Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Review pubs.acs.org/CR

Recent Advances in Ultrathin Two-Dimensional Nanomaterials Chaoliang Tan,† Xiehong Cao,†,‡ Xue-Jun Wu,† Qiyuan He,† Jian Yang,† Xiao Zhang,† Junze Chen,† Wei Zhao,† Shikui Han,† Gwang-Hyeon Nam,† Melinda Sindoro,† and Hua Zhang*,† †

Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore ‡ College of Materials Science and Engineering, Zhejiang University of Technology, 18 Chaowang Road, Hangzhou 310014, China ABSTRACT: Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocatalysis, batteries, supercapacitors, solar cells, photocatalysis, and sensing platforms. Finally, the challenges and outlooks in this promising field are featured on the basis of its current development.

CONTENTS 1. Introduction 2. Advances of Ultrathin 2D Nanomaterials 3. Composition and Crystal Structures 3.1. Graphene 3.2. Hexagonal Boron Nitride (h-BN) 3.3. Graphitic Carbon Nitride (g-C3N4) 3.4. Transition Metal Dichalcogenides (TMDs) 3.5. Black Phosphorus (BP) 3.6. III−VI Layered Semiconductors 3.7. MXenes 3.8. Metal Phosphorus Trichalcogenides 3.9. Layered Double Hydroxides (LDHs) 3.10. Metal Oxides 3.11. Transition Metal Oxyhalides 3.12. Metal Halides 3.13. Perovskites and Niobates 3.14. Silicates and Hydroxides (Clays) 3.15. Metal−Organic Frameworks (MOFs) 3.16. Covalent−Organic Frameworks (COFs) and Polymers 3.17. Metals 3.18. Nonlayer Structured Metal Oxides 3.19. Nonlayer Structured Metal Chalcogenides 4. Synthetic Methods 4.1. Micromechanical Cleavage 4.2. Mechanical Force-Assisted Liquid Exfoliation 4.2.1. Sonication-Assisted Liquid Exfoliation 4.2.2. Shear Force-Assisted Liquid Exfoliation 4.3. Ion Intercalation-Assisted Liquid Exfoliation

© 2017 American Chemical Society

4.4. Ion Exchange-Assisted Liquid Exfoliation 4.4.1. Cation Exchange-Assisted Liquid Exfoliation 4.4.2. Anion Exchange-Assisted Liquid Exfoliation 4.5. Oxidation-Assisted Liquid Exfoliation 4.6. Selective Etching-Assisted Liquid Exfoliation 4.7. Chemical Vapor Deposition 4.8. Wet-Chemical Syntheses 4.8.1. Hydro/Solvothermal Synthesis 4.8.2. 2D-Oriented Attachment 4.8.3. Self-Assembly of Nanocrystals 4.8.4. 2D-Templated Synthesis 4.8.5. Hot-Injection Method 4.8.6. Interface-Mediated Synthesis 4.8.7. On-Surface Synthesis 4.8.8. Other Wet-Chemical Synthesis Methods 5. Characterization 5.1. Optical Microscopy 5.2. Scanning Probe Microscopy 5.2.1. Atomic Force Microscopy 5.2.2. Conductive Atomic Force Microscopy (CAFM) 5.2.3. Electrostatic Force Microscopy (EFM) and Kelvin Probe Force Microscopy (KPFM)

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Chemical Reviews 5.2.4. Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STS) 5.3. Scanning Electron Microscopy 5.4. Transmission Electron Microscopy/Scanning Transmission Electron Microscopy 5.5. X-ray Absorption Fine Structure Spectroscopy 5.6. X-ray Photoelectron Spectroscopy 5.7. Raman Spectroscopy 6. Applications 6.1. Electronic/Optoelectronic Devices 6.1.1. Ultrathin 2D TMDs 6.1.2. Ultrathin 2D BP Nanosheets 6.1.3. Ultrathin 2D h-BN Nanosheets 6.1.4. van der Waals Heterostructures 6.2. Electrocatalysis 6.2.1. Hydrogen Evolution Reaction (HER) 6.2.2. Oxygen Evolution Reaction (OER) 6.2.3. Oxygen Reduction Reaction (ORR) 6.2.4. Other Electrocatalytic Reactions 6.3. Batteries 6.3.1. Li-Ion Batteries 6.3.2. Non-Li-Ion Batteries 6.4. Supercapacitors 6.4.1. Graphene-Based Supercapacitors 6.4.2. Other 2D Nanomaterial-Based Supercapacitors 6.4.3. Ultrathin 2D Nanomaterial-Based Flexible Supercapacitors 6.5. Solar Cells 6.5.1. Graphene-Based Transparent Conductive Electrodes 6.5.2. 2D TMD-Based Solar Cell Devices 6.6. Photocatalysis 6.6.1. 2D Metal Oxide Nanomaterial-Based Photocatalysts 6.6.2. 2D Metal Chalcogenide NanomaterialBased Photocatalysts 6.6.3. Other 2D Nanomaterial-Based Photocatalysts 6.7. Sensing Platforms 6.7.1. Electronic Sensors 6.7.2. Fluorescent Sensors 6.7.3. Electrochemical Sensors 7. Concluding Remarks and Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References

Review

Geim, and co-workers successfully exfoliated graphene from graphite using Scotch tape,2 currently classified as the micromechanical cleavage technique. The 2D feature is unique and indispensable to access unprecedented physical, electronic, and chemical properties due to electron confinement in two dimensions.3 Graphene, a one-atom-thick and crystalline carbon film, is an exemplary model due to its unexpected properties including ultrahigh room-temperature carrier mobility,2 quantum hall effect,4 ultrahigh specific surface area,5 high Young’s modulus,6 excellent optical transparency,7 and excellent electrical2 and thermal8 conductivities. The explorations of other graphene-like ultrathin 2D nanomaterials are also growing.9−11 To name a few, hexagonal boron nitride (hBN),12−14 transition metal dichalcogenides (TMDs),15−18 graphitic carbon nitride (g-C3N4),19−21 layered metal oxides,22,23 and layered double hydroxides (LDHs)23,24 are typical graphene-like ultrathin 2D nanomaterials that exhibit versatile properties due to their similar structural features but different compositions from graphene. Promising research on graphene and graphene-like 2D nanomaterials further enriched the exploration of 2D ultrathin family members, such as MXenes,25 noble metals,26−29 metal−organic frameworks (MOFs),30−32 covalent−organic frameworks (COFs),33 polymers,34−37 black phosphorus (BP),38,39 silicene,40−42 antimonene,43−45 inorganic perovskites,46,47 and organic−inorganic hybrid perovskites.48,49 Demands for property modulations greatly stimulate the development of various synthetic methods to prepare ultrathin 2D nanomaterials.1 The well-established synthetic methods include the micromechanical cleavage,2,50−52 mechanical forceassisted liquid exfoliation,53−59 ion intercalation-assisted liquid exfoliation,60−67 ion exchange-assisted liquid exfoliation,68−71 oxidation-assisted liquid exfoliation,72−76 selective etchingassisted liquid exfoliation,77−80 chemical vapor deposition (CVD),81−88 and wet-chemical syntheses.89−96 All of the aforementioned methods can be categorized into two categories: top-down and bottom-up methods. Because ultrathin 2D nanomaterials prepared by different synthetic methods might exhibit different structural characteristics, that is, different physical, electronic, chemical, and surface properties, a comprehensive characterization is crucial. Obtaining precise sizes, compositions, thicknesses, crystal phases, doping, defects, vacancies, strains, electronic states, and surface properties of these synthesized ultrathin 2D nanomaterials is important to understand the correlation between the structural characteristics and properties/functionalities.97 Therefore, these synthesized ultrathin 2D nanomaterials have been widely characterized by a host of advanced techniques, such as optical microscopy,98−100 scanning probe microscopy (SPM),101−105 electron microscopy,106−111 X-ray absorption fine structure spectroscopy (XAFS),112−115 X-ray photoelectron spectroscopy (XPS),116−118 and Raman spectroscopy.119−124 More importantly, the appealing properties of ultrathin 2D nanomaterials render them very promising for a wide range of applications, such as electronics/optoelectronics,125−132 catalysis,133−143 energy storage and conversion,144−156 sensors,157−168 and biomedicine.169−177 Given their unique structural features, outstanding properties, and promising applications, ultrathin 2D nanomaterials have now became one of the hottest research topics in condensed matter physics, material science, chemistry, and nanotechnology. Bearing this in mind, we believe that offering a timely critical Review on this topic toward a wide range of readers is of great importance for the future development of this promising field.

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1. INTRODUCTION Ultrathin two-dimensional (2D) nanomaterials represent an emerging class of nanomaterials that possess sheet-like structures with the lateral size larger than 100 nm, or up to a few micrometers and even larger, but the thickness is only single- or few-atoms thick (typically less than 5 nm).1 Although their explorations dated back a few decades, 2004 marked the year of ultrathin 2D nanomaterials resurgence when Novoselov, 6226

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means that electrons can be considered as massless Dirac Fermions.3,185 This unique electronic property of graphene renders it an ideal platform for the fundamental condensed matter research to experimentally study the relativistic effects, allowing observation of phenomena such as quantum Hall effect and Klein paradox, where this unique electronic band structure disappears when the thickness of graphene is thicker than three layers.3,186,187 Another unique advantage is that graphene allows electrons to travel in a distance up to few micrometers without scattering.3,188 This property guarantees graphene with ultrahigh charge carrier mobility at room temperature and excellent electrical conductivity.3 Note that the measured room-temperature charge carrier mobility of graphene is as high as ∼10 000 cm2 V−1 s−1.2,3 Even so, one of the major drawbacks of graphene is the lack of a bandgap, making it unsuitable as a candidate for high-performance lowpower digital transistors.3 Ultrathin 2D TMD nanosheets, such as MoS2, WS2, MoSe2, and WSe2, are semiconductors with large bandgaps (∼1−2 eV) as well as moderate carrier mobilities (a few to hundreds of cm2 V−1 s−1).125,126 The BP nanosheets have been demonstrated to exhibit a wide range of bandgap (∼0.3−2.1 eV) with carrier mobility up 1000 cm2 V−1 s−1.132 Hence, the aforementioned ultrathin 2D nanosheets are ideal channel materials for the construction of high-performance electronic/optoelectronic devices.125,126 Benefiting from the strong in-plane covalent bond and atomic thickness, ultrathin 2D nanomaterials usually exhibit excellent mechanical strength and flexibility. As a typical example, the measured breaking strength and Young’s modulus of graphene are as high as 42 N m−1 and 1.0 TPa, respectively,6 meaning that graphene is the thinnest material with the strongest mechanical strength. Moreover, graphene can endure more than 20% of the elastic deformation without breaking, suggesting its excellent flexibility.6 Nevertheless, some other ultrathin 2D nanomaterials have also been proven to possess excellent mechanical properties. For example, the measured Young’s modulus for the single-layer MoS2 nanosheet is about 270 GPa, which is superior to that of the bulk MoS2 (∼240 GPa) and steel (∼205 GPa).189 Recent theoretical calculations also revealed that plenty of covalently bonded ultrathin 2D nanomaterials are expected to have excellent mechanical properties comparable to those of the graphene or singlelayer MoS2 nanosheet.190 Because they all have 2D structural characteristics similar to those of graphene, it is reasonable to predict that all of the ultrathin 2D nanomaterials might have superior mechanical properties. On top of mechanical strength and flexibility, the atomic thickness can also endow ultrathin 2D nanomaterials with excellent optical transparency, which increases as thickness decreases. Therefore, the atomic thickness ensures the ultimate optical transparency of ultrathin 2D nanomaterials. Graphene only absorbs ∼2.3% of the white light with a negligible reflectance (∼0.1%), which means that more than 97% of the white light can be transmitted from graphene.7 This result proved that graphene is a highly optical transparent material. It can be predicted that many other ultrathin 2D nanomaterials with atomic thickness should also have excellent optical transparency comparable or close to that of graphene. The coexistence of excellent mechanical strength, high flexibility, light transmittance, as well as the outstanding electronic properties on ultrathin 2D nanomaterials makes them compelling in the construction of high-performance electronic/optoelectronic devices that are wearable, flexible, and transparent.191,192

Although a number of Reviews concerning ultrathin 2D nanomaterials have been published, almost all of them focused on a selected ultrathin 2D nanomaterial (e.g., graphene), a particular type of ultrathin 2D nanomaterials (e.g., just TMDs, metal oxides, noble metals, MXenes, or polymers), or specific properties/applications of some ultrathin 2D nanomaterials (refs 1,3,9,10,16,17,24,38,53,90,133,178). None has made an effort on a thorough overview in this hot research field. Additionally, we have witnessed the great progress made in the past three years, and the most novel 2D ultrathin structures have not been reviewed. Hence, our aim is to critically summarize the state-of-art progress on all types of ultrathin 2D nanomaterials with particular emphasis on recent advances. We start briefly from a discussion of the unique advantages of ultrathin 2D nanomaterials in relation to their composition and crystal structures. Various synthetic methods for the preparation of ultrathin 2D nanomaterials are then described with comments on their advantages and limitations. After that, we introduce the major techniques to characterize the ultrathin 2D nanomaterials, followed by examining their great potential applications in the electronics/optoelectronics, electrocatalysis, batteries, supercapacitors, solar cells, photocatalysis, and sensing platforms. Finally, on the basis of the current progress, we conclude this Review with some personal insights on the challenges and outlooks in this promising field.

2. ADVANCES OF ULTRATHIN 2D NANOMATERIALS New properties can be achieved after a material is altered chemically and/or physically. Stemming from this principle, ultrathin 2D nanomaterials show many unprecedented physical, electronic, chemical, and optical properties that are unattainable in its counterparts. In this section, we briefly summarize the uniqueness of ultrathin 2D nanomaterials from different aspects. Generally, there are a number of unique advances that have now been identified or uncovered for ultrathin 2D nanomaterials. First, the confinement of electrons in two dimensions in the ultrathin region, especially for single-layer 2D nanomaterials, facilitates their compelling electronic properties, which are ideal for the fundamental study in condensed matter physics and electronic/optoelectronic devices.3 Second, the strong in-plane covalent bond and atomic thickness enable their excellent mechanical strength, flexibility, and optical transparency, which are important for utilization in the nextgeneration devices.3,125,126 Third, the large lateral size, while still maintaining atomic thickness, endows them with ultrahigh specific surface area.5 This is exceedingly attractive for those surface-related applications such as catalysis and supercapacitors.133,179 Fourth, the solution-based processability of ultrathin 2D nanomaterials for fabrication of high-quality freestanding thin films via simple methods, such as vacuum filtration, spin coating, drop casting, spray-coating, and inkjet printing, is essential for some practical applications, supercapacitors and solar cells.180,181 Last, high exposure of surface atoms allows easy regulation to the properties and functionalities by means of surface modification/functionalization, element doping, and/or defect/strain/phase engineering.182−184 One of the most striking properties of graphene is its unprecedented electronic properties, enabling great potential in the fundamental study of its intrinsic properties and the application of electronic/optoelectronic devices.3 Because of its peculiar 2D structural features and electronic band structure, electrons mimic the relativistic particles in graphene, which 6227

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fabrication of a single-layer MOF nanosheet-based membrane, that is, Zn2(benzimidazole)4, on a porous α-Al2O3 substrate by a simple hot-drop coating method.30 When used as the molecular sieving, the fabricated 2D MOF membrane exhibits an ultrahigh H2 gas permeance (up to several thousand gas permeation units) with the H2/CO2 selectivity greater than 200.30 Excellent film-forming ability also shows promising results as the active layers in functional devices, such as solar cells and memory devices,181,198 in which the quality of active thin films plays a crucial role in the realization of highperformance devices. Ultrathin 2D nanomaterials, with all atoms exposed for modification, act as ideal platforms to engineer their properties and functionalities at the atomic level through surface modification/functionalization, element doping, and/or defect/vacancy/strain engineering.201 This is unattainable in other types of nanomaterials and their bulk counterparts. Graphene, a zero bandgap material, unsuitable for high-performance lowpower field-emission transistors (FETs), can open its bandgap by functionalizing with organic molecules on its surface.202 The inert graphene, unsuitable for the electrocatalytic oxygen reduction reaction (ORR), can be doped with heteroatoms (such as N, S, P, and/or B) on its backbone to form N, S, P, and/or B-doped graphene sheets, which are highly efficient electrocatalysts.203 The element doping in ultrathin 2D nanomaterials can significantly alter their catalytic properties.203,204 The catalytic activities of ultrathin 2D nanomaterials can also be remarkably tuned by the defect/vacancy engineering.205−208 Recent work to increase the abundance of defects on the MoS2 nanosheet backbone obviously demonstrated its enhanced catalytic activity toward the electrocatalytic HER.205,206 The introduction of sulfur vacancies on the basal planes of MoS2 nanosheets also promises a way to promote its electrocatalytic activity toward HER.207,208 The generation of pits in single-layer 2D CeO2 nanosheets can significantly enhance their catalytic performance toward the CO oxidation.209 Pit-confined single-layer CeO2 nanosheets showed higher conversion efficiency and lower activation energy as compared to other counterparts, that is, single-layer CeO2 nanosheets without pits and bulk CeO2 crystals.209 It suggested that the unsaturated coordination of Ce, induced by pits, gave rise to the superior catalytic performance for CO oxidation.209

Large lateral size and atomic thickness endow ultrathin 2D nanomaterials with high specific surface area and ultimate exposure of their surface atoms, making them exceedingly desirable in a number of practical applications, such as supercapacitors and catalysis.133,180 The calculated result suggested that graphene has a theoretical specific surface area as high as 2630 m2 g−1.5 Other ultrathin 2D nanomaterials are also expected to possess high specific surface area comparable to that of graphene because of their similar structural characteristics. Because of their high specific surface area, ultrathin 2D nanomaterials have been proven to be very attractive for those surface-related applications. It has been demonstrated that the graphene thin film possesses a much higher specific capacitance when used as the electrode in a supercapacitor as compared to that of the graphite due to its ultrahigh specific surface area.7 Ultrathin 2D nanomaterials also exhibited excellent catalytic activities in a number of catalytic reactions, such as organic catalysis, electrocatalysis, and photocatalysis, due to their high specific surface area and ultimately exposed surface atoms.133 As a typical example, single-layer rhodium (Rh) nanosheet has been demonstrated to be a highly efficient catalyst for catalytic organic reactions due to the 100% surface exposure of Rh atoms.29 Single-layer CoNi and CoFe LDH nanosheets exhibit much higher catalytic performance toward the electrochemical oxygen generation as compared to their bulk comperparts due to the more exposed active sites and enhanced electronic conductivity after being exfoliated into few-layer nanosheets.141 Note that their catalytic activity and stablility are even better than those of the commercial iridium dioxide catalyst.141 Because of their relatively large lateral size (up to few micrometers) and atomic thickness, these solution-dispersed ultrathin 2D nanomaterials can be easily fabricated into freestanding thin films with high film quality, flexibility, and transparency via various simple fabrication approaches, such as vacuum filtration, spin coating, drop casting, spray-coating, and inkjet printing.181,193−197 The thickness of fabricated thin films can be easily tuned by controlling the concentration of 2D nanosheets and/or the volume of the nanosheet suspension.181,193 This excellent film-forming ability of ultrathin 2D nanomaterials makes them very attractive in a number of applications, such as flexible and transparent electronic, optoelectronic devices, as well as electrodes for energy storage and conversion. An example is the reduced graphene oxide (rGO) thin films that could be used as flexible and transparent electrodes in some electronic devices (e.g., solar cells and memory devices) due to their superior electrical conductivity, flexibility, and transparency.181,198 The excellent film-forming ability and electrical conductivity of some ultrathin 2D nanomaterials, such as rGO, MXenes, and 1T-MoS2, also make them very attractive as alternatives for electrode materials of high-performance supercapacitors.179,180,199 For example, the single- or few-layer Ti3C2 nanosheet, one of the members in the MXene family, can be easily fabricated into thin films by a simple vacuum filtration or roller milling technique.199 The fabricated freestanding thin films were used as electrodes in supercapacitors with the high capacitance up to 900 F cm−3 as well as the excellent rate performance and cycling stability.199 Cui and co-workers also demonstrated that a hybrid electrode made from graphene and few-layer BP nanosheets exhibits a specific capacity of 2440 mA h g−1 at current density of 0.05 A g−1, when used as an electrode for sodium-ion battery (SIB).200 In another example, Yang and co-workers reported the

3. COMPOSITION AND CRYSTAL STRUCTURES To date, large quantities of ultrathin 2D nanomaterials have been prepared by various synthetic methods. Even though the composition and crystal structures vary in different materials, they all can be categorized into two types: layered and nonlayer structured materials. For layered materials, the in-plane atoms connect to each other by strong chemical bonding in each layer, while these layers stack together though the weak van der Waals interaction to form bulk crystals.53 Graphite is a typical example of layered materials in which graphene layers weakly stack together, forming the bulk graphite crystal.210 Besides graphite, there are many other layered materials, such as h-BN, TMDs, g-C3N4, BP, transition metal oxides (TMOs), and LDHs. Their layered nature renders them ready to be exfoliated into ultrathin 2D nanosheets by means of top-down exfoliation methods, such as micromechanical cleavage, mechanical forceassisted liquid exfoliation, and ion intercalation-assisted liquid exfoliation. In contrast, other materials crystallize in three dimensions via atomic or chemical bonding, forming bulk crystals, such as metals, metal oxides, metal chalcogenides, and 6228

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polymers.90 Especially, they can crystallize into various crystal phases depending on the coordination modes between atoms, arrangement of atoms, or stacking orders between layers.211−213 These crystal phases can significantly affect the materials’ properties and functionalities.211−217 In this section, we mainly introduce the crystal structures of these widely explored ultrathin 2D nanomaterials based on the composition in detail.

represents a chalcogen (e.g., S, Se, or Te).16,225−227 The prepresentive crystal structures of TMDs are shown in Figure 2.

3.1. Graphene

Graphene is a single-atom-thick graphite, an allotrope of carbon in the form of 2D structure.2 It is composed of a hexagonalclose-packed carbon network, in which each atom covalently bonds to three neighboring ones through the σ-bond (Figure 1a).218,219 The distance between two neighboring carbon atoms

Figure 2. Crystal structures of MoS2 with different polymorphisms. Reproduced with permission from ref 211. Copyright 2015 The Royal Society of Chemistry.

Similar to the graphite, TMDs have a layered structure, in which TMD monolayers stack together through the van der Waals force. Each TMD monolayer consists of three atomic layers, in which a transition metal layer is sandwiched between two chalcogen layers.16 One of TMDs’ unique features is their capability to form different crystal polytypes. Taking MoS2 as an example, it crystallizes with four different crystal structures, that is, 2H, 1T, 1T′, and 3R, depending on the different coordination models between Mo and S atoms and/or stacking orders between layers (Figure 2).211 The 2H structure has an atomic stacking sequence (S−Mo−S′) ABA in a hexagonal closed packing symmetry and trigonal prismatic coordination. The 1H structure is used to describe the single-layer structure of 2H phase (Figure 2). The 1T structure has an octahedral coordination with tetragonal symmetry in which each layer has an atomic stacking sequence of (S−Mo−S′) ABC. The distorted 1T (denoted as 1T′) structure also has an octahedral coordination, similar to that of the 1T structure, but it contains a superstructure in each layer, such as a tetramerization (2a × 2a), a trimerization ( 3a × 3a ), and a zigzag chain (2a × a) (Figure 2). Note that ReS2 and ReSe2 have a naturally zigzag 1T′ structure.228,229 The 3R structure has three layers per primitive cell with a different stacking sequence between individual layers as compared to the 2H phase (Figure 2).211 The 2H-type MoS2 is dominant because it is thermodynamically stable in nature.211

Figure 1. Crystal structures of (a) graphene, and (b) h-BN. (c) sTriazine, and (d) tri-s-triazine structure models of g-C3N4.

in a single sheet is about 1.42 Å. Individual layers stack together through the van der Waals force to form the graphite, in which the distance between adjacent layers is about 3.35 Å.219 3.2. Hexagonal Boron Nitride (h-BN)

The bulk h-BN has a layered crystal structure similar to that of graphite. It consists of equal numbers of boron and nitrogen atoms arranged in a hexagonal structure (space group = P63/ mmc).220 Within each layer, the boron and nitrogen atoms are bonded by covalent bonds, while these layers stack together by the van der Waals force to form the bulk crystal (Figure 1b).221 As compared to graphite, the bulk h-BN exhibits similar lattice constants (a = 2.504 Å; a is the distance between two neighboring atoms) and interlayer distances (3.30−3.33 Å).220,221 A single-layer h-BN nanosheet can be regarded as a graphene analogue, which is commonly known as the “white graphene”.

