Toward High-Performance Lithium-Ion Batteri - ACS Publications

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Constructing Highly Oriented Configuration by Few-Layer MoS2: Toward High-Performance Lithium-Ion Batteries and Hydrogen Evolution Reactions Sanpei Zhang,† B. V. R. Chowdari,‡ Zhaoyin Wen,*,† Jun Jin,† and Jianhua Yang† †

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People's Republic of China and ‡Department of Physics, National University of Singapore, Singapore 117542

ABSTRACT Constructing three-dimensional (3D) architecture with oriented configurations by

two-dimensional nanobuilding blocks is highly challenging but desirable for practical applications. The well-oriented open structure can facilitate storage and efficient transport of ion, electron, and mass for high-performance energy technologies. Using MoS2 as an example, we present a facile and effective hydrothermal method to synthesize 3D radially oriented MoS2 nanospheres. The nanosheets in the MoS2 nanospheres are found to have less than five layers with an expanded (002) plane, which facilitates storage and efficient transport of ion, electron, and mass. When evaluated as anode materials for rechargeable Li-ion batteries, the MoS2 nanospheres show an outstanding performance; namely, a specific capacity as large as 1009.2 mA h g
1 is delivered at 500 mA g
1 even after 500 deep charge/ discharge cycles. Apart from promising the lithium-ion battery anode, this 3D radially oriented MoS2 nanospheres also show high activity and stability for the hydrogen evolution reaction. KEYWORDS: oriented configurations . two-dimensional materials . electrochemical performance . molybdenum . nanoparticles

R

ecently, global warming and the energy crisis have accelerated the development of alternative energy technologies for traditional fossil fuels, such as Li-ion batteries (LIBs) and hydrogen production through water electrolysis.1,2 The performances of these electrochemical energy technologies are strongly dependent on efficient intercalation of electrons and ions.3,4 Therefore, recent studies have focused on designing novel nanostructured materials with enhanced electrochemical response.5
7 Precisely controlling the orientation of the void channel and the nanobuilding blocks of the channel in a three-dimensional (3D) open architecture has already led to significant improvement in various fields of science and technology.8
14 Accompanied by the emergence of graphene, two-dimensional (2D) transition metal dichalcogenides such as molybdenum disulfide (MoS2) have attracted increasing ZHANG ET AL.

interest due to their exotic structure
property relationships in catalysis, energy storage, electronics, optoelectronics, etc.15
19 However, owing to the interlayer van der Waals forces and high surface energy,20 it is inevitable for 2D nanosheets to restack when used for any practical applications, causing a significant decrease in the performance in many applications.21,22 To date, various strategies have been put forward to retain the individual nanosheets with largely exposed sites to take advantage of the active nanomaterials. One effective strategy is to synthesize MoS2/carbon composites, including graphene nanosheets,23,24 carbon nanotubes,25 polyaniline nanowires,26 carbon nanosheets,27 carbon nanofibers,28 and mesoporous carbon.29 It has been reported that these conductive carbon layers on the surface of MoS2 nanosheets can avoid aggregation of the active nanosheets and enhance the electrical conductivity. VOL. XXX

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* Address correspondence to [email protected]. Received for review September 18, 2015 and accepted November 7, 2015. Published online 10.1021/acsnano.5b05891 C XXXX American Chemical Society

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Scheme 1. Schematics depicting the formation of the 3D radially oriented MoS2 nanospheres.

