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Journal Name ARTICLE Vertically aligned MoS2 nanosheets on graphene for highly-stable electrocatalytic hydrogen evolution reaction Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/

Paulraj Gnanasekara, Dharmaraj Periyanagoundera and Jeganathan Kulandaivela* Efficient hydrogen evolution reaction (HER) using two-dimensional layered materials as electrocatalyst with highpermormance remains a challenging task due to the insufficient edge active sites. In this regard, herein, molybdenum disulphide nanosheets with rich active sulphur sites are vertically grown on graphene surface by chemical vapour deposition process. The direct integration of vertically aligned MoS2 nanosheets on graphene forms van der Waals (vdW) heterojunction which facilitates a barrier free charge transport towards the electrolyte as a result of unique and well-matched energy band alignment at the interface. The prospective combination of Ohmic graphene/MoS2 heterostructure and high electrocatalytic edge activity of sulphur deliver an incredible and small turn on potential of 0.14 V vs. RHE in acid electrolyte solution. Most importantly, the use of vertical vdW device architexture exhibits nearly 8X improvement in HER than that of layered counterpart. Besides, the HER reaction is highly stable over 50 hours of continuous run even after 150 days. The combined analysis of our study make certain that the graphene/MoS2 heterostructure will be an efficient alternative electrode for the low-cost and large scale electrochemical applications.

abundant, and eventually (vi) Low fabrication cost for commercialization.

1. Introduction Hydrogen (H2), the simplest element in our expanding universe, is known to be the spotless, clean and promising fuel for the future generation and potential source of renewable energy available next to the sun.1 H2 is the alternative energy to overcome the centuries of reliance on conventional fossil fuels and will be a better solution for existing crucial environmental issues such as global warming, climate change and CO2 emission.2,3 Even though numerous techniques are in practice to harvest ultra-pure hydrogen energy,4-7 electrocatalytic assisted water splitting is considered as one of the superior approaches to overcome the extending global energy demand.8 This approach is deeply investigated because of its low production cost, straight forward processing and high efficiency with low operational voltage. In electrocatalytic hydrogen production, electric potential is applied between anode and cathode to produce O2 and H2 from our limitless source of water via two half-cell reactions of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).1,2,9 In the electrocatalytic H2 production, the electrodes must possess (i) high active catalytic sites, (ii) superior conductivity, (iii) rich acidstability, (iv) ability to withstand high current density, (v) earth

Centre for Nanoscience and Nanotechnology, Department of Physics, Bharathidasan University, Tiruchirappalli-620024, Tamil Nadu, INDIA. *Corresponding Author E-mail: [email protected] / [email protected] Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x

Till date, rare earth materials such as Platinum (Pt) is known to be the best electrode material for HER which exhibits almost zero Gibbs free energy and low turn on potential upon the electrocatalytic reaction.8,10 In contrast, Pt and Pt based materials are expensive and have a drawback of dissolution and therefore limit device viability.11,12 Hence, there is a need to develop a low-price, ecofriendly, earth ample and highly efficient electrode materials for direct commercial implementation of electrocatalytic HER. At the same time, with years of continuous investigation and tremendous efforts by the research community, it is found that the transition metal based chalcogenides, carbides, phosphides and nitrates exhibit superior electrocatalytic performance that may replace the conventional Pt based electrodes.13-18 Excitingly, molybdenum disulphide (MoS2) a kind of transition metal dichalcogenides (TMDC), emerged as a promising material and regarded as the most efficient material among all kind of transition metal based materials for electrocatalytic HER.3,8 The three-atom thick MoS2 provokes extremely active sulphur terminated edge sites and displays robust nature even in strong acid medium which are the indispensable pre-requirement for high efficient electrodes for electro catalytic reactions.1,12,19 Importantly, MoS2 exhibits near zero positive Gibbs free energy (ΔGH ~ 0.06eV) with high exchange current density in the volcano plot of electrocatalytic water splitting along with low turn on potential, which make certain that MoS2 is the ideal material for future day-to-day applications of HER.3,12

