Interface Engineering in Two-Dimensional Heterostructures: Towards an Advanced Catalyst for Ullmann Couplings

Article abstract of DOI:10.1002/anie.201508571

The design of advanced catalysts for organic reactions is of profound significance. During such processes, electrophilicity and nucleophilicity play vital roles in the activation of chemical bonds and ultimately speed up organic reactions. Herein, we demonstrate a new way to regulate the electro- and nucleophilicity of catalysts for organic transformations. Interface engineering in two-dimensional heteronanostructures triggered electron transfer across the interface. The catalyst was thus rendered more electropositive, which led to superior performance in Ullmann reactions. In the presence of the engineered 2D Cu₂S/MoS₂ heteronanostructure, the coupling of iodobenzene and para-chlorophenol gave the desired product in 92 % yield under mild conditions (100 °C). Furthermore, the catalyst exhibited excellent stability as well as high recyclability with a yield of 89 % after five cycles. We propose that interface engineering could be widely employed for the development of new catalysts for organic reactions.

Full text of DOI:10.1002/anie.201508571

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International Edition: DOI: 10.1002/anie.201508571
German Edition: DOI: 10.1002/ange.201508571
Heterogeneous Catalysis Very Important Paper
Interface Engineering in Two-Dimensional Heterostructures: Towards
an Advanced Catalyst for Ullmann Couplings
Xu Sun+, Haitao Deng+, Wenguang Zhu, Zhi Yu, Changzheng Wu,* and Yi Xie
Abstract: The design of advanced catalysts for organic
reactions is of profound significance. During such processes,
electrophilicity and nucleophilicity play vital roles in the
activation of chemical bonds and ultimately speed up organic
reactions. Herein, we demonstrate a new way to regulate the
electro- and nucleophilicity of catalysts for organic trans-
formations. Interface engineering in two-dimensional hetero-
nanostructures triggered electron transfer across the interface.
The catalyst was thus rendered more electropositive, which led
to superior performance in Ullmann reactions. In the presence
of the engineered 2D Cu2S/MoS2 heteronanostructure, the
coupling of iodobenzene and para-chlorophenol gave the
desired product in 92% yield under mild conditions (1008C).
Furthermore, the catalyst exhibited excellent stability as well as
high recyclability with a yield of 89% after five cycles. We
propose that interface engineering could be widely employed
for the development of new catalysts for organic reactions.
metal–organic frameworks (MOFs), have been intensively
investigated; their electro/nucleophilicity is controlled either
by induction or by conjugation effects.[8–13] In this regard, an
ideal catalyst for organic reactions should display high
stability, fine dispersity, and optimized electro/nucleophilicity.
Transition-metal chalcogenides (TMCs), with rich d elec-
tron configurations leading to controllable electronic struc-
tures, have experienced major development in the pursuit of
novel catalyst designs.[14–20] Much effort has been devoted to
engineering the surface and interface of TMC catalysts to
enable diverse catalytic processes;[14,17,18–23] however, how to
precisely manipulate their electro/nucleophilicity to be appli-
cable for catalysis in organic reactions still remains a grand
challenge. Interface engineering by forming 2D heteronano-
structures of TMC catalysts with 2D nanomaterials[24,25]
provides a new opportunity for optimizing the electro/
nucleophilicity by interfacial effects.[26,27] Herein, we highlight
a general route for the fabrication of 2D heteronanostruc-
tures, including Cu2S/MoS2, CdS/MoS2, and FeS/MoS2, by
domain-matching epitaxial growth[28,29] of TMC materials on
2D MoS2 nanosheets to form well-defined interfaces. We find
that the 2D Cu2S/MoS2 heteronanostructure triggered a spon-
taneous electron transfer across the interface, which enhances
the electron affinity of the catalytic surface and thus favors
the attack of nucleophiles during the catalyzed reaction. As
a proof of concept, the 2D heteronanostructure was tested in
Ullmann couplings; the catalyst showed superior activity at
low temperatures, excellent stability, durability, and high
recyclability. Interface engineering in 2D heteronanostruc-
tures is thus a promising approach for optimizing the electro/
nucleophilicity of the catalysts to improve their performance
in organic reactions.
