Two-Dimensional Metal Nanomaterials: Synthesis, Properties, and

Review Cite This: Chem. Rev. 2018, 118, 6409−6455

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Two-Dimensional Metal Nanomaterials: Synthesis, Properties, and Applications Ye Chen, Zhanxi Fan, Zhicheng Zhang, Wenxin Niu, Cuiling Li, Nailiang Yang, Bo Chen, and Hua Zhang*

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Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore ABSTRACT: As one unique group of two-dimensional (2D) nanomaterials, 2D metal nanomaterials have drawn increasing attention owing to their intriguing physiochemical properties and broad range of promising applications. In this Review, we briefly introduce the general synthetic strategies applied to 2D metal nanomaterials, followed by describing in detail the various synthetic methods classified in two categories, i.e. bottom-up methods and top-down methods. After introducing the unique physical and chemical properties of 2D metal nanomaterials, the potential applications of 2D metal nanomaterials in catalysis, surface enhanced Raman scattering, sensing, bioimaging, solar cells, and photothermal therapy are discussed in detail. Finally, the challenges and opportunities in this promising research area are proposed.

CONTENTS 1. Introduction 2. Synthetic Methods 2.1. Bottom-up Methods 2.1.1. Organic Ligand-Assisted Growth 2.1.2. Small Molecules and Ions Mediated Synthesis 2.1.3. 2D Template-Confined Growth 2.1.4. Polyol Method 2.1.5. Seeded Growth 2.1.6. Photochemical Synthesis 2.1.7. Hydro-/Solvothermal Methods 2.1.8. Crystal Phase Transformation 2.1.9. Biological Synthesis 2.1.10. Nanoparticle Assembly 2.1.11. Other Bottom-up Methods 2.2. Top-down Methods 2.2.1. Mechanical Compression Method 2.2.2. Exfoliation Method 2.2.3. Nanolithography 3. Properties 3.1. Stability 3.2. Crystal Structure 3.3. Plasmonic properties 3.4. Magnetic Properties 3.5. Electrical Properties 4. Applications 4.1. Catalysis 4.1.1. Electrocatalysis 4.1.2. Heterogeneous Catalysis 4.2. Surface Enhanced Raman Scattering (SERS) 4.3. Sensing 4.4. Photothermal Therapy 4.5. Bioimaging © 2018 American Chemical Society

4.6. Solar Cells 5. Conclusion and Perspective Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References

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1. INTRODUCTION Compared to their bulk counterparts, two-dimensional (2D) nanomaterials often possess unusual physiochemical properties owing to their high aspect ratio, unique surface chemistry, and quantum-size effect.1−6 Since the discovery of graphene,7−11 the vast library of 2D materials, such as transition metal dichalcogenides (TMDs),12−15 hexagonal boron nitride,16−18 graphitic carbon nitride,19,20 black phosphorus,21,22 layered double hydroxides (LDHs),23,24 silicene,25,26 2D metal oxides/ sulfides,24,27 transition metal carbides, nitrides and carbonitrides (MXenes),28−30 2D metals,31−36 2D polymers,37−40 2D metal−organic frameworks,41−43 2D covalent-organic frameworks,44,45 and 2D perovskites,46−48 has attracted tremendous enthusiasm among researchers to explore the intriguing properties and applications.49 Among various kinds of 2D nanomaterials mentioned above, 2D metallic nanomaterials have recently received extensive research interest because of their anisotropic structure, fascinating properties, and promising applications in catalysis,31,50−54 surface enhanced Raman Special Issue: 2D Materials Chemistry Received: December 7, 2017 Published: June 21, 2018 6409

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scattering (SERS),55−58 sensing,59−63 bioimaging,64−66 photothermal therapy,56,67,68 solar cells,69,70 and so on. The development of metal nanomaterials represents one of the pioneering areas in the history of modern nanoscience. As early as 1857, Faraday first reported the successful chemical synthesis of Au nanoparticles.71 With the development of electron microscope which allows the observation of nanostructures, extensive studies on the controlled synthesis of colloidal Au nanomaterials have emerged.72 In the meantime, tremendous efforts have been devoted to exploring the controlled synthesis of other kinds of metal nanostructures and their fascinating properties. To date, great progress has been achieved in the syntheses of metal nanostructures with controlled sizes,73−75 shapes/facets,76−92 compositions,93−96 and architectures.97−102 Recently, significant progress in the crystal-phase-controlled synthesis of various metal nanostructures has also been reported.103−108 In this Review, we mainly focus on the research progress of various 2D and quasi-2D metal nanostructures. Notably, these 2D metal nanostructures vary from their thicknesses, sizes, shapes, compositions, and crystal phases, which have been named as nanoprisms,55,109−111 nanoplates,57,112−114 nanodisks,115−117 nanosheets,31,52,118 nanorings,119,120 nanowheels,121−123 nanoribbons,107 nanobelts,124,125 nanocones,51 and so on. Importantly, unlike the intrinsically layered materials, most metals have a highly symmetric crystal lattice.126 This suggests that 2D morphology is not thermodynamically favored during the growth of metal nanocrystals.127 Therefore, to facilitate the 2D anisotropic growth, kinetic control over the growth pathway is often applied by either slowing down the atom addition process or lowering the total free energy of the metal nanostructures.35 As the surface energy could dominate in the total free energy of 2D nanostructures with high surface-tovolume ratio, the latter strategy is often achieved by reducing the surface energy of 2D metals with capping agents.128 Therefore, to obtain anisotropic 2D metal nanomaterials, most synthetic strategies rely on a solution-based colloidal synthesis, a bottom-up growth pathway induced by thermal decomposition, or chemical reduction of a metal precursor. In this process, isotropic nuclei are usually formed in the early stage, after which a symmetry-breaking event (e.g., addition of intermediate species, altering of reaction conditions) takes place to trigger the anisotropic growth manner. Then, the ultrasmall metal nuclei are either driven to merge into 2D morphologies or grow only in 2D directions. Many parameters have been invoked in the past decades to explain the asymmetric morphology evolution during the bottom-up nanocrystal growth.129 Some commonly adopted formation mechanisms in wet-chemical syntheses are template effects,106,130,131 facet-selective adsorptions of ligands,111,132 and micellar arrangements of surfactants.122,133 Interestingly, these factors could affect the 2D morphology cooperatively in some complicated growth systems, resulting in the delicate control over the final products.134,135 Although there have been a few reviews focusing on certain types of 2D noble metal nanomaterials35,109,136,137 and ultrathin metal nanomaterials,32,33,138 a summary on the various kinds of 2D metal nanomaterials by comprehensively introducing their synthetic methods, properties, and applications is still lacking. This Review aims to provide a whole picture of the research progress made on 2D metal nanomaterials to date, highlight recent advances, and share some perspectives (Scheme 1). First, we introduce the various

Scheme 1. Overview of this Review

synthetic methods for synthesis of 2D metal nanomaterials in detail. Specifically, we distinguish the various methods based on the critical reacting species or the key experimental conditions resulting in the final 2D morphology. The bottom-up methods include organic ligand-assisted growth, small-molecules-and-ions-mediated synthesis, 2D templateconfined synthesis, polyol method, seeded growth, photochemical synthesis, hydrothermal and solvothermal methods, crystal phase transformation, biological synthesis, and nanoparticle assembly. Meanwhile some novel top-down methods and special techniques such as mechanical compression, exfoliation method, and nanolithography are also introduced. Next, the unique physicochemical properties including stability, crystal structures, plasmonic properties, magnetic properties, and electrical properties are discussed. After that, various promising applications of 2D metal nanomaterials in catalysis, solar cells, surface enhanced Raman scattering (SERS), sensing, bioimaging, and photothermal therapy are summarized. Finally, on the basis of the current research status, we highlight the challenges and potential opportunities lying in this field and provide some personal perspectives and future research directions.

2. SYNTHETIC METHODS 2.1. Bottom-up Methods

2.1.1. Organic Ligand-Assisted Growth. Capping ligands referenced in this context are the long carbon-chain molecules that are able to coordinate with the metal ions/ atoms and modulate the crystal growth during a colloidal synthesis. Typically, a ligand is composed of one or multiple functional groups that can bind to the 2D metal surface and also a chemically inert body that stabilizes the metal−ligand complex sterically. These surface ligands are widely used to direct the 2D growth of nanocrystals in bottom-up wetchemical synthesis. Normally, ligands might have more than one role in the colloidal synthesis of 2D metal nanostructures.139 The formation of metal−ligand affinities can contribute to the growth of 2D morphology in three ways: 6410

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i.e., (i) the adsorption of ligand lowers the surface energy of the exposed metal surface, which greatly helps to minimize the total free energy; (ii) the spontaneous 2D packing of some types of metal−ligand complexes leads to a “soft templating” effect that promotes the formation of 2D shapes;140 (iii) the long-chain organic ligands provide excellent steric stabilization. Importantly, different functional groups exhibit different binding strengths and facet preferences with the metal ions/ atoms, which enables them to act in multiple roles, such as mild reducing agents,105 facet-directing agents,141 surface property modulators,142 and highly viscous solvents.143 Usually, based on the type of ligand−metal interactions, we can divide the ligands into three categories, i.e. ionic organic ligands, nonionic organic ligands, and polymer ligands. Ionic organic ligands are among the earliest reported capping ligands in the colloidal synthesis of anisotropic metal nanomaterials, including cetyltrimethylammonium bromide (CTAB),122,123,144−147 octadecyltrimethylammonium bromide (OTAB), 1 2 1 octadecyltrimethylammonium chloride (OTAC),121 sodium di(2-ethyl-hexyl)sulfosuccinate (Na(AOT)),115,148,149 sodium dodecyl sulfate (SDS),150 sodium dodecylsulfonate (SDSn),151 dodecane-1,12-bis(trimethylammonium bromide),151 docosylpyridinium bromide,141 etc. Usually, ionic organic ligands are composed of a hydrophilic “head” and a long-chain hydrophobic “tail”. Therefore, proper combination and ratio of ionic ligands and solvents can lead to the formation of 2D micelles, in which 2D metal nanostructures are confined to grow.122,123,148,152,153 For example, in aqueous solution containing equal molar amount of CTAB and sodium perfluorooctanoate (FC7), 2D micelle assembly, named as bicelle, can form. Within the bicelle, by reducing K2PtCl4 with ascorbic acid (AA) at 25 °C, Pt nanowheels with diameter of around 500 nm were successfully obtained (Figure 1a,b).122 Interestingly, by varying the platinum concentration, temperature, and total concentration of surfactants (CTAB + FC7), various platinum nanostructures including circular dendritic nanosheets with tunable diameters were produced (Figure 1c,d).123 The obtained nanowheels were unstable under the irradiation of an electron beam. As a result, the dendritic nanosheet portion of the nanowheels transformed into ripening-resistant holey sheets. Besides the bicelle system, ionic organic ligands can directly participate in the shape control of metal nanostructures. For example, Huang et al. recently reported the high-yield preparation of heterostructured Au@Pd core−shell nanowheels (Figure 1e− g).121 The coreduction of Na2PdCl4 and HAuCl4 with AA was carried out with OTAC as capping ligand. Control experiments showed that without OTAC, aggregates of nanoparticles were formed, and no AuPd nanoplates were formed if OTAC was changed to OTAB. Their result suggested that the presence of OTAC, especially its Cl− ion, played an important role in the formation of nanowheels with good dispersity and a welldefined shape. Meanwhile, Cu nanodisks and Ag nanodisks were also successfully obtained using the 2D micelle directiongrowth strategy.148,149 In addition, it is worth mentioning that some ionic organic ligands can also affect the growth direction of metal nanostructures. For example, Qi et al. reported the synthesis of various shapes of Au nanobelts by reducing HAuCl4 with AA in a mixed ligand system of CTAB and SDSn.151 The single crystalline nanobelts can be grown along either the ⟨110⟩ or ⟨211⟩ direction by simply changing the reaction temperature. They proposed that the cooperative

Figure 1. (a) Low-magnification and (b) high-magnification SEM images of Pt nanowheels. Reproduced with permission from ref 122. Copyright 2008 American Chemical Society. (c) TEM image of Pt nanowheels. (d) High-angle annular dark-field scanning-TEM (HAADF-STEM) image of Pt nanowheels. Inset: Pt density profile across the indicated rectangular region. Reproduced with permission from ref 123. Copyright 2011 Royal Society of Chemistry. (e) The ideal model of an Au@Pd nanowheel. (f) TEM and (g) HAADFSTEM images of individual AuPd nanowheels. Reproduced with permission from ref 121. Copyright 2013 John Wiley & Sons, Inc.

effect of mixed surfactants played a subtle role in controlling the growth direction of Au nanobelts. Compared to the ionic organic ligands, nonionic organic ligands generally have weaker binding with metal atoms. Some frequently used nonionic ligands include oleic acid (OA),50 oleylamine (OAM),141,143,154 ortho-phenylenediamine,155 trioctylphosphine (TOP),156 trioctylphosphine oxide (TOPO), thiolates,157,158 ethylene glycol (EG),143 bis(amidoethylcarbamoylethyl) octadecylamine (C18N3),159 etc. A common nonionic organic capping ligand is OAM. Muscat et al. reported the synthesis of ultrathin Ag nanosheets in OAM using Cu+ ions as reducing agent, and the thickness and edge length of obtained Ag nanosheets are 25 ± 15 nm and 3−8 μm, respectively.154 Control experiments suggested that the slow reduction process by Cu+ ions and OAM and the cooperative effect between OAM and hexane contributed to the nanosheet morphology. In another work, Son et al. reported that ultrathin Rh nanoplates were obtained by using OAM as the capping ligand, reducing agent, as well as solvent.143 The mixture of [Rh(CO)2Cl]2 and OAM was kept 6411

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undisturbed at 50 °C for 10 days to allow the 2D packing of Rh-OAM complex and gradual reduction to form nanoplates. Recently, our group reported the first synthesis of 4H hexagonal Au nanoribbons by using OAM as a capping ligand at mild conditions (Figure 2a−d).107 The obtained Au nanoribbons have thickness of 2−6 nm, width of 15−61 nm, and length of 0.5−6 μm. Interestingly, it was found that the binary solvent system of 1,2-dichloropropane and hexane is also critical for the successful synthesis of 4H Au nanoribbons. In the absence of 1,2-dichloropropane, only face-centered cubic (fcc) Au spherical nanoparticles were obtained. Mean-

while, the molar ratio of the nonionic organic ligands and metal precursor could be critical to obtain 2D metal nanostructures. For example, Wang et al. reported the synthesis of micrometer-scale single crystalline Au nanoplates by reducing HAuCl4 in an aqueous solution of orthophenylenediamine.155 It was found that the concentration of ortho-phenylenediamine in the solution can control the morphology of the obtained Au nanostructures. With the ratio of ortho-phenylenediamine and Au increased from 1:1 to 3:1 and 8:1, the final products changed from Au nanoplates to aggregated and isolated spherical Au particles, respectively. Moreover, different nonionic ligands can also be combined in a binary solvent system to yield different morphology or crystal phases. For instance, Alivisatos et al. reported the various Co nanostructures obtained by mixing two ligands, chosen from OA, TOPO, o-dodecylamine (DDA), hexadecylamine (HDA), and octadecylamine (ODA), as surfactant mixture.116 Amines with longer chains were found to favor the formation of Co nanodisks rather than Co nanoparticles. Similarly, by using a mixture of TOP and OAM, Peng et al. demonstrated a facile one-pot strategy to obtain NiCo alloy nanoplates from Ni(acac)2 and CuCl2·2H2O.156 Polymeric ligands possess long molecular chains that can slow down the reaction process and prevent particle coalescence. In the synthesis of 2D metal nanomaterials, there have been reports on using poly(vinylpyrrolidone) (PVP),111,113,140,160−167 poly(vinyl alcohol) (PVA),168 polyethylene (100) stearyl ether (Brij 700),169 poly(vinyl alcohol),168 polyethylenimine,170 and polyamine,171 as the polymer ligand. PVP has been extensively used as an efficient steric stabilizer in the aqueous synthesis of various 2D metal nanostructures.140,161 Thanks to the long-chain polymeric backbone as a natural agglomeration blocker and the mild reducing capability from the hydroxyl (−OH) group at the molecule ends, PVP serves as an ideal type of reductant for the kinetically controlled synthesis of metal nanostructures.167 As a typical example, Xia et al. found that PVP served as dual functional ligand in the kinetically controlled synthesis of noble metal nanoplates including Au, Ag, Pd, and Pt.172 By tuning either the ratio of PVP and metal precursor or the molecular weight of PVP, the reduction kinetics of PVP could be tuned to result in different shape evolution pathways. Similar shape control of Au nanoplates with PVP resulted in the formation of single crystalline Au nanoplates with high purity and narrow size distribution (Figure 2e−g).140 Significantly, experimental studies suggested that PVP could play even more roles in the synthesis of 2D metal nanostructures, such as influencing the growth pathways of Ag nanoplates between bimodal and unimodal.173 Besides Ag and Au, other 2D metal nanostructures, including Bi nanoplates,164 Pd nanoplates,172 Pt nanoplates,172 and Cu nanoplates160 have also been obtained by using PVP as the polymer ligand. Moreover, other commercialized polymers were also found to be useful as a stabilizer. For instance, Yoo et al. reported the usage of Brij 700 surfactant as the reducing, capping, and shape-directing agent in the synthesis of multilayered Au spirangles (Figure 2h−j).169 The geometrically unique multilayered Au plates were about 5−10 μm in edge length. The 2D morphology of Au spirangles could be altered by simply varying the reaction temperature. By increasing the temperature from 50 to 90 °C, Au plates with holes in their center regions were obtained. In contrast, by decreasing temperature from 50 to 25 °C, singlelayered Au nanoplates were found to be the main products. As

Figure 2. (a) Low-magnification TEM image of 4H Au nanoribbons. Inset: A typical TEM image of a folded Au nanoribbon, showing the typical thickness and flexibility of Au nanoribbons. (b) Simulated unit cell of 4H Au. (c) Selected area electron diffraction (SAED) pattern taken along the [110]4H zone axis of a typical Au nanoribbon (shown in the inset). (d) Aberration-corrected high-resolution TEM (HRTEM) image of the center region of an Au nanoribbon. Reproduced with permission from ref 107. Copyright 2015 Nature Publishing Group. (e) Field-emission SEM (FESEM) image of Au nanoplates with average width and thickness of ∼450 nm and ∼39 nm, respectively. (f) TEM image of a triangular Au nanoplate. Inset: The SAED pattern recorded along the [111] zone axis. (g) FESEM images of edges of individual hexagonal (top) and triangular (bottom) Au nanoplates. Reproduced with permission from ref 140. Copyright 2005 American Chemical Society. (h) SEM image of Au spirangles. (i) TEM image of a typical Au spirangle. (j) SAED pattern of a typical area in an Au spirangle. Reproduced with permission from ref 169. Copyright 2011 The Royal Society of Chemistry. 6412

