Electrochemistry of Nanostructured Layered Transition-Metal

Review pubs.acs.org/CR

Electrochemistry of Nanostructured Layered Transition-Metal Dichalcogenides Xinyi Chia, Alex Yong Sheng Eng, Adriano Ambrosi, Shu Min Tan, and Martin Pumera* Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore electron transfer (HET) rates of many redox probes are much higher at the graphite edge compared to the basal plane.3 As a result of its versatility, graphite and its derivatives have numerous important industrial applications, from solid-state lubricants to electrode materials. A survey of the literature from the past decade reveals the intensive research on graphene and its derivatives and demonstrates the effects of nanoscale confinement which alter the properties of graphite after exfoliation into a monolayer or few layers.4,5 Several other materials6 possess a layered structure similar to graphite and have gained enormous attention in recent years include transition-metal dichalcogenides (TMDs), metal carbides,7 layered oxides,8,9 black phosphorus,10−12 and layered structure halides.13 From this wide spectra of layered inorganic CONTENTS materials, the layered transition-metal dichalcogenides which have the general chemical formula MX2, where M is a transition 1. Introduction A metal (i.e., Mo, W, Re) and X is a chalcogen, generally S, Se, or 2. Composition and Structure A Te, exhibit the most favorable electrochemical properties. 3. Inherent Electrochemistry of Transition-Metal Studies of the electrochemical behavior of TMDs potentially Dichalcogenides (TMDs) G advance their development for various electrochemical 4. Electrochemistry at TMDs J applications. While there is a large volume of literature 4.1. Electrochemistry of Molecular Probes at especially dedicated to reviewing the applications of TMDs, TMD Electrodes J particularly useful for hydrogen production,14−20 supercapaci4.2. Electrochemical Hydrogen Evolution Reactors,20,21 or batteries,15 we wish to focus our discussion here tion (HER) at TMD Electrodes L rather on their fundamental electrochemical properties. 4.2.1. Hydrogen Evolution Mechanism L Focusing our attention on MoS2, WS2, MoSe2, and WSe2 as 4.2.2. Key Factors Governing HER at TMD the most researched TMD materials, we will discuss, in Electrodes L particular, their inherent electrochemistry, highlighting the 4.2.3. Activation of TMDs for HER O often overlooked fact that such materials possess important and 4.3. Oxygen Reduction Reaction (ORR) at TMD well-defined redox properties. We then move on to discuss the Electrodes R use of these TMDs as electrode-modifier materials for 4.4. TMD Materials for Electrochemical Capaciimportant electrochemical applications such as (I) the probe tors R and detection of redox-active molecules in solution, (II) 5. Conclusion and Future Perspectives U electrocatalytic hydrogen generation, (III) the oxygen reduction Author Information V reaction, and (IV) the charge storage capabilities for the Corresponding Author V fabrication of supercapacitors. Along with providing useful Notes V direct comparisons of MoS2, WS2, MoSe2, and WSe2, which are Biographies V generally missing in the literature, we also evaluate the Acknowledgments W emerging trends in research endeavored to enhance both the References W electrochemical and the catalytic properties of TMDs. In that regard, we provide a summary of the reviewed articles in Table 1, including materials, properties, and applications. 1. INTRODUCTION Layered inorganic systems exhibit many technologically important and scientifically interesting properties, often linked to their anisotropy. A typical example is graphite, which consists of stacked graphene monolayers.1 The electrical and thermal conductivities of graphite along the graphene basal plane are 3− 5 orders of magnitude higher than across the planes in the graphite crystal.2 In a similar manner, the heterogeneous © XXXX American Chemical Society

2. COMPOSITION AND STRUCTURE A single sheet of TMD consists of one plane of transition metals sandwiched between two layers of chalcogens, forming a Received: May 12, 2015

A

DOI: 10.1021/acs.chemrev.5b00287 Chem. Rev. XXXX, XXX, XXX−XXX

HYDROGEN EVOLUTION REACTION (HER)

catalysis/application/ inherent property

commercially obtained followed by chemical intercalation

nanosheets

B

nanosheets strained 1T phase nanosheets WS2-composites WS2/rGO Co-doped WS2/carbon WS2(1−x)Se2x NT/carbon fiber

MoS2 NP/rGO vertically aligned MoS2/GC Co-doped MoS2/carbon V-doped MoS2 nanosheets MoS2 NP/MGF WS2-homogeneous nanosheets

defect-rich nanosheets MoS2-composites MoS2 NP/activated carbon MoS2 NP/Au(111) MWMoS2/MWCNTs

73 83 77 36 89

350 mVa (85 mV/dec)b WS2-unt: 390,a −140 mVd (93 mV/dec)b WS2-ox: 590,a −230 mVd (164 mV/dec)b WS2-red: 420,a −180 mVd (104 mV/dec)b 142 mVa (70 mV/dec)b 250 mVa,j (60 mV/dec)b 260 mVa,j (58 mV/dec)b 350 mVa,j (132 mV/dec)b 270 mVa,j (105 mV/dec)b (2.9 × 10−2 mA/cm2geometric)c

CVD followed by microwave-assisted chemical intercalation commercially obtained followed by Li intercalation hydrothermal followed by annealing wetness impregnation followed by sulfidization with H2S CVD

32 70

64 79 36 84 85

commercially obtained followed by Li intercalation

solvothermal sulfurization of Mo film wetness impregnation followed by sulfidization with H2S solid-state reaction followed by liquid exfoliation hydrothermal

66 35 76

−100 to −200 mVd (55−60 mV/dec)b (1.3 × 10−7−3.1 × 10−7 A/cm2geometric)c −150 to −200 mVd (109 mV/dec)b (4.0 × 10−6 mA/ cm2geometric)c 150 mVa,j −100 mVd (41 mV/dec)b (110−119 mV/dec)b (2.2 × 10−6 A/cm2surface area)c 350 mVa,j (101 mV/dec)b −130 mVd (60−75 mV/dec)b 140 mVa,j (42 mV/dec)b

wetness impregnation followed by sulfidization with H2S sputter annealing precipitation followed by reduction

72 78

41

31 32 70

41

59

ref

80

edge: 656−839 mVa (167−224 mV/dec)b basal: (149−362 mV/dec)b MoS2-untr: 620 mV,a −460 mVd (152 mV/dec)b MoS2-ox: 780 mV,a −500 mVd (220 mV/dec)b MoS2-red: 610 mV,a −420 mVd (139 mV/dec)b 200 mVa,j (40 mV/dec)b 550 mVa (99 mV/dec)b MoS2-unt: 530,a −210 mVd (110 mV/dec)b MoS2-ox: 570,a −230 mVd (161 mV/dec)b MoS2-red: 470,a −210 mVd (95 mV/dec)b MoS2-untr: 440 mV,a −310 mVd (81 mV/dec)b MoS2-ox: 600 mV,a −360 mVd (116 mV/dec)b MoS2-red: 400 mV,a −260 mVd (72 mV/dec)b 187 mVa (43 mV/dec)b 150−200 mVd (50 mV/dec)b

operational/experimental parameters

120 mVd (50 mV/dec)b

CVD followed by chemical intercalation electrodeposition onto silica template followed by sulfidization with H2S and etching of template hydrothermal

commercially obtained

bulk powder

double gyroid

natural molybdenite crystal

synthesis method/treatment

MoS2-homogeneous bulk crystal

material

Table 1. Comprehensive Table of Electrochemistry of TMD Nanosheets Including Materials, Properties, Synthetic Method, Applications, and Key Electrochemical Data

Chemical Reviews Review


OXYGEN REDUCTION REACTION (ORR)


Table 1. continued material

C

Co3S4 nanoparticles on carbon CoSe2 nanoparticles on carbon FeS2-based thin film carbon-supported Ru-based chalcogenide (e.g., Ru1S0.6Mo0.36) Pt/C

MoS2 bulk microparticles nanoparticles (2−10 nm) WS2 bulk exfoliated sheets MoSe2 bulk exfoliated sheets WSe2 bulk exfoliated sheets others/related materialsi CoS2 nanocrystals

WSe2-composites edge-terminated WSe2/carbon fiber

MoSe2-composites vertically aligned MoSe2/GC edge-terminated MoSe2/carbon fiber S-doped MoSe2 nanosheets perpendicularly oriented MoSe2 nanosheets/3D graphene network WSe2-homogeneous nanosheets nanosheets

MoSe2-homogeneous nanosheets

40 40

−0.48 V vs Ag/AgClf (0.1 M KOH) −0.45 to −0.46 V vs Ag/AgClf,j (0.1 M KOH)

commercially obtained commercially obtained followed by Li intercalation

commercially obtained

Co nanoparticles heated with sulfur Co nanoparticles heated with selenium magnetron sputtering from FeS2 target chemical reduction of RuCl3 with ammonium tetrathiomolybdate followed by heat treatment

ca. −0.3 V vs Ag/AgCl,f 61−105 mV/dec,g ne ≈ 4 (0.1 M KOH 40, or NaOH) 94, 98

96 96 97 94

95

40 40



0.4 V vs Ag/AgCle,j at −1 mA cm−2, ne = 2−4 depending on potential (0.1 M HClO4) 0.66−0.68 V vs RHEh (0.5 M H2SO4) 0.69 to 0.72 mV/decg vs RHE,h 125 mV/decg (0.5 M H2SO4) 0.78 V vs RHE,h 200 mV/decg (0.1 M HClO4) 0.8 V vs RHEe,j at −1 mA cm−2, 48−79 mV/dec,g ne ≈ 4 (0.1 M NaOH)

40 40



hydrothermal synthesis

98 98

−0.44 V vs SCE,f ne ≈ 2 at −0.5 V (0.1 M KOH) −0.35 to −0.4 V vs SCE,f,j ne ≈ 4 at −0.5 V (0.1 M KOH)

commercially obtained commercially obtained followed by ultrasonication and centrifugation

86

32 70

800 mVa (>240 mV/dec)b WSe2-unt, ox, red: 760−790,a −270 to −310 mVd (>120 mV/ dec)b

commercially obtained followed by Li intercalation commercially obtained followed by Li intercalation

