Two-dimensional transition metal dichalcogenide nanosheet-based composites.

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Two-dimensional transition metal dichalcogenide nanosheet-based composites Chaoliang Tan and Hua Zhang* Ultrathin two-dimensional (2D) nanosheets of layered transition metal dichalcogenides (TMDs), such as MoS2, TiS2, TaS2, WS2, MoSe2, WSe2, etc., are emerging as a class of key materials in chemistry and electronics due to their intriguing chemical and electronic properties. The ability to prepare these TMD nanosheets in high yield and large scale via various methods has led to increasing studies on their hybridization with other materials to create novel functional composites, aiming to engineer their chemical, physical and electronic properties and thus achieve good performance for some specific applications. In this critical review, we will

Received 21st May 2014

introduce the recent progress in hybrid nanoarchitectures based on 2D TMD nanosheets. Their synthetic

DOI: 10.1039/c4cs00182f

strategies, properties and applications are systematically summarized and discussed, with emphasis on those new appealing structures, properties and functions. In addition, we will also give some perspectives on

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the challenges and opportunities in this promising research area.

1. Introduction Since many unconventional properties have been observed in graphene,1–6 other two-dimensional (2D) inorganic nanomaterials, particularly mono- and multi-layered nanosheets of layered transition metal dichalcogenides (TMDs), such as MoS2, TiS2, TaS2, WS2, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. E-mail: [email protected]; Web: http://www.ntu.edu.sg/home/hzhang/

Chaoliang Tan received his BE degree in Applied Chemistry from the Hunan University of Science and Technology in 2009. After he got his ME degree in Applied Chemistry from South China Normal University, he moved in 2012 to the School of Materials Science and Engineering of Nanyang Technological University in Singapore where currently he is pursuing his PhD degree under the supervision of Professor Hua Chaoliang Tan Zhang. His research interests focus on the synthesis, assembly, and applications of twodimensional nanosheets (e.g. graphene and transition metal dichalcogenides) and their composites.

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MoSe2, WSe2, etc., have received increasing research interest in recent years.7–13 Featuring 2D morphology and ultrathin thickness, these TMD sheets present some unusual physical, chemical or electronic properties compared to their bulk counterparts and therefore hold great promise for a variety of applications.7–13 Until now, a wide range of strategies have been developed for the preparation of single- or few-layer TMD nanosheets, such as mechanical cleavage method, chemical vapor deposition (CVD) growth, chemical Li-intercalation and exfoliation, electrochemical Li-intercalation and exfoliation, liquid phase exfoliation and

Hua Zhang obtained his BS and MS degrees from Nanjing University in 1992 and 1995, respectively, and completed his PhD with Prof. Zhongfan Liu at Peking University 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 in 2001. After working at NanoInk Inc. Hua Zhang (USA) and the Institute of Bioengineering and Nanotechnology (Singapore), he joined Nanyang Technological University in July 2006. His current research interests focus on synthesis of two-dimensional nanomaterials and carbon materials (graphene and carbon nanotubes), and their application in nano- and bio-sensors, clean energy, water remediation, etc.

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wet chemical syntheses.7,8,14–19 Being chemically and electronically active, these 2D nanostructures have shown great potential applications in opto-electronic/electronic devices, electrocatalysis, sensors and energy storage.7,8,20–26 Engineering advanced nanocomposites by hybridization of two or more materials is the most fascinating approach to overcome the weaknesses of individual counterparts as well as optimize their performance or even potentially generate new function for practical applications.27,28 The recent success in the preparation of these 2D TMD nanosheets via various solutionprocessable and scalable techniques offers great opportunities for constructing structurally defined functional hybrid nanostructures by using them as building blocks.7,8 Graphene-based sheet structures, such as graphene oxide (GO) and reduced graphene oxide (rGO), the most studied 2D nanomaterials in the last decade, have been proved to be fascinating templates or fillers for construction of functional composites for numerous applications.27–41 Inspired by the success of graphene-based composites, a lot of effort has been devoted to the incorporation of these ultrathin TMD nanosheets with a number of materials as well as the exploration of their potential for various applications. In general, based on the roles or existing formats of these TMD nanosheets, their composite materials can be classified into three categories: (1) TMD nanosheet-templated composites, (2) in-plane 2D alloyed or doped hetero-structures, and (3) hierarchical nanostructures by synthesis or assembly of TMD sheets on, in or together with other kinds of nanostructures. In this review, we aim to give an overview of this rapidly emerging research direction and a clear understanding of the basic knowledge, challenges, and opportunities in this promising field. First of all, the synthetic methods for the preparation of TMD nanosheetbased composites are introduced. Then their potential applications in catalysis, electronic devices, batteries, and so on are described. The emphasis is placed on those interesting structures, properties or function of these hybrid nanomaterials. Finally, based on the current studies, we give some personal insights into the future research direction in this hot area.

2. Preparation of TMD nanosheet-based composites One of the most striking characteristics of 2D nanomaterials, which benefits from their morphology and ultrathin thickness, is the large specific area, making them ideal building blocks or fillers for construction of hybrid nanomaterials. Great progress has been made in the last decade in engineering functional hybrid nanostructures based on 2D graphene-based sheets (e.g. GO and rGO) to enhance their physical, chemical, and electronic properties and thus to realize good performance for various kinds of applications.27,28 As a class of newly emerging 2D nanomaterials, the exploration of hybridizing TMD nanosheets with other materials has been a subject of intensive studies in the last three years. Till now, a number of materials, such as noble metals,42–53 oxides,54–62 polymers,63–68 metal chalcogenides,71–86,97 carbonaceous materials,88–96,98–110 and so on,69,70,87 have been demonstrated

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to hybridize with TMD nanosheets to generate functional hybrid nanoarchitectures. It is noteworthy that despite that they possess a similar 2D lamellar structure as graphene, the physical (e.g. mechanical property), chemical (e.g. surface state or thermal oxidation) and electronic (e.g. conductivity or band gap) properties of TMD nanosheets are different from those of graphene sheets.7,8 For example, unlike GO or rGO that has many oxygencontaining functional groups on its surface,111,112 there are no such functional groups on the surface of TMD nanosheets.7 Therefore, the design of hybrid nanomaterials based on TMD nanosheets should be different from that of graphene-based composites. In this section, the synthetic methods for the preparation of various kinds of TMD nanosheet-composites will be introduced and discussed, with emphasis on those appealing structures and synthetic routes. 2.1.

Preparation of TMD nanosheet-templated composites

2.1.1. TMD nanosheet-templated noble metal composites. Noble metal nanostructures, a class of materials whose properties are highly dependent on their size, shape, composition and morphology, have shown great potential applications in catalysis, electronics, sensors, and biomedicine.113–115 However, two of the unavoidable barriers for noble metals are their scarcity and expensiveness when they are used for practical applications. Although great efforts have been devoted to the exploration of other costeffective and abundant materials, it still remains a big challenge to find materials that can fully replace the noble metals for certain applications (e.g. catalysis). Alternatively, the deposition of noble metal nanocrystals on other cheap or sustainable materials could be a feasible and promising way to reduce their consumption, and at the same time optimize their performance.35 Due to their large specific surface area, good thermal-stability and conductivity, graphene-based sheets, especially GO and rGO, have been demonstrated to be striking templates for directing the growth of various noble metal nanostructures for many applications.35 Inspired by these studies, the attempt to grow noble metal nanostructures on TMD nanosheets has attracted considerable attention recently.42–53 To date, several kinds of noble metal nanocrystals, including Au, Pd, Pt and Ag, have been grown on TMD nanosheets, such as MoS2, TiS2, TaS2 and WS2, by different approaches. One of the most commonly used methods for the growth of noble metal nanocrystals on TMD nanosheets is the solution-based chemical reduction with reduction agents or directly by the TMD sheets themselves. As a typical example, our group demonstrated the epitaxial growth of several noble metal nanocrystals including Pd, Pt and Ag on dispersible singlelayer MoS2 nanosheets (Fig. 1).42 Specifically, Pd nanoparticles (NPs) with a size of ca. 5 nm (Fig. 1A and B) were epitaxially grown on MoS2 nanosheets by reduction of K2PdCl4 with ascorbic acid using poly(vinylpyrrolidone) (PVP) as the surfactant.42 Pt NPs with sizes of 1–3 nm were also epitaxially grown on the MoS2 nanosheets by photochemical reduction of K2PtCl4 in the presence of sodium citrate (Fig. 1C).42 In addition to the spherical NPs, the growth of Ag triangle nanoplates was also realized on MoS2 sheets (Fig. 1E and F).42 Notably, it is the first time to realize the epitaxial growth of noble metal nanocrystals on dispersible


