Metallic-Phase MoS2 Nanopetals with Enhanced Electrocatalytic

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Metallic-Phase MoS2 Nanopetals with Enhanced Electrocatalytic Activity for Hydrogen Evolution Jing Wang, Nan Wang, Yanzhen Guo, Jianhua Yang, Jianfang Wang, Fang Wang, Jie Sun, Hua Xu, Zong-Huai Liu, and Ruibin Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03324 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 7, 2018

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ACS Sustainable Chemistry & Engineering

Metallic-Phase MoS2 Nanopetals with Enhanced Electrocatalytic Activity for Hydrogen Evolution Jing Wang,† Nan Wang,† Yanzhen Guo,‡ Jianhua Yang,‡ Jianfang Wang,‡ Fang Wang,† Jie Sun,† Hua Xu,† Zong-Huai Liu,† and Ruibin Jiang*,† †

Key Laboratory of Applied Surface and Colloid Chemistry, Shaanxi Key Laboratory for

Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, West Chang’an Road 620, Xi’an 710062, China ‡

Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China

Email: [email protected]

KEYWORDS: Metallic phase, Electrocatalysis, Hydrogen-evolution reaction (HER), MoS2, Hydrothermal method

ABSTRACT: MoS2 shows a great promise as a low-cost and highly active alternative to platinum-based catalysts for electrochemical hydrogen production from water. The activity of MoS2 is believed from the edge, defect sites and the basal plane of metallic phase (M-MoS2). Here we report on a facile hydrothermal method for the preparation of MoS2 nanopetals that are in metallic phase and with abundant edges. The amount of sulfur precursors are found to play a

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critical role on the formation of MoS2 nanopetals. The MoS2 nanopetals exhibit remarkable electrocatalytic activities for the hydrogen evolution reaction (HER), with a low overpotential of 210 mV at the current density of 10 mA cm-2 and a Tafel slope of 44 mV per decade. The MoS2 nanopetals also display very good durability, with a very small negative shift of 11 mV at the current density of 10 mA cm-2, and negligible shift at the current density of 50 mA cm-2 after 2000 cycles. Our facile preparation method and high electrocatalytic activity of MoS2 nanopetals for HER pave a new avenue for the development of highly efficient MoS2-based electrocatalysts for HER.

INTRODUCTION Hydrogen is a promising energy carrier to substitute traditional fossil fuel because of its clean and sustainable advantage. Electrocatalytic hydrogen evolution reaction (HER) is considered as one of the most important pathways to obtain hydrogen efficiently.1 Ideal electrocatalysts for HER concurrently should possess tiny overpotentials, small Tafel slopes, and large exchange current density at low catalyst loading.2 To have widespread impact, such catalytic materials must also be low cost, electrochemically stable, and synthetically scalable with facile methods. Although Pt nanoparticles represent the current benchmark for HER electrocatalysis,3 the elemental scarcity, high cost, and poor stability against nanoparticle coarsening and agglomeration impose practical limitations and motivate the need for a next generation of alternative HER catalysts. In the past few years, many earth-abundant alternatives to Pt, including transition metal carbides,4−8 nitrides,9 borides,10,11 sulfides,12−25 phosphides,26 and selenides,27 have been explored for electrocatalytic HER.


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Among these earth-abundant HER catalysts, MoS2 has been discovered theoretically and experimentally as a very promising HER catalyst owing to its elemental abundance, high catalytic activity, and electrochemical stability.12−23 Despite the various merits of MoS2, its functional performance remains inferior to that of Pt. This is because, in its semiconducting phase which is the highest stable phase of MoS2, only the limited edges are active for HER while the large basal planes are inert for HER.12,13,20,28 To elevate the catalytic active of semiconducting phase MoS2 (S-MoS2), many efforts have been devoted to increase the edge sites through nano-structuring techniques.29,30 Besides the edge sites, the inert basal planes of S-MoS2 can be activated for HER by introducing S-vacancies, strains, and hetero-atom dopants.31−35 In comparison with S-MoS2, the metallic phase MoS2 (M-MoS2) has high conductivity and its basal planes are active for electrocatalytic HER.15,16,36 However, the metastability of M-MoS2 renders difficulty to its preparation. The preparation of M-MoS2 mainly relies on lithium intercalation and exfoliation of S-MoS2.16,36 The M-MoS2 prepared by such method suffers from the coexistence with S-MoS2 in a relative proportion of 50~80%.36 In addition, the intermediary LixMoS2 and intercalator of n-butyllithium are both dangerous materials that may self-heat and are highly pyrophoric in air.31 To data, there is only one chemical synthesis approach for the preparation of pure M-MoS2, where the M-MoS2 is prepared through hydrothermal method by using MoO3, thioacetamide and urea as presusors.16 The prepared M-MoS2 by this method is stable and have comparatively large lateral size, giving rise to limited edge sites. Nevertheless, both in S-MoS2 and M-MoS2, activities of edge sites for HER are higher than the basal planes.22 Therefore, an efficient synthesis method that can produce pure M-MoS2 with abundant edge sites in large scale is still highly desirable.


