**1. Introduction**

Water pollution is one the fundamental problems that have got the serious concerns of the researchers. Water poluution arises due to a number of reasons including domestic, industrial, agricultural, scinec and technology. The contribution of industrial sector in water pollution is significant. A number of industries like textile industry, leather industry, food industry, pharmaceutical industry, printing industries etc. release their wastewater to the environment. These wastewater of these industries contain a variety of organic and inorganic pollutants. Hence, the wastewater released from these induries cause water pollution. The water pollution due to dyes is very serious issue. The textile industry is the main industry that

releases the dyes contaminated wastewater to the environment. Textile industry uses a huge amount of dyes annualy. It is estimated that about 700000 tons of the various dyes are produced annualy around the world. These dyes have been classifies as azo dyes, basic dyes, direct dyes, vat dyes, reactive dyes etc. More than 15% of the globally produced dyes are released to the environment in wastewater from textile and other industries. These dyes released in aqueous system are highly toxic and carcinogeneic in nature. These dyes badly affect the living organisms. Dyes are highly colored substances that impart intense color to the aqueous body when dyes contaminated wastewater is released into it. Due to intense color of aqueous body, the sunlight is inhibited by penetration into the interior of the aqueous body. As a result of non-penetration of sunlight, the aqueous ecosysytem is badly affected by dyes [1–6].

A varities of protocols have been attempeted for the removal of dyes from aqueous body. Techniques like biological degradation technique, membrane filteration technique, adsorption atechniques, sedimentation technique, physical coagulation technique and chemical coagulation technique has been attempted for the elimination of dyes from aqueous medium. However, these techniques are not successful in the removal of dyes. As these techniques do not degrade the dyes molecules, therefore, these techniques fail to remove the dyes from aqueous medium. These techniques only transform the dyes pollutants from one form to another form. Furthermore, these techniques give rise secondary pollution [7–10]. Hence, there is an urgent need to develop effective techniques for the removal of these dyes and other organic pollutants from aqueous medium.

The advanced oxidation processes are considered as effective alternative technique for the removal of dyes from aqueous medium. Advanced oxidation processes are chemical processes which are based on production of hydroxyl radicals. These hydroxyl radicals are highly reactive spectices. These species take part in complete degradation of organic pollutants molecules. The advanced oxidation processes include heterogeneous photocatalysis, electrochemical oxidation and Fenon reactions. **Table 1** shows various advanced oxidation proceses. The advanced oxidation processes have many advantages compared to conventional techniques as follows [11–13].


Due to the above mentioned merits of advanced oxidation processes, the advanced oxidation processes have got considerable interest of the scientists. The photocatalysis using heterogeneous metal oxide semicondutors as photocatalyst in the presence of ultra violet or visible light is one of the techniques of advanced oxidation proesses. The photocatalytic treatment of organic pollutants has got significant attention of the researchers recently. The photocatalytic technique doesnot need any extra ordinary energy requirement. Furthermore, there is no formation of hazardous by-products in photocatalysis. A large number of reports are available of photocatalytic degradation of dyes in aqueous medium. **Figure 1** shows the total number of photocatalytic reports for various dyes reported during 2000–2017.


#### *Photocatalytic Applications of Titanium Dioxide (TiO2) DOI: http://dx.doi.org/10.5772/intechopen.99598*

**Table 1.**

*Various advanced oxidation processes.*

**Figure 2** shows the total number, percentage and number of reports under visible and ultra violet radiation for various dyes.

The data given in **Figure 2** shows that researchers have studidied the tiazine dyes mostly. The xanthenes dyes are at second position in the mostly investigated dyes in the subject of photocatalytic degradation.

The applications and fundamentals of photocatalysis developed tremendously during the last four decades. Photocatalysis can be defined as a reaction induced by irradiation of light in the presence of a substance called a catalyst. The photocatalytic reactions are initiated by the absorption of light having energy greater than or equal to the band gap energy of the photocatalyst. The energy difference between the highest filled energy level (valence band) and lowest vacant

#### **Figure 1.**

*Number of publications on photocatalytic treatment of wastewater. This figure is reproduced with permission from Environ. Sci. Technol. 2019, 53, 29372947 Copyright 2019 American Chemical Society.*

### *Photocatalytic Applications of Titanium Dioxide (TiO2) DOI: http://dx.doi.org/10.5772/intechopen.99598*

energy level (conduction band) of photocatalytic substance is called band gap energy of the cited substance [14, 15]. The absorption of light causes the excitation of electrons from the highest filled energy level (valence band) to the lowest vacant energy level (conduction band) of the photocatalyst. This photoinduced excitation creates a positive hole (h<sup>+</sup> ) in the valence band and electrons (e) in conduction band of the substance. After formation of positive holes and electrons, two types of processes may proceed further.


Hence, for photocatalytic reactions, process 1 mentioned above (recombination of positive holes and electrons) must be prevented to favor the photocatalytic reactions. The goal of photocatalysis is to initiate reactions of positive holes and electrons with the reductants and oxidants to produce oxidized and reduced products, respectively [16].

Presently, photocatalysis is used in several emerging fields like photodegradation of aqueous organic pollutants, production of hydrogen by water splitting, treatment of gaseous environmental pollutants like NOx, treatment of halogenated hydrocarbon, inactivation of microorganisms, treatment of pesticides and organohalide compounds, oxidation of micropollutants and many more [17–21].
