Hydrogen Evolution Reaction on Hybrid Catalysts of Vertical MoS2

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Hydrogen Evolution Reaction on Hybrid Catalysts of Vertical MoS2 Nanosheets and Hydrogenated Graphene Xiuxiu Han, Xili Tong, Xingchen Liu, Ai Chen, Xiaodong Wen, Nianjun Yang, and Xiang-Yun Guo ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03316 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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Hydrogen Evolution Reaction on Hybrid Catalysts of

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Vertical MoS2 Nanosheets and Hydrogenated Graphene

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Xiuxiu Han,†,‡ Xili Tong, †,* Xingchen Liu, † Ai Chen,§ Xiaodong Wen, † Nianjun Yang,†,ǁ,* Xiang-Yun

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Guo†

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State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences,

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Taiyuan 030001, China ‡

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§

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The University of Chinese Academy of Sciences, Beijing 100049, China

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The First Hospital, Shanxi Medical University, Taiyuan 030001, China

Institute of Materials Engineering, University of Siegen, Siegen 57076, Germany

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EMAIL ADDRESS: [email protected] (X. T.), [email protected] (N. Y.)

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ABSTRACT: Two-dimensional (2D) molybdenum sulfide (MoS2) is an attractive noble-metal-free

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electrocatalyst for the hydrogen evolution (HER) in acids. Tremendous effort has been made to engineer

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MoS2 catalysts with either more active sites or higher conductivity to enhance their HER activity.

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However, little attention has been paid to synergistically structural and electronic modulations of MoS2.

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Herein, 2D hydrogenated graphene (HG) is introduced into MoS2 ultrathin nanosheets for the

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construction of a highly efficient and stable catalyst for HER. Owing to synergistic modulations of both

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structural and electronic benefits to MoS2 nanosheets via the HG supporting, such a catalyst has an

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improved conductivity, more accessible catalytic active sites, and moderate hydrogen adsorption energy.

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On the optimized MoS2/HG hybrid catalyst, HER occurs with an overpotential of 124 mV at 10 mA ACS Paragon Plus Environment

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cm−2, a Tafel slope of 41 mV dec−1, and a stable durability for 24 h continuous operation at 30 mA cm−2

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without observable fading. The high performance of the optimized MoS2/HG hybrid catalyst for HER

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was interpreted with Density Functional Theory (DFT) calculations. The simulation results reveal that

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the introduction of HG modules electronic structure of MoS2 to increase of the number of the active

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sites and simultaneously optimizes the hydrogen adsorption energy at S edge atoms, eventually

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promoting the HER activity. This study thus provides a strategy to design and develop high performance

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HER electrocatalysts by employing different 2D materials.

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KEYWORDS: Hydrogen evolution reaction, MoS2 nanosheets, Hydrogenated graphene, Solvothermal

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synthesis, Hydrogen adsorption energy

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INTRODUCTION

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As a green renewable source, hydrogen has been widely considered as one of the most promising

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alternatives to fossil fuels in future. Over past years, the generation of hydrogen via hydrogen evolution

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reaction (HER: 2H+ + 2e− →H2) from water splitting has attracted extensive attention and significant

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achievements have been obtained.1 Water splitting in acids has been emerged as the primary method of

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hydrogen production.2 In acidic media different reactions involved in three mechanisms have been

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suggested for HER, namely (i) Volmer reaction (H3O+ + e− →Hads) with a Tafel slope of 120 mV dec−1,

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(ii) Heyrovsky reaction (H3O+ + Hads + e− →H2) with a Tafel slope of 40 mV dec−1, and (iii) Tafel

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reaction (2Hads →H2) with a Tafel slope of 30 mV dec−1.3-4 It is quite obvious that the hydrogen

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adsorption (Hads) is of vital importance for the HER kinetics since Hads takes part in each

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electrochemical reaction step in the course of HER. Moreover, it has been demonstrated in Sabatier

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principle that optimal catalysts feature moderate Hads energies for HER.5 For example, due to the

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suitable Hads energy on the catalytic active sites of Pt-based materials, Pt-based materials have been

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proven to be the most active catalysts for HER up to date. On the other side, their high cost, limited ACS Paragon Plus Environment

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crustal abundance, and low durability hinder their industrial mass applications. Therefore, cheap and

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abundant HER catalysts with the outstanding performance are highly needed. To design and develop

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those catalysts, how to optimize Hads energy is of the first consideration.

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With respect to cheap and abundant HER catalysts, various kinds of noble-metal-free catalysts have

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been explored very recently, including phosphides,6-8 carbides,9-10 nitrides,11-12 selenides,13-15 borides,16

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and sulfides.17-20 Among them, the employment of the unique and indispensable two-dimensional (2D)

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materials as HER catalysts has gained great attention.21 Taking MoS2 as an example, it is a typical 2D

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transition-metal-dichalcogenides and owns several S-Mo-S layers through van der Waals interactions.

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Due to its natural abundance, good catalytic activity, and edge-terminated structure, it has been

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considered as a representative and potential HER catalyst.22-23 On the other side, the catalytic

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performance of MoS2 is significantly hindered by several severe obstacles: i) sluggish reaction kinetics

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at high current densities, due to its low electrical conductivity; ii) poor utilization of active sites,

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resulting from the severe stacking of nanosheets via van der Waals attractions; iii) low Hads energy on

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the S edge sites of MoS2, resulting from a high electron number on S atom.22, 24-25 Note that, the Hads

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energies for most of MoS2 based catalysts have not well-clarified so far.

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Herein, we introduce the employment of vertically ultrathin MoS2 nanosheet arrays grown on

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hydrogenated graphene (HG) as a noble-metal free catalyst for HER. In this way, the adequate

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ferromagnetism of HG optimizes the electronic structure of MoS2. This originates from the formation of

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unpaired electrons in graphene, together with the remnant delocalized π bonding network.26-27 Due to its

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unique nanostructure, and the most importantly the suitable Hads energy on MoS2, this MoS2/HG hybrid

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catalyst is expected to be one of the most powerful, efficient, and stable catalysts for HER. To the best

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of our knowledge, there are no reports about the design of such a MoS2/HG hybrid catalyst. The

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catalytic ability of such a hybrid towards HER and its composition influence on HER has not been

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shown.

