**2. Properties of 2D TMDs and advantages for electrocatalysis**

2D TMDs are usually denoted as MX2, where M is a transition metal of groups 4–10 (e.g. Ti, V, Co, Ni, Nb, Mo, Hf, Ta, and W) and X is a chalcogen (S, Se, and Te). MX2 constructs layered structures by the formation of X‐M‐X, three layers of atoms with the chalcogen atoms in two hexagonal planes separated by a plane of metal atoms, and the valence states of metal (M) and chalcogen (X) atoms are +4 and -2, respectively (**Figure 1a**) [30–32]. There are two combination modes of metal and chalcogen elements in MX2, trigonal prismatic or octahedral phases, which are referred to as monolayer 2H and 1T‐MX2, respectively (**Figure 1b**–**d**). Transition metals in different groups have different numbers of *d*‐electrons, which fill up the non‐bonding *d* bands to different levels, resulting in varied electronic properties ranging from insulators such as HfS2, semiconductors such as MoS2 and WS2, semimetals such as WTe2 and TiSe2, to true metals such as NbS2 and VSe2. A few bulk TMDs such as NbSe<sup>2</sup> and TaS2 exhibit low‐temperature phenomena including superconductivity, charge density wave (CDW), and Mott transition [14]. When these materials are exfoliated into mono‐ or few layers, their properties largely preserve and also present additional characteristics due to confinement effects [14, 15, 27, 33]. The mono‐ or few‐layer TMDs have thickness at the atomic level or significantly lower than their edge lengths and thus appear like sheets (namely, nanosheets). The atomic‐thin nature endows them with many distinctive properties with respect to their bulk counterparts, such as high specific surface area owing to planar structures, abundant uncoordinated surface atoms, excellent solution dispersity, and mechanical flexibility. These features make the 2D TMDs ideal candidates (or component parts for hybrid structures) with improved electroca‐ talytic performance to substitute their parent materials.

crucial challenges faced with our society to develop reliable and "green" approaches for energy

Electrocatalytic energy conversion utilizing renewable power sources (e.g. solar and wind energy) is regarded as one of the most efficient and cleanest energy conversion pathways [2– 5]. Furthermore, the converted energy is easy to store and use as clean energy or chemical stock. Specifically, the involvement of the electrocatalytic hydrogen evolution reaction (HER) in the cathode and the oxygen evolution reaction (OER) in the anode can efficiently drive water splitting and finally convert the electrical energy into chemical form, that is, hydrogen energy [6–8]. When CO2 is reduced in the cathode while OER happens in the anode, which is the scheme of so‐called artificial photosynthesis, it converts the electrical energy into chemical forms stocked in CO or hydrocarbons [9–11]. Hence, in such context, it is urgently required in both academic and industrial fields to build our power‐supply systems based on electrocatal‐ ysis, amongst which developing efficient electrocatalysts for the aforementioned reactions is

Two‐dimensional (2D) materials have been widely studied for their important physical and chemical properties over the last several decades [12]. Since the recent discovery of graphene [13], 2D materials have gained extensive attention since they exhibit novel and unique physical, chemical, mechanical, and electronic properties [3, 14–19]. In the abundant family of 2D materials, transition metal dichalcogenides (TMDs) have attracted significant interest and become the focus of fundamental research and technological applications due to their unique crystal structures, a wide range of chemical compositions, and a variety of material properties [5, 14, 20–24]. Recently, TMDs have emerged as one kind of efficient electrocatalysts for energy‐

2D TMDs are usually denoted as MX2, where M is a transition metal of groups 4–10 (e.g. Ti, V, Co, Ni, Nb, Mo, Hf, Ta, and W) and X is a chalcogen (S, Se, and Te). MX2 constructs layered structures by the formation of X‐M‐X, three layers of atoms with the chalcogen atoms in two hexagonal planes separated by a plane of metal atoms, and the valence states of metal (M) and chalcogen (X) atoms are +4 and -2, respectively (**Figure 1a**) [30–32]. There are two combination modes of metal and chalcogen elements in MX2, trigonal prismatic or octahedral phases, which are referred to as monolayer 2H and 1T‐MX2, respectively (**Figure 1b**–**d**). Transition metals in different groups have different numbers of *d*‐electrons, which fill up the non‐bonding *d* bands to different levels, resulting in varied electronic properties ranging from insulators such as HfS2, semiconductors such as MoS2 and WS2, semimetals such as WTe2 and TiSe2, to true metals such as NbS2 and VSe2. A few bulk TMDs such as NbSe<sup>2</sup> and TaS2 exhibit low‐temperature phenomena including superconductivity, charge density wave (CDW), and Mott transition [14]. When these materials are exfoliated into mono‐ or few layers, their properties largely preserve and also present additional characteristics due to confinement effects [14, 15, 27, 33]. The mono‐ or few‐layer TMDs have thickness at the atomic level or significantly lower than their edge lengths and thus appear like sheets (namely, nanosheets). The atomic‐thin nature

related reactions, such as the HER and CO2 reduction reaction [15, 21, 25–29].

**2. Properties of 2D TMDs and advantages for electrocatalysis**

conversion and storage.

the most fundamental but vital task in this endeavour.

64 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

**Figure 1.** Structures of TMDs and schematic illustration for the advantages of TMDs as the electrocatalyst. (a) Structure of TMDs. Reproduced with permission from Radisavljevic et al. [32]; copyright 2011 Nature Publishing Group. Sche‐ matic representations of a typical TMD structure with trigonal prismatic (b) and octahedral (c) coordinations from *c*‐ axis (upper) and section view (middle). Reproduced with permission from Chhowalla et al. [14]; copyright 2013 Nature Publishing Group, (d) side view of the structure. Reproduced with permission from Wang et al. [33]; copyright 2012 Nature Publishing Group and (e) schematic illustration for the advantages of 2D TMDs as the electrocatalyst.

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

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

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

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

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

(5) Highly tunable properties. One major difference between 2D TMDs and other 2D materials, such as graphene, is their high anisotropy and unique crystal structure. For this reason, the material properties of 2D TMDs can be effectively tuned in a wide range through different methodologies including reducing dimensions, intercalation, heterostructure, alloying, gating, pressure, lighting, and so forth [27, 40–44]. For example, through the intercalation of guest ions, the carrier densities of 2D TMDs can be tuned by multiple orders of magnitude, which will affect the electron transfer rate in the electrocatalytic reactions [45]. 2D TMDs provide a great platform of tuning material properties towards desired activity and selectivity of a specific electrochemical reaction.