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In this work, a facile wet-chemistry method is developed for the synthesis of M-MoS2 with abundant edge sites. The M-MoS2 is synthesized through a hydrothermal method with thioacetamide (TAA) and sodium molybdate as S and Mo sources, respectively. The prepared M-MoS2 has petal-like shape with lateral size of ~75 nm and thickness smaller than 5 layers. The metallic phase and the small lateral size endow the prepared MoS2 with very high electrocatalytic activity for HER with a low overpotential of 210 mV at the current density of 10 mA cm-2 and a Tafel slope of 43 mV per decade. Our prepared M-MoS2 also exhibits very good durability for electrocatalytic HER. The ratio between S and Mo sources is found to play a critical role for the formation of M-MoS2 nanopetals. EXPERIMENTAL METHODS Synthesis of MoS2 Samples. TAA and sodium molybdate were employed to hydrothermally synthesize MoS2 samples. For the synthesis of M-MoS2 nanopetals, 0.25 mmol of Na2MoO4·2H2O and 1.6 mmol of TAA were dissolved in 40 mL of deionized water (with resistivity of 18.1 MΩ cm) under vigorous stirring for 45 min to form a homogeneous solution. The obtained solution was then transferred into a 50-mL Teflon-lined stainless steel autoclave and maintained at 200 °C for 24 h. The prepared M-MoS2 nanopetals were collected and washed with deionized water several times, followed by storage in deionized water before use. The same procedure was used to synthesize other MoS2 samples with TAA of 0.4, 0.5, and 2.4 mmol. The exfoliated MoS2 was prepared through the ultrasonication of bulk MoS2 (Aladdin, 99.95% metal basis) in the presence of cetyltrimethylammonium bromide (CTAB).37 Specifically, 50 mg of bulk MoS2 was added into aqueous CTAB solution with mass concentration of 1%. The suspension was then set into a 110-W bath sonicator for ultrasonic exfoliation. The suspension turned into blackish green liquid after 8 h ultrasonic treatment. Blackish green liquid


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was centrifuged at 1086g. The supernatant of the solution was gently decanted into another centrifugation tube and the precipitate was discarded. The supernatant was then centrifuged at 11826g to precipitate the exfoliated MoS2. The obtained exfoliated MoS2 was finally redispersed in 5 mL of deionized water for use. Characterizations. Transmission electron microscopy (TEM) imaging was performed on a JEOL JEM-2100 microscope operated at 200 kV. The high-resolution TEM (HRTEM) imaging was carried out on an FEI Tecnai G2 F20 operated at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Axis Ultra DLD (Kratos) system with Al Kα irradiation (hν = 1486.6 eV). Ultraviolet–visible–infrared absorption spectroscopy of the all samples dispersions in water were recorded using a Hitachi U-3900. The atomic force microscopy (AFM) measurements were carried out on a Bruker Dimension ICON microscope under the contact mode. Raman spectroscopy was measured using a Renishaw in Via Reflex Raman microscopy and spectrometer, and a 532-nm laser was employed for excitation. The Xray diffraction (XRD) patterns were acquired on a Rigaku Smartlab diffractometer with Cu Kα radiation (λ = 1.5406 Å). During the measurements of Raman spectroscopy and XRD patterns, the MoS2 dispersions were first deposited on glass slides and the measurements were performed before the samples were completely dried. Electrocatalytic Hydrogen Evolution. The electrochemical measurements were performed with a standard three-electrode electrochemical station (CHI 660E Instruments) using Ag/AgCl (in 3.5 M KCl solution) as the reference electrode, a graphite rod as the counter electrode, and glassy carbon electrode (5 mm in diameter) coated with catalysts as the working electrode in a rotating disk electrode (RDE) operating at 2000 rpm. The catalysts were ultrasonically dispersed


