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Monolayer MoS2 Films Supported by 3D Nanoporous Metals for High-Efficiency Electrocatalytic Hydrogen Production Yongwen W. Tan, Pan Liu, Luyang Y. Chen, Weitao T. Cong, Yoshikazu Ito, Jiuhui H. Han, Xianwei W. Guo, Zheng Tang, Takeshi Fujita, Akihiko Hirata, and Mingwei W. Chen* As an important energy carrier, hydrogen holds tremendous promise for sustainable green energy innovation.[1] Hydrogen production by water electrolysis recently gains increasing attention because of the high efficiency and eco-friendliness.[2–4] The key step in water electrolysis is the hydrogen evolution reaction (HER) at a thermodynamic overpotential of ~1.23 V.[2,5–7] Appropriate catalysts, mostly Pt and alloys, can dramatically improve the energy conversion efficiency by significantly dropping the overpotential.[8] However, the low natural abundance and high material cost of Pt restrict the practical implementation of electrochemical hydrogen production. Recently, earth-abundant and low-cost transition metal dichalcogenides, such as MoS2 and WS2, are emerging as a promising replacement of Pt catalysts for HER.[5,9] It has been suggested that the electrocatalysis originates from the undercoordinated atoms at edge sites of the 2H sulfide compounds.[5,10] Enormous efforts have recently been dedicated to fabricate various nanostructured dichalcogenides to exposure more edge sites for amplifying HER catalysis.[11–29] On the other hand, this often results in the less stability and the inefficient electrical conductivity of the dispersed nanostructured 2D catalysts.[9,10,17,18] As an alternative approach, lattice strains have been suggested to be an effective way to enhance the catalytic activity of 2H sulfide compounds, although the underlying mechanisms have not been fully understood.[11] In this study, we developed a continuous monolayer MoS2 film grown on the curved internal surface of 3D nanoporous gold Dr. Y. W. Tan, Dr. P. Liu, Dr. L. Y. Chen, Dr. W. T. Cong, Dr. Y. Ito, J. H. Han, Dr. X. W. Guo, Dr. T. Fujita, Dr. A. Hirata, M. W. Chen WPI Advanced Institute for Materials Research Tohoku University Sendai 980-8577, Japan E-mail: [email protected] M. W. Chen State Key Laboratory of Metal Matrix Composites School of Materials Science and Engineering Shanghai Jiao Tong University Shanghai 200030, P.R. China Dr. W. T. Cong, Prof. Z. Tang Key Laboratory of Polar Materials and Devices East China Normal University Shanghai 200062, P.R. China M. W. Chen CREST, JST, 4–1–8 Honcho Kawaguchi Saitama 332-0012, Japan
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(NPG). Different from the conventional wisdom, this “edgefree” monolayer MoS2 with significant out-of-plane strains by lattice bending shows a high catalytic activity towards HER, which may pave a new way to fabricate 2D catalysts with a large effective surface area and high catalytic activity. Figure 1a illustrates the fabrication procedure of the 3D continuous MoS2 films by using chemically dealloyed NPG (Figure S1, Supporting Information)[30] as substrates for chemical vapor deposition (CVD) of monolayer MoS2.[31,32] Free-standing monolayer MoS2@NPG composites were used as catalytic electrodes for electrocatalytic hydrogen production, in which the continuous MoS2 film acts as active catalysts and the conductive NPG provides a short path for fast charge transfer and transport (Figure 1b). Figure 2a,b shows the scanning electron microscope (SEM) images of the monolayer MoS2@NPG composite, which has a pore size of ~100 nm. By optimizing CVD time and the distance between the MoO3 precursor and the NPG substrate, the layer number of MoS2 can be well controlled from monolayer to few layers as evidenced by Raman spectra statistically taken from a large area (Figure 2c). The frequency difference (Δ) of two characteristic Raman modes (E12g and A1g) has been widely used as the identity of the MoS2 layer number. The Δ values were measured to be ≈20, 22, and 23 cm−1 for the synthetic 1 layer (1L), 2L, and 3L MoS2@NPG films, which are fully consistent with previous reports in the literature.[33] To confirm the continuity and uniformity of the synthesized MoS2 films, we performed Raman characterization by stochastic measurements of the MoS2 characteristic modes from different regions spanning the entire centimeter-sized samples (Figure S3, Supporting Information). The well-consistent Raman spectra demonstrate that the CVD films with the defined MoS2 layers are homogeneous in the large-scale samples. Separate photoluminescence (PL) experiments further corroborate the monoand few layer features of the CVD MoS2 films. Figure 2d presents the PL spectra of 1L, 2L, and 3L MoS2@NPG samples along with bulk MoS2 as a reference. The monolayer and few layer MoS2 samples exhibit PL peaks corresponding to the direct band gap at ~660 nm, whereas the bulk MoS2 only shows a visible PL peak of the indirect band gap. Moreover, the PL intensity of the 1L MoS2@NPG is much stronger than those of the 2L and 3L MoS2@NPG samples, consistent with reported direct band gap properties of 2D crystalline MoS2.[34,35] The continuous monolayer MoS2 film on the internal surface of NPG is directly elucidated by high-resolution scanning transmission electron microscope (STEM). Figure 3a is
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two layers of packed sulfur atoms (Figure 3b). The structure of the S Mo S sandwich layer is in accordance with the simulated image (the inset of Figure 3b) based on the standard 2H structure of MoS2 (Figure 3c) as well as the theoretical thickness (~0.667 nm) of monolayer MoS2.[36] The consistence of the experimental observation with the simulation demonstrates that the monolayer MoS2 has a 2H structure and the (001)2H plane is approximately parallel to the topmost surface of the gold substrate on the flat region. Moreover, the ABF-STEM image (Figure 3d) reveals significant lattice distortions of the Figure 1. Monolayer MoS2@NPG towards catalytic HER. a) Schematic diagram of the fabrica- monolayer MoS2, particularly at the curved tion process of monolayer MoS2@NPG hybrid materials by a nanoporous metal-based CVD regions of the NPG substrate. The variation approach.[32] b) Schematic HER catalyzed by the monolayer MoS2@NPG hybrid material. of the S Mo S bonding angle α in the ABFSTEM image is plotted in Figure 3e. Except the film on flat regions of NPG, the monolayer MoS2 film expea high-resolution STEM image taken by a high-angle annular dark-field (HAADF) detector, which shows the side view image riences serious out-of-plane lattice strains on the curved NPG of the MoS2 films on NPG. It can be clearly seen that the substrate. The large lattice strains of the monolayer MoS2 film MoS2 film with a single unit cell thickness uniformly sits on were also confirmed by the top view HAADF-STEM image and selected area electron diffraction (Figure S4, Supporting the top of a curved gold ligament. Since annular bright-field Information). Both curved lattice and elongated diffraction (ABF) STEM is capable of imaging both the heavy element of pattern demonstrate that there are large lattice distortions in molybdenum and the light element of sulfur, it was employed the monolayer MoS2 film, which is consistent with the curved to image the distribution of Mo and S atoms in the monolayer crystal. The zoom-in ABF-STEM image from a flat region show morphology of the monolayer film as seen from the side view a row of molybdenum atomic columns