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a unique advantage, the high viscosity and linearity of some polymers can form “soft templates” to enable large-scale growth of micrometer-sized nanoplates. Wang et al. synthesized microsized, high-yield, single-crystalline Au nanoplates by heating the aqueous solution of linear polyethylenimine and HAuCl4 at 100 °C.171 The obtained nanoplates were as large as up to ∼40 μm in lateral size and showed a preferential growth direction along the plane. The linear polyethylenimine was believed to significantly contribute to the kinetic control over the growth rates of different crystal facets, although the exact mechanism was not clear. Furthermore, one can also use the strategy of combinatorial ligand system in polymer ligand mixtures. For example, Lee et al. systematically studied how a mixture of similar polymer ligands affected the morphology of metal nanocrystals.174 By screening from 24 synthetic and natural polymers with different molecular structures, chemical functionalities and average molar masses, they identified several factors for controlling the subsequent anisotropic growth of Ag nanoplates, such as the charge type of the polymer, the polymer length, and the amphiphilic behavior of the polymer. 2.1.2. Small Molecules and Ions Mediated Synthesis. Small molecules and ions, which are also termed as additives or small adsorbates in some literature, can play big roles in modulating the growth of 2D metal nanocrystals.175,176 In a wet-chemical synthetic system, besides the metal precursor, reducing agent, and protecting ligands, a relatively small amount of small molecules or ions, such as oxygen (O2),177 carbon monoxide (CO),54,175 hydrogen peroxide (H2O2),178 halide ions (mainly refer to Cl−, Br−, and I−),179−183 and metal ions (e.g. Ag+, Cu2+, and Fe3+), is sometimes introduced to promote the anisotropic growth. These small molecules/ions act actively in tuning the redox potentials of metal precursors and the surface energy of different crystal facets. Recently, CO has been discovered as an effective shapemodulating agent in the colloidal synthesis of metal nanocrystals. 175 It has been reported that CO molecules preferentially adsorb on some low-index facets of metals by forming relatively strong coordination bonding with the surface atoms.184−186 Therefore, a sufficient presence of CO can efficiently promote the anisotropic growth of 2D metal nanostructures and stabilize them. In 2009, Remita et al. reported the synthesis of ultrathin hexagonal Pd nanosheets in a water/toluene emulsion purged with CO gas.187 The emulsion was prepared by mixing toluene solution of Pd precursor and aqueous solution of surfactant. Ultrathin Pd nanosheets with thickness of ∼2 nm gradually formed after purging CO gas into the emulsion at a slow rate for 15 min. The slow diffusion of CO led to the slow nucleation of Pd and slow surface protection by CO, which were critical in defining the nanosheet morphology. Other than using an emulsion system, Zheng et al. developed a facile approach to produce uniform hexagonal Pd nanosheets by reducing Pd(acac)2 in a solution containing solvent (dimethylformamide or benzyl alcohol), PVP, a halide salt, and water under CO gas charging with a pressure of 1 bar (Figure 3a,b).31 The reaction vessel was heated to 100 °C and maintained for 3 h under stirring. As a result, uniform Pd nanosheets with tunable edge lengths from 20 to 160 nm and thickness of less than 10 atomic layers were obtained. The lateral surface of Pd nanosheets was enclosed by {111} facets, which was believed to be attributed to the strong adsorption of CO molecules on the basal (111) planes of Pd nanosheets.31 Using a similar strategy, they also reported the

Figure 3. (a) TEM image of ultrathin Pd nanosheets. Inset: Photo of an ethanol dispersion of Pd nanosheets in a vial. (b) SAED pattern of a single Pd nanosheet (shown in the inset). Reproduced with permission from ref 31. Copyright 2011 Nature Publishing Group. (c) TEM image of Rh nanosheets with edge length of 880 nm. (d) SAED pattern of an Rh nanosheet (shown in the inset). Reproduced with permission from ref 54. Copyright 2015 John Wiley & Sons, Inc. (e) TEM image of multilayered Pd nanosheets with a rotational mismatch of 8°. (f) The corresponding SAED pattern of the multilayered Pd nanosheets in (e). Reproduced with permission from ref 189. Copyright 2014 American Chemical Society. (g) TEM image of ultrathin PdCu alloy nanosheets. (h) HRTEM image of a PdCu alloy nanosheet. Reproduced with permission from ref 50. Copyright 2017 John Wiley & Sons, Inc.

synthesis of atomically thick Rh nanosheets with controllable size in a glass vessel containing Rh(II) acetate, PVP, N,Ndimethylformamide (DMF), and CO gas, and heated at 150 °C for 3 h (Figure 3c,d).54 By controlling the CO pressure in the vessel from 0.5 to 1.0, 1.5, and 2.0 atm pressure, the edge length of Rh nanosheets can be tuned from 500 to 880, 1050, and 1300 nm, respectively. In addition to the freestanding 2D nanostructures, orderly stacked Pd nanosheets have also been obtained with CO molecules.188,189 Interestingly, Yang et al. reported a multilayered ultrathin Pd nanosheet architecture with a Hanoi Tower shape, formed by reducing Pd(acac)2 dissolved in acetic acid with CO gas for 24 h (Figure 3e,f).189 6413

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Figure 4. (a) Photo of the green solution of Au nanoprisms obtained with NaI added in the red seed solution. (b) SEM image of Au nanoprisms. (c) TEM image of a single Au nanoprism. Reproduced with permission from ref 180. Copyright 2013 American Chemical Society. (d) SEM image of Ag nanoplates formed in the absence of I−. (e−f) SEM images of Ag nanoplates obtained in the presence of a given concentration of I− which was added at (e) t = 4 h and (f) t = 3 h after the reaction had started. The corresponding schemes of Ag nanoplates are also shown in (d−f). Reproduced with permission from ref 183. Copyright 2014 The Royal Society of Chemistry.

ature.116 Very recently, our group reported a facile method for synthesizing ultrathin PdCu alloy nanosheets with controllable composition (Figure 3g,h).50 By the coreduction of Pd(acac)2 and Cu(acac)2 with Mo(CO)6 in a mixture of TOPO and OA at 60 °C for 18 h, ∼3-nm-thick PdCu nanosheets were obtained. Besides PdCu nanosheets, Pd and Ni nanosheets were also successfully obtained by using W(CO)6 as the COreleasing agent.192,193 Different from CO, metal ions and halide ions are mostly responsible for the oxidative etching of certain metal atoms or crystal facets to slow down the additional nucleation process during the crystal growth.194 Typically, halide ions are proved to bind strongly with Au and Ag nanocrystals and play critical roles in modulating their anisotropic morphology.180,183,195 For example, Chung et al. studied the effect of halide ions during the growth of Au nanoparticles. They found that with a small amount of I− ions in the growth solution, the majority of Au products were triangular nanoprisms. One year later, Mirkin et al. demonstrated that I− ions can bind strongly and selectively to the Au (111) crystal facet and thus promote the formation of (111)-oriented Au nanoprisms.181 Wei et al. also showed that when only Br− ions were used, the formation of Au nanorods dominated. However, when the proper combination of Br− and I− ions was applied, high-quality Au nanoprisms could be synthesized (Figure 4a−c).180 In the case of Ag, the activation energy of etching reaction between halide ions and Ag has been calculated, indicating the order of their etching ability to be Cl− < I− < Br−.182 Byun et al. synthesized Ag nanoplates with tunable shapes by systematically studying the effect of halide ions on the formation pathways of Ag nanoplates (Figure 4d−f).183 Their results suggested that halide ions could promote the development of {100} crystal planes. Among the halide ions, I− ions dramatically affect the

The stacked Pd nanosheets shared a growth center and had a layer thickness of less than 1 nm. The formation pathway was believed to be closely related to the strong adsorption of the intermediate, Pd4(CO)4(OAc)4 complex, on the Pd(110) surface. Indeed, the intermediate formation resulted from the Pd-CO interaction plays an essential role in the synthesis of 2D Pd nanostructures. In a two-step synthetic strategy reported by Zheng et al., a Pd carbonyl complex, [Pd2(μ-CO)2Cl4]2−, was prepared prior to the formation of Pd nanosheets by stirring a mixture of 30 μL of 1 M H2PdCl4 aqueous solution and 10 mL of anhydrous DMF under the CO atmosphere with 1 atm pressure for 15 min. After the formation of [Pd2(μ-CO)2Cl4]2− carbonyl complex, the CO atmosphere was removed, and then the formation of Pd nanosheets was triggered by adding 1 mL of water into the aforementioned mixture.190 The as-obtained Pd nanosheets are surface-clean with thickness of ∼2 nm. It was deduced that during the formation of Pd nanosheets, some of the bridging CO ligands acted as reducing agents to reduce Pd2+ to Pd0, while the other CO ligands were bound to Pd {111} surfaces and promoted the formation of the 2D morphology. In addition to monometallic 2D nanostructures, recently, Han et al. reported a successful preparation of Pd− Pt−Ag ternary-alloy nanosheets by coreduction of the metal precursors in a suitable molar ratio in the aqueous solution purged with CO.191 Only the sheetlike morphology was formed when the CO content in the solution was saturated. However, due to safety concerns, the direct usage of CO gas is often restricted. To find alternatives, CO-releasing agents such as metal carbonyl compounds (e.g., W(CO)6, Mo(CO)6, Co(CO)8) have become increasingly popular in the synthesis of 2D metal nanomaterials.50,192,193 These solid compounds are relatively stable at ambient conditions and can gradually decompose to release CO molecules at elevated temper6414

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Figure 5. (a) Schematic illustration of the formation of an hcp Au square sheet (AuSS) on a GO sheet. (b) Low-magnification TEM image of the formed AuSSs. (c) HRTEM image of a typical region of AuSS oriented perpendicular to [110]h. (d) SAED pattern of an AuSS on a GO sheet, showing diffraction rings of GO and spots of AuSS along the the [110]h zone axis. (e) Scheme of the crystallographic model of a typical hcp AuSS with its basal plane along the [110]h zone axis, showing “AB” stacking along the [001]h direction. Reproduced with permission from ref 106. Copyright 2011 Nature Publishing Group. (f) Schematic illustration of the shape and phase evolution of an AuSS to Au squarelike plate (AuSP). hcp is denoted with light gray color, and fcc is denoted with dark gray color. (g) TEM image of AuSPs on GO synthesized via the secondary growth of AuSSs. (h) AFM image and profile analysis of a typical AuSP. (i) HRTEM image of a center area of AuSP. Inset: Fast Fourier transform (FFT) pattern of the corresponding HRTEM image in (i). Reproduced with permission from ref 202. Copyright 2011 John Wiley & Sons, Inc.

successfully synthesized intermetallic PtBi nanoplatelets with a surprising hexagonal close-packed (hcp) structure. Moreover, the combined usage of small molecules and ions has also been developed as an effective confining growth strategy. A typical example is the combined usage of CO and Fe3+ in the synthesis of mesocrystalline Pd nanocorolla.68 It was suggested that while Fe3+ ions were involved in the etching-induced modification over laterals of the Pd nanostructures at the early stage, CO was more decisive in confining the 2D growth of nanosheets via its strong binding on Pd (111) crystal planes. Similarly, ultrathin Ni nanosheets have also been successfully synthesized by using CO/Fe3+ pairs.197,198 2.1.3. 2D Template-Confined Growth. In principle, any 2D nanomaterials or 2D substrates can potentially serve as

2D morphology by promoting vertical growth over lateral growth via the selective deposition of Ag onto the nanoplate seeds. Notably, the oxidative etching can effectively prevent the formation of multiply twinned structures by controlling the crystallinity of nanocrystals.194 The lattice distortion generated at twin boundaries of the nanocrystals makes them much more prone to etching, which explains the fact that single-crystalline nuclei often survive as a major intermediate product and thus yield high-quality nanoplates. In addition, halide ions can also promote the formation of alloy nanoplates. Hou et al. demonstrated a one-pot synthetic strategy to obtain PtBi nanoplatelets with controllable size and composition.196 By slowly heating the mixture of Pt(acac)2, Bi(NE)3, NH4Br, and OAM in a four-neck flask under N2 atmosphere, they 6415

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Figure 6. (a) Simulated atomic structure of a suspended mono/atomic Fe layer in a graphene pore. (b) Low-voltage CS-corrected TEM image of a monatomic Fe layer with the square unit cell. Inset: The interatomic spacing of the square unit cell. (c) Smoothed image of (b). (d) Image simulation of a monatomic Fe layer. Reproduced with permission from ref 207. Copyright 2014 American Association for the Advancement of Science. (e) Schematic illustration of the synthetic procedure of 2D Au nanosheets in LDH. Reproduced with permission from ref 131. Copyright 2015 Nature Publishing Group. (f) The structure of HGM lamellar bilayer and water layer. (g,h) Photos of HGM solution at room temperature (g) and 50 °C (h). (i) SEM image of a typical Au membrane. Reproduced with permission from ref 150. Copyright 2014 Nature Publishing Group.

nm. It is the first wet-chemical synthesis of pure hcp Au nanostructures with ambient stability. Importantly, GOsupported Au square-like plates with hcp/fcc crystal phase heterostructures can be formed via a shape and phase evolution from the ultrathin Au square sheets (Figure 5f−i).202 After the secondary growth of Au, the nonuniform thickness increase successfully induced the formation of alternating hcp and fcc crystal domains, i.e. so-called crystal phase heterostructures,224 and a slight shape change. In addition to Au nanosheets, GO or r-GO templated growth of other 2D metal nanomaterials, such as Ag nanoprisms,206 Pd square nanoplates,205 and Pt nanosheets,203 has also been achieved. Interestingly, other than using the graphene surface, Rümmeli et al. fabricated single-atom-thick Fe membranes with ordered 2D lattice in the pore defects of graphene (Figure 6a−d).207 They found that the porous graphene produced under the electron beam irradiation can be used to accommodate Fe atoms, in which single atomic crystalline Fe layer could be formed and sealed. Moreover, some 2D templates are able to constrain the growth of 2D metal nanostructures within their interlayer spaces.131,150,199−201,225 By using this strategy, metal ions are first diffused into the interlayer space, in which they are then in situ reduced by the insertion of reducing agent or thermal treatment. For instance, Au precursors (AuCl4−) could be introduced into the interlayer space of Mg/Al-LDH via the anion exchange. Then LDH/Au nanosheets were produced by a subsequent chemical reduction (Figure 6e).131 The asprepared Au nanosheets are single crystalline with (001) basal planes and thickness of a few atomic layers. In addition, by

templates for the growth of other 2D nanomaterials. They can either interact with the metals (or their precursors, reduction intermediates) to chemically confine the 2D growth or simply confine the 2D growth physically within their interlayers or on their surfaces. To date, a variety of 2D materials or substrates including graphite,199−201 graphene oxide (GO),106,202−206 reduced GO (r-GO),199−202 graphene,207 TMD nanosheets,208 titanium dioxide (TiO2),209 LDH,131 lamellar hydrogel or membrane,130,150 liquid crystal,210−212 ionic liquid,213,214 zwitterionic vesicle,215 indium tin oxide (ITO) coated surfaces,216,217 n-type GaAs substrate,218,219 and interfaces of liquid/liquid, liquid/gas, and liquid/solid220−223 have been utilized as directing templates for the growth of 2D metal nanomaterials. Presynthesized 2D nanomaterials can also be called “hard” templates, as they are stable during the synthesis and can provide relatively strong support to the 2D metal nanostructures. Graphene and its derivatives (GO and r-GO) are popular growth templates owing to their solution processability, ultrathin and flexible structure, and modifiable surfaces. For example, the surface of solution processed GO is usually rich in active functional groups, which can facilitate the nucleation and 2D growth of metal precursors on its surface. In 2011, our group reported the successful synthesis of GOsupported ultrathin Au square sheets with a unique hcp crystal phase (Figure 5a−e).106 In this synthesis, OAM was applied as a mild reducing agent, by which HAuCl4 was gradually reduced into Au clusters on GO and eventually grew into Au square sheets with thickness of ∼2.4 nm and edge length of 200−500 6416

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Figure 7. (a) Scheme of the vertical seeded growth of Ag on Ag nanoplates along the vertical direction. (b) TEM image of triangular Ag nanoplates used as the initial seeds with mean edge length of 45 ± 15 nm. Inset: TEM image of a number of nanoplates stacked together, showing the thickness of Ag nanoplates to be 5.0 ± 0.5 nm. (c,d) SEM images of Ag thin plates obtained by repeating the seeded growth using PVP as a capping agent for 4 and 8 times. (e) Scheme of the lateral seeded growth of Ag on Ag nanoplates along the lateral direction. (f) TEM image of triangular Ag nanoplates used as the initial seeds, with inset showing their thickness. (g,h) SEM images of Ag thin plates obtained by repeating the seeded growth using sodium citrate as a capping agent for 4 and 8 times. Reproduced with permission from ref 254. Copyright 2011 John Wiley & Sons, Inc. (i) Schematic illustration of the synthetic procedure of Pd@Ru nanosheets. NS: nanosheet. (j) Low-magnification TEM image of Pd seed nanosheets. Inset: Statistical analysis of the lateral size of Pd nanosheets. (k) Low-magnification TEM image of Pd@Ru nanosheets. Inset: Statistical analysis of the lateral size of Pd@Ru nanosheets. (l) STEM-EDS elemental mapping of a typical Pd@Ru NS. Reproduced with permission from ref 262. Copyright 2016 John Wiley & Sons, Inc.

thick water layer sandwiched by two bilayer membranes formed by self-assembly of two surfactants, i.e. hexadecylglyceryl maleate (HGM) and SDS (Figure 6f−i).150 The wide water-filled spacing provided excellent 2D interspace for the constrained growth of polymer-free, ultrathin, single crystalline, and atomically flat Au membranes with surface area over 100 μm2 . Another bilayer lamellar assembly composed of dodecylglyceryl itaconate was realized by Jin et al. to fabricate ultrathin single crystalline Au nanosheets with tunable thickness.130 By changing the concentration of AuCl4− ions, the thickness could be well tuned from 5 to 10 nm to 10−20

using graphite or graphene membranes as 2D templates and the thermal treatment as reduction process, Sn nanoflakes, Pt nanosheets, and 2D Pd nanoparticles have also been obtained.199−201,225 The interfaces of liquid/liquid, liquid/solid, or liquid/gas can be utilized as “soft” 2D templates owing to the selfassembly of molecules into a 2D structure at these interfaces. For example, bilayer lamellar structures resulted from the 2D assembly of surfactants at the liquid/liquid interface are an excellent soft template to confine the 2D growth of metal nanostructures.130,150 For example, Niu et al. built a 100-nm6417

