2D Transition Metal Chalcogenides and van der Waals Heterostructures:

Fundamental Aspects of their Electrochemistry

Robert A.W. Dryfe

School of Chemistry and National Graphene Institute,

University of Manchester,

Oxford Road,

Manchester M13 9PL

U.K.

robert.dryfe@manchester.ac.uk

Abstract: The explosion of research on graphene has prompted a similar level of activity on

materials with related structures, i.e. two-dimensional materials formed from solids with a

laminar structure, which can be isolated in their monolayer form. The main focus of activity

beyond graphene, in electrochemical and other contexts, has focussed on the transition metal

dichalcogenides. This review will highlight important advances in the use of these materials

in an electrochemical context. The existence of a “family” of two-dimensional materials has

led to recent interest in the creation of “van der Waals heterostructures”, where dissimilar

two-dimensional materials are assembled in a specific fashion to produce structures with

distinct electronic properties. The application of these materials in electrochemistry is in its

infancy but very recent works suggest this will be an extremely important area of research in

the coming years.

Figure 1 Schematic illustration of an idealised van der Waals heterostructure, formed from the controlled

stacking of layers of distinct 2D materials, and the electrochemical intercalation of a cation into such a material.

Graphene is commonly cited as the prototypical two-dimensional material, although evidence

for the formation of molybdenum disulfide in its monolayer form was presented in 1980s [1].

MoS2 had, in fact, attracted the interest of electrochemists around the same period because its

layered structure meant it was recognised as a plausible anode material for lithium ion

batteries [2]. The importance distinction between the most common transition metal

dichalcogenides (TMDCs, including MoS2) and graphene is that the former are

semiconductors in their natural form, whereas graphene is a zero-band gap semiconductor

[3••]. The finite band gap (generally in the range 1-2 eV) found for the most stable TMDCs

meant that the photoelectrochemistry of the bulk forms of these materials was widely studied,

generally in the context of water splitting, in the 1970s and 80s [4]. Bulk MoS2 in particular

has been recognised as an excellent catalyst for hydrogen evolution, with numerous

experimental and theoretical studies of the mechanism of this process reaching a consensus

that the edge plane of the material is the active reaction site [5].

The interest in the application of bulk forms of TMDCs in various electrochemical contexts,

coupled with the realisation that mono- or “few-layer” forms of these materials could be

prepared readily, has prompted the recent avalanche of work on the electrochemical

properties of the 2D forms of TMDCs. Of course, the electronic structure of the 2D forms

varies from the bulk forms, hence the fundamental electrochemical behaviour of the

materials, compared to the bulk materials, is expected to differ. An excellent review

describing the physical properties of TMDCs from an electrochemical perspective is

recommended as background reading [6••]. The existence of a number of polytypes of the

TMDCs should also be noted, again see references [3••,6••] for further background. The

thermodynamically stable form of most group 6 TMDCs is the semiconducting 2H phase

(hexagonal, where the number denotes the number of MX2 units per unit cell). A metastable,

metallic 1T phase (tetragonal) also exists for this group, as does a 3R (rhombohedral) phase,

which is a semiconductor [7].

One complication in understanding the “intrinsic” electrochemical properties of 2D materials

has been the general reliance of the (electro-)chemical community on samples prepared via

solution phase methods, such as ultrasonic exfoliation. Although useful as a route to large

(gram-scale) amounts of material, the variations in exfoliate thickness, lateral size and surface

state that are inherent to these methods, coupled with a “random” assembly of the materials

on some kind of conducting substrate to form an electrode, make quantitative studies of

electrochemical phenomena (e.g. electron transfer kinetics) difficult [8]. It is worth briefly

considering the different routes to prepare 2D forms of TMDCs and the subsequent modes of

assembly/contact to form electrodes.

TMDCs, as is the case for other 2D materials, can readily be prepared by solution-based

exfoliation, frequently involving ultrasound [9]. This method, although scalable, produces

dispersions with a statistical distribution of flake thicknesses and rather small particles

(lateral dimensions are typically below 1 micron in samples with a high “few-layer” content).

An alternative solution phase preparation involves n-doping (e.g. by lithiation) of the bulk

material, the negatively charged material being readily exfoliated to the few-layer material,

frequently in the 1T phase, in high yield [1,10]. Chemical lithiation requires inert (glove box)

conditions, due to the instability of the organo-lithium reagents. Clearly, the lithiation can be

achieved electrochemically, and a number of articles have described such approaches [11],

with a recent one from our laboratory outlining a particularly simple approach, where

electrochemical lithiation of MoS2 in an aprotic solvent is followed by immersion of the

lithiated material in water [12•]. The electrocatalytic function of MoS2 with respect to

hydrogen evolution produces a rapid exfoliation of the lithiated material, as lithium displaces

hydrogen from water (note the equivalent process fails with lithiated graphite), producing

large quantities of graphene with a thickness maximum corresponding to tri-layer material.

