Recent Advances in the Solution-Based Preparation of Two

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Cite This: Chem. Rev. 2018, 118, 6151−6188

Recent Advances in the Solution-Based Preparation of TwoDimensional Layered Transition Metal Chalcogenide Nanostructures Jae Hyo Han,†,‡,§ Minkyoung Kwak,†,‡,§ Youngsoo Kim,†,‡ and Jinwoo Cheon*,†,‡,§ †

Center for Nanomedicine, Institute for Basic Science (IBS), Seoul 03722, Republic of Korea Yonsei-IBS Institute and §Department of Chemistry, Yonsei University, Seoul 03722, Republic of Korea

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‡

ABSTRACT: The precise control in size/thickness, composition, crystal phases, doping, defects, and surface properties of two-dimensional (2D) layered transition metal chalcogenide (TMC) is important for the investigation of interwoven relationship between structures, functions, and practical applications. Of the multiple synthetic routes, solution-based top-down and bottom-up chemical methods have been uniquely important for their potential to control the size and composition at the molecular level in addition to their scalability, competitive production cost, and solution processability. Here, we introduce an overview of the recent advances in the solution-based preparation routes of 2D layered TMC nanostructures along with important scientific developments.

CONTENTS 1. Introduction 2. Overview of Top-down Solution-Based Exfoliation Strategies 2.1. Solvent/Surfactant-Assisted Exfoliation 2.1.1. Sonication-Assisted Exfoliation in Solution 2.1.2. Size Sorting of Exfoliated Nanosheets 2.2. Molecular Intercalation/Exfoliation Methods 2.2.1. Intercalation of Alkali Metals and Gaseous Molecules for Exfoliation 2.2.2. Intercalation of Interlayer-Spacing-Tunable Organic Molecules for Exfoliation 2.2.3. Intercalation for Inorganic/Organic Hybrid Composite Nanostructures 3. Overview of Bottom-up Chemical Synthetic Strategies 3.1. Synthetic Methods for 2D Layered TMC Nanosheets 3.1.1. Hot Injection Method 3.1.2. One-Pot Heat-up Method 3.1.3. Hydro/Solvothermal Method 3.1.4. Template-Assisted Method 3.2. Methodologies for Controlling the Size/ Thickness, Crystal Geometry, and Chemical Composition 3.2.1. Control of the Lateral Size and Thickness 3.2.2. Control of the Crystal Geometry 3.2.3. Control of the Chemical Composition in Alloys 3.2.4. Substitutional Doping 4. 2D Layered TMC-Based Multicomponent Heterostructures © 2018 American Chemical Society

4.1. TMCs−Metal−Chalcogenides/Oxides 4.2. TMCs−Noble Metals 4.3. TMCs−Carbonaceous Materials 4.4. TMCs−CNT/Graphene 5. Summary and Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References

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1. INTRODUCTION Two-dimensional (2D) layered transition metal chalcogenide (TMC) nanostructures are emerging rapidly originated from their exceptional chemical and physical properties that are not present in other nanostructures including graphene. With a typical chemical formula of MX2 (M = transition metals of groups IV−X and X = chalcogen), TMCs exhibit a variety of anisotropic characteristics ranging from electron mobility to magnetism, intercalation, catalytic, and optical properties.1−5 Within each layer of TMC nanostructures, six chalcogen atoms are covalently bonded to a transition metal center while individual layers intercoupled by van der Waals interactions (vdWs). Two different crystallographic geometries exist as in octahedral (1T) and trigonal prismatic (2H) forms based on their atomic

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Received: April 25, 2018 Published: June 21, 2018 6151

DOI: 10.1021/acs.chemrev.8b00264 Chem. Rev. 2018, 118, 6151−6188

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Figure 1. Solution-based preparation of 2D layered transition metal chalcogenide (TMC) nanosheets based on top-down and bottom-up approaches. (a) Unique characteristics of 2D layered TMC nanostructures. Within each layer of TMC nanostructures, metal and chalcogen atoms are covalently bonded, whereas the individual layers are intercoupled by van der Waals (vdWs) interaction. (b) Solution-based syntheses of freestanding 2D layered TMC nanosheets with the desired size/thickness and chemical compositions with high dispersity.

expand the scope of their practical usability. To date, conventional synthetic pathways including chemical vapor deposition (CVD), mechanical/chemical exfoliations, and bottom-up chemical synthetic approaches have been explored for synthesizing 2D TMC nanostructures. Notably, the solutionbased synthetic routes of sonication-assisted liquid-phase exfoliation, intercalation chemistry, and colloidal synthesis have progressed rapidly within the past decade and been spun off to large-scale low-cost production and colloidal processing in various substrates with the ability to control of the size and composition (Figure 1b).14−17 Several review papers have already been devoted to this research area;18−31 especially, those published in 2016 and 2017 have emphasized on the doped 2D TMC nanostructures,18 TMC nanostructures for energy storages and conversion,21,30 MoS2 chemistry and its applications,25 and exfoliation-based solution processing of 2D TMCs,31 general aspects of ligand engineering and self-assembly of nanocrystal platelets but with limited coverage of layered TMCs,19,20 and the specific properties/applications associated

configuration in the individual slab. Unique structural features, including a high surface-to-volume ratio, unsaturated coordination at edge, and open structure make 2D TMCs sensitive to their chemical environments (Figure 1a). In addition, 2D TMCs have versatile physicochemical properties; for instance, their electronic structures range from metallic to semimetallic or semiconducting subjective to their compositional/structural combination and atomic coordination. For example, MoS2 nanosheets are known for their exceptional performance in optoelectronics, valleytronics, and electrochemical energy storage.6−10 Moreover, nanoscale confinement, such as singlesheet formation or lateral size reduction, has led to new scientific understanding and interesting potential applications.11−13 From the synthetic perspective, the ability to modulate the size and thickness of these materials allows one to identify new phenomena associated with their electronic/optical/catalytic properties. In addition, synthetic methods for defect engineering, crystal geometry control, alloying, doping, and heterostructure formation are essential for tuning the properties of TMCs to 6152


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with its morphology and crystallographic features.29 While these progressive enterprises on solution-based preparation of 2D TMC nanostructures have clearly proven the benefits of tuning properties and improving performance, the accounts on their preparation strategies and growth mechanisms with chemistry perspectives have not been adequate to fully encompass current advances and developments. Therefore, it is of significance to offer a comprehensive review of solution-based preparation strategies of 2D layered TMC nanostructures. In this review, we overview the recent advances in the solution-based preparation of 2D layered TMC nanosheets as effective protocols for singleto few-layered nanosheets with control of their chemical compositions, crystal geometries, and heterostructures. We first discuss the solution-based top-down approaches for exfoliation and inorganic−organic hybrid nanostructures. Next, the bottomup approach is highlighted with a specific focus on hot-injection, heat-up, hydrothermal, and template-assisted methods. Finally, outlook for current challenges and new directions of the field is provided.

and inverse gas chromatography measurements, which indicated that group IV TMCs possess surface energies in the range of 65− 75 mJ/m2, and a series of solvents with similar surface energies were surveyed to identify a good solvent. According to their free energy of mixing model, the energy cost for exfoliation was at a minimum when the solvents had similar surface energy to that of 2D TMCs.40−42 Along the trial-and-error-based solvent screening process, the experimental parameters of the starting mass and sonication power/time were varied to understand the relationship between them.43−45 The concentration of exfoliated nanosheets scaled linearly as a function of initial mass and sonication time; however, extensive sonication produced high heterogeneity in size and defect-rich nanoflakes, which are obviously detrimental for certain applications. Recently, there have been a number of efforts to rationalize the experimental data, and the HSP have been adopted as an explanation.46 However, there were significant case-by-case deviations to determine the ideal exfoliation solvent in a predictable manner. The Owens, Wendt, Rabel, and Kaelble (OWRK) theory with Young’s equation has been rationalized to explain the explicit relations between the material and the solvent and improve the yield and monodispersity of the exfoliated flakes. Ajayan et al. derived the dispersive component ratios of 2D materials and obtained values of 0.471 for graphene, 0.563 for WS2, 0.449 for MoS2, and 0.450 for h-BN.47 The exfoliation efficiency was enhanced according to the component ratio, as the polar-to-dispersion ratio of the surface tension of a mixed solvent (two solvents mixed with a volume fraction that ideally matches the polar and dispersive components) matches that of the targeted 2D TMCs. For example, a solution of 45% (v/v) ethanol in water was effective for exfoliating MoS2 (Figure 2a and 2b), whereas 35% (v/v) was

2. OVERVIEW OF TOP-DOWN SOLUTION-BASED EXFOLIATION STRATEGIES Solution-based top-down approaches for exfoliation, organic− inorganic heterostructure, crystal geometry control, and colloidal processing typically include sonication-assisted liquid-phase exfoliation, guest molecule intercalation, and surface reactions. The merit of the solution-based top-down approach is the ability to produce single- to few-layered 2D TMCs in large scale. The chemistry behind these protocols initially comprises solubility parameter screening for matching of the material−solvent dispersive energy, intercalation of reactive alkali metal reagents via solvothermal or electrochemical routes, and in situ H2 gas generation. More recently, by employing the principles of molecular intercalation chemistry, a novel exfoliation strategy was developed under mild conditions to produce nano- (nm) to micrometer (μm)-sized single-layer nanosheets, which are crucial for establishing high crystallinity and controllability in the lateral dimensions. The production of a few layered nanosheets enriched dispersion can be incorporated into cutting-edge devices. For example, the inkjet printing process has been demonstrated recently to provide a high-speed device manufacturing process for transistors, photodetectors, and photovoltaics.16,17,32−34 2.1. Solvent/Surfactant-Assisted Exfoliation

2.1.1. Sonication-Assisted Exfoliation in Solution. Liquid-phase exfoliation (LPE) is a simple and straightforward protocol that involves the sonication of layered bulk materials in organic solvents. The experimental parameters for control of the size/thickness and optimization of the yield are the starting mass of the bulk composite, the sonication power/time, the centrifugation conditions, and most importantly the choice of solvent.25 In liquid-phase exfoliation, ultrasonication generates jet cavitation through pressure fluctuations, and the cavitation acts on the bulk material to induce fragmentation into small flakes and exfoliation.35 During exfoliation, the choice of an appropriate dispersant is critical for reducing the potential energy between adjacent layers that are intercoupled by van der Waals interactions.36 Studies identifying superb exfoliation solvents have emerged in the evaluation of physical characteristics of solvent, such as surface tension and the Hildebrand and Hansen solubility parameters.37−39 One of the primary approaches was experimental fitting of the Hansen solubility parameters (HSP)

Figure 2. Photographs of sonication-assisted liquid-phase exfoliation of (a) MoS2 and (c) WS2 in various ethanol/water mixture ratios. Absorbance of (b) MoS2 and (d) WS2 suspensions in different ethanol/ water weight ratios are shown as dots, and calculated Ra values are shown as solid lines. Reproduced with permission from ref 48. Copyright 2011 Wiley-VCH.

effective for exfoliating WS2 in an exceptionally high yield (Figure 2c and 2d).48,49 Water and ethanol individually was a poor solvent for exfoliation due to the large difference in their surface energies from that of bulk MoS2. Similarly, Duan et al. demonstrated a contact angle measurement approach for the quantification of the material−solvent interfacial energy.50 The addition of organic surfactants with high binding affinities on the basal planes of the TMCs is another feasible route for enhancing the exfoliation yields. For example, ionic sodium cholate was 6153


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Figure 3. (a) Schematic describing the cascade centrifugation technique. Size selection is accomplished by discarding sediments at increasing centrifugation speeds. Adapted with permission from ref 57. Copyright 2016 American Chemical Society.

added as a surfactant to stabilize exfoliated nanosheets, reducing the probability of restacking.44,51 In addition, P-123,52 bovine serum albumin (BSA),53 PVP,13 and other polymers54−56 also have offered improved exfoliation efficiencies. 2.1.2. Size Sorting of Exfoliated Nanosheets. The liquidphase-exfoliated TMCs nanosheets have polydispersity in the lateral size (L) and the thickness (N) in the ranges of 50 nm ≤ L ≤ 500 nm and 1 ≤ N ≤ 20. Because the physicochemical properties of TMC nanosheets are strongly dependent on their size confinement effect, size sorting is a mandatory prerequisite for their practical applications. Two strategies for sorting exfoliated TMCs flakes are sedimentation-based separation and density-gradient ultracentrifugation.31,43,57,58 Sedimentation-based separation is arguably one of the universal separation techniques in industry and the laboratory for the isolation of specific components (i.e., particle) in terms of its size, shape, and density from a heterogeneous mixture. In solution, particles in different sizes possess different sedimentation rates at different centrifugation speeds because heavy weighted larger nanoflakes precipitate under centrifugation while smaller or ultrathin flakes with light weights remain in dispersion. For example, through the repetitive cycles of centrifugation and discarding, denoted as liquid cascade centrifugation, the size selection of a monolayer-enriched solution was achieved.57 Because the thicker nanoflakes can be removed in each step of the cascade performed at different centrifugation speeds, the resulting supernatants become increasingly enriched with monolayer flakes (Figure 3a). Density-gradient ultracentrifugation is another effective method to isolate a solution of flakes with a specific number of layers based on their different buoyant densities.59−64 The central idea is that because the buoyant density of surfactant-encapsulated nanoflakes varies as a function of their size and thickness, polydispersed nanoflakes in solution can be refined to a spatially varying density gradient profile in response to the ultrafast centrifugation. In fact, TMC possesses intrinsically high buoyant densities, so additional surfactant is required to generate effective density variations as a function of size. For example, an amphiphilic block copolymer (Pluronic F68) can reduce the overall buoyant density and enable the sorting of multiple distinct color bands at corresponding isopycnic points (Figure 4a− e).58 As a result, single- to trilayer enriched solutions were obtained as a histogram of the nanoflake thicknesses collected at the specific bands (Figure 4f). The resulting size-sorted single-layer MoS2 exhibited decent photoluminescence.

Figure 4. Density gradient ultracentrifugation (DGU) separation of polydispersed nanoflakes. (a−e) DGU separation of (a) MoS2, (b) WS2, (c) MoSe2, (d) WSe2, and (e) ReS2 with polymeric surfactants that produce distinct buoyant density profiles depending on nanoflakes size. (f) Thickness histograms of separated MoS2 at various isopycnic points, showing the successful isolation of single-, bi-, and trilayer enriched dispersion. (a−d and f) Reproduced with permission from ref 58. Copyright 2014 Nature Publishing Group. (e) Reproduced with permission from ref 62. Copyright 2016 American Chemical Society.

2.2. Molecular Intercalation/Exfoliation Methods

Intercalation chemistry is full of potential for low-dimensional and 2D layered TMC structures, while graphite intercalation compounds have been extensively studied for decades.65−67 More recently, the field of intercalation systems has expanded widely; new host materials ranging from inorganic clays to transition metal oxides (TMOs) and TMCs with various guest intercalates from atomic ions to organic molecules have been investigated for (i) the exfoliation of bulk crystals toward a single layer nanosheets, (ii) the synthesis of metastable inorganic/ organic hybrid nanostructures with weak or strong perturbation of the electronic structure, and (iii) the establishment of reversible intercalation/deintercalation reactions for advanced material properties in battery electrodes, electrochromic displays, 6154


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Figure 5. Solvothermal intercalation of n-butyl lithium for manual shaking exfoliation. (a) Schematic illustration for the manual shaking exfoliation process of partially lithiated LixMX2 into single layers.(b) Optical microscopy image of exfoliated single-layer TaS2 nanosheets on a SiO2/Si substrate. (c) Lattice distortion resulting from lithium intercalation into TMCs. Dashed spheres represent the new atomic position after lithium insertion. (d) Relationship between the initial crystal size and the strain in the lateral size of exfoliated sheet: (blue) crystal size variations, (red) lateral size distributions of exfoliated sheets, and (black) calculated lattice strain upon lithium intercalation. Reproduced with permission from ref 84. Copyright 2017 American Chemical Society.

Figure 6. Electrochemical exfoliation of 2D layered transition metal chalcogenide. (a) Schematic illustration of the setup of electrochemical exfoliation station. First, bulk layered materials were deposited onto a Cu foil and used as the cathode and lithium foil as the anode. After electrochemical intercalation, the composite on the Cu foil was sonicated in water to obtain the nanosheet dispersion. (b) AFM images of the exfoliated TMC nanosheets Si/SiO2 substrates. Insets show the corresponding height profiles at the indicated sites (white dotted boxes). (a) Adapted with permission from ref 86. Copyright 2012 Wiley-VCH. (b) Reproduced with permission from ref 87. Copyright 2011 Wiley-VCH.

catalysis, and sensors. It is believed that a better understanding of the mechanisms of intercalation or intercalated systems can yield valuable information for the structure/property correlations in

host−guest solid-state reactions with the desired structural and electronic complexity. One of the possibilities, the rational design of smart intercalates that can facilitate the molecular doping for 6155


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Figure 7. Exfoliation into few-layered VS2 by effusion of intercalated NH3 from the as-synthesized VS2·NH3 composite. (a) (i) Preparation of VS2·NH3 precursor. (ii) Effusion of NH3 molecules from stacked layers, disrupting interlayer interactions and periodicity in the c axis. (iii) Thin films assembly of few-layered VS2 by vacuum filtration. (b and c) Time-dependent ex situ XRD analyses of VS2·NH3 for different sonication times. (b) Magnified plots of the circled area in c, with the peak shifts to higher angle, indicating the crystallographic retention from VS2·NH3 into VS2. Reproduced with permission from ref 89. Copyright 2011 American Chemical Society.

MoS2. While the initial 2H phase can be retained by the thermal treatment and irradiation of the infrared (IR) laser, the mechanisms for this transformation have been extensively studied.74−76 Upon intercalation, Li+ ions reside in the interlayer S−S tetrahedron sites, where a large strain energy induces drifts of the sulfurs in intralayer plane direction and results in a phase shift from 2H to 1T. Notably, exfoliated MoS2 nanosheets were composed of multiple localized domains of 2H, 1T, and metastable 1T′ that is a distorted form of the pristine 1T phase. Atomic level imaging by STEM has shown that the distorted 1T′ phase exhibits a superlattice structure resulted by the clusterization of Mo or W atoms in zigzag chains.77−81 Other methods, including the modification of organolithium sources such as lithium halides82 and sodium potassium alloy (NaK),83 were also reported to prevent the phase transition and carry out the successful exfoliation. There have been modifications in the intercalation methods and chemical reagents to improve the yield and monodispersity in the thickness and lateral size. The solvothermal reaction-based intercalation of n-BuLi at 100 °C provided an effective alternative route for overcoming the relatively high thermodynamic energy barrier for the intercalation of Li+ in a much shorter time of 2 h at a desired concentration (i.e., LixMX2 (0 < x < 0.83)).84 In particular, the lithiation of LixTaS2 at 0.55 was optimal for manual shaking exfoliation into submillimeter-sized TaS2 monolayers within a short time (Figure 5a and 5b). A strategy of mild manual shaking showed promise for minimizing the fragmentation of nanosheets where the lateral sizes of exfoliated nanosheets were

reversible Fermi-level pinning and related electronic properties tuning, has not yet been demonstrated. 2.2.1. Intercalation of Alkali Metals and Gaseous Molecules for Exfoliation. Solution-based exfoliation was demonstrated as early as 1975 with TaS2 and NbS2 by intercalation of hydrogen and water molecules. Since then the library of intercalates has evolved to alkali-metal-containing organic reagents.68−70 A typical procedure involves the immersion of a large quantity of bulk 2D layered TMC crystals in an n-butyl lithium (n-BuLi)/hexane solution for 3 days with vigorous stirring under an inert atmosphere. The intercalation of lithium ions (Li+) into TMC is a type of a topotactic chargetransfer redox reaction.71 After completion of intercalation, the composite is transferred to water. At this stage, the formation of H2 gas and possibly undesirable H2S and Li2S byproducts in the vdWs gaps readily facilitates exfoliation with average lateral sizes of 300−800 nm and a thickness under 1 nm, which corresponds to the theoretical value of a monolayer. Since the first report in 1986,72 this method has been underutilized for decades until the photoluminescence of single-layer MoS2 nanosheets was observed by this chemical exfoliation method.73 In addition, detailed TEM analyses show that group VI TMCs undergo a first-order phase shift from trigonal-prismatic 2H to a metastable octahedral 1T phase upon Li intercalation.68 Excess electron injections to the metal center of group VI TMC (from d2 to d3) by Li+ is one of the primary driving forces for this phase shift. Consequently, the phase shift induces a reconstruction of the density of states, which generates catalytically active metallic 1T6156


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Figure 8. Tandem molecular intercalation (TMI) for exfoliation of 2D layered TMC. (a) Schematic illustration of sequential intercalation of short and long alkylamines in random bilayered arrangements. (b and c) Cross-section TEM image and 1H NMR spectrum of intercalated TiS2 with (b) propylamine and (c) propylamine and hexylamine. (d−g) Microscopic characterization of exfoliated TiS2 nanosheets. (d) Top-view and (e) highmagnification TEM images. (f) HRTEM image and FFT pattern (inset). (g) AFM image. Adapted with permission from ref 96. Copyright 2015 Nature Publishing Group.

each S−V−S layers by the weak vdWs force and hydrogen bonding. The ex situ XRD patterns showed a peak shift to higher angles for VS2·NH3 from 9.59° to 15.38° which features a reduction of interlayer spacing from 9.21 to 5.73 Å by effusion of NH3 (Figure 7b and 7c). Other chemical moieties such as supercritical carbon dioxide (CO2), hydrazine, and polymers also have been utilized because of their high diffusion rates for intercalation and effusion.92−95 2.2.2. Intercalation of Interlayer-Spacing-Tunable Organic Molecules for Exfoliation. Current exfoliation methods for the preparation of single-layer 2D TMC typically require external energies such as sonication with frequently harsh preparation conditions to overcome vdWs interaction energy between the adjacent layers. As alternatives to avoid fragmentations and defects from the usage of excessive energies, the development of mild exfoliation strategies for obtaining single-layer TMCs with high efficiency and reproducibility is currently being pursued.96−98 Tandem molecular intercalation (TMI) for producing single-layer TMC nanosheets was recently developed by Cheon et al., in which short “door opener” molecules first intercalate to expand the interlayer spacing for the entering of long “primary” molecules to form random bilayered arrangement of intercalates with different alkyl lengths between the layers.96 In the course of intercalation, the interlayer spacing of TiS2 (6.5 Å) was expanded to 9.6 and 21 Å (Figure 8b and 8c), suggesting that alkylamine intercalation proceeded to form a bilayer arrangement between the layers. The computational modeling showed that the voids in the bilayer intercalate mixture (Figure 8a(iii)) are essential for reducing the total interaction energy below 2kBT; therefore, a spontaneous exfoliation into

nearly identical to the original domain size of bulk crystals. Furthermore, mechanistic insights on the tensile and compressive strains determined by theoretical computations show that compressive strain is the predominant factor for excessive lattice distortion, which eventually causes cracking and fragmentation at grain boundaries (Figure 5d). TMCs can also be intercalated electrochemically85−87 by using the bulk TMC as the cathode and Li metal as the anode (Figure 6a).86,87 With this method, large-scale production of single-layer TMC nanosheets in high-yield was achieved in various chemical compositions of MoS2, WS2, TiS2, and TaS2 (Figure 6b).87 The controlled exfoliation of TMCs into specific trilayered nanosheets was demonstrated by the stoichiometric reaction of MoS2 with n-BuLi and exfoliation in a mixture of 45% (v/v) ethanol/ water solution. Although the exact mechanism of this process has yet to be determined, its exfoliation efficiency was 11−15%. With the high concentration of intercalation (i.e., LixMX2 (x > 1)), ionic solutions of TMCs in polar solvents were prepared.88 In addition to organolithium intercalation, intercalation of alkylamines and gaseous molecules such as NH3 has also been successful for inducing exfoliation.89−91 For example, few-layered VS2 nanosheets were prepared by the effusion of intercalated gas molecules from bulk composites.89 VS2·3NH3 was hydrothermally prepared by reaction between ammonium metavanadate (NH4VO3) and thioacetamide (TAA) where thermolysis of TAA is the source for in situ intercalation of NH3 in the synthesis. VS2·3NH3 was ultrasonicated in water, resulting in exfoliation to ultrathin VS2 nanosheets accompanied by the effusion of NH3 (Figure 7a). Ammonia (NH3) is volatile, so that no residues were found after exfoliation, while NH3 molecules are weakly bound to 6157


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Figure 9. High-yield exfoliation of 2D layered TMC by intercalation of a sodium naphthalenide adduct. (a) Bulk MoS2 was pre-expanded by the thermal decomposition of N2H4. (b) Pre-expanded MoS2 reacts with A+C10H8− forming a stage I intercalated sample, which can be exfoliated to a single layer with mild sonication. (c−e) Photographs of (c) bulk MoS2 crystal, (d) pre-expanded MoS2, and (e) exfoliated single-layer MoS2 dispersion. (f) AFM images, corresponding EDS spectra, and photographs of exfoliated nanosheets dispersed in water: (i) TiS2, (ii) TaS2, and (iii) NbS2. Reproduced with permission from ref 98. Copyright 2014 Nature Publishing Group.

