MoS2 Nanosheet Assembling Superstructure with a Three

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MoS2 Nanosheet Assembling Superstructure with Three-dimensional Ion Accessible Site: A New Class of Bifunctional Material for Battery and Electrocatalysis Jiabao Ding, Yu Zhou, Yanguang Li, Shaojun Guo, and Xiaoqing Huang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04815 • Publication Date (Web): 16 Mar 2016 Downloaded from http://pubs.acs.org on March 17, 2016

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Chemistry of Materials

Abstract: The Layered molybdenum disulfide (MoS2) nanostructured materials are of great interest for electrochemical energy storage/conversion and electrocatalytic water splitting. However, they still show the very limited performance due to their limited active sites. In order to create more efficient MoS2 materials, herein, we develop a simple yet efficient approach to a unique column-like MoS2 superstructure composed of edge-terminated MoS2 nanosheets (CLET MoS2). These MoS2 nanosheets as building blocks with fully exposed active edges are oriented in a preferred manner, rendering CLET MoS2 exhibit excellent electrochemical performance in both lithium ion storage and hydrogen evolution reaction (HER). Compared with commercial MoS2, these hierarchical MoS2 superstructures possess much higher specific capacity and superior cycling performance for lithium ion storage, and excellent electrocatalytic activity and stability for HER with a very low Tafel slope of 39 mVdecade-1, showing their great potential applications in lithium-ion batteries and water splitting. Keywords: Column-Like, Edge-Terminated, MoS2, Lithium Ion Storage, Hydrogen Evolution Reaction INTRODUCTION Since the emergence of graphene, two dimensional (2D) layered materials have been in flush in recent years due to their amazing physical and chemical properties.[1] As one of the most famous 2D materials, MoS2 possesses a layered structure with each layer consisting of molybdenum atoms sandwiched between sulfur atoms by covalent bonds (S-Mo-S), while its interlayers are interacted by Van der Waals forces. Such particular structure is extremely important for achieving their applications in catalysis, energy storage devices, electronics, optoelectronics, lubricants, and so on.[2-7] Due to the weak interlayer interaction, nanostructured MoS2 can accommodate Li ions with less volume change than those of some other conversion materials.[8] As indicated in the previous studies, there are two kinds of surface sites on MoS2 layers, i.e. terrace sites of the basal plane and edge sites at the border. MoS2

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nanomaterials with rich edge sites at the border are proved to be active for hydrogen evolution reaction (HER).[9-10] Compared with its bulk materials, nanostructured MoS2 has higher specific surface area and better electrochemical performances owing to its size effect.[2] It inspires a great deal of works on designing various MoS2 to improve its properties by controlling the morphologies and sizes. Among all the MoS2 nanomaterials, MoS2 nanosheets are of more interest because they exhibit more active edge site for lithium ion batteries (LIBs) and HER. Unfortunately, during the electrochemical process, MoS2 nanosheets tend to form the aggregated materials due to their property toward minimizing the surface energy, making the edge site not completely exposed, which leads to the reduced performance. In this regard, assembling MoS2 nanosheets into three-dimensional (3D) layer-by-layer superstructure with maximized accessibility to reactants and exposed active edges in the preferred oriented manner may be the best strategy to achieve the very high performance for both LIBs and HER, but remains a tremendous challenge (Scheme 1). Up to now, there are very limited papers about using the complicated approaches to make hierarchical MoS2 with worm-like structures for applications only in sodium or lithium batteries at low current density. [11,12]


