In-situ electrochemical SPR study of gold surface smoothing by repetitive cathodic deposition and anodic dissolution of copper in an ionic liquid

We performed in-situ analysis of the initial process of copper electrodeposition in an ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (C4mimTFSA), using electrochemical surface plasmon resonance (ESPR), where the SPR resonance angle (Δθ) was recorded simultaneously with cyclic voltammograms (CVs) at the gold interface of C4mimTFSA. Δθ shifted at the potentials where faradaic current appears in the CVs, indicating that SPR probes the redox processes of copper between Cu2+, Cu+, and Cu metal. In stark contrast to the fact that the CVs are not cycle dependent, the amount of the Δθ shift due to the copper electrodeposition decreased with increasing the CV cycle. A model reflectivity analysis using the Bruggeman effective medium approximation predicted that this decrease in Δθ is due to the decrease in the roughness of the gold surface, and it was actually confirmed by ex-situ atomic force microscopy observation. Since the dissolution of gold was found to be negligible from the atomic absorption measurement of IL after the ESPR measurements, the roughness decrease of the gold surface is likely due to the surface diffusion of gold atoms promoted during copper electrodeposition and dissolution processes. It was found that ESPR can track the surface roughness change of the gold electrode in an in-situ manner in the order of Å. •Electrodeposition of Cu in an ionic liquid has been studied by electrochemical SPR.•ESPR is sensitive to the electrode surface roughness on the order of angstrom.•The electrodeposition of Cu induces the decrease in the electrode surface roughness.•ESPR is proven to be an in-situ surface roughness sensor during electrodeposition.