Ultrafine Pd nanoparticles loaded benzothiazole-linked covalent organic framework for efficient photocatalytic C-C cross-coupling reactions

Article abstract of DOI:10.1039/d0ra03739g

We proposed a strategy that a benzothiazole-linked covalent organic framework (TTT-COF) was used as a substrate to prepare metal composite photocatalyst Pd NPs?TTT-COF. Firstly, benzothiazole linked TTT-COF exhibited superior chemical stability and photoresponse. Moreover, a finer particle size (2.01 nm) and more uniform distribution of Pd NPs were observed in Pd NPs?TTT-COF owing to the binding interaction between Pd NPs and S in benzothiazole groups. Pd NPs?TTT-COF exhibited superior efficiency and reusability in photocatalytic C-C cross-coupling reactions. Mechanism study suggested that photogenerated electrons and holes on TTT-COF played important roles in these reactions. This journal is

Full text of DOI:10.1039/d0ra03739g

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PAPER
Ultrafine Pd nanoparticles loaded benzothiazole-
linked covalent organic framework for efficient
photocatalytic C–C cross-coupling reactions†
Cite this: RSC Adv., 2020, 10, 29402
a
a
Yongliang Yang,ab Hongyun Niu,
Weijia Zhao,ab Lin Xu,a Hui Zhang
acd
*
and Yaqi Cai
We proposed a strategy that a benzothiazole-linked covalent organic framework (TTT-COF) was used as
a substrate to prepare metal composite photocatalyst Pd NPs@TTT-COF. Firstly, benzothiazole linked
TTT-COF exhibited superior chemical stability and photoresponse. Moreover, a finer particle size (2.01
nm) and more uniform distribution of Pd NPs were observed in Pd NPs@TTT-COF owing to the binding
interaction between Pd NPs and S in benzothiazole groups. Pd NPs@TTT-COF exhibited superior
efficiency and reusability in photocatalytic C–C cross-coupling reactions. Mechanism study suggested
that photogenerated electrons and holes on TTT-COF played important roles in these reactions.
Received 26th April 2020
Accepted 3rd August 2020
DOI: 10.1039/d0ra03739g
rsc.li/rsc-advances
surface energy and result in the deactivation of the catalysts.10
Therefore, stable and uniform supporting substrates are
Introduction
necessary for metal NPs catalysts, and COFs are suitable for
being adopted as a kind of substrates. On the one hand, on the
basis of regular and tunable pore structure as well as high
surface area of COFs, ultrane metal clusters can be embedded
on the pores of COFs with conned nano sizes uniformly. On
the other hand, heteroatoms can be articially introduced into
the COFs frameworks owing to the designability of COFs
building blocks, thus metal clusters can be anchored in the
pores via the binding interactions between metal atoms and
heteroatoms. Zhang's group has reported some metal NPs-COFs
hybrid materials as stable and efficient heterogeneous cata-
lysts,11,12 whose COFs substrates were synthesized from the
precursors containing heteroatoms (S, P).
Covalent Organic Frameworks (COFs) are a class of conjugated
porous organic materials constructed with organic building units
via strong covalent bonds.1–3 To date, most research on the design
of COFs has been focused on regulating the chemical structure of
the organic modules. It's worth noting that the linkage of COFs is
also crucial to the properties of COFs. Dynamic covalent linkages
between building blocks are necessary for the construction of
crystalline and ordered COFs architectures, but are not conducive
to the robustness of COFs,2 which is a major impediment to the
applications. Recently, the exploration of novel linkages has
become the focus in the development of COFs, a series of novel
covalent linkages such as olen linkage,4 benzimidazole5/ben-
zoxazole6/benzothiazole7 linkages, amide linkage,8 dioxin
linkage9 have been reported, which could not only enhance the
chemical stability compared to the COFs based on boronate ester
linkage and imine linkage, adopted by a majority of known COFs,
but also exhibited more fascinating and unique properties.
Metal nanopaticles (NPs) are widely used as heterogeneous
catalysts in chemical synthesis due to their high surface area
and ultrane size distribution. However, the ultrane metal
clusters tend to gradually accumulate on account of their high
Here in, we propose an ingenious strategy to load Pd NPs on
photoactive benzothiazole-linked TTT-COF as a Mott–Schottky
aState Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center
for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China.
