Hydrogen Evolution Reaction Activity of GrapheneâMoS2 van der

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Letter

On the Hydrogen Evolution Reaction Activity of Graphene-MoS2 van der Waals Heterostructures Ravi Kumar Biroju, Deya Das, Rahul Sharma, Shubhadeep Pal, Larionette P. L. Mawlong, Kapil Bhorkar, Giri P K, Abhishek K. Singh, and Tharangattu N. Narayanan ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 10, 2017

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ACS Energy Letters

On the Hydrogen Evolution Reaction Activity of Graphene-MoS2 van der Waals Heterostructures Ravi K. Biroju1, #, Deya Das2, #, Rahul Sharma1, #, Shubhadeep Pal1, #, Larionette P. L. Mawlong3, Kapil * Bhorkar1,. Giri P. K. 3, Abhishek K. Singh2,*, and Tharangattu N. Narayanan1 1

TIFR-Center for Interdisciplinary Sciences (TCIS), Tata Institute of Fundamental Research, 21 Brundavan Colony, Narsingi, Hyderabad- 500075, India. 2 Materials Research Centre, Indian Institute of Science, Bangalore- 560012, India. 3 Center for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati -781039, India.

ABSTRACT. Finding out a suitable noble metal free catalyst for hydrogen evolution reaction (HER) by photo-electrocatalytic (PEC) water splitting is an enduring challenge. Here, molecular origin of number of layers and stacking sequence dependent PEC HER performance of MoS2/graphene (MoS2/GR) van der Waals (vdW) vertical heterostructures is studied. Density functional theory (DFT) based calculations show that the presence of MoS2 induces p-type doping in GR, which facilitates hydrogen adsorption in GR side compared to MoS2 side with ∆GH more close to 0 eV in MoS2/GR bi-layer vertical stacks. The activity maximizes in graphene with monolayer MoS2 and reduces further for bi-layer and multi-layers of MoS2. The PEC HER performance is studied in various electrodes namely single layer graphene, single and few layered MoS2, and their two different types of vertical heterojunctions having different stacking sequences. Graphene on top of MoS2 sequence showed the highest photoresponse with large reaction current density and lowest charge transfer resistance towards HER, in tune with the DFT calculations. These findings establish the role of stacking sequence in electrochemistry of atomic layers, leading to the designing of new electrocatalysts by combinatorial stacking of minimal number of layers.

Corresponding Author: [email protected] or [email protected]; [email protected]. # Equally Contributing Authors. *

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Graphene-MoS2 atomic layers based vertical heterojunctions, with two different types of stacking sequences having graphene on top of MoS2 and vice versa are demonstrated for hydrogen evolution reaction (HER). Molecular origin of the high activity of top layer graphene structure towards HER is established through density functional theory based calculations.


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Recent advances in van der Waals (vdW) solids of atomic layers invoke the possibility of new types of vertical quantum hetero-junctions having atomically sharp interfaces between dissimilar materials.1 Strong covalent bonds provide sufficient in-plane stability to two dimensional (2D) crystals, whereas relatively weak vdW interactions are sufficient to keep the stack together.1 These vertical structures are found to have novel interface induced physical and chemical properties.2-3 Recently, these heterostructures are also studied for their efficacy towards electrocatalytic activities.4 It is established that in most of the 2D layers, basal planes (defect free) are inactive towards any catalytic reactions4. But it is found that, formation of heterostructures, where the atomic layers are placed one over the other with dissimilar atomic layer, can drastically change the inherent electrocatalytic activities of individual layers towards certain reactions - even leading to the activity of basal plane otherwise it is inactive, and it has been reported that this is because of the effect of built in electric field formed among the dissimilar layers or their metallic substrates.5,6,7 The authors have recently reported that in vdW solids based photo-electrochemistry, not only the selection of layers but their stacking order (layer sequencing or which layer is exposed to the electrolyte/analyte) also matters in deciding the net catalytic performance.7 The photo-electrocatalytic (PEC) response of a few layered MoS2 and single layer graphene was studied, and it is found that the graphene on top of MoS2 (MSGR) has the highest PEC efficacy towards hydrogen evolution reaction (HER),

