Recent progress in two-dimensional nanomaterials: Synthesis, engineering, and applications

FlatChem 18 (2019) 100133

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Recent progress in two-dimensional nanomaterials: Synthesis, engineering, and applications

T

Fa Yanga,b, Ping Songa, Mingbo Ruana, Weilin Xua,

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a

State Key Laboratory of Electroanalytical Chemistry, & Jilin Province Key Laboratory of Low Carbon Chemical Power, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China b University of Science and Technology of China, Anhui 230026, China

ARTICLE INFO

ABSTRACT

Keywords: Two-dimensional nanomaterials Synthesis Energy ORR CO2RR

With the discovery of mechanically exfoliated graphene in 2004, two-dimensional (2D) nanomaterials have emerged as one of the most promising candidates in the fields of chemistry, material science, physics, and nanotechnology due to their unique physical, chemical, optical and electronic properties. In this Review, we briefly introduce the general synthetic strategies applied to 2D nanomaterials, followed by describing some important and newly developed members of the 2D family. Then, we discuss in detail the engineering strategies to enhance their intrinsic performance for extensive applications among the electrocatalysis, photocatalysis, energy storage, and bioimaging. Finally, the challenges and outlooks in these fields are also addressed.

1. Introduction After Novoselov and Geim successfully exfoliated graphene from graphite using Scotch tape in 2004 [1], the development of two-dimensional (2D) nanomaterials in the fields of chemistry, material science, physical, and nanotechnology has been getting more and more attention [2]. Compared with other types of nanomaterials (including bulk counterparts, zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanowires, and three-dimensional (3D) networks) [3], 2D materials possess unique properties and performance due to their high aspect ratio, quantum-size effect, and plain surface conformation [4–7]. Specifically in the following five aspects: First, the reaction electrons could be confined in 2D structure, especially on single-layer nanosheets, which enables extremely compelled transport of electrons without interlayer interactions [8]. Second, the ultrathin atomic thickness and strong in-plane chemical bonds give them high mechanical strength, flexibility and optical transparency properties [3]. Third, the large horizontal size offers them with ultrahigh specific surface area, which facilitates them in catalysis, supercapacitors, and rechargeable batteries [8]. Fourth, the large fraction of surface atoms allows easy surface engineering to enhance intrinsic material properties through more active sites construction [9]. Fifth, the ultrathin 2D geometry structure provides a simple and ideal model to modulate the electronic-state, and establish clear structure–property relationships. Due to these appealing advantages, 2D materials have become the

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Corresponding author. E-mail address: [email protected] (W. Xu).

https://doi.org/10.1016/j.flatc.2019.100133 Received 16 July 2019; Accepted 12 August 2019 Available online 13 August 2019 2452-2627/ © 2019 Elsevier B.V. All rights reserved.

research hotspots among the wide applications of chemistry [10], biology [11], physical [12,13]. Meanwhile, the family of 2D materials is getting more and more booming, including hexagonal boron nitride (hBN) [14], graphitic carbon nitride (g-C3N4) [15], transition metal dichalcogenides (TMDs: MoS2, TiS2, TaS2, WS2, MoSe2, WSe2, etc.) [16–18], layered metal oxides [19,20], layered double hydroxides (LDHs) [21], and black phosphorus (BP) [22] as well as many newly developed members, such as metal–organic frameworks nanosheets (MOFs) [23–25], covalent-organic frameworks (COFs) [26,27] and polymers[28,29], metals [30,31], silicene [32], metal phosphorus trichalcogenides [33], perovskites [34,35], and niobates [36]. Recently, organic–inorganic hybrid perovskites [37], silicates [38], and MXenes [39,40] have also been developed (Scheme 1). Driven by their extraordinary properties, a large number of synthetic methods have been explored to obtain these 2D nanomaterials. These methods could be categorized into two categories: top-down and bottom-up methods [2,41]. The top-down methods mainly depend on driving forces (e.g. exfoliation) to break the van der Waals interaction among their stacked bulk counterparts to obtain thin layer crystals. These methods mainly include mechanical cleavage [42], mechanical force-assisted liquid exfoliation [43–45], ion intercalation-assisted method [46], and selective etching-assisted method [47]. It is worth noting that the top-down categories are only applicable to those layered bulk materials. On the other hand, many other 2D nanomaterials are prepared via bottom-up methods through covalent or ionic bonding.

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hand, the promising application prospects accelerate and enrich the development of synthetic methods. Until now, most of the ultrathin 2D nanomaterials could be obtained from exfoliation of layer 2D crystals. For example, the known bulk graphite is composed of weakly stacked graphene single-layer [41]. While the bottom-up methods, which are dependent on the precursors under given chemical reaction conditions, are more practical. In this section, we are not going to summarize all reported preparation methods in detail, but only highlight some regularly used and newly developed methods. 2.1. Top-down methods 2.1.1. Mechanical exfoliation route The mechanical exfoliation method is a common strategy to obtain ultrathin sheets by using scotch tape to exfoliate layered compounds without breaking the in-plane covalent bonds of each layer [64]. In 2004, Novoselov and co-workers first successfully obtained single-layer graphene from highly oriented pyrolytic graphite through mechanical exfoliation route [1]. This method has been widely used to prepare various ultrathin 2D materials with different layers, including h-BN [65], 2D TMDs (MoS2, NbSe2, TiS2, TaS2, WS2, WSe2, and TaSe2) [66,67], metal oxides, Black phosphorus (BP), MOFs, etc. In a typical process, the fresh surface of a bulk crystal is firstly attached to a piece of scotch tape and then peeled into a thin sheet by using another adhesive, then leave nanosheets on the substrate by stripping the scotch tape [43]. Since these ultrathin nanosheets are directly exfoliated from their layered compounds without occurring chemical reactions, the nanosheets kept the perfect crystal quality and remain stable under ambient conditions. C.R.Dean and co-workers used a kind of mechanical transfer process to fabricate high-quality mono- and bilayer graphene on single-crystal h-BN substrates (Fig. 1a). The graphene devices on hBN substrates have better mobilities and carrier inhomogeneities than devices on SiO2 [65]. Although the mechanically exfoliated nanosheets have many advantages, including high crystal quality, clean surface, and large lateral size; some deficiencies of this method limit their practical application. First, the low production yield restricts large-scale industrial application. Second, a substrate always needs to support the exfoliated nanosheets. Third, the size and shape of nanosheets are hard to control accurately. Fourth, this method is more suitable for lager layered crystals [3,55]. Based on above considerations, improving the production rate and yield is a necessary issue to meet large-quantities practical demand.

Scheme 1. Schematic illustration of different kinds of typical 2D nanomaterials, such as graphene, BP, TMDs, MOFs, COFs, MXenes, LDHs, and Metals.

Chemical vapor deposition (CVD) [48,49] and wet-chemical methods [50,51] are two typical bottom-up methods, including 2D templateconfined growth [52], seed-growth [53], hydrothermal method [54], ligand-assisted growth, as well as nanoparticle assembly [7]. Unlike the limited conditions of top-down methods, the bottom-up methods are more widely used in practical applications. That means, almost all kinds of 2D materials could be synthesized through bottom-up methods [2,13,55,56]. Meanwhile, the regular instruments for characterizing 2D nanomaterials were scanning tunneling microscopy (STM), atomic force microscopy (AFM), and high resolution transmission electron microscopy (HRTEM). Although various reviews on 2D nanomaterials have emerged, most authors focused on the advantages and disadvantages of synthetic methods and materials properties, a summary on the various 2D materials by comprehensively introducing the strategies for engineering electronic states and material properties to enhance novel functionalities is still lacking, which is of particular importance for the development and applications of 2D nanomaterials. Meanwhile, the understanding of size [57], thicknesses [58], surface compositions [59], lattice strain [60], crystal phase [61], edge defects [62], vacancies [63], and electronic properties of these 2D materials is essential to explore the conjunction between the structural and intrinsic characteristics. In this review, we start briefly introducing various synthetic strategy of 2D materials. Generally, combined with previous literatures and the reaction intermediate species, we divide these various methods into topdown and bottom-up methods. Then, we summarize some recently revealed members of the 2D family. After that, a detailed comparison and analysis of thickness/size regulation, elemental doping, pit/pore creation, edge/vacancy engineering, strain/phase/interface engineering, and 2D heterostructure construction are made in extensive applications among the electrocatalysis (such as oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), CO2 reduction reaction (CO2RR), oxygen evolution reaction (OER), and nitrogen reduction reaction (NRR), photocatalysis, energy storage (supercapacitors and lithium ion batteries (LIBs)), and bioimaging, etc. Finally, the new opportunities, challenges, and further development of 2D nanomaterials are also addressed.

2.1.2. Liquid exfoliation route Mechanical solid exfoliation method has been proved to be an effective way to exfoliate bulk crystals into single- or few-layer nanosheets by applying external force. Inspired by this route, people realized that the layered crystals could also be exfoliated into thin nanosheets in liquid phase (e.g. N-methyl-pyrrolidone and dimethyl-formamide) [43,45,68]. Researchers have developed lots of mechanical force-assisted liquid exfoliation methods to prepare high-yield and -quality ultrathin nanosheets, such as graphene [69], h-BN [70], TMDs [71,72], metal oxides [73], metal hydroxides [70], MOFs [24,74], etc. Similar to the mechanical solid exfoliation method, an external force is also required to break the van der Waals interaction in liquid method. Typically, the liquid exfoliation route has two main means: sonication-assisted liquid exfoliation and shear force-assisted liquid exfoliation [69], while the sonication-assisted strategy is more simple and practical. Here, the main liquid exfoliation techniques for layered materials were briefly introduced as shown in Fig. 1b, c. Moreover, the efficiency of exfoliation process could be increased by matching the surface tension of solvent with that of bulk compounds [43,75]. Meanwhile, the solvent could stabilize the exfoliated nanosheets by preventing stacking and aggregation; researches have also shown that the polymer and surfactant could stabilize them under the sonication process [43]. For this reason, organic solvents are widely used to exfoliate crystals efficiently.

2. Synthetic methods In recent years, researchers have made enormous efforts to explore various facile, feasible, and reliable methods to prepare 2D nanomaterials, and investigate their properties and functions simultaneously. On one hand, the preparation of 2D materials with expected sizes, thickness, compositions, and defects is critical to further study their physical, chemical, electronic, and optical properties. On the other 2

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Fig. 1. (a) Schematic illustration of mechanical transfer process used to fabricate graphene-on-BN devices [65]. Copyright © 2010 Macmillan Publishers Limited. (b) Sonication-assisted exfoliation: The layered crystal is sonicated in a solvent, resulting in exfoliation and nanosheet formation. In “good” solvents—those with appropriate surface energy—the exfoliated nanosheets are stabilized against reaggregation. Otherwise, for “bad” solvents reaggregation and sedimentation will occur [43]. Copyright © 2013 American Association for the Advancement of Science. (c) Schematic illustration of solvothermal-assisted exfoliation and optical photo of dispersion of BP sheets in ACN [75]. Copyright © 2018 Journal of Materiomics.

In 2008, the exfoliation of graphite into graphene was first developed by Coleman’s group, but limited by the low concentration of graphene suspension (0.01 mg/mL) [76]. After that, researchers have been trying various methods by exploring more functional volatile solvents. Coleman and coworkers increased the concentration of graphene suspensions by using low boiling point solvents (e.g. isopropanol and chloroform) [77]. Meanwhile, Blake et al. utilized N,N-Dimethylformamide (DMF) as solvent to obtain a high-quality graphene suspension [78]. The exfoliation of graphite in aqueous solutions (e.g. sodium dodecyl benzene sulfonate (SDBS) and sodium cholate) was also developed [79,80]. However, it is worth noting that the use of additional exfoliants may change the original characteristic of exfoliated nanosheets [81]. Currently, compared with mechanical exfoliation method, the liquid exfoliation method might be more efficient to prepare dispersed ultrathin 2D nanomaterials due to the high-yield and large-scale production. By controlling the sonication time, power, temperature, additional solvent, and the shape of vessels, the concentration, lateral size, and thickness of synthesized nanosheets could be regulated precisely, and the obtained volume of sheets alters from hundreds of milliliters to liters [82]. However, some disadvantages of the liquid exfoliation method still exist. For example, the yield of single-layer nanosheets is relatively low, and the lateral size is small due to the strong sonication force. More unfavorable is that the used polymers and surfactants are usually harmful. Therefore, the exfoliation process needs further optimization to prepare single-layer nanosheets with high-yield and large lateral sizes.

the reactive precursors decompose on the surface of substrate to form large-scale ultrathin flakes under high temperature and high vacuum. The obtained nanosheets have advantages of flexible size, high crystal quality, and tunable thickness. In 1897, De Lodyguine et al. first utilized CVD technique to reduce tungsten hexachloride with hydrogen to coat tungsten on carbon filament for lamps [83]. Recently, Novoselov et al. have prepared a type of high-quality and large-area single/few layer graphene [84]. Significantly, Lee et al. synthesized a kind of large area uniform MoS2 nanosheets on SiO2/Si substrates by using MoO3 and S powders as precursors [85]. In 2015, Sungjoo Lee et al. prepared layercontrolled and large-area MoS2 films by using oxygen plasma to deal with the surfaces of SiO2 substrates (Fig. 2a–c). The produced MoS2 films are mono, bi, or trilayer thicknesses by varying the period (90 s, 120 s, or 300 s) of the oxygen plasma treatment on the SiO2 substrate [86]. Recently, Jiang et al. described a controlled synthesis of uniform monolayer ReSe2 flakes with variable morphology (sunflower- or truncated-triangle-shaped) on SiO2/Si substrates using different ambient-pressure chemical vapor deposition (CVD) setups [87] (Fig. 2d–f). To date, CVD method has been adopted successfully for the preparation of a wide variety of nanosheets including graphene, TMDs, h-BN, and metal oxide [48,50,51,88,89]. Previous researches have shown that CVD process is a promising method to produce ultrathin 2D nanomaterials, but some disadvantages still exist currently. First, a desired substrate is necessary to support the growth of 2D nanomaterials; avoiding the complicated transfer process is also difficult. In addition, the high temperature and high vacuum make the CVD method complicated and low efficient.

2.2. Bottom-up methods 2.2.1. Chemical vapor deposition (CVD) The CVD method is the widely used bottom-up strategy to synthesis high-purity nanosheets or thin films on substrates [49]. In this route,

2.2.2. Wet-Chemical method Wet-chemical method is another typical and widely used bottom-up strategy for the synthesis of 2D materials with high quality and yield, 3

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Fig. 2. Layer-controlled CVD growth of large-area two-dimensional MoS2 films. (a) Schematic diagram of the MoS2 films after the surface plasma treatment. TEM images of folded layers for (b) Monolayer MoS2. (c) Bilayer MoS2. APCVD syntheses of monolayer ReSe2 on SiO2/Si substrates through different experimental setups [86]. Copyright © 2015 The Royal Society of Chemistry. (d) Schematic illustration of CVD synthesis of sunflower-shaped ReSe2 by placing ReO3 in the upstream, adjacent location of the growth substrate. (e) OM images of sunflower-shaped ReSe2 grown on SiO2/Si substrates under 650 °C. (f) OM images of truncated triangleshaped ReSe2 grown on SiO2/Si substrates under 650 °C [87]. Copyright © 2017 Tsinghua University Press and Springer-Verlag GmbH Germany. Au nanoprisms obtained with NaI in CTABr growth solution: (g, h) SEM image of Au nanoprisms [53]. Copyright © 2012 American Chemical Society. (i) TEM image of the palladium nanosheets. (j) TEM image of the assembly of palladium nanosheets perpendicular to the TEM grid. Inset: thickness distribution of the palladium nanosheets [94]. Copyright © 2010 Nature Publishing Group.

depending on the chemical reactions of certain precursors at proper solution phase conditions, including templated synthesis, solvothermal synthesis, self-assembly of nanocrystals, and soft colloidal synthesis [90–93]. In this route, surfactants are usually added to control and stabilize the size, thickness and morphology of nanosheets due to their powerful controllability. They have been widely used to synthesize various types of 2D materials, such as graphene, h-BN, g-C3N4, TMDs (e.g., MoS2, TiS2, TaS2, WS2, and ZrS2), metals (e.g., Au, Pd, and Rh), metal oxides (e.g., TiO2, CeO2, In2O3, SnO2, and Fe2O3), metal chalcogenides (e.g., PbS, CuS, SnSe, ZnSe, ZnS, and CdSe), LDHs, MOFs, COFs, and polymers [90,94–96]. Joseph et al. reported a facile one-pot synthetic strategy of Au nanoprisms with high purity (Fig. 2g, h), demonstrating that appropriate combinations of halide anions can provide many possibilities to regulate the morphology diversification of Au nanostructures [53]. Huang et al. utilized carbon monoxide (CO) as surface confining agent to synthesize freestanding hexagonal palladium (Pd) nanosheets (Fig. 2i, j) [94]. In all, the wet-chemical method has

advantages of high reaction yield, easier control of size and morphology compared with other methods. More importantly, the obtain materials have very good dispersion in organic or aqueous media, which is beneficial for further applications [97,98]. However, it is difficult to control the final morphology of the desired product because the whole process is easily affected by reaction parameters, including synthesis temperature, synthesis time, surfactants, solvents, and concentration of precursors. Meanwhile, single-layer nanosheets for most nanomaterials are also hard to achieve due to the intrinsical complication of synthesis process. 3. Composition and crystal structures As we summarized above, various synthetic methods have been developed to produce various 2D materials. Due to the differences in the composition and crystal structures, these 2D materials could be divided into two categories: layered and non-layered nanomaterials. 4

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For layered materials, including graphene, h-BN, g-C3N4, BP, TMDs, and LDHs, the in-plane atoms interconnected with each other to form bulk crystals through strong chemical bonding. In contrast, some other compounds can crystallize into various crystal structures (e.g., metals, metal oxides, metal chalcogenides, and polymers) by the coordination between atoms or chemical bonding in two dimensions [61,99,100]. In this section, we highlight some newly-developed members of 2D nanomaterial.

