Core-shell Cu2S:NiS2@C hybrid nanostructure derived from a metal-organic framework with graphene oxide for photocatalytic synthesis of N-substituted derivatives

Article abstract of DOI:10.1039/d1ta00159k

Recently, photocatalysis has attracted considerable interest, leading to opportunities for using renewable energy sources. A Cu:Ni metal-organic framework (MOF) modified with graphene oxide (GO) was fabricated by a facile and convenient process. Subsequen

Full text of DOI:10.1039/d1ta00159k

Highlighting a study about Hybrid Nanostructure Derived
from a Metal–Organic Framework for Photocatalytic
Synthesis by Mohammad Yusuf from IOM group under Prof.
Kang Hyun Park, Pusan National University.
As featured in:
Core–shell Cu2S:NiS2@C hybrid nanostructure derived
from a metal–organic framework with graphene oxide for
photocatalytic synthesis of N-substituted derivatives
A Cu:Ni metal–organic framework (MOF) modified with
graphene oxide (GO) was fabricated by a facile and
convenient process. Subsequently, the Cu:Ni sulfide
encapsulated on porous C with reduced GO (Cu2S:NiS2@C/
rGO) as a core–shell nanostructure was achieved using a
MOF (Cu:Ni-BTEC/GO). The Cu2S:NiS2@C/rGO with hybrid
structure showed superior photocatalytic activity for
producing N-substituted derivatives through three kinds of
oxidative coupling reactions. Moreover, the MOF-derived
photocatalyst exhibited good reusability and stability.
See Kang Hyun Park et al.,
J. Mater. Chem. A, 2021, 9, 9018.
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Core–shell Cu2S:NiS2@C hybrid nanostructure
derived from a metal–organic framework with
graphene oxide for photocatalytic synthesis of N-
substituted derivatives†
Cite this: J. Mater. Chem. A, 2021, 9,
9018
a
Mohammad Yusuf,
Shamim Ahmed Hira,a Hyeonhan Lim,a Sehwan Song,b
Sungkyun Parkb and Kang Hyun Park
a
*
Recently, photocatalysis has attracted considerable interest, leading to opportunities for using renewable
energy sources. A Cu:Ni metal–organic framework (MOF) modified with graphene oxide (GO) was
fabricated by a facile and convenient process. Subsequently, the Cu:Ni sulfide encapsulated on porous C
with reduced GO (Cu2S:NiS2@C/rGO) as a core–shell nanostructure was achieved using a MOF (Cu:Ni-
BTEC/GO). Cu2S:NiS2@C/rGO with a hybrid structure showed superior photocatalytic activity for
producing N-substituted derivatives through three kinds of oxidative coupling reactions. Moreover, the
MOF-derived photocatalyst exhibited good reusability and stability. The excellent photocatalytic behavior
of MOF-derived bimetallic chalcogenides would channel further interest in the design, synthesis, and
modification of MOFs as photocatalysts for diverse applications.
Received 7th January 2021
Accepted 22nd February 2021
DOI: 10.1039/d1ta00159k
rsc.li/materials-a
aggregation, enhanced dispersion, and increased catalytic
activity.10 Furthermore, inorganic metals can be used as
Introduction
precursors of metals or metal oxides, and the organic linker can
act as a template for establishing a nanoporous carbon support.
Hence, a MOF-based multifunctional structure can be obtained
by deriving the component of the MOF. MOF-based derivatives
overcome the limitations of the raw stability of MOFs at high
temperatures and harsh chemical environments.4,5,11
Multifunctional hybrid catalysts contain metallic nanoparticles,
and the support represents an effective strategy for wide-
ranging applications, especially in organic and photochemical
reactions. Numerous hybrid catalysts, particularly core–shell,
yolk–shell, double-shell, hollow, and their relative structures,
have been developed and utilized for energy storage, sensing,
water treatment, catalysis, and biomedical applications.1,2
However, well-dened structures with effective methods and
excellent catalytic performance are still being studied.
