Interface-Assisted Synthesis of 2D Materials: Trend and Challenges

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Interface-Assisted Synthesis of 2D Materials: Trend and Challenges Renhao Dong, Tao Zhang, and Xinliang Feng*

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Department of Chemistry and Food Chemistry & Center for Advancing Electronics Dresden, Technische Universität Dresden, 01062 Dresden, Germany ABSTRACT: The discovery of graphene one decade ago has triggered enormous interest in developing two-dimensional materials (2DMs)that is 2D allotropes of various elements or compounds (consisting of two or more covalently bonded elements) or molecular frameworks with periodic structures. At present, various synthesis strategies have been exploited to produce 2DMs, such as top-down exfoliation and bottom-up chemical vapor deposition and solution synthesis methods. In this review article, we will highlight the interfacial roles toward the controlled synthesis of inorganic and organic 2DMs with varied structural features. We will summarize the state-of-the-art progress on interfacial synthesis strategies and address their advancements in the structural, morphological, and crystalline control by the direction of the arrangement of the molecules or precursors at a confined 2D space. First, we will provide an overview of the interfaces and introduce their advantages and uniqueness for the synthesis of 2DMs, followed by a brief classification of inorganic and organic 2DMs achieved by interfacial synthesis. Next, the currently developed interfacial synthesis strategies combined with representative inorganic and organic 2DMs are summarized, including the description of method details, the corresponding structural features, and the insights into the advantages and limitations of the synthesis methods, along with some recommendable characterization methods for understanding the interfacial assembly of the precursors and crystal growth of 2DMs. After that, we will discuss several classes of emerging organic 2DMs with particular emphasis on the structural control by the interfacial synthesis strategies. Note that, inorganic 2DMs will not be categorized separately due to the fact that a number of review articles have covered the synthesis, structure, processing, and applications. Finally, the challenges and perspectives are provided regarding the future development of interface-assisted synthesis of 2DMs with diverse structural and functional control.

CONTENTS 1. Introduction 2. Overview of Interfacial Synthesis 2.1. Classification of Interfaces 2.2. Advantages and Uniqueness of Interfaces 2.2.1. Gas/Solid and Liquid/Solid Interfaces 2.2.2. Layered Structures as Interfaces 2.2.3. Air/Liquid and Liquid/Liquid Interfaces 2.2.4. Surface Roughness 2.2.5. Molecular Orientation 2.2.6. Chemical Reactivity and Selectivity 2.2.7. 2D-Confined Template-Directed Synthesis 3. Classification of 2DMs 3.1. Inorganic 2DMs 3.1.1. Graphene 3.1.2. Hexagonal Boron Nitride (h-BN) 3.1.3. Transition Metal Dichalcogenides (TMDs) 3.1.4. Metal Chalcogenides 3.1.5. Metals 3.1.6. Metal Oxides 3.2. Organic (Organometallic) 2DMs 3.2.1. 2D Polymers (2DPs) 3.2.2. 2D Supramolecular Polymers (2DSPs)

© 2018 American Chemical Society

3.2.3. 2D Covalent-Organic Frameworks (2D COFs) 3.2.4. 2D Metal−Organic Frameworks (2D MOFs) 4. Interfacial Synthesis Strategies 4.1. Air/Water Interface 4.2. Liquid/Liquid Interface 4.3. Liquid/Solid Interface 4.3.1. Graphene Templating 4.3.2. Salt Templating 4.3.3. Metal-Directed Synthesis 4.4. Gas/Solid or Vacuum/Solid Interface 4.5. Layered Structure Templating 4.5.1. Surfactant Bilayer Templating 4.5.2. Layered Double Hydroxide Templating 4.6. Self-Sacrificial Templates 5. Interfacial Synthesis of 2DPs 5.1. 2DPs by Photopolymerization 5.2. 2DPs by Imine Bonds 5.3. 2DPs by Cyclotrimerization 5.4. 2DP by Homocoupling 6. Interfacial Synthesis of 2DSPs

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Special Issue: 2D Materials Chemistry Received: January 26, 2018 Published: June 18, 2018 6189

DOI: 10.1021/acs.chemrev.8b00056 Chem. Rev. 2018, 118, 6189−6235

Chemical Reviews 7. Interfacial Synthesis of 2D COF Films 8. Interfacial Synthesis of 2D MOF Films 8.1. None-Conjugated 2D MOFs 8.2. Conjugated 2D MOFs 9. Interfacial Synthesis of Other Sheetlike Materials 9.1. Polymer Nanosheets 9.1.1. Liquid/Ice Interface 9.1.2. Surfactant Bilayer Templating 9.1.3. Other Layered Structures 9.2. None-Graphitic Carbon Nanosheets 9.2.1. Salt Templating 9.2.2. Montmorillonite Templating 10. Structural and Compositional Characterization 10.1. Imaging Methods 10.2. Spectroscopic Methods 11. Conclusions and Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments 12. Abbreviations and Terminology References

Review

(M: early transition metal; A: group A element (mostly IIIA and IVA); X: C and/or N; n = 1−3)) have also gained great interest. In addition to inorganic 2DMs, organic and organometallic 2DMs have also attracted growing attention, which include 2D covalent organic frameworks (2D COFs),66−71 2D metal− organic frameworks (2D MOFs), 19,72−80 2D polymers (2DPs), 5,10,18,81−94 and 2D supramolecular polymers (2DSPs).77,95−107 Their structures, properties, and functions can be feasibly tuned at the molecular level, due to the structural diversities of organic ligands and the various linkage chemistries. Thus, tailor-made organic 2DMs can be achieved by varying the ligands with π-conjugation, electron-donating/-accepting properties, selective functional groups, catalytic and photoactive centers, etc. Thus far, the capabilities for the preparation of 2DMs with desired composition, size, thickness, crystal phase, defect, and surface property have been of the core importance for the further study of their physical and chemical properties as well as exploration of potential applications. Various synthetic protocols based-on “top-down” and “bottom-up” strategies have been established for the synthesis of 2DMs. The “top-down” strategies include the physical mechanical exfoliation method from their corresponding bulk layered materials3,12 and the chemical exfoliation of bulk layered crystals in solution assisted by mechanical sonication,4,6,108−124 shear force,125,126 ion intercalation,127−129 and exchange.130−132 as well as electrochemical methods133−137 However, the former protocol is practically not scalable. Despite the latter one offering a potential tool to produce large-scale single-layer or few-layer 2DMs at the ambient condition, the current limitation remains the broad distribution of sheet thickness and lateral size as well as low yield, low structure integrity, and high tendency of restacking. In contrast, the “bottom-up” strategies, including chemical vapor deposition (CVD)8,23,138−148 and wet-chemistry synthesis, can overcome these limitations. Importantly, the interface has been playing a key role in most “bottom-up” synthesis methods and is advantageous for directing the orientation of the molecules or precursors. Typically, the reactions at the interfaces between air− water (or gas−liquid), liquid−liquid, liquid−solid, and gas−solid (or vacuum-solid) have been explored to offer paramount control over the morphology and the structure of inorganic 2DMs, such as carbon sheets, metal sheets, metal oxide sheets, transition metal dichalcogenides, and metal chalcogenides as well as organic (organometallic) 2DMs including 2DPs, 2DSPs, 2D COFs, and 2D MOFs. For instance, the CVD method relying on the substrates (metal or carbon) yields an interface between precursor vapor and substrate for the subsequent adsorption, nucleation, arrangement, and polymerization of corresponding precursors or intermediates (such as carbon radicals generated from CH4 for graphene synthesis). In this respect, the solid substrate provides a confined 2D geometry for the growth or polymerization of precursors. Thus far, the controlled synthesis of single-layer or multilayer 2DMs with tunable lateral size and thickness has been realized through interfacial synthesis in the past decade. Nevertheless, the in-time summary about the interfacial synthesis strategies toward emerging 2DMs has still been missing. Hence, in this review article, we aim to provide an overview on the current efforts on the development of interfacial synthesis methodologies, understanding the role of interfaces and structural control of 2DMs, including inorganic, organic, and organometallic materials. It should be noted that this review does not encompass ordered covalent or noncovalent networks on a