CN also hamper its practical applications. Subject to preparation solidstate conditions of pristine CN at high temperatures, the synthesized bulky material show high recombination rate of charge, insolubility in most solvents, and low electrical conductivity [114,115]. Therefore, tremendous effort has been made for regulating molecular structure of CN. The polymeric features (polymeric feature, rich chemistry and facile synthesis) allow easily regulating its structure by engineering hydrogen bonds, modifying the polymerization degree, adjusting coordination interaction, doping and copolymerization at molecular level [116]. For example, the modification of the aromatic p-conjugated framework could control its molecular structure and introduce a variety of functional groups into its interfacial structure, to meet task-specific applications. Che et al. designed a kind of Cring-C3N4 in-plane heterostructure for fast electrons transfer (Fig. 3b), which could promote photocarrier diffusion length and lifetime by 10 times compared to pristine g-C3N4 [117]. Meanwhile, Song et al. reported a type of ultrathin carbon nitride nanosheets with graphited carbon ring (CN-GP) by thermal polymerization of polyvinyl butyral and melamine membrane, which could greatly cover the range of visible/near-infrared light and weaken the barrier of the photocarrier transfer [118]. Therefore, the rational design of the molecular structure would not only improve the intrinsic activity relative to bulk counterpart, but also provide new insights to create high-performance CN materials.

3.1. Metal carbides and nitrides (MXenes) Since the discovery of Ti3C2 in 2011, a new family of 2D transitionmetal carbides and nitrides (MXenes) has emerged [101]. Owing to the advantages of excellent electrical conductivity, multiple surface terminations, hydrophilic property, and solvent compatibility [39,102,103], MXenes have attracted a lot of attention and are expected to become the next-generation energy materials [104]. The preparation of MXenes is usually done by selective etching the raw (MAX) phases and connecting to each layer by strong metallic and covalent bonds to form Mn+1AXn (n = 1, 2, or 3) structure, where M is the transition metal (Ti, V, Cr, Nb, etc.), A is another element from group IIIA or IVA (Al, Si, Sn, In, etc.), and X stands for carbon or nitrogen [105]. So they possess the metallic conductivity and hydrophilic property to surface hydroxyl or oxygen. For example, HF has been used as a strong etching solution to produce MXenes of three different structures, the structures of M2X, M3X2, and M4X3 corresponds to Ti2AlC, Ti3AlC2, and Ta4AlC3, respectively [105]. Alexey Lipatov et al. developed a modified synthetic method to produce high-quality monolayer Ti3C2Tx flakes (Fig. 3a). Following the original procedure (Route 1), Ti3C2Tx was synthesized by immersing Ti3AlC2 powder into a LiF–HCl solution. By comparison, we can see that the Ti3C2Tx flakes produced by the modified synthesis method (Route 2) have higher quality and larger size than that of produced by Route 1 [106]. This improvement relies on the altar of the etching solution, which facilitated both etching of aluminum and intercalation of lithium. Unlike many other 2D materials, the resistance of Ti3C2Tx structure is higher than individual sheets, showing an excellent ability of electron transport through the surface terminations of different flakes. Recently, a new family of stable two-dimensional crystals with an open-channel tetrahedral bonding network emerged, named as TXene [107]. The atomic arrangements of TXene and MXene are shown in Fig. 3c, d. To date, more than 20 types of MXenes have been successfully synthesized with the structures and properties being theoretically predicted [39,108]. Moreover, due to the availability of large amount of solid solutions and surface terminations, more new kinds of MXenes could be produced in future.

3.3. 2D metals Different from other 2D nanomaterials, 2D metals usually have a highly symmetric crystal lattice due to their intrinsic metal plasticity, suggesting that they are not thermodynamically favored during the growth of nanocrystals [7]. Researchers usually take the pathway of reducing the total free energy of the metal structures with capping agents to control the overall growth pathway [119,120]. At present, solution-based colloidal method, thermal decomposition, and chemical reduction of a metal precursor are often applied to synthesize 2D Metals. In a typical process, nucleation takes place first; then intermediate species are added to initiate the anisotropic growth; lastly, metal nuclei grow into 2D-structure [121]. For wet-chemical syntheses, some commonly adopted formation strategies are based on template methods with surfactants arrangements [122,123]. Moreover, they could delicately control final products by affecting the 2D morphology. To date, the synthesis of 2D metals with controlled sizes, compositions, shapes/ facets, and crystal-phase have been achieved. Generally, metal crystals can be clarified into three types: face-centered cubic (fcc), hexagonal close-packed (hcp), and body-centered cubic (bcc) [124]. The fcc structure shows a stacking sequence of “ABC”, the hcp structure shows an “ABAB” stacking sequence, and as for the bcc structure, one lattice point lies in the center of the cubic cell [125]. Reis et al. reported the synthesis of monolayer Bismuthene on a SiC substrate via a CVD method [126]. As shown in Fig. 4a–c, the monolayer Bismuthene possesses a gap of ~0.8 eV and conductive edge states. Recently, Han et al. synthesized ultrathin bismuth nanosheets with single crystallinity and enlarged surface areas by in situ topotactic transformation of BiOI (Fig. 4d) [31]. Wu et al. also prepared highly crystalline Bi nanosheets with thickness of about 2 nm on silicon substrates by facile hot-pressing method (Fig. 4e) [127]. Lee et al. synthesized single-crystalline copper nanoplates (Cu NPLs) via a simple hydrothermal method (Fig. 4f), which show very low resistance to electron transfer process [128].

3.2. Polymeric carbon nitride (CN) As a promising 2D conjugated polymer, polymeric carbon nitride (CN) is getting more and more attention due to the facile synthesis condition, abundant and inexpensive raw materials, and wide applications [109,110]. To date, five different phases of CN were predicted and produced: a-C3N4, b-C3N4, g-C3N4, cubic C3N4, and pseudocubic C3N4 [111,112]. Among these phases, g-C3N4 has attracted tremendous attention due to the unique graphite layered skeleton through sp2 hybridization of carbon and nitrogen atoms [113]. It is worth noting that most of the synthesized g-C3N4 is not a perfect C3N4 stoichiometry, but a defect-rich N-bridged ''poly (triazine)' because of the incomplete demineralization or polymerization of the nitrogen-containing precursors. g-C3N4 in two different structure models could be synthesized via selecting proper precursors and condensation methods: (1) striazine constructed through the condensed s-triazine units (Fig. 3e), and (2) condensed tri-s-triazine subunits connected through planar tertiary amino groups (Fig. 3f) [15]. The conjugated polymeric network endows polymeric carbon nitride with high physicochemical stability, unique optical properties. While some disadvantages and pristine features of

3.4. Metal–organic frameworks nanosheets (MOFs) Recently, 2D metal–organic frameworks nanosheets (MOFs) emerged as a new type of crystalline porous compounds, which are produced via coordination bonds between organic ligands and metal atoms [129]. It is worth noting that MOFs could also serve as a diverse source of 2D MOF nanosheets if they could form into nanometer-scale thickness not only into 3D structures. MOFs have some unique 5

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Fig. 3. Synthesis route of Ti3C2Tx flakes. (a) Summary of Routes 1 and 2 and schematic structures of Ti3AlC2 and Ti3C2Tx [106]. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Synthetic route for the in-plane heterostructure of (Cring)–C3N4 [117]. Copyright © 2017 American Chemical Society. (c, d) Atomic arrangements of transition metal carbonitride (MXene) and tetragen chalcogenide (TXene) [107]. Copyright © 2019 American Chemical Society. Scheme of (e) s-triazine and (f) tri-s-triazine based connection in g-C3N4. Blue and gray spheres represent nitrogen and carbon atoms, respectively [15]. Copyright © 2012 The Royal Society of Chemistry.

characteristics, such as large surface area, highly ordered pores, and abundant active sites on their surfaces [130]. In addition, MOFs can be tuned by size, connectivity and dynamic interaction with target guest molecules through the selection of the organic building blocks. The MOFs show great potential applications in electrochemical and biological areas [24]. By changing the coordination mode between ligands and metal ions, MOFs can form various crystal structures. For synthesis methods, the top-down route is simple via sonication or shaking to dissolve the interlayer interaction [130]. Although researchers have successfully prepared MOF nanosheets on specific substrates, the direct synthesis of MOFs is still difficult. Meanwhile, owing to the effect of isotropic micrometre-sized crystals and dispersible nanoparticles, the subsequent incorporation or integration of the two components is usually restricted [131]. For instance, the exfoliation of bulk MOFs into nanosheets have been achieved in H2O [74], acetone [132], methanol [133], and ethanol [134]; but the synthesized nanosheets are not stable and uniform due to their restacking. Noting that most reported MOFs are layer structure, which have been produced on substrates based on layer-by-layer growth method. Here, changing MOFs into nanosheets (UMOFNs) is an efficient experimental route at the atomic/molecular levels because nanometer thicknesses and the distinct surface atomic structures of UMOFNs are easily identifiable and tunable. Zhao et al. have successfully synthesized a type of NiCo bimetal organic framework nanosheets (NiCo-UMOFNs) through a mixed solution of Ni2+, Co2+ and BDC (Fig. 5a, b) [129]. In addition, it is also hard to synthesize

freestanding MOF nanosheets because of blending them into uniform composites. Rodenas et al. reported a bottom-up strategy to produce highly crystalline and evenly dispersed MOF nanosheets, which showed well-defined cubic crystals with edge size ranging from 2 to 10 μm and thickness from 6 to 8 nm (Fig. 5c–e). Generally, the produce process of MOFs depends on the diffusion-mediated kinetics [23]. 3.5. Covalent–organic frameworks nanosheets (COFs) Since the first report of covalent organic frameworks (COFs) in 2005 [135], they have become a new type of important crystalline polymers with ordered porous structures and tunable topology structures, which follow reticular chemistry protocols similar to that of MOFs nanosheets [136]. COFs are formed by strong covalent connection of organic units that consisting of light elements, such as C, N, B, H, Si and O. Compared with traditional polymers, COFs could integrate the building unites into 2D or 3D ordered polymeric skeletons [137]. A periodic porous COFs framework can be formed by covalent integration and then crystallize into an ordered layered structure. COFs have shown great potential for chemical sensing, electrocatalysis, energy storage, proton conduction, and drug delivery due to their unique aesthetic architectures [27]. Moreover, COFs provide ideal platforms to transport guest molecules and open channels into react active sites where charges could rapidly move. In fact, individual COFs layers could form into the bulk crystal by van der Waals stacking. Therefore, thinner COFs usually exhibit more 6

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Fig. 4. Bismuthene on SiC (0001) structural model. (a) Sketch of a bismuthene layer placed on the threefold-symmetric SiC (0001) substrate in R30° commensurate registry. (b) Topographic STM overview map showing that bismuthene fully covers the substrate. The flakes are of ~25-nm extent, limited by domain boundaries. (c) Close-up STM images for occupied and empty states. They confirm the formation of Bi honeycombs [126]. Copyright © 2017 American Association for the Advancement of Science. (d) SEM image of topotactically reduced BiNSs [31]. Copyright © 2018 Springer Nature. (e) FESEM images of BiNSs transferred on carbon tape directly from Si/SiO2 [127]. Copyright © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (f) Optical microscopy image of the as-prepared large-size Cu NPLs, the inset shows the SEM image of the Cu NPLs with a triangular shape [128]. Copyright © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

excellent activity due to larger surface area and more accessible active site compared to stacked bulk counterparts [138]. Generally, COFs are produced directly from their bulk counterparts by exfoliation and mechanical delamination based on the top-down method. At present, the high yield of producing few-layered nanosheets is still limited by the strong interlayer π–π bond. Therefore, reducing the interlayer stack by integrating building units into the skeleton contributes to the easy formation of 2D COFs [139]. Recently, Zhao et al. synthesized a type of azine, imine-linked COFs named NUS-30 by the aggregation of monomers containing induced TPE moieties [140]. They first demonstrated that isostructural bulk COF powders can be exfoliated into ultrathin 2D nanosheets with thickness of 2.4 nm by a gas exfoliation approach (Fig. 6a–c). In addition, Zhang et al. synthesized a hexagonal sheetstructure (namely TPA-COF) by choosing two flexible molecules with C3v molecule as building units (Fig. 6d, e). Because the flexible building units are integrated into the COFs skeleton, the interlayer stacking could be weakened and then leads to the easy exfoliation of TPA-COF into ultrathin nanosheets [27].

the structural and electronic properties make 2D nanomaterials very promising for electrochemistry applications. Meanwhile, most of 2D nanomaterials are non-precious metal and metal-free catalysts, such as transition metal dichalcogenides (TMDs), graphene derivatives, and porous carbon nanosheets (PCNs) are a kind of promising materials [146,147]. Recent researches have shown that atomic engineering of 2D materials is an efficient way to achieve superior performance [148]. Compared to bulk counterparts, it is more easily to regulate surface and interfacial electronic structure, including size/thickness adjusting, pit/ pore creation, vacancy engineering, elemental doping, and 2D heterostructure construction (Scheme 2). In addition, the control of surface strain/phase could also improve the electron transfer capacity. For example, when surface reconstruction occurs on the surface of 2D transition-metal sulfides and nitrides at strong alkaline solution, an interesting core-shell structure with electron transfer channel will form [149]. Therefore, choosing proper engineering strategies is very important to improve their performance, and could also guide the design of 2D materials.

4. Engineering and applications

4.1. Applications in electrocatalysis

The distinctive physical, electronic, and chemical features endow 2D materials great promising applications. In addition, the exotic and easily manipulated electronic states usually increase the electron density around the Fermi energy, which leads to the improvement of optimized band gap and enhancement of carrier mobility [141–143]. It is well known that their large specific surface areas can provide abundant active sites by properly engineering [13]. For example, 2D materials as supports could avoid nanoparticle aggregation, retaining the number of active sites during long-term cycling [144]. Additionally, 2D materials are ideal model catalysts to establish a correlation between structure and performance due to their single crystalline structure [145]. Both

With increasing concern over the serious environmental problems and severe fossil fuel shortage, developing sustainable and low energyconsuming electrochemical conversion routes to produce fuels and chemicals has become especially important [150,151]. Electrocatalytic reactions (e.g., HER, ORR, HOR, OER, CO2RR and NRR, etc.) lie at the center of clean energy because of mild reaction conditions and excellent compatibility. Many attempts have been made to develop highly-efficient and durable catalysts. Over the past few years, 2D materials have shown excellent performance in electrocatalysis due to ultrahigh specific surface area and pore structure. Moreover, 2D nanomaterials are pursued as economical alternatives for expensive precious-metal 7

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Fig. 5. (a) TEM image of NiCo-UMOFNs. The inset shows the Tyndall light scattering of NiCo-UMOFNs in an aqueous solution. (b) AFM image of as-prepared NiCoUMOFNs, showing measured dimensions of individual flakes [129]. Copyright © 2016 Macmillan Publishers Limited, part of Springer Nature. (c) Scanning electron micrograph of bulk-type CuBDC MOF crystals. (d, e) Scanning electron micrograph and atomic-force micrograph (with corresponding height profiles), respectively. The height profiles, color-coded red and blue, are measured along the corresponding tracks shown in the atomic-force micrograph [23]. Copyright © 2014 Macmillan Publishers Limited.

catalysts. For example, MoS2 have shown good electroactivity for HER [152]; Graphene emerged as a highly-efficient metal-free catalysts applied to various reactions [153]. In addition, 2D metals and metaloxides have shown excellent performance at carbon dioxide reduction reaction (CO2RR) [58,154]. Considering reaction process and mechanism, the number of active sites, electrical conductivity, mass transport and reaction energy barrier are important parameters for achieving excellent electrocatalytic activities [145]. Generally, regulating surface atomics and size could create more active sites to full react with substrates; adjusting surface species of electrode materials could lower reaction energy barrier; and introducing impurities and defects such as dopants or even additional functional groups can maximize the intrinsic activity of 2D materials. Therefore, how to utilize the synergistic effect of these key factors is of great importance for promoting electrocatalytic activity.

