Enhanced Photocatalytic Degradation of Methylene Blue and Methyl Orange Dyes via Transition Metal-Doped Titanium Dioxide Nanoparticles

Herein, we reviewed the photocatalytic degradation of methylene blue and methyl orange using titanium dioxide nanoparticles. The nanoparticles normally face challenges due to electron-hole recombination effects and low surface area. Doping with transition metals such as copper (Cu), chromium (Cr), manganese (Mn), cobalt (Co) and nickel (Ni) is an alternative way of reducing the electron-hole recombination effects, improves structural and electronic properties but also reduces on the band gap energy below 3.0 eV. Photocatalyst which is an advanced oxidation process (AOP) is used as an alternative to remove both methylene blue and methyl orange dyes. This process produces highly reactive oxygen species (ROS) such as hydroxyl radical, sulphates and superoxide radical which create alternative pathways for the degradation of synthetic dyes to less toxic by-products which are water, hydrochloric acid and carbon dioxide. This review further discusses the synthesis techniques which include chemical vapor deposition, hydrothermal synthesis, sonochemical synthesis, sol-gel method, spray pyrolysis, etc that are commonly used in the preparation of pure and doped titanium dioxide nanoparticles.