300 mVa (77 mV/dec)b

87 88

160 mV,a,j −90 mVd (58 mV/dec)b 159 mV,a −50 mVd (61 mV/dec)b

mixing of solutions of MoCl5, S and Se powders, followed by heating CVD

selenization of W film

79 86

32 70

ref

(105−120 mV/dec)b (2.0 × 10−6 A/cm2surface area)c 250 mVa (60 mV/dec)b

350 mVa (82 mV/dec)b MoSe2-unt, ox, red: 360 ± 10,a −150 ± 20 mVd (78−86 mV/ dec)b


selenization of Mo film selenization of Mo film

commercially obtained followed by Li intercalation




CAPACITANCE


Table 1. continued

D

hydrothermal synthesis hydrothermal synthesis hydrothermal synthesis liquid-phase exfoliation in 1-dodecyl-2-pyrrolidinone (N12P) chemical exfoliation with chemical exfoliation with chemical exfoliation with commercially obtained chemical exfoliation with chemical exfoliation with

nanosheets spherical nanoclusters nanospheres nanosheets

exfoliated sheets exfoliated sheets exfoliated sheets bulk powder exfoliated sheets exfoliated sheets

WS2-homogeneous bulk powder exfoliated sheets

PEDOT/MoS2 nanocomposite

PPy/MoS2 nanocomposite

microporous C-wrapped MoS2 sheets MoS2 sheets/PANI nanocomposite PANI/MoS2 composite

tubular C/MoS2

MoS2-composites MoS2 nanosheets/MWCNT MoS2 nanosheet/rGO MoS2/rGO microfiber

hydrothermal synthesis hydrothermal synthesis

nanospheres mesoporous

commercially obtained chemical exfoliation with t-BuLi

hydrothermal synthesis of MoS2 in the presence of MWCNTs hydrothermal synthesis of MoS2 in the presence of graphene oxide (GO) exfoliation of bulk MoS2 by electrochemical Li intercalation; mixing with GO solution; wet-spinning fiber formation in KOH solution hydrothermal synthesis of C/MoS2 from glucose + (NH4)2MoS4 precursors in the presence of AAO film liquid-phase exfoliation of bulk MoS2 in NMP; wrapping MoS2 sheets with ZIF-8 metal organic framework; calcination at 900 °C to get porous C-wrapped MoS2 chemical exfoliation of bulk MoS2 with n-BuLi; chemical polymerization of aniline with APS in the presence of MoS2 sheets hydrothermal synthesis of MoS2; chemical polymerization of aniline with APS in the presence of MoS2 sheets hydrothermal synthesis of MoS2; chemical polymerization of pyrrole with APS in the presence of MoS2 sheets chemical exfoliation of bulk MoS2 with n-BuLi; chemical polymerization of EDOT with APS in the presence of MoS2 sheets

t-BuLi n-BuLi

MeLi n-BuLi t-BuLi

CVD hydrothermal synthesis hydrothermal synthesis hydrothermal synthesis


MoS2-homogeneous nanowall films nanosheets nanoflowers nanospheres

material

122

575 F/g at 20 mV/s scan rate in 1 M H2SO4

2.5 F/g at 0.5 A/g current density in 0.1 M PBS 40 F/g at 0.5 A/g current density in 0.1 M PBS

405 F/g at 1 A/g current density in 1 M H2SO4

113 113

124

123

121

390 F/g at 0.8 A/g current density in 1 M H2SO4

554 F/g at 1 A/g current density in 1 M KCl

120

119

115 116 118

39 39 39 113 113 114

109 110 111 112

107 108

102 104 105 106

ref

189 F/g at 1 A/g current density in 1 M H2SO4

210 F/g at 1 A/g current density in 3 M KOH

453 F/g at 1 A/g current density in 1 M Na2SO4 243 F/g at 1 A/g current density in 1 M Na2SO4 30 F/cm3 at 0.1 μA current in PVA−H2SO4 gel electrolyte

70 mF/cm2 (100 F/g) at 1 mV/s scan rate in 0.5 M H2SO4 8 mF/cm2 at 10 mV/s scan rate in 1 M NaOH 168 F/g at 1 A/g current density in 1 M KCl 106 F/g at 5 mV/s scan rate (93 F/g at 0.5 A/cm2) in 1 M Na2SO4 122 F/g at 1 A/g current density in 1 M KCl 376 F/g at 1 mV/s in 1 M Na2SO4, 406 F/g at 1 mV/s in 1 M KCl 129.2 F/g at 1 A/g current density in 1 M Na2SO4 1220 F/g at 0.5 A/g in 1 M Na2SO4 368 F/g at 5 mV/s scan rate in LiCl−PVA gel 2 mF/cm2 at 10 mV/s in 6 M KOH; 2.25 F/cm2 at 10 mV/s in Et4NBF4; 2.4 F/cm2 at 10 mV/s in BMIM−PF6 ionic liquid 18 F/g at 0.5 A/g current density in 1 M KOH 40 F/g at 0.5 A/g current density in 1 M KOH 36 F/g at 0.5 A/g current density in 1 M KOH 2.5 F/g at 0.5 A/g current density in 0.1 M PBS 12 F/g at 0.5 A/g current density in 0.1 M PBS 400−550 F/cm3 at 20 mV/s scan rate in aqueous ionic electrolytes




INHERENT ELECTROCHEMISTRY


Table 1. continued material

E

wetness impregnation followed by sulfidation with H2S hydrothermal synthesis

others/related materialsi CoSx/carbon paper CoS2 nanocrystals

Eox = 1.14 V vs NHE (0.5 M H2SO4) Ered = −0.1 V vs Ag/AgCl

36 95

40 40, 70


WSe2-homogeneous bulk material (fine powder) exfoliated sheets

Eox = 0.85 V vs Ag/AgCl (PBS, pH 7) Eox ≥ 0.85 V vs Ag/AgClj (PBS, pH 7)

40 40, 70

Eox ≥ 0.9 V vs Ag/AgClj (PBS, pH 7) variable; Eox from 0.6 to 0.9 V vs Ag/AgClj (PBS, pH 7)


40 40, 70 36 36

Eox = 1.0 V vs Ag/AgCl (PBS, pH 7) Eox ≥ 1.0 V vs Ag/AgClj (PBS, pH 7) Eox ≥ 1.0 V vs NHE (0.5 M H2SO4) Eox ≥ 1.1 V vs NHE (0.5 M H2SO4)


WS2-homogeneous bulk material (fine powder) exfoliated sheets

36 38

36

wetness impregnation followed by sulfidation with H2S wetness impregnation followed by sulfidation with H2S

wetness impregnation followed by sulfidation with H2S electrochemical reduction of MoS2 on 3-aminopropyltriethoxysilane

Co-doped MoS2/carbon paper APTES-MoS2

Eox = 0.7 V vs NHE (assigned to edge), Eox = 0.98 V vs NHE (assigned to basal plane); (0.5 M H2SO4) Eox ≥ 1.1 V vs NHE (0.5 M H2SO4) Ered = −0.71 V vs Ag/AgCl (0.5 M NaCl) Eox = 1.87 V (acetonitrile/0.5 M tetrabutylammonium tetrafluoroborate)

39 39, 70

59 59

103

113 113

113 113

117

ref

WS2-composites WS2/carbon paper Co-doped WS2/carbon paper MoSe2-homogeneous bulk material (fine powder) exfoliated sheets

wetness impregnation followed by sulfidation with H2S

MoS2-composites MoS2/carbon paper


4760 μF/cm2 (317 F/cm3) at 0.1 A/m2 current density in BMIMBF4 ionic liquid

hydrothermal synthesis of VS2·NH3 intermediate followed by exfoliation

bulk material (fine powder) nanosheets



Eox ≥ −0.3 V, Ered ≤ −0.7 V vs Ag/AgClj (PBS, pH 7) stable potential window within approximately ± 1 V (PBS, pH 7) Eox ≥ 1.3 V, Ered ≤ −1.0 V vs Ag/AgCl (0.1 M KCl) Eox ≥ 1.2 V, Ered ≤ −0.8 V vs Ag/AgClj (0.1 M KCl; PBS, pH 7)



natural molybdenite crystal natural molybdenite crystal

350 F/g at 0.5 A/g current density in 1 M Na2SO4


hydrothermal synthesis of WS2 in the presence of graphene oxide (GO)


MoS2-homogeneous edge (large crystal) basal (large crystal)

WS2-composites WS2 nanosheet/rGO MoSe2-homogeneous bulk powder exfoliated sheets WSe2-homogeneous bulk powder exfoliated sheets VS2-homogeneous nanosheets



Review

HER overpotential at −10 mA/cm2. bHER Tafel slope. cExchange current density. dHER onset potential at −0.1 mA cm−2. eORR onset potential. fORR peak reduction potential. gORR Tafel slope. Open circuit potential. iNon-layered structures. ne = number of transferred electrons. Eox = peak oxidation potential. Ered = peak reduction potential. jApproximate values.

commercially obtained WO2

trilayer. Multiple trilayers are stacked and held together by weak out-of-plane van der Waals interactions, forming the bulk material. This unique structure of TMDs results in two distinctive orientations: basal and edge planes, which have been reported to exhibit anisotropic properties, e.g., surface inertness of the basal plane in contrast with high surface energy of the edge plane.22 As such, the electrochemical activity of the basal and edge planes of TMDs is also anticipated to be anisotropic. To gain further insight into the electrochemical properties of TMDs, it may be of interest to inspect the example of graphite. The electrical resistivity (reciprocal of electrical conductivity) of graphite measured parallel to the graphene layer is 0.04 mΩ cm, which is significantly lower than that measured perpendicular to the layers (150 mΩ cm).23 In addition, the marked differences in the heterogeneous electron transfer rate constants (k0obs) between the edge (k0 = 2.2 × 10−2 cm s−1) and the basal (k0obs ≈ 10−9 cm s−1) planes of graphite with a Fe(CN)63−/4− redox probe3,24 suggest that both properties exhibit highly anisotropic characteristics which may be extended to the case of TMDs, being structurally similar to graphite.20 For TMDs, the electrical conductivity along the layer is ca. 2200 times higher than that across the van der Waals forces between layers.25 On the basis of the above observation, the heterogeneous electron transfer (HET) on the edge plane of TMDs is predicted to be considerably faster than that on the basal plane and anticipated to have higher catalytic activity toward the hydrogen evolution reaction (HER). Besides the highly anisotropic properties encountered for the material in relation to the crystal orientation, it is also important to mention that differences in properties can also arise depending on the transition-metal coordination by the chalcogen and the stacking sequence of multiple layers. A single-layer TMD can have a trigonal prismatic or octahedral metal coordination phase (generally referred to as 2H and 1T, respectively) as shown in Figure 1. In multiple structures, since each individual layer could possess any of the two coordination phases, many different polymorphic structures could be exhibited by the same TMD material giving rise to a large variety of different properties. However, the most commonly found polymorphs are the so-defined 1T, 2H, and 3R, where the digit indicates the number of layers in the crystallographic unit cell and the letter indicates the type of symmetry with T standing for tetragonal (D3d group), H for hexagonal (D3h 5 group), and R rhombohedral (C3v group). The stacking sequence along the z axis is given by a sequence of three letters indicating the relative positions of the chalcogen− metal−chalcogen atoms in each layer. A single layer in the 1T phase has a stacking sequence AbC, and due to its symmetry, multiple layers in such octahedral phase would have AbC AbC AbC as the only possible sequence. The 2H phase has a stacking sequence AbA BaB with the sulfur atoms overlapping along the z axis with the metal atoms of the adjacent layer. The rhombohedral symmetry is given by layers all in trigonal prismatic phase but with the position of metal and chalcogen atoms shifted resulting in a stacking sequence AbA CaC BcB in the unit cell. Depending on the metal, one of the two phases is thermodynamically more stable and therefore represents the most commonly found one as natural bulk TMD. Group 4 TMDs (e.g., Ti and Zr) show mostly octahedral coordination, while Group 6-based TMDs (e.g., Mo and W) present the 2H phase as the most stable.