catalyst surface is close to zero, where hydrogen is bound neither too strongly nor too weakly [156]. Among various metals, Pt and Ru are the most widely used catalysts for HER due to their low ΔGH, while the high price and low storage hinder their development. However, the ΔGH values of resource-abundant materials are too high with low intrinsic activity, it is of great importance to develop cheap and abundant electrocatalysts to replace precious metal [157]. Therefore, taking some strategies to adjust electronic structure and active sites of resourceabundant materials for improving activity is highly concerned. Then, a class of graphenic carbon-based materials have been developed with performance comparable to precious metal catalysts [158,159]. Asefa et al. reported metal-free B-doped graphene with efficient HER activity [160]. Chen et al. reported that nitrogen and sulfur co-doping nanoporous graphene with low operating potential for HER [161]. Qiao et al. investigated HER on a series of heteroatom-doped graphene by mutually corroborating electrochemical reaction rate measurements and theoretically computed adsorption energetics [162]. They found that the value of hydrogen adsorption free energy (ΔGH) controls the performance of a catalyst in addition to particular physicochemical properties. Similar to the graphene, 2D layered metal chalcogenide materials could act as highly competitive earth-abundant catalysts for the HER. For instance, Song et al. synthesized the flowerlike 1T-MoS2 nanosheets achieving excellent HER activity and stability (Fig. 7a, b), confirming that the MoS2 possesses facile electrode kinetics and improved number of active sites [163]. Recent research shows that vertically aligned ultrasmall monolayer MoS2 are abundant in S-edge sites and vacancies, resulting enhanced HER activity with a low overpotential of 126 mV at 10 mA/cm2 and 140 mV at 100 mA/mg, which provides a new strategy to develop well-dispersed ultrasmall 2D

4.1.1. HER/HOR Hydrogen evolution reaction (HER) has been developed as an important source of high power-density fuel [56]. The process involves the protons of solution contacting with the electrons of electrode to produce H2 via the reaction: 2H+ + 2e− → H2, which can be divided into two steps: Volmer step: H+ + e− + * → H* (1); Heyrovsky step: H* + H+ + e− → H2 + * or Tafel step: 2H* → H2 + 2* (2); where H* is intermediate and * regards as active site of catalyst [13,155]. HOR has the same steps as HER except in reverse. Previous researches have shown that the overall rate of HER is limited by the hydrogen adsorption free energy (ΔGH) and the electronic structures of materials. DFT calculations have demonstrated that an excellent HER catalyst should meet this requirement: the ΔGH of hydrogen atomic adsorbed on 8

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Fig. 6. (a) AFM images of 2D nanosheets obtained by temperature-swing exfoliation of NUS-30. Inset: The theoretical thickness of NUS-30 (5 layers, 2.3 nm) based on the AA stacking structures. (b) TEM images of exfoliated ultrathin 2D nanosheets of NUS-30. (c) View of the slipped AA stacking crystal structures of NUS-30 [140]. Copyright © 2018 American Chemical Society. (d) AFM image of TPA-COF NSs with the thickness indicated. (e) Left: lattice-averaged, P3-symmetry imposed and CTF-corrected HRTEM image. Right: simulated projected potential map with a point spread function width of 4 Å, in which the projected structural model in green and unit cell in red are embedded [27]. Copyright © 2017 American Chemical Society.

4.1.2. OER The electrocatalytic water splitting involves two key half reactions: the cathodic HER: 2H+ + 2e− → H2 and the anodic OER: 2H2O → O2 + 4H+ + 4e− in acidic electrolytes or 4OH− → 2H2O + O2 + 4e− under neutral or alkaline conditions [167]. Compared with the HER, the slow kinetics of the multistep proton-coupled electron transfer in OER limit the overall efficiency of water splitting [168]. So developing highly-efficient OER catalysts to improve the overall reaction rate is necessary. In addition, OER is the main resources for metal-air batteries and fuel cells. The theoretical oxidation potential of OER is 1.23 VRHE under the 298.15 K and 1.013 atm, but some unfavorable factors of electrode and catalysts materials increased 1.5 times for practical operating voltage compared to theoretical value [167]. Therefore, developing high-efficient catalysts with low overpotential is promising to solve these problems. Precious metals and their oxide materials (Ir, Ru, IrO2 and RuO2) are considered to be the best OER materials with relatively low overpotential and high stability [169]; their commercial applications are severely impeded by their shortage and high cost. In this respect, exploring low-cost and abundant OER catalysts with low overpotential should be of great importance for practical applications [170]. In the last few decades, transition metals (Fe, Co, Ni, and oxides), TMOs/transition metal hydroxides and LDHs have been widely studied in alkaline solution for OER [155]. For example, Kuang et al. produced a Fe–Mn–O hybrid nanosheet enabling a low overpotential of 273 mV at 10 mA/cm2, a small tafel slope of 63.9 mV/dec, and high durability. Detailed electrochemical test and mechanism analysis revealed that the superior OER performance can be attributed to the lowcrystallinity structure with thickness down to 1.4 nm, which can expose large amounts of active sites [171]. Yan et al. synthesized NiMn-LDHs

Scheme 2. Schematic illustration of engineering strategies for improving activity of two-dimensional materials toward widely applications.

materials [164]. In addition, Zheng et al. reported ultrathin PtCu alloy nanosheets exhibiting higher HER activity at 0.05 M H2SO4 solution [165]. Wang et al. prepared ultrathin PtAgCo nanosheets achieving a high current density up to 705 mA/cm2 at −400 mV [166]. Above experiments all proved the importance of proper engineering strategies for improving HER activity of 2D materials. 9

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Fig. 7. Applications of 2D materials in electrocatalysis. Enhanced HER from chemically exfoliated metallic MoS2 nanosheets: (a) Schematic diagram of the 1T-MoS2; (b) Polarization curves at lower potentials and corresponding Tafel plots [163]. Copyright © 2013 American Chemical Society. Few-layer graphdiyne doped with sphybridized nitrogen atoms for ORR: (c) AFM image and thickness of FLGDYO nanosheet; (d) ORR polarization curves of NFLGDY-700, NFLGDY-800, NFLGDY-900c and Pt/C catalysts at 1,600 r.p.m. in O2-saturated 0.1 M KOH [180]. Copyright © 2018 Springer Nature. Ultrathin Cu/Ni(OH)2 nanosheets for CO2RR: (e) TEM image of the cross-sectional nanosheets obtained by microtoming; (f) FEs of CO and H2 at different potentials [186]. Copyright © 2017 American Association for the Advancement of Science. Efficient NRR with MXene (Ti3C2Tx) under Ambient Conditions: (g) Top-view TEM images of an MXene nanosheet; (h) Faradic efficiency and NH3 yield rate under cycling stability at −0.1 V versus RHE [203]. Copyright © 2018 Elsevier Inc.

with thickness of 1.3 nm, exhibiting two times improvement of the activity with overpotential low to 80 mV at 10 mA/cm2 compared to the traditional NiMn-LDHs in 0.1 M NaOH. DFT calculation revealed that the enhanced OER activity originated from the highly exposed reactive sites with a nearly optimal intermediates (*OH and *O) adsorption energy [172]. In addition, Tang et al. synthesized an ultrathin NiCo bimetal-organic framework nanosheet acquiring an overpotential of 250 mV at 10 mA/cm2 for OER performance in alkaline conditions. They found that the coordination effect between the unsaturated Ni and Co metals is important to the largely improved electrocatalytic activity [129].

some strategies to increase their atomic utilization, including alloying Pt with transition metals, and loading single-atomic and so on. Meanwhile, the high-performance M-N-C and N-doped carbon materials have also been developed [177]. Currently, various 2D materials (graphene and its derivatives, MOFs, TMOs and TMHs, etc.) exhibit excellent ORR activity, but still can’t meet the practical requirements [178,179]. Exploring strategies based on the optimization of chemical composition and surface structure for obtaining a superhigh ORR activity for 2D materials is becoming more and more important, including thickness/ size control, defect/interfacial engineering, elemental doping, and 2D heterostructure construction [155]. In fact, all these methods follow the principle of tuning the electronic structures to reduce the free energy of intermediates, and creating more reaction sites for oxygen adsorption. Take graphene as an example, Song et al. synthesized S, N dual-doped graphene-like carbon nanosheets by a simple pyrolysis of a mixture of melamine and dibenzyl sulfide for ORR. Such 2D material shows enhanced activity and durability compared to the commercial Pt/C catalyst in both alkaline and acidic solution [178]. Recently, Wang et al. synthesized a kind of sp-N-doped graphdiyne catalyst exhibiting excellent ORR activity with a tafel slope of 60 mV dec−1 and E1/2 of 0.87 V (Fig. 7c, d). Experimental and DFT analysis show that the high catalytic activity could be attributed to the sp-N dopant, which facilitates O2 adsorption and electron transfer on the surface of the sp-Ndoped graphdiyne [180].

4.1.3. ORR Developing highly efficient and low-cost oxygen reduction reaction (ORR) catalysts is of great importance for commercial applications of fuel cells and metal–air batteries [173]. Unfortunately, as the cathode reaction of PEMFCs, the ORR always suffers from very sluggish kinetics in comparison with the anodic HOR, limiting the overall PEMFCs device performance. ORR is also a multistep reaction process through two pathways: (1) 2e− pathway with formation of intermediate species of H2O2 in acidic medium or HO2− in alkaline medium, (2) through a more efficient 4e− process directly forming H2O in acidic medium or OH− in alkaline medium. The option of the 2e− or 4e− is mainly affected by the adsorption energy of intermediates and reaction barriers, which can be divided into dissociative (O* and OH*) and associative (O*, OOH*, and OH*) to adjust the free energy pathway [2,174]. Generally, the dissociative pathway involves the oxygen atomic transform into oxygen intermediates, while the associative pathway forms oxygen intermediates by three steps. Moreover, the diffusion of oxygen species into the electrolyte is another key factor [175,176]. Therefore, selecting an appropriate oxygen path to accelerate the sluggish kinetics of ORR is necessary. Noble-metal catalysts (Pt, Ru) have displayed excellent activities for ORR, but practical applications have been restricted by their high price and shortage. Researchers have developed

4.1.4. CO2RR With increasing concerns in greenhouse gas (CO2) emission and the fast consumption of fossil fuels, electrocatalytic reduction of CO2 into value-added products, including CO, formate, methanol, formaldehyde, methane, ethanol, and ethylene, provides an effective way to solve these environment and energy problems [54]. The structure of CO2 is extremely stable, linear and centrally symmetric, with two carbonoxygen double bonds (C]O, 116.3 pm) [181]. Therefore, it is hard to be activated under normal reaction. A typical CO2RR process is usually 10

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intermediates and final products [2,184]. The group I is Sn, Hg, Pb, Bi, and In preferring producing formate; Au, Ag, Zn, and Pd (group II) prefer producing CO; and Cu (group III) is capable of producing hydrocarbons [185]. However, limiting by low activity of sites and subject to some strongly bound intermediates among bulk metals, designing them into 2D thin sheets may be a good solution. Xie et al. synthesized a type of partially oxidized atomic-layered Co nanosheets by a ligandconfined growth method. Such nanosheets exhibit excellent CO2RR activity and selectivity to formate (HCOO−) at a low overpotential of only 0.24 V [54]. Meanwhile, Zheng et al. reported ultrastable atomic copper nanosheets to produce CO with faradaic efficiency of 92% at a low overpotential of 390 mV (Fig. 7e, f) [186]. These results demonstrated the importance of 2D metal catalysts. The atomic thickness of 2D materials could provide abundant low-coordinated active sites for efficient CO2 adsorption, and the following easy tailoring of the surface electronic structure could dramatically facilitate CO2 activation by stabilizing the intermediates. In addition, the increased DOS and electron charge density near the conduction band edge promote carrier transport along the 2D conducting channels and enable fast reduction kinetics. These advantages undoubtedly prove 2D-structure broad application prospects for CO2RR. Typical 2D electrocatalysts for CO2RR are summarized in Table 2.

Table 1 Standard Redox Potentials of CO2RR to different products. electron transfer

Half-electrochemical thermodynamic reactions

E0 (V vs SHE)

e− 2e−

CO2 + e− → CO2%− CO2(g) + 2H+ + 2e− → HCOOH(l) CO2(g) + 2H2O(l) + 2e− → HCOO− (aq) + OH− CO2 + 2H+ + 2e− → CO(g) + H2O(l) CO2(g) + 2H2O(l) + 2e− → CO(g) + 2OH− 2CO2(g) + 2H+ + 2e− → H2C2O4(aq) 2CO2(g) + 2e− → C2O42− (aq) CO2(g) + 4H+ + 4e− → C(s) + 2H2O(l) CO2(g) + 2H2O(l) + 4e− → C(s) + 4OH− CO2(g) + 4H+ + 4e− → HCHO(l) + H2O(l) CO2(g) + 3H2O(l) + 4e− → HCHO(l) + 4OH− CO2(g) + 6H+ + 6e− → CH3OH(l) + H2O(l) CO2(g) + 5H2O(l) + 6e− → CH3OH(l) + 6OH− CO2(g) + 8H+ + 8e− → CH4(g) + 2H2O(l) CO2(g) + 6H2O(l) + 8e− → CH4(g) + 8OH− 2CO2(g) + 12H+ + 12e− → CH2CH2(g) + 4H2O(l) 2CO2(g) + 8H2O(l) + 12e− → CH2CH2(g) + 12OH− 2CO2(g) + 12H+ + 12e− → CH3CH2OH(l) + 3H2O(l) 2CO2(g) + 9H2O(l) + 12e− → CH3CH2OH(l) + 12OH− 2CO2 + 14H+ + 14e− → CH3CH3 + 4H2O 3CO2 + 18H+ + 18e− → CH3CH2CH2OH + H2O

−1.90 −0.25 −1.078 −0.106 −0.934 −0.50 −0.59 0.21 −0.627 −0.07 −0.898 0.016 −0.812 0.169 −0.659 0.064 −0.764 0.084 −0.744 −0.27 −0.31

4e−

6e− 8e− 12e−

14e− 18e−

4.1.5. NRR Ammonia (NH3) is one of the most important industrial chemical products due to its wide application in fertilizer production, energy conversion, and pharmaceutical [194]. Compared to other energy carriers, ammonia is an ideal hydrogen storage medium due to staying 17.6% hydrogen in liquid solution. More importantly, NH3 is the only carbon-free energy carrier without CO2 emission, which can be utilized as a good alternative to hydrogen [195]. At present, the production of NH3 is more than 145 million metric tons mainly dependent on the traditional Haber-Bosch process (N2 + 3H2 ↔ 2NH3, 300–550 °C and 200–350 atm) [195]. However, the process is energy-intensive and consuming mass fossil fuel (about 1–3% of the world’s annual fossil energy output). A large quantity of H2 used in this process is mainly from the steam-reformed natural gas (CH4 + 2H2 → 4H2 + CO2), generating about 300 metric tons of CO2 into the atmosphere every year, and leading to notable environmental issues. Therefore, it is well worth searching for an alternative sustainable, green, and cost-effective approach to replace energy-intensive Haber-Bosch process [196]. Recently, the electrocatalytic nitrogen reduction reaction (NRR), producing ammonia from water and atmospheric nitrogen in ambient conditions is getting more and more attention for N2 fixation [195,204]. Tamelen and Seeley started the early research on the reduction of N2 with a titanium-aluminum system in 1969 [205]. Typically, the NRR is carried out in a three-electrode system with oxygen evolution as the

divided into three steps: (1) the chemical adsorption of CO2 on the surface of catalysts; (2) the transfer of electrons and protons migration to dissociate C]O bond(s) or form CeH and CeO bond(s); (3) the desorption of products from the catalyst surface and release to electrolyte [182]. Compared to ORR with only two products (H2O and H2O2), the CO2RR in aqueous solutions is a multiple-proton-coupled electron transfer process involving two-, four-, six-, eight-, twelve-, fourteen-, or eighteen-electron reaction pathways to form different products (Table 1) [183]. To date, some key parameters are adopted to assess CO2RR activity, involving: (1) faradaic efficiency (FE), representing the proportion of electrons used to form a specific reducing product; (2) overpotential, the potential of detecting desired products; (3) current density, the total reaction geometric current; (4) Tafel slope, indicating the rate-determining step of CO2RR and the intrinsic chemical reaction rate; and (5) turnover frequency (TOF), which is the number of molecules reacting per active site [184]. In addition, HER can also happen under reduction potentials. In view of this, improving selectivity and faradaic efficiency toward specific products among various carbon-based products is a major challenge in CO2RR. Various strategies have been adopted to improve the selectivity and activity of CO2RR, involving alloying and surface structure modification. Generally, metals are the most widely used CO2RR materials, which can be divided into three groups based on binding energy of

Table 2 A summary of 2D material-based CO2RR electrocatalysis. 2D materials

Products

Performance or parameters

Ref.