sandwiched between images (Figure 3). Importantly, undercoordinated surface steps and lattice defects (dislocations and vacancies), which are suggested to be the catalytically active sites for HER, are rarely seen in the continuous monolayer film. The HER catalytic activities of the MoS2@ NPG hybrid materials were evaluated in a 0.5 M H2SO4 solution using a three-electrode electrochemical system. Figure 4a shows the cathodic polarization curves of 1L, 2L and 3L MoS2@NPG together with Pt and pure NPG as references. The kinetic currents in the plots are normalized by effective surface areas of the electrodes (Figure S5, Supporting Information). The monolayer MoS2@NPG film possesses the lowest onset overpotential (η) of 118 mV (Figure 4b), while the onset overpotentials gradually increase to 128 and 132 mV for the 2L and 3L samples, respectively. Although the density of the undercoordinated edge sites and lattice defects of the continuous monolayer MoS2 film is much less than those of nanostructured MoS2, the onset overpotential of HER Figure 2. Structure characterizations of MoS2@NPG. a) Top view SEM image of the monolayer is noticeably lower than the reported values MoS2@NPG material. The inset shows a zoom-in image. The MoS2 film is too thin to be seen of MoS2 nanoparticles,[10,37,38] mesopores,[17] by SEM. b) Side view SEM image showing the 3D nanoporous structure of the monolayer nanowires,[18] nanosheets,[14,15,19,39] and flat MoS2@NPG material. c) Raman spectra of bulk MoS2 and the synthesized monolayer (1L), MoS films by CVD (Table S2, Supporting 2 bilayer (2L), and trilayer (3L) MoS2@NPG materials. The characteristic Raman modes (E12g Information).[12,17,20] Cathodic current denand A1g) of MoS2 are labeled in the spectra. The difference between the out-of-plane (A1g) and in-plane (E12g) Raman peaks is used to characterize the layer number of MoS2. d) Photo- sity at a low applied potential is an important luminescence spectra of 1L, 2L, and 3L MoS2@NPG, and bulk MoS2 reference with 514.5 nm criterion for evaluating catalytic activities of electrocatalysts. The monolayer MoS2@NPG laser excitation. 2
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to 2L and 3L MoS2@NPG (Figure S7, Supporting Information), suggesting a much faster electron transfer and a high Faradaic efficiency during HER. This apparently benefits from the unique electronic properties of the curved monolayer MoS2 and the low-resistance Ohmic contact of the monolayer MoS2 with the highly conductive NPG substrate.[40,41] Moreover, the obvious layer dependence of the series resistance also indicates that the lower catalytic activity of 2L and 3L MoS2@NPG may be associated with the indirect contact of the topmost active MoS2 layer with the conductive NPG substrate, which underlines the importance of conductivity in the electrocatalysis of MoS2. Besides the high electrocatalytic activity, the lifetime is also an important criterion to evaluate an electrocatalyst. We investigated the cycling stability of MoS2@NPG in the acidic electrolyte by continuous cyclic voltammetry (CV) testing at a scan rate of 50 mV s−1 (Figure S8, Supporting Information). At either constant applied potential (250 mV) or constant cathodic current (0.25 mA cm−2), the MoS2@NPG electrodes show excellent cycling retention with less than 20% degraFigure 3. Atomic structure of monolayer MoS2 on gold ligaments of NPG. a) Cross-section dation after 1000 cycles. Moreover, at a conHAADF-STEM image of monolayer MoS2@NPG. The single layer contrast of MoS2 from the stant overpotential of 250 mV, the catalytic heavy Mo atoms. b) Higher resolution STEM images of monolayer MoS2@NPG from the flat hydrogen production can continuously take region of a gold ligament. A row of molybdenum atomic columns are sandwiched by two layers place for over 9 h with very slow current of sulfur atoms. The inset shows the simulated STEM image based on the standard structure decrease (Figure S8, Supporting Information). model of 2H MoS2 in (c). d) The annular bright-field STEM image of monolayer MoS2@NPG It has been theoretically predicted and taken from a curved region of NPG substrate. e) The variation of the S Mo S bonding angles (α) at the curved region shown in (d). The dash line indicates the S Mo S bonding angles experimentally demonstrated that the cata(120°) of perfect 2H MoS2. Scale bars: 1 nm in (a), (b), and (d). lytic activity of MoS2 catalysts originates from the chemically active undercoordinated atoms at edge sites.[5,10] For this reason, 2D MoS2 films by CVD usually do not have adequate HER activiexhibits a high current density of 0.52 mA cm−2 at 250 mV, which is ~2, 4, and 15 times higher than those of the 2L, 3L ties because the continuous films lack sufficient undercoordiMoS2@NPG, and bare NPG, respectively. Moreover, the signifinate edge atoms.[16] Different from the common wisdom, in cant hydrogen evolution (J = 10 mA cm–2) can be observed at a this study we found that the continuous monolayer MoS2 films low operating voltage of –226 mV (Table S2, Supporting Inforwith scarce edge sites present high catalytic activity toward mation). The high catalytic activity of the monolayer MoS2@ HER. Unlike conventional CVD using flat substrates, the continuous monolayer MoS2 films were grown on the internal surNPG is also validated by Tafel plots, which correspond to the cathodic current change with an increase in applied potentials face of dealloyed NPG that is constructed by highly curved gold and reflect the rate limitation of electrocatalyzed reactions. ligaments containing both negative and positive curvatures and The Tafel slope (46 mV/decade) of the monolayer MoS2@NPG high curvature gradients.[42,43] The MoS2 films naturally inherit is noticeably lower than those of the 2L (52 mV/decade), 3L the 3D curvature of NPG by continuous lattice bending (Figure 3; (55 mV/decade) MoS2@NPG, and pure NPG (95 mV/decade). Figure S4, Supporting Information). Considering the continuous films are deficient of catalytically active edge sites, the In comparison with the best MoS2-based catalysts reported in high catalytic activity of the “edge-free” films appears to come the literature (Table S2, Supporting Information),[10,14–18,37–39] from the bent MoS2 lattices with large out-of-plane strains. It the “edge-free” monolayer MoS2@NPG shows comparable catalytic activity in the onset potential and Tafel slope. has been noticed that in-plane tensile strains can dramatically decrease the free energy of atomic hydrogen absorption on disThe high electrode reaction kinetics of the monolayer torted 1T WS2 nanosheets, whereas the effect of the in-plane MoS2@NPG is more evident in electrochemical impedance spectra (EIS). Figure 4d is the Nyquist plots of the MoS2@NPG strains is minor for semiconductor 2H phase.[11] For MoS2, electrodes at the overpotential of 250 mV. The EIS experiment possible electronic structure changes caused by lattice strains reveals the low charge transfer resistance (Rct) and internal have also been discussed before[44,45] but the strain-induced resistance of the monolayer MoS2@NPG electrode, compared catalysis of 2D MoS2 has not been systematically investigated.
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Figure 4. Electrocatalytic performances of platinum, pure NPG, and MoS2@NPG. a) Polarization curves; b) derivative polarization curves showing the onset potentials; c) corresponding Tafel slopes; and d) Nyquist plots showing the electrode kinetics of MoS2@NPG.
