Background Azo dyes represent the most commonly used group of dyes in the textile industry. These organic dyes are mainly resistant to biodegradation and may exhibit toxic and carcinogenic properties. The purpose of this study was to investigate the effects of doping zinc oxide (ZnO) nanoparticles (NPs) with transition metals (silver, manganese, and copper) on the photocatalytic efficiency of ZnO NPs in the removal of Direct Blue 15 dye from aqueous environments under ultraviolet (UV) radiation and visible light irradiation. Methods One or two metals were used for doping the NPs. In total, seven types of undoped and transition metal-doped NPs were synthesized using the thermal solvent method with ZnO precursors and transition metal salts. The characteristics of the synthesized NPs were determined based on the scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Xray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, dynamic light scattering (DLS), atomic force microscopy (AFM), and zeta potential measurements. Results The produced ZnO NPs did not exhibit any particular photocatalytic activities under UV radiation and visible light irradiation. The highest removal efficiency under UV radiation was about 74% in the presence of silver-doped ZnO NPs, while the maximum efficiency under visible light was 70% in the presence of copper-doped ZnO NPs. The lowest removal efficiency Highlights • Transition metal-doped ZnO NPs was used for photocatalytic removal of Direct Blue 15 dye. • Photocatalytic activity of ZnO NPs was improved after doping with Ag, Mn, and Cu. • SEM, XRD, FTIR, and AFM corroborated the synthesis of transition metal-doped NPs. • Photocatalysis using Ag-doped ZnO NPs could degrade 74% of dye under UV radiation. • About 70% of dye could be removed using Cu-doped ZnO NPs under visible light.

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