Highly Efficient Hydrogen Evolution Reaction Using Crystalline

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Highly Efficient Hydrogen Evolution Reaction Using Crystalline Layered Three Dimensional Molybdenum Disulfides Grown On Graphene Film Amirhossein Behranginia, Mohammad Asadi, Cong Liu, Poya Yasaei, Bijandra Kumar, Patrick Phillips, Tara Foroozan, Joseph C Waranius, Kibum Kim, Jeremiah Abiade , Robert F. Klie, Larry A Curtiss, and Amin Salehi-Khojin Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03997 • Publication Date (Web): 18 Dec 2015 Downloaded from http://pubs.acs.org on December 21, 2015

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Chemistry of Materials

Highly Efficient Hydrogen Evolution Reaction Using Crystalline Layered Three Dimensional Molybdenum Disulfides Grown On Graphene Film Amirhossein Behranginia1,ψ, Mohammad Asadi1,ψ, Cong Liu2, Poya Yasaei1¥, Bijandra Kumar1, 3¥, Patrick Phillips4, Tara Foroozan5, Joseph C. Waranius1, Kibum Kim1,6, Jeremiah Abiade1, Robert F. Klie4, Larry A. Curtiss2, Amin Salehi-Khojin1* 1

Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, IL, 60607, USA 2

Materials Science Division, Argonne National Laboratory, Argonne, IL, 60439, USA

3

Conn Center for Renewable Energy Research, University of Louisville, Louisville, KY, 40292, USA 4

Department of Physics, University of Illinois at Chicago, Chicago, IL, 60607, USA

5

Department of Civil and Material Engineering, University of Illinois at Chicago, Chicago, IL, 60607, USA 6

Department of Mechanical Engineering, Chungbuk National University, Cheongju, 361-763, South Korea [ψ] Authors equally contribute to this work [¥] Authors equally contribute to this work [*] Corresponding author, [email protected],

Abstract: Electrochemistry is central to applications in the field of energy storage and generation. However, it has advanced far more slowly over the last two decades, mainly due to lack of suitable and affordable catalysts. Here, we report the synthesis of highly crystalline layered three dimensional (3D) molybdenum disulfide (MoS2) catalysts with bare Mo edge atoms and demonstrate their remarkable performance for the hydrogen evolution reaction (HER). We found that Mo edge terminated 3D MoS2 directly grown on graphene film exhibits a remarkable exchange current density (18.2 µA.cm-2) and turnover frequency (> 4 S-1) for HER. The obtained exchange current density is 15.2 and 2.3 times, respectively, higher than that of MoS2/graphene and MoS2/Au catalysts, both with sulfided Mo edge 1 ACS Paragon Plus Environment


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atoms. An easily scalable and robust growth process on a wide variety of substrates along with prolonged stability suggests this material as a promising catalyst in the energy related applications. 1. Introduction Recently, molybdenum disulfides (MoS2) is being widely used in a broad range of applications from electronics1,2 and optoelectronics3,4 to energy conversion5,6 and storage systems.7 Although enormous attention has been dedicated to produce atomically thin two dimensional (2D) MoS 2 films,8–13 layered three dimensional (3D) structures of MoS2 with a high density of active edge atoms for electrochemical applications have not yet been explored. To date, several synthetic structures, such as MoS2 nanoparticles on Au (111),14 vertically aligned MoS2 nanoflakes,15 ordered double-gyroid MoS2 bi-continuous networks,6 defect-rich MoS2 nanosheets,16 and chemically synthesized thiomolybdate [Mo3S13]2- clusters17 with sulfided Mo edge atoms have been developed and tested for HER. In this report, we employed the chemical vapor deposition (CVD) method to synthesize crystalline 3D structured MoS2 with bare Mo edge atoms on graphene film, and demonstrate its outstanding activity and stability for HER. In particular, the electrochemical impedance spectroscopy (EIS) experiments and density functional theory (DFT) calculations reveal that Mo edge terminated MoS2 and graphene show strong synergy for HER resulting in significant enhancement in the electron transfer to the reactants and desorption of intermediates from catalyst surface. 2. Experimental Details: Materials: Molybdenum trioxide (MoO3) powder (99.98%) and sulfur (S) powder (99.98%) were purchased from Sigma-Aldrich. Platinum (Pt) gauze 52 mesh was purchased from Alfa Aesar. Ag/AgCl and 3M KCl were bought from BASi. 2 ACS Paragon Plus Environment

