Microscopic characterization of electrochemical interfaces

Surface-enhanced Raman spectroscopy (SERS) and infrared reflection-absorption spectroscopy (IRAS) have been employed for in-situ probing molecular interaction and structure at electrochemical interfaces. The first technique was applied to Pt-group transition metal surfaces by an “overlayer” SERS strategy involving electrodeposition as ultrathin films on SERS-active gold, thereby entirely chemically modifying the substrate yet largely retaining the giant Raman enhancement. A new deposition procedure utilizing constant-current, rather than constant-potential, deposition was developed, yielding pinhole-free films. This improvement opens up important opportunities for studying interfacial chemistry on transition-metal surfaces. Rich SER spectra for chemisorbed benzene and monosubstituted benzenes, specifically toluene and benzonitrile, were observed in aqueous solutions. The spectral fingerprints indicate that benzene and toluene adsorb “flat”, while benzonitrile binds via the nitrile group with the benzene ring pendant. Significant Raman enhancement was also found on these transition-metal films deposited on SERS-inactive substrates, including carbon and unroughened gold, utilizing either green or red excitation, indicating transition metals can also support SERS. The overlayer-SERS strategy was also utilized for characterizing lattice vibrations (phonons) for cadmium chalcogenide ultrathin films and superlattices formed by electrochemical atomic-layer epitaxy. The band broadening and frequency redshifts with decreasing film thickness for these semiconductor films signal the occurrence of quantum confinement effects. The large phonon softening observed for CdS in CdS/CdSe superlattices provides strong evidence for tensile strain in the CdS layers. Intermolecular interactions at ordered electrochemical interfaces were investigated by means of IRAS, utilizing carbon monoxide and nitric oxide adsorbed on Pt-group, single crystal electrodes as model systems. Detailed comparison of the vibrational fingerprints with corresponding data obtained in ultrahigh vacuum shed light on the influence of the double-layer solvation on the adlayer structure