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Nanoporous Metal Enhanced Catalytic Activities of Amorphous Molybdenum Sulfide for High-Efficiency Hydrogen Production Xingbo Ge, Luyang Chen, Ling Zhang, Yuren Wen, Akihiko Hirata, and Mingwei Chen* Converting renewable energy into a chemical form is one of the most efficient ways for energy storage. As a high-energydensity clean chemical fuel, hydrogen can be conveniently utilized in proton exchange membrane fuel cells for high-energy applications. Accordingly, hydrogen produced by water splitting has recently been emerging as a key technology component of hydrogen economy.[1,2] The central step of water splitting is the hydrogen evolution reaction (HER). In the absence of catalysts, a large overpotential, corresponding to the thermodynamic potential of water catalysis, is needed to split water into hydrogen. Platinum and its alloys are the most active catalysts for HER and can dramatically enhance the reaction rate at a very low overpotential for high efficiency energy conversion.[3,4] However, the high material costs and poor natural abundance of platinum restrict the practical applications of the electrochemical water splitting technique.[5] Since Hinnemann and co-workers first reported that hexagonal MoS2 nanoparticles supported on graphite are active for HER in acidic media and the catalytic activity of MoS2 is predicted to be comparable with that of Pt,[6] developing highly active molybdenum sulfide composites as non-Pt HER catalysts has been the recent topic of intense studies.[7–9] Since the electrocatalytic activity of MoS2 has been identified to arise from the edge sites of the 2H sulfide compound,[6,10] many approaches have recently been developed to construct molybdenum sulfide with an optimized structure and morphology to expose the edge sites that are electrochemically active for HER.[11–17] In addition to the edge effect from the crystalline MoS2, amorphous molybdenum sulfide has also been found to have an excellent HER activity in a wide pH range.[18–20] The high catalytic activity is suggested to result from the intrinsic surface defect sites of the amorphous molybdenum sulfide, i.e. coordinately and structurally unsaturated sulfur atoms on the Prof. M. W. Chen WPI Advanced Institute for Materials Research Tohoku University Sendai 980–8577, Japan, CREST, JST 4–1–8 Honcho Kawaguchi, Saitama 332–0012, Japan State Key Laboratory of Metal Matrix Composites School of Materials Science and Engineering Shanghai Jiao Tong University Shanghai 200030, P.R. China E-mail: [email protected] Dr. X. Ge,[+] Dr. L. Chen,[+] Dr. L. Zhang, Dr. Y. Wen, Dr. A. Hirata WPI Advanced Institute for Materials Research Tohoku University, Sendai 980–8577, Japan [+]These
authors contributed equally to this work.
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amorphous surface.[18] Thus, not limited to the edge sites of crystalline MoS2, the entire surface of amorphous molybdenum sulfide is active to catalyze HER for a higher reaction rate. However, the molybdenum sulfide has a low electric conductivity which suppresses charge transport and hence efficiency of electrocatalysis.[7] Therefore, a highly conductive support with a large effective surface area is technically desirable to improve the electrocatalytic activity of the amorphous molybdenum sulfide. Dealloyed nanoporous gold (NPG) possesses high electric conductivity, a large surface area and high chemical stability.[21–23] These unique properties make NPG an ideal support material to construct functional nanostructure in electrochemical energy conversion and storage.[24–27] In the present work, we utilize NPG as the highly conductive large-surface-area support of amorphous molybdenum sulfide. It was found that the NPG substrate can dramatically improve electrocatalytic activity of the amorphous molybdenum sulfide in both acidic and neutral media for high efficiency hydrogen production. Dealloyed Au/Ag alloy membranes have a three-dimension nanoporous structure, consisting of gold ligaments and nanopore channels.[28] The internal surface of NPG is available for chemical modification and catalysis applications because of the open porosity. The precursor (MoS42−) and reagent were first used to wet the internal surface of the as-prepared NPG and then exposed to the hydrazine that acts as the reductant for the plating reaction. The slow wet chemical deposition allows us to uniformly plate a thin layer of molybdenum sulfide on the internal surface of nanoporous metals. Figure 1a shows a bright-field transmission electron microscope (TEM) image of the as-prepared molybdenum sulfide@NPG composite. The NPG substrate has the pore sizes ranges from 20–30 nm. A layer of molybdenum sulfide with a bright contrast and uniform thickness can be observed on the internal surface of nanopores. The molybdenum sulfide layer is ∼4 nm thick and tightly bonds to the ligament surface of NPG by forming a core-shell nanoporous structure. The high-resolution TEM (HRTEM) micrograph (Figure 1b) reveals that any crystal lattice cannot be found in the molybdenum sulfide layer, demonstrating that the as-plated molybdenum sulfide has an amorphous structure. The fast Fourier transform (FFT) pattern (inset of Figure 1b) taken from the plated layer further confirms the amorphous characteristic of the molybdenum sulfide. However, in the high-angle annular dark field (HAADF) images (Figure S1) in which the contrast is basically proportional to the square of atomic numbers, the Mo rich clusters with a size of 1–2 nm can be frequently observed in the amorphous phase. XPS was employed to investigate the chemical state and composition of the amorphous molybdenum sulfide. The
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survey spectrum (Figure 2a) shows the predominant signals of Au, Cu, Mo, C and S. The Cu and C are from the specimen holder and environment contaminations, respectively. The welldefined peaks at 84 and 87.7 eV (Figure 2b) correspond to the 4f7/2 and 4f5/2 of metallic Au of the nanoporous support.[27] The Mo 3d spectrum (Figure 2c) exhibits two peaks correspond to the 3d5/2 (229.2 eV) and 3d3/2 (232.4 eV) of the Mo4+ ions, a common state of Mo in MoS2 and MoS3.[20] The S 2p spectrum shows a broad and complex peak (Figure 2d), which consists of
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Figure 1. Microstructure of the molybdenum sulfide@NPG composite. (a) Bright field TEM image showing a core-shell nanoporous structure; and (b) HRTEM micrograph of the atomic structures of the gold ligament and molybdenum sulfide layer. The Inset of Figure 1(b) is the FFT pattern taken from the labeled area.
