The autocatalytic reduction of copper(II) by cobalt(II) in aqueous diethylenetriamine solutions studied by EQCM

Abstract

Calculations show that cobalt(II) complexes with diethylenetriamine ( dien) are rather strong reducing agents and are capable to reduce copper(II) ions to the metallic state and, therefore, can be used in electroless plating systems for the deposition of copper coatings. A thermodynamic analysis of equilibria in system Cu–Cu(II)–Co(II)–Co(III)-diethylenetriamine was carried out and the conditions of Cu(II) reduction to metal by Co(II) were predicted. Copper(II) reduction is supposed to occur in pH region from ca. 5 to ca. 9.5 (20 °C). The reduction process is predicted to be strongly dependent on the solution pH and ligand ( dien) concentration. The most negative difference of a redox potential (Δ E) between Cu(II)/Cu and Co(III)/Co(II) couples could reaches values of −200 mV. The autocatalytic copper(II) reduction by cobalt(II) in diethylenetriamine solutions (electroless copper plating) was studied by measuring the instantaneous copper deposition rate, open-circuit potential and electrochemical parameters of partial reactions. The Cu(II) reduction process depended largely on the solution’s pH and the anions presenting in the plating solution. A comparison of copper deposition under the same conditions using chloride, tetrafluoroborate, sulfate, acetate salts and corresponding acids (to adjust solution pH) showed that the process rate decreased in the following sequence: chloride≫tetrafluoroborate>sulfate>acetate. The process investigation by EQCM measurements provided data on the rate of partial electrochemical reactions both in the separate Cu(II) and Co(II), and the complete electroless plating solutions; these data indicated the sensitivity of the partial anodic reaction to solution pH, anion nature and to ligand concentration. Co(II)- dien complexes are very active in the copper(II) reduction and high process rates can be obtained.