3.5. Black Phosphorus (BP)

The bulk BP crytallizes into a layered orthorhombic crystal structure with the space group Cmca (64). The distance between adjacent layers is 5.4 Å, and individual layers stack together through the van der Waals force (Figure 3a). A BP monolayer is composed of a puckered honeycomb structure, in which one P atom bonds with the other three (Figure 3b). Among the four P atoms, three of them are located in the same plane, while the fourth one is located at the parallel adjacent plane (Figure 3b,c).230−232

3.3. Graphitic Carbon Nitride (g-C3N4)

The g-C3N4 is another analogue of graphite with a van der Waals layered structure.222−224 The crystal structure of g-C3N4 can be regarded as N-substituted graphite framework formed through the sp2 hybridization of carbon and nitrogen atoms. There are two different structural models for the g-C3N4: (1) striazine constructed by the condensed s-triazine units with a periodic array of single-carbon vacancies (Figure 1c), and (2) condensed tri-s-triazine subunits connected through the planar tertiary amino groups with larger periodic vacancies in the lattice (Figure 1d).222−224

3.6. III−VI Layered Semiconductors

The III−VI layered semiconductors are a class of layered metal chalcogenides with a general formula of MX (M = Ga, In; X = S, Se, Te).233,234 GaSe is an example of layered materials from the III−VI metal chalcogenide family composed of vertically stacked Se−Ga−Ga−Se layers via the van der Waals interaction (Figure 3d,e).235 Each layer has a hexagonal structure with D3h symmetry. The distance between two neighboring layers is about 0.84 nm, and the lattice constant along the axes is about 0.40 nm.235

3.4. Transition Metal Dichalcogenides (TMDs)

TMDs are layered compounds with the general chemical formula of MX2, where M is a transition metal element and X 6229

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Figure 3. Crystal structures of (a−c) BP and (d,e) GaSe. (f) Crystal structures of MAX phases. Part (f) is reproduced with permission from ref 237. Copyright 2015 Elsevier Ltd.

Figure 4. Crystal structures of (a,b) NiPS3, (c) LDHs, (d) MoO3, and (e) α-V2O5.

3.7. MXenes

3.8. Metal Phosphorus Trichalcogenides

MXenes are a class of 2D layered transition metal carbides and/ or nitride produced by selective etching of the raw MAX phases with a general formula of Mn+1AXn (n = 1, 2, or 3), where M is the transition metal (e.g., Ti, V, Cr, Nb, etc.), A is another element from group IIIA or IVA (e.g., Al, Si, Sn, In, etc.), and X stands for carbon and/or nitrogen.25 MAX phases have a layered, hexagonal structure with P63/mmc symmetry, in which M layers are nearly hexagonally close-packed together and X atoms fill the octahedral sites (Figure 3f).25,236,237 The element A is metallically bonded to the M element and interleaved in the Mn+1Xn layers. The A layer can be selectively etched from the MAX phases using strong etching solutions, for example, HF, forming MXenes with three different structures, that is, M2X, M3X2, or M4X3.25

Metal phosphorus trichalcogenides with the chemical formula of MPS3 (where M = Mn, Fe, Ni, Zn, etc.) are layered materials with monoclinic structures.238−240 The in-plane coordination number of a single sheet is 3. Each metal ion has a phosphorus dimer at the center, and the metal layer is sandwiched by distorted octahedral S layers. Figure 4a,b shows the crystal structure of NiPS3,241 in which individual layers stack together through the van der Waals force to form bulk crystals.241 3.9. Layered Double Hydroxides (LDHs)

LDHs, with a common formula of [Mz+1−xM3+x(OH)2]m+[An−]m/n·yH2O, are a class of layered materials with positively charged layers and weakly bounded charge-balancing anions or solvation molecules and interlayer water molecules.242−244 In most cases, Mz+ and M3+ represent the divalent (e.g., Mg2+, Zn2+, Ni2+; z = 2) and trivalent metal ions (e.g., Al3+, Ga3+, Fe3+, Mn3+), respectively, giving m = x. 6230

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Figure 5. Crystal structures of (a) FeOCl, (b) CrCl3, and (c) CaTiO3. (d) Crystal structure of organic−inorganic pervoskites. Part (d) is reproduced with permission from ref 261. Copyright 2010 The Royal Society of Chemistry.

structure in which each layer is constructed by a corrugated metal−oxygen plane sandwiched by two halide layers.253,254 FeOCl, as an example, has a layered structure with the orthorhombic space group Pmnm (59), as shown in Figure 5a.255 The Fe3+ is surrounded by four oxygen and two chlorine ions to form the octahedral structure, sharing their edges with each other to form in-plane layers. Likewise, the bulk crystal is constructed by stacking individual layers together through the van der Waals force.253−255

These cations occupy octahedral holes in a brucite-like layer; An− is a nonframework charge-balancing anion or solvation molecule, such as CO32−, Cl−, SO42−, and RCO2−, locating in the gap of hydrated interlayer.242−244 In one case, Mz+ represents Li+ (z = 1) and M3+ represents Al3+, thus giving m = 2x − 1. The value of x is varied and normally in the range of 0.2−0.33, while the value of y is dependent on the anion, the water vapor pressure and temperature. The typical crystal structure of LDHs is schematically shown in Figure 4c. The metal cations occupy the centers of octahedra whose vertexes, containing hydroxide ions, are shared and connected together to form 2D layers. Because of the diversity of cations (Mz+ or M3+), the interlayer anion (An−), together with the value of x, LDHs involve a large class of isostructural materials.

3.12. Metal Halides

Metal halides are a class of inorganic compounds with the chemical formula of MXn, where M is a metal and X refers to a halogen.256 Taking CrCl3 as an example, the Cl− ions arranged in a pseudocubic close packed mode and the Cr3+ ion filled two-thirds of the octahedral holes in the alternating Cl layers (Figure 5b).257 The resulting CrCl6 octahedra share their edges with the adjacent octahedra, forming a 2D layer.257

3.10. Metal Oxides

Metal trioxides have a layered structure with a general formula of MO3 (M = Mo, Ta, W, etc.).245−250 For example, MoO3 has a layered structure in which each layer is predominantly composed of distorted MoO6 octahedra in an orthorhombic crystal (space group Pcmn, a = 3.963 Å, b = 3.696 Å, c = 13.855 Å) (Figure 4d).245,247 The octahedra shares edges with its neighbors to form 2D layers. The bulk crystal is constructed by stacking different layers together along the y-axis through the van der Waals force.246,247 The α-V2O5 is the most stable phase in the vanadium oxide family due to its highest oxidation state of vanadium.251 Figure 4e shows the crystal structure of α-V2O5. It has a layered structure and crystallizes with an orthorhombic unit cell structure (space group: Pmmn, a = 11.510 Å, b = 3.563 Å, and c = 4.369 Å).251 Each layer is composed of distorted trigonal bipyramidal polyhedral where O atoms locate around V atoms. The polyhedra shares their edges by forming (V2O4)n zigzag double chains along the (001) direction and shares their corners by cross-linking along (100), thus forming 2D layers. Different layers stack together through the van der Waals force to form a bulk crystal.251,252

3.13. Perovskites and Niobates

Inorganic perovskites are one type of compounds with the general chemical formula of AMX3, where A and M are both cations while X represents an anion (O, Cl, Br, I, etc.).258 M is octahedrally coordinated with X by forming the MX6 octahedra building blocks, M is located at the center of the octahedra, while X occupies the corner around M. The as-formed MX6 octahedra are connected together by sharing corners, thus giving rise to an extended three-dimensional (3D) network. A is a metal or organic cation that fills the hole of the octahedra networks and balances the charge of the compound. As an example of inorganic perovskites, the crystal structure of CaTiO3 is schematically shown in Figure 5c.259 Besides inorganic perovskites, organic−inorganic perovskites have also attracted considerable attention in recent years due to their promising applications in solar cells and optoelectronic devices.260 Organic−inorganic perovskites are a class of unique compounds in the perovskite family due to their unique structures; that is, organic and inorganic components are built by alternate stacking from the molecular scale.261−263 Different from inorganic perovskites, the A cations in organic−inorganic perovskites are replaced by organic cations to compensate for the charges of the overall compounds. Because of the limited

3.11. Transition Metal Oxyhalides

Transition metal oxyhalides are a class of inorganic compounds with the general formula of MOX, where O is oxygen, X is halogen, and M is Fe, Cr, V, or Ti. MOX has a layered crystal 6231

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Figure 6. Crystal structures of (a) aluminum phyllosilicates and (b) the Zn-TCPP MOF. Part (b) is reproduced with permission from ref 273. Copyright 2009 The Royal Society of Chemistry. The schematic illustration of the crystal unit cell of (c) fcc, (d) hcp, and (e) 4H of gold. Parts (c,d) reproduced with permission from ref 280. Copyright 2015 Springer International Publishing AG. Part (e) reproduced with permission from ref 285. Copyright 2015, Nature Publishing Group. Crystal structures of (f) CeO2 and (g) PbS.

tetrahedral and octahedral layers requires the layer distortion to match with each other and minimize the overall bond-valence distortions of the crystallite. Because the tetrahedral layer is more flexible than the octahedral layer, the tetrahedral layer becomes corrugated or twisted, while the octahedral layer is flattened.264

spaces, only short chain organic cations, consisting of three or less C−C or C−N bonds, can be integrated into the perovskite networks. The organic−inorganic perovskites have a general formula of MAMX3 (MA = CH3NH3; M = Pb, Sn; X = Cl−, Br−, I−).261,262 In these compounds, the [MX6]4− octahedra could form chained, layered, or 3D networks depending on the organic ammonium cations at A sites (Figure 5d).261,262 The ⟨100⟩ oriented layered organic−inorganic halide perovskites, with a general formula of (R−NH3)2MAn−1MnX3n+1 (R−NH3+ is an alkylammonium or phenethylammonium species, and n is the number of perovskite sheets), alternate between bilayers of R−NH3+ organic cations. The NH3+ heads in the organic cations interact with the halogens in the perovskite layers, thereby forming hydrogen/ionic bonds, while the hydrocarbon tails R reach out to the sheet surface and extend into the space between the adjacent perovskite layers.

3.15. Metal−Organic Frameworks (MOFs)

MOFs are one kind of crystalline porous compounds in which the metal ions or clusters are linked by coordinating organic ligands to form bulk crystals.271,272 Depending on the different coordination modes between ligands and metal centers, MOFs can form various kinds of crystal structures in different space groups. Even having the same ligand and metal center, MOFs are able to crystallize into different structures on the basis of different coordination modes. It is worth pointing out that MOFs not only can crystallize into 3D structures but also layered structures. There is no general fomula for MOF crystals. One case in point for MOF crystal structure is introduced by Zn-TCPP (TCPP = tetrakis(4-carboxyphenyl)porphyrin).273,274 The Zn-TCPP MOF has a space group of I4/mmm, and its crystal structure is shown in Figure 6b.273 Four Zn paddlewheel metal nodes, that is, Zn2(COO)4, are linked by one TCPP ligand to form an in-plane layer. Individual layers stack together in an AB packing pattern,273,274 in which the zinc atoms in the center of porphyrin rings align with the zinc atoms in the paddlewheel metal nodes. The network of metal nodes and organic linkers contains nanopores paralleling to the stacking direction. Note that most of the reported 2D MOF nanosheets have a layered structure.30−32,274

3.14. Silicates and Hydroxides (Clays)

Clay minerals are hydrous aluminum phyllosilicates which can be classified as 1:1 and 2:1 clays according to the configuration of tetrahedral silicate layers and octahedral hydroxide layers, respectively.264−270 The 1:1 clays consist of one tetrahedral layer linked with one octahedral group in each single layer, known as kaolinite and serpentine.264 The 2:1 clays have an octahedral layer sandwiched between two tetrahedral layers, such as talc, vermiculite, and montmorillonite.264 Clay minerals have a layered structure connected by sharing SiO4 as tetrahedra and/or AlO4 as octahedra.264 Each silica tetrahedron provides three oxygen atoms as vertexes to share with another tetrahedral, forming an extended 2D layer (Figure 6a). The fourth vertex, located at the same side and pointing to a similar direction, is not shared. Usually, an octahedral layer exists in clays and bonds with the tetrahedral layer. Thus, the unshared vertex from the tetrahedral layer participates in the formation of octahedral layer. The additional oxygen atom in the tetrahedral layer, located above the gap, forms an OH group by bonding to a hydrogen atom. In most cases, the interlayer cation, such as Na+ or K+, is intercalated to compensate and balance the charges. The bonding mismatch between the

3.16. Covalent−Organic Frameworks (COFs) and Polymers

COFs are porous crystalline materials, covalently connected by organic units that are made of light elements, such as H, B, C, N, and O. Organic building block units are covalently integrated into an ordered structure to form a periodic porous COF framework.275,276 2D COFs normally crystallize into a layered structure. A single-layer COF sheet is formed via strong covalent bonds. Individual layers can stack via van der Waals 6232

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3.19. Nonlayer Structured Metal Chalcogenides

interaction to form the bulk crystal with periodically aligned channels parallel to the stacking direction. For example, the 2D HHTP-DPB COF has a porous structure (P63/mmm structure) with 4.7 nm wide hexagonal pores.277 Polymers are a class of materials that consist of macromolecules,278 which might form crystals, depending on the molecular structure and stacking orders of molecular building blocks. Note that 2D polymer nanosheets are normally derived from layered bulk polymers.34,35,37

Unlike layered TMDs, nonlayer structured metal chalcogenides are a class of metal chalcogenides whose bulk crystallizes in three dimensions.288,289 They have a general formula of MX (where M = Cd, Pb, Sn, Zn, Ag, etc.; X = S, Se, Te). PbS is one example of a nonlayer structured metal chalcogenide. As shown in Figure 6g, the bulk PbS crystallizes in a halite cubic structure with the space group of Fm3m.290 Each ion is surrounded by six other ions of the opposite charge. Because the surrounding ions are located at the vertices of the octahedral, they form a regular octahedral arrangement where another ion fills the space in the voids.290

3.17. Metals

Metal atoms are closely positioned to neighboring ones in one of two common arrangements.279 Normally, metals crystallize into a face-centered cubic (fcc), hexagonal close-packed (hcp), or body-centered cubic (bcc) structure.279 In the fcc structure, there are six lattice points at the center of the six faces and eight ones at corners of the cubic unit cell, in which each face lattice point and corner lattice point are shared by two and eight unit cells, respectively. It shows a stacking sequence of “ABC” along the [111] close-packed direction (Figure 6c). In the hcp structure, there are three layers of lattice points in the unit cell. In both the top and the bottom layers, six lattice points arrange themselves in a hexagonal shape with a seventh lattice point located at the middle of the hexagon. In the middle layer, three atoms fill in the triangular “grooves” of the top and bottom layers. It has alternating layers with an “ABAB” stacking sequence along the [001] close-packed direction (Figure 6d), meaning that the third atomic layer is in exactly the same position as the first layer.27,212,279,280 In the bcc structure, one lattice point is located at the center of the cubic unit cell, and eight lattice points are located at the corners, in which each corner atom is shared by eight unit cells.212,279,280 Different from Ru and Os with hcp phase, the other noble metals, that is, Au, Ag, Pt, Pd, Rh, and Ir, usually crystallize in the common fcc structure.212,279,280 Note that ultrathin 2D metal nanostructures normally exhibit the same crystal structures as their corresponding bulk materials.26−29,281 In addition, Weiss, Andrews, and co-workers successfully cleaved monolayer Au structure by chemical lift-off lithography from the bulk Au substrate.282,283 It is worth pointing out that some unusual crystal phases have also been observed in ultrathin noble metal nanostructures. For example, in 2011, the unconventional hcp phase was observed in Au square sheets with thickness of ∼2.4 nm, which are stable under ambient conditions at room temperature.284 Moreover, a recent study has demonstrated that the 4H phase with a stacking sequence of “ABCB” along the [001] close-packed direction can be stabilized in Au nanoribbons with thickness of 2.0−6.0 nm (Figure 6e).285

4. SYNTHETIC METHODS The capabilities for the preparation of ultrathin 2D nanomaterials with desired composition, size, thickness, crystal phase, defect, and surface property are of particular importance for the further study of their physical, chemical, electronic, and optical properties, as well as exploration of varying potential applications. On the other hand, in return, the compelling properties and promising applications can promote the rapid development of various reliable synthetic methods for preparing ultrathin 2D nanomaterials. These methods include the micromechanical cleavage, mechanical force-assisted liquid exfoliation, ion intercalation-assisted liquid exfoliation, ion exchange-assisted liquid exfoliation, oxidation-assisted liquid exfoliation, selective etching-assisted liquid exfoliation, CVD, and wet-chemical syntheses. All of the aforementioned methods can be divided into two categories: top-down and bottom-up methods. The top-down methods include mechanical cleavage, mechanical force-assisted liquid exfoliation, ion intercalationassisted liquid exfoliation, ion exchange-assisted liquid exfoliation, oxidation-assisted liquid exfoliation, and selective etchingassisted liquid exfoliation, all of which rely on the exfoliation of thin layer 2D crystals from their parent layered bulk crystals. Note that the top-down methods are only applicable to those materials whose bulk crystals are layered compounds. In contrast, CVD growth and wet-chemical syntheses belong to the bottom-up methods, which are based on chemical reactions of certain precursors at given experimental conditions. Unlike the top-down limitation, bottom-up approaches are more versatile in principle. That is, all types of ultrathin 2D nanomaterials might be obtainable by bottom-up methods. In this section, we focus on the introduction of various welldeveloped synthetic methods to prepare ultrathin 2D nanomaterials. The advances and limitations of each method are discussed along with addition of personal insights. Of note, we do not intend to systematically summarize all of the reported preparation methods for ultrathin 2D nanomaterials in detail here, but only introduce those methods that have been widely used and/or been proven to be applicable to a wide range of materials.

3.18. Nonlayer Structured Metal Oxides

Nonlayer structured metal oxides are formed via chemical bonding in three dimensions.286 Examples of nonlayer structured metal oxides are CeO2, TiO2, In2O3, SnO2, and Fe2O3. They do not have a general formula, and their crystal structures could be different from one another.90 CeO2 is one example of nonlayer structured metal oxides. CeO2 has its fluorite structure with the space group of Fm3m.287 The fluorite structure is formed by a fcc unit cell of cations and the anions occupying the space of the octahedral. The crystal structure of CeO2 is shown in Figure 6f.287 In the CeO2 structure, each Ce4+ ion is coordinated to 8 equivalent nearest-neighbors of O2−, while each O2− ion is coordinated to four nearest neighboring Ce4+.287

4.1. Micromechanical Cleavage

The micromechanical cleavage technique is a conventional method to fabricate thin flakes by exfoliation of layered bulk crystals, using Scotch tape.2,50−52 The original idea of this technique is to apply mechanical force via Scotch tape to weaken the van der Waals interaction between the layers of bulk crystals without breaking the in-plane covalent bonds of each layer, hence peeling off single- or few-layers of 2D crystals. In a typical process, the bulk crystal (e.g., graphite) is first attached to the adhesive on the Scotch tape and then peeled 6233

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Figure 7. Schematic illustration of the typical process by the modified micromechanical cleavage method for exfoliation of graphene from graphite via Scotch tape. Reproduced with permission from ref 312. Copyright 2015 American Chemical Society.

into a thin flake by using another adhesive surface.2,50,51 This process can be repeated several times to obtain an appropriately thin flake. The freshly cleaved thin flake on the Scotch tape is then attached to a clean, flat target surface (e.g., SiO2/Si), and rubbed using tools such as plastic tweezers to further cleave it. Finally, single- or few-layers of nanosheets left over on the substrate can be obtained by peeling off the Scotch tape. Mechanically exfoliated ultrathin 2D crystals can be observed and identified by an optical microscope when the suitable substrate are used, as discussed in detail in section 5.1. In 2004, Novoselov, Geim, and co-workers first successfully cleaved a single-layer graphene nanosheet from graphite by using the micromechanical cleavage technique.2 Later, the same group demonstrated the extension of this technique for the exfoliation of other ultrathin 2D nanomaterials, including h-BN, MoS2, NbSe2, and Bi2Sr2CaCu2Ox, from their parent layered bulk crystals.50 Since then, this method has been widely used to cleave various kinds of ultrathin 2D nanomaterials with varying layer numbers from their layered bulk crystals by many other groups. The exfoliated nanosheets range from TMDs (e.g., TiS2, TaS2, TaSe2, MoSe2, WS2, WSe2, MoTe2, ReS2, MoxW1−xS2, ReS2xSe2(1−x), etc.),52,291−300 topological insulator (e.g., Bi 2Te 3, Bi 2Se3 , and Sb 2Te 3),301−303 CuInP 2S 6,304 BP,132,305−307 antimonene,44 and metal phosphorus trichalcogenides (e.g., MPS3: M = Fe, Mn, Ni, Cd, and Zn)308 to hBN.50,309−311 Theoretically speaking, this method is capable of producing all kinds of ultrathin 2D nanomaterials whose bulk crystals are layered compounds. It is believed that more new types of ultrathin 2D crystals will be produced in this manner. This approach can be categorized as a nondestructive technique because neither chemicals nor chemical reactions were necessary during the fabrication process. Therefore, the exfoliated single- or few-layer nanosheets kept the “perfect” crystal quality, normally defined as pristine, from their layered bulk crystals. The sizes of the produced 2D crystals can be up to a few to tens of micrometers, and these surfaces are very clean because no chemicals were introduced during the exfoliation process. The relatively large lateral size (up to tens of micrometers), clean surface, and excellent crystal quality with minimum defects make the mechanically cleaved ultrathin 2D nanomaterials compelling candidates for the fundamental study of the intrinsic physical (e.g., mechanical property), optical

(e.g., photoluminescence), and electronic properties (layerdependent bandgaps), as well as the demonstration of highperformance electronic and/or optoelectronic devices (e.g., transistors and phototransistors). Although the micromechanical cleavage technique has many advantages, such as wide applicability, high crystal quality, clean surface, and large lateral size, there are still several disadvantages that restrict its practical application in its current form. First, the production yield of this technique is quite low, and thick flakes always coexist on the substrate along with the single- or few-layer nanosheets. Second, the production rate is quite slow, which is not competitive to the CVD growth and those solution-based methods. Both the low yield and the slow production rate make it difficult to realize the demands for various practical applications, high yield and large-scale production. Third, the size, the thickness, and the shape of the produced ultrathin 2D nanomaterials are difficult to control because the exfoliation process is operated manually by hands, which lack the precision, controllability, or repeatability. Sutter and co-workers recently demonstrated an effective way to enhance the exfoliation yield and increase the area of obtained graphene and Bi2Sr2CaCu2Ox nanosheets by slightly modifying the micromechanical cleavage technique (Figure 7).312 Briefly, the substrate was first treated by the oxygen plasma to remove ambient adsorbates. An additional heat treatment was then introduced during the exfoliation process to ensure a more uniform interface contact between the substrate and bulk crystals. Such a modified technique improved the production yield, and increased the nanosheet area (Figure 7),312 which might be further used to produce other 2D nanomaterials with large area and improved production yield. Javay and co-workers also reported that the area of exfoliated TMD nanosheets can be significantly enlarged by evaporation of Au films onto various substrates, such as SiO2/Si and quartz, before the exfoliation.313 Because the interfacial Au atoms have strong affinity with the top chalcogen layer of bulk TMD crystals, the contact interaction between the substrate and bulk crystals is enhanced, thus enlarging the area of exfoliated nanosheets.313 Last, a substrate is a prerequisite to support the produced 2D crystals in the exfoliation process, which eliminates the possibility of producing freestanding or solution-dispersed nanosheets. 6234

