VS2 Lamellar Heterostructure for

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In-situ Engineering MoS2 NDs/VS2 Lamellar Heterostructure for Enhanced Electrocatalytic Hydrogen Evolution Cuicui Du, Dongxue Liang, Mengxiang Shang, Jinling Zhang, Jianxin Mao, Peng Liu, and Wenbo Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03929 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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ACS Sustainable Chemistry & Engineering

In-situ Engineering MoS2 NDs/VS2 Lamellar Heterostructure for Enhanced Electrocatalytic Hydrogen Evolution Cuicui Du,† Dongxue Liang,† Mengxiang Shang,† Jinling Zhang,† Jianxin Mao,† Peng Liu† and Wenbo Song*† †

College of Chemistry, Jilin University, Changchun 130012, P.R. China

* Corresponding author: E-mail: [email protected]; Fax: +86-431-85168420

ACS Paragon Plus Environment

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ABSTRACT

Layered transition-metal dichalcogenides (TMDs) have provided new opportunities for developing inexpensive and earth-abundant catalysts for electrochemical hydrogen evolution reaction (HER). As the typical family members, semiconducting MoS2 has been widely researched for HER electrocatalysis, while it has been rarely investigated for the emerging metallic VS2, despite of high interest and great potential in this area. Herein, by using ultra-small monolayer MoS2 nanodots as size-controlled precursor, a lamellar heterostructure of MoS2 nanodots and metallic VS2 nanosheets (MoS2 NDs/VS2) is in-situ hydrothermally integrated. Via tuning the hybridizing level of MoS2 nanodots, the morphological and structural evolution of the product is explored, and the electrocatalytic HER performances are researched. The optimized hybrid is featured with thinner lamellar heterostructure, upgraded electrical conductivity, increased catalytically active sites and improved intrinsic activity, thus exhibiting significantly enhanced performance for HER electrocatalysis in acidic media. The possible synergistic interaction between the two counterparts is elucidated preliminarily. This work may pave the way for further investigations on TMDs-based heterostructure containing metallic VS2 nanosheets, which shows great potential in broadening the horizon of electrocatalysis and materials chemistry.

KEYWORDS: lamellar heterostructure, VS2 nanosheets, MoS2 nanodots, hydrogen evolution, electrocatalysis.


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INTRODUCTION Increasingly serious environmental issues such as the energy crisis and global warming have prompted people to develop renewable energy sources as the alternative of traditional fossil fuels to meet future global energy demands. Among various new clean and renewable energy, hydrogen (H2), as the most promising next-generation energy carriers for constructing future energy infrastructure, is expected to provide humans with a sustainable and pollution-free energy supply system. Hydrogen generation is one of the key technologies to convert sustainable energy to H2 for realizing the hydrogen economy. Among different hydrogen production methods, electrochemical water splitting is a highly promising and attractive means with unique advantages of unlimited natural resource, good production safety, high H2 purity and so on. For water electrolysis technology, an effective hydrogen evolution reaction (HER) electrocatalyst is urgently required, which can significantly reduce the overpotential, thereby improving energy conversion efficiency. Typically, the state-of-the-art HER electrocatalysts are Pt-group metals. Nevertheless, the high cost and low reserves extremely hinder their extensive usage. Thus, searching for effective and economical substitutes for Pt-based catalysts has been actively pursed currently toward the realization of sustainable H2 economy.1-5 The emergence of transition metal dichalcogenides (TMDs) as a kind of typical twodimensional (2D) layered materials, has promoted the vigorous development of materials science and related disciplines. The unique properties derived from their novel structural features including the presence of d electrons and high specific surface area etc., make TMDs-based materials of important applications in different fields such as field emission, chemical sensing and electrocatalysis. As one of the representative TMDs, MoS2 has achieved great research progress as a promising candidate for HER electrocatalysis owing to its admirable catalytic


