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Amorphous carbon supported MoS2 nanosheets as effective catalyst for electrocatalytic hydrogen evolution
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Amorphous carbon supported MoS2, which was elaborately prepared by using the facile hydrothermal method followed by annealing, is first employed as catalyst for hydrogen evolution reaction (HER). Herein, we demonstrate a preparation strategy, by which the MoS2 and carbon materials could be formed in-situ and simultaneously. The MoS2 nanosheets are vertically on the carbon nanosphere, as illustrated in the scanning electronic micrograph. The unique morphology can expose abundant edge of MoS2 layer as active sites for HER, while the underlying amorphous carbon effectively improves the conductivity. By means of employing amorphous carbon as substrate, an optimized catalyst was developed, exhibited enhanced catalyst activity for electrocatalytic HER with an onset potential as low as 80 mV, extremely large cathodic current density and excellent stability. Notably, a Tafel slop of 40 mV/ decade was measured, which exceeds by far the activity of previous MoS2 catalysts and suggests the VolmerHeyrovsky-mechanism for the MoS2-catalyzed HER.
1 Introduction
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Hydrogen, with a wide range of applications, has been proposed as a major energy carrier in the future.1 As for the production of hydrogen, electrocatalytic hydrogen evolution reaction (HER) has been considered as one of the sustainable and efficient approaches to generate hydrogen, in which Pt-based materials are always used as the catalysts. However, the low abundance and high cost of Pt have limited its large scale application.2,3 Considering this, various noble-metal-free catalyst materials, such as transition metal sulfides,4-6 cobalt compounds7,8 and , macromolecular compounds,9 10 have been creatively developed to partially substitute the role of Pt in HER. These electrocatalysts not only exhibit a high activity close to Pt, but also are abundant throughout the world.11 However, there still remains a large challenge to improve their catalytic efficiency to meet the requirements of practical application.12,13 Among these low-cost candidates, MoS2-based materials have been considered as one of the most promising alternatives.14 As a typical lamellar compound, MoS2 is composed of three atom layers that are held together by weak van der Waals interactions: a Mo layer sandwiched between two S layers.15,16 Both the computational and experimental results have confirmed that the electrocatalytic HER activity of MoS2 mainly stems from the sulfured Mo-edge of MoS2 plates rather than the basal planes, and it is proportional to the number of those active edge sites.17,18 However, there are two disadvantages which confine the practical application of MoS2. To enhance the electrocatalytic ability of MoS2, the following two aspects have to be considered: (1) the strong van der waals interactions existed among the lamellar crystals will inevitably result in the aggregation phenomena, which decreases the number of the active sites as well as the This journal is © The Royal Society of Chemistry [year]
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whole electrocatalytic activity;19 (2) the poor conductivity of MoS2, which is confined to the laterally transfer electron along the lamella structure of MoS2 nanosheets, restricted the efficient electron transfer as well as the related electrochemical kinetic.20,21 Taking these factors into accounts, designing MoS2-based materials with more active edge sites and good conductivity would be an effective way to improve electrocatalytic HER. As for the choosing of substrates, the carbons, including graphene,2123 carbon nanotubes,24,25 activated carbon,26 carbon papers,27 and so on, have appeared as one of the most attractive substrates for loading the MoS2 catalysts, which could offer wide electrochemical path, high surface-to-volume ratio and excellent chemical stability.28 Up to now, several strategies have been proposed to create MoS2-carbon composite materials. For example, the report of synthesizing MoS2 on reduced graphene oxide (RGO), by firstly preparing GO suspended in solution and a subsequent solvothermal reaction, represents a promising technique to promote the MoS2-based HER.22 However, there returns the cost obstacle which hinders its practical application, because the stepwise preparation of these composites is complicated and expensive.29 Besides, there have been few reports on the preparation strategies, by which the MoS2 and carbon materials could form in-situ and simultaneously. Thus, there still remains a challenging task to rationally design low-cost, easy prepared and highly efficient MoS2-based electrocatalysts which can not only expose more active edge sites but also exhibit efficient charge conductivity. Herein, we presented a facile and low-cost hydrothermal method to prepare amorphous carbon supported MoS2 (denoted as MoS2/AC). Amorphous carbon act as the substrate which not only can disperse the MoS2 nanosheets to guarantee the exposure of active edge sites but also can facilitate the electron transfer [journal], [year], [vol], 00–00 | 1
Nanoscale Accepted Manuscript
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Xue Zhao, a,b Hui Zhu,a and Xiurong Yang*a
Nanoscale
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3 Results and discussion 3.1 Characterization of MoS2/AC 65
2 Experimental 2.1 Catalyst Synthesis
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In a typical synthesis, 0.15g of Na2MoO4·2H2O, 0.50 g of glucose and 0.20 g NH2CSNH2 were dissolved in 30 mL deionized water. After stirring for a few minutes, the obtained solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, and heated at 240 oC for 24h. After cooling naturally, the black precipitates were collected by centrifugation, washed with deionized water and ethanol, and dried in a vacuum oven at 80 oC for 24h. To increase the electron conductivity of MoS2/AC, the obtained composites were annealed in a tube furnace at 800 oC for 2 h under the protection of argon. For the control experiment, the pure MoS2 nanoparticles were prepared using the same process in the absence of glucose.
spectroscopy (XPS) analysis was carried out on an ESCALAB MKII X-ray photoelectron spectrometer using mono-chromated Al Ka X-rays and the energy values were calibrated by using the C1s level of 284.8 eV. The specific surface areas were determined from nitrogen adsorption using the Brunauer–Emmett–Teller method.
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Glucose was applied as carbon source to establish the amorphous carbon substrate. With the attendance of glucose, colloidal carbons will be in-situ produced. Simultaneously, MoS2 was formed onto the surface of the carbon, which herein may significantly inhibit the aggregation of MoS2 nanosheets.30 After the following calcinations, amorphous carbon was formed, forming the MoS2/AC composites. As shown in Fig. 1a, the
2.2 Electrochemical measurements
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Electrochemical measurements of various samples were performed on a glassy carbon electrode in 0.5M H2SO4 solution using a typical three-electrode setup. Generally, 4 mg of catalyst and 80 µl of 5 wt% Nafion solution were dispersed in 1 ml of 4:1 v/v water/ethanol by at least 30 min sonication to form a homogeneous ink. Then 15 µL of the catalyst ink was loaded onto a glassy carbon electrode (GCE, 5 mm in diameter). The HER activity was evaluated by linear sweep voltammetry on rotating disk electrode with a rotation rate of 1600 rpm, by a scan rate of 100 mV·s-1. The polarization curves were obtained after iRcompensation. In all measurements, we used Ag/AgCl electrode as the reference and the potential values are corrected to reverse hydrogen electrode (RHE) (see ESI, Fig. S1†). In 0.5 M H2SO4, E (RHE) = E (Ag/AgCl) + 0.204 V. The current density was calculated by geometric area of the glassy carbon electrode which is 0.196 cm2. Prior to any electrochemical measurement, the electrolyte solution was purified with N2 for 1h to remove completely the oxygen, and stable polarization performance cures were recorded after 10 cycles.
