Efficient visible-light-driven Suzuki coupling reaction over Co-doped BiOCl/Ce-doped Bi2O2CO3composites

Article abstract of DOI:10.1039/d0gc04206d

Suzuki coupling reaction is a widely practiced protocol in organic synthesis for the formation of C-C bonds. The conventional process for this reaction usually involves high temperatures and noble metals. Hence, the development of a green, cost-effective photocatalytic system is an attractive and challenging strategy for the reaction. Here, we report a modified palladium-free Co-doped BiOCl/Ce-doped Bi₂O₂CO₃(CBCB) composite, which shows high photocatalytic activity under white LED irradiation. At room temperature, an excellent yield (91%) of the desired cross-coupling product biphenyl was obtained in environmentally friendly solvents. Density functional calculations, together with the experimental results, show that the presence of Co and Ce ions results in the appearance of some impurity levels near the Fermi level of pure BiOCl and Bi₂O₂CO₃, which decreases their forbidden bandwidth, thus ensuring higher light absorption and superior electronic conductivity. Moreover, the intimate interfacial contact between Co-doped BiOCl and Ce-doped Bi₂O₂CO₃planes has a significant impact on the separation and transfer of photoinduced charge carriers, which ultimately leads to a remarkable increase in visible-light-driven photocatalytic activity.

Full text of DOI:10.1039/d0gc04206d

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Efficient visible-light-driven Suzuki coupling
reaction over Co-doped BiOCl/Ce-doped
Bi2O2CO3 composites†
Cite this: Green Chem., 2021, 23,
1823
Dan-Xia Zhao, Guo-Ping Lu
and Chun Cai
*
Suzuki coupling reaction is a widely practiced protocol in organic synthesis for the formation of C–C
bonds. The conventional process for this reaction usually involves high temperatures and noble metals.
Hence, the development of a green, cost-effective photocatalytic system is an attractive and challenging
strategy for the reaction. Here, we report a modified palladium-free Co-doped BiOCl/Ce-doped
Bi2O2CO3 (CBCB) composite, which shows high photocatalytic activity under white LED irradiation. At
room temperature, an excellent yield (91%) of the desired cross-coupling product biphenyl was obtained
in environmentally friendly solvents. Density functional calculations, together with the experimental
results, show that the presence of Co and Ce ions results in the appearance of some impurity levels near
the Fermi level of pure BiOCl and Bi2O2CO3, which decreases their forbidden bandwidth, thus ensuring
higher light absorption and superior electronic conductivity. Moreover, the intimate interfacial contact
between Co-doped BiOCl and Ce-doped Bi2O2CO3 planes has a significant impact on the separation and
transfer of photoinduced charge carriers, which ultimately leads to a remarkable increase in visible-light-
driven photocatalytic activity.
Received 12th December 2020,
Accepted 27th January 2021
DOI: 10.1039/d0gc04206d
rsc.li/greenchem
structures as catalysts in Suzuki coupling reactions, and the
yield obtained under illumination at 809 nm was ∼2 times
Introduction
Suzuki coupling of aryl halides with organoboron species is higher than that obtained for the reaction heated to the same
one of the most powerful tools in organic chemistry for the for- temperature (Scheme 1b).9 Apart from plasmonic metals, a
mation of C–C bonds.1–3 Typically, Suzuki reactions are per- high turnover frequency of 1053 h−1 was reported for the
formed under homogeneous catalytic systems using a variety Mott–Schottky type Pd/SiC heterojunction in the coupling of
of Pd(II) or Pd (0) complexes.4,5 Although typical homogeneous
catalysts are highly efficient and selective, the recovery of
potentially toxic palladium complexes and ligands is not in
line with the development of green chemistry. In contrast,
heterogeneous Pd catalysts are favored over homogeneous cata-
lysts because of their ease of handing and reusability.6,7
Nonetheless, many of these reported Suzuki coupling pro-
cesses with heterogeneous catalysts are performed under
heating conditions, which is energy intensive (Scheme 1a).
Thus, visible light-driven Suzuki coupling reactions based on
noble metal/semiconductor heterogeneous photocatalysis is a
hot topic in the field of sustainable chemistry, because it com-
bines catalytic efficiency and potential sunlight utilization.8
Sun and co-workers studied the use of plasmonic Au–Pd nano-
Chemical Engineering College, Nanjing University of Science & Technology, 200
Xiaolingwei, Nanjing 210094, P. R. China. E-mail: c.cai@njust.edu.cn;
Fax: +86-25-84315030; Tel: +86-25-84315514
†Electronic supplementary information (ESI) available: Complementary charac-
Scheme 1 Suzuki coupling reactions carried out under different
terisation and catalytic data. See DOI: 10.1039/d0gc04206d
heterogeneous catalysts.
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iodobenzene with phenylboronic acid under visible light importantly, CBCB exhibits excellent recyclability, and can be
irradiation at 30 °C (Scheme 1c).10 Lim et al. showed that reused at least five times without an obvious decrease of cata-
modification of MoS2 nanosheets with Pd nanodots is an lytic performance. We believe that this catalytic system meets
effective way to promote the Suzuki C–C coupling reaction the requirement of green chemistry. Furthermore, theoretical
under sunlight illumination at room temperature calculations and photoelectrochemical properties identified
(Scheme 1d).11
that doping heteroatoms in pure BiOCl and Bi2O2CO3 phases
Although supported palladium or other noble-metal-based can accelerate the separation of the interfacial electron–hole
catalysts show reasonable performance in C–C bond for- pairs, and the synergistic effect of favourable components in
mation, their expensive price and scarce reserves limit their the two phases could be the key factor in improving the photo-
practical applications. Moreover, compared with their semi- catalytic activity.
conductor counterparts, photocatalysts based on plasmonic
metal nanoparticles show relatively low light absorption.
