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ReseaRch aRticle
A Polarization Boosted Strategy for the Modification
of Transition Metal Dichalcogenides as Electrocatalysts
for Water-Splitting
Guanyu Chen, Chang Zhang, Shuyan Xue, Jiwei Liu, Yizhe Wang, Yunhao Zhao, Ke Pei,
Xuefeng Yu, and Renchao Che*
Dr. G. Chen, Dr. C. Zhang, Dr. S. Xue, Dr. Y. Zhao, Dr. K. Pei,
Dr. X. Yu, Prof. R. Che
Laboratory of Advanced Materials
Department of Materials Science and Collaborative Innovation
Center of Chemistry for Energy Materials (iChem)
Fudan University
Shanghai , P. R. China
E-mail: [email protected]
Dr. J. Liu
School of Materials Science and Engineering
Changzhou University
Changzhou, Jiangsu , P. R. China
Dr. Y. Wang
Materials Genome Institute
International Centre of Quantum and Molecular Structures, and
Physics Department
Shanghai University
Shanghai , P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/./smll..
DOI: 10.1002/smll.202100510
1. Introduction
As a promising hydrogen (H) produc-
tion method, water-splitting powered by
electricity has been regarded as an ideal
strategy in the future.[–] However, the
sluggish reaction kinetics of hydrogen
evolution reaction (HER) and oxygen
evolution reaction (OER) hinder the fur-
ther application.[] Recently, D transi-
tion metal dichalcogenides (TMDs) such
as MoS and WS have drawn increasing
attention as ecient bifunctional electro-
catalysts for OER and HER due to their
earth-abundance, low cost, and ecient
catalytic performance.[–] The unique
layer structures of TMDs are beneficial to
the electrocatalysis with the edges demon-
strated to provide more active sites com-
pared with bulk materials.[] Nevertheless,
some deficiencies such as low electronic
conductivity caused by van der Waals
force between the layers and limited active
sites induced by basal plane inactiveness
impede their direct applications.[]Various
strategies including morphology regula-
tion,[] crystal phase engineering,[–]
and doping with other elements[] have been put forward to
solve the problems and to fabricate the benign bifunctional
electrocatalysts. As expected, many modified TMDs-based
composites, especially WS-based electrocatalysts have been
prepared, such as, WS@graphene,[] Mo.W.S,[] T’-D
WS,[] WS nanodots,[] Co-doped WS/WO nanotubes[]
and N,P co-doped exfoliated WS.[] Among them, tuning the
polarization eect of the components to achieve better conduc-
tivity has drawn increasing attentions for the enhancement of
water-splitting performance. By the strong polarization between
the heterojunction interface, the formed built-in electric field
and the charge accumulation on the surface will show posi-
tive influence on the water-splitting performance.[] Although
these progresses are encouraging, the main hindering factors,
as the limited conductivity and few numbers of catalytic sites
still exist, which hampers the performance of TMDs-based
catalysts.[]Furthermore, the lack of OER active sites pre-
vents the application for water-splitting of TMDs, making it
an urgent problem to be solved.[] Therefore, to fabricate the
The design and fabrication of transition metal dichalcogenides (TMDs) are
of paramount significance for water-splitting process. However, the limited
active sites and restricted conductivity prevent their further application.
Herein, a polarization boosted strategy is put forward for the modification
of TMDs to promote the absorption of the intermediates, leading to the
improved catalytic performance. By the forced assembly of TMDs (WS2
as the example) and carbon nanotubes (CNTs) via spray-drying method,
such frameworks can remarkably achieve low overpotentials and superior
durability in alkaline media, which is superior to most of the TMDs-based
catalysts. The two-electrode cell for water-splitting also exhibits perfect
activity and stability. The enhanced catalytic performance of WS2/CNTs
composite is mainly owing to the strong polarized coupling between CNTs
and WS2 nanosheets, which significantly promotes the charge redistribution
on the interface of CNTs and WS2. Density functional theory (DFT)
calculations show that the CNTs enrich the electron content of WS2, which
favors electron transportation and accelerates the catalysis. Moreover, the
size of WS2 is restricted caused by the confinement of CNTs, leading to
the increased numbers of active sites, further improving the catalysis. This
work opens a feasible route to achieve the optimized assembling of TMDs
and CNTs for ecient water-splitting process.
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TMDs for enhanced conductivity and improved water-splitting
performance are of paramount significance for water-splitting
process.
