Formation of Ni-doped MoS2 nanosheets on N-doped carbon nanotubes towards superior hydrogen evolution

Electrochimica Acta 338 (2020) 135885

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Formation of Ni-doped MoS2 nanosheets on N-doped carbon nanotubes towards superior hydrogen evolution Tao Dong a, Xiao Zhang b, Peng Wang a, Hsueh-Shih Chen c, Ping Yang a, * a

School of Material Science and Engineering, University of Jinan, Jinan, 250022, PR China Fuels and Energy Technology Institute and Department of Chemical Engineering, Curtin University, Perth, WA6845, Australia c Department of Materials Science & Engineering, National Tsing Hua University, Hsinchu City, 300, Taiwan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 May 2019 Received in revised form 3 February 2020 Accepted 10 February 2020 Available online 17 February 2020

Molybdenum disulfide (MoS2) is one of the promising candidates for hydrogen evolution reaction (HER), it’s HER properties can be extremely enhanced by the construction of nanostructure and the regulation of electronic structure. In this paper, hierarchical composites with MoS2 and carbon nanotubes (CNTs) as the noble metal-free electrocatalysts for HER were fabricated through calcination and hydrothermal processes, in which the MoS2 nanosheets with Ni atoms incorporation were vertically grown on the outsides of nitrogen (N)-doped CNTs. The constructed hierarchical structures provide large specific surface area of about 150 m2 g
1 and high structure stability. Importantly, the MoS2 nanosheets with enlarged interlayer spacing possess ultrathin structure of about 4 nm in thickness. Owing to the incorporation of Ni atoms, abundant unsaturated S atoms are obtained in the basal plane, which leading to more active sites exposed during HER processes. Meanwhile, N-doped CNTs can act as the substrate for MoS2 nanosheets growing and simultaneously improve the conductivity of catalyst. Based on the synergy of hierarchical structure, increased active sites and enhanced conductivity, the as-prepared electrocatalysts exhibit superior HER activity with low overpotentials, small Tafel slopes and long-time durability in both acidic and alkaline electrolytes. The overpotentials of 158 and 179 mV were required to drive HER current density up to 10 mA cm
2 in 0.5 M H2SO4 and 1.0 M KOH, respectively. This work reports a highly efficient MoS2-based electrocatalyst for HER via fabricating hierarchical structure and doping Ni atoms. © 2020 Elsevier Ltd. All rights reserved.

Keywords: MoS2 Carbon nanotube Hydrogen evolution reaction Ni-doping Electrocatalyst

1. Introduction Electrocatalytic water splitting is an efficient and environmentfriendly route to produce hydrogen among various H2 production technologies [1e4]. Noble metal Pt-based electrocatalyst shows the highest HER activity, but their further application is limited due to high cost and scarcity [5]. Therefore, the exploration of noble metal-free and high efficient catalysts is an important research direction. As a typical two-dimensional layered material, MoS2 is considered as a promising HER electrocatalyst due to its high catalytic activity with hydrogen adsorption energy (DGH) near to Pt, excellent stability and low cost [6]. Previous reports revealed that the HER activity of MoS2 is derived from the S-site containing unsaturated MoeS bonds at the edge of layer [7e9]. However, the

* Corresponding author. E-mail address: [email protected] (P. Yang). https://doi.org/10.1016/j.electacta.2020.135885 0013-4686/© 2020 Elsevier Ltd. All rights reserved.

MoS2 synthesized via conventional methods always possess low conductivity and limited surface area, which leading to poor HER perfoamances. Therefore, improving the HER activity of MoS2 is a research hotspots. Design and preparation of MoS2 with special nanostructures is one of the effective strategies to improve HER activity. Various nano-structured MoS2-based electrocatalysts, such as, nanosheets with rich defects, thin films with large pore structure, double helix structure with abundant mesoporous, three-dimensional columnlike or flower-like structure, complex hollow structures composed of nanosheets had been prepared, and obtained enhanced HER activity compares with the bulk MoS2 material [10e15]. The unique structure allows the unsaturated S atoms at the edges of MoS2 to be preferentially exposed, which can greatly increase the number of active sites for HER. Furthermore, the poor conductivity of MoS2 limits the conduction rate of electron during HER process, which further hinders the HER performances. Hence, the combination with highly conductive materials (such as graphene, carbon

