Unprecedented Structural Sensitivity toward Average Terrace Width: Nafion Adsorption at Pt{hkl} Electrodes

Ultrathin Nafion overlayers ranging from 0 to 3.3 nm in thickness have been deposited on platinum single crystal electrodes in order to study their voltammetric behavior as a function of both Nafion surface coverage and platinum surface step density. Unusual structural sensitivity of Nafion-induced voltammetric peaks ascribable to Nafion interactions with step sites is observed as a function of average terrace width. For stepped surfaces containing both {111} and {100} terraces, average terrace widths of 2–3 atoms give rise to the most intense Nafion-step electrosorption peaks. Rapid quenching of the Nafion-step voltammetric peaks is observed as the average terrace width is gradually increased until for >5 atom wide terraces all such electrochemical features are completely attenuated. This rapid diminution of the step-Nafion voltammetric peak, within just three atoms of terrace width, is unprecedented, and a parallel with the “splitting” of hydrogen underpotential deposition (H UPD ) step peaks in perchloric acid aqueous electrolyte for closely spaced steps is noted. On the basis of these measurements, it is suggested that Nafion is a molecular probe of adsorbed OH species on Pt electrodes.