, in acidic medium than MoS2 on top of graphene (GRMS), though MoS2 is the

photo-active material and visible light has little effect on the PEC performance of graphene.7 Similar stacking sequence effect has been observed in WS2/MoS2 vdW vertical stacks by another research group.7 The mechanism leading to this new parameter called 'stacking sequence' is not completely understood yet, and here a molecular mechanism for these observed effects is proposing based on density functional theory (DFT) calculations, and subsequent layer dependent HER studies. Graphene-MoS2 hybrids were identified as potential candidates in electrocatalysis.4,

7

For

example, a recent DFT study showed that the existence of graphene layer below the MoS2 has a 3 ACS Paragon Plus Environment

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noticeable influence on the charge density distribution of MoS2.4 The built in electric field in MoS2graphene hybrid forms an excessive negative charge density in the structure, which eventually enhances the HER process.4 Moreover, this sandwich structure can also make the basal plane of MoS2 close to the thermo-neutral (∆GH~0), enabling the basal plane of MoS2 to be active towards the HER process. Further, the high optical transparency (in visible region) and high electronic conductivity of graphene can provide efficient current injection into the MoS2 layers - augmenting the PEC process.7 But the observed stacking sequence dependent PEC performance of MoS2/graphene hybrids indicates that other than the interface induced potential modification at the hetero-junction resulting to an enhanced HER process, proton adsorption-desorption kinetics itself can be modified/tuned with the layer sequencing. It has been found from the recent studies that the interface induced effects of layer stacking are limited only to the adjacent layers, and hence many layered stacked solids may not observe promising effects like in single layered stacks.7 Further, high quality crystals are important to see these effects to avoid the recombination of photocarriers related effects.8 Hence in this study, the white light PEC performances of single layer graphene (GR), MoS2 (MoS2 monolayer, MSM), a few layered (3-4 layers) MoS2 (MS), and their various stacking sequences (MSGR (graphene on top of MS), GRMS (MS on top of GR), MSMGR (graphene on top of MSM), and GRMSM (MSM on top of GR)) are studied. Detailed structural and morphological characterization using field emission scanning electron microscope (FESEM), transmission electron microscope (TEM, and high resolution (HR) TEM) and atomic force microscopy (AFM) along with spectroscopic techniques such as photoluminescence (PL) and microRaman analysis are conducted to establish the stacking of layers and influence of sequencing in the photophysical phenomena. The growth and transfer of large area single layer graphene to a graphite patterned polydimethyl siloxane (PDMS) substrate have been established in our previous report.7 The same method has been utilized in this work too to obtain graphene transferred electrodes (detailed synthesis protocol is given in the Methods section, Supporting Information). MoS2 transferred electrodes are also developed using the