Therefore, the development of a green and sustainable visible
Experimental section
light active semiconductor photocatalyst based on non-noble
Synthesis of photocatalysts
metals is of high interest for both academia and research.
A Co-doped BiOCl/Ce-doped Bi2O2CO3 composite photocatalyst
was synthesized by a solvothermal method, and it was labeled
as CBCB (Scheme S1†). For comparison, the BiOCl/Bi2O2CO3
composite was prepared according to the same procedure
except for the precursor without cobalt and cerium ions and
marked as BB. The detailed preparation methods of BiOCl,
Bi2O2CO3, Co-doped BiOCl, Ce-doped Bi2O2CO3, BB and CBCB
catalysts are provided in the ESI.†
Bismuth-based materials are important visible-light-respon-
sive photocatalysts and have generated a great deal of inter-
est.12 Among them, bismuth subcarbonate (Bi2O2CO3) and
bismuth oxychloride (BiOCl) with a particular Sillen structure
are Earth-abundant compounds with low toxicity.13 Their
inherent two-dimensional or three-dimensional layered crystal
structure shows excellent photocatalytic activity due to its pro-
longed photocarrier lifetime and large specific surface area
Despite these advantages, several shortcomings, such as low
visible light absorption ability and quantum efficiency, should
be kept in mind. To address these issues, several strategies
have recently attracted attention. Heteroatom doping is con-
sidered as an effective method because the electronic structure
and implicitly the photocatalytic activity can be rationally con-
trolled. For example, rare earth elements (Y,14 Eu,15 Er,16 etc.)
which have a 4f electron shell with a multi-electron configur-
ation can act as shallow potential trapping centers for photo-
generated electron–hole pairs. Therefore, doping with rare
earth elements can prolong the separation time of carriers,
thereby improving their photocatalytic activity.17 Bi-based
semiconductors doped with transition metal ions (Fe,18 Cu,19
Mn,20 Zn,21 etc.) exhibit a narrow bandgap and extended light
absorption to the visible and even infrared light
regions.20However, the formation of heterojunctions between
Bi-based semiconductors and other materials could greatly
influence their photocatalytic performance. Considering that
BiOCl and Bi2O2CO3 are similar in layered structure,22 BiOCl/
Bi2O2CO3 composites have the advantage of easily matching
the structure and forming heterostructures.23 To our knowl-
edge, this heterostructure has only been investigated for pollu-
tant elimination. We were motivated to explore its application
in photocatalytic Suzuki coupling reactions.
Characterization of photocatalysts
The as-prepared samples were characterized via X-ray powder
diffraction (XRD), X-ray photoelectron spectroscopy (XPS),
scanning electron microscopy (SEM), transmission electron
microscopy (TEM)-X-ray spectroscopy (EDS), high-resolution
transmission electron microscopy (HRTEM), UV-vis diffuse
reflectance spectroscopy (DRS), photoluminescence (PL) spec-
troscopy, electrochemical impedance spectroscopy (EIS), and
transient photocurrent response. Detailed information about
experimental instruments and setting parameters is provided
in the ESI.†
Photocatalytic activity tests
For typical reactions, organohalides (0.2 mmol), boronic acids
(0.22 mmol), K2CO3(0.45 mmol) and CBCB (25 mg) were
added to 3 mL of EtOH/H2O (1 : 1) mixture in a glass vial
under an argon atmosphere. The mixture was irradiated with a
white LED lamp (15 W) for 16 h. The products were analyzed
by GC-MS and GC.
Theoretical calculation method
First-principles calculations within the framework of density
functional theory (DFT) were performed by employing the
Vienna ab initio simulation package (VASP). The computational
details of calculation modes are provided in the ESI.†
Herein, a Co-doped BiOCl/Ce-doped Bi2O2CO3 (CBCB) het-
erojunction photocatalyst with a sphere-like morphology was
successfully prepared by a simple hydrothermal method. The
photocatalytic activity of the CBCB composite for the Suzuki
coupling reaction between bromobenzene and phenylboronic
acid was evaluated at room temperature and atmospheric
pressure. As expected, an excellent yield (91%) of the desired
Results and discussion
Characterization of samples
product was obtained in environmentally friendly solvents Fig. 1 shows the XRD patterns of different photocatalysts. The
(EtOH/H2O) under white LED irradiation (Scheme 1e). More diffraction peaks of Bi2O2CO3 and BiOCl can be indexed to
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Fig. 1 XRD patterns of (a) BiOCl, Co-doped BiOCl, Bi2O2CO3 and Ce-doped Bi2O2CO3; (b) BB and CBCB.