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Both experimental and theoretical investigations substantiate that HER performance of MoS2 is purely depend on electrocatalytic activity of edge sulfur sites only, since in-plane sites are inert to electrocatalytic activity.20,21 Therefore, increasing the number of edge active sites without compromising stability and excellent electrical properties are the current challenges faced by scientific community. To overcome the difficulties, numerous efforts have been made to improve the HER performance of MoS2.15,22,23 Engineering the morphologies of manifold structures such as flat layers, nanospheres, vertical sheets and nanowires of MoS2 are the ultimate ways to manipulate the number of catalytically active sulphur sites.20,24-30 Among them, vertically aligned MoS2 NS possess high surface area as compare to other low dimensional counterparts which provides more catalytic active edge sites, making vertical MoS2 NS as the preferred architecture for electrocatalytic HER.31,32 In contrast, the major hindrance in implementation of MoS2 for HER application is the poor inter-layer electrical charge transport, which eventually hampers the device performance.33,34 Therefore, it is essential to combine rich electrocatalytically active MoS2 with highly conducting materials having fruitful band alignment to reduce Ohmic loss at the junction interface.34 In addition to that, the cost of commonly used conducting electrodes/layers such as FTO, ITO, glassy carbon electrode, metal layer (Au, Ag and Ni) and ionic binder (Nafion) are highly expensive in terms of commercialization and thus making MoS2 based HER device not viable for low cost applications. In order to circumvent the challenges, we employed chemical vapor deposition (CVD) grown large scale graphene as conducting electrode to support the electrocatalytic active vertical MoS2 NS. Here, graphene not only forms a superior van der Waals (vdW) contact to MoS2 but also provides an unhindered charge transport toward the active sulphur sites of MoS2 which advances to supremacy negative charge density at MoS2 for enhanced HER performance. The implementation of MoS2/graphene electrodes significantly reduces the contact resistance owing to well-matched interface band alignment, where the interface is barrier free and highly Ohmic, and thus profoundly replaces the use of electrode materials such as Au, Ag and ionic binders.35,36 Moreover, recent theoretical elucidations based on MoS2-Au and MoS2-graphene heterostructures exhibit comparable hydrogen absorption Gibbs free energy value of ΔGH ~ 0.12 (graphene) ΔGH ~ 0.14 (Au) for HER reactions, which crystalize the charge transport properties of graphene is similar to that of metal electrodes.12 Thus, interfacing MoS2-graphene electrode is an impressive approach to improve the HER performance. To the best of our knowledge this is the first attempt to fabricate the vertical MoS2 nanosheets directly on graphene 2D layer electrode to form vdW heterostructure by CVD for HER application. This unique vdW heterostructures results in nearly 8 times higher electrocatalytic HER performance as compared

to MoS2 layered counterpart. The device demonstrates stable and enhanced electrocatalytic HER performance with stability over half year.

2. Experimental Section: 2.1 Graphene synthesis and Transfer onto SiO2 Substrate: Graphene was synthesized using atmospheric pressure CVD (APCVD) of acetylene on copper foil as reported in our earlier work.37 To transfer graphene onto the SiO2 substrate, a thin layer of poly methyl methacrylate (PMMA) was deposited on top of graphene/Cu foil by spin coating. The underlying Cu foil was entirely removed using 0.5 M concentrated ammonia persulfate solution. Prior to the transfer, SiO2 substrate was sonicated in acetone, ethanol and DI water for 15 minutes each. Then, the substrate is treated with Piranha solution (piranha solution is a 3:1 ratio mixture of H2SO4 and H2O2) for two hours at 70⁰C. The isolated PMMA/graphene rinsed several times with DI water until metal etchant contaminations are completely removed and stack was transferred on to SiO2 substrate effectively without any damage of fragile graphene. Transferred substrate was dried in vacuum ambient for 4 hours and the polymer residual PMMA on top of graphene/SiO2 stack was removed by acetone treatment.

Figure 1: Schematic diagram of two-zone CVD furnace utilized to synthesis of vertical MoS2 NS.

2.2 Synthesis of Vertical aligned MoS2 NS on Graphene/SiO2 Substrate: The growth of vertical MoS2 NS was performed using homebuilt two-zone CVD reactor. The schematic diagram of CVD furnace and vertical mounting of substrate are shown in figure 1. The precursors of MoO3 and sulphur are taken as 10 mg and 80 mg respectively and placed on the respective heating zones as shown in figure 1. Initially, the atmospheric residuals present inside the quartz tube was eliminated by pumping the chamber to 10-3 mbar followed by building of inert Ar gas atmosphere at which the entire reaction was carried out. The substrate was placed perpendicular to the precursor at a distance of 1 cm from MoO3 and 20 cm from sulphur source. For the synthesis of vertical MoS2 NS, the heating zone of MoO3 precursor was

