XPS and ARXPS study of silver underpotential deposition on platinum in acid solution

The underpotential deposition (upd) of silver on polycrystalline platinum electrodes has been studied using electrochemical techniques and X-ray photoelectron spectroscopy (XPS). Under the electrochemical experimental conditions used for deposition, the cathodic (qc) voltammetric charge density is around 200 μC m−2 which is equivalent to ∼1.2 ML of deposited silver. XPS and angle-resolved X-ray photoelectron spectroscopy (ARXPS) were used to determine the chemical composition of the deposited films. Two different approaches have been used to extract the concentration depth profiles from ARXPS measurements: (a) a simple parametric model with some hypotheses about the form of the depth profile and (b) a general algorithm that uses regularization and singular value decomposition (SVD) techniques. The structure and composition of the films is rather complex. The upd deposition of silver yields films with concentration gradients that can be modeled by a multilayer structure involving silver islands. Starting from the outer surface, the predominant species are C, O+S+Ag, and the platinum substrate.