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1. INTRODUCTION Two-dimensional (2D) materials (2DMs) refer to laterally infinite, one to a few atom- or monomer-unit thin, atomically ordered networks with strong in-plane bonding and weak outplane bonding resulting in unique chemical reactivity and physical properties.1,2 In a more general view, 2DMs can be categorized as 2D allotropes of various elements or compounds (consisting of two or more covalently bonded elements) or molecular frameworks with periodic structures.1−20 Thus, 2DMs possess a high degree of anisotropy with layered structure. Currently, 2DMs have gained great interest due to their unique physical and chemical properties as well as potential applications in (opto)electronics, catalysis, energy storage and generation, sensing, separation, and other related fields.2,21 The discovery of graphene one decade ago,3 a truly 2D “aromatic” monolayer of carbon atoms, has demonstrated exceptional physical properties including ultrahigh electron mobility,3 quantum Hall effect,22 ballistic charge carrier transport, etc.1,23 The rapid rising of graphene research is a strong evidence not only in terms of basic scientific interest but also for its potential technological impact. Taking into consideration that graphene is just the tip of the iceberg in the whole family of 2DMs, one can imagine the great fundamental interest and potential for the whole class of 2DMs. Typical examples of inorganic 2DMs beyond graphene include functional derivatives of graphene (e.g., graphene oxide, GO),24,25 hexagonal boron nitride (h-BN),8,26−28 transitionmetal dichalcogenides (TMDCs, such as MoS 2 , WS 2 , WSe2),7,12,29−32 Xenes (such as silicene and phosphorene),9,14,33−35 graphitic carbon nitrides,36−39 noble metals,40−46 metal oxides,47−53 etc. The flexibility of the monolayers enables them to be stacked on top of one another in a chosen sequence to yield unique, heterogeneous multilayers, leading to a class of artificial materials, so-called van der Waals heterostructures,20,54,55 where each 2DM can possess a specific function. Additionally, homogeneous multilayer materials, such as 2D (layered) hybrid perovskites,56−60 and MXenes61−65 (a group of layered ternary compounds with the general formula Mn+1AXn 6190

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Figure 1. Schematic illustration of various interfaces for chemical synthesis, including gas/liquid, liquid/liquid, gas/solid (or vacuum/solid), and liquid/ solid interfaces.

Figure 2. Schematic illustration of the organization of molecules at gas/solid and liquid/solid interfaces.

2DMs by interfacial synthesis and present the details on the control of their crystalline growth, structure, and morphology. In section 9, we will specifically discuss several representative examples which do not lead to structurally defined 2DMs but materials with novel 2D morphology, for which these examples make an important contribution to the development of interfacial synthesis. Finally, in the Conclusion and Outlook, we will highlight the current limitations and challenges of the interfacial synthesis strategies as well as their future developments.

solid surface under vacuum (like metal or highly oriented pyrolytic graphite) by means of scanning tunneling microscopy (STM) due to the intrinsic challenging lift of these networks from the surface. 149−151 The covalent or noncovalent modification of 2DMs2,152,153 and the organization of nanoparticles into 2D arrays154−156 are also excluded in this review, although the interface also contributes to these examples. Additionally, the emulsion polymerization at the liquid/liquid interfaces will not be considered due to the obstacles in obtaining structurally defined 2DMs. This review article will start from the overview of interfacial synthesis methods (section 2), including the introduction of interface types, the advantages, and the uniqueness. After that, a brief definition of the involved 2DMs using interfacial synthesis methods will be provided in section 3. In the next section, through a few examples, we will present the method details and mechanisms about the reaction at the air/water interface (and the Langmuir−Blodgett (LB) method), the liquid/liquid interface, liquid/solid interface (such as the surfaces of graphene, salt, and metal), and gas/solid and/or vacuum/solid interface (such as chemical vapor deposition). The layered structures as the templating role for directing growth of 2DMs at the interfaces will be especially discussed, such as the soft colloid templates based on the bilayer structure of surfactants and the hard templates of montmorillonites and layered double hydroxides (LDHs). Additionally, some self-sacrificial templates not only play as an interface for the direction of 2DM growth but also provide the reactant sources, which will also be discussed. Given that there have already been several excellent reviews focusing on the structure, property, and functions of inorganic 2DMs,2,15,157−161 we will not separately make a summary about these inorganic 2DMs but rather incorporate them in section 4 where each interfacial methodology has been introduced. Subsequently in sections 5, 6, 7, and 8, we will respectively demonstrate emerging organic (organometallic)

2. OVERVIEW OF INTERFACIAL SYNTHESIS 2.1. Classification of Interfaces

An interface is a flat or curved space (or a phase boundary) between two different matters, or two different phases in one matter. The thickness of the interface space can range from several angstroms to nanometer and even to micrometer size. The interface between air and matter, or vacuum and matter, is called a surface. Once the area-to-thickness ratio is high enough in a flat interface, we call it a two-dimensional interface. In chemistry and material fields, interface is more active for multireactions at the phase boundaries than those in the bulk phase. Generally, interfaces involved in chemical reactions or materials synthesis include air/liquid,162 liquid/liquid (flat interface or emulsion droplets),163,164 liquid/solid, and air (or vacuum)/solid interfaces (Figure 1). The interface is well-known to direct the assembly of the precursors or the nucleation and growth of the intermediates in a confined 2D space, which plays a crucial role on controlling the morphology and the structure of the final products. Additionally, some interfaces, like metal (Au, Cu) or surfactant surfaces, not only play as the templates but also contribute to the reactions as catalysts. Even though the interface has contributed into various applications in material synthesis, catalysis, energy or matter transport, nanotechnology, optical 6191

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Figure 3. Schematic illustration of the preorganization of molecules at surfactant bilayer surfaces.