E-mail: caiyaqi@rcees.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
cInstitute of Environment and Health, Jianghan University, Wuhan 430056, China
dInstitute of Environment and Health, Hangzhou Institute For Advanced Study, UCAS,
China
Scheme 1 Graphical representation of the synthesis of Pd NPs@TTT-
COF.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra03739g
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heterojunction. Heteroatoms were introduced into the COFs crystal structure of TTT-COF aer reduction by NaBH4 and
linkage bonds through a post-synthesis modication scheme. loading of Pd NPs (Fig. S2†).
The as-designed hybrid materials Pd NPs@TTT-COF exhibited
The chemical transformation of TTI-COF to TTT-COF was
remarkable photocatalytic activities as well as high stability and veried by solid-state 13C cross-polarization magic angle spin-
recyclability in a series of C–C cross-coupling reactions (Suzuki– ning nuclear magnetic resonance (CP-MAS NMR) spectroscopy
Miyaura, Stille, Heck and Sonogashira) (Scheme 1).
and Fourier transform infrared (FT-IR) spectroscopy. As shown
in Fig. 1d, the imine carbon 2 signal at 158 ppm was shied to
169 ppm corresponding to thiazole carbon 20 signal and merged
into one peak with triazine carbon 10 signal. Carbon 3 signal at
151 ppm was shied to 30 signal at 158 ppm and carbon 4 signal
at 115 ppm disappeared due to integration of 40 signal into the
benzene rings range of 120–140 ppm. All these variations
conrmed the formation of thiazole structures. A weak 151 ppm
signal indicated trace amounts of unreacted imine structures.
In the FT-IR spectra (Fig. 1f), the disappearance of the charac-
teristic vibration of imine HC]N at 1625 cmꢁ1 in TTI-COF and
the presence of C]N vibration at 1608 cmꢁ1 corresponding to
thiazole units in TTT-COF conrmed successful conversion of
TTI-COF to TTT-COF.13 All characteristic peaks in TTT-COF were
preserved in Pd NPs@TTT-COF, which manifested the retention
of chemical structure aer treatment.
The thermogravimetric analysis (TGA) curves (Fig. S3†)
showed that benzothiazole-linked TTT-COF exhibited better
thermal stability in comparison to imine-linked TTI-COF.
Additionally, the chemical stability was signicantly enhanced
by the transformation of linkage. Both TTI-COF and TTT-COF
can resist neutral and strong acidic (10 M HCl) conditions,
however, structural collapses of TTI-COF were observed under
strongly alkaline (10 M NaOH) and strongly reductive (NaBH4)
conditions whereas TTT-COF exhibited high tolerance to such
extreme conditions (Fig. 2a and b). Considering that the
reduction process via NaBH4 was essential to prepare Pd
NPs@TTT-COF and alkaline conditions were generally used in
cross-coupling reactions, it is of great signicance to improve
the chemical stability of the substrates by locking the reversible
imine linkage.
Results and discussion
Imine-linked TTI-COF was converted to benzothiazole-linked
TTT-COF by a post-synthetic sulfuration strategy,7 Pd NPs
were introduced to the TTT-COF substrates subsequently via an
impregnation-reduction method to obtain Pd NPs@TTT-COF.
The content of Pd in the composite material is 6.01 wt%
determined by inductively coupled plasma mass spectrometry
(ICP-MS).