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Fig. 1 (A) Transmission electron microscopy (TEM) image of a Pd nanoparticle (NP) decorated MoS2 nanosheet. (B) Selected area electron diffraction (SAED) pattern of a Pd–MoS2 hybrid nanosheet. (C) TEM image of a Pt NP decorated MoS2 nanosheet. (D) TEM image of a Ag nanoplate decorated MoS2 nanosheet. (E) TEM image of a typical Ag nanoplate on a MoS2 nanosheet. (F) Fast Fourier transform (FFT)-generated SAED pattern of (E). Reproduced with permission from ref. 42. Copyright 2013, Nature Publishing Group.

nanosheets in aqueous solution via wet chemical syntheses. Unlike the growth of other noble metals, we found that Au NPs can be easily deposited on MoS2 sheets by simply mixing the HAuCl4 and MoS2 sheets in the presence of sodium citrate,42 while most of the Au NPs randomly adsorbed on MoS2 without epitaxial growth. Similarly, Huang et al. reported spontaneous deposition of Au NPs on MoS2 and WS2 nanosheets due to the direct reaction of them with HAuCl4.43 It is suggested that the high work function of MoS2 (+5.2 eV) makes its Fermi level higher than the reduction potential of AuCl4 (+1.002 V), leading to the reduction of AuCl4 by the MoS2 sheets.43 Note that Au NPs would preferentially grow at defective sites such as edges or line defects, since these sites possess high energy.43 Recently, Yang et al. presented that the selective deposition of Au NPs can also be achieved on CVD-grown MoS2 sheets at edge or defective sites by a simple solution dip casting method.44 Moreover, Berry et al. have demonstrated the incorporation of Au and Ag NPs in both mechanically and chemically exfoliated MoS2 sheets by direct reduction or microwave routes, in which the electrical, thermal, and structural properties of MoS2 sheets can be modulated and its gate capacitance can be significantly enhanced by 9-fold.45 Since the high-yield production of singlelayer TiS2 and TaS2 nanosheets has been achieved by our group, recently, we demonstrated that TiS2 and TaS2 nanosheets were successfully used as dispersible templates for the synthesis of Au and Pt NPs on their surface with high surface coverage and narrow size distribution.46 Very intriguingly, it was found that by depositing Au NPs on MoS2 monolayers, a reversible 2H and 1T phase transition in MoS2 sheets could be achieved due to the resonant excitation of plasmon modes of Au NPs. This strategy offered a facile way for phase engineering of ultrathin 2D TMD nanosheets for specific applications.47 2.1.2. TMD nanosheet-templated other composites. Besides noble metals, other materials including metal oxides (e.g. MoO3,


WO3 and Fe3O4),54–58 metal chalcogenides (e.g. PbSe and MoS2),71,72 polymers,63–68 metal–organic frameworks (MOFs),69 and organic molecules70 have also been composited with TMD nanosheets via different methods. Note that the most straightforward and effective way for preparation of metal oxide-based TMD hybrid nanostructures is to partially oxidize the TMD sheets themselves. As a representative example, our group reported the formation of MoO3 fragments on MoS2 sheets by thermal annealing of mechanically exfoliated few-layer MoS2 sheets in air.54 By annealing two-layer MoS2 at 330 1C in air, the top layer MoS2 could be oxidized to MoO3 fragments and the bottom layer MoS2 sheets still remained intact, and thus the single-layer MoS2 decorated with MoO3 was successfully obtained. Similarly, WO3 decorated WSe2 hybrid nanosheets can be prepared by using high power laser-induced oxidation of mechanically exfoliated few-layer WSe2 nanosheets under ambient conditions.55 The WO3 can form on WSe2 sheets with partial oxidation of single- to five-layer WSe2 nanosheets by laser irradiation during Raman mapping measurements. Interestingly, the as-prepared WO3 on WSe2 thin layers (i.e. single- to three-layer) has a hexagonal structure, while that formed on thick layers (i.e. four- to five-layer) has a monoclinic structure. More importantly, the oxidation sites on the WSe2 sheets can be controlled by controlling the location of laser irradiation. The aforementioned two examples used mechanically exfoliated TMD sheets as the starting materials, so they are limited for large scale production by the low yields of TMD sheets. Very recently, we developed a facile method to prepare MoS2–MoO3 hybrid nanomaterials from solution-processed lithium-exfoliated MoS2 nanosheets.56 The as-prepared MoS2 sheets were in situ oxidized by a heat-assisted spray-coating procedure in air during the film preparation, followed by thermal-annealingdriven crystallization to form hybrid nanostructures composed of (100)-dominated MoS2 and (021)-dominated a-MoO3.

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This method might be further extended to prepare other kinds of TMD–oxide hybrid nanostructures. Alternatively, a simple twostep method proposed by Joh et al. was used for the synthesis of MoO3 NP-decorated MoS2 sheets.57 First of all, H2O2 was used as an oxidizing agent to oxidize bulk MoS2 crystals to form expanded bulk MoS2 decorated with MoO3 NPs. Then the ultrathin hybrid nanosheets composed of MoO3 NP modified MoS2 sheets were obtained by ultrasound exfoliation in aqueous solution. Additionally, a two-step hydrothermal method was developed by Xue et al. to prepare MoS2 sheets decorated with Fe3O4 NPs.58 After the preparation of MoS2 sheets using Na2MoO4 and C2H5NS as precursors via the hydrothermal method, Fe3O4 NPs with a size of B3.5 nm were deposited on MoS2 sheets by a second hydrothermal step. Notably, the layer number of MoS2 sheets prepared by the hydrothermal method is difficult to be controlled, so most of the obtained MoS2 sheets are multi-layers instead of single layers. In addition, TMD nanosheet-based metal chalcogenide composites have also been prepared.71,72 For example, Zaumseil et al. developed a facile wet-chemical method for the preparation of 2D hybrid nanostructures composed of PbSe quantum dot (QD) decorated MoS2 or WS2 nanoflakes, in which the PbSe QDs less than 10 nm were epitaxially grown on the sonication-exfoliated MoS2 or WS2 flakes (Fig. 2).71 As an interesting example, Shaijumon et al. reported the fabrication of a single component heterodimensional hybrid nanostructure of MoS2 QD loaded nanosheets.72 MoS2 QDs with a diameter of 1–4 nm interspersed with fewlayer MoS2 sheets were prepared by a modified liquid exfoliation technique. Moreover, the TMD nanosheets have been composited with polymers and MOFs or used for direction of the assembly of organic molecules. Generally, hybridizing TMD nanosheets with polymers is to simply mix specific polymers with TMD sheets or their precursors together, by which the polymers can

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be functionalized on the surface of TMD nanosheets through physical adsorption or weak non-covalent chemical bonding. For instance, a simple technique was proposed by our group for the preparation of polyvinylpyrrolidone (PVP)-coated few-layer MoS2 nanosheets by direct exfoliation of MoS2 bulk crystals with the aid of PVP with sonication.63 The as-synthesized PVP– MoS2 hybrid can be easily dispersed in the low-boiling ethanol solvent, making it suitable for thin film preparation and device fabrication by a solution processing technique. This facile and feasible method could be a general approach for fabrication of polymer-based 2D TMD composites from their bulk crystals. As an alternative, Liu et al. presented a simple method for functionalization of lipoic acid conjugated polyethylene glycol (LA–PEG) on Li-intercalated WS2 and MoS2 sheets.64,65 The two S atoms in the disulfide group of LA–PEG enable its strong binding affinity to WS2 or MoS2 compared to other weak interactions such as van der waals force and physical adsorption, thereby significantly enhancing their physiological stability. In addition, very recently, we demonstrated that MOFs can be easily coated on MoS2 nanosheets by a facile approach.69 By simply mixing aqueous solution of 2-methylimidazole and zinc acetate in the presence of MoS2 sheets with mild shaking and followed by 2 h undisturbed reaction, zeolitic imidazolate frameworks (i.e., ZIF-8) could be uniformly coated on MoS2 sheets to form 2D core–shell hybrid nanostructures (Fig. 3). More importantly, this facile and general procedure can also be used to coat ZIF-8 on other 2D nanomaterials, such as GO, Pt NP-decorated rGO and MoS2 sheets. Besides, we also found that single-layer TMD nanosheets including MoS2, TiS2 and TaS2 can be used as templates for directing the assembly of organic aggregationinduced emission (AIE) molecules (4,6-di(9H-carbazol-9-yl)-N,Ndiphenyl-1,3,5-triazin-2-amine, referred to as DDTA).70 With the assistance of TMD monolayers, almost all the AIE molecules could be assembled into organic sheet structures with a size of

Fig. 2 (A) SAED pattern of the PbSe–MoS2 hybrid material. (B) Schematic illustration of the epitaxial relationship between the PbSe QDs and MoS2 flakes, showing the three equivalent orientation variants of the PbSe QDs and (C) schematic illustration of the elongation of the diffraction points in reciprocal space (diffraction pattern) by rotation of the PbSe unit cell in real space. (D) HRTEM image of the PbSe–MoS2 hybrid showing the three orientation variants marked in red, green and yellow and (E) enlarged HRTEM images of the QDs at positions 1, 2, and 3 of panel (D). (F) SAED pattern of PbSe QDs simply mixed with exfoliated MoS2 flakes. Reproduced with permission from ref. 71. Copyright 2014, John Wiley & Sons, Inc.