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In this contribution, we report about the synthesis of such a hybrid catalyst and its performance for HER.

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The weight-ratio of MoS2 to HG and the morphology of MoS2 are controlled during the synthesis ACS Paragon Plus Environment

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process. The optimized MoS2/HG hybrid catalyst delivers an overpotential of 124 mV at 10 mA cm−2

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and a Tafel slope of 41 mV dec−1, as well as excellent stability. For example, no current decay was

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found over 24 h at 30 mA cm−2 for continuous operation in acidic conditions. To interpret the high HER

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performance of these hybrid catalysts, Density Functional Theory (DFT) calculations were conducted,

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focusing on the electronic structures of MoS2 on HG and the Hads energy of the optimized hybrid

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catalysts. The simulation results prove that HG adjusts Hads energy of MoS2 to a suitable degree as well

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as modules the electronic structure between MoS2 and HG, namely gives a rise of the additional active

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sites.

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EXPERIMENTAL

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Chemicals and reagents

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All chemicals were purchased from Shanghai Chemical Reagent Co. Ltd (Shanghai, China). The

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reagents were of analytical grade and used as received without further purification. Graphene oxide was

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synthesized from natural graphite by a modified Hummer’s method.28 HG was prepared through a

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modified cathodic electrochemical exfoliation method (see the supporting material).29

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Synthesis of the MoS2-based catalysts

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The synthesis process of the MoS2/HG hybrid catalyst is briefly illustrated in Scheme S1. First, 25 mg of

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HG was dispersed in 10 mL of dimethylformamide (DMF) under sonication. Then, 55 mg of

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(NH4)2MoS4 was added into the aforementioned solution and sonicated at room temperature for 1 h until

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a black homogeneous slurry was achieved. Subsequently, the resulting mixture was transferred to a 100

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mL Teflon-lined autoclave and heated at 200 °C in an oven for 12 h, followed by natural cooling to

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room temperature. In the next step, the black product was collected by centrifugation at 10 000 rpm for 3

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min. The collection was further washed with distilled water and ethanol repeatedly for 6 times. The

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obtained MoS2/HG was dried at 60 °C in vacuum oven for overnight. For comparison, MoS2/RGO was

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prepared once graphene oxides instead of HGs were added. ACS Paragon Plus Environment

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Characterization The structure and morphology analysis of the catalysts were conducted using a JEOL JEM-2100F highresolution transmission electron microscope (HRTEM) as well as a JSM-7001F field emission scanning electron microscope (FESEM) equipped with an energy dispersive spectroscopic (EDS) detector. The phases of the catalysts were determined using a Bruker D8 advance A25X-ray diffraction (XRD) system and Cu-Kα radiation in the 2θ range from 5o to 90o. Raman spectra were collected on a HORIBA Jobin Yvon LabRAM HR800 spectrometer. A 514.5-nm Ar laser was used as the excitation source. The total H content of the prepared HG and oxygen content of the obtained RGO were measured by a Vario EL cube elementary analysis system (EA). The Mo contents in the catalysts were determined by a Thermo iCAP 6300 inductively coupled plasma optical emission spectrometer (ICP-OES). X-ray photoelectron spectroscopy (XPS) was performed at a Kratos Axis Ultra DLD spectrometer. An Al Kα (hk = 1486.6 eV) X-ray source was employed as the excitation source. The fittings of the obtained XPS spectra were performed using the XPSpeak41 simulation software. The baseline of the XPS spectrum was corrected by the Shirley-type background with a zero slope. Peaks in the high-resolution XPS spectra for C 1s, Mo 3d and S 2p were de-convoluted using a Gaussian-Lorentzian mixed function (Gaussian: 70%, Lorentzian: 30%). Thermogravimetry-mass spectrometry (TG-MS) was recorded with a SETARAM SETSYS Evolution 16/18 TG and PFEIFFER OMNI star MS at a heating rate of 5 oC min−1 in air. Fourier transform infrared (FTIR) spectrometry was obtained on a Bruker Tensor 27 spectrometer. Hydrogen temperature programmed desorption (H2-TPD) experiments were performed in a tubular quartz reactor. The outlet gases were detected by an HPR20 mass spectrometer. About 200 mg sample was reduced at 200 °C for 2 h in a pure H2 atmosphere with a heating rate of 10 °C min−1. They were then cooled down to room temperature in the same atmosphere. The temperature was kept isothermal for 30 min. To remove physisorbed and/or weakly bound species, the sample was cleaned with nitrogen gas flow at a flow rate of 30 sccm for 30 min. TPD was performed by heating the sample from room temperature to 800 °C at a ramp rate of 5 °C min−1 under nitrogen atmosphere. ACS Paragon Plus Environment

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Electrochemical Measurements HER electrocatalytic activity of the catalysts was measured at room temperature using a CHI 760E electrochemical workstation (Shanghai, China) in a standard three-electrode mode. Saturated calomel electrode (SCE) and graphite rod acted as the reference and counter electrodes, respectively. All potentials throughout this paper were calibrated to the reversible hydrogen electrode (RHE) scale (see supporting materials). An ink coated glassy carbon disk electrode (5 mm in diameter) was served as the working electrode. To form a homogeneous ink, 5 mg catalysts and 20 µL of 5 wt% Nafion solution were mixed in 1 mL of water/ethanol (v/v = 4:1) by sonication for 30 min. Then, 5 µL of the ink was loaded onto a rotating glassy carbon disk electrode by hand dropping. The loading density of the catalyst was about 0.127 mg cm−2. Linear sweep voltammetry (LSV) was conducted in the range of 150 to −300 mV (vs. RHE) at a sweep rate of 5 mV s−1 in 0.5 M H2SO4 solution. Prior to experiments, the solution was purged with pure H2 for at least 30 min. The overpotentials were obtained at the reduction current density of 10 mA cm−2. Tafel plots were drawn from the overpotentials (η) as a function of the logarithm scales of the current density (logj). Tafel slopes (b) were obtained by fitting the linear portions of the Tafel plots according to Tafel equation (η = a + blogj). All polarization curves were iR-corrected with a compensation level of 90%. The durability tests of the catalysts were conducted by use of chronoamperometry. The running time was 24 h and the used overpotential was 176 mV. To investigate the cycling stability of the catalysts, cyclic voltammetry (CV) was conducted in the potential range of −300 to 30 mV (vs. RHE). The scan rate was 50 mV s−1 and the cycle number was 20 000. The electrochemical impedance spectroscopy (EIS) was carried out on a Zahner electrochemical workstation (Germany) over a frequency range from 105 to 1 Hz with a 5 mV amplitude. The impedance spectra were recorded at an overpotential of 124 mV and further fitted using simplified Randles circuit for the estimation of the series and charge-transfer resistances. ACS Paragon Plus Environment

