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One Pot Assembly of Vertical Embedded MoS2/Graphene Heterostructure and Its High Performance for Hydrogen Evolution Reaction Liang Li, Jinxin Li, Lili Liu, Xinran Wang, Ying Guo, and Yajun Zhou ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01993 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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ACS Applied Energy Materials

One Pot Assembly of Vertical Embedded MoS2/Graphene Hetero-structure and Its High Performance for Hydrogen Evolution Reaction Liang Li,* Jinxin Li, Lili Liu, Xinran Wang, Ying Guo and Yajun Zhou Laboratory for Low Dimensions Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China. * E-mail: [email protected]; Fax: +86-021-64250740; Tel: +86-021-64252599

ABSTRACT Two-dimensional molybdenum disulfide has great potential to replace rare precious metals for   hydrogen evolution reaction. Herein, we report a one pot assembly strategy for in-situ growing vertical embedded MoS2 nanocrystals on graphene surface. The resultant sandwiched architecture could not only expose more active sites within MoS2 edge, but also reduce stacking tendency of the layer-structured graphene support, leading to a significant improvement in HER performance. The materials showed an over-potential of 142 mV at 10 mA·cm−2 and a Tafel slope of 68 mV·dec−1, which are among the best results ever obtained by the similar materials. Moreover, this method is facile, environmental friendly and suitable for mess production, making it attractive for practical applications. KEYWORDS: MoS2, Graphene, Sandwiched structure, Hydrogen evolution reaction, Electrocatalysis 1. INTRODUCTION Hydrogen has the largest energy density over any other fuel in the world and possesses all the advantages of green renewable energy.1,

2

Accordingly, with the depletion of fossil fuels and the

continued increasing of public awareness for environmental protection, hydrogen produced from water by the hydrogen evolution reaction (HER) has recently been emerged as a key technology for green energy economy.3,

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At present, the mainly used electrode material for HER is platinum catalyst.

Unfortunately, the high cost and rare nature abundance restrict its large-scale applications. Only hydrogen production on a large scale at low cost could realize its destiny as an alternative energy

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source to fossil fuels. Therefore, much effort have been devoted to explore other cheap and abundant materials which are also very active for HER to replace platinum catalyst.5-7 Molybdenum disulfide (MoS2) is a kind of layered structure semiconductor. Study found that MoS2 with more exposed edge sites is a good catalyst candidate for HER, which may be comparable with platinum.4, 5, 8, 9 Effective methods have been developed to expose the edge sites of MoS2 as many as possible.10,

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Moreover, highly conductive materials, such as reduced graphene oxide (RGO),

multi-walled carbon nanotube (CNT) and carbon fiber paper, etc. have been used as support for MoS2 catalyst to increase the charge transport from the conductive support to active sites and enhance the efficiency of electro-catalysis.12-15 Even with these catalyst-design engineering effort, however, due to its layered structure and intrinsically restacking nature, the active edge sites of crystalline MoS2 are hardly exposed enough for HER, resulting in a relatively lower catalytic activities. Additionally, the organic solvent used in the synthesis of MoS2 may bring a lot of environmental problems during large scale production.16 Green synthesis of MoS2 composites with high catalytic activities for HER is still remain a challenge. Herein, we propose a simpler, in-situ reduction approach for synthesis of sandwiched MoS2/Graphene composite with huge number of exposed MoS2 active sites for HER only using water as a solvent. In this protocol, amorphous MoS3 thin layer was firstly fabricated on the both surfaces of the layer-structured graphene oxide. After in-situ reduction under hydrothermal condition, the resultant ultrathin MoS2 nanocrystals were homogeneously and vertically anchored on the graphene surface to form sandwiched MoS2-nanocrystal/Graphene hetero-structure. This one pot assembly strategy not only offers high density of exposed active edge sites from MoS2-nanocrystals, but also guarantees efficient charge flow from the highly conductive graphene support to active surface sites due to the high intra-layer conductivity, which cannot be easily achieved by other method. The resulting materials shows excellent catalytic activity for HER. At a current density of 10 mA·cm-2, the over-potential is only 142 mV and the Tafel slope is 68 mV·dec-1. More importantly, this method is facile, environmental friendly and suitable for mass production, making it very attractive for practical application. 2. EXPERIMENTAL 2.1 Chemicals and Reagents 2 / 15


