chen2021

Research Article
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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*
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 efficient water-splitting process.
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 200438, P. R. China
E-mail: [email protected]
Dr. J. Liu
School of Materials Science and Engineering
Changzhou University
Changzhou, Jiangsu 213164, P. R. China
Dr. Y. Wang
Materials Genome Institute
International Centre of Quantum and Molecular Structures, and
Physics Department
Shanghai University
Shanghai 200444, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202100510.
DOI: 10.1002/smll.202100510
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1. Introduction
As a promising hydrogen (H2) production method, water-splitting powered by
electricity has been regarded as an ideal
strategy in the future.[1–3] However, the
sluggish reaction kinetics of hydrogen
evolution reaction (HER) and oxygen
evolution reaction (OER) hinder the further application.[4] Recently, 2D transition metal dichalcogenides (TMDs) such
as MoS2 and WS2 have drawn increasing
attention as efficient bifunctional electrocatalysts for OER and HER due to their
earth-abundance, low cost, and efficient
catalytic performance.[5–8] The unique
layer structures of TMDs are beneficial to
the electrocatalysis with the edges demonstrated to provide more active sites compared with bulk materials.[9] 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.[10]Various
strategies including morphology regulation,[11] crystal phase engineering,[12–16]
and doping with other elements[17] have been put forward to
solve the problems and to fabricate the benign bifunctional
electrocatalysts. As expected, many modified TMDs-based
composites, especially WS2-based electrocatalysts have been
prepared, such as, [email protected],[18] Mo0.5W0.5S2,[19] 1T’-D
WS2,[20] WS2 nanodots,[21] Co-doped WS2/W18O49 nanotubes[22]
and N,P co-doped exfoliated WS2.[23] Among them, tuning the
polarization effect of the components to achieve better conductivity 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 positive influence on the water-splitting performance.[24] 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.[25]Furthermore, the lack of OER active sites prevents the application for water-splitting of TMDs, making it
an urgent problem to be solved.[26] Therefore, to fabricate the
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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 conductivity, and low weight, carbon nanotubes have been considered
as one of the most attractive substrates to be assembled with
active electrocatalysts to form efficient electrocatalysts.[27–29]
By combining TMDs with carbon nanotubes (CNTs), the conductivity and electrocatalytic performance have been promoted.[30–32] However, when assembled with CNTs, the size
of TMDs is hard to be controlled, leading to the loss of the
activity.[33]The mismatched ratios of length to diameter between
them make it difficult to achieve and maintain the effective connection, which blocks the mass transport and electron transfer
of the catalysts.[34] Therefore, only limited works have reported
the composite consisting of TMDs and CNTs with less attractive
catalytic performance.[35–37] Furthermore, the polarization effect
between WS2 and CNTs is rarely mentioned and discussed
before, which hinders the development of the WS2-based electrocatalysts. Apparently, to design a novel assembly strategy for
the highly efficient 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 WS2/CNTs hollow microsphere with superior water-splitting catalytic performance has been successfully
synthesized via a spray-drying method. Many 2D WS2 layers
have been confined onside the surface of CNTs shell to form a
unique 3D conductive framework. By the confinement of CNTs
shell, the obtained WS2 exhibits decreased size and increased
exposed edge sites, both of which promote the electrocatalytic
performance. The overpotential can be optimized to 148 mV
(at 10 mA cm−2) for HER and 316 mV (at 50 mA cm−2) for OER
in 1 m KOH, which surpasses almost all the 2H-WS2 based nanocomposites, even some noble metal materials. Benefitting from
the simple procedure, industrial output and the effective force
assembly property, the WS2/CNTs composite exhibits excellent
water-splitting performance (1.65 V for 10 mA cm−2). The polarization effect between WS2 and 3D CNTs networks accelerates the
electronic conductivity, leading to the efficient electrocatalysis,
which is confirmed by electron holography and density functional theory (DFT) calculations. Therefore, our results provide a
guidance for the modification of TMD-based bifunctional watersplitting catalysts through the polarization boosted strategy.
2. Results and Discussion
The synthesis process of WS2 nanosheets grown on carbon
nanotube hollow microsphere as illustrated in Figure 1a and
Scheme S1, Supporting Information. At first, the ammonium
tungstate was dissolved into the mixture of distilled water and
CNTs to form a stable solution. Under the effect of ultrasonic
crushing, the agglomerated CNTs were dispersed, and continuously stirred for 1h 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 uniform liquid drops, consisting of tungstate, CNTs, and water.
