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As a subtype of catalysis, photocatalytic reactions are carried out under ambient conditions of temperature and pressure to produce costly chemical products. IUPAC defined photocatalytic reactions as a “change in the rate of chemical reaction under the action of ultraviolet, visible or infrared radiation in the presence of a substance—named as “Photocatalyst” with its main function which is to absorb light radiation for the transformation of reactants [5].

For simple example, Chlorophyll is an essential photocatalyst which exists in plants as a natural substance. In photocatalysis process, the chlorophyll allows plants to absorb energy from light which is used to convert carbon dioxide and water into glucose together with nutrients. In brevity, photocatalysis in plants is a photochemical reaction which provides oxygen with the help of a photocatalyst, released by plants or trees into air. A schematic diagram for this process in which facilitation of chemical reaction from solar light radiation due to photocatalyst is given is shown in Figure 1.3 [13].

In short, photocatalysis is the activity in which light radiation intersects on the surface of a specific substance to carry out chemical reactions such as oxidation and reduction reactions.

Here, specific substance is known as “Photocatalyst” which is quite responsible to attain enough energy level to absorb those incident heat waves for modifying the state of reacting molecules into valuable chemical products. Photocatalysis has two types:

1 (A) Homogeneous photocatalysis/photochemical reactions

2 (B) Heterogeneous photocatalysis/photochemical reactions.

Figure 1.3 Photosynthesis process in plants [13].

In homogeneous photocatalysis, the reactants and photocatalysts are available in the same phase. Acid catalysis, organo-metallic catalysis, and enzymatic catalysis are more common homogeneous photocatalysis. Ozone and photo-Fenton systems are prominent homogeneous photocatalysts in nature.

In heterogeneous photocatalytic reaction, the reactant and photocatalyst are present in different phases. These reactions include dehydrogenation, metal depositing, removal of gaseous pollutants, water detoxification, oxidation hydrogen atom transfer, etc. Most commonly heterogeneous photocatalysts are semiconductors and transition metal oxides. In heterogeneous photocatalysis, titanium dioxide TiO₂ is the most recognized and studied photocatalyst. It is highly efficient for removing severely toxic and non-biodegradable organic contaminates of air or water. TiO₂ is well-known as superior photocatalysts compared to others due to its versatile characteristics, such as cost effectivity, safety in use, highly stability, high photocatalytic activity at ambient conditions, i.e. temperature.