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2.3 Characterization
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Scanning electron microscope (SEM) images were taken with a FE-SEM XL30 ESEM-FEG at an accelerating voltage of 20.0 kV. Transmission electron microscopy (TEM) images were taken using a Tecnai G2 F20 with an accelerating voltage of 100 kV. Xray powder diffraction (XRD) patterns for the various samples were recorded using a D8-Advance system (Bruker, Germa) with Cu Kα radiation (λ =0.154 nm). Energy-dispersive X-ray spectroscopy (EDS) analysis and Element Mapping of the material was along with the TEM. X-Ray photo-electron 2 | Journal Name, [year], [vol], 00–00
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Fig. 1 SEM images (A) for the as-prepared MoS2/AC showing nanosized MoS2 with highly exposed edges stacked on the carbon nanosphere. TEM image (B) and HRTEM images (C, D) of MoS2/AC. (E) The high angle annular dark-field scanning TEM image and corresponding elemental mapping images of the MoS2/AC.
MoS2/AC composites exhibit a 3D sphere-like architecture, with a diameter of about 500 nm. Besides, on the surface of the nanospheres, many nanosheets are uniformly dispersed with an average thickness of 20 nm. Approximately, the nanosheets stack perpendicularly on the underlying substrate, processed highly exposed edges with slightly folded dentations on the rim, which significantly increase the exposure of active edge sites.31 In strong contrast, in the absence of amorphous carbon, the exact same synthesis method produced pure MoS2 coalesced into layered particles of large sizes (see ESI, Fig. S2†). The drastic morphological difference highlights the important role of amorphous carbon as a novel support material for decentralizing This journal is © The Royal Society of Chemistry [year]
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during the electrocatalysis and result in highly improved conductivity of the composite. It was observed that the MoS2 nanosheets were vertically sticking into the amorphous carbon substrates. Most importantly, after an anneal treatment, the obtained composite exhibited an outstanding HER performance with a onset potential of 80 mV, Tafel slope of 40 mV/dec, exchange current density of 0.474 mA·cm-2 and also conductivity of 217 Ω. To our knowledge, it is the first report of employing the amorphous carbon as substrate to improve MoS2 for enhanced catalysis HER and has catalytic properties comparable with platinum.
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the MoS2 nanosheets. The unique morphology of the MoS2/AC was further verified by the TEM image as shown in Fig. 1b. The MoS2 nanowhiskers, corresponding to the parallel lines in Fig. 1c, are clearly dispersed in the amorphous carbon substrate, which are counted from one to about ten layers. Moreover, the distinguishable lattice fringes for MoS2 can be seen clearly in the HRTEM image (Fig. 1d). Since the MoS2 nanosheets are slightly folded in some region, the field of vision contains both images in top (d=0.27) and side (d=0.62) view. Both of them are consistent with the d spacing of (100) and (002) planes of hexagonal MoS2, respectively. In addition, as illustrated in Fig. 1e, the Mo and S elements reveal evenly distribution of across the whole composites. It is found that the image of carbon is not exactly the same as Mo and S because of the disturbance by the carbon from the TEM grid.
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Fig. 2 XRD patterns of (a) MoS2/AC, (b) the unannealed MoS2 by the same hydrothermal route, (c) pure MoS2.
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Besides the morphological investigation, the crystal structure of MoS2/AC was detected by XRD technology. As shown in Fig. 2a, all the three samples display similar diffraction peaks which correspond well to the hexagonal structure (JCPDS 37-1492), thus certified the high purity of the product. Comparing with the unannealed MoS2 in Fig. 2b, both MoS2/AC and pure MoS2 (Fig. 2c) show improved crystallinity with more obvious peaks and higher intensity. The increase of crystallinity is one of the benefits from the annealing process. Only three diffraction peaks (2θ) at 33.7° (100), 59.3° (110) and 69.6° (201) are found for the MoS2/AC, revealing single phase of MoS2 in the hybrid. The distinctive strong (002) peak of pure MoS2 at 2θ=14.8° signifies a well-stacked layered structure, while the disappear of (002) peak indicates that stacking of the single layers doesn’t take place.32 We can infer that the MoS2 in the composites should have the structure of single layer or few layers and expose the catalytic active site, which was named graphene-like structure by Rao et al.30 This conclusion agrees well with the clearly observation from the microscopy imaging. The amorphous carbon and chemical state of MoS2 was further confirmed by XPS analysis. As shown in Fig. 3A, the peaks of C, O, S, and Mo can be obviously detected. The binding energy of C 1s at 285 eV is identified to amorphous carbon (Figure 3B). In the peaks of S 2p (Figure 3C), the peaks at about 161.85 eV and
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Fig. 3 XPS of MoS2/AC. (A) Survey spectrum. (B) C 1s spectrum. (C) S 2p spectrum. (D) Mo 3d spectrum. 162.8 eV are related to S 2p3/2 and S 2p1/2 binding energies, respectively. In the peaks of Mo 3d (Figure 3D), the peaks at about 228.8 eV and 232.1 eV can be attributed to Mo 3d 5/2 and Mo 3d 3/2 binding energies, respectively. Besides, the spectrum indicates the dominant existence of the 1T phase, proved by the deconvolution of XPS Mo3d bands (see ESI, Fig. S3†).33,34 Note that the 1T phase of MoS2 is meta-stable, the MoS2 nanosheets are composed of 2H and major 1T polymorphs, providing an prediction of better HER performance. 35 3.2 Evaluation of electrocatalytic activity To demonstrate electrochemical performance of the MoS2/AC composites for HER, the LSV was performed. As shown in Fig. 3A, polarization curves were obtained after iR-compensation in 0.5M H2SO4. As control experiments, polarization curves of bare glass carbon electrode, pure MoS2, and commercial bulk MoS2 exhibit poor HER performance with high onset overpotential (about 230, 180 mV, and 170 mV) as well as low cathodic current density. In comparison, the MoS2/AC exhibits excellent HER activity. In electrochemical process, onset potential stems from the extent of the barrier in energy conversion and means the extra energy to overcome. The onset potential of MoS2/AC is 80 mV, which is comparable to that of Pt and means low energy consumption in catalytic process. This onset potential is smaller than the reported results of MoS2-based electrocatalysts, which suggests the good catalytic activity of the product.36 The comparison between MoS2/AC and unannealed MoS2 indicates that the annealing process improves the catalysis activity (see ESI, Fig. S4A†). The eletrocatalytic performance of MoS2-free AC has scarcely any catalytic activity, suggesting that catalytic activity is directly origined from the MoS2 nanosheet (see ESI, Fig. S4B†). In addition, the current density of MoS2/AC is among the highest report for noble-metal-free catalyzing HER, with a large current density of 91 mA·cm-2 at overpotential =200 mV, which is proportional to the quantity of evolved hydrogen. For comparison, a recently reported defect-rich MoS2 based HER catalyst gave a current density of 13 mA·cm-2 at the same overpotential.37 Therefore, the MoS2/AC Journal Name, [year], [vol], 00–00 | 3
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Fig. 4 (A) Polarization performance curves on a rotating glassy carbon disk electrode recorded in 0.5M H2SO4 for various electrocatalysts. The curves are after several inceptive polarization scans for the freshly prepared samples; (B) The Tafel plots in which only the linear portions were showed to give a clear vision.
