**3.1 Application of TiO2 in dye-sensitized solar cell (DSSC)**

The solar energy is a clean, abundant and renewable energy [25]. The current technology for the conversion of sunlight to electrical power is predominately silicon-based solid state solar cells. In recent years, the new semiconducting material-based solar cells have emerged to offer the possible alternative photovoltaic technology with prospect of cheap fabrication and flexibility [26–28]. Nanostructured TiO2 has been the main semiconducting material for this new generation of solar cells. In this technology, an electron sensitizer absorbing in the visible is used to inject charge carriers across the semiconductor-electrolyte junction into TiO2 to enhance the conversion efficiency from solar energy, because TiO2 with its band gap of 3.2 electronvolt (eV) absorbs only the ultraviolet part of the solar energy. This type of solar cells is therefore called dye-sensitized solar cells (DSSCs). The dye-sensitized solar cells (DSSCs) have exhibited high performance and have the potential to be low-cost [29–33].

**Figure 1** illustrates the working principle of a dye-sensitized solar cell. The dye-sensitized solar cell consists of two electrodes, a dye-sensitized nano-structured TiO2 mesoporous layer, and a liquid electrolyte containing redox system (I/I <sup>3</sup> ). The nano-structured TiO2 mesoporous layer with a monolayer of the charge transfer dye at its surface is placed in contact with a redox electrolyte. Under solar

#### **Figure 1.**

*Working principle of a dye-sensitized solar cell.* S*,* S<sup>+</sup> *, and* S\* *represent dye sensitizer, oxidized dye sensitizer, and excited dye sensitizer, respectively.* ΔV *is the difference between Fermi level and electrochemical potential of the electrolyte.*

#### *Applications of Titanium Dioxide Materials DOI: http://dx.doi.org/10.5772/intechopen.99255*

irradiation, the charge transfer dye injects electrons into the conduction band of TiO2, and the electrons are conducted to the external circuit to produce electric power. The original state of the dye is subsequently restored by an electron donation from the electrolyte (for example, an organic solvent containing a redox system of iodide/triiodide couple).

The nano-structured TiO2 mesoporous layer in a dye-sensitized solar cell has a much larger surface area available for the dye-chemisorptions. The kinetic processes occurring in a dye-sensitized solar cell have been profoundly changed as a result of using nano-structured TiO2. Solar energy-to-electricity conversion efficiencies of DSSCs have been increased. The record for the highest certified single cell and DSSCs module efficiencies are 11.9% and 8.8%, respectively [34].

More recently, TiO2 is used in a new type of solar device so-called pervoskite solar cells. As in DSSCs, TiO2 is used as a mesoporous layer. However, instead of using organic dye in DSSCs, organic lead complex (for example, CH3NH2PbI3) is used to inject electrons into the conduction band of TiO2. In a short period of the recent few years, the reported efficiency of pervoskite solar cells was 9.7% initially, and then 12.0% [35, 36]. Further progress was made with efficiencies above 15.0% [37]. The record for the highest certified single cell and minimodule efficiencies are 20.9% and 16.0%, respectively [34].

### **3.2 Application of TiO2 in hydrogen production**

In 1972, Fujishima and Honda discovered the phenomenon of photocatalytic splitting of water on a TiO2 electrode under UV light [6, 38, 39]. Compared with other photocatalysts, TiO2 is much more promising as it is stable, non-corrosive, environmentally friendly, abundant and cost effective. **Figure 2** illustrates the mechanism of the photocatalytic hydrogen production by TiO2 semiconducting materials. When excited by photons which have energy equal to or higher than their band gap (*Eg*), electrons (e�) in valence band (VB) of TiO2 are promoted from to the conduction band (CB). Simultaneously, Holes (h<sup>+</sup> ) create in VB of TiO2. The process of generating (e�) and (h<sup>+</sup> ) in TiO2 with excitation by photons is described with Eq. (2):

$$TiO\_2 + hv \rightarrow e^- + h^+ \tag{2}$$

The photo-generated (e�) and (h<sup>+</sup> ) in TiO2 can recombine, releasing energy in the form of heat or photons. The photo-generated (e�) and (h<sup>+</sup> ) that migrate to the surface of TiO2 without recombination can reduce and oxidize H2O molecules adsorbed on the surface of TiO2 to generate H2 and O2.

