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

Abstract

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°( θ sc ) 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°(θ sc ) 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°(θ sc ) 0.33) overlayer of BT sulfur atoms residing in the 3-fold hollow sites of the UPD monolayer.

X-ray absorption spectroscopy characterization of Zn underpotential deposition on Au(111) from phosphate supporting electrolyte

“…An improvement in fitting is accompanied by a reduction in R and, as a consequence, structural refinement using EXCURV98 was conducted with the intent of minimizing this parameter, while also ensuring that the model was chemically and physically viable. The model development was carried out according to established protocols that are described in greater detail in the literature [13,41]: the theoretical structure is refined to optimize R after the inclusion of each 'shell' of symmetry-related atoms, which are added sequentially in order of proximity to the absorbing atom (shells at comparable distances were added simultaneously).…”

“…A thorough description of the composition and operation of an equivalent cell during electrochemical and XAS studies of UPD adlayers is available in the literature [13]. We note, however, that the cell allows for exposure of the Au(111) electrodes to bulk electrolyte during electrochemical measurements and UPD adlayer deposition.…”

“…A detailed description of the beamline and storage ring properties during the course of this study are available in the literature [13]. The energy of the incident X-ray photons was calibrated to the maximum in the first derivative of the XAS spectrum obtained from the 5µm zinc metal foil.…”

“…Therefore, XAS represents a complementary or alternative tool to techniques that require longer range order, which include X-ray diffraction (XRD)/surface X-ray scattering (SXS). One can also conduct in situ XAS measurements of electrode materials with the use of synchrotron radiation [7,8,9], including in situ structural studies of the electrode surface [10,11,12,13]. This is an important capability because it eliminates the potential for rearrangement of the interfacial structure upon removal of the working electrode from the electrolyte and, by extension, electrochemical control.…”

“…Despite the apparent strengths of XAS for investigation of the electrode surface structure, use of the technique to study UPD systems was largely discontinued after the late 1990's because of the extended data collection periods required to obtain high quality spectra. Significantly, a recent study of electrodeposited Cu monolayers on Au from sulfate buffer illustrates that improvements in flux and detection efficiency at synchrotron sources has mitigated this problem for UPD adlayers with high surface coverages (surface coverage, θ sc , ≥0.67) [13]. Zn UPD on Au(111) from phosphate buffer is an excellent system with which to build upon this preceding Cu(UPD)/Au(111) study for several reasons: (1) voltammetric experiments indicate that the Zn adlayer has a considerably reduced surface coverage (θ sc ∼0.33) [14,15,16] with respect to Cu UPD and, therefore, provides an opportunity to demonstrate the suitability of XAS for the investigation of low concentration surface species; (2) the specific geometrical arrangement of the Zn adlayer and co-adsorbed anion overlayer remains unresolved, despite prior research conducted for the phosphate/Zn(UPD)/Au(111) system using a range of characterization techniques; (3) the mechanisms of Zn electrodeposition and structure of the resulting deposit are of relevance for important industrial applications, particularly in understanding the formation and properties of anti-corrosion galvanic coatings.…”