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nanotubes, carbon fibers, conductive polymers, etc.) has become an effective way to solve the above issue [16e19]. Dai’s group [20] reported a method of supporting MoS2 nanoparticles on reduced graphene oxide (rGO) by solvothermal reaction. The presence of rGO greatly increases the electron conduction rate during the HER process, thereby effectively reducing the onset overpotential and Tafel slope of the MoS2 electrocatalyst, also achieving excellent electrochemical stability. In addition, growing nanostructure of MoS2 (such as nanosheets) on the conductive substrate to form a three-dimensional hierarchical structure can not only improve the overall conductivity of composite, but also prevent MoS2from agglomerating to exposure a large amount of active sites. It is well known that hetero-element doping can adjust the electronic structure of material, which further regulates its intrinsic conductivity [21,22]. Therefore, elemental doping strategy has received enormous attention to improve the HER activity of MoS2 [23,24]. Xie’s group [25] successfully prepared vanadium-doped MoS2 ultrathin nanosheet via solid state reaction and exfoliation methods. The electronic structure of MoS2 was regulated by the introduction of vanadium atoms, which can improve the conductivity and active site concentration at the same time. In addition, experimental and theoretical research revealed that the incorporation of hetero-elements (such as Fe, Co, Ni) into MoS2 can also improve the HER activity by increasing additional active sites and reducing adsorption free energy of hydrogen. The exchange current density of the electrocatalyst is significantly increased after heteroelement doping, which indicates that the effective active sites number is enormous increasing [26e29]. From the practical application point of view, the electrolysis of water in alkaline solution to generate H2 is a more valuable method. Unfortunately, water molecules are more difficult to activate in alkaline conditions than in acidic, which results in a higher overpotential for HER in alkaline electrolytes [30]. There is still great challenge to obtain excellent HER performance under alkaline conditions for MoS2-based electrocatalysts. Based on previous studies, we propose that the synergistic effect of fabricating hierarchical structure, increasing conductivity and regulating electronic structure can construct the advanced MoS2-based electrocatalysts with excellent HER activity both in acid and alkaline solution. Herein, the MoS2/CNTs composites were controllably synthesized by a calcination and hydrothermal route, in which ultrathin MoS2 nanosheets with incorporation of nickel atoms are vertically grown on N-doped CNTs to form the hierarchical structure. The asprepared electrocatalyst exhibited excellent HER performance both in acidic (0.5 M H2SO4) and alkaline (1.0 M KOH) solutions due to the synergy of structural and electronic effects. 2. Experiment section 2.1. Preparation of samples All chemicals were used without any further purification. Firstly, the Ni nanoparticles/N-doped CNTs composites were synthesized by calcination process. 50 mg of Ni(NO3)26H2O and 1.0 g of dicyandiamide were ground in a mortar. After uniformly mixed, the powders were placed into a covered crucible and calcination under Ar atmosphere at 800  C for 2 h with heating rate of 5  C min
1. The black products were Ni nanoparticles/N-doped CNTs, which were noted as Ni/NCNTs. Then, the Nickel-doped MoS2/N-doped CNTs composites were synthesized by a hydrothermal method. Typically, 363 mg of Na2MoO42H2O and 726 mg of thioacetamide (TAA) were dissolved in 30 mL of water, then 50 mg of Ni/NCNTs added into the above solution. After ultrasonic dispersion, the solution was transferred into a sealed Teflon-lined stainless steel autoclave and kept at 220  C for 12 h. After reaction, the product was

