Efficient Hydrogen Evolution by Mechanically Strained MoS2

Jul 29, 2014 - Sung-Min Kim , Sang-Hoon Jin , Yu-Jin Lee , Min Hyung Lee .... Jin Young Oh , Ji Hoon Lee , Sun Woong Han , Soo Sang Chae , Eun Jin Bae...
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Efficient Hydrogen Evolution by Mechanically Strained MoS2 Nanosheets Ji Hoon Lee,† Woo Soon Jang,† Sun Woong Han, and Hong Koo Baik* Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea S Supporting Information *

ABSTRACT: We demonstrated correlations between mechanically bent tensile-strain-induced two-dimensional MoS2 nanosheets (NSs) and their electrochemical activities toward the hydrogen evolution reaction (HER). The tensile-straininduced MoS2 NSs showed significantly steeper polarization curves and lower Tafel slopes than the strain-free ones, which is consistent with the simple d-band model. Furthermore, the mechanical strain increased the electrochemical activities of all the NSs toward the HER except those loaded with high MoS2 mass. Mechanically bending MoS2 NSs to induce tensile strain enables the production of powerful, efficient electrocatalysis systems for evolving hydrogen.

1. INTRODUCTION Bulk molybdenum disulfide (MoS2), a transition metal dichalcogenide (TMDC), is a multilayer compound (S−Mo− S) that exhibits an indirect band gap of ∼1.2 eV, while monolayer MoS2 shows a direct band gap of ∼1.9 eV owing to the quantum confinement effect.1−3 In addition to its tunable band structure, two-dimensional (2D) monolayer MoS2 shows unique electrical, physical, and chemical properties such as excellent charge carrier mobility,4 an extremely high Young’s modulus,5 and numerous catalytically active sites at the metallic edge.6 Thus, 2D monolayer MoS2 has recently attracted significant attention for application in a wide range of promising applications such as optoelectronics, thermoelectrics, transistors, and catalysts.4,6−11 Further, it has recently drawn significant attention as a potential electrocatalyst for hydrogen evolution because it shows numerous catalytically active edge sites and an advantageous band gap aligned with the hydrogen redox potential and because it is naturally abundant and inexpensive.6,10,12 Most researchers have previously developed MoS2 compounds with more exposed edge sites where hydrogen is evolved to achieve high-performance hydrogen evolution. Designing efficient nanostructures such as MoS2 films showing vertically aligned layers,13,14 MoS2 nanosheet (NS) showing high-stacked structures,15 MoS2 NS basal planes showing defect structures,16 and MoS2 nanoparticles on 3D structures has been the main focus of research.17 However, according to simplified hydrogen evolution at each catalytically active site,12,18 the hydrogen evolution reaction (HER; 2H+ + 2e− → H2) is affected by not only the number of accessible active sites but also the number of electrons supplied to them. Supplying enough electrons from the electrode to the active sties is as important as increasing the number of exposed active © 2014 American Chemical Society

sites where the protons/electrons transfer and is directly related to the hydrogen evolution efficiency of electrocatalysis systems. Mechanically deformed electronic structures are correlated with the surface electrochemical activity of catalysts. Inducing strain in catalysts by alloying noble metals or coating the surfaces with an overlayer of noble metals has shown that tensile or compressed strain changes the densities of states (DOS) and the electrochemical activities of catalysts.19−21 Therefore, mechanical deformation is a powerful method of supplying electrons to catalyst active sites, thereby increasing electrochemical activity. There has also been some research on the correlations between mechanical deformation and the electrochemical activities of other catalysts such as metal oxides,22,23 carbon nanotubes (CNTs),24 and graphene.25,26 Consequently, it is critical to understand how mechanical deformation affects the electrochemical activity of MoS2 NSs to produce desirable electronic structures, thereby optimizing MoS2 NS electrochemical activity. We experimentally demonstrated the correlations between mechanically bent strain-induced MoS2 NSs and their electrochemical activities toward the HER. According to d-band theory,20,21 tensile-strain-induced Mo atoms increase the number of electron states near the Fermi-level and enhance the electrochemical activity toward the HER. Our electrochemical measurements show that the mechanically bent tensile-strain-induced MoS2 NSs show steeper polarization curves and lower Tafel slopes than the strain-free MoS2 NSs, which is consistent with the theoretical hypothesis. However, high mass loading offset the beneficial effect of mechanical Received: April 8, 2014 Revised: July 23, 2014 Published: July 29, 2014 9866