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MoS2 synthesis: The S and MoO3 powder were used as precursors for MoS2 growth. The temperature profiles at different zones of the furnace were precisely controlled to synchronize the S and MoO3 vaporization rates to prevent forming of intermediate products. The maximum temperature was set at 850 oC for 15 minutes. Direct growth on different substrates such as GC and graphene/GC were performed to facilitate the electrode fabrication process as well as to improve the efficiency of the HER. More details are provided in the Supplementary information. Physicochemical characterization: Scanning Electron Microscope (SEM) images are acquired by the In-lense detector of a Carl Ziess electron microscope integrated in a Raith e-LiNE plus electron beam lithography system at 20 kV acceleration voltage and 10 mm working distance. Acton TriVista CRS confocal Raman with 11 mw power and 0.5 µm spot size was utilized to get Raman spectra from MoS2 structures and graphene flakes. The Atomic Force Microscopy (AFM) topography maps are acquired in tapping mode by an Icon Bruker system. Scanning transmission electron microscopy (STEM) characterization was performed on a JEOL JEM-ARM200CF, operated at 200 kV, equipped with an Oxford X-Max^N 100TLE silicon drift detector (SDD) for energy dispersive X-ray (EDX) analysis. EDX spectra were acquired from 0-10 keV with 2048 total channels. X-ray Photoelectron Spectroscopy (XPS) Analyses were performed on a monochromatic Al Kα source instrument (Kratos, Axis 165, England) operating at 12 kV and 10 mA, for an X-ray power of 120 W. Spectra were collected with a photoelectron takeoff angle of 90° from the sample surface plane, energy steps of 0.1 eV, and a pass energy of 20 eV for all elements. All spectra were referenced to the C1s binding energy at 284.6 eV. Three-electrode electrochemical setup: All experiments were performed inside the threeelectrode electrochemical cell using 0.5 M H2SO4 as an electrolyte. Synthesized catalysts, platinum (Pt) gauze 52 mesh and Ag/AgCl were used as a working, counter and reference electrode, 3 ACS Paragon Plus Environment


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respectively. The reference electrode was calibrated with respect to RHE using platinum mesh for both working and counter electrodes in the same electrolyte (0.5 M H2SO4) bubbled with pure H2 (99.99%). The calibration results in 0.164 V shift versus RHE. All CV experiments were obtained by sweeping potential between +0.1 to -0.7 V vs RHE with the scan rate of 5 mV.s-1 using a CHI600D potentiostat. 2. Results and Discussion The 3D structure of MoS2 were synthesized through a dual precursor CVD method with two separated zones to precisely control the temperature profile of the growth process. Seven milligrams of MoO3 powder was uniformly dispersed inside of the crucible to produce a uniform concentration of MoO3 vapor all over the target substrate. The upside-down target substrate also mounted on top of the MoO3 crucible. A sufficient amount of sulfur precursor (one gram) is also located in the upstream of the flow to guarantee complete sulfurization. Independent two-stage temperature profiles (Figure S1, Supporting Information) were applied to each zones to synchronize the evaporation time of the S and MoO3 powders to avoid the formation of intermediate structures (section S1, Supporting Information). Figure 1a shows the low magnification SEM image of the 3D MoS2 structures. XPS results (Figure S2, Supporting Information) show standard Mo 3d5/2 (~229.0 eV) and S 2p3/2 (~162 eV) peaks consistent with the presence of MoIV and the S2- present in MoS2 structures. The XPS spectrum without any peaks at ~236 (a characteristic for MoVI) illustrates absence of molybdenum oxides.18In addition, Figure S3 (section S2, Supporting Information) demonstrates Raman point spectra obtained from the 3D MoS2 structure up to 800 cm-1. Results indicate that MoS2 characteristic peaks associated with E2g and A1g vibrational modes for 3D structured MoS219 without any other peaks up to 800 cm-1 which further supports the absence of oxides formation.10,20