Table 1 Preparation parameters of different samples. Sample

Na2MoO42H2O

TAA

NieMoS2/NCNTs-1 NieMoS2/NCNTs-2 NieMoS2/NCNTs-3 NieMoS2/NCNTs-4

0.121 0.242 0.363 0.484

0.242 0.484 0.726 0.968

g g g g

Ni-CNTs g g g g

50 50 50 50

mg mg mg mg

centrifuged, washed with DI water, and dried. Four groups of samples were prepared by changing the adding amount of Na2MoO42H2O and TAA. The detailed parameters are listed in Table 1, the products were noted as NieMoS2/NCNTs-1, NieMoS2/ NCNTs-2, NieMoS2/NCNTs-3, NieMoS2/NCNTs-4, respectively. Furthermore, pure MoS2 nanosheets (noted as MoS2 NSs) were prepared for comparison. Typically, 0.242 g of Na2MoO42H2O and 0.484 g of TAA were dissolved in 30 mL of water, the solution was transferred into a sealed Teflon-lined stainless steel autoclave and kept at 220  C for 12 h. 2.2. Material characterization The crystal structure and phase composition were measured using powder X-ray diffraction (XRD) meter (Bruker D8-Advance)at 40 kV and 40 mA using Cu Ka radiation (l ¼ 0.15406 nm). The morphology and microstructure were examined by transmission electron microscopy (TEM, JEM-2010, JEOL) at an accelerating voltage of 200 kV. The Raman spectra were tested using Raman spectrometer (Raman, LabRAMHR Evolution, Horiba) at the laser wavelength of 532 nm, the objective of  50 was used. Surface element analysis was performed using X-ray photoelectron spectrometer (XPS, Escalab 250 XI, Thermo) with monochromatic Al Ka radiation (1486.6 eV). The overall resolution is 0.48 eV (Ag 3d5/2), the pass energy for the survey and for the region is 100 eV and 30 eV, respectively. All binding energies were charge-corrected by the C 1s peak at 284.6 eV [18]. The background subtraction and curve fitting were accomplished using XPS Peak 4.1 software. The specific surface area and pore structure were characterized using nitrogen adsorption/desorption test on multi-function adsorption instrument (MFA-140) at 77K. The morphology was also observed by scanning electron microscopy (SEM, QUANTA250 FEG, FEI) without sputtering of the surface, the elemental distribution was analyzed by energy dispersive spectrometer (EDS). 2.3. Electrochemical measurement Linear sweep voltammetry (LSV), cyclic voltammetry (CV), i-t curve and electrochemical impedance spectroscopy (EIS) were performed in a three-electrode system at room temperature using an electrochemical workstation (CHI 660E). The acidic electrolyte used in the electrochemical test was 0.5 M H2SO4, and the alkaline electrolyte was 1.0 M KOH. The counter electrode is graphite rod, the reference electrodes are Ag/AgCl (acidic solution) and Hg/HgO (alkaline solution) electrodes, respectively. Detailed preparation process of working electrode were as follows: Firstly, 2 mg of the prepared catalyst was dispersed in 1 ml of solution (containing 990 ml of ethanol and 10 ml of Nafion solution (5%)), after sonication for 30 min, 5 ml of the dispersion was dropped on the pre-treated glassy carbon electrode with diameter of 3 mm (geometric area is 0.071 cm2), then dried at room temperature for testing. The voltages appearing in the text are corrected by iR drop and calibrated to the reversible hydrogen electrode (RHE). The corresponding calculation formulas for different reference electrodes are as follows [31,32]:

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E(RHE) ¼ E(Ag/AgCl) þ 0.1976 þ 0.059  pH E(RHE) ¼ E(Hg/HgO) þ 0.0977 þ 0.059  pH 3. Results and discussion 3.1. Morphology and microstructure The morphology and microstructure of samples were characterized by SEM and TEM. Fig. 1(a) and Fig. 1(b) show the typical SEM and TEM images of Ni/NCNTs, which can be observed that CNTs with average diameter of about 100 nm and length of several micrometers are interconnected to form the three-dimensional loose network. In addition, Ni nanoparticles with diameter of 20e60 nm are randomly embedded in the CNTs. The Ni/NCNTs acted as onedimensional skeleton for the growth of MoS2 nanosheets, which beneficial to uniformly disperse of MoS2 nanosheets and largely enhanced the conductivity. SEM images of the product are shown in Fig. 1(c). Obviously, the one-dimensional structure of CNTs can be maintained after the growth of MoS2, the average diameter is increased to about 400 nm. The ultrathin MoS2 nanosheets are interdigitated and vertically grown on CNTs, which is beneficial to full exposure of the active sites at edge of plane. Observed from the cross-section image, MoS2 nanosheets are only grown on the outside of CNTs. The microstructure is further confirmed by TEM measurement. As Fig. 1(d) shown, ultrathin MoS2 nanosheets with size of about 100e150 nm are vertically grown on the outside of CNTs. HRTEM images of NieMoS2/NCNTs-3 are shown in Fig. 1(e),