Under the heat exchange of the hot wind, the water is quickly
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evaporated and the CNTs are assembled to form a hollow
sphere, which is ascribed to the low diffuse speed of CNTs
inward compared with water molecules and ions outward. By
adding H2SO4 (0.5 m) into Na2S, H2S has been obtained and
then pumped into the tube furnace to form WS2/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 WS2. This structure not only
limits the size of WS2 which is beneficial to acquire more
active sites, but also improves the WS2 conductivity by the effective connection between CNTs and WS2, both of which promote
the electrocatalysis performance of WS2.
The morphology and structure of both CNTsHMS and WS2/
CNTsHMS is measured by scanning electron microscopy
(SEM) and transmission electron microscope (TEM). The CNTsHMS exhibits a hollow yarn-like structure (Figure S1a, Supporting Information), which is corresponding to the previous
works.[38] The microparticle sizes are measured from 2.5 to 24
µm. After calcined at 750 °C, the WS2/CNTsHMS-750 exhibits a
combined feature of yarn-like CNTs hollow shells and 2D-WS2
confined in the twining CNTs (Figure 1b–d). Under a lower
temperature of 650 °C, WS2/CNTsHMS-650 shows a composite feature of CNTs and 2D 2H-WS2 (Figure S1b,d, Supporting Information). Its size distribution has an average size
of 114.1 nm, compared with 412.9 nm of WS2/CNTsHMS-750
(Figure S1b,c, Supporting Information). Under a higher calcination temperature of 850 °C, however, the size of WS2 reaches
1.16 µm (Figure S1d, Supporting Information). The increased
size of WS2 makes it difficult to form the uniform hollow
microsphere for the mismatched size of WS2 and CNTs, leading
to the unsuccessful assembling of WS2/CNTsHMS (Figure S1d,
Supporting Information).
As the unique assembling of CNTs and WS2 exhibits strong
confinement effect,[39] the size of WS2 is obviously decreased
compared with commercial WS2, which leads to the increased
active sites of WS2 (Figure S2, Supporting Information).
Figure 1e–g shows the TEM images of WS2/CNTsHMS-750.
This structure is prepared via forced assembly method by
1D-CNTs and 2D-WS2.The high-angle annular dark fieldscanning transmission electron microscope element mapping
(STEM)-mapping exhibits that the W and S elements mainly
mapped onside the 2D hexagonal WS2 layers, while the C element dispersed on the surface of CNTsHMS, which confirms
the successful synthesis of WS2/CNTsHMS (Figure 1h–k). To
observe the inside structure of WS2/CNTsHMS, a thin slice of
WS2/CNTsHMS cut by FIB is shown in Figure S3, Supporting
Information. The 2D WS2 layers are confined at the CNTs
shell (Figure S3b–d, Supporting Information), which is corresponding to the TEM images. Importantly, many layers are
grown inside the CNTs shell (Figure S3d, Supporting Information). Supposedly, the tungstate is adsorbed onside the CNTs
via the spray-drying method. During the sulphuration process,
the tungstate gradually transforms into WS2 and then grows
larger toward every orientation to form WS2/CNTsHMS. Furthermore, via almost the same method, other TMDs-based
composites have been successfully prepared including MoS2
(Figure S4, Supporting Information) and VS2 (Figure S5, Supporting Information), confirming the possible widely application for the modification of TMDs. Moreover, the spray-drying
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Figure 1. a) Simplified schematic illustration of the formation processes of prepared WS2/CNTsHMS-750; b–d) The SEM images of WS2/CNTsHMS-750; e–g) The TEM images of WS2/CNTsHMS-750, the inset of (g) is the FFT view of WS2/CNTsHMS-750; h–k) The STEM element mapping
of WS2/CNTsHMS-750.
method is conducive to industrialization due to the benefits of
low-cost, high output, easy operation (Figure S6, 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 WS2/CNTsHMS.
