The global challenge of water pollution caused by hazardous chemicals and dyes necessitates the development of eco-friendly wastewater treatment technologies. Investigations are currently focused on nanomaterials owing to their exceptional capability to eliminate both inorganic and organic pollutants from wastewater. This study investigates the synthesis of zinc oxide nanoparticles ZnO NPs and zinc oxide \(\Vert \) copper oxide nanocomposite ZnO \(\Vert \) CuO NC with a 0.65:0.35 ratio via a low-cost, environmentally friendly method using Retama monosperma L. (Boiss). Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy with energy-dispersive spectroscopy (FESEM-EDS), UV–visible diffuse reflectance spectroscopy (UV–Vis DRS), and micro-Raman spectroscopy were used to study the biomedical nanoparticles. These analyses confirmed the successful formation of ZnO NPs and ZnO \(\Vert \) CuO NC. The FTIR tests showed strong peaks in the 400–600 \(cm^{-1}\) range. These were the metal oxide bonds, such as Zn-O and Cu-O. UV–Vis DRS showed that the bandgap energy decreased from 3.29 eV for ZnO NPs to 3.03 eV for ZnO \(\Vert \) CuO NC. FESEM images revealed a spherical morphology for ZnO (49 nm) and a combination of spherical and hexagonal shapes for ZnO \(\Vert \) CuO NC (61 nm). In an open environment and under natural conditions, the photocatalytic performance of ZnO \(\Vert \) CuO NC demonstrated superior efficiency in removing rhodamine b (Rh-b) and crystal violet (CV). Over 120 min, the efficiency increased from 74.66 to 95.58% for Rh-b and from 97.39 to 99.35% for CV, outperforming pristine ZnO NPs. Kinetic analysis revealed that ZnO NPs showed pseudo-first-order kinetics, whereas ZnO \(\Vert \) CuO NC followed pseudo-second-order kinetics.