typical tetragonal Bi2O2CO3 (JCPDS card No. 41-1488)24 and
The morphologies of samples were analyzed by SEM. As
BiOCl (JCPDS card No. 06-0249),25 respectively. For Co-doped shown in Fig. 2a, the pristine BiOCl is mainly composed of
BiOCl and Ce-doped Bi2O2CO3, the sample still has the pure smooth nano-sheets with a size of 150–400 nm. The mor-
phase structure of BiOCl and Bi2O2CO3; no other compounds phology of Co-doped BiOCl is similar to BiOCl, except that the
are observed, and the intensities of the diffraction peaks have nanosheets become rougher, and the size of Co-doped BiOCl
become weaker, which is because the presence of metal ions remains in the range of 130–400 nm, indicating that the incor-
affects the crystal size and limits the growth of the crystal in a poration of metal Co ions has little effect on the morphology
specific lattice plane (Fig. 1a).26 While for the BiOCl/Bi2O2CO3 of Co-doped BiOCl (Fig. 2b). Bi2O2CO3 (Fig. 2c) presents a
heterojunction, compared with the standard card, it can be large number of self-assembled flower-like microspheres with
seen that the peak of Bi2O2CO3 or other compounds cannot be a width of 0.5–7 μm and a thickness of 20–70 nm, whereas Ce-
detected because the content of Bi2O2CO3 is low and the high doped Bi2O2CO3 exhibits loose agglomerated micro-flowers
diffraction intensity of BiOCl covers the peak of Bi2O2CO3 with a smaller diameter, suggesting that the presence of Ce
(Fig. 1b). Additionally, compared with pure BiOCl, an ions inhibits the growth and agglomeration process of
enhanced (102) peak emerged in the BiOCl/Bi2O2CO3 (BB) Bi2O2CO3(Fig. 2d). The BB composite samples are regular
phase. However, after the introduction of Co and Ce ions, the nanosheets with thicknesses of 5–60 nm, and finally aggre-
(002) and (003) peaks of CBCB became weaker and its (102) gated together to form flower-like microspheres (Fig. 2e).23
peak showed a small shift. The reason for such differences After the coupling of Co-doped BiOCl and Ce-doped Bi2O2CO3,
might be owing to the more intimate contact of the two phases the size and thickness of the CBCB composite (Fig. 2f) become
of Co-doped BiOCl and Ce-doped Bi2O2CO3 in the hetero- smaller and thinner compared to the BB composite. In the
structure vicinity.27
flower-like CBCB sample, the open pores formed by the inter-
Fig. 2 SEM images of as-prepared samples: (a) BiOCl, (b) Co-doped BiOCl, (c) Bi2O2CO3, (d) Ce-doped Bi2O2CO3, (e) BB and (f) CBCB.
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secting ultra-thin nanosheets allow the reactant to enter the Fig. S2a,† it can be seen that Bi, Cl, O, C, Co, and Ce elements
inner surface of the photocatalyst and provide more active are all existing in the full survey spectrum, which is in agree-
sites. Also, the open pores promote the multiple scattering of ment with the elemental mapping results. Two Bi peaks at
UV-vis light, thereby improving the light-harvesting ability.28 159.2 eV and 164.5 eV are ascribed to Bi 4f7/2 and Bi 4f5/2 from
The detailed microstructure of CBCB was further explored by Bi3+ ions in [Bi2O2]2+ (Fig. S2b†).32 The O 1s spectra (Fig. S2c†)
TEM. Fig. 3a clearly shows that the size of the CBCB is mainly deconvolved into three peaks with binding energies
nanosheets is 0.15–0.5 μm with an average thickness of about of 529.85, 531.17, and 532.3 eV, which can be assigned to the
20 nm. The Co-doped BiOCl nanosheets are well wrapped and lattice oxygen of Bi–O, the surface hydroxyl groups, and chemi-
infiltrated with Ce-doped Bi2O2CO3, indicating that the two sorbed oxygen, respectively.33 In Fig. S2d,† the C has three
components are well coordinated in the heterojunction.29 The chemical states, which are identified by the peaks at 284.8,
HRTEM image in Fig. 3b exhibits the lattice orientation and 286.2, and 288.78 eV, respectively. The peak fitted at 284.8 eV
heterojunction interface of CBCB. The clear lattice fringes of comes from the standard reference carbon, while the peak
0.275 nm and 0.273 nm are respectively ascribed to the (110) observed at 286.2 and 288.6 eV should be ascribed to the C–O
lattice planes of BiOCl and the (110) lattice planes of bond in [CO3]2−. The XPS signal of Cl 2p (Fig. S2e†) shows two
Bi2O2CO3.30 This result undoubtedly confirmed that the CBCB major peaks around 197.9 and 199.5 eV, corresponding to Cl
heterostructure has been successfully constructed as expected. 2p3/2and Cl 2p1/2 of Cl− in the CBCB sample, respectively.34
Similarly, a clear heterojunction interface can also be seen in The high-resolution Ce 3d spectrum is shown in Fig. S2f.†
the TEM image of BB (Fig. 1S†). Furthermore, similar layered Multiple peaks between 880 eV and 915 eV indicate the co-
structures of Co-doped BiOCl and Ce-doped Bi2O2CO3 facilitate existence of Ce3+ and Ce4+ in the CBCB nanocomposite. The
the formation of CBCB heterostructure nanocomposites. In peaks at 899.0 eV, 888.6 eV, and 881.6 eV are attributed to the
the microspherical CBCB material, Ce-doped Bi2O2CO3 retains 3d5/2 level of Ce(IV), while the diffraction peaks at 906.6 eV and
the original flower-like shape. Meanwhile, Ce-doped Bi2O2CO3 902.2 eV are ascribed to the 3d3/2 level of Ce(IV). The observed
was used as a precursor, and then Co-doped BiOCl nanosheets peaks at 903.9 and 885.5 eV belong to Ce(III).35,36 In the spec-
were grown on the surface. It is conjectured that the interface trum of Co 2p, six peaks appear (Fig. S2g†), and the peaks at
contact layer between Co-doped BiOCl and Ce-doped Bi2O2CO3 778.1 and 795.3 eV are designated as Co0 2p3/2 and Co0 2p1/2
,
nanosheets can be constructed through sharing the (Bi2O2)2− respectively.37 The binding energy of the Co 2p1/2 peaks are
sheets from Bi2O2CO3.31 In Scheme S1,† the possible growth located at 804.8 and 796.5 eV, and the Co 2p3/2 peaks are
+
mechanism of a heterojunction is proposed. The corres- observed at 781.2 and 780.3 eV, corresponding to the Co2
ponding HAADF-STEM mode shows that Bi, Cl, O, C, Co, and species.38 The strong interaction between Ce/Co elements and
Ce are well distributed in CBCB. Co and Ce elements are pre- the composite accelerates the transfer of electrons between the
ferably located near the heterojunction interface, which proves interfaces, thereby greatly improving the photocatalytic activity
that Co and Ce are successfully doped in the CBCB heterojunc- of the nanocomposite.
