Enhanced Efficiency of Electron–Hole Separation in Bi2O2CO3 for Photocatalysis via Acid Treatment

Article abstract of DOI:10.1002/cctc.201800101

We herein prepared defective Bi₂O₂CO₃ microspheres via a simple nitric acid treatment method. Upon generation from Bi₂O₂CO₃, electrons and holes were superbly separated with the existence of defects. Combined with the advantages of multiple scattering and reflection, a large surface area and increased range of visible-light absorption, the defective Bi₂O₂CO₃ dispalyed a high efficiency for the photodegradation of HCHO gases and methyl orange. The defective Bi₂O₂CO₃ also possessed a superior stability, which is important for practical applications. This work opens up new opportunities for the design and fabrication of high-activity Bi-based photocatalysts for environmental protection.

Full text of DOI:10.1002/cctc.201800101

Accepted Article
Title: Enhanced efficiency of electron-hole separation in Bi2O2CO3 for
photocatalysis via acid treatment
Authors: Yongchao Huang, Kunshan Li, Ying Lin, Yexiang Tong, and
Hong Liu
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To be cited as: ChemCatChem 10.1002/cctc.201800101
Link to VoR: http://dx.doi.org/10.1002/cctc.201800101
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10.1002/cctc.201800101
ChemCatChem
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Enhanced efficiency of electron-hole separation in Bi2O2CO3 for
photocatalysis via acid treatment
Yongchao Huang[a], Kunshan Li[a], Ying Lin[b], Yexiang Tong*[b] and Hong Liu*[a,c]
Dedication ((optional))
Abstract: We herein prepared defective Bi2O2CO3 microspheres via
simple nitric acid treatment method. Upon generation from
of photoinduced carriers to a certain extent. However, it does not
change the intrinsic properties of Bi2O2CO3 itself. Thus, there is need
to further intrinsically enhance the photocatalytic performance of
Bi2O2CO3 by a simple and environmentally friendly method.
a
Bi2O2CO3, electrons and holes were superbly separated with the
existence of defects. Combined with the advantages of multiple
scattering and reflection, large surface area and enhancement of
visible light absorption range, the defective Bi2O2CO3 demonstrated
high efficiency for the photodegradation of HCHO gases and methyl
Previous studies demonstrated that the photocatalytic activity of
semiconductors may be closely related to their surface nature. For
example, Li et al.[16] revealed that acid treatment could change the
orange. The defective Bi2O2CO3 also demonstrated superior stability, surface property of hematite, which substantially improved the
which is important for practical application. This work opens up new
opportunities for the design and fabrication of high activity Bi-based
photocatalysts for environmental protection.
efficiency of photoelectrons moving out of traps enhanced the
separation of electron-hole pairs and boosted the
photoelectrochemical performance. Inspired by Li’s work, we used a
simple acid treatment method to improve the photocatalytic activity
of Bi2O2CO3 catalyst. Subsequently, we found that a simple acid
treatment process could availably improve the photocatalytic
performance of Bi2O2CO3 by decreasing the recombination of
electron-holes and increasing the visible light absorption. The acid
treated Bi2O2CO3 microspheres exhibited 10 time higher and 5 times
higher photocatalytic performance than the pristine Bi2O2CO3
microspheres for HCHO gas and MO degradation respectively,
under the irradiation of visible light (λ ≥ 420 nm). Moreover, the
mechanism of photodegradation was revealed by electron
paramagnetic resonance. The enhanced separation of electron-hole
pairs, the large surface area and the enhancement of visible light
absorption range were attributed to the enhanced photoacitivty of
Bi2O2CO3 microspheres as a result of formation of surface defects
and caverns.
The increasingly severe environmental regulations for organic dyes
emissions from industrial wastewaters require the development of
economically feasible and effective technologies.[1] Simultaneously,
photocatalysis is one of the promising ways to eliminate organic
dyes into nontoxic small molecule (CO2 and H2O) for energy
consumption at lower cost.[2] Nevertheless, the slow development of
catalysts with nontoxicity and high efficiency limit their practical
applications.[3] Therefore, it is imperative to seek high effective
catalysts to meet the requirements of potential industrial applications.
