Oxidation-Sulfidation Approach for Vertically Growing MoS2 Nanofilms

Oct 25, 2016 - Molybdenum disulfide (MoS2) has emerged as a promising electrocatalyst for hydrogen evolution reaction (HER). However, the performance ...
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Oxidation-Sulfidation Approach for Vertically Growing MoS2 Nanofilms Catalysts on Molybdenum Foils as Efficient HER Catalysts Tingsong Hu,†,‡,∥ Kan Bian,†,∥ Guoan Tai,*,† Tian Zeng,†,‡ Xufeng Wang,†,‡ Xiaohua Huang,§ Ke Xiong,† and Kongjun Zhu*,† †

The State Key Laboratory of Mechanics and Control of Mechanical Structures, Key Laboratory for Intelligent Nano Materials and Devices of Ministry of Education, College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China ‡ School of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China § College of Physics & Electronic Engineering, Taizhou University, Taizhou 318000, China S Supporting Information *

ABSTRACT: Molybdenum disulfide (MoS2) has emerged as a promising electrocatalyst for hydrogen evolution reaction (HER). However, the performance of the catalyst suffers from the scarce active sites and poor electrical conductivity. Here we grow vertical MoS2 films on Mo foils to achieve highly catalytic active sites and enhanced electrical conductivity for facilitating high efficient HER catalysis. The ultrathin nanofilm with a thickness of around 4 nm on molybdenum foils is grown by a two-step method: (1) the molybdenum oxide (MoO2) nanofilm is achieved by oxidizing the surface of the Mo foil under a low pressure condition and (2) a MoS2 nanofilm is obtained by sulfurizing the MoO2 nanofilm in sulfur vapor at 700 °C within 1 min. Furthermore, the vertically aligned MoS2 nanofilm on Mo foils exhibit excellent stability in acidic solution and the electrochemical measurements show an onset overpotential of as low as 18 mV and a small Tafel slope of 55 mV/dec. The excellent HER catalysis originates from the synergistic effect of the dense catalytic active sites at the vertical MoS2 surface and superior electron transport along the Mo foil. This study opens a novel avenue for the development of earth-abundant, low-cost electrocatalysts with high HER activities.

1. INTRODUCTION Hydrogen has huge potential as a clean and sustainable energy source to meet our future energy demands.1−4 Hydrogen is usually produced by electrochemical water splitting, namely, hydrogen evolution reaction (HER) via electrochemical reduction of protons to hydrogen. It is an important half reaction used to couple with oxygen evolution reaction in electrochemical water splitting for clean energy applications.3,4 To proceed smoothly without consuming extra energy, a catalyst must meet the requirement to initiate proton reduction with minimal overpotential.5,6 Although traditional Pt- or Ptbased materials are the reported best electrocatalysts with low reduction overpotentials and fast reduction kinetics in acidic media, the costliness and scarcity substantially hinder their large-scale applications in HER.7−9 Consequently, replacing the precious and low abundant catalysts with cost-effective alternatives such as Mo, W, and their molecular derivatives has attracted extensive interest toward making the production of hydrogen cheaper.10−12 Molybdenum sulfide (MoS2), an earth abundant lamellar solid with the relatively weak van der Waals forces between these layers with a spacing of around 0.65 nm, has shown prominent HER catalysis owing to a hydrogen binding energy nearing Pt-group metals.13−17 However, the limitation of © 2016 American Chemical Society

catalytic active sites, electrical conductivity, and special surface area of MoS2 nanostructures has impeded the application of MoS2-based electrocatalysts for high efficient HER. Thereby, most studies aimed at the achievement of more exposed active sites and higher special surface area to gain high efficient HER. Both theoretical and experimental studies have shown that the HER performance arose from the molybdenum edge sites of MoS2 sheets, while their basal planes were catalytically inert.13,18 Thus, nanostructured MoS2 with exposed edge sites should be more active than the bulk counterpart for HER electrocatalysis. Previously, various MoS2 nanostructures, including nanosheets,19 mesoporous nanofilms,20−23 metal or graphene nanocomposites,24−28 and amorphous nanoparticles,29 have been developed to improve their HER performance. Besides, MoS2 nanostructures grown on different substrates such as Au(111),13 carbon paper,14 and highly orientated pyrolytic graphite (HOPG)30 have exhibited high HER catalysis arising from a large number of active edges on their surface. Consequently, maximizing catalytic MoS2 active edges and fabricating the edge nanostructure have become a Received: August 11, 2016 Revised: September 28, 2016 Published: October 25, 2016 25843