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Figure 1. (a) Schematic depicting chemical exfoliation. (b) SEM image of layered bulk MoS2. (c) AFM image of few MoS2 NS layers. Mo XPS spectra for (d) as-exfoliated and (e) annealed MoS2 NSs. represents the PET thickness and R represents the mechanical bending radius of the substrate. Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) were performed at 4 mV s−1 while the working electrodes were immersed in a 0.5 M H2SO4 electrolyte in a potential stat (VMP2, Biologic), which was used to measure the electrochemical potentials of three-electrode cells each consisting of a fabricated working electrode, a saturated calomel reference electrode (Accumet), and a Pt counter electrode, which was used to calibrate the reference electrode for the reversible hydrogen potential. ERHE = ESCE + 0.241 V.

strain on the electrochemical activities of the MoS2 NSs. Scanning electron microscopy (SEM) images and electrochemical impedance spectroscopy (EIS) showed that increasing the distance from an electrode to the outer MoS2 active sites decreased the electron mobility even though it increased the beneficial effect of the mechanical strain.

2. EXPERIMENTAL SECTION Molybdenum(IV) sulfide powder and n-butyllithium solution (1.5 M in hexanes) were purchased from Aldrich Chemical. Chemically exfoliated MoS2 NSs were prepared according to the methods described in the literature.7,10,29 LixMoS2 was synthesized by intercalating lithium ions into the bulk MoS2. Typically, 0.1 g of bulk MoS2 powder was dissolved in 3 mL of n-butyllithium solution diluted with 20 mL of chemically inert hexane. The lithium ions were intercalated at 90 °C under N2 purging for about 2 days. The LixMoS2 was centrifuged and washed with hexane to remove excess lithium and organic residues. Immediately after that, the LixMoS2 was exfoliated by hydrolyzing it and ultrasonically treating it in DI water for 90 min. The resulting solution was centrifuged several times with DI water. The suspension (2 μg/mL) was vacuum-filtrated through a mixed cellulose ester membrane with 100 nm pores (Advantec) and was contact-printed onto a working electrode (Ag/PET substrate) fabricated by thermally evaporating a Ag source on a polyethylene terephthalate (PET) substrate (0.2 mm) by 100 nm thick. Ag paste was then attached on top of the working electrode to protect it from damage during mechanical bending, which can disturb current flow. Field-emission scanning electron microscopy (FE-SEM, JSM7001F, JEOL) was used to image the bulk MoS2 powder. The thickness and lateral size of the exfoliated MoS2 NSs and MoS2 NS films were measured using atomic force microscopy (AFM, XE-100, Park systems). The chemical structure of the MoS2 NS films was examined using X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo UK) with 1486.6 eV Al Kα radiation. The crystal structure of the bulk and chemically exfoliated MoS2 NS film was analyzed using high-resolution X-ray diffraction (HR-XRD, Ultima IV, Rigaku) with Cu Kα radiation. The MoS2 NSs were mechanically bent to investigate the effect of mechanical strain on them. Each fabricated (MoS2 film/Ag/PET) working electrode was manually bent under a constant force and speed for about 3 s. The tensile-strain ratio for each working electrode was calculated using the simple kinetic equation ε = T/2R, where T

3. RESULTS AND DISCUSSION 3.1. Chemical Exfoliation and Characterization. We prepared bulk MoS2 powder as a raw material to obtain 2D MoS2 NSs (Figure 1b), which were subsequently chemically exfoliated with n-butyllithium (Bu-Li) solution to intercalate the Li ions into the NSs, as shown in Figure 1a. Bu-Li molecules were diffused into octahedral sites in the bulk MoS2 interlayers because Mo atoms show high electron affinity to Li atoms. Bu-Li was then used to produce the Li x MoS 2 compounds. The Li ions in the LixMoS2 compounds reacted with hydroxyl groups in deionized (DI) water, producing LiOH. Hydrolysis expanded the space between the bulk MoS2 layers and exfoliated them.27 Moreover, ultrasonication improved the exfoliation by vibrating the bulk MoS2 and increasing the diffusion of hydroxyl groups. The AFM images show that numerous exfoliated MoS2 NSs were laterally 300− 500 nm and 1−4 nm thick (Figure 1c). Li intercalation induces chemically exfoliated MoS2 NSs to undergo a unique phase transition. Li intercalating into bulk MoS2 changes and destabilizes the d-band structure of Mo. The local phase transitions from trigonal prismatic (2H-MoS2) into octahedrally coordinated Mo (1T-MoS2) occur to stabilize the structure.28 The chemically exfoliated local-phase-transitioned MoS2 NSs exhibit coexisting 2H-1T polymorphs. Figure 1d shows the Mo 3d XPS spectrum for the chemically exfoliated MoS2 NSs. The spectrum shows 2H-MoS2 peaks (pink line) around 229.2 and 232.3 eV, corresponding to the Mo4+ 3d5/2 and Mo4+ 3d3/2 components, respectively. It also reveals 1TMoS2 peaks (green line) around 228.3 and 231.5 eV, corresponding to structure distortion and binding energies 9867

