Exploration of Nanostructured Functional Materials Based on

Feature Article pubs.acs.org/JPCC

Exploration of Nanostructured Functional Materials Based on Hybridization of Inorganic 2D Nanosheets Jayavant L. Gunjakar, In Young Kim, Jang Mee Lee, Yun Kyung Jo, and Seong-Ju Hwang* Center for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano Sciences, Ewha Womans University, Seoul 120-750, Korea ABSTRACT: The 2D nanosheets of layered inorganic solids prepared by soft-chemical exfoliation reaction can be used as effective building blocks for hybridization with inorganic, organic, bio-, and polymer molecules/nanostructures. In comparison with graphene nanosheets, the 2D inorganic nanosheets boast much higher tunability in their chemical composition and physicochemical properties, leading to the creation of unexpected novel functionalities upon hybridization. Despite such unique and intriguing advantages of inorganic nanosheets, there are still only limited numbers of studies regarding the inorganic nanosheet-based hybrid materials. This Feature Article focuses on fundamental aspects of diverse synthetic strategies of the 2D nanosheet-based nanohybrids such as electrostatically derived reassembling, layer-by-layer deposition, crystal growth on the surface sites of nanosheets, and so on. Also, diverse functionalities of these 2D nanohybrid materials are discussed with an emphasis on the energy and environmental applications such as Li-ion batteries, supercapacitors, photocatalysts, fuel cells, and greenhouse gas capture. A prospect for the exploration of novel inorganic 2D nanosheet-based functional materials is provided along with new strategies to optimize the functionality of 2D inorganic nanosheets and their nanohybrids. reaction of layered inorganic solids.4,6−10 These inorganic materials are readily applicable for the hybridization with various guest species including inorganic, organic, polymeric, and biomolecules/nanostructures.1−3,6,16−18 In comparison with graphene nanosheets, the exfoliated 2D nanosheets of inorganic solids have much greater diversity in their crystal structure, chemical composition, surface properties, and so on.19,20 Of prime importance is that the 2D nanosheets of inorganic solids boast a broad spectrum of physicochemical properties from insulating to metallic and from diamagnetic to ferromagnetic, and moreover these properties are tunable by changing the chemical composition of the pristine compound.21−25 The application of these 2D nanostructured materials as building blocks makes possible the design and synthesis of novel hybridtype materials with versatile and tailorable physicochemical properties. Furthermore, the hybridization of 2D inorganic nanosheets with guest species can lead to the formation of highly porous structure and the controlled modification of electronic structure, which are caused by the house-of-cards-type stacking of sheet-like crystallites and an electronic coupling between the hybridized components, respectively.26−32 This enables optimization of the physicochemical properties and applicability of nanohybrid materials. These 2D nanosheet-based hybrid materials show wide applications such as heterogeneous catalysts,27−29,33−35 high-Tc superconductors,36−38 ferromagnetic/ferroelectric films,25,39 porous electrode materials,40−43 fuel cells,44−46 electrochemical sensors,47 biomolecule/drug

1. INTRODUCTION Hybridization between different chemical species attracts a great deal of research activity because of its effectiveness not only in controlling the physicochemical properties of each component but also in creating unexpected functionality via a synergistic coupling between the components.1−3 This synthetic method provides an effective way to overcome the limitations in the conventional design of functional materials using a single component. Since most advantages of the hybridization technique originate from the chemical interaction between hybridized components on their interface, nanostructured materials with expanded surface area become promising building blocks for the synthesis of hybrid materials. In comparison with other nanostructured materials, the 2D nanosheets prepared by the exfoliation process of layered materials are unique in terms of unusually high anisotropy in their crystal structure and morphology.4−10 The large surface area and thin thickness of the exfoliated nanosheets render them very useful candidates for the synthesis of nanohybrid material with unique physicochemical properties. This is due to the fact that all the component ions in the subnanometer-thick 2D nanosheet are exposed to its surface and can be remarkably modified by the interaction with hybridized foreign species. The representative example of exfoliated 2D nanosheets is a reduced graphene oxide (rG-O) nanosheet prepared by the exfoliation of graphite oxide, i.e., a 2D hexagonal monolayer of oxidized graphite lattice.11,12 Currently this 2D nanostructured carbon and its nanohybrids attract intense research interest because of their unique physicochemical properties and valuable functionalities.13−15 Like these chemically prepared rG-O nanosheets, the exfoliated 2D inorganic nanosheets can be synthesized by soft-chemical exfoliation © XXXX American Chemical Society

Received: October 28, 2013 Revised: December 8, 2013

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reservoirs,16,17 photoluminescence materials,48−51 and inorganic−polymer composite films.18,52,53 In this Feature Article, we review current research trends on the synthesis of 2D inorganic nanosheet-based nanohybrids and their applications for energy and environmental technologies such as Li-ion batteries, photocatalysts, supercapacitors, CO2 capture, and fuel cells. The purpose of this article is not only to provide a brief survey on laboratory experiments about 2D nanohybrids but also to expand the cognitive domain of the research of 2D nanosheet-based nanohybrids via the provision of future research prospects on this topic.

of LDH materials can be achieved by the solvation of LDH materials in polar organic solvents like formamide.70,71 Although the exfoliation of LDH is very facile and effective, the resulting colloidal suspension of the LDH nanosheets is not fairly stable enough for long-term storage for longer than several months. Depending on the composition of the LDH materials, the resulting colloidal suspension can last for several weeks to several months.10,70,72 This colloidal stability of the LDH nanosheet is significantly weaker than those of other exfoliated nanosheets like layered metal oxides and aluminosilicate clays. Most of the 2D inorganic nanosheets prepared by these chemical exfoliation methods are obtained in the form of colloidal suspension and possess sufficient surface charge, which are fairly suitable for the hybridization with guest species. The surface charge of the exfoliated inorganic nanosheets reflects the original layer charge of the corresponding host materials. Many inorganic nanosheets such as layered metal oxides, layered metal chalcogenides, and aluminosilicate clays show negative surface charge, which is similar to chemically prepared rG-O nanosheets.68,73,74 Conversely, the exfoliated LDH nanosheets are cationic species because of the positive layer charge of the pristine LDH materials.27,72 In addition to these chemical methods, some of the inorganic nanosheets can be synthesized by physical methods such as mechanical separation with adhesive tapes and chemical vapor deposition.54,68 However, the resulting nanosheets are mainly obtained in the form of thin film on the substrate and thus not applicable as precursors for hybridization with other chemical species. Even though there are many classes of inorganic materials that can form exfoliated nanosheets, most of the studies ever performed deal with the nanosheets of layered metal oxides, LDH, and layered metal chalcogenides because of their high structural stability and remarkable diversity in property and functionality. This Feature Article also focuses mainly on these layered materials and their chemically prepared nanosheets and nanohybrids. 2.1. Layered Metal Oxide. There are many structure types of layered metal oxides applicable as host materials of exfoliation reaction: rocksalt-type layered structure (LiCoO 2 , Li[Mn1/3Co1/3Ni1/3]O2, K0.45MnO2, etc.),8,73,75 layered Perovskite structure (Dion−Jacobson phase, KCa2Nb3O10, KSr2Nb3O10, KCa 2Ta 3O 10, KSr2 Ta 3O10, KLaNb 2O 7, K(Ca,Sr) 2Nb3 O10 , etc.),76,77 lepidocrocite-type structure (CsxTi2−x/4□x/4O4; □ = vacancy),33 and puckered layered titanoniobate structure (KTiNbO5, KTi2NbO7, K3Ti5NbO14, etc.),78−80 and so on. The layer charge density of layered metal oxide is crucial for an effective exfoliation into individual nanosheets since the increase of layer charge reinforces the electrostatic interaction between host layers and hence prevents the separation of the host layer into the single nanosheets. Since the host metal oxides are generally prepared by conventional solid state reaction, their chemical compositions and layer charges are readily controllable. Since the exfoliation of layered transition metal oxides can be induced only by the intercalation of bulky amine molecules or the ion exchange of ammonium cations into the protonated derivative of the pristine materials,7,8 first the interlayer alkali metal ions of the host materials should be exchanged with protons or hydronium ions. The exchange of alkali metal ions with protons is achieved by a simple dispersion of the pristine materials in aqueous solutions of strong acids. Then the obtained protonated materials are reacted with bulky organic cations to replace the interlayer protons/hydronium ions with these larger cations. In the course of the ion-exchange process, the swelling of the host lattice simultaneously occurs as a consequence of an