Another one is to assemble the 2D MoS2 nanosheets into a 3D hierarchical architecture, which can retain the large contact surface and avail the advantage of active nanomaterials.30
35 For instance, Wang's group reported a 3D hollow tubular architecture constructed by ultrathin MoS2 nanosheets, which resulted in excellent electrochemical properties as anode materials for LIBs.31 Recently, Lou's group reported a templateassisted strategy for the synthesis of hierarchical MoS2 microboxes constructed by ultrathin nanosheets.30 The hollow MoS2 microboxes show enhanced lithium storage and hydrogen evolution reaction (HER) performance. These excellent performances are attributed to the 3D microstructure constructed by ultrathin MoS2 nanosheets, which might facilitate the transport and storage of the Li ion and provide enough active sites. Nonetheless, constructing a highly oriented 3D architecture integrated with 2D nanobuilding blocks has rarely been reported. In this article, we demonstrate a simple and effective method to synthesize 3D, open, radially oriented MoS2 nanospheres with ultrathin nanosheets as walls. The as-prepared nanospheres have a highly oriented open microstructure with preserved ultrathin MoS2 layers. Furthermore, the obtained 3D radially oriented MoS2 nanospheres deliver an increased d spacing of the (002) crystal plane, which can provide enough space for fast lithium-ion intercalation.36 As expected, when evaluated as LIB anode materials, these MoS2 nanospheres manifest a specific capacity as large as 1009.2 mA h g
1 at 500 mA g
1 even after 500 cycles, which may be the best record for pure MoS2.16,30
32,36
40 Also, they show high activity and stability for HER. ZHANG ET AL.

RESULTS AND DISCUSSION The fabrication of 3D radially oriented MoS2 nanospheres is schematically depicted in Scheme 1. The process starts by dissolving (NH4)6Mo7O24 3 4H2O, NH2CSNH2, and poly(vinylpyrrolidone) (PVP) in distilled water to obtain a homogeneous solution. In the first step, the (NH4)6Mo7O24 3 4H2O is decomposed to ultrathin MoOx nanosheets and reacts with NH2CSNH2 to obtain MoS2 nanosheets. Meanwhile, due to the high surface energy of few-layer MoS2, the PVP is tightly absorbed on the (002) plane of the MoS2 nanosheets. The absorbed PVP surfactant can efficiently protect the 2D nanosheet structure from restacking. In the second step, the PVP on the surface of MoS2 drives the radially oriented assembly of 2D nanosheets into 3D nanospheres. After being thoroughly washed with water, ethanol, and acetone, the 3D open, radially oriented MoS2 nanospheres integrated with few-layer MoS2 are obtained. The high crystallinity and phase purity of the obtained products were confirmed by the X-ray diffraction (XRD) technique. As shown in Figure 1a, all of the identified peaks can be perfectly indexed to the standard pattern of hexagonal MoS2 (JCPDS card No. 37-1492), revealing the high purity of the product. Compared with the XRD pattern of bulk MoS2 powder (Figure S1), the obvious broadness of all the peaks in the diffraction pattern shown in Figure 1a suggests that the crystallites are at the nanoscale level. Notably, the 2θ of the (002) diffraction peak was shifted from 14.378° (the standard angle) to a low-angle of 12.48°, revealing the increased interlayer distance of the (002) VOL. XXX

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ARTICLE Figure 1. (a) XRD pattern of the 3D radially oriented MoS2 nanospheres, showing a clear shift of the (002) peak. (b) Lowmagnification FESEM and (c) TEM images of the 3D radially oriented MoS2 nanospheres. (d) High-magnification TEM image of MoS2 nanospheres. (e,f) HRTEM images corresponding to area 1 and area 2 in (d), respectively. (g) Structural model of ultrathin MoS2 viewed from the [110] and [001] directions. The Mo and S atoms are colored purple and green, respectively.

plane from 0.63 to 0.707 nm (calculated from the Bragg equation). This interesting phenomenon may be related to the involvement of PVP during the formation process. The thickness of the sheet along the c axis, which was determined from the half-width of the (002) peak using the Debye
Scherrer equation, was approximately 3.9 nm, corresponding to five S
Mo
S layers (with the expanded interlayer distance of 0.707 nm). The morphology of the obtained hexagonal MoS2 was characterized by field-emission scanning electron microscopy (FESEM), as shown in Figure 1b. These MoS2 nanospheres are highly uniform, with an average diameter of ∼200 nm. A corresponding transmission electron microscopy (TEM) image (Figure 1c) reveals that the nanospheres are shown to adopt uniform radial orientation and are constructed of ultrathin nanosheets. In the high-magnification TEM graph (Figure 1d), it is apparent that the entire sphere is indeed composed of well-organized MoS2 nanosheets. The phase purity and elemental composition of the ZHANG ET AL.