Finally, the exfoliation was completed and readily dispersed in aqueous solution by mild ultrasonication (Figure 9e). The exfoliation protocol is generally applicable to a wide range of TMCs, including TiS2, TaS2, and NbS2, as well as TiSe2, NbSe2, and MoSe2 (Figure 9f). While there has been much progress in the intercalation of interlayer spacing-tunable molecules induced exfoliation for single-layer TMC nanosheets over the past few years, many challenges still exist. It is still quite difficult to utilize the mild exfoliation strategy in bulk materials due to the limited penetration depth of intercalates in reasonably adequate amounts. In addition, the rates of diffusions in the edges of a crystal and the inner basal plane substantially differ from each other. The driving forces for stage-controlled intercalation also remain to be understood.99−102 To address these challenges, further studies may proceed with comprehensive characterization, such as in situ real time techniques, for tracking the intercalation processes and probing structural evolution. Realtime measurements that can provide important kinetic information are expected to provide a realistic model for dynamic intercalation processes. Comprehensive molecular dynamics (MD) simulations are also helpful to obtain complementary understanding in order to resolve difficult experimental situations. 2.2.3. Intercalation for Inorganic/Organic Hybrid Composite Nanostructures. Intercalation chemistry can serve not only as a powerful method for isolation of singlelayer TMCs but also as a route to synthesize inorganic/organic hybrid nanostructures with structural flexibility. Two-dimensional layered TMCs have been known as versatile intercalation

single-layer TMCs at room temperature can occur (Figure 8a and 8d−g). Furthermore, it was discovered that functional groups in intercalates are critical for driving their effective intercalation into TMCs. Because the intercalation is based on Lewis acid (TMCs) and base (intercalates) interactions, being close in energy levels between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (ΔEHOMO−LUMO) promotes kinetically controlled intercalation under ambient conditions. For example, the LUMOs of group IV TMCs range from −6.0 to −5.5 eV, which are close to the HOMO energy levels (−6.2 eV) of alkylamine bases. On the other hand, the LUMOs of group VI TMCs typically fall between −4.2 and −3.5 eV, relatively stronger base such as alkoxides (HOMO energy level of −4.2 eV) was needed. With the selection of appropriate intercalates, group IV (TiS2, ZrS2), V (NbS2), and VI (WSe2, MoS2) TMCs were exfoliated down to single-layer nanosheets. Using a pre-expanded bulk TMC, the intercalation of naphthalenide adducts of Na produced single-layer MoS2 nanosheets with an unprecedented lateral size of up to 400 mm2 in high quality and yield (∼90%).98 The pre-expansion of the interlayer spacing by reacting bulk MoS2 with hydrazine (N2H4) under hydrothermal conditions took place by intercalation of N2H4 and subsequent decomposition to N2, NH3, and H2 at elevated temperatures (Figure 9a). Consequently, the volume of MoS2 crystal was expanded by more than 100 times in the vertical direction relative to their original volume (Figure 9c and 9d). In the next step, the pre-expanded MoS2 crystals were intercalated by an alkali naphthalenide solution (Figure 9b). 6158


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Table 1. Intercalation Compounds of Organic Molecules with Group IV and V TMCsa intercalate ammonia hydrazine alkylamine (RnNH2, n = 1−18) aniline formamide alkylamide (RnCONH2, n = 1−7,17) N-alkylformamide (HCONHRn, n = 1,2) N,N-dimethylformamide urea N,N-dimethylurea N,N′-Dimethylurea N,N-Diethylurea (or N,N′) pyridine (Py) 2-alkyl(Rn)-Py (n = 1−3) 3-alkyl(Rn)-Py (n = 1,2) 4-alkyl(Rn)-Py (n = 1−3) imidazole N,N-dimethyl sulfoxide trimethylphosphine oxide phosphine oxide pyridine N-oxide (PyO) 2-methyl-PyO 3-methyl-PyO 4-methyl-PyO

host TMCs amines TiS2, TiSe2, ZrS2, HfS2, VSe2, NbS2, NbSe2, TaS2, TaSe2 TiS2, TiSe2, ZrS2, ZrSe2, HfS2, VSe2, NbS2, NbSe2, TaS2, TaSe2 TiS2, NbS2, NbSe2, TaS2 TiS2, NbS2, TaS2 amides TiS2, NbS2, TaS2 TiS2, NbS2, TaS2 TiS2, NbS2, TaS2 TiS2, NbS2, TaS2 TiS2, NbS2, TaS2 TiS2 TiS2 TiS2, NbS2, TaS2 N-heterocycles TiS2, NbS2, NbSe2, TaS2, TaSe2 TiS2, NbS2, TaS2 TiS2, NbS2, TaS2 TiS2, NbS2, TaS2 TiS2, NbS2, TaS2 S-, P-, and N-oxides TiS2, NbS2, TaS2 TiS2, NbS2, TaS2 TiS2, TaS2 TiS2, NbS2, TaS2 TiS2, NbS2, TaS2 TiS2, NbS2, TaS2 TiS2, NbS2, TaS2

δ (Å)b

methodc

3.0−4.5 3.0−6.7 3.0−52.3 11.8−12.1

A, B A, B, D A, C, D A, B, C

7.2−7.6 5.8−5.8, 51.0 3.9−12.9 3.7−3.9 6.6−6.7 12.0 3.9 3.7−4.0

A A A A E A A A

5.9−6.1 5.4−6.5 5.3−7.1 5.7−5.9 5.7−5.8 11.8−12.6 11.6−13.0 7.7−12.6 5.9−6.8 6.6−12.9 6.8−11.8 5.4−5.8

A, B, C A, C A, C A, C A A, A A A, C A, C A, C C

Reproduced with permission from ref 66. Copyright 1979 D. Riedel publishing. bδ corresponds to the value of increase in interlayer spacings of host TMCs. cMethods: (A) Sealed tube conditions. (B) Electrolysis method with corresponding salt as the electrode. (C) Pre-expansion with NH3 or N2H4 following by treatment with intercalate. (D) Using benzene as solvent. (E) Using n-butanol as solvent. a

compositional change into TiS2[(HA)x(H2O)y(DMSO)z] was also achieved by exchanging DMSO with H2O by immersing in DIW (Figure 10a). HAADF-STEM observations showed that the TiS2 and organic layers stacked alternatingly, forming a stage 1 compound (Figure 10b and 10c). The hybrid superlattice of TiS2[(HA)x(H2O)y(DMSO)z] showed a large increase in electrical conductivity of 790 S·cm−1 and a power factor of 0.45 mW·m−1· K−2 attributed to the externally injected electrons from H2O. For group VI TMCs, the intercalation of dipole molecules such as H2O in the electrolyte and carbonaceous materials has been widely studied as a method for increasing the ion diffusivity via expansion of the vdWs gap.111−113 Recently, the search for new intercalates has evolved where poly(ethylene) was chosen as an effective intercalate because of its high electrochemical stability, good ionic conductivity, and dielectric permittivity. For the synthesis of MoS2−PEO nanocomposites with expanded interlayer distances, modified chemical delamination, and reassembly technique was performed (Figure 11a).109,110 Commercially available bulk MoS2 (com-MoS2) was first exfoliated by chemical lithiation to obtain single-layer dispersion. The negative charges on MoS2 generated by Li+ ions provided reactive chemical sites for functionalization of poly(ethylene oxide) (PEO).82 Then a different molar amount of PEO was added to the dispersion to obtain a single-layer MoS2−PEO nanocomposite. The nanocomposite was reassembled into multilayered composite by drying in vacuum. After intercalating the PEO with molar ratios of 4:1 and 1:1 (MoS2:PEO), the

hosts that can accommodate a wide variety of guest species, and their host−guest interactions are energetically in part associated with their electronic and steric effects. It was reported that the molecules with pKa values greater than four are pivotal in the context of intercalation.103 For example, group IV and group V TMCs can readily form intercalated composites with a wide variety of guest species by electrochemical or chemical methods, especially with electron-donating Lewis bases or chemical reducing agents (Table 1).66 Acrivos et al. reported that the act of intercalation is based on Mulliken’s electron donor (intercalates)−acceptor (TMC) concept, where the charges from the functional group of the intercalate transfer to the d orbital of the metal atoms through sulfur p orbitals.104,105 Therefore, the ionization potential, effective dipole moment, size, and symmetry of the intercalates should be considered with regard to the success of intercalations with the desired molar ratios. The presence of a low-lying dz2 band in group VI TMCs limits the choice of intercalates, which are mainly restricted to alkali metals or pre-expansion at high temperatures (>700 °C) in the gas phase for intercalation of sterically disfavored molecules (e.g., substituted aromatics, polymers, and ferrocene).106 In group IV TMCs, there have been major advances in the preparation of inorganic/organic superlattice nanostructures and dielectric constant variations for thermoelectric applications.107−110 For example, hexylammonium (HA) chloride dissolved in dimethyl sulfoxide (DMSO) was successfully intercalated via an electrochemical reaction in bulk TiS2, forming a hybrid superlattice of TiS2[(HA)x(DMSO)w].107,108 The 6159


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Figure 10. Electrochemical intercalation and conversion of TiS2 into inorganic/organic hybrid nanostructures. (a) Illustration of electrochemical intercalation of hexylammonium ions generated in situ in bulk TiS2 crystal to form TiS2[(HA)x(DMSO)y] and the final product of TiS2[(HA)x(H2O)y(DMSO)z] through solvent exchange. (b) HAADF-STEM image of the TiS2[(HA)x(H2O)y(DMSO)z] hybrid showing a wavy structure. (c) Magnified HAADF-STEM image of TiS2[(HA)x(H2O)y(DMSO)z] with an expanded van der Waals gap of 9.65 Å. It reveals that the intercalated hexylammoniums are in bilayer arrangement. Reproduced with permission from ref 107. Copyright 2015 Nature Publishing Group.

structure from an indirect to a direct band gap in intercalated MoS2 (Figure 12g). This study opened up a new direction into the class of bulk monolayer materials that are particularly attractive for electronics applications without the need for singlelayer preparation. Despite the considerable attention given to intercalated composites on 2D layered TMC nanostructures, a current survey of the literature shows that studies on group V TMCs are largely those from the 1970 to 1980s, which focus on the intercalation of linear alkylamines with a superconducting transition temperature (Tc) between 1.7 and 4.2 K depending on the chain length.66 To date, there are more than 100 group V TMC−organic complexes reported, and these compounds require detailed analyses with modern characterization techniques, which may provide a proper understanding of the chargetransfer-induced effect on the host TMC and may lead to higher Tc values and manipulation of Mott insulators in intercalated 2D TMC nanostructures.

interlayer distance increased to 1.19 and 1.45 nm, respectively, corresponding to a single- and bilayer arrangement of PEO between the layers of MoS2. In the absence of a PEO functionalization step, the restacked MoS2 (res-MoS2) showed no interlayer expansion, proving that the delamination− reassembly did not impact the gap expansion (0.61 nm) (Figure 11b−e). The interlayer-expanded MoS2−PEO composite exhibited remarkable Mg diffusion kinetics and an exceptional rate capability, increased specific capacity, and Coulombic efficiency. Recently, Rajamathi et al. reported ammonium (NH4+)-intercalated MS2 (M = Mo and W) by soaking the lithiated TMCs (LixMS2) in a saturated ammonium chloride (NH4Cl) solution.90 Duan et al. reported a pioneering work on a molecular intercalation approach to form a new class of stable inorganic−organic superlattice MoS2 , WSe 2 , and NbSe 2 nanostructures.114 By electrochemically intercalating trimethylammonium bromides with variable carbon chain lengths and substitutional groups, tailored expansion of the interlayer distance in MoS2 from 6.14 to 14, 19, and 24 Å was achieved (Figure 12a−f). The expansion of the vdWs gap by intercalation with relatively large molecules could decouple the interlayer interaction and considerably modulate the electronic structure. As a proof-of-concept study, intercalated MoS 2 showed prominent photoluminescence emission at 720 nm, multiple orders of magnitude stronger than that of pristine multilayer MoS2, suggesting a transformation of the electronic band

3. OVERVIEW OF BOTTOM-UP CHEMICAL SYNTHETIC STRATEGIES Development of synthetic techniques for producing nanostructures that are uniform in composition, size, and shape has been a critical step in the identification, understanding, and exploitation of the material properties. In this regard, bottom-up chemical synthesis has become a state-of-the art technique for preparing 6160


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Figure 11. Interlayer expanded MoS2 composites with poly(ethylene oxide) (PEO) intercalates. (a) Schematic illustration of the delamination− restacking process for desired interlayer gap expansion. (b−e) Cross-sectional TEM images of (b) commercially available MoS2 (com-MoS2), (c) restacked MoS2 (res-MoS2) with only H2O molecules trapped between the layers, (d) peo1-MoS2 with an interlayer distance of 1.19 nm, and (e) peo2MoS2 with interlayer distance of 1.45 nm. Adapted with permission from ref 110. Copyright 2015 American Chemical Society.

electrochemical energy storage, nonlinear photonics, and photodetectors.122,123 In addition, these multilayered colloidal TMC nanoplates offer unique opportunities for manipulation and assembly to create new structures with controlled orientation by an external electric or magnetic field.124−128 Even though there are unique advantages of colloidal 2D TMCs, a number of understandings and challenges must be addressed in the bottom-up chemical synthesis of 2D layered TMC nanostructures to realize their practical applications in industry. These challenges include (1) the relationship between the surface energy and surfactants, (2) effective ligand stripping methods, (3) the kinetic relationship on precursor decomposition rates and the growth of nanostructures, and (4) largescale production in the industrial level. In this part, a general discussion of the synthetic schemes and point-by-point strategies for control in size/thickness, crystal geometry, composition, and doping of nanosheets in 2D layered TMCs will be provided with some representative examples.

high-quality freestanding metallic, semiconducting, and magnetic nanocrystals. Since the successful demonstration of the quantum confinement effects of CdSe quantum dot nanocrystals in the early 1990s,115 the bottom-up chemical synthetic approach has become one of the most important synthetic protocols.27 The solution-based synthesis of 2D layered TMCs was developed following a general synthetic scheme:116 a reaction medium is first heated to a sufficient temperature (usually from 150 to 300 °C) where the precursors decompose into monomers. As the population of the monomer reaches the supersaturation level, spontaneous crystal growth starts to occur by the burst nucleation of seeds and the consumption of monomers. Finally, the desired size and shape of nanostructures are achieved by surface stabilization by organic surfactants. One of the key assets of bottom-up chemical synthesis is the simplicity in control of size, shape, and composition by analytical alteration of the reaction parameters including time, temperature, concentration, and choice of molecular precursors. For example, the size control of nanostructures is achieved by extending the reaction time or elevating the reaction temperature.117,118 Appropriate selection of organic surfactants, in which one binds tightly to the specific surfaces of nanocrystal, thereby hindering growth, results in shape control of the final nanostructure (e.g., platelets and cubes).119−121 The nucleation−growth theory proposed by LaMer is based on the classical nucleation theory originated by Becker and Döring in the 1920s.20 Even though ultrathin nanosheets with negligible thickness have been particularly important in their use for optoelectronics, catalysis, and valleytronics, there are also studies on relatively thicker multilayered nanoplates showing their utilization in

3.1. Synthetic Methods for 2D Layered TMC Nanosheets

3.1.1. Hot Injection Method. The hot injection method is one of the most distinguished chemical synthetic schemes for the preparation of single-crystalline, size- and shape-controlled nanocrystals, where an anionic precursor solution is rapidly injected into a hot mixture (150−350 °C) solution of metal precursors and capping surfactants. For the successful preparation of 2D layered TMCs, close control of the reaction parameters (e.g., temperature, molar ratio, surfactants, and time) and the choice of appropriate precursors (e.g., metal and chalcogen reactants) is imperative for directing the reaction pathway to a kinetic route, thereby maintaining the intrinsically 6161


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Figure 12. Electrochemical intercalation for 2D layered TMC composites with interlayer expanded gap tunability. (a) Schematics of the electrochemical intercalation process for interlayer van der Waals gap expansion. (b) XRD patterns of pristine- and intercalated-MoS2 with quaternary ammonium molecules with different carbon chain lengths (lauryl-, cetyl-, or octadecyl-trimethylammonium bromide, that is, LTAB, CTAB, or OTAB). (c and d) Cross-sectional TEM images of (c) pristine MoS2 and (d) MoS2/CTAB heterostructure. Scale bars: 2 nm. (e) XRD patterns of MoS2 and intercalated MoS2 with quaternary ammonium molecules with four equivalent substitutional groups of increasing chain length (tetrabutyl-, tetraheptyl-, or tetradecyl-ammonium bromide, that is, TBAB, THAB, or TDAB). (f) XPS spectra of MoS2, MoS2/CTAB and MoS2/THAB superlattice. (g) Photoluminescence observed from pristine MoS2, MoS2/CTAB, and MoS2/THAB heterostructure. Reproduced with permission from ref 114. Copyright 2018 Nature Publishing Group.

identical thickness of 1.6 nm, respectively.130 According to ab initio calculations on the structurally optimized TiS2 and ZrS2 nanodiscs models, the anisotropic growth in 2D nanodisc shape attributes to the large surface free energy difference between the edge and the basal plane. As Wulff theorem states that the crystal tends to grow at a much faster rate in the high-energy direction in order to minimize the total surface energy, the growth rate in the lateral direction is dominant in this case. This synthetic method generally operates in other TMCs such as VS2, NbS2, TaS2, and HfS2. 3.1.2. One-Pot Heat-up Method. In the one-pot heat-up method, all of the molecular precursors are first mixed at low temperature, typically below 120 °C, and then heated to facilitate chemical reactions and growth of nanosheets. All members of the group IV136−139 V140,141 and VI142−144 2D TMC compounds have been synthesized using this synthetic scheme. Typically, metal chlorides and elemental sulfur (S) or selenium (Se) powder chalcogen precursor are utilized. TiS2 nanosheets with a

favored anisotropic planar growth. Typically, metal−chloride, −carbonyl, −acetylacetonate, and −carboxylate precursors have been employed as effective metal precursor.97,129−135 For example, a generalized synthetic protocol was reported for establishing lateral size controllability.129 In the synthesis of TiS2 nanosheets, carbon disulfide (CS2) was swiftly injected to a solution of titanium(IV) tetrachloride (TiCl4) in oleylamine at 300 °C to form single-crystalline TiS2 nanodiscs with a lateral dimension of 150 nm (Figure 13a−d). For the size control, a sequential increase in the concentration of both TiCl4 and CS2 by 1.3, 1.6, and 2.4 times produced TiS2 nanosheets with smaller sizes of 100, 60, and 40 nm, respectively (Figure 13e). Thus, the results show that the reactant concentration is an important factor that governs the lateral size of nanosheets, where fast nucleation at a high reactant concentration affords a large number of seeds, which yield small particles. Alternatively, extending the reaction time concomitantly increased the lateral diameter of ZrS2 nanosheets from 20 to 35 and 60 nm with an 6162


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Figure 13. Colloidal synthesis of TiS2 nanodiscs by a hot-injection method. (a) Chemical equation for the synthesis of TiS2 nanocrystals. (b−c) TEM images of TiS2 nanodiscs, (b) low magnification, (c) side-view HRTEM image. Inset is the FFT pattern. (d) Ball-and-stick model of TiS2 slab. (e) Size control of TiS2 nanodiscs: (i−iv) TEM images with different lateral dimensions of (in nm) (i) 150, (ii) 100, (iii) 60, and (iv) 40. (f) Schematic illustration of anisotropic growth in lateral direction. Adapted with permission from ref 129. Copyright 2012 American Chemical Society.

lateral dimension of ∼500 nm and a thickness of ∼5 nm were synthesized by injecting TiCl4 into a oleylamine containing elemental S at 100 °C, followed by heating to 300 °C.138 Although elemental S is a cost-effective and synthetically reliable molecular precursor for the large-scale production of various metal sulfide quantum dots, it was recently reported that elemental S produces poor quality 2D layered TMC nanosheets compared to nanosheets synthesized with alternative chalcogen sources, such as CS2 (Figure 14a).129,136,137 In contrast to the problems associated with elemental S, elemental Se can be employed in the preparation of group IV and V metal selenide nanocrystals. For example, a mixture of TiCl4 and elemental Se in oleylamine produced high-quality TiSe2 nanocrystals (Figure 14b−e).141 The homolytic cleavage of elemental S and Se in oleylamine generated the radical species at elevated temperatures, whereas no radical species were generated in CS2 (Figure 14f). The control experiment showed no noticeable changes in the elemental Se, whereas structural degradation obviously occurred with the S powder, which is attributed to the fact that the reactivity of generated radical species by elemental Se is relatively soft compared to its S counterpart. (Figure 14g).129,145 This synthetic protocol was generalized to other transition metal selenide nanosheets of TiSe2, ZrSe2, HfSe2, VSe2, NbSe2, and TaSe2. The successful preparation of high-quality 2D nanosheets by Se powder has been observed in other classes of 2D systems,

for instance, 2D CdX (X = S, Se, and Te) nanoplatelets. While CdSe nanoplatelets prepared with Se powder have a distinct size, thickness, and shape,146 CdS nanoplatelets synthesized with sulfur powder showed polydispersity in size and irregular shapes.147,148 On the other hand, when 1-octanethiol or bis(trimethylsilyl)sulfide (TMS2S) was used as an alternative sulfur precursor, high-quality 2D CdS nanoplatelets were obtained.149−152 For 2D layered TMCs, one limited study has thus far been carried out on the factors affecting the structural integrity of synthesized nanosheets and on their plausible mechanism.129 Therefore, it is essential to survey various metal and chalcogen precursors to obtain a tunable library of controlled precursor reactivity and quantitative conversion kinetics and intermediate metal complex species for the establishment of generalized synthesis of 2D layered TMCs in safe, common, and low-cost pathways.153−156 In addition to elemental sulfur powder, CS2 and 1-dodecanethiol (1-DDT) is another appropriate chalcogen precursor, especially for the preparation of ultrathin single-layer TMCs owing to its controlled rate of decomposition into H2S gas through the course of the reaction period, which will be discussed further in section 3.2.1. A survey of molybdenum-based single-source precursors that contain either xanthate or dithiocarbamate ligands, i.e., [Mo2O4(S2CNEt2)2], [Mo2O2S2(S2CNEt2)2], [Mo2S4(S2CNEt2)2], [Mo2O2S2(S2COEt)2], and 6163


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Figure 14. Synthesis of 2D layered TMC nanocrystals by a one-pot heat-up method. (a) Schematic illustrating the choice of appropriate chalcogen precursor is important for yielding high-quality 2D TMC nanocrystals. Reactive radical species from elemental S degrades the nanocrystals during the synthesis. (b) Reaction equation for the synthesis of TiSe2 nanocrystals. (c) Low-magnification, (d) top-view, and (e) side-view TEM images of TiSe2 nanocrystals. (f) EPR spectra of elemental S and Se and CS2 in oleylamine. No radical generation signal was detected for CS2. (g) TEM images of TiSe2 nanodiscs treated with elemental S and Se in oleylamine at 300 °C. Reproduced with permission from ref 129. Copyright 2012 American Chemical Society.

the radial orientation (Figure 15a−c).161,168 Each nanosheet was mostly comprised of five layers or less with an interlayer spacing of 7.1 Å, which is a slight expansion of the vdWs gap (Figure 15d−f). In the hydrothermal synthesis, ultrathin defect-rich TMCs were synthesized by adjusting the concentrations of the precursors. Recently, the gram-scale synthesis of defect-rich MoS2 from a high concentration of precursors and different amounts of thiourea was reported.169−172 To achieve a defectrich structure, excess thiourea was added to inhibit the oriented attachment crystal growth with multiple dislocations and distortions (Figure 16b−f).169 In contrast, defect-free MoS2 nanosheets can be obtained via a quantitative reaction with high concentrations of precursors (Figure 16a). A high precursor concentration is critical for producing ultrathin nanosheets, as reducing the concentration of the metal precursor produced thicker nanosheet assemblies. The in situ intercalation of the oxidized solvent (i.e., DMF),173 Na+, NH4+, or oxygen molecules during the hydrothermal synthesis of group VI TMCs has been observed with expansion of interlayer spacings of 6.7 and 9.5 Å.173−177 Because the intercalation of small molecules can induce

[Mo2S4(S2COEt)2], showed that [Mo2O2S2(S2COEt)2] is ideal for the production of single-layer MoS2 due to its fast thermolysis kinetics in homogeneous intermediate species.122,157 3.1.3. Hydro/Solvothermal Method. Hydrothermal synthesis is a process in which the formation and growth of crystals occurs in a sealed autoclave at elevated temperatures and pressures with reaction parameters of concentration, pH, time, pressure, organic additives, and reducing agents.158,159 The hydrothermal synthesis of 2D layered TMCs produced 3D radially oriented nanospheres and corrugated nanoflowers.160−167 It is worth noting that only group VI TMCs are suitable for hydrothermal synthesis, as group IV and V TMCs would oxidize to metal oxides during the synthetic process. For example, a mixture of hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O), thiourea (NH2CSNH2), and poly(vinylpyrrolidone) (PVP) was prepared in deionized water (DIW). During synthesis, ultrathin MoOx nanosheets were first produced by decomposition of ((NH4)6Mo7O24·4H2O) and then sulfurized by NH2CSNH2, facilitating chemical transformation into MoS2 nanosheets. The control study without the PVP showed that the PVP served as a shape-directing agent for 6164


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Figure 15. Hydrothermal synthesis of radially oriented 3D MoS2 nanospheres. (a) Schematic illustration of the radial shape orientation route for the 3D MoS2 nanospheres. (b−e) TEM images of MoS2 nanospheres. (b) Low-magnification, (c) high-magnification, and (d, e) high-resolution images of area 1 and area 2 in c, respectively. (f) Ball-and-stick slab model of MoS2. Purple and green circles denote Mo and S, respectively. Reproduced with permission from ref 161. Copyright 2015 American Chemical Society.