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displays prominent hydrogen evolution behaviour with low energy consumption as well as large quantity of hydrogen evolution. Tafel behaviour was observed from the polarization curves at the overpotential of 10 to 60 mV. The i-E date fit well with the Tafel equation at different overpotential ranges (Fig. 4B), yielding Tafel slopes of 30, 40, 107 and 160 mV/decade for Pt, MoS2/AC, commercial MoS2, and pure MoS2, respectively (see ESI, Table S1†). A smaller Tafel slope means a faster increment of HER velocity with the increasing of potential, catalysts with smaller Tafel slope are advantageous for practical applications. Therefore, the MoS2/AC displays the best activity with the smallest slope value of 40 mV/decade which is in line with MoS2/graphene,22 and outperforms most of the MoS2-based catalysts. Many studies have devoted to explain the mechanism of hydrogen revolution, upon which Tafel slope values can be used to determine which HER reaction path predominates for a catalyst.22 According to reported theory, a Tafel slope of 40 mV per decade is obtained for the MoS2/AC, suggesting the VolmerHeyrovsky mechanism of HER, which occurs via a fast discharge reaction (Equation (1)) and then a rate determining ion and atom reaction (Equation (2)).13,25 However, Pt catalyzed HER is
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Fig. 5 Durability test for the MoS2/AC. Negligible HER current was lost after 1000 cycles of cycle voltammogram from -0.4 to 0.1V vs. RHE. proceed through the Volmer-Tafel mechanism (Equation 1 and 3) with fast reaction rate and a Tafel slope of 30 mV per decade.38,39 Volmer reaction H3O+ + e + cat → cat-H + H2O (1) Heyrovsky reaction H3O++ e + cat-H → cat + H2 +H2O (2) Tafel reaction cat-H + cat-H → 2cat + H2 (3) The most inherent measure of catalytic activity for HER is the exchange current density (j0)40, which is obtained by fitting the logi-E data to the Tafel equation and using extrapolation methods. In detail, the 20% Pt/C reveals a j0 of 3.39 mA·cmG-2 and the MoS2/AC yields a j0 at 0.474 mA·cm-2, which is larger than most previous reports for MoS2-based catalysts (see ESI, Table S1 and Fig. S5†). The large j0 of 0.474 mA·cm-2, that is 1.67 A g−1 normalized by the loading weight, suggests superior catalytic activity and confirms the enrichment effect of the amorphous carbon. Stability is another important requirement of catalyst. As shown in Fig. 4, a long-term cycling test of the MoS2/AC was per formed. Only a slight activity loss was observed after 1000 cycles, indicating that the MoS2/AC is of good durability in a long-term electrochemical process. In addition, the catalytic performance of the catalyst ink stored for half a year is identical to that of fresh prepared, showing well stability under air atmosphere (see ESI, Fig. S2C, Fig. S4C†). 3.3 Discussion on the role of amorphous carbon
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As far as we know, the catalytic activity observed on the MoS2/AC is in line with MoS2/graphene and MoS2/CNT reported recently. 22,25 In this work, excellent catalytic activity results from not only the intrinsic property of MoS2 (∆GH = 0.08eV),17 but the amorphous carbon substrate. Several good properties are based on the amorphous carbon, such as the high edge-plain-ratio, the large specific surface area, the good conductivity, and the improvement of stability. First, the amorphous carbon significantly inhibit the aggregation of MoS2 nanosheets, thus increased the edge-plainratio of MoS2. Since the active sites of MoS2 for HER are proposed to be on the edges which have coordinate unsaturated Mo and/or S atoms, a high edge-plain-ratio can bring in more additional active edge sites as well as high catalytic activity. This journal is © The Royal Society of Chemistry [year]
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functional groups and MoS2 contributes to the catalysis stability to some extent. This conclusion is accord with the report that the combination of inorganic substances and carbon material shows excellent performance, due to chemical coupling effect and the electronic coupling effect.28 Therefore, the MoS2/AC exhibited enhanced stability, again highlighting the role of amorphous carbon as a novel substrate in improving the stability of the nanomaterials.
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In conclusion, we have presented a highly active electrocatalyst of MoS2/AC where the MoS2 nanosheets were formed vertically on amorphous carbon nanospheres via a facile and versatile approach. As an effective substrate, amorphous carbon inhibit the aggregation of MoS2 nanosheets, expose abundance catalytic edge sites and render MoS2/AC excellent conductivity, thus effectively enhanced the catalytic performance of MoS2 based HER electrocatalyst. The MoS2/AC exhibited superior catalyst activity for HER with an onset potential as low as 80 mV, extremely large cathodic current density and excellent stability, showing a great potential as a low cost alternative to Pt in practical application. Further, the synthesis strategy may inspire the researches for other applications, such as photoelectroncatclytic water splitting.
