CC BY-NC-ND
0# stifl CrossMark
ELSEVIER
Graphene-like layered metal S dichalcogenide/graphene composites: : synthesis and applications in energy 1 storage and conversion
«1 •y «■ <1 "J 1
Dongyuan Chen1'2, Weixiang Chen1'*, Lin Ma1, Ge Ji2, Kun Chang1 and Jim Yang Lee2
1 Department of Chemistry, Zhejiang University, Hangzhou 310027, China
2 Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, 119260 Singapore, Singapore
The unusual properties of graphene and graphene-like (GL-) layered metal dichalcogenides (LMDs, MoS2, WS2 and SnS2) have stimulated strong interest in GL-LMD/graphene composites. Heterostructures which are constructed by stacking GL-LMD and graphene together are expected to extend the usability of these 2D materials beyond graphene alone. This review will focus on recent progress in the synthesis and applications of GL-LMD/graphene composites in energy storage and conversion. The remarkable electrochemical properties of GL-LMD/graphene for reversible lithium storage are highlighted in particular. The applications of these composites in electrochemical and photochemical water splitting for hydrogen generation are also discussed.
Introduction
Two-dimensional (2D) nanosheets have drawn widespread interest because of their uncommon electronic, chemical and physical properties [1-3]. Among 2D nanocrystals, graphene nanosheets (GNS) have shown the most application potential because of a suite of properties (e.g. large surface area, excellent electrical conductivity and material flexibility) which can easily find uses in Li-ion batteries, supercapacitors, fuel cells, photovoltaic devices and others [3-8]. Since the discovery of graphene, other 2D graphene-like (GL-) nanosheets with single-layer or few-layer structure, mostly notably the layered metal dichalcogenides (LMDs, MoS2, WS2, etc.), have also been synthesized with their application potential explored [9-13]. A typical layered LMD such as MoS2 (or WS2) is formed by van der Waals stacking of the covalently bonded S-M-S (M = Mo, W) layers into a close-packed hexagonal lattice. The electronic structure of layered LMDs depends largely on the layer number due to quantum confinement effects. Bulk MoS2 is a semiconductor with an indirect bandgap of ~1.2 eV while monolayer MoS2 is a ~1.8 eV direct bandgap semiconductor [14] with high photoluminescence quantum yield and other unusual properties [15]. There have been many studies on these GL-LMDs where properties suitable for a diversity of
*Corresponding author:. Chen, W. (ieixiangchen@zju.edu.cn)
applications, such as electronic devices [16], energy storage and conversion [17-19], phototransistors [20], optical identification [21] and sensors [22,23], were demonstrated. The synthesis of GL-LMDs can be divided into two strategies: (1) top-down methods based on the exfoliation of bulk-LMDs using mechanical cleavage, chemical Li-intercalation and exfoliation [18], electrochemical lithiation-assisted exfoliation [24], modified liquid exfoliation [25-27], laser thinning [28]; and (2) bottom-up methods such as solution chemistry preparations (e.g. hydrothermal synthesis [29], oleylamine-assised wet-chemical approach [30], van der Waals epitaxy [31] and chemical vapor deposition (CVD) [32].
The construction of heterostructures and hybrid devices by stacking different 2D layered materials together is an emerging research area. A number of intriguing physical phenomena with application values have been demonstrated [33-35]. Since both GL-LMD and graphene have analogous microstructure and morphology, Heterolayered GL-LMD/graphene composites which maximize structural compatibility may synergize the GL-LMD and GNS interaction to result in favorable outcomes greater than the sum of individual components. Since the synthesis and applications of graphene and 2D GL-LMDs have been reviewed elsewhere [2,36-39], we will focus this review on the synthesis of GL-LMD/graphene composites and their applications in energy storage and conversion (Scheme 1).
1369-7021/© 2014 Elsevier Ltd. All rights reserved. http://dx.doi.Org/10.1016/j.mattod.2014.04.001
SCHEME 1
Schematic illustration of the applications of the GL-LMD (MoS2, WS2, SnS2 or SnSe2)/GNS composites in energy storage and conversion (Li-ion battery, hydrogen production and solar energy conversion).