Acknowledged by the large specific surface, good conduc-
tivity, and low weight, carbon nanotubes have been considered
as one of the most attractive substrates to be assembled with
active electrocatalysts to form ecient electrocatalysts.[–]
By combining TMDs with carbon nanotubes (CNTs), the con-
ductivity and electrocatalytic performance have been pro-
moted.[–] However, when assembled with CNTs, the size
of TMDs is hard to be controlled, leading to the loss of the
activity.[]The mismatched ratios of length to diameter between
them make it dicult to achieve and maintain the eective con-
nection, which blocks the mass transport and electron transfer
of the catalysts.[] Therefore, only limited works have reported
the composite consisting of TMDs and CNTs with less attractive
catalytic performance.[–] Furthermore, the polarization eect
between WS and CNTs is rarely mentioned and discussed
before, which hinders the development of the WS-based elec-
trocatalysts. Apparently, to design a novel assembly strategy for
the highly ecient TMDs-CNTs composites, and to study the
catalytic mechanism, are of great importance for the following
research on the TMDs-based catalysts.
Herein, a novel WS/CNTs hollow microsphere with supe-
rior water-splitting catalytic performance has been successfully
synthesized via a spray-drying method. Many D WS layers
have been confined onside the surface of CNTs shell to form a
unique D conductive framework. By the confinement of CNTs
shell, the obtained WS exhibits decreased size and increased
exposed edge sites, both of which promote the electrocatalytic
performance. The overpotential can be optimized to  mV
(at  mA cm−) for HER and mV (at  mA cm−) for OER
in   KOH, which surpasses almost all the H-WS based nano-
composites, even some noble metal materials. Benefitting from
the simple procedure, industrial output and the eective force
assembly property, the WS/CNTs composite exhibits excellent
water-splitting performance (.V for mA cm−). The polari-
zation eect between WS and D CNTs networks accelerates the
electronic conductivity, leading to the ecient electrocatalysis,
which is confirmed by electron holography and density func-
tional theory (DFT) calculations. Therefore, our results provide a
guidance for the modification of TMD-based bifunctional water-
splitting catalysts through the polarization boosted strategy.
2. Results and Discussion
The synthesis process of WS nanosheets grown on carbon
nanotube hollow microsphere as illustrated in Figure1a and
Scheme S, Supporting Information. At first, the ammonium
tungstate was dissolved into the mixture of distilled water and
CNTs to form a stable solution. Under the eect of ultrasonic
crushing, the agglomerated CNTs were dispersed, and con-
tinuously stirred for h to acquire a homogeneous suspension.
Afterward, via a facilely forced assembly spray-drying method,
a composite consisting of CNTs shells and tungstate ions has
been obtained. The stable solution was broken up to form uni-
form liquid drops, consisting of tungstate, CNTs, and water.
Under the heat exchange of the hot wind, the water is quickly
evaporated and the CNTs are assembled to form a hollow
sphere, which is ascribed to the low diuse speed of CNTs
inward compared with water molecules and ions outward. By
adding HSO (. ) into NaS, HS has been obtained and
then pumped into the tube furnace to form WS/CNTsHMS
composite. Finally, the hollow microspheres wound by CNTs
are obtained, and the inner and outer interfaces of the balls are
loaded with a large amount of WS. This structure not only
limits the size of WS which is beneficial to acquire more
active sites, but also improves the WS conductivity by the eec-
tive connection between CNTs and WS, both of which promote
the electrocatalysis performance of WS.
The morphology and structure of both CNTsHMS and WS/
CNTsHMS is measured by scanning electron microscopy
(SEM) and transmission electron microscope (TEM). The CNT-
sHMS exhibits a hollow yarn-like structure (Figure Sa, Sup-
porting Information), which is corresponding to the previous
works.[] The microparticle sizes are measured from . to 
µm. After calcined at °C, the WS/CNTsHMS- exhibits a
combined feature of yarn-like CNTs hollow shells and D-WS
confined in the twining CNTs (Figure b–d). Under a lower
temperature of  °C, WS/CNTsHMS- shows a com-
posite feature of CNTs and D H-WS (Figure Sb,d, Sup-
porting Information). Its size distribution has an average size
of . nm, compared with . nm of WS/CNTsHMS-
(Figure Sb,c, Supporting Information). Under a higher calci-
nation temperature of °C, however, the size of WS reaches
. µm (Figure Sd, Supporting Information). The increased
size of WS makes it dicult to form the uniform hollow
microsphere for the mismatched size of WS and CNTs, leading
to the unsuccessful assembling of WS/CNTsHMS (Figure Sd,
Supporting Information).