Recently, synthesizing various hybrid nanostructures from
metal–organic frameworks (MOFs) has generated considerable
interest for diverse applications, especially in catalysis.3,4
Nevertheless, MOF-based multifunctional nanostructures still
have some limitations, which generally restrict their applica-
tions.5 Therefore, tailoring MOFs with other functional mate-
rials, such as polymers, semiconductors, and graphene oxide
(GO), can be an excellent alternative to overcome the limited
abilities of single MOFs.6–9 Following recent studies, incorpo-
rating MOFs and graphene-based materials can integrate the
benets of the individual components owing to their improved
stability, modied structure, enlarged surface area, diminished
MOF derivatives are usually achieved through chemical
reduction and thermal treatment or a combination of both
methods. In addition, thermal treatment (direct pyrolysis),
including carbonization, phosphorization, and sulfurization, is
an effective and easy method. Direct pyrolysis by sulfurization
can transform the structure into an S-doped multifunctional
structure consisting of a metal and porous carbon. Further-
more, this method is a promising strategy for fabricating metal-
sulde-based multifunctional structures.4,12–14 Metal suldes
(chalcogenides) have gained research attention for fuel cells,
environmental remediation, and photocatalysis owing to many
materials with readily controllable mechanical, optical, phys-
ical, and chemical properties. Metal suldes have various
structures that can be enriched by adding different metal ions
and templates; moreover, these suldes have an appropriate
band gap that can be controlled through the composition. This
ability is suitable for developing photocatalysts using metal-
sulde-based materials.15,16 In addition, the development of
photocatalysts based on the multistructural design of metal
chalcogenides, especially for organic reactions, remains a chal-
lenge. Among the numerous structural engineering options,
aDepartment of Chemistry and Chemistry Institute for Functional Materials, Pusan
National University, Busan, 46241, Republic of Korea. E-mail: chemistry@pusan.ac.kr
bDepartment of Physics, Pusan National University, Busan, 46241, Republic of Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ta00159k
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semiconductor TiO2 with rened core–shell and arranged OES) using an optical system (Optima 8300, PerkinElmer Inc.).
structural designs provides an example of the advantages of The properties of the surface structure were identied using N2
structural modication and multicomponent architecture sorption isotherms using a Brunauer–Emmett–Teller (BET)
owing to critical developments including photostability, surface area analyzer (ASAP 2020, Micromeritics Instrument
charge–carrier separation, light absorption, and surface prop- Corp.). Additional characterization was performed via UV-Vis
erties.17 Therefore, the opportunities for the structural devel- diffuse reectance spectroscopy (DRS) using a UV-Vis/near
opment of metal suldes utilized for highly effective infrared spectrophotometer (V-770, JASCO Corp.) and a micro-
photocatalysis are remarkably abundant based on this concept. Raman spectrometer (FEX, NOST), respectively. Subsequently,
Moreover, metal–metal oxide catalysts with a Cu- or Ni-based the photocatalyst was identied using a gas chromatography–
multistructural design have been consistently utilized for mass spectrometry (GC-MS) system (QP2010 SE, Shimadzu
organic transformations to reduce or replace noble metals and Corp.).
to enlarge hybrid material catalysts.18–24 In addition, multi-
component engineered metal hybrid nanocatalysts have been
developed to achieve highly active catalysts with excellent
stability.1,18 However, intense temperature and pressure levels
remain a risk that must be addressed. Light-harvesting mate-
rials for photocatalyst systems can overcome this limitation to
achieve mild conditions and open opportunities for using
renewable energy sources.25,26
In contrast, N-containing organic compounds are indis-
pensable components in modern synthetic organic chemistry.
Among these compounds, N-heterocyclic and N-aryl species are
promising biologically active compounds, pharmaceuticals,
and ne chemicals with tremendous applications from labora-
tory- to industrial-scale production.27–31 Various approaches for
updating N-heterocyclic and N-aryl derivatives have been
implemented with convenient and validated synthetic routes.
One promising technique is the oxidative coupling reaction.32–37
However, the search for an efficient method and catalyst system
for oxidative coupling reactions is ongoing.
Based on the above considerations, developing a multifunc-
tional core–shell nanostructure from a MOF derivative con-
taining non-noble metals (Cu and Ni) and tailored with GO
could be a promising strategy for oxidative coupling reactions
forming C–N bonds. A hybrid nanostructure of Cu and Ni
chalcogenides was fabricated from a MOF derivative through
a simple method. In addition, the structures of Cu2S:NiS2@C/
rGO increased the photocatalytic activity for the synthesis of
N-substituted derivatives.
Materials synthesis
Synthesis of Cu:Ni-BTEC/GO. A GO suspension was prepared
by continuously dispersing GO powder (50 mg) in deionized
water via ultrasonication at an amplitude of 30% for 30 min.
Approximately 1.25 mmol of both Cu(NO3)2$2.5H2O and
Ni(NO3)2$6H2O was then dissolved in 25 mL of dime-
thylformamide (DMF). Subsequently, 1.25 mmol of 1,2,4,5-
benzenetetracarboxylic acid (H4BTEC) was added to the solu-
tion. The reaction system was combined with the GO suspen-
sion, transferred to a Teon-lined autoclave (100 mL), and the
ꢀ
nanoparticles were continuously grown at 150 C for 3 h. The
reaction was then naturally cooled at room temperature.
