Functionalized Hybridization of 2D Nanomaterials

Abstract

The discovery of graphene and subsequent verification of its unique properties have aroused great research interest to exploit diversified graphene-analogous 2D nanomaterials with fascinating physicochemical properties. Through either physical or chemical doping, linkage, adsorption, and hybridization with other functional species into or onto them, more novel/improved properties are readily created to extend/expand their functionalities and further achieve great performance. Here, various functionalized hybridizations by using different types of 2D nanomaterials are overviewed systematically with emphasis on their interaction formats (e.g., in-plane or inter plane), synergistic properties, and enhanced applications. As the most intensely investigated 2D materials in the post-graphene era, transition metal dichalcogenide nanosheets are comprehensively investigated through their element doping, physical/chemical functionalization, and nanohybridization. Meanwhile, representative hybrids with more types of nanosheets are also presented to understand their unique surface structures and address the special requirements for better applications. More excitingly, the van der Waals heterostructures of diverse 2D materials are specifically summarized to add more functionality or flexibility into 2D material systems. Finally, the current research status and faced challenges are discussed properly and several perspectives are elaborately given to accelerate the rational fabrication of varied and talented 2D hybrids.

Keywords: 2D nanomaterials; functionalization; heterostructures; hybrid; modification.

Figure 1

Functionalized hybridization of various 2D nanomaterials via diverse strategies to combine different species.

Figure 2

a–c) Schematic strategies for top‐down production of 2D nanomaterials. a) Mechanical cleavage by using Scotch tape. Reproduced with permission.37 Copyright 2012, Institute of Physics. b) Sonication‐driven exfoliation via 1) the addition of exfoliating/stabilizing agents, 2) sonication, and 3) increment of sonication time. Reproduced with permission.38 Copyright 2014, American Chemical Society. c) Ion intercalation‐assisted exfoliation. Reproduced with permission.39 Copyright 2018, The Royal Society of Chemistry. d) Schematic strategies for bottom‐up production of 2D nanomaterials: hydrothermal and solvothermal synthesis of MoS₂ nanosheets. Reproduced with permission.43 Copyright 2015, The Royal Society of Chemistry; and with permission.5 Copyright 2011, American Chemical Society. e) Chemical vapor deposition of WS₂ nanosheets. Reproduced with permission.44 Copyright 2013, American Chemical Society.

Figure 3

Element doping of TMD layers. a) Schematic doping of various ions and atoms into/onto MoS₂ layers. b) Phase engineering of MoS₂ nanosheet after intercalated by Li⁺ or Na⁺. Reproduced with permission.54 Copyright 2015, American Chemical Society; and with permission.55 Copyright 2014, American Chemical Society. c) TiS₂ nanosheet doped with H. Reproduced with permission.58 Copyright 2013, American Chemical Society. d) MoS₂ nanosheet doped with O. Reproduced with permission.59 Copyright 2013, American Chemical Society.

Figure 4

Sulfur vacancy passivation by alkanethiol molecules. a) Schematic drawings of a MoS₂ field effect transistor before and after the treatment with alkanethiol molecules. b) Raman spectra of sulfur vacancies. c) I _DS–V _G curves before and after thiol treatment with logarithmic scale. The inset shows I _DS–V _G curves with linear scale. Reproduced with permission.67 Copyright 2015, American Chemical Society.

Figure 5

C—S bonding‐mediated surface functionalization of MoS₂ nanosheets. a) Schematic modification of MoS₂ nanosheets with 2‐iodoacetamide or iodomethane. Reproduced with permission.69 Copyright 2014, Nature Publishing Group. b) Schematic basal‐plane functionalization of MoS₂ with 4‐methoxyphenyldiazonium tetrafluoroborate. Reproduced with permission.70 Copyright 2015, American Chemical Society.

