Gamma-Radiation Induced Redox Reactions and Colloidal Formation of Chromium and Cobalt Oxide Nanoparticles

The main goal of this thesis research is to develop a mechanistic understanding of radiation-induced chromium oxide and cobalt oxide nanoparticle formation in aqueous solutions containing initially dissolved metal ions. When exposed to ionizing radiation, water decomposes to form a range of chemically reactive radical and molecular products. This redox agents can readily change the oxidation state of dissolved metal ions. The solubility of a transition metal ion can vary by several orders of magnitude depending on its oxidation state and the solution pH. Thus, reactions that can alter the oxidation state of a dissolved ion can lead to the condensation of insoluble species and the formation of solid particles. The formation of Co3O4 nano-scale colloid particles by gamma irradiation of CoSO4 solutions was investigated as a function of pH, initial CoII concentration and radical scavenger environment. Particle formation was observed only in aerated solutions. Analysis of the particle formation as a function of irradiation time and information from the scavenger studies shows that the particles evolve from Co(OH)2 to CoOOH and then to Co3O4 with oxidation of Co2+ to Co3+ by OH being the most important process. Transmission electron microscopy (TEM) images show that the final particles size depend on the initial conditions of the solution. The formation of chromium oxide nanoparticles by gamma radiolysis of CrVI solutions was investigated as a function of pH, initial CrVI concentration and scavenger environment. The results show that CrVI is easily reduced to CrIII by a homogeneous aqueous reaction with eaq-, but, due to the stability of CrIII colloids, the growth of the Cr(OH)3 particles is very slow. However, after some time the Cr(OH)3 is converted to Cr2O3. Again for this system, the sizes of the particles formed depend on the solution conditions