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

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 &z.rad;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 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 &z.rad;OH and H 2 O 2 , and reactions of Cr VI with &z.rad;e aq − and H 2 O 2 . The size distribution of the particles that are formed is controlled by these solution-solid interface reactions. The kinetics of chromium oxide formation by γ-radiolysis of initially dissolved Cr VI were investigated. The results show that Cr VI is easily reduced to Cr III by homogeneous aqueous reaction with &z.rad;e aq − , but 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.