DOI: 10.1021/acs.jpcc.6b08120 J. Phys. Chem. C 2016, 120, 25843−25850

Article

The Journal of Physical Chemistry C

Figure 1. (a) Schematic illustration of the preparation of MoS2 nanofilms. First, MoO2 nanofilm is produced on a Mo foil at 1000 °C for 30 min in O2/Ar mixed gas. Then, the vertically aligned MoS2 nanofilm is in situ generated on a Mo foil at 700 °C within 1 min in S/Ar gas. (b) Lowmagnification SEM image of a Mo foil. (c) SEM image of a MoO2 nanofilm on the Mo foil. (d) SEM image of a MoS2 nanofilm on the Mo foil.

gas; then, set the temperature up to 700 °C for 1 min; after the reaction, open the furnace for rapid cooling with 100 sccm Ar gas flow. To characterize the thickness and structure of the samples, vertically aligned MoS2 layer at one side of the Mo foil was carefully removed by a 800-grit SiC paper. A poly(methyl methacrylate) (PMMA) layer was spin-coated onto another side of the foil. The underside Mo foil can be dissolved using 2 M FeCl3 solution for over 24 h. Then, the MoS2 film with the PMMA coating layer was transferred onto a 285 nm SiO2/Si substrate, and the supported PMMA layer was removed by immersing into hot acetone at 80 °C for 1 h. 2.3. Structural and Morphological Characterization. The morphology of the MoS2 films was characterized by optical microscopy (VHX-5000, Keyence). Field emission scanning electron microscopy (FESEM) images were captured on a FEI Sirion 200 at 10 kV. Transmission electron microscopy (TEM) images were carried out on a FEI Tecnai G20 system at 200 kV. The thickness of the MoS2 films was characterized by atomic force microscopy (AFM, DI Nanocope 8). Elemental composition was analyzed using X-ray photoelectron spectroscopy (ESCALAB 250, Thermo Fisher Scientific Inc.) with focused monochromatized Al Ka radiation. Raman spectroscopy was performed on a Raman microscopy (Horiba Jobin Yvon LabRAM HR-Evolution) with an excitation laser of 514 nm and a spot size of 2 μm. 2.4. Electrochemical Measurements. Electrochemical measurements were performed in a three-electrode cell on a computer-controlled potentiostat (CHI660C) using a saturated calomel electrode (SCE) as the reference electrode, a graphite rod as the counter electrode, and a MoS2 nanofilm on the Mo foil as the working electrode. Since all the measured potentials refer to that of the reversible hydrogen electrode (RHE), the overpotential for the HER catalysis was given directly in this study. In each experiment, the electrode was immersed in a solution saturated with Ar gas at 50 mV (versus RHE). The performance of the HER catalyst was measured in 0.5 M H2SO4 solution using linear sweep voltammetry from −0.60 to 0 V versus RHE with a scan rate of 5 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were performed using an electrochemical analyzer (Princeton Applied Research, Parstat 4000). The EIS was investigated within the frequency from 1 to 10 MHz under a single modulated 10 mV alternating current (AC) potential in 0.5 M H2SO4 at various potentials.

momentous topic.23,31−34 Vertically grown 2D metal chalcogenides are being investigated for various electrocatalytic applications due to their ultralarge chemically active edge sites. The vertical orientation also allows for maximizing exposure of the edge sites and, therefore, presents unique opportunities for high-performance HER.33,35−38 The design of MoS2 catalysts with well-defined active sites on suitable supports is crucial to expose and stabilize more active edges at the external surface. Herein, we demonstrate that MoS2 nanofilms with molecular layers perpendicular to Mo foils can be synthesized by a rapid sulfurization process in sulfur vapor at 700 °C within 1 min at atmospheric pressure, in which the layers are vertically aligned on the surface of Mo foil to maximally expose the active sites for HER. Significant improvement of MoS2 HER activity is observed on the Mo foil in comparison with the other substrates. Strong chemical bonds are formed between the vertically aligned layers and the underlying Mo substrate, which result in high electrical conductivity and outstanding electrochemical stability.