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All chemicals were purchased from Shanghai Chemical Reagent Co. Ltd. (Shanghai, China). The reagents were of analytical grade and used as received without any further purification. 2.2 Synthesis of GO colloidal Graphene oxide colloidal was synthesized by a modified Hummer’s method. In a typical synthesis. 1 g graphite powder, 4 ml deionized water, 46 ml concentrated H2SO4 and 3.0 g KMnO4 were add into 250 ml three-necked flask. After reacted under stirring at 40 oC for 120 min, the mixture was cooling with ice-bath. Then, 300 ml deionized water and 5 ml of H2O2 (30%) were added under stirring and reacted for another 15 min. The reactant was filtered and washed with 1:10 HCl solution. The solid substance was evenly dispersed within 500 ml deionized water. The graphene oxide colloidal could be finally obtained after dialyzed for 7 days to eliminate the residual impurity ions. 2.3 Synthesis of the sandwiched MoS2/Graphene composite 22.5 mg of (NH4)2MoS4 was dispersed in 5 ml deionized water, 7.5 ml colloidal contain 12.5 mg of graphene oxide was added under stirring until a black homogeneous slurry was achieved. Then, 3 ml 1 M HCl was added under stirring. After centrifugation and washing thoroughly, the solid was freeze-drying overnight to get MoS3/Graphene oxide composite. 8 mg ascorbic acid was added into above mentioned MoS3/Graphene-oxide slurry under stirring. The mixture was then transferred into a 100 ml autoclave and kept at 180 °C for 10 h. After centrifugation and washing thoroughly, the solid was freeze-drying overnight to get MoS2/Graphene composite. 2.4 Materials Characterization Powder X-ray diffraction (XRD) analysis was performed on Bruker D8 advance X-ray diffraction (XRD) system with Cu Kα radiation (λ = 0.154 05 nm) at 40 kV, 40 mA. The morphology analysis was conducted using a JEOL JEM-2100F high resolution transmission electron microscope (HRTEM) as well as a JSM-7001F field emission scanning electron microscope (FESEM) equipped with an energy dispersive spectroscopic (EDS) detector. X-ray photoelectron spectroscopy (XPS) spectra were collected on a VG Micro MK II instrument using monochromatic Al Kα X-rays as the excitation source. The elemental binding energy calibration was referenced to the C (1s) line signal at 284.6 eV. Raman spectra were obtained on a LabRAM HR800 spectrometer. A laser beam with wavenumber of 514.5 nm was used for excitation. 3 / 15



2.5 Electrochemical Measurements Electrochemical measurements were performed on a CHI 760E electrochemical workstation in a standard three-electrode mode, saturated calomel electrode (SCE) and graphite rod were used as the reference and counter electrodes, respectively. All potentials throughout this paper were calibrated to the reversible hydrogen electrode (RHE) scale. An ink-coated glassy carbon electrode (6 mm in diameter) served as the working electrode. A typical ink-coating procedure was as follows. 5 mg catalyst and 50 ul 5 wt % Nafion solution were mixed with 500 μl absolute ethanol by sonication for 60 min to form a homogeneous ink. After that, 20 μl of the ink was loaded onto a glassy carbon electrode and dried at room temperature naturally. The loading density of the catalyst was about 0.701 mg·cm-2. Linear sweep voltammetry (LSV) was conducted in the range of + 100 to - 300 mV at a sweep rate of 5 mV·s−1 in 0.5 M H2SO4 solution. The over-potentials were obtained at a reduction current density of 10 mA cm−2. To investigate the stability of the catalysts, the time-dependent current density curve was performed under the over-potential of -40 mV. The electrochemical impedance spectroscopy (EIS) was collected within the frequency range from 0.01 Hz to 105 Hz under the overpotential of - 40 mV.