Preparation of GL-LMD/graphene composites
GL-LMD/graphene composites can be fabricated by synthesizing GL-LMD and graphene separately followed by a simple restacking operation. For example, Coleman et al. has prepared MS2/GNS (M = Mo, W) hybrid films by filtering-mix-restacking the dispersions of exfoliated monolayer MS2 and graphene. The hybrid produced as such exhibited enhanced mechanical properties and conductivity. However, the simple stacking method does not have good uniformity control and the GL-LMD and graphene may not be synergized enough to deliver the best properties possible. Most recent efforts are focused on the preparation of GL-LMD/GNS composites by different methods such as hydrothermal processing, cationic surfactant-assisted wet-chemical reduction and CVD based on van der Waals epitaxy. These different synthetic routes to GL-LMD/GNS are reviewed in this section.
GL-MoS2/GNS composites
In our previous work, single-layer MoS2@amorphous carbon (a-C) composites were prepared by a glucose-assisted hydrothermal route;using thiacetamide as the sulfur source and reductant [29]. The success of that work motivated us to synthesis single-and few-layer MoS2/GNS composites by a similar hydrothermal approach in the presence of graphene oxide sheets (GOS) [40,41]. In addition, the better results were obtained when L-cysteine was used as the sulfur source and reductant in the hydrothermal synthesis [42]. This was believed to be due to the abundance of functional groups (-NH2, -SH and COOH) in L-cysteine. The few-layer MoS2/GNS composites have been successfully fabricated by the reaction between Na2MoO4 and L-cysteine under hydrothermal conditions in the presence of GOS followed by heat treatment (Fig. 1) [42]. It was demonstrated that GNS (or GOS) could restrain the layered MoS2 from well-stacking in the c-direction, favoring
the formation of few-layer MoS2/GNS composites instead. Recently, we have also presented a facile wet-chemical approach to synthesize GL-MoS2/GNS composites based on the simultaneous reduction of (NH4)2MoS4 and GOS with hydrazine under reflux conditions with the assistance of the cationic surfactants [43,44]. GL-MoS2/GNS could also be prepared by the CTAB-assisted hydrothermal route [45] (Fig. 2). The microstructures and the electrochemical properties of GL-MoS2/GNS composites for reversible lithium storage were dependent on the structure of the cationic surfactants (cetyltrimethylammonium bromide, CTAB; dodecyltrimethylammonium bromide, DTAB; octocyltrimethy-lammonium bromide, OTAB; tetrabutylammonium bromide TBAB) and their concentrations in the synthesis [43-45]. Furthermore, the layer number of GL-MoS2 nanosheets in the composites could also be varied by using different cationic surfactants and concentrations. The preparation of ultrathin MoS2/N-doped-gra-phene composites was reported by Chang et al. using N-doped-graphene for the hydrothermal reactions [46]. N-doped-graphene, which has a rougher and more wrinkled surface than graphene, could allow the MoS2 nanosheets to disperse more effectively and uniformly.
Other methods have also been developed lately to improve the synthesis of layered MoS2/GNS (or rGO) composites. Zhou et al. presented a facile method to synthesis MoS2/GNS hybrids by the combination of an electrochemical lithiation-assisted exfoliation process and a hydrazine monohydrate vapor reduction [47]. Gong et al. also used the bottom-up approach to fabricate 3D MoS2/GNS hybrids from exfoliated-MoS2 nanosheets and GOS by hydrothermal treatment [48]. The same group also reported the synthesis of 3D porous MoS2/GNS architectures from (NH4)2MoS4 and GOS by hydrothermal synthesis and assembly [49]. These 3D MoS2/GNS architectures had high surface area, a multilevel porous structure
(1)M0S2 (a)
10 20 30 40 50 60 70 80 2 6 /degree
Cycle Number Z (ii)
FIGURE 1
(a) XRD patterns of the layered MoS2/graphene (MoS2/G) composites prepared by L-cysteine-assisted hydrothermal route after heat treatment, showing that the (0 0 2) peaks of MoS2 become broaden and lower with increasing of GNS content in the composites. (b) and (c) Typical TEM and HRTEM images of the few-layer MoS2/G (1:2) composite. (d) Rate capability of the MoS2 and MoS2/G electrodes at different current densities. (e) Nyquist plots of MoS2 and MoS2/G (1:2) electrodes. Reprinted (adapted) with permission from Ref. [42]. Copyright 2011, American Chemical Society.
and high conductivity [49]. CVD is another strategy for the growth of MoS2 nanosheets on the graphene [31,50]. Shi etal. developed a CVD method for 2D-MoS2/GNS heterostructures based on van der Waals epitaxial growth on a Cu-foil template covered with gra-phene [31]. This simple method provides an expedient method to produce MoS2/graphene heterolayered films for optical and electronic nanodevices.