Acknowledgements Fig. 6 (A) Equivalent circuit model used to model the HER process of the studied system. (B) Nyquist plots of MoS2/AC and pure MoS2 modified GCE 5
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Second, the amorphous carbon improves the specific surface area, which is confirmed by SEM and BET analysis of between the MoS2/AC and pure MoS2 (see ESI, Fig. S2 and S6†). The N2 adsorption desorption characterization and pore size distribution (see ESI, Table S2†) show that MoS2/AC have a high specific surface area of 43.9 m2·g−1 and desorption average pore width of 14.4 nm, thus lots of microporous structure. Note that the ionic interchange at the catalyst surface is one of the key factors to the catalytic reaction41. Thus the large surface area and the microstructure, which actually stem from the formation of amorphous carbon nanoparticles, attribute to high catalytic efficiency. Comparisons of LSV and Tafel plot between the MoS2/AC and pure MoS2 prove this conclusion. Third, the amorphous carbon improves the conductivity of MoS2/AC, which can be proved by EIS measurement measured in 0.5 M H2SO4 (Figure 6). The unannealed MoS2 shows larger charge-transfer resistance than MoS2/AC. The improvement of conductivity also benefits from the annealing process. Comparison on the Nyquist plots of the EIS response between the MoS2/AC and pure MoS2 on GCE shows that the MoS2/AC has lower charge-transfer resistance, illustrating the superior electrocatalytic activity of the MoS2/AC. Finally, the amorphous carbon supporter can stabilize the disordered structure of the MoS2 nanosheets throughout the polarization cycles. The residual oxygen-containing functional groups on the amorphous carbon are confirmed by 8.46w% of oxygen (see ESI, Fig. S7†), providing sufficient functional groups on carbon surface to afford intimate interactions with MoS2. The coupling effect between the oxygen-containing This journal is © The Royal Society of Chemistry [year]
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This work was financially supported by the National Natural Science Foundation of China (No. 21175124), the National Key Basic Research Development Project of China (No. 2010CB933602).
Notes and references a
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State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China. E-mail: [email protected] b University of the Chinese Academy of Sciences, Beijing 100049, China. † Electronic Supplementary Information (ESI) available: Experimental details, additional data and discussions. See DOI: 10.1039/b000000x/ 1 Dresselhaus, M. S.; Thomas, I. L. Nature 2001, 414, 332. 2 Chen, W.-F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Angewandte Chemie International Edition 2012, 51, 6131. 3 Fang, B.; Kim, M.-S.; Kim, J. H.; Song, M. Y.; Wang, Y.-J.; Wang, H.; Wilkinson, D. P.; Yu, J.-S. Journal of Materials Chemistry 2011, 21, 8066. 4 Zeng, Z.; Tan, C.; Huang, X.; Bao, S.; Zhang, H. Energy & Environmental Science 2014, 7, 797. 5 Yang, J.; Voiry, D.; Ahn, S. J.; Kang, D.; Kim, A. Y.; Chhowalla, M.; Shin, H. S. Angewandte Chemie International Edition 2013, 52, 13751. 6 Karunadasa, H. I.; Montalvo, E.; Sun, Y.; Majda, M.; Long, J. R.; Chang, C. J. Science 2012, 335, 698. 7 Kleingardner, J. G.; Kandemir, B.; Bren, K. L. Journal of the American Chemical Society 2013, 136, 4. 8 Hu, X.; Brunschwig, B. S.; Peters, J. C. Journal of the American Chemical Society 2007, 129, 8988. 9 Wiese, S.; Kilgore, U. J.; DuBois, D. L.; Bullock, R. M. ACS Catalysis 2012, 2, 720. 10 Zhuo, J.; Wang, T.; Zhang, G.; Liu, L.; Gan, L.; Li, M. Angewandte Chemie International Edition 2013, 52, 10867. 11 Li, Y.; Somorjai, G. A. Nano Letters 2010, 10, 2289. 12 Wu, Z.; Fang, B.; Wang, Z.; Wang, C.; Liu, Z.; Liu, F.; Wang, W.; Alfantazi, A.; Wang, D.; Wilkinson, D. P. ACS Catalysis 2013, 2101.
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4 Conclusions
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13 Wu, Z.; Fang, B.; Bonakdarpour, A.; Sun, A.; Wilkinson, D. P.; Wang, D. Applied Catalysis B: Environmental 2012, 125, 59. 14 Merki, D.; Fierro, S.; Vrubel, H.; Hu, X. Chemical Science 2011, 2, 1262. 15 Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. Journal of the American Chemical Society 2005, 127, 5308. 16 Balendhran, S.; Ou, J. Z.; Bhaskaran, M.; Sriram, S.; Ippolito, S.; Vasic, Z.; Kats, E.; Bhargava, S.; Zhuiykov, S.; Kalantar-Zadeh, K. Nanoscale 2012, 4, 461. 17 Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Science 2007, 317, 100. 18 Karunadasa, H. I.; Montalvo, E.; Sun, Y.; Majda, M.; Long, J. R.; Chang, C. J. Science 2012, 335, 698. 19 Firmiano, E. G. S.; Cordeiro, M. A. L.; Rabelo, A. C.; Dalmaschio, C. J.; Pinheiro, A. N.; Pereira, E. C.; Leite, E. R. Chemical Communications 2012, 48, 7687. 20 Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang, R.; Lei, Y.; Pan, B.; Xie, Y. Journal of the American Chemical Society 2013, 135, 17881. 21 Liao, L.; Zhu, J.; Bian, X.; Zhu, L.; Scanlon, M. D.; Girault, H. H.; Liu, B. Advanced Functional Materials 2013, n/a. 22 Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. Journal of the American Chemical Society 2011, 133, 7296. 23 Yan, Y.; Xia, B.; Qi, X.; Wang, H.; Xu, R.; Wang, J. Y.; Zhang, H.; Wang, X. Chem Commun (Camb) 2013, 49, 4884. 24 Lin, T.-W.; Liu, C.-J.; Lin, J.-Y. Applied Catalysis B: Environmental 2013, 134, 75. 25 Yan, Y.; Ge, X.; Liu, Z.; Wang, J. Y.; Lee, J. M.; Wang, X. Nanoscale 2013, 5, 7768. 26 Laursen, A. B.; Vesborg, P. C. K.; Chorkendorff, I. Chemical Communications 2013, 49, 4965. 27 Conway, B. E.; Tilak, B. V. Electrochimica Acta 2002, 47, 3571. 28 Liang, Y.; Li, Y.; Wang, H.; Dai, H. Journal of the American Chemical Society 2013, 135, 2013. 29 Wu, S.; Zeng, Z.; He, Q.; Wang, Z.; Wang, S. J.; Du, Y.; Yin, Z.; Sun, X.; Chen, W.; Zhang, H. Small 2012, 8, 2264. 30 Chang, K.; Chen, W.; Ma, L.; Li, H.; Li, H.; Huang, F.; Xu, Z.; Zhang, Q.; Lee, J.-Y. Journal of Materials Chemistry 2011, 21, 6251. 31 Kong, D.; Wang, H.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y. Nano Letters 2013, 13, 1341. 32 Ramakrishna Matte, H. S. S.; Gomathi, A.; Manna, A. K.; Late, D. J.; Datta, R.; Pati, S. K.; Rao, C. N. R. Angewandte Chemie International Edition 2010, 49, 4059. 33 Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M. Nano Letters 2011, 11, 5111. 34 Huang, X.; Zeng, Z.; Bao, S.; Wang, M.; Qi, X.; Fan, Z.; Zhang, H. Nat Commun 2013, 4, 1444. 35 Voiry, D.; Salehi, M.; Silva, R.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M. Nano Lett 2013, 13, 6222. 36 Wang, T.; Zhuo, J.; Du, K.; Chen, B.; Zhu, Z.; Shao, Y.; Li, M. Advanced Materials 2014, 1. 37 Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Advanced Materials 2013, 25, 5807. 38 Thomas, J. G. N. Transactions of the Faraday Society 1961, 57, 1603. 39 Conway, B. E.; Tilak, B. V. Electrochimica Acta 2002, 47, 3571. 40 Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U. Journal of The Electrochemical Society 2005, 152, J23. 41 Xiang, Q.; Yu, J.; Jaroniec, M. Journal of the American Chemical Society 2012, 134, 6575.
