Improved Hydrogen Evolution Reaction Performance using MoS2

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Improved hydrogen evolution reaction performance using MoS2-WS2 heterostructures by physico-chemical process Dhanasekaran Vikraman, Sajjad Hussain, Kamran Akbar, Linh Truong, Adaikalam Kathalingam, Seung-Hyun Chun, Jongwan Jung, Hui Joon Park, and Hyun-Seok Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00524 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Sustainable Chemistry & Engineering

Improved hydrogen evolution reaction performance using MoS2-WS2 heterostructures by physicochemical process Dhanasekaran Vikramana, Sajjad Hussainb,c, Kamran Akbarb,d, Linh Truongb,e, Adaikalam Kathalingamf, Seung-Hyun Chunb,e, Jongwan Jungb,c, Hui Joon Parkg,h, Hyun-Seok Kima* a.

Division of Electronics and Electrical Engineering, Dongguk University-Seoul, 30, Pildong-ro

1 gil, Jung-gu, Seoul 04620, Korea. b.

Graphene Research Institute, Sejong University, 209, Neungdong-ro, Gwangjin-gu, Seoul

05006, Korea. c.

Institute of Nano and Advanced Materials Engineering, Sejong University, 209, Neungdong-ro,

Gwangjin-gu, Seoul 05006, Korea. d.

Department of Energy Science, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon

16419, Korea. e.

Department of Physics, Sejong University, 209, Neungdong-ro, Gwangjin-gu, Seoul 05006,

Korea. f.

Millimeter-wave Innovation Technology (MINT) Research Center, Dongguk University-Seoul,

30, Pildong-ro 1 gil, Jung-gu, Seoul 04620, Korea. g.

Department of Energy Systems Research, Ajou University, 206, Worldcup-ro, Suwon 16499,

Korea. h.

Department of Electrical and Computer Engineering, Ajou University, 206, Worldcup-ro,

Suwon 16499, Korea. *Corresponding author - Email: [email protected] KEYWORDS. Heterostructures; Electrocatalyst; Hydrogen evolution; MoS2; WS2

ACS Paragon Plus Environment

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ABSTRACT. This report describes synthesis of layered molybdenum disulphide (MoS2)tungsten disulphide (WS2) heterostructure onto fluorine doped tin oxide covered glass substrates using a combination of chemical bath deposition and RF sputtering techniques. FESEM images revealed that MoS2-WS2 heterostructure surface consisted of cauliflower structured array of grains with the spherical structures. The vertically aligned atomic layers were explored by transmission electron microscopy images for MoS2-WS2 heterostructure. Hydrogen evolution reaction (HER) kinetics show overpotentials of 151 and 175 mV @ 10 mA/cm2 with Tafel slope values of 90 and 117 mV/decade for pristine MoS2 and WS2 electrocatalysts, respectively. Improved electrocatalytic activity for HER was established with overpotential 129 mV @ 10 mA/cm2 and Tafel slope 72 mV/decade for the MoS2-WS2 heterostructure. The MoS2-WS2 heterostructure electrocatalyst showed robust continuous HER performance over 20 hours in an acidic solution. This improved electrochemical performance emerges from the elevation of electron-hole separation at the layer interfaces, and sharing of active edge sites through the interface. This study provides the basis to develop new applications for transition-metal dichalcogenides heterostructures in future energy conversion systems.

INTRODUCTION The unremitting reduction of Earth’s environmental health makes it crucial to explore alternate sustainable energy resources to diminish the consumption of fossil fuels1-4. Since water is an abundant natural resource, a good alternative green and renewable energy source is hydrogen (H2) production by splitting water, reducing our dependency on other non-abundant natural energy resources3, 5. Compared with other routes, electrochemical water splitting via the hydrogen evolution reaction (HER) is a simple and non-polluting methodology to produce H2 on a large scale with high efficiency6, 7. However, this route is limited primarily due to the cost of


