The electrodeposition of copper on Au(111) and on HOPG from the 66/34 mol% aluminium chloride/1-butyl-3-methylimidazolium chloride room temperature molten salt: an EC-STM study

Abstract

The electrodeposition of Cu on flame annealed Au(111) films and on highly ordered pyrolytic graphite (HOPG) obtained from the Lewis acid room temperature molten salt 1-butyl-3-methylimidazolium chloride/aluminium chloride (34/66 mol%) has been investigated by electrochemical STM. On Au(111) the visible deposition of Cu starts at + 450 mV s. Cu/Cu⁺ at the steps between the Au terraces, at + 350 mV an 8 × 8 superstructure is observed which closes to a Cu monolayer at + 250 mV where charge integration gives a value of 280 ± 30 μC cm⁻², which correlates with 1 monolayer of Cu on Au (roughness factor 1.3) for a one electron step. Between + 250 and + 10 mV, Cu clusters with an average height of 3–4 monolayers of Cu grow both at the edges and on the Cu monolayer, at + 50 mV a second Cu monolayer seems to be closed. A surface alloy between Cu and Au is observed, overpotential deposition (OPD) follows the Stranski–Krastanov growth mode. The defects left on Au after dissolving the Cu heal on a time scale of 2 h. Upon oxidation of the Cu there are still Cu clusters at + 300 mV with heights between 2–3 monolayers. On HOPG, only OPD of Cu is observed. It starts at an overvoltage of 100 mV and yields small Cu clusters that grow preferentially on the basal planes of HOPG. At low overvoltages the growth is described by the model of instantaneous nucleation, at higher overvoltages the nucleation changes from progressive at a limited number of active sites to progressive.