Pt Monolayer Creation on a Au Surface via an Underpotentially Deposited Cu Route

Abstract

Kinetics of platinum monolayer formation via the redox replacement of underpotentially deposited copper on a Au(111) electrode surface by a platinum tetrachloride complex (PtCl42-) was studied by voltammetry, X-ray photoelectron spectroscopy (XPS), electrochemical quartz crystal microbalance (QCM), energy-dispersive spectroscopy (EDS), and scanning tunneling microscopy (STM). The Pt 4f(7/2) peak intensities of XPS and voltammetric responses of H adsorption/desorption increased as the incubation time increased in PtCl42- solutions. The resonance frequency of the Cu/Au QCM in 0.05 M H2SO4 solution was observed to quickly decrease within 1 min when it contacted with PtCl42- solutions, and then it remained unchanged for hours. EDS data showed that Cu was not found at high PtCl42- solutions. Ex situ STM images revealed a largely uncovered Au(111) surface with mountain like Pt nanoparticles at a replacement time of 10 min, and mostly covered Au(111) one with plain-like Pt nanoparticles of 4 h. The average size of Pt particles decreased approximately logarithmically as a function of time, whereas the number of Pt particles increased. Thus, after replacement, Pt atoms were dynamic in hours, resulting in a flattened Pt monolayer. A mechanism that includes reverse Ostwald ripening is provided to rationalize the observations.