Figure 16. Hydrothermal synthesis of defect-rich ultrathin MoS2 nanosheets. (a) Synthetic pathways to obtain defect-rich or defect-free MoS2 nanosheets. (b) XRD pattern of as-synthesized defect-rich MoS2. (c) Low-magnification FESEM and (d) TEM images of the defect-rich MoS2 nanosheets. (e) HRTEM image and the corresponding Fourier transform pattern of area 1 in d. (f) Cross-sectional HRTEM image of area 2 in d. Reproduced with permission from ref 169. Copyright 2013 Wiley-VCH.

around the SiO2 nanospheres. Finally, mesoporous MoS2 foam was obtained by etching the SiO2 template with an HF solution (Figure 17b). Notably, other types of scaffolds have been utilized to create porous architectures, where honeycomb-like MoS2 was synthesized on three-dimensional graphene foam (3DGF) as a skeleton (Figure 17c).179 By a hydrothermal reaction between sodium molybdate and thiourea with P123 as a surfactant at 200 °C, interconnected and porous 3D honeycomb-like architectures with pore size from 50 nm to several hundred nanometers composed of MoS2 nanosheets with a lateral size of ∼100 nm and thickness of ∼3 nm were synthesized (Figure 17d). The density of MoS2 in the 3D architecture was easily controlled by adjusting the concentrations of both reactants. Molecular reagents that induce structural conversion from presynthesized template crystals to 2D TMC nanostructures is an appealing synthetic route for creating a hierarchical

a change in the crystal geometry from 2H to 1T, a further discussion is provided in section 3.2.2. 3.1.4. Template-Assisted Method. Self-assembly is a spontaneous process that coordinates the creation of organized structures or patterns from pre-existing disordered components through weak local interactions (e.g., van der Waals forces, hydrogen bonding, π−π interactions). Recently, atomically thin 2D TMC nanosheets were assembled in a shape-directing scaffold to create a porous geometric architecture.178−184 For example, a uniform mesoporous MoS2 foam was prepared according to the synthetic procedure illustrated in Figure 17a.178 In the synthetic process, (NH4)6Mo7O24 was homogeneously coated onto presynthesized SiO2 nanospheres via a wet impregnation method. Then a chemical reaction with CS2 on the SiO2 surface converted the Mo precursor into small MoS2 domains that self-assembled into vertically aligned MoS2 layers 6165


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Figure 17. Atomically thin 2D TMC nanosheets assembled in a shape-directing scaffold for pore geometric architecture. (a) Schematic illustration of the fabrication of mesoporous MoS2 foam (mPF-MoS2). (b) Microscopic characterizations of mPF-MoS2. (i) SEM image (ii) TEM image and (iii) HRTEM image of mPF-MoS2, with the inset showing a typical MoS2 layer distance of 0.62 nm and a distinct mesopore. (c) Schematics of the fabrication of the honeycomb-like MoS2 (HC-MoS2) on three-dimensional graphene foam (3DGF). (d) Microscopic characterization of HC-MoS2 (i) and (ii) FESEM images at different magnifications. (iii) High-magnification image of HC-MoS2 showing the wire is composed of ultrathin MoS2 nanosheets. (a and b) Reproduced with permission from ref 178. Copyright 2017 Nature Publishing Group. (c and d) Reproduced with permission from ref 179. Copyright 2014 Wiley-VCH.

morphology as the final product (Figure 18b and 18c). In this regard, the size of the template is critical for the transformation of the structure into freestanding multilayered nanosheets. By reducing the length of the 1D metal oxide template from the microscale to the nanometer scale, a morphological transformation to multilayered platelet nanosheets was achieved.186 With a tungsten oxide (W18O49) nanorod that is 30 nm long and a diameter of 5 nm, the sulfidation reaction transformed the structure into multilayered tungsten sulfide (WS2) nanosheets with dimensions of 40 nm. WS2 nanosheets with dimensions from 40 to 85 nm were obtained from W18O49 nanorods with lengths from 30 to 75 nm.

structure.185,186 For example, sulfidation-based conversion from presynthesized 1D metal oxides has been an effective pathway not only from the synthetic perspective but also for providing fundamental insights into the selection of appropriate chemical reagents (i.e., sulfidation sources) for establishing the desired morphology and chemical composition. The energetic instability of the interfaces between the pre-existing metal oxide and the newly formed sulfide region during the intermediate stage spontaneously lowers the activation energy barrier for the completion into a new type of 2D nanostructures.187 For example, hierarchical 2H-MoS2 nanotubes were successfully synthesized by an anion-exchange reaction between 10-μm-long Mo3O10−C2H10N2 (ethylenediamine trimolybdate) nanowires and L-cysteine at 200 °C for 14 h, from which the morphological transformation from clean nanowires to an intermediate core− shell structure and 2H-MoS2 nanotubes occurred (Figure 18a).185 On the basis of the analyses of time-dependent EDX line scan elemental profiling, a mechanism was proposed in which the continuous dissolution of ethylenediamine in the solvent and the concentration gradient of incoming sulfur ions due to the large micrometer-scale dimensions of the Mo3O10− C2H10N2 template nanowires direct the formation of a tubular

3.2. Methodologies for Controlling the Size/Thickness, Crystal Geometry, and Chemical Composition

3.2.1. Control of the Lateral Size and Thickness. The ability to control and manipulate the physical and chemical properties of materials is a key topic in chemistry and materials science. Yet, the challenges certainly remain since the solutionbased prepared TMC nanosheets are usually in crumpled and/or aggregated form. Although there are ample opportunities in many potent applications; for example, discrete nanoflowers are 6166


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Figure 18. Synthesis of hierarchical 2H-MoS2 nanotubes via sulfurization of Mo3O10−C2H10N2 nanowires. (a) SEM images and (b) elemental line scans of the anion-exchanged intermediates at different reaction times (i) 0, (ii) 6, and (iii) 12 h. (c) Schematic representation of anion-exchange reaction pathways for hierarchical MoS2 nanotubes. Adapted with permission from ref 185. Copyright 2013 Wiley-VCH.

Figure 19. Synthesis of single-layer ZrS2 nanosheets by the diluted chalcogen continuous influx (DCCI). (a) Synthetic strategy for single-layer vs multilayer ZrS2 nanosheets. (b) TEM image and (C) AFM image of the single-layer ZrS2 nanosheets synthesized through a continuous supply of the chalcogen source in diluted concentration. (d) TEM image and (e) high-magnification side-view TEM image of the multilayer ZrS2 nanodiscs synthesized through a rapid and highly concentrated supply of the chalcogen source. Reproduced with permission from ref 132. Copyright 2014 American Chemical Society.

highlighted in section 3.1.1.132 Briefly, the distinct kinetic difference between 1-DDT and CS2 in the generation of H2S to the reaction medium governed the distinct crystal growth modes between 2D and 3D. Because of the sufficiently higher surface energies of the edge facets, the crystal growth rates in the lateral direction should be much faster than the vertical direction but only when growth is in a critical window to maintain the energy differences between the facets during reaction. By further indepth correlation studies of the relationship between molecular structure and chemical reactivity, precise kinetical control of chalcogen precursor conversion rates can be rationalized, similar to diluted carrier gas flow in the CVD method. Other freestanding TMC nanosheets have been synthesized by modulating the reactivity of the metal precursors. Notably, the key for the synthesis of single-layer VSe2 nanosheets was the use

of interest in catalysis because of their highly exposed catalytically active edge sites, for an understanding of the intrinsic nanoscale properties, such as the size-associated quantum confinement effects (QCE), freestanding structurally genuine platelet nanosheets are needed. To achieve this, the reactivity of precursors has to be modulated to confine the growth trajectory within the lateral a and b axes.97,131−134,188 It has been reported that a diluted chalcogen continuous influx (DCCI) strategy is feasible as a chemical synthetic protocol for preparing single-layer TMC nanosheets.132 When 1-DDT was reacted with ZrCl4 in oleylamine at 245 °C, single-layer nanosheets with a lateral size of ∼200 nm were produced (Figure 19b and 19c). However, when CS2 as an alternative sulfur precursor was used in an identical reaction condition, multilayered ZrS2 nanodiscs with a lateral size of ∼20 nm were produced (Figure 19d and 19e), as 6167


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Figure 20. Colloidal synthesis of single- to multilayered WSe2 nanosheets. (a) Reaction scheme for thickness-controlled WSe2 nanosheets. Reducing the binding affinity of capping ligands is effective for thickness reduction. (b) TEM images of WSe2 nanosheets synthesized with different capping ligands of (i) oleylamine, (ii) oleyl alcohol, and (iii) oleyl acid. (i′, ii′) Cross-section TEM images of the multilayered nanosheets and (iii′) pseudocolored image of the single-layer nanosheets. (c) XRD patterns of WSe2 nanosheets synthesized with (i) oleylamine, (ii) oleyl alcohol, and (iii) oleic acid. Reproduced with permission from ref 131. Copyright 2015 American Chemical Society.

There have been extensive studies on the exfoliation of numerous 2D structures, while the bottom-up synthetic strategy for 2D layered TMC nanosheets is still in a premature stage, and thus, other potentially important compounds, such as PtS2 and ReS2, are yet to be synthesized.190−192 Although reliable synthetic methods for lateral size control at fixed thickness or vice versa remain to be developed and the exact crystal growth mechanism of colloidal TMC nanosheets remains elusive, some of the key concepts can be outlined for future works. The thickness of TMC nanosheets is sensitive to its nucleation and the subsequent growth rate, controlled either by the continuous release of the precursors into the reaction media by a programmed syringe pump or by the selection of molecular reagents that support a kinetically controlled release of a reactive species (e.g., H2S) in the course of the reaction. Another important parameter is adaptation of a cosurfactants strategy that has specific tendencies to stabilize edges or planar facets, so that the growth can occur exclusively at preferred direction. For example, bulky surfactants with high binding affinities can gradually hinder growth in all direction at which ultrasmall (sub10 nm)-sized nanoflakes can be synthesized. Nevertheless, more efforts are needed toward systematic studies and understanding of size-governing factors by both experimental and theoretical approaches. 3.2.2. Control of the Crystal Geometry. In 2D layered TMCs, there has been a growing demand for wet chemistrybased synthetic protocols for producing metastable 1T-MX2 (M = Mo, W and X = S, Se, and Te) because of its novel physicochemical properties resulting from the metallic electronic structure.193 Although alkali-metal-intercalated exfoliation of

of vanadyl acetylacetonate (VO(acac)2) as an efficient V precursor.133 In addition, the different coordinating strengths of the functional groups on the capping ligands can shift the growth trajectory from exclusively two-dimensional growth toward single-layer nanosheets to three-dimensional growth to form multilayered nanosheets. By choosing a capping ligand with low affinity (i.e., oleic acid), single-layer WSe2 nanosheets with lateral sizes of 200 nm can be obtained. On the other hand, multilayered nanosheets with lateral sizes below 20 nm were produced with relatively high affinity capping ligands, such as oleyl alcohol and oleylamine (Figure 20a−c).131 Computational analysis of a simplified model of a nanosheet revealed ligand binding affinities toward the metal centers on the edge facets of −1.475, −1.161, and −0.848 eV for methyl-amine, -alcohol, and formic acid, respectively. Thus, it was experimentally and theoretically demonstrated that the binding affinity of the capping ligands to the edge facets can provide a degree of control in thickness from single to multilayers. This methodology was further developed for the lateral-size-controlled synthesis of WSe2 multilayered quantum dots (MQDs).97 To produce WSe2 quantum dot nanodiscs with a diameter (d) of 5.8 nm on average, a mixture solution of Ph2Se2 and trioctylphosphine oxide (TOPO) was rapidly injected into a solution of W(CO)6 in TOPO. For the size of sub-10 nm regime, the usage of a bulky surfactant ligand (i.e., TOPO) was critical, as the long-chain trialkyl groups served to block incoming monomers, thereby producing smaller particles, as previously reported in the case of 0D II−VI semiconducting quantum dots. The lateral size of WSe2 MQDs can be controlled from 2.5 to 9.7 nm by gradually reducing the molar ratio of the surfactant to the metal precursor. 6168


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Figure 21. Colloidal synthesis of 1T′-WS2 and 2H-WS2 nanosheets. (a) Chemical additives (e.g., hexamethyldisilazane (HMDS)) determine the crystal geometry of single-layer WS2 nanosheets in either 1T′ or 2H form. (b) (i) Image of 1T′-WS2 colloidal dispersion, (ii) TEM image of 1T′-WS2 nanosheets, and (iii) HAADF-STEM image of a single-layer 1T′-WS2 nanosheets. Distorted 1T′ structure is evident by the zigzag pattern of the tungsten atoms. (c) (i) Image of 2H-WS2 colloidal dispersions, (ii) TEM image of 2H-WS2 nanosheets, and (iii) HAADF-STEM image of a single-layer 2H-WS2 nanosheet. Reproduced with permission from ref 194. Copyright 2014 American Chemical Society.

Figure 22. Synthesis of stable metallic 1T′-WS2 nanosheets by in situ intercalation of ammonium ions during the synthesis. (a) TEM image of ammonium ion-intercalated MoS2 nanosheets in a crumpled morphology. (b) HAADF-STEM image of 1T′-MoS2 nanosheets showing an obvious zigzag chain superlattice. (c) Side-view TEM image of 1T′-MoS2 nanosheets with an extended interlayer distance ranging from 9.01 to 9.80 Å. (d) XRD patterns of fresh 1T′-MoS2 and a specimen aged for 6 months. (e) Schematic illustration of 1T′-WS2 layers with the expanded interlayer spacing induced by ammonium ions intercalation. (a−d) Reproduced with permission from ref 162. Copyright 2015 Wiley-VCH. (e) Reproduced with permission from ref 163. Copyright 2015 Wiley-VCH.

MX2 can produce the 1T phase, the crystal geometry was actually in a mixture of 1T and 2H phases and was thermally unstable, transforming back to 2H within a month. Therefore, there have been efforts to develop synthetic protocols for the preparation of pure 1T-MX2 that does not have the tendency to return to the thermally stable 2H phase.162−164,194 Control of the crystal geometry between the 1T and the 2H phases is possible by

utilizing additives such as oleic acid and hexamethyldisilazane (HMDS) (Figure 21a).194 For the synthesis of 1T′-WS2, WCl6, oleic acid, and oleylamine were first mixed in a flask. Then CS2 was rapidly introduced. This precursor mixture was injected in a dropwise manner into an oleylamine solution at 320 °C using a syringe pump (Figure 21b). The crystal structure of the sheets, as determined by STEM imaging, was a distorted octahedral (1T′) 6169


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in selected stoichiometries of MoxW1−xSe2 alloys.213 For the synthesis of MoxW1−xSe2 alloy nanostructures with x = 0−1, for example, hexamethyldisilazane (HMDS) was first injected into a vacuum-flushed mixture of diphenyl diselenide in oleylamine. To this solution, a stoichiometric mixture solution of MoCl5 and WCl6 in oleic acid was drop injected using a syringe pump. MoxW1−xSe2 nanosheets typically had agglomerated nanoflowerlike morphologies (Figure 23a−f). Although the Mo/W ratio

structure (Figure 21b(ii)). Introducing hexamethyldisilazane (HMDS) into the oleylamine solution produced 2H-WS2 nanosheets in an aggregated form (Figure 21c). Although further systematic studies are required to assay the crystal geometry governing pathway during the crystal growth, the reaction pathways of the tungsten precursor appear to be the primary factor. To be specific, the binding of oleic acid to the tungsten nuclei significantly reduces the reactivity such that the reaction is kinetically driven to 1T phase in single layers. On the other hand, addition of HMDS readily reacts with oleate ligands to complex with the tungsten nuclei, thereby promoting the reactivity that results in the formation of thermodynamically favored flower-like aggregated 2H-WS2 multilayered nanosheets. Large-scale synthesis of multilayered 1T′-MoS2 was achieved by employing the hydrothermal method.162−164,195 For the synthesis of 1T′-MoS2, ammonium-intercalated molybdic acid ((NH4)6Mo7O24·4H2O) and thiourea (CS(NH2)2) were dissolved in DIW and subsequently autoclaved.162 In the reaction pathway, the amine molecules in (NH4)6Mo7O24·4H2O hydrolyze and the excess CS(NH2)2 is protonated by water under the hydrothermal conditions at 200 °C to form positively charged ammonium ions (NH4+). The in situ-generated positive NH4+ ions readily intercalate in the layers and form an ionic interaction with sulfur atoms, which may stimulate charge imbalances, resulting in the glide of the sulfur atoms toward the 1T crystal geometry, as seen in the Li intercalation method. Overall, corrugated 1T′-MoS2 nanosheets with an average lateral size of 200 nm were synthesized (Figure 22a). As-synthesized MoS2 nanosheets exhibit clustered Mo atoms in zigzag chain superlattice with the nearest Mo−Mo distance of 2.72 Å, similar to 1T-MoS2 produced by the Li-intercalation method. (Figure 22b). The side-view TEM image revealed interlayer distances ranging from 9.01 to 9.80 Å, which are in agreement with the lowangle shift in the peaks associated with the c axis observed in the XRD pattern (Figure 22c−e). Moreover, the similarities in the XRD patterns between half-year-aged and fresh samples supported the high stability of the intercalated NH4+, therefore, implying that the 1T phase was preserved. Theoretical computation additionally supported the premise that the direct intercalation of NH4+ into 2H-MoS2 did not cause a transformation to 1T-MoS2. Others have further investigated the origin of the favored interlocking metastable 1T′ over the 2H structure that the compressive lattice strain of 1% exerted by the aggregated nanoflower morphology, small grain sizes, and polycrystallinity inhibits the recovery to the thermodynamically stable 2H crystal geometry.196 Current research progress in the crystal geometry shift from 1T to 2H particularly falls into the nanoscale manipulation of charge density waves (CDWs) by chemical doping,197 photoexcitation,198 pressure,199 carrier injection,200 and thickness reduction201 rather than crystal geometry change to 2H or 3R. However, earlier literature reports showed that the crystal geometry change from 1T-TiS2 and TaS2 to 3R is possible by the intercalation of ethylenediamine and Na+ ions.202−205 3.2.3. Control of the Chemical Composition in Alloys. Substitutional 2D TMC alloys can alter band structures, excitonic transitions, photoluminescence emissions, carrier types, and favorable hydrogen adsorption for catalysis.206−209 To date, wet chemical synthetic strategies have been successful in the preparation of MoxW1−xSe2,210−213 WS2ySe2(1−y),213 and MoS2xSe2−2x166,214,215 alloys upon the selection of appropriate precursors. Because of the high miscibility and similar reactivity, TMC alloys with different transition metals have been prepared

Figure 23. Solution synthesis of MoxW1−xSe2 alloys. (a) HAADF-STEM image, (b) TEM images, and (c−f) elemental mapped images (scale bar: 100 nm) of Mo0.35W0.65Se2 nanostructure. (g) Elemental composition percentage (green) and chalcogen/metal ratio (blue) with respect to x in the MoxW1−xSe2 alloy nanostructure. Adapted with permission from ref 213. Copyright 2017 American Chemical Society.

within the alloy nanosheets can be tuned by the use different molar ratios of metal precursors, abundant chalcogen defect sites were present as the chalcogen/metal ratio shortly falls to 1.8 rather than 2 (Figure 23g). Through hydrothermal reactions, Mo1−xWxS2 nanosheets with different compositions were also synthesized by using (NH4)10W12O41·xH2O and (NH4)6Mo7O24·4H2O as metal precursors and thiourea as a sulfur source.165 TMC alloys with different chalcogens have been prepared in a number of different approaches. For example, the sulfurization of solution-prepared MoSe2 nanoflakes at 600 °C and hot injection of a mixture solution of chalcogen precursors have been reported. Because of the different thermolysis kinetics and compatibility of commercially available chalcogen precursors, chalcogen alloys in desired stoichiometries are more synthetically challenging.166,215,216 To resolve this issue, a number of chalcogen sources have been screened, and diaryl−chalcogens have been shown to be the most effective choice. However, issues remain for selenium incorporation beyond the stoichiometry of WS1.20Se0.80 both in colloidal synthetic pathways and in the 6170


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Figure 24. Synthesis of Mn-doped MoS2 nanosheets via a supercritical hydrothermal method. (a) Schematic illustration of the Mn-doped MoS2 synthetic process. (b) Low-magnification TEM image; inset is a high-magnification of the white-boxed region. (c) HRTEM image and corresponding FFT pattern. (d−g) EDX elemental mapping images. (h) HAADF-STEM image, and (i) intensity spectra of the selected area of the Mn-doped MoS2 nanosheets. Reproduced with permission from ref 226. Copyright 2017 Wiley-VCH.