Our density functional theory (DFT) calculations indicate that, obviously different from in-plane strains, out-of-plane lattice bending can evidently change the band gap of monolayer MoS2 (Figures S9 and S10, Supporting Information). The distinct out-of-plane strains result in continuous changes of S Mo S bonding angles (α), which can be expressed as relative S and/ or Mo atomic migration in the theoretical model, as depicted in Figure S9 a, b, and c (Supporting Information). When the S Mo S bonding angle α is larger than 140°, the band gap of the monolayer 2H MoS2 is close to zero (Figure S9d, Supporting Information), indicating that there is a local semiconductorto-metal transition caused by the out-of-plane lattice bending. Since the catalytic activity of MoS2 edges is mainly from the edge S atoms with dangling bonds, we also analyzed the density of states of the S atoms as the function of the bending angles (Figure S10, Supporting Information). As shown in Figure S9e (Supporting Information), the lattice bending significantly contributes to the charge density of S atoms, which is in striking contrast to that of the pristine 2H MoS2 (α = 120°). The enriched charge density of S atoms at the bent regions is very analogous to that of edge S atoms with dangling bonds. We
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also evaluated the free energy of atomic hydrogen adsorption at the bent lattices of MoS2 (Figure S9f, Supporting Information). Our calculations show that the free energy is associated with the S Mo S bonding angle α. The lattice bending gives rise to smaller differential binding free energy than that of the pristine MoS2. The lowest energy of ≈0.2 eV appears at α = 155°. The local semiconductor-to-metal transition,[12,46] 12 the lower adsorption energy, and enriched charge density at the curved regions of the “edge-free” MoS2 films fairly explain the superior catalysis of the curved monolayer MoS2 without abundant edge sites. Consequently, our finding certainly provides new insights into the strain effect on catalytic properties of 2D catalysts and may pave a new way to tailor the catalytic activity of 2D catalysts by lattice stain engineering. Besides the out-of-plane strains, the dependence of HER activity on the MoS2 layer number implies the importance of the conductive NPG substrate because the high interlayer potential barrier may give rise to the lower interlayer electron hopping efficiency.[16] The direct contact between the monolayer MoS2 and highly conductive gold leads to the low-resistance Ohmic contact for fast charge transfer and high HER efficiency.[28,41]
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Experimental Section Materials Fabrication: NPG membranes used in this study were prepared by chemically dealloying.[30] Approximately1-µm thick Au35Ag65 alloy sheets were etched by 69 vol.% HNO3 at room temperature for 6 h. The silver component was selectively leached away from the alloy and the remained gold forms a 3D bicontinuous open porous structure by self-assembly. After dealloying, the residual acid within the nanopore channels was removed by water rinsing. The NPG samples have a uniform porous structure with an average pore size of about 20–30 nm. The monolayer MoS2 films were grown on the internal surface of NPG by CVD. MoO3 and pure sulfur were used as reactant materials and argon was used as the carrier gas. The NPG membranes were placed on the insulating SiO2/Si substrates, and the film growth was performed at the reaction temperature of 650 °C, whereas the pore and ligament size of NPG substrates increases from 20–30 to ≈100 nm. Materials Characterization: Raman and PL spectra were recorded using a Renishaw Raman microscope (Renishaw InVia RM 1000) with an incident laser wavelength of 514.5 nm. The microstructure of the MoS2@ NPG hybrid materials was investigated using a field-emission SEM (JEOL JIB-4600F) and a field-emission transmission electron microscope (JEOL JEM-2100F, 200 keV) equipped with double spherical aberration (Cs) correctors for both the probe- and image-forming objective lenses. The Cs correctors were optimized for image observations and the pointto-point resolution of STEM is better than 1.0 Å. The HAADF detector was set to collect electrons scattered between 100 and 267 mrad. The STEM image simulations were performed using the Win HREM software (HREM Research Inc). In the calculations, the probe convergence angle is 25 mrad and the HAADF detector inner and outer angles are set as 100 and 267 mrad, respectively. Electrochemical Measurements: The electrocatalytic experiments were performed using an electrochemical workstation (Ivium Technology) in a standard three-electrode mode. The electrolyte is a 0.5 M H2SO4 water solution. A saturated calomel and platinum wire were used as the reference electrode and counter electrode, respectively. The potential values reported in this study are versus the reversible hydrogen electrode. The HER activities of the samples were evaluated by linear sweep voltammetry at 5 mV s−1. The electrolyte was purged with high pure N2 for 30 min prior to measurements. CV was conducted at 50 mV s−1 to investigate the cycling stability. The EIS were collected from 105 to 0.1 HZ with an amplitude of 10 mV under various overpotentials. The
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Because the catalytically active sites are the strained lattices, the degeneration of the MoS2@NPG catalysts possibly results from strain relaxation during HER, rather than the loss of active sites of dispersed catalysts by agglomeration. Since the lattice strains are introduced by the 3D nanoporous morphology of stable NPG, this explains the excellent catalytic retention of the MoS2@NPG catalysts. In conclusion, we have successfully grown a large-scale 3D monolayer MoS2 on the internal surface of dealloyed NPG by a CVD method. The novel monolayer MoS2@NPG electrode exhibits high catalytic activities with a smaller Tafel slope of 46 mV/decade and a lower onset potential of −118 mV. Moreover, as an important finding, the catalytic activity of 2D MoS2 can be activated by introducing out-of-plane strains, providing an effective way to developing highly active 2D catalysts by lattice strain engineering. Although the NPG substrate used in this study is still too expensive for large-scale practical applications, the idea and concept reported in this work can be realized by using cheap nanoporous metals, such as nanoporous Cu and Ni, for economic MoS2-based catalysts toward highefficiency electrochemical hydrogen production.
