Morphological Engineering of Winged Au@MoS2 Heterostructures for

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Morphological Engineering of Winged Au@MoS2 Heterostructures for Electrocatalytic Hydrogen Evolution Yuan Li, Marek B Majewski, Saiful M. Islam, Shiqiang Hao, Akshay A. Murthy, Jennifer G. DiStefano, Eve D Hanson, Yaobin Xu, C. Wolverton, Mercouri G. Kanatzidis, Michael R. Wasielewski, Xinqi Chen, and Vinayak P. Dravid Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03109 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Morphological Engineering of Winged Au@MoS2 Heterostructures for Electrocatalytic Hydrogen Evolution Yuan Li,†,‡ Marek B Majewski,¶ Saiful M. Islam,⊥ Shiqiang Hao,† Akshay A. Murthy,†, § Jennifer G. DiStefano,†,§ Eve D. Hanson,†,§ Yaobin Xu,†,‡ Chris Wolverton,† Mercouri G. Kanatzidis,⊥ Michael R. Wasielewski,¶,⊥ Xinqi Chen,*,‡,∥ Vinayak P. Dravid *,†,‡,§

†Department

of Materials Science and Engineering, ‡Northwestern University Atomic and

Nanoscale Characterization Experimental (NUANCE) Center, ¶Argonne-Northwestern Solar Energy Research (ANSER) Center,

⊥Department

of Chemistry, §International Institute for

Nanotechnology (IIN), and ∥Department of Mechanical Engineering, Northwestern University, Evanston, Illinois 60208, USA

*Corresponding authors Xinqi Chen: [email protected], Vinayak Dravid: [email protected]

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ABSTRACT Molybdenum disulfide (MoS2) has been recognized as a promising cost-effective catalyst for water-splitting hydrogen production. However, the desired performance of MoS2 is often limited by insufficient edge-terminated active sites, poor electrical conductivity, and inefficient contact to the supporting substrate. To address these limitations, we developed a unique nanoarchitecture, namely winged Au@MoS2 heterostructures enabled by our discovery of the “seeding effect” of Au nanoparticles for the chemical vapor deposition synthesis of verticallyaligned few-layer MoS2 wings. The winged Au@MoS2 heterostructures provide an abundance of edge-terminated active sites and are found to exhibit dramatically improved electrocatalytic activity for the hydrogen evolution reaction. Theoretical simulations conducted for this unique heterostructure reveal that the hydrogen evolution is dominated by the proton adsorption step, which can be significantly promoted by introducing sufficient edge active sites. Our study introduces a new morphological engineering strategy to make the pristine MoS2 layered structures as highly competitive earth-abundant catalysts for efficient hydrogen production.

Keywords: winged Au@MoS2, heterostructure, seeding effect, chemical vapor deposition, hydrogen evolution reaction


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Electrochemical water splitting is an efficient and sustainable method for the large-scale production of hydrogen gas, a promising clean and renewable future energy source.1 The hydrogen evolution reaction (HER) can be achieved by various precious and noble metals such as Pt, Pd and Rh; however, the scarcity and costly nature of these metals has severely limited their utility in commercial hydrogen production. Recently MoS2 has been recognized as a promising nonprecious material for HER due to its natural abundance, low cost and promising catalytic capability.2 However, bulk MoS2 is not naturally active for HER and also shows poor conductivity for electrochemical applications; as a result, many chemical approaches including chemical exfoliation,3 metallic 1T-MoS2 conversion,4,5 and defect engineering6,7 have been explored to improve the HER performance. However, these processes generally deform the crystal structure of MoS2, which can induce the formation of metastable phases and diminish its chemical stability for long-term electrolysis.3 The most active sites of MoS2 for HER have been demonstrated to be present at the edges of the layers, due to the affinity between hydrogen atoms and unsaturated sulfur atoms.8-10 As a result, morphological and structural engineering methods have been outlined as another promising strategy to improve the catalytic performance of MoS2 without sacrificing its crystalline nature.11 Representative studies, for instance, include the development of 3D mesoporous MoS2 networks with a high degree of surface curvature,12-14 edge-terminated and interlayer-expanded colloidal MoS2 nanostructures,15 and vertically-aligned few-layered MoS2 films on glassy carbon,16 graphite paper,17 or graphene.18 The latter provides a potential strategy for developing large-scale hydrogen-evolving catalysts since the products are directly grown on a conductive substrate and thereby allow for efficient in-plane electron transport towards the edge-terminated active sites. However, the catalytic performance for the materials on these substrates, although improved, was 3 ACS Paragon Plus Environment

