The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets

Journal name:
Nature Chemistry
Volume:
5,
Pages:
263–275
Year published:
DOI:
doi:10.1038/nchem.1589
Published online

Abstract

Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs — obtained either through exfoliation of bulk materials or bottom-up syntheses — are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.

At a glance

Figures

  1. Structure of monolayered TMDs.
    Figure 1: Structure of monolayered TMDs.

    a, About 40 different layered TMD compounds exist. The transition metals and the three chalcogen elements that predominantly crystallize in those layered structure are highlighted in the periodic table. Partial highlights for Co, Rh, Ir and Ni indicate that only some of the dichalcogenides form layered structures. For example, NiS2 is found to have apyrite structure but NiTe2 is a layered compound. b,c, c-Axis and [11–20] section view of single-layer TMD with trigonal prismatic (b) and octahedral (c) coordinations. Atom colour code: purple, metal; yellow, chalcogen. The labels AbA and AbC represent the stacking sequence where the upper- and lower-case letters represent chalcogen and metal elements, respectively. d,e, Dark-field scanning transmission electron microscopy image of single-layer MoS2 showing the contrast variation of 1H (d) and 1T (e) phases. Blue and yellow balls indicate Mo and S atoms, respectively. f, Zigzag chain clusterization of W atoms due to Jahn–Teller distortion in single layer WS2. The clustered W atoms are represented by orange balls. The √3a x a unit cell of the superstructure is indicated with a white rectangle. Images in d and e reproduced with permission from ref. 12, © 2012 ACS. Image in f courtesy of T. Fujita.

  2. d-Orbital filling and electronic character of various TMDs.
    Figure 2: d-Orbital filling and electronic character of various TMDs.

    a, Qualitative schematic illustration showing progressive filling of d orbitals that are located within the bandgap of bonding (σ) and anti-bonding states (σ*) in group 4, 5, 6, 7 and 10 TMDs. D3h and D3d refer to the point group associated with the trigonal prismatic and the octahedral coordination of the transition metals (that is, the two monolayer polymorphs described in Fig. 1b). It should be noted that this simple model assumes ideal coordination; structural distortion often seen in many tellurides and group 7 TMDs lead to deviation in the electronic structure. The filled and unfilled states are shaded with dark and light blue, respectively. According to ligand field theory, D3d compounds form two non-bonding d orbitals, dyz,xz,xy (bottom) and dz2, x2–y2 (top), while D3h (or Oh) compounds exhibit three d orbitals whose character is predomintly dz2, dx2–y2,xy, and dxz,yz (from bottom to top). When an orbital is partially filled (such as in the case of group 5 and 7 TMDs), the Fermi level (EF) is within the band and the compound exhibits a metallic character. When an orbital is fully filled (such as in group 6 TMDs), the Fermi level is in the energy gap and a semiconducting character is observed. b, Energy dispersion (energy versus wavevector k) in bulk, quadrilayer (4L), bilayer (2L) and monolayer (1L) MoS2 from left to right. The horizontal dashed line represents the energy of a band maximum at the K point. The red and blue lines represent the conduction and valence band edges, respectively. The lowest energy transition (indicated by the solid arrows) is direct (vertical) only in the case of a single layer. Indirect transition in monolayer (dashed arrow in 1L plot) is greater in energy than the direct band edge transition (solid arrow). c, Band structure of MoS2 showing six valleys and opposite spin–orbit splitting of the valence band at the K and K′ (−K) points. The red and blue surfaces represent spin–orbit-split valence band maxima, each of which is associated with a particular electron spin. The green surfaces represent the conduction band minima or the valleys. Figures reproduced with permission from: b, ref. 6, © 2010 ACS; c, ref. 36, © 2012 APS.

  3. Chemical exfoliation of monolayered TMDs.
    Figure 3: Chemical exfoliation of monolayered TMDs.

    a, Schematic representation of the electrochemical lithiation process for synthesis of 2D nanosheets from layered bulk materials47. The cut-off voltage used to optimize the electrochemical lithiation conditions for preparation of few-layer BN, NbSe2, WSe2, Sb2Se3 and Bi2Te3 is also indicated. b, (i) Photographs of 2D nanosheet dispersions and (ii) the corresponding atomic force microscope images on SiO2 substrates42, 47. The insets of the AFM images represent height profiles from the substrate onto the nanosheets. The height of the step at the edge indicates the thickness of the nanosheets. Figures reproduced with permission from: a, b(i,ii) NbSe2, ref. 47, © 2012 Wiley; b(i) MoS2, WS2, TiS2, TaS2, b(ii) WS2, TiS2, TaS2, ZrS2, ref. 42, © 2011 Wiley.