products were proven by energy-dispersive X-ray analysis and elemental mapping images (Figure S2), which showed strong Mo and S signals. Further high-resolution transmission electron microscopy (HRTEM) images at the center and side of the nanospheres are shown in Figure 1e,f, respectively. Obvious lattice fringes can be observed across the whole nanospheres, indicating the well-preserved nanosheet structures in the nanospheres. Interplanar spacing of 7.1 Å can be clearly observed in the organized nanosheets, which is consistent with the expanded d spacing of the (002) planes of hexagonal MoS2 (Figure 1g). Remarkably, most of the nanosheets in the 3D radially oriented nanospheres are found to be less than five S
Mo
S layers, confirming the preserved ultrathin nanosheet structure. Raman spectroscopy measurements have been reported to be a reliable diagnostic tool to confirm the ultrathin nature of MoS2 nanosheets.41 Figure 2a shows the Raman spectra of the MoS2 nanospheres excited by a 532 nm laser line at room temperature. Compared VOL. XXX

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ARTICLE Figure 2. (a) Raman spectra of MoS2 nanospheres and commercial MoS2. (b) Atomic vibrations of the E12g and A1g vibrational modes. High-resolution XPS spectra of the Mo 3d and S 2s peaks (c) and S 2p peak (d) of MoS2 nanospheres.

with bulk MoS2, the frequencies of the E12g and A1g peaks shift from 381 and 409 cm
1 for the bulk MoS2 to 381.5 and 407 cm
1 for the 3D MoS2 nanospheres. As shown in Figure 2b, the in-plane E12g mode is attributed to an opposite vibration of the Mo atom with respect to two S atoms, and the A1g mode is related to the out-of-plane vibration of only S atoms along opposite directions.42 The decrease in layer number with diminished interlayer van der Waals force results in an obviously stronger out-of-plane atom vibration (A1g) along the c axis (corresponding to lower force constants) but has a minor impact on the in-plane vibration (E12g).43 Thus, the observed change of peak frequencies of E12g and A1g clearly confirmed the ultrathin nature of nanosheets in the MoS2 nanospheres. X-ray photoelectron spectroscopy (XPS) was used to study the chemical state of Mo and S in the nanospheres. The 3d3/2 (232.2 eV) and 3d5/2 (229 eV) peaks in the high-resolution XPS spectra of Mo are attributed to Mo4þ in MoS2 (Figure 2c).31 The peaks located at 161.9 and 163 eV in the high-resolution S 2p spectra (Figure 2d) are associated with the S 2p3/2 and S 2p1/2 components of MoS2, respectively.31 From the combined analysis of XRD, FESEM, TEM, Raman, and XPS spectroscopy, it is conclusively demonstrated that the pure 3D radially oriented open MoS2 nanospheres are constructed by ultrathin nanosheets. To understand the radially oriented process and role of PVP additive, a series of designed experiments were performed. First, when the mixture NH2CSNH2
(NH4)6Mo7O24 without PVP was treated under the ZHANG ET AL.