Finally, the prepared catalyst was collected, rinsed with DMF
and methanol (MeOH), and subsequently dried. Cu-BTEC/GO,
Ni-BTEC/GO, and Cu:Ni-BTEC were synthesized using the
same analog process for comparison.
Synthesis of Cu2S:NiS2@C/rGO. The as-prepared Cu:Ni-
BTEC/GO and S powder in a 1 : 2 ratio were placed in two
separate alumina crucible boats in a tube furnace. The S powder
was xed upstream following the sample at a distance of
ꢁ10 cm. The samples were annealed stepwise in a owing N2
atmosphere at 250 ꢀC for 1 h and then continuously at 350 ꢀC for
1 h at a heating rate of 2 ꢀC minꢂ1. The product was collected for
further analyses.
Photocatalytic activity
Chan–Lam coupling reactions of phenylboronic acid and
imidazole. A mixture of phenylboronic acid (0.5 mmol), imid-
azole (0.5 mmol), catalyst (5 mg), and solvent (4 mL; MeOH-to-
H2O ratio was 3 : 1) was placed in a 25 mL quartz round-bottom
Experimental
Materials and instrumentation
All chemicals were purchased from Sigma-Aldrich, TCI, Acros, ask and stirred at room temperature (25 ꢀC). Aer the reaction
Alfa–Aesar, or Daejung Company and were used as obtained was completed, MeOH (5 mL) was added, and the catalyst was
without purication. The properties of the catalysts were separated by centrifugation. Subsequently, the product was
analyzed via X-ray diffraction (XRD) using a diffractometer extracted with CH2Cl2, dehydrated with MgSO4, and concen-
(X'Pert3 MRD, Malvern Panalytical Ltd.). The morphologies of trated using a rotary evaporator.
the catalysts were characterized via eld emission scanning
Cyclization of o-phenylenediamine and aldehyde. In a 25 mL
electron microscopy (SEM) using a microscope (SUPRA 40VP, quartz round-bottom ask with a magnetic stirrer, 1,2-phenyl-
Carl ZEISS) and transmission electron microscopy (TEM) using enediamine (0.25 mmol), aromatic aldehyde (0.25 mmol),
a high-resolution microscope (TALOS F200X, FEI) at 200 keV ethanol (EtOH) (3 mL), and catalyst (2.5 mg) were added. Aer
with integrated energy dispersive X-ray (EDX) mapping. Surface the reaction was completed, EtOH (5 mL) was added. Subse-
chemical state data were obtained via X-ray photoelectron quently, the catalyst was separated, extracted with ethyl acetate,
spectroscopy (XPS) using a spectrometer (AXIS Supra, Kratos dehydrated, and concentrated to obtain the desired product.
Analytical Ltd.). The elemental composition was determined via
Oxidative homocoupling of benzylamine. Benzylamine (0.25
inductively coupled plasma optical emission spectroscopy (ICP– mmol), catalyst (2.5 mg), and acetonitrile (CH3CN, 3 mL) were
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placed in a quartz round-bottom ask. Aer the reaction was of the bimetallic sulde trapped in the C matrix tailored with
completed, CH3CN (5 mL) was added. Subsequently, the catalyst rGO (Cu2S:NiS2@C/rGO) was obtained by in situ growth of
was separated and concentrated using a rotary evaporator.
nanoparticles derived from H4BTEC ligands and GO from direct
All the reactions were conducted under irradiation from carbonization and sulfurization.
a xenon arc lamp 300 W (Newport Corp.) with a cut-off of 400–
The morphologies of the MOF and its derivative were iden-
800 nm and an intensity of 200 mW cmꢂ2. The lamp was tied via SEM and TEM. The nanorod form of the Cu:Ni-BTEC
equipped with specic longpass lters (cut-on of 400, 455, 515, MOF was observed using a solvent consisting of DMF and
and 610 nm) to suppress the transmission at selected water (H2O) in a 1 : 1 volume ratio (Fig. 1a). The MOF exhibited
frequencies (for example, a cut-on of 400 nm blocks <400 a size distribution of approximately 32 nm (Fig. S2†). However,
wavelengths), a liquid lter to minimize heat and remove IR a ower-like structure was obtained when only DMF was used as
wavelengths (>900 nm), an IR cut-off to obstruct >800 nm the solvent (Fig. 1c). Moreover, H2O has a high affinity for
wavelengths, and an Air Mass 1.5G lter to match AM1.5g coordinating with the metal centers compared to DMF; hence,
natural solar conditions (solar spectrum). The reaction was the cage grew big when only H2O was used as the solvent, as
monitoring by thin layer chromatography. Subsequently, conrmed by a micro urchin-like shape (Fig. 1e). For compar-
product conversion was measured via GC-MS.
ison, the metals Cu and Ni were identied as well (Fig. 1g and i).