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Figure 8. Schematic illustration of sonication-assisted liquid exfoliation of graphite into graphene. Reproduced with permission from ref 314. Copyright 2014 The Royal Society of Chemistry.

graphene at low-cost in liquid phase. In their first report, the concentration of graphene suspension was 0.01 mg mL−1, which is relatively low for further applications. Later, a nonpolar solvent, that is, ortho-dichlorobenzene (ODCB), was also proven to be effective for production of homogeneous graphene suspension.316 Although having high effectivity, the aforementioned solvents have a high boiling point and are relatively toxic. Bearing this in mind, efforts have been devoted to the exploration of volatile solvent for the sonication-assisted liquid exfoliation. As a typical example, Coleman and coworkers demonstrated the sonication of graphite in some low boiling point solvents, including isopropanol and chloroform, to obtain a relatively high concentration of graphene nanosheet suspensions.317 Some other low boiling point solvents, such as propanol and acetonitrile, were also used for sonication-assisted liquid exfoliation of graphite.318,319 Besides graphene, in 2011, Coleman and co-workers further extended this method for the exfoliation of layered bulk crystals into 2D nanosheets, including MoS2, WS2, MoSe2, NbSe2, TaSe 2 , NiTe 2, MoTe 2, h-BN, and Bi 2Te 3 .55 Both the experimental and the theoretical results suggested that the good matching of surface tension between the layered crystals, not only the solvents choice, is also a key factor for the efficient exfoliation to minimize the energy of exfoliation.55 The solvent is also important in stabilizing the exfoliated nanosheets and prohibiting their restacking and aggregating. In another example, Zhang and co-workers demonstrated that the mixture of water and ethanol is effective for exfoliating and dispersing TMD nanosheets.320 It is noteworthy to remark that pure water or ethanol is inefficient to exfoliate TMDs due to the large surface energy. Although pure water was always believed to be insufficient for the exfoliation of layered bulk crystals at room temperature, a recent study has proven that pure water can be a promising solvent in the sonication-assisted exfoliation method by simply heating the water at elevated temperature (e.g., 60 °C).321 At

4.2. Mechanical Force-Assisted Liquid Exfoliation

The micromechanical cleavage technique proved that applying mechanical force on layered bulk crystals is an effective way to exfoliate them into single- or few-layer 2D nanomaterials. Taking inspiration from this, layered bulk crystals could also be exfoliated into ultrathin 2D nanosheets in liquid phase if proper mechanical forces are applied on the layered bulk crystals dispersed in liquid media. Bearing this in mind, a number of mechanical force-assisted liquid exfoliation methods were developed for the high-yield and large-scale exfoliation of layered bulk crystals in liquid. On the basis of the mechanical forces, exfoliation method in liquid can be divided into two main categories: sonication-assisted liquid exfoliation and shear force-assisted liquid exfoliation. 4.2.1. Sonication-Assisted Liquid Exfoliation. Sonication is the simplest and most common mechanical force that has been used for the exfoliation of layered bulk crystals into ultrathin 2D nanosheets in liquid media. In a typical process, layered bulk crystals were dispersed in a specific solvent (e.g., N-methyl-pyrrolidone (NMP)) before being treated with sonication at a certain time (Figure 8).53,59,314,315 After sonication, the suspension was purified via centrifugation to get a nanosheet suspension. The basic idea is that sonication can induce liquid cavitation, which in turn induces bubbles in the solution. Microjets and shock waves passed through the layered bulk crystals dispersed in solution when these bubbles collapse. In this case, an intensive tensile stress will be generated on the layered bulk crystals, thus leading to the exfoliation of layered bulk crystals into thin layers of sheets. The key factor for achieving efficient exfoliation of layered bulk crystals is matching the surface energy between the layered bulk crystal and the solvent system. First developed in 2008 by Coleman’s group for the exfoliation of graphite into graphene,54 this method is quite simple and effective, which does not need any complicated equipment and expensive chemicals. It paves a new way for the high-yield and large-scale production of 6235

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Figure 9. (a) A Silverson model L5M high-shear mixer with mixing head in a 5 L beaker of graphene dispersion. (b) Close-up view of a DD32 mm mixing head and (c) a DD16 mm mixing head with the rotor (left) separated from its stator. (d) Photograph of graphene-NMP dispersions. Reproduced with permission from ref 377. Copyright 2014 Nature Publishing Group.

(2-vinylpyridine)] (PS-b-P2VP), and poly(isoprene-b-acrylic acid) (PI-b-PAA); and (3) biomolecules such as bovine serum albumin (BSA).326−335 Nowadays, the sonication-assisted liquid exfoliation method has been widely utilized for exfoliation of numerous layered compounds into ultrathin 2D nanosheets, such as graphene,54,315 h-BN,12,56,336,337 many TMDs (e.g., MoS2, WS2, MoSe2, NbSe2, TaSe2, NiTe2, and MoTe2),55,338−345 TMOs (e.g., MoO3, WO3, MnO2, etc.),57,346−350 topological insulators (e.g., Bi 2Te3 , Bi2 Se3 , and Sb2 Te 3),351−353 BP,58,354−359 MOFs,30,360−364 COFs,365−368 coordination polymers,369−372 antimonene,373,374 etc.375,376 The concentration, lateral size, in addition to thickness of the obtained nanosheets can be roughly tuned by controlling the sonication time, solvent system, polymers additive, ultrasonic power, sonication temperature, and the shape of vessels.49 We highlight this method for the high-yield and massive production of ultrathin 2D nanomaterials in solution at low cost due to its simple process. Currently, the sonication-assisted liquid exfoliation method might be the most widely used approach for the preparation of solutiondispersed ultrathin 2D nanomaterials. Having mentioned the above, there are several disadvantages for the sonicationassisted liquid exfoliation method. First, the yield of the singlelayer nanosheets in the exfoliation suspension is low. As known, some of the extraordinary properties of 2D nanomaterials only can be observed in its single-layer form. Second, the lateral size of the produced nanosheets is relatively small because the sonication force breaks down the big nanosheets into small fragments. Third, for the sonication in aqueous polymer/ surfactant solution, the residual polymer/surfactant absorbed on the exfoliated nanosheets is undesirable for some further applications, such as electronics, optoelectronics, electrocatalysis, and energy storage. Last, the sonication process may induce some defects on the exfoliated nanosheets, which will affect the properties of the exfoliated nanosheets. 4.2.2. Shear Force-Assisted Liquid Exfoliation. The sonication-assisted liquid exfoliation method has been well established for a wide spectrum of ultrathin 2D nanomaterials, and the production rate is much higher as compared to the micromechanical cleavage method. Although the sonicationassisted liquid exfoliation can achieve the production of ultrathin 2D nanomaterials with concentrations up to ∼1 mg mL−1 at optimized conditions, its production rate still cannot meet the requirement for industrial applications. To further promote the production rate, shear force-assisted liquid exfoliation was developed for scaling up the production of ultrathin 2D nanomaterials. On the basis of their rich

this point, water becomes both effective for the exfoliation of layered bulk crystals of graphite, h-BN, MoS2, WS2, and MoSe2 into nanosheets in the sonication process and the stabilization of the exfoliated nanosheets after the exfoliation because of the presence of platelet surface charges induced by edge functionalization or intrinsic polarity. It was suggested that the edge functionalization or intrinsic polarity-induced platelet surface charging can be attributed to the good solubility of exfoliated nanosheets in water. The successful exfoliation of layered bulk crystals in pure water makes this method promising for the preparation of ultrathin 2D nanomaterials for practical applications in the near future. Note that for the sonication-assisted liquid exfoliation in solvents, effective exfoliation can only be achieved in solvents with matching surface energy to the layered bulk crystals. The surface energy varies for different bulk crystals, making it difficult to find a suitable solvent system for each layered bulk crystal. Alternatively, sonication of layered bulk crystals in aqueous solution with addition of polymers or surfactants is another promising way for exfoliation of them into ultrathin 2D nanosheets (Figure 8).314 The surface tension of the aqueous solution can be easily tuned by addition of polymers or surfactants in it, thereby matching it to the surface energy of layered bulk crystals and achieving efficient exfoliation of layered materials.53 As a typical example, Coleman and coworkers first reported the exfoliation of graphene from graphite with a concentration of 0.1 mg mL−1 by direct sonication of graphite powder in the sodium dodecylbenzenesulfonate (SDBS) aqueous solution.322 Later, they also reported the sonication of graphite in sodium cholate (SC) aqueous solution to obtain a graphene nanosheet suspension with an increased yield up to 0.3 mg mL−1 by increasing the sonication time to 400 h.323 Some other ionic surfactants, such as sodium deoxycholate (SDC), cetyltrimethylammonium bromide (CTAB), and acetic acid, have also been used to assist the exfoliation of graphite by sonication.324,325 Besides ionic surfactants, several agents for exfoliating 2D nanosheets in aqueous solution have been employed: (1) nonionic surfactants such as P-123-polyoxyethylenesorbitanmonooleate (Tween 80), polyoxyethylenesorbitantrioleate (Tween 85), polyoxyethylene(4)dodecyl ether (Brij 30), polyoxyethylene(100)octadecylether (Brij 700), polyoxyethyleneoctyl (9−10) phenylether (Triton X-100), gum Arabic from acacia tree, Pluronic P-123, and n-dodecyl β-D-maltoside (DBDM); (2) polymers such as polystyrene (PS), polyvinyl chloride (PVC), poly(methyl methacrylate) (PMMA), polyvinylpyrrolidone (PVP), ethyl cellulose (EC), poly[styrene-b6236

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Figure 10. Schematic illustration of the two-step chemical ion intercalation-assisted liquid exfoliation method for the preparation of single-layer TMDs nanosheets by using the N2H4 and metal naphthalenide (metal = Li, Na, or K) as the intercalators. Reproduced with permission from ref 66. Copyright 2014 Nature Publishing Group.

rotating blade stirred tank reactors are promising for the largescale graphene production. Overall, shear exfoliation is a very promising technique, which has been commercialized as a graphene production technique.

experience in sonication-assisted liquid exfoliation, Coleman and co-workers demonstrated high yield and massive production of graphene flakes by using a high-shear rotorstator mixer (Figure 9).377 It was believed that under high speed rotation, the mixer can generate high shear rates in liquid containing layered bulk crystals.51,377 The exfoliation of layered bulk crystals can be trigged by the shear forces induced by the high shear rates. Similar to the sonication-assisted liquid exfoliation, the use of proper solvent and polymer can reduce the energy cost of the exfoliation process and stabilize the exfoliated nanosheets, thus making this exfoliation process more efficient. The shear-force apparatus has a mixing head constituting a rotor and a stator, a simple and widely available setup. By using this device, graphite can be exfoliated into graphene with lateral size of 300−800 nm in NMP solvent using high speed rotation of the mixing head.377 This method was further extended to exfoliate BP bulk crystal into few-layer nanosheets.354,378,379 Of note, the diameter of the rotator can be tuned to control the production amount of graphene sheets. The influences of the rotor diameter and the mixer-induced shear force characteristics were systematically explored to further investigate the shear force-assisted exfoliation mechanism.380 Unsurprisingly, shear rate acts as the key factor in exfoliating the layered materials. When the shear rate is lower than 104 S−1, the exfoliation efficiency of graphene is very poor, but wellexfoliation can be obtained if the shear rate is higher than 104 S−1. Therefore, the exfoliation of graphene might be realized under a shear rate above 104 S−1 generated by any apparatuses. Currently, the exfoliation of graphite can be achieved in liquid volumes up to hundreds of liters with a production rate of 1.44 g h−1, which is much faster than any other reported prepared methods. To generate high shear rate in all regions, two different groups independently reported the exfoliation of graphite into graphene in aqueous surfactant solution by using a kitchen blender.381,382 A kitchen blender is a simple and commercially available rotating-blade mixer, which can generate turbulence in the whole container. By optimizing the mixing parameters including the nanosheet concentration, liquid volume, rotor speed, and mixing time, the production rate could reach the order of mg min−1 scale. The obtained nanosheet dimensions are in the range of 2−12 layers for thickness and 40−200 nm for length. Recently, Coleman and co-workers also produced h-BN, MoS2, and WS2 nanosheets by using the kitchen blender.383 These results imply that industrial

4.3. Ion Intercalation-Assisted Liquid Exfoliation

The ion intercalation-assisted liquid exfoliation is another representative top-down approach for production of ultrathin 2D nanomaterials. The basic idea of this method is to intercalate cation ions with small ionic radius (e.g., Li+, Na+, K+, or Cu2+ ions) into the interspacing of layered bulk crystals to form ion-intercalated compounds, in which the ion intercalation can significantly expand the interspacing and weaken the van der Waals interaction between adjacent layers in layered bulk crystals. The ion-intercalated compounds could be easily exfoliated into single- or few-layer nanosheets under mild sonication treatment in a specific solvent (e.g., water) for short time (e.g., 10 min). In most cases, intercalated ions (e.g., Li+ or Na+) can react with the solvent (e.g., water) to generate hydrogen gas, which can also help to separate the adjacent layers during the sonication process and thus further promote the exfoliation efficiency.60,61 High yield of single- or few-layer nanosheets can be obtained after further purification to remove thick flakes via centrifugation. The typical example of this method is the chemical Li intercalation-assisted exfoliation method by using oganometallic compounds (e.g., n-butyllithium) as intercalators.60,61 In a typical process, layered bulk crystal was first refluxed in hexane solution containing nbutyllithium for several days (e.g., 3 days), in which the nbutyllithium reacted with layered bulk crystals to form the Liintercalated compound. The Li-intercalated compound was transferred into common solvent (e.g., water) and then treated with subsequent sonication to obtain the nanosheet suspension. It is worth pointing out that the Li ions can react with water to generate hydrogen gas during the process, which can further promote the success of exfoliation process. 60,61 After purification via centrifugation, single- or few-layer nanosheets can be obtained in high yield. Of particular interest is the early attempt to exfoliate layered TMDs by the chemical Li intercalation-assisted liquid exfoliation method, which dates back to the 1980s when Morrison and co-workers used the n-butyllithium as the intercalator to exfoliate the bulk MoS2 crystal into single-layer MoS2 nanosheet.61 Besides the n-butyllithium, some other compounds (e.g., LiBH4) have also been proven to be effective 6237

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As known, Li+ ions were intercalated into the cathode during the discharge process in the LIB system where the Li foil and layered materials are used as the anode and cathode, respectively. Taking the LIB as inspiration, Zhang and coworkers developed an electrochemical Li intercalation-assisted liquid exfoliation method for the exfoliation of layered bulk crystals into ultrathin 2D nanosheets (Figure 11).63 The basic

intercalators for the exfoliation of ultrathin 2D nanomaterials.228 Although a high yield of single-layer 2D nanomaterials can be obtained, the lateral size of the produced nanosheets is relatively small, that is, less than 1 μm. Bearing this in mind, Loh and co-workers recently combined the hydrothermal method and the chemical ion intercalation-assisted liquid exfoliation method to produce large size nanosheets (Figure 10).66 In their experiment, the layered bulk crystals were first reacted with hydrazine (N2H4) by a hydrothermal method, in which the N2H4 intercalation can significantly expand the volume of layered bulk crystals after decomposition of the N2H4 molecule. The expanded layered bulk crystals were then intercalated by metal naphthalenide (metal = Li, Na, or K) to form intercalated compounds. High yields of single-layer nanosheets were obtained after sonication and purification. For MoS2, the yield of the single layer was up to 90%, and the lateral size was up to 400 μm2.66 As mentioned above, another major disadvantage of the ion intercalation-assisted exfoliation method is that the Liintercalation process needs long reaction times (e.g., 3 days) and high temperature (e.g., 100 °C) for some compounds.60,61 Bearing this in mind, Wang and co-workers recently modified the Li intercalation process to address the aforementioned two main disadvantages.384 They discovered that under sonication, bulk TMDs can react with the n-butyllithium within 1 h at room temperature, making the exfoliation of TMDs much more efficient. To date, the chemical ion intercalation-assisted liquid exfoliation method has been widely used for the preparation of various ultrathin 2D nanomaterials, such as MoS2, MoSe2, TiS2, WS2, ReS2, and Cu2WS4.60,61,66,228,384−387 The lateral size, amount of defects, layer number, and concentration of produced nanosheets can be roughly tuned by tuning the experimental conditions, such as the particle size of bulk crystals, reaction time, temperature, initial concentration of bulk crystals, intercalating agents, and sonication time. One of the unique advantages of this method is that the ionintercalation into layered bulk crystals of some of the TMDs (e.g., MoS2 and WS2) can induce the phase transformation from semiconducting hexagonal (2H) and metallic octahedral (1T) phase, offering a powerful way for the phase engineering of 2D TMDs.211 Another obvious advantage of this method is that the produced nanosheets are positively charged with a clean surface, making them promising for some further applications, such as electrocatalysis and energy storage.211 Of note, currently, this method has only been proven to be effective for the exfoliation of layered metal sulfide bulk crystals and graphite. On the basis of our experience, although metal ions can also be intercalated into layered metal selenide or telluride bulk crystals, the structure of metal selenide or telluride bulk crystals will decompose during the sonication process because the intercalated compounds are very reactive and increase the instability of metal selenide or telluride bulk crystals. Therefore, it is hard to extend this method to exfoliate layered metal selenide or telluride bulk crystals. This method cannot be used for the exfoliation of other layered bulk crystals, such as metal oxides, metal hydroxides, MOFs, and COFs. It is worth pointing out that organometal compounds used in this method are highly explosive and very sensitive to moisture and oxygen, so the experiment needs to be operated in a glovebox with extreme caution. The ion intercalation process is hard to control in this method, making it difficult to avoid the insufficient ion-intercalation or over ion-intercalation.

Figure 11. Schematic illustration of the electrochemical Li intercalation-assisted liquid exfoliation method for preparation of single- or few-layer TMD nanosheets. Reproduced with permission from ref 63. Copyright 2011 John Wiley & Sons, Inc.

idea is the same as the chemical Li intercalation-assisted liquid exfoliation method. The major difference is that the Liintercalation was driven by electrochemical force and Li foil rather than the organometallic compounds used as the Li source. In a typical process, powders of layered bulk crystals coated on metal foils (e.g., Cu) were used as cathodes to assemble in Li ion battery cells, in which Li foils were used as anodes (Figure 11).63 The discharge process was applied to intercalate Li ions into layered bulk crystals to form Liintercalated compounds (Figure 11). After that, the Liintercalated compound-coated electrodes were then taken out, washed, and sonicated in water or ethanol to obtain nanosheet suspensions (Figure 11). A high yield of single- or few-layer nanosheets can be obtained after purification via centrifugation. The electrochemical Li intercalation-assisted exfoliation method has been successfully used to prepare many ultrathin 2D nanosheets from their layered bulk crystals such as graphene, h-BN, many TMDs (e.g., MoS2, WS2, TiS2, TaS2, ZrS2, NbSe2, WSe2, MoS2xSe2(1−x), and MoxW1−xS2), some other metal chalcogenides (e.g., Sb2Se3, Bi2Te3, Ta2NiS5, and Ta2NiSe5), and 2D heteronanostructures.63−65,388−390 It is noteworthy that the amount of Li ions and the intercalation speed in this method can be finely tuned by controlling the cutoff voltage and discharge current in the discharge process to achieve more efficient exfoliation for different types of layered bulk crystals. The production yield of this method is very high. Among them, the yields of single-layer MoS2, TaS2, and Ta2NiS5 nanosheets are over 90%.63,65,389 Loh and co-workers used a similar method to intercalate Li ions into graphite by using the graphite as the electrode in an electrolyte of Li salts and propylene carbonate.391 After sonication of the intercalated graphite in solution, few-layer graphene with yield up to 70% was obtained. The electrochemical Li intercalation-assisted liquid exfoliation method is much more complicated as compared to the chemical ion intercalation-assisted liquid exfoliation method because it needs the assembly of battery 6238

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Figure 12. Schematic illustration of the exfoliation of a typical-layered perovskite (KCa2Nb3O10) into Ca2Nb3O10 nanosheet by the cation exchangeassisted liquid exfoliation method. Reproduced with permission from ref 394. Copyright 2015 Nature Publishing Group.

weaken the interaction between adjacent layers in layered bulk crystals. The expanded compounds can be easily exfoliated into single- or few-layer nanosheets with the assistance of mechanical forces, such as mechanical shaking and sonication. On the basis of the type of exchanged ions, this method can be divided into two typical methods: cation exchange-assisted liquid exfoliation and anion exchange-assisted liquid exfoliation. 4.4.1. Cation Exchange-Assisted Liquid Exfoliation. Cation exchange-assisted liquid exfoliation was developed for the preparation of ultrathin 2D nanosheets from layered metal oxides and metal phosphorus trichalcogenides.23,394−398 Layered metal oxides, such as Cs0.7Ti1.825O4 (or K0.8Ti1.73Li0.27O4), K0.45MnO2, and KCa2Nb3O10, have been proven to be easily exfoliated into nanosheets by the cation exchange-assisted exfoliation method (Figure 12).394−398 This class of layered metal oxides consists of corner- or edge-shared MO6 octahedra (M = Ti, Mn, Nb, etc.) with a plate-like structure and interlayer alkali metal cations (e.g., K+, Rb+, Cs+, etc.).395−398 It was found that by simply immersing them in an acid aqueous solution, the interlayer alkali metal cations can be exchanged by H+ cation to form hydrated protonic compounds, such as H0.7Ti1.825O4·H2O (or H1.07Ti1.73O4·H2O), H0.13MnO2·0.7H2O, and HCa2Nb3O10· 1.5H2O. After that, organoammonium ions can further replace the interlayer protons via a cation exchange process by dispersing them in an aqueous base solution, tetrabutylammonium hydroxide (TBA+OH−; (C4H9)4N+OH−), because of their Brønsted solid acidity. After this cation exchange process, the interlayer spacing of metal oxide layered bulk crystals was significantly expanded due to the relatively large radius of organoammoniumions as compared to that of the protons. The expanded compounds can be easily exfoliated into welldispersed metal oxide nanosheets with positive charge on their surface, which are Ti0.91O20.36− (or Ti0.87O20.52−), MnO20.4−, or Ca2Nb3O10−, via a subsequent mechanical shaking or sonication treatment. It was found that layered metal phosphorus trichalcogenides, such as MnPS3 and CdPS3, can also be exfoliated into nanosheets by the cation exchange-assisted liquid exfoliation method.399,400 In a typical process, the powder of MPS3 (M = Mn and Cd) layered bulk crystals was stirred in KCl aqueous solution a given time (e.g., 1 h), in which the M cations can be partially replaced by the potassium ion to form intermediate compounds, that is, M0.8K0.4PS3(H2O)y.400 After the first cation exchange, the interlayer spacing of MPS3 layered bulk crystals

cells. Another disadvantage of this method is that additional additives, such as polyvinylidenefluoride (PVDF) and activated carbon, were mixed with layered bulk crystals during the electrode fabrication process to enhance the conductivity and quality of electrodes. Therefore, the residual additives may absorb on the exfoliated nanosheets, leaving them undesirable for some specific applications. It is worth pointing out that the aforementioned intercalators used for ion intercalation-assisted liquid exfoliation methods, such as n-butyllithium, LiBH4, and Li foil, are very sensitive to oxygen and moisture, making the exfoliation process complex and dangerous. Recently, Zheng and co-workers demonstrated that some common inorganic salts, such as NaCl and CuCl2, can also be used as intercalators for the exfoliation of layered bulk crystals into ultrathin 2D nanomaterials.392,393 In graphene preparation, graphite powder was first dispersed in water containing saturated inorganic salts, that is, NaCl or CuCl2, and the mixture was then heated at 100 °C to evaporate the water, in which cation ions (e.g., Na+ or Cu2+) can intercalate into interlayer spacing of graphite.392 The intercalated graphite was then sonicated in organic solvent (DMF, ethanol, NMP, or toluene) for a short time to obtained single- or few-layer graphene nanosheets. This method can achieve yield up to 65% (1−5 layers), and the lateral size of graphene sheets can be up to tens of micrometer. Later, the same group extended this method for the preparation of single- or few-layer MoS2, MoSe2, WS2, and WSe2 nanosheets from their layered bulk crystals.393 As compared to the intercalating agents, including nbutyllithium, LiBH4, and Li foil, these inorganic salts here are much safer and cheaper, making it more suitable for usage in practical applications. This method needs to use a large amount of inorganic salts, and the exfoliated nanosheets were dispersed in high boiling point solvents, that is, NMP or DMF. 4.4. Ion Exchange-Assisted Liquid Exfoliation

