This study aimed to investigate the effect of Cu doping on the structural and photocatalytic properties of ZnO. The ultrasonic-assisted sol-gel method was employed to fabricate Cu/ZnO photocatalysts with Cu loadings of 0, 1, 3, 5, and 7 wt.%. Doping ZnO with Cu (up to 5 wt.%) resulted in smaller particles, more homogeneous dispersion, and a higher surface area. Moreover, increasing Cu content led to increased compressive strain, reduced crystal size, a lower energy bandgap, a diminished rate of electron-hole recombination, and a smoother surface. The average particle sizes of pure ZnO (ZnCu0), ZnO doped with 5 wt.% Cu (ZnCu5), and ZnO doped with 7 wt.% Cu (ZnCu7) were 36.5, 31.5, and 35.1 nm, respectively, while their energy bandgaps were 3.2, 2.9, and 3.05 eV. ZnCu5 exhibited superior properties due to the synergistic effect of the synthesis method and optimal Cu doping. Photocatalytic degradation of rhodamine B (RhB) and methylene blue (MB) under simulated sunlight showed that ZnCu5 had the highest efficiency, achieving 93.6 and 91.2% degradation of RhB and MB after 3 h, respectively, compared to 59 and 55% for ZnCu0. Furthermore, ZnCu5 retained 93% of its photocatalytic activity after 5 cycles (900 min). Further Cu loading beyond 5 wt.% led to undesirable properties, including larger particles, non-uniform distribution, intensified tensile strain, increased energy bandgap, and reduced photocatalytic performance. The optimal ZnCu5 concentration was determined to be 1 g/L. The degradation process was primarily driven by hydroxyl (OH∙) and superoxide (O2∙−) radicals, along with photo-generated electrons.