GL-WS2/graphene composites
Another typical LMD, WS2, which has a higher intrinsic electrical conductivity than MoS2, was also composited with graphene and investigated. Its good conductivity allowed the interaction
between graphene and LMD to be analyzed with less interference from conductivity effects. Rao's group fabricated GL-WS2/GNS by reducing GOS dispersion with hydrazine hydrate under reflux in the presence of preformed few-layer WS2 [51]. The few-layer WS2/ graphene could be prepared through the hydrothermal reaction between WCl6 and thioacetamide in the presence of GOS [52,53]. Recently, we demonstrated a facile synthesis of few-layer WS2/N-doped-GNS by one-pot hydrothermal reduction with hydrazine hydrate (Fig. 3) [54]. The high-temperature high-pressure environment of hydrothermal processing could result in the effective N-doping of graphene. The electrochemical performance of GL-WS2/ N-doped-GNS composites was improved by the higher electrical
FIGURE 2
(a) XRD patterns of the few-layer MoS2 and GL-MoS2/G prepared by a CTAB-assisted hydrothermal route using Na2MoO4, L-cysteine and GOS after heat treatment. HRTEM of (b) GL-MoS2/G-CT01, (c) GL-MoS2/G-CT02 and (d) GL-MoS2/G-CT05, showing that the layer number of GL-MoS2 in the composites is 3-4,
2-3 layer and monolayer, respectively, for 0.01, 0.02 and 0.05 mol L 2013, John Wiley & Sons, Inc.
CTAB in hydrothermal solution. Reproduced with permission from Ref. [45]. Copyright
conductivity of N-doped-GNS. The hydrothermal treatment to incorporate nitrogen into graphene is a simple, gentle and effective alternative to the gas-phase method for producing GL-LMD/ N-doped-GNS composites [54].
Few-layer SnX2/GNS (X = S, Se) composites Other layered metal dichalcogenides (such as SnS2, SnSe2) have also been combined with GNS to improve their electrochemical performance. SnX2/GNS (X = S, Se) composites with the enhanced electrochemical performance could be synthesized by wet-chemical methods [56-62]. For instance SnSe2/graphene composites were prepared by the reduction of GOS in solution in the presence of SnSe2 nanosheets [60]. SnS2/GNS composites have also been synthesized by a one-step hydrothermal reaction between thioa-cetamide and tin (IV) in the presence of GOS [58]. We have also developed a facile one-pot hydrothermal process using L-cysteine as the sulfur source and reductant for preparing few-layer SnS2/ GNS hybrids [57]. Fig. 4 shows that few-layer SnS2 with defects or a disorder structure were dispersed well in the graphene surface [55]. Lou et al. reported a two-step approach for SnS2/GNS hybrids where SnO2 nanoparticles were transformed into 2D-SnS2 nano-plates directly on/between graphene [58]. Recently, SnS2/GNS composites were prepared by a one-step microwave-assisted technique for electrochemical lithium storage and photocatalytic applications [62].
Roles of GOS in GL-LMD/GNS synthesis
The potential of graphene in the construction of novel hetero-structures and future hybrid devices has been reviewed elsewhere [3,39,63,64]. While the use of graphene at the composite level may
be more prosaic, graphene nonetheless plays a deterministic role in application performance enhancements. GNS have been used as a versatile template to grow other 2D-nanosheets by CVD. However, GNS does not have a large number of functional groups on its surfaces for further modifications with guest materials. Due to van der Waals interactions, agglomeration and restacking are common in GNS leading to the loss of effective surface areas and poorer properties than expected. On the contrary, GOS, with an abundance of oxygen-containing functional groups to provide innumerable modification possibilities [63-66], is a good platform for composite preparation [3,67-69]. In addition, GOS also disperses better than GNS in solution and it can be easily reduced to GNS by wet-chemical methods. More importantly, due to the crumpling and cross-linking of GOS during the hydrothermal treatment [70], a 3D-GNS network may be generated to provide the GL-LMD/GNS composites with a large specific area, multilevel porous structure and high electrical conductivity [49]. Therefore, GOS dispersion is a good starting material for the large-scale and low-cost production of GL-LMD/GNS by the wet-chemical methods for energy storage and conversion applications.