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synthesis of Pd triangular and hexagonal nanoplates via a polyol process.57 In their report, the reduction rate of Pd salts could be manipulated through the addition of FeCl3 in the growth solution. By fine-tuning the concentration of FeCl3, Pd triangular and hexagonal nanoplates can be selectively synthesized. In another study, the same group found that polyacrylamide (PAM) could act as a dual functional polymer in the polyol method for the synthesis of Ag nanoplates.237 PAM can serve as a stabilizer for Ag nanoplates, and also as a ligand to bind with Ag+ ions. The formation of coordination complexes of Ag+ ions and PAM dramatically decreased the reduction rate of Ag+ ions, resulting in the formation of Ag nanoplates. In addition to Ag and Pd, Rh nanoplates have also been synthesized using 1,4-butanediol as the reducing agent and solvent.230 2.1.5. Seeded Growth. Seeded growth is a two-step synthetic strategy that effectively separates the nucleation and the subsequent growth processes. By using presynthesized seeds to focus only on the kinetics control of growth, the seeded growth can achieve fine-tuned morphology with a narrow size distribution.246−250 One important factor in the seed-mediated synthesis is to ensure high-quality seeds. For example, Kelly et al. reported the effect of poly(sodium styrenesulfonate) (PSSS) on suppressing the diversity of seed sizes and shapes for the high-yield growth of Ag nanoprisms.251 With the presence of PSSS in the seed production, the yield of Ag nanoprisms with abundant stacking faults can be as high as 95%. More importantly, the overgrowth pathway in a seeded growth needs to be well controlled. For instance, it was found that citrate ligands can effectively block the overgrowth of Ag on the {111} facets and only allow the lateral growth.252,253 Therefore, via slow reduction of Ag-citrate complex, thin Ag nanoplates with thickness of less than 10 nm and lateral size up to 4 μm was successfully obtained. Importantly, the seeded growth can have different growth modes. For instance, Xia et al. reported the lateral and vertical overgrowth of Ag using triangular Ag nanoplates as seeds.254 When different capping agents, such as sodium citrate and PVP, are used, Ag can selectively deposit on different facets of the seeds, resulting in Ag nanoplates with thickness of ∼200 nm (Figure 7a−d) and Ag nanosheets with a lateral size of ∼5 μm (Figure 7e−h), respectively. Moreover, seeded growth techniques have also been widely employed for the controlled synthesis of 2D Au nanostructures. For example, Mirkin et al. demonstrated a three-step growth strategy to obtain a mixture of spherical and triangular Au nanoparticles from small Au seed nanoparticles.255 In a further study of this seeding methodology, Au nanoprisms in narrow size distribution were obtained with edge length ranging from 100 to 300 nm. With a low reducing rate, the growth of Au nanoprism could be initiated, stopped, and reinitiated at different stages, while the original shape, thickness, and crystallinity of the nanoprisms were well maintained.247 In addition, with fine control of the growth steps, the edge length and thickness of Au nanoplates could be well tailored, and their uniformity and yield were simultaneously improved as well.146 Critical parameters to realize such uniform growth include the surfactant concentration, the time interval between multiple growth steps, and the control of overgrowth. Furthermore, seed-mediated growth also provides the possibility of growing metal nanoplates directly on semiconductor substrates by implanting the seeds on substrate and a subsequent growth of nanoplates.256

nm and 20−40 nm. Notably, by making use of the large area of the liquid/liquid and liquid/solid interfaces, the synthesis of microscale atomically flat Au nanosheets has also been well demonstrated using various surfactants at the interfaces.220,222,223 Importantly, a mixed liquid system with ordered lamellar structures can also be utilized as 2D templates. For example, lyotropic liquid crystals, composed of proper concentrations of surfactants, solvents, and metal precursors, can serve as “soft” 2D templates to guide the growth of Pt nanosheets with 4−10 nm in thickness and up to 500 nm in diameter, and microsized Au nanoplates as well.210−212 Semiconductor substrates, such as GaAs substrate and ITOmodified substrate, can also assist in the formation of Au and Ag nanoplates.216−219,226 This strategy is especially useful to prepare well-dispersed metal nanoplates for subsequent substrate-based applications, such as SERS. For example, Sun et al. fabricated surfactant-free Ag nanoplates with thickness of 50−70 nm and edge length ranging from 200 nm to 1 μm on n-type GaAs wafers.219 The formation of Ag nanoplates was achieved via a simple galvanic reaction between AgNO3 and the GaAs surface at room temperature. 2.1.4. Polyol Method. The polyol method, using polyols as the solvent and/or the reducing agent, has been widely used to obtain metal nanostructures with controllable morphologies.227,228 Many different polyols, such as ethylene glycol, 1,4butanediol, 1,5-pentanediol, di(ethylene glycol), glycerol, and poly(ethylene glycol), have been employed.229−234 Besides polyols, polymer ligands are also added to stabilize the assynthesized metal nanostructures.235−237 In the past two decades, various 2D metallic nanostructures have been synthesized via the polyol method. Particularly, the polyol method has been extensively used for the synthesis of Au nanoplates.238−242 Au nanoplates with different sizes, thicknesses, and shapes have been prepared by tuning the reaction parameters in polyol methods. For example, ultralarge Au nanoplates with tens of micrometers in lateral size can be simply synthesized by heating the ethylene glycol solution of HAuCl4 and PVP.239 These Au nanoplates with thickness of ∼90 nm are either triangular or hexagonal in shape. In another work, Hojo et al. developed a rapid heating process to synthesize smaller Au nanoplates with a lateral size range of 100−1000 nm.243 In this process, the fast injection of precursors into preheated ethylene glycol led to a quick nucleation and therefore the formation of smaller Au nanoplates. Furthermore, Majlis et al. developed a process to control the thickness of Au nanoplates in a ternary surfactant mixture of CTAB, PVP, and poly(ethylene glycol) (PEG).244 By simply tuning the concentration ratio of the aforementioned three surfactants in the growth solution, the high yield of Au nanoplates with thickness as small as 5 nm could be obtained. While the aforementioned Au nanoplates were either triangular or hexagonal in shape, Shi et al. devoted their efforts to synthesize Au nanoplates with other shapes by precisely controlling the temperature variation in the polyol process.245 They found that a temperature variation in the early stage of nanoplate growth could promote the preferential growth of the {111} plane along ⟨211⟩ and other high-index directions, leading to the formation of well-defined shield-like and star-like Au nanoplates. Besides Au nanoplates, the polyol method has also been successfully used to synthesize other metal nanoplates, such as 2D Pd, Ag, and Rh nanostructures. For example, Xia et al. demonstrated that kinetic control could be utilized in the 6418

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Figure 8. (a) HAADF-STEM image of three platinum circular nanosheets grown on liposomes with diameter of 100 nm. (b) TEM image of a large dendritic Pt nanosheet and its SAED pattern (inset). Reproduced with permission from ref 273. Copyright 2004 American Chemical Society. (c) SEM image and (d) TEM image of star-shaped Au nanoparticles. (e) Side view of two six-star Au nanoparticles. (f) TEM image of a typical six-star Au nanoparticle. Reproduced with permission from ref 269. Copyright 2010 American Chemical Society. (g−i) TEM images of Ag nanoprisms with average edge lengths of (g) ∼38 nm, (h) ∼72 nm, and (i) ∼120 nm, prepared by varying the primary excitation wavelength coupled with a fixed secondary wavelength. (j) The edge length of nanoprisms as a function of the primary excitation wavelength. Reproduced with permission from ref 283. Copyright 2003 Nature Publishing Group. (k) Schematic illustration of the photochemical growth process of an Au nanoprism. Under visible-light irradiation, hot electrons (e−) generated in the Au nanoprism help to reduce Au precursors (red arrow) with the assistance of PVP adsorbed along the edges of the nanoprism, while hot holes (h+) are scavenged by the donor (D) methanol (white arrow). Reproduced with permission from ref 110. Copyright 2016 Nature Publishing Group. (l) SEM image of Au hexagonal nanoprisms obtained after 2 h of plasmon irradiation. Insets: (i) High-magnification SEM image of a single hexagonal nanoprism and (ii) nanoscale secondary-ion mass spectrometry (NanoSIMS) image showing the elemental distribution of 12C14N− signals (green) from adsorbed PVP on a hexagonal nanoprism. (m) SEM image of Au triangular nanoprisms obtained after 2 h of irradiation with the presence of I− during the growth. Insets: (i) High-magnification SEM image of a single triangular nanoprism and (ii) NanoSIMS image showing the elemental distribution of 12C14N− signals (green) and 127I− signals (blue) from a triangular nanoprism. Reproduced with permission from ref 111. Copyright 2016 Nature Publishing Group.

Significantly, the seeded growth provides a general strategy to synthesize bimetallic and multimetallic 2D nanostructures. Preformed 2D metal seeds can act as heterogeneous nucleation sites for the deposition of other metal atoms through the chemical reduction or thermal decomposition of the metal precursors.257 There are three approaches to deposit a second layer of metal on a metal core, i.e. direct reduction of metal precursors, galvanic replacement between the metal core and the secondary metal precursors, and under a potential deposition process. When the reduction potential of the seed metal is higher than that of the shell metal, a reductant is needed to induce the seeded growth. Using this approach, bimetallic core−shell Au@Ag nanoprisms, Pd@Ag nanoplatelets, Au@Ag hexagonal nanoplates, Au@Cu nanoplates, and Au@Ni nanoprisms have been successfully synthesized by

using Au nanoprisms, Pd nanosheets, Au nanodisks, Au nanoplates, and Au nanprisms as seeds, respectively.56,258−261 Recently, our group has demonstrated the seeded growth of Pd@Ru nanosheets via the underpotential deposition (UPD) of submonolayered Ru on ultrathin Pd nanosheets (Figure 7i− l).262 As a special type of seeded growth, the UPD process usually occurs when the standard reduction potential of the shell metal is lower than that of the core metal. Another similar work was reported, in which Pd@Ru nanoribbons with atomically dispersed Ru were synthesized.263 When the secondary metal has a higher reduction potential than the core metal, galvanic replacement spontaneously takes place, which however often results in a nonuniform overgrowth of the shell metal.59 One effective way to minimize the galvanic reaction is to add protective surface ligands onto the core 6419

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citrate ions, resulting in the conversion of spherical Ag nanoparticles into triangular Ag nanoprisms. In the meantime, smaller Ag nanoparticles were simultaneously oxidized into Ag+ ions which were used as a source for the growth of Ag nanoprisms. In other studies, Mirkin et al. showed that the monodisperse Ag nanoprisms with controllable edge lengths have been synthesized by using dual-beam illumination or changing the pH of the growth solutions (Figure 8g−j).283,284 Moreover, a similar strategy was also used to synthesize Au@ Ag core−shell nanoprisms with plasmonic Au nanoparticles as seeds.285 Very recently, plasmon-mediated synthesis has also been used to synthesize 2D Au nanostructures (Figure 8k).110,111 Wei et al. demonstrated the plasmon-driven synthesis of Au nanoprisms and investigated their growth mechanism at the single-nanoprism level (Figure 8l,m).111 In their work, the formation of hexagonal Au nanoprisms was realized by using visible light to irradiate an aqueous solution containing Au nanocrystal seeds, HAuCl4, PVP, and methanol. Upon irradiation, surface plasmon excitation generated hot electron−hole pairs. While the hot holes were quickly scavenged by methanol, the hot electrons reduced Au precursors toward the growth of nanoprisms. Interestingly, the selective adsorption of PVP played an important role in the kinetics control of this photochemical process. The PVP ligands were observed to concentrate aqueous AuCl4− from solution along the prism perimeter of twinned defects, attract the extra electrons from methanol oxidation, and thus promote the growth of nanostructure in a 2D manner. Furthermore, the introduction of iodide ions could promote the growth of triangular nanoprisms by passivating the Au {111} facets. 2.1.7. Hydro-/Solvothermal Methods. Owing to the simple setup and facile operation, the hydrothermal and solvothermal methods have been widely used in the synthesis of various nanomaterials.286−288 Typically, the synthesis is performed above ambient temperature and pressure in aqueous (hydrothermal method) or nonaqueous (solvothermal method) solutions sealed in Teflon-lined stainless-steel autoclaves. Importantly, the reaction can be carried out at a temperature beyond the normal boiling point of solvents, which is inaccessible in other synthesis methods. At relatively high temperature and high pressure, the reactivity and solubility of reactants can be greatly increased, resulting in variations of physiochemical properties of solvents.289 Accompanied with the optimum ratio of reacting species, such as precursors, reductants and ligands, the nucleation and growth processes of metal nanocrystals could be regulated to form uniform products with fine-tuned morphology and size.289 The shortage of this method lies in its difficulty in real-time observation for the shape evolution process of metal nanocrystals. Moreover, compared to other wet-chemical methods, hydrothermal and solvothermal methods are relatively heavy energy consuming. Hydrothermal method, which uses water as a solvent, has been successfully applied to synthesize various 2D metal nanomaterials, such as Ag nanoplates,290,291 Rh nanosheets,292,293 Co nanoplatelets,294 Co nanosheets,295 Ni nanoplatelets and nanobelts,125,294 Cu nanoplatelets,294 and Ru nanoplates.58 Typically, Li et al. reported the synthesis of Co nanoplatelets in an aqueous solution including CoCl2, sodium dodecyl sulfate, sodium hydroxide and sodium hypophosphite monohydrate at 110 °C for 24 h. This strategy can also be applied to synthesize Ni and Cu nanoplatelets by simply changing the reaction temperature and time.294

metal surface and incorporate shell metal ions with capping ligands. For example, Yin et al. reported the deposition of a thin layer of Au on Ag nanoplates by slowly introducing HAuCl4 growth solution into a mixture of PVP, diethylamine, ascorbic acid, and Ag nanoplates.59 In the same year, Xue et al. also demonstrated the coating of Au layer on preformed Ag nanoprism via a surfactant-free approach, in which Au precursor ions were reduced directly with hydroxylamine at a mild reaction rate.264 In addition to the Au@Ag nanoplates mentioned above, Pd@Pt nanoplates with hexagonal and triangular shapes, porous Au@Pd triangle nanoplates, and triangular Au/Pd nanoplates have also been prepared.93,265,266 2.1.6. Photochemical Synthesis. Photochemical synthesis has been used as a unique and efficient method for the synthesis of 2D metallic nanostructures in recent years. It utilizes light irradiation as a powerful tool to reduce metal precursors and induce the formation of 2D metal nanostructures.267−271 For example, high-energy UV light can cause generation of superoxide and ethoxy radicals from ethanol molecules, which act as strong reducing agents for the reduction of metal ions and convert AuCl4− ions to metallic Au.272 By introducing Ag+ ions as a shape-directing agent, the growth of Au triangular nanoplates can be realized in mesoporous silica shells.271 In addition, more complicated photosensitizers could be introduced to assist the reduction of metal salts. For example, Shelnutt et al. reported the usage of tin-porphyrin (SnP) as a unique photosensitizer. The absorption of visible or UV light by SnP will yield the excited triplet π−π* state, SnP*, which is rapidly reduced by ascorbic acid to form radical anion SnP−•. Note that SnP−• is a strong reducing agent and capable of reducing a variety of metal ions.273 Based on this idea, dendritic platinum nanosheets could be synthesized by using unilamellar liposomes as the template (Figure 8a,b). Our group reported the synthesis of platelet-like Au nanorods and six-star Au nanoparticles through the TiO2-assisted photochemical reduction process (Figure 8c−f).269 In this process, the colloidal TiO2 sol was used as the photocatalyst to initiate the reaction and also the stabilizing agent for the produced Au nanostructures. Moreover, highenergy UV light can also be used to reshape Ag nanoplates.274 When exposed to UV light, the triangular Ag nanoplates in aqueous solution can undergo a gradual shape transformation. In this process, their sharp tips became rounded, and their thickness increased. As a result, the LSPR wavelength could be tuned from 870 to 450 nm by controlling the irradiation time. Recently, visible light has been used in the synthesis of 2D metallic nanostructures based on the plasmon-mediated processes, which is originated from the unique plasmonic properties of noble metal nanostructures.275 Upon the plasmonic excitation of Ag nanomaterials by visible light, Ag nanoparticles can undergo a shape transformation to Ag nanoprisms.112,117,276−280 In 2001, the Mirkin group discovered that the monodispersed Ag nanoprisms were synthesized by the plasmonic excitation of Ag nanoparticles.277 Specifically, after spherical Ag nanoparticles stabilized by trisodium citrate and bis(p-sulfonatophenyl) phenylphosphine dihydrate dipotassium salt were irradiated with visible light, the shape conversion of Ag nanostructures from spherical to triangular was achieved. A plasmon-induced electron transfer mechanism was proposed to explain the formation of Ag nanoprisms.281,282 First, energetic electron−hole pairs were generated from plasmon excitation of spherical Ag nanoparticles. Then, these electron−hole pairs could catalyze the reduction of Ag+ ions by 6420

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The solvothermal method adopts nonaqueous solvents such as benzyl alcohol, OAM, formamide, ethylene glycol, and DMF. Until now, monometallic nanosheets of Ag,296−299 Rh,52,300 Ir,301 and Ru302 have been obtained by using this method. For example, Li et al. prepared single atomic layered Rh nanosheets with edge length of ca. 500−600 nm from Rh(acac)3 in the presence of PVP, formaldehyde, and benzyl alcohol at 180 °C for 8 h (Figure 9c,d).52 More recently, Huang et al. demonstrated the solvothermal synthesis of Ir superstructures, composed of ultrathin Ir nanosheets, from IrCl3 mixed with citric acid, glyoxal, and PVP at 200 °C for 5 h (Figure 9e,f).301 Besides monometallic nanosheets, bimetallic nanosheets, such as BiRh nanoplates,303 PtCu nanosheets,51 PdNi nanosheets,304 and PtAg and PtAgCo nanosheets305 have also been synthesized by the solvothermal method. For example, Wang et al. reported the controlled synthesis of ultrathin PtCu alloy nanosheets with thickness of 4−6 atomic layers (Figure 9g) and PtCu nanocones (Figure 9h) via a twostep solvothermal approach.51 Typically, a gel-like material was first prepared from a mixture of PVP, formaldehyde, and tris(hydroxymethyl)aminomethane. Then the gel was added to the formamide solution containing Cu(acac)2, Pt(acac)2, and KI to synthesize PtCu alloy nanosheets at 130 °C for 3 h. The lateral size of obtained PtCu nanosheets was tuned from 8 to 50 nm by simply increasing the KI concentration in the growth solution. Importantly, this strategy can also be applied to synthesize PtAg and PtAgCo ultrathin nanosheets.305 Moreover, by using the presynthesized PtCu alloy nanosheets as seeds, a series of PtCu-based core−shell nanosheets, such as PtCu@Pd, PtCu@PdRu, PtCu@PdIr, and PtCu@PdRh, can be synthesized by the solvothermal method.306 2.1.8. Crystal Phase Transformation. Unlike conventional bottom-up synthetic approaches, crystal phase transformation can generate novel 2D metal nanostructures from presynthesized 2D metal nanostructures with an unusual crystal structure.307 Importantly, these unusual 2D nanostructures can be used as active templates, on which surface modification processes, such as ligand exchange and metal coating, take place to induce a partial or complete crystal phase transformation.307 Recently, for the first time, our group has successfully applied this strategy on some 2D Au nanostructures with unusual crystal phases to produce novel Au nanostructures as well as Au-based bimetallic and multimetallic nanostructures.107,114,307−309 Notably, it was found that ligand exchange is a simple and effective method to induce crystal phase transformation of 2D Au nanostructures with unusual crystal phases. As a typical example, the hcp Au square sheets protected with OAM ligands underwent a complete phase transformation to the fcc phase when the OAM ligands were exchanged with octadecanethiol ligands (Figure 10a−d). Note that the square shape did not change during the phase transformation, and the obtained fcc Au square sheets on GO were enclosed with the {100} planes. The interaction between the thiol functional groups and Au atoms played a critical role in the atomic reconstruction of the Au nanosheet, leading to the formation of a unique (100)fccoriented nanosheet. Indeed, the same crystal phase transformation phenomenon was also observed when octadecanethiol was changed to various kinds of other thiols, such as 1dodecanethiol, 1-octanethiol, 1-propanethiol, benzeneethanethiol, and 11-mercapto-1-undecanol. The ability of thiol molecules to induce crystal phase transformation of Au nanostructures with unusual phases can be applied on 4H

Recently, Yan et al. reported the controlled synthesis of ultrathin Ru nanoplates with different morphology and size (Figure 9a,b).58 The triangular Ru nanoplates with edge length

Figure 9. (a,b) TEM images of triangle Ru nanoplates. Inset of (b): a geometric model of the nanoplate. Reproduced with permission from ref 58. Copyright 2012 American Chemical Society. (c) Lowmagnification and (d) high-resolution TEM image of the PVP-capped Rh nanosheets. Reproduced with permission from ref 52. Copyright 2014 Nature Publishing Group. (e) HAADF-STEM image of 3D Ir superstructures composed of ultrathin Ir nanosheets. (f) HRTEM image of an individual 3D Ir superstructure. Reproduced with permission from ref 301. Copyright 2016 American Chemical Society. (g) TEM image of PtCu nanosheets. (h) TEM image of PtCu nanocones. Inset: Different orientations of the nanocone with corresponding cone model. Reproduced with permission from ref 51. Copyright 2013 American Chemical Society.

of 23.8 ± 4.6 nm and thickness of ∼3 nm were obtained in a mixture of RuCl3·xH2O, formaldehyde (40 wt%) and PVP at 160 °C for 4 h. With an increasing amount of RuCl3·xH2O and PVP, irregular-shaped Ru nanoplates with edge length of ∼15 nm and thickness of ∼1.5 nm were obtained. 6421

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Figure 10. (a) Schematic illustration of the ligand exchange-induced phase transformation of hcp Au square sheet. (b) High-magnification TEM image of a typical fcc Au square sheet obtained after ligand exchange. (c) SAED pattern and (d) HRTEM image of a typical fcc Au square sheet taken along the [100]fcc zone axis. Reproduced with permission from ref 307. Copyright 2015 Nature Publishing Group. (e) Schematic illustration of ligand exchange induced phase transformation of 4H Au nanoribbon. (f) TEM image of an Au nanoribbon after the ligand exchange. (g) The corresponding SAED pattern of the region inside the rectangle in (f), showing the fcc crystal phase oriented along the [001]fcc zone axis. (h) HRTEM image taken from the center of the nanoribbon in (f). Reproduced with permission from ref 107. Copyright 2015 Nature Publishing Group.