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the clear wrinkled sheets possess nearly transparent condition, which indicates the ultrathin structure of MoS2 nanosheets. The layer numbers of MoS2 nanosheets are only 3e6 layers and the thickness is about 2e4 nm. In addition, the sharp edges reveal the high crystallinity [33]. As the HRTEM image (Fig. 1(f)) shown, the lattice spacing with 0.27 nm corresponds to the (100) plane of 2H phase MoS2, while the lattice spacing with 0.69 and 0.70 nm corresponds to the interlayer (002) plane. The results indicate that the as-prepared MoS2 possess enlarged layer spacing, which contribute to the improvement of HER performance. Elemental distributions of Mo, S, Ni, C and N elements of NieMoS2/NCNTs-3 are exhibited in Fig. 1(g). The results demonstrate that Mo and S elements are uniformly distributed on outer side of the one-dimensional structure, and Ni element is uniformly distributed on MoS2. N element is uniformly distributed on carbon nanotube at the inner side of whole structure, indicating that N element is uniformly doped into CNTs. For comparison, MoS2 with 2H phase was obtained via hydrothermal method. Their SEM image (Fig. S1) shows that the nanosheets structure with serious agglomeration situation is present. In addition, the adding amount of Mo and S sources during the hydrothermal process were regulated to obtain a series of products. The detailed parameters are listed in Table 1, and SEM images of the product are shown in Fig. S2. The results exhibit that the full surface of CNTs cannot be completely covered by MoS2 nanosheets with small adding amount of Mo and S sources. However, a large number of MoS2 nanosheets coated on CNTs with excessive adding amount,

Fig. 1. (a) and (b) SEM and TEM images of Ni/NCNTs, (c) SEM image of NieMoS2/NCNTs-3, inset shows the cross-section SEM image. (d) TEM image of NieMoS2/NCNTs-3, (e) and (f) HRTEM image of NieMoS2/NCNTs-3, (g) element mapping of Mo, S, Ni, C and N.

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Fig. 2. (a) XRD patterns of MoS2 and Ni–MoS2/NCNTs-3, (b) Raman spectra of MoS2, Ni/NCNTs and Ni–MoS2/NCNTs-3, (c) Raman spectra of E12g (in-plane) and A1g (out-plane) vibration modes of MoS2 and Ni–MoS2/NCNTs-3, (d) Schematic diagram of E12g and A1g.

while MoS2 nanosheets are assembled into sphere-like structure, which extremely reduce their dispersity. The crystal structure and phase composition were investigated by XRD. The XRD pattern of Ni/NCNTs is shown in Fig. S3. The result shows that the product contains only metal Ni and graphitic carbon. No other impurities exist, such as nickel oxide. Fig. 2(a) shows the XRD patterns of MoS2 and NieMoS2/NCNTs-3. The diffraction peak located at 26.5 can be indexed to the (002) crystal plane of graphitic carbon, indicating the presence of CNTs. The diffraction peaks appeared at 13.8, 33, 38.4 and 58.3 correspond to the (002), (100), (103) and (110) crystal faces of 2H MoS2 (JCPDS card no. 37e1492), respectively. The (002) crystal planes of MoS2 are stacked along the c-axis, the expanded interlayer spacing (shown in Fig. 2(d)) of (002) planes is benefit to the improvement of electrocatalytic performance [13]. Compared with pure MoS2, the diffraction peak of (002) crystal plane of NieMoS2/NCNTs-3 shifts to lower angle, indicating its interlayer spacing is enlarged, which is consistent with the result of HRTEM. In addition, the weak diffraction peaks appeared at 44.3, 51.5 and 76 can be indexed to (111), (200) and (220) crystal faces of the cubic Ni metal (JCPDS card no. 04e0850). The large-sized Ni particles in Ni/NCNTs (circled in Fig. S4) cannot be completely etched during the hydrothermal process, resulting in the existence of Ni in the final product. Apart from metal Ni, there are two weak diffraction peaks appearing at 31.5 and 53.5 , which belong to the cubic NiS2 (JCPDS card no. 03e0734). NiS2 may be generated from Ni and excess TAA. Both Ni and NiS2 have HER activity [34,35], so their existing cannot weaken the HER performance of Ni-doped MoS2/N-doped CNTs composites.

