Supporting Information Hydrogen Evolution Reaction on Hybrid Catalysts of Vertical MoS 2 Nanosheets and Hydrogenated Graphene Xiuxiu Han,, Xili Tong,,* Xingchen Liu, Ai Chen, Xiaodong Wen, Nianjun Yang,,,* Xiang-Yun Guo State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China University of Chinese Academy of Sciences, Beijing 100049, China The First Hospital, Shanxi Medical University, Taiyuan 030001, China Institute of Materials Engineering, University of Siegen, Siegen 57076, Germany EMAIL ADDRESS: tongxili@sxicc.ac.cn (X. T.), nianjun.yang@uni-siegen.de (N. Y.) Table of Content Experimental details S2 Schemes S3 Figures S4-S13 Tables S14-S16 Supporting references S17 S1
EXPERIMENTAL DETAILS Synthesis of hydrogenated graphene 50 mg graphite powders were put into a porous ceramic tube. A platinum wire inserted into graphite powder was applied as the cathode. A platinum flake was used as the anode. Both electrodes were immersed in 20 ml propylene carbonate (PC) solution with 0.1 M tetrabutylammonium hexafluorophosphate. To obtain exfoliated graphite powders, a voltage of 30 V was applied for 4 h. Then the exfoliated graphite powder was dispersed in N, N-dimethylformamide (DMF) solution and washed with DMF through centrifugation at 13 000 rpm for 5 times. To further separate graphitic material, the obtained graphite powders were dispersed again in DMF via mild sonication for 15 min, and the suspension was subjected to centrifugation at 4000 rpm for 10 min. HG were collected with a 100 nm porous filter by vacuum filtration and washed with deionized water, and subsequent dried overnight at 60 C in vacuum oven. Measurements of double layer capacitances Double layer capacitances (Cdl) of the MoS2/HG, MoS2/RGO, and MoS2 catalysts were obtained by cyclic voltammetric experiments in the potential range of 0 to 0.3 V vs. RHE in 0.5 M H2SO4 at the scan rates of 2, 4, 6, 8 and 10 mv s 1. The working electrode was held at each potential vertex for 20 s prior to next cycles. Conversion between the SCE and the RHE The following equation was applied ERHE ESCE 0.059 ph E θ SCE (Eq. S1) where ERHE is the converted potential versus RHE, ESCE is the experimental potential measured against the SCE, and E θ SCE is the standard potential of SCE at 25 C (246 mv). The electrochemical measurements were carried out in 0.5 M H2SO4 solution (ph 0) saturated with high purity H2 at room temperature, thus ERHE (mv) ESCE (mv) 246 (mv). S2
SCHEMES Scheme S1. Schematic illustration of the synthesis of the MoS2/HG hybrid catalyst and HER on the MoS2/HG hybrid catalyst. The synthesis of the MoS2/HG hybrid catalysts is schematically illustrated in Scheme S1. In the first step, HG is prepared by electrochemical reduction exfoliation from commercial graphite powders. Then, MoS2 nanosheets are vertically grown on the surface of HG via a solvothermal synthesis method. In this step, (NH4)2MoS4 is reduced by hydrogens from HG, accompanying the restoration of conjugated hexatomic rings of carbon. Finally, hydrogen both from the electrolyte and the surface of HG is adjacent to MoS2 and further transfers to the S edge sites and Mo sites in Mo-C bonds of MoS2, leading to the occurrence of hydrogen evolution, namely a HER process. S3
FIGURES Figure S1. (A, C) SEM and (B, D) TEM images of HGs prepared using a cathodic electrochemical exfoliation approach and RGO obtained from GO using a thermal reduction approach. S4