As shown in Fig. 1a and b, the powder X-ray diffraction
(PXRD) patterns exhibited six distinct diffraction peaks at 4.08ꢀ,
6.88ꢀ, 8.16ꢀ, 10.62ꢀ, 14.54ꢀ and 25.64ꢀ for TTI-COF and
4.16ꢀ,7.14ꢀ, 8.32ꢀ, 10.96ꢀ, 14.44ꢀ and 25.82ꢀ for TTT-COF, cor-
responding to the crystal planes of (100), (110), (200), (210),
(310) and (001). The structure models of TTI-COF and TTT-COF
were simulated by Material Studio (v. 7.0), and the experimental
patterns of both showed good agreement with the each simu-
lated PXRD pattern of eclipsed AA stacking models. (Fig. S1†)
Aer Pawley renement of experimental PXRD patterns, a unit
ꢀ
ꢀ
ꢀ
cell with parameters a ¼ b ¼ 25.54 A, c ¼ 3.51 A, a ¼ b ¼ 90 , g ¼
120ꢀ and agreement factors Rwp ¼ 6.42%, Rp ¼ 4.67% for TTI-
ꢀ
ꢀ
ꢀ
ꢀ
COF and a ¼ b ¼ 24.88 A, c ¼ 3.51 A, a ¼ b ¼ 90 , g ¼ 120 with
Rwp ¼ 3.87% and Rp ¼ 2.68% for TTT-COF was obtained. An
ordered sliding of the PXRD patterns was observed aer sulfu-
ration (Fig. 1c), which could be explained by the decrease of cell
ꢀ
ꢀ
parameters (25.54 A vs. 24.88 A) from TTI-COF to TTT-COF
according to Bragg's law. An almost undifferentiated PXRD
pattern for Pd NPs@TTT-COF conrmed the retention of the
Fig. 1 Comparison of the experimental PXRD patterns of TTI-COF (a) and TTT-COF (b) with the Pawley refined pattern (black), simulated pattern
for AA stacking (blue) and difference plot (green). Comparison of PXRD (c) and 13C NMR (d) patterns between TTI-COF and TTT-COF. (e)
Assignment of the 13C signals to the respective 13C nuclei in the structures. (f) Comparison of IR patterns between TTI-COF, TTT-COF and Pd
NPs@TTT-COF.
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Fig. 2 PXRD patterns of TTI-COF (a) and TTT-COF (b) after processing under relevant conditions. HR-TEM images of Pd NPs@TTT-COF (c) and
Pd NPs@TTI-COF (d), the insets are the size distribution profiles of Pd NPs. (e) S 2p region in the XPS spectra of TTT-COF before and after Pd
loading.
The permanent porosities of COFs were assessed by NPs and Pd NPs supported by TpPa-1 previously reported.14,15 In
measuring nitrogen adsorption isotherms at 77 K. All of TTI- addition, Pd NPs in Pd NPs@TTI-COF exhibited higher contrast
COF, TTT-COF and Pd NPs@TTT-COF showed typical type-IV and clearer lattice fringes in HR-TEM images than those in Pd
isotherms, which are characteristic of mesoporous materials. NPs@TTT-COF, revealing that Pd NPs tend to be distributed on
The Brunauer–Emmett–Teller (BET) surface areas of TTI-COF, the surface of the TTI-COF substrate, whereas in Pd NPs@TTT-
TTT-COF and Pd NPs@TTT-COF were found to be 1659, 1640 COF, more Pd NPs were encapsulated in the pores and inter-
and 1025 m2 gꢁ1, with total pore volumes of 0.7024, 0.6989, and layer of TTT-COF. This could be attributed to the addition of
0.434 cm3 gꢁ1, respectively (Fig. S4†). The similar BET surface sulfur element. Sulfur atoms in benzothiazole groups could
areas of TTI-COF and TTT-COF indicated that sulfuration boost the anchoring effect of COFs substrates on Pd NPs and
process did not signicantly change the porous structure, and conne them in the internal pores of COFs to restrict their
an evident decrease of that in Pd NPs@TTT-COF conrmed that aggregation, thus Pd NPs can be distributed in the substrates
Pd NPs were distributed in pores within COFs. Pore size with a ner and more uniform size.
distribution (PSD) of TTI-COF and TTT-COF were derived from
X-ray photoelectron spectroscopy (XPS) was used to investi-
NLDFT as 2.34 and 2.16 nm, respectively, which were consistent gate the oxidation state of the Pd NPs and interactions between
with the reduction of cell parameters aer sulfuration deduced Pd species and COFs substrates. In the XPS spectra of Pd
by PXRD.