tion photocatalyst (Fig. 3c).
UV-vis diffuse reflectance spectroscopy (DRS) is used to
The chemical state and elemental compositions of the determine the optical properties of the as-prepared samples.
CBCB composite were identified by XPS. As shown in As shown in Fig. 4, Bi2O2CO3 displays a sharp absorption edge
Fig. 3 (a) TEM and (b) HRTEM images of CBCB; (c) EDS elemental mapping images of Bi, Cl, C, O, Co and Ce elements.
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and 2.90 eV, respectively (Fig. S3, ESI†). According to the evalu-
ation of the XPS valence band (VB) spectrum, the VB potentials
of BiOCl, Co-doped BiOCl, Bi2O2CO3, and Ce-doped Bi2O2CO3
are approximately 2.80, 2.65, 1.70, and 2.24 V, respectively
(Fig. S4, ESI†). The corresponding conduction-band (CB)
potentials of samples are −0.22, −0.16, −1.46, and −0.66 eV,
respectively. The above results show that doping elements (Co
and Ce) have a great influence on the band structure of BiOCl
and Bi2O2CO3.
Photocatalytic Suzuki cross-coupling reaction
The photocatalytic activity of the synthesized materials (BiOCl,
Co-doped BiOCl, Bi2O2CO3, Ce-doped Bi2O2CO3, BB and
CBCB) was evaluated for the Suzuki coupling reaction between
bromobenzene and phenylboronic acid at room temperature
under visible light irradiation using a white LED lamp (15 W).
As shown in Fig. 5a, among all as-prepared photocatalysts, the
CBCB heterojunction photocatalyst exhibited the highest
activity and achieved an excellent yield (91%) of the desired
cross-coupling product, i.e., biphenyl, with 16 h of reaction.
Pure BiOCl and Bi2O2CO3 catalysts led to low yields of the
coupling product. When the Co and Ce ions were inserted in
the materials (Co-doped BiOCl and Ce-doped Bi2O2CO3
samples), the activity increased obviously, indicating that
these catalysts can effectively activate the Suzuki coupling reac-
tion of bromobenzene. In addition, compared with purely
bismuth-based catalysts (i.e., BiOCl, Co-doped BiOCl,
Bi2O2CO3 and Ce-doped Bi2O2CO3), the yield of biphenyl pro-
ducts obtained by composite photocatalysts is higher. This
indicates that the formed heterojunction can generate more
effective light-excited electron–hole pairs to participate in the
reaction. The control experiment performed in the dark
showed that under identical experimental conditions, the yield
Fig. 4 UV-vis diffuse reflectance spectra of the samples.
of about 380 nm. The Ce-doped Bi2O2CO3 has a significant
absorbance in the visible light region after introducing Ce to
Bi2O2CO3. Impressively, the absorption edge of BiOCl can
extend to the visible light, almost covering the full solar spec-
trum. After anchoring Co@BiOCl, the absorbance of Co-doped
BiOCl is enhanced in the range from 480 to 800 nm. Due to
the essential bandgap absorption of BiOCl, both the BB and
CBCB nanocomposites present similar absorption character-
istics and induce a broad absorption in the visible light
region. Therefore, it is reasonable that the heterojunction has
more effective photocatalytic activity under visible light.39 The
bandgap of as-prepared photocatalysts can be calculated as
follows:
n=2
αhv ¼ A ðhv ꢀ EgÞ
where α, h, ν, Eg, and A are the absorption coefficient, Planck for the coupling product was only 8% (Table 1, entry 2). No
constant, light frequency, bandgap, and a constant, respect- reaction was observed in the absence of a photocatalyst
ively. BiOCl and Bi2O2CO3 are indirect and direct semi- (Table 1, entry 3). Similarly, no biphenyl product in the
conductors, respectively, and their corresponding n values are absence of bromobenzene or phenylboronic acid could be
4 and 2.30 Hence, the bandgaps (Egs) of BiOCl, Co-doped found, indicating that there was no self-coupling phenomenon
BiOCl, Bi2O2CO3, and Ce-doped Bi2O2CO3 are 3.02, 2.81, 3.16, in the reaction (Table 1, entries 7 and 8).