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Scheme 1. A schematic illustration of 3D MoS2 superstructure with maximized accessibility to reactants and exposed active edges. Herein, we demonstrate a simple yet efficient wet-chemical approach to prepare 3D column-like MoS2 superstructures composed of edge-terminated MoS2 nanosheets (designated as CLET MoS2). Such interesting superstructure with 3D accessible sites, maximized edges and proper layer distance makes them work as efficient bifunctional materials for LIBs and HER. As a result, the as-prepared CLET MoS2 delivers the high reversible specific capacity of around 840 mAhg-1 at a current density of 200 mAg-1, superior cycling durability up to 500 cycles, and excellent rate performance for the LIBs. In particular, they also show enhanced performance for HER with low onset overpotential and small Tafel slopes of 39 mVdecade-1. To the best of our knowledge, our CLET MoS2 is the best MoS2 material for both LIBs and HER reported to date. EXPERIMENTAL SECTION Chemicals. Molybdenum chloride (MoCl5, 99.6%) was purchased from Alfa Aesar. Cysteine (HSCH2CH(NH2)COOH, 98%), molybdic acid (H2MoO4, ACS reagent), thiourea (H2NCSNH2, 99%) and1-methyl-2-pyrrolidinone (NMP, 99%) were obtained from J&K Scientific Ltd. (Shanghai, China). Sodium molybdate dehydrate (Na2MoO4•2H2O, AR), thiosemicarbazide (NH2NHCSNH2, AR), thioacetamide (CH3CSNH2, AR), commercial MoS2, N,N-dimethylformamide (DMF, AR) and ethylene glycol (EG, AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All the chemicals were used as received without further purification. The water (18 MΩ/cm) used in all experiments was prepared by an ultra-pure purification system. Preparation of CLET MoS2. In a typical preparation of CLET MoS2, 61 mg of sodium molybdate dehydrate and 121.2 mg of cysteine were added into 10 mL NMP. After stirring for 30 min, the solution


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was then transferred to a Teflon-lined stainless-steel autoclave with a capacity of 25 mL. The sealed vessel was then heated at 220 ℃ in an oven for 18 h before it was naturally cooled to room temperature. The black product was collected by centrifugation and washed with ethanol/water mixture for three times. Characterizations. Low-magnification transmission electron microscopy (TEM) was conducted on a HITACHI HT7700TEM at an acceleration voltage of 120 kV. High-magnification TEM and scanning transmission electron microscopy (STEM) were conducted on an FEI Tecnai F20 transmission electron microscope at an acceleration voltage of 200 kV. Scanning electron microscopy (SEM) images were taken with a HITACHI S-4700 cold field emission scanning electron microscope operated at 15 kV. Xray diffraction (XRD) pattern was collected on X’Pert-Pro MPD diffractometer (Netherlands PANalytical) with a Cu Kα X-ray source (λ = 1.540598 Å). X-ray photoelectron spectroscopy (XPS) spectra were conducted on a Thermo Scientific ESCALAB 250 XI X-ray photoelectron spectrometer. Fourier transform infrared (FT-IR) spectra were collected on Bruker Optics VERTEX70 FT-IR Spektrometer. Raman spectrums were conducted on HORIBA HR800. Lithium-ion batteries (LIB). To prepare the working electrodes for battery measurements, MoS2 active materials, multi-walled carbon nanotubes (MWCNT) and polyacrylic acid (PAA) with 7:2:1 mass ratio were mixed together in water to form a homogenous slurry, and then casted onto a copper foil and finally dried in vacuum at 60°C for 12 h. Standard 2032 type coin cells were assembled in an Ar-filled glovebox by sequentially stacking together the working electrode, a Celgard 2400 polypropylene membrane and a Li metal disk in 1:1 v/v ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte containing 1 M LiPF6. Galvanostatic charge-discharge experiments were performed in the voltage range of 0.01-3.0 V under various current densities at room temperature on a MTI Battery Testing System (CT-3008). Specific capacities were calculated based on the mass of active materials. Cyclic voltammetry (CV) was