h

a

broad oxidation peak; Eox ≥ 0.4 V vs Ag/AgClj (PBS, pH 7)

commercially obtained MoO2

material catalysis/application/ inherent property

Table 1. continued

others/related materialsi


Eox = 0.6 V vs Ag/AgCl (0.1 M HClO4) Eox from 0.7 to 0.9 V vs Ag/AgCl (PBS, pH 7)


40, 70 40, 70

ref

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F


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Figure 1. Different metal coordination and stacking sequence of TMD structural unit cells. Metal coordination can be either octahedral or trigonal prismatic. The octahedral coordination allows stacking sequences such as AbC AbC AbC, etc., with tetragonal symmetry; trigonal prismatic single layers can be stacked in two different ways giving hexagonal symmetry (2H) or rhombohedral symmetry (3R).

the oxygen reduction reaction (ORR). The final section of this review will be also dedicated to the application of TMD materials for the fabrication of supercapacitors as efficient energy storage devices.

Considering MoS2 or WS2, for example, it has been shown that a phase transition 2H → 1T is possible through alkali intercalation due to the introduction of extra electrons and rearrangements of the d orbitals.26−28 Such phase transition is very important because it completely alters the electronic properties of the materials; 2H-MoS2 (or 2H-WS2) is semiconducting, while 1T-MoS2 (or 1T-WS2) is metallic.29 Within the same material, therefore, it is possible to have completely different properties depending on the metal coordination. Semiconducting 2H-MoS2 offers interesting optical properties when down-sized to a monolayer,30 while the 1T phase has been proven to possess enhanced catalytic abilities for the hydrogen evolution reaction.31 With regard to the TMDs based on Mo and W with S and Se, their structural polymorph is an aspect particularly significant to be taken into account when evaluating their properties. This is because one of the most commonly used methods of exfoliation requires organolithium compounds as intercalators and such a chemical method produces a 2H → 1T phase transition, although with different efficiencies depending on the compound.32 It has been recently shown, for example, that WS2 is particularly sensitive to exfoliation by means of tert-butyllithium (t-BuLi) intercalator with the consequent 2H → 1T transition.32 As mentioned, TMD has the general chemical formula of MX2 where each metal is coordinated to two chalcogens; hence, the metal is in the + 4 oxidation state and the chalcogen in the −2 state. We will show that it is possible to electrochemically oxidize/reduce the metal/chalcogen, resulting in the inherent electrochemistry of the material itself. Surprisingly, this inherent electrochemistry of TMDs is rarely discussed in literature and is presented here as first aspect within the electrochemical properties of TMDs. We then proceed in the following paragraphs to discuss the electrochemistry of redoxactive molecules at TMD-based electrodes as well as their electrocatalytic properties toward the hydrogen generation and

3. INHERENT ELECTROCHEMISTRY OF TRANSITION-METAL DICHALCOGENIDES (TMDs) The recent proliferation of research into TMDs is, to a great extent, the result of promise that they hold in applications including energy storage33 and conversion34 but most prominently as a catalyst for HER.35 In spite of this, the current level of fundamental knowledge of inherent TMD properties is not in keeping with the fast pace of development in TMD-based applications. We believe that this is a crucial aspect of study since changes that TMD materials can undergo during operational use will inadvertently alter the efficacy of such devices. Herein, the term “inherent electrochemistry” is used to describe the characteristic behavior of different TMDs when used as electrode materials ranging from electrochemical sensing to catalysis. Importantly, these properties arise due to redox reactions which the TMD electrode surface itself undergoes as a result of the applied oxidative/reductive potential and is independent of the analyte in solution. Observations can however be influenced by various conditions such as pH, the potentials applied, and the nature of the electrolyte. A first report by Chorkendorff and co-workers36 demonstrated the inherent electrochemical behavior of MoS2 particles supported on carbon paper under typical conditions employed in HER electrocatalysis. The voltammogram in Figure 2a illustrates the inherent oxidation of MoS2 beginning at 0.6 V vs NHE and which reaches a maximum at 0.98 V. This oxidative process was observed to be chemically irreversible and consequently resulted in a decrease in HER activity. Interestingly, the authors were also able to correlate the peak G


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Figure 2. Inherent electrochemistry of transition-metal dichalcogenides after various treatment and experimental conditions. Cyclic voltammogram of (a) MoS2 under typical hydrogen evolution conditions of 0.5 M H2SO4 saturated with N2, scan rate of 2 mV s−1. (Inset) Decrease in the oxidation peak during the second sweep. Reprinted with permission from ref 36. Copyright 2009 Royal Society of Chemistry. (b) Cyclic voltammogram of APTES-MoS2-modified electrode in aqueous 0.5 M NaCl saturated with N2, scan rate of 50 mV s−1; potentials given with respect to the Ag/AgCl reference electrode. Reprinted with permission from ref 38. Copyright 2012 John Wiley and Sons. Cyclic voltammograms of (c) as-received bulk MoS2 and (d−f) MoS2 after chemical exfoliation with methyllithium, n-butyllithium, and tert-butyllithium. Conditions: Starting potential 0 V vs Ag/ AgCl scanned in the anodic direction; 0.1 M KCl; scan rate of 100 mV s−1. Reprinted with permission from ref 39. Copyright 2015 John Wiley and Sons. Cyclic voltammograms of (g) bulk MoSe2, (h) WS2, and (i) WSe2. Conditions: Starting potential at 0 V vs Ag/AgCl, scanned in the anodic direction; 50 mM PBS at pH 7.2, scan rate of 100 mV s−1. Reprinted with permission from ref 40. Copyright 2014 American Chemical Society. Cyclic voltammograms of (j) cobalt sulfide (CoSx), (k) cobalt-promoted MoS2, and (l) cobalt-promoted WS2 deposited on carbon paper. Conditions: 0.5 M H2SO4 saturated with N2, scan rate of 5 mV s−1. Reprinted with permission from ref 36. Copyright 2009 Royal Society of Chemistry.

mechanism as the sulfur in MoS2 is found to only undergo partial oxidation during anodic scans. More recently, electrochemical reductions of MoS2 have also been performed.38,39 Zhang and co-workers report an irreversible reduction occurring at −0.71 V vs Ag/AgCl in aqueous electrolyte (Figure 2b), but this reduced state can be reoxidized at ca. 1.87 V if conducted in an organic electrolyte of acetonitrile and tetrabutylammonium tetrafluoroborate.38 The authors proposed from XPS data that the reduction process arises from a phase change to the material. Further, we then

oxidation potentials at 0.7 and 0.98 V to oxidation of the MoS2 edge and basal planes, respectively. The larger oxidation potential required for the basal planes corroborates with their greater resistance toward corrosion.37 Additionally, it was determined from X-ray photoelectron spectroscopy (XPS) that the observed electrochemical oxidation process was due to changes in the surface chemical composition, primarily the conversion of MoS2 to MoO3, SO42−, and S22−. Nonetheless, there remains some difficulty in obtaining a conclusive reaction H


Chemical Reviews

Review

Figure 3. Potential−pH diagrams illustrating the thermodynamic stabilities of the (a) molybdenum−sulfur−oxygen−hydrogen system, (b) tungsten−sulfur−oxygen−hydrogen system, (c) sulfur−oxygen−hydrogen system, and (d) selenium−oxygen−hydrogen system. Reprinted with permission from ref 43. Copyright 1988 Springer-Verlag.

suggesting that the −1.0 V wave corresponds to the reduction of the oxidized state formed at 1.3 V. In contrast to MoS2, less is known regarding the inherent electrochemical behavior of other TMDs. In the same discussed work on inherent MoS2 electrochemistry, Bonde et al. also explored the activity of WS2, cobalt sulfide (CoSx), cobaltpromoted MoS2 (Co−Mo−S), and cobalt-promoted WS2 (Co−W−S) (Figure 2j−l).36 These materials similarly exhibit irreversible oxidations which lower their respective HER catalytic activities. Cobalt-incorporated MoS2 and WS2 also experienced shifts in the inherent oxidation peaks toward higher potentials and are further postulated to also have improved HER activity. We then observed that the inherent electrochemistry depends intrinsically on the constituent elements making up the TMD itself (Mo/W and S/Se).40 Minor variations were also reported depending on the extent of

investigated the effects of chemical exfoliation on MoS2 using organolithium reagents where both n-BuLi and t-BuLi were found to be most effective.39 Both oxidation and reduction processes were noted in all MoS2 materials with their different exfoliation extents. An oxidation wave at 1.3 V vs Ag/AgCl and a broad reduction peak at ca. −1.0 V were seen for bulk MoS2 (Figure 2c). In comparison, peak current intensities were largest after BuLi exfoliations, attributed to the efficient exfoliation process giving rise to larger specific surface areas (Figure 2e and 2f). In all cases, both oxidation and reduction waves also diminished in current intensities with each potential sweep; the most rapid decrease occurred with the n-BuLi- and t-BuLi-exfoliated materials. More importantly, a clue to the origins of the processes responsible may lie in the fact that the reduction peak only occurs after an initial anodic sweep, thus I


Chemical Reviews

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can undergo reaction in parallel, and even then it is likely that both reactions will only proceed partially. Hence, we believe that a general awareness of such TMD character is essential for applications utilizing TMDs as electrode materials.