Fig. 5 (a) Photocatalytic activity of various catalysts under the same conditions. (b) Recycling ability of CBCB for a photocatalytic Suzuki coupling
reaction. Reaction conditions: bromobenzene (0.2 mmol); phenylboronic acid (0.22 mmol); K2CO3 (0.45 mmol); CBCB (25 mg); 1 : 1 EtOH/H2O,
3 ml; RT; atmospheric pressure; 16 h; a 15 W white LED. Yields were determined by GC-MS and GC.
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Table 1 Screening and mechanistic study of the photocatalytic Suzuki
coupling reaction using CBCBa
Table 2 Influence of the reaction conditions for Suzuki cross-coupling
reactionsa
Yieldb (%)
Entry
Base
Solvent
Atmos
Yieldb (%)
Entry
Reaction condition variations
Light
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
K2CO3
CS2CO3
Na2CO3
KOH
NaOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
EtOH : H2O (1 : 1)
EtOH : H2O (1 : 1)
EtOH : H2O (1 : 1)
EtOH : H2O (2 : 1)
EtOH : H2O (1 : 1)
EtOH : H2O (1 : 1)
EtOH : H2O (1 : 1)
EtOH : H2O (1 : 1)
EtOH
H2O
DMF
DMSO
Dioxane
Ar
O2
Air
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
91
8
59
30
79
43
30
25
39
28
11
Trace
Trace
Trace
Trace
30
1
2
3
4
5
6
7
8
CBCB
CBCB
No CBCB
+
−
+
+
+
+
+
+
91
8
Trace
Trace
22
Trace
Trace
Trace
e
− scavenger (BQ)
Hole scavenger (h+)
h+/e−/radical scavenger
No bromobenzene
No phenylboronic acid
9
a Reaction conditions: bromobenzene (0.2 mmol); phenylboronic acid
(0.22 mmol); K2CO3 (0.45 mmol); CBCB (25 mg); 1 : 1 EtOH/H2O, 3 ml;
RT; atmospheric pressure; 16 h; a 15 W white LED. b Yield of the target
product (mol %) was determined by GC-MS and GC analyses.
10
11
12
13
14
15
16
17
Acetonitrile
Toluene
DMF : H2O
DMSO : H2O
8
Compared with some reported reaction conditions of
heterogeneous catalysts for Suzuki coupling (Tables S1 and
2†), it can be seen that both high temperatures and visible
light drive the Suzuki coupling relying on various Pd-based cat-
alysts. However, CBCB without Pd atoms can also achieve high
conversion under mild conditions, indicating that the present
work provides a sustainable approach for the formation of C–C
bonds.
a Reaction conditions: bromobenzene (0.2 mmol); phenylboronic acid
(0.22 mmol); K2CO3 (0.45 mmol); CBCB (25 mg); 1 : 1 EtOH/H2O, 3 ml;
RT; atmospheric pressure; 16 h; a 15 W white LED. b Yields were deter-
mined by GC-MS.
Cs2CO3 showed good activity for the Suzuki coupling reaction
Under visible light irradiation, the effects of the different (Table 2, entry 5). However, Na2CO3 and hydroxides (KOH and
reaction conditions (solvents, bases, and reaction atmosphere) NaOH) led to relatively low selectivities and yield of the
on the coupling reaction of bromobenzene with phenylboronic desired product (Table 2, entries 6–8). Moreover, without the
acid were investigated. First, the influence of several common protection of an inert gas, the yield of the corresponding
solvents on the reaction results was studied. Polar aprotic sol- product is significantly reduced (Table 2, entries 1–3).
vents, such as N,N-dimethylformamide (DMF), dimethyl sulf-
After the optimization studies, the photocatalytic Suzuki
oxide (DMSO), dioxane, and acetonitrile (ACN), were not appro- coupling reaction was performed for different substrates
priate for this reaction (Table 2, entries 11–14). The nonpolar (Table S3†). Good yields of biphenyl could be obtained by coup-
solvent toluene led to a trace yield of product (Table 2, entry ling various aryl halides with arylboronic acids (Table S3, entries
15). By contrast, much higher yields were obtained in the polar 1–3†). As expected, bromobenzene with electron-donating
protic solvents such as C2H5OH and H2O (Table 2, entries 9 groups (i.e., CH3, NH2, and OH, see entries 4–6) exhibited lower
and 10). These results are completely different from those product yields, as compared to those with electron-withdrawing
usually obtained for conventional Suzuki reactions catalyzed groups (i.e., Cl and F). Phenylboronic acid with electron-donat-
by heterogeneous catalysts. Interestingly, a mixture of solvents ing substituents (Table S3, entries 7 and 10†) exhibited a higher
led to a significant increase in photocatalytic activity. Hence, reactivity than those with electron-withdrawing ones (Table S3,
when EtOH-H2O (1 : 1) was used as the solvent, excellent biphe- entry 11†). Moreover, steric hindrance had a negative impact on
nyl yield and selectivity were obtained (Table 2, entries 1, 4, 16 the coupling reaction and the ortho-substituted substrates
and 17), which is in line with the principles of green chem- leading to relatively low yields (Table S3, entries 7–9†).