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in a water−ethanol solution (v/v 3:1) containing 0.1 wt% Nafion, and a drop of the catalysts (10 µL, 2.0 mg/mL) was then transferred onto the glassy carbon electrode serving as a working electrode. The catalyst loading was calculated to be 0.1 mg/cm2 on the glassy carbon electrode with a geometric area of 0.196 cm2. All of the measurements were performed in a N2-saturated 0.5-M H2SO4 electrolyte and measured using a linear sweep with a scan rate of 5 mV/s. Cyclic voltammograms at various scan rates (2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 mV/s) were collected in the 0.1−0.2 V vs. RHE range and used to estimate the double-layer capacitance. The electrochemical impedance spectroscopy (EIS) measurements were carried out at 220 mV overpotential with the frequency ranging from 10 MHz to 1 Hz. RESULTS AND DISCUSSION TAA and sodium molybdate are employed as S and Mo sources to synthesize MoS2 through facile hydrothermal method. In the synthesis, the amount of TAA is excess with the ratio between S and Mo sources of 6.4. Figure 1a displays the TEM image of the synthesized MoS2. The prepared MoS2 has a petal-like shape with lateral size of ~75 nm. Such a small lateral size confers numerous edge sites on the prepared MoS2, which can be observed clearly on the enlarged TEM image (Figure 1b). Because of the thermodynamic fluctuation, the obtained MoS2 nanopetals possess frizzy edges (Figure 1b). The curled edges can induce strain within MoS2, which not only can stabilize the metallic phase of MoS2, but also can enhance the electrocatalytic activity for HER.21,33 Figure 1c shows the HRTEM taken on the edge of the MoS2 nanopetals. The fringes of MoS2 layers can be clearly observed. The spaces between layers are about 0.63 nm and 0.62 nm, which are consistent with the value in bulk MoS2. One can find from the fringes of layers that number of the MoS2 layers is less than five. The lattice fringes


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corresponding to {100} facets of MoS2 can also be observed clearly on the HRTEM image (Figure 1d). AFM was employed to identify the thickness of our prepared MoS2 nanopetals (Figure 1e,f and Figure S1 in the Supporting Information). It can be found from AFM images that the prepared MoS2 has exceedingly abundant edges and crumpled surface, which can enhance the MoS2 intrinsic electrocatalytic activity for HER regardless of continuous operation or elevated temperature.21

Figure 1. Morphology characterization of the prepared M-MoS2 nanopetals. a,b) TEM images with different magnifications. c,d) HRTEM images taken on M-MoS2 nanopetals at different regions. e) AFM image taken on a M-MoS2 nanopetal. f) Height profile extracted from the solid line indicated in (e). In order to ascertain the phase of our prepared MoS2 nanopetals, the atomic resolution TEM was carried out on the MoS2 nanopetals. From the HRTEM image (Figure 2a), the MoS2 nanopetals contain both semiconducting and metallic phase MoS2. The fast Fourier transform (FFT) on the HRTEM image displays two sets of hexagonal spots. One set of hexagonal spots


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rotates 30° with respect to the other (Figure S2 in the Supporting Information), indicating that the MoS2 nanopetals are composed of semiconducting and metallic phase.38,39 Two phases of MoS2 can be clearly observed on the inverse FFT image (Figure 2b). The metallic phase of MoS2 on the inverse FFT image has trigonal lattice areas owing to the octahedral coordination of Mo atoms, whereas the semiconducting phase shows honeycomb lattice on the inverse FFT because of the trigonal prismatic coordination. The metallic phase of MoS2 is a metastable state and gradually transforms into semiconducting phase under dry state by annealing or irradiation of Xray, laser, and electron beam.16,40 Therefore, the presence of semiconducting phase in the MoS2 nanopetals may arise from the phase transformation induced by the electron beam irradiation.

