CoSe nanostructured thin films were synthesized using the chemical bath deposition method. We investigated the role of deposition time on the evolution of the nanoparticle shape, configuration and optical band gap in Cobalt selenide (CoSe) nanoparticle preparation process. Rutherford Backscattering Spectrometry was utilized, as an accurate technique, for obtaining an insight into the depth profile and the stoichiometry variation of the constituent concentration during the deposition time. The Scanning Electron Microscopy result exhibited that nanorods are formed by increasing the deposition time. Derivation Ineffective Thickness Method (DITM) was employed to determine the optical band gap energy and transitions index. The band gap energies resulted from DITM were compared with the Tauc model and it was found that there is no need for any presumption about the natural transition in DITM. The lower band gap energy of prepared thin films was achieved at the longer deposition time. The chemical structure of the azo dye Congo Red were decolorized to 90% using solar light