The typical diffraction peaks appearing at 14.3° and 28.9° for
WS2/CNTsHMS are assigned to (002) and (004) planes of
WS2.[40,41] Furthermore, the other peaks are corresponding to
2H-WS2 (JCPDS Card No. 08–0237), which confirms the successful reaction of tungstate ions and H2S to form WS2 during
the synthesis process. From the enlarged XRD pattern at 2θ =
20–30°(Figure S7, Supporting Information), the peak at 26° is
assigned to the (002) plane of graphite, indicating the existence of CNTs. Raman measurements have been carried out
to investigate the graphitization degree of the composites
(Figure 2b). The sharp peaks at 1350 and 1600 cm−1 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.[42] With the temperature increasing
from 650 to 850 °C, the ratio of ID/IG shows a rising tendency.
The abundant defects caused by the reaction of WS2 and CNTs
are beneficial to regulate the adsorption of the intermediates
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and facilitate the reaction kinetics. According to the TGA data
(Figure 2c), the content of CNTs in WS2/CNTsHMS reaches
22.4%. Therefore, the content of WS2 in the composite is 77.6%.
The combustion temperature of CNT can be obtained by DTG
curve analysis, which is about 650 °C. To evaluate the pore
structures in WS2/CNTsHMS, the Brunauer-Emmert-Teller
(BET) N2 absorption-desorption measurement has been executed (Figure 2d and Figure S8, Supporting Information). The
results show that when the relative pressure at 0.4–1.0, the hysteresis loops between adsorption and desorption reveal a typical
mesoporous structure of WS2/CNTsHMS.[43] 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 WS2/CNTsHMS derived
from the desorption branch of the isotherm are in the range
of 20–80 nm (inset of Figure 2d and Figure S8, Supporting
Information), further revealing the mesoporous structure of the
composites, which is beneficial for the transportation of electron and the access to the electrolyte.
X-ray photoelectron spectroscopic (XPS) was used to probe
the surface chemical environment of WS2/CNTsHMS-750.
The wide XPS spectrum (Figure 3a) reveals that the W, O, C,
S elements coexist on the surface of WS2/CNTsHMS-750.
The appearance of the peaks of oxygen is contributed to the
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Figure 2. The a) XRD pattern and b) Raman spectrum of WS2/CNTsHMS; c) TGA and DTG of WS2/CNTsHMS-750; d) Isothermal plots of N2 absorption and desorption and pore size distribution of WS2/CNTsHMS-750.
Figure 3. XPS spectra of WS2/CNTsHMS-750: a) Wide scan, b) C 1s spectra, c) W 4f spectra, and d) S 2p spectra.
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Figure 4. Electrochemical characterizations for HER. a) Polarization curves, b) Tafel plots, c) Cdl values, and d) Long cycle test of WS2/
CNTsHMS-750.
absorption of H2O and O2. No other peaks are detected, which
prove the purity of WS2/CNTsHMS-750. The fitted C1s spectrum (Figure 3b) shows the peak located at 284.8 eV, which
can be attributed to CC or C = C in the CNTs. As shown in
Figure 3c, the peaks at 32.8 and 35.2 eV indicate the W4f7/2 and
W4f5/2 states of W, respectively. The peak appeared at 38.8eV
can be attributed of WO3, caused by the oxidation of WS2,
which is corresponding to the previous works.[44] The peaks at
162.1 and 163.9 eV are corresponding to the S 2p3/2 and S 2p1/2
states of S, respectively (Figure 3d). The results above confirm
the successful formation of WS2 in WS2/CNTsHMS-750, which
is corresponding to the SEM and TEM images.
The electrocatalytic activities of WS2/CNTsHMS, the commercial WS2 (CWS2) and the composite of CWS2 and CNTsHMS (CWS2/CNTsHMS) toward HER were tested. Figure 4a
shows the linear sweep voltammetry (LSV) polarization
curves of HER. Compared to other catalysts (Figure S7, Supporting Information), the WS2/CNTsHMS-750 displays the
lowest overpotential of 148 mV (10 mA cm−2). The Tafel plots
were used to evaluate the kinetics of HER as Figure 4b. The
comparison of the Tafel slope of different catalysts is as follows: CWS2 > CWS2/CNTsHMS > WS2/CNTsHMS-850 > WS2/
CNTsHMS-650 > WS2/CNTsHMS-750. Therefore, the WS2/
CNTsHMS-750 exhibits the smallest Tafel slope of 77.3 mV
dec−1. In terms of overpotentials and Tafel slopes, WS2/
CNTsHMS-750 shows the best performance, which can be
attributed to the strong interaction between WS2 and CNTs.