exfoliation as in the MoS2 case, with the exfoliations confirmed based on surface area measurements. Comparing between various TMDs, an important overall observation was a generally increasing trend in oxidation potentials in the order WSe2 < MoSe2 < WS2 < MoS2.39,40 There was also an interesting distinction between bulk sulfide and selenide TMDs (Figure 2c and 2g−i); both selenides show an almost immediate disappearance of its inherent oxidation peak after just the first anodic sweep, while the decrease was gradual for the sulfides requiring repeated potential cycling. Such differences understandably exist between TMDs because of the variety of species involved, their differing reactivities, and also the nature of products formed after oxidation. Considering that such redox behaviors occur as surface processes, X-ray photoelectron spectroscopy is thus highly suitable for studying compositional changes in the materials.36,40−42 Other than XPS, one might gain preliminary insights on the elemental composition by considering the thermodynamic stability of the investigated TMD. Potential−pH or Pourbaix diagrams may be useful in obtaining an idea of the products generated from inherent oxidations/reductions occurring at specific potentials. Under acidic HER conditions (pH ≈ 0.5), it is suggested that TMDs like MoS2 and WS2 (Figure 3) are fully stable only within a small potential window just positive of potentials where hydrogen evolution occurs.43 In practice however, variations in the “operating window” (i.e., potential range where electrode material is inert) exist as seen earlier from Figure 2. The different species predicted to exist at more extreme anodic or cathodic potentials thus represent probable identities of actual products formed. On the basis of XPS data, significant dissolution of material was discovered after oxidation and a number of solid-state and aqueous products were proposed including molybdenum and tungsten oxides, sulfates, and selenates.40 Dissolution of sulfur or selenium can also occur under cathodic potentials, giving H2S and H2Se or HS− and HSe− ions depending on the solution pH (Figure 3c and 3d). Another important consideration is the reactivity of native state TMDs prior to electrochemical treatment. Jaegermann and Schmeisser correlated TMD reactivity toward atmospheric oxygen with their electronic structure, with sulfides being the most resistant to oxidation followed by selenides, while MoTe2 fully undergoes corrosion.42 Most chalcogenides exhibit some thermodynamic instability in the presence of O2, and oxidation can occur during processing steps such as crushing or grinding.44 This results in some extent of passivation from surface oxides which themselves possess distinctive electrochemical activities. One difference between molybdenum and tungsten dichalcogenides is the far poorer solubility of passivating WO3 compared to MoO337 and is therefore likely to account for the observation where WS2 requires numerous potential sweeps for its inherent oxidation peak to disappear40 and its HER current to be adversely affected.36 In summary, TMDs and their surface passivation layers have been shown to be responsible for a variety of inherent electrochemical activities. Such properties are however often overlooked even in contemporary literature, despite the fact that they pose limitations to TMD applications. Additional factors that influence inherent electrochemistry vary from experimental conditions such as pH, applied potential, and the choice of electrolyte. It should nevertheless be emphasized that much difficulty remains in conclusive determination of reaction mechanisms corresponding to the observed behavior. Key reasons for this complexity are that a TMD and its surface oxide

4. ELECTROCHEMISTRY AT TMDs In the previous section we discussed the inherent electrochemistry of TMDs, showing that these materials can themselves be oxidized/reduced. Such inherent electrochemical behavior undeniably limits its electrochemical window, e.g., for sensing.45 On the other hand, the electrochemical oxidation/ reduction of the TMDs surfaces may present a viable way to tune both their electrochemical and their catalytic properties.41 In this section we will focus on the electrochemistry at the surfaces of TMDs. First, we discuss the electrochemistry of molecular probes (excluding H+), and later we will move to hydrogen evolution and oxygen reduction reactions. Subsequently, we will also be extending our discussions to the capacitive behavior of TMDs. 4.1. Electrochemistry of Molecular Probes at TMD Electrodes

Electrode kinetics represents a fundamental subject in electrochemistry. Investigations on the electron transfer between the electrode surface and the inorganic redox system in solution can facilitate a better knowledge on the mechanism involved in such process, the role played by the reactive sites at the interface, and the electronic state of the solid material (electrode). Several different redox systems can be used to study the electron transfer mechanism. Some of them such as the aquated Fe3+/2+, Eu3+/2+, and V3+/2+, the coordination complex Fe(CN)63−/4−, and the organic compound ascorbic acid (AA) undergo a so-called inner-sphere electron transfer mechanism requiring the formation of covalent bridges with the electrode surface.46 They are therefore particularly useful to study the surface conformation of the electrode material and the presence of specific reactive sites (e.g., oxygenated groups, defects, etc.). An outer-sphere redox probe, on the contrary, does not require such linkage with the surface and the electron transfer event is strongly dependent on the electronic state of the solid rather than its surface conformation. Outer-sphere redox probes, such as IrCl62−/3− and Ru(NH3)63+/2+, can then provide information on the electronic properties of the material, the density of electronic states (DOS), and correspondingly the number of electrons and/or holes of a certain energy to allow the electron transfer process. These different redox probes have been extensively employed to study carbon-based electrodes47 such as glassy carbon,48 highly ordered pyrolytic graphite (HOPG),49,50 carbon nanotubes,51,52 and graphene.2,53−55 In particular, the role of the oxygen functionalities as well as the density of defects on the basal plane of the graphitic material has been highlighted to strongly influence the electron transfer kinetics.53,55,56 Electron transfer events occur at very different rates at the pristine basal plane compared to the edge planes with the former several orders of magnitude lower than the latter.3 Similarly to carbon electrodes, semiconductor electrode materials were also tested for electron transfer processes.57 In particular, MoS2 is known for its photosensitivity to UV−vis light with a band gap of ca. 1.75 eV and also for its strong structural anisotropy. Being a layered material, it possesses two distinct surface planes: the basal, perpendicular to the c axis, and the edge surface, parallel to the c axis. Ahmed and J


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Figure 4. Potential of MoS2 platforms for biosensing. (a) Simultaneous oxidation of ascorbic acid (AA), dopamine (DA), and uric acid (UA) at exfoliated MoS2 surface. Reprinted with permission from ref 38. Copyright 2012 John Wiley and Sons. (b) Detection of H2O2 released from the cells via its electrochemical reduction on MoS2 nanosheets. Reprinted with permission from ref 60. Copyright 2013 American Chemical Society.

Gerischer used Fe(CN)63−/4− and other redox probes to study the mechanism of electron transfer at these two distinct surfaces of MoS2,58 providing a correlation with the electronic structure of the material. It was shown that the electron transfer events occur at much higher rates at the edge surface of MoS2 crystals than at the basal surface due to a greater overlapping of the dxy and dx2−y2 orbitals of the conduction band of the metal (Mo) with the orbitals of the redox probe. We found that the edge plane of a macroscopic crystal of MoS2 exhibits fast electron transfer rates (k0obs = 4.96 × 10−5 cm s−1) toward Fe(CN)63−/4−, while the pristine basal plane of a MoS2 crystal shows transfer rates limiting to zero. It was found that if mechanically damaged, the basal plane shows apparent fast HET, reflecting the fact that for Fe(CN)63−/4− the edges are the active sites. In the case of Ru(NH3)63+/2+, edge and basal plane heterogeneous electron transfer rate constants were found to be similar, with k0obs = 1.1 × 10−3 cm s−1 for the edge plane and 9.3 × 10−4 cm s−1 for the basal plane.59 A comprehensive electrochemical study was recently carried out by Zhang and co-workers, who prepared single- to few-layer MoS2 sheets from bulk natural MoS2 powder by means of an electrochemical Li intercalation followed by exfoliation in water.38 After the preparation, an APTES-MoS2 film was deposited on GC electrode for electrochemical studies. Testing the material with both Fe(CN)63−/4− and Ru(NH3)63+/2+ redox probes, the authors show that a preliminary reduction of the film is necessary to confer good conductivity to the film and thus obtain the standard redox response of both probes but with electron transfer rates comparable to the bare GC electrode.38 Using the same electrode platform, the same group showed better performance for the detection of AA, dopamine (DA), and uric acid (UA) compared to the bare GC electrode, whereby the three redox signals of a mixture sample can be selectively distinguished (Figure 4a). MoS2 has also been shown to be catalytic to the reduction of hydrogen peroxide. This allowed the determination of nanomolar levels of H2O2 secreted by living cells using nanoplatelets of MoS2 (Figure 4b).60 Separately, graphene combined with TMDs were fabricated and individually employed in various applications; graphene/MoS2 composites were utilized for acetaminophen oxidation for sensing purposes, 61 while graphene/WS 2 composites were used for voltammetric oxidation and detection of catechol, resorcinol, and hydroquinone.62 From previous studies, it is evident that MoS2-based electrodes can undergo structural and electronic alteration by

means of electrochemical pretreatments. With this in mind, we investigated this aspect in greater detail using a chemically exfoliated MoS2-based electrode.41 In this work, a precise tuning of the electrochemical and hence catalytic properties of MoS2 sheets was demonstrated by means of preliminary reductive or oxidative electrochemical treatments (Figure 5).

Figure 5. Influence of anodic/cathodic pretreatment of MoS2 films on heterogeneous electron transfer (HET) rates. Cyclic voltammograms of 5 mM Fe(CN)63−/4− on (a) bulk MoS2 and (b) exfoliated MoS2 after electrochemical treatment at PBS pH 7. (c) Summary of peak-topeak separations of treated and untreated materials at PBS pH 7 and their corresponding error bars. Conditions: supporting electrolyte, KCl (0.1 M); scan rate, 100 mV s−1; all potentials are vs Ag/AgCl reference electrode. Reprinted with permission from ref 41. Copyright 2014 John Wiley and Sons.

For example, the electron transfer rate between the modified electrode and the Fe(CN)63−/4− probe was increased by approximately one order of magnitude upon a preliminary reductive treatment, resulting in k0obs = 2.15 × 10−3 cm s−1 compared to 2.26 × 10−4 cm s−1 of the untreated material. Density functional theory (DFT) calculations demonstrated that such electrochemical improvement, as well as the better HER catalytic activity, can be correlated to the stabilization of the 1T phase by electron doping during the reductive K


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treatment.41 In another work we examined and compared different TMDs, i.e., MoSe2, WS2, and WSe2, using a Fe(CN)63−/4− probe.40 Interestingly, a first important observation made was the fact that the inherent electrochemical behavior of the TMDs hindered the redox signals coming from the Fe(CN)63−/4− during the first voltammetric scan. Successive potential scans substantially decreased the inherent redox activity of the materials reaching a more stable structural and chemical conformation. Only at this point the redox peaks of the Fe(CN)63−/4− probe could be clearly recorded. Electron transfer rates at these TMD electrodes followed the trend MoSe2 ≫ WSe2 > WS2 with k0obs of ca. 9 × 10−4, 1.2 × 10−5, and 2.7 × 10−6 cm s−1, respectively, after exfoliation. The rapid electron transfer at the MoSe2 electrode was superior to the bare GC electrode and correlated with its higher conductivity compared with the other TMDs. MoSe2 therefore presents itself as a promising material to be used for electrochemical applications.