Figure 4. Schematic illustration of the preorganization of molecules (monomers and catalyst) in bulk liquid and at air−liquid and liquid−liquid interfaces.

interface, substantial attention should be paid, especially to the catalytic activity of the metal substrate and molecular diffusion at the interface, besides the surface energy of substrates, reactivity, and geometry of monomer precursors. 2.2.2. Layered Structures as Interfaces. Surfactant bilayers are 2D organized molecular structures spontaneously assembled in the water phase.171,172 The surfactant bilayers in solution (e.g., water) normally possess hydrophobic (inner surface) and hydrophilic (outer surface) structured surfaces. These soft bilayer structures can serve as interfaces for the synthesis of materials with defined morphology and ordering (Figure 3).172 The ordering either occurs through the simple templating of monomer precursors on bilayer surfaces to form organic/inorganic replicas of the underlying surfactant membranes; or monomer precursors are incorporated in an ordered manner into the hydrophobic inner surface of bilayer matrices, where restructuring may occur through the cooperative assembly of the surfactant−precursor pair.172 A notable advantage of this method is to obtain individual nanosheets, since the surfactant layers act as breaking points in organic solvent and stabilize agent preventing the reaggregation of the nanosheets. In addition, due to the dynamic phase separation and transition of surfactant bilayers in solution,171 the molecular diffusion, exchange, and chemical reaction dynamics can be regulated during their organization at the surfactant layer surfaces by the phase behavior of the bilayers, e.g., through pH, temperature, ionic strength of solvent, as well as surfactant structures. Similar to surfactant bilayers, layered double hydroxides (LDHs) also offer local microenvironments of 2D confinement and templating effect for chemical reaction and crystallization.173 However, the hard nature of LDH is unfavorable for molecular diffusion and exchange. In addition, such hard templates are difficult to remove, compared to the soft surfactant layers. 2.2.3. Air/Liquid and Liquid/Liquid Interfaces. At the microscopic level, chemical reactions at the liquid interfacial region have some unique properties in contrast to that in bulk solution, due to the strong asymmetry in the intermolecular interactions (e.g., monomer−monomer, monomer−solvent, and monomer−catalyst), as shown in Figure 4. For example, there is

engineering, and biomedicine, we will focus on the discussion of interface-assisted synthesis of 2DMs in this review article. 2.2. Advantages and Uniqueness of Interfaces

2.2.1. Gas/Solid and Liquid/Solid Interfaces. Reaction/ polymerization on solid surfaces involves several kinetic procedures including nucleation, diffusion, and growth (Figure 2). To achieve highly ordered structures on a solid surface, a low density of nuclei is required to ensure large crystalline domain size. Moreover, a high diffusion rate or mobility of monomers/ precursors will enhance the reaction efficiency, and a slow polymerization rate will also facilitate the formation of wellordered structures. Synthesis on solid surfaces under ultrahigh vacuum (UHV) condition is an efficient strategy to prepare 2D networks with atomic thickness and highly defined periodic structure.165 Scanning tunnelling microscopy (STM) can be subsequently employed to image and characterize the resulting materials with atomic resolution.166 However, this method has limitation in terms of small lateral size or crystalline domains (typically tens of nanometers) and high defect density, due to the low mobility of monomers and reactivity at the vacuum-solid interfaces. Despite the fact that a substantial improvement by optimizing the interplay between nucleation and growth is conceivable since the rapid progress in graphene synthesis by CVD growth,167 fundamental studies are needed to improve the synthesis generality. In the case of the liquid/solid interface, single-crystal solid surfaces can offer an ideal atomically flat interface to direct the polymerization reactions in liquid media and simultaneously support the resulting film.67,168 Appreciable substrate−molecule interactions can help the adsorption and organization of precursors into ordered structures,169 which is beneficial for reducing the interactions between precursor molecules. However, unlike the gas/solid interface, the employment of metal− substrate catalyzed reactions at a liquid/solid interface cannot guarantee monolayer reaction on the metal surface. A slight etching/corrosion of a metal substrate will yield catalytic species in the liquid media, which therefore induces solution-based reaction surrounding the metal surface and results in hierarchical nanostructures rather than 2D ordered networks.170 Therefore, in order to gain 2D controlled reactions at a solid/liquid 6192

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2.2.6. Chemical Reactivity and Selectivity. The historical assumption that solubility is required for high catalytic efficiency has been challenged by the observation of large rate and yield increases for a variety of reactions at interfaces in comparison to the reactions occurring in homogeneous media.188 While the mechanism behind the unique interfacial reactivity remains poorly understood, the use of different interfaces for selectively solubilizing the partners (e.g., monomer−monomer or monomer−catalyst) of a reaction is a convenient and efficient pathway, which has been widely utilized in both organic synthesis and polymerizations.189 Breslow et al. reported the utilization of aqueous dispersions (i.e., hydrocarbon−water interface) for the Diels−Alder reactions and found that the reaction rate was dramatically increased in comparison to that in bulk solution.190 Subsequent studies have elucidated that this behavior results from enforced hydrophobic interactions and stabilization of the activated complex by hydrogen-bond formation at the interface.188,191 The adjustment of the reactivity of each monomer at the interface is essential to guarantee high yield reactions and even stereoselectivity. Recently, Kumar et al. reported that, in ionic liquid/n-hexane interface, the Diels−Alder reaction shows large rate enhancements of the order of 106 to 108 times and high stereoselectivity as compared to in homogeneous media.192 The hydrophobicity of ionic liquids was considered to be the factor in controlling stereoselectivity. However, the interactions and orientation of the molecules governed by hydrogen-bonding abilities and polarities of the interface may play a crucial role in determining the reaction (accelerating) mechanism.188,191 These advantages promise that the interfacial synthesis methodology can be used for the development of novel materials with controlled morphologies and chemical and physical properties. On the other hand, the development of new chemistry methodologies also relies on the uniqueness of a particular interface. The unprecedented power of an interface to confine and order molecules in a defined geometry is attractive for the bottom-up synthesis of 2DMs.5 As such, the interfacial synthesis, although it has been extensively studied both experimental and theoretically in the past decades,182,193 will earn more attraction in the future. 2.2.7. 2D-Confined Template-Directed Synthesis. In the past decades, template-directed synthesis has served as an efficient method for the preparation of various inorganic and organic materials with diversified dimensionalities. It has advantages in the control of the size, morphology, and composition as well as designed functions.172,194−198 Template-directed synthesis can be regarded as supramolecular assistance to covalent synthesis, which relies on the use of noncovalent interactions, such as hydrogen bonding, electrostatic interactions, metal coordination, and π−π stacking interactions, to preorganize the monomers/precursors into a certain geometry to afford the thermodynamically preferred product. For instance, the aggregates generated from amphiphilic surfactants and block copolymers can be used as templates to achieve highly ordered mesoporous materials, with large surface areas and uniform pore sizes.199,200 Interface-assisted synthesis of 2DMs can also be ascribed to a template-directed synthesis in a 2D-confined space. In a typical templating insight, the monomers/precursors are concentrated along the interfaces between air−water, liquid−liquid, liquid−solid, and gas−solid, and stack to hold the reactive sites close together, facilitating the desired reaction conformation, wherein the template effect translates information from the size and shape of an interface to the finally obtained 2D structures.