The ion exchange-assisted liquid exfoliation is another effective method used for the exfoliation of some layered bulk crystals, such as metal oxides, LDHs, and metal phosphorus trichalcogenides. The original idea for this method is to use different ions with a relatively large radius to replace the existing ions with a relatively small radius that are in the presence of the layered bulk crystals via an ion exchange process. After the ion exchange process, the interlayer spacing of layered bulk crystals can be largely expanded, which can 6239

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Figure 13. Schematic illustration of the anion exchange-assisted liquid exfoliation method for exfoliation of bulk LDHs into nanosheets. Reproduced with permission from ref 141. Copyright 2014 Nature Publishing Group.

anion exchange with dodecyl sulfate anions.406−408 Sasaki and co-workers recently demonstrated the exfoliation of LDHs in water to get well-dispersed nanosheets after the anion exchange with aliphatic carboxylates or short-chain organic sulfonates (RSO3−), further extending the exfoliation of other LDH layered bulk crystals in aqueous solution.409 Overall, the ion exchange-assisted liquid exfoliation method is quite effective for exfoliation of a number of layered bulk crystals into ultrathin 2D nanosheets in solution, including metal oxides, LDHs, and metal phosphorus trichalcogenides. This method is potentially able to achieve high yield and largescale production. It is worth pointing out that the chemical formula of exfoliated nanosheets is normally a little bit different from that of their layered bulk crystals due to the occurrence of chemical reactions during the exfoliation process. This method has only been proven to be effective for the exfoliation of a specific type of layered compounds, making it hard to be further extended for exfoliation of other layered materials, such as graphite, h-BN, TMDs, TMOs, MOFs, and COFs. For the anion exchange-assisted exfoliation method, most of the LDH nanosheets were exfoliated and dispersed in organic solvents with formamide being the most effective solvent.23,24 These organic solvents are toxic, and formamide has a high boiling point, which makes it hard to remove from the exfoliated nanosheets or to process further in other applications.

was expanded from 6.49 to 9.43 Å. The potassium ions were further exchanged by lithium ions by stirring the M0.8K0.4PS3(H2O)y in a 3 M LiCl aqueous solution for a given time (e.g., 1 h). After that, M0.8Li0.4PS3(H2O)x compound was obtained with its interlayer spacing of about 12.0 Å. By simply dispersing the M0.8Li0.4PS3(H2O)x compound in water or a PVP aqueous solution, single-layer M0.8PS3 nanosheets (e.g., Mn0.8PS3 and Cd0.8PS3) can be obtained in high yield and large scale.400 The obtained nanosheet suspensions were stable and do not settle for several months. 4.4.2. Anion Exchange-Assisted Liquid Exfoliation. Besides cation exchange-assisted liquid exfoliation, anion exchange-assisted liquid exfoliation has also been established for the exfoliation of layered bulk crystals of LDHs.23,24 LDHs have a general formula of M2+1−xM3+x(OH)2An−x/n·yH2O (M2+ = Mg2+, Fe2+, Co2+, Ni2+, Zn2+, etc.; M3+ = Al3+, Fe3+, Co3+, etc.; and A = CO32−, Cl−, NO3−, ClO4−, etc.).24 LDH layered bulk crystals are composed of octahedral hydroxide layers of divalent and trivalent (M2+ and M3+) metal cations, accommodating charge-balancing anions (An−) in the interlayer gallery.24 Generally, LDH layered bulk crystals can be synthesized by simply mixing the constituent divalent and trivalent metal salts in an alkali solution.23 Alternatively, plate-like LDH layered bulk crystals with a regular hexagonal shape could be prepared by a homogeneous precipitation method from a mixed solution of MCl2 and AlCl3, in which the urea (CO(NH2)2) or hexamethylenetetramine (C6H12N4) was used as the hydrolysis agent.68,244 All transition-metal LDHs with high crystallinity can be synthesized by the soft-chemical oxidation of homogeneously precipitated brucite-type M 2+ (OH) 2 (e.g.,Co1−xFex(OH)2, Co1−xNix(OH)2) plates.69 These asprepared LDH layered bulk crystals were then ready to be exfoliated into nanosheets via the anion exchange-assisted liquid exfoliation method (Figure 13).68,69,141,142,244,401 Note that the anion interlayers in LDH layered bulk crystals can be exchanged by other anions to enlarge the interlayer spacing. Early attempts used organophilic anions (e.g., amino acids and dodecylsulfate) to exchange the original anions (e.g., Cl− and Br−) in LDH layered bulk crystals.402 The exchanged LDHs were then exfoliated in nanosheets by heating or sonication treatment in organic solvents, such as 1-butanol, CCl4, 1hexanol, 1-octanol, and 1-decanol, to obtain colloidal suspensions.402 Later, it was found that several inorganic anions, such as NO3− and ClO4−, are also effective to replace anions in LDH layered bulk crystals, leading to an efficient exfoliation into single-layer nanosheets in formamide (HCONH2).68−70,403−405 A recent study revealed that formamide is also a promising solvent for the exfoliation of LDHs based on trivalent rare-earth cations into nanosheets after the

4.5. Oxidation-Assisted Liquid Exfoliation

For the exfoliation of graphite, there is another widely used method, which is the oxidation-assisted liquid exfoliation method, also known as the modified Hummers’ method.72,73 The basic idea underlying this method is to use strong oxidizing agents to oxidize graphite to form graphite oxide. The oxidation of graphite can generate abundant oxygen-containing functional groups on the surface of each graphene layer, which can remarkably expand the interlayer spacing of bulk graphite crystals and thus significantly weaken the van der Waals interaction between adjacent layers.72,73 With subsequent sonication treatment, the expanded graphite oxide can be exfoliated into single-layer graphene oxide (GO) nanosheets.72−74 The most widely used oxidizing agent is a mixture of potassium permanganate and concentrated sulfuric acid.74 This method allows for the preparation of single-layer GO nanosheets in high yield and large amount in the solution phase. One of the major disadvantages is the usage of strong oxidizing agents in this process, making this method not very safe as compared to other methods, for example, the sonicationassisted liquid exfoliation method. Although it is very effective for the preparation of GO nanosheets, it is hard to further extend this method to other layered materials. 6240

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exfoliation method for preparing MXenes from bulk Mn+1AXn phases.25,77 The basic idea is to use a strong etching agent (e.g., HF) to etch away the A layers without breaking the bonds in Mn+1Xn layers using selective experimental conditions (Figure 14).25,78 As a result, after the etching of A layers, the remaining

Of particular interest is that abundant oxygen-containing functional groups, such as carboxyl, epoxy, and hydroxyl groups, will be generated on the GO surface due to the oxidation.410 These oxygen-containing groups on GO nanosheets can be partially removed after reduction to produce reduced GO (rGO) nanosheets. The reduction of GO nanosheets can be achieved by many strategies, such as chemical reduction via reducing agents, thermal annealing, photochemical reduction, electrochemical reduction, and so on.411−417 Residual functional groups on the surface of rGO nanosheets may still exist even after the reduction.310 Both GO and rGO nanosheets have different physical, chemical, and electronic properties as compared to the pristine graphene or CVD-grown graphene. One of the obvious differences is that graphene has an excellent electrical conductivity,418−420 while GO nanosheet is electrically insulating.418−420 Even after reduction back to its rGO form, the electrical conductivity of rGO sheets is still not as good as those obtained from mechanical exfoliation or CVD-grown. Moreover, GO nanosheet is amphiphilic and can be well dispersed in water,421,422 while graphene is highly hydrophobic.423 Because of their distinctive properties from graphene, GO and rGO were defined as graphene derivatives rather than graphene, although graphene has been widely used to name rGO nanosheets in many published papers. In this case, GO and rGO nanosheets can be regarded as a unique class of carbonaceous ultrathin 2D nanomaterials, with interesting properties unattainable from other ultrathin 2D nanomaterials. The abundant functional groups on GO nanosheets allow them to be easily modified with organic or polymeric molecules via covalent bonds.424 The electrical conductivity of GO nanosheets can be easily tuned from insulating to highly conductive by precisely controlling the amount of oxygen-containing functional groups on its surface.410 The rich functional groups on the GO surface can be used as nucleation sites to facilitate the growth of other nanocrystals (noble metals, metal oxides, metal chalcogenides, or MOFs) on their surface, making them ideal dispersible templates for the construction of functional composites in the solution phase.425−437

Figure 14. Schematic illustration of the synthesis process of MXenes from MAX phases. Reproduced with permission from ref 78. Copyright 2012 American Chemical Society.

etched crystals consist of loosely packed Mn+1Xn layers, which can be easily exfoliated into single- or few-layer nanosheets via a subsequent sonication treatment (Figure 14).25,78 The obtained nanosheets have a general formula of Mn+1Xn, named as MXenes by Gogotsi and co-workers.25 It is worth pointing out that the chemical composition of the obtained nanosheets by this method is different from that of the originally used bulk crystals, which is also different from other exfoliation methods. In 2011, Gogotsi and co-workers first reported this method for the production of the Ti3C2 nanosheet from the Ti3AlC2 bulk crystal.77 In a typical process, the powder of Ti3AlC2 bulk crystal was dispersed in a 50% concentrated HF solution at room temperature for a given time (e.g., 2 h) to etch away the Al layers. After being washed several times with water, the etched intermediate product was sonicated in water to obtained a dispersed Ti3C2 nanosheet.77 Notably, the experimental conditions, such as the concentration of HF aqueous solution, the reaction time, and temperature, need to be tuned to find the optimized conditions. MXenes prepared by this method normally possess some functional groups (e.g., OH, F, O, H, etc.) on their surface, which are favorable for stabilizing them in solution and some specific applications. This method is a simple and general method for the high yield and massive production of MXenes from MAX phases in solution. Note that this method needs to use strong corrosive chemical, that is, HF, as etchant, which makes the preparation process relatively dangerous. Gogotsi and co-workers reported that the mixture of fluoride salts (e.g., LiF, NaF, KF, and CsF) and HCl or H2SO4 can be used to replace the strong corrosive HF as the etchant for the MAX phases to prepare MXenes.199 This finding provided a much safer way to produce MXenes by using the selective etching-assisted liquid exfoliation, greatly promoting its potential for practical applications. To date, this method has been successfully applied to prepare many other MXenes, such as Ti2C, Nb2C, Ti4N3, V2C, (Ti0.5,Nb0.5)2C, (V0.5,Cr0.5)3C2, Ti3CN, Ta4C3, Mo2TiC2, Mo2Ti2C3, Cr2TiC2, Mo2CTx, and Al3C3.25,77−80,199,439−447 Because there are more than 60 different compounds in MAX phases, it is believed that more

4.6. Selective Etching-Assisted Liquid Exfoliation

The selective etching-assisted liquid exfoliation method was developed by Gogotsi’s group for the preparation of ultrathin 2D nanosheets of early transition metal carbides and/or carbonitrides, that is, MXenes, by using bulk MAX phases as the starting materials.25,77 MAX phases have a general formula of Mn+1AXn (n = 1, 2, or 3), in which M, A, and X represent early transition metal, element of group IIIA or IVA, and C and/or N, respectively.438 In bulk crystals of Mn+1AXn, the M layers are nearly close-packed with filled X atoms at the octahedral sites, and the layers of A atoms are sandwiched between the Mn+1Xn layers. The in-plane M−X bond is a mixed covalent/metallic/ionic, while the out-of-plane M−A bond is metallic. Considering its crystal structure, Mn+1AXn phases can be also regarded as a kind of layered materials, in which the 2D Mn+1Xn layers are connected by the A layers.25 Different from those traditional van der Waals layered materials, the metallic bonding between Mn+1Xn layers in Mn+1AXn phases is much stronger than the van der Waals interaction in conventional layered compounds such as graphite and TMDs. Therefore, aforementioned exfoliation methods are ineffective for exfoliation of MAX phases. Bearing this in mind, Gogotsi and co-workers developed an elective etching-assisted liquid 6241

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MoS2 nanosheets on insulating substrates by the thermal decomposition of ammonium thiomolybdates that was dipcoated onto the substrates with a subsequent sulfurization using sulfur vapor (Figure 15).475 Alternatively, the CVD growth of

MXenes can be prepared by using this method in the near future. There are several limitations to this method. This method cannot be used to prepare all MXenes from the MAX phases.25 It is hard to further extend this method for the preparation of other ultrathin 2D nanomaterials, such as graphene, TMDs, and metal oxides. 4.7. Chemical Vapor Deposition

CVD is a traditional technique for the preparation of highpurity materials or thin films such as W, Ti, Ta, Zr, and Si on substrates.81 The first utilization of CVD technique in industry dates back to 1897, when de Lodyguine first used it to reduce tungsten hexachloride with hydrogen to coat tungsten on carbon filament for lamps.448 Currently, the CVD method has been used for the massive production of highly purified polycrystalline silicon in industry. The key step for the production of single crystal silicon, CVD is widely used for many modern technologies, such as electronics and solar cell devices.449 Over the past decade, CVD method has also been continuously developed and recognized as a reliable and powerful technique for producing a large number of ultrathin 2D nanomaterials.81 In a typical process, the preselected substrates are put in a furnace chamber, and one or more gas/ vapor precursors are cycled through the chamber, in which the precursors can react and/or decompose on the surface of substrates.81 In this case, ultrathin 2D nanosheets can be obtained on the substrate in proper experimental conditions. In some growth processes, catalysts need to be used in the reaction process, for example, for growing graphene.82 In 2006, Somani and co-workers first demonstrated the growth of thick graphene from camphor pyrolysis on a Ni substrate by the CVD technique.450 Although the graphene resulted from this work is about 30 layers, it proved the possibility for the growth of single- or few-layer graphene by the CVD technique.450 Inspired by this work, many efforts have been devoted to optimizing experimental conditions to achieve the growth of single- or few-layer graphene sheets. Beton and co-workers achieved the growth of single-layer graphene by CVD method on polycrystalline Ni film deposited on the SiO 2 /Si substrate.451 Note that the Ni film here not only acted as the substrate to support the growth of graphene, but also as the catalyst to facilitate the nucleation of precursors to form graphene sheets. Significantly, Ruoff and co-workers demonstrated the growth of large-area single-layer graphene film up to 0.5 mm on copper foil by the CVD method by using methane and hydrogen as gas sources.84 It is worth pointing out that the precursors, substrates, catalysts, temperature, and atmospheres are among the key factors in determining the structure features of the final graphene products in the CVD growth.82 By finetuning those experimental parameters, the controlled growth of graphene with tunable layer number, crystallinity, and lateral size can be achieved on different substrates with different precursors by the CVD technique.81,82 Likewise, CVD can be extended for the growth of many other 2D nanosheets on various substrates, such as h-BN nanosheets,452−456 topological insulators (e.g., In2Se3 and Bi2Se3),457−465 metal carbides,466,467 silicene,42,468 borophenes,469−471 and antimonene.472 Not only limited to the aforementioned nanosheets, singleor few-layer TMDs have also been grown by the CVD technique on various substrates.87,473 Although the growth of TMDs dated back to the 1980s,474 the growth of ultrathin 2D TMD nanosheets was only achieved a few years ago. In 2012, Li and co-workers first demonstrated the growth of few-layer

Figure 15. Schematic illustration of the CVD method for the synthesis of MoS2 thin layers on insulating substrates. Reproduced with permission from ref 475. Copyright 2012 American Chemical Society.

large-area few-layer MoS2 nanosheets was achieved by sulfurization of Mo metal film by sulfur vapor, in which the Mo film was predeposited on the SiO2 substrate using an electronic beam evaporator.476 The size and thickness of the film can roughly be tuned by controlling the size of the substrate and the thickness of Mo metal film. TMOs and transition metal chlorides (e.g., MoO3 and MoCl5) can also be used as the Mo sources to produce MoS2 nanosheets by the CVD technique.477−481 Up to now, many ultrathin 2D nanosheets of TMDs, including MoS2,474−481 WS2,482−485 MoSe 2 , 48 6−4 88 WSe 2 , 48 9−4 92 ZrS 2 , 49 3, 49 4 ReS 2 , 4 95 , 49 6 MoTe2,497−499 etc., have been grown by the CVD technique from different precursors at different temperatures on various substrates under different atmosphere. CVD technique has the highest level of control among all other synthetic methods for ultrathin 2D nanomaterials. Recent studies have proven that not only pure TMD nanosheets, but also alloyed TMD nanosheets (e.g., MoS2xSe2(1−x), MoxW1−xS2, and WS2xSe2(1−x)) have also been prepared by controlling the experimental conditions in the CVD technique.500−511 The bandgap of alloyed TMD nanosheets can be finely tuned by adjusting the ratio of elemental compositions. It has been demonstrated significantly that 2D TMD heteronanostructures (e.g., WSe2−WS2, MoSe2−MoS2, and MoSe2−WSe2) can also be prepared by epitaxial growth of another TMDs on an existing TMD seed.512−521 By tuning the experimental conditions, the epitaxial mode, that is, lateral and vertical growth, can also be controlled in the growth of 2D TMD heteronanostructures to obtain lateral and vertical specifications.512 Because of different band gaps of different TMD nanosheets, ultrathin 2D lateral and vertical TMD heteronanostructures can be regarded as the natural p−n junctions with atomic thickness, which are ideal candidates for the construction of high-performance electronic devices in modern electronics/optoelectronics.522,523 The CVD technique allows researchers to prepare ultrathin 2D nanomaterials with high crystal quality, high purity, and limited defects on certain substrates with scalable size and 6242

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example, Xie and co-workers used the solvothermal method to prepare freestanding 4-atom-thick Co nanosheets with and without surface cobaltoxide by using dimethylformamide as the reductant.526 As another interesting example, Li and co-workers prepared single-layer Rh nanosheets with an edge length of 500−600 nm through a facile solvothermal synthetic approach by using PVP as the surfactant.29 Remarkably, this report is the first example for the realization of the synthesis of single-layer metal nanosheets. In addition to metal nanosheets, a lot of ultrathin 2D nanosheets of metal oxides and metal chalcogenides have also been synthesized by this method.209,527−531 For instance, Dou and co-workers demonstrated a general solvothermal method for the synthesis of ultrathin 2D transition metal oxide nanosheets, including TiO2, ZnO, Co3O4, WO3, Fe3O4, and MnO2, by using the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (P123) and ethylene glycol as surfactants in ethanol.527 Using this strategy, TMOs of TiO2 (∼200 nm), ZnO (1−10 μm), Co3O4 (1−10 μm), WO3 (1−10 μm), Fe3O4, and MnO2 nanosheets with thickness ranging from 1.6 to 5.2 nm (corresponding to 2−7 stacking monolayers) have been prepared. Driven by the hydrophobic effect of the oleate tails, Xie and co-workers prepared ultrathin CeCO3OH nanosheets.209 The CeCO3OH nanosheet could be transformed into the CeO2 nanosheet with 3-atom-layer thickness via the thermal treatment at 400 °C for 2 min in air. As a typical member of TMDs, the MoS2 nanosheet can also be synthesized by a facile hydrothermal method.205,532−534 Xie and co-workers reported the preparation of few-layer defect-rich MoS2 nanosheet with the assistance of excess thiourea.205 The same group also prepared the oxygenincorporated MoS2 nanosheet in water at 200 °C using a similar hydrothermal method, which showed an enlarged interlayer spacing of 9.5 Å as compared to the pristine 2H-MoS2 at 6.15 Å.532 Later, Wei and co-workers prepared 2H-MoS2 nanosheet by using a similar synthetic procedure.533 The 2H-MoS2 nanosheet can be partially transformed into metallic 1T phase by a second solvothermal treatment.533 Because of the presence of metallic 1T phase, the MoS2 nanosheet with mixed 2H and 1T phases showed robust ferromagnetism at room temperature. The hydro/solvothermal synthesis method is a simple and scalable method for the synthesis of ultrathin 2D nanomaterials in high yield and low cost. However, it is hard to figure out the growth mechanism for the hydro/solvothermal synthesis process because all of the reactions occur in a sealed autoclave, thus making it difficult to design experiments for other material systems. The hydro/solvothermal synthesis is quite sensitive to the experimental conditions, such as the concentration of precursors, solvent systems, used surfactants or polymers, and temperature, making it difficult to precisely control the structure in different batches and laboratories. Most of the 2D nanosheets synthesized by the hydro/solvothermal synthesis method were few-layer rather than single-layer. 4.8.2. 2D-Oriented Attachment. The 2D-oriented attachment is another typical wet-chemical synthesis method used for the synthesis of ultrathin 2D nanomaterials.90 Unlike the classical mechanism, the 2D-oriented attachment shows a peculiar growth process and achieves nanostructures with welldefined morphology.535,536 In the 2D-oriented attachment, adjacent nanocrystals are connected with each other and fused together to form single-crystalline 2D nanosheets by sharing a common crystallographic facet to eliminate the high energy facets and interfaces.537−539 In 2010, Weller and co-workers developed this method for the synthesis of PbS nanosheets

controllable thickness. Significantly, the electronic properties of ultrathin 2D nanomaterials, such as graphene and TMDs, are approaching those of mechanically exfoliated thin layers. Therefore, ultrathin 2D nanomaterials grown by the CVD technique are also promising candidates for the fabrication of high-performance electronic and optoelectronics devices. Unlike the low yield and low production rate of the micromechanical cleavage technique, CVD is capable of producing materials in industry scale, a proven case for materials such as polycrystalline silicon. It is believed that CVD technique is a promising method with potential to produce ultrathin 2D nanomaterials in industry applications for electronics and optoelectronics. The CVD method still has some disadvantages at its current form. Ultrathin 2D nanomaterials grown by the CVD technique are always deposited on the substrates, needing to be transferred to other substrates for further investigation and applications. The CVD technique normally needs high temperature and inert atmosphere, leading to relatively high cost of production as compared to the solution-based methods. 4.8. Wet-Chemical Syntheses

Wet-chemical syntheses, which belong to the bottom-up approaches, are also good choices for the preparation of ultrathin 2D nanomaterials in high yield and large amount.89,90 Wet-chemical syntheses represent all of the synthetic methods that rely on the chemical reactions of certain precursors at proper experimental conditions conducted in solution phase.524 Because of their powerful controllability, wet-chemical syntheses have been considered as a class of convenient and reproducible strategies for the preparation of ultrathin 2D nanomaterials with controlled size and thickness, which are potentially scalable for industrial applications. Particularly, wetchemical syntheses have been widely used for preparing various nonlayer structured ultrathin 2D nanomaterials because they are unable to be prepared by those top-down methods.90 The synthesized 2D nanomaterials can easily be dispersed in organic or aqueous media, which make them very suitable for various applications. Wet-chemical syntheses can be also used for the synthesis of ultrathin 2D nanomaterials with layered structural features.89 Unlike all of the methods discussed above, there are no general principles underlying each wet-chemical synthesis method. One wet-chemical synthesis method could be as different from the other. To classify, hydro/solvothermal synthesis, 2D-oriented attachment, 2D-templated synthesis, self-assembly of nanocrystals, hot-injection method, interfacemediated synthesis, and on-surface synthesis are a few routines. In this section, we focus on the introduction of these typical wet-chemical protocols for the preparation of 2D ultrathin nanosheets and provide some representative examples for each protocol. The advances and limitations of each synthetic route are also discussed. 4.8.1. Hydro/Solvothermal Synthesis. The hydro/ solvothermal synthesis method is a typical wet-chemical synthesis strategy, using water or organic solvent as the reaction medium in a sealed vessel, in which the used reaction temperature is higher than the boiling point of the solvent.525 When the reaction temperature of the closed system is heated above the boiling point of the solvent system, the solvent will be autogenerated in high pressure to promote the reaction and improve the crystallinity of the as-synthesized nanocrystals. This method has been widely used to prepare ultrathin 2D nanomaterials, especially inorganic materials.90 As a typical 6243