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Fig. 3 (A) TEM image of MoS2@ZIF-8 hybrid structures. (B) TEM image of a curled MoS2@ZIF-8 structure, showing the MoS2 nanosheet and ZIF-8 coating. (C) STEM image and the corresponding energy-dispersive X-ray spectroscopy (EDS) mapping of a typical MoS2@ZIF-8 hybrid structure. Reproduced with permission from ref. 69. Copyright 2014, American Chemical Society.

Fig. 4 TEM images of DDTA (2 mM) in the presence of (A) MoS2, (B) TiS2 or (C) TaS2 at the concentration of 1.6 mg mL 1. (D) AFM height image and (E) height profiles of DDTA (2 mM) in the presence of MoS2 (1.6 mg mL 1) in the mixture of THF and H2O (v : v = 1 : 99). Reproduced with permission from ref. 70. Copyright 2014, John Wiley & Sons, Inc.

about 0.2–2 mm and a thickness of about 9–20 nm (Fig. 4).70 More importantly, the fluorescence intensity of the AIE fluorophore can be significantly enhanced, rather than quenched by these single-layer TMD nanosheets. 2.2. Preparation of in-plane alloyed or doped 2D TMD heteronanosheets In-plane hybrid nanosheets such as alloyed or doped heteronanosheets are a class of hybrid nanomaterials that can keep the morphology, topology and crystalline feature without phase separation of 2D nanomaterials.116,117 These unique advantages make them suitable for engineering the intrinsic physical or electronic properties of 2D nanostructures, such as defects,


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band gap, conductivity, etc. To date, several methods have been used to fabricate the in-plane hybrid TMD nanosheets such as alloyed (e.g. MoS2(1 x)Se2x and Mo1 xWxS2)73–83 and doped (e.g. H-TiS2, O-MoS2 and S-MoSe2)84–87 hetero-structures, in order to modulate their band gaps or conductivity, or achieve higher electrocatalytic activity. Notably, until now, all the reported methods for production of alloyed TMD sheets are based on the CVD-growth, by which high-quality and large-size alloyed single- or multi-layers can be obtained. As a typical example, Suenaga et al. reported the preparation and characterization of alloyed TMD monolayers.73 Single crystals of Mo1 xWxS2 monolayers were prepared by using Mo, W and S elements as precursors via the CVD-growth.73 The crystal structure and atom distribution of single-layered Mo1 xWxS2 were characterized and visualized using an atomically resolved scanning transmission electron microscope (STEM) and the ratio of alloyed Mo or W was also successfully quantified. This study provided a reliable way for the synthesis and characterization of alloyed TMD monolayers. As an another example, Xie et al. prepared a series of TMD monolayer alloys of Mo1 xWxS2 (x = 0 to 1 with a step of B0.1) by cleavage of their bulk crystals.74 The band gap of Mo1 xWxS2 monolayers is tunable by the W component, which is evidenced by both PL experiments and DFT simulations. It was found that these 2D alloys exhibited first-order Raman modes and second-order Raman modes in the range of 100–480 cm 1 when these hybrid nanosheets were systematically investigated by Raman spectroscopy.75 Alternatively, Liu et al. reported the synthesis of Mo1 xWxS2 alloys by sulfurizing as-prepared Mo1 xWx thin films through the CVD-process, and the obtained alloy thin films were about 50 nm on a lateral scale and 2–4 layers in thickness.76 Similarly, the preparation of single-layer MoS2(1 x)Se2x has also been achieved through similar CVD-growth strategies.77–83 For example, Ajayan et al. developed a one-step, direct synthesis of atomic single- and bi-layer MoS2(1 x)Se2x with tunable compositions (Fig. 5).77 The preparation of isolated triangular single-crystal sheets as well as continuous films was realized by tuning the growth parameters. Similar to Mo1 xWxS2, by controlling selenium doping of MoS2, the optical band gap could be continuously tuned by over 200 meV. Very recently, Li et al. systematically investigated the two typical ways for the preparation of monolayer MoS2xSe2(1 x) alloys, i.e. selenization of MoS2 and sulfurization of MoSe2.82 It was demonstrated that S atoms can be randomly and then homogeneously replaced by Se atoms in the selenization of MoS2, while S atoms preferentially replaced the Se atoms along the crystalline orientation of MoSe2 to form biphases in the sulfurization of MoSe2. Therefore, the optical band gaps of the monolayer MoS2xSe2(1 x) alloys prepared by different methods are much different. This study offered a simple strategy to control the atomic structure of monolayer TMD alloys to achieve a tunable band gap. Besides the CVD method, high temperature solid state reaction has also been reported for the preparation of few-layer MoS2(1 x)Se2x alloys, in which the ratio of Se and S can be easily controlled by tuning their atomic ratio in precursors.83 Besides alloyed nanosheets, doping another element in the TMD sheets is an alternative way to prepare in-plane

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Fig. 5 Atom-by-atom dopant analysis in monolayer MoS2. (A) Annular dark field (ADF) image of pristine MoS2. (B) ADF image of Se-doped MoS2 (Se ratio of 12%). (C) Higher magnification ADF image showing Se dopants. The green and cyan circles highlight single- and double-Se substituted S2 sites, respectively. (D) Comparison of experimental and simulated image intensity line profiles from single- and double-Se substitution at S2 sites. (E) Siteseparated image intensity histogram analysis of pristine and Se-doped MoS2 monolayers. (F) Structure model obtained from histogram analysis showing the distribution of single- and double-Se substituted S2 sites. Reproduced with permission from ref. 77. Copyright 2014, American Chemical Society.

doped heterostructures.84–87 Typically, Xie et al. developed a facile one-pot hydrothermal method for the preparation of oxygen incorporated MoS2 sheets.84 The band gap of MoS2 can be reduced by incorporation of oxygen in it, and thus its intrinsic conductivity can be enhanced. Ultrathin hydric titanium disulfide (H-TiS2) nanosheets were also successfully prepared by the same group through an Li-intercalation and exfoliation method.85 It has been proved that the conductivity of TiS2 sheets can also be significantly enhanced by hydrogen incorporation, making it a promising candidate as electrode material for future electronics. Recently, Yan et al. demonstrated the synthesis of ultrathin S-doped MoSe2 nanosheets by using Se, S and MoCl5 as precursors via a simple wet-chemical synthesis.86 The experimental data suggested that the catalytically active sites and electrical conductivity of MoSe2 sheets can be increased by the S doping in the backbone. As an interesting example, Suenaga et al. reported that Re or Au (0.5–1 at%) atoms can be doped in the host MoS2 lattice by the chemical vapor transport (CVT) method.87 All the aforementioned results indicated that incorporation of another element in the backbone of TMD sheets to form doped in-plane hybrid nanosheets is an effective way for tuning their intrinsic properties, such as band gap, conductivity, and/or defects, to realize better performances for diversified applications. 2.3. Preparation of hierarchical hybrid nanostructures based on 2D TMD nanosheets Some of the attractive properties or functions of ultrathin 2D nanosheets profit from their large specific surface area. However, when they are used as electrodes or thin films for device

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applications, a significant decrease of their effective surface area occurs. Fortunately, the synthesis or assembly of 2D nanosheets on, in or with other nanostructures, such as one-dimensional (1D) nanofibers, nanobelts, or nanotubes, 2D nanosheets or threedimensional (3D) networks, to construct hierarchical hybrids is an effective strategy to retain their large specific surface area to a certain degree. Bearing this in mind, TMD sheets have been synthesized or assembled onto, with or in several nanostructures such as TiO2 nanobelts/tubes, carbon nanotubes (CNTs), carbon fibers, graphene and 3D graphene, to form various hybrid nanostructures for a wide range of applications.59–62,88–110 The most commonly used materials for hybridization with TMD nanosheets to form hierarchical nanostructures are carbonaceous nanomaterials, including graphene, CNTs and carbon fibers. Chemically converted graphene-based sheets, such as GO and rGO, possess abundant oxygen-containing groups that can facilitate the immobilization of other components grown on their surface, making them promising templates for the preparation of composites.111,112 The synthesis or assembly of layered TMD sheets on or with graphene to form TMD–graphene composites has been reported by several groups.88–96,98–100 The most popular strategy used for the preparation of TMD–graphene composites is the hydrothermal method. As a typical example, Chen et al. developed a facile process for the synthesis of layered MoS2–graphene composites by an L-cysteine-assisted hydrothermal method with subsequent annealing treatment by using sodium molybdate, GO and L-cysteine as precursors (Fig. 6A–C).88 Besides MoS2 sheets, the growth of WS2 sheets on graphene has also been explored recently. For instance, a one-pot hydrothermal reaction