Figure 2. Structural characterization of M-MoS2 nanopetals. a) HRTEM image. b) Inverse FFT image of (a). The region in the orange lines is the metallic phase. The region in the red triangle is semiconducting phase. c) XRD patterns of M-MoS2 and exfoliated MoS2. d) Raman spectra of M-MoS2 and exfoliated MoS2. e) XPS of Mo 3d electrons in M-MoS2 and exfoliated MoS2. f) Normalized absorption spectra of M-MoS2 and exfoliated MoS2.


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Although the metallic and semiconducting phases of MoS2 have very similar XRD patterns in dry state, their XRD patterns in wet state are distinctly different from each other.16,41 The wet metallic phase MoS2 exhibits two distinct diffraction peaks at angle smaller than 20°, while only one diffraction peak appears in this angle region on the diffraction pattern of semiconducting phase MoS2.16,41,42 We therefore carried out XRD characterization on the prepared MoS2 nanopetals and exfoliated MoS2 under wet state. The wet exfoliated MoS2 shows a diffraction peak at 13.9° (Figure 2c), which corresponds to the diffraction from (001) facets. In contrast, the wet MoS2 nanopetals display two distinct diffraction peaks at 8.8° and 17.8° (Figure 2c). The peak at 8.8°, which corresponds to a spacing of 10.04 Å, is identified as (001)-H2O for MoS2H2O. Since the layer thickness of MoS2 is ~6.2 Å, the space between two adjacent MoS2 layers would be 3.84 Å. As the thickness of a monolayer of water is 2.6 Å, the space between two adjacent MoS2 layers is larger than the thickness of a monolayer of water but smaller than the thickness of a double layer of water. Hence, the double layer of adsorbed water is broken and partial water molecules are evaporated during the preparation of XRD sample and XRD measurements. The peak located at 17.8° is assigned to the second-order diffraction of MoS2H2O. When the M-MoS2 is dried, only one weak X-ray diffraction peak appears at angle smaller than 20° (Figure 2c), indicating the desorption of the adsorbed water.16 As a result, the XRD characterization definitely indicates that our prepared MoS2 nanopetals are metallic phase. The most notable difference between M-MoS2 and S-MoS2 is the symmetry of the sulfur in their structures. The structural variations result in significant differences in their characteristic Raman features. Figure 2d shows the Raman spectra of the prepared MoS2 nanopetals and the exfoliated MoS2, which TEM image is shown in Figure S3 in the Supporting Information. The Raman spectrum of the exfoliated MoS2 shows two strong peaks at 381 cm-1 and 406 cm-1 and a


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weak peak at 285 cm-1, which are the characteristic, E12g , A1g and E1g modes of semiconducting MoS2, respectively.43 Compared with the exfoliated MoS2, the E12g mode is absent on the Raman spectrum of the MoS2 nanopetals, and the A1g mode becomes weak (Figure 2d). The E12g mode has been found to be inactive in octahedrally coordinated MoS2,43 i.e. the M-MoS2. Moreover, besides the weak A1g mode, two strong Raman bands are observed at 152 cm-1 and 324 cm-1, and one additional weak band appears at 221 cm-1. These three Raman bands have been demonstrated theoretically and experimentally in metallic phase MoS2.43,44 Both the absence of the E12g mode and the appearance of the three bands substantiate that our prepared MoS2 nanopetals belong to metallic phase. In addition, it is found that the Raman spectrum of the MMoS2 nanopetals after stored in water for 70 days is almost identical to that of the fresh prepared ones (Figure S4 in the Supporting Information), indicating that the M-MoS2 nanopetals are very stable in water. The remarkable stability of M-MoS2 in water is attributed to the stabilizing function of the adsorbed water molecules on the surface of the M-MoS2 layers.16,41 The phase of the prepared MoS2 nanopetals was further identified by using XPS (Figure 2e). For exfoliated MoS2, Mo 3d spectrum consists of peaks around 229.9 eV and 233.1 eV corresponding to the Mo 3d5/2 and 3d3/2 components, respectively. In contrast, the two peaks of Mo 3d in the MoS2 nanopetals are red shift to ~228.9 eV and ~232.1 eV, with shifting energy ~1 eV. This result is consistent with the relaxation energy of 1.0 eV for the conversion of semiconducting phase to metallic phase.42 The shift of Mo 3d toward lower binding energy confirms the metallic phase of the prepared MoS2 nanopetals.16,45 In addition, the Mo 3d peaks of the MoS2 nanopetals exhibit weak shoulders at the positions as those in the exfoliated MoS2. The details for the peak deconvolution are shown in Figure S5 in the Supporting Information. Similar to the HRTEM results, the presence of semiconducting phase in the M-MoS2 nanopetals results