2. SYNTHETIC APPROACHES FOR 2D INORGANIC NANOSHEETS Since the exfoliative synthesis of 2D inorganic nanosheets heavily relies on an intercalation mechanism,4,7−10 the corresponding host materials should possess highly anisotropic 2D crystal structure with strong in-plane bonds and relatively weak out-ofplane bonds, which provides expandable interlayer gallery space for accommodating bulky guest species or solvent molecules.20,53−57 A variety of inorganic layered materials including layered metal oxides, layered double hydroxides (LDHs), aluminosilicate clays, layered metal chalcogenides, layered metal trihalides, and layered class III−VI/V−VI semiconductors satisfy this requirement and thus are applicable as host materials for the exfoliative synthesis of inorganic 2D nanosheets.4,7−10,58−61 Most of these materials have exchangeable cations or anions in the interlayer space or weak van der Waalstype bonds between the host layers. From the viewpoint of synthesis, the host layered materials can be prepared by conventional synthetic methods such as solid state reaction, coprecipitation, ion-exchange, and calcination− reconstruction.7−9,56−64 There are several chemical methods to exfoliate these materials into individual nanosheets. First, the exfoliation of layered metal oxides and layered metal hydroxides including LDHs can be achieved by the intercalation of bulky guest species, causing the weakening of interlayer interaction and the subsequent delamination into individual layers assisted by an infinite solvation process. The intercalation of bulky guest species can occur by several routes like ion-exchange and interlayer complexation.7,8,10,38 The exfoliated nanosheets prepared by this method form stable aqueous colloidal suspensions, in which the exfoliated nanosheets are welldispersed. Second, the exfoliated 2D nanosheets of layered metal chalcogenides and Aurivillius-type layered metal oxides can be synthesized by the interlayer evolution of H2 gas, giving rise to the mechanical separation of layered lattice into a single nanosheet.64,65 In this method, lithium ions are first intercalated into the host materials under anhydrous conditions. Then, the obtained Li-intercalation compound is reacted with water molecules, leading to the hydroxylation of interlayer lithium ions and the formation of H2 molecules. The evolution of H2 gas in the interlayer space causes the drastic volume expansion of the host lattice and the subsequent physical separation of the host lattice into individual nanosheets.64−68 Third, a dispersion of layered inorganic solids in polar solvent yields the exfoliated inorganic nanosheets via the effective solvation of individual host layers. The representative example is aluminosilicate clay mineral.69 This material experiences an automatic solvation of interlayer alkali metal ions by water molecules, resulting in the formation of the highly stable colloidal suspension of exfoliated clay nanosheets. Similarly the exfoliation B

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Figure 1. Schematic structural models of (a) lepidocrocite-type layered titanate, (b) MnO2, (c) LDH, (d) MoS2, and (e) graphene nanosheets.

infinite swelling of the LDH layers in formamide solvent, which requires neither the use of any surfactant/organophilic anions nor heat treatment.71 For the effective exfoliation process, the nitrate form of the LDH materials is generally used because the monovalent state of nitrate ions minimizes the interlayer interaction between the LDH layers and thus facilitates the exfoliation into individual LDH monolayers.10,71,72 Alternatively, the exfoliation of LDH in water solvent can be achieved by the intercalation of long-chain organic anions like dodecylsulfate ions, leading to the aqueous colloidal suspension of LDH nanosheets.84,85 2.3. Layered Metal Chalcogenide. A layered metal chalcogenide with a general formula of MX2 (M = Ti, Zr, Hf, Mo, Sn, W, V, Nb, etc.; X = S, Se, and Te) is one of the most investigated host materials not only for intercalation reaction but also for the exfoliation process.86,87 The crystal structure of these materials consists of the stacked layers composed of edge-shared MX6 octahedra, in which the chalcogen atoms in two hexagonal planes are separated by a plane of metal atoms. In contrast to the layered metal oxides and LDH having charge-compensating interlayer ions, the layered metal chalcogenides do not have any species in their interlayer space since each layer of MX2 is electrically neutral. The adjacent layers of metal chalcogenide are held together by weak van-der-Waals-type interactions to form a variety of 3D crystal structures. Depending on the trigonal prismatic and octahedral coordination of metal ions and the stacking orders of layers, the layered metal chalcogenides crystallize in hexagonal/rhombohedral and tetragonal symmetry.88,89 One of the most effective methods to exfoliate this layered material is the interlayer evolution of H2 gas via the hydroxylation reaction of preintercalated lithium ions.64,66−68 The intercalation of Li ions into the layered metal chalcogenide can be achieved by the reaction with lithiation agents such as nbutyllithium64,66 and by an electrochemical discharge process with an electrochemical cell composed of layered metal chalcogenide cathode, lithium metal anode, and Li-containing

automatic solvation of intercalated organic cations and host layers. The resulting weakening of interlayer interaction in the host material gives rise to the exfoliation of the host lattice into individual nanosheets under gentle shaking or ultrasonication, as demonstrated in Figure 2.7,55,75 In general, the exfoliated nanosheets of layered metal oxides are obtained in the form of aqueous colloidal suspension. Since all the layered metal oxides possess negative layer charge, the exfoliation of these materials yields negatively charged colloidal nanosheets. Figures 1a and 1b display the representative examples of exfoliated layered titanate and MnO2 nanosheets, respectively. These materials are one of the most widely used metal oxide nanosheets for basic building blocks for novel heterostructured nanohybrids. 2.2. LDH. Another representative host material for the synthesis of inorganic nanosheets is LDH. This material is classified as a unique family of layered material having anionexchange capacity with a general chemical formula of [M2+1−xM3+x(OH)2]x+[An−x/n]x−·mH2O (M2+ = Mg2+, Fe2+, Co2+, Ni2+, Zn2+, Cu2+, etc.; M3+ = Al3+, Cr3+, Fe3+, Co3+, etc.; x = 0.2−0.33).56 The prototype of this material is the brucitetype Mg(OH)2 phase. The crystal structure of the brucite-type LDH nanosheet is illustrated in Figure 1c. The partial substitution of divalent metal ions with trivalent metal ions creates the positively charged LDH lattice with charge-balancing anions in its interlayer space. Since the charge density of the brucite [M2+1−xM3+x(OH)2]x+ layer is determined by a molar fraction of trivalent metal ions (0.2 ≤ x ≤ 0.33), the surface charge of the exfoliated LDH nanosheets is tailorable by the control of the cation composition of the host material. The LDH materials can be synthesized by a simple hydrolysis reaction of metal ions, in which the coprecipitation of two different metal ions occurs at controlled pH conditions.81−83 Of prime importance is that the particle size, crystallinity, chemical composition, and layer charge are controllable in a wide range without altering the crystal structure of the LDH lattice. The most convenient exfoliation method for LDH material is an C