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Figure 2. (a) MoS2 NSs dispersed in aqueous solution. (b) Vacuum-filtrated MoS2 NS film on mixed cellulose ester membrane. (c) Fabricated test sample: MoS2 NS film/Ag electrode/PET substrate. (d) AFM image of the transferred MoS2 NS film. (e) Schematic illustrating the electrochemical measurement process with the mechanical bending. (f) Calculation of tensile strain ratio for MoS2 NS.

shifting to lower by ∼0.9 eV because the local phase had transitioned from 2H-MoS2 into 1T-MoS2. Additionally, the small peak in the Mo 3d spectrum around 235.1 eV, corresponding to Mo6+ 3d5/2, indicates that the Mo had slightly oxidized in ambient air. The S 2p spectrum (Figure S1a, Supporting Information) shows that the local phase similarly transitioned from 2H-MoS2 into 1T-MoS2. We annealed the chemically exfoliated MoS2 NS at 300 °C to compare its distorted structure with the original 2H-MoS2 structure. Annealing reverted 1T-MoS2 into 2H-MoS2, which is a metastable phase.7 Figure 1e shows the Mo 3d XPS spectrum for the annealed MoS2 NS. The spectrum shows the 2H-MoS2 peaks (pink line) around 229.2 and 232.8 eV, corresponding to the Mo4+ 3d5/2 and Mo4+ 3d3/2 components, respectively. It also shows very small 1T-MoS2 peaks (green line) around 228.3, 231.5, and 234.6 eV, meaning that annealing the MoS2 NS to revert 1T-MoS2 into 2H-MoS2 produced an almost completely undistorted structure. In addition, the peak around 236 eV indicates that the Mo had partly oxidized during annealing. The S 2p spectrum (Figure S1b) showed similar tendencies. The XPS spectra suggest that about 50% of the local phase had transitioned from 2H-MoS2 into 1T-MoS2 in the chemically exfoliated MoS2 NS. According to the literature,6,10,28 the 2H-MoS2 and 1T-MoS2 phases coexist in the chemically exfoliated MoS2 NS, leading to lattice-mismatchinduced zigzag strain at the polymorph boundary. Zigzag strain converts semiconducting MoS2 NS into metallic MoS2 NS, lowering the energy barrier toward hydrogen evolution. Consequently, intrinsic strain by coexistence of 2H-MoS2 and

1T-MoS2 phases in the chemically exfoliated MoS2 NS we obtained shows an advantageous potential aligned with the HER. 3.2. Sample Preparation and Bending Test. The chemically exfoliated MoS2 NSs were suspended in DI water (Figure 2a), and the concentration of the suspension was 2 μg/ mL, as measured using a microbalance. The suspension was vacuum-filtered through the cellulose membrane (Figure 2b), and the resulting MoS2 NS thin film was contact-printed onto an arbitrary substrate. The AFM image in Figure 2d shows the surface morphology (root-mean-square roughness = 0.6 nm) of the transferred MoS2 NS film and the in-plane alignment of the MoS2 NSs. Additionally, HR-XRD was used to analyze the alignment of the MoS2 NS film. Figure 2S shows that the MoS2 NSs were highly oriented along the c-axis. The vacuum-filtered MoS2 film was then contact-printed onto a 0.2 mm thick Ag-coated PET substrate (Figure 2c) to determine how mechanical bending affected the electrochemical activities of the strain-induced MoS2 NSs toward the HER. A MoS2 film/Ag/PET working electrode was fabricated for the mechanical bending tests and was immersed in a 0.5 M H2SO4 electrolyte in an electrochemical measurement system. LSV was used to characterize the electrochemical activity of the electrode toward the HER (Figure 2e). A saturated calomel electrode was used as a reference electrode and was calibrated with a Pt counter electrode for the reversible hydrogen potential. We note that all polarization curves are not corrected for iR loss. 9868

