Open access peer-reviewed chapter

Two‐Dimensional Transition Metal Dichalcogenides for Electrocatalytic Energy Conversion Applications

Written By

Fengwang Li and Mianqi Xue

Submitted: 23 November 2015 Reviewed: 26 April 2016 Published: 31 August 2016

DOI: 10.5772/63947

From the Edited Volume

Two-dimensional Materials - Synthesis, Characterization and Potential Applications

Edited by Pramoda Kumar Nayak

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Abstract

Electrocatalytic energy conversion using renewable power sources is one of the most promising ways for energy storage and energy utilization in the new century. Specific catalysts are needed to improve the electrocatalytic reactions. Two‐dimensional transition metal dichalcogenides (2D TMDs) have attracted considerable interest as alternatives to noble metal catalysts due to their unique electronic structure and high catalytic activity. Over the past years, a great number of 2D TMD‐based catalysts have been explored for various electrocatalytic reactions, such as the hydrogen evolution reaction (HER) as a half reaction of water splitting and CO2 reduction reaction as part of artificial photosynthesis. This chapter provides an overview of recent progress on TMD‐based electrocatalysts, including mechanism understanding of the advantages of 2D materials, especially 2D TMDs and the up‐to‐date synthesizing approaches of TMDs, and state‐of‐the‐art applications of TMDs in electrocatalytic reactions, and finally outlines the current challenges and future opportunities.

Keywords

  • 2D materials
  • transition metal dichalcogenide
  • electrocatalysis
  • water splitting
  • hydrogen evolution reaction
  • CO2 reduction reaction
  • artificial photosynthesis

1. Introduction

Owing to the rapid development of modern society, the enormous demand for energy has become one of the most important issues affecting human life since twentieth century. However, the excessive reliance on the combustion of nonrenewable fossil fuels, such as coal, petroleum, and natural gas, brings not only ecological and environmental problems but also harsh ongoing impacts on the global economy and society [1]. Hence, it has become one of the crucial challenges faced with our society to develop reliable and “green” approaches for energy conversion and storage.

Electrocatalytic energy conversion utilizing renewable power sources (e.g. solar and wind energy) is regarded as one of the most efficient and cleanest energy conversion pathways [25]. Furthermore, the converted energy is easy to store and use as clean energy or chemical stock. Specifically, the involvement of the electrocatalytic hydrogen evolution reaction (HER) in the cathode and the oxygen evolution reaction (OER) in the anode can efficiently drive water splitting and finally convert the electrical energy into chemical form, that is, hydrogen energy [68]. When CO2 is reduced in the cathode while OER happens in the anode, which is the scheme of so‐called artificial photosynthesis, it converts the electrical energy into chemical forms stocked in CO or hydrocarbons [911]. Hence, in such context, it is urgently required in both academic and industrial fields to build our power‐supply systems based on electrocatalysis, amongst which developing efficient electrocatalysts for the aforementioned reactions is the most fundamental but vital task in this endeavour.

Two‐dimensional (2D) materials have been widely studied for their important physical and chemical properties over the last several decades [12]. Since the recent discovery of graphene [13], 2D materials have gained extensive attention since they exhibit novel and unique physical, chemical, mechanical, and electronic properties [3, 1419]. In the abundant family of 2D materials, transition metal dichalcogenides (TMDs) have attracted significant interest and become the focus of fundamental research and technological applications due to their unique crystal structures, a wide range of chemical compositions, and a variety of material properties [5, 14, 2024]. Recently, TMDs have emerged as one kind of efficient electrocatalysts for energy‐related reactions, such as the HER and CO2 reduction reaction [15, 21, 2529].

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2. Properties of 2D TMDs and advantages for electrocatalysis

2D TMDs are usually denoted as MX2, where M is a transition metal of groups 4–10 (e.g. Ti, V, Co, Ni, Nb, Mo, Hf, Ta, and W) and X is a chalcogen (S, Se, and Te). MX2 constructs layered structures by the formation of X‐M‐X, three layers of atoms with the chalcogen atoms in two hexagonal planes separated by a plane of metal atoms, and the valence states of metal (M) and chalcogen (X) atoms are +4 and -2, respectively (Figure 1a) [3032]. There are two combination modes of metal and chalcogen elements in MX2, trigonal prismatic or octahedral phases, which are referred to as monolayer 2H and 1T‐MX2, respectively (Figure 1bd). Transition metals in different groups have different numbers of d‐electrons, which fill up the non‐bonding d bands to different levels, resulting in varied electronic properties ranging from insulators such as HfS2, semiconductors such as MoS2 and WS2, semimetals such as WTe2 and TiSe2, to true metals such as NbS2 and VSe2. A few bulk TMDs such as NbSe2 and TaS2 exhibit low‐temperature phenomena including superconductivity, charge density wave (CDW), and Mott transition [14]. When these materials are exfoliated into mono‐ or few layers, their properties largely preserve and also present additional characteristics due to confinement effects [14, 15, 27, 33]. The mono‐ or few‐layer TMDs have thickness at the atomic level or significantly lower than their edge lengths and thus appear like sheets (namely, nanosheets). The atomic‐thin nature endows them with many distinctive properties with respect to their bulk counterparts, such as high specific surface area owing to planar structures, abundant uncoordinated surface atoms, excellent solution dispersity, and mechanical flexibility. These features make the 2D TMDs ideal candidates (or component parts for hybrid structures) with improved electrocatalytic performance to substitute their parent materials.

Figure 1.

Structures of TMDs and schematic illustration for the advantages of TMDs as the electrocatalyst. (a) Structure of TMDs. Reproduced with permission from Radisavljevic et al. [32]; copyright 2011 Nature Publishing Group. Schematic representations of a typical TMD structure with trigonal prismatic (b) and octahedral (c) coordinations from c‐axis (upper) and section view (middle). Reproduced with permission from Chhowalla et al. [14]; copyright 2013 Nature Publishing Group, (d) side view of the structure. Reproduced with permission from Wang et al. [33]; copyright 2012 Nature Publishing Group and (e) schematic illustration for the advantages of 2D TMDs as the electrocatalyst.

To advance the catalysis, especially electrocatalysis research, it is imperative to deepen the understanding on the underlying mechanisms involved in catalytic processes. As heterogeneous electrocatalysis essentially occurs at the interface of electrode (including catalyst) and bulk solution, the surface of catalysts should play a key role in determining species adsorption and electron transfer and, in turn, hold promise to tailoring reaction activity and selectivity in catalysis. In this regard, the 2D TMDs hold the advantages to catalyse the electrochemical reactions for the following reasons, except their unique intrinsic properties (Figure 1e).