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Figure 1b shows the atomic structure of the 3D MoS2 and its edge structure using STEM. A multi-layered stacking of MoS2 nanosheets shown in Figure 1b with Mo terminated edges along the (100) and (010) crystallographic planes is similar to the previously reported edge terminations in mechanically exfoliated MoS2.21 The corresponding fast Fourier transforms (FFTs) taken from multi-layers area (inset of Figure 1b) shows sharp hexagonal benzene-like patterns indicative of highly crystalline 3D structures with epitaxial stacking of the MoS2 layers. EDX results (Figure S4, Supporting Information) obtained from the 3D structures reveals an approximate composition of 32% Mo and 68% S, which is consistent with the stoichiometric ratio of MoS2. 22 In order to visualize the layered construction of the CVD grown 3D MoS2, we quenched the CVD chamber by force cooling to rapidly reduce the temperature and terminate the growth of the top layers. High magnification SEM images of the resulting structure (Figure 1c) not only show the layered construction of the 3D grains, but also reveal an epitaxial conformance between the layers, as evidenced by the parallel edges in the stacked layers. This is consistent with the FFT patterns of the atomic resolution images obtained from STEM. To gain insight into the growth mechanism of the 3D MoS2, we deliberately placed MoO3 powder (2.5 mg) in the corner of the crucible (Figure S5a, Supporting Information) to visualize the transition from 2D to 3D MoS2. An optical image of the grown structure on a Si/SiO2 substrate is shown in Figure S5b (section S3, Supporting Information). Variation in the color spectrum of the sample corresponds to different concentrations and morphology of the MoS2 structures. Detailed morphological characterization was performed by optical and scanning electron microscopy (SEM) as shown in Figure S5c (section S3, Supporting Information), in which the concentration increases as one moves from left to right. Starting from the low concentration side, initially small triangular MoS2 monolayer flakes appear on the substrate (Figure S5c-1, Supporting



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Information). As the concentration increases, the flakes grow to larger sizes with some smaller MoS2 islands growing on top of the flakes (Figure S5c-2, Supporting Information). As shown in Figure S5c-3 (section S3, Supporting Information), the flakes keep growing in size, reaching together and making grain boundaries, meanwhile smaller MoS2 islands begin to grow in the outof-plane direction on top of the existing flakes. Next, the exiting islands merge together and eventually form a polycrystalline film with numerous islands of multilayer MoS 2 (Figure S5c-4, Supporting Information). In higher concentration regions (Figure S5c-5, Supporting information), white lines representing thicker MoS2 structures appear on the polycrystalline film, which are believed to form due to higher local growth rate on the defect sites such as grain boundaries.23 Moving to even higher concentration regions (Figure S5c-6, Supporting Information), the grown structures rapidly convert to 3D granular structures with a high surface roughness and spatial heterogeneity. This abrupt and localized transition from the 2D to 3D structures resembles the Stranski-Krastanov (SK) growth mechanism of thin films, in which the growth mode suddenly changes as the layer thickness exceeds a critical level.23–26 One possible explanation of such sudden transition can be the release of a residual strain induced by thermal and intrinsic stresses between the MoS2 and the substrate. To test this hypothesis, we performed Raman spectroscopy on the grown structures from monolayers to 3D structures (Figure 1d). Interestingly, we observed that the peak positions of our multilayer films have 3 cm-1 positive shift relative to the bulk spectrum. This shift of spectrum is known to be indicative of an induced strain in the grown structure.27 However, the peak positions of the 3D structured MoS2 shift back to the peak positions of the bulk MoS2, which implies the release of the induced strain. Here, we note that the difference in the positions of the E2g and A1g peaks is known to be dependent on the thickness of the MoS2 structure19 which is changing from