Figure 6

Noncovalent hybridization of BSA on MoS₂ nanosheets. a) Schematic binding of BSA on MoS₂ layer via benzene rings and disulfides. b) AFM image of single‐layer MoS₂ bonded with BSA. c) Biocompatibility and d) specific capacitance of MoS₂‐BSA nanosheets and other MoS₂ products. As observed, the surface‐hybridized BSA endows a better biocompatibility and a larger specific capacitance due to its biological and insulative nature. Reproduced with permission.73 Copyright 2015, American Chemical Society.

Figure 7

Schematic illustration of BSA‐induced exfoliation of WSe₂ nanosheets and then loading with photosensitizer methylene blue on nanosheets for synergistic photothermal/photodynamic cancer therapy. Reproduced with permission.99 Copyright 2017, The Royal Society of Chemistry.

Figure 8

Epitaxial growth of Au nanoparticles on MoS₂ nanosheets by using BSA‐caged Au₂₅ nanoclusters. a) Schematic exfoliation of MoS₂ nanosheets and subsequent surface growth of Au₂₅ clusters into Au_m nanoparticles. b) Low‐resolution and c) high‐resolution TEM images of Au_m/MoS₂ nanosheets, and d) their enlarged TEM image from the highlighted area in (c). e) Schematic growth of Au_m to Au_{m + n} nanoparticles on MoS₂ nanosheets with time upon the addition of H₂O₂, and the corresponding color evolution. Reproduced with permission.122 Copyright 2018, The Royal Society of Chemistry.

Figure 9

The hierarchical growth of MoS₂ nanosheets on the inner surface of TiO₂ nanocavities. a) Schematic illustration for the fabrication process of MoS₂@TiO₂ heterostructures. b) Top‐view SEM image, c) TEM image and d) high‐resolution TEM image of MoS₂@TiO₂ heterostructure with a MoS₂ thickness of 10 nm. e) Schematic diagram of the energy band structure, plasmonic resonance, and electron transfer pathway in the MoS₂@TiO₂ heterojunction. Reproduced with permission.129 Copyright 2018, The Royal Society of Chemistry.

Figure 10

Graphene growth along Zigzag (z) and Klein (k) edges on Ni(111). a) Zigzag and Klein edges of a top‐fcc epitaxial graphene layer. b,c) High‐speed scanning tunneling microscopy acquired in quasi‐constant height mode at the z edge b) and the k edge c). White arrows indicate the position of C atoms in fcc‐hollow sites near the kink. Reproduced with permission.143 Copyright 2018, Science.

Figure 11

Hybridization of metal oxides on RGO nanosheets. a) Schematic representation of the sol–gel strategy toward ultradispersed TiO₂ nanocrystals on graphene. HRTEM image (right) showing nanosized TiO₂ with highly exposed edges stacked on graphene nanosheets. Reproduced with permission.153 Copyright 2013, American Chemical Society. b) Schematic processes for synthesizing the hybrid of graphene and phosphorized SnO₂. Reproduced with permission.160 Copyright 2018, The Royal Society of Chemistry.

Figure 12

The growth of W₁₈O₄₉ nanograsses on g‐C₃N₄ nanosheets. a) Synthetic approach to W₁₈O₄₉/g‐C₃N₄ heterostructure. b) SEM images of top and side view of the heterostructure. c) TEM image and the corresponding elemental mapping images of the heterostructure. Reproduced with permission.175 Copyright 2017, Wiley‐VCH.

Figure 13

Chemical passivation of BP layers via covalent modification. a) Reaction scheme of benzene‐diazonium tetrafluoroborate derivatives on BP layer. b,c) AFM characterization and d) histograms of the surface roughness of pristine BP flake immediately after exfoliation and after ten days of ambient exposure. e,f) AFM characterization and g) histograms of the surface roughness of the BP morphology immediately after functionalization with 4‐nitrobenzene‐diazonium (4‐NBD) for 30 min and after ambient exposure for ten days. Scale bars are 2 µm. Reproduced with permission.190 Copyright 2016, Nature Publishing Group.