no obvious difference in the density of monomers in interfacial liquid and bulk liquid. However, the surface roughness and the molecular orientation and interaction are completely different in the two systems.174 These differences can significantly influence or modify the migration of molecules and the equilibria as well as the rate of chemical reactions. Ordered arrangement of the monomers at the interface can be further triggered by their dynamic self-assembly. For instance, amphiphilic molecules can form 2D periodic order on scales much larger than molecular dimensions.175 Additionally, the liquid interfacial region offers a 2D confined space for the facile lateral mobility of the monomers, which enables the growth of large-area continuous 2D structures, while the solid substrates having fixed terraces and grain boundaries generally lead to the formation of short-range ordered networks on their surfaces. 2.2.4. Surface Roughness. It has been experimentally demonstrated that air−liquid is extremely smooth with a rootmean-square roughness of about 3 Å in the case of water surface,174 in contrast to bulk liquid, which is less ordered.176 On one hand, surface roughness, as a microscopic dynamical aspect of the surface, is relevant to an understanding of the (re)arrangement of molecules (i.e., solutes) at interfaces. For example, using fluorescence depolarization experiments, Wirth et al.177 have demonstrated that the reorientation dynamics and the lateral diffusion of acridine orange at liquid/liquid (water/ alkanes) interfaces are strongly affected by the roughness of the interface. There is also evidence from molecular dynamics simulations that smooth interface determines the orientational distribution and the initial fate of colliding molecules.178,179 On the other hand, the diffusion of molecules across the interface and at the interface may be affected by surface roughness.180,181 Given the importance of both orientations and mass transfer of molecules to chemical reactions, the roughness of a specific interface exerts a substantial role in controlling the dynamics of chemical reactions at an interface. 2.2.5. Molecular Orientation. The geometrical arrangement of molecules at an interface, which is referred to as the orientational structure, is of key importance to any microscopic description of the chemical and physical properties of interfaces.182 The interface structure is determined by the intermolecular forces among the solvents and the molecules in the adjacent phases. The orientation-determining forces range from electrostatic force and hydrogen-bonding to hydrophilic solvation and hydrophobic effect.183 As a result, certain molecular orientations can be preferred at interfaces. This characteristic is of significant importance because steric requirements for chemical reactions at interfaces may lead to strong dependence of reactivity on molecular orientation at the interface to form 2D ordered networks.5 Moreover, molecular orientation also determines liquid surface potential, which is important for understanding the chemical reaction dynamics at interfaces. Specifically, infrared visible sum-frequency generation (SFG) studies have been used to prove that the water molecules are polar oriented at the air−liquid (i.e., water) interface, in which more than 20% of surface water molecules have a non-hydrogenbonded OH (free OH bond) pointing out of the liquid and the other OH (hydrogen bonded) pointing into the bulk liquid.184,185 These dangling OH bonds have been observed in experiments and simulations of ice.186 Their existence at the water surface can be both interesting and significant for elucidating the phenomena such as the transfer and assembly of molecules and chemical reactions at the interface.187 6193

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3. CLASSIFICATION OF 2DMS In the past few years, different classes of single-layer or layerstacked 2DMs have been synthesized by the interfacial methodologies. The layer-stacked 2DMs generally exhibit strong in-plane interaction and weak van der Waals force between layers. In this section, we will first provide a literature-relevant definition of the 2DMs which are especially obtained by interfacial synthesis.

phases. In sphalerite, the sulfide atoms pack in a cubic symmetry and the Zn2+ ions occupy half of the tetrahedral holes.224,225 Another crystalline structure is the hexagonal form known as the mineral wurtzite.226 Lead sulfide (PbS) is another typical example of nonlayer structured metal chalcogenide with cubiccrystal symmetry. To orientate their growth into 2D sheets using solution synthesis, surfactants have been commonly employed to yield a 2D confined space for the nucleation and growth,227,228 which will be discussed in section 4.5.1. 3.1.5. Metals. Metal atoms are closely positioned to neighboring ones. Normally, metal atoms arrange into three crystalline phases, i.e., face-centered cubic ( fcc), hexagonal closepacked (hcp), or body-centered cubic (bcc) structure.229 The 2D metals generally exhibit the same crystal phases as their corresponding bulk materials. In this review, some noble metal nanosheets, such as Au,42,44,45,230 Pd,41 and Rh231 which have been synthesized by layered structure templating methods, will be presented in section 4.5. 3.1.6. Metal Oxides. Metal oxides are chemical compounds that contain metal and oxygen atoms linked by chemical bonding. Metal oxides are also a kind of nonlayer structured materials in three dimensions. In the past decade, a number of metal oxide nanosheets have been prepared by interfacial synthesis strategies, such as TiO2,232,233 ZnO,232 Co3O4,232 WO3,232 Fe2O3,49 Fe3O4, CuO,47 In2O3,234 ZnO,52 MoO3,51 WO3,51 MoO2,51 MnO,51 etc. The interfacial methods usually include salt templating and surfactant-layer templating, which will be discussed in sections 4.3 and 4.5, respectively. Apart from the above-discussed examples which could be synthesized at the interfaces, other inorganic 2DMs such as black phosphorene,9,14,33−35 2D (layered) hybrid perovskites,56−60 and MXenes61−65 have also received much attention. The current synthesis methods for these 2DMs mainly focus on the mechanical and chemical exfoliation of their bulk layered structures. The interface-assisted synthesis of single-layer or few-layer structures remains under development.

3.1. Inorganic 2DMs

3.1.1. Graphene. Graphene as the allotrope of carbon in the form of a two-dimensional, atomic scale hexagonal lattice1,3,23 has been a star on the horizon of chemistry, materials science, condensed-matter physics, and nanotechnology.201 Currently, various methods have been developed for the graphene synthesis, such as mechanical exfoliation,3 chemical exfoliation,4 electrochemical exfoliation,134−137 epitaxial growth,202−204 chemical vapor deposition (CVD) method,23,145,205 and organic synthesis.206−209 In this article, the synthesis of graphene at a vapor/ solid interface by a CVD method will be briefly discussed in section 4.4. 3.1.2. Hexagonal Boron Nitride (h-BN). 2D h-BN nanosheets are structural analogues of graphene with sublattices being occupied by equal numbers of boron and nitrogen atoms alternatingly arranged in a honeycomb configuration.140,142,210−212 h-BN has attracted much attention recently due to its high temperature stability, high mechanical strength, large thermal conductivity, and low dielectric constant. The hexagonal crystal structure of h-BN with crystallographic parameters of a = 0.250 nm and c = 0.666 nm and the interlayer spacing of 0.333 nm enable excellent interaction with graphene. Because of the partially ionic character of the B−N bonds, h-BN is an insulator with a direct band gap of 5−6 eV. In this article, the synthesis of h-BN at a vapor/solid interface by a CVD method will be briefly discussed in section 4.4. 3.1.3. Transition Metal Dichalcogenides (TMDs). Transition metal dichalcogenides (TMDs) are layered compounds with the general chemical formula of MX2, where M is a transition metal element and X represents a chalcogen (e.g., S, Se, or Te). Generally, TMDs have a layered structure, in which monolayers stack by the van der Waals forces. Each TMD monolayer consists of three atomic layers, in which a transition metal layer is sandwiched between two chalcogen layers.2,7 One TMD can also have different crystal polytypes. For instance, MoS2 crystallizes with four crystalline phases, including 2H, 1T, 1T′, and 3R, relying on the different coordination models between Mo and S atoms and/or stacking orders between layers.29,213,214 The 2H-type MoS2 is dominant due to its thermodynamical stability in nature. At present, a number of methods including top-down liquid exfoliation6 and bottom-up solvothermal synthesis,215−219 hot-injection method,220,221 and CVD growth143,146,222 have been established for the preparation of TMDs. In this review, interfacial synthesis strategies toward various TMD nanosheets with different crystalline structures, sizes, and thickness, including the surfactant-assisted synthesis (in section 4.5.1), self-sacrificial templating (in section 4.6), and CVD methods (in section 4.4), will be discussed. 3.1.4. Metal Chalcogenides. Unlike layered TMDs, metal chalcogenides are a class of nonlayer structured inorganic materials. They have a general formula of MX (where M = Cd, Pb, Sn, Zn, Ag, etc.; X = S, Se, Te).2,223 For instance, zinc sulfide (ZnS) is one example of a nonlayer structured metal chalcogenide. The bulk ZnS exists in two major crystalline