Au nanoribbons as well (Figure 10e−h).107 Remarkably, the ligand exchange-induced crystal phase transformation from 4H Au to fcc Au nanoribbon enclosed with {001}fcc facets has been successfully realized. Another efficient way to induce crystal phase transformation of 2D Au nanostructures with unusual crystal phases is via the coating of a secondary metal.114,307−309 This metal layer can be grown by reducing metal precursor in the presence of the presynthesized nanostructures or via a galvanic reaction with the presynthesized nanostructures. In either way, the outer layer metal could generate surface strain at the Au-metal interface due to the lattice mismatch between the Au core and the metal shell, which results in crystal phase transformation of the Au core. For example, by coating a thin layer of Ag on the hcp Au square sheets with or without using OAM, partial phase transformation to form hcp/fcc Au@Ag core−shell square sheets or complete phase transformation to form fcc Au@Ag core−shell square sheets was achieved, respectively (Figure 11a−e).307 Besides coating Ag, other metals like Pt and Pd were also successfully grown on hcp Au square sheets.114 The

as-obtained fcc Au@Pt nanostructures were found to have two types of shapes, i.e. a major product of fcc Au@Pt rhombic nanoplate, and a minor product of fcc Au@Pt square nanoplate (Figure 11f−j). In addition, galvanic reaction could be used to synthesize multimetallic 4H/fcc nanoribbons. Recently, our group reported the synthesis of 4H/fcc trimetallic Au@PdAg core−shell nanoribbons from 4H/fcc Au@Ag nanoribbons as seeds via galvanic reaction (Figure 11k−n).309 Moreover, this strategy was also utilized to synthesize the 4H/fcc trimetallic Au@PtAg and quatermetallic Au@PtPdAg core−shell nanoribbons. Notably, these epitaxially grown alloy shells, i.e. PdAg, PtAg, and PtPdAg, on the 4H/fcc Au core were successfully synthesized with novel 4H hexagonal phase for the first time. Significantly, the strategy discussed here can not only induce crystal phase transformation of the core metal but also generate new crystal phases of the shell metal and has been successfully employed to obtain a novel crystal phase of various metals.308 2.1.9. Biological Synthesis. Biological approaches utilize natural substances, biological molecules, or biomimetic synthetic strategies to produce metal nanostructures. Unlike 6422

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Figure 11. (a) Schematic illustration of Ag coating induced crystal transformation of hcp Au square nanosheet. (b) TEM image of fcc Au@Ag square sheets prepared by coating Ag on hcp Au square sheets with ascorbic acid or NaBH4. (c) HRTEM image of an fcc Au@Ag square sheet taken in the center area. (d) TEM image of hcp/fcc Au@Ag square sheets prepared by coating Ag with oleylamine (OAM) on hcp Au square sheets. (e) HRTEM image of an hcp/fcc Au@Ag square sheet taken in the center area. Reproduced with permission from ref 307. Copyright 2015 Nature Publishing Group. (f) Schematic illustration of the preparation of (101)fcc-oriented fcc Au@Pt rhombic nanoplates from hcp Au square sheets (top). Minor products of (100)fcc-oriented fcc Au@Pt square nanoplates (bottom) were also observed. (g) TEM image of a typical fcc Au@Pt rhombic nanoplate with (101)fcc orientation. (h) Typical HRTEM image of an fcc Au@Pt rhombic nanoplate taken in the center region. Inset: the corresponding FFT pattern. (i) TEM image of a typical fcc Au@Pt square nanoplate with (100)fcc orientation. (j) HRTEM image of an fcc Au@Pt square nanoplate taken in the center region. Inset: the corresponding FFT pattern. Reproduced with permission from ref 114. Copyright 2015 John Wiley & Sons, Inc. (k) Schematic illustration of the synthetic procedure for 4H/fcc Au@PdAg core−shell nanoribbon via the galvanic reaction of Pd(NO3)2 with Ag from 4H/fcc bimetallic Au@Ag core−shell nanoribbon. (l,m) Magnified TEM images of a representative 4H/fcc Au@PdAg nanoribbon. (n) HRTEM image of the obtained 4H/fcc trimetallic Au@PdAg nanoribbon collected from its edge area. Reproduced with permission from ref 309. Copyright 2016 American Chemical Society.

mixing chloroaurate ions and amyloid fibrils with proper ratios at 60 °C and pH of 2 (Figure 12a−c).315 By optimizing the concentration of the amyloid fibrils, which act as reducing and stabilizing agents as well as nucleation sites, fine-tuning over the size of Au microflakes could be realized. The abundant resource of biosubstance makes them suitable for large-scale synthesis of 2D metal nanostructures. For example, Qian et al. demonstrated a facile route for the mass production of hexagonal Au nanosheets with thickness of ∼50 nm based on polysaccharide chitosan, a cellulose-like biopolymer that can act as the stabilizing agent and reducing agent (Figure 12d).312 Additionally, proteins extracted from the human body could also be utilized in the synthesis of 2D metal nanostructures. As

synthetic chemicals, the bioderived substances usually possess complicated structure and/or composition, and own advantages in easy accessibility, low cost, and low environmental impact.310−313 Giant biomolecules such as proteins and fibrils can contribute to the fabrication of macroscale 2D metal nanostructures. As a typical example, Mezzenga et al. reported the templating effect of β-lactoglobulin amyloid-like fibrils in the synthesis of triangular and hexagonal Au nanoplates with lateral size in micrometers.314 The protein-based β-lactoglobulin filaments are twisted nanofibrils with diameter of 1−6 nm and length over 1.0 μm. In a further study, the research group successfully prepared single-crystal Au microflakes with planar area over 104 μm2 and thickness larger than 100 nm by 6423

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Figure 12. (a) Schematic illustration of the preparation procedure for macroscopic single-crystal Au microflakes using amyloid fibrils, and the corresponding visual observation (b) and optical image (c) of the final products (inset: a typical dark field optical image). Reproduced with permission from ref 315. Copyright 2015 John Wiley & Sons, Inc. (d) SEM images of Au nanoplates synthesized by reducing a solution of HAuCl4 with chitosan at 100 °C. Inset: Enlarged SEM image. Reproduced with permission from ref 312. Copyright 2007 Elsevier Ltd. (e) TEM image of Ag nanodisks and nanotriangles prepared using the extract of unicellular green alga Chlorella vulgaris. The arrows indicate overlapped areas of several flat nanodisks or nanotriangles. Insets: SEM images of circular and triangular Ag nanocrystals. Reproduced with permission from ref 313. Copyright 2007 American Chemical Society.

induce the formation of monolayer islands, which eventually merge into continuous superlattice films via a successive drying process (Figure 13a).326 Based on this strategy, the assembly behavior of nanoparticles can be regulated by using different surface ligands to balance the attraction and repulsion forces between nanoparticles.335,336 For example, Luo et al. successfully fabricated a monolayered free-standing Au nanoparticle superlattice by employing ssDNA as a “dry ligand” in a microhole-confined, drying-mediated self-assembly process.325 Interestingly, the internal order of the assembled nanoparticles was determined by the DNA number density and length. Benefiting from the wide-range controllability of DNA length, the superlattice was featured with tunable internal gaps from 0.9 ± 0.2 nm to 19.6 ± 1.7 nm by increasing DNA length from DNA-T5 to DNA-T90, which was beyond the scopes of alkyl molecular ligands. In addition, various liquid−liquid interfaces also provided many possibilities in confined assembly of nanoparticles to form 2D constructions. As a typical example, Zhang et al. obtained single-cluster-thick sheets derived from the 2D self-assembly of Au nanoclusters at the microscale lamellar interface of liquid paraffin and dibenzyl ether at an evaporation temperature of 140 °C.330 Another solution-based approach to construct anisotropic assembly is the oriented attachment of nanoparticles.337−339 In a modified polyol method, Ag nanoplates with unusual jagged edges were formed from the oriented 2D attachment of smaller Ag nanoplates, driven by the higher reactivity of the nanoplate edges.339 Further fusion of the ordered nanocrystal assemblies under exposure to exterior energy (e.g., heat and light) can lead to continuous 2D nanosheets/nanoplates.203,221,324,337,338,340 Such transition is commonly observed in metals that have low fusion temperature, such as Au and Ag. By annealing the ordered Ag nanocrystal assemblies on

reported by Lee et al., the biomineralization ability of serum albumin protein can be applied to the synthesis of single crystalline Au nanoplates at good yield.316 Moreover, many plant extracts also contain functional substances that can act as reducing agents, shape-directing agents, and/or stabilizers. In 2004, Sastry et al. discovered that the extract from the lemongrass plant could react with aqueous chloroaurate ions to produce thin, flat, single-crystalline Au nanotriangles.310 Lee et al. found that the protein-containing extract of unicellular green alga Chlorella vulgaris can be used to synthesize singlecrystalline Ag nanoplates (Figure 12e) and high-yield Au nanoplates at room temperature.313,317 They identified that the hydroxyl groups in Tyr residues were responsible for the reduction of Ag+ ions, and the carboxyl groups in Asp and/or Glu residues were responsible for the growth of 2D Ag nanoplates. Based on this result, a simple bifunctional tripeptide with one Tyr residue and two carboxyl groups was designed and employed to produce Ag nanoplates with high yield. Using a similar biological approach, the synthesis of Au nanotriangles with Aloe vera plant extract, Au nanoplates with natural humic substance, Au nanoplates assisted with pear fruit extract, and Au nanoplates with brown seaweed extract has also been achieved.318−321 2.1.10. Nanoparticle Assembly. Distinct from the aforementioned approaches, nanoparticle assembly constructs 2D metallic nanostructures by using metal nanocrystals as building blocks.322,323 The 2D self-assembly of nanoparticles is usually triggered by controlling the anisotropic interactions between nanoparticles at interfaces, such as the liquid−air interface,324−326 liquid−liquid interface,327−332 and solid− liquid interface.333,334 Taking solvent evaporation at the liquid−air interface as an example, the subtraction of solvent can break the equilibrium of the colloidal suspension and 6424

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Figure 13. (a) Schematic illustration of the self-assembly process of Au nanocrystals captured by a quickly receding liquid−air interface. Reproduced with permission from ref 326. Copyright 2006 Nature Publishing Group. (b) 2D Ag nanocrystal assembled before annealing. Inset: The corresponding fast Fourier transform pattern. (c) 2D Ag nanosheets obtained after annealing samples in (b) for 20 h. (d) Top: HRTEM image of one Ag triangular particle. Bottom 1: Low-magnification image of the particle. Bottom 2: The FFT pattern of the HRTEM image. Reproduced with permission from ref 340. Copyright 2007 Nature Publishing Group. (e) TEM image of AuAg nanowires with small amount of nanoparticles as byproducts. (f) HAADF-STEM image of the assembled AuAg nanosheets. (g) AFM image of an AuAg nanosheet. Inset: The height profile of the dotted line. Reproduced with permission from ref 341. Copyright 2015 American Chemical Society.

different substrates at 50 °C, Pileni et al. prepared triangular Ag single crystals (Figure 13b−d).340 It was also observed that Ag with larger crystal size and higher crystallinity was formed on highly oriented pyrolytic graphite (HOPG) than on amorphous carbon. The formation of single-crystal Ag triangular nanostructures was found to involve three stages, i.e. formation of nanocrystal arrays, annealing of nanoarrays, and recrystallization of the coalesced nanocrystals. In addition to nanoparticles, the building blocks for assembled 2D nanosheets/nanoplates can also be nanowires. Our group demonstrated that AuAg nanowires with diameter of 2−3 nm and length up to 1 μm could be well patterned and fused to novel 2D AuAg nanosheets guided by the block copolymer Pluronic P123 (Figure 13e−g).341 The resulted uniform 2D AuAg nanosheets had length of ∼1.50 μm, width of 510 ± 160 nm, and thickness of ∼100 nm. 2.1.11. Other Bottom-up Methods. Besides the aforementioned synthetic methods, there are also some unique bottom-up approaches to obtain 2D metal nanomaterials. For example, besides chemical reductant, various external energy sources have been employed to reduce metal ions and form 2D nanostructures. For example, sonochemical energy has been applied to synthesize Ag nanoplates, ringlike Au nanocrystals, and Au nanobelts.120,124 By sonicating an aqueous solution of

HAuCl4 and α-D-glucose, Zhang et al. synthesized highly flexible single-crystalline Au nanobelts with thickness of ∼10 nm, width of 30−50 nm, and length of several micrometers (Figure 14a).124 Magnetic field can also be applied to synthesize 2D metal nanostructures with magnetic properties. By applying a magnetic field of ca. 0.1 T at the center of an autoclave vessel, Liao et al. synthesized thin Co nanosheets with tunable size and thickness by varying the intensity of magnetic field.342 Microwave energy was also found useful in preparing Au nanoplates and Ag nanoplates.342−346 Moreover, the electrochemical deposition method has also been proven a feasible approach to obtain 2D metal nanostructures. Via galvanic cell reactions, large-scale Ag nanosheet assembles (Figure 14c,d), ultralong Ag nanobelts, Ag nanosheet arrays, and ultrathin Ni nanosheets have been successfully obtained.347−350 Some other types of special synthesis methods involve the usage of special nanostructures as supports leading to the formation of 2D metal nanomaterial based hybrid structures.351,352 For instance, Xia et al. reported the successful preparation of Au nanoplates simultaneously with the dedoped polypyrrole nanofiber film via the in situ reduction of AuCl4− on the film surface.351 In another work, Hupp et al. fabricated Ag nanodisks in an aqueous suspension of carboxylate6425

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Figure 14. (a) SEM image of Au nanobelts produced via a sonochemical method. Reproduced with permission from ref 124. Copyright 2006 John Wiley & Sons, Inc. (b) TEM image and enlarged image (inset) of freshly prepared Ag/polystyrene sphere hybrids. Reproduced with permission from ref 352. Copyright 2002 American Chemical Society. (c) Schematic illustration of the cell set up for the synthesis of Ag nanosheet-assembled films on a Si substrate through galvanic cell reaction. (d) SEM image and enlarged image (inset) of the Ag nanosheet-assembled films. Reproduced with permission from ref 347. Copyright 2013 Elsevier Ltd.

applied to hard metals. Very recently, a hot-pressing method of fabricating free-standing ultrathin Bi nanosheets from Bi nanoparticles was developed by the same group (Figure 15d,e).356 The resultant Bi nanosheets exhibited high crystallinity and small thickness as low as ∼2 nm. A similar method is also applied to obtain ultrathin 2D Pt nanosheets.357 2.2.2. Exfoliation Method. Although most metallic materials are nonlayered structures, very few metals, such as Sb and As, possess allotropes with layered structures that are stable under ambient conditions.358 Taking Sb as an example, the gray allotrope of Sb crystallizes in a distorted hcp structure, in which the interatomic distances in the intralayers are 10− 20% shorter than that between neighboring layers.359 Very recently, Yang et al. successfully applied the liquid-phase exfoliation method to obtain Sb nanosheets from bulk gray Sb with thickness of ∼3.0−4.3 nm.359 In a typical exfoliation process, gray Sb powders were dispersed in isopropyl alcohol solution containing sodium hydroxide and then sonicated under ambient conditions (Figure 16a,b). Interestingly, some 2D metal nanostructures can be derived from exfoliated 2D metallic compounds. For instance, Matsumoto et al. demonstrated that monolayer Pt nanosheets could be obtained by reducing the Pt oxide monolayers exfoliated from the layered platinum oxide (Figure 16c).360 Moreover, Fukuda et al. also fabricated Ru nanosheets by reducing the exfoliated ruthenate layers.361 2.2.3. Nanolithography. Nanolithography is a powerful technique that can fabricate nanoscale patterns with finecontrolled shape, size, and spacing.363,364 It can be used to construct well-defined 2D metal arrays on substrates via a

functionalized polystyrene mesospheres and observed the polystyrene-originated nucleation and anisotropic growth of Ag (Figure 14b).352 Furthermore, Gautam et al. showed that free-standing Pt nanosheets could be synthesized by mechanochemical transformation of Te nanorods under magnetic stirring through galvanic displacement with H2PtCl6 in ethylene glycol medium.353 Although the underlying mechanism of these studies is not very clear, they provided alternative pathways to obtain novel 2D metal nanomaterials. 2.2. Top-down Methods

2.2.1. Mechanical Compression Method. Alternatively, 2D metal nanomaterials can be obtained by reducing the dimension of bulk metals via a mechanical compression process. Thanks to the excellent ductility of metal materials, the thickness of metal sheets can be reduced simply by compressing and folding them repeatedly at room temperature. Recently, this strategy was utilized by Wu et al. to fabricate Au, Ag, and Fe nanosheets (Figure 15a−c).354 In a typical process, an Au (or Ag, Fe) foil with thickness of ∼70 μm was stacked with a sacrificial metal foil, and the stacked binary metal sheet was repeatedly folded and calendered at room temperature with a rolling machine. After 20 times of rolling, the thickness of each metal layer was reduced down to only a few nanometers (Figure 15b). Then, the freestanding ultrathin 2D metal nanosheets can be obtained by selective etching of the sacrificial metal (Figure 15c). This method was later applied to fabricate multiatomic layered Al nanosheets.355 Moreover, at elevated temperature, this strategy can also be 6426

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Figure 15. (a) Schematic illustration of the fabrication procedure for metal nanosheets via mechanical compression. (b) Photograph of the binary metal plate consisting of alternately arranged metallic nanolayers by repeating the folding and calendering process. Inset: A typical cross-sectional SEM image of the bimetallic nanolayers. (c) SEM image of the free-standing 2D metallic nanosheets obtained by selective etching of one metal from the as-prepared binary metal plate. Inset: Photograph of the metal nanosheets after etching. Reproduced with permission from ref 354. Copyright 2016 John Wiley & Sons, Inc. (d) Schematic illustration of the fabrication process of Bi nanosheets using hot-pressing method. (e) SEM image of Bi nanosheets transferred on a carbon tape from Si/SiO2 substrate. Reproduced with permission from ref 356. Copyright 2017 John Wiley & Sons, Inc.