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Figure 2: (a-d) FESEM images of vertically grown MoS2 NS on graphene, and (e-h) show the EDS elementary mapping of vertical MoS2 (the mapping element is mentioned in bottom right corner).

ramped up to 650 ⁰C at a heating rate of 15 ⁰C/min, and subsequently the temperature is slowly increased to 750 ⁰C at a rate of 2 ⁰C/min and kept at 750 ⁰C for 10 minutes. For sulphurization, temperature of sulphur zone was rapidly increased to 200 ⁰C at a heating rate of 20 ⁰C/min. 2.3 Characterization Techniques: The morphological features of MoS2 on graphene surface were analyzed using field emission scanning electron microscope (FESEM) (Carl-Zeiss Σigma). Elementary compositions are confirmed by Energy Dispersive Spectroscopy (EDS) mapping (Oxford instruments EDS Inca Act-X). The optical properties of MoS2 was investigated by Raman scattering and Photoluminescence (PL) spectroscopy using Renishaw invia Raman Spectrometer with excitation laser source of 532nm and spot size of 1µm. 2.4 Electrochemical Studies: For electrocatalytic and impedance measurements, graphene layer was used as electrode and a Cu strip contact was provided using silver paste. Except the active area of MoS2 NS, entire substrate including the metal contact was completely insulated using epoxy isolation solution. Electrochemical measurements were carried out using Bio-Logic

potentiostat SP-150 workstation. A 3-electrode system was incorporated for all electrochemical measurements in which vertical MoS2 NS on graphene was used as a working electrode, Pt rod as a counter electrode and Ag/AgCl as a reference electrode. We used 0.5 M H2SO4 electrolyte for electrochemical measurements. All the potentials were calibrated using reversible hydrogen electrode (RHE), assessed using equation ERHE = EAg/Agcl+0.059 × pH + Eo. The iR polarized linear sweep voltammetry (LSV) studies were carried out at a scan rate of 50mVs-1 for cathodic potential range of 0 to -0.8V vs. RHE.

3. Results and Discussion: The vertical MoS2 NS was successfully synthesized on graphene/SiO2 substrate by facile CVD approach. The growth of MoS2 NS along vertical direction was achieved by fine controlling the parameters such as the gas flow, temperature and positioning the substrate vertically relative to the precursors which follows the Mullins-Sekerka instability mechanism as revealed by Fu Zhang et al and R A Vila et al.35,36 In contrast, for the growth of layered MoS2, we placed the substrate parallel to the precursors and various gas flow rate is maintained as reported in our earlier work.37 Briefly, at the growth temperature (750 ⁰C) MoO3 (in zone–II) is reduced into volatile MoO(3-x) vapors. Here, graphene does the major role in initial nucleation for the growth of vertical MoS2 nanosheets on

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graphene (VMN) that greatly reduces the required free energy for the primary nucleation and promote the subsequent growth of MoO(3-x) NS.40 During the initial process of sulphurization, the surface of MoO(3-x) NS are sulphurized into MoS2 while the innerlayers remain as MoO(3-x), thus forming core-shell like MoS2/MoO(3-x) structure. The prolonged sulphurization for 30 minutes completely render the core-shell structure into MoS2 NS39 as evidenced by the experimental observation.

the characteristic vibrations of D (1349 cm-1), G (1584 cm-1) and 2D (2698 cm-1) and full Raman spectrum of VMN is shown in figure S3.37 The full Raman spectrum of VMN and pristine graphene on SiO2 is shown in figure S4. PL spectrum was recorded to probe the optical nature of grown VMN. The characteristic B1 and A1 excitations of MoS2 are observed at 650 and 667 nm respectively (figure 3b).40,41 The dominant excitation A1 present at 1.85 eV evidences the direct band gap of VNM and comparable to that of monolayer MoS2. The decrease in PL intensity of VMN is attributed to the inter-layer charge trapping and also may be due to the change in ᴦ point of valence band maximum. The wide full width at half maximum (FWHM) of VMN (200 meV) can be ascribed to the presence of multiple MoS2 NS.42,43 The structural and electrical properties of CVD grown graphene was briefly discussed in our previous reports.37,40 XPS analysis was carried out to investigate the chemical states of MoS2/graphene heterostructure after a period of 90 days stored in laboratory ambient. The XPS survey spectrum of VMN is shown in figure S4. The presence of core level peaks of S 2p at 161.2 and 165.4eV (figure 3c), S 2s peak at 226.3eV, and Mo 3d peaks at 288.5 and 287.1 eV (figure 3d) ensure the presence of MoS2. In addition, the appearance of strong peaks at 284.4 and 285.2 eV evidence the C-C of sp2 and C-O, respectively (figure S5), in which the C-O is attributed to the surface oxidation as a result wet chemical transfer of graphene from Cu foil to SiO2. Our observation validates the absence of oxygen incorporation in MoS2 lattice, indicating highly stable nature of MoS2 structure, consistence with earlier reports. 23,29,36

