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Bulk layered heterojunction as an efficient electrocatalyst for hydrogen evolution

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Science Advances  31 Mar 2017:
Vol. 3, no. 3, e1602215
DOI: 10.1126/sciadv.1602215
  • Fig. 1 The anisotropic charge transport in the bulk and the spontaneous formation of Cu–Mo–S–based dense nanocomposites during deposition of MoS2 directly on Cu foils.

    (A) Illustration of a typical bulk heterojunction (BHJ). A bulk heterojunction consists of particulate/granular structures connected to the electrodes with different chemical potentials (green and red plates). When either electrons or holes are generated, they are then separated isotropically (black arrows). (B) Illustration of the anisotropic charge transport in bulk. The BLHJ concept uses layered chalcogenides inside the bulk chalcogenides. When charges are injected into the layered chalcogenides from one electrode (for example, red plate), they would undergo fast transfer because of the strong anisotropy in the transport properties. (C) Image of the as-grown CMS layers on Cu foil. (D) Plan-view SEM of the as-deposited CMS. (E) Cross-sectional scanning transmission electron microscopy (TEM) image of the resulting CMS layer. (F) Bright-field TEM image of the cross-sectioned TiO2/CMS/Cu, where the topmost layers consist of Pt particles and carbon deposited as protection for focused ion beam sectioning. (G) High-resolution TEM (HR-TEM) image of the Chevrel phase of Cu2.76Mo6S8 formed at the interface of MoS2 and Cu2S. The inset shows that the electron diffraction patterns collected in the dashed orange circle confirm formation of the Chevrel phase. ZA, zone axis.

  • Fig. 2 Electrochemical analysis and HER of the CMS layers.

    (A) Linear sweep voltammetry. (B) Onset HER potentials. (C) Tafel curves of the CMS layers (as-grown, cyan; annealed, orange; annealed with TiO2, red) together with those of the control materials (Cu, black; MoS2, green; Pt, blue). (D) Nyquist plots. (E) Chronogalvanometry of MoS2 and CMS/TiO2. The legend colors in (E) are common to all.

  • Fig. 3 Selectivity of transport properties of the resulting CMS BLHJs.

    (A) AFM height image of CMS. (B) Corresponding current map by C-AFM at a sample bias of −0.5 V. (C) Simplified model of BLHJ structures, highlighting localized transport paths of 2D materials (blue). (D) Proposed mechanisms on energy (e) barrier lowering by the localized electric fields.

  • Fig. 4 Electrochemical analysis and HER of the NMS layers.

    (A) Polarization curves. (B) Onset HER potentials. (C) Tafel analysis (NMS, red) together with those of the control materials (bare Ni form, gray; sulfurized Ni form, green; Pt, blue). (D) Nyquist plots. The legend colors in (A) are common to all, except for (E). (E) Stability tests of NMS materials both at 10 mA/cm2 and at −0.1 V versus RHE when using Pt as counter electrode for about 100 hours. (F) Long-term stability of our NMS samples for about 1 month where Pt was replaced by the other sheet of NMS (see the inset image).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/3/e1602215/DC1

    fig. S1. Schematic illustration of the deposition system.

    fig. S2. Sequential gas-phase reaction of MoS2 on Si wafers.

    fig. S3. Influence of tp of MoCl5 on the uniform growth zone.

    fig. S4. Elemental and structural analyses of thin MoS2 films.

    fig. S5. Elemental analysis of annealed CMS layers.

    fig. S6. High-resolution XPS spectra of the annealed CMS layers in Mo (3d), S (2p), Cu (2p), and Cl (2p) regions.

    fig. S7. Raman spectra of MoS2 (300 cycles) grown on Au.

    fig. S8. The Mott-Schottky measurement of MoS2 on an Au/Si substrate.

    fig. S9. The Hall effect measurements of MoS2 on 500-nm-thick SiO2/Si.

    fig. S10. Arrhenius plot of the resistivity from Hall effect measurements on ALD-grown MoS2/SiO2 (500 nm)/Si.

    fig. S11. SEM images of the as-grown CMS layers.

    fig. S12. Thickness dependence of ALD films and Mo contents as a function of position.

    fig. S13. HER activities from the CMS samples with thickness gradient.

    fig. S14. Schematic illustration of the structural evolution of BLHJs upon annealing.

    fig. S15. Low-magnification TEM micrograph of our CMS layers upon annealing (500°C for ~1 hour under N2 flow) to give an overview of the structures that consist of layered MoS2 and the superstructures of Chevrel clusters (marked by yellow and blue arrows, respectively).

    fig. S16. XRD patterns of our CMS on Cu subjected to different thermal treatments.

    fig. S17. EDX elemental analysis of our TiO2/CMS/Cu structures.

    fig. S18. EDX line scan results for the TiO2/CMS/Cu structures.

    fig. S19. Detailed elemental maps of our CMS layers shown in Fig. 1E (main text), indicative of the origin of local variations in the detected elements.

    fig. S20. HR-TEM image of annealed CMS to give an overview of the local surface termination.

    fig. S21. Schematic of our three-electrode cell used for HER experiments.

    fig. S22. Surface morphology of TiO2-coated annealed CMS.

    fig. S23. Estimation of electrochemically active surface area of our CMS material by double-layer capacitance measurements.