**Figure 2.** *Illustration of mechanism of photocatalytic hydrogen production by TiO2.*

As can be seen in **Figure 2**, the conduction band level of TiO2 is more negative than the hydrogen production level (*EH*2*=H*2*O*), and the valence band is more positive than water oxidation level (*EO*2*=H*2*O*). ΔE, representing the energy difference between hydrogen production level and water oxidation level, is 1.23 eV. The conduction and valence band levels of TiO2 meet the requirement for hydrogen production. As in dye-sensitized solar cell, nano-structured TiO2 can enhance the photocatalytic reactions for the generation of hydrogen by a number of ways. For example, much more surface area per mass is available for adsorption of water due to the decreased particle sizes of TiO2 nano-particles. The surface of TiO2 nano-particles can also become more reactive because much higher portion of atoms exists at the surface. Furthermore, the quantum effect of TiO2 nanoparticles becomes more significant as particle size of TiO2 is getting very small. Nano-structured TiO2 has been known to be stable, chemically inert, and low cost.

Despite many advantages of using TiO2 for photocatalytic hydrogen production, the efficiency using solar energy for water-splitting by TiO2 is still low, and is currently not been used for industrial scale of hydrogen production. The low energy conversion efficiency of TiO2 in water-splitting is believed to be caused by the wasteful recombination of electron/hole pairs, backward reaction of combining hydrogen and oxygen into water, and limitations for TiO2 to utilize visible light due to its large band gap. Research has been carried out to produce nano-structured TiO2 with a narrower band gap in order to utilize visible-light energy more efficiently. Progresses have been made in modifying the band gap of nano-structured TiO2 by means of metal loading, ion doping, metal ion-implantation, dye sensitization and composite TiO2. Noble metals, such as Pt, Au, Pd, and Ag, have been reported to be very effective in enhancing TiO2 photocatalysis [40–43]. Carbondoped nano-structured TiO2 have showed much more efficient water splitting under visible-light illumination [44]. A study using a dye sensitizer for photocatalytic hydrogen production was investigated [45]. A visible light absorber, C3N4, has been coupled to many wide-band gap semiconductors to improve solar harvesting. A 50 wt % C3N4/TiO2 junction was found to double H2 evolution compared to pure C3N4 under visible irradiation [46].

### **3.3 Application of TiO2 in energy storage**

Lithium-ion batteries are a type of rechargeable batteries commonly used in consumer electronics. Lithium ion battery system and technology has been a revolutionary change in the field of power supply battery. Anode materials based on titanium oxides are the promising candidates as alternative materials to carbonaceous anodes due to advantages in terms of cost, safety and toxicity [47, 48]. TiO2 also exhibits excellent structural stability, high discharge voltage plateau (more than 1.7 V versus Li<sup>+</sup> /Li), and excellent cycling stability [49, 50].

Typically the Li<sup>+</sup> insertion–extraction reaction for TiO2 polymorphs occurs according to reaction (3):

$$\text{x}Li^{+} + \text{TiO}\_{2} + \text{xe}^{-} = \text{LixTiO}\_{2} \tag{3}$$

x can range between 0 and 1, depending strongly on the polymorph, particle size, and morphology of TiO2. The maximum theoretical capacity is 335 mAh g�<sup>1</sup> which corresponds to x = 1. This makes TiO2 a highly competitive alternative to graphite anodes having a theoretical capacity of 372 mAh g�<sup>1</sup> [51–53]. However, TiO2 has limitations, such as low capacity, low electrical conductivity, and poor rate capability. Strategies have been developed to address the issues of TiO2-based

#### *Applications of Titanium Dioxide Materials DOI: http://dx.doi.org/10.5772/intechopen.99255*

anodes. These include the use of multi-dimensional nanostructured TiO2, composite and coating materials, and element doping.

One dimensional anatase TiO2 nanofiber anodes were used as an anode active material in Li ion batteries and exhibited a high lithium storage capacity, a stable cycle life, and good rate capability [54]. Two dimensional TiO2 nanosheets have been shown to exhibit the superior capacities, improved cycling stability and rate capabilities, owing to unique exposed facets, shortened path, and reserved porous structures [55–57]. Nanostructured TiO2 is a low voltage insertion host for Li and a fast Li insertion/extraction host [58, 59]. These characteristics provide nanostructured TiO2 a potential anode material for high-power Li-ion batteries. Studies on the use of nanostructured TiO2 as anode with LiCoO2 cathode demonstrated specific capacity of 169 mAh g<sup>1</sup> [60]. Xu, et al. investigated electrochemical performance of TiO2-coated LiCoO2 and LiMn2O4 in different potential regions [61]. Mechanically blended composite of nanosized TiO2 and carbon nanotubes (CNTs) has been used as potential anode materials for Li-ion batteries. It was found that the TiO2/ CNTs nanocomposites exhibited an improved cycling stability and higher reversible capacity than CNTs [62, 63]. Metal oxide coatings containing TiO2 can efficiently improve the capacitive performance of the materials through synergistic effects in an electrode system [64–67].