3. RESULTS AND DISCUSSION 3.1 Structure and compositional characterization

Figure 1. Schematic illustration for the formation of sandwiched MoS2/Graphene hetero-structure

The typical synthetic procedure for sandwiched MoS2/graphene hetero-structure is illustrated in Figure 1. SEM and TEM analysis were employed in the experiment to investigate the morphology and composition changes during the fabrication procedure. When graphene oxide colloidal was mixed with 4 / 15


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(NH4)2MoS4 solution, the carbonyl group on the surface of graphene oxide was firstly served as initial active site to form MoS3 quantum dot immediately based on the reaction: (NH4)2MoS4+2H+ →MoS3↓+2NH4++H2S↑ Sequential treatment with HCl aqueous solution produced a plate-like morphology with smooth surface and increased thickness when compared with original bared GO nanosheet, as showing in the SEM images (Figure 2a, 2b). EDS mapping analysis (Figure S1) clearly indicates the uniformly dispersed Mo and S species on the GO surface. Further TEM and HRTEM analysis (Figure 3a, 3c) clearly prove that something with regular structure are evenly dispersed on the GO surface. Moreover, select area diffraction analysis in the same area proves an amorphous structure. Combined with the decomposition reaction of (NH4)2MoS4 under acidic condition and the chain structure of amorphous MoS3, it may be concluded that a thin layer of amorphous MoS3 could be gradually formed around aforementioned quantum dot. Similar morphology and structure had also been reported in other MoS3 grafted materials.17-19 When in-situ reduction with ascorbic acid under hydrothermal condition, amorphous MoS3 was turned to MoS2 nanocrystals and graphene oxide support was simultaneously reduced to graphene, resulting in the sandwiched MoS2-nanocrystal/graphene hetero-structure. Figure 2c shows the SEM image after reduction. The original flat and smooth surface become wrinkled, and a lot of small sheet-like materials with obviously exposed edge could be found in the image. High angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and EDS mapping analyses (Figure 2d-g) clearly reveal the uniformly dispersed and configured Mo and S species on the surfaces of graphene. Layered materials with inter-layer spacing of 0.65 nm could be easily found in HRTEM image (Figure 3d), which is in well accordance with that of MoS2 layered structure, confirming the vertical embedded nano-crystals of MoS2.13,

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The average crystal size collected from over 10

different particles yielded an average value of 5.1 nm. The formation of vertically embedded MoS2 structure may be originated from the in-situ reduction of MoS3 on the GO surface. During reduction process, the original parallel array of MoS3 chain was in-situ reduced by ascorbic acid and produced a parallel array of line-like MoS2 seed. Under hydrothermal condition, the MoS2 nano-crystalline grew along the direction perpendicular to the graphene support, resulting in the vertically embedded MoS2 nano-crystalline. This vertical embedded architecture can ensure much exposed edge of MoS2 and 5 / 15



excellent electron transfer between the highly conductive graphene support to active surface sites, thus, resulting in the significantly enhanced catalytic activity for HER.

Figure 2. SEM images of GO (a), MoS3/GO composite (b), MoS2/Graphene composite (c); HAADF-STEM image of MoS2/Graphene composite (d) and corresponding EDS mapping images for C (e), Mo (f) and S (g) elements.

Figure 3. TEM and HRTEM images of MoS3/GO composite (a, c) and MoS2/Graphene composite (b, d).

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Figure 4. XRD patterns (a) and Raman spectra (b) of MoS3/GO, MoS2/Graphene and amorphous MoS3