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Figure 3. (a) Schematic illustrating mechanism through which mechanical strain affects Mo atoms. (b) Tensile-strain-ratio-dependent polarization curves. Scan rat, 4 mV/s; electrolyte, 0.5 M H2SO4; ESCE = ERHE + 0.241 V. (c) Tafel plots-based on polarization curves acquired at (b). (d) Fluctuation of tensile-strain-ratio-dependent current density over time. Current density was measured at constant 0.3 V. All samples were loaded with 6 μg/cm2 MoS2.

We fabricated NS test samples loaded with 6 μg/cm2 MoS2 NS and mechanically bent them under the same applied voltage to measure how mechanical strain affected the electrochemical activities of the samples. Polarization curves shown in Figure 3b indicate how the electrochemical activity depends on various tensile strain ratios. The Ag working electrode (gray line) showed almost no electrochemical activity from 0 to −0.7 V, which means it was relatively stable in the electrochemical measurement system to test HER activities. The strain-free (0% strain) chemically exfoliated MoS2 NS (red line) and strain-free annealed MoS2 NS (black line) showed 31 and 19 mA/cm2 at −0.6 V, respectively, suggesting that 1T and 2H MoS2 coexisted in the chemically exfoliated MoS2 NS, which lowered energy barrier and aligned the electrochemical potential of the NS well with that for the HER. The 0.005% tensile strain MoS2 NS (green line) shows a polarization curve drastically steeper than the strain-free MoS2 NS (red line) does and shows 41 mA/cm2 at −0.6 V. The 0.01 and 0.02% tensile strain MoS2 NSs (blue and pink lines, respectively) also show polarization curves steeper than the strain-free MoS2 NS (red line) does and show 44 and 48 mA/cm2 at −0.6 V, respectively. To understand the detailed mechanical strain effect toward to HER activity with the simple d-band model, we acquired Tafel plots based on polarization curves and exchange current density, j0, by the extrapolation method as shown in Figure 3c and Table 1. We obtained all Tafel slopes of various test samples according to the Tafel equation (η = b(log j − log j0), where η is the overpotential, j is the current density, and b is the Tafel slope).14,15 The low Tafel slope, b, means rapid increase of HER rate with overpotential and leads to high efficient

3.3. Mechanism and Electrochemical Measurement for Mechanical Strain Effct. According to d-band theory, mechanical-strain-induced changes in atomic distances can significantly affect the density of d-states and changes in the DOS can enhance the electrochemical activities of material surfaces. Figure 3a briefly illustrates how tensile strain affects dynamic bands. Mo atoms move far away from each other when MoS2 NSs acquire mechanical-bending-induced tensile strain. The degree of d-state overlap between neighboring Mo atoms decreases because of the expanded Mo lattice; hence, the bandwidth narrows. The density of d-states near the center of the d-band level (Ed) increases simultaneously and shows upshifts of Ed to maintain the degree of d-filling. The increased density of d-states near the Fermi level (EF) facilitates the supply of electrons from the electrode to the active-edge sites, thereby increasing the electrochemical activity toward the HER. We experimentally determined how various tensile strain ratios induced the change of Mo atomic distances and affected the electrochemical activities with the amounts of current density when the NSs loaded with various MoS2 masses were mechanically strained under various applied voltages. Figure S4a shows the mechanically strained sample for XPS measurement, and Figure S4b, c with Table S1 indicates that the relative fraction of 1T and 2H components for 0.02%strained MoS2 showed 65% and 35%, respectively, while those one for strain-free MoS2 showed both 50%. Increasing 15% fraction of 1T component for 0.02%-strained MoS2 means that the mechanical strain induces the change of Mo atomic distances, which enhanced the electrochemical activities at the active sites. 9869