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Computational Method Theoretical calculations were performed within DFT framework implemented in the Vienna ab initio simulation package (VASP).30-31 The electron-electron interactions were treated with the generalized gradient approximation (GGA) exchange-correlation functional in the Perdew-Burke-Ernzerhof (PBE) form.32 The electron-ion interactions were calculated using the projector augmented wave method.33 The Brillouin zone was sampled using a Monkhorst-Pack k-point set of 7×7×1 for HG and oxidized graphene (OG), and a set of 2×2×1 for Mo6S12 supported on HG and OG. All atoms are relaxed with any constraint. To account for the potential magnetic nature of the materials, spin-polarization was included. The wave functions were built from a plane-wave set with a maximum energy cutoff of 400 eV. The convergence criterions for the electronic and geometric relaxation were set to be 1.0×10−5 eV and 0.02 eV Å−2, respectively. Monolayers of HG and OG were modeled using the zigzag configuration with the ratio of H:C to O:C of 2:8 (25%).34 The adsorption energy (Eads) of one H atom is defined as Eads = EH/cat − (Ecat + EH), where EH/cat is the total energy of the catalyst with one H atom adsorption, Ecat is the total energy of the bare catalyst and EH is the total energy of a free H atom. These adsorption energies were derived from full geometry optimizations. Therefore, they reflect any structural changes in catalysts induced by adsorption.

RESULTS AND DISCUSSION Construction of the MoS2/HG hybrid catalyst For the construction of the MoS2/HG hybrid catalyst, graphene can be obtained from the reduction of graphene oxide, or dehydrogenation from HG. The morphological and compositional analysis of the synthesized HGs and RGO are summarized in Figure S1. The hydrogen content on the HG, determined using combustible elemental analysis, is 3 wt%. The oxygen content on the reduced graphene oxide (RGO) is as high as 20 wt%, determined by means of pyrolytic elemental analysis. From their SEM images (Figure S1-A, C), one can see that RGO possesses a rougher and more curved surface than HG, 7 ACS Paragon Plus Environment

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probably resulting from RGO aggregation. As determined from their TEM images, the number of graphene layers for HG is only 5 (Figure S1-B), less than that (7 layers) for RGO (Figure S1-D). Therefore, HG features more advantages over RGO. First, the synthesis of HG avoids the destruction of hexatomic rings and the loss of conjugated electron.29 Second, the super hydrophobic surface of HG substrate obviously benefits the release of gas bubble, which accelerates the HER kinetics.35 Third, the adequate ferromagnetism of HG optimizes the electronic structure of MoS2 when MoS2 is combined with HG.

Structure of the MoS2/HG hybrid catalyst The morphology and properties of constructed MoS2/HG hybrid catalysts were characterized using electron microscopes (Figure 1) and compared with the MoS2/RGO catalysts. Figure 1A shows the SEM image of as-prepared MoS2 ultrathin nanosheets grown on the surface of HGs. MoS2 nansheets are highly ordered and uniformly distributed on the surface of HG, leading to the formation of a porous structure. The agglomerating and stacking of MoS2 nanosheets are not visible, indicating that the generated reactive seeds are abundant and uniformly distributed. As revealed in the high-magnification SEM image (inset of Figure 1A), wrinkled MoS2 nanosheets are vertically oriented and interconnected with each other. The lateral sizes are around 100-200 nm and the sizes of the formed nanopores are about 100 nm. The interconnected porous structure is promising to provide efficient direct routes for ion/electron transport and then is possible to ensure the participant of every nanosheet in the HER process. In contrast, MoS2 nanosheets on the RGO surface tend to stack into the flower-like and nonuniformed agglomerations, originating from the nonhomogeneous distribution of oxygen defects on RGO (Figure S3).36-37 Without HG or RGO, pure MoS2 nanosheets assemble into microspheres via van der Waals attractions (Figure S4). The representative TEM image of the MoS2/HG hybrid catalyst is shown in Figure 1B, where reveals similar observation with SEM image shown in Figure 1A. The light contrast in various areas of the TEM image proves the vertically aligned nature of thin nanosheets. This is because the surface energy of the ACS Paragon Plus Environment

