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 ˙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 ˙OH and H(2)O(2), and reactions of Cr(VI) with ˙e(aq)(-) and H(2)O(2). The size distribution of the particles that are formed is controlled by these solution-solid interface reactions.