Ferroelectric polarization and thin-layered structure synergistically promoting CO2 photoreduction of Bi2MoO6

Article abstract of DOI:10.1039/d0ta02102d

Photocatalytic CO₂ reduction for producing solar fuels is promising but severely restricted by the fast recombination of photogenerated electrons and holes and insufficient reactive sites of photocatalysts. Formation of an internal electric field is an effective route for facilitating charge separation, and two-dimensional structure construction is beneficial to increasing catalytic sites. Herein, ultrathin Bi₂MoO₆ nanosheets with strong ferroelectricity were prepared by a combined CTAB-assisted hydrothermal and corona poling post-treatment process for synergistically improving the CO₂ photoreduction activity. Without sacrificial agents and co-catalysts, the polarized Bi₂MoO₆ ultrathin nanosheets demonstrate a remarkable CO₂ reduction activity for CO production with a rate of 14.38 μmol g^-1 h^-1 in the gas-solid system, over 10 times enhancement than that of bulk Bi₂MoO₆. The combined strategies considerably promote the separation of photogenerated electrons and holes and enrich the reactive sites for CO₂ adsorption, which co-boost the photocatalytic CO₂ reduction performance of Bi₂MoO₆. In addition, a synergistically enhanced effect between corona poling and thin-layered structure was disclosed. This work provides corona poling as an efficient route for promoting charge separation of particulate photocatalysts, and offers new insights into synergistically improving the CO₂ photoreduction activity.

Full text of DOI:10.1039/d0ta02102d

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Ferroelectric polarization and thin-layered structure
synergistically promoting CO2 photoreduction of Bi2MoO6
Shuguan Li, Liqi Bai, Ning Ji, Shixin Yu, Sen Lin, Na Tian,* Hongwei Huang*
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid
Wastes, National Laboratory of Mineral Materials, School of Materials Science and
Technology, China University of Geosciences, Beijing 100083, China
*Corresponding author: hhw@cugb.edu.cn (H.W. Huang); tiannacugb@163.com (N.
Tian)
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ABSTRACT: Photocatalytic CO2 reduction for producing solar fuels is promising but
severely restricted by the fast recombination of photogenerated electrons and holes and
insufficient reactive sites of photocatalysts. Formation of internal electric field is an
effective route for facilitating the charge separation, and two-dimensional structure
construction is beneficial to increasing catalytic sites. Herein, ultrathin Bi2MoO6
nanosheets with strong ferroelectricity were prepared by a combined CTAB-assisted
hydrothermal and corona poling post-treatment process for synergistically improving
the CO2 photoreduction activity. Without sacrificial agents and co-catalysts, the
polarized Bi2MoO6 ultrathin nanosheets demonstrate a remarkable CO2 reduction
activity for CO production with a rate of 14.38 μmol g-1 h-1 in the gas-solid system, over
10 times enhancement than the bulk Bi2MoO6. The combined strategies considerably
promote the separation of the photogenerated electrons and holes and enrich the
reactive sites for CO2 adsorption, which co-boost the photocatalytic CO2 reduction
performance of Bi2MoO6. In addition, a synergistically-enhanced effect between
corona poling and thin-layered structure was disclosed. This work provides corona
poling as an efficient route for promoting charge separation of particulate
photocatalysts, and offers new insights into synergistically improving the CO2
photoreduction activity.
Keywords: Bi2MoO6, ultrathin structure, ferroelectric polarization, charge separation,
CO2 reduction.