(1) High surface areas. Since the electrocatalytic reactions occur on the surface of catalysts via electron transfer, species adsorption, and activation, it is the surface atom that mainly participates in the reactions. For this reason, boosting the surface‐to‐volume ratio would expose more atoms to the reaction species and thus help increase the probability of “active sites” to interact with reaction species. In comparison with conventional nanocatalysts, 2D materials possess significantly higher ratio of surface atom number to total atom number, thereby promoting their catalytic activities. This ratio increases with the reduction in the number of atomic layers and hypothetically reaches its maximum value in monolayer.

(2) Abundant uncoordinated surface atoms. During a catalytic process, the reaction substrates are absorbed onto the surface atoms of the catalyst and then dissociated into highly reactive intermediates, which makes these surface atoms catalytically active sites. Certainly not all the surface atoms can efficiently participate in catalytic reactions. As a matter of fact, the adsorption and dissociation often take place on the coordinately unsaturated sites that are thermodynamically instable [16]. In the 2D materials, there are much more uncoordinately unsaturated atoms on the surface, which enhances their activities in catalysis. It is worth noting that the basal surfaces of catalysts might be catalytically inert in some certain reactions [14, 16]. Nevertheless, this feature does not diminish the advantage of 2D materials since catalytic activity can still arise from the active sites located along the edges of the nanosheets. For example, MoS2 is a typical and efficient electrocatalyst for HER with higher activity at the edge compared to the basal part. The research indicates that atoms at the edges have lower coordination number than those on the surface, thereby providing reaction sites with higher catalytic activity [34, 35].

(3) Planar structure with atomic thickness. In general, the electrocatalytic performances of a catalyst are not only governed by the intrinsic nature and the number of active sites but also determined by the electron transfer between the active site and the supporting electrode [36]. From this point of view, the unique planar structure with atomic thickness of 2D materials is a decisive advantage for catalysis. The density of states should be significantly increased due to surface distortion of 2D materials, which favours the electron transport along the 2D conducting channels with high mobility as well as between its interface with other components or media, facilitating the electron diffusion between the catalytically active sites and the supporting electrode [37]. Furthermore, the unique planar structure of 2D materials also makes them ideal loading substrates for the assembly or growth of various novel hybrid catalysts. Other building blocks can be readily loaded on the flat surface or stacked layer by layer for various catalytic applications [38].

(4) Excellent solution dispersity. 2D materials often show excellent dispersity in certain solution whose type depends on their synthetic method. Thus far, exfoliation in aqueous solution is the most widespread method to produce 2D materials [39]. Most of the resulted materials are thus dispersible in water to facilitate further processing and applications towards green chemistry. In general, their superior dispersity to other counterparts is enabled by high surface area as well as large portion of uncoordinated surface atoms. The high specific surface area keeps nanosheets from precipitation, while electrostatic repulsion between the nanosheets further prevents their agglomeration.

(5) Highly tunable properties. One major difference between 2D TMDs and other 2D materials, such as graphene, is their high anisotropy and unique crystal structure. For this reason, the material properties of 2D TMDs can be effectively tuned in a wide range through different methodologies including reducing dimensions, intercalation, heterostructure, alloying, gating, pressure, lighting, and so forth [27, 4044]. For example, through the intercalation of guest ions, the carrier densities of 2D TMDs can be tuned by multiple orders of magnitude, which will affect the electron transfer rate in the electrocatalytic reactions [45]. 2D TMDs provide a great platform of tuning material properties towards desired activity and selectivity of a specific electrochemical reaction.

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3. Preparation methods for 2D TMDs

2D TMDs provide excellent model systems to fundamentally establish structure‐property relationship in electrocatalysis given their unique structural characteristics. Certainly the synthesis and fabrication of related materials are the central part of this research and both physical and chemical approaches have been utilized effectively for the synthesis of TMDs.

3.1. Exfoliation

Exfoliation is a top‐down assembly method, in which physical and chemical driving forces are used to achieve separation of bulk materials. In the exfoliation of 2D materials, the precursors are usually their bulk counterparts. During the exfoliation, external or internal driving force is needed to weaken and eventually overcome the van der Waals force between adjacent layers. It can use mechanical force, such as friction or shear forces, or chemical force, in which the free energy or externally added electrochemical energy provides the driving force. Exfoliated sheets must typically be stabilized to prevent aggregation and re‐stacking using surfactants, polymers, solvents, or liquid‐liquid interfaces that trap and stabilize the exfoliated sheets [39, 46, 47].

It is the successful isolation of graphene from graphite using scotch tape that sparks the tremendous interest in exfoliating 2D TMD materials [13]. This mechanical exfoliation possesses the advantages of being highly reproductive and is quite suitable to fabricate single devices for research purposes and build devices based on all‐layered materials [12, 48]. Nevertheless, mechanical exfoliation is not suitable for large‐scale production due to the absence of layer number and lateral size control capability and it also suffers from low yield and contaminates monolayer surfaces with the adhesive polymer [32]. The limitations on throughput can be overcome by exfoliation in the liquid phases [46, 47, 49]. In general, direct sonication of a layered host is carried out in a solvent chosen to stabilize the exfoliated sheets and sometimes selected based on matching surface tension to solid surface energies. Although this method can partially exfoliate TMDs into few‐layer materials, only a very low yield of monolayer TMDs can be produced.

In addition to the direct dry or liquid‐phase exfoliation, a two‐step process, ion intercalation followed by exfoliation, is able to produce TMDs with a higher yield. Lithium, sodium, or potassium ions are intercalated into the interlayer space and form ion‐intercalated compounds, which can be further sonicated in water or organic solvents to form TMD dispersions [50]. Exfoliation of the bulk TMD crystals can also be achieved using organolithium compounds. For example, the n‐BuLi reacts chemically with TMDs, forming Li‐intercalated compounds [51]. The compounds are further exfoliated by the reaction of Li with water. A variety of TMD sheets, including MoS2, TiS2, TaS2, and WS2, can be produced by this method with the lateral size up to few microns (Figure 2a).

Figure 2.

Preparation methods for TMDs. (a) Chemical exfoliation process of TMDs. Reproduced with permission from Zheng et al. [50]; copyright 2014 Nature Publishing Group, (b) CVD growth of TMDs. Reproduced with permission from Shi et al. [59]; copyright 2014 Royal Society of Chemistry, (c) hydrothermal growth of MoS2 on reduced graphene oxide. Reproduced with permission from Li et al. [83]; copyright 2011 American Chemical Society, and (d) colloidal synthesis of TMDs. Reproduced with permission from Yoo et al. [85]; copyright 2014 American Chemical Society.

Electrochemical exfoliation has been used for several decades for exfoliation and restacking of layered materials to generate novel compounds [14]. It proceeds through the electrochemical insertion of an ion (such as Li+) into the host crystal. This destabilizes the crystal while inducing a phase change at the same time (Eq. (1)).