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same technique (polymer assisted transfer) reported in reference7, but here care has been taken to the selective transfer of single and a few layered MoS2 samples separately. The structure of MoS2 and the number of layers of MoS2 on Si/SiO2 are assessed from the FESEM and micro-Raman analyses, respectively. Figure 1(a) shows the FESEM images of a few layered (MS) and monolayer MoS2 (MSM). Most of the few layered MoS2 have polygon morphology while in-case of the single layer MoS2, perfect triangular shape is observed - which is significant for the trigonal prismatic structure (2H phase) and usually it forms in-case of CVD growth.9-10 Figure 1(a) (top right inset) shows the TEM image of a few layered MoS2 (MS) flake and bottom right inset is a high resolution TEM (HRTEM) image of the MS. The thickness of the MS (inferred from the HRTEM) is found to be ~ 2-2.5 nm indicates the presence of 3-4 layers of MoS2, which later confirmed by Raman and AFM analyses. The corresponding selective area electron diffraction (SAED) shows the presence of hexagonal lattice diffraction spots. Before going to fabricated the MoS2-graphene vdW stacks, the crystalline quality of as transferred CVD graphene layer is estimated from HRTEM as shown in the figure 1(b), which shows the lattice image of defect free GR and corresponding inverse fast Fourier transform (IFFT) (left inset) and fast Fourier transform (FFT) (right inset) are shown in the figure. Figure 1(c-d) shows the HRTEM image of MSGR sample. Both the images are taken at the same location on the MSGR with different focusing/defocusing to get top (graphene) and bottom (MS) layers individually. Note that the top and bottom insets corresponds to IFFT and FFT images of graphene and MoS2, respectively. Blue and red lining in the SAED pattern denote the diffraction patterns of graphene and MoS2, respectively. Additional FESEM images of MoS2 and graphene vdW heterostructures having different stacking sequences with a few layer and monolayer MoS2 are shown in the supporting information (see figure S1). In addition, AFM topographical features of a few layer MoS2 (MS) and MSGR are shown in the supporting information, figure S2. MS has an average thickness of ~2 nm indicating the presence of 3-4 layers, as inferred from the Raman analysis, discussed in the later section. The MSGR has found to be unwashed polymer residues, which has occurred during wet transfer of graphene.


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Figure 1(e) represents the Raman fingerprints of MoS2 and graphene, and the corresponding Raman spectral line shape analysis is given in the Table T1, supporting information. The Raman spectra of pristine layers and MSGR hybrids with monolayer and few layered MoS2 are taken on Si/SiO2 substrates. Typical Raman signatures of GR - such as D, G and 2D- bands are found in both the samples whereas in the case of MoS2, E2g and A1g peak positions reveal the nature of trigonal prismatic structure (2H- phase) of MoS2 crystals and their peak separations (∆k) show that the number of atomic layers in case of a few layered MoS2 is ~ 3-4 layers (see Table T1).11 The average grain sizes of mono and a few layered MoS2 are similar but the microstructure is changing from triangular (monolayer) to polygon shape (a few layered). This is confirmed via optical and FESEM images of MSGR and MSMGR samples taken at the same magnification and these are shown in figure S3, supporting information. Further confirmation on the structural and excitation energy differences in the MS and MSM samples are clear from the areal Raman mappings of E2g and A1g as shown in figure S4, supporting information. The PL spectra of pristine MSM and MS, and their heterostructures with top graphene configuration are shown in the figure 1(f). All the spectra are recorded with 532 nm laser excitation where the samples are kept on Si/SiO2 substrates at room temperature. Figure 1(f) shows the strong PL bands at around 622 and 669 nm (peak position varies depending on the number of layers)11-12, those are ascribed to 'B' exciton and 'A' exciton, respectively. 'A' excitation peak is derived from the direct band gap of MoS2 and 'B' exciton peak arises from the direct gap transition between the minima of the conduction band and the lower level valence band maxima, where it is created by strong valence band spin-orbit splitting at the K point.13, 11, 14 It is evident from the figure 1(f), that the PL intensities (622 and 669 nm) of single layer MoS2 (MSM) is enormously strong in comparison to a few layer system in both pristine and vdW heterostructures. A detailed description on the stacking sequence dependent PL evolution of MoS2-graphene vdW structures was reported elsewhere.7 Gaussian line shape analysis of PL bands ‘A’ and ‘B’ reveal the information about the inter layer interactions and spin-orbit coupling when they form heterostructures15, and it is shown in the table T2 of the supporting information. The integrated intensity ratio of ‘A’ and ‘B’ PL peaks (IA/IB) in MSMGR is ~ 5 times higher than that of all the samples including MSGR, indicates formation of significant amount of 6 ACS Paragon Plus Environment