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1. Introduction
With the rapid development of technology, more and more energy is required to meet
the needs of human life and activities.1,2 As the massive consumption of fossil energy,
the issues of energy depletion and environmental pollution are becoming increasingly
serious, which now many researchers are paying extensive attention to.3,4,5
Semiconductor-based photocatalysis has emerged as one of the most promising ways
to solve the above problems. In particular, the photocatalytic CO2 reduction shows huge
potentials for converting CO2 into carbon-containing solar fuels.6,7 However, the low
charge separation and insufficient reactive sites seriously impairs the photocatalytic
CO2 reduction performance of semiconductors.8
Internal electric field has received wide spread attention due to the large potential in
driving the directional movement of the photogenerated charge carriers.9 For example,
the carrier separation efficiency and photocatalytic ability of BiOI can be enhanced by
optimizing crystal structure to increase the internal electric field.10 As a new type of
electric field, polarization electric field was recently regarded as an efficient approach
to inhibit the charge recombination.11-15 Due to the spontaneous or piezo- polarization,
there will generate the positive and negative polarized charges at the two ends of the
polar materials, which offers a strong driving force to drive the photogenerated
electrons and holes to separate and transport along opposite directions. Our group
reported that the macroscopic polarization enhancement can considerably strengthen
the bulk charge separation of BiOIO3, which largely improves its photocatalytic activity
for production of reactive oxygen species and CO2 reduction.11,12 Besides, it was also
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demonstrated that the annealing post-treatment causes the increased ferroelectric
polarization of SrBi4Ti4O15 nanosheets, thus resulting in high charge separation and
prominent photocatalytic CO2 reduction activity.14 In addition, it has been reported that
fabrication of two-dimensional (2D) thin-layered structure was capable of exposing
more unsaturated atoms as surface catalytic sites, thus enhancing the photocatalytic
activity.16 For instance, the construction of 2D ultrathin structure of Bi2WO6 was
revealed to provide a large amount of reactive sites for improving the CO2 reduction
activity.17
Among the numerous semiconductors, bismuth-based photocatalytic materials are
considered to be excellent candidates because of their inherent physical and chemical
properties.18 As a typical Aurivillius-phase compound, Bi2MoO6 has attracted
substantial attentions in the field of photocatalysis due to its layered crystal structure
and moderate band gap,19 which has been widely surveyed for degradation of
pollutants20,21, oxygen evolution22, nitrogen fixation23 and reduction of CO2 to solar
fuels24,25. Bi2MoO6 is structurally composed of alternating (Bi2O2)2+ and perovskite-
type (MoO4)2- layers, and oxygen atoms are shared between the layers.26,27 The internal
electric field formed between (Bi2O2)2+ and (MoO4)2- layers in Bi2MoO6 can effectively
separate the photogenerated charge carriers.28 Owing to the unique layered structure of
Bi2MoO6, it is very conducive to the formation of nanosheets, and the thickness can be
regulated from dozens of layers to an atomic layer. More importantly, Bi2MoO6 is a
typical ferroelectric belonging to the Bi2WO6 family with the asymmetric space group
of Pna2,29 and thus the ferroelectric polarization field in Bi2MoO6 may be employed as
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the driving force to restrain the charge recombination. Besides, the ferroelectric
polarization of Bi2MoO6 can be enhanced by applying an external electric field, which
may further promote the charge separation. However, the influence of ferroelectricity
on the photocatalytic activity of Bi2MoO6 has not been investigated so far to the best of
our knowledge.
In this work, we first prepared ultrathin Bi2MoO6 nanosheets by a CTAB-assisted
hydrothermal process, and then employed the corona poling post-treatment to enhance
the ferroelectricity of Bi2MoO6. It was found that the corona poling and formation of
thin-layered structure greatly enhance the separation and transfer of photogenerated
charge carriers and CO2 adsorption, which co-promote the photocatalytic CO2
reduction performance. The ultrathin Bi2MoO6 nanosheets treated by corona poling
demonstrate a highly enhanced CO2 reduction activity into CO with a production rate
of 14.38 μmol g-1 h-1, over 10 times higher than that of the bulk Bi2MoO6 sample.
Besides, the microstructure, chemical states and ferroelectricity were investigated in
detail to elucidate the photocatalytic mechanism.
2. Experimental section
2.1 Sample preparation and poling treatment
2.1.1 Synthesis of ultrathin Bi2MoO6 (BMO-U)
All chemicals were of analytic grade and were used without further treatment. The
ultrathin Bi2MoO6 nanosheets were synthesized by a typical hydrothermal method.
