Tailoring ZnO-Based Photocatalysts with Dopants for Enhanced Environmental Remediation: A Critical Review

ZnO-based nanomaterials have attracted considerable attention for the degradation of persistent organic pollutants because of their low cost, high stability, and excellent photocatalytic properties. Among advanced oxidation processes (AOPs), heterogeneous photocatalysis is particularly attractive due to its operational simplicity, cost-effectiveness, and potential for large-scale application. This review critically summarizes recent advances in ZnO-based photocatalysts for the degradation of organic pollutants under visible and solar-light irradiation. Particular emphasis is placed on synthesis approaches, doping strategies, and surface modifications that improve light harvesting, charge separation, and photocatalytic stability. Comparative analysis indicates that metal and semiconductor dopants enhance visible-light activity by regulating the band structure, suppressing electron–hole recombination, and facilitating interfacial charge transfer. The roles of various dopants—including alkali metals, alkaline earth metals, transition metals, noble metals, rare earth elements, and semiconductors—are systematically discussed in relation to their impact on crystal structure, electronic properties, morphology, and photocatalytic performance. Overall, this review provides a comprehensive assessment of metal- and semiconductor-doped ZnO nanocomposites for the efficient degradation of dyes and pharmaceutical pollutants, highlighting key design principles for the development of visible-light-active ZnO-based photocatalysts.