Chloride-Induced Surface States In Cu(110)/Liquid Interfaces

While surface states (SSs) have been widely exploited under ultrahigh vacuum conditions, their appearance and role in metal-electrolyte interfaces are still a controversial debate. The existence of SSs, similar to those shown here, permits control and tunability of electronic properties of functional metal-electrolyte interfaces. Resonant excitations among them could enhance, for example, photocatalytic reactions or permit further investigations of the surface chemistry of such reactions with surface resonance Raman spectroscopy. On the one hand, SSs and other properties are readily adjustable via an applied electrical potential, that is, by promoting changes in the chemical potential favoring adsorption of ionic species. On the other hand, the presence of the electrolyte induces additional scattering and screening effects, so that the electron charge distributions can differ considerably in the presence of high electric fields as compared to the respective surfaces in vacuum. Hence, a straightforward comparison is challenging. In this work, we report on a systematic study, by means of electrochemical impedance spectroscopy (EIS) jointly with in situ reflectance anisotropy spectroscopy (RAS), which aimed to assess the evolution of surface properties and SSs occurring at Cu(110) in contact with an HCl solution. Thereafter, by modeling the RAS response and in comparison with electrochemical scanning tunneling microscopy measurements, specific surface structures have been identified and ascribed to the optical response. In a specific potential range, three additional resonances are detected in RAS that can be explained by two-dimensional confined SSs. This work renders both in situ RAS and EIS as useful tools to study and tune SSs in a systematic way by applied electrical potentials, thereby stabilizing thermodynamically preferred surface structures.