The Cu-MOF and Ni-MOF grew toward the tetragonal plate and
hexagonal rod, respectively.38
Moreover, GO was used to modify the structure and enhance
the stability, photo-properties, and catalytic activity.10,39 The GO-
modied MOF was identied by the GO layer, as shown in
Fig. 1b, 2a, b and 3(i). For comparison, the morphologies of
Cu:Ni-BTEC/GO (with H2O or DMF as the solvent), Cu-BTEC/
GO, and Ni-BTEC/GO are presented in Fig. 1 (right side). Aer
heat treatment, the MOF transformed into Cu and Ni sulde
embedded in the C matrix of the rGO layer (Cu2S:NiS2@C/rGO).
The rod shape was maintained; however, the surface changed
from smooth to rough. Fig. 3 shows the additional morpho-
logical details; such details were evaluated via TEM with inte-
grated EDX mapping. Cu and Ni sulde nanoparticles were
incorporated into the C shell with a size of approximately
10 nm. Furthermore, the porous C shell and the graphene layer
were clearly conrmed by high-angle annular dark-eld
(HAADF), as shown in Fig. 3(ii)c. The presence of Cu, Ni, S,
and C was conrmed from the elemental mapping (Fig. 3(i–ii)d–
Results and discussion
The synthesized bimetallic (Cu and Ni) sulde with a multi-
functional structure was obtained by simple derivation of
a MOF. The synthetic strategy for Cu:Ni-MOF/GO and its
derivative Cu2S:NiS2@C/rGO is illustrated in Scheme 1. First,
a novel bimetallic GO-modied Cu:Ni-MOF was fabricated
through a hydrothermal method. The new ligand for this
bimetallic MOF was applied using H4BTEC, which is rich in
carboxylate groups incorporated with the metal center and O-
containing functional groups from GO (epoxy, hydroxyl, and
carboxyl). The seeding of the MOF was rst achieved under
reux conditions, conrmed by TEM (Fig. S1, ESI†). The
continuous mixing with GO by the hydrothermal method
formed a Cu:Ni-MOF/GO structure. Subsequently, the trans-
formation of the MOF composite into metal sulde was ach-
ieved through in situ carbonization and sulfurization with S
powder under inert (N2) conditions. The remarkable structure
Scheme 1 Schematic illustration of Cu:Ni-BTEC/GO and Cu2S:NiS2@C/rGO.
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h). In addition, the Cu, Ni, and S contents (wt%) in the catalyst
before and aer calcination were conrmed by ICP–OES (Table
S1†). Here, a hybrid core–shell nanostructure of a bimetallic (Cu
and Ni) sulde encapsulated on porous C and modied with GO
was successfully formed from an MOF derivative.
The XRD patterns in Fig. 4a indicate that the bimetallic
sulde was obtained aer the calcination of Cu:Ni-BTEC/GO.
For comparison, the other MOFs and MOFs without GO
loading are shown in Fig. S3.† The peaks at 26.67ꢀ, 30.00ꢀ,
46.65ꢀ, and 54.99ꢀ for Cu2S and 27.26ꢀ, 31.59ꢀ, 35.46ꢀ, 38.94ꢀ,
45.24ꢀ, 53.43ꢀ, 56.00ꢀ, 58.79ꢀ, and 60.95ꢀ for NiS2 were attrib-
uted to the hexagonal Cu2S (ICSD#020560) and triclinic NiS2
(ICSD#022369), respectively. The broad peak at around 15–25ꢀ
indicated the C and rGO peaks. Moreover, the successful
synthesis of bimetallic sulde (Cu2S:NiS2@C/rGO) was
conrmed by the Fourier transform (FT) patterns from the high-
resolution TEM images, as shown in Fig. 4b–d. The interplanar
distances were approximately 0.28 and 0.32 nm, which were in
agreement with the (200) lattice point of the NiS2 and the (002)
lattice point of Cu2S structures, respectively.
The chemical surface and composition states were analyzed
via XPS, and the results are shown in Fig. 5. Cu, Ni, S, O, and C
appeared in the survey scan spectra of the samples (Fig. S4†).
