NATURE OF SURFACE BONDING ON VOLTAMMETRICALLY OXIDIZED NOBLE-METALS IN AQUEOUS-MEDIA AS PROBED BY REAL-TIME SURFACE-ENHANCED RAMAN-SPECTROSCOPY

The nature of the metal-oxygen bonding formed during the initial phase of voltammetric electrooxidation of platinum, rhodium, ruthenium, and gold surfaces in aqueous media has been examined by means of surface-enhanced Raman spectroscopy (SERS). The first three surfaces were formed by electrodeposition as ultrathin (ca. three monolayer) films on a SERS-active gold substrate, enabling intense Raman spectra to be obtained for oxidation of the transition-metal overlayers. Sequences of SER spectra were typically obtained in real time during cyclic potential excursions in acidic (0.1 M HClO4) and basic (0.1 M KOH) media, enabling the evolution of the surface vibrational properties to be correlated with the simultaneous voltammetric (current-potential) response. Several Raman bands are evident upon surface electrooxidation within the frequency range ca.250-850 cm-1, corresponding to metal-oxygen vibrational modes. On gold, platinum, and rhodium, abroad vibrational band at 500-600 cm-1 appears close to the onset of irreversible surface oxidation as discerned voltammetrically and is removed upon oxide reduction during the subsequent reverse voltammetric sweep. Together with this feature, ascribed to metal-oxygen stretching within a place-exchanged oxide/hydroxide film, a narrower band at ca. 300 cm-1 is observed on platinum and rhodium in acid, assigned to a bending vibration involving terminally bound oxygen atoms. On gold in base, but not on platinum and rhodium, a separate feature at 420-490 cm-1 is also observed at lower potentials, ascribed to specifically adsorbed hydroxide ions, i.e., not involving metal-oxygen place exchange. Ruthenium electrooxidation yielded more complex spectral behavior, featuring the potential-dependent appearance of several bands between 300 and 800 cm-1, attributed to the formation of ruthenium oxides of differing oxidation state. A distinction between M-O and M-OH vibrations was undertaken by means of deuterium solvent isotope shifts. The tendency to form predominantly surface metal oxides (M-0) rather than hydroxides (M-OH) increases in the sequence Pt < Au < Rh < Rh and to a greater extent in acidic solution. These vibrational features are compared with related spectral observations for metal surfaces dosed with oxygen in gas-phase (and vacuum) environments and for bulk-phase metal oxides. The similarities and differences in the oxygen surface bonding are assessed.