO4 and CuOx/CoOx /BiVO4
samples are estimated to be 2.34, 2.30 and 2.16 eV. They are slightly smaller than the reported
values of pure BiVO4. The decrease in band gap energy indicates that the nanocomposite has
a much greater optical absorption region than pure BiVO4, which can be excited to produce
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more electron–hole pairs under the same visible light illumination and then result in higher
photocatalytic activity.
(a)
BiVO4
CuO /BiVO
x
4
CuO /CoO /BiVO
x
x
4
300
400
500
600
 (nm)
(b)
4
2
0
BiVO4
CuO /BiVO
x
4
CuO /CoO /BiVO
4
x
x
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4
hv (eV)
Figure 6 a. UV-Vis spectra of CuOx/CoOx /BiVO4-0.20M sample b. [F(R∞)hν]1/2 versus hν
for CuOx/CoOx /BiVO4-0.20M sample
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Table 1. Fraction of {010} facets, specific surface areas, bandgap energies of BiVO4
samples with different NaCl concentration.
Fraction of
Sample
Surface area
(m2 g-1)
1.3
Bandgap energy
{010} facets (%)
(eV)
2.3
BiVO4-0M
BiVO4-0.04M
BiVO4-0.10M
BiVO4-0.20M
BiVO4-0.50M
BiVO4-2.0M
/
26
2.2
2.8
2.9
2.6
1.4
2.3
2.3
2.3
2.3
2.3
44
64
75
/
ZnOx, WOx, FeOx and CuOx were loaded onto series of BiVO4 samples through photo-
reduction deposition technique (Figure 7). SEM images show that CuOx nanoparticles were
selectively deposited onto the {010} facets of BiVO4 crystal (Figure 8b). Several oxide co-
catalysts MnOx, PbO2 and CoOx have been deposited by photo-oxidation using NaIO3 as
sacrificial agent on the surface of CuOx/BiVO4 catalyst. SEM analysis demonstrates that these
oxides are located on the {110} facets of BiVO4 crystal (Figure 7, 8c).
Figure 7. Schematic of the synthesis of CuOx/CoOx/BiVO4 catalyst.
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Figure 8. SEM images of BiVO4-0.20M sample with selective photo-deposited metal oxides:
(a) CuOx/BiVO4, (b) CoOx/BiVO4, (c) CuOx/CoOx/BiVO4.
To provide further insights into the copper and cobalt state in CoOx/CuOx/BiVO4 catalyst
was characterized by XPS (Figure S2, SI). The binding energies of Cu 2p3/2 for the catalyst
was about 932.7 eV, which could be attributed to either Cu0 or CuI [37-39]. The positions of the
Cu L3VV Auger lines for these samples were centered at 916.7 eV, corresponding to that of
CuI in Cu2O [40]. Figure S2, SI represents the Co 2p XPS spectra of the catalyst. Co 2p3/2
binding energies at 781.5 eV, absence of the satellite structure and 2p3/2-2p1/2 spin-orbit
splitting (ΔE) of 15.1 eV indicates on the presence of Co3O4 phase [41-43]
.
3.2 Catalytic performance
Figure 9 demonstrates the effect of photo-reductive deposition of different metal oxides
over BiVO4-0.20M sample, which according to our characterization study demonstrates the
most efficient charge separation. Methane, ethane, propane, formaldehyde and methanol were
formed as the products of the reaction of CO2 reduction by H2O. The deposition of W and Fe
does not lead to significantly improve of the catalytic performance in comparison with the
parent BiVO4-0.20M which is almost inactive. The main products of the reaction were CO
and methane. In the presence of ZnOx activity significantly increases to 2 μmolg-1h-1 with
increase of the contribution of methane in the products in comparison with CO. The highest
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activity (6.8 μmolg-1h-1) has been observed over CuOx/BiVO4-0.20M with formation of broad
range of hydrocarbons (methane, ethane and propane) and oxygenates (formaldehyde and
methanol).
