Defect-Rich Bi12O17Cl2 Nanotubes Self-Accelerating Charge Separation for Boosting Photocatalytic CO2 Reduction

Article abstract of DOI:10.1002/anie.201809492

Solar-driven reduction of CO₂, which converts inexhaustible solar energy into value-added fuels, has been recognized as a promising sustainable energy conversion technology. However, the overall conversion efficiency is significantly limited by the inefficient charge separation and sluggish interfacial reaction dynamics, which resulted from a lack of sufficient active sites. Herein, Bi₁₂O₁₇Cl₂ superfine nanotubes with a bilayer thickness of the tube wall are designed to achieve structural distortion for the creation of surface oxygen defects, thus accelerating the carrier migration and facilitating CO₂ activation. Without cocatalyst and sacrificing reagent, Bi₁₂O₁₇Cl₂ nanotubes deliver high selectivity CO evolution rate of 48.6 μmol g^?1 h^?1 in water (16.8 times than of bulk Bi₁₂O₁₇Cl₂), while maintaining stability even after 12 h of testing. This paves the way to design efficient photocatalysts with collaborative optimizing charge separation and CO₂ activation towards CO₂ photoreduction.

Full text of DOI:10.1002/anie.201809492

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Defect-Rich Bi12O17Cl2 Nanotubes Self-Accelerating Charge
Separation for Boosting Photocatalytic CO2 Reduction
Authors: Jun Di, Chao Zhu, Mengxia Ji, Meilin Duan, Ran Long, Cheng
Yan, Kaizhi Gu, Jun Xiong, Yuanbin She, Jiexiang Xia,
Huaming Li, and Zheng Liu
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201809492
Angew. Chem. 10.1002/ange.201809492
Link to VoR: http://dx.doi.org/10.1002/anie.201809492
http://dx.doi.org/10.1002/ange.201809492

10.1002/anie.201809492
Angewandte Chemie International Edition
Defect-Rich Bi12O17Cl2 Nanotubes Self-Accelerating Charge
Separation for Boosting Photocatalytic CO2 Reduction
2
1
3
3
2
4
Jun Di1,2‡, Chao Zhu , Mengxia Ji , Meilin Duan , Ran Long , Cheng Yan , Kaizhi Gu ,
‡
Jun Xiong1, Yuanbin She5, Jiexiang Xia1,*, Huaming Li1, Zheng Liu2,*
1
School of Chemistry and Chemical Engineering, Institute for Energy Research,
Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, P. R. China
2
Center for Programmable Materials, School of Materials Science & Engineering,
Nanyang Technological University, Singapore 639798, Singapore
3
Hefei National Laboratory for Physical Sciences at the Microscale, School of
Chemistry and Materials Science, and National Synchrotron Radiation Laboratory,
University of Science and Technology of China, Hefei, Anhui 230026, P. R. China
4
School of Chemistry and Molecular Engineering, East China University of Science
and Technology, Shanghai 200237, P. R. China
5
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou,
310032, P. R. China
‡ These authors contributed equally to this work.
*Corresponding author: z.liu@ntu.edu.sg; xjx@ujs.edu.cn
1
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10.1002/anie.201809492
Angewandte Chemie International Edition
Abstract
Solar-driven reduction of CO2, that converts inexhaustible solar energy to value-added
fuels, has been recognized as a promising sustainable energy conversion technology.
However, the overall conversion efficiency is significantly limited by the inefficient
charge separation and sluggish interfacial reaction dynamic resulted from lacking
sufficient active sites. Herein, Bi12O17Cl2 superfine nanotubes with bilayer thickness of
tube wall are designed to achieve structural distortion for the creation of surface oxygen
defects, thus accelerate carrier migration and facilitate CO2 activation simultaneously.