Figure 14

The selective growth of Co₂P nanocrystals on the BP edges. a) TEM image of one BP/Co₂P nanosheet. b) HR‐TEM image of Co₂P corresponding to the edge location and c) HR‐TEM image of BP corresponding to the center position. d) AFM image and line‐scan of one BP/Co₂P nanosheet. e) Schematic synthesis of BP/Co₂P heterostructures. Reproduced with permission.205 Copyright 2018, Wiley‐VCH.

Figure 15

Controlled exfoliation of WO₃ nanosheets in a pH 4 solution and subsequent hole creation on resulting nanosheets in a pH 8 solution. a) Schematic illustration for the electrostatic binding of BSA on WO₃ surface. b) Calculated binding energies. c) High‐resolution TEM images of WO₃ nanosheets. Reproduced with permission.42 Copyright 2017, Wiley‐VCH. d) Schematic illustration from the WO₃ bulk to WO₃ nanosheets to porous nanosheets. e) pH‐dependent evolution of the absorption intensity of WO₄ ²⁻ ions after WO₃ powder is dissolved in aqueous solutions at different pH values for 24 h. f) TEM image of porous WO₃ nanosheets and the comparison on catalytic ability of different samples (inset). Reproduced with permission.216 Copyright 2018, American Chemical Society.

Figure 16

Chemical synthesis of Pt‐SnO₂ nanosheets. a) Schematic illustration of synthetic process. b) TEM image of SnO₂ nanosheets functionalized by Pt nanoparticles. c) High resolution TEM image of Pt‐SnO₂ nanosheets. Reproduced with permission.219 Copyright 2018, Wiley‐VCH.

Figure 17

Hybridization of different TMD nanosheets. a) Schematic of lateral epitaxial growth of WS₂–WSe₂ heterostructures. b) AFM image of WS₂–WSe₂ lateral heterostructure. Inset is optical image of a triangular domain. Scale bars = 5 µm. c) Raman and d) photoluminescence characterization in the center region and the peripheral region of WS₂–WSe₂ heterostructures: 1,2) are Raman mapping at 419 and 256 cm⁻¹, respectively; 3,4) are photoluminescence mapping at 665 and 775 nm, respectively. Reproduced with permission.236 Copyright 2014, Nature Publishing Group. e) SEM image of the ternary MoS_{2 x}Se_{2(1− x )} nanosheets (x = 0.48). f) Normalized photoluminescence spectra of the MoS_{2 x}Se_{2(1− x )} nanosheets excited with a 488 nm argon ion laser. Curves (a–l) are the MoS_{2 x}Se_{2(1− x )} nanosheets synthesized at temperature of 830 to 796 °C, respectively. Reproduced with permission.238 Copyright 2014, American Chemical Society.

Figure 18

a) Schematic illustration for synthesizing sandwiched VG/MoSe₂/N‐C core/shell arrays and b–d) SEM images of typical products. VG represents vertical graphene and N‐C represents N‐doped carbon in shell. Reproduced with permission.250 Copyright 2017, Wiley‐VCH.

Figure 19

A van der Waals heterojunction p–n diode based on BP and MoS₂ nanosheets. a) Schematic device structure. b) Optical image of device structure. The dark purple region is monolayer MoS₂ while the blue flake is few‐layer BP. Scale bar, 10 µm. c) Gate tunable I–V characteristics of the p–n diode. The top inset shows the I–V characteristics under semilog scale. The bottom inset shows the rectification ratio as a function of back gate voltage V _g. Reproduced with permission.254 Copyright 2014, American Chemical Society.

Figure 20

Surface hybridization of WO₃ and MoS₂ nanosheets into WO₃/MoS₂ hybrid by simultaneously exfoliating them in the presence of BSA. a) Schematic formation for WO₃/MoS₂ nanosheet sandwiched with BSA. b) Low‐resolution TEM image of a WO₃/MoS₂ hybrid prepared at the weight ratio of WO₃/MoS₂ = 4:1, accompanied with the high‐resolution TEM images on its individual nanosheets. Reproduced with permission.42 Copyright 2017, Wiley‐VCH.