    fig. S24. XPS analyses of CMS materials to check possible contamination of noble metals.

    fig. S25. Stability against scanning of the present CMS system (10,000 times).

    fig. S26. XPS analyses of TiO2/CMS after stability tests.

    fig. S27. Reproducibility tests in the electrocatalytic performance of CMS and NMS materials.

    fig. S28. Electrochemical analysis and HER of the CMS layers with a non-Pt counter electrode (that is, graphite).

    fig. S29. UPS spectra.

    fig. S30. Optical absorption of amorphous TiO2 grown on quartz glass.

    fig. S31. Energy band diagrams and the corresponding circuit models of various structures.

    fig. S32. HER measurements of our 40-nm-thick TiO2 with different control samples.

    fig. S33. KPFM study of our CMS on Cu.

    fig. S34. Local transport study of our annealed CMS/Cu samples.

    fig. S35. Comparison between our CMS/TiO2 and various HER materials (32) in the electrocatalytic performance.

    fig. S36. TEM image of the NMS layer as prepared to give an overview of the structures that densely consist of layered MoS2.

    fig. S37. ALD cycle–dependent HER performance of annealed CMS.

    fig. S38. HR-TEM image of a CMS layer annealed at 700°C.

    fig. S39. Linear sweep voltammetry of the CMS layers annealed at different temperatures.

    table S1. Summary of the catalytic performance of various materials for the hydrogen evolution reaction, reported in the literature.

    Reference (48)

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Schematic illustration of the deposition system.
    • fig. S2. Sequential gas-phase reaction of MoS2 on Si wafers.
    • fig. S3. Influence of tp of MoCl5 on the uniform growth zone.
    • fig. S4. Elemental and structural analyses of thin MoS2 films.
    • fig. S5. Elemental analysis of annealed CMS layers.
    • fig. S6. High-resolution XPS spectra of the annealed CMS layers in Mo (3d), S (2p), Cu (2p), and Cl (2p) regions.
    • fig. S7. Raman spectra of MoS2 (300 cycles) grown on Au.
    • fig. S8. The Mott-Schottky measurement of MoS2 on an Au/Si substrate.
    • fig. S9. The Hall effect measurements of MoS2 on 500-nm-thick SiO2/Si.
    • fig. S10. Arrhenius plot of the resistivity from Hall effect measurements on ALD-grown MoS2/SiO2 (500 nm)/Si.
    • fig. S11. SEM images of the as-grown CMS layers.
    • fig. S12. Thickness dependence of ALD films and Mo contents as a function of position.
    • fig. S13. HER activities from the CMS samples with thickness gradient.
    • fig. S14. Schematic illustration of the structural evolution of BLHJs upon annealing.
    • fig. S15. Low-magnification TEM micrograph of our CMS layers upon annealing (500°C for ~1 hour under N2 flow) to give an overview of the structures that consist of layered MoS2 and the superstructures of Chevrel clusters (marked by yellow and blue arrows, respectively).
    • fig. S16. XRD patterns of our CMS on Cu subjected to different thermal treatments.
    • fig. S17. EDX elemental analysis of our TiO2/CMS/Cu structures.
    • fig. S18. EDX line scan results for the TiO2/CMS/Cu structures.
    • fig. S19. Detailed elemental maps of our CMS layers shown in Fig. 1E (main text), indicative of the origin of local variations in the detected elements.
    • fig. S20. HR-TEM image of annealed CMS to give an overview of the local surface termination.
    • fig. S21. Schematic of our three–electrode cell used for HER experiments.
    • fig. S22. Surface morphology of TiO2-coated annealed CMS.
    • fig. S23. Estimation of electrochemically active surface area of our CMS material by double-layer capacitance measurements.
    • fig. S24. XPS analyses of CMS materials to check possible contamination of noble metals.
    • fig. S25. Stability against scanning of the present CMS system (10,000 times).
    • fig. S26. XPS analyses of TiO2/CMS after stability tests.
    • fig. S27. Reproducibility tests in the electrocatalytic performance of CMS and NMS materials.
    • fig. S28. Electrochemical analysis and HER of the CMS layers with a non-Pt counter electrode (that is, graphite).
    • fig. S29. UPS spectra.
    • fig. S30. Optical absorption of amorphous TiO2 grown on quartz glass.
    • fig. S31. Energy band diagrams and the corresponding circuit models of various structures.
    • fig. S32. HER measurements of our 40-nm-thick TiO2 with different control samples.
    • fig. S33. KPFM study of our CMS on Cu.
    • fig. S34. Local transport study of our annealed CMS/Cu samples.
    • fig. S35. Comparison between our CMS/TiO2 and various HER materials (32) in the electrocatalytic performance.
    • fig. S36. TEM image of the NMS layer as prepared to give an overview of the structures that densely consist of layered MoS2.
    • fig. S37. ALD cycle–dependent HER performance of annealed CMS.
    • fig. S38. HR-TEM image of a CMS layer annealed at 700°C.
    • fig. S39. Linear sweep voltammetry of the CMS layers annealed at different temperatures.
    • table S1. Summary of the catalytic performance of various materials for the hydrogen evolution reaction, reported in the literature.
    • Reference (48)

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