Dominant role of trioctylphosphine on the particle size and various properties of CoO nanoparticles

The dominant role of trioctylphosphine (TOP) on the particle size and various properties of CoO nanoparticle (NP) samples having Scherrer size in the range 9.4–42.8 nm is reported here; samples have been characterized in detail systematically. Remarkably, while nanosized CoO or CoO/Co nanocomposites with monoclinic or face-centered cubic (fcc) phase can be prepared depending on the synthesis conditions and particle size systematically decreases with increase in concentration of TOP, there is an increase in optical band gap and electrical resistivity with decrease in particle size. The highly semiconducting electrical resistivity is explained with the variable range hopping and small polaron hopping models that indicate in turn enhanced localized density of states, but decrease in Debye temperature (θ_D) as the particle size decreases. Curiously, 25.9 nm sample shows anomalous behaviour. Moreover, the silica-coated 9.4 nm CoO NP and bare 10.8 nm CoO NP samples exhibit ferromagnetism at 300 K even though their bulk counterpart is antiferromagnetic, with attractive indications of possible potential biomedical applications of the former.