2. EXPERIMENTAL SECTION 2.1. Materials. Mo foil with thickness of 20 μm (99.95%) was purchased from Shengyuan Co., Ltd., China. Sulfur powder was purchased from Alfa Aesar. O2 and Ar gases were received from Nanjing Electronic Devices Institute. Other reagents were obtained from the Sinopharm Chemical Reagent Co., Ltd., China. 2.2. Growth and Transfer of MoS2 Nanofilms. Typically, a Mo foil with length and width of 80 and 30 mm was pretreated according to the reported procedure.39 After annealing, the Mo foil was cut into some pieces with size of 25 × 8 mm. After that, the MoO2 nanofilm composed of rough nanoparticles on the Mo foil were prepared by an oxidation procedure in the conditions of a vacuum and 1000 °C for 30 min, where 0.5 sccm oxygen (O2) and 50 sccm ultrahigh-purity argon (Ar) were used as the oxidation gas and the carrier gas, respectively. Furthermore, the vertically aligned MoS2 nanofilm on the Mo foil was generated by a vapor transport process at the atmospheric pressure using 10 sccm Ar gas. A crucible containing 500 mg of sulfur powder was located upstream from the MoO2 nanofilm on the Mo foil; this zone was heated to 150 °C with a ceramic heating element. The detailed growth process of the MoS2 nanofilm is as follows: set the furnace temperature of 300 °C under 100 sccm Ar gas for 10 min; ramp to 700 °C with a heating rate of 30 °C min−1 under 10 sccm Ar 25844

DOI: 10.1021/acs.jpcc.6b08120 J. Phys. Chem. C 2016, 120, 25843−25850

Article

The Journal of Physical Chemistry C

Figure 2. (a) Optical image of as-grown vertically aligned MoS2 nanofilm transferred onto a 285 nm SiO2/Si substrate. (b) AFM topographical image of the MoS2 nanofilm transferred onto 285 nm SiO2/Si substrate. Height profile along the white line in the inset shows a thickness of 3.9 nm, indicating that the film is indeed ultrathin. (c) 3D height profile of the AFM scanned in (b). (d) Raman spectrum of the vertically aligned MoS2 nanofilm, normalized to the peak intensity of silicon.

Figure 3. (a) Low-magnification TEM image of the MoS2 nanofilm covering TEM grids. (b) High-magnification TEM image of the MoS2 nanofilm over a hole of the TEM grid. (c) Enlarged TEM image of the MoS2 monolayer. Inset is the corresponding SAED pattern. (d) High-resolution TEM (HRTEM) image of the nanofilm, showing the vertically aligned structure of MoS2 with a layer spacing of 0.65 nm. 25845

DOI: 10.1021/acs.jpcc.6b08120 J. Phys. Chem. C 2016, 120, 25843−25850

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The Journal of Physical Chemistry C

Figure 4. (a) XPS spectrum of Mo 3d and S 2s peaks. (b) XPS spectrum of S 2p peaks.

Figure 5. (a) The unit cell structures of elemental Mo (left), MoO2 (middle), and 2D MoS2 monolayer (right). (b) Schematic diagram of the proposed growth mechanism. The oxidization reaction requires the diffusion of oxygen into the Mo lattice on the surface of Mo substrate and converts it into MoO2 at high temperature, then the MoO2 is converted into vertical aligned MoS2 layers within a very short time, originating from enough volume expansion in the film, finally surface reconstruction driven by surface free energy leads to the formation of horizontal growth of MoS2.

3. RESULTS AND DISCUSSION

transferred onto diverse substrates such as silicon and glass by removing the underlying Mo foil using a 2 M FeCl3 solution. The optical contrast between the MoS2 nanofilm and SiO2/Si substrate shows that the film is continuous and large-scale (Figure 2a). Atomic force microscopy (AFM) measurement shows that the thickness of the MoS2 nanofilm is around 4 nm, suggesting that the film is ultrathin (Figure 2b). The 3D height profile of the AFM image reveals that the nanofilm is composed of MoS 2 nanoparticles, in accordance with the high magnification SEM image (Figure 2c). The MoS2 nanofilm was further characterized by Raman spectroscopy, as shown in Figure 2d. Raman spectrum of the sample showed that the two characteristic peaks located at 383 and 409 cm−1 are assigned to be E2g1 and A1g modes of MoS2.39,40 The E2g1 and A1g modes can be ascribed to the inplane vibration of S−S atoms in relation to the Mo atom and the out-of-plane vibration of S atoms, respectively.39,40 The yielded frequency difference between the two modes (A1g − E2g1) is 26 cm−1, in accordance with those observed for fewlayer MoS2.40 The same Raman spectra are also achieved for the samples sulfurized at 700 °C for 30 and 60 min, suggesting that the thickness of MoS2 nanofilms is independent of the reaction duration, as shown in Figure S3(b,d). No obvious difference is observed from the optical contrast between the 1 min-, 30 min- or 60 min-sulfurized MoS2 nanofilms and the SiO2/Si substrate (Figure S3(a,c).