The general synthetic procedures for the 2D TMC/MoS2
heteronanostructures are illustrated in Figure 1a and b. After
sonication in N,N-dimethylformamide (DMF), bulk MoS2 was
exfoliated into nanosheets with various sizes, among which
the negatively charged large nanosheets provided the basic
framework for the construction of the heteronanostructures.
At first, the metal ions, Mn+, are attracted to the surface of the
larger nanosheets owing to electrostatic interactions. Then,
these Mn+ ions will further react with S2À to produce MxS on
the large MoS2 nanosheets. It is understandable that these S2À
ions originate from the dissolution of the smaller MoS2
nanosheets, which is assisted by the strong dipolar aprotic
solvent DMF: the smaller nanosheets usually have a higher
chemical potential and thus tend to dissolve; the dissolved
ions will then diffuse and be transported to and grow on the
surface of the larger nanosheets, which is similar to the
Ostwald ripening process.[30] When the solution was heated
with the magnetic stirrer set at 2508C, MxS was easily
T
he catalysis of organic reactions remains a vibrant field of
scientific research and has attracted tremendous attention
owing to its promising application in the synthesis of
pharmaceuticals, agrochemicals, and organic electronic devi-
ces.[1–4] During most organic reactions, the electrophilic or
nucleophilic surface of a catalyst activates chemical bonds and
boosts the formation of intermediates, resulting in a decrease
in activation energy of the reaction,[5–7] which accelerates the
reaction. Various excellent and monodisperse catalysts for
organic reactions, including enzymes, organocatalysts, and
[*] Dr. X. Sun,[+] H. T. Deng,[+] Z. Yu, Prof. C. Z. Wu, Prof. Y. Xie
Hefei National Laboratory for Physical Sciences at the Microscale
iChEM (Collaborative Innovation Center of Chemistry for Energy
Materials), Hefei Science Center (CAS), and CAS Key Laboratory of
Mechanical Behavior and Design of Materials
University of Science and Technology of China
Hefei, Anhui 230026 (P. R. China)
E-mail: czwu@ustc.edu.cn
Prof. W. G. Zhu
International Center for Quantum Design of Functional Materials
(ICQD), Hefei National Laboratory for Physical Sciences at the
Microscale (HFNL)
Synergetic Innovation Center of Quantum Information and Quantum
Physics, University of Science and Technology of China
Hefei, Anhui 230026 (P. R. China)
and
Key Laboratory of Strongly-Coupled Quantum Matter Physics
Chinese Academy of Sciences, School of Physical Sciences
University of Science and Technology of China
Hefei, Anhui 230026 (P. R. China)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508571.
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image of the interface area of Cu2S/
MoS2 shows two sets of lattice fringes,
which correspond to MoS2 and Cu2S,
respectively, further confirming that
the two sides of the interface are
composed of Cu2S and MoS2 (Fig-
ure 1 f). The lattice distance of approx-
imately 2.7 Š corresponds to the (100)
and (010) lattice planes of hexagonal
MoS2. For the in situ formed Cu2S
nanocrystals, the measured lattice spac-
ing was about 0.34 nm, which is in good
agreement with the (100) planes of
hexagonal Cu2S, and sixfold symmetry
similar to that of the MoS2 substrate
was observed. The fast Fourier trans-
form pattern shows two sets of diffrac-
tion spots, which correspond to MoS2
and Cu2S (Figure 1 f, inset). The first
set of spots with well-defined hexago-
nal symmetry could be assigned to the
(100) and (010) planes of MoS2. The
second set of spots with a lattice spac-
ing of approximately 0.34 nm could be
assigned to the (100) planes of Cu2S,
revealing the [001] orientation of the
Cu2S nanocrystals. The aforemen-
tioned analyses revealed that the
h100i directions of Cu2S are aligned
with the h100i directions of MoS2 while
Figure 1. Synthesis and characterization of the 2D heteronanostructures. a,b) Synthetic route
towards MxS/MoS2. c) XRD pattern of Cu2S/MoS2. d) TEM image, showing highly dispersed Cu2S
anchored onto MoS2, resulting in the formation of the Cu2S/MoS2 2D heteronanostructure.