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activity and low cost.6-14 Whereas, the limitation of catalytically active sites and the poor electrical conductivity of MoS2 has hindered the development and wide applications of MoS2based HER catalysts. On the one hand, the catalytic active sites of MoS2 nanostructure are mainly its edges, while its basal plane is inert for HER catalysis, which has been proved by the previous theoretical calculations and experimental research.15-17 Thus exposing more edge sites in synthesis of MoS2-based nanostructure has been pursued with vast effort. On the other hand, the poor electrical conductivity of semiconducting MoS2 is believed to have greatly negative effects on its electrochemical performance. To further improve the electrocatalytic HER performance, enormous attempts have also been devoted to tuning the electric properties of MoS2-based nanostructure.9-10, 18-21 VS2 is another typical family member of 2D layered TMDs, which has a similar hexagonal structure with MoS2, consisting of triple layers of S−V−S with an interlayer spacing of 5.76 Å stacking together by weak van der Waals (vdW) interactions. VS2 is emerging as a promising electrocatalyst and gradually attracting increasing attention due to its unique electronic structure. As is well-known, the electrical conductivity plays an important role in determining electrochemical performance. However, the traditional TMDs are semiconductors, which will inhibit their high-efficient electrocatalytic performance. Different from other TMDs, VS2 shows an intrinsic metallic electronic ground state that provides an essential advantage for superior performance as a promising electrode material. Currently, the research of VS2 mainly involves sensors,22-26 electrochemical energy storage and conversion27-31 and so on. Specially, the unique electronic structure and high specific surface area of layered VS2 make it a potential active electrocatalyst for HER.32-34 An et al. predicted that VS2 possesses higher HER catalytic activity compared with MoS2 and WS2 monolayers in triangular morphology.35 The theoretical feasibility


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to obtain VS2 nanomaterials with ultrathin nanostructure has been revealed by the structural characteristics and calculation results. However, the layered VS2 nanosheets have rarely been reported for HER electrocatalysis during the past decades due to the difficulty in synthesis, which has been regarded as a long-standing challenge.36 In previous reports for the preparation of VS2, it is inevitable to utilize the rigid synthetic methods by high-temperature solid-state reaction under the atmosphere of H2S. This caused great difficulties for the systematic research works of scientists, resulting in a long-term absence of the practical investigation on VS2. Until 2011, a simple synthesis method to prepare layered VS2 nanosheets was developed by Feng et al.,37 which greatly promoted the research progress of VS2 nanomaterials. However, similar to the other TMDs, the pristine layered VS2 tends to stack and aggregate because of the high surface energy as well as the interlayer vdW attractive forces.38 Nanosheets re-stacking and aggregation may decrease the exposed active sites and conductivity. As a consequence, intensive search should be dedicated to designing and synthesizing VS2 nanomaterials with well-controlled structures to improve the electrocatalytic performance for HER. For example, Liang et al. reported a hydrothermal route to vertically grow high-density VS2 nanoplates array on the surface of the conductive substrate for efficient HER electrocatalysis in 2016.39 Previous work demonstrated that the vdW heterostructures with unprecedented properties beyond the corresponding individual components can be constructed through interlayer vdW interactions by interfacing two or more layers of 2D layered materials such as TMDs, boron nitride and graphene etc..40 In this concern, the MoS2/VS2 heterostructure with perspective on HER catalysis can be designed and fabricated due to the structural similarity of the two counterparts. Herein, a novel hierarchical MoS2 NDs/VS2 lamellar heterostructure by smart coupling engineering of ultra-small MoS2 nanodots with metallic layered VS2 nanosheets is