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the scarce and precious noble metal platinum (Pt) and its alloys to use as electrocatalysts8, 9. Hence, many studies have explored capable and robust electrocatalysts from earth-abundant and cost-effective elements to replace the scarce Pt-group metals to provide high current density at low overpotential for efficient energy production10-13. The graphene innovation makes attraction of two dimensional (2D) materials because of their wide range of potential applications, including electrochemical, transistors, photonic, and optoelectronic devices14. Among the numerous 2D materials, transition metal dichalcogenides (TMDCs) have emerged as the most promising candidate, possessing tunable energy band gaps and advantageous optoelectronic properties3, 10, 15. TMDCs denote a group of layered materials, with distinct and flexible optical, mechanical, and electrical behaviors. Their elemental makeup is generally denoted as MX2, where M is a transition metal and X is a chalcogen2, 6, 16, and they have fascinated research interest owing to their abundance and superior HER electrochemical activity16. Previous studies have shown that TMDCs have tunable electronic structure which can enhance their intrinsic HER electrochemical properties to provide efficient catalysts17-19. In particular, nanocrystalline TMDC structures are having exposed sulfur or selenide edge sites to catalyze the hydrogen evolution rather than the inert bulk form15, 20. MoS2 and WS2 are highly featured materials from TMDC family that have direct band gap transitions and active edge sites6, 21, 22, and similar electronic band structure23. Recent theoretical studies demonstrated that MoS2 and WS2 valence band edges are more positive compared with the water oxidation potential, in contrast to its bulk form which lacks to attain the thermodynamic necessities for electrochemical water splitting24. Voiry et al.25, 26 demonstrated improved HER properties for 1T-polytype phase MoS2 and WS2 than 2H phase. As predicted, S 2p valence band orbitals are having nearby value of water oxidation potential, which suggests


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their capability to use as HER electrocatalysts27-29. Many attempts have been made to synthesize well-ordered MoS2 and WS2 edge sites for efficient HER electrocatalysts29-31, but no suitable HER catalysts from cost effective and abundant materials that are comparable to Pt based catalysts have resulted. Recent interest has turned to heterostructures, which are essential building blocks in present semiconductor industries, and show an important part in electrochemical devices32, 33. TMDC layers are stacked to form heterostructures with van-der Waals interactions, which is arranged with layer by layer using different kinds of TMDC materialsto construct the heterojunctions. Various desirable characteristics are demonstrated on such TMDC heterostructures, and validated in applications with better performance32, 34. The heterojunctions might be generate fascinating novel properties and applications, enabling creation of atomically sharp interfaces and providing a broad range of semiconductor heterojunctions with interesting properties23, 33. This study demonstrates chemical bath deposited (CBD) MoS2 and RF sputtered WS2 heterostructures as efficient HER electrocatalysts. MoS2 was prepared on fluorine doped tin oxide glass (FTO) substrates by CBD and post-annealing as reported in previously6, and WS2 was layered by sputtering and sulfurization to obtain MoS2-WS2 heterostructures, delivering more active edges for HER. Electrocatalytic properties are considerably enhanced because of the materials’ interfacing, and the resultant MoS2-WS2 heterostructure electrocatalyst showed outstanding HER performance with the overpotential ~129 mV @10 mA/cm2 and the small Tafel slope (72 mV/decade), with robust stability over 20 h HER performance.

