Review on the removal of organic dyes by photocatalytic degradation using nanocomposite of transitional metal oxides under visible-light

Varieties of wide organic and inorganic pollutants especially pesticides, poisonous metals, and dyes are introduced in to the water system from various sources such as: industrial effluents, agricultural runoff and chemical spills. The new treatment methods (Photocatalytic degradation method) are necessary for the removal of persistent dye chemicals or converting them into harmless compounds in water. The photocatalytic technique with metal oxide semi conductors has become one of the most promising methods for waste water treatment. In particular, ZnO has attracted much attention with respect to the degradation of various pollutants due to its being relatively cheap, stable, high photocatalysis, non-toxicity, and wide band gap. One of the main problems associated with the feasibility and viability of ZnO Semiconductor photocatalytic oxidation is: Recombination of photo-generated electrons and holes in the semiconductor and high band gap energy (>3.0 eV) that requires radiation in the UV range for electron excitation from VB to CB. To overcome this problem it can be achieved by: doping transition metal, quantum dot photosensitization, carbon nano structure and improving the catalytic efficiency of the semiconductor photocatalyst by minimizes the electron-hole recombination. The factors affect photocatalytic degradation and synthesis methods for nanocomposites were summarized. For example Ternary system of Fe2O3-ZnO-MnO2, to improve the photocatalytic activity of wide band gap photocatalysts was discussed. But the difficulties in recycling and preconcentration restricted the utilization of finer nanoparticles. To overcome this limitation, the nanoparticles have been supposed to be fixed on the inert supports such as zeolite for practical applications in order to improve the recovery efficiency of photocatalysts.