Nanoscale phenomena in surface electrochemistry: some insights from scanning tunneling microscopy and infrared spectroscopy

Examples of nanoscale phenomena at electrochemical interfaces, examined recently in our laboratory by means of in-situ scanning tunneling microscopy (STM) and infrared reflection-absorption spectroscopy (IRAS), are discussed with the overall objective of assessing some spheres of applicability of these approaches for exploring substrate and adlayer atomic-/molecular-level properties which propagate over nanoscale distances. While the direct spatial structural information obtainable by in-situ STM makes this technique of obvious importance for exploring nanoscale phenomena, the infrared probe offers not only chemical specificity, but also the fast (similar to 10(-11) s) spectral timescale enables fluxional adlayer structures to be probed. Four types of applications along these lines are considered. First, recent STM evidence for adsorbate-induced nanoscale restructuring on ordered monocrystalline electrodes is discussed, specifically for Pt(100) and Au(110) surfaces. The occurrence of restructuring with retention of the substrate unit cell (i.e. without reconstruction) is outlined. Apparent similarities in the long-range Br-induced restructuring observed on Au(110) with the adsorbate-induced reconstruction of (110) surfaces in UHV are also noted. Second, the likely occurrence of CO-induced reconstruction of Pd(110) electrodes, as probed by IRAS, is described as an example of the value of spectral comparisons between electrochemical and UHV systems for elucidating surface structure in the former environment. Third, the utility of dipole-dipole coupling analysis for elucidating the nanoscale spatial structure of both CO adlayers and metal substrates is discussed. The specific topics chosen, CO islands formed during adlayer electrooxidation and the microscopic nature of Pt/Ru alloy catalysts, illustrate the usefulness of such analyses in both ordered monocrystalline and polycrystalline surface environments. Lastly, applications of UHV-based IRAS measurements for exploring double-layer charge-solvent interactions are briefly outlined. The results uncover a long-range influence of ionic charge upon inner-layer solvent orientation and also clarify the roles of the solvent molecules in curtailing short-range ion-adlayer interactions and screening the otherwise-complex electrostatic-field effects upon adsorbate vibrational properties. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.