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from the transformation of M-MoS2 into S-MoS2 in dry state under X-ray illumination.40,41 Similarly, the S 2p3/2 and 2p1/2 peaks of the prepared MoS2 nanopetals are located at 161.9 eV and 162.9 eV, respectively, which also shift toward lower binding energy ~1 eV with respect to the corresponding peaks in the exfoliated MoS2 (Figure S6 in the Supporting Information). Therefore, the XPS spectra of Mo 3d and S 2p also show that our prepared MoS2 nanopetals are metallic phase. Since the energy band structures between semiconducting and metallic phase MoS2 are different from each other, their optical properties are therefore clearly distinct. Figure 2f gives the absorption spectra of the exfoliated MoS2 and the prepared MoS2 nanopetals. The exfoliated MoS2 shows four absorption peaks, which is typical absorption characteristics of S-MoS2.46 The two peaks located at high energy originate from the interband transitions, while the two peaks at 610 nm and 672 nm result from the energy split of valence-band and spin-orbital coupling.46,47 In contrast, the prepared MoS2 nanopetals do not exhibit any absorption peaks with wavelength longer than 300 nm, which is the characteristic absorption feature metals. Therefore, all of above structural characterizations, including HRTEM, XRD, Raman, XPS, and UV-visible light absorption, indicated definitely that our prepared MoS2 nanopetals are M-MoS2. Because of their metallic phase and abundant edges, the prepared MoS2 nanopetals should have high electrocatalytic activity for HER. We therefore carried out HER measurements on the MoS2 nanopetals and exfoliated MoS2 deposited on glassy carbon electrodes using a threeelectrode cell in a 0.5-M sulfuric acid electrolyte. Figure 3a show the polarization curves of the MoS2 nanopetals and the exfoliated MoS2 compared with those of bulk MoS2 and Pt/C. At the geometric catalytic current density (j) of 10 mA/cm2, the overpotential of the MoS2 nanopetals is 210 mV, which is exceedingly smaller than those of the exfoliated MoS2 (472 mV) and bulk


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MoS2 (553 mV). From the extrapolation of the linear region of overpotential versus log[j] (Figure 3b), we obtained Tafel slopes of 44, 192, 294, and 34 mV per decade for the M-MoS2 nanopetals, exfoliated MoS2, bulk MoS2, and Pt/C, respectively. Such low Tafel slope value of 44 mV/decade for M-MoS2 nanopetals suggest a two-electron transfer process,48,49 which is very close to the values obtained on previously reported M-MoS2.16,37 The small overpotential and Tafel slope evince that the prepared M-MoS2 nanopetals have high electrocatalytic activity for HER.

Figure 3. Electrocatalytic HER performance of the synthesized M-MoS2 nanopetals. a) Polarization curves after iR correction for M-MoS2 in comparison with bulk MoS2, exfoliated MoS2 and Pt/C. b) Corresponding Tafel plots obtained from the polarization curves. c) Charging current density differences plotted against scan rates. The linear slop, equivalent to twice of the double-layer capacitance, Cdl, was used to represent the ECSA. d) Durability measurement of MMoS2. The polarization curves were recorded initially and after 1,000 sweeps between -0.1 V