Figure S2. The volume and color changes of the natural graphite powders before and after cathodic electrochemical exfoliation. The natural graphite powders exhibit dark grey with metallic luster. After cathodic exfoliation, hydrogenated graphene (HG) is formed. It is in black purely and has enlarged volume. Figure S3. (A) SEM and (B) TEM images of MoS2/RGO. MoS2 nanosheets stack into spheres with the minimum diameter of 100 nm. This demonstrates that MoS2 nanosheets are not stable and cannot disperse uniformly on the surface of RGO (Figure S3-A, B). The layer number of MoS2 is more than 10 (inset in Figure S3-B). S5
Figure S4. (A) SEM and (B) TEM images of pure MoS2. The free-standing MoS2 is tightly aggregated into microspheres in various diameters and coalesced together. Figure S5. Energy dispersive spectrum of the MoS2/HG hybrid catalyst. A ratio of 1:2.1 is obtained for the amount of Mo to that of S, approximate the stoichiometry of MoS2. S6
Figure S6. Raman mapping of the MoS2/HG hybrid catalyst. The spots marked with red circles represent MoS2 nanosheets. It is clear that MoS2 nanosheets are uniformly distributed on the surface of HG. Figure S7. Infrared absorption spectra of HG sheets, MoS2/HG, GO, and MoS2/RGO. The region of C- H stretching vibrations is highlighted with dashed lines. The bonds in the range from 2850 to 3000 cm 1 are obviously decreased after solvothermal reaction, indicating that some C-H bonds undergo the chemical transformation during the solvothermal synthesis process. S7
Figure S8. XPS spectra of the MoS2/RGO catalyst: (A) survey spectrum and (B-D) high-resolution spectra for (B) C 1s, (C) Mo 3d, and (D) S 2p. In (B-D), the solid black lines are experimental results, while the red and the dashed lines are simulated ones. The solid grey lines are background. The fitting parameters of (B) C 1s, (C) Mo 3d, and (D) S 2p are shown in Table S2. S8
Figure S9. (A) Linear sweep voltammograms at a sweep rate of 5 mv s 1 in 0.5 M H2SO4 solution for the MoS2/HG hybrid catalysts with a MoS2 weight ratio of 6.3% (blue curve), 13.8% (purple curve), 27.5% (red curve), and 36.5% (black curve). (B) SEM images of these hybrid catalysts. Figure S10. Fitted exchange current densities (j0) for MoS2 (blue line), MoS2/RGO hybrid (green line), and MoS2/HG (red line) catalysts. S9
Figure S11. Calculated turnover frequency (TOF) for MoS2 (blue line), MoS2/RGO hybrid (green line), and MoS2/HG (red line) catalysts. The per-site TOF (s 1 ) calculated according to the method shown in the literature S1-S3 was employed to explore the intrinsic catalytic power of the MoS2/HG and MoS2/RGO catalysts. The calculated TOF of the MoS2/HG hybrid catalyst for each active site was 7.8 s at the potential of 220 mv, much higher than MoS2 and MoS2/RGO catalysts, indicating better intrinsic catalytic activity of the MoS2/HG hybrid catalyst. S10
Figure S12. Cyclic voltammograms of (A) MoS2/HG, (B) MoS2/RGO, and (C) MoS2 based films at scan rates from 2 to 10 mv s 1 in 0.5 M H2SO4. For all catalysts, only capacitive currents are observed. (D) Estimation of Cdl by plotting the current density variation (Δj ( ja jc ) at 150 mv vs. RHE) against scan rate. After extracting the Cdl from the fitted linear regressions, we cancalculated the electrochemically active surface area (EASA) from the equation of EASA Cdl Cs, where Cs are assumed to be same for all electrodes. S4-S5 Thus, the variation trend of EASA is in line with that of Cdl. S11
