A systematic review on photocatalytic activities of metal oxide semiconductors (MOS) for wastewater: dye degradation

For over two decades, global ecosystem has been negatively affected by pollutions from wastewater effluents from various sources (textile, paper, pharmaceutical, paper, leather, batteries, rubber, cosmetics, and plastics industries). These pollutants are usually in form of dyes (Methylene Blue, Methyl Orange, Eosin Y, etc.) or organic substance (ciprofloxacin, organochlorines, organophosphorus, and chloropyridines, etc.). The effects produce by these pollutants on our health and environment are enormous. To solve the problems caused by the release of the wastewater into our ecosystem, scientists have established that photocatalysis degradation of wastewater pollutants using metal oxide semiconductor (MOS) photocatalysts is most promising method for effective degradation. This paper review metal oxide semiconductors photocatalyst for wastewater treatment, highlighting ways of improving efficiency of metal oxide semiconductors photocatalyst in order to address the environmental challenges. It also presents overview of dye pollution from wastewater with discussion on metal oxide semiconductors photocatalysts focusing on the methods of producing MOS (co-precipitate methods, hydrothermal methods, solvothermal methods, sol–gel methods, electro-spinning methods, thermal evaporation methods, chemical vapor deposition methods, etc.). Then, a systematic review on the most important metal oxide semiconductors (TiO2, ZnO, WO3, etc.,) with mechanism of photocatalytic process was discussed. Finally, enhancement methods focusing on the optimization of photocatalytic activities of metal oxide semiconductors under visible light irradiation is presented.