DFT and In-Situ Spectroelectrochemical Study of the Adsorption of Fluoroacetate Anions at Gold Electrodes

The adsorption of mono-, di-, and trifluoroacetate anions at gold electrodes is studied in perchloric acid solutions by cyclic voltammetry and in situ infrared and Raman spectroscopies. Surface enhanced infrared reflection absorption spectroscopy experiments under attenuated total reflection conditions (ATR-SEIRAS) and surface enhanced Raman scattering (SERS) experiments were carried out with gold thin-film electrodes sputtered on silicon and on a polycrystalline gold substrate, respectively. Theoretical harmonic vibrational frequencies of trifluoroacetate species adsorbed with different geometries on Au clusters with (111), (100), and (110) orientations, as well as IR and Raman intensities, have been obtained from B3LYP/LANL2DZ, 6-31+G(d) calculations. This theoretical study was extended to di- and monofluoroacetate anions adsorbed on Au(111) clusters. The theoretical and experimental results confirm the bidentate bonding of fluoroacetate anions with the OCO plane perpendicular to the metal surface irrespective of the surface crystallographic orientation. No significant effect of either the surface crystallographic orientation or the total charge of the metal cluster-adsorbate on the vibrational frequencies of adsorbed fluoroacetate species can be deduced from the DFT calculations. The ATR-SEIRA spectra show the existence of nonhydrogen-bonded water molecules coexisting with adsorbed fluoroacetate anions, in an extent that depends on both the fluoroacetate coverage and the number of fluorine atoms in the fluoroacetate anion. Step-scan ATR-SEIRAS measurements carried out with the gold thin-film electrodes have allowed for the monitoring of the time-dependent behavior of trifluoroacetate adsorption in potential step experiments, showing a Langmuir-type adsorption kinetics.