Without cocatalyst and sacrifice reagent, Bi12O17Cl2 nanotubes deliver a high selectivity
CO evolution rate of 48.6 µmol g-1 h-1 in water, roughly 16.8 times than that of bulk
Bi12O17Cl2. Meanwhile the nanotubes maintain their photocatalytic activity even after
12 h testing. This excellent activity is ascribed to better adsorption and activation of
CO2, increased separation efficiency of electron-hole pairs for both bulk and surface,
as well as better CO liberation from catalyst. This work paves the way to design
efficient photocatalysts with collaborative optimizing charge separation and CO2
activation towards CO2 photoreduction.
Keywords: Bi12O17Cl2 nanotube; Oxygen defects; CO2 reduction; Photocatalytic;
Charge separation
2
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10.1002/anie.201809492
Angewandte Chemie International Edition
With the drastic consumption of traditional fossil fuels, the energy crisis and
environmental pollution have become serious problems worldwide.[1] Photocatalytic
reduction of CO2 has been considered as a promising alternative solution in the manner
by converting inexhaustible solar energy to value-added fuels. During the past decades,
numerous efforts have been devoted to the development of efficient photocatalysts with
different structures for efficient CO2 photoreduction.[2, 3] Despite certain progresses
have been acquired, the CO2 conversion efficiency is still fairly low, principally due to
the high CO2 activation energy, and the sluggish kinetics of involved multiple electrons
and protons transfer processes. In order to achieve an efficient CO2 conversion rate,
boosting the charge separation efficiency and building active sites for facilitating CO2
activation may be feasible approaches.
With the intense studies of layered materials, some functional 2D structures have
attracted massive interest not only in electronic devices but also in catalysis
application.[4-7] The high surface area enables better harvesting of solar energy, and the
atom-thick layers allow the photogenerated charge carriers to migrate faster from the
interior to the surface with less bulk recombination.[8] Benefit from these advantages, a
series of ultrathin materials, such as g-C3N4,[9] In2O3,[10] Bi2WO6,[11] ZnAl-LDHs[12] and
ZnIn2S4,[13] have been produced and widely investigated in the photocatalysis field.
However, in spite of numerous researches on the optimization of ultrathin layered
structures, the photocatalytic behavior is still unsatisfactory due to the high surface
charge recombination rate and the lack of sufficient surface active sites. For instance in
CO2 reduction process, electrons generated in the interior of semiconductor first
migrate to the surface and then activate the adsorbed CO2 at active sites. Although the
ultrathin structure reduces the recombination in the bulk, the surface-reached electrons
will still recombine with holes, lying in the fact that insufficient active sites at surface
can cause the sluggish interfacial reaction. To address aforementioned issues, building
surface active sites for CO2 activation and accelerating surface charge separation are
necessary. Previous reports have demonstrated that the oxygen defects on the surface
of photocatalysts are helpful for promoting the adsorption and activation of target
molecules (e.g. N2, O2, CO2).[14-16] By virtue of the increased local charge density at the
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10.1002/anie.201809492
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defect sites, these target molecules can create powerful interaction at oxygen defects,
lower the bond energy of molecules and facilitate the transfer of photoexcited electrons
to reaction species.[14, 17] Therefore, building surface oxygen defects is an important
way to realize more active sites for high-efficiency CO2 photoreduction.
Considering the intrinsic layered structure feature of 2D materials, if the 2D
materials can be curved to form tubular structure, it will inevitably result in surface
structure distortion. This structural distortion will lead to the escape of partial surface
oxygen atoms to form oxygen defects. Generally, the coordination-unsaturated atoms
at the steps, corners, kinks and edges of materials with more dangling bonds are prone
to act as active sites.[18] The atoms with abundant dangling bonds along the crooked
surface or near oxygen defects on the surface favors the increase of active sites.
Simultaneously, the surface oxygen defects can also serve as surface separation centers
for charge carriers, promoting the utilization efficiency of surface-arrived charge
carriers.