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electrolyte (LiPF6).90,91 The crystal structure of exfoliated MoS2 nanosheets is illustrated in Figure 1d. Alternative attempts are made to exfoliate the layered metal chalcogenide by ultrasonication in solvents with specific surface energy of ∼40 mJ m−1.9 However, the resultant materials are not monolayered nanosheets but stacked nanosheets with several tens of nanometers thickness.

host layers.85,93 Most of the intercalation processes occur on the basis of diverse chemical interactions between the host and guest such as ion-exchange, acid−base reaction, hydrogen bonding, redox reaction, and electrochemical redox reaction.1,92 These chemical interactions provide a driving force to diffuse guest species into the gallery space of the host lattice. Since the intercalation reaction occurs by the diffusion of guest species in the 2D limited space of the gallery of host materials, the size and shape of guest species as well as the charge density of host layers are fairly crucial in forming the intercalation compounds. The elevation of reaction temperature and pressure and the loading of ultrasonication/microwave radiation are frequently useful in expediting the intercalation reaction and also in increasing the degree of hybridization.1−3,17,18,82−94 3.2. Exfoliation−Reassembling. Despite the usefulness of the intercalation reaction in synthesizing 2D hybrid materials, this method suffers from several limitations such as a strict requirement for the high mobility of guest species. Such drawbacks of the intercalation method can be circumvented by adapting the exfoliation−reassembling strategy, in which the host nanosheets and the guest species are freely reassembled from initially separated states.3 Since the exfoliated 2D nanosheets possess a sufficient surface charge, these materials can be hybridized with oppositely charged guest species in terms of electrostatic interaction. This exfoliation−reassembling method allows us to hybridize a wide spectrum of guest species including bulky nanoparticles and biological macromolecules with the exfoliated 2D nanosheets. No requirement of this method for the high diffusivity of guest species provides a remarkably wide opportunity to hybridize diverse couples of host and guest.17,18,27−35 Of prime importance is that the chemical composition and stacking pattern of the reassembled nanohybrids can be precisely tailored by controlling the ratio of reactants and the condition of reaction, respectively. 3.3. Anchored Assembly. The exfoliated 2D nanosheets can play a role as anchoring sites for guest nanoparticles and as nucleation sites for the growth of nanoparticles. Although a lot of effort is devoted to the graphene-assisted growth of nanoparticles,28,95 the anchoring of guest nanoparticles on the surface of exfoliated 2D inorganic nanosheets is seldom investigated.81 In contrast to the intercalation and exfoliation−reassembling methods, the anchoring of guest species can be achieved by the adsorption of guest species or the direct growth of guest nanoparticles on the surface of 2D inorganic nanosheets. The homogeneous anchoring of guest species or the uniform nucleation of precursor ions followed by the crystal growth plays a crucial role in the successful application of this synthetic strategy. Since the exfoliated 2D nanosheets possess large surface area, these materials can provide useful anchoring sites for guest species and nucleation centers for precursor ions. Hence, the exfoliated nanosheet serves as a powerful platform for the synthesis of anchored assembly accommodating diverse types of guest species such as nanoparticles, nanosheets, organic molecules, nanoclusters, and biological/drug molecules. In one instance, the anchoring of metal nanoparticles on the transition metal oxide nanosheets with photocatalytic activity can occur by the photodeposition of metal cations caused by the photoinduced excited electrons.96 The surface functionalization of inorganic nanosheets with surfactant molecules makes possible the homogeneous adsorption of nanoparticles on their surface.97,98 The controls of electrostatic interaction between nanosheets and ionic precursors and the chemical environment

3. SYNTHETIC STRATEGIES FOR 2D NANOSHEET-BASED HYBRID MATERIALS As illustrated in Figure 2, there are diverse synthesis routes to 2D inorganic nanosheet-based hybrid materials like intercalation,

Figure 2. Representative synthesis strategies for 2D inorganic nanosheet-based hybrid materials: (a) intercalation, (b) exfoliation− reassembling, (c) anchored assembly, and (d) LbL film deposition.

exfoliation−reassembling, anchored assembly, layer-by-layer (LbL) deposition, etc.1−3,92 Using these lattice engineering techniques, the 2D inorganic nanosheets can be intimately hybridized with various guest species such as organic and inorganic ions, inorganic nanoparticles/nanoclusters, polycations/polyanions, and bioactive molecules.1−3 The form of the obtained nanohybrids including heterolayered nanohybrids, hierarchically assembled nanostructures, and multilayered hybrid films is strongly dependent on the synthetic method applied.26−43 Most of the present synthetic methods heavily rely on electrostatic interaction between exfoliated 2D nanosheet and hybridized species. Hence, the anionic nanosheets of layered metal oxides and layered metal chalcogenides are generally hybridized with cationic species, whereas the hybridization with anionic species can occur for the positively charged LDH nanosheets. 3.1. Intercalation. The intercalation reaction can provide a conventional synthetic method to a heterolayered nanohybrid composed of 2D inorganic nanosheets and guest species without an intermediate step of the exfoliation of the host lattice.92 During the intercalation process, the intralayer structure of individual host layers remains intact, whereas there are significant modifications in the interlayer distance and stacking mode of the D

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Figure 3. (A) Diffuse reflectance UV−vis spectra for (a) the pristine layered titanium oxide, (b) the as-prepared Fe2O3-layered titanate nanohybrid, and its derivatives calcined at (c) 200, (d) 300, and (e) 400 °C. (B) Visible-light-induced (λ > 420 nm) photocatalytic degradation of MB by the Fe2O3layered titanate nanohybrid (○) and its derivatives calcined at 300 (◊) and 400 °C (△), the pristine layered titanate (●), and α-Fe2O3 (▲). Reprinted with permission from ref 31. (C) Cross-sectional HR-TEM images of the Zn−Cr−LDH−W7O24 (ZCW) and Zn−Cr−LDH−V10O28 (ZCV) nanohybrids at high magnifications with (right) their schematic representation. (D) Visible light-induced (λ > 420 nm) O2 generation by (a) the pristine Zn−Cr−LDH, (b) ZCV-1, (c) ZCV-2, (d) ZCW-1, and (e) ZCW-2 nanohybrids calcined at 200 °C. Reprinted with permission from ref 29.