MoS2+xLi++xeLixMoS2E1
LixMoS2+xH2OMoS2+x2H2+xLiOHE2

Placing the intercalated material in polar solvents forces hydrolysis of the lithiated species and formation of single‐sheet colloidal suspensions (Eq. (2)) [52, 53]. The yield of this method is nearly 100% but requires long reaction times and careful exfoliation to prevent destruction. This method may be one of the most promising for large‐scale fabrication of true monolayer materials [14, 52, 54, 55].

3.2. Chemical vapour deposition

Chemical vapour deposition (CVD) is an important and widely used technique for growing inorganic materials, which yields large, high‐quality single crystals of oxide and chalcogenide materials with morphologies ranging from nanoribbons, plates, to monolayers [5658]. In a typical CVD process, source powder(s) or molecular precursor in solution is heated. A carrier gas (e.g. argon, nitrogen, or forming gas) transports the vapour‐phase precursors downstream to substrates that are placed in a region of appropriate temperature for nucleation of TMDs (Figure 2b). Optimization of substrate choice, molecular precursors, and reaction geometry can facilitate growth of monolayers [59]. Compared with chemical exfoliation, the CVD method is more efficient in growing TMD monolayer films on substrates (SiO2/Si [60] or Au [61]), with high quality and controllable thickness [46, 62].

Several CVD synthesis methods for TMDs have been studied, such as sulphurization of a transition metal or metal oxide thin film, thermal decomposition of thio‐salts, and vapour‐phase transport method [6370]. Furthermore, Li and co‐workers developed a growth method of TMDs via vapour‐phase chemical reaction of transition metal oxide and chalcogen, which can control the thickness and crystallinity of TMDs [71, 72]. Specifically, metal oxide MoO3 is used as transition metal source and it undergoes a two‐step reaction. The suboxide MoO3-x is firstly produced during the reaction, which further serves as an intermediate to react with chalcogen vapour (sulphur) and forms the monolayer TMDs with a triangular shape.

Further studies show that the formation of TMDs is controlled by the surface energy of substrate. The aromatic molecules can significantly enhance the wetting between precursors and the substrate surfaces and thus promote the nucleation and lateral growth of TMDs [73]. TMDs’ single‐crystal domain with lateral size up to several hundred micrometres can be produced by optimising the vapour‐phase reaction conditions [74]. The direct vapour‐phase reaction of transition metal oxide and sulphur/selenium has been widely adopted to produce TMDs including MoS2, WS2, MoSe2, and WSe2 [62, 7577].

3.3. Wet chemical approaches

Wet chemical approaches are bottom‐up methods which offer a potentially powerful alternative to exfoliation and CVD. It can be used to synthesize TMDs with thicknesses ranging from the monolayer to hundreds of layers [78, 79]. Compared with CVD method, the reaction temperatures are much lower, and the produced materials are exceptional uniform and with low defect density. Thanks to diverse wet‐chemical methods, the materials can be doped by adding other reagents during growth and one can also use ligand chemistry to cap the material's surface in order to modify or protect the surface [8082]. Moreover, wet‐chemical methods are often easily translated into larger‐scale manufacturing processes, which may facilitate the commercialization of TMD materials. By selection of environmentally precursors and solvents, solution‐based methods can be adapted to adhere to principles of green chemistry and manufacturing [80].

A traditional wet‐chemical approach to chalcogenides involves hydrothermal or solvothermal growth (Figure 2c) [83]. Taking the synthesis of MoS2 nanoflakes as an example, in a typical procedure, (NH4)6Mo7O24·4H2O and thiourea have been utilized as the precursors for the Mo and S elements in hydrothermal reactions [84]. After reaction in a Teflon‐lined stainless steel autoclave, the low‐quality MoS2 flakes with abundant active sites are obtained.

Colloidal synthesis is a well‐established technique for synthesizing TMDs [80]. In a typical process, similar to other colloidal synthetic routes, a cold solution of precursor chemicals is injected into a hot solvent or a one‐pot route can also be adopted where precursors are mixed together and then heated (up to 320 °C). Recently, it was reported that monolayer TMDs such as TiS2, HfS2, and ZrS2 could be synthesized by a novel colloidal referred to as “diluted chalcogen continuous influx” [85]. In this method, the delivery rate of a chalcogen source (such as H2S or CS2) to a transition metal halide precursor in solution was controlled to be slow enough to favour the lateral (2D) growth over 3D growth (Figure 2d).

Although the wet‐chemical approaches may unavoidably alter the lattice structure of thin TMDs and introduce extrinsic defects during exfoliation process, these defects may be helpful in electrocatalytic reactions [14, 16, 22].

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4. Applications of 2D TMDs for electrocatalytic energy conversion

4.1. Hydrogen evolution reaction

H2 is considered as one of the most promising energy carriers owing to its high energy density and environmentally benign character. Nevertheless, it is still a great challenge to produce H2 efficiently [7, 9]. Amongst various H2 production pathways, the electrocatalytic hydrogen evolution reaction is attracting tremendous attention due to its high energy‐converting efficiency and the abundant raw material, namely, water. As is well known, hydrogen molecules can be generated from a process named water splitting, which can be divided into two independent half‐reactions, HER to generate hydrogen in the cathode and OER to produce oxygen in the anode. However, the large electrolytic window of water means that appropriate electrocatalysts are required to lower the overpotential for water splitting.

Figure 3.

TMDs as electrocatalyst for HER. (a) Plot of exchange current density as a function of DFT‐calculated Gibbs free energy of absorbed atomic hydrogen for MoS2 and pure metals. Reproduced with permission from Jaramillo et al. [34]; copyright 2007, American Association for the Advancement of Science, (b) nitrogenase (left) and hydrogenase (middle) inorganic compounds designed to mimic the edge sites of MoS2 (right). Reproduced with permission from Hinnemann et al. [29]; copyright 2005 American Chemical Society, (c) HRTEM image and the Fourier transform pattern (inset) of the defect‐rich MoS2 ultrathin nanosheets, (d) polarization curves of various MoS2‐based samples as indicated. Reproduced with permission from Xie et al. [87]; copyright 2013 Wiley, (e) schematics of MoO3‐MoS2 core‐shell nanowires as catalysts for HER. Reproduced with permission from Chen et al. [90]; copyright 2011 American Chemical Society, (f) scheme for the vertically aligned MoS2 layers. Reproduced with permission from Kong et al. [43]; copyright 2013 American Chemical Society, (g) exfoliation of 2H MoS2 to 1T MoS2 by lithium intercalation. Reproduced with permission from Lukowski et al. [91]; copyright 2013 American Chemical Society, and (h) scheme for the MoSx/NCNT forest hybrid catalyst. Reproduced with permission from Li et al. [94]; copyright 2014 American Chemical Society.

