Abstract
The formation of Cr2O3 nanoparticles by γ-irradiation of dichromate solutions was investigated in the presence of radical scavengers, tert-butyl alcohol, nitrous oxide, and dissolved oxygen. The concentrations of CrVI and CrIII species and the chemical composition and sizes of particles that were formed were measured as a function of irradiation time. A mechanism is proposed that is consistent with the observed effects of scavengers on the kinetics of reduction of CrVI in solution to solid CrIII and the final size of Cr2O3 nanoparticles that are formed. The chromium reduction occurs in three stages: Stage I involves homogeneous aqueous reduction of CrVI(aq) to CrIII(aq) followed by spontaneous condensation of Cr(OH)3, providing nucleation sites onto which CrVI and CrIII coprecipitate and form mixed CrVI/CrIII oxide/hydroxide particles. In Stage II, the aqueous reduction of CrVI(aq) and CrIII(aq) reaches steady state but the adsorption of CrVI continues, growing the mixed CrVI/CrIII oxide/hydroxide particles. Stage 3 involves the solid-state conversion of CrVI and CrIII, and the particles slowly convert from a mixed CrVI/CrIII oxide/hydroxide to Cr(OH)3 and then to Cr2O3. Adding a radical scavenger in solution affects the kinetics of particle formation and growth at different stages differently. This information can be used to tailor the final sizes of particles.
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