  4. Chemical vapour deposition of ultrathin TMDs.
    Figure 4: Chemical vapour deposition of ultrathin TMDs.

    a, Schematic of MoS2 layer deposited by two-step thermolysis, and the films obtained on a sapphire and silica sustrate53. b, MoS2 nanosheets on CVD graphene/Cu substrates through van der Waals epitaxy55. c, Schematic illustration of single- to few-layered MoS2 by sulfurization of Mo thin film56. d, Alternative method for the growth of a MoS2 monolayer, also on a substrate (sample) from MoO3 and S powders through a gas-phase reaction. The substrate has been treated with aromatic molecules to assist the growth of larger flakes, as shown by the optical micrograph and atomic force microscopic images of the triangular nanosheets that are obtained57. The red circles represent the heating reaction chamber. e, Wafer-scale deposition of mono- to few-layered MoS2 films obtained by direct sulfurization of MoO3 thin films, and their transfer onto arbitrary substrates60. f,g, CVD of ultrathin TMDs by vaporization and decomposition of a single precursor in solution (f) and by vaporization and decomposition of metal and chalcogen precursors in solid forms (g). Figures reproduced with permission from: a, ref. 53, © 2012 ACS; b, ref. 55, © 2012 ACS; d, ref. 57, © 2012 Wiley; e, ref. 60, © 2012 RSC.

  5. Surface chemistry probed by adsorption of DBT.
    Figure 5: Surface chemistry probed by adsorption of DBT.

    a, STM image (52 × 52 Å2) of a Mo-edge-terminated cluster after dosing of DBT at 300 K. The white arrows indicate S vacancies. b, S-edge-terminated cluster after dosing of DBT at 300 K. The STM image is 22 × 22 Å2. c, Illustrations of the adsorption of DBT on sites of Mo-edge-terminated (edge, left) and S-edge-terminated (corner, right) clusters with S vacancies70. The red curves in the case of the Mo-edge cluster show steric hindrance. d, STM images (15 × 15 Å2) of the corner sites of a Mo- and S-edge S vacancy sites interacting with a DBT molecule. The distances between the S atom of DBT and the corner Mo atom show that the shape of the MoS2 material greatly influences its interaction with DBT, and the efficiency of its desulfurization. Figure reproduced with permission from ref. 70, © 2012 ACS.

  6. Hydrogen evolution reaction catalysis with TMDs.
    Figure 6: Hydrogen evolution reaction catalysis with TMDs.

    a, Volcano plot of exchange current density (i0) as a function of DFT-calculated Gibbs free energy (ΔGH*) of adsorbed atomic hydrogen for MoS2 and pure metals82. b, [(PY5Me2)MoS2]2+ compound designed to mimic the edges of MoS2. Colour code of the atoms: green, Mo; yellow, S; light blue, F; red, O; blue, N; and grey, C. H atoms are omitted for clarity83. c, (i,ii) Schematics of solvothermal synthesis (i) with graphene oxide sheets for MoS2/RGO hybrid synthesis and (ii) without graphene oxide sheets, resulting in large, free MoS2 particles. (iii,iv) SEM and TEM (inset) images of the MoS2/RGO hybrid (iii) and of the free MoS2 particles (iv)75. d, SEM images of graphene-protected Ni foam on which MoS2 catalysts were grown84. Left, low magnification image of Ni foam coated with graphene and MoS2. Middle, the region indicated by the red circle in the left image has been magnified to show the skeleton structure of the foam. Right, magnified region from the red circle in the central image, showing the graphene/MoS2 films on the polycrystalline Ni foam. Figures reproduced with permission from: a, ref. 82, © 2007 AAAS; b, ref. 83, © 2012 AAAS; c, ref. 75, 2011 ACS; d, ref. 84, © 2012 Wiley.

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Affiliations

  1. Materials Science and Engineering, Rutgers University, 607 Taylor Road, Piscataway, New Jersey 08854, USA

    • Manish Chhowalla
  2. Interdisciplinary School of Green Energy and Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 689-798, Republic of Korea

    • Hyeon Suk Shin
  3. Department of Physics, National University of Singapore, Singapore 117542

    • Goki Eda
  4. Department of Chemistry, National University of Singapore, Singapore 117543

    • Goki Eda &
    • Kian Ping Loh
  5. Graphene Research Centre, National University of Singapore, Singapore 117546

    • Goki Eda &
    • Kian Ping Loh
  6. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan

    • Lain-Jong Li
  7. School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

    • Hua Zhang

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The authors declare no competing financial interests.

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