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photo excitons in MSMGR7. Hence, Raman and PL analyses show that photo physical phenomena of MoS2 can be varied with respect to the number of layers and their stacking with other layers such as graphene. The HER performance of various working electrodes (graphite patterned PDMS substrates, schematic of the working electrodes are given in the inset, figure 2) modified with atomic layers (MS, MSM, and GR) and their hybrids (GRMS, MSGR, MSMGR) are studied using three-electrode linear sweep voltammetry (LSV, details are given in the Methods section, Supporting Information). All the LSV measurements both in dark and light (white light, wavelength range from 370–730 nm) are carried out in 0.5 M H2SO4 solution at a scan rate of 10 mVs-1. Figure 2(a) shows the LSVs of PEC HER reaction using MSGR and GRMS, and comparison of their photocurrent enhancement with respect to dark. The HER Tafel slopes with different electrodes are plotted in the supporting information, figure S5, and they show variation with respect to different electrodes as well as light/dark conditions. But it has shown (in our earlier report and other recent reports)7 that Tafel slopes cannot give proper measure of kinetics in the case of an evolution reaction. Here the current densities are calculated using geometrical area of the working electrode containing the transferred 2D layers. The charge transfer kinetics of the inner redox probe [Fe(CN)6]4-/3- on various working electrodes are calculated (supporting information, figure S6) and the electrochemical area of the electrodes are calculated using Randles-Sevcik equation (details in supporting information). The LSVs, where the current densities are calculated using electrochemical surface area (instead of geometrical area), are given in the supporting information, figure S7. These indicate that the drastic enhancement in the reaction current densities are not from the differences in the geometrical area of the electrodes. As observed before7, MSGR is providing the best PEC performance compared to other geometries. Due to high surface area of MSGR, a high Faradaic contribution is seen while considering the geometrical area for current density calculations (figure 1a), but it is not there while taking electrochemical surface area, figure S7. The HER LSV experiments are extended to MSM, MSMGR, and GRMSM, where single layer MoS2 (MSM) is used. The results are shown in figure 2b. The LSV of benchmarked HER catalyst, Pt/C, is compared in both the cases. 7 ACS Paragon Plus Environment

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As it has reported, in the case of MS, HER reaction starts at an onset potential of ~ -0.45 V (RHE).7 In the presence of light (LSV in light is taken after 45 min of light exposure in all the cases), the onset potential shifts to a lower potential (~ 50 mV change) along with a drastic enhancement in the current density, indicating the generation of photocarriers in MS after the exposure of light and the augmented reduction process of adsorbed proton. The GR electrode has only a nominal HER response having an onset potential of ~ 0.55 mV with very low current density, and has no light induced carrier generations. GRMS electrode shows similar onset potential (with a little drop in the current density) as that of MS while in MSGR, the current density is drastically enhanced upon illumination of white light. In the cases of MSM samples, the same trend is observed with a little decrease in the current density (calculated using geometrical area). The decrease in the current density is due to the fewer amount of MoS2 in MSM in a given electrode area, as it is shown in figure 1a and figure S1 (areal density of layers is less, as inferred from FESEM data). The light response of individual structures is further distinguished by calculating the ratio of current densities (Jlight/Jdark) at -0.6 V (RHE) of each geometries (the values are given in supporting information, table T2) and the corresponding values are given in the bar diagram, figure 2c. It can be seen from the figure 2c that the light response is highest for MSMGR, and this is much higher than that of MSGR. This indicates that the light induced activities are much higher in heterojunctions having less number of layers (here bi-layer), which is further established in the DFT analysis. To further probe in to the HER performance of various samples, electrochemical impedance spectroscopy (EIS) measurements are performed on each samples in dark and light at -0.4 V (vs RHE, with an input sine wave having 10 mV amplitude) where the HER is supposed to be just initiated (onset potential). EIS is identified as a powerful technique to probe the heterogeneous electron transfer rate both in light and dark HER conditions.7 Figure 2d shows the Nyquist plots of GRMS and MSGR in dark and light illumination. Inset shows the magnified view of the MSGR in light exposure, showing the charge transfer resistance (Rct). This impedance plot is fitted with Randles circuit (the dotted lines correspond to experimental values and solid line corresponds to the fitting). Randles circuit consists of solution 8 ACS Paragon Plus Environment