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from tiny PbS nanocrystals.540 It was found that PbS nanocrystals were attached in a 2D orientation to form single-crystalline nanosheets with size of 0.8−2 μm and thickness of about 2.2 nm (Figure 16).537,540 The dense

4.8.3. Self-Assembly of Nanocrystals. Impelled by the development of nanocrystal synthesis and surface modification technologies, self-assembly of nanocrystals has been developed as one of the efficient ways to create nanoarchitectures with inner nanocrystals in ordered and steady manner, in which presynthesized nanocrystals spontaneously organize with each other by noncovalent interactions, such as van der Waals interactions, electrostatic interactions, and/or hydrogen bonds.545 It has been demonstrated that the assembly of lowdimensional nanocrystals, such as nanoparticles and nanowires, is a promising strategy to prepare ultrathin 2D nanomaterials.90 The assembly process of nanocrystals mostly involves the fusion of low-dimensional nanocrystals to form larger crystals in two dimensions. Although the 2D-oriented attachment method discussed in section 4.8.2 also belongs to the assembly strategy, it is worthy to discuss it in a separate section because it has been well studied and widely used for a number of materials. In 2006, Kotov and co-workers first reported the preparation of polycrystalline 2D CdTe nanosheet with thickness of ∼3.4 nm by the self-assembly of CdTe nanocrystals (3.4 nm in diameter).546 It was suggested that the self-assembly process was driven by the dipole moment, small positive charge, and directional hydrophobic attraction. Wu and co-workers prepared ultrathin Cu nanosheet by self-assembly of dodecanethiol capped Cu nanoclusters.547,548 The assembled Cu nanosheet gives strong luminescent intensity, although the original Cu nanoclusters are nonluminescent.548 Later, the same group reported that the 2D Au nanosheet could be also obtained by self-assembly of dodecanethiol capped Au nanoclusters.549 The thickness of the assembled Au nanosheets could be tuned from 1.5 to 100 nm by adjusting the concentration of Au nanoclusters and reaction temperature. In another example, Cheng and co-workers prepared an artificial membrane based on bilayered Au nanosheets by selfassembly of polystyrene capped Au nanoparticles at the liquid/ air interface.550 Besides nanoparticles, nanowires have also been used as building blocks to prepare ultrathin 2D nanosheets via the assembly strategy. Acharya and co-workers prepared an ultrathin 2D PbS nanosheet via the coalescence of PbS nanowires.551 Initially, the supercrystalline-assembled PbS nanowires were obtained with the assistance of the capping ligand trioctylamine. The supercrystalline nanowires were then assembled into single-crystalline nanosheets with regularly rectangular shape at the air/water interface with the assistance of surface pressure and temperature. By using a similar strategy, Yao and co-workers synthesized Eu2O3 nanosheet by selfassembly of Eu2O3 nanowires.552 In their experiment, the Eu2O3 nanowire bundles were first prepared by using 1,5pentanediol as the capping and structure directing agents. After being soaked in water, the nanowire bundles gradually selfassembled into sheet-like porous structures, and finally transformed into ultrathin rectangle Eu2O3 nanosheet with thickness of ∼3.8 nm. The lateral size of Eu2O3 nanosheets could be tuned from several hundred nanometers to 10 μm by simply adjusting the soaking time of the bundles in water. 4.8.4. 2D-Templated Synthesis. Templated synthesis is an effective strategy for growth of anisotropic nanostructures, which refers to the use of the presynthesized nanomaterials or bulk substrates as templates to confine/direct the growth of specific nanostructures.553,554 Over the past few years, tremendous efforts have been made in 2D-templated synthesis of ultrathin 2D nanosheets.555−558 For instance, by using the GO nanosheet as template, Zhang and co-workers achieved the

Figure 16. Schematic demonstration for the oriented attachment process of PbS nanoparticles to form the nanosheet. Reproduced with permission from ref 537. Copyright 2011 American Chemical Society.

packing of oleic acid ligands on {100} surface of PbS nanocrystals is the key point for this growth process. The presence of chlorine-containing compounds also assists in triggering the oriented attachment process by exposing the highly reactive {110} facets (Figure 16). On the basis of this work, Sun and co-workers prepared PbS nanosheets with different thicknesses by tuning the reaction temperature and the corresponding chlorine-containing compounds.541 Vanmaekelbergh and co-workers reported a two-steps method (assembly and oriented attachment) for the synthesis of 2D PbSe nanosheets with a honeycomb superlattice.542 In their experiments, PbSe nanocrystals terminated with {100}, {110}, and {111} facets were first prepared. They found that the PbSe nanocrystals attached together via the {100} facets to form a 2D honeycomb superlattice with octahedral symmetry. The 2D PbSe honeycomb structure could be transformed into a 2D CdSe superlattice through a cation exchange reaction. Nowadays, 2D-oriented attachment is considered as the exclusive way to prepare 2D nanocrystals with nonlayered structures. Various 2D nanosheets with layered structures can also be prepared by the 2D-oriented attachment method. Jeong and co-workers synthesized Bi2Se3 nanosheet by a simple and quick solution process.543 The growth mechanism involved in oriented attachment of small nanocrystals followed by epitaxial recrystallization into nanosheets. The driving force for this kind of 2D oriented attachment was the intrinsic tendency of a layered structure with negatively charged Se atoms. In their experiment, they found that the amine-containing surfactants preferred to enlarge the lateral dimension and get larger sized nanosheets. By using the oleylamine as solvent and surfactants, Wang and co-workers prepared a single-layer SnSe nanosheet with the lateral size of ∼300 nm and thickness of ∼1 nm by the coalescence of the SnSe nucleus in an oriented attachment mechanism.544 6244

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Figure 17. Schematic illustration of the synthesis process of the hcp Au square nanosheet on a GO template. Reproduced with permission from ref 284. Copyright 2011 Nature Publishing Group.

of cadmium chalcogenide nanocrystals, is a very attractive approach to prepare monodispersed colloidal nanocrystals with uniform size, shape, and high purity.567 The hot-injection method driven by high initial supersaturation involved the rapid injection of the highly reactive reactants into a hot solution containing long-chain oleyl amine and/or oleic acid surfactants. This method has been extended to synthesize ultrathin 2D nanomaterials, especially 2D metal chalcogenide nanostructures.568−575 As a typical example, a single-layer CdSe nanosheet was synthesized using this method by heating the reactants of CdCl2 and Se powder in a mixture solution of octylamine and oleylamine.576 Ultrathin 2D CdSe and CdTe nanosheets were also obtained by slight modification of the experimental conditions. Single-crystalline GeS and GeSe nanosheets with the average lateral dimensions of 2−4 μm × 0.5−1 μm and thickness of ∼5 nm were also synthesized by heating the mixture of GeI4, hexamethyldisilazane, oleylamine, oleic acid, and dodecanethiol or trioctylphosphine selenide to 320 °C for 24 h.577,578 In 2012, Zhang and co-workers used the hot-injection method to achieve the generalized synthesis of ultrathin metal sulfide nanocrystals.579 Particularly, CuS nanosheet with a regular hexagonal shape was synthesized in gram amount. The resultant CuS nanosheet is ∼3.2 nm (two unit cells) and ∼453 nm in thickness and lateral size, respectively. Yan and co-workers synthesized a series of 2D trivalent rare earth (RE)-selenium nanosheets, that is, RESe2 (RE = La, Nd) and RE4O4Se3 (RE = Nd, Sm, Gd, Ho), in which all of the nanosheets contain Se with −1 valence in planar Se layers.580 Note that all of the aforementioned metal chalcogenide nanosheets are nonlayer structured materials. 2D nanosheets of layered metal chalcogenides, for example, TMDs, can be synthesized by this method. As a typical example, Cheon and co-workers reported the synthesis of single-layer MoSe2 and WSe2 nanosheets with lateral size of 200−400 nm by using the hot-injection method.581 In their experiments, they found that the capping ligands were very important for controlling the number of layer in nanosheets. By changing the capping ligands from oleic acid to oleyl alcohol and oleylamine, the thickness of the nanosheets could be tuned from single layer to 2−3 layers and 4−8 layers. The same group also prepared a series of groups IV and V transition metal sulfide and selenide nanosheets, such as TiS2, ZrS2, HfS2, VS2, NbS2, TaS2, TiSe2, ZrSe3, HfSe3, VSe2, NbSe2, and TaSe2, by using the CS2 and Se powder as the injection chalcogen precursors.582−585 As an interesting example, Ozin and co-workers demonstrated the synthesis of 2D WS2 nanosheets with controllable crystal phase by simply tuning the injecting precursors. By injecting the CS2 and WCl6 precursors into hot oleylamine solution (320 °C), single-layer metallic WS2 nanosheet with a distorted octahedral (1T′) phase was obtained, and the as-prepared nanosheet has a

growth of hexagonal close-packed (hcp) Au square nanosheet with size between 200 and 500 nm and thickness of ∼2.4 nm via wet-chemical synthesis method (Figure 17).284 Normally, the face-centered cubic ( fcc) structure is the most stable phase of the gold and can be easily obtained by wet-chemical methods. To highlight, this is the first time that pure hcp Au structure that is stable under ambient conditions has been prepared by a solution-based method. The hcp Au nanosheet can be transformed to the fcc phase via phase transformation induced by the e-beam irradiation in a TEM, a secondary growth step, surface ligand exchange, or coating of a metal thin layer.559,560 As another example, Wei and co-workers demonstrated the synthesis of a freestanding half-unit-cell αFe2O3 nanosheet by using CuO nanoplate as the template.561 After the synthesis of layered iron hydroxide nanosheet on the surface of CuO nanoplate, freestanding iron hydroxide nanosheet can be obtained by slowly etching away the CuO template. After that, the iron hydroxide nanosheet was further transformed into freestanding α-Fe2O3 nanosheet by a simple thermal annealing treatment. The obtained α-Fe2O3 nanosheet has size of up to ∼1 μm and thickness of 0.55−0.59 nm. By using layered α-Ni(OH)2 nanosheet as the template, NiO nanosheet with size of up to micrometer and thickness I (2.66). Considering the atomic size of halogen dopants, the authors suggested that Br and I elements can form partially ionized bonds of −Br+− and −I+− to enhance the charge transfer, which has been verified by the first-principle DFT calculation. The charge transfer weakens the O−O bond strength of the adsorbed oxygen with the most prominent case found in IGnP.921 Huang and co-workers reported that the I-doped graphene shows comparable onset potential but better current density than the commercial Pt/C catalyst. The XPS and Raman spectroscopy confirm the formation of I3− in I-doped graphene, which is considered as the key factor for the enhancement of the ORR activity.922 Although great progress has been achieved in the usage of heteroatom doped-graphene sheets as metal-free catalysts, the mechanism underlying the dopant-induced ORR enhancement is still not completely clear. The main obstacle results from the lack of control of the configuration of the dopants based on current synthesis methods; thus the correlation between the true activity and the dopant configuration cannot be well understood. Another important issue about the doped graphene as ORR catalysts is that the effects of the trace amount of metal impurity remaining in the catalyst cannot be overlooked. The achievement gained in M−N−C (M = Fe and/or Co) as ORR catalyst in acid media has highlighted the importance of the metal, as the M−N4 moieties are proposed as the active sites.923,924 The proposed active sites have been identified as the FeN4 porphyrinic architectures in these doped carbons.925,926 It should be noted that there is no obvious crystalline iron structures in the doped carbon after etching,925 which means that common XRD is not enough to preclude the effect of the metal impurity. Wang et al. reported that trace Mn metallic residue originated from the Hummers’ oxidation method during the synthesis of GO or the impurity in graphite is sufficient to alter the electrocatalytic properties of the obtained rGO, as even the residue Mn is at ppm level.927 The finding emphasizes the importance of elemental analysis in revealing the active catalytic sites in doped graphene, especially for these samples declared as “metal-free” electrocatalysts. Note that the effect of the metal impurity can be precluded by the ORR poison tests, such as using CN− to poison the active sites.924,928 Besides doped graphene sheets, some other ultrathin 2D nanosheets have also been studied as ORR catalysts. For

conditions, while exhibiting a higher selectivity for oxygen reduction with the electron transfer numbers close to 4 (Figure 39d).903 By using a similar approach, Yang et al. reported the synthesis of S-doped graphene by using the GO-porous silica sheet as the starting material. The S doping is achieved by thermal annealing under H2S-rich atmosphere.910 In this method, the usage of porous silica can effectively prevent the aggregation of graphene and facilitate the doping process by assisting the gas transport. The obtained S-doped graphene also shows good activity for ORR with good resistance against crossover effect by methanol.910 The edge-selectively sulfurized graphene nanoplatelet also can perform as a good ORR catalyst.904 The authors of this work found that the oxidization of the doped graphene can further improve its capability to surpass the commercial Pt/C catalyst. They attributed the origin of high ORR activity to the spin density increase.904 It should be noted that the exact role of the S on the ORR activity of doped graphene is still unclear, and further study is needed to reveal the correlation between the catalytic activity and the configuration of S moieties. The codoped S with other heteroatoms, for example, N, in graphene can further improve their activity for ORR.911−914 Xu et al. reported that the codoped graphene shows improved onset potential and higher current density as compared to that of individually N- or S-doped graphene.911 A similar result is presented by Qiao and co-workers on the mesoporous N,Scodoped graphene by the post-treatment method.912 The obtained mesoporous N,S-codoped graphene has proven to be a remarkable metal-free catalyst for ORR with the electron transfer number of up to 3.6.912 The DFT calculation revealed that a synergistic effect in codoped graphene results from the changes of charge and spin densities at different sites of dually doped graphene, which lead to more active sites than singly doped samples.912 Furthermore, the 3D N,S-co-doped graphene framework can also be used as an excellent catalyst for ORR under alkaline conditions.913 Up to now, the studies on ORR activity of P-doped graphene are still scarce. Although the P atom has the same valence electron number with the N, it is still unclear whether it can be substituted into the sp2 carbon lattice, considering its large size as compared to N. Liu et al. reported that the P-doped graphite layer is capable of working as a metal-free catalyst for ORR in alkaline medium.915 The XPS confirms that the P atom has been incorporated into graphene layers through formation of P−C and P−O bonding sites. The P-doped catalyst showed superior activity as compared to the nondoped reference, confirming the importance of the heteroatom doping. However, its performance is still lower than that of the commercial Pt/C catalyst, and the electron transfer number is only about 3, which may result from its lower surface area and active sites.915 The thermal annealing of GO and triphenylphosphine has been used to synthesize P-doped graphene, which was reported by Hou and co-workers.916 The obtained P-doped graphene shows outstanding ORR stability and tolerance to methanol crossover effect, as well as excellent selectivity with the electron transfer number larger than 3.916 Using an ionic liquid 1-butyl-3methlyimidazolium hexafluorophosphate as P precursor, Li et al. reported the post-thermal annealing method for the synthesis of P-doped graphene with large surface area and relatively high P-doping level (1.16 atom %). The P-doped graphene exhibits performance comparable to that of the commercial Pt/C under alkaline condition. The authors attributed the origin of the excellent activity to the partial 6271

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Figure 40. (a) The crystal structure of Ni3(HITP)2 structure. (b) Polarization curves of Ni3(HITP)2 and the blank glassy carbon electrode under N2 versus O2 atmosphere. (c) Activation-controlled Tafel plot for Ni3(HITP)2. (d) Potential-dependent Faradaic efficiency of Ni3(HITP)2 catalyst for H2O2 production and %H2O2 production by Ni3(HITP)2. Reproduced with permission from ref 936. Copyright 2016 Nature Publishing Group.

Figure 41. (a) High-resolution TEM image and (b,c) enlarged high-resolution TEM images of partially oxidized Co nanosheets. (d,e) The related schematic atomic models, clearly showing distinct atomic configuration corresponding to hexagonal Co and cubic Co3O4. (f) Linear sweep voltammetric curves in a CO2-saturated (solid line) and N2-saturated (dashed line) 0.1 M Na2SO4 aqueous solution. (g) Faradaic efficiencies of formate at each given potential for 4 h. Data are shown for partially oxidized Co 4-atom-thick layers (red), Co 4-atom-thick layers (blue), partially oxidized bulk Co (violet), and bulk Co (black). Reproduced with permission from ref 526. Copyright 2016 Nature Publishing Group.

electrocatalysts.931−933 For instance, Wang et al. reported that the Pd@Pt nanosheet with atomically smooth Pt skin on Pd shows excellent catalytic activity and stability toward the oxygen reduction reaction in an acidic electrolyte. The mass activity of the nanosheet is nearly 7-fold higher that of the commercial Pt/ C catalyst, which provides an efficient way for Pt utilization.931 The DFT calculation has predicted that 2D π-conjugated MOF nanosheets can be applied as ORR electrocatalysts, and their

instance, Song and co-workers reported that oxygen-incorporated and P-doped MoS2 nanosheets show good activities for ORR.929,930 The onset potentials of electrocatalysts are improved with the current density several fold higher than that of pristine MoS2. It is found that doping of heteroatoms in the MoS2 nanosheet induces the ORR selectivity from twoelectron (pristine MoS2) to nearly four-electron process.929,930 2D metal nanoplates are also good candidates for ORR 6272

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6.3. Batteries

catalytic activities are highly dependent on the metal centrals.934 In fact, the graphene-porphyrin MOF composite has been reported with enhanced activity for ORR in 2011.935 Miner et al. reported that the intrinsically conductive MOF, Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), works as a well-defined catalyst for ORR in alkaline media with the activity competitive with those most active nonplatinum group metal electrocatalysts (Figure 40).936 Note that the conductive MOF contains square planar Ni−N4 sites, the active moieties in the M−N−C catalyst. Following that finding, thinning the MOF to nanometer scale can increase the exposed active sites and further enhance their activities. MOFs themselves have been widely used as precursors for the synthesis of high-performance ORR electrocatalysts by the pyrolysis method.937−939 COFs are another flexible precursor for the synthesis of ORR catalysts due to their well-defined and tunable structure.940−943 Dai and co-workers reported that COF-derived graphene analogues by carbonization of the metal-incorporated COFs act as efficient electrocatalysts for ORR in both alkaline and acid media with a good stability and remarkable tolerances to methanol-crossover and CO-poisoning effects.940 6.2.4. Other Electrocatalytic Reactions. The anisotropic synthesis of ultrathin 2D metal nanosheets has been achieved in recent years, and their potential applications as electrocatalysts have been also widely explored.28,29,944−946 For instance, Zheng and co-workers reported that the Pd nanosheet shows excellent electrocatalytic activity for the oxidation of formic acid, and its activity is 2.5 times as active as that of the commercial Pd black.28 Freestanding Co with a thickness of 4 atomic layers has been synthesized by Xie and co-workers. They found that partially oxidized Co 4-atom-thick layers showed outstanding intrinsic activity and selectivity for CO2 reduction, producing formate in aqueous solution with a low overpotential (Figure 41).526 Note that the catalytic activity of the partially oxidized catalyst is better than that of the pure Co atomic layers, which indicates the importance of manipulating the surface oxidation state of metal catalysts. The MoS2 edge sites have been confirmed as active sites for CO2 reduction.947 The MoS2 shows superior CO2 reduction performance as compared to noble metal-based catalysts with a high current density and low overpotential (54 mV), and the CO Faradaic efficiency increases correspondingly with the decrease of the applied potentials.947 Furthermore, the selective reduction of CO2 to CO has also been achieved on MOFs and COFs,948,949 which contain the active Co-porphyrin moieties.950 Specifically, the Co-porphyrin that contained COF shows high Faradaic efficiency (90%) and turnover numbers (up to 290 000 h−1) at pH 7 with an overpotential of −0.55 V. Note that the COF catalyst possesses a 26-fold improvement in activity as compared to the molecular Co-porphyrin complex and enhanced stability without degradation over 24 h.949 The tunable structure of MOFs and COFs enables the precise manipulation of the spatial arrangement of catalytic active centers, and the intrinsic pore structure around the active sites provides access for mass transfer. These advantages demonstrate the great potential of MOFs and COFs used as electrocatalysts, and their activity could be further enhanced if the rational design and construction of frameworks with high conductivity and dimensionality-controlled synthesis of these nanomaterials, such as ultrathin 2D nanosheets, can be achieved.