3.1. Catalyst Synthesis and Characterization. To attain highly catalytic active sites and enhance electrical conductivity of the MoS2 catalysts, we grew molybdenum dioxide (MoO2) by oxidizing the surface of a preannealing Mo foil at 1000 °C in an oxidizing environment (O2/Ar) under low pressure conditions (the presence of the MoO2 nanofilm on Mo foil was verified by Raman spectroscopy (Figure S1)), and then a vertically aligned MoS2 nanofilm was achieved by further sulfurizing MoO2 on the Mo foil in a sulfur vapor (S/Ar) at 700 °C for 1 min (Raman spectrum of the MoS2 nanofilm on Mo foil was characterized to confirm the complete transformation from MoO2 to MoS2, as shown in Figure S2). The entire growth process is demonstrated, as shown in Figure 1a. Scanning electron microscopy (SEM) was employed to characterize the microstructure of the as-prepared nanofilms. First, a nearly atomically smooth surface of Mo foil are obtained by annealing it at 1400 °C for 10 h under a high-purity H2 condition (Figure 1b).39 Second, the surface became rough and nanoparticle-like after it was oxidized at 1000 °C in an O2/Ar environment (Figure 1c). Finally, the surface was reconstructed by sulfurizing the MoO2 nanofilm, which was confirmed by the formation of smaller nanoparticles on the surface of Mo foil (Figure 1d). The vertically aligned MoS2 nanofilm can be easily 25846

DOI: 10.1021/acs.jpcc.6b08120 J. Phys. Chem. C 2016, 120, 25843−25850

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The Journal of Physical Chemistry C

Figure 6. (a) Cathodic polarization curves of MoS2 nanofilms on the Mo foil compared with a pure Mo foil and a standard Pt electrode. (b) Corresponding Tafel plot (log current versus potential) of the samples. (c) Electrochemical impedance spectroscopy (EIS) Nyquist plots for the electrodes. (d) Cycling stability tests. Red line: polarization curve of the sample before cycling. Green line: polarization curve of the sample before cycling. Blue line: polarization curve of the sample before cycling.

1:1.75, which is close to the chemical stoichiometric ratio of MoS2. 3.2. Growth Mechanism. The growth mechanism of vertically aligned MoS2 layers on the Mo foil can be understood as shown in Figure 5a. The atomic distance of elemental Mo− Mo is enlarged from 0.32 to 0.485 nm after the oxidation, as shown in Figure 5a. The formation of MoO2 nanoparticles facilitates the vertical MoS2 layers grown on the Mo foil surface (Figure 5b), resulting from an unconstrained free volume expansion in the vertical direction during the sulfurization of very thin, discontinuous MoO2 nanoparticles. With prolonging the reaction duration, vertically oriented layers are transformed into horizontally grown ones, owing to surface reconstruction (Figure S6). It is noted that the surface energy of the active edge sites is almost 2 orders of magnitude higher than that of the basal plane.43 Reduction of surface energy is compensated by releasing strain energy to form horizontally grown 2D layers.36 To validate the proposed grown mechanism, a thick MoO2 film was prepared by oxidizing a Mo foil in the mixed gas of 2 sccm O2 and 50 sccm ultrahigh-purity Ar at 1000 °C for 1 h. Then, the corresponding MoS2 nanofilm was produced by sulfurizing the MoO2 nanofilm on Mo foil in sulfur vapor at 700 °C for 1 min. AFM measurement shows that the thickness of the obtained MoS2 film was 12.4 nm (Figure S7(a,b)). TEM and HRTEM images indicate that the thick film has a horizontal alignment with polycrystalline structure (Figure S7(c,d)). Therefore, the change from the vertical to horizontal direction could be attributed to the high surface energy of the vertically aligned MoS2. 3.3. Electrocatalytic HER Performance. To evaluate the catalytic activity of the MoS2 nanofilms for the water splitting reaction, the catalytic properties are investigated in 0.5 M