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resistance (R1), charge transfer resistance (R2 or Rct), constant phase element (Q2) and diffusion of ions in electrolyte solution represented by Warburg region (W2). After fitting with the above circuit, Rct has been estimated. Among all the samples (MSGR and GRMS), MSGR showed the lower Rct value in dark condition. Upon illumination, the Rct value of MSGR is further reduced to 0.08 kΩ (80 Ω) indicating the augmented charge transfer process in the presence of light. The Rct values of GRMS were also decreased upon illumination, indicating the presence of light generated carriers in these electrodes but charge transfer resistance is quite high compared to MSGR. The EIS studies of single layer MoS2 based samples are shown in figure 1e, and all the measurements are carried out at -0.4 V (vs RHE). The corresponding Randles circuit fitting is also shown in each curve. It is observed that Rct values of MSMGR and GRMSM (both in light and dark) are much lower than the corresponding MSGR and GRMS samples. MSMGR is having lowest Rct (30 Ω). For further comparison, Rct values of various electrodes under light exposure are shown in figure 1e. it can be seen from the figure 1e that MSMGR (MSGR is next highest) is having the lowest Rct among various geometries. This indicates that both the number of layers and their stacking sequence matters the efficacy of HER process. In order to understand the mechanism behind the enhanced catalytic activity in graphene/MoS2 heterostructures, theoretically hydrogen adsorption has been studied using first principle based DFT calculations. The heterostructure having one layer of MoS2 and graphene (bi-layer system) is shown in figure 3a, and the band structure of this system along with density of states (DOS) is plotted in figure 3b. When graphene forms vertical heterostructure with MoS2, Dirac cone at K point remains unaltered making the system metallic. However, Fermi level shifts lower leading to p-type doping in graphene.16 The conduction band at Γ point comes from MoS2 layer and it touches the Fermi level denoting that charge transfer happens from graphene to MoS2. Hydrogen adsorption has been studied in the bilayer heterostructure both in graphene side and in MoS2 side as shown in figure 3(c-d). Gibbs free energy (∆GH) for hydrogen adsorption on a catalyst should be closer to zero to ensure best electro-catalytic activity. In the graphene side, hydrogen adsorbs 9 ACS Paragon Plus Environment

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on the top of a carbon atom having a bond length of 1.12 Å and as a result, the corresponding carbon atom protrudes out of the graphene plane. On the other hand, in the case of MoS2 side, hydrogen prefers to adsorb almost at the center of MoS2 hexagon. The values of ∆GH are plotted in figure 3e for both the cases and it is found that hydrogen adsorption is preferable on graphene side having ∆GH of 1.45 eV than MoS2 side (2.01 eV). In order to find out the reason behind the preference of hydrogen adsorption on graphene side, the band structures and DOS for both the cases have been calculated as plotted in figure 4(a-b). When hydrogen is adsorbed in graphene side, the sp2 bonded carbon becomes sp3 hybridized, which destroys the linear band dispersion of pristine graphene (see figure 3a). The conduction band of pristine graphene, lying at the Fermi level, almost becomes dispersionless. Similarly, in the case of hydrogen adsorption on MoS2 side, the conduction band coming from MoS2 in the heterostructure shifts low towards the Fermi level and becomes almost dispersionless (figure 3b). The linear dispersion of graphene at K point remains unchanged. From the DOS studies, H-1s states are found at the Fermi level, similar to the previous case. Graphene being p-type doped takes electron from atomic hydrogen, thereby shows strong adsorption energy than the adsorption on MoS2 side. It has been further confirmed by calculating charge accumulation and depletion as shown in figure S8. Being a vdW heterostructure, there is no charge transfer between the layers, as expected. When hydrogen is adsorbed on the graphene side, hydrogen depletes more compared to the case of hydrogen adsorption on MoS2 side. Moreover, in the case of hydrogen adsorption on MoS2 side, Mo atoms only accumulates the charge from hydrogen. Therefore, hydrogen adsorbs strongly on graphene side due to more charge transfer compared to the other case. The effect of number of MoS2 layers in the hydrogen adsorption is further investigated by increasing the number of MoS2 layers. In the presence of more MoS2 layers, Dirac cone of graphene splits (shown in figure 4c for two layers of MoS2), as the sub-lattice symmetry of graphene breaks due to the interaction with MoS2 layers. However, the graphene remains p-type doped and due to the change in symmetry, Brillouin zone changes leading to changes in high symmetry points. In graphene with two 10 ACS Paragon Plus Environment