same conditions as that for the preparation of 3D MoS2 nanospheres, we found that the reaction was easy to obtain as pure hexagonal MoS2 (Figure S3a), which is consistent with the previous reports.44 SEM and TEM images (Figure S3b
d) verified the ultrathin nanosheet morphology of the products. When increasing amounts of PVP surfactant were added in the precursor solution, the MoS2 nanosheets started to assemble into a compact sphere (Figure S4a
c). However, too high of a concentration of PVP will increase the viscosity of the precursor solution and confine the radial assembly process (Figure S4d). Interestingly, the lateral size of the nanosheets in the MoS2 nanospheres is close to the size of the former MoS2 nanosheets fabricated without PVP. Therefore, it can be discerned that the main role of PVP is to enable the radially oriented process without confining the growth of MoS2 sheets. In addition, we also compared the XRD patterns of MoS2 nanospheres before and after washing with PVP. As shown in Figure S5, with no change in other diffraction peaks, the stronger intensity of the (002) peak shows that PVP is mainly anchored on the (002) plane, which is attributed to the high surface energy. The favorably selective location on the (002) plane further confirms that the role of PVP is in controlling the radially oriented assembly process without confining the 2D lateral size of the (002) planes. Besides, it should be noted that the (002) peak of MoS2 nanospheres (Figure 1a) is broader than that of the corresponding peak of MoS2 nanosheets (Figure S3a), indicating less layers of MoS2 sheets in the nanospheres. VOL. XXX

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ARTICLE Figure 3. (a) Discharge
charge voltage profiles of 3D radially oriented MoS2 nanospheres. (b) Cycling performance of MoS2 nanospheres and MoS2 nanosheets, and Coulombic efficiency of MoS2 nanospheres within a voltage range of 0.01
3.0 V vs qLi/Liþ at a current density of 100 mA g
1. (c) Cycling performance of MoS2 nanospheres at a current density of 500 mA g
1. (d) Rate capability of MoS2 nanospheres.

Furthermore, comparison of Raman spectra (Figure S6) between MoS2 nanospheres and nanosheets also verified the result with the shift of A1g and 1E2g. This suggests that PVP anchored on the (002) plane can also efficiently preserve the nanosheet structure from restacking during the hydrothermal process. Full nitrogen sorption isotherms of the 3D radially oriented MoS2 nanospheres and MoS2 nanosheets were recorded to obtain information on the pore size distribution and specific surface area before and after assembly. As shown in Figure S7, the isotherms of MoS2 nanospheres belong to a typical IV isotherm characteristic of mesoporous materials. Compared with the isotherm characteristic of MoS2 nanosheets, a distinct capillary condensation step at P/P0 = 0.3 to 0.8 reflects uniform mesopores resulting from the radial mesopore channels of the MoS2 nanospheres. Accordingly, the Brunauer
Emmett
Teller specific surface area of MoS2 nanospheres is calculated to be 43 m2 g
1, which is higher than that of MoS2 nanosheets (32 m2 g
1). Moreover, the pore size distribution (Figure S8) calculated using the Barrett
Joyner
Halenda model shows a sharp peak at 3.3 nm, which is attributed to mesoporous channels in the MoS2 nanospheres resulting from the uniform assembly of MoS2 nanosheets. According to above results and previous report, we affirmatively deduced that the formation process of our 3D radially oriented MoS2 nanospheres may be as follows (as shown in Scheme 1): ZHANG ET AL.

during the solvothermal process, the (NH4)6Mo7O24 3 4H2O is decomposed to MoOx nanosheets and the NH2CSNH2 is hydrolyzed to NH3, H2S, and CO2. The released H2S acts as a sulfide source as well as a reducing agent to reduce MoOx precursors to MoS2 nanosheets (step 1).27,45,46 Due to the high surface energy of the (002) plane, PVP is tightly absorbed on the surface of MoS2 ultrathin nanosheets and then drives the radially oriented assembly into a 3D nanospheres (step 2). However, the mechanism for the detailed assembly process of 3D radially oriented open MoS2 nanospheres should require more careful studies and in-depth systematic investigation. Hoping that these interesting 3D nanosheetorganized nanospheres may improve the Li storage performance and provide faster Li-ion diffusion, we have used them as anode materials for LIB anodes and lithium metal foil as the counter/reference electrode. Figure 3a shows the representative discharge
charge profiles for the first five cycles at 100 mA g
1 in the voltage window of 0.01 to 3 V. In agreement with previous reports, there are two voltage plateaus located at around 1.2 and 0.6 V in the discharge process of the first cycle. The former might be assigned to the insertion of Liþ ions into the interlayer of MoS2 to form LixMoS2, while the latter is ascribed to a conversion reaction involving the decomposition of LixMoS2 to Li2S and Mo.47 After the first discharge cycle, a different discharge profile is observed, which is in accordance VOL. XXX