3.2. Organic (Organometallic) 2DMs

Organic 2D materials are crystalline, layer-structured polymers with building blocks (organic monomers) linked via covalent bonds (like CC, CN bonds) or noncovalent interactions (such as metal coordination bonds, host−guest interactions, hydrogen or halogen bonds).17 3.2.1. 2D Polymers (2DPs). 2D polymers are a class of organic 2D materials comprising free-standing, single-atom/ monomer-thick, planar, covalent networks with well-defined periodicity along two orthogonal directions, as recently redefined by Schlüter and co-workers.5,10,18,83,84,89,90,235 There have been numerous impressive attempts to synthesize free-standing sheetlike polymers,5 which can date back to the first report of interfacial polymerization of elaeostearin and maleic anhydride at the air/water interface in a LB trough by Gee and Rideal in 1935. In 1958, Blumstein and co-workers realized the cross-linked polymerization to form covalent monolayer within the interlayers of montmorillonite clays, which presented a 2D space confined cross-linking polymerization using hard templates for the first time.5 In 1993, Stupp et al.236 achieved bulk synthesis of free-standing polymer nanosheets with a thickness of approximately 5 nm and lateral extensions of 102−103 nm within surfactant bilayers. However, the synthesis of 2DPs meeting the above criteria remained inaccessible until 2012 when the first example was achieved by exfoliation of a lamellar polymer single crystal.235 Subsequently, a series of single-layer and few-layer 6194

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3.2.4. 2D Metal−Organic Frameworks (2D MOFs). MOFs are crystalline, porous coordination polymers made by connecting inorganic metal ions or clusters with ditopic or polytopic organic ligands.274,275 MOFs stand out among other framework materials due to their tailorable structures and functions, ultrahigh porosities, and large surface areas.276 The majority of MOFs are networks with coordinative bonds oriented to form a 3D network. These features afford MOFs great potential in various applications,275,277−279 such as gas storage,280,281 separation,122,282 catalysis,283−285 sensors,286 and biomedicine.287 Originally developed for the above applications, in recent years it has been shown that MOFs also have huge potential applications in (opto-)electronics,288−293 information processing,294−297 energy storage,298−300 and conversion.301−303 2D MOFs are featured with layered structures with strong inplane coordination but weak out-plane interactions such as van der Waals interaction and hydrogen bonding. Thus far, 2D MOFs have been prepared in the forms of layer-stacked powders and single-layer and few-layer nanosheets.19 In this review article, we will present single-layer and few-layer 2D MOFs produced by interfacial methods (section 8), including the air/water, liquid/ liquid, and surfactant templating.

2DPs have been prepared by the similar exfoliation strategy.82−84,89,237 To a certain extent, the exfoliated single-layer 2D COFs111,113−115,118,238 and covalent triazine frameworks (CTFs)239,240 or graphitic carbon nitrides (GCNs)241−243 can also be considered as 2DPs, although their structural elucidation remains elusive. Apart from the top-down exfoliation method, the interfacial synthesis strategy has also been exploited to prepare crystalline single-layer and few-layer 2DPs.89,92,93 In this review article, we will introduce the synthesis of 2DPs at the air/ water (section 4.1 and 5) and liquid/liquid (section 5) interfaces. 3.2.2. 2D Supramolecular Polymers (2DSPs). Supramolecular polymers are characterized by the monomeric units holding together via highly directional and reversible noncovalent interactions.244−247 Currently, a large number of supramolecular polymers have been developed by self-assembly of monomers along 1D and 3D directions in solution.248−251 These supramolecular polymers exhibit attractive applications in self-healing, optoelectronics, and biomedicine. However, despite numerous reports on the synthesis of supramolecular networks on solid surfaces by scanning tunneling microscopic (STM) observation,151,252−255 the supramolecular assembly of organic monomers into a 2D plane to form free-standing, periodic networks remains less explored in solution96,97,100,107,256−270 and at the interfaces.77,95,99,271 Analogous to the definition of 2DPs, 2D supramolecular polymers (2DSPs) can be regarded as a type of organic 2DMs comprising free-standing, single-atom/ monomer-thick, planar, noncovalent networks with periodicity along two orthogonal directions. In this article, we will focus on the discussion of 2DSPs synthesized at the air/water and liquid/ liquid interfaces (section 6). Other organic 2D nanosheets achieved via the self-assembly of organic small molecules,272 DNA, protein, and polymers17 as well as the supramolecular networks on solid surfaces, which do not meet the above definition, are excluded from 2DSPs in this review. Note that, the formation of 2DSPs can be driven by various noncovalent interactions, including metal-coordination, host−guest, and hydrogen and halogen bonds. Thus, there can be inevitable overlap between coordination-based 2DSPs and 2D metal− organic framework (2D MOF) nanosheets. Thus, to simplify the discussion, we will introduce the coordination-based 2DSPs in the section of 2D MOFs (section 8). 3.2.3. 2D Covalent-Organic Frameworks (2D COFs). COFs are a class of organic crystalline porous polymers that allow the atomically precise integration of organic units into extended structures with periodic skeletons and ordered nanopores by strong covalent bonds.68−70,273 In 2005, Yaghi and co-workers reported the first example of layer-stacked crystalline 2D COF composed of repeating benzene rings66 linked by boroxine. Since then, more than one hundred COFs with various porous structures have been developed employing abundant organic building blocks and rich linkage chemistries, such as boronate, imine, hydrozone, azine, squaraine, phenazine, imide, benzimidazole, and triazine linkages.68−70,273 Generally, 2D COFs are synthesized from planar building blocks to provide layer stacked structures, which feature the extended planar network within a 2D layer and periodically columnar π-arrays aligned with an atomic precision in the vertical direction, i.e., strong in-plane covalent bonding and weak out-plane van del Waals force. In this review, we will focus on multilayer-stacked 2D COF films prepared at the air/water and liquid/liquid interfaces (section 7). Note that, as clarified in section 3.2.1, single-layer 2D COF can also be considered as 2DP, which will be presented in section 5.

