Surface-Alloying during Pb Underpotential Deposition on Au

Abstract

Surface-alloys can form spontaneously during the deposition of one metal onto another, especially in systems with a large lattice mismatch that do not have bulk counterparts (so-called immiscible systems). The spontaneous surface alloying has been widely studied in Ultra High Vacuum (UHV) [1], but these materials have not been exploited much in practical applications. Surface-alloying in an electrochemical environment is an ideal route to the design of new functional materials. Electrochemically controlled surface-alloying would allow room-temperature conditions and better scalability making it more viable and versatile for practical applications.The focus of our study was the surface-alloying during underpotential deposition (UPD) of Pb on Au(111) [2,3]. As shown in Fig 1.a, the cyclic voltammetry (CV) of Pb UPD on Au (111) has asymmetric anodic and cathodic peaks (labelled in the order of their associated appearance). The most positive anodic peak A2 is often associated with the dealloying of Pb incorporated in the Au surface. The electrochemical measurements and the in-situ Scanning Tunnelling Microscopy (STM) characterisation during Pb UPD on Au(111) have shown that the Pb ML stripping is followed by a significant Au surface roughening [2]. During Pb ML formation and dissolution, the top-surface Au atoms get displaced, creating highly mobile Au ad-islands and vacancy islands [2]. The in-situ surface stress measurements have shown a tensile stress relaxation caused by the alloying/dealloying of Pb-Au during Pb ML formation and dissolution [2,4-5].In this study, we used two different electrochemical protocols to examine the surface alloying process: 1) repeated potential cycling within the UPD range and 2) polarisation at a fixed potential in the UPD region. By following the changes in electrochemical behaviour, we established the range of potentials and conditions at which the surface-alloys form. In addition to that, we investigated the structural and compositional changes by surface sensitive techniques such as X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS), Energy-Filtered Photoemission Electron Microscopy (EF-PEEM) and Work Function (WF) mapping.To study the 'dynamic' alloying, the potential was cycled repeatedly between 0.0 V and 0.5 V (vs Pb/Pb2+), as marked in Fig 1.a, over the potential range negative from the A2 anodic peak. The potential was cycled up to 400 times. After each 100th cycle, the potential was extended to the whole Pb UPD region to observe the changes in the cathodic and anodic peaks. For illustration, Fig 1.b and Fig 1.c show the changes of peaks A2 and A3 with the number of cycles. While both peaks show changes of the shape and total charge, we observe a significant shift of the peak A2 suggesting dramatic changes associated with the morphology of the surface and the amount of alloyed Pb in the top surface layer. The Pb-Au surface alloy composition, based on the charge under the A2 peak, increase from 17% up to 25% of Pb (after 400 cycles) as shown in Fig 1.d. Similar changes of the CV peaks have been observed after the polarisation at the fixed potential of 0.2 V (vs Pb/Pb2+) in the Pb UPD region for different times (15 min - 60 min).Furthermore, the Pb UPD on Au(111) has been studied in the presence of chloride. These results show gradual changes in positions and shapes of all characteristic peaks of the Pb UPD leading to complete disappearance of the A2 dealloying peak, which have not been reported before.