Pt is currently the most efficient electroactive and electrochemically stable catalyst for HER, but its high cost and rare existence limit its wide application. Hence, exploring other earth abundant materials with high catalytic activities has attracted intensive interest. The experimental explorations of TMDs as the electrocatalyst for HER were motivated by theoretical calculations on MoS2 materials, which demonstrated that the hydrogen‐binding energy of MoS2 was close to that of metals such as Pt, Rh, Re, and Ir (Figure 3a) [29, 34, 86]. The density functional theory (DFT) calculations showed that metallic edges of trigonal prismatic (2H) MoS2 clusters were highly active compared to the basal plane of the chalcogenide, where it remained inert from the electrochemical point of view [14]. The surface‐active sites of MoS2 was also probed by biomimicry of Mo(IV)‐disulphide inorganic and organic (Figure 3b) [29, 35].

The activities of TMDs are usually limited by the proportion of active edge sites [16]. To tackle this problem, Lou and co‐workers fabricated the defect‐rich MoS2 ultrathin nanosheets by adding excess thiourea in the precursors (Figure 3c and d) [87]. The excess thiourea played a key role in the formation of defect‐rich MoS2, which not only worked as a reductant to reduce VIMo to IVMo but also worked as a capping agent to stabilize the morphology of MoS2 nanosheets. The resultant defect‐rich MoS2 showed outstanding electrocatalytic activity towards HER. It held a low overpotential of 120 mV, a large current density, and a small Tafel slope of 50 mV decade-1. They attributed the superior performance to the additional active edge sites exposed on defect‐rich MoS2 ultrathin nanosheets. To increase the active surface of MoS2, Kibsgaard et al. fabricated a 3D MoS2 porous network [88]. Chen et al. synthesized vertically oriented MoO3‐MoS2 core‐shell nanowires, in which the MoS2 shell contributes to the outstanding catalytic response as well as to protection against corrosion (Figure 3e) [89, 90]. Cui's group synthesized MoS2 films with vertically aligned layers [43]. The structure predominantly exposes the edges on the film surface maximally (Figure 3f). The edge‐terminated surface is obtained by overcoming the free energy barrier kinetically through rapid sulphurization.

Besides the active sites, the electric conductivity of MoS2 is another crucial factor to affect its electrocatalytic activity. Jin and co‐workers reported metallic 1T‐MoS2 nanosheets, which were prepared by chemical exfoliation via lithium intercalation to from semiconducting 2H‐MoS2 nanostructures (Figure 3g) [91]. This catalyst exhibited metallic conductivity and achieved a current density of 10 mA cm-2 at an overpotential of -187 mV vs. RHE. Additionally, a small Tafel slope of 43 mV decade-1 was reported for this catalyst. Xie's group studied the influence of active sites and conductivity of MoS2 on the electrocatalytic activity and achieved the balance between them by controlling disorder engineering and oxygen incorporation in MoS2 ultrathin nanosheets. This oxygen‐doped MoS2 with synergistically structural and electronic modulations achieved high‐efficient HER activity [84].

In order to further improve the catalytic efficiency and stability of TMD‐based electrocatalysts, enormous research efforts have been devoted to the incorporation of TMDs with other materials, such as noble metals and carbon materials (Figure 3h) [9296]. Zhang and co‐workers demonstrated the wet‐chemical synthesis of noble metal nanostructures epitaxially grown on TMD nanosheets. The noble metal‐TMD composites exhibit good electrocatalytic activity in hydrogen evolution reaction [97, 98].

4.2. CO2 reduction reaction

Despite the tremendous efforts being made to implement renewable energy sources, there remains a need in the longer term to be able to sustainably generate liquid fuels for applications including aviation and mining [9, 11]. Electrochemical CO2 reduction, recycling CO2 back to fuels, and commodity chemicals utilizing renewable energy as a power source could potentially provide a solution to this problem [99]. However, CO2 is very stable under environmental conditions and HER often prevails over CO2 reduction in aqueous electrolytes under cathodic polarization [100, 101], making it essential to find a suitable catalyst to achieve cost‐effective CO2 reduction with high efficiency and selectivity. Metals and especially nanostructured metals derived from metal oxide have been widely studied as electrocatalysts for CO2 reduction [100, 102106]; however, these systems generally show low activities and/or selectivity for a solo product (such as CO, formate, methanol, methane, ethylene, and ethanol) or need nonaqueous solvents which may limit practical application.

Figure 4.

TMDs as electrocatalysts for electrochemical reduction of CO2. (a) Binding energies Eb(COOH) vs. Eb(CO) for transition metals and Mo and S edges of MoS2. Reproduced with permission from Shi et al. [108]; copyright 2014 Royal Society of Chemistry, (b) raw greyscale HAADF and false‐colour low‐angle annular dark‐field (LAADF) image (inset) of MoS2 edges (scale bar, 5 nm), (c) cyclic voltammetric (CV) curves for bulk MoS2, Ag nanoparticles (Ag NPs), and bulk Ag in CO2 environment. The electrolyte is a mixture of 96 mol% water and 4 mol% EMIM‐BF4, (d) CO and H2 Faradaic efficiency (FE) at different applied potentials. Reproduced with permission from Asadi et al. [109]; copyright 2014 Nature Publishing Group, (e) CVs of rGO‐PEI‐MoS2‐modified GCE in N2‐saturated (black curve) and CO2‐saturated (red curve) 0.5 M aqueous NaHCO3 solution. Scan rate was 50 mV s-1, (f) Faradaic efficiency for CO (red bars) and H2 (blue bars) as a function of potential, (g) amount and Faradaic efficiency of H2 (circles) and CO (squares). Potentiostatic electrolysis at -0.4 V in CO2‐saturated 0.5 M aqueous NaHCO3 solution and (h) Tafel plot of CO production partial current density vs. overpotential on rGO‐PEI‐MoS2. Reproduced with permission from Li et al. [110]; copyright 2016 Royal Society of Chemistry.

Recently, Nørskov et al. demonstrated theoretically that MoS2 or MoSe2 could possibly be electrocatalysts for CO2 reduction by DFT calculation [107, 108]. Their results indicate the edge site of MoS2 or MoSe2 is active for electrochemical CO2 reduction due to the different scaling relationships of adsorption energies between key reaction intermediates (*CO and *COOH) on the edges of MoS2 or MoSe2 compared to transition metals (Figure 4a). Experimental results of MoS2 as electrocatalyst for CO2 reduction were firstly reported by Asadi et al. [109] (Figure 4bd). They uncovered that MoS2 showed superior CO2 reduction performance compared with the noble metals with a high current density and low overpotential (54 mV) in an ionic liquid. They also utilized DFT calculations to reveal the catalytic activity mainly arises from the molybdenum‐terminated edges of MoS2 due to their metallic character and a high d‐electron density. The experimental result that vertically aligned MoS2 showed an enhanced performance compared to bulk MoS2 crystal supported their calculations.