istry. The superior performance obtained in the mixed solvent
To elucidate the electron transfer mechanism in the photoca-
can be attributed to the increase in the solubility of the reac- talyzed Suzuki cross-coupling reactions, the coupling of bromo-
tants and bases. The bases dissolved in water can activate benzene and phenylboronic acid in the presence of electron and
phenylboronic acid while enhancing the reaction rate in the hole scavengers was studied. P-Benzoquinone (BQ) was used as
aqueous medium.8 In addition, compared with aprotic sol- an electron scavenger to trap photoinduced electrons on the
vents, protic solvents may be oxidized more easily through surface of the catalyst. Therefore, the use of 0.4 mmol BQ
photoinduced holes transferred from CBCB due to their lower resulted in only trace amounts of product, highlighting the key
oxidation potentials.40 We also studied the influence of role played by the electrons in this reaction (Table 1, entry 4).
various bases (K2CO3, Cs2CO3, and Na2CO3) in a mixture of Triethanolamine (TEA) is a well-known hole scavenger. Yet,
EtOH–H2O, K2CO3 led to the best yield (Table 2, entry 1) while when 0.4 mmol TEA was added, the yield of the desired biphenyl
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Fig. 6 Atomic-layer stacking of a BiOCl, Bi2O2CO3, Co-doped BiOCl, Ce-doped Bi2O2CO3 and Co-doped BiOCl (001)/Ce-doped Bi2O2CO3 (001)
heterojunction model. Green, purple, red, brown, blue and yellow atoms are Cl, Bi, O, C, Co and Ce, respectively.
decreased to 22% (Table 1, entry 5). We speculate that the photo- states, the mechanism using CBCB as an efficient photo-
generated holes can diffuse into the adsorption sites of phenyl- catalyst in the coupling reaction was explored. Fig. 6 shows the
boronic acid, and the adsorbed molecules could be oxidized atomic models of bulk BiOCl, Bi2O2CO3, Co-doped BiOCl, and
with the cleavage of the C–B bond.40,41 Moreover, the strong Ce-doped Bi2O2CO3 structures. The lattice constants were
paramagnetic resonance (EPR) signal at a g value of 2.0 was found to be a = b = 3.917 Å and c = 7.408 Å for BiOCl and a =
detected on the CBCB (Fig. S5†), indicating that oxygen 4.014 Å, b = 4.011 Å, and c = 14.204 Å for Bi2O2CO3, which are
vacancies were generated in CBCB.42 Although these results consistent with the experimental values.43,44 When doped with
suggest the importance of the electron–hole pairs in the photo- metal ions (Ce or Co), the lattice parameters of BiOCl are a =
catalytic Suzuki reaction, the exact role of holes remains unclear. b = 3.83 Å and c = 7.244 Å. For Bi2O2CO3, the lattice constants
The reusability of the CBCB photocatalyst was investigated are a = 4.033 Å, b = 4.029 Å, and c = 14.28 Å. The interface
to evaluate the stability of the designed architecture. After the model employed the average size of both BiOCl and
photocatalytic Suzuki coupling, the CBCB photocatalyst was Bi2O2CO3 slabs to make a small mismatch (∼0.50%) along
separated from the reaction solution by centrifugation. the a- and b-axis. Experimentally, the Co-doped BiOCl (001)
Afterwards, it was used in a new catalytic cycle under identical and Ce-doped Bi2O2CO3 (001) facets will generate plenty of
conditions. As shown in Fig. 5b, the recovered photocatalyst oxygen vacancies under UV/visible light. These oxygen
exhibited high catalytic performance for the five cycles without vacancies increase the separation efficiency of carriers, result-
obvious loss of activity. Furthermore, the inductively coupled ing in the (001) crystal planes of BiOCl and Bi2O2CO3 exhibit-
plasma mass spectrometry (ICP-MS) test shows that the ing excellent photoactivity.45–47 Therefore, it would be ben-
content of Co/Ce in CBCB remains above 0.95%/0.2% after eficial to construct a heterojunction model by contacting the
photocatalysis (Table S4†). There is no significant change in BiOCl (001) plane and the Bi2O2CO3 (001) plane.
the crystalline structure and morphology of the recovered cata-
To explore the interaction between the components in the
lyst observed through XRD and SEM, respectively (Fig. S6†). CBCB heterojunction, we analyzed their electronic structure. As
From the HRTEM image of the recovered photocatalyst, a clear shown in Fig. S8a,† the bandgaps of pure BiOCl and Bi2O2CO3
heterojunction interface can be observed, which further proves are 2.5 and 0.3 eV, respectively, which are smaller than the
that there exists intimate interfacial contact between Co-doped experimental value. This phenomenon stems from the limit-
BiOCl and Ce-doped Bi2O2CO3 planes (Fig. S7†). These results ation of DFT calculations; the discontinuity of the exchange-
demonstrate that CBCB exhibited good stability and was suit- related potential is not considered in the DFT framework.48
able as a photocatalyst for Suzuki coupling reactions.
Compared with bulk BiOCl, after Co ion doping, some impurity
levels appear near the Fermi level of BiOCl, which reduces the
bandgap of Co-doped BiOCl to 1.6 eV. Fig. 7a shows that the
Theoretical exploration
Based on the first-principles calculation of the crystal structure impurity energy level mainly appears near the top of the valence
properties, the energy band structure, and the density of band, indicating that Co-doped BiOCl is a p-type semi-
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Fig. 7 (a) Band structures, and (b) total density of states and local density of states of Co-doped BiOCl, Ce-doped Bi2O2CO3 and a Co-doped BiOCl
(001)/Ce-doped Bi2O2CO3 (001) heterojunction model. The positive DOS is for spin up while the negative one is for spin down. (c) The electron
difference density maps of the Co-doped BiOCl (001)/Ce-doped Bi2O2CO3 (001) interface. Green, purple, red, brown, blue and yellow atoms are Cl,
Bi, O, C, Co and Ce, respectively.