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MoS2 edge sites is almost 2 orders of magnitude higher than that of the terrace sites. The interfacial energy between vertical MoS2 nanosheets and the HG substrate is much smaller than that of the horizontal MoS2 with the HG substrate. Ultimately, the competition between the surface energy and interfacial energy of the MoS2 structure determines the preferred growth orientation.38 Meanwhile, the growth of a vertically aligned MoS2 nanosheet on the surface of HG is energetically preferable to the growth of a larger-area horizontal structure. This is because the strain energy induced by 2D growth is released by the expansion in vertical direction.39 The parallel lines in the high-resolution TEM image of the curled and folded edge (Figure 1C) correspond to the layers of MoS2 nanosheets. For those MoS2 nanosheets, they are composed of 4-7 layers. The distance between the two S-Mo-S layers is about 0.65 nm at the edges. This value is in good agreement with the thickness of one single MoS2 layer, corresponding to the (002) plane of 2H MoS2.40 Note that, RGO support is much thicker (e.g., more than 12 layers, Figure S3-B) than MoS2 layers. A thinner MoS2 layer is thus expected to provide more accessible S edge sites in the MoS2/HG hybrid catalyst for HER.41 Meanwhile, the MoS2/HG hybrid catalyst contains two sets of diffraction signals (inset of Figure 1C): the diffraction rings (assigned to polycrystalline MoS2) and the diffraction spots (indexed as good crystalline graphene sheets).42-43 Furthermore, the EDS element mappings (Figure 1D) reveal the uniform distribution of Mo, S, and C elements on the whole surface of the MoS2/HG hybrid catalyst. The atomic ratio of Mo element to that of S element, calculated from the corresponding EDS spectrum (Figure S5), is about 1:2.1, fully in line with the stoichiometry of MoS2. All these electron microscope results confirm the successful introduction of high quality vertical and ultrathin MoS2 nanosheets to the entire surface of HG, namely the successful synthesis of the MoS2/HG hybrid catalyst with optimized structures and interfacial energies.

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Figure 1. (A) FE-SEM, (B) TEM, and (C) HRTEM images of the MoS2/HG hybrid catalyst, (D) SEM image and corresponding EDS elemental mapping of C (red), Mo (green), and S (yellow) elements in the MoS2/HG hybrid catalyst. The insets of (A) and (C) show the high-magnification SEM image and the corresponding SAED pattern for the MoS2/HG hybrid catalyst, respectively.

The phase of the MoS2/HG hybrid catalyst was obtained by the XRD measurements (Figure 2). In the XRD patterns of the MoS2/HG hybrid catalyst (Figure 2A), the 2θ peak at 25.2° is ascribed to the stacked HG sheets, while four broad diffraction peaks located at 17.6°, 33.5°, 43.3°, and 56.9° are assigned to the (002), (100), (006), and (110) planes of MoS2 (2H-MoS2, JCPDS 37-1492),42 respectively. These support again the successful synthesis of crystalized MoS2 on the surface of HG. The low peak intensities of MoS2 are due to their ultrathin feature and poor crystallinity.

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Figure 2. (A) XRD patterns of the MoS2/HG hybrid catalyst. The pattern with a star key (＊) is from the HG substrate; (B) Raman spectra of (a) HG, (b) MoS2/HG, (c) GO, and (d) MoS2/RGO. The peaks marked with pound keys (#) indicate the Resonance Raman (RR) scattering of MoS2. Those with star keys (＊) indicate the characteristic bonds of Mo-C.

The used HG and the MoS2/HG hybrid catalyst were characterized using Raman spectrometer as well. In its Raman spectrum shown in Figure 2B-a, the characteristic Raman peaks of HG are noticed: D band at 1346.5 cm−1, G band at 1587 cm−1, and symmetric 2D bands at 2695 cm−1. The D band is associated with sp3-hybridized carbon atoms and defects within the carbon atom plane, while G band is ascribed to sp2-hybridized carbon atoms in the graphene sheet. The intensity of G band is less than that of D band, an indication of more sp3 hybridized carbon atoms in HGs. In the case of the MoS2/HG hybrid catalyst, of which optical photographs are shown in Figure S6, two Raman peaks located at 374 and 408 cm−1 are associated with the in-plane E12g and out-of-plane A1g vibrational modes of the hexagonal MoS2, respectively.44-45 The relatively high intensity of the A1g suggests an edge-terminated structure for vertically aligned MoS2 nanosheets. These results are in line with SEM and TEM observations. More attractively, the characteristic bonds of Mo-C are observed, located at 829 and 1008 cm−1.46 They are the direct evidences of the combination of MoS2 nanosheets with HGs. The Resonance Raman (RR) scattering of MoS2 appeared for the MoS2/HG hybrid catalyst does not show up in the Raman spectrum of the MoS2/RGO catalyst, where a broad peak is noticed ACS Paragon Plus Environment

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(Figure 2B-D). The reason behind is that MoS2 nanosheets are much disordered in the MoS2/RGO catalyst in comparison to those in the MoS2/HG hybrid catalyst, as shown in their SEM and TEM images (Figure S3). Moreover, compared with HG, the ratio of the intensity of D band (ID) to that of G band (IG) for the MoS2/HG hybrid catalyst is reduced. This hints the occurrence of hydrogen stripping in HGs and the restoration of delocalized π conjugation. In contrast, the ratio of ID/IG in the MoS2/RGO hybrid catalyst is close to that in GO (Figure 2B-c, d), suggesting that the recovery of the delocalized π conjugation in the MoS2/RGO hybrid catalyst is relatively low. Furthermore, the infrared absorption spectra of HG and the MoS2/HG hybrid catalyst were recorded and compared with those for the MoS2/RGO hybrid catalyst. For example, the infrared absorption spectra of HG shows the strong bonds in the wavelength between 2850 and 3000 cm−1, resulting from sp3 carbonhydrogen vibrations (Figure S7).47 In the case of MoS2/HG hybrid catalyst, the intensities of these aliphatic C-H stretching bonds decrease remarkably. Some bonds even disappear. These facts indicate again the occurrence of hydrogen stripping on the C-H bonds. However, the C-H stretching bonds are negligible in the case of the MoS2/RGO hybrid catalyst. As the band gap of HG can be decreased by reducing the H content of HG. Its conductivity is thus improved. Therefore, the electrical conductivity of the MoS2/HG hybrid catalyst is expected to be increased and its charge transportation will be promoted after hydrogen stripping.

