Over- and underpotential deposition of copper from a deep eutectic solvent: Pt(1 1 1) single crystal versus polycrystalline Pt substrates

Abstract

• Deposition/dissolution of Cu on Pt from ethaline occur in one-electron reactions. • A full (1x1) monolayer of Cu adatoms is formed on Pt(111) in ethaline. • No voltammetric signals of Cu UPD were observed for Pt(poly) in ethaline. • Surface structure determines the mechanism and rate of Cu OPD at the initial stages. • Cu OPD on Pt(111) and Pt(poly) in ethaline occurs via 2D and 3D growth, respectively. Deep eutectic solvents (DESs) attract ever greater interest as nonaqueous media for the electrodeposition of metals and alloys. This work investigates the initial stages of Cu electrodeposition from DES representing a mixture of choline chloride and ethylene glycol (ChCl:EG) with a (1:2) molar ratio that contains CuCl or CuCl 2 salts. These studies were performed by means of cyclic voltammetry and scanning probe microscopy on single-crystalline Pt(111) and polycrystalline Pt(poly) electrodes. Additional experiments on Cu deposition were carried out using an aqueous Cl - -rich solution (3 M NaCl + 0.01 M CuCl). The process of Cu underpotential deposition (UPD) on the surface of Pt(111) in DES is observed for the first time. It is proposed that a complete (1x1) monolayer of Cu adatoms is formed at potentials more positive than the onset of overpotential deposition (OPD). The comparison of results for Pt(111) and Pt(poly) in ChCl:EG (1:2) and in the aqueous 3 M NaCl solution confirms that the surface morphology and (co)adsorption of solution components play an important role in the Cu UPD process. In DES, as well as in the aqueous solutions containing CuCl, Cu OPD occurs via the reduction of Cu(I) ions. The Cu(I)/Cu(II) redox transition was observed at significantly more positive potentials. We conclude on the basis of voltammetry and in situ scanning tunneling microscopy (STM) data that the mechanism of Cu electrodeposition in DES is determined by the surface structure. STM data demonstrate the layer-by-layer growth of a two-dimensional Cu deposit on Pt(111), while the resulting Cu deposit on Pt(poly) is characterized by a grain-like morphology. We also demonstrate that the mechanism of Cu OPD affects the initial deposition rate under conditions of diffusion limitations.