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carried out on a CHI660E potentiostat at a scan rate of 0.1 mVs-1. Hydrogen evolution reaction (HER). All electrochemical tests were conducted using a electrochemical workstation (CHI660E) with the glassy carbon electrode as the working electrode, a saturated calomel electrode as the reference electrode and Pt wire as the counter electrode, respectively. The MoS2 (2.2 mg), carbon black (1 mg) and a Nafion solution (5 wt %，10μL) were dispersed in a water/ethanol mixture (195 μL water and 200 μL ethanol) with the assistance of sonication to form a homogeneous ink. 10 μL of the catalyst ink were then loaded onto a glassy carbon electrode with a diameter of 5 mm (catalyst loading ca. 0.40 mgcm-2). Linear sweep voltammetry was performed in 0.5 M H2SO4 solution purged with nitrogen at the scan rate of 5 mVs-1. All the potentials were transformed into reversible hydrogen electrode (RHE). The Nyquist plots were performed at overpotential of 160 mV over a frequency range from 100 kHz to 0.1 Hz.

RESULTS AND DISCUSSION A simple wet-chemical approach was used to create the CLET MoS2 with high yield, in which sodium molybdate dehydrate (Na2MoO4•2H2O) and cysteine (HSCH2CH(NH2)COOH) were used as Mo and S precursors, respectively, and 1-methyl-2-pyrrolidinone (NMP) was applied as solvent (see the details in Supporting Information). We have optimized a variety of synthetic parameters, such as solvents, precursor, and reaction time for the optimal preparation of CLET MoS2 (Figure S1-4). The asprepared product was first characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Representative low-magnification SEM images (Figure 1a&S4) clearly show that the product consists of uniform nanostructures with column-like morphology. These column-like structures have the average diameter of around 108 nm and length from about 0.5 µm to 1 µm, calculated based on 50 random selected column-like structures. Importantly, each column-like nanostructure is comprised of


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many building blocks with highly ordered feature (Figure 1b). Figure 1c presents the XRD pattern of the as-prepared CLET MoS2. Compared with the commercial MoS2, there are two new peaks at the lowangle region, whose d-spacings are 0.94 nm and 0.49 nm, respectively, indexed to (001) and (002) reflections, respectively.[13-14] The d spacing (0.94 nm) of CLET MoS2 is larger than that (0.61 nm) of the pristine 2H-MoS2 (JCPDS Card No.77-1716), indicating the emergence of a different layered structure, which may be due to the intercalation of NMP or its oxidized species into the initially formed MoS2 nanosheets. Since NMP can be used as a stripping agent to exfoliate the bulk two dimensional (2D) layered materials into single or several layered nanosheets, we infer that when NMP is used as solvent, it may play a significant role in preventing the aggregation of MoS2 during the synthesis.[15-17] Obviously, the peaks at high-angle region (32.7o and 57.2o) can be attributed to (100) and (110) planes of 2H-MoS2, showing no change in the atomic arrangement along the basal planes. The asymmetric diffraction peak at near 32.7o may show the existence of stacking faults between the nanosheets, which may result from a-b plane gliding.[13]


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Figure 1. Representative (a) low-magnification SEM image, (b) high-magnification SEM image of CLET MoS2, and (c) XRD patterns of CLET MoS2 and commercial MoS2.

The structure of CLET MoS2 was further characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and high-angle annular dark-field scanning TEM (HAADF-STEM) images (Figure 2). Figure 2a shows a representative TEM image of a CLET MoS2 with an obvious presentation of assembling structure. Each CLET MoS2 consists of many monolayer MoS2 nanosheets with the thickness of around 0.63 nm as building blocks (Figure 2b). The monolayer MoS2 nanosheets in CLET MoS2 are stacked face to face, and branch out from the center. The HAADF-STEM image of CLET MoS2 further clearly demonstrates the thin and lamellar structure with sharp contrast (Figure 2c). Energy Dispersive X-ray spectroscopy (EDX) reveal the molar ratio of Mo to S in the CLET MoS2 is


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close to 1:2 over a large area. Figure 2d shows the elemental mapping for Mo and S, showing the uniform distribution of both Mo and S throughout the CLET MoS2. Both the high-resolution X-ray photoelectron spectra (XPS) and Raman spectra of CLET MoS2 display the characteristic peaks of MoS2, similar to those of the commercial MoS2 (Figure S5-6). The Brunauer-Emmett-Teller (BET) surface areas of the CLET MoS2, determined from N2 adsorption-desorption isotherms, is 67.9 m2g-1, which is 17 times higher than commercial MoS2 (3.9 m2g-1 ) (Figure S7).