Transmission electron microscopy (TEM) image shows a large-area, textured MoS2 film whose structure is polycrystalline (Figure 3a−c), which is revealed by the corresponding fast Fourier transform (FFT) image (left inset, Figure 3c). Highresolution TEM (HRTEM) image reveals that the film is composed of highly dense, dominantly vertically aligned 2D MoS2 layers. Most of the grains consist of a large number of self-assembled MoS2 layers with a spacing of around 0.65 nm (white arrow), while amorphous regions also occur in the MoS2 nanofilm. The elemental mapping images of sulfur and molybdenum are shown in Figure S4, confirming that the nanofilm is uniformly composed of MoS2. With prolonging the reaction duration over 30 min, horizontally grown 2D layers with single-crystalline grains are observed (Figures S4(a,b)), showing shows hexagonal single-crystalline lattice fringes of MoS2 oriented in (001) zone axis (left, bottom inset). The X-ray photoelectron spectroscopy (XPS) was adopted to reveal the chemical nature and bonding state of the MoS2 nanofilm (Figure 4a,b). The XPS survey-scan spectrum of the film on a 300 nm-SiO2/Si substrate for binding energies from 0 to 1000 eV is shown in Figure S5. Besides the Mo and S peak, peaks corresponding to C and O were also detected. The C peak may arise from impurities in the vacuum vessels and O peak dominantly comes from the SiO2/Si substrate. The XPS spectrum shows the binding energy of two principal peaks for Mo 3d5/2 = 229 eV and Mo 3d3/2 = 232 eV, which indicates the oxidation state of Mo4+.39,41 The absence of a characteristic peak at around 235.8 eV, which is indexed to Mo6+ 3d5/2 of MoO3, indicated that the nanofilm is completely converted into MoS2. Additionally, the binding energies at 161.6 and 162.8 eV can be ascribed to the S2− 2p of MoS2 (Figure 4b).42 From XPS spectra, the sulfur-to-molybdenum molar ratio (S:Mo) is 25847

DOI: 10.1021/acs.jpcc.6b08120 J. Phys. Chem. C 2016, 120, 25843−25850

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The Journal of Physical Chemistry C

the horizontally oriented nanofilm grown at 700 °C for 60 min. The results demonstrated that the electrocatalysts afford remarkedly faster HER kinetics because of the presence of the Mo support. The stability of the vertically aligned MoS2 nanofilm is probed by cyclic voltammetry in the potential window from −0.6 to 0 V vs RHE at a scan rate of 50 mV/s (Figure 6d). After 500 cycling, an attenuation of cathodic current is observed. The stability was also evaluated using a constant overpotential of 120 mV, and the current showed a slight degradation during a long period of 10 000 s, as shown in Figure S8. From a theoretical point of view, the hydrogen adsorption free energy has been shown to be a good descriptor for the rate of HER with an optimal binding energy of ΔGH ≅ 0 eV, with the optimal HER catalyst having a thermo-neutral ΔGH. Density functional theory (DFT) calculations have shown that the hydrogen adsorption ΔGH values of (101̅0) Mo-edge and (1̅010) S-edge are +0.06 and −0.45 eV, respectively.48,49 So, vertically aligned MoS2 thin film on Mo foils with a large number of exposed Mo edges have higher HER property than the lateral grown counterpart. Besides, the catalytic stability and activity of the supported catalyst are strongly correlated to the catalyst−support interaction.13,48,50 It was reported that the electronic properties and catalytic activity of MoS2-based 2D atomic crystals can be tuned by metal substrates. DFT calculations were utilized to investigate the electronical and chemical properties of a monolayer MoS2 supported on Au(111), Ir(111), Pd(111), and Ru(0001).13,51−53 These theoretical studies showed that the catalytic activity of the MoS2 layer can be tuned by the introduction of a metal substrate, and the improved binding of hydrogen can be ascribed to electron transfer from the metal substrate to the MoS2 layer.51 Besides, the reduction of the layer number of 2D crystals significantly promoted the electron transfer and enhanced the electrocatalytic activity.54 Therefore, we expect a further reduction in thickness of MoS2 nanofilms to improve the stability and the catalytic activity.