Active sites of the MoS2/HG hybrid catalyst To further characterize the chemical nature (e.g., element chemical state) and bonding state of the MoS2/HG hybrid catalyst, its survey spectrum and high-resolution XPS spectra for different elements (e.g., C 1s, Mo 3d, and S 2p) were recorded (Figure 3). The fitting results, including the positions, fullwidth-at-half-maximum (FWHM) values, and areas for different peaks, are listed in Table S1. As control experiments, the corresponding XPS spectra for the MoS2/RGO catalyst and the related fitting results are shown in Figure S8 and Table S2, respectively. For their full survey XPS spectra, the ACS Paragon Plus Environment

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existence of C, Mo, and S elements is testified (Figure 3A). Three peaks are found in the C 1s highresolution XPS spectrum of the MoS2/HG hybrid catalyst: two strong peaks located at 284.8 and 285.4 eV (assigned to the non-hydrogenated C=C bond and the hydrogenated C-C bond, respectively)48 and a relatively weak peak located at 289.1 eV (assigned to the -COO functional group). This weak peak results from remained formic acid on the surface of the MoS2/HG hybrid catalyst. Formic acid is probably generated from the decomposition of DMF (Figure 3B, Table S1). Differently, the highresolution XPS spectrum for C 1s of the MoS2/RGO catalyst exhibits four characteristic peaks. They are located at 285.0, 286.1, 287.2, and 289.1 eV, indicating the presence of functional groups of C=C, -C-O, C=O, and -COOH on the surface of the MoS2/RGO catalyst (Figure S8-B, Table S2). Note that, the XPS peak intensity of the C=O bond for the MoS2/RGO catalyst is stronger than that for the MoS2/HG catalyst. This is because this signal is from both C=O bonds on the surface of RGO and from formic acid remained on the surface of the MoS2/RGO catalyst. The XPS spectra of Mo 3d for both catalysts were then carefully analyzed. Two main characteristic peaks at the binding energies of 228.8 and 232 eV, corresponding to Mo(IV), are noticed for the MoS2/HG hybrid catalyst (Figure 3C, Table S1). The peaks appeared at 229.9, 233.2, 232.5, and 235.6 eV are related with Mo(V) and Mo(VI). Similar peaks appear in the XPS spectra of the MoS2/RGO catalyst (Figure S8, Table S2). the The peak of Mo(VI) probably results from the oxidation of the catalyst into MoO3 or MoO42− in air and the peak of Mo(V) is from the reduction of Mo(VI) into of Mo(V).40 Note that, these Mo(VI) and Mo(V) components unfortunately deteriorate the HER activity of MoS2.49 For both catalysts, the percentages of Mo(IV) were then further estimated. For the MoS2/HG hybrid catalyst, a percentage of 68% of the total Mo amount is found for Mo(IV), while an integrated dose of Mo(V) and Mo(VI) only occupies 32% of the total Mo amount (Table S1). In the case of the MoS2/RGO catalyst, Mo(IV) occupies 61% of the total Mo amount. Both components of Mo(V) and Mo(VI) have a percentage of 39% of the total Mo amount (Figure S8, Table S2). The composition of Mo(IV) is thus the dominant oxidation states in both catalysts, despite the components of Mo(V) and Mo(VI) do exist. The difference (7%) of the percentages of Mo(IV) in two catalysts proves that HG ACS Paragon Plus Environment

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helps MoS2 to prevent the transformation of Mo(IV) into Mo(VI). In other words, the high activity of MoS2 towards HER is possible to be remained. In addition, the XPS peak for S 2s is seen around 226.1 eV. In high-resolution XPS spectra of S 2p, the doublets located at 161.7 and 162.8 eV are attributed to the binding energies of S 2p3/2 and S 2p1/2 in MoS2, respectively. Two shoulders at 163.0 and 164.1 eV are assigned to bridging S22- and/or apical S2ligands, respectively (Figure 3D, Table S1).50 Based on the peak areas for each element in these XPS spectra, the estimated atomic ratio of Mo to S is 1:1.9, in line with the element ratio obtained from EDS measurements. More importantly, the unsaturated S atoms are well-known as active sites for HER.51 The unique characteristics in the MoS2/HG hybrid catalyst then make vertically aligned MoS2 nanosheets feature more accessible catalytic active sites and ultrafast electron transfer from the HG substrate to the MoS2 edges within one S-Mo-S layer. Consequently, the accelerated HER kinetics is expected on the surface of the MoS2/HG hybrid catalyst.

Figure 3. XPS of the MoS2/HG hybrid catalyst: (A) survey spectrum and (B-D) high-resolution spectra for (B) C 1s, (C) Mo 3d, and (D) S 2p. In (B-D), the solid black lines are experimental results, while the red and the dashed lines are simulated ones. The solid grey lines are background. The fitting parameters of (B) C 1s, (C) Mo 3d, and (D) S 2p are shown in Table S1. ACS Paragon Plus Environment

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HER performance of the MoS2/HG hybrid catalyst The MoS2/HG hybrid catalyst is expected to have following features towards HER. First, the combination of vertically aligned MoS2 nanosheets with HG by such a convenient and efficient approach will accelerate the electron transfer of HER. This is because the ideal HG support has an extraordinary electronic conductivity and a large surface to load MoS2 catalysts.34 Taking MoS2 nanosheets deposited on RGO as an example, they exhibit high HER activity with a small overpotential of 160 mV at 10 mA cm−2.52 Second, the porous structure associated with MoS2 nanosheets on the super-aerophobia surface of HG not only provides lots of pathways/tunnels for fast transfer of ions/charges, but also brings in more active electro-catalytic sites. Since the S edge sites of MoS2 are identified as active sites for HER,53 vertical and ultrathin MoS2 nanosheets offer for sure more available electrocatalytic active sites and timely repel of as-formed H2 bubbles. In other words, a constant active electrode area will be remained during HER process. Third, the electron structure of MoS2 is optimized on the HG support. The favorite/moderate hydrogen adsorption energy is expected to be achieved on the S edge sites. The electrocatalytic properties of HG, MoS2, MoS2/HG, MoS2/RGO, and Pt/C catalysts towards HER were evaluated and compared using LSV (Figure 4). The obtained LSV curves are shown in Figure 4A. In comparison to MoS2-based catalysts, the HER catalytic activity of HG is negligible (purple curve), judging from the reduction current density and reduction potential for HER. Among used MoS2-based catalysts in this study, the MoS2/HG hybrid catalyst delivers the smallest overpotential (~124 mV) at the cathodic current density of 10 mA cm−2 (red curve). This overpotential is inferior to that (~42 mV) obtained on the commercial 20 wt% Pt on Vulcan carbon black (black curve), but highly superior to that (~293 mV) achieved on the pure MoS2 catalyst (blue curve). This is ascribed to the good conductivity of MoS2, derived from the occurrence of more conjugated electron through hydrogen stripping from HG support and unique structure of vertically aligned ultrathin MoS2 nanosheets. Moreover, the overpotential for HER on the MoS2/HG hybrid catalyst is lower than that (~172 mV) obtained on the MoS2/RGO catalyst (green curve). The comparison of the activity of the MoS2/HG ACS Paragon Plus Environment