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found to be not significant unless a further interlayer expanding or intercalation process was applied.16 This is mainly due to the natural poor conductivity of MoS2, particularly when the entire substrate is covered by MoS2 films.5 In this letter, we introduce a novel concept that utilizes the seeding effect of metal nanoparticles to control and modulate the morphology of vertically-aligned few-layer MoS2 wings. We contribute new insights into the metal-seeding principle and report its successful application in the growth of free-standing MoS2 wings surrounding the Au nanostructures, namely winged Au@MoS2 (w-Au@MoS2) heterostructures. The MoS2 wings provide an abundancy of edgeterminated active sites for the hydrogen evolution reaction, while simultaneously providing low resistance electron transport pathways mediated through the Au nanoparticle cores. We observed significantly improved catalytic performance on these winged heterostructures with a hydrogenevolution overpotential approaching ~120 mV. Our work paves a new 2D morphological engineering approach for future electrochemical energy applications.

RESULTS AND DISCUSSION Structural evolution of winged Au@MoS2 heterostructures The CVD nucleation and growth process can be exploited to induce preferential growth and gain morphological control. In conventional CVD-grown MoS2 monolayers, a clear fullerenelike MoOxSy nucleus is always observed at the center of each flake (Figure S1a, Supporting Information), inducing the initial formation of lateral MoS2 layers19,20 Our recent study demonstrates the possibility to replace these fullerene-like nuclei with Au nanoparticles that serve as heterogeneous nucleation sites. 21 Subsequent growth leads to the formation of Au@MoS2 core4 ACS Paragon Plus Environment

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shell heterostructures with laterally-grown MoS2 monolayers spreading from the nanoparticles. 22 The Au seeds are first patterned on the Si substrate. As partially present in Figure S1 (Supporting Information), we observe the constant formation of lateral MoS2 nanosheets from individual nanoparticles (Figure S1b and c, Supporting Information) as well as the relatively-larger Au clusters with a predesigned geometry (Figure S1d-f, Supporting Information), clearly demonstrating a so-called seeding effect. Such seeding phenomenon can be explained by the natural affinity of gold and sulfur atoms, which leads to the initial formation of MoS2 shells on the Au nanoparticles (Figure S1g-j, Supporting Information).22 These MoS2 shell-encapsulated Au nanoparticles (Au@MoS2) induce the subsequent lateral growth of MoS2 layers.20,22 More interestingly, the morphology of MoS2 structures surrounding the Au nanoparticles can be effectively modulated by controlling the CVD conditions. The evolution of free-standing MoS2 wings on the Au@MoS2 heterostructures can be successfully achieved by increasing the vapor pressure of sulfur in the growth chamber. It has been demonstrated that the formation of vertically-aligned MoS2 flakes can be potentially originated from the collision and distortion areas of the substrates or supporting materials.23,24 Here the unique curved geometry in our Au@MoS2 system leads to an abundance of strained and/or defect sites, and thus, free-standing MoS2 wings on spherical Au nanoparticles are more favorable compared to a flat substrate. Micrographs of the resultant winged Au@MoS2 (w-Au@MoS2) heterostructures are shown in Figure 1. In this growth process, the MoS2 wings are consistently observed on each individual Au@MoS2 heterostructure and exhibit a similar morphology (Figure 1a and b). With increasing growth time, the products form a flower-like structure surrounding the Au@MoS2 core (Figure 1c). Additional SEM images are provided in Figure S2 (Supporting Information). The EDS maps in Figure 1d-g confirm the presence of elemental Au, Mo, and S. TEM is used to probe the morphology and atomic structure, 5 ACS Paragon Plus Environment