Dispersions of TMDCs produced by such liquid phase routes are readily filtered and dried on

conducting substrates for use as electrodes in energy storage/conversion applications

(batteries, supercapacitors, hydrogen-based fuel cells) [13]. By way of contrast, there are far

fewer papers on the electrochemical response of “well-defined” mono- or few-layer TMDC

samples, which is surprising given the layer-dependence of the electronic structure of these

materials. The reasons for this are probably derived from the relative complexity of sample

preparation/characterisation and the relatively small size (sub-mm) of the resultant flakes,

necessitating the use of “micro-scale” electrochemical techniques. Large (mm, or larger,

lateral dimensions) monolayer samples of 2D materials can be prepared via chemical vapour

deposition (CVD) methods, although grain boundaries and pinholes are always present to

some extent. Mechanical exfoliation produces the highest quality MoS2 samples and has

been used by Velický et al to prepare samples ranging in thickness from monolayer to bulk,

and thereby study the photo-electrochemistry of these materials in aqueous solution [14••]. A

complex dependence of electron transfer rate on illumination intensity was found for

monolayer MoS2 in particular. Also, the rate of electron transfer was found to decrease as

sample thickness decreased, indicating that the interfacial (solid/molecule) charge transfer,

rather than charge carrier transport between the layers of the solid, was the limiting factor.

This work investigated simple, nominally “outer-sphere” redox couples, whereas a more

recent paper on photoelectrochemical hydrogen evolution on mechanically exfoliated MoS2

has suggested that the electron transfer rate is highest for the thinnest samples [15]. Well-

defined MoS2 samples prepared via the CVD method have been used by Zhang et al in a

quantitative study of hydrogen evolution kinetics, where the rate of hydrogen evolution was

found to be higher on edges than on the basal plane of the monolayer material, and higher on

the metallic (1T) phase than the 2H phase [16], although no correlation to thicker (including

bulk) samples was presented. This means that at the present stage no general conclusions can

be drawn about the dependence of (photo-)electrochemical activity on TMDC sample

thickness. The above works represent “the exception that proves the rule”: generally the

literature on the electrochemical properties of TMDCs, as with the case of graphene, is

dominated by films produced from solution-phase exfoliation and there is little or no

electrochemical data on CVD or mechanically exfoliated 2D TMDCs, other than MoS2.

Some consideration will now be given to electrochemical studies of van der Waals

heterostructures, i.e. structures formed from well-defined assemblies of more than one 2D

material [3••,17••]. The word heterostructure is taken to have a restricted meaning in this

context, where it applied to “deliberate” assemblies, in a specific sequence and sometimes

with a specific relative orientation (see below) of 2D materials, whereas we treat “random”

mixtures of more than one 2D material as a 2D composite. Given the preponderance of work

to date on graphene and TMDCs, such heterostructures have generally been formed by the

combination of these two materials, with the other commonly found component being the 2D

form of hexagonal boron nitride (h-BN). Electrochemical studies of h-BN are confounded by

the insulating nature of this material [3••], however it can be studied in its monolayer form as

part of a larger structure, as discussed in the following section on heterostructures. The

insulating properties of h-BN drove the first interest in van der Waals heterostructures, for

example by creating light-emitting devices from “sandwiching” quantum wells of

semiconducting TMDCs between h-BN layers, with graphene used as the transparent

conductor [18]. The perturbation of the electronic structure is governed not only by the

combination of materials, but also the by the relative alignment of the 2D lattices, creating a

highly “tunable” superlattice [17••]. This concept gives rise, in the electrochemical context, to

the attractive idea that a “designer” electrode material could be created, with properties

optimised (Fermi level, adsorption energy on external surface, intercalation energy on

internal surfaces) to optimise the thermodynamics (in the context of energy storage, i.e.

secondary batteries) or kinetics (in the context of energy conversion, i.e. electrocatalysis) of a

specific process.

Modified electrodes have been prepared that consist of monolayers of h-BN “covering” Au

electrodes, which were found to impart otherwise absent catalytic function toward oxygen

reduction to these electrodes [19]. The more specific case of heterostructures formed from

layers of two (or more) 2D materials has begun to be explored in the electrochemical context

very recently. Two papers of note appeared in 2016, which expanded the palette of 2D

materials by showing that layered minerals, specifically mixed metal chalcogenides such as

franckeite (Pb5Sn2FeSb2S14), could be exfoliated to monolayers, which consisted of a natural

heterostructure of two distinct layers (Pb/Sb rich, Sn/Fe rich): the electrochemical properties

of such structures have been investigated [20•,21•]. Subsequently, electrochemical methods

have been used to create heterostructures, by cathodic intercalation of laminar materials

(including MoS2) with tetraalkylammonium ions to produce so-called monolayer atomic

crystal molecular superlattices [22••], although the concept of inclusion of (neutral) organic

molecules within MoS2 can be traced back to the pioneering work of Frindt et al [23•], and

has also re-appeared in the context of MoS2-based membranes [24]. The presence of the

organic ions decouples the layers of the bulk material from one another, such that the optical

properties (e.g. photo-luminescence) of the intercalated material differ markedly from those

of the bulk material, see Figure 2 for details of the distinct response of the electrochemically-

generated ion-2D material structure [22••]. Tunability arises both from potential-induced

control over the degree of intercalation (staging) and the identity (size) of the intercalated ion.