conductors can provide a powerful method not only to control the band structure and optical property of semiconducting materials but also to depress the charge recombination of electrons and holes via their spatial separation.106−109 The application of exfoliated 2D inorganic nanosheets as a building block makes possible the efficient widening of the light absorption region, the remarkable depression of charge recombination, the minimization of migration distance to surface reaction sites, and the expansion of surface area via the formation of porous stacking structure. Our group clearly demonstrates that an electrostatically derived reassembling of anionic exfoliated 2D titanate nanosheets and cationic guest species can provide a powerful synthetic methodology to a series of highly active visible-light photocatalysts of guest-layered titanate nanohybrids (guest = metal ions, metal hydroxide, quantum dots, and so on). In one instance, the hybridization between the layered titanate nanosheet and iron oxide nanocluster is achieved by the reassembling of exfoliated titanate nanosheets with iron hydroxide nanoclusters.31 As illustrated in Figure 3A, the obtained Fe2O3-layered titanate nanohybrid shows distinct absorption in the visible-light region, a result of electronic coupling between wide bandgap titanate and narrow bandgap iron oxide. This nanohybrid has an Eg value of ∼2.3 eV, which is much smaller than that of the pristine layered titanate (3.6 eV). Along with the red shift of the absorption edge, the hybridization with iron oxide generates a shoulder peak at ∼2.5 eV corresponding to the d−d transition of iron ions. The as-

of reaction media are highly crucial in synthesizing the anchored assembly of nanosheets and nanoparticles.99,100 3.4. LbL Film Deposition. The exfoliated 2D inorganic nanosheets are applicable as effective building blocks for the LbL fabrication of multilayered hybrid film.8,39,73,101 This LbL deposition method relies on the charge compensation mechanism between nanosheets and guest species.102 As illustrated in Figure 2, the exfoliated 2D nanosheets are alternatively deposited with oppositely charged guest species like polyelectrolytes (PEs), biomolecules, proteins, dyes, nanoparticles, and nanosheets in terms of electrostatic attraction.102,103 After each dipping cycle, the surface charge of the obtained LbL films becomes reversed. The LbL deposition method allows us to finely control the film thickness and stacking patterns at a nanometer-scale precision. This simple procedure can provide one of the most powerful routes to nanostructured thin films with nanoarchitectured organization.

4. APPLICATIONS OF 2D NANOSHEET-BASED HYBRID MATERIALS FOR ENERGY AND ENVIRONMENTAL TECHNOLOGIES 4.1. Photocatalysts. The semiconductor-assisted photocatalytic reactions for water splitting and sanitization of organic/ inorganic pollutants attract intense research interest because of their usefulness as an eco-friendly option for the depletion of fossil energy and the deepening of environmental pollution.104,105 The hybridization between two types of semiE

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the metallic 1T polytype MoS2 nanosheet shows extraordinary photocatalytic activity for H2 evolution with a high generation rate of 30 mmol h−1 g−1.119 Additionally, this type of nanosheet can be used as a building block for the synthesis of hybrid-type photocatalysts. The hybridization of the exfoliated MoS2 nanosheets with guest species like CdSe yields novel hybridtype photocatalysts fairly active for visible-light-induced H2 generation.120 The photocatalytic activity of the obtained CdSe−MoS2 nanocomposite for H2 production is 3.7 times larger than that of nanocrystalline CdSe. 4.2. Electrodes for Li-Ion Batteries. The exploration of novel efficient electrode materials for lithium-ion batteries attracts intense research interest because of the increasing importance of battery technology as an environmentally benign power source.121,122 Since low-dimensional nanostructured materials can provide a short Li+ diffusion path and expanded surface area for efficient intercalation/insertion of Li+ ions, intense research efforts are devoted to the exploration of various nanostructured electrode materials such as 1D nanowires/ nanotubes, 2D nanosheets, and 3D mesoporous structures.123−126 Like graphene nanosheets, the 2D nanosheets of redoxable inorganic solids such as layered metal oxide, layered chalcogenide, and LDH can be promising candidates as electrode materials for Li-ion batteries. Among them, the layered 3d transition metal oxide nanosheets are applicable as both cathode and anode materials in terms of the intercalation/deintercalation and conversion mechanisms, respectively.127−131 Conversely layered metal chalcogenide nanosheets can be used solely as anode materials because of their low working potentials for the electrochemical redox process.132 In the case of LDH nanosheets, the presence of hydroxide ions prevents the direct application of these materials as electrodes for lithium secondary batteries. Instead, the heat treatment of these materials yields efficient hybrid-type electrode materials since the LDH materials experience a phase transformation to redoxable metal oxides at elevated temperature.133,134 There are several synthetic approaches to 2D inorganic nanosheet-based electrode materials for lithium-ion batteries as follows: (1) the solidification of colloidal nanosheets to powdery electrode materials, (2) the synthesis of 2D nanostructured electrode materials via the phase transition of exfoliated nanosheets, and (3) the synthesis of porous electrode material via the hybridization between exfoliated inorganic nanosheets and guest species. First, several methods are developed for the solidification of colloidal nanosheets such as adsorption on a solid template, freeze drying, hydrothermal treatment, and so on. In one instance, the hollow spheres of layered [Mn1/3Co1/3Ni1/3]O2 can be synthesized via the adsorptive LbL coating of the nanosheets and polycations on the polystyrene beads and the subsequent heat treatment.135 Alternatively the freeze-drying of exfoliated titanate nanosheets yields the cotton-like flakes of layered titanate.136 This material shows discharge capacity of 165 mAh g−1 with a smooth potential profile. The average working potential of titanate nanosheets is lower than anatase TiO2 and Li4Ti5O12, indicating the advantage of titanate nanosheets for anode application. The precipitation of colloidal MoS2 nanosheets can be achieved by hydrothermal treatment at 180 °C, leading to the synthesis of sheet-shaped MoS2 powdery materials with a high reversible lithium storage capacity and excellent rate capability.137 Alternatively, the MoS2/PEO/graphene nanocomposite is synthesized with the exfoliated nanosheets of MoS2 and graphene. This material displays promising electrode

prepared Fe2O3-layered titanate nanohybrid and its derivatives calcined at 300 and 400 °C can induce distinct photodegradation of methylene blue (MB) molecules under visible-light irradiation (λ > 420 nm) (see Figure 3B). The photocatalytic activity of these nanohybrids is much stronger than those of the pristine cesium titanate and iron oxide, underscoring notable beneficial effect of hybridization on the photocatalytic activity of iron oxide and layered titanium oxide. The observed improvement of the photocatalytic activity upon the hybridization is attributable to the strong electronic coupling between subnanometer-thick layered titanate nanosheet and iron oxide nanoclusters, resulting in the enhancement of visible-light absorptivity and the extended lifetime of photogenerated electrons and holes. Also, the expanded surface area induced by the mesoporous stacking structure of hybrid crystallites makes another contribution to the improvement of photocatalytic activity via the provision of surface reaction sites. In addition to semiconducting metal oxides, several LDH phases are reported to show promising photocatalytic activity for the visible-light-induced O2 generation.27−29,110−118 Considering the facile exfoliation of the LDH phase into a positively charged nanosheet, this material is supposed to be fairly suitable for the hybridization with negatively charged species in terms of an exfoliation−reassembling strategy.27,29,80 For example, the electrostatically derived reassembling of cationic Zn−Cr−LDH nanosheets with anionic titanate nanosheets leads to the formation of heterolayered Zn−Cr−LDH-layered titanate nanohybrids with remarkably high photocatalytic activity for visible-light (λ > 420 nm)-induced O2 generation.27 The hybridization with layered titanate induces an additional crucial advantage of the improvement of the chemical stability of LDH material in acidic media. Similarly, the 2D nanohybrid composed of cationic LDH nanosheets and anionic graphene nanosheets (rG-O and graphene oxide) can be synthesized by an exfoliation−reassembling method.80 The resulting LDH− graphene nanohybrids also exhibit higher visible-light photocatalytic activity for visible-light (λ > 420 nm)-induced O2 generation than does the pristine LDH material, confirming the beneficial effect of hybridization. Recently, our group reported that the hybridization of Zn−Cr−LDH 2D nanosheets with polyoxometalate (POM) 0D nanoclusters (W7O246− and V10O286−) is fairly effective in enhancing the visible-light photocatalytic activity of the Zn−Cr−LDH phase for O2 generation.29 In this study, two kinds of nanohybrids of Zn− Cr−LDH−W7O246− and Zn−Cr−LDH−V10O286− were synthesized with a charge-balanced ratio and a 3-fold POM-excess ratio (ZCW-1/ZCV-1 and ZCW-2/ZCV-2) to probe the effect of stoichiometry on the photocatalytic activity of the resulting nanohybrids. As illustrated in the cross-sectional high-resolution transmission electron microscopy (HR-TEM) analysis of Figure 3C, the LDH nanosheets are regularly interstratified with POM nanoclusters. After the hybridization with POM nanoclusters, the photocatalytic activity of the Zn−Cr−LDH is remarkably enhanced for all of the applied ratios of Zn−Cr−LDH/POM, as plotted in Figure 3D. Like the case of titanate nanosheet-based hybrid photocatalysts, the enhanced photocatalytic activity of the LDH-based nanohybrid can be interpreted as a result of an unusually strong electronic coupling between subnanometerthick LDH nanosheets and guest species, leading to the optimization of visible-light harvesting ability and the lifetime of electrons and holes. Like the exfoliated metal oxide and LDH nanosheets, some of the layered metal dichalcogenide nanosheets possess promising photocatalytic activity. For instance, F