e) HAADF-STEM image and elemental mapping of Mo, Cu, and S for Cu2S/MoS2. f) HRTEM
images of Cu2S/MoS2 and fast Fourier transform electron diffraction (FFT ED) patterns (inset).
g) Domain-matching epitaxy of the as-formed Cu2S/MoS2 heteronanostructure.
sharing
a common [001] direction,
which is due to the fact that the sulfur
layers in these two samples hold the
anchored onto the larger MoS2 nanosheets, forming 2D TMC/
MoS2 heteronanostructures with tunable electronic struc-
tures. In short, the epitaxial growth of TMC materials on 2D
MoS2 nanosheets constitutes a general route for the fabrica-
tion of 2D heteronanostructures, such as Cu2S/MoS2, CdS/
MoS2, and FeS/MoS2, with well-defined interfaces.
same hexagonal symmetry (see Figure 1g), favoring in-plane
epitaxial growth.
The epitaxial relationship during the heterogeneous
growth process can be explained in terms of domain match-
ing.[28,29] For Cu2S/MoS2, the (100) spacing ratio between Cu2S
and MoS2 was about 1.26, which is close to 5/4, implying that
there are four Cu2S unit cells per five MoS2 unit cells. As
a result, the mismatch[33] was as low as about 1.2%, thus
enabling the in situ growth of Cu2S nanocrystals on MoS2
substrates by domain matching. Notably, the presence of 2D
MoS2 as a growth matrix also rendered hexagonal Cu2S, which
is usually observed at high temperatures, stable at room
temperature. At room temperature, Cu2S usually exists in the
monoclinic phase known as chalcocite, and it would convert
into hexagonal Cu2S at temperatures above 1058C. In Cu2S/
MoS2, the stabilization of the high-temperature hexagonal
Cu2S phase is due to domain-matching epitaxial growth on the
hexagonal MoS2 lattice framework. Therefore, domain-
matching epitaxy can stabilize hexagonal Cu2S phases when
immobilized on 2D nanosheets.
Composition and morphology data of the as-synthesized
Cu2S/MoS2 heteronanostructure are shown in Figure 1. The
powder X-ray diffraction (XRD) patterns (Figure 1c) show
two sets of diffraction peaks that could be readily indexed to
MoS2 (in turquoise, JCPDS No. 37-1492) and hexagonal Cu2S
(in blue, JCPDS No. 84-0206), which confirmed the successful
growth of Cu2S nanocrystals on the MoS2 nanosheets. A
transmission electron microscopy (TEM) image of Cu2S/
MoS2 revealed that individual Cu2S nanocrystals with a width
of about 20 nm had been deposited on the MoS2 nanosheets
with no apparent aggregation (Figure 1d). Most of the
nanocrystals were located at the edges of the MoS2 nano-
sheets.[31,32] A HAADF-STEM image as well as the elemental
mapping data confirmed the presence of elemental Cu and
the uniform distribution of Cu2S on the surface of the MoS2
nanosheets, forming 2D Cu2S/MoS2 heteronanostructures
(Figure 1e).