As a proof-of-concept demonstration, the layered Bi12O17Cl2 is selected as a model
system to perform investigation. The oxygen defect modified Bi12O17Cl2 superfine
nanotube with bilayer thickness of tube wall is constructed. Taking advantage of
ultrathin tube wall for fast bulk charge diffusion, tubular structure to accelerate surface
charge separation and oxygen defects for effective CO2 activation, the Bi12O17Cl2
nanotubes delivers a striking 16.8 times improved photocatalytic activity than bulk
Bi12O17Cl2 towards converting CO2 to CO.
Transmission electron microscopy (TEM) results of Bi12O17Cl2 nanotubes reveal
a clean, freestanding tube-like morphology with the average tube diameter around 6.2
nm (Figure S1a, b). From the enlarged image, the thickness of most tube wall was 2
layers (Figure S1c, d). Particularly, the surface microstructure and local atomic
structure are further studied by aberration-corrected scanning transmission electron
microscopy using the high-angle annular dark field imaging technique (STEM-
HAADF). As depicted in Figure 1a and 1b, this system displays a freestanding superfine
tube-like morphology. The tube diameter is about 6 nm and the near transparency in
tube center shows its ultrathin thickness of tube wall (Figure 1b, 1c). The partly
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10.1002/anie.201809492
Angewandte Chemie International Edition
distorted lattice can be observed due to the curved structure and existed oxygen defects
(Figure 1d).[7] From the enlarged image, the thickness of tube wall is 2 layers (Figure
1c, 1e). The measured exterior bismuth interatomic distance is 0.408 nm, much larger
than that of 0.389 nm in BiOCl (Figure 1e, 1f). The HAADF-STEM-EDS elemental
mapping of Bi12O17Cl2 nanotubes further suggests the uniformly distribution of Bi, O,
Cl elements (Figure S2). These results give solid direct evidence for the artificial
synthesis of clean and freestanding Bi12O17Cl2 nanotubes with bilayer tube wall
thickness.
The XRD pattern can be readily indexed to the tetragonal Bi12O17Cl2 (JCPDS No.
37-0702), suggesting the Bi12O17Cl2 materials have been prepared successfully (Figure
2a).[19] The weak peak intensity and larger half peak width meaning the poor
crystallinity, in good agreement with the superfine nanotubes with ultrathin thickness.
To determine the formation of oxygen defects, the low-temperature electron
paramagnetic resonance (EPR) analysis is performed as it can afford valuable
fingerprinting information regarding the trapped electrons and surface defects in
materials. The Bi12O17Cl2 samples displays an EPR signal at g = 2.001 (Figure 2b),
which can be assigned to the electrons trapped on oxygen defects.[10] However, the
Bi12O17Cl2 nanotubes exhibits the greatly enhanced signal intensity than bulk
Bi12O17Cl2 (characterizations of bulk Bi12O17Cl2 are shown in Figure S3-S5), revealing
the Bi12O17Cl2 nanotubes possess much higher oxygen defect concentration. XPS O 1s
core level spectra is shown in Figure 2c with two peaks located at 529.7 and 531.0 eV.