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Amorphous carbon supported MoS2 nanosheets as effective catalyst for electrocatalytic hydrogen evolution
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Amorphous carbon supported MoS2, which was elaborately prepared by using the facile hydrothermal method followed by annealing, is first employed as catalyst for hydrogen evolution reaction (HER). Herein, we demonstrate a preparation strategy, by which the MoS2 and carbon materials could be formed in-situ and simultaneously. The MoS2 nanosheets are vertically on the carbon nanosphere, as illustrated in the scanning electronic micrograph. The unique morphology can expose abundant edge of MoS2 layer as active sites for HER, while the underlying amorphous carbon effectively improves the conductivity. By means of employing amorphous carbon as substrate, an optimized catalyst was developed, exhibited enhanced catalyst activity for electrocatalytic HER with an onset potential as low as 80 mV, extremely large cathodic current density and excellent stability. Notably, a Tafel slop of 40 mV/ decade was measured, which exceeds by far the activity of previous MoS2 catalysts and suggests the VolmerHeyrovsky-mechanism for the MoS2-catalyzed HER.
1 Introduction
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Hydrogen, with a wide range of applications, has been proposed as a major energy carrier in the future.1 As for the production of hydrogen, electrocatalytic hydrogen evolution reaction (HER) has been considered as one of the sustainable and efficient approaches to generate hydrogen, in which Pt-based materials are always used as the catalysts. However, the low abundance and high cost of Pt have limited its large scale application.2,3 Considering this, various noble-metal-free catalyst materials, such as transition metal sulfides,4-6 cobalt compounds7,8 and , macromolecular compounds,9 10 have been creatively developed to partially substitute the role of Pt in HER. These electrocatalysts not only exhibit a high activity close to Pt, but also are abundant throughout the world.11 However, there still remains a large challenge to improve their catalytic efficiency to meet the requirements of practical application.12,13 Among these low-cost candidates, MoS2-based materials have been considered as one of the most promising alternatives.14 As a typical lamellar compound, MoS2 is composed of three atom layers that are held together by weak van der Waals interactions: a Mo layer sandwiched between two S layers.15,16 Both the computational and experimental results have confirmed that the electrocatalytic HER activity of MoS2 mainly stems from the sulfured Mo-edge of MoS2 plates rather than the basal planes, and it is proportional to the number of those active edge sites.17,18 However, there are two disadvantages which confine the practical application of MoS2. To enhance the electrocatalytic ability of MoS2, the following two aspects have to be considered: (1) the strong van der waals interactions existed among the lamellar crystals will inevitably result in the aggregation phenomena, which decreases the number of the active sites as well as the This journal is © The Royal Society of Chemistry [year]
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whole electrocatalytic activity;19 (2) the poor conductivity of MoS2, which is confined to the laterally transfer electron along the lamella structure of MoS2 nanosheets, restricted the efficient electron transfer as well as the related electrochemical kinetic.20,21 Taking these factors into accounts, designing MoS2-based materials with more active edge sites and good conductivity would be an effective way to improve electrocatalytic HER. As for the choosing of substrates, the carbons, including graphene,2123 carbon nanotubes,24,25 activated carbon,26 carbon papers,27 and so on, have appeared as one of the most attractive substrates for loading the MoS2 catalysts, which could offer wide electrochemical path, high surface-to-volume ratio and excellent chemical stability.28 Up to now, several strategies have been proposed to create MoS2-carbon composite materials. For example, the report of synthesizing MoS2 on reduced graphene oxide (RGO), by firstly preparing GO suspended in solution and a subsequent solvothermal reaction, represents a promising technique to promote the MoS2-based HER.22 However, there returns the cost obstacle which hinders its practical application, because the stepwise preparation of these composites is complicated and expensive.29 Besides, there have been few reports on the preparation strategies, by which the MoS2 and carbon materials could form in-situ and simultaneously. Thus, there still remains a challenging task to rationally design low-cost, easy prepared and highly efficient MoS2-based electrocatalysts which can not only expose more active edge sites but also exhibit efficient charge conductivity. Herein, we presented a facile and low-cost hydrothermal method to prepare amorphous carbon supported MoS2 (denoted as MoS2/AC). Amorphous carbon act as the substrate which not only can disperse the MoS2 nanosheets to guarantee the exposure of active edge sites but also can facilitate the electron transfer [journal], [year], [vol], 00–00 | 1
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Xue Zhao, a,b Hui Zhu,a and Xiurong Yang*a
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3 Results and discussion 3.1 Characterization of MoS2/AC 65
2 Experimental 2.1 Catalyst Synthesis
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In a typical synthesis, 0.15g of Na2MoO4·2H2O, 0.50 g of glucose and 0.20 g NH2CSNH2 were dissolved in 30 mL deionized water. After stirring for a few minutes, the obtained solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, and heated at 240 oC for 24h. After cooling naturally, the black precipitates were collected by centrifugation, washed with deionized water and ethanol, and dried in a vacuum oven at 80 oC for 24h. To increase the electron conductivity of MoS2/AC, the obtained composites were annealed in a tube furnace at 800 oC for 2 h under the protection of argon. For the control experiment, the pure MoS2 nanoparticles were prepared using the same process in the absence of glucose.