The C–C binding energy (BE; 284.5 eV) was used for calibration
to further deconvolute the spectra. As shown in Fig. 5a, the Cu
2p peak at a BE of 934.96 eV was assigned to the Cu–O bond of
Cu-BTEC, and this peak was validated with the Cu2+ satellite
peak. The shi of Cu 2p3/2 to the lower energy (932.29 eV) and
the disappearance of the satellite peak conrmed the successful
sulfurization process, which produced the Cu2S structure. The
deconvolution of Ni 2p (Fig. 5b) for the derived MOF at BEs of
853.73 and 855.53 eV demonstrated a BE that was considerably
lower than that of the underived MOF. This result conrms that
Ni-BTEC peaks were transformed into those of the NiS2 struc-
ture and impurities (Ni(OH)2). In addition, as shown in Fig. 5c,
Fig.
1 SEM images of (a) Cu:Ni-BTEC (b) Cu:Ni-BTEC/GO in
DMF:H2O, (c) Cu:Ni-BTEC (d) Cu:Ni-BTEC/GO in DMF, (e) Cu:Ni-BTEC the S 2p at BEs from 160 eV to 168 eV was attributed to the
(f) Cu:Ni-BTEC in H2O, (g) Cu-BTEC (h) Cu-BTEC/GO in DMF:H2O,
and (i) Ni-BTEC (j) Ni-BTEC/GO in DMF:H2O.
presence of Cu–S and Ni–S 2p3/2 (2p1/2) at a BE of 162.25
(163.43) eV, C–S 2p3/2 (2p1/2) at a BE of 163.70 (164.88) eV, and
impurities (SO32ꢂ) at a BE of 167.00 eV. The C 1s deconvolution
spectrum exhibited the features of C]O–OH, C–O–C, C]C
(sp2), and C–C/C–H (sp3) for Cu:Ni-BTEC/GO, indicating the
characteristic of C from the BTEC ligand and GO (Fig. 5d). Aer
the calcination process, the decreased number of carboxyl and
epoxy functional groups and the increased C]C (sp2) and C–C/
C–H (sp3) of Cu2S:NiS2@C/rGO were attributed to the trans-
formation of the ligand and GO into the C matrix and rGO
structure, respectively.40
In addition, Raman spectroscopic analyses were performed
to examine and classify the structural and electronic conju-
gation states of GO aer thermal treatment. The two specic
features of the G band (1575 cmꢂ1) and D band (1350 cmꢂ1
)
were obtained, and these features were assigned to the E2g
symmetry of sp2 C atoms and the breathing mode of A1g
symmetry, respectively (Fig. 6a). The intensity ratio of the D to
the G band (ID/IG) for Cu2S:NiS2@C/rGO (0.70) was higher than
that of Cu:Ni-BTEC/GO (0.46), implying that the thermal
process enhanced the reduction and generated defects in the
Fig. 2 SEM image of (a and b) Cu:Ni-BTEC/GO and (c and d) Cu2-
S:NiS2@C/rGO.
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Fig. 3 TEM image of (i) Cu:Ni-BTEC/GO and (ii) Cu2S:NiS2@C/rGO.
GO structure.41 To further verify the features of the surface
structures of the as-prepared catalysts, N2 sorption isotherms
were obtained, as shown in Fig. 6b. All the catalysts displayed
Fig. 4 (a) XRD patterns of Cu:Ni-BTEC/GO and Cu2S:NiS2@C/rGO, (b
and c) high-resolution TEM image of Cu2S:NiS2@C/rGO, inset: FT
patterns of Cu2S and NiS2.
Fig. 5 (a) XPS spectra of Cu:Ni-BTEC/GO and Cu2S:NiS2@C/rGO (a)
Cu 2p lines, (b) Ni 2p lines, (c) S 2p lines and (d) C 1s lines.
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aggregation of the active NPs.8 For comparison, the BET
surface area, pore volume, and average pore size of the other
prepared materials are displayed in Table S2.† Using the Bar-
rett–Joyner–Halenda method, the average pore size distribu-
tion was calculated for all the materials assigned to the
mesoporous material.42
UV-Vis DRS was used to identify the spectral characteristics
of the as-synthesized catalysts, as shown in Fig. 7a. The as-
prepared catalysts before the sulfurization process showed UV
adsorption in the <400 nm region and increased adsorption in
the visible region at >500 nm. However, aer sulfurization, the
adsorption expanded from the UV to the visible area for Cu2-
S:NiS2@C/rGO, highlighting its superior optical properties
compared to other catalysts. The photogenerated charge
transfer properties were then evaluated through electro-
chemical impedance spectroscopy (EIS) (Fig. 7b). Compared to
the other materials, Cu2S:NiS2@C/rGO exhibited small semi-
circles, indicating low charge transfer resistance. This result
was related to large charge separation efficiency, resulting in
remarkably decreased photorecombination of electron–hole
pairs.
