This title appears in the Scientific Report : 2000 

Summenfrequenz-Schwingungsspektroskopie an der Platin/Flüssigkeit-Grenzfläche
Dederichs, F. (Corresponding author)
Institut für Grenzflächenforschung und Vakuumphysik; IGV
Jülich Forschungszentrum, Zentralbibliothek 2000
Aachen, Techn. Hochsch., Diss., 2000
Dissertation / PhD Thesis
Struktur und Dynamik der Grenzflächen
Berichte des Forschungszentrums Jülich 3758
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The last years have witnessed a tremendous advance in understanding electrochemical interfaces. These insights have been mode possible by the impmvement of existing and the invention of new experimental techniques, respectively . One of these methods is optical sumfrequency generation (SFG), which in this work is applied to the investigation of molecular vibrations at electrolyte/platinum interfaces . As a second-order nonlinear optical effect, SFG is due to its inherent interface sensitivity ideally suited for the spectroscopy of electrochemical interfaces. In order to lay the foundations for the experimental applications we start with a theoretical description of sum-frequency generation and discuss the experimental setup utilized in this work. The first experimental chapter deals with the chemisorption of carbon monoxide (CO) onto (111) and (110) platinum (Pt) single-crystal faces in a CO-saturated 0 .1 M HC1O4 aqueous electrolyte . Whereas CO adsorbs on Pt(110) only on terminal sites as indicated by a single vibrational band around 2075 cmil in our sum-frequency spectra, we observe different adsorption geometries on Pt(111) . For potentials below 0 .37 V/RHE CO adsorbs on terminal and hollow sites of the (111) face while for higher potentials up to electro-oxidation of the carbon monoxide at about 0 .55 V/RHE it occupies terminal and Bridge sites, respectively . We discuss in detail the influence of the electrochemical thin layer electrolyte in our spectrochernical cell on the electro-oxidation of CO. We investigate the adsorption of cyanide (CN) on Pt(111) surfaces by dissociation of acetonitrile (CH 3CN) molecules from the vapor phase above liquids containing acetonitrile, followed by irmnersion of the sample into the liquid . Using optical sum-frequency generation and cyclic voltammetry we can identify the adsorbed Spezies unambiguously as cyanide by the characteristic potential dependencies of the C-N stretching vibration frequencies and the voltammetric profile in a (0.1 M HC1O4 + 25 M CH3CN) aqueous electrolyte . In neat acetonitrile we observe two adsorbed states of CN with vibrational bands at 1861 uni (hollow site) and 2073 cm-' (on-top site), distinctly below and above that of the isolated molecule, demonstrating a covalent CN-platinum bond . We discuss duster calculations which show that the weakening and strengthening of the C-N bond at the hollow and on-top sites is due to a surface-induced depletion of the Bonding In and antibonding 4o orbitals, respectively. In the last poft of this work we present a sum-frequency study of the electrochemical interface forrned by (111), (110) and (100) platinum faces, respectively, with aqueous electrolytes containing 0.1 M HCIO4. Despite of strong IR absorption due to water molecules in the bulk electrolyte, interfacial water vibrations are observed in our SFG spectra because their bands are dramatically broadened towards lower frequencies, containing components which are redshifted as much as 1000 cri ' . We conclude that these frequency shifts are caused by the strong, inhomogeneous eiearie field at the electrochemical interface, which weakens the O- -bonds . A detailed analysis of our sum-frequency spectra, using an inhomogeneous broadened model function and taking into account the linear-optical properties of the three-layer-system CaF 2 (laser window)/aqueous electrolyte/Pt electrode, allows us to deduce the potential dependence of the SFG amplitude of the O-H-vibrations. Since this amplitude of the water vibrations correlates with the charge density on the platinum electrode surface, we are able to estimate the potential of zero charge (PZC) of the Pt electrodes . For Pt(111) in 0 .1 M HCIO4 we obtain a PZC value of 0 .86 +20-.0128 V/RHE.