two doublets, i.e. S22− 2p3/2 (163.2 eV), S2− 2p3/2 (161.5 eV) and S22− 2p1/2 (164.5 eV), S2− 2p1/2 (162.4 eV). The XPS spectra suggest the existence of two kinds of S ligands, in accordance with the reported amorphous MoS3.[20,29] The doublets at the higher binding energy have been attributed to the bridging S22− and/ or apical S2− ligands as well as the terminal S2− ligands.[29] The intensity ratio of the two doublets is about 5:5.1, slightly lower than typical value (5:4) of amorphous MoS3. Thus, the doublet at the lower binding energy is supposed to contain the basal plane S2− ligands, which have a binding energy very close to that of terminal S2− ligands (161.8 eV). Because only MoS2 has the basal plane S2− ligands,[29] the appearance of these ligants indicates the existence of local MoS2 ordered structure in the amorphous molybdenum sulfide, which is consistent with the HAADF-STEM images (Figure S1). The formation of the MoS2 clusters could be associated with the usage of the reducing agent (N2H4) in the plating process. Therefore, the chemically plated amorphous molybdenum sulfide is the mixture of MoS3 and MoS2. Although it is impossible to accurately quantify the ratio of the two components only from the S 2p peaks, we can still estimate the composition of the amorphous molybdenum sulfide by XPS quantification. The measured S/Mo ratio is ∼2.7, which further confirms the amorphous molybdenum sulfide indeed contains a small fraction of MoS2 as imaged in the real-space HAADF micrographs (Figure S1). On the basis of the XPS study, we assign the fabricated electrode as amorphous MoS2.7@NPG. Figure 3a shows the HER polarization curves of bare NPG, MoS2.7@NPG and a reference sample of amorphous MoS2.7 deposited on a glass carbon electrode (GCE) (MoS2.7@GCE) in a 0.5 M H2SO4 aqueous solution at room temperature. All three electrodes exhibit obvious catalytic activities towards HER. Although gold is known as a relatively active catalyst for HER,[30] the amorphous MoS2.7 composites (MoS2.7@NPG and MoS2.7@ GCE) have higher catalytic activities than the bare NPG, indicating the intrinsically higher catalytic activity of the amorphous MoS2.7 than that of gold. Moreover, the amorphous MoS2.7@ NPG shows a much higher activity than the MoS2.7@GCE, demonstrating the additional catalytic enhancement from the NPG substrate. The onset HER potentials of NPG, MoS2.7@GCE and MoS2.7@NPG, above which the current increases quickly, were determined to be −145, −143, −125 mV vs. RHE, respectively (Figure 3b). The lower overpotential (∼18 mV) of the amorphous MoS2.7@NPG composite suggests an obvious improvement in the intrinsic catalysis of the amorphous molybdenum sulfide towards HER, compared to pure molybdenum sulfide. Such low overpotential is only observed from molybdenum sulfide grown on highly conductive graphene and the low overpotential has been attributed to the chemical and electronic coupling between the active molybdenum sulfide and graphene substrate.[17,31] For our case, the ultralow overpotential can still be achieved with the absence of graphene. This probably arises from the excellent charge transfer between the amorphous MoS2.7 and the NPG support. The HER current densities recorded at −0.2 V vs. RHE are −0.2, −0.9, −5.7 mA/cm−2 for pure NPG, MoS2.7@GCE, MoS2.7@NPG electrodes, respectively. More than 6 times improvement in the current density is realized on MoS2.7@NPG compared to the MoS2.7@GCE. The dramatically improved catalytic performance mainly origins from the high
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Figure 2. XPS spectra of amorphous molybdenum sulfide@NPG. (a) The survey spectrum of the entire sample; (b) Au 4f; (c) Mo 3d; and (d) S 2p core levels spectra.