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achieved for the first time. The proposed heterostructure is synthesized through a facile hydrothermal process by using water-soluble ultra-small monolayer MoS2 nanodots as sizecontrolled precursor, sodium orthovanadate and thioacetamide as V and S sources, respectively. By varying the hybridizing level, the effect of MoS2 nanodots on the layered nanostructure is explored. The possibility of utilizing the proposed heterostructure as an efficient HER catalyst under acidic conditions has been demonstrated. The interaction between MoS2 nanodots and VS2 nanosheets is preliminarily probed. EXPERIMENTAL SECTION Materials Sodium orthovanadate (Na3VO4·12H2O), ammonium molybdate ((NH4)6Mo7O24·4H2O), Nacetyl-L-cysteine (NAC) and thiourea (CN2H4S) were commercially available from Sinopharm Chemical Reagent Co., Ltd. Thioacetamide (C2H5NS, TAA) was obtained from Shanghai Yuanye Biotechnology Co., Ltd. The commercial 20 wt% Pt/C catalyst was obtained from Johnson Matthey Corporation. All the chemicals used in the experiments were of analytical grade as received without further purification. Synthesis of MoS2 nanodots (MoS2 NDs) precursor The water-soluble ultra-small monolayer MoS2 NDs with uniform nano-size (～2 nm) was obtained via hydrothermal synthesis according to our previous research.41 In a typical procedure, the reaction mixture was prepared in an ice-water bath by firstly dissolving (NH4)6Mo7O24·4H2O (12.5 mM) and NAC (30 mM) in 40 mL of ultrapure water. Subsequently, the final mixed solution was got by adding CN2H4S (25 mM) under vigorous stirring and then transferred to an autoclave to keep at 200 °C for 4 h. The product was separated from the reaction mixture by


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centrifugation and then purified by column chromatography at ambient temperature to obtain a uniform MoS2 NDs solution for further use. Synthesis of MoS2 NDs/VS2 heterostructure The MoS2 NDs/VS2 heterostructure was prepared referring to the previous synthetic method of pristine VS2 with some modification.37 Typically, Na3VO4·12H2O (75 mM) and TAA (375 mM) were added into 40 mL of MoS2 NDs solution (1.25 mg mL-1), which was stirred for 1 h and then transferred to a Teflon-lined stainless-steel autoclave. The sealed vessel was maintained at 160 °C in an oven for 24 h. After cooled down to ambient temperature, the product was centrifuged, and then the precipitate was collected and washed to re-disperse into 20 mL ultrapure water. To avoid the potential oxidation of V4+ to V5+, the suspension was saturated with N2 to expel the dissolved O2. Above suspension was then ultrasonicated for 3 h in an ice-water bath. Finally, the product of MoS2 NDs/VS2 heterostructure was collected by centrifugation and obtained via vacuum drying. For comparative study, three different samples were synthesized by varying the concentration of MoS2 NDs precursor solution (0.63, 1.25 and 1.88 mg mL-1) and denoted respectively as MoS2 NDs/VS2-1, MoS2 NDs/VS2-2 (MoS2 NDs/VS2 heterostructure) and MoS2 NDs/VS2-3. Synthesis of pristine VS2 By replacing MoS2 NDs precursor solution with ultrapure water, pristine VS2 was obtained by utilizing the similar synthetic method. RESULTS AND DISCUSSION The typical synthetic process of the MoS2 NDs/VS2 lamellar heterostructure is schematically described in Figure 1. The water-soluble size-controlled precursor of ultra-small monolayer MoS2 NDs was firstly hydrothermally synthesized according to our previous report.41 By using


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sodium orthovanadate as V source and thioacetamide as S source, in-situ hydrothermal synthesis was performed to precisely confine MoS2 NDs within layered VS2 nanosheets. The MoS2 NDs/VS2 heterostructure prepared by this synthetic approach potentially have the following advantages: Firstly, the water-solubility of MoS2 NDs precursor facilitates the formation of homogeneous solution with sodium orthovanadate and thioacetamide, ensuring that ultra-small MoS2 nanodots well disperse on VS2 nanosheets; Secondly, during the in-situ hydrothermal process, MoS2 nanodots act as separator to inhibit the re-stacking of VS2 nanosheets, which is beneficial to generating thinner nanosheet structure; Thirdly, layered VS2 nanosheets act as supporting skeleton to effectively hinder the agglomeration of MoS2 nanodots, potentially beneficial to the high-activity and structure stability of the catalysts. Benefiting from the unique hierarchical architecture, the as-prepared MoS2 NDs/VS2 lamellar heterostructure is expected to achieve an improved electrocatalytic activity for HER.