The oxygen-containing groups on the GOS surface promote better interaction with metal cations such as Sn4+, which has been used to advantage in the synthesis of SnS2/GNS [55]. However, the oxygen-containing groups also render the GOS surface negatively charged, thwarting the approach of metal precursors in anionic forms (MS4~ and MO4~, M = Mo, W). The native charge incompatibility between GOS and MS4~ (or MO4~) could be averted by a facile cationic surfactant-assisted wet-chemical route which we have demonstrated for the facile preparations of GL-MoS2/GNS and GL-WS2/N-doped-GNS. Cationic surfactants (CTAB, OTAB,
FIGURE 3
(a) Schematic illustration of the preparation of the WS2/N-doped-graphene (NG) composites by a facile one-pot hydrothermal route and the Li+ store in the composites. TEM and HRTEM images of (b) WS2/NGC1, (c) WS2/NGC2 and (d) WS2/NGC5 composites prepared by hydrothermal route with assistance of 0.01, 0.02 and 0.05 mol L-1 CTAB. (e) Cycle performance and rate capability of the WS2/NGC1 and WS2/NGC1 composites at different current densities. (c) Nyquist plots of WS2, WS2/NG and WS2/NGC2 composite electrodes, in which an equivalent circuit model of the studied system is inserted. CPE represents the constant phase element [54].
DTAB and OTAB) could easily be adsorbed on the negatively charged GOS surface through electrostatic interaction, and effectively mediated the charge incompatibility between GOS and MS42~ (or MO4-). More importantly, these cationic surfactants have also shown some ability to control the microstructure (layer number) of GL-MS2 (M = Mo, W) in the composites [43-45,54].
Applications in energy storage and conversion
Energy and environment are two most important considerations in the sustainable development of human society. The consumption of conventional fossil fuels could be reduced by the use of
clean and renewable energies derived from the sun and wind. New materials which can effectively convert and store renewable energies are the key to clean technology. 2D nanoarchitectures, such as graphene and GL-LMD nanosheets, have shown some promises in energy storage and conversion. The GL-LMD/GNS composites, in particular, could offer greater versatility and opportunity in developing truly multifunctional materials for applications. It is therefore not surprising that such hybrids are receiving increasingly attention in the research community. This section will briefly review the applications of GL-LMD/GNS composites in high-performance Li-ion batteries, hydrogen production and solar energy conversion.
FIGURE 4
(a) Schematic diagram for L-cysteine-assisted hydrothermal synthesis of few-layer SnS2/graphene (FL-SnS2/G) hybrid. (b) XRD pattern, (c) TEM and (d) HRTEM image of the FL-SnS2/G hybrid. (e) Rate capability of SnS2 and FL-SnS2/G electrodes at the different current densities. (f) Nyquist plots of SnS2 and FL-SnS2/G electrodes, in which an equivalent circuit model of the studied system is inserted. CPE represents the constant phase element [55].
Li-ion batteries
Next-generation LIBs for vehicle electrification and grid-scale energy storage have to provide high energy and power densities, excellent cycle stability and a strong rate performance. The capability of LIBs depends mostly on electrode materials, especially the anode materials. Graphite anodes, which are ubiquitous in current designs, are limited by a low theoretical capacity of 372 mAh g_1. 2D-LMD nanosheets such as MoS2, WS2, SnS2 are able to reversibly store Li+ at higher capacities [18,41,5457,59,71,72] but they are deficient in rate and cycle performance [18,71]. We have recently demonstrated that GL-LMD(MoS2, WS2 or SnS2)/GNS composites where the two 2D nanosheets were tightly coupled to promote synergistic interaction could significantly improve the electrochemical performance of lithium storage. The structural and morphological similarity between GL-LMD and GNS was the enabling factor of synergy. Specifically few-layer MoS2/GNS and SnS2/GNS composites were produced by L-cysteine-assisted hydrothermal synthesis [42,55]. The few-layer MoS2/GNS and SnS2/GNS composite delivered a reversible specific capacity as high as 900-1200 mAh g_1 with excellent cycle stability and enhanced rate capability (Figs. 1 and 4) [42,55]. A variant of the composite, exfoliated-MoS2/GNS/PEO nanocomposite, also
displayed high capacity (1080 mAh g-1) and enhanced rate performance [73]. The remarkable electrochemical performance of GL-LMD/GNS was also demonstrated in other recent works [48,49,58]. A 3D porous MoS2/GNS architecture prepared by hydrothermal reaction and subsequent freeze-drying exhibited a high specific capacity (1200 mAh g-1) of lithium storage with enhanced rate capability and long cycle life (3000 cycles) [49]. The WS2/graphene composite prepared by hydrothermal route and freeze-drying also exhibited good cycling stability and outstanding high-rate capability of lithium storage [52].
