ZnO-Based Hybrid Photocatalysts for Pharmaceutical Micropollutant Degradation: From Doped Nanoparticles to CNT Aerogels

The continuous release of pharmaceutical residues into aquatic environments has become a major environmental concern due to their persistence, bioaccumulation, and resistance to conventional wastewater treatment technologies. Among semiconductor photocatalysts, ZnO has attracted considerable attention because of its low cost, chemical stability, and strong oxidative capability. However, its practical application remains limited by rapid electron–hole recombination, photocorrosion, and poor visible-light utilization associated with its wide band gap (~ 3.2 eV). This review provides a comprehensive overview of advanced ZnO-based photocatalysts for pharmaceutical micropollutant degradation, highlighting the evolution from doped ZnO nanoparticles to multifunctional nanocomposites and three-dimensional ZnO–CNT aerogel systems. The effects of metal doping (e.g., Ca, Cu, Ga, In, and Al) on band-gap narrowing (~ 3.2 to ~ 2.7 eV), oxygen-vacancy generation, enhanced visible-light absorption, and reduced charge-carrier recombination are critically discussed. Particular attention is devoted to ZnO–CNT aerogels, which exhibit surface-area enhancement from ~ 2 to 132 m² g⁻¹, interconnected conductive porous networks, and improved mass-transfer properties. These systems frequently achieved degradation efficiencies above 90–99% together with high mineralization efficiency and excellent reusability. Among them, In-doped ZnO/CNT aerogels demonstrated near-complete degradation and mineralization under combined activation processes, highlighting the strong potential of defect-engineered aerogel architectures for advanced wastewater treatment. Finally, current challenges related to large-scale fabrication, reactor integration, long-term stability, and real wastewater applications are discussed to outline future directions for sustainable ZnO-based photocatalytic systems.