Research Article www.small-journal.com 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 Small 2021, 2100510 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 2100510 (1 of 10) © 2021 Wiley-VCH GmbH www.small-journal.com www.advancedsciencenews.com 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 Small 2021, 2100510 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 2100510 (2 of 10) © 2021 Wiley-VCH GmbH www.small-journal.com www.advancedsciencenews.com 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 Small 2021, 2100510 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 2100510 (3 of 10) © 2021 Wiley-VCH GmbH www.small-journal.com www.advancedsciencenews.com 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. Small 2021, 2100510 2100510 (4 of 10) © 2021 Wiley-VCH GmbH www.small-journal.com www.advancedsciencenews.com 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 CC 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 Small 2021, 2100510 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 2100510 (5 of 10) © 2021 Wiley-VCH GmbH www.small-journal.com www.advancedsciencenews.com 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 Small 2021, 2100510 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 2100510 (6 of 10) © 2021 Wiley-VCH GmbH www.small-journal.com www.advancedsciencenews.com 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 Small 2021, 2100510 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. 2100510 (7 of 10) © 2021 Wiley-VCH GmbH www.small-journal.com www.advancedsciencenews.com 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. Small 2021, 2100510 2100510 (8 of 10) © 2021 Wiley-VCH GmbH www.small-journal.com www.advancedsciencenews.com 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: [1] C. Wang, X. Shao, J. Pan, J. Hu, X. Xu, Appl. Catal., B 2020, 268, 118435. [2] M. Qu, Y. Jiang, M. Yang, S. Liu, Q. Guo, W. Shen, M. Li, R. He, Appl. Catal., B 2020, 263, 118324. [3] Z. Peng, J. Liu, B. Hu, Y. Yang, Y. Guo, B. Li, L. Li, Z. Zhang, B. Cui, L. He, M. Du, ACS Appl. Mater. Interfaces 2020, 12, 13842. 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