The band energy at 529.7 eV is deemed as the oxygen bond of Bi-O-Bi while the peak
at 531.0 eV can be assigned to a high number of defect sites with a low oxygen
coordination.[20] The area of the peak at 531.0 eV for Bi12O17Cl2 nanotubes is larger
than that of bulk counterparts, revealing the Bi12O17Cl2 nanotubes have more oxygen
defects, further testified by surface charge revealed by zeta-potentials. The zeta
potentials change from -4.8 mV for bulk Bi12O17Cl2 to +24.0 mV for the Bi12O17Cl2
nanotubes, which suggests the increased oxygen defect concentrations can attract more
positive charge on the surface.[21] In order to determine the structure differences of the
samples, Raman spectroscopy has been carried out (Figure 2d). The A1g and Eg modes
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10.1002/anie.201809492
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of Bi-Cl bond stretching mode at about 131 cm-1 in bulk Bi12O17Cl2 shifts to 139 cm-1
for Bi12O17Cl2 nanotubes. The peak at 95 cm-1 is ascribed to the A1g internal Bi-Cl
stretching mode, while its ratio in bulk Bi12O17Cl2 is much higher than that in Bi12O17Cl2
nanotubes. Together with the result of STEM-HAADF, TEM, XRD, EPR and XPS, it
suggests the bilayer Bi12O17Cl2 nanotubes with abundant oxygen defects have been
obtained which provide an ideal structure to boost the CO2 photoreduction activity. The
tubular structure will inevitable result in surface structure distortion and atoms with
more dangling bonds be formed. At the same time, the partial higher electron density in
oxygen defects is favorable to adsorption and activation of CO2. Therefore, the atoms
with abundant dangling bonds along the crooked surface or near oxygen defects are
preferable to be active sites for CO2 reduction.
It has been widely accepted that energy band structure of semiconductors will
greatly affect the photocatalytic behavior. The photoresponse range of the Bi12O17Cl2
nanotubes and bulk counterpart is determined by UV-Vis absorption spectra (Figure 3a).
Due to the quantum confinement effect, the onset absorption edge of Bi12O17Cl2
nanotubes displays blue shift relative to bulk Bi12O17Cl2.[11] However, the shoulder and
tail absorption in Bi12O17Cl2 nanotubes appears, which is derived from the abundant
surface oxygen defects.[22] The band gap of the Bi12O17Cl2 nanotubes is calculated to
be about 2.36 eV, which is slightly larger than that of bulk Bi12O17Cl2 (2.25 eV) (Figure
3b). The relative positions of valence band (VB) maximum for samples are determined
by XPS valence spectra. As shown in Figure 3c, the Bi12O17Cl2 nanotubes displays more
positive location (1.56 eV) than that of bulk Bi12O17Cl2 (1.40 eV). Thus, it is reasonable
to infer that its conduction band (CB) minimum is -0.8 and -0.85 eV of Bi12O17Cl2
nanotubes and bulk Bi12O17Cl2, respectively, as schematically illustrated in Figure 3d.
It is important to note that the downshifted CB position of Bi12O17Cl2 nanotubes still
meet the thermodynamic demands for CO2 reduction. Generally, the electrons on the
higher CB position have stronger reduction power, thus the CO2 is easier to be activated
to produce carbon-based fuels. In this system, the Bi12O17Cl2 nanotubes possess more
positive CB potential, thereby the increased CO2 photoreduction activity may be not
dominated by the CB potential.
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To disclose the role of unique tubular structure with bilayer thickness and surface
oxygen defects in affecting photocatalysis, the solar CO2 reduction experiments are
performed in water without cocatalyst and sacrifice reagent under the irradiation of 300
W Xe lamp. The dominant product is determined to be CO accompanied by a trace
amount of methane. The CO yield gradually improved with the illumination time, and
the total yield of CO achieved during 4 h reaction is 194.5 µmol g-1 (Figure 4a). The
average CO-generation rate of Bi12O17Cl2 nanotubes is up to 48.6 µmol g-1 h-1, which
is 16.8 times higher than that of the bulk Bi12O17Cl2 (2.9 µmol g-1 h-1), revealing the
giant increased photocatalytic activity of the Bi12O17Cl2 nanotubes. During the CO2
photoreduction process, Bi12O17Cl2 nanotubes can simultaneously realize H2O
oxidation into O2 with an average O2 evolution rates of about 23 µmol g-1 h-1. The
control experiments in dark, without photocatalysts or in Ar did not show the generation
of CO, demonstrating the CO is formed by CO2 reduction over Bi12O17Cl2 under light
irradiation. Moreover, the 13C isotope labelling experiment is performed over
Bi12O17Cl2 nanotubes using 13CO2 as substrate to explore the source of carbon. As
shown in Figure 4c, the peak at m/z = 29 in the mass spectra can be assigned to 13CO,
suggesting that the carbon source of CO is derive from the used CO2. This superior CO-
generation rate over Bi12O17Cl2 nanotubes is also outperform to many reported results,
such as oxygen vacancies-rich BiOCl,[16] zinc vacancies-rich one-unit-cell ZnIn2S4,[21]
partially oxidized SnS2 atomic layers,[23] defect-rich ZnAl-LDH,[12] carbon-doped BN
nanosheets,[24] α-Fe2O3/g-C3N4,[25] and UiO-66/C3N4 nanosheets.[26] More importantly,
no significant loss of photocatalytic activity can be observed after three cycle testing,
suggesting the stability (Figure 4b). XRD, TEM, XPS and EPR analysis in Figure S6
further demonstrate the structure of the Bi12O17Cl2 nanotubes does not occur any
obvious variation after photoreduction cycle test, certifying its favorable photostability.