spectroscopy (XPS) analysis was carried out on an ESCALAB MKII X-ray photoelectron spectrometer using mono-chromated Al Ka X-rays and the energy values were calibrated by using the C1s level of 284.8 eV. The specific surface areas were determined from nitrogen adsorption using the Brunauer–Emmett–Teller method.
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Glucose was applied as carbon source to establish the amorphous carbon substrate. With the attendance of glucose, colloidal carbons will be in-situ produced. Simultaneously, MoS2 was formed onto the surface of the carbon, which herein may significantly inhibit the aggregation of MoS2 nanosheets.30 After the following calcinations, amorphous carbon was formed, forming the MoS2/AC composites. As shown in Fig. 1a, the
2.2 Electrochemical measurements
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Electrochemical measurements of various samples were performed on a glassy carbon electrode in 0.5M H2SO4 solution using a typical three-electrode setup. Generally, 4 mg of catalyst and 80 µl of 5 wt% Nafion solution were dispersed in 1 ml of 4:1 v/v water/ethanol by at least 30 min sonication to form a homogeneous ink. Then 15 µL of the catalyst ink was loaded onto a glassy carbon electrode (GCE, 5 mm in diameter). The HER activity was evaluated by linear sweep voltammetry on rotating disk electrode with a rotation rate of 1600 rpm, by a scan rate of 100 mV·s-1. The polarization curves were obtained after iRcompensation. In all measurements, we used Ag/AgCl electrode as the reference and the potential values are corrected to reverse hydrogen electrode (RHE) (see ESI, Fig. S1†). In 0.5 M H2SO4, E (RHE) = E (Ag/AgCl) + 0.204 V. The current density was calculated by geometric area of the glassy carbon electrode which is 0.196 cm2. Prior to any electrochemical measurement, the electrolyte solution was purified with N2 for 1h to remove completely the oxygen, and stable polarization performance cures were recorded after 10 cycles.
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2.3 Characterization
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Scanning electron microscope (SEM) images were taken with a FE-SEM XL30 ESEM-FEG at an accelerating voltage of 20.0 kV. Transmission electron microscopy (TEM) images were taken using a Tecnai G2 F20 with an accelerating voltage of 100 kV. Xray powder diffraction (XRD) patterns for the various samples were recorded using a D8-Advance system (Bruker, Germa) with Cu Kα radiation (λ =0.154 nm). Energy-dispersive X-ray spectroscopy (EDS) analysis and Element Mapping of the material was along with the TEM. X-Ray photo-electron 2 | Journal Name, [year], [vol], 00–00
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Fig. 1 SEM images (A) for the as-prepared MoS2/AC showing nanosized MoS2 with highly exposed edges stacked on the carbon nanosphere. TEM image (B) and HRTEM images (C, D) of MoS2/AC. (E) The high angle annular dark-field scanning TEM image and corresponding elemental mapping images of the MoS2/AC.
MoS2/AC composites exhibit a 3D sphere-like architecture, with a diameter of about 500 nm. Besides, on the surface of the nanospheres, many nanosheets are uniformly dispersed with an average thickness of 20 nm. Approximately, the nanosheets stack perpendicularly on the underlying substrate, processed highly exposed edges with slightly folded dentations on the rim, which significantly increase the exposure of active edge sites.31 In strong contrast, in the absence of amorphous carbon, the exact same synthesis method produced pure MoS2 coalesced into layered particles of large sizes (see ESI, Fig. S2†). The drastic morphological difference highlights the important role of amorphous carbon as a novel support material for decentralizing This journal is © The Royal Society of Chemistry [year]
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during the electrocatalysis and result in highly improved conductivity of the composite. It was observed that the MoS2 nanosheets were vertically sticking into the amorphous carbon substrates. Most importantly, after an anneal treatment, the obtained composite exhibited an outstanding HER performance with a onset potential of 80 mV, Tafel slope of 40 mV/dec, exchange current density of 0.474 mA·cm-2 and also conductivity of 217 Ω. To our knowledge, it is the first report of employing the amorphous carbon as substrate to improve MoS2 for enhanced catalysis HER and has catalytic properties comparable with platinum.
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the MoS2 nanosheets. The unique morphology of the MoS2/AC was further verified by the TEM image as shown in Fig. 1b. The MoS2 nanowhiskers, corresponding to the parallel lines in Fig. 1c, are clearly dispersed in the amorphous carbon substrate, which are counted from one to about ten layers. Moreover, the distinguishable lattice fringes for MoS2 can be seen clearly in the HRTEM image (Fig. 1d). Since the MoS2 nanosheets are slightly folded in some region, the field of vision contains both images in top (d=0.27) and side (d=0.62) view. Both of them are consistent with the d spacing of (100) and (002) planes of hexagonal MoS2, respectively. In addition, as illustrated in Fig. 1e, the Mo and S elements reveal evenly distribution of across the whole composites. It is found that the image of carbon is not exactly the same as Mo and S because of the disturbance by the carbon from the TEM grid.
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Fig. 2 XRD patterns of (a) MoS2/AC, (b) the unannealed MoS2 by the same hydrothermal route, (c) pure MoS2.