Figure 1. Schematic representation of the preparation of MoS2 NDs/VS2 lamellar heterostructure and pristine VS2.


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Characterization of the as-prepared samples To confirm the structural information and chemical composition of the samples, X-ray diffraction (XRD) analysis was firstly performed. The XRD pattern of pristine VS2 (Figure S1) exhibits several characteristic peaks corresponding respectively to the (001), (011), (012), (003) and (110) etc. planes of hexagonal VS2 (ICDD, reference number, 01-089-1640) free of impurity. After incorporation of MoS2 NDs, the main peaks at about 15.6o, 35.8o, 45.4o, 47.1o, 57.3o indexed respectively to the (001), (011), (012), (003) and (110) planes of VS2 are also clearly presented in the XRD patterns of series MoS2 NDs/VS2 samples with different levels of MoS2 NDs (Figure 2a). These diffraction peaks match well with those of pristine VS2, illustrating the successful formation of hexagonal VS2 in the presence of ultra-small monolayer MoS2 NDs precursor. However, the characteristic peaks for MoS2 are difficult to discern from the baseline, which is possibly because the ultra-small nano-structure, low-crystallinity or relatively low percentage of the MoS2 nanodots in MoS2 NDs/VS2 samples.

Figure 2. (a) XRD patterns of MoS2 NDs/VS2 samples with tunable MoS2 NDs loading and (b) Raman spectrum of MoS2 NDs/VS2-2 (100-1100 cm-1).


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Furthermore, Raman spectroscopy analysis of relevant samples was conducted to determine the structure. As displayed in Figure 2b, the Raman spectrum of MoS2 NDs/VS2 heterostructure in the range of 100-1100 cm-1 presents six characteristic peaks of VS2 at 138, 190, 280, 405, 685 and 990 cm-1, corresponding to the rocking and stretching vibrations of V–S bonds or their combination.42 In addition, the Raman spectra in the range of 200-500 cm-1 for pristine VS2 and series of MoS2 NDs/VS2 samples with tunable MoS2 NDs loading are shown in Figure S2. All samples exhibit two typical peaks of layered VS2 expected for the in-plane and out-of-plane vibration modes at 280 and 405 cm-1, respectively.39 Above results further indicate the in-situ generation of VS2 upon introducing ultra-small monolayer MoS2 NDs precursor. Generally, hexagonal MoS2 exhibits two dominant Raman modes of the in-plane vibration (E12g) and out-ofplane vibration (A1g) located at ～379 cm-1 and ～405 cm-1, respectively.43 And the relative intensity of E12g and A1g is related to their terminal structures: a lower intensity ratio of E12g and A1g for nanoscale MoS2 possibly reveals an edge-terminated nanostructure.44 Therefore, the absence of E12g and A1g characteristic peaks of MoS2 in the Raman spectrum for various samples may be related to the ultra-small nano-structure or relatively low percentage of MoS2 NDs. The morphology and structure of electrocatalytic materials is a key factor affecting their HER activity. To probe the morphological and structural evolution, scanning electron microscope (SEM) was performed on pristine VS2 and series MoS2 NDs/VS2 samples. For pristine VS2, thick nanosheets can be observed in Figure 3a-b, originating from serious stacking of the 2D layered structure because of its poor structural stability.45 After hybrid with different levels of MoS2 NDs, the morphology of MoS2 NDs/VS2 samples is shown in Figure 3c-f. Compared with pristine VS2, Figure 3c also displays a thick layered structure for MoS2 NDs/VS2-1, indicating that the introduction of a relatively lower level of MoS2 NDs does not significantly affect the