Raman spectroscopy was used to further characterize the structure, the results are shown in Fig. 2(b). Two peaks appearing at 1350 and 1600 cm
1 correspond to the defect-induced (D) and graphitic-induced (G) bands of graphitic carbon, while the peaks appearing around lower wave number of 400 cm
1 correspond to the E12g (in-plane) and A1g (out-plane) vibration modes of the MoS2 [36], their schematic diagram are shown in Fig. 2(d). The intensity ratio of D and G peaks refers to ID/IG, which reflects the defect degree of graphitic carbon [37]. Compared with Ni/NCNTs, ID/IG of NieMoS2/NCNTs-3 show no clearly change, indicating that the grown of MoS2 have no significant effects on CNTs. The Raman spectra around 400 cm
1 of MoS2 and NieMoS2/NCNTs-3 were amplified, the result is shown in Fig. 2(c). Two peaks appeared at 378.4 and 401.4 cm
1 belong to E12g and A1g vibration modes ofMoS2, respectively. Compare to pure MoS2 of 379.8 and 406.6 cm
1, both of them are shifted to the lower wave number, as well as the intensity is weakened. This is because that the incorporating of Ni atoms weakens the vibration of the MoeS bond, which further lead to the decrease of vibration frequency and intensity [23]. In addition, the difference value (D) between the wave numbers of E12g and A1g is related to the number of layers [28]. D value of NieMoS2/NCNTs-3 (23 cm
1) is smaller than that of MoS2 (26.8 cm
1), indicating the fewer layers of the MoS2 grown on CNTs. The chemical composition and element valence of NieMoS2/ NCNTs-3 were further studied by XPS. Fig. 3(a) shows a typical XPS survey spectrum, which demonstrates that the product mainly composed of Ni, Mo, S, C, N and O, which atomic concentrations are 0.72, 17.28, 40.99, 33.1, 5.62, and 2.29 at %. The Ni 2p spectrum is

T. Dong et al. / Electrochimica Acta 338 (2020) 135885

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Fig. 3. XPS spectra of NieMoS2/NCNTs-3: (a) survey spectrum, (b) Ni 2p, (c) Mo 3d, (d) S 2p, (e) C 1s, (f) N 1s.

shown in Fig. 3(b), which includes two spin orbital and corresponding satellite peaks. Two characteristic peaks of Ni (II) 2p3/2 and Ni (II) 2p1/2 with the binding energy of ~854.9 and ~872.8 eV can be observed, reveals that Ni successfully doped into MoS2 [28]. The obvious signal noise is ascribed to the low concentration and high dispersion of Ni. In addition, Ni0 signal cannot be detected due to the limited detection depth of the XPS. The Mo 3d spectrum is shown in Fig. 3(c), two peaks appearing at~229.5 and ~232.6 eV correspond to Mo 3d 5/2 and Mo 3d 3/2, indicating the presence of Mo4þ, and the weak peak appeared at 235.9 eV belongs to Mo6þ

[31]. In addition, the XPS peak shifts toward higher binding energy compared with pure MoS2 due to the incorporation of Ni atoms [26]. Furthermore, the results obtained by S 2p spectrum (Fig. 3(d)) are consistent with that of Mo 3d spectrum, the XPS peaks are shifted to higher binding energy due to the Ni doping [28]. The C 1s spectrum (Fig. 3(e)) confirms three types of carbon species: CeC (~284.6 eV), CeN (~286.3 eV) and CeO (~287.8 eV) [34]. The peak appeared at ~394.2 eV in N 1s spectrum (Fig. 3(f)) belongs to Mo 3p [31]. The fitting results show that three types of N species, namely: pyridinic N (~395.1 eV), pyrrolic N (~397.9 eV) and graphitic N

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Fig. 4. Preparation scheme of nickel-doped MoS2/N-doped CNTs composite.