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Figure 4. (A) Cross-sectional HR-TEM images and (B) schematic models of structural change upon calcination for (a) NiO-layered titanate and (b) Fe2O3-layered titanate nanohybrids. (C) Dependence of discharge capacity on cycle numbers for the pristine cesium titanate (▲), NiO-layered titanate nanohybrids calcined at 200 (●) and 300 °C (○), and Fe2O3-layered titanate nanohybrids calcined at 200 (⧫) and 300 °C (◊). Reprinted with permission from ref 130. (D) Schematic diagram for the synthesis of the Li4Ti5O12−SnO2 nanocomposite. (E) Potential profiles of the first discharge process and (F) discharge capacity vs cycle number plots for the Li4Ti5O12−SnO2 nanocomposites with the Li4Ti5O12/SnO2 ratios of (a) 1, (b) 2, (c) 4, and (d) 8, and (e) Li4Ti5O12 2D nanosheets, and (f) SnO2 nanocrystals. Reprinted with permission from ref 40.

nanosheets leads to the formation of a porous 2D array of anatase TiO2 nanoparticles.129 Upon the calcination process, the lepidocrocite-type layered titanate nanosheets are changed into anatase TiO2 nanoparticles with improved electrochemical activity. The hybridized graphene nanosheet acts as a template to keep the 2D morphology of titanium oxide crystallites during the phase transformation. As a consequence, the porous 2D array of anatase TiO2 nanoparticles can be obtained. The enhancement of ion transport in this porous 2D array improves the electrode performance of TiO2 nanoparticles. Additionally, the 2D nanostructure of lithiated metal oxide can be synthesized by the reaction between exfoliated metal oxide nanosheets and Li precursors; the 2D Li4Ti5O12 nanosheets with cubic spinel structure are obtained by the reaction between exfoliated titanate nanosheet and n-BuLi, which is followed by the heat treatment at elevated temperatures.127 Although this material possesses 3D cubic spinel structure, the highly anisotropic 2D morphology of the precursor titanate nanosheets remains unchanged upon calcination. The Li4Ti5O12 nanosheet prepared by heat treatment

performance for lithium-ion batteries with an excellent rate capability (>250 mAh g−1 at 10 A g−1, >600 mAh g−1 at 50 mA g−1).138 The exfoliated WS2 nanosheet subjected to the treatment with superacid demonstrates an exceptionally large first-cycle reversible capacity of 470 mAh g−1 at 25 mA g−1.139 Second, the exfoliated nanosheets can be used as precursors for other 2D nanostructured electrode materials via phase transformation at elevated temperature. The heat treatment of the exfoliated LDH nanosheets provides a facile and effective way to synthesize intimately coupled mixed metal oxide nanocomposites with diverse chemical compositions. Upon the heat treatment, the Zn−Fe−LDH material experiences a phase transition to the homogeneously mixed nanocomposites of ZnO−ZnFe2O4 with anode functionality.82 The selective etching of ZnO nanoparticles from the nanocomposite materials gives rise to the formation of the porous assembly of spinel-structured ZnFe2O4 nanoparticles with promising anode functions for lithium secondary batteries. Also, the heat treatment of 2D nanostructured layered titanate hybridized with graphene G

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Figure 5. (A) Photoimages of mixed colloidal suspensions of rG-O and layered MnO2 nanosheets and (B) FE-SEM image of the Li−rG-O-layered MnO2 nanocomposite. (C) Galvanostatic charge/discharge curves of the first few cycles and a few cycles around the 1000th cycle at a constant current density of 0.5 mA cm−2 for the Li−rG-O-layered MnO2 nanocomposites with the rG-O/MnO2 ratios of (a) 0.1, (b) 0.2, and (c) 0.3, (d) the Li-layered MnO2 nanocomposite, and (e) the Li−rG-O nanocomposite. (f) Variations of the capacitance retention as a function of the number of cycles for the Li/ rG-O-layered MnO2 nanocomposites with the rG-O/MnO2 ratios of 0.1 (●), 0.2 (▲), and 0.3 (■), the Li-layered MnO2 nanocomposite (⧫), and the Li−rG-O nanocomposite (▽). Reprinted with permission from ref 42. (D) Crystal structure and (E) photograph of the exfoliated Zn−Co−LDH nanosheet. (F) CV curves of the film composed of the restacked Zn−Co−LDH nanosheets. Reprinted with permission from ref 72.

at 600 °C possesses promising electrochemical functionality as an anode material for lithium secondary batteries. Similarly, olivine-type LiMPO4 nanosheets can be synthesized by a liquidphase exfoliation and the following solvothermal lithiation process in high-pressure, high-temperature supercritical fluids.140 Due to the 2D morphology of the resulting olivine nanosheets with high surface area, this material shows a fast lithium transport and thus high energy density and good rate capability. Third, the hybridization of redoxable inorganic nanosheets with electrochemically active species leads to the formation of hybrid-type electrode materials with synergistically improved electrode performance. In one instance, mesoporous MOx−layered titanate (M = Fe and Ni) nanohybrids are synthesized by electrostatically derived reassembling between negatively charged titanate nanosheets and positively charged metal oxide nanoclusters.130 As shown in Figure 4A, these nanohybrids demonstrate an assembly of parallel dark lines representing the titanate layers, indicating the layer-by-layer ordering of the layered titanate and metal oxide nanoclusters. The Fe2O3-layered titanate nanohybrid displays excellent thermal stability of the hybridized iron oxide species, which surely contrasts with a distinct phase transition of nickel species in the NiO-layered titanate caused by the calcination process

(Figure 4B). The mesoporous nanohybrids are superior to the pristine titanate in terms of electrode performance for lithiumion batteries, indicating the merit of hybridization in improving the electrochemical property of titanium oxide. Similarly, the hybridization of exfoliated MoO 3 nanosheets with TiO 2 nanoparticles yields highly efficient electrode materials with mesoporous structure.141 As shown in Figure 4D, an anchored assembly of SnO2 nanoparticles on the Li4Ti5O12 nanosheets leads to the synthesis of the Li4Ti5O12−SnO2 nanocomposite.40 The composite formation with SnO2 nanoparticles is effective in enhancing the electrode performance of Li4Ti5O12 nanosheets. In addition, the exfoliated metal oxide nanosheet can be used as dopant for layer-structured electrode materials. The heterolayered nanocomposites of MnO2 and [Mn1/3Co1/3Ni1/3]O2 nanosheets are synthesized by the reassembling of two kinds of metal oxide nanosheets and lithium ions.41 Upon the following calcination process, the hydrated layered structure of the asprepared nanocomposites is transformed into a dehydrated layered structure at 200−400 °C. The partial replacement of MnO2 layers with more stable [Mn1/3Co1/3Ni1/3]O2 layers results not only in the increase of the discharge capacity of the Li−MnO2 nanocomposite but also in the prevention of the phase transition of layered manganese oxide to spinel structure during H