conductivity and large surface area provided by the NPG substrate. The high HER kinetics of MoS2.7@NPG is also demonstrated by the lower Tafel slope of 41 mV/decade at low overpotentials, which is much lower than that of NPG (87 mV/decade) and MoS2.7@GCE (58 mV/decade) electrodes (Figure 3c). The Tafel slope of MoS2.7@NPG is also much lower than those of nanocrystalline MoS2 (50–60 mV/decade),[10,14] and equal to or better than those of graphene supported MoS2 nanoparticles and the carbon nanotubes blended amorphous MoS3 composites.[17,20] The observed Tafel slope of ∼41 mV/decade suggests that the electrochemical desorption of hydrogen is the rate-limiting step in the HER on the MoS2.7@NPG electrode.[3] Considering that the total MoS2.7@NPG loading is only ∼100 µg/cm2 with ∼6 µg/cm2 MoS2.7, the designed catalyst show a promising route for fabricating highly active HER catalysts with ultra-low catalyst loading. The electrocatalytic hydrogen production from neutral water is of significance in practical applications. Several catalysts, including amorphous molybdenum sulfide films,[19] cobalt/phosphate composites,[32] are active for HER in neutral solutions. Figure 4a shows the polarization curves of NPG, MoS2.7@GCE, and MoS2.7@NPG samples in PBS at pH = 7. The activity trend of the catalysts is similar to that in the acid solution. Compared to NPG and MoS2.7@GCE, MoS2.7@ NPG displays a superior catalytic activity with the overpotentials of 120 mV vs. RHE. At −0.2 V, the current density of the MoS2.7@NPG sample is 0.48 mA/cm2, which is ∼2 and 4 times higher than that of MoS2.7/GCE (0.27 mA/cm2) and pure NPG (0.12 mA/cm2), respectively. The ultralow H3O+ concentration 3102
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in PBS (pH = 7) makes H2O as the main source for H2 production as in alkaline solutions, thus results in the very lower current densities compared to that in the acid solution. Tafel analysis suggests that the pure NPG has a Tafel slope of ∼132 mV/decade at low overpotentials (Figure 4b), which is similar to those of metals in alkaline solutions.[33] MoS2.7/GCE and MoS2.7@NPG show much lower Tafel slopes of 72 and 60 mV/ decade, respectively (Figure 4b), indicating a rate-determining electrochemical desorption of hydrogen with a large coverage of adsorbed hydrogen. The observed catalytic activity and efficiency of MoS2.7@NPG in neutral water are also much higher than that of the reported amorphous MoS3 film (∼0.2 mA/cm2 at −0.2 V)[19] and cobalt/phosphate composite (∼0.08 mA/cm2 at −0.2 V, 140 mV/decade).[32] In summary, we have successfully developed a new type of nanoporous electrocatalyst by chemically plating a thin layer of amorphous molybdenum sulfide on the internal surface of dealloyed nanoporous gold to form a core-shell molybdenum sulfide@NPG composite. The novel MoS2.7@NPG electrode exhibits 6-fold higher catalytic activity compared to conventional molybdenum sulfide catalyst in a 0.5 M H2SO4 solution with a Tafel slope of 41 mV/decade and MoS2.7 loading of ∼6 µg/cm2. MoS2.7@NPG also shows much higher HER activity in neutral media than conventional molybdenum sulfide and the Cobased material that is known the best catalysts for HER beside Pt catalysts. The improved catalytic performance of the amorphous molybdenum sulfide mainly origins from the interplay between the thin amorphous sulfide coating and the highly conductive and large-surface nanoporous gold. Possible charge
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can be realized by utilizing economic nanoporous transition metals, such as Cu and Ni, as demonstrated by our preliminary results shown in Figure S2.
Experimental Section
Figure 3. (a) HER polarization curves, (b) the onset HER potentials and (c) Tafel curves of NPG, MoS2.7@GCE, and MoS2.7@NPG in 0.5 M H2SO4.