Figure 6. Comparisons of HER parameters between MoS2 and various metallic and biological catalysts, as well as between the two plane orientations of MoS2. (a) Volcano curve of exchange current density plotted against DFT-calculated Gibbs free energy of adsorbed atomic hydrogen for nanoparticulate MoS2 and the pure metals. Reprinted with permission from ref 35. Copyright 2007 The American Association for the Advancement of Science. (b) DFT-calculated free energy diagram for HER. Reprinted with permission from ref 66. Copyright 2005 American Chemical Society. Exchange current density plotted against (c) MoS2 area coverage and (d) MoS2 edge length. Reprinted with permission from ref 35. Copyright 2007 The American Association for the Advancement of Science.

4.2. Electrochemical Hydrogen Evolution Reaction (HER) at TMD Electrodes

As a large volume of the current literature focused mainly on the electrochemical production of hydrogen at various TMDs, this warrants a brief discussion on its mechanism. Following that key factors that drive HER will be examined, with strong emphasis on strategies that enhance the HER electrocatalytic activity of TMDs. 4.2.1. Hydrogen Evolution Mechanism. The HER mechanism has been thoroughly studied, and there are two widely accepted mechanisms.63,64 Both of them are two-step mechanisms that begin with the adsorption of a proton onto the electrode surface through an electrochemical reduction process (Volmer step):

apparent Tafel slope of the polarization curve in linear sweep voltammetry. 4.2.2. Key Factors Governing HER at TMD Electrodes. To improve the efficiency of TMDs as HER catalysts, the identification of their active sites is vital. Differing HER catalytic activities have been reported for TMDs of various structures; hence, it is essential to relate the HER activity to the TMD structure. Three main factors govern the HER performance of the TMDs: (I) HER active sites, (II) the intrinsic catalytic activity of TMDs, and (III) their conductivity. Earlier studies have suggested the active sites for HER at TMDs to be the edges. In 2005, Hinnemann et al. used the DFT calculations to show that the edges of MoS2 are the active sites.66 On the basis of DFT calculations, the hydrogen adsorption energy of the edges, as depicted by the nitrogenase model, were close to that of the hydrogenase model and Pt which nears the thermoneutral energy (Figure 6a and 6b). Later, Jaramillo and co-workers investigated the exchange current density dependence on the presence of edges and the basal plane of MoS2 for nanoparticles presented.35 While direct linear dependence of exchange current density on the length of the active sites was observed, there was no correlation of the exchange current density with the surface area of MoS2 (Figure 6c and 6d). Using large centimeter-sized pristine natural MoS2 crystal, we found that indeed the edge plane is highly active for HER while the basal plane does not produce a noticeable reaction.59 These studies on the HER active sites of TMDs can be supplemented with more detailed investigations that examine specific aspects that influence the HER catalytic activity of TMDs, to be discussed in the ensuing paragraphs. A deeper insight into the HER performance of TMDs is gained by examining their intrinsic catalytic activity. The intrinsic activity of the TMDs toward HER is determined in terms of ΔGH and the coverage of H (θH) at each metal or

H+(aq) + e−(m) → H•(ads)

This step is followed either by the recombination of two hydrogen atoms adsorbed on the surface (Tafel step): H•(ads) + H•(ads) → H 2(g)

or by the direct bonding of a hydrated proton with the adsorbed hydrogen atom which includes an electron transfer from the electrode surface (Heyrovsky step). Here, it is important to underline that the second proton does not adsorb on the catalytic surface. H•(ads) + H+(aq) + e−(m) → H 2(g)

It is important to highlight the fact that HER is a two-step reaction involving adsorption of hydrogen ion and desorption of hydrogen molecule. Depending on the electrode surface, either the first or the second step can be rate determining. Nørskov et al. showed that the exchange current densities of materials for hydrogen evolution are directly related to the Gibbs free energy of the adsorbed H (ΔGH). It was found that if the bond between hydrogen and metal is too strong, the second step of the reaction (Heyrovsky or Tafel step) is rate limiting; if the bond is too weak, the first step (Volmer step) is rate limiting. Ideally, the interaction of hydrogen with the surface should be thermoneutral (ΔGH ≅ 0).65 From Figure 6a, it can be observed that Pt is situated near the peak of the curve, which indicates that high catalytic activity toward HER occurs for ΔGH ≅ 0, i.e., when the ΔGH of the product and reactants is similar. It is possible to use Tafel analysis to determine whether the first or the second step is rate determining, based on the L


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Figure 7. Intrinsic catalytic activity of TMD as a key factor in HER. (a) Theoretical study of the influence of TMD composition on hydrogen evolution reaction. Structures and hydrogen adsorption free energies for MoS2, WS2, MoSe2, and WSe2. The most stable edge configurations are as shown. Reprinted with permission from ref 69. Copyright 2014 Royal Society of Chemistry. (b) Experimental data on the influence of TMD composition. (Left) Polarization curves of tert-butyllithium exfoliated MoS2, WS2, MoSe2, and WSe2 measured in 0.5 M H2SO4 electrolyte at a scan rate of 2 mV s−1. (Right) Derived Tafel slopes and error bars of the TMD materials corresponding to the polarization curves. Reprinted with permission from ref 70. Copyright 2015 American Chemical Society.

edge is active. Among the four TMDs, MoSe2 is predicted to be the most active catalyst, followed by WS2 because they possess the most thermoneutral energy at both edges and then MoS2 and WSe2. This is consistent with the experimental trends obtained from our HER studies performed on t-BuLi-exfoliated TMDs32,70 as seen in Figure 7b.70 The conductivity of the TMDs also plays a crucial factor to their HER efficiency. This arises from the chemical nature of the TMD when existing in various polymorphs,71 with 2H and 1T phases as the most frequently encountered, and also from the electrical transport between the active site and the electrode. Lukowski et al. demonstrated that the conversion of semiconducting 2H-phase MoS2 nanostructures, which were directly grown on graphite, to the metallic 1T phase by exfoliation using lithium intercalation improved the HER performance.72 During the intercalation process, the reducing agent n-butyllithium destabilized the thermodynamically favored 2H polymorph into the octahedral 1T structure. The resulting metallic 1T-phase MoS2 nanosheets yielded improved HER performance with an overpotential of −0.19 V (vs RHE) at a current density of −10 mA cm−2 and a low Tafel slope of

chalcogen edge, both of which are characteristic to the type of TMD. Since the first step of HER involves hydrogen binding to the catalyst surface, ΔGH is a suitable descriptor for the rate of reaction. As mentioned above, optimal ΔGH occurs at the thermoneutral value of 0 eV,67,68 where the binding is neither weak nor strong. The other parameter θH is defined as the fraction of a monolayer that is HER active. Specific to the TMDs, θH is the fraction of chalcogen atoms on the edge or basal plane that enables hydrogen binding. Tsai et al. studied the inherent catalytic activity of the TMDs by DFT calculations.69 As illustrated in Figure 7a, the ΔGH for the basal planes is ca. 2 eV across TMDs, which indicates that the basal plane is inert regardless of the type of transition metal or chalcogen. All edges are noted to have ΔGH close to zero except for the W edge in WSe2 and S edge in MoS2. The W edge in WSe2 binds weakly to hydrogen, which limits the adsorption step (Volmer process), while the S edge of MoS2 binds too strongly to hydrogen, which limits the desorption step (Heyrovsky or Tafel processes). Briefly, the primary active site for MoS2 is the Mo edge; for WS2 and MoSe2, both the metal and the chalcogen edges are active; for WSe2, only the Se M


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Figure 8. Conductivity of TMD is largely attributed to its chemical nature and the electrical transport ability. (a) (Left) Structure of 2H- and 1TMoS2. (Center) Polarization curves of 2H- and 1T-MoS2; shown in the inset are their Tafel slopes. (Right) Nyquist plots of 2H- and 1T-MoS2. Reprinted with permission from ref 72. Copyright 2013 American Chemical Society. (b) (Left) Solvothermal synthesis to obtain MoS2/rGO hybrid. (Right) Polarization curves of the MoS2/rGO hybrid and control measurements (MoS2 nanoparticles and rGO). Pt was included as a yardstick to assess performance of the catalyst. Reprinted with permission from ref 64. Copyright 2011 American Chemical Society.

43 mV dec−1, which was assigned to better conductivity (Figure 8a). Electrochemical impedance spectroscopy (EIS) conducted on the MoS2 materials under HER conditions confirmed fast electrode kinetics upon exfoliation as depicted in a significant decline in charge-transfer resistance from 232 Ω in 2H-MoS2 to 4Ω in the metallic 1T-MoS2. Inspired by their HER findings with 1T-MoS2, Lukowski et al.73 replicated the study on metallic WS2. Similar to their previous work with MoS2, lithium intercalation chemistry was also employed to facilitate the transition of WS2 from 2H to 1T phase, albeit at a higher temperature of 80 °C because intercalation is more challenging for WS2 due to the larger susceptibility of WS2 to oxidation than MoS2.74 To overcome this problem, the group devised a microwave-assisted method that afforded chemically exfoliated 1T-WS2 nanosheets after just 20 min, which effectively accelerated the intercalation process. Conventionally, the process requires 48 h of heating in an oven. Exfoliated 1TWS2 nanosheets produced via both the conventional oven method and the microwave-assisted intercalation showed improved HER catalytic activity with low overpotentials of −0.14 and −0.15 V (vs RHE), respectively, at a current density of −10 mA cm−2, and both had Tafel slopes of 70 mV dec−1, considerably lower than the as-grown 2H-WS2 nanostructures (85 mV dec−1). Charge-transfer resistance for the exfoliated 1T-WS2 nanosheets was found to be 6 Ω for oven intercalation and 5 Ω for microwave-assisted interaction, dramatically lower than the as-grown 2H-WS2 nanostructures (200 Ω). As such, in accordance with the observations of metallic MoS2 nanosheets, higher conductivity of metallic 1T-WS2 nanosheets translates into faster HER kinetics. However, this prompts us to question if the intrinsic HER activity of the 1T phase stems from the number of edges or its conductivity. On the basis of the electrochemical oxidation of the edges of 2H- and 1T-MoS2, Chhowalla and co-workers31 discovered oxidized 2H-MoS2 edges had weaker catalytic activity while the HER performance

