2D/2D Heterojunctions for Catalysis

Abstract

2D layered materials with atomic thickness have attracted extensive research interest due to their unique physicochemical and electronic properties, which are usually very different from those of their bulk counterparts. Heterojunctions or heterostructures based on ultrathin 2D materials have attracted increasing attention due to the integrated merits of 2D ultrathin components and the heterojunction effect on the separation and transfer of charges, resulting in important potential values for catalytic applications. Furthermore, 2D/2D heterostructures with face-to-face contact are believed to be a preferable dimensionality design due to their large interface area, which would contribute to enhanced heterojunction effect. Here, the cutting-edge research progress in 2D/2D heterojunctions and heterostructures is highlighted with a specific emphasis on synthetic strategies, reaction mechanism, and applications in catalysis (photocatalysis, electrocatalysis, and organic synthesis). Finally, the key issues and development perspectives in the applications of 2D/2D layered heterojunctions and heterostructures in catalysis are also discussed.

Keywords: 2D nanojunctions; electrocatalysis; organic synthesis; photocatalysis; synthesis.

Figure 1

Schematic structures of bulk heterojunction and 2D/2D heterojunction with coupled interface.

Figure 2

a–d) Schematic illustration of typical 2D/2D heterostructures with different contact interfaces.

Figure 3

a) Schematic illustration of the components of MoS₂/WSe₂ heterostructures (MoS₂ monolayers are stacked onto WSe₂ monolayers); b) optical micrograph of MoS₂/WSe₂ heterostructures after thermal treatment; and c) AFM image of the MoS₂/WSe₂ heterostructures (the thickness of MoS₂ or WSe₂ monolayers is 0.6–0.7 nm according to the cross‐sectional height profile). Reproduced with permission.49 Copyright 2014, American Chemical Society.

Figure 4

a) Schematic diagram and b) atomic model of WS₂/MoS₂ heterostructures, h‐BN layers are introduced to control the separation distance between WS₂ and MoS₂ layers. Reproduced with permission.45 Copyright 2018, WILEY‐VCH.

Figure 5

a) Schematic illustration of the MoSe₂‐templated WSe₂ growth. Reproduced with permission.55 Copyright 2015, American Chemical Society. b) Synthetic paths for preparing graphene/WS₂ layered heterostructures (dashed boxes: the corresponding optical images of the samples). Reproduced with permission.53 Copyright 2017, American Chemical Society. c) Synthetic paths for preparing h‐BN/G layered heterostructures. Reproduced with permission.21 Copyright 2018, WILEY‐VCH.

Figure 6

a–f) Schematic illustration of electronic band structures of typical S–S junctions and M–S junctions.

Figure 7

a) HRTEM images of MoS₂‐CP heterostructures with different layer numbers (yellow arrows: defects; scale bar: 5 nm); b) The distribution of MoS₂ with different layer number in MoS₂‐CPs. Reproduced with permission.16 Copyright 2015, Elsevier.

Figure 8

Schematic illustration of the oxidation mechanism of GSCN heterostructures. Reproduced with permission.10 Copyright 2011, American Chemical Society.

Figure 9

a) TEM image (arrows: overlapped interfaces), b) HAADF‐STEM images and elemental mapping (Pt‐labeled C₂N and Co‐labeled aza‐CMP), c) overall water‐splitting performance, and d) schematic illustration of the electronic band structures of aza‐CMP/C₂N heterostructures. Reproduced with permission.69 Copyright 2018, WILEY‐VCH.

Figure 10

Photocatalytic H₂ evolution based on different catalysts with a) >420 nm and b) >780 nm light irradiation; c) schematic diagram for photocatalytic H₂ evolution using BP/CN. Reproduced with permission.68 Copyright 2017, American Chemical Society.

Figure 11

a) Photocatalytic degradation kinetics of RhB by the as‐prepared photocatalysts under visible‐light irradiation; b) schematic illustration of charge separation at the interface of AgIO₃/g‐C₃N₄‐NS under visible‐light irradiation. Reproduced with permission.81 Copyright 2015, WILEY‐VCH.

Figure 12

a) The ln (C/C₀) versus time curves of MB with various photocatalysts: ZnO/MoS₂ heterostructures with different loading amounts of MoS₂ (mass ratio is 0, 0.01, 0.1, and 1 wt%) and commercial P25; b) the apparent rate constants of MB photodegradation with various photocatalysts; and c) schematic illustration of photocatalytic mechanisms of ZnO/MoS₂ heterostructures. Reproduced with permission.85 Copyright 2014, WILEY‐VCH.

Figure 13

Polarization curves of MoS₂‐CPs and bare CPs in N₂‐saturated a) 0.5 mol L⁻¹ H₂SO₄ and b) 1.0 mol L⁻¹ KOH electrolyte with a sweep rate of 0.5 mV s⁻¹; c) schematic illustration of MoS₂‐CPs; and d) electrochemical impedance spectroscopy (EIS) spectra of MoS₂‐CPs and bare CPs at low frequency with −0.1 V versus reversible hydrogen electrode (RHE) in N₂‐saturated 0.5 mol L⁻¹ H₂SO₄ electrolyte. Inset: corresponding EIS spectra at high frequency. Reproduced with permission.16 Copyright 2015, Elsevier.

Figure 14

a) Polarization curves of Ni‐Fe LDH‐NS@DG as bifunctional catalyst for overall water splitting in 1 m KOH (loaded on Ni foam with 2 mg cm⁻²), inset: comparison of different catalysts (i: Ni‐Fe LDH‐NS@DG, 2 mg cm⁻² loading; ii: Ni‐Fe LDH‐NS@DG with 1 mg cm⁻² loading; iii: Ni‐Fe LDH‐NS@NG with 2 mg cm⁻² loading; iv: Ni‐Fe LDH‐NS@G with 2 mg cm⁻² loading; and v: bare Ni foam electrode); b) comparison of the required voltage (current density: 20 mA cm⁻²) of various noble metal free bifunctional electrocatalysts for overall water splitting; c) picture of a water‐splitting device assisted by solar; and d) schematic illustration of electrocatalytic mechanism of Ni‐Fe LDH‐NS@DG for overall water splitting based on the density functional theory (DFT) calculation results (pink sphere: electron, purple sphere: hole). Reproduced with permission.95 Copyright 2017, WILEY‐VCH.

Figure 15

a) Schematic illustration of synthetic process of CdS‐(GR‐M) (M: metal ions of Cr³⁺, Mn²⁺, Ca²⁺, Ni²⁺, Cu²⁺, Fe²⁺, Co²⁺, and Zn²⁺); photocatalytic performance of CdS‐(GR‐M) (M: metal ions of Cr³⁺, Mn²⁺, Ca²⁺, Ni²⁺, Cu²⁺, Fe²⁺, Co²⁺, and Zn²⁺) with different weight addition ratio of GR, CdS‐GR, and CdS for b) photocatalytic oxidation of benzyl alcohol and c) reduction of 4‐nitroaniline under visible light irradiation (λ > 420 nm; 2 h and 80 min, respectively). Reproduced with permission.104 Copyright 2014, American Chemical Society.

Figure 16

a) TOF values of h‐BNC/G‐x (x = 1.6, 15, 20; weight percentage of glucose in reactants) and pristine BN; b) schematic illustration of reaction mechanism over the h‐BN‐C/G. Reproduced with permission.21 Copyright 2018, WILEY‐VCH.