Bismuth subcarbonate (Bi2O2CO3) is
a typical Sillén phase
compound with high earth abundance and low toxicity.[4] It has drawn
remarkable attention in the environmental field due to its layered
2-
structure. CO3 groups interleaved into the stacking of (Bi2O2)2+
sheets, which is beneficial for photocatalytic performance.[5]
However, the low light absorption ability (visible light) and the rapid
recombination of photogenerated electron-hole pairs inhibited the
photocatalytic performance of Bi2O2CO3.[6] Therefore, many
strategies were exploited to enhance the light absorption and
increase separation efficiency of photogenerated electron-hole pairs.
These strategies include combining with other narrow bandgap
The pristine Bi2O2CO3 (BOC) microspheres were prepared
according to our previous works.[4d] BOC-50, BOC-100 and BOC-
200 were obtained by using 0.01 M nitric acid (50 mL, 100 mL
and 200 mL) treatment with BOC sample, respectively. The
scanning electron microscopy (SEM) demonstrated that BOC
possess diameters of 1.5 μm microspheres (Figure 1a). After
treatment with nitric acid, the morphology of BOC-200 also
maintained the microspheres. However, the surface became rough
and some caverns appeared, suggesting the enhancement of light
harvest due to multiple scattering and reflection. The surface areas
gradually increase with the increase of the acid amount. The surface
areas of the BOC, BOC-50, BOC-100, BOC-200 were measured to
be 9.5, 13.2, 15.2, and 19.9 cm2/g, respectively (Figure S1a).
Subsequently, the microspheres were also confirmed to be 1.5 μm
according to the transmission electron microscopic image (TEM)
(Figure 1c). The HR-TEM image displayed a lattice spacing of 0.685
nm corresponding to the {002} facet of tetragonal Bi2O2CO3, which is
shown in Figure 1d and 1e. Figure 1f shows the results of
corresponding selected area electron diffraction (SAED) pattern
collected from BOC-200. The diffraction rings centered at {123},
{110}, {002} and {011} are identified for the tetragonal phase of
Bi2O2CO3.[4d]
semiconductors
(e.g.,
Ag2O/Bi2O2CO3,[7]
BiVO4/Bi2O2CO3,[8]
Bi2O2CO3/Bi2MoO6, N-BOC/GQDs[10]), doping (e.g., CO32--doped,[11]
N-doped,[4a, 4b, 12] Sb-doped,[5a] C-doped[13], I-doped[14]) and exposing
facets.[6, 11, 15] Although these methods could enhance the separation
[9]
[a] Y C. Huang, K S. Li, H. Liu*
Research Institute of Environmental Studies at Greater Bay
Guangzhou University
Guangzhou Higher Education Mega Center,Outer Ring Road No. 230,
Guangzhou 510006, China
E-mail: liuhong@gzhu.edu.cn
[b] Y. Lin, Prof. Y X. Tong*
MOE Laboratory of Bioinorganic and Synthetic Chemistry, the Key Lab
of Low-Carbon Chemistry and Energy Conservation of Guangdong
Province, School of Chemistry
Sun Yat-sen University
Guangzhou 510275, China
The X-ray diffraction measurement (XRD) was performed to study
the phase and composition of all the samples. As displayed in Figure
S1b, the peaks collected for pristine BOC microspheres corresponds
to the tetragonal phase (JCPDS card number # 41-1488). After
E-mail: chedhx@mail.sysu.edu.cn
[c] Chongqing Institute of Green and Intelligent Technology, Chinese
Academy of Sciences, Chongqing, 401122, China
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10.1002/cctc.201800101
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treatment with acid, the phase hasn’t change. However, the intensity
of peak {002} increases along with the content of acid increases.
After treatment with the acid, the surface became rough and some
caverns appeared, which will expose more {002} surface. So the
{002} peak increased in XRD. These results are consistent with the
TEM, where the {002} facet are observed. Furthermore, the Raman
spectra and FTIR spectra also demonstrated that the phase has not
changed (Figure S1c and S1d). The samples exhibited peaks at
about 551 and 848 cm-1, which are assigned to the stretching
vibrations of the Bi-O. And the peak at 1400 cm-1 is attributed to the
stretching vibrations of C-O and C=O.[17] All these results
demonstrated that treatment with the acid will not change the phase
of BOC.
Scheme 1 Scheme suggesting the formation of oxygen vacancies induced by
nitric acid treatment.
Figure 2a displays the EPR spectra of all the catalysts. The
pristine BOC did not show any signal in the window. While the BOC-
200 exhibited the symmetric peaks (g=2.0), which is a typical signal
of defects.[18a, 20] The defects in the samples can be related to the
peak intensity increment. Combined with the results of the XPS, the
2-
defects may be come from the lack of carbonate ions in the CO3
layers. Furthermore, the optical properties of the samples are
analysed by the UV-vis diffuse reflection spectra, which is shown in
Figure 2b. BOC-acid shows more absorption in the UV region than
the pristine BOC and exhibits absorption in the visible region.
