Radiation-induced formation of Co3O4 nanoparticles from Co2+(aq): probing the kinetics using radical scavengers

Abstract

The effects of the Co²⁺ 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 [Co²⁺]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 Co²⁺(aq) is proposed. The main cobalt oxidation reaction changes with time. Oxidation of Co²⁺(aq) to Co³⁺(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.