Hydrogen Evolution Reaction on Hybrid Catalysts of Vertical MoS2

Jan 18, 2018 - Herein, 2D hydrogenated graphene (HG) is introduced into MoS2 ultrathin nanosheets for the construction of a highly efficient and stabl...
0 downloads 0 Views 2MB Size
Subscriber access provided by University of Florida | Smathers Libraries

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1

Hydrogen Evolution Reaction on Hybrid Catalysts of

2

Vertical MoS2 Nanosheets and Hydrogenated Graphene

3

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

4

Guo†

5



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

6

Taiyuan 030001, China ‡

7

§

8 9

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

ǁ

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

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

10

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

11

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

12

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

13

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

14

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

15

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

16

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

17

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

18

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

19

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

1

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

20

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

21

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

22

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

23

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

24

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

25

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

26

HER electrocatalysts by employing different 2D materials.

27

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

28

synthesis, Hydrogen adsorption energy

29 30

INTRODUCTION

31

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

32

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

33

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

34

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

35

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

36

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

37

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

38

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

39

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

40

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

41

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

42

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

43

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

2

Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

44

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

45

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

46

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

47

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

48

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

49

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

50

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

51

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

52

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

53

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

54

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

55

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

56

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

57

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

58

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

59

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

60

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

61

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

62

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

63

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

64

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

65

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

66

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

67

shown.

68

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

69

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

3

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 29

70

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

71

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

72

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

73

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

74

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

75

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

76

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

77

sites.

78 79

EXPERIMENTAL

80

Chemicals and reagents

81

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

82

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

83

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

84

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

85 86

Synthesis of the MoS2-based catalysts

87

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

88

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

89

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

90

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

91

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

92

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

93

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

94

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

95

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

4

Page 5 of 29

ACS Catalysis

96

1 2 3 97 4 5 98 6 7 99 8 9 100 10 11 12 101 13 14 102 15 16 103 17 18 104 19 20 21 105 22 23 106 24 25 107 26 27 28 108 29 30 109 31 32 110 33 34 35 111 36 37 112 38 39 113 40 41 42 114 43 44 115 45 46 116 47 48 49 117 50 51 118 52 53 119 54 55 56 120 57 58 121 59 60

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

5

ACS Catalysis

Page 6 of 29

122

1 2 3 123 4 5 124 6 7 125 8 9 126 10 11 12 127 13 14 128 15 16 129 17 18 130 19 20 21 131 22 23 132 24 25 26 133 27 28 134 29 30 31 135 32 33 136 34 35 137 36 37 38 138 39 40 139 41 42 140 43 44 45 141 46 47 142 48 49 143 50 51 52 144 53 54 145 55 56 146 57 58 59 60

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

6

Page 7 of 29

ACS Catalysis

147

1 2 3 148 4 5 149 6 7 150 8 9 151 10 11 12 152 13 14 153 15 16 154 17 18 155 19 20 21 156 22 23 157 24 25 26 158 27 28 159 29 30 31 160 32 33 161 34 35 162 36 37 163 38 39 40 164 41 42 165 43 44 166 45 46 47 167 48 49 168 50 51 169 52 53 170 54 55 56 171 57 58 172 59 60

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

ACS Catalysis

173

1 2 3 174 4 5 175 6 7 176 8 9 177 10 11 12 178 13 14 179 15 16 180 17 18 181 19 20 21 182 22 23 183 24 25 184 26 27 28 185 29 30 186 31 32 187 33 34 188 35 36 37 189 38 39 190 40 41 191 42 43 44 192 45 46 193 47 48 194 49 50 195 51 52 53 196 54 55 197 56 57 198 58 59 60

Page 8 of 29

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

8

Page 9 of 29

199

1 2 3 200 4 5 201 6 7 202 8 9 203 10 11 12 204 13 14 205 15 16 206 17 18 207 19 20 21 208 22 23 209 24 25 210 26 27 28 211 29 30 212 31 32 213 33 34 214 35 36 37 215 38 39 216 40 41 217 42 43 44 218 45 46 219 47 48 220 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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.

ACS Paragon Plus Environment

9

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 221 222 29 30 223 31 32 224 33 34 225 35 36 226 37 38 39 227 40 41 42 228 43 44 229 45 46 230 47 48 49 231 50 51 232 52 53 233 54 55 56 57 58 59 60

Page 10 of 29

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.

