**4. Hierarchical nanostructure**

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

multidimensional nanostructure advantages, including high reaction area, benefit electron transfer, avoiding aggregation, enhance electrolyte diffusion, and offer directional electron pathways.

Herein, we list partial literature with hierarchical TMCs nanostructure. **Figure 10** shows SEM of Ni3Se4 with sea urchins-like structure, TiO1.1Se0.9 with nanospheres and 1D nanorods, NiCo0.2 with hollow structure and nanoclusters, NiCo2S4 with ball-in-ball structure, NiS@MoS2 with feather duster-like hierarchical structure, CoSe2/CoSeO3 with hierarchical urchin-like structure, CuO/Co3O4 with core-shell structure and CoS2/NC@Co-WS2 with yolk-shell structure by Lee et al., Li et al., Jiang et al., Jiang et al., Su et al., Huang et al., Liao et al., and Huang et al., respectively [49–56]. And their efficiency parameters are listed in **Table 3**. Lee et al. synthesized the Ni3Se4 sea urchins-like structure (**Figure 10a**) through one-step and low temperature hydrothermal process [49]. It reveals *η* of 8.31%, which attribute to the high active electrocatalytic surface area. Li et al. obtained TiO1.1Se0.9 with nanospheres and 1D nanorods (**Figure 10b**) *via* a simple dip-coating process and rapid thermal annealing (RTA) process [50]. The TiO1.1Se0.9 exhibits *η* of 9.47%, which is better than the Pt. The result is established that the nanospheres can work as electro-catalytic active sites, and the nanorods can function not only as electro-catalytic active sites but also as fast electron transport channels, as shown in **Figure 11a**. Jiang et al. synthesized NiCo0.2 hollow structure and nanoclusters, having uniform spherical particles with an average diameter of about 2 μm and shell thickness of around 200 nm (**Figure 10c**), *via* a thermal method [51]. It shows *η* of 9.30% and displays that the novel spherical structures can efficiently promote the transfer of electrons from the conductive carbon frameworks to metal nanoparticles, thus resulting in high electrocatalytic activity for the reduction. Jiang et al. acquired NiCo2S4 ball-in-ball structure (**Figure 10d**) by a thermal method [52]. Its efficiency is 9.49%, which is attributed to the rougher surface, higher surface area, and high diffusion coefficient for redox. Su et al. obtained NiS@MoS2 feather duster-like hierarchical structure, which has *η* of 8.58% [53]. They propose that feather duster-like hierarchical structure array can support the fast electron transfer and electrolyte diffusion channels, moreover, it also can render abundant active catalytic sites and large electron injection efficiency from CE to the electrolyte. Huang et al. gained CoSe2/CoSeO3 hierarchical urchin-like structure (**Figure 10g**), the nanoparticle-composed sphere is the central core with a diameter of about 50 nm surrounded by several hexagonal prisms, through a one-step hydrothermal method [54]. The CoSe2/CoSeO3 reveals *η* of 9.29% and is mention that the

**Figure 10.**

*The SEM of hierarchical nanostructure with (a) Ni3Se4, (b) TiO1.1Se0.9, (c) NiCo0.2, (d) NiCo2S4, (e) CoSe2/ CoSeO3, (f) CuO/Co3O4, and (g) CoS2/NC@Co-WS2 [49–56].*

**359**

**5. Conclusion**

**Table 3.**

**Figure 11.**

*Nanostructured Transition Metal Compounds as Highly Efficient Electrocatalysts…*

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

**Materials** *η* **(%)** *V***OC (V)** *J***SC (mA cm−2) FF** *η***/***η***Pt Ref** Ni3Se4 8.31 0.75 16.27 0.69 1.03 [49] TiO1.1Se0.9 9.47 0.79 17.22 0.70 1.22 [50] NiCo0.2 9.30 0.78 17.80 0.67 1.16 [51] NiCo2S4 9.49 0.84 17.40 0.647 1.14 [52] NiS@MoS2 8.58 0.77 16.64 0.67 1.05 [53] CoSe2/CoSeO3 9.29 0.82 16.09 0.70 1.12 [54] CuO/Co3O4 8.34 0.73 18.13 0.63 1.06 [55] CoS2/NC@Co-WS2 9.21 0.82 16.50 0.67 1.13 [56]

urchin-like structure possessing the hexagonal prism structure and nanoparticles to provide both rapid electron transport routes and a reasonably high surface area for electro-catalytic reactions, as shown in **Figure 11b**. Liao et al. obtained CuO/ Co3O4 core-shell structure (**Figure 10f**) *via* a facile self-templated method [55]. The CuO/Co3O4 has *η* of 8.34% and an excellent electronic transmission channel and more adsorption sites for the redox couple, which greatly enhances the subsequent redox process. Huang et al. acquired CoS2/NC@Co-WS2 with yolk-shell structure (**Figure 10g**) [56]. By virtue of larger surface area and more effective active sites,

*The mechanism of hierarchical nanostructure with (a) TiO1.1Se0.9 and (b) CoSe2/CoSeO3 [50, 54].*

In this section, it can be found that the hierarchical TMCs nanostructure has better performance than the Pt in CE. In other words, they can efficiently raise the TMCs performance, so the hierarchical TMCs nanostructure could replace Pt directly.

