MECHANISM OF FORMATION OF COPPER THIOCYANATE ON THE COPPER ANODE

A primary (barrier) film forms on the copper anode at an underpotentia l relative to the secondary (porous) film and exhibits a pre-peak or s houlder at -0.19 V (vs. SHE), for a 0.1 mol dm(-3) KSCN electrolyte. T he anodic peak current for the primary film is linearly dependent upon the sweep rate, while potential steps into the primary film region pr oduce monotonic current decays with j = kt(-1), consistent with a plac e-exchange mechanism for the initial formation of the barrier film. Up on stirring, the size of the primary film peak decreases as hydrogen e volution competes with the film-formation process. A porous CuSCN film begins to form at potentials 50-100 mV more positive than the barrier film, producing a larger peak at 0.01 V (0.1 mol dm(-3) KSCN), equiva lent to a film of 15-20 monolayers, with thicker films formed in more concentrated thiocyanate solutions. The anodic peak current for the po rous film and the potential change to reach the peak are both proporti onal to the square root of the sweep rate, which is consistent with a model for film growth controlled by the resistance across the underlyi ng barrier film. Raman spectroscopy reveals at least two distinct S-bo nded CuSCN species, one of which is lost upon partial reduction of the film, and is due to the barrier film. The remaining species has the s ame Raman spectrum as crystalline CuSCN.