Figure 16. (a) Schematic illustration of the fabrication process of Sb nanosheets. Bulk Sb powders were first exfoliated in isopropyl alcohol (IPA) to obtain ultrathin Sb nanosheets. (b) TEM image of Sb nanosheets, Reproduced with permission from ref 362. Copyright 2017 John Wiley & Sons, Inc. (c) Schematic illustration of the synthesis of Pt nanosheets from the exfoliation of Pt oxide. Reproduced with permission from ref 360. Copyright 2014 The Royal Society of Chemistry.

“parent” layer with a focused electron beam.364,365 Using this technique, Au nanohole and nanodisk arrays were obtained with precisely controlled size and spacing (Figure 17a,b).366 By

multistep process. As a typical branch of nanolithography, electron beam lithography (EBL) has been widely used to construct metal nanostructures by scaling down a predeposited 6427

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Figure 17. (a,b) SEM images of Au nanodisk arrays with diameters of 60 and 200 nm, and gratings of 100 and 250 nm, respectively. The heights of all nanodisks are 50 nm measured by AFM. Reproduced with permission from ref 366. Copyright 2008 American Chemical Society. (c) SEM image of NSL triangular Al nanocrystal array. Reproduced with permission from ref 367. Copyright 2008 American Chemical Society. (d) SEM image of NSL triangular Cu nanocrystal array. Reproduced with permission from ref 368. Copyright 2007 American Chemical Society. (e) AFM image of NSL triangular Ag nanocrystal array. Reproduced with permission from ref 369. Copyright 2005 American Chemical Society.

varying the diameter of holes and the distance between holes, the nanohole array served as an excellent model to study the dimension-dependent SERS effect. Another typical example is the nanosphere lithography (NSL) technique developed by Van Duyne et al, which is a multistep process to produce surface-confined nanostructure arrays.136,367,368 NSL uses a monolayer self-assembly of polystyrene spheres with controlled diameter as the mask to deposit various 2D nanostructures. Using this strategy, ordered arrays of triangular Cu, Al, and Ag nanocrystals have been successfully fabricated (Figure 17c− e).367−369 In addition to EBL and NSL, nanoimprint lithography and hole-mask lithography were also effective toward fabricating 2D metal nanostructures.370,371 Moreover, via top-down nanofabrication techniques such as focused ion beam milling, one can also obtain high-definition metal nanostructures based on large, chemically grown, and single crystalline metal flake.372 This strategy is a combination of bottom-up and top-down nanofabrication and thus can yield homogeneous, reproducible, and well-defined metal nanostructures.

For instance, the ultrathin Au square sheets synthesized on the GO surface (Figure 5a−e) were found stable in solution at ambient conditions, but they can be destroyed when exposed to high energy electron beam irradiation within several seconds, resulting in crystal phase transformation and formation of pores in the nanosheets (Figure 18a,b).106 Moreover, Kan et al. studied the thermal stability of Au nanoplates and found that fragmentation started at 450 °C.374 The XRD spectra taken at different annealing temperatures suggested that the melting and breaking started mainly from the edges with (110) facets, while the {111} surfaces remained quite stable. On the contrary, Ma et al. observed a localized corrosion initiated from the {111} surfaces of large Au nanoplates.376 This counter-Ostwald ripening process was believed to be associated with the combined effect from dodecylbenzenesulfonate and AuCl4− anions. Normally, chemically active species like halide ions can gradually sculpt the sharp edges of metal nanoprisms at room temperature and lead to a shape rounding process.182 Kelly et al. found that Ag nanoprisms went through a shape transition from triangular to disklike when modified with thiol-terminated poly(ethylene glycol) (PEG-SH) with a concentration of 0.1 mM (Figure 18c).132 The Ag nanodisks could be further deformed into irregular Ag nanoparticles when the concentration of PEG-SH is increased to 1.0 mM. In a following stability and degradation study, they also found out that adverse conditions like thiols, amines, halide ions, and high temperatures could all result in the irreversible degradation of Ag, Au, and Ag@Au nanoprisms.373 In the meantime, countermeasures to increase the stability of 2D metal nanostructures have been developed. Coating inert protective layers or alloying with another metal have been demonstrated to effectively help 2D metal nanostructures preserve their shapes during chemical and physical postprocessings.56,59,377−379 For example, Ag@Pd core−shell nanoplates exhibited superior photochemical stability as

3. PROPERTIES 3.1. Stability

Although various 2D metal nanostructures have been successfully prepared by controlling the growth kinetics, the stability issues arisen during characterizations and applications of the as-prepared 2D metal nanomaterials are unneglectable. From a thermodynamic perspective, 2D morphology is not the most stable state of metal materials due to the high surface energy and high ratio of unsaturated surface atoms of 2D structures. Theoretically, all metal nanostructures tend to transform into isotropic shape to minimize the surface-tovolume ratio if given sufficient energy.127 The external energy source could be in the form of heat, high-energy electrons, or chemical attacks from oxidizers or other harsh chemical environments.56,106,182,274,373−375 6428

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Figure 18. (a) Schematic illustration of the e-beam-induced crystal phase transformation of an Au square sheet from hcp to fcc. (b) TEM image of the resultant porous Au square sheet after e-beam irradiation for ∼20 s. Reproduced with permission from ref 106. Copyright 2011 Nature Publishing Group. (c) Photos and corresponding schematic illustration of the shape deformation of Ag nanoprisms exposed to thiol-terminated molecules. Reproduced with permission from ref 132. Copyright 2013 American Chemical Society. (d) Schematic illustration of the four plasmon resonance modes involved in Ag nanodisks. They are (i) the dipolar in-plane mode, (ii) the dipolar out-of-plane mode, and (iii, iv) the corresponding quadrupolar modes, respectively. Reproduced with permission from ref 395. Copyright 2005 American Chemical Society. (e) Photos and corresponding extinction spectra of Ag nanoprism solutions prepared at pH = 11.2 with various excitation wavelengths. Reproduced with permission from ref 284. Copyright 2007 John Wiley & Sons, Inc. (f) The hysteresis loops of Co nanoplatelets with the magnetic field applied parallel (∥) and perpendicular (⊥) to the nanoplatelets at 300 K. Reproduced with permission from ref 294. Copyright 2007 American Chemical Society. (g) I−V curve of an ultrathin Au membrane spanned on two gold electrodes with a gap of 10 μm. Inset: Optical image of the device. Reproduced with permission from ref 150. Copyright 2014 Nature Publishing Group.

compared to pure-Ag nanoplate.378 Yin et al. also observed the stability increment of Ag nanoplates against H2O2 oxidation after the epitaxial Au coating.59 Interestingly, Xue et al. showed that the nanoprism edges could be selectively coated with Au, the resultant edge-Au-coated Ag nanoprisms exhibited remarkable stability during chemical etching.377 Besides the addition of a nonactive metal, nonmetals could also act as protective layers. As a typical example, Mirkin et al. demonstrated that the stability of Ag nanoprisms can be greatly increased by coating them with silica.378

example, most 2D Au nanostructures possess fcc crystal phase, with a typical stacking sequence of “ABC” along its closely packed direction of [111]fcc. As a result, the various fcc Au 2D structures were usually enclosed by the low-index facet of {111}fcc, in order to minimize the surface energy. Recently, our group synthesized thin Au nanostructures with two new crystal structures, i.e. hcp and 4H, with characteristic stacking sequences of “AB” and “ABCB” along their close-packed direction of [001]hcp and [001]4H, respectively.106,107 The lateral surfaces of the obtained 2.4 nm-thick square-shaped hcp Au nanosheets were exposed with a (110)hcp crystal plane, whereas the 4H Au nanoribbons were found to have a thickness of 2−6 nm and a (110)4H basal plane. In the case of Ru, the unusual crystal phase of fcc was observed in triangular Ru nanoplates by Yan et al.302 They suggested that stacking faults generated during the growth of the triangle Ru nanoplates were responsible for the crystal lattice altering and the resultant stacking ABC sequence.

3.2. Crystal Structure

Because of the strong and isotropic nature of metallic bonds, the majority of metal nanomaterials adopt the most closely packed crystal structures as in their bulk forms, i.e. either fcc or hcp crystal structure. However, theoretical studies and experimental results suggested that metal nanostructures with unusual crystal phases could also be obtained.103,104,380−382 As summarized in Table 1, to date, 2D nanostructures of Au, Ru, and Rh and Au@M (M = Ag, Pd, Pt, Ir, Pt, Ag, etc.) with unusual crystal phases have been reported. Take Au as a typical 6429

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Table 1. Summary of Some Representative 2D Metal Nanomaterials with Corresponding Synthetic Methods, Crystal Structures, and 2D Morphologies Synthetic method Organic ligandconfined growth

Small moleculemediated synthesis

2D templateconfined growth

Polyol method

Seeded growth

Photochemical synthesis

Hydro-/ solvothermal methods

Metal

2D morphology

Crystal structure

Nanoplates, nanobelts, nanospirangles Nanoribbons Nanosheets, nanoplates Nanoplates Nanowheels Nanowheels Nanodisks, nanosheets, nanoplates Nanoplates Nanodisks Nanoplates Nanosheets, nanodisks, nanoplates Nanosheets Nanosheets Nanosheets Nanoplates Nanosheets, nanorings Nanosheets Nanoplatelets Nanosheets Nanoplates Nanosheets, nanoplates, nanoprisms

fcc

Ag Pd Pt Fe Sn Au Pd Ag Rh Au Ag Pd@Ag Au@Ag Au@Cu Au@Ni Ag@Au Ag/Pd Pd@Ru Pd@Pt Au Ag

Nanoprisms, nanoplates Nanoplates Nanosheets Single atomic membrane Nanoflakes Nanosheets, nanoplates Nanoplates Nanoplates Nanoplates Nanoprisms, nanoplates Nanoprisms, nanoplates Nanoplates Nanoplates, nanodisks Nanoplates Nanoprisms Nanoplates, nanoprisms Nanoplates Nanosheets, nanobelts Nanoplates Nanoprisms Nanoprisms, nanodisks, nanoplates

fcc fcc fcc bcc/hcp tetrb fcc fcc fcc fcc fcc fcc fcc fcc fcc fcc fcc fcc fcc fcc fcc fcc

Au@Ag Ag Rh

Nanoprisms Nanoplates, nanosheets, nanoflakes Nanosheets Nanosheets Nanoplatelets, nanobelts Nanoplatelets, nanosheets Nanoplatelets Nanoplates Nanosheets Nanosheets Nanosheets, nanocones Nanosheets Nanoplates Nanosheets Nanosheets

fcc fcc fcc hcp fcc hcp fcc fcc hcp fcc fcc fcc hcpc fcc fcc

Au

Pd Rh Pt AuPd Ag NiCu Co Bi Cu PdCu Rh Ni Au Pd PtPdAg PtBi Au

Ni Co Cu Ru Ir PtCu PdNi BiRh PtAg, PtAgCo PtCu@PdRu, PtCu@PdIr, PtCu@PdRh, PtCu@Pd 6430

4H fcc fcc fcc fcc fcc fcc hcp rhoa fcc fcc fcc fcc fcc fcc fcc hcp hcp hcp/fcc fcc

Reference 140, 145, 151, 155, 169, 171, 173 107 141, 172 143 122, 123, 172 121 148, 154, 167, 172, 174 156 116 164 144, 149, 160 50 54 193 180 31, 187−190, 192 191 196 106 202 130, 131, 210, 211, 216, 220, 222, 223 206, 218, 219 205 199, 203, 212 207 225 162, 238−245 57 237 230 146, 247, 255 251−254 56 258, 261 259 260 59, 264 93, 266 67, 263 265 110, 111 112, 117, 277, 281 −284 285 290, 291, 296−299 52, 293, 300 52 125, 294 294, 295 294 58 302 301 51 304 303 305 306

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Table 1. continued Synthetic method

Metal

2D morphology

fcc fcc hcp/fcc fcc 4H/fcc

107, 307 307 307 114 107, 308, 309

fcc

Au

Nanoflakes, nanoplates, nanosheets, nanoprisms Nanoplates Nanosheets Nanoplates Superlattice sheets Nanosheets Nanoplates, nanosheets, nanodisks, nanobelts, nanosheet arrays Nanorings, nanobelts, nanoplates

Co Ni Pt Au, Ag, Al Fe Bi Sb Pt Ru Au, Pd, Pt Cu, Al, Ag

Nanosheets Nanosheets Nanosheets Nanosheets Nanosheets Nanosheets Nanosheets Nanosheets Nanosheets Nanohole array, nanodisk array Triangular nanocrystal array

hcp

241, 310, 314, 318, 320, 321 313 203 324, 338, 340 325 341 120, 343, 345, 347, 348, 350, 352 120, 124, 344−346, 351 342 349 353 354, 355 354 356 362 360 361 366, 370, 371 367−369

Au Au@Ag

Biological synthesis

Au@Pt, Au@Pd Au@Ag, Au@Pd, Au@AgPd, Au@AgPt, A@AgPdPt, Au@Ir, Au@Ru, Au@Rh, Au@Ir, Au@Os, Au@Cu Au

Other bottom-up methods

Mechanical compression method Exfoliation method

Nanolithography

Reference

Nanosheets, nanoribbons Nanosheets Nanosheets Nanoplates Nanoribbons

Crystal Phase Transformation

Nanoparticle assembly

Crystal structure

Ag Pt Ag Au AuAg Ag

fcc fcc fcc fcc fcc fcc

fcc fcc bcc rho hcpd fcc hcp

a

rho stands for rhombohedral. btetr stands for tetragonal. cNiAs type. dDistorted hcp.

half antiparallel to the incident electric field (Figure 18d).34,395 In 2001, Mirkin et al. first observed the distinct quadrupole plasmon resonance peaks in Ag nanoprisms and an unusual Rayleigh scattering in the red region, a phenomenon that had not been observed in conventional spherical nanoparticles.277 In a following report, the quadrupole plasmon mode in Au nanoprisms was also observed.255 As the scattering mode is closely related to the size of 2D nanocrystals, the position and intensities of LSPR peaks can be readily tuned over a wide range by simply controlling the thickness and edge length.192,251,283,284,396,397 As a typical example, Ag nanoprism suspension with fine-tuned prism edge length was able to exhibit various colors with excitation wave ranging from 470 to 633 nm (Figure 18e).284 As a result, the plasmonic properties can be used to indicate the shape evolution during a 2D crystal growth. For instance, during the shape transformation of Au nanoplates from triangle to hexagon, the gradual red shifts of the peak position of the in-plane dipole resonance were observed by Shuford et al.388 Constructing core−shell architectures could further enhance the plasmonic properties of 2D metal nanostructures.56,261,264,398 For instance, a thin-layer coating of Au on Ag nanoprism could effectively alter the environmental refractive index, leading to a significant change of the LSPR peak position.264,398,399 Notably, besides Au and Ag, attractive plasmonic properties such as broad spectral and strong NIR plasmon absorptions have also been observed from other 2D metal nanostructures. For example, the ultrathin and uniform Pd nanosheets prepared by Zheng et al. exhibited a welldefined and tunable LSPR peak in the NIR region with a relatively high extinction coefficient.31 When the Pd nano-

3.3. Plasmonic properties

One of the prominent characteristics of metal nanostructures is the localized surface plasmon resonance (LSPR) phenomenon, which is associated with the induction of the collective oscillation of conduction electrons under electromagnetic (EM) irradiation.227 At a certain range of EM wavelength (which corresponds to a certain excitation frequency), a strong oscillation of the surface electrons is induced in resonance with the incident light, resulting in a phenomenon known as localized SPR (LSPR).383 The LSPR behavior is dependent on the material chemistry (type of metals), size, shape, composition, and dielectric environment of the metal nanomaterials.367,384−386 The characteristic plasmonic properties of 2D metal nanomaterials are particularly attractive for their applications in photothermal therapy, LSPR-based sensing, solar cells, bioimaging, optical wave guiding, etc. To date, a number of 2D metal nanostructures have been reported to have LSPR absorption in the visible (vis) and near-infrared (NIR) regions, such as Au,61,130,387,388 Ag,276,389,390 Cu,294,368 Ni,197 Pd,187,192 and Al.367,391 Particularly, Au and Ag nanostructures are most widely studied thanks to their wide range of spectral absorption, strong shape-dependent plasmonic behaviors, and low energy dissipation.155,251,255,261,277,283,389,392 Unlike 0D and 1D, 2D morphology with tunable aspect ratio enables plasmonic metals to possess both dipole and quadrupole plasmon modes.393,394 In brief, the dipole mode is induced by the oscillation of electrons parallel or antiparallel to the incident electric field, while the quadrupole mode is generated by the oscillation of electrons that is half-parallel and 6431

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Figure 19. (a) Cyclic voltammogram (CV) curves of Pd nanosheets and Pd black recorded in an aqueous solution of 0.5 M H2SO4 and 0.25 M HCOOH at a scan rate of 50 mV s−1. Reproduced with permission from ref 31. Copyright 2011 Nature Publishing Group. (b) Peak current densities of EN-treated PdCu alloy nanosheets with different Cu/Pd ratios (Samples 1−5 correspond to Cu/Pd = 0.14, 0.24, 0.37, 0.45, and 0.56, respectively) in electrocatalytic formic acid oxidation recorded in aqueous solution of 0.5 M H2SO4 and 0.25 m HCOOH at scan rate of 50 mV s−1. Reproduced with permission from ref 50. Copyright 2017 John Wiley & Sons, Inc. (c) Peak mass activities of PtCu nanocone alloy, PtCu nanosheet alloy, Pt black, and Pt/C catalyst in electrocatalytic ethanol oxidation recorded in the aqueous solution of 0.5 M H2SO4 and 0.1 M CH3CH2OH at a scan rate of 50 mV s−1. Reproduced with permission from ref 51. Copyright 2013 American Chemical Society. (d) Specific and mass activities of PtPb nanoplates/C, PtPb nanoparticles/C, and commercial Pt/C catalysts for ORR. Reproduced with permission from ref 53. Copyright 2016 American Association for the Advancement of Science. (e) Polarization curves for PtAg, AgCo, and PtAgCo in reference to 20% Pt/C with the same metal loading in a 0.5 M H2SO4 aqueous electrolyte. Reproduced with permission from ref 305. Copyright 2017 American Chemical Society. (f) Faradaic efficiencies of formate formed from the electroreduction of CO2 for 4 h by partially oxidized Co ultrathin layers (red), Co ultrathin layers (blue), partially oxidized bulk Co (violet), and bulk Co (black). Reproduced with permission from ref 295. Copyright 2016 Nature Publishing Group.