The electrocatalytic ability of catalysts is usually determined by below two points: one is the intrinsic electrocatalytic activity, and another is the nanostructure. The nanostructure of TMCs can briefly be classified into 0D, 1D, 2D, and hierarchical nanostructures; those have different properties and could obviously affect the electrocatalytic ability. Herein, the partial reports about DSSCs with the electrocatalysts having 1D, 2D, or hierarchical nanostructures are selected for introduction and discussion. 1D nanostructure possesses several advantages, including the 1D electron transfer pathways, promoting electrolyte penetration, avoiding stack problem, and high reaction area. However, not all the electrocatalysts with 1D nanostructure show better performance than the Pt in DSSC application. Some of them lied down

the CoS2/NC@Co-WS2 (*η* of 9.21%) has better performance than the Pt.

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



#### **Table 3.**

*Solar Cells - Theory, Materials and Recent Advances*

directional electron pathways.

multidimensional nanostructure advantages, including high reaction area, benefit electron transfer, avoiding aggregation, enhance electrolyte diffusion, and offer

*The SEM of hierarchical nanostructure with (a) Ni3Se4, (b) TiO1.1Se0.9, (c) NiCo0.2, (d) NiCo2S4, (e) CoSe2/*

*CoSeO3, (f) CuO/Co3O4, and (g) CoS2/NC@Co-WS2 [49–56].*

Herein, we list partial literature with hierarchical TMCs nanostructure. **Figure 10** shows SEM of Ni3Se4 with sea urchins-like structure, TiO1.1Se0.9 with nanospheres and 1D nanorods, NiCo0.2 with hollow structure and nanoclusters, NiCo2S4 with ball-in-ball structure, NiS@MoS2 with feather duster-like hierarchical structure, CoSe2/CoSeO3 with hierarchical urchin-like structure, CuO/Co3O4 with core-shell structure and CoS2/NC@Co-WS2 with yolk-shell structure by Lee et al., Li et al., Jiang et al., Jiang et al., Su et al., Huang et al., Liao et al., and Huang et al., respectively [49–56]. And their efficiency parameters are listed in **Table 3**. Lee et al. synthesized the Ni3Se4 sea urchins-like structure (**Figure 10a**) through one-step and low temperature hydrothermal process [49]. It reveals *η* of 8.31%, which attribute to the high active electrocatalytic surface area. Li et al. obtained TiO1.1Se0.9 with nanospheres and 1D nanorods (**Figure 10b**) *via* a simple dip-coating process and rapid thermal annealing (RTA) process [50]. The TiO1.1Se0.9 exhibits *η* of 9.47%, which is better than the Pt. The result is established that the nanospheres can work as electro-catalytic active sites, and the nanorods can function not only as electro-catalytic active sites but also as fast electron transport channels, as shown in **Figure 11a**. Jiang et al. synthesized NiCo0.2 hollow structure and nanoclusters, having uniform spherical particles with an average diameter of about 2 μm and shell thickness of around 200 nm (**Figure 10c**), *via* a thermal method [51]. It shows *η* of 9.30% and displays that the novel spherical structures can efficiently promote the transfer of electrons from the conductive carbon frameworks to metal nanoparticles, thus resulting in high electrocatalytic activity for the reduction. Jiang et al. acquired NiCo2S4 ball-in-ball structure (**Figure 10d**) by a thermal method [52]. Its efficiency is 9.49%, which is attributed to the rougher surface, higher surface area, and high diffusion coefficient for redox. Su et al. obtained NiS@MoS2 feather duster-like hierarchical structure, which has *η* of 8.58% [53]. They propose that feather duster-like hierarchical structure array can support the fast electron transfer and electrolyte diffusion channels, moreover, it also can render abundant active catalytic sites and large electron injection efficiency from CE to the electrolyte. Huang et al. gained CoSe2/CoSeO3 hierarchical urchin-like structure (**Figure 10g**), the nanoparticle-composed sphere is the central core with a diameter of about 50 nm surrounded by several hexagonal prisms, through a one-step hydrothermal method [54]. The CoSe2/CoSeO3 reveals *η* of 9.29% and is mention that the

**358**

**Figure 10.**

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

**Figure 11.**

*The mechanism of hierarchical nanostructure with (a) TiO1.1Se0.9 and (b) CoSe2/CoSeO3 [50, 54].*

urchin-like structure possessing the hexagonal prism structure and nanoparticles to provide both rapid electron transport routes and a reasonably high surface area for electro-catalytic reactions, as shown in **Figure 11b**. Liao et al. obtained CuO/ Co3O4 core-shell structure (**Figure 10f**) *via* a facile self-templated method [55]. The CuO/Co3O4 has *η* of 8.34% and an excellent electronic transmission channel and more adsorption sites for the redox couple, which greatly enhances the subsequent redox process. Huang et al. acquired CoS2/NC@Co-WS2 with yolk-shell structure (**Figure 10g**) [56]. By virtue of larger surface area and more effective active sites, the CoS2/NC@Co-WS2 (*η* of 9.21%) has better performance than the Pt.

In this section, it can be found that the hierarchical TMCs nanostructure has better performance than the Pt in CE. In other words, they can efficiently raise the TMCs performance, so the hierarchical TMCs nanostructure could replace Pt directly.
