Radiation-induced formation of Co sub(3)O sub(4) nanoparticles from Co super(2+)(aq): probing the kinetics using radical scavengers

The effects of the Co super(2+) content and different radical scavengers on the kinetics of gamma -radiation-induced Co sub(3)O sub(4) nanoparticle formation and growth were investigated. There are four distinct stages of particle formation with different oxidation rates. Scavengers and [Co super(2+)] sub(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 Co sub(3)O sub(4) particle formation by gamma -irradiation of solutions containing Co super(2+)(aq) is proposed. The main cobalt oxidation reaction changes with time. Oxidation of Co super(2+)(aq) to Co super(3+)(aq) by radiolytically produced &z.rad; OH occurs first in the solution phase. This is followed by spontaneous co-precipitation of mixed Co super(II)/Co super(III) hydroxide nucleate particles. Adsorption of Co super(II)(ad) followed by surface oxidation of Co super(II)(ad) to CoOOH(ad) by H sub(2)O sub(2) grows particles with a solid CoOOH(s) phase. In parallel, the solid-state transformation of CoOOH(s) and Co super(II)(ad) to form Co sub(3)O sub(4)(s) occurs.