1.940 g of Bi(NO3)3·5H2O was added in 30 ml of distilled water containing 10% molar
ratio of CTAB. After sufficient ultrasonic stirring for 30 min, 0.484 g of
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Na2MoO4·2H2O was added into the mixture solution followed by continuous ultrasonic
stirring for 30 minutes, and then the mixture was transferred into a 50 ml Teflon-lined
autoclave. The autoclave was heated in an oven at 180 °C for 20 h. After cooling down
to room temperature, the obtained products were collected by centrifugation and
washed with hot distilled water and absolute ethanol for ten times. The resulting
products were dried at 80 °C for 12 h, and denoted as BMO-U. Bulk Bi2MoO6 was
synthesized according the same preparation procedure without the addition of CTAB,
and the as-obtained products were named as BMO.
2.1.2 Corona poling
A self-made instrument was employed to exert corona poling on the photocatalysts.
150 mg of the as-prepared BMO and BMO-U was put into the mold, and then the
voltage of the instrument was increased to achieve an electric field of 20 kV/cm.
Afterwards, the samples were treated under this electric field for 30 min. The as-
obtained polarized BMO-U and BMO were denoted as BMO-U-P and BMO-P,
respectively.
2.2 Characterization
X-ray powder diffraction (XRD) test was used to survey the phases of photocatalysts
on an XRD powder diffraction instrument with monochromatized Cu Ka radiation (l =
1.5406 nm) at 40 kV and 40 mA at room temperature. Transmission electron
microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM)
were conducted to investigate the morphologies and microstructures of the series of
Bi2MoO6 samples with using a JEM-2100 electron microscopy (JEOL, Japan), and
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energy dispersive spectroscopy (EDS) attached to the TEM was used to examine the
composition of the products at the same time. Nitrogen adsorption BET method was
utilized to characterize the specific surface area of the samples on a Micromeritics 3020
instrument. X-ray photoelectron spectroscopies (XPS, ESCALAB 250Xi) were
conducted to analyze the surface elemental composition and states of the samples. UV–
vis diffuse reflectance spectra (DRS) were recorded on a Cary 5000 (America Varian)
spectrophotometer to determine the light absorption properties of the samples. The
photoluminescence (PL) spectra of the samples were measured by using a Hitachi F-
4600 fluorescence spectrophotometer.
2.3 Photocatalytic CO2 reduction measurement
The photocatalytic CO2 reduction activities of the as-obtained Bi2MoO6 samples
were assessed in a gas-solid reaction system with a closed quartz reactor (PerfectLight,
China) of 500 ml in volume. 10 mg of the photocatalyst was put in a petri dish with an
area of about 28 cm2 containing a small amount of distilled water. The above mixture
was dispersed sufficiently by ultrasonic treatment, and then the petri dish was put into
an oven for drying. After that, the above petri dish was put in the reaction cell with a
triangular bracket. 1.7 g of NaHCO3 was placed in the bottom of the reaction cell,
followed by sealing and vacuum treatment. Then, 15 ml of H2SO4 (0.7 M) was injected
into the cell to produce CO2 gas via the reaction between NaHCO3 and H2SO4. At every
1 h, 1 ml of generated gas was taken by a syringe and then qualitatively analyzed by
the gas chromatograph. The production yield was calculated on the basis of a calibration
curve. The blank test was conducted without adding NaHCO3 or with purging argon in
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the reaction cell.
2.4 Photoelectrochemical measurement
The electrochemical impedance spectroscope (EIS) and Mott-Schottky plots were
measured on an electrochemical analyzer (CHI660E, Shanghai) equipped with a
standard three-electrode system. Ag/AgCl (saturated KCl) electrode and platinum (Pt)
wire are employed as the reference electrode and the counter electrode, respectively.
