Potentiodynamic electrochemical impedance spectroscopy of lead upd on polycrystalline gold and on selenium atomic underlayer

Pb upd on polycrystalline Au and on Au coated with Se atomic layer was investigated by potentiodynamic electrochemical impedance spectroscopy. Faradaic and double layer responses have disclosed two distinct stages in Pb upd on Au: a partly irreversible stage, attributed to formation and growth of Pb 2D islands, and a reversible phase transition in the final stage of a monolayer deposition. The completion of a continuous monolayer formation in the potential scan was signalised by a sharp minimum in double layer pseudocapacitance Q _dl. Pb²⁺ reduction, which was monitored concurrently by parameters of Faradaic response, continued shortly after the Q_dl minimum and showed sharp maxima of adsorption capacitance and inverse Warburg constant at 40 mV below Q_dl minimum. This was explained by surface free energy minimisation that forced continuous atomic layer formation with inclusion of some lead cations into Pb monolayer. The two-stage Pb upd transformed into a single-stage strongly irreversible upd as a result of Se atomic underlayer deposition on Au.