Radiation-Induced Formation of Chromium Oxide Nanoparticles: Role of Radical Scavengers on the Redox Kinetics and Particle Size

The formation of Cr2O3 nanopartides by gamma-irradiation of dichromate solutions was investigated in the presence of radical scavengers, tert-butyl alcohol, nitrous oxide, and dissolved oxygen. The concentrations of Cr-VI and Cr-III 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 Cr-VI in solution to solid Cr-III and the final size of Cr2O3 nanopartides that are formed. The chromium reduction occurs in three stages: Stage I involves homogeneous aqueous reduction of Cr-VI(aq) to Cr-III(aq) followed by spontaneous condensation of Cr(OH)(3), providing nucleation sites onto which Cr-VI and Cr-III coprecipitate and form mixed Cr-VI/Cr-III oxide/hydroxide particles. In Stage II, the aqueous reduction of Cr-VI(aq) and Cr-III(aq) reaches steady state but the adsorption of Cr-VI continues, growing the mixed Cr-VI/Cr-III oxide/hydroxide particles. Stage 3 involves the solid-state conversion of Cr-VI and Cr-III, and the particles slowly convert from a mixed Cr-VI/Cr-III 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.