The formation of MoS2-nanocrystal/graphene hetero-structure also can be confirmed by X-ray diffraction and Raman analysis. Figure 4a presents the variation of XRD patterns before and after reduction. Generally, MoS3 possesses an amorphous structure, it does not show any diffraction peak except a hole located at 2 theta value of about 20 o, as shown in Figure 4a. However, a broadened diffraction peak emerged at about 14

o

after reduction procedure. This value is correspond well with

(002) diffraction peak of MoS2, confirming the successful reduction process. The estimate MoS2 crystal particle size by Scherrer equation is only about 5.0 nm, in well agreement with that of HRTEM analysis. It should be noted that small crystal size and vertical arrangement are two important factors to ensure enough active edge sites for HER. Figure 4b shows the Raman spectra for the composites. The Raman peaks at about 378.5 and 403.8 cm-1 can be ascribed to the vibration modes of Mo-S, indicating the existence of molybdenum sulfide.13, 21, 22 On the other hand, the G and D band form GO support is located at 1598 cm-1 and 1356 cm-1, respectively. When treatment with ascorbic acid under hydrothermal condition, D and G band become sharp and have a slight shift (at 1587 and 1353 cm-1, respectively). Generally, GO contains a certain fraction of sp3 carbon which leads to a broader G and D bands as showing in Figure 4b. When treatment with ascorbic acid under hydrothermal condition, the reduction of GO leaves behind the most of topological defects. The D and G bands become sharp and display a shift to lower frequencies as reported in the literatures.23-25 Besides, the peak intensity ratio of the D and G band (ID/IG) was always used to determine the disorder degree of the graphite materials.26, 27 An increased ID/IG ratio could also be found in the Raman spectra, clearly proves the simultaneous reduction of GO. High conductivity of the graphene support is also a key factor to ensure the high catalytic activity of the composite for HER. 7 / 15



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Figure 5. XPS survey (a) and high-resolution spectra for C 1s (b), Mo 3d (c) and S 2p (d).

XPS analysis was used to determine the surface chemical nature and bonding state of the prepared MoS2-nanocrystal/graphene hetero-structure. XPS survey and high-resolution XPS spectra for C 1s, Mo 3d and S 2p were recorded as shown in Figure 5. To better understanding the reaction process during synthesis procedure, the corresponding spectra for the intermediate, MoS3/GO, was also presented. XPS survey (Figure 5a) clearly revealed the existence of Mo, C, S and O element. In Figure 5b, the high resolution spectrum of C 1s could be deconvoluted into three peaks at 284.6, 285.6 and 287.3 eV presenting for C=C, C-O, and –COO-, respectively, according to the literatures.28,

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The

almost disappeared –COO- binding energy for MoS2/Graphene composite clearly indicates the successful reduction process, corresponding well with the above Raman analysis. In Mo 3d region, the two sharp and strong binding energy located at 229.6 and 232.7 eV could be assigned as Mo 3d 5/2 and Mo 3d 3/2 of Mo+4, respectively. There are also exist some high valence state Mo ions (236.0 eV for Mo6+ and 231.0 eV for Mo5+), which may be formed during preparation process. However, the mole ratio of high valence state Mo ion presents obviously decrease after reduction. Besides, S 2s electrons have a closed binding energy and also locate at this region at 227.0 eV. Figure 5d depicts the S 2p spectra with a broad and complex appearance. It consists of two doublets after deconvolution, suggesting the existence of two kinds of S ligands according to the literatures.30, 8 / 15


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The doublet

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centered at binding energy of 161.9 and 163.3 eV corresponds to the S 2p3/2 and S 2p1/2 of S2respectively. Meanwhile, the doublet at relative higher binding energy could be ascribed to the S22ligands of bridging and/or terminal S2- species.32-34 After reduction, the doublet at lower binding energy become predominant indicating the successful transmission from MoS3 to MoS2.

3.2 Electrocatalytic activity and stability toward HER

Figure 6. (a) LSV polarization curves; (b) The corresponding Tafel plots; (c) EIS spectra of MoS2 and MoS2/Graphene composite; (d) Time-dependent current density curves of MoS2/Graphene composite under the static overpotential of 10 mV