transfer between the insulator sulfide and free-electron gold may be the underlying mechanism of the high catalytic activity of MoS2.7@NPG. The excellent catalytic performance is also associated with the large surface area of the nanoporous electrode for effectively utilization of the active materials. Although NPG is expensive, the idea and concept developed in this study
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Fabrication of Amorphous Molybdenum Sulfide@NPG: NPG membranes with a thickness of 100 nm and a pore size of 20–30 nm were fabricated by chemically dealloying Au35Ag65 films in concentrated HNO3 (70%) for 30 min at room temperature. The residual acid and impurity ions in NPG pore channels were removed by repeated water rinsing. A thin layer of amorphous molybdenum sulfide was plated onto the internal surface of NPG by chemical deposition. In a typical chemical plating procedure, the as-prepared NPG membrane was first loaded on a glass slide for drying at room temperature for one hour. After fully wet by a 5 mM (NH4)2MoS4 solution (solvent: Dimethylformamide:Water = 1:3), the NPG membrane was carefully transferred into a diluted hydrazine solution (2%) to finalize the amorphous molybdenum sulfide plating on the internal surface of NPG. For comparison, the amorphous molybdenum sulfide was also deposited on a glass carbon electrode (GCE) using the same method. Before electrochemical tests, all the as-prepared molybdenum sulfide samples were rinsed with distilled water. TEM and XPS Characterization: The microstructure of molybdenum sulfide@NPG composites was investigated using a field-emission
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www.MaterialsViews.com transmission electron microscope (TEM) (JEOL JEM-2100F, 200 keV) equipped with double spherical aberration (Cs) correctors for both the probe-forming and image-forming objective lenses. The chemical state and composition of the as-prepared molybdenum sulfide@NPG composite loaded on Cu sheet were analyzed using X-ray photoelectron spectroscopy (XPS, AxIS-ULTRA-DLD) with Al Kα (mono) anode at energy of 150 W. Electrochemical Measurements: The electrocatalytic activities of the as-prepared electrodes toward HER were evaluated on the electrochemical workstation (Ivium Technology) in a standard threeelectrode mode in a 0.5 M H2SO4 and 0.2 M phosphate buffer solution (PBS, pH = 7). SCE and a Pt wire were selected as the reference electrode and counter electrode, respectively. The potential values reported in this study are versus the reversible hydrogen electrode (RHE). The electrochemical experiments were carried out at room temperature (∼20 °C). The HER activities were evaluated by linear sweep voltammetry (LSV) at 5 mV/s. The electrolytes were purged with high pure N2 for 30 min prior to measurements.
Acknowledgements This work is sponsored by JST-CREST “Phase Interface Science for Highly Efficient Energy Utilization”, JST; “World Premier International (WPI) Research Center Initiative for Atoms, Molecules and Materials”, MEXT, Japan; and the State Key Laboratory for Advanced Metals and Materials (2013-ZD03). Received: November 16, 2013 Published online: February 19, 2014 [1] A. J. Bard, M. A. Fox, Acc. Chem. Res. 1995, 28, 141. [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] B. E. Conway, B. V. Tilak, Electrochim. Acta 2002, 47, 3571. [4] J. Greeley, T. F. Jaramillo, J. Bonde, I. Chorkendorff, J. K. Norskov, Nat . Mater. 2006, 5, 909. [5] A. Le Goff, V. Artero, B. Jousselme, P. D. Tran, N. Guillet, R. Metaye, A. Fihri, S. Palacin, M. Fontecave, Science 2009, 326, 1384. [6] B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jorgensen, J. H. Nielsen, S. Horch, I. Chorkendorff, J. K. Norskov, J. Am. Chem. Soc. 2005, 127, 5308. [7] Y. H. Lee, X. Q. Zhang, W. Zhang, M. T. Chang, C. T. Lin, K. D. Chang, Y. C. Yu, J. T. W. Wang, C. S. Chang, L. J. Li, T. W. Lin, Adv. Mater. 2012, 24, 2320. [8] L. B. Laursen, S. Kegnæs, S. Dahla, I. Chorkendorff, Energy Environ. Sci. 2012, 5, 5577.