Fig. 6 (a) Raman spectra of Cu:Ni-BTEC/GO and Cu2S:NiS2@C/rGO,
(b) N2 sorption isotherms of various Cu:Ni-BTEC/GO and its
derivatives.
a characteristic type IV isotherm and were close to hysteresis
type H3. The BET surface area of Cu2S:NiS2@C/rGO was larger
than that of Cu:Ni-BTEC/GO. The calcination process
increased the BET surface area of the Cu:Ni-MOF due to the
exposure of more surface-active sites as far as possible up to
The photocatalytic efficiencies of the synthesized materials
were initially utilized for C–N bonds via the Chan–Lam
coupling reaction using the coupling of arylboronic acid and
imidazole as a model reaction. Table 1 shows the various
conditions for optimization. A conversion of >97% was ach-
ieved through Cu2S:NiS2@C/rGO in entry 4. The photocatalytic
activities of the materials followed the order Cu2S:NiS2@C/
rGO > Cu:Ni-BTEC/GO > Cu-BTEC/GO > Ni-BTEC/GO (N. R.)
¼ Cu:Ni-BTEC (N. R.) (entries 4–11). The combination of GO
and Cu:Ni-MOF provided a synergetic effect for better reaction,
as indicated by entries 8 (Cu:Ni-BTEC/GO) and 9 (Cu:Ni-BTEC).
Fig. 7 (a) UV-Vis DRS spectroscopy and (b) EIS spectra of various
Cu:Ni-BTEC/GO and its derivatives.
Table 1 The photocatalytic activity of Cu2S:NiS2@C/rGO and its derivatives for C–N coupling of phenylboronic acid and imidazolea
Entry
Catalyst
Solvent
Time (h)
Conv.b (%)
Selec.b (%)
1
2
3
4
5
6
7
8
Cu2S:NiS2@C/rGO
Cu2S:NiS2@C/rGO
Cu2S:NiS2@C/rGO
Cu2S:NiS2@C/rGO
Cu2S:NiS2@C/rGO
Cu2S:NiS2@C/rGO
Cu2S:NiS2@C/rGO
Cu:Ni-BTEC/GO
MeOH
MeOH
3 h
6 h
6 h
6 h
6 h
6 h
6 h
6 h
6 h
6 h
6 h
6 h
76
88
94
97
N.R
74
99
60
98
80
94
—
94
94
94
MeOH : H2O (1 : 1)
MeOH : H2O (3 : 1)
H2O
EtOH : H2O (3 : 1)
CH3CN : H2O (3 : 1)
MeOH : H2O (3 : 1)
MeOH : H2O (3 : 1)
MeOH : H2O (3 : 1)
MeOH : H2O (3 : 1)
MeOH : H2O (3 : 1)
24
9
Cu:Ni-BTEC
N.R
15 (72)
N.R (15)
N.R
—
10
11
12d
Cu-BTEC/GO (Sc)
Ni-BTEC/GO (Sc)
GO, GO AHT, GO ACT
94 (94)
— (90)
—
a
Reaction conditions: 0.5 mmol of 1a and 1b, 4 mL solvent, 5 mg of photocatalyst Cu2S:NiS2@C/rGO, 10 mg of entry 8–12. The reactions were
conducted in air at room temperature (r.t) under Xe-lamp irradiation (400–800 nm) with a light intensity of 200 mW cmꢂ2
selectivity determined by GC-MS analysis. Sulfurization of Cu and Ni-MOF. AHT is aer hydrothermal (reaction) treatment, ACT is aer
.
Conversions and
b
c
d
calcination treatment.
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The Cu-MOF aer sulfurization process tends to be more irradiated at 200 mW cmꢂ2. Second, to conrm the effect of the
active compared to the sulfurized Ni-MOF (entry 10–11). Cu irradiation wavelength, various longpass lters (cut-on) were
and Ni are widely used for C–N coupling reactions as an applied to remove transmission below a specic wavelength
alternative to noble metal Pd. However, few studies were (Fig. 8b). Filters of 400, 455, 515, and 610 nm were used, and the
conducted using Ni or Cu:Ni catalysts relative to Cu catal- conversions at 400–800, 455–800, 515–800, and 610–800 nm
ysis.33,43 The bimetallic system of Cu:Ni displayed better pho- wavelengths were 97%, 86%, 72%, and 52%, respectively. By
tocatalytic efficiency compared to the single Cu or Ni dividing the conversion under the light and dark conditions
attributed to the synergetic effect. In addition, by introducing (25%) as calculation [(97ꢂ86)/(97ꢂ29) ꢃ 100%], the overall
a bimetallic MOF, the downsizing of single Cu and Ni micro- light-induced conversion under specic wavelengths of 400–
MOF was developed to produce a Cu:Ni nano-MOF (Fig. 1), 455, 455–515, 515–610, and 610–800 nm was obtained to be
which improves the catalytic selectivity and activity. This result 16%, 21%, 29%, and 34%, respectively. These data were
provides an encouraging approach to the further synthesis of consistent with the UV-Vis DRS absorption of Cu2S:NiS2@C/
nano-MOFs, as a nano-MOF is still a challenge.44,45 For rGO, as shown in Fig. 8c. Therefore, the light-driven capacity
comparison, the bare GO aer hydrothermal and calcination of Cu2S:NiS2@C/rGO through the synergetic effect of the
treatment was identied (entry 12) and no conversion was composition and multifunctional structure considerably inu-
observed. Moreover, the solvent played a major role in the enced the photocatalytic activity for the N-arylation of imidazole
photocatalytic reaction. Excellent conversion was achieved with arylboronic acid.