impedance data were fitted to a simplified Randles circuit to extract the series and charge-transfer resistances. The effective electrochemical surface area of annealed NPG and MoS2@NPG hybrid materials were evaluated by the CV method. DFT Calculations: The DFT calculations were performed by using the Vienna ab initio simulation package based on the spin-polarized DFT within the generalized gradient approximation (GGA). The electron– ion interactions are presented by the Troullier–Martins-type normconserving pseudopotentials with a partial core correction and the GGA exchange correlation potential in the form of Perdew–Burke–Ernzerhof functional is adopted. An energy cutoff of 400 eV was used for plane wave basis expansion.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was sponsored by the JST-CREST “Phase Interface Science for Highly Efficient Energy Utilization,” JST, Japan and “World Premier International (WPI) Research Center Initiative for Atoms, Molecules and Materials,” MEXT, Japan. Z. T. was supported by National Science Foundation of China. Received: August 20, 2014 Revised: September 26, 2014 Published online:
[1] J. A. Turner, Science 2004, 305, 972. [2] M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 2010, 110, 6446. [3] Y. Zheng, Y. Jiao, Y. Zhu, L. H. Li, Y. Han, Y. Chen, A. Du, M. Jaroniec, S. Z. Qiao, Nat. Commun. 2014, 5, 3783. [4] Y. Zheng, Y. Jiao, L. H. Li, T. Xing, Y. Chen, M. Jaroniec, S. Z. Qiao, ACS Nano 2014, 8, 5290. [5] B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, J. H. Nielsen, S. Horch, I. Chorkendorff, J. K. Nørskov, J. Am. Chem. Soc. 2005, 127, 5308. [6] J. K. Norskov, T. Bligaard, J. Rossmeisl, C. H. Christensen, Nat. Chem. 2009, 1, 37. [7] Y. Zheng, Y. Jiao, M. Jaroniec, S. Z. Qiao, Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201407031R1. [8] J. Greeley, T. F. Jaramillo, J. Bonde, I. B. Chorkendorff, J. K. Norskov, Nat. Mater. 2006, 5, 909. [9] A. B. Laursen, S. Kegnaes, S. Dahl, I. Chorkendorff, Energy Environ. Sci. 2012, 5, 5577. [10] T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, Science 2007, 317, 100. [11] D. Voiry, H. Yamaguchi, J. Li, R. Silva, D. C. B. Alves, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda, M. Chhowalla, Nat. Mater. 2013, 12, 850. [12] M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. Li, S. Jin, J. Am. Chem. Soc. 2013, 135, 10274. [13] D. Voiry, M. Salehi, R. Silva, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda, M. Chhowalla, Nano Lett. 2013, 13, 6222. [14] J. Xie, J. Zhang, S. Li, F. Grote, X. Zhang, H. Zhang, R. Wang, Y. Lei, B. Pan, Y. Xie, J. Am. Chem. Soc. 2013, 135, 17881. [15] J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X. W. Lou, Y. Xie, Adv. Mater. 2013, 25, 5807.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5
www.advmat.de
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www.MaterialsViews.com [16] Y. Yu, S.-Y. Huang, Y. Li, S. N. Steinmann, W. Yang, L. Cao, Nano Lett. 2014, 14, 553. [17] J. Kibsgaard, Z. Chen, B. N. Reinecke, T. F. Jaramillo, Nat. Mater. 2012, 11, 963. [18] Z. Chen, D. Cummins, B. N. Reinecke, E. Clark, M. K. Sunkara, T. F. Jaramillo, Nano Lett. 2011, 11, 4168. [19] Z. Lu, W. Zhu, X. Yu, H. Zhang, Y. Li, X. Sun, X. Wang, H. Wang, J. Wang, J. Luo, X. Lei, L. Jiang, Adv. Mater. 2014, 26, 2683. [20] H. Wang, D. Kong, P. Johanes, J. J. Cha, G. Zheng, K. Yan, N. Liu, Y. Cui, Nano Lett. 2013, 13, 3426. [21] D. Merki, S. Fierro, H. Vrubel, X. Hu, Chem. Sci. 2011, 2, 1262. [22] Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong, H. Dai, J. Am. Chem. Soc. 2011, 133, 7296. [23] H. I. Karunadasa, E. Montalvo, Y. Sun, M. Majda, J. R. Long, C. J. Chang, Science 2012, 335, 698. [24] X. Ge, L. Chen, L. Zhang, Y. Wen, A. Hirata, M. Chen, Adv. Mater. 2014, 26, 3100. [25] Y.-H. Chang, C.-T. Lin, T.-Y. Chen, C.-L. Hsu, Y.-H. Lee, W. Zhang, K.-H. Wei, L.-J. Li, Adv. Mater. 2013, 25, 756. [26] Z. Wu, B. Fang, Z. Wang, C. Wang, Z. Liu, F. Liu, W. Wang, A. Alfantazi, D. Wang, D. P. Wilkinson, ACS Catal. 2013, 3, 2101. [27] L. Liao, J. Zhu, X. J. Bian, L. N. Zhu, M. D. Scanlon, H. H. Girault, B. H. Liu, Adv. Funct. Mater. 2013, 23, 5326. [28] D. J. Li, U. N. Maiti, J. Lim, D. S. Choi, W. J. Lee, Y. Oh, G. Y. Lee, S. O. Kim, Nano Lett. 2014, 14, 1228. [29] J. Kibsgaard, T. F. Jaramillo, F. Besenbacher, Nat. Chem. 2014, 6, 248. [30] J. Erlebacher, M. J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki, Nature 2001, 410, 450. [31] S. Najmaei, Z. Liu, W. Zhou, X. Zou, G. Shi, S. Lei, B. I. Yakobson, J.-C. Idrobo, P. M. Ajayan, J. Lou, Nat. Mater. 2013, 12, 754. [32] a) Y. Ito, H.-J. Qiu, T. Fujita, Y. Tanabe, K. Tanigaki, M. W. Chen, Adv. Mater. 2014, 26, 4145; b) Y. Ito, Y. Tanabe, H.-J. Qiu, K. Sugawara,
6
wileyonlinelibrary.com
[33] [34] [35] [36] [37] [38] [39] [40] [41]
[42]
[43] [44]
[45] [46]
S. Heguri, N. H. Tu, K. K. Huynh, T. Fujita, T. Takahashi, K. Tanigaki, M. W. Chen, Angew. Chem. Int. Ed. 2014, 53, 4822. C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone, S. Ryu, ACS Nano 2010, 4, 2695. G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen, M. Chhowalla, Nano Lett. 2011, 11, 5111. A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, F. Wang, Nano Lett. 2010, 10, 1271. G. Eda, T. Fujita, H. Yamaguchi, D. Voiry, M. Chen, M. Chhowalla, ACS Nano 2012, 6, 7311. J. Bonde, P. G. Moses, T. F. Jaramillo, J. K. Norskov, I. Chorkendorff, Faraday Discuss. 2009, 140, 219. H. Vrubel, D. Merki, X. Hu, Energy Environ. Sci. 2012, 5, 6136. J. Kim, S. Byun, A. J. Smith, J. Yu, J. Huang, J. Phys. Chem. Lett. 2013, 4, 1227. T. Fujita, H. Okada, K. Koyama, K. Watanabe, S. Maekawa, M. W. Chen, Phys. Rev. Lett. 2008, 101, 166601. C. Gong, C. Huang, J. Miller, L. Cheng, Y. Hao, D. Cobden, J. Kim, R. S. Ruoff, R. M. Wallace, K. Cho, X. Xu, Y. J. Chabal, ACS Nano 2013, 7, 11350. T. Fujita, P. Guan, K. McKenna, X. Lang, A. Hirata, L. Zhang, T. Tokunaga, S. Arai, Y. Yamamoto, N. Tanaka, Y. Ishikawa, N. Asao, Y. Yamamoto, J. Erlebacher, M. W. Chen, Nat. Mater. 2012, 11, 775. T. Fujita, L. H. Qian, K. Inoke, J. Erlebacher, M. W. Chen, Appl. Phyl. Lett. 2008, 92, 251902. A. Castellanos-Gomez, R. Roldán, E. Cappelluti, M. Buscema, F. Guinea, H. S. J. van derZant, G. A. Steele, Nano Lett. 2013, 13, 5361. E. Scalise, M. Houssa, G. Pourtois, V. Afanas’ev, A. Stesmans, Nano Res. 2012, 5, 43. T. Fujita, Y. Ito, Y. W. Tan, H. Yamaguchi, D. Hojo, A. Hirata, D. Voiry, M. Chhowalla, M. W. Chen, Nanoscale 2014, 6, 12458.