Figure S13. Thermogravimetric analysis of the MoS2/HG hybrid catalyst (red) under O2 atmosphere at a heating rate of 5 C min 1 coupled with mass spectrometry (MS). The outlet gases decomposed from MS are H2O (blue), SO2 (olive), and CO2 (magenta). Figure S14. TEM (A) and HRTEM (B) images of the MoS2/HG catalysts after 24 h continuous HER process in 0.5 M H2SO4 solution. Negligible changes are noticed in the morphology and structure of the MoS2/HG catalysts, comparing with the freshly synthesized MoS2/HG catalysts. These results confirm superior stability of the MoS2/HG catalysts in strong acidic condition. S12
Figure S15. The top view of the optimized structure of MoS2 on HG. Dark cyan, yellow, white, grey, and blue spheres denote for Mo, S, H, C, and C with the ferromagnetism, respectively. Figure S16. H2-TPD experiments using MoS2/HG, MoS2/RGO, MoS2, and commercial 20 wt% Pt/C catalysts. H2-TPD measurements show the hydrogen desorption temperature (~370 C) on the MoS2/HG hybrid catalyst is higher than that (~310 C) on the MoS2/RGO catalyst and that (~310 C) on the MoS2 catalyst. These results indicate that the hydrogen adsorption energy of the MoS2/HG hybrid catalyst becomes observably strengthened compared with that for the MoS2 and MoS2/RGO catalysts. Moreover, the hydrogen desorption temperature on the MoS2/HG hybrid catalyst is much closer to that (~410 C) on 20 wt% Pt/C in comparison with that on the MoS2/RGO catalysts. These facts suggest the optimized hydrogen adsorption energy in the MoS2/HG hybrid catalyst. S13
TABLES Table S1: Fitting results of XPS spectra of the MoS2/HG hybrid catalyst in Figure 3. C 1s: Peak Label Position (ev) FWHM (ev) Area C=C 284.8 1.05 3730 C-C 285.4 1.05 1865 -COO 289.1 1.9 771 Mo 3d: Peak Label Position (ev) FWHM (ev) Area % Conc. +4 228.8 1.1 6981 41 +4 232 1.1 4654 27 +5 229.9 1.7 2231 13 +5 233.2 1.7 1487 9 +6 232.5 1.9 994 6 +6 235.6 1.9 663 4 S 2s 226.1 2.24 3474 S 2p: Peak Label Position (ev) FWHM (ev) Area S 2-161.7 1.08 2539 S 2-162.8 1.08 1270 S2 2-163 1.49 1542 S2 2-164.1 1.49 771 Sulfate species 168.9 2.05 328 FWHM: full width half maximum. S14
Table S2: Fitting results of XPS spectra of the MoS2/RGO catalyst in Figure S8. C 1s: Peak Label Position (ev) FWHM (ev) Area C=C/C-C 285 1.35 11265 -C-O 286.1 1.35 2897 C=O 287.2 1.08 728 -COO 289.1 1.03 1435 Mo 3d: Peak Label Position (ev) FWHM (ev) Area % Conc. +4 228.8 1 8393 37 +4 232 1 5607 24 +5 229.9 1.9 3487 15 +5 233 1.9 2326 10 +6 232.8 1.25 1928 8 +6 235.9 1.25 1274 6 S 2s 226.2 2.34 4316 S 2p: Peak Label Position (ev) FWHM (ev) Area S 2-161.7 1.19 3523 S 2-162.8 1.19 1762 S2 2-163. 1.12 1396 S2 2-164.1 1.12 698 Sulfate species 168.8 1.58 576 FWHM: full width half maximum. S15
Table S3. Comparison of HER performance on different catalysts in 0.5 M H2SO4. Catalyst m / mg cm 2 j0 / ma cm 2 j0/m / ma mg 1 η * / mv Tafel slope / mv dec 1 Defect-rich MoS2 nanosheets 0.285 0.009 0.0312 195 50 S6 Stepped edge MoS2 sheet 3.200 0.070 0.0219 104 59 S7 Cu7S4@MoS2 nanoframe 0.280 0.019 0.0679 133 48 S8 Mo2C/CNT 2.000 0.014 0.07 152 55 S9 MoS2/CoSe2 hybrid 0.280 0.073 0.2607 68 36 S10 MoS2/RGO-1 0.200 / / 165 41 S11 MoS2/RGO-2 0.280 / / 160 41 S12 MoS2/CNS 0.318 0.003 0.0082 200 53 S13 MoSx/GO 0.318 0.003 0.0100 220 48 S3 ET&IE MoS2 0.280 0.010 0.0357 150 49 S14 MoSx-NCNT 0.102 0.033 0.3235 110 40 S15 MoC-Mo2C 0.140 0.011 0.0786 126 43 S16 Co9S8@MoS2/CNFs 0.212 / / 190 110 S17 MoS2 HG 0.127 0.019 0.1496 124 41 This work m: catalyst loading density; j0: exchange current density; * The values of η were obtained at the current density of 10 ma cm -2. Defect-rich MoS2 nanosheets: defect-rich MoS2 ultrathin nanosheets; Stepped edge MoS2 sheet: stepped edge surface terminated MoS2 sheet array; Cu7S4@MoS2 nanoframe: the ultra-small donut shaped Cu7S4@MoS2 hetero-nanoframes; Mo2C/CNT: carbon nanotube-supported Mo2C nanoparticles; MoS2/CoSe2 hybrid: quasi-amorphous MoS2-coated CoSe2 hybrid; MoS2/RGO-1: the layered hybrid of MoS2 nanosheets and graphene; MoS2/RGO-2: MoS2 nanoparticles grown on reduced graphene oxide sheets; MoS2/CNS: The ultrathin MoS2-coated carbon nanospheres; MoSx/GO: MoSx/GO using the medium degree of oxidation of GO as matrix; ET&IE MoS2: a class of colloidal MoS2 nanostructure with edge-terminated (ET) and interlayer-expanded (IE) features; MoSx-NCNT: MoSx/vertical N-doped carbon nanotube forest; MoC-Mo2C: MoC-Mo2C heteronanowires composed of well-defined nanoparticles; Co9S8@MoS2/CNFs: a cubic cobalt sulfide-layered molybdenum disulfide coreshell/carbon nanofibers hybrid system. Ref Table S4 HER performances on various MoS2 based catalysts. Catalyst Tafel slope / mv dec j0 / μa cm η1 * / mv η2 * / mv Rct / Ω MoS2/HG 41 19.2 124 143 3.65 MoS2/RGO 50 4.98 172 195 14.94 MoS2 93 1.95 293 / 20.06 * The values of η1 and η2 were obtained the current densities were 10 and 20 ma cm, respectively. S16
SUPPORTING REFERENCES (S1) Dai, X.; Du, K.; Li, Z.; Liu, M.; Ma, Y.; Sun, H.; Zhang, X.; Yang, Y. ACS Appl. Mater. Interfaces 2015, 7, 27242-27253. (S2) Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. J. Am. Chem. Soc. 2014, 136, 7587-7590. (S3) Hu, W.-H.; Shang, X.; Han, G.-Q.; Dong, B.; Liu, Y.-R.; Li, X.; Chai, Y.-M.; Liu, Y.-Q.; Liu, C.-G. Carbon 2016, 100, 236-242. (S4) McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2013, 135, 16977-16987. (S5) Yang, Y.; Fei, H.; Ruan, G.; Tour, J. M. Adv. Mater. 2015, 27, 3175-3180. (S6) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Adv. Mater. 2013, 25, 5807-5813. (S7) Hu, J.; Huang, B.; Zhang, C.; Wang, Z.; An, Y.; Zhou, D.; Lin, H.; Leung, M. K. H.; Yang, S. Energy Environ. Sci. 2017, 10, 593-603. (S8) Xu, J.; Cui, J.; Guo, C.; Zhao, Z.; Jiang, R.; Xu, S.; Zhuang, Z.; Huang, Y.; Wang, L.; Li, Y. Angew. Chem. Int. Ed. 2016, 55, 6502-6505. (S9) Chen, W. F.; Wang, C. H.; Sasaki, K.; Marinkovic, N.; Xu, W.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Energy Environ. Sci. 2013, 6, 943. (S10) Gao, M. R.; Liang, J. X.; Zheng, Y. R.; Xu, Y. F.; Jiang, J.; Gao, Q.; Li, J.; Yu, S. H. Nat. Commun. 2015, 6, 5982. (S11) Zheng, X.; Xu, J.; Yan, K.; Wang, H.; Wang, Z.; Yang, S. Chem. Mater. 2014, 26, 2344-2353. (S12) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. J. Am. Chem. Soc. 2011, 133, 7296-7299. (S13) Hu, W.-H.; Han, G.-Q.; Liu, Y.-R.; Dong, B.; Chai, Y.-M.; Liu, Y.-Q.; Liu, C.-G. Int. J. Hydrog. Energy. 2015, 40, 6552-6558. (S14) Gao, M. R.; Chan, M. K.; Sun, Y. Nat. Commun. 2015, 6, 7493. (S15) Li, D. J.; Maiti, U. N.; Lim, J.; Choi, D. S.; Lee, W. J.; Oh, Y.; Lee, G. Y.; Kim, S. O. Nano Lett. 2014, 14, 1228-1233. (S16) Lin, H.; Shi, Z.; He, S.; Yu, X.; Wang, S.; Gao, Q.; Tang, Y. Chem. Sci. 2016, 7, 3399-3405. (S17) Zhu, H.; Zhang, J.; Yanzhang, R.; Du, M.; Wang, Q.; Gao, G.; Wu, J.; Wu, G.; Zhang, M.; Liu, B.; Yao, J.; Zhang, X. Adv. Mater. 2015, 27, 4752-4759. S17