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our proposed mechanisms toward to the HER activities, the Volmer−Heyrovsky reaction is suitable for our electrochemical test systems. In detail, the Volmer reaction associated with proton absorption is not the main HER rate-determining step due to the sufficient supply of H+ protons from strong acidic electrolyte, while Heyrovsky reaction associated with electrochemical desorption is the one and directly relates to the amount of electrons to the active sites facilitated by the mechanical tensile strain, which leads to a lower Tafel slope, b. The Tafel plot shown in Figure 3c indicates how mechanical strain facilitates the supply of electrons, leading to affect HER rate-determining. The Ag working electrode (gray line) shows the highest Tafel slope of 159 mV/decade and smallest exchange current density of 2.75 μA/cm2 as listed in Table 1, which means almost no electrochemical activity toward to HER and very small accessible active sites. Tafel slopes with its exchange current density for the strain-free chemically exfoliated MoS2 NS (red line) and the strain-free annealed MoS2 NS (black line) were 145 mV/decade with 10.23 μA/cm2 and 154 mV/decade with 4.90 μA/cm2, respectively, suggesting that intrinsic strain only existing in the chemically exfoliated MoS2 NS makes advantageous electrochemical activity toward HER. The 0.005, 0.01, and 0.02% tensile strain MoS2 NSs (green, blue, and pink lines, respectively) show much lower Tafel slopes of 141, 138, and 135 μA/cm2, respectively, than the strain-free MoS2 NS (red line) does while all of exchange

Table 1. Electrochemical Analysis for Various TensileStrained MoS2 NSsa test samples Ag only annealed MoS2/Ag MoS2/Ag strain 0% MoS2/Ag strain 0.005% MoS2/Ag strain 0.01% MoS2/Ag strain 0.02%

Tafel slope, b (mV/decade)

log(|j|) (mA/cm2)

159 154

−2.56 −2.31

2.75 4.90

0.89 1.95

145

−1.99

10.23

5.95

141

−1.95

11.22

7.62

138

−1.96

10.96

8.78

135

−1.99

10.23

9.24

j0 (μA/cm2)b j (mA/cm2)c

a All samples loaded with 6 μg/cm2 of MoS2 were tested in a 0.5 M H2SO4 electrolyte in an electrochemical measurement system. b Exchange current densities (j0) were acquired from Tafel curves by using extrapolation methods. cCurrent densities (j) were recorded at η = 400 mV.

hydrogen evolution. In general, HER is determined by three principal steps, commonly named Volmer, Heyrovsky, and Tafel reactions, and it can be simply expressed as 2H+ + 2e− → H2.16 According to the previous literature,14−16 the Volmer step associates with proton absorption and the Heyrovsky and Tafel steps associate with electrochemical desorption. In the case of

Figure 4. (a) Polarization curves for unstrained samples loaded with various amounts of MoS2 and for corresponding loaded samples tensile-strained at 0.01%. (b) Tafel plots based on polarization curves acquired at (a). (c) Film thickness and morphology for samples loaded with various amounts of MoS2. (d) Electrochemical impedance spectra of various amounts of MoS2 at −0.3 V versus RHE from 200 kHz to 10 mHz. 9870

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Table 2. Electrochemical Analysis for Unstrained and Tensile Strained Samples Loaded with Various Amounts of MoS2a test samples

Tafel slope, b (mV/decade)

log(|j|) (mA/cm2)

j0 (μA/cm2)b

j (mA/cm2)c

MoS2/Ag-2 μg strained MoS2/Ag-2 μg MoS2/Ag-6 μg strained MoS2/Ag-6 μg MoS2/Ag-12 μg Strained MoS2/Ag-12 μg

152 142 142 138 143 140

−2.09 −2.08 −1.99 −1.96 −1.79 −1.77

8.12 8.32 10.23 10.96 16.22 16.98

3.45 5.48 5.95 8.78 9.98 12.14

a All 0.01% tensilestrained samples were tested in a 0.5 M H2SO4 electrolyte in an electrochemical measurement system. bExchange current densities (j0) were acquired from Tafel curves by using extrapolation methods. cCurrent densities (j) were recorded at η = 400 mV.