(~400.4 eV) [34]. Fig. S5 shows the structure of these three N species. The pyridinic and pyrrolic nitrogen can destroy the ordered structure of carbon and create defects, which not only improve the conductivity, but also help to the nucleation and close contact of MoS2 on CNTs, resulting in largely enhanced of HER performance [38]. Both of the XPS and Raman results reveal that the successfully incorporated of Ni atoms into MoS2. Large surface area and stable pore structure are the important factors for electrocatalysts. Fig. S6 shows the N2 absorption/ desorption isotherms and pore size distribution of NieMoS2/ NCNTs-3. The specific surface area of NieMoS2/NCNTs-3 was calculated to be 151.4 m2 g
1. The isotherms exhibit a typical type IV curves with a distinct hysteresis loop at high relative pressure (P/ P0), which reveal the mesoporous structure. The mesoporous feature was further confirmed by the pore size distribution curve, the average pore size of as-prepared composites is 5 nm. Large specific surface area and abundant mesoporous are beneficial to the exposure of active sites and rapid mass transfer process during HER process.

3.2. Formation mechanism Fig. 4 shows the synthesis scheme of nickel-doped MoS2/Ndoped CNTs composite with a two-step method, which including calcination and hydrothermal process. The low-cost nickel nitrate and dicyandiamide were used as nickel and carbon source, respectively. After uniformly mixed via grinding process, the precursor powder was gradually annealed to high temperature under Ar atmosphere. With the calcination temperature increasing, nickel nitrate is decomposed and turned into nickel nanoparticles, while dicyandiamide (containing rich nitrogen element) is gradually pyrolysized and turned into graphitic carbon nitride. With calcination temperature increased to 800  C, the graphitic carbon nitride is decomposed and forms N-doped CNTs under the catalytic effect of nickel [36]. The product (Ni/NCNTs) with Ni nanoparticles embedded in the CNTs is formed. With sodium molybdate and TAA used as the Mo and S source, MoS2 nanosheets were grown on the Ni/NCNTs framework through a hydrothermal process. One-dimensional CNTs are acted as the substrate for the nucleation and growth of MoS2 nanosheets. In addition, the acidic solution is formed because of the hydrolysis of TAA, and then the Ni nanoparticles were dissolved into Ni2þ. During the hydrothermal process, Ni2þ can be incorporated into the crystal lattice of MoS2 to form Ni-doped MoS2 nanosheets. It is worth noting that the MoS2 nanosheets were interconnected and vertically arrayed on the outside of CNTs. Finally, the Ni-doped MoS2/Ndoped CNTs (NieMoS2/NCNTs) composites with hierarchical structure were obtained. The doping of Ni atoms can change the electronic structure of MoS2, resulting in the increase of intrinsic conductivity [25,28]. In addition, more defects on the basal plane of MoS2 can be created due to the incorporation of Ni atoms, which leads to abundant unsaturated S atoms [39].