Surprisingly, the light absorption is continuously increased from
BOC-50 to BOC-200. This may be attributed to the surface defects
and light trapping effect derived from the surface caverns of 3D
microspherical architectures. In order to study the surface defects
influence on the surface property of BOC, we performed the TPR
measurement. As displayed in Figure 2c, pristine BOC microspheres
have an intensity peak at about 410 oC, which may due to the
reduction of Bi3+ to Bi. In comparison to untreated BOC sample, the
reductive peak of BOC-200 reveals a shift of about 30 oC toward the
low temperature, suggesting the existent of surface defects in BOC-
200 microspheres.
Figure 1 SEM images of (a) pristine BOC and (b) BOC-200, (c, d, e) TEM
images and (f) SAED image of BOC-200.
The surface composition and chemical state of the entire BOC
samples are analysed via the X-ray photoelectron spectroscopy
(XPS). As displayed in Figure S2a, only Bi, C and O are detected for
all the samples, indicating there is no impurities. The peaks located
at 159.0 and 164.3 eV can be attributed to Bi 4f7/2 and Bi 4f5/2 and
corresponded to Bi3+ (Figure S2b).[18] The peaks have not change
after acid treatment, indicating the valence of bismuth did not
change. Besides, the high-resolution C 1s XPS spectra of pristine
BOC microspheres demonstrated two intensity peaks at 284.8 and
288.7 eV. The peak at 284.8 eV can be ascribed to adventitious
carbon species and the peak at 288.7 eV can be assigned to
carbonate ion in BOC. After the acid treatment, the peak at 284.8 eV
remain unchanged, while the peak at 288.7 eV shifted up to 288.9
eV, suggesting that acid treatment could have effect on the
2-
electronic distribution of C in the CO3 layers.[5b] This is very
important for high performance of BOC-200. The nitric acid maybe
2-
react with the CO3 into CO2 and H2O, leading to the lack of
2-
carbonate ions in the CO3 layers (Scheme 1). Obviously, the high-
resolution O 1s will change due to the change of CO32-. Figure S2d
shows the O 1s XPS spectra of all the samples, displaying slight
asymmetry.
A new broad peak at 534.8 eV was appeared,
associating with oxygen on the surface of H2O. This was because
the defects affected the surface properties of BOC-200, which could
be the same as Xie’s literature.[19]
Figure 2 (a) EPR spectra, (b) UV−vis diffuse reflectance spectra and (c)
TPR spectra of all the samples.
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The photocatalytic performance of the prepared BOC samples are
evaluated by means of HCHO removal in the gas phase. Previous
degradation can be observed after 150 min, suggesting that •OH
and h+ were the main active radicals during the photocatalytic
degradation. Moreover, the efficiency slightly decreased with the
reports have demonstrated that the photolysis of HCHO by itself can
be negligible upon visible light irradiation.[4d,
21]
−
Furthermore, TiO2
addition of benzoquinone (BQ, •O2 scavenger), revealing that •O2−
was performed as a comparison and it can remove only 8% HCHO
after 120 min. The pristine BOC can only remove 10% HCHO after
120 min under visible light irradiation, as displayed in Figure 3a. It is
obvious that the treated samples show higher photocatalytic
performance than that of pristine BOC. The BOC-200 completely
degraded HCHO gases at 100 min under the visible light irradiation,
which is 10 time higher than the pristine BOC. Furthermore, BOC-
200 displays superior cycling stability and with not any decay after
800 min irradiation (Figure S3), which can be confirmed by the
strong resistance to morphology and composition changes (Figure
S4). Interestingly, BOC samples could not only remove HCHO in the
gas phase, but also degrade the organic pollutions in aqueous
solution. For example, methyl orange (MO) as a typical organic dyes
pollutant is used to evaluate the activity of photocatalysts under
simulated solar irradiation.[22] The content of MO in aqueous for
BOC-200 decreases more quickly with time of solar irradiation for
acid treated samples than for pristine BOC, which can be seen in
Figure 3b. The degradation rate of MO in the presence of BOC-200
reaches 98% after 180 min under simulated solar irradiation, while
only less than 35% of MO molecules are decomposed with pristine
BOC in the same condition. All these above data demonstrated that
treating with acid could effectively enhance the photocatalytic
performance of BOC.