ACS Paragon Plus Environment

10

Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 234 235 13 236 14 15 16 237 17 18 238 19 20 21 239 22 23 24 240 25 26 241 27 28 242 29 30 31 243 32 33 244 34 35 245 36 37 38 246 39 40 247 41 42 248 43 44 45 249 46 47 250 48 49 251 50 51 52 252 53 54 253 55 56 254 57 58 59 60

ACS Catalysis

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

11

ACS Catalysis

255

1 2 3 256 4 5 257 6 7 258 8 9 259 10 11 12 260 13 14 261 15 16 17 262 18 19 263 20 21 264 22 23 24 265 25 26 266 27 28 267 29 30 268 31 32 33 269 34 35 270 36 37 271 38 39 40 272 41 42 273 43 44 274 45 46 275 47 48 49 276 50 51 277 52 53 278 54 55 56 279 57 58 280 59 60

Page 12 of 29

(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

12

Page 13 of 29

281

1 2 3 282 4 5 283 6 7 284 8 9 285 10 11 12 286 13 14 287 15 16 288 17 18 289 19 20 21 290 22 23 291 24 25 292 26 27 28 293 29 30 294 31 32 295 33 34 296 35 36 37 297 38 39 298 40 41 299 42 43 44 300 45 46 301 47 48 302 49 50 303 51 52 53 304 54 55 305 56 57 306 58 59 60

ACS Catalysis

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

13

ACS Catalysis

307

1 2 3 308 4 5 309 6 7 310 8 9 311 10 11 12 312 13 14 313 15 16 314 17 18 315 19 20 21 316 22 23 317 24 25 318 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 319 50 51 320 52 53 54 321 55 56 322 57 58 323 59 60

Page 14 of 29

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

14

Page 15 of 29

324

1 2 3 325 4 5 326 6 7 327 8 9 328 10 11 12 329 13 14 330 15 16 331 17 18 19 332 20 21 333 22 23 334 24 25 26 335 27 28 336 29 30 337 31 32 338 33 34 35 339 36 37 340 38 39 341 40 41 342 42 43 44 343 45 46 344 47 48 345 49 50 51 346 52 53 347 54 55 348 56 57 58 349 59 60

ACS Catalysis

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

15

ACS Catalysis

350

1 2 3 351 4 5 352 6 7 353 8 9 10 354 11 12 355 13 14 356 15 16 17 357 18 19 358 20 21 359 22 23 360 24 25 26 361 27 28 362 29 30 363 31 32 33 364 34 35 365 36 37 366 38 39 367 40 41 42 368 43 44 369 45 46 370 47 48 371 49 50 51 372 52 53 373 54 55 56 57 58 59 60

Page 16 of 29

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.

ACS Paragon Plus Environment

16

Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 374 26 27 28 375 29 30 376 31 32 377 33 34 35 378 36 37 38 379 39 40 380 41 42 381 43 44 382 45 46 47 383 48 49 384 50 51 385 52 53 54 386 55 56 57 58 59 60

ACS Catalysis

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.

ACS Paragon Plus Environment

17

ACS Catalysis

387

1 2 3 388 4 5 389 6 7 390 8 9 10 391 11 12 392 13 14 393 15 16 394 17 18 19 395 20 21 396 22 23 397 24 25 26 398 27 28 399 29 30 400 31 32 401 33 34 35 402 36 37 403 38 39 404 40 41 42 405 43 44 406 45 46 407 47 48 49 408 50 51 409 52 53 410 54 55 56 411 57 58 412 59 60

Page 18 of 29

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

Page 19 of 29

413

1 2 3 414 4 5 415 6 7 416 8 9 10 417 11 12 418 13 14 419 15 16 17 420 18 19 421 20 21 422 22 23 423 24 25 26 424 27 28 425 29 30 426 31 32 33 427 34 35 428 36 37 429 38 39 40 430 41 42 431 43 44 432 45 46 433 47 48 49 434 50 51 435 52 53 436 54 55 56 437 57 58 59 60

ACS Catalysis

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

ACS Paragon Plus Environment

19

ACS Catalysis

438

1 2 3 439 4 5 440 6 7 441 8 9 442 10 11 12 443 13 14 444 15 16 445 17 18 446 19 20 21 447 22 23 448 24 25 449 26 27 28 450 29 30 451 31 32 452 33 34 453 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

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.