3.3. Electrocatalytic HER performance The electrocatalytic HER activity of NieMoS2/NCNTs-1, NieMoS2/NCNTs-2, NieMoS2/NCNTs-3 and NieMoS2/NCNTs-4 was evaluated under a typical three-electrode system in 0.5 M H2SO4 and 1 M KOH electrolyte, the HER polarization curves and corresponding Tafel slopes are shown in Fig. S7. NieMoS2/NCNTs-3 exhibits the best HER activity in acidic and alkaline electrolyte among these four as-prepared catalysts. In addition, the lowest Tafel slope indicates the fastest reaction kinetics. Due to the mainly active sites being provided by MoS2, the HER activity can gradually improve with the amount of MoS2 increasing. However, when the MoS2 loading is excessive (sample NieMoS2/NCNTs-4), the agglomeration phenomenon is serious, which reduces the exposure of active sites, affects the transmission of electrons and protons, further hinders their HER activity. In addition, commercial Pt/C, MoS2 NSs (preparation, see experimental section), bulk MoS2 (purchase from Macklin) and Ni/ NCNTs were used as catalysts to perform electrocatalytic HER measurements under the same conditions. Their HER performances are compared with NieMoS2/NCNTs-3, the corresponding results are shown in Fig. 5. Fig. 5(a) and (c) show the HER polarization curves of NieMoS2/NCNTs-3, Ni/NCNTs, MoS2 NSs, bulk MoS2 and commercial Pt/C (20%) in both acidic and alkaline solution. Commercial Pt/C catalysts exhibit the highest HER activity with almost zero onset overpotential in acidic and alkaline electrolytes. Ni/ NCNTs exhibited inferior HER activity in both of two electrolytes, the required overpotential exceed 300 mV to reach a current density of 10 mA cm
2. Compared with bulk MoS2, the HER activity of MoS2 NSs is largely enhanced due to the increased number of exposed active sites. Thanks to the synergy of micro-morphology and electronic structure, the HER activity of as-prepared NieMoS2/NCNTs-3 catalysts shows significant improvement in both acidic/alkaline electrolytes compared with MoS2 NSs. Overpotentials of 158 and 179 mV are delivered to reach a current density of 10 mA cm
2 in 0.5 M H2SO4 and 1 M KOH electrolyte, respectively. Besides, only 203 and 232 mV overpotential are needed to achieve 50 mA cm
2. The unique morphology, enlarged interlayer spacing, doping of Ni atoms and composited with CNTs are the main reasons for the high HER activity of NieMoS2/NCNTs3. Tafel slope was used to evaluate the kinetics of HER reaction process, the mechanism and reaction pathway were further explored. Combined with HER polarization curve, the Tafel curve is obtained by following formula [32]:Where h is overpotential, j is current density and b is Tafel slope. The Tafel slope was obtained by fitting the linear portion and the results are shown in Fig. 5(b) and (d). Compared with bulk MoS2, MoS2 NSs and Ni/NCNTs, NieMoS2/NCNTs-3 catalysts show the smallest HER Tafel slope, which is as low as 69.3 and 62.3 mV dec
1 in the acidic and alkaline solutions, respectively. The smaller Tafel slope indicates the faster the reaction kinetics and better hydrogen production performance. As well known, electrocatalytic HER is

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Fig. 5. (a) LSV curves of Ni–MoS2/NCNTs-3, Ni/NCNTs, MoS2 NSs, bulk MoS2 and commercial Pt/C (20%) in acidic solution (0.5 M H2SO4), (b) Corresponding Tafel plots. (c) LSV curves of Ni–MoS2/NCNTs-3, Ni/NCNTs, MoS2 NSs, bulk MoS2 and commercial Pt/C (20%) in alkaline solution (1.0 M KOH), (d) Corresponding Tafel plots.

mainly divided into two basic reaction processes: Volmer-Tafel mechanism and Volmer-Heyrovsky mechanism [4,8]. According to previous reported, hydrogen generation conforms to VolmerHeyrovsky mechanism when the Tafel slope located in the range of 40e120 mV dec
1. Therefore, the rate limiting and determining step during HER reaction process of the NieMoS2/NCNTs-3 catalysts is the Heyrovsky step, which is the electrochemical desorption of hydrogen [40].

The electrochemical active surface area (ECSA) is directly related to the HER performance of the catalyst. Due to the linear relationship between ECSA and double-layer capacitance (Cdl) at the solidliquid interface, ECSA of catalyst can be evaluated through Cdl, which is obtained from CV curves [41]. Fig. S8(a)e(c) show the CV curves at different scan rates of bulk MoS2, MoS2 NSs and NieMoS2/ NCNTs-3 catalysts in 0.5 M H2SO4. Fig. 6(a) shows the linear fitting of current density vs scan rates at the potential of
0.15 V (vs RHE)

Fig. 6. (a) Linear fitting of capacitive currents vs scan rates at −0.15 V (vs RHE) for bulk MoS2, MoS2 NSs and Ni–MoS2/NCNTs-3, (b) Nyquist plots of bulk MoS2, MoS2 NSs and Ni– MoS2/NCNTs-3.

