Phase- and Size-Dependent Optical and Magnetic Properties of CoO Nanoparticles

A simple pyrolysis method has been developed to synthesize microstructure-controlled CoO nanoparticles from cobalt acetylacetonate in oleylamine at or above 200 °C. XRD, SEM, and HRTEM analyses indicate that the cubic and hexagonal CoO nanoparticles with different morphologies, viz. spherical, quasi-cubic, and pyramidal, could be obtained via varying the precursor concentration, and the average size of hexagonal CoO nanoparticles increases with increasing reaction time or reaction temperature. XPS, TG-DTA, and FTIR analyses reveal that the as-synthesized nanoparticles are pure CoO with good thermal stability. Raman and UV–vis absorption spectra show that the optical properties of CoO nanoparticles are of obvious size effect, which revealed their characteristic feature. Whatever the crystal structure and particle shape are, the CoO nanoparticles with sizes of 33, 59, and 85 nm exhibit two band gaps, and the corresponding band gap differences are 1.84, 1.62, and 1.42 eV, respectively. The pure hexagonal CoO nanoparticles display complete room temperature paramagnetism, while the CoO nanoparticles that contain cubic phase show interesting magnetic behavior due to intrinsic antiferromagnetic structure and uncompensated surface spins, which were confirmed by VSM and ESR studies.