CVD method.217 In addition, the alloy composites with telluride (Te) have not been achieved. In the future, a kinetic understanding of chalcogen precursor decomposition rates and their size and stoichiometry, with close attention to the multiple reaction parameters, will provide an uninterrupted expansion of the designed molecular reagents for the successful preparation of structurally flat 2D TMC alloys. 3.2.4. Substitutional Doping. As dopants are introduced into 2D TMC nanostructures, the emergence of novel properties and advances in the material performance are anticipated.218,219 For example, nitrogen (N) doping has been extensively examined in order to reconfigure the electronic structures of 2D layered nanostructures (i.e., graphene) for tunable materials properties in catalysis, electrochemical energy storage, and electronics. N dopants can add extra charge carriers in the Fermi level and also serve as additional catalytic sites for extraordinary electrochemical properties.220,221 For 2D layered TMCs, it has been proposed that the p orbitals of the N atom can hybridize with the d orbitals of the neighboring transition metal atoms and the p orbitals of the S atoms at the Fermi level, injecting more charge carriers into p-type semiconductors and improve intrinsic conductivity.221,222 The expected outcomes include weak ferromagnetism, improved HER efficiency with the additional active sites, and enhanced conductivity. However, “selfpurification”, whereby extrinsic impurities are expelled from the crystal lattice to release high strain energy, remains a primary hurdle that synthetic chemists must overcome.223,224 Recently, it was discovered that Zn, Mn, Fe, Co, Ni, Cu, Pt, Sn, P, N, and F ions can be substitutionally doped into MoS2 nanosheets by the hydrothermal reaction.225−235 For example, the high supersaturation of the reaction solutions under high-temperature (400 °C) and high-pressure (22 MPa) conditions allows one to overcome the self-purification effect to produce metallic impurities in MoS2 nanosheets (Figure 24a).226 The product is corrugated nanosheets with an average thickness of five layers

and an interlayer distance of 0.63 nm in 2H geometry (Figure 24b and 24c). Energy-dispersive X-ray (EDX) mapping and inductively coupled plasma atomic emission spectroscopy (ICPAES) analysis revealed a Mn/Mo molar ratio of ∼0.03:0.97, corresponding to a chemical composition of Mn0.03Mo0.97S2 (Figure 24d−g). According to the HAADF-STEM imaging and cross-sectional intensity mapping of the atoms, the substitution of Mn atoms (darker spots marked in yellow circles) in the crystal lattice of MoS2 (the brighter white Mo atoms) was achieved (Figure 24h and 24i).

4. 2D LAYERED TMC-BASED MULTICOMPONENT HETEROSTRUCTURES When 2D TMC nanostructures meet other structures, they not only enable the realization of fascinating architectures that were impossible to build before but also achieve materials properties that were not possessed by the pristine structures alone. Owing to the research efforts, the synthesis of 2D TMC-based multicomponent heterostructures has significantly flourished and enabled a variety of interesting studies in catalysis, electrochemical energy storage, and photovoltaics.28,29 Through ion exchange, epitaxial growth, template-based multistep growth, and surface functionalization reaction pathways, various 2D layered TMC heterostructures coupled with metal chalcogenides,236−243 metal oxides,244−251 noble metals,252−256 carbonaceous materials,257−262 and CNT/graphene263−267 have been prepared. Despite the recent advances in this research topic, additional efforts are needed in both the fundamental understanding and the rational design of 2D TMC-based multicomponent heterostructures at a highly controllable level. Currently, reported studies mostly focus on MoS2. Because TMCs constitute a wide range of different chemical compositions, such as TiS2, HfS2, NbS2, and VSe2, and they can be insulators, semiconductors, semimetals, or metals, the hybridization of TMC nanosheets can create new opportunities 6171


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Figure 25. Edge-initiated anion-exchange reaction of TiS2 with H2O to form an intermediate TiS2−TiO2 heterostructure and a product TiO2 toroid. (a) Nucleophilic H2O as chemical stimulus to induce structural transformation. (b) Time-dependent monitoring of morphological transformation of TiS2 nanodiscs to toroid TiO2 nanocrystals. (c) Schematic illustration of morphological transformations of TiS2. (d) Elemental analysis of (ii) TiS2(disc)− TiO2(shell) nanocrystal with sulfur mapped in yellow (disc) and oxygen mapped in red (shell). (e) Nucleophilic attack by the H2O on edge titanium dangling bonds (S, yellow; partially coordinated Ti, green; O, red; H, blue) proceeded to hydrolysis and condensation into Ti−OH and Ti−O−Ti. Reproduced with permission from ref 244. Copyright 2013 American Chemical Society.

TiS2(disc)−TiO2(shell) heterostructure (Figure 25d). After 36 h, a TiS2(disc)−TiO2(toroid) structure with thin gaps (∼3−5 nm) at the interface was observed (Figure 25b(iii)). The formation of voids between TiS2−TiO2 is attributed to Ti4+ ion diffusion via a vacancy-exchange-induced Kirkendall process.270 Eventually, this intermediate structure became toroidal TiO2 after 60 h (Figure 25b(iv)). On the basis of the XRD analysis and later confirmed by theoretical calculations,271 it was shown that the nucleophilic attack of H2O to the unsaturated Ti dangling edge atoms converts into Ti(OH)4−x (x = 1−3) and eventually transformed in amorphous TiO2 as a product (Figure 25e). This structural transformation results in the change of the optical and electronic properties. The TiS2(disc)−TiO2(shell) heterostructure provides the increased solar energy uptake from the UV to the near-infrared region and forms the staggered band alignment (type II heterojunction) with a remarkably enhanced charge separation rate in photocurrent measurements. The peripheral edges could also serve as regioselective nucleation sites for cation-exchange reactions. Treating a TiS2 template with copper(II) chloride transformed into Cu2S−TiS2 heterostructure and a product of toroidal-shaped Cu2S nanocrystals.236 The observed ion-exchange reaction pathway is almost identical to the edge-initiated anion-exchange reaction, where copper ions readily deposit onto coordinatively unsaturated chalcogens ions on a peripheral edge of TiS2 and heteroepitaxially grown into Cu2S. At that time, interfacial release of Ti ions to the media and sulfur ion migration toward Cu2S at the edge can occur. The general applicability of this method was further demonstrated by expanding it to other cationic metal sources (i.e., Ag, Mn, and

for novel functional composites. Furthermore, studies on the hybridization of exfoliated nanosheets have relied on the restacking of the charged nanosheets in colloidal dispersion.90,268 Therefore, it is desirable to develop simple and potential generality of the hybridization routes for the self-assembly, template-assisted assembly, and molecular cross-linking of 2D TMC building blocks utilizing various chemical interactions, such as hydrogen, covalent, and coordination bonding. 4.1. TMCs−Metal−Chalcogenides/Oxides

2D layered TMCs coupled with metal−chalcogenide/oxide heterostructures are essential for establishing fast transfer of photogenerated excitons for photocatalysis, an increased number of reactions sites for the hydrogen evolution reaction (HER) and heterojunctions for band gap engineering.269 Because TMCs are composed of transition metals and chalcogens, ion-exchange reactions with external chemical stimuli are possible, which provide a sophisticated design route for new TMC-based heterostructures with new geometrical shapes and interesting chemical principles in behind. For example, the peripheral edges of 2D layered nanostructures are composed of unsaturated metals and chalcogenides dangling bonds and proven to be reactive sites for ion-exchange reations.23 TiS2−TiO2 (disc− shell) heterostructured nanocrystals were synthesized in an anion-exchange reaction between TiS2 nanocrystals and H2O as an anion source (Figure 25a−c).244 In the first 12 h, the formation of a few-nanometer-thin shell on the edge of TiS2 disc was observed ((Figure 25b(i and ii), as shown by the elemental analysis that the transformed thin shell was composed of TiO2 (red) while the core was TiS2, indicating the formation of a 6172


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Figure 26. Synthesis of Cu2S nanoparticles on the basal plane of MoS2 via an ion-exchange reaction. (a) Schematic illustration of the synthetic route toward Cu2S−MoS2. (b) XRD pattern of Cu2S/MoS2. (c) TEM image showing highly dispersed Cu2S anchored onto MoS2, resulting in the formation of the Cu2S−MoS2 2D heteronanostructure. (d) HAADF-STEM image and elemental mapping of Mo, Cu, and S for Cu2S−MoS2. (e) HRTEM images of Cu2S−MoS2 and fast Fourier transform electron diffraction (FFT ED) patterns (inset). (f) Domain-matching epitaxy of the as-formed Cu2S−MoS2 heteronanostructure. Reproduced with permission from ref 237. Copyright 2016 Wiley-VCH.

Cd) which produced disc−shell intermediates of TiS2−Ag2S, −MnS, and −CdS heterostructures, respectively. Not only the peripheral edge sites but also the basal plane can serve as an ion-exchange reaction site for the synthesis of heterostructured nanosheets.237,242 Negatively charged MoS2 was used as a basic framework onto which the introduced copper metal ions (Cu+) were readily deposited via electrostatic interactions. These Cu+ ions spontaneously reacted with sulfur atoms in the media that dissoluted from the highly fragmented MoS2 during the sonication-assisted exfoliation process, forming CuxS intermediate species (Figure 26a).237 Under the elevated reaction temperature of 250 °C, Cu2S was epitaxially grown on MoS2 in the [001] direction, as these two samples possess the same hexagonal symmetry (Figure 26b−e). The epitaxial growth on MoS2 nanosheets constitutes a general route for the preparation of 2D heteronanostructures, such as Cu2S/MoS2, CdS/MoS2, and FeS/MoS2. The small lattice mismatch of 1.2% in the (100) spacing between MoS2 and Cu2S enabled epitaxial growth by domain matching (Figure 26f). The epitaxial growth of PbSe quantum dots on sonication-exfoliated MoS2 or WS2 nanoflakes was also demonstrated by hot injection of PbO and selenium powder.239 However, the majority of PbSe QDs on the MoS2 showed a random orientation because of the abundant defects on the sonicated nanoflake surface. An alternative mechanistic pathway has been demonstrated for the synthesis of CuS nanoprisms on electrochemically exfoliated TiS2 nanosheets.242 In this synthetic pathway, Li+ was first electrochemically intercalated into bulk TiS2 crystals deposited on Cu foil to form LixTiS2, which resulted in expansion of the interlayer spacing and formation of Li2S as a byproduct. The

highly reactive Li2S and Cu foil then reacted to form CuS nanocrystals on the TiS2 nanosheets. This synthetic process has been further extended to the growth of ZnS and Ni3S2 on TiS2 nanosheets by using Zn and Ni foil instead of Cu. Other studies have designed unique 2D TMC-based heterostructures such as ultrasmall donut-shaped Cu7S4@MoS2 and WOn−WX2 (n = 2.7, 2.9; X = S, Se) by utilizing ion-exchange reactions between the molecular precursors or presynthesized templates.243,245 In addition to ion-exchange reactions, a multistep hydrothermal reaction is another synthetic route where CdS,272,273 ZnS,274 In2S3,275 ZnIn2S4,276 Fe3O4,246 TiO2,247−249,277,278 BiPO4,279 and SnO250,251,280 have been decorated on TMCs or vice versa. In the synthesis of TiO2/MoS2/graphene multicomponent heterostructures, a MoS2/graphene hybrid was first prepared by reacting Na2MoO4 with H2CSNH2 in an aqueous solution of graphene oxide (GO).248 Then the subsequent reaction of Ti(OC4 H 9) 4 and MoS2 /graphene led to the successful production of a TiO2/MoS2/graphene composite. The chalcogen ions exposed on the surface 2D layered TMCs have served as the reactive sites for growth of noble metals or semiconducting nanocrystals. More recently, alternative approaches have been made to epitaxially grow 2D TMCs on existing templates, such as nanowires. As discussed previously, 2D TMC nanosheets have been widely employed as a substrate or the sacrificial agents for the heteroepitaxial growth of various nanostructures. The epitaxially grown TMC nanosheets on other nonlayered nanostructures remain rather unexplored despite the benefits of complementary efficient charge separation and the intellectual interest. For the edge epitaxial growth on a onedimensional (1D) Cu2‑xS nanowire substrate, vertically standing 6173


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Figure 27. Epitaxial growth of MoSe2 nanosheets on Cu2−XS nanowires. (a) Cu2−xS−MoX2 (X = S or Se) heterostructure was obtained through seemediated growth method. (b) TEM image of Cu2−xS−MoSe2 heterostructure. Marked regions correspond to (i) Cu1.94S, (ii) Cu2S, and (iii) heteroepitaxially grown MoSe2. Inset is the FFT pattern of region iii. (c) Crystallographic visualization of heteroepitaxial Cu2−xS−MoSe2 nanostructure. (d) HAADF- and (e) ABF-STEM images of Cu2−xS−MoSe2 heterostructures. (f−h) TEM images of the Cu2−xS−MoSe2 heterostructures with increased average lateral sizes of MoSe2: (f) 1, (g) 5, and (h) 10 nm. (i) Linear dependence of MoSe2 weight percentage (wt %) in MoSe2 in Cu2−xS−MoSe2 heterostructures with respect to the volume of the Se precursor. Reproduced with permission from ref 240. Copyright 2017 American Chemical Society.

nm Pd nanoparticles were epitaxially grown on MoS2 nanosheets by chemical reduction of K2PdCl4 with ascorbic acid (Figure 28a).252 According to HR-TEM analyses, it was revealed that the Pd NPs were epitaxially grown in the major orientations of the (111) and (101) planes on the (001) plane of the MoS2 surface (Figure 28b−d). Mild reducing conditions for preferential nucleation on the 2D substrate rather than in solution and the appropriate use of surfactants, such as sodium citrate, PVP or CTAB, are essential. The noble metals were grown on other 2D layered TMCs, such as TiS2 and TaS2, in a similar approach (Figure 28e−g).255 The Pt−MoS2 hybrid nanostructure showed competitive electrocatalytic properties compared to commercial Pt−C, whereas MoS2 showed little activity. Recently, it was reported that a sonication-assisted polyol method can be employed to synthesize Pd−WS2 hybrid nanostructures at room temperature without the addition of a reductant.256 The energy output by sonication is adequate for the oxidation of ethylene glycol at room temperature, promoting Pd

MoX2 (X = S or Se) nanosheets were grown epitaxially on nanowires (Figure 27a, 27b, 27d, and 27e).240 Since the lattice fringe of the (002) planes of MoSe2 (0.66 nm) was twice that of the (002) planes of Cu2−xS (0.33 nm), epitaxial growth was ensured (Figure 27c). The density and lateral sizes of MoX2 were gradually tuned by the continuous injection of chalcogen precursors (Figure 27f−i). Recently, it was reported that single-layer MS2 (M = W or Mo) can selectively grow on wurtzite CdS to form MS2−CdS nanohybrids.241 It was proposed that the anionic precursor (i.e., thiomolybdate) preferentially deposited onto the Cd-rich (0001) surface, promoting regioselective nucleation and growth. 4.2. TMCs−Noble Metals

Epitaxial growth of noble metal nanocrystals on large-area singlelayer MoS2 nanosheets has been realized by choosing appropriate reduction conditions including chemical or photochemical reduction or electroless deposition with the precursors of K2PtCl4, K2PdCl4, HAuCl4, or AgNO3.252−255 For example, ∼5 6174


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Figure 28. Solution-based epitaxial growth of noble metal nanoparticles (NPs) on single-layer TMC nanosheets. (a−d) TEM analyses of Pd NPs epitaxially grown on a MoS2 nanosheet. (a) Low-magnification TEM image. (b) SAED pattern. (c and d) HRTEM images. (Inset in d) Photograph of the Pd−MoS2 colloidal dispersion. (e) TEM images of the Pt−MoS2 nanosheet:(i) Low-magnification and (ii) high-resolution image. (f) TEM images of the Pt−TiS2 nanosheet: (i) Low-magnification and (ii) high-resolution image. (g) TEM images of the Pt−TaS2 nanosheet: (i) Low-magnification and (ii) high-resolution image. (a−e) Reproduced with permission from ref 252. Copyright 2013 Nature Publishing Group. (f and g) Reproduced with permission from ref 255. Copyright 2015 Royal Society of Chemistry.

growth of TMC nanosheets.261 Thermodynamically unfavorable yolk−shell MoS2@C nanospheres were prepared by the synthetic strategy illustrated in Figure 30a. A hollow mesoporous carbon sphere (HMCS) reaction chamber was initially produced by co-condensation of oligomers and the surface calcination of SiO2 particles. Then the SiO2 core was selectively etched away by NaOH (Figure 30b). MoS2 nanosheets were grown hydrothermally inside of the HMCSs. The growth of yolk−shell MoS2@C nanospheres is associated with the ability of the precursors to penetrate into the carbon spheres (Figure 30c). When mesopores were absent from the carbon shell, MoS2 growth was limited to the outside of the carbon shells. This method was further extended to obtain other chemical compositions of yolk−shell TMCs@C nanospheres, such as WS2@C and SnS2@C. 2D layered TMCs could also serve as a core material for encapsulation by zeolitic imidazolate frameworks (ZIFs) (Figure 31a).262,284 For the synthesis, aqueous solutions of 2methylimidazole (C4H6N2) and zinc acetate (Zn(CH3COO)2· 2H 2 O) were added to the solution of exfoliated MoS 2 nanosheets. TEM images of the product clearly present curled MoS2 sheets embedded in a ZIF shell (Figure 31b).284 Thermal treatment of hydro/solvothermally synthesized 2D layered TMCs is another active method in which carbonization of in situ intercalated organic surfactants, such as PVP,168,285,286 octylamine,287 and dopamine,288−290 produces TMCs@C composites. Lee et al. recently demonstrated the synthesis of hierarchical MoS2@C nanotubes consisting of alternative monolayers of 1T-MoS2 and carbon by solvothermal reaction.287 In the synthesis, MoO3 and sulfur powders were dissolved in octylamine to form aggregated MoS2 nanosheets with an

nanoparticles to grow on the surface of the template WS2 nanosheets. Although the epitaxial growth of Pd nanoparticles was not confirmed in this paper, the hybrid structure exhibited a high turnover frequency in photocatalyzed Suzuki reactions. 4.3. TMCs−Carbonaceous Materials

A multistep synthetic scheme was employed to form core−shell TMC heterostructures, where the cavities in the hollow core could effectively alleviate the strain from the large volume entanglement during the multiple cycle of charge/discharge and the decorated shell on TMC nanosheets possess a high surface area for electrolytes and a reduced diffusion mean free path for electrons and ions.281−283 All of these merits contribute to the high electrochemical performance. The hollow carbonaceous core can be synthesized in a variety of ways involving multistep compositional change and subsequent etching. Then 2D layered TMCs can be grown on the hollow core via a seed-mediated growth method (Figure 29a).257 For example, cube-shaped αFe2O3 with edge dimensions of 500 nm was synthesized by a coprecipitation method. Then a smooth layer of polydopamine (PDA) was deposited on the surface (Figure 29b and 29c). In step II, these core−shell α-Fe2O3@PDA particles were transformed into α-Fe2O3@C nanocubes by carbonization in a N2 atmosphere. Then the α-Fe2O3 core was etched away with HCl to obtain carbon nanoboxes. Finally, MoS2 nanosheets were readily grown on the surface by a solvothermal reaction, producing C@MoS2 nanoboxes (Figure 29d and 29e). A similar approach has yielded three-layered TiO2@C@MoS2 tubular nanowires.258 A presynthesized template can serve not only as a substrate for nucleation sites but also as a reaction chamber for the confined 6175


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Figure 29. Synthesis of MoS2 nanosheets on N-doped carbon nanoboxes. (a) Illustration of the synthetic process to form C@MoS2 nanoboxes. TEM images of (b, c) carbon nanobox template and (d, e) C@MoS2 nanoboxes. Reproduced with permission from ref 257. Copyright 2015 Wiley-VCH.

Figure 30. Synthesis of MoS2 nanosheets in space-confined hollow mesoporous carbon spheres. (a) Synthetic approach for the yolk−shell MoS2@C nanospheres. TEM images of (b) hollow mesoporous carbon spheres and (c) MoS2@C nanospheres. Reproduced with permission from ref 261. Copyright 2017 American Chemical Society.

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Figure 31. Encapsulation of MoS2 in microporous carbon polymers (MPC). (a) Schematic illustration of MoS2@MPC preparation. (b) TEM images of (i) MoS2 nanosheets, (ii) MoS2@ZIF-8, and (iii) MoS2@MPC. Reproduced with permission from ref 284. Copyright 2014 Royal Society of Chemistry.

Figure 32. Solvothermal synthesis of MoS2 NPs on reduced graphene oxide (rGO). (a) Synthetic scheme for MoS2/rGO hybrid structure. (b−d) Microscopic characterization of MoS2/rGO hybrid nanosheets. (b) SEM image. (c) TEM image. High-magnification image of the folded edge of a MoS2 NPs. (d) High-resolution TEM image. Reproduced with permission from ref 264. Copyright 2011 American Chemical Society.

synthesis of MoS2/rGO hybrid nanosheets, one-step solvothermal protocols have been reported using (NH4)2MoS4 and hydrazine as chemical precursors in the graphene oxide solution. The product showed randomly distributed MoS2 nanoparticles with fold edges on graphene oxide (Figure 32b−d).264 The (NH4)2MoS4 precursor was first deposited on the surfaceexposed oxygen functional groups of GO and became thermally reduced to MoS2. Notably, the aggregated MoS2 nanosheets were formed without the presence of GO. Furthermore, by carefully controlling the reaction conditions, the synthesis of edge-oriented vertical MoS2 nanosheets on rGO was also demonstrated (Figure 33a−c).176,291,292 In addition, a higher density of MoS2 with longer edge length was grown on rGO by gradually increasing the reaction temperature (Figure 33d).176 It is proposed that the difference in surface wettability between rGO and MoS2 compelled the nanosheets to grow vertically on the rGO surfaces. Other reports have utilized nitrogen (N)doped CNT and rGO for MoS2 deposition owing to the attraction of the anionic thiomolybdate MoS2 precursor to the protonated lone pair of electrons at the pyridinic N-doped site.293 Control experiments showed that small MoSx patches were unevenly distributed on the pristine CNT surface, whereas

interlayer gap spacing of 9.8 Å, indicating the intercalation of octylamine between the layers. Finally, the intercalated molecules could be carbonized to amorphous carbon under solvothermal conditions, resulting in a MoS2@C composite. Because of the expanded interlayer spacing produced by the carbonaceous shell, the TMCs@C composite electrodes exhibited a high reversible specific capacity with a long cycle life in electrochemical energy storage applications. 4.4. TMCs−CNT/Graphene

To further increase the conductivity, 2D layered TMCs have been incorporated in various conductive backbones, including carbon nanotubes (CNT), graphene, and conductive polymers.263−267 In particular, graphene has been widely utilized for its high electrical conductivity, flexibility, and chemical robustness. To facilitate these properties, TMC−graphene hybrid nanostructures have been synthesized, especially on reduced graphene oxides (rGO) in which the MoS2 nanosheets either lie flat or stand vertically on the rGO. The abundant oxygen functional groups (e.g., −COO−, −COOH, and −COH) on rGO sheets serve as MoS2 nucleation sites due to their high chemical interactions with Mo precursors (Figure 32a). For the 6177


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Figure 33. Synthesis of vertically standing MoS2 on reduced graphene oxide (rGO). (a) Schematic illustration of the vertical of MoS2 with expanded interlayer spacing on rGO. (b and c) TEM images of vertical MoS2/rGO nanosheets. (b) Low-magnification and (c) HRTEM images. (d) SEM images of MoS2/rGO synthesized at various reaction temperatures for increased density with longer edge lengths of MoS2 nanosheets: (i) 200, (ii) 220, (iii) 240, and (iv) 260 °C. Adapted with permission from ref 176. Copyright 2017 American Chemical Society.

