Formation, Absorption Spectrum, and Chemical Reactions of Nanosized Colloidal Cobalt in Aqueous Solution

The radiation−chemical reduction of Co 2+ ions in aqueous solution of Co(ClO 4 ) 2 and sodium formate is described. Stable metal sols containing spherical particles with a diameter of 2−4 nm are formed under γ-irradiation in the presence of polyacrylate as a stabilizing additive. Their optical absorption increases smoothly in the UV region without a maximum up to 200 nm (ε 200 = 1.3 × 10 4 M -1 cm -1 ). Pulse radiolysis is used to study the reduction of Co 2+; the subsequent formation of colloidal Co occurs via an autocatalytic mechanism. The electrochemical potentials of Co 2+ /Co 0 and Co + /Co 0 are determined to be −2.3 and −2.9 V, respectively.