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Fig. 6 (A) Scanning electron microscopy (SEM), (B) TEM and (C) HRTEM images of the MoS2–graphene composite. Reproduced with permission from ref. 88. Copyright 2011, American Chemical Society. (D) TEM image of as-prepared WS2–rGO hybrid nanosheets and (E) magnified image of the marked area in (D). The inset in (D) indicates the diffraction pattern of WS2. (F) HTREM image of folded edges of WS2 sheets on rGO. The inset shows a magnified image of some folded edges of WS2 sheets. Reproduced with permission from ref. 93. Copyright 2014, John Wiley & Sons, Inc.

process at low temperature was developed by Chhowalla et al., which was used for the synthesis of WS2 nanosheets with tungsten chloride and thioacetamide as precursors to functionalize rGO (Fig. 6D–F).93 3D graphene networks grown by CVD have been proved to be attractive substrates to accommodate various materials for preparing functional composites.118,119 Recently, layered MoS2 flakes were grown on 3D graphene networks by a CVD method from (NH4)2MoS4, and the resultant composite can be directly used as an anode for Li ion batteries.94 In addition to the synthesis of TMD sheets by using precursors, the layer-by-layer assembly of TMD sheets of various kinds in the as-prepared state

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or together with GO or rGO is a promising alternative approach for fabrication of TMD sheet-based hybrid nanostructures. The amount of each component is easily controlled during the assembly process. As an example, Singh et al. reported that the layered free-standing MoS2–GO paper can be prepared by a simple vacuum filtration of as-prepared GO and acid-exfoliated few-layer MoS2 dispersed in solution, and the MoS2–rGO paper was obtained after the subsequent thermal reduction.96 Almost at the same time, layer-by-layer stacking of MoS2 and WS2 composites was also achieved via a similar method by Rajamathi et al.97 In addition, CNTs, carbon fibers/tubes and amorphous carbon have also been widely used as templates or matrices for growth of or hybridization with TMD nanosheets.101–109 For example, a simple glucose-assisted hydrothermal method was developed by Lou et al. for the growth of MoS2 nanosheets on the CNT surface using sodium molybdate hexahydrate and thiourea as precursors (Fig. 7A–C).101 Alternatively, another two-step method was developed for the preparation of MoS2 sheet-coated multiwalled CNTs. First of all, the intermediate product, i.e. MoS3, was synthesized on acid-treated multi-walled CNTs by simple mixing of (NH4)2MoS4 and HCl in CNT suspension.102 After washing with distilled water and drying in air, the MoS2–CNT composite was obtained by annealing the MoS3–CNT sample at 650 1C for 1 h in a H2 (10%) flow. As an another example, a simple dissolution and sintering method was developed to prepare the ultrathin MoS2 nanosheet-coated active carbon fibers.104 The as-prepared active carbon fibers were immersed in (NH4)2MoS4 containing dimethylformamide (DMF) solution (1.25 wt%) and then the obtained sample was annealed at 750 1C for 2 h in a 5% H2/Ar atmosphere. Besides TMD nanosheet-coated hybrid nanostructures, MoS2 nanosheet-embedded carbon nanofibers or nanotubes have also been successfully prepared.105–107 As a typical example, Maier et al. developed a simple electrospinning

Fig. 7 (A) TEM image of the MoS2–CNT hybrid nanostructures. (B) TEM image of a single MoS2–CNT hybrid nanostructure. (C) HRTEM image of MoS2 nanosheets. Reproduced with permission from ref. 101. Copyright 2011, John Wiley & Sons, Inc. (D) SEM, (E) TEM and (F) HRTEM images of MoS2 sheet-decorated TiO2 nanobelts. Reproduced with permission from ref. 59. Copyright 2013, John Wiley & Sons, Inc.


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process for fabrication of single-layer MoS2 sheet-embedded carbon fibers.106 In their experiment, (NH4)2MoS4 and PVP with desired concentrations were dissolved in DMF to form a homogeneous solution, and then the as-prepared solution was used to produce fibers consisting of the precursors by a electrospinning process. The small-size, single-layered MoS2 sheet-embedded carbon fibers were obtained by pyrolysis of the as-synthesized hybrid electrospun fibers at 450 1C for 2 h under a H2/Ar (5 vol%/95 vol%) atmosphere and then at 800 1C for 6 h under Ar, since (NH4)2MoS4 and PVP were decomposed to MoS2 monolayers and carbon matrix, respectively. This method provides a facile and effective way for in situ preparation of TMD nanosheet-embedded composites with a homogeneous distribution and controllable components. Alternatively, another similar two-step electrospinning process was proposed by Lu et al. for the preparation of MoS2 nanoflake-embedded carbon fibers, in which the MoS2 flakes were first prepared by a hydrothermal method.107 In addition to binary composites, the synthesis of TMD nanosheetbased ternary hybrid nanostructures has also been reported recently. Ye et al. presented the fabrication of CdS NP-loaded MoS2–graphene hybrid nanostructures by a solution-chemistry method. Specifically, the MoS2–graphene hybrid was first prepared by a hydrothermal method and then used as a template for the growth of CdS NPs to form ternary composites.110 More interestingly, the synthesis of TMD nanosheets on TiO2 nanobelts/tubes, SnO2 nanotubes, and SrTiO3 single crystals has also been achieved via hydro/solvothermal methods or CVD growth.59–62 For example, coating few-layer MoS2 sheets on TiO2 nanobelts was realized by our group by a hydrothermal reaction with Na2MoO42H2O and C2H5NS as starting materials (Fig. 7D–F).59 Unlike GO or rGO, the pristine TiO2 nanobelt has a very smooth surface, and therefore acid treatment is required to create the rough surface that offers rich nucleation sites for the growth of MoS2 sheets. Similarly, MoS2 nanosheets have also been grown on TiO2 and SnO2 nanotubes by a similar solvothermal process.60,61

3. Applications of TMD nanosheetbased composites The aim of engineering functional composites with welldefined structures, morphology, crystal phase and composition is to modulate or optimize their physical, chemical, and/or electronic properties, thereby achieving better performance for specific applications. Actually, most of the resultant TMD nanosheet-based hybrid nanostructures do present advances in a wide range of applications including electrocatalysis,42,43,46,49,61,62,84,86,93,95,109 electronic devices,48,54,56,63,69,71,99 Li or Na ion batteries,58,60,88–94,96–98,100–103,105–108 photocatalysis,48,59,110 biomedicine,64–67 and so on.51,52,57,104 In this section, we will focus on the discussion about the potential of the aforementioned prepared composites for a number of applications, with a highlight on those composites with outstanding performance or new function.

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3.1.

Electrocatalysis

The global energy shortage and environmental deterioration has driven an intensive search for sustainable and clean energy. Being a clean, efficient, and durable energy fuel, hydrogen generated by water splitting is one of the most promising alternatives for future energy.120 Pt-based catalysts are the most efficient electrocatalysts for hydrogen evolution reaction (HER).121 Due to the high cost and scarcity of Pt, great efforts have been devoted to the exploration of earth-abundant materials with high catalytic activity in order to replace Pt-based catalysts. Among all the studied alternatives, TMD-based nanomaterials, especially MoS2 and WS2 nanosheets, have been demonstrated to be one of the most promising non-precious electrocatalysts for HER.122–125 Two key factors of TMD nanosheets for HER are the conductivity and effectively active sites, i.e. better conductivity and more active sites result in higher catalytic activity. To this end, a variety of strategies have been developed to engineer functional hybrid nanomaterial based TMD nanosheets, in order to optimize their conductivity and/or enrich active sites. For example, Xie et al. reported that oxygen incorporation in the MoS2 sheets can enhance their intrinsic conductivity.84 Moreover, the introduction of oxygen in the MoS2 crystals can induce structural disorder that can offer abundant unsaturated sulfur atoms as active sites. Therefore, such oxygen incorporated MoS2 hybrid nanosheets exhibited higher activity for HER compared to pure MoS2 sheets. As an alternative, Yan et al. presented that the introduction of S element in MoSe2 nanosheets to form S-doped MoSe2 hybrid nanosheets, referred as S-MoSe2, can also improve the intrinsic electrical conductivity of the composite and thus lead to higher catalytic activity for HER.86 Moreover, the unsaturated Se-edge in MoSe2 was found to be electrocatalytically active and beneficial for the HER process. Interestingly, although graphene is chemically inert for HER, the growth of TMD nanosheets on graphene can significantly enhance the catalytic activity because of the excellent conductivity.93,95 As a typical example, the WS2 sheet-decorated rGO composite presented much higher activity for HER compared to pure WS2 sheets.93 The better performance is attributed to the formation of an interconnected conducting rGO network, leading to the rapid electron transfer from the electrode to the catalyst. Another possible reason for the enhanced activity of the WS2–rGO composite might arise from the random orientation of WS2 sheets aligned on rGO surface which can avoid the retaking of WS2 sheets to some extent during the electrode fabrication, thus ultimately keeping the effectively active sites of the catalyst. In a similar fashion, the functionalization of MoS2 sheets on rGO surface to form MoS2–rGO composites has also been proved to be a good way to achieve highly active catalysts for HER.95 The enhanced electrocatalytic activities of HER can also be achieved by growth of TMD nanosheets on other substrates, such as amorphous carbon, SnO2 nanotubes and SrTiO3 single crystals.61,62,109 As for the catalytic activity, although optimized performance has been realized through the aforementioned strategies, the catalytic efficiency of the hybrids still cannot be compared with