EXPERIMENTAL DETAILS 1. Synthesis of MoS2–WS2 on FTO substrates


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Chemical bath deposition (CBD), an ease and low-cost methodology, was employed to prepare the MoS2 thin films onto FTO substrates. Prior to the film preparation, the FTO substrates were ultrasonically cleansed with acetone, isopropyl alcohol (IPA), and deionized (DI) water, followed by drying and baking for 5 min. The MoS2 thin films were deposited using CBD route with a bath solution encompassing 0.5 M thiourea (CH4N2S) and 30 mM ammonium molybdate ((NH4)6Mo7O24). The bath temperature was fixed at 90°C, and solution pH (10 ± 0.1) was maintained using hydrochloric acid (HCl). Different thickness MoS2 layers were grown by applying different deposition times (20 and 40 min). The reaction occurred in the presence of hydrazine hydrate (N2H4, 1.0 M). As-deposited MoS2 thin films were subjected to S environment annealing at 450°C for 60 min to increase crystallinity. The chamber pressure (2 × 10-2 Torr) and the rate of carrier gas (100 sccm) were maintained constantly. The tungsten (W) layer was deposited on FTO and MoS2/FTO substrates using an RF magnetron sputtering system, in Ar atmosphere. The gas flow rate, space amongst the source and substrate, and pressure were fixed constantly for all the experiments. The pre-sputtering was conducted for 5–10 min to establish stable sputtering conditions before the film deposition. Assputtered W-MoS2/FTO and W/FTO samples were subjected to sulfurization process at 500°C for 1 h using CVD furnace. 2. Characterization of MoS2, WS2 and MoS2-WS2 films The synthesized MoS2, WS2, and MoS2-WS2 films were characterized by Renishaw inVia RE04, Raman spectroscopy (Source - 512-nm Ar laser). X-ray diffraction (XRD) studies were carried out using a Rigaku D/max-2500 diffractometer with Cu-Kα radiation. Mo, W, and S photoelectron reflection spectra and depth profile analyses were performed by X-ray photoelectron spectroscopy (XPS) (PHI 5000 Versa Probe II, ULVAC-PHI). Argon ion


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sputtering was used to perform surface etching. JEOL JSM-6700F, field emission scanning electron microscopy (FESEM), was used to characterize the film morphology, and Vecco Dimension 3100, atomic force microscopy (AFM) was used to characterize the film topography. The MoS2-WS2 atomic layer properties were confirmed using a JEOL-2010F transmission electron microscopy (TEM) with the support of Gatan software (DigitalMicrograph, version 3.21). 3. Hydrogen evolution reaction performance Electrochemical HER analysis was accomplished using a three electrode electrochemical system (CHI 660D). The graphite rod served as the counter electrode, Ag/AgCl served as the reference electrode, and MoS2, WS2, and MoS2-WS2 films served as the working electrode. Linear sweep voltammetry (LSV) was recorded in 0.5 M H2SO4 electrolyte at room temperature with 10 mV/s scan rate. The observed potentials were standardized in terms of reversible hydrogen electrode (RHE). Electrochemical impedance spectroscopy (EIS) was carried out with the frequency ranging from 0.01 Hz to 100 kHz.

RESULTS AND DISCUSSION The detailed simple CBD technique is discussed in experimental section, and a schematic representation of the resultant MoS2-WS2 heterostructure is shown in Figure 1. Figure 2a shows the Raman scattering spectra for MoS2, WS2, and MoS2-WS2. MoS2 shows E12g and A1g peaks at 380.54 and 406.34 cm-1, respectively6, whereas WS2 shows E12g and A1g peaks at 354.78 and 418.47 cm-1, respectively35. The observed peak difference values for MoS2 (25.8 cm-1) and WS2 (63.7 cm-1) are confirmed the few layer thicknesses of prepared films6, 35. In contrast, the MoS2-WS2 heterostructure peaks appear to be associated to the out of plane Mo−S