of the oxidized edges of 1T phase was unchanged. Clearly, the HER activity of 2H-MoS2 is dependent on the reduced, while the edges of 1T phase are not the predominant active sites for HER. The importance of conductivity to HER catalytic nature of the 2H and 1T phases was affirmed by the addition of singlewalled carbon nanotubes (SWNTs). 2H-MoS2/SWNT showcased higher conductivity and faster HER kinetics with a lowered overpotential compared to 2H-MoS2. In contrast, the HER performance of 1T-MoS2 remained unaffected by the addition of SWNTs. Therefore, it has been determined that conductivity is the key property toward the inherent HER activity of 2H- and 1T-polymorphs wherein 1T phase possesses intrinsically higher charge-transfer kinetics that facilitates HER. Apart from the chemical nature of the TMD, the supporting platform for electrical transport is another crucial aspect of conductivity that influences HER of TMDs. Various TMD catalyst supports, such as Au,35 activated carbon,66 carbon paper,36 graphite,75 carbon nanotubes,76 and graphene-based materials,64,77 have been experimented with, the majority of which with MoS2. Among them, graphene-based materials have garnered much success in achieving remarkable HER catalysis, postulated to be associated with the large surface area, electrical and chemical coupling effect, and conductivity of graphene. In one example, Li et al.64 synthesized MoS2 on reduced graphene oxide (rGO) via a solvothermal reaction involving (NH4)2MoS4 and N2H4 in GO to obtain a MoS2/rGO hybrid endowed with outstanding catalytic activity (Figure 8b). The resulting hybrid exhibited selective growth of MoS2 nanoparticles that were well dispersed on GO because GO prevented aggregation of MoS2 nanoparticles. In doing so, the homogeneous dispersion of MoS2 on GO engendered numerous accessible sites for HER. Furthermore, electrical coupling to the graphene sheets also provided a conductive network to facilitate electron transfer from MoS2 to the electrode. Hence, abundant catalytic sites and the conductive N


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Figure 9. Methods to enhance the active sites for HER are by maximizing exposed edge sites and inducing defects in TMDs. (a) Synthesis procedure and structural model for mesoporous MoS2 with a double-gyroid (DG) morphology. Reprinted with permission from ref 78. Copyright 2012 Nature Publishing Group. (b) (Left) Synthesis of defect-free and defect-rich MoS2 nanosheets. (Right) SEM image of obtained defect-rich MoS2 nanosheets. Reprinted with permission from ref 80. Copyright 2013 John Wiley and Sons.

4.2.3. Activation of TMDs for HER. Studies on TMDs have shown that their HER electrocatalytic activities can be improved upon activation; hence, there is a strong motivation in enhancing their catalytic properties via various strategies. Current strategies to activate the TMDs for HER focus on the three factors considered above and attempt to increase the number of active sites, enhance the intrinsic activity of the TMD, and improve the conductivity between active sites and the catalyst substrate. The first strategy toward improving the electrocatalytic activity of TMDs is to increase the amount of active sites. Increasing the number of active sites can be achieved through maximizing exposure of the edges or inducing defects in the TMDs. Jaramillo and co-workers created a highly ordered double-gyroid MoS2 bicontinuous network, first, by electrodepositing Mo onto a silica template and, subsequently, sulfidizing using H2S.78 This double-gyroid MoS2 structure exhibited a high surface curvature preferentially exposing a

underlying graphene-based catalyst support system participate in conferring the MoS2/rGO hybrid with excellent HER performance, having a low overpotential of 0.1 V and Tafel slope of 41 mV dec−1. Likewise, a WS2/rGO hybrid also having better HER catalytic performance was synthesized using hydrothermal reaction in a one-pot process by Shin and coworkers.77 Upon annealing, WS2/rGO hybrid had a Tafel slope of 58 mV dec−1 and faster charge transfer confirmed by electrochemical impedance studies. In addition, the removal of oxidized impurities in WS2 during annealing may also contribute to its better catalytic activity. It was concluded that the enhanced HER performance of the WS2/rGO hybrid was an interplay of conductivity and crystallinity reasons. In particular, conductivity had a bigger slice of the pie. Interconnected conducting rGO network provided efficient electrical contact between the WS2 catalyst and the electrode which promoted HER kinetics. O


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Figure 10. Doping and strain influence the intrinsic catalytic activity of TMDs. (a) Hydrogen adsorption energies (ΔGH) of each transition-metaldoped MoS2. Reprinted with permission from ref 81. Copyright 2015 Royal Society of Chemistry. (b) Effect of varying tensile strain of 1T WS2 on its free energy. The free energy of H+ + e− is by definition equal to that of 1/2H2 at standard conditions (1 bar of H2 and pH = 0 at 300 K). Free energies for the intermediate adsorbed state H* are calculated by using DFT and corrected for zero-point energies and entropy. Reprinted with permission from ref 83. Copyright 2013 Nature Publishing Group.

Figure 11. Better conductivity in TMDs is achievable by doping, electrochemical pretreatment, and also providing a catalyst support. (a) (Top) Polarization curves recorded on GC electrode of V-doped MoS2 nanosheets and undoped MoS2. (Bottom) Nyquist plot of the V-doped MoS2 nanosheets; (inset) pure MoS2. Reprinted with permission from ref 84. Copyright 2014 Royal Society of Chemistry. (b) Effect of applied pretreatment potentials of bulk (2H) and exfoliated (2H + 1T) MoS2 on HER overpotential at −10 mA cm−2. Reprinted with permission from ref 41. Copyright 2014 John Wiley and Sons. (c) (Top) Synthesis procedures of MoS2/MGF hybrid. (Bottom) Nyquist plot of electrodes modified with MoS2/MGF, physical mixture containing MoS2 and MGF, and pure MoS2 particles. Reprinted with permission from ref 85. Copyright 2013 John Wiley and Sons.

ensued (Figure 9b). The defect-rich MoS2 nanosheets exposed additional active edge sites which were the defect-induced cracks on the basal plane, eventually engendering a high density of active sites at 1.8 × 10−3 mol g−1. As anticipated, the exceptional HER performance of these defect-rich MoS2 nanosheets was noted with low onset potential at 0.12 V and a Tafel slope of 50 mV dec−1. Another activation strategy is to enhance the intrinsic catalytic activity of TMDs. This has been done by Chorkendorff and co-workers by doping the S edge of MoS2 and WS2 with Co.36 As determined by computational studies, doping decreased the ΔGH of the S edge of both TMDs, from 0.18 to 0.1 eV in MoS2 and from 0.22 to 0.07 eV in WS2. This lowered ΔGH correlates to enhanced intrinsic catalytic activity of the edges for HER. It has also been experimentally confirmed that both Co-doped TMDs exhibited better HER performance than its undoped counterpart. More recently,

significant portion of edge sites (Figure 9a). In doing so, this catalyst showed excellent catalytic behavior as manifested in low onset potentials of 0.15−0.2 V (vs RHE) and a Tafel slope of 50 mV dec−1. Cui et al. developed stable MoS2 and MoSe2 films with vertically aligned layers, so that the edges were best exposed on the surface.79 Despite their considerably large Tafel slopes ranging from 105 to 120 mV dec−1, these edgeterminated films were found to have favorable HER exchange current densities of 2.2 × 10−6 and 2.0 × 10−6 A cm−2 for MoS2 and MoSe2, respectively. In another example, the number of active sites can be increased by introducing defects to the basal plane, which is otherwise inert to HER. Xie et al. engineered a defect-rich MoS2 structure by varying precursor concentration and thiourea amounts.80 During the fabrication process of defect-rich MoS2 nanosheets, thiourea was added in excess because adsorption of thiourea molecules on the surface led to partial inhibition of crystal growth, and a defective structure P


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foam (MGF) has been proposed by Liu and co-workers as a catalyst support to boost the conductivity of TMDs.85 This group fabricated MoS2 nanoparticles embedded on MGFs, resulting in MoS2/MGF hybrid with highly dispersed MoS2 nanoparticles on MGFs without aggregation (Figure 11c), akin to reported observations in MoS2/rGO hybrid.64 A high specific surface area and conductive graphene skeleton enabled rapid electron transfer in MoS2/MGF, resulting in commendable catalytic HER performance with a Tafel slope of 42 mV dec−1. Finally, a multipronged approach combining two or more strategies has also emerged in the field to activate TMDs for HER. Cui and co-workers combined two TMD activation strategies, increasing number of active sites and enhancing its intrinsic activity, on vertically aligned MoSe2 and WSe2 nanofilms.86 Edge-terminated MoSe2 and WSe2 were perpendicularly aligned on rough and curved carbon fiber surfaces to maximize the exposure of active edge sites for HER. They were demonstrated to be efficient HER electrocatalysts having low overpotentials of −0.25 and −0.3 V (vs RHE) and Tafel slopes of 60 and 77 mV dec−1 for MoSe2 and WSe2, respectively. Ni atoms were doped onto edge-terminated MoSe2 nanofilms and found to enhance the HER activity (exchange current density increased to 2.8 × 10−3 mA cm−2) in a manner resembling Co doping, whereby the intrinsic activity of the edge sites improved.36 Another multipronged approach, undertaken by Yan et al., reported that enhanced HER catalysis of S-doped MoSe2 nanosheets of low overpotential at 0.16 V and Tafel slope at 58 mV dec−1 stemmed from the increase in active sites and better electrical conductivity.87 Incorporating S into MoSe2 causes stacking faults and plane defects and gives rise to edge plane curvature and nonuniform spacing between planes. Exposed edge sites and unsaturated S and Se lining the nanodomain walls served as active sites for hydrogen adsorption. Additionally, the enhanced charge-transfer kinetics for HER was inferred from the impedance measurement wherein Nyquist plots exhibited substantially lower impedance for S-doped MoSe2 (36 Ω) than undoped MoSe2 (145 Ω). Tapping into increasing the number of active sites and conductivity, Chen and co-workers fabricated a MoSe2/ graphene hybrid featuring perpendicularly oriented MoSe2 nanosheets on a 3D graphene network.88 The perpendicular orientation of MoSe2 nanosheets engendered a higher density of active sites and accessible edges. The graphene network bridges the graphite disc and MoSe2 and improves the electrical contact at the electrode interface, thereby reducing resistance and facilitating HER. Combined effects of active sites and good conductivity elicited an impressive HER performance of a low onset potential at −0.05 V. He and co-workers synthesized component-controllable WS2(1−x)Se2x nanotubes on carbon fibers conferred with desirable electrocatalytic properties for HER.89 WS2(1−x)Se2x nanotubes were synthesized in the presence of Se and S powders at high temperatures to ensure their simultaneous reaction with WO3 nanowires. In the process, the high temperature created a violent reaction producing defects and exposed edges in WS2(1−x)Se2x nanotubes deemed as the active sites. Nyquist plots revealed fast charge-transfer resistance in WS2(1−x)Se2x nanotubes, demonstrating its enhanced electrical conductivity. Therefore, the synergistic effect of increased active sites and good conductivity of WS2(1−x)Se2x nanotubes led to lower overpotentials of HER than WS2 and WSe2 nanotubes.

