Photochemistry is a field which investigates of beam effect between the visible and near ultraviolet region and initiated by absorption of a photon is called photochemical reaction. Some specific photochemical reactions are given specific names. For examples, photosynthesis is a mechanism which produces oxygen from CO2 and water using Chlorophyll like a catalyst Photocatalysis is an opposite mechanism of the photosynthesis.
Definition of Photocatalyst
The photocatalyst is a semiconductor material that forms a strongly oxidizing environment on its surface under the influence of ultraviolet (UV) light. When UV light is applied, photocatalyst absorbs light and increases energy. This photon energy is transformed the chemical energy and degrades the microbes, molds, bad odors and produce harmful organic chemicals into carbon dioxide and water by containing high oxidation power.
The process of the photocatalysis and photosynthesis are shown in Figure 1. Important advantages of this technology; renewable and provide a good alternative for using pollution-free solar energy treatment method, unlike the transfer of impurities from one side to another like classical treatment method, it produces harmless products.
The yield of the photocatalyst depends on the wavelength and intensity of the light source. The photocatalyst needs higher energy photon than band energy for activation. Figure 2 shows the region where titanium dioxide can absorb in the light spectrum. As can be seen from the figure, this region corresponds to about 5% of the light spectrum. Therefore, a small shift in the band structure can further absorb the solar spectrum and thus increase the performance of the photocatalyst. Similarly, the absence of ultraviolet light in indoor lighting conditions is the biggest obstacle to the use of photocatalyst in closed areas.
If the photocatalyst surface area increases, the greater the number of organic substances that will be adsorbed on the surface and thus the activity of the photocatalyst increases. One disadvantage of thin films is that they have a lower surface area than the powder form of TiO2. The recombination of electrons and holes in any chemical reaction is the most dominant photocatalytic reaction. Because 95% of the charge carriers (electrons and holes) are recombined without reaction. This situation is the biggest limitation in semiconductor photocatalysis system. The recombination may be on the photocatalyst surface or in the center of the bulk, while the rate of recombination determines the efficiency of the photocatalyst.
TiO2 as a Photocatalyst
Titanium dioxide is one of the preferred industrial or consumer materials in our daily life. Titanium is a very rough, silvery white, low corrosion and very bright element. The melting point is 3,020 oF, boiling point 5,949 oF and density is 4,5 g/cm3 at 20 oC Oxidation states are +2, +3, and +4, as in the oxygen compounds titanium monoxide, TiO, dititanium trioxide, Ti2O3, and titanium dioxide, TiO2, respectively. The +4 oxidation state is the most stable.
Titanium dioxide (TiO2) has three different polymorphs. These are Rutile, Anatase and Brookite. Anatase is the most studied TiO2 polymorph among the three natural phases (anatase, rutile and brookite). Most common phases are Anatase (3.2 eV 387 nm) and Brookite (3.0 eV 413 nm). Although, anatase phase of TiO2 is more toxic than the rutile phase but, when particles become smaller and surface area increase, rutile phase of TiO2 become harmful. Surface area increase, TiO2 nanoparticles can absorb UV radiation easily and get great photocatalytic activity. Surface modifications cause changes in photocatalytic activity also.
The anatase crystal phase of TiO2 shows the highest photocatalytic activity than other TiO2 crystal structures. That’s why Anatase type of titanium dioxide is most commonly used as a photocatalyst in the industry. The anatase band gap is higher than rutile. This phase can raise the electron from valence band to conduction band to higher energy levels of the redox potentials of the adsorbed molecules when the absorbable light is low.
This case increases the molecule’s oxidation power transfer of the electrons and facilitates the transfer of electrons from the TiO2 to the adsorbed molecules. This statement has also been extended to illustrate activities related to orientation to the surface, suggesting that different surfaces offer different band gaps.
Anatase electron-hole pair life is longer than rutile for charge carrier in surface reactions. The capture of electrons and holes of the photocatalyst (by molecular oxygen and hydroxyl ion) determines the quantum yield at the surface. On the anatase surface, the holes are joined more tightly than the rutile with the hydroxyl ion. Therefore, the life of the charge carriers of the anatase is higher than that of the rutile. As a result, anatase shows better photocatalytic activity than rutile. Rutile is a thermodynamically stable phase between the three natural phases of TiO2. Due to its high optical, high chemical stability, high refractive index, high dielectric constant and excellent scattering efficiency, it has a wide range of application areas.
Anatase, rutile and brookite structures can be identified by TiO6 octahedral chains surrounded by 6 O-2 ions octahedral of each Ti+ 4 ions are shown in Figure 3. The octahedral in the anatase, rutile and brookite structures are connected by corners, corners edges and edges.
The difference between these phases depends on the distortion of the octahedral structures. Rutile structure shows orthorhombic distortion which is not regular. On the other hand, anatase is less distortion than rutile and brookite. The Ti-Ti distances in anatase are more than rutile. However, Ti-O distances in anatase are shorter than rutile. These differences are reflected in both mass densities and band structures of anatase and rutile.
Photoelectrochemical studies of important inorganic compound TiO2 were initiated in the late 1960s. Fujishima and Honda (1972) used rutile titanium dioxide (TiO2) and platinum electrode to separate the water molecule photocatalytically into oxygen and hydrogen. After this discovery, heterogeneous photocatalysis has been started to intense studies against potential environmental pollution problem.
Heterogeneous photocatalysis can be considered as a brightest method in advanced oxidation processes. This process uses molecular oxygen as oxidizing agents and converts impurities into harmless substances. Heterogeneous photocatalysis has been utilized air and water treatment systems, self-cleaning surfaces, solar cells and hydrogen production.
Heterogeneous photocatalysis is based on the interaction of semiconductor material with light. The idea of photocatalysis has emerged as the use of solar light as an energy source for environmental cleaning. The heterogeneous photocatalysis method is an ideal and promising approach.
Titanium dioxide (TiO2) is the most commonly studied and used as a photocatalyst among other semiconductors with photocatalytic activity owing to its high oxidation potential, comparatively low cost and high efficiency when irradiated with ultraviolet (UV) light. When TiO2 is irradiated via ultraviolet (UV) light, electron-hole pairs are formed. Electron-hole pairs produce ·OH radicals and O2. super oxides to decompose contaminants on the photocatalyst surface.
In literature, TiO2 is used in powder form generally for many applications, whereas there are a lot of essential and practical applications to use it as a thin film. These applications should not be limited to only self-cleaning surfaces or antibacterial surfaces. High-efficiency self-cleaning is important for titanium dioxide-coated self-cleaning surfaces, as well as the commercial success of solar panels in hard-to-reach places and the difficulty cleanliness of larger glass surfaces at high-rise buildings.