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that of the commercial Pt/C catalyst. Recently, our group demonstrated that the combination of Pt NPs with TMD nanosheets is an attractive way for the implementation of high catalytic efficiency for HER.42 We discovered that Pt–MoS2 hybrid sheets, obtained by epitaxial growth of well-dispersed 1–3 nm Pt NPs on single-layer MoS2, exhibited excellent catalytic activity towards HER, which is higher than that of the commercial Pt/C catalyst on the basis of same Pt loading (Fig. 8A and B). The excellent performance of this Pt–MoS2 composite may be ascribed to the strong coupling effects between Pt NPs and MoS2 sheets. The exposed high-index facets of Pt NPs induced by epitaxial alignment on MoS2 surface may also partially contribute to the high activity (Fig. 8C–F). Additionally, the large specific surface area of the 2D hybrid nanostructure makes the electron transfer efficient, thereby further enhancing its catalytic activity. It has also been found that Pt-based TiS2 and TaS2 composites from the growth of Pt NPs on their single layers also exhibited much enhanced activities for HER, in which the catalytic activity of Pt/TiS2 is close to that of the commercial Pt/C catalyst.46 As an interesting example, Huang et al. reported that, despite that Au is chemically inert for HER, the deposition of small amounts of Au NPs on MoS2 or WS2 sheets can improve their catalytic activities in HER.43 In addition to the HER, TMD sheets functionalized with noble metals also hold promise for other electrocatalytic reactions. As an interesting example, Wang et al. reported that the Pd–MoS2 composite resulting from the growth of Pd NPs on MoS2 sheets could be a good electrocatalyst for methanol oxidation.50 Notably, the resultant catalyst showed higher catalytic activity compared to the commercial Pd/C catalyst. The good conductivity of 1T-phase and the large surface area of MoS2 sheets could contribute to the excellent catalytic performance. Moreover, the strong coupling effect between MoS2 sheets and homogeneously decorated Pd NPs may also contribute to the enhanced catalytic activity.

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3.2.

Electronic devices

Single- or few-layer TMD nanosheets, especially the direct band gap single-layer MoS2, have been demonstrated to be very attractive for fabrication of electronic devices, such as transistors and phototransistors. Recently, hybrid nanostructure based TMD sheets have been extensively studied for electronic device applications.48,54,56,63,69,71,99 For example, our group first demonstrated the fabrication of single-layer MoS2 based phototransistors.23 Intriguingly, Chen et al. demonstrated that the photocurrent of MoS2-based phototransistors can be significantly increased by deposition of Au NPs on the MoS2 sheet, giving maximum photocurrent enhancement at the wavelength corresponding to the Au NP plasmon resonance.48 The localized surface plasmon in Au NPs gives rise to enhancement of the local optical field in the vicinity of Au NPs and thus the light absorption of the underneath MoS2 layer, resulting in wavelength-dependent photocurrent enhancement of the MoS2 device. Moreover, these individual localized surface plasmon oscillations in neighboring Au NPs can effectively couple together to further enhance the photocurrent. Importantly, Zaumseil et al. reported that the solution-processed PbSe QD-deposited MoS2 flakes can be used to fabricate a photodetector with a near-infrared photosensitivity with long-term stability in air (Fig. 9).71 Of note, even after weeks of storage in air and under illumination, no degradation of its performance was observed on these devices. The PbSe–MoS2 based device also exhibited excellent mechanical stability on a PET substrate upon repeated bending. Recently, our group demonstrated that the field-effect transistor (FET) with channel material of MoO3-modified MoS2 nanosheets presented a p-type semiconducting property.54 This electronic behavior is much different from that of the pristine singlelayer MoS2-based FET device that shows an n-type semiconducting property. The conversion of n-type to p-type of MoS2 sheets can be attributed to the doping effect of MoO3, which acts as the hole injection layer. This example provides a simple and

Fig. 8 (A) Polarization curves of Pt/MoS2, Pt/C and MoS2. (B) The corresponding Tafel plots. (C–F) HRTEM images of (101)-oriented Pt NPs on MoS2 (scale bar, 2 nm). Reproduced with permission from ref. 42. Copyright 2013, Nature Publishing Group.


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Fig. 9 (A) Schematic illustration of a PbSe–MoS2 photodetector. (B) Photoresponse of a PbSe–MoS2 (black line) and MoS2 (red line) photodetector to incoming light (l Z 1200 nm). The ON–OFF switching time of the light source was 10 s, Vbias = 14 V. (C) 48 cycles of repeated ON–OFF switching of a PbSe–MoS2 photodetector showing excellent stability in air without degradation. The ON–OFF switching time of the light source was 5 s, Vbias = 10 V. (D) Photoresponse of a PbSe–MoS2 photodetector at different voltages, varied from 4 V to 18 V in steps of 2 V. Reproduced with permission from ref. 71. Copyright 2014, John Wiley & Sons, Inc.

effective way for modulation of the electronic properties of the TMD nanosheets. Recently, our group reported that PVP-coated MoS2 sheets can be used as an active layer for flexible memory devices by using the rGO film as the conductive bottom electrode, displaying a nonvolatile rewritable memory effect with a switching threshold voltage of B3.5 V and an ON/OFF ratio of B102.63 It is worth pointing out that MoS2 sheets themselves are semiconductors and have no switching behavior. The dielectric PVP functionalized on MoS2 surface played a crucial role in the device and the electrical switching effect is attributed to the charge trapping and detrapping behavior of MoS2 in the PVP. Alternatively, a hybrid thin film by simply mixing MoS2 and GO sheets has also been successfully applied as the active layer for rewritable memory devices, where the GO insulator serves as the dielectric material in the system.99 Although MOFs have been considered inert in electronic devices, it has been demonstrated that MoS2@ZIF-8 core–shell hybrid nanosheets can be used as an active layer for the fabrication of flexible memory devices (Fig. 10A and B).69 Unlike previous MoS2-based memory diodes, the MoS2@ZIF-8 exhibited a write-once-read-many-times (WORM) switching behavior rather than a flash memory effect. More importantly, the ON/OFF ratio of this device is as high as 7.0 104, which is much higher than that of MoS2-based memory devices hybridized with PVP63 or GO.99 In addition, it has been demonstrated that MoS2–MoO3 hybrid nanostructures can be used to fabricate light-emitting diodes (LEDs) with a configuration of Au/Ti/n-SiC/p-MoS2–MoO3/ ITO/glass with intense light emission (Fig. 10C and D).56 The turn-on voltage of this diode is approximately 4.5 V. Its current

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could reach about 15 mA when the applied forward bias was increased to 10 V. It was found that the MoS2–MoO3 hybrid nanostructure acts as an efficient p-type hole injection layer in the device. Based on the fitting of its spectrum taken at 18 V, the electroluminescence (EL) spectra display broad emission profiles with four sub-bands located at l = 411, 459, 553, and 647 nm, respectively. In addition to the aforementioned TMD nanosheet-based hybrid nanomaterials, a new class of hybrid nanostructures, prepared by simple stacking of various 2D materials, such as graphene, hexagonal boron nitride (h-BN) and TMDs, in the vertical or lateral directions via van der Waals force, have been the subject of intensive studies very recently.131 It has been proved that unusual properties and new phenomena can be achieved using this kind of hetero-structures for electronic devices.132–135 However, in this review, we do not cover the studies on this kind of heretostructures for electronic devices, since two recent excellent reviews focusing on this topic have been published.136,137 3.3.