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phonon A1g mode at 406.46 cm-1, and in-plane Mo−S phonon E12g mode at 380.66 cm-1, in addition to two WS2 peaks at 356.51 (in-plane E12g mode) and 416.5 cm-1 (out of plane A1g mode)15, 36, which are slightly shifted to higher wavenumbers23, 37. Figure 2b shows the XRD patterns of MoS2, WS2, and MoS2-WS2 heterostructure films, indexed with standard JCPDS patterns (MoS2: 872416, and WS2: 872417). MoS2 exhibited (002), (004), (102), and (105) lattice oriented peaks with (002) being the preferential orientation. WS2 also exhibited the (002) lattice peak as the preferential orientation along, with weaker (101), (103), (105), (107), (114), (202), and (109) peaks. The MoS2-WS2 heterostructure (002) was preferentially oriented and the lattices planes could not be distinguished for MoS2 and WS2. The lattices exhibited hexagonal phase primitive lattice structures with only slight peak position difference between the MoS2 and WS2. Figures 3a–c show MoS2, WS2, and MoS2-WS2 heterostructure surface morphologies, from FESEM. MoS2 film consist of uniformly sized spherical grains with smooth surfaces (Fig. 3a). The MoS2 film shows dense morphology, and the surface is consisted without pinholes, voids and cracks. The image confirms that MoS2 CBD growth produced homogeneous nucleation over the FTO substrate. On the other hand, a dense array of nanograins were observed for RF sputtered WS2 with voids (Fig. 3b). In contrast, the MoS2-WS2 heterostructure surface shows a dense array of grains without pinholes or cracks (Fig. 3c). Higher magnification (Fig. 3c, inset) shows the cauliflower structured array of grains consisting of spherical structures. This morphological growth offers valuable understandings of surface formation routines during heterostructure film growth. Figure S1 (Supporting Information) shows the MoS2-WS2 heterostructure FESEM cross-sectional interface structure. Figures 4a–c show MoS2, WS2, and MoS2-WS2

heterostructure

AFM

topology,

respectively.

All

films

show

uniform


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nanomorphology with spherical grains. Larger bright grains are evident for MoS2-WS2 heterostructure due to agglomeration, which is highly dependable with the FESEM results. Furthermore, Figure 5a shows the typical transmission electron microscopy (TEM) image of MoS2-WS2 heterostructure film, which consists of vertically aligned layers, with their FFT pattern as an inset. From their surface profile in Fig. 5b, the layer thickness is extracted at ~ 0.64 nm. Moreover, a reduced FFT pattern by spot mask mode is provided in Fig. 5c and their phase profile spectrum confirmed the width of ~0.64 nm which is well consisted with d-spacing value of (002) lattice orientation. A TEM image of MoS2-WS2 heterostructure with the low magnification is given in Fig. S2. Figures 6a–d show MoS2 and WS2 chemical compositions from XPS. MoS2 (Fig. 6a) exhibits sulfur (S) 2p peaks at 163.83 (S 2p1/2) and 162.66 eV (S 2p3/2) binding energies; Mo characteristic peaks at 232.87 and 229.65 eV (assigned to Mo 3d3/2 and Mo 3d5/2, respectively); and a smaller peak at 226.6 eV (S2- 2S). WS2 exhibits W (W 4f5/2 and W 4f7/2 at 32.61 and 34.71 eV, respectively), and S (2p1/2 and S 2p3/2 at 162.65 and 163.86 eV, respectively) related peaks (Figs. 6c and d). XPS depth profile was performed to analyze MoS2-WS2 heterostructure interfacial structure and chemical composition, and MoS2/SiO2 interfacial structure, using a 1keV Ar ion beam for sputtering, as shown in Figure 7(a). Figure S3 shows XPS survey spectra for different etched MoS2-WS2 heterostructure sputtering times: 0, 15.5, and 37 min. W and S core peaks contribute the main peaks for MoS2-WS2 heterostructure with 0 min sputtering time (i.e., no etching), whereas when the heterostructure film is etched by ion beam, W atomic concentration reduces linearly with exposure time, and vanishes after ~13 min sputtering (Fig. 7a), which confirms there was no diffusion of WS2 into MoS2.