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with the results reported previously for MoS2 anodes.31,40 The plateau around 2.3 V is ascribed to the oxidation of Li2S into Liþ ions and S.47 The above voltage plateaus in the discharge
charge process are also verified by cyclic voltammetry (CV) studies (Figure S9). In the first cycle of 3D radially oriented MoS2 nanospheres, the initial discharge and charge specific capacities are found to be approximately 1498 and 1170 mA h g
1, respectively, showing a Coulombic efficiency of ∼78%. The initial irreversible capacity loss might be related to the inevitable formation of a solid electrolyte interphase and decomposition of the electrolyte.23,48 The discharge
charge cycling performance was evaluated at the current density of 100 mA g
1 and is shown in Figure 3b. For the MoS2 nanospheres, a high discharge specific capacity of ∼1095.7 mA h g
1 has been retained even after 110 cycles. The Coulombic efficiency rapidly increases to ∼97% in the third cycle and is maintained above 98% after the 15th cycle, indicating efficient transport of electrons and reversible insertion and extraction of Liþ in the radially oriented MoS2 nanospheres. In contrast, the MoS2 nanosheets show continuous and progressive capacity decay along with cycling at the same current density. After 100 cycles, the specific discharge capacity of MoS2 nanosheet electrodes can only deliver 203 mA h g
1. Furthermore, our MoS2 nanospheres also exhibit excellent cycling stability at 500 mA g
1 (Figure 3c). The capacity can be maintained as large as 1009.2 mA h g
1 even after 500 cycles. Meanwhile, as shown in Figure 4a,b, the radially oriented structures in the electrode are still detectable even after 500 cycles under the current density of 500 mA g
1, indicating the good stability of the radially oriented structure. More importantly, compared to the fresh electrode (Figure 4c), the electrode after 500 cycles (Figure 4d) can be retained well without any cracks. The results suggest that the highly open structure can effectively buffer the volume change during cycling. To evaluate the rate performance, hierarchical MoS2 nanospheres were cycled at different current densities ZHANG ET AL.

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Figure 4. (a) TEM image of the MoS2 nanosphere electrode after 500 cycles at the current density of 500 mA g
1. (b) Schematic depicting the lithiation and delithiation. (c) SEM image of the MoS2 nanosphere electrode before testing. (d) SEM image of the MoS2 nanosphere electrode after 500 cycles at the current density of 500 mA g
1.