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Figure 6. (A) (Left) Powder XRD patterns of (a) protonated layered titanate, (b) the as-prepared zirconium complex-layered titanate and its derivatives calcined at (c) 200, (d) 300, (e) 400, (f) 500, and (g) 600 °C. (Right, top) FE-SEM image and (right, bottom) cross-sectional HR-TEM image of the asprepared zirconium complex-layered titanate. (B) CO2 adsorption behaviors of (a) the as-prepared zirconium complex-layered titanate and its calcined derivatives at (b) 200 and (c) 300 °C. Reprinted with permission from ref 156. (C) (a) FE-SEM image, (b) schematic model, and (c) cross-sectional HR-TEM images for the as-prepared CdO/Cr2O3-layered titanate nanohybrid. (D) The CO2 adsorption behaviors of the as-prepared CdO/Cr2O3layered titanate nanohybrid (●) and its derivatives calcined at 200 (▲), 300 (■), 400 (⧫), 500 (▼), 600 (○), and 700 °C (△), the protonated layered titanate (▽), the CdO/Cr2O3-layered titanate nanohybrids calcined at 400 (□) and 500 °C (◊), and the nonpillared mixture of Cr2O3 and CdO (+). All the present data are measured at 0 °C up to 101.325 kPa. Reprinted with permission from ref 157.

unchanged. The resulting nanocomposite materials exhibit a large specific capacitance of ∼160 F g−1 and excellent cyclability. Mn K-edge X-ray absorption near-edge structure (XANES) and field-emission scanning electron microscopy (FE-SEM) analyses for the nanocomposites after repeated cycling clearly demonstrate that the electronic structure and porous morphology of these materials are well-maintained during the electrochemical cycling. Such a promising electrode performance of the restacked Li−MnO2 nanocomposite can be further enhanced by the incorporation of rG-O nanosheets. As demonstrated in Figure 5A, the homogeneous mixture colloidal suspensions of the exfoliated rG-O and layered MnO2 nanosheets can be formed because of the similar chemical natures of both nanosheets. The following addition of Li+ ions into the mixture colloidal suspensions of negatively charged nanosheets yields the Li− rG-O-layered MnO2 nanocomposites showing highly porous morphology. The Li−rG-O-layered MnO2 nanocomposites display much larger specific capacitance of 210 F g−1 than does the Li-layered MnO2 nanocomposite (160 F g−1), highlighting the beneficial effect of the rG-O incorporation on the electrode performance of layered MnO2 nanocomposites.42 Both of the MnO2 nanosheet-based nanocomposites show good retention ability of ∼95−97% up to 1000 cycles. The present results

electrochemical cycling. The exfoliation−reassembling strategy provides an opportunity to achieve layer doping for lithium metal oxide electrode material. 4.3. Electrodes for Supercapacitors. Supercapacitors attract prime attention as auxiliary power sources to secondary batteries because of their fast charging/discharging capability, high power density, and excellent cyclability.142 Since the storage of electrical energy by the supercapacitors occurs near the interface between electrode and electrolyte, the optimization of the surface structure of electrode materials is very crucial in improving the energy storage capacity of supercapacitors. The hybridization with the exfoliated 2D nanosheets is effective in exploring highly efficient electrode materials for supercapacitors since the 2D nanosheet-based nanohybrids possess highly porous structure with expanded surface area. Furthermore, the subnanometer-level thickness of the exfoliated 2D nanosheets makes possible an optimization of the power density of 2D nanosheet-based hybrid materials via a strong electronic coupling with hybridized conductive materials. The reassembling of exfoliated MnO2 nanosheets with alkali metal ions yields highly porous alkali metal−MnO2 nanocomposites showing promising pseudocapacitance behaviors.75 Upon the reassembling process, the 2D morphology and layered crystal structure of exfoliated MnO2 nanosheets remain I

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Figure 7. (A) Schematic representation of the synthesis route of Pt−rG-O-layered titanate nanocomposites with improved ORR activity. (B) CV curves of the Pt−rG-O-layered titanate nanocomposites with the Pt/Ti ratios of 0% (solid lines), 0.5% (dotted lines), 1% (dashed lines), and 2% (dot-dashed lines) obtained in Ar-saturated 0.1 M HClO4 solution at a sweep rate of 20 mV s−1. Reprinted with permission from ref 44. (C) TEM images of (a) PtRu/ C, (b) layered titanate nanosheet (0.05)−PtRu/C, (c) layered titanate nanosheet (0.25)−PtRu/C, and (d) layered titanate nanosheet (0.50)−PtRu/C. (D) Chronoamperograms for (a) PtRu/C, (b) layered titanate nanosheet (0.05)−PtRu/C, (c) layered titanate nanosheet (0.25)−PtRu/C, and (d) layered titanate nanosheet (0.5)−PtRu/C in 0.5 M H2SO4 + 1 M CH3OH (60 °C) at 500 mV versus RHE. Fresh catalysts (dotted lines) and catalysts after accelerated durability tests (solid lines). Reprinted with permission from ref 45.

underscore that layered MnO2 nanosheets can be used as useful building blocks for efficient supercapacitor electrode material. In addition to the layered metal oxide nanosheets, the exfoliated LDH nanosheets are other promising candidates as electrode materials for supercapacitors. In one instance, the exfoliated Zn−Co−LDH nanosheet is synthesized by the dispersion of the pristine Zn−Co−LDH material in formamide. This Zn−Co−LDH nanosheet displays excellent pseudocapacitance performances with the specific capacitance of ∼160−170 F g−1 and good capacitance retention.72 This result demonstrates that the exfoliated Zn−Co−LDH nanosheet has a potential applicability for a supercapacitor electrode. Similarly, the

exfoliated VS2 nanosheet shows excellent electrode performance for a supercapacitor with a large specific capacitance of 4760 μF cm−2.143 4.4. CO2 Adsorbents. A great deal of research efforts are devoted to the synthesis of porous inorganic solids and their application for the capturing of greenhouse gas and the storage of gaseous fuel.144,145 The sequestration of CO2 gas is one of the most important issues in mitigating the increasing threats of global warming. There are several factors for the optimization of the gas adsorption function of porous materials such as high proportion of micropores, high reactivity for gas molecules, and readily accessible pore structure. Taking into account their J