With respect to the preparation of working electrodes, 10 mg of BMO series of samples
was dispersed in 1 ml of ethanol. Subsequently, the obtained suspension liquid was
dropped on the indium tin oxide glass (25 mm × 40 mm) and then dried at 70 °C for 12
h in an oven. 0.1 M Na2SO4 aqueous solution was used as the electrolyte solution, and
a 300 W Xe lamp was used as the light source.
2.5 Ferroelectric property measurements
The ferroelectric property of the series of BMO samples was measured on a
ferroelectric tester (AixACCT, TF Analyzer 2000, Germany). The samples were firstly
pressed in a tablet (∼10 mm in diameter and ∼1.10 mm in thickness), and then were
annealed at 250 ℃ for 3 h and the silver paste was applied to both sides of the tablets.
Afterwards, the samples were treated under an electric field of 150 kV/cm at 1000 Hz.
An atomic force microscopy (AFM) combined with Kelvin probe force microscopy
(KPFM) (Bruker Multimode 8) was used to provide the KPFM images. The samples
were dispersed in absolute ethanol and then dropped on a silicon wafer to form a thin
film.
3. Results and discussion
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3.1 Characterization of photocatalysts
The morphology, phase and crystal structure of the series of Bi2MoO6 samples were
studied. Figure 1a shows the XRD patterns. All the samples show diffraction peaks at
10.8°, 23.4°, 28.2°, 32.5°, 33.1°, 35.9°, 46.6°, 47.1°, 55.5°, 56.2° and 58.4°,
corresponding to the (020), (111), (131), (200), (060), (151), (202), (260), (331), (191)
and (262) planes of Bi2MoO6 (JCPDS No. 21-0102), respectively. Compared with the
bulk BMO sample, no extra peaks were detected in the XRD spectra of BMO-U, BMO-
P and BMO-U-P, which demonstrates that the introduction of CTAB and corona poling
do not change the crystalline phase of Bi2MoO6.30 The diffraction peak intensity ratio
of the (131)/(200) crystal planes of BMO is about 3.57, which is higher than that of
BMO-U (⁓3.18). The decrease in this peak intensity ratio may indicate the presence of
selective growth of specific facet of Bi2MoO6 by the addition of CTAB. From the XRD
data, it was found that the lattice parameters were altered in BMO-U, BMO-P and
BMO-U-P, which indicates the distortion of crystal lattice and stronger polarization in
these samples (Table S1). The unit cell parameters for a, b and c axis of BMO-U-P are
changed by 0.161%, 0.028% and -0.104% compared to that of BMO, respectively,
which suggest that the crystal lattices undergo larger prolongation along a axis and
compression along c axis.
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Figure 1. (a) XRD patterns of the BMO, BMO-P, BMO-U and BMO-U-P samples. (b)
TEM, (c) HRTEM (inset: the SAED pattern) and (d) EDS elemental mapping of the
BMO-U-P sample. (e) AFM image of BMO-U-P and (f) the corresponding height
profile.
TEM image shows the nanosheet morphology of the as-prepared BMO-U-P (Figure
1b), revealing a lamellar structure. Figure 1c shows the HRTEM image of the BMO-
U-P nanosheet, and the legible interplanar lattice with a space of 0.275 and 0.274 nm
corresponds well with the (200) and (002) planes of Bi2MoO6, suggesting the exposure
of {020} facet. The selected area electron diffraction (SAED) pattern (inset of Fig. 1c)
further clearly reveals the single crystalline nature of the BMO-U-P nanosheet. The
elemental mapping shows that Bi, Mo, O are evenly distributed in BMO-U-P (Figure
1d). The thickness of as-obtained series samples was measured by the AFM, and the
results were shown in Figure 1e, 1f and S2. The thickness of BMO and BMO-P is
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approximately 40 nm, while that of BMO-U and BMO-U-P is about 3 nm. The
thickness of BMO and BMO-U-P is consistent with their specific surface area (17.10
m2/g and 35.08 m2/g, respectively). The AFM results show that the ultrathin Bi2MoO6
nanosheets were successfully obtained and the corona poling treatment has no obvious
effect on the thickness of Bi2MoO6.