The FESEM images of VMN on graphene were depicted in figure 2 (a-d) which reveal the morphological evolution of MoS2 with high density over centimeter scale on graphene surface. The average thickness and length of VMN were calculated to be 56 nm and 5 µm, respectively. In addition, the elemental distribution of Mo, S and C are identified from the EDS elementary mapping which substantiates the homogeneous coverage of vertical MoS2 NS on graphene (figure 2e-h) and the presence of C corresponds to the graphene underlying layer on scanned area. Figure 3a displays the characteristic Raman spectrum of VMN with comparison of monolayer MoS2 synthesized by CVD. The presence of two characteristic dominant E12g and A1g vibrations at 377 and 404 cm-1 respectively, represent the formation of MoS2. The E12g and A1g peaks are assigned to the in-plane and out-of-plane vibration of Mo-S atoms respectively. The peak difference (ΔK) of VMN is noted to be 25.5 cm-1, which is in line with the previous reports of vertical sheet morphology of MoS2.23 The strong shift in Raman vibrational modes (E12g and A2g) for vertical sheets of MoS2 is attributed to the Coulombic interactions and stacking induced bonding charges in between layers. In addition, one can roughly estimate the number of edge active sites via intensity of Raman A1g mode as it originates from vibration of sulphur sites along the c-axis.40 Thus, the enhanced intensity of A1g mode of VMN strongly suggest the presence of numerous edge terminated sulphur sites. Raman spectrum of graphene recorded after transfer onto SiO2 substrate (Figure S2) displays

3.1 HER Characterization: To evaluate the HER performance of VMN, we carried out electrocatalytic water splitting measurements and compared with layered MoS2 on graphene (LMG) and layered graphene (LG). A schematic structure of the fabricated device is shown in inset of figure 4a. The iR polarized LSV curves of LG, LMG, VMN and Pt rod are shown in figure 4a. The comparative results of turn on potential and over potential (η10) at 10mA/cm2 of all devices are drawn graphically in figure 4b. The LG, LMG, VMN and Pt exhibit the turn on potential (potential to reach 1 mA/cm2) of -394, -310, -188, and -18 mV vs. RHE, respectively. Further, the VMN device shows an over potential (η10) at -421 mV vs. RHE, which is significantly lower than LMG (η10 = 590 mV vs. RHE) and LG (η10 = 1300 mV vs. RHE) samples. A maximum current density of 280 mA/cm2 is achieved on the VMN sample. At The negative potential of -0.8V vs. RHE. Such an unprecedented performance of VMN is attributed to the presence of high density of sulphur site which are electrocatalytically active towards HER. On the other hand, the LG device exhibits negligible electrocatalytic activity owing to inert nature of graphene towards HER and poor charge interface transport between graphene and electrolyte solution which ends in high turn on potential and low current density. Despite LMG sample does not possess high active sulphur site as compared to vertical counterpart, an unhindered charge

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Figure 4: Electrocatalytic measurements (a) Polarized LSV curve at scan rate of 50mV/S on 0.5M H2SO4 electrolyte (b) comparative graph of turn on potential and overpotential at 10mA/cm2 (c) Tafel slope of LG, LMG, VMN, and Pt and (d) Nyquist plot of LG, LMG and VMN electrodes recorded at the applied bias of 0.0V vs. RHE.