Ultrathin bismuth nanosheets

formate

[31]

Ultrathin Co3O4 Layers Partially oxidized atomic cobalt layers

formate formate

Ultrastable atomic copper nanosheets Hexagonal Pd nanosheets 2D Mesoporous Bi Nanosheets Metal Oxyhalide-Derived Catalysts

CO CO formate formate

E-MoS2 Nanosheets Atomically thin Ni-doped SnS2 nanosheets

CO carbonaceous

boron-doped graphene Nitrogen-doped graphene

formate formate

High selectivity (~100%) and large current density are measured over a broad potential, excellent durability for > 10 h 1.72 nm thick Co3O4 layers showed formate Faradaic efficiency of over 60% in 20 h with approximately 90% formate selectivity at an overpotential of only 0.24 V, outperforms previously reported electrodes a current density of 4.3 mA/cm2 with a CO faradaic efficiency of 92% at a low overpotential of 0.39 V reached CO faradaic efficiency of 94% at −0.5 V, without any decay after a stability test of 8 h enable the selective to formate with large current density, excellent Faradaic efficiency (≈100%) current densities of up to 200 mA/cm2—more than a twofold increased compared to previous best Bi catalysts shows a high current density, 61 mA/cm2 at −1.1 V, and the highest CO FE of 81.2% at −0.9 V achieved a remarkable FE of 93% for carbonaceous product with a current density of 19.6 mA/cm2 at −0.9 V FE of 66%, DFT shows that boron-doping in graphene introduces asymmetric spin density Faradaic efficiencies reached a maximum (73%) at a moderate negative potential (−0.84 V)

11

[58] [54] [186] [187] [188] [189] [190] [191] [192] [193]

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anodic reaction. The detailed cathodic and anodic reactions under different pH conditions are listed below: Acidic conditions: N2 + 6H+ + 6e− → 2NH3 (cathodic), 3H2O → 3/2O2 + 6H+ + 6e− (anodic); Basic conditions: N2 + 6H2O + 6e− → 2NH3 + 6OH− (cathodic), 6OH− → 3H2O + 3/2O2 + 6e− (anodic) [196]. A major challenge for NRR is low faraday efficiency, high overpotential and ultralow selectivity for NH3. Though current researches have made some breakthroughs with Faradaic efficiency increased since 2016, while ammonia production is still far away from practical applications. Currently, metal-based catalysts (e.g., Au, Pt, Ru, Mo, Ni, and Fe2O3) are the most widely studied materials. Generally, transition metals among them are taken as promising materials to activate the strong N-N bond because of the availability of d-orbital electrons [206]. Unfortunately, the low faradaic efficiency and poor resistance hinder the demand of practical applications. This is mainly due to the poor adsorption of N2 molecules on metals surface. Moreover, the bond energy between nitrogen and metal is too weak for N2 activation and the dorbital electrons prefer forming metal-H bonds, resulting the occurrence of HER and decrease of FENH3 [207]. A thorough understanding and promising strategies are necessary for the design of new classes of electrocatalysts. The tunable electronic structure and ultrahigh surface area endow 2D materials easier to engineer for NRR. Based on these unique structures, some meaningful researches have been reported. For instance, Wang et al. synthesized a few-layer black phosphorus nanosheets (FL-BP NSs) for NRR [201], achieving a high ammonia yield of 31.37 μg h−1 mgcat−1. DFT calculations showed that the diff-zigzag type edges of FL-BP NSs could selectively catalyze the reduction of N2 to NH3 by an hydrogenation pathway. Recently, they also demonstrated that MXene (Ti3C2Tx) nanosheets attached to a vertically aligned metal host can exhibit a high FENH3 of 5.78% and 4.72 μg h−1 cm−2 at an ultrasmall overpotential for NRR (Fig. 7g, h) [203]. The optimal active sites on Ti3C2Tx for NRR are Ti atoms. Accordingly, the typical 2D electrocatalysts for NRR are summarized in Table 3.

in LIBs and supercapacitors for 2D nanomaterials and discuss their future opportunities and challenges. 4.2.1. LIBs Rechargeable LIBs are the most widely used energy storage devices, and become the primary power source of digital device due to their higher energy density (store 2–3 times the energy per unit volume), less environmental impact, and longer lifetime compared with other batteries [212]. Most commercial LIBs involve a graphite negative electrode (anode) and a layered LiCoO2 positive electrode (cathode). More detailed description of LIBs is listed: on charging, lithium ions are released from the layered LiCoO2, pass through the electrolyte, and insert the anode graphite layers. Discharging is its reverse process, while the electrons will pass across the external circuit [209]. The theoretical energy density is ~372 W h kg−1 and practical values between 120 and 150 W h kg−1 for LIBs, which are much higher than those of other batteries [213]. However, exploring the alternative anode materials with better storage capacity is urgent because the theoretical capacity of graphite is also relatively low to meet the demand of commercialized anode material. In the past few years, although some materials (Co, Fe, and Sn) present higher theoretical capacities, these materials usually suffer from poor cycling life and slow charging rate [214,215]. In addition, Black phosphorus (BP) has a theoretical capacity of 2596 mAh/ g, which is seven times that of graphite, but lifetime is shorter due to their large volume expansions during Li+ ion intercalation [216]. Various complicated nanostructures (such as hollow, core-shell, and porous nanostructures) have been constructed to solve these shortages. In comparison with bulk components, 2D nanomaterials show higher specific Li-storage capacity as both cathodes and anodes in LIBs due to their larger specific surface area. In addition, the layer structure could shorten the Li+ diffusion and intercalation/deintercalation distances [217]. 2D materials can attain more lithium storage and high specific capacity due to the feature of easy structural engineering (surfaces, defects, doping, or edges). For instance, the bare graphene has a reversible capacity of 650 mAh/g1 [218]. Notably, a high reversible capacity of ~1327 mAh/g−1 at a low rate of 50 mA/g−1 was obtained for the B-doped graphene [219]. Another effective way is to hybridize an ultrathin nanosheet with another type sheet-material for forming hybrid-structure; this treatment could avoid the aggregation of catalysts during charging and discharging process. In 2010, Dai et al. reported that the Mn3O4 nanoparticles loaded at RGO (Mn3O4/RGO hybrid) can exhibit a high specific capacity of 900 mAh/g (close to their theoretical capacity) and a remarkable cycling stability [220]. Recently, Konrad et al. designed a composite nanostructure with MoS2 nanosheets grown on graphene sheets (MoS2/G) via hydrothermal method (Fig. 8a–c). As an anode LIBs material, the MoS2/G composite electrode presents a stable cycling performance (1077 mAh g−1 at 100 mA g−1 after 150 cycles) and a long cycle life (907 mAh g−1 at 1000 mA g−1 after 400 cycles) [221]. Recently, Man et al. also observed an interface synergistic effect from layered MoS2/SnS2 heterostructure for the enhancement of Li-Ion storage performance (Fig. 8d, e) [208]. It indicates that the rational designs of 2D materials are promising to enhance their performance in lithium ion batteries.

4.2. Applications in energy storage In addition to the energy conversion, the development of suitable materials for highly efficient energy storage is of great importance to fulfill the increasing demand of energy due to their large specific capacitance, high power density, and fast charging-discharging rates [3]. Recently, 2D materials have shown great promising for high-performance energy storage devices, such as lithium ion batteries (LIBs) and supercapacitors due to their high surface-to-volume ratios, good conductivity, and thermal stability under harsh operational environments [13]. Moreover, the large interlayer spacing could store and release lithium ions and electric charges, which is essential for LIBs. 2D nanomaterials including graphene, MXenes and TMDs have been explored to enhance LIBs and supercapacitors electrochemical performance [208,209]. For example, the most famous 2D material, graphene, is extremely attractive for energy storage applications with a theoretical specific surface area (2630 m2 g−1) [210], larger than that of activated carbon (~900 m2 g−1) or carbon nanotubes (usually from 100 to 1000 m2 g−1) [211]. In this section, we highlight the recent researches Table 3 A summary of 2D material-based NRR electrocatalysis. 2D materials Rh nanosheet B-doped graphene MoO3 nanosheets Mo2C/C nanosheets Black Phosphorus nanosheets B4C nanosheets MXene (Ti3C2Tx) nanosheets MoS2/CC

Electrolytes 0.1 M KOH 0.05 M H2SO4 0.1 M HCl 0.5 M Li2SO4 0.01 M HCl 0.1 M HCl 0.5 M Li2SO4 0.1 M Na2SO4

FENH3 (%) 0.217 10.8 1.9 7.8 5.07 15.95 5.78 1.17

12

Production rate of NH3 −1

−1

23.88 μg h mgcat 9.8 μg h−1 cm−2 −1 29.43 mg h gcat−1 11.3 μg h−1 mgcat−1 31.37 μg h−1 mgcat−1 26.57 μg h−1 mgcat−1 4.72 μg h−1 cm−2 8.08 × 10−11 mol s−1 cm−1

Reference [197] [198] [199] [200] [201] [202] [203] [204]

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Fig. 8. Applications of 2D materials in energy storage. MoS2 Nanosheets Vertically Grown on Graphene Sheets for Lithium-Ion Battery Anodes: (a) HRTEM images of the MoS2/G sample; (b) Schematic illustration of the synthesis procedure of MoS2/G; (c) Cycling performance of MoS2/G, MoS2, and rGO electrodes at a current density of 100 mA g−1 for 150 cycles [221]. Copyright © 2016 American Chemical Society. Layered Metal Sulfides of MoS2/SnS2 enhance Li-Ion storage performance: (d) Schematic illustrations of the horizontal heterostructure of MoS2/SnS2; (e) Cycling performance of MoS2-rGO, SnS2-rGO, and MoS2/SnS2-rGO at 200 mA g−1 [208]. Copyright © 2018 American Chemical Society. Metallic 1T phase MoS2 nanosheets as supercapacitor electrode materials: (f) High-magnification image of restacked MoS2 nanosheets; (g) Evolution of the volumetric capacitance of the 1T phase MoS2 electrodes with scan rate for different electrolytes and 1-μmand 5-μm-thick films. The concentration of the cations in the electrolyte solutions was fixed at 1 M [231]. Copyright © 2015 Macmillan Publishers Limited. Multiheteroatom doped ultrathin porous carbon nanosheets for high performance supercapacitors: (h) SEM images of gelatin@KCs; (i) Volumetric capacitance of all the PCNS electrodes at the current densities from 1 to 200 A g−1 [232]. Copyright © 2018 Elsevier Ltd.

4.2.2. Supercapacitors Supercapacitors are another type energy storage devices with high power density (~103 W kg−1), ultrafast charging/discharging rate (~1 s), excellent cycling life (~104 cycles), and safe operation environmental [222]. According to the Ragone plot, for an ideal highperformance supercapacitor, how to realize a proper balance between power density and energy density is of great importance for the research of supercapacitors [223]. Based on their working mechanism, supercapacitors are divided into two types: electrochemical doublelayer capacitors (EDLCs) and pseudo-capacitors. EDLCs store energy by reversible ion adsorption/desorption, while the energy storage of pseudo-capacitors relies on reversible redox species [224]. Activated carbon (AC) with an ultrahigh surface area (~3000 m2/g) is one of the first studied electrode materials for supercapacitors based on EDLCs mechanism. However, the broad pore size distribution and inaccessible pores of ACs lead to a low specific capacitance (< 10 μF/cm2), which is much lower than the theoretical capacitance of EDLCs (15–25 mF/cm2) [225,226]. Generally, specific surface area, surface terminates, chemical stability, and electrical conductivity are four key factors to evaluate the performance of supercapacitors. Therefore, by tuning these factors, various strategies have been adopted to develop high-efficient materials, including defect engineering, heteroatom doping, thickness/

size control, and 2D heterostructure construction, etc. Graphene, MXenes, and TMDs are promising candidates for supercapacitors due to their good resistance to oxidation. For example, the theoretical capacitance of graphene is as high as 550 F g −1, but graphene nanosheets tend to be restricted by van der Waals interactions, resulting in a deterioration of energy storage performance. Thus, preventing restacking of graphene is of great importance [3]. Wu et al. prepared a type of chemically modified graphene and polyaniline (PANI) nanofiber composites through in situ polymerization, which exhibited a specific capacitance of 480 F/g at a current density of 0.1 A/g [227]. Recently, Yan et al. synthesized a flexible and conductive MXene/graphene heterostructure through electrostatic self-assembly between rGO and MXene nanosheets [228]. After electrostatic assembly, rGO inserted into the layers of MXene, increasing the interlayer spacing and lead to a capacitance of 1040 F cm−3 at a scan rate of 2 mV s−1. Such result proves the effect of interlayer spacing on the improvement of EDLCs performance of graphene. MXenes (e.g., Ti4C3), 2D TMDs (e.g., MoS2), and metal hydroxides (e.g., β-Co(OH)2) have also shown excellent performance [229,230]. The volumetric capacitance of Ti3C2Tx in neutral and basic electrolytes is about 300–400 F cm−3, comparable with the recently reported value of 350 F cm−3 from graphene-based materials. In addition, the 1T-phase MoS2 nanosheets can also achieve 13

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high volumetric energy density (Fig. 8f, g) [231]. This material can be intercalated electrochemically by the ions such as H+, Li+, Na+, and K+ and achieve high capacitance values of 400–700 F cm−3 as well as high coulombic efficiencies (95%) and stability over 5,000 cycles even at high-voltage and non-aqueous organic operation condition. The excellent performance is attributed to the intrinsic hydrophilicity and high electrical conductivity of 1T MoS2. Recently, Liu et al. reported a kind of multi-heteroatom doped ultrathin porous carbon nanosheets for high performance supercapacitors (Fig. 8h, i) [232]. Based on these researches, we expect that other TMDs and carbon materials can also exhibit excellent properties for energy storage applications once forming suitable active sites.

[246]. By controlling the evaporation angle, the channel length of the transistors can be reproducibly controlled to be anywhere between 20 and 70 nm (Fig. 9e–g). Due to the adoption of the smallest channel length (20 nm), the devices exhibit the highest on-state current (174 μA/μm) to date at a small source drain bias of 100 mV. In addition, 2D h-BN nanosheet is also an ideal electronic and optoelectronic material with a bandgap of 5.9 eV, which has been studied as the tunneling junction, gate dielectric, and electrical packaging [247]. Novoselov et al. investigated the electronic properties of h-BN crystalline layers with different conducting materials (graphite, graphene, and gold) on either side of the barrier layer [248]. They demonstrated that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field. As shown in Fig. 9i, the zero-bias conductivity for each type of device scales exponentially with the BN barrier thickness and is of the order of 1 kΩ−1 μm−2 for a monolayer BN sandwiched between gold and graphite electrodes, decreasing to approximately 0.1 GΩ−1 μm−2 for devices with 4 BN atomic layers. Besides contact electrodes, dielectrics are also important components for electronics. 2D h-BN has been used to form heterostructures with graphene, MoS2 and BP to improve the carrier mobility of 2D semiconducting materials or semimetals [249]. It has been studied that h-BN has strong in-plane ionic bonding in a planar hexagonal lattice structure. Moreover, the construction of heterostructures can suppress rippling and reduce scattering from the substrate in superimposed 2D semiconductors or semimetals [250]. For example, Mishchenko et al. reported vertical field-effect transistor for flexible and transparent electronics based on graphene-WS2 heterostructures [251]. The combination of tunnelling (under the barrier) and thermionic (over the barrier) transport allows for unprecedented current modulation exceeding 1 × 106 at room temperature and very high ON current (Fig. 9k). They have also demonstrated that WS2 is a material that acts as an ideal vertical transport barrier in graphene heterostructures. The photoelectrochemical (PEC)-type photodetector is a newly developed photoresponsive device which can be used to detect signals without external power sources [252]. Benefitting from its thicknessmodulated optical energy gap, various PEC-type photodetectors have been produced based on various materials including semiconductor, P-N junction, schottky junction, and semiconductor/graphene hybrid [253,254]. The far infrared region and ultra-fast luminescence make graphene an appealing material for photodetector, but the major challenges of graphene-based photodetectors are limited by low absorption cross-section and zero-band gap [255,256]. 2D TMDs have great potential as light harvesting photocatalyst since the enhanced photoluminescence (PL). However, most researches on the direct use of TMDs as visible light photocatalyst are limited by the thickness of single-layer [257]. Recently, the creation of 2D heterostructures assembled by graphene and other 2D crystals has been proven to be an important solution for the realization of novel photodetector [258]. Zhang et al. fabricated a novel sunlight photo-detector based on MoS2/graphene heterostructure [259], which shows superior photoresponse activities under the illumination of sunlight in contrast with bare MoS2 and graphene. Recently, He et al. designed a graphene/silicon (Gr/Si) (2D/3D) van der Waals heterostructure for high-performance photodetectors, where graphene acts as an efficient carrier collector and Si as a photon absorption layer [260]. In addition, metal oxide semiconductors are useful building blocks in the photoelectric devices due to their low-dimensional nanostructures with suitable bandgap, unique conduction properties, and confined carrier conduction pathways [261,262]. Moreover, the photoelectric performance based on metal oxide semiconductors could be enhanced by charge-carrier engineering [143].