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further confirming the presence of free-standing few-layer MoS2 wings on each heterostructure (Figure 1h-j). High-resolution TEM image (Figure 1k) indicates a layer-to-layer spacing of ~0.65 nm for the wings and a typical hexagonal in-plane atomic structure of MoS2 (Figure 1k inset). The corresponding diffraction pattern (Figure 1l) also proves the presence of dominant facets: Au (111), MoS2 (100), and MoS2 (110). We further extend the CVD growth process to a highly-ordered pyrolytic graphite (HOPG) substrate, for the purpose of performing electrochemical hydrogen evolution. It is first worth noting that when utilizing the conditions of growing lateral MoS2 monolayers, we observe the same seeding effects on HOPG as on the Si substrates, where the planar MoS2 flakes are consistently formed at each Au nanoparticle site (Figure 2a). The w-Au@MoS2 heterostructures are further obtained by increasing the sulfur vapor pressure as we described above. As shown in Figure 2b, free-standing MoS2 wings are consistently formed on each Au nanoparticle and the inset in Figure 2b shows a representative individual w-Au@MoS2 heterostructure. Additional experimental results are demonstrated in Figure S3 (Supporting information). Besides the growth on the Au nanoparticles, we further realized that the MoS2 wings can be grown on island-like Au seeds (as representatively shown in Figure 2c). Due to this morphological flexibility in placement of Au seeds, we can accordingly modulate the density of MoS2 wings on the HOPG substrate by effectively varying the growth time (see Figure S4 and S5, Supporting Information). From highresolution images (e.g., Figure S4a and Figure S5a, Supporting Information), we clearly observe that each individual MoS2 wing is preferentially grown from the Au island, as opposed to the HOPG substrate. A low magnification TEM image of the island-seeded w-Au@MoS2 heterostructures is shown in Figure 2d. The yellow arrows indicate the edges of various MoS2 wings grown on 6 ACS Paragon Plus Environment

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Au@MoS2 islands. The enlarged image in Figure 2e confirms that the entire Au island seed is encapsulated in few-layer MoS2 shell. Moreover, individual w-Au@MoS2 heterostructures are shown in Figure 2f and g, where one can clearly observe the formation of MoS2 wings from each Au seed. High-resolution images in Figure 2h-j further demonstrate the multilayered structure of the MoS2 shell on the Au island (Figure 2h) as well as the typical few-layered nature (~2-3 layers) of the MoS2 wings (Figure 2i), with a typical hexagonal in-plane atomic structure consistent with the 2-H MoS2 for the latter (Figure 2j). More details about the TEM demonstration are provided in Figure S6 (Supporting Information). Figure 2k outlines the basic atomic structure of an individual MoS2 wing, which is expected to be few-layered MoS2 flakes free-standing on the Au@MoS2 islands, and typically terminated with the preferential zigzag or armchair edges,25 as further proved by the STEM images in Figure S7 (Supporting Information). Spectroscopic characterizations were conducted for the w-Au@MoS2 heterostructures grown on both HOPG (Figure 3) and Si substrates (Figure S8, Supporting Information), indicating very similar composition and elemental states of the products from different substrates. Raman spectra of the Au@MoS2 and w-Au@MoS2 heterostructures (Figure 3a) indicate the presence of the two typical vibration modes, E12g and A1g.34,26 The E12g mode is attributed to the in-plane vibration of Mo and S atoms, while the A1g mode is related to the out-of-plane vibration of S atoms.26 For the Au@MoS2 heterostructures, the two vibration modes are centered at 380.7 cm-1 and 405.6 cm-1, respectively, while those of the w-Au@MoS2 are slightly redshifted by ~3.6 cm-1, which is indicative of reduced strain in the winged heterostructures.27,28 In addition, in both materials, the E2g-to-A1g frequency difference (ca. 24.7 cm-1) remains constant. This value is slightly smaller than that of bulk MoS2, indicating the multi/few-layered nature of MoS2 shells and wings.29 XPS spectra of the heterostructures (Figure 3b-e) indicate the presence of the expected S, 7 ACS Paragon Plus Environment

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Mo, and Au peaks. The S 2p peak can be deconvolved into two peaks at 163.9 eV and 162.7 eV (Figure 3b), corresponding to the 2p1/2 and 2p3/2 orbitals. We observed a slight broadening of both 2p peaks for the w-Au@MoS2 heterostructures, with the emergence of two new additional peaks at high binding energy after the deconvolution. This is likely due to the presence of different S valence states originating from the edge-terminated MoS2 wings.30-32 The Mo 3d peaks at 233.04 eV and 229.87 eV correspond to 3d5/2 and 3d3/2 doublet,33,34 confirming the expected Mo4+ and S2states in MoS2 (Figure 3c). In addition, we also observe Au 4f5/2 and 4f7/2 peaks at 88.00 eV and 84.29 eV (Figure 3d), respectively, as well as the C 1s peak around 284. 6 eV (Figure 3e) for the sample grown on HOPG.

