**3. Two-dimensional (2D)**

Geim and Grigorieva classified 2D materials into three groups [35]. First group, graphene type contains graphene, fluorographene, graphene oxide, hBN, *etc.*; second group, 2D chalcogenides (transition metal) type includes MoS2, NbS2, NbSe2, CoSe2, MoSe2, ZrSe2, GaSe, GaTe, InSe, Bi2Se3, Bi2Te3, *etc.*; final group, 2D oxides type involves TiO2, MnO2, V2O5, RuO2, perovskite-based materials (LaNb2O7, Ba4Ti3O12, Ca2Ta2TiO10 *etc.*), hydroxides (Ni(OH)2, Eu(OH)2, *etc.*), *etc*. Research of 2D TMCs

nanostructure is intensified in recently [16, 36]. The bandgap energy is reduced by decreasing the layer of the TMCs [16, 35, 37–40]. In other words, a single or a few layers of the 2D TMCs nanostructure presents excellent electrocatalytic ability. Besides that, the 2D TMCs nanostructure has advantages including enhancing the diffusion of electrolyte, vertical electron channel, and special optical property.

In this section, the partial works of literature are chosen depending on the electrocatalytic performance and structure. Their corresponding SEM images and efficiency parameters are shown in **Figures 7** and **8**, and **Table 2**, respectively. In **Figure 7**, Ibrahem et al., Huang et al., and Mohammadnezhad et al. applied the horizontal 2D nanostructure with NbSe2, MoSe2, and Cu2ZnSnSxSe4-x in CE for DSSCs [41–43]. Ibrahem et al. reported that the NbSe2 nanosheet (**Figure 7a**) has the best performance among nanosheet, nanorod, and nanoparticle [41]. They mention that nanosheet could provide high surface area and coverage. And the NbSe2 nanosheet existed *η* of 7.73%, which is better than the Pt CE. The result indicates that NbSe2 nanosheet substitutes to the noble metal Pt in DSSCs. Following the idea, MoSe2 and Cu2ZnSnSxSe4-x nanosheet show the *η* of 7.58% and 5.73%, respectively. However, both of their values of *η* are lower than the Pt. To increase the performance of 2D TMCs nanostructure, the pseudo-vertical 2D nanostructure was synthesized and provide the vertical electron channel. The pseudo-vertical 2D nanostructure with MoS2, CoSe2, MoS2, CuxZnySnzS, and CoNi2S4 were obtained by Antonelou et al., Chiu et al., Raj et al., Chiu et al., and Patil et al., respectively [44–48]. Antonelou et al. obtained the MoS2 nanosheet with *η* of 8.40%, which has thicknesses down to the 1-2 nm scale. Chiu et al. acquired the nanoclimbing-wall-like CoSe2 (**Figure 8a**) through an electrodeposition process, by using bathes with different pH values.

Its performance is 8.92%. They mentioned that vertical nanowall provides conducting charge for electrocatalytic reduction, as shown in **Figure 9a**. Raj et al. synthesized reflectivity of MoS2 nanosheet (**Figure 8b**), which has *η* of 7.50%, through chemical vapor deposition (CVD). The reflectivity of MoS2 nanosheet is

**Figure 7.**

*The SEM of 2D nanostructure with (a) NbSe2, (b) MoSe2, (c) Cu2ZnSnSxSe4-x [41–43].*

**357**

as an electrocatalyst.

**Table 2.**

**Figure 9.**

**4. Hierarchical nanostructure**

*Nanostructured Transition Metal Compounds as Highly Efficient Electrocatalysts…*

*A partial list of literature on the DSSCs with 2D TMCs nanostructure based CEs.*

*The mechanism of 2D nanostructure with (a) CoSe2 and (b) MoS2 [45, 46].*

**Materials** *η* **(%)** *V***OC (V)** *J***SC (mA cm−2) FF** *η***/***η***Pt Ref** NbSe2 7.73 0.74 16.85 0.62 1.10 [41] MoS2 8.40 0.74 22.60 0.50 0.97 [44] CoSe2 8.92 0.73 18.03 0.67 1.08 [45] MoSe2 7.58 0.70 15.97 0.67 0.97 [42] MoS2 7.50 0.71 15.20 0.70 1.03 [46] CuxZnySnzS 7.44 0.67 16.57 0.66 1.03 [47] CoNi2S4 8.86 0.66 19.21 0.70 0.98 [48] Cu2ZnSnSxSe4-x 5.73 0.69 12.60 0.66 0.99 [43]

raised their high reflectivity facilitates the absorbance of more photons, and more active edge sites exposed to redox couple in the electrolyte, as shown in **Figure 9b**. Chiu et al. gained CuxZnySnzS nanowall-structure by thermal solvent method, and it shows performance 7.44%. The performance is attributed to improves the carrier transport pathway and effectively reduces the interface resistance. Patil et al. utilized a simple one-step solution-based fabrication method for CoNi2S4 interconnected nanosheet (**Figure 8c**). The CoNi2S4 exhibits *η* of 8.86%, which attributes to a larger active surface area with favorable charge transport. The pseudo-vertical 2D nanostructure has obviously improvement of electrocatalytic ability than the normal 2D nanostructure. It is not only providing vertical transport pathways and active surface area but also contributes to reflection photon. Those properties make 2D TMCs nanostructure have the potential to alternative Pt