4.3. Electronic/Optoelectronic devices In the past few decades, 1D nanomaterials (such as carbon nanotubes and silicon nanowires) have shown great promising for applications in electronic and optoelectronic devices [233,234]. Since the discovery of ultrathin 2D nanomaterials, these 2D materials have been applied extensively in these devices due to their remarkable mechanical flexibility, tunable electronic properties, and high optical transparency, all of which are desirable for flexible devices [13,235]. Moreover, the free of dangling bond on the surfaces of 2D nanomaterials can alleviate the surface scattering effects. Although graphene has been studied for long time, its electronic application is hindered by the lack of bandgap. Recently, the 2D semiconducting nanomaterials, such as TMDs [236238], h-BN [65], and black phosphorus (BP) [239], show excellent performance in these devices due to their relatively high carrier mobility and tunable band structures. Meanwhile, the construction of heterostructures could adjust atomic-sharp interfaces and lead to exceptional performance. 2D TMDs are considered as promising materials for next-generation electronics and optoelectronic devices because of its large bandgap and superior resistance to short-channel effect [18], while MoS2 is one of the typical representatives of semiconducting TMDs. The most important basic building block of modern electronic circuits is the fieldeffect transistor (FET), mostly used as a switch in digital circuits [240]. The first implementation of a switchable single-layer MoS2 transistor was demonstrated by Kis et al to achieve a superhigh mobility of at least 200 cm2 V−1 s−1 and room-temperature current on/off ratios of 1 × 108 for tunnel FETs [241]. After that, Kis et al. studied the intrinsic mobility and conductivity of single-layer MoS2 sandwiched in the dualgated geometry [242]. The dependence of mobility on temperature shows clear evidence of the strong suppression of charged-impurity scattering in dual-gate devices with a top-gate dielectric. In the past few years, the advancement of electronics has always been limited by energy dissipation. Based on this problem, Yalon et al. reported the first direct measurement of spatially resolved temperature in functioning 2D monolayer MoS2 transistors by using Raman thermometry in 2017 [243]. As shown in Fig. 9a, b, the thermal boundary conductance of the MoS2 interface with SiO2 is one order magnitude larger than previously thought, yet near the low end of known solid–solid interfaces. In addition, MoS2 is also a direct-bandgap semiconductor due to quantummechanical confinement with large absorption coefficiency, making it popular in optoelectronics. For example, Wang et al. reported straingated flexible optoelectronics based on monolayer piezoelectric-semiconductor MoS2 [244]. Fig. 9d shows the photoresponse in single-layer device when there is no strain applied; while the photodetection is systematically tuned by substrate-induced strain. Similar to TMDs, black phosphorus (BP) is also a p-type semiconductor and useful building block for 2D electronics [245]. The first BP transistor was demonstrated by Zhang et al in 2014, which is achieved with drain current modulation on the order of 105 and well-developed current saturation in the I–V characteristics [239]. Miao et al. reported highperformance top-gated BP field-effect transistors with channel lengths down to 20 nm fabricated using a facile angle evaporation process

4.4. Other applications In addition to applications in electronics, catalysis, and energy storage and conversion, 2D nanomaterials have become important candidates for SERS, sensing, bioimaging and solar cells applications. 14

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Fig. 9. Energy Dissipation in Monolayer MoS2 Electronics: (a) Topography image overlaid on schematic device structure and experimental setup. The Raman signal is measured while electrical bias is applied. The device is capped with a thin AlOx layer which enables stable operation during extended testing. (b) Current versus drain voltage and corresponding temperature maps at back-gate VGS = 25 V. Colored circles mark the bias point of each temperature color map [243]. Copyright © 2017 American Chemical Society. Single-Atomic-Layer MoS2 for Strain-Gated Flexible Optoelectronics: (c) A flexible two-terminal single-atomic layer MoS2 device. (d) Electrical transport in single-layer device in the dark and under different illumination intensities (wavelength = 442 nm) [244]. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Ultrashort channel length Black Phosphorus field-effect transistors: (e) Schematic diagrams illustrating the fabrication process of the ultrashort channel length BP FETs. (f) Comparison of transfer curves for top-gated BP FETs with various channel lengths. (g) Device on-current density plotted as a function of 1/L [246]. Copyright © 2015 American Chemical Society. Electron Tunneling through Ultrathin Boron Nitride Crystalline Barriers: (h) An optical image of one of our graphite/BN/graphite devices. The bottom graphite layer is shaped by reactive plasma etching into several stripes of 2.5 μm width (horizontal purple lines). Thin BN layers have high transparency so the edges of BN crystallites with different numbers of layers are marked by black lines. (i) Characteristic I–V curves for graphite/BN/graphite devices with different thicknesses of BN insulating layer: black curve, monolayer of BN; red, bilayer; green, triple layer; and blue, quadruple layer [248]. Copyright © 2012 American Chemical Society. Vertical field-effect transistor based on graphene-WS2 heterostructures: (j) Optical images of device in reflection and transmission modes. (k) I–V plot for the device with Vg = 0 V and bending applied. Curvature = 0.05 mm−1; T = 300 K [251]. Copyright © 2012 Nature Publishing Group.

4.4.1. SERS Surface-enhanced Raman scattering (SERS) is a phenomenon in which the molecular cross section of Raman scattering or near the surfaces of certain MNMs are enhanced by factors up to ~1014 [263]. As a useful and high-sensitivity detection tool, SERS has been applied in many fields, including cancer detection, food quality analysis, and compound analysis, etc. In order to obtain a reliable, stable, and reproducible SERS signal for detecting the target, two important factors should be taken into consideration. One is activity and another is reproducibility. At present, Noble metal (e.g., Ag and Au) have been widely used as the most effective SERS substrates, but these substrates have poor reproducibility because of the difficulty of controlling the aggregation process [264,265]. Generally, the morphology of the metal substrates could affect the spectral ranges of SERS. Therefore, controlled self-assembly of 2D nanostructures with well-defined

morphologies provides a proper strategy to prepare a substrate with high activity and reproducibility. For example, Zheng et al. strongly enhanced the photothermal stability of plasmonic metal nanoplates with the help of a core-shell structure (hexagonal Pd@Ag) by applying uniform Pd nanosheets as the seeds (Fig. 10a, b) [266]. The SERS signals on Pd@Ag could retain exceed 90% of initial intensity after 40 min laser irradiation, but the signal of Ag nanoprisms have reduced to ~60%, proving the superiority of the 2D core-shell nanoplates as an NIR-SERS substrate. In comparison to the conventional noble-metalbased SERS technique driven by plasmonic electromagnetic mechanism (EM), the family of 2D layered materials (e.g., graphene and TMDs) have been recently applied in the game of Raman enhancer due to their atomic uniformity, chemical stability, and biocapability [267,268]. Recently, 1T′ transition metal telluride semimetals (e.g. WTe2 and MoTe2) have attracted attention due to their intriguing semimetal 15

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Fig. 10. The photothermal stability of the plasmonic Pd@Ag core–shell bimetallic nanoplates upon irradiation for 30 min with a 2 W, 808 nm laser: (a) the top and bottom images are TEM images of the nanoplates before and after the laser irradiation, respectively. (b) The SPR changes before and after the laser irradiation [266]. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. FL quenching and SERS effects on the 1T′ transition metal tellurides: (c) Schematic diagram of the 2D transition metal tellurides as platforms for FL quenching and Raman enhancing of analytes. (d) Raman-FL spectra of the dye R6G coated on bare SiO2 (black line) and on 2D 1T′-WTe2 (red line). The signal integration times were 0.5 s for the black line and 4 s for the red line, respectively. The plotted intensities are normalized with respect to the integration times. The Raman peaks labeled with “#” denote the R6G signatures [270]. Copyright © 2018 American Chemical Society. N-doped graphene through enhanced Raman scattering: (e) HRTEM images of monolayer NG sheets on TEM grids. The inset is the corresponding selected-area electron diffraction pattern of NG. (f) Raman signals of RhB molecules on PG and NG sheets with 5 × 10−5 M. The laser excitation line is 2.41 eV, and the integration time is 10 s for all cases, where the arrows indicate graphene G and D bands [275]. Copyright 2016 © The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Room temperature gas sensing of two-dimensional titanium carbide (MXene): (g) Schematic representation of Ti3C2Tx structure and different functional groups on the surface of Ti3C2Tx nanosheets. (h) Gas-sensing results of a device based on Ti3C2Tx toward 100 ppm acetone and ammonia gas bubbling at room temperature (25 °C) [108]. Copyright © 2017 American Chemical Society. Core–Shell Pd@Au nanoplates as theranostic agents for invivo photoacoustic imaging: (i) Scheme show the fabrication process of Pd@Au nanoplates. (j) Photoacoustic imaging of Pd@Au-PEG in tumor sites and quantification photoacoustic signals of tumor site at different time [291]. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Enhancing efficiency of perovskite solar cells via N-doped graphene: (k) Cross-sectional SEM images of perovskite/N-RGO hybrid solar cells. (l) Current density–voltage characteristics for pristine perovskite (control) and perovskite/N-RGO hybrid solar cell (N-RGO) [306]. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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nature and extremely large magnetoresistance [269]. In 2018, Xu et al. demonstrated the WTe2 and MoTe2 as ultrasensitive platforms for CMbased SERS [270]. As shown in Fig. 10c, d, with the adoption of 1T′WTe2 atomic layers, the signal ratios of Raman/FL features are greatly augmented, allowing the analyte Raman peaks to be distinguishable from the background. They also revealed that the ultrasensitive SERS effects depend on the strong analyte-telluride interaction of 1T′-W (Mo)Te2 and the large DOS near the Fermi level. In addition to TMDs, graphene-enhanced Raman scattering (GERS) has been studied in recent years [271,272]. In 2009, Liu et al. reported graphene as a substrate to suppress fluorescence in resonance Raman spectroscopy [273]. In 2010, they also discussed the possibility that graphene can be used as a substrate for enhancing Raman signals of adsorbed molecules, and found that the enhanced efficiencies are quite different on monolayer, few-layer, multilayer graphene, graphite, and highly ordered pyrolytic graphite (HOPG) [274]. Recently, Feng et al. presented a comprehensive study on the GERS effect of pristine graphene and nitrogen-doped graphene [275]. Fig. 10f shows the enhanced Raman scattering effect of RhB for both NG and PG samples, showing that the molecules on NG exhibit a higher Raman intensity than that on PG substrates. These results indicate that graphene and TMDs are promising candidates for surface-enhanced Raman scattering (SERS).

will alter. For example, Yu et al. demonstrated a simple sensing method to detect inorganic anions in aqueous solution based on silver nanoplates with edge length of 70 nm and thickness of 2 nm [287]. This asprepared silver nanoplates show a high sensitivity on the order of 1 × 10−6 M in distinguish individual anions (e.g., Cl−, Br−, I−, H2PO4−, and SCN−) from other anions (e.g., F−, SO42−, CH3COO−, NO3−, and ClO4−) and inorganic cations (e.g., Zn2+, Cd2+, and Cu2+). It was proposed that the surface electron charging is mainly determined by the interaction tendency between silver atoms and various inorganic anions in water. 4.4.3. Bioimaging Fluorescent probe nanomaterials are highly concerned because of their superior performance in cancer imaging and therapy [288]. Inorganic QDs and heavy metals have a good fluorescent signal for bioimaging due to their tunable wavelength, excellent photostability and high quantum yields, but high toxicity severely restricted their practical applications [289]. Notably, 2D nanomaterials could be used as agents by labeling specific tissues or cells to improve imaging sensitivity and diagnostic ability, and the tunable LSPR properties of metal nanostructure show strong light absorbing or scattering capabilities [290]. Typical examples include the utilization of metal nanoplates in photoacoustic imaging, CT imaging, and photothermal therapy. In 2014, Zheng et al. reported a type of core-shell Pd@Au nanoplates as theranostic agent (Fig. 10i, j), which showed an obvious enhancement with a significantly higher CT value [291]. In addition, MoS2 have also been found to show good biocompatibility, high stability in physiological fluids, and good single- and two-photon fluorescence imaging properties. MoS2 has Raman active modes which are thickness sensitive and offer a reliable way for determining number of layers specifically [292]. Chou et al. reported a kind of exfoliated MoS2 as near-infrared photothermal agents, displaying about 7.8 times strengthen absorbance with an extinction coefficient at 800 nm of 29.2 L g−1 cm−1 for NIR [293]. These characteristics provide a foundation to explore the use of MoS2 for a wide range of biological applications. Naturally, apart from metal materials, graphene and graphene-like 2D nanomaterials have been proved to the promising metal-free fluorescent nanomaterials. For example, Liu et al. firstly developed functionalized graphene nanosheets (RGO–IONP–PEG) as efficient probes for in vivo multimodal tumor imaging [294]. The surface temperature of tumors in mice treated with RGO–IONP–PEG increased to 48 °C after laser exposure, leading to a complete elimination. This study highlights the great potential of graphene-based functional nanocomposites for cancer theranostic applications.

4.4.2. Sensing Sensors have attracted enormous attention due to their wide applications in process control, environmental monitoring, and medical diagnostics, etc [276]. In the past few decades, most researches are focused on metal oxide-based sensors, but increased power consumption and safety problems hinder their practical application. Thus, it is desirable to explore sensing materials with high surface-to-volume ratios and excellent surface activities under low or room temperatures [277]. Due to the unique localized surface plasmon resonance (LSPR) properties of 2D materials, low-power-consumption LSPR-based sensors have been explored by integrating molecular identification techniques [278]. At present, the 2D nanomaterials have been used as sensing platforms for three sensing systems (electronic sensors, fluorescent sensors, and electrochemical sensors). The electronic sensors are based on the conductance change of the channel material in FET devices or chemiresistors induced by the interaction between the target analytes and the channel material via two major sensing mechanisms: the electrostatic gating effect and the doping effect [279,280]. In terms of ultrathin 2D nanomaterials, their atomic thickness can ensure the ultimate exposure of surface atoms to analytes in the detection process, thus promising ultimate sensitivities for given materials [2]. The realization of fluorescence sensors is based on the fluorescence responses induced by the interaction between target analytes and fluorescent probes, the high specific surface area of 2D materials enables them ultimate surface interaction with fluorescent dye molecules, thus yielding a quenching efficiency superior to those of other morphologies or their bulk counterparts [281]. The detection in electrochemical sensors is mainly based on electron transfer between the active material coated on a working electrode and the target analytes in a three-electrode working system in an electrolyte solution. 2D nanomaterials have been widely explored as active materials in electrochemical sensors due to their advantages such as enhanced mass transport enabled by small size, high surface area, and enhanced signal-to-noise ratio as compared to their bulk counterparts [282,283]. Until now, graphene, metal dichalcogenides (e.g. MoS2, MoSe2, WS2, SnS2, and VS2), black phosphorus (BP), h-BN, and g-C3N4 have been explored as sensing materials [284–286]. In 2017, Lee et al. synthesized 2D Ti3C2Tx sheets (MXene) for room temperature gas sensing by a simple solution casting strategy (Fig. 10g, h), through which they successfully detected ethanol, methanol, acetone, and ammonia gas at room temperature [108]. Moreover, many other LSPR-based sensors have been prepared by observing the changes of LSPR peak. Once the detected molecules confined to the surface of 2D nanomaterials, their dielectric constant of surroundings

4.4.4. Solar cells Among all the renewable energy (wind energy, hydrogen energy, tidal energy, solar cells, and biomass-fired electricity generation), the usage of solar energy is an efficient pathway [295,296]. Solar cells could directly convert solar radiation into electricity by utilizing the energy from the sun. In 1954, based on a p–n junction type solar cell, researchers first achieved the commercial conversion of solar into electric energy with 6% efficiency at the Bell Telephone Laboratories [297]. After that, photovoltaic cells became the power source with conversion efficiencies of 15–20%. However, the relatively high cost and the toxic chemicals have prevented their widespread use and promoted people to seek environmentally friendly and low-cost alternatives. The unique optical and electrical properties of 2D materials can be applied in solar cell devices. Up to now, graphene and TMDs have been explored as functional photovoltaic devices [260,298–302]. Recent years people have witnessed the wide development of graphene replacing traditional transparent conductive oxide electrodes (e.g., ITO and FTO) in solar cell devices due to its high optical transparency and electrical conductivity. Furthermore, the impurity doping or hybridization of graphene can increase the conductivity while maintaining its high transparency, making it suitable for flexible 17

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Fig. 11. Manipulation of catalytic activity through thickness and size control. Ultrathin Co3O4 layers realizing optimized CO2 Electroreduction to formate: (a) Calculated density of state (DOS) and charge-density distribution for Co3O4 atomic layer slab with thickness of 1.72 nm and (b) bulk Co3O4 slab. The yellow shaded parts represent the increased DOS at the conduction band edges of Co3O4 atomic layer. (c) Comparison of linear sweep voltammetric curves in 0.1 M KHCO3 aqueous solution for Co3O4 with different thicknesses [58]. Copyright © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Monolayer-precision synthesis of MoS2 and their nanoscale size effects in the HER: (d) Representative layer models for 1L, 2L, and 3L MoS2. The slab models for 2L and 3L are stacked with a layer center-to-center distance of 6.19 Å, and a vacuum space is introduced above and below the layer models. (e) TOFs with respect to potential and (f) TOFs at −200 mV (vs RHE) with respect to the number of layers (inset: Atomic resolution TEM images of corresponding MoS2@OMC nanostructures) [57]. Copyright © 2015 American Chemical Society. The layer-dependent catalytic activity of PtSe2 atomic crystals for HER: (g) Optical image of as-grown PtSe2 flakes, exhibiting a narrow thickness distribution. (h) Diagram on thickness of the as grown few-layer PtSe2 as a function of the growth temperature and the amount of reactants. (i) Polarization curves for the HER obtained on 1–5L and ~20L PtSe2 flakes and a commercial Pt/C catalyst [310]. Copyright © 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Controllable growth and transfer of monolayer MoS2 on Au foils and application in HER: (j) Schematic view illustrating the edges of monolayer MoS2 functioning as the active catalytic sites for HER. (k) SEM images of monolayer MoS2 flakes with 40% coverage. (l) Coverage-dependent polarization curves and corresponding Tafel plots [313]. Copyright © 2014 American Chemical Society. 18