X-ray photoelectron spectroscopy (XPS) and Auger
electron spectroscopy (AES) gave the valence states of the
elements in the heteronanostructure. The XPS peaks corre-
sponding to Mo 3d and S 2p are in accordance with previous
reports,[34,35] demonstrating that no obvious changes in their
valence states have occurred (Figure 2a,b). The high-reso-
To obtain microstructural information on the heterostruc-
ture, high-resolution transmission electron microscopy
(HRTEM) was carried out. A representative HRTEM
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Furthermore, systematic char-
acterization studies, for example,
by Raman, UV/Vis, and AES spec-
troscopy, revealed the presence of
interfacial effects in the 2D hetero-
nanostructure. The Raman spectra
revealed changes in the vibration
modes for the 2D Cu2S/MoS2 het-
eronanostructure, confirming the
above-mentioned hypothesis (Fig-
ure 2e). In detail, after immobiliza-
tion of Cu2S onto MoS2, the in-
1
plane vibration mode E2g and the
out-of-plane vibration mode A1g
had been blue-shifted compared to
pristine MoS2 (from 379 to 381 cmÀ1
1
for E2g and from 404 to 410 cmÀ1
for A1g); the deviation in the A1g
mode was larger. This could be
attributed to the in situ growth of
Cu2S, which exploits the S layer of
MoS2 as an external S source, hence
changing the primitive vibration
À
mode of the Mo S bonds, of
which the out-of-plane vibration
mode is altered more significantly.
UV/Vis spectroscopy also provided
evidence for electronic interactions
(Figure 2 f), as the absorption edge
of Cu2S/MoS2 showed band-gap
narrowing in the combined system.
The Cu LMM AES was much more
intriguing (Figure 2d); the charac-
teristic peak of Cu for Cu2S/MoS2
was found at 917.9 eV, whereas the
peak for a mechanical mixture of
Cu2S and MoS2 is located at approx-
imately 917.6 eV. The slight positive
deviation of 0.3 eV for the hetero-
nanostructure compared to the
Figure 2. Spectroscopic characterization of the Cu2S/MoS2 heteronanostructure. a–c) XPS spectra for
Mo, S, and Cu, respectively. d) LMM spectra of a mechanical mixture of Cu2S and MoS2 and the
Cu2S/MoS2 heteronanostructure, revealing a slight positive deviation for the heteronanostructure
(highlighted in yellow, magnified in the inset). e) Raman spectrum, showing blue shifts in the
vibration modes. f) UV/Vis spectrum. A red shift of the absorption edge of Cu2S/MoS2 was observed,
indicating substantial band-gap narrowing.
mechanical
mixture
indicates
a change in charge density on CuI,
which could be related to interfacial
effects. All of the spectra revealed
the presence of strong interfacial
lution core spectrum of Cu 2p shows two obvious peaks with
binding energies of approximately 932.4 eV and 952.6 eV,
which could be assigned to Cu 2p1/2 and Cu 2p3/2, respectively
(Figure 2c), excluding the possibility of Cu2+ formation
because no satellite peaks were detected. However, as the
Cu+ and Cu 2p peaks correspond to the same binding energy,
the valence state of Cu could not be determined by the core
2p spectrum alone. The Auger electron spectrum (AES) in
Figure 2d revealed the valence state to be + 1, as the
characteristic peak associated with LMM (Figure 2d, blue
line) was close to that of Cu1+ according to a previous
report.[35] This result confirmed the formation of a Cu2S/MoS2
heterostructure.
effects in the Cu2S/MoS2 heterostructure.
To gain more comprehensive insight into the changes in
the spectroscopic properties of the Cu2S/MoS2 heteronano-
structure, we performed density functional theory (DFT)
calculations to investigate the electronic structure upon
interface formation. To model the Cu2S/MoS2 heterostruc-
ture, we constructed a supercell containing a 4 ” 4 Cu2S cell
matching with a 5 ” 5 MoS2 cell, and a vacuum region of about
20 Š. The calculated density of states (Figure 3b) indicates
that the Fermi level of the combined Cu2S/MoS2 system lies
above the bottom of the conduction band of MoS2, resulting
in the injection of electrons from Cu2S into MoS2 upon
contact. This is also consistent with the band diagram
(Figure 3a) that was estimated by calculations on the isolated
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electron depletion around the Cu atoms, rendering the CuI
atoms more electropositive.