XPS is used to characterize the surface element composition and chemical state of
BMO, BMO-P, BMO-U and BMO-U-P.31 All the XPS survey spectra clearly
demonstrated the existence of Bi, Mo, and O in all samples (Figure 2a). The C 1s peak
is because of the widespread presence of carbon in the environment.32 Figure 2b-d show
the high-resolution XPS of Bi 4f, Mo 3d, and O 1s, respectively. The Bi 4f spectrum
exhibits two peaks located at 164.67 and 159.37 eV, resulting from the spin-orbit
splitting of Bi 4f5/2 and Bi 4f7/2, respectively (Figure 2b). Similarly, the Mo 3d spectrum
also shows two peaks at 235.80 and 232.63 eV, which are attributed to the Mo 3d3/2 and
Mo 3d5/2, respectively (Figure 2c). The O 1s peak is divided into two peaks at 531.73
and 530.17 eV, corresponding to the surface adsorbed -OH and lattice oxygen atom,
respectively (Figure 2d). In comparison with BMO and BMO-U, it can be clearly seen
that the binding energies of Bi 4f, Mo 3d and O 1s peaks shift slightly to a lower position
in BMO-P and BMO-U-P, which demonstrates that the corona poling changes the
coordination environments of these atoms. It provides an obvious evidence for the
presence of lattice distortion due to the strong external electric field, confirming the
successful ferroelectric polarization.18,33 Moreover, the binding energies of the Bi 4f,
Mo 3d and O 1s in BMO-U-P show larger right-shifts than that in BMO-P, indicating
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that the ultrathin structure may enhance the lattice distortion under externalDOelI:e1c0.t1r0i3c9/D0TA02102D
field. The XPS results clearly disclose that corona poling causes the lattice distortion
of Bi2MoO6, and a larger distortion occurs in ultrathin Bi2MoO6 nanosheets.
Figure 2. (a) XPS survey spectra and the high-resolution XPS spectra of (b) Bi 4f, (c)
Mo 3d and (d) O 1s of the BMO, BMO-P, BMO-U and BMO-U-P samples.
UV-vis DRS were measured to compare the light absorption property of BMO,
BMO-P, BMO-U and BMO-U-P.34 Figure 3a and 3b show the UV–vis DRS spectra of
the samples and their bandgap energy level from the plots (αhv)1/2 estimated by the
energy of absorbed light, respectively.35 From the steep absorption edge of the spectra,
not the impurity level, there are relationships between the visible light absorption and
the intrinsic band transition.36,37 Compared to BMO and BMO-P, the light absorption
edges of BMO-U and BMO-U-P shift from 489 nm to 478 nm (a magnified view of the
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absorption edge is shown in the inset of Figure 3a), revealing that the thinD-OlaI:y10e.r10e3d9/D0TA02102D
structure causes the blue shift of absorption edge, and the polarization treatment has no
effect on the light absorption properties of the samples. The formula was used to
determine the band gap (Eg)38:
αhv = A (hv - Eg)n/2
(1)
and the α, v, h and Eg correspond to coefficient, light frequency, Planck constant and
band gap energy, respectively, and the A is a constant. The band gap of BMO-U and
BMO-U-P was estimated to be 2.32 eV, which is slightly wider than that of BMO and
BMO-P (2.30 eV) (Figure 3b).
The band gap level plays a very important role in understanding the photocatalytic
activity because the redox reaction types of a semiconductor are determined by the
valence band (EVB) and conduction band (ECB) positions.39 In order to determine the
flat band potential (Efb) of BMO, BMO-P, BMO-U and BMO-U-P, the Mott-Schottky
plots were measured under three different frequencies (500, 800 and 1000 Hz) as shown
in Figure 3c, 3d, S3a and S3b.40 At the given different test frequencies, it is obvious
that three tangents of the Mott-Schottky curves intersect at the same point on the X axis.