NPs@TTT-COF, the characteristic peaks of Pd 3d5/2 and 3d3/2
Scanning Electron Microscopy (SEM) was used to charac- were located at 335.9 eV and 341.2 eV respectively (Fig. S8†),
terize the morphology of COFs. The characteristic petal-like which were slightly higher than the binding energy (BE) values
morphology of TTI-COF formed by p–p stacking of the COF of metallic Pd (335.0 eV for Pd 3d5/2 and 340.3 eV for Pd 3d3/2
)
layers was maintained in Pd NPs@TTT-COF aer sulfuration but much lower than the BE values of Pd(II) species, conrming
under high temperature, reduction by NaBH4 and loading of Pd that Pd(II) precursor was successfully reduced to Pd(0) by
NPs (Fig. S5†). SEM-EDX elemental mapping exhibited homo- NaBH4. The BE center of S 2p in Pd NPs@TTT-COF was located
geneous distribution of sulfur and palladium in COF substrate at 163.75 eV, exhibiting a negative shi of 0.3 eV compared with
(Fig. S6†). Long-ordered channels of TTT-COF can be clearly that in TTT-COF. Such positive and negative shi of BE for Pd
observed in the high resolution transmission electron micro- 3d and S 2p were probably caused by the charge transfer from
scope (HR-TEM) image (Fig. S7†). The characteristic d-spacing Pd NPs to S atoms, which conrmed the metal–ligands inter-
of p–p stacking, which is equal to the distance between face-to actions between Pd NPs and S atoms of TTT-COF substrates.16
face 2D TTO-COF plane. Another d-spacing of 2.16 nm is The S 2p XPS spectra of TTT-COF exhibited two extra peaks in
observed in the vertical direction. The d-spacing of 0.34 and 168.3 and 162.8 eV, which is caused by the residual sulfur in
2.16 nm are corresponding to the diffraction peaks of (001) and TTT-COF aer the sulfuration reactions. However, no changes
(100) facets in the PXRD pattern according to Bragg's law (nl ¼ were observed in BE of N 1s aer Pd NPs loading in TTT-COF.
2d sin q). In the Fig. 2c, Pd NPs were distributed in TTT-COF These results revealed that sulfur atoms have a stronger inter-
substrate uniformly with an average particle size of 2.01 nm. action with Pd NPs than N atoms (Fig. S9†), which is an
The interplanar spacing of 0.23 nm in the region of Pd NPs important factor for ner particle size of the Pd NPs in
corresponded to Pd (111) crystal plane (Fig. S7†). However, benzothiazole-linked TTT-COF than that in imine-linked TTI-
a larger and uneven size of Pd NPs (average size: 6.3 nm) was COF.
observed in Pd NPs@TTI-COF (Fig. 2d), which was similar to Au
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Fig. 3 (a) UV-DRS patterns of TTI-COF and TTT-COF, the insets are the photographs of the samples. (b) Steady-state photoluminescence (PL)
spectra (b) and transient fluorescence spectra (c) of TTI-COF and TTT-COF. (d) Photocurrent measurements of TTI-COF and TTT-COF. (e) VB
XPS spectra of TTI-COF and TTT-COF. (f) Band alignment of TTI-COF and TTT-COF.
In the UV-DRS spectrums, TTT-COF exhibited a broader of band structure in TTI-COF aer sulfuration. The VB edges of
absorption band in the visible region than TTI-COF (Fig. 3a) TTI-COF and TTT-COF were located at 2.38 and 1.38 eV (Fig. 3e),
with a band narrowing of 0.3 eV (Fig. S10†), revealing that the and the conduction band (CB) potential were calculated as
sulfuration of COFs can improve the light-harvesting efficiency ꢁ0.27 and ꢁ0.96 eV, respectively, according to the formula ECB
in the visible region and narrow the optical bandgap. Compared ¼ EVB ꢁ Eg.