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~20 cm-1 in single layer to ~25 cm-1 in our multilayer and 3D structures. This is independent of the observed blue shift in the multilayer spectra, which is due to the induced strain. Based on above discussions, we propose a dual-step Stranski-Krastanov mode for the growth of 3D MoS2. First, growth initiates from grain boundaries or ordered defects that have high surface energies. Next, the 3D structures form on the film with the thickness beyond the critical point in a less or more compact manner depending on the concentration of Mo and S (Figure S6, Supporting Information). We also synthesized uniform 3D MoS2 on (i) amorphous glassy carbon (GC), (ii) graphene covered GC and (iii) Si/SiO2 substrate (transferred to GC for electrochemical experiment). The glassy carbon and graphene substrates are intentionally selected due to their high conductivity, inert nature and wide applicability in various electrochemical systems.5,28 For easier characterization, we initially synthesized MoS2 on partially covered graphene flakes, which was then transferred to Si/SiO2. Figure S7 and S8 (section S4 and S5, supporting Information) show the SEM and Raman spectra of the 3D structure MoS2 on graphene film. SEM image illustrates that the growth is more compact on top of the hexagonally shaped graphene flakes compared to the bare Si/SiO2. This is attributed to the presence of wrinkles and defects on graphene film which act as nucleation sites and accelerate the growth rate of the 3D structures.29 This process is also extended to fully covered GC with graphene film for the electrochemical experiments. Cyclic voltammetry (CV) experiments are then carried out for these catalysts and compared with platinum which is the most efficient catalyst known for HER. All experiments are performed at identical conditions using 0.5 M H2SO4 as the electrolyte. The current densities are normalized with respect to the geometrical surface area and reported based on RHE (reference hydrogen electrode) scale. As shown in Figure 2a, both MoS2 on SiO2 (transferred to GC) and directly grown on GC exhibit low onset potentials of 175 and 140 mV, respectively. Interestingly, the HER takes



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place at a much smaller onset potential (70 mV vs RHE) for the MoS2 on graphene sample. In particular, 10 mA.cm-1 current density is achieved at ~100 mV overpotential. A similar HER current density was recorded at ~180 mV overpotential for the recently studied efficient thiomolybdate clusters.17 Additionally, the linear part of Tafel plot for three different catalysts was studied to further explore their catalytic properties and HER mechanisms.5,17 In general, three elementary reaction steps that can control the rate of HER are: (i) adsorption of reactant (H+) on catalyst active sites (Volmer step), (ii) intermediate formation (Had, Tafel step), and (iii) product desorption (Heyrovsky step) from active sites.25–27 H+ + e-  Had

(Volmer, rxn 1)

Had + Had  H2

(Tafel, rxn 2)

H+ + e- + Had  H2

(Heyrovsky, rxn 3)

Tafel slops of 120, 40, and 30 mV.dec-1 are correlated to Volmer, Heyrovsky and Tafel step, respectively, as a rate determining step (RDS) for HER.27 The obtained Tafel slope and exchange current density for different studied catalysts are shown in Figure 2b, Figure S9 and Table S1 (section S6, Supporting Information). The Tafel slop for MoS2 on graphene is ~41 mV.dec-1 proposing the Volmer-Heyrovsky (rxn 1 and 2) as a dominant mechanism and Heyrovsky step as a RDS.5,17,30–32 Tafel slop for MoS2 on GC (~55 mV.dec-1) and transferred MoS2 (~68 mV.dec-1) also suggest similar mechanism with Heyrovsky step as a RDS. However, the calculated lower Tafel slope (~41 mV.dec-1) and higher exchange current density (18.2 µA.cm-2) for the MoS2 on graphene (Table S1, Supporting Information) implies a remarkable improvement in the charge carrier mobility of the MoS2. To further explore the reaction mechanism and effect of graphene in catalytic performance of