properties of Ni nanosheets and nanobelts have also been well studied. Leng et al. studied the size-dependent magnetic properties of triangular and hexagonal Ni nanosheets.197,198 The coercive forces of Ni nanosheets at 300 K were measured to be 180 Oe, which were much higher than that of bulk Ni (ca. 0.7 Oe) and Ni hollow nanospheres (ca. 102 Oe) previously reported.198 Moreover, they further discovered that Ni nanosheets showed a transition from superparamagnetism to ferromagnetism and a decrease in blocking temperature with increasing edge lengths.197 Peng et al. also observed the ferromagnetic characteristics of Ni nanosheets at room temperature and their evident magnetic anisotropy compared to Ni nanoparticles.193 In another work, ferromagnetic Ni nanobelts prepared by Qian et al. showed a remarkably strong coercivity value of 640 Oe at room temperature, which could be attributed to the belt-like morphology with thicknesses of only ∼15 nm.117 Meanwhile, Fe- and Ni- based 2D nanostructures, such as NiCu alloy nanoplates and FeAl binary nanosheets, were also found to exhibit enhanced magnetic properties.156,354 For instance, Peng et al. observed that triangular NiCu alloy nanoplates possess larger coercivity and higher divergence temperature of magnetization compared to hexagonal NiCu alloy nanoplates.156

sheets were further coated with a thin layer of Ag, tunable LSPR properties could be achieved over a wide spectral range from 477 to 971 nm by simply changing the Ag layer thickness.56 3.4. Magnetic Properties

The magnetic properties of nanomaterials highly depend on the volume, surface, and step atoms of the nanostructure.400 Generally, anisotropic magnetic properties are desired for applications in memory devices where the nanomaterials act as ferromagnets. However, isotropic nanostructures of magnetic metals, such as Fe, Co, and Ni, are usually superparamagnetic at room temperature due to the nanosize confinement.294 An effective strategy to enhance the magnetic anisotropy is to fabricate magnetic nanocrystals with shape anisotropy.400 Therefore, Fe-, Co-, and Ni-based 2D nanostructures could be ideal candidates to enhance the magnetic anisotropy feature.125,156,193,197,198,294,342 For example, Co and Ni nanoplatelets were found to possess much enhanced coercivity forces and blocking temperatures with increased shape anisotropy.294 Impressively, the Co nanoplatelets exhibited distinctly enhanced coercivities of 218 and 176 Oe with magnetic fields applied parallel and perpendicular to the nanoplatelets at 300 K, respectively (Figure 18f). Another report demonstrated an even higher coercivity in Co nanosheet-assembled thin films with coercivities of 321.7 Oe in a magnetic field parallel to the film plane and 220.5 Oe in a magnetic field perpendicular to the film plane.342 The magnetic

3.5. Electrical Properties

Metals generally possess excellent electrical conductivity and high electrical stability. However, the conductivity of the metal may suffer a decrease at reduced sizes especially at nanoscale. 6432

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commercial Pd black (Figure 19a).31 The maximum current density of Pd nanosheets for the FOR was 1380 mA mgPd−1 at the peak potential of 0.14 V (vs saturated calomel electrode (SCE)), which was about 2.5 times that of commercial Pd black. The outstanding electrocatalytic activity of Pd nanosheets arises from their greater electrochemically active surface area (67 m2 g−1) as compared to that of the commercial Pd black (47 m2 g−1). In order to further enhance the electrocatalytic activity for the FOR, Pd nanosheets with clean surfaces were prepared since the organic capping agents (e.g., PVP) could block the active sites on the Pd surface and decrease the catalytic performance.190 Impressively, compared to PVP-capped Pd nanosheets, the surface-clean Pd nanosheets demonstrated a peak current density of 1.42 A mgPd−1 at a much more negative peak potential (0.04 V vs SCE). Due to the high cost and natural scarcity of noble metals, alloying them with earth-abundant transition metals becomes an effective strategy to reduce the usage of noble metals and further improve their electrocatalytic activity.50,196 Recently, our group prepared a series of ultrathin PdCu alloy nanosheets with tunable Cu/Pd atomic ratios.50 Importantly, after ligand exchange with ethylenediamine (EN), the obtained PdCu alloy nanosheets can serve as highly efficient electrocatalysts for FOR (Figure 19b). Since EN is an electron donor, the surface of EN-treated PdCu alloy nanosheets becomes electron-rich, which is beneficial for the absorption of electron-deficient reactants like formic acid. Notably, the EN-treated PdCu alloy nanosheets with Cu/Pd atomic ratio of 0.19 showed the highest current density of 1656 mA mgPd−1 at the peak potential of 0.12 V (vs Ag/AgCl) in the FOR. Beside the FOR, the methanol oxidation reaction (MOR), ethanol oxidation reaction (EOR), and glycerol oxidation reaction (GOR) have also received extensive attention owing to their distinct advantages, such as high energy density, high conversion efficiency, and safe for storage and transportation.51,141,216,409,413 To date, many 2D metallic alloy nanostructures have been employed as promising electrocatalysts with excellent catalytic activities and stabilities in these reactions. For example, Huang et al. reported that the PtPb@Pt core−shell nanoplates supported on the commercial carbon exhibited excellent electrocatalytic activity and stability toward MOR and EOR under acidic conditions.53 The asprepared nanoplates showed a mass activity of 1.5 A mgPt−1 for MOR, 2.4 and 7.9 times that of the PtPb nanoparticles supported on commercial carbon and the commercial Pt/C, respectively. For the EOR, the nanoplates also exhibited a specific activity of 2.5 mA cm−2 and mass activity of 1.4 A mgPt−1, 1.9 and 2.5 times that of the PtPb nanoparticles supported on commercial carbon, and 10.4 and 8.8 times that of the commercial Pt/C, respectively. Another work reported by Wang et al. demonstrated that the mass activity of ultrathin PtCu alloy nanosheets for the EOR under acidic condition was 19 and 2.7 times that of commercial Pt black and Pt/C, respectively (Figure 19c).51 Very recently, Zheng et al. reported that the 3D PdCu alloy nanosheets with Pd/Cu molar ratio of 45/55 (referred to as Pd45Cu55) exhibited superior activity and stability toward the EOR under alkaline condition.414 The mass activity of 3D PdCu alloy nanosheets was 4.3 A mg−1, which is 2.8, 3.1, and 7.2 times that of PdCu nanoparticles, Pd nanosheets, and commercial Pd black, respectively. Moreover, 96% of the initial activity of the 3D Pd45Cu55 alloy nanosheets were retained after 2 h of stability test at 0.7 V (vs reversible hydrogen electrode (RHE)),

Surprisingly, the conductivity of many reported 2D metal nanomaterials was found to be the same magnitude as that of their bulk counterparts, making them promising conducting materials for future electronic devices. Recently, Jin et al. investigated the electrical properties of ultrathin and ultralarge Au membranes with atomically smooth surface (Figure 6i).150 The ultrathin Au membrane with ∼5 nm still exhibited a low resistivity of 4.0 × 10−8 Ωm (Figure 18g), which was comparable to that of bulk Au film (2.2 × 10−8 Ωm) and only a few orders of magnitude lower than that of thermally evaporated Au film. It suggested that the ultrathin and ultralarge single-crystalline Au membrane could be potentially applied to next-generation conducting electronic devices. In fact, many Au and Ag 2D nanostructures have been incorporated into flexible substrates or conductive matrix to act as electrodes or conductive adhesives.297,401,402 As a typical example, Jeong et al. reported highly stretchable electrodes made of multilayers of Au nanosheets.401 The electrical resistivity of the electrode film was highly dependent on the number of Au nanosheet transferred and reached a minimal resistivity of 1 × 10−5 Ωm after transferring six layers of nanosheets. Moreover, the patterned electrode displayed excellent electrical stability at 100% strain. Other hybrid conducting structures were also developed, such as the nanocomposite films composed of Au nanoplatelets and amyloid fibrils402 and the conductive adhesives made of Ag flakes and polymer resin.297

4. APPLICATIONS 4.1. Catalysis

Given the intriguing structural and electronic properties, 2D metal nanomaterials, especially those of noble metals, have recently drawn substantially increasing attention in various catalytic applications.33,403 The high surface-to-volume ratio in the 2D metal nanostructures endow them with abundant catalytically active sites exposed on the surface.33 In the Catalysis section, we summarize the recent progress on the utilization of 2D metal nanomaterials in various catalytic applications, such as electrocatalysis and heterogeneous catalysis. 4.1.1. Electrocatalysis. 2D metal nanomaterials are outstanding electrocatalysts in a wide range of applications, including fuel cells, metal−air batteries, water splitting, CO2 reduction, etc., and have demonstrated great potentials in substituting the present commercial electrocatalysts. Direct liquid fuel cells, which are generally powered by the oxidation of small organic molecules, such as acids, alcohols, and ethers, are promising renewable energy devices. Their advantages include lower operating temperature, less pollution, and safer transport, as compared to conventional energy conversion carriers.404,405 Among various liquid cells, the direct formic acid fuel cell represents a potentially promising power source owing to its clean and efficient energy supply.406−408 2D metal nanostructures can be excellent catalysts for the anodic reaction, i.e. formic acid oxidation reaction (FOR). Particularly, 2D noble metals, such as Pt and Pd, have demonstrated superior catalytic properties toward a variety of electrocatalytic reactions.273,409−412 Pd-based 2D nanostructures are potential candidates as electrocatalysts for FOR due to their outstanding antipoisoning capacity.301,411 For example, Zheng et al. demonstrated that the Pd nanosheets exhibited superior electrocatalytic activity for the FOR as compared to the 6433

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Electrocatalytic water splitting has been considered as a promising strategy to generate a renewable and clean hydrogen source.428,429 Among various metals, to date, Pt is considered the most promising catalyst for hydrogen evolution reaction (HER) owing to its near-zero Gibbs free energy of adsorbed hydrogen.415 However, the high cost and low reserve of Pt severely hinder its widespread application in HER. In order to gain a sustainable hydrogen production, alloying Pt with cheap yet catalytically active metal materials becomes a cost-effective strategy.305,429 For example, Wang et al. reported that the trimetallic PtAgCo ultrathin nanosheets exhibited superior electrocatalytic activity and stability toward HER as compared to the commercial Pt/C catalyst.305 Particularly, by tuning the composition of PtAgCo ultrathin nanosheets, a current density up to 705 mA cm−2 at a potential of −400 mV in 0.5 M H2SO4 aqueous solution was achieved (Figure 19e). As known, the electrocatalytic water splitting process consists of HER at the cathode and oxygen evolution reaction (OER) at the anode. However, the sluggish kinetics of the multistep proton-coupled electron transfer in OER has been constraining the overall reaction rate of water splitting.430 Therefore, it is desired to develop more efficient OER electrocatalysts to improve the overall efficiency of water splitting. Currently, Ir and Ru are considered to be the state-ofthe-art OER electrocatalysts with relatively low overpotential, small Tafel slope, and high stability.301 Recently, Huang et al. prepared 3D Ir superstructures composed of ultrathin Ir nanosheets.301 The obtained superstructure has several structural advantages, such as 3D accessible active sites, large surface area, and proper layer distance. As a result, the Ir superstructure exhibited superior electrocatalytic performance toward the OER in both alkaline and acidic conditions as compared to the Ir nanoparticles. Specifically, the 3D Ir superstructure showed a low onset potential of 1.43 V (vs RHE) and very low Tafel slope of 32.7 mV decade−1 in alkaline condition, and an onset potential of 1.45 V (vs RHE) and small Tafel slope of 40.8 mV decade−1 in acidic condition. With the increasing concerns in greenhouse gas (CO2) emission and the fast consumption of fossil fuels, electrocatalytic reduction of CO2 into value-added products provides an effective strategy to solve the environment and energy issues at the same time.431,432 In the electrocatalytic reduction of CO2, water usually serves as the source of proton and CO2 is reduced to carbon compounds, such as CO, formate, methanol, formaldehyde, methane, and ethylene at the cathode, while the OER occurs at the anode.432 Currently, the development of efficient CO2 reduction catalysts focuses on increasing the Faradaic efficiency and selectivity toward specific carbon compounds, as they can be readily used as industrial fuels and chemicals.415,432 Recently, Xie et al. prepared the freestanding Co nanosheets with thickness of 4 atomic layers using a ligand-confined growth strategy.295 They found that partially oxidized Co nanosheets exhibited excellent activity and selectivity for the CO2 reduction reaction, producing formate in aqueous solution with a low overpotential (Figure 19f). Impressively, the catalytic performance of partially oxidized Co nanosheets is better than that of the pure Co nanosheets, the partially oxidized bulk Co, and the bulk Co materials. It was speculated that the oxidation of Co could facilitate the rate-determining chemical reaction, resulting in lowering of the onset potential from 0.73 to 0.68 V upon partial oxidation. This work demonstrated the importance of manipulating the surface oxidation state of

indicating their excellent catalytic stability. In addition, Hou et al. reported that the ordered intermetallic PtBi nanoplatelet exhibited excellent catalytic activity and stability for the MOR in acidic electrolyte, giving a final current density 6.4 times that of the commercial Pt/C after 4000 s in the chronoamperometric measurement.196 Importantly, it was found that wellstructured trimetallic nanosheets could further boost the catalytic activities due to the synergistic effect between the constituent metals. For example, Han et al. reported that the ultrathin free-standing PdPtAg trimetallic alloy nanosheets showed enhanced electrocatalytic activity and CO tolerance compared to the PdPt bimetallic alloy nanosheets, PdPt bimetallic nanodendrites, PdPtAg trimetallic nanodendrites, commercial Pd/C, and Pt/C catalysts in basic electrolyte.191 The incorporation of Ag was believed to contribute to the higher catalytic activity of the PdPtAg nanosheets compared to the PdPt nanosheets by enhancing the OHads formation and CO tolerance. Developing highly efficient oxygen reduction reaction (ORR) catalysts is the key to commercialize future generation of fuel cells and metal−air batteries.415−417 However, the sluggish reaction kinetics of ORR and the poor durability and high cost of state-of-art catalysts severely hinder the large-scale commercialization of fuel cells and metal−air batteries.416,417 To date, Pt-based catalysts have been widely used in the ORR and various strategies have been developed to improve their activity, including alloying Pt with transition metals,405,418−420 forming Pt-skin or skeleton surface,97,421,422 engineering lattice strain,53,423−425 and so on. For example, Huang et al. reported that biaxially strained PtPb@Pt core−shell nanoplates supported on the commercial carbon exhibited exceptional ORR performance in O2-saturated 0.1 M HClO4 aqueous solution (Figure 19d).53 Specifically, the specific activity of PtPb@Pt core−shell nanoplates supported on the commercial carbon could reach up to 7.8 mA cm−2 at the potential of 0.9 V (vs RHE), which is 4.1 and 33.9 times that of the PtPb nanoparticles supported on the commercial carbon and commercial Pt/C, respectively. Significantly, the mass activity of PtPb@Pt core−shell supported on the commercial carbon was 4.3 A mgPt−1 at the potential of 0.9 V (vs RHE), which is 9.8 times that of the 2020 U.S. Department of Energy target.426 The DFT calculations revealed that the stable Pt(110) facets exposed on the surface of PtPb@Pt core−shell nanoplates had large tensile strains, which were beneficial for optimizing the Pt−O bond strength. Moreover, the durability of PtPb@Pt core−shell nanoplates supported on the commercial carbon could be greatly enhanced originating from the intermetallic PtPb core and uniform atomic layered Pt shell. After 50,000 sweeping cycles, the PtPb@Pt core−shell nanoplates supported on the commercial carbon showed negligible mass activity decay of 7.7% as compared to the PtPb nanoparticles supported on the commercial carbon (37.0%) and commercial Pt/C (66.7%). Another work reported by Ding et al. revealed that coating ultrathin Pd nanosheets with single-layered Pt could produce a specific activity of 0.438 A cmPt−2 and mass activity of 0.717 A mgPt−1 at the potential of 0.9 V (vs RHE) in O2-saturated 0.1 M HClO4 aqueous solution, which was 3.4 and 6.6 times that of the commercial Pt/C, respectively.427 Moreover, it was found that the ultrathin nanosheets were more durable than the commercial Pt/C. After the accelerated durability test for 5,000 cycles, the nanosheets showed a smaller negative shift for half-wave potential (18 mV) compared to that of the commercial Pt/C (59 mV). 6434

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Figure 20. (a,b) Comparisons of catalytic ability of PVP-capped Rh nanosheets, PVP-capped Rh nanoparticles, and commercial Rh/C in (a) hydrogenation of phenol and (b) hydroformylation of 1-octene. Reproduced with permission from ref 52. Copyright 2014 Nature Publishing Group. (c) The plot of conversion of the hydrogenation of nitrobenzene vs time using Pd nanosheets and C2H2 treated Pd nanosheets as catalysts. Reproduced with permission from ref 443. Copyright 2014 American Chemical Society. (d) The plot of conversion of 1-octyne vs reaction time using Pd@Ru nanosheets and Pd nanosheets as catalysts in the semihydrogenation of 1-octyne. Reproduced with permission from ref 262. Copyright 2016 John Wiley & Sons, Inc. (e) The conversion of benzyl alcohol (left axis, filled symbols) and selectivity for the formation of benzaldehyde (right axis, open symbols) as a function of time, plotted from the oxidation of benzyl alcohol by AuPd nanowheels. (f) The conversion of iodobenzene as a function of time during the Suzuki coupling reaction under light irradiation and conventional heating conditions. Reproduced with permission from ref 121. Copyright 2013 John Wiley & Sons, Inc.

involves the adsorption of reactants on the surface of catalysts, surface reaction of adsorbed species, and desorption of products from the surface of catalysts.435 It has been reported that the catalytic activity and selectivity of metal nanomaterials toward specific reactions are dependent on their size, morphology, composition, surface structure, and crystal phase.437−439 Importantly, the high surface area-to-volume ratio and high-density unsaturated atoms exposed on the surface have made the ultrathin 2D metal nanomaterials promising catalyst candidates in various heterogeneous reactions. Hydrogenation reactions represent crucial industrial processes, producing key feedstock for fine-chemical synthesis.440,441 2D metal nanomaterials are promising catalysts in a variety of hydrogenation reactions, such as styrene,442

metal catalysts. Very recently, Zheng et al. prepared ultrathin hybrid Cu/Ni(OH)2 nanosheets with ultrastable atomic Cuenriched surface. Importantly, the formate on the surface of nanosheets could protect metallic Cu against oxidation, making them promising in the electrochemical conversion of CO2 and water into syngas. At a low overpotential of 0.39 V, these nanosheets show a current density of 4.3 mA/cm2 with a CO Faradaic efficiency of 92%.433 This study indicates the importance of surface coordination chemistry in determining the chemical properties of metal nanomaterials. 4.1.2. Heterogeneous Catalysis. Heterogeneous catalytic reactions represent a common group of chemical reactions in chemical, pharmaceutical, petroleum, energy, and automobile industries, in which solid phase nanocatalysts are desired.102,434−436 Typically, a heterogeneous catalytic reaction 6435