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Mo atomic concentration increases linearly with etching time up to ~12 min, then decreases linearly and disappears at ~36 min etching. Sulfur concentration slowly decreased with etching time, which confirmed formation of MoS2 and WS2, and almost disappeared after etching ~36 min (Figs. 7a and S3). Figure 7b shows an expanded view of the W 4f core peak variations for 0 and 15.5 min etching time. Complete etching of the WS2 structure is confirmed, which further confirms heterostructure formation. The 2p1/2 and 2p3/2 peaks are exhibited at 0 and 15.5 min etching, as predicted in Figs. 7c and S3. The well-established Mo 3d peaks are observed at 15.5 min etching (Fig. 7d). Figure S3 shows the maximum etched XPS spec depth profile for complete removal of the heterostructure from the FTO surface. Thus, XPS depth profiling analysis strongly confirms heterostructure formation from the proposed physico-chemical approach for layered MoS2-WS2. Figure 8 shows electrocatalytic activities towards HER evaluated by (LSV), as described above. Longer time (40 min) CBD deposited MoS2-WS2 heterostructure results are included for comparison. Surface morphology and topography of MoS2 (40 min)-WS2 heterostructure are shown in Figs. S4 and S5, respectively, and clearly exhibit circular agglomerated nanograins. Figure 8a shows iR corrected LSV curves for MoS2, WS2, MoS2 (20 min)-WS2 and MoS2 (40 min)-WS2 films using graphite rod as the counter electrode (CE). The superior and high cost commercial electrocatalytic Pt material exhibits minimal overpotential = 25 mV@10 mA/cm2, whereas MoS2 (20 min)-WS2 and MoS2 (40 min)-WS2 heterostructure films exhibit low overpotential = 129 and 137 mV (@10 mA/cm2, respectively. In contrast, pure MoS2 and WS2 exhibit inferior HER activity with large overpotentials = 151 and 175 mV (@10mA cm2), respectively.


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The observed MoS2(20 min)-WS2 heterostructure/FTO HER catalytic overpotential (129 @10mA/cm2) is lower than previously reported for TMDCs, such as MoS2 nanosheets with different phase structures (153–343 mV @10 mA/cm2)38; MoS2 nanosheet composites (140 mV @10 mA/cm2)39, Ti foil supported MoS2 (150 mV @10 mA/cm2)40, CoSe2 (137 mV @10 mA/cm2)41, WS2 nanocomposites (180 mV @10 mA/cm2)42, 1T-WS2 (142 mV @10 mA/cm2)43, WS2(1-x)Se2x (170 mV @10 mA/cm2)44, and MoS2(1-x)Se2x and WS2(1-x)Se2x (141–214 mV@10 mA/cm2)15, which suggests that the current MoS2-WS2 heterostructure could be an excellent HER catalyst. The substrate is crucial for the electrocatalytic activity, which serves as the supporting electrode. The high conductivity of FTO offers the fast transfer of electrons among the active edge sites underneath the electrode for excellent electrochemical kinetics44, 45. Bonde et al.46 demonstrated free energy of hydrogen adsorption (∆GH) at 0.10 and 0.07 eV for S-edge MoS2 and WS2, respectively. The outstanding MoS2-WS2 heterostructure catalytic activity may arise from the MoS2 QD spherical grains combined with WS2 nanograins providing a characteristic defect-rich structure that carries more active edge sites for HER, and the disordered stacking of the CBD MoS2-RF sputtered WS2 on the FTO electrode surface may enhance electron transfer efficiency between the active edge sites and underlying electrode23. Previous studies have shown that strain induced defective lattice structures provide positive influence on HER properties for WS2 and MoS219, 25. Commercial electrocatalytic Pt material showed low Tafel slope = 34 mV/decade, with MoS2 (20 min)-WS2 heterostructure = 72 mV/decade (Figure 8b), which was smaller than MoS2 (40 min)-WS2 heterostructure (93 mV/decade), pure MoS2 (90 mV/decade), and WS2 (117 mV/decade). A larger Tafel slope for thicker (longer deposition time) MoS2 (40 min)-WS2


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heterostructure could be due to detachment of the interface region and thicker MoS2. The MoS2 (20 min)-WS2 heterostructure Tafel slope was lesser than other TMD based hybrids, such as dendritic structured MoS2 (73 mV/decade)47, Pt doped MoS2 nanosheets (96 mV/decade)18, transition metals incorporated MoS2 (109–118 mV/decade)48, Co-doped MoS2 and WS2 (101– 132 mV/decade)

