OICC Press Iranian Journal of Catalysis 2345-4865 5 2 2024 02 03 Optical and photocatalytic properties Undoped and Mn-doped ZnO nanoparticles synthesized by hydrothermal method: Effect of annealing temperature EN Maryam Bordbar Department of Chemistry, Faculty of Science, University of Qom, Qom, Iran. Seyed Mohammad Vasegh Faculty of Technical & Engineering, Islamic Azad University, Tehran South Branch, Tehran, Iran. Somaye Jafari Faculty of Science, Payame Noor University, Qom, Iran. Ali Yeganeh Faal Department of Chemistry, Payame Noor University, Iran. Journal Article 2024 02 03 Undoped and Mn-doped ZnO nanoparticles were successfully prepared by the hydrothermal method with different annealing temperature conditions. Structural, chemical and optical properties of the samples were studied by X-ray diffraction (XRD), Field Emission scanning electron microscopy (FESEM), UV-Vis spectrophotometry and Fourier transform infrared (FT-IR) spectroscopy. The phase purity was confirmed by X-ray powder diffraction (XRD) and XRD spectra showed that all the samples were hexagonal wurtzite structure and as the annealing temperature increases the material becomes less crystalline. In addition, the average crystal size was found to be 15-30 nm from SEM and XRD. It is seen that the optical band gap increases when the ZnO is doped with manganese and decreases when annealing temperature increases. The photoacatalytic activity of Undoped and Mn-doped ZnO nanoparticles was tested by the degradation of metyl orang (MO) under UV light and indicated that Mn-doped ZnO has higher photocatalytic activity relative to ZnO nanoparticles and photocatalytic activity decreases when annealing temperature increases.