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Figure 21. (a) Raman spectra of 4-mercaptopyridine molecules adsorbed on Pd triangular nanoplates, hexagonal nanoplates, and 8 nm cuboctahedra. Reproduced with permission from ref 57. Copyright 2005 American Chemical Society. (b) Comparison of SERS spectra recorded from the aqueous solution of 1,4-benzenedithiol-adsorbed Ag nanoplates with straight edges, curved, and wavy edges, respectively. Reproduced with permission from ref 291. Copyright 2014 John Wiley & Sons, Inc. (c) LSPR spectra of an Au nanoplates-coated glass substrate before (black) and after (red) functionalization with human anti-IgG, and after exposure to human IgG (green). Reproduced with permission from ref 256. Copyright 2010 American Chemical Society. (d) Extinction spectra for the monolayer of Ag nanoprisms. The maximum wavelength shifted from 630 to 550 nm upon drying. Inset: The corresponding observance of color change. Reproduced with permission from ref 398. Copyright 2005 John Wiley & Sons, Inc.

nitrobenzene,429 cyclohexene,292 phenol,292 anthracene,54 and acetylene303 hydrogenation reactions. For example, Li et al. showed that single-layered Rh nanosheets exhibited remarkably excellent catalytic activity in the hydrogenation of phenol as compared to the Rh nanoparticles and the commercial Rh/ C (Figure 20a).52 Specifically, the PVP-capped Rh nanosheets achieved >99.9% conversion of phenol within 4 h at 30 °C under low H2 pressure (1.0 MPa). The conversion of phenol using Rh nanosheets as the catalyst was 7 and 4 times those of PVP-capped Rh nanoparticles and commercial Rh/C, respectively. Besides, the Rh nanosheets also exhibited enhanced catalytic activity and selectivity in the hydroformylation of 1-octene as compared to the Rh nanoparticles and the commercial Rh/C under mild reaction conditions (Figure 20b). The superior catalytic performance of Rh nanosheets in both organic reactions arose from the complete exposure of Rh atoms in the single-layer nanosheets. In addition, controlling the surface wettability of noble metal nanosheets via regulating the interaction between the reactants and metal surfaces is an effective strategy to improve their catalytic performance. For example, Zheng et al. treated Pd nanosheets with acetylene (C2H2) to induce the formation of polyacetylene on the Pd nanosheet surfaces, which then turned hydrophobic.443 As a result, in the hydrogenation reaction of styrene, the C2H2-treated Pd nanosheets could achieve 100% conversion at ∼56 min, while the untreated Pd nanosheets

required 500 min to completely hydrogenate styrene into ethylbezene under the same conditions (Figure 20c). The structural regulation of noble metal nanostructures at atomic scale can significantly change their catalytic performance.444,445 Very recently, our group demonstrated that decorating the ultrathin Pd nanosheets with submonolayered Ru (referred to as Pd@Ru nanosheets) showed enhanced catalytic activity toward the hydrogenation of 1-octyne compared to the pure ultrathin Pd nanosheets (Figure 20d).262 To be specific, 100% conversion of 1-octyne was obtained after 40 min of reaction at room temperature under 1 atm H2 pressure catalyzed by Pd@ Ru nanosheets. In comparison, under the same reaction conditions, only around 60% conversion could be obtained with Pd nanosheets as the catalyst. Moreover, almost similar selectivity toward 1-octene was obtained over Pd@Ru nanosheets and Pd nanosheets at the same conversion of 1octyne. This result indicated that the Pd@Ru nanosheets displayed improved catalytic activity without compromising the selectivity compared to the pure Pd nanosheets. Importantly, the Pd@Ru nanosheets also showed superior catalytic activity in the reduction of 4-nitrophenol as compared to the pure ultrathin Pd nanosheets. The excellent catalytic properties of Pd@Ru nanosheets toward the selective hydrogenation of 1-octyne and the reduction of 4-nitrophenol may be attributed to their ultrathin 2D feature, modification of the 6436

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and curved edges, respectively (Figure 21b). Yan et al. also found that stronger SERS signals were generated from the Ru capped columns and Ru triangle nanoplates as compared to the Ru nanoparticles.58 Meanwhile, as the sharp corners and edges in 2D metal nanomaterials could be unstable, 2D metal nanomaterials based heterostructures or alloys have been developed to increase their stability as SERS substrates.56,257,452,461 For example, Zheng et al. demonstrated the excellent photothermal stability of hexagonal Pd@Ag core−shell nanoplates by monitoring the Raman signals of 4pyridinethiol on Pd@Ag nanoplates and Ag nanoprisms with continuous NIR irradiation.56 The SERS signals on Pd@Ag were able to maintain more than 90% of their original intensity after 40 min of laser irradiation, while the signal on Ag nanoprisms already decreased by ∼60%, indicating the advantage of the core−shell nanoplates over the pure-Ag nanostructure as an NIR-SERS substrate. In another work, Jin et al. reported the good reproducibility and stability of the GOsupported Ag nanoplates as the SERS substrate.462 Besides the sharp edges and corners of isolated 2D metal nanostructures, the Raman hot spots can also be generated from properly arranged 2D metal nanostructures with tunable gaps between adjacent metal nanostructures, such as assemblies of 2D metal nanomaterials and 2D metal nanostructure arrays.366,390,463 Recently, Hu et al. examined theoretically and experimentally the effect of gap distance of Ag nanoplate dimer on SERS activity.390 The simulated local electromagnetic field distribution suggested that a dramatic resonance enhancement resulted from the strong plasmon coupling effect could be observed when the gap between the two Ag nanoplates decreased to as small as 2 nm. Indeed, the observed SERS signal of adenine molecules adsorbed on a film substrate made of Ag nanoplates was measured to be several orders of magnitude stronger than that of more isolated Ag nanoparticles. Another more ideal model to study the effect of internanostructure distance on SERS properties is to build up a series of Au nanodisk arrays with different diameters using lithography methods.366 Indeed, the Au nanodisk arrays with smaller diameter exhibited a stronger SERS signal of 4mecaptopyridine due to the stronger electromagnetic coupling effect between the Au nanodisks. Moreover, by modulating the orientation mode of Ag nanoprism assemblies, Han et al. obtained different SERS properties with opposite excitation wavelength dependency.463 They believe that the different adsorption characteristics of 4-aminobenzenethiol on different exposed crystal planes are responsible for the selective SERS enhancement.

electronic structure of Pd, formation of crystal defects, and synergistic effects between Pd and Ru components. In some oxidation reactions, such as the oxidation reactions of benzyl alcohol,121 ethylbenzene,131 toluene,84 and CO,300 2D metal nanomaterials can also serve as highly efficient catalysts. For example, the 2D Au nanosheets synthesized by Xiao et al. on an Al−Mg LDH template were negatively charged with a thickness of a single to a few atomic layers.131 The Au/LDH hybrid exhibited superior catalytic activity in the solvent-free aerobic oxidation of ethylbenzene and toluene compared to Au nanoparticles supported on conventional substrates. Impressively, the morphology of Au nanosheets could be well maintained after the catalytic reaction due to the confined effect of Al−Mg LDH, showing excellent catalytic stability. Interestingly, LSPR properties arising from controllable lateral size of 2D noble metal nanosheets, such as Au,446 Ag,291 Pd,31 etc., can also be utilized for light-triggered catalytic reactions. In this regard, light irradiation can be applied to replace conventional heating in the liquid-phase catalytic reactions. For example, Huang et al. demonstrated that the Au−Pd nanowheel exhibited enhanced catalytic activity in the Suzuki coupling reactions and the oxidation of benzyl alcohol under light irradiation as compared to that measured with conventional heating conditions (Figure 20e,f).121 In addition to the aforementioned works, many other 2D metal nanomaterials are also found promising in heterogeneous reactions, such as the reduction reactions of 4nitrophenol,119,133,379,443 nitrobenzene,447 2-nitroaniline,448 and methylene blue,156 the Heck coupling reactions,200 the selective dehydrogenation of formic acid,449 and so on. 4.2. Surface Enhanced Raman Scattering (SERS)

SERS is a highly sensitive molecule-detecting technique that captures the signal enhancement of Raman scattering originated from the molecule absorbed by the SERS substrate.219,390,450−452 As a powerful detection tool with single-molecule accuracy, SERS has been found useful in many fields, such as explosives detection, cancer detection, food quality analysis, and so on.453−456 Noble metal nanomaterials, especially Ag and Au, have been extensively applied as the most effective SERS substrates.384,451,452 Importantly, the spectral ranges and enhancement factors in SERS have been proven to strongly depend on the morphology of the metal substrates.457−459 2D metal nanostructures with well-defined morphologies, such as nanoprisms, nanoplates, and nanosheets, could serve as excellent SERS substrates with high activities.55,58,291 The presence of sharp corners and edges generates greatly enhanced local electric fields and can amplify the Raman scattering signals as Raman hot spots.55,219,460 As a typical example, Xia et al. reported the shape dependency of SERS substrate in the detection of 4-mercaptopyridine.57 They found that the SERS signal intensity on triangular and hexagonal Pd nanoplate substrates was around 4 and 3 times that of Pd cuboctahedra, respectively (Figure 21a). A recent study reported by Qin et al. observed that the increment of sharp features in Ag nanoplates could greatly enhance the SERS signal in detecting 1,4-benzenedithiol molecules.291 The rough edges and sharp corners in Ag nanoplates with wavy edges could generate much stronger SERS signals of 1,4benzenedithiol compared to those of straight and curved edges, respectively. Specifically, the peak at 1564 cm−1 for Ag nanoplates with wavy edges was measured to be 6.5 and 13 times stronger than that of the nanoplates with straight edges

4.3. Sensing

Benefiting from the unique LSPR properties of 2D metal nanomaterials, LSPR-based sensors have been developed by combining LSPR principles with complementary molecular identification techniques.464 As the LSPR oscillation is strongly dependent on the dielectric constant of the surrounding environment of the metal nanostructures, LSPR-based sensors can be used to detect many chemical and biological species by monitoring the LSPR peak changes when the analyte molecules bind to the surface of 2D metal nanostructures and alter the dielectric constant of their surroundings.465,466 Au- and Ag-based 2D metal nanomaterials have been most intensively investigated in their LSPR-based sensing abilities of a wide range of biomolecules, such as glucose,476 human antiIgG,256 prostate-specific antigen,370 streptavidin,59,467 amino6437

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Figure 22. (a−c) Photothermal effect of the Au nanoplates on cancer cells. (a) Hela cancer cells with green fluorescent protein were covered by a layer of hydrogel and Au nanoplates (λLSPR = 808 nm). Inset: Schematic illustration of the irradiation experiment. (b) Hela cancer cells after irradiation with an 808 nm laser beam for 4 min. (c) Hela cells covered by the hydrogel and Au nanoplates (λLSPR = 969 nm) and irradiated under the same condition for 4 min. Reproduced with permission from ref 396. Copyright 2015 The Royal Society of Chemistry. (d) The plot of temperature vs time recorded at different concentrations of Pd nanosheets under laser irradiation. (e,f) Micrographs of human hepatoma cells after (e) 2 min and (f) 5 min of irradiation by a 1 W laser. Dead cells are stained in blue. Reproduced with permission from ref 31. Copyright 2011 Nature Publishing Group. (g) Infrared thermal images of tumor-bearing mice with and without the injection of PEG-modified Pd@Au nanoplates under an 808 nm laser. (h) Photograph of a mouse with PEG-modified Pd@Au nanoplates injection. (i) Photograph of a mouse with PEGmodified Pd@Au nanoplates injection and photothermal cancer therapy at 24 h postinjection. The black tumor is an evidence for the effective accumulation of Pd@Au nanoplates in tumor sites. Reproduced with permission from ref 474. Copyright 2014 John Wiley & Sons, Inc.

thiol,142 bovine serum albumin,399 alkanethiols,61 C-reactive protein,63 and antibiontin.467 As a typical example, Zamborini et al. demonstrated that the LSPR peaks of Au nanoplates redshifted by 22−68 nm after the edge functionalization with human anti-IgG (Figure 21c).256 The red shift of the peak was up to 8 times greater than the human anti-IgG functionalized Au nanospheres. Importantly, besides biomolecules, Ag nanoplates were also highly sensitive toward a series of inorganic ions, such as halide, phosphate, and thiocyanate ions.468 These ions were able to interact with the metal surface and modulate the surface electron charging, which thus affected the LSPR band shift. Moreover, core−shell nanostructures have been proposed to enhance the stability of the nanoplates under various chemical modifications that are required for the LSPR sensing.399 For example, Yin et al. showed the high specificity of the highly stable Ag@Au nanoplates in the LSPR sensing of streptavidin.59 Moreover, the recognition of LSPR sensitivity on the change of local medium refractive index can also be utilized to build ultrasensitive sensors.256,398,465 For instance, Mirkin et al. reported the change of color appearance of the monolayer 2D Ag nanoprism assembly on a glass substrate when the substrate was changed from wet state to dry state (Figure 21d).398 Notably, this color change of the Ag-nanoprism-coated

substrate is a reversible process and highly sensitive to slight change in humidity, like a breath on the substrate. While the aforementioned sensing techniques are based on the change of local dielectric constant of the 2D metal nanostructures, some chemical species can affect the LSPR spectra by inducing a shape change of the 2D metal nanostructures.469,470 For instance, Chen et al. reported the Hg2+ sensing capability of Ag nanoprisms based on the Hg2+-induced morphology transformation of Ag nanoprisms from triangular to oblate spheroidal shapes.469 The addition of Hg2+ caused the Ag nanoprisms to gradually change into round nanodisks, and the detection limit of Hg2+ could be as low as 3.3 nM. Similarly, Ag nanoplates were also found capable of detecting poly(styrene4-sulfonate) molecules via an irreversible shape degradation of the Ag nanoplates.470 In addition to LSPR-based sensors, the ultralarge surface-tovolume ratio and catalytic activity of 2D metal nanomaterials could also be utilized for other sensing applications.62,471 For instance, Sun et al. reported the H2O2 sensor by taking advantage of the catalytic activity of Au nanoplates toward both the oxidation and reduction of H2O2.471 This sensor was constructed on the surface of a glassy carbon electrode, on which a dispersion of Au nanoplate was dried to form an Au nanoplate film. The as-constructed H2O2 sensor exhibited a 6438

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Figure 23. (a) TEM image of Ag nanoplates with average edge length of ∼218 nm used in the photoacoustic imaging. (b) Dark-field microscopy of pancreatic cancer cells after incubation with EGFR-conjugated Ag nanoplates. (c) 2D cross-sectional photoacoustic image of an orthotopic pancreatic tumor in a mouse, showing the accumulation of conjugated Ag nanoplates in yellow, oxygenated blood in red, and deoxygenated blood in blue. Reproduced with permission from ref 64. Copyright 2012 American Chemical Society. (d−h) PA images of PEG-modified Pd@Au nanoplates in tumor and (i) intensity of PA signals of tumor at different times after injection. Reproduced with permission from ref 474. Copyright 2014 John Wiley & Sons, Inc.

modification of Au nanoprisms toward biocompatible photothermal functions.158 By tuning the nanoprism size, the LSPR band was set along the NIR region. By functionalizing the nanoprisms with heterobifunctional PEG and 4-aminophenyl β-D-glucopyranoside chains, the stability and biocompatibility of Au nanorprisms were much enhanced. As a result, the tailored Au nanoprisms showed remarkable performance as photothermal agents in the cell cultures. Pd nanosheets with the NIR absorption also possess excellent photothermal properties.31,67,68,472,473 For example, Zheng et al. examined the photothermal effects of ultrathin Pd nanosheets by irradiating aqueous solutions of Pd nanosheets with different concentrations using a NIR laser.31 At a concentration of 27 ppm and irradiation time of 10 min, the solution temperature increased by more than 20 °C from 28 °C. In comparison, the temperature increment in the absence of Pd nanosheets was only 0.5 °C (Figure 22d). Meanwhile, the Pd nanosheets demonstrated improved photothermal stability compared to Au and Ag nanoprisms, by well maintaining the sheetlike morphology after 30 min irradiation with 2 W, 808 nm laser. The excellent biocompatibility and photothermal cell-killing efficacy of Pd nanosheets were also confirmed. Remarkably, the polyethylenimine-exchanged Pd nanosheets were able to kill ∼100% of the liver cancer cells within 5 min of 808 nm laser irradiation (1.4 W cm−2) (Figure 22d,f). In a following study, they further demonstrated the excellent photothermal effect and biocompatibility of corollalike Pd nanosheets.68 Moreover, surface functionalization of Pd nanosheets using a photosensitizer, i.e. chlorin e6 (Ce6), could further enhance their photothermal conversion efficiency and

fast response time of less than 3 s and a detection limit of as low as 0.1 mM. In another work by Yang et al., an effective H2 sensor was successfully realized by modulating the stacking configuration of flexible Pd nanosheets.62 Both theoretical and experimental results suggested that Pd nanosheets with a higher order of nanostructures could promote the H2 detection sensitivity and reduce the response time. 4.4. Photothermal Therapy

Photothermal therapy uses electromagnetic radiation to treat various medical conditions such as cancer. In a typical photothermal tumor therapy, a photosensitizer is excited upon irradiation to generate local hyperthermia, which is utilized to destroy targeted cancer cells. As the IR light could achieve excellent tissue penetration, metal nanostructures with strong LSPR absorption in the NIR region are particularly useful for photothermal therapies.67,288 2D metal nanomaterials have been demonstrated to be excellent candidates for photothermal therapy due to their high photothermal stability, outstanding photothermal conversion efficiencies, and excellent biocompatibilities.31,67,158,396,472 The shape, size, and surface ligand of 2D metal nanomaterials are important parameters for improving their photothermal properties. For example, Zhou et al. reported the tuning of NIR plasmonic modes of Au nanoplates for enhanced photothermal therapy by precisely tuning the shape of Au nanoplates with O3.396 By tuning the plasmon mode into one resonant with the excitation source (λ = 808 nm), the Au nanoplates displayed an improved performance in the photothermal treatment on Hela cancer cells compared to those without plasmon mode tuning (Figure 22a−c). Fuente et al. also studied the synthesis and 6439