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hybrid catalyst with those reported MoS2-based catalysts is summarized in Table S3. For example, under a current density of 10 mA cm−2 the Tafel slope for HER on the MoS2/HG hybrid catalyst is only 41 mV dec−1. This value is same as that for another two MoS2/RGO catalysts, but smaller than that other MoS2 based catalysts. Moreover, the overpotential for HER on the MoS2/HG hybrid catalyst is 124 mV, lower than that for two MoS2/RGO catalysts. Furthermore, the required loading density of the MoS2/HG hybrid catalyst is smaller than that for two MoS2/RGO catalysts. Therefore, HER performance of the MoS2/HG hybrid catalyst is better than that of those reported MoS2-based catalysts. Such better HER performance originates partially from more exposed active sites at the edges of well-formed and vertically aligned arrays. Namely, more accessible catalytic active sites are available for HER. Note that, HG surface is super hydrophobic. The adhesion of generated hydrogen gas bubbles during the course of HER will be thus much reduced on the surface of the MoS2/HG hybrid catalyst. Another fact for better HER performance of the MoS2/HG hybrid catalyst might come from reduced hydrogen adsorption energy on the MoS2/HG hybrid catalyst, as described in later sessions. For example, the calculated hydrogen adsorption energy (-2.79 eV) is much reduced on our MoS2/HG hybrid catalyst, in comparison to that (-2.22 eV) on the MoS2/OG catalyst. To further explore the effect of the loading ratios of MoS2 to HG on the HER performance, the MoS2/HG hybrid catalysts were constructed with different MoS2 loading weights. The morphologies of MoS2 nanosheets are found to be controlled by the MoS2 loading ratios. This is verified from the SEM images of these MoS2/HG hybrid catalysts (the insets in Figure S9). The highest HER activity is obtained with an optimal MoS2 loading ratio of 27.5 wt%, concluded from the overpotential and the reduction current density of HER (Figure S9). Lower HER activity with a MoS2 loading ratio of less than 27.5 wt%, is due to less catalytic active species, while lower HER activity with a higher MoS2 loading ratio than 27.5 wt% is probably ascribed to less efficient electron and ion diffusion on the thicker MoS2 nanosheets.

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Figure 4. (A) linear sweep voltammograms in the potential range from 150 to −300 mV at a scan rate of 10 mV s−1, (B) corresponding Tafel plots, and (C) Nyquist plots obtained on different MoS2-based catalysts. (D) Current-time plot obtained on the MoS2/HG hybrid catalyst at a static overpotential of 176 mV. The inset shows the polarization curves at the first and 20 000th cycles.

Tafel slope is well-known to be an inherent parameter for the evaluation of the rate determining steps in the course of HER.54 In the cases of MoS2, MoS2/RGO, and MoS2/HG catalysts, the Tafel slopes, as shown in Figure 4B, are 93, 50, and 41 mV dec−1, respectively. Therefore, a classical Volmer-Heyrovsky mechanism occurs on the MoS2-based catalysts towards HER. For bulk MoS2, a higher Tafel slope is obtained. This is due to the poor conductivity as well as low accessible active sites.55 For the MoS2/HG hybrid catalyst, its Tafel slope is lower than that for the MoS2/RGO hybrid catalyst, indicating faster proton discharge kinetics on the MoS2/HG hybrid catalyst than that on the MoS2/RGO hybrid catalyst.

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To evaluate the inherent HER activity, the exchange current densities (j0) were obtained by the extrapolation of Tafel plots (Figure S10). As listed in Table S4 the value of j0 (19.2 µA cm−2) for the MoS2/HG hybrid catalyst is about 4 times larger than that for the MoS2/RGO catalyst. With the same loading amount, the value of j0 for the MoS2/HG hybrid catalyst is also better than those for most of catalysts reported (see the Table S3). In addition, the highest turnover frequency (TOF) for HER is achieved on the MoS2/HG hybrid catalyst (Figure S11). A deeper insight into the interface reactions and electrode kinetics of the MoS2/HG hybrid catalyst during the HER process was shed by employing EIS. Figure 4C shows the experimental Nyquist plots using above catalysts (points) and the fitted curves using related electrical equivalent circuit (lines).56 The MoS2/HG hybrid catalyst exhibits one capacitive semicircle without Warburg impedance. This indicates that the corresponding equivalent circuit for HER is characterized by one time-constant and the reaction is kinetically controlled (inset of Figure 4C).57 Table S4 lists the fitted results, where Rct is charge transfer resistance of the catalysts. The MoS2/HG hybrid catalyst shows the lowest Rct (3.65 Ω). This reflects a fastest electron transfer rate for HER on the MoS2/HG hybrid catalyst.58 Additionally, the electrochemical double-layer capacitances (Cdl) measured by the cyclic voltammetric method (Figure S12) are employed to evaluate the electrochemically active surface area (EASA) of the catalysts. Notably, the Cdl value of the MoS2/HG hybrid catalyst (12.3 mF cm−2) is much larger than that of the MoS2/RGO catalyst (8.1 mF cm−2), and 12 times higher than that of MoS2 (1 mF cm−2). Provided that these catalysts have similar normalized capacitances, these values of the obtained capacitances clarify clearly the biggest EASA of the MoS2/HG hybrid catalyst. Apart from the high activity and fast kinetics, the stability of the electrocatalysts is another important criterion to evaluate the performance of a HER catalyst. Thermal stability of the MoS2/HG hybrid catalyst was first tested using TG-MS (Figure S13). The recorded TG curve shows three main weightloss domains for the MoS2/HG hybrid catalyst once the temperature is increased. The first weight loss is in the temperature range from room temperature to 200 °C, due to the volatilization of adsorbed solvent. The second weight loss is seen in the temperature domain from 200 to 450 °C, resulting from the 18 ACS Paragon Plus Environment