As the unique assembling of CNTs and WS exhibits strong
confinement eect,[] the size of WS is obviously decreased
compared with commercial WS, which leads to the increased
active sites of WS (Figure S, Supporting Information).
Figure e–g shows the TEM images of WS/CNTsHMS-.
This structure is prepared via forced assembly method by
D-CNTs and D-WS.The high-angle annular dark field-
scanning transmission electron microscope element mapping
(STEM)-mapping exhibits that the W and S elements mainly
mapped onside the D hexagonal WS layers, while the C ele-
ment dispersed on the surface of CNTsHMS, which confirms
the successful synthesis of WS/CNTsHMS (Figure h–k). To
observe the inside structure of WS/CNTsHMS, a thin slice of
WS/CNTsHMS cut by FIB is shown in Figure S, Supporting
Information. The D WS layers are confined at the CNTs
shell (Figure Sb–d, Supporting Information), which is cor-
responding to the TEM images. Importantly, many layers are
grown inside the CNTs shell (Figure Sd, Supporting Informa-
tion). Supposedly, the tungstate is adsorbed onside the CNTs
via the spray-drying method. During the sulphuration process,
the tungstate gradually transforms into WS and then grows
larger toward every orientation to form WS/CNTsHMS. Fur-
thermore, via almost the same method, other TMDs-based
composites have been successfully prepared including MoS
(Figure S, Supporting Information) and VS (Figure S, Sup-
porting Information), confirming the possible widely applica-
tion for the modification of TMDs. Moreover, the spray-drying
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method is conducive to industrialization due to the benefits of
low-cost, high output, easy operation (Figure S, Supporting
Information). Confirmed by the results above, a universal
forced assembly strategy has been put forward to modify the
TMDs for decreasing particle size and increasing conductivity
via spray-drying.
Figure2a exhibits the crystal phase of WS/CNTsHMS.
The typical diraction peaks appearing at .° and .° for
WS/CNTsHMS are assigned to () and () planes of
WS.[,] Furthermore, the other peaks are corresponding to
H-WS (JCPDS Card No. –), which confirms the suc-
cessful reaction of tungstate ions and HS to form WS during
the synthesis process. From the enlarged XRD pattern at θ=
–°(Figure S, Supporting Information), the peak at ° is
assigned to the () plane of graphite, indicating the exist-
ence of CNTs. Raman measurements have been carried out
to investigate the graphitization degree of the composites
(Figureb). The sharp peaks at  and cm− represent
the D band and G band, respectively. The intensity ratio of D
band and Gband (ID/IG) could be a symbol of the defects and
imperfection of the carbon.[] With the temperature increasing
from  to °C, the ratio of ID/IG shows a rising tendency.
The abundant defects caused by the reaction of WS and CNTs
are beneficial to regulate the adsorption of the intermediates
and facilitate the reaction kinetics. According to the TGA data
(Figure c), the content of CNTs in WS/CNTsHMS reaches
.%. Therefore, the content of WS in the composite is .%.
The combustion temperature of CNT can be obtained by DTG
curve analysis, which is about  °C. To evaluate the pore
structures in WS/CNTsHMS, the Brunauer-Emmert-Teller
(BET) N absorption-desorption measurement has been exe-
cuted (Figured and Figure S, Supporting Information). The
results show that when the relative pressure at .–., the hys-
teresis loops between adsorption and desorption reveal a typical
mesoporous structure of WS/CNTsHMS.[] The twinning of
CNTs to form a sphere is the mainly reason of the emergence
of mesoporous structure. Using the Barrett-Joyner-Halenda
model, the pore size distributions of WS/CNTsHMS derived
from the desorption branch of the isotherm are in the range
of – nm (inset of Figure d and Figure S, Supporting
Information), further revealing the mesoporous structure of the
composites, which is beneficial for the transportation of elec-
tron and the access to the electrolyte.
X-ray photoelectron spectroscopic (XPS) was used to probe
the surface chemical environment of WS/CNTsHMS-.
The wide XPS spectrum (Figure3a) reveals that the W, O, C,
S elements coexist on the surface of WS/CNTsHMS-.