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formation of VS2 nanosheets. In contrast, increasing the amount of MoS2 NDs generates MoS2 NDs/VS2-2 with a larger-scale thinner and looser uniform lamellar nanostructure without any aggregation (Figure 3d-e). However, by further increasing the hybridizing level of MoS2 NDs, aggregation occurs for MoS2 NDs/VS2-3 via stacking of smaller flakes (Figure 3f), probably due to the interference and hindrance of excess MoS2 nanodots to the large-scale growth of layered VS2 nanosheets during the in-situ hydrothermal process. The SEM characterization and analysis indicate that MoS2 NDs/VS2-2 has the optimal 2D lamellar structure. Thereafter, we focused our study on the MoS2 NDs/VS2-2 sample. In comparison with the pristine VS2 nanosheets, MoS2 NDs/VS2-2 minimizes the re-stacking of nanosheets, which potentially provides a facile access even to the deep inside of the assembly, contributing more catalytically active sites and better electrical conductivity.

Figure 3. SEM images of (a-b) pristine VS2, (c) MoS2 NDs/VS2-1, (d-e) MoS2 NDs/VS2-2 and (f) MoS2 NDs/VS2-3. The microstructure of MoS2 NDs/VS2-2 was characterized and probed by transmission electron microscopy (TEM). The 2D lamellar structure of the as-synthesized sample can be reconfirmed


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by Figure 4a. The ultra-small MoS2 nanodots are uniformly distributed on the lamellar structure forming intimate contact between the two 2D components as shown in the high-resolution TEM image (Figure 4b), which is significantly different from that of pristine VS2 nanosheets (Figure 4c). Above observation proves that the proposed in-situ synthetic approach can effectively embed ultra-small MoS2 nanodots within VS2 nanosheets to form a homogeneous layered heterostructure. The in-situ growth of thinner VS2 lamellar structure in the presence of ultrasmall MoS2 NDs possibly originates from chemical interactions between the two counterparts with structural similarity.

Figure 4. TEM images of (a-b) MoS2 NDs/VS2-2 and (c) pristine VS2; High-resolution XPS spectra of (d) V 2p, (e) S 2p and (f) Mo 3d for MoS2 NDs/VS2-2. Finally, the formation of the MoS2 NDs/VS2 heterostructure was further explored and quantified by X-ray photoelectron spectroscopy (XPS) measurements, in which V, S and Mo elements exist together. Totally, the estimated loading of MoS2 in the sample is about 15.0 At.%.


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The atomic valence states of each element were further probed based on the high-resolution XPS spectra of V 2p, S 2p and Mo 3d. In Figure 4d, the high-resolution V 2p spectrum can be split into two characteristic peaks at 517.1 and 524.6 eV, assigned to the binding energy of V 2p3/2 and V 2p1/2 orbitals of V4+, respectively.39, 46-47 Figure 4e shows two characteristic peaks of S2− species at the binding energy of 161.1 and 163.0 eV, which correspond to S 2p3/2 and S 2p1/2, respectively. In the high-resolution Mo 3d spectrum of the MoS2 NDs/VS2 heterostructure (Figure 4f), the dominating characteristic peaks located at 232.8 and 235.9 eV confirm the presence of MoS2 in the materials.48 Recently, a density functional theory calculation about MoS2@VS2 nanocomposite49 shows that the redistribution of charge is beneficial to stabilizing VS2 and boosting the electrical conductivity to present metallic characteristics. In addition, theory and experiments have also proved that MoS2 and VS2 can form a highly lattice matched heterojunction with electron transfer due to the orbital interaction between various atoms, but not just stack on the macroscopic scale.25 It is well known that the transfer of electrons will result in the shift of the binding energy. The electron transfer between VS2 and MoS2 in the proposed heterostructure was investigated by XPS characterization. For the MoS2 NDs/VS2 heterostructure, the binding energies of Mo 3d5/2 and Mo 3d3/2 orbital are 232.8 eV and 235.9 eV, which are much higher than 230.6 eV and 233.4 eV for pristine MoS2 ND.41 Meanwhile, compared to pristine VS2, the characteristic peaks of S 2p for the MoS2 NDs/VS2 heterostructure also show obvious shift toward lower binding energy (Figure S3). The binding energy shift for Mo 3d and S 2p orbital indicates a possibility of electronic interaction between the two components in the heterostructure via electron transport from MoS2 to VS2 at the two-phase interface, potentially facilitated by increasing MoS2 NDs loading (Figure S4).