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Figure 24. (a) Schematic illustration of the dual interfacial layer strategy and device configuration incorporating Ag nanoprisms (λext = 535 nm) into PEDOT:PSS hole transporting layer and Ag nanoprisms (λext = 450 nm) into C60-bis layer, respectively. Insets: High-magnification TEM images of Ag nanoprisms obtained with λext = 535 nm (front) and λext = 450 nm (rear), respectively. (b) Current−voltage characteristics of the fabricated devices with and without Ag nanoprisms. Reproduced with permission from ref 70. Copyright 2014 John Wiley & Sons, Inc. (c) TEM and STEM images of the synthesized triangular nanostructures at stage A (tip-gold-coated Ag nanoprism), stage B (edge-gold-coated Ag nanoprism), stage C (etched Ag nanoprism), and stage D (triangular Au nanoframe). (d) Schematic illustration of the self-assembled triangular Ag@Au nanostructures on a glass slide covered by the bulk heterojunction blend. (e) Representative photoinduced absorption spectra of the OPV films with and without edge-gold-coated silver nanoprisms (GSNP). Reproduced with permission from ref 377. Copyright 2014 American Chemical Society.

biocompatibility.472 Then in vivo photothermal treatment on tumor-bearing mice confirmed the double phototherapy effect contributed by the Pd nanosheets-mediated photothermal elimination and the photodynamic destruction effect of Ce6. Moreover, the size effect of Pd nanosheets on their photothermal efficiency was also investigated.67 The photothermal conversion efficiency of Pd nanosheets with sub-10 nm edge lengths at 808 nm irradiation was measured to be as high as 52.0%, much greater than that of large Pd nanosheets with edge length of 41 nm. Constructing core−shell heterostructures could further improve the photothermal efficiency and stability of 2D metal nanomaterials.56,473−475 One effective approach is to composite metal nanosheets with silica.475,476 For example, Zheng et al. reported that silica coating could greatly enhance the photothermal efficacy of Pd nanosheets by increasing their uptake by cancer cells. Interestingly, by attaching Pd nanosheets on mesoporous silica nanospheres, the therapeutic efficiency was also greatly enhanced due to the dramatically increased amount of cellular internalization by ∼11 times compared to the unloaded Pd nanosheets.473 In addition, metal coating on Pd nanosheets was found to have enhanced photothermal stability compared to the pure shell metal nanostructures.56,474 Impressively, by using PEG-modified Pd@Au nanoplates as the NIR photothermal agent, efficient photothermal ablation of tumors in vivo was realized under the irradiation of low-power-density NIR laser (808 nm, 0.5 W cm −2 ) (Figure 22g−i).56

cells of interest.477 2D metal nanostructures with tunable LSPR properties could be alternative contrast agents in different bioimaging techniques due to their potentially strong light absorbing or scattering capabilities.477 A typical example is the usage of metal nanoplates in photoacoustic (PA) imaging.64,474,478 Emelianov et al. introduced Ag nanoplates as a novel PA contrast agent for in vivo molecular PA image (Figure 23a−c).64 After modifying the surface of Ag nanoplates with epidermal growth factor receptor (EGFR), a commonly used antibody to induce directional conjugation of the nanoplates, the EGFR-conjugated Ag nanoplates could accumulate selectively in pancreatic cancer cells in vitro (Figure 23b). The conjugated Ag nanoplates were then injected into the mouse vain for in vivo imaging using a combined ultrasound and PA imaging system (Figure 23c). This work suggested the ability of Ag nanoplates to provide enhanced contrast for the tumor region that can be distinguished easily from other surrounding tissues such as blood. Such ability was also observed in Au nanoplates by Emelianov et al., who modified the Au nanoplates with PEG-SH molecules followed by silica coating and used them for the PA imaging of sentinel lymph node.478 Recently, Zheng et al. reported the significant PA contrast effect of core−shell Pd@Au nanoplates.474 During the in vivo PA imaging, the PA signals of tumor sites were found more than 4 times enhanced at 24 h after the injection of PEG modified Pd@Au nanoplates, whereas only major blood vessels could be seen when no injection was made (Figure 23d−i). In addition to PA imaging, 2D metal nanomaterials have also been utilized in some other bioimaging techniques, such as Au nanoplates for optical coherence imaging,396 Au nanoprisms for multispectral optoacoustic tomography,66 Au nanoplates for two-photon-induced luminescence,65 and Pd@Au nanoplates for computed tomography imaging.474

4.5. Bioimaging

Notably, 2D metal nanostructures can act as contrast agents in biological imaging. Contrast agents can improve the imaging sensitivity and diagnostic ability by labeling specific tissues or 6440

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4.6. Solar Cells

the different mechanisms involved in the formation of 2D morphology, we categorized the synthetic methods into bottom-up and top-down approaches and explained several methodologies under each category in detail. Although the investigation on the physical and chemical properties of 2D metal nanomaterials is still primitive, we sorted out the most frequently studied properties including stability, crystal structure, plasmonic properties, magnetic properties, and electrical properties. More explorations of the distinct properties of 2D metal nanomaterials, arising from the 2D features, have been well supplemented by evaluating their performances in a wide range of applications. The outstanding performance of 2D metal nanomaterials has proven themselves to be promising candidates for next-generation functional nanomaterials. In spite of the great progress made in this intriguing field, many challenges lie ahead in the systematic and comprehensive study on 2D metal nanomaterials. First, theoretical studies on the formation mechanism of many 2D metal nanostructures and their structure−property relationships are heavily lacking and need to be substantially strengthened, although huge experimental efforts have been devoted to the controlled synthesis of different 2D morphologies and the explanation of the formation mechanism. Second, most of the previous reports only provided the simple correlation between one or two synthetic parameters and the final morphology by examining the intermediate products formed at certain reaction time intervals or based on the specific control experiments. Therefore, the in situ study of the shape evolution toward 2D is urgently desired especially for the bottom-up methods. Third, from the application point of view, few of the as-obtained 2D nanomaterials were readily suitable for large-scale and highyield syntheses toward practical applications. Instead, many of the applications with excellent performances were merely proof-of-concept and carried out in laboratories. Few reports have taken into account of cost, processability, and scale-up possibility of the 2D products. Moreover, some of the 2D metal nanomaterials, especially the ultrathin ones, might lack long-term stability and durability, restricting their roles in real applications. Therefore, there is still a long way to go before implementing the 2D metal nanomaterials for practical applications, especially when the working condition is harsh. Despite the aforementioned challenges, there are also many opportunities in the study of 2D metal nanomaterials. Here, we would like to highlight some important directions that are worth pursuing. Although there have been many reports on 2D metal nanomaterials so far, in the periodic table of elements, many metal elements are yet to be obtained in 2D morphology, not to mention the huge number of possible combinations of different metal elements which can result in the formation of metal alloys and multimetallic heterostructures. One strategy to access the 2D shape of more metal elements is probably via the epitaxial overgrowth of secondary metals onto faceted 2D metal nanocrystal seeds (an extended method from 2D-template confined growth).308,393 In this way, the 2D structure of the shell metal could be well constrained due to the strong intermetallic bonds. If the shell metal is thin enough, it could adopt the crystal structure of the core metal even if the adopted crystal structure is not the same as the shell metal’s bulk form. Significantly, with proper match of lattice parameters, novel 2D metal nanostructures could be epitaxially grown on nonmetal templates as well.208 Moreover, these 2D metal nanomaterial-based hybrids may also possess other

The excellent plasmonic and electrical properties of 2D metal nanomaterials can also be exploited in solar cell devices. The incorporation of 2D metal nanomaterials could enhance the performances of photovoltaic devices in several aspects. For instance, the excellent conductivity of metal nanosheets could be used to improve the charge transport efficiency in perovskite solar cells, thus facilitating the increment of overall conversion efficiency.225 In the case of organic photovoltaics (OPV), plasmonic metal nanostructures could be embedded into the interfacial layer between the electrode and active layer to enhance the photon harvesting.69 In 2010, Ginger et al. studied the ability of Ag nanoprisms to enhance absorption and charge generation in OPV films and found that the plasmon-resonant Ag nanoprisms could greatly enhance the generation of long-lived charge carriers. In a following study, they investigated the effect of the aspect ratio of Ag nanodisks in their plasmonic absorption enhancement in the buffer layer of OPV.479 They identified that the dipole-like surface plasmon mode played a main role in enhancing the absorption of the active layer, which was tunable by modulating the aspect ratio of Ag nanodisks. The theoretical calculation also predicted that with optimized nanodisk size, the total absorption in the active layer could be 16% higher than that without the nanodisks. Based on these findings, Ginger et al. reported the incorporation of Ag nanoprisms into OPV devices via a dual interfacial layer strategy, which resulted in a dramatic increment of the power conversion efficiency of bulk heterojunction OPV from 7.7% to as high as 9.0% (around 17% improvement) (Figure 24a,b).70 It is worth mentioning that superior to nanospheres, the Ag nanoprisms with tunable edge length possessed tailorable LSPR bands, which allows the optimization of device performance by choosing the most device-compatible absorption range.70 Moreover, by coating Au at the edge of Ag nanoprisms, Xue et al. demonstrated the impressive enhancement of charge carrier generation by ∼7 times, which was believed to be attributed to the improved shape asperity of the as-prepared edge-gold-coated Ag nanoprisms (Figure 24c−e).377

5. CONCLUSION AND PERSPECTIVE Benefiting from the advancement in various characterization tools and the growing demands for future technologies, the development of 2D metal nanomaterials dramatically expanded in the past decades. In this Review, we have summarized the research progress made to date on 2D metal nanomaterials from the synthetic strategies, to the property studies, and application explorations. Compared to the 0D and 1D metal nanostructures, 2D metal nanomaterials are the most difficult to obtain due to the intrinsically isotropic crystal nature of metals and high surface energy in 2D morphology. Yet different synthetic methods have been developed to prepare relatively stable 2D metal nanostructures with well controlled parameters including shape, size, composition, and crystal structure. Taking the shape and size of 2D metal nanostructures for typical examples, the 2D shapes can be in the form of disk, triangle, square, hexagon, ring, cone, belt, ribbon, and so on, while their thicknesses can vary from single atom to hundreds of nanometers, and their lateral sizes range from tens of nanometers to hundreds of micrometers. Meanwhile, various 2D metal nanomaterial-based heterostructures, alloys, and assemblies have been successfully prepared. To understand 6441

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AUTHOR INFORMATION

advantages compared to bare 2D metal nanostructures, such as increased stability, novel physicochemical properties, and promising applications arising from the synergistic effect. Another promising direction is to explore the thickness limit of various 2D metal nanomaterials, i.e. to obtain metal nanosheets with single atom thickness, which can be called as “Metallene”.52 The unique coordination environment of metal atoms in a single atomic layer could result in significant change in the electronic structure, and therefore fascinating new properties are highly expected. The difficulty of obtaining “metallene” is, however, obvious, i.e. to simultaneously overcome the strong interaction of metallic bonds to form aggregation and minimize the surface energy in a single-atom metal layer. Nonetheless, we may solve this problem step by step. For instance, although free-standing “metallene” is quite difficult to be obtained, coating or growth of monolayered metals on external supports has been realized.207,259,427 In the meantime, many methods have been developed to subtract certain metal elements in multimetallic nanostructures.97,480 Therefore, by using a two-step method, it is probable to obtain monolayer metal nanosheets. Last but not the least, a novel class of 2D metal nanomaterials with unusual crystal structures could be synthesized via the crystal phase engineering. Very recently, some novel synthetic methods have been developed for the crystal-phase-controlled synthesis of thin metal nanomaterials. These methods are generally divided into two types, i.e. direct synthesis and crystal phase transformation.103 For example, via the ligand exchange at ambient conditions, the crystal phase transformation of 2D Au nanostructures from hcp to fcc and from 4H to fcc have been realized.107,307 Crystal phase engineering also enables us to obtain 2D metal nanomaterialbased heterostructures with novel crystal phases via a template synthetic strategy, in which a variety of metals with unusual crystal structures could be obtained by their epitaxial growth on presynthesized 2D metal nanostructures with novel crystal phases.2,114,308,481,482 These novel and unusual crystal phases of metals can further enrich the crystal phase library in the periodic table of elements. In addition to the crystal phase control of crystalline nanomaterials, the phase engineering also includes the crystallinity control. Very recently, amorphization of layered crystalline materials using our previously reported electrochemical lithiation method483 has been achieved.484 Given the high surface energy of 2D metal nanomaterials, it is feasible to realize fine-tuned crystallinity of 2D metal nanosheets by adjusting the synthetic conditions via wetchemical methods. The random atomic arrangement in amorphous metal nanomaterials can affect the electronic structures and produce interesting physicochemical properties. Therefore, developing methods to prepare amorphous 2D metal nanomaterials is desired.485 More importantly, via the phase engineering, one can also obtain heterophase nanostructures in which domains with different degrees of crystallinity coexist485 or crystal-phase heterostructures in which different crystal phases coexist.2,224 Based on the aforementioned description, we believe that (crystal) phase engineering has opened a new research direction to invoke fascinating chemical and physical properties of 2D metal nanomaterials and their hybrid nanomaterials for a wider range of applications.

Corresponding Author

*E-mail: [email protected]. ORCID

Wenxin Niu: 0000-0002-0835-3295 Cuiling Li: 0000-0003-2283-579X Bo Chen: 0000-0001-6743-9251 Hua Zhang: 0000-0001-9518-740X Notes

The authors declare no competing financial interest. Biographies Ye Chen received her B.S. degree in Engineering (Materials Engineering) from the School of Materials Science and Engineering in Nanyang Technological University (Singapore). Currently, she is a Ph.D. student at School of Materials Science and Engineering in Nanyang Technological University (Singapore) under the supervision of Prof. Hua Zhang. Her research interests focus on the synthesis, characterization, and applications of noble metal nanomaterials. Zhanxi Fan received his B.S. degree in Chemistry from Jilin University in 2010 and completed his Ph.D. under the supervision of Prof. Hua Zhang at Nanyang Technological University in 2015. After that, he worked as a Project Officer and subsequently Research Fellow in the same group until early 2017. Currently, he is a Postdoctoral Fellow at Lawrence Berkeley National Laboratory, USA. His research interests include the wet-chemical synthesis, characterization and applications of low-dimensional noble metal based nanomaterials. Zhicheng Zhang received his B.S. degree in College of Chemistry and Chemical Engineering from Southwest Petroleum University (China) in 2007 and completed his Ph.D. in College of Chemical Engineering from China University of Petroleum, Beijing (China) in 2012. Then he worked as a postdoc in Prof. Xun Wang’s group from Department of Chemistry, Tsinghua University (2012−2014). After that, he joined Prof. Hua Zhang’ group in School of Materials Science and Engineering, Nanyang Technological University, Singapore (2014− Present). His current research interests focus on the designed synthesis and catalytic/electrocatalytic applications of noble metalbased nanostructures with controlled crystal phase, morphology, size, and composition. Wenxin Niu received a B.S. degree from University of Science and Technology of China in 2004. He then obtained a Ph.D. degree from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 2010. After postdoctoral work in University of Florida and National University of Singapore, he is currently a Research Fellow at the School of Materials Science and Engineering in Nanyang Technological University (Singapore) in Prof. Hua Zhang’s group. His research interests include the synthesis and assembly of noblemetal and semiconductor nanomaterials and their applications in catalysis, sensing, and energy conversion. Cuiling Li received her Ph.D. degree from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CAS, China) in 2012. Then she started her postdoctoral work with Prof. Yusuke Yamauchi, supported by the JSPS Postdoctoral Fellowship in the National Institute for Materials Science (NIMS), Japan. She is currently a Research Fellow under the supervision of Prof. Hua Zhang at the School of Materials Science and Engineering in Nanyang Technological University, Singapore. Her research interests focus on the tailored design and synthesis of nanostructured materials and their applications in energy conversion systems, electrocatalysis, and analytical applications. 6442

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Nailiang Yang received his B.S. degree in Material Chemistry from Sun Yat-sen University, China, in 2007. He obtained his Ph.D. degree from the University of Chinese Academy of Sciences in 2013 under the supervision of Prof. Dan Wang. After graduation, he joined Prof. Pileni’s group as a postdoctoral fellow at Université Pierre et Marie Curie, France. Since 2015, he has been working as a Research Fellow at the School of Materials Science and Engineering in Nanyang Technological University (Singapore) in Prof. Hua Zhang’s group. His research interests include the synthesis and assembly of 2D nanomaterials and the study of their intrinsic properties related to crystal phase. Bo Chen received his B.S. degree and M.S. degrees from Lanzhou University in 2009 (China) and Shandong University in 2012 (China), respectively, and completed his Ph.D. with Prof. Hua Zhang at Nanyang Technological University in 2017 (Singapore). Currently, he is a Postdoctoral Fellow at School of Materials Science and Engineering, Nanyang Technological University in Prof. Hua Zhang’s group. His research work focuses on the preparation of novel functional materials, especially two-dimensional nanomaterials, for water remediation and water splitting. Hua Zhang obtained his B.S. and M.S. degrees from Nanjing University (China) in 1992 and 1995, respectively, and completed his Ph.D. study under the supervision of Professor Zhongfan Liu at Peking University (China) in 1998. As a Postdoctoral Fellow, he joined Prof. Frans C. De Schryver’s group at Katholieke Universiteit Leuven (Belgium) in 1999 and then moved to Prof. Chad A. Mirkin’s group at Northwestern University (U.S.) in 2001. After he worked at NanoInk Inc. (U.S.) and the Institute of Bioengineering and Nanotechnology (Singapore), he joined Nanyang Technological University (Singapore) in July 2006. His current research interests focus on the synthesis of ultrathin two-dimensional nanomaterials (e.g., metal nanosheets, graphene, metal dichalcogenides, metal− organic frameworks, etc.) and their hybrid composites for various applications, e.g., nano- and biosensors, clean energy, (opto)electronic devices, catalysis, and water remediation; the controlled synthesis, characterization, and application of novel metallic and semiconducting nanomaterials, and complex heterostructures, etc.

ACKNOWLEDGMENTS This review work was supported by MOE under AcRF Tier 2 (ARC 19/15, No. MOE2014-T2-2-093; MOE2015-T2-2-057; MOE2016-T2-2-103; MOE2017-T2-1-162) and AcRF Tier 1 (2016-T1-001-147; 2016-T1-002-051; 2017-T1-001-150; 2017-T1-002-119), and NTU under Start-Up Grant (M4081296.070.500000) in Singapore. We would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Technological University, Singapore, for use of their electron microscopy facilities. REFERENCES (1) Tan, C.; Cao, X.; Wu, X.-J.; He, Q.; Yang, J.; Zhang, X.; Chen, J.; Zhao, W.; Han, S.; Nam, G.-H.; et al. Recent Advances in Ultrathin Two-Dimensional Nanomaterials. Chem. Rev. 2017, 117, 6225−6331. (2) Zhang, H. Ultrathin Two-Dimensional Nanomaterials. ACS Nano 2015, 9, 9451−9469. (3) Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutiérrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.; et al. Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene. ACS Nano 2013, 7, 2898−2926. (4) Nasilowski, M.; Mahler, B.; Lhuillier, E.; Ithurria, S.; Dubertret, B. Two-Dimensional Colloidal Nanocrystals. Chem. Rev. 2016, 116, 10934−10982. 6443

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Chemical Reviews

Review

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DOI: 10.1021/acs.chemrev.7b00727 Chem. Rev. 2018, 118, 6409−6455