Well-defined ultrathin Pd films on Pt(111): electrochemical preparation and interfacial chemistry

摘要

Well-defined ultrathin films of palladium, with coverages ranging fromsubmonolayer, ΘPd = 0.5 monolayer (ML), to multilayer, ΘP d = 8 ML, wereelectrochemically deposited on Pt(111) using potentiostatic and potentiodynamicmethods. In both methods, between the coverage regimes studied, the growth of the Pdfilms follows the Stranski-Krastanov mechanism.The interfacial electrochemical properties associated with the film-to-bulktransition were characterized by conventional voltammetric techniques in combinationwith low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES).The voltammetric peaks associated with H-atom adsorption and desorption on terracesites indicate that the Pd electrodeposit starts to exhibit bulk-like properties at a coverageof 3 ML.Voltammetric cycling, in sulfuric acid solution, between the hydrogen evolutionand the double-layer regions, was found to exert minimal influence on the annealing(smoothening) of the electrodeposited Pd films. However, cycling within the samepotential region in the presence of bromide anions (at which Br- adsorption/Brdesorption takes place) smoothens the initially rough Pd films essentially as well ashigh-temperature annealing.The influence of chemisorbed bromine on the anodic dissolution of Pd was alsostudied; this was for comparison with previous work on the anodic dissolution of Pd, ininert electrolyte, catalyzed by chemisorbed iodine. The present studies indicated that asmall but measurable amount of bromine was desorbed along with dissolution of the Pdstep atoms; bromine at the Pd terrace behaved identically to iodine in that the coverageof iodine is maintained regardless of the amount or origin of the of anodically strippedPd.Atomically smooth, well-defined ultrathin Pd films were prepared by a constantpotential deposition (CPD) method followed by multiple potential cycles, in dilute Brsolution,within the double-layer region and reductive removal of Brads, by simpleemersion at a potential just before the hydrogen evolution reaction potential (EHER). Apreviously adapted method for the same purpose involved the chemisorption of iodineonto ultrathin PdCPD films, from dilute I- solution, followed by reductive desorption ofIads in iodide-free solution at pH 10 and at a potential just before EHER.