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oxidation of the HG and MoS2. In this temperature domain, the formation of H2O and SO2 is accompanied, respectively. The third weight loss occurs from 450 to 800 °C, ascribed to the oxidative decomposition of graphene.59-60 To evaluate the electrochemical durability of the MoS2/HG hybrid catalyst, the amperometric curves (namely, current-time plots) were then recorded in 0.5 M H2SO4 with an operation time of 24 h at a high current density of 30 mA cm−2. As shown in Figure 4D, the current density remains unchanged in the measured time range, manifesting strong long-term durability of the MoS2/HG hybrid catalyst for HER. Moreover, TEM and HRTEM images of the MoS2/HG catalyst after the durability test for 24 h (Figure S14) reveal that the morphology and vertical array structure of MoS2 nanosheet have negligible changes. Once again, the catalyst of MoS2 nanosheet features the excellent stability under the long-term HER process in strong acidic condition (0.5 M H2SO4).61-62 To evaluate the cycling stability of the MoS2/HG hybrid catalyst, the cyclic voltammograms were recorded in the range of −300 and 30 mV for 20 000 cycles at a scan rate of 50 mV s−1. As shown in the inset of Figure 4D, the initial LSV curve is almost overlapped with the one after the 20 000th cycles, further suggesting excellent cycling stability of the MoS2/HG hybrid catalyst. This is probably ascribed to the strong interaction between HG and MoS2 and enough room of porous structures for H2 bubble release.

Simulation of HER kinetics on the MoS2/HG hybrid catalyst For the electrocatalysts, their electrocatalytic activities depend sensitively on the energetics of the interactions between the reactive surface and the key reaction intermediates (e.g., their adsorption/desorption, bond formation/breaking).63 In this study, HER follows a Volmer-Heyrovsky mechanism on the MoS2/HG hybrid catalyst. In other words, the content of Hads on the surface of the MoS2/HG hybrid catalyst is thus of vital importance for the HER kinetics. From the calculated Gibbs energies it has been proved that an increase of the bond strength of the adsorbed hydrogen on MoS2

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nanosheets stabilizes Hads, enlarges Hads coverage on the edge, and eventually enhances significantly the HER activity of MoS2.22,53 To figure out the situation of Hads in this study, DFT calculations were then applied to the MoS2/OG and the MoS2/HG hybrid catalysts. Note here that, a simple OG model, instead of a complicated RGO model was employed for the simulations in this study. This is because the number of O atoms is more controllable on the surface of OG. Moreover, the ratio and location of diverse functional groups (e.g., hydroxyl, carbonyl, etc.) are unclear on the surface of RGO. On the other hand, the most representative functional group of C=O in both RGO and OG determines the formation of the C-O-Mo bond in the catalyst. This is partially due to the highest content of the C=O bond in both RGO and OG, partially due to the optimal binding conditions (e.g., a smaller space obstruction) between MoS2 and the C=O bond. In other words, the binding sites to anchor MoS2 are expected to be the C=O bonds for both RGO and OG. Excluding the effects of other functional groups (e.g., hydroxyl, etc.) on the surface of RGO during the course of the simulation of Hads will make more sense and will be more time-saving as well. In this way, the key role of the C=O bonds, namely the use of HG for the construction of the HER catalysts is expected to be clarified. Furthermore, it is generally agreed that the S edge sites are the active ones for the HER.64 Therefore, Hads energy was calculated at the S edge sites using a MoS2 model.

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Figure 5. Optimized electronic structure of MoS2 on HG (A) and on OG (C) as well as optimized adsorption energy of MoS2 on HG (B) and on OG (D) when one H atom is combined to the S edge site. Dark cyan, yellow, white, red, and grey spheres denote for Mo, S, H, O, and C atoms, respectively. Figure 5 presents simulated optimized electronic structure and adsorption energy of MoS2 on HG and OG. According to the optimized structure model of MoS2 on HG (Figure 5A) and the optimized adsorption energy (namely, one H atom combined to the S edge site of MoS2 on HG) (Figure 5B), the Hads energy for MoS2 on HG was calculated to −2.79 eV. Similarly, the Hads energy for MoS2 on OG was calculated to −2.22 eV (Figure 5C, D). Remind that Sabatier principle suggests the volcano-type relation between the HER activity of the catalyst and Hads energy. In other words, the optimal HER electrocatalysts are those featuring moderate Hads energy.65 For example, the theoretical Hads energy of Pt, the best catalyst for HER up to date, is −3.3 eV.66 In our case, the Hads energy on the MoS2/HG hybrid catalyst (−2.79 eV) is larger than that on the MoS2/OG catalyst and shifts closer to that for the Pt catalyst (−3.3 eV). When the ferromagnetism in HG is not present, the Hads energy for the MoS2/HG catalyst was calculated to −2.13 eV. This significant difference between the MoS2/HG hybrid catalyst in