As electrochemical performance generally benefits from conductivity increases, N-doped-GNS, a more conductive form of GNS, was also used in the preparation of MS2/N-doped-GNS (M = Mo, W) composites. Chang et al. demonstrated that the MoS2/N-doped-GNS could provide a high reversible capacity of 1020 mAh g_1 with enhanced cycle and rate performance. We have also recently prepared GL-WS2/N-doped-GNS composites by a facile one-pot hydrothermal route and demonstrated their excellent electrochemical performance for lithium storage (Fig. 3) [54].
The good electrochemical performance of GL-LMD/GNS composites for reversible lithium storage is due to a combination of
factors: good transport properties of GNS, a robust heterolayered structure of GL-LMD/GNS composites and the synergistic interaction between GL-LMD and GNS. The interaction between GNS and MoS2 layers has been predicted in theory and confirmed by experiments to contribute to the improvements in the electrochemical lithium storage [74-77]. Rapid electron transfer, low transport resistance and good structural stability are essential for electrochemical lithium storage. The presence of highly conductive and flexible GNS reduces the contact resistance and stabilizes the composite microstructure. More importantly, GNS can greatly enhance electron transfer in the electrode reaction [78]. The much lower charge-transfer resistance of GL-LMD/GNS electrodes for the electrochemical lithiation/lithiation has been confirmed by EIS (Figs. 1(e), 3(f) and 4(f)) [42,55]. GL-LMD/GNS with a 3D architecture could leverage on their extensive surface area, multilevel porous structure and high electrical conductivity to facilitate lithium diffusion, fast electron-transfer and electrolyte access [42,48,49,55].
Hydrogen generation and solar energy conversion
The efficient and low-cost production of hydrogen from water splitting is the only way mankind can claim independence from the reliance on fossil fuels. This is often perceived as a futuristic goal in view of the numerous technological challenges to overcome. While Pt-group metals (PGM) are the most effective catalysts for the hydrogen evolution reaction (HER), the cost and
scarcity considerations render the PGM impractical for deployment in industrial-scale hydrogen production by water splitting. Thus, alternative catalysts based on nonprecious metals and metal-free materials are being actively pursued. Nanosized MS2 (X = Mo, W) has been reported to exhibit high electrochemical HER activity comparable to the Pt-group metals [79-85]. The exposed edges of MS2 (X = Mo, W) nanosheets are the active sites [81-83,85-88], but the overall HER activity can be affected by the extremely low conductivity between adjacent S-M-S (M = Mo, W) layers in a layered MoS2 structure. Consequently increasing the number of active edge sites and enhancing the charge transport in the layered structures are salient strategies to improve the HER activity of MS2 (M = Mo, W) nanosheets.
MoS2/GNS composites have also been shown as a promising HER electrocatalyst with good and stable activity [88-91]. Dai et al. demonstrated the high electrocatalytic HER activity of a MoS2/ rGO composite prepared by a solvothermal method (see Fig. 5) [88]. The good electrocatalytic activity was attributed to MoS2 nanoparticles with an abundance of active edge sites which grew selectively on the graphene surface by solvothermal synthesis, and the chemical and electronic coupling between MoS2 and graphene which enabled rapid electron transfer from the MoS2 layers to the electrode [88]. The high electrocatalytic activity of MoS2/GNS also find uses in the counter electrode (CE) of dye-sensitized solar cells (DSSC), where power conversion efficiency comparable to that of DSSC with Pt-CE was reported [92-94]. Recently, MoSe2/GNS and
FIGURE 5
(a) Polarization curves of HER obtained with several catalysts as indicated, (b) corresponding Tafel plots recorded on glassy carbon electrodes with a catalyst loading of 0.28 mg/cm2, (c) TEM image of MoS2/RGO showing folded edges of MoS2 particles on RGO in the hybrid and (b) HRTEM image showing nanosized MoS2 with highly exposed edges stacked on a RGO sheet. Reprinted (adapted) with permission from Ref. [88]. Copyright 2011, American Chemical Society.
FIGURE 6
SEM images obtained from (a) the rGO sheets, (b) the p-MoS2/n-rGO; (c) TEM image of the p-MoS2/n-rGO, and (d) HRTEM image of the p-MoS2/n-rGO for the area marked in the red square in image (c). (e) UV-vis absorption spectra of the solitary MoS2, the MoS2/rGO, and the p-MoS2/n-rGO and (f) hydrogen generated by the MoS2, the MoS2/rGO, and the p-MoS2/n-rGO photocatalysts. Reprinted (adapted) with permission from Ref. [97]. Copyright 2011, American Chemical Society.