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layers of MoS2, the direct band gap of MoS2 at Γ point reduces to 1.12 eV from 1.59 eV for graphene with monolayer MoS2. The reduction in band gap with increasing number of MoS2 layers is in well agreement with PL measurements, where A peak has been found to be shifted towards higher wavelength region (figure 2f). The values of ∆GH have been plotted as a function of number of layers as shown in figure 4d. For hydrogen adsorption on graphene side, the values of ∆GH decrease with increasing number of MoS2 layers. On the other hand, it increases for the adsorption on MoS2 side. However, the values of ∆GH almost remain same for two and three layers of MoS2, denoting the bulk limit. Hence, hydrogen adsorption becomes the most preferable on the graphene side for the bi-layer graphene/MoS2 heterostructure. In conclusion, HER process in acidic medium is studied with atomic layers of graphene (single layer) and MoS2 (single (MSM), and a few layered (MS)) and their van der Waals vertical heterostructures of different stacking sequences (MSGR, GRMS, MSMGR, and GRMSMS), and these geometries are characterized using electron microscopy. All the electrochemical measurements are carried out in graphite patterned PDMS substrate to avoid the interference from the substrate. In both the singleand multi layered MoS2 samples, graphene on top of MoS2 (ie. MSMGR and MSGR) showed the highest photoresponse and HER activity, indicating the role of stacking sequence in HER performance of MoS2/graphene heterostructures. MSMGR showed the higher photo-response than MSGR, with highest Jlight/Jdark (at -0.6 V) and lowest Rct value (30 Ω for MSMGR and 80 Ω for MSGR) in EIS measurements. DFT calculations explained the underlying mechanism of this enhancement in catalytic activity in MSMGR as the charge transfer from graphene to MoS2 leads to p-type doping in graphene, leading to ∆GH for hydrogen adsorption on graphene side closer to zero. This study shows that the band engineering and tuning of the HER kinetics of layered structures can obtained via non-covalent interactions, unlike the case of other structures such carbon nanotubes where covalent functionalization is found to be changing the HER kinetics and dispersion at the Fermi level, aiding an augmented HER process.17


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Acknowledgement RKB, SP, RS, and TNN acknowledge TCIS for financial support. Authors also thank Prof. P. V. Satyam, IoP Bhubaneswar, India for helping to conduct HRTEM measurements for the MoS2-graphene heterostructures. D.D. and A.K.S. acknowledge DST Nanomission for funding and thank ”SahasraT”, Cray XC40 Cluster of Supercomputer Education and Research Center and Materials Research Center, Indian Institute of Science, Bangalore for providing the required computational facilities. Supporting Information Available: The following things are available in the supporting information: FESEM imaging of pristine MoS2 andMoS2-graphene systems, AFM topography profiles of MS and MSGR vdW heterostructure, comparison of grain size in CVD grown few and mono layered MoS2, Raman mappings of E2g and A1g Raman modes in mono and few layered MoS2, Raman spectral line shape analysis of pristine MoS2 and MoS2-graphene heterostructures, charge transfer resistance (Rct) and current density (j) values of pristine mono and a few layer MoS2 and graphene- MoS2 vdW solids, Tafel plots for the polarization curves in GRMS, MSGR, GRMSM and MSMGR, calculation of Electrochemical Surface Area (ECSA) of the electrode using Ramdles-Savick Equation, the PEC HER activity in graphene- MoS2 systems estimated with respect to the geometrical surface area, top and side view of Charge accumulation and depletion after hydrogen adsorption on bilayer graphene-MoS2 heterostructure, schematic of a two zone CVD setup and detailed Methods on the experimental and theoretical parts..