WS2/CNTsHMS-650 shows the harsh kinetics for the less
defects and lower degree of crystallization as the calcination
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temperature is lower than WS2/CNTsHMS-750. However, when
the calcination temperature reaches 850 °C, the overgrowth of
WS2 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 S8 and Table S1, Supporting
Information) further prove the favorable HER kinetics and fast
catalytic rate for WS2/CNTsHMS-750 because of the smallest
charge transfer resistance than others, which can be contributed to the formation of 3D CNTs conductive networks and the
interaction between them and WS2. To evaluate the ECSA of the
samples, the CV tests have been performed in a small potential
range of 0.04–0.15 V with various scan rates from 20 to 100 mV
(Figure S9, Supporting Information). WS2/CNTsHMS-750 also
achieves the largest Cdl value (13.8 mF cm−2), showing the most
exposed active sites on the surface. The rising number of active
sites can be attributed to the confinement effect of CNTsHMS,
leading to the decreased size of WS2. To evaluate the intrinsic
HER performance of the samples, the ECSA-normalized LSV
curves have been in Figure S10, Supporting Information. The
WS2/CNTsHMS-750 has exhibited the lowest overpotentials,
showing the most efficient intrinsic HER catalysis. Stability is
another key factor for the practical application of the catalysts.
After 3000 cycles of CV test, no obvious current density change
has been found (Figure 4d), indicating the admirable durability
of WS2/CNTsHMS-750. The morphology and structure of the
WS2/CNTsHMS-750 after stability test is shown in Figure S11,
Supporting Information. The mesoporous sphere morphology
of WS2/CNTsHMS-750 still remains after the long-term
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Figure 5. Electrochemical characterizations for OER. a) Polarization curves, b) Tafel plots, c) Cdl values, and d) Long cycle test of WS2/CNTsHMS-750;
The e) LSV test and f) stability test of WS2/CNTsHMS-750 for water-splitting process. The inset is the photograph of the water-splitting process.
stability test. The favored stability is mainly contributed to the
strong interaction and tight confinement between CNTs shells
and WS2 layers. Overall, benefitting from the unique forced
assembly method of WS2 and CNTs, the promoted HER performance and perfect long-term stability of WS2/CNTsHMS-750
have been achieved, which outperforms most WS2-based works
(Table S2, Supporting Information).
OER is the other half-reaction of water-splitting, which
always exhibits harsh kinetics because of the multi-proton process.[45,46] To analyze the electrocatalytic OER activity of WS2/
CNTsHMS, the polarization curves were tested (Figure 5a).
As the overpotentials at the current density of 10mA cm−2 of
the samples are hardly to be distinguished for the oxidation
of Ni, the overpotentials at the current density of 50mA cm−2
(η50) have been compared to confirm the OER performance.
WS2/CNTsHMS-750 exhibits the lowest overpotential, the
smallest Tafel slope and the smallest charge transfer resistance (Figures S10 and S11, Supporting Information, Figure 5b),
confirming the best OER catalysis property. The Cdl value of
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WS2/CNTsHMS-750 reaches 9.2 mF cm−2, higher than other
samples, which is corresponding to the HER performance
(Figure S12, Supporting Information, and Figure 5c). The
intrinsic OER performance has also been revealed (Figure S14,
Supporting Information). The WS2/CNTsHMS-750 shows
enhanced OER intrinsic performance. In brief, by the combination of WS2 and CNTs via a spray-drying method, both the
intrinsic performance and ECSA have been improved, confirming the effectiveness of our strategy. As expected, WS2/
CNTsHMS-750 also achieves superb durability during the
OER process. The current density shows almost no difference
after 3000 CV test cycles and the I–T curve reveals that after
100 000 s the current density has maintained 96.1%, both of
which confirms the perfect stability of WS2/CNTsHMS-750
toward OER process (Figure 5d). The OER performance of
WS2/CNTsHMS-750 also surpasses most works of TMDs-based
catalysts (Table S4, Supporting Information). For studying
the bifunctional catalytic activity of HER and OER of WS2/
CNTsHMS-750, a two-electrode electrolysis cell is constructed