Figure 2. (a) TEM, (b) HRTEM, and (c) HAADF-STEM images of CLET MoS2. (d) HAADF-STEM image and the corresponding EDX elemental mappings of CLET MoS2.

The production of CLET MoS2 with fully exposed active edges is the most striking feature of the synthesis reported herein. To reveal the growth process of our CLET MoS2, we investigated the growth


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intermediates obtained at different reaction times. TEM result shows that there was no lamellar structure after 3 h (Figure 3a). As the reaction proceeded, the lamellar structures appeared in the aggregated products at 6 h (Figure 3b). With further increasing the reaction time, the lamellar structures of these intermediates continued to evolve (Figure 3c). Together with the appearance of a primary anisotropic self-assembly behavior, a further increase in the size of the lamellar nanostructures was observed after 11 h (Figure 3 d). When the reaction time reached 12 h, the typical CLET MoS2 with column-like architecture assembled by distinctive layered structure was obtained (Figure 3e). There was no change on the structure of the intermediates if we further prolonged the reaction time to 18 h, indicating that the reaction has been completed (Figure 3f). Based on these time-dependent studies, it looks the formation mechanism of CLET MoS2 includes two steps: (a) the initial formation of primary building blocks with lamellar nanostructures at the early stage and (b) the subsequent anisotropic self-assembling into column-like superstructures to minimize their surface energy at high temperature. Furthermore, we found that, a variety of synthetic parameters, such as solvents, precursors, and reaction time have much impact on the formation of high-quality of CLET MoS2 (Figure S1-4), which is due to the high sensitivity of self-assembling process to the types of solvent, precursors, and also the duration of reaction time.


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Figure 3. Representative TEM images of CLET MoS2 obtained at different reaction times: (a) 3 h, (b) 6 h, (c) 9 h, (d) 11 h, (e) 12 h, and (f) 18 h.

The CLET MoS2 was paired with MWCTs and a Li metal disk in standard coin cells for cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) studies. The reason for the addition of MWCNTs herein is to promote the electric conductivity of CLET MoS2. MWCNTs themselves have a very small specific capacity, whereas CLET MoS2 without the addition of MWCNT has much inferior electrochemical performance (Figure S8). Figure 4a shows the CVs of CLET MoS2 from the first three cycles. During the initial cathodic sweep, two reduction peaks can be distinguished with a minor one centered at ~1.5 V and a major one at ~0.5 V (vs. Li+/Li, the same hereafter). The former is attributed to the intercalation of Li+ ions into the interlayer space of MoS2 to form LixMoS2(x75%. To our best knowledge, this represents one of the best long-term cycling performances ever measured for MoS2 electrode materials-even more impressive considering that it is achieved at the large current rate of 1 Ag-1.[29-30]


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Figure 4. (a) The first three CVs of CLET MoS2. (b) GCD profiles and (c) cycling performance of CLET MoS2 and commercial MoS2 at 0.2 Ag-1. (d) Cycling performance of CLET MoS2 at different current densities. (e) Long-life cycling performance and Coulombic efficiency of CLET MoS2 for LIBs at 1 Ag-1.