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[9] 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, Na. Mater. 2013, 12, 850. [10] T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, Science 2007, 317, 100. [11] V. W. Lau, A. F. Masters, A. M. Bond, T. Maschmeyer, ChemCatChem 2011, 3, 1739. [12] T. F. Jaramillo, J. Bonde, J. Zhang, B. L. Ooi, K. Andersson, J. Ulstrup, I. Chorkendorff, J. Phys. Chem. C 2008, 112, 17492. [13] J. Bonde, P. G. Moses, T. F. Jaramillo, J. K. Norskov, I. Chorkendorff, Faraday Discuss. 2008, 140, 219. [14] Z. Kibsgaard, B. Chen, N. Reinecke, T. F. Jaramillo, Nat. Mater. 2012, 11, 963. [15] Z. Chen, D. Cummins, B. N. Reinecke, E. Clark, M. K. Sunkara, T. F. Jaramillo, Nano Lett. 2011, 11, 4168. [16] X. Zong, H. Yan, G. Wu, G. Ma, F. Wen, L. Wang, Can Li, J. Am. Chem. Soc. 2008, 130, 7176. [17] Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong, H. Dai, J. Am. Chem. Soc. 2011, 133, 7296. [18] D. Merki, X. Hu, Energy Environ. Sci. 2011, 4, 3878. [19] D. Merki, S. Fierro, H. Vrubel, X. Hu, Chem. Sci. 2011, 2, 1262. [20] H. Vrubel, D. Merki, X. Hu, Energy Environ. Sci. 2012, 5, 6136. [21] Y. Ding, Y. J. Kim, J. Erlebacher, Adv. Mater. 2004, 16, 1897. [22] T. Fujita, H. Okada, K. Koyama, K. Watanabe, S. Maekawa, M. W. Chen, Phys. Rev. Lett. 2008, 101, 166601. [23] T. Fujita, P. F. Guan, K. McKenna, X. Y. 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. [24] Y. Ding, M. W. Chen, J. Erlebacher, J. Am. Chem. Soc. 2004, 126, 6876. [25] X. Lang, A. Hirata, T. Fujita, M. Chen, Nat. Nanotech. 2011, 6, 232. [26] J. L. Kang, A. Hirata, H.-J. Qiu, L. Y. Chen, X. B. Ge, T. Fujita, M. W. Chen, Adv. Mater. 2014, 26, 269. [27] X. Ge, X. Yan, R. Wang, F. Tian, Y. Ding, J. Phys. Chem. C 2009, 113, 7379. [28] T. Fujita, L. H. Qian, K. Inoke, J. Erlebacher, M. W. Chen, Appl. Phys. Lett. 2008, 92, 251902. [29] T. Weber, J. C. Masers, J. W. Niemantsverdriet, J. Phys. Chem. 1995, 99, 9194. [30] L. A. Kibler, ChemPhysChem 2006, 7, 985. [31] 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. [32] S. Cobo, J. Heidkamp, P. A. Jacques, J. Fize, V. Fourmond, L. Guetaz, B. Jousselme, V. Ivanova, H. Dau, S. Palacin, M. Fontecave, V. Artero, Nat. Mater. 2012, 11, 802. [33] R. Subbaraman, D. Tripkovic, D. Strmcnik, K. C. Chang, M. Uchimura, A. P. Paulikas, V. Stamenkovic, N. M. Markovic, Science 2011, 334, 1256.
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Nanoporous Metal Enhanced Catalytic Activities of Amorphous Molybdenum Sulfide for High-Efficiency Hydrogen Production Xingbo Ge, Luyang Chen, Ling Zhang, Yuren Wen, Akihiko Hirata, and Mingwei Chen* Converting renewable energy into a chemical form is one of the most efficient ways for energy storage. As a high-energydensity clean chemical fuel, hydrogen can be conveniently utilized in proton exchange membrane fuel cells for high-energy applications. Accordingly, hydrogen produced by water splitting has recently been emerging as a key technology component of hydrogen economy.[1,2] The central step of water splitting is the hydrogen evolution reaction (HER). In the absence of catalysts, a large overpotential, corresponding to the thermodynamic potential of water catalysis, is needed to split water into hydrogen. Platinum and its alloys are the most active catalysts for HER and can dramatically enhance the reaction rate at a very low overpotential for high efficiency energy conversion.[3,4] However, the high material costs and poor natural abundance of platinum restrict the practical applications of the electrochemical water splitting technique.[5] Since Hinnemann and co-workers first reported that hexagonal MoS2 nanoparticles supported on graphite are active for HER in acidic media and the catalytic activity of MoS2 is predicted to be comparable with that of Pt,[6] developing highly active molybdenum sulfide composites as non-Pt HER catalysts has been the recent topic of intense studies.[7–9] Since the electrocatalytic activity of MoS2 has been identified to arise from the edge sites of the 2H sulfide compound,[6,10] many approaches have recently been developed to construct molybdenum sulfide with an optimized structure and morphology to expose the edge sites that are electrochemically active for HER.[11–17] In addition to the edge effect from the crystalline MoS2, amorphous molybdenum sulfide has also been found to have an excellent HER activity in a wide pH range.[18–20] The high catalytic activity is suggested to result from the intrinsic surface defect sites of the amorphous molybdenum sulfide, i.e. coordinately and structurally unsaturated sulfur atoms on the Prof. M. W. Chen WPI Advanced Institute for Materials Research Tohoku University Sendai 980–8577, Japan, CREST, JST 4–1–8 Honcho Kawaguchi, Saitama 332–0012, Japan State Key Laboratory of Metal Matrix Composites School of Materials Science and Engineering Shanghai Jiao Tong University Shanghai 200030, P.R. China E-mail: [email protected] Dr. X. Ge,[+] Dr. L. Chen,[+] Dr. L. Zhang, Dr. Y. Wen, Dr. A. Hirata WPI Advanced Institute for Materials Research Tohoku University, Sendai 980–8577, Japan [+]These
authors contributed equally to this work.