Figure 34. Synthesis of carbon nanotube (CNT)-wired hierarchical MoS2 tubular structures. (a) Synthetic preparation scheme for the CNT/ polyacrylonitrile (PAN) nanowire template. (b) Multistep synthetic process for the CNT-in-tubular MoS2 structure. (i) Deposition of a protective CoSx layer on the CNT/PAN nanowires. (ii) Hydrothermal growth of MoS2 nanosheets on the nanowire. In this step tubular nanowire is obtained by thermal dissolution of PAN in the core. (iii) Thermal reduction of CoSx layer to Co NPs. (iv) Selective acid etching of Co NPs to obtain product CNT/MoS2 tubular structure. (c) FESEM and TEM images of the CNT-in-PAN nanofibers (i and i′), CNT/PAN-CoSx nanofibers (ii and ii′), CNT/MoS2−CoSx tubular structures (iii and iii′), and CNT/MoS2−Co tubular structures (iv and iv′). Reproduced with permission from ref 259. Copyright 2016 AAAS.

an ultrathin MoSx layer was densely coated on the N-doped CNT surface. The hybridization of TMCs and CNTs was also reported to enhance the electrical conductivity.259,294,295 For example, CNTwired hierarchical MoS2 nanotubes were successfully prepared by

the following synthetic approach, as schematically shown in Figure 34b.259 First, carboxyl-group-functionalized multiwalled CNTs were coated with polyacrylonitrile (PAN) nanofibers by electrospinning (Figure 34a). During this process, the nitrile functional groups of PAN allowed homogeneous deposition by 6178


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and/or aggregated, which has hindered a better understanding of their materials functionalities including electrical, optical, piezoelectric, and valley properties. With the advances of the field, it is expected that the versatility of solution synthetic methodologies will provide new and important development such as solution-based heteroatom inclusion chemistry to generate proper doping and alloying on specific sites of structurally flat 2D TMC nanosheets. Another important aspect of 2D layered TMCs that has been poorly understood is their surface chemistry. To comprehend and establish the basic principles of these chemical phenomena at the surface such as chemisorption of molecules, redox chemistry, atomic exchanges, and corrosion for practical applications, it is anticipated to explore the relations between TMCs and reactive molecules. The explicit understanding of chemical principles will not only benefit the design of advanced functional properties but also contribute to attaining stable TMC-based nanodevices. In addition, as the guiding principles of chemical substitution, diffusion, corrosion, and extraction are developed, it will be possible to engineer edge and basal coordination state, porosity, doping level, and surface-modified 2D TMC nanostructures. On the basis of the recent progress regarding the science of 2D layered TMCs, we foresee an enormous potential impact of chemical solution-based methodologies in establishing fundamentally new and unexpected phenomena and properties of 2D nanostructures. All of these capabilities provide a bridge for the practical incorporation of 2D layered TMC nanostructures in cutting-edge high-performance electronic, optoelectronic, and electrochemical devices and biological applications.

coordinating with the carboxyl groups on the CNTs (Figure 34c(i and i′)). Then a protective CoSx layer was coated onto the CNT/PAN nanowire to avoid morphological deformation and obtain a 1D morphology in the final product (Figure 34c(ii and ii′)). The hydrothermal growth of ultrathin MoS2 nanosheets on the composite was then performed, followed by complete removal of the PAN to form a CNT-wired tubular MoS2 structure as the product (Figure 34c(iii and iii′)). Heating and acid treatments were necessary to remove the CoSx layers at the end (Figure 34c(iv and iv′)). The glucose-assisted synthesis of MoS2 nanosheets on CNTs has also been reported. It was proposed that glucose might serve as an efficient binder of MoS2 to CNTs.295 The CNT-wired tubular MoS2 structure exhibited outstanding lithium ion battery (LIB) performance with a high charge capacity, rate capability, and ultralong cycle life. Recently, there have been enormous efforts to utilize solution-prepared 2D TMC nanosheets in electrochemical energy storage applications, such as super- and pseudocapacitors, where promising results have been shown through high energy and power densities and longer cycling life.189,296 It is believed that additional efforts on 2D TMC-based electrodes with miniaturization to the nanoscale and porosity generated by surface activations may prove to be interesting as they facilitate high surface area and multiple electrolyte diffusion pathways that can significantly boost the electrochemical energy storage property of 2D layered materials.297−300

5. SUMMARY AND OUTLOOK In this review, we outlined the solution-based preparation strategies for 2D layered TMC nanostructures as efficient methods for producing single- or few-layered 2D TMCs with control of the size/thickness, chemical composition, crystal geometry, and heterostructure. There is plethora of opportunities for new chemical preparation protocols for producing next-generation 2D TMCs. The advantages and disadvantages of the presented approaches are as follows: In the top-down approaches, the sonication-assisted exfoliation is a simple method with scalability; however, extensive sonication can produce heterogeneity in size and defect-rich nanoflakes. The alkali metal intercalation-induced exfoliation process has been effective and led to the interesting discovery of the crystal geometry shift from 2H to the 1T. However, the harsh reaction condition and formation of undesirable byproducts (such as Li salts) can generate defects and crystal deformations. Alternatively, intercalation of organic molecules for inorganic/organic hybrid nanostructures with structural flexibility and functionality has recently emerged. The molecular design of proper intercalates of organic−inorganic hybrid nanostructures for achieving high permeability and effective exfoliation to enhance device performance is yet to be demonstrated. There is no doubt that intercalation chemistry has new opportunities in the rational design of intercalates for stage-controlled intercalation for new properties such as fine tuning of the band structure, Fermi level, and carrier density for next-generation energy storage and conversion. Currently, solution-based bottom-up approaches are in development, and understanding of the mechanisms underlying the 2D crystal growth remains limited. For example, the lateral sizes of colloidal 2D TMC nanosheets are in the regime of a few hundred nanometers, and their size controllability limitation is one of the disadvantages involved when comparing CVD-grown 2D nanostructures. At present, most of the solution-prepared doped and alloy nanosheets are typically crumpled, buckled,

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Jae Hyo Han: 0000-0003-1905-6971 Jinwoo Cheon: 0000-0001-8948-5929 Notes

The authors declare no competing financial interest. Biographies Jae Hyo Han is currently a postdoctoral fellow at the Institute for Basic Science (IBS) Center for Nanomedicine. He graduated with a B.S. degree from the University of Wisconsin−Madison and received his Ph.D. degree from Yonsei University under the supervision of Prof. Jinwoo Cheon. His research interest encompasses unveiling the chemical reactivity of 2D layered nanostructures. Minkyoung Kwak is currently a graduate student at Yonsei University under the supervision of Prof. Jinwoo Cheon. Her research is focused on the development of surface functionalization methods for 2D layered transition metal chalcogenide nanostructures. Youngsoo Kim is currently a research scientist at the Institute for Basic Science (IBS) Center for Nanomedicine. He graduated with a B.S. degree from Korea University and received his Ph.D. degree from there as well. His research is on nanoscale interface engineering between plasmonic nanoparticles and 2D layered structures. Jinwoo Cheon is the Horace G. Underwood Professor at Yonsei University and the director of the Institute for Basic Science (IBS) Center for Nanomedicine. He graduated with a B.S. degree from Yonsei University and received his Ph.D. degree from the University of Illinois at Urbana−Champaign. After his postdoctoral training at the University 6179


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Casiraghi, C. Heterostructures Produced from Nanosheet-Based Inks. Nano Lett. 2014, 14, 3987−3992. (18) Tedstone, A. A.; Lewis, D. J.; O’Brien, P. Synthesis, Properties, and Applications of Transition Metal-Doped Layered Transition Metal Dichalcogenides. Chem. Mater. 2016, 28, 1965−1974. (19) Li, X.; Shan, J.; Zhang, W.; Su, S.; Yuwen, L.; Wang, L. Recent Advances in Synthesis and Biomedical Applications of Two-Dimensional Transition Metal Dichalcogenide Nanosheets. Small 2017, 13, 1602660. (20) Nasilowski, M.; Mahler, B.; Lhuillier, E.; Ithurria, S.; Dubertret, B. Two-Dimensional Colloidal Nanocrystals. Chem. Rev. 2016, 116, 10934−10982. (21) Cao, X. H.; Tan, C. L.; Zhang, X.; Zhao, W.; Zhang, H. SolutionProcessed Two-Dimensional Metal Dichalcogenide-Based Nanomaterials for Energy Storage and Conversion. Adv. Mater. 2016, 28, 6167− 6196. (22) Voiry, D.; Mohite, A.; Chhowalla, M. Phase Engineering of Transition Metal Dichalcogenides. Chem. Soc. Rev. 2015, 44, 2702− 2712. (23) Bhimanapati, G. R.; Lin, Z.; Meunier, V.; Jung, Y.; Cha, J.; Das, S.; Xiao, D.; Son, Y.; Strano, M. S.; Cooper, V. R.; et al. Recent Advances in Two-Dimensional Materials beyond Graphene. ACS Nano 2015, 9 (12), 11509−11539. (24) Zhang, X. D.; Xie, Y. Recent Advances in Free-Standing TwoDimensional Crystals with Atomic Thickness: Design, Assembly and Transfer Strategies. Chem. Soc. Rev. 2013, 42, 8187−8199. (25) Zhang, X.; Lai, Z. C.; Tan, C. L.; Zhang, H. Solution-Processed Two-Dimensional MoS2 Nanosheets: Preparation, Hybridization, and Applications. Angew. Chem., Int. Ed. 2016, 55, 8816−8838. (26) Guo, Y. Q.; Xu, K.; Wu, C. Z.; Zhao, J. Y.; Xie, Y. Surface Chemical-Modification for Engineering the Intrinsic Physical Properties of Inorganic Two-Dimensional Nanomaterials. Chem. Soc. Rev. 2015, 44, 637−646. (27) Han, J. H.; Lee, S.; Cheon, J. Synthesis and Structural Transformations of Colloidal 2D Layered Metal Chalcogenide Nanocrystals. Chem. Soc. Rev. 2013, 42, 2581−2591. (28) Tan, C. L.; Zhang, H. Two-Dimensional Transition Metal Dichalcogenide Nanosheet-Based Composites. Chem. Soc. Rev. 2015, 44, 2713−2731. (29) Zhang, X.; Cheng, H.; Zhang, H. Recent Progress in the Preparation, Assembly, Transformation, and Applications of LayerStructured Nanodisks beyond Graphene. Adv. Mater. 2017, 29, 1701704. (30) Xu, J.; Zhang, J. J.; Zhang, W. J.; Lee, C. S. Interlayer Nanoarchitectonics of Two-Dimensional Transition-Metal Dichalcogenides Nanosheets for Energy Storage and Conversion Applications. Adv. Energy Mater. 2017, 7, 1700571. (31) Kang, J.; Sangwan, V. K.; Wood, J. D.; Hersam, M. C. SolutionBased Processing of Monodisperse Two-Dimensional Nanomaterials. Acc. Chem. Res. 2017, 50, 943−951. (32) Kelly, A. G.; Hallam, T.; Backes, C.; Harvey, A.; Esmaeily, A. S.; Godwin, I.; Coelho, J.; Nicolosi, V.; Lauth, J.; Kulkarni, A.; et al. AllPrinted Thin-Film Transistors from Networks of Liquid-Exfoliated Nanosheets. Science 2017, 356, 69−73. (33) Hu, G. H.; Albrow-Owen, T.; Jin, X. X.; Ali, A.; Hu, Y. W.; Howe, R. C. T.; Shehzad, K.; Yang, Z. Y.; Zhu, X. K.; Woodward, R. I.; et al. Black Phosphorus Ink Formulation for Inkjet Printing of Optoelectronics and Photonics. Nat. Commun. 2017, 8, 278. (34) McManus, D.; Vranic, S.; Withers, F.; Sanchez-Romaguera, V.; Macucci, M.; Yang, H. F.; Sorrentino, R.; Parvez, K.; Son, S. K.; Iannaccone, G.; et al. Water-Based and Biocompatible 2D Crystal Inks for All-Inkjet-Printed Heterostructures. Nat. Nanotechnol. 2017, 12, 343−350. (35) Bonaccorso, F.; Lombardo, A.; Hasan, T.; Sun, Z. P.; Colombo, L.; Ferrari, A. C. Production and Processing of Graphene and 2D Crystals. Mater. Today 2012, 15, 564−589. (36) Wang, M.; Xu, X. W.; Ge, Y. C.; Dong, P.; Baines, R.; Ajayan, P. M.; Ye, M. X.; Shen, J. F. Surface Tension Components Ratio: An

of California−Berkeley and UCLA, he joined KAIST and later moved to Yonsei University. He is a recipient of numerous awards, including the Ho−Am Prize, POSCO TJ Park Prize, Inchon Prize, and Song-gok Science Award. Currently. He is a Senior Editor of Accounts of Chemical Research and a fellow of the American Chemical Society and the Royal Society of Chemistry.

ACKNOWLEDGMENTS This work was supported by the Institute for Basic Science (IBSR026-D1). REFERENCES (1) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263−275. (2) Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; et al. TwoDimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568−571. (3) Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Liquid Exfoliation of Layered Materials. Science 2013, 340, 1226419. (4) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100−102. (5) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Engineering the Surface Structure of MoS2 to Preferentially Expose Active Edge Sites for Electrocatalysis. Nat. Mater. 2012, 11, 963−969. (6) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (7) Lee, J.; Wang, Z.; Xie, H.; Mak, K. F.; Shan, J. Valley Magnetoelectricity in Single-Layer MoS2. Nat. Mater. 2017, 16, 887− 891. (8) Zeng, H.; Dai, J.; Yao, W.; Xiao, D.; Cui, X. Valley Polarization in MoS2 Monolayers by Optical Pumping. Nat. Nanotechnol. 2012, 7, 490− 493. (9) Mak, K. F.; He, K.; Shan, J.; Heinz, T. F. Control of Valley Polarization in Monolayer MoS2 by Optical Helicity. Nat. Nanotechnol. 2012, 7, 494−498. (10) Acerce, M.; Akdogan, E. K.; Chhowalla, M. Metallic Molybdenum Disulfide Nanosheet-Based Electrochemical Actuators. Nature 2017, 549, 370−373. (11) Gopalakrishnan, D.; Damien, D.; Shaijumon, M. M. MoS2 Quantum Dot-Interspersed Exfoliated MoS2 Nanosheets. ACS Nano 2014, 8, 5297−5303. (12) Ha, H. D.; Han, D. J.; Choi, J. S.; Park, M.; Seo, T. S. Dual Role of Blue Luminescent MoS2 Quantum Dots in Fluorescence Resonance Energy Transfer Phenomenon. Small 2014, 10, 3858−3862. (13) Zhang, X.; Lai, Z. C.; Liu, Z. D.; Tan, C. L.; Huang, Y.; Li, B.; Zhao, M. T.; Xie, L. H.; Huang, W.; Zhang, H. A Facile and Universal Top-Down Method for Preparation of Monodisperse Transition-Metal Dichalcogenide Nanodots. Angew. Chem., Int. Ed. 2015, 54, 5425−5428. (14) Varrla, E.; Backes, C.; Paton, K. R.; Harvey, A.; Gholamvand, Z.; McCauley, J.; Coleman, J. N. Large-Scale Production of Size-Controlled MoS2 Nanosheets by Shear Exfoliation. Chem. Mater. 2015, 27, 1129− 1139. (15) Li, J. T.; Naiini, M. M.; Vaziri, S.; Lemme, M. C.; Ostling, M. Inkjet Printing of MoS2. Adv. Funct. Mater. 2014, 24, 6524−6531. (16) Finn, D. J.; Lotya, M.; Cunningham, G.; Smith, R. J.; McCloskey, D.; Donegan, J. F.; Coleman, J. N. Inkjet Deposition of Liquid-exfoliated Graphene and MoS2 Nanosheets for Printed Device Applications. J. Mater. Chem. C 2014, 2, 925−932. (17) Withers, F.; Yang, H.; Britnell, L.; Rooney, A. P.; Lewis, E.; Felten, A.; Woods, C. R.; Sanchez Romaguera, V.; Georgiou, T.; Eckmann, A.; Kim, Y. J.; Yeates, S. G.; Haigh, S. J.; Geim, A. K.; Novoselov, K. S.; 6180


Chemical Reviews

Review

Efficient Parameter for Direct Liquid Phase Exfoliation. ACS Appl. Mater. Interfaces 2017, 9, 9168−9175. (37) Shen, J. F.; Wu, J. J.; Wang, M.; Dong, P.; Xu, J. X.; Li, X. G.; Zhang, X.; Yuan, J. H.; Wang, X. F.; Ye, M. X.; et al. Surface Tension Components Based Selection of Cosolvents for Efficient Liquid Phase Exfoliation of 2D Materials. Small 2016, 12, 2741−2749. (38) May, P.; Khan, U.; Hughes, J. M.; Coleman, J. N. Role of Solubility Parameters in Understanding the Steric Stabilization of Exfoliated TwoDimensional Nanosheets by Adsorbed Polymers. J. Phys. Chem. C 2012, 116, 24390−24391. (39) Coleman, J. N. Liquid-Phase Exfoliation of Nanotubes and Graphene. Adv. Funct. Mater. 2009, 19, 3680−3695. (40) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699−712. (41) Cunningham, G.; Lotya, M.; Cucinotta, C. S.; Sanvito, S.; Bergin, S. D.; Menzel, R.; Shaffer, M. S. P.; Coleman, J. N. Solvent Exfoliation of Transition Metal Dichalcogenides: Dispersibility of Exfoliated Nanosheets Varies Only Weakly between Compounds. ACS Nano 2012, 6, 3468−3480. (42) Gaur, A. P. S.; Sahoo, S.; Ahmadi, M.; Dash, S. P.; Guinel, M. J. F.; Katiyar, R. S. Surface Energy Engineering for Tunable Wettability through Controlled Synthesis of MoS2. Nano Lett. 2014, 14, 4314− 4321. (43) Backes, C.; Smith, R. J.; McEvoy, N.; Berner, N. C.; McCloskey, D.; Nerl, H. C.; O’Neill, A.; King, P. J.; Higgins, T.; Hanlon, D.; et al. Edge and Confinement Effects Allow in situ Measurement of Size and Thickness of Liquid-Exfoliated Nanosheets. Nat. Commun. 2014, 5, 4576. (44) Smith, R. J.; King, P. J.; Lotya, M.; Wirtz, C.; Khan, U.; De, S.; O’Neill, A.; Duesberg, G. S.; Grunlan, J. C.; Moriarty, G.; et al. LargeScale Exfoliation of Inorganic Layered Compounds in Aqueous Surfactant Solutions. Adv. Mater. 2011, 23, 3944−3948. (45) O’Neill, A.; Khan, U.; Coleman, J. N. Preparation of High Concentration Dispersions of Exfoliated MoS2 with Increased Flake Size. Chem. Mater. 2012, 24, 2414−2421. (46) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z. Y.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’ko, Y. K.; et al. High-Yield Production of Graphene by Liquid-Phase Exfoliation of Graphite. Nat. Nanotechnol. 2008, 3, 563−568. (47) Shen, J. F.; He, Y. M.; Wu, J. J.; Gao, C. T.; Keyshar, K.; Zhang, X.; Yang, Y. C.; Ye, M. X.; Vajtai, R.; Lou, J.; et al. Liquid Phase Exfoliation of Two-Dimensional Materials by Directly Probing and Matching Surface Tension Components. Nano Lett. 2015, 15, 5449−5454. (48) Zhou, K. G.; Mao, N. N.; Wang, H. X.; Peng, Y.; Zhang, H. L. A Mixed-Solvent Strategy for Efficient Exfoliation of Inorganic Graphene Analogues. Angew. Chem., Int. Ed. 2011, 50, 10839−10842. (49) Kim, J.; Kwon, S.; Cho, D. H.; Kang, B.; Kwon, H.; Kim, Y.; Park, S. O.; Jung, G. Y.; Shin, E.; Kim, W. G.; et al. Direct Exfoliation and Dispersion of Two-Dimensional Materials in Pure Water via Temperature Control. Nat. Commun. 2015, 6, 8294. (50) Halim, U.; Zheng, C. R.; Chen, Y.; Lin, Z. Y.; Jiang, S.; Cheng, R.; Huang, Y.; Duan, X. F. A Rational Design of Cosolvent Exfoliation of Layered Materials by Directly Probing Liquid-Solid Interaction. Nat. Commun. 2013, 4, 2213. (51) De-Mello, G. B.; Smith, L.; Rowley-Neale, S. J.; Gruber, J.; Hutton, S. J.; Banks, C. E. Surfactant-Exfoliated 2D Molybdenum Disulphide (2D-MoS2): the Role of Surfactant Upon the Hydrogen Evolution Reaction. RSC Adv. 2017, 7, 36208−36213. (52) Guardia, L.; Paredes, J. I.; Rozada, R.; Villar-Rodil, S.; MartinezAlonso, A.; Tascon, J. M. D. Production of Aqueous Dispersions of Inorganic Graphene Analogues by Exfoliation and Stabilization with Non-Ionic Surfactants. RSC Adv. 2014, 4, 14115−14127. (53) Guan, G. J.; Zhang, S. Y.; Liu, S. H.; Cai, Y. Q.; Low, M.; Teng, C. P.; Phang, I. Y.; Cheng, Y.; Duei, K. L.; Srinivasan, B. M.; Zheng, Y. G.; Zhang, Y. W.; Han, M. Y. Protein Induces Layer-by-Layer Exfoliation of Transition Metal Dichalcogenides. J. Am. Chem. Soc. 2015, 137, 6152− 6155.