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Monolayer MoS2 Films Supported by 3D Nanoporous Metals for High-Efficiency Electrocatalytic Hydrogen Production Yongwen W. Tan, Pan Liu, Luyang Y. Chen, Weitao T. Cong, Yoshikazu Ito, Jiuhui H. Han, Xianwei W. Guo, Zheng Tang, Takeshi Fujita, Akihiko Hirata, and Mingwei W. Chen* As an important energy carrier, hydrogen holds tremendous promise for sustainable green energy innovation.[1] Hydrogen production by water electrolysis recently gains increasing attention because of the high efficiency and eco-friendliness.[2–4] The key step in water electrolysis is the hydrogen evolution reaction (HER) at a thermodynamic overpotential of ~1.23 V.[2,5–7] Appropriate catalysts, mostly Pt and alloys, can dramatically improve the energy conversion efficiency by significantly dropping the overpotential.[8] However, the low natural abundance and high material cost of Pt restrict the practical implementation of electrochemical hydrogen production. Recently, earth-abundant and low-cost transition metal dichalcogenides, such as MoS2 and WS2, are emerging as a promising replacement of Pt catalysts for HER.[5,9] It has been suggested that the electrocatalysis originates from the undercoordinated atoms at edge sites of the 2H sulfide compounds.[5,10] Enormous efforts have recently been dedicated to fabricate various nanostructured dichalcogenides to exposure more edge sites for amplifying HER catalysis.[11–29] On the other hand, this often results in the less stability and the inefficient electrical conductivity of the dispersed nanostructured 2D catalysts.[9,10,17,18] As an alternative approach, lattice strains have been suggested to be an effective way to enhance the catalytic activity of 2H sulfide compounds, although the underlying mechanisms have not been fully understood.[11] In this study, we developed a continuous monolayer MoS2 film grown on the curved internal surface of 3D nanoporous gold Dr. Y. W. Tan, Dr. P. Liu, Dr. L. Y. Chen, Dr. W. T. Cong, Dr. Y. Ito, J. H. Han, Dr. X. W. Guo, Dr. T. Fujita, Dr. A. Hirata, M. W. Chen WPI Advanced Institute for Materials Research Tohoku University Sendai 980-8577, Japan E-mail: [email protected] M. W. Chen State Key Laboratory of Metal Matrix Composites School of Materials Science and Engineering Shanghai Jiao Tong University Shanghai 200030, P.R. China Dr. W. T. Cong, Prof. Z. Tang Key Laboratory of Polar Materials and Devices East China Normal University Shanghai 200062, P.R. China M. W. Chen CREST, JST, 4–1–8 Honcho Kawaguchi Saitama 332-0012, Japan
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(NPG). Different from the conventional wisdom, this “edgefree” monolayer MoS2 with significant out-of-plane strains by lattice bending shows a high catalytic activity towards HER, which may pave a new way to fabricate 2D catalysts with a large effective surface area and high catalytic activity. Figure 1a illustrates the fabrication procedure of the 3D continuous MoS2 films by using chemically dealloyed NPG (Figure S1, Supporting Information)[30] as substrates for chemical vapor deposition (CVD) of monolayer MoS2.[31,32] Free-standing monolayer MoS2@NPG composites were used as catalytic electrodes for electrocatalytic hydrogen production, in which the continuous MoS2 film acts as active catalysts and the conductive NPG provides a short path for fast charge transfer and transport (Figure 1b). Figure 2a,b shows the scanning electron microscope (SEM) images of the monolayer MoS2@NPG composite, which has a pore size of ~100 nm. By optimizing CVD time and the distance between the MoO3 precursor and the NPG substrate, the layer number of MoS2 can be well controlled from monolayer to few layers as evidenced by Raman spectra statistically taken from a large area (Figure 2c). The frequency difference (Δ) of two characteristic Raman modes (E12g and A1g) has been widely used as the identity of the MoS2 layer number. The Δ values were measured to be ≈20, 22, and 23 cm−1 for the synthetic 1 layer (1L), 2L, and 3L MoS2@NPG films, which are fully consistent with previous reports in the literature.[33] To confirm the continuity and uniformity of the synthesized MoS2 films, we performed Raman characterization by stochastic measurements of the MoS2 characteristic modes from different regions spanning the entire centimeter-sized samples (Figure S3, Supporting Information). The well-consistent Raman spectra demonstrate that the CVD films with the defined MoS2 layers are homogeneous in the large-scale samples. Separate photoluminescence (PL) experiments further corroborate the monoand few layer features of the CVD MoS2 films. Figure 2d presents the PL spectra of 1L, 2L, and 3L MoS2@NPG samples along with bulk MoS2 as a reference. The monolayer and few layer MoS2 samples exhibit PL peaks corresponding to the direct band gap at ~660 nm, whereas the bulk MoS2 only shows a visible PL peak of the indirect band gap. Moreover, the PL intensity of the 1L MoS2@NPG is much stronger than those of the 2L and 3L MoS2@NPG samples, consistent with reported direct band gap properties of 2D crystalline MoS2.[34,35] The continuous monolayer MoS2 film on the internal surface of NPG is directly elucidated by high-resolution scanning transmission electron microscope (STEM). Figure 3a is
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two layers of packed sulfur atoms (Figure 3b). The structure of the S Mo S sandwich layer is in accordance with the simulated image (the inset of Figure 3b) based on the standard 2H structure of MoS2 (Figure 3c) as well as the theoretical thickness (~0.667 nm) of monolayer MoS2.[36] The consistence of the experimental observation with the simulation demonstrates that the monolayer MoS2 has a 2H structure and the (001)2H plane is approximately parallel to the topmost surface of the gold substrate on the flat region. Moreover, the ABF-STEM image (Figure 3d) reveals significant lattice distortions of the Figure 1. Monolayer MoS2@NPG towards catalytic HER. a) Schematic diagram of the fabrica- monolayer MoS2, particularly at the curved tion process of monolayer MoS2@NPG hybrid materials by a nanoporous metal-based CVD regions of the NPG substrate. The variation approach.[32] b) Schematic HER catalyzed by the monolayer MoS2@NPG hybrid material. of the S Mo S bonding angle α in the ABFSTEM image is plotted in Figure 3e. Except the film on flat regions of NPG, the monolayer MoS2 film expea high-resolution STEM image taken by a high-angle annular dark-field (HAADF) detector, which shows the side view image riences serious out-of-plane lattice strains on the curved NPG of the MoS2 films on NPG. It can be clearly seen that the substrate. The large lattice strains of the monolayer MoS2 film MoS2 film with a single unit cell thickness uniformly sits on were also confirmed by the top view HAADF-STEM image and selected area electron diffraction (Figure S4, Supporting the top of a curved gold ligament. Since annular bright-field Information). Both curved lattice and elongated diffraction (ABF) STEM is capable of imaging both the heavy element of pattern demonstrate that there are large lattice distortions in molybdenum and the light element of sulfur, it was employed the monolayer MoS2 film, which is consistent with the curved to image the distribution of Mo and S atoms in the monolayer crystal. The zoom-in ABF-STEM image from a flat region show morphology of the monolayer film as seen from the side view a row of molybdenum atomic columns