radicals did not appear in the photocatalysis. Electron spin
resonance (ESR) spin-trip technology is used to observe the active
−
radicals (such as •O2 and •OH.) during the reaction. Figure 3d
shows the results of ESR over BOC-200 samples under the dark
and visible light irradiation at room temperature. There are no
signals under the dark with BOC and BOC-200 samples, suggesting
that no •OH radicals are generated under the dark. While turning the
light on, four ESR signals can be seen (DPMO-•OH adducts signals),
revealing that •OH radicals are generated during photocatalytic
degradation. However, no signals appeared for BOC-200 sample no
matter under the dark or visible light irradiation, suggesting that no
−
•O2 generated during the photocatalysis (Figure S5). This is
−
because the redox potential of O2/•O2 is -0.33 V, which is more
negative than that of the CBM of BOC, revealing the CBM electrons
cannot reduce the O2 to •O2−.[4d] And the potential of the holes at the
VBM of BOC (3.53 eV) is more positive than the redox oxidize OH-
/•OH (1.99 eV), and therefore the holes can oxidize OH- into •OH
radicals. Then, •OH could oxidize organic pollutants into CO2 and
H2O.[24] These results reveal that h+ and •OH are the main active
species in the photocatalytic degradation.
Figure 3 (a) Photocatalytic performance of HCHO gases oxidation under
the irradiation of visible light (λ
≥420 nm), (b) Photocatalytic degradation
of MO (pH=7) with different time under the visible light irradiation (λ
≥
420 nm) and (c) Photocatalytic performance of degradation of MO with
different scavengers in the present of BOC-200. (d) ESR spectra of
DMPO-·OH over BOC and BOC-200 sample under visible light
irradiation and in the dark at room temperature.
Figure 4 (a) EIS Nyquist plots, (b) Transient photocurrent densities
under the visible light irradiation, (c) Photoluminescence spectra excited
by 300 nm, and (d) Nanosecond-level time-resolved fluorescence
spectra of all the samples. (e) The proposed migrated mechanism of
photoelectrons and holes.
In order to study the photocatalytic mechanisms over BOC-200
sample, we added some different reagents during the
photodegradation of MO under the visible light, as displayed in
Figure 3c. The pH values of pristine MO solution is 7.0. After adding
−
benzoquinone (BQ, •O2 scavenger), tert-butylalcohol (TBA, •OH
The photoinduced electrons, holes separation and charge carrier
dynamics are very important for the photocatalysis. Electrochemical
impedance spectroscopy (EIS) is firstly used to observe the charges
migration process. As displayed in Figure 4a, the arc radius of acid
treated BOC samples are much smaller than that of pristine BOC,
suggesting that the defects have a positive effect on the separation
scavenger) and triethanolamine (TEOA, h+ scavenger), the pH
values are 7.0, 7.1 and 8.2, respectively. The influences of pH
(between 7.0-8.5) can be ignored according to previous reports.[23]
After adding TBA and TEOA into the photocatalytic system, the
photodegradation rate decreased apparently. Only about 10%
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and transfer efficiency of photogenerated electron-hole pairs.[25] And
the BOC-200 sample has the smallest arc radius, which is consistent
with the photocatalytic performance. Furthermore, photocurrent-time
response is performed to study the interfacial charge separation and
transfer dynamics of semiconductors. BOC-200 shows a rapid and
dramatic photocurrent generation with superb reproducibility under
the light illumination, revealing the generation of photoexcited
electrons (Figure 4b). Subsequently, photoluminescence (PL) is
carried out to confirm the enhanced separation efficiency of
photogenerated charge carriers, which is shown in Figure 4c. All the
treated samples demonstrate lower PL emission than that of pristine
BOC, indicating that recombination emission by surface traps is
reduced by surface defects.[26] Figure 4d shows the nano-second-
level time-resolved fluorescence decay spectra of BOC and BOC-
200. The radiative lifetime of BOC and BOC-200 were 3.851 and
4.494 ns, which were obtained by the decay spectra (Table S1). This
demonstrated that defects can increase the radiative lifetimes of
BOC.[21b] Based on the above analysis, there are two main reasons
for the efficient photocatalytic performance of acid treated BOC
samples. First, the surface defects narrow the bandgap, thereby
expanding absorption in the visible region. Thus, some caverns
enhance the light harvest due to multiple scattering and reflection.