Abild-Pedersen et al. adopted DFT study to evaluate the effect of doping the S edge of MoS2 with various transition metals.81 DFT calculations revealed that 14 metal-doped MoS2 structures have ΔGH closer to thermoneutral than the S edge as marked in the gray bands in Figure 10a. Of these, six metal dopants, namely, Ru, Rh, Co, Fe, Mn, and Ta, exhibited notable improvements in ΔGH toward the thermoneutral value, even prevailing over the HER active Mo edge. At present, only a few of them such as Co, Fe, and Ni had been experimentally tested to show improved HER performance, hence verifying the computed trend.36,82 Besides doping, Chhowalla and coworkers showed that the HER performance of metallic 1TWS2 nanosheets can be improved by strain engineering.83 They noticed that the conversion from 2H- to 1T-WS2 was associated with distortion and strain in the resulting 1T phase, which prompted their study into the effect of strain on WS2 toward HER. Their DFT study established that an optimal strain value of 2.7% on 1T-WS2 resulted in ΔGH that was close to thermoneutral (Figure 10b), while no effect of strain on the catalytic activity of 2H-WS2 was observed. The third activation approach seeks to improve the conductivity of TMDs and has been materialized by doping, electrochemical means, or using a catalyst support. Wu and coworkers84 showed that V doping of MoS2 nanosheets can enhance the conductivity of the material and, hence, improved catalytic activity for HER. The intralayer-vanadium-doped samples were prepared by a solid-state reaction in a mixture of vanadium, molybdenum, and sulfur powders and then obtained by liquid exfoliation. ICP analysis deduced the Vdoped samples to be V0.09Mo0.91S2 and V0.05Mo0.95S2. Both Vdoped MoS2 displayed low onset potentials at ca. −0.13 V and improved Tafel slopes of 69 and 80 mV dec−1 from 90 mV dec−1 in undoped MoS2. Clearly, V-doped MoS2 had better HER ability than unmodified MoS2 (Figure 11a). This improved catalytic activity was attributed to the decreased charge-transfer resistance of V-doped MoS 2 (26 Ω in V0.09Mo0.91S2 and 44 Ω in V0.05Mo0.95S2) than undoped MoS2 (2.7 kΩ). It was inferred that higher doping of V atoms optimized the electrical properties of the MoS2 system, leading to enhanced conductivity and faster electron transport. Moreover, tweaking the ratio of V and Mo has shown potential in tuning the catalytic activity for HER. Recently, by employing an electrochemical method, we precisely tuned the electron transfer kinetics and catalytic properties of exfoliated (2H + 1T) and bulk (2H) MoS2 films by varying the pretreatment potential.41 In general, electrochemical oxidative treatment of MoS2 film reduces the HER catalytic activity of both 2H- and (2H + 1T)-MoS2 films, while subjecting them to a reductive pretreatment exhibited improved HER performance. Such effect was more prominent in the exfoliated material. The optimal pretreatment potential applied to attain improved HER activity was determined to be + 0.4 V and below (Figure 11b). DFT calculations attributed this phenomenon to the structural destabilization of 2H-MoS2 by electron doping into its conduction band when reduced. In contrast, reduction stabilizes the 1T phase due to crystal field effects. Following this work we recently investigated the effects of electrochemical pretreatments to the catalytic and charge-transfer properties of more TMD materials such as MoS2, WS2, MoSe2, and WSe2.70 Various research groups have also been actively exploring suitable catalyst supports for TMDs to enhance their conductivity and, hence, HER performance. In a strategy that extends from the use of 2D rGO, 3D mesoporous graphene Q


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Figure 12. Oxygen reduction reaction on layered transition-metal dichalcogenides and related nanostructured chalcogenide materials. (a) ORR polarization curves on chalcogen-modified ruthenium catalysts performed on rotating disc electrode in 0.1 M NaOH at 900 rpm and 20 mV/s and the corresponding Tafel and Levich plots. Reprinted with permission from ref 94. Copyright 2011 American Chemical Society. (b) Linear sweep voltammograms of MoS2 particles obtained after centrifugation at differing speeds. Conditions: O2-saturated 0.1 M KOH at 900 rpm and 10 mV s−1. (c) Electrons transferred for ORR at different potentials for bulk MoS2 and MoS2 particles collected after centrifugation. Reprinted with permission from ref 98. Copyright 2013 John Wiley and Sons.

4.3. Oxygen Reduction Reaction (ORR) at TMD Electrodes

importantly, the number of transferred electrons was found to vary between two and four electrons depending on the applied potential for bulk or large particles (Figure 12c). The smallest MoS2 particles (ca. 2 nm diameter) however showed ORR with the number of electrons remaining close to four regardless of the potential, suggesting a size effect on the observed reduction mechanism. We also studied the ORR on chemically exfoliated MoSe2, WS2, and WSe2 but observed no significant enhancements with increased surface areas after exfoliation.40 As a whole, only MoSe2 demonstrated slightly better performance over WS2 and WSe2, but as in the case of HER, it is postulated that this is primarily the result of its intrinsically higher electrical conductivity.

Another industrially important electrochemical process is the ORR, which is the typical cathode reaction employed in fuel cell technologies today. Similar to the HER, Pt materials are currently the most efficient electrocatalysts available, thus limiting the commercial viability of large-scale fuel cell applications.90 The search for alternatives then led to a first report by Alonso-Vante and Tributsch on Mo4.2Ru1.8Se8 exhibiting activities comparable to Pt.91 Subsequent investigations focused on metal chalcogenide compounds consisting primarily of Ru-based sulfide and selenide materials with varying amounts of Mo doping92−94 and also cobalt95,96 or iron97 chalcogenides. However, it should be noted that these materials are not strictly layered in structure and usually adopt pyrite-type structures. Most ORR studies employ rotating disk voltammetry, which allows derivation of the number of electrons transferred and therefore the electrochemical reduction mechanism. As illustrated in Figure 12a, the Mukerjee group had previously shown that the ORR on Ptand Ru-based catalysts proceeds via transfer of four electrons corresponding to the direct reduction of O2 to H2O. In contrast, few have investigated pure 2D-layered TMD materials, and the same group demonstrated that ORR on sulfurdecorated molybdenum proceeds instead through the 2e− reduction process (Figure 12a).94 Further, Li and co-workers investigated pure MoS2 particles which showed enhancement in ORR with decreasing particle sizes (Figure 12b).98 More

4.4. TMD Materials for Electrochemical Capacitors

The constant increasing energy demand in modern society has triggered massive research efforts toward the development of increasingly efficient energy storage devices. Of these, electrochemical capacitors (ECs), also called supercapacitors, are receiving enormous attention due to the higher power densities and longer life cycle demonstrated over batteries.99,100 ECs store energy through two different mechanisms: (i) nonfaradaic charge separation at the electrode/electrolyte interface which generates the double-layer capacitance and (ii) fast and reversible faradaic redox process of an electroactive material which generates the so-called pseudocapacitance.101 Two important characteristics should be met by the electrode materials for the development of efficient ECs: high surface R


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Figure 13. Exfoliation of metallic VS2 and its application to in-plane supercapacitors. (a) VS2·NH3 precursor with NH3 molecules intercalated into the S−V−S layers. (b) Effusion of NH3 molecules away from the stacked layers, breaking down the c axis periodicity and resulting in the formation of ultrathin VS2 nanosheets. (c) Vacuum-filtration assembly of the as-synthesized VS2 nanosheets into transferrable thin films on mixed cellulose membrane. (d) Schematic illustration of the in-plane supercapacitor. (e) Planar ion migration pathways for the in-plane supercapacitor. (f) Cyclic voltammograms at different scanning rates of 20, 100, and 200 mV/s. (g) Galvanostatic cycling behavior and IR drop illustration. (h) Galvanostatic charge/discharge curves of the as-fabricated in-plane supercapacitor. (i) Cycle life investigation of the supercapacitor, showing negligible degradations in the coulomb efficiency and specific capacitance. Reprinted with permission from ref103. Copyright 2011 American Chemical Society.

flexible all-solid supercapacitors, obtaining a planar capacitance of 4760 μF/cm2 using the water-soluble ionic liquid BMIMBF4 as electrolyte. The supercapacitor demonstrated also excellent cycling stability with negligible loss of capacitance after 1000 charge/discharge cycles (Figure 13d−i).103 More recently, planar microsupercapacitors were fabricated using laser-patterned thin films of MoS2 obtaining a planar capacitance of 8 mF/cm2.104 In order to increase the active surface area and the density of the edge sites of MoS2, several hydrothermal procedures have been adopted to produce different nanostructures of MoS2 and then applied to the fabrication of supercapacitors.105−111 Flower-like nanostructures were obtained by Wang and co-workers using MoO3 nanorods as precursor. The MoS2 electrode showed a specific capacitance of 168 F/g at 1 A/g current density.105 MoS2 nanospheres were fabricated in two different works by Zhou et al. and Krishnamoorthy et al., who then tested the synthesized MoS2 nanomaterial as supercapacitor electrode, obtained in aqueous electrolytes a specific capacitance of 106 F/g at a 5 mV/s scan rate for the former106 and 122 F/g at 1 A/g current density from charge/discharge studies for the latter.107 A mesoporous MoS2 nanostructure was obtained by Ramadoss et al. which possessed remarkable capacitive properties, giving specific capacitances of 376 and 403 F/g at a 1 mV/s scan rate in 1 M Na2SO4 and KCl, respectively.108 MoS2 nanosheets were produced hydrothermally by Huang and co-workers, achieving a specific capacitance of 129.2 F/g at 1 A/g current density in aqueous electrolyte.109 More recently, spherical nanostructures

area and high conductivity. In this regard, 2D materials and, in particular, TMD materials can provide promising features since they can be produced at nanometer scale combining possible inherent redox processes which account for the pseudocapacitive behavior. One of the first studies on the use of TMD materials for capacitors was carried out in 2007 by Soon and Loh, who investigated the capacitive behavior of edge-oriented MoS2 films grown by chemical vapor deposition.102 The authors measured excellent charge storage capacity for the MoS2 film, reaching values as high as 70 mF/cm2 in 0.5 M H2SO4 at 1 mV/s scan rate. This was translated in about 100 F/ g (at 1 mV/s) as specific capacitance, comparable to the capacitance measured using carbon nanotubes electrodes. This value decreased to 20 F/g at a scan rate of 50 mV/s. The authors demonstrated that faradaic processes implying the intercalation of protons or alkali metals in the MoS2 interlayers occur only at slow scan rates because such process is slow. At high scan rates only proton adsorption and nonfaradaic processes occur.102 With the aim of applying a highly conductive TMD material for supercapacitors, Feng et al. proposed the use of VS2 to fabricate a flexible planar supercapacitor.103 Similar to MoS2, a single trilayer of VS2 consists of one plane of vanadium sandwiched between two sulfur atoms with weak van der Waals forces holding together multiple trilayers (Figure 13a). It possesses metallic character and therefore represents an optimal electrode material for capacitors. The authors prepared thin films of VS2 exfoliated by NH3 (Figure 13a−c) intercalation and applied them to build S