Batteries

Lithium-ion battery (LIB) is one of the most attractive rechargeable batteries that is widely used for powering electronic devices in our daily life.126–128 The ultrathin thickness and lateral morphology of the 2D TMD nanosheet contribute to its high surface-to-volume ratio and short diffusion path, rendering it a brilliant electrode material for LIBs.129,130 Due to the relative low conductivity, low theoretical specific capacity and easy restacking character of the TMD sheet, the hybridization of it with other materials, such as graphene, 3D graphene networks, CNTs, Fe3O4 NPs, TiO2 nanotubes, and carbon fibers, is one of


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Fig. 10 (A) The current–voltage (I–V) characteristics of the MoS2@ZIF-8 based flexible memory device. Inset: the schematic of the device. (B) The retention-ability test of the memory device at a reading voltage of 0.5 V in the ON and OFF states. Reproduced with permission from ref. 69. Copyright 2014, American Chemical Society. (C) The I–V curve of LEDs with the device configuration Au/Ti/n-SiC/p-MoS2–MoO3/ITO/glass. Inset: lighting photo of the LED taken at 18 V. (D) EL spectra of the LED device biased at different forward voltages. Inset: fitting of sub-bands for the EL spectrum taken at 18 V. Reproduced with permission from ref. 56. Copyright 2014, John Wiley & Sons, Inc.

the most attractive strategies to overcome its weaknesses and thus further optimize its performance in LIBs.58,60,88–94,97,98,101–103,105–107 The results of these TMD nanosheet-based composites used as anodes for LIBs are summarized in Table 1. The most studied materials that are used to hybridize with TMD sheets for LIBs are graphene-based nanomaterials. For example, Chen et al. reported that hybrids of few-layer MoS2 coated graphene sheets prepared via a hydrothermal method

Table 1

were used as an anode for LIB.88 This MoS2–rGO composite gave a high specific capacity of ca. 1100 mA h g 1 at a current density of 0.1 A g 1, which is higher than that of pure MoS2 sheets and graphene sheets. It also showed an excellent stability even after 100 cycles. The highly conductive rGO sheets significantly enhanced the conductivity of the hybrid electrode, and also served as a matrix to accommodate volume changes during the charging and discharging process. Until now, several other reports have

List of TMD nanosheet-based composites used for LIBs and their performances

Materials MoS2 sheets grown on graphene MoS2 sheets grown on graphene MoS2 sheets grown on amorphous carbon MoS2 sheets grown on graphene WS2 sheets grown on graphene MoS2 flakes grown on 3D graphene MoS2 sheets grown on graphene MoS2 sheets grown on CNT MoSX sheets grown on CNT MoS2 sheets grown on carbon fibers MoS2 sheets embedded in graphitic carbon nanotubes MoS2 monolayers embedded in carbon fibers MoS2 flakes embedded in carbon fibers Fe3O4 NPs grown on MoS2 sheets MoS2 sheets grown on TiO2 nanotubes

Preparation method Hydrothermal synthesis with thermal annealing Hydrothermal synthesis with thermal annealing Hydrothermal synthesis with thermal annealing Hydrothermal synthesis Hydrothermal synthesis CVD growth Lithiation assisted exfoliation and hydrazine vapour reduction Hydrothermal synthesis Solvothermal synthesis Wet chemical synthesis with thermal annealing Electrospinning process with thermal annealing Electrospinning process with thermal annealing Hydrothermal and electrospinning process with thermal annealing Hydrothermal synthesis Solvothermal synthesis


Capacity and cyclic performance 1

1

1187 mA h g at 0.1 A g after 100 cycles; B900 mA h g 1 at 1 A g 1 after 50 cycles 940–1020 mA h g 1 at 0.1 A g 1 after 100 cycles 912 mA h g

1

1290 mA h g 905 mA h g 1 877 mA h g 1 915 mA h g 1

at 0.1 A g 1

1

at 0.1 A g at 0.1 A g 1 at 0.1 A g 1 at 0.5 A g 1

1

at 0.5 A g

88 89

after 100 cycles

90

1

91 92 94 98

after 50 cycles after 100 cycles after 50 cycles after 700 cycles

698 mA h g 1 at 0.1 A g 1 after 60 cycles B1000 mA h g 1 at 0.05 A g 1 after 45 cycles 917 mA h g 1 at 0.1 A g 1 after 90 cycles 1150 mA h g

Ref.

1

after 100 cycles

101 103 104 105

1007 mA h g 1 at 1 A g 1 after 100 cycles; 661 mA h g 1 at 10 A g 1 after 1000 cycles 1150 mA h g 1 at 0.05 A g 1 after 100 cycles

107

1200 mA h g 1 at 0.5 A g 1 after 560 cycles 472 mA h g 1 at 0.1 A g 1 after 100 cycles

58 60

106

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demonstrated the preparation of MoS2–rGO or WS2–rGO composites for LIBs with improved capacities and/or cycling stabilities (Table 1).89,91,92,98 Note that the MoS2–rGO composite prepared by the combination of a lithiation-assisted exfoliation process and a hydrazine monohydrate vapor reduction method displayed excellent cycle life and its specific capacity could still be maintained at 915 mA h g 1 at 0.5 A g 1 even after 700 cycles. In addition to 2D graphene sheets, 3D graphene networks have also been used as templates for the growth of MoS2 flakes by the CVD method in our group.94 The resultant MoS2-coated 3D graphene composite can be directly used as an anode in a LIB cell and it showed enhanced specific capacity and cycling stability. Besides graphene, hybrid nanostructures from the growth of TMD nanosheets on or in other kinds of carbonaceous nanomaterials, such as CNTs and carbon fibers/tubes, have also been applied as electrodes for LIBs, and most of them showed enhanced specific capacity and/or cycling stability compared to their individual counterparts (Table 1).90,101,103,104 Similar to graphene, the good conductivity of CNTs or carbon fibers and the high specific surface area of TMD nanosheets grown on them could be attributed for the improved performance. In addition, the composites of Fe3O4 NP-decorated MoS2 sheets and MoS2 nanosheet-coated TiO2 nanotubes have also been prepared and applied as anodes for LIB with enhanced performances (Table 1).58,60 However, all the TMD sheets prepared in the aforementioned composites are multilayers instead of single layers, leading to the much reduced surface area for lithium storage.

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Besides, the aggregation of TMD or graphene sheets during the electrode fabrication and structure collapse during the discharging and charging process are inevitable, which are harmful to their performance. Very recently, hybrid nanofibers of small MoS2 monolayers homogeneously embedded in carbon fibers have been successfully prepared as an anode for LIB (Fig. 11A).106 Amazingly, such a novel hybrid nanostructure exhibited a fascinating rate performance and cycling stability for LIB.106 Its initial discharge and charge capacities are 1712 and 1267 mA h g 1 at 0.1 A g 1, respectively (Fig. 11B). More impressively, the capacity of composite fibers can be maintained at 661 mA h g 1 even after 1000 cycles at a very high current density of 10 A g 1 (Fig. 11C and D). Generally, for most of the nanostructures used for LIBs, although the high initial discharge and charge capacity can be achieved, their cycling performances are not good enough because the structure expansion or collapse normally occurs during the Li-ion diffusion in them during the discharging and charging processes, particularly at a high current density, resulting in a dramatic decrease of their capacities during the cycling performances. Obviously, the excellent rate performance and cycling stability for LIB of this hybrid nanofiber can be ascribed to its unique nanostructure. First, the atomic thickness feature of randomly well-dispersed MoS2 sheets makes this composite very efficient of Li-ion storage and beneficial to prevent the Ostwald ripening and coarsening. Second, the excellent mechanical property of carbon fibers wrapped around the

Fig. 11 (A) Schematic representation based on TEM modeling studies to demonstrate the unique morphology of a composite of single-layered ultrasmall MoS2 nanosheets embedded in a thin carbon nanofiber. The large black sphere, small black sphere, and white sphere correspond to Mo, C, and S, respectively. (B) Charge and discharge voltage profiles for the first three cycles at 0.1 A g 1 of the MoS2–carbon fiber composite. (C) High rate performance of the MoS2–carbon fiber composite. (D) Excellent cycling performance of the MoS2–carbon fiber composite. Black sphere: charge; gray sphere: discharge. Reproduced with permission from ref. 106. Copyright 2014, John Wiley & Sons, Inc.