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Figure Captions Figure 1: Structural characterization of MoS2 and graphene van der Waals heterostructures. (a) FESEM images of a few layered (MS) and single layer MoS2 (MSM) flakes. TEM image of a few layered flake and corresponding HRTEM image and SAED pattern is shown below for MS sample. (b) Hexagonal crystalline lattice of single layer large area graphene (GR) and inset shows the IFFT and FFT pattern of SLG. (c-d) HRTEM images of MSGR from the same place with different focusing (top layer and bottom layer separately). Evidence for the defect free graphene and MoS2 heterostructure is revealed from the HRTEM and corresponding IFFT and FFT indicate the diffraction pattern of graphene and MoS2, respectively. (e) Raman signatures of pristine MoS2 and MoS2-graphene heterostructures. (f) PL spectra of various samples. Note that that the Raman and PL measurements were performed inside the Lincom cell and symbol ‘*’ corresponds to the fluorescence background coming from the transparent glass of the cell. Figure 2: The PEC HER activity in (a) graphene- few layered MoS2 systems and (b) graphene- mono layer MoS2 systems. (c) A bar diagram shows the ratio of HER current density (J) in dark and light of MoS2 and MoS2-graphene samples at -0.6 V (RHE). (d-e) EIS characteristics of graphene-MoS2 van der Waals solids with different stacking sequences of a few and mono layer MoS2. Note that the data points represent experimental data while solid black line is a simulation using the Randle’s circuit to estimate the charge transfer resistance (Rct) in dark and light conditions. (f) Rct values of various electrodes in dark and light conditions. . Figure 3: (a) Bi-layer heterostructure of graphene and MoS2 where cyan, yellow and black color denote Mo, S and C atoms, respectively; (b) corresponding band structure and density of states. Side and top view of hydrogen adsorption (c) on graphene side and (d) MoS2 side, where hydrogen is denoted by red color sphere. (e) Plot of ∆GH for hydrogen adsorption in both of these cases. Figure 4: Band structure and density of states of bilayer graphene/Mos2 heterostructure when hydrogen is adsorbed (a) on graphene side and (b) on MoS2 side. (c) Band structure and density of states of heterostructure having graphene with three layers of MoS2. (d) Plot of ∆GH as a function of number of layers of MoS2 while graphene layer is kept fixed to one (red color plot represents the hydrogen adsorption on MoS2 side and green color plot represent the same on graphene (GR) side).


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Figure 1:


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GR

Figure 3:


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ACS Energy Letters

MoS2 side GR- side

Figure 4:


ACS Energy Letters

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References (1)

Ajayan, P.; Kim, P.; Banerjee, K. Two-dimensional van der Waals materials. Phys Today 2016, 69 (9), 38-44.

(2)

Geim, A. K.; Grigorieva, I. V. Van der Waals heterostructures. Nature 2013, 499 (7459), 419425.

(3)

Shi, Y.; Zhang, H.; Chang, W.-H.; Shin, H. S.; Li, L. J. Synthesis and structure of twodimensional transition-metal dichalcogenides. MRS Bulletin 2015, 40 (07), 566-576.

