**3. Semiconductors photocatalysts**

Several semiconductors were reported as photocatalysts such as TiO2, ZnO, CeO2, ZnSe, ZrO2, Nb2O5, WO3, SiC, and CdS. The bandgap of each material determine the energy needed from light to be activated, i.e. high bandgap materials need high energy and low wavelength light such as UV, while materials with small bandgap need low energy and higher wavelength light such as visible light. Generally speaking, the photocatalysts should be stable, cost-effective, abundant, non-toxic, active, and operate under different conditions. The photocatalytic process implies the absorption of a photon with a higher energy than the bandgap, hence the electron will be excited from the valence band to the conduction band and electron/hole pairs will be formed. If both reach the surface, electron can participate in reduction reaction and the hole will participate in oxidation reaction. TiO2 is one of the most interesting materials, and the most studied one in photocatalysis research. This is because TiO2 is abundant, nontoxic, stable, and very active under UV illumination. However, due to its wide bandgap (3.2 eV), it cannot utilize visible light to be activated. Several attempts have been reported to shift the adsorption band of TiO2 toward visible light region, such as doping TiO2 with other metal or metal oxide, creating sub-energy level in TiO2 lattice, decreasing the crystal size of TiO2 to nano-level, or forming composite with another material. Several transition elements were reported as dopant for TiO2, and it showed a shift in the bandgap toward the visible light region such as Cr6+, V5+, and Fe3+. Moreover, nobel metals such as Ag, Au, and Pt were also reported as an electron trap in TiO2, however, this system is difficult to commercialize due to high cost of

the materials. Creating a sub-energy level also attracts several researchers to increase the activity of TiO2 in visible light. ZnO is a white powder with a bandgap of 3.2 eV, it has been studied as an active photocatalyst, and it exhibited higher photoactivity in several reactions. Moreover, the degradation of several antibiotic compounds is present in water such as amoxicillin, ampicillin, and cloxacillin. The comparison between ZnO and TiO2 in the fever of ZnO was discussed. It has been shown that the degradation of cellulose bleaching effluent was investigated by using ZnO and TiO2 as photocatalysts, ZnO showed better activity than TiO2. ZnO, again, showed better activity than TiO2 in the degradation of Acid Red 14 dye. Furthermore, high activity is also reported for ZnO than TiO2 (Degussa P25) in the degradation of Acid Brown 14 dye under different operating conditions. WO3 is a pale yellow semi-conductor metal oxide with a bandgap of 2.8 eV. Here, WO3 differs from TiO2 and ZnO in its light adsorption capacity, it absorbs light up to 500 nm, which indeed gives an advantage over TiO2 and ZnO. Many authors reported the photocatalytic activity of WO3 with a certain co-catalyst [4].