Rechargeable batteries, one type of important energy storage devices in our daily life, have lower cost and less environmental impact as compared to that of primary batteries. With growing demands for more powerful electronic devices, currently developed rechargeable batteries are still limited by their low energy density, short cycle life, slow charge rate, relatively high cost, fire risk, and so on. Because the properties of electrode materials significantly affect the performance of rechargeable batteries, developing novel electrode materials with novel structures and surface properties is of great importance to improve the performance of rechargeable batteries. This has been demonstrated by research in the last decades with graphite as an example.951 As the most commonly used anode for LIBs, graphite has a theoretical capacity of 372 mAh/g, which, after exfoliation, the mono/few-layer graphene nanosheet exhibited double its theoretical capacity (744 mAh/g).952 This is because both sides of the graphene nanosheet can be effectively utilized. The 2D crystalline structure of graphene holds a number of advantages as compared to graphite, such as facilitating the intercalation/deintercalation of electrolyte ions, buffering the volume expansion of electrode materials due to the elasticity of 2D nanosheet, and enhancing surface/interface lithium storage properties.953,954 Besides, the generated surface defects and sheet edges of graphene during the exfoliation process can serve as active sites for the lithium storage, which leads to a specific capacity of graphene higher than its theoretical value. Lian et al. demonstrated the graphene nanosheet exhibited a high reversible capacity of 848 mAh/g after 40 charge/discharge cycles at the current density of 100 mA/g.955 Amazingly, the boron-doped graphene showed a ultrahigh capacity up to 1327 mAh/g after 30 cycles at 50 mA/g, which is partially due to generated heteroatomic defects on doped-graphene nanosheet.956 Because of the large surface area and high electrical conductive nature of graphene, it can also be used as a matrix to prevent aggregation of other high-capacity active materials, such as metal oxides/sulfides, by forming composite electrodes, which have shown enhanced specific capacity, rate capabilities, and cycling performances.957,958 Encouraged by the successful application of graphene in LIBs, many other graphene-like 2D nanomaterials have been prepared and explored in rechargeable batteries. As a typical graphite analogue, MoS2 has a layered structure with higher theoretical capacity (670 mAh/g) based on the following lithium storage mechanism (MoS2 + 4Li+ + 4e− ↔ Mo + 2Li2S), which has been intensively studied as the anode material for LIBs.959 Although possessing high initial capacity, bulk MoS2 normally shows a quick decay of its capacity in the subsequent charge/discharge cycles or at increased charge/ discharge rate. This phenomenon is believed due to pulverization of electrode that was caused by the large volume expansion of bulk MoS2 as well as the high energy barriers for intercalation of Li+ ions into the interlayer space of bulk MoS2. In contrast, 2D MoS2 nanosheet showed much enhanced LIB performance due to the shorter diffusion path length for Li+ ions and enlarged surface active sites.960−963 A recent report demonstrated that the 2D MoS2 nanosheet prepared by exfoliation of bulk MoS2 exhibited a reversible capacity of ∼750 mAh/g after 50 charge/discharge cycles at the current density of 50 mA/g. Similar results have also been found in other 2D nanomaterials, such as WS2,964 VS2,965 SnS2,966−968 and SnO2,969 clearly indicating the advantages and promising future of using 2D nanomaterials in rechargeable batteries. In 6273

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Figure 42. (a,b) TEM images of the mesoporous graphene nanosheet. (c) Galvanostatic discharge/charge curves and (d) rate capability of the mesoporous graphene nanosheet. Reproduced with permission from ref 1009. Copyright 2013 American Chemical Society.

this section, we introduce the recent development of rechargeable batteries based on ultrathin 2D nanomaterials, with emphasis on the advantages of novel 2D nanomaterials and their composites in applications like LIBs and other nonlithium-ion batteries, such as SIBs and lithium−sulfur (Li− S) batteries. 6.3.1. Li-Ion Batteries. LIBs are one type of typical and common energy storage devices in our daily life, being well established and widely used for portable electronic devices as well as electrical vehicles.970−975 The relatively low theoretical capacity of graphite (372 mAh/g), a commercialized anode material of LIBs, has triggered the intensive research on searching the alternative anode materials with better lithium storage property. Many materials with higher theoretical capacities, such as cobalt, iron, tin-based oxides, and sulfides, have been investigated for LIBs in the past years.961,976−979 For instance, SnO2 has a theoretical capacity of 790 mAh/g, which is over twice that of graphite.979,980 Although having high theoretical capacities, those materials normally suffer from poor cycling performance, due to their large volume expansions during Li+ ion intercalation. To solve the common problem, a number of strategies have been developed to construct various complicated nanostructures, such as hollow, core−shell, and porous nanostructures. Jiang et al. reported that ultrathin SnO2 nanosheet delivered a reversible capacity of 534 mAh/g after 50 cycles at a current density of 156 mA/g when used as an anode in a LIB, which is much higher than the SnO2 nanoparticle (355 mAh/g) and hollow sphere counterparts (177 mAh/g).981 This clearly indicates the structure of electrode materials has a significant effect on their performance and also demonstrates the promising application of 2D nanomaterials for LIBs. Electrodes based on 2D nanomaterials can be classified into restacked ultrathin 2D nanosheets, hierarchical structures assembled by 2D nanosheets, and their composites. Ultrathin inorganic 2D nanosheets with single- or few-layer thickness are typical 2D nanomaterials for LIBs.964,982,983

Electrodes fabricated by restacking those ultrathin inorganic 2D nanosheets possess high surface area and edge sites that result from their unique 2D morphologies. Restacked nanosheets normally exhibited larger interlayer distance than their bulk counterparts, which not only facilitates Li+ ion insertion/ extraction, but also has more tolerance to the volume change of the electrode, therefore leading to the enhanced specific capacity and electrochemical stability. The electrode fabricated by restacking exfoliated MoS2 nanosheet delivered a reversible capacity of ∼750 mAh/g after 50 cycles at current density of 50 mA/g, much better than that of the bulk MoS2 electrode (226 mAh/g).982 The good cycling performance of restacked MoS2 electrode should be related to the enlarged spacing of MoS2 layers (0.635 nm) and high specific surface area. Zhang and coworkers reported that, when used as anode in a LIB, the CuS nanosheet delivered a specific capacity of ∼642 mAh/g after 360 cycles at 0.2 A/g, surpassing other CuS nanostructures.579 Many other inorganic 2D nanosheets have also been explored as anodes for LIBs, such as MXenes,79,984,985 metal-based nanosheets,986,987 and 2D heteronanostructures.390 Because of the vdW interaction, 2D nanomaterials tend to restack together, forming irreversible agglomerates during the electrode fabrication process, resulting in the loss of some special properties arising from their ultrathin 2D structural characteristics, such as high density of active sites and large accessible surface area. Previous studies on graphene have revealed that the fabrication of 3D porous architectures is able to effectively prevent aggregation of graphene, while in the meantime preserving the superior intrinsic properties of 2D nanosheets.988−998 The resultant 3D graphene-based materials have exhibited promising performances in a wide range of applications.992,993,997,998 Hence, creation of 3D architectures by using 2D nanomaterials as building blocks is highly expected to address the problem of aggregation, providing a promising pathway to realize practical applications of 2D nanomaterials.999,1000 It is an appealing strategy to assemble 2D nanosheets 6274

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Figure 43. (a) Schematic illustration of the preparation process for expanded graphite. (b) Cycling performance and (c) rate capability of the expanded graphite electrode. Reproduced with permission from ref 1032. Copyright 2014 Nature Publishing Group.

50 mAh/g even at a relatively low rate of 10 C. A good capacity retention of ∼93.8% was also achieved on the V2O5 nanosheet electrode after 50 charge/discharge cycles at 0.2 C, as compared to the bulk counterpart (46%). The unique 2D structure provides the enhanced rate capability and electrochemical stability of V2O5 nanosheet when used as cathodes in LIBs. Similarly, electrodes based on Li2MnSiO4 and Li2FeSiO4 nanosheets exhibited good LIB performances, for example, discharge capacities of 350 and 340 mAh/g for the Li2MnSiO4 and Li2FeSiO4 nanosheet, respectively, which are much higher than those of their nanoparticle counterparts.1013 Although ultrathin 2D nanomaterials exhibited enlarged specific capacity as compared to their bulk counterparts, the high density of edge sites on 2D nanomaterials increases not only the contact resistance between active materials and current collectors, but also the electron transfer resistance among nanosheets. This results in the limited cycling performances for practical applications. Fabrication of 2D hybrid nanocomposites is another effective way to further improve the performance of ultrathin 2D nanosheets for LIBs.997,1014−1019 Zhang and co-workers reported that MoS2-coated 3D graphene showed a high reversible capacity of 877 mAh/g at 100 mA/g after 50 cycles.997 A capacity of 466 mAh/g persists even at a high current density of 4 A/g. The in situ growth of 2D MoS2 nanoplate on the graphene surface enabled an intimate contact between them, guaranteeing an effective and fast electron transfer process. In contrast, the electrode of MoS2 powdercoated 3D graphene composite suffered from a quick decay of its capacity from 638 mAh/g to less than 100 mAh/g after 50 cycles, which can be attributed to the crack and drop of active materials caused by the large volume expansion of MoS2 powder during cycling process. It has also been reported that graphene could prevent the aggregation of 2D nanomaterials, resulting in the formation of thinner and smaller grain size of synthesized 2D nanomaterials.1016,1018 This led to the reduced energy barrier for Li+ ion intercalation and also enhanced surface/interface lithium storage properties.

into hierarchical 3D architectures with a variety of morphologies, for example, wires, 1001 tubes,1002 boxes, 1003,1004 spheres,1005−1007 and flower-like structures.1008 These 3D structures are able to effectively prevent the aggregation of 2D nanosheets, as well as largely exposing their surface area and active edge sites. In addition to preserving the intrinsic features of 2D nanomaterials, new properties also emerged from assembled 3D architectures. As a typical example, Wang and co-workers reported that the MoS2 nanotube assembled from monolayer MoS2 exhibited excellent electrochemical properties as anode material in a LIB, delivering a reversible specific capacity of 839 mAh/g after 50 cycles at 100 mA/g as well as a good rate capability of 600 mAh/g at a high current density of 5 A/g.1002 Alternatively, engineering pores on ultrathin 2D nanomaterials is also an effective way to prevent the destruction of electrode materials caused by their volume expansions as well as to shorten the Li+ ion diffusion length for improved rate capability. As a typical example, Zhao et al. reported the utilization of porous graphene nanosheet as anode in a LIB, which exhibited a highly reversible capacity of 1040 mAh/g at 100 mA/g, better than the other porous carbon-based anodes (Figure 42).1009 This strategy can also be applied to prepare other porous inorganic 2D nanomaterials. As another example, 2D macroporous Co3O4 platelet exhibited an excellent cycling performance with no obvious decrease of capacity after 30 charge/discharge cycles.1010 In contrast, the other morphologies of Co3O4, that is, nanoparticles and rod-like structure, showed quick decays of their capacities during cycling. The Co3O4 macroporous platelet also showed a good rate capability with a reversible capacity of 811 mAh/g at a discharge rate of 2 C, which benefits from the large surface area and short diffusion length for Li+ ions. Besides application for anode materials, ultrathin 2D nanomaterials have also been explored as cathodes for LIBs.1011 Few-layer V2O5 nanosheet with thickness of ∼3 nm has been used as a cathode in a LIB, showing an excellent rate capability of 117 mAh/g at ultrahigh discharge rate of 50 C.1012 In contrast, the bulk V2O5 exhibits a specific capacity less than 6275

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Figure 44. (a) Schematic illustrations of BP, Na-intercalated BP, and Na3P. (b−d) TEM and HRTEM images of the BP-graphene hybrid. (e) Cycling performance of the BP-graphene hybrid anode. Reproduced with permission from ref 200. Copyright 2015 Nature Publishing Group.

6.3.2. Non-Li-Ion Batteries. LIBs have been the main power sources for portable electronics, such as cellphones, laptops, and digital watches, since its first demonstration in the 1990s. Because of their clean and sustainable features, LIBs are also promising power sources for electrical vehicles. Although the perspective of LIBs is promising, it should be pointed out that there are limited amounts of lithium-containing minerals, which are unevenly distributed on Earth. This directly leads to the increased price of lithium elements, which in turn significantly affects the further extension of commercialized LIBs for electrical vehicles as well as the large-scale stationary electrical storage devices that required large lithium concentration. Therefore, the development of other types of batteries based on the cost-effective electrode materials is of great importance and priority for large-scale renewable energy storage devices. Up to now, a number of rechargeable nonLi-ion batteries have been developed due to the relatively lowcost and earth-abundant elements, such as Na-ion,1020−1024 Mgion,1025−1028 and Al-ion batteries.1029 Sodium ion batteries (SIBs) are promising and attractive alternatives to LIBs.1030 The abundant resource of sodium makes SIBs cost-effective, potentially competitive with LIBs in large-scale energy storage systems.1020,1023 The large size of sodium ions (0.102 nm in radius) renders many electrode materials for LIBs not suitable for SIBs.1031 Graphite is considered as an inactive anode material for SIBs due to its small interlayer distance that restricts the intercalation of Na+

ions. Wen et al. reported that graphite could be a promising anode material for SIBs with long cycle life after it is expended (Figure 43a), which exhibited a reversible capacity of 284 mAh/ g at 20 mA/g and a capacity retention of ∼74% after 2000 charge/discharge cycles at 100 mA/g (Figure 43b,c).1032 This work provides a cost-effective way to develop electrode materials for SIBs on the basis of the earth-abundant graphite resource. The specific capacity of the expanded graphite (284 mAh/g at 20 mA/g) is still relatively low, which is not comparable with other electrode materials, such as the exfoliated MoS2 nanosheet (386 mAh/g at 40 mA/g).1033 In addition to expanded graphite, ultrathin 2D nanomaterials have been also explored as electrode materials for SIBs, such as TMDs, MXenes,441 MoO3−x nanosheet,1021 and BP.200 As a typical example, Chen et al. revealed that the interlayer distance of MoS2 nanosheets plays an essential role in its SIB performance.1034 The electrochemical results showed that the MoS2 nanosheet with the largest interlayer distance (0.69 nm) exhibited the best SIB performance, for example, reversible capacities of 300 and 195 mAh/g at current densities of 1 and 10 A/g after 1500 cycles, respectively. This indicates that the enlarged interlayer distance of MoS2 benefits the enhanced electrochemical stability of the electrode and also leads to the fast reaction kinetics, which allows for an easier intercalation/ deintercalation of Na+ ions into the MoS2 structure. The BP nanosheet has also been demonstrated for SIBs.200 Note that BP has a high electrical conductivity (∼300 S/m) and large 6276

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composite showed a reversible capacity of ∼505 mAh/g at rate of 1C (1680 mA/g) and a capacity retention of 75% over 100 cycles.1038 In addition, the functional groups on GO sheets play an important role in anchoring S atoms through forming chemical bonds between them, such as O−S bonds. The oxygenated functional groups can effectively prevent the formed Li2Sn species from dissolving into the electrolyte during cycling.1040 It is also found that graphene sheets doped by heteroatoms, such as N, B, and S atoms, have strong ionic interactions with Li2Sn, thus inhibiting its dissolution in the electrolyte.1041 Apart from forming chemical bonds with S, ultrathin 2D nanomaterials can also serve as a physical trapper to stabilize the cathode materials of Li−S batteries by wrapping them. Wang et al. demonstrated that the graphene-wrapped sulfur nanoparticle exhibited a high specific capacity of ∼600 mAh/g after 100 charge/discharge cycles.1042 Later, a similar strategy was also applied to TMDs, in which TMD nanosheets served as host materials.1043 In this work, TMD thin films, that is, TiS2, ZrS2, and VS2, were coated onto Li2S nanoparticles to form corresponding Li2S@TMD core−shell nanostructures (Figure 45a,b).1043 The resultant TMD films acted as the physical

theoretical specific capacity (2596 mAh/g). It has a channel length of 3.08 Å, large enough for the diffusion of Na+ ions, which is also much larger than that of graphite (1.86 Å) (Figure 44a).200 Cui and co-workers reported the utilization of a fewlayer BP nanosheet with thickness of 2−5 nm as the electrode for SIBs.977 The electrode was prepared by mixing phosphorene with graphene to form a BP-graphene hybrid with a sandwiched structure (Figure 44b−d).200 The incorporated graphene can serve as an elastic buffer to prevent the large volume expansion of the electrode material (∼500%) due to the formation of Na3P alloy during the sodiation process. As a result, the hybrid electrode showed an ultrahigh capacity of 2440 mAh/g at a rate of 0.02 C (Figure 44e),200 which is close to the theoretical specific value of BP for SIBs (2596 mAh/g). The hybrid electrode exhibited a good cycling property with ∼85% retention of its initial discharge capacity at 0.02 C after 100 cycles. Even at a large rate of 10 C, a capacity retention of 77% still remained, indicating the good electrochemical stability. Ultrathin 2D nanomaterials have also been investigated in lithium−sulfur (Li−S) batteries. Li−S batteries possess a high energy density up to 2600 Wh/kg, making it a promising candidate for non-LIB energy storage applications with high theoretical capacity of 1675 mAh/g based on the redox reaction of S + 2Li+ + 2e− ↔ Li2S.1035,1036 The natural abundance of sulfur resources also makes Li−S batteries cost-effective and sustainable as compared to LIBs. The performance of currently developed Li−S batteries still suffered from poor cycling properties despite the concept of Li−S batteries that has been demonstrated for over 30 years. One significant issue that limits the practical application of Li−S batteries is the polysulfide dissolution during the charge/discharge process, that is, the “shuttle effect”. In a typical Li−S battery, the intermediate lithium polysulfide (Li2Sn, n = 4−6) was formed during the charge/discharge process, which can be dissolved in the electrolyte and transported back and forth between electrodes. This side reaction not only induces the loss of electrode material, but also leads to the passivation of cathode surface, resulting in a low Coulombic efficiency and rapid capacity decrease during cycling process of Li−S batteries. To solve this “shuttle effect”, the development of novel electrode materials/ structures that have strong interaction with S and firmly anchor S on the electrode is essential to achieve a high cycling property of Li−S batteries. Ultrathin 2D nanomaterials featuring high surface area and plenty of active sites on both basal planes and edges can effectively confine S through chemical bonding or physical adsorption. Ultrathin 2D nanomaterials with large lateral sizes could also suppress shuttle effects by entrapping the polysulfide and preventing it from dissolving in the electrolyte, which makes ultrathin 2D nanomaterials promising for Li−S batteries. Carbon-based 2D nanomaterials have been demonstrated as promising host materials for Li−S batteries due to the affinity of carbon to sulfur.1037 Graphene with a high specific surface area (∼2600 m2/g), ultrahigh electrical conductivity, and excellent structure stability has been intensively studied for Li−S batteries. Ultrathin 2D nanomaterials can restrict the polysulfide dissolution by the formation of chemical bonds or physical adsorption between ultrathin 2D nanomaterials and sulfur element. Ultrathin 2D nanomaterials are able to trap the formed Li2Sn within the interlayer spacing, which serve as good physical entrappers.1038,1039 Taking graphene as a typical example, the formed sandwich-like structure of graphene/S

Figure 45. Schematic illustration (a) and TEM image (b) of the Li2S@ TiS2 composites. (c) Schematic illustration of the interaction between Li2S and TiS2. (d) Rate capability of the Li2S@TiS2 composite electrode. Reproduced with permission from ref 1043. Copyright 2014 Nature Publishing Group.

barrier of Li2S to prevent its dissolution in the electrolyte. Different from the graphene shell, a chemical bonding between Li2S and TiS2 with a high binding energy of 2.99 eV can be formed (Figure 45c), minimizing the “shuttle effect”.1043 As a result, the cathode of Li2S@TiS2 composite showed an excellent cycling behavior with a capacity retention of 77% after charging and discharging it at 583 mA/g for 400 cycles. It also exhibited a good rate capability, for example, reversible capacity of 700 and 503 mAh/g at rates of 0.2 and 4 C, respectively (Figures 45d).1043 Because of the large members of 2D TMD nanosheets, it can be expected that promising host materials for Li−S batteries could be found in the family of 2D TMDs. MXenes have also been used for Li−S batteries.443 After incorporation of S element into Ti2C using a heat treatment, a strong interaction between them was established by forming 6277

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Figure 46. Schematic illustration of the chemical structure (a) and SEM image (b) of OCN material. (c) Cycling performances of OCN and g-C3N4 electrodes. Reproduced with permission from ref 1046. Copyright 2015 American Chemical Society.

species. Each pore can act as a “micro-reactor” that accommodates the volume expansion of sulfur, giving rise to improved cycling stabilities, for example, a capacity retention of 74% at 1 C after 100 cycles. Mesoporous 2D nanomaterials are not only limited to graphene. Zhang et al. reported the use of 2D oxygenated carbon nitride (OCN) nanosheet for Li−S batteries.1046 The resultant OCN nanosheet is of mesoporous structure with a pore size of 1−2 nm thus and has a high surface area up to 605.97 m2/g. The OCN nanosheet is also full of oxygenated functional groups (e.g., −C−OH, −COOH, and −C−O−C) on its surface (Figure 46a,b).1046 The N concentration in the OCN nanosheet (20.49 wt %) is much higher than those of the N-doped graphene sheet, therefore having substantive active sites for anchoring the polysulfide. As expected, after the incorporation of S on the OCN nanosheet, the fabricated cathode of S/OCN hybrid with S concentration of 56 wt % delivered a reversible discharge capacity of 447.3 mAh/g after charging/discharging for 500 cycles, which is better than the g-C3N4 (Figure 46c).1046 After the S/OCN cathode was tested for 2000 cycles, a reversible capacity of 230.7 mAh/g still remained. It is noteworthy that mesoporous TMD nanosheets have not been reported for Li−S batteries yet. The nanosized pores within TMD nanosheets may provide abundant edge sites of metallic centers that can form strong interaction with sulfur through metal−S bonds, which may effectively confine the polysulfide within these nanopores. Considering the large number of familiar members of TMDs, there might be many potential host materials for Li−S batteries. Ultrathin 2D nanomaterials have shown promising applications in energy storage devices with much improved specific capacity, rate capability, and cycling performance as compared to their bulk counterparts. There are still some challenges in

the S−Ti−C bonding, which was confirmed by XPS.443 The resultant S−Ti2C hybrid with a sulfur concentration of 70 wt % delivered a high reversible capacity of 723 mAh/g after 650 charge/discharge cycles. It is worthy to note that the cycling performance is comparable to those of other reported host materials, such as N,S-doped graphene.443 At a high rate of 4 C, a specific capacity of 660 mAh/g still remained for the S−Ti2C hybrid electrode. It was believed that the good performance of the hybrid electrode is attributed to a combined factor of high stability and electrical conductivity of the S−Ti2C hybrid.443 On the one hand, the strong interaction between Ti2C and formed Li2Sn inhibits the loss of active materials, leading to the improved cycling performance. The metallic properties of Ti2C also benefit the high-rate performance of S−Ti2C hybrid electrode due to its high electrical conductivity. Mesoporous 2D nanomaterials are another kind of promising host materials for Li−S batteries, which have shown improved performances.1044,1045 Sulfur nanoparticles trapped within the pores of 2D nanomaterials are more stable, as compared to those coated on the exterior surface of 2D nanomaterials and directly exposed to the electrolyte. Apart from the minimized “shuttle effect” of Li2Sn by forming strong interaction between pore edges and sulfur, the high electrical conductivity and elastic matrix of some mesoporous 2D nanomaterials (e.g., graphene) can also promote the high-rate performance of Li−S batteries.1045 The nanoporous graphene with pore size of ∼3.8 nm was fabricated by chemical activation of rGO using KOH and then used as the host material for Li−S batteries.1044 Sulfur can be encapsulated into pores of graphene by a melt diffusion process. Because of the strong affinity of sulfur to carbon, sulfur nanoparticles were constrained inside the nanopores of the graphene sheet, which limits the diffusion of formed Li2Sn 6278

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this research field. Low initial Coulombic efficiencies are commonly observed for most of the rechargeable batteries based on electrodes constructed from ultrathin 2D nanomaterials, leading to a great loss of specific capacity. Although ultrathin 2D nanomaterials exhibited improved capacity as compared to many traditional electrode materials, their electrochemical stabilities are still required to be further explored when considering their practical applications. Because most of the reported ultrathin 2D nanomaterials were tested in limited charge/discharge cycles (e.g., 200 nm), the restacked aggregation, with a 10-fold enhancement of the surface area, showed a higher activity for photocatalytic hydrogen evolution than the starting KCa2Nb3O10 compound. When RuOx was intercalated between the layers during the exfoliation-restacking route, the overall photocatalytic water splitting was achieved. The activity of tetrabutylammonium hydroxide-supported (TBA-supported) HCa2Nb3O10 nanosheet produces 16 times more hydrogen and 8 times more oxygen than the nonexfoliated bulk niobate. Doping transition metals in perovskite nanosheets is also an effective method to improve their photocatalytic activities by controlling the electron concentration and/or improving the light harvesting capability. Without cocatalysts, dopants in bulk materials, such as Zr-doped KTaO3 and Rh-doped SrTiO3, do not significantly improve the catalytic activity, because dopants are present within catalysts rather than on the surface, and thus have an indirect effect on the catalytic reaction taking place on the catalyst surface.1189,1196,1197 In contrast, dopants in ultrathin 2D nanomaterials (thickness of ∼1 nm) are located very close to the surface. In this case, most of the dopants in ultrathin 2D nanosheets can be directly involved in the catalytic reaction that occurred on the surface. As a typical example, Okamoto et al. reported that a Rh-doped KCa2Nb3O10 nanosheet synthesized by exfoliating layered KCa2Nb3−xRhxO10−δ exhibited a high photocatalytic activity for H2 production from a water/ methanol system without the cocatalyst loading (Figure 54).1189 The maximum H2 production rate of the Rh-doped nanosheet was 165 times larger than that of the bulk Rh-doped layered oxide (Figure 54b). The quantum efficiency at 300 nm was 65%. The significant improvement of H2 evolution activity is attributed to the RhO6 units in the nanosheet in direct contact with the reactants and that act as H2 evolution sites. The efficiency of solar energy conversion with the HCa2Nb3O10 nanosheet is limited by the large bandgap (3.5 eV) that restricts its utilization of solar energy. Maeda et al. reported band-edge tunable perovskite nanosheets of HCa2−xSrxNb3O10 and HCa2Nb3−yTayO10.1198 These nanosheets exhibited very high photocatalytic activity for H2 evolution from a water/methanol mixture under band gap irradiation. For the HCa2Nb3−yTayO10, the H2 evolution activity was first increased with the Ta content up to y = 1, and then decreased slightly due to a compromise between the rise of the conductive band potential and the efficiency of light harvesting. The highest activity was obtained with the HCa2Nb2TaO10 nanosheet reaching an apparent quantum yield of approximately 80% at 300 nm. In addition to exfoliated nanosheets of layered perovskites, exfoliated nanosheets of LDHs are another class of 2D nanomaterials, which can be used as photocatalysts for water splitting. Silva et al. first synthesized a series of nanosheets from Zn/Ti, Zn/Ce, and Zn/Cr LDHs at different Zn/metal atomic

Figure 54. (a) The crystal structure of Rh-doped KCa2Nb3O10 and photocatalytic reaction model in water/methanol system. (b) Timedependent hydrogen evolution over different Rh-doped KCa2Nb3O10 nanosheets. Reproduced with permission from ref 1189. Copyright 2011 American Chemical Society.