transport at the interface yields an intermediate HER performance. The combination of abundant edge active sites of vertically aligned MoS2 NS with unhindered transport due to graphene electrode results in remarkable HER activity 23,34,40 which is ~8 times higher than LMG and ~35 times higher than LG. We have also calculated the Tafel slope from the LSV studies, which is the intimately related to electrocatalytic activity of a material and key to conclude whether the HER follows Volmer-Heyrovsky or Volmer-Tafel mechanism. Tafel plot of respective devices is shown in figure 4c. The characteristic Tafel slope was calculated using the equation η=b log(j)+a, where η is the over potential, b is the Tafel slope, a is the constant and j is the current density (mA/cm2).44 The electrocatalytic HER follows two steps: first, discharging reaction, that is proton (H+) was absorbed on MoS2. This surface absorbed H+ ions combine with electrons from cathode, yields Hads (H+ + e- = Hads), Hads is the absorbed hydrogen atom on the catalytic sites. This initial discharge reaction is known as Volmer mechanism with Tafel slope of 116 mV/dec. Second, the desorption/combination reaction, this would be either Heyrovsky mechanism (Hads+ H+ + e- = H2) or Tafel mechanism (Hads+ Hads = H2) with Tafel slope value of 38 and 29 mV/dec respectively.45 The LG, LMG, VMN and Pt exhibits Tafel slope of 320, 104, 84 and 30 mV/dec respectively. The VMN and LMG structures are expected to follow Volmer - Heyrovsky rate determination step with high Tafel slope value and this might

be due to the restricted electron-proton recombination by outturn hydrogen atoms.44 Electrochemical Impedance spectroscopy (EIS) was performed to probe the interface charge migration and electrolyte-MoS2 charge transfer resistance. The Nyquist plots of the VMN, LMG and LG are shown in figure 4d, all measurements were carried out in the range of 1 Hz to 104 Hz at applied bias of 0 V vs. RHE. An equivalent circuit model is illustrated from EIS measurements with electronic phase element and found to be RS-RCTC, where RS is the resistance at graphene-MoS2 junction, C and RCT is the differential capacitance and resistance at the MoS2–electrolyte junction, respectively.10 Absence of multiple semicircles in Nyquist plot indicates the barrier free unhindered charge transport across graphene-MoS2 heterostructure. The 2.5X reduction in RCT value of VMN (80 Ω) as that of layer counterpart (193 Ω) substantiates the rapid electron transfer kinetics toward the electrolyte as a result of intrinsic highly dense active sulphur sites. The graphene as interlayer not only promotes the growth of vertical sheets but also provides a resistance free and highly Ohmic transport, thereby boosting the number of carriers moving toward the electrolyte. Hence, the high performance of VMN device can be attributed to the prospective combination of MoS2/graphene hybrid nano-structures with reduced contact resistance and having high population of active edge sulphur sites.

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Journal Name HER measurements in Table T1 (supporting information). In comparison, with notable exemptions, despite large area, VMN devices exhibits similar turn on potential with very high stability (50K sec in CA and 150days). This is very significant and highest among the reported results including glassy carbon electrodes. However, the slightly high value of η10 of VMN device is purely due to the strong coupling between graphene and MoS2 which results in weakening of hydrogen binding energy and thus nominally shifts Gibbs free energy of MoS2, as theoretically demonstrated by Tsai et al.12 From our observations, graphene as electrode and the direct growth of VMN on graphene by CVD has several benefits such as (i) well-aligned MoS2 growth along the vertical direction, (ii) abundant edge active sulphur sites, (iii) barrier free, unhindered charge transport towards electrolyte, and (iv) enhanced HER performance due to the reduced RCT. In general, the 2D vdW combination of high electrocatalytic active VMN with rich electron transport layer of graphene not only advances to significant improvement in electrocatalytic HER performance but also paves a way for the future integration of graphene with various TMDC to form heterojunction for diverse applications. Further, we are working on enhancing the HER activity of large area MoS2/Graphene devices functionalizing with single atom catalyst, the results will be published in the forthcoming article.

In addition, the durability is the important criteria next to the performance in search of better electrocatalytic HER electrodes for the day-to-day applications. Here, the superlative non-destructive HER performance of VMN device on strong acid medium was evidenced by continuous 1000 cycle LSV studies and 50 hours chronoamperometry (CA) after 24 hours immersion in 0.5M H2SO4 electrolyte solution as shown in figure 5. The long-term stability of VMN was perceptible from continuous 1000 cycle LSV studies and only imperceptible variation in 1000th cycle from the primary cycle is identified as shown in figure 5a. Further, to evaluate the long-term stability, the VMN device was kept under open ambient for 150 days and afterwards HER characterizations were repeated. The LSV results of day 150 displays negligible change in onset potential with minimum variation in the maximum current density (j) at -0.8 V vs. RHE of day 01 (figure 5b). The stability is further corroborated via CA measurements for 50 hours at an applied bias voltage of -0.5V vs. RHE for different days and the results are shown in figure 5d. The VMN device exhibits remarkable stability with negligible variations even after 150 days which indicates the long-term stability and sustainability in acidic environment. Eventually, in order to highlight the figure-of-merit, we have drawn the uncertainty of the experiments based on the data collected from turn on potential and η10 of different devices under same conditions as shown in figure 5c. The average turn on potential and η10 of the devices are found to be -0.14 ± 0.005 and -0.40 ± 0.02 V vs. RHE, respectively (figure 5c), demonstrating the impressive consistency, long time repeatability and robustness for future HER applications. In addition to that, we compared our VMN device performances with reported MoS2/graphene electrocatalytic