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the thermal stability of the heterolayered structure. As shown in Figure 6D, the CdO/Cr2O3-layered titanate nanohybrid calcined at 500 °C exhibits very large CO2 adsorption capacity (1.78 mmol g−1 measured at 100 kPa and 273 K), which is comparable to those of MOFs with much larger surface area. This fact clearly demonstrates that the cohybridization of a 2D inorganic nanosheet with two kinds of guest nanoclusters is fairly effective in exploring the efficient CO2 adsorbents. 4.5. Electrocatalysts for Fuel Cells. The development of efficient and economic electrocatalyst materials is of prime importance in the commercialization of fuel cell technologies.158,159 The use of expensive Pt elements as electrocatalysts casts serious economic problems in the practical use of a lowtemperature fuel cell. Intense research efforts are devoted to the minimization of the amount of expensive Pt metal used. Taking into account the fact that catalytic reaction occurs on the surface of solid catalysts, the decrease of particle size helps to minimize the amount of Pt metal used. However, the metal nanoparticle suffers from a strong tendency to agglomerate into bigger particles during the repeated use, leading to the severe degradation of catalyst performance.160 To circumvent this problem, many attempts are made to immobilize and stabilize the Pt metal nanoparticles with expanded surface area on the solid matrix (Figure 7).44,45,92,161−164 Since the exfoliated 2D nanosheets possess large surface area and many anchoring surface sites, these nanostructured materials can act as promising solid supports for Pt nanoparticles. In fact, the layered titanate nanosheet is applied for solid supports for PtRu electrocatalysts, resulting in the improvement of the long-term stability of nanocrystalline PtRu electrocatalysts.45 This study strongly suggests the effectiveness of exfoliated 2D inorganic nanosheets as a solid matrix for the immobilization of metallic electrocatalysts. However, the inferior electrical conductivity of a semiconducting titanate nanosheet is not advantageous for the significant improvement of the on-set potential and current of the PtRu electrocatalyst.

extremely high anisotropic 2D morphology and the existence of many active sites on the surface, 2D inorganic nanosheets can be promising building blocks for the formation of porous and highly active inorganic solids applicable as CO2 adsorbent. Diverse porous materials such as metal−organic framework (MOF) and zeolite-related materials are applied as CO2 gas adsorbents.146−148 Yan et al. report 2D MOFs functionalized with amine and carboxylic acid.149 Despite their relatively small surface areas, the 2D layered MOFs still have good CO2 adsorption capability compared with other typical MOFs. The polar functional groups dispersed on the surface of the 2D layer are key factors for the efficient uptake of CO2 molecules, indicating the beneficial role of the 2D layer as a support of the amine and carboxyl functional groups. The delaminated zeolite 2D nanosheets are also good candidates for CO2 adsorbents.150 The zeolitic ITQ-6 nanosheet functionalized with amine displays much better CO2 adsorption capability than does mesoporous SBA-15 silica functionalized with the same ligands. This study suggests the advantage of delaminated zeolite nanosheets in supplying an accessible external surface and supporting the amine ligands.151 However, the efficient CO2 adsorption in the MOF and zeolite materials can be achieved only by the surface functionalization with amine and carboxylic acid. Additionally, the boron nitride (BN) nanosheet shows a promising functionality as CO2 absorbent with high CO2 selectivity, which can be drastically enhanced by introducing electrons. The absorbed CO2 can be spontaneously released from BN nanosheets without any reaction barrier by removing the electrons.152 Another layered material of LDHs can be used for adsorbing Lewis acidic CO2 because they possess both high surface area and abundant basic sites at the surface. The heat treatment of LDH at elevated temperature induces the dehydroxylation and decarbonation, followed by the formation of an active basic site.153 Reddy et al. claim that the Mg−Al− LDH calcined at 400 °C has the highest adsorption capability.154 The further improvement of the CO2 capture functionality of Mg−Al−LDH is attempted via the control of chemical composition.155 However, the low long-term stability of LDH during CO2 adsorption/desorption limits its use as a gas adsorbent for practical application. Alternatively, the pillaring of basic inorganic guest species into layered metal oxide nanosheets provides a breakthrough for the synthesis of highly porous CO2 adsorbent with good thermal stability. A reassembling between exfoliated layered metal oxide nanosheets and nanosized inorganic guest species yields very well-developed microporous and mesoporous structure. In our group, porous zirconium complex-layered titanate nanohybrids are synthesized by reassembling reaction between negatively charged layered titanate nanosheets and the positively charged zirconium complex.156 As presented in Figures 6A and 6B, the zirconium complex-layered titanate nanohybrids show the expanded (010) plane and the house-of-cards-type porous morphology, indicating the existence of both micropores and mesopores in these materials. The zirconium complex-layered titanate nanohybrids possess promising CO2 adsorption capability of 0.9 mmol g−1 measured at 100 kPa and 273 K, which is contributable to the expanded surface area (∼176 m2 g−1) and active sites induced from zirconium species. To optimize the CO2 adsorption capability of porous pillaring nanohybrids, the copillaring effect of basic cadmium oxide with chromium oxide is examined (see Figure 6C).157 The copillaring of two different metal oxide nanoclusters leads to the further increase of micropore volume as well as to the improvement of

5. FUTURE SCOPES The present Feature Article focuses on the validity of the 2D inorganic nanosheet as building blocks for novel nanohybrid materials with useful functionalities for diverse fields. The applications of the 2D nanosheet-based hybrid materials are not restricted to the energy and environmental technologies, a main subject of this article, but include many other technologies such as nanobiotechnology including controlled/targeted drug delivery and tissue engineering. Despite the remarkable usefulness of the 2D inorganic nanosheets, studies on these 2D nanosheet-based hybrid materials are not sufficiently investigated yet. Thus, there is wide room to explore novel hybrid materials with the exfoliated 2D nanosheets. Since most of the hybridization methods using exfoliated nanosheet precursors rely on an electrostatic interaction between the components, most of the nanohybrid materials ever-reported are composed of oppositely charged nanosheets and guest species. To circumvent this limitation, it is desirable to develop new synthetic methods based on other chemical interactions like hydrogen bonding, covalent bonding, and coordination bonding. Another methodology to expand the area of hybrid materials is to modify the surface charge of the exfoliated nanosheet by surface modification, e.g., positively charged metal oxide nanosheets and negatively charged LDH nanosheets. Taking into account the fact that most functionality of the present nanohybrid materials such as electrodes and electroK

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catalysts can be further improved by the increase of electrical conductivity, the coupling of 2D inorganic nanosheets with highly conductive carbon, metal, and polymeric nanostructures can provide a powerful tool to optimize the functionality of the 2D nanosheet-based hybrid materials. Although the chemical composition of the exfoliated 2D nanosheets is readily controllable, there are only a few studies regarding the chemical substitution of 2D nanosheets and their nanohybrids. The application of chemically substituted inorganic nanosheets as a precursor would allow us to further improve the functionality of the 2D nanosheet-based hybrid materials. Additionally the LbL ordering of different 2D nanosheets can provide a unique opportunity to synthesize novel multifunctional materials. In one instance, the interstratification of ferromagnetic and ferroelectric nanosheets would yield multiferroic materials with tailorable property and stacking structure. The heterolayered nanohybrid composed of superconducting and insulating nanosheets is expected to show a Josephson junction effect. Considering that many kinds of nanosheets (more than two) can be assembled into a single solid lattice, the present hybridization strategy with 2D inorganic nanosheets provides a novel methodology to synthesize a great number of novel nanohybrid materials with unprecedented intriguing properties.

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Gunjakar L. Jayavant received a B.S. degree in physics (2001), a M.S. degree in physics (2004), a B. Ed. degree in general science and mathematics (2005), and a Ph.D. degree (2009) in physics from Shivaji University, Kolhapur (India). He is currently a research professor in Ewha Womans University, Korea (Supervisor: S. -J. Hwang). His research interests include the synthesis and characterization of layered double hydroxide (LDH) nanosheet-based nanohybrids for applications in energy storage and conversion devices such as supercapacitors, photocatalysts, and Li secondary batteries.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-2-3277-4370. Fax: +82-2-3277-3419. E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

In Young Kim received a B.S. degree in chemistry (2008), a M.S. degree in inorganic chemistry (2010), and a Ph.D. degree in inorganic chemistry (2014) from Ewha Womans University (Korea). She has focused on the synthesis and characterization of layered metal oxide nanosheet-based nanohybrids applicable for Li secondary batteries, photocatalysts, gas adsorbent, and solar cells. She has research interests on X-ray absorption spectroscopic analysis for nanomaterials.