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Besides the morphological investigation, the crystal structure of MoS2/AC was detected by XRD technology. As shown in Fig. 2a, all the three samples display similar diffraction peaks which correspond well to the hexagonal structure (JCPDS 37-1492), thus certified the high purity of the product. Comparing with the unannealed MoS2 in Fig. 2b, both MoS2/AC and pure MoS2 (Fig. 2c) show improved crystallinity with more obvious peaks and higher intensity. The increase of crystallinity is one of the benefits from the annealing process. Only three diffraction peaks (2θ) at 33.7° (100), 59.3° (110) and 69.6° (201) are found for the MoS2/AC, revealing single phase of MoS2 in the hybrid. The distinctive strong (002) peak of pure MoS2 at 2θ=14.8° signifies a well-stacked layered structure, while the disappear of (002) peak indicates that stacking of the single layers doesn’t take place.32 We can infer that the MoS2 in the composites should have the structure of single layer or few layers and expose the catalytic active site, which was named graphene-like structure by Rao et al.30 This conclusion agrees well with the clearly observation from the microscopy imaging. The amorphous carbon and chemical state of MoS2 was further confirmed by XPS analysis. As shown in Fig. 3A, the peaks of C, O, S, and Mo can be obviously detected. The binding energy of C 1s at 285 eV is identified to amorphous carbon (Figure 3B). In the peaks of S 2p (Figure 3C), the peaks at about 161.85 eV and
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Fig. 3 XPS of MoS2/AC. (A) Survey spectrum. (B) C 1s spectrum. (C) S 2p spectrum. (D) Mo 3d spectrum. 162.8 eV are related to S 2p3/2 and S 2p1/2 binding energies, respectively. In the peaks of Mo 3d (Figure 3D), the peaks at about 228.8 eV and 232.1 eV can be attributed to Mo 3d 5/2 and Mo 3d 3/2 binding energies, respectively. Besides, the spectrum indicates the dominant existence of the 1T phase, proved by the deconvolution of XPS Mo3d bands (see ESI, Fig. S3†).33,34 Note that the 1T phase of MoS2 is meta-stable, the MoS2 nanosheets are composed of 2H and major 1T polymorphs, providing an prediction of better HER performance. 35 3.2 Evaluation of electrocatalytic activity To demonstrate electrochemical performance of the MoS2/AC composites for HER, the LSV was performed. As shown in Fig. 3A, polarization curves were obtained after iR-compensation in 0.5M H2SO4. As control experiments, polarization curves of bare glass carbon electrode, pure MoS2, and commercial bulk MoS2 exhibit poor HER performance with high onset overpotential (about 230, 180 mV, and 170 mV) as well as low cathodic current density. In comparison, the MoS2/AC exhibits excellent HER activity. In electrochemical process, onset potential stems from the extent of the barrier in energy conversion and means the extra energy to overcome. The onset potential of MoS2/AC is 80 mV, which is comparable to that of Pt and means low energy consumption in catalytic process. This onset potential is smaller than the reported results of MoS2-based electrocatalysts, which suggests the good catalytic activity of the product.36 The comparison between MoS2/AC and unannealed MoS2 indicates that the annealing process improves the catalysis activity (see ESI, Fig. S4A†). The eletrocatalytic performance of MoS2-free AC has scarcely any catalytic activity, suggesting that catalytic activity is directly origined from the MoS2 nanosheet (see ESI, Fig. S4B†). In addition, the current density of MoS2/AC is among the highest report for noble-metal-free catalyzing HER, with a large current density of 91 mA·cm-2 at overpotential =200 mV, which is proportional to the quantity of evolved hydrogen. For comparison, a recently reported defect-rich MoS2 based HER catalyst gave a current density of 13 mA·cm-2 at the same overpotential.37 Therefore, the MoS2/AC Journal Name, [year], [vol], 00–00 | 3
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Fig. 4 (A) Polarization performance curves on a rotating glassy carbon disk electrode recorded in 0.5M H2SO4 for various electrocatalysts. The curves are after several inceptive polarization scans for the freshly prepared samples; (B) The Tafel plots in which only the linear portions were showed to give a clear vision.
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displays prominent hydrogen evolution behaviour with low energy consumption as well as large quantity of hydrogen evolution. Tafel behaviour was observed from the polarization curves at the overpotential of 10 to 60 mV. The i-E date fit well with the Tafel equation at different overpotential ranges (Fig. 4B), yielding Tafel slopes of 30, 40, 107 and 160 mV/decade for Pt, MoS2/AC, commercial MoS2, and pure MoS2, respectively (see ESI, Table S1†). A smaller Tafel slope means a faster increment of HER velocity with the increasing of potential, catalysts with smaller Tafel slope are advantageous for practical applications. Therefore, the MoS2/AC displays the best activity with the smallest slope value of 40 mV/decade which is in line with MoS2/graphene,22 and outperforms most of the MoS2-based catalysts. Many studies have devoted to explain the mechanism of hydrogen revolution, upon which Tafel slope values can be used to determine which HER reaction path predominates for a catalyst.22 According to reported theory, a Tafel slope of 40 mV per decade is obtained for the MoS2/AC, suggesting the VolmerHeyrovsky mechanism of HER, which occurs via a fast discharge reaction (Equation (1)) and then a rate determining ion and atom reaction (Equation (2)).13,25 However, Pt catalyzed HER is
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Fig. 5 Durability test for the MoS2/AC. Negligible HER current was lost after 1000 cycles of cycle voltammogram from -0.4 to 0.1V vs. RHE. proceed through the Volmer-Tafel mechanism (Equation 1 and 3) with fast reaction rate and a Tafel slope of 30 mV per decade.38,39 Volmer reaction H3O+ + e + cat → cat-H + H2O (1) Heyrovsky reaction H3O++ e + cat-H → cat + H2 +H2O (2) Tafel reaction cat-H + cat-H → 2cat + H2 (3) The most inherent measure of catalytic activity for HER is the exchange current density (j0)40, which is obtained by fitting the logi-E data to the Tafel equation and using extrapolation methods. In detail, the 20% Pt/C reveals a j0 of 3.39 mA·cmG-2 and the MoS2/AC yields a j0 at 0.474 mA·cm-2, which is larger than most previous reports for MoS2-based catalysts (see ESI, Table S1 and Fig. S5†). The large j0 of 0.474 mA·cm-2, that is 1.67 A g−1 normalized by the loading weight, suggests superior catalytic activity and confirms the enrichment effect of the amorphous carbon. Stability is another important requirement of catalyst. As shown in Fig. 4, a long-term cycling test of the MoS2/AC was per formed. Only a slight activity loss was observed after 1000 cycles, indicating that the MoS2/AC is of good durability in a long-term electrochemical process. In addition, the catalytic performance of the catalyst ink stored for half a year is identical to that of fresh prepared, showing well stability under air atmosphere (see ESI, Fig. S2C, Fig. S4C†). 3.3 Discussion on the role of amorphous carbon
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As far as we know, the catalytic activity observed on the MoS2/AC is in line with MoS2/graphene and MoS2/CNT reported recently. 22,25 In this work, excellent catalytic activity results from not only the intrinsic property of MoS2 (∆GH = 0.08eV),17 but the amorphous carbon substrate. Several good properties are based on the amorphous carbon, such as the high edge-plain-ratio, the large specific surface area, the good conductivity, and the improvement of stability. First, the amorphous carbon significantly inhibit the aggregation of MoS2 nanosheets, thus increased the edge-plainratio of MoS2. Since the active sites of MoS2 for HER are proposed to be on the edges which have coordinate unsaturated Mo and/or S atoms, a high edge-plain-ratio can bring in more additional active edge sites as well as high catalytic activity. This journal is © The Royal Society of Chemistry [year]
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functional groups and MoS2 contributes to the catalysis stability to some extent. This conclusion is accord with the report that the combination of inorganic substances and carbon material shows excellent performance, due to chemical coupling effect and the electronic coupling effect.28 Therefore, the MoS2/AC exhibited enhanced stability, again highlighting the role of amorphous carbon as a novel substrate in improving the stability of the nanomaterials.
