Extending surface Raman spectroscopy to transition metal surfaces for practical applications .1. Vibrational properties of thiocyanate and carbon monoxide adsorbed on electrochemically activated platinum surfaces

By using confocal microprobe Raman spectroscopy and a unique electrochemical pretreatment procedure for the Pt surface, we are able to extend the detailed surface Raman studies, for the first time, to bare Pt electrodes in a wide potential region (e.g. -1.0 to +1.4 V vs SCE) and to more general adsorbates such as SCN- and CO having small Raman scattering cross sections. Taking advantage of being capable of observing the adsorbate-metal vibrational bands in the low-frequency region over IR and SFG techniques, the surface Raman spectroscopy has demonstrated the virtues of yielding detailed information on the surface bending affected by surface coverage, coadsorbate, electrolyte ions, and electrode potential. The potential-dependent Raman spectra of SCN- reveal that the N-bound adsorbate is favored at the more negative potentials. The orientation conversion to the S-bound species occurs in the more positive region, depending on the SCN- concentration. In a solution of 10(-5) M NaSCN and 0.1 M NaClO4, a new C-N stretching band at ca. 2010 cm(-1) was assigned to the bridge-bound SCN- due to the low surface coverage. A preliminary study on CO irreversibly adsorbed on Pt, from -1.4 to 0.2 V in 0.1 M NaOH, infers that the interactions of CO with adjacent coadsorbed oxygen-containing species and with the surface are even more complex. At -0.8 V, the hydroxyl species starts to coadsorb with CO to the surface, then is partially oxidized at more positive potentials, and finally forms the oxidized sites at potentials positive of 0.0 V. In comparison with the roughened Ag, Cu, and Au electrodes, the dispersed Pt surface by electrochemical activation has shown better stability and reproducibility for the Raman measurements. This further provides a good reason to be optimistic that surface Raman spectroscopy will contribute on an increasingly broad frontier to the development of a true molecular-level probe of interfaces, particularly for practical applications. Finally, it is worth emphasizing that the present results could promote new efforts to sort out the surface selection rule and SERS mechanism(s).