H2SO4 using a three-electrode system. Electrodes are swept from negative to positive potential at 5 mV s−1 on the reversible hydrogen electrode (RHE) scale. A graphite rod with a diameter of 0.3 cm is employed as the counter electrode and a calomel electrode is adopted as the reference. Bare Mo foil and commercial Pt foil with 1 × 1 cm2 are also tested for comparison. The polarization curves are shown in Figure 6a. The vertically aligned MoS2 film on the Mo foil synthesized at 700 °C for 1 min exhibits a remarkably high activity with an onset overpotential of 18 mV. The overpotential under a constant current density of 10 mA/cm2 is frequently employed as a typical reference base for the electrocatalytic performance.44 To achieve the HER current density, an overpotential of less than 200 mV is needed, and here we obtain a 175 mV overpotential which is higher than the overpotential of Pt foil (109 mV). In comparison, the MoS2 films synthesized on the Mo foil at 700 °C for 30 and 60 min have overpotentials of 193 and 187 mV, respectively, while the bare Mo foil requires 352 mV at 10 mA cm−2. In comparison with the reported MoS2based electrocatalysts, the vertically aligned MoS2 nanofilm on Mo foil shows a lower onset overpotential and a small Tafel slope (Table S1). To elucidate the underlying mechanism of the HER activity, Tafel plots derived from the electrochemical polarization curves are acquired, as shown in Figure 6b. The linear regions of the plots were fitted to the Tafel equation (η = b log(j) + a, where b is the Tafel slope and j is the current density). The Tafel slope of a commercial Pt foil is 30 mV dec−1, in good agreement with the reported result,27 which supports the validation of our electrochemical measurements. Accordingly, the calculated Tafel slopes of MoS2/Mo synthesized for 1 min, MoS2/Mo for 60 min, and the bare Mo foil are 55, 58, and 142 mV/decade, respectively. The results further confirm that the MoS2/Mo synthesized for 1 min has a high HER activity. It is noted that the small Tafel slope is preferable to the practical catalytic application because it gives rise to a high HER rate with overpotential.45 Generally, three possible principal steps are suggested for the HER process in acidic condition, as noted by Volmer, the Heyrovsky, and the Tafel steps.3,4,46 The Volmer reaction is a primary discharge step, which is associated with proton absorption (H3O++e− → Hads+H2O). It is a ratedetermining step and a Tafel slope (b) of ∼120 mV/decade should be obtained (b = 2.3RT/αF ≈ 120 mV, where α ≈ 0.5 is the symmetry coefficient, F is the Faraday constant, R is the ideal gas constant, and T is the absolute temperature).27 This step is followed either by an electrochemical desorption stage of H2 gas (Heyrovsky reaction, Hads + H3O+ + e− → H2 + H2O, b = 2.3RT/(1 + α)F ≈ 40 mV) or a recombination step (Tafel reaction, Hads + Hads → H2, b = 2.3RT/2F ≈ 30 mV).27 The obtained Tafel slope of 55 mV/decade in our work is small, suggesting that the Volmer-Heyrovsky HER mechanism is followed in the HER process. The small Tafel slope of the electrocatalyst can be ascribed to the strong electronic coupling between the ultrathin MoS2 nanofilm and the Mo surface. To gain further insight into the HER kinetics on the surface of the catalysts, the fluent charge transport is analyzed by electrochemical impedance spectroscopy (EIS), as shown in Figure 6c. The low-frequency section of the EIS is correlated with hydrogen adsorption onto the catalysts to form hydridetype species, while the high-frequency section is associated with the charge-transfer resistance.47 The vertically aligned MoS2 nanofilm on the Mo foil shows much lower charge transfer resistance than the bare Mo foil, while it is slightly higher than

4. CONCLUSION In conclusion, we develop a facile method of fabricating vertically aligned MoS2 nanofilms on Mo foils to achieve highly catalytic active sites and enhance electrical conductivity. The nanofilms with abundant catalytic active sites represent a novel metastable structure of 2D materials. The synergistic interplay, originating from the strong chemical and electronic coupling between the vertical MoS2 layer and the metal Mo substrate, facilitates the high efficient electron transfer and promotes the proton adsorption and fast reduction kinetics. Electrical coupling to the underlying Mo substrate afforded a rapid electron transport from the semiconducting MoS2 nanosheets to the Mo electrode. This study can be extended to vertically oriented nanofilms of extensive other 2D atomic layers on metal substrates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08120. Optical microscopic images, Raman spectra, HRTEM, XPS, time- dependent electrocatalytic current density 25848

DOI: 10.1021/acs.jpcc.6b08120 J. Phys. Chem. C 2016, 120, 25843−25850

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The Journal of Physical Chemistry C

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and summary of electrocatalytic performance of the reported MoS2-based catalysts. (PDF).

AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected]. Phone: 86-25-84896326. Fax: 86-25-84896326. * E-mail: [email protected]. Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (11502109, 11372132, 61474063, 11302100, and 51372114), Jiangsu NSF (SBK2015022205), the Innovation Fund of NUAA (NZ2015101 and NE2015102), SKL Funding of NUAA (0415G02), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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DOI: 10.1021/acs.jpcc.6b08120 J. Phys. Chem. C 2016, 120, 25843−25850