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MoS2 nanosheets eliminates the expansion (shrinkage) problems during charging (discharging), making them very stable during cycling performances. Last, the good conductivity and 1D character of the carbon matrix that is perfectly coupled with MoS2 sheets allow an easier transport for Li ions. As a promising alternative for LIB, sodium-ion battery (SIB) is receiving increasing attention in recent years due to the relative abundance of sodium compared to lithium. Recently, TMD nanosheet-based composites used for SIBs have also been explored.96,100,106,108 Surprisingly, the aforementioned unique hybrid nanoarchitecture composed of single-layer MoS2 sheets embedded in carbon fibers also showed excellent performance when it was used as an anode for SIB, which is the best among all the reported anode materials for SIBs.106 The specific capacities of this composite are 854 and 623 mA h g 1 at current densities of 0.1 and 1 A g 1, respectively. Its specific capacity can reach up to 331 mA h g 1 even at a current density as high as 10 A g 1. More intriguingly, it also exhibited excellent cycling performance for sodium storage. Its specific capacity still remained at 484 mA h g 1 at a current density of 1 A g 1, and the value was 253 mA h g 1 even at a very high current density of 10 A g 1 after 100 cycles. This striking performance of such hybrid nanomaterial makes it an ideal candidate as anode material for SIBs in real application. In addition, the hybrid nanostructures resulting from the incorporation of MoS2 or WS2 with graphene have also been used as anode materials for SIBs with enhanced performance.96,100,108 3.4.

Photocatalysis

It has been demonstrated that some of the TMD nanosheetbased composites could be promising photocatalysts for water splitting and photodegradation.48,59,110 For example, our group reported that the decoration of a small amount of Au NPs on MoS2 nanosheets can significantly enhance their light energy absorption due to the plasmon resonance of Au NPs, which is confirmed by monochromated electron energy-loss spectroscopy (EELS) in a STEM.48 The enhanced light energy absorption is expected to give enhanced photocatalytic activity of MoS2. When deposited on fluorine doped tin oxide (FTO) and used as a photoanode for photoelectrochemical cells (PECs), the Au–MoS2 hybrid sheets with a small Au loading (B3%) presented a much higher photocurrent (370 mA cm 2) at 0.8 V compared with that of pure MoS2 sheets (100 mA cm 2), which can be further enhanced to 790 mA cm 2 after heat treatment at 350 1C, indicating the high potential of the composite as a photocatalyst for water splitting. It has also been reported that the heterostructure of MoS2 nanosheet decorated TiO2 nanobelts is a multifunctional photocatalyst for hydrogen production and dye degradation.59 The photocatalytic activity of MoS2–TiO2 composites for hydrogen production is dependent on the loading amount of MoS2 sheets, and the highest performance was obtained when the loading amount of MoS2 sheets is B50 wt%. The hydrogen production rate of the resultant hybrid (50 wt% of MoS2) is as high as 1.6 mmol h 1 g 1, which is better than that achieved with pure TiO2, MoS2 and other loading amounts of MoS2. More importantly, the hybrid


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nanostructure displayed a strong adsorption ability and an enhanced photocatalytic degradation rate towards organic dyes (e.g. Rhodamine B). The large specific surface area of 2D MoS2 sheets and the strong coupling effect between MoS2 sheets and TiO2 belts could contribute to the good adsorption and photocatalytic degradation properties, respectively. Very recently, Ye et al. demonstrated that the ternary CdS–MoS2–graphene composite could be used as a highly efficient photocatalyst for hydrogen generation.110 As shown in Fig. 12, the CdS/MoS2/ graphene hybrid catalyst gave the highest photocatalytic activity for water splitting in comparison with CdS, CdS–MoS2 and CdS–graphene in lactic acid solution. It is worth pointing out that the catalytic rate of the ternary composite is about 1.6 mmol h 1 in the first hour and 9 mmol h 1 after 5 h of irradiation, which are better than that of Pt/CdS (B0.276 mmol h 1). The excellent performance of this composite may be attributed to the high conductivity of graphene, the exposed rich edge sites of MoS2 and the synergistic effect between the three components. 3.5.

Biomedicine

The high specific surface area and unique optical properties of TMD nanosheets make them very promising for biomedical applications.64–67 Generally, the functionalization of TMD nanosheets with certain polymers (e.g. PEG and chitosan) is necessary to enhance their physiological stability and biocompatibility.64–67 As a typical example, Liu et al. discovered that the 2D PEG–WS2 composite with excellent physiological stabilities and strong NIR absorbance holds great potential for in vivo dual-modal computed tomography (CT)/photoacoustic imaging guided photothermal therapy (PTT) (Fig. 13).64 The cell toxicity test indicated that such hybrid nanostructure showed negligible toxicity to cells. Particularly, it was found that 100% of in vivo photothermal tumor elimination could be achieved after either intratumoral injection with a low dose of PEG/WS2 or intravenous injection with a moderate dose of this nanoagent with NIR laser irradiation at a relatively low power density. Later, they also found that PEG–MoS2 hybrid nanosheets could be used as a promising carrier in drug delivery for combined photothermal and chemotherapy of cancer.65 It was found that the PEG–MoS2 composite gave highly efficient loading of therapeutic molecules, such as chemotherapy drugs doxorubicin (DOX), 7-ethyl-10-hydroxycamptothecin(SN38), and a photodynamic agent chlorine e6 (Ce6), which is ascribed to the high surface-area-to-mass ratio of ultrathin MoS2 sheets. An excellent synergistic anti-cancer effect was observed in inhibiting tumor growth in animal experiments both in vitro and in vivo, by using DOX loaded PEG/MoS2 nanosheets as a typical example. In contrast to conventional photothermal agents (e.g. gold nanomaterials), such 2D PEG/MoS2 nanosheets exhibited an intrinsic high drug loading ability, making them very useful for combination therapy of cancer. Recently, Zhao et al. also demonstrated that the chitosan-modified MoS2 nanosheets can be used as contrast agents for CT imaging, presenting enhanced performance in in vitro imaging than commercial Iopromide.66 These fascinating studies may open a new way for the use of

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Fig. 12 (A) Photocatalytic H2 production activities of CdS, CdS/MoS2, optimized CdS/MoS2/graphene, CdS/graphene, and Pt/CdS (0.5 wt%) with the highest H2 generation for 1 h and (B) their amount of H2 generation in 5 h; (C) photocatalytic cycling behavior of samples per hour. (D) Schematic illustration of the microstructure of MoS2 and (E) its cocatalytic mechanism of H2 generation in lactic acid solution. Reproduced with permission from ref. 110. Copyright 2014, American Chemical Society.

TMD nanosheet-based polymer hybrid nanomaterials in biomedical applications. 3.6.

Other applications

In addition to the aforementioned applications, other kinds of potential applications, such as organic solar cells (OSCs),57 dye-sensitized solar cells (DSSCs),102 and electrochemical sensors,51,52 of TMD nanosheet-based composites have also been demonstrated recently. Intriguingly, it was demonstrated that the MoO3 NP loaded MoS2 nanosheets can be used as hole extraction layers (HELs) for OSCs.57 In contrast to MoS2 sheets and MoO3 NPs, in OSCs, MoO3/MoS2 as HEL exhibited superior performance with a power conversion efficiency (PCE) of 6.9% compared to that of MoS2 sheets (1.5%) and MoO3 NPs (5.2%), because of its better matched ohmic contact than MoS2 and more homogeneous film quality than MoO3 NPs. It is worth pointing out that the high PEC of MoO3/MoS2 as HEL was achieved via a solution-processed procedure without thermal annealing that was always required for solution-processed HELs reported before. More significantly, the MoO3–MoS2 composite-based device possessed higher long-term stability than MoO3 NP-based device. The results indicated that the MoO3–MoS2 hybrid nanosheets that were prepared by a simple method could be a promising hole-transporting layer for highly efficient and stable OSCs. More importantly, the MoS2–CNT composite can be used as a counter electrode (CE) catalyst in DSSC with excellent performance.102 The cyclic voltammogram

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(CV) results exhibited that the cathodic current density of such hybrid CE was higher than that of MoS2 and CNT, indicating the high activity for the reduction of triiodide (I3 ) to iodide (I ). It is noteworthy that the performance of this hybrid is even better than that of the sputtered Pt CE. Moreover, the peak current densities of this hybrid CE presented excellent electrochemical stability, and no sign of degradation was observed after consecutive 100 CV tests. Impressively, the MoS2/CNT CE showed a very low charge transfer resistance (1.69 U cm2) for the reduction of I3 . The DSSC device with MoS2/CNT as CE gave a high power conversion efficiency of 6.45%, which is a little higher than that of the DSSC device using Pt as CE (6.41%). The excellent conductivity of CNT and the large surface area of MoS2 grown on its surface may mainly contribute to the significant improvement of the catalytic activity of this hierarchical hybrid nanostructure. In addition, the synergistic effect between MoS2 sheets and CNTs may also contribute to the enhanced activity. Electrochemical sensors are one kind of important and effective sensing systems for detection of various chemical and biological species. The high surface area and good conductivity of 2D TMD nanosheets render them promising materials for electrochemical detection. It has been proved that the deposition of noble metal NPs on certain substrates (e.g. graphene) is a facile and feasible way to enhance their performance in electrochemical sensors.35 Bearing this in mind, the electrochemical sensing properties of TMD nanosheet-based