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using WS2/CNTsHMS-750 as both the anode and cathode electrodes (inset of Figure 5e). Noticeably, the water-splitting cell
only requires a voltage of 1.65 V to provide a current density of
10 mA cm−2, which surpass most reported TMDs-based nanomaterials (Table S5, Supporting Information). The stability of
WS2/CNTsHMS-750 toward water-splitting was tested via the
I–T curve (Figure 5f). By the continuous test of 40 h, the current density of WS2/CNTsHMS-750 has maintained 73.9%, confirming the fantastic durability of WS2/CNTsHMS-750, same as
the excellent stability toward the bifunctional electrocatalysis,
which is induced by the unique assembly method we have
put forward. To reveal the effect of composition ratio between
CNTs and TMDs, three samples with 20, 30, and 40 mL CNTs
dispersion solution added (named as WS2/CNTsHMS-20,
WS2/CNTsHMS-30, and WS2/CNTsHMS-40) have been prepared via the same spray-drying method. The electrocatalytic
performance has been tested for the samples (Figure S15,
Supporting Information). The overpotentials of these three
samples at 10mA cm−2 (η10) for HER process are measured as
182 mV (WS2/CNTsHMS-20), 146 mV (WS2/CNTsHMS-30),
and 191 mV (WS2/CNTsHMS-40), revealing the most effective HER catalysis for WS2/CNTsHMS-30. As expected, the
OER performance has shown the same tendency. The overpotentials at 50 mA cm−2 (η50) are measured as 348 mV (WS2/
CNTsHMS-20), 326 mV (WS2/CNTsHMS-30), and 367 mV
(WS2/CNTsHMS-40). The low amount of CNTs will increase
the difficulty of the assembling and weaken the polarization
between CNTs and WS2. However, when large amount of CNTs
is supplied, the decreasing ratio of WS2 will lead to the limited
catalytic performance. So when 30 mL CNTs dispersion solution were added, the samples have demonstrated the most
attractive bifunctional electrocatalysis.
To explain the influence of interfacial charge distribution
between WS2 and adjacent CNTs on the electrocatalysis, the
off-axis electron holography reveals the charge density mapping
of WS2/CNTsHMS-750 (Figure 6). By hologram reconstruction
images, the charge density difference between WS2 and CNTs
can be obtained (Figure 6b,e). The charge density line profiles
in the region of the black arrows (Figure 6b,e) exhibit that it
is difficult for the interfacial charge between the adjacent WS2
(≈0.5 e nm−1) or CNTs (≈0.7 e nm−1) to be redistributed because
of the similar composition and electronegativity (Figure 6c,f).
Therefore, no strong polarization was formed locally.
Contrastively, when WS2 and CNTs are tightly entwined,
intense charge redistribution (≈4 e nm−1) happens at the interface (Figure 6h), much higher than each of them. It should
be noted that the electrons transfer from CNTs to WS2 driven
by the polarization effect[47,48] (Figure 6i), revealing that the
built-in electric field emerges along the interfaces. The asformed electric field promotes the charge accumulation on the
WS2, especially on the edge of 2D-WS2 nanosheets, which is
widely believed to be the active sites for electrocatalysis.[49,50]
Figure 6j shows the charge density of disconnected WS2 and
CNTs (≈20 nm). The charge density is calculated of −1.1 e nm−1
at the WS2 surface (Area 1) and 1.3 e nm−1 at the CNTs surface (Area 2). Only limited polarization is observed, indicating
the rapid decay of the charge accumulation effect (Figure 6k).
The polarization can be attributed to the work function distinctions between WS2 and CNTs. Work function is described as
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the energy required to move an electron from the bulk to the
vacuum outside the surface. It can be defined theoretically as:
Φ = Vvac − E F (1)
where Vvac is macroscopically averaged electrostatic potential in
the vacuum, and EF is the Fermi level of the metallic system.
The work function of CNTs and WS2 reaches 4.80 and 4.95 eV,
respectively, according to the previous works.[51,52] When WS2
and CNTs exhibit a close connection effectively, the Fermi
levels of them are aligned to the same energy (Figure 6l). The
charge will transfer from CNTs to WS2 along the CNTs conductive network to achieve the level alignment. However, with
the distance of WS2 and CNTs increased, the transportation of
electrons become more difficult and the intensity of the polarization shows a decreased tendency. The electron redistribution of WS2/CNTsHMS-750 is simulated by DFT calculation.