Basically, 0D nanostructure possesses a high reaction area; 1D and 2D nanostructure offers directional electron pathways and enhance electrolyte penetration. But they have their own weakness. For example, 0D nanostructure is easy aggregation and has larger heterogeneous resistance; 1D and 2D nanostructure have lower reaction area. A hierarchical nanostructure consists of the nanostructure with multidimensional subunits (0D, 1D, and 2D). It merges various subunits, so it has

*DOI: http://dx.doi.org/10.5772/intechopen.94021*

**Figure 8.**

*The pseudo-vertical 2D nanostructure with (a) CoSe2, (b) MoS2, and (c) CoNi2S4 [45–48].*

*Nanostructured Transition Metal Compounds as Highly Efficient Electrocatalysts… DOI: http://dx.doi.org/10.5772/intechopen.94021*


#### **Table 2.**

*Solar Cells - Theory, Materials and Recent Advances*

nanostructure is intensified in recently [16, 36]. The bandgap energy is reduced by decreasing the layer of the TMCs [16, 35, 37–40]. In other words, a single or a few layers of the 2D TMCs nanostructure presents excellent electrocatalytic ability. Besides that, the 2D TMCs nanostructure has advantages including enhancing the diffusion of electrolyte, vertical electron channel, and special optical property. In this section, the partial works of literature are chosen depending on the electrocatalytic performance and structure. Their corresponding SEM images and efficiency parameters are shown in **Figures 7** and **8**, and **Table 2**, respectively. In **Figure 7**, Ibrahem et al., Huang et al., and Mohammadnezhad et al. applied the horizontal 2D nanostructure with NbSe2, MoSe2, and Cu2ZnSnSxSe4-x in CE for DSSCs [41–43]. Ibrahem et al. reported that the NbSe2 nanosheet (**Figure 7a**) has the best performance among nanosheet, nanorod, and nanoparticle [41]. They mention that nanosheet could provide high surface area and coverage. And the NbSe2 nanosheet existed *η* of 7.73%, which is better than the Pt CE. The result indicates that NbSe2 nanosheet substitutes to the noble metal Pt in DSSCs. Following the idea, MoSe2 and Cu2ZnSnSxSe4-x nanosheet show the *η* of 7.58% and 5.73%, respectively. However, both of their values of *η* are lower than the Pt. To increase the performance of 2D TMCs nanostructure, the pseudo-vertical 2D nanostructure was synthesized and provide the vertical electron channel. The pseudo-vertical 2D nanostructure with MoS2, CoSe2, MoS2, CuxZnySnzS, and CoNi2S4 were obtained by Antonelou et al., Chiu et al., Raj et al., Chiu et al., and Patil et al., respectively [44–48]. Antonelou et al. obtained the MoS2 nanosheet with *η* of 8.40%, which has thicknesses down to the 1-2 nm scale. Chiu et al. acquired the nanoclimbing-wall-like CoSe2 (**Figure 8a**) through an electrodeposition process, by using bathes with different pH values. Its performance is 8.92%. They mentioned that vertical nanowall provides conducting charge for electrocatalytic reduction, as shown in **Figure 9a**. Raj et al. synthesized reflectivity of MoS2 nanosheet (**Figure 8b**), which has *η* of 7.50%, through chemical vapor deposition (CVD). The reflectivity of MoS2 nanosheet is

*The SEM of 2D nanostructure with (a) NbSe2, (b) MoSe2, (c) Cu2ZnSnSxSe4-x [41–43].*

*The pseudo-vertical 2D nanostructure with (a) CoSe2, (b) MoS2, and (c) CoNi2S4 [45–48].*

**356**

**Figure 8.**

**Figure 7.**

*A partial list of literature on the DSSCs with 2D TMCs nanostructure based CEs.*

#### **Figure 9.**

*The mechanism of 2D nanostructure with (a) CoSe2 and (b) MoS2 [45, 46].*

raised their high reflectivity facilitates the absorbance of more photons, and more active edge sites exposed to redox couple in the electrolyte, as shown in **Figure 9b**. Chiu et al. gained CuxZnySnzS nanowall-structure by thermal solvent method, and it shows performance 7.44%. The performance is attributed to improves the carrier transport pathway and effectively reduces the interface resistance. Patil et al. utilized a simple one-step solution-based fabrication method for CoNi2S4 interconnected nanosheet (**Figure 8c**). The CoNi2S4 exhibits *η* of 8.86%, which attributes to a larger active surface area with favorable charge transport. The pseudo-vertical 2D nanostructure has obviously improvement of electrocatalytic ability than the normal 2D nanostructure. It is not only providing vertical transport pathways and active surface area but also contributes to reflection photon. Those properties make 2D TMCs nanostructure have the potential to alternative Pt as an electrocatalyst.