with TTI-COF, a signicant decrease of uorescence of TTT-COF
Palladium catalysts have been widely used in C–C cross-
meant lower recombination rate of photogenerated electron– coupling reactions, but the conditions of heating and reux
hole pairs (Fig. 3b), and a shorter transient uorescence lifetime are necessary for most of these reactions. Considering that NPs
of TTT-COF conrmed higher conduction efficiency of photo- with ultrane size would generally exhibit better catalytic
carriers17,18 (Fig. 3c). In addition, higher photocurrent response performance and TTT-COF is a photosensitive semiconductor,
of TTT-COF also indicated better separation and migration of the Suzuki–Miyaura cross-coupling reaction of iodotoluene and
photocarriers in COFs with benzothiazole linkages (Fig. 3d), phenylboronic acid under visible light was used as a model
which could be attribute to the enhanced p-conjugation within reaction to investigate the photocatalytic activity of Pd
the COF planes from imine linkages to benzothiazole linkages. NPs@TTT-COF. The reaction system included iodotoluene (0.3
A similar result was also conrmed in previous report of mmol), phenylboronic acid (0.4 mmol), excess potassium
benzoxazole-linked COFs.5 The valence band X-ray photoelec- carbonate and 5 mg photocatalyst in ethanol/H2O, an excellent
tron spectroscopy (VB XPS) was used to investigate the variation yield (99%) of desired product biphenyl was obtained within an
hour under visible light. The turnover frequency (TOF) of the
composite photocatalyst for the model reaction was calculated
as 344 hꢁ1, which is signicantly higher than that of previous Pd
NPs loaded photocatalysts (Table S3†). A series of control
Table 1 Control experiments for the model reaction
experiments were carried out (Table 1), which conrmed that
illumination, Pd NPs and TTT-COF substrates were all neces-
sary for this photocatalytic Suzuki–Miyaura reaction. Addition-
ally, p-benzoquinone (BQ, an electron scavenger) and
diisopropylethylamine (iPr2NEt, a hole scavenger) were added
to the reaction system respectively to investigate the mechanism
of the photocatalytic reaction. The photocatalytic cross-
coupling reaction was greatly blocked in the presence of BQ
and iPr2NEt, respectively. When both were added to the reaction
system, the reaction was almost blocked. It can be seen that
photogenerated electrons and holes were both signicant to the
photocatalytic Suzuki–Miyaura cross-coupling reaction.
Entry
Photocatalyst
Light
Additiveb
Yieldc/%
1
2
3
4
5
6
7
Pd NPs@TTT-COFa
TTT-COF
-
Pd NPs@TTT-COF
Pd NPs@TTT-COF
Pd NPs@TTT-COF
Pd NPs@TTT-COF
3
3
3
7
99
N.D.
N.D.
22
31
37
3
3
3
BQ
iPr2NEt
BQ + iPr2NEt
Trace
a
In the light of the above results and relevant studies,19 we
proposed a possible reaction mechanism of this photocatalytic
Suzuki–Miyaura reaction as shown in Scheme 2. Upon the
Reaction conditions: 4-iodotoluene (0.3 mmol), phenylboronic acid
(0.35 mmol), K2CO3 (0.6 mmol), 6 mL ethanol/H2O, Xe lamp (>420
b
nm), room temperature, N2 atmosphere. The amount of additives
was 1 mmol. c Yields were determined by GC-MS.
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Table
2
Pd NPs@TTT-COF photocatalyzed C–C cross-coupling
reactions with different aryl iodides. Values in parentheses are the TOF
values (hꢁ1).
Scheme 2 Proposed mechanism for the photocatalytic model.
irradiation of visible light, electrons on the VB of TTT-COF were
excited to jump to the CB and migrate to Pd NPs which act as
electron sink. C–I bonds of the adsorbed iodotoluene will
become longer when excited electrons on the surface of the
electron-rich Pd NPs enter the unoccupied orbital of iodoto-
luene, which facilitated the cleavage of the C–I bond to generate
phenyl radicals and forming the intermediate of organometallic
complex. The B(OH)2 groups of phenylboronic acid were con-
verted into B(OH)3ꢁ groups under the alkaline atmosphere, and
the C–B bonds were cleaved by photogenerated holes to
generate another phenyl radical. Aer the process of reductive
elimination, the desired product biphenyl was generated and
Pd catalysts were regenerated.
Pd NPs@TTT-COF exhibited superior stability and reus-
ability. Almost no loss of activity was found in the model reac-
tion among the four recyclings of photocatalyst (Fig. S11†). No
signicant changes were observed in the PXRD and IR patterns
of Pd NPs@TTT-COF before and aer the fourth run of the
photocatalytic reaction (Fig. S12†) which beneted from the
good chemical stability of benzothiazole-linked TTT-COF.
Moreover, the porosity of Pd NPs@TTT-COF was also well
maintained aer four cycles, the BET surface areas was found to
be 1003.2 cm3 gꢁ1. As shown in the TEM images, there is no
signicant increase in the size of Pd NPs aer the fourth run,
(Fig. S13†) whereas in previous reports of metal NPs@TpPa-1,
the size of NPs was increased with the number of cycles.14 The
loading rate of Pd in the composite photocatalyst (5.88 wt%)
dropped slightly aer four cycles determined by ICP-MS. These
results can be attributed to the high chemical stability of TTT-
COF and the strong anchoring effect of the benzothiazole
groups in TTT-COF to Pd NPs.