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MoS2, we performed binding energy measurement in 0.1M perchloric acid (HClO4) solution at potential ranging from -0.1 to 0.4 V vs Ag/AgCl with the scan rate of 10 mV.s-1 (Figure 2c). In principal, a lower over-potential corresponds to a higher binding energy at the constant current density.33,34 The binding energy measurement shows strong adsorption (lower over-potential) for Pt compared to MoS2 on graphene suggesting intermediate formations (Tafel step) as a RDS. This is consistent with Tafel slope of 33 mV.dec-1 measured for Pt (Figure 2b). However the desorption peak shows opposite trend where MoS2 on graphene exhibits higher desorption energy compared to Pt and MoS2 on GC. This further verifies the Heyrovsky step as a RDS where desorption of the produced species from the catalyst surface is known to be the rate determining step for the reaction.5,17,30–32 Furthermore, the binding energy experiment shows strong adsorption and desorption (lower over-potential) for MoS2 in presence of graphene. We attribute this to an enhanced contact resistance between graphene and MoS232,35,36 which could boost electrons transfer toward active edge sites. We also performed electrochemical impedance spectroscopy (EIS) experiments to study the charge transfer resistances (Rct) in the MoS2 catalyst with and without the graphene layer. The Nyquist plots were recorded at overpotential of 150 mV to ensure that the HER takes place on both catalysts (Figure 2d). An equivalent Randles circuit model was applied to calculate Rct value for each system (inset of Figure 2d). The MoS2 on graphene exhibits much smaller Rct (~65Ω) compared to that of without graphene layers (>150Ω). Figures S10a and b (section S7, Supporting Information) also show the EIS results for the MoS2 on graphene and GC during HER at different over-potentials. Results indicate a smaller charge transfer resistance5,37 at all over-potentials due to enhanced contact between MoS2 and graphene further supporting the results of binding energy measurements.



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The turnover frequency is also calculated (TOF) for different MoS2 catalysts using the roughness factor (RF) method6. The RF number of our catalyst is determined by comparing the double layer capacitor (Cdl) of this catalyst with flat standard MoS2 (60 µF.cm-1). The CV experiment at different scan rates was used to calculate the Cdl of catalysts38 (Figure S11a, Supporting Information). The extracted Cdl values from the slope of current density-scan rate graphs at +0.2 V (Figure S11b, Supporting Information) are equal to 2.13 and 1.68 mF.cm-1 for MoS2 on graphene and GC, respectively. The calculated number of active sites for MoS2 on graphene (4.07×1016 sites.cm-2) is ~ 1.2 times higher than those of amorphous GC (3.21×1016 sites.cm-2, Supporting Information, equation S1). Moreover, the calculated TOFs (Supporting Information, equation S2) for MoS2 on graphene show higher values at entire range of overpotentials (Figure 3a and Figure S11c, Supporting Information). In particular, TOF at 200 mV overpotential is > 4 (s-1) which suggest a remarkable activity of the catalyst for HER. The extended TOFs calculated at higher over-potentials (>200 mV) is also presented in the Figure S11c (section S8, Supporting Information). The stability of MoS2 on graphene catalyst was also examined after 1000 continues cycles of CV experiments (section S9, Supporting Information). As shown in Figure 3b, approximately 5% current density decay at the over-potential of 0.15 mV confirms high stability of this catalyst during HER. Density functional theory (DFT) calculations were carried out to investigate the catalytic properties of the bare Mo edge of MoS2 and the support effect of graphene (section S10, Supporting Information). The unsupported MoS2 nanoribbon was calculated to represent MoS2 transferred onto the glassy carbon in the experiment, due to the weak effect from the support. The MoS2 nanoribbon supported on graphene was calculated to represent MoS2 on graphene. We 10 ACS Paragon Plus Environment