MoS2 mass could inhibit the beneficial effect of the mechanical strain on the electrochemical activity and render it inefficient. (2) Increasing the number of intrinsically semiconducting NSs loaded with high MoS2 mass could disturb the electrons supplied from the electrode to the exposed active catalytic sites. Figure 4c shows cross-sectional SEM images of the NS films loaded with various MoS2 masses. The films exhibit different thicknesses and morphologies. The NS films loaded with 2 and 6 μg/cm2 of MoS2 are about 20 and 50 nm thick, respectively, and show relatively smooth morphologies; however, one loaded with 12 μg/cm2 of MoS2 is about 80 nm thick shows rougher morphology. The degree of electron transport from the working electrode to the exposed active catalytic sites is determined by how many electrons goes through the NS films with various MoS2 masses and inefficiently supplied electrons inhabit the charge transfer process. EIS was conducted to investigate the electrochemical charge transfer resistance (Rct) for HER at the surface of active materials. According to our equivalent circuit model, Rohmic is negligible and the diameter of semicircle in Nyquist plot means the charge transfer resistance (Rct). In Figure 4d, NS films loaded with 2 μg/cm2 (pink line) and 6 μg/cm2 (blue line) of MoS2 showed much lower charge transfer resistances than one loaded with 12 μg/cm2 of MoS2 (green line). Additionally, Ag (gray line) as a working electrode in our system showed much larger diameter than others loaded with MoS2, which was caused by its own large potential barrier toward HER even though Ag is well-known as conductive metal. In other words, inefficiently supplied electrons caused by the thicker MoS2-overloaded NSs inhibit the hydrogen/electron transfer reaction and offset the beneficial effect of mechanical strain on the electrochemical activity of the NSs toward the HER due to poor in-plane alignment and high charge transfer resistance. Stability is another important factor for the HER catalyst. We measured the electrochemical stability of NS films loaded with 6 μg/cm2 of MoS2 in our system ranging from 0 to −0.3 V at a scan rate of 15 mVs−1. Through the cycling test, we found out the MoS2/Ag (Figure S5a) became more and more unstable and its current density increased gradually after about 35 cycles. On the other hand, when Ag (Figure S5b) was used as the working electrode, the catalyst was very stable throughout the cycling test and its current density was negligible ranging from 0 to −0.3 V due to its redox potential toward HER. We cautiously hypothesize that the phenomena would be caused by multistacked NS film which has weak van der Waals bonding. Initially, enough protons from the strong acidic electrolyte (0.5 M H2SO4) are supplied to the exposed active sites of MoS2 NS film. Subsequently, the protons could pass through the NS film inducing structural changes that loosen the packing of adjacent layers and resulting in exposure of more

current densities are nearly same. These results indicate that mechanically tensile-strained NSs loaded with the same MoS2 mass make HER rate-determining associated with Tafel reaction faster and show better electrochemical activities under the same applied voltage. Furthermore, we evaluated how the tensile strain ratio affected the amount of current density that the MoS2 NSs produced. Figure 3d shows that the current density and total area increased with increasing tensile strain ratio when the NSs loaded with 6 μg/cm2 of MoS2 were mechanically bent under a constant 0.3 V. The calculated total areas corresponding to the electrochemical activities are shown in Figure S3. We fabricated test samples loaded with 2, 6, and 12 μg/cm2 of MoS2 and mechanically bent them under the same applied voltage to estimate how mechanical strain affected the electrochemical activities of NSs loaded with different masses of MoS2. Figure 4a shows the polarization curves for the electrochemical activities of strain-free NSs loaded with 2 (dark green dash line), 6 (red dash line), and 12 μg/cm2 (dark red dash line) of MoS2. The mechanical strain increased the electrochemical activities of the NSs loaded with 2 μg/cm2 (green line) and 6 μg/cm2 (blue line) of MoS2 toward the HER, showing 25 and 44 mA/cm2 at −0.6 V, respectively, which are much higher than the current densities and electrochemical activities that the strain-free counterparts showed toward the HER. However, the electrochemical activity of the mechanically strained NS loaded with 12 μg/cm2 of MoS2 (pink line) did not show the same tendency: it showed a current density of 41 mA/cm2 at −0.6 V, which is not significantly different from the strain-free counterpart. To investigate further the mechanical strain effect of NSs loaded with different masses of MoS2, Tafel plots based on polarization curves and exchange current density obtained by extrapolation method are shown in Figure 4b and Table 2. Tafel slopes of 0.01% tensilestrained NSs loaded with 2 μg/cm2 (green line), 6 μg/cm2 (blue line), and 12 μg/cm2 (pink line) of MoS2 show 142, 138, and 140 mV/decade, respectively, which are lower than those of strain-free NSs, and it means that mechanical strain effect to the various masses of MoS2 increased HER rate-determining associated with Heyrovsky reaction. However, Tafel slope of 0.01% tensilestrained NSs loaded with 12 μg/cm2 of MoS2 was decreased by only 3 mV/ decade, which is rarely different from strain-free one, whereas Tafel slopes of 0.01% tensilestrained NSs loaded with 2 and 6 μg/cm2 of MoS2 were decreased by 10 and 7 mV/decade, respectively. The results inconsistent with the theoretical hypothesis suggest that the mechanical strain effect is offset to the MoS2-overloaded NSs. There are two main reasons the experimental results are inconsistent with the hypothesis for the mechanical strain effect. (1) Poor in-plane alignment of NSs loaded with high 9871

