Gamma-radiation induced formation of chromium oxide nanoparticles from dissolved dichromate

Abstract

The formation of chromium oxide nanoparticles by gamma radiolysis of Cr^VI (CrO₄²⁻ or Cr₂O₇²⁻) solutions was investigated as a function of pH and initial Cr^VI concentration by measuring [Cr^VI], the particle concentration ([Cr^III(col)]) and [H₂], 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 ˙e_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 Cr₂O₃ 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 ˙OH and H₂O₂, and reactions of Cr^VI with ˙e_aq⁻ and H₂O₂. The size distribution of the particles that are formed is controlled by these solution–solid interface reactions.