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In conclusion, we have presented a highly active electrocatalyst of MoS2/AC where the MoS2 nanosheets were formed vertically on amorphous carbon nanospheres via a facile and versatile approach. As an effective substrate, amorphous carbon inhibit the aggregation of MoS2 nanosheets, expose abundance catalytic edge sites and render MoS2/AC excellent conductivity, thus effectively enhanced the catalytic performance of MoS2 based HER electrocatalyst. The MoS2/AC exhibited superior catalyst activity for HER with an onset potential as low as 80 mV, extremely large cathodic current density and excellent stability, showing a great potential as a low cost alternative to Pt in practical application. Further, the synthesis strategy may inspire the researches for other applications, such as photoelectroncatclytic water splitting.
Acknowledgements Fig. 6 (A) Equivalent circuit model used to model the HER process of the studied system. (B) Nyquist plots of MoS2/AC and pure MoS2 modified GCE 5
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Second, the amorphous carbon improves the specific surface area, which is confirmed by SEM and BET analysis of between the MoS2/AC and pure MoS2 (see ESI, Fig. S2 and S6†). The N2 adsorption desorption characterization and pore size distribution (see ESI, Table S2†) show that MoS2/AC have a high specific surface area of 43.9 m2·g−1 and desorption average pore width of 14.4 nm, thus lots of microporous structure. Note that the ionic interchange at the catalyst surface is one of the key factors to the catalytic reaction41. Thus the large surface area and the microstructure, which actually stem from the formation of amorphous carbon nanoparticles, attribute to high catalytic efficiency. Comparisons of LSV and Tafel plot between the MoS2/AC and pure MoS2 prove this conclusion. Third, the amorphous carbon improves the conductivity of MoS2/AC, which can be proved by EIS measurement measured in 0.5 M H2SO4 (Figure 6). The unannealed MoS2 shows larger charge-transfer resistance than MoS2/AC. The improvement of conductivity also benefits from the annealing process. Comparison on the Nyquist plots of the EIS response between the MoS2/AC and pure MoS2 on GCE shows that the MoS2/AC has lower charge-transfer resistance, illustrating the superior electrocatalytic activity of the MoS2/AC. Finally, the amorphous carbon supporter can stabilize the disordered structure of the MoS2 nanosheets throughout the polarization cycles. The residual oxygen-containing functional groups on the amorphous carbon are confirmed by 8.46w% of oxygen (see ESI, Fig. S7†), providing sufficient functional groups on carbon surface to afford intimate interactions with MoS2. The coupling effect between the oxygen-containing This journal is © The Royal Society of Chemistry [year]
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This work was financially supported by the National Natural Science Foundation of China (No. 21175124), the National Key Basic Research Development Project of China (No. 2010CB933602).
Notes and references a
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State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China. E-mail: [email protected] b University of the Chinese Academy of Sciences, Beijing 100049, China. † Electronic Supplementary Information (ESI) available: Experimental details, additional data and discussions. See DOI: 10.1039/b000000x/ 1 Dresselhaus, M. S.; Thomas, I. L. Nature 2001, 414, 332. 2 Chen, W.-F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Angewandte Chemie International Edition 2012, 51, 6131. 3 Fang, B.; Kim, M.-S.; Kim, J. H.; Song, M. Y.; Wang, Y.-J.; Wang, H.; Wilkinson, D. P.; Yu, J.-S. Journal of Materials Chemistry 2011, 21, 8066. 4 Zeng, Z.; Tan, C.; Huang, X.; Bao, S.; Zhang, H. Energy & Environmental Science 2014, 7, 797. 5 Yang, J.; Voiry, D.; Ahn, S. J.; Kang, D.; Kim, A. Y.; Chhowalla, M.; Shin, H. S. Angewandte Chemie International Edition 2013, 52, 13751. 6 Karunadasa, H. I.; Montalvo, E.; Sun, Y.; Majda, M.; Long, J. R.; Chang, C. J. Science 2012, 335, 698. 7 Kleingardner, J. G.; Kandemir, B.; Bren, K. L. Journal of the American Chemical Society 2013, 136, 4. 8 Hu, X.; Brunschwig, B. S.; Peters, J. C. Journal of the American Chemical Society 2007, 129, 8988. 9 Wiese, S.; Kilgore, U. J.; DuBois, D. L.; Bullock, R. M. ACS Catalysis 2012, 2, 720. 10 Zhuo, J.; Wang, T.; Zhang, G.; Liu, L.; Gan, L.; Li, M. Angewandte Chemie International Edition 2013, 52, 10867. 11 Li, Y.; Somorjai, G. A. Nano Letters 2010, 10, 2289. 12 Wu, Z.; Fang, B.; Wang, Z.; Wang, C.; Liu, Z.; Liu, F.; Wang, W.; Alfantazi, A.; Wang, D.; Wilkinson, D. P. ACS Catalysis 2013, 2101.
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