sandwiched between images (Figure 3). Importantly, undercoordinated surface steps and lattice defects (dislocations and vacancies), which are suggested to be the catalytically active sites for HER, are rarely seen in the continuous monolayer film. The HER catalytic activities of the MoS2@ NPG hybrid materials were evaluated in a 0.5 M H2SO4 solution using a three-electrode electrochemical system. Figure 4a shows the cathodic polarization curves of 1L, 2L and 3L MoS2@NPG together with Pt and pure NPG as references. The kinetic currents in the plots are normalized by effective surface areas of the electrodes (Figure S5, Supporting Information). The monolayer MoS2@NPG film possesses the lowest onset overpotential (η) of 118 mV (Figure 4b), while the onset overpotentials gradually increase to 128 and 132 mV for the 2L and 3L samples, respectively. Although the density of the undercoordinated edge sites and lattice defects of the continuous monolayer MoS2 film is much less than those of nanostructured MoS2, the onset overpotential of HER Figure 2. Structure characterizations of MoS2@NPG. a) Top view SEM image of the monolayer is noticeably lower than the reported values MoS2@NPG material. The inset shows a zoom-in image. The MoS2 film is too thin to be seen of MoS2 nanoparticles,[10,37,38] mesopores,[17] by SEM. b) Side view SEM image showing the 3D nanoporous structure of the monolayer nanowires,[18] nanosheets,[14,15,19,39] and flat MoS2@NPG material. c) Raman spectra of bulk MoS2 and the synthesized monolayer (1L), MoS films by CVD (Table S2, Supporting 2 bilayer (2L), and trilayer (3L) MoS2@NPG materials. The characteristic Raman modes (E12g Information).[12,17,20] Cathodic current denand A1g) of MoS2 are labeled in the spectra. The difference between the out-of-plane (A1g) and in-plane (E12g) Raman peaks is used to characterize the layer number of MoS2. d) Photo- sity at a low applied potential is an important luminescence spectra of 1L, 2L, and 3L MoS2@NPG, and bulk MoS2 reference with 514.5 nm criterion for evaluating catalytic activities of electrocatalysts. The monolayer MoS2@NPG laser excitation. 2
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to 2L and 3L MoS2@NPG (Figure S7, Supporting Information), suggesting a much faster electron transfer and a high Faradaic efficiency during HER. This apparently benefits from the unique electronic properties of the curved monolayer MoS2 and the low-resistance Ohmic contact of the monolayer MoS2 with the highly conductive NPG substrate.[40,41] Moreover, the obvious layer dependence of the series resistance also indicates that the lower catalytic activity of 2L and 3L MoS2@NPG may be associated with the indirect contact of the topmost active MoS2 layer with the conductive NPG substrate, which underlines the importance of conductivity in the electrocatalysis of MoS2. Besides the high electrocatalytic activity, the lifetime is also an important criterion to evaluate an electrocatalyst. We investigated the cycling stability of MoS2@NPG in the acidic electrolyte by continuous cyclic voltammetry (CV) testing at a scan rate of 50 mV s−1 (Figure S8, Supporting Information). At either constant applied potential (250 mV) or constant cathodic current (0.25 mA cm−2), the MoS2@NPG electrodes show excellent cycling retention with less than 20% degraFigure 3. Atomic structure of monolayer MoS2 on gold ligaments of NPG. a) Cross-section dation after 1000 cycles. Moreover, at a conHAADF-STEM image of monolayer MoS2@NPG. The single layer contrast of MoS2 from the stant overpotential of 250 mV, the catalytic heavy Mo atoms. b) Higher resolution STEM images of monolayer MoS2@NPG from the flat hydrogen production can continuously take region of a gold ligament. A row of molybdenum atomic columns are sandwiched by two layers place for over 9 h with very slow current of sulfur atoms. The inset shows the simulated STEM image based on the standard structure decrease (Figure S8, Supporting Information). model of 2H MoS2 in (c). d) The annular bright-field STEM image of monolayer MoS2@NPG It has been theoretically predicted and taken from a curved region of NPG substrate. e) The variation of the S Mo S bonding angles (α) at the curved region shown in (d). The dash line indicates the S Mo S bonding angles experimentally demonstrated that the cata(120°) of perfect 2H MoS2. Scale bars: 1 nm in (a), (b), and (d). lytic activity of MoS2 catalysts originates from the chemically active undercoordinated atoms at edge sites.[5,10] For this reason, 2D MoS2 films by CVD usually do not have adequate HER activiexhibits a high current density of 0.52 mA cm−2 at 250 mV, which is ~2, 4, and 15 times higher than those of the 2L, 3L ties because the continuous films lack sufficient undercoordiMoS2@NPG, and bare NPG, respectively. Moreover, the signifinate edge atoms.[16] Different from the common wisdom, in cant hydrogen evolution (J = 10 mA cm–2) can be observed at a this study we found that the continuous monolayer MoS2 films low operating voltage of –226 mV (Table S2, Supporting Inforwith scarce edge sites present high catalytic activity toward mation). The high catalytic activity of the monolayer MoS2@ HER. Unlike conventional CVD using flat substrates, the continuous monolayer MoS2 films were grown on the internal surNPG is also validated by Tafel plots, which correspond to the cathodic current change with an increase in applied potentials face of dealloyed NPG that is constructed by highly curved gold and reflect the rate limitation of electrocatalyzed reactions. ligaments containing both negative and positive curvatures and The Tafel slope (46 mV/decade) of the monolayer MoS2@NPG high curvature gradients.[42,43] The MoS2 films naturally inherit is noticeably lower than those of the 2L (52 mV/decade), 3L the 3D curvature of NPG by continuous lattice bending (Figure 3; (55 mV/decade) MoS2@NPG, and pure NPG (95 mV/decade). Figure S4, Supporting Information). Considering the continuous films are deficient of catalytically active edge sites, the In comparison with the best MoS2-based catalysts reported in high catalytic activity of the “edge-free” films appears to come the literature (Table S2, Supporting Information),[10,14–18,37–39] from the bent MoS2 lattices with large out-of-plane strains. It the “edge-free” monolayer MoS2@NPG shows comparable catalytic activity in the onset potential and Tafel slope. has been noticed that in-plane tensile strains can dramatically decrease the free energy of atomic hydrogen absorption on disThe high electrode reaction kinetics of the monolayer torted 1T WS2 nanosheets, whereas the effect of the in-plane MoS2@NPG is more evident in electrochemical impedance spectra (EIS). Figure 4d is the Nyquist plots of the MoS2@NPG strains is minor for semiconductor 2H phase.[11] For MoS2, electrodes at the overpotential of 250 mV. The EIS experiment possible electronic structure changes caused by lattice strains reveals the low charge transfer resistance (Rct) and internal have also been discussed before[44,45] but the strain-induced resistance of the monolayer MoS2@NPG electrode, compared catalysis of 2D MoS2 has not been systematically investigated.
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Figure 4. Electrocatalytic performances of platinum, pure NPG, and MoS2@NPG. a) Polarization curves; b) derivative polarization curves showing the onset potentials; c) corresponding Tafel slopes; and d) Nyquist plots showing the electrode kinetics of MoS2@NPG.
