Radiation-induced formation of Co3O4 nanoparticles from Co(2+)(aq): probing the kinetics using radical scavengers

The effects of the Co(2+) content and different radical scavengers on the kinetics of γ-radiation-induced Co3O4 nanoparticle formation and growth were investigated. There are four distinct stages of particle formation with different oxidation rates. Scavengers and [Co(2+)]0 affect the oxidation kinetics in the different stages and consequently the final size of the particles formed. Radiolysis model calculations were performed to obtain the time-evolution of the concentrations of key oxidants and reductants, and the effect of scavengers on those concentrations. Based on the model results and experimental data a reaction mechanism for Co3O4 particle formation by γ-irradiation of solutions containing Co(2+)(aq) is proposed. The main cobalt oxidation reaction changes with time. Oxidation of Co(2+)(aq) to Co(3+)(aq) by radiolytically produced ˙OH occurs first in the solution phase. This is followed by spontaneous co-precipitation of mixed Co(II)/Co(III) hydroxide nucleate particles. Adsorption of Co(II)(ad) followed by surface oxidation of Co(II)(ad) to CoOOH(ad) by H2O2 grows particles with a solid CoOOH(s) phase. In parallel, the solid-state transformation of CoOOH(s) and Co(II)(ad) to form Co3O4(s) occurs.