The HER activity of the as-synthesized vertical embedded MoS2/Graphene composite was investigated using 0.5 M H2SO4 as electrolyte. Figure 6a depicts the linear sweep voltammetry (LSV) polarization curve of MoS2/Graphene composite within a cathodic potential window of 0.1 to - 0.3 V. The HER catalytic activities of MoS2 (synthesized with same acidification and reduction method) and commercial 20% Pt/C catalyst are also provided for comparison. Obviously, the commercial Pt/C catalyst possesses the highest HER activity while the MoS2 has nearly no HER activity within the potential window. The MoS2 material from direct reduction of MoS3 has a relative lower catalytic 9 / 15



activity for HER, which may concern with the coexistence of sulfur within its structure. Surprisingly, when embedded on the surface of graphene, the HER preference had a significant improvement, as shown in Figure 6A. The HER performance could take place at a much smaller onset potential (~ 60 mV) over the as-synthesized MoS2/Graphene composite. Particularly, a current density of 10 mA·cm-1 could be achieved at an over-potential of 142 mV. All these data are among the best results ever obtained by the similar materials, indicating the better electro-catalytic properties of sandwiched MoS2 /Graphene hetero-structure.35-40 The improved HER catalytic activity certainly could be attributed to the synergy between MoS2 nanocrystal and graphene support as well as the increased amount of active site within the vertical embedded MoS2 nanoparticles. Moreover, to further explore catalytic activity and understand the detailed reaction mechanism of HER, the Tafel plots and exchange current density were also carefully investigated in the experiment. As shown in Figure 6b, the Tafel plot of the sandwiched MoS2/Graphene composite presents the slope of 68 mV·dec-1, which proposes the mixed Volmer-Heyrovsky mechanism for HER. The Tafel plots also indicate that both H3O+ discharging and desorption of Had to form molecular H2 determine the HER rate.41 Meanwhile, the exchange current density obtained from Tafel equation are 11.8 mA·cm-2 for pure MoS2 and 120 mA·cm-2 for the sandwiched MoS2/Graphene composite, respectively. Obviously, the simultaneously reduced graphene oxide support and tightly vertical embedded MoS2 architecture promote the charge transfer during HER process. In addition, electrochemical impedance spectroscopy (EIS) measurement was performed at an over-potential of - 40 mV to further investigate the charge transfer resistance for HER. As shown in the Nyquist plot (Figure 6c), the charge transfer resistance (Rct) were attributed to the semicircle recorded at the identical over-potential. The pure MoS2 presents much larger Rct due to its intrinsic semiconductor properties. On the contrary, the as-synthesized sandwiched MoS2/Graphene composite exhibited lower Rct (20.8 Ω). Even compared with other MoS2/graphene composites reported before, the Rct is still among the relative lower group.37, 42 Generally, higher charge transportation from active surface sites to the conductive support could promote the efficiency of electro-catalysis. This result is corresponding well with above exchange current density analysis. The turnover frequency (TOF) of the as-synthesized sandwiched MoS2/Graphene composite for HER was also calculated for comparison, which could give the direct evidence for the catalytic activity 10 / 15


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of the composite. The number of active sites was quantified by the electrochemical method according to literatures.36,

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Under the over-potentials of 150 mV and 200 mV, the TOF value of the MoS2/

Graphene composite are 33.7 and 118.7, respectively. These data are much higher than that reported before under the same conditions, suggesting a remarkable improved catalytic activity of the sandwiched MoS2/Graphene catalyst for HER. As another important factor for a good HER catalyst, the electrochemical catalytic stability of the as-synthesized sandwiched MoS2/Graphene composite was evaluated in 0.5 M H2SO4 solution. Figure 6d presents the time-dependent current density curve under the static over-potential of 10 mV. It clearly shows that the current density remained almost unchanged for 8000 s, implying the relative high stability of the catalyst for HER. Combined with the structural analysis, the high HER catalytic activity of the as-synthesized vertical embedded MoS2/Graphene hybrid may originate from its unique sandwiched architecture and in-situ grown strategy. (1) The vertical embedded MoS2 nanocrystals on the graphene support could supply enough active sites within its edges during HER performance. (2) The sandwiched structure of the composite could not only take full advantage of high surface area of the graphene support, but also reduce its stacking tendency. Thus, the effective contact between active sites and electrolyte could be readily achieved for HER. (3) The in-situ synthetic strategy of the composite could form strong coordination between MoS2 nanocrystal and graphene support, and promote the charge transfer between them and enhance the efficiency of electro-catalysis. All these positive effect for HER cannot be easily fully achieved by other methods. 4. CONCLUSION In summary, a simpler, environmental friendly strategy for in-situ growing vertical embedded MoS2 nanocrystals on graphene surface was presented. The resultant sandwiched architecture not only expose more active sites within MoS2 edge, but also reduce stacking tendency of the layer-structured graphene support, leading to a significant improvement for HER performance. Electrochemical measurements revealed that the MoS2/Graphene composite exhibited ultrahigh catalytic activities for HER. The materials showed an over-potential of 142 mV at 10 mA·cm−2 and a Tafel slope of 68 mV·dec−1, which are among the best results ever obtained by the similar materials. Moreover, this method is facile, environmental friendly and suitable for mess production, making it very attractive for practical applications. 11 / 15