4. INTERFACIAL SYNTHESIS STRATEGIES 4.1. Air/Water Interface

The air/water interface in a Langmuir−Blodgett (LB) trough has to be employed to confine water-insoluble monomers or even nanoparticles (Figure 5), which can freely rotate and move

Figure 5. Schematic illustration of chemical synthesis at air−water interface by Langmuir−Blodgett (LB) method.

around in lateral directions unless they are densely packed to form a monolayer.304 The synthesis of organic monolayers on a water surface was first realized by Rayleigh in 1890. In 1917, Langmuir developed the experimental and theoretical concepts which underlie our modern understanding of the behavior of molecules in insoluble monolayers.305 This technology was later named with Langmuir and Blodgett, and Langmuir was awarded the Nobel Prize in 1932 for his research on surface chemistry. The LB technology enables fabrication of a single-monomerthick organic film with control over the packing density of monomers by tailoring the surface pressure of water.305−309 Thereby, this method provides the opportunity to control the composition and the structure of 2DMs by noncovalent and covalent bonds at the molecular level. When one repeats the transfer process, layer-stacked 2D materials can be prepared. Currently, this strategy has been particularly utilized for the synthesis o f o rganic 2 DMs such as s ingle-layer 2DPs, 82,85,87−89,310 single-layer 2DSPs, 99 multilayer 2D COFs,311,312 and single-layer or few-layer 2D MOFs72,77,95,102,271,313−319 as well as other organic/metal− organic films with low structural ordering. In the following, we 6195

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the axial positions. Pyridine also coordinated to the Co atoms of the porphyrin unit. The 4-fold symmetry of the porphyrin ligand facilitated the construction of a grid structure. The 2D arrays were deposited onto a solid substrate, such as Si(100) or quartz, by the horizontal dipping method. The repetitive process of successive sheet deposition resulted in the formation of a layerby-layer film with controlled thickness. π−π stacking between two pyridine molecules helped to stabilize the stacking structure. The stacked MOF film was characterized by GIXRD, which showed crystalline ordering in both in-plane and out-of-plane directions. The average crystalline domain size was estimated to be 20 nm from the full width at half-maximum of the GIXRD peak by using Scherrer’s equation.320 Another interesting 2D MOF nanosheet was reported by Nishihara and co-workers in 2013.74,316 They synthesized a single-layer nickel bis(dithiolene) nanosheet at the air/water interface (Figure 8a) under atmospheric pressure rather than the compressed pressure used for the LB method (Figure 8b). An ethyl acetate solution of benzenehexathiol (BHT) was gently spread onto the surface of an aqueous solution of Ni(OAc)2 and NaBr. After the evaporation of ethyl acetate, the monolayer nanosheets were obtained and could be transferred onto HOPG substrate. Atomic force microscopy (AFM) measurement revealed the thickness of ∼0.6 nm for the single-layer nanosheet. The same group further achieved the synthesis of other 2D MOF nanosheets comprising bis(dipyrrinato)-metal complexes at the air/water interface102 for photoelectric conversion, which will be discussed in section 8.1. In 2015, our group demonstrated the synthesis of a large area, single-layer, fully conjugated 2DSP (or conjugated 2D MOF) consisting of nickel bis(dithiolene) complexes at the air/water interface by the LB method.77 In this work, 1,2,5,6,9,10hexathioltriphenylene (HTTP), a π-conjugated monomer, was employed as the key building block. Figure 9a illustrates the typical fabrication process of large-area Ni3(HTTP)2 sheets by the LB method at the air/water interface. A submonolayer of HTTP monomers was spread over the water surface in a LB trough. After the close packing of the monomers into a dense film upon compressing to 10 mN m−1, a solution of nickel salts was

will introduce the synthesis procedures at the air/water interface with several representative examples. In 2002, Talham and co-workers synthesized a crystalline monolayer with square patterned structures by metal coordination at the air/water interface.271 An iron−nickel cyanidebridged network within a 2D plane was obtained after spreading amphiphilic pentacyanoferrate(III) complex on the surface of an aqueous solution of Ni2+ ions in a LB trough (Figure 6).

Figure 6. Assembly of a coordination monolayer based on 4(didodecylamino)pyridine/iron(III) complexes at the air−water Interface. Reproduced with permission from ref 271. Copyright 2002 American Chemical Society.

Synchrotron grazing incidence X-ray diffraction (GIXRD) measurements confirmed the formation of a face-centered square grid structure and revealed their average crystalline domain size of ∼36 nm2. In contrast, such a 2D structure could not be obtained by solution synthesis under similar conditions. Hence, confinement of the reactants within a 2D interface would be advantageous to induce an anisotropic growth due to the restricted diffusion. In 2010, Kitagawa et al. reported the synthesis of MOF monolayer on the water surface72 using [5,10,15,20-tetrakis(4carboxyphenyl)porphyrinato]cobalt(II) as carboxylic acid ligand and pyridine as ancillary ligand molecule (Figure 7). The CHCl3/ethanol solution of the monomers was spread onto the aqueous solution of Cu2+ ions. Pressing the surface with barrier walls led to the formation of a copper-mediated 2D array, which possessed a paddle-wheel linkage comprising two Cu2+ and four COO− groups and was capped with two pyridine molecules at

Figure 7. Formation of a 2D MOF monolayer via coordination of [5,10,15,20-tetrakis(4-carboxyphenyl)porphyrinato]cobalt (II) and pyridine with Cu2+ ion at air/water interface by LB method. Reprinted by permission from Macmillan Publishers Ltd.: Nat. Mater. ref 72, copyright 2010. 6196

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cell size of ∼2 nm. Such 2D MOF can be transferred onto glassy carbon electrodes (∼20 mm2) for electrochemical studies, such as the electrode materials for electrocatalytic hydrogen generation. Due to its well-defined structure and monolayer feature, this 2D MOF can be used as a model electrocatalyst for addressing the fundamental catalytic active sites.78,321 Besides the MOF nanosheets, 2DPs85,87−89 and layer-stacked 2D COFs89,311 have also been prepared at the air/water interface. In 2014, Schlüter and co-workers demonstrated the synthesis of a 2DP at the air/water interface (Figure 10).85 A dilute chloroform

Figure 8. (a) Chemical structure of monolayer nickel bis(dithiolene) complex nanosheet. (b) Illustration of the synthesis of BHT-Ni monolayer at the air/water interface. Reproduced with permission from ref 74. Copyright 2013 American Chemical Society.

injected into the water phase. With the diffusion of nickel ions from the bulk phase to the interface, polymerization was triggered by the coordination of nickel ions with dithiolene units, resulting in targeted 2DSP with a large area on the order of square millimeters (Figure 9b), and the sheet thickness was ∼0.7 nm (Figure 9c) which is associated with single-layer feature. In addition, multilayer structures with a controllable number of layers could be readily deposited on substrates by a repeated LB transfer process (Figure 9d). The obtained sheets exhibited freestanding structural feature and high mechanical strength, which could span over hexagonal holes with 18 μm side length on copper grids (Figure 9e). As the single-layer sheets showed low stability under electron irradiation, the selected area electron diffraction (SAED) was performed by cryogenic transmission electron microscopy (cryo-TEM) at −175 °C, exhibiting a typical hexagonal diffraction pattern (Figure 9e, inset). This result at least suggests the local crystallinity presented in the single-layer 2DSP with a hexagonal ordered network having the

Figure 10. (a) Chemical structure of anthracene-based monomer. (b) Side view of simulated monomer conformation at the interface giving thickness of ∼1.4 nm. (c) AFM image of polymer film after photodimerization of anthracene at air/water interface. (d) Structural model of 2DP through [4 + 4] anthracene dimerization. Reproduced with permission from ref 85. Copyright 2014 John Wiley & Sons, Inc.