Li and co‐workers reported amorphous MoS2 on a polyethylenimine‐modified reduced graphene oxide substrate as an effective catalyst for electrocatalytic CO2 reduction (Figure 4eh) [110]. The catalyst is capable of producing CO at an overpotential as low as 140 mV and reaches a maximum Faradaic efficiency (FE) of 85.1% at an overpotential of 540 mV. Another interesting point is that at an overpotential of 290 mV with respect to the formation of CO, it catalyses the formation of syngas with high stability, which could be readily utilized in the current Fischer‐Tropsch process and produce liquid fuels, such as ethanol and methanol. Their detailed mechanism investigation indicated that the efficiency and selectivity towards CO2 reduction rather than hydrogen evolution at the optimal applied potential were attributed to the synergetic effect of MoS2 and PEI: (1) the intrinsic properties of MoS2 that it can selectively bind the intermediate during the CO2 reduction reaction path is the principal factor contributing the CO2 reduction and (2) PEI, an amine containing polymer with outstanding CO2 adsorption capacity, can stabilize the intermediate and thus lower the energy barrier by hydrogen bond interaction.

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5. Summary and outlook

Energy issue is one of the most urgent and critical topics in our modern society. Recently, there is increasing demand for cost‐effective, efficient, and environmental‐friendly energy conversion and storage devices to reduce the excessive reliance on nonrenewable fossil fuels. Due to the unique physicochemical properties of 2D TMDs, they have shown enormous potential for wide‐ranging and diversified fundamental and technological applications, which include intensive research on electrocatalytic energy conversion applications, especially hydrogen evolution reaction and CO2 reduction reaction. In these electrocatalytic reactions, the maximization of active edges and the conductivity are identified as the core issues for further development of TMD‐based catalysts. A large number of synthetic strategies have been focused on maximizing the exposure of edge sites; phase structure tuning has also been as a potential tool for enhancing the electrical transport properties of TMDs.

Overall, the rich chemistry of TMDs builds an extensive platform for the study of fundamental and practical scientific phenomena in the development of real electrocatalysts for energy conversion applications. There is still much room to further improve the electrocatalytic performance of TMDs. Specifically, the fine tune of band structure and Fermi level could provide as powerful tools. Hence, a combination of theoretical, fundamental, and electrocatalysis‐based applications should be explored in order to make a guidance to the developing directions. Furthermore, the mass‐productive synthesis of high‐quality TMDs should emerge as an urgent issue to adapt to the widely application of them in an industry level.