(4)

Honglin Li, K. Y.; Chao, L.; Zheng, T.; Bangjun, G.; Xiang, L.; Hao, F.; Ziqiang, Z. ChargeTransfer Induced High Efficient Hydrogen Evolution of MoS2/graphene Cocatalyst. Sci Rep 2015, 5, 18730.

(5)

Araujo, P. T.; Terrones, M.; Dresselhaus, M. S. Defects and impurities in graphene-like materials. Mater. Today 2012, 15 (3), 98-109.

(6)

Shi, J.; Tong, R.; Zhou, X.; Gong, Y.; Zhang, Z.; Ji, Q.; Zhang, Y.; Fang, Q.; Gu, L.; Wang, X.. et al. Temperature-Mediated Selective Growth of MoS2/WS2 and WS2/MoS2 Vertical Stacks on Au Foils for Direct Photocatalytic Applications. Adv. Mater. (Weinheim, Ger.) 2016, 28 (48), 10664-10672.

(7)

Ravi, K. B.; Shubhadeep, P.; Rahul, S.; Giri, P. K.; Tharangattu, N. N. Stacking sequence dependent photo-electrocatalytic performance of CVD grown MoS2 /graphene van der Waals solids. Nanotechnology 2017, 28 (8), 085101.

(8)

Britnell, L.; Ribeiro, R. M.; Eckmann, A.; Jalil, R.; Belle, B. D.; Mishchenko, A.; Kim, Y. J.; Gorbachev, R. V.; Georgiou, T.; Morozov, S. V. et al. Strong Light-Matter Interactions in Heterostructures of Atomically Thin Films. Science 2013, 340 (6138), 1311-1314.

(9)

Govind Rajan, A.; Warner, J. H.; Blankschtein, D.; Strano, M. S. Generalized Mechanistic Model for the Chemical Vapor Deposition of 2D Transition Metal Dichalcogenide Monolayers. ACS Nano 2016, 10 (4), 4330-4344.

(10)

Wang, S.; Rong, Y.; Fan, Y.; Pacios, M.; Bhaskaran, H.; He, K.; Warner, J. H, Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition. Chem. Mater. 2014, 26 (22), 6371-6379.

(11)

Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C.-Y.; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2. Nano Lett. 2010, 10 (4), 1271-1275.

(12)

Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M. Photoluminescence from Chemically Exfoliated MoS2. Nano Lett. 2011, 11 (12), 5111-5116.


Page 18 of 19

Page 19 of 19

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ACS Energy Letters

(13)

Chow, P. K.; Jacobs-Gedrim, R. B.; Gao, J.; Lu, T. M.; Yu, B.; Terrones, H.; Koratkar, N. Defect-Induced Photoluminescence in Monolayer Semiconducting Transition Metal Dichalcogenides. ACS Nano 2015, 9 (2), 1520-1527.

(14)

Kuc, A.; Zibouche, N.; Heine, T. Influence of quantum confinement on the electronic structure of the transition metal sulfide. Phys. Rev. B 2011, 83 (24), 245213.

(15)

Zhang, Y.; Li, H.; Wang, H.; Liu, R.; Zhang, S. L.; Qiu, Z. J. On Valence-Band Splitting in Layered MoS2. ACS Nano 2015, 9 (8), 8514-8519.

(16)

Pandey, T.; Nayak, A. P.; Liu, J.; Moran, S. T.; Kim, J. S.; Li, L. J.; Lin, J. F.; Akinwande, D.; Singh, A. K. Pressure-Induced Charge Transfer Doping of Monolayer Graphene/MoS2 Heterostructure. Small 2016, 12 (30), 4063-4069.

(17)

Pal, S.; Sahoo, M.; Veettil, V. T.; Tadi, K. K.; Ghosh, A.; Satyam, P.; Biroju, R. K.; Ajayan, P. M.; Nayak, S. K.; Narayanan, T. N. Covalently Connected Carbon Nanotubes as Electrocatalysts for Hydrogen Evolution Reaction through Band Engineering. ACS Catalysis 2017, 2676-2684.


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