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Figure 14. Molybdenum sulfide nanospheres for flexible supercapacitors. (a, b) Low- and high-magnification SEM images and (c, d) low- and highresolution TEM images of the synthesized MoS2 spheres. (e) Photographs of solid-state supercapacitor bent in different angles showing the flexibility of the device. (f) Specific capacitance as a function of scan rate. (g) Charging curve of three supercapacitors connected in series charged up to 3 V at different current values. (h) Photograph of 8 commercial red color LEDs lit by three charged supercapacitors connected in series. Reprinted with permission from ref 111. Copyright 2015 Elsevier.

were again fabricated.110,111 Javed et al. applied the material as a flexible supercapacitor, obtaining a specific capacitance of 368 F/g at a 5 mV/s scan rate. The capacitor demonstrated also good stability over 5000 cycles, unaltered performance upon bending, and a high power density of 128 W/kg at an energy density of 5.42 Wh/kg (Figure 14).111 A different preparation method was employed by Winchester and co-workers.112 In their recent work, commercial bulk powder of MoS2 was treated ultrasonically in the presence of 1dodecyl-2-pyrrolidinone (N12P), obtaining a liquid-phase exfoliation of the material into single- to few-layer MoS2 sheets. Thin films of the material were tested in a twoelectrode parallel plate setup for capacitance measurements in aqueous electrolyte (KOH), organic electrolyte (Et4NBF4 in PC), and ionic liquid (BMIM-PF6). The best capacitive behavior was measured in BMIM-PF6 electrolyte with a planar capacitance of 2.4 mF/cm2 at a 10 mV/s scan rate with a voltage window applied of 3.5 V. A more efficient exfoliation procedure to obtain single-to-few layers TMD materials starting from the abundantly available natural bulk powders consists of the use of organolithium compounds such as methyllithium (MeLi), n-buthyllithium (n-BuLi), and tert-buthyllithium (tBuLi). After Li-ion intercalation in organic solvent, the addition of water causes a violent reaction which splits apart the material layers. Such chemical exfoliation is very commonly used to obtain thin sheets of the material for several different applications as seen in previous sections of this review. It has

been demonstrated that using this procedure favors the transition from the trigonal prismatic 2H polymorph to the octahedral 1T phase of the Group 6-based TMD materials. Such structural conversion significantly alters the electronic properties of the materials with consequences, in particular, on their conductivities.26−28 The 2H polymorph is in fact semiconducting, while the 1T phase has metallic character. We recently investigated the influence of different organolithium intercalants on the electrochemical properties of chemically exfoliated MoS2, WS2, MSe2, and WSe2 including capacitive performances.32,39,113 Interestingly, t-BuLi was extremely efficient in the exfoliation of the bulk materials, in particular, with respect to WS2, which demonstrated better catalytic properties toward the HER and better capacitive behavior after chemical exfoliation due to the increased active surface area as well as the material conductivity.32,113 Another brilliant example of using the chemical exfoliation with organolithium compounds to obtain a restacked thin film of a metallic 1T phase of MoS2 for enhanced capacitive properties was recently demonstrated by Chhowalla’s group.114 In this work the 1T-MoS2 showed excellent abilities to intercalate ions such as H+, Na+, K+, and Li+, achieving volumetric capacitance values between 400 and 700 F/cm3 in both aqueous and organic electrolytes (Figure 15). As said, the high conductivity of the material is a key factor to obtain significant capacitive properties, and in this regard, approaches consist of the preparation of composite material T


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Figure 15. Metallic 1T-phase molybdenum sulfide as supercapacitor electrode material. (a, b) Photographs of electrodes consisting of a thick film of chemically exfoliated 1T-MoS2 prepared by vacuum filtration and transferred onto rigid glass (a) and a flexible polyimide substrate (b). (c) Side view of the electrode observed by scanning electron microscopy (SEM) showing the layered nature of the film made by restacking exfoliated MoS2 nanosheets. (d) SEM image of as-exfoliated monolayer 1T-phase MoS2 nanosheets. (e) High-angle annular dark-field scanning transmission electron microscope image of monolayer 1T-phase MoS2. (Inset) Atomic structure of 1T-phase MoS2 (Mo and S atoms are displayed in blue and yellow, respectively). (f) High-resolution X-ray photoelectron spectrum from the Mo 3d region of as-exfoliated 1T-MoS2 (black). Contributions from 1Tand 2H-phase components in the Mo 3d spectrum are indicated by blue and red curves, respectively. (g) Comparison of the CV curves of 1T MoS2 in 0.5 M K2SO4 and 1 M KCl. The absence of a noticeable difference between the two electrolytes supports the fact that only cations are being stored. (h) Galvanostatic cycles from 0.5 to16 A/g in K2SO4. (i) Capacitance retention after 5000 cycles in 0.5 M Li2SO4, H2SO4, and 1 M TEA BF4 in acetonitrile. Reprinted with permission from ref 114. Copyright 2015 Nature Publishing Group.

biomarkers. More recently, there is also increasing knowledge on their inherent electrochemistry, whereby the metals can undergo electrochemical reduction or be oxidized to higher valence states, both of which alter their properties. Such reductive or oxidative pretreatments can tailor the electrochemical activity of TMDs toward the oxidation/reduction of electrochemically active substances in solution. Significant efforts have also been carried out to create catalytic systems for hydrogen evolution with the eventual aim of replacing platinum catalysts for such reactions. We discussed various strategies for activation and improving HER on TMDs, including maximizing the exposure of active sites, fabricating TMD hybrids on a catalyst support, and doping TMDs. Given the fact that TMDs are highly versatile materials in terms of chemical composition and crystal structure, one can expect that efforts toward further enhancing the electrochemical properties will lie in this direction. One can envision that doping and chemical modifications of the surface/edges of TMDs will lead to its tailored properties for further applications. In summary, we believe that the core of future development in the electrochemistry of TMDs lies with achieving greater control

combining the high surface area of nanostructured (or exfoliated) TMD materials with highly conductive materials. Combining TMDs with carbon materials is certainly a wellexplored strategy with examples of composites using CNTs,115 graphene,116−118 and microporous carbon119,120 in combination with TMD materials. Other groups proposed the combination of TMD materials with conductive polymers such as PANI,121,122 PPy,123 and PEDOT.124 It is worth mentioning the high specific capacitance of 575 F/g at 1 A/g current density obtained using PANI/MoS2 composite as electrode material in aqueous electrolyte.121

5. CONCLUSION AND FUTURE PERSPECTIVES Layered transition-metal dichalcogenides have very interesting electrochemical properties which depend on their elemental composition, crystal structure, size, and defects. Their electrochemistry is strongly anisotropic and largely driven by the edge sites. These edge sites display rapid electron transfer kinetics toward various redox probes, such as Fe(CN)64−/3− and Ru(NH3)62+/3+; specifically, MoS2 has been demonstrated to be electrochemically active for the detection of multiple U


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over their chemical and physical properties, such as (1) exploiting their unique anisotropic structures for electrochemical sensing and catalysis, (2) improved control of transitions between their semiconducting and metallic phases as both phases hold promise for very distinct types of applications, and (3) achieving a better understanding of inherent electrochemical properties of TMDs such that we may limit their negative effects or even go further to manipulate them toward catalytic applications such as the HER.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] . Fax: (+65)6791-1961

Adriano Ambrosi received his Ph.D. degree from Dublin City University, Ireland, in 2007. As a postdoctoral researcher he first worked for 2 years at ICN (Spain) and then in 2009 at NIMS (Japan). In 2010 he joined the research group of Prof. Martin Pumera at Nanyang Technological University, Singapore, where he currently works as Senior Research Fellow. His research interests include the application of nanomaterials to electrochemical biosensors, synthesis, and fundamental electrochemical studies of graphene and other 2D materials for biosensing and energy storage devices and synthetic nanomotors.

Notes

The authors declare no competing financial interest. Biographies

Xinyi Chia graduated with her B.Sc. (Hons) degree in Chemistry and Biological Chemistry from Nanyang Technological University (NTU), Singapore, in 2014. At present, she is a Ph.D. student in Prof. Martin Pumera’s research group in NTU. Her research interests are in electrochemistry and 2D layered nanomaterials such as transitionmetal chalcogenides.

Shu Min Tan received her B.Sc. (Hons) degree in Chemistry and Biological Chemistry from Nanyang Technological University, Singapore, in 2013. Presently, she is pursuing her Ph.D. degree in Prof. Martin Pumera’s group, with her research interest focusing on the electrochemistry of layered nanomaterials for energy-related applications.

Alex Yong Sheng Eng completed his B.Sc. (Hons) degree in Chemistry and Biological Chemistry from Nanyang Technological University (NTU) Singapore in 2012 and is currently pursuing his Ph.D. in Prof. Martin Pumera’s group. His research interests encompass the fundamental electrochemistry of graphenes and nanostructured materials, their application in catalytic systems, and also the effects of chemical functionalization on these materials.

Martin Pumera is a faculty member at Nanyang Technological University, Singapore, since 2010. He received his Ph.D. at Charles University, Czech Republic, in 2001. After two postdoctoral stays (in V


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the United States and Spain), he joined the National Institute for Materials Science, Japan, in 2006 for a tenure-track arrangement and stayed there until Spring 2008, when he accepted a tenured position at NIMS. In 2009, he received the ERC-StG award. He has broad interests in nanomaterials and microsystems, in the specific areas of electrochemistry and synthetic chemistry of carbon nanomaterials, nanomotors, nanotoxicity, and energy storage devices. He is associate editor of the Science and Technology of Advanced Materials, a member of Editorial board of Chemistry A European Journal, Electrochemistry Communications, Electrophoresis, Electroanalysis, The Chemical Records, ChemElectroChem, and eight other journals. He has published over 370 peer-reviewed articles and has a h-index of 44.

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Chemical Reviews

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Electrochemistry of Nanostructured Layered Transition-Metal

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