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studied the relationship between the theoretical MoS2 hydrogen binding energy and the experimental HER exchange current density (Figure 4). Previous studies of HER on pure metal surfaces39 have suggested that the experimental exchange current density is directly correlated with the adsorption free energy of hydrogen (Figure 4). Too strong/weak hydrogen binding leads to a low activity of HER, while a material with the hydrogen adsorption energy close to 0 gives the highest activity, i.e., Pt surface. We noticed that the Mo terminated MoS2 system shows a similar relationship between hydrogen binding energy and activity. Both unsupported MoS2 and MoS2 on graphene show negative hydrogen adsorption energies. This indicates that formation of the adsorbed hydrogen, Had, is thermodynamically downhill (the Volmer step), while the second step of HER, the formation of H2, is the RDS. Thus, given the reaction follows the Volmer-Heyrovsky mechanism for the MoS2 systems, the calculated hydrogen adsorption energies confirms the experimental results that the Heyrovsky step is the RDS. The hydrogen desorption on MoS2 on graphene became slightly weaker than that on the unsupported MoS2. The slightly weaker hydrogen binding energy induces a remarkable change in the exchange current density as well as the TOF of HER (Figure 4). It is notable that the measured exchange current density of the bare Mo edge of MoS2 on graphene is higher than the previously reported MoS2/graphene and MoS2/Au(111), in both of which the Mo edges were sulfided. This suggests that the MoS2 with bare Mo edge synthesized in this study has unique catalytic properties, compared to previously studied systems. 4. Conclusion In conclusion, using CVD method, we synthesized 3D structured MoS2 with Mo edge atoms and with high degree of crystallinity on the graphene film with the aim of enhancing the HER performance. The observed improvement is mainly attributed to high number of active edge sites



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of 3D structured MoS2, enhanced charge transfer toward active edge atoms and optimized hydrogen binding energy due to the presence of graphene. The ability to directly grow these 3D structures over a wide range of substrates enables ready assimilation of these new materials in any arbitrary platform for various energy conversion and storage technologies.


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Associated Content: Supporting Information: Experimental Details of MoS2 Growth and Transfer Processes, XPS, Raman and EDX characterization of the 3D structured MoS2, 2D to 3D MoS2 transition, SEM characterization of the 3D Structured MoS2 on partially covered graphene flakes, Raman characterization of the 3D Structured MoS2 on graphene/SiO2 substrate, Three-electrode electrochemical setup,, Exchange current density calculation for different catalyst systems, Electrochemical Impedance Spectroscopy (EIS) experiment, Turn over frequency (TOF) calculation of MoS2/graphene catalyst active sites, Stability of MoS2 on graphene catalyst and DFT calculation details. Acknowledgements: A.S.K. work was supported by National Science Foundation (Grant #NSF-CBET-1512647). The authors acknowledge the MRSEC Materials Preparation and Measurement Laboratory shared user facility at the University of Chicago (Grant# NSF-DMR-1420709). The authors also acknowledge the EPIC facility (NUANCE Center-Northwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the Nanoscale Science and Engineering Center (NSF EEC–0647560) at the International Institute for Nanotechnology; and the State of Illinois, through the International Institute for Nanotechnology. The computational work was supported by the U.S. Department of Energy, Office of Science, BES-Division of Materials Science and Engineering and BES-Scientific User Facilities under Contract DE-AC02-06CH11357. We acknowledge the computing resources operated by the Laboratory Computing Resource Center (ANL), and the ANL Center for Nanoscale Materials. We also thank the support of the Argonne Director's Fellowship to Cong Liu.



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Author Contributions: A.S.K., A.B., M. A. conceived the idea. A.B. synthesized the CVD MoS2 3D structure. M.A. and B.K. performed the electrochemical experiments. P.Y., B.K. and K.K. synthesized the CVD graphene. P.Y., B.K., T.F. and J.C.W helped in CVD MoS2 synthesis. A.B., M.A., B.K., and P.Y carried out the characterizations. P. P., and R. K. carried out STEM and EDX. A.S.K. and J.A. jointly supervised the M.A. efforts. All of the authors contributed to the manuscript before submission. A.S.K. work was supported by University of Illinois at Chicago through the Start-up budget and Chancellor Proof of Concept award. Additional Information: A.S.K., A.B., M.A., P.Y. and T. F. submitted a patent disclosure to UIC office of technology and management.


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b

a (a)

d

c

Bulk SL

ML

3D

380 400 420 Raman Shift (cm-1) Figure 1. (a) Low magnification SEM image from 3D structured MoS2 (scale bar is 15 µm). (b) Atomicresolution ADF STEM image, showing the layered structure of the 3D crystals (scale bar is 3 nm). The inset shows the fast Fourier transform (FFT) obtained from the multilayer region in which the unique hexagonal pattern of MoS2 crystal implies the epitaxial stacking of the layers. (c) SEM image of the 3D structured MoS2 from a rapid cooling experiment (scale bar is 0.5 µm). (d) Raman point spectra obtained from a monolayer flakes (red), multilayer MoS2 film (blue) and 3D structured MoS2 (black).