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amorphous surface.[18] Thus, not limited to the edge sites of crystalline MoS2, the entire surface of amorphous molybdenum sulfide is active to catalyze HER for a higher reaction rate. However, the molybdenum sulfide has a low electric conductivity which suppresses charge transport and hence efficiency of electrocatalysis.[7] Therefore, a highly conductive support with a large effective surface area is technically desirable to improve the electrocatalytic activity of the amorphous molybdenum sulfide. Dealloyed nanoporous gold (NPG) possesses high electric conductivity, a large surface area and high chemical stability.[21–23] These unique properties make NPG an ideal support material to construct functional nanostructure in electrochemical energy conversion and storage.[24–27] In the present work, we utilize NPG as the highly conductive large-surface-area support of amorphous molybdenum sulfide. It was found that the NPG substrate can dramatically improve electrocatalytic activity of the amorphous molybdenum sulfide in both acidic and neutral media for high efficiency hydrogen production. Dealloyed Au/Ag alloy membranes have a three-dimension nanoporous structure, consisting of gold ligaments and nanopore channels.[28] The internal surface of NPG is available for chemical modification and catalysis applications because of the open porosity. The precursor (MoS42−) and reagent were first used to wet the internal surface of the as-prepared NPG and then exposed to the hydrazine that acts as the reductant for the plating reaction. The slow wet chemical deposition allows us to uniformly plate a thin layer of molybdenum sulfide on the internal surface of nanoporous metals. Figure 1a shows a bright-field transmission electron microscope (TEM) image of the as-prepared molybdenum sulfide@NPG composite. The NPG substrate has the pore sizes ranges from 20–30 nm. A layer of molybdenum sulfide with a bright contrast and uniform thickness can be observed on the internal surface of nanopores. The molybdenum sulfide layer is ∼4 nm thick and tightly bonds to the ligament surface of NPG by forming a core-shell nanoporous structure. The high-resolution TEM (HRTEM) micrograph (Figure 1b) reveals that any crystal lattice cannot be found in the molybdenum sulfide layer, demonstrating that the as-plated molybdenum sulfide has an amorphous structure. The fast Fourier transform (FFT) pattern (inset of Figure 1b) taken from the plated layer further confirms the amorphous characteristic of the molybdenum sulfide. However, in the high-angle annular dark field (HAADF) images (Figure S1) in which the contrast is basically proportional to the square of atomic numbers, the Mo rich clusters with a size of 1–2 nm can be frequently observed in the amorphous phase. XPS was employed to investigate the chemical state and composition of the amorphous molybdenum sulfide. The
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survey spectrum (Figure 2a) shows the predominant signals of Au, Cu, Mo, C and S. The Cu and C are from the specimen holder and environment contaminations, respectively. The welldefined peaks at 84 and 87.7 eV (Figure 2b) correspond to the 4f7/2 and 4f5/2 of metallic Au of the nanoporous support.[27] The Mo 3d spectrum (Figure 2c) exhibits two peaks correspond to the 3d5/2 (229.2 eV) and 3d3/2 (232.4 eV) of the Mo4+ ions, a common state of Mo in MoS2 and MoS3.[20] The S 2p spectrum shows a broad and complex peak (Figure 2d), which consists of
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Figure 1. Microstructure of the molybdenum sulfide@NPG composite. (a) Bright field TEM image showing a core-shell nanoporous structure; and (b) HRTEM micrograph of the atomic structures of the gold ligament and molybdenum sulfide layer. The Inset of Figure 1(b) is the FFT pattern taken from the labeled area.
two doublets, i.e. S22− 2p3/2 (163.2 eV), S2− 2p3/2 (161.5 eV) and S22− 2p1/2 (164.5 eV), S2− 2p1/2 (162.4 eV). The XPS spectra suggest the existence of two kinds of S ligands, in accordance with the reported amorphous MoS3.[20,29] The doublets at the higher binding energy have been attributed to the bridging S22− and/ or apical S2− ligands as well as the terminal S2− ligands.[29] The intensity ratio of the two doublets is about 5:5.1, slightly lower than typical value (5:4) of amorphous MoS3. Thus, the doublet at the lower binding energy is supposed to contain the basal plane S2− ligands, which have a binding energy very close to that of terminal S2− ligands (161.8 eV). Because only MoS2 has the basal plane S2− ligands,[29] the appearance of these ligants indicates the existence of local MoS2 ordered structure in the amorphous molybdenum sulfide, which is consistent with the HAADF-STEM images (Figure S1). The formation of the MoS2 clusters could be associated with the usage of the reducing agent (N2H4) in the plating process. Therefore, the chemically plated amorphous molybdenum sulfide is the mixture of MoS3 and MoS2. Although it is impossible to accurately quantify the ratio of the two components only from the S 2p peaks, we can still estimate the composition of the amorphous molybdenum sulfide by XPS quantification. The measured S/Mo ratio is ∼2.7, which further confirms the amorphous molybdenum sulfide indeed contains a small fraction of MoS2 as imaged in the real-space HAADF micrographs (Figure S1). On the basis of the XPS study, we assign the fabricated electrode as amorphous MoS2.7@NPG. Figure 3a shows the HER polarization curves of bare NPG, MoS2.7@NPG and a reference sample of amorphous MoS2.7 deposited on a glass