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toward direct bandgap in 2D semiconductors: MoSe2 versus MoS2. Nano Lett. 2012, 12, 5576−5580. (4) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS2 transistors. Nat. Nanotechnol. 2011, 6, 147−150. (5) Gomez, A. C.; Poot, M.; Steele, G. A.; Herre, S. J.; Zant, V. D.; Agrait, N.; Bollinger, G. R. Elastic properties of freely suspended MoS2 nanosheets. Adv. Mater. 2012, 24, 772−775. (6) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.; Jin, S. Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets. J. Am. Chem. Soc. 2013, 135, 10274−10277. (7) Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M. Photoluminescence from chemically exfoliated MoS2. Nano Lett. 2011, 11, 5111−5116. (8) Li, Y.; Li, Y. L.; Araujo, C. M.; Luo, W.; Ahuja, R. Single-layer MoS2 as an efficient photocatalyst. Catal. Sci. Technol. 2013, 3, 2214− 2220. (9) Lee, H. S.; Min, S. W.; Chang, Y. G.; Park, M. K.; Nam, T. W.; Kim, H. J.; Kim, J. H.; Ryu, S. M.; Im, S. G. MoS2 nanosheet phototransistors with thickness-modulated optical energy gap. Nano Lett. 2012, 12, 3695−3700. (10) 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. Nat. Mater. 2013, 7, 1−6. (11) Zeng, Z.; Sun, T.; Zhu, J.; Huang, X.; Yin, Z.; Lu, G.; Fan, Z.; Yan, Q.; Hng, H. H.; Zhang, H. An effective method for the fabrication of few-layer-thick inorganic nanosheets. Angew. Chem. 2012, 51, 9052−9056. (12) Laursen, A. B.; Kegnaes, S.; Dahl, S.; Chorkendorff, I. Molybdenum sulfides-efficient and viable materials for electro- and photoelectrocatalytic hydrogen evolution. Energy Environ. Sci. 2012, 5, 5577−5591. (13) 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 Lett. 2013, 13, 1341−1347. (14) Wang, H.; Lu, Z.; Xu, S.; Kong, D.; Cha, J. J.; Zheng, G.; Hsu, P. C.; Yan, K.; Bradshaw, D.; Prinz, F. B.; Cui, Y. Electrochemical tuning of vertically aligned MoS2 nanofilms and its application in improving hydrogen evolution reaction. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 19701−19706. (15) Wu, Z.; Fang, B.; Wang, Z.; Wang, C.; Liu, Z.; Liu, F.; Wang, W.; Alfantazi, A.; Wang, D.; Wilkinson, D. P. MoS2 nanosheets: A designed structure with high active site density for the hydrogen evolution reaction. ACS Catal. 2013, 3, 2101−2107. (16) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Defect-rich MoS2 ultrathin nanosheets with additional active edge sites for enhanced electrocatalytic hydrogen evolution. Adv. Mater. 2013, 25, 5807−5813. (17) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 nanoparticles grown on graphene: An advanced catalyst for the hydrogen evolution reaction. J. Am. Chem. Soc. 2011, 133, 7296−7299. (18) Vrubel, H.; Moehl, T.; Gratzel, M.; Hu, X. Revealing and accelerating slow electron transport in amorphous molybdenum sulphide particles for hydrogen evolution reaction. Chem. Commun. 2013, 49, 8985−8987. (19) Mavrikakis, M.; Hammer, B.; Nørskov, J. K. Effect of strain on the reactivity of metal surfaces. Phys. Rev. Lett. 1998, 81, 2819−2822. (20) Hammer, B.; Morkawa, Y.; Nørskov, J. K. CO chemisorption at metal surfaces and overlayers. Phys. Rev. Lett. 1996, 76, 2141−2144. (21) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Nørskov, J. K. Surface electronic structure and reactivity of transition and noble metals. J. Mol. Catal. A: Chem. 1997, 115, 421−429. (22) Shu, D. J.; Ge, S. T.; Wang, M.; Ming, N. B. Interplay between external strain and oxygen vacancies on a rutile TiO2 (110) surface. Phys. Rev. Lett. 2008, 101, 116102-1−116102-4. (23) Sayle, T. X. T.; Parker, S. C.; Catlow, C. R. A. The role of oxygen vacancies on ceria surfaces in the oxidation of carbon monoxide. Surf. Sci. 1994, 316, 329−336.