(54) Zhao, M.; Chang, M. J.; Wang, Q.; Zhu, Z. T.; Zhai, X. P.; Zirak, M.; Moshfegh, A. Z.; Song, Y. L.; Zhang, H. L. Unexpected Optical Limiting Properties from MoS2 Nanosheets Modified by A Semiconductive Polymer. Chem. Commun. 2015, 51, 12262−12265. (55) Xuan, D. D.; Zhou, Y. F.; Nie, W. Y.; Chen, P. P. Sodium AlginateAssisted Exfoliation of MoS2 and Its Reinforcement in Polymer Nanocomposites. Carbohydr. Polym. 2017, 155, 40−48. (56) Wang, D.; Song, L.; Zhou, K. Q.; Yu, X. J.; Hu, Y.; Wang, J. Anomalous Nano-Barrier Effects of Ultrathin Molybdenum Disulfide Nanosheets for Improving the Flame Retardance of Polymer Nanocomposites. J. Mater. Chem. A 2015, 3, 14307−14317. (57) Backes, C.; Szydlowska, B. M.; Harvey, A.; Yuan, S. J.; VegaMayoral, V.; Davies, B. R.; Zhao, P. L.; Hanlon, D.; Santos, E. J. G.; Katsnelson, M. I.; et al. Production of Highly Monolayer Enriched Dispersions of Liquid-Exfoliated Nanosheets by Liquid Cascade Centrifugation. ACS Nano 2016, 10, 1589−1601. (58) Kang, J.; Seo, J. W. T.; Alducin, D.; Ponce, A.; Yacaman, M. J.; Hersam, M. C. Thickness Sorting of Two-Dimensional Transition Metal Dichalcogenides via Copolymer-Assisted Density Gradient Ultracentrifugation. Nat. Commun. 2014, 5, 5478. (59) Sun, X. M.; Luo, D. C.; Liu, J. F.; Evans, D. G. Monodisperse Chemically Modified Graphene Obtained by Density Gradient Ultracentrifugal Rate Separation. ACS Nano 2010, 4, 3381−3389. (60) Ghosh, S.; Bachilo, S. M.; Weisman, R. B. Advanced Sorting of Single-Walled Carbon Nanotubes by Nonlinear Density-Gradient Ultracentrifugation. Nat. Nanotechnol. 2010, 5, 443−450. (61) Green, A. A.; Hersam, M. C. Solution Phase Production of Graphene with Controlled Thickness via Density Differentiation. Nano Lett. 2009, 9, 4031−4036. (62) Kang, J.; Sangwan, V. K.; Wood, J. D.; Liu, X.; Balla, I.; Lam, D.; Hersam, M. C. Layer-by-Layer Sorting of Rhenium Disulfide via HighDensity Isopycnic Density Gradient Ultracentrifugation. Nano Lett. 2016, 16, 7216−7223. (63) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Sorting Carbon Nanotubes by Electronic StructureUsing Density Differentiation. Nat. Nanotechnol. 2006, 1, 60−65. (64) Green, A. A.; Hersam, M. C. Processing and Properties of Highly Enriched Double-Wall Carbon Nanotubes. Nat. Nanotechnol. 2009, 4, 64−70. (65) Whittingham, M. S.; Jacobson, A. J. Intercalation Chemistry; Academic Press: New York, 1982; p 595. (66) Lévy, F. Intercalated Layered Materials; D. Reidel Pub. Co.: Dordrecht, Holland; Boston, 1979; p 578. (67) Dresselhaus, M. S. Intercalated Graphite: Symposium, Boston, MA, Nov 1982; North-Holland: New York, 1983; p 414. (68) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.; Jin, S. Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets. J. Am. Chem. Soc. 2013, 135, 10274−10277. (69) Yuwen, L. H.; Yu, H.; Yang, X. R.; Zhou, J. J.; Zhang, Q.; Zhang, Y. Q.; Luo, Z. M.; Su, S.; Wang, L. H. Rapid Preparation of Single-Layer Transition Metal Dichalcogenide Nanosheets via Ultrasonication Enhanced Lithium Intercalation. Chem. Commun. 2016, 52, 529−532. (70) Ambrosi, A.; Sofer, Z.; Pumera, M. Lithium Intercalation Compound Dramatically Influences the Electrochemical Properties of Exfoliated MoS2. Small 2015, 11, 605−612. (71) Schollhorn, R. Reversible Topotactic Redox Reactions of Solids by Electron-Ion Transfer. Angew. Chem., Int. Ed. Engl. 1980, 19, 983− 1003. (72) Joensen, P.; Frindt, R. F.; Morrison, S. R. Single-Layer MoS2. Mater. Res. Bull. 1986, 21, 457−461. (73) Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M. Photoluminescence from Chemically Exfoliated MoS2. Nano Lett. 2011, 11, 5111−5116. (74) Chou, S. S.; Huang, Y. K.; Kim, J.; Kaehr, B.; Foley, B. M.; Lu, P.; Dykstra, C.; Hopkins, P. E.; Brinker, C. J.; Huang, J.; Dravid, V. P. Controlling the Metal to Semiconductor Transition of MoS2 and WS2 in Solution. J. Am. Chem. Soc. 2015, 137, 1742−1745. 6181


Chemical Reviews

Review

Catalytic Hydrodeoxygenation of 4-Methylphenol. Chem. - Eur. J. 2016, 22, 2910−2914. (95) Daeneke, T.; Clark, R. M.; Carey, B. J.; Ou, J. Z.; Weber, B.; Fuhrer, M. S.; Bhaskaran, M.; Kalantar-zadeh, K. Reductive Exfoliation of Substoichiometric MoS2 Bilayers Using Hydrazine Salts. Nanoscale 2016, 8, 15252−15261. (96) Jeong, S.; Yoo, D.; Ahn, M.; Miro, P.; Heine, T.; Cheon, J. Tandem Intercalation Strategy for Single-Layer Nanosheets as an Effective Alternative to Conventional Exfoliation Processes. Nat. Commun. 2015, 6, 5763. (97) Jin, H.; Ahn, M.; Jeong, S.; Han, J. H.; Yoo, D.; Son, D. H.; Cheon, J. Colloidal Single-Layer Quantum Dots with Lateral Confinement Effects on 2D Exciton. J. Am. Chem. Soc. 2016, 138, 13253−13259. (98) Zheng, J.; Zhang, H.; Dong, S. H.; Liu, Y. P.; Tai Nai, C.; Suk Shin, H.; Young Jeong, H.; Liu, B.; Ping Loh, K. High Yield Exfoliation of Two-Dimensional Chalcogenides using Sodium Naphthalenide. Nat. Commun. 2014, 5, 2995. (99) Shih, C. J.; Vijayaraghavan, A.; Krishnan, R.; Sharma, R.; Han, J. H.; Ham, M. H.; Jin, Z.; Lin, S. C.; Paulus, G. L. C.; Reuel, N. F.; et al. Biand Trilayer Graphene Solutions. Nat. Nanotechnol. 2011, 6, 439−445. (100) Mittal, J.; Inagaki, M. Intercalation of MoCl5 into GraphiteDetermining Factor to Control Stage Structure. Synth. Met. 1999, 99 (1), 79−84. (101) Wang, F.; Chen, G. R.; Li, W.; Wang, Y. G.; Wang, C. X.; Zhang, Y. B.; Xia, Y. Y. Layer Controllable Graphene Using Graphite Intercalation Compounds with Different Stage Numbers through Li Conversion Reaction. Adv. Mater. Interfaces 2016, 3, 1500496. (102) Zhao, S. Y. F.; Elbaz, G. A.; Bediako, D. K.; Yu, C.; Efetov, D. K.; Guo, Y. S.; Ravichandran, J.; Min, K. A.; Hong, S.; Taniguchi, T.; et al. Controlled Electrochemical Intercalation of Graphene/h-BN van der Waals Heterostructures. Nano Lett. 2018, 18, 460−466. (103) Gamble, F. R.; Osiecki, J. H.; Cais, M.; Pisharody, R.; Disalvo, F. J.; Geballe, T. H. Intercalation Complexes of Lewis Bases and Layered Sulfides: A Large Class of New Superconductors. Science 1971, 174, 493−497. (104) Eckert, H.; Herber, R. H. A Charge-Transfer Model of Lewis Base Intercalation Compounds of FeOCl. J. Chem. Phys. 1984, 80, 4526. (105) Meyer, S. F.; Acrivos, J. V.; Geballe, T. H. Insertion Compounds of 2H-TaS2·NH3. Proc. Natl. Acad. Sci. U. S. A. 1975, 72 (2), 464−468. (106) Golub, A. S.; Zubavichus, Y. V.; Slovokhotov, Y. L.; Novikov, Y. N. Single-Layer Dispersions of Transition Metal Dichalcogenides in the Synthesis of Intercalation Compounds. Russ. Chem. Rev. 2003, 72, 123. (107) Wan, C.; Gu, X.; Dang, F.; Itoh, T.; Wang, Y.; Sasaki, H.; Kondo, M.; Koga, K.; Yabuki, K.; Snyder, G. J.; et al. Flexible n-type Thermoelectric Materials by Organic Intercalation of Layered Transition Metal Dichalcogenide TiS2. Nat. Mater. 2015, 14, 622−627. (108) Wan, C.; Kodama, Y.; Kondo, M.; Sasai, R.; Qian, X.; Gu, X.; Koga, K.; Yabuki, K.; Yang, R.; Koumoto, K. Dielectric Mismatch Mediates Carrier Mobility in Organic-Intercalated Layered TiS2. Nano Lett. 2015, 15, 6302−6308. (109) Li, Y. F.; Liang, Y. L.; Robles Hernandez, F. C.; Deog Yoo, H.; An, Q. Y.; Yao, Y. Enhancing sodium-ion battery performance with interlayer-expanded MoS2-PEO nanocomposites. Nano Energy 2015, 15, 453−461. (110) Liang, Y. L.; Yoo, H. D.; Li, Y. F.; Shuai, J.; Calderon, H. A.; Robles Hernandez, F. C.; Grabow, L. C.; Yao, Y. Interlayer-Expanded Molybdenum Disulfide Nanocomposites for Electrochemical Magnesium Storage. Nano Lett. 2015, 15, 2194−2202. (111) Geng, X. M.; Zhang, Y. L.; Han, Y.; Li, J. X.; Yang, L.; Benamara, M.; Chen, L.; Zhu, H. L. Two-Dimensional Water-Coupled Metallic MoS2 with Nanochannels for Ultrafast Supercapacitors. Nano Lett. 2017, 17, 1825−1832. (112) Tang, Y. C.; Zhao, Z. B.; Wang, Y. W.; Dong, Y. F.; Liu, Y.; Wang, X. Z.; Qiu, J. S. Carbon-Stabilized Interlayer-Expanded Few-Layer MoSe2 Nanosheets for Sodium Ion Batteries with Enhanced Rate Capability and Cycling Performance. ACS Appl. Mater. Interfaces 2016, 8, 32324−32332.

(75) Wang, X. F.; Shen, X.; Wang, Z. X.; Yu, R. C.; Chen, L. Q. AtomicScale Clarification of Structural Transition of MoS2 Upon Sodium Intercalation. ACS Nano 2014, 8, 11394−11400. (76) Wang, L. F.; Xu, Z.; Wang, W. L.; Bai, X. D. Atomic Mechanism of Dynamic Electrochemical Lithiation Processes of MoS2 Nanosheets. J. Am. Chem. Soc. 2014, 136, 6693−6697. (77) Voiry, D.; Salehi, M.; Silva, R.; Fujita, T.; Chen, M. W.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M. Conducting MoS2 Nanosheets as Catalysts for Hydrogen Evolution Reaction. Nano Lett. 2013, 13, 6222− 6227. (78) Eda, G.; Fujita, T.; Yamaguchi, H.; Voiry, D.; Chen, M. W.; Chhowalla, M. Coherent Atomic and Electronic Heterostructures of Single-Layer MoS2. ACS Nano 2012, 6, 7311−7317. (79) Yang, D.; Sandoval, S. J.; Divigalpitiya, W. M. R.; Irwin, J. C.; Frindt, R. F. Structure of Single-Molecular-Layer MoS2. Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 43, 12053. (80) Qin, X. R.; Yang, D.; Frindt, R. F.; Irwin, J. C. Real-Space Imaging of Single-Layer MoS2 by Scanning Tunneling Microscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 44, 3490. (81) Heising, J.; Kanatzidis, M. G. Structure of Restacked MoS2 and WS2 Elucidated by Electron Crystallography. J. Am. Chem. Soc. 1999, 121, 638−643. (82) Ghorai, A.; Midya, A.; Maiti, R.; Ray, S. K. Exfoliation of WS2 in the Semiconducting Phase Using a Group of Lithium Halides: a new method of Li intercalation. Dalton Trans. 2016, 45, 14979−14987. (83) Feng, H.; Hu, Z.; Liu, X. Facile and Efficient Exfoliation of InorganicLayered Materials Using Liquid Alkali Metal Alloys. Chem. Commun. 2015, 51, 10961−10964. (84) Peng, J.; Wu, J.; Li, X.; Zhou, Y.; Yu, Z.; Guo, Y.; Wu, J.; Lin, Y.; Li, Z.; Wu, X.; Wu, C.; Xie, Y. Very Large-Sized Transition Metal Dichalcogenides Monolayers from Fast Exfoliation by Manual Shaking. J. Am. Chem. Soc. 2017, 139, 9019−9025. (85) Zhu, G.; Liu, J.; Zheng, Q.; Zhang, R.; Li, D.; Banerjee, D.; Cahill, D. G. Tuning Thermal Conductivity in Molybdenum Disulfide by Electrochemical Intercalation. Nat. Commun. 2016, 7, 13211. (86) Zeng, Z. Y.; Sun, T.; Zhu, J. X.; Huang, X.; Yin, Z. Y.; Lu, G.; Fan, Z. X.; Yan, Q. Y.; Hng, H. H.; Zhang, H. An Effective Method for the Fabrication of Few-Layer-Thick Inorganic Nanosheets. Angew. Chem., Int. Ed. 2012, 51, 9052−9056. (87) Zeng, Z. Y.; Yin, Z. Y.; Huang, X.; Li, H.; He, Q. Y.; Lu, G.; Boey, F.; Zhang, H. Single-Layer Semiconducting Nanosheets: High-Yield Preparation and Device Fabrication. Angew. Chem., Int. Ed. 2011, 50, 11093−11097. (88) Cullen, P. L.; Cox, K. M.; Bin Subhan, M. K.; Picco, L.; Payton, O. D.; Buckley, D. J.; Miller, T. S.; Hodge, S. A.; Skipper, N. T.; Tileli, V.; Howard, C. A. Ionic Solutions of Two-Dimensional Materials. Nat. Chem. 2017, 9, 244−249. (89) Feng, J.; Sun, X.; Wu, C. Z.; Peng, L. L.; Lin, C. W.; Hu, S. L.; Yang, J. L.; Xie, Y. Metallic Few-Layered VS2 Ultrathin Nanosheets: High Two-Dimensional Conductivity for In-Plane Supercapacitors. J. Am. Chem. Soc. 2011, 133, 17832−17838. (90) Anto Jeffery, A.; Nethravathi, C.; Rajamathi, M. Two-Dimensional Nanosheets and Layered Hybrids of MoS2 and WS2 through Exfoliation of Ammoniated MS2 (M = Mo,W). J. Phys. Chem. C 2014, 118, 1386−1396. (91) Jeffery, A. A.; Nethravathi, C.; Rajamathi, M. Scalable Large Nanosheets of Transition Metal Disulphides through Exfoliation of Amine Intercalated MS2 [M = Mo, W] in organic solvents. RSC Adv. 2015, 5, 51176−51182. (92) Wang, Y.; Zhou, C. H.; Wang, W. C.; Zhao, Y. P. Preparation of Two Dimensional Atomic Crystals BN, WS2, and MoS2 by Supercritical CO2 Assisted with Ultrasound. Ind. Eng. Chem. Res. 2013, 52, 4379− 4382. (93) Qi, Y. H.; Xu, Q.; Wang, Y.; Yan, B.; Ren, Y. M.; Chen, Z. M. CO2Induced Phase Engineering: Protocol for Enhanced Photoelectrocatalytic Performance of 2D MoS2 Nanosheets. ACS Nano 2016, 10, 2903− 2909. (94) Liu, G. L.; Ma, H. L.; Teixeira, I.; Sun, Z. Y.; Xia, Q. N.; Hong, X. L.; Tsang, S. C. E. Hydrazine-Assisted Liquid Exfoliation of MoS2 for 6182


Chemical Reviews

Review

Electrocatalysts for the Hydrogen Evolution Reaction. Chem. Commun. 2016, 52, 9228−9231. (134) Son, D.; Chae, S. I.; Kim, M.; Choi, M. K.; Yang, J.; Park, K.; Kale, V. S.; Koo, J. H.; Choi, C.; Lee, M.; Kim, J. H.; Hyeon, T.; Kim, D. H. Colloidal Synthesis of Uniform-Sized Molybdenum Disulfide Nanosheets for Wafer-Scale Flexible Nonvolatile Memory. Adv. Mater. 2016, 28, 9326−9332. (135) Cheng, L.; Huang, W. J.; Gong, Q. F.; Liu, C. H.; Liu, Z.; Li, Y. G.; Dai, H. J. Ultrathin WS2 Nanoflakes as a High-Performance Electrocatalyst for the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. 2014, 53, 7860−7863. (136) Park, K. H.; Choi, J.; Kim, H. J.; Oh, D. H.; Ahn, J. R.; Son, S. U. Unstable Single-Layered Colloidal TiS2 Nanodisks. Small 2008, 4, 945− 950. (137) Prabakar, S.; Bumby, C. W.; Tilley, R. D. Liquid-Phase Synthesis of Flower-like and Flake-like Titanium Disulfide Nanostructures. Chem. Mater. 2009, 21, 1725−1730. (138) Plashnitsa, V. V.; Vietmeyer, F.; Petchsang, N.; Tongying, P.; Kosel, T. H.; Kuno, M. Synthetic Strategy and Structural and Optical Characterization of Thin Highly Crystalline Titanium Disulfide Nanosheets. J. Phys. Chem. Lett. 2012, 3, 1554−1558. (139) Qian, X. X.; Shen, S. D.; Liu, T.; Cheng, L.; Liu, Z. TwoDimensional TiS2 Nanosheets for in vivo Photoacoustic Imaging and Photothermal Cancer Therapy. Nanoscale 2015, 7, 6380−6387. (140) Stender, C. L.; Sekar, P.; Odom, T. W. Solid-State Chemistry on a Surface and in a Beaker: Unconventional Routes to Transition Metal Chalcogenide Nanomaterials. J. Solid State Chem. 2008, 181, 1621− 1627. (141) Sekar, P.; Greyson, E. C.; Barton, J. E.; Odom, T. W. Synthesis of Nanoscale NbSe2 Materials from Molecular Precursors. J. Am. Chem. Soc. 2005, 127, 2054−2055. (142) Huang, J. M.; Kelley, D. F. Synthesis and characterization of MoSe2 and WSe2 nanoclusters. Chem. Mater. 2000, 12, 2825−2828. (143) Altavilla, C.; Sarno, M.; Ciambelli, P. A Novel Wet Chemistry Approach for the Synthesis of Hybrid 2D Free-Floating Single or Multilayer Nanosheets of MS2@oleylamine (M = Mo, W). Chem. Mater. 2011, 23, 3879−3885. (144) Antunez, P. D.; Webber, D. H.; Brutchey, R. L. Solution-Phase Synthesis of Highly Conductive Tungsten Diselenide Nanosheets. Chem. Mater. 2013, 25, 2385−2387. (145) Tsuchii, K.; Tsuboi, Y.; Kawaguchi, S.; Takahashi, J.; Sonoda, N.; Nomoto, A.; Ogawa, A. Highly Selective Double Chalcogenation of Isocyanides with Disulfide-Diselenide Mixed Systems. J. Org. Chem. 2007, 72, 415−423. (146) Ithurria, S.; Dubertret, B. Quasi 2D Colloidal CdSe Platelets with Thicknesses Controlled at the Atomic Level. J. Am. Chem. Soc. 2008, 130, 16504−16505. (147) Bouet, C.; Laufer, D.; Mahler, B.; Nadal, B.; Heuclin, H.; Pedetti, S.; Patriarche, G.; Dubertret, B. Synthesis of Zinc and Lead Chalcogenide Core and Core/Shell Nanoplatelets Using Sequential Cation Exchange Reactions. Chem. Mater. 2014, 26, 3002−3008. (148) Li, Z.; Qin, H. Y.; Guzun, D.; Benamara, M.; Salamo, G.; Peng, X. G. Uniform Thickness and Colloidal-Stable CdS Quantum Disks with Tunable Thickness: Synthesis and Properties. Nano Res. 2012, 5, 337− 351. (149) Rossinelli, A. A.; Riedinger, A.; Marques-Gallego, P.; Knusel, P. N.; Antolinez, F. V.; Norris, D. J. High-Temperature Growth of ThickShell CdSe/CdS Core/Shell Nanoplatelets. Chem. Commun. 2017, 53, 9938−9941. (150) Li, Z.; Peng, X. G. Size/Shape-Controlled Synthesis of Colloidal CdSe Quantum Disks: Ligand and Temperature Effects. J. Am. Chem. Soc. 2011, 133, 6578−6586. (151) Mahler, B.; Nadal, B.; Bouet, C.; Patriarche, G.; Dubertret, B. Core/Shell Colloidal Semiconductor Nanoplatelets. J. Am. Chem. Soc. 2012, 134, 18591−18598. (152) Tessier, M. D.; Mahler, B.; Nadal, B.; Heuclin, H.; Pedetti, S.; Dubertret, B. Spectroscopy of Colloidal Semiconductor Core/Shell Nanoplatelets with High Quantum Yield. Nano Lett. 2013, 13, 3321− 3328.

(113) Liu, Y. C.; Fan, L. Z.; Jiao, L. F. Graphene Intercalated in Graphene-Like MoS2: A Promising Cathode for Rechargeable Mg Batteries. J. Power Sources 2017, 340, 104−110. (114) Wang, C.; He, Q.; Halim, U.; Liu, Y.; Zhu, E.; Lin, Z.; Xiao, H.; Duan, X.; Feng, Z.; Cheng, R.; et al. Monolayer Atomic Crystal Molecular Superlattices. Nature 2018, 555, 231−236. (115) Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodisperse Cde (E = S, Se, Te) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706− 8715. (116) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies. Annu. Rev. Mater. Sci. 2000, 30, 545−610. (117) Law, M.; Luther, J. M.; Song, O.; Hughes, B. K.; Perkins, C. L.; Nozik, A. J. Structural, Optical, and Electrical Properties of PbSe Nanocrystal Solids Treated Thermally or with Simple Amines. J. Am. Chem. Soc. 2008, 130, 5974−5985. (118) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V. Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdSe-TiO2 Architecture. J. Am. Chem. Soc. 2008, 130, 4007−4015. (119) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Shape Control of CdSe Nanocrystals. Nature 2000, 404, 59−61. (120) Sun, Y. G.; Xia, Y. N. Shape-Controlled Synthesis of Gold and Silver Nanoparticles. Science 2002, 298, 2176−2179. (121) Jun, Y. W.; Choi, J. S.; Cheon, J. Shape Control of Semiconductor and Metal Oxide Nanocrystals through Nonhydrolytic Colloidal Routes. Angew. Chem., Int. Ed. 2006, 45, 3414−3439. (122) Yu, S. L.; Wu, X. Q.; Wang, Y. P.; Guo, X.; Tong, L. M. 2D Materials for Optical Modulation: Challenges and Opportunities. Adv. Mater. 2017, 29, 1606128. (123) Zhu, Z. F.; Zou, Y. S.; Hu, W. D.; Li, Y. B.; Gu, Y.; Cao, B. Q.; Guo, N.; Wang, L.; Song, J. Z.; Zhang, S. L.; et al. Near-Infrared Plasmonic 2D Semimetals for Applications in Communication and Biology. Adv. Funct. Mater. 2016, 26, 1793−1802. (124) Rossi, D.; Han, J. H.; Jung, W.; Cheon, J.; Son, D. H. Orientational Control of Colloidal 2D-Layered Transition Metal Dichalcogenide Nanodiscs via Unusual Electrokinetic Response. ACS Nano 2015, 9, 8037−8043. (125) Rossi, D.; Camacho-Forero, L. E.; Ramos-Sanchez, G.; Han, J. H.; Cheon, J.; Balbuena, P.; Son, D. H. Anisotropic Electron-Phonon Coupling in Colloidal Layered TiS2 Nanodiscs Observed via Coherent Acoustic Phonons. J. Phys. Chem. C 2015, 119, 7436−7442. (126) Rossi, D.; Han, J. H.; Yoo, D.; Dong, Y. T.; Park, Y.; Cheon, J.; Son, D. H. Photoinduced Separation of Strongly Interacting 2-D Layered TiS2 Nanodiscs in Solution. J. Phys. Chem. C 2014, 118, 12568− 12573. (127) Sano, K.; Kim, Y. S.; Ishida, Y.; Ebina, Y.; Sasaki, T.; Hikima, T.; Aida, T. Photonic Water Dynamically Responsive to External Stimuli. Nat. Commun. 2016, 7, 12559. (128) Liu, M. J.; Ishida, Y.; Ebina, Y.; Sasaki, T.; Hikima, T.; Takata, M.; Aida, T. An Anisotropic Hydrogel with Electrostatic Repulsion Between Cofacially Aligned Nanosheets. Nature 2015, 517, 68−72. (129) Jeong, S.; Yoo, D.; Jang, J. T.; Kim, M.; Cheon, J. Well-Defined Colloidal 2-D Layered Transition-Metal Chalcogenide Nanocrystals via generalized synthetic protocols. J. Am. Chem. Soc. 2012, 134, 18233−6. (130) Jang, J. T.; Jeong, S.; Seo, J. W.; Kim, M. C.; Sim, E.; Oh, Y.; Nam, S.; Park, B.; Cheon, J. Ultrathin Zirconium Disulfide Nanodiscs. J. Am. Chem. Soc. 2011, 133, 7636−7639. (131) Jung, W.; Lee, S.; Yoo, D.; Jeong, S.; Miro, P.; Kuc, A.; Heine, T.; Cheon, J. Colloidal Synthesis of Single-Layer MSe2 (M = Mo, W) Nanosheets via Anisotropic Solution-Phase Growth Approach. J. Am. Chem. Soc. 2015, 137, 7266−7269. (132) Yoo, D.; Kim, M.; Jeong, S.; Han, J.; Cheon, J. Chemical Synthetic Strategy for Single-Layer Transition-Metal Chalcogenides. J. Am. Chem. Soc. 2014, 136, 14670−14673. (133) Zhao, W.; Dong, B.; Guo, Z.; Su, G.; Gao, R.; Wang, W.; Cao, L. Colloidal Synthesis of VSe2 Single-Layer Nanosheets as Novel 6183