ratio and tested them for the visible-light photocatalytic oxygen generation.1199 All of the prepared LDH nanosheets exhibited quite remarkable photocatalytic activity with those that contain Zn and Cr composite as the most active. As compared to the Zn/Ti or Zn/Ce LDH nanosheet, the Zn/Cr LDH nanosheet exhibited much greater light absorption in the visible region. Its apparent quantum yields for oxygen generation were 60.9% and 12.2% at 410 and 570 nm, respectively. In another example, Lee et al. synthesized a series of nanosheets of titanium-embedded LDHs, including Ni/Ti LDH and Cu/Ti LDH. The highsurface-area Ni/Ti-LDH nanosheet with two visible absorption bands in the blue and red wavelength regions was found to show a higher O2 generation rate than that of the Cu/Ti-LDH nanosheet under visible excitation (49 vs 31 mmol of O2) by using 200 mg of the photocatalyst and 1 mmol of AgNO3 as a sacrificial agent, indicating good water oxidation activity of Tiembedded LDHs.1200 These nanosheets of Ti-embedded LDHs also showed a high photocatalytic H2 evolution activity. Taking the advantage of the large number of surface Ti3+−O defects as trapping site that improves charge separation, the TiO6 octahedra confined in the 2D matrix of a series of Ti-embedded LDHs, such as Zi/Ti LDH, Zn/Ti LDH, and Mg/Ti LDH, can suppress the carrier recombination under visible light irradiation.1201 As a result, the Ti-embedded LDHs showed a H2 evolution rate of 31.4 mmol h−1, which is 18 times higher than that of the control K2Ti4O9 sample. Apart from photocatalytic water splitting, photoreduction of CO2 into hydrocarbon fuels over semiconductor photocatalysts has been considered as an optional technique to solve the global energy crisis. Tu et al. reported that hollow spheres composed of exfoliated Ti0.91O2 and graphene fabricated by a layer-by-layer assembly technique can be used as the photocatalyst for CO2 reduction.1202 The graphene-Ti0.91O2 shows a 9 times increase for the photocatalytic CO2 conversion to produce valuable fuels CO and CH4 than that of the commercial P25 TiO2.1202 It is believed that the ultrathin nature of Ti0.91O2 nanosheet composited with graphene significantly increased the lifetime of photoexcited charge 6289

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Figure 55. (a,b) TEM images and (c) HRTEM image of WS2−CdS heteronanostructure. (d) Time-dependent photocatalytic hydrogen evolution for WS2−CdS, MoS2−CdS, and pure CdS nanostructures. (e) Comparison of the H2-evolution rate under visible light irritation. (f) Schematic illustration of the photocatalytic process of MS2−CdS hybrid nanostructure in lactic acid solution. Reproduced with permission from ref 1212. Copyright 2015 John Wiley & Sons, Inc.

amount of noble metals, such as Pt, Ru, and Rh, as the cocatalyst. A CdS−WS2 composite was also reported by Hao et al.; the activity of CdS was increased by up to 28 times with only 1 wt % of WS2, suggesting that WS2 was also a good cocatalyst for photocatalytic H2 evolution.1209 To improve interfacial charge transfer, Ye and co-workers reported a hybrid photocatalyst consisting of CdS nanocrystal grown on the surface of the nanosized MoS2-graphene hybrid for hydrogen evolution under visible light irradiation.1213 Through optimization of each component proportion, the highest photocatalytic H2 evolution activity of MoS2/graphene-CdS was achieved when the content of the MoS2/graphene cocatalyst is 2.0 wt % and the molar ratio of MoS2 to graphene is 1:2. A 1.8 mmol/h H2 evolution rate was recorded in lactic acid solution. This result is not only higher than MoS2/graphene-CdS in Na2S− Na2SO3 solution, but also much higher than that of Pt/CdS in lactic acid solution. Although considerable research efforts have been made to develop TMD-based heterogeneous for photocatalytic H2 evolution, precise control of chemical composition and morphology for better hydrogen evolution performance still remains a challenge. Considering that the active sites for H2 evolution are actually located at the edge, rationally designing TMD-based heterogeneous heterostructures to expose a large amount of active edge sites is critical for the improvement of catalytic performance. Zhang and co-workers synthesized MS2− CdS (M = W, Mo) nanohybrids composed of single-layer MS2 nanosheets with lateral size of 4−10 nm selectively grown on the Cd-rich (0001) surface of wurtzite CdS nanocrystals for the photocatalytic hydrogen evolution (Figure 55a−c).1212 The WS2−CdS and MoS2−CdS nanohybrids exhibited excellent activities for photocatalytic hydrogen evolution under the visible light irradiation (>420 nm) (Figure 55d), which were about 16 times and 12 times higher than that of the pure CdS nanocrystal (Figure 55e), respectively. It was believed that the improved catalytic activities of these MS2−CdS nanohybrids were mainly attributed to the large number of active sites on the extremely small lateral size (from 4 to 10 nm) of MS2 nanosheets (Figure 55f). The single-layer structure of MS2 and

carriers, which leads to the large enhancement in the photocatalytic activity. Nevertheless, the efficiency of valuable chemical products is still at a considerably low level because the photoreduction of CO2 is considered to be more difficult than hydrogen evolution from water, as it requires a high conduction band potential. CO2 reduction is a multielectron transfer process, for example, two electrons for the production of carbon monoxide (CO) and formic acid (HCOOH), and six and eight electrons for the methanol (CH3OH) and methane (CH4) production, respectively.1203 Thus, to efficiently drive the multielectron reduction reaction for the production of hydrocarbon fuels, the surface modification of photocatalysts with cocatalysts is necessary. Yin et al. decorated the Cu(II) nanocluster as the cocatalyst onto the Nb3O8− nanosheet. Under UV-light irradiation, photoexcited electrons in the conduction band of the Nb3O8− nanosheet are injected into the Cu(II) nanocluster through interface and efficient reduction of CO2 molecules to CO.1204 6.6.2. 2D Metal Chalcogenide Nanomaterial-Based Photocatalysts. To date, earth-abundant layered 2D TMDs, such as MoS2 and WS2, are being developed at a rapid pace for electrocatalytic water splitting.1205,1206 MoS2 and WS2 nanosheets are not active toward photocatalytic water splitting when directly used as photocatalysts. Yet in the presence of photosensitizers, such as CdS and TiO2, they are very good cocatalysts for photocatalytic H2 production, which not only act as active sites for H2 generation but also improve the photogenerated charge separation and increase the stability of photocatalysts. Until now, many TMD nanosheets, such as MoS2, WS2, and NiS2, have been reported as excellent costeffective cocatalysts.1207−1213 Because of its high mobility for charge transport, using MoS2 as the cocatalyst can enhance the separation of photogenerated electrons and holes as well as decrease the activation potentials for H2 generation. Zong et al. directly deposited MoS2 on the surface of CdS and found that by loading with only 0.2 wt % of MoS2, the H2 production rate can increase up to 36 times as compared to that using pure CdS.1208 The catalytic activity using MoS2 as the cocatalyst is even higher than using the same 6290

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Figure 56. (a) TEM image of TiO2 NPs combined with layered MoS2/graphene hybrid. (b) Schematic illustration of the charge transfer in the TiO2−MoS2/graphene hybrid. (c) Photocatalytic H2 evolution of the TiO2−MoS2/graphene hybrid. Reproduced with permission from ref 1225. Copyright 2012 American Chemical Society.

small particle size of MS2−CdS shortened the diffusion pathway of charge carriers and thus decreased the recombination probability of charge carriers, which further improved the photocatalytic activity. Benefiting from the unique atomic structure, ultrathin 2D metal chalcogenides could achieve enhanced solar light harvesting critical to enhance the photocatalytic properties. Taking the synthetic all-surface-atomic SnS as an example, the surface atomic elongation and structural disordering as revealed by XAFS spectroscopy endowed the all-surface-atomic SnS nanosheet with excellent structural stability and much increased density of states at the valence band edge.1214 This suggests that the enhanced light-harvesting capability leads to the improvement of their efficiency in visible light water splitting. The all-surface-atomic SnS nanosheet-based photoelectrode exhibits a 67.1% IPCE at 490 nm, much higher than that of the bulk counterpart (1.66%). The photocurrent density could reach up to 5.27 mA cm−2, which is 2 orders of magnitude higher than that of the bulk counterpart. Similar results have also been achieved in the synthetic 3-atomic-thick SnS2 and 4atomic-thick ZnSe single-layers.530,1215 6.6.3. Other 2D Nanomaterial-Based Photocatalysts. g-C3N4, a metal-free polymeric semiconductor, was reported to be active for H2 or O2 evolution from water under visible light irradiation with the assistance of a sacrificial reagent. Pristine gC3N4 prepared by solid-state reaction at high temperature (500−600 °C) is a typical bulk material with a rather low surface area (ca. 10 m2 g−1), which seriously limits its performance. 19 As compared to its bulk form, g-C 3 N 4 nanosheet has many advantages benefiting from the reduced thickness, including enlarged surface area, shortened charge carriers migration from the bulk to the surface, as well as accelerating electrons transport along the in-plane direction.1216,1217 g-C3N4 nanosheet with a thickness of ∼2 nm was synthesized by the thermal oxidation etching of the bulk C3N4 in air. The obtained g-C3N4 nanosheet possesses a high specific surface area of 306 m2 g−1, a larger bandgap (by 0.2 eV), and increased lifetime of photoexcited charge carriers because of the quantum confinement effect.1218 As a result, the g-C3N4 nanosheet showed photocatalytic activities superior to those of the bulk g-C3N4 under both UV−visible and visible light irradiation. The average hydrogen evolution rate of the gC3N4 nanosheet under UV−visible light irradiation is 170.5 μmol h−1, which is 5.4 times higher than that of the bulk counterpart. Pure g-C3N4 suffers some intrinsic shortcomings, such as insufficient solar energy harvesting (bandgap of ca. 2.7 eV), a rather high exciton binding energy and a high electron−

hole recombination rate due to the defective polymerization of organic networks, and the rather poor crystallinity.1219 Bandgap engineering of g-C3N4 to control its light harvesting ability and redox potential is essential to enhance its photocatalytic performance. The main strategies to narrow the bandgap or improve the redox ability include elemental doping (such as sulfur, boron, iodine, and/or oxygen doping and incorporation of Fe 3+ , Mn 3+ , Co 3+ , Ni 3+ , and Cu 2+ into the g-C 3 N 4 framework) and molecular doping. Besides bandgap engineering, the construction of g-C3N4−semiconductor and g-C3N4− metal heterojunctions could improve the charge separation. All of them can achieve the significant enhancement of photocatalytic hydrogen evolution activity of g-C3N4. Graphene is another one of the ultrathin 2D materials that has been widely used in photocatalysis. Although a variety of semiconductor photocatalysts have been developed for photocatalytic water splitting, their practical application is limited due to the low efficiency as a result of rapid recombination of photogenerated electrons and holes within photocatalysts. Graphene can be used as an efficient electron acceptor to enhance the photoexcited charge transfer and to inhibit the backward reaction by separation of the evolution sites of hydrogen and oxygen for improved photocatalytic H 2 production activity. In this regard, graphene has been examined in combination with semiconductor photocatalysts, which resulted in improved photocatalytic activity.1220−1222 The combination of graphene with semiconductor photocatalysts can offer two potential benefits. First, due to the high work function of graphene (4.42 eV), photogenerated electrons can transfer from semiconductors to graphene, and thus the recombination of photogenerated charges will be efficiently suppressed. Taking the graphene−CdS composite as an example, a picosecond ultrafast electron transfer process from the excited CdS to the graphene has been reported by Cao et al., using time-resolved fluorescence spectroscopy.1223 This work demonstrated that graphene can act as the electron acceptor to facilitate the charges separation in the photoexcited electron−hole pairs in CdS. Li et al. reported a high efficiency of photocatalytic H2 production in graphene−CdS photocatalyst using Pt as cocatalyst.1224 Photocatalytic hydrogen production experiments under visible light irradiation showed that graphene−CdS composite contents of 1.0 wt % rGO and 0.5 wt % Pt possess a high H2-production rate of 1.12 mmol h−1, which is 4.87 times higher than the that of CdS−Pt catalyst.1224 The high photocatalytic H2-production activity can be predominantly attributed to the presence of graphene that serves as an electron collector and transporter to efficiently 6291

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environments, gas leakage monitoring, diagnosis of diseases, and health care. Ultrathin 2D nanomaterials, for example, graphene and TMDs, have been widely used as novel sensing platforms to construct different sensing systems for the detection of various guest species due to some extraordinary properties, such as excellent conductivity, high specific surface area, high sensitivity to external stimulants, excellent fluorescence quenching ability, and so on. In this section, we focus on the introduction of the utilization of ultrathin 2D nanomaterials as sensing platforms for the development of a number of sensing systems, including electronic sensors, fluorescent sensors, and electrochemical sensors. 6.7.1. Electronic Sensors. The realization of detection in electronic sensors is 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.164 A typical FET device consisted of a semiconducting channel between the drain/source electrodes, in which the conductance of the channel can be modulated by varying the gate voltage through the dielectric layer. Without the gate electrode, the device consisting of a channel material between two electrodes can be considered as a chemiresistor, which possesses a more simplified device structure as compared to the FET device. Upon exposure of the channel material to the guest species, the interaction between guest species and the channel can induce the conductance change in FETs channel or chemiresistors, rendering them promising platforms for sensing applications.164 There are two major sensing mechanisms for electronic sensors, that is, the electrostatic gating effect and the doping effect. In the real sensing applications, both mechanisms might contribute to the sensing effect coupled with other complicated mechanisms.164 One of the obvious advantages of ultrathin 2D nanomaterials, especially the single-layer ones, is that their atomic thickness can ensure the ultimate exposure of their surface atoms to analytes in the detection process, thus promising ultimate sensitivities for given materials, which is unattainable with other forms of nanomaterials or their bulk counterparts. To date, the most widely used channel materials of ultrathin 2D nanomaterials for electronic sensors are graphene and its derivatives. Because we already summarized the research work related to graphene-based electronic sensors,164 here we mainly focus on the introduction of ultrathin 2D nanomaterials beyond graphene for electronic sensors. Many other ultrathin 2D nanomaterials, such as TMDs, BP, and metal oxides, have also been used as channel materials for the fabrication of electronic sensors to detect a wide range of analytes. Zhang and co-workers first demonstrated the utilization of mechanically exfoliated MoS2 nanosheets from single layer to four layers as channel materials to fabricate FET sensors for the detection of NO.291 The absorption of NO on MoS2 surface caused the p-doping effect of the channel of the n-type of MoS2 semiconductor, thus resulting in a current decrease. All of the multilayer MoS2-based FET sensors exhibited stable responses with a detection limit of 0.8 ppm in contrast to the unstable one based on single-layer MoS2.291 Besides mechanically exfoliated MoS2 nanosheets, Zhang and co-workers also demonstrated that the solution-dispersed MoS2 nanosheet prepared by the electrochemical Li intercalationassisted exfoliation method can be used as the channel material for electronic sensors.706 The as-exfoliated MoS2 nanosheet can be easily fabricated as a thin film on a solid substrate (e.g.,

lengthen the lifetime of photogenerated charge carriers from the CdS nanoparticles. Second, because graphene possesses an extremely high conductivity, the accepted electrons can migrate rapidly across its 2D backbone, which serves as an electron highway, to the reactive sites for H2 evolution. Xiang et al. reported a composite material consisting of TiO2 nanocrystal grown on a layered MoS2/graphene hybrid as a high-performance photocatalyst for H2 evolution (Figure 56a,b).1225 The TiO2/MoS2/graphene composite reaches a high H2 production rate of 165.3 μmol h−1 when the content of the MoS2/graphene cocatalyst is 0.5 wt % and the content of graphene in this cocatalyst is 5.0 wt %, and the apparent quantum efficiency reaches 9.7% at 365 nm (Figure 56c). In this case, graphene acted as a conductive electron transport “highway” to transfer the photogenerated electrons in the CB of TiO2 to the MoS2 nanosheet. The electrons then reacted with the adsorbed H+ ions at the edges of MoS2 to form H2. Besides CdS and TiO2, graphene has also been hybridized with other semiconductors to improve their photocatalytic performance, such as NaTaO3,1226 Sr2Ta2O7,1227 BiVO4,1228 ZnS,1229 g-C3N4,1230 and so on. One thing that should be pointed out is that the amount of graphene in photocatalytic systems based on graphene−semiconductor composites should be neither too low nor too high. A too small amount of graphene may be not enough to efficiently disperse the semiconductor photocatalysts, while too much graphene will lead to shielding of the surface active sites, rapidly decreasing the intensity of light through the reaction solution, called a “shielding effect”. GO can be also used as a photocatalyst for H2 evolution from a methanol/water mixture under UV or visible-light irradiation even without using Pt as the cocatalyst. Yeh et al. reported a GO semiconductor with an apparent bandgap of 2.4−4.3 eV by a modified Hummers’ procedure.1231 The bandgap of graphite oxide was dependent on the oxidized level, and was decreased during photocatalytic reaction because of the GO reduction. Regardless of the bandgap reduction, the energy level of the conductive band edge of GO is high enough to supply an overpotential for H2 evolution.1231 The BP nanosheet has also attracted increasing interest due to its high carrier mobility (up to 1000 cm2 V−1 s−1) and distinguished optoelectronic properties. When the BP crystal is exfoliated into BP nanosheets, they may be used for water splitting, because the band gap of BP increases from 0.3 eV (bulk) to 2.1 eV (monolayer).1232 DFT calculation shows that the conduction band of single-layer BP nanosheet is more negative than that of the H+/H2 redcution potential, indicating the possibility of using a single-layer BP nanosheet as a photocatalyst for hydrogen evolution.1233 However, the valence band edge of single-layer BP nanosheet is not more positive than that of the redox potential of O2/H2O.1233 Until now, only the BP@TiO2 hybrid material has shown the enhanced photocatalytic performance under light irradiation for the dye degradation.1234 6.7. Sensing Platforms

The development of simple and reliable sensing systems that are to rapidly detect specific chemical or biological analytes, including gas molecules, toxic ions, chemical molecules, and biomolecules, with high sensitivity, selectivity, and stability in a given environment, is essential in a wide range of real applications. Detection is applicable for toxic substances in 6292

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Figure 57. (a) Schematic illustration of the MoS2-based FET biosensor. (b) Optical image of a MoS2 flake on a SiO2/Si substrate. (c) Optical image of the MoS2-based FET biosensor device. (d) Photograph and scheme (inset figure) of the fabricated chip. Reproduced with permission from ref 707. Copyright 2012 American Chemical Society.

the surface of the dielectric layer covered on the MoS2 channel to capture the streptavidin. After exposure to the streptavidin, the FET sensor showed a significant current decrease due to the gating effect induced by the negative charge of the streptavidin (Figure 57a).707 The FET sensor exhibited excellent sensitivities to both the pH values and the biomolecules. Later, MoS2 nanosheet-based FET sensors have also been fabricated by other groups to detect the prostatespecific antigen.1246,1247 It is expected that more electronic sensors will be fabricated using new ultrathin 2D nanomaterials as channel materials for the detection of various target analytes because abundantly new ultrathin 2D nanomaterials have been synthesized in the past few years.1248 Although excellent sensitivities have been achieved on electronic devices fabricated using ultrathin 2D nanomaterials, there are still some disadvantages on those electronic sensors. One is the lack of good selectivity toward specific target analytes due to their high sensitivity to external stimulants. This issue might be solved by surface modification or sieving a thin layer coating of the channel 2D nanomaterials. MOFs have been widely used as functional materials for gas separation due to their ability to selectively sieve different gas molecules enabled by their well-defined pore sizes. Therefore, it is expected to significantly enhance the selectivity of FET sensors for gas molecules by coating a thin MOF layer on the channel 2D nanomaterial without compromise of its sensitivity because the MOF layer can selectively sieve gas molecules with a diameter less than its pore size. Another possible way to enhance the selectivity for electronic devices is to modify the channel surface with specific functional groups and thus to strengthen the specific interaction between the target analytes to the channel material, thus achieving enhanced selectivity. Another obvious disadvantage of electronic sensors based on ultrathin 2D nanomaterials is lack of long-term stability due to

SiO2/Si) and then used as the channel material to fabricate a chemiresistor for NO detection. The resultant sensor gave a low detection limit of 190 ppt.706 The as-exfoliated MoS2 nanosheet can be easily coated as a thin film on a flexible polyethylene terephthalate (PET) substrate to fabricate a flexible gas sensor for NO2 detection, in which rGO thin films were used as flexible electrodes. The sensitivity of the sensor device can be further increased by decoration of Pt nanoparticles on the surface of the MoS2 thin film, yielding a low detection limit of 2 ppb for NO2 detection. Kalantar-Zadeh and co-workers reported a highly selective electronic sensor based on the SnS2 flake for NO2 detection.1235 The high selectivity of the SnS2-based sensor to NO2 can be attributed to the strong affinity and favorable position of the partially occupied molecular orbital of NO2 and Fermi level of SnS2. This new mechanism might open a new avenue for the fabrication of highly sensitive and selective electronic sensors for gas molecules based on ultrathin 2D nanomaterials. Nowadays, the construction of electronic sensors for gas detection has been achieved on many other ultrathin 2D nanomaterials, such as WS2,1236−1238 MoSe2,1239 BP,167,1240 WO3,1241 ZnO,1242,1243 and NiO.1244 We would like to refer our reader to the recent review paper focusing on gas sensors-based ultrathin 2D nanomaterials beyond graphene for details.1245 Besides gas detection, electronic sensors based on MoS2 nanosheets have also been fabricated for the detection of biomolecules. Banerje and co-workers have reported the fabrication of a FET sensor by using mechanically exfoliated MoS2 nanosheet as the channel material for pH and biomolecule detection (Figure 57).707 The fabricated FET sensor gave obvious current changes when pH values diverge due to the surface charge changing induced by the protonation/ deprotonation of the OH groups on the gate dielectric. Note that for the biomolecule detection, biotin was used to modify 6293

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other ultrathin 2D nanomaterials as fluorescence quenchers. As a typical example, Zhang and co-workers first reported the development of a single-layer MoS2-based fluorescent sensor for biomolecule detection.1252 As shown in Figure 58a, the

the low stability of those ultrathin 2D nanomaterials induced by the surface oxidation and moisture absorption. One possible way to solve this issue is to coat some stable thin layer materials (e.g., metal oxides and polymers) on the surface of the channel 2D nanomaterials to block the ultrathin 2D nanomaterials from the ambient conditions and thus to enhance the stability of ultrathin 2D nanomaterials. 6.7.2. Fluorescent Sensors. The detection capability in fluorescent sensors is based on the fluorescence responses, that is, enhancement or quenching of fluorescent sensors, induced by the interaction between target analytes and fluorescent probes. Most of the reported ultrathin 2D nanomaterials, such as graphene, TMDs, h-BN, and LDHs, normally have none or negligible fluorescence emission themselves, rendering them unable to be directly used as fluorescent probes for sensing applications. Nevertheless, these ultrathin 2D nanomaterials are efficient fluorescent quenchers to fluorescent dyes. Note that the high specific surface area and extreme atom exposure from ultrathin dimenssion enable them ultimate surface interaction with fluorescent dye molecules, thus yielding a quenching efficiency superior to those of other morphologies or their bulk counterparts. Because of the excellent fluorescence quenching ability and selectivities, some ultrathin 2D nanomaterials, such as GO, TMDs, Ta2NiS5, g-C3N4, and MOFs, have been widely used as sensing platforms to develop turn-on fluorescent sensors for selective and sensitive detection of various target analytes, for example, DNA. As a typical example, Yang and coworkers first reported the construction of a fluorescent sensor based on GO nanosheet for the detection of biomolecules, including DNA and proteins.160 The principle of the designed sensor is quite simple.160 The dye-labeled ssDNA was first mixed with the GO nanosheet, where the dye-labeled ssDNA was absorbed on the GO surface and its fluorescence was quenched by the GO nanosheet due to the strong noncovalent interaction. After that, the target DNA was added to the system. Because of the strong affinity between the dye-labeled ssDNA and the target DNA, the dye-labeled ssDNA spontaneously hybridized with the target DNA. The hybridization resulted in the release of the dye-labeled ssDNA from the GO nanosheet, leading to restoration of the fluorescence of the dye molecule. The fluorescence enhancement can be used for quantitative detection of the target DNA. This design is based on the target hybridization-induced probe liberation. The resultant fluorescent sensor exhibited good sensitivity and selectivity. A similar design was used to construct a fluorescent sensor for protein detection, which exhibited a detection limit as low as 2 nM with good selectivity. In contrast, Fan and co-workers slightly changed the design of fluorescent sensors based on the GO nanosheet.161 During the detection process, the target DNA was first hybridized with the dye-labeled ssDNA and then mixed with the GO nanosheet, which is based on the discrimination ability of GO to the ssDNA and dsDNA. On the basis of this design, the resultant sensor gave excellent selectivity and sensitivity for the DNA detection with a detection limit of 100 pM, lower than that of the previous design. Multiplexed DNA detection toward multiple DNA targets in the same solution was also achieved in this study. Later, the GO nanosheet was also used as the sensing platform to develop fluorescent sensors via similar strategies for selective and sensitive detection of various analytes, including nucleic acids, proteins, metal ions, and small molecules.1249−1251 Inspired by the success on GO-based fluorescent sensors, fluorescent sensors have now been constructed using many