WS2/GNS prepared by hydrothermal method were also demonstrated to exhibit enhanced electrocatalytic activity for HER [53,95].
When photocatalytic TiO2 nanoparticles were incorporated in the composite formulation, MoS2/GNS also functioned as a co-catalyst in photocatalytic H2 generation. The TiO2/MoS2/GNS prepared by two-step hydrothermal process could demonstrate a high H2 production rate of 165.3 mmol/h [96]. The enhanced photoca-talytic activity was once again attributed to the synergetic interaction between MoS2 and graphene which improves the interfacial charge transfer and supplies a large number of active adsorption sites [96]. Recently, a p-MoS2/n-RGO heterostructure formed by depositing p-MoS2 nanoplatelets on n-type N-doped-RGO was found to exhibit significant photocatalytic HER activities in the wavelength range from ultraviolet to near-infrared (Fig. 6) [97]. The p-MoS2/n-rGO catalyst exhibited an average hydrogen production rate of 160.6 mmol g_1 h_1 under the natural sunlight. The photoelectro-chemical test demonstrates that the p-MoS2/n-rGO junction greatly enhances the charge generation and suppresses the charge recombination, which is responsible for enhancement of solar hydrogen generation [97]. More recently, Rao's group also demonstrated the
highly effective visible-light-induced H2 generation by a nanocom-posite of few-layer 2H-MoS2 with heavily nitrogenated graphene duo to that the heavily N-doped-GNS was effective in acting as an electron channel to few-layer 2H-MoS2 [98].
Conclusions and outlook
GL-LMD/graphene heterostructures formed by stacking or compositing different 2D-nanosheets in ways which synergize their interaction can extend the usability of 2D nanosheets beyond simply graphene or 2D-LMDs. The purpose is to create more possibilities by combining the unusual properties of 2D nanosheets for applications exploring a wide range of technological areas (electronic and photoelectronic devices, sensors, and advanced energy storage and conversion systems). The recent increase in the number of papers in the design and fabrication of 2D heterostructures is testimonial of the interest and relevance of this new research direction.
With global energy consumption and CO2 emission increasing nearly exponentially in recent times, it is crucial to develop more effective technologies for harvesting, conversion and storage of clean and renewable energy;and materials innovations are a key
enabler. In this regard the GL-LMD/GNS composites are a new generation of hybrid nanomaterials that have shown good promise. Recent research has shown that GL-LMD (MoS2, WS2, SnS2 and others)/GNS composites are competent Li-ion battery anode materials. Their effectiveness for reversible lithium storage could be attributed to a number of factors: the unique structure and outstanding properties of GNS, a robust heterolayered composite structure and the synergistic interaction between GL-LMD and GNS. The tight coupling between GL-MS2 (M = Mo, W) and graphene also contributes to the activity of the composite in electrochemical HER. Hence the GL-MS2/GNS (M = Mo, W) composites can be used for hydrogen production by electrochemical water splitting. The composite can also function as a co-catalyst for the photocatalytic generation of hydrogen in the presence of a photocatalyst. Of particular interest is a p-MoS2/«-doped-RGO composite which can be used directly as a photocatalyst for solar hydrogen production form water splitting. Since p-MoS2/«-doped-RGO only contains earth-abundant, nontoxic and inexpensive elements, it has good potential to become a highly-active and low-cost catalyst for industrial-scalable hydrogen production and efficient solar energy conversion.
Even with the demonstration of synergistic interaction between GL-LMD and GNS in several cases that led to enhanced performance in applications, more efforts are still needed to well characterize the electronic structure of the heterostructure for a better understanding of the synergistic interaction. It remains to be a challenge to develop synthesis routes which can produce hetero-layered structures with good control of the size and layer number of GL-LMDs based on van der Waals epitaxy on a graphene surface or layer-by-layer assembly from the dispersions of the 2D crystal nanosheets. The GL-LMD/GNS composites should also be assimilated by facile and scalable methods to promote industrial acceptance. In this regard the bottom-up approach based on the solution chemistry reduction of GOS produced on a relatively large scale could be a promising route if the synthesis can provide good control of the LMD size and layer number. Thus far the GL-LMD/GNS heterostructures have been investigated mainly for energy storage and conversion applications;there may be new opportunities to be uncovered in other application areas such as electronic and photoelectronic devices.