The apparent quantum efficiency of Bi12O17Cl2 nanotubes at 400 nm is calculated to be
0.14% within 6 h.
To insight the origin of increased CO yield, the interfacial adsorption, charge
separation and CO desorption process are detail studied since they are crucial factors
essentially determining the efficiency of photocatalysis. It is widely accepted that a
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10.1002/anie.201809492
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larger specific surface area can adsorb more reactant. The Bi12O17Cl2 nanotubes
displays much higher BET specific surface area than bulk Bi12O17Cl2 (16.36 vs 3.73
m2/g) (Figure S7), which favors the photocatalytic process. The CO2 adsorption is
consider as the prerequisite for CO2 photoreduction, therefore the CO2 adsorption
isotherms are further determined (Figure 4d). The Bi12O17Cl2 nanotubes displays a 3.4
times higher CO2 adsorption amount than that of bulk Bi12O17Cl2, suggests the effective
adsorption of CO2. Moreover, the abundant surface oxygen defects in Bi12O17Cl2
•−
nanotubes are conducive to the CO2 adsorption and activation to form CO2
intermediate.[12, 16] Contact-angle measurement suggest hydrophilicity is not the
dominated factor to affect CO2 photoreduction process (Figure S8).
To study the migration and recombination process of photogenerated charge
carriers, the time-resolved transient photoluminescence (PL) spectra are performed
(Figure 4d).[26] The decay kinetic of Bi12O17Cl2 nanotubes is much slower than that of
bulk Bi12O17Cl2, in which the average lifetimes are determined to be 11.38 and 3.55 ns,
respectively. This greatly prolonged lifetime suggests the Bi12O17Cl2 nanotubes have
higher charge separation efficiency. Three features may account for the greatly
improved charge separation efficiency, namely bilayer thickness of tube wall, tubular
structure and abundant surface oxygen defects. The ultrathin thickness of tube wall
guarantees the rapid migration of charge carriers from the material interior to the
surface, while the charge carriers in bulk materials will be more likely to be lost due to
recombination.[18] At the same time, the generated electrons will to migrate along the
[Cl2] slices to [Bi12O17] slices (perpendicular to nanotube direction) due the internal
static electric fields, while the holes will transfer along the [Cl2] slices to the material
edges.[19, 27] Since the nanotube structure and the [Cl2] slices is along with the tube
direction (from STEM image), the generated holes will self-accelerating transfer along
the nanotubes. The directional conduction function of Bi12O17Cl2 nanotubes for holes
enable the effective charge separation. In addition, the surface oxygen defects can serve
as surface separation centers for charge carriers and further increases the lifetime of the
carriers. Moreover, the greatly decreased PL intensity of Bi12O17Cl2 nanotubes in
steady-state PL spectra and smaller resistance in electrochemical impedance spectra
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also suggest the better electrical conductivity and more efficient charge separation
(Figure. S9).