carbon electrode (GCE) (MoS2.7@GCE) in a 0.5 M H2SO4 aqueous solution at room temperature. All three electrodes exhibit obvious catalytic activities towards HER. Although gold is known as a relatively active catalyst for HER,[30] the amorphous MoS2.7 composites (MoS2.7@NPG and MoS2.7@ GCE) have higher catalytic activities than the bare NPG, indicating the intrinsically higher catalytic activity of the amorphous MoS2.7 than that of gold. Moreover, the amorphous MoS2.7@ NPG shows a much higher activity than the MoS2.7@GCE, demonstrating the additional catalytic enhancement from the NPG substrate. The onset HER potentials of NPG, MoS2.7@GCE and MoS2.7@NPG, above which the current increases quickly, were determined to be −145, −143, −125 mV vs. RHE, respectively (Figure 3b). The lower overpotential (∼18 mV) of the amorphous MoS2.7@NPG composite suggests an obvious improvement in the intrinsic catalysis of the amorphous molybdenum sulfide towards HER, compared to pure molybdenum sulfide. Such low overpotential is only observed from molybdenum sulfide grown on highly conductive graphene and the low overpotential has been attributed to the chemical and electronic coupling between the active molybdenum sulfide and graphene substrate.[17,31] For our case, the ultralow overpotential can still be achieved with the absence of graphene. This probably arises from the excellent charge transfer between the amorphous MoS2.7 and the NPG support. The HER current densities recorded at −0.2 V vs. RHE are −0.2, −0.9, −5.7 mA/cm−2 for pure NPG, MoS2.7@GCE, MoS2.7@NPG electrodes, respectively. More than 6 times improvement in the current density is realized on MoS2.7@NPG compared to the MoS2.7@GCE. The dramatically improved catalytic performance mainly origins from the high
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Figure 2. XPS spectra of amorphous molybdenum sulfide@NPG. (a) The survey spectrum of the entire sample; (b) Au 4f; (c) Mo 3d; and (d) S 2p core levels spectra.
conductivity and large surface area provided by the NPG substrate. The high HER kinetics of MoS2.7@NPG is also demonstrated by the lower Tafel slope of 41 mV/decade at low overpotentials, which is much lower than that of NPG (87 mV/decade) and MoS2.7@GCE (58 mV/decade) electrodes (Figure 3c). The Tafel slope of MoS2.7@NPG is also much lower than those of nanocrystalline MoS2 (50–60 mV/decade),[10,14] and equal to or better than those of graphene supported MoS2 nanoparticles and the carbon nanotubes blended amorphous MoS3 composites.[17,20] The observed Tafel slope of ∼41 mV/decade suggests that the electrochemical desorption of hydrogen is the rate-limiting step in the HER on the MoS2.7@NPG electrode.[3] Considering that the total MoS2.7@NPG loading is only ∼100 µg/cm2 with ∼6 µg/cm2 MoS2.7, the designed catalyst show a promising route for fabricating highly active HER catalysts with ultra-low catalyst loading. The electrocatalytic hydrogen production from neutral water is of significance in practical applications. Several catalysts, including amorphous molybdenum sulfide films,[19] cobalt/phosphate composites,[32] are active for HER in neutral solutions. Figure 4a shows the polarization curves of NPG, MoS2.7@GCE, and MoS2.7@NPG samples in PBS at pH = 7. The activity trend of the catalysts is similar to that in the acid solution. Compared to NPG and MoS2.7@GCE, MoS2.7@ NPG displays a superior catalytic activity with the overpotentials of 120 mV vs. RHE. At −0.2 V, the current density of the MoS2.7@NPG sample is 0.48 mA/cm2, which is ∼2 and 4 times higher than that of MoS2.7/GCE (0.27 mA/cm2) and pure NPG (0.12 mA/cm2), respectively. The ultralow H3O+ concentration 3102
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in PBS (pH = 7) makes H2O as the main source for H2 production as in alkaline solutions, thus results in the very lower current densities compared to that in the acid solution. Tafel analysis suggests that the pure NPG has a Tafel slope of ∼132 mV/decade at low overpotentials (Figure 4b), which is similar to those of metals in alkaline solutions.[33] MoS2.7/GCE and MoS2.7@NPG show much lower Tafel slopes of 72 and 60 mV/ decade, respectively (Figure 4b), indicating a rate-determining electrochemical desorption of hydrogen with a large coverage of adsorbed hydrogen. The observed catalytic activity and efficiency of MoS2.7@NPG in neutral water are also much higher than that of the reported amorphous MoS3 film (∼0.2 mA/cm2 at −0.2 V)[19] and cobalt/phosphate composite (∼0.08 mA/cm2 at −0.2 V, 140 mV/decade).[32] In summary, we have successfully developed a new type of nanoporous electrocatalyst by chemically plating a thin layer of amorphous molybdenum sulfide on the internal surface of dealloyed nanoporous gold to form a core-shell molybdenum sulfide@NPG composite. The novel MoS2.7@NPG electrode exhibits 6-fold higher catalytic activity compared to conventional molybdenum sulfide catalyst in a 0.5 M H2SO4 solution with a Tafel slope of 41 mV/decade and MoS2.7 loading of ∼6 µg/cm2. MoS2.7@NPG also shows much higher HER activity in neutral media than conventional molybdenum sulfide and the Cobased material that is known the best catalysts for HER beside Pt catalysts. The improved catalytic performance of the amorphous molybdenum sulfide mainly origins from the interplay between the thin amorphous sulfide coating and the highly conductive and large-surface nanoporous gold. Possible charge
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can be realized by utilizing economic nanoporous transition metals, such as Cu and Ni, as demonstrated by our preliminary results shown in Figure S2.