Apart from its excellent property for Li ion storage, MoS2 is also well known for its electrocatalytic activity toward HER. Here, we also explore the electrocatalytic activity of our CLET MoS2 along with benchmark Pt/C (JM, Figure S10) and commercial MoS2 toward HER. To evaluate its electrocatalytic activity for HER, the CLET MoS2 was mixed with carbon black (CABOT VXC-72), and then dispersed


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into a mixture of ethanol, water and Nafion by sonication. This mixture was drop cast onto glass carbon electrodes to prepare the working electrode. All the measurements were conducted in nitrogen-saturated 0.5 M H2SO4 solution (see Supporting Information). As revealed in Figure 5a, the CLET MoS2 exhibit an onset potential of approximately -0.125 V versus reversible hydrogen electrode (RHE), which was followed by a rapid increase in the cathodic current density when the potential shift from onset potential to more negative potentials. Due to the unique hierarchical structure with optimized exposed active edges, the CLET MoS2 possesses a Tafel slope value of 39 mVdec-1 (Figure 5b), much better than that of commercial MoS2 (109 mVdec-1), and comparable with that of the commercial Pt/C (29 mVdec-1), and even better than those of previously reported results.[31-33] Compared with commercial MoS2, the EIS Nyquist plots of CLET MoS2 (Figure 5c) reveals a smaller charge transfer resistance (Rct), indicating a fast Faradaic process. The long-term stability of our CLET MoS2 in HER was also tested, showing there are little changes on the polarization curves before and after 1000 cycles (Figure 5d), which indicates it has very high electrochemical stability for HER.


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Figure 5. (a) Polarization curves of CLET MoS2, Pt/C (JM) and commercial MoS2. (b) Tafel plots for the three catalysts derived from (a). (c) EIS Nyquist plots of CLET MoS2 and commercial MoS2. The inset shows Nyquist plots at high-frequency range. (d) Polarization curves of CLET MoS2 before and after 1000 potential cycles.

CONCLUSIONS To summarize, a simple yet efficient wet-chemical approach was developed for the preparation of well-defined hierarchical MoS2 nanoassembly superstructure composed of edge-terminated MoS2 nanosheets. This approach leads to favorable column-like MoS2 superstructure with active edges thoroughly exposed. When applied as the anode material of LIBs, the CLET MoS2 exhibit very high reversible specific capacity of 840 mAhg-1 at a current density of 200 mAg-1 and much better cyclic stability than commercial MoS2. As for the HER application, the CLET MoS2 shows very high electrocatalytic activity and stability for HER, with an impressively small overpotential and a Tafel slope


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of 39 mVdec-1. Given its excellent performance in LIBs and HER, these edge-terminated MoS2 superstructure may find broad applications in many important catalytic reactions, including electrochemical, photoelectrochemical processes and beyond. ASSOCIATED CONTENT Supporting Information. Figure S1-9. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author [email protected]; [email protected]; [email protected] ACKNOWLEDGMENT This work was financially supported by the start-up funding from Soochow University and Peking University, Young Thousand Talented Program, the National Natural Science Foundation of China (No. K110902615), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References 1. Novoselov, K. S.; Geim, A. K.; Morozov, S.; Jiang, D.; Zhang, Y.; Dubonos, S.; Grigorieva, I.; Firsov, A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666. 2. Gao, Q.; Giordano, C.; Antonietti, M. Biomimetic Oxygen Activation by MoS2/Ta3N5 Nanocomposites for Selective Aerobic Oxidation. Angew. Chem. Int. Ed. 2012, 124, 11910. 3. Seger, B.; Laursen, A. B.; Vesborg, P. C.; Pedersen, T.; Hansen, O.; Dahl, S.; Chorkendorff, I. Hydrogen Production Using a Molybdenum Sulfide Catalyst on a Titanium-Protected n+p-Silicon Photocathode. Angew. Chem. Int. Ed. 2012, 51, 9128. 4. Xu, X.; Liu, W.; Kim, Y.; Cho, J. Nanostructured Transition Metal Sulfides for Lithium Ion Batteries: Progress and Challenges. Nano Today 2014, 9, 604.


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MoS2 Nanosheet Assembling Superstructure with a Three

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