Chemical Reviews

Review

(171) Cai, L.; He, J. F.; Liu, Q. H.; Yao, T.; Chen, L.; Yan, W. S.; Hu, F. C.; Jiang, Y.; Zhao, Y. D.; Hu, T. D.; et al. Vacancy-Induced Ferromagnetism of MoS2 Nanosheets. J. Am. Chem. Soc. 2015, 137, 2622−2627. (172) Yang, L. J.; Zhou, W. J.; Lu, J.; Hou, D. M.; Ke, Y. T.; Li, G. Q.; Tang, Z. H.; Kang, X. W.; Chen, S. W. Hierarchical Spheres Constructed by Defect-Rich MoS2/Carbon Nanosheets for Efficient Electrocatalytic Hydrogen Evolution. Nano Energy 2016, 22, 490−498. (173) Gao, M. R.; Chan, M. K. Y.; Sun, Y. G. Edge-Terminated Molybdenum Disulfide with a 9.4-Angstrom Interlayer Spacing for Electrochemical Hydrogen Production. Nat. Commun. 2015, 6, 7493. (174) Hu, Z.; Wang, L. X.; Zhang, K.; Wang, J. B.; Cheng, F. Y.; Tao, Z. L.; Chen, J. MoS2 Nanoflowers with Expanded Interlayers as HighPerformance Anodes for Sodium-Ion Batteries. Angew. Chem., Int. Ed. 2014, 53, 12794−12798. (175) Hwang, H.; Kim, H.; Cho, J. MoS2 Nanoplates Consisting of Disordered Graphene-like Layers for High Rate Lithium Battery Anode Materials. Nano Lett. 2011, 11, 4826−4830. (176) Sun, Y. G.; Alimohammadi, F.; Zhang, D. T.; Guo, G. S. Enabling Colloidal Synthesis of Edge-Oriented MoS2 with Expanded Interlayer Spacing for Enhanced HER Catalysis. Nano Lett. 2017, 17, 1963−1969. (177) Zhou, J.; Fang, G. Z.; Pan, A. Q.; Liang, S. Q. OxygenIncorporated MoS2 Nanosheets with Expanded Interlayers for Hydrogen Evolution Reaction and Pseudocapacitor Applications. ACS Appl. Mater. Interfaces 2016, 8, 33681−33689. (178) Deng, J.; Li, H.; Wang, S.; Ding, D.; Chen, M.; Liu, C.; Tian, Z.; Novoselov, K. S.; Ma, C.; Deng, D.; Bao, X. Multiscale Structural and Electronic Control of Molybdenum Disulfide Foam for Highly Efficient Hydrogen Production. Nat. Commun. 2017, 8, 14430. (179) Wang, J.; Liu, J.; Chao, D.; Yan, J.; Lin, J.; Shen, Z. X. SelfAssembly of Honeycomb-Like MoS2 Nanoarchitectures Anchored Into Graphene Foam for EnhancedLithium-Ion Storage. Adv. Mater. 2014, 26, 7162−7169. (180) Yang, X.; Zhang, Z.; Fu, Y.; Li, Q. Porous Hollow Carbon Spheres Decorated with Molybdenum Diselenide Nanosheets as Anodes for Highly Reversible Lithium and Sodium Storage. Nanoscale 2015, 7, 10198−203. (181) Zhang, L.; Wu, H. B.; Yan, Y.; Wang, X.; Lou, X. W. Hierarchical MoS2 Microboxes Constructed by Nanosheets with Enhanced Electrochemical Properties for Lithium Storage and Water Splitting. Energy Environ. Sci. 2014, 7, 3302−3306. (182) Guo, B. J.; Yu, K.; Song, H. L.; Li, H. L.; Tan, Y. H.; Fu, H.; Li, C.; Lei, X.; Zhu, Z. Q. Preparation of Hollow Microsphere@Onion-Like Solid Nanosphere MoS2 Coated by a Carbon Shell as a Stable Anode for Optimized Lithium Storage. Nanoscale 2016, 8, 420−430. (183) Choi, S. H.; Ko, Y. N.; Lee, J. K.; Kang, Y. C. 3D MoS2-Graphene Microspheres Consisting of Multiple Nanospheres with Superior Sodium Ion Storage Properties. Adv. Funct. Mater. 2015, 25, 1780− 1788. (184) Jin, J.; Kim, B.; Kim, M.; Park, N.; Kang, S.; Lee, S. M.; Kim, H. J.; Son, S. U. Template Synthesis of Hollow MoS2-Carbon Nanocomposites Using Microporous Organic Polymers and Their Lithium Storage Properties. Nanoscale 2015, 7, 11280−11285. (185) Zhuo, S. F.; Xu, Y.; Zhao, W. W.; Zhang, J.; Zhang, B. Hierarchical Nanosheet-Based MoS2 Nanotubes Fabricated by an Anion-Exchange Reaction of MoO3-Amine Hybrid Nanowires. Angew. Chem., Int. Ed. 2013, 52, 8602−8606. (186) Seo, J. W.; Jun, Y. W.; Park, S. W.; Nah, H.; Moon, T.; Park, B.; Kim, J. G.; Kim, Y. J.; Cheon, J. Two-dimensional nanosheet crystals. Angew. Chem., Int. Ed. 2007, 46, 8828−8831. (187) Sadtler, B.; Demchenko, D. O.; Zheng, H.; Hughes, S. M.; Merkle, M. G.; Dahmen, U.; Wang, L. W.; Alivisatos, A. P. Selective Facet Reactivity During Cation Exchange in Cadmium Sulfide Nanorods. J. Am. Chem. Soc. 2009, 131, 5285−93. (188) Sun, W. Y.; Li, P.; Liu, X.; Shi, J. J.; Sun, H. M.; Tao, Z. L.; Li, F. J.; Chen, J. Size-Controlled MoS2 Nanodots Supported on Reduced Graphene Oxide for Hydrogen Evolution Reaction and Sodium-Ion Batteries. Nano Res. 2017, 10, 2210−2222.

(153) Hendricks, M. P.; Campos, M. P.; Cleveland, G. T.; Jen-La Plante, I.; Owen, J. S. A Tunable Library of Substituted Thiourea Precursors to Metal Sulfide Nanocrystals. Science 2015, 348, 1226− 1230. (154) Smith, D. K.; Luther, J. M.; Semonin, O. E.; Nozik, A. J.; Beard, M. C. Tuning the Synthesis of Ternary Lead Chalcogenide Quantum Dots by Balancing Precursor Reactivity. ACS Nano 2011, 5, 183−190. (155) Allen, P. M.; Walker, B. J.; Bawendi, M. G. Mechanistic insights into the formation of InP quantum dots. Angew. Chem., Int. Ed. 2010, 49, 760−762. (156) Campos, M. P.; Hendricks, M. P.; Beecher, A. N.; Walravens, W.; Swain, R. A.; Cleveland, G. T.; Hens, Z.; Sfeir, M. Y.; Owen, J. S. A Library of Selenourea Precursors to PbSe Nanocrystals with Size Distributions near the Homogeneous Limit. J. Am. Chem. Soc. 2017, 139, 2296−2305. (157) Savjani, N.; Lewis, E. A.; Bissett, M. A.; Brent, J. R.; Dryfe, R. A. W.; Haigh, S. J.; O’Brien, P. Synthesis of Lateral Size-Controlled Monolayer 1H-MoS2@Oleylamine as Supercapacitor Electrodes. Chem. Mater. 2016, 28, 657−664. (158) Shi, W.; Song, S.; Zhang, H. Hydrothermal Synthetic Strategies of Inorganic Semiconducting Nanostructures. Chem. Soc. Rev. 2013, 42, 5714−5743. (159) Feng, S.; Xu, R. New Materials in Hydrothermal Synthesis. Acc. Chem. Res. 2001, 34, 239−247. (160) Geng, X.; Sun, W.; Wu, W.; Chen, B.; Al-Hilo, A.; Benamara, M.; Zhu, H.; Watanabe, F.; Cui, J.; Chen, T. P. Pure and Stable Metallic Pphase Molybdenum Disulfide Nanosheets for Hydrogen Evolution Reaction. Nat. Commun. 2016, 7, 10672. (161) Zhang, S.; Chowdari, B. V.; Wen, Z.; Jin, J.; Yang, J. Constructing Highly Oriented Configuration by Few-Layer MoS2: Toward HighPerformance Lithium-Ion Batteries and Hydrogen Evolution Reactions. ACS Nano 2015, 9, 12464−12472. (162) Liu, Q.; Li, X. L.; He, Q.; Khalil, A.; Liu, D. B.; Xiang, T.; Wu, X. J.; Song, L. Gram-Scale Aqueous Synthesis of Stable Few-Layered 1TMoS2: Applications for Visible-Light-Driven Photocatalytic Hydrogen Evolution. Small 2015, 11, 5556−5564. (163) Liu, Q.; Li, X. L.; Xiao, Z. R.; Zhou, Y.; Chen, H. P.; Khalil, A.; Xiang, T.; Xu, J. Q.; Chu, W. S.; Wu, X. J.; et al. Stable Metallic 1T-WS2 Nanoribbons Intercalated with Ammonia Ions: The Correlation between Structure and Electrical/Optical Properties. Adv. Mater. 2015, 27, 4837−4844. (164) Liu, Q.; Sun, C. Y.; He, Q.; Khalil, A.; Xiang, T.; Liu, D. B.; Zhou, Y.; Wang, J.; Song, L. Stable Metallic 1T-WS2 Ultrathin Nanosheets as a Promising Agent for Near-Infrared Photothermal Ablation Cancer Therapy. Nano Res. 2015, 8, 3982−3991. (165) Yang, K.; Wang, X.; Li, H.; Chen, B.; Zhang, X.; Li, S.; Wang, N.; Zhang, H.; Huang, X.; Huang, W. Composition- and Phase-Controlled Synthesis and Applications of Alloyed Phase Heterostructures of Transition Mmetal Disulphides. Nanoscale 2017, 9, 5102−5109. (166) Zhang, J.; Kang, W.; Jiang, M.; You, Y.; Cao, Y.; Ng, T. W.; Yu, D. Y.; Lee, C. S.; Xu, J. Conversion of 1T-MoSe2 to 2H-MoS2xSe2−2x Mesoporous Nnanospheres for Superior Sodium Storage Performance. Nanoscale 2017, 9, 1484−1490. (167) Liu, Z. P.; Gao, Z. C.; Liu, Y. H.; Xia, M. S.; Wang, R. W.; Li, N. Heterogeneous Nanostructure Based on 1T-Phase MoS2 for Enhanced Electrocatalytic Hydrogen Evolution. ACS Appl. Mater. Interfaces 2017, 9, 25291−25297. (168) Shao, J.; Qu, Q. T.; Wan, Z. M.; Gao, T.; Zuo, Z. C.; Zheng, H. H. From Dispersed Microspheres to Interconnected Nanospheres: Carbon-Sandwiched Mono layered MoS2 as High-Performance Anode of Li-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 22927−22934. (169) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Defect-Rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution. Adv. Mater. 2013, 25, 5807−5813. (170) Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang, R.; Lei, Y.; Pan, B.; Xie, Y. Controllable Disorder Engineering in Oxygen-Incorporated MoS2 Uultrathin Nanosheets for Efficient Hydrogen Evolution. J. Am. Chem. Soc. 2013, 135, 17881−17888. 6184


Chemical Reviews

Review

Alloys: A First-Principles Prediction. J. Phys. Chem. Lett. 2014, 5, 285−291. (208) Xie, L. M. Two-Dimensional Transition Metal Dichalcogenide Alloys: Preparation, Characterization and Applications. Nanoscale 2015, 7, 18392−18401. (209) Li, H. L.; Duan, X. D.; Wu, X. P.; Zhuang, X. J.; Zhou, H.; Zhang, Q. L.; Zhu, X. L.; Hu, W.; Ren, P. Y.; Guo, P. F.; et al. Growth of Alloy MoS2xSe2(1‑x) Nanosheets with Fully Tunable Chemical Compositions and Optical Properties. J. Am. Chem. Soc. 2014, 136, 3756−3759. (210) Gong, Q. F.; Sheng, S. D.; Ye, H. L.; Han, N.; Cheng, L.; Li, Y. G. MoxW1‑x(SySe1‑y)2 Alloy Nanoflakes for High-Performance Electrocatalytic Hydrogen Evolution. Part. Part. Syst. Char. 2016, 33, 576−582. (211) Lei, Y.; Pakhira, S.; Fujisawa, K.; Wang, X. Y.; Iyiola, O. O.; Lopez, N. P.; Elias, A. L.; Rajukumar, L. P.; Zhou, C. J.; Kabius, B.; et al. Low-temperature Synthesis of Heterostructures of Transition Metal Dichalcogenide Alloys (WxMo1‑xS2) and Graphene with Superior Catalytic Performance for Hydrogen Evolution. ACS Nano 2017, 11, 5103−5112. (212) Yang, K.; Wang, X. S.; Li, H.; Chen, B.; Zhang, X.; Li, S. Z.; Wang, N.; Zhang, H.; Huang, X.; Huang, W. Composition- and PhaseControlled Synthesis and Applications of Alloyed Phase Heterostructures of Transition Metal Disulphides. Nanoscale 2017, 9, 5102− 5109. (213) Sun, Y. F.; Fujisawa, K.; Lin, Z.; Lei, Y.; Mondschein, J. S.; Terrones, M.; Schaak, R. E. Low-Temperature Solution Synthesis of Transition Metal Dichalcogenide Alloys with Tunable Optical Properties. J. Am. Chem. Soc. 2017, 139, 11096−11105. (214) Gong, Q. F.; Cheng, L.; Liu, C. H.; Zhang, M.; Feng, Q. L.; Ye, H. L.; Zeng, M.; Xie, L. M.; Liu, Z.; Li, Y. G. Ultrathin MoS2(1‑x)Se2x Alloy Nanoflakes For Electrocatalytic Hydrogen Evolution Reaction. ACS Catal. 2015, 5, 2213−2219. (215) Zhang, J.; Wu, M. H.; Shi, Z. T.; Jiang, M.; Jian, W. J.; Xiao, Z.; Li, J.; Lee, C. S.; Xu, J. Composition and Interface Engineering of Alloyed MoS2‑xSe2(1‑x) Nanotubes for Enhanced Hydrogen Evolution Reaction Activity. Small 2016, 12, 4379−4385. (216) Meiron, O. E.; Kuraganti, V.; Hod, I.; Bar-Ziv, R.; Bar-Sadan, M. Improved Catalytic Activity of Mo1‑xWxSe2 Alloy Nanoflowers Promotes Efficient Hydrogen Evolution Reaction in Both Acidic and Alkaline Aqueous Solutions. Nanoscale 2017, 9, 13998−14005. (217) Fu, Q.; Yang, L.; Wang, W. H.; Han, A.; Huang, J.; Du, P. W.; Fan, Z. Y.; Zhang, J. Y.; Xiang, B. Synthesis and Enhanced Electrochemical Catalytic Performance of Monolayer WS2(1‑x)Se2x with a Tunable Band Gap. Adv. Mater. 2015, 27, 4732−4738. (218) Buonsanti, R.; Milliron, D. J. Chemistry of Doped Colloidal Nanocrystals. Chem. Mater. 2013, 25, 1305−1317. (219) McFarland, E. W.; Metiu, H. Catalysis by Doped Oxides. Chem. Rev. 2013, 113, 4391−4427. (220) Zhang, J. T.; Xia, Z. H.; Dai, L. M. Carbon-Based Electrocatalysts for Advanced Energy Conversion and Storage. Sci. Adv. 2015, 1, e1500564. (221) Li, R. C.; Yang, L. J.; Xiong, T. L.; Wu, Y. S.; Cao, L. D.; Yuan, D. S.; Zhou, W. J. Nitrogen Doped MoS2 Nanosheets Synthesized via A Low-Temperature Process as Electrocatalysts with Enhanced Activity for Hydrogen Evolution Reaction. J. Power Sources 2017, 356, 133−139. (222) Azcatl, A.; Qin, X. Y.; Prakash, A.; Zhang, C. X.; Cheng, L. X.; Wang, Q. X.; Lu, N.; Kim, M. J.; Kim, J.; Cho, K.; et al. Covalent Nitrogen Doping and Compressive Strain in MoS2 by Remote N-2 Plasma Exposure. Nano Lett. 2016, 16, 5437−5443. (223) Erwin, S. C.; Zu, L. J.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Doping Semiconductor Nanocrystals. Nature 2005, 436, 91−94. (224) Dalpian, G. M.; Chelikowsky, J. R. Self-Purification in Semiconductor Nanocrystals. Phys. Rev. Lett. 2006, 96, 226802. (225) Xiao, W.; Liu, P. T.; Zhang, J. Y.; Song, W. D.; Feng, Y. P.; Gao, D. Q.; Ding, J. Dual-Functional N Dopants in Edges and Basal Plane of MoS2 Nanosheets Toward Efficient and Durable Hydrogen Evolution. Adv. Energy Mater. 2017, 7, 1602086. (226) Tan, H.; Hu, W.; Wang, C.; Ma, C.; Duan, H.; Yan, W.; Cai, L.; Guo, P.; Sun, Z.; Liu, Q.; et al. Intrinsic Ferromagnetism in Mn-

(189) Sun, W. Y.; Li, P.; Liu, X.; Shi, J. J.; Sun, H. M.; Tao, Z. L.; Li, F. J.; Chen, J. Size-Controlled MoS2 Nanodots Supported on Reduced Graphene Oxide for Hydrogen Evolution Reaction and Sodium-Ion Batteries. Nano Res. 2017, 10, 2210−2222. (190) Liu, E. F.; Fu, Y. J.; Wang, Y. J.; Feng, Y. Q.; Liu, H. M.; Wan, X. G.; Zhou, W.; Wang, B. G.; Shao, L. B.; Ho, C. H.; et al. Integrated Digital Inverters Based on Two-Dimensional Anisotropic ReS2 FieldEffect Transistors. Nat. Commun. 2015, 6, 6991. (191) Zhang, Q.; Tan, S. J.; Mendes, R. G.; Sun, Z. T.; Chen, Y. T.; Kong, X.; Xue, Y. H.; Rummeli, M. H.; Wu, X. J.; Chen, S. L.; et al. Extremely Weak van der Waals Coupling in Vertical ReS2 Nanowalls for High-Current-Density Lithium-Ion Batteries. Adv. Mater. 2016, 28, 2616−2623. (192) Gao, J.; Li, L.; Tan, J. W.; Sun, H.; Li, B. C.; Idrobo, J. C.; Singh, C. V.; Lu, T. M.; Koratkar, N. Vertically Oriented Arrays of ReS2 Nanosheets for Electrochemical Energy Storage and Electrocatalysis. Nano Lett. 2016, 16, 3780−3787. (193) Wypych, F.; Schollhorn, R. 1T-MoS2, a New Metallic Modification of Molybdenum-Disulfide. J. Chem. Soc., Chem. Commun. 1992, 0, 1386−1388. (194) Mahler, B.; Hoepfner, V.; Liao, K.; Ozin, G. A. Colloidal Synthesis of 1T-WS2 and 2H-WS2 Nanosheets: Applications for Photocatalytic Hydrogen Evolution. J. Am. Chem. Soc. 2014, 136, 14121−14127. (195) Wang, D. Z.; Zhang, X. Y.; Bao, S. Y.; Zhang, Z. T.; Fei, H.; Wu, Z. Z. Phase Engineering of a Multiphasic 1T/2H MoS2 Catalyst for Highly Efficient Hydrogen Evolution. J. Mater. Chem. A 2017, 5, 2681− 2688. (196) Sun, Y. F.; Wang, Y. X.; Sun, D.; Carvalho, B. R.; Read, C. G.; Lee, C. H.; Lin, Z.; Fujisawa, K.; Robinson, J. A.; Crespi, V. H.; et al. Low-Temperature Solution Synthesis of Few-Layer 1T′-MoTe2 Nanostructures Exhibiting Lattice Compression. Angew. Chem., Int. Ed. 2016, 55, 2830−2834. (197) Yu, Y. J.; Yang, F. Y.; Lu, X. F.; Yan, Y. J.; Cho, Y. H.; Ma, L. G.; Niu, X. H.; Kim, S.; Son, Y. W.; Feng, D. L.; et al. Gate-Tunable Phase Transitions in Thin Flakes of 1T-TaS2. Nat. Nanotechnol. 2015, 10, 270−276. (198) Stojchevska, L.; Vaskivskyi, I.; Mertelj, T.; Kusar, P.; Svetin, D. Brazo Ultrafast Switching to a Stable Hidden Quantum State in an Electronic Crystal. vskii, S.; Mihailovic, D. Science 2014, 344, 177−180. (199) Sipos, B.; Kusmartseva, A. F.; Akrap, A.; Berger, H.; Forro, L.; Tutis, E. From Mott State to Superconductivity in 1T-TaS2. Nat. Mater. 2008, 7, 960−965. (200) Cho, D.; Cheon, S.; Kim, K. S.; Lee, S. H.; Cho, Y. H.; Cheong, S. W.; Yeom, H. W. Nanoscale Manipulation of the Mott Insulating State Coupled to Charge Order in 1T-TaS2. Nat. Commun. 2016, 7, 10453. (201) Yoshida, M.; Suzuki, R.; Zhang, Y. J.; Nakano, M.; Iwasa, Y. Memristive Phase Switching in Two-Dimensional 1T-TaS2 Crystals. Sci. Adv. 2015, 1, e1500606. (202) Boller, H.; Blaha, H. Intercalation Phases of TiS2, 2H-NbS2, and 1T-TaS2 with Ethylenediamine and Trimethylenediamine - A Crystal Chemical and Thermogravimetric Study. J. Solid State Chem. 1982, 45, 119−126. (203) Bouwmeester, H. J. M.; Wiegers, G. A. A Neutron Powder Diffraction Study of ND3 Intercalated Titanium Disulfide, 3R-TiS2ND3. J. Solid State Chem. 1988, 76, 391−397. (204) Wiegers, G. A. Physical-Properties of 1st-Row Transition-Metal Dichalcogenides and Their Intercalates. Physica B+C 1980, 99, 151− 165. (205) Wiegers, G. A.; Haange, R. J.; Vanbruggen, C. F. The Formation of 1T and 2H Titanium Disulfide by Deintercalation of NaxTiS2. Synth. Met. 1984, 9, 9−17. (206) Wang, G.; Robert, C.; Suslu, A.; Chen, B.; Yang, S. J.; Alamdari, S.; Gerber, I. C.; Amand, T.; Marie, X.; Tongay, S.; et al. Spin-Orbit Engineering in Transition Metal Dichalcogenide Alloy Monolayers. Nat. Commun. 2015, 6, 10110. (207) Xi, J. Y.; Zhao, T. Q.; Wang, D.; Shuai, Z. G. Tunable Electronic Properties of Two-Dimensional Transition Metal Dichalcogenide 6185