Electrocatalytic hydrogen evolution reaction Electrochemical hydrogen evolution experiments were conducted on both the Si-supported and HOPG-supported w-Au@MoS2 structures. The electrodes were properly sealed to only expose the front side to the electrolyte. As shown in Figure S9 (Supporting Information), the w-Au@MoS2 grown on Si substrates yields a remarkable increase of electrocatalytic activity as compared with the Au nanoparticles and Au@MoS2 on the same substrate. However, the HER performance is moderate likely due to the low conductivity of the Si substrate. The samples on the HOPG substrates show significantly improved HER activity. As shown in Figure 4a, the w-Au@MoS2 electrodes exhibit impressive and consistently higher activity when compared with the Au@MoS2 electrode. Meanwhile, the island-seeded w-Au@MoS2 sample with a moderate wing density (usually present a nice violet color) shows the best catalytic performance. The corresponding Tafel plots are shown in Figure 4b. The w-Au@MoS2 heterostructures show decreased Tafel slopes (59.6 ± 6.4 mV dec-1 from multiple repeating samples). 8 ACS Paragon Plus Environment

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Comparison of hydrogen evolution overpotential and Tafel slopes of various recently reported MoS2 catalysts with our w-Au@MoS2 heterostructures (Figure 4c) indicates that pristine or commercial MoS2 (Figure 4c, black) catalysts exhibit moderate HER activity;3-5 metallic 1TMoS2 (Figure 4c, green) exhibits a significantly decreased overpotential and Tafel slope;3-5,35 while materials employing the recently developed defect engineering methods (Figure 4c, pink) show a further decreased overpotential, albeit with larger Tafel slopes.7,10,36 One recent method for increasing HER performance involves growing MoS2 on Au foils2,37 or forming composites of MoS2 and Au (Figure 4c, yellow),38 while the studies on the structural engineering of MoS2 (Figure 4c, red), are perhaps the most promising approach to realize the best HER performance with MoS2based materials.12,13,15 Compared with these materials, our island-seeded w-Au@MoS2 heterostructures exhibit an overpotential of 145.9 ± 24.5 mV, with a possibility to reach overpotential as low as ~120 mV with favorable Tafel slopes. To date this is likely one of the lowest overpotential values achieved on pristine MoS2 structures, which exhibit greater crystalline quality and chemical stability as compared to chemically-exfoliated or defect-engineered materials.14-16 As indicated in Figure 4d, when using a constant cathodic potential, we observe an steady cell current during hydrogen evolution of a 10 h reaction window (inset in Figure 4d). The polarization curves before and after the 10 h electrolysis show insignificant difference, indicating that the integrity of the electrode was maintained over the course of the experiment. This is confirmed by the electron microscopic images of the sample after electrolysis, which, despite the presence of slight contamination and atomic distortion, generally retains a comparable crystal lattice to the pristine structure (Figure S10, Supporting Information). To comprehensively understand the impact of our morphological engineering strategy on the HER activity of MoS2 structures, density functional theory (DFT) calculations were conducted 9 ACS Paragon Plus Environment