And the Efb of BMO, BMO-P, BMO-U and BMO-U-P was determined to be -0.61, -
0.61, -0.65 and -0.65 eV (versus to Ag/AgCl electrode), respectively. Thus versus to a
standard hydrogen electrode (NHE), the Efb of BMO, BMO-P, BMO-U and BMO-U-P
was -0.41, -0.41, -0.45 and -0.45 eV, respectively.41 It indicates that the polarization
treatment does not change the position of the energy band. The series of BMO samples
are obvious n-type semiconductors, because of the positive slopes of the Mott-Schottky
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curves.42 In general, for n-type semiconductors, the ECB position is about 0.1⁓0.3 eV
more negative than Efb.43 Therefore, the ECB of BMO, BMO-P, BMO-U and BMO-U-
P was approximately determined to be -0.51 -0.51, -0.55 and -0.55 eV, respectively.
The EEB can be calculated by the following formula 44:
EVB = ECB + Eg
(2)
the VB position of BMO, BMO-P, BMO-U and BMO-U-P is calculated to be 1.79, 1.79,
1.77 and 1.77 eV, respectively. As it is well known, there is a close relationship between
the photoreduction ability and CB position for a semiconductor, and electrons on a more
negative CB potential show a stronger driving force for reduction. 45,46 Thus, BMO-U
and BMO-U-P have a stronger CO2 reductive ability.
Figure 3. (a) UV–vis absorption spectra of a series of BMO samples and (b) the
corresponding Tauc plots. The Mott-Schottky plots of the (c) BMO and (d) BMO-U-P
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under three different frequencies.
3.2 Photocatalytic activity for CO2 reduction
The photocatalytic activity of BMO, BMO-P, BMO-U and BMO-U-P was
investigated by CO2 reduction under simulated solar irradiation in a gas-solid system.
As demonstrated in Figure 4a, BMO shows the poorest CO2 photoreduction
performance among all the samples with a CO production of only 5.45 μmol g-1 within
4 h. Both BMO-P and BMO-U samples exhibit improved photocatalytic performance,
and the CO production reaches 22.06 and 16.34 μmol g-1, respectively. BMO-U-P
displays the highest photocatalytic CO2 reduction performance with a CO production
of 57.51 μmol g-1 within 4 h. Figure 4b shows the corresponding CO production rate of
these samples. The CO evolution rate of the BMO-U-P sample is 14.38 μmol g-1 h-1,
which is over 10 times higher than that of BMO (1.36 μmol g-1 h-1). This outstanding
CO evolution rate is also higher than that of most of the previously reported bismuth-
based catalysts, such as Bi24O31Cl10-OV (0.90 μmol g-1h-1), BiOI (5.18 μmol g-1h-1),
and even some other state-of-the-art photocatalysts, such as Mg-In LDH (4.00 μmol g-
1h-1) (detailed comparison with more photocatalyts are shown in Table S2).47-49 The
control experiments of the BMO-U-P sample under different conditions were shown in
Figure 4c. It can be clearly seen that there is no CO detected in the darkness or without
photocatalyst, which demonstrates the necessity of both light and photocatalyst. A
slight of CO was detected over the BMO-U-P sample under simulated solar irradiation
in an Ar atmosphere. It is understandable that trace of organics were attached on the
surface though the samples were alternately washed with anhydrous ethanol and hot
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distilled water. Figure 4d shows the results of CO2 reduction of BMO-U-P for three
cycles. It can be seen that the CO production still maintains 44.36 μmol g-1 after three
cycles, revealing the good stability of BMO-U-P.
Figure 4. (a) Photocatalytic CO production and (b) production rate of the BMO, BMO-P,
BMO-U and BMO-U-P samples under simulated sunlight. (c) CO production rate of
the BMO-U-P sample under different conditions or without catalyst. (d) Cycling
experiment of the BMO-U-P sample.