References

  1. 1. Dresselhaus M, Thomas I. Alternative energy technologies. Nature. 2001;414:332–337.
  2. 2. Hong WT, Risch M, Stoerzinger KA, Grimaud A, Suntivich J, Shao‐Horn Y. Toward the rational design of non‐precious transition metal oxides for oxygen electrocatalysis. Energy and Environmental Science. 2015;8:1404–1427.
  3. 3. Bonaccorso F, Colombo L, Yu G, Stoller M, Tozzini V, Ferrari AC, et al. 2D materials. Graphene, related two‐dimensional crystals, and hybrid systems for energy conversion and storage. Science. 2015;347:1246501.
  4. 4. Xie J, Xie Y. Transition metal nitrides for electrocatalytic energy conversion: opportunities and challenges. Chemistry – A European Journal. 2015;22:3588–3598.
  5. 5. Zhang G, Liu H, Qu J, Li J. Two‐dimensional layered MoS2: rational design, properties and electrochemical applications. Energy and Environmental Science. 2016;9;1190–1209. DOI: 10.1039/c5ee03761a.
  6. 6. Mallouk TE. Water electrolysis: divide and conquer. Nature Chemistry. 2013;5:362–363.
  7. 7. Turner JA. Sustainable hydrogen production. Science. 2004;305:972–974.
  8. 8. Norskov JK, Christensen CH. Toward efficient hydrogen production at surfaces. Science. 2006;312:1322–1323.
  9. 9. Gasteiger HA, Markovic NM. Just a dream‐ or future reality? Science. 2009;324:48–49.
  10. 10. House RL, Iha NYM, Coppo RL, Alibabaei L, Sherman BD, Kang P, et al. Artificial photosynthesis: where are we now? Where can we go? Journal of Photochemistry and Photobiology C: Photochemistry Reviews. 2015;25:32–45.
  11. 11. Lackner KS. A guide to CO2 sequestration. Science. 2003;300:1677–1678.
  12. 12. Geim AK, Novoselov KS. The rise of graphene. Nature Materials. 2007;6:183–191.
  13. 13. Novoselov KS, Geim AK, Morozov S, Jiang D, Zhang Y, Dubonos Sa, et al. Electric field effect in atomically thin carbon films. Science. 2004;306:666–669.
  14. 14. Chhowalla M, Shin HS, Eda G, Li LJ, Loh KP, Zhang H. The chemistry of two‐dimensional layered transition metal dichalcogenide nanosheets. Nature Chemistry. 2013;5:263–275.
  15. 15. Das S, Robinson JA, Dubey M, Terrones H, Terrones M. Beyond graphene: progress in novel two‐dimensional materials and van der Waals solids. Annual Review of Materials Research. 2015;45:1–27.
  16. 16. Sun Y, Gao S, Lei F, Xie Y. Atomically‐thin two‐dimensional sheets for understanding active sites in catalysis. Chemical Society Reviews. 2015;44:623–636.
  17. 17. Wang Z, Zhu W, Qiu Y, Yi X, von dem Bussche A, Kane A, et al. Biological and environmental interactions of emerging two‐dimensional nanomaterials. Chemical Society Reviews. 2016;45:1750–1780.
  18. 18. Zhang X, Xie Y. Recent advances in free‐standing two‐dimensional crystals with atomic thickness: design, assembly and transfer strategies. Chemical Society Reviews. 2013;42:8187–8199.
  19. 19. Luo B, Liu G, Wang L. Recent advances in 2D materials for photocatalysis. Nanoscale. 2016;8:6904–6920. DOI: 10.1039/c6nr00546b.
  20. 20. Lv R, Robinson JA, Schaak RE, Sun D, Sun Y, Mallouk TE, et al. Transition metal dichalcogenides and beyond: synthesis, properties, and applications of single‐ and few‐layer nanosheets. Accounts of Chemical Research. 2015;48:56–64.
  21. 21. Gao MR, Xu YF, Jiang J, Yu SH. Nanostructured metal chalcogenides: synthesis, modification, and applications in energy conversion and storage devices. Chemical Society Reviews. 2013;42:2986–3017.
  22. 22. Wang H, Yuan H, Sae Hong S, Li Y, Cui Y. Physical and chemical tuning of two‐dimensional transition metal dichalcogenides. Chemical Society Reviews. 2015;44:2664–2680.
  23. 23. Li H, Shi Y, Chiu M‐H, Li L‐J. Emerging energy applications of two‐dimensional layered transition metal dichalcogenides. Nano Energy. 2015;18:293–305.
  24. 24. Wang H, Feng H, Li J. Graphene and graphene‐like layered transition metal dichalcogenides in energy conversion and storage. Small. 2014;10:2165–2181.
  25. 25. Bang GS, Choi S‐Y. Graphene and two‐dimensional transition metal dichalcogenide materials for energy‐related applications. In: Kyung C‐M, editor. Nano Devices and Circuit Techniques for Low‐Energy Applications and Energy Harvesting. KAIST Research Series. Netherlands: Springer; 2016. p. 253–291.
  26. 26. Zhang X, Liang J, Ding S. The application of nanostructure MoS2 materials in energy storage and conversion. In: Wang MZ, editor. MoS2. Lecture Notes in Nanoscale Science and Technology. Switzerland: Springer; 2014. p. 237–268.
  27. 27. Jariwala D, Sangwan VK, Lauhon LJ, Marks TJ, Hersam MC. Emerging device applications for semiconducting two‐dimensional transition metal dichalcogenides. ACS Nano. 2014;8:1102–1120.
  28. 28. Bai S, Xiong Y. Recent advances in two‐dimensional nanostructures for catalysis applications. Science of Advanced Materials. 2015;7:2168–2181.
  29. 29. Hinnemann B, Moses PG, Bonde J, Jørgensen KP, Nielsen JH, Horch S, et al. Biomimetic hydrogen evolution: MoS2 nanoparticles as catalyst for hydrogen evolution. Journal of the American Chemical Society. 2005;127:5308–5309.
  30. 30. Huang X, Zeng Z, Zhang H. Metal dichalcogenide nanosheets: preparation, properties and applications. Chemical Society Reviews. 2013;42:1934–1946.
  31. 31. Sun Y, Gao S, Xie Y. Atomically‐thick two‐dimensional crystals: electronic structure regulation and energy device construction. Chemical Society Reviews. 2014;43:530–546.
  32. 32. Radisavljevic B, Radenovic A, Brivio J, Giacometti V, Kis A. Single‐layer MoS2 transistors. Nature Nanotechnology. 2011;6:147–150.
  33. 33. Wang QH, Kalantar‐Zadeh K, Kis A, Coleman JN, Strano MS. Electronics and optoelectronics of two‐dimensional transition metal dichalcogenides. Nature Nanotechnology. 2012;7:699–712.
  34. 34. Jaramillo TF, Jørgensen KP, Bonde J, Nielsen JH, Horch S, Chorkendorff I. Identification of active edge sites for electrochemical H2 evolution from MoS2 nanocatalysts. Science. 2007;317:100–102.
  35. 35. Karunadasa HI, Montalvo E, Sun Y, Majda M, Long JR, Chang CJ. A molecular MoS2 edge site mimic for catalytic hydrogen generation. Science. 2012;335:698–702.
  36. 36. Lai J, Li S, Wu F, Saqib M, Luque R, Xu G. Unprecedented metal‐free 3D porous carbonaceous electrodes for water splitting. Energy and Environmental Science. 2016;9:1210–1214. DOI: 10.1039/c5ee02996a.
  37. 37. Sun Y, Cheng H, Gao S, Sun Z, Liu Q, Liu Q, et al. Freestanding tin disulfide single‐layers realizing efficient visible-light water splitting. Angewandte Chemie International Edition. 2012;51:8727–8731.
  38. 38. Huang X, Tan C, Yin Z, Zhang H. 25th anniversary article: hybrid nanostructures based on two-dimensional nanomaterials. Advanced Materials. 2014;26:2185–2204.