Chemical Reviews

Review

Nanoframes with Abundant Active Edge Sites for Ultrahigh-Performance Hydrogen Evolution. Angew. Chem., Int. Ed. 2016, 55, 6502−6505. (244) Han, J. H.; Lee, S.; Yoo, D.; Lee, J. H.; Jeong, S.; Kim, J. G.; Cheon, J. Unveiling Chemical Reactivity and Structural Transformation of Two-Dimensional Layered Nanocrystals. J. Am. Chem. Soc. 2013, 135, 3736−3739. (245) Han, S.; Yang, X.; Zhu, Y.; Tan, C.; Zhang, X.; Chen, J.; Huang, Y.; Chen, B.; Luo, Z.; Ma, Q.; et al. Synthesis of WOn -WX2 (n = 2.7, 2.9; X = S, Se) Heterostructures for Highly Efficient Green Quantum Dot Light-Emitting Diodes. Angew. Chem., Int. Ed. 2017, 56, 10486−10490. (246) Chen, Y.; Song, B. H.; Tang, X. S.; Lu, L.; Xue, J. M. Ultrasmall Fe3O4 Nanoparticle/MoS2 Nanosheet Composites with Superior Performances for Lithium Ion Batteries. Small 2014, 10, 1536−1543. (247) Zhou, W. J.; Yin, Z. Y.; Du, Y. P.; Huang, X.; Zeng, Z. Y.; Fan, Z. X.; Liu, H.; Wang, J. Y.; Zhang, H. Synthesis of Few-Layer MoS2 Nanosheet-Coated TiO2 Nanobelt Heterostructures for Enhanced Photocatalytic Activities. Small 2013, 9, 140−147. (248) Xiang, Q. J.; Yu, J. G.; Jaroniec, M. Synergetic Effect of MoS2 and Graphene as Cocatalysts for Enhanced Photocatalytic H2 Production Activity of TiO2 Nanoparticles. J. Am. Chem. Soc. 2012, 134, 6575−6578. (249) Xu, X.; Fan, Z. Y.; Ding, S. J.; Yu, D. M.; Du, Y. P. Fabrication of MoS2 Nanosheet@TiO2 Nanotube Hybrid Nanostructures for Lithium Storage. Nanoscale 2014, 6, 5245−5250. (250) Huang, Y. P.; Miao, Y. E.; Zhang, L. S.; Tjiu, W. W.; Pan, J. S.; Liu, T. X. Synthesis of Few-Layered MoS2 Nanosheet-Coated Electrospun SnO2 Nanotube Heterostructures for Enhanced Hydrogen Evolution Reaction. Nanoscale 2014, 6, 10673−10679. (251) Chen, Y.; Lu, J.; Wen, S.; Lu, L.; Xue, J. M. Synthesis of SnO2/ MoS2 Composites with Different Component Ratios and Their Applications as Lithium Ion Battery Anodes. J. Mater. Chem. A 2014, 2, 17857−17866. (252) Huang, X.; Zeng, Z. Y.; Bao, S. Y.; Wang, M. F.; Qi, X. Y.; Fan, Z. X.; Zhang, H. Solution-Phase Epitaxial Growth of Noble Metal Nanostructures on Dispersible Single-Layer Molybdenum Disulfide Nanosheets. Nat. Commun. 2013, 4, 1444. (253) Kim, J.; Byun, S.; Smith, A. J.; Yu, J.; Huang, J. X. Enhanced Electrocatalytic Properties of Transition-Metal Dichalcogenides Sheets by Spontaneous Gold Nanoparticle Decoration. J. Phys. Chem. Lett. 2013, 4, 1227−1232. (254) Shi, Y. M.; Huang, J. K.; Jin, L. M.; Hsu, Y. T.; Yu, S. F.; Li, L. J.; Yang, H. Y. Selective Decoration of Au Nanoparticles on Monolayer MoS2 Single Crystals. Sci. Rep. 2013, 3, 1839. (255) Zeng, Z. Y.; Tan, C. L.; Huang, X.; Bao, S. Y.; Zhang, H. Growth of Noble Metal Nanoparticles on Single-Layer TiS 2 and TaS 2 Nanosheets for Hydrogen Evolution Reaction. Energy Environ. Sci. 2014, 7, 797−803. (256) Raza, F.; Yim, D.; Park, J. H.; Kim, H. I.; Jeon, S. J.; Kim, J. H. Structuring Pd Nanoparticles on 2H-WS2 Nanosheets Induces Excellent Photocatalytic Activity for Cross-Coupling Reactions under Visible Light. J. Am. Chem. Soc. 2017, 139, 14767−14774. (257) Yu, X. Y.; Hu, H.; Wang, Y.; Chen, H.; Lou, X. W. Ultrathin MoS2 Nanosheets Supported on N-doped Carbon Nanoboxes with Enhanced Lithium Storage and Electrocatalytic Properties. Angew. Chem., Int. Ed. 2015, 54, 7395−7398. (258) Wang, S.; Guan, B. Y.; Yu, L.; Lou, X. W. D. Rational Design of Three-Layered TiO2 @Carbon@MoS2 Hierarchical Nanotubes for Enhanced Lithium Storage. Adv. Mater. 2017, 29, 1702724. (259) Chen, Y. M.; Yu, X. Y.; Li, Z.; Paik, U.; Lou, X. W. Hierarchical MoS2 tubular structures internally wired by carbon nanotubes as a highly stable anode material for lithium-ion batteries. Sci. Adv. 2016, 2, e1600021. (260) Wang, Y.; Yu, L.; Lou, X. W. Synthesis of Highly Uniform Molybdenum-Glycerate Spheres and Their Conversion into Hierarchical MoS2 Hollow Nanospheres for Lithium-Ion Batteries. Angew. Chem., Int. Ed. 2016, 55, 7423−7426. (261) Zhang, X.; Zhao, R.; Wu, Q.; Li, W.; Shen, C.; Ni, L.; Yan, H.; Diao, G.; Chen, M. Petal-like MoS2 Nanosheets Space-Confined in Hollow Mesoporous Carbon Spheres for Enhanced Lithium Storage Performance. ACS Nano 2017, 11, 8429−8436.

Substituted MoS2 Nanosheets Achieved by Supercritical Hydrothermal Reaction. Small 2017, 13, 1701389. (227) Yang, Y.; Wang, S. T.; Zhang, J. C.; Li, H. Y.; Tang, Z. L.; Wang, X. Nanosheet-Assembled MoSe2 and S-doped MoSe2‑x Nanostructures for Superior Lithium Storage Properties and Hydrogen Evolution Reactions. Inorg. Chem. Front. 2015, 2, 931−937. (228) Du, C. C.; Huang, H.; Jian, J.; Wu, Y.; Shang, M. X.; Song, W. B. Enhanced Electrocatalytic Hydrogen Evolution Performance of MoS2 Ultrathin Nanosheets via Sn Doping. Appl. Catal., A 2017, 538, 1−8. (229) Deng, J.; Li, H. B.; Xiao, J. P.; Tu, Y. C.; Deng, D. H.; Yang, H. X.; Tian, H. F.; Li, J. Q.; Ren, P. J.; Bao, X. H. Triggering the Electrocatalytic Hydrogen Evolution Activity of the Inert Two-Dimensional MoS2 Surface via Single-Atom Metal Doping. Energy Environ. Sci. 2015, 8, 1594−1601. (230) Wang, Y. Z.; Liu, S. S.; Hao, X. F.; Zhou, J. S.; Song, D. D.; Wang, D.; Hou, L.; Gao, F. M. Fluorine- and Nitrogen-Codoped MoS2 with a Catalytically Active Basal Plane. ACS Appl. Mater. Interfaces 2017, 9, 27715−27719. (231) Dai, X. P.; Du, K. L.; Li, Z. Z.; Liu, M. Z.; Ma, Y. D.; Sun, H.; Zhang, X.; Yang, Y. Co-Doped MoS2 Nanosheets with the Dominant CoMoS Phase Coated on Carbon as an Excellent Electrocatalyst for Hydrogen Evolution. ACS Appl. Mater. Interfaces 2015, 7, 27242− 27253. (232) Sun, C. Q.; Zhang, J. Y.; Ma, J.; Liu, P. T.; Gao, D. Q.; Tao, K.; Xue, D. S. N-doped WS2 Nanosheets: A High-Performance Electrocatalyst for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2016, 4, 11234−11238. (233) Liu, P. T.; Zhu, J. Y.; Zhang, J. Y.; Xi, P. X.; Tao, K.; Gao, D. Q.; Xue, D. S. P Dopants Triggered New Basal Plane Active Sites and Enlarged Interlayer Spacing in MoS2 Nanosheets toward Electrocatalytic Hydrogen Evolution. ACS Energy Lett. 2017, 2, 745−752. (234) Liu, P. T.; Zhu, J. Y.; Zhang, J. Y.; Tao, K.; Gao, D. Q.; Xi, P. X. Active Basal Plane Catalytic Activity and Conductivity in Zn Doped MoS2 Nanosheets for Efficient Hydrogen Evolution. Electrochim. Acta 2018, 260, 24−30. (235) Shi, Y.; Zhou, Y.; Yang, D. R.; Xu, W. X.; Wang, C.; Wang, F. B.; Xu, J. J.; Xia, X. H.; Chen, H. Y. Energy Level Engineering of MoS2 by Transition-Metal Doping for Accelerating Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2017, 139, 15479−15485. (236) Jeong, S.; Han, J. H.; Jang, J. T.; Seo, J. W.; Kim, J. G.; Cheon, J. Transformative Two-Dimensional Layered Nanocrystals. J. Am. Chem. Soc. 2011, 133, 14500−14503. (237) Sun, X.; Deng, H.; Zhu, W.; Yu, Z.; Wu, C.; Xie, Y. Interface Engineering in Two-Dimensional Heterostructures: Towards an Advanced Catalyst for Ullmann Couplings. Angew. Chem., Int. Ed. 2016, 55, 1704−1709. (238) Yin, X. L.; Li, L. L.; Jiang, W. J.; Zhang, Y.; Zhang, X.; Wan, L. J.; Hu, J. S. MoS2/CdS Nanosheets-on-Nanorod Heterostructure for Highly Efficient Photocatalytic H2 Generation under Visible Light Irradiation. ACS Appl. Mater. Interfaces 2016, 8, 15258−15266. (239) Schornbaum, J.; Winter, B.; Schiessl, S. P.; Gannott, F.; Katsukis, G.; Guldi, D. M.; Spiecker, E.; Zaumseil, J. Epitaxial Growth of PbSe Quantum Dots on MoS2 Nanosheets and their Near-Infrared Photoresponse. Adv. Funct. Mater. 2014, 24, 5798−5806. (240) Chen, J. Z.; Wu, X. J.; Gong, Y.; Zhu, Y. H.; Yang, Z. Z.; Li, B.; Lu, Q. P.; Yu, Y. F.; Han, S. K.; Zhang, Z. C.; et al. Edge Epitaxy of TwoDimensional MoSe2 and MoS2 Nanosheets on One-Dimensional Nanowires. J. Am. Chem. Soc. 2017, 139, 8653−8660. (241) Chen, J. Z.; Wu, X. J.; Yin, L. S.; Li, B.; Hong, X.; Fan, Z. X.; Chen, B.; Xue, C.; Zhang, H. One-pot Synthesis of CdS Nanocrystals Hybridized with Single-Layer Transition-Metal Dichalcogenide Nanosheets for Efficient Photocatalytic Hydrogen Evolution. Angew. Chem., Int. Ed. 2015, 54, 1210−1214. (242) Tan, C. L.; Zeng, Z. Y.; Huang, X.; Rui, X. H.; Wu, X. J.; Li, B.; Luo, Z. M.; Chen, J. Z.; Chen, B.; Yan, Q. Y.; et al. Liquid-Phase Epitaxial Growth of Two-Dimensional Semiconductor Hetero-nanostructures. Angew. Chem., Int. Ed. 2015, 54, 1841−1845. (243) Xu, J.; Cui, J. B.; Guo, C.; Zhao, Z. P.; Jiang, R.; Xu, S. Y.; Zhuang, Z. B.; Huang, Y.; Wang, L. Y.; Li, Y. D. Ultrasmall Cu7S4@MoS2 Hetero6186


Chemical Reviews

Review

(280) Huang, Y. P.; Miao, Y. E.; Fu, J.; Mo, S. Y.; Wei, C.; Liu, T. X. Perpendicularly Oriented Few-Layer MoSe2 on SnO2 Nanotubes for Efficient Hydrogen Evolution Reaction. J. Mater. Chem. A 2015, 3, 16263−16271. (281) Klankowski, S. A.; Rojeski, R. A.; Cruden, B. A.; Liu, J. W.; Wu, J.; Li, J. A High-Performance Lithium-Ion Battery Anode Based on the Core-shell Heterostructure of Silicon-Coated Vertically Aligned Carbon Nanofibers. J. Mater. Chem. A 2013, 1, 1055−1064. (282) Cui, L. F.; Ruffo, R.; Chan, C. K.; Peng, H. L.; Cui, Y. CrystallineAmorphous Core-Shell Silicon Nanowires for High Capacity and High Current Battery Electrodes. Nano Lett. 2009, 9, 491−495. (283) Wu, F.; Chen, J. Z.; Chen, R. J.; Wu, S. X.; Li, L.; Chen, S.; Zhao, T. Sulfur/Polythiophene with a Core/Shell Structure: Synthesis and Electrochemical Properties of the Cathode for Rechargeable Lithium Batteries. J. Phys. Chem. C 2011, 115, 6057−6063. (284) Weng, Q. H.; Wang, X.; Wang, X. B.; Zhang, C.; Jiang, X. F.; Bando, Y.; Golberg, D. Supercapacitive Energy Storage Performance of Molybdenum Disulfide Nanosheets Wrapped with Microporous Carbons. J. Mater. Chem. A 2015, 3, 3097−3102. (285) Zhou, K. Q.; Jiang, S. H.; Bao, C. L.; Song, L.; Wang, B. B.; Tang, G.; Hu, Y.; Gui, Z. Preparation of Poly(vinyl alcohol) Nanocomposites with Molybdenum Disulfide (MoS2): Structural Characteristics and Markedly Enhanced Properties. RSC Adv. 2012, 2, 11695−11703. (286) Zhao, X. Y.; Hu, C. W.; Cao, M. H. Three-Dimensional MoS2 Hierarchical Nanoarchitectures Anchored into a Carbon Layer as Graphene Analogues with Improved Lithium Ion Storage Performance. Chem. - Asian J. 2013, 8, 2701−2707. (287) Shi, Z. T.; Kang, W. P.; Xu, J.; Sun, Y. W.; Jiang, M.; Ng, T. W.; Xue, H. T.; Yu, D. Y. W.; Zhang, W. J.; Lee, C. S. Hierarchical Nanotubes Assembled from MoS2-Carbon Monolayer Sandwiched Superstructure Nanosheets for High-Performance Sodium Ion Batteries. Nano Energy 2016, 22, 27−37. (288) Zhao, C. Y.; Wang, X.; Kong, J. H.; Ang, J. M.; Lee, P. S.; Liu, Z. L.; Lu, X. H. Self-Assembly-Induced Alternately Stacked Single-Layer MoS2 and N-doped Graphene: A Novel van der Waals Heterostructure for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8, 2372− 2379. (289) Jiang, H.; Ren, D. Y.; Wang, H. F.; Hu, Y. J.; Guo, S. J.; Yuan, H. Y.; Hu, P. J.; Zhang, L.; Li, C. Z. 2D Monolayer MoS2-Carbon Interoverlapped Superstructure: Engineering Ideal Atomic Interface for Lithium Ion Storage. Adv. Mater. 2015, 27, 3687−3695. (290) Wang, R. T.; Wang, S. J.; Peng, X.; Zhang, Y. B.; Jin, D. D.; Chu, P. K.; Zhang, L. Elucidating the Intercalation Pseudocapacitance Mechanism of MoS2-Carbon Monolayer Interoverlapped Superstructure: Toward High-Performance Sodium-Ion-Based Hybrid Supercapacitor. ACS Appl. Mater. Interfaces 2017, 9, 32745−32755. (291) Chatti, M.; Gengenbach, T.; King, R.; Spiccia, L.; Simonov, A. N. Vertically Aligned Interlayer Expanded MoS2 Nanosheets on a Carbon Support for Hydrogen Evolution Electrocatalysis. Chem. Mater. 2017, 29, 3092−3099. (292) Liu, A. P.; Zhao, L.; Zhang, J. M.; Lin, L. X.; Wu, H. P. SolventAssisted Oxygen Incorporation of Vertically Aligned MoS2 Ultrathin Nanosheets Decorated on Reduced Graphene Oxide for Improved Electrocatalytic Hydrogen Evolution. ACS Appl. Mater. Interfaces 2016, 8, 25210−25218. (293) Zhao, L.; Hong, C. C.; Lin, L. X.; Wu, H. P.; Su, Y. W.; Zhang, X. B.; Liu, A. P. Controllable Nanoscale Engineering of Vertically Aligned MoS2 Ultrathin Nanosheets by Nitrogen Doping of 3D Graphene Hydrogel for Improved Electrocatalytic Hydrogen Evolution. Carbon 2017, 116, 223−231. (294) Ren, D. Y.; Jiang, H.; Hu, Y. J.; Zhang, L.; Li, C. Z. SelfAssembling Few-Layer MoS2 Nanosheets on a CNT Backbone for High-rate and Long-Life Lithium-Ion Batteries. RSC Adv. 2014, 4, 40368−40372. (295) Ding, S. J.; Chen, J. S.; Lou, X. W. Glucose-Assisted Growth of MoS2 Nanosheets on CNT Backbone for Improved Lithium Storage Properties. Chem. - Eur. J. 2011, 17, 13142−13145.

(262) Huang, X.; Zheng, B.; Liu, Z. D.; Tan, C. L.; Liu, J. Q.; Chen, B.; Li, H.; Chen, J. Z.; Zhang, X.; Fan, Z. X.; et al. Coating TwoDimensional Nanomaterials with Metal-Organic Frameworks. ACS Nano 2014, 8, 8695−8701. (263) Yang, J.; Voiry, D.; Ahn, S. J.; Kang, D.; Kim, A. Y.; Chhowalla, M.; Shin, H. S. Two-Dimensional Hybrid Nanosheets of Tungsten Disulfide and Reduced Graphene Oxide as Catalysts for Enhanced Hydrogen Evolution. Angew. Chem., Int. Ed. 2013, 52, 13751−13754. (264) Li, Y. G.; Wang, H. L.; Xie, L. M.; Liang, Y. Y.; Hong, G. S.; Dai, H. J. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296−7299. (265) Li, D. J.; Maiti, U. N.; Lim, J.; Choi, D. S.; Lee, W. J.; Oh, Y.; Lee, G. Y.; Kim, S. O. Molybdenum Sulfide/N-Doped CNT Forest Hybrid Catalysts for High-Performance Hydrogen Evolution Reaction. Nano Lett. 2014, 14, 1228−1233. (266) Chang, K.; Chen, W. X. L-Cysteine-Assisted Synthesis of Layered MoS2/Graphene Composites with Excellent Electrochemical Performances for Lithium Ion Batteries. ACS Nano 2011, 5, 4720−4728. (267) Liao, L.; Zhu, J.; Bian, X. J.; Zhu, L. N.; Scanlon, M. D.; Girault, H. H.; Liu, B. H. MoS2 Formed on Mesoporous Graphene as a Highly Active Catalyst for Hydrogen Evolution. Adv. Funct. Mater. 2013, 23, 5326−5333. (268) Liu, Y. B.; Liu, J. W. Hybrid Nanomaterials of WS2 or MoS2 Nanosheets with Liposomes: Biointerfaces and Multiplexed Drug Delivery. Nanoscale 2017, 9, 13187−13194. (269) Lu, Q. P.; Yu, Y. F.; Ma, Q. L.; Chen, B.; Zhang, H. 2D Transition-Metal-Dichalcogenide-Nanosheet-Based Composites for Photocatalytic and Electrocatalytic Hydrogen Evolution Reactions. Adv. Mater. 2016, 28, 1917−1933. (270) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Formation of Hollow Nanocrystals through the Nanoscale Kirkendall Effect. Science 2004, 304, 711−714. (271) Cucinotta, C. S.; Dolui, K.; Pettersson, H.; Ramasse, Q. M.; Long, E.; O’Brian, S. E.; Nicolosi, V.; Sanvito, S. Electronic Properties and Chemical Reactivity of TiS2 Nanoflakes. J. Phys. Chem. C 2015, 119, 15707−15715. (272) Liu, M. M.; Li, F. Y.; Sun, Z. X.; Ma, L. F.; Xu, L.; Wang, Y. H. Noble-Metal-Free Photocatalysts MoS2-Graphene/CdS Mixed Nanoparticles/Nanorods Morphology with High Visible Light Efficiency for H2 Evolution. Chem. Commun. 2014, 50, 11004−11007. (273) Zhang, J.; Zhu, Z. P.; Feng, X. L. Construction of TwoDimensional MoS2/CdS p-n Nanohybrids for Highly Efficient Photocatalytic Hydrogen Evolution. Chem. - Eur. J. 2014, 20, 10632−10635. (274) Zhu, B. L.; Lin, B. Z.; Zhou, Y.; Sun, P.; Yao, Q. R.; Chen, Y. L.; Gao, B. F. Enhanced Photocatalytic H2 Evolution on ZnS Loaded with Graphene and MoS2 Nanosheets as Cocatalysts. J. Mater. Chem. A 2014, 2, 3819−3827. (275) Jiang, W. J.; Liu, Y. F.; Zong, R. L.; Li, Z. P.; Yao, W. Q.; Zhu, Y. F. Photocatalytic Hydrogen Generation on Bifunctional Ternary Heterostructured In2S3/MoS2/CdS Composites with High Activity and Stability under Visible Light Irradiation. J. Mater. Chem. A 2015, 3, 18406−18412. (276) Tian, G. H.; Chen, Y. J.; Ren, Z. Y.; Tian, C. G.; Pan, K.; Zhou, W.; Wang, J. Q.; Fu, H. G. Enhanced Photocatalytic Hydrogen Evolution over Hierarchical Composites of ZnIn2S4 Nanosheets Grown on MoS2 Slices. Chem. - Asian J. 2014, 9, 1291−1297. (277) Shen, M.; Yan, Z. P.; Yang, L.; Du, P. W.; Zhang, J. Y.; Xiang, B. MoS2 Nanosheet/TiO2 Nanowire Hybrid Nanostructures for Enhanced Visible-Light Photocatalytic Activities. Chem. Commun. 2014, 50, 15447−15449. (278) Chen, B.; Zhao, N. Q.; Guo, L. C.; He, F.; Shi, C. S.; He, C. N.; Li, J. J.; Liu, E. Z. Facile Synthesis of 3D Few-Layered MoS2 Coated TiO2 Nanosheet Core-Shell Nanostructures for Stable and HighPerformance Lithium-Ion Batteries. Nanoscale 2015, 7, 12895−12905. (279) Lv, H.; Liu, Y. M.; Tang, H. B.; Zhang, P.; Wang, J. J. Synergetic Effect of MoS2 and Graphene as Cocatalysts for Enhanced Photocatalytic Activity of BiPO4 Nanoparticles. Appl. Surf. Sci. 2017, 425, 100−106. 6187


Chemical Reviews

Review

(296) Muller, G. A.; Cook, J. B.; Kim, H. S.; Tolbert, S. H.; Dunn, B. High Performance Pseudocapacitor Based on 2D Layered Metal Chalcogenide Nanocrystals. Nano Lett. 2015, 15, 1911−1917. (297) Liu, Y. P.; Wang, H. T.; Cheng, L.; Han, N.; Zhao, F. P.; Li, P. R.; Jin, C. H.; Li, Y. G. TiS2 Nanoplates: A High-Rate and Stable Electrode Material for Sodium Ion Batteries. Nano Energy 2016, 20, 168−175. (298) Zhang, X.; Zhang, H. T.; Li, C.; Wang, K.; Sun, X. Z.; Ma, Y. W. Recent Advances in Porous Graphene Materials for Supercapacitor Applications. RSC Adv. 2014, 4, 45862−45884. (299) Zhu, Y. W.; Murali, S.; Stoller, M. D.; Ganesh, K. J.; Cai, W. W.; Ferreira, P. J.; Pirkle, A.; Wallace, R. M.; Cychosz, K. A.; Thommes, M.; et al. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science 2011, 332, 1537−1541. (300) Sun, H. T.; Mei, L.; Liang, J. F.; Zhao, Z. P.; Lee, C.; Fei, H. L.; Ding, M. N.; Lau, J.; Li, M. F.; Wang, C.; et al. Three-dimensional holeygraphene/niobia composite architectures for ultrahigh-rate energy storage. Science 2017, 356, 599−604.

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