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to examine the thermodynamic driving force for the hydrogen generation process. Considering the morphological differences between the MoS2 catalysts obtained via seeded growth in this study, we modeled the Gibbs free energy of hydrogen adsorption (ΔGºH), which has proven to serve as an important indicator of HER activity, 39 for various catalyst geometries including (1) a flat basal plane (underlying MoS2 sheet and wing surface), (2) curved basal plane (MoS2 shell on Au nanoparticles and islands), (3) zigzag edges and (4) armchair edges (wing edges) in Figure 2k. The ΔGºH was calculated as function of monolayer (ML) coverage of hydrogen atoms as representatively depicted in Figure S11a-d (Supporting Information). In general, a material with ΔGºH close to zero is usually the most favorable for HER since very negative ΔGºH will lead to slow release of hydrogen molecules from the catalyst surface, whereas the protons will be hard to absorb onto the catalyst when the ΔGºH is too positive.40 Figure 5a shows the variation of ΔGºH of hydrogen adsorption on different MoS2 planes and edges. The flat basal plane of MoS2 exhibits a ΔGºH of 1.2-2.0 eV depending on the hydrogen coverage, which is consistent with the previous study.40 The curved basal plane (with a curvature of 0.0338 m-1) leads to an increased activity since the ΔGºH decreased. This is probably due to the presence of strain and lattice distortion in the MoS2 shell structures.36 The ΔGºH on the edge sites of MoS2 wings approaches zero when the hydrogen coverage increased to 50%, indicating significantly increased HER activity as compared with the basal plane. This further confirms the importance of our experimental strategy of producing high-density MoS2 wing-like structures by using our unique discovery of such metal-seeding method. Besides, it is worth noting that for the first time our calculation reveals that the zigzag edge sites have a slight positive ΔGºH, which is favorable for hydrogen adsorption, while the armchair edges sites are favorable for hydrogen


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desorption due to their negative ΔGºH. The co-existence of both type of active sites might further improve the overall activity of MoS2 wings by involving an inter-lattice charge transfer process.41 To gain more insight into the HER mechanism, we further calculated the combination of two adsorbed hydrogen atoms via a typical Tafel step in HER. Figure 5b shows the Tafel step reaction pathway of HER on the zigzag edge sites of MoS2 wing. We found that weakly absorbed hydrogen atoms (0.23 eV) form the product (H2 molecules) by crossing a transition state (TS) with H-H distance of 1.16 Å. The calculated activation barrier energy is 0.21 eV, and the free energy drops to -2.8 eV in the final state, which indicates a highly favorable driving force for hydrogen formation.39 In addition, it is worth mentioning that the activation barrier and overall driving force for this Tafel step of H2 formation is very similar for all structures (1-4) modeled above (Figure S11e and f, Supporting Information), indicating the overall HER activity (the control step) is dominantly determined by the hydrogen adsorption/desorption process, rather than the charge transfer step. To collaborate our theoretical results with experimental data, Electrochemical Impedance Spectroscopic (EIS) studies were further conducted using the same HER electrodes. The obtained impedance spectra are shown in Figure 5c and d. All the impedance spectra show one capacitive arc in the high-frequency region and two capacitive arcs in the low-frequency region. The former corresponds to impedance arising from the electrode barrier layer, while the latter is probably associated with the electrocatalytic hydrogen evolution processes such as (a) adsorption of proton on catalysts, (b) charge-transfer Tafel step reaction, (c) release of hydrogen molecules.42 The impedance parameters (Table S1, Supporting Information) obtained from the impedance spectra provide direct insights into the electrochemical capacitance and resistance associated with these steps.43,44 The charge-transfer resistance (Rct) is much smaller than the adsorption resistance (Ra) 11 ACS Paragon Plus Environment

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overall, which supports our DFT simulation results that the HER process fundamentally depends on the hydrogen adsorption step. Moreover, compared with the Au@MoS2 electrodes, we find that Ra of the w-Au@MoS2 samples is significantly decreased. This is consistent with our simulation results in Figure 5a, indicating the improvement of hydrogen absorption efficiency on our winged structures. In addition, it is worth noting that we observed the barrier layer resistance, Rf, is much smaller on HOPG electrodes as compared with those on the Si substrates; this increase in substrate conductivity explains the significant increase of the HER activity (compare Figure 4a to Figure S9 in Supporting Information). Finally, based on our DFT and EIS study, we believe the excellent HER activity observed from the w-Au@MoS2 on HOPG substrate can be attributed to the abundance of active edge sites engineered through the MoS2 wings, which induces a small chargetransfer resistance (Rct) of HER; and also due to the improved conductivity originating from the contact between MoS2 and the conductive substrate mediated by the Au nanoparticle cores.