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photovoltaic devices. For example, Ruoff et al. reported a method using a fluoropolymer as both the supporting and doping layer, which could increase the carrier density significantly, and the resulting monolayer graphene film exhibits a sheet resistance of ~320 Ω/sq. Sang et al. prepared a N-doped reduced graphene film by spin-casting method, which act as a transparent cathode for high-performance PLEDs with the resistance achieved 300 Ω/sq at 80% transmittance [303]. With the development of dye-sensitized solar cells (DSCs) based on the fast regenerative photoelectrochemical mechanism, conventional solid-state photovoltaic technologies are challenged [304]. The main difference between DSCs and conventional cells is that the functional element of DSCs is separated by the charge carrier transport. In 2008, Wang et al. firstly reported the graphene films as transparent window electrodes for DSSCs by subsequent thermal reduction of GO, which exhibited a higher conductivity of 550 S/cm and a transparency of more than 70% among 1000–3000 nm [305]. Recently, Anders et al. improved conversion efficiency of solar cells by introducing N-RGO into the perovskite layer (Fig. 10k, l). An increase in short circuit photocurrent density JSC (20.77–21.80 mA cm−2), VOC (1.12–1.15 V), and slightly in FF (0.73–0.74) were observed [306]. The high performance is mainly attributed to the larger grains and thicker perovskite layer, which reduces the grain boundaries. Meanwhile, 2D TMDs are another attractive materials as interlayers or active layers in photovoltaic devices due to their metallic, semiconducting, and superconducting behaviors [307]. Therefore, the incorporation of 2D metals could improve the charge transport efficiency and increase overall conversion efficiency.

via a fast-heating method. They deduced that the ultrahigh fraction of low-coordinated surface Co atoms may be the potential active sites for efficient CO2RR. DFT calculations shown in Fig. 11a, b clearly indicate that the ultrathin Co3O4 nanosheet possesses a higher density of states (DOS) at the conduction band edges compared to bulk counterpart. Furthermore, the 1.72 nm layers showed a current density of 0.68 mA cm−2 at −0.88VSCE, which is 1.5 and 20 times higher electrocatalytic activity than that of 3.51 nm thick Co3O4 layers and bulk counterpart, respectively (Fig. 11c). Combining experimental and DFT calculations revealed that the thinner Co3O4 nanosheets have more active sites and higher dispersed charge density. Single-layered 2D nanomaterials have the highest theoretical specific density of active sites for electrochemical reaction. Notably, Joo et al. reported the preparation of MoS2 with different thickness and the size-dependent catalytic activity for HER. The model structures of one-, two-, and three-layer MoS2 were established based on the ΔG of DFT calculations [57]. Fig. 10d shows the representative slab models of single-, double-, and triple-layer MoS2 (named as 1L, 2L, and 3L). It can be seen that the 1 L- and 2L-MoS2@OMC catalysts show the best performance, with a TOF value of 2.32 s−1 at 200 mV. Experimental and DFT calculation (Fig. 11e, f) show that the turnover frequency of HER increases with the decrease of layer numbers due to the exposing of more sulfur sites in the edges. More interestingly, when size reduces from micron to nanometer scales, the lattice distortion occurs and introduces a large number of active sites for catalytic process. Recently, Jiao et al. investigated the layer-dependent HER activity of PtSe2 flakes based on theoretical and experimental results [310]. As shown in Fig. 11g–i, the HER activity of PtSe2 flakes increases gradually with the increase of layer numbers, as indicated by the small over potential (~60 mV) and Tafel slope (~41 mV/decade). The thickness, size and edge of graphene also play important roles in the ultimate properties. Impressively, Bao et al. reported the size effect of graphene on ORR catalysis with the catalytic activity increased for the smaller size of graphene at different fixed voltages [311]. Dai et al. developed a chemical method to control the size of various graphene nano-structures by controlled etching, a high on/off ratio up to ~104 was achieved for field-effect transistors built with sub-5-nm-wide graphene nanoribbon [312]. Therefore, controllable synthesis of monolayer materials is an efficient way to enhance catalytic activity. Liu et al. synthesized high quality monolayer MoS2 flakes on Au foils with tunable size (edge length from 200 nm to 50 μm) via CVD strategy (Fig. 11j, k) [313]. Meanwhile, the comparison of polarization curves and corresponding tafel plots of monolayer MoS2 were established in Fig. 11l. It can be seen that the monolayer MoS2 flakes with 80% coverage exhibited small overpotential of 300 mV and high current density of 50.5 mA cm−2, which is 25 times higher than that of bulk MoS2. This enhancement is attributed to the gradual increase of density of the active sites at the edge of MoS2. Following this work, thickness and size regulation have been applied to a wide variety of catalytic reactions.

4.5. Strategies of engineering It has been known that abundant active sites and high catalytic activity are significant for chemical properties of catalytic materials [56]. Although 2D materials have some natural properties originating from their pristine active sites, the number of active sites is limited and the activity is mild. The surface and interfacial engineering strategies can maximize the intrinsic activity of 2D materials and endow them with favorable composition, thickness, defects, and surface properties for practical application. Compared with the bulk counterparts, a key advantage of 2D nanomaterials is that all the catalytic active sites in the atomic-thickness nanosheets can be sufficiently exposed, making the chemical modification easy [308]. To date, many strategies have been adopted to modulate the electronic and physical structures, including size and thickness regulation, elemental doping, vacancy engineering, strain/phase engineering, and 2D heterostructure construction, etc [155]. For example, making the 2D materials thinner and smaller is often implemented to increase density of active edge sites. Doping or attaching functional groups at the edges could amplify variation of properties; modulation of basal sites could adjust the intrinsic activity of inert basal planes; endowing edges with low coordination numbers can create the dangling bonds and atom vacancies. In this section, we will discuss these strategies of engineering in detail by providing some typical examples.

4.5.2. Elemental doping Elemental doping is an effective and controllable method to alter electron transfer properties and manipulate surface chemistry composition of 2D nanomaterials for electrochemistry applications, which is generally classified into two categories: non-metal-elemental doping and metal-elemental doping [56,145,314]. Elemental doping can improve catalytic activity of 2D nanomaterials by disturbing the basal plane, enlarging the interlayer spacing, enhancing the rate of charge transfer during reaction. Non-metal-elemental doping generally includes the doping of N-, B-, O-, S-, F- and P-atoms into 2D nanomaterial, including graphene, MoS2, and g-C3N4 [315,316]. Metal-elemental doping is mainly based on transition metals for synthesizing singleatom catalysts on certain supports [191]. At electrocatalysis process, the pristine activity of a 2D material is limited by the number of active sites and mild characteristic; while dopants can maximize the intrinsic activity of 2D materials for catalyses. A typical example is that the

4.5.1. Thickness and size regulation Thickness and size of 2D nanomaterials have great implications on their chemical properties. With thickness down to the atomic scale, catalytic activity enhancement may result from in-plane defects generated by structural disordering and the change of the electronic states [55,158]. Moreover, for some ultrathin 2D nanomaterials, the active sites mainly originate from unsaturated metal edges or nonmetal edges. The number of active sites increases dramatically with the decrease of layer number and horizontal size, where the fully exposed edges could enhance the catalytic activity [309]. Therefore, one of the most reasonable ways to engineer the electronic structure is to regulate the thickness and size of the nanosheets. Xie et al. reported ultrathin Co3O4 nanosheets used for CO2 electroreduction to formate [58]. The nanosheets with thickness of 1.72 and 3.51 nm were successfully prepared 19

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graphene sheets and h-BN materials doped with heteroatoms (N, S, P, F and B) are highly favorable for improving their electrocatalytic activities toward ORR, OER, HER, CO2RR, and NRR [317]. Amongst all heteroatom-doped graphene, N-doped graphene has been most extensively investigated. For example, Jeon et al. developed N-doped edge-functionalized graphene, which exhibited better ORR activity than its counterparts [141]. They have also found that B- and N-codoped nanoribbons could serve as efficient ORR catalysts, the excellent electrocatalytic properties were attributed to the abundant edges sites created by the doped boron and nitrogen atoms. Therefore, the low-cost doped graphene-based catalysts have great potential to replace the precious Pt-based catalysts for electrocatalytic reactions [176,318]. In addition, doping at the edges and basal sites could also amplify their catalytic activity by tweaking the intrinsic activity of inert basal planes. Dai et al. developed edge-rich dopant-free graphene as highly efficient ORR electrocatalyst. The onset potential of the ORR on the untreated graphene electrode is 0.76 VRHE, shifted positively to around 0.87 VRHE on the edge-rich graphene (P–G). The positive shift of onset potential clearly demonstrated a significant enhancement of the edge-rich graphene relative to the pristine graphene electrode [319] (Table 4).

active sites where O2 molecules are adsorbed as the initial step of the ORR. Similarly, it has been known that non-metal atom doping is also an efficient way to improve other properties, including perovskite solar cells and polymer light-emitting diodes (PLEDs). For example, Mahboubeh et al. enhanced efficiency of perovskite solar cells with N-doped graphene (Fig. 12j) [306]. Jin et al. reported a kind of N-doped reduced graphene transparent electrodes to enhance performance of polymer light-emitting diodes (Fig. 12k)[335]. Besides the graphene-based materials, non-metal-atom doping has been extended to other materials for various reactions. For instance, Huang et al. prepared P-doped 2H-MoS2 film on conductive substrate by a low-temperature heat method [330]. Interestingly, it was found that P-doping could dramatically decrease Mo valence charge. The P2H-MoS2 material shows improved HER activity with low onset potential (130 mV) and small tafel slope of 49 mV dec−1 [330]. Except for MoS2 alloys, S-doped MoP layer, achieved through a facile electrochemical anodic process and a two-step chemical vapor deposition treatment, also exhibited enhanced performance. Yang et al. proved that the H2 formation (via Volmer–Heyrovsky mechanism) on a metal (Mo) absorbed hydride is favored over S-MoP than MoS2 by the calculation [333]. In addition, two-dimensional metal carbides (MXenes) are also found to be promising candidates. Handoko et al. synthesized Ti3C2Tx via different fluorine-containing etchants and found that those with higher fluorine coverage on the basal plane have lower HER activity [336]. The HF etchant-Ti3C2Tx achieved −10 mA/cm2 at 189 mV overpotential in acid solution. Non-metal doped 2D nanomaterials have shown excellent catalytic activity for the ORR, OER, HER, CO2RR and NRR by optimizing electronic states and adsorption energies.

4.5.2.1. Non-metal-elemental doping. With the confinement of dimension to atomic-scale thickness, surface incorporation of nonmetal atom provides an efficient way to engineer the electronic structure of electrocatalysts [145]. Nitrogen (N) has greater electronegativity compared to carbon atom and other heteroatoms; nitrogen-doped graphene was one of the first studied materials with improved chemical reactivity. After that, other heteroatoms such as O, B, S, F and P have also been reported [56]. Qiao et al. developed a class of heteroatom-doped graphene as efficient HER electrocatalysts based on experiments and DFT calculations. The investigated models were established via the synergistic coupling effect between N and other heteroatoms (phosphor (P), sulphur (S) and boron (B)) [162]. As shown in Fig. 12a, a wide range of doping samples (N,S-G, N,P-G and N,B-G) were evaluated as HER electrocatalysts. From DFT calculation shown at Fig. 12b, the optimal dual-doped graphene model is N,S-G with a low ΔGH* value of 0.23 eV compared to N,P-G of 0.53 eV and N,B-G of 1.10 eV. In agreement well with the predicted trend: N,S-G and N,P-G exhibit a lower overpotential than that of the N-G (Fig. 12c). Different non-metal atoms can form different types of active sites on graphene surface. Boron (B) is also an important doping element that adjusting electron deficiency of graphene. In 2018, Zheng et al. reported a kind of boron-doped graphene for NRR [198]. The much smaller electronegativity of boron (2.04) than that of carbon (2.55) resulted in an obvious difference of electron densities of carbon ring structure (Fig. 12d). The positively charged boron atoms are the optimal active centers for the formation of NH3 (Fig. 12e). The BC3 structure presents the lowest energy barrier for NRR with a NH3 production rate of 9.8 μg hr−1 cm−2 and higher faradic efficiencies of 10.8% at −0.5 VRHE (Fig. 12f). Generally, these doped-heteroatoms exist in the form of substitutional dopants at the edges or in the basal plane, disrupting the atom network, and creating defect sites. Therefore, doping mode could also significantly affect their electrocatalytic performance [308]. To further understand the observed high activity of these doped 2D materials, Nakamura et al. designed a highly oriented pyrolitic graphite (HOPG) model catalysts with well-defined π conjugation and well-controlled pyridinic N to study the ORR active site via Ar+ etching mask method [334]. Fig. 12g shows that the surface uniform structures were distributed in a wide range. The N 1s spectra (Fig. 12h) show clearly that the undoped samples (clean-HOPG and edge-HOPG) are indeed free of N, whereas the graphitic N (401.1 eV) and pyrrolic N (400.1 eV) are observed for grap-HOPG and pyri-HOPG). Fig. 12i shows that the pyriHOPG model catalyst displays higher ORR activity than the N-free model catalysts. They further found that pyridinic N is the optimal ORR active sites, carbon atoms next to pyridinic N are proposed to be the

4.5.2.2. Metal-Elemental doping. Incorporating metal atoms into the crystal lattice of 2D nanomaterials is also an effective approach to manipulate the electronic structures and enhance the intrinsic catalytic activity by causing the fine deformation of atomic arrangement and redistribution of electron density [55,155,337]. Zeng et al. modified atomically thin SnS2 nanosheets by doping Ni for efficient CO2RR. The 5% Ni-doped SnS2 nanosheets achieved a highest faradaic efficiency of 93% for carbonaceous product with 19.6 mA cm−2 at −0.9 VRHE. The electronic structures of pristine SnS2 and Ni-doped SnS2 nanosheets were studied to explain the dramatic difference of charge-transfer process [191]. Fig. 13a shows the calculated density of states (DOS) of a single-layer SnS2 slab and a Ni-doped SnS2 slab, the band gap from 2.3 eV of pristine SnS2 decreased to 1.9 eV of Ni-doped SnS2, which was attributed to the Ni doping (Fig. 13b). Meanwhile, the doped transitionmetals have been expected to enhance HER activity of MoS2 by adjusting the free energy of hydrogen adsorption (ΔGH) on the surface. Xing et al. successfully activated the MoS2 surface basal plane by doping with a low content of atomic palladium (Pd) based on a redox technique [338]. As illustrated in Fig. 13d, they found that Pd substitution occurs at the molybdenum site, simultaneously introducing sulfur vacancy and converting the 2H into the stabilized 1 T structure, a trigonal lattice structure of the 1T- and 2H- are both clearly shown in Fig. 13e. The 1% Pd-doped MoS2 exhibited 805 μA cm−2, overpotential of 78 mV at 10 mA cm−2 (Fig. 13f). In addition, Dai et al. reported cobalt-doped FeS2 nanosheets–carbon nanotubes for highly-efficient HER [329]. Xie et al. synthesized a Mndoped CoSe2 ultrathin nanosheets exhibited excellent HER activity with a small overpotential of 174 mV, a small tafel slope of 36 mV/dec, and a large current density of 68.3 μA cm−2 [331]. Zhu et al. developed onepot synthesis of Co-doped VSe2 nanosheets for enhanced HER [332]. DFT calculations show that Co dopants dramatically reduce the Gibbs free energy for hydrogen adsorption (ΔGH) and promote electron transfer and HER kinetics. Yan et al. synthesized a type of telluriumdoped black phosphorus (Te-doped BP) single crystals for OER, which demonstrated that the onset-potential decreased from 1.63 V for undoped BP to 1.49 V for Te-doped BP [326]. In addition, Lei et al. synthesized a Fe(III) modified BiOCl ultrathin nanosheet (Fe(III)@BOC 20

21

Boron-doped graphene Boron carbide nanosheet

N-doped graphene nanosheets B- and N-codoped graphene porous boron carbon nitride nanosheets nitrogen, oxygen-doped graphene nanosheets Te-doped black phosphorus g-C3N4 coordinated transition metals N- and O-codoped carbon hydrogel film Co-doped FeS2 nanosheets-CNTs P-doped 2H-MoS2 film Mn-doped CoSe2 ultrathin nanosheets Oxygen-incorporated MoS2 nanosheets Co-doped VSe2 Nanosheets S-doped MoP nanoporous layer

Phosphorus-Doped g-C3N4 N-doped carbon nanosheets

Prepared by graphene nanosheets with NH3 two-step doping strategy polymer sol–gel method micro-wave plasma enhanced CVD system liquid exfoliation; ultrasonication annealing at 600 °C in N2; polycondensation reactions filtration process followed by N-doping with ammonia Hydrothermal method low-temperature heat treatment in N2 liquid exfoliationmethod Hydrothermal method Hydrothermal method Electro-anodic process; a two-step CVD thermal reduction of H3BO3 with GO in a mixed H2/Ar Commercially obtained

Hydrothermal method High-temperature pyrolysis of GO and H3BO3 under Ar High-temperature pyrolysis of L-cysteine and melamine hydrothermal process; under heating in N2 High-temperature pyrolysis of citric acid and NH4Cl

Ni-doped SnS2 nanosheets Boron-doped graphene

Nitrogen doped graphene nanoribbon

Synthesis

Catalyst

Table 4 A summary of heteroatom doping of 2D materials.