The appearance of the peaks of oxygen is contributed to the
Figure 1. a) Simplified schematic illustration of the formation processes of prepared WS/CNTsHMS-; b–d) The SEM images of WS/CNT-
sHMS-; e–g) The TEM images of WS/CNTsHMS-, the inset of (g) is the FFT view of WS/CNTsHMS-; h–k) The STEM element mapping
of WS/CNTsHMS-.
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Figure 3. XPS spectra of WS/CNTsHMS-: a) Wide scan, b) C s spectra, c) W f spectra, and d) S p spectra.
Figure 2. The a) XRD pattern and b) Raman spectrum of WS/CNTsHMS; c) TGA and DTG of WS/CNTsHMS-; d) Isothermal plots of N absorp-
tion and desorption and pore size distribution of WS/CNTsHMS-.
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absorption of HO and O. No other peaks are detected, which
prove the purity of WS/CNTsHMS-. The fitted Cs spec-
trum (Figure b) shows the peak located at . eV, which
can be attributed to CC or C = C in the CNTs. As shown in
Figurec, the peaks at . and . eV indicate the Wf/ and
Wf/ states of W, respectively. The peak appeared at .eV
can be attributed of WO, caused by the oxidation of WS,
which is corresponding to the previous works.[] The peaks at
. and . eV are corresponding to the S p/ and S p/
states of S, respectively (Figured). The results above confirm
the successful formation of WS in WS/CNTsHMS-, which
is corresponding to the SEM and TEM images.
The electrocatalytic activities of WS/CNTsHMS, the com-
mercial WS (CWS) and the composite of CWS and CNT-
sHMS (CWS/CNTsHMS) toward HER were tested. Figure4a
shows the linear sweep voltammetry (LSV) polarization
curves of HER. Compared to other catalysts (Figure S, Sup-
porting Information), the WS/CNTsHMS- displays the
lowest overpotential of  mV ( mA cm−). The Tafel plots
were used to evaluate the kinetics of HER as Figure b. The
comparison of the Tafel slope of dierent catalysts is as fol-
lows: CWS> CWS/CNTsHMS > WS/CNTsHMS- > WS/
CNTsHMS- > WS/CNTsHMS-. Therefore, the WS/
CNTsHMS- exhibits the smallest Tafel slope of . mV
dec−. In terms of overpotentials and Tafel slopes, WS/
CNTsHMS- shows the best performance, which can be
attributed to the strong interaction between WS and CNTs.
WS/CNTsHMS- shows the harsh kinetics for the less
defects and lower degree of crystallization as the calcination
temperature is lower than WS/CNTsHMS-. However, when
the calcination temperature reaches °C, the overgrowth of
WS decreases the number of edge active sites as well as breaks
the conductive networks of CNTs, both of which lead to the
decay of catalytic performance. The electrochemical impedance
spectroscopy (EIS) results (Figure S and Table S, Supporting
Information) further prove the favorable HER kinetics and fast
catalytic rate for WS/CNTsHMS- because of the smallest
charge transfer resistance than others, which can be contrib-
uted to the formation of D CNTs conductive networks and the
interaction between them and WS. To evaluate the ECSA of the
samples, the CV tests have been performed in a small potential
range of .–. V with various scan rates from  to mV
(Figure S, Supporting Information). WS/CNTsHMS- also
achieves the largest Cdl value (. mF cm−), showing the most
exposed active sites on the surface. The rising number of active
sites can be attributed to the confinement eect of CNTsHMS,
leading to the decreased size of WS. To evaluate the intrinsic
HER performance of the samples, the ECSA-normalized LSV
curves have been in Figure S, Supporting Information. The
WS/CNTsHMS- has exhibited the lowest overpotentials,
showing the most ecient intrinsic HER catalysis. Stability is
another key factor for the practical application of the catalysts.
After  cycles of CV test, no obvious current density change
has been found (Figured), indicating the admirable durability
of WS/CNTsHMS-. The morphology and structure of the
WS/CNTsHMS- after stability test is shown in Figure S,
Supporting Information. The mesoporous sphere morphology
of WS/CNTsHMS- still remains after the long-term
Figure 4. Electrochemical characterizations for HER. a) Polarization curves, b) Tafel plots, c) Cdl values, and d) Long cycle test of WS/
CNTsHMS-.
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