X-ray Absorption Spectroscopy Characterization of Cu Underpotential Deposition on Au(111) and Organothiol-Self-Assembled-Monolayer-Modified Au(111) Electrodes from Sulfate Supporting Electrolyte

• Published in: Journal of physical chemistry. C 2009-07, Vol.113 (28), p.12260-12271
• Main Authors: Lee, Jonathan R. I, O’Malley, Rachel L, O’Connell, Timothy J, Vollmer, Antje, Rayment, Trevor
• Format: Article
• Language: English
• Subjects: C: Surfaces, Interfaces, Catalysis
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp8099412

Cu K-edge X-ray absorption spectroscopy (XAS) has been used for the in situ structural characterization of Cu monolayers prepared on both bare Au(111) and butanethiol self-assembled monolayer (BT-SAM)-modified Au(111) electrodes by underpotential deposition (UPD) from a sulfate supporting electrolyte. In conjunction with electrochemical measurements, the XAS studies enable assignment of the evolution in interfacial structure of both systems as a function of the applied electrochemical potential. The measurements of Cu adlayer formation on unmodified Au(111) from sulfate supporting electrolyte, an extensively studied and model UPD system, allow a reevaluation of XAS as an alternative or complementary technique to more established methods for the structural investigation of the electrified interface, such as X-ray diffraction (XRD) and surface X-ray scattering (SXS). The experiments reported in this paper indicate that recent technological advances enable XAS studies of UPD monolayers to be conducted on a time scale comparable to that of XRD and SXS. Meanwhile, the XAS data are consistent with the accepted model for the evolution in structure of the Cu adlayer under increasingly reducing conditions: following the formation of a commensurate (√3×√3)R30° (θsc = 0.67) adlayer in which the Cu adatoms occupy the 3-fold hollow sites, additional UPD results in a Cu-(1×1) monolayer structure. Theoretical modeling of the XAS data also allows a structural assignment for the coadsorbed anion overlayer and illustrates that the sulfate ions adopt a (√3×√3)R30° (θsc = 0.33) arrangement on the surface of both Cu adlayers. The XAS characterization of Cu UPD on BT-SAM-modified Au electrodes indicates that the Cu adatoms reside in face-centered cubic 3-fold hollow sites of an unreconstructed Au(111) surface. This is significant because it suggests that formation of the Cu UPD adlayer lifts, at least in part, the organothiol-SAM-induced reconstruction of the Au substrate surface. The local environment of the Cu adatoms presents the possibility that adlayer regions with a Cu-(1×1) structure form on the Au surface, which support a coadsorbed (√3×√3)R30° (θsc = 0.33) overlayer of BT sulfur atoms residing in the 3-fold hollow sites of the UPD monolayer.