Semiconductor photocatalysts

electrons. Thus the improvement of the photocatalytic activity can be noticed upon the irradiation of the catalyst in the visible region [94].

1.4

Scientists have made rapid and significant advances in the field of semiconductor materials, which has attracted significant interest due to its numerous practical applications [95-99]. Essential characteristics of a photocatalyst include stability and the absence of resulting secondary pollution.

It is well known that morphological and structural characteristics affect the photocatalytic activity of a semiconductor [100].

Many researchers have been reported that titanium dioxide (TiO2), zinc oxide (ZnO), and tin dioxide (SnO2) are the most prominent photocatalysts. Semiconductors as (TiO2) with a BG of (3.2 eV), (ZnO 3.3 eV) and (SnO2) which has a large BG energy (3.6 eV), are the most ideal photocatalysts for the degradation of dyes, phenols and pesticides [29, 98, 101]. By doping the metal oxide with impurities reduces the Ebg and electromagnetic radiation with lower energy (to be fit in the visible light region) and can be utilized to activate the catalyst, possibly increasing photocatalytic activity. Below, In the Table 2 shows the BGs of a variety of metal oxides [82, 102, 103].

Table 2: Band-gap energy of some photocatalysts [94-96, 104].

Photocatalyst Band-gap energy (eV) Photocatalyst Band-gap energy (eV)

Si 1.1 ZnO 3.3

TiO2 (rutile) 3.0 TiO2 (anatase) 3.2

SnO2 3.6 SnO ~ 2.5 - 3.0

WO3 2.7 CdS 2.4

ZnS 3.7 ZnO 3.2

These semiconductors have been recognized as preferable materials for photocatalytic processes due to their high photosensitivity, nontoxic nature, low cost and chemical stability [105-107]. Exposure to UV or solar irradiation during photolysis may initiate

organic degradation. Green plants use solar lights to produce O2 and H2O through photosynthesis. Dead bodies also undergo chemical transformation to produce oil [108].

Energy supplied by absorbing a photon of light enables excitation of reactant molecules to promote degradation reactions [59]. TiO2 photocatalysts have received the most attention from a number of researchers [109].

Scientists put additional interest in the progress of photocatalysis research which is indicated by the huge number of research publications exhibited over the previous years.

Photocatalysis research literature survey with other metal oxides key words is summarized in Figure 2 (source, Scopus, May 2017). According to this investigation, photocatalysis research has generated approximately > 42,713 articles about the application of TiO2, ZnO and SnO2. As photocatalysts which have been published to date, the most publications were dealt with TiO2. Solar irradiation or visible light irradiation is also widely investigated. As a survey nearly 14,334 articles listed in Scopus, TiO2 and ZnO are the most popular and widely studied photocatalyst materials, with more than half of the literature reporting the use of TiO2, while only over 3,593 paper on ZnO have been published to date. Unlike other photocatalysts, SnO2 has not been thoroughly studied about 531 articles in Scopus. In spite of the countless benefits of SnO2 and the complete absence of secondary pollution due to photoerosion [110]. In addition, the rutile structure of SnO2 is very similar to that of TiO2, but as photocatalyst material is not widely reported as it shown in Figure 2 [111, 112].

Figure 2: Comparison of the search term “Photocatalysis” in Scopus articles

1.4 Semiconductor photocatalysts 37 In a typical photocatalysis process, the breakdown of an organic compound (phenol) in an aerated solution can be summarized as in equation 1.

The reaction takes place when UV radiation photoexcites a semiconductor catalyst in the presence of oxygen, ^•OH is generated to attack oxidizable contaminants, breaking down molecules yielding CO2, and H2O. The reaction is applied for the oxidization of almost any organic substance due to its positive oxidation potential [113].

Unfortunately, the swift recombination rate of the photogenerated eˉ - h⁺ pairs hinders the industrial application of these semiconductors as it will be discussed in the following subjects [114, 115].

Observing the environment is essential to protect the neighborhood and the surroundings from the previously mentioned toxins. Nanotechnology can identify these pollutants, and give clues about removing them from the water. By controlling different contaminations in the environment, would require less labor and not much energy in the future [116]. Nanotechnology has the capability of creating functional materials, devices and systems with new properties through manipulating of matter in the range of about 0.1-100 nm [117].

The quality of these Nps completely depends on their phase, size, shape and dimension [118, 119]. Improving scalable and simple routes to construct nanomaterials with a convenient size and microstructure is very important in nanotechnology and synthetic chemistry [120].

Nanotechnology covers many knowledge areas such as chemistry, physics, engineering, material science and biology. Nanomaterials have unique electrical, physical, chemical, and magnetic properties, which can be manipulated [121, 122]. These Nps have been studied from both experimental and theoretical points of view due to their potential application in solar energy conversion and photocatalysis [123, 124]. Nanotechnologists have the ability to produce controlled Nps, which are mainly applied in catalysis to

improve chemical reactions, cut down the amount of catalytic materials, getting good results, saving money and reducing pollutants, such as materials that supply clean water from polluted ones.

Water is important for human life and if it is not clean would create many environment disaster applications. Researchers could synthesize Nps and decide how to apply their chemical and physical properties for various kinds of toxic site remediation. Monitoring the environment at the beginning of the pollution created is necessary to prevent pollution, discover solutions or when to degrade environmentally dangerous toxins. The use of Nps to detect water contamination and remediate through degradation of poisonous materials to safe minerals is getting appeal from researchers everywhere.

Nanotechnology can be used to reduce the cost through different aspects. Priority comes by breaking down of big molecules (chemically or physically) into smaller materials of desired shapes and sizes, also by building up nanostructures, by bringing in individual atoms and molecules together.