C.Durai Vathi, cobalt oxide is a known p-type semiconductor that has various stable cubic phases, including CoO, Co2O3 , and Co3O4 with a wide range of direct band gaps extending from 1.48 eV to 2.19 eV.

The blue shift in the UV-visible spectrum typically occurs due to a decrease in the size of nanoparticles, leading to an increase in the band gap and higher energy transitions.

The optical band gap of pure cobalt oxide nanoparticles typically lies in the 2.1 to 2.9 eV range. However, these values vary depending on the parameters, such as synthesis method and particle size. The blue shift may be the reason for the defects, strain, or surface-to-volume ratio variation.