CONCLUSIONS In this contribution, we demonstrate the seeding effect of Au nanostructures on the CVD synthesis of winged Au@MoS2 heterostructures with promising HER performance. The key advantage of this technique is the highly-controllable growth of free-standing MoS2 wings with desired morphology, which, on the one hand, leads to the creation of abundant edge-terminated active sites to drive the formation of hydrogen molecules, and on the other hand, enables efficient charge transport within the w-Au@MoS2 structures through their favorable contact with the Au core. Compared with conventional approaches such as metallic 1-T conversion and defect engineering, our method demonstrates the possibility to realize high-efficiency hydrogen production by using pristine MoS2 material, which is a stable compound more amenable for future 12 ACS Paragon Plus Environment

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commercial development of water-splitting techniques. Moreover, our discovery of such seeding effect may also be applicable to other non-precious metal seed systems and thereby enable a broad strategy for designing and developing novel 2D layered structures for future heterogeneous catalysis, energy storage/conversion, and high-performance electronics.

ASSOCIATED CONTENT Supporting Information Experimental details; Au-seeding effect on lateral MoS2 monolayer growth; w-Au@MoS2 growth on island-like cores and characterization; DFT simulation details; impedance characterizations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION *Corresponding Author Xinqi Chen, E-mail: [email protected] Vinayak Dravid, E-mail: [email protected]

Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS This material is based upon work supported by the National Science Foundation (NSF) under Grant No. DMR-1507810. This work made use of the EPIC, Keck-II, and/or SPID facilities of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. Electrochemistry experiments were supported by the Argonne-Northwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the U.S. DOE, Office of Science, Office of Basic Energy Sciences, under award no. DE-SC0001059 (M.B.M.). J.G.D. gratefully acknowledges support from the National Science Foundation Graduate Research Fellowship Program (NSF-GRFP). A. A. M. gratefully acknowledges support from the Ryan Fellowship and the IIN at Northwestern University.

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(44) Singh, R. K.; Ramesh, R.; Devivaraprasad, R.; Chakraborty, A.; Neergat, M. Electrochim. Acta 2016, 194, 199-210.


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FIGURES

Figure 1. Discovery of Au-seeded growth of w-Au@MoS2 (on Si substrate). (a,b) SEM images of w-Au@MoS2 grown for 10 min. (c) Representative SEM image of w-Au@MoS2 grown for 30 min. (d-g) EDS elemental mapping for w-Au@MoS2. Scale bar: 100 nm. (h-j) TEM images of wAu@MoS2. (k) High-resolution TEM image and (l) diffraction pattern for w-Au@MoS2.


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Figure 2. Morphological engineering on HOPG substrate. (a) SEM image showing the seeded growth of planar MoS2. (b) SEM image of the nanoparticle-seeded w-Au@MoS2. The inset shows an individual w-Au@MoS2. (c) SEM and (d) TEM images of the island-seeded w-Au@MoS2. The yellow arrows in (d) show the presence of MoS2 wings. (e) Enlarged TEM image demonstrating the entire encapsulation of Au island with MoS2 shell (see green arrows). (f,g) TEM images of individual island-seeded w-Au@MoS2. (h-j) High-resolution TEM images for location (1) and (2) in (f). (k) Schematic outlining the atomic structure of MoS2 wings seen in the TEM demonstration.


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Figure 3. Spectroscopic characterizations of the w-Au@MoS2 on HOPG. (a) Raman spectra of Au@MoS2 and w-Au@MoS2. (b-e) XPS spectra indicating the presence S 2p (b), Mo 3d (c), Au 4f (d), and C 1s (e) in the heterostructures.


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Figure 4. HER performance of the w-Au@MoS2. (a) Linear polarization curves of Au@MoS2 and w-Au@MoS2 (nanoparticles/islands) on HOPG and (b) the corresponding Tafel plots. (c) Comparison of the Tafel slopes and overpotentials (at 10 mA cm-2) of various MoS2-based catalysts reported in literature. (d) Polarization curves show that the w-Au@MoS2 (islands) continue to exhibit excellent catalytic activity after ~10 h of electrolysis.


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Figure 5. HER mechanism. (a) DFT calculated Gibbs free energy diagram revealing the driving force of hydrogen adsorption on four types of MoS2 surface/edge: (1) flat basal plane, (2) curved basal plane, (3) zigzag edge, (4) armchair edge. The Mo atoms on the surface or edge are all stabilized by disulfide ligands, and calculations are conducted for 1/8, 2/8, 3/8, and 4/8 monolayer (ML) coverage of hydrogen atoms. (b) Energy diagram for the Tafel-step reaction pathway of HER on the zigzag edge of MoS2 wings. (c,d) Impedance spectra of various samples obtained on Si substrate (c) and HOPG substrate (d).


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