(Figure 3d). Due to the unique structure, the MoS2 nanospheres exhibit excellent rate performance. As the current rates increase from 100 to 200, 500, 1000, and 2000 mA g
1, the MoS2 nanospheres show stable capacities of 1184.8, 1165.2, 882.7, 601.5, and 353.6 mA h g
1, respectively. Importantly, when the current density decreases to 100 mA g
1 after cycling at high current densities, a high discharge capacity of 1050.2 mA h g
1 can be recovered in another 35 cycles. The enhanced Li storage property and excellent cycling performance for MoS2 nanospheres should be associated with its unique microstructure that offers the following advantages. First, the 3D open void space in the nanospheres distributing regularly from the center to the surface along the radial direction can significantly facilitate rapid and efficient 3D transport of electrons and ions into the deep locations of the overall electrode, leading to enhanced capacities and rate capability. Meanwhile, the highly open structure can effectively buffer the volume change during cycling. Second, it has been previously reported that the expanded (002) interlayer can provide sufficient space for high rate lithium intercalation.36 Moreover, the preserved ultrathin nanosheets can provide a large number of active sites for hosting Liþ and also can greatly shorten the diffusion distance for electrons and ions, which is beneficial for the good rate cycling capacity and high Coulombic efficiency. In addition to being used as a high-energy-density battery electrode, the electrocatalytic activity of asprepared MoS2 nanospheres for the HER is also investigated. The HER activity of the MoS2 nanospheres is characterized by the typical cathodic polarization curves and corresponding Tafel plots. As shown in Figure S10a, compared to the MoS2 nanosheets, the MoS2 nanospheres exhibit a higher HER activity, with a small onset overpotential of ∼110 mV. To prove the enrichment of active sites provided by the highly oriented structure, the number of active sites for MoS2 nanospheres and nanosheets was estimated.49 As shown in Table S1, the radially oriented MoS2 nanospheres deliver a density of active sites of 0.63  10
3 mol g
1, which is higher than that of MoS2 nanosheets. More active sites in the radially oriented MoS2 nanospheres are mainly attributed to the thinner nanosheets in the spheres and the higher surface area of the highly oriented architecture. Furthermore, the turnover frequency (TOF) for each active site of the MoS2 nanospheres was calculated to be 0.89 s
1 at η = 350 mV and pH 0, which is much higher than the TOF value of MoS2 nanosheets, indicating the better intrinsic catalytic activity. The corresponding Tafel slope of the MoS2 nanospheres is 72 mV decade
1 (Figure S10b), whereas the MoS2 nanosheets possess much higher Tafel slopes of 80 mV decade
1. Exchange current density values were obtained by employing the extrapolation method

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EXPERIMENTAL SECTION Preparation of 3D Radially Oriented MoS2 Nanospheres. In a typical reaction, 1 mmol hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24 3 4H2O, Sigma-Aldrich), 14 mmol thiourea (NH2CSNH2, Sigma-Aldrich), and 0.01 mmol (0.6 g) surfactant PVP (K29
K32, Sigma-Aldrich) were dissolved in 35 mL of deionized water under vigorous stirring to form a homogeneous solution. Then, the solution was transferred into a 45 mL Teflon-lined stainless steel autoclave and kept at 220 °C for 18 h. After being cooled to room temperature, the generated precipitates were centrifuged and washed with water, ethanol, and acetone several times. The final products were dried at 80 °C under vacuum for 12 h. In contrast, PVP-free MoS2 nanosheets were synthesized by a hydrothermal method following the procedure described elsewhere.44 In short, 0.5 mmol hexaammonium heptamolybdate tetrahydrate and 7 mmol thiourea were dissolved in 35 mL of deionized water under vigorous stirring to form a homogeneous solution. Then, the solution was transferred into a Teflon-lined stainless steel autoclave with a volume of 45 mL and maintained at 220 °C for 18 h. After being cooled to room temperature, the final product was washed with water and ethanol several times and dried at 80 °C under vacuum. Structural Analyses. X-ray photoelectron spectroscopy analysis was conducted using a twin anode gun, Mg KR (1253.6 eV) (Microlab 310F Scanning Auger Microprobe, VG Scientific Ltd.). The FESEM images were taken on a JEOL JSM-6700F SEM. TEM images were taken on JEOL JEM-2100F microscopes. The discharge products were analyzed by X-ray photoelectron spectroscopy (ESCALAB 250). Thermogravimetric analysis was conducted on a TA-Q50 instrument. XRD patterns were collected on a Bruker D8 Advanced X-ray diffractometer with Ni-filtered Cu KR radiation (λ = 1.5406 Å) at a voltage of 40 kV and a current of 40 mA. Raman spectra were detected by a DXR Raman microscope with a 532 nm wavelength excitation source (Thermal Scientific Corporation, USA). Electrochemical Measurements. Lithium-ion batteries: active material (70 wt %), conductive carbon black (20 wt %, Super-P, Timcal), and poly(vinylidene fluoride) binder (10 wt %, Aldrich) in N-methylpyrrolidone were mixed into a homogeneous slurry. The obtained slurry was pasted on Cu foil by an automatic film applicator (AFA-II automatic film applicator, Shanghai Pushen Chemical Machinery Co, Ltd.), followed by drying in a vacuum oven for 12 h at 80 °C. After the electrode disks were cut to 1.4 cm diameter, the disk was used as the anode of LIBs without calendared before testing, and the