4
3
2
1
8
6
4
2
0
CO
CH4
C2H6
C3H8
HCHO
CH3OH
H2
0
4
4
4
4
M
iVO
/B
x
iVO
/B
x
iVO
/B
x
-0.2
/BiVO
x
4
O
uO
WO
FeO
ZnO
C
BiV
Figure 9. Effect of metal oxides on the performance of BiVO4-0.20M for photocatalytic
reduction CO2 with H2O.
Further we have studied effect of CuOx deposition over BiVO4 catalyst with
systematically changed fraction of {010} facets (Figure 10). As compared to BiVO4 with an
irregular morphology, which was synthesized without addition of NaCl, CuOx/BiVO4
catalysts with a uniform truncated tetragonal bipyramidal morphology exhibited higher
formation rate of hydrocarbon fuels. Interestingly, the photocatalytic performance depends on
the fraction of {010} facets. The formation rate of hydrocarbons has bell shaped curve with
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maximum at the fraction of exposed {010} facets 64 % with formation rate 6.8 μmolg-1h-1.
CuOx/BiVO4-010 and CuOx/BiVO4-110 with predominantly {010} and {110} facets
demonstrate lower formation rate of hydrocarbons in comparison with catalysts containing
both facets.
Our characterizations clarified that the specific surface areas change slightly with an
increase in the fraction of {010} facets. These cannot explain the change in the catalytic
behaviours with an increase in the fraction of {010} facets. Furthermore, the bandgap energy
values calculated from the UV-vis spectroscopy for this series of catalysts were the same,
being 2.3 eV (Table 1). Thus, the observation that the BiVO4 single crystal with an 64%
fraction of {010} and {110} facets shows the highest catalytic performance (Table 1) suggests
that both facets play crucial roles in the photocatalytic reduction CO2.
5
4
3
2
1
0
8
6
4
2
0
CO
CH4
C2H6
C3H8
HCHO
CH3OH
H2
a
a
x
x
x
4
4
-010
4
-110
x
x
O
O
x
x
-0.2M
-0.5M
CuO
-0.1M
4
-0.04M
O
O
4
4
CuO
4
O
O
CuO
/BiV
O
CuO
CuO
BiV
CuO
4
CuO
/BiV
O
/BiV
/BiV
/BiV
/BiV
/BiV
Figure 10. Effect of the fraction of {010} facets on the photocatalytic performance of
a
CuOx/BiVO4 catalyst for reduction CO2 with H2O. BiVO4 with an irregular morphology
(NaCl concentration: 0M).
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Recent studies have demonstrated that photogenerated electrons and holes in monoclinic
BiVO4 crystals will migrate to the {010} and {110} facets respectively [23]. Li and coworker
[44]
evaluated the energy levels of {010} and {110} facets of BiVO4 crystals by DFT
calculation method. They observed a slight difference in energy of the valence and conductive
band between the {010} and {110} facets which suggested that the electron transfer from {110}
to {010} facets is feasible thermodynamically. Thus, it can be expected that the coexistence
of the {010} and {110} facets with the appropriate ratio will favor the separation of
photogenerated electrons and holes. This order fits well with obtained photoluminescence
measurement, confirming that the {010} and {110} facet coexistence and appropriate ratio
facilitates the separation of electron-hole pairs. Furthermore, the order of the efficiency of
electron-hole pairs separation is the same as that of the formation rate of hydrocarbon fuels
for the photocatalytic reduction of CO2 with H2O (Figure 10). Therefore, we believe that the
ability of electron-hole pairs separation is a key factor in determining the catalytic behavior
of CuOx/BiVO4 with different fraction of {010} facets.
Table 2 displays the photocatalytic performance data of CuOx/BiVO4 with different
CuOx contents over BiVO4-0.20M. Note that the increase in Cu content from 1 to 5 wt%
enhances the formation rate of hydrocarbons with further stabilization at 8 wt%. The surface
area of BiVO4 is quite low and saturation of {010} facets takes place already at relatively low
Cu content. Hydrogen production follows the same trend as hydrocarbon production and
indicates on not full utilization of hydrogen for CO2 reduction during photocatalysis. For other
catalysts hydrogen production was also related to production of hydrocarbons.
