Temperature-Dependence of the Under-Potential Deposition of Ag on Pt(111) in H2SO4

The under-potential deposition (UPD) of metallic and semiconducting species on noble-metal substrates has been studied over the past two decades using numerous electrochemical and spectroscopic techniques as well as scanning probes. Among various substrate-adsorbate systems, the electrodeposition of silver on single-crystal and polycrystalline Pt electrodes has attracted a lot of attention owing to its simplicity. Specifically, the UPD of Ag is a single-electron process, the substrate (Pt) and adsorbate (Ag) have the same cubic face centered (cfc) crystallographic structure, and possess similar lattice constants (aAg = 4.09 A and aPt = 3.92 A, respectively). The difference of the lattice constant of Ag and Pt is only 4.3%, yet the difference of the volumes of the Ag and Pt crystallographic units is 13.6% (aAg 3 = 68.42 A3 and aPt 3 = 60.24 A3). Thus, for a purely crystallographic reason, the overlayer of AgUPD on the Pt( 111) substrate is always compressed. The UPD Ag on Pt(hkl) electrodes from various aqueous electrolytes has been studied using a variety of techniques such as cyclic-voltammetry (CV), in-situ scanning tunnelling microscopy (STM), atomic force microscopy (AFM), and surface X-ray scattering (SXS)1–20. The first investigation oi the UPD Ag on an ordered Pt(111) electrode as well as on other low-index faces of Pt was reported by El Omar et al.2. They observed that the deposition of AgUPD resulted in a two-layer deposit, but did not report any surface structure. A few years later, Franc et al. using angular distribution Auger microscopy (ADAM) concluded that the AgUPD overlayer was commensurate with the Pt(111) substrate11, but this structural assignment later generated a great deal of controversy. Subsequently, it was observed that in order to accomplish a complete suppression of the under-potential deposition of H (UPD H) an overlayer of ca. 1.5 equivalent monolayers (ML) of Ag was required 12.