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.