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and -0.5 V (versus RHE) at 100 mV/s. All the HER measurements were conducted in an N2saturated 0.5-M H2SO4 electrolyte at 25 °C. To further probe the HER performance of the prepared M-MoS2 nanopetals, we measured the double-layer capacitance (Cdl) derived from the cyclic voltammogrametry measurement results (Figure S7 in the Supporting Information), which can be used to reflect the effective electrochemically active surface area. Capacitive current was plotted as a function of scan rate to extract the Cdl values (Figure 3c). The M-MoS2 nanopetals have a Cdl value of 10.5 mF/cm2, which is two orders of magnitude larger than that of the exfoliated MoS2. The larger Cdl value indicates that the M-MoS2 nanopetals expose more catalytically active sites than the exfoliated MoS2 with the same catalyst loading. This result is consistent with the fact that our prepared MoS2 nanopetals are in metallic phase and have abundant edges. Both the abundant edges and the basal planes of M-MoS2 are active sites for electrocatalytic HER.22,29,31,36 Electrochemical impedance spectroscopy was also employed to provide further insight into the electrode kinetics during HER. The Nyquist plots for the different MoS2 samples are shown in Figure S8 in the Supporting Information. The charge transfer resistances (Rct) are extracted by fitting the Nyquist plots using a Randles equivalent circuit. A much smaller Rct of 7 Ω is obtained on the M-MoS2 nanopetals in comparison with the exfoliated and bulk MoS2 (Table S1 in the Supporting Inforamtion), suggesting a much faster electron transfer and a higher Faradaic efficiency and thus superior HER kinetics. Such smaller Rct of the M-MoS2 nanopetals is attributed to their metallic phase and abundant edges. Furthermore, the long-term cycling stability of the M-MoS2 nanopetals for HER was investigated. After 2,000 cycles of continuous operation, the M-MoS2 nanopetals exhibit a slightly decay of the HER performance (Figure 3d). At the current density of 10 mA/cm2, the overpotential shows a small cathodic shift of ~11 mV, which only corresponds to a decay of 5%. It should be noted that the decay of the HER performance is even smaller at


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larger current density (Figure 3d). The very good long-term stability of the M-MoS2 nanopetals for HER is also reflected by the chronopotentiometry at the current density of 10 mA/cm2 (Figure S9 in the Supporting Information). We further studied the effect of the molar ratio between S and Mo sources (S/Mo ratio) on the formation of M-MoS2 nanopetals and the HER performances of the obtain MoS2 samples (Figure 4). The S/Mo ratios of 9.6, 6.4, 2.0, and 1.6 were investigated. The S/Mo ratio of 6.4 gives the M-MoS2 nanopetals mentioned above. When the S/Mo ratio is further increased to 9.6, the prepared MoS2 has broken petal-like shape (Figure 4a). In contrast, when the S/Mo ratios are 2.0 and 1.6, MoS2 nanosheets form flower-like nanoparticles (Figure 4c and 4d). The size of nanoparticles increases with the reduction of S/Mo ratio. Although the S/Mo ratios of the sources are much different, XPS analysis shows that the ratios of the prepared MoS2 samples are very close to 2. They are 2.16, 2.02, 1.86, and 1.81 for the MoS2 samples obtained from S/Mo ratios of 9.6, 6.4, 2.0, and 1.6, respectively. To identify the structures, we carried out XRD, Raman, XPS, and UV-visible absorption characterizations on the prepared MoS2 samples (Figure 4e,f, Figure S10−S12 in the Supporting Information). The characterization results indicate clearly that MoS2 samples obtained with S/Mo ratios of 9.6 and 6.4 are in metallic phase, while the samples prepared with S/Mo ratios of 2.0 and 1.6 are in semiconducting phase. Therefore, as the S/Mo ratio is increased, the obtained MoS2 gradually changes from semiconducting phase to metallic phase (Figure 4g). When the S source is excess, the hydrolysis of TAA creates many nuclei in the growth solution. During the growth of these nuclei, the surplus S could bind at the formed MoS2 edges and thus hinders the lateral growth of the MoS2 nanosheets.16 That is why the lateral size of the obtained MoS2 samples decrease with the increase of S/Mo ratio. Owing to the fluctuation of thermodynamics, the edges of very small MoS2 nanosheets would curl up, giving


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rise to petal-like shape. Simultaneously, the strain induced by the curl of MoS2 nanopetals can stabilize the metallic phase,21,32,33 leading to M-MoS2 nanopetals. In contrast, as the S source is reduced, the number of the nuclei created by the hydrolysis is small. The edges of MoS2 nuclei cannot be hindered any more. The MoS2 nuclei would grow up and form follower-like nanoparticles with large size. Owing to the lack of strain stabilization, the M-MoS2 transforms into S-MoS2 during the growth.