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10.1002/cctc.201901115
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Table 2. Effect of Cu content on the performance of CuOx/BiVO4-0.20M for
photocatalytic reduction CO2 with H2O
Product formation rate
Hydrocarbon
formation
rate
(μmol g-1 h-1)
Catalyst
CO
CH4
C2H6 C3H8 HCHO CH3OH
H2
(μmol g-1 h-1)
1% CuOx/BiVO4 0.51 0.73
0
0
0
0
0
0.73
4.4
3% CuOx/BiVO4 0.82
2.5
3.5
3.3
0.61
1.1
1.0
0.42
0.80
0.63
0.35
0.60
0.51
0.55
0.82
0.60
0.42
0.91
0.96
5% CuOx/BiVO4
8% CuOx/BiVO4
1.4
1.3
6.8
6.0
The catalytic properties after deposition of co-catalysts by photo-oxidation over
CuOx/BiVO4 have been studied by catalytic tests at the same conditions (Figure 11). The
catalytic activity was gradually increasing in the row: CuOx/BiVO4<MnOx/CuOx/BiVO4
<PbO2/CuOx/BiVO4 <CoOx/CuOx/BiVO4. It is interesting to note that deposition of co-
catalysts does not lead to significant change in the selectivity to hydrocarbons (Figure S3, SI).
Methane was still responsible for about 40 % of carbon utilization with selectivity about 20 %
to ethane and propane and rest to heavier hydrocarbons. Afterwards the catalytic performance
of CoOx/CuOx/BiVO4 has been optimized by variation of CoOx content. The catalytic behavior
is similar to those of Cu with increase of the catalytic activity till 5 wt% of CoOx and stable
performance at higher loading (Table S1, SI). Thus, the highest catalytic activity for CO2
reduction by water is achieved for the CuOx/CoOx/BiVO4 catalyst with selective photo-
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10.1002/cctc.201901115
ChemCatChem
deposited CuOx and CoOx nanoparticles over {010} and {110} facets, respectively. For
comparison, the CuOx/CoOx/BiVO4 (imp) synthesized by impregnation method with non-
selective loading of CuOx and CoOx demonstrates much lower catalytic activity and
significantly higher contribution of methane in the products (Figure 11, Figure S3, SI). This
result indicates that the photocatalytic performance can be significantly enhanced when the
reduction/oxidation co-catalysts were selectively deposited on the corresponding
reduction/oxidation reaction facets, which can minimize electron-hole pairs recombination.
This assumption has been confirmed by the fact that intensity of photoluminescence band
decreased significantly by loading CuOx and CoOx onto the BiVO4-0.20M crystal (Figure 5).
8
7
6
5
4
3
2
1
0
12
9
CO
CH4
C3H8
CH3OH
C2H6
HCHO
H2
6
3
0
4
2
x
x
x
O
O
O
p)
oO
iV
oO
iV
Pb
x
C
/
C
x
Mn
(im
4
/
B
x
/
x
/
4
/
4
4
O
O
iV
O
x
O
iV
iV
uO
uO
uO
uO
uO
/B
C
C
/B
C
C
/B
C
/B
Figure 11. Effect of cocatalyst on the performance of CuOx/BiVO4-0.20M for
photocatalytic reduction CO2 with H2O.
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The stability of optimized catalyst containing 5 wt% Cu and 5 wt% Co over BiVO4-
0.20M has been studied by repeated tests with degassing and CO2 reintroduction after 7 h of
reaction. Similar activities for the formations rate of hydrocarbon fuels were observed after
five cycles (Figure 11).
8
CO
CH4
C2H6
C3H8
15
12
9
7
6
5
4
3
2
1
0
HCHO
CH3OH
H2
6
3
0
1
2
3
4
5
Figure 12. Repeated uses of the 5 wt% CuOx/5 wt% CoOx /BiVO4-0.20M catalyst for
photocatalytic reduction of CO2 with H2O.