Seong-Ju Hwang obtained a B.S. degree in chemistry (1992) and a M.S. degree in inorganic chemistry (1994) from Seoul National University (Korea) and a Ph.D. degree in inorganic chemistry from Université Bordeaux I (France) in 2001. He worked as a postdoctoral researcher at Michigan State University in 2001 and 2002. Prof. Hwang joined the Department of Applied Chemistry at Konkuk University in 2002 and moved to the Department of Chemistry & Nano Sciences at Ewha Womans University in 2005. He became a full professor in 2012 and also serves as an associate editor of the Journal of Solid State Chemistry. Prof. Hwang has been working in the field of “Exploratory synthesis and characterization of nanostructured transition metal oxides applicable for energy production, energy storage, and environmental purification”.

Jang Mee Lee received a B.S. degree in chemistry (2011) and a M.S. degree in inorganic chemistry (2013) from Ewha Womans University (Korea). She is a currently a Ph.D. student in the Department of L

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(10) Ma, R.; Liu, Z.; Li, L.; Iyi, N.; Sasaki, T. Exfoliating Layered Double Hydroxides in Formamide: A Method to Obtain Positively Charged Nanosheets. J. Mater. Chem. 2006, 16, 3809−3813. (11) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable Aqueous Dispersions of Graphene Nanosheets. Nat. Nanotechnol. 2008, 3, 101−105. (12) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228−240. (13) Huang, X.; Qi, X.; Boey, F.; Zhang, H. Graphene-Based Composites. Chem. Soc. Rev. 2012, 41, 666−686. (14) Mao, H. Y.; Laurent, S.; Chen, W.; Akhavan, O.; Imani, M.; Ashkarran, A. A.; Mahmoudi, M. Graphene: Promises, Facts, Opportunities, and Challenges in Nanomedicine. Chem. Rev. 2013, 113, 3407−3424. (15) Kamat, P. V. Graphene-Based Nanoassemblies for Energy Conversion. J. Phys. Chem. Lett. 2011, 2, 242−251. (16) Park, D. -H.; Choy, J. -H. Emerging Strategies in Infohybrid Systems. Eur. J. Inorg. Chem. 2012, 32, 5145−5153. (17) Oh, J. -M.; Park, D. -H.; Choy, J. -H. Integrated Bio-Inorganic Hybrid Systems for Nano-Forensics. Chem. Soc. Rev. 2011, 40, 583−595. (18) Leroux, F.; Besse, J. -P. Polymer Interleaved Layered Double Hydroxide: A New Emerging Class of Nanocomposites. Chem. Mater. 2001, 13, 3507−3515. (19) Xu, M.; Liang, T.; Shi, M.; Chen, H. Graphene-Like TwoDimensional Materials. Chem. Rev. 2013, 113, 3766−3798. (20) Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutierrez, 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. (21) Osada, M.; Takanashi, G.; Li, B. -W.; Akatsuka, K.; Ebina, Y.; Ono, K.; Funakubo, H.; Takada, K.; Sasaki, T. Controlled Polarizability of One-Nanometer-Thick Oxide Nanosheets for Tailored, High-k Nanodielectrics. Adv. Funct. Mater. 2011, 21, 3482−3487. (22) Fukuda, K.; Sato, J.; Saida, T.; Sugimoto, W.; Ebina, Y.; Shibata, T.; Osada, M.; Sasaki, T. Fabrication of Ruthenium Metal Nanosheets via Topotactic Metallization of Exfoliated Ruthenate Nanosheets. Inorg. Chem. 2013, 52, 2280−2282. (23) Jang, E. S.; Chang, J. J.; Gwak, J.; Ayral, A.; Rouessac, V.; Cot, L.; Hwang, S. -J.; Choy, J. -H. Asymmetric High-Tc Superconducting Gas Separation Membrane. Chem. Mater. 2007, 19, 3840−3844. (24) Gwak, J.; Ayral, A.; Rouessac, V.; Cot, L.; Grenier, J. C.; Jang, E. S.; Choy, J. H. Synthesis and Characterization of Porous Inorganic Membranes Exhibiting Superconducting Properties. Mater. Chem. Phys. 2004, 84, 348−357. (25) Osada, M.; Sasaki, T.; Ono, K.; Kotani, Y.; Ueda, S.; Kobayashi, K. Orbital Reconstruction and Interface Ferromagnetism in SelfAssembled Nanosheet Superlattices. ACS Nano 2011, 5, 6871−6879. (26) Kim, T. W.; Hur, S. G.; Hwang, S. -J.; Choy, J. -H. Layered Titanate−Zinc Oxide Nanohybrids with Mesoporosity. Chem. Commun. 2006, 220−222. (27) Gunjakar, J. L.; Kim, T. W.; Kim, H. N.; Kim, I. Y.; Hwang, S. -J. Mesoporous Layer-by-Layer Ordered Nanohybrids of Layered Double Hydroxide and Layered Metal Oxide: Highly Active Visible Light Photocatalysts with Improved Chemical Stability. J. Am. Chem. Soc. 2011, 133, 14998−15007. (28) Kim, T. W.; Ha, H. -W.; Paek, M. -J.; Hyun, S. -H.; Choy, J. -H.; Hwang, S. -J. Unique Phase Transformation Behavior and Visible Light Photocatalytic Activity of Titanium Oxide Hybridized with Copper Oxide. J. Mater. Chem. 2010, 20, 3238−3245. (29) Gunjakar, J. L.; Kim, T. W.; Kim, I. Y.; Lee, J. M.; Hwang, S. -J. Highly Efficient Visible Light-Induced O2 Generation by SelfAssembled Nanohybrids of Inorganic Nanosheets and Polyoxometalate Nanoclusters. Sci. Rep. 2013, 3, 2080. (30) Jung, T. S.; Kim, T. W.; Hwang, S. -J. Mesoporous Assembly of Layered Titanate with Well-Dispersed Pt Cocatalyst. Bull. Korean Chem. Soc. 2009, 30, 449−453. (31) Kim, T. W.; Ha, H. W.; Paek, M. -J.; Paek, I. H.; Hyun, S. -H.; Choy, J. -H.; Hwang, S. -J. Mesoporous Iron Oxide−Layered Titanate

Chemistry and Nano Sciences at Ewha Womans University (Supervisor: S. -J. Hwang). Her research focuses on the synthesis and characterization of layered transition metal oxide and metal dichalcogenides for applications in photocatalysis and Li secondary batteries.

Yun Kyung Jo received a B.S. degree in chemistry (2011) and a M.S. degree in inorganic chemistry (2013) from Ewha Womans University (Korea). She is currently a Ph.D. student in the Department of Chemistry and Nano Sciences at Ewha Womans University (Supervisor: S. -J. Hwang). Her research focuses on the layered transition metal oxide and its applications in photocatalysis, supercapacitor, and Li secondary batteries.

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ACKNOWLEDGMENTS This work was supported by the Solvay, the RP-Grant 2011 of Ewha Womans University, the Global Frontier R&D Program (2013-073298) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning, and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2010-C1AAA0012010-0029065).

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