According to the previous reports, the reaction mechanism of CO2 reduction to
yield CO in H2O can be presented as follows:[23]
CO2 (g) → CO2*
H2O → H+ + OH-
CO2* + H+ + e- → COOH*
COOH* + H+ + e- → CO* + H2O
CO* → CO
(1a)
(1b)
(2)
(3)
(4)
where “*” means the corresponding adsorption state on the surface. To deeply insight
into CO2 photoreduction process over Bi12O17Cl2 nanotubes, in situ Fourier transform
infrared spectroscopy (FTIR) is carried out (Figure 4f). The peaks at 1609 and 1340
cm-1 were assigned to the asymmetric and symmetric OCO stretches of b-CO3 - groups,
2
respectively. A new IR peak around 1565 cm-1 appears and the intensity gradually
increased with the extended irradiation time, in which it can be ascribed to COOH*
intermediate, revealing the activation and reaction of adsorbed CO2. Additionally, CO
desorption process is also an important factor to determine whole CO2 photoreduction
performance. To assess the CO desorption performance, the CO temperature-
programmed desorption (TPD) measurements are performed (Figure S10). The lower
CO onset desorption temperature and higher total amount of detected CO for Bi12O17Cl2
nanotubes suggests the formed CO* molecules can deviate from the surface of
Bi12O17Cl2 nanotubes much easier, which enables the efficient and sustaining
production of CO.
In conclusion, bilayer Bi12O17Cl2 nanotubes with abundant surface oxygen defects
has been prepared. Compared to the bulk counterparts, the Bi12O17Cl2 nanotubes
displays improved CO2 reduction property under solar light irradiation. The unique
tubular structure with ultrathin thickness and defects-rich enable the Bi12O17Cl2
nanotubes with improved adsorption and activation of CO2, increased charge separation
efficiency, and better CO liberation from catalyst, optimizing the CO2 photoreduction
activity to yield CO. In brief, this study may open new opportunities for designing other
catalysts with similar structure for highly efficient solar driven CO2 reduction.
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Acknowledgements
This work is supported by the National Natural Science Foundation of China (No.
21676128, 21576123, 21476098, 11504046). This work also supported by MOE Tier 1
grant RG4/17, MOE Tier 2 grant MOE2016-T2-1-131, Singapore National Research
Foundation under NRF award number NRF-RF2013-08.
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Figure 1. (a-e) Aberration-corrected HAADF-STEM images of Bi12O17Cl2 nanotubes,
(f) intensity profile corresponding to the dark cyan arrow in e.
Figure 2. Characterizations for Bi12O17Cl2 nanotubes and bulk Bi12O17Cl2: (a) XRD
patterns, (b) EPR, (c) O 1s XPS spectra, (d) Raman spectra.
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Figure 3. (a) UV-vis spectra, (b) (αEphoton)
1/2 vs Ephoton curves, (c) XPS valence spectra
of the Bi12O17Cl2 samples, (d) schematic band structure obtained according to the
results in (b) and (c).
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Figure 4. (a) Photoreduction of CO2 into CO over Bi12O17Cl2 samples, (b)
photocatalytic stability of Bi12O17Cl2 nanotubes, (c) mass spectra of 13CO (m/z = 29)
produced over Bi12O17Cl2 nanotubes, (d) CO2 adsorption isotherms, (e) time-resolved
transient PL decay, (f) in situ FTIR spectra for the adsorption and activation of CO2
on Bi12O17Cl2 nanotubes.
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TOC
Defect-rich Bi12O17Cl2 superfine nanotubes were prepared for the photocatalytic
reduction of CO2. Benefiting from the superfine nanotube structure to accelerate charge
separation and oxygen defects to facilitate CO2 activation, the Bi12O17Cl2 nanotubes
displayed a CO formation rate of 48.6 µmol g-1 h-1 in water without cocatalyst and
sacrifice reagent, roughly 16.8 times than that of bulk Bi12O17Cl2.
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