**1. Introduction**

Energy crisis and environmental pollution arising from the burning of carbonbased fossil fuel in the past decades facilitate people to reconsider the way we utilized the resource on earth. Hydrogen as an ideal energy source came up to the stage due to its high energy density and environmental benignity [1, 2]. Electrochemical water splitting is not only regarded as the cleanest technique for hydrogen generation but also suitable to perform on a large scale. The appropriate electrocatalysts are developed to boost the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) because of the sluggish kinetics of these two core reactions [overpotential (η), **Figure 1a**]. Although platinum-based and iridium/ruthenium-based catalysts, respectively, have shown very promising performance in HER and OER, the industrial application is restricted due to their high cost and limited availability [3, 4]. Thus, developing alternative electrocatalysts based on non-noble metals or earth-abundant elements is still highly demanded.

The realization of HER and OER mechanisms plays a crucial role to design the efficient electrocatalysts, so the brief discussions on their mechanisms were introduced.

Two kinds of HER mechanisms have been completely studied and widely accepted [5, 6]. The first step, a proton from the solution adsorbs onto the catalytic sites of the electrode with a reduction process, is called as Volmer step:

**Figure 1.**

*(a) Polarization curves for HER and OER [9] and (b) volcano plot of exchange current density (j0) as a function of DFT-calculated Gibbs free energy (ΔGH) of adsorbed atomic hydrogen on pure metals [7].*

$$\text{H}^+\_{\text{(aq)}} + \text{e}^- \rightarrow \text{H}^\*\_{\text{(ads)}} \tag{1}$$

Then, either the recombination of two adsorbed hydrogen atoms on the electrode surface is called as Tafel step:

$$\text{H}^{\bullet}\_{\text{(ads)}} + \text{H}^{\bullet}\_{\text{(ads)}} \to \text{H}\_{2(g)} \tag{2}$$

or the reaction of an adsorbed hydrogen atom with the hydrated proton, which proceeds with an electron transfer from the electrode surface, is called as Heyrovsky step:

$$\rm H^{\bullet}\_{\rm (ads)} + H^{+}\_{\rm (aq)} + e^{-} \to H\_{2(g)} \tag{3}$$

Nørskov et al. proposed a Volcano curve showing that experimental exchange currents of materials are as a function of the Gibbs free energy of the adsorbed hydrogen (ΔGH) (**Figure 1b**) [7, 8]. Ideally, the interaction of hydrogen with the electrode surface should be thermoneutral (ΔGH ffi 0), otherwise either Heyrovsky or Tafel step (strong bonding) or Volmer step would become the rate-determining step. In the case of the OER, many researchers have proposed possible mechanisms at the anode in acidic electrolyte (Eqs. (4)–(8)). However, there are some differences around the reaction of forming oxygen. One route of forming oxygen is through the direct recombination of two MO (M represents Mn, Fe, Co, and Ni metals) (Eq. (6)), while the other route of forming oxygen is through the decomposition of the MOOH intermediate (Eq. (8)).

$$\text{M} + \text{H}\_2\text{O}\_{(l)} \to \text{MOH} + \text{H}^+ + \text{e}^- \tag{4}$$

$$\rm{MOH} + \rm{OH}^- \rightarrow \rm{MO} + \rm{H\_2O\_{(l)}} + \rm{e}^- \tag{5}$$

$$\text{2MO} \rightarrow \text{2MS} + \text{O}\_{2(g)}\tag{6}$$

prepared from their bulk material by mechanical exfoliation are called the topdown method, while the TMC layers were produced from the elemental precursors on the target substrate in the bottom-up method, such as chemical vapor deposition (CVD) approach. The CVD method for TMC synthesis has been widely used due to its unique advantages such as scalable size, high crystallinity, and controllable thickness of TMCs [10–13]. One of the most classic TMCs, molybdenum sulfide (MoS2), has been developed via CVD synthesis to replace the zero bandgap

*Two routes of the TMC layer preparation: (top) a schematic representation of the top-down method – mechanical exfoliation and (bottom) a schematic illustration of the bottom-up method – CVD [22].*

*Transition Metal Chalcogenides for the Electrocatalysis of Water*

*DOI: http://dx.doi.org/10.5772/intechopen.92045*

**Figure 2.**

**79**

graphene. [14] There are two routes to synthesize MoS2 layer by CVD (**Figure 3a**). One is a two-step growth route, where the Mo-based precursors are first deposited and then conducted the sulfurization or decomposition process to form MoS2 (route 1, **Figure 3a**). The other growth way of MoS2 layer is an one-step growth, where the gaseous Mo- and S-based precursors are simultaneously introduced and react to form MoS2 on a substrate (route 2, **Figure 3a**). Recent reports have also demonstrated to grow MoS2 on other kinds of insulating substrates such as quartz, mica, and sapphire (**Figure 3b**) [15, 16]. **Figure 3c** shows a typical experimental setup for the low-pressure CVD synthesis of MoS2. The formation of entire MoS2 would be prevented according to the ternary ModOdS phase diagram (**Figure 3d**) if the reducing atmosphere is too weak [17]. **Figure 3e** displays two possible mechanisms for the growth of MoS2. The well-established CVD synthesis of MoS2 is regarded as a prototype for the synthesis of other TMCs such as MoSe2 and WS2 [18, 19]. Except from the VI B group metals such as molybdenum and tungsten, the research works

$$\text{MO} + \text{H}\_2\text{O}\_{(l)} \to \text{MOOH} + \text{H}^+ + \text{e}^- \tag{7}$$

$$\text{MOOH} + \text{H}\_2\text{O}\_{(l)} \to \text{M} + \text{O}\_{2(g)} + \text{H}^+ + \text{e}^- \tag{8}$$

