Gamma-radiolysis-assisted cobalt oxide nanoparticle formation

The formation of Co 3 O 4 nano-scale colloid particles by gamma irradiation of CoSO 4 solutions was investigated. Solutions of 0.2-0.3 mM CoSO 4 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 , H 2 O 2 , 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 CoSO 4 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 Co 3 O 4 . 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 Co 3 O 4 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 γ-FeOOH nanoparticles. Gamma-irradiation of CoSO 4 solutions produced highly uniform, nanometer-sized Co 3 O 4 particles. The mechanistic study using a variety spectroscopy and TEM analyses shows that hydration of Co II species forms insoluble Co(OH) 2 and this provides nucleation sites on which further oxidation of Co II by water radiolysis products (&z.rad;OH, H 2 O 2 ) can occur. The resulting CoOOH then converts to more stable Co 3 O 4 nanoparticles.