Transition metal oxide nanoparticles have attracted significant attention due to their tunable electronic structures and potential applications in visible-light-driven photocatalysis. In the present study, CuO, α-Fe₂O₃, and CuFe₂O₄ nanoparticles were synthesized via a simple and cost-effective co-precipitation method followed by controlled calcination at 400–600 °C. The formation of respective oxide phases was confirmed through Fourier Transform Infrared (FTIR) spectroscopy, which revealed characteristic metal–oxygen stretching vibrations corresponding to monoclinic CuO, hematite (α-Fe₂O₃), and spinel CuFe₂O₄ structures. The optical properties of the synthesized nanoparticles were investigated using UV–visible diffuse reflectance spectroscopy (DRS). The reflectance data were transformed using the Kubelka–Munk function, and optical band gaps were determined through Tauc plot analysis for both direct and indirect electronic transitions. The results demonstrated distinct band gap values for each oxide system, highlighting the influence of composition and crystal structure on electronic properties. CuO exhibited the narrowest band gap among the studied materials, indicating strong visible-light absorption capability. Furthermore, the photocatalytic activity of CuO nanoparticles was evaluated through the degradation of methyl orange dye under visible light irradiation in the presence of H₂O₂. A significant reduction in dye concentration was observed, confirming the correlation between narrow band gap and enhanced photocatalytic performance. This study provides a systematic comparative evaluation of CuO, Fe₂O₃, and CuFe₂O₄ nanoparticles synthesized under identical conditions, offering insights into synthesis–structure–optical property relationships and their relevance in environmental remediation applications.

Keywords: Metal Oxide NPs; Optical Properties; CuO Fe₂O₃; CuFe₂O₄;