An in situ STM study of cobalt electrodeposition on Au(111) in BMIBF4 ionic liquid

Abstract

The initial stage of Co electrodeposition on Au(111) from CoCl2 and Co(BF4)2 in a room-temperature ionic liquid of BMIBF4 is investigated by in situ STM. Preferential nucleation of Co at structure imperfections of the reconstructed Au(111) surface is observed for deposition from CoCl2 at a surprisingly negative potential (−2.05 V vs. Pt wire). Monoatomic-height Co clusters of 2–3 nm in diameter are formed, which develop into Co islands with size confinement of 3–4 nm after prolonged deposition. The large driving force required for Co deposition from CoCl2 is attributed to the molecular type of the salt in the ionic liquid together with the tip shielding effect. This is proved by the pronounced difference of Co deposition from Co(BF4)2, which takes place at a much less negative potential and proceeds in a 3D progressive nucleation and growth mode without preference in nucleation sites. Surface alloying accompanies the island formation, which is confirmed by pit generation upon stripping of the Co islands. The results are discussed in comparison with features of Co deposition in UHV and in aqueous solutions.