Revisiting Defect-Engineered M(III)-Doped ZnO Photocatalysts for Emerging Pollutant Photodegradation and Mineralization: A Mini-Review.

Zinc oxide (ZnO) is a widely studied semiconductor for photocatalytic applications due to its suitable bandgap, chemical stability, and high electron mobility. Its photocatalytic efficiency is often limited by rapid electron-hole recombination. Metal doping has emerged as an effective strategy to overcome this limitation as it introduces lattice distortions, modifies electronic properties, and creates oxygen vacancies. Trivalent metal dopants substitute Zn2+ in ZnO lattice, inducing oxygen vacancies that act as intrinsic defects, trap charge carriers, extend their lifetime, and enhance charge separation efficiency. Moreover, the presence of VO facilitates the generation of reactive oxygen species (ROS), such as superoxide radicals (•O2 -) and hydroxyl radicals (•OH), under UV or visible light irradiation, which radicals are responsible for the oxidative degradation of organic pollutants. The synergy between metal dopants and oxygen vacancies not only tunes the optical and electronic properties of ZnO but also significantly improves its photocatalytic performance. This makes doped ZnO an attractive candidate for water treatment applications, including the degradation of pharmaceuticals, dyes, and other persistent organic contaminants. Understanding the interplay between dopant type, vacancy concentration, and ROS generation is crucial for designing highly efficient, stable, and recyclable photocatalysts for sustainable water purification.