ABSTRACT
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.
ABSTRACT
The formation of chromium oxide nanoparticles by gamma radiolysis of Cr(VI) (CrO(4)(2-) or Cr(2)O(7)(2-)) solutions was investigated as a function of pH and initial Cr(VI) concentration by measuring [Cr(VI)], the particle concentration ([Cr(III)(col)]) and [H(2)], and by characterizing the particles using TEM, Raman, FTIR and XPS. The results show that Cr(VI) is easily reduced to Cr(III) by a homogeneous aqueous reaction with Ëe(aq)(-), but, due to the stability of Cr(III) colloids, the growth of the Cr(OH)(3) particles is very slow. As the particles grow the interior of the particle dehydrates to form Cr(2)O(3) while the outer layer remains hydrated. When most of the Cr(VI) that is initially present in the solution is converted to Cr(OH)(3) further redox reactions of chromium species occur on the particle surfaces. The redox system reaches a pseudo-equilibrium state due to cyclic reactions of Cr(III) with ËOH and H(2)O(2), and reactions of Cr(VI) with Ëe(aq)(-) and H(2)O(2). The size distribution of the particles that are formed is controlled by these solution-solid interface reactions.
ABSTRACT
Gamma-irradiation of deaerated aqueous solutions containing FeSO(4) leads to the formation of uniform-sized colloidal particles of γ-FeOOH. At short irradiation times, or in solutions with a low initial [Fe(2+)](0), spherical particles with a size less than 10 nm are formed. These primary particles grow to form a dendritic structure upon longer irradiation, and the final size of the large particles is â¼60 nm with a very narrow size distribution. Further prolonged irradiation does not change the final particle size. The narrow size distribution is attributed to rapid homogeneous radiolytic oxidation of soluble Fe(2+) to relatively insoluble Fe(3+) hydroxides [Fe(H(2)O)(6-n)(OH)(n)](3-n) leading to particle nucleation by spontaneous condensation. These primary particles then grow into γ-FeOOH particles with a dendritic structure. The final size reached at long times is regulated by the steady-state redox conditions established during long-term irradiation at the aqueous-solid interface.