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Electrocatalytic activity evaluation

Figure 5. (a) LSV polarization curves and (b) corresponding Tafel plots of different samples; (c) Stability tests after 1000 cycles of continuous operation and (d) time-dependent current density plot at static overpotential of 300 mV for MoS2 NDs/VS2-2. The above characterization results suggest that the MoS2 NDs/VS2 lamellar heterostructure may be a promising candidate for electrocatalytic hydrogen generation. The HER performances for series of MoS2 NDs/VS2 samples with tunable MoS2 NDs loading (MoS2 NDs/VS2-1, MoS2 NDs/VS2-2 and MoS2 NDs/VS2-3) were assessed in acidic media. Their typical LSV curves and corresponding Tafel plots are presented in Figure 5a and 5b. For comparison, the performances of pristine VS2 and 20 wt% Pt/C catalyst are also included. The pristine VS2 presents an inferior


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catalytic activity for HER. By contrast, series of MoS2 NDs/VS2 samples exhibit distinctive catalytic activity relative to the hybridizing level of MoS2 NDs. In the range of relatively low hybridizing level (MoS2 NDs/VS2-1 and MoS2 NDs/VS2-2), the HER activity is dramatically enhanced with the increase of MoS2 NDs loading. By further increasing MoS2 NDs content, the catalytic activity of MoS2 NDs/VS2-3 deteriorates. An optimal catalytic activity is achieved for MoS2 NDs/VS2-2 sample that displays a thinner lamellar heterostructure, exhibiting an onset overpotential of 220 mV. To reach a current density of 10 mA cm-2, only 291 mV of overpotential is required. The Tafel slope is 58.1 mV dec-1, which is lower than that of pristine VS2 and the other MoS2 NDs/VS2 samples, indicating facile kinetics for HER. Evidently, the ingenious incorporation of ultra-small MoS2 nanodots with metallic VS2 nanosheets to form a hierarchical lamellar heterostructure induces significantly enhanced catalytic activity for HER, compared to the counterpart of pristine VS2 nanosheets. Tafel slope is usually used to assess the reaction kinetics for HER. A small Tafel slope of a HER electrocatalyst generally suggests its high charge transfer ability. Furthermore, Tafel slope as an inherent property indicator can also give additional insight into its rate-determining step and dominant mechanism for HER. In acidic media, HER involves three typical elementary electrochemical reactions:50-51 Volmer reaction [H3O+ + e- → Hads + H2O (a primary discharge step to generate an Hads atom)], followed by either Heyrovsky reaction [Hads + H3O+ + e- → H2 + H2O (an electrochemical desorption step)] or Tafel reaction [Hads +Hads → H2 (a chemical desorption step)] to produce molecular hydrogen, resulting in either the Volmer−Heyrovsky or Volmer−Tafel mechanism. In which, Hads represents an absorbed H atom at the active site of a catalyst. According to previous research,52 a Tafel slope of ～30 or ～40 mV dec−1 is normally invoked in the case of high surface coverage of Hads, assigning the rate-controlling step to the