4. Conclusions The integration of vertically aligned MoS2 NS with graphene by CVD resulted in highly stable, robust and high-performance metal free hybrid electrocatalyst for HER reaction. The structural and morphological studies corroborate the homogeneous distribution of VMN NS with highly active sulphur sites. The hybrid electrocatalyst synergistically promotes high active barrier free electron transport exhibiting an outstanding high electrocatalytic activity including a lower turn on potential, lower Tafel slope (84 mV/dec) which is ~35 times higher than LG. The hybrid vdW heterojunction retains a stable performance even after half-year. The high and stable performance of the VMN structure can be attributed to the prospective combination of the pronounced electrocatalytic active sites with rapid charge transfer kinetics towards exposed active surface area. Thus, the findings highlight the significance of interface engineering and tuning the active sites by controlling the morphologies for the improved electrocatalytic HER performance.

Conflicts of interest There are no conflicts to declare.

Acknowledgements The K.J. thanks the Department of Science and Technology (DST), Govt. of India for the financial support to develop the infrastructural facility under the scheme of FIST and Nanomission (Contract No.SR/NM/NS-1502/2014). KJ also acknowledges Ministry of Human Resource Development under Rashtriya Uchchatar Shiksha Abhiyan (RUSA), Alexander von

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Humboldt (AvH) foundation and DRDO for partial financial support. G.P acknowledges DST-INSPIRE, Govt. of India for the award of Junior Research Fellowship (DST INSPIRE-JRF IF#170198 DST/INSPIRE Fellowship/2016).

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(13) Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang, R.; Lei, Y.; Pan, B.; Xie, Y. Controllable Disorder Engineering in Oxygen-Incorporated MoS2 Ultrathin Nanosheets for Efficient Hydrogen Evolution. J. Am. Chem. Soc. 2013, 135 (47), 17881– 17888. (14) Voiry, D.; Yamaguchi, H.; Li, J.; Silva, R.; Alves, D. C. B.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M. Enhanced Catalytic Activity in Strained Chemically Exfoliated WS2 Nanosheets for Hydrogen Evolution. Nature Materials 2013, 12 (9), 850–855. (15) Kong, D.; Wang, H.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y. Synthesis of MoS2 and MoSe2 Films with Vertically Aligned Layers. Nano Letters 2013, 13 (3), 1341–1347. (16) Vrubel, H.; Hu, X. Molybdenum Boride and Carbide Catalyze Hydrogen Evolution in Both Acidic and Basic Solutions. Angewandte Chemie - International Edition 2012, 51 (51), 12703– 12706. (17) Chen, W. F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Hydrogen-Evolution Catalysts Based on Non-Noble Metal Nickel-Molybdenum Nitride Nanosheets. Angewandte Chemie - International Edition 2012, 51 (25), 6131–6135. (18) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. Journal of the American Chemical Society 2013, 135 (25), 9267–9270. (19) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317 (5834), 100–102. (20) Yu, M.; Zhao, S.; Feng, H.; Hu, L.; Zhang, X.; Zeng, Y.; Tong, Y.; Lu, X. Engineering Thin MoS2 Nanosheets on TiN Nanorods: Advanced Electrochemical Capacitor Electrode and Hydrogen Evolution Electrocatalyst. ACS Energy Letters 2017, 2 (18), 1862– 1868. (21) Chatti, M.; Gengenbach, T.; King, R.; Spiccia, L.; Simonov, A. N. Vertically Aligned Interlayer Expanded MoS2 Nanosheets on a Carbon Support for Hydrogen Evolution Electrocatalysis. Chemistry of Materials 2017, 29 (7), 3092–3099.

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