solution of anthracene-based amphiphilic monomer (Figure 10a) was spread onto the water surface in a LB trough and the monomers were able to preorganize into closely packed monolayer through compressing. This compression step ensured that the anthracene units stacked into a face-to-face manner. As a result, the monomers were bonded together via a [4 + 4]cycloaddition reaction upon UV light irradiation, providing the single-layer 2DP. The AFM image revealed the thickness of ∼1.7

Figure 9. (a) Schematic illustration of the synthesis of a single-layer 2D MOF sheet composed of triphenylene-fused nickel bis(dithiolene) complexes through the Langmuir−Blodgett method at an air/water interface. (b) Optical microscopy (OM) image revealing large-area single-layer sheets. (c) Tapping-mode AFM height images and the corresponding cross-sectional analysis demonstrating a single-layer sheet measuring ∼0.7 nm in thickness. (d) OM image of bilayer sheets. (e) Transmission electron microscopy image of monolayer on copper meshes. Inset: selected area electron diffraction pattern of monolayer. Reproduced with permission from ref 77. Copyright 2015 John Wiley & Sons, Inc. 6197

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Figure 11. (a) Chemical structures of the three-way dipyrrin ligand molecule and the bis(dipyrrinato)zinc(II) complex nanosheet. (b) Schematic illustration and photographs of the liquid/liquid interfacial synthesis and multilayers transferred onto an ITO substrate. Scale bars, 5 and 1 mm, respectively. (c) Photograph of the multilayer nanosheet transferred onto an ITO substrate. Scale bar: 20 μm. (d) AFM height image and its crosssectional analysis along the magenta line. Scale bar: 20 μm. Reprinted by permission from Macmillan Publishers Ltd.: Nat. Commun. ref 102, copyright 2015.

Figure 12. (a) Structure of CuBDC MOF. Copper, oxygen, and carbon atoms are shown in blue, red, and gray, respectively. The insets on the right-hand side show views along the b (top) and c (bottom) crystallographic axes showing the stacking direction and the pore system, respectively. (b) Picture showing the spatial arrangement of different liquid phases during the synthesis of CuBDC MOF nanosheets. Phases labeled as i, ii, and iii correspond to a 1,4-benzenedicarboxylate solution, the solvent spacer layer and the solution of Cu ions, respectively. (c) XRD curves. (d) SEM image of bulk-type CuBDC MOF crystals. (e and f) SEM and AFM images of CuBDC MOF nanosheets. Reprinted by permission from Macmillan Publishers Ltd.: Nat. Mater. ref 323, copyright 2015.

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Figure 13. (a) Schematic illustration of graphene directing formation of Au square sheets. (b) TEM image of ∼2.4 nm thick Au nanohseets on a GO surface (scale bar, 500 nm). Inset: structural models with its basal plane along the [110]h zone axis, showing ABAB stacking along the [001]h direction. (c) HRTEM image oriented normal to [110]h (scale bar, 2 nm). (d) SAED pattern of an Au nanosheet on GO. (e) SAED of [320]h zone axis was collected by tilting an Au nanosheet. Reprinted by permission from Macmillan Publishers Ltd.: Nat. Commun. ref 42, copyright 2011.

prepared by vertically aligning a mixture of dimethylformamide (DMF) and acetonitrile (CH3CN) in different ratios. The top phase contained a mixture of 1 mL of DMF and 2 mL of CH3CN, the middle phase contained a mixture of 1 mL of DMF and 1 mL of CH3CN, while the bottom phase was composed of a mixture of 2 mL of DMF and 1 mL of CH3CN. Then Cu(NO3)2 was dissolved into the top phase, and the 1,4-benzenedicarboxylate (BDC) ligand was dissolved in the bottom phase. The MOF sheets could be obtained when Cu2+ and the BDC ligands slowly diffused into the middle phase. The crystal structure of obtained CuBDC is shown in Figure 12a, in which one copper dimer with square-pyramidal coordination geometry is connected by four BDC ligands to form the layers that stack along the [−201] direction. Compared with the bulk CuBDC MOF, the 2D CuBDC nanosheets show only three reflection peaks due to the strong preferential orientation of CuBDC along the basal plane (Figure 12c). These peaks are indexed to the (−201), (−402), and (−804) planes of the CuBDC crystal, indicating the successful formation of CuBDC nanosheets. SEM and AFM investigations confirmed the square-shape CuBDC nanosheets with lateral dimensions of 0.5−4 μm and thicknesses ranging from 5 to 25 nm (Figures 12d−f). The developed three phase synthesis strategy is universal and can be applied to produce other MOF nanosheets, such as CoBDC, ZnBDC, copper 1,4naphthalenedicarboxylate (Cu(1,4-NDC)), and copper 2,6naphthalenedicarboxylate (Cu(2,6-NDC)).323

nm for monolayer 2DP (Figures 10b−d). Nevertheless, the internal structure of such monolayer 2DP remains to be disclosed.94 4.2. Liquid/Liquid Interface

Liquid/liquid interface comprising two immiscible solvents also provides a 2D space at the phase boundaries for the confined growth of 2DMs. One example is the synthesis of a few-layer 2D MOF film, which can be obtained via diffusion of various ligands and salts to meet at the liquid/liquid interface.19,322 For instance, Nishihara and co-workers reported layer-stacked 2D MOF sheets comprising bis(dipyrrinato)-metal 102,105 and bis(terpyridine)-metal complexes,104,106 respectively, at the dichlorombethane/water interface. Figure 11a depicts a framework structure of the dipyrrin-based 2D MOF film, which was obtained from three-functional dipyrrin and Zn(OAc)2. The liquid/liquid interfacial synthesis involved layering a CH2Cl2 solution of the dipyrrin ligand with top-phase aqueous Zn(OAc)2 under ambient conditions (Figure 11b). Spontaneous complexation between the dipyrrin ligand and Zn2+ occurred at the liquid/liquid interface, affording layer-stacked 2D MOF films. The multilayer nanosheets exhibited a macroscopic sheet morphology over the centimeter scale (Figure 11c). The film thickness can be tuned from 6 to 800 nm (corresponding to 5− 670 layers) by varying the concentration of the dipyrrin ligand. Figure 11d illustrates a flat, smooth film with the thickness of ∼700 nm. The multilayer 2D MOF films exhibited high crystallinity with hexagonal structures as verified by the TEM/ SAED investigations. In addition to the two-phase interface, in 2015, Gascon and coworkers developed a three-phase synthesis strategy to prepare copper 1,4-benzenedicarboxylate (CuBDC) MOF nanosheets with high-aspect ratio (Figure 12a).323 As shown in Figure 12b, the synthesis medium consisted of three immiscible liquid phases

4.3. Liquid/Solid Interface

The liquid/solid interface provides a confined space between reactant solution and substrate surface. The precursors in the solution will adsorb on the solid surface by physical or chemical interactions and are preorganized under the direction of solid substrate. The liquid/solid interfaces presented here mainly involve the graphene templating (section 4.3.1), salt templating 6199