3.3 Mechanism investigation on the enhanced photocatalytic activity
Due to ferroelectric polarization, a strong polarization electric field will be formed
between the two sides of BMO-U-P nanosheets, which can provide a stronger driving
force for improving the separation and transfer efficiency of photogenerated charge
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carriers.50-53 The surface potential of the samples was investigated by AFM and KPFM.
Figure 5 shows the AFM topographic and KPFM potential images of the as-obtained
samples, where the black and red lines represent for their thickness and surface potential,
respectively. For more accurate comparison, the samples with the similar stacking
thickness (31-33 nm) were selected for potential difference measurement, as shown by
the height plots (the black lines) in Figure 5c, f, i and l. As revealed by the red lines,
the potential difference of BMO, BMO-P, BMO-U and BMO-U-P is measured to be
28.25, 43.82, 56.02, and 108.25 mV, respectively. Thus, BMO-U-P has the largest
surface potential difference, which indicates its strongest internal electric field, thus
resulting in substantially strengthened separation and transfer efficiencies of
photogenerated charge carriers.54
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Figure 5. (a, d, g, j) AFM topographic images, (b, e, h ,k) KPFM potential images and
(c, f, i, l) the corresponding height curves (black lines) and surface ferroelectric
potential (red lines) of BMO, BMO-P, BMO-U, BMO-U-P, respectively.
The polarization-electric field hysteresis curves of the as-obtained samples were
measured to study the importance role of ferroelectricity in improving the
photocatalytic performance (Figure 6a). When an electric field of -150⁓150 kV/cm
was applied, both the saturation polarization (Ps) and remnant polarization (Pr) display
an order of BMO < BMO-P < BMO-U < BMO-U-P. BMO-P and BMO-U-P show
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larger Ps and Pr than their non-polarized counterparts (BMO and BMO-U), which may
be due to that corona poling causes the aligned arrangement of ferroelectric domains,
thus enhancing the ferroelectricity, as confirmed by the surface potential difference of
BMO-P and BMO-U-P (Figure 5). It was worth noting that the ferroelectricity of BMO-
U is stronger than that of BMO-P, which may be attributed to that the fabrication of
ultrathin structure increases the asymmetry of the structural units in Bi2MoO6. As
shown in Table S1, the lattice parameter a of BMO-U has prolonged to be 5.49556 Å
from 5.48338 Å for BMO, which clearly proves that lattice distortion has occurred in
BMO-U. It can be explained as that the construction of thin-layer structure causes the
absence of a large amount of surface atoms, which results in larger lattice distortion,
contributing to enhanced spontaneous polarization of BMO-U. The same phenomenon
was also observed for the ultra-thin SrTiO3 material.55 It was also confirmed by the
above XPS analysis that the largest shift in binding energy was observed for BMO-U-
P. Based on the above analysis, it can be inferred that the microstructure change of
ferroelectric materials also affects their ferroelectric properties. Due to the synergistic
effect of thin-layered structure and corona poling, BMO-U-P shows the strongest
ferroelectricity, consistent with KPFM observation.
To investigate the relationship between ferroelectric properties and charge separation,
PL was used to study the charge recombination rate in Bi2MoO6 series samples.56 The
recombination efficiency of photogenerated electrons and holes is demonstrated by the
intensity of the PL emission peak, and a lower PL emission intensity means a lower
recombination rate. Figure 6b displays that BMO shows the highest emission intensity
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at the emission peak position of approximately 480 nm, which matches wellDwOIi: t1h0.10it3s9/D0TA02102D
optical band gap. It should be noted that the emission intensity of BMO-P and BMO-U
is significantly lower than that of the BMO, which indicates that both corona poling
treatment and fabrication of ultrathin structure can reduce the recombination rate of
photogenerated charge carriers. Evidently, BMO-U-P demonstrates the weakest
emission intensity among all the samples, revealing its lowest recombination rate of
photogenerated electrons and holes.