Our density functional theory (DFT) calculations indicate that, obviously different from in-plane strains, out-of-plane lattice bending can evidently change the band gap of monolayer MoS2 (Figures S9 and S10, Supporting Information). The distinct out-of-plane strains result in continuous changes of S Mo S bonding angles (α), which can be expressed as relative S and/ or Mo atomic migration in the theoretical model, as depicted in Figure S9 a, b, and c (Supporting Information). When the S Mo S bonding angle α is larger than 140°, the band gap of the monolayer 2H MoS2 is close to zero (Figure S9d, Supporting Information), indicating that there is a local semiconductorto-metal transition caused by the out-of-plane lattice bending. Since the catalytic activity of MoS2 edges is mainly from the edge S atoms with dangling bonds, we also analyzed the density of states of the S atoms as the function of the bending angles (Figure S10, Supporting Information). As shown in Figure S9e (Supporting Information), the lattice bending significantly contributes to the charge density of S atoms, which is in striking contrast to that of the pristine 2H MoS2 (α = 120°). The enriched charge density of S atoms at the bent regions is very analogous to that of edge S atoms with dangling bonds. We
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also evaluated the free energy of atomic hydrogen adsorption at the bent lattices of MoS2 (Figure S9f, Supporting Information). Our calculations show that the free energy is associated with the S Mo S bonding angle α. The lattice bending gives rise to smaller differential binding free energy than that of the pristine MoS2. The lowest energy of ≈0.2 eV appears at α = 155°. The local semiconductor-to-metal transition,[12,46] 12 the lower adsorption energy, and enriched charge density at the curved regions of the “edge-free” MoS2 films fairly explain the superior catalysis of the curved monolayer MoS2 without abundant edge sites. Consequently, our finding certainly provides new insights into the strain effect on catalytic properties of 2D catalysts and may pave a new way to tailor the catalytic activity of 2D catalysts by lattice stain engineering. Besides the out-of-plane strains, the dependence of HER activity on the MoS2 layer number implies the importance of the conductive NPG substrate because the high interlayer potential barrier may give rise to the lower interlayer electron hopping efficiency.[16] The direct contact between the monolayer MoS2 and highly conductive gold leads to the low-resistance Ohmic contact for fast charge transfer and high HER efficiency.[28,41]
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Experimental Section Materials Fabrication: NPG membranes used in this study were prepared by chemically dealloying.[30] Approximately1-µm thick Au35Ag65 alloy sheets were etched by 69 vol.% HNO3 at room temperature for 6 h. The silver component was selectively leached away from the alloy and the remained gold forms a 3D bicontinuous open porous structure by self-assembly. After dealloying, the residual acid within the nanopore channels was removed by water rinsing. The NPG samples have a uniform porous structure with an average pore size of about 20–30 nm. The monolayer MoS2 films were grown on the internal surface of NPG by CVD. MoO3 and pure sulfur were used as reactant materials and argon was used as the carrier gas. The NPG membranes were placed on the insulating SiO2/Si substrates, and the film growth was performed at the reaction temperature of 650 °C, whereas the pore and ligament size of NPG substrates increases from 20–30 to ≈100 nm. Materials Characterization: Raman and PL spectra were recorded using a Renishaw Raman microscope (Renishaw InVia RM 1000) with an incident laser wavelength of 514.5 nm. The microstructure of the MoS2@ NPG hybrid materials was investigated using a field-emission SEM (JEOL JIB-4600F) and a field-emission transmission electron microscope (JEOL JEM-2100F, 200 keV) equipped with double spherical aberration (Cs) correctors for both the probe- and image-forming objective lenses. The Cs correctors were optimized for image observations and the pointto-point resolution of STEM is better than 1.0 Å. The HAADF detector was set to collect electrons scattered between 100 and 267 mrad. The STEM image simulations were performed using the Win HREM software (HREM Research Inc). In the calculations, the probe convergence angle is 25 mrad and the HAADF detector inner and outer angles are set as 100 and 267 mrad, respectively. Electrochemical Measurements: The electrocatalytic experiments were performed using an electrochemical workstation (Ivium Technology) in a standard three-electrode mode. The electrolyte is a 0.5 M H2SO4 water solution. A saturated calomel and platinum wire were used as the reference electrode and counter electrode, respectively. The potential values reported in this study are versus the reversible hydrogen electrode. The HER activities of the samples were evaluated by linear sweep voltammetry at 5 mV s−1. The electrolyte was purged with high pure N2 for 30 min prior to measurements. CV was conducted at 50 mV s−1 to investigate the cycling stability. The EIS were collected from 105 to 0.1 HZ with an amplitude of 10 mV under various overpotentials. The
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Because the catalytically active sites are the strained lattices, the degeneration of the MoS2@NPG catalysts possibly results from strain relaxation during HER, rather than the loss of active sites of dispersed catalysts by agglomeration. Since the lattice strains are introduced by the 3D nanoporous morphology of stable NPG, this explains the excellent catalytic retention of the MoS2@NPG catalysts. In conclusion, we have successfully grown a large-scale 3D monolayer MoS2 on the internal surface of dealloyed NPG by a CVD method. The novel monolayer MoS2@NPG electrode exhibits high catalytic activities with a smaller Tafel slope of 46 mV/decade and a lower onset potential of −118 mV. Moreover, as an important finding, the catalytic activity of 2D MoS2 can be activated by introducing out-of-plane strains, providing an effective way to developing highly active 2D catalysts by lattice strain engineering. Although the NPG substrate used in this study is still too expensive for large-scale practical applications, the idea and concept reported in this work can be realized by using cheap nanoporous metals, such as nanoporous Cu and Ni, for economic MoS2-based catalysts toward highefficiency electrochemical hydrogen production.