Experimental Section
Figure 3. (a) HER polarization curves, (b) the onset HER potentials and (c) Tafel curves of NPG, MoS2.7@GCE, and MoS2.7@NPG in 0.5 M H2SO4.
transfer between the insulator sulfide and free-electron gold may be the underlying mechanism of the high catalytic activity of MoS2.7@NPG. The excellent catalytic performance is also associated with the large surface area of the nanoporous electrode for effectively utilization of the active materials. Although NPG is expensive, the idea and concept developed in this study
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Fabrication of Amorphous Molybdenum Sulfide@NPG: NPG membranes with a thickness of 100 nm and a pore size of 20–30 nm were fabricated by chemically dealloying Au35Ag65 films in concentrated HNO3 (70%) for 30 min at room temperature. The residual acid and impurity ions in NPG pore channels were removed by repeated water rinsing. A thin layer of amorphous molybdenum sulfide was plated onto the internal surface of NPG by chemical deposition. In a typical chemical plating procedure, the as-prepared NPG membrane was first loaded on a glass slide for drying at room temperature for one hour. After fully wet by a 5 mM (NH4)2MoS4 solution (solvent: Dimethylformamide:Water = 1:3), the NPG membrane was carefully transferred into a diluted hydrazine solution (2%) to finalize the amorphous molybdenum sulfide plating on the internal surface of NPG. For comparison, the amorphous molybdenum sulfide was also deposited on a glass carbon electrode (GCE) using the same method. Before electrochemical tests, all the as-prepared molybdenum sulfide samples were rinsed with distilled water. TEM and XPS Characterization: The microstructure of molybdenum sulfide@NPG composites was investigated using a field-emission
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www.MaterialsViews.com transmission electron microscope (TEM) (JEOL JEM-2100F, 200 keV) equipped with double spherical aberration (Cs) correctors for both the probe-forming and image-forming objective lenses. The chemical state and composition of the as-prepared molybdenum sulfide@NPG composite loaded on Cu sheet were analyzed using X-ray photoelectron spectroscopy (XPS, AxIS-ULTRA-DLD) with Al Kα (mono) anode at energy of 150 W. Electrochemical Measurements: The electrocatalytic activities of the as-prepared electrodes toward HER were evaluated on the electrochemical workstation (Ivium Technology) in a standard threeelectrode mode in a 0.5 M H2SO4 and 0.2 M phosphate buffer solution (PBS, pH = 7). SCE and a Pt wire were selected as the reference electrode and counter electrode, respectively. The potential values reported in this study are versus the reversible hydrogen electrode (RHE). The electrochemical experiments were carried out at room temperature (∼20 °C). The HER activities were evaluated by linear sweep voltammetry (LSV) at 5 mV/s. The electrolytes were purged with high pure N2 for 30 min prior to measurements.
Acknowledgements This work is sponsored by JST-CREST “Phase Interface Science for Highly Efficient Energy Utilization”, JST; “World Premier International (WPI) Research Center Initiative for Atoms, Molecules and Materials”, MEXT, Japan; and the State Key Laboratory for Advanced Metals and Materials (2013-ZD03). Received: November 16, 2013 Published online: February 19, 2014 [1] A. J. Bard, M. A. Fox, Acc. Chem. Res. 1995, 28, 141. [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] B. E. Conway, B. V. Tilak, Electrochim. Acta 2002, 47, 3571. [4] J. Greeley, T. F. Jaramillo, J. Bonde, I. Chorkendorff, J. K. Norskov, Nat . Mater. 2006, 5, 909. [5] A. Le Goff, V. Artero, B. Jousselme, P. D. Tran, N. Guillet, R. Metaye, A. Fihri, S. Palacin, M. Fontecave, Science 2009, 326, 1384. [6] B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jorgensen, J. H. Nielsen, S. Horch, I. Chorkendorff, J. K. Norskov, J. Am. Chem. Soc. 2005, 127, 5308. [7] Y. H. Lee, X. Q. Zhang, W. Zhang, M. T. Chang, C. T. Lin, K. D. Chang, Y. C. Yu, J. T. W. Wang, C. S. Chang, L. J. Li, T. W. Lin, Adv. Mater. 2012, 24, 2320. [8] L. B. Laursen, S. Kegnæs, S. Dahla, I. Chorkendorff, Energy Environ. Sci. 2012, 5, 5577.
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