Oscillatory Electrodeposition and Formation of Alloy Multilayers Induced by a Phase Transition of Adsorbed Surfactants at the Electrode Surface

Abstract

The electrodeposition reaction in an acidic solution containing Cu²⁺ and Sn²⁺ shows, in the presence of a cationic surfactant, a negative differential resistance (NDR) and a current oscillation in a potential region where SnCu alloy is deposited. Depth profiles obtained by Auger electron spectroscopy (AES) have indicated that alloy films deposited during the oscillation have a structure of nano-period multilayers composed of alloys of different compositions. Detailed investigations have revealed that the NDR arises from adsorption of a cationic surfactant (acting as an inhibitor for diffusion of Cu²⁺ and Sn²⁺ ions) on the electrode (alloy) surface and the oscillation comes from coupling of the NDR with the ohmic drop in the electrolyte. A notable point is that the NDR appears in a very narrow region of the potential (of about 5 mV or less). This fact strongly suggests that the NDR (and the oscillation) arise from a sudden structural change in an adsorbed surfactant layer such as a phase transition from a gaseous adsorbed layer to a condensed liquid-like one. This conclusion was supported by the experiments on effects of the concentration and the chain length of the surfactant on the NDR.