  39. 39. Nicolosi V, Chhowalla M, Kanatzidis MG, Strano MS, Coleman JN. Liquid exfoliation of layered materials. Science. 2013;340:1226419.
  40. 40. Fuhrer MS, Hone J. Measurement of mobility in dual‐gated MoS2 transistors. Nature Nanotechnology. 2013;8:146–147.
  41. 41. Koski KJ, Cui Y. The new skinny in two‐dimensional nanomaterials. ACS Nano. 2013;7:3739–3743.
  42. 42. Lopez‐Sanchez O, Lembke D, Kayci M, Radenovic A, Kis A. Ultrasensitive photodetectors based on monolayer MoS2. Nature Nanotechnology. 2013;8:497–501.
  43. 43. Kong D, Wang H, Cha JJ, Pasta M, Koski KJ, Yao J, et al. Synthesis of MoS2 and MoSe2 films with vertically aligned layers. Nano Letters. 2013;13:1341–1347.
  44. 44. Yang Z, Liang H, Wang X, Ma X, Zhang T, Yang Y, et al. Atom‐thin SnS2-xSex with adjustable compositions by direct liquid exfoliation from single crystal. ACS Nano. 2016;10:755–762.
  45. 45. Dresselhaus MS. Intercalation in Layered Materials. Netherlands: Springer; 2013.
  46. 46. Coleman JN, Lotya M, O’Neill A, Bergin SD, King PJ, Khan U, et al. Two‐dimensional nanosheets produced by liquid exfoliation of layered materials. Science. 2011;331:568–571.
  47. 47. Smith RJ, King PJ, Lotya M, Wirtz C, Khan U, De S, et al. Large-scale exfoliation of inorganic layered compounds in aqueous surfactant solutions. Advanced Materials. 2011;23:3944–3948.
  48. 48. Li H, Lu G, Wang Y, Yin Z, Cong C, He Q, et al. Mechanical exfoliation and characterization of single- and few-layer nanosheets of WSe2, TaS2, and TaSe2. Small. 2013;9:1974–1981.
  49. 49. Paton KR, Varrla E, Backes C, Smith RJ, Khan U, O’Neill A, et al. Scalable production of large quantities of defect‐free few‐layer graphene by shear exfoliation in liquids. Nature Materials. 2014;13:624–630.
  50. 50. Zheng J, Zhang H, Dong S, Liu Y, Nai CT, Shin HS, et al. High yield exfoliation of two‐dimensional chalcogenides using sodium naphthalenide. Nature Communications. 2014;5:2995.
  51. 51. Eda G, Yamaguchi H, Voiry D, Fujita T, Chen M, Chhowalla M. Photoluminescence from chemically exfoliated MoS2. Nano Letters. 2011;11:5111–5116.
  52. 52. Zeng Z, Yin Z, Huang X, Li H, He Q, Lu G, et al. Single-layer semiconducting nanosheets: high-yield preparation and device fabrication. Angewandte Chemie International Edition. 2011;50:11093–11097.
  53. 53. Auerbach SM, Carrado KA, Dutta PK. Handbook of Layered Materials. Boca Raton: CRC Press; 2004.
  54. 54. Benavente E, Santa Ana M, Mendizábal F, González G. Intercalation chemistry of molybdenum disulfide. Coordination Chemistry Reviews. 2002;224:87–109.
  55. 55. Golub AS, Zubavichus YV, Slovokhotov YL, Novikov YN. Single‐layer dispersions of transition metal dichalcogenides in the synthesis of intercalation compounds. Russian Chemical Reviews. 2003;72:123–141.
  56. 56. Miró P, Audiffred M, Heine T. An atlas of two‐dimensional materials. Chemical Society Reviews. 2014;43:6537–6554.
  57. 57. Zhi C, Bando Y, Tang C, Kuwahara H, Golberg D. Large-scale fabrication of boron nitride nanosheets and their utilization in polymeric composites with improved thermal and mechanical properties. Advanced Materials. 2009;21:2889–2893.
  58. 58. Cai G, Jian J, Chen X, Lei M, Wang W. Regular hexagonal MoS2 microflakes grown from MoO3 precursor. Applied Physics A. 2007;89:783–788.
  59. 59. Shi Y, Li H, Li L‐J. Recent advances in controlled synthesis of two‐dimensional transition metal dichalcogenides via vapour deposition techniques. Chemical Society Reviews. 2015;44:2744–2756.
  60. 60. Ji Q, Zhang Y, Gao T, Zhang Y, Ma D, Liu M, et al. Epitaxial monolayer MoS2 on mica with novel photoluminescence. Nano Letters. 2013;13:3870–3877.
  61. 61. Shi J, Ma D, Han G‐F, Zhang Y, Ji Q, Gao T, et al. Controllable growth and transfer of monolayer MoS2 on Au foils and its potential application in hydrogen evolution reaction. ACS Nano. 2014;8:10196–10204.
  62. 62. Najmaei S, Liu Z, Zhou W, Zou X, Shi G, Lei S, et al. Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers. Nature Materials. 2013;12:754–759.
  63. 63. Zhan Y, Liu Z, Najmaei S, Ajayan PM, Lou J. Large-area vapor-phase growth and characterization of MoS2 atomic layers on a SiO2 substrate. Small. 2012;8:966–971.
  64. 64. Lin Y‐C, Zhang W, Huang J‐K, Liu K‐K, Lee Y‐H, Liang C‐T, et al. Wafer‐scale MoS2 thin layers prepared by MoO3 sulfurization. Nanoscale. 2012;4:6637–6641.
  65. 65. Liu K‐K, Zhang W, Lee Y‐H, Lin Y‐C, Chang M‐T, Su C‐Y, et al. Growth of large‐area and highly crystalline MoS2 thin layers on insulating substrates. Nano Letters. 2012;12:1538–1544.
  66. 66. Wu S, Huang C, Aivazian G, Ross JS, Cobden DH, Xu X. Vapor–solid growth of high optical quality MoS2 monolayers with near‐unity valley polarization. ACS Nano. 2013;7:2768–2772.
  67. 67. Ubaldini A, Jacimovic J, Ubrig N, Giannini E. Chloride‐driven chemical vapor transport method for crystal growth of transition metal dichalcogenides. Crystal Growth & Design. 2013;13:4453–4459.
  68. 68. Ubaldini A, Giannini E. Improved chemical vapor transport growth of transition metal dichalcogenides. Journal of Crystal Growth. 2014;401:878–882.
  69. 69. Shaw JC, Zhou H, Chen Y, Weiss NO, Liu Y, Huang Y, et al. Chemical vapor deposition growth of monolayer MoSe2 nanosheets. Nano Research. 2015;7:511–517.
  70. 70. Nayak PK, Lin F‐C, Yeh C‐H, Huang J‐S, Chiu P‐W. Robust room temperature valley polarization in monolayer and bilayer WS2. Nanoscale. 2016;8:6035–6042.
  71. 71. Lee YH, Zhang XQ, Zhang W, Chang MT, Lin CT, Chang KD, et al. Synthesis of large-area MoS2 atomic layers with chemical vapor deposition. Advanced Materials. 2012;24:2320–2325.
  72. 72. Lee Y‐H, Yu L, Wang H, Fang W, Ling X, Shi Y, et al. Synthesis and transfer of single‐layer transition metal disulfides on diverse surfaces. Nano Letters. 2013;13:1852–1857.
  73. 73. Ling X, Lee Y‐H, Lin Y, Fang W, Yu L, Dresselhaus MS, et al. Role of the seeding promoter in MoS2 growth by chemical vapor deposition. Nano Letters. 2014;14:464–472.
  74. 74. van der Zande AM, Huang PY, Chenet DA, Berkelbach TC, You Y, Lee G‐H, et al. Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide. Nature Materials. 2013;12:554–561.
  75. 75. Huang J‐K, Pu J, Hsu C‐L, Chiu M‐H, Juang Z‐Y, Chang Y‐H, et al. Large‐area synthesis of highly crystalline WSe2 monolayers and device applications. ACS Nano. 2013;8:923–930.
  76. 76. Zhang Y, Zhang Y, Ji Q, Ju J, Yuan H, Shi J, et al. Controlled growth of high‐quality monolayer WS2 layers on sapphire and imaging its grain boundary. ACS Nano. 2013;7:8963–8971.
  77. 77. Chang Y‐H, Zhang W, Zhu Y, Han Y, Pu J, Chang J‐K, et al. Monolayer MoSe2 grown by chemical vapor deposition for fast photodetection. ACS Nano. 2014;8:8582–8590.