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the absence and presence of ferromagnetism in HG evidences the optimization of electronic structure of MoS2 induced by the ferromagnetism in HG (Figure S15). Simultaneously, the fact that the Hads energy on the MoS2/HG hybrid catalyst is larger than that on the MoS2/RGO catalyst and close to that on the Pt/C catalysts is also testified with H2-TPD measurements (Figure S16). This agreement supports well the enhanced HER kinetics on the MoS2/HG hybrid catalyst. Namely, the high HER performance of the MoS2/HG hybrid catalyst mainly originates from the stronger hydrogen adsorption. Optimized electronic structure (or more active sites) of the MoS2/HG hybrid catalyst is another source for its high HER performance. For example, two H-atoms on HG close to the C-Mo bond transfer to the Mo site of Mo-C at the interface of MoS2 and HG (Figure 5A, B). This Mo site further acts as the highly active site for HER.67 The structure change of MoS2/HG benefits to the charge balance on MoS2. During the HER process, H-atoms will transfer from HG to the Mo sites of C-Mo bonds for H2 evolution. Simultaneously, HG is ceaselessly regained under the electrochemical reduction conditions. Therefore, Mo sites in C-Mo bonds generate the additional active sites in the MoS2/HG hybrid catalyst. This eventually results in the enhanced HER kinetics on the MoS2/HR hybrid catalyst. CONCLUSIONS Aligned ultrathin MoS2 nanosheets with tunable morphology and content are vertically grown on the entire surface of HG via a simple solvothermal process. The HER performance achieved on the optimized MoS2/HG hybrid catalyst includes a low overpotential of 124 mV at a current density of 10 mA cm−2 and a small Tafel slope of 41 mV dec−1. The Tafel slope for HER obtained on the MoS2/HG hybrid catalyst is much better than the MoS2/RGO catalyst and most of reported MoS2-based catalysts. Furthermore, the MoS2/HG hybrid catalyst shows long-term durability in continuous operating HER at high current densities and many cycles (e.g., up to 20 000 cycles). Such high HER performance was interpreted by means of DFT calculations, namely analyzing the electronic structure of this hybrid catalyst and estimating the Hads energy on the S edge sites of MoS2. The simulation results confirm that the improvement of the content of Hads on MoS2 by HG support as well as the enhanced active sites ACS Paragon Plus Environment

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from C-Mo bonds at the interface of MoS2 and HG. Consequently, enhanced HER activity on the MoS2/HG hybrid catalyst originates the high conductivity of this catalyst, its large number of active sites and large surface area, super hydrophobic surface of HG substrate, and reduced hydrogen adsorption energy on this catalyst. Synergistic effects between these facts are expected. To figure out which aspect(s) mainly dominate(s) such improved HER activity, further studies are required in future. In summary, combining different noble-metal free 2D materials with their optimized interface properties (e.g., structure, surface energy) is an efficient and promising approach to construct HER electrocatalysts. This strategy has the potential to be employed for the design and development of other electrocatalysts, such as for oxygen reduction and evolution as well as for the reduction of carbon dioxide and nitrogen into useful chemicals in the future.

SUPPORTING INFORMATION: Experimental methods, scheme, results (including SEM/TEM characterizations, IR spectra, XPS spectra, electrochemical measurements, TG-MS, and TPD plots), and tables are available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT: The authors thank Prof. Jiangang Chen (State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences) for helping perform H2-TPD experiments, Dr. Yongqi Zhang (Division of Physics and Applied Physics, Nanyang Technological University, Singapore) for the valuable discussion. X.T. acknowledges the financial support from the National Natural Science Foundation of Youths (21403275), open project by SKLCC (J16-17-909), and coal-based key scientific and technological project of Shanxi province (MC-2014-01), X. G. acknowledges the financial support from the National Natural Science Foundation (21673271, 21473232), N.Y. acknowledges the financial support from the German Research Foundation (DFG) under project YA344/1-1.

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REFERENCES (1) Zheng, Y.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Angew. Chem. Int. Ed. Engl. 2015, 54, 52-65. (2) Deng, S.; Zhong, Y.; Zeng, Y.; Wang, Y.; Yao, Z.; Yang, F.; Lin, S.; Wang, X.; Lu, X.; Xia, X.; Tu, J. Adv. Mater. 2017, 29, 1700748. (3) Yan, Y.; Ge, X.; Liu, Z.; Wang, J. Y.; Lee, J. M.; Wang, X. Nanoscale 2013, 5, 7768-7771. (4) Guo, X.; Cao, G.-l.; Ding, F.; Li, X.; Zhen, S.; Xue, Y.-f.; Yan, Y.-m.; Liu, T.; Sun, K.-n. J. Mater. Chem. A 2015, 3, 5041-5046. (5) Laursen, A. B.; Varela, A. S.; Dionigi, F.; Fanchiu, H.; Miller, C.; Trinhammer, O. L.; Rossmeisl, J.; Dahl, S. J. Chem. Educ. 2012, 89, 1595-1599. (6) Wang, X.-D.; Xu, Y.-F.; Rao, H.-S.; Xu, W.-J.; Chen, H.-Y.; Zhang, W.-X.; Kuang, D.-B.; Su, C.-Y. Energy Environ. Sci. 2016, 9, 1468-1475. (7) Yan, H.; Jiao, Y.; Wu, A.; Tian, C.; Zhang, X.; Wang, L.; Ren, Z.; Fu, H. Chem. Commun. (Camb) 2016, 52, 9530-9533. (8) Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X. Angew. Chem. Int. Ed. Engl. 2014, 53, 6710-6714. (9) Wu, H. B.; Xia, B. Y.; Yu, L.; Yu, X. Y.; Lou, X. W. Nat. Commun. 2015, 6, 6512. (10) Liu, Y.; Yu, G.; Li, G. D.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. Angew. Chem. Int. Ed. Engl. 2015, 54, 10752-10757. (11) Chen, W.-F.; Iyer, S.; Iyer, S.; Sasaki, K.; Wang, C.-H.; Zhu, Y.; Muckerman, J. T.; Fujita, E. Energy Environ. Sci. 2013, 6, 1818. (12) Cao, B.; Veith, G. M.; Neuefeind, J. C.; Adzic, R. R.; Khalifah, P. G. J. Am. Chem. Soc. 2013, 135, 19186-19192. (13) Tang, H.; Dou, K.; Kaun, C.-C.; Kuang, Q.; Yang, S. J. Mater. Chem. A 2014, 2, 360-364. (14) Lei, X.; Yu, K.; Li, H.; Tang, Z.; Zhu, Z. J. Phys. Chem. C. 2016, 120, 15096-15104.

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