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ACKNOWLEDGEMENT ASSOCIATED CONTENT Supporting Information: Synthesis of MoS2 catalyst; XRD, Raman spectra, SEM and TEM images of MoS2; EDS mapping for the MoS3/GO composite; Comparison of HER performance over different catalysts reported in the literatures; TEM image and XRD pattern of MoS2/Graphene composite after long term durability test; LSV polarization curves of the MoS2/Graphene composites with different graphene content; LSV polarization curve of MoS2 catalyst; Electric double layer capacitor of MoS2/Graphene composite and MoS2; Calculation of turn over frequency (TOF); Calculation of exchange current density; Calculation of charge transfer resistance (Rct); Reference (1) Service, R. F. Bringing Fuel Cells Down to Earth. Science 1999, 285, 682-685. (2) Deng, J.; Li, H.; Wang, S.; Ding, D.; Chen, M.; Liu, C.; Tian, Z.; Novoselov, K. S.; Ma C. Deng, D. and Bao, X. Multiscale Structural and Electronic Control of Molybdenum Disulfide Foam for Highly Efficient Hydrogen Production. Nat. Commun. 2017, 8, 14430.   (3) Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28,141-145. (4) Maeda, K.; Teramura, K.; Lu, D.; Takata, T. Saito, N.; Inoue, Y.; Domen, K. Photocatalyst Releasing Hydrogen from Water. Nature 2006, 440, 295-295. (5) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296–7299. (6) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.;

Schaak, R. E .

Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135. 9267-9270. (7) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles. Angew. Chem. Int. Ed. 2014, 53, 5427-5430. (8) Ren, X.; Ma, Q.; Fan, H.; Pang, L.; Zhang, Y.; Yao, Y.; Ren, X.; Liu, S. A Se-doped MoS2 Nanosheet for Improved Hydrogen Evolution Reaction. Chem. Commun. 2015, 51, 15997–16000. (9) Cui, Z.; Chu, H.; Gao, S.; Pei, Y.; Ji, J.; Ge, Y.; Dong, P.; Ajayan, P. M.; Shen, J.; Ye, M. Large-scale Controlled Synthesis of Porous Two-dimensional Nanosheets for the Hydrogen Evolution Reaction through a Chemical Pathway. Nanoscale 2018, 10, 6168–6176. (10) Chung, D. Y.; Park, S. K.; Chung, Y. H.; Yu, S. H.; Lim, D. H.; Jung, N.; Ham, H. C.; Park, H. Y.; Piao, Y.; Yoo, S. J.; Sung, Y. E. Edge-exposed MoS2 Nano-assembled Structures as Efficient Electrocatalysts for Hydrogen Evolution Reaction. Nanoscale 2014, 6, 2131-2136. (11) Zhang, N.; Gan, S.; Wu, T.; Ma, W.; Han, D.; Niu, L. Growth Control of MoS2 Nanosheets on Carbon Cloth for Maximum Active Edges Exposed: An Excellent Hydrogen Evolution 3D Cathode. ACS Appl. Mater. Interfaces 2015, 7, 12193-12202.

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