2−
conductor. With the addition of Co ions, BiOCl and the Co mainly composed of O 2p orbitals in the CO3 layer, and the
2+
elements are likely to form a homogenous junction locally, and CBM is mainly composed of Bi 6p orbitals in the Bi2O2 layer
the built-in electric field formed in the Co-doped BiOCl can (Fig. S8b†). The electron transition mode excited by light is O
effectively separate the photoinduced carriers generated by 2p → Bi 6p. The doping of the Ce ions greatly affects the total
BiOCl, thus greatly enhancing the photocatalytic activity of density of states (TDOS) and the local density of states (LDOS)
BiOCl. As shown in Fig. 7a, Ce is embedded in Bi2O2CO3 with a of Bi2O2CO3, and the system shows the spin-polarized DOS.
spin state. There are two spin states near the Fermi level of Ce- The VBM of Ce-doped Bi2O2CO3 is still mainly determined by
doped Bi2O2CO3, and the bandgap value of the spin-down part O 2p orbitals; while the CBM is mostly contributed by Bi 6p
is close to 0, making the system exhibit metallic characteristics. and Ce 4f orbitals, of which Ce 4f orbitals account for a larger
Due to the presence of Ce-doped Bi2O2CO3, the band structure proportion (Fig. 7b). Additionally, it can be observed that the
of the CBCB heterojunction also presents
a spin state. 4f orbital of Ce crosses the Fermi level, that is, the 4f orbital of
Compared to pure Co-doped BiOCl and Ce-doped Bi2O2CO3, the Ce appears at the bottom of the CB and the top of the VB. The
impurity energy level of CBCB near the Fermi level increases sig- main reason for this phenomenon is that it has two valence
nificantly and becomes denser, indicating the enhancement of states of Ce4+ and Ce3+ (see the analysis of XPS in Fig. S2†).
hybridization between Co-doped BiOCl and Ce-doped Bi2O2CO3, Ce4+ easily captures photogenerated electrons and prevents the
which is conducive to charge transfer at the interface.
recombination of electron–hole pairs, thus improving the cata-
To explain the change in the energy band from a micro- lytic activity of Bi2O2CO3. Consistent with the analysis results
scopic point of view, the density of states of different samples of the above band structure, the existence of Ce-doped
was calculated and analyzed. It can be seen from Fig. S8b† Bi2O2CO3 makes CBCB exhibit a spin-polarized total density of
that the valence band maximum (VBM) of bulk BiOCl is states (TDOS). The Ce oxidation state (+3 or +4) in the CBCB
mainly determined by the O 2p and Cl 3p states, while the Bi composite significantly affects the spin state, which in turn
6s and Bi 6p state contribution is small. The conduction band may regulate the photocatalytic performance.49 Moreover, the
minimum (CBM) is mostly dominated by the Bi 6p states. coupling of Co-doped BiOCl and Ce-doped Bi2O2CO3 showed
After Co atom doping, the VBM at the Fermi level is mainly an increased charge density near the Fermi level. Therefore,
provided by the Cl 3p and Co 3d states, and the CBM is mainly we speculated that the CBCB heterojunction will exhibit
contributed by the Bi 6p states (Fig. 7b). Moreover, resonance enhanced photoresponsivity.50 On the other hand, the charge
hybridization occurs near the Fermi level of BiOCl, causing the in the heterojunction CBM is mostly localized on Ce-doped
electrons there to be more active. The formed intermediate Bi2O2CO3, and the charge in the VBM is mainly localized on
energy level greatly reduces the forbidden bandwidth, thereby Co-doped BiOCl, indicating that CBCB is a type II heterojunc-
making the optical excitation easier and generating more tion. The VBM and CBM states are located in different com-
charge carriers. For bulk Bi2O2CO3, the VBM of Bi2O2CO3 is ponents of the CBCB heterostructure, which can realize the
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Fig. 8 Photoluminescence (a) and EIS-Nyquist plots (b) of Co-doped BiOCl, Ce-doped Bi2O2CO3 and a Co-doped BiOCl/Ce-doped Bi2O2CO3
heterojunction.
effective separation of electrons and holes in space, and of CBCB is relatively small, indicating a faster interface charge
improve the photocatalytic performance of the photocatalyst.
The electron difference density and Bader charge analyses
are used to investigate the charge transfer of the CBCB inter-
transfer of the heterojunction photocatalyst.