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Tafel or Heyrovsky reaction; while for the case of low Hads surface coverage, it gives a Tafel slope of ～120 mV dec-1, manifesting the dominated Volmer reaction. In this study, a Tafel slope of 58.1 mV dec−1 for the proposed lamellar heterostructure implies the dominated electrochemical desorption process of Hads and the Volmer-Heyrovsky mechanism. The interfacial properties and HER kinetics were explored by conducting electrochemical impedance spectroscopy (EIS) measurements under operating conditions. It has been well established that the interface charge transfer resistance (Rct) strongly correlates with the electrochemical performance, which can be determined from the diameter of semicircles in the Nyquist plots. The Nyquist plots of MoS2 NDs/VS2 heterostructure electrode at various overpotentials were recorded and shown in Figure S5a. By fitting these plots, an equivalent circuit is obtained as shown in the inset of Figure S5a, which consists of two parallel combinations of the resistors and the constant phase elements in series with a Rs resistor. Rct is overpotential-dependent, which decreases by improving the overpotential because of the facilitated charge transfer with the increase of cathodic bias during hydrogen evolution. The differences in HER electrocatalytic activities of pristine VS2 and series of MoS2 NDs/VS2 samples are further elucidated by Rct as revealed by the Nyquist plots at the same overpotential in Figure S5b. Among them, the MoS2 NDs/VS2 lamellar heterostructure (MoS2 NDs/VS2-2) exhibits a smaller Rct, which indicates faster charge transfer kinetics for HER electrocatalysis. This result is consistent with that of LSV tests. To better understand the origin of the enhanced HER activity for the MoS2 NDs/VS2 lamellar heterostructure, the amount of catalytically active sites for pristine VS2 and series of MoS2 NDs/VS2 samples with tunable MoS2 NDs loading was indicated and estimated by monitoring the change of electrochemical double-layer capacitance (Cdl), which is typically positive


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correlated with the electrochemically active surface area (ECSA).53,54 The cyclic voltammograms of pristine VS2 and series of MoS2 NDs/VS2 samples at different scan rates under non-Faradaic potential are shown in Figure S6. Obviously, the MoS2 NDs/VS2 lamellar heterostructure (MoS2 NDs/VS2-2) generates much higher capacitance current. Through the calibration of the total current density at +0.2 V for anode and cathode versus the scan rate, Figure S7 is obtained according to the current−voltage relationship, in which the halves of the slopes are the corresponding Cdl values. Consistent with the LSV results, the MoS2 NDs/VS2 lamellar heterostructure displays a much larger Cdl value (2.33 mF cm-2) than pristine VS2 (0.14 mF cm2

), MoS2 NDs/VS2-1 (1.02 mF cm-2) and MoS2 NDs/VS2-3 (0.17 mF cm-2), indicating a

significant increase in the exposed active sites and thus dramatically improved HER activity. Moreover, the intrinsic activity of the proposed MoS2 NDs/VS2 heterostructure is evaluated via normalizing the electrocatalytic current by Cdl.55 As displayed in Figure S8, the specific HER activity of MoS2 NDs/VS2-2 is obviously higher than the others, confirming the intrinsic optimization of the active sites in the composite. As a result, the exposure of active sites at MoS2 NDs/VS2 interfaces is promoted, and the intimate contact between MoS2 NDs and VS2 nanosheets at the interface maximizes the intrinsically catalytic activity for HER. Overall, above observation reveals that the incorporation of ultra-small MoS2 nanodots can upgrade the electrical conductivity, increase the accessible active sites and improve the intrinsic activity for HER, ultimately enhancing the catalytic performance. In the practically catalytic application, stability is a significant criterion for evaluating a catalyst. Firstly, cyclic voltammetry measurements and continuously electrocatalytic hydrogen evolution at static overpotential by chronoamperometry were conducted for MoS2 NDs/VS2 lamellar heterostructure in 0.5 M H2SO4 solution. As shown in Figure 5c, the LSV polarization