As discussed above, electron transfer across the interface
could render CuI more electropositive, resulting in enhanced
eletrophilicity, which might facilitate the attack of nucleo-
philes during catalysis. For a proof-of-concept application, the
Cu2S/MoS2 heteronanostructure was used as a catalyst in an
organic reaction to understand how interfacial electron
transfer influences the catalytic performance. The transi-
tion-metal ion Cu+, with abundant 3d electrons, is a common
catalyst for organic reactions.[36–40] The copper-catalyzed
arylation of nucleophiles, the so-called Ullmann reaction, is
a useful and practical method for the synthesis of various
targets for the life sciences and polymer industries, and is
pivotal in the formation of aryl carbon–carbon as well as aryl
carbon–heteroatom bonds.[36,37] This process thus provides an
opportunity to understand and manipulate the profound
relationship between interfacial electron transfer and cata-
lytic activity. In our case, the Cu2S/MoS2 heteronanostructure
was found to benefit from interfacial electron transfer from
CuI to MoIV, rendering CuI more electropositive, and thus
holds great promise as a catalyst for Ullmann couplings
(Figure 4a).
The catalytic performance of 5 wt% Cu2S/MoS2 in the
coupling reaction of iodobenzene and para-chlorophenol was
investigated in toluene solution in air at 1008C (for results
obtained under other conditions, see the Supporting Infor-
mation, Table S1), and CsCO3 was used as the base. After
heating the reaction mixture to reflux at 1008C for ten hours,
the desired product had been formed in 92% yield (Table 1,
entry 3), demonstrating that the Cu2S/MoS2 heterostructure is
a good catalyst of this reaction and could even be a new type
of ligand-free heterogeneous catalyst, which is significant for
practical drug synthesis.[37] For comparison, the catalytic
activities of a mechanical mixture of Cu2S and MoS2 and
the individual components were tested as well, but they
showed much lower or even no activity in this reaction
(entries 1, 5, and 6). The catalytic efficiencies of 3 and 10 wt%
Figure 3. Calculated electronic structures of the Cu2S/MoS2 heterona-
nostructure. a) Band diagrams of MoS2 and Cu2S/MoS2. b) The calcu-
lated total density of states of a pristine MoS2 monolayer (pink) and
the projected density of states for MoS2 in the Cu2S/MoS2 hetero-
nanostructure (blue). The Fermi level of the combined system is
shifted to zero, as indicated by the dashed line, and the energy levels
of the two systems are aligned with respect to the vacuum level. An
increase in the density of states at the Fermi level is highlighted by the
green region. c) The charge density difference in the 2D heteronano-
structure of Cu2S/MoS2; dark red and dark blue represent regions of
electron accumulation and depletion, respectively. The electron transfer
from Cu2S to MoS2 is indicated by the blue arrow.
Table 1: Screening of various heterostructures as catalysts of the
Ullmann reaction between iodobenzene and para-chlorophenol.
Entry
Catalyst
Yield [%]
Cu2S and MoS2 systems. The projected density of states on
MoS2 from the heterostructure (Figure 3b, blue) shows
substantial band-gap narrowing (in accordance with the
UV/Vis spectroscopy results) and partial filling of the
conduction band of MoS2, in comparison with that of pristine
MoS2 nanosheets (pink). A more intuitive understanding can
be obtained from the charge density difference image in
Figure 3c. The accumulation and depletion of electrons
mostly happen at the interface region, and the overall
direction of such transfers is from Cu2S to MoS2 as indicated
by the blue arrow. Such electron transfer ultimately leads to
a reduced electron density in Cu2S, which is revealed by the
1
2
3
4
5
6
7
8
Cu2S+MoS2
3 wt% Cu2S/MoS2
5 wt% Cu2S/MoS2
10 wt% Cu2S/MoS2
MoS2
Cu2S
Cu
20
77
92
82
n.d.