ACS Paragon Plus Environment

20

Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 454 25 455 26 27 456 28 29 457 30 31 32 458 33 34 35 459 36 37 460 38 39 461 40 41 42 462 43 44 463 45 46 464 47 48 49 465 50 51 466 52 53 54 467 55 56 468 57 58 59 60

ACS Catalysis

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

ACS Paragon Plus Environment

21

ACS Catalysis

469

1 2 3 470 4 5 471 6 7 472 8 9 473 10 11 12 474 13 14 475 15 16 476 17 18 477 19 20 21 478 22 23 479 24 25 480 26 27 28 481 29 30 482 31 32 483 33 34 484 35 36 37 485 38 39 486 40 41 487 42 43 44 488 45 46 489 47 48 490 49 50 51 491 52 53 492 54 55 493 56 57 494 58 59 60

Page 22 of 29

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

22

Page 23 of 29

495

1 2 3 496 4 5 497 6 7 498 8 9 499 10 11 12 500 13 14 501 15 16 502 17 18 503 19 20 21 504 22 23 505 24 25 26 506 27 28 507 29 30 31 508 32 33 34 35 509 36 37 510 38 39 511 40 41 42 512 43 44 513 45 46 514 47 48 515 49 50 51 516 52 53 517 54 55 56 518 57 58 59 60

ACS Catalysis

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.

ACS Paragon Plus Environment

23

ACS Catalysis

519

1 2 3 520 4 5 6 521 7 8 522 9 10 523 11 12 13 524 14 15 525 16 17 526 18 19 527 20 21 22 528 23 24 529 25 26 530 27 28 29 531 30 31 532 32 33 533 34 35 534 36 37 38 535 39 40 536 41 42 537 43 44 45 538 46 47 539 48 49 540 50 51 52 541 53 54 542 55 56 57 58 59 60

Page 24 of 29

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.

ACS Paragon Plus Environment

24

Page 25 of 29

543

1 2 3 544 4 5 545 6 7 546 8 9 547 10 11 12 548 13 14 549 15 16 550 17 18 19 551 20 21 552 22 23 553 24 25 554 26 27 28 555 29 30 556 31 32 557 33 34 35 558 36 37 559 38 39 560 40 41 42 561 43 44 562 45 46 563 47 48 564 49 50 51 565 52 53 566 54 55 567 56 57 58 568 59 60

ACS Catalysis

(15) Zhou, H.; Yu, F.; Liu, Y.; Sun, J.; Zhu, Z.; He, R.; Bao, J.; Goddard, W. A.; Chen, S.; Ren, Z. Energy Environ. Sci. 2017, 10, 1487-1492. (16) Vrubel, H.; Hu, X. Angew. Chem. Int. Ed. Engl. 2012, 51, 12703-12706. (17) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. Nat. Chem. 2013, 5, 263-275. (18) Cummins, D. R.; Martinez, U.; Sherehiy, A.; Kappera, R.; Martinez-Garcia, A.; Schulze, R. K.; Jasinski, J.; Zhang, J.; Gupta, R. K.; Lou, J.; Chhowalla, M.; Sumanasekera, G.; Mohite, A. D.; Sunkara, M. K.; Gupta, G. Nat. Commun. 2016, 7, 11857. (19) Wang, D. Y.; Gong, M.; Chou, H. L.; Pan, C. J.; Chen, H. A.; Wu, Y.; Lin, M. C.; Guan, M.; Yang, J.; Chen, C. W.; Wang, Y. L.; Hwang, B. J.; Chen, C. C.; Dai, H. J. Am. Chem. Soc. 2015, 137, 15871592. (20) Guo, J.; Zhu, H.; Sun, Y.; Tang, L.; Zhang, X. Electrochim. Acta. 2016, 211, 603-610. (21) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183-191. (22) Deng, J.; Li, H.; Wang, S.; Ding, D.; Chen, M.; Liu, C.; Tian, Z.; Novoselov, K. S.; Ma, C.; Deng, D.; Bao, X. Nat. Commun. 2017, 8, 14430. (23) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. J. Am. Chem. Soc. 2005, 127, 5308-5309. (24) Li, G.; Zhang, D.; Qiao, Q.; Yu, Y.; Peterson, D.; Zafar, A.; Kumar, R.; Curtarolo, S.; Hunte, F.; Shannon, S.; Zhu, Y.; Yang, W.; Cao, L. J. Am. Chem. Soc. 2016, 138, 16632-16638. (25) Laursen, A. B.; Kegnæs, S.; Dahl, S.; Chorkendorff, I. Energy Environ. Sci. 2012, 5, 5577. (26) Xie, L.; Wang, X.; Lu, J.; Ni, Z.; Luo, Z.; Mao, H.; Wang, R.; Wang, Y.; Huang, H.; Qi, D.; Liu, R.; Yu, T.; Shen, Z.; Wu, T.; Peng, H.; Özyilmaz, B.; Loh, K.; Wee, A. T. S.; Ariando; Chen, W. Appl. Phys. Lett. 2011, 98, 193113. (27) González-Herrero, H.; Gómez-Rodríguez, J. M.; Mallet, P.; Moaied, M.; Palacios, J. J.; Salgado, C.; Ugeda, M. M.; Veuillen, J.-Y.; Yndurain, F.; Brihuega, I. Science 2016, 352, 437-441. (28) Stankovich, S.; Dikin, D. A.; Dommett, G. H.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282-286. ACS Paragon Plus Environment