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(section 4.3.2) and metal templating (section 4.3.3), which have been used for the synthesis of metal nanosheets and 2D COF films. 4.3.1. Graphene Templating. Graphene is chemically inert.324 However, the oxidization of graphene can render graphene with rich oxygen functional groups, such as −COOH and −OH, which makes the oxidized graphene sheets promising templates for the nucleation and growth of noble metal nanomaterials.25,325 As a typical example, Au square nanosheets with thickness of 2.4 ± 0.7 nm (∼16 Au atomic layers) and size of 200−500 nm were produced by reducing HAuCl4 with 1-amino9-octadecene on graphene oxide (GO) template (Figures 13a and b).42 The Au nanosheet owned unique hexagonal-close packed (hcp) phase rather than the common face-centered cubic (fcc) phase (Figures 13c−e). This was the first report about the pure hcp Au nanostructure, which was stable under ambient conditions. In contrast, the synthesis without involving GO sheets by heating a mixture of HAuCl4, 1-amino-9-octadecene, hexane, and ethanol resulted in the formation of Au nanosheets with a large size distribution (30−500 nm) coexisted with Au nanowires and nanoparticles. Thus, GO played a crucial role in tuning the morphology. The formation process of the Au nanosheets follows: (I) after mixing oleylamine and Au3+ in hexane together with GO sheets, Au3+ was partially reduced to Au+; (II) the resulted oleylamine−AuCl complexes adsorbed and self-assembled into square-like structures on the GO surface due to the electrostatic interaction with abundant negatively charged oxygen-containing functional groups; (III) after slow reduction, small Au seeds appeared and merged into square-like structures until the formation of a large-sized nanosheet. Additionally, by replacing the oleylamine with thiol molecules, the hcp phase could be transformed into (100)f-oriented fcc Au square nanosheets.45 The same group further demonstrated that the coating of Ag thin layer on the hcp Au square nanosheet could generate Au@Ag core−shell square sheets.45,326 Similarly, the growth of Pt or Pd thin layer on Au square nanosheet induced the core−shell Au@Pt or Au@Pd nanoplates.327 In addition to the inorganic nanosheets, graphene has been used as a substrate for the heterogeneous growth of 2D COF films. In 2011, Dichetel and co-workers reported the growth of COF-5 films on single-layer graphene (SLG) on copper substrate by solvothermal condensation of 1,4-phenylenebis(boronic acid) (PBBA) and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) in a mesitylene-dioxane (1:1 v/v) mixed solvent at 90 °C (Figure 14).67 The COF-5 film completely covered the graphene surface and exhibited a thickness of 195 ± 20 nm, corresponding to ∼580 layers. The synchrotron grazing incidence X-ray diffraction was used to confirm the crystallinity of the COF-5 films and indicated that the hexagonal lattice of the COF-5 grains was aligned parallel to the substrate surface. Moreover, crystalline films of triphenylene-pyrene (TP)-COF and Ni phthalocyaninePBBA COF were also successfully grown on SLG/SiO2 substrates. Thereby, this fabrication technique facilitated the study of the effect of the substrates on the growth of COF films and provided a promising route for the integration of COFs into a variety of devices, e.g., organic photovoltaics. 4.3.2. Salt Templating. The salt crystals, such as NaCl and Na2SO4, have been reported as efficient templates for the synthesis of 2DMs such as metal oxide 51 and metal disulfide,328−330 because these thermally stable crystals can be uniformly coated by the precursors, which further direct the 2D confined reaction. Figure 15 reveals a general synthesis procedure using salt templating. The precursors and the salt

Figure 14. Solvothermal condensation of HHTP and PBBA in the presence of a substrate-supported SLG surface provides COF-5 as both a film on the graphene surface, as well as a powder precipitated in the bottom of the reaction vessel. Reproduced with permission from ref 67. Copyright 2011 American Association for the Advancement of Science.

Figure 15. Schematic illustration of the synthesis of 2D nanosheets by salt templating.

are first mixed to form fine composite powder by thorough grinding. After that, heating treatment is carried out to yield products mixed with salts. The final products are readily obtained by washing the salts with water. The use of excessive salts in the synthesis can effectively avoid the agglomeration of the obtained nanosheets. In 2016, Gogotsi and co-workers reported a general strategy of using salt (NaCl or KCl) crystal surfaces as templates for the synthesis of various transition metal oxide nanosheets, such as hexagonal-MoO3, MoO2, MnO, and WO3.51 Regarding the synthesis of 2D MoO3, a precursor solution was first prepared by stirring Mo powder and H2O2 in ethanol. Then, the precursor solution was mixed with a large volume of NaCl powder (Figures 16a and b). Next, the solution was dried and the mixture was annealed at 280 °C to produce 2D oxides, through an intermediate hydroxide phase. The salts were removed by water and the MoO3 nanosheets were dispersed in ethanol for further assembly into a restacked film (Figures 16c and d). In this synthesis of 2D MoO3, the NaCl microcrystals served as the templates to guide the oxide growth at elevated temperatures. The as-produced MoO3 sheets exhibited a large lateral size (up to 100 μm) and few-layer thickness of 0.25 cm2 and 1.8 nm in thickness (Figures 28c and d). Moreover, the crystallinity of a resultant multilayer film was confirmed with hexagonal patterns by grazing incidence wideangle X-ray scattering (GIWAXS) analysis.

5.4. 2DP by Homocoupling

Li and co-workers reported the synthesis of graphdiyne via homocoupling polymerization at liquid/solid interface in 2010,331 in which they dissolved hexaethynylbenzene (HEB) in pyridine with the presence of copper foil. Copper ions, dissociated from the metallic copper, could catalyze the oxidative homocoupling of HEB, providing graphdiyne on Cu foil in a stacked form that was 1 μm thick. Several subsequent efforts have also been sought to produce graphdiyne with better quality; however, the realization of a thin graphdiyne nanosheet (10−3 S cm−1).292,481 In 2012, Yaghi et al.73 reported the first fully π-d conjugated, layer-stacked 2D MOF powders based-on triphenylene-fused Cu-catecholates by employing 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) as building block via solvothermal synthesis. This 2D MOF showed the conductivity as high as ∼0.2 S cm−1 at room temperature. Since then, it has been commonly accepted that the development of highly conductive 2D MOFs strongly relies on the design of the linking units, metal centers, and the organic ligands for the construction of a π-conjugated system. Notably, the recent reports revealed that the metal-bis(dithiolene)/bis(diimino) complex-based 2D MOFs presented further improved conductivity. These reported 2D MOFs were based on benzene (hexathiolbenzene, HTB; hexaiminobenzene, HAB)74,79,316,483 or triphenylene (HHTP; 2,3,6,7,10,11-hexaaminotriphenylene, HATP; 2,3,6,7,10,11-hexathioltriphenylene, H T T P ) d e r i v a t i v e s 7 3 , 7 5 , 7 7 , 7 8 , 8 0 , 2 9 8 , 3 1 7 , 4 8 4 , 4 8 5 w it h thiol,74,77,316,317,483 hydroxyl,73 or amino75,78,79,484 groups linked via Ni or Cu metal centers. These materials have been integrated as electrode materials for applications in transistor,293 electrocatalysis,77,78,317,485 chemiresistive sensor,484,486−488 energy storage,298,299 and superconductivity.489 However, the incorporation of conjugated 2D MOFs into highly efficient electronic devices remains challenging due to the difficulty in processing the powder samples. The interfacial strategy opens up a bright path to synthesize macroscopic (from μm2 to cm2), crystalline, single-or-multilayer 2D MOF film with adjustable thickness (from