NRR NRR

ORR/OER ORR HER OER ORR ORR ORR OER OER OER OER HER HER HER HER HER HER

CO2RR

CO2RR CO2RR

Application

[198] [202]

[322] [323] [324] [325] [326] [327] [328] [329] [330] [331] [149] [332] [333]

specific activity at 0.5 V: 0.07 to 0.14 electron s−1 pyri-N−1 The Jk value is13.87 mA cm−2, selectivity 98.5% for four-electron pathway Eonset is 0.94 VRHE, E1/2 is 0.82 V in alkaline; Eonset is 0.84 V in acidic. with an overpotential of 351 mV and a Tafel slope of 38 mV dec−1 in alkaline solution the OER Eonset of undoped and Te-doped BP nanosheets was 1.63 and 1.49 V, respectively Eonset is 1.5 VRHE, and a 1.61 V to deliver an anodic current density of 10 mA/cm2 reached 5 mA cm−2 at the overpotential of 368 mV and 14.8 mA cm−2 at 564 mV ~0.12 V at 20 mA/cm2, Tafel slope of ~46 mV dec−1, and long-term durability over 40 h with low onset potential (130 mV) and small Tafel slope of 49 mV dec−1 a overpotential of 174 mV, Tafel slope of 36 mV/dec, and current density of 68.3 mA cm−2 exhibits onset overpotential as low as 120 mV, accompanied by extremely large current density a low overpotential of 230 mV at 10 mA/cm2, a small Tafel slope of 63.4 mV/dec with a low overpotential of 86 mV at 10 mA cm−2 and low Tafel slope of 34 mV dec−1 achieves a NH3 production rate of 9.8 μg h−1 cm−2 and FE of 10.8% at −0.5 VRHE achieve a NH3 yield of 26.57 μg h−1 mg−1 cat , and a fairly high FE of 15.95% at –0.75 VRHE

[193]

Exhibits 15.4 A gcatalyst−1 with FECO of 87.6% at overpotential of 0.49 V

[320] [321]

[191] [192]

achieved a FE of 93% for carbonaceous with a current density of 19.6 Ma cm−2 at −0.9 VRHE achieved a FE of 66% for formate at −1.4 VSCE

onset potential of 0.94 VRHE, E1/2 of 0.67 V for ORR; onset potential of 1.53 V for OER charge/discharge voltage gap (0.77 V), energy density (806 Wh kg−1), and cycling life (over 330 h) were obtained at 10 mA cm−2

Ref.

Performance parameters

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Fig. 12. Manipulation of catalytic activity through non-metal-elemental doping. Dual-doped graphene models and electrochemical measurements for HER: (a) Atomic configurations of three dual-doped models with the lowest computed |ΔGH*|. Green, pink, blue, red, gold, purple and white represent: C, B, N, O, S, P and H atoms, respectively. (b) The three-state free energy diagram for the pure, single- and dual-doped graphene models. (c) Electrochemical measurements on various graphene-based materials in 0.5 M H2SO4 showing polarization curves with the benchmark MoS2 under the same conditions for direct comparison [162]. Copyright © Macmillan Publishers Limited, part of Springer Nature. Boron-Doped Graphene for NRR: (d) Schematic of the atomic orbital of BC3 for binding N2. (e) LUMO (blue) and HOMO (red) of undoped G (G) and BG (right). The position of a single doped boron atom was labeled. (f) The FENH3 values of BG-1, BOG, BG-2, and G at different applied potentials [198]. Copyright © 2018 Elsevier Inc. Structural characterization of four types of N-HOPG model catalysts and their ORR performance. (g) Top: optical image of patterned edge-N+-HOPG; Bottom: the AFM image obtained for the region indicated by the yellow rectangle in top. (h) N 1s XPS spectra of model catalysts. (i) ORR results for model catalysts corresponding to (h). Nitrogen contents of the model catalysts are shown as the inset in (h) [334]. Copyright © 2017 American Association for the Advancement of Science. (j) Time-resolved PL decay plots of pristine perovskite, perovskite/RGO, and perovskite/NRGO films. The measurements were performed from the perovskite film side [306]. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (k) Luminous efficiency vs luminance (ηEL-L) curves of iPLEDs [335]. Copyright © 2011 American Chemical Society.

NS) by a facile solvothermal method [339]. The Fe(III) doping resulted in a 0.58 eV down-shift of the BiOCl and extended the light absorption from ultraviolet light to visible light while promoting the interfacial charge transfer. As shown in Fig. 13h–j, the modification of Fe(III) narrows the band gap of BiOCl, while keeping its photocatalytic ability.

surface sites on MoS2 sitting on Au (1 1 1). Based on the quantitative analysis of scanning tunneling microscopy, the HER activity is correlated linearly with the number of edge sites on MoS2 catalyst. In 2012, Jaramillo et al. successfully engineered the surface structure of MoS2 to expose more edge sites by preparing highly ordered large-area thin MoS2 films with nanoscale pores [343]. The high surface curvature of mesostructure exposes a large fraction of edge sites as well as increasing surface area, which sharply enhance HER activity. By DFT calculations, it was found that the electronic state could be strongly enhanced at the edges of graphene compared to in-plane sites. By plasma etching, Dai et al. also developed a kind of edge-rich graphene as a highly efficient ORR electrocatalyst [319]. With a similar strategy, edge-rich carbon nanotubes and graphite can also be obtained with enhanced ORR activity. In 2014, Asadi et al. discovered that MoS2 edges have superior CO2RR activity with a high current density and low overpotential [344]. After that, they also reported WSe2 nanosheets exhibiting enhanced CO2 reduction efficiency compared to their bulk counterparts due to the generation of a large number of edges. A higher current density of 18.95 mA/cm2 was observed on WSe2 nanosheets compared to that of 3.4 mA/cm2 on bulk MoS2 at the same overpotential of 54 mV (Fig. 14d) [345]. Based on DFT calculations for TMDCs (Fig. 14e), they found that COOH* formation is exergonic due to the strong binding to metal edge sites, and the CO* is also much more stable on the TMDCs than on Ag, residing at lower energy than COOH*. Moreover, the calculated density of the edge metal atom (Mo or W) further proves the strong binding interaction of the adsorbed intermediates with the TMDCs (Fig. 14f). In addition, Zhang et al. reported a kind of few-layer black phosphorus (BP) nanosheets as electrocatalysts for highly efficient OER [346]. More importantly, they are aware that reduction of thickness of BP nanosheets could generate extra edge active sites on the ultrathin planar structure. The edge sites could also enhance SERS properties. Qin et al. investigated the role of etching in the formation of Ag nanoplates with straight, curved and wavy edges and then compared their SERS properties (Fig. 14g). It was found that Ag nanoplates with wavy edges embraces a SERS enhancement factor at least 6 and 13 times stronger than those with straight and curved edges, respectively [347].

4.5.3. Creating active sites It is generally accepted that the exposure of more surface active sites can greatly improve the electrocatalytic activity. Effective pit/pore creation and edge/vacancy engineering on the basal surfaces have been explored to create more active sites, such as coordination number, edge sites, and oxygen vacancies. 4.5.3.1. Pit/pores creation. Most researches have shown that the lowcoordination surface atoms on 2D materials are disordered. Recently, it has been realized that the pit/pores creation on the surface and edges of catalysts can significantly decrease the coordination number and cause huge variations in the electronic states of the surrounding atoms [308]. For instance, Xie et al. produced a type of atomically-thick porous Co3O4 sheets with a thickness of 0.43 nm and about 30% pore occupancy affording low-coordinated Co3+ atoms to serve as the catalytically active sites for OER [340]. Due to the obviously increased density of states at the valence band and conduction band, the porous Co3O4 sheets exhibit an electrocatalytic current density up to 341.7 mA cm−2, roughly 50-times larger than that of the bulk counterpart. Afterwards, they successfully developed pore-rich WO3 ultrathin nanosheets with nearly fully exposed highly crystal facets for oxygen-evolving photoanode [341]. Compared to bulk counterpart, the Raman spectrum of WO3 nanosheets with numerous surface pore is broadening, which is attributed to the enhanced electron–phonon effect (Fig. 14a). Therefore, as shown in Fig. 14b, the pore-rich WO3 nanosheets showed a large current density of 2.14 mA cm−2 at 1.0 V, about 18 times higher than that of bulk counterpart. In order to clarify the variation of electronic structure, it was further found that the state density of pore-rich WO3 nanosheets dramatically increased compared to bulk WO3 (Fig. 14c). The significantly increased DOS endows more photogenerated holes and apace carrier migration rate. Recently, Cao et al. also synthesized a type of porous Co3O4 nanosheets via a selfsacrificing template method for reversible electrochemical lithium storage. Impressively, the porous structure shorten the diffusion path of ions/electrons and increase the contact surface between the electrolyte and electrode material, obtaining a high discharge capacity of 1000 mA h g−1 at 400 mA g−1 after 100 cycles [342]. Therefore, pit/pores creation is indeed an efficient method to improve the performance of 2D materials.

4.5.3.3. Vacancy engineering. Vacancies presenting in 2D nanomaterials are commonly recognized as the active sites for electrocatalytic processes due to their adjustable atomic scale and high concentration. Incorporation of vacancies is a more efficient way to decrease the coordination number and manipulate the electronic states of surface atoms compared with pits/pores creation and edge engineering [348]. Therefore, vacancy engineering has been widely adopted to increase their catalytic activities. However, proper engineering methods are required to precisely study vacancies and their effect on reaction mechanisms. For example, Xie et al. successfully solved the problem of insufficient active sites in bulk CoSe2 by reducing its thickness into the atomic scale [349]. The positron annihilation spectrometry and Xray absorption fine structure (XAFS) spectra clearly show that a large number of VCo″ vacancies formed in the ultrathin nanosheets. DFT calculations revealed that these VCo″ vacancies can serve as efficient

4.5.3.2. Edge engineering. It is well known that the exposed atoms with more unsaturated coordination sites on the edges of 2D materials have greater propensity than other parts of the materials (basal planes) [7]. Because the edge sites play important role in catalytic activity, it becomes crucial to design the edge structure to enhance their performance. In 2007, Chorkendorf et al. have identified active edge sites for HER [62]; they systematically studied the distribution of 23

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active sites of OER with overpotential as low as 320 mV at 10 mA/cm2. After that, they synthesized rich- and poor-VO In2O3 porous sheets by a fast-heating strategy [63]. As expected, the O-vacancy-rich In2O3 nanosheet exhibits a photocurrent of 1.73 mA/cm2, which is over 2.5 and 15 times larger than that of the O-vacancy-poor In2O3 and bulk

In2O3 material. In addition, the reducible VO site can also activate the electron and atomic structure of CO2 molecules. For example, Zeng et al. reported an efficient strategy to activate CO2 by introducing oxygen vacancies into ZnO nanosheets [350], which exhibited FECO of 83% with a current density of −16.1 mA cm−2 (Fig. 14h). DFT

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Fig. 13. Manipulation of catalytic activity through metal-elemental doping. Ni-doped SnS2 nanosheets for CO2RR: (a) Calculated DOS of SnS2 and Ni-doped SnS2 slabs. (b) The distribution of charge density at the defect level of Ni-doped SnS2 slab. (c) Faradaic efficiencies for carbonaceous product (C-product) over pristine SnS2 and Ni-doped SnS2 nanosheets [191]. Copyright © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Design of a Pd–MoS2 material for HER: (d) Schematic illustration of the spontaneous MoS2/Pd (II) redox reaction. (e) Dark-field scanning transmission electron microscopy image of the 1%Pd–MoS2. Blue and yellow balls indicate Mo and S atoms, respectively. Scale bar: 1 nm. (f) LSV polarization curves of MoS2, 1%Pd–MoS2, 1%Pd–MoS2/CP, 1%Pd–C, and 20%Pt–C (with iR correction). (g) Free energy versus the reaction coordinates of different active sites based on DFT calculation for the energetics of Pd doping to promote the HER activity [338]. Copyright © 2018 Springer Nature. Fe(III) modified BiOCl ultrathin nanosheet towards high-efficient visiblelight photocatalyst: (h) Valence-band XPS spectra of the Fe(III)@BOC NS and BOC NS samples, and the schematic illustration of the band structure of the Fe(III)@BOC NS and BOC NS. (i) The long-term of hydrogen evolution with Fe(III)@BOC NS and BOC NS photocatalysts under visible light irradiation. (j) Transient photocurrent densities of Fe(III)@BOC NS and BOC NS electrodes with light on/off cycles under visible light irradiation in 0.1 M Na2SO4 electrolyte solution at an applied potential of 0.5 V versus Ag/AgCl reference electrode [339]. Copyright © 2016 Elsevier Ltd.

calculations show that the oxygen vacancies can increase the charge density of ZnO around the valence band enhancing activation of CO2 (Fig. 14i). Meanwhile, Xie et al. also reported a kind of atomic layer with confined vacancies for CO2RR, and demonstrated that the main defect is the oxygen (II) vacancy [351]. The oxygen vacancy concentrations of these materials were evaluated by X-ray absorption fine structure spectroscopy. As expected, the oxygen (II) vacancies reduced the rate-limiting activation barrier from 0.51 to 0.40 eV through stabilizing the formate anion radical intermediate. Recently, Sun et al. developed a new vacancy engineering strategy for OER, they deliberately introduced base-soluble Zn (II) or Al (III) sites into NiFe LDHs, then the Zn (II) or Al (III) sites were selectively etched to form atomic M(II)/M(III) defects with improved OER activity. DFT calculations revealed that the form of dangling Ni–Fe sites could efficiently lower the Gibbs free energy of the oxygen evolution process [352]. Recently, Wei et al. reported a high-index faceted porous Co3O4 nanosheets with oxygen vacancies for highly efficient water oxidation [353], which were successfully synthesized by a simple hydrothermal method followed by NaBH4 reduction. These surface oxygen vacancies can substantially enhance the charge transfer rate and increase the number of active sites for OER.

CO2RR by constructing the atomic models of Pd octahedron and icosahedron (Fig. 15f). It showed that the icosahedron surface has a distinct strain with +1.8%, while the octahedron is that of −0.5%. It is worth noting that most TMDs (e.g., MoS2, WS2, etc.) have two main crystal phases (2H and 1T), some results indicated that metallic 1T-phase MoS2 exhibits higher performance than 2H-phase MoS2, mainly due to the excellent conductivity of 1T-phase [60,61]. In most experiments on phase engineering of TMDs, lithium (Li) was usually used to intercalate TMDs, which then leads to an electron transfer from the reducing agent to the structure of TMDs and increases the electron density of the d orbital, inducing the phase transform from pristine 2H into metallic 1T [231]. For instance, Manish et al. developed a kind of phase-engineered low-resistance MoS2 transistors [356]. They discovered that the metallic 1T-phase can be locally induced on semiconducting 2H-phase nanosheets, thus decreasing their resistances. HRTEM shows that the phase boundary between the 1T- and 2H-phases is atomically sharp with no visible defects (Fig. 15g). Moreover, the transfer characteristics of electrodes with or without 1T phase are shown in Fig. 15h. In 2015, they also reported a type of metallic 1T phase MoS2 nanosheets as supercapacitor electrode materials attaining capacitance values of 400 to 700 F cm−3 in aqueous electrolytes [231]. The enhanced performance is attributed to the intrinsic hydrophilicity and high electrical conductivity of 1T-phase. Their work shows that phase engineering is a very promising method to sharply enhance the performance of MoS2 devices.