Acknowledgements
This work was financially supported by the Natural Science Foundation of China (21173190), International Sci-Tech Cooperation Program of China (2012DFG42100), Singapore A*STAR Project 1220203049 (R279-000-370-305), Doctoral Program of China (2011010113003), International Sci-Tech Cooperation Project of Zhejiang Province (2013C24011) and Postdoctoral Fund of China (2013M540485).
References
[1] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183.
[2] X. Huang, et al. Small 7 (2011) 1876.
[3] M. Pumera, Energy Environ. Sci. 4 (2011) 668.
[4] E. Pollak, et al. Nano Lett. 10 (2010) 3386.
[5] G.X. Wang, et al. Carbon 47 (2009) 2049.
[6] E. Yoo, et al. Nano Lett. 8 (2008) 2277.
[7] P.V. Kamat, J. Phys. Chem. Lett. 2 (2011) 242.
[8] L.W. Ji, et al. Energy Environ. Sci. 4 (2011) 2682.
[9] C.N.R. Rao, A. Nag, Eur. J. Inorg. Chem. (2010) 4244.
[10] G. Eda, et al. Nano Lett. 11 (2011) 5111.
[11] H. Li, et al. Adv. Funct. Mater. 22 (2012) 1385.
[12] S.X. Wu, et al. Small 8 (2012) 2264.
[13] R.S. Sundaram, et al. Nano Lett. 13 (2013) 1416.
[14] E.S. Kadantsev, P. Hawrylak, Solid State Commun. 152 (2012) 909.
[15] A. Splendiani, et al. Nano Lett. 10 (2010) 1271.
[16] B. Radisavljevic, et al. Nat. Nano 6 (2011) 147.
[17] G. Du, et al. Chem. Commun. 46 (2010) 1106.
[18] J. Xiao, et al. Chem. Mater. 22 (2010) 4522.
[19] H. Hwang, H. Kim, J. Cho, Nano Lett. 11 (2011) 4826.
[20] Z.Y. Yin, et al. ACS Nano 6 (2012) 74.
[21] H. Li, et al. Small 8 (2012) 682.
[22] D.J. Late, et al. ACS Nano 7 (2013) 4879.
[23] C.F. Zhu, et al. J. Am. Chem. Soc. 135 (2013) 5998.
[24] Z.Y. Zeng, et al. Angew. Chem. Int. Ed. 50 (2011) 11093.
[25] J.N. Coleman, et al. Science 331 (2011) 568.
[26] R.J. Smith, et al. Adv. Mater. 23 (2011) 3944.
[27] K.-G. Zhou, et al. Angew. Chem. Int. Ed. 50 (2011) 10839.
[28] Y. Liu, et al. ACS Nano 7 (2013) 4202.
[29] K. Chang, et al. J. Mater. Chem. 21 (2011) 6251.
[30] C. Altavilla, M. Sarno, P. Ciambelli, Chem. Mater. 23 (2011) 3879.
[31] Y.M. Shi, et al. Nano Lett. 12 (2012) 2784.
[32] Y.H. Lee, et al. Adv. Mater. 24 (2012) 2320.
[33] Q.H. Wang, et al. Nat. Nanotechnol. 7 (2012) 699.
[34] T. Georgiou, et al. Nat. Nanotechnol. 8 (2013) 100.
[35] A.K. Geim, I.V. Grigorieva, Nature 499 (2013) 419.
[36] S.J. Guo, S.J. Dong, Chem. Soc. Rev. 40 (2011) 2644.
[37] M.S. Xu, et al. Chem. Rev. 113 (2013) 3766.
[38] S.Z. Butler, et al. ACS Nano 7 (2013) 2898.
[39] Z.S. Wu, et al. Nano Energy 1 (2012) 107.
[40] K. Chang, W.X. Chen, J. Mater. Chem. 21 (2011) 17175.
[41] K. Chang, W.X. Chen, Chem. Commun. 47 (2011) 4252.
[42] K. Chang, W.X. Chen, ACS Nano 5 (2011) 4720.
[43] Z. Wang, et al. J. Mater. Chem. A 1 (2013) 2202.
[44] Z. Wang, et al. RSC Adv. 3 (2013) 21675.
[45] G. Huang, et al. Small 9 (2013) 3693.
[46] K. Chang, et al. Adv. Energy Mater. 3 (2013) 839.
[47] X.S. Zhou, L.J. Wan, Y.G. Guo, Chem. Commun. 49 (2013) 1838.
[48] Y. Gong, et al. Adv. Funct. Mater. 24 (2014) 125, http://dx.doi.org/10.1002/ adfm.201300844.