25

ACS Catalysis

569

1 2 3 570 4 5 571 6 7 572 8 9 573 10 11 12 574 13 14 575 15 16 576 17 18 19 577 20 21 578 22 23 579 24 25 26 580 27 28 581 29 30 582 31 32 583 33 34 35 584 36 37 585 38 39 586 40 41 42 587 43 44 588 45 46 589 47 48 590 49 50 51 591 52 53 592 54 55 593 56 57 58 594 59 60

Page 26 of 29

(29) Zhao, M.; Guo, X.-Y.; Ambacher, O.; Nebel, C. E.; Hoffmann, R. Carbon 2015, 83, 128-135. (30) Kresse, G.; Furthmiiller, J. Phys. Rev. B 1996, 54, 11169-11186. (31) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758-1775. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865-3868. (33) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953-17979. (34) Huang, H.-C.; Lin, S.-Y.; Wu, C.-L.; Lin, M.-F. Carbon 2016, 103, 84-93. (35) Yan, Y.; Xia, B.; Li, N.; Xu, Z.; Fisher, A.; Wang, X. J. Mater. Chem. A 2015, 3, 131-135. (36) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc. Rev. 2010, 39, 228-240. (37) Pei, S.; Cheng, H.-M. Carbon 2012, 50, 3210-3228. (38) Fei, L.; Lei, S.; Zhang, W. B.; Lu, W.; Lin, Z.; Lam, C. H.; Chai, Y.; Wang, Y. Nat. Commun. 2016, 7, 12206. (39) Jung, Y.; Shen, J.; Liu, Y.; Woods, J. M.; Sun, Y.; Cha, J. J. Nano. Lett. 2014, 14, 6842-6849. (40) Zheng, X.; Xu, J.; Yan, K.; Wang, H.; Wang, Z.; Yang, S. Chem. Mater. 2014, 26, 2344-2353. (41) Min, S.; Lu, G. J. Phys. Chem. C. 2012, 116, 25415-25424. (42) Chang, K.; Chen, W. X. Acs Nano 2011, 5, 4720-4728. (43) Elias, D. C.; Nair, R. R.; Mohiuddin, T. M. G.; Morozov, S. V.; Blake, P.; Halsall, M. P.; Ferrari, A. C.; Boukhvalov, D. W.; Katsnelson, M. I.; Geim, A. K.; Novoselov, K. S. Science 2009, 323, 610-613. (44) Chen, J. M.; Wang, C. S. Solid State Commun. 1974, 14, 857-860. (45) Zhang, X.; Zhang, Q.; Sun, Y.; Zhang, P.; Gao, X.; Zhang, W.; Guo, J. Electrochim. Acta. 2016, 189, 224-230. (46) Xiao, T.-C.; York, A. P. E.; Al-Megren, H.; Williams, C. V.; Wang, H.-T.; Green, M. L. H. J. Catal. 2001, 202, 100-109. (47) Dischler, B.; Bubenzer, A.; Koidl, P. Solid State Commun. 1983, 48, 105-108. (48) Nikitin, A.; Näslund, L.-Å.; Zhang, Z.; Nilsson, A. Surf. Sci. 2008, 602, 2575-2580. (49) Chen, Z.; Cummins, D.; Reinecke, B. N.; Clark, E.; Sunkara, M. K.; Jaramillo, T. F. Nano Lett. 2011, 11, 4168-4175. ACS Paragon Plus Environment