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curves were measured before and after 1000 CV cycles for MoS2 NDs/VS2-2 sample. The negligible difference indicates that the MoS2 NDs/VS2 lamellar heterostructure is stable during HER electrocatalysis. The long-term stability is also assessed by prolonged electrolysis at constant overpotential of 300 mV over extended periods. From Figure 5d, one can see that the current density decreases slightly in the initial stage and then remains stable for HER electrolysis over 16 h, demonstrating the promise for practical applications of the MoS2 NDs/VS2 lamellar heterostructure catalyst. To investigate the role of VS2 nanosheets in MoS2 NDs/VS2 lamellar heterostructure, HER catalytic performance of the pristine MoS2 NDs was also evaluated under the same conditions. As shown in Figure S9a, the HER catalytic activity of the proposed lamellar heterostructure is significantly superior than that of MoS2 NDs. Figure S9b shows the comparative Tafel plots of MoS2 NDs/VS2 lamellar heterostructure and MoS2 NDs. Obviously, the heterostructure exhibits a much smaller Tafel slope compared with MoS2 NDs, suggesting faster electrocatalytic reaction kinetics. The interfacial properties and charge transfer kinetics of MoS2 NDs were also probed by EIS measurements, and the Nyquist plot is provided in Figure S9c for comparison with the MoS2 NDs/VS2 lamellar heterostructure. The charge transfer resistance of the heterostructure is smaller than that of MoS2 NDs, indicating more facile charge transport during HER electrocatalysis. The amount of the active sites for MoS2 NDs is estimated from the Cdl calculated from CV measurements (Figure S10). By contrast, the MoS2 NDs/VS2 heterostructure shows a remarkably higher Cdl value (2.33 mF cm-2) than that of MoS2 NDs (0.26 mF cm-2, Figure S9d), suggesting unambiguously richer effective active sites for HER. As a consequence, the introduction of VS2 lamellar structure is favorable for improving electrical conductivity and


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increasing accessible active sites, finally boosting the HER catalytic performance of the heterostructure. Above analysis indicates that the HER catalytic performance of the MoS2 NDs/VS2 lamellar heterostructure with upgraded electrical conductivity, increased active sites and improved intrinsic activity, has been significantly enhanced compared with both pristine VS2 nanosheets and MoS2 nanodots. This is attributed to a possible synergistic effect of metallic VS2 nanosheets and ultra-small MoS2 nanodots, stemming from the strong interfacial chemical/electronic interaction between the two components. On one hand, the metallic VS2 nanosheets not only compensate the poor electrical conductivity of MoS2 NDs and facilitate the mass transport for a higher accessibility of active sites, but also provides additional catalytically active sites for HER. On the other hand, the ultra-small MoS2 nanodots not only provide abundant edge sites for HER catalysis, but also act as separators to inhibit re-stacking of VS2 nanosheets, generating thinner nanosheet structure with upgraded electrical conductivity and more exposed active sites. Additionally, the interface effect between MoS2 and VS2 is believed to maximize the intrinsically catalytic activity for HER. CONCLUSIONS In summary, in-situ engineering MoS2 NDs/VS2 lamellar heterostructure is hydrothermally realized by using ultra-small monolayer MoS2 nanodots as size-controlled precursor. The morphology, structure and electrochemical performance of the proposed heterostructure have been revealed to be associated with the hybridizing level of MoS2 NDs. Compared with pristine VS2 nanosheets, the heterostructure displays a thinner lamellar morphology and possesses upgraded electrical conductivity, increased catalytically active sites and improved intrinsic activity. Significantly enhanced HER electrocatalytic activity is achieved in acidic media,


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exhibiting a lower overpotential and a smaller Tafel slope with a good stability. The improved performance is ascribed to the possible synergistic interaction between metallic VS2 nanosheets and ultra-small MoS2 nanodots. This work highlights the significance of smart design and precise synthesis of novel TMDs-based hierarchical heterostructure for versatile applications, especially paving the way for further investigations on metallic VS2 nanosheets-based electrocatalysts for sustainable hydrogen production. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The experimental detail including Physical Characterization and Electrochemical Measurements; Additional images and data including XRD pattern, Raman spectra, Highresolution XPS spectra of S 2p and Mo 3d, Nyquist plots, CV curves and corresponding linear plots of ∆j versus scan rate, LSV curves normalized by Cdl, LSV curves and Tafel curves for various samples (PDF). AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was supported by the National Natural Science Foundation of China (No. 21475051) and the Science and Technology Development Project of Jilin Province, China (No. 20180414022GH). REFERENCES (1)

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Graphical Abstract (For Table of Contents Use Only)

Synopsis The lamellar heterostructure by smartly coupling MoS2 nanodots with metallic VS2 nanosheets is in-situ synthesized as an enhanced electrocatalyst for sustainable hydrogen production.


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VS2 Lamellar Heterostructure for

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