**2. General principle of photocatalysis**

The term "Photocatalysis" refers to change in the rate of a chemical reaction or its initiation under the action of light in the presence of a catalyst. This process works just like natural photosynthesis, takes place in presence of photosynthetic organisms that converts carbon dioxide into sugars (chemical fuels) using the light energy from sunlight. It is a green process and mainly divided into two types: (i) Homogeneous photocatalysis, and (ii) heterogeneous photocatalysis. In homogeneous photocatalysis, the reactants and the photocatalysts exist in the same phase (e.g., photocatalysis of ozone where all reactants come under one phase, i.e., gas phase). Whereas in heterogeneous photocatalysis, the reactants and the photocatalysts exist in different phases (e.g., semiconductor photocatalysis.). As shown in **Figure 3**, when a photocatalyst absorbs light irradiation from sunlight or an illuminated light source, the electrons in the valence band of semiconductor are excited to the conduction band, whereas the holes are left in the valence band. This creates electron (e− ) and hole (h+ ) pairs called as semiconductor's "photo-excited" state and the energy difference between the

*Graphene Related Materials and Composites: Strategies and Their Photocatalytic Applications… DOI: http://dx.doi.org/10.5772/intechopen.102404*

valence band and conduction band is referred as "band gap". After photoexcitation, the excited electrons and holes migrate to the surface of photocatalyst to carry out photochemical reactions. For example, in photocatalytic removal of organic pollutants, the photogenerated e− /h+ take part in oxidation/reduction process and produces reactive oxygen species (OH, O2 \_ and H2O2), which can eventually decompose organic pollutants [21, 22]. The ideal photocatalyst requires several key factors such as (i) a suitable band gap to allow the utilization of a significant fraction of the solar spectrum; (ii) optimal band edges relative to the water redox levels; (iii) high mobilities of electrons and holes thereby to reach the surface and reduce/oxidize the targeted molecules before recombining; and (iv) chemical/structural stability.