active sites and better HER. The instability of the materials as electrocatalyst imposes significant limitations on the practical uses for evolving hydrogen and should be improved by further work, such as using the pressure-assisted method30 or hydrogen conductive passivation film method10 to enhance stability. Nevertheless, our results demonstrate that introduction of mechanical strained in MoS2 nanosheet films can lead to significant increases in electrochemical activities, specifically with respect to hydrogen evolution.

4. CONCLUSION We demonstrated correlations between mechanically bent tensile-strain-induced MoS2 NSs and their electrochemical activities toward the HER. The LSV results indicated that the tensile-strain-induced MoS2 NSs showed steeper polarization curves and lower Tafel slopes than the strain-free MoS2 NSs, which is consistent with the theoretical hypothesis. Furthermore, the mechanical strain increased the electrochemical activities of all the NSs toward the HER except those loaded with high MoS2 mass. Mass overloading and poor in-plane alignment disturbed the mobility of electrons from the electrode to the exposed active sites, thereby offsetting the beneficial effect of the mechanical strain on electrochemical activity. Ultimately, further studies are required for optimizing the MoS2 loading to maximize the beneficial effect of mechanical strain on electrochemical activity. However, mechanically bending MoS2 NSs to induce tensile strain enables the production of practical, efficient electrocatalysis systems for evolving hydrogen.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details as described in the text. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: thinfi[email protected]. Tel: +82-2-2123-2838. Fax: +82-2-315-5375. Author Contributions †

J.H.L. and W.S.J. equally contributed to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) Grant (2010-0029207) founded by the government of Korea (MSIP).

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REFERENCES

(1) Wang, Q. H.; Zadeh, K. K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat. Nanotechnol. 2012, 7, 699−712. (2) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nat. Chem. 2013, 5, 263−275. (3) Tongay, S.; Zhou, J.; Ataca, C.; Lo, K.; Matthews, T. S.; Li, J.; Grossman, J. C.; Wu, J. Thermally driven crossover from indirect 9872

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(24) Song, X.; Liu, S.; Yan, H.; Gan, Z. Effects of mechanical deformation on outer surface reactivity of carbon nanotubes. IEEE Electron. Compon. Technol. Conf. 2008, 58, 2091−2095. (25) Poetschke, M.; Rocha, C. G.; Torres, L. E. F. F.; Roche, S.; Cuniberti, G. Modeling graphene-based nanoelectromechanical devices. Phys. Rev. B 2010, 81, 193404−1-4. (26) Lee, J. K.; Yamazaki, S.; Yun, H.; Park, J. W.; Kennedy, G. P.; Kim, G. T.; Pietzsch, O.; Wiesendanger, R.; Lee, S. W.; Hong, S. L.; Weglikowska, U. D.; Roth, S. Modification of electrical properties of graphene by substrate-induced nanmodulation. Nano Lett. 2013, 13, 3494−3500. (27) Zeng, Z.; Yin, Z.; Huang, X.; Li, H.; He, Q.; Lu, G.; Boey, F.; Zhang, H. Single-layer semiconducting nanosheets: High-yield preparation and device fabrication. Angew. Chem., Int. Ed. 2011, 50, 11093−11097. (28) Eda, G.; Fujita, T.; Yamaguchi, H.; Voiry, D.; Chen, M.; Chhowalla, M. Coherent atomic and electronic heterostructures of single-layer MoS2. ACS Nano 2012, 6, 7311−7317. (29) Matte, H. S. S. R.; Gomathi, A.; Manna, A. K.; Late, D. J.; Datta, R.; Pati, S. K.; Rao, C. N. R. MoS2 and WS2 analogues of grapheme. Angew. Chem. 2010, 122, 4153−4156. (30) Shin, K. H.; Jang, Y. J.; Kim, B. S.; Jang, J. S.; Kim, S. H. Highly conductive reduced graphene oxide produced via pressure-assisted reduction at mild temperature for flexible and transparent electrodes. Chem. Commun. 2013, 49, 4887.

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