Gamma-radiolysis-assisted cobalt oxide nanoparticle formation

The formation of Co3O4 nano-scale colloid particles by gamma irradiation of CoSO4 solutions was investigated. Solutions of 0.2-0.3 mM CoSO4 at pH 6.0 and 10.6 (air-saturated and Ar-purged) were irradiated at an absorbed dose rate of 5.5 kGy h (1). The resulting concentrations of H-2, H2O2, Co-II and Co-III species in solution and the chemical composition and sizes of particles that were formed were measured as a function of irradiation time. Particle formation was observed only for initially air-saturated CoSO4 solutions at pH 10.6. Analysis of the particle formation as a function of irradiation time shows that the particles evolve from Co(OH)(2) to CoOOH and then to Co3O4. The radiolytic oxidation of Co-II to Co-III was completed in 100 min and the chemical composition of the final particles was identified as Co3O4 by XPS, Raman and UV-Vis spectroscopy. Transmission electron microscopy (TEM) images show the final particles are approximately uniform in size, ranging from 8 to 20 nm. A mechanism is proposed to explain the particle formation. A key factor is the low solubility of Co(OH)(2) in air-saturated solutions at high pH. This mechanism for particle formation is compared with the mechanism previously reported for the radiolytic formation of gamma-FeOOH nanoparticles.