26

Page 27 of 29

595

1 2 3 596 4 5 597 6 7 598 8 9 599 10 11 12 600 13 14 601 15 16 602 17 18 19 603 20 21 604 22 23 605 24 25 606 26 27 28 607 29 30 608 31 32 609 33 34 35 610 36 37 611 38 39 612 40 41 42 613 43 44 614 45 46 615 47 48 616 49 50 51 617 52 53 618 54 55 619 56 57 58 620 59 60

ACS Catalysis

(50) Fu, W.; He, H.; Zhang, Z.; Wu, C.; Wang, X.; Wang, H.; Zeng, Q.; Sun, L.; Wang, X.; Zhou, J.; Fu, Q.; Yu, P.; Shen, Z.; Jin, C.; Yakobson, B. I.; Liu, Z. Nano Energy 2016, 27, 44-50. (51) Li, D. J.; Maiti, U. N.; Lim, J.; Choi, D. S.; Lee, W. J.; Oh, Y.; Lee, G. Y.; Kim, S. O. Nano Lett. 2014, 14, 1228-1233. (52) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. J. Am. Chem. Soc. 2011, 133, 7296-7299. (53) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Science 2007, 317, 100-102. (54) Conway, B. E.; Tilak, B. V. Electrochim. Acta. 2002, 47, 3571-3594. (55) Jaegermann, W.; Tributsch, H. Prog. Surf. Sci 1988, 29, 1-167. (56) Rammelt, U.; Reinhard, G. Electrochim. Acta. 1995, 40, 505-511. (57) Liao, L.; Zhu, J.; Bian, X.; Zhu, L.; Scanlon, M. D.; Girault, H. H.; Liu, B. Adv. Funct. Mater. 2013, 23, 5326-5333. (58) Li, X.; Han, G.-Q.; Liu, Y.-R.; Dong, B.; Shang, X.; Hu, W.-H.; Chai, Y.-M.; Liu, Y.-Q.; Liu, C.-G. Electrochim. Acta. 2016, 205, 77-84. (59) Altavilla, C.; Sarno, M.; Ciambelli, P. Chem. Mater. 2011, 23, 3879-3885. (60) Guo, J.; Li, F.; Sun, Y.; Zhang, X.; Tang, L. J. Power Sources 2015, 291, 195-200. (61) Yang, L.; Zhou, W.; Lu, J.; Hou, D.; Ke, Y.; Li, G.; Tang, Z.; Kang, X.; Chen, S. Nano Energy 2016, 22, 490-498. (62) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Adv. Mater. 2013, 25, 5807-5813. (63) Strmcnik, D.; Kodama, K.; van der Vliet, D.; Greeley, J.; Stamenkovic, V. R.; Markovic, N. M. Nat. Chem. 2009, 1, 466-472. (64) Hu, J.; Huang, B.; Zhang, C.; Wang, Z.; An, Y.; Zhou, D.; Lin, H.; Leung, M. K. H.; Yang, S. Energy Environ. Sci. 2017, 10, 593-603. (65) Deng, J.; Li, H.; Xiao, J.; Tu, Y.; Deng, D.; Yang, H.; Tian, H.; Li, J.; Ren, P.; Bao, X. Energy Environ. Sci. 2015, 8, 1594-1601. ACS Paragon Plus Environment

27

ACS Catalysis

621

1 2 3 622 4 5 623 6 7 624 8 9 10 625 11 12 13 626 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 29

(66) Luo, Y.-R. Comprehensive Handbook of Chemical Bond Energies; CRC Presss: Boca Raton, FL, 2007, 1-1633. (67) Shi, Z.; Nie, K.; Shao, Z.-J.; Gao, B.; Lin, H.; Zhang, H.; Liu, B.; Wang, Y.; Zhang, Y.; Sun, X.; Cao, X.-M.; Hu, P.; Gao, Q.; Tang, Y. Energy Environ. Sci. 2017, 10, 1262-1271.

ACS Paragon Plus Environment

28

Page 29 of 29

627

1 2 3 4 5 6 7 8 9 10 11 12 13 14 628 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

SYNOPSIS TOC

ACS Paragon Plus Environment

29