Figure 58. (a) Schematic illustration of the single-layer MoS2-based fluorescent DNA sensor. (b) Fluorescence spectra of proble DNA (P1) and proble DNA and target DNA (P1/T1) duplex with and without MoS2. (c) Fluorescence spectra of P1 with addition of different concentrations of T1. (d) Calibration curve for DNA detection. Reproduced with permission from ref 1252. Copyright 2013 American Chemical Society.

basic idea is similar to that of the GO-based sensor proposed by Fan and co-workers,161 which is based on the discrimination ability of MoS2 toward the ssDNA and dsDNA. The resultant sensor exhibited good selectivity and sensitivity for DNA detection, yielding a detection limit of 500 pM (Figure 58b−d). Later, Zhang and co-workers reported the systematic study and comparison of single-layer MoS2, TiS2, and TaS2 as sensing platforms for constructing fluorescent sensors for DNA detection.1253 The results revealed that the TaS2-based sensor gave the best performance and a lower sensitivity of 0.05 nM as compared to sensors based on MoS2 (0.1 nM) and TiS2 (0.2 nM) at the same conditions. The multiplexed fluorescent sensor was also developed on the basis of the single-layer TaS2 for the multiplexed DNA detection. Fluorescent sensors have been also developed on the basis of ultrathin 2D nanomaterials of MoS2,1254−1256 WS2,1257,1258 Ta2NiS5,389 g-C3N4,1259,1260 and MnO21261 for selective and sensitive detection of a number of target species, including DNA, proteins (e.g., prostate specific antigen), metal ions (e.g., Ag+), and small molecules (e.g., 2,4,6trinitrophenoland ascorbic acid). Besides ultrathin 2D inorganic nanomaterials, Zhang and co-workers further extended the concept of construction of fluorescent sensors based on organic−inorganic 2D MOF nanosheets.274 It was found that, similar to graphene and TMDs, 2D MOF nanosheets also exhibited excellent quenching ability to the probe DNA and thus could be used as sensing platforms to fabricate fluorescent sensors for DNA detection. All of the 2D MOF-based sensors exhibited good selectivity and sensitivity, among which the 2D Cu-TCPP nanosheet-based sensor gave the lowest detection limit of 20 pM. It is anticipated that the fluorescent sensor design can be further extended to some new ultrathin 2D nanomaterials, such as COFs, MXenes, BP, and polymers. 6294

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To highlight, there are several advantages of fluorescent sensors based on ultrathin 2D nanomaterials. First, all of the reported fluorescent sensors are constructed on the basis of solution-processed ultrathin 2D nanomaterials that can be produced in high yield and large amount to reduce the fabrication cost. Second, the detection mechanism for fluorescent sensors is quite simple and fast; the reaction finishes within minutes. Third, multiple target analytes can be detected in the same solution. Last, ultrathin 2D nanomaterialbased fluorescent sensors generally exhibited superior sensitivity as compared to that of fluorescent sensors based on other types, due to 2D structural feature-induced superior fluorescence quenching efficiency. Having mentioned the above, there are also several disadvantages of ultrathin 2D nanomaterial-based fluorescent sensors. One is the complex preparation of the dye-labeled sDNA, that is, the probe DNA; thus, dyelabeled sDNA is quite expensive as are its derived fluorescent sensors. Another disadvantage is that fluorescent sensors normally have relatively low long-term stability due to the relatively low stability of fluorescent dyes labeled on DNA. 6.7.3. Electrochemical Sensors. Electrochemical sensors have been used for decades for the detection of various kinds of chemical species because of their simplicity, low cost, good sensitivity, and selectivity.1262 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. Three kinds of signals can be generated from three different techniques, for example, cyclic voltammetry, amperometry, and potentiometry. Electrochemical activity, specific surface area, and electrical conductivity are major key factors of a material to determine its performance as the electrode material in electrochemical sensors. 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.1263,1264 Ultrathin 2D nanomaterials have also been widely used as electrode materials for electrochemical sensors because of their good electrical conductivity, superior electrochemical properties, and extremely high surface to volume ratios.165,1265 Graphene and its derivatives, for example, rGO, are the most widely used ultrathin 2D nanomaterials for electrochemical sensors. Because we already summarized the research work related to graphenebased electrochemical sensors,165 we mainly focus on the introduction of ultrathin 2D nanomaterials beyond graphene for electrochemical sensors in this section. Besides graphene, many other ultrathin 2D nanomaterials, such as MoS2, WS2, SnS2, FeS2, and Ni3S2, have been also used to fabricate electrodes in electrochemical sensors for the detection of various target analytes.1248 As a typical example, Zhang and co-workers first demonstrated the use of single-layer MoS2 nanosheet as electrode material in the electrochemical sensor for the detection of glucose and dopamine.166 The MoS2 or chitosan-MoS2 was first modified on the glass carbon electrode (GCE) and then electrochemically reduced to form reduced MoS2 (rMoS2). The fabricated electrode was used as a sensor to detect glucose based on the CV signal. The fabricated electrode can also be used for selective detection of dopamine in the presence of ascorbic acid and uric acid, based on the signal responses of differential pulse voltammograms. It should be pointed that pure TMD nanosheets normally did not show excellent sensing performance due to their relatively low

electrical conductivity. The most commonly used strategy to further enhance the electrochemical sensing performance is to decorate noble metal nanoparticles, such as Au, Ag, Pt, and Pd, on their surface to form hybrid nanosheets, exhibiting enhanced sensitivity as compared to that of pure TMD nanosheets.1248 This principle is applicable to other ultrathin 2D nanomaterials, which do not have excellent electrical conductivity for electrochemical sensors. Because of their excellent electrical conductivity, good electrochemical property, and high surface area, MXenes have been also utilized as electrode materials to develop electrochemical sensors for the detection of a number target analytes. Zhu and co-workers reported the use of Ti3C2 nanosheet, one of the typical MXenes, as the electrode material to construct an electrochemical biosensor.1266 The Ti3C2 nanosheet-based sensor exhibited good performance for the nitrite detection with a detection limit as low as 0.12 μM. Besides ultrathin 2D inorganic nanomaterials, Zhang and coworkers reported the utilization of 2D MOF nanosheets as electrode materials in electrochemical sensors for H2O2 detection.1267 These M-TCPP(Fe) MOF nanosheets (M = Co, Cu, and Zn) were assembled as thin films on electrodes via a Langmuir−Schäfer method (Figure 59a). The electrodes

Figure 59. (a) Scheme showing the assembly process for the electrode fabrication based on the 2D MOF nanosheet. (b) Typical amperometric response of electrode based on 2D Co-TCPP(Fe) nanosheet and bare GCE with successive addition of H2O2. (c) Amperometric responses of electrode based on 2D Co-TCPP(Fe) nanosheet with the addition of 10 μM fMLP and 300 U mL−1 catalase in the absence (black curve) and presence (red curve) of MDA cells. The inset is the bright-field microscopy image of MDA cells. Reproduced with permission from ref 1267. Copyright 2016 John Wiley & Sons, Inc.

based on 2D MOF nanosheets exhibited good catalytic activity toward the reduction of H2O2 and thus were used for detection of H2O2 (Figure 59b), in which the Co-TCPP(Fe) nanosheetbased electrode gave a low detection limit of 0.15 × 10−6 M. The real-time tracking of H2O2 that was secreted by live cells was also achieved on the electrochemical sensor based on the 2D Co-TCPP(Fe) nanosheet (Figure 59c). It is anticipated that more electrochemical sensors will be developed on the basis of new ultrathin 2D nanomaterials in the near future. It should be pointed out that there are several disadvantages of electrochemical sensors using ultrathin 2D nanomaterials as electrode materials. First, although a lot of ultrathin 2D 6295

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applications. However, a controlled synthesis of ultrathin 2D nanomaterials with desirable structures is still difficult to realize by most current well-developed methods. Therefore, the preparation of ultrathin 2D nanomaterials with desired structural characteristics in a highly controllable manner is still one of the challenges in this field. Second, from the characterization point of view, understanding the growth mechanism of ultrathin 2D nanomaterials is critically important, but it is not easy. Therefore, another challenge in this field lies in the identification or development of effective characterization techniques to explore the growth mechanism of ultrathin 2D nanomaterials. Promisingly, some in situ characterization techniques have been developed in recent years, such as the in situ TEM, in situ XPS, and in situ Raman spectroscopy, with potential relevance for studying the growth mechanism of ultrathin 2D nanomaterials. Third, from the application point of view, most current ultrathin 2D nanomaterials lack the longterm stability and durability due to their relatively low physical and/or chemical stability, thus restricting their potential applications. Their relatively low stability mainly arises from the following situations: (1) Ultrathin 2D nanomaterials dispersed in liquid solution cannot be stored for long time. High tendency to irreversible aggregation leads to the significant loss of advances arising from their 2D structural features. (2) Most of the ultrathin 2D nanomaterials are easy to oxidize in ambient conditions, resulting in the structural decomposition/degredation. (3) Structural change, decomposition, or collapse may occur during the chemical reaction in applications, such as electrocatalysis and LIBs. Therefore, it is clear that the stability of 2D nanomaterials is critical, and required to be maintained not only during their storage and processing but also in applications. Hence, one of the most critical challenges in this field lies in the exploration of simple but reliable methods to stabilize these ultrathin 2D nanomaterials to dramatically prolong their stability. The current research on ultrathin 2D nanomaterials, especially those beyond graphene, is far from mature despite the fact that great progress has been made in this exciting field. First, because ultrathin 2D nanomaterials are defined from their dimensionality rather than material compositions, we believe that any kind of ultrathin 2D nanomaterials could be prepared if their growth can be confined into two dimensions and down to single- or few-atomic layers using proper experimental conditions. As a result, the most straightforward idea is to use those well-developed methods or develop new ways to prepare new ultrathin 2D nanomaterials with varying single-composition or multicompositions, which can be expected to exhibit new properties and functionalities. The well-developed exfoliation methods, such as the micromechanical cleavage, sonication-assisted liquid exfoliation, and ion intercalationassisted liquid exfoliation, might be also used to exfoliate new layered compounds into single- or few-layer nanosheets. Second, it is worth pointing out that each material has its own disadvantages. The most straightforward way to overcome the drawback of a material is to hybridize it with other materials to form composites or heteronanostructures. Graphene has been widely used as a highly conductive matrix to hybridize with other low conductive materials, such as TMDs and metal oxides, to enhance their electrical conductivity, thus optimizing their performance in some specific applications (e.g., electrocatalysis, LIBs, etc.).427−430 More intriguingly, the synergistic effect between different components could bring some new appealing properties or functionalities. It has been reported that

nanomaterials have been used as electrode materials for electrochemical sensors, the relatively low electrical conductivity of most 2D nanomaterials normally leads to a relatively low detection limit. A common way to solve this issue is to grow noble metal nanoparticles on the surface of these ultrathin 2D nanomaterials to enhance their electrical conductivity and electrochemical activity, giving rise to enhanced sensing performance. However, the use of noble metal nanoparticles will increase the overall cost of fabricated sensors. For 2D TMD nanomaterials, another method to enhance electrical conductivity is engineering the crystal phase of TMDs. Metallic 1T phase TMD nanosheets have been proven to generate higher electrical conductivity as compared to the 2H phase nanosheets. Another disadvantage is that electrochemical sensors by using ultrathin 2D nanomaterials as electrode materials normally have low long-term stability because most of the ultrathin 2D nanomaterials have relatively low stability due to their ultrathin nature.

7. CONCLUDING REMARKS AND OUTLOOK In the past decade, explosive progress has been achieved in the research area of ultrathin 2D nanomaterials from fundamental study to the development of next generation technology. Without a doubt, this great stride has revolutionized the unique role of dimensionality in determining the nanomaterials’ intrinsic properties as well as their wide potential applications. In this Review, we categorized the recent progress from diverse aspects, that is, the composition and crystal structures, synthetic methods, characterization techniques, and promising potential applications. Moving on from the graphene, ultrathin 2D nanomaterials have become a new class of nanomaterials, allowing researchers to choose suitable materials with desirable properties for required applications at will to some extent. These ultrathin 2D nanomaterials can be prepared via wide ranges of well-developed synthetic methods, which have their own advantages and limitations. Of particular interest is that different synthetic methods can be used to prepare ultrathin 2D nanomaterials with varying structural features including size, thickness, crystallinity, crystal phase, defect, doping, strain, and surface property, which are favorable for various applications. With combination of appropriate characterization techniques, the composition and structural information on these ultrathin 2D nanomaterials can be clearly visualized down to the atomic level, which are useful for understanding the correlations between structures and properties. Their unprecedent properties, due to their ultrathin 2D structural characteristics, have been explored for a variety of applications, and with some ultrathin 2D nanomaterials showed excellent performance with the potential to replace the current commercial sources/ technologies. Their performance in applications has been demonstrated to be a function of their precisely engineered structural features. Extensive exploration in the field of ultrathin 2D nanomaterials also brings new challenges. First, from the material synthesis point of view, the current production yield, quality, quantity, and production rate of ultrathin 2D nanomaterials are still far from the criteria that are required for industry or commercialization. Therefore, one of the major challenges in this hot field is to realize the high-yield and massive production of ultrathin 2D nanomaterials to meet the industry requirements. The physical, chemical, and electronic properties of ultrathin 2D nanomaterials are highly dependent on their structural features, which determine their performances in their 6296

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both the cobalt metal nanosheets and the cobalt oxide nanosheets exhibited low electrocatalytic activities, but their composite gave rise to excellent activity toward the electrocatalytic CO2 reduction.526 Bearing this in mind, one of the promising research directions in the near future is to construct hybrid nanomaterials or well-defined heteronanostructures by using ultrathin 2D nanomaterials as building blocks, thus further optimizing their properties and/or functionalities. Third, the research on the preparation of materials is always prior to the exploration of their potential applications. Although many kinds of ultrathin 2D nanomaterials have been prepared in recent years, the potential applications of most of them still remain to be explored. For example, the excellent catalytic activity of MoS2 toward the electrochemical HER was only identified about 10 years ago,638,1268 despite that the research on MoS2 can be dated back decades ago.60,61 Given that these newly synthesized ultrathin 2D nanomaterials potentially have excellent performance for some unexplored applications, another promising future direction is to identify the most suitable application for each ultrathin 2D nanomaterial.

Qiyuan He received his B.S. degree in Applied Chemistry from Wuhan University (China) in 2005, and then obtained his M.S. degree in Polymer Science under the supervision of Prof. Wei Huang in Fudan University (China) in 2008. He completed his Ph.D. degree under the supervision of Prof. Hua Zhang at School of Materials Science and Engineering in Nanyang Technological University (Singapore) in 2012. He joined Prof. Xiangfeng Duan’s group at UCLA as a Research Fellow in 2013. Since 2016, he has been working as a Research Fellow in Prof. Hua Zhang’s group at Nanyang Technological University. His research interests include nanoelectronics and thin-film electronics, and their applications in next-generation electronics, flexible electronics, chemical and biological sensors, energy, etc. Jian Yang obtained his B.S. degree from Fudan University (China) in 2012. He is currently pursuing his Ph.D. under the supervision of Professor Hua Zhang at the School of Materials Science and Engineering, Nanyang Technological University (Singapore). His research focuses on the electronics of layered semiconductors, such as molybdenum disulfide, black phosphorus, etc., and their heterostructures. Xiao Zhang received his B.E. and M.E. degrees at Harbin Engineering University (China) in 2010 and 2013, respectively. He then moved to the Interdisciplinary Graduate School of Nanyang Technological University in Singapore as a Ph.D. student under the supervision of Prof. Hua Zhang (2013). His research interest focuses on the synthesis and applications of novel two-dimensional nanomaterials.

AUTHOR INFORMATION Corresponding Author

*Fax: (+65) 67909081. E-mail: [email protected]. ORCID

Junze Chen received his B.E. and M.E. degrees from Southwest Jiaotong University (China) in 2009 and Sichuan University (China) in 2012, respectively. Currently, he is a Ph.D. student at the School of Materials Science and Engineering of Nanyang Technological University (Singapore) under the supervision of Prof. Hua Zhang. His research interest focuses on the synthesis of colloidal nanocrystals and hierarchical nanostructures, and the study of their applications in photonics and energy.

Hua Zhang: 0000-0001-9518-740X Notes

The authors declare no competing financial interest. Biographies Chaoliang Tan received his B.E. and M.E. degrees in Applied Chemistry from Hunan University of Science and Technology (China) in 2009 and South China Normal University (China) in 2012, respectively. He then moved to the School of Materials Science and Engineering of Nanyang Technological University (Singapore) where he completed his Ph.D. study under the supervision of Professor Hua Zhang in 2016. Since then, he has been working a Research Follow in the same group. His research interests focus on the synthesis, characterization, and applications of ultrathin two-dimensional nanomaterials (e.g., transition metal dichalcogenides) and their functional heterostructures/composites.

Wei Zhao obtained his B.E. degree in Material Physics and Chemistry at Wuhan University (China) in 2009. He then completed his Ph.D. under the supervision of Prof. Fuqiang Huang at Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), in 2014. From 2014 to 2016, he worked as a Research Fellow in Prof. Hua Zhang’s group. He currently holds a position at SICCAS, working on lowdimensional nanomaterials for solar energy conversion and storage. Shikui Han received his Ph.D. degree at the University of Science and Technology of China in 2013. Since 2013, he has been working as a Research Fellow at the School of Materials Science and Engineering of Nanyang Technological University (Singapore) in Professor Hua Zhang’s group. His research interests focus on the wet-chemical synthesis and applications of two-dimensional nanosheets (e.g., transition metal dichalcogenides and transition metal oxides) and their heteronanostructures.

Xiehong Cao received his B.E. degree in Polymer Materials and Engineering from Zhejiang University (China) in 2008, and then completed his Ph.D. under the supervision of Prof. Hua Zhang at the School of Materials Science and Engineering in Nanyang Technological University in Singapore in 2012. After working as a Research Fellow in the same group, he joined Zhejiang University of Technology (China) in 2015. His current research interests include the synthesis of two-dimensional nanomaterials (e.g., graphene and transition metal dichalcogenides) and their applications in energy, environment, sensors, etc.

Gwang-Hyeon Nam received a B.S. degree (2008) and M.S. degree (2010) from Ajou University (South Korea). Currently, he is a Ph.D. student at Nanyang Institute of Technology in Health and Medicine of Nanyang Technological University (Singapore) under the supervision of Professor Hua Zhang. His research interests focus on nanoelectronics and electronic skins based on two-dimensional materials.

Xue-Jun Wu received his B.S. in Applied Chemistry from Wuhan University of Technology (China) in 2005 and his Ph.D. in Physical Chemistry from Peking University (China) in 2010. Since 2010, he has been working as a Research Fellow at the School of Materials Science and Engineering in Nanyang Technological University (Singapore) in Prof. Hua Zhang’s group. His research interests include the synthesis of colloidal nanocrystals and hierarchical nanostructures, and the study of their applications in photonics and energy.

Melinda Sindoro received her B.S. in Chemistry and Biological Chemistry from Nanyang Technological University (Singapore) in 2010. After she completed her Ph.D. in Materials Chemistry with focus on the metal−organic frameworks at the University of Illinois at Urbana−Champaign (U.S.) in 2016, she joined the School of Materials Science and Engineering at Nanyang Technological 6297

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University (Singapore) as a Research Fellow, working with Professor Hua Zhang on novel nanomaterials. Prof. Hua Zhang obtained his B.S. and M.S. degrees from Nanjing University (China) in 1992 and 1995, respectively, and completed his Ph.D. study under the supervision of Professor Zhongfan Liu at Peking University (China) in 1998. As a Postdoctoral Fellow, he joined Prof. Frans C. De Schryver’s group at Katholieke Universiteit Leuven (Belgium) in 1999, and then moved to Prof. Chad A. Mirkin’s group at Northwestern University (U.S.) in 2001. After he worked at NanoInk Inc. (U.S.) and the Institute of Bioengineering and Nanotechnology (Singapore), he joined Nanyang Technological University (Singapore) in July 2006. His current research interests focus on the synthesis of ultrathin two-dimensional nanomaterials (e.g., metal nanosheets, graphene, metal dichalcogenides, metal−organic frameworks, etc.) and their hybrid composites for various applications in nano- and biosensors, clean energy, (opto-)electronic devices, catalysis, and water remediation; controlled synthesis, characterization and application of novel metallic and semiconducting nanomaterials, and complex heterostructures; etc.

ACKNOWLEDGMENTS This work was supported by the MOE under AcRF Tier 2 (ARC 19/15, no. MOE2014-T2-2-093; MOE2015-T2-2-057), NTU under Start-Up Grant (M4081296.070.500000) and iFood Research Grant (M4081458.070.500000), and the Singapore Millennium Foundation in Singapore. REFERENCES (1) Zhang, H. Ultrathin Two-Dimensional Nanomaterials. ACS Nano 2015, 9, 9451−9469. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (3) Geim, A. K.; Novoselov, K. S. The Rising of Graphene. Nat. Mater. 2007, 6, 183−191. (4) Zhang, Y.; Tan, Y.-W.; Stormer, H. L.; Kim, P. Experimental Observation of the Quantum Hall Effect and Berry’s Phase in Graphene. Nature 2005, 438, 201−204. (5) Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R. S. GrapheneBased Ultracapacitors. Nano Lett. 2008, 8, 3498−3502. (6) Lee, C.; Wei, X. D.; Kysar, J. W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385−388. (7) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Fine Structure Constant Defines Visual Transparency of Graphene. Science 2008, 320, 1308− 1308. (8) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior Thermal Conductivity of Single-Layer Graphene. Nano Lett. 2008, 8, 902−907. (9) Xu, M. S.; Liang, T.; Shi, M. M.; Chen, H. Z. Graphene-Like Two-Dimensional Materials. Chem. Rev. 2013, 113, 3766−3798. (10) Gupta, A.; Sakthivel, T.; Seal, S. Recent Development in 2D Materials Beyond Graphene. Prog. Mater. Sci. 2015, 73, 44−126. (11) Ferrari, A. C.; Bonaccorso, F.; Fal’ko, V.; Novoselov, K. S.; Roche, S.; Bøggild, P.; Borini, S.; Koppens, F. H. L.; Palermo, V.; Pugno, N.; et al. Science and Technology Roadmap for Graphene, Related Two-Dimensional Crystals, and Hybrid Systems. Nanoscale 2015, 7, 4598−4810. (12) Lin, Y.; Williams, T. V.; Connell, J. W. Soluble, Exfoliated Hexagonal Boron Nitride Nanosheets. J. Phys. Chem. Lett. 2010, 1, 277−283. (13) Weng, Q.; Wang, X.; Wang, X.; Bando, Y.; Golberg, D. Functionalized Hexagonal Boron Nitride Nanomaterials: Emerging Properties and Applications. Chem. Soc. Rev. 2016, 45, 3989−4012. 6298

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