A model of the WS2/CNTs interface is constructed to calculate
the charge density difference for WS2 and CNTs (Figure 6m).
Concentrating on the interface of WS2 and CNTs, it can be
found that the electrons (≈1.5e) have been accumulated on
the S sites of WS2 (red area), which are transferred from the
surface of CNTs (blue area). Based on the results above, it can
be concluded that the strong polarization on the interface and
charge redistribution between the adjacent WS2 and CNTs can
significantly activate the edge active sites of WS2, leading to the
enhanced performance for water-splitting process. However,
this effect shows a quick decay with the distance of WS2 and
CNTs increasing. So, the close connection of WS2 and CNTs via
this forced assembly method makes WS2/CNTsHMS a satisfying candidate as the catalyst of water-splitting.
3. Conclusion
In this work, a polarization boosted strategy for the modification of TMDs (WS2 as the example) with industrial outputs
via a spray-drying method has been put forward. The unique
WS2/CNTsHMS is assembled by the confined WS2 layers and
the hollow CNTs microspheres. Benefiting from strong polarization and efficient combination between WS2 and CNTs,
the conductivity has been improved and more active sites are
exposed of the composite. The WS2/CNTsHMS achieves the
superior HER/OER performance. Impressively, the overpotential can be improved to 148 mV (at 10 mA cm−2) for HER and
316 mV (at 50 mA cm−2) for OER with superior stability, which
outperforms almost all reported 2H WS2-based catalysts. Using
WS2/CNTsHMS as the catalyst of water-splitting, an efficient
overpotential of 1.65 V has been achieved. Electron holography
also evaluates that the conductivity can be improved by polarized interfaces and facilitate the transfer of electrons from CNT
to WS2, leading to the activation of the active sites on WS2. The
DFT calculation indicates that the charge transfer from CNTs
to WS2, which enhance the catalytic performance of WS2. This
work puts forward a new idea to achieve the efficient assembling of 2D-TMDs and 1D-CNTs and successfully reveals the
enhanced mechanism, which provide a credible method for
the modification of TMDs for more efficient water-splitting
catalysis.
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Figure 6. The off-axis electron holography of WS2/CNTsHMS-750. a,d,g) TEM images of WS2/CNTsHMS-750; b,e,h,j) Charge distribution of WS2/CNTsHMS-750; c,f,i,k) The profile of charge distribution of WS2/CNTsHMS-750; l) The energy level alignment scheme of WS2 and CNTs; m) The model of
WS2/CNTsHMS and n) the calculated charge density difference of WS2/CNTsHMS.
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4. Experimental Section
Chemical: Ammonium tungstate, commercial WS2, and potassium
hydroxide were all purchased from Sinopharm Chemical Reagent Co.,
Ltd. CNTs dispersion solution (TNWDM-M8), DI water were obtained
from a Milli-Q system. The CNTs dispersion solution (TNWDM-M8)
used was purified and acidized before the synthesis process to remove
metal catalysts and amorphous carbon. All reagents were analytical grade
in this work, and could be used directly without further purification.
Synthesis of 3D WS2/CNTs Hybrid Microspheres: A facial spray-drying
method was used to synthesize the mixture of wolframate and CNTs
hollow microsphere. First, 2 mmol ammonium tungstate was dissolved
into the mixture solution of 500 mL distilled water and 30 mL CNTs
dispersion solution. Then, the solution was treated by ultrasonicator
(YH-9000LD, 900 W) to disperse the agglomerated CNTs, with a cyclic
program of 10 s on followed by 5 s for 10 min, and under continuous
stirring for 1 h to acquire a uniformly dispersed solution. Subsequently,
the suspension was transformed into a spray-drying device. The spray
parameters were set as follows: Spray drying in air, inlet temperature was
180 °C, and the feed rate was 1000 mL h−1. The black precursor power
was collected and further sulphurated. The sulphuration treatment was
conducted in a mixture atmosphere of N2 and H2S. The temperature
rose to 750 °C with the rate of 5 °C min−1, H2S gas was been pumped
into tube furnace with N2 for 2 h, and finally cooled down to room
temperature with N2 atmosphere, denoting as WS2/CNTsHMS-750. The
effects of different temperature on WS2 growth were also studied, so the
product prepared at 650 and 850 °C can been called WS2/CNTsHMS-650
and WS2/CNTsHMS-850. Through the same spray-drying procedure, a
composite can be acquired by commercial WS2 and CNTs dispersion
solution directly, which is named CWS2/CNTsHMS.