  78. 78. Miró P, Han JH, Cheon J, Heine T. Hexagonal transition-metal chalcogenide nanoflakes with pronounced lateral quantum confinement. Angewandte Chemie International Edition. 2014;53:12624–12628.
  79. 79. Bianco E, Butler S, Jiang S, Restrepo OD, Windl W, Goldberger JE. Stability and exfoliation of germanane: a germanium graphane analogue. ACS Nano. 2013;7:4414–4421.
  80. 80. Han JH, Lee S, Cheon J. Synthesis and structural transformations of colloidal 2D layered metal chalcogenide nanocrystals. Chemical Society Reviews. 2013;42:2581–2591.
  81. 81. Kong D, Koski KJ, Cha JJ, Hong SS, Cui Y. Ambipolar field effect in Sb‐doped Bi2Se3 nanoplates by solvothermal synthesis. Nano Letters. 2013;13:632–636.
  82. 82. Li J, Chen Z, Wang R‐J, Proserpio DM. Low temperature route towards new materials: solvothermal synthesis of metal chalcogenides in ethylenediamine. Coordination Chemistry Reviews. 1999;190:707–735.
  83. 83. Li Y, Wang H, Xie L, Liang Y, Hong G, Dai H. MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction. Journal of the American Chemical Society. 2011;133:7296–7299.
  84. 84. Xie J, Zhang J, Li S, Grote F, Zhang X, Zhang H, et al. Controllable disorder engineering in oxygen‐incorporated MoS2 ultrathin nanosheets for efficient hydrogen evolution. Journal of the American Chemical Society. 2013;135:17881–17888.
  85. 85. Yoo D, Kim M, Jeong S, Han J, Cheon J. Chemical synthetic strategy for single‐layer transition‐metal chalcogenides. Journal of the American Chemical Society. 2014;136:14670–14673.
  86. 86. Li T, Galli G. Electronic properties of MoS2 nanoparticles. The Journal of Physical Chemistry C. 2007;111:16192–16196.
  87. 87. Xie J, Zhang H, Li S, Wang R, Sun X, Zhou M, et al. Defect-rich MoS2 ultrathin nanosheets with additional active edge sites for enhanced electrocatalytic hydrogen evolution. Advanced Materials. 2013;25:5807–5813.
  88. 88. Kibsgaard J, Chen Z, Reinecke BN, Jaramillo TF. Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis. Nature Materials. 2012;11:963–969.
  89. 89. Voiry D, Salehi M, Silva R, Fujita T, Chen M, Asefa T, et al. Conducting MoS2 nanosheets as catalysts for hydrogen evolution reaction. Nano Letters. 2013;13:6222–6227.
  90. 90. Chen Z, Cummins D, Reinecke BN, Clark E, Sunkara MK, Jaramillo TF. Core–shell MoO3–MoS2 nanowires for hydrogen evolution: a functional design for electrocatalytic materials. Nano Letters. 2011;11:4168–4175.
  91. 91. Lukowski MA, Daniel AS, Meng F, Forticaux A, Li L, Jin S. Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets. Journal of the American Chemical Society. 2013;135:10274–10277.
  92. 92. Hong X, Liu J, Zheng B, Huang X, Zhang X, Tan C, et al. A universal method for preparation of noble metal nanoparticle-decorated transition metal dichalcogenide nanobelts. Advanced Materials. 2014;26:6250–6254.
  93. 93. Zheng X, Xu J, Yan K, Wang H, Wang Z, Yang S. Space‐confined growth of MoS2 nanosheets within graphite: the layered hybrid of MoS2 and graphene as an active catalyst for hydrogen evolution reaction. Chemistry of Materials. 2014;26:2344–2353.
  94. 94. Li DJ, Maiti UN, Lim J, Choi DS, Lee WJ, Oh Y, et al. Molybdenum sulfide/N‐doped CNT forest hybrid catalysts for high‐performance hydrogen evolution reaction. Nano Letters. 2014;14:1228–1233.
  95. 95. Liao L, Zhu J, Bian X, Zhu L, Scanlon MD, Girault HH, et al. MoS2 formed on mesoporous graphene as a highly active catalyst for hydrogen evolution. Advanced Functional Materials. 2013;23:5326–5333.
  96. 96. Yu XY, Hu H, Wang Y, Chen H, Lou XWD. Ultrathin MoS2 nanosheets supported on N-doped carbon nanoboxes with enhanced lithium storage and electrocatalytic properties. Angewandte Chemie International Edition. 2015;54:7395–7398.
  97. 97. Huang X, Zeng Z, Bao S, Wang M, Qi X, Fan Z, et al. Solution‐phase epitaxial growth of noble metal nanostructures on dispersible single‐layer molybdenum disulfide nanosheets. Nature Communications. 2013;4:1444.
  98. 98. Zeng Z, Tan C, Huang X, Bao S, Zhang H. Growth of noble metal nanoparticles on single‐layer TiS2 and TaS2 nanosheets for hydrogen evolution reaction. Energy and Environmental Science. 2014;7:797–803.
  99. 99. Qiao J, Liu Y, Hong F, Zhang J. A review of catalysts for the electroreduction of carbon dioxide to produce low‐carbon fuels. Chemical Society Reviews. 2014;43:631–675.
  100. 100. Hori Y. Electrochemical CO2 reduction on metal electrodes. In: Vayenas CG, White RE, Gamboa‐Aldeco ME, editors. Modern Aspects of Electrochemistry, vol. 42. New York: Springer; 2008. p. 89–189.
  101. 101. Schneider J, Jia H, Muckerman JT, Fujita E. Thermodynamics and kinetics of CO2, CO, and H+ binding to the metal centre of CO2 reduction catalysts. Chemical Society Reviews. 2012;41:2036–2051.
  102. 102. Chen Y, Kanan MW. Tin oxide dependence of the CO2 reduction efficiency on tin electrodes and enhanced activity for tin/tin oxide thin‐film catalysts. Journal of the American Chemical Society. 2012;134:1986–1989.
  103. 103. Costentin C, Robert M, Savéant J‐M. Catalysis of the electrochemical reduction of carbon dioxide. Chemical Society Reviews. 2013;42:2423–2436.
  104. 104. Tripkovic V, Vanin M, Karamad M, Bjo¨rketun ME, Jacobsen KW, Thygesen KS, et al. Electrochemical CO2 and CO reduction on metal‐functionalized porphyrin‐like graphene. The Journal of Physical Chemistry C. 2013;117:9187–9195.
  105. 105. Chen Y, Li CW, Kanan MW. Aqueous CO2 reduction at very low overpotential on oxide‐derived Au nanoparticles. Journal of the American Chemical Society. 2012;134:19969–19972.
  106. 106. Zhang S, Kang P, Meyer TJ. Nanostructured tin catalysts for selective electrochemical reduction of carbon dioxide to formate. Journal of the American Chemical Society. 2014;136:1734–1737.
  107. 107. Chan K, Tsai C, Hansen HA, Nørskov JK. Molybdenum sulfides and selenides as possible electrocatalysts for CO2 reduction. ChemCatChem. 2014;6:1899–1905.
  108. 108. Shi C, Hansen HA, Lausche AC, Nørskov JK. Trends in electrochemical CO2 reduction activity for open and close‐packed metal surfaces. Physical Chemistry Chemical Physics. 2014;16:4720–4727.
  109. 109. Asadi M, Kumar B, Behranginia A, Rosen BA, Baskin A, Repnin N, et al. Robust carbon dioxide reduction on molybdenum disulphide edges. Nature Communications. 2014;5:4470.
  110. 110. Li F, Zhao S‐F, Chen L, Khan A, MacFarlane DR, Zhang J. Polyethylenimine promoted electrocatalytic reduction of CO2 to CO in aqueous medium by graphene‐supported amorphous molybdenum sulphide. Energy and Environmental Science. 2016;9:216–223.

Written By

Fengwang Li and Mianqi Xue

Submitted: 23 November 2015 Reviewed: 26 April 2016 Published: 31 August 2016