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Fig. 7. (a) HER polarization curves of initial and 1000th cycles of NieMoS2/NCNTs-3 in 0.5 M H2SO4 electrolyte, inset shows the chronoamperometry curve at overpotential of 150 mV (vs RHE). (b) HER polarization curves of initial and 1000th cycles of NieMoS2/NCNTs-3 in 1.0 M KOH electrolyte, inset shows the chronoamperometry curve at overpotential of 180 mV (vs RHE).

for these three catalysts, Cdl is calculated from the slope of the line. The results show that the Cdl of NieMoS2/NCNTs-3 is much higher than the other two catalysts. The higher Cdl value means the larger ECSA with more active site, which can be contributing to the significant improvement of HER performance. In addition, the HER performance and reaction kinetics of catalyst are further explored by the conductivity of catalysts. The EIS plots of bulk MoS2, MoS2 NSs and NieMoS2/NCNTs-3 are shown in Fig. 6(b), in which the semicircle at high frequency region corresponds to the charge transfer resistance (Rct). Compared with bulk MoS2 and MoS2 NSs, NieMoS2/NCNTs-3 exhibits the smallest Rct, indicating the fastest HER kinetics process, which is mainly due to the enhanced conductivity via incorporating of Ni atoms and integrating with CNTs.

Furthermore, the NieMoS2/NCNTs-3 catalyst has the largest slope in the low-frequency region, indicating the smallest diffusion impedance, which is also beneficial to the rapid reaction kinetics process and the improvement of HER performance [42]. Long-term stability is another key factor for electrocatalysts. Continuous CV and chronoamperometry were used to test the HER stability of the catalyst in acidic and alkaline solutions. As shown in Fig. 7(a) and (b), the polarization curve after 1000th CV cycles both in acidic and alkaline electrolytes shows no significant change compared with that initial one, indicating the good electrochemical stability. The current density show no significant loss after continuous electrocatalytic HER process for 10 h in the acidic and alkaline solutions, further demonstrates the durability of the

Fig. 8. Schematic diagram for NieMoS2/NCNTs during HER process.

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catalyst. Furthermore, the stability of the constructed structure was further characterized by the morphology of the catalyst after electrochemical testing. As shown in Fig. S9, negligible change can be observed from the morphology of catalyst after 10 h of continuous HER process, indicates the superior stability of as-prepared hierarchical structure. Tables S2 and S3 show the HER performance of the reported MoS2-based electrocatalysts in 0.5 M H2SO4 and 1.0 M KOH electrolytes, respectively. The results demonstrate that the as-prepared NieMoS2/NCNTs catalysts possess excellent HER activity in acidic and alkaline electrolyte. As shown in Fig. 8, the excellent HER performance of NieMoS2/NCNTs is mainly attributed to the following aspects: (1) Hierarchical structure provides a large specific surface area with more active sites exposed and high structural stability. (2) The CNTs can not only prevent the agglomeration of MoS2 nanosheets, but also facilitate the electron conduction of electrocatalysts. (3) Ultrathin structure of MoS2 nanosheets with enlarged layer spacing are vertically grown on CNTs, which is beneficial to the exposure of more active sites. (4) The incorporation of Ni atoms into MoS2 can regulate the electronic structure of basal plane to increase the intrinsic conductivity and it is favorable to provide additional active sites.

4. Conclusions In summary, the composite of nickel-doped MoS2 nanosheets and N-doped CNTs were synthesized by calcination and hydrothermal process. The ultrathin MoS2 nanosheets have an enlarged layer spacing, which grows vertically and tightly on the outside of CNTs to form the hierarchical structure. In addition, Ni atoms are uniformly incorporated into MoS2 nanosheets. The experimental results show that the as-prepared NieMoS2/NCNTs-3 catalysts exhibit excellent electrocatalytic HER activity and long-term stability in both acidic and alkaline electrolytes. The improved electrocatalytic performance is attributed to the synergy of structure engineering and electronic structure regulation for MoS2: Ni atom doping can regulate the electronic structure of MoS2 to increase its intrinsic conductivity while providing additional active sites; CNTs acted as one-dimensional substrate can prevent the agglomeration of MoS2, while increase the conductivity. This work provides a new insight to design and preparation the non-precious metal MoS2based electrocatalysts with enhanced HER performance.

CRediT authorship contribution statement Tao Dong: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software. Xiao Zhang: Investigation, Methodology, Project administration, Resources, Writing - original draft, Writing - review & editing. Peng Wang: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Writing - original draft. Hsueh-Shih Chen: Investigation, Methodology, Project administration, Resources, Writing - original draft, Writing review & editing. Ping Yang: Investigation, Methodology, Project administration, Resources, Writing - original draft, Writing - review & editing.

Acknowledgment This work was supported by the National Natural Science Foundation of China (grant no. 51572109 and 51772130).

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