The photocatalytic performance of Pd NPs@TTT-COF in
photocatalytic C–C cross-coupling reactions was further inves-
tigated (Table 2). Satisfactory conversions (82–99%) of various
substrates were observed in the photocatalytic Suzuki–Miyaura
reactions. Iodobenzenes with electron-donating substituents
showed lower conversions compared to the electron-
withdrawing ones, which can be explained by the fact that the
electron-rich aryl iodides are not easily attacked by the electrons
on the Pd NPs to result in the cleavage of C–I bond. On the
contrary, phenylboronic acid with electron-withdrawing
substituents exhibited lower conversion because electron-
decient phenylboronic acids were not favorable to react with
positive holes (h+). In addition, meta-methyl substituted iodo-
benzene and phenylboronic acid showed lower conversions
which could be attribute to electronic effect and steric
hindrance.20 Surprisingly, Pd NPs@TTT-COF has also shown
excellent efficiency of photocatalysis in a series of other classic
C–C cross-coupling reactions (Stille, Heck and Sonogashira).
The reactants with electron-withdrawing substituents still
exhibited higher conversions in such reactions, suggesting that
the series of C–C cross-coupling reactions could probably follow
similar reaction mechanisms. The TOF of each reaction was
calculated to be higher than that of other reported photo-
catalysts, (Table S3†) which was mainly attributed to two
factors: (1) the ultrane size and uniform distribution of Pd NPs
resulted in higher contact area and more catalytic sites of Pd
catalysts; (2) benzothiazole-linked p-conjugated COFs frame-
works exhibited a strong photoelectric response, indicating
high photoelectron density on Pd NPs and the increased the
catalytic efficiency. Interestingly, it is noted that cis-stilbene
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accounted for a large part of the products of the photocatalytic
Heck reactions whereas trans-stilbene was the main product in
previous reports.15 Control experiments have been carried out to
prove that isomerization of stilbene from trans (E) form to cis
(Z) form can be catalyzed by Pd NPs@TTT-COF under the visible
light irradiation (Table S4†). Recently, Banerjee's group re-
ported a triazine functionalized COF (TpTt) for the E to Z
isomerization of trans-stilbene via a process of triplet state
excitation,21 and triazine groups have also been found to play an
important role in the photoisomerization of stilbene.22 Hereby,
we believed that the cis products of the photocatalytic Heck
reactions were obtained by photoisomerization of the original
trans products catalyzed by triazine-based TTT-COF substrate
via a similar mechanism.
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Conclusions
In summary, we have developed a hybrid material Pd NPs@TTT-
COF photocatalyst. Several advantages of benzothiazole-linked
COFs substrate were proved as follows: (1) superior chemical
stability makes the COFs substrate resistant to extreme condi-
tions in the preparation and application of catalysts; (2)
benzothiazole-linked conjugated framework exhibited higher
conduction efficieney of photocarriers; (3) Pd NPs were
anchored by S atoms of benzothiazole groups in the pores of
COFs with an ultrane and uniform particle size. Pd NPs@TTT-
COF showed superior photocatalytic performance and reus-
ability in a series of C–C cross-coupling reactions. Considering
the designability of COFs as well as the diversity of metal NPs,
this strategy will provide great opportunities for the design and
synthesis of photocatalysts for a wide range of applications.
This work would inspire researchers that the linkage could also
bring some fascinating properties to COFs.
Conflicts of interest
There are no conicts to declare.
18 Q. Jia, S. Zhang, X. Jia, X. Dong, Z. Gao and Q. Gu, Catal. Sci.
Technol., 2019, 9, 5077–5089.
19 Z. J. Wang, S. Ghasimi, K. Landfester and K. A. I. Zhang,
Chem. Mater., 2015, 27, 1921.
20 D. Sun and Z. Li, J. Phys. Chem. C, 2016, 120, 19744–19750.
21 M. Bhadra, S. Kandambeth, M. K. Sahoo, M. Addicoat,
E. Balaraman and R. Banerjee, J. Am. Chem. Soc., 2019,
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Acknowledgements
This work was supported by the National Key Research and
Development Program (2016YFA0203100), the National Natural
Science Foundation of China (21537004, 21621064, 21777169).
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