25
12
–
n.d.
Reaction conditions: 1a (244 mg, 1.2 mmol, 1.2 equiv), 2 f (1 mmol,
1 equiv), toluene (2 mL), catalyst (10 mg), Cs2CO3 (388 mg, 1.2 mmol,
1.2 equiv), 1008C, 10 h. Yields of isolated products were calculated
based on the amount of 2 f. n.d.=not determined.
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Cu2S/MoS2 were also determined under otherwise identical
conditions (entries 2 and 4). Overall, 5 wt% Cu2S/MoS2
exhibited the best catalytic activity for the coupling of
iodobenzene and para-chlorophenol. Therefore, all further
reactions were conducted with this catalyst.
To explore the influence of different substituents on the
coupling process, we analyzed the reactions of various phenol
derivatives with iodobenzene. In all cases, the reactions
proceeded with good efficiency, rendering the corresponding
products in 70–95% yield (Scheme 1). The optimized electro-
philicity, which is due to interfacial electron transfer, has
greatly improved the catalytic activity of Cu2S, leading to high
reactivity at low temperatures.
The operational lifetime is also an important property of
a catalyst. Compared to homogeneous catalysts, heteroge-
neous ones can be easily recycled, enabling sustainable
industrial applications.[37] The durability of the Cu2S/MoS2
heteronanostructure was thus analyzed (Figure 4b). After
recycling the catalyst five times, no apparent loss of activity
was observed, with yields of around 90% in all five cycles.
TEM images (Figure S5) of a catalyst sample that had been
recycled five times (reactions at 1008C) showed no obvious
changes in morphology or aggregation of the Cu2S nano-
particles, further confirming the high recyclability as well as
dispersibility and substantiating the potential practical appli-
cations of this heteronanostructured catalyst. Compared with
other catalysts,[36–40] apart from the high reactivity, which
benefits from interfacial electron transfer, high recyclability
was achieved owing to the use of 2D MoS2 nanosheets as the
catalyst support, which constitutes a more cost-effective
approach for the design of catalysts of the Ullmann reaction
and various other organic transformations.
In conclusion, engineering the interface of TMCs by
forming 2D heteronanostructures has been shown to be a new
route for controlling their electro/nucleophilicity. The hetero-
structure was shown to be an efficient catalyst for Ullmann
couplings under mild conditions and displayed excellent
stability and recyclability. The induced interfacial electron
transfer in the 2D heteronanostruc-
Figure 4. a) Cu2S/MoS2 as a catalyst of Ullman couplings. b) Yields
obtained for different cycles and the Ullmann reaction that was carried
out to test durability.
ture increases the electron affinity
of the catalytic surface, favoring the
nucleophilic attack of the reactant
during the catalytic process. We
anticipate that interface engineer-
ing by the formation of heterostruc-
tures will be a powerful method for
optimizing catalysts for various
organic reactions and find various
pharmaceutical and industrial ap-
plications.
Acknowledgements
This work was financially supported
by the National Basic Research
Scheme 1. Substrate scope of the Cu2S/MoS2 catalyzed Ullman coupling. Reaction conditions: 1a
(244 mg, 1.2 mmol, 1.2 equiv), 2 (1 mmol, 1 equiv), toluene (2 mL), 5 wt% Cu2S/MoS2 (10 mg,
0.1 equiv), Cs2CO3 (388 mg, 1.2 mmol, 1.2 equiv), 1008C, 10 h. Yields of isolated products were
calculated based on the amount of 2.
Program
of
China
(2015CB932302), the National Nat-
ural Science Foundation of China
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Keywords: catalysis · electrophilicity · nucleophilicity ·
interface engineering · Ullmann reaction
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Received: September 13, 2015
Published online: December 16, 2015
Angew. Chem. Int. Ed. 2016, 55, 1704 –1709
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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