Characterization: Powder X-ray diffraction (XRD, with Ni-filtered Cu Kα
radiation) measurements were used to characterize the crystal structure.
Thermogravimetric analysis can evaluate the carbon content, and the
heating rate arrived at 10 °C min−1 in air from 25 to 900 °C (Pyris 1 TGA207). Field emission scanning electron microscope (Hitachi S-4800,
Japan) and transmission electron microscope (TEM) (JEOLJEM-2100F)
can be used to observe the morphology. Moreover, TEM(JEOL JEM2100F) can been used to conduct high resolution transmission electron
microscope, selected are electron diffraction, STEM, and electron
holography, which design with a post-column Gatan imaging filter (GIFTridium). X-ray photoelectron spectroscopy (ESCALAB210) was used to
analyze the elemental composition and valence. The wavelength of the
excitation laser of Raman spectra was 633 nm. The BET surface area and
pore properties were measured by ASAP 2420 analyzer at 77K.
Electrode Fabrication: All the Ni foam substrates (10 mm × 20 mm ×
1 mm) were pretreated with HCl (2mol L−1), distilled water, and anhydrous
ethanol under the ultrasonication for few minutes before utilization. All
the electrodes were prepared according to a reported method.[31] Briefly,
80 wt% sample, 10 wt% polymer binder (polyvinylidene fluoride), and
10 wt% carbon black were dispersed in a N-methyl-pyrrolidone solvent
to acquire a homogeneous slurry. After being grinded evenly, the slurry
was spread on the treated Ni foam substrates, and the loading mass
was about 2 mg cm−2, and dried overnight at 70 °C under vacuum.
Electrochemical Measurements: All electrochemical measurements
were conducted on the electrochemical workstation (CHI 660e) with a
three-electrode system. The electrolyte was N2-saturated KOH solution
(1 m, pH = 13.6). The counter electrode used carbon rod, the reference
electrode used saturated calomel electrode (SCE). The Ni foam coated
with WS2/CNTsHMS or other catalysts were served as the working
electrode. Before the electrochemical measurements, the electrolyte
needed to inlet N2 for 30 min. According to the Nernst equation: E(RHE)
= E(SCE) + 0.242 + 0.0592 × pH, the potentials were calibrated in regard
to the reversible hydrogen electrode (RHE), and all the polarization
plots were IR corrected. The LSV measurements were carried out at a
scan rate of 5 mV s−1. EIS measurements were performed at potential of
−1.15 V (HER) or 0.48 V (OER) and frequency range from 106 to 0.01 Hz.
The electrochemical active area was estimated by CV curves, which used
to measure Cdl under a potential of −1.1 to −1 V (HER) and 0 to 0.1 V
Small 2021, 2100510
(OER). The catalysts stability measurement used 3000 circles of cyclic
voltammetry (CV) curves from −0.4 to 0 V of HER and 1 to 1.4 V of OER
at a scan rate of 50 mV s−1. The chronopotentiometry technique was
tested at −0.4 V (HER) and 1.6 V (OER).
Computation Details: Cambridge Sequential Total Energy Package
was used to conduct the DFT calculations. The generalized gradient
approximation of Perdew-Burke-Ernzerhof function was employed in
this investigation. The electron wave function is expanded by planewave
basis with a kinetic energy cutoff of 500 eV. The K-points mesh is set to
3 × 3 × 1. The periodic images were separated by vacuum layers 30 Å to
eliminate image interactions.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
G.C. and C.Z. contributed equally to this work. This work was supported
by the Ministry of Science and Technology of China (973 Project No.
2018YFA0209102) and the National Natural Science Foundation of China
(11727807, 51725101, 51672050, and 61790581).
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
Research data are not shared.
Keywords
electron holography, polarization,
dichalcogenides, water-splitting
spray-drying,
transition
metal
Received: January 26, 2021
Revised: March 8, 2021
Published online:
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