30

b

a Pt MoS2 grown on graphene

15

MoS2 grown on GC

MoS2 transfered on GC

MoS2 transferred on GC

(v)

0.3

-15

/dec

0.1

-45

0.0

-0.2

0.0

v 68 m

0.2

-30

-0.4

Pt MoS2 grown on graphene

0.4

MoS2 grown on GC

0

-4

-3

log (iH /Acm-2)

d 0.3 0.2 0.1 0.0

0.24

0.30

0 Desoption

-0.2 -0.3 -0.4 -0.5

- Zim (ohm)

MoS2 on Graphene

150 Adsoption

Pt MoS2 on GC

-1

-2

2

c 1

v/dec 55 m /dec 41 mv

ec 33 mv/d

Potential (V vs RHE )

Current Density (mA/cm2)

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MoS2 grown on graphene MoS2 grown on GC

120 90

Cd

60 30

Rs

0

0.10 0.15

-0.1 0.0 0.1 0.2 0.3 0.4 Potential (V vs Ag/AgCl)

0

50

Rct

100 150 Zreal (ohm)

200

Figure 2. (a) Cyclic voltammetry curve for Platinum, MoS2 grown on graphene, grown on GC and Transferred to GC. (b) Tafel plot obtained for different catalysts. 3D MoS2 grown on graphene shows lower Tafel slop than that of grown on GC or grown on Si/SiO2 and transferred on GC. (c) Binding energy measurement for Pt, MoS2 on graphene and GC in 0.1M HClO4 solution. The results confirm higher adsorption and desorption energy for MoS2 on graphene at the constant current density compared to direct grown MoS2 on GC. (d) Nyquist plots collected from EIS experiments at overpotential of 150 mV for 3D MoS2 grown on graphene and GC, evidencing higher charge transfer rate for 3D MoS2 grown on graphene. An equivalent Randles circuit was fitted to the data to calculate Rct value for each catalyst system (inset).


a MoS2 grown on graphene

4

MoS2 grown on GC

40

MoS2 transferred on GC

3 2

20

1 0 50

100

150

200

0


60

TOF(S-1)

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b MoS2 grown on graphene

0

After 1000 cycles

-15 -30 -45 -0.15

 (mV)

-0.10

-0.05

0.00

Potential (V vs RHE )

Figure 3. (a) Calculated TOF at different current densities with respect to the over-potentials smaller than 200 mV. (b) Stability of MoS2 grown on graphene after 1000 continues CV cycles. This catalyst shows negligible shift (< 5%) of current densities for increased potential cycling.


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Pd

Bare Mo edge of MoS2/Graphene Bare Mo edge of MoS2 Mo

Pt Pt

Rn

Ni Co

Ir Sulfided Mo edge of MoS2/Au (111) Cu

Sulfided Mo edge of MoS2/Graphene

Au Ag

ΔGH (eV)

Figure 4. Activity relationship for the hydrogen evolution reaction (HER) showing the exchange current density as a function of the calculated free energy of hydrogen adsorption, ΔGH. The black points are experimental data of exchange current density and computational ΔGH 39. The blue points represent measured rates on sulfided Mo edge of MoS2 nanoparticles on either Au(111)40 or graphene41 plotted at the calculated ΔGH42. The red points are the experimental data and the calculated ΔGH from this study, in which the ΔGH of bare Mo edge of MoS2 (without a support) was calculated to represent the MoS2 transferred onto glassy carbon, due to the weak interactions between MoS2 and the support. It was assumed that surfaces with negative ΔGH have high coverage (θH = 1 ML) and surfaces with positive ΔGH have low coverage (θH = 0.25 ML), where θH is defined as the fraction of a monolayer with respect to the number of edge metal atoms41,42. Thus in this study we use the same assumptions for the coverages.



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Highly Efficient Hydrogen Evolution Reaction Using Crystalline

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