Mechanism
face. In Fig. 7c, the yellow and blue regions represent charge Unlike the homogeneous catalysts used for the Suzuki coup-
accumulation and depletion, respectively. For the Co-doped ling reaction for which the mechanism is easier to establish,
BiOCl layer, part of the electrons of Co atoms are transferred for a heterogeneous catalyst with multiple active sites, the
to the BiOCl phase, resulting in the formation of a built-in mechanism is difficult to understand. On the basis of the pre-
electric field in Co-doped BiOCl, which is in line with the viously reported works, DFT calculations, and experimental
energy band analysis. Compared with Co-doped BiOCl, Ce- results obtained herein, the photocatalytic mechanism of the
doped Bi2O2CO3 accumulates more charges on the surface. Suzuki coupling reaction over CBCB hybrids under visible
When a heterojunction is formed between Co-doped BiOCl light irradiation was proposed (Fig. 9). Upon light irradiation,
and Ce-doped Bi2O2CO3, charge shift and redistribution will the photocatalyst CBCB can absorb a part of the visible light,
occur at the interface of the heterojunction, indicating a generating electron–hole pairs in Co-doped BiOCl and Ce-
strong interaction between the two components.51 Meanwhile, doped Bi2O2CO3 layers. Due to the intimate interfacial contact
Bader charge analysis found that the charge flows from the Ce- between the Co-doped BiOCl and Ce-doped Bi2O2CO3, the elec-
doped Bi2O2CO3 layer to the Co-doped BiOCl layer (0.2 e), trons in the conduction band of Ce-doped Bi2O2CO3 are easily
which following our expected experimental results.
transferred to the conduction band of Co-doped BiOCl, which
To further investigate the separation efficiency of the photo- activates the C–X bond of the aryl halides and reduces it to the
generated charge carriers of catalysts, the PL spectra were corresponding aryl anion radical.40,52 Simultaneously, the
recorded (Fig. 8a). Clearly, compared with Co-doped BiOCl and holes in the valence band of Co-doped BiOCl migrate to the
Ce-doped Bi2O2CO3, the PL signal intensity of hybrid CBCB valence band of Ce-doped Bi2O2CO3, and the photogenerated
was suppressed. That is, under visible light irradiation, the holes activate the C–B bond of phenylboronic acid.
electrons migrate from the LUMO of Ce-doped Bi2O2CO3 to the Phenylboronic acid captures OH− from the alkaline reaction
conduction band (CB) of Co-doped BiOCl, and Ce4+ can also medium to produce B(OH)3−, which is then converted into a
act as the electron capture trap, leading to the spatial separ- phenyl radical cation with the assistance of photoinduced
ation of charge carriers,27 and thus delaying the electron–hole holes.53 Finally, the phenyl radical cation and aryl anion
pair recombination. The result is in good agreement with the undergo a cross-coupling reaction to obtain the desired biphe-
above theoretical calculation. The electron transfer mechanism nyl product. On the other hand, compared to the heterojunc-
of the photocatalyst was also studied through transient photo- tion constructed from BiOCl and Bi2O2CO3, the heterojunction
current and EIS experiments. As expected, Co-doped BiOCl interface made of Co-doped BiOCl and Ce-doped Bi2O2CO3 can
and Ce-doped Bi2O2CO3 exhibited
a lower photocurrent effectively shorten the diffusion length of charge carriers and
density. However, when the Co-doped BiOCl and Ce-doped improve the transmission efficiency of electron–hole pairs. All
Bi2O2CO3 formed a heterojunction, the photocurrent density these results indicate that the introduction of Co and Ce ions
of CBCB increased remarkably, indicating that more photo- and the construction of a Co-doped BiOCl/Ce-doped Bi2O2CO3
generated electrons in the composite were transferred to ITO heterojunction can significantly inhibit the recombination of
(Fig. S9†). The EIS spectrum of the prepared samples is shown photoinduced charge carriers and enhance the photocatalytic
in Fig. 8b. Similar to the results of PL analysis, the arc radius activity of the Suzuki cross-coupling reaction.
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Fig. 9 Proposed mechanism of the photocatalytic Suzuki cross-coupling of bromobenzene and phenylboronic acid.
Conclusions
Acknowledgements
We gratefully acknowledge the Key Laboratory of Biomass
Energy and Material, Jiangsu Province (JSBEM201912) and the
Chinese Postdoctoral Science Foundation (2015 M571761 and
2016T90465) for financial support. This work was funded by
the Priority Academic Program Development of Jiangsu Higher
Education Institution and Instrument and Equipment
Foundation of Nanjing University of Science & Technology.
In summary, a modified BiOCl/Bi2O2CO3 (CBCB) composite
was successfully prepared, which acts as a highly efficient
palladium-free heterogeneous photocatalyst for the Suzuki
coupling reaction. As expected, an excellent yield (91%) of
the desired product biphenyl was obtained under mild con-
ditions. The highly efficient photocatalytic performance over
CBCB can be attributed to the following points: (1) the incor-
poration of Co ions makes Co-doped BiOCl exhibit a nar-
rowed bandgap and enhances the light absorption. Moreover,
a built-in electric field directed from Co to BiOCl is formed
in the Co-doped BiOCl layer, resulting in an accelerated
charge transfer process; (2) when Bi2O2CO3 is doped with Ce,
the low-lying empty 4f orbital of Ce4+ easily captures light-
excited electrons, thereby preventing the recombination of
electron–hole pairs. On the other hand, the spin state of Ce
in Ce-doped Bi2O2CO3 can greatly regulate the photocatalytic
performance; and (3) the type II heterojunction constructed
by the Co-doped BiOCl and Ce-doped Bi2O2CO3 facets effec-
tively promotes the separation and transfer of photogenerated
electron–hole pairs due to intimate interfacial contact and
appropriate energy level positions. Thus, the utilization of
visible light as the energy source, EtOH/H2O as environmen-
tally friendly solvents, high photocatalytic activity and re-
usability is expected to make the modified palladium-free
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