impedance data were fitted to a simplified Randles circuit to extract the series and charge-transfer resistances. The effective electrochemical surface area of annealed NPG and MoS2@NPG hybrid materials were evaluated by the CV method. DFT Calculations: The DFT calculations were performed by using the Vienna ab initio simulation package based on the spin-polarized DFT within the generalized gradient approximation (GGA). The electron– ion interactions are presented by the Troullier–Martins-type normconserving pseudopotentials with a partial core correction and the GGA exchange correlation potential in the form of Perdew–Burke–Ernzerhof functional is adopted. An energy cutoff of 400 eV was used for plane wave basis expansion.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was sponsored by the JST-CREST “Phase Interface Science for Highly Efficient Energy Utilization,” JST, Japan and “World Premier International (WPI) Research Center Initiative for Atoms, Molecules and Materials,” MEXT, Japan. Z. T. was supported by National Science Foundation of China. Received: August 20, 2014 Revised: September 26, 2014 Published online:
[1] J. A. Turner, Science 2004, 305, 972. [2] M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 2010, 110, 6446. [3] Y. Zheng, Y. Jiao, Y. Zhu, L. H. Li, Y. Han, Y. Chen, A. Du, M. Jaroniec, S. Z. Qiao, Nat. Commun. 2014, 5, 3783. [4] Y. Zheng, Y. Jiao, L. H. Li, T. Xing, Y. Chen, M. Jaroniec, S. Z. Qiao, ACS Nano 2014, 8, 5290. [5] B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, J. H. Nielsen, S. Horch, I. Chorkendorff, J. K. Nørskov, J. Am. Chem. Soc. 2005, 127, 5308. [6] J. K. Norskov, T. Bligaard, J. Rossmeisl, C. H. Christensen, Nat. Chem. 2009, 1, 37. [7] Y. Zheng, Y. Jiao, M. Jaroniec, S. Z. Qiao, Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201407031R1. [8] J. Greeley, T. F. Jaramillo, J. Bonde, I. B. Chorkendorff, J. K. Norskov, Nat. Mater. 2006, 5, 909. [9] A. B. Laursen, S. Kegnaes, S. Dahl, I. Chorkendorff, Energy Environ. Sci. 2012, 5, 5577. [10] T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, Science 2007, 317, 100. [11] D. Voiry, H. Yamaguchi, J. Li, R. Silva, D. C. B. Alves, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda, M. Chhowalla, Nat. Mater. 2013, 12, 850. [12] M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. Li, S. Jin, J. Am. Chem. Soc. 2013, 135, 10274. [13] D. Voiry, M. Salehi, R. Silva, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda, M. Chhowalla, Nano Lett. 2013, 13, 6222. [14] J. Xie, J. Zhang, S. Li, F. Grote, X. Zhang, H. Zhang, R. Wang, Y. Lei, B. Pan, Y. Xie, J. Am. Chem. Soc. 2013, 135, 17881. [15] J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X. W. Lou, Y. Xie, Adv. Mater. 2013, 25, 5807.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5
www.advmat.de
COMMUNICATION
www.MaterialsViews.com [16] Y. Yu, S.-Y. Huang, Y. Li, S. N. Steinmann, W. Yang, L. Cao, Nano Lett. 2014, 14, 553. [17] J. Kibsgaard, Z. Chen, B. N. Reinecke, T. F. Jaramillo, Nat. Mater. 2012, 11, 963. [18] Z. Chen, D. Cummins, B. N. Reinecke, E. Clark, M. K. Sunkara, T. F. Jaramillo, Nano Lett. 2011, 11, 4168. [19] Z. Lu, W. Zhu, X. Yu, H. Zhang, Y. Li, X. Sun, X. Wang, H. Wang, J. Wang, J. Luo, X. Lei, L. Jiang, Adv. Mater. 2014, 26, 2683. [20] H. Wang, D. Kong, P. Johanes, J. J. Cha, G. Zheng, K. Yan, N. Liu, Y. Cui, Nano Lett. 2013, 13, 3426. [21] D. Merki, S. Fierro, H. Vrubel, X. Hu, Chem. Sci. 2011, 2, 1262. [22] Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong, H. Dai, J. Am. Chem. Soc. 2011, 133, 7296. [23] H. I. Karunadasa, E. Montalvo, Y. Sun, M. Majda, J. R. Long, C. J. Chang, Science 2012, 335, 698. [24] X. Ge, L. Chen, L. Zhang, Y. Wen, A. Hirata, M. Chen, Adv. Mater. 2014, 26, 3100. [25] Y.-H. Chang, C.-T. Lin, T.-Y. Chen, C.-L. Hsu, Y.-H. Lee, W. Zhang, K.-H. Wei, L.-J. Li, Adv. Mater. 2013, 25, 756. [26] Z. Wu, B. Fang, Z. Wang, C. Wang, Z. Liu, F. Liu, W. Wang, A. Alfantazi, D. Wang, D. P. Wilkinson, ACS Catal. 2013, 3, 2101. [27] L. Liao, J. Zhu, X. J. Bian, L. N. Zhu, M. D. Scanlon, H. H. Girault, B. H. Liu, Adv. Funct. Mater. 2013, 23, 5326. [28] D. J. Li, U. N. Maiti, J. Lim, D. S. Choi, W. J. Lee, Y. Oh, G. Y. Lee, S. O. Kim, Nano Lett. 2014, 14, 1228. [29] J. Kibsgaard, T. F. Jaramillo, F. Besenbacher, Nat. Chem. 2014, 6, 248. [30] J. Erlebacher, M. J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki, Nature 2001, 410, 450. [31] S. Najmaei, Z. Liu, W. Zhou, X. Zou, G. Shi, S. Lei, B. I. Yakobson, J.-C. Idrobo, P. M. Ajayan, J. Lou, Nat. Mater. 2013, 12, 754. [32] a) Y. Ito, H.-J. Qiu, T. Fujita, Y. Tanabe, K. Tanigaki, M. W. Chen, Adv. Mater. 2014, 26, 4145; b) Y. Ito, Y. Tanabe, H.-J. Qiu, K. Sugawara,
6
wileyonlinelibrary.com
[33] [34] [35] [36] [37] [38] [39] [40] [41]
[42]
[43] [44]
[45] [46]
S. Heguri, N. H. Tu, K. K. Huynh, T. Fujita, T. Takahashi, K. Tanigaki, M. W. Chen, Angew. Chem. Int. Ed. 2014, 53, 4822. C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone, S. Ryu, ACS Nano 2010, 4, 2695. G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen, M. Chhowalla, Nano Lett. 2011, 11, 5111. A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, F. Wang, Nano Lett. 2010, 10, 1271. G. Eda, T. Fujita, H. Yamaguchi, D. Voiry, M. Chen, M. Chhowalla, ACS Nano 2012, 6, 7311. J. Bonde, P. G. Moses, T. F. Jaramillo, J. K. Norskov, I. Chorkendorff, Faraday Discuss. 2009, 140, 219. H. Vrubel, D. Merki, X. Hu, Energy Environ. Sci. 2012, 5, 6136. J. Kim, S. Byun, A. J. Smith, J. Yu, J. Huang, J. Phys. Chem. Lett. 2013, 4, 1227. T. Fujita, H. Okada, K. Koyama, K. Watanabe, S. Maekawa, M. W. Chen, Phys. Rev. Lett. 2008, 101, 166601. C. Gong, C. Huang, J. Miller, L. Cheng, Y. Hao, D. Cobden, J. Kim, R. S. Ruoff, R. M. Wallace, K. Cho, X. Xu, Y. J. Chabal, ACS Nano 2013, 7, 11350. T. Fujita, P. Guan, K. McKenna, X. Lang, A. Hirata, L. Zhang, T. Tokunaga, S. Arai, Y. Yamamoto, N. Tanaka, Y. Ishikawa, N. Asao, Y. Yamamoto, J. Erlebacher, M. W. Chen, Nat. Mater. 2012, 11, 775. T. Fujita, L. H. Qian, K. Inoke, J. Erlebacher, M. W. Chen, Appl. Phyl. Lett. 2008, 92, 251902. A. Castellanos-Gomez, R. Roldán, E. Cappelluti, M. Buscema, F. Guinea, H. S. J. van derZant, G. A. Steele, Nano Lett. 2013, 13, 5361. E. Scalise, M. Houssa, G. Pourtois, V. Afanas’ev, A. Stesmans, Nano Res. 2012, 5, 43. T. Fujita, Y. Ito, Y. W. Tan, H. Yamaguchi, D. Hojo, A. Hirata, D. Voiry, M. Chhowalla, M. W. Chen, Nanoscale 2014, 6, 12458.
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