Figure 4. MoS2 samples prepared with different amounts of TAA. a−d) TEM images of MoS2 samples obtained with ratios between S and Mo sources of 9.6, 6.4, 2.0, and 1.6, respectively. e) XRD patterns of the MoS2 samples shown in (a−d). f) Raman spectra of the MoS2 samples shown in (a−d). g) Schematic of the phase change of MoS2 with the increase of TAA amount. h)


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Polarization curves for the MoS2 samples shown in (a−d). For comparison, Polarization curve of Pt/C is also shown. i) Corresponding Tafel plots of the MoS2 samples shown in (a−d). The HER electrocatalytic performances of the prepared MoS2 samples were also investigated. Figure 4h shows the polarization curves of the four MoS2 samples. At the geometric catalytic current density of 10 mA/cm2, the overpotentials of MoS2 samples synthesized from S/Mo ratios of 9.6, 6.4, 2.0, and 1.6 are 210, 210, 393, and 442 mV, respectively. The Tafel slopes are 48, 44, 108, and 133 mV per decade for the MoS2 samples prepared from S/Mo ratios of 9.6, 6.4, 2.0, and 1.6, respectively (Figure 4i). Clearly, the M-MoS2 samples exhibit higher electrocatalytic activity for HER than the S-MoS2. In addition, the M-MoS2 samples have larger Cdl values than the S-MoS2 samples (Figure S12 in the Supporting Information), indicating that the M-MoS2 nanopetals expose more catalytically active sites than the S-MoS2 ones. We also studied the current density calculated according to the electrochemically active surface area (ECSA). The ECSA-corrected current densities of the M-MoS2 nanopetals are still higher than the S-MoS2 samples (Figure S14 in the Supporting Information), indicating that the M-MoS2 nanopetals have higher intrinsic activity than the S-MoS2 samples. The impedance analysis shows that both M-MoS2 samples have smaller Rct than those of the two S-MoS2 samples (Figure S15 and Table S1 in the Supporting Information), suggesting a much faster electron transfer and a higher Faradaic efficiency on the M-MoS2 samples. CONCLUSIONS In conclusion, we report a new facile approach for the synthesis of M-MoS2 nanopetals in large scale and study their electrocatalytic activity for HER. TAA and sodium molybdate are employed as S and Mo sources, respectively. When TAA amount is excess, the obtained MoS2 nanosheets have petal-like shape with curled edges. The lateral size of the MoS2 nanopetals is


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smaller than 100 nm and therefore their edges are numerous. The MoS2 nanopetals are found in metallic phase. Both the metallic phase and abundant edges endow the MoS2 nanopetals with superior electrocatalytic activity for HER. The very small overpotential of 210 mV is obtained at the geometric current density of 10 mA/cm2. The corresponding Tafel slop is 44 mV per decade, which is exceedingly smaller than S-MoS2. The M-MoS2 nanopetals also display excellent durability during the HER process. In addition, the amount of S source is found to play a pivotal importance on the formation of M-MoS2 nanopetals. Only when the amount of TAA is excess, can the M-MoS2 nanopetals be produced. Our findings provide a facile method for the preparation of M-MoS2 with abundant edges and very good stabilities in large scale and thereby will be very helpful for the applications of MoS2 in renewable energy and heterogeneous catalysis. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxx. AFM imges, TEM images, Raman spectra, XPS, cyclic voltammetric cureves, impedence curves. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: 86-29-85310780 Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (61775129 and 61505102), Fundamental Research Funds for the Central Universities (GK201602004). REFERENCES (1)

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Table of Contents Graphic

Synopsis: MoS2 nanopetals with metallic phase and abundant edges are prepared via a facile hydrothermal method. They exhibit high activity and stability for hydrogen evolution reaction.


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Metallic-Phase MoS2 Nanopetals with Enhanced Electrocatalytic

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