Influence of particle size on the electrocatalytic activity and optical properties of NiO nanoparticles

•The crystallite size is directly proportional to the calcination temperature.•The size of the nanocrystals influenced their structural, morphological, optical and electrocatalytic properties.•UV–vis-NIR measurements show that optical properties are dependent on the crystallite size and lattice imperfections/defects.•The simulation of the electronic and optical properties was satisfactory, despite the difficulty in reproducing the dynamics of the system due to its strongly correlated states.•The electrochemical evaluation shows that the samples are promising electrocatalysts for the oxygen evolution reaction in an alkaline medium.