**7. Semiconductor and new applications for photocatalysis**

The term photocatalyst is a combination of two words such as **Photo,** which is related to photon, and **Catalyst**, which are substances that alter the reaction rate of materials, mainly semiconductors in the presence of light [33]. This is done through chemical reactions known as photocatalythic reaction or photocatalysis that generate an electron-hole pair, when a semiconducting material is exposed to light. The photocatalysis can be categorized into two types depending on the physical state of reactant and semiconductor.

If both (the semiconductor and reactant) are in the same phase such as gas, solid, or liquid, we refer to homogeneous photocatalysis. Otherwise, if semiconductor and reactant are in different phases, they are called heterogeneous photocatalysis [33, 34].

Photocatalysis is a branch of chemistry and it is referred to as a green chemical pathway, as the main applications studied are environmental applications, biotechnology, and health care such as solar cells, water splitting, and purification, wastewater treatment, pollutant degradation, cancer treatment, antibacterial, air purifying, antifogging, self-cleaning, batteries and conservation and storage of energy, etc. [35].

When considering the main materials compatible for photocatalysis, some parameters need to be taken into account such as photocatalyst loading, pH, surface area and morphology, reaction temperatures, contaminant concentration, and calcination temperature of photocatalysts [36]. Many materials are used for these applications, and among them TiO2, TiN, Pt, Si, MoS2, MoS2/TiO2 are the most basic materials used as photocatalyst and they are at low-cost, abundance in nature, chemically stable, and transparent to visible light [33, 36–39].

TiO2 seems to be an excellent photocatalyst and the most used one. The concept to use TiO2 as photocatalytic for water purification was introduced in 1977 by Frank and Bard. They notice that if you put catalytically active TiO2 powder into a pool filled with polluted water and then illuminate it with sunlight, the water will gradually become purified. This concept is very interesting and still used by researchers nowadays for purifying water and then extended to purifying air, cancer treatment, etc. [40].

As reported in many scientific works and in the previous paragraphs, environmental pollution and energy shortage are becoming the big challenge for the developed and in developing countries. Photocatalysis, especially more advanced artificial

systems used to directly convert solar energy into storable energy, might be of interest to solve these issues. The idea is to develop photocatalyst with high efficiency, stability, and low cost using a single-material component.