Table 3 shows that the formation rate of the CO2 reduction over CoOx/CuOx/BiVO4
catalyst under visible light range (382-600 nm) is only slightly lower compared to the results
with light range before filtration (240-600 nm). The main advantage of BiVO4 as
semiconductor in comparison with TiO2 is its ability to work under visible light irradiation
Table 3. Performances of the 5 wt% CuOx/5 wt%CoOx /BiVO4-0.20 M catalyst for
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10.1002/cctc.201901115
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photocatalytic reduction CO2 with H2O measured under irradiation at different spectral
ranges.
Product formation rate
Hydrocarbon
formation
rate
Spectral
(μmol g-1 h-1)a
range of
irradiation
(nm)
(μmol g-1 h-
CO
CH4 C2H6 C3H8 HCHO CH3OH
H2
1
)
dark
0
0
0
0
0
0
0
0
240-600
382-600
1.5
0.60
3.8
2.0
1.1
0.52
0.50
0.21
1.1
0.30
0.81
0.41
1.0
0.2
7.3
3.1
a
Reaction conditions: catalyst, 0.1 g; Gas phase pressure, CO2 0.2 Mpa; H2O 15 mL;
irradiation time, 7h; light source, Hamamatsu LC8-06 Hg-Xe stabilized irradiation lamps
with a spectral irradiance in the range 240-600; Cut-off filter: visible light, 382 nm < λ < 600
nm.
3.3 Z-scheme mechanism of the CuOx/CoOx /BiVO4-0.20M catalyst for photocatalytic
reduction CO2 with H2O
The valence band (VB) of Cu2O is very close to the conduction band (CB) of BiVO4.
This fact suggests a possibility of construction of the Z-scheme photocatalysis system for
reduction of CO2 similar to the previous reports [34, 45-47]. The significant enhancement of
photocatalytic CO2 reduction can be explained by Z-scheme charge-transfer mechanism
(Figure 13).
In summary, when both Cu2O and BiVO4 were excited by the irradiation, the
photogenerated electrons in the CB of BiVO4 recombine with the photogenerated holes in the
VB of Cu2O owing to their close band position. Finally, the photogenerated electrons in the
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10.1002/cctc.201901115
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CB of Cu2O are used for the reduction of CO2 produce hydrocarbon fuels, while the
photogenerated holes in the VB of BiVO4 transfer to Co3O4 of oxidation co-catalyst, which
used for the oxidation reaction of water (Figure 13).
The effect of close position of CB of BiVO4 and VB of Cu2O has been verified by
deposition of other metal oxides (WOx, FeOx and ZnO) by photo-reduction technique with
different band positions instead of CuOx (Figure 9). The activity correlates well with band
position on the Figure 13. Thus, FeOx and WOx almost did not affect catalytic activity of pure
BiVO4 due to low CB potential in comparison with the CO2/CH4 reduction potential. However,
ZnO with higher reduction potential results in significant increase of the catalytic activity.
Figure 13. (a) Comparison of the band structures of various semiconductor with respect to
the redox potentials of reduction of CO2, (b) Schematic of the Z-scheme of CuOx/CoOx /BiVO4-
0.20M catalyst.
4. Conclusion
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10.1002/cctc.201901115
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The present work focuses on the design efficient photocatalyst construction for the
preferential photocatalytic reduction of CO2 with H2O. The monoclinic BiVO4 crystals with
truncated tetragonal bipyramidal morphology and controllable exposed {010} and {110}
facets ratios have been synthesized. The fraction of {010} facets could be varied in the range
from 26 to 75 %. The facet-selective photo-deposition of the metal oxides can significantly
enhance the photocatalytic reduction of CO2. BiVO4 crystals containing 64% of {010} facets
fraction with Cu2O and Co3O4 nanoparticles photo-deposited onto {010} and {110} facets,
respectively, were the most efficient for the separation of photogenerated electron-hole pairs.
This catalyst demonstrated the highest stable catalytic activity toward production of
hydrocarbons even under visible light irradiation. The construction of Z-scheme for CO2
reduction by water using oxidation and reduction co-catalysts played a key role in enhancing
of the efficiency of charge separation.
Keywords: photocatalysis; structural sensitivity; reduction of CO2; bismuth vanadate; Z-
scheme
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