Structural and optical investigations of cobalt oxide nanoparticles

Abstract

Two stable phases of cobalt oxide nanoparticles of controlled sizes have been synthesized using environ- mentally friendly inorganic precursor. Structural charac- terization using X-ray diffraction (XRD) shows a single- phase spinal Co3 O4 structure up to annealing temperature of 800 °C and a mixed phase of Co3 O4 and CoO particles for T > 900 °C. Single-phase CoO nanoparticles are also obtained by annealing the particles at a temperature >900 °C and cooling in inert atmosphere. Average macro- and micro- strain were estimated using XRD data. Macrostrain was found to be the minimum for particles annealed at 600 °C, whereas microstrain was found to decrease with increasing annealing temperature up to 900 °C. A correlation between the density of localized states (DOS) in the band gap and strain is expected because the origin of both strain and DOS are defects and bond length distortions. Sub-gap absorption measurement and model calculations have been used for the determination of DOS. For cobalt oxide nanoparticle sam- ples we find a correlation between estimated strain and den- sity of states in the band gap.