#### **2. Preparations of TMCs**

#### **2.1 CVD synthesis of TMCs**

The preparation methods of TMCs layers can be categorized into two main approaches: top-down and bottom-up methods (**Figure 2**). The TMC layers

*Transition Metal Chalcogenides for the Electrocatalysis of Water DOI: http://dx.doi.org/10.5772/intechopen.92045*

#### **Figure 2.**

Hþ

H•

H•

position of the MOOH intermediate (Eq. (8)).

**2. Preparations of TMCs**

**2.1 CVD synthesis of TMCs**

**78**

ð Þ ads <sup>þ</sup> <sup>H</sup>•

proceeds with an electron transfer from the electrode surface, is called as

ð Þ ads þ H<sup>þ</sup>

trode surface is called as Tafel step:

*Advanced Functional Materials*

Heyrovsky step:

**Figure 1.**

ð Þ aq <sup>þ</sup> <sup>e</sup>� ! <sup>H</sup>•

*(a) Polarization curves for HER and OER [9] and (b) volcano plot of exchange current density (j0) as a function of DFT-calculated Gibbs free energy (ΔGH) of adsorbed atomic hydrogen on pure metals [7].*

Then, either the recombination of two adsorbed hydrogen atoms on the elec-

or the reaction of an adsorbed hydrogen atom with the hydrated proton, which

Nørskov et al. proposed a Volcano curve showing that experimental exchange currents of materials are as a function of the Gibbs free energy of the adsorbed hydrogen (ΔGH) (**Figure 1b**) [7, 8]. Ideally, the interaction of hydrogen with the electrode surface should be thermoneutral (ΔGH ffi 0), otherwise either Heyrovsky or Tafel step (strong bonding) or Volmer step would become the rate-determining step. In the case of the OER, many researchers have proposed possible mechanisms at the anode in acidic electrolyte (Eqs. (4)–(8)). However, there are some differences around the reaction of forming oxygen. One route of forming oxygen is through the direct recombination of two MO (M represents Mn, Fe, Co, and Ni metals) (Eq. (6)), while the other route of forming oxygen is through the decom-

The preparation methods of TMCs layers can be categorized into two main approaches: top-down and bottom-up methods (**Figure 2**). The TMC layers

ð Þ ads (1)

ð Þ ads ! H2 gð Þ (2)

ð Þ aq <sup>þ</sup> <sup>e</sup>� ! H2 gð Þ (3)

M þ H2Oð Þ<sup>l</sup> ! MOH þ H<sup>þ</sup> þ e� (4) MOH þ OH� ! MO þ H2Oð Þ<sup>l</sup> þ e� (5)

MO þ H2Oð Þ<sup>l</sup> ! MOOH þ H<sup>þ</sup> þ e� (7) MOOH þ H2Oð Þ<sup>l</sup> ! M þ O2 gð Þ þ H<sup>þ</sup> þ e� (8)

2MO ! 2M5 þ O2 gð Þ (6)

*Two routes of the TMC layer preparation: (top) a schematic representation of the top-down method – mechanical exfoliation and (bottom) a schematic illustration of the bottom-up method – CVD [22].*

prepared from their bulk material by mechanical exfoliation are called the topdown method, while the TMC layers were produced from the elemental precursors on the target substrate in the bottom-up method, such as chemical vapor deposition (CVD) approach. The CVD method for TMC synthesis has been widely used due to its unique advantages such as scalable size, high crystallinity, and controllable thickness of TMCs [10–13]. One of the most classic TMCs, molybdenum sulfide (MoS2), has been developed via CVD synthesis to replace the zero bandgap graphene. [14] There are two routes to synthesize MoS2 layer by CVD (**Figure 3a**). One is a two-step growth route, where the Mo-based precursors are first deposited and then conducted the sulfurization or decomposition process to form MoS2 (route 1, **Figure 3a**). The other growth way of MoS2 layer is an one-step growth, where the gaseous Mo- and S-based precursors are simultaneously introduced and react to form MoS2 on a substrate (route 2, **Figure 3a**). Recent reports have also demonstrated to grow MoS2 on other kinds of insulating substrates such as quartz, mica, and sapphire (**Figure 3b**) [15, 16]. **Figure 3c** shows a typical experimental setup for the low-pressure CVD synthesis of MoS2. The formation of entire MoS2 would be prevented according to the ternary ModOdS phase diagram (**Figure 3d**) if the reducing atmosphere is too weak [17]. **Figure 3e** displays two possible mechanisms for the growth of MoS2. The well-established CVD synthesis of MoS2 is regarded as a prototype for the synthesis of other TMCs such as MoSe2 and WS2 [18, 19]. Except from the VI B group metals such as molybdenum and tungsten, the research works

of TMCs based on VIII B group metals such as cobalt [20] and nickel [21] are also widely studied in recent years.
