Two-dimensional synthetic templates

The isolation of atomic-thin graphene sheets from bulk graphite has led to the discovery of a number of novel properties of graphene that have enormously benefited the development of nanotechnology [1], which has inspired the exploration of other 2D materials, such as transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN), etc. [2]. To exfoliate a layered bulk crystal, the van der Waals interaction between the adjacent layers should be overcome, which can be realized by techniques like mechanical exfoliation [1] and solvent-assisted ultrasound exfoliation [3]. These two methods are relatively non-destructive so that the intrinsic properties of 2D materials could be preserved. However, the former method is mostly applied for fundamental studies and the latter one is difficult to produce high-yield single-layer nanosheets. The intercalation-based methods, on the other hand, are more effective and reliable in mass production of single-layer nanosheets. It was demonstrated decades ago that many chemical species, like alkali metals, transition-metal halides and amine molecules, can be intercalated into various layered materials [4]. As one of the most intensively studied intercalates, lithium, for example, can be inserted into the interlayer spaces of a layered crystal by soaking the crystal in n-butyl lithium solution at an elevated temperature (e.g. 100°C) for a long time (e.g. 3 days). Alternatively, the electrochemical intercalation method (Fig. 1a–c), which employs electric potential to drive the insertion of Li ions [5], can shorten the reaction time and lower the temperature required in the aforementioned soaking process. It should be noted that the intercalation-based methods might generate crystal defects in the exfoliated nanosheets, even leading to their structural transformation. For example, the MoS2 nanosheets prepared from chemical or electrochemical intercalation method are partially or completely transformed from the semiconducting hexagonal 2H phase to the metallic trigonal 1T polymorph [6]. Nevertheless, the 1T-MoS2, which can be transformed back to 2H-MoS2 via thermal annealing at inert atmosphere, is in fact an attractive material with much higher electrocatalytic activity in hydrogen evolution reaction compared to the 2H-MoS2 [6]. After exfoliation, these functional 2D materials can be subsequently used for electronic/optoelectronic devices, electrocatalysis, photocatalysis and energy storage/conversion devices. (a–c) Schematic illustration of the electrochemical intercalation and exfoliation process. (a) Li ions from a Li foil as anode are inserted into a layered bulk crystal used as cathode in a battery cell. The Li-intercalated compound in (b) is ultrasonicated in water or ethanol to give isolated nanosheets in (c). Reprinted with permission from [5]. Copyright 2011, John Wiley & Sons, Inc. (d, e) Schematic illustration of the epitaxial growth of metal nanoparticles on MoS2, showing azimuthal orientation of a metal nanoparticle on the MoS2(001) surface in the (111) and (101) orientations. Only the first S layer and the second Mo layer are drawn in the scheme. (f, g) Transmission electron microscopy images showing Pd nanoparticles epitaxially deposited on an exfoliated MoS2 nanosheet, showing (111) or (101) orientation. Reprinted with permission from [8]. Copyright 2013, Nature Publishing Group. Importantly, besides the large specific surface area, the exfoliated 2D materials are also lightweight and highly flexible, and could be dispersed in various solvents depending on their surface functionalities. These features are especially appealing, as versatile wet-chemical reactions can proceed on these dispersible 2D templates, leading to the construction of functional hybrids with possibly enhanced properties and novel functions for various applications. Although metallic structures can be deposited in perfect epitaxial registry on a semiconductor substrate via the solid-state epitaxial growth, in a colloidal synthesis, the nucleation of metallic domains on a semiconductor nanostructure generally occurs on defects, edges or surfaces with a large curvature [7]. Thanks to the high-yield production of dispersible MoS2 nanosheets with flat and extendable surfaces, the epitaxial growth of various noble metals such as Pd, Pt and Ag on MoS2 has been achieved in solution under mild conditions [8]. It should be emphasized that the epitaxial interface between the 2D template and deposited nanostructures is beneficial to the charge transfer between them, which was also demonstrated in the PbSe quantum dot/MoS2 flake hybrid system recently used for photodetector [9]. In addition, the low cost and easy implementation make the solution-phase epitaxial growth attractive for scalable production of functional hybrids. As mentioned above, besides defects generated during the exfoliation process, 2D materials also carry surface charges or surfactant molecules which are required for their dispersion. These factors add elements of complexity to colloidal synthetic processes, and thus unfortunately, solution-phase epitaxial growth on 2D templates is not always achieved. When the lattice structure of a 2D material cannot directly guide the orientation and alignment of surface anchored nanocrystals, the 2D material can act as a platform for planner assembly of surfactant/ligand molecules, which in turn assist the assembly and growth of inorganic crystals. This has been successfully demonstrated in graphene-templated synthesis [10]. However, unlike chemically derived graphene materials, which can be linked to various kinds of molecular species via covalent/non-covalent interactions, other types of inorganic 2D materials, such as MoS2, mainly interact with ligands/surfactants via the electrostatic interaction. This has greatly limited their use as templates to guide the growth of other functional materials in a controllable fashion. Recently, the hybridization of TMDs with polymers has been reported [11]. Considering the rich chemistry of polymer materials, it can be expected that the TMD/polymer hybrids can be further used for controlled deposition of other metallic and semiconducting nanostructures. The recent successful demonstration of oriented growth of covalent organic framework films on chemical vapor deposited graphene [12] suggests that in addition to inorganic structures, the controlled growth of organic crystals on various 2D templates may also be viable to bring up exciting properties and applications of novel inorganic–organic hybrid systems. Moreover, besides graphite and TMDs, with the recent advances in exfoliation techniques, practically any 3D layered compounds like clays, layered oxides, halides, carbides, nitrides, hydrides, hydroxides, phosphates and phosphonates can be exfoliated to give isolable 2D materials. While these materials can be insulators, metals, semimetals and semiconductors with direct and indirect band gaps, making them attractive building blocks for construction of functional hybrid materials, they also differ in crystal structure and surface chemistry which make them promising templating materials to potentially direct the synthesis and assembly of novel nanostructures for a broad range of applications. This work was supported by Singapore 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, the National Natural Science Foundation of China (51322202), and the Natural Science Foundation of Jiangsu Province (BK20130927). 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.