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Fig. 13 In vivo dual-model imaging in 4T1-tumor bearing mice. (A) CT images of PEG/WS2 solutions with different concentrations. (B) Hounsfield units (HU) of PEG/WS2 as a function of its concentration. (C) CT images of mice before and after i.t. injection of PEG/WS2 (5 mg mL 1, 20 mL). (D) CT images of mice before and after i.v. injection of PEG/WS2 (5 mg mL 1, 200 mL). The CT contrast was obviously enhanced in the mouse liver (green dashed circle) and tumor (red dashed circle). (E) Photoacoustic tomography (PAT) images of tumors on mice before and after i.t. or i.v. injection of PEG/WS2. (F) Photoacoustic signals in the tumors from mice before and after i.t. or i.v. injections of PEG/WS2 solution. For PAT imaging, 20 or 200 mL of PEG/WS2 at the concentration of 2 mg mL 1 was i.t. or i.v. administered, respectively. Reproduced with permission from ref. 64. Copyright 2014, John Wiley & Sons, Inc.

noble metal composites have been investigated recently.51,52 For instance, Wang et al. reported that the electrode modified with hybrid nanosheets of Au NP functionalized MoS2 sheets gave excellent electrocatalytic activity toward dopamine (DA).51 The current of its oxidation peak is linearly dependent on the concentration of DA in the range of 0.1–200 mM, with a detection limit of as low as 80 nM. More importantly, the detection of DA based on Au/MoS2 modified electrode can be realized in the presence of a large excess of ascorbic acid that is regarded as the most annoying interferent for DA detection. Note that the detection limit of this Au–MoS2 hybrid for DA is lower than that of the pure MoS2 monolayer. Moreover, a ternary composite composed of Ag NPs, MoS2 and chitosan was prepared for electrochemical sensing of tryptophan with a detection limit of 0.05 mM.52 It is believed that TMD nanosheet-based composites, especially noble metal composites, could be a new promising platform for electrochemical sensing applications.


4. Conclusions and perspectives In summary, the unique properties of TMD nanosheets together with the success of massive production have aroused great research interest in the usage of them as building blocks to construct structurally defined functional hybrid nanomaterials. As a new class of fascinating templates, the creation of novel functional composites has been realized by hybridization of them with a variety of materials such as noble metals, metal oxides, metal chalcogenides and carbonaceous materials. A number of synthetic methods have been developed for the preparation of TMD nanosheet-based composites, trying to control the components in the hybrid composites. It is worth pointing out that when using TMD nanosheets as building blocks, especially under extreme conditions (e.g. high temperature or strong oxidizing agents), they are easy to be oxidized in contrast to graphene.54–57 However, taking advantage of this feature, 2D heterostructures

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of metal oxides-metal dichalcogenides (e.g. p–n junctions) might be constructed by partially oxidizing the TMD sheets if the oxidation process can be controlled. Significantly, these resultant composites did present superior performance in a wide range of applications, like electronic devices (e.g. phototransistors, memory devices, etc.), catalysis (e.g. HER), batteries (e.g. LIBs and SIBs) and biomedicine (e.g. photothermal therapy). Some appealing examples with excellent performance have been highlighted, such as epitaxially grown noble metals on MoS2 monolayers for HER,42 PEG–WS2 hybrid nanosheets for biomedicine,64 and single-layer MoS2 sheets embedded in carbon fibers for LIBs and SIBs.106 Despite the pioneering studies done in this appealing research area, challenges still remain both in the fundamental understanding and in the rational design of TMD nanosheetbased hybrid nanomaterials at a highly controllable level. The properties and functions of composites are not only related to their composition, crystal phase, and structure, but are also dependent on the spatial organization/assembly, surface exposure, distribution and interaction of/between each component. For example, the hybrid nanomaterials composed of MoS2 nanosheets and carbon fibers, including MoS2 nanosheets grown on carbon fibers,90 few-layer MoS2 nanoflakes embedded in carbon fibers,107 and small single-layer MoS2 nanosheets embedded in carbon fibers,106 have been used for LIBs. Although they possess same components, their performances as anodes for LIBs are much different from each other. It was found that the MoS2 monolayer embedded carbon fibers presented much better performance (e.g. capacity and cycling stability) than the other two. However, at the current stage, most of the studies are focusing on the simple hybridization of TMD nanosheets with other materials, without too much concern on their hybrid formats (e.g. spatial organization, interaction, component distribution, etc.). One of the most challenges lies in how we can achieve excellent performance towards specific applications by rational design of a composite in a highly controllable fashion. It should be admitted that the study of TMD nanosheetbased functional composites is still in the infant stage. On the one hand, currently, most of the materials used for hybridization with TMD nanosheets are inorganic nanomaterials, such as noble metals, metal oxides and carbonaceous nanomaterials. Previous studies have demonstrated that the incorporation of 2D graphene sheets into polymers could be a fascinating strategy to optimize the mechanical, chemical and electronic properties of polymers for the realization of optimal performance for applications in supercapacitors and photovoltaic devices.29 One of the future directions lies in the combination of TMD nanosheets with polymers to create TMD nanosheet-based polymer composites for a wide range of applications. On the other hand, the mainstream of the examples focuses on the usage of MoS2 sheets as building blocks for construction of hybrid nanomaterials. It is worth pointing out that unlike graphene, TMDs constitute a big family that includes a wide range of compounds such as MoS2, TiS2, TaS2, WS2, MoSe2, WSe2, HfS2, WTe2, and so on, depending on the combination of chalcogen and transition metals.7,8 More intriguingly, although these TMD sheets possess similar structural features, their chemical, physical and electronic properties

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are versatile. For example, the TMDs can be insulators (e.g. HfS2), semiconductors (e.g. MoS2 and WS2), semimetals (e.g. WTe2 and TiSe2), and metals (e.g. NbS2 and VSe2), making them versatile for a wide range of applications.7 Along with the development of advanced synthetic methods, one of the most promising opportunities in this area is the hybridization of other kinds of TMD nanosheets, such as WS2, WSe2, ZrS2 and HfS2, with a variety of materials to generate novel functional composites for various applications.

Acknowledgements This work was supported by MOE under AcRF Tier 2 (ARC 26/13, No. MOE2013-T2-1-034), AcRF Tier 1 (RG 61/12, RGT18/13, and RG5/13), and Start-Up Grant (M4080865.070.706022) in Singapore. This research is also conducted by NTU-HUJ-BGU Nanomaterials for Energy and Water Management Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), that is supported by the National Research Foundation, Prime Minister’s Office, Singapore.

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Chem. Soc. Rev., 2015, 44, 2713--2731 | 2731

Two-dimensional transition metal dichalcogenide (TMD) nanosheets.

Spin-orbit engineering in transition metal dichalcogenide alloy monolayers.

Flexible transition metal dichalcogenide nanosheets for band-selective photodetection.

Transition metal dichalcogenide growth via close proximity precursor supply.

Intrinsic disorder in graphene on transition metal dichalcogenide heterostructures.

Two-Dimensional Transition Metal Dichalcogenide Alloys: Stability and Electronic Properties.

Epoxy nanocomposites with two-dimensional transition metal dichalcogenide additives.

A universal method for preparation of noble metal nanoparticle-decorated transition metal dichalcogenide nanobelts.

Metal-semiconductor barrier modulation for high photoresponse in transition metal dichalcogenide field effect transistors.

Monolayer PtSe₂, a New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially Grown by Direct Selenization of Pt.

Flexible n-type thermoelectric materials by organic intercalation of layered transition metal dichalcogenide TiS2.

Chromatic Mechanical Response in 2-D Layered Transition Metal Dichalcogenide (TMDs) based Nanocomposites.

Quantum wells formed in transition-metal dichalcogenide nanosheet-superlattices: stability and electronic structures from first principles.

Ferroelasticity and domain physics in two-dimensional transition metal dichalcogenide monolayers.

Direct exciton emission from atomically thin transition metal dichalcogenide heterostructures near the lifetime limit.

Low-temperature thermal transport and thermopower of monolayer transition metal dichalcogenide semiconductors.

Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers.

Vacancy-induced formation and growth of inversion domains in transition-metal dichalcogenide monolayer.

Lorentz-violating type-II Dirac fermions in transition metal dichalcogenide PtTe2.

Second harmonic generation from artificially stacked transition metal dichalcogenide twisted bilayers.

Evidence for Chemical Vapor Induced 2H to 1T Phase Transition in MoX2 (X = Se, S) Transition Metal Dichalcogenide Films.

A high-performance complementary inverter based on transition metal dichalcogenide field-effect transistors.

Coherent Control of Nanoscale Ballistic Currents in Transition Metal Dichalcogenide ReS2.

Size-dependent nonlinear optical properties of atomically thin transition metal dichalcogenide nanosheets.