using a 3 : 1 mixture of the protic solvent (MeOH) and H2O.
Photocatalysis was substantially related to the electron–
Decreasing the polarity of the solvent to EtOH reduced the hole transfer of the material via light irradiation. Therefore,
conversion, whereas increasing the polarity of the aprotic the effects of the electron and hole scavengers were investi-
solvent CH3CN enhanced the conversion (entry 3–7). However, gated (Fig. 8d).47 Carbon tetrachloride (CCl4) is an electron
MeOH is an environmentally preferable solvent to CH3CN;46 quencher for capturing the electrons of Cu2S:NiS2@C/rGO.
hence, this condition was used in the next step. Furthermore, With the addition of CCl4, no desired C–N coupling product
another solvent condition was determined for crosschecking was achieved. Moreover, the addition of the hole quencher
(entry 1–7). A comparative study with a previously reported triethanolamine (TEA) resulted in considerably decreased
catalyst for C–N coupling is shown in the ESI (Table S3†). The conversion. This phenomenon indicates that the photo-hole
high conversion achieved by the free base, free additional production was successfully blocked. The suggested reac-
oxidation agent, and green solvent provided a convenient tion mechanism is shown in Fig. 9. The numerous electrons
operational protocol and minimized the environmental issues in the conduction band and the photogenerated holes in the
for the C–N oxidative coupling reaction.
valence band can activate the imidazole and phenylboronic
The effect of light on the enhanced catalytic activity was acid in the MeOH solution to form the cross-coupling
explored in terms of intensity and wavelength dependency. product.48–50
First, as shown in Fig. 8a, the intensity was varied from 100 mW
The above results demonstrate that the optimized condi-
cmꢂ2 to 200 mW cmꢂ2 with 20 intervals. The conversion at 100 tions were catalysis by Cu2S:NiS2@C/rGO using MeOH : H2O
mW cmꢂ2 (75%) was better than that under dark conditions (3 : 1) as the solvent at ambient temperature (25 ꢀC) for 6 h
(29%). Furthermore, increasing the intensity improved the under open-air conditions. The optimized Cu2S:NiS2@C/rGO-
conversion of substrates; the optimum conversion was when promoted photocatalytic C–N coupling of phenylboronic acid
Fig. 8 (a) The dependence of (a) light intensity, (b) wavelength, (c) UV-
Vis DRS absorption of Cu2S:NiS2@C/rGO and (d) electron–hole aspect Fig. 9 Plausible mechanism for the visible light activated photo-
photocatalyst.
catalytic C–N coupling of phenylboronic acid and imidazole.
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Table 2 The photocatalytic performance of Cu2S:NiS2@C/rGO for the C–N coupling of phenylboronic acid and imidazole with various moietiesa
Entry
1
1a
1b
Product
Conv.b (%)
97
Selec.b (%)
96
2
3
4
5
6
7
91
96
>99
95
97
97
95
98
98
97
98
96
8
>99
>99
a
Reaction conditions: 0.5 mmol of 1a and 1b, 4 mL solvent, 5 mg of photocatalyst. The reactions were conducted in air at r.t for 6 h, under Xe-lamp
irradiation (400–800 nm) with a light intensity of 200 mW cmꢂ2
.