Second, the acid treatment also introduces surface defects, which
facilitate charge transfer. Hence, according to Figure 4e, the caverns
can effectively reduce the migration path of holes that is beneficiary
for improving holes transfer efficiency. All these can hinder
recombination of charge carriers to accelerate the photocatalytic
reactions. The synergistic effects of the above two factors enhance
the photocatalytic performance of acid treated BOC samples.
Avatar 370) and Transmission electron microscopy (TEM,
JEM2010-HR) were carried out to characterize the samples.
Furthermore, UV-vis diffuse reflectance spectra were recorded on a
UV-vis spectrophotometer (UV-2450). The photoluminescence
spectrometer were performed by Fluorospectro photometer (F-
4500). The specific surface was analysed by the measurement
instrument (ASAP 2020). Electron spin resonance tests were
carried out in the X-band with 5.00 G modulation amplitude. The
electrochemical testing were carried out with
electrochemical station in a standard three electrode configuration.
Detailed steps can be seen in our previous works.[4d, 21]
a CHI 660 C
.
Photocatalytic activity measurements
：
0.01 g materials was used in the photocatalysis. HCHO gas was
produced by injecting the purified air flow (N2/O2 = 4,100 mL min-1)
into 37% HCHO solution (kept at 0 oC). The gas hourly space
velocity was maintain at 60,000 mL h-1. A 300 W Xe lamp (a 420
nm UV light cut filter) was used as the light source. The products
were analyzed by gas chromatograph (Agilent 7890A). Using the
following formula to calculated the HCHO conversion:
HCHO conversion (%) = [CO2]out/[HCHO]in×100
[CO2]out and [HCHO]in are the CO2 concentration in the products
and the HCHO concentration in the reactor, respectively.
Methyl orange (MO) (100 mL, 10 mg L-1) solution was used to
evaluate the photocatalytic performance of catalysts (0.01 g).
Before the light on, the solution was stirred ceaselessly for 60 min
to eliminate the absorption ability of catalysts. Using the 300 W Xe
lamp as the light source with a 420 nm UV light cut filter and making
sure the light intensity is 100 mW cm-2. With different illuminating
time, 3.0 mL reactive solution was taken out. Using UV-vis
spectrophotometer to analyze the residual dyes concentration. The
absorption wavelength of MO was 655 nm. Finally, the
photocatalytic efficiency was calculated by the C/C0 × 100% (C is
the final dyes concentrations and C0 is the initial concentration of
dyes).
In this work, we demonstrated that surface defects and caverns
could be induced into BOC microspheres via acid treatment. The
structural properties, the morphology and photocatalytic
performances of the pristine BOC and acid treated BOC samples
were studied systematically. In comparison with pristine BOC
microspheres, BOC-200 microspheres exhibited
5 folds higher
photocatalytic performance for MO degradation under the irradiation
of visible light ( λ ≥ 420 nm). The BOC-200 microspheres also
displayed the superior stability after 180 min under simulated solar
irradiation. Furthermore, the mechanism of photodegradation was
revealed by electron paramagnetic resonance analyses. The
enhanced separation of electron-hole pairs, the large surface area
and the enhancement of the visible light absorption range were
attributed to the enhanced photoactivity of BOC-200 microspheres
as a result of formation of surface defects and caverns. We believe
that this work not only opens new opportunities for the design of high
performance BOC for photocatalytic degradation, but also offers a
new method to fabricate defective materials.
Acknowledgements ((optional))
This work was supported by the Natural Science Foundation of
China (51525805, 21706295, 51378494, 21461162003, 21773315
and 21476271), Natural Science Foundation of Guangdong Province
(2017A030313055 and 2015B010118002) and the Fundamental
Research Funds for the Central Universities (17lgjc36).
Keywords: photocatalysis • Bi2O2CO3 microspheres • acid
treatment • charge separation • environmental protection
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10.1002/cctc.201800101
ChemCatChem
COMMUNICATION
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COMMUNICATION
Upon generation from
Bi2O2CO3, electrons and
Yongchao Huang,
Kunshan Li, Ying Lin,
Yexiang Tong* and Hong
Liu*
holes
separated
existence of defects. The
were
superbly
with the
Page No. –Page No.
defective
demonstrated
efficiency
Bi2O2CO3
high
Enhanced efficiency of
electron-hole separation
in Bi2O2CO3 for
for
the
photocatalysis.
photocatalysis via acid
treatment
This article is protected by copyright. All rights reserved.