[49] Y. Gong, et al. Adv. Mater. 25 (2013) 3979.
[50] X. Cao, et al. Small 9 (2013) 3433.
[51] K. Shiva, et al. Nano Energy 2 (2013) 787.
[52] X.D. Xu, et al. J. Mater. Chem. A 1 (2013) 14548.
[53] J. Yang, Angew. Chem. Int. Ed. 52 (2013) 1.
[54] D. Chen, et al. Nanoscale 5 (2013) 7890.
[55] K. Chang, et al. J. Power Sources 201 (2012) 259.
[56] L.H. Zhuo, et al. RSC Adv. 2 (2012) 5084.
[57] Z.F. Jiang, et al. J. Mater. Chem. 22 (2012) 9494.
[58] B. Luo, et al. Energy Environ. Sci. 5 (2012) 5226.
[59] M. Sathish, et al. J. Phy. Chem. C 116 (2012) 12475.
[60] J. Choi, et al. Chem. Commun. 47 (2011) 5241.
[61] X. Jiang, et al. J. Power Sources 237 (2013) 178.
[62] P. Chen, ACS Appl. Mater. Interfaces 5 (2013) 12073.
[63] X. Huang, et al. Chem. Soc. Rev. 41 (2012) 666.
[64] S. Bai, X.P. Shen, RSC Adv. 2 (2012) 64.
[65] X. Cui, et al. Nanoscale 3 (2011) 2118.
[66] M. Zhang, et al. J. Mater. Chem. 21 (2011) 1673.
[67] W.I. Park, et al. Nanoscale 3 (2011) 3522.
[68] L. Mao, et al. J. Mater. Chem. 22 (2012) 1845.
[69] C.X. Peng, et al. ACS Nano 6 (2012) 1074.
[70] Y.X. Xu, et al. ACS Nano 4 (2010) 4324.
[71] Q. Wang, J.H. Li, J. Phy. Chem. C 111 (2007) 1675.
[72] S.K. Park, et al. Dalton Trans. 42 (2013) 2399.
[73] J. Xiao, et al. Adv. Funct. Mater. 21 (2011) 2840.
[74] Y.D. Ma, et al. Nanoscale 3 (2011) 3883.
[75] J. Park, et al. Electrochim. Acta 92 (2013) 427.
[76] V.O. Koroteev, et al. J. Phys. Chem. C 115 (2011) 21199.
[77] X.D. Li, et al. J. Phys. Chem. C 117 (2013) 15347.
[78] W. Li, et al. ACS Nano 5 (2011) 2264.
[79] B. Hinnemann, et al. J. Am. Chem. Soc. 127 (2005) 5308.
[80] T.F. Jaramillo, et al. Science 317 (2007) 100.
[81] Y.G. Li, et al. J. Am. Chem. Soc. 133 (2011) 7296.
[82] D. Merki, et al. Chem. Sci. 2 (2011) 1262.
[83] J. Kibsgaard, et al. Nat. Mater. 11 (2012) 963.
[84] T.Y. Wang, et al. Energy Environ. Sci. 6 (2013) 625.
[85] D. Voriy, et al. Nat. Mater. 12 (2013) 850.
[86] V.W.H. Lau, et al. ChemCatChem 3 (2011) 1739.
[87] H.I. Karunadasa, et al. Science 335 (2012) 698.
[88] Y. Li, et al. J. Am. Chem. Soc. 133 (2011) 7296.
[89] S.X. Min, G.X. Lu, J. Phys. Chem. C 116 (2012) 25415.
[90] Y.-H. Chang, et al. Adv. Mater. 25 (2013) 756.
[91] E.G.S. Firmiano, et al. Chem. Commun. 48 (2012) 7687.
[92] G. Yue, et al. Electrochim. Acta 85 (2012) 162.
[93] C.-J. Liu, et al. J. Mater. Chem. 22 (2012) 21057.
[94] J.-Y. Lin, C.-Y. Chan, S.-W. Chou, Chem. Commun. 49 (2013) 1440.
[95] H. Tang, et al. J. Mater. Chem. A 2 (2014) 360.
[96] Q. Xiang, J. Yu, M. Jaroniec, J. Am. Chem. Soc. 134 (2012) 6575.
[97] F. Meng, et al. J. Am. Chem. Soc. 135 (2013) 10286.
[98] U. Maitra, et al. Angew. Chem. Int. Ed. 52 (2013) 13057.