4.5.4. Strain, phase, and interface engineering 4.5.4.1. Strain and phase engineering. Lattice strain and crystal phase play a great role in electrochemical process by tuning the surface electronic structure as well as the adsorption ability [155]. It is generally accepted that surface strain is the force caused by mismatch between lattice of different components or twin structures, while phase engineering has a highly conductive matrix. Owing to the atomic thickness and high resilience of 2D materials, the d-band electronic structure could be optimized to approach the Fermi level, increasing the electron-transfer rate in reaction process [184]. Therefore, it was considered to be an efficient strategy to improve electrocatalytic activities. For instance, Zheng et al. reported a method to activate and optimize the basal plane of monolayer 2H-MoS2 for HER by introducing strain and sulphur (S) vacancies in the basal plane [354]. Their experimental and DFT calculation results indicated that the formed S-vacancies and strain could allow hydrogen to bind directly to Mo atoms. Recently, Wang et al. made a new breakthrough on tunable intrinsic strain. Generally, surface strain formed depends on external stress, but the effect is often hidden by interfacial reconstructions and materials shape. They firstly explored intrinsic surface stresses in Pd nanosheets synthesized by using CO as both the reducing agent and stabilizing ligand [355]. As shown in Fig. 15a and 15b, surface metal atoms often form charge redistribution to alter the structure of ultrathin nanosheets, leading to a corresponding change in the distance between metal layers. The compressive strain strongly depends on the slab thickness of freestanding transition metal varying from 1 ML (~0.2 nm) to 12 ML (~2.5 nm). Experimentally, Pd nanosheets with (1 1 0) terminations have the best ORR activity compared to Pd/C and Pt/C catalysts (Fig. 15e), showing that generating and tuning of intrinsic surface strain is an efficient way to engineer and enhance electrocatalysis activity. In 2017, Zeng et al. use Pd nanostructures as an ideal platform to understand strain effects on

4.5.4.2. Interface engineering. Interface engineering is another efficient strategy to enhance performance of electrocatalysts [357]. Many researches have shown that interface interactions between two different components will alter their electronic states and chemical properties. To make sure that the multicomponent electrocatalysts can present improved electrochemical performance, these issues should be taken into consideration: (1) the compatibility of components, (2) the conductivity and electronic transmission after the composition, and (3) the solution dispersion of multicomponent [145]. Generally, interface engineering is divided into two categories: heterostructure engineering and synergistic interaction. Heterostructure engineering usually includes chemical bonding between two different materials, while synergistic interaction is the physical contact of two materials with confined electron transport. Both methods have advantages and disadvantages, and complement each other [55]. Yu et al. synthesized an efficient molybdenum disulfide/cobalt diselenide hybrid catalyst (MoS2/CoSe2) by in situ growth of MoS2 on the surface of CoSe2 [358]. HRTEM images (Fig. 15i) revealed that the layered MoS2 nanosheet has an interlayer space of 0.63 nm. Fig. 15j shows that MoS2/CoSe2 electrode has a low overpotential of 11 mV for HER, while pure CoSe2 or MoS2 nanosheets exhibited inferior HER activity. This enhancement could be attributed to the synergistic effects between MoS2 and CoSe2 materials. Zhang et al. also reported that the support graphene can promote the activity of amorphous MoS2 for CO formation from CO2RR [359]. In addition, Qiao et al. synthesized a hybrid with graphitic-carbon nitride and N-doped graphene (C3N4@NG hybrid) as a metal-free catalyst. It exhibited an excellent HER activity with low overpotential [360]. Experimental and DFT calculations 25

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Fig. 14. Manipulation of catalytic activity through creating active sites. Pore-rich WO3 ultrathin nanosheets for efficient oxygen-evolving photoanode: (a) Raman spectrum. (b) The photocurrent versus applied potential curves. (c) The DOS of pore-rich WO3 nanosheets [341]. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Nanostructured transition metal dichalcogenide electrocatalysts for CO2 reduction in ionic liquid: (d) Cyclic voltammetry (CV) curves for WSe2 nanosheets, bulk MoS2, Ag nanoparticles (Ag NPs), and bulk Ag in CO2 environment. Inset shows the current densities in low overpotentials. (e) Calculated free energy diagrams for CO2 electroreduction to CO on Ag (1 1 1), Ag55NPs, MoS2, WS2, MoSe2, and WSe2 at 0 VRHE. (f) Top: calculated partial density of states of the d band (spin-up) of the surface Ag atom of Ag55. Bottom: the surface baremetal edge atom (W) of the WSe2 [345]. Copyright © 2017 American Association for the Advancement of Science. (g) A schematic diagram showing a possible mechanism for the formation of Ag triangular nanoplates with straight edges and enneahedral nanoplates with curved edges during a hydrothermal synthesis [347]. Copyright © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Oxygen vacancies in ZnO nanosheets enhance CO2RR to CO: (h) Current densities for CO production. (i) Gibbs free energy diagrams for CO2 reduction to CO on ZnO slab with/without oxygen vacancy. * indicates an adsorption site [350]. Copyright © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. 26

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Fig. 15. Manipulation of catalytic activity through strain, phase, and interface engineering. Fine tuning of intrinsic strain to optimize ORR catalytic reactivity on Pd (1 1 0) nanosheets: (a) Mechanism of the generation of intrinsic strain in 2D transition metal nanosheets. h is the height of an atomic layer; ε is lattice strain of platinum group and coinage metal slabs. (b) Potential energy profile of strained versus unstrained Pd (1 1 1) slabs with thicknesses of 8 ML, 4 ML, and 2 ML. (c) Intrinsic in-plane strain of fcc (1 1 1) and hcp (0001) with thicknesses from 1 ML to 12 ML. (d) Pd bulk lattice (yellow dots) on the HRTEM images of Pd nanosheets of 5 ML. (e) ORR polarization curves of Pd nanoparticles, as well as Pd nanosheets with average thickness of 3 ML, 5 ML, and 8 ML in 0.1 M KOH (inset shows the halfwave potential). Using Pd nanostructures as an ideal platform to understand strain effects in CO2RR: (f) TOP: surface strain fields of a Pd octahedron and icosahedron. Color indicates strain labeled in the color map. The surface strain is calculated based on the equilibrium bond length in bulk Pd. Bottom: projected d-density of states (PDOS) of surface atoms on Pd (1 1 1) surfaces with different surface strains. The calculated d-band centers are marked with white lines [355]. Copyright © 2019 American Association for the Advancement of Science. Phase engineering of MoS2: (g) High-resolution transmission electron microscope image of an atomically thin phase boundary (indicated by the arrows) between the 1T and 2H phases in a monolayered MoS2 nanosheet. Scale bar, 5 nm. (h) Properties of field-effect transistors with 1T and 2H contacts. Blue curves represent devices with 1T phase electrodes and the black curves are with Au on the 2H phase. The red curve is the Id 1T channel device [356]. Copyright © 2014 Macmillan Publishers Limited. MoS2/CoSe2 hybrid for HER: (i) HRTEM images of MoS2/CoSe2 hybrid showing distinguishable microstructures of MoS2 and CoSe2. Scale bars, 5 nm. (j) Polarization curves for HER on bare GC electrode and modified GC electrodes comprising MoS2/CoSe2 hybrid, pure MoS2, pure CoSe2 and a high-quality commercial Pt/C catalyst. Catalyst loading is about 0.28 mg cm−2 for all samples. Sweep rate: 2 mV s−1 [358]. Copyright © 2015 Macmillan Publishers Limited. 2D/2D BiOCl-g-C3N4 ultrathin heterostructure nanosheets for enhanced visible-light-driven photocatalytic activity in environmental remediation: (k, l) High-magnification of TEM image of 50CN-50BC composite nanosheet. (m) Photocatalytic degradation of 4-CP under visible light irradiation over different samples. (b) Pseudo first-order kinetic fitting and the determined apparent rate constants (k) [361]. Copyright © 2017 Elsevier B.V.

indicated that the unique electrocatalytic properties originate from the synergistic enhancement of proton adsorption and reduction kinetics. Recently, Cui et al. reported that the 2D BiOCl-g-C3N4 ultrathin heterostructure nanosheet can enhance visible-light-driven

photocurrent response in environmental remediation [361]. As shown in Fig. 15l, the corresponding HRTEM image exhibits good crystalline and clear lattice fringes of the tetragonal BiOCl. Moreover, the formation of the interface between g-C3N4 layer and BiOCl nanosheet 27

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revealed the strong interfacial interaction between them. Such catalyst exhibited 95% removal efficiency of 4-chlorophenol within 2 h, which is about 12.5, 5.3 and 3.4 times of that on pure BiOCl, g-C3N4 and OVspoor heterostructure, respectively (Fig. 15m, n).

heterojunction (HJ) photocathode for PEC H2 production [375]. As shown in Fig. 16h, the bare Si photocathode exhibits poor PEC performance, while MoS2/Si-HJ photocathode achieves an excellent PEC H2 production performance with a positive onset potential of 0.5 VRHE and a maximum photocurrent density of 36.33 mA cm−2 at 0 VRHE, indicating an exceptional catalytic activity of 2D heterostructure structure. Fu et al. integrated the active OER and HER components as the heterostructures for the efficient overall water splitting. The highefficient catalysts could be constructed by coupling active OER (Ni3N) and HER (NiMoN) catalyst as the heterostructures [376]. In addition, the 2D heterostructure could greatly improve detection sensitivity of specific DNA, which is of great importance among environmental monitoring, gene therapy, biomolecular analysis and other biomedical applications. For example, Chen et al. reported a kind of graphene/ MoS2 heterostructures for ultrasensitive detection of DNA hybridisation [377]. As shown in Fig. 16i, the integrated PL peak area significantly increased with the concentration of the added complementary DNA solutions for graphene/MoS2 stack film. More importantly, the proposed graphene/MoS2 heterostructure is able to sense the aM concentration of target DNA (Fig. 16j). It is expected that the application of 2D heterostructure could be expanded to other area, including labelfree detection of protein, metal-ion contaminants and intercellular.

4.5.5. 2D heterostructure construction The single-crystalline nature and extraordinary aspect ratios of 2D nanomaterials provide an ideal platform for the construction of 2D heterostructure from different types of ultrathin nanosheets through van der Waals interactions or chemical bonds [3]. 2D heterostructure construction usually could enhance electrochemical performances through the increase of density states, the variety of valence positions and conduction bands [362]. More importantly, it is possible to engineer microsized 2D p-n heterojunctions at the atomic/nanometer scale to change their electronic states and chemical properties [363–365]. The prerequisite for good applications is the construction of various kinds of 2D heterostructures with well-defined structures, compositions, crystal phases, and interfaces. Generally, design principles for constructing 2D heterostructures follow these parameters: (I) different materials should have similar crystal structures; (2) at least one component should have potential electrocatalytic activity; (3) the complex should have good conductivity; (4) both could complement each other. To date, five types of 2D heterostructures have been constructed: (I) two different types of 2D nanomaterials form vertical 2D heterostructures via layer-by-layer deposition/growth [366]. In particular, the epitaxial growth of one 2D nanosheet onto another is special, some researchers have reported the construction of type (I) 2D vertical heterostructures by in situ epitaxial growth 2D TMD/semiconductor vertical heterostructures [367]. (II) The edge of ultrathin nanosheet grows with another edge to form an in-plane 2D heterostructure. For example, Li et al. reported epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface, where the edge of WSe2 induces the epitaxial MoS2 growth [368]. As shown in Fig. 16a, b, an atomically sharp interface between the WSe2-MoS2 junctions was formed. To study the electrical properties, the prepared WSe2-MoS2 heterojunction contacted with Pd and Ti/Au. Fig. 16c clearly shows the symmetric I–V curves and photovoltaic effect for individual WSe2 (contacted with Pd) and MoS2 (contacted with Ti/Au), indicating that the heterojunction is predominant rather than the small schottky barriers between metal and TMDCs. In addition, WSe2-WS2 and MoSe2WSe2 heterostructures have also been constructed by them. (III) The vertical growth of aligned 2D nanoarrays on another substrate to form hierarchical heterostructures [369,370], the construction components are different from that of type (I). (IV) Partially engineering the crystal phase of TMD nanosheets. For example, after transforming its 2H phase to 1T phase in a part of the MoS2 nanosheet, the 2D in-plane 2H-1T MoS2 heterostructure was formed [371]. (V) The binary, ternary, and multiple phase of 2D metal nanostructures could coexist 2D heterostructure [372]. Therefore, the construction of 2D well-defined heterostructure is very desirable in electrocatalytic process. Graphene and its derivatives are the most widely used materials to complex with atomically thin 2D nanomaterials. The incorporation of graphene not only improves the electrical conductivity, but also enhances the intrinsic properties of the host materials. For example, Dou et al. reported a kind of atomic layer-by-layer Co3O4/graphene composite (ATMCNsGE) for high performance lithium-ion batteries, which was synthesized by hybridization of ATMCNs and graphene under stirring and heating conditions (Fig. 16d) [373]. The structure of ATMCNs-GE was further explored by Raman spectroscopy and XPS, as shown in Fig. 16e, f, an extra peak (530.2 eV) of O 1s belong to the Co-O-C linkage; meanwhile, these typical Raman peaks (F2g, Eg and A1g mode vibrations) are strongly enhanced for ATMCNs-GE. As expected in Fig. 16g, the ATMCNs-GE exhibits much higher rate capability (5.62 C) compared with ATMCNs (0.11 C). The construction of 2D heterostructure could also enhance HER and OER performance [374]. Recently, He et al. reported MoS2/Si-

5. Conclusions and outlook In the past few decades, researchers have witnessed the emergence, development, and superiority of 2D nanomaterials. These atomicthickness nanomaterials undoubtedly exhibit some fascinating structural, electronic, and physical properties. In the near future, we believe that 2D nanomaterials can make a huge contribution in both fundamental studies and practical applications. In this review, we briefly introduce the general synthetic strategies and some important and newly developed members of the 2D family. Then, we discuss in detail the engineering strategies to enhance their intrinsic performance for wide ranges of applications among the electrocatalysis (HER, OER, ORR, CO2RR, NRR), batteries, supercapacitors, photocatalysis, etc. The 2D nanostructures can serve as ideal platforms to resolve the structure–property relationships, their intrinsic activities could be further enhanced by various engineering strategies due to the atomic thickness plasticity and chemical compatibility. Generally, these engineering strategies could be divide into two main aspects: one is the engineering of the intrinsic structure of 2D nanomaterials, including thickness/size regulation for increasing the number of active sites, and edge/vacancy engineering or pit/pores creation for creating more active sites; the other is to enhance efficiently the kinetics of charge transfer, including elemental doping (non-metal-elemental and metal-elemental doping), strain/phase/interface engineering, and 2D heterostructure construction for designing synergetic composites. In all, the surface and electronic structure engineering is of great importance for achieving novel physical and chemical properties by optimizing the spin configuration, electrical conductivity, active site exposure, and reaction energy barrier. Although great progresses have been made in this fantastical field, many new challenges have emerged for the systematic and comprehensive study of 2D nanomaterials. First, it is still difficult to produce ultrathin (single- or two- layer) 2D nanomaterials with high yield and quality. Considering current preparation methods accordingly, wetchemical method is more effective due to the solution compatibility and controllability. Second, the insightful characterization and understanding of 2D nanostructure are still at an initial stage; while the understanding of growth mechanism is critically important for designing new kinds of 2D nanomaterials and regulating their intrinsic activity precisely. Therefore, the development of economic, effective, and advanced in situ characterization techniques is very necessary. Third, it should be noted that the ultrathin 2D nanomaterials must have a dynamic structure variation over long working time under reaction 28

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conditions. The construction of carbon-loaded ultrathin 2D nanomaterials or mixing ultrathin metal sheets with stable carbon materials (e.g. GO, g-C3N4, and BCN, etc.) could be an efficient solution for the longterm stability problem. Fourth, to date, although many strategies

have been adopted to modulate the electronic and physical structures for enhancing their performance, they always regulated only one aspect of intrinsic material, which inevitably will destroy their excellent structure. We believe that the hybridization of 2D nanomaterials is a

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Fig. 16. Manipulation of catalytic activity through 2D heterostructures construction. Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface: (a) High-resolution STEM images taken from the WSe2-MoS2 in-plane heterostructure. (b) Atomic model showing the interface structure between WSe2 and MoS2. (c) Electrical transport curves for Ti-contacted MoS2 and Pd-contacted WSe2 separately [368]. Copyright © 2015 American Association for the Advancement of Science. Atomic layer-by-layer Co3O4/Graphene composite for high performance lithium-ion batteries: (d) High-resolution STEM image of ATMCNs-GE. (e) O 1s XPS spectra for ATMCNs-GE. (f) Raman spectra of ATMCNs-GE, ATMCNs, and graphene, with the inset showing details of the G band. (g) Rate capabilities and capacity ratio (inset) of ATMCNs-GE and ATMCNs for LIBs electrochemical performance [373]. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (h) LSV curve of bare Si cell and MoS2/Si-HJ photocathode in 0.5 M H2SO4 at a scan rate of 20 mV/s upon AM 1.5G illumination [375]. Copyright © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement. Graphene/MoS2 heterostructures for ultrasensitive detection of DNA hybridisation: (i) Schematic illustration for the charge distribution of DNA on graphene/MoS2. (j) The real-time photoluminescence response of the graphene/ MoS2 to the target-DNA with increased concentration [377]. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

promising strategy to fully utilize the unique properties of different 2D structure and generate new properties. In addition, the intrinsic defects (e.g. elemental composition, surface functional groups, and lattice distortion) of 2D materials are being ignored. The role of intrinsic defects on materials performance should also be investigated deeply in future. Fifth, theoretical studies on the formation and working mechanism of many 2D nanostructures are heavily lacking and need to be strengthened. In summary, with the help of advanced characterization techniques, a combination of theoretical computation and experimental measurements, it is very promising to design and develop new functional 2D nanomaterials.

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