**4. Oxygen-defected semiconductors**

The oxygen-defected semiconductors attracted a lot of interest in the last decade. The idea behind the oxygen-defected semiconductors is modifying the semiconductor lattice through the presence of few oxygen vacancies. This can be achieved *via* either synthesis of the semiconductor in limited oxygen environment (two-step synthesis) or extracting some oxygen from the lattice of the semiconductor (two-step synthesis). The high photocatalytic activity of the oxygen-defected semiconductor can be related to the creation of a sub-energy level below the conduction band of the semiconductor. This sub-energy level can be used for electron relaxing after electron/ hole pair formation, therefore, this relaxing minimizes the recombination between the photo-generated electrons and holes. In **Figure 3**, the creation of sub-energy level and its role in photocatalysis process is illustrated.

TiO2 was the first reported semiconductor that can create oxygen-defected sites in the crystals lattice. Mao et al. reported in Science [5] that reduced titania (TiO2 − x), which contains oxygen vacancies sites, VO-Ti3+, is much more active under the visible light illumination than the equivalent TiO2. Mao used the hydrogenation technique at elevated temperature to reduce the commercially available TiO2. Later, it was reported the one-step *in-situ* reduction of TiO2 by NO and CO as reducing gases to produce blue titania, the produced material showed high photocatalytic performance in water

#### *Introductory Chapter: Photocatalysis – Principles, Opportunities, and Applications DOI:http://dx.doi.org/10.5772/intechopen.110420*

splitting reaction than neat titania. It has been pointed out that the high activity of the hydrogenated titania in the decomposition of water contaminants (sulfosalicylic acid and phenol) under the illumination of UV. Several reports were published describing different techniques to create Ti3+ in TiO2 such as the thermal treatment under vacuum or poor oxygen environment and thermal treatment at elevated temperature with reducing agents. More complicated methods were also reported such as laser treatment at elevated temperature > 500 K or bombardment with high-energy particles such as neutrons or γ-ray.

Moreover, the oxygen-defected ZnO was synthesized by several techniques such as the reduction of ZnO thin films by biogenic tactic. The photocatalytic performance of the prepared material was evaluated in the degradation of different dyes and 4 nitrophenol, results showed higher degradation rate than neat ZnO, however, stability of the oxygen-defected ZnO was not discussed. Furthermore, it was presented a computational study about the oxygen-defected sites in ZnO. In a third study, it was discussed the synthesis of oxygen-defected ZnO nanorods by thermal treatment for zinc acetate as a precursor, and although the photocatalytic activity was higher than neat ZnO in the degradation of methylene blue dye, stability was not discussed. It is interesting to mention that sometimes oxygen-defected semiconductors can create colored material such as blue titania black ZnO. The synthesis of black ZnO was achieved by hydrogenation technique. Black ZnO showed higher photocatalytic activity than the corresponding ZnO.

Oxygen-defected WO3 as a photocatalyst was less explored. Only few studies demonstrated the computational calculations about the presence of oxygen-defected WO3, stability was not discussed. In a recent report, it has been reported the electronic structure of WO3 was changed by incorporating different metals, however, the discussion about oxygen-defected and the stability were not discussed.