Figure 6. (a) Electric hysteresis loop, (b) PL spectra, (c) EIS spectra and (d) CO2
adsorption isotherms of BMO, BMO-P, BMO-U and BMO-U-P.
Electrochemical impedance spectroscopy (EIS) can indicate the migration and
transfer of photogenerated charge carriers in semiconductors, in which the arc can
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reflects the interface layer resistance at the surface of electrode.57 Figure 6c shows the
Nyquist plots of BMO, BMO-P, BMO-U and BMO-U-P under irradiation of simulated
sunlight. It can be seen that the arc radius of BMO-P, BMO-U and BMO-U-P electrodes
is all smaller than that of BMO, which indicates that the charge transfer rate can be
increased by corona poling and fabrication of ultrathin structure. The smallest arc radius
was observed for BMO-U-P, suggesting the fastest charge transfer efficiency.
It is well known that difference in CO2 adsorption capability has an impact on the
photocatalytic CO2 reduction activity. Figure 6d shows the CO2 adsorption isotherms
of BMO, BMO-P, BMO-U and BMO-U-P. Compared to BMO and BMO-P, the thin-
layered structure in BMO-U and BMO-U-P greatly enhanced the CO2 adsorption,
which is due to exposure of more surface reactive sites. It has been reported that the
photoreduction reaction mainly occurs on the {020} crystal plane of the Bi2MoO6.58
From Figure 1c, the exposed facet of the as-prepared BMO-U-P sample is {020} facet.
Thus, the large exposure proportion of {020} facet in BMO-U-P is beneficial for
increasing the surface catalytic sites for CO reduction. In addition, the polarized
samples (BMO-P and BMO-U-P) also show slightly higher CO2 adsorption ability than
un-polarized counterparts. It may be because the corona poling results in the lattice
distortion and asymmetric distribution of surface charges, benefiting the CO2
adsorption.
Based on the above analyses, the mechanisms for substantially boosted charge
separation and CO2 reduction reaction over BMO-U-P were proposed, as illustrated in
Figure 7. Under the strong polarization electric field, the photogenerated electrons and
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holes are promptly separated and transferred to opposite directions along a oDrOcI: 1a0x.10is3.9/D0TA02102D
The thin-layered structure is also favorable for the fast migration of charge carriers from
bulk phase to surface of BMO-U-P. Moreover, the surface reactive sites on the top
facets were enriched, which greatly benefits CO2 adsorption and activation. Especially,
the ferroelectric polarization and thin-layered structure take a synergistic part in
promoting the above advantages, thus co-boosting the CO2 photoreduction activity of
Bi2MoO6.
Figure 7. The schematic illustration for charge separation mechanism and CO2
reduction reaction over BMO-U-P.
4. Conclusion
In conclusion, ultrathin Bi2MoO6 nanosheets with strengthened ferroelectricity were
synthesized by a one-step hydrothermal reaction with the addition of CTAB followed
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by corona poling. Corona poling induces the formation of strong polarized elecDtrOicI: 1f0i.e10l3d9/D0TA02102D
along a and c axis in Bi2MoO6, which largely depresses the recombination of
photogenerated electrons and holes between layers. Fabrication of thin-layered
structure not only promotes the charge separation, but also greatly enhances the
adsorption of CO2 on Bi2MoO6. The thin-layered structure was demonstrated to foster
the polarization degree under corona poling because of the larger lattice distortion.
Compared to bulk Bi2MoO6, the polarized ultrathin Bi2MoO6 nanosheets demonstrated
drastically enhanced CO2 reduction activity for CO production with a rate of 14.38
μmol g-1 h-1 in the gas-solid system, which is over 10 times higher than that of bulk
Bi2MoO6. This study pioneers a cooperative strategy to regulate the charge separation
and reactive sites on the basis of polarization and microstructure control.
Acknowledgements
This work was jointly supported by the National Natural Science Foundation of China
(No. 51972288 and 51672258), the Fundamental Research Funds for the Central
Universities (2652018290).
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