46

, and edge enriched MoS2 pyramid platelets (140–145 mV/decade-1)

49

. The

complete HER reaction could be proceed through a discharge step (Volmer reaction), H 3O

+

+ e− → H

ads

+ H 2O ,

(1)

followed by either an ion and atom reaction (Heyrovsky reaction), H ads + H 3 O + + e − → H 2 + H 2 O ,

(2)

or combination reaction (Tafel reaction),

H

ads

+ H

ads

→ H2.

(3)

The MoS2-WS2 Tafel slope (72 mV/decade) indicates that HER occurs through the VolmerHeyrovsky reaction, where rapid proton discharge (equation 1) is followed by the Heyrovsky or Tafel reaction steps (equations 2 or 3)6, 50. Exchange current density (j0) was estimated by the fitting of Tafel plot to the cathodic current, and j0 for MoS2 (20 min)-WS2 heterostructure film ~4.36×10-1 mA/cm2, which is higher than that of pristine MoS2 (3.07×10-1 mA/cm2), WS2 (2.05×10-1 mA/cm2), or MoS2 (40 min)-WS2 heterostructure (3.73×10-1 mA/cm2); and also exceeds that of MoS2(1-x)Se2x alloy (1.12×10-1 mA/cm-2)15, WS2(1-x)Se2x alloy (8.91×10-2 mA/cm2)15, Au/MoS2 (1.9×10-1 mA/cm-2) 6, carbon paper supported MoS2 particles (4.6×10-3 mA/cm2)

46

, and Au supported MoS2 (3.1×10-4 mA/cm2)51. The observed HER parameters for

MoS2, WS2 and MoS2-WS2 heterostructure are tabulated in Table 1. We analyzed electrode kinetics in HER using electrochemical impedance spectroscopy (EIS) from 0.01 Hz to 100 kHz, as shown in Figure 8c for all catalysts. The very low series resistances


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confirm the necessity of the highly conducting FTO substrate to effectively reduce ohmic loss15. The heterostructure films show charge transfer resistance Rct ≈ 2.6 Ω and 3.7 Ω for MoS2 (20 min)-WS2 and MoS2 (40 min)-WS2, respectively, and it is considerably inferior to pure MoS2 (4.3 Ω) and WS2 (11.5 Ω). These low Rct suggests rapid reaction processes between the electrode and electrolyte, which are attributed to enriched sulfur active edge sites, providing higher HER activity. Reduced MoS2-WS2 heterostructure Rct can be attributed to high electron transport through the interface and active sites into sulfur sites. Stability is another vital criteria for a noble HER catalyst. Therefore, continuous HER performance with constant overpotential was recorded to explore the robustness in an acidic electrolyte. Time dependent (j-t) chronoamperometric curve was recorded for 20 h with a constant 129 mV overpotential for the MoS2 (20 min)-WS2 heterostructure electrode (Fig. 9a). Although the current density shows a slight variation after 20 h operation, which is due to consumption of H+ ions or accumulation of H2 bubbles on the electrode surface, hindering the reaction15, 50, the MoS2-WS2 heterostructure shows high stability overall. Long term MoS2-WS2 heterostructure cycling stability was further confirmed from polarization between -0.5 and 0.2 V vs RHE in 0.5 M H2SO4, after 20 h of continuous hydrogen production, as shown in Fig. 9b. The MoS2-WS2 heterostructure electrode polarization curve afterward 20 h constant performance is almost overlay the preliminary curve. CONCLUSION MoS2-WS2 heterostructure film was synthesized by a combined CBD, magnetron sputtering, and sulfurization process. Raman and XRD results were obviously elucidated the formation of MoS2-WS2 heterostructure. XPS depth profile analysis was evidently discovered MoS2-WS2 heterostructure formation from the proposed physico-chemical approach. Moreover, TEM