Conversions (base on 1b) and selectivity determined by GC-MS analysis.
b
and imidazole was then applied to various substrates, as shown of imine was conducted for 4 h using 0.25 mmol of reactants
in Table 2. The explored diverse range of reactants with modi- and CH3CN as the solvent (Table 3, entries 4–6). A satisfactory
ed electron-donating groups did not signicantly affect the completed reaction and the synthesis of N-heterocyclic deriva-
reaction (entries 1–4). For electron-donating groups (entries 5– tives bearing electron-donating or electron-withdrawing moie-
7), the conversion and selectivity remained >90%. Aromatic and ties were achieved.
heteroatomic boronic acids, such as N-containing hetero-
In addition to catalytic activity, reusability is also a key
aromatic boronic acids (entry 8), achieved high conversion and consideration in organic synthesis using heterogeneous cata-
selectivity to the desired product. These phenomena led to lysts. With the model reaction and optimized conditions in
a photocatalytic activity with remarkable functional group Table 1, the catalyst was reused for ve cycles aer being
selectivity under mild conditions.
recovered through simple centrifugation and then washed with
In addition to enhancing the catalytic performance, Cu2- EtOH. The results in Fig. 10a demonstrate good recyclability
S:NiS2@C/rGO was used for the synthesis of other N-compound with high conversion and selectivity values. Moreover, the
derivatives, such as N-heterocyclic and N-aryl derivatives. The N- stability of the Cu2S:NiS2@C/rGO catalyst was evaluated. The
heterocyclic product was obtained through the oxidative cycli- surface structures before and aer usage were analyzed via XPS.
zation of o-phenylenediamine and aromatic aldehyde. The The C 1s, Cu 2p, Ni 2p, and S 2p spectra of the photocatalyst
benzimidazole product was obtained by using 0.25 mmol of before and aer 5 cycles of usage were insignicantly different
both substrates with EtOH as the solvent at room temperature (Fig. 10b). The XRD-veried crystalline structure showed
for 1 h (Table 3, entries 1–3). Moreover, the presence of electron- minimal changes in intensity without changing the main
donating and electron-withdrawing substituents resulted in structure (Fig. S5a†). The morphology was characterized via
excellent conversion and high selectivity. The oxidation of TEM (Fig. S5b and c†), and results showed the tendency of the
benzylamine to imines was selected as an example of generating photocatalyst to preserve its form.
N-aryl derivatives. Photo-activated Cu2S:NiS2@C/rGO synthesis
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Journal of Materials Chemistry A
Paper
Table 3 The photocatalytic performance of Cu2S:NiS2@C/rGO for the N-substituted derivatives with various moietiesa
Entry
1a
1b
Product
Conv.b (%)
Selec.b (%)
1
2
98
92
99
95
3
98
99
4
5
95
93
99
99
6
95
99
a
Reaction conditions: entry 1–3: 0.25 mmol of 1a and 1b, 3 mL EtOH, 2.5 mg of the photocatalyst, reactions for 1 h, conversions base on 1b. Entry 4–
6: 0.25 mmol reactant, 3 mL CH3CN, 5 mg of the photocatalyst, reactions for 6 h. The reactions were conducted in air at r.t, under Xe-lamp
b
irradiation (400–800 nm) with a light intensity of 200 mW cmꢂ2
.
Conversions and selectivity determined by GC-MS analysis.
Conclusions
A series of bimetallic MOFs using Cu and Ni as metal centers
connected with a carboxylate-group-rich H4BTEC ligand was
synthesized through a facile and simple method. Subsequently,
a hybrid core–shell nanostructure of bimetallic Cu and Ni sulde
encapsulated on porous C tailoring with GO was fabricated from
an MOF derivative. Direct carbonization and sulfurization of the
MOF (Cu:Ni-BTEC/GO) produced the bimetallic sulde Cu2-
S:NiS2@C/rGO. Cu2S:NiS2@C/rGO exhibited excellent photo-
catalytic properties and enhanced photocatalytic activity. The
light-activated material Cu2S:NiS2@C/rGO photogenerated
holes and electrons, resulting in improved products of the C–N
coupling of phenylboronic acid with imidazole, cyclization of o-
phenylenediamine with aldehyde, and oxidative homocoupling
reactions of benzylamine. The synergetic combination of non-
noble metals Cu and Ni in the multifunctional structure of the
bimetallic sulde enhanced the photocatalytic activity of N-
derivative compounds synthesized through oxidative coupling
reactions. In addition, the recyclability and stability as important
aspects of the heterogeneous catalyst were established. This
hybrid structure can offer additional options for synthesizing
core–shell structures through MOF derivation and lead to wide-
ranging applications, such as in organic synthesis.
Fig. 10 (a) Reusability study for the photocatalytic activity of Cu2-
S:NiS2@C/rGO and (b) XPS analysis of Cu2S:NiS2@C/rGO photocatalyst
after the reusability test.
Conflicts of interest
There are no conicts to declare.
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Acknowledgements
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) grant
funded by the Korea Government (NRF-2020R1I1A3067208 and
NRF-2018R1D1A1B07045663). S. Song was supported by the
Global Ph.D. Fellowship Program (NRF-2018H1A2A1062253).
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