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CONCLUSION In summary, well-organized, uniform 3D open MoS2 nanospheres from 2D MoS2 nanobuilding blocks with radially oriented space have been successfully synthesized by a novel and effective hydrothermal method. The preserved few-layer MoS2 in the nanospheres with an expanded (002) plane and the highly oriented structure are beneficial for the ion and mass transport and provide enough active sites. Such intriguing architecture makes it work as a promising candidate for use in various applications. As expected, this 3D architecture shows greatly enhanced capacity and remarkable long cycling performance at high rates when used as an anode material for LIBs and high electrocatalytic activity for the HER. The success of constructing an oriented configuration integrated with 2D nanobuilding blocks may pave the way to further study and application of 2D materials.

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to the Tafel curves. As listed in Table S1, the radially oriented MoS2 nanospheres display a higher exchange current density of 5.01 μA cm
2, which is 5.3 times higher than that of the MoS2 nanosheets. The high value of j0 verifies the efficiency of the unique highly oriented structure with few-layer MoS2. To evaluate the durability of the MoS2 nanospheres in an acidic environment, potential sweeps were conducted. As shown in Figure S11, the current density exhibits only negligible loss even after 500 cycles, which might be related to the consumption of Hþ or the accumulation of H2 bubbles on the surface of the electrode that hindered the reaction.24 The enhanced electrocatalytic performance is mainly ascribed to the special structure organized by preserved few-layer MoS2 nanosheets that can offer more active edge sites and provide interconnected pathways for ion and mass transport.

average thickness of the coating is ∼0.02 mm. The Swageloktype cells were assembled in an argon-filled glovebox (O2 e 0.1 ppm, H2O e 0.1 ppm) with the coated Cu disk as working electrode and lithium metal foil as the counter/reference electrode. The electrolyte is 1 M LiPF6 in a mixture of ethylene carbonate and diethyl carbonate (1:1 by weight). Glass fiber (Whatman) was used as the separator. The typical mass loading of active materials is about 1 mg cm
2. The charge
discharge tests were performed on a LAND battery tester. CVs were obtained on a CHI 660D electrochemical workstation. Hydrogen Evolution Reaction. All electrochemical measurements were performed in a three-electrode system at an electrochemical station (CHI660B). Four milligrams of catalyst and 80 μL of 5 wt % Nafion solution were dispersed in 1 mL of 4:1 v/v water/ethanol by at least 30 min sonication to form a homogeneous ink. Then, 5 μL of the catalyst ink (containing 20 μg of catalyst) was loaded onto a glassy carbon electrode of 3 mm in diameter (loading ∼0.285 mg/cm2). Linear sweep voltammetry with a scan rate of 5 mV s
1 was conducted in 0.5 M H2SO4 (purged with pure N2) using a Ag/AgCl (in 3 M KCl solution) electrode as the reference electrode, Pt foil (4.0 cm2) as the counter electrode, and the glassy carbon electrode with various samples as the working electrode. The working electrode was mounted on a rotating disc electrode with a rotating speed of 1000 rpm during the test. All potentials were calibrated to a reversible hydrogen electrode. Conflict of Interest: The authors declare no competing financial interest. Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b05891. EDS spectra and elemental mapping images, TEM images, Raman spectra and XRD patterns of MoS2 nanospheres with different amounts of PVP, CV profiles of 3D radially oriented MoS2, and HER performance of as-prepared hierarchical MoS2 nanospheres and MoS2 nanosheets (PDF) Acknowledgment. This work was supported by the National Natural Science Foundation of China under Grant No. 51432020 and fundamental research project from the Science and Technology Commission of Shanghai Municipality No. 14JC1493000.

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