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images explored the vertically aligned layers on the surface of MoS2-WS2 heterostructure with the layer thickness of 0.64 nm. MoS2-WS2 heterostructure electrocatalysts exposed excellent catalytic performance with 129 mV @ 10 mA/cm-2 overpotential, high exchange current density (4.36×10-1 mA/cm2), small Tafel slope (72 mV/decade), and excellent long term stability compared with pristine MoS2 and WS2. This study provides a potential pathway of heterostructure formation using TMDC layers as an auspicious candidate to use in HER electrochemical reactions.


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Figure 1. CBD prepared MoS2 with RF sputtered WS2 heterostructure


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Figure 2. (a) Raman and (b) XRD spectra of MoS2, WS2, and MoS2-WS2 heterostructure


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Figure 3. Field emission scanning electron microscope images of (a) MoS2, (b) WS2, and (c) MoS2-WS2 heterostructure


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Figure 4. Atomic force microscope topography of (a) MoS2, (b) WS2, and (c) MoS2-WS2 heterostructure


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Figure 5. High resolution transmission electron microscopy images of MoS2/WS2 heterostructure. (a) Vertically aligned surface with inset of FFT pattern. (b) Surface profile to estimate the thickness of the layers in (a). (c, d) R-FFT pattern by spot mask mode and their phase profile.


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Figure 6. X-ray photoluminescence spectra of (a) and (b) Mo 3d and S 2p binding energies for CBD deposited MoS2, and (c) and (d) W 4f and S 2p binding energies for RF sputtered WS2


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Figure 7. X-ray photoluminescence depth profile spectra of (a) MoS2-WS2 heterostructure, and (b)–(d) W 4f, S 2p and Mo 3d binding energies at 0 and 15.5 min sputter time for MoS2-WS2 heterostructure.


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Figure 8. Electrochemical performance (a) Linear sweep voltammetry curves with 10 mV/s sweep rate, (b) Tafel plots obtained from the polarization curves, and (c) electrochemical impedance spectra (Inset: Pt spectrum).


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Figure 9. MoS2-WS2 heterostructure electrocatalytic stability: (a) polarization curves before and after 20 h HER performance, and (b) chronoamperometric profile at a constant overpotential for 20 h.


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Table 1. Electrochemical parameters for different electrocatalysts

Exchange current

Overpotential Tafel slope Electrocatalyst

density

(mV vs RHE) at 10 mA/cm2

(mV/decade)

(j0, mA/cm2)

Pt

25

34

3.25

CBD MoS2

151

90

3.07 x 10-1

Sputtered WS2

175

117

2.05 x 10-1

MoS2 (20 min) -WS2

129

72

4.36 x 10-1

MoS2 (40 min) -WS2

137

93

3.73 x 10-1

ASSOCIATED CONTENT Figure S1. Cross sectional FESEM image for MoS2-WS2 heterostructure Figure S2. Low magnification TEM image for MoS2 -WS2 heterostructure Figure S3. Survey spectra of XPS depth profile for MoS2-WS2 heterostructure recorded at 0, 15.5 and 37 min etching time. Figure S4. FESEM image for MoS2 (40 min) -WS2 heterostructure Figure S5. AFM image for MoS2 (40 min) -WS2 heterostructure


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AUTHOR INFORMATION Corresponding Author Email: [email protected]; Tel.: +82-2-2260-3996; Fax: +82-2-2277-8735 ACKNOWLEDGMENTS This work was supported by the Ministry of Trade, Industry and Energy (MOTIE, Korea) under Sensor Industrial Technology Innovation Program (No. 10063682) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2017R1D1A1A09000823), and the research program of Dongguk University in 2017. REFERENCES

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For Table of Contents Use Only

A simple physico-chemical approach using prepared MoS2-WS2 heterostructure provides robust electrocatalytic properties towards HER.


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Improved Hydrogen Evolution Reaction Performance using MoS2

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