Figure 2 Electrochemical intercalation of the cetyltrimethylammonium cation into black

phosphorus to form a so-called “monolayer phosphorene molecular superlattice” (MPMS). a,

Current-potential response with (inset) a false-colour scanning electron micrograph (SEM) of

the MPMS transistor, scale bar = 5 μm. b, Use of the starting BP (black) and the final MPMS

(green) as an ionic gate transistor at 0.01 V drain–source bias. c, Evolution of the photo-

luminescent signals response as a function of electrochemical intercalation. d, Evidence of

the correlation between the bandgap of the material and the applied electrochemical potential.

e, Map of photoluminescence (at 550.0 nm) showing the uniformity of the response (inset:

corresponding SEM image, Scale bars = 3 μm. f, AFM images of the BP flake and g AFM of

the resulting MPMS after cation intercalation, scale bars = 300 nm. Reprinted by permission

from: Springer Nature, C. Wang et al, Nature, Vol. 555, pages 231–236, Copyright 2018.

A related, but distinct, approach is the electrochemical intercalation of ions into specifically

desgined heterostructures (layers of distinct 2D materials), as opposed to randomly oriented

composites of 2D materials. This concept was first exemplified in the work of Zhen and co-

workers, who described electrochemical lithiation of a heterostructure formed from

monolayers of graphene and MoS2, again demonstrating substantial changes in the photo-

luminescent energy and intensity, compared to isolated MoS2, i.e. exploiting the relatively

weak van der Waals coupling between the graphene and TMDC layers [25]. Very recently,

Kim and co-workers have expanded upon this approach by stacking layers of 2D materials

and investigated the intercalation of ions (Li+) into the resultant heterostructures. Their initial

study used monolayer graphene sandwiched between two h-BN layers [26], with an expanded

range of structures formed from h-BN, graphene and TMDCs (MoS2 and MoSe2) explored in

subsequent work [27••]. Significantly, from the perspective of energy storage, the Li+

intercalation capacity and potential are respectively higher and lower for the graphene/ MoS2

interface, compared to intercalation into the MoS2/MoS2 interface, see Figure 3 for detailed

microscopic and computational studies of one of the heterostructures studied. The proof-of-

concept established by such works clearly allows for the intriguing possibility that such

artificial heterostructures can be designed for optimal uptake of ions of higher charge, e.g.

Mg2+ or Al3+: development of anode materials based on the reversible intercalation of such

ions would be an important breakthrough. Normally, alkali metal intercalation of MoS2

results in decomposition to form the group 1 metal sulphide and metallic Mo at low

potentials: despite the interest in the use of exfoliated (i.e. few-layer) MoS2 and composites

thereof as anodes in sodium ion batteries [28], the decomposition reaction hinders the

reversibility of the battery [29]. The work of Kim and co-workers suggests that this

decomposition process is suppressed in the heterostructure, possibly by the dimensional

confinement resulting from the sulphide existing on only one “side” of the intercalated

species [27••].

Figure 3 a, Schematic of van der Waals heterostructure for electron microscopic analysis.

Inset, optical micrograph of a representative device, scale bar = 10 μm. b, High-angle annular

dark-field (HAADF) STEM images of vertically stacked h-BN, MoS2, graphene, h-BN, h-BN

devices before intercalation and c (HAADF) STEM images after one potentiostatic lithiation

cycle, scale bars = 5 nm, insets: the corresponding selected-area electron diffraction patterns.

d, Computed lithium-atom binding energy as a function of the number of lithium atoms in the

supercell. e, Computed relaxed structures (top) and density-of-states plots (bottom) for

pristine (left), and lithium-intercalated (middle, 1 Li; right, 4 Li) heterostructures. f, Proposed

mechanism of van der Waals heterostructure intercalation. Reprinted by permission from:

Springer Nature, D. Kwabena Bediako et al, Nature, Vol. 558, pages 425–429, Copyright

2018.

In a similar vein, the heterostucture approach has been extended to the optimisation of the

hydrogen evolution reaction, using CVD grown layers of MoS2 and graphene, with graphene

on MoS2 found to be the optimal structure, a conclusion backed up with density functional

theory calculations.[30]. Clearly, the formation of such devices is laborious, but when backed

up by calculation to act as a guide to reduce the “materials parameter space”, the controlled

formation of heterostructures promises to be an excellent way to discover new electrodes, for

applications both in the battery and fuel cell contexts. 2D composites, with an uncontrolled

orientation and contacts between the constituent 2D layers, have shown promise as electrode

materials [31], however the basis of their enhanced performance is generally not well

understood. Deliberate construction of heterostructures, where the geometry of material

contact is defined, offers a rational way to understand the electrochemistry of these complex

systems, as well as revealing new physical and chemical phenomena